An isotopic mixing model for the origin of granitic rocks in southeastern Australia

Abstract

A simple mixing process was a significant element in the genesis of many southeastern Australian granitic rocks: major and trace element abundances are near-linearly correlated, Nd and Sr initial isotopic compositions define a simple hyperbolic trend, and many initial 87Sr/ 86Sr ratios are a regular function of Rb/Sr ratios. Geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members. The high-Si end member is meta-sedimentary-derived magma comparable in composition to the Cooma Granodiorite, the one pluton in the region surrounded by, and clearly derived from, regional metamorphic rocks. The low-Si end member is basaltic material with similarities to rare gabbros also found in the area. Isotope dilution calculations based upon initial Sr and Nd isotopic compositions deduced for the end members yield realistic hypothetical Sr and Nd concentrations for the end members in a demonstration of the internal consistency of the model. All of the granitic rocks, from hornblende tonalites to cordierite granodiorites, are a single broad family, products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement.