The Geochemistry, Metasomatism and Petrogenesis of the Granites of the English Lake District

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 variablemetasomatic perturbations of theirprimary 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 fromthat 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.

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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 in prior tothe extrusion of the Borrowdale Volcanic the Lower Ordovician, followed by the Eskdale and Group (Wadge 1972; Jeans 1972). Although most are then the Ennerdale intrusions in the late Ordovician to

Ordovlclon Silurian Devontan Silurian Ordovlclon -T' 500490 480 470 460 450 440 430 420 410 380390400 370 360

slates Upper OrdovtclanSklddawUpper slates and Post -Tectonic cove1 Sedlmentatlon Eycott B~~~~~~~I~Silurian sediment5 Volconlctty - - volcanlcsvoIcanIcs Deformotlon ------A B FG Majorlntruslves I I m

FIG.2. Time-scale of the major events in the geological evolution of the Lake District. A, Carrock Fell Gabbro; B, Threlkeldmicrogranite; C,Eskdale; D, Ennerdale; E, Carrock Fell granophyre andferrogabbro; F, Shap; G. Skiddaw.

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early Silurian (although crosscutting intrusive contacts by drilling, it should be noted that the Eskdale pluton atGreat HallGill, Wastwater, indicate thatthe is apparentlyolder (429 * 4 Ma;Rundle 1979) and Eskdale intrusion may be later than Ennerdale), and precedes the widespread mineralization in theLake the Shap andSkiddaw granites in the Lower Devo- District by some 40 Ma. The probable age and nature nian. On the basis of interpretation of Bouguer gravity of theLake District batholiththus needs tobe data, a large granite batholith is thought to underlie resolved. much of theLake District (Bott 1974) andmore Petrographicaspects of theLake District granites detailed gravity modelling (Lee 1984) has suggested are summarized in Table 2. Texturally, the smaller that it might be representedat the surface by the intrusions, the Carrockand Ennerdale granophyres Eskdalegranite, with additionalcupolas at Wasdale and the Threlkeld microgranite, are fine grained with Head and Birker Fell. Although the gravity expression small phenocrysts of plagioclase andpyroxene or is similar to that of the buried Wensleydale [400 f 10 biotite, suggesting that they arenear to liquid Mal (Dunham 1974) andWeardale [410 f 10 Mal compositions. The Eskdale,Skiddaw and Shap plutons (Holland & Lambert 1970) which have been sampled are coarsergrained and appear to represent various TABLE2: A summary of the petrography of Lake District granites Name Felakpars Majics Accessory Texture Secondary Ski'ddaw Plagioclase Biotite Apatite Granitic Pyrite and perthitic Zircon Fluorite K-feldspar Magnetite Muscovite in roughly Monazite Calcite equal Xenotime Epidote proportions Sphene Chlorite Rutile Shap Plagioclase Biotite Zircon Porphyritic K-feldspar and Hornblende Apatite (K-feldspar) Pyrite perthitic (minor) Allanite granite Chalcopyrite K-feldspar Sphene Fluorite in roughly Magnetite Muscovite equal Chlorite proportions Baryte Hematite Carrock Fell Plagioclase Clino- Apatite Porphyritic Chlorite granophyre phenocrysts, pyroxene Zircon (plagioclase) Muscovite alkali feldspar Magnetite granophyre groundmass Ilmenite Ennerdale Plagioclase Clino- Apatite Porphyritic Chlorite phenocrysts, pyroxene Magnetite (plagioclase) Muscovite alkali feldspar Zircon granophyre Epidote groundmass Biotite Allanite Eskdale Plagioclase Biotite Zircon Granitic Hematite and Garnet Apatite Muscovite perthitic Monazite Chlorite K-feldspar Epidote in equal Calcite proportions Rutile Threlkeld Plagioclase Garnet Magnetite Fine-grained, Muscovite phenocrysts, (biotite) slightly Chlorite subordinate porphyritic Pyrite alkali (plagioclase) Calcite feldspar granite Skiddaw type (A) granite (referred to in Figs 3, 4 and 5) has a greater proportion of plagioclase and biotite than the type (B) granite. The Eskdale granodiorite (a) has a greater proportion of plagioclase, garnet and biotite than the Eskdale granite (b), cf. Table 4.

