Boyce, A.J.

Shail, RK

Stuart, F.M.

Wilkinson, J.J.

Applied Earth Science

Open Research Exeter

THE ROLE OF POST-VARISCAN EXTENSIONAL TECTONICS AND MANTLE MELTING IN THE GENERATION OF THE LOWER PERMIAN GRANITES AND  GIANT W-AS-SN-CU-ZN-PB OREFIELD OF SW ENGLAND  R.K. Shail1, F.M. Stuart2, J.J. Wilkinson3 and A.J. Boyce2  1Camborne School of Mines, University of Exeter, Redruth, Cornwall, TR15 3SE, UK (R.K.Shail@csm.ex.ac.uk) 2Scottish Universities Environmental Research Centre (SUERC), East Kilbride, Glasgow, G75 0QF, UK 3Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London, SW7 2AZ   Introduction:  The Cornubian batholith of SW England is the largest in the British Isles and one of the most intensively investigated in the world. Pro-duction from the associated giant W-As-Sn-Cu-Zn-Pb orefield ceased with the closure of South Crofty Mine in 1998. Total historical output is estimated at ~1.75 Mt Sn, 1 Mt Cu and 0.13 Mt Zn, and the prov-ince played a major role in the development of early magmatic-hydrothermal mineralization models and the science of mining geology. Many of the petrological, geochemical and geo-chronological characteristics of the batholith, and the distribution, parageneses and broad fluid characteris-tics of the associated hydrothermal mineralisation have been elucidated. However, published models do not satisfactorily explain: (1) the generation and em-placement of a very substantial volume of granite within a relatively external part of the European Var-iscides, and (2) the complementary factors that fa-voured the development of a giant orefield.  We present here the integrated results of tecton-ic/structural geological and helium/sulphur isotope studies. These data indicate that post-collisional lith-ospheric extension and mantle partial melting exerted a major control upon magmatism and associated mineralization. These processes are considered in the wider context of Permo-Carboniferous magmatism in Europe, and generic controls upon Sn-Cu mineraliza-tion.  Tectonics/structural geology:  The metasedi-mentary and metavolcanic rocks that host the batho-lith accumulated in marine sedimentary basins along the northern flank of an ~E-W trending Devonian rift zone in which oceanic lithosphere had locally devel-oped (Lizard ophiolite). The passive margin was subsequently overridden during Variscan conver-gence (Carboniferous) and the sedimentary basins were progressively incorporated into a northwards propagating thrust stack above a mid-crustal détach-ment.  Mesoscopic thrust-related deformation is repre-sented by two generations of folds and cleavage that are generally compatible with a top sense of shear to the NNW (e.g. Shackleton et al. 1982). The crust underwent only moderate thickening and the associ-ated regional metamorphism does not usually exceed epizone (Warr et al. 1991). A third generation of folds and cleavage has pre-viously been interpreted to have formed during forceful diapiric emplacement of the Cornubian batholith (e.g. Rattey and Sanderson 1984). Our data do not support this hypothesis and instead indicate that these structures formed during regional souther-ly-directed shear. We attribute this episode of defor-mation to the extensional reactivation of major thrust faults and crustal thinning prior to granite emplace-ment (Shail and Alexander, 1997).   Granite magmatism:  The granites of the Cor-nubian batholith were generated and emplaced dur-ing a 25 Ma period between c. 295-270 Ma (Chen et al. 1993). Gravity modelling indicates the batholith has dimensions of ~200×50×13 km and a volume of 68,000 km3 (Willis-Richards and Jackson 1989). Coarse-grained two mica peraluminous granite pre-dominates and has initial 87Sr/86Sr ratios of 0.710-0.716 and εNd values of -4.7 to –7.1 (Darbyshire and Shepherd 1994). Although a dominant lower crustal source is implied, a mantle component has tentatively been inferred on the basis of wholerock geochemis-try, Sm-Nd isotope systematics, enclave studies and a close spatial and temporal association with lampro-phyres and basalts (e.g. Leat et al. 1987, Darbyshire and Shepherd 1994, Stimac et al. 1995). Magmatic fabrics developed within the granites are commonly compatible with NW-SE extension during emplace-ment (Mintsa Mi Nguema et al. 2002).  Mineralization:  There is a clear spatial associa-tion between the granite batholith and W-As-Sn-Cu-Zn mineralization. Principal styles include pegma-tites (W, As, Sn), greisen-bordered sheeted veins (W, As, Sn), tourmaline-quartz breccia veins (Sn) and chlorite-tourmaline-sulphide veins (Sn, Cu, As, Zn, Pb). The latter two styles accounted for the majority of production and, in most areas, are hosted by steep-ly dipping extensional faults and/or tensile fractures that formed in response to regional NW-SE to N-S extension (Moore 1975, Shail and Alexander 1997). The fracture network allowed variable mixing be-tween magmatic-hydrothermal, meteoric and meta- morphic fluids (Alderton and Harmon 1991, Wil-kinson et al. 1995)  Helium and sulphur isotope study:  In order to test whether a mantle-derived component can be rec-ognized, we have analysed: (1) He isotopes in vola-tiles hosted by fluid inclusions in wolframite, arseno-pyrite and fluorite separates from magmatic-hydrothermal veins, and (2) S isotopes in the arseno-pyrite from the same veins. The specimens are from the Land’s End, Carnmenellis, Cligga Head and Hingston Down granites.  