Mafic granulites of the Schirmacher region, East Antarctica: fluid inclusion and geothermobarometric studies focusing on the Proterozoic evolution of the crust

• Published in: Transactions of the Royal Society of Edinburgh: Earth Sciences
• Main Authors: Rao, D. Rameshwar, Sharma, Rajesh, Gururajan, N. S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1997
• Subjects: Antarc*; Antarctica; East Antarctica
• Online Access: http://dx.doi.org/10.1017/s0263593300002285; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263593300002285

Abstract In the Proterozoic complex of the Schirmacher region of East Antarctica, a retrograde pressure–temperature (P–T) history has been inferred through quantitative geothermobarometry and fluid inclusion studies of the mafic granulites. Microthermometric investigations of the fluid phases trapped in quartz and garnet identified three types of inclusions, namely, earliest pure CO 2 inclusions (0·987–1·057 g cm −3 ), CO 2 –H 2 O inclusions and aqueous inclusions. The temperature and pressure of metamorphism have been estimated through different calibrations of geothermometers and geobarometers. The mineral reactions and compositional zoning in the minerals record P–T conditions from nearly 837 ± 26°C, 7·1±0·2 kbar to 652 ± 33°C, 5·9 ± 0·3 kbar. A good correlation between the fluid and mineral data is observed. The isochores typical of highdensity CO 2 fluids fall well within the P–T box estimated by mineral thermobarometry. The abundance of primary CO 2 inclusions in early metamorphic minerals (notably quartz and primary garnet) and the general correspondence between fluid and mineral P–T data indicate a ‘fluid-present’ carbonic regime for the high-grade metamorpism; however, from the present data largescale CO 2 advection could not be envisaged. The subsequent stages involved a decrease in CO 2 density, a progressive influx of hydrous fluids and the generation of retrograde amphibolite facies metamorphism in the area. The estimated P–T conditions of the region suggest that the rocks were metamorphosed at a depth of 19–24 km, with a geothermal gradient of c. 3°5C km −1 . The estimated P–T conditions of the rocks imply a clockwise P–T–t path with a gradual decrease in temperature of around 250°C and a decrease in pressure of around 1700 bar. They have a d P /d T gradient of ≈7 ± l bar °C −1 , arguing for an isobaric cooling history of the terrane under normal thickened crust after the underplating of mantle-derived material.