Primary lithological variation in the kaolinized St Austell Granite, Cornwall, England

Abstract

Geological mapping of existing and redundant kaolin workings within the St Austell Granite has identified a suite of granitic rocks which show evidence of complex late-stage magmatic and hydrothermal processes. Coarse porphyritic biotite granites, like those which predominate in southwest England, occur much more widely than previously acknowledged, and are intruded by an apparently cogenetic suite of lithium-mica granites and tourmaline granites. The tourmaline granites characteristically exhibit very variable textures, with coarse quartz grains set within a fine grained, tourmaline-rich matrix. A highly evolved fine-grained tourmaline granite represents the most evolved of this suite. Topaz granites intrude the earlier granite varieties, and all are intruded by rhyolite porphyry dykes (elvans). Major and trace element chemical data suggest that the biotite granite–lithium-mica granite–tourmaline granite suite represents the product of crystallization of a granitic magma within which B (but not F) became progressively enriched until water saturation was achieved. Water exsolution effectively quenched any remaining granitic melt, resulting in the very variable textures shown by the tourmaline granites. The topaz granites are chemically distinct from their predecessors, showing marked enrichment in F, Li and P 2 O 5 (but not B). Instead of being products of differentiation of biotite granite magma, the topaz granite melts may have been derived separately in a later episode of partial melting of the same source. Kaolinization is widespread throughout the western part of the St Austell Granite, and deposits worked at present tend to be located in granite varieties other than biotite granite. The geochemical parameters used to distinguish the primary granite types (particularly Nb v. Zr and Ga–Nb–Zr plots) are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material.

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

“…The major and trace element geochemistry of the Cornubian granites has been fairly well established and this study is broadly in agreement with previous studies (e.g. Charoy, 1986;Stone, 1992;Manning et al, 1996;Stone, 2000;Chappell and Hine, 2006;Müller et al, 2006a) (Supplementary Table 2). …”

“…Barium vs. Rb shows a curved trend from high Ba / low Rb in G1 and G3 granites through G2 and G4 to G5 granites which have abundant Rb (Figure 6g). Classification of biotite, tourmaline and topaz granites using a Zr vs. Nb plot (Manning et al, 1996) can be replicated in this study, showing that the G1, G2…”

“…The tourmaline granites can be further subdivided into three textural subtypes: globular quartz, equigranular and lithium-mica variants; each named for their distinctive textural characteristics (Manning et al, 1996). These classifications are adapted for this study, providing five granite types that have unique textural, mineralogical and geochemical characteristics (Table 1, Figure 1).…”

“…G3 granites are fine-to coarse-grained, with microperthitic alkali feldspar phenocrysts >25 mm, approximately 10% modal biotite, plagioclase and accessory tourmaline, cordierite, zircon, monazite, apatite and rutile ( Figure 2c). Tourmaline (G4) granites have distinct textural variants, described in detail by Manning et al (1996); globular quartz (Figure 2d), equigranular and a coarse-grained Li-mica porphyritic facies that is texturally similar to coarse-grained porphyritic two mica (G1a) granites. G4 granites are usually characterised by tourmaline as the dominant ferromagnesian mineral, with Li-rich biotite group micas and accessory topaz, apatite, columbite and Nb-rich rutile / ilmenite.…”

“…Accessory minerals are low in abundance, but diverse, comprising amblygonite, Nb-rich rutile, Mn-rich apatite, columbite-tantalite, cassiterite, zircon, fluorite and very rare wolframite. Topaz-rich aplites and pegmatites are associated with the Tregonning and St. Austell G5 granites (Stone, 1975(Stone, , 1992Manning et al, 1996).…”