Petrology and Isotopic Chronology of Tagong Granite in Eastern Songpan-Ganzi Fold Belt and Its Tectonic Significance

The Triassic-Jurassic intermediate-acid granites in the eastern Songpan-Ganzi fold belt, such as the Tagong granite, provide important information for the collisional orogeny of the Late Triassic and the tectonic evolution in Yanshanian. The regional geological survey map shows that the Tagong granite consists of quartz diorite, granodiorite, biotite monzonitic granite. Its geochemical characteristics indicate that the main body of the rock belongs to high-potassium-calcium-alkaline series and quasi-aluminum-over-aluminum transition. The rare earth element pattern shows that it is enriched in light rare earth and depleted in heavy rare earth. It has the characteristics of crust-derived magma. The rare earth-trace elements dosing results indicate that granite was formed in the post-collision stage of the orogenic period. The isotope samples collected from the Tagong granite obtained high-quality isotopic age, namely 214±1 Ma, 214±1 Ma, 215±1 Ma, 216±1 Ma, which can be used as evidences to limit the collision time among the Yangtze, North China and Changdu plates in the Triassic. Based on the geochemical characteristics and isotopic chronology of the Tagong granitoids, combined with the regional research data, it is considered that the Late Triassic granitoids similar to the Tagong granite in the eastern Songpan-Ganzi fold belt were formed due to the lithospheric delamination in the post-collision stage of Changdu, Yangtze and the North China plates.