Ultrahigh–temperature metamorphism and melt inclusions from the Sør Rondane Mountains, East Antarctica

Abstract

This paper reports the first outcrop occurrence of an ultrahigh–temperature (UHT) metamorphic rock from the Sør Rondane Mountains (SRM), East Antarctica. A pelitic gneiss from Balchenfjella, eastern SRM, contains mesoperthite that gave UHT condition (>900 °C) by ternary feldspar thermometry. The UHT mesoperthite is present both in the matrix and as an inclusion in garnet. The garnet also has nanogranitoid inclusions next to the mesoperthite, which are interpreted to be an UHT melt. The re–integrated nanogranitoid composition is plotted in the primary phase region of quartz and classified as granite. Even crystallized nanogranitoids can provide appropriate original melt composition in the An–Ab–Or and Qz–Ab–Or spaces, whereas Mg concentration is enriched due to local retrograde Fe–Mg exchange reaction between the nanogranitoid inclusions and the host garnet. Although metamorphic rocks in the SRM are highly retrogressed, this study revealed that the microstructural evidence of UHT condition is partially preserved. Further investigation of timing and areal extent of UHT metamorphism helps us to understand the tectonic model of the SRM.