The magmatic evolution of the Land's End pluton, Cornwall, and associated pre-enrichment of metals

The peraluminous Land's End granite is one of the composite plutons forming the late-Variscan Cornubian batholith. The western part of the pluton hosts the Sn-Cu mineralisation of the St. Just mining district. The pluton consists of early megacrystic biotite (Mg-siderophyllite) granites and albite microgranites and younger Li-siderophyllite granites, and tourmaline granites together with fine-grained massive quartz-tourmaline rocks (MQT). Most of the granite varieties evolved by fractional crystallisation from a common crustal magmatic reservoir, apart from the albite microgranite, which has a different source. Trace element analyses of K-feldspar megacrysts in the older biotite granites made using laser ablation ICP-MS reveal that the megacrysts were derived from moderately evolved sources and now reside in separate granite sub-stages with variable degrees of fractionation, so that equilibrium between phenocrysts and host no longer persists. The younger granites show a strong in situ fractionation at the cm- to 100-m-scale, whereby boron-rich melts are concentrated in the apical parts of the individual intrusive units. Geological, textural and mineralogical data suggest that the tourmaline granites and MQT evolved mainly from fractionated Li-siderophyllite granites. Cathodoluminescence, trace element and melt inclusion studies of quartz from the MQT reveal that the MQT was formed during the transition from the magmatic to hydrothermal states from a mixture of immiscible phases. The Li-siderophyllite granites, tourmaline granites and MQT appear to be the immediate magmatic precursors from which the hydrothermal fluids responsible for the mineralisation in the St Just district originated. (c) 2005 Elsevier B.V. All rights reserved.