Geochemical investigations of microgranitoid enclaves in the S-type Cowra Granodiorite, Lachlan Fold Belt, SE Australia

The Cowra Granodiorite is a relatively mafic, enclave-rich, S-type pluton in the Lachlan Fold Belt, which has been cited as a type example of a restitic origin for all varieties of enclaves in Lachlan Fold Belt S-type granites. Microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage: pyroxene microtonalites and biotite microgranites. No geochemical or isotopic distinction can be made between the two varieties. Petrographic evidence (acicular apatites, xenocrysts from the host granite) suggests an origin as mingled, more mafic, magmas, which have been variably contaminated by the more felsic host magma. This is supported by the fact that the microgranitoid enclaves have isotopic compositions (87Sr/86Sr(i)=0.7095 to 0.7144, εNd(i)=−9.2 to −6.9) that are generally more primitive than, or similar to, those of the host granite (87Sr/86Sr(i)=0.7142, εNd(i)=−8.8). The spread in isotopic compositions, like their trace element compositions, is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling. Several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite. Instead, they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement.