西藏班公湖-怒江缝合带中段东巧豆荚状铬铁矿特征及构造背景  

The feature and tectonic setting of Dongqiao podiform chromitite in the central segment of the Bangong Lake-Nujiang suture zone in Tibet

作　　者：曹楚奇 杨经绥[1,2] 杨胜标 董玉飞 陈晓坚 熊发挥 卢雨潇[1] CAO ChuQi;YANG JingSui;YANG ShengBiao;DONG YuFei;CHEN XiaoJian;XIONG FaHui;LU YuXiao(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China;State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]南京大学地球科学与工程学院,南京210023 [3]中国科学院地球化学研究所,矿床地球化学国家重点实验室,贵阳550081

出　　处：《岩石学报》2022年第11期3391-3410,共20页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(92062215、41720104009);中国地质调查局地质调查项目(DD20221630)联合资助.

摘　　要：早侏罗世东巧蛇绿岩位于班公湖-怒江缝合带(班怒带)东段,蕴含较为丰富的豆荚状铬铁矿资源。东巧地幔橄榄岩主体由方辉橄榄岩组成,铬铁矿赋存在其内部的纯橄岩脉中。方辉橄榄岩和纯橄岩均显示出弧前橄榄岩的特征。方辉橄榄岩中橄榄石的Fo值为89.8~92.2,斜方辉石的和单斜辉石的Mg^(#)值分别变化于89.7~92.0和92.7~95.1,铬尖晶石的Cr^(#)值(Cr^(#)=100×Cr/(Cr+Al))为60.8~75.9;纯橄岩中橄榄石的Fo值为91.7~92.5,斜方辉石Mg^(#)值变化于91.7~92.1,单斜辉石的Mg^(#)值变化于94.0~94.6,铬尖晶石的Cr^(#)值为69.0~83.1。铬铁矿主要呈致密块状和浸染状构造,其中铬尖晶石的矿物包裹体有橄榄石、斜方辉石、单斜辉石、角闪石和铂族矿物等。矿石中的铬尖晶石与橄榄岩中的铬尖晶石相比,具有较高的Cr^(#)值(72.5~86.9)和Mg^(#)值(52.8~70.5),较低的Al_(2)O_(3)(6.25%~13.6%)、TiO_(2)(0.06%~0.16%)和Zn(518×10^(-6)~714×10^(-6)),属于高铬型铬铁矿,平衡熔体与玻安质熔体有亲缘性。方辉橄榄岩中铂族元素(PGE)总含量(14.01×10^(-9)~32.81×10^(-9))近似于原始地幔,IPGE(Os、Ir和Ru)/PPGE(Rh、Pt和Pd)的比值均大于1;纯橄岩的PGE总量(13.36×10^(-9)~16.08×10^(-9))略低于原始地幔,IPGE和PPGE富集程度近似;铬铁矿的铂族元素总量(108.4×10^(-9)~645.7×10^(-9))远远高于原始地幔和地幔橄榄岩中PGE的含量,且IPGE以及Rh相对原始地幔富集,而Pt和Pd相对亏损,具明显右倾特征的配分模式,指示东巧地幔橄榄岩和铬铁矿形成过程经历了熔体抽取和交代作用。通过与全球典型豆荚状铬铁矿矿床的特征对比,认为班怒带的蛇绿岩应该有良好的铬铁矿成矿背景。The Early Jurassic Dongqiao ophiolite is located in the central segment of the Bangong Lake-Nujiang suture zone,which contains comparatively abundant podiform chromitite resources.The Dongqiao mantle peridotite is dominated by harzburgite,which chromitite is endowed in the vein dunite inside it.Both harzburgite and dunite indicate the characteristics of fore-arc mantle peridotite.In harzburgite,the Fo value of olivine is 89.8~92.2,the Mg^(#) value of orthopyroxene is 89.7~92.0,while clinopyroxene is 92.7~95.1,and the Cr^(#) value(Cr^(#)=100×Cr/(Cr+Al))of Cr-spinel is 60.8~75.9;In dunite,the Fo value of olivine is 91.7~92.5,the Mg^(#) value range of orthopyroxene is 91.7~92.1,while the Mg^(#) value range of clinopyroxene is 94.0~94.6,and the Cr^(#) value of Cr-spinel is 69.0~83.1.The Dongqiao chromitite outcrops with dense massive and disseminated structures contain various mineral inclusions specifically including olivine,orthopyroxene,clinopyroxene,amphibole and platinum group minerals in the Cr-spinel.The Cr^(#) value(72.45~86.92)and Mg^(#) value(52.8~70.52)of Cr-spinel in Dongqiao chromitite are relatively high,while Al_(2)O_(3)(6.25%~13.55%),TiO_(2)(0.06%~0.16%)and Zn(518×10^(-6)~714×10^(-6))content are relatively low,indicating the high-Cr type,and the equilibrium melts show boninitic affinities.The whole-rock platinum group elements(PGE)range of harzburgite(14.01×10^(-9)~32.81×10^(-9))approximate the primitive mantle,while all values of IPGE(Ir,Ru and Pt)/PPGE(Os,Rh and Pd)>1;PGE contents of dunite(13.36×10^(-9)~16.08×10^(-9))are a little lower than the primitive mantle,while IPGE and PPGE contents are enriched to an approximate extent;PGE contents of chromitite(108.4×10^(-9)~645.7×10^(-9))are significantly higher than the PGE contents in the primitive mantle and mantle peridotite,which enriched IPGE and Rh as well as deficient Pt and Pd compared to the original mantle,show a negative slope distribution pattern,and indicating the melt extraction and metasomatism process during formation of the Dong

关 键 词：东巧豆荚状铬铁矿 班公湖-怒江缝合带 铂族元素 包裹体 玻安质熔体 

分 类 号：P588.125[天文地球—岩石学] P618.33[天文地球—地质学]