The Petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith - lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England
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The Early Permian Cornubian Batholith was generated during an extensional regime following Variscan convergence within the Rhenohercynian Zone of SW England. Its component granites can be classified, using mineralogical, textural and geochemical criteria, into five main types, all of which are peraluminous (A/CNK >1.1): G1 (two-mica), G2 (muscovite), G3 (biotite), G4 (tourmaline) and G5 (topaz). G1 granites formed through up to 20% muscovite and minor biotite dehydration melting of a metagreywacke source at moderate temperatures and pressures (731-806°C, >5 kbar). Younger G3 granites formed through higher temperature, lower pressure (768-847°C, <4 kbar) biotite-dominated melting of a similar source. Partial melting was strongly influenced by the progressive lower-mid crustal emplacement of mafic igneous rocks during post-Variscan extension and a minor (<5- 10%) mantle-derived component in the granites is possible. Two distinct fractionation series, G1-G2 and G3-G4, are defined using whole rock geochemical and mineral chemical data. Variations in the major elements, Ba, Sr and Rb indicate that G1 and G3 granites underwent 15-30% fractionation of an assemblage dominated by plagioclase, alkali feldspar and biotite to form, more evolved G2 and G4 granites respectively. Decreasing whole rock abundances of Zr, Th and REE support fractionation of zircon, monazite, apatite and allanite. Subsolidus alteration in G2 and G4 granites is indicated by non-primary muscovite and tourmaline and modification of major and trace element trends for G3-G4 granites, particularly for P2O5 and Rb. Topaz (G5) granites show low Zr, REE and extreme enrichment in Rb (up to 1530 ppm) and Nb (79 ppm) that cannot be related in a straightforward manner to continued differentiation of the G1-G2 or G3-G4 series. Instead, they are considered to represent partial melting, mediated by granulite facies fluids, of a biotite-rich restite following extraction of G1 and/or G3 magmas; they do not exhibit the typical geochemical characteristics of intraplate A-type granites.
This study was supported by the European Regional Development Fund and European Social Fund as part of the convergence funding for Cornwall and the Isles of Scilly (Combined Universities in Cornwall project number11200NCO5), supporting a PhD for BS, and the European Union (Horizon 2020 project 641650 FAME). The Natural History Museum, London, UK are thanked for the loan of samples. We thank Richard Scrivener and Nicholas LeBoutillier for help with sampling. Steve Pendray, Sharon Uren and Joe Pickles assisted with the sample preparation and analysis. Axel Müller and Karel Breiter are thanked for discussions about topaz granites. We gratefully acknowledge Romain Tartèse and an anonymous reviewer for their constructive comments which helped to improve the manuscript.
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.