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

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 CO2 inclusions (0·987–1·057 g cm−3), CO2–H2O 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 CO2 fluids fall well within the P–T box estimated by mineral thermobarometry. The abundance of primary CO2 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 CO2 advection could not be envisaged. The subsequent stages involved a decrease in CO2 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 dP/dT 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.

A high-resolution record of Mid- to Late-Holocene environmental changes from a land-locked lake in Schirmacher Oasis, East Antarctica

“…Precambrian crystalline terrain forms the basement of the SO, which is a fragment of the East Antarctic craton (Sengupta, 1986). The geology of SO is dominantly metamorphosed quartz of eldspathic gneisses alternating with mafic granulites, garnet-bearing leucogranite and veins of leucogneiss (Rao et al, 1997; Sengupta, 1986). Lake L6 (Lat: 70°45’21.4" S and Long: 11°35’52.6" E) (Figure 1) is landlocked located in the Schirmacher Oasis.…”

Contrasting geological background based on the geochemistry of the mafic metamorphic rocks in central Dronning Maud Land

“…The highgrade metamorphic rocks are divided into the following units (Fig. 2a): (1) quartzo-feldspathic gneiss, (2) augen gneiss, (3) a mixed zone including metapelite and calc-silicate gneiss, mafic granulite and charnockite, (4) garnet-biotite gneiss, and (5) banded gneiss (Sengupta, 1993;Rameshwar Rao et al 1997).…”

“…(a) Schirmacher Hills (modified after Sengupta, 1993; Rameshwar Rao et al . 1997); (b) Hochlinfjellet and (c) Jutulsessen (modified after Ohta, 1993); (d) Filchnerfjella (Owada et al . 2003).…”