The Nature and Timing of Orogenic Activity in the Caledonian Rocks of the British Isles (Geological Society Memoir No. 9)

The Nature and Timing of Orogenic Activity in the Caledonian Rocks of the British Isles

The Nature and Timing of Orogenic Activity in the Caledonian Rocks of the British Isles EDITED

BY

A. L. H A R R I S University of Liverpool

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Memoir Number 9 1985 Published for The Geological Society by Blackwell Scientific Publications OXFORD BOSTON

LONDON PALO ALTO

EDINBURGH MELBOURNE

Published by Blackwell Scientific Publications Editorial offices: Osney Mead, Oxford, OX2 0EL 8 John Street, London, WC1N 2ES 23 Ainslie Place, Edinburgh, EH3 6AJ 52 Beacon Street, Boston, Massachusetts 02108, USA 744 Cowper Street, Palo Alto, California 94301, USA 107 Barry Street, Carlton, Victoria 3053, Australia

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First published 1985

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The Nature and timing of orogenic activity in the Caledonian rocks of the British Isles.-(Memoir/Geological Society, ISSN 0435-4052; no. 9) 1. Geology, Stratigraphic--Paleozoic 2. Geology--Great Britain I. Harris, A . L . II. Series 551.7'2 QE654 ISBN 0-632-01298-6

Contents

Preface, vii Caledonian igneous rocks of Britain and Ireland, 1 G. C. Brown, E. H. Francis, P. Kennan & C. J. Stillman

Introduction and tectonic setting, 1 Volcanic rocks, 2 Plutonic rocks, 5 Summary and conclusion, 8 References, 9

Time and deformation in the Caledonide Orogen of Britain and Ireland, 17 D. PoweIl & W. E. A. Phillips

The NW Foreland, 17 The Northern Highland zone, 18 The Dalradian zone, 20 The Midland Valley zone, 24 The Southern Upland zone, 27 Longford-Down zone, 27 The Lake District zone, 28 The Leinster zone, 31 The Welsh zone, 33 Southwest England zone, 35 Summary and conclusion, 35 References, 36

Grade and time of metamorphism in the Caledonide Orogen of Britain and Ireland, 41 D. J. Fettes, C. B. Long, R. E. Bevins, M. D. Max, G. J. H. Oliver, T. J. Primmer & L. J. Thomas

Foreland to the Caledonides, 41 Orthotectonic zone of the Caledonides, 41 Highland Boundary Complex, 46 Paratectonic zone of the Caledonides, 46 Hercynides, 50 Granites and their contact effects, 51 References, 51

Plates, inside front and back cover

Preface

The maps (Plates 1-3) and the explanatory text which make up this memoir form an important part of the United Kingdom and Ireland contribution to International Correlation Programme Project 27 'The Caledonide Orogen'. Project 27 which was initiated by Professor B. A. Sturt (Bergen) had as an early objective the production of maps which would aid correlation throughout the Caledonide/ Appalachian orogen. Studies of the orogen have involved Sedimentation & Stratigraphy, Faunas & Floras Volcanism, Plutonism, Metamorphism, Deformation and Cover-Basement relationships. Plates 1-3 of this memoir represent the UK and Ireland work on Plutonism, Volcanism, Metamorphism and Deformation. The different aspects of the orogen have been the responsibility of international working groups, who have determined internationally acceptable symbolisation and colours; Plates 1-3 conform to the schemes agreed.

The Appalachian orogenic zone of North America incorporates Alleghanian rocks. These are broadly coeval with the Hercynian rocks of the British Isles, and therefore are treated on the maps and in the text which comprise this memoir. The authors of each section of the memoir acknowledge the help they have received from many workers in Britain and Ireland, and the names of those who have helped are listed at the end of each section. I for my part am glad to acknowledge the help and encouragement received from Richard Hewish and Colin Earle of E.M.R. who produced the plates and from John Robson and the staff of Blackwells who are publishing the memoir for the Society.

A. L. Harris

Caledonian igneous rocks of Britain and Ireland G. C. Brown, E. H. Francis, P. Kennan & C. J. Stillman

Introduction and tectonic setting The British sector of the Caledonide-Appalachian fold belt, which bridges the Iapetus plate closure zone, has a range of plutonic, volcanic and volcaniclastic lithologies with characteristic space-time variations. The map of Caledonian igneous rocks in the British Isles (Plate 1) is compiled from many sources, some of which predate the use of methods of classification which are based largely on chemical data. In such cases, whilst extensive re-mapping continues, the compilers have had to resort to the collection and analysis of representative samples from the areas in question, the collection sites being based on existing maps. Thus, whilst the references to the mapped areas indicate specific publications, the chemical data on which the coding is based are commonly derived from compilations such as Stillman & Williams (1979) and Stillman & Francis (1979) for volcanic rocks and Sutherland & Pankhurst (1982), Le Bas (1982), Hall (1972) and Brown et al. (1981) for plutonic rocks. The coding system, which is explained fully in the Plate 1 legend, is that accepted by the Plutonics-Volcanics International Working Group of IGCP Project 27 and is intended to provide a shorthand description by which the user of the map may ascertain as much information as possible about the igneous rocks of each locality, many of which cover too small an area on the 1:2 million scale of the map to be reconizable on any conventional system of colour and symbol designation. Given the small scale of Plate 1 the chemical symbols portrayed are a compromise in some cases, and cannot do full justice to the wide petrochemical diversity of certain small extrusive complexes or intrusive centres. Limitations were also imposed by a rigid colour scheme denoting age and whilst, for some igneous bodies, high precision radiometric data are unambiguous, other suites have ages falling across the systematic colour boundaries, and yet others are either poorly dated or have attracted no radiometric work. All R b - S r dates have been adjusted, where necessary, to accord with the S t e i g e r - Jager (1977) decay constant as recommended by the IUGS Subcommission for Geochronology, viz. (,,l Rb = 1.42 • 10 11 a-l). Plate 1 does not indicate the more conjectural interpretations used in most modern plate-tectonic magmatic models, but reference to such interpretations may be gathered from the appropriate publications (notably papers such as Stillman & Williams (1979), Ryan et al. (1980), Stillman & Francis (1979), Max & Long (1979) and Fitton et al. (1982)). A feature common to all such interpretations is the relationship between Caledonian magmatism and the history of the Iapetus ocean. However, there is no conclusive indication on the map of the suture which is believed to represent the junction between the leading edges of the two

continental plates that underwent accretion, both sedimentary and igneous, whilst being brought together by the closure of the ocean throughout the Lower Palaeozoic (Phillips et al. 1976). A wide variety of evidence indicates that suturing took place in late Ordovician times approximately through southern Scotland and Central Ireland (Leggett et al. 1979). The closure is believed to have been effected by oceanic lithosphere subduction both northwards and southwards and to have been accompanied and followed by substantial and repeated lateral transgressive movement, all of which has resulted in the almost total loss of oceanic crust except for the Ballantrae ophiolite complex of southern Scotland. No unequivocal ophiolite fragments have yet been recognized in Ireland though recent studies suggest that some may exist in Tyrone and in western Ireland (Ryan et al. 1983). The proposed trace of the Iapetus suture is largely concealed beneath the Lower Carboniferous of the Irish midlands, yet there is an increasing body of evidence, both structural and geophysical, to confirm its existence. Its position, running northeastward across Ireland from the Shannon estuary to the east coast at Clogher Head some 55 km north of Dublin, can hardly now be regarded as conjectural. Similarly, geophysical, structural and sedimentological data indicate that the suture passes south of the Southern Uplands of Scotland from the Solway Firth to the coast near Berwick. The two plates display different Proterozoic and Phanerozoic histories up to the late Silurian-early Devonian structural and plutonic events which essentially mark the end of Caledonian activity, and this difference is mirrored in the volcanic rocks. To the north of the suture, the Orthotectonic Caledonides, or Laurentian Plate, is underlain by Archaean-Proterozoic basement (Hamilton et al. 1979) which occurs as refractory granulites in the extreme northwest of Scotland. Seismic evidence (e.g. Bamford et al. 1978) indicates that this basement continues beneath Moine-Dalradian cover, south to the Midland Valley and to Central Ireland where granulite xenoliths in Carboniferous volcanic vents (Upton et al. 1983, Strogen 1974) also testify to Proterozoic basement at depth. In contrast, there is no evidence of continental basement older than c. 800 Ma (Hampton & Taylor 1983) beneath the southern Paratectonic Caledonides or Celtic Plate. It is likely, therefore, that the exposed low-grade and strongly folded greywacke-shale sequences with generally calc-alkaline magmatism in the Southern Caledonides are the exposed part of thickened accretionary prisms of marginal and back-arc basin volcaniclastics intruded by Caledonian plutonic complexes (cf. Brown et al. 1981). These basins lay along the northern margin of the European Proterozoic continental foreland prior to suturing. In the text which follows, separate consideration is given

G. C. B r o w n , E. H. Francis, P. K e n n a n & C. J. Stillman

to volcanic and plutonic rocks, first from the Irish area and then from the rest of the British province, viz. Scotland, England and Wales. The geographical separation has been retained for convenience, since many users of the map will wish to relate more closely to one area than the other. Attention is drawn to links between the two provinces, and between intrusive and extrusive events where this seemed relevant to the compilers. A brief summary concludes the section on magmatic rocks to draw attention to similarities and differences between Britain and Ireland.

Volcanic rocks Ireland The great majority of Irish Caledonian volcanic rocks are submarine and found in the Lower Palaeozoic marine sediments of the Paratectonic Caledonides. The principal areas are in the Murrisk-Connemara region of western Ireland, the Wicklow, Wexford, Waterford region of southeastern Ireland, the Longford-Down region extending part way across the north-central part of the country, and the Tyrone region of Northern Ireland. They are also found in smaller inliers appearing through the post Caledonide cover which blankets the centre of the country. On the northern (Laurentian, or North American) plate, significant volcanism is also seen in the Dalradian basin within the Orthotectonic zone. Here volcanic rocks were erupted in late Proterozoic to Cambrian times and were deformed and metamorphosed by the Cambro-Ordovician Grampian (early Caledonian) orogenic event. Now seen as green beds (Max & Long 1980), they were Southern Highland (Upper Dalradian) Group tholeiitic pillow basalts and tufts, apparently associated with a tholeiitic dyke swarm cutting Appin (Lower Dalradian) Group rocks. They are thought to have been emplaced during a period of crustal extension associated with the opening of the Iapetus ocean. When extension gave way to Grampian compression, synorogenic magmatism occurred in what is now Connemara, producing a suite of ultrabasic-to-acid bodies with volcanic arc geochemistry (Yardley & Senior 1982). Marginal now to this continental mass, though perhaps themselves 'exotic' terrains, are a number of areas of early Ordovician island-arc volcanics. The Lough Nafooey volcanics, now situated just to the north of the Connemara migmatites, are submarine island-arc tholeiites which later became calc-alkaline (Ryan et al. (1980). The~e were erupted on the flanks of the developing South Mayo trough in Tremadoc to Arenig times. Younger sequences continuing up to the Llanvirn are bimodal, basic and acid. Similar bimodal volcanics and intrusives are found some 70 km northeast at Charlestown (Charlesworth 1960a,b). Between Cookstown and Omagh in Co. Tyrone are Lower Ordovician basaltic and andesitic submarine volcanics and associated gabbroic and ultrabasic intrusions, in tectonic contact with Dalradian, and possibly even older metasediments and volcanics (Hartley 1933, 1936; Angus 1977). The relationship here bears comparison with that of the Highland Border Series in Scotland. A similar relationship is postulated

for the mafic volcanics associated with serpentinites found on the southern side of Clew Bay, Co. Mayo (Ryan et al. 1983). The Longford-Down Zone, the westward extension of the Southern Uplands zone of Scotland, an accretionary prism of sediments stacked onto the Laurentian continental margin by obduction from the southeast, also contains volcanic components (Phillips & Skevington 1968, Sharpe 1970, O'Connor 1975, Morris 1979). In Scotland, the Ballantrae ophiolite reveals evidence of pre-Arenig marginal basin ocean crust obducted between 490 and 470 Ma ago and other components derived from oceanic volcanoes and ocean-floor volcanism are seen in the tectonic slices of the northern edge of the Southern Uplands. In Ireland the basic volcanic rocks seen south of Belfast Lough, at Strokestown and in South Connemara are similarly believed to be the products of spreading-axis volcanism, tectonically emplaced aboved subduction zones but there is as yet no certain evidence for direct equivalence with the Scottish rocks. South of the suture, Caledonian volcanic rocks are found predominantly in east and southeast Ireland and are largely restricted to the Ordovician. They can be grouped into two belts, north and south of Dublin (Stillman & Williams 1979). In the area north of Dublin small inliers of Lower Palaeozoic rocks reveal a history of volcanism which started in the Llanvirn and ended in Ashgill times (France 1967, Harper & Rast 1964, Manistre 1952). These eruptions of basalt and basaltic andesite are geochemically classified as island-arc tholeiites and calc-alkaline basic volcanics which were erupted as oceanic islands that started as primitive outer-arc islands but evolved to very potassic compositions (Stillman & Williams 1979). Petrographically and chemically they equate well with the Eycott Volcanics of the northern Lake District in England, which are of Arenig to Llanvirn age. South of Dublin is a major belt of Lower Palaeozoic rocks which contains an earlier late-Cambrian or Tremadocian phase of submarine basalts occurring as minor pillow lavas (Brindley & Millan 1973; Bruck et al. 1978). There is evidence of Lower Ordovician volcanism in the region now flanking the Leinster granite (Br/ick 1976, BrOck et al. 1980) but these rocks have been much affected by the metamorphic aureole of the granite and as yet their geochemical affinities are not clear. In the region south and east of the granite the main Ordovician volcanicity started in Llanvirn-to-Llandeilo times with eruptions of basalts and andesites of both tholeiitic and calc-alkaline chemistry (Stillman & Williams 1979). These were followed by Caradoc-Ashgill eruptions of calc-alkaline andesite and rhyolite (Gardiner 1974, BrOck et al. 1980, Shannon 1979. which were in places associated with substantial C u - F e - Z n - P b sulphide mineralization (Platt 1977, Sheppard 1980). This volcanism was apparently subduction-related and erupted in an arc situated on the northwestern margin of the Southern Britain micro-continental plate above a southeastward dipping subduction zone (Stillman & Williams 1979). The acid volcanics may owe their origin, at least in part, to late Ordovician crustal remelting (Stillman 1980). Possible correlatives along strike to the northeast are the Borrowdale Volcanics of the Lake District. Small outcrops of Llanvirn-Caradoc volcanics of similar

Caledonian igneous rocks o f Britain and Ireland

geochemical affinities are seen in the Slieve Aughty inlier of counties Clare and Galway (Holland 1981). Throughout Ireland the peak of Caledonian volcanic activity was passed by the end of the Ordovician, and the Silurian period was one of limited and sporadic activity. South of the suture, in the Dingle Peninsula of southwest Ireland, and in one of the Blasket islands, a small amount of Wenlock rhyolite and tholeiitic andesite occurs (Parkin 1976, Stillman 1981). North of the suture there is rather more; in South Mayo and Clare Island, Llandovery to Wenlock acid volcanics are found; these are largely pyroclastic--some being ignimbrites, and may indicate crustal remelts. In the Llandovery of the eastern end of the Longford-Down massif, particularly on the Ards Peninsula, the metabentonites of the Birkhill Shales represent widespread, though volumetrically minor pyroclastic eruptions (Cameron & Anderson 1980). The late-orogenic Lower Devonian period which in Scotland provides evidence of renewed large-scale volcanism is, in Ireland, notable mainly for its plutonism; volcanic rocks are of minor importance and occur only north of the suture. Lower Old Red Sandstone tuffs and lavas of andesitic and dacitic composition are seen in the eastern half of the Charlestown inlier (Charlesworth 1960a,b) and at Cushendall on the northeast Antrim coast. The middle Devonian, as in Britain, is absent, and the post Caledonian Upper Devonian to Permian period provides only two distinct but minor suites of volcanic rocks; Upper Old Red Sandstone acid tuffs, felsites and dolerite intrusions emplaced in a fluviatile continental environment form the Lough Guitane Volcanics south of Killarney (Avison 1982). In the Lower Carboniferous epicontinental seas further north the Limerick volcanics were erupted as submarine piles which sometimes built up to subaerial vents (Str6gen 1973). These were dominantly basaltic eruptions of lava and tuff. Occasional isolated island volcanoes are known elsewhere in the midlands--the only one depicted on the map is at Crochan Hill in Co. Meath. Britain

From the ornament on Plate 1 across mainland UK three main sectors can be distinguished. The northernmost, virtually the whole of Scotland plus the Cheviots of Northeast England, forms part of the former Laurentian plate, whilst the median sector, or Celtic Plate, is represented by most of northern and central England and all of Wales. The third, southern sector, formed of southwest England only, is essentially Hercynian. Since the review by StiUman & Francis (1979) new data are available for reinterpretation of all three sectors.

Scotland and the Cheviots

Cambro-Ordovician basic complexes interpreted as ophiolites occur in Shetland, along the Highland Border zone and at Ballantrae, in Ayrshire. Together with basic volcanics of the same age span within the Dalradian, they have been cited as remnants of former oceanic lithosphere in various plate-tectonic reconstructions, though the most recently adduced evidence has not been entirely corroborative.

Graham & Bradbury (1981) find the Dalradian tholeiitic metabasites to have affinity with oceanic spreading centres, but as this conflicts with the sedimentary pattern, they assume development of an oceanic basin marginal to Iapetus. Henderson & Fortey (1982) similarly invoke a restricted oceanic basin in which to generate the Highland Border rocks and this seems to be reconcilable with the presence of both oceanic tholeiites and within-plate alkali basalts (Ikin 1983). Henderson & Robertson (1982), however, do not relate this basin with that of the Dalradian tholeiites; instead, they infer that the Highland Border rocks were emplaced allochthonously from farther north, though it is not clear whether their marginal basin was the same as that which is believed to have given rise still farther to the north to the Shetland ophiolites--also tectonically emplaced (Flinn 1979). A marginal basin interpretation has also been invoked by Bluck et al. (1980) for the Arenig Ballantrae Complex, though here, too, the complications first noticed by Wilkinson & Cann (1974) throw doubt on the simple concept of obducted oceanic lithosphere. In particular, Thirlwall & Bluck (1984) advance Sr-Nd evidence of polygeneticism implying at least four tectonically juxtaposed units. Similarly, tectonic slicing of the neighbouring Southern Uplands Ordovician sediments obscures the palaeogeographic significance of the minor volcanics included within the postulated accretionary wedge. However, the effusion, within a relatively short post Arenig age span (Harris et al. 1975, Gale et al. 1980), of tholeiitic, calc-alkaline, alkaline and peralkaline volcanics (Lambert et al. 1981, Thirlwall 198 la, Bloxam 1982) is at least consistent with the presence of an island-arc system related to a destructive plate margin. Major advances have been made in the interpretation of the late Caledonian volcanic-plutonic manifestations. In addition to the existence of a NW-dipping Benioff zone implied by the spatial variation in trace elements and Sr-Nd isotopes (Thirlwall 1981b, 1982), a solution to the previously unexplained apparent discrepancy between subduction-related magmatism and the accumulation of post orogenic (LORS) molasse has been offered by new radiometric ages. These indicate that, from the Midland Valley northwards, the extrusive and intrusive rocks are late Silurian or at youngest Siluro-Devonian. Their previously assumed equivalents in the Southern Uplands and Cheviots are younger (early Devonian) as well as showing significant differences in chemistry (Thirlwall 1981b, 1983). Subsequent volcanism was essentially within-plate continental and alkaline. It started with late Devonian basalts in the Shetlands (Fitton et al. 1982) and continued virtually throughout the Carboniferous, most voluminously in the Midland Valley, where it was controlled by Caledonoid lineaments (Francis 1978). The basalts and their subordinate more acid differentiates show signs of fractionation (MacDonald & Whyte 1981) and of a heterogenous mantle source (MacDonald 1975, 1980). This 80 Ma timespan occupied by mainly mildly alkaline volcanic activity was interrupted only briefly, late in the Carboniferous, by a single episode of tholeiitic injection in the form of E - W dykes (MacDonald et al. 1981) and a major sill complex (Francis 1982).

