High Fe-Ti mafic magmatism and tectonic setting of the Broken Hill Inlier, N.S.W. Australia

Abstract

We present petrographic, geochemical (major, traces and REE) and isotopic (Sm-Nd and Rb-Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Inlier 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 by variable degrees of partial melting of a depleted mantle source, only weakly more enriched in incompatible elements compared to present day N-type MORB. This was followed by simple crystal fractionation or by an AFC process involving only small degrees of crustal assimilation (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 Fe2O3 and 4.2 wt % of TiO2. Some intermediate rocks were derived from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Alma, Rasp Ridge and Potosi Gneisses) are not linked by fractional crystallization to the mafic melt that produced the meta-igneous amphibolites, and are products of anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bi-modal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Inlier (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 Inlier, in contrast, that imply smaller degrees of partial melting, relatively thicker lithosphere and more crustal contamination, are consistent with placing this Inlier on a 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.