Oriented Microinclusions of Al–Fe–Mg–Ti Oxides in Quartz from Metapelitic Granulites of the Bunger Hills, East Antarctica

Abstract

The Bunger Hills, East Antarctica occupies an area of about 300 km² and is underlain by large Neoarchean tonalite–granite orthogneiss bodies in the southern part and a Paleoproterozoic volcanosedimentary sequence, buckled into isoclinal folds and metamorphosed to the granulite facies, in the northern. The sequence is intruded by large syncollisional charnockite plutons and consists predominantly of strongly migmatized garnet–sillimanite–cordierite paragneisses (metapelitic granulites). A characteristic feature of the mineral composition of these rocks is the paragenesis of Zn-poor spinel and quartz. The P–T conditions for the equilibrium of these two minerals are 6–7 kbar and 970–1030°C as estimated with the method of isochemical phase diagrams and 985–1005°C, as calculated with the aid of Ti-in-quartz thermobarometry (Huang and Audétat, 2012). Quartz contains needle-shaped microinclusions less than 1 μm in thickness aligned along the [0001] direction and evenly distributed throughout the quartz grains. According to data of electron-probe analysis, they are heterogeneous and consist of rutile and spinel (hercynite) intergrowths. The available data suggest that the genesis of the microinclusions is related to the exsolution process and the diffusion of structural impurities in quartz (Al, Fe, and Ti) in the direction of parallel translation-slip zones (10\(\bar {1}\)10), along which the high-temperature plastic deformations of quartz took place during the postpeak stage. The appearance of spinel in the microparagenesis of the oxides of the Al–Fe–Mg–Ti system is interpreted as a result of the processes synchronous with the intracrystal diffusion: the open-system diffusion of Mg atoms along the weakness zones of the quartz lattice or the infiltration of Mg-rich fluids along the microscopic cleavage planes formed as a result of hydrofracturing.