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SHRIMP U-Pb dating of zircons from granitoid bodies in the Songpan-Ganzi terrane and its implications.

HU JianMin, MENG QingRen, SHI YuRuo and QU HongJie Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Beijing SHRIMP Center, Beijing 100037  
The Songpan-Ganzi terrane is in general a triangular fold belt, bounded on the north, southeast and east by the eastern Kunlun-western Qinling tectonic belt, the Jinsha river tectonic belt and the Longmenshan tectonic belt, respectively. It is mainly composed of Triassic turbidite, which is intruded by many granitoid bodies. It is obvious that the composition, sources and time of the granitic magma are very important for studing the basement and tectonic evolution of the Songpan-Ganzi terrane. This paper reports some SHRIMP U-Pb dating results for zircons from granitoids in the Songpan-Ganzi terrane. The dating results are as follows: (1) The granitoid intrusions occurred in the Late Triassic and Early Jurassic; (2) The earl-stage magmatism was nearly simultaneous with the folding of Triassic strata, suggesting that the magmatism might have related with a large detachment under the Triassic strata; (3) Some of the crystal zircons contain rounded or unregular cores in shape, and it is thought that the rounded zircons were captured from the wall rocks during the upwelling of the granitic magma or detrital zircons from partial melting of the lower crust. Based on the SHRIMP dating, the core zircons yield ages of the Permian, early Paleozoic, late Paleozoic, and early Proterozoic, respectively. The present of Proterozoic detrital zircons indicate that the Songpan-Ganzi terrane is presumably floored with Precambrian basement rocks.
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