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proportions of early crystallized minerals (plagioclase may be related tothat hostedinCarboniferous and biotite) within a matrix formed by late crystalliza- limestone in S Cumbria, is associated with the tion of K-feldspar andquartz. Although oxygen Ennerdale and Eskdale intrusions. isotope data (Table 1) suggest that some of these Many of the granites show visible effects of granites may be S-type according tothe criteria of metasomatism(Table 2), thedegree and nature of Chappell & White 1974 , then:. petrographic charac- which varies, in part depending onthe exposed teristics and initial 6Sr/"Sr ratlos indicate that none is structural level of the intrusions and of the related unequivocally of S-type. geothermal field. Skiddaw, Shap and, to some extent, Eskdale show various high-temperature effects of water-rock interaction (P. B. Greenwoodpers. Metalliferous mineralization comm.;Shepherd et al. 1976) with greisenization at associated with Lake District Skiddaw and Eskdale,and subsolidus K-feldspar growth at Shap. Such phenomenaare attributed to granites high-temperature acid leaching andpostmagmatic K-metasomatism, respectively (Korzhinskii 1953, The Lake District contains an important orefield with 1964; Beus 1963). TheThrelkeld, Ennerdale and numerous mineral veins of hydrothermal origin (Stan- Eskdaleintrusions have been variably affected by ley & Vaughan 1982; Dagger 1977; Shepherd et al. low-temperature alteration, with the development of 1976). Shepherd et al. postulated,on the basis of sericite,chlorite and calcite, the Threlkeld intrusion isotopic and fluid inclusion evidence, that the tungsten having been pervasively altered. ores of the province weredeposited soon afterthe consolidation of the granites from saline solutions of mixed magmatic and circulating ground water origin, Geochemistry and Firman (1978) suggested a similar origin for the other vein systems in Lower Palaeozoic rocks. Sampling and analytical techniques Radiometricages indicate thatthe first episodes of hydrothermal activity in theLake District, which More than 150 rock samples were collected from granites (sensu lato) and their host rocks in the Lake District province deposited W-MO minerals in and around granites and duringthe regional geochemical survey undertaken by the Cu in volcanics andslates, took place 390 Ma ago BGS in 1979. The complete analytical dataand details of (Shepherd et al. 1976). Pb-Zn mineralization occurred sampling localities are lodged in the UK-IGBA system within later at c. 360 Ma, contemporaneous with the deposi- the National Geochemical Data Bank, whence they can be tion of the basal Carboniferousconglomerates (Fir- retrieved on application to the Data Bank Manager of the man 1978), whereas K-Ar studies of clays suggest that British GeologicalSurvey. Wherever possible, thesamples hydrothermal activity continuedintermittently until wereobtained from quarries,underground mines and new Jurassic times (Ineson & Mitchell 1974). roadcuttings. However, additionalsamples were collected from natural outcrop to ensure adequateregional cover. Care Recently,a link has been proposedbetween the was taken to ensure thatall materials collected lacked visible orefield generally and the buried Lake District indications of surfaceweathering. Rocks exhibiting various Batholith which has been inferred to have high heat types of hydrothermal alteration, such assericitization, production and hence to have beencapable of hematitization and greisenization,were also collected in maintaining hydrothermal convective cells in periods order to assess associated chemical effects. of increased tectonism and increased mantle heat flow Sampleswere jaw crushed and ground in agate prior to (Simpson et al. 1979; Brown et al. 1980; Moore 1982). analysis. Approximately 30 elementswere determined by Theextant geothermal field Qf Wairakei in New X-ray fluorescence spectrometry using the Philips PW1600 Zealand is considered to provide anappropriate instrument at Leicester University. Major elementswere analysed using fused disc techniques (Saunders et al. 1982). modelfor theLake Districtmineralization. The Trace element analysis was carried out by Rhand W complex distribution of veins in the Lake District is excitation of powder briquettes using the method of Marsh et interpretedto have beenproducta of several al. (1982) with tube lineinterference corrected using the generations of single pass hydrothermal cells with procedure of Bougault et al. (1977) and corrected for mass varying metal sources. Moreover, erosion of the Lake absorption using the method of Nesbitt et al. (1976). Analysis District is thought to have exposed structural levels in by optical emission spectrometry was undertaken using the the fossil hydrothermal systems several kilometres procedures outlined in the Geochemical Atlas of the Hebrides below those visible in active geothermal fields. (British Geological Survey 1984). Uranium was determined The most important mineralization associated with by thedelayed neutron method (Bowie et al. 1971) at the HERALD reactor centre, AWRE, Aldermaston. Rare earth granites is the W mineralization at Carrock Mine, near elements were determined by instrumental neutron activation tothe Skiddaw granite(Shepherd et al. 1976). The by ICIon contractthrough theBGS and by inductively Shap granite is associated with Cu and MO mineraliza- coupled plasma spectrometry(ICP) at King's College, tion although this has always beenuneconomic, London according to the method described by Walsh et al. whereas (later)Fe (hematite) mineralization which (1981).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1139/4888936/gsjgs.142.6.1139.pdf by guest on 30 September 2021 1144 C. O'Brien et al. Effects of metasomatism and hydrothermal 1ooO- alteration on the major and trace element Q composition of the intrusions 0 a Before discussing the magmatic geochemistry of the 800 - Lake Districtgranites, the effect andextent of the metasomatism onthe primary majorand traceelement geochemistrymust be considered. In general, - the elements most likely to have been redistributed in - 600 aqueous systems are those of low ionic potential (<3) k such as CS, Rb, K, Na, Li, Ba, Sr, Ca, and Mg, which - D m behave as hydrated cations, as well as those of high 400- ionic potential (HIP) (>12) such as B, P, and S, which behave assoluble complex oxyanions. Theformer l group includes some important large ionic lithophile (LIL)elements which are commonly used as pet- rogeneticindicators. Theelements of intermediate ionic potential such as Y and the REE, Zr, Ti, Hf, Ta, Nb,Th, and U, are generally held inrefractory accessory minerals and are normally the most resistant 300 400 500 to alteration. However, where there is a high activity Rb (ppm) of acidic ligands such as F- and Cl- (usually during 0 Eskdolegranlte m Eskdalegranodlorlte initial high-temperature water-rock interaction), redis- Eskdole greisen + Sklddawgronlte (A) tribution of elements such as Sn, W, Be, Ta, U and 0 Sklddow granlte (8) 0 Sklddawgrelsen Q Shop Ennerdale REE may occur (Eugster 1985). Uranium is subse- 0 Threlkeld A granophyreCarrockFell quently readily remobilized from easily leachable sites A Corrock Fellgabbra A CarrockFell hybrld suite A Carrock Fell leached as a result of the number and stability of its complexes granophyre over a wide range of Eh,pH andtemperature FIG. 3. Rb v. Ba variationdiagram for the Lake conditions (Langmuir 1978). District intrusions. The effects of extensivehydrothermal activity on the geochemistry of the Lake District granites can be examined atthe simplest level by considering the relationship between pairs of elements which, on the @I, basis of their similar chemistry in silicate systems, would normally show relatively simpleigneous fractionation trends, for example, plots of Rb v. Sr, Rb v. Ba, K v. Rb, and U v. Th all exhibit simple igneous trendsfor Caledonian granitesin northern Scotland (Pankhurst 1979; Plant et al. 1980) but such diagrams show a wide range of values forthe Lake District 0 province generally (Fig. 3), and for the Eskdale and A Skiddawintrusions inparticular, although other individual intrusions display a closer grouping. On the other hand plots of HFS elementssuch as Ti02v. Y or V should display igneous fractionation trends despite intense alteration;for example, the variably altered Eskdale, Skiddaw and Shap granites fall on a simple igneous trendon aplot of Ti02 v. V (Fig. 4). Enhanced B/Ga and B/Si02ratios have been shown to be an indicator of B metasomatism, and can be linked 0 0.2 0.4 0.6 0.8 1.0 to shale-derived fluids interacting with granites at high TlO2 temperatures (Simpson et al. 1979; Lister 1979). Most 0 Eskdolegranlte Eskdole gronodlorlte P Eskdole grelsen + Sktddawgranite (A) of the Lake District granites have elevated levels of B 0 Sklddawgronlte (B) 0 Sklddow grelsen 0 Shap Ennerdole compared with their Scottish equivalents,and B/Ga 0 Threlkeld A CorrockFell granophyre (Fig. 5 and Table 3) and B/Si02 ratios show a large A CarrockFell gabbro A CarrockFell hybrid suite A Carrock Fell leached range. The Ennerdale and Carrock Fell granophyres gronophyre are an exception, but these are the least metasoma- FIG. 4. TiOz v. V variationdiagram for the Lake tized on the basis of other isotopic, geochemical and District intrusions.

