New constraints on metamorphism in the Highjump Archipelago, East Antarctica

Abstract

The age and conditions of metamorphism in the Highjump Archipelago, East Antarctica, are investigated using samples collected during the 1986 Australian Antarctic expedition to the Bunger Hills–Denman Glacier region. In situ U-Pb dating of monazite from three metasedimentary rocks yields ages between c. 1240–1150Ma and a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1183 ±8 Ma, consistent with previous constraints on the timing of metamorphism in this region and Stage 2 of the Albany–Fraser Orogeny in south-western Australia. This age is interpreted to date the development of garnet ± sillimanite ± rutile-bearing assemblages that formed at c. 850–950°C and 6–9 kbar. Peak granulite facies metamorphism was followed by decompression, evidenced largely by the partial replacement of garnet by cordierite. These new pressure–temperature determinations suggest that the Highjump Archipelago attained slightly higher temperature and pressure conditions than previously proposed and that the rocks probably experienced a clockwise pressure–temperature evolution.