Thermal evolution of the Tseel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular-rich garnet in the garnet zone to zoned garnet with grossular-rich cores and pyrope-rich rims in the staurolite zone, and pyrope-rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high-P and low-T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520-580 degrees C and 4.5-7kbar in the kyanite stability field and (ii) a low-P and high-T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570-680 degrees C and 3.0-6.0kbar in the sillimanite stability field. The earlier high-P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U-Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510Ma; (ii) the low-P and high-T metamorphism occurred at 377 +/- 30Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 +/- 7Ma. The age of the earlier low-T and high-P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450-400Ma. The low-P and high-T metamorphism continued for c. 100Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high-P and low-T to low-P and high-T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.