Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 • BASEMENT-COVER RELATIONSHIPS Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 BASEMENT-COVER RELATIONSHIPS FLINN ET AL~g~ JOHNSTONE ET AL RATHBONE ~ HARRIS~'~ RAMSAY & STURT SANDERSi I & VAN BREEMEN BREWER ET AL" 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 Lewisian complex 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 Suture 84 e The Lake District-Isle of Man-Leinster Belt 84 f The Irish Sea Horst 85 g The Welsh Basin and its eastern borders 85 h Eastern England 86 j The Midland Craton 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 gneisses) 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 orogeny 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 crust. 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 continental crust 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- metamorphism 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 Precambrian (<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 Dalradian 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 ;i!i!!!!iiiiiiiiii!i!ii!ii!i!!!!i!i!i!i!i!iiiiiililili!i!i!/ / I :~ii::!i!::!::!::i~!~!~::!)i~::i::i::ilili::i::i::i~'~' / .:i!iiiiiiiiiiiiiiii!i!ii~ .,:.;,:.:-:.:.:.;.:1.-i!!!!:::!:!:!:. .:.:.:,:.:.:.:.:...:.:.:.:.:.:,:.:.:.:.:.:. iii!iiiiiiiiiiiiii!iiii!iiiiiiiiiii ::iiii)iiii)iiiii.C.BA]'.ON.iiiii :::::::::::::::::::::::::::::::::::::::::::::::::::::::: .:.:.:.:.:.:.:,:.:.:.:.:.:, ............. .. ::::::::::::::::::::::::::::: iiiii!MIDLAND:!:!:!:!:!:!:??!iiiiiiiii!!i +:.:.:.:.:........-.....-...:...:.:+:,:.: : : :::::::::::::::::::::::::::::::::::: iii!iii!i!i!! CR ATO N :iii~ -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 shear-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 Proterozoic times (-1750 Ma). There is no Laxfordian granitic gneiss relatively rich in K and record of later Proterozoic activity on the craton, Rb, while elsewhere pegmatite and granite but a number of lines of evidence suggest that the formed more irregular vein-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 fault. 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 granites 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).