Origin of microgranitoid enclaves in the s-type wilson’s promontory batholith, victoria: Evidence for magma mingling

A feature of the S-type Wilson’s Promontory Batholith (Lachlan Fold Belt, Australia) is the presence of microgranitoid enclaves. These enclaves are fine-grained and have rounded or irregular shapes, with diameters varying from 1 to 150 cm. Most contain megacrysts (mainly quartz, plagioclase and K-feldspar), many of which show overgrowths of material formed at higher temperature. ‘Double enclaves’ (consisting of either darker microgranitoid enclaves or homfelsic xenoliths enclosed in another microgranitoid enclave) are common. The enclaves contain similar minerals to the host rocks, but contain more biotite and less garnet. They have poikilitic or equigranular textures, and acicular apatite is common. The major and trace element geochemistry of enclaves and host overlap, but initial Sr and Nd isotopic ratios are different. The available data can most satisfactorily be explained by an origin of the enclaves involving mingling between a more mafic ‘enclave’ magma and a more felsic host magma. Origins for enclaves as fragments of restite or accumulations of early-formed minerals less satisfactorily fit the available evidence.