柴达木盆地北缘开屏沟纯橄岩中铬铁矿环带成因及其构造意义

Genesis of the zonal texture of chromite from the Kaipinggou peridotite in the northern margin of Qaidam basin and its tectonic significance

开屏沟纯橄岩位于柴达木盆地北缘西段,岩石发生强烈的蛇纹石化,因此很难解析其性质和形成环境,本文通过对橄榄岩内发育的铬铁矿进行研究,探讨纯橄岩的性质及形成过程。岩相学观察和电子探针测试表明,铬铁矿具有明显核边结构,核部为铝铬铁矿,具有相对高Al_2O_3,低FeO^T、TiO_2、Cr~#特点,指示寄主原岩形成于俯冲带(SSZ)环境,寄主原岩为俯冲带橄揽岩。核部铝铬铁矿为岩浆型铬铁矿,通过计算得到其结晶温度平均为1372℃,结晶压力平均为2.96GPa,Δlgf_(O2)平均为-1.42,表明其形成于地幔软流圈。边部为高铁铬铁矿,具有低Al_2O_3,高FeO^T、TiO_2、Cr~#特征,指示铬铁矿边部受到蛇纹石化蚀变与富Fe流体的共同作用。综上,推测开屏沟纯橄岩原岩形成于SSZ环境,后期受到流体交代改造发生蛇纹石化,最终暴露地表。

The Kaipinggou peridotite is located in the western part of the northern Qaidam Basin. The peridotite experienced strong serpentinization, and therefore, it is difficult to determine its property and formation environment. Furthermore, the local enrichment of Ni in the peridotite with Ni=0.15%~0.23% indicates that metallogenic mechanism is rather complicated. A detail analysis was performed on chromite from the peridotite to understand the properties of the peridotite and the mechanism of nickel mineralization. Petrographic observation and electron microprobe analyses show that chromite is characterized by a distinct core-rim structure with alumchromite at the core, and relatively high Al2O3, low FeOT, TiO2 and Cr#. These features indicate that the host rock formed in the subduction zone(SSZ). Alumchromite at the core is magmatic in origin. Calculation shows that it has an average crystallization temperature of 1372℃, an average crystallization pressure of 2.96 GPa and an average ΔlgfO2 of-1.42, indicating that it formed within the mantle asthenosphere. High-iron chromite around the rim is characterized by low Al2O3, high FeOT, TiO2 and Cr#, indicating that the rim of the chromite was affected by serpentinization alteration and Fe-rich fluids. In summary, it can be speculated that the protolith of the Kaipinggou peridotite was formed in the subduction zone, subsequently transformed into serpentine due to fluid metasomatism and modification, and finally uplifted to the surface.