ORIGIN OF MICROGRANITOID ENCLAVES IN THE S-TYPE WILSONS PROMONTORY BATHOLITH, VICTORIA - EVIDENCE FOR MAGMA MINGLING

A feature of the S-type Wilson's Promontory Batholith (Lachlan Fold Be lt, Australia) is the presence of microgranitoid enclaves. These encla ves are fine-grained and have rounded or irregular shapes, with diamet ers varying from 1 to 150 cm. Most contain megacrysts (mainly quartz, plagioclase and K-feldspar), many of which show overgrowths of materia l formed at higher temperature. 'Double enclaves' (consisting of eithe r darker microgranitoid enclaves or hornfelsic xenoliths enclosed in a nother microgranitoid enclave) are common. The enclaves contain simila r 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 ho st overlap, but initial Sr and Nd isotopic ratios are different. The a vailable 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.