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Geochemical effects associated with greisenization and high-temperature alkaline metasomatism Additional information on the more extreme effects of high-temperaturealteration can beobtained by I," comparing the traceelement compositions of the l: 0 Skiddaw andEskdale greisens with those of the respective less-altered granites. This can also be done for the Carrock granophyre using samples adjacent to mineralized veins. Figure 4 demonstrates that with respect to the immobile elements Ti and V, the greisens deviate only slightly from the main fractionation trend of the least alteredsamples. Mineralogical studiesindicate that 00 most of the TiOz is retained in secondary rutile in the l: greisens. Hence, aplot of amore mobile element against TiOz can, toa first approximation, indicate the magnitude of the loss or gain of LIL elements. In Figs 6 and 7, for instance, plots of Sr and of Rb against B(ppm) TiOzdemonstrate that, particularly .for Skiddaw, 0 Eskdolegronlte Eskdolegronodlorlte significant Sr loss and Rb dispersion occurs during the P Eskdolegrelsen + Skiddawgronlte (A) 0 Sktddow gronlte (6) 0 Sklddowgreisen formation of greisens, a result of replacement of sodic 0 Shop Ennerdole plagioclase by abundant secondary muscovite. There is 0 Threlkeld A Carrock Fell gronophyre A CorrockFell gobbro A Corrock Fell hybrldsulte also some suggestion fromthese plots that the high A Corrock Fellleached RblSr ratios of the Eskdale and Skiddaw granites may granophyre have beenenhanced by hydrothermal activity, now FIG. 5. B v. Ga variationdiagram for the Lake reflected by pervasive sericitization in the granites. District intrusions. (Detection limit of B is 1 ppm). The behaviour of other elements during hydrothermal alteration can be illustrated by comparing mantle- mineralogical criteria. Shap contains the lowest aver- normalized trace element plots for average unaltered age B levels of the metasomatized and/or mineralized granite and average greisen compositions for the three granites of the Lake District and the significance of intrusions (Fig. 8). Sr andthe light REEare most this is discussed further below. affected and have decreased in abundance as a result

TABLE3: A summary of the geochemical effects of mineralization

LIL andHIP LIP HFS RbLi Ba Sr K B P LREENb V ThIU KIRbBalRb SrlRb BIGa BISiO, Skiddaw granite 4.6 146 0.2 1.2 0.9 0.4 Skiddaw greisen ++----++ = -- 2.0 129 0.4 0.13.2 12.1 Shap granite 0.7 1.4 2.8 163 2.9 0.2 Shap mineralized -- +-++= - --- 3.4 0.4137 3.7 1.247.3 Carrock 4.6 2130.4 4.7 0.0 0.0 granophyre Carrock ++--=+= - -- 4.4 154 0.2 3.2 1.8 0.6 microgreisen Eskdale granite 173 3.4 1.2 0.3 0.8 0.2 Eskdale greisen +---=+-- - -- 1.8 99 0.2 0.02 1.8 0.6 Threlkeld 2.2 187 2.6 0.7 1.6 0.4 Ennerdale 3.7 249 2.9 0.3 0.01 0.0

LIP = low ionic potential; HIP = high ionic potential. +, Element increased in greisens or mineralized granites. -, Element depleted in greisens or mineralized granites. =, Element not noticeably affected by water-rock interaction.

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3oo. Eskdole Sklddow of hydrothermal activity. The effects onthe abund- B ances of many other elements appear to be relatively

B + i small, andthe distinctive geochemical signatures of - 2oc- +- BB +e the three intrusions on such diagrams are not changed B -g IL- significantly by greisenization, suggesting thatthe m observed geochemical features of these intrusions are 100 0 more a reflection of primary magmatic processes than of secondary hydrothermal activity. The Skiddaw greisens contain particularly high 0 0.1 0.2 04 0.5 06 0 I 0 2 0.3 0.4 0.5 0.3 0 0.6 concentrations of B (Fig. 5 and Table 3), but there is T102 TIO, no correlation of B content with any other element FIG. 6. TiOz v. Sr variationdiagrams for the which might be used as a magma fractionation index. Eskdaleintrusion and the Skiddaw intrusion. Moreover, muscovite, which is in this case the main Symbols as Fig. 3. B-bearing phase, appears in all cases to be secondary. The addition of B is thus considered to be dueto high-temperaturemetasomatism (cf. Simpson et al.

Eskdale Sktddaw 1979; Lister 1979), the probable source of B-enriched 5001 Q fluids being the Skiddaw slates. The Eskdale greisens I D@ and granite, which are chemically similar to Skiddaw but which are emplaced into Borrowdale volcanic rocks having much lower B contents than the Skiddaw slates,contain relatively little B. The Carrock Fell granophyre shows no evidence of pervasive metasomatism such as occurs in the Skiddaw and Eskdale intrusions, although incipient greisening occurs adja- 0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.2 0.3 04 0.5 0.6 cent to mineral veins-for example, adjacent to the TIOZ TIO, west vein at Brandy Gill, near Carrock Mine. Samples FIG. 7. TiOa v. Rbvariation diagrams for the of this microgreisen show many of the geochemical Eskdaleintrusion and the Skiddaw intrusion. features of the Skiddaw greisen with elevated B, Rb, Symbols as Fig. 3. Li and reduced Sr contents. Moreover, the light REE

BaRbUThKNbLaCeSrNdPZrTI Y BaRb U Th K NbLaCeSr Nd P ZrTI Y BaRb U Th K NbloCe SrNd P Zr Ti Y i -Average grani te -Averagegranite -Average granophyre __Average greisen -- Average greisen -- Average leached granophyre

FIG. 8. Mantlenormalized trace element foraverage granite compositions with respective average greisen compositions for (a) the Skiddaw intrusion, (b) the Eskdale intrusion, and (c) the Carrock Fell granophyre.

