Evidence of isotopic equilibration between microgranitoid enclaves and host granodiorite, Warburton Granodiorite, Lachlan Fold Belt, Australia

Abstract

Microgranitoid enclaves in the S-type Warburton Granodiorite (Lachlan Fold Belt, Australia) display a range in mineralogical and isotopic compositions. Many show obvious similarities to the host granodiorite: they contain biotite as the sole ferromagnesian phase and have isotopic signatures similar to the host. Other enclaves contain cummingtonite ± actinolite and rare orthopyroxene, mainly in the core of the enclaves. These enclaves typically have higher ϵ Ndi and lower sol 87 Sr 86 Sr i than the host rocks. Reversely zoned plagioclase megacrysts in some enclaves contain anhedral cores which are similar in composition to An-poor plagioclase crystals in the host rock (An45); these megacrysts are jacketed by An-rich plagioclase (An70-An80). The enclaves are interpreted to be globules of a more mafic magma which mingled with the host while it was still partially liquid. Megacrysts within the enclaves are thought to have been derived from the granodioritic magma. The mafic “enclave magma” is likely to have been derived from a mantle source, but equilibration between enclaves and host rock has changed the mineralogy and (isotope) geochemistry of all enclaves. The equilibration process was incomplete for some enclaves, resulting in mineralogical zoning and distinctions in isotopic signature between these enclaves and the host rock. This equilibration process is likely to have been arrested as a result of rapid cooling of the pluton, due to its small size and shallow emplacement level. It is uncertain whether the compositions of the least equilibrated enclaves are truly representative of the original mafic “enclave magma” or whether their trace element signature has been affected by diffusional processes within a stratified magma chamber.