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• BASEMENT-COVER RELATIONSHIPS Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASEMENT-COVER RELATIONSHIPS

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JOHNSTONE ET AL

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0 km 100 I I WATSON & DUNNING- GENERAL REVIEW KENNAN ET AL-- PARATECTONIC IRELAND BAMFORD-- SEISMIC CONSTRAINTS Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

The Caledonides of the British Isles--reviewed. 1979. Geological Society of London.

Basement-cover relations in the British Caledonides

Janet Watson & F. W. Dunning

CONTENTS 1. Introduction 67 2. The Metamorphic Caledonides 68 a The and related rocks 68 b Pre-Caledonian cover units 70 c Other possible basement units 72 d The Caledonian orogenic front 73 e Grenville activity in the northern Caledonian province 74 3. The Non-metamorphic Caledonides 76 a Basic facts relating to the belt in general 76 b The Midland Valley Transition Zone 77 c The Southern Uplands-Longford-Down-Clare Inliers Belt 83 d The Iapetus 84 e The Lake District-Isle of Man-Leinster Belt 84 f The Irish Sea 85 g The Welsh Basin and its eastern borders 85 h Eastern England 86 j The Midland 86 4. Conclusions 87 5. Acknowledgements 88 6. References 88

1. Introduction underlying the Metamorphic Caledonides (which Although the conventional regional subdivi- consists mainly of ) and that underlying sion of the British and Irish Caledonides takes the Non-metamorphic Caledonides (which is little account of the pre-Caledonian basement, it dominated by rocks of low metamorphic grade is nevertheless important to consider the base- penetrated by acid plutonic complexes) are ment in this context, for two principal reasons: already well known and, as Dewey (1974) has firstly, because regional chemical and structural pointed out, these contrasts support the idea that variations in the basement might be expected to the Caledonian led to the union of two have exerted an influence on the tectonic evolu- or more plates at a crustal suture passing through tion of the Caledonian belt; and secondly, Southern Scotland. Geophysical evidence shows because crustal units which were brought that the contrasts observed in surface outcrops together by plate movements during the Caledo- extend to the deep parts of the . Thus, high nian cycle might be expected to differ from each seismic velocities (>6.4km/sec) appear to other with respect to the character of their pre- characterise the crust beneath the Metamorphic Caledonian basement. In considering these Caledonides at depths of no more than 6-12 km, points, one can ask three principal ques- whereas similar velocities are not recorded---or tions-what are the broad lithological and chem- occur only at much greater depths--to the south ical characteristics of the basement, both at the of the inferred suture (Bamford et al. 1977, level of the present outcrops and at depth? What 1978). Heat flow/heat production studies suggest are the main features of its pre-Caledonian struc- that the basement under much of England and ture? What are the ages of the rocks and of their Wales consists of low-grade metamorphic rocks pre-Caledonian structural patterns? Information with acid plutons to depths of about 15 km on these questions comes not only from pub- (Richardson & Oxburgh 1978). These findings lished studies of exposed basement areas but also suggest that the at depth was from regional geophysical and geochemical maps moderately uniform in terms of lithology and and from seismic studies. Our objective is to metamorphic grade within each terrain, but that attempt an assessment of the evidence so far as it the characters of northwestern and southeastern concerns the significance of regional variations in terrains differed significantly from one another in the basement (Fig. 1). these terms. Such an inference does not, of The general differences between the basement course, necessarily imply that each terrain rep- 67 Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

68 Janet Watson & F. W. Dunning resents a single pre-Caledonian age province. as a cover unit, since its structure and Discussion of the basement structure is com- date from Palaeozoic times. Until plicated by uncertainties concerning the defini- the significance of such differences in timing tion of the start of the Caledonian cycle. In between the Metamorphic and Non- England and Wales, rocks and structures of late metamorphic Caledonides becomes clearer, it (<800Ma) age are generally seems best to avoid attaching a uniform age- assigned to the basement ~and the unconformity significance to the basement/cover boundary. which separates them from overlying Lower Palaeozoic (plus Arvonian) strata is taken as the 2. The Metamorphic Caledonides dividing surface between basement and cover. This practice is in line with the observed contrasts a. The Lewisian complex and associated of tectonic response to Caledonian deformation rocks between rocks above and below the unconfor- The principal pre-Caledonian unit of the mity. In the Metamorphic Caledonides, on the basement in NW Britain is the Lewisian complex other hand, the Supergroup which of gneisses and granulites which underlies both includes considerable thicknesses of late Pre- the Hebridean craton and the adjacent parts of cambrian (~< 700 Ma) sediments is usually treated the Caledonides. On the craton, and at certain

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-5~-q FIG. 1. Main geotectonic units of the British Caledonides. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 69 localities within the Scottish Caledonides, rocks development of large ductile -zones of NW of the complex have yielded Archaean isochron and NE trend on which both lateral and vertical ages (-2700 Ma) and their earliest tectonic and displacements took place. Some of these shear- metamorphic patterns are Archaean. The com- zones--notably the Laxford zone on the Scottish plex was subjected to intermittent tectonic and mainland and the South Harris zone in the Heb- metamorphic reworking down to early rides-were invaded by concordant sheets of times (-1750 Ma). There is no Laxfordian granitic relatively rich in K and record of later Proterozoic activity on the craton, Rb, while elsewhere pegmatite and but a number of lines of evidence suggest that the formed more irregular -complexes in the basement underlying the Metamorphic older gneisses, these granitic rocks occupy a Caledonides was subjected to mid-Proterozoic restricted area in the basement of the craton (Fig. (1700-1000 Ma) plutonism, tectonism or 2). The main (late Laxfordian) phase of granitic metamorphism (see section 2e). injection appears to have been associated with Taking into account submarine outcrops iden- uplift of the deep crustal gneisses and was fol- tified by geophysical and other means on the lowed by stabilisation and erosion. Although the continental shelf, gneisses and granulites of supracrustal gneisses and meta-igneous com- Lewisian type appear to underlie an area of at plexes frequently carry disseminated pyrite with least 60 000 km 2, extending from West Shetland minor amounts of copper, no significant mineral- to Inishtrahull and from the edge of the continen- isation is recorded in the basement. tal slope at least to the Great Glen . Gneisses The characteristics which would seem to be of Lewisian type are tectonically interleaved with important in the context of Caledonian tectonism metasediments of the Moine throughout the are: (a) the predominance of coarse, massive Northern Highlands of Scotland. Although crystalline rocks and the virtual absence of bed- basement rocks of this type are not exposed in ded series with well defined layering, (b) the the area occupied by the Dalradian Supergroup, presence of steeply dipping discontinuities in the they appear from seismic evidence (Bamford et form of ductile shear-zones some of which appear al. 1977) to be present at depth in the Grampian to extend to considerable depths and (c) the very Highlands. refractory nature of the assemblage and the low Through this large area, the basement of content of volatile and heat-producing elements Lewisian type maintains a remarkably uniform in all rocks but the late and pegmatites. character. It is invariably dominated by gneisses A basement with these features might be and granulites, the average major-element com- expected to be relatively inert, to break along positions of which resemble those of intermedi- pre-determined lines and to prove somewhat ate to acid calc-alkaline igneous rocks (e.g. barren as a source of partial melts, of volatiles Sheraton 1970, Bowes 1972, Tarney et al. 1972). and of mineralising solutions. From the evidence Minor amounts of Archaean metasedimentary so far mentioned, it would seem to follow that and metavoicanic gneiss, together with material of this refractory kind forms the metamorphosed intrusive complexes of various lower crust under much of the Metamorphic kinds, are incorporated in the gneisses. The Caledonides. A new light on this anomaly is heat-producing elements U, K, Th, Rb are gen- provided by Pidgeon & Aftalion's (1978) study erally present in very small amounts, a feature of zircons in Caledonian granites of the commonly attributed to depletion in volatiles and Metamorphic Caledonides in Scotland. The incompatible elements during deep-seated isotopic compositions of uranium and lead in metamorphism (e.g. Moorbath et al. 1969, Tar- zircons from a dozen granites suggest that sub- ney et al. 1972). High seismic velocities, high stantial proportions of the parent of densities and 'noisy' aeromagnetic patterns these plutons were derived from partial melting characterise the Lewisian basement, especially of basement rocks of mid-Proterozoic where granulites are present. (> 1000-1700 Ma) age, or of younger metasedi- Throughout the Hebridean craton, the oldest ments containing detrital zircons from such a structural patterns in the Lewisian date from an basement (Fig. 2). This evidence raises the possi- Archaean phase of gneiss-forming metamorph- bility (see section 2e) that the Lewisian basement ism -2700 Ma (the early Scourian or Badcallian underlying the Metamorphic Caledonides was phase). These patterns were extensively modified more strongly affected by Laxfordian or post- during late Scourian and Laxfordian Laxfordian felsic magmatism than the corres- (-2600-1800Ma) phases of inhomogeneous ponding basement of the Hebridean craton (cf. deformation and metamorphism leading to the Johnstone et al. this volume). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