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appearto havesuffered depletion (Figs 8 and g), probably reflecting the breakdown of apatite in hydrothermal fluids in which the activity of F was high; indeed, fluorite is presentin many samples of the adjacent mineralized Skiddaw granite. The Shap graniteis unusual in having relatively high levels of lithophile elements K, Rb, U and Th (Table

A 4, Figs 3 and lO), suggesting it is magmatically moderately evolved; but in termsof the concentrations of elements such as Sr, Ba, Mg, Cr, Ni, Ti, V (Table 4, Figs 3 and 10) and poorly developed Eu anomalies (Fig. ll), it could be considered as one of the least evolved plutons. Shap, of course, is well known for the spectacular subsolidus growth of large K-feldspars in xenoliths and autoliths within the granite and in the adjacent country rocks, which implies considerable K

0 Eskdalegranlte Eskdalegranodiorite P Eskdolegrelsen + Sklddowgranite (A) 0 Sklddowgrantte (8) 0 Sklddawgreisen 0 Shap Ennerdole 0 Threlkeld A CarrockFell granophyre A CorrockFell gabbro A Carrock Fell hybrldsuite A Carrock Fell leached granophyre FIG.9. TiO, v. Ce variation diagram for the Lake District intrusions.

5001 0 0

4001

- 300 - k m - A

v) 200r A A AA

KZ0

0 Eskdalegranlte Eskdalegronodiorite P Eskdolegreisen + Skiddawgranite (A) La Ce Pr Nd PrnSmEu Gd Tb Dy Ho Er TmYb Lu 0 Sklddow granlte (8) 0 Skiddowgreisen 0 Shop Ennerdole 11. Chondrite normalized rare earth element pat- 0 Threlkeld A granophyreCorrockFell terns for the Lake District intrusions. Numbers in A CarrockFell gabbro A CorrockFell hybrld suite A CorrockFell leached parentheses on the right-hand side of the diagram granophyre give a reference point on thescale for the FIG. 10. K,O v. Sr variation diagram for the Lake individualplots which are separated by one log District intrusions. scale.

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Carrockgrano-Carrock leached Eskdalegrano-Eskdale granite Ennerdalegranophyre phyre hybridCarrock gabbroCarrock Threlkeld diorite(b)t (a)? n=18 an-l n=16 on-l n=6 m-l n=12 an-l n=3 m-l n=13 an-l n=9 m-l n=31 an-l

SiO, 74.3 1.2 51.5 2.1 65.9 5.2 75.4 2.0 76.1 1.8 72.3 1.7 67.9 2.1 76.3 1.3 TiO, 0.32 0.04 2.07 0.7 052 0.2 0.25 0.04 0.22 0.05 0.34 0.01 0.43 0.02 0.13 0.1 Ab03 13.7 0.3 18.2 1.4 13.8 0.9 12.9 0.3 15.0 2.8 15.0 0.6 16.5 0.8 13.1 0.8 Fe203 1.8 0.5 10.6 1.2 7.9 3.5 3.0 0.7 2.0 0.9 3.0 0.4 4.1 0.2 1.4 0.4 MnO 0.03 0.01 0.25 0.04 0.27 0.l 0.09 0.04 0.02 0.01 0.08 0.02 0.11 0.02 0.03 0.01 MgO 0.4 0.1 4.7 2.6 0.3 0.2 0.2 0.1 0.2 0.1 1.1 0.3 0.9 0.2 0.2 0.1 CaO 0.6 0.2 8.3 1.9 2.6 1.3 0.9 0.5 0.1 0.03 1.2 0.5 2.2 0.4 0.5 0.1 Na,O 4.2 0.7 2.9 1.1 4.6 0.2 4.5 0.2 4.2 1.1 4.0 0.9 3.2 0.3 3.2 0.5 K20 5.2 1.1 1.2 0.4 4.1 0.6 3.6 0.2 2.9 0.8 2.9 0.4 4.7 0.3 4.9 0.5 p205 0.05 0.02 0.14 0.07 0.12 0.07 0.03 0.004 0.02 0.01 0.18 0.01 0.20 0.01 0.17 0.3 L01 0.55 0.1 1.15 0.4 0.54 0.2 0.71 0.4 1.71 0.0 2.14 0.6 1.13 0.1 0.62 0.1 Total 101.2 101.0 100.7 101.6 101.6 102.2 101.4 100.6

Nb 15 2 7.7 1 20 6 26 8 26 l 11 0 14 1 12 2 Zr 279 46 121 25 412 186 424 48 375 68 136 3 199 7 79 29 Y 36 8 27 5 85 10 1l7 52 98 5 l7 1 45 6 21 6 Sr 55 16 223 27 156 71 62 20 30 4 86 32 208 35 26 14 U 5.1 1 2.1 0.4 3.5 1 4.0 0.2 3.9 0.5 3.5 0.7 3.4 0.8 3.8 2.2 Rb 167 41 44 14 99 23 129 16 161 32 132 16 170 13 290 56 Th 20 3 3 2 11 4 14 1 18 4 7 1 15 3 9 4 Pb 9 5 12 3 19 4 27 5 30 13 14 l0 30 6 16 5 Ga 17 1 23 3 28 3 24 l 25 3 17 0 21 1 21 2 Zn 13 7 84 13 162 36 82 27 91 60 39 13 42 l1 20 9 Ni 2 1 27 35 2 l 3 l 3 1 14 1 5 l 2 1 Cr 5 2 82 102 3 1 4 1 4 5 24 2 18 2 6 3 V 13 2 350 111 3 1 2 2 2 2 42 1 41 4 8 6 La 38 12 18 4 42 7 93 90 25 20 23 1 40 7 12 8 Ce 84 19 35 9 95 16 128 41 48 39 47 3 85 11 31 13 Nd 32 8 20 4 53 8 91 69 34 26 20 1 37 5 15 6 Ba 439 108 230 62 524 86 591 52 470 95 329 109 633 49 143 99 Li 6 l 34 12 20 10 25 3 46 39 51 16 71 45 42 50 Be 2.6 0.6 1.1 0.2 2.3 0.3 2.4 0.6 2.9 2 1.2 0.2 2.4 0.6 2.5 1 B

