藏南雅拉香波穹窿角闪片岩的年代学、地球化学及地质意义  

Geochronology, Geochemistry and Their Geological significances of Hornblende Schists from the Yardoi dome, Southern Tibet, China

作　　者：董磊[1] 李光明[1] 王燚 向安平[1] 曹华文[1] 黄波 DONG Lei;LI Guang-ming;WANG Yi;XIANG An-ping;CAO Hua-wen;HUANG Bo(Chengdu Center,China Geological Survey,Chengdu 610083,China;Sichuan Institute of Geological Survey,Chengdu 610081,China)

机构地区：[1]中国地质调查局成都地质调查中心,四川成都610083 [2]四川省地质调查院,四川成都610081

出　　处：《矿物学报》2020年第5期556-568,共13页Acta Mineralogica Sinica

基　　金：国家重点研发计划(编号:2018YFC060403,2016YFC0600308);中国地质调查局地质调查项目(编号:DD20190147)。

摘　　要：雅拉香波穹隆位于特提斯喜马拉雅构造带东部,其拆离系地层为曲德贡岩组,主要由千枚岩、片岩、变粒岩及少量片麻岩组成,通常认为其沉积时代为古生代。本文通过详细的野外工作,在其中新发现的一套与石榴子石绢云千枚岩互层产出的角闪片岩,薄片鉴定主要组成矿物为角闪石(65%~70%)、石英(25%~30%)与斜长石(约5%)。为查明角闪片岩的原岩性质、成岩时代以及构造背景,本文对角闪片岩展开了系统的岩石学、锆石U-Pb定年及全岩地球化学的研究。锆石测点的U-Pb同位素加权平均年龄可划分为>1000Ma、(525.4±8.7)Ma、(234.5±2.7)Ma及(47.1±0.5)Ma等4组,前2组代表了继承锆石的年龄,后2组分别代表了原岩结晶年龄及后期变质事件的年龄。岩石地球化学分析表明,角闪片岩具有较高的FeOT(13.46%~13.74%)、P2O5(0.38%~0.51%)以及Ti O2(4.26%~4.68%),轻重稀土具中等程度分异((La/Yb)N平均7.87),Ti/V和Nb/Y比值较高,富集高场强元素(如Nb、Zr),具有较弱的Ce负异常(δCe=0.89~0.96)与Eu异常(δEu=0.94~1.07)。原岩恢复及构造判别显示,角闪片岩的原岩是具有OIB型地球化学特征的板内碱性玄武岩。结合曲德贡岩组的年龄资料和区域大地构造背景,认为中晚三叠世的(234.5±2.7 Ma)角闪片岩原岩结晶年龄限定了曲德贡岩组顶部的沉积时限,显示出曲德贡岩组上岩段的部分地层可能解体为中生代地层;角闪片岩原岩形成于被动大陆边缘裂谷背景中,可能是由于岩石圈发生伸展-减薄,软流圈上涌与岩石圈地幔物质混合而形成的产物。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%), P2 O5(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 Mesozoi

关 键 词：雅拉香波 曲德贡岩组 角闪片岩 年代学 地球化学 板内玄武岩 藏南 

分 类 号：P581[天文地球—岩石学] P597[天文地球—地质学]