The Entire anorthositic gneiss, eastern Arunta Inlier, Central Australia: Geochemistry and petrogenesis

Abstract

A cumulus igneous origin is proposed for anorthositic gneisses, anorthositic metagabbros and meta‐ultramafic rocks comprising the Entire anorthositic gneiss member of the Early Proterozoic Harts Range meta‐igneous complex (HRMC), eastern Arunta Inlier. The ultramafic and anorthositic gneisses possess distinctive whole‐rock chemistries which are the result of cumulus components with widely different proportions of variably evolved intercumulus liquid. A comagmatic relationship between these rocks and their host HRMC amphibolites is supported by a strong similarity between the observed trace element chemistry of the amphibolites and the trace element compositions of calculated equilibrium liquids from which the cumulates crystallized. In addition, some garnet‐bearing anorthositic metagabbros have absolute trace element abundances similar to the amphibolites. The parental magma for the ultramafic‐anorthositic rocks was a depleted aluminous tholeiitic melt selectively re‐enriched in large ion lithophile (LIL) elements and with lower Mg/(Mg + Fe2+) ratios and Ni contents than expected in primary melts, due to early olivine‐controlled fractionation. Extensive plagioclase+clinopyroxene‐controlled low‐pressure fractionation of the evolved melt abstracted bulk mineral assemblages comprising the anorthositic rocks and yielded the tholeiitic fractionation trend of the host basaltic amphibolites. Periodic removal of olivine+pyroxene‐dominated assemblages gave rise to the ultramafic cumulates and also contributed to the concomitant iron enrichment trend in residual liquids. The Entire anorthositic and ultramafic gneisses show mineralogical and geochemical features more closely analogous to Archaean stratiform anorthosite complexes than to Mid‐Proterozoic massif‐type anorthosite bodies. The suite is interpreted as the metamorphosed remnant of an extensive layered stratiform complex. Intrusion probably took place in a rifting continental margin environment where thin continental crust facilitated rapid ascent of the mantle‐derived tholeiitic magma to a high level, permitting fast but limited differentiation to yield the layered sequence.