70 Janet Watson & F. W. Dunning b. Pre-Caledonian cover-units were deposited directly on deeply The Lewisian basement is separated from the eroded gneisses and granulites. The predomi- overlying stratigraphical units by a major uncon- nance in the Moine of feldspathic and argillace- formity. On the Hebridean craton, the cratonic ous psammites and the prevalence of cross- cover includes two Proterozoic groups (the Stoer bedding suggest deposition in fluviatile or deltaic and the Torridon divisions of the Torridonian), environments, while the thickness of the succes- dated respectively at -1000 Ma and -800 Ma sion indicates active subsidence (Johnstone (Moorbath 1969), as well as Palaeozoic and 1975). The tectonic environment may have been -Tertiary successions. In this region, the one of extensional block-faulting similar to that basement/cover division falls unequivocally bet- envisaged by Stewart (1975) for the accumula- ween the Lewisian gneisses and the Stoer Group, tion of the Torridon cover-unit on the adjacent and as effects of both mid-Proterozoic and craton. Tanner et al. (1970) infer that the basin of Caledonian tectonic activity ]are extremely slight, deposition in the Northern Highlands may have all post Lewisian strata are virtually undisturbed. been subdivided by horst blocks and Watson In the Metamorphic Caledonides, the Lewisian (1977) has suggested that the faulting which gneisses in Northern Scotland are unconformably defined the basin may have followed the lines of overlain by psammites and pelites of the Moine deep shear-zones built into the basement prior to which themselves suffered repeated folding and 1800 Ma (p. 69). metamorphism after deposition. The relations of Through much of the Northern Highlands, these two units are unquestionably those of Lewisian rocks are tectonically interleaved with basement and cover and it is clear that Moine the Moine metasediments to form a number of

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Fio. 2. The distribution of granites containing inherited zircon fractions in the Scottish Highlands (based on Pidgeon & Aftalion 1978). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the Brit&h Caledonides 71

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FI6. 3. Sketch map to show the inferred extent of Archaean to early Proterozoic autochthonous basement on the Hebridean craton and beneath the Moine nappe in Northern Scotland and the relationships of late-Caledonian granitic and alkaline plutons. The insert map shows positive Bouguer gravity anomalies thought to relate to pre-Caledonian basement units (Bott & Watts 1970, Talwani & Eldholm 1972) in relation to Hossack's (1978) placing of the source region of the Jotun . Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

72 Janet Watson & F. W. Dunning thin but laterally extensive sheets which have a in the NW Caledonides have been tentatively general NNE elongation. The interleaving of identified as basement units distinct from the basement and cover began during the first undoubted cover-succession of 'young Moine' identifiable phases of compressive deformation and Dalradian. Details of units within the Mid- which followed the deposition of the cover. land Valley block in Ireland are given on There are many indications that the style of pp. 80--83. Two assemblages which occur north deformation in the basement differed from that of the Highland Boundary in Scotland are men- in the cover during these early phases: the base- tioned briefly here. ment yielded mainly by , with the (i) In the district of Buchan, NE Scotland, development of thick belts and the samples from the highly deformed gneisses (of definition of anticlinal wedges and detached Cowhythe, Ellon, Inzie Head, etc.) which under- thrust-slices, whereas the cover yielded mainly by lie supposed Dalradian metasediments of the folding and penetrative deformation; some Banff nappe (Read 1955) have yielded isochron basement slices were probably derived from ages of -700 Ma (Sturt et al. 1977). The horst blocks in the basin (Johnstone et al. 1969). metasediments immediately below and above The regional structures resulting from the early these gneisses are assigned to the Argyll (Middle tectonic phases in the zone which was to become Dalradian) Group which is regarded as largely the Caledonian belt were, therefore, strongly Cambrian, and these dates therefore suggest that influenced by the contrasts between Lewisian the tectonised gneiss layer was already crystalline basement and Moine cover; the effects of later when they were deposited; high initial 87Sr/86Sr phases, which probably took place at higher ratios of the gneisses suggest, indeed, that their metamorphic grade, are displayed in much the metasedimentary parent rocks were considerably same style by basement and cover. older than 700 Ma. Sturt et al. (1977) regard The early tectonic events leading to the inter- these gneisses as forming an allochthonous base- leaving of Lewisian and Moine units have been ment slice emplaced along with its cover on folded dated as Precambrian by age determination of Dalradian and possibly derived from a Cadomian pegmatites emplaced after deformation. Giletti et source area to the south of the Highland region. al. (1961) and van Breemen et al. (1974) (ii) The basement units discussed so far are all obtained isotopic ages of -750 Ma for pegma- continental in character. The broad uniformity of tites of this suite from the Morar-Glenfinnan basement types, the stratigraphical relationships region, a result which has been interpreted in two of Lewisian and Moine and the continuity of alternative ways: as evidence of a (late Precam- layers defined by seismic velocities suggest that, brian) pre-Caledonian orogenic event (the with the possible exception of the gneisses men- Morarian of Lambert 1969); or as a record of tioned onder (i), all these units formed part of a early events in a long Caledonian cycle. Brook single continental plate on the northwestern side et al. (1976) and Brook et al. (1977), using of the Iapetus ocean. The possibility that frag- isotopic data from Moine metapelites and from ments of are incorporated in the an early granitic intrusion (the Ardgour granitic structure was raised by Garson & Plant (1973) gneiss), have obtained significantly older isoc- who pointed to the occurrence of trains of hron ages (-1000 Ma) which lead them to con- mafic-ultramafic complexes which might mark clude that the earliest tectonic and metamorphic the sites of former ocean basins. Of particular events affecting the Moine should be assigned to interest is the train enclosed by Dalradian the Grenville cycle. So far as the Ardgour gneiss metasediments which Garson & Plant (1973) is concerned, this inference conflicts with zircon traced from NE Shetland via Portsoy to Tayval- U/Pb studies from which Pidgeon & Aftalion lich. In Fetlar and Unst, Shetland, the nappes of (1978) conclude that the parent granitic rocks serpentine, gabbro and associated rocks were were emplaced at -545 Ma, along with the Carn emplaced at the earth's surface (Flinn 1958, Chuinneag granite. The significance of these con- 1974) and could represent oceanic crustal mater- flicting results is one aspect of the more general ial obducted on crystalline complexes of the problem of the extent of Grenvillian northwestern plate; and although the extent of (-1000 Ma) tectonothermal activity in the Brit- the basin from which these nappes were derived ish Caledonian province and is discussed in sec- remains doubtful, the possibility that a sea tion 2e which deals with this topic. floored by oceanic crust opened during the period of deposition of the Southern Highland c. Other possible basement units (Upper Dalradian) Group is worth considering Several metamorphic or igneous assemblages (p. 77). 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Basement-cover relations in the British Caledonides 73 d. The Caledonian orogenic front the Laxford front and of exposed or buried areas At the present level of erosion, the western of granulites in the Lewisian basement. Some of limit of late Proterozoic and early Palaeozoic these anomalies extend eastward for 20-30 km deformation in Northern Scotland is marked by from the thrust-front and may be presumed to the Moine thrust-zone at which the Moine nappe delineate basement structures in the buried auto- overlies the Hebridean craton. From Loch chthon. The aeromagnetic and gravity gradients Eriboll to Glen Torridon, the eastward dip of the related to the Laxford front (cf. Bott et al. 1972) thrust-zone is low (<20 ° ) and it follows that the prolong the southeasterly line of the shear-zone autochthon which is over-ridden at the thrust- to a point 20-30 km east of the thrust-front, near zone must lie at no great depth below the surface the inferred source of the Glencoul thrust. The throughout the western portion of the Moine pronounced aeromagnetic anomalies which are nappe. Two lines of evidence enable one to thought to mark buried granulites north of the estimate the position of the orogenic front (i.e. Laxford front (Flinn 1969, Bott et al. 1972) the effective western limit of deformation and, extend considerably further eastward, and are or, metamorphism) in this buried autochthon traversed by linear NNE features which might (Figs. 3 and 4). represent thrusts or zones of retrogression in the Lewisian wedges carried westward in the autochthonous basement. Moine thrust-zone provide direct samples from Taking these indications together (Figs. 2 and the autochthonous basement. In Assynt, Kin- 3) it seems reasonable to conclude that the lochewe and Lochalsh, the rocks of these wedges western limit of effective late Proterozoic and retain structural and metamorphic patterns of early Palaeozoic deformation in the autochthon- Scourian to Laxfordian age (-2700-1800 Ma) ous basement beneath the Moine nappe lies at and show only localised effects of subsequent least 25 km east of the thrust-front. West of this penetrative deformation. The Glencoul nappe of limit, the structural patterns of the autochthon- Assynt, in which the ductile shear-zone of the ous basement are continuous with those of the Laxford front (p. 69) can be identified, has Hebridean craton and are of Archaean to early suffered a westward displacement estimated by Proterozoic age. From the fact that aeromagnetic Bailey (1935) at 21 km and by Coward (pers. anomalies indicate the occurrence of granulites at comm.) at 30 km. depth, it can be inferred that post Lewisian West of the Moine thrust, aeromagnetic and metamorphism was insufficient to bring about Bouguer gravity anomalies mark the location of regional retrogression. These inferences suggest

