摘要
Genesis of metasedimentary-volcanics outcroped in Kangxian-Pipasi-Nanping （康县-琵琶寺-南坪） tectonic zone is closely related with tectonic evolution of the Mianlue （勉略） Ocean. In the west end of Kangxian-Pipasi-Nanping tectonic zone, there are hundred meters of andesitic ignimbrites and tuffites. Zircon U-Pb dating on these volcanics has great significance to the tectonic implication of the volcanic rocks. Zircons from an ignimbrite sample （JZG-L） have been collected from Yazhagou （牙扎沟） in the east end of the Tazang-Longkang （塔藏-隆康） Section for LA-ICP MS U-Pb dating. The zircons from Longkang andesitic ignimbrites are magmatic genesis with oscillatory zoning and high U （（35-750） ppm）, Th （（311-717） ppm） contents with high Th/U （0.44-1.30） ratios. The measured 206pb/238u ratios are in good analytical precision, yielding a weighted mean age of （246±3） Ma （MSWD=2.6, n=12, 2a）. Some detrital zircons have also been observed, they have maximal concordia age of （2 481±33） Ma, which indicates the existence of Precambrian material in the Jiuzhaigou （九寨沟） area. Combined with regional geology and the volcanic rocks in the studied area, the （246±3） Ma zircon U-Pb age suggests a Late Permian orogenic volcanism and provides important geochronology evidence for the Mianlue suture westward extension.
Genesis of metasedimentary-volcanics outcroped in Kangxian-Pipasi-Nanping （康县-琵琶寺-南坪） tectonic zone is closely related with tectonic evolution of the Mianlue （勉略） Ocean. In the west end of Kangxian-Pipasi-Nanping tectonic zone, there are hundred meters of andesitic ignimbrites and tuffites. Zircon U-Pb dating on these volcanics has great significance to the tectonic implication of the volcanic rocks. Zircons from an ignimbrite sample （JZG-L） have been collected from Yazhagou （牙扎沟） in the east end of the Tazang-Longkang （塔藏-隆康） Section for LA-ICP MS U-Pb dating. The zircons from Longkang andesitic ignimbrites are magmatic genesis with oscillatory zoning and high U （（35-750） ppm）, Th （（311-717） ppm） contents with high Th/U （0.44-1.30） ratios. The measured 206pb/238u ratios are in good analytical precision, yielding a weighted mean age of （246±3） Ma （MSWD=2.6, n=12, 2a）. Some detrital zircons have also been observed, they have maximal concordia age of （2 481±33） Ma, which indicates the existence of Precambrian material in the Jiuzhaigou （九寨沟） area. Combined with regional geology and the volcanic rocks in the studied area, the （246±3） Ma zircon U-Pb age suggests a Late Permian orogenic volcanism and provides important geochronology evidence for the Mianlue suture westward extension.
基金
the National Natural Science Foundation of China (Nos. 40572050, 40272042, and 40234041)
the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of Ministry of Education, China.
