Regional high‐pressure metamorphism during intracratonic deformation: the Petermann Orogeny, central Australia

Abstract

The Petermann Orogeny is a late Neoproterozoic to Cambrian (c. 560–520 Ma) intracratonic event that affected the Musgrave Block and south‐western Amadeus Basin in central Australia. In the Mann Ranges, within the central Musgrave Block, Mesoproterozoic granulite facies gneisses, granites and mafic dykes have been substantially reworked by deep crustal non‐coaxial strain of late Neoproterozoic to early Cambrian age. Dolerite dykes have recrystallized to garnet granulite facies assemblages, associated with the development of a mylonitic fabric at P=12–13 kbar and T =700–750 °C. Migmatization is restricted to discrete shear zones, which represent conduits for hydrous fluids during metamorphism. Peak metamorphism was followed by decompression to c. 7 kbar, reflecting exhumation of the terrane along the south‐dipping Woodroffe Thrust. In scattered outcrops north of the Mann Ranges, peak metamorphism occurred at P=9–10 kbar and T =c. 700 °C. The Woodroffe Thrust separates these deep crustal mylonites from granites that were metamorphosed during the Petermann Orogeny at P=c. 6–7 kbar and T =c. 650 °C. The similarity in peak temperatures at different crustal levels implies an unusual thermal regime during this event. The existence of a relatively elevated geotherm corresponding with Th‐ and K‐enriched granites that were in the mid‐crust during the Petermann Orogeny suggests that radiogenic heat production may have substantially contributed to the thermal regime during metamorphism. This potentially has implications for the mechanisms by which intra‐plate strain was localized during this event.