West Outcrop Of Source_region Of East Coast Thrust_front Glencoul Nappe Coast

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lO~rnent "reia(r~s • ~ "~.~', ' " ~~~." '~ - ' KM .~l~-_ere'nyil[e .s! " " " Late /oz~)'ic/"" don "] ..... 2,,~;/~~.'/• .'? ? ?.'"~,~.~.~.~2~X\structureandmetamorphisrn'J ~se,sm,ove,oc, t,es u~/..;//.. ././../. /.i<.,x. y Late eroterozoic/: ioverprints earlier structure?.l ) =,6.4km/sec t ? ? "~ 9 Caledonianshear_ ...... ; ; ;I / zones? L I L J I J 1 J L J J I 30 20 10 0 10 20 30 40 50 60 70 80 KM -..- NO LATE CALEDONIAN GRANITES -.,.-~ LATE CALEDONIAN GRANITES....- Plutonism Originating In .,,.LATE CALEDONIAN..,,. Lower Crust~Upper Mantle ALKALINE COMPLEXES _4 O0 Ma Fie. 4. Diagrammatic cross-section of the Caledonian orogenic front in NW Scotland, illustrating the discussion in section 2d. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

74 Janet Watson & F. W. Dunning that tectonic and thermal conditions at depth in Hossack's (1978) placing of the source of the the area now occupied by the western part of the Jotun nappes, which should lie within the Moine nappe were closer to those of the craton Caledonides (Fig. 3, inset). than to those of the orogen. The absence from the western tract of late-orogenic or post- e. Grenville activity in the northern orogenic granites and the occurrence of alkaline Caledonian province complexes not found elsewhere in the Cale- The idea that the Metamorphic Caledonides donides emphasise this contrast (Figs. 3 and 4). incorporate assemblages of metamorphic rocks To north and south of the region discussed which came into existence during three separate above, the orogenic front has a somewhat differ- geological cycles has gained support in recent ent character. From Skye southward, basement years. The assemblages referred to can be units near the thrust-zone in Iona and Islay show specified as follows: extensive retrogression and penetrative deforma- tion. A typical late orogenic granite appears very (i) Lewisian complex (gneisses and granu- close to the thrust-front in Mull and indirect lites): earliest metamorphic events Archaean evidence (Beckinsale & Obradovich 1973) sug- (-2700 Ma). gests that the dip of the thrust-zone is -30 ° . In (ii) 'Old Moines' and Ox Mountains metased- west Shetland, where several post-orogenic gra- iments: earliest metamorphic events Grenville or nites lie within 10 km of the probable thrust- late Proterozoic (-1000 Ma or -750 Ma). front, shear-zones thought to correspond to the (iii) Dalradian Supergroup + 'Young Moines' Moine thrust-zone (Pringle 1970) dip eastward at (metasediments): earliest metamorphic events about 60 ° . In these regions, the western limit of Caledonian (-500 Ma?) penetrative deformation and of metamorphism in the underlying autochthon coincides roughly with The structural and metamorphic distinction the thrust-front, a coincidence which suggests between Lewisian and Moine is well established either that a deeper section of the orogenic and is reflected in the style of early tectonic margin is exposed or that displacements at the events within the Caledonian province. On the thrust-front were small. other hand, the distinction between assemblages Evidence concerning the course of the (ii) and (iii) is obscure. In Scotland, the position orogenic front between Shetland and mainland of their mutual boundary (which, on the assump- Scotland comes from geophysical studies (Flinn tion that the initial metamorphism in (ii) was a 1969, Bott & Watts 1970) of the west Shetland Grenville event, represents the true basement/ shelf. A linear zone of positive Bouguer cover junction in the Metamorphic Caledonides) anomalies, high seismic velocities and strong remains unknown. The distribution of pegmatites magnetic anomalies extends SSW for over yielding Precambrian ages and of granite gneiss 200 km to terminate 50 km northeast of the bodies equated with the Ardgour granite gneiss Lewisian islets of Sule Skerry. This zone ('high (p. 72) with respect to the Morar, Glenfinnan A' of Bott & Watts 1970) is considered to mark a and Loch Eil structural divisions of Moine rocks NNE horst block of Lewisian-like granulites and (cf. Johnstone et al. 1969, Johnstone 1975) gneisses between basins filled with post Caledo- implies that virtually the whole Moine outcrop of nian sediments. Although it has been tacitly the Northern Highlands represents Old Moines. assumed that the basement of the horst belongs If this vast metamorphic complex was even par- to the Hebridean craton, the eastern boundary of tially unroofed and cooled after a Grenville stage the zone of anomalies is out of alignment with the of tectonism, it might be expected to have trace of the orogenic front in northern Scotland responded to Caledonian reworking as a strong (Fig. 3). This misfit suggests that the orogenic and dry basement--by rupture, development of front is stepped back eastward by warping or by shear-zones and inhomogeneous deformation, faulting near the southern end of 'high A'; a and by detachments near the basement-cover southerly downthrow on faults in this region boundary. Structural and metamorphic studies could account for the differing character of the (Rathbone & Harris this volume) which might orogenic front in Shetland and Northern Scot- test these possibilities are only in their early land. The zone of positive Bouguer anomalies stages (e.g. Harris and Watterson, in discussion west of Shetland continues northward to a point of Powell 1974, Winchester 1974, Phillips et al. off Norway as an apparently unbroken feature 1975). Problems of equal interest arise from the (Talwani & Eldholm 1972). If it is assigned to the remarkable coincidence between metamorphic craton, an interesting discrepancy is revealed by zoning attributable to Precambrian and to Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 75

Caledonian events. In western Inverness-shire, intrusions in the Grampian Highlands only one for example, the sillimanite-zone pelitic gneisses gives a figure >1500 Ma. Furthermore, Dalra- of the Glenfinnan Division appear to have dian metasediments in Connemara contain detri- undergone veining and migmatisation in both tal zircon populations derived from source rocks episodes; and the grade of both metamorphic probably between 1300 and 1675 Ma in age events decreased westward from the gneisses (Pidgeon & Aftalion 1978). towards the present orogenic front. Such coinci- These results imply that the deep crust from dences suggest a continuity in thermal conditions which the granites in question came did not at depth which may best fit the alternative view exactly resemble the Lewisian basement of the that the initial metamorphism of the Old Moines Hebridean craton. Although the intrusive Lax- was an early event (-750 Ma) in the Caledonian fordian granite sheets of Harris (Fig. 2) carry cycle. zircons not incompatible with those inherited by A second line of evidence concerning this a number of Caledonian granites, these rocks problem arises from isotopic studies of the pre- form an inconsiderable proportion of the com- Moine basement and of Caledonian granites plex and the dominant Archaean gneisses and which appear to have been derived from the granulites carry zircon of very different isotopic pre-Caledonian crust. Tectonic wedges and slices composition (Pidgeon & Aftalion 1978). The of Lewisian basement which are interleaved with Lewisian of the craton and that of the basement Moine metasediments of Northern Scotland have slices interleaved with Moine metasediments yielded Rb-Sr isochron ages of --2700 Ma in one would therefore be unlikely sources for partial area (the Scardroy slice: Moorbath 8,: Taylor melts of the type required, or for detrital zircons 1974) and a scatter of mineral ages from over of the kind recorded in the Connemara Dalra- 2200 Ma to above 400 Ma (e.g. Moorbath 1967, dian. Taking into account the evidence from the Miller et al. 1963). These results, though incom- L.I.S.P.B. seismic profile (p. 76) which indicates plete, are consistent with the accepted view that that the basement under both Northern and the basement tract from which the interleaved Grampian highlands has seismic velocities slices in the Morar Division of Moine came was appropriate for granulites and gneisses, two essentially similar to the Lewisian complex of the alternative interpretations can be suggested. Hebridean craton. They neither rule out nor support the possibility that this basement had (1) The basement under the Caledonides rep- been subjected to tectonothermal reworking dur- resents an extension of the Lewisian complex ing the Grenville cycle. which incorporates a high proportion of late More positive evidence for the reality of Gren- Laxfordian granites or subvolcanic complexes. ville activity comes from (i) dates within a Gren- An excellent analogue would be the Ketilidian ville time-span from the Annagh Gneiss complex province of South Greenland in which Archaean of Co. Mayo, western Ireland, which have earlier to early Proterozoic gneisses and granulites are been compared with the Laxfordian gneisses of penetrated by abundant late-tectonic granites the craton (van Breemen et al. 1976 and in dated at 1900-1800 Ma and also by younger press), (ii) an Rb-Sr isochron of -1050 Ma for anorogenic complexes of the rapakivi- part of the Ardgour granitic gneiss in Scotland anorthosite suite (Allaart 1976). (Brook et al. 1976) and (iii) the analysis of (2) The basement consists partly or mainly of inherited zircons from thirteen Caledonian gra- a Grenville structural province in which crust- nites in the Northern and Grampian highlands forming or reworking processes not active on the (Pidgeon & Aftalion 1978). The U-Pb isotopic craton led to the production of gneisses and systems of zircons from these granites (Fig. 2) are granulites and the emplacement of granites considered to suggest derivation by partial melt- within the time-span 1800-1000 Ma. A Gren- ing of continental crustal material of mid- ville front delimiting this province against the Proterozoic age, or of sedimentary sequences older Lewisian complex would certainly lie east carrying detrital zircons of this age. With the of the Caledonian front in the autochthon single exception of the Helmsdale granite (the (p. 73). If Precambrian metamorphism in the inherited zircons from which suggest contribu- Old Moines was of Grenville age, the front in the tions from material as old as 2000 Ma), the Moine nappe must be roughly coincidental with inferred ages for inherited zircons fall in the the Caledonian front, although remnants of range 1800-1000 Ma. Of seven intrusions in the Lewisian rocks unaffected by Grenvillian rework- Northern Highlands, five suggest the involve- ing must survive well within the province (Scar- ment of crustal material > 1500 Ma in age; of six droy slice, etc.). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

