High Fe–Ti mafic magmatism and tectonic setting of the Paleoproterozoic Broken Hill Block, NSW, Australia

Abstract

We present petrographic, geochemical (major, traces and rare earth elements (REE)) and isotopic (Sm–Nd and Rb–Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Block of western NSW, Australia. The mafic rocks occur throughout the lower Willyama Supergroup stratigraphy and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600–1580 Ma). Our data indicate that the metabasites originated as a result of variable degrees of partial melting of a depleted mantle source, but slightly enriched with incompatible elements compared to present day N-MORB. This was followed by simple crystal fractionation or by an assimilation-fractional crystallisation (AFC) process involving only small degrees of crustal assimilation (rate of assimilation to rate of crystal fractionation, r = 0.05–0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt% of total iron as Fe 2O 3 and 4.2 wt% of TiO 2. Volumetrically minor intermediate rocks evolved from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Rasp Ridge and Hores/Potosi Gneisses) are not linked by fractional crystallisation to the mafic melt that produced the meta-igneous amphibolites and are products of the anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bimodal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Block (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Domain, in contrast, imply smaller degrees of partial melting, higher degrees of crustal contamination and the presence of a subcontinental lithospheric mantle source, which suggests relatively thicker lithosphere and places the Olary Domain on the rift margin. Active faulting during the rift stage, coupled with submarine sedimentation and an anomalous geothermal gradient driven by lithospheric thinning, provide an ideal theoretical environment for the formation of the Broken Hill Pb–Zn–Ag orebody.