TANG Hong-wei1, 2, HOU Ming-cai1, YANG Wei1,2, YAN Lin2, ZHANG Jie2, CHEN Bei-zhan2, SHI Jun-bo2, YU Huai2, CHEN Lin2
(1. Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China;
2. 282 Brigade of Sichuan Nuclear Geology Bureau, Deyang 618000, China)
Abstract: The main Late Triassic intrusive rocks are porphyritic monzonitic granite and fine to medium grain sized granodiorite in the study area. The zircon U-Pb dating yielded an age of 204±1.7 Ma for the porphyritic monzonitic granite, indicating a product of Late Triassic magmatism. The lithogeochemical characteristics of these rocks reveal that they have Rittmann Index values (σ) of 1.70–2.71, The A/NK values of 1.22–1.85, and A/CNK values of 1.03–1.24, respectively. They belong to the high-K calc-alkalic- shoshonite series and are peraluminous granite. They are relatively enriched in LILE (Rb, K), obviously enriched in LREE, and depleted in Ba, Nb, Ta, Sr, P, Ti, with the relatively strong fractionation between LREE and HREE and negative Eu anomaly. The lithological and geochemical characteristics of intrusive rocks indicate that they belong to S-type granite. The εHf(t) values of -15.83–-3.24 and Hf isotopic two-stage model ages of 1444–2240 Ma for zircons from porphyritic monzonitic granite indicate that the granite could be formed by partial melting of the Mesoproterozoic-Paleoproteozoic crust with source rocks of major sandstone and minor mudstone. The fractional crystallization occurred during the magma evolution process. It is believed that these Late Triassic intrusive rocks could be products of the northward subduction of the Neo-Tethyan ocean. In the subduction process, the continuous upwelling of mantle-derived magma into the lower crust resulted in the partial melting of ancient crust materials to form the granitic magma. The time of subduction of the Neo-Tethyan ocean should be no later than 204 Ma.
Keywords: Gangdese belt; Late Triassic; intrusive rock; zircon U-Pb age; geochemistry; Hf isotope
ACTA MINERALOGICA SINICA Vol. 40, No.1, 2020, page 25-40
