Constraints on interpretations of lower crustal structure, tectonic setting and metallogeny of the Eastern Goldfields and Southern Cross Provinces provided by granite geochemistry

Abstract

New trace-element geochemical data on granites fromthe eastern Southern Cross Province and the western Eastern Goldfields Province of the Yilgarn Block provide additional constraints on tectonic and metallogenic syntheses: the new data also enables estimates of the nature and composition of the lower crust to be made. All granites were derived by partial melting of the lower crust and all appear to have had a significant crustal prehistory. Using multi-element, primordial-mantle-normalised trace-element abundance patterns, the granites can be divided into four broad groups, three of which are predominantly Sr-undepleted and Y-depleted, whilst the fourth group is always Sr-depleted and Y-undepleted. The first three groups are believed to have had garnet, but not plagioclase stable at some stage of their prehistory, whilst the fourth group had plagioclase but not garnet. Seismic data does not show any evidence of a significant garnet-bearing layer within the crust. As the present-day trace-element signatures of the granites were probably derived from earlier melting events, interpretations of tectonic setting existing at the time of their emplacement based solely on their geochemistry are questionable. The ultimate source of granites of Groups 1, 2 and 3 may have been derived by partial melting associated with subduction. Group 4 granites, which have never had garnet as a significant component in their source regions, closely resemble the rift-related Proterozoic anorogenic suites. The geochemistry of the granites also predicts the sharp crustmantle boundary shown by seismic data. Some plutons are enriched in Rb, U and Y and also have decreasing K Rb ratios with increasing SiO 2; similar trends are found in granites which have associated Sn, W and Au mineralisation in other Australian provinces. Alteration is common in some parts of plutons, and regional mapping of these altered granites could define regional fluid circulation systems, which may be related to mineralisation.