Multi-Stage Metamorphism of the UHP Pelitic Gneiss from the Southern Altyn Tagh HP/UHP Belt, Western China: Petrological and Geochronological Evidence

The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2-M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz +/- ilmenite, recording the P-T condition of >1.12 GPa and approximate to 850-930 oC. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill(1))+biotite (Bt(1))+plagioclase (Pl(1))+ilmenite+quartz, and confined under P-T conditions of 0.5-0.8 GPa and approximate to 770-795 oC. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl(2), Sill(2) and Bt(2) in the matrix, and the P-T conditions were 0.4-0.6 GPa and <675 oC. A clockwised P-T path was obtained for the pelitic gneiss in the P-T pseudosection, which showed a deep subduction/collision processes with subsequent exhumation and cooling. Combined with the corresponding multistage metamorphic assemblages, the age dating results implied that the zircons from the gneiss have integrated the recording peak metamorphic (M1, 484 +/- 3 Ma) and retrograded metamorphic ages (M2 to M3, 450 +/- 2 Ma). There was about 32 Ma interval during the first exhumation from the upper mantle depth (>350 km) to the lower crust depth (approximate to 40-20 km), resulting in an average exhumation rate of 9.11-9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at approximate to 500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.