Anatexis and metamorphic history of Permian pelitic granulites from the southern Chinese Altai: Constraints from petrology, melt inclusions and phase equilibria modelling

Abstract

Pelitic granulites from the southern Chinese Altai record a prominent Permian high–temperature reworking event. The lack of solid constraints on anatexis and metamorphic P–T path of these rocks has hindered our understanding of the thermo–tectonic history of Chinese Altai orogen. Here, we present a detailed field and petrological study of the pelitic granulites cropping out at Wuqiagou area, Fuyun county. They occur as lenses or interlayers hosted within biotite–plagioclase gneisses, with a rutile–bearing assemblage composed of garnet ± orthopyroxene + cordierite + biotite ± K–feldspar + plagioclase + quartz ± spinel ± sillimanite + ilmenite. These pelitic granulites preserve diagnostic features of anatexis, such as small patches of leucosomes at macroscopic scale, melt inclusions in garnet porphyroblasts and highly cuspate quartz at microscopic scale. Melt inclusions (up to ~40 μm) are now crystallized into a cryptocrystalline aggregate of feldspars, quartz and biotite (i.e. they are nanogranitoids). Phase equilibria modelling in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) 10–compositional model system suggests a possible clockwise P–T path involving a post–peak isobaric cooling (IBC) process from 800 to 850 °C to near solidus conditions (700–720 °C) at 5.0–6.0 kbar. SIMS zircon UPb geochronology indicates that the granulite–facies metamorphism took place at 284.1 ± 2.6 Ma, which is coeval with spatially associated mantle–derived mafic intrusions. Therefore, we propose that the Permian granulite–facies metamorphic event occurred in a post–orogenic extensional tectonic setting, which was associated with the input of external heat, related to underplating and intruding of mantle–derived magmas possibly as a result of the Tarim mantle plume activity.