PRIMARY LITHOLOGICAL VARIATION IN THE KAOLINIZED ST-AUSTELL GRANITE, CORNWALL, ENGLAND

Geological mapping of existing and redundant kaolin workings within th e St Austell Granite has identified a suite of granitic rocks which sh ow 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 gr anites and tourmaline granites. The tourmaline granites characteristic ally exhibit very variable textures, with coarse quartz grains set wit hin a fine grained, tourmaline-rich matrix. A highly evolved fine-grai ned tourmaline granite represents the most evolved of this suite. Topa z 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 a ny remaining granitic melt, resulting in the very variable textures sh own by the tourmaline granites. The topaz granites are chemically dist inct from their predecessors, showing marked enrichment in F, Li and P 2O5 (but not B). Instead of being products of differentiation of bioti te granite magma, the topaz granite melts may have been derived separa tely in a later episode of partial melting of the same source. Kaolini zation is widespread throughout the western part of the St Austell Gra nite, and deposits worked at present tend to be located in granite var ieties 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.