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Crustal Growth and Late Precambrian-Early J. geol. Soc. London, Vol. 141, 1984, pp. 521-536, 9 figs, 3 tables. Printed in Northern Ireland Crustal growth and late Precambrian-early Palaeozoic plate tectonic evolution of England and Wales R. S. Thorpe, R. D. Beckinsale, P. J. Patchett, J. D. A. Piper, G. R. Davies & J. A. Evans SUMMARY: Comparison of the basement rocks of Britain reveals major contrasts to the N and S of the Iapetus suture (Solway-Shannonline). North of the suture, polymetamorphic high-gradebasement schists and gneissesand igneous rocks date from about 2900Mato c. 400Ma, the latter age corresponding to closureof the Iapetus ocean. By contrast, S of the suture, inEngland and Wales,the basement is dominantlylow grade metamorphic rock, together with intrusive and extrusive igneous, and sedimentary rocks. There is little geological or geochemical evidence for the presence of basement older than c. 900 Ma below this area and the predominant expression of crustal growth in England and Wales is indicated by radiometric ages between c. 700 Ma and 450 Ma, reflecting late Precambrian-Palaeozoic magmatism and metamorphism. Palaeomagnetic data from Scotland demonstrate that the crust in northern Britain developed as an integral part of the Laurentian Shield during mid-Proterozoic times (1600-1000 Ma). The palaeomagnetic record from the pre-Ordovician rocks of England and Wales shows that this area was unrelated to the Laurentian Shield prior to closure of the Iapetus Ocean. The apparent polar wander (a.p.w.) path is that of a distinct microplate involved in rapid growth of Iapetus during early Cambrian times, and emplaced in its present configuration adjacent to northern Britain as a result of subduction of the oceanbasin between Cambrian and Silurian times. We conclude that whereas the basement to the north of the Iapetus suture formed an essentially continental unit from 2900 to 400 Ma, the basement of England and Wales is a result of crustal growthfrom about 900Ma to 400Ma and formed by accretion ofisland arcs,associated accretionary prisms and fore-arc basin sediments within the Iapetus Ocean. Precambrianand younger basement rocks crop out RosslareComplex is composed of paragneissesand overlarge areas in Scotland, but occur as small basic to acid orthogneisses which have experienced a outcrops in Englandand Wales. Scattered borehole complex metamorphic history. Max (1975) has argued occurrences in N Wales, central and eastern England from unpublished Rb-Sr whole-rock isochron data that indicate that such rocks may form the largest part of ‘a minimumage of c. 1700-1600Mawould be the crust below much of southern Britain (Fig. 1). In appropriatefor the second metamorphic episode in contrast to the predominantly high-grade metamorphic the Rosslare Complex, but that there is some evidence basement rocks of Scotland, the basement of southern of incomplete rejuvenation of the oldest part of the Britainconsists largely of intrusiveand volcanic complex which would probably be no more than about igneousrocks, low-grade metamorphic rocks and 2400Ma’(Max 1975, p. 101). Winchester & Max sedimentary rocks. Recent studies of these rocks from (1982) have reported geochemical data relevant to the southernBritain have been reviewed by Dunning origin of theRosslare Complex and argued that a (1975), Shackleton (1975) and Thorpe (1979, 1982). In complex metanlorphic history ‘pre-dates the intrusion, this contribution we review the geology, geochemistry, metamorphismand cooling from amphibolite facies isotopicand palaeomagnetic data for exposed base- conditions at about 650 Ma’. The Rosslare Complex is ment rocks in southern Britain and use these data to adjacentto meta-sedimentary rocks termed the Cul- argue that much of the basement of southern Britain lenstown Group. This is much less deformed than the formed during the time interval c. 700-450 Ma by an RosslareComplex and is correlated with themeta- episode of crustalgrowth due toformation and sedimentary rocks of the Mona Complex in Anglesey accretion of islandarcs and associated accretionary (seebelow). The Rosslare Complex was intruded by prisms (subduction complexes). theCarnsore granodiorite dated by themineral whole-rock Rb-Sr intersection method at S20 f 6 Ma Geological characteristics of (Leutwein et al. 1972) and the Salteesgranite which Precambrian-early Palaeozoic has a Rb-Sr whole-rock isochron age of 436 + 7 Ma rocks S of the Iapetus suture (Max et al. 1979). SE Ireland (Fig. 1: areas 1 and 2) Anglesey-Lleyn (Fig. 1: area3) The Precambrian rocks of SE Ireland are termed the The largestoutcrop of Precambrianrock within S RosslareComplex and the Cullenstown Group. The Britain inis Angleseyand Lleyn (Greenly 1919; Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/3/521/4888109/gsjgs.141.3.0521.pdf by guest on 25 September 2021 522 R.al. S. Thorpe et TECTONIC UNIT LITHOLOGY GEOLOGICAL AGE I I Church Bay Iuffs New Harbour ,:BNew Harbour GP U"lt Pre-Cambrian South Slack Unil { m:"u",s:~::'s:,"lKh:;:c',",, i Fm. FIG. 2. Stratigraphical and structuralsequence in theMona Complex, based on Barber & Max (1979) and Barber er al. (1981). The spaces between the units correspond to possible tectonic discordances. 