中喜马拉雅陈塘地区基性麻粒岩变质作用、原岩特征及其地质意义  

Metamorphic evolution,protolith and tectonic implications of mafic granulite from the Chentang area,Central Himalaya

作　　者：王淑真 张贵宾[1] 刘帅奇 WANG ShuZhen;ZHANG GuiBin;LIU ShuaiQi(MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871

出　　处：《岩石学报》2024年第5期1603-1627,共25页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42372061、91755206、41972056、41622202)资助

摘　　要：本文对中喜马拉雅陈塘地区高压基性麻粒岩进行了岩石学、地球化学和锆石年代学及Hf同位素研究,确定了其变质P-T演化、原岩性质和构造背景。陈塘地区基性麻粒岩发生了明显部分熔融,基质中保存有单斜辉石和斜长石后成合晶,指示其可能经历了榴辉岩相变质作用。根据基性麻粒岩的矿物组合识别出三个变质演化阶段:高压麻粒岩相矿物组合为石榴子石+单斜辉石+斜长石+金红石+石英,温压条件为1.0~1.3GPa、730~800℃;中压麻粒岩相矿物组合为石榴子石+单斜辉石+斜方辉石+角闪石+斜长石+钛铁矿+金红石+石英,温压条件为0.78~0.82GPa、820~890℃;而角闪岩相阶段以角闪石的大量出现为特征,变质温压条件为0.6~0.7GPa、660~740℃。锆石U-Pb年代学指示基性麻粒岩高压麻粒岩相的变质时代为15~14Ma,原岩时代为~1800Ma。另外,基于TiO_(2)含量可将基性麻粒岩划分为两组,一组具有低的TiO_(2)和高的ε_(Nd)(t),稀土元素配分模式与E-MORB一致,原岩为弧后盆地玄武岩。第二组基性麻粒岩相比于第一组,具有高的TiO_(2)、REE、Th/Ta和低的Mg^(#)、Nb/La和ε_(Nd)(t),其稀土元素配分模式与OIB一致。地球化学特征表明高Ti基性麻粒岩的原岩经历了显著的地壳混染,其形成与低Ti组基性麻粒岩原始岩浆上升过程混染壳源物质有关。中喜马拉雅Ama Drime地体-陈塘地区-Arun河谷沿南北方向上均出露的古元古代岩浆记录可能与新生代裂谷活动导致喜马拉雅造山带古元古代基底剥露至地表有关。另外,本研究证实印度大陆北缘在古元古代(1.9~1.8Ga)处于汇聚型板块边缘,伴生有广泛的弧/弧后岩浆作用,而非裂谷环境。This paper presents petrography,bulk-rock geochemistry,zircon U-Pb dating,and Hf isotope studies on high-pressure mafic granulites from the Chentang area,Central Himalaya.We identify metamorphic evolution and protolith features of mafic granulites and thus explore the tectonic significance.The mafic granulites showed obvious melting features in outcrops,symplectites of clinopyroxene and plagioclase were preserved which might be the decompression products of eclogite-facies metamorphism.The metamorphism of the mafic granulites was divided into three stages:(1)high-pressure granulite-facies metamorphism consists of Grt-Cpx-Pl-Rt-Qz under P-T conditions of 1.0~1.3GPa and 730~800℃;(2)mid-pressure granulite-facies metamorphism consists of Grt-Cpx-Opx-Pl-Ilm-Rt-Qz at 0.78~0.82GPa and 820~890℃;(3)amphibolite-facies metamorphism was characterized by wide occurrence of amphibole,under 0.6~0.7GPa and 660~740℃.Zircon U-Pb dating indicates that the high-pressure granulite-facies metamorphism occurred at 15~14Ma and the protolith age was~1800Ma.Based on the bulk-rock TiO_(2)content,the mafic granulites can be divided into two groups.Group I exhibits low TiO_(2)but highε_(Nd)(t)and shows E-MORB REE patterns,which were associated with the back-arc setting.GroupⅡshows OIB alike REE pattern and has TiO_(2),REE,Th/Ta,and lower Mg^(#),Nb/La,andε_(Nd)(t).Geochemical evidence suggests that GroupⅡshows a similar magma source to GroupⅠbut underwent significant crustal contamination during magma ascent.The Paleoproterozoic magmatic records were spread along the north-south direction,from the north Ama Drime Massif to Chentang,to the south Arun Valley.The exposure of the Paleoproterozoic records which were widely distributed in the Lesser Himalayan Sequence were related to the Cenozoic rift activity.Besides,this study confirms that the northern Indian margin was at the convergent plate boundary with widespread arc/back-arc magmatism rather than in a rift setting during the Paleoproterozoic(1.9~1.8Ga)era.

关 键 词：中喜马拉雅 基性麻粒岩 变质作用 原岩特征 构造意义 

分 类 号：P588.347[天文地球—岩石学] P597.3[天文地球—地质学]