摘要
High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement,central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene + plagioclase ± spinel and hornblende + plagioclase around garnet,which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860-920°C and 1.46-1.56 GPa,which retrogressed from post-peak phase at 820-890°C and 0.88-1.15 GPa to amphibolite facies at 550-670°C and 0.52-0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of ～50 km and then were rapidly exhumed to a shallow crustal level (～20 km). The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic,which is a crucial step to the tectonic evolution of the Tibetan Plateau.
High-pressure mafic granulites with a peak mineral assemblage of garnet ＋ clinopyroxene ＋ rutile ＋ quartz were found in the Amdo basement, central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene ＋ plagioclase ± spinel and hornblende ＋ plagioclase around garnet, which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860-920℃ and 1.46-1.56 GPa, which retrogressed from post-peak phase at 820-890℃ and 0.88-1.15 GPa to amphibolite facies at 550-70℃ and 0.52-0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of -50 km and then were rapidly exhumed to a shallow crustal level （-20 km）. The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic, which is a crucial step to the tectonic evolution of the Tibetan Plateau.
基金
supported by the National Natural Science Foundation of China (40972056 and 40672051)
the project entitled "The evolution of Tethys suture in south Tibet" of the Institute of Geology,Chinese Academy of Geological Sciences (1212010918013)
