P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

摘要

Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M_1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M_2) consists of garnet+clinopyroxene+hornblende+plagioclase in the malic granulites. The peak metamorphism was followed by near-isothermal decompression (M_3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M_4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905 deg C and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M_3) were estimated at 720-800 deg C and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M_4) at 594-708 deg C and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M_1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.