Zircon U-Pb dating of the Nanyili granodiorite in the Pingwu area, northwestern Sichuan, and its geological significance

Abstract：Plenty of Indosinian intrusive rocks outcrop in the Bikou massif in the Wuping area, northwestern Sichuan, but accurate chronological data are lacking. The authors analyzed the interior structures of zircons from the Nanyili granodiorite at the southwest margin of the Bikou massif by using the CL technique, and on that basis, they performed the in-situ zircon U-Pb dating using the LA-ICP-MS and obtained reliable age data. The results show that： the zircons from the granodiorite have higher Th/U ratios, and the CL images display oscillatory zoning in the interior of the zircons, suggesting the features of magmatic origin. The crystallization age of the Nanyili granodiorite is 223.1±2.6 Ma (MSWD=1.4), indicating that the granodiorite formed in the early Late Triassic. The Nanyili granodiorite was intruded later than the main collision period of the Mianxian-Lueyang tectonic belt and emplaced in a post-collision setting or at least in a transition stage from syncollision to postcollision.