Petrogenesis and Tectonic Implication of Late−Triassic Granitoids in the West−Central Part of Songpan−Ganze Block

Through various methods such as petrography, zircon U−Pb geochronology, petrogeochemistry and Lu−Hf isotopes, the petrological and geochemical characteristics of the granitic rocks from the south entrace of Bayan Har Mountain and Dari area have been systematically compared, in order to find out its petrogenesis, magma source area and basement attributes. The magmatic zircon U−Pb ages of the granitic rocks from the south entrace of Bayan Har Mountain and Dari area are (212.0±2.2) Ma, (213.3±1.7) Ma, (217.0±1.9) Ma and (215.4±6.4) Ma. Studies on major and trace elements show that the former belongs to high−potassium calcium−alkaline peraluminous I−type granodiorite, while the latter belongs to potash basalt and high−potassium calc−alkaline, peraluminous S−type quartz monzonite and granite. The characteristics of the trace elements of the granitic rocks from the south entrace of Bayan Har Mountain and Dari area are: enrichment of large ion lithophile elements such as Rb, Th, U, depletion of high field strength elements such as Nb and Ta, and slight Zr. Hf negative anomaly, but the former Nb, Ta and other elements are significantly more depleted than the latter. Eu anomaly is also more obvious. The granitic rocks from the south entrace of Bayan Har Mountain area are both light rare earth−enriched rare earth element distribution models, but the content of light and heavy rare earths in the samples from the Dari area are higher than those of the Bayan Har Mountain samples. The zircon Hf isotopic data of granitic rocks from the Bayan Har Mountains area show that εHf(t)=−3.62～2.92, the average is −0.54, and the age of the zircon Hf two−stage model is between 1.07 and 1.48 Ga. Combining previous research data and the composition of major, trace and Hf isotopes in this paper, it is inferred that the source areas of the granitic rocks from the south entrace of Bayan Har Mountain and Dari area are lower crust mafic rocks and middle crust sandstones, respectively. The Songpan−Garze block has a Neoproterozoic basement, and its basement is related to the basement of the Yangtze block. It is speculated that the granitic rocks in the study area are the products of partial melting of different crustal rocks induced by lithospheric delamination under the background of post−collision. The granitic rocks in the study area are the products of partial melting of different crustal rocks induced by lithospheric delamination under the background of post−collision.