* From Fitton ef al. (1982). n = number of analyses. t See Table 2.

and Rb mobility. The large feldspars within the granite has fundamentally enhanced the LIL element levels in frequently contain anhedral inclusions of other miner- the rocks is difficult to assess. The relatively low levels als. Moreover, somesamples of theShap granite of B in the Shap granite suggests that the surrounding which have very high levels of Ba are pervasively sedimentswere not the source of the metasomatic mineralized, with the development of microveins and fluids, and a magmatic water origin for these fluids has secondary overgrowths of baryte. These features, plus been suggested on the basis of stable isotope studies the fairly extensive sulphide development in both the by S. Caunt (pers.comm.). A more appropriatemodel, graniteand aureole, indicate thatthe plutonhas whereby the Shap granite was the site of discharge of suffered metasomatism, but the extent to which this high temperature fluids fromdepth, is suggested to

Borrowdale volcanics Skiddaw granite Basaltic Eskdale greisen Shap (a)t (b)t Skiddaw greisen Skiddaw slates andesite. Rhyolite' n=4 on-l n=14 an-l n=9 an-l n=16 a-l n=8 an-l n=lO m-l

81.9 0.6 69.8 0.7 71 0.9 74.9 1.4 78.2 4.5 54.2 2.6 55.7 72.6 SiO, 0.13 0.01 0.56 0.04 0. 15 0.05 0.20 0.07 0.16 0.1 1.16 0.1 0.71 0.19 TiO,

13.1 0.4 14.9 0.4 15.0 0.4 13.6 0.3 13.5 0.9 24.5 2.5 19.9 14.9 A1203 0.9 0.1 2.3 0.4 2.6 0.3 1.2 0.5 1.1 0.6 9.2 2.0 6.7 2.0 Fe203 0.01 0.0 0.05 0.01 0.08 0.01 0.06 0.02 0.04 0.04 0.143 0.W3 0.16 0.03 MnO 0.2 0.1 1.2 0.3 0.9 0.1 0.4 0.2 0.3 0.3 2.5 0.1 3.4 0.6 MgO 0.3 0.0 1.8 0.5 1.3 0.2 0.5 0.2 0.5 0.6 0.4 0.4 3.7 0.4 CaO 0.2 0.0 3.3 0.3 3.4 0.1 3.3 0.4 0.1 0.3 0.4 0.1 2.1 0.7 Na,O 3.9 0.2 5.6 0.6 4.9 0.2 5.1 0.4 4.9 1.1 5.4 0.3 4.1 6.4 K20 0.18 0.03 0.24 0.02 0.16 0.03 0.10 0.03 0.22 0.2 0.25 0.1 0.20 0.21 p205 1.28 0. l 0.76 0.4 0.81 0. l 0.72 0.3 1.50 0.4 nd __ __ L01 102.1 100.5 101.0 100.1 100.5 99.2 96.7 98.0 TOT

11 0 13 l 16 1 16 2 16 2 18 2 18 16 Nb 69 3 169 l1 158 15 70 26 62 18 189 51 204 170 Zr 11 1 12 l 18 2 17 3 17 3 35 6 42 40 Y 5 2 408 49 187 23 46 26 13 7 42 31 257 91 Sr 3.6 0.4 9.2 1.9 3.4 0.7 9.8 2.9 9.2 4.8 2.2 0.7 3.7 __ U 323 13 408 27 249 l0 304 37 345 70 163 51 178 25 1 Rb 7 2 25 3 20 2 13 4 15 8 11 7 16 20 Th 4 1 41 7 33 5 64 79 23 17 5 10 27 6 Pb 25 2 23 1 20 1 19 1 22 4 24 10 __ __ Ga 10 l 26 4 45 6 135 194 48 89 54 54 __ __ Zn 1 0 14 2 9 1 4 2 2 1 26 34 12 5 Ni 6 2 32 4 24 3 10 5 9 2 69 87 41 1 Cr 9 l 46 544 7 16 8 18 8 102 75 _- __ V 3 1 52 5 46 6 19 8 14 4 37 15 39 41 La 12 1 95 9 91 12 42 15 42 22 85 35 86 65 Ce 8 0 36 4 34 4 18 5 14 2 34 9 _- __ Nd 79 3 801 145 579 69 170 90 125 59 586 267 728 1679 Ba 8 2 117 32 159 18 105 55 194.9 99 34 17 __ __ Li 2.2 0. l 7.4 1 3.8 l 9.2 5 3.3 1 2.0 l __ __ Be 46 8 16 9 11 5 36 16 306 137 36 8 _- __ B 1 1 6 1 9 5 5 3 3 3 26 16 __ __ CO <0.1 0 0.1 0 10.1 0 0.3 l 0.2 0 <0.1 0 __ __ Cd

explain the enrichments of incompatible elements such largely unaffected by metasomatism.Whereas the as Rb and Li. effects of metasomatism can be demonstrated,the In summary, the Skiddaw, Eskdale and Shap scale is more difficult to assess. The implication from intrusions appear to be the intrusions most affected by Fig. 8 is thatthe essential primary geochemical late-stage,high-temperature hydrothermal activity, features of the plutons are preserved. the Threlkeldbody has suffered from pervasive Magmatic geochemistry low-temperaturehydrothermal activity which caused some redistribution and depletion in Ba and Sr, but Attempts at modelling the petrogenetic processes the Carrock and Ennerdale plutons seem to have been involved in the evolution of the Lake District granites

