Metamorphic P-T-t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton (Dunhuang block): Evidence for early Paleozoic continental subduction

摘要

The Dunhuang block, which constitutes the northeastern segment of the Tarim Craton, is located between the Central Asian Orogenic Belt (CAOB) to the north and the Altyn-Qilian Orogenic Belt to the south. The present study reveals that the early Paleozoic HP mafic granulites from the Dunhuang block underwent four stages of metamorphism: prograde amphibolite-facies (Ml), peak high-pressure (HP) granulite-facies (M2) and two late stages of amphibolite-facies retrograde metamorphism. Based on phase equilibrium modeling, P-T conditions of the four stages of metamorphism are estimated at 720-750 °C and 11-13 kbar, 760-800 °C and 14-16 kbar, 690-720 °C and 8-8.5 kbar, and at <6 kbar and < 640 °C respectively. Peak granulite-facies metamorphism is characterized by a low geothermal gradient of ca. 16°C/km. Zircon U-Pb dating shows that the HP granulite-facies metamorphism occurred at ca. 431 Ma and the early retrograde amphibolite-facies overprint at ca. 403 Ma Thus the investigated mafic rocks here reveal a clockwise P-T-t path involving burial heating before peak granulite-facies metamorphism and subsequent decompression-cooling with an uplift rate of ca. 0.8 km/Ma. This, together with a continental affinity of the HP metamorphic rocks, indicates that the Dunhuang block experienced a collisional orogenesis during the early Paleozoic