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.

关于S型花岗岩中的微花岗岩包体（"镁铁质夹杂物``）的起源的争论部分围绕着对微构造的解释。微观证据表明，包体是岩浆成因，而不是变质或部分熔融变质成因，后者得到花岗岩形成残留岩混溶假说的一些支持者的青睐。S型花岗岩中的微花岗岩包体对残留岩混溶假说没有关系，因为它们是主岩浆形成后并入岩浆室的混合岩浆球。微观构造和同位素证据表明在一般岩浆系统中,酸性S型地幔和来源于包体形成岩浆的地幔之间的混合。然而，对岩浆混合影响S型花岗岩的化学成分的程度仍然是有争议的。

KEY WORDS:

• Lachlan Fold Belt
• magma mixing
• microgranitoid enclave
• microstructure
• S-type granitoid
• restite

ACKNOWLEDGEMENTS

I thank Cal Barnes, Ian Nicholls, Scott Paterson and Simon Turner for helpful comments.