110.. ported an Rb-Sr whole-rock isochron age of 595 12 200 kilometres 12- f Ma for anepisode of almandine-amphibolitefacies metamorphism at Holland Arms. The initial "Sr/'% FIG. 1. Precambrian rocks of England and Wales ratio of these gneisses (0.7061 f 3) was considered to showing outcrops referred to in text. 1, Wexford precludea premetamorphic history for these rocks (Cullenstown Formation); 2, Wexford(Rosslare extending back beyond c. 750 Ma. This conclusion was Complex); 3, Anglesey-Lleyn (Mona Complex); 4, recognizedasbeing consistent with Shackleton's N Wales (Arvonian Volcanic Group); 5, Shrop- shire (Uriconian volcanics and Longmyndian sedi- (1969, 1975) interpretation of the fieldevidence that ments); 6, Stanner-Hanter Complex; 7, Leicester- the gneissesrepresent more highly metamorphosed shire (Charnian and S Leicestershire diorites); 8, S parts of the Bedded Succession. Wales (Dimetian Complex and Pebidianvolca- To investigate further the age of metamorphism and nics); 9, S Wales (Johnston Complex and Bento- magmatismin the Mona Complex, samples of the nianvolcanics); 10, Malvern Hills (Malvernian gneisses and the Coedana granite analysed by Beckin- Complex and WarrenHouse volcanics); 11, sale & Thorpe(1979) were selected for Sm-Nd Guernsey (Pentevrian and Cadomian); 12, Jersey analysis by G. R.Davies. One sample wasselected (Cadomian). Circles represent boreholeswhich have penetrated rocksof presumed Precambrian from both the micaceous paragneiss and orthogneiss age. groups in Gaerwen quarry, and these samples lie on the Rb-Sr whole-rock isochrons published by Beckin- sale & Thorpe (1979; Fig. 2). Sm and Nd contents and Shackleton 1969, 1975; Barber & Max 1979; Barber et 143Nd/'44Nd ratios are shown in Table 1. The samples al. 1981; Fig. 2). In the Mona Complex of Anglesey have a narrow range of SmiNd ratios from 0.1795 to andLleyn,thick succession a of flysch-type 0.1978 and also little spread in present day 143Nd/'44Nd sedimentaryrocks (the 'bedded succession') was ratios with values between 0.51211 and 0.51220. These deposited during the late Precambrian, and, according 143Nd/'44Ndratios are lower than present-day values to Barber & Max (1979), the early Palaeozoic. These forthe chondritic uniform reservoir (CHUR;pre- rockswere folded, metamorphosed and intruded by sumedtobe representative of the bulk Earth; igneousrocks during the Precambrian, although DePaolo & Wasserburg 1979) andthe model age, according toBarber & Max(1979), much of their termed TcHUR, or time at which the SdNd ratio of deformation is Caledonian.The succession contains theprecursors was fractionatedfrom CHUR to the chert and manganiferous shale,and mafic andultra- measured values, may be calculated. Values of TCHUR mafic igneousrocks which resemble successively are about 840Ma and 890Ma for the gneisses. Since deepercomponents of oceaniccrust (Wood 1974; many crustal rocks appear to have been derived from Thorpe 1974,1978). These rocks were folded and mantle sources with higher Sm/Nd than CHUR, the metamorphosedto schists and intruded by igneous ages of Sm/Ndfractionation relative tosuch a rocks of intermediate and acid composition during the MORB-likesource region (TM~RB)areshown in Precambrian. They are associated with gneisses meta- Table 1. These extend the possible crustal ages of the morphosedunder high T-high Pconditions (sillima- gneiss precursors to c. 1300 Ma and the Sm-Nd data nite-almandine facies). thereforepreclude total generation of thesegneiss Evidence bearing upon the age of the gneisses was samples from the mantle prior to c. 1300 Ma. It would, reviewed by Beckinsale & Thorpe (1979) whore- of course, be a simple exercise to erect a model with Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/3/521/4888109/gsjgs.141.3.0521.pdf by guest on 25 September 2021 Early crustal evolution of England and Wales 523 TABLE 1. Sample number LocationRocktype Splz [ppm] Nd [ppm] SmlNdx 143Nd/"NdtTc,uR[Mal TMORB[M~I M onian graniticMonian 5.8210.512171 0.1978 29.43 f22 1300 890 2622 Gaerwen quarry } gneiss$ AS16 [GRSH 480 728] micaceous6.092Monian 31.00 0.1965 0.512198f28 1260 840 gneiss: Gw alchmai quarry Coedana granite 2.140 11.92 0.1795 0.512109f200.1795 11.92 2.140 graniteCoedana Gwalchmaiquarry 1270 890 [GR SH372 7661 260126090.1959 1 19.34 3.788 0.512149f20 920 1325 BDN 1549 IGS borehole Rushton schist 0.171947.81 8.218 0.511841110 1260 1550 1, ,, BDN 1573BDN } '' )) 7.2320.511934*28 0.2008 36.01 1670 1350 * Sm/Nd ratios determined to f0.1% (Davies 1983). t The BCR-1 value for '"Nd/"'Nd measured at the Open University during the period of this study was 0,51262 52. $ Sample descriptions are given in Beckinsale and Thorpe (1979; Table 1). TMORB parameters '43Nd/'44Nd present day= 0.513100, Sm/Nd = 0.3772. an 'enriched mantle' composition in terms of SmiNd that would yield SdNd model ages concordant with the Rb/Sr whole-rock isochron ages.
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