A tripartite approach to unearthing the duration of high temperature conditions versus peak metamorphism: An example from the Bunger Hills, East Antarctica

We present LA-ICP-MS U-Pb monazite and zircon geochronology, trace element chemistry and phase equilibria forward modelling to constrain the P-T-t evolution of the Bunger Hills, East Antarctica. Metasedimentary rocks in the Bunger Hills record evidence for a protracted metamorphic history during the Mesoproterozoic. Taken in isolation, zircon and monazite ages suggest an extremely long duration of high-temperature conditions (ca. 200 Myr). Calculated P-T models indicate metamorphism involved medium pressures of 5.5-7.1 kbar and high to ultrahigh temperatures of 800-960 °C, and that the P-T path likely tracked along a down-pressure to isobaric cooling trajectory. Integrating trace element data from zircon, monazite and garnet indicates that, despite the spread in U-Pb ages, peak metamorphism essentially occurred over the interval ca. 1220-1180 Ma. The age and conditions of Mesoproterozoic metamorphism are consistent with the high-grade metamorphic evolution proposed previously for Stage-2 of the Albany-Fraser Orogeny in southwestern Australia. The P-T-t conditions are interpreted to reflect extension, potentially associated with unloading and exhumation of a collisional orogen following Stage-1 of the Albany-Fraser Orogeny. This is the first study to integrate geochronology, trace element chemistry and P-T modelling to constrain the metamorphic evolution of the Bunger Hills and to interpret these constraints within the context of the now separate terranes of the Musgrave-Albany-Fraser Orogen. The three-way approach adopted in this study demonstrates that zircon and monazite may grow and modify through a number of processes. An integrated petrochronologic approach is therefore essential for investigations on high-grade terranes.