4

G . C . Brown, E. H. Francis, P. Kennan & C. J. Stillman

Caledonian volcanic rocks o f England and Wales By contrast with the northern sector, there is no unequivocal evidence of rocks older than late Precambrian. Extrusive and intrusive complexes form scattered outcrops in Wales, the Welsh Borderland and the English Midlands, are seen in borehole cores in central England and, debarably, in the Lizard Complex in Southwest England. All fall in the 700-500 Ma age range (Thorpeetal. 1984) and R B - S r and U - P b data suggest that they represent new additions to the crust, containing little recycled material. It appears that the late Precambrian and Eocambrian was a period of crustal growth by widespread calc-alkaline activity related to island arcs, mostly with thin, slightly older, sialic basement. The consequent implication of subduction and micro-continental collision is consistent with the view that the basement of mainland Wales, eastern Ireland, Anglesey and Rosslare may have been significantly different. North Wales experienced late Proterozoic Monian and Eocambrian Arvonian calc-alkaline activity in an island-arc or continental margin setting. On Anglesey, the Monian spilitic pillow lavas and associated serpentinites have oceanic crustal trace element compositions and they have been interpreted as an allochthonous ophiolite (Thorpe 1982) and their associated volcaniclastics as trench deposits (Harris, in press), though this view is not universally accepted. These rocks were metamorphosed before the Arvonian was unconformably deposited upon them. Andesites, dacites and rhyolites form this calc-alkaline suite (Thorpe 1979), whose youngest tufts may be partly Cambrian in age (Wood 1969). Andesitic and rhyolitic lavas and volcaniclastics in Southwest Wales are approximately of the same age as the Malvern volcaniclastic suite which has ocean-floor tholeiite trace-element chemistry (Thorpe 1974). A bimodal association in the Warren House Group gives way to a somewhat younger suite (558 + 16 Ma, i.e. up to possibly early Cambrian according to Patchett et al. (1980)) of Uriconian lavas and tufts of basic intermediate and acid compositions. These have a trace-element composition which Thorpe (1974) interprets as indicating a basalt-andesite-rhyolite association of continental margin type. In Central England the late Precambrian Charnian volcaniclastics and porphyroids seen in Leicestershire have calc-alkaline chemistry (Thorpe 1972), while further south near Nuneaton are other pyroclastics cut by mafic intrusions (Evans 1968). Following this Eocambrian to early Cambrian activity, magmatism was absent in England and Wales until the Tremadoc, when eruptive activity started at Trefgarne and Rhobell Fawr. Activity from then on until early Silurian was, however, extensive and is well documented. Though low-grade metamorphism has tended to inhibit geochemical research, the balance is redressed by Fitton et al. (1982) who discern two distinct sectors. The first, introduced in the Irish volcanics section, extends from eastern and southeastern Ireland to the Lake District and is thought to represent island-arc activity along the southeastern side of Iapetus. It shows variation both southward and with time from transitional tholeiitic/calc-alkaline to truly calc-alkaline. The

other sector, represented by Wales, shows a crude variation with time, from tholeiitic varieties during Tremadoc-Llanvirn, through calc-alkaline during Llandeilo-Caradoc, to eventual alkaline activity in the Llandovery--a variation compatible with the development nowadays assumed by most workers (e.g. Bevins 1982), of a marginal basin. The model of Fitton et al. (1982) assumes southeastward subduction along at least two zones and stages throughout this period but, otherwise, it follows the long held assumption that there was no end-Caledonian activity corresponding to the calc-alkaline Siluro-Devonian magmatism of Scotland and the Cheviots. This view is called into question by new evidence of Plinean activity in the form of a basal Devonian ashfall, the Townsend Tuff, which extends over at least 10 ~ km 2 in South Wales (Allen & Williams 1981). By comparison with isopachs of modern Plinean activity, the recorded thicknesses of up to 3 m suggest the former presence of sizeable stratovolcanoes within, or no great distance from, the Welsh Basin; none as yet has been located though it may be significant that small intrusions of microgranite have been recognized cutting late Wenlock sediments in the Welsh Borders (Sanderson & Cave 1980). There is no record of subsequent Devonian volcanism. When activity was resumed during the Carboniferous, it followed a similar pattern to the northern Scottish part of what had become the same craton; i.e. it continued sporadically throughout the period and was dominantly within-plate alkaline basaltic, with a late phase of tholeiitic dyke-and-sill injection in northern England. By way of minor contrast, there is also evidence of magmas transitional to tholeiites in early Carboniferous sills and lavas of Derbyshire (Ineson et al. 1983, MacDonald et al. 1984), and in early Westphalian lavas and sills in the East Midlands (Kirton 1984). Later Westphalian basalts of the East Midlands and of the concealed Oxfordshire coalfield are essentially alkaline.

Hercynian volcanic rocks o f Southwest England Pre-Devonian evidence is restricted to the Lizard complex, the interpretation of which continues to be controversial. For instance, Kirkby (1979) views it as an ophiolite, a remnant of late Silurian oceanic crust obducted early in the Devonian, whereas Floyd (1983b) still holds to the more traditional concept (Green 1964) of a Precambrian basement relic intruded by a high-temperature peridotite diapir, possibly during the late Devonian as indicated by the K - A r age of the metamorphism. The most recent S m - N d isochron of 375 ~ 34 Ma (Davies 1984) suggests that the complex crystallised in mid-Devonian times, that is, not long before the time of the metamorphism. Elsewhere in the trough, basaltic piles of limited thickness and extent accumulated, mostly subaqueously, throughout the late Devonian and early Carboniferous. The rocks have undergone low-grade metamorphism, but trace-elements and fractionation indices give a general pattern of tholeiitic magmatism during the Devonian of southern Cornwall, giving way to both tholeiitic and alkaline activity elsewhere (Floyd 1982, 1983a). There is no agreement as to whether these rocks are entirely within-plate

Caledonian igneous rocks o f Britain and Ireland and continental-rift related (Floyd & E1 Samman 1980), or whether they have such ocean-floor affinity as to indicate a setting within a small oceanic basin (Chandler & Isaac 1982). All such basaltic activity ended before the beginning of the Namurian. Evidence of the last eruptive episode is found near Plymouth in the form of rhyolitic plugs and at least one flow resting on deformed Devonian sediments. They are assumed to be the sole remnants, apart from pebbles in the younger Permo-Triassic molasse, of extrusions capping the potassic Cornubian granite batholith (Cosgrove & Elliott 1976). The granite marks the close of the Hercynian orogeny; it has been dated at 295 -+ 10 Ma, i.e. late Westphalian, by Exley & Stone (1982). As the emplacement of the Cornubian batholith is not related to Caledonian tectonic events in the UK it will receive no further consideration in this account.

Plutonic rocks Ireland A map of plutonic rocks in the Irish Caledonides is concerned with, inter alia, five major granite intrusions (classified as 'Caledonian I-Type' by Pitcher, 1983) and to a less obvious but no less important degree, with major centres of basic magmatism. Of the major granite intrusions, the most widely known are the granites of Donegal (Pitcher & Berger 1972). These comprise a suite of eight contemporaneous post tectonic high-level intrusions representing a single magmatic event. As originally recognized for British Caledonian intrusive complexes in general by Read (1961) they are remarkable in their range of intrusive mechanisms--the individual intrusions reflect a range that includes permitted emplacement, forceful diapiric intrusion and wedging into a shear zone. In complete contrast, the various individual units of the Leinster batholith in Southeast Ireland are zoned, dome-like diapirs accommodated by forceful distension of their slate envelopes (Brindley 1973). The most northerly of the contiguous units has been shown to comprise a range of petrographic types ranging from quartz diorite to granite. These are thought to have been emplaced in a succession of magmatic pulses from a progressively fractionated reservoir at depth (BrOck & O'Connor 1977). The Leinster granite is unique among Irish granites in the very marked negative Bouguer gravity anomaly associated with it; downward extension to some 8 km is indicated (Cassidy & Locke 1982). However, in its structural features, the Leinster granite shows many features that are common to the Newry and Ox Mountain intrusions and to the Ardara and Main intrusions in Donegal. A full understanding of the relationships between the varied components of the major Galway granite remains to be determined. Though upward movement of granite in individual parts of this intrusion is recognized, a near-roof situation is indicated by the disposition of late granites, by the dip of outer contacts and by clear evidence, locally, of stoping. Leake (1978) has postulated that the generation and emplacement of the various Irish granite intrusions might be related to major known tectonic lineaments. This con-

cept has been developed by Hutton (1982) into a plausible model that relates the emplacement and the deformation history of the Donegal granites to a sinistral shear zone. A similar model may apply to the Leinster intrusion and to some of the others. The most important basic intrusions, dated at 510 Ma (Pidgeon 1969), and thus older than all of the major granite intrusions, may represent the dispersed fragments of an even larger early Caledonian intrusion in Connemara (Leake 1970). They comprise a suite of basic and ultrabasic rocks, often beautifully layered, belonging to one period of intrusion and to one magma series. Their aspect is reminiscent of parts of ophiolitic sequences though they have not been classed as such. However, metabasic and ultrabasic intrusive rocks south of Clew Bay, Co. Mayo have been recognized recently as ophiolites (Ryan et al. 1983). A great suite of early sills in Donegal may represent a period of continental rifting and crustal separation--the opening of Iapetus (Graham 1976). In a more local sense, crustal extension is indicated by the dolerite dyke swarm of Tallaght near the northwest margin of the Leinster granite (Angus & Brindley 1970). The dating of the various intrusions, particularly the granites, remains a problem. With the exception of the age determinated by Pankhurst et al. (1976) for the Ox Mountains granites--a now unique age of c. 480 Ma--all other Rb-Sr isochron whole-rock dates published are based on data showing very high degrees of non-experimental scatter. There is, in most cases, good corroborative evidence that the isotopic ages are essentially correct, but the isotopic data do warn that whole-rock samples of the Irish granites may not always support simple straight-line isochrons. It follows that comparison of initial 87Sr-S6Sr ratios for the various Irish granites may be premature. However, it is deduced provisionally from isotopic evidence that most magmas were produced by melting of upper mantle and-or juvenile lower crust with contamination by, or incorporation of, upper crustal material. As with many of the younger (Siluro-Devonian) Scottish and English granites the large volume of some of the intrusions (e.g. Leinster) suggests that an origin solely by crustal anatexis is unlikely. Also as in Scotland, the role of precursor intermediate-basic magmas is clearly signalled by the appinitic bodies that cluster around some of the intrusions; most of these are too small to indicate on Plate 1. However, the incorporation of crustal rocks is reflected in the presence of inherited zircon in the Donegal Thorr intrusion. A model for magma origin, incorporating the essential crust-mantle elements proposed for Southern Uplands granites by Harmon & Halliday (1980) seems applicable to most Irish granites. With one exception, the granites postdate regional deformation events and the closure of Iapetus. They were emplaced during post orogenic uplift and their present relative positions may reflect later extensive lateral movements. The cause of this widespread contemporaneous magmatism must relate to combinations of circumstances identified by Brown et al. (1981) in an earlier review; viz: (a) post collision rebound resulting in adiabatic melting, (b) tectonically increased thickness of sialic insulation and-or (c) continued subduction beneath the now sutured continent.

G. C. B r o w n , E. H. Francis, P. K e n n a n & C. J. S t i l l m a n Britain

As in earlier sections, a distinction is drawn between plutonic complexes occurring in the northern and southern Caledonides. Separate consideration is given to a group of intrusions south of the Southern Uplands fault in Scotland which, whilst they have many characteristics in common with the southern group, lie to the north of the Iapetus suture. Northern and central Scotland

The map shows sixty-four intrusions in this area which range in age from c. 1000 Ma (mid-Proterozoic) to c. 360 Ma (Lower Carboniferous) although the vast majority were emplaced during a 100 Ma period from early Ordovician to lowermost Devonian. The rock types vary from early Ordovician basic-ultrabasic plutons in Aberdeenshire, through a wide range of calc-alkaline diorite-granodiorite (tonalite)-granite plutons (with associated appinites) across the province, to the end-Silurian mildly alkaline silicic plutons of the Cairngorm-Aberdeen area. Several Ordovician-Silurian alkaline syenitic complexes occur along and to the east of the Moine Thrust in northwest Scotland. The style of intrusive events changes from early pre-tectonic migmatitic gneisses and well-defined, pre-deformational intrusions (the 'Older Granites' of Read 1961), through syn-to-post tectonic intrusions (the 'Newer Granites' of Read). Most of the earlier post tectonic intrusions appear to have been forcefully emplaced in that their margins have been deformed by intrusion--generally these are Ordovician and early Silurian plutons--whilst many of the younger intrusions have passive or 'permitted' styles of emplacement. A contemporary analysis of these changes, which follow a general rather than a strict chronological sequence across the Caledonian metamorphic climax at c. 485 Ma, is given by Pankhurst & Sutherland (1982). However, the post tectonic evolution of intrusive style may reflect a progressive relaxation of the crust following collision (a theme developed by Simpson et al. (1979) and Brown et al. (1981)). The earliest phase of intrusive magmatism is recorded by large areas of regional migmatite and granite gneiss north of the Highland Boundary fault, e.g. the Strath Halladale and Ardgour granite gneiss complexes shown and referenced on Plate 1. Whilst there are uncertainties about the original ages of these complexes due to resetting during later metamorphism, minimum ages ranging up to 1030 Ma for the Ardgour gneiss have been determined (Brook et al. 1976). Other zones of intermittent 'granitized' Moine and Dalradian occur in the area around Loch Coire, immediately to the west of Strath Halladale, in the Central Highlands between the Findhorn, Strathspey and Cairngorm intrusions, and across south-central Aberdeenshire from Ben Vurich in the southwest to Peterhead in the northeast. Individual pre-tectonic Cambrian intrusions in which foliated hornblende-biotite granite gneiss and biotite augen gneiss are common include the Carn Chuinneag complex (which contains some peralkaline riebeckite granite) north of the Great Glen (Shepherd 1973), and a group of intrusions around Ben Vurich in the Southern Highlands

(Bradbury et al. 1976). Also part of this early phase of Caledonian magmatism in northern Scotland are small epidiorite complexes (reviewed by Wadsworth 1982) scattered throughout the Moines and Dalradian, Notable among these is the Portsoy sill of Aberdeenshire. The main period of gabbro emplacement, broadly coincident with Grampian metamorphism, is dated at 489 +- 17 Ma (Pankhurst 1970) and takes the form of six major intrusive complexes that form a discontinuous arc around the Boyndie syncline in Aberdeenshire. The form and Cambro-Ordovician age of these complexes resembles that of the Connemara intrusions discussed earlier in that they comprise layered successions ranging from peridotites through gabbros to syenogabbros (Wadsworth 1970). They also contain granular gabbros, noritic gabbro cumulates and the occasional pegmatite. Because these plutons have been affected by a complex post emplacement tectonic history, comments on their origins are sparse, yet there exists a possibility that they are tectonically-emplaced fragments of ocean lithosphere. By far the majority of the individual plutons in this northern province are post tectonic granites ranging in age from early Ordovician to Siluro-Devonian. Attempts to classify and subdivide these intrusions are virtually impossible without gross generalization (but see Brown & Locke (1979) for an attempt based on geophysical signatures and, see Pankhurst & Sutherland (1982) for an attempt based on age and petrological criteria). Like the gabbros before them, most of the early to mid-Ordovician plutons (440-480 Ma) are confined to Aberdeenshire and surrounding areas, e.g. Aberchirder, Kennethmont and Strichen (Pankhurst 1974). They take the form of small outcrops of mainly biotite granite with occasional diorite and granodiorite; sometimes they show signs of late-tectonic flow foliation even at 440-450 Ma as is the case with the Glen Kyllachy granite (van Breemen & Piasecki 1983). From available isotopic, field and geochemical data (Pankhurst 1979, Halliday et al. 1979, 1981, Plant et al. 1980) these granites appear to have been derived by partial melting of mainly crustal source rocks, probably upper crustal metasediments first metamorphosed during the Grenville event (van Breemen & Piasecki 1983). Late Ordovician-early Silurian granites (440-410 Ma) are more widespread throughout the Scottish Caledonides and generally are more diverse in petrology and composition. Many have an early basic 'appinitic' component, though hornblende-biotite diorites, granodiorites and tonalites are ubiquitous; see Sutherland (1982a) and Pankhurst & Sutherland (1982) for details. Many are substantial, complex, multiple intrusions (e.g. Strontian, Foyers, Rogart) with intimate magmatic reaction relationships between successive magma pulses; in this connection a classic description of the Garabal Hill-Glen Fyne intrusion is given by Nockolds (1941). The geophysical expression of these granites on regional gravity and aeromagnetic maps is virtually negligible indicating, in contrast to the Siluro -Devonian intrusions that there is no significant contrast in density or magnetic properties between the intrusions and their enveloping country rocks (Brown & Locke 1979, Brown 1982, Locke & Brown 1978). Thus there is a possibility that the three-dimensional volume of these

Caledonian igneous rocks o f Britain and Ireland

intrusions is small and there is no volumetric difficulty in furnishing partial melts from a range of crustal anatectic sources (see discussions by Pankhurst 1979, Harmon & Halliday 1980, Harmon 1983) during and following ultrametamorphism. Indeed, low initial neodymium isotope ratios (Hamilton et al. 1980) and the presence of small amounts of inherited Proterozoic zircon in many of these intrusions (Pidgeon & Aftalion 1978) argue for an important crustal contribution to magma genesis, although some low initial strontium ratios may indicate a component of recent mantle derivation. The latest Siluro-Devonian group of Caledonian intrusions (415-396 Ma) in central and northern Scotland is also the most extensive in outcrop area and volumetric extent, to judge from their large negative Bouguer gravity anomalies; compare also the Leinster granite in southern Ireland. They comprise an extensive suite of granitic ring complexes and discordant plutons, the largest of which follow a N E - S W line from the Aberdeenshire coast to Glen Etive, parallel to, but some 200 km NW of the Caledonian suture. Recent geochemical studies of four large plutons from the Cairngorm-Aberdeen area (Brown et al. 1984) identify these as uniformly alkali-calcic peraluminous granites which carry the greatest enrichments of LIL and HFS elements in the northern Caledonian area. They have chemical compositions resembling the granitoids of modern mature continental arcs from which it is deduced that an enriched sub-crustal magma source was involved, perhaps even crustal underplate adiabatically remelted under post collision rebound conditions, as suggested by Simpson et al. (1979). Clearly, these last granites have the strongest link to Caledonian subduction processes and it is significant that they were emplaced over the same time interval that the major end-Caledonian calc-alkaline Siluro-Devonian arc-related volcanics, described earlier, were being erupted. It is significant also that slightly older and contemporaneous peralkaline intrusive complexes such as Glen Dessary, Ben Loyal and Borrolan (Sutherland 1982b) were being emplaced further north. A recent multi-isotope study of the Etive igneous complex (Clayburn et al. 1983) has confirmed the existence of a significant mantle-derived melt component, particularly in the earlier diorite -granodiorite phases of this complex body. The later granitic pulses which now occupy the centre of the complex contain an increasingly significant contribution from a late Proterozoic (Grenville?) lower crustal source, notably of relatively short crustal residence time compared with the Lewisian basement. It is possible that this genetic model might be more widely applicable in the case of other Caledonian granites; however, the model remains to be tested elsewhere in northern Scotland. Other late Caledonian multiple intrusions from northern Scotland which vary from dioritic margins to granitic cores include Ben Nevis and Lochnagar and several of the earlier granites such as Ballachulish and Strontian, whilst most of the Siluro -Devonian Aberdeenshire granites, especially Cairngorm and Ballater, are uniform biotite granites. Southern Uplands o f Scotland

