新疆洪古勒楞蛇绿岩中铬尖晶石的化学成分变异及其意义

本文应用了洪古勒楞蛇绿岩中铬尖晶石的电子探针分析数据,讨论该蛇绿岩中不同岩石中铬尖晶石化学成分特征,显示出铬尖晶石的成分与寄主岩石有关。基本可将其分为三组:造矿铬尖晶石、堆积岩副矿物铬尖晶石和地幔橄榄岩副矿物铬尖晶石。它们在主要成分和微量成分方面均存在区别,化学成分的相关性也不一样。共生橄榄石-铬尖晶石对的元素分配说明二者基本为平衡矿物。

VARIATIONS IN CHEMICAL COMPOSITION OF CHROME SPINELS FROM HONGGULELENG OPHIOLITE, XINJIANG AND THEIR SIGNIFICANCE

Variations in chemical composition of 116 chrome spinel samples from the ophiolite are discussed in this paper. The chemical composition of associated chrome spinels from the ophiolite are related to their host-rocks. The chrome spinels in cumulates are rich in FeO and TiO_2, but those in harzburgites are rich in MgO and Al_2O_3.The chemical composition of ore-forming chrome spinels is similar to that of harzburgites, but the former is richer in MgO, Cr_2O_3 and Al_2O_3 than the latter. In the Cr_2O_3-MgO-Al_2O_3-TFeO diagram, chrome spinels from the ophiolite show two variation trends and so do,ore-forming and associated chrome spinels in the harzburgites. However,chrome spinels in the cumulates show an opposite trend. Statistical analysis indicates that in terms of their chemical composition chrome spinels can be divided into three groups:(1) Mg-Al, (2) Fe-Ti and (3) Cr. On the (FeO+ TiO_2)- (MgO+ Al_2O_3)-Cr_2O_3 diagram, chrome spinels from different host-rocks are distinguished from one another. The authors have proposed the discriminating equations for ore-forming and associated Cr-spinels in the cumulates and harzburgites.The variation in chemical composition of coexisting olivine-Cr-spinel indicates that they are minerals in equilibrium with each other.The cumulates formed in the magma chamber are different from peridotite of upper mantle origin.