76 Janet Watson & F. W. Dunning

tions between Caledonian non-metamorphic 3. The Non-metamorphic Caledonides cover and metamorphic basement. a. Basic facts relating to the belt in general Fortunately there are a few basic facts and The problems of basement-cover relations in constraints within which speculation as to the Caledonides are immediately compounded basement-cover relations must operate. The least south of the Highland Boundary fault and its equivocal are the xenoliths of granulite-facies correlative structures in Ireland. Excluding the basement rock discovered in Carboniferous vol- problematical 'Old Moines', whose southerly canic agglomerates in central Ireland (Strogen extent is unknown, there are now two basements 1974) and in Carboniferous vents in the Scottish to consider: (1) the older crystalline Precambrian Midland Valley (Graham & Upton 1978). The basement of the Metamorphic (early) Scottish rocks are K-, Rb- and Y-depleted granu- Caledonides and (2) the basement of the Non- lites of Lewisian type. Some of the Irish rocks are metamorphic (late) Caledonides. The latter may also Lewisian-type hypersthene-bearing garnet be the Metamorphic Caledonides themselves or granulites, and new samples recently collected by their older crystalline basement, or metamorphic the Geological Survey of Ireland are richly gar- and non-metamorphic Precambrian rocks of netiferous metasedimentary granulites identical many types beyond the domain of the with the Leverburgh paragneisses from South Metamorphic Caledonides. Harris (Dr R. Dearnley pers. comm.). It is there- Each of the main geotectonic zones of the fore certain that the Scottish Midland Valley and Non-metamorphic Caledonides has its own set of the Irish end of the Southern Uplands massif are problems, not least of which is that the nature of underlain at depth by Archaean continental the basement may be completely unknown. The crust. In the Scottish Midland Valley, this base- geotectonic zones from northwest to southeast ment lies as little as 7 km below the surface, are: the Midland Valley transition zone; the according to the L.I.S.P.B. profile (Bamford et al. Southern Uplands-Longford-Down-Central Irish 1977); Graham & Upton (1978) infer that it is (Clare) inliers belt; the Lake District-Isle of directly overlain by Ordovician and younger Man-Leinster belt; the Irish Sea horst; and the rocks. In Ireland the granulite basement could Welsh basin and its eastern borders (Fig. 1). In have a younger pre-Caledonian complex inter- addition, the zone of Caledonian folding under posed between it and the Lower Palaeozoic eastern England north and south of the Thames sequence, but the large size of some of the Estuary may or may not be a separate geotec- granulite clasts and the absence of lithic clasts tonic zone. other than Lower Palaeozoic and Carboniferous On a NW-SE traverse of the British rocks suggests that the granulite basement may Caledonides, unequivocal pre-Caledonian base- directly underlie the Caledonian sequence. ment is last seen on the northwestern side of the The L.I.S.P.B. profile established the existence Highland Boundary fault in northwest Mayo in of continental crust, albeit rather thinner than the Annagh Gneiss (Erris) complex (Sutton & normal, across the entire width of the British Max 1969), recently dated as Grenvillian (van Caledonides from Eriboll to the Austwick inliers. Breemen et al. 1976 and in press). It is not seen The authors suggested that the region north of again before the Irish Sea horst is reached, where the Southern Upland fault might be underlain by the Rosslare and Mona complexes come to the Lewisian granulite basement at depths between 6 surface. It is again seen in the Welsh Borders and 14 km, while south of the Stublick faults the where both metamorphic and non-metamorphic basement below 8 to 14 km might be analogous Precambrian are exposed along the Church Stret- to the lower grade metamorphic basement in ton fault zone. A wide belt from the Southern Brittany. Beneath the Southern Uplands, the first Upland fault to the Irish Sea horst is underlain by clear reflecting interface lies at a depth of approx- very thick non-metamorphic Lower Palaeozoic imately 20 km; a basement-cover interface com- formations whose relations to the underlying parable to that to the north and south could not pre-Caledonian basement are quite unknown. be located. While obviously highly significant, Between the Highland Boundary fault and the this has to be considered in the context of the presumed trace of the Southern Upland fault in Irish granulite xenoliths. Other relevant geophys- Ireland, the non-metamorphic/metamorphic/ ical data will be discussed under the headings of crystalline basement relations are highly con- the various Caledonian geotectonic zones. It is troversial, there being strong disagreement as to worth noting that Pidgeon & Aftalion (1978), in whether or not certain rocks constitute pre- their study of zircon U-Pb systems in Caledonian Caledonian basement and concerning the rela- granites, found that, whereas granites north of Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 77 the Highland Boundary fault had dominantly Kennedy's--the difficulty of deriving clastic sed- Proterozoic source rocks, those to the south had iments across the Durness carbonate dominantly middle Palaeozoic source rocks. (Smith 1976)--and on the basis of the granulite While this does not necessarily indicate the xenoliths (Graham & Upton 1978). A valuable absence of a Precambrian basement south of the new piece of evidence in support of a landmass is Highland Boundary fault, it does suggest some the discovery by Mr R. P. Tripp of a fundamental difference to the extent that the boulder in the early Caradoc Benan Conglomer- granite-generating mechanism is different, i.e. ate containing an unusual Lower Canadian Palaeozoic crust is being selectively fused or the (Tremadoc) fauna including a new species of granites have a subcrustal source. trilobite. Dr A. W. A. Rushton, who has deter- A third important factor is the provinciality of mined the fossils, considers derivation from the Cambrian and early Ordovician shelly faunas Durness sequence unlikely, partly on account of (Williams 1972, McKerrow & Cocks 1976). the quite different limestone lithology and mode Although other explanations of provinciality of preservation and partly on the difficulty of such as climatic zonation and land barriers are bringing a limestone boulder a long distance possible, the most acceptable is that of a deep across the Moine-Dalradian -belt. The most ocean of fairly substantial width (7600 km at its likely source would seem to be a pocket of maximum'according to McKerrow & Cocks; shelf-facies Tremadoc possibly downfaulted into 1800 km according to Failer & Briden 1978) a part of the Midland Valley landmass which between the Scottish-NW Irish and Anglo- formed an island in the Caradocian sea. Further Welsh-SE Irish realms. This must mean that support for the notion of a Midland Valley oceanic crust formed the basement to some landmass comes from the well known analogy Lower Palaeozoic sequences (even if they are between the low degree of late Caledonian fold- now allochthonous) and that of ing displayed in the Silurian of the Lesmahagow oceanic crust, including the spreading ridge and Hagshaw Hills inliers and the gently folded (Lambert & McKerrow 1976), must have occur- Silurian with conformably overlying Old Red red, with various attendant tectonomagmatic over the Precambrian horsts of the effects. For the wide zone between the Southern Welsh Borderland. Upland fault and the Irish Sea horst, discussion On the assumption that a Precambrian granu- of basement-cover relations must to a large lite basement exists under the Scottish Midland extent rest on geotectonic speculations which, if Valley, which supplied to the Upper the oceanic explanation of faunal provinciality is Dalradian, we can visualise the basement/cover accepted, are likely to be plate- synth- relations as a Dalradian onlap sequence rapidly eses. thinning southwards against a Lewisian-like mas- sif on which there may be pockets of late Cam- b. The Midland Valley transition zone brian shelf-sediment, and which is mantled by Scodand. Although customarily thought of as Ordovician and younger rocks. Of course, it is terminating the Metamorphic Caledonides in highly likely that a large part of this Dalradian Scotland, the Highland Boundary fault evidently onlap sequence has been cut out by southward- does not do so: Harris & Fettes (1972) have directed thrusting on the basal slide of the Tay shown that in the region of Dunkeld the main Nappe which must surface beneath the Midland downward-facing hinge of the Tay Nappe prob- Valley (Fig. 5, sections 2, 3, 4). The relationship ably lies under the Old Red Sandstone sediments between the Midland Valley massif and the pre- and volcanics of the Strathmore . How sumed Lewisian basement of the Moine- far south the Dalradian metasediments might be Dalradian prism is uncertain. It may have been considered to extend beneath the younger essentially contiguous, though separated by Palaeozoic rocks depends on the view taken of faults--including an ancestral Highland Bound- Cambrian palaeogeography in the Highland ary fault--initiated upon uplift prior to the Border region. Kennedy (1958, p.l19) assumed deposition of the Upper Dalradian (Fig. 5, sec- that the clastic Upper Dalradian of the southern tion 1). Or the Midland Valley massif may have Grampian Highlands was derived from a Pre- been separated off by marginal basin formation cambrian landmass occupying the position of the before Upper Dalradian , to return Midland Valley and the region to the south. later when the basin closed by subduction along Though T. N. George (1960, p.42) considered the Dunrossness-Portsoy-Tayvallich line as this landmass an improbable structure it has proposed by Garson & Plant (1973). received recent support on similar grounds to From our present knowledge of Andean geol- Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