Although this region lies to the north of the Caledonian

suture, it is generally regarded as a Lower Palaeozoic accretionary prism along the northern margin of Iapetus. Thus, in lacking any clear evidence of an older crustal basement, it is an exceptional part of the northern province. Seven intrusions ranging in age from 392 to 408 Ma, all Siluro-Devonian, are shown on Plate 1. Had these been emplaced before closure they would have been regarded as outer-arc granites; however, their post closure ages suggest that they must have evolved above the buried ridge zone. Although they are late-Caledonian granites, unlike contemporary intrusions to the north all three of the large plutons in the Southern Uplands show some evidence of forceful emplacement. Considerable petrochemical variation is displayed by these three intrusions, and also by the Carsphairn complex; indeed the Loch Doon pluton, which varies from quartz diorite margins to a granite core, is the most strongly zoned intrusion in the British Caledonides (Brown et al. 1979, Tindle 1983) and displays, almost completely, a smooth fractionation sequence from margin to core, in contrast to the multiple intrusion at Carsphairn where hybrid rocks form part of the gabbro-diorite-granite sequence (Deer 1935). With margins of biotite granite and a hydrothermally affected core of two-mica granite, the Cairnsmore of Fleet intrusion (Parslow 1968) is the most evolved granite in the Southern Uplands, followed by the Criffell granodiorite-granite mass with its adjacent copper-mineralized subvolcanic granodiorite stocks, sheets and breccia pipes at Black Stockarton Moor to the west (Leake & Cooper 1983). Isotopic work by Stephens & Halliday (1979), Halliday et al. (1981) and Halliday (1983) indicates an age progression of activity from Doon to Criffell to Fleet that relates to their respective degrees of magmatic evolution. Moreover, initial strontium isotope data are used to argue that, to initial partial melts from mantle or juvenile lower crust, are added progressively greater amounts of crustal partial melt, reflecting the isotopic composition of Southern Uplands sediments, as the magmas evolved by a process of combined assimilation and fractional crystallization. As with the Etive complex, incipient melting seems to have been related to thermally anomalous mantle associated with ocean closure; clearly, parallel processes are being invoked for the origin of melts for Siluro-Devonian intrusions throughout the British Caledonides. On recent observations by M. F. Thirlwall (pers. comm) other smaller plutons in the eastern part of the Southern Uplands fall into the same broad category of age and petrological type; they are the surface representatives of large buried intrusions in this area that have been inferred to be present from gravity data Lagios & Hipkin (1979). England and Wales

Recent studies of intrusive complexes in this part of the Caledonides have led to reviews of their geology and petrology by Le Bas (1982), of their age by Rundle (1981) and Hampton & Taylor (1983) respectively for the Lake District and Southern Britain, and of their relationship to the late Precambrian-early Palaeozoic evolution of England and Wales by Thorpe et al. (1984). Plate 1 shows

8

G. C. Brown, E. H. Francis, P. Kennan & C. J. Stillman

twenty-two intrusions in this area (including two in the Isle of Man); for the most part these are calc-alkaline granodiorite-granite plutons of late Caledonian age, though earlier, late Precambrian-Cambrian foliated and deformed diorite-tonalite-granite intrusions occur in Wales and the Welsh Borderlands (i.e. the Malverns, Ercall, Twt Hill, Coedana, Sarn and St. Davids intrusions and the Johnston complex--see detailed references on Plate 1). From borehole evidence (Thorpe 1982) combined with small outcrops (Cribb 1975), Cambrian diorites and granodiorites are inferred to underlie much of the English Midlands. Like the Southern Uplands, most of England and Wales is thought to have formed as an accretionary margin on the flanks of Iapetus, in this case a northwards accreting margin on the south flank. Magmatic events related to subduction commenced soon after accretion and isotopic analysis of the accessible early intrusive rocks reveal short pre-magmatic residence times for the crustal component (Thorpe et al. 1984). Whilst the main outcrops of intrusive rocks in the southern Caledonides occur in the Lake District there is a prominent late Caledonian diorite-granophyric granite intrusion closely associated with the calc-alkaline andesite lavas at Cheviot, and major batholiths are known from gravity data and boreholes to underlie the north Pennines (Bott et al. 1978). From limited borehole samples, the Weardale intrusion is a mineralized late Silurian two-mica granite whilst the Wensleydale intrusion is a chloritized biotite granite of late Ordovician age. Gravity evidence also suggests that there are several other granitic bodies beneath parts of eastern and central England (Rollin 1982), but these have yet to be proved by drilling. Bott (1974) has shown that an E - W granite batholith also underlies the entire Lake District at shallow depth, yet the earliest intrusive rocks exposed in this area are the Llanvirn ilmenite gabbros of the Carrock Fell igneous complex, which intrude part of the Skiddaw Slate-Borrowdale Volcanics sequence that is only marginally older than the gabbro itself. A later granophyric intrusive phase at Carrock Fell lies in the middle of the age range (438-394 Ma) for granitic intrusions in the Lake District. The oldest exposed granitoid plutons, of Llandeilo age at Threlkeld and St. John's, are porphyritic microdiorites with similar compositions to parts of the associated Borrowdale Volcanics; they may well record the location of sub-volcanic igneous centres. Two large end-Ordovician intrusions occur in the western Lakes as the granophyric granite at Ennerdale and the biotite and two-mica granite intrusion with associated haematite at Eskdale--this immediately post volcanic intrusive phase is thought to mark the main 'plutonic constructive phase of the Lake District batholith' (Le Bas 1982). A single Ordovician intrusion of this type also occurs much further south in Leicestershire: the Mountsorrel hornblende-biotite granodiorite. Although no further post Ordovician volcanic activity is recorded in the Lake District area, the ubiquitous end-Caledonian intrusions are represented in the form of the high-K megacrystic biotite adamellite at Shap and the biotite leucogranite at Skiddaw (Webb & Brown 1984). Both carry highly distinctive hydrothermal alteration and

mineralization: molybdenite-chalcopyrite-fluorite-barite at Shap and wolfram-apatite-arsenopyrite together with greisen at Skiddaw. As with the Siluro-Devonian granites of the Eastern Highlands, geochemical data (Brown et al. 1984) show that fresh granite compositions from Shap and Skiddaw resemble those of modern continental arcs, whilst the Ordovician Lake District plutons and volcanics have affinities with more primitive arcs. Therefore, the growth and evolution of the continental crust in this part of the Southern Caledonides by subduction-related magmatism during the Ordovician-Silurian period appears to be recorded in the geochemistry of the igneous rocks.

Summary and conclusions Caledonide eruptive magmatism was essentially controlled by the plate-tectonic evolution of the margins of the Iapetus ocean. Tectonostratigraphic zones run sub-parallel to the Caledonide suture and most current interpretations imply that the volcanic rocks were erupted in arcs--either island-arcs or on active continental margins--or in marginal basins. The implication is that the paratectonic Caledonides represent the accretion of such arcs and basins which were compressed and swung into parallelism with the suture before and during collision. It appears that the volcanism in most of the zones maintains its character along the strike-length that can be observed, and the principal variations relate to differences in the nature of the volcanism in different zones and to changes as the orogen evolved. This continuity along strike is particularly obvious in the Tremadoc-to-Ashgill volcanism which dominates the southeast and central inliers of the Irish paratectonic zones, and the median sector or Celtic plate of Britain. It is also true for that sector of the Laurentian plate that borders the suture, in the Southern Uplands and the Longford Down areas. On the other hand the Welsh Basin, with its well developed marginal basin character, is distinctly different and does not extend along strike. Within the Laurentian plate, volcanism is recorded over a much longer time span, w,~th its major activity at the beginning and end of orogenesis. Widespread Dalradian volcanism was related initially to the stretching and rifting of the pre-Iapetus crust and to early marginal basin development on that crust. End-Caledonian Siluro-Devonian volcanism and subsequent post Caledonian within-plate volcanism of Carboniferous age is more restricted in occurrence, with only minor basaltic suites being recorded in areas outside Scotland and northern England. Volcanic activity within the Hercynian sector is almost entirely restricted to Southwest England; there is just one limited phase of late Devonian rhyolite eruption in Southwest Ireland. Mafic plutonism, which was not uncommon in the early stages of the Caledonian orogen, likewise relates to the development of Iapetus margins and indeed some bodies may represent fragments of the ocean floor itself. However, by far the greatest volume of Caledonide plutonic rocks are intermediate-to-acid and were emplaced as widespread and often extensive end-Caledonian granitic intrusions. For variation amongst these, the suture and its marginal zones appear to have had much less relevance. Very similar intrusions occur in all zones and many more are exposed in

Caledonian igneous rocks of Britain and Ireland the north and east than in the south and west. In Ireland, both north and south of the suture, there is a small number of coeval intrusive complexes, some of batholithic dimensions, most of which possibly relate to a common magmatic event which took place on both sides of the suture over a relatively short time span. In Britain, on the other hand, there seems to be a considerable difference between the magmatism within the Laurentian plate in north and central Scotland, which produced over sixty intrusions, and that of the region south of the suture in England and Wales where there are only approximately twenty intrusions, though it is appreciated that this may be a function of erosion level. In both regions, the plutons are more diverse in composition and were emplaced over a longer time span than those in Ireland. Even the post tectonic granites range from Ordovician to Siluro-Devonian in age. Whilst correlatives of the Irish plutons may well be found amongst the intrusions in England, Wales and the Southern Uplands, it would seem that the thermal history of the deep crust in Britain, particularly under the Laurentian plate, differs significantly from that of the crust under Ireland. Acknowledgments

The authors are glad to acknowledge advice and assistance from Dr. N. H. Gale, Dr. M. le Bas, Dr. R. J. Pankhurst, Dr. C. C. Rundle, Dr. M. S. Thirlwall, Dr. R. S. Thorpe and Dr. P. C. Webb. References Alderton, D. H. M. & Rankin, A. H. 1983. The character and evolution of hydrothermal fluids associated with the kaolinized St. Austell granite, S. W. England. J. geol. Soc. London, 140, 297-309. Allan, D. A. 1940. The geology of the Highland Border: Stonehaven to Arran. Trans. R. Soc. Edinburgh, 60, 171-93. Allan, W. C. 1970. The Morven-Cabrach intrusion. Scott. J. Geol. 6, 53-72. Allen, J. R. L. & Williams, B. P. J. 1981. Stratigraphy of the Townsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh Borders. J. geol. Soc. London, 138, 15-29. AI-Turki, K. I. S. 1972. A petrographical and chemical study of the Carnmenellis Granite and associated rocks, Thesis, Ph.D. Univ. Exeter (unpubl.). Angus, N. S. 1977. The Craigballyharky Granitic Complex within the Tyrone Igneous series. Proc. R. Irish Acad. 77, 181-99. Brindley, J. C. 1970. A swarm of Caledonian dolerite intrusions in the Talleght Hills, Co. Dublin. Proc. R. Irish Acad. 69B, 165-78. Arthurton, R. S. & Wadge, A. J. 1981. Geology of the country around Penrith. Mem. geol. Surv. G.B. Ashcroft, W. A. & Boyd, R. 1976. The Belhelvie mafic igneous intrusion, Aberdeenshire--a re-investigation. Scott. J. Geol. 12, 1-14. Astin, T. R. 1983. Reply to: Thirlwall, M. F. 1981a. Implications of Caledonian plate tectonic models of chemical data from volcanic rocks of the British Old Red Sandstone. J. geol. Soc. London, 140, 315-18. Avison, M. 1982. The geology o f the Lough Guitane Volcanic Complex and associated sediments at County Kerry Island. Ph.D. thesis, University of Keele (unpubl.). in press. Contemporaneous faulting and the eruption and preservation of the Lough Guitane volcanic complex, Co. Kerry. J. geol. Soc. London. Bailey, E. B. 1960. The geology of Ben Nevis and Glen Coe. Mem. geol. Surv. G. B. 2nd Ed. Bamford, D., Nunn, K., Prodehl, C. & Jacob, B. 1978. LISPB-IV: Crustal structure of northern Britain. Geophys. J. R. astr. Soc. 54, 43-60. Barritt, S. D. 1983. The controls o f radio--element distribution in the Etive and Cairngorm granites: implication for heat production. Thesis, Ph.D. Open University (unpubl.). -

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1975. Precambrian rocks of Wales. ln: Harris, A. L. et al. (eds) A correlation o f Precambrian rocks in the British Isles. Spec. Rep. geol. Soc. London 6, 78-82. Shannon, P. M. 1979. The Petrology of the Ordovician volcanic rocks of County Wexford, Ireland. J. Earth Sci. R. Dublin Soc. 2, 41-59. Sharpe, E. N. 1970. An occurrence of pillow lavas in the Ordovician of County Down. Ir. Nat. J. 16, 299-301. Shepherd, J. 1973. The structure and structural dating of the Carn Chuinneag intrusion, Ross-shire. Scott. J. Geol. 9, 63-88. Shepherd, T. J., Beckinsale, R. D., Rundle, C. C. & Durham, J. 1976. Genesis of Carrock Fell tungsten deposits, Cumbrian fluid inclusion and isotopic study. Trans. lnstn Ming Metall. B85, 63-74. Sheppard, W. A. 1980. The ores and Host Rock Geology of the Avoca Mines, Co. Wickiow, Ireland. Nor. geol. Unders. 360, 369-83. Simpson, A. 1965. The syntectonic Foxdale--Archallagan granite and its metamorphic aureole, Isle of Man. Geol. J. 4, 415-34. Simpson, P. R., Brown, G. C., Plant, J. A. & Ostle, D. 1979. Uranium mineralisation and granite magmatism in the British Isles. Phil. Trans. R. Soc. London, A291, 385-412. --, Plant, J. & Cope, M. J. 1977. Uranium abundance and distribution in some granites from the northern Scotland and southwest England as indicators of uranium provinces, h~: Jones, M. H. (ed.) Geology, Mining and Extractive Processes o f Uranium. Instn. Ming Metall. London 126-39. Skiba, W. 1952. The contact phenomena on the NW of the Crosscloney complex, Co. Cavan. Trans. geol. Soc. Edinburgh 15, 322-45. Smith, T. E. 1970. The structural characteristics of the Strathspey complex, Inverness-shire. Geol. Mag. 107, 201-15. Soper, N. J. 1963. The structure of the Rogart Igneous complex, Sutherland. J. geol. Soc. London, 119, 445-78. Stamp, L. D. & Wooldridge, S. W. 1923. The igneous and associated rocks of Llanwrtyd (Brecon). Q. J. geol. Soc. London 79, 16-46. Steiger, R. H. & Jager, E. 1977. Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth planet. Sci. Lett. 36, 359-63. Stephens, W. E. & Halliday, A. N. 1979. Compositional variation in the Galloway plutons. In: Atherton, M. P. & Tarney, J. (eds) (q.v.), 9-17. Stevenson, I. P. 1971. The Ordovician rocks of the country between Dwygyfylchi and Dolgarrog, Caernarvonshire. Proc. Yorks. geol. Soc. 38, 517-46. -1980. Caledonide volcanism in Ireland. In: Wones, D. R. (ed.) The Caledonides in the U.S.A. (q.v.), 279-88. 1981. Caledonian igneous activities. In: Holland, C. H. (ed.) (q.v.), 83-106. 1982. Lower Palaeozoic volcanism in Ireland. In: Sutherland, D. S. (ed.) (q.v.) & Maytham, D. 1973. The Ordovician volcanic rocks of Arklow Head Co. Wicklow. Sci. Proc. R. Irish Acad. 73B, 61-77. & Williams, C. T. 1979. Geochemistry and tectonic setting of some Upper Ordovician volcanic rocks in east and southeast Ireland. Earth planet. Sci. Lett. 42, 288-310. & Francis, E. H. 1979. Caledonide Volcanism in Britain and Ireland. In: Harris, A. L., Leake, B. E. & Holland, C. H. (eds) (q.v.), 557-77. Strong, D. F., Stevens, R. K., Malpas, J. & Badham, J. P. N. 1975. A new tale for the Lizard. Proc. Ussher Soc. 3, 252. Stone, M. 1975. Structure and petrology of the Tregonning Godolphin granites. Proc. Geol. Assoc. London, 86, 155-70. -

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Caledonian igneous rocks o f Britain and Ireland -

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1982. The behaviour of tin and some other trace elements during granite differentiation, West Cornwall, England. In: Evans, A. M. (ed.) Metallization associated with acid magmatism. Wiley, 339-55. Strogen, P. 1973. The volcanic rocks of the Carrigogunnel area, County Limerick. Sci. Proc. R. Dublin Soc. AS, 1-26. 1974. The sub-Palaeozoic basement in central Ireland. Nature 250, 562-3. Styles, M. T. & Rundle, C. C. 1984. The Rb-Sr isochron age of the Kennack gneiss and its bearing on the age of the Lizard Complex, Cornwall. J. geol. Soc. London, 141, 15-20. Sutherland, D. S. 1982a. Petrography of Caledonian intrusive rocks. In: Sutherland, D. S. (ed.) (q.v.) 496-510. 1982b. Alkaline intrusions of north-western Scotland. In: Sutherland, D. S. (ed.) (q.v.), 203-14. & Pankhurst, R. J. 1982. Analyses of Caledonian intrusive rocks of Scotland and Ireland. In: Sutherland, D. S. (ed.) (q.v.), 203-14. Summerhayes, C. P. 1966. A geochronological and strontium isotope study on the Garrabal Hill - Glen Fyne igneous complex, Scotland. Geol. Mag. 103, 153-65. Sutton, J. S. 1970. The Terman Granite and associated minor intrusives Co. Mayo, Ireland. Sci. Proc. R. Dublin Soc. 3A, 293-302. Taylor B. J., Burgess, I. C., Land D. H., Mills, D. A. C., Smith, D. B. & Warren, P. T. 1971. Northern England. Br. reg. Geol. 4th edn. Thorpe, R. S. 1978. Tectonic emplacement of ophiolitic rocks of Precambrian Mona Complex of Anglesey. Nature 275, 57-8. Tilley, C. E. 1924. Contact-metamorphism in the Comrie area of the Perthshire Highlands. Q. J. geol. Soc. London, 80, 22-71. Tindle, A. G. 1982. Petrogenesis o f the Loch Doon granite intrusion, Southern Uplands o f Scotland. Thesis, Ph.D. Open University (unpubl.). Thirlwall, M. F. 1979. The petrochemistry o f the British Old Red Sandstone province. Thesis, Ph.D. Univ. Edinburgh (unpubl.). 1981a. Implications for Caledonian plate tectonic models of chemical data from volcanic rocks of the British Old Red Sandstone. J. geol. Soc. London, 138, 123-38. 1981b. Peralkaline hyolites from the Ordovician Tweedale Lavas, Peebleshire, Scotland. Geol. J. 16, 41-44. 1982. Systematic variation in chemistry and Nd-Sr isotopes across a Caledonian calc-alkaline volcanic arc: implications for source material. Earth planet. Sci. Lett. 58, 27-50. 1983. Discussion on implications for Caledonian plate tectonic models of chemical data for volcanic rocks of the British Old Red Sandstone. J. geol. Soc. London, 140, 315-318. & Fitton, J.G. 1983. Sm-Nd garnet age for the Borrowdale Volcanic Group, English Lake District. J. geol. Soc. London 140, 511-18. & Bluck, B. J. 1984. Sr-Nd isotope and chemical evidence that the Ballantrae 'ophiolite', S. W. Scotland, is polygenetic. In: Shelton, A. W. (ed.) Ophiolites and oceanic lithosphere. Spec. Publ. geol. Soc. London, (in press). Thomas, J. D., Dodson, M. H., Rex, D. C. & Ferrara, G. 1966. Caledonian magmatism in North Wales. Nature, London 209, 866-68. Thorpe, R. S. 1972. The geochemistry and correlation of the Warren House, the Uriconian and the Charnian Volcanic Rocks from the English Pre-Cambrian. Proc. Geol. Assoc. London. 83, 269-85. 1974. Aspects of magmatism and plate tectonics in the Precambrian of England and Wales. Geol. J. 9, 115-36. 1979. Late Precambrian igneous activity in southern Britain. In: Harris, A. L., Leake, B. E. & Holland, C. H. (eds) (q.v.), 579-84. 1982. Precambrian igneous rocks of England, Wales and south-eastern Ireland. In: Sutherland, D. S. (ed.) (q.v.) 19-35. - - , Beckinsale, R. D., Patchett, P. J., Piper, J. D. A., Davies, G. R. & Evans, J. A. 1984. Crustal growth and late Precambrian-early Palaeozoic plate tectonic evolution of England and Wales. J. geol. Soc. London, 141,521-36. Townend, R. 1966. The geology of some granite plutons from western Connemara, Co. Galway. Proc. R. Irish Acad. 65B, 157-202. Tremlett, W. E. 1969. Caradocian volcanicity in the Lleyn Peninsula. In: Wood, A. (ed.) (q.v.), 357-85. -

