An exhumation pressure–temperature path and fluid activities during metamorphism in the Tseel terrane, SW Mongolia: Constraints from aluminosilicate-bearing quartz veins and garnet zonings in metapelites

Abstract

The Tseel terrane of the Central Asian Orogenic Belt, SW Mongolia, contains a record of amphibolite-facies (locally granulite-facies) metamorphism and multiple igneous intrusions. However, the metamorphic history of the terrane and the relation of metamorphism to the intrusion of granitoids remain uncertain. In this study, we investigated pelitic gneisses and aluminosilicate-bearing quartz veins located at various distances from a granitoid body in the central part of the Tseel terrane.The pelitic gneisses are composed mainly of garnet (Grt)+biotite (Bt)+plagioclase (Pl)+sillimanite (Sil)+quartz (Qtz). Garnet in the pelitic gneiss located far from the granitoid shows compositional zoning, characterized by decreases in Ca and Mn from core to rim, and increases in Fe and Mg, along with minor retrograde zoning at the outermost rim. The P–T path during garnet growth was estimated by garnet isopleth thermobarometry based on a P–T pseudosection and by conventional Grt–Bt geothermometry and Grt–Bt–Pl–Qtz geobarometry. The two approaches yield similar P–T paths, with the decompression P–T path extending from the kyanite (Ky) stability field (560±10°C and 6.5±0.5kbar) to the sillimanite stability field (600±5°C and 3.8±0.5kbar), accompanied by a slight increase in temperature (by 40–50°C). Garnet grains in pelitic gneisses located near the granitoid body lack a high-Ca core and yield P–T conditions of the sillimanite stability field, suggesting the local thermal effect of a granitoid intrusion at shallow crustal depths (<∼20km).Aluminosilicate-bearing quartz veins in the pelitic gneisses contain kyanite, sillimanite, and andalusite (And) at their margins. The textural relations among aluminosilicate polymorphs in the veins indicate their formation in the order of Ky→Sil→And. Microthermometric analyses of fluid inclusions reveal that these veins formed in the kyanite stability field, that corresponds to the growth of high-Ca garnet cores in the pelitic gneiss. The precipitation of coarse-grained kyanite in fractures was probably assisted by the focused flow of fluid produced by a dehydration reaction (i.e., the garnet-in reaction). After the main period of vein growth, the replacement of kyanite by sillimanite and andalusite occurred in association with the formation of muscovite in the veins. These reactions were probably facilitated by fluid supply from granitoids at shallow crustal depths.