Genetic significance of multiple enclave types in a peraluminous ignimbrite suite, Lachlan Fold Belt, Australia

Abstract

The Violet Town Volcanics (Lachlan Fold Belt, Australia) are an S-type ignimbrite suite containing microgranitoid enclaves, basaltic andesite enclaves and enclaves of high-silica rhyolite. The microgranitoid enclaves are similar to those in peraluminous granites. They typically have lower initial 87Sr/86Sr and higher εNd than the host, and represent globules of a mafic, mantle-derived magma, which was hybridized by mixing and diffusional exchange with the host magma. The basaltic andesite enclaves were incorporated into the ignimbrite as xenoliths, but their parental magma may have been similar to that of the microgranitoid enclaves. They are isotopically less depleted than other mantle-derived rocks from the Lachlan Fold Belt, reflecting contamination by crustal material, or derivation from less depleted mantle sources. The high-silica rhyolite enclaves, previously interpreted to be related to the ignimbrite by crystal fractionation, have εNd values up to 3 units higher than their host, and cannot be related to their host by crystal fractionation or assimilation-fractional crystallization (AFC) processes. The coexistence of S-type magmas and mantle-derived magmas suggests that the latter may have played a role in the Palaeozoic magmatism of the Lachlan Fold Belt, acting as a heat source for melting and perhaps also contributing chemical components to the crustally derived magmas.