南天山满大勒克蛇绿岩铬铁矿矿物学特征及其意义

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,并具有豆英状特征.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2.76～3.00 GPa,平均2.87 GPa;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-2.14 log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慢,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 Crspinel crystal represents the provenance where the mineral crystallized.The authors thus chose cores of the Crspinel 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 1359℃ to 1394℃ , 1379℃ 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 reach the crustal surface, where they were subjected to regional metamorphism (higher oxygen fugacity).