-,4 OO

1 IM'DDLE CAMBR,AN]

Midland Valley So'uthern Uplands Lake DistrJct-Leinster Upper DalradJan Basin Massif Basement CAMBRIAN IAPETUS Basin

21ARENIG-LLANVIRNJ

*"4° Grampian Orogen Ballantrae Complex Skiddaw Slates Irish Sea Welsh D1-D2 (island~... arc~_stage)A ARENIG-LLANVIRN IAPETUS Basin Horst Basin Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

31CA~ADOCl

Grampian Orogen Girvan Tappins Moffat ~" RAD~IAN IAPETUS 8orrowdale Shelf D3 Shelf Trough Rise L;A Ut, Snowdonia

...... ~i~i~::~;,~l~;~::~~iiiii' ~iii~i~iil;~ii~! i!~ii~...... +;~';iOffset|~'o.,~{~".~ :ll!

o FI~ km ~p Ioo 41END-S~LUR,ANI 25 h I

Grampian Orogen Ballantrae lapetus Snowdon Church D4 Suture Suture Lake District Syncline Stretton ',1 e~ Continental basement ~.~°

Oceanic crust

r~

FIG. 5. Plate-tectonics scheme for the British Caledonides illustrating some of the possible interactions discussed in this paper, notably the Grampian orogeny produced by the collision of the Southern Uplands basement with the Midland Valley massif and the impaction of the latter into the Moine-Dalradian pile and its thinned, ductile basement. Based on Dewey, Moseley, Roberts & Treagus, Kelling, A. G. Smith, Lewis & Bloxam and other authors. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

80 Janet Watson & F. W. Dunning ogy (Cobbing 1978), it is certain that the almost were presumably erupted through the closed incredible folding of the Dalradian nappes suture onto the surface of the extended conti- (Roberts & Treagus 1977) was not achieved by nent; they arose from a subduction zone which simple subduction of oceanic crust on the must have surfaced in a trench on the southern Andean pattern. Very strong lateral compression margin of this extended . The subduc- causing multiple overlaps seems mandatory to tion zone probably migrated slowly southwards, explain the extreme crustal shortening rep- eventually to become the site of the Iapetus resented by the Dalradian nappes. Such overlap suture. The final configuration closely resembles of cover units is only possible if the underlying that pictured by Moseley (1977), with a steeply basement had become highly ductile before dipping ophiolite suture separating sialic blocks impaction into it of the flanking massif. This which are surmounted by a continuous uncon- degree of ductility may have resulted from self- formable cover of Arenig volcanics and younger heating in the sedimentary pile (Richardson & Ordovician-Silurian sediments. It might just be Powell 1976) supplemented perhaps by hot possible to bring Dalradian formations down as rising from a subducting slab. If the far south as the Ballantrae suture if the Midland Upper Dalradian Basin was a marginal basin, Valley massif is assumed to have been buried by then impaction resulted from its closure; if on the Cambrian sediments which were not stripped off other hand the Midland Valley massif has in the Arenig transgression envisaged by Neville remained contiguous with the sub-Dalradian George (1960). sialic basement, or has been emplaced by trans- Ireland. For the purpose of this discussion, the form faulting, the compression of the Dalradian Tyrone igneous complex, the Ox Mountains prism could have been brought about by a inlier and South Connemara will be included in 'domino' effect in which was "transmitted the Midland Valley transition zone, though opin- through the Midland Valley massif as a result of ions differ on the siting of the Highland Bound- collision on its southern side with the sialic base- ary and Southern Upland faults; for example, ment of the Southern Uplands (Fig. 5). Lambert & McKerrow (1976) run the Highland The basement/cover relations in the critical Boundary fault south of the Tyrone igneous Girvan area are difficult to resolve while the complex while Phillips and co-workers (1976) precise structure and origin of the Ballantrae take the Southern Upland fault north of the ophiolite complex remain in dispute. Opinion Connemara Dalradian outcrop. ranges from the view that the complex mainly High-grade sillimanite and gneisses in comprises obducted oceanic crust and mantle the central inlier of the Tyrone igneous complex, surmounted by younger island arc volcanics regarded as Dalradian by Hartley (1933) and (Church & Gayer 1973), to the view that the Cobbing (1964), are equated with allegedly rocks are primarily island-arc volcanics (Lewis & pre-Caledonian high-grade metamorphic rocks in Bloxam 1977) associated with older and very the Ox Mountains by Phillips and co-workers subordinate ocean-floor now in the form (1976). These authors also consider the basaltic of glaucophane and amphibolite (Wilkin- pillow lavas, andesitic tufts and rhyolites which son & Cann 1974). Dewey & Pankhurst (1970) flank the metamorphic inlier on the northwest to interpret the complex as an upthrust wedge of be part of the Dalradian sequence, though the pre-Arenig oceanic lithosphere containing eclo- topmost volcanic rocks are of lowest Caradoc gite and glaucophane schist surmounted by age. Cobbing and co-workers (1965) have shown Arenig lavas produced by frictional melting dur- that the volcanic sequence is probably uncon- ing subduction. Despite the oceanic or quasi- formable on the metamorphic inlier and also on a oceanic origin of the complex, it is today almost deformed layered basic complex which they certainly underlain by continental crust compar- would equate with basic intrusions in the Dalra- able to that under the Southern Uplands. The dian of Connemara and the Scottish Highlands. smoothed aeromagnetic map of Great Britain Phillips (pers. comm.) now asserts that the schists (Hall & Dagley 1970) shows a consistent pattern of the inlier are thrust over the Ordovician of broad, low-amplitude residual anomalies over volcanics from which they are separated by a the entire Girvan-Southern Uplands region. This mylonite zone. The writer's (F.W.D.) opinion is continental basement must have been thrust in that the volcanics are too young to be Dalradian under the Ballantrae complex from the south and the layered basic complex seems more likely after closure of an ocean basin during which the to be a Dalradian feature along with the schists older glaucophane-bearing part of the complex which it intrudes. may have formed. The later island-arc volcanics A wide difference of opinion also exists as to Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 81 the age of the high-grade metamorphic rocks that ever, Ryan & Archer (1977) assert that the compose large parts of the Ox Mountains inlier. oldest formation of the Ordovician sequence, the Phillips and co-workers (1975) believe that the Lough Nafooey Group palaeontologically dated high-grade rocks, including in the northeast Ox as late Tremadoc-early Arenig, is affected by a Mountains rocks which they regard as high- single phase of upright folding and low-grade pressure granulites, form part of an older com- pumpellyite facies metamorphism in total con- plex on which the Southern Highland (Upper trast to the intense polyphase deformation and Dalradian) Group was unconformably deposited high-grade metamorphism of the Connemara before the unconformity was obliterated by Dalradian. Yardley (1976) relates the formation thrusting. They think that the high-grade of the South Mayo Trough to adjacent uplift in metamorphic rocks may be either a pre- Connemara during the main metamorphic peak, Caledonian basement or possibly sediments dated by Leggo & Pidgeon (1970) at about deformed in an early, intra-Dalradian orogenic 520 Ma, i.e. pre-Ordovician. Although the phase and subsequently uplifted and eroded to Ordovician of South Mayo is commonly thought contribute sediment to the Upper Dalradian of as resting on Dalradian, some authors see the cover. They also compared the Ox Mountains formation of the South Mayo trough in terms of sequence with the 'Old' (Morarian) Moines and crustal rifting so that Ordovician sedimentation with the Grenvillian. Other workers (Taylor occurred on a basement of oceanic crust, perhaps 1968, Lemon 1971, Long & Max 1977) regard in an intra-arc (Dewey 1971) or in a back the Ox Mountains rocks, and particularly the continental arc basin comparable to the Gulf of high-grade rocks in the SW Ox Mountains, as California (Ryan & Archer 1977), which opened Caledonian (i.e. Moine-Dalradian or Dalradian) out westwards to the Iapetus Ocean. This latter with a 'Grampian' history. Van Breemen and mode of origin implies a ridge-trench collision of co-workers (1978) have obtained essentially the sort proposed by Lambert & McKerrow Caledonian dates from Rb-Sr mineral and (1976), who suggest (p.280) that the whole whole-rock age determinations on rocks high in trough and its contents might be allochthonous, the Ox Mountains succession, whereas the granu- having been moved in from the east along trans- lites have yielded an 11-point Rb-Sr whole-rock current faults. While asserting that at one time it isochron of 860 Ma (Long & Yardley, this vol- was part of the Iapetus Ocean floor, Lambert & ume). Ten of the points give an isochron of McKerrow maintain that it subsequently 895 Ma (C. B. Long, pers. comm.) which might developed as a (? in continental crust) located suggest a younger Grenvillian or Sveco- over the cooling and contracting subducted Norwegian age. Iapetus spreading ridge. Basement/cover relations are also controver- The Caledonian structure and sequence of sial in South Mayo and Connemara, where the events in the Scottish Midland Valley are difficult very thick (>15 km) early Ordovician and dis- to reconcile with those in Ireland. There is no cordant Silurian sequence of the South Mayo clear Irish analogue to the Midland Valley granu- Trough overlies metamorphic Dalradian, though lite massif. According to Phillips and co-workers the Ordovician/Dalradian contact is concealed by (1976) it would be an Ox Mountains horst sup- Silurian overstep. As in the Ox Mountains, it has plying sediment to the Upper Dalradian, but the been suggested that pre-Caledonian basement is Ox Mountains are part of a geotectonic unit exposed within the Caledonian sequence. Phillips bounded on the south by the South Mayo trough (1973) thought that the Deer Park complex of and the Connemara Dalradian, whereas the amphibolite-facies metasediments and metaba- Scottish Midland Valley granulite massif is sites exposed in Clare Island and on the south flanked on the south by the quite different side of Clew Bay might be older, pre-Caledonian Southern Uplands geotectonic unit. The question basement comparable to the Ox Mountains suc- immediately arises: does the Midland Valley cession or alternatively, deep-level Moine or granulite massif terminate westwards before Appin (Lower Dalradian) Group deformed and reaching South Mayo and Connemara? Or does metamorphosed before lower grade Upper Dal- it underlie that region or skirt it to the south? If radian had been tectonically emplaced above it. Max & Ryan's (1975) contention that the north- Phillips also regarded the case for pre-Arenig ern, offshore boundary fault of the South Con- deformation and metamorphism of the Dalradian nemara Group is the Southern Upland fault is as unsafe on the grounds that pre-Upper Llan- correct, then we might expect to find the granu- dovery folding in the Ordovician approached in lite basement on its southern side. Since Upper style and intensity that in the Dalradian. How- Dalradian clastic rocks probably occur in Con- Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