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15

Tweedie, J. R. 1979. Origin of uranium and other metal enrichments in the Helmsdale Granite, eastern Sutherland, Scotland. Trans. Instn Ming Metall. B88, 145-53. Turneil, H. B. 1982. Palaeomagnetism of L o w e r Palaeozoic Intrusions in the Scottish Highlands. Thesis, Ph.D. Univ Leeds, (unpubl.). Upton, B. J. G. 1982. Carboniferous to Permian volcanism in the stable foreland. In: Sutherland, D. S. (ed.) (q.v.), 255-75. --, Aspen, P. & Chapman, N. A. 1983. The upper mantle and deep crust beneath the British Isles: evidence from inclusions and volcanic rocks. J. geol. Soc. London 140, 105-21. Van Breemen, O. & Boyd, R. 1972. A radiometric age for pegmatite cutting the Belhelvie mafic intrusion, Aberdeenshire. Scott. J. Geol. 8, 115-20. - - , Aftalion M., Pankhurst, R. J. & Richardson, S. W. 1979. Age of the Glen Dessary Syenite, Inverness-shire: diachronous Palaeozoic metamorphism across the Great Glen. Scott. J. GeoL 15, 49-62. - - , Aftalion, M., & Johnson, M. R. W. 1979. Age of the Loch Borrolan complex, Assynt, and late movements along the Moine Thrust zone. J. geol. Soc. London, 136, 489-95. & Piasecki, M. A. J. 1983. The Glen Kyllachy Granite and its bearing on the nature of the Caledonian Orogeny in Scotland. J. geol. Soc. London, 140, 47-62. Wadge, A. J., Gale, N. H., Beckinsale, R. D. & Rundle, C. C. 1978. A Rb-Sr isochron for the Shap Granite. Proc. Yorks. geol. Soc. 42, 297-305. Wadsworth, W. J. 1970. The Aberdeenshire layered intrusion of north-east Scotland. Spec. Publ. geol. Soc. South Afr. 1,565-75. 1982. The basic plutons. In: Sutherland, D. S. (ed.) (q.v.), 135-148. Walker, F. 1925. Four granitic intrusions in south-eastern Scotland. Trans. geol. Soc. Edinburgh, 11, 357-65. Walker, G. P. L. & Leedal, G. P. 1954. The Barnesmore granite complex, Co. Donegal. Sci. Proc. R. Dublin Soc. 26, 207-44. Watson, J. V. & Plant, J. 1979. The regional geochemistry of uranium as a guide to deposit formation. Phil. Trans. R. Soc. London, A291, 321-38. Webb, P. C. & Brown, G. C. 1983. Lake District granites--heat production and related geochemistry. Geothermal investigations of the U.K. British Geological Survey Report Series. Williamson, J. M. & Cann, J. R. 1974. Trace elements and tectonic relationships of basaltic rocks in the Ballantrae igneous complex, Ayrshire. Geol. Mag. 111, 35-41. Williams, T. G. 1934. The Pre-Cambrian and Lower Palaeozoic Rocks of the Eastern End of the St. David's Pre-Cambrian Area, Pembrokeshire. Q. J. geol. Soc. London, 90, 32-75. Wilson, H. E. 1972. Regional geology of Northern Ireland. Geol. Surv. N. Ireland. & Robbie, J. A. 1966. The geology of the country around Ballycastle. Mere. geol. Surv. N. Ireland. Wilson, J. R. & Leake, B. E. 1972. The petrochemistry of the epidiorites of the Tayvallich Peninsula, North Knapdale, Argyllshire. Scott. J. Geol. 8, 215-52. Wiseman, J. D. H. 1934. The central and south-western Highland epidiorites: a study in progressive metamorphism. Q. J. geol. Soc. London, 90, 354-417. Wood, D. S. 1969. The base and correlation of the Cambrian rocks of North Wales. In: Wood, A. (ed.) (q.v.) 47-66. Yardley, B. W. D. & Senior, A. 1982. Basic magmatism in Connemara, Ireland, evidence for a volcanic arc? J. geol. Soc. London 139, 67-70. Ziegler, A. M., McKerrow, W. S., Burne, W. V. & Baker, P. 1969. Correlation and environmental setting of the Skomer Volcanic Group, Pembrokeshire. Proc. Geol. Assoc. London, 80, 407-39. -

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G. C. Brown Department o f Earth Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA. E. H. Francis Department o f Earth Sciences, University o f Leeds. P. Kennan Department o f Geology, University College, Dublin. C. J. Stiliman Department o f Geology, Trinity College, Dublin.

Time of deformation in the Caledonide Orogen of Britain and Ireland D. Powell & W. E. A. Phillips

Considerable problems attach to any attempt to define precisely the timing of such geological events as deformation. The biostratigraphical evidence may be imperfect or lacking, the isotopic evidence may be imprecise or conflicting, and where both lines of evidence are available, disagreement over definition of the Geological time scale creates problems of wider correlation. The last situation is exemplified by the English Lake District wherein the biostratigraphical and isotopic data appear to be reliable and compatible but the isotopic data conflict with some recently published time scales. Of particular importance to the present study are disagreements over the ages of the Lower Palaeozoic period boundaries. Estimates for the Devonian-Silurian boundary vary from 400 to 412 Ma (Odin 1982, cf McKerrow et al. in press); the Silurian-Ordovician boundary 418 to 438 Ma (Odin 1982, cf Harland et al. 1982); the Ordovician-Cambrian boundary 495 to 513 Ma (Odin 1982, cf McKerrow et al. in press); and the Cambrian-Vendian boundary 530 to 590 Ma (Odin 1982, cf Harland et al. 1982). Such differences evidently become critical where comparisons of timing based only on biostratigraphic evidence are made with those based solely on isotopic data. Particular difficulties also arise from the problems involved in determining the biostratigraphic base of the Devonian within rocks of Old Red Sandstone facies. Throughout this work reference has been made to the Special Reports of the Geological Society of London numbers 1, 2, 3, 6, 7, 8 and 10, which provide summaries of the lithostratigraphic and biostratigraphic record in Britain and Ireland from the Precambrian to the base of the Permian. Thus, references herein to series and stages are largely on the basis of these reports to which the reader is referred for detail, and an assessment of reliability of the palaeontological control. Papers of particular importance within these reports are referred to where relevant in the text. Whilst reference is made to some sources of isotopic data, where this is particularly important in assessing the timing of deformation, references for much of the data concerning intrusive and extrusive igneous rocks can be found on Plate 1. The opportunity has been taken to present a considerable quantity of new data relating to the geometry of structures in Ireland which is less well known than the geometry of structures in Britain. Hence both the form and age of Irish structures are treated, while the timing of deformation only in Britain is emphasized. For descriptive convenience the history of deformation of the orogen is discussed within the context of a number of geographically, and in part geologically, defined zones, the limits of which are outlined in Figure 1. Earlier reviews of the timing of deformation in the British and Irish Caledonides can be found in McKerrow (1962), George (1963) and Powell (1983). Reviews of the structure and structural development of parts of the British

Caledonides are given in Johnson et al. (1979), Coward & Siddans (1979), Craig (1983) and Duff & Smith (in prep).

The NW Foreland (Fig. 2) The northwest margin of the Caledonide orogen in the British Isles is normally regarded as the Moine thrust zone and its possible correlatives in Shetland and Ireland (Fig. 1 .). Recent field, isotopic, and geophysical evidence (Sibson 1977, Smythe et al. 1982, Brewer & Smythe 1983, IGS 1:100000 map: The Outer Hebrides 1981), however, suggests that zones of Caledonian reworking associated with major thrusts, may extend as far west as the Outer Isles and, possibly, Flannan thrusts. Between these major dislocations lie large areas of Archaean to Lower Proterozoic Lewisian gneisses and Upper Proterozoic Torridonian sediments that are unaffected by Caledonian deformation and are, to the east, unconformably overlain by tilted but otherwise undeformed Cambro-Ordovician shelf sediments. A break in sedimentation within the Durness Group of this shelf sequence, may represent all of the Middle and Upper Cambrian (Palmer et al. 1980) but no folding or tilting has been recognized relating to this top Lower Cambrian-basal Arenig interval. Thus, if this hiatus reflects the Grampian orogenic events recognized as affecting the rocks of the Dalradian zone (Fig. 1, see below), these relationships demonstrate that the northwestern part of the N Highlands was probably well outside the Grampian mobile belt. Within the Moine thrust zone (Fig. 1), moderate to intense folding, cleavage formation and the development of mylonitic fabrics associated with thrusts comprise a polyphase, but probably in part, progressive, deformation sequence, formed under low-grade metamorphic conditions. The deformation sequence affects rocks of the foreland together with Upper Proterozoic Moine metasediments. Displacements on the thrusts are towards the northwest and involved minimum movements of about 70 km (Elliot & Johnson 1980, Coward 1980). The timing of movement on at least some of the thrusts is bracketed by the upper Arenig to possibly lower Llanvirn age of the Durine Formation of the Durness Group and the age of 430 ___4 Ma for intrusion, during or after thrusting, of the Loch Borralan syenite (van Breemen et al. 1979; Parsons & McKirdy 1983). Late Silurian to early Devonian movements on the thrust zone have been postulated (Elliot & Johnson 1980) and are indicated by the occurrence of Rb-Sr and K - A r mica ages from Moine metasediments near the thrust zone, in the range 420 to 405 (Brook & Powell, unpublished results, Kelley, pers. comm.), if such ages reflect uplift due to overthrusting. K - A r data from mylonites of the Outer Isles thrust zone give ages between 471 and 394 Ma (Sibson 1977) which may indicate partial resetting at 394 Ma, that is during the Lower Devonian. 17

18

D. P o w e l l & W. E. A . Phillips

~~HET 60 ~

MOINE THRL

OUTER ISLES

/t

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Fig. 1. Time of deformation and tectonic zones of the British Caledonides. G-Girvan, B-Ballantrae.

The Northern Highland Zone (Fig. 2) Between the Moine thrust and the Great Glen fault the Moine schists, comprising predominantly metasediments, together with Lewisian basement inliers, form an extensive tract (Fig. 1) that has undergone polyphasal deformation and metamorphism. Outside the thrust zone, stratigraphical equivalents of the Cambro-Ordovician foreland shelf sequence are absent, but non-metamorphic sediments of Lower Devonian (upper Siegenian to upper Emsian) age unconformably overlie the metamorphic complex. Rb-Sr and U-Pb isotopic studies of Moine rocks imply a minimum age of sedimentation for the Moine schists of c. 1004 Ma

(Brewer et al. 1979, Aftalion & van Breemen 1980). An age equivalence of Moine sediments with at least part of the Torridonian sequence of the foreland is implied by recent palaeomagnetic work (Smith et al. 1983) but is not supported by older isotopic data for Torridonian sediments (Moorbath 1969). Over the last decade the recognition of major ductile shear zones, the Sgurr Beag slide, amongst others, (Tanner 1970, Tanneretal. 1970, Mendum 1979, Rathbone & Harris 1979, Powell et al. 1981, Rathbone et al. 1983), which on deep seismic evidence appear to be major crustal structures (Soper & Barber 1982, Smythe et al. 1982, Coward 1983) has led to a clearer understanding of the tectonic characteristics and poss-

19

T i m e d e f o r m a t i o n in the C a l e d o n i d e O r o g e n N. HIGHLANDS

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Fig. 2. Sedimentary, tectonic and igneous history of the Foreland and N Highland zone. In this and following tables, gaps in the stratigraphic record are indicated by vertical lines, unconformities by wavy lines; moderate to weak folding by the unornamented fold symbol, intense folding by the black fold symbol, thrusting by horizontal half arrows; granite intrusion by pecked balloons, gabbroic intrusions by black balloons, pegmatite emplacement by pecked oblique dykes. Isotopic ages where underlined, relate to extrusive or metamorphic events; where not, to intrusive events.

ible evolution, of the Moine sector. The major structure is dominated by a number of thrust nappes which, because of the syn-metamorphic nature of their bounding slide zones (Powell et al. 1981), must have developed at depths of more than 15 km in the continental crust. Their regional disposition and apparent geometry at depth, together with isotopic evidence for the age of movement on some (Breweret al. 1979, Powell et al. 1981, Brook & Powell unpublished results), accords with their being members of a family of Caledonian thrusts. Overthrust movements, which by implication were toward the northwest for the Sgurr Beag slide, involve minimum displacement of 15 to 50 km (Powell et al. 1981, Kelley, per. comm.). The Sgurr Beag and other slides effect ductile, synmetamorphic, reworking of pre-existent high-grade, mig-

matitic, Moine metasediments (Rathbone & Harris 1979, Powell et al. 1981) which bear evidence for at least two phases of deformation, metamorphism and a phase of regional, though sporadic, pegmatite emplacement, that are Precambrian in age (Gilletti et al. 1961, Brewer et al. 1979, van Breemen et al. 1974, Piasecki & van Breemen 1983, Powell et al. 1983). Consequently, identification of Caledonian, as against Precambrian, orogenic phenomena is difficult. Because of the lack of biostratigraphical control, assessment of the timing of deformation events within the Moine sector relies on isotopic data for intrusive and metamorphic rocks and correlation of structures found within the Moine thrust zone with those of the Moine 'hinterland'. In the southwestern and eastern Moine areas, mid-to late Ordovi-

20

D. P o w e l l & W. E. A . P h i l l i p s

cian metamorphic activity, up to middle amphibolite facies, is indicated by whole-rock Rb-Sr isochrons for metasediments of 467 ___20 Ma (Brewer et al. 1979) and 453-Ma (Rock et al. pers. comm.) and by the age of 456- 5 Ma from the Glen Dessary syenite (van Breemen et al. 1979) which, whilst being deformed and metamorphosed, itself intruded already deformed and metamorphosed Moine rocks (van Breemen et al. 1979, Roberts et al. in press) K - A r and R B Sr mica ages, ranging from 462 to 400 Ma from the southwestern Moine (Brook & Powell, unpublished results), probably reflect, not only cooling from this metamorphism, but also partial resetting of an older Precambrian or early Caledonian event. Assuming that rocks of the northern Moine area underwent a similar sequence of Caledonian events to those in the southwest, the age of 555 __- Ma for intrusion of the deformed and metamorphosed Carn Chuinneag granite (Pidgeon & Johnson 1974) provides a maximum age for Caledonian metamorphic activity and for movement on the Sgurr Beag slide (Rathbone 1980, S. Kelley, pers. comm., cf Lambert et al. 1979). In general, such a history is in accord with the ages of 405 +_- 15 Ma and 414 _ 3 Ma given for intrusion of the post tectonic, post metamorphic Grudie and Ross of Mull granites; with intrusion of the Cluanie granite into hot country rocks at 425 +_ 4 Ma; and with intrusion of the early tonalitic phase of the Strontian granite into ductile country rocks, at 435 --- 10 Ma. A prolonged period of cooling from c. 453 to c. 405 Ma is implied, that is from mid-late Ordovician to late Silurian-Lower Devonian. In the N Highland zone, deposition of late Siegenian to upper Emsian post metamorphic, molasse deposits demonstrate emergence of the N Highlands during the Lower Devonian. Further earth movements are, however, indicated by folding and faulting of Lower Devonian rocks before the unconformable deposition of over-lying Middle Devonian, early Eifelian to upper Givetian, sediments (Mykura 1983). Open, gentle folding followed in the late Givetian or early Frasnian, before deposition of the overlying, unconformable, Upper Devonian. To the northeast of mainland Scotland, in the possible equivalent of the N Highland zone in west Mainland Shetland (Fig. 1), Middle Devonian sediments unconformably overlying Precambrian metamorphic rocks underwent polyphase deformation and low-grade metamorphism before intrusion of the Sandsting granite at or before 371 - 10 Ma (Myurka 1976); that is during late Middle or Upper Devonian time.

major nappes and their translation southeastwards (Bradbury et al. 1979) or southwestwards (Shackleton 1979, Coward 1983). These events preceded intrusion of the Ben Vurich granite at 514 --- 6 Ma (Bradburyetal. 1976) and thus took place during the late Cambrian. In Shetland, the Colla Firth permeation belt forms part of the East Mainland succession that has been correlated with the Dalradian (Flinn et al. 1972). It contains schistose granitic rocks for which an Rb-Sr whole rock age of 514 +-- 25 Ma has been interpreted as a minimum age for early metamorphism (Flinn & Pringle 1976). The Dalradian zone, and its correlatives, thus underwent regional, compressive deformation during the late Cambrian but the orogenic belt was not necessarily emergent (Dewey 1982). Subsequent deformation (D3), peak metamorphism, and intrusion of the gabbros in NE Scotland all appear to be closely related in time (Pankhurst 1970), and are dated by the age of 489 +-- 17 Ma given for the syn-metamorphic intrusion of the Insch Gabbro. Depths of burial are thought

Upper

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The varied group of sediments and subsidiary volcanics that comprise the Dalradian Supergroup are on isotopic and biostratigraphic evidence of Vendian to Middle Cambrian age. Their stratigraphical relationships to the adjacent Highland Boundary Complex (Henderson & Robertson 1982), which contains rocks of Ordovician-lower Arenig age (Curry et al. 1982), are not, however, clear. Early phases of deformation (D~ & D z) and burial metamorphism of the Dalradian involved the formation of

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Fig. 3. Sedimentary,tectonic and igneous history of the Dalradian zone (Scotland) (symbols as Fig. 2).

Time deformation in the Caledonide Orogen

21

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.

St Davids Caerfai

~

,,

L"I' ,

.

,

i i :' ; iI

I

533 Caerfai LONGMYNDIAN

URICONIAN

558 667

RUSHTON SCHIST MONA COMPLEX

Fig. 16. Sedimentary, tectonic and igneous history of North Wales

Fig. 17. Sedimentary, tectonic and igneous history of Central Wales and the Welsh Borders.

34

D. Powell & W. E. A . Phillips Stephanian Westphalian

Namuri an

Dinantian

.........

,

IK

. . . . . . . . . . . . .

Upper

i

FRASNIAN

Ol

Middle

VETIAN

!

EIFELIAN i i

E MSIAN

Lower

i

,

,

SIEGENIAN ........ GEDINNIAN

Pridoli Ludlow Wenlock

I

L1andovery

..... IDWIAN

. . . . . . .

'

I

Ashgill

I

I

____CAUTLE_Y_AN ...... !

Z

i

Caradoc U

0

0

.

.

.

.

.

UPPER

i .

.

.

.

.

.

.

l

.