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are constrained by two important factors. The first is the uncertainty over the magnitude of the metasomatic A effects on LILelement abundances, emphasisthus I needing to be placed on the more immobile high field strength (HFS) elements and the REE. The second is the limited range in composition observed in severalof A the plutons, which makes identification of the mineral A A phases controlling the fractionating processes difficult, i.e. only the end product is observed. ""1 A Onan AFM diagram (Fig. 12) theLake District m intrusions follow a typical calc-alkaline trend, with the m exception of theCarrock granophyre and related ferrogabbrosand dolerites, all of which follow a tholeiitic trend(Hunter 1980). TheCarrock granophyre also differs from the other LakeDistrict granites in its trace element characteristics. HFS elements such as Zr and Nb, the heavy REE and Y,as well as many LIL elements(e.g. Ba, Fig. 3) were evidently behaving incompatibly in the magma and reach high I concentration levels in the more silicic granophyres. 0 0.2 0.4 0.6 0.8 1.0 Total REE levels are high (Table 5), but the TIOZ chondrite-normalized patterns are relatively flat (Fig. 0 Eskdale granite Eskdale granodlorlte 11). The marked negative Eu anomaly is consistent U Eskdale greisen 4 Skiddow gronlte (A) with extensive plagioclase fractionation, as is the 0 Skiddaw granlte (B1 4 Sklddow grelsen 0 Shop Ennerdale progressivedecline in Srcontent (Table 4). The 0 Threlkeld A CarrockFell gronophyre A CarrockFell gabbro A CarrockFell hybrld sulte marked increase in heavy REE (Fig. 13) would imply A Carrock Fell leached that the other crystallizing phases were minerals with qranophyre low partition coefficients for these elements, such as FIG. 13. TiOz v. Y variation diagram for the Lake pyroxene (and/or olivine), butnot hornblende; the District intrusions. reductionin TiOzand P205 indicatesilmenite and

apatite crystallization. All these phases are common in F the associated ferrogabbros and hybrid suite. The Carrockgranophyre has the highest Ba/Rb ratios of all the Lake District granitoids. Zr/Nb ratios are also high, discounting any alkaline affinity. The closest geochemical analogues of these rock types are thosein ensialic marginal basin environments along the Andean Cordillera, such as Sarmiento (Saunders et al. 1979), Bransfield Strait (Weaver et al. 1979) and Aysen(Bartholomew & Tarney 1984), all of which comprise basic-acid rock associations and where the A silicic rocks have high levels of Zr, Y and the heavy 0 REE, relatively flat REE patterns and high Ba/Rb and Zr/Nb ratios, much as in the Carrock granophyre. The Sarmiento rocks were emplaced during an extensional phase which preceded the emplacement of the volumi- A M nous Mesozoic calc-alkaline Patagonian batholith. The Carrock complex may haveformed in a similar extensional tectonic environment. It is tempting also O Eskdalegranlte Eskdale gronodlorlte 0 Eskdale qreisen 4 Sklddow granite (AI to relate the Eycott volcanics to such an extensional 0 Skiddaw granlie (81 4 Skfddawgrelsen 0 Shop Ennerdole phase: the extensiveCasma volcanics in Peru which 0 Threlkeld A CorrockFell granophyre precede the development of the CoastalBatholith A Corrock Fell qabbro A CarrockFell hybrld sulte A CarrockFell leached were extruded in an extensional environment (Ather- granophyre ton et al. 1983) and have broadly similar characteris- FIG. 12. AFM variationdiagram forthe Lake tics. D istrict intrusions.District If thegranophyres Carrock have developedtheir

SH33 SD37 SD44SD37 SH33 TK27EN26 ED23 ED16 ED2 SD32 CF.5

La 36.8 24.2 30.6 22.8 36.08 12.52 13.54 2.93 24.0 34.5 Ce 62.7 48.8 62.1 47.7 74.98 22.06 37.63 7.59 44.0 120.0 Pr 6.1 5.2 6.54 n.d. 8.1 3.2 3.12 0.89 n.d. n.d. Nd 23.8 20.8 27.07 18.9 36.53 14.2813.13 3.9 27.2 92.0 Sm 4.2 3.2 4.01 4.03 6.63 2.372.95 0.9 n.d. n.d. Eu 1.02 0.7 0.83 0.45 1.35 0.310.34 0.05 0.78 2.96 Gd 2.96 2.8 3.24 n.d. 6.73 3.162.37 1.02 n.d. n.d. Tb n.d. n.d. n.d. 0.49 n.d. n.d. n.d. n.d. n.d. n.d. DY 1.62 1.90 2.38 n.d. 5.73 2.482.41 1.00 4.50 24.0 Ho 0.29 0.33 0.42 0.35 1.09 0.30 0.14 0.48 n.d. n.d. Er 0.71 0.99 1.23 n.d. 3.01 0.69 0.36 1S0 n.d. n.d. Yb 0.79 0.91 1.11 1.70 2.75 0.43 0.28 1.88 1.59 10.5 Lu 0.07 0.14 0.16 0.26 0.42 0.09 0.05 0.33 n.d. n.d. Localities of samples indicated on Fig. 11.

distinctive geochemical characteristics through frac- intrusion, but the difference in the shape of the heavy tional crystallization of mafic magmas, then the very REE patterns (Fig. 11) and the marked difference in different trace element patterns of the remaining Lake phosphorus content (Fig. 14) suggests thatapatite District granitoids cannot be explained without invok- fractionation may havebeen responsible for the ing totally different fractionating mineral assemblages 'dished' shape of the Ennerdale heavy REE pattern. or considering alternativesource characteristics, The sample suite analysed for the Shap granite did amongst which a crustal source is inherently possible. not include the minor more hornblendic facies. Shap Forcomparison, analyses of the Skiddaw Slateand REE patterns have the highest La/Yb ratios of all the Borrowdale Volcanic country rocks are included in Lake District granites (Fig. ll),which implies either a Table 4, and are plotted on Fig. 14.Genesis of the sourceenriched in light REEand/or with residual granitoidsthrough melting of the Skiddaw Slates is garnet or involvement of a fractionating phase such as unlikely for a variety of reasons. The slates havea hornblendecapable of retaining the heavy REE. marked negative P anomaly on mantle-normalized However,Eu anomalies (Fig. 11) and Sr, P, or Ti plots (Fig. 14), which is not seen in the compositions anomalies (Fig. 14) are minor compared with those in of any of theLake Districtgranites apart from theother Lake Districtgranites, and such simple Ennerdale and Carrock. B levels in the slates are high, patterns limit the amount of plagioclase, hornblende, whereas B concentrations in the granites are generally apatite or ore fractionation that might have taken low, except where there has been late-stage hydrother- place. High V contents (Fig. 4) also limit the extent of mal exchan e between granites and slates. Reported ore fractionation.Although simultaneous separation initial 87Sr/'Sr ratios forthe granites have a fairly of plagioclase and hornblende, perhaps under condi- limited range around 0.707, which is not perhaps as tions of high oxygen fugacity, may have inhibited the high as may beexpected for the melting of mature development of negative Sr and Eu anomalies, higher continental sediments (no data for the Skiddaw Slates oxygen fugacities would then have led to increased are available). In any case, it is unlikely that the range negative V and Ti anomalies, which are not observed. of geochemical types of granite could begenerated Hornblende separation would be expected to produce through melting of otherwise fairly uniform sediments. concave heavy REE patterns, which are not observed. The Borrowdale volcanics may potentially be a rather In geochemical terms Shap, surprisingly, appears to better source on geochemical grounds, but this would have suffered the least crystal fractionation of all the be geologically unrealistic. Lake District granites. In view of the limited evidence The geochemical data indicate two main types of for fractional crystallization, it would appear that the magmas (Fig. 4): Threlkeld and Ennerdale each show steep rare earth patterns must reflect source composi- a very restricted range of composition; Eskdale and tion or melting conditions. The REE data would be Skiddaw display clear magmatic trends; Shap shows consistent partial melting of a mafic source with some characteristics of both types. The chemical residual garnet. uniformity within the Threlkeld and Ennerdale intru- The least metasomatized samples from the Skiddaw sions, together with their finer grain size, suggests they and Eskdale intrusions show a clear magmatic trend approximate liquid compositions. The small negative on most bi-element variation diagrams (Figs 3,4,6,7, Eu anomalies (Fig. 11) indicates that these magmas 9, 10 and 13). These systematic chemical variations are may have undergone plagioclase fractionation before consistent either with in situ fractionation or with

