| dc.description.abstract | 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. | en_GB |
| dc.description.sponsorship | 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. | en_GB |
