西藏雅鲁藏布江缝合带普兰地幔橄榄岩铬尖晶石成因研究  

Genesis of chromian spinels in the Purang mantle peridotites of the Yarlung-Zangbo Suture Zone,Tibet

作　　者：赵斌 龚小晗 黄启帅[1] 史仁灯[1,4] ZHAO Bin;GONG XiaoHan;HUANG QiShuai;SHI RenDeng(Key Laboratory of Continental Collision and Plateau Uplift,Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,China;University of Chinese Academy of Sciences,Beijing 100049,China;China University of Geosciences,Beijing 100083,China;CAS Center for Excellence in Tibetan Plateau Earth Sciences,Beijing 100101,China)

机构地区：[1]中国科学院青藏高原研究所大陆碰撞与高原隆升重点实验室,北京100101 [2]中国科学院大学,北京100049 [3]中国地质大学,北京100083 [4]中国科学院青藏高原地球科学卓越创新中心,北京100101

出　　处：《岩石学报》2020年第2期455-468,共14页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41672054、41972052)资助

摘　　要：普兰蛇绿岩位于雅鲁藏布江缝合带西段,其中地幔橄榄岩由方辉橄榄岩、含单斜辉石方辉橄榄岩以及少量二辉橄榄岩及纯橄岩组成。尖晶石是地幔橄榄岩中常见的副矿物,可以作为重要的岩石学成因指示剂。在野外地质调查基础上,通过岩相观察、电子探针、尖晶石成分面分析、电子背反射衍射分析,可将普兰地幔橄榄岩铬尖晶石分为三类:第一类铬尖晶石呈自形,粒径较小(<100μm),或包裹于斜方辉石中,或杂乱分布于橄榄石和辉石之间,具有高Cr^#(>0.6)、低Mg^#(0.43~0.57)的特征,为部分熔融+玻安质熔体交代成因;第二类铬尖晶石呈半自形-他形,粒径较大(>100μm),常含有橄榄石、辉石包裹体,具有中Cr^#(0.17~0.42)、高Mg^#(0.63~0.77)的特点,主要受部分熔融作用影响;第三类铬尖晶石呈他形蠕虫状与辉石交生在一起构成后成合晶结构,粒径变化较大,具有低Cr#(0.17~0.28)、高Mg^#(0.67~0.77)的特点。EBSD分析结果显示尖晶石、辉石的结晶学优选方位(CPO)较为相似,表明为同一矿物分解而来,单斜辉石与大陆岩石圈地幔捕掳体中石榴子石的稀土元素对比表明构成后成合晶结构的辉石和铬尖晶石为具有大陆岩石圈地幔属性的高压石榴子石退变分解而成。综合分析表明:普兰蛇绿岩的地幔橄榄岩体在从石榴子石相深度上升过程中发生了石榴子石退变、岩石部分熔融及熔体渗透作用,岩体经历了威尔逊旋回初期的大陆裂谷阶段,主体经历了中-低程度的部分熔融,类似大洋中脊环境,局部受到了富硅、富镁玻安质熔体的影响。The Purang mantle peridotites,located in the western Yarlung Zangbo Suture Zone,are mainly composed of harzburgite,Cpx-harzburgite and a small amount of lherzolite and dunite.As an accessory mineral in the mantle peridotites,the chromian spinel can be used to be an important petrological indicator.Based on geological survey,the chromian spinels from Purang mantle peridotites are divided into three categories through lithographic observation in detail,electron probe,elemental distribution mapping of spinels and Electron Back Scattered Diffraction(EBSD)analysis.Type I chromian spinels are euhedral crystal with a small size(<100μm),which are distributed in large particles of orthopyroxenes or between olivines and pyroxenes.With the character of high Cr^#(>0.6)and low Mg^#(0.43~0.57),they were caused by boninitic melt metasomatism after partial melting.TypeⅡchromian spinels show the characters of subhedral-irregular crystal and large particle size(>100μm).The typeⅡspinels with medium Cr^#(0.17~0.42)and high Mg^#(0.63~0.77)were shaped by partial melting of the mantle.Type Ⅲ spinels that show the characteristics of low Cr^#(0.17~0.28)and high Mg^#(0.67~0.77)are part of symplectite(rod-shaped,vermicular spinels closely associated with pyroxenes),which form crystal structure with a large range in particle size.EBSD analysis for the constituent minerals of the symplectite shows similar Crystallographic Preferred Orientation(CPO),which suggests that they represent the decomposition products of a precursor mineral.The similarity character of REE distribution between clinopyroxenes in symplectite and garnets from continental lithospheric mantle xenolith suggests that both the spinels and pyroxenes in symplectite were decomposed by high-pressure garnet(majorite)which show the character of ancient sub-continental lithospheric mantle.As a result,it is speculated that the process of garnet degeneration,partial melting and melt permeation occurred in the upwelling process of the Purang mantle peridotites from the depth of

关 键 词：铬尖晶石 部分熔融 熔体渗透 石榴子石分解 普兰地幔橄榄岩 

分 类 号：P578.46[天文地球—矿物学] P588.125[天文地球—地质学]