Discovery of high-pressure pelitic granulite in Ailaoshan orogenic belt, southeastern Tibet, and its tectonic implications
青藏高原东南缘哀牢山构造带泥质高压麻粒岩的发现及其构造意义

High-pressure pelitic granulite containing porphyroblasts of garnet(Grt),sillimanite(Sil),K-feldspar(Kf) and plagioclase(Pl),and inclusions of spinel(Spl),kyanite(Ky),sapphirine(Spr) and rutile(Rut) have been found from the Ailaoshan orogenic belt,southeastern Tibet,regarded as a key indicator for identification of the boundary between the Indochina and South China blocks.The temperatures and pressures calculated by garnet-biotite-plagioclase-quartz geothermobarometery and hightemperature equilibrium spinel + quartz assemblage show the formation and evolution of the graunlite had experienced clockwise P-T path from high-pressure/-temperature to mid-temperature and low-pressure metamorphism.That are:1) the high-pressure/-temperature metamorphism(HP/HT) is characterized by an assemblage of Ky + Sil + Grt1 + Kf1 + Pl1 + Spr + Ter(Kf + Pl) + Bt1 + Spl + Qtz + Ilm1 + Rut1,and the peak P-T conditions of ≥ 10.4kbar at 850 ～ 919℃;2) the mid-temperature and low-pressure retrogressive metamorphism characterized by an assemblage of Grt2 + Bt2 + Pl2 + Ms + Qtz + Ilm2 + Rut2,and two stage P-T conditions of 4.9 ～ 6.5kbar at 664 ～ 754℃ and 3.5 ～ 3.9kbar at 572 ～ 576℃ respectively.The dynamic metamorphic path was that the metapelitic rock experienced high-pressure/-temperature granulite-facies metamorphism in lower continental crust(≥30km) exhumated to mid-upper crust rapidly,it might be related to the continental collision and underthrusting between the Indochina and South China blocks,and exhumation and slip shearing lately.