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Zircon U-Pb Geochronology and Geological Significance of Guangtoushan Granitoids from Western Qinling,Central China

WU Fenghui~1, LIU Shuwen~ 1, LI Qiugen~1, WANG Zongqi~2, SU Li~3, YANG Kai~1, ZHANG Fan~1, YAN Quanren~4, YAN Zhen~41. The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871; 2. Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037; 3. The Geohogical Lab Center, China University of Geosciences, Beijing 100083; 4. Institute of Geology,Chinese Academy of Geological Sciences,Beijing,100037  
Guangtoushan granitoids are located in the north of Mianlue suture zone, which consist of tonalites and monzogranites. The tonalites display gneissic feature, which are locally mylonitized into granitoid mylonites. The weakly gneissic to massive monzogranites with a little garnet emplaced after the mylonite zone. LA-ICPMS zircon U-Pb isotope dating reveals that Guangtoushan granitoid pluton experienced two stages of emplacement. The earlier gneissic tonalite, sample GT18-01, emplaced at 221±6 Ma and the later monzogranite, sample GT11-01, yields a crystalline age of 199±4 Ma. Integrated with the previous studies in regional tectonic setting and geochemical characteristics, the early tonalites of the Guangtoushan granitoids were most likely to emplace at ~221 Ma, which possibly represents the arc magmatism of oceanic crust subduction before Mianlue ocean basin closed. Then, Mianlue ocean basin closed and the Yangtze Craton was amalgamazed with Qinling micro-block, which formed Mianlue suture zone. The S-type monzogranitic magmatism probably occured in the syn-collision tectonic setting during the main orogenic stage of the Qinling orogenic belt at ~199 Ma. The closure event of the Mianlue ocean basin, which formed the Mianlue suture zone,occured between ~221 Ma and ~199 Ma.
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