Two Types of Symplectites in a Lower Crust Granulite Xenolith from the Zarnitsa Kimberlite (Yakutia): A Record of Si-Metasomatism and Decompression

Abstract

Abstract ––The paper presents data on a granulite xenolith from the Zarnitsa kimberlite pipe (Yakutia, Russia), which stores a record of two metasomatic events in addition to the main stage of metamorphism. The granulitic mineral assemblage consists of garnet, clinopyroxene, and plagioclase as main phases. The granulite xenolith contains kyanite–clinopyroxene and later orthopyroxene–plagioclase symplectites. Kyanite–clinopyroxene symplectites appear as short veins inside or between grains of rock-forming minerals. Orthopyroxene–plagioclase symplectites form kelyphite rims in all garnets or occur as veins in garnet grains. The P–T conditions for granulite in the lower crust reconstructed by Perple_X phase equilibrium modeling are 700–750 ℃ and 1.2–1.3 GPa. According to TWQ thermodynamic calculations, the kyanite–clinopyroxene symplectites were produced by Si-metasomatism at invariable Р–Т parameters. The growth of orthopyroxene–plagioclase symplectites after garnet was maintained by Ca inputs upon heating and decompression (200 ℃ temperature increase and 0.6 GPa pressure decrease) while the xenolith was transported by ascending kimberlite melt.