ENCLAVES AND THEIR BEARING ON THE ORIGIN OF THE CORNUBIAN BATHOLITH, SOUTHWEST ENGLAND

Enclaves of diverse origin are present in minor amounts in the coarse- grained biotite granites of the Cornubian batholith, southwest England . The most common enclave type is layered, rich in biotite, cordierite and aluminosilicates, and has textures and compositions that reveal v ariable degrees of melt extraction from metasedimentary source rocks. Rare sillimanite-bearing enclaves represent residual material, either from the region of magma generation or its ascent path, but most such enclaves were probably derived from the contact aureole closer to the present level of exposure. These non-igneous enclaves (NIE) and their disaggregation products are present in all major plutons, comprising f rom < 2 to 5 vol.% of the granites. Enclaves of igneous origin are als o present in all major plutons except Carnmenellis, generally comprisi ng < 1 vol.% of the granites. The most common type is intermediate in composition, with microgranular texture, and mineral compositions and textures consistent with an origin by magma mixing. Large crystals of K-feldspar, plagioclase and quartz, common in these microgranular encl aves (ME) but absent in NIE, represent phenocrysts derived from the si licic end-member during magma mixing events rather than products of me tasomatism as suggested previously. Although the composition of the ma fic end-member (basaltic or lamprophyric) involved in the mixing proce ss is poorly constrained, the presence of ME in the granites, and the preponderance of mantle-derived mafic rocks in the coeval Exeter Volca nics, indicate that mafic magma injection into the crust was a factor in the generation of the batholith. Advection of sub-crustal heat prov ides an explanation for large-volume crustal melting in regions of rel atively thin crust such as southwest England.