班公湖-怒江缝合带东段丁青地幔橄榄岩成因:来自钻孔ZK02地幔橄榄岩矿物学及地球化学特征约束  

Genesis of Dingqing mantle peridotite in the eastern segment of the Bangong-Nujiang suture zone:Evidence from mineralogy and geochemistry of mantle peridotite from borehole ZK02

作　　者：徐向珍 熊发挥[1,2] 张承杰 陈建[4] 张然 闫金禹 杨经绥 XU XiangZhen;XIONG FaHui;ZHANG ChengJie;CHEN Jian;ZHANG Ran;YAN JinYu;YANG JingSui(Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;CARMA(Center for Advanced Research on Mantle),Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;The Second Geo-Exploration Institute of CMGB,Fuzhou 350108,China;The 4 th Geological Team of Hebei Geology and Mining Bureau,Chengde 067000,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China)

机构地区：[1]南方海洋科学与工程广东省实验室(广州),广州511458 [2]地幔研究中心,自然资源部深地动力学重点实验室,中国地质科学院地质研究所,北京100037 [3]中国冶金地质总局第二地质勘查院,福州350108 [4]河北省地矿局第四地质大队,承德067000 [5]南京大学地球科学与工程学院,南京210023

出　　处：《岩石学报》2021年第10期2944-2970,共27页Acta Petrologica Sinica

基　　金：南方海洋科学与工程广东省实验室(广州)人才团队引进重大专项(GML2019ZD0201);国家自然科学基金项目(41720104009,92062215,42172069);青藏高原第二次科学考察(2019QZKK0801);自然资源部深地动力学重点实验室自主研究课题(J1901-28);中国地质调查局地质调查项目(DD20190060)联合资助.

摘　　要：丁青蛇绿岩位于班公湖-怒江缝合带东段,是该缝合带出露面积最大的蛇绿岩。为查明岩体成因,在丁青东岩体中实施了一口165.19m的钻孔。除最顶部有约0.5m厚的第四系残坡积物外,其余均为地幔橄榄岩。结合显微镜鉴定将岩心划分出17个岩性单元层,岩性主要以方辉橄榄岩为主,夹少量纯橄岩和含铬铁矿纯橄岩。地幔橄榄岩中橄榄石的Fo变化于88.79~93.73,铬尖晶石的Cr^(#)变化于44.33~81.66,揭示丁青地幔橄榄岩可能经历过约20%~40%的中高度部分熔融作用;全岩地球化学分析表明其具有富镁(MgO=45.98%~49.45%)、贫铝(Al_(2)O_(3)=0.19%~1.37%)和贫钙(CaO=0.28%~0.70%)的特点,属于熔融程度较高的地幔残余物质。岩石具有明显不同于阿尔卑斯蛇绿岩的轻稀土元素富集特征,指示区内地幔橄榄岩先经历了较强程度的部分熔融,后经历了俯冲消减过程中的流体交代。利用地幔橄榄岩中的铬尖晶石成分计算母熔体Al_(2)O_(3)含量对应的FeO/MgO值,与不同构造环境原始岩浆成分相比较,发现丁青地幔橄榄岩母熔体大多处于玻安岩中。纯橄岩氧逸度估算FMQ=-3.05~-0.71,方辉橄榄岩氧逸度FMQ=-3.89~+1.47,显示丁青地幔橄榄岩有俯冲作用的参与。通过丁青钻孔岩心的研究,提出丁青东岩体可能形成于俯冲带之上的弧前环境这一观点。Dingqing ophiolite is located in the eastern segment of the Bangong-Nujiang suture zone,which is the largest exposed ophiolite in the suture zone.A 165.19m-deep borehole has been drilled in the eastern of the Dingqing ophiolite to help understanding the genesis of the mantle peridotite.The cores are consisted of about 0.5m thick Quaternary sediments in the upper part and the rest mantle peridotite which can be divided into 17 lithologic unit layers according to the microscopic identification.The mantle peridotites are mostly harzburgites mixed with a small amount of dunites and chromite-containing dunite.The Fo values of olivine(88.79~93.73)and the Cr^(#)of chromium spinel(44.33~81.66)suggest that the Dingqing mantle peridotite may have experienced about 20%~40%partial melting.All the petrochemical features,including rich MgO(45.98%~49.45%)and poor aluminum(Al_(2)O_(3)=0.19%~1.37%)and calcium(CaO=0.28%~0.70%),are attribute to the highly-melted residual mantle.Judging from the LREE enrichment that the mantle peridotite is obviously different from the Alpine ophiolite since it may experience strong partial melting and the subsequent fluid metasomatism during the subduction.The FeO/MgO_(2)value corresponding to the Al_(2)O_(3)content of the parent melt is calculated using the chromium spinel composition in the mantle peridotite,demonstrating that they mainly belong to the boninite comparing to the primitive magma in various tectonic environments.The FMQ values of oxygen fugacities of dunite and harzburgite are-3.05~-0.71 and-3.89~+1.47 respectively,indicating they have experienced the subduction process.Basing on the data of drilling core,it is proposed that the eastern of the Dingqing ophiolite may be formed in the forearc environment in the supra subduction zone.

关 键 词：地幔橄榄岩 全岩地球化学 氧逸度 丁青蛇绿岩 班公湖-怒江缝合带 

分 类 号：P588.125[天文地球—岩石学]