Multi-stage metamorphism in the Rayner–Eastern Ghats Terrane: P–T–t constraints from the northern Prince Charles Mountains, east Antarctica

Abstract

Metapelitic rocks from the northern Prince Charles Mountains in the Rayner Complex in east Antarctica record evidence for a protracted metamorphic history during the late Mesoproterozoic to early Neoproterozoic. In situ LA–ICP–MS U–Pb monazite geochronology yields ages in the interval 1030–880Ma. There is a spread in U–Pb ages both between and within individual samples. Two samples record monazite populations at c. 1020Ma, which have been variably reset. The remaining samples contain single monazite populations with 206Pb/238U weighted mean ages of 940–900Ma. Calculated metamorphic phase diagrams for a sample preserving a defined late Mesoproterozoic monazite population suggest this early part of the metamorphic history may reflect a higher-pressure phase of metamorphism. This stage was overprinted by a cordierite-bearing assemblage, texturally accompanied by monazite growth at 950–900Ma. The conditions of the second event are consistent between samples, and suggest that it involved lower pressures of 6–7kbar and temperatures of 850–880°C. The geochronology and metamorphic conditions for the Neoproterozoic metamorphism obtained in this study are consistent with the evolution proposed for elsewhere in the Rayner Complex and also the contemporaneous and formerly contiguous UHT metamorphism in the Eastern Ghats Province in India. This is the first study to integrate metamorphic constraints from the now separate terranes, and it suggests that that Rayner–Eastern Ghats terrane as a whole records prolonged high temperatures over a spatially large (>500,000km2) area. This has implications for the timescales and footprint of geodynamic processes involving the mid-to-deep crust.