Abstract

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Title:: Metamorphic Evolution Processes and Their Geological Implications of Gneisses from Duoba Area of Tibet

Author(s):: LI Peng; ZHANG Cong; LIU Xiao-yu; YANG Jing-sui; FU Yi-wen; 1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China; 2. National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing 100037, China; 3. Linzhi Bureau of Land and Resources, Linzhi 860100, Tibet, China

Keywords:: metamorphism; granitic gneiss; pressure-temperature conditions; Neoproterozoic; zircon geochronology; Nyainqentanglha Group; Lhasa block; Tibet

Abstract:: A suit of metamorphic rocks, which experience amphibolite and partly granulite facies metamorphism exposes on Lhasa block, are recognized as the basement of Lhasa block and named as Nyainqentanglha Group in the former literatures. Although the rocks are thought to be part of Nyainqentanglha Group, however, it is still lack of reliable geochronological data in some area. By means of the petrological and mineralogical investigations on the gneisses exposed in Duoba area of the north part of Lhasa block, it shows that the gneiss suit is composed of mica schist and garnet-bearing plagioclase gneiss, which has experienced amphibolite facies metamorphism. The garnet in the gneiss with homogeneous composition does not have core-rim structure. The traditional mineral-pair geothermobarometer shows that the metamorphic pressure-temperature conditions are 640 ℃-660 ℃ and 0.60-0.67 GPa, which belong to amphibolite facies metamorphism. LA-ICP-MS zircon U-Pb dating technique is used for the first time to date the metamorphic age of the gneiss in Duoba area and yield the Proterozoic metamorphic age of (847.0±5.7)Ma, which can be compared with Nyainqentanglha Group in the area, exhibiting strong evidence that the gneiss in Duoba area is also part of Nyainqentanglha Group.