82 Janet Watson & F. W. Dunning nemara and since the main tract of Lower and suture). The net effect must be to diminish the Middle Dalradian in Connemara was almost cer- role of, or even cast doubt on the existence of, tainly continuous with that of Donegal and the Ballantrae suture. An alternative point of Perthshire (Shackleton in discussion of Phillips view is that the Scottish Southern Uplands had 1973), it would seem that the granulite basement no granulite basement (see below) and that the of the Irish end of the Southern Uplands- granulite basement of the central Irish Car- Longford-Down-Clare inliers belt played the boniferous volcanoes is the Scottish Midland same role in relation to Dalradian sedimentation Valley granulite basement diverted southwards and tectonics as the Midland Valley granulite across the line of the Southern Upland fault, basement in Scotland (which, we have seen, may which would require the Ballantrae suture to have been separated from the Scottish Southern merge with the Iapetus suture in SW Ireland. Uplands sialic basement by the Ballantrae One rather startling solution would be to take the # / l ~t

r/ /

Highland Boundary Fault

• × × × ~k'-'~A'ZGr a n u I it e xenoliths

LeannaD Ettrk:ic~,Val~ey Thrust

Annagh Gneiss %

Dalradian

T /

FIG. 6. Location and discussion map showing some of the features mentioned in the text, including a proposed new extrapolation of the Leannan Fault and its consequences. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 83

Leannan fault through the southwest end of the oceanic crust. However, if one accepts the prop- Ox Mountains inlier, through Castlebar to Gal- osition of Lambert & McKerrow (1976) that the way Bay, lining up the c.500 Ma-old Oughterard Grampian orogeny was triggered by the subduc- granite with the similarly old Ox Mountains tion of the Iapetus spreading ridge in the Arenig, granodiorite, and so eliminating the awkward then the oceanic crust subducted subsequently Connemara salient, while admittedly creating a would have been that which lay to the south of host of other problems (Fig. 6), such as the the spreading ridge. This means that the crust location of the South Mayo Trough east of the subducted during Caradoc to Wenlock times fault. The southerly continuation of the fault into under the Southern Uplands must have become counties Clare and Kerry in Fig. 6 is drawn to progressively older during the process of subSuc- accord with anomalous trends in the Bouguer tion; depending on subduction rates, the sub- anomaly map in Phillipsetal. (1976). Dr B. W. D. ducted layer and the sedimentary sequence Yardley (pers. comm.) also thinks that the Lean- above it could ultimately have been of quite low nan fault may pass through the SW Ox Moun- Cambrian age. The inference must be that either tains, so accounting for a major metamorphic pre-Arenig sediments and oceanic crust are pres- contrast, but he could not accept that there was ent at depth in the imbricate slices in the South- any substantial transcurrent movement. Accord- ern Belt or d~collement always occurred at a ing to Phillips et al. (1976) the Connemara specific horizon (the Moffat ) and all older Dalradian massif was brought in from the west or strata were selectively subducted along with the northwest by sinistral transcurrent movement oceanic crust. along a fault-zone which they consider bounds To overcome the problem posed by Powell's the Connemara massif on the north and is poss- geophysical evidence, Lambert & McKerrow ibly the Southern Upland fault. propose that the seismic data can be interpreted in terms of a gradual downward increase in c. The Southern Uplands-Longford- velocity consistent with increasingly metamor- Down- Clare inliers belt phosed Lower Palaeozoic sediments at depth. Scotland. Fairly general agreement exists that The L.I.S.P.B. profile, it could be argued, sup- the Southern Uplands massif has an imbricate ports this interpretation to a point, though there thrust structure on both the major and minor remain 15 km of crust below the 20 km interface, scale. The chief differences within this interpreta- i.e. more than twice the normal thickness of the tion concern the deeper Caledonian structure oceanic crust. The generation of large amounts of and the composition of the deep crust. Weir granite has also to be satisfactorily accounted for (1974a) and Fyfe & Weir (1976) postulate a in a location far too near the trench to be d~collement in the Northern and Central belts in attributed to the subduction zone beneath it. which the Ettrick Valley thrust functions as the Phillips (1978) argues that the entire - sole-thrust riding on mechanically incompetent ary prism of Iapetus sediments was obducted Moffat Shales. The Ettrick Valley thrust is equ- onto the granulite basement. A quite different ated with the seismic, magnetic and conductivity view is that of Weir (1974b) who suggests that discontinuity which lies at a depth of 12 km metamorphic clasts in the Hawick rocks of poss- under the central Southern Uplands, beneath ible Telychian (mid-Silurian) age on the north- which Powell (1971) postulated the existence of east coast of Wigtown Bay were derived from a Lewisian-type basement rocks. The L.I.S.P.B. presumed Dalradian (i.e. Vendian-Cambrian) profile did not detect this discontinuity beneath terrain ('Telychian Cockburnland') with soda- the central Southern Uplands, where the main granites and very subordinate ophiolites on the refracting interface lies at a depth of 20 km site of the Northern belt of the Southern (Bamford et al. 1977, p.487). The structural Uplands. This must imply the stripping off of any model favoured by McKerrow and co-workers older Silurian and Ordovician deposits resting on (1977) is that of an accretionary prism composed the Dalradian of this Telychian Cockburnland of thrust-wedges of greywacke with soles of Mof- and the subsequent concealment of the Dalra- fat . The thrust-wedges are interpreted as dian massif under the Ettrick Valley d~collement. successively younger strips of ocean bottom sed- The situation in Ireland is appreciably differ- iment 'buttered' with trench and deep-sea fan ent. The structure of the Lower Palaeozoic rocks greywackes and stacked one above the other in in the equivalent zone which straddles the Clare the inner trench wall above the subduction zone. inliers (Weir 1974a) involves diapiric upfolding In this model, the deep basement of the Southern and southeastward thrusting, but can hardly be Uplands is, of course, Ordovician and Silurian described as imbricate. Weir's section (p.108) Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

