新疆萨尔托海铬铁矿特征及大地构造环境

Characteristics and Tectonic Environment of Chromites from the Sartokay Ophiolite in Xinjiang

为探讨新疆萨尔托海蛇绿岩形成的物化条件和大地构造环境,选取萨尔托海蛇绿岩中块状铬铁矿内未蚀变的铬尖晶石进行矿物学和矿物地球化学分析。结果表明,铬铁矿成分均一,不发育环带结构;铬铁矿中的Cr_(2)O_(3)含量为33.68%~41.52%,Al_(2)O_(3)含量为2.82%~5.75%,TFeO含量为31.01%~47.59%,Cr^(#)为82.56~90.24,Mg#为52.22~70.89,Yfe为30.65~50.09,Fe^(2+#)为29.11~47.78,为富铁铬铁矿,其寄主岩石为蛇绿岩;铬铁矿结晶平均温度为1655.24℃,平均压力4.44 GPa,推断其形成深度约137.78 km;地幔氧逸度为FMQ+2.18~FMQ+2.63 log单位(平均值为FMQ+2.46log单位);地幔熔融程度23.17%~23.55%(平均23.40%)。推测萨尔托海赋矿岩石原岩石榴石二辉橄榄岩形成于亏损地幔。萨尔托海蛇绿岩形成的大地构造环境为洋内弧后盆地。

In order to address the physicochemical condition and tectonic environment for the formation of the Sarto-kay ophiolite in Xinjiang,The unaltered chromite spinel in massive chromite from the Sartokay ophiolite was selected for mineralogical and mineral geochemical analysis.Chromite is homogeneous in composition and it has no banded structure.The content of Cr_(2)O_(3)(33.68~41.52%),Al_(2)O_(3)(2.82%~5.75%)and FeOT(31.01%~47.59%),with Cr^(#)values of 82.56~90.24,Mg#values of 52.22~70.89,Yfe values of 30.65~50.09 and Fe^(2+#)values of 29.11~47.78.They are aluminumrich chromites.The estimated average temperature and pressure for crystallization of the chromite are 1655.24℃and 4.44 GPa,respectively,inferring its formation depth of about 137.78 km.It is inferred that the Sartokay chromite hosting peridotite could be derived from the garnet lherzolites in the depleted upper mantle with oxygen fugacities from FMQ+2.18 to FMQ+2.63 log units(average FMQ+2.46log units)and degrees of partial melting of the mantle from 23.17%to 23.55%(average23.40%).It is speculated that garnet lherzolite was formed in depleted mantle.The tectonic setting of the Sartokay ophiolite is an intra oceanic backarc basin.