尖晶石是地幔橄榄岩中一种非常重要的矿物,虽然含量很低,却可以作为其寄主岩石——地幔橄榄岩——的成因指示剂.普兰岩体东部铬尖晶石分布广泛,几乎在所有岩石类型中都有出现.通过矿物学特征的研究,认为尖晶石可分为两类,一类较自形,颜色较深；另一类多呈不规则状和蠕虫状,颜色较浅.根据其化学特征,又可分为富铬型的尖晶石(Cr#＞60)和富铝型的尖晶石(Cr#＜60),富铬型尖晶石主要出现在纯橄岩、蛇纹岩和辉石岩中,富铝型尖晶石主要出现在二辉橄榄岩中.其中,方辉橄榄岩中的尖晶石形态、化学成分变化比较大,说明它经历了较为复杂的形成和变化过程.通过对尖晶石矿物学和化学特征的综合研究,认为该区尖晶石具有深海橄榄岩和上部俯冲板片(SSZ)构造环境橄榄岩特征,普兰岩体可能先后经历了MOR和SSZ两种构造环境.%Spinel is an important mineral in mantle peridotite, and probably acts as a petrogenetic indicator of its host rock though it's low in volume. Chrome spinel are widely distributed in almost all rocks of the eastern Purang ultramafic rocks, Xizang ( Tibet) . In terms of mineralogy characteristics, spinels can be divided into two groups. One group exhibits euhedral to subhedral grains with dark brown color, the other one shows irregular and vermicular shape with light brown color. On the other hand, the spinels can also be divided into chromium-rich spinels( Cr# > 60) and aluminum-rich spinels( Cr#<60)in the light of their chemical compositions. The former one mainly occurs in dunite, serpentinite and pyroxenite, and the latter species exists in lherzolite. However, the spinel in harzburgite is apparently different with regard to the morphology and chemical characteristics from that in other rocks, which indicates that it experienced complicated and variable geological processes. Based on the comprehensive studies on spinel mineralogy and chemical composition, we conclude that spinels in the Purang ultramafic rocks are of characteristics of both abyssal and supra-subduction zone ( SSZ) setting.
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