Early Paleozoic granulite-facies metamorphism and anatexis in the northern West Qinling orogen： Monazite and zircon U-Pb geochronological constraints  

作　　者：MAO XiaoHong ZHANG JianXin YU ShengYao LI YunShuai YU XingXing LU ZengLong 

机构地区：[1]Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China [2]School of Earth and Space Sciences,Peking University,Beijing 100871,China [3]Submarine Geosciences and Prospecting Techniques,MOE and College of Marine Geosciences,Ocean University of China,Qingdao 266100,China [4]Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology,Qingdao 266235,China [5]Institute of Surface-Earth System Science,Tianjin University,Tianjin 300072,China

出　　处：《Science China Earth Sciences》2017年第5期943-957,共15页中国科学（地球科学英文版）

基　　金：supported by the National Natural Science Foundation of China (Grant Nos. 41630207, 41572180);the China Geological Survey Project (Grant No. 12120115027001)

摘　　要：In the northern West Qinling orogen(WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma(MSWD=0.88) and 433±4 Ma(MSWD=0.27), respectively.Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups:424±3 Ma(MSWD=0.45) and 402±3 Ma(MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma(MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.In the northern West Qinling orogen（WQO）, granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma（MSWD=0.88） and 433±4 Ma（MSWD=0.27）, respectively.Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups：424±3 Ma（MSWD=0.45） and 402±3 Ma（MSWD=1.4）, which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma（MSWD=0.3）, which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.

关 键 词：West Qinling Granulite Migmatite U-Pb age dating Metamorphism Early Paleozoic 

分 类 号：P588.3[天文地球—岩石学] P618.51[天文地球—地质学]