Zircon grain shape holds provenance information: A case study from southwestern Australia

Abstract

Detrital zircon geochronology is a powerful tool to address a range of geological questions related to sedimentary provenance. Nonetheless, non-unique answers may result when igneous rocks of the same age are sourced from disparate locations. In an effort to resolve some of this issue in zircon provenance investigations, we explore the potential of detrital zircon grains to be linked to their original magmatic source through their grain shape. In order to develop this grain shape measure as a provenance tool, we first examine the relationships between chemistry and a range of different grain shape descriptors. We find grain shape to show a linear relationship to U-concentration on a lithologic unit level. We interpret this relationship to be a function of crystal chemical control in igneous samples. Principal component analysis demonstrates the potential of simple grain shape descriptors (major axis, minor axis, and effective diameter) to characterize grains from particular rock units. Applying these same shape measures to sedimentary basins in the Proterozoic Albany-Fraser Orogen in southwest Australia allows us to closely replicate the findings of previous traditional U-Pb geochronological investigations in terms of grain provenance. In addition, we apply this technique to Mesozoic sediment on the southern margin of Australia and show for the first time that its detritus is more likely derived from the underlying crystalline basement rather than surrounding orogens that share similar magmatic ages.