Metamorphism of the Mougooderra Formation: Implications for Neoarchean tectonics in the western Youanmi Terrane, Yilgarn Craton

Abstract

In the Yilgarn Craton, the Mougooderra Formation is an extensive sedimentary sequence that preserves low-variance metamorphic mineral assemblages and unconformably overlies older greenstone sequences within the Youanmi Terrane. Here we present garnet Lu–Hf and Sm–Nd geochronology in combination with pressure–temperature (P–T) estimates of the Mougooderra Formation and the underlying Polelle Group to give insights into the geodynamics of this region at the time of metamorphism. Metamorphic rocks from the Mougooderra Formation have variable metamorphic mineral assemblages that reflect bulk rock concentrations of Fe, Mg, Ti, Al, and K at constant P–T. Based on these variations, the metamorphic rocks of this formation can be separated into a high Fe–Mg group, a high Al group and an average pelitic group that each preserve low-pressure mineral assemblages. Thermobarometry and phase equilibrium modelling indicate that the Mougooderra Formation experienced P–T conditions of 545–580 °C and ~2–3.5 kbar, while the Polelle Group reached 610–650 °C at ~4–7.2 kbar. Garnet Lu–Hf and Sm–Nd geochronology returned respective ages of 2686 ± 18 Ma and 2611 ± 35 Ma for the Mougooderra Formation and ages of 2685 ± 15 Ma and 2590 ± 21 Ma for the Polelle Group. Lu–Hf garnet ages are interpreted to record the timing of late prograde metamorphism and indicate simultaneous metamorphism, whereas the geological significance of the Sm–Nd ages is unclear. Garnet Lu–Hf ages obtained in this study are similar to emplacement ages of syntectonic granites in the region. Although these distal contemporaneous granites may have contributed to elevated heat flow, the high apparent thermal gradients calculated here, as well as synchronous craton-wide magmatism, reflect a regional thermal event at this time. The ages, P–T conditions, and high apparent thermal gradients from the study area suggest metamorphism occurred as the result of high heat flow into thinned crust of the Youanmi Terrane and indicates a complex thermal history within this region of the Yilgarn Craton. These results are consistent with the hypothesis that metamorphism during the Archean was more closely related to heat from mantle-derived magmatism.