Tectonothermal history recorded in mafic dykes and enclaves of gneissic basement in the Schirmacher Hills, East Antarctica

Abstract

There are successive generations of mafic dykes and other mafic and ultramafic bodies within the gneissic rocks of the Schirmacher Hills in East Antarctica. Three major groups of events can be recognised by comparison of their internal structures with those of their charnockitic or granitic host, and by identification of their successive stages of transformation by metamorphism and migmatization. These events are: (1) granulite facies metamorphism (M1) syntectonic with D1; (2) a second phase of granulite facies metamorphism (M2) and associated charnockitization syntectonic with transposition of D1 foliation by isoclinal folding during D2; and (3) emplacements of granites and pegmatites associated with amphibolite facies metamorphism (M3), broadly syntectonic with two generations of isoclinal folding (D3). The oldest mafic and ultramafic rocks occur as isolated enclaves and preserve a discordant D1 foliation marked by metamorphic minerals characteristic of the first granulite facies metamorphism. Mafic bodies that postdate M1 occur as dykes that were emplaced either: (1) syn M2, (2) syn M3, or (3) post M3. It is suggested that M1, M2 and M3 took place towards the end of the Neo-, Meso- and Palaeoproterozoic, respectively.