A geochemical study of zoned inclusions in granitic rocks

Abstract

Six selected zones enclaves found within four granitic plutons in southern Victoria, Australia were analyzed to determine the nature and cause of the zonation. Enclaves identified as restitic, autolithic, and xenolithic were analyzed having a wide range of compositions including calcic and peraluminous compositions. Bulk chemical analyses of the various zones present and the granitic rocks at the margins of the enclaves as well as of the constituent minerals were done to test whether the zonations observed can be explained by melt + restite mixing or by diffusional processes. Both the mineralogical and chemical data suggest the formation and composition of a reaction rim resulting from diffusion controlled mass transfer across the inclusion-magma interface. The driving force necessary for this transfer is the chemical potential difference seen by the mineralogical and compositional differences between the core and the magma. -Authors