L]andeilo L1anvi rn Aren i g LOWER

Tremadoc

Merioneth

I St Davids ac

7 (.J

Caerfai

;-/-S

E587 643

Fig. 18. Sedimentary,tectonic and igneous history of South Wales. in Anglesey as, in part, Cambrian, and the unconformity at the base of the overlying Arenig-to-Llandovery sequence as, in places, recording only a short period of time, Barber & Max (1979) are led to conclude that low-grade metamorphism and much of the deformation of the Bedded Succession is Caledonian in age, post Llandovery rather than Precambrian. This conclusion is at variance with the observation of Gibbons (1983) and earlier workers, that the Penmynydd Zone schists in Llyn are derived by deformation and metamorphism of Gwna Group materials. In Anglesey, the schists are intruded by the Coedana granite which, on isotopic and biostratigraphic evidence, is pre-Ordovician. The lowest Cambrian to uppermost Silurian rocks of

North Wales show evidence for polyphase deformation (Helm et al. 1963, Dewey 1969, Coward & Siddans 1979), but the distribution and timing of the constituent events are not clearly constrained. Over much of North Wales, Arenig strata unconformably overlie older rocks (George 1963), and folding in the late Tremadoc is reported at Rhobell Fawr (Kokelaar 1979). The Llandeilo is not represented in Snowdonia, Llyn or Anglesey, perhaps suggesting deformation during this time, and an intra-Ashgill hiatus indicates post-Onnian, pre-Rawtheyan earth movements which, throughout most of the Welsh zone, may relate in part to fault movements (George 1963). Silurian strata, which share the folding of conformably underlying late Ordovician sediments in Anglesey and the Denigh-Conway area, attest to post Llandovery and post middle Ludlow deformation respectively. Devonian rocks are unfortunately only present in Anglesey where supposed Gedinnian sediments were folded and cleaved before deposition of the overlying Asbian stage limestones of the Dinantian. The intensity of deformation displayed by the Lower Devonian is, however, much less than that of nearby Siluro-Ordovician rocks. Carboniferous sedimentation in North Wales was, in part, controlled by fault movements, but further folding and faulting was post Westphalian but pre-Permian in age. Precambrian tectonic, igneous al~d metamorphic activity is recorded in the Welsh Borderland (Figs 1 & 17) in the inliers of the Longmynd, Wrekin and Malvern amongst others. Cambrian-to-Silurian strata unconformably overlie folded and, in places, metamorphosed rocks which have yielded isotopic ages of 702 _+ 8 Ma (Stanner Hill volcanics), 670 -+ 10 Ma (Malvernian gneisses), 667 -+ 20 Ma (Rushton Schists), and 448 -+ 16 Ma (Uriconian volcanics) Patchett et al. 1980, Beckinsale (pers. comm.). Unconformities and non-sequences in the Cambrian succession of the Welsh Borders and English Midlands indicate minor earth movements. The Ordovician succession is incomplete and Llandovery rocks resting unconformably upon folded Caradoc and older Ordovician rocks (George 1963), and a complete Arenig-to-Caradoc succession is only displayed in the Shelve inlier. The Caradoc transgresses eastwards across folded and faulted Tremadoc and Cambrian strata (George 1963). Thus, post Tremadoc, pre-Caradoc (Costonian) deformation, and that in the top Caradoc to middle Llandovery interval, involved production of major folds and faults. In southeast Central Wales, adjacent to the Towy anticline and Builth inliers, intra-Ashgill, pre-Llandovery folding and faulting was pronounced (George 1963). Further major breaks in sedimentation do not occur in Central Wales and the Welsh Borders until the Middle Devonian. End-Emsian to beginning-Famenian strata are absent, but movements during this time did not produce folds except along the Towy anticline. Thus, the last major Caledonian deformation phases occurred during Ashgill and Llandovery times (George 1963). In South Wales (Fig. 1), Lower Cambrian sediments rest unconformably on Precambrian volcanic and intrusive rocks from which isotopic dates of 643 --- 5 and 587 - 14 Ma are reported (Fig. 18). Vendian events are indicated. In the southwest, the major part of the Merioneth and the entire

35

T i m e d e f o r m a t i o n in the C a l e d o n i d e O r o g e n

Tremadoc series are absent, a hiatus which, taken with the unconformable relationships of the overlying Arenig, may indicate late Cambrian deformation. Ordovician rocks, ranging from basal Arenig to upper Caradoc, form a continuous sequence suggesting a period of tectonic quiescence. The absence of lower Ashgill strata, and the occurrence of a basal Cautleyan unconformity, attest to intra-Ashgill deformation. Ashgill sediments pass without break into Llandovery, but the Wenlock is locally unconformable and probably postdates folding in the late Llandovery (George 1963). Downtonian to Gedinnian sediments overlie Silurian-to-Precambrian rocks with marked unconformity in Pembrokeshire, recording strong, late Silurian folding, but further west the unconformity is much less pronounced and in places probably absent. This phase of deformation is probably restricted to the vicinity of the Towy anticline. In South Wales, Middle Devonian rocks are again absent except in Pembrokeshire. The Upper Devonian (Famennian) is unconformable, but Middle Devonian folding is not apparent. Carboniferous rocks conformably overlie the upper Devonian except in the southwest, where Namurian strata overstep folded Ordovician and Llandovery sediments. During the Carboniferous, intra-Dinantian, pre-Namurian, and intra-Westphalian movements influenced sedimentation (Dunne 1983). Strong folding in the Namurian-lower Westphalian interval is apparent in the Forest of Dean, but the main northward-directed folding and thrusting of Hercynian age occurred during late Westphalian to early Permian times.

Stephan 13q i

d e s t p q a : ',m

Namurian

Dinantiar,

[AM[NN

The structural evolution of the Devonian and Carboniferous rocks of Devon and Cornwall has recently been interpreted as involving the progressive northward propagation of major thrusts and attendant folding, followed by southward back-thrusting and folding (Shackleton et al. 1982). Early thrust movements appear to have commenced in the south, during the Lower Devonian, with emplacement of the Start-Lizard thrust nappe. Sedimentation during the late Visean and Namurian as influenced by northward propogating thrusts (Isaac et al. 1982). In the central area, thrusting accompanied by folding and cleavage formation is dated as Namurian in age (Isaac et al. 1982), whilst to the northeast, in the Mendips, such deformation is post Lower Stephanian (Shackleton et al. 1982). Subsequent folding, back-thrusting, followed by normal faulting is variously interpreted as of Westphalian or Lower Permian age (Isaac et al. 1982, cf Shackleton et al. 1982). Thus early deformation appears largely to have accompanied sedimentation and was diachronous from south to north. Emplacement of the Southwest England granite batholith is dated at 286 +--4 Ma, and is thought to have reached high crustal levels by Westphalian (Isaac et al. 1982) or Lower Permian times (Shackleton et al. 1982). Cooling ages for slates range from 365 to 345 Ma in SW Cornwall, to 340 to 320 Ma in the central belt, and 290 to 270 Ma in the Culm synclinorium (Dodson & Rex 1971), indicating diachroneity of uplift.

AN

Upper FRASNIAN

Middle

EMSIAN

....... SIEGENIAN

Lower

~!

Pridoli

oc

Ludlow

.~

Wenlock

Llandovery

Ashgi 11

IIiliq!bIiil !i,i1'iI!q!II]

'

/ Llanvirn

o~

Arenig

o

!l ~I ~ 11''

-1

=.

o

I

Southwest England zone (Fig. 19)

.L

I!' i" i

I L!liL:![!i 1

!

IIHJ

l~g. 19. Sedimentary, tectonic and igneous history of Southwest England.

Summary and conclusion The main Caledonian tectono-metamorphic activity affecting the rocks of the N Highland zone took place at 450-460 Ma; that is, during the Llandeil-Caradoc interval. There is no isotopic or other evidence to suggest the occurrence of Grampian orogenic activity. Early movements on the Moine thrust zone and on 'slide' zones within the orogenic belt may relate to this Ordovician deformation, but deformation and metamorphism involving overthrusting also took place in mid-to-late Silurian times; that is, at around 420 Ma. Final uplift and cooling appear to coincide with intrusion of post metamorphic, post tectonic granites. In contrast, orogenic activity in the Dalradian zone, is late Cambrian to Ordovician in age (c. 540 to 460 Ma) with peak metamorphic conditions achieved during the Lower Ordovician (Arenig?) at c. 490 Ma. Uplift took place during Upper Ordovician times (460 to 440 Ma); that is, at the time of peak Caledonian metamorphic activity in the N Highland zone. A further contrast is the apparent absence of mid-to-late Silurian tectono-metamorphic events in the Dalradian zone except in Shetland. Thus, despite arguments supporting continuity of the N Highland and Dalradian zones during the development of the Caledonian orogenic belt (van Breemen & Piasecki

36

D . P o w e l l & W. E. A . P h i l l i p s

1983, Coward 1983), and suggestions of diachroneity across the zones (van Breemen et al. 1979), it remains possible that the two zones evolved as separate entities until late Silurian to Devonian times. The earliest common Caledonian events appear to be intrusion of the 'younger granites' between 415 and 400 Ma (Figs. 2 & 3). The apparent absence of Dalradian rocks beneath the Midland Valley zone, the occurrence of undeformed Tremadoc limestone and 560 Ma granite clasts in Ordovician conglomerates at Girvan, and the presence of relics of Ordovician oceanic rocks along the Highland Border, may imply considerable separation of the Dalradian and Midland Valley zones during Vendian and Cambrian times. There is considerable doubt as to the validity of lithological correlations with the Dalradian of metasediments in the central and southwest Ox Mountain (cf. Pankhurst et al. 1976, Long & Max 1977). The low greenschist-facies metagreywackes of 'upper' Dalradian aspect were deformed considerably later (477 Ma) than the main deformation of the Dalradian (pre-514 Ma). This younger (mid-Ordovician event) is readily equated with mid-Ordovician deformation in south Mayo and Tyrone. The silicious granulites of the northeast Ox Mountains are plausibly correlated with the gneissic basement of the Midland Valley. These factors are consistent with major displacements being inferred on the Highland Boundary fault and its continuation into Ireland. However, the temporal coincidence of obduction-related metamorphism and deformation at Ballantrae with the peak of metamorphic activity in the Dalradian zone during the early Ordovician (Arenig?), suggests that basin closure and-or collision of a microcontinent (Watson & Dunning 1979) had occurred by early Ordovician times. Initiation of the Southern Uplands accretionary prism might have been a consequence of such a collision. In the Longford-Down and Southern Uplands zone, sedimentation spans the period Llanvirn-Wenlock and there is no direct expression of the important mid-Ordovician tectonism of the zone to the north. Influx of volcanic, plutonic and metamorphic detritus is the only indication of events to the north during the late Ordovician. This contrast in deformation history favours allochthonous models for the Southern Upland-Longford Down zone, involving imbricate thrusting and-or transcurrent displacements. Caledonian deformation in the Southern Upland zone, if related to development of an accretionary prism, may coincide in time, with the main Caledonian tectono-metamorphic activity in the N Highland zone. It seems unlikely, however, that the latter was due directly to subduction. There is a marked contrast between the timing and nature of deformation of the Lake District and the Southern Uplands in Britain. In the latter there is an absence of the Ordovician deformation and granite emplacement of the Lake District. In Ireland Ordovician tectonism died out, particularly in the northwest of the Leinster zone. In most of central Ireland Silurian rocks are juxtaposed across the suture zone. Though there is no evidence of different timing of deformation on either side of the suture, recumbent F1 structures are restricted to the northwest side. South of the Iapetus suture, sporadic outcrops of Pre-

cambrian basement provide evidence for a fundamental change in crustal structure in as much as, in contrast to the terrain to the north, Vendian orogenic events are recorded. Late Tremadoc, pre-Arenig deformation affected North Wales and parts of the South Wales area but were not widespread throughout the Welsh sector. The base of the Arenig is not seen in the Lake District. The earliest widespread Caledonian deformation appears to be that in the late Ordovician, in the Caradoc-Ashgill interval. Folding in the early Silurian is restricted to south Central and South Wales and the Welsh Borders. In Ireland the early Cambrian (535 Ma) and early Silurian (436 Ma) granites of the Rosslare c o m p l e x p l u s the major pre-Arenig unconformity show a marked contrast in deformation history from the rest of the Leinster zone. The major faults which separate the two areas suggest that important displacements have brought the two areas together in Silurian or later times. In Britain and Ireland, deformation that may relate to collision following the closure of Iapetus, appears to overlap the Silurian-Devonian boundary whether defined biostratigraphically or isotopically. Thus, intense folding in the late Silurian is recorded in parts of the Midland Valley zone, in North Wales, and in the western part of the South Wales area. In the Lake District zone, possibly the Southern Upland zone, and parts of the Midland Valley, such deformation may be earliest Devonian (Gedinnian) in age. Crustal readjustments following collision are presumably responsible for the intra-Devonian deformation recorded throughout the British Caledonides.

Acknowledgments Encouragement and advice, (in some cases unbeknown to them), during the preparation of this report and the accompanying 1:1000,000 map, has been offered by: Bernard Anderson, Peter Banham, Maureen Brook, David Coller, Mike Coward, Douglas Fettes, Derek Flinn, Howel Francis, Tony Harris, Stuart McKerrow, Grace Page, David Sanderson, Alec Smith and Jack Soper. We are, however, responsible for any mistakes or misinterpretation.

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37

Duff, P. McL. D. & Smith, A. J. (in prep). Geology o f England & Wales. Scottish Academic Press, Edinburgh. Dunne, W. M. 1983. Tectonic evolution of SW Wales during the Upper Paleozoic. J. geol. Soc. London, 140, 257-66. Elliott, D. & Johnson, M. R. W. 1980. Structural evolution in the northern part of the Moine Thrust Zone. Trans. R. Soc. Edinburgh, 71, 69-96. Emo, G. T. & Smith, D. G. 1978. Palynological evidence for the age of the Lower Palaeozoic rocks of slieve Aughty, Counties Clare and Galway. Proc. R. Irish Acad. 78B, 281-92. Flinn, D., May, F., Roberts, J. L. & Treagus, J. E. 1972. A revision of the stratigraphic succession of the East Mainland of Shetland. Scott. J. Geol. 8, 335-43. & Pringle, I. R. 1976. Age of the migmatization in the Dalradian of Shetland. Nature 259, 299-300. Flinn, D., Robinson, T. & Thomas, R. E. 1984. The Caledonides of Shetland. In: Gee, D. G. & Sturt, B. A. (eds) The Caledonide orogen Scandinavia and related areas Wiley Interscience. Francis, E. H. 1983. Carboniferous. In: Craig, G. Y. (ed.) (q.v.), 253-96. Gale, N. H., Beckinsale, R. D. & Wadge, A. J. 1980. Discussion of a paper by McKerrow, Lambert and Chamberlain on the Ordovician, Silurian and Devonian time-scales. Earth. planet. Sci. Lett. 51, 9-17. George, T. N. 1963. Palaeozoic growth of the British Caledonides. In Johnson, M. R. W. & Stewart, F. H. (eds) The British Caledonides. Oliver & Boyd, Edinburgh & London, 1-34. Gibbons, W. 1983. The Monian 'Penmyndd Zone of Metamorphism' in Llyn, North Wales. Geol. J. 18, 21-41. Gill, W. D. 1962. The Variscan fold belt in Ireland. In: Coe, K. (ed.) Some aspects o f the Variscan fold belt. Manchester University Press, 44-64. Gilletti, B. J., Moorbath, S. & Lambert, R. St. J. 1961. A geochronological study of the metamorphic complexes of the Scottish Highlands. Q. J. geol. Soc. London, 118, 233-72. Graham, A. M. & Upton, B. G. J. 1978. Gneisses in diatremes, Scottish Midland Valley: petrology and tectonic implications. J. geol. Soc. London, 135, 219-28. Graham, J. R., Richardson, J. B. & Clayton, G. 1983. Age and significance of the Old Red Sandstone around Clew Bay, NW Ireland. Trans. R. Soc. Edinburgh, 73, 245-49. Gray, J. R. & Yardley, B. W. D. 1979. A Caledonian blueschist from the Irish Dalradian. Nature, London, 278, 736-37. Halliday, A. N., Aftalion, M. & Leake, B. E. 1980. A revised age for the Donegal granites. Nature, London, 284, 542-43. Harland, W. B., Cox, A. V., Llwellyn, P. G., Pickton, C. A. G., Smith, A. G. & Waiters, R. 1982. A geologic time scale. 131pp. Cambridge University Press. Harris, A. L., Holland, C. H. & Leake, B. E. (eds) 1979. The Caledonides o f the British Isles--reviewed. Spec. Publ. geol. Soc. London, 8, 768pp. , Parson, L. M., Highton, A. J. & Smith, D. I. 1981. New/Old Moine relationships between Fort Augustus and Inverness (abstr). J. Struct. Geol. 3, 187-88. Hartley, J. J. 1933. The Geology of northeastern Tyrone. Proc. R. Irish Acad. 41B, 218-85. 1936. The age of the igneous series of Slieve Gallion, Northern Ireland. Geol. Mag. 73, 226-8. Helm, D. G., Roberts, B. & Simpson, A. 1963. Polyphase folding in the Caledonides south of the Scottish Highlands. Nature, London, 200, 1060-62. Henderson, W. G. & Robertson, H. F. 1982. The Highland Border rocks and their relation to marginal basin development in the Scottish Caledonides. J. geol. Soc. London, 139, 435-52. Isaac, K. P., Turner, P. J. & Stewart, I. J. 1982. The evolution of the Hercynides of central SW England. J. geol. Soc. London, 139, 523-34. 1Johnson, M. R. W., Sanderson, D. J. & Soper, N. J. 1979. Deformation in the Caledonides of England, Ireland and Scotland. In : Harris, A. L., Holland, C. H. & Leake, B. E. (eds) (q.v.), 165-86. Kokelaar, B. P. 1979. Tremadoc to Llanvirn volcanism on the southeast side of the Harlech Dome (Rhobell Fawr), N. Wales. In: Harris, A. L., Holland, C. H. & Leake, B. E. (eds) (q.v.), 591-96. Lambert, R. St. J. & McKerrow, W. S. 1976. The Grampian orogeny. Scott. J. Geol. 12, 271-92. , Winchester, J. A. & Holland, J. G. 1979. Time, space and intensity relationships of the Precambrian and lower Palaeozoic metamorphisms

38

D. Powell & W. E. A.

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39

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D. Poweil, Department o f Geology, Bedford College (University o f London), Regent's Park, London NW1 4NS W. E. A. Phillips, Department o f Geology, Irinity College, Dublin

Grade and time of metamorphism in the Caledonide Orogen of Britain and Ireland D, J . F e t t e s , C. B. L o n g , R. E. B e v i n s , M . D. M a x , G. J . H . O l i v e r , T . J. P r i m m e r , L. J . Thomas, & B. W . D. Yardley

areas, however, as in the Scottish Moine, the different peak metamorphic effects cannot be distinguished, and in these area the facies indicated relates to the highest grade for which evidence is preserved in the rocks, regardless of its age. For further discussion of the distribution of the metamorphic facies particularly in the orthotectonic zone the reader is referred to Fettes (1983) and to Long et al. (1983) and the references therein.

