西藏羌塘中部荣玛地区含石榴石蓝片岩变质特征及其构造意义  

Garnet blueschist from Rongma area,central Qiangtang,North Tibet:metamorphic characteristics and implications of the tectonic evolution

作　　者：孟献真[1] 牛文超[2] 王根厚[3] 王行军 陶宏[5] 

机构地区：[1]安徽省地质测绘技术院,合肥230022 [2]天津地质调查中心,天津300170 [3]中国地质大学(北京),北京100083 [4]有色金属矿产地质调查中心,北京100012 [5]中国地震局地球物理勘探中心,郑州450002

出　　处：《地质调查与研究》2017年第3期169-177,共9页Geological Survey and Research

基　　金：国家地质调查项目"西藏1/5万双湖区角木日地区区域地质调查项目(1212010818014)"

摘　　要：荣玛地区含石榴石蓝片岩产出于西藏尼玛县荣玛乡北蓝岭一带。含石榴石蓝片岩中的石榴石具有核部富锰,边部富铁、镁的特点,蓝闪石具有环带结构,以此推断,本区的含石榴石蓝片岩先期经历了硬柱石榴辉岩相的变质,后期经历了退变质的蓝片岩相的叠加变质。根据温压估算,含石榴石蓝片岩的变质压力为0.69~0.75 GPa,变质温度为420~460℃。含石榴石蓝片岩的^(40)Ar/^(39)Ar测年年龄在221±6 Ma,证明此区的含石榴子石蓝片岩形成于二叠-晚三叠时期南北羌塘陆块之间的洋壳俯冲过程中,这一俯冲最终使得古特提斯洋闭合、南北羌塘陆块成为统一。A metamorphic belt exists within Qiangtang basin in northern Tibet, and blueschist and eclogite were distinguished within this belt. Rongma garnet blueschist is taken from Lanling mountains Nima Rongrna town in Tibet. The area where garnet blueschist is existed is a high-pressure metamorphic belt, presented as a whole strip, and the rock is composed by high pressure metamorphic rocks, mainly on glaucophane and garnet. The garnet in garnet blueschist has the characteristics of Mn-rich in core and Fe, Mg-rich in the edge. The glaucophane has band structure, so we can conclude that the early garnet blueschist experienced lawsonite eclogite facies metamorphism, and in later period garnet blueschist experienced retrograde superposition of blueschist facies metamorphism. According to the temperature and pressure texting, this area of garnet blueschist＇ s formation pressure is 0.69 - 0.75 GPa and formation temperature is 420 ~ 460 ℃. And the 40Ar/39Ar dating of garnet blueschist show that the age of the garnet blueschist is 221 ± 6 Ma, so we suggest that the garnet blueschist formed in the Permian - Late Triassic period when there is a cold subduction of an oceanic plate between northern and southern Qiangtang blocks, and this subduction of oceanic crust ultimately makes the Paleo-Tethys Ocean closed diving, and the North and the South Qiangtang landmass form a single land mass.

关 键 词：羌塘地体 石榴石蓝片岩 变质 俯冲 

分 类 号：P588.3[天文地球—岩石学]