Microstructures of microgranitoid enclaves and the origin of S-type granitoids

Abstract

Arguments about the origin of microgranitoid enclaves (‘mafic inclusions’) in S-type granitoids partly revolve around the interpretation of microstructures. Microstructural evidence indicates that the enclaves are of igneous origin, rather than the metamorphic or partly melted metamorphic origin favoured by some proponents of the restite unmixing hypothesis for granitoid formation. Microgranitoid enclaves in S-type granitoids have no bearing on the restite unmixing hypothesis, because they are globules of hybrid magma incorporated into the magma chamber after the formation of the host magma. Microstructural and isotopic evidence indicates mixing between felsic S-type and mantle-derived enclave-forming magmas in the general magmatic system. However, the extent to which magma mixing contributes to the chemical composition of S-type granitoids remains controversial.