Plate 3 depicts three major belts distinguished by their style and age of metamorphism: the orthotectonic and paratectonic zones of the Caledonides and the Hercynides. Age of metamorphism, shown on the inset map (Plate 3), in part reflects the areal distribution of these zones and refers to the timing of the main phase or peak of metamorphism. The orthotectonic zone is, therefore, shown to be metamorphosed during the earliest Ordovician although recrystallization was almost certainly taking place from the late or even mid-Cambrian to the Silurian. In the paratectonic belt the main metamorphism is shown as occurring in the period Lower Ordovician-to-Silurian although recrystallization may well have continued into the Lower Devonian. The Lower Devonian rocks within the central and southern Caledonides have suffered folding and weak metamorphism normally before Middle Devonian times and have been differentiated on the map. The southern limit of significant Caledonian deformation is normally taken as the northern edge of the Midland Block (F. W. Dunning, pers. comm.). However, it is believed that weak Caledonian metamorphism affected rocks south of this line. Similarly, recrystallization associated with the Hercynian extended well to the north of the so-called Hercynian front, the northern limit of strong deformation (F. W. Dunning, pers. comm.). There is, therefore, some doubt as to whether the main recrystallization in pre-Carboniferous rocks of South Wales and the English Midlands relates to the late-Caledonian or to the Hercynian. Metamorphic grade shown on Plate 3 is represented by means of the facies scheme outlined in Table 1. The term 'weak metamorphism' is used for all rocks below the greenschist facies; the zeolite and prehnite-pumpellyite facies are distinguished where known. Because of the difficulty in defining where metamorphism begins and diagenesis ends, the term weak metamorphism is used to cover all low-grade rocks involved in the orogen even though these are effectively non-metamorphic. An example of this is the Carboniferous of the Midland Valley of Scotland. Facies is determined by the highest mineral assemblage preserved in the rock even though this may have been largely retrograded. For example, in polyphasal metamorphic terrains where a kyanite + staurolite metamorphism is followed by a sillimanite overprint and a later partially retrogressive chlorite-grade phase, the facies shown on Plate 3 is based on the sillimanite-bearing assemblages. In polymetamorphic terrains the earlier peak metamorphism is represented by coloured dots, for example granulite-facies relics of Lewisian basement. In some

Foreland to the Caledonides The foreland rocks of the Caledonide orogen comprise Lewisian crystalline basement with Torridonian and Cambro-Ordovician sedimentary cover. The Lewisian has suffered polymetamorphism during at least three tectonothermal events, namely, the Scourian (c. 2600 Ma), Inverian (c. 2200 Ma) and Laxfordian (c. 1600 Ma). The metamorphism was generally in the granulite or upper amphibolite facies. Rocks similar to those of the foreland are involved in the Caledonides as thrust slices in the Moine thrust zone and the Lewisian occurs as tectonically introduced inliers within the Moine rocks of the orogen. West of the Moine thrust, low-grade Caledonian recrystallization can be identified in the foreland rocks, particularly in the Torridonian of Southern Skye. In the Outer Hebrides the formation of mylonite on the Outer Hebrides thrust zone is believed to be of Caledonian age. An extensive belt of mainly hydrous retrogression thought to be associated extends into the Lewisian basement producing epidote-chlorite-sericite-quartz-albite assemblages. In the southeast the nature of the foreland is less well known although fragments of old crust occur locally (see Anderton et al. 1979).

Orthotectonic zone of the Caledonides The orthotectonic zone of the Caledonides is bounded to the north and west in Scotland by the Moine thrust zone and to the south by the Highland Boundary fault. In Ireland the NE Antrim, Donegal-Fermanagh, Tyrone, Ox Mountains, NW Mayo and Connemara inliers form part of the orthotectonic zone. The zone is compoased of two lithological assemblages: 1) the Moine, occupying the northern two-thirds of the zone in the Northern and Central Highlands of Scotland, is composed predominantly of psammites and pelites both of which commonly contain calc-silicates and metabasic rocks: 2) the Dalradian forming the southern part of the zone passes stratigraphically upwards from the Moine and comprises a diverse assemblage consisting of 41

42

D. J. Fettes et al.

Table

1, Classification of facies. Diagnostic Minerals Facies Zeolite

Weak

Symbol

W

Pelite

Basic Rocks

Kaolinite

Analcime Laumontite

PrehnitePumpellyite

GS

Epidote Amphibolite

E

Lower

E
10), Prehnite + Pumpellyite

Plagioclase (Anl0-An30)

Biotite, Chlorite, Muscovite

Staurolite, Kyanite, Andalusite, Cordierite

Plagioclase (An3o-An65)

Garnet, Biotite, Muscovite, Cordierite Hornblende

Sillimanite Ortho- + Clinopyroxene Chlorite + Actinolite Cordierite + Garnet

Andalusite

AA

Forbidden Minerals

Chlorite, Saponite, Dolomite + Quartz 'White Mica' Ankerite

Prehnite + Pumpellyite

Greenschist

Common Minerals

Calc-Silicate

Kyanite Plagioclase (An > 65)

Sillimanite + K-Feldspar

Andalusite Sillimanite + K-Feldspar

Plagioclase (An > 65)

Orthopyroxene + Clinopyroxene Glaucophane + Lawsonite, Jadeite + Quartz Aragonite

quartzites, limestones, pelites including black schists, greywackes and basic volcanics. Recent work (Piasecki et al. 1981, Brewer et al. 1979, Harris et al. 1981) has shown the Moine to consist of two series which have a complex cover-basement relationship. Harris et al. (1978) suggested that the term Moine should be restricted to the basement rocks, which occur in the area northwest and immediately southeast of the Great Glen fault in Scotland and in the NE Ox Mountains and Lough Derg areas of Ireland. They referred the cover to the Grampian Group of the Dalradian into which it can locally be seen to pass by sedimentary passage. The sedimentary age of the Moine is uncertain but may be late-Middle to early-Upper Proterozoic. The rocks were subjected to two pre-Caledonian thermal events, namely, the Grenville (c. 1000 Ma) and the Knoydartian/ Morarian (c. 750 Ma); the latter has not been recognized in Ireland. The Dalradian sediments range in age from Upper Proterozoic through to lowermost Middle Cambrian. In Shetland the metamorphic rocks are dived by the N - S trending Walls Boundary fault. West of the fault foreland rocks (Western Series of Pringle 1970) pass eastwards into metasedimentary rocks of uncertain age (Eastern Series of Pringle 1970). East of the fault, the East Main-

Garnet, Cordierite, K-Feldspar, Sillimanite, Hornblende, Calcic Plagioclase

Staurolite, Muscovite, Epidote, Zoisite

Stilpnomelane Pumpellyite

Andalusite

land succession is composed of a thick (22-27 km) series of near vertical beds (Mykura 1976) which has been correlated with the Dalradian (sensu Harris et al. 1978) sequence of the Scottish Highlands (Flinn et al. 1972). Basis of facies

In the Dalradian rocks the grade is generally assessed in terms of assemblages in pelitic rocks which can be referred to the classical Barrovian (biotite --, garnet ~ staurolite kyanite --~ sillimanite) and Buchan (biotite ~ cordierite --~ andalusite ~ sillimanite) zones. Considerable data exist on the distribution of these index minerals and associated isogradic surfaces (see summaries in Atherton 1977, Fettes 1979, Long et al. 1983). In the Moine rocks of Scotland the widespread development of index minerals in pelitic rocks apparently has been inhibited by the unsuitable chemical composition of pelites and, following the pioneering work of Kennedy (1949), the grade has traditionally been determined in terms of mineral assemblages in calc-silicates. Kennedy's scheme was expanded by Tanner (1976) to give the following sequence; albite + zoisite + calcite + biotite --~ oligoclase + zoisite + calcite + biotite ~ andesine +

Metamorphism

in the C a l e d o n i d e O r o g e n

zoisite + biotite andesine + zoosite + hornblende bytowite/anorthite + hornblende ~ bytowite/anorthite + pyroxene. Various workers, but primarily Winchester (1974), have refined the zonal scheme by reference to the chemical composition of the calc-silicates in particular the CaO:AI203 value. However, in accord with the arguments of Powell et al. (1981) the present map is based on the anorthite content of plagioclase as detailed in Table 1. Data have been drawn from a range of sources including (Tanner 1976, Winchester 1974, Soper & Brown 1971, Powell et al. 1981). In constructing Table 1, correlations have been made between the zonal scheme based on pelites and that based on calc-silicates. Although a practical necessity, such correlations are not wholly satisfactory. Theoretical considerations and the detailed work of Powell et al. (1981) in western Inverness-shire suggest that where a pelite-based isograd can be defined it may well intersect the calc-silicate based isograds. Equally in the Moine of the far north of Scotland albite + epidote assemblages (Soper & Brown 1971), defining the greenschist facies, coexist with garnetiferous pelites although it should be stated that the two assemblages may be of different age. The base of the upper amphibolite facies, based on sillimanite + K-feldspar assemblages in pelites, cannot be satisfactorily defined in calc-silicate assemblages. This is because the highest grade assemblage in calc-silicates, namely, anorthite + pyroxene, coexists with both middle and upper amphibolite assemblages, the pyroxene isograd lying within the sillimanite zone (Powell et al. 1981, Soper & Brown 1971).

Polymetamorphic (Moine) terrains Lewisian basement slices occur along the length of the zone, mainly in the north and west, where the post Lewisian cover was presumably thinnest. Relic assemblages of granulite and high amphibolite facies can be identified in a number of these inliers. For example, in Shetland in the Western Series of Pringle (1970), at Borgie (Moorhouse 1976), the Glenelg-Attadale inlier (Barber & May 1976, Sanders 1979) which also shows relics of eclogite facies (Sanders 1979), at Inishtrahull (Long et al. 1983, MacIntyre et al. 1975) and in the Erris complex of Co. Mayo (Long et al. 1983). In the Moine rocks northwest of the Great Glen the grade rises rapidly from the greenschist facies in the west to the middle amphibolite facies. In Inverness-shire and southern Ross-shire the high-grade rocks lie in a narrow belt, the grade falling off eastwards to the lower amphibolite facies which occupies a broad belt across to the Great Glen. How far this pattern reflects the polymetamorphic effects of the Grenville, the Knoydartian/Morarian and the Caledonian is still uncertain. This problem is partly caused by the absence of reliable regional correlations of locally determined structural sequences and the lack of any recognized regional event which would define the onset of the Caledonian. In southern Inverness-shire Powell et al. (1981) have suggested that the pre-Caledonian metamorphic grade rose from garnet grade in the west to middle amphibolite facies

43

in the east, and that this metamorphic pattern was foreshortened by sliding (ductile thrusting) and associated folding at the beginning of the Caledonian. These movements, Powell et al. (1981) believe, were broadly synchronous with the main Caledonian overprint, which rose in a few kilometres from low greenschist (chlorite zone) facies in the west to middle amphibolite facies in the east. The high-grade nature of the Caledonian effects is supported by the disposition of the pyroxene isograd (in calc-silicates) which cuts across folds of the early Caledonian slides. This contrasts with the views of Roberts et al. (in press) who suggest that in Glendessary the Caledonian appears to be a retrogressive overprint of lower amphibolite facies during the imposition of steep structures the eastern limit of which marks the eastern edge of the high-grade belt. In the Moines of the northernmost Highlands the grade rises from west to east. The low-grade metamorphism is later than the formation of mylonites on the Moine thrust and of related slides within the Moine (Soper & Brown 1971, Mendum 1979) and is probably of Caledonian age. However, the middle to upper amphibolite-facies rocks in NE Sutherland appear to be cut by a pre-Caledonian granite (Lintern et al. 1982, see below) and it is possible that the Naver slide (Moorhouse & Moorhouse 1983) running along the western side of the high-grade rocks is responsible for juxtaposing two terrains of different age. If so, the Caledonian is present only as a weak overprint on these high-grade rocks. It should be noted, however, that Soper & Brown (1971) described an apparent metamorphic transition into the high-grade rocks and that the Naver Slide is poorly defined in the south. East of the Great Glen polymetamorphic terrains have been described (Piasecki 1980, Harris et al. 1981) where a retrogressive overprint has been imposed on a middle amphibolite basement. The exact limits of the polymetamorphic rocks are uncertain. In Ireland, polymetamorphic terrains with complex histories have been identified in the NW Mayo, Lough Derg, E Rosses Point and NE Ox Mountains inliers. In these areas the rocks show evidence of Grenville granulite-facies metamorphism partially overprinted by a low amphibolite Caledonian metamorphism (Long et al. 1983). Age of metamorphism

In the Moine of the N Highlands of Scotland the pre-metamorphic Carn Chuinneag granite (550--- 10 Ma Pidgeon & Johnson 1974) gives a maximum age for the main Caledonian metamorphism which is here in the lower amphibolite facies (kyanite + staurolite). This contrasts with an age of 649--- 32 Ma (Lintern et al. 1982) on the Strath Halladale granite (some 50 km north of Carn Chuinneag) which apparently cuts the high-grade amphibolite-facies (sillimanite-muscovite-K-feldspar) country rocks. A late low-grade overprint possibly records the Caledonian effects. If these dates are accepted it follows that post metamorphic movements on the Naver slide (Moorhouse & Moorhouse 1983), bounding the highgrade rocks to the west, have transported these rocks from the east and juxtaposed them against the lower amphibo-

44

D. J. F e t t e s et al.

lite-facies Caledonian rocks. These arguments, if correct (see above), have two major implications; firstly that movements on the Naver slide are late in the Caledonian and secondly that because these rocks must have come from a general eastward direction it follows that the grade of Caledonian metamorphism fell to the east. In the Moine of S Inverness-shire an age of 1004 -+ 28 Ma has been suggested for the pre-Caledonian metamorphism (Brook et al. 1976) and an age of 467 + 20 Ma as the minimum age of Caledonian metamorphism (Brewer et al. 1979). A further constraint on the age of the Caledonian metamorphism is provided by the date of 456 -+ 5 Ma (van Breemen et al. 1979) on the Glendessary syenite which pre-dates the main metamorphism. Powell et al. (1981) suggest a spread of ages of 4 7 0 - 4 0 0 Ma for recrystallization. They argue that the main metamorphism was synchronous with the early sliding and associated folding and that the rocks were at high temperatures before the sliding. Smith (1979) has demonstrated the existence of regional thermal gradients at the time of intrusion of a regional microdiorite suite between 4 5 0 - 4 2 0 Ma. In Ireland a preliminary age of c. 895 Ma has been derived (Long & Yardley 1979) for the probable Grenville rocks of the NE Ox Mountains and Lough Derg inliers, though supporting data have not yet been published. The rocks in these inliers have a strong similarity to rocks of the Grenville inliers of northern Newfoundland (Church 1969). The Annagh Division gneisses of the Erris Complex on the Mullet peninsula and adjacent mainland of NW Co. Mayo have yielded Grenville ages of C. 1050-950 Ma, though even older c. 2000 Ma basement is thought likely. The Inishkea Division is a younger c. 750 Ma part of the same complex (Winchester & Max pers. comm.).

P - T estimates

Barr (1983) has suggested the following conditions of peak metamorphism, in the Moine west of the Great Glen: P = 6.5 kbar, T = 520-585~ in the epidote-amphibolite facies; P = 6 . 5 - 7 k b a r , T = 580-615~ in the lower amphibolite facies and P - 6-6.5 kbar, T -- 640~ in the middle amphibolite facies. It should be emphasized that these results probably relate to the early metamorphism. East of the Great Glen, Highton (pers. comm.) has reported P -- 6.5 kbar and T = 650vC for the upper part of the lower amphibolite facies. In the NE Ox Mountains P > 10 kbar and T > 800~ have been reported (Phillips et al. 1975).

Non-polymetamorphic (Dalradian) terrains The distribution of facies in these areas is well documented (see summaries in Fettes 1979, 1983, Yardley 1980, Long et al. 1983). In Scotland the grade rises quickly northwards from the Highland Boundary fault, the greater part of the belt lying in the amphibolite facies. In Ireland the pattern is less obvious, the orthotectonic zone being exposed only in a few inliers. Rocks within these show no clear zonal progression though grade generally increases towards areas of

pre-Caledonian basement. Recognition of the prograde facies is particularly difficult in NW Donegal due to extensive greenschist facies retrogression. In Shetland the grade generally rises towards a central belt. The highest grade rocks generally lie in the middle amphibolite facies locally rising to the upper amphibolite and in one case to the granulite facies. In Scotland, including Shetland, the highest grade rocks are marked by the development of migmatites and sillimanite (commonly fibrolite) crystallization. Considerable debate has taken place as to whether or not these features are the result of a separate later phase of metamorphism, although now it is accepted that in most cases they are part of the general progressive sequence (cf. Flinn 1967, May 1970). The highest grade rocks in Scotland appear in the NE Highlands. The metamorphic pattern is complicated by the contact effects of the basic intrusions but Harte & Hudson (1979) have attempted to define a 'regional' sillimanite isograd. Within this zone sillimanite + K-feldspar assemblages are found locally. Recently, Baker & Droop (1983) have described garnet + clinopyroxene and garnet + sillimanite + K-feldspar assemblages from metabasic rocks and metapelites in Deeside, that is within the 'regional' sillimanite zone. Baker & Droop (1983) have calculated peak conditions in these rocks as P = 8-+ 1.5 kbar and T -- 820 + 50~ They also suggest conditions in the kyanite zone to the west as P = 6 - 7 kbar and T = 500-600~ This accords well with the results of Sivaprakash (1982) who reported T = 500-580~ and P = 8-11 kbar for epidote-amphibolite and lower amphibolite facies rocks from the Central Highlands of Scotland. In the Central Highland Moine upper amphibolite facies migmatites have been described by Ashworth (1979b), although these may relate to pre-Caledonian metamorphism. In Ireland the highest grade rocks are in Connemara where the granulite facies is locally attained with sillimanite + cordierite + K-feldspar gneisses and garnet + cordierite + sillimanite + plagioclase gneisses. Intrusive migmatites were emplaced in this zone. Partial melting gave rise to local migmatization in the high amphibolite facies zone. Estimated conditions for these rocks have been reported as P = 4 - 7 kbar and T = 700~ (Long et al. 1983). Yardley et al. (1979) suggest conditions of P = 6-7 kbar and T = 600~ for lower amphibolite-facies (kyanite + staurolite) rocks adjacent to the Lough Derg and NE Ox Mountains pre-Caledonian inliers. In Scotland the facies series shows evidence of decreasing pressure from southwest to northeast (Fettes et al. 1976). This is reflected in the work of Harte & Hudson (1979) who proposed a four-fold division of the eastern Dalradian, namely; 1) Barrovian, biotite ---, garnet ~ staurolite kyanite; 2) Stonehavian, biotite --* garnet ~ chloritoid + biotite ~ staurolite --* sillimanite; 3) W Buchan, biotite --~ cordierite ~ andalusite ~ staurolite ~ kyanite; 4) E Buchan, biotite ~ cordierite ~ andalusite ~ silliamite. Harte (pers. comm.) has also argued that chloritoid + biotite pairs in the Central Highlands may represent an early phase in the progressive metamorphic sequence, the later phase being at a higher pressure (Barrovian) (cf. Atherton

M e t a m o r p h i s m in the Caledonide Orogen & Smith 1979, see Fettes 1983). There is, therefore, a probable variation in facies in both space and time. In the south of NW Co. Mayo, on Achill Island, an early Grampian 'blueschist' assemblages with crossite, considered most likely to represent high pressure greenschist-facies conditions, is preserved in an epidosite band at one locality. Overprinting by late medium pressure (Barrovian) assemblages has apparently destroyed this early assemblage elsewhere in the same basic volcanic formation, which can be traced inland intermittently for c. 35 km (Gray & Yardley 1979). The crossite is variably replaced directly by a barroisitic amphibole, and actinolite may mantle either amphibole. In Connemara evidence of an early, possibly widespread, Barrovian metamorphism is preserved at a few localities in the south of the area of Dalradian rocks, where staurolite and kyanite have been found as armoured inclusions in higher grade rocks (J. Barber pers. comm.) whose assemblage relates to a later, lower pressure (particularly in the south) Barrovian metamorphism which rises in grade to granulite facies in the south. Regional uplift and steepening of the thermal gradient gave rise (especially in north Connetnara) to an even later (D3) low pressure (Buchan) metamorphism with late andalusite and cordierite, and sillimanite which replaces andalusite (Yardley 1976, 1980). More recent work has identified prehnite-pumpellyite assemblages resulting from late extensive hydration (Yardley, pers. comm.). Through the small tectonic window of the Delaney Dome in SW Connemara epidote-amphibolite-facies rocks, chiefly of rhyolitic composition though of uncertain affinity, are seen structurally to underlie the parautochthonous high-grade Dalradian schists and gneisses and Connemara gabbro suite.

