东昆仑清水泉蛇纹岩中铬铁矿环带成因

东昆仑造山带清水泉超基性岩已蚀变成蛇纹岩,部分浸染状铬铁矿颗粒具有环带结构,这将为研究铬铁矿形成后所经历的变质过程及其寄主岩体的构造演化提供可靠的信息.通过对铬铁矿进行显微结构观察和电子探针分析得出:所研究铬铁矿的环带结构从核部到边缘依次为铝铬铁矿、高铁铬铁矿和铬磁铁矿,被绿泥石所包裹.从铝铬铁矿到高铁铬铁矿,Cr2O3,Al2O3和MgO含量下降,Fe2O3和FeO含量升高;Cr#,TiO2含量,YFe值以及Fe2+#值明显具有升高的趋势,而Mg#值却急剧下降.以上变化规律表明本文所研究的铬铁矿经历了由高温到低温,同时氧逸度上升的过程,并伴随有蛇纹石化作用、热液流体以及区域变质作用等改造,最终形成环带结构.此过程与铬铁矿寄主岩体经历的由地幔抬升至浅部地壳以及相关的变质作用过程相对应.

The genesis of zonal texture of chromites from the Qingshuiquan serpentinite in East Kunlun Mountains

Abstract: The ultrabasic rocks in the Qingshuiquan area of Eastern Kunlun Mountains have been altered into serpentinite, in which the zonal texture of some chromites provides important information concerning the alteration and evolutionary process of the rocks. The result of electron microprobe analysis shows that the girdle of chromite is composed of three parts, i.e., alumo-chromite, ferri-chromite and chrome-magnetite, from core to rim, respectively, and all of them were wrapped in chlorite. From the alumo-chromite to the ferri-chromite, the Cr2O3, Al2O3, MgO and Mg# values decrease, while the Fe2O3, FeO, TiO2 as well as Cr# and YFe values increase. The authors hold that, along with the serpentinization and regional metamorphism as well as hydrothermal fluid activity, the chromite went through a process in which the temperature was decreasing and the oxygen fugacity was increasing. And then the zonal texture was formed. This process corresponded to another one during which the host rock of the chromite moved upward from the mantle to the crust, accompanied by metamorphism.