J. geol. Soc. London, Vol. 142, 1985, pp. 1139-1157, 15 figs, 5 tables. Printed in Northern Ireland The geochemistry, metasomatism and petrogenesis of the granites of the English Lake District C. O’Brien, J. A. Plant*, P. R. Simpson* & J. Tarney Department of Geology, The University, Leicester LE1 7RH, UK and *British Geological Survey, London, UK SUMMARY: New geochemical data for the Carrock, Threlkeld, Ennerdale, Shap, Skiddaw and Eskdale granites of the Lake District are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions. The Caledonian granites of the Lake District have more associated hydrothermal activity and mineralizationthan their equivalents north of the Iapetussuture in Scotland. Pervasive high-temperaturemetasomatism which affects the Shap, Skiddaw and Eskdaleintrusions is accompanied by remobilization of the large ionic lithophile elements (K, Rb, Sr, U) and Li and B, although high field strength elements, including the rare earth elements, remain unaffected except near greisen and mineral veins. The Threlkeld intrusion appears to have suffered loss of Baand Sr duringa low-temperature event. However, there is no consistentrelationship between granite composition, hydrothermal activity and mineralization which could be used to supportthe granitecupola model for mineralization inthe province. Rather, the episodic mineralization which affects the Lake District may be related more to the broad geothermal field consequent upon the emplacement of the deeper Lake District batholith, identified from geophysical data. Despite the variable metasomatic perturbations of their primary chemistry, three distinct groups of intrusions can be recognized on geochemical grounds. The Carrock granophyre is a product of tholeiitic fractionation and the Threlkeld and Ennerdale bodies are more typical calc-alkaline intrusions, whereas the Eskdale, Shap and Skiddaw plutons show a progressive evolution towards more complex geochemical patterns, suggesting decreasing hornblende but increasing plagioclase control of fractionating processes. The culmination of this trend may have been the emplacement of a large, high heat production granite beneath the Lake District at the end of the Caledonian orogeny. The suggestion made on geophysical grounds that the Eskdale intrusion is a cupola of the Lake District batholith cannot be simply reconciled with geochemical modelling.None of theLake District graniteshas geochemical or isotopic characteristics which are convincingly S-type (i.e. formed by partial melting of a sedimentary protolith), and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate. Renewedinterest in the British Caledoniangranites mineralization in the Caledonian. Hence a systematic has led to a number of detailed studies of intrusions in study of the trace element geochemistry and petrology Scotland, north of the Iapetus suture, using modern of the Carrock, Threlkeld, Ennerdale, Shap, Skiddaw trace and isotope data to understand their petrogene- and Eskdalegranites has been carried out in a sis, and particularly to constrain the extent of crustal collaborative study between Leicester University and involvement (e.g. Halliday 1984). In contrast, except the British Geological Survey (BGS).The study for some K-Ar and Rb-Sr geochronological data, few concerns the magmatic evolution of the Lake District studies have been made of the Caledonian granitoids granites in relationto models for the tectonic setting of of the southern plate, most of which outcrop in the the Lake District province during the Caledonian. The Lake District. Although broadly contemporaneous, geochemical effects of water-rock interaction, result- these granites were emplaced in a terrain related to a ing from the hydrothermal activity in the LakeDistrict different,southerly dipping subduction zone during geothermal field (which persistedfrom Devonian to the closure of Iapetus,and wereintruded into possibly as late as the Jurassic) has also been continental crust apparently considerably younger in investigated, with implications for the associated age and differentin character from that underlying mineralization. most of Scotland (Thorpe et al. 1984). An additional feature is that the LakeDistrict Caledonian province is Tectonic setting of the Lake considerably more mineralized thanthe Scottish District province. Comparisons between the two provinces may prove During the Caledonianorogeny theLake District useful in constraining models for granite genesis and formed part of theEuropean (or‘southern’) plate. