南天山满大勒克蛇绿岩铬铁矿矿物学特征及其意义  

Mineralogy of chromites in Mandaleke ophiolite of South Tianshan Mountains and its geological implications

作　　者：张炜斌[1] 张东阳[1] 张招崇[1] 黄河[1] 赵莉[1] 

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：《岩石矿物学杂志》2011年第2期243-258,共16页Acta Petrologica et Mineralogica

基　　金：国家科技支撑项目(2007BAB25B05);111计划(B07011);教育部创新团队计划资助项目

摘　　要：满大勒克蛇绿岩属于南天山晚古生代蛇绿岩带,主要由蛇纹石化橄榄岩、玄武岩和含放射虫硅质岩组成。其中蛇纹岩富含铬铁矿,并具有豆荚状特征。显微镜观察和电子探针分析表明,铬铁矿具有明显的环带结构,从核部到边部呈现出由铬尖晶石到铬铁矿到磁铁矿的变化特征,元素组成上具有由富Al和Cr、贫Fe到贫Al和Cr、富Fe的变化趋势。利用铬铁矿(铬尖晶石)核部矿物的化学成分,得出铬尖晶石的结晶温度为1359～1394℃,平均1379℃;压力为2.76～3.00GPa,平均2.87GPa;相对于FMQ缓冲剂的地幔氧逸度为FMQ-1.50～FMQ-2.92log单位,平均值为FMQ-2.14log单位;地幔熔融程度F为16.7%～18.0%。结合已有研究成果,推测满大勒克蛇绿岩的基底橄榄岩单元源区为石榴石二辉橄榄岩,形成于亏损的软流圈地幔,对应的大地构造位置为弧前盆地。研究区铬尖晶石的环带特征显示其经历了温度由高到低、氧逸度由低到高的演变,反映了满大勒克蛇绿岩套的橄榄岩单元经历了高温高压的地幔环境(低氧逸度),之后随构造变动被推覆到地壳,定位之后又经历了区域变质作用的演化过程(高氧逸度)。The Mandaleke ophiolite belongs to the late Paleozoic ophiolite belt in South Tianshan Mountains and is composed of serpentinized peridotites,basalts and radiolarian cherts.The chromite or Cr-spinel occurs in the serpentinized peridotites as a ubiquitous accessory mineral,some of which has the podiform texture.Petrographic observation and electron microprobe analysis indicate that the Cr-spinel grains display obvious composition zoning,suggesting the path of such thermal events as crystallization,upwelling and alteration.The core of the Cr-spinel crystal represents the provenance where the mineral crystallized.The authors thus chose cores of the Cr-spinel to trace the nature of the mantle and the whole crystals to deduce the thermal events that the host rocks have experienced.On the basis of olivine-spinel geothermometry,the crystallized temperature of the anhedral core is estimated to be from 1 359℃ to 1 394℃,1 379℃ on average.Similarly,according to the Cr-spinel geobarometer,the mantle pressure is estimated to be from 2.76 GPa to 3.00 GPa,2.87 GPa on average,while the oxygen fugacity relative to FMQ buffer is FMQ-1.50～ FMQ-2.92 log units,FMQ-2.14 log units on average.Furthermore,the degree of partial melting of the mantle is estimated to be from 16.7% to 18.0%,17.3% on average,in the light of its relationship with Cr#.These parameters provide some clues of the physicochemical conditions：these peridotites were derived from garnet lherzolites located in the asthenosphere of the upper mantle,and the incompatible element-depleted mantle might have resulted from previous partial melting.The tectono-discrimination diagrams imply that the Mandaleke ophiolite was formed in a fore-arc setting.The compositional zoning suggests that the peridotites have undergone a thermal path of decreasing temperature and increasing oxygen fugacity.It can be inferred that the mantle peridotites experienced the process from a high temperature and high pressure mantle（lower oxygen fugacity）stage to a thrusting slice stage to

关 键 词：铬尖晶石 成分分带 物理化学条件 构造背景 

分 类 号：P578.46[天文地球—矿物学] P588.125[天文地球—地质学]