Zircon LA-ICP MS U-Pb Dating of the Longkang Andesitic Ignimbrites from Jiuzhaigou： Evidence of 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.