Zircon LA-ICP-MS U-Pb dating of the Longkang ignimbrite in the Jiuzhaigou area, Sichuan, China: Evidence of the westward extension of the Mianlüe suture

A suite of metasedimentary-volcanic rocks exposed in the Kangxian-Pipasi-Nanping tectonic zone is closely related to the tectonic evolution of the Mianlüe ocean. Andesitic ignimbrite and tuffite nearly a hundred meters thick are exposed at the eastern end of the Tazang-Longkang section in the western segment of the Kangxian-Pipasi-Nanping tectonic belt. An ignimbrites sample (JZG-L) was collected near Yazagou at the eastern end of the Tazang-Longkang section, from which more than one hundred zircon grains were selected. The zircon LA-ICP-MS U-Pb analysis shows that the zircons are of typical magmatic origin with oscillatory zoning, higher U (35×10-6-750×10-6), Th (31×10-6-717×10-6) and Th/U (0.44-1.30). The 206Pb/238U ratios are in good agreement with the analytical precision, yielding a weighted mean age of 246.2±2.8 Ma (MSWD=2.6; n=12, 2 σ). Some detrital zircons have also been observed, which have the oldest concordia age of 2481±33 Ma, indicating the existence of Precambrian basement in the Jiuzhaigou area. According the regional geology and characteristics volcanic rocks in the study area, the 246.2±2.8 Ma zircon U-Pb age suggests the occurrence of Late Permian orogenic volcanism and provides important isotope geochronological evidence for the westward extension of the Mianlüe suture to the Nanping area.