怒江构造带东段同卡蛇绿混杂岩中变玄武岩地球化学特征和角闪石^(40)Ar-^(39)Ar年龄  

Geochemical analysis and amphibole ^(40)Ar- ^(39)Ar dating for meta-basalts from Tongka ophiolitic mélange,eastern Nujiang belt.Geological Bulletin of China

作　　者：李化启[1] 李天福[1] 吉风宝 LI Huaqi;LI Tianfu;JI Fengbao(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Tibet Museum of Natural Science,Lhasa 825000,Tibet,China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]西藏自然科学博物馆,西藏拉萨825000

出　　处：《地质通报》2021年第8期1279-1290,共12页Geological Bulletin of China

基　　金：国家自然科学基金项目《藏东八宿变质杂岩带:对怒江构造带中东段早侏罗世弧-陆造山的启示》(批准号:41772216);中央级公益性科研院所基本科研业务费项目《西藏安多微地体早中生代构造热事件探究》(编号:J1910);中国地质调查局项目《拉萨-腾冲构造岩浆带区域地质调查》(编号:DD20190060)。

摘　　要：青藏高原中部班公湖-怒江缝合带所代表古洋盆的性质和早期演化仍存在很多争议。为探讨怒江构造带中-东段早侏罗世构造-岩浆事件的性质和微地体构造、造山格局,对怒江带东段分割八宿微地体的同卡蛇绿混杂岩中的变玄武岩进行了岩石地球化学分析和角闪石^(40)Ar-^(39)Ar年代学研究。同卡蛇绿混杂岩中的变玄武岩经历了强烈变质变形,显示绿帘角闪岩相的矿物学组成。岩石地球化学分析表明,变玄武岩的稀土元素配分模式为左倾的轻稀土元素亏损型,显示正常洋中脊玄武岩(N-MORB)特征。但在N-MORB标准化的微量元素配分图解上,样品又显示Rb、Ba、Th、U、Pb、Sr的富集和Nb、Ta的亏损,表明变玄武岩样品具有N-MORB和岛弧的双重特征,形成于不成熟的弧后盆地扩张中心,为SSZ型蛇绿岩的组分。同卡变玄武岩获得角闪石^(40)Ar-^(39)Ar坪年龄为161±2 Ma,与邻近同卡微陆块中石榴矽线黑云斜长片麻岩和怒江带中段安多黑云二长片麻岩中的黑云母^(40)Ar-^(39)Ar冷却年龄(165~167 Ma)相近,从蛇绿岩构造就位的角度,为约束怒江构造带东段次级洋盆的闭合时限和嘉玉桥变质地体于侏罗纪早期沿同卡—俄学一线与同卡微陆块碰撞拼贴提供了直接证据。通过对怒江构造带北缘早、晚侏罗世构造-岩浆事件的综合分析,认为班公湖-怒江构造带不同区段、不同分支的演化、俯冲造山极性及造山时限可能有所不同。这个类似多岛-增生造山的模式表明,青藏高原腹地的造山可能是一系列不同规模、不同性质的地质体沿不同层级的洋盆渐次消减、渐次拼贴的结果,并不一定存在那种古特提斯大洋关闭后中特提斯大洋再接着打开的过程。The nature and early evolution of the ancient ocean basin represented by the Bangongco-Nujiang suture zone in the central Qinghai-Tibetan Plateau are still controversial.To identify the nature of the Early Jurassic tectono-magmatic events and micro-continental tectonics and orogenic processes along the mid-eastern Nujiang suture,geochemical study on meta-basaltic rocks and^(40)Ar-^(39)Ar dating of amphibole from the Tongka ophiolitic mélange,eastern Nujiang belt,were conducted.The meta-basalts in the Tongka ophiolitic mélange witnessed strong metamorphic deformation,indicating the mineralogical composition of the epidote-amphibolite facies.Rare earth element(REE)patterns of the meta-basaltic rocks on the chondrite-normalized REE diagram display LREE depletions indicative of N-MORB,whereas on the N-MORB-normalized spider diagram,the samples indicate Rb,Ba,Th,U,Pb and Sr enrichments and Nb and Ta depletions,which suggest a mixing characteristic of arc and N-MORB,and an origin from back-arc basin spreading center,indicating a SSZ affinity.The^(40)Ar-^(39)Ar dating of the amphibole from a meta-basalt sample yields a plateau age of 161±2 Ma,analogous to^(40)Ar-^(39)Ar cooling ages(165~167 Ma)of biotite from the Tongka and Amdo gneisses,which provides direct evidence from ophiolite emplacement for local closure of the eastern Nujiang Ocean and Early Jurassic collision along the Tongka-Exue suture between the Jiayuqiao and Tongka micro-blocks.On the basis of comprehensive analyses of the early and late Jurassic magmatic rocks north of the Bangong-Nujiang belt,the Bangong-Nujiang belt is assumed to have distinct evolutionary histories,subduction polarities and collisional timing at different segments and ophiolitic branches.The multi-island-accretive orogeny model suggests that the orogeny in the hinterland of the Qinghai-Tibet Plateau might be the result of varying level convergences by diverse-sized terranes/blocks with consumption of different oceanic basins,rather than the re-opening process of the Meso-Tethyan Oc

关 键 词：怒江构造带 变玄武岩 八宿微地体 安多微地体 早侏罗世 

分 类 号：P534.52[天文地球—古生物学与地层学] P597[天文地球—地质学]