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Eskdole id1

FIG. 15. LILelement variationdiagrams for the Skiddawintrusion (a-c) and for the Eskdale intrusion (d-f) showing modelled liquid and corres- :loo) ponding cumulate compositions; symbols as Fig. 3.

progressive emplacement of components of a deeper 100) level magma chamber which was undergoingfractiona- tion.Compositional variations were therefore modelled initially using an in situ cumulate/liquid fractional crystallization model similar tothat proposed by 100) Tindle & Pearce (1981) for the Loch Doon intrusion in the Southern Uplands. Both the Skiddaw and Eskdale plutons have compositionalbreaks between the less evolved (a) and more evolved (b) facies. Assuming the parental granitoid magma composition lies within this compositional gap, modelling of theLIL element

FIG.14. Mantle normalized trace element patterns for the Lake Districtgranites, together with representative analyses of some Borrowdale volcanics and SkiddawSlates. Numbers on the right- hand side of the diagram give a reference point on 1RbU ThK NbLaCeSrNdP ZrTl Y the scale for the individual plots which are separated by one log scale.

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In the Skiddaw intrusion, For the Skiddaw intrusion for example, the best-fit however, modelling suggests theREE are mainly result suggests an initial cumulate assemblage of a controlled by monazite andfeldspar, resulting in a minimum of 30% plagioclase, and a maximum of 26% relative depletion of the light REE but moderate biotite and a second assemblage of a minimum of 43% enrichmentin the heavy REE duringfractional plagioclase and8% potassium feldspar with a max- crystallization. Minor phases can thus play an impor- imum of 27% biotite (Fig. 15). A similar good tant role in shaping REE patterns in granite systems, agreement was found forthe Eskdaleintrusion, as emphasized by Watson & Harrison (1984). supporting the proposition thatthe Eskdalegranite andgranodiorite are genetically related(Rundle Discussion 1979). Although such in situ fractionation can be modelled Previously published information on theU-Pb sys- successfully for both Skiddaw and Eskdale, it is tematics of zircons (Pidgeon & Aftalion 1978) and on apparent from the mantle normalized patterns in Fig. the initial Sr isotoperatios of Lake District plutons 14 that the range of compositions within each intrusion (Table 1) give little support for a crustal origin for the have similar trace element characteristics which differ granite magmas, although Harmon & Halliday (1980) significantly fromthose of Shap,for instance. This have suggested that some are ‘S’-type granites on the implies that the immediate parental magmas of basis of high 601’ values. Eskdaleand of Skiddaw had undergoneearlier The Carrock complex has tholeiitic characteristics, fractionation, eitheratdepth, or during magma and may be temporally related to the Eycott volcanics generation, beforeemplacement at higher levels. In as suggested by Hunter (1980). It may have been both intrusions, negative Eu anomalies (Fig. 11) emplaced in a ‘back-arc’ extensional environment suggest plagioclase removal and the strong negative Ti during the earliest stages of magmatic evolution of the anomalies (Fig. 14) suggest sphene or ilmenitefrac- southern continental margin of Iapetus as were similar tionation. The depleted heavy rare earth pattern for complexes at theAndean margin (Bartholomew & the least geochemically evolved samples (that with the Tarney 1984). The reported younger Rb-Sr age of the smallest negative Eu anomaly) from Skiddaw (Fig. 11) Carrock granophyre (which has a large error) should is indicative of derivationfrom a mafic source with perhaps be reconsidered in relation to possible residual garnet, similar to that for the Shap granite. resetting associated with the emplacement of the The progressive enrichment of the heavy rare earths Skiddaw granite. The granophyre appearsto have with an increasing in size of the negative Eu anomaly close geochemical links with the Carrock ferrogabbros indicates that hornblende cannot have been part of the and hybrid suite but not with the gabbro. fractionating assemblage; a back projection of this The I-type nature of the Threlkeld microgranite is trend towards more primitive liquids would result in a supported by its geochemistry and low initial s7Sr/s6Sr steep rare earth pattern very similar to that observed ratios,and it has been suggested by Wadge et al. in theShap granite, which has noEu anomaly. (1974) that it may be comagmatic with the Borrowdale Interestingly,garnet is presentas a primary mineral volcanics. Allowing for the pervasive alteration of the phase in both the Eskdale granodiorite and granite, Threlkeldintrusion, thereare indeedsome broad but garnetcannot have been a significant earlier similarities in trace element composition (Fig. 14). fractionatingphase because there is no observed However, there are some significant differences with depletion of Y or the heavy REE in theEskdale respect to P, Y, Nb, and Zr between the microgranite granodiorite (Fig. 11). The large range of variation in and published data (Fitton et al. 1982) for Borrowdale theabundance of both heavy REE and light REE lavas with comparable silica contents, which suggests within theEskdale intrusion might suggest that that either the source or the fractionating phases may monazite andior garnet, both of which concentrate the have beendifferent. Similar comments can be made REEand are petrographically common, may have concerning theEnnerdale granophyre whichis geo- been important in silu crystallizing phases at Eskdale. chemically similar to Threlkeld and has low initial The larger negative Ti anomalies in the Eskdale s7Sr/s6Srratios. intrusion compared with Skiddaw suggest that ilmenite Whereas thereare compositional similarities be- may have been a more important early fractionating tween theEskdale, Skiddaw andShap granites, the phase. patterns for Skiddaw andEskdale shown in Figs 11 The observedvariation in behaviour of the REE and 14 are significantly more complex than those for during in situ fractional crystallization reflects the Shap,and imply morepre-emplacement fractional differing mineral assemblages involved. In the Eskdale crystallization of phases such as pla ioclase and intrusion theREE distributions are controlled not ilmenite. Although reported initial s7Sr/sH Sr ratios are