84 Janet Watson & F. W. Dunning shows a lowermost chert and shale sequence e. The Lake District-Isle of Man-Leinster equivalent to the Moffat Shales resting on base- belt ment of unknown character, which of course The difficult question of the likely basement to need not necessarily be of Precambrian age. the Lake District Lower Palaeozoic sequence is 80 km to the northeast are the Carboniferous compounded by the problem of the thickness of agglomerates with their granulite boulders, and the Skiddaw Slates and their relation to the Manx there is nothing in the gravity anomaly map Slates. If the Skiddaw Slates are 9 km thick as (Phillips et al. 1976) or the magnetic map (Max & proposed by Simpson (1967) and especially if the Riddihough 1975) to suggest that the granulite succession is to any degree augmented by the basement does not extend southwards beneath 7.5 km thick Manx Slates (Simpson 1968), then the Clare inliers. It seems clear that the likely some form of marginal or epi-oceanic accretio- structure of the southwest end of the Southern nary prism as proposed by Dewey (1969) and Uplands-Longford-Down-Clare inliers belt can- others seems inevitable. More recently, Moseley not plausibly be interpreted in terms of the (1977) has suggested that a continental margin imbricate accretionary prism many would accept with a superposed volcanic chain of Cascades as reasonable for the Southern Uplands end. The type might better fit the geochemical data. This southwest end seems nearer to some form of implies a basement of older continental crust arc-trench gap flysch-apron, though this is rather beneath the Lake District sequence. Dunning & difficult to reconcile with the more distal charac- Max (1975) have proposed that an older crystal- ter of the turbidites. A major problem in the line basement is necessary to account for the Southern Uplands is the underground course of large volume of intermediate-acid Borrowdale the Ettrick Valley d~collement northwards volcanics. If the Skiddaw Slates are only 1800 m towards the Southern Upland fault. Possibly it thick, as originally proposed by Rose (1954), or might curve down into either the Southern c.2000 m as estimated by Jackson (1978), then Upland fault or perhaps the Ballantrae suture clearly a large thickness of pre-Arenig continen- (Fig. 5). tal crust must be present to make up the 30 km or so thickness of the crust in the eastern Lake District shown on the L.I.S.P.B. profile (Bam- ford et al. 1976). In fact, a large part of the Lake District seems to be underlain by Caledonian d. The lapetus suture granite (Bott 1974). The age of the pre-Arenig The contrast in velocities of the pre- basement is open to speculation. At depth it may Caledonian (sub-al) crustal layers on either side be Pentevrian but nearer the surface it might of the Southern Uplands in the L.I.S.P.B. profile more likely be of Mona or even Cambrian age (Bamford et al. 1977) is the most telling single (see below). piece of evidence in favour of a Lewisian-type In Ireland, the Ribband Group of Co. Wexford basement north of the Southern Uplands and a has been compared with the Skiddaw Slate Pentevrian-Cadomian basement to the south, as Group by Brenchley & Treagus (1970) who proposed by Dewey (1974). The most likely concluded that the 'Manx-Skiddaw Trough' location for the Iapetus suture would be under (Simpson 1968) extended into Ireland. The Rib- the Solway Firth as indicated by Moseley (1977): band Group mudstones and siltstones of Middle a distinct WSW-trending group of anomalies on Cambrian to Llandeilo age rest conformably on the smoothed aeromagnetic map runs down the the Lower to Middle Cambrian Bray Group Solway Firth separating the major Scottish zone (BriJck et al. this volume). According to Crimes of Caledonoid (WSW) magnetic trends from an & Crossley (1968), the deep-water Cahore English zone of Charnoid (NW) magnetic trends. (Bray) Group rocks show evidence of derivation Moreover, a linear conductivity anomaly has from the north or northeast, and not from the been discovered in the same position by Grimes southeast, in which direction the Precambrian (1977). In Ireland, Phillips et al. (1976) have basement is presently exposed. The greywackes identified the suture (as the Navan-Shannon of the Lower to Middle Cambrian Bray Head fault) from Salterstown on the coast of Louth to Formation in County Dublin were also derived the central region of the Clare inliers in which from a northerly source similar in composition to there are contrasts of structure and stratigraphy the Rosslare complex (Brfick & Reeves 1976). across the line of the suture. The suture thereaf- This poses a major problem if the Cambrian ter follows the line of the Shannon Estuary out to Iapetus lay between the Southern Uplands belt the continental margin. and the Irish Sea horst. A possible solution may Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 85 be afforded by the results of recent drilling by clearly elucidated, but is presumed to be an Glomar Challenger on D.S.D.P. legs 56 and 57 unconformity. The high-grade metamorphic across the Japan trench (Geotimes April 1978). rocks of Greenly's (1919) Penmynydd Zone are From the drilling and from seismic profiling, it regarded as an extension of his 'Basal Gneisses'. has been established that a landmass (named the The main effect of these discoveries, if Barber & Oyahsio landmass) occupied the site of the Japan Max's interpretation is correct, is a new role for trench for much of Cenozoic time, contributing the Cambrian in the Caledonian sequence of the sediment to a Cenozoic sequence deposited in Southern Irish Sea area and a diminution of the the arc-trench gap. It is suggested that a landmass significance of the Irish Sea horst. similarly situated between the Cadomian-folded Mona complex and the Iapetus ocean floor may g. The Welsh Basin and its eastern have supplied sediment to the Cahore Group and borders the Bray Group. This would accord with the In the peripheral regions of the Welsh Basin, suggestion by Phillips (1978) that the Leinster the Palaeozoic/Precambrian stratigraphical rela- basin was ensialic. An alternative proposed by tions are still very much as T. N. George de- Barber & Max (1979) is that the landmass was picted them in several classical reconstructions the granulite basement of the Longford-Down (George 1963, Figs. 6, 8, 9, 11, 13). In NW belt before it was thrust in under the Longford- Wales, Wood (1969) has demonstrated the con- Down belt. A similar suggestion was made by formable or nearly conformable relationship Mitchell & McKerrow (1975) for the Southern between the Cambrian and Arvonian, which Uplands region. might thus be an Eocambrian formation. The The precise nature of the contact of the Cahore Arvonian is overstepped northwards by the Group with the late Precambrian Cullenstown Cambrian between Llanberis and central Ang- Formation (Max & Dhonau 1974) is obscure. lesey (Wood 1974), though precisely what the The stratigraphicai relations in SE Ireland sug- relationships would be if the Gwna Group is gest that the Skiddaw Slate Group in the Lake Cambrian is uncertain. The base of the presumed District might possibly be conformably underlain Arvonian in the Harlech was not reached by Cambrian rocks resting on older or younger during the recent IGS drilling. Precambrian of 'southern' aspect. Mitchell & Reading (1971), Ziegler & McKer- row (1972) and Dewey (1974) have suggested f. The Irish Sea horst that the Welsh basin might be a small marginal Pre-Caledonian basement is widely exposed in ocean basin of Sea of Japan type formed behind Anglesey, the Lleyn peninsula and SE Ireland. an island arc. Ridgway (1976) agrees that this (or Prior to the researches of Barber & Max (1979 better still, a northward-dipping subduction zone and pers. comm.), the generally accepted view of under Wales) would best accord with the overall the stratigraphical relations (Wood 1974, Shack- compositional variations in Welsh Ordovician leton 1975) was of the late Precambrian Mona volcanic rocks. However, the second L.I.S.P.B. complex, possibly incorporating an older high- profile (Bamford et al. 1976) suggests normal grade metamorphic basement, folded and continental crust under NE Wales. Given the metamorphosed around 600 Ma and overlain by reduced thickness of lower Palaeozoic sediments Arenig to early Caradoc sediments, the Cam- in Wales (a maximum of 10 km in NE Wales), brian being almost entirely absent as a result of the bulk of the crust under Wales must be mid- to late Cambrian uplift and erosion. Now, Precambrian basement. Dunning & Max (1975) however, Barber and Max have found structural assume that pre-Uriconian metamorphic base- and stratigraphical evidence, supported by new ment must underlie a belt running northeast microfossil discoveries, to suggest that a large across central Wales as a consequence of the part of the Mona complex, and in particular the eastward overturning of the Longmynd syncline. very thick Gwna Group, is of Cambrian age. These metamorphic rocks are assumed to be The deformation of these rocks is allegedly older than the 'Bedded Succession' of the Mona Caledonian, and the discordance between them complex and to equate with the older crystalline and the overlying Ordovician is ascribed to 'Basal Gneiss'. In the Welsh Borderland (Neville deposition on the highly irregular top of the George 1963), various lower Palaeozoic forma- Gwna olistostrome (Greenly's (1919) 'autoclas- tions overstep a horst of late Precambrian vol- tic melange'). The relationship between these canic and sedimentary rocks strongly folded Cambrian rocks and the residual, more com- around 600 Ma ago. The horst was raised, to the plexly folded Mona and older rocks has yet to be accompaniment of significant deformation of the Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