45

tion (Leslie & Kneller, pers. comm.). Leslie (pers. comm.) has suggested conditions of P = 3-4 kbar and T = 720-750~ for garnet + cordierite hornfels close to the contact bf the eastern basic masses. In Connemara a c. 500 Ma complex gabbroic suite (earliest date 510 Ma) was intruded broadly coevally with the peak metamorphic conditions (Leake 1970a) and followed by the extensive development of intrusive and migmatitic gneisses (Leake 1970b). It is generally concluded that, although local aureoles (Evans 1964, Evans & Leake 1970, Leake & Skirrow 1960) can be defined, the migmatite development is too widespread to relate directly to the gabbros and it is probable that both may relate to a common cause (Yardley 1980, Long et al. 1983). The gabbroic bodies were subsequently broken up during continuing and later deformation phases (Leake 1970a). Estimates of P = c.5 kbar and T up to 850~ as aureole conditions, with T = c.1000~ for pelitic xenoliths have been given (Evans & Leake 1970, Treloar 1981). In central Tyrone a suite of basic rocks, possibly related to those of Connemara and NE Scotland, occupies much of the inlier. The country rock contains high-grade upper amphibolite facies rocks. The extent to which these represent regional rather than thermal conditions (Long et al. 1983) is uncertain. In summary it may be stated that in a number of localities across the belt magmas were intruded between c. 510-480 Ma penecontemporaneously with the peak conditions of metamorphism. The intrusions are associated spatially with a number of high-grade metamorphic rocks and although they may be linked through some common fundamental cause in that they coincide with low pressure metamorphic regimes they undoubtedly had a strong influence on the metamorphic facies pattern.

Basic Intrusives

Age o f metamorphism

Major suites of basic and allied rocks are found at three localities within this zone, namely, the NE Dalradian, Tyrone and Connemara. In each case they have strongly influenced the local metamorphic pattern. It is texturally difficult in all three regions to separate the broader aureole effects of the basic suite from the 'background' regional metamorphism and the full extent of the thermal contribution of the basic rocks is not clearly known. In NE Scotland a series of basic and dioritic rocks, dated at 489 + 17 Ma (Pankhurst 1970) closely followed the peak of regional metamorphism (Fettes 1970). Their aureole effects include sillimanite crystallization and partial melting of the country rocks (Ashworth 1976, 1979b, Gribble 1968). It is uncertain how far the intrusives relate to the development of the high grade sillimanite-bearing, oligoclase-biotite gneisses of the region although Harte & Hudson (1979) have attempted to draw the silimanite isograd for the schists, prior to the emplacement of the basic rocks. This uncertainty is further complicated by a phase of shearing which disrupts both the basic masses and their aureoles (Munro & Gallacher, in press). This can be clearly seen in E Aberdeenshire where a zone of xenolithic gneisses associated with the basic rocks shows evidence of tectonic disrup-

Stratigraphical control on the age of metamorphism is limited. In much of the Dalradian, maximum ages are given by the youngest beds which are probably of lowermost Middle Cambrian age, c. 530-540 Ma. A minimum age is fixed at 410-420 Ma by the 'non-metamorphic' late Caledonian granites and the Lower Old Red Sandstone. The deformation in the Dalradian was polyphasal. In general terms the peak metamorphic conditions followed the primary deformation with its associated tectonic thickening (see Fettes 1979, 1983, Yardley 1980). The basic rocks in the Dalradian of NE Scotland whose intrusion is thought shortly to postdate the climax of metamorphism have yielded an age of 489 +- 17 Ma (Pankhurst 1970). In the Central Highlands granites believed to be broadly coeval with the metamorphic climax have given ages of 514 + 5 / - 7 Ma and 481 -+ 15 Ma (Bradbury et al. 1976). These dates seem to suggest an age of 480-510 Ma for the peak of metamorphism. Secondary deformation is generally associated with uplift and retrogression. This phase of uplift which appears to have been at different rates and times throughout the belt occurred in the span 460-440 Ma (Harte et al., in press). Recrystallization in this terrain therefore extended from

46

D. J. Fettes et al.

520 to 440 Ma, the progressive phase from 520 to 480 and the retrogressive from 460 to 440 Ma. In Shetland the metamorphic sequence is generally similar to that of the rest of the orthotectonic zone, although there are local differences (Flinn 1967, May 1970, summary in Mykura 1976). An age of 530 + 25 Ma Flinn & Pringle (1976) has been proposed for the Colla Firth migmatites which are generally accepted to be synchronous with or later than the peak of metamorphism. In Connemara the evidence suggests that the Barrovian metamorphism was mid-to-late Cambrian and most probably reached its c. 500 Ma MP2 peak prior to, or during the initial deposition of the basic volcanic Lough Nafooey Group of south Co. Mayo. A graptolite horizon near the top of this group has yielded a graptolite fauna that has been interpreted as Lancefield 3 and correlated with the Tetragraptus approximatus zone (Ryan et al. 1980), though this is then referred to as being Tremadocian. Harland et al. (1982), however, regarded this zone as being lowermost Arenig or largely Arenig, rather than Tremadocian as is claimed by Ryan et al. (1980). Following the Barrovian events in Connemara, uplift caused a fall in pressures while temperatures continued to rise during development of the Connemara (gabbro suite)/South Mayo (Lough Nafooey Group) island arc (Yardley et al. 1982, Ryan et al. 1980). The Buchan metamorphism is thus probably Arenig (and later?), overlapping in time the deposition of much of the Ordovician of south Co. Mayo. Discussion

Thus the orthotectonic zone constitutes an area of fairly uniform metamorphic grade, the grade rising quickly from the margins and lying generally in the amphibolite facies. Local peaks of middle-upper amphibolite facies can be identified. Although the facies series varies in both space and time the greater part of the zone exhibits medium pressure metamorphism; the exceptions to this lie at opposite ends of the zone in Connemara and in NE Scotland where low pressure facies assemblages predominate. These areas are also characterized by numerous basic intrusions closely associated with the peak metamorphic conditions. Various models have been proposed to account for the regional metamorphic pattern in Scotland and Ireland; these are summarized by Fettes (1979, 1983), Yardley (1980), Yardley et al. (1982). Notwithstanding the apparent similarities a particular point of interest is the contrast in age of metamorphic activity between the polymetamorphic and nonpolymetamorphic terrains. The former appears to have undergone recrystallization in the general span of 470-400 Ma, the latter in the span 520-440 Ma. In the former terrain peak metamorphic conditions were probably close to the beginning of regional deformation, a tectonic regime dominated by ductile thrusting and associated folding; later movements were largely responsible for uplift and retrogression. In the non-polymetamorphic (Dalradian) terrains, however, peak metamorphic conditions were coeval with and later than a tectonic regime involving tectonic thickening associated with major nappe and recum-

bent fold development; later uplift and retrogression was broadly contemporaneous with peak metamorphism in the polymetamorphic terrain. Unfortunately the contact between these two terrains lies in a poorly exposed zone where relevant data on the age of Caledonian metamorphism is scarce and it is uncertain if the contact between the terrains is gradational or sharp.

Highland Boundary Complex Lying along the southern border of the orthotectonic zone in Scotland (the Highland Boundary fault zone) there is a strip of exotic rocks collectively termed the Highland Boundary Complex (Henderson & Robertson 1982). This unit includes grits, lavas, cherts, serpentinites and black shales and has been interpreted as a tectonically emplaced ophiolitic slice (Henderson & Robertson 1982). Fossils from limestones within the sequence indicate a lower Arenig age (Curry et al. 1982, in press). The series contains rocks of anomalously high-grade, garnetiferous hornblende-schists being juxtaposed against low greenschists-facies Dalradian. In addition, the rocks locally appear thermally to metamorphose the adjacent Dalradian (Henderson & Robertson 1982). This has been interpreted as metamorphism by the sole of an obducting slice (op. cit.). In the Clew Bay (Westport) inlier of the west of Ireland structures are complex. A northerly greenschist-facies area of Southern Highland Group Dalradian schists is in tectonic contact with the fragmented, sheared and mylonitized Deer Park Complex ophioilitic suite within which there are smaller areas of probably low amphibolite-facies, dominantly semi-pelitic metasediments, with striped amphibolites possibly representing metavolcanics (Ryan etal. 1983). Similar rocks are probably represented in the Kill inlier of Clare Island (Phillips 1973).

Paratectonic zone of the Caledonides The paratectonic zone comprises Cambrian, Ordovician and Silurian rocks. These range in metamorphic grade from weak to greenschist facies. Their mineral assemblages alone Table 2. Correlations between various indicators of metamorphic grade. 1 Weaver index of illite crystallinity (Weaver 1960) 2 Weber index (Hbrel) of illite crystallinity (Weber 1972a,b) 3 Peak width values of illite crystallinity (Kisch 1980) 4 Conodont colour alteration index (CAI) (Bergstr6m 1980). N.B.: correlation and temperatures assumed for the purposes of this map. 1

2

150

2.3-12.1

150-110

> 12.1

< 110

3

>0.38 ~ 0.38~176 . cit; Simplon, P.R. et el. 1977. otl). ¢it. 514 ± 25 Rb-Sr Flinn. O. Er Pringle, J.R. 1976 Nature 2 1 8 371 + 10 K-Ar Miller, J.A. b Flinn. 0.1966, of). cit; Simplon atal. 1977. of). Pit. Mykure, W. 1976. op. Pit. 398 ± 5 K-Ar Miller J.A. b Flinn, D. 1966. op. Pit: May. F. In: Mykura. W. 1976. op. cit; Simpson el aL 1977, oil). cat. 368 ± 8 K-Ar Hallidey, A.N. st as. 1977. Scott. J. GeoL, 13, recalculated by Gale. N.H. st el. 1980. Earth planet. Sci. LeSt. 81; Kellock, E. 1969. Scott J. Geol. 8; Thirlwsll, M.f. 1981a. o11).cit. Kellork. E. 1969, of). ¢it; Thirlwell, M.F. 19810. op, Pal. Gslleghlr, M.J. et el. 1971. Trans. Inatn. Ming. Metal/. 1180; Robertson, R.C.R. 8 Parsons. I. 1974 Scott. J. Geol. 10. 649 ± 30 Rb-Sr BrOok. M,. quoted by Psnkhursl, RJ. 1982 In: igneous rocks o f the BHtish /ales red. Sutherlend. D.) Wiley; Brown, P.E. 1971. Minerelog. Meg. London INf. 4 3 0 ± 4 U-Pb Van 8reemen. O. et as. 1979. J. l~ol, Sere, Lend. 186. 405 ± 15 Rb-Sr Derbyshire. D.P.F., quoted by Pankhurst R.J. 1982. op. cit; Gallsgher, M.J. st el. 1971. op. cat. GalleGhar, M J . et aL 1971. op. cat; Read, H.H. el el. 1928. Mere, geol. Surf. Scotland. c. 420 K-Ar Brown. P.E. el as. 1968. Prec. ~brks. geol. Sac., I I . Soper. N.J. 1963. Q J. geol. Sac. Lend. 111. c. 420 U-Pb Pidgeon. R.T. 8 Aftalion. M. 1978 In: Crustelevolutlon in NW Britein, etc. reds Sowes. D.R. end Leeks, B.E.} Geol. J, Spat. tssuelO; Hall, A. 1972. Minerelo9. Meg. London. ~NI; Plant, J.A. et as. 1980. ]pans. instn MingMetatl. 1}81}; Tweedme,J.R. 1979. Trans. Inxtn Ming Me/all. B i l l . c. 400 U-Pb Pidgeon. R.T. 1~Aflslion, M. 1978, op, Pit; Read. H.H. el el. 1926, op. cit. c. 400 U-Pb PidGeOn, R.T. ~ Aftelion, M. 1978, op. cit; Pesch. B.N. st el. 1912. Mere. geol. Sun~. Scotland. 550 ± 10 Rb-Sr Lon9. L.E. ~ Lambert, R.St J. 1963 In: The British Celodonides reds M.R.W. Johnson b Stewart, F.H.) Oliver I~ Boyd; 555 ± 10 U-Pb Pidgeon. R.J. ~ Johnson, M.R.W. 1974. Eanhplonet. Sci. Left. 14; Harker. R.k 1 9 6 2 J, Petrol+ 3. Gains, T. et as. 1971. Nature. Phy#.Sci. 834, 425 ± 4 Rb-Sr Brook. M./)ere. comm; c. 417 U-I~o Pldgeon, R.T. b Ahalion, M. 1878, op. Pit; Leedal. G.P. 1952. QJ. geol. Sac. Lend. 108. 415 ± 5 Rb-Sr Turnell, H.S. 1982. PhD. Thesis, Univ. Leeds (unpubl.): Nicholls, G.D. 1951. (1,/. geol. Sac. Lend. 180. 456 ± 5 U-Pb Van 8reemen. O. el el. 1979. Scott. J. Geol. 18; Richardson, S.W. 1968. (~J. geol. Sac. Lend. 124. 1030 ± 45 Rb-Sr Brook, M. el el. 1976. Nature. 2 8 0 ; Harry, W.T. 1964. GJ. geol. Sac. Lend. 1 0 9 ; Marcy, E.L.R 1963 In: British Celodonides reds Johnson. M,R.W, ~ Slowirt, F.H.) Oliver ~ 8oyd, 435 ± 10 U-Pb Pidgeon. R.T. b AftaGon, M. 1978. oil). Pal; Sebirm, PA. 1963. Bull• geol. Surf. G.B. 2 0 ; Munro, M. 1965. Scott. J. Geol. 1. 414 ± 3 Rb-Sr Hlllidey, A.N. et el. 1979 In: The Calodonides oaths British/ales - ray/awed reds Harris, A.L eras.) Spec. Pub/. 9wol Soc. Lend. 8. Lee, G.W. ~ Bailey. EB. 1925. The pre-Terliarv geology of Muff. etc. Mere. 9ool. Surf. Scotland. 400 ± 7 K-At Thirwell, M.F. 1979. I=h.D+ Thesis, Univ. Edinburgh (unpubl.) Grooms, D.R. b Hall, A. 1974. Minerelog. Meg. London. 8 9 ; Fitton. J.G. el el. 1982 In: Andesites: orogenic andesites end related rocks red. Thorpe, R.S.). Wiley. (s) 401 ± 6 Rb-Sr (b) 396 + 12 Rb-Sr Cleybum, J.A.P. eras. 1983. Nature, :1~8; Bailey, E.B. 1960. Geology o f Ben Naris ~ Glances, Mere. geol. S u ~ Scot/end; Berritt. S,D. 1983. PhD. Thesis. Open University (unpubl.) c. 401 K-Ar Brown, P.E. eta/. 1968, op. cit; Bsiley. E.B. 1960, op. Pit. Fitton, J.G. eta/. 1982, Oil).cit; Bailey, E.B. 1960,op.cit; Roberts, J.L 1974. Scott. J. Geol. 10, Fitton. J.G. e~ as. 1982, oil)- Pit; Bailey, E.B. 1960, of). c~t: Heslem. H.W. 1968. J. Petrel. 9. Heslem. H.W. 1968. op. Pit; Bailey. E.B. 1960. op. Pit. 402 ± 18 K-At Brown, RE. eras. 1968. op. c~t; Hinxmen. LW. etel. 1923. Geology o f Car:our end Moor of Rennorh, Mere. gaol, Sun~. Scotland. 4 0 0 ± 10 U-Pb PidGeon, R.T. b Aftslion, M. 1978, op. cit; Hinxmen, LW. et as. 1923. op. Pit.

COMPILED BY: G.C. BROWN (BRITAIN),E.H. FRANCIS (BRITAIN), P.S. KENNAN (IRELAND) & C.J. STILLMAN (IRELAND) PROJECTION: LAMBERTS CONFORMAL 20

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KEY TO SYMBOLS USED IN LOCALITY - CITATION LIST: PERMIAN

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135 ' 136

t .

,.

:"

, '2•, ' :r " I

e

________53ON

.:...%. + .'

j

+

86. ISlurntislend

: I

I .

87. E. Lothian

88. Arthur's Seat 89. Pentlands •

275

_

90. Bethgeta Hills

: . , ,, .;

279

• ~80

2131 -

91. Campsies

-

2ON

+

+

-!-

162

92. Clyde Plateau (RInfrew)

94.

169 X

d

95. 170 96. 97. 98+ 98.

-51°N

I00.

/

;~

+

/

+

17 i

101.

+

102.

+

+

103. 104.

MA" 3h>8h

4h>Gh>lOh

M1, f p Z l '

12>M2 f* 12 > M 2 - F 2 fp °

3mah

107. CriftellOslbeattm

3h > 8h

108. Sirrlnswsrks

M3f

109. Eildon Hills 110. Kelso Traps

F3i M3I

111. Priestlew

8h

112. Cockburn Law

8h

113, Eyemouth 114. Cheviol Granite

I2 f p * 3b>9h

Tromletl. W.E. 1969. In: W~/~ red. Wood. A.) Univ.

115. Cheviot levis

12 fp *

J. gaol. So¢. Lend, 141; R.M. 1956. Geol. J. 1.

116. Whin Sill

MI s

117. Weerdele

3 b > 3m

118. Cross Fell, north

3b>8h

119. Cross Fell. south

F2fap°Z~

120. Wensleydsle

3(b) chloritised 3b

50°N

10°W

9'~W

8°W

247. Bellewstown

F2 a *

248. Rock, bill

3m

249. 6albriggan

( M : > I ) 2 > [ M > I ) I France, O.S. 1967. Geol. J. 6. fe~ (M>I)1> ( M > [ ) 2 Stillmen, C.J. b Williams, C.T. 1979, op. cir. fs~ i 3h 401 ± 17 Rb-Sr Leggo, P.J. etal. 1966. O,J, g~ol. Sac. Lend. 122; Townend. R. 1966. Prec. R. Irish Acid. 881}. 16 u" 510 ± 10 Zircon U- Pb Pidgeon, R+T,1969. Scott. J. Geol. 3; KsnsrisSoutiriou. R. ~ Angus, N.S. 1976. J. geol. Sac. Lend+ 122.

250. Lambay/Portrene 251, Omey

252• DawrosCurrywongaunDoughraugh 253. inish 3h 254. Emsmore, R o ~ l s l o ~ e 10> 16 u' Gowls b Ceshel. Laugh Wheelaun 255. Roundstone 3b 256• OuGhterard

3m

257. S. connemarsGroup ( M > l > f ) O f p eli 258. Gilwey 259, Crochan Hill

3>4h M3 fpZ~

260, Tslleght Hills 261. Leinster. N. Units

M0d 4>3b

262, Kildare, Hill of Allen 263. Roundwood 284. West Wicklow

(M>|) 2 fpA MOfA (M>])OfIZ~

265. Au0hrim-Bellinadash 5b 266. Relhdrum (F>])2 fep,~*i 267. Cerrigmoreoiorite 10>4h 268. Avoca 269. Arklow Head 270. Leinster S. Units 271• Wezford 272. Slieve Aughty 273. 274. 275. 276. 277.

Umlrick Dingle Inishvikilieun Laugh Guitene Wsterlord (upper volcanica) 278. Wsterford (lower volcanics) 279. ouncannon 280. Seltees

281. Cernsore 282. Bellycastle

~

Harper, J.C. E~Rest, N. 1964, Prec. Ft. Irish Acid. (1411; Stillmsn, C.J. 8 francis. E.H. 1979. Op. cit. 8rindley, J.C. i~ Kennan. P.S. 1972. Prec. R. Irish Acid. 7211.

4 1 8 ± 8 Rb-Sr LOGgo. RJ. et aL 1966, op. cir. Kansris-Soutiriou. R. ~ Angus. N.S. 1976. op. cil; PidGeon. R+T. 1 9 6 9 . op. Pal. 398 ± 12 Rb-Sr Leggo, P.J. e/aL 1966, op. cil; Max. M.D•etal. 1978. Bull+ geol. Surf, Ireland+ 8. 459 ± 7 Rb-Sr Leeks, B.E. 1978, op. cat; Bradshaw. R. etal. 1969. PreC. ft. Irish A r i d . III1}. Ryan, P.D. et el. 1983, Geol. Meg. 120. 398 ± 12 Rb-Sr Max, M.D. et a/. 1978. op. cat. Uplou, B.J.G. 1982. In: Igneous recks of the British isles red. Suthlrlend. O.S.) Wiley. 6ruck. PM. 1976. Bull. gaol. Surf. Ireland ~. 404 ± 24 Rb-Sr O'Connor. RJ. b 6tuck. RM. 1978. J. Earth Sci. R Dublin Sac. 1. Stillmsn, C.J. ~ Williams. C.T. 1979. op. cat. Bruck. P.M. 1976, op. cat; Brindle,/. J .C. ~ Millen, S. 1973. Sci. Prec. Ft. Dublin Sac. 4 A , Bruck. P.M. 1976. oil). Pit. Brindley. J+C. b conner, 8.P+ 1972, Prec. R, Irish Acid. 721}. Shepperd, W.A. 1980. Nor. geol. Unders. 8 8 0 . 412 ± 21 Rb-Sr O'connor, RJ. ~ Reeves, T.J. 1980• Bull. geol. Surf. Ireland 2.