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On the basis of these observa- rothermal activity in the Lake District from Devonian tions it is unlikely that either granite is ‘S’-type. to Jurassic times has disturbed the abundance patterns A feature worth noting is that mantle and chondrite of the more mobile lithophile elements, and isotopic normalized patterns for Shap in Figs 11 and 14 are the systems dependent on these elements may also have least complex of all the granites (the negative Nb beenaffected. However,the patterns of the less anomaly is a feature of all subduction related magmas; mobile trace elementsand the broad primary geo- cf. Saunders et al. 1980). This begs the question of chemical signatures of eachgranite pluton arepre- why, when elements are arranged in order of incom- served. patibility formantle assemblages on such diagrams, A trendfrom early tholeiitic magmatism to in- the patterns are relatively simple. Is it purely coinci- creasingly evolved-calc-alkaline granites with time is dence, or does it imply that the essential geochemical recognized, culminating perhaps in the emplacement features of granites such as Shapare an inherited of the buried high heat production Lake District mantle characteristic? The more complex patterns of batholith which is responsible for the geothermal field theother graniteshave distinct negativeanomalies and the related episodic mineralization. superimposed, most of which can be attributed to the Some of the geochemical variations observed within fractionation effects of distinct mineral phases. individual intrusions are explicable interms of frac- .TheEskdale granite has a geochemical signature tional crystallization (either in situ or at deep crustal similar to the Skiddaw granite (Fig. 14) buta more levels), hence marked negative anomalies for Eu, Sr, extended fractional crystallization history is indicated P, Ti,and V which reflect the removal of phases by the greater magnitude of the negative Eu, Sr andTi containing these elements, are apparent in the normal- anomalies. Thereare, however,clear differences ized patterns for the Carrock, Eskdale and Skiddaw between REE patterns of the two intrusions (Fig. 11) intrusions (Fig. 14). The different Lake District in thatEskdale is characterized by aflatter REE intrusions appear to have undergone varying degrees pattern in relation tothe Skiddaw granite, and the of fractional crystallization, with Shap providing little latter shows rather more heavy REE depletion. The directevidence of anextended history of fractional steep REE patterns characterizing theShap and crystallization. Skiddawintrusions cannotbe related to the flatter Changes in the geochemistry of plutons also reflects REE pattern of the Eskdale intrusion by variation in changes in source composition and melting conditions. realistic early cumulus phases. This would imply that It is possible for example to recognize a distinct theEskdale sourcehad differenta chemistry or temporal variation with respect to the REE. Theearly mineralogy. TheREE patterns of theShap and tholeiitic to calc-alkaline granitoids have flat to Skiddaw granites would be compatible with derivation moderately light REE-enriched patterns, whereas the of these magmas from a mafic source with residual younger Lower Devonian granites have more fraction- garnet, whereas those of the Eskdale intrusion, along ated REE patterns with heavy REE depletion.This with those of Threlkeld,Ennerdale and Carrock change cannot be modelled by fractional crystalliza- would be consistent with derivationfrom a source tion froma single magma type using any realistic without residual garnet. mineralassemblage. Thetrend toward increasingly Finally, the changetowards increasingly fraction- fractionated REE patterns with time may be depen- ated REE patterns with time is also accompanied by dent on the thermal conditions of magma generation, an increase in the level of radioactive elements U, Th, and would be consistent with garnet being present as a Rb,and K so thatthe late granites(Shap and residual phaseduring generation of thelater Lake Skiddaw) have greater heat production capability than District magmas. theearlier intrusions. The pervasive hydrothermal None of theLake District granitesappears to be activity, and the close spatial association of mineral- S-type, although some such as Eskdale display some ized veins with the subsurface geometry of the buried isotopic features of S-type granites; this is most easily Lake District Batholith,together with the unusually accounted for by later hydrothermal effects. high regional heat production of theLake District The parental magmas of the granites must have had suggest that the buried Lake District Batholith is also asubcrustal origin, relatedto the southerlydipping a high heat production granite, possibly similar in subduction zone during and following the closure of

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Iapetus; but how granitic magmas such as Shap, which the Lake District Batholith (Lee 1984) conflicts with appear to have suffered little fractional crystallization, age relationsand geochemical data:the moderately can be generated in a subduction or post-subduction fractionated REE patterns of the Eskdale granite and environment remains to be elucidated. granodiorite are consistent with it being emplaced at The nature of the buried Lake District Batholith is an intermediatestage in the magmatic sequence, as uncertain. A high heat productiongranite with suggested by its Rb-Sr age (Rundle 1979). similarities to Shap, Skiddaw or the buried Weardale pluton is required to drive the hydrothermal system and account for the subsequent mineralization, and for present day high heat flow (Wheildon et al. 1984). ACKNOWLEDGEMENTS.Theauthors thank K. Holmesand N. Equivalent high heat production granites were em- G. Marsh for the provision of analytical data and Dr I. Luff for making available computer data processing programs; C. placed in the Scottish Caledonides(e.g. Cairngorm) in O,Brien acknowledges the receipt of a NERCCASE the late stages Of magmatic evolution of the northern studentship. The paper is published by the permission of the plate also duringLower Devonian times. Director of the British Geological Survey, Sir Malcolm Geophysical evidence linking the Eskdalepluton to Brown, FRS.

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Received 11 October 1984, revised typescript accepted 8 June 1985.

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