THE P-T HISTORY OF THE NORTHEASTERN GAWLER CRATON, SOUTH AUSTRALIA FROM MONAZITE CHIME ANALYSIS, 40AR/39AR BIOTITE GEOCHRONOLOGY AND THERMAL HISTORY MODELLING

The geologic record of the Proterozoic period of the Gawler Craton in South Australia has attracted much recent interest as it is the time of generation of the giant Olympic Dam Cu-Au-U-REE deposit and numerous smaller, economically viable deposits (e.g. Prominent Hill, Carapateena). Despite this economic interest, the geological history of the Gawler Craton at this time is poorly understood, primarily due to the poorly exposed nature of the terrane.

The Mount Woods Inlier (MWI) is located in the northeastern Gawler Craton. The prograde and post-peak metamorphic history of the Inlier is of interest as it may have temporal and structural associations with generation of the ca. 1582 Ma (Belperio et al., 2007, Econ. Geol., 102, 1499-1510) Prominent Hill IOCG deposit. In this study, the prograde metamorphic history of the inlier has been constrained using metamorphic analysis and monazite CHIME geochronology of an exposed garnet-cordierite-spinel metapelite. This shows the inlier underwent prograde metamorphism through temperatures of 500-600°C at ca. 1615 Ma. Peak metamorphism occurred at ca. 1595 Ma (e.g. Chalmers, 2007, PIRSA Report Book 2007/20) and attained conditions of ~750°C at ~4.5kbar. These results were combined with ⁴⁰Ar/³⁹Ar biotite geochronology and garnet-biotite Fe-Mg exchange modelling using the program THERMAL HISTORY (Robl et al., 2007, Comp. & Geosc., 33, 760-772) to constrain the post-peak metamorphic P-T path of the terrane, cooling rates and a temperature-time (T-t) path for the Inlier. Consistently broad plateaus in the biotite cooling age data shows that the terrane cooled through ~300°C at ~1535 Ma. Cooling rates are constrained to ~3-9°C/Ma from peak temperatures to ~1535 Ma, and subsequently decreased to ~3-4°C/Ma or ~0.2°C/Ma depending on whether the inlier was exhumed at ca. 1450 Ma or during the Permian respectively. The T-t path for the MWI has a concave shape. Cooling of the inlier is suggested to have begun due to cessation of voluminous magmatism and mantle heat input that was also the mechanism for peak metamorphic temperatures, in combination with initiation of exhumation along the southeastern margin of the inlier ca. 1592-1582 Ma.