XU Chang-hao¹, REN Fei^2*, LU Biao³

(1. Department of Geology College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
2.Chengdu Center of China Geological Survey, Chengdu 610081, China;
3.Hydrogeology and Engineering Geology Brigade of Chengdu, Sichuan Bureau of Geology & Mineral Resources, Chengdu 610072, China

Abstract: The northern Lancangjiang conjunction belt is believed to be an ideal natural laboratory to study the expansion, subduction, and closure of the paleo-Tethys ocean. The Jitang, Niuduo, and Dongdashan plutons,whose distribution is basically parallel to the conjunction belt, are thought to have recorded evolution history of the paleo-Lancangjiang ocean and thus are very important for understanding the evolution of the paleo-Tethys ocean. In this paper, we have carried out detailed petrography, zircon U-Pb geochronology (LA-ICP-MS), and geochemistry of the Niuduo pluton in the Lancangjiang conjunction belt. The results show that the weighted average zircon LA-ICP-MS U-Pb age of the Niuduo fine-grained monzonitic granite is 248.9±1.3 Ma（MSWD=1.1，n=30）, indicating that the monzonitic granite was formed in the Olenekian stage in Early Triassic. The IA/CNK ratios ranging from 1.06 to 1.45 and corundum molecule numbers ranging from 1.05 to 4.96 suggest that the monzonitic granite belongs to the peraluminous and high-K calc-alkaline series. The contents of trace elements of samples show that the Niuduo monzonitic granite is characterized with the relative enrichment of Rb, K, Th, U, Nd, Sm, and LREE but the relative depletion of Ba, Sr, Nb, Zr, Hf, P, Ti, and HREE. This feature is consistent with that of the S-type granite. The variation of U-Pb ages, varying from 262±1.3 Ma to 243.6±1.4 Ma, of zircons from the Niuduo monzonitic granite, may reveal the continuous evolution of magma during the subduction of the paleo-Tethys ocean. The comparison of the Lincang granite, whose zircon U-Pb ages varying from 261 Ma to 203 Ma, with the Newduo granite shows that these granites have similar diagenetic ages(e.g., Lincang: 261-203 Ma, Niuduo: from 262±1.3 to 243.6±1.4 Ma), petrogenesis (e.g., S-type), and magmatic evolution process, inferring that they have similar tectonic evolution process. Thus, it is believed that the paleo-Tethys ocean was closed in the eastern Tibet at 248.9±1.3 Ma. In addition, it is believed that the Niuduo fine-grained monzonitic granite could be formed by magma derived from the partial melting of metamorphosed mudstone resulted from the thickening and warming of the crust caused by the collisional orogeny, as well as the deep-seated heat flow caused by deep-seated fusion during the lithospheric shear and extensional period.

Keywords: the northern Lancangjiang conjunction belt; Tethys Ocean; fine-grained monzonitic granite; petrogenesis; tectonic significance

ACTA MINERALOGICA SINICA Vol. 40, No.3, 2020, page 237-247