Proterozoic Granulites from the Rauer Group, East Antarctica. I. Decompressional Pressure-Temperature Paths Deduced from Mafic and Felsic Gneisses

• Published in: Journal of Petrology
• Main Author: HARLEY, SIMON L.
• Format: Text
• Language: English
• Published: Oxford University Press 1988
• Subjects: Articles; East Antarctica; Rauer Group; Antarc*; Antarctica
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/29/5/1059; https://doi.org/10.1093/petrology/29.5.1059

Granulites from the Rauer Group, East Antarctica, were metamorphosed at 860�40�C during a high-grade tectonothermal episode younger than 1400 Ma and probably close to 1000 Ma in age. A spatial variation of pressures of metamorphism at the thermal peak is calculated for felsic and mafic granulites preserving garnet-orthopyroxene-plagioclase assemblages with or without additional clinopyroxene and quartz. Pressures of 6 to 7.5 kb are derived for the northern parts of the Rauer Group, whereas 7–8�5 kb pressures are calculated for similar granulites some 10–20 km further south. Post-deformational reaction textures including orthopyroxene-plagioclase symplectites after garnet in basic granulites and plagioclase moats or rims on garnet and orthopyroxene in felsic granulites indicate a decompressional pressure-temperature-time evolution ( P-T-t ) which is confirmed by garnet-orthopyroxene-plagioclase-quartz and garnet-orthopyroxene barometry of zoned and regrown minerals. A pervasive decompression through c . 2 to 3–5 kb in the northern Rauer Group and to 5–6 kb in the southern part of the region occurred at temperatures above 700�C and probably in excess of 750�C. This P-T evolution, which indicates a uniform unroofing of some 6–9 km while quite high mid- to lower-crustal temperatures only decreased by c . 100�C, is consistent with the later stages of a prolonged collision-related thermal evolution. Comparisons of the P-T-t paths of the late Proterozoic granulites from the Rauer Group and elsewhere in East Antarctica with calculated P-T paths for simple collisional models where erosion terminates the heating phase show that externally- derived magmatic additions and an enhanced total heat budget are necessary to produce the observed high-temperature evolution.