地质学报2017，Vol.91Issue(1)：111-131,21.
羌塘中西部红脊山地区蓝片岩Sr-Nd同位素与锆石U-Pb年龄特征及其构造意义
Zircon U-Pb Dating and Sr-Nd Isotopic Compositions of Hongjishan Blueschists in the Qiangtang Terrane, Northern Tibet and their Tectonic Implications
摘要
Abstract
Hongjishan mélange belt is an important part of the late Triassic high-pressure metamorphic belt in central Qiangtang,which was fromed due to the collision between Paleo-tethys and Qiangtang Block.In this paper we present the results of geochemical and isotopic geochronological investigations on Hongjishan blueschists from this belt.Geochemical analyses show that the two rock series,alkaline and tholeiitic basalts,have grossly similar bulk compositions,but they can be subdivided into two groups based on their TiO2contents.These metamafic rocks are alkaline basalts and have high TiO2 (2.86％ ～ 4.84％),are enriched in light rare earth elements (LREEs) with (La/Yb)N values of 11.42 ～ 20.05 and high field strength elements (HFSEs),REE distribution patterns and trace element abundances are similar to ocean island basalt (OIB).The low TiO2 (1.74％ ～ 1.81％) rocks are tholeiitic and have low rare earth element abundances(67.27 × 10-6～ 68.59 × 10-6) and display light REE (LREE) slightly enriched patterns with (La/Yb)N values of 2.49 ～ 2.81,are more akin to enriched mid-ocean ridge basalts (E-MORB).Sr-Nd isotopic compositions of the protolith are εNd(t)=-0.1 ～ 3.9,(87Sr/86Sr)i=0.704812 ～ 0.708365,suggests that the basaltic protoliths of blueschists were derived from a depleted mantle source.LA-ICP-MS Zircon U-Pb dating of blueschists from Hongjishan mélange belt in order to determine the age and to discuss the geotectonic background and tectonic magmatic events that Hongjishan complex has experienced.Results show zircons from analyzed sample present available Th/U ratio between 0.33 ～1.33,and these zircons show oscillatory and sector-zoning,typical of magmatic origin.Zircons from the blueschist show U-Pb ages in the range 283 ～ 312 Ma,that there are two well defined group ages,i.e.weighted average 206Pb /238U ages are 288.3 ±-1.9 Ma (n=15,MSWD=0.39) and 304.2 ± 2.3 Ma (n=14,MSWD=0.54),thus the age probably represents the formation age of the protolith of blueschis.Therefore the formation time of the protolith of Hongjishan blueschists are equal to the late Carboniferousearly Permian.Coupled with the regional geological facts and results of previous studies,it can be concluded that basaltic protoliths formed in the rift valley setting,with a plume-related source,have the same tectonic setting and dynamic mechanism as the Permian mafic dyke swarms in southern Qiangtang.This paper is more inclined to believe the Hongjishan simultaneously appeared magmatic activity association with the South Qiangtang mafic dyke incidents.The blueschist mafic protolith zircon U-Pb ages are rarely reported not only in the Hongjishan but also in the whole Qiangtang region,therefore determination of the magmatic activity time provides crucial evidence for describing the Paleo-tethys tectonic evolution process precisely in the Qiangtang area.关键词
羌塘/蓝片岩/地球化学/锆石U-Pb年龄/Sr-Nd同位素Key words
Qiangtang/Blueschist/Geochemistry/U-Pb geochronology/Sr-Nd isotopes引用本文复制引用
刘奎,鲁如魁,陈文郁,南国卫,谢林泽..羌塘中西部红脊山地区蓝片岩Sr-Nd同位素与锆石U-Pb年龄特征及其构造意义[J].地质学报,2017,91(1):111-131,21.基金项目
本文为国家自然科学基金项目(编号:41072159,41190073,41421002)联合资助的成果. （编号:41072159,41190073,41421002）
