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 tectonic zone is closely related with tectonic evolution of the Mianlue 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 in the east end of the Tazang-Longkang 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, 2σ). 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 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.