Thermal evolution of the central Halls Creek Orogen, northern Australia

The Halls Creek Orogen in northern Australia records the Palaeoproterozoic collision of the Kimberley Craton with the North Australian Craton. Integrated structural, metamorphic and geochronological studies of the Tickalara Metamorphics show that this involved a protracted episode of high-temperature, low-pressure metamorphism associated with intense and prolonged mafic and felsic intrusive activity in the interval ca 1850-1820 Ma. Tectonothermal development of the region commenced with an inferred mantle perturbation event, probably at ca 1880 Ma. This resulted in the generation of mafic magmas in the upper mantle or lower crust, while upper crustal extension preceded the rapid deposition of the Tickalara sedimentary protoliths. An older age limit for these rocks is provided by a psammopelitic gneiss from the Tickalara Metamorphics, which yield a (Pb/Pb)-Pb-207-Pb-206 SHRIMP age of 1867 +/- 4 Ma for the youngest detrital zircon suite, Voluminous layered mafic intrusives were emplaced in the middle crust at ca 1860-1855 Ma, prior to the attainment of lower granulite facies peak metamorphic conditions in the middle crust. Locally preserved layer-parallel D-1 foliations that were developed during prograde metamorphism were pervasively overprinted by the dominant regional S-2 gneissosity coincident with peak metamorphism Overgrowths on zircons record a metamorphic Pb-207/Pb-206, of 1845 +/- 4 Ma. The S-2 fabric is folded around tight folds and cut by ductile shear zones associated with D-3 (ca 1830 Ma), and ail pre-existing structures are folded around large-scale, open F-4 folds (ca 1820 Ma). Construction of a temperature-time path for the mid-crustal section exposed in the central Halls Creek Orogen, based on detailed SHRIMP zircon data, key field relationships and petrological evidence, suggests the existence of one protracted thermal event (>400-500 degrees C for 25-30 million years) encompassing two deformation phases. Protoliths to the Tickalara Metamorphics were relatively cold (similar to 350 degrees C) when intruded by the Fletcher Creek Granite at ca 1850 Mo, but were subsequently heated rapidly to 700-800 degrees C during peak metamorphism at ca 1845 Ma. Repeated injection of mafic magmas caused multiple remelting of the metasedimentary wail rocks, with mappable increases in leucosome volume that show a strong spatial relationship to these intrusives. This mafic igneous activity prolonged the elevated geotherm and ensured that the rocks remained very hot (greater than or equal to 650 degrees C) for at least 10 million years The Mabel Downs Tonalite was emplaced during amphibolite facies metamorphism, with intrusion commencing at ca 1835 Ma. its compositional heterogeneity, and the presence of mutual cross-cutting relations between ductile sheer zones and multiple injections of mingled magma suggest that it was emplaced syn-D-3. Broad-scale folding attributable to F-4 was accompanied by widespread intrusion of granitoids, and F-4 fold limbs are truncated by large, mostly brittle retrograde Sa shear zones.