蛇绿岩型铬铁矿床包壳纯橄榄岩中的流体过程印记:来自西藏雅鲁藏布江缝合带罗布莎和泽当岩体的地质学、岩石学和橄榄石晶体化学证据  

Imprints of fluid process of shell dunite in ophiolitic chromite deposits:evidences from geology,petrology and crystal chemistry of olivine found in Luobusa and Zedang ophiolites in the Yarlung Zangbo suture zone,Tibet

作　　者：罗照华[1] 江秀敏[1] 刘晓[1] 李重[1] 吴宗昌 井文超 LUO Zhaohua;JIANG Xiumin;LIU Xiao;LI Zhong;WU Zongchang;JING Wenchao(State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083

出　　处：《地学前缘》2019年第1期272-285,共14页Earth Science Frontiers

基　　金：中国地质调查局地质大调查项目(121201102000150069-01;1212011220921);青海省地质调查研究院项目(QHDDY201401)

摘　　要：岩浆型矿床一般认为是岩浆分异的产物,因为这类矿床通常缺乏强烈的近矿围岩蚀变。蛇绿岩中的豆荚状铬铁矿被认为是一种典型的岩浆型矿床,流行的成因模型包括岩浆通道模型和熔体-岩石反应模型。深部晶体群的大量发现,表明铬铁矿成矿系统不是一种理想系统,而是至少由两类子系统组成的复杂性动力系统。因此,流行模型不再适用,必须构建能够整合新证据的成因模型。这类矿床的典型地质特征是具有从方辉橄榄岩围岩经包壳纯橄榄岩到铬铁矿石的分带,且包壳纯橄榄岩与铬铁矿之间为渐变接触关系,包壳纯橄榄岩与方辉橄榄岩之间既可以为渐变接触关系,也可以为截然接触关系或侵入接触关系。因此,阐明纯橄榄岩的成因是理解豆荚状铬铁矿形成机制的关键。西藏雅鲁藏布江缝合带中罗布莎和泽当两个代表性超镁铁质杂岩体的新观察揭示:(1)包壳纯橄榄岩的出露宽度变化于厘米级到百米级,但岩石具有均匀的细粒结构,流行模型难以解释;(2)包壳纯橄榄岩可以划分为至少两种构造类型:块状纯橄榄岩和片理化纯橄榄岩,暗示了纯橄榄岩形成过程的多阶段特点;(3)包壳纯橄榄岩主要由变晶橄榄石组成,仅含有少量由熔体或流体析出的橄榄石晶体;(4)与方辉橄榄岩相比,包壳纯橄榄岩中的橄榄石具有高MgO、Cr2O3、CaO和低MnO、Al2O3的特点,展示了矛盾的晶体化学特征;(5)邻近铬铁矿体的纯橄榄岩中常见反豆状结构,类似于多相稀释流体流体制中紊流产生的中尺度结构。上述看似矛盾的证据表明包壳纯橄榄岩的形成过程有大量深部流体的参与,因而流体过程可以作为构建一个新的整合模型的基础。据此,文中提出一个熔体-流体流模型,其基本机制是溶解-沉淀反应Opx+Fluid→Ol±Sp±Cpx±Pl±SiO2(fluid),而基本前提则是深部还原流体的持续供给和熔体-流体流的快速上升�Magmatic deposits are generally regarded as the product of magmatic differentiation for lack of strong near-ore wall rock alteration.Ophiolitic podiform chromite deposits are typical magmatic deposits.Popular genetic models of podiform chromitities include the magma channel model and melt-rock interaction model.However,the discovery of a massive deep seated crystal population,indicated the chromite ore system is not an ideal but complex dynamic system with at least two subsystems.A different genetic model,therefore,is needed to account for the new evidences.One characteristic feature of chromite deposits is the zoning from harzburgite wall rock,through shell dunite,to chromitite,with progressive contacts between dunite and chromitite,and abrupt,progressive or intrusive contacts between shell dunite and harzburgite.Thus,the origin of dunite is key to the understanding of formation mechanism of podiform chromitities.The recent observations of two typical ultramafic complexes in Luobusa and Zedang in the Yarlung Zangbo suture zone revealed:(1)the width of shell dunite varies from centimeters to hundred meters,but the rock’s uniform fine-grained structure can not be explained by the popular models;(2)shell dunite can be classified into massive and schistose structures,which implies a multi-stage process in dunite formation;(3)shall dunite mainly consists of blastic olivine,with few olivine crystals crystallized from melt or fluid;(4)in contrast to harzburgite,olivine crystals in shell dunite are characterized by high MgO,Cr2O3,CaO and low MnO,Al2O3contents-two contradictory features;and(5)the anti-podiform texture,commonly found in dunite near the chromite ore body,resembles the mesoscale texture formed by turbulence in the multi dilute fluid flow.These seemingly contradictory observations suggest that dunite formation involves a deep seated fluid mass,hence fluid process could be a basis for building a new model.Based on this assumption,we proposed here a melt-fluid flow model,its mechanism is the dissolution-prec

关 键 词：包壳纯橄榄岩 豆荚状铬铁矿 橄榄石 熔体-流体流 西藏罗布莎蛇绿岩 

分 类 号：P588.125[天文地球—岩石学] P611.11[天文地球—地质学]