Late Paleozoic-Early Mesozoic tectonic evolution of the Paleo-Asian Ocean： geochronological and geochemical evidence from granitoids in the northern margin of Alxa, Western China  

作　　者：Xin SHA Jinrong WANG Wanfeng CHEN Zheng LIU Xinwei ZHAI Jinlong MA Shuhua WANG 

机构地区：[1]School of Earth Sciences, Gansu Key Laboratory of Mineral Resources in Western China Lanzhou University, Lanzhou 730000, China [2]State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210000, China

出　　处：《Frontiers of Earth Science》2018年第1期191-214,共24页地球科学前沿（英文版）

基　　金：During the research work, we appreciated advice from Senior Engineer Yang Bin of the Gansu Province Geology and Mining Bureau and Zhang Xiang, Engineer, of Gansu Province Geological Survey. We thank the editors and anonymous reviewers for comments that greatly improved the presentation of the paper. Thanks to Shuangshuang Wang for the help during the laboratory work. This study was supported by the National Natural Science Foundation of China （Grant No. 41173014）, the Central Universities Fundamental Research Project （Lzu-Jbky-2012-128）, and Gansu Key Laboratory of Mineral Resources in Western China （Lanzhou University） funded project.

摘　　要：The Paleo-Asian Ocean （Southern Mongolian Ocean） ophiolitic belts and massive granitoids are exposed in the Alxa block, in response to oceanic subduction processes. In this work, we report petrographic, geochem- ical, and zircon U-Pb age data of some granitoid intrusions from the northern Alxa. Zircon U-Pb dating for the quartz diorite, tonalite, monzogranite, and biotite granite yielded weighted mean 206pb/238U ages o f 302 ±9.2 Ma, 246.5±54.6 Ma, 235±4.4 Ma, and 229.5±5.6 Ma, respectively. The quartz diorites （-302 Ma） exhibit geochemical similarities to adakites, likely derived from partial melting of the initially subducted Chaganchulu back-arc oceanic slab. The tonalites （-246.5 Ma） display geochemical affinities of I-type granites. They were probably derived by fractional crystallization of the modified lithospheric mantle-derived basaltic magmas in a volcanic arc setting. The monzo- granites （-235 Ma） are characterized by low A1203, but high Y and Yb with notably negative Eu anomalies. In contrast, the biotite granites （-229.5 Ma） show high A1203 but low Y and Yb with steep HREE patterns and the absence of negative Eu anomalies. Elemental data suggested that the biotite granites were likely derived from a thickened lower crust, but the monzogranites originated from a thin crust. Our data suggested that the initial subduction of the Chaganchulu oceanic slab towards the Alxa block occurred at - 302 Ma. This subduction process continued to the Early Triassic （-246 Ma） and the basin was finally closed before the Middle Triassic （-235 Ma）. Subsequently, the break-off of the subducted slab triggered asthenosphere upwelling （240-230 Ma）.The Paleo-Asian Ocean （Southern Mongolian Ocean） ophiolitic belts and massive granitoids are exposed in the Alxa block, in response to oceanic subduction processes. In this work, we report petrographic, geochem- ical, and zircon U-Pb age data of some granitoid intrusions from the northern Alxa. Zircon U-Pb dating for the quartz diorite, tonalite, monzogranite, and biotite granite yielded weighted mean 206pb/238U ages o f 302 ±9.2 Ma, 246.5±54.6 Ma, 235±4.4 Ma, and 229.5±5.6 Ma, respectively. The quartz diorites （-302 Ma） exhibit geochemical similarities to adakites, likely derived from partial melting of the initially subducted Chaganchulu back-arc oceanic slab. The tonalites （-246.5 Ma） display geochemical affinities of I-type granites. They were probably derived by fractional crystallization of the modified lithospheric mantle-derived basaltic magmas in a volcanic arc setting. The monzo- granites （-235 Ma） are characterized by low A1203, but high Y and Yb with notably negative Eu anomalies. In contrast, the biotite granites （-229.5 Ma） show high A1203 but low Y and Yb with steep HREE patterns and the absence of negative Eu anomalies. Elemental data suggested that the biotite granites were likely derived from a thickened lower crust, but the monzogranites originated from a thin crust. Our data suggested that the initial subduction of the Chaganchulu oceanic slab towards the Alxa block occurred at - 302 Ma. This subduction process continued to the Early Triassic （-246 Ma） and the basin was finally closed before the Middle Triassic （-235 Ma）. Subsequently, the break-off of the subducted slab triggered asthenosphere upwelling （240-230 Ma）.

关 键 词：Paleo-Asian Ocean ALxa GRANITE geochemistry 

分 类 号：P588.1[天文地球—岩石学] P732.6[天文地球—地质学]