(F>I)2 tapS1* f2 p ~ . - 1 3 i 4:> 3b

Sheppard, W.A. 1980. op. Pit. Stiltmen. C.J. ~ Maythem, D. 19?3. Prec. R. Irish Acid. 7 3 8 . 8findley, J.C. 1973~ Sci. Pn~c. R. Dublin Sac. 4 A ; Bruck. P.M. Reeves. T.J. 1983. gull. geol. Surv. Ireland 8. (F>[)2fapAi Shsnnon, PM.1979.J. Earth Sci. R. Dublin $oc. 2. M1~:3 > M3 tp 11 Emo, G. 1978. The Lower PaIaeozoic ... Sliave Aught~ - Clare Gahea~ Ph.D. Thesis, Univ, Dublin (unpubl.) (M > [)3 fp ,~* Slrogen, R 1973, Sci. Prec. R. Dublin Sac. 8A. FOe* S1illmen, C.J. b francis, E.H. 1979, op. Pil. |1 fpZ~ Perkin, J. 1976. Sci. Prec. R. Dublin SoM2fep ° Av~son, M.1984J. geoI. Sac. Lend. 141. F 2 > I 2 > M 2 tap Sliilman, C.J. I~ Williams, C.T. 1979. op. cir. ( M > ] ) I > ( M > ] ) 2 Stillman. C.J. ~ Williams, C.T. 1979, op. cir. fa ZXi I>FOp~. 4b

3b MOfp

Gerdiner. RR+R. 1974• Buff. geol. Sur~ Ireland1, 436 ± 7 Rb-Sr Max. M.O. at el. 1979. In: The Calodonides of the British i s i ~ - reviewed (ed s Hsr ri s, A.L. et el.) epic. Pubs. geol. Sac. Lend. B. 538 1. 8 Rb-Sr Leutwein, E etal. 1972. Bull. geol. Sur~ Ireland. 1. Wilson. H,E. ~ Rabbis, J.A. 1966. The geology . . . . 8ell~castla, Mare. geol. Surf. N.I.

Prolect 27 The Ca/edomcle Of().~+,~l U m t e d KmqcJom & ire/anc~ Working G r o u p s

7°W ~

6°W L

3b Mlds" 5h

Angus, N. 1977. Prec. R. Irish Acid. 7711.

173. Meldon 174. Newton Abbot

MI. M 3 s f p A M3 f

( M > u ) O tap

Angus, N. 1977. op. tit; Wilson, H.E. 1972. Regional Geolngy of N. Ireland. G.S.N.I+

175. S. Devon

M3 pfL1> 13 pf

( M > I ) O tap Zli 5b

Wilson,

176. E. Cornwall

M1. M3 s

5>4h

Wilson. H.E. 1972, O9. cil.

I O fp

Geological Survey map of Northern Irelsnd 1977.

178. Badman Moor

3m4b

MI IpL~

Shsrpe, E.N. 1970• Irish Nat. J. 18; Stillmen, C.J. ~ Francis, E.H 1979. In: The Catedonides of the British Isles - reviewed reds Harris, A.L el el.) Spec. PubL geol. Sac. Lend. 8. Cameron, D.TJ. ~ / ~ d t r s o n , T.B, 1980. Geol. J. 16. 403 ± 3 Rb-Sr (Slie~egarron intermediate rocks); 399 ± 3 Rb-Sr (NE pluton granodio¢ile); Meighsn, I.G. I~ Neeson, J.G In: The Caledonides of the B~'ti#h Isles - reviewed reds Hsms. A,L. et a/J S1~¢, Pub/. gaol, S ~ . Lend. 8. 390 ± 21 Rb-Sr (granodiorite) O'Connor, PJ. 19?5. SCL Prec. R. Dublin Sac. BA. MeiGhsn, I.G. El. Neeson, J.G. 1979, op. cit.

179. Tintsgel

M3fp~,

183. csrnmenallis

3m4b

7381 ± 10 Rb-Sr Leutwein, F. 1970./rishNe/. J, 18; ; Sutton, J.S 1970. Sci. Prec. R. Dublin Sac. 2A. Phillips, W.E.A. e/aL 1975. Sci. Prec. R. Dublin Sac. 8 A ; Lemon. G.G. 1971. Geol. Mat. 101. 477 ± 6 Rb+Sr Pankhurst, R.J. eta/. 1976. J. geol. Sac. Lend. 132; 489 ± 18 Rb-Sr Max. M,D. etel. 1979. Bu/Igeol. Sur~. Ireland, 2. Pankhurst. R.J. e/e~ 1976, op. cit; Max, M.D. et el. 1979, op. Pit. O'connor, E. 1975, Prec. R. Irish Acid. 76B. Phillips, W.E.A. b Skevin91on. D. lg68, Sci. Prec. R. Dublin Sac. SA. Morris. J.H. 1980. J. Earth Sci. Ft. Dub/. Sac. B; Morris. J.H. 1979, Ph.D. Thesis. Univ. Dubl. Max. M.O. ~ Long. C.B. 1979. op. Pal.

184. The Lizsrd complex

M1,16u

214. Barnsmore 215. S, Done981 matedoleriles 216. Craigballyherky Tyrone 217. Tyrone igneous complex (basic plutonics) 218• Tyrone igneous complex ( Volcanics 8' Tonalite) 219• Pomeroy granites (+ Slieve Gellion) 220• Fermenagh ORS Volcanics 221. Helen'sBsy (8ellygrOt|

H.E. 1972, op. cil.

Townsend Tuff Tortworthinlier Sunnyhiil, Mendips Dsrtmoor

FO p M2 f 12f 3m4b

177. Hercynican rhyolites F3f

222. Ards metabentonites 0 p 223. Newry 4h

224. Newry (NE u l t r a i s t i c ) 225. Blacksod

2b

226. Slash Wood

16 u

227. Slieva Gamph

4h

228, Slieve Gamph 229. Shercock 230. Lou9 h Acsnon

3b MOpZl MOfpA

231. Strokastown

M I > M2 fp Z~

16u

232. NW. Mayo green beds 233. Charlestown

M 0 fp

234• 235. 236. 237.

( | - M ) 0 fp 10h F0 p * M l f R > u' 16 u 4b

E. Murrisk Tourmakeedy Mweelraa connemare slriped amphibolita 243. KilbridePeninsula 244. Laugh Nalooey 245. North+West Galway

[

3°W ~

169. 170. 171• 172.

MO f p L l ' 4-5h 16

239. 240• 241. 242.

I

4°W L

Max, M.O. b Long, (;.B. 1979, op. cat. 405 ± 5 Rb-S¢ Hellidey. A.N. et as. 1980. op. cat. French, W.J+ 1977. Prec. R+IriehAcad, 771}; Pilcher. W.S. Et BerBer. A.R. 1972. op. Pit. Walker. G.P.L. ~ Leedsl. G.P. 1954. ScL Prec. R. Dublin SorM>I)OfsZI p ,

M 0 fpZl FOfp FOs °(Z~) 16 u" MI IpZ~ ( M > F)2 fp F3 fp

246. Collon-Grangegeeth ( M > I ) 2 > ( M > I ) I lap A

Chsrlesworth. H.A.K. 19606. Prec. R. Irish Acid. 618; Stillman, C.J. 1982 In: Igneous reclcl of the British Isles red. Sulherland, D.S.) Wiley. Chsrlesworth, H.A.K. 196Ob. Prec. R. Irish Acid. 618. Skibs. W. 1952. Trens. geol. Sac. Edinburgh. 111. Phillips. W.E.A. 1974, J. geol. Sac. Lend. 130. Rysn. P.D. el a/+ 1983. Nature ~Ki2; Phillips, W+E.A et el. 1975. op, Pit; Max. M.D. b Long. C.B. 1979, op. Pit. Insider. D.D. 8' Kelly, TJ. 1979. Bull. geol. Surf. Ireland 2. L u k e , B.E. 1978. In: Crf#taI evolution in northwestern Britain and adjacent region, reds Bowes. D.R. b Leeka. B.E.) Geol. J. Spec. issue 10. McMenus, J. 1972. Prec. R. Irish Acid. 7 2 8 . Stillmsn. C.J. 1982, oil). cir. Dewey, J.F. 1963• Q.J. geol. $o3b

185, Tregonning-Godoq)hin 3 m > 3 b

186. S.W. cornwall 187. Lancls End

M1 sl ~, 3m < 3 b

188• Spilly Isles

3m 12> F2 sfp

122. Stockdele 123. 8orrowdeleGroup

3b 12>F2>M2 fep "11

124. Eskdell

3m > 3b

125. Ennerdsle 126. Threlkald+St.Johns

3b 4b M2 fep' 558 ± 16 Rb-Sr PMchetl. P.J+st el+ 1980. J, geol. So¢. Lend. 187: c. 600 RboSr Bath, A.H. 1974. J. geol. Sac. Lend. 1~O; 632 ± 32 and 677 ± 72 K-Ar Fitch. F.J. else. 1967, oil)- Ot; Greig, D.C. e/e/. 1968. C~ology of the countly around Church Stre/1on etc. Mere. geol. surv. G.B.; Thorpe, R.S. 1972. Prec. geol. Assoc. London 8~; 1982, op. eL% 3 (h) 533 + 13 Rb-Sr PatcheS1, P.J. eta/. 1980. op. cat;Thorpe, R.S. 1982. op. cit. M3 fp > M1 fs Ineson. PR+ et el. 1983• Prec. Yorkshire geol. So(:. 4 4 ; MacDonald, R, eta/. 1984, J. geol. Sac. Lend, 141. M3 sfp > M1 f Kitten, S+R. 1984. J+ ~ / + Sac, Lend. 141. 3 b < 4 h > 9h 423 ± 17 Rb-Sr Cribb, S+J. 1975. J, geol. Sac. Lend. 131; 4 3 0 ± 7 Rb-Sr Hsmpton, C.M. ~ Taylor, P.N. 1983. J. g~ol. So(:+Lend. 140; 452 ± 8 U-Pb. Pidgeon, R.T. I~ Aftalion, M. 1978. op. Pit; La Bes. M+J. 1972. Prec. ~brlc~hire ~ L So 9h 534 ± 22 Rb-Sr Cribb, S.J. 1975, oil). Pit; Le Bas. M.J. 1982. op~ cir. M3 s 308 ± IO K+Ar Fitch, F.J. alas. 1970, op. (:;it: Kirlon.S.R.1984.op.cit. 3 b < 4 h > 9h' 681 ± 53 Rb-Sr Beckinsale, R.D. else. 1981, J, gaoL Sac. Lend. 181; Thorpe, R.S. 1982, oil). cil. 3b9h 702 ± 8 Rb-Sr Beckinule, R.O. e/e/. 1981. oil). Pal; Holgete, N. ~ Hellowos. K.A.K. 1941, Geol. Meg. 7 t . F2-M2 fpZl Janes, O.T. ~ PuGh, W.J. 1949. O,J, geol. Sac. Lend. 108; Nicholls, GD, 1959. (2,1. geol. SO 13 f

106. Csirnsmore of Fleet

o~

406 ± 4 Rb-Sr Summerhlye. C.P~1966. Gaol Meg. 108; Norko~ds, S.R. 1941, (~J. geol. So(:. Lend. 1}8. Simpson, P.R. etaL 1979, Phil. Trene. R. Sac. Lend. 8~11A. Plant, J. of iI. 1980, oil)- Pit, Borredaile, J.G. 1973. ]pens. R. Sac. E d i n b u ~ . (111.

Thirlwell, M.F. 1979, oil). Pal; frenpis, E.H. e/el, 1970 The geology o f the St#ling dietricL Mere. gaol Su~. O. R 304 ± 12 K-Ar Fore,h, I.H. ~ Chisholm. J.I. 1977. The geology o f East Fife. Mere. geol. S u ~ G.B. MeoOonald. R. ef eL 1981. Tren~. R. Sac. Edinburg~ Earth Sci. 7 8 ; M1S Frenpis. E.H. 1982. J. geol. So¢. Lend. 131}. MmoOonmld. R e/e/. 1976. Scott. J. Geol. 11; MecOoneld. R e~ el. M3f>p 1977. Soott. J. Geol. 15; Allan, O,A. 1924. Trens R. Sac. Edinburgh. 83. M3 f ~ 3 fpa 353 ± 7 K-Ar De Souse, H.A.f. 1982. In: Numerical doting in strttigraphy(ed. Odin, G.S.) Wiley; Upton, B.G.J. 1982. In:Igneous rocks o f the British Isles red. Sutherfend, D.) Wiley. M3 f 354 ± 5 Fitch. f.J. e/as. 1970 C.R. 6tb InL Con~. Carb, StreL Geol. 2; Blare. G.R 1868. Arthurb Seat, Edinburgh• M 2 - ] 2 > F 2 fp * Mykure, W. 1980. gull. geol. Surf. G.B. 18; Thirwell, M.F. 1979, op. Pit. M3 f > p 320 ± 5 K-At Da Souza.. H.A.F. 1982. op. cir. MacDonald ef el. 1977 op. cir. M3 f > 1 3 f,p ° 3 3 5 - 3 2 5 K-Ar De Souze H.A.F. 1982. op. tit; Frenpis ef el, 1970, op. cat; MacDonald, J.G. ~ Whyte, F. 1981. Trans. R. Sac. Edinburgh Earth Sci. 7 2 . M 3 > [ 3 > F 3 f : > p * 310-325 K-Ar De Souse, H.A.F.. 1992, oil)- Pit. Johnstone, G.S. 1965. BU//geoL Sut~ G.& 88.

M1 f p A . 1 6 u

188 ~ "

408 ± 5 Rb-Sr Tumell. H.B. 1982. op. oil; Tilley. C.E. 1824. O.J. g~ol. Sac. Lend. 1}0. Henderson. W.G. I~ Farley, N.J. 1992. Scott. J. Geol. 18.

M3s

105. Bellentree

177

514 ± 7 U-Pb Penkhurst, R.J. ~ Pidgeo~, R.T. 1976, oil). Pi{: 8redbury, H.J. et el. 1976. J. geol. Sac. Lend., 182. 481:1215 Rb-Sr P~nkhorst, R.J. b Pidgeon, R.T 1976. Eerthplanet. Sci. L~t. 31. 411 ± 8 Rb-Sr Thidwall, M.F. 1983. J. geol. Sac. Lend. 140. Thirlwell. M.F. 1979. op. Pal. Henderson. e.G. i~ Roloartson. A.H.F. 1982. J. gaol So¢. Lend. 131}.

]2 > M 2 - F 2 fp *

79 • .

186

Plant, J. el el. 1980, oil), Pit. Wadsworth. 1970. 1982. of). Pit; Ashcroh, W.A. b Boyd. R. 1976. Scott. J. Geol. 12. 462 ± 5 Rb-Sr Van Breemen, O. ~ Boyd. R. 1972. Scott. J. Geol., 8. 489 t- 17 Rb-Sr Penkhurst, R.J. 1970. Scoff. J. Geol. l ; Wldeworth, W.J. 1970. 1982. of). cat. 453 ± 4 Rb-Sr Psnkhurst, R.J. 1974, op. cat: Busrewil, M.T. e/el. 1975. Minerelog. Meg. London. 40. 404 ± 5 K-Ar Brown. P.E. e/Is. 1968. ot). Pit; Hell. A. 1972, oil). Pit; Webb. P.C. b Brown. G.C. Unpubl. data. Wadsworth, W.J. 1970, 1982. op. cat; Allen, W.C. 1970. Scott. J. Geol. 8. Plant. J. et as. 1980, oil). cit; Brown. G.C. 1979. In: The Calodonides o f the British le/es - raw'awed (ede Harris, A•L. st e l i Sl~lc. Publ. 9eel. So 9 h 408 t- 8 Rb-Sr Thidwsll, M.F.. unpublished dlta,-Richey. J.E. e/el. Distinkhorn 1930. of). c~I. F2,]2,M2,f>p Thirlwell. M.F. 1979, of). c~t; Read. H.H. 1927, Prec. Geol. Assoc, Tinto Lend. 18. F3p Thirlwell, M.F. 1981b. Geol. J. 10; Bloxem, T.W. 1982. In: Igneous Winkleslon Hill rock# of the Briticb Isles. red. 8ulherlend, D.) Wiley. Lambert, R. St. J. e/M. 1981. J. l~ml. So¢'. Lend. 188: 81axis, T.W. S.Uplends Ordovipien M1.M2,M3, fp 1982. op. (:it. vol¢anics Grsig, D.C. 1971. The south of Sootlond. Br re~ Geol. HMSO; Span9o Water 8h > 9h Brown, G.C. unpublished date. 453 ± K-Ar Gale, N.H. e~el. 1880. op. (:it; Bloxem, T.W. 1982. op. Bml Hill F2.12 f > p Pit. Ceinsmoreof 3 h < 4 h > G h > 1 0 h 406 ± 4 Rb-Sr Thirlwell, M.F. unpublished date: Deer, W.A. 1935. (1,1. geol. Sac. Lend. M . Carlphsirn Lath Dean 3 b < 8 h > 9 h > l O h 408 ± 2 Rb-Sr Stephenl, W.E. ~ Hellidey. A.N. 1978. In: Orig/ns of granite batlmlitl~ reds Atherton, M.R b Tamey. J.). Shivl Publ.. Orpin9ton. Brown, G.C. e/el. 1979. J. geol. Sac. Lend. 118.

93. Mechrihenish 16.___4163

• ,~.~

3b'

54°N

4 0 4 Rb-Sr Brewn, J.F.1975.Ph.D.Thesis.Oz/ordUniv. (unpubl.) Smith. T.E.1970Geol MeB 107. c• 385 U-Pb Pidgeon. R.T.El Aflelion, M. 1978. oil). Pal; 409 ± 18 K-Ar Brown, P.E. e/as. 1868, oil). Pit; Marston. R.J. 1871. GJ. 9woe. Sac. Lend. 128. 413 ± 5 Rb-Sr Van BrNmen, O. b Pieampiti, M.A.J. 1983. JgeoL SoM1>F1 f ?a p ?°A

130. Cockarmouth laves

M3 f

131. Dhoon

4b

132. Foxdlle 133. Scarlets volcanics

3b>3m M3 p f

134. Mane complex

F1, I1 p

135. Mona uttremefics

16u

136. Coedana (chloritised)

3(b) .

/ P

O~ngle

f

+

g

+

+

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q

pU

~

qlb

./

/

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) J

/IW~GEL

Start Point schists of ? Oevonlsn age. ' L;~,;~, ¢oml~ex ~ * locally with upc~r Irnphibf~ll It, hiSS.

mKtusnce

CITATION UST Ashworth, J.R. 1979. In: Caledonldes of the BrlUSh Isles - reviewed (ads. I lanls. A.L. at el). STUN:. Publ, geol. So¢, Lend, 8. Barber, A.J, l May F, 1976: Scott. J. Geol.12 Chlnner, G.A. 1960 J. Petrol. Harle, S. & Hudson, N.F.C. 1979. in: CItedonides of the British o Isles - reviewed (edL Harris, A.L. el el). , SpeC. Publ. geol. Soc. Lend. 8. Moorhoule. S.J. 1976. Scott. J. Geol. 12 , Poweil, D. sial. 1981J. geol. Soc. Lend. 138. ~ Sanders. I.S. 1979. In: C,411edonidesOf the BriUsh isles '~ reviewed (eda. Hardlk A.L. ot el,). I 8~t¢. Pull)l. 9eot, S(I¢, Lend. 8 Watham. K.P. 1983 J.geol. Soc. Lend. 143

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