Changes in the [formula omitted] ratios of fluids as evidence for different metamorphic episodes in high grade gneisses from the Konovalov Mountains area (Rayner Complex, East Antarctica)

Abstract

Rocks of the Rayner complex in East Antarctica have suffered a complex multiphase metamorphic evolution. Radiogenic age constraints taken from the literature suggest at least two metamorphic events, the `main Rayner event' accompanied by charnockite emplacement 963 Myr ago and an amphibolite facies overprint 500–550 Myr ago. Some authors have suggested that a second high temperature metamorphic event was connected with the widespread intrusion of granitic magmas at about 770 Myr. Petrological and stable isotope thermometry is consistent with three metamorphic events, (i) a first granulite facies metamorphism, as preserved in the cationic equilibria of minerals in enderbitic gneisses yielding 810°C and 6.9 kbar; (ii) a regionally pervasive overprint, also at high temperatures (730°C and 5.9 kbar), previously regarded as the `main Rayner' event and; (iii) an amphibolite facies overprint followed by a greenschist facies retrogression, which is especially pronounced in late shear zones. We show that the two older events can be distinguished by the isotopic composition of the metamorphic fluids. During the first granulite facies event, the rocks preserved their inherited isotopic signatures, whereas the second granulite facies overprint was accompanied by pervasive flux of fluids of magmatic (possibly mantle) origin. During the third, amphibolite grade event, the fluids trapped during the second event were remobilised and led to partial reequilibration.