Geochronology of a rapid 1.85–1.86 Ga tectonic transition: Halls Creek orogen, northern Australia

Abstract

Early Proterozoic orogenic rocks of the Halls Creek orogen are characterized by widespread, ensialic orogeny and magmatism, and linear, post-tectonic igneous complexes several hundred kilometres long. Conventional UPb zircon techniques are applied to erect a precise chronological framework quantifying the tectonic evolution of this fold belt. Volcanism associated with early rifting (Ding Dong Downs Volcanics, lowermost Halls Creek Group) whose age has not yet been determined, is followed by a quartz-rich, arenaceous formation, and in turn by fine-grained, variably volcanogenic, phyllitic deposits with abundant carbonates. Stratabound, high-level ‘rhyolitic’ sills are emplaced into younger parts of this Biscay Formation (∼ middle Halls Creek Group). These have distinct incompatible trace element compositions, with large enrichments in Zr, Nb and Y, and are regarded as second stage A-type melts derived from a Rb-depleted granulite source. They are possibly coeval with tuffs in the same unit and, if so, the sill's UPb zircon age of 1856 ± 5 Ma closely dates this sequence, specifically the onset of turbidite deposition and commencement of orogenic activity in inferred provenance zones to the immediate west. However, confirmation of such an age interpretation depends on substantiation of this inferred coeval link between the dated sill and the Biscay Formation tuffs, and until such the 1856 ± 5 Ma result remains a minimum for the age of the Halls Creek Group. Subsequent compressional orogeny of the basinal trough sediments was accompanied by deformation, high-temperature polymetamorphism, and syn-tectonic plutonism. Anatectic pegmatite, believed to have formed as a melt product of granulite-facies metamorphism, has a UPb zircon age of 1854 ± 6 Ma. This age for high-grade metamorphism is in agreement with reassessed RbSr whole-rock ages measured previously. It closely controls the timing of the Barramundi orogeny as being somewhat younger in this subprovince than elsewhere in northern Australia, and may imply a relatively short interval, of a few to 13 million years, between supracrustal deposition and deep crustal orogenesis. A better constrained and even more rapid tectonic transition is evident from the Whitewater Volcanics, late tectonic, felsic volcanism dated at 1850 ± 5 Ma. The onset of this unconformably younger volcanism is marked by a concomitant change in tectonic style. This changeover from a pre-cratonic to cratonic setting took place in a few million years or less.