DONG Lei1, LI Guang-ming1*, WANG Yi2, XIANG An-ping1, CAO Hua-wen1, HUANG Bo2
(1. Chengdu Center, China Geological Survey, Chengdu 610083, China;
2. Sichuan Institute of Geological Survey, Chengdu 610081, China)
Abstract: The Yalaxiangbo (Yardoi) dome is located in the eastern part of the Tethyan Himalaya tectonic belt, with its detachment system of an undivided Paleozoic Qudegong Rock Formation, which mainly consists of phyllite, schist, granulite and minor gneiss. In this paper, based on the detailed field work, a suite of hornblende schist interbedded with garnet sericite phyllite in the Qudegong Rock Formation has been identified. The hornblende schist is mainly composed of hornblende (65%－70%) and quartz (25%－30%) with minor plagioclase (5%±). Systematic petrology, geochemical analyses, and LA-ICP-MS zircon U-Pb dating of the are carried out to investigate the protolith, formation age, and tectonic setting of the hornblende schist. The LA-ICP-MS zircon U-Pb dating results show that four weighted mean ages including ＞1000 Ma, 525.4±8.7 Ma, 234.5±2.7 Ma and 47.1±0.5 Ma have been obtained. It is believed that the ages of ＞1000 Ma and 525.4±8.7 Ma could represent ages of inherited zircons, while ages of 234.5±2.7 Ma and 47.1±0.5 Ma could represent the crystallization age of metabasalt and the age of the later metamorphic event, respectively. Analyses of rock geochemistry indicate that the hornblende schists are characterized with relatively high contents of major elements including FeOT (13.46%－13.74%), P2O5 (0.38%－0.51%), and TiO2 (4.26%－4.68%), with medium fractionation of LREE and HREE ((La/Yb)N average of 7.87), enriched HSFE (such as Nb, Zr), relatively high ratios of Ti/V and Nb/Y, relatively weak Ce (δCe values of 0.89－0.96) and Eu (δEu values of 0.94－1.07) anomalies. The study reveals that the protolith of the hornblende schists belong to the intraplate alkaline basalt with geochemical features of the OIB-type basalt. Combined with age data and regional tectonic setting of the Qudegong Rock Formation, we have believed that the data of the 234.5±2.7 Ma limits the sedimentary time of the top part of Qudegong Rock Formation, indicating that part of the upper member of the Qudegong Rock Formation could be sourced from Mesozoic strata. The protolith of hornblende schist could be formed in a rift setting of continental margin, as a product of the mixing of upwelling asthenosphere-derived melts and lithospheric mantle-derived components due to the extension and thinning of the lithosphere.
Keywords: Yardoi dome, Qudegong Rock Formation, hornblende schist, geochronology, geochemistry, intraplate basalt, Southern Tibet
ACTA MINERALOGICA SINICA Vol. 40, No.5, 2020, page 556-568