86 Janet Watson & F. W. Dunning flanking Cambrian and Ordovician, in the late span must overlap with the period of deposition Ordovician-early Silurian, prior to overstep and of the Dalradian Supergroup (usually assigned to burial by the younger Silurian, commencing with the cover) and it would be equally reasonable to the upper Llandovery (Telychian) (Bridges relate them to early tectonic and magmatic 1975). episodes within the Caledonian cycle. Such an interpretation is supported by the fact that very h. Eastern England similar felsic volcanic assemblages of late Pre- From the steep dips in Silurian and Ordovician cambrian age are found close to the southeastern rocks encountered in boreholes, a Caledonian- margin of the Appalachians from Newfoundland folded lower Palaeozoic massif can be assumed (Avalon platform) as far as eastern Mas- to exist in eastern England north and south of the sachusetts. These indications (e.g. Rast et al. Thames Estuary (Dunning & Watson 1977, fig. 1976) that felsic characterised a long 1). Steep dips have also been recorded in tract near the southern border of the Iapetus boreholes near the northeast coast of East Anglia Ocean suggest a genetic connection with the (Wills 1978). This massif is tentatively inter- developing orogen and several authors (e.g. preted by some authors as the infilling of an Thorpe 1972) have outlined interpretations aulacogen (e.g. Evans 1979). The basement of involving early stages of subduction at the margin this extension of the Brabant massif is completely of Iapetus or of a subsidiary ocean basin. unknown (as indeed it is in the Brabant massif Volcanic and more localised sedimentary for- itself). But it is more likely than not to consist of mations of late Precambrian age have a wide younger Precambrian, probably within the zone lateral extent and are seen at intervals from SW of the Cadomian orogen. The basal Caledonian Wales (Pebidian) to East Anglia. Their distribu- formation may be a deep-water facies of the tion bears little relation to that of the northwes- Cambrian not seen in the many boreholes which terly geophysical anomalies mentioned above proved Cambrian in central England. and for this and other reasons it seems probable that the source of the anomalies is located in an j. The Midland craton older basement unit. Fragments of retrogressed The small and apparently isolated Midland metamorphic complexes (Malvernian, etc.) craton (Fig. 1) appears to be a remnant of a caught up in major fault zones, and metamorphic larger tectonic unit, parts of which have been detritus in several formations of the cratonic modified by Caledonian tectonism. The deep cover provide the only accessible samples poss- structure revealed by geophysical data is domi- ibly coming from this underlying basement. nated by a number of NW and NNW linear Seismic velocities are lower than those charac- anomalies (clearly displayed on aeromagnetic teristic of the Lewisian complex (Bamford et al. maps) which appear to terminate near the east- 1978) and heat flow-heat production studies sug- ern margins of the Welsh basin and Lake District gest that the predominant metamorphic grade and which are separated by crustal strips in which down to about 15 km is low (Richardson & a weak east-west grain is sometimes detectable. It Oxburgh 1978). An assemblage of low- to is suggested below that the northwesterly medium-grade metasediments and, or, metavol- anomalies mark long-lived basement fault zones. canics rather than a complex of granulites and The exposed pre-Caledonian rocks, seen only gneisses therefore seems indicated. in scattered inliers and boreholes, represent The age of this older basement is unknown. almost exclusively a very late Precambrian The exposed metamorphic rocks have yielded assemblage of tectonised but essentially only isotopic dates <800 Ma, which are consi- unmetamorphosed acid-intermediate pyroclas- dered to relate to late tectonic events. The few tics, lavas and subvolcanic complexes, together granites penetrating the craton whose zircons with clastic sedimentary formations (Uriconian, have been investigated do not contain inherited Charnian, Longmyndian, etc.). The limited zircons substantially older than the date of emp- palaeontological and isotopic evidence implies lacement (Pidgeon & Aftalion 1978) but galena that the rocks of this unit are no more than from post Caledonian mineral veins in Durham 600-700 Ma in age (see Dunning 1975 for bib- contains lead whose isotopic composition sug- liography). Where seen in contact with uncon- gests derivation from crustal material > 1700 Ma formable Cambrian strata, the rocks of the unit in age (Wedepohl et al. 1978). The northwesterly show a relatively high degree of and alignment of basement dislocations parallels that for this reason are usually assigned to a pre- of long-lived dislocations widely developed in the Caledonian basement. Nevertheless, their time- main part of the European craton to the east, Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Basement-cover relations in the British Caledonides 87 where the basement structure dates mainly from beneath the northwestern Caledonides, though early Proterozoic (Svecofennide, >1800 Ma) still inconclusive, suggests that the significance of events (see Watson 1976 for bibliography). superposition of structure and metamorphic These slight indications support th6 views of zones in the 'old Moines' of Northern Scotland Shackleton (1969) and Baker (1971) that the requires re-examination. deep basement is at least early Proterozoic in (d) The intense deformation of the Dalradian age; but there is no compelling reason to elimi- Supergroup, with its extreme crustal shortening, nate a Grenvillian or Brioverian (<1000 Ma) must have been the result of plate collision acting age. on a sedimentary prism and crystalline basement Whatever was the initial age of the crust rendered highly ductile by self-heating sup- beneath the Midland craton, it appears that the plemented by heating from below. That deforma- basement dislocations have had a long history of tion was brought about purely by subduction on activity. The Malvern line and portions of the the Andean pattern now seems very unlikely. northwesterly zones which traverse the eastern (e) The impaction of the Midland Valley and northern parts of the craton coincide with granulite basement into the Dalradian prism may zones of structural disturbance affecting have been caused by a 'domino'-style collision Palaeozoic or post Palaeozoic strata (cf. Dunning with the continental basement of the Southern & Max 1975). These may have defined fault- Uplands, resulting from subduction at the Ballan- troughs such as the early Caledonian aulacogen trae suture. mentioned on p.86. Several of the known granitic (f) Continental basement thrust in from the and other intrusions (ranging from -550 to south presumably now underlies the Ballantrae -300 Ma in age) are located on these zones (e.g. ophiolite complex, though a large part of the Bott 1974, Bott et al. 1978), as are several zones complex may have been erupted after the under- of post Caledonian lead-baryte-fluorite mineral- thrusting. isation (cf. Evans & Maroof 1976). They have (g) Basement/cover relations in the Tyrone therefore influenced crustal developments during inlier-Ox Mountains-South Mayo zone are not and after the Caledonian cycle. They may be capable of resolution while present controversy compared in this respect with parallel (probably continues over the age of the high-grade related) lineaments in and east of the southern metamorphic rocks, though older basement of North Sea which controlled such features as the possible late Grenvillian age may exist in the NE eastern border of the Brabant massif, the Sole Pit Ox Mountains. trough and the Danish-Polish furrow or Torn- (h) The westward extension of the Scottish quist line. Midland Valley granulite massif is highly uncer- tain. It may skirt the Connemara Dalradian mas- Conclusions sif on the south, especially if the latter has been emptaced by transcurrent movements, possibly It would be rash of us to claim that we have on the Leannan or other faults. Otherwise the arrived at any positive conclusions in what granulite massif presumably lies beneath or ter- amounts to a discussion paper. Nevertheless, minates against the Connemara Dalradian. opinions have been expressed on a number of (i) The imbricate accretionary prism theory topics and for what they are worth, we list them proposed by McKerrow et al. (1977) for the here. structure of the central Southern Uplands (a) The Lewisian basement underlying the becomes, despite its many attractions, increas- Caledonian orogen may be more extensively ingly difficult to apply in the direction of Ireland. invaded by early Proterozoic granitic material It is also difficult to reconcile satisfactorily with than is the corresponding basement of the Hebri- the theory of major d~collement on the Ettrick dean craton. Valley thrust. (b) The western limit of late Proterozoic and (j) Good geophysical evidence exists for plac- Caledonian tectonism in the autochthonous ing the Iapetus suture under the Solway Firth as basement of Northern Scotland lies at least proposed by Moseley (1977). 25 km east of the orogenic front defined by the (k) The problem of basement type in the Lake Moine thrust-zone. Late-orogenic Caledonian District is complicated by the problem of the granites formed by crustal melting are not found thickness of the Skiddaw Slates. If Simpson's west of this limit. (1967) thickness estimate is accepted, particu- (c) Evidence concerning the occurrence of a larly if augmented by his estimate for the Manx zone of Grenville tectonothermal activity Slates, then the Skiddaw-Manx sequence must be Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

88 Janet Watson & F. W. Dunning

an epi-oceanic accretionary prism. If the sequ- (1) A landmass, possibly analogous with the ence is much thinner, as in early Geological Cenozoic Oyahsio landmass that formerly Survey estimates, then basement/cover relations occupied the Japan trench, must have flanked the would be similar to those in the Leinster basin, Leinster basin on the northwest to supply sedi- i.e. Ordovician resting conformably on Cambrian ment to the Cambrian Bray Head formation and resting unconformably on younger and older the Cahore Group. Precambrian.

ACKNOWLEDGEMENTS. The authors have been able to make substantial improvements to the paper as a result of discussions with Drs Eales, Long, Max and Weir (F.W.D.) and Dr R. Pidgeon and Mr G. S. Johnstone (J.W.). Dr W. E. A. Phillips made many useful critical comments in correspondence with F.W.D.

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JANET WATSON, Department of Geology, Imperial College of Science and Technol- ogy, London. F. W. DUNNING, Geological Museum, Exhibition Road, London.