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1139/4888936/gsjgs.142.6.1139.pdf by guest on 30 September 2021 1140 C. O’Brien et al. This is believed to havebecome joined onto the Thesouthern province of the Caledonides is less unstable southeastern margin of the American-Green- well understood than the Scottish Caledonides, owing landic (or ‘northern’)continental plate (Williams mainly to the younger sedimentary cover over most of 1969) in a collision event which culminated during the England (Fig. 1). Seismic refraction studies (Bamford late Silurian-early Devonian, the suture being located 1979) and isotopicstudies of granites (Hampton & along the Solway line in southern Scotland. The Lake Taylor 1983; Pidgeon & Aftalion 1978) provide little District now lies immediately south of the suture and evidence in favour of widespreada Precambrian represents the most northerly exposure of the Caledo- gneissose basement comparable with that of northern nian rocks of the southern plate. To the north, in the Scotland. Instead, studies of isolated inliers indicate SouthernUplands of Scotland, is a thick prism of thatthe pre-Caledonianbasement of the southern LowerPalaeozoic sediments with ophiolitic compo- Caledonides consists of calc-alkaline volcanics and nents thought to have been tectonically accreted onto sediments,ranging in age from c. 700 to c. 500 Ma thenorthern continental plate duringnorthward (Patchett et al. 1980; Bath 1974; Beckinsale & Thorpe subduction of an oceanic plate (Leggett et al. 1979). 1979; Thorpe et al. 1984); and it has been suggested Post CaledonianPost cover.Borrowdale volcanics. Caledonian granites. .. .. ......... Silurlan sediments. HSkiddaw slates. 0. .. FIG. 1. Simplified geological map of the English Lake District with inset of schematic tectonic map of the British Caledonides. HBF = Highland Boundary Fault; SUF = Southern Uplands Fault. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1139/4888936/gsjgs.142.6.1139.pdf by guest on 30 September 2021 GranitesEnglish of the Lake District 1141 TABLE1: Published isotopic dates for the major intrusions of the Lake District Intrusion Age (Mal Age Intrusion Method "Srta6Sr 60l8 Reference Skiddaw 392 * 4 K-Ar (biotite) 5.9-9.9 Shepherd et al. (1976) Shap 393 f 3 Rb-Sr 0.7077 11.0* Wadge et al. (1976) Carrock Fell 416 f 20 Rb-Sr 0.7071 Rundle (1979) granophyre Ennerdale 420 f 4 Rb-Sr 0.7057 Rundle (1979) Eskdale 429 f 4 Rb-Sr 0.7076 10.9* Rundle (1979) Threlkeld Rb-Sr 445 f 15 0.7055 Wadge et al. (1974) Carrock Fell K-Ar 468 f .l0 (biotite) Rundle (1979) gabbro * for Shap andEskdale from Harmon & Halliday (1980). (Le Bas 1982) that these rocks evolved as a series of exposed in a broad belt in the northern Lake District, island arcs and marginal basins. Evidence of a minor they also occur in isolated inliers south of the collision at c. 600 Ma in Anglesey, possibly due to the Borrowdale volcanics, suggesting that they may also closure of a marginal basin (Wright 1976; Barber & underliethe Borrowdale group. The Borrowdale Max 1979), may represent the final consolidation of volcanics are subaerial to subaqueous calc-alkaline the southern plate, which appears to have behaved lavas and tuffs compatible with a relatively evolved coherently from Cambrian times. continental margin environment(Fitton & Hughes During the Cambro-Ordovician, the relatively 1970); they have given a Sm-Nd mineral isochron age young andprobably thin southern continentalplate of c. 457 Ma (Thirlwall & Fitton 1983), consistent with became the site of sedimentation in the Welsh Basin a stratigraphic age of Llandeilo to Caradoc (Harland et and in theLake District,where the oldest rocks al. 1982). The Skiddaw andBorrowdale Groupsare presently exposed arethe Skiddaw Group. These unconformably overlain by limestones,shales, mud- consist of more than 3 km of graptoliticmudstones stones and greywackes of Upper Caradoc and Ash7ill with subsidiary greywacke and sandstones of Arenig to age. The rocks of theLake District are at lower Upper Llanvirn age (Wadge 1978) and have been greenschist metamorphic grade (Oliver et al. 1984) and interpreted as turbiditesequences deposited in re- are only weakly deformed in contrast to the Caledo- latively deep water(Jackson 1978). Inthe northern nides of N Scotland where Moine and Dalradian rocks Lake District, the Eycott Hill volcanics, which repre- are strongly deformedat greenschist to upper sent the earliest volcanism in the area, have tholeiitic amphibolite grade. affinities (Fitton & Hughes 1970) and are regarded as Granite magmatism occurred episodically in both primitive island arc lavas possibly extruded onan provinces of the Caledonides. Inthe Lake District immature, thinned continental crust. activity apparently commenced (see Table 1 and Fig. Sediments of the Skiddaw Group weredeformed 2) with the emplacement of the Threlkeld intrusion