Application of He isotopes.  Where fluid inclu-sions are trapped by dense U- and Th-poor minerals, the outward diffusion rate of He is low and isotope ratios are essentially unmodified from the time of trapping (e.g. Stuart et al. 1995). Subcontinental lith-ospheric mantle is characterized by 3He/4He of ~7 R/Ra (atmospheric ratio), whereas crustal radiogenic He is characterized by 3He/4He of ~0.05 R/Ra. As a consequence, 3He/4He is a sensitive indicator of man-tle-derived volatiles in crustal fluids. Results.  A mantle-derived component is recog-nized within the majority of specimens and from all the granites, including the oldest and youngest within the batholith. Analyses of sensibly coeval minerals from a paragentically early assemblage (CRXC6) indicate He retention in arsenopyrite>> wolframite> fluorite. The sulphur isotope data are consistent with a dominantly magmatic source. Collectively, the data provide the first unequivocal evidence for a mantle source of volatiles in the magmatic-hydrothermal fluids associated with the Permian granites of  SW England.    Discussion:  Variscan convergence in SW Eng-land brought about modest thrust-related thickening of a thermally young passive margin. Whilst this undoubtedly increased the transient geothermal gra-dient within the lower plate, Permian granite magma-tism and associated magmatic-hydrothermal mineral-ization was ultimately initiated, and subsequently controlled, by c.30 Ma of post-collisional lithospher-ic extension. Mantle melts underplated and/or inject-ed during this episode provided the additional heat necessary to generate a large volume of granite from the lower part of a relatively thin crust.   Comparison with European Variscides.  HT-LP metamorphism and widespread granite magmatism across the Variscides of mainland Europe have been attributed to orogenic collapse (Ménard and Molnar 1988). In SW England, it is more likely that latest Carboniferous changes in plate boundary stresses (e.g. Henk 1999) permitted differential lithospheric extension west of the Bray Fault, although mantle melt generation may have been aided by anomalous temperatures established following the creation of Pangea (e.g. Doblas et al. 1998).  Generic controls.  Post-collisional lithospheric extension provides a favourable regime for the gen-eration of enhanced volumes of granite and so poten-facilitates greater accumulation of Sn during differ-entiation. Our work is compatible with earlier sug-gestions that the transfer of mantle-derived volatiles (+/-melts) into the magmatic system may be a pre-requisite for the Cu and S enrichment of the sul-phide-cassiterite association displayed by many giant tin deposits (Clark et al. 1993). Continued litho-spheric extension during and after granite emplace-ment is critical for the creation of steeply dipping upper crustal fault systems in which magmatic-hydrothermal and other fluids can be efficiently transported and mixed.   References:  Alderton D.H.M. and Harmon R.S. (1991) Min. Mag., 55, 605-611. Clark A.H. et al. (1993) Proc. Ussh. Soc., 8, 112-116. Chen Y. et al. (1993) J. Geol. Soc. London, 150, 1183-1191. Dar-byshire D.P.F. & Shepherd T.J. (1994) J. Geol. Soc. London, 151, 795-802. Doblas et al. (1998) J. Afri-can Earth Sci., 26, 89-99. Henk (1999) Tectonics, 18, 774-792. Jackson et al. (1989) Econ. Geol., 84, 1101-1133. Leat et al. (1987) Trans. R. Soc. Edin-burgh: Earth Sci., 77, 349-360. Ménard and Molnar (1988) Nature, 334, 235-237. Mintsa Mi Nguema T. et al. (2002) Phys. and Chem. of the Earth, 27, 1281-1287. Rattey P.R. and Sanderson D.J. (1984) J. Geol. Soc. London, 141, 87-95. Shackleton et al. (1982) J. Geol. Soc. London, 139, 533-541. Shail R.K. and Alexander A.C. (1997). Jour. Geol.  Soc. London, 154, 163-168. Stimac et al. (1995) Min. Mag., 59, 273-296. Stuart et al. (1995) Geochim. Cosmochim. Acta, 59, 4663-4673. Warr et al. (1991) J. Met. Geol., 9, 751-764. Wilkinson J.J. et al. (1995) Chem. Geol., 123, 239-254. Willis-Richards J. and Jackson N.J. (1989) Econ. Geol., 8484, 1078-1100. Table 1. Helium and sulphur isotope data Name Mineral R/Ra mantle He % δ 34S ‰ C95-1  wolframite  0.36 4.4  C95-2 wolframite  0.16 1.6  C95-3  arsenopyrite  0.75 10.0 +3.1 C95-4  arsenopyrite  2.22 31.2 +3.2 C95-5  wolframite  0.05 0.0  CRXC6 wolframite  0.23 2.5  CRXC6 arsenopyrite  1.01 13.9 +4.7 CRXC6 fluorite  0.06 0.2  RC1 wolframite  0.15 1.5  RC1  fluorite  0.11 0.8  RC1 arsenopyrite  0.99 13.5 +1.5 D54  fluorite  0.68 9.0  HD1  arsenopyrite  0.73 9.8 +1.4  

The role of post-Variscan extensional tectonics and mantle melting in the generation of the Lower Permian granites and the giant W-As-Sn-Cu-Zn-Pb orefield of SW England

The role of post-Variscan extensional tectonics and mantle melting in the generation of the Lower Permian granites and the giant W-As-Sn-Cu-Zn-Pb orefield of SW England

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