Finding of high-pressure mafic granulites in the Amdo basement,central Tibet  

作　　者：ZHANG XiaoRan SHI RenDeng HUANG QiShuai LIU DeLiang CIDAN SuoLang YANG JingSui DING Lin 

机构地区：[1]Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China [2]School of Sciences, Tibetan University, Lhasa 850000, China [3]Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

出　　处：《Chinese Science Bulletin》2010年第32期3694-3702,共9页

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)

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.

关 键 词：高压基性麻粒岩 西藏中部 地下室 高压变质作用 矿物组合 变质条件 角闪岩相 PT轨迹 

分 类 号：P588.3[天文地球—岩石学] S158.1[天文地球—地质学]