中天山地块科马提质超镁铁岩体的发现:新疆峡东岩体

Discovery of komatiitic ultramafic intrusion in Mid-Tianshan terrain; Xiadong intrusion, Xinjiang.

新疆和甘肃交界处的峡东岩体是以纯橄岩为主的超镁铁质杂岩体,经过全岩主量元素分析和橄榄石的探针分析,发现纯橄岩具有很高的MgO含量(36%～39%),橄榄石的镁值(Fo值)在90～96之间。通过反演原始岩浆成分的方法得到原始岩浆中MgO=28%、FeO=9%,属于科马提质岩浆。通过模拟初始岩浆的结晶分异过程,峡东岩体是科马提质岩浆发生高程度结晶分异的产物。利用MELTS软件计算原始岩浆的演化过程,得到初始岩浆的液态温度为1703℃,在1649℃和1555℃时分别开始结晶铬尖晶石和橄榄石,在1093℃开始结晶单斜辉石,这一结晶顺序与峡东岩体的矿物组合是一致的,也说明峡东岩体是科马提质岩浆高度结晶分异的产物。通过计算得到初始岩浆为原始地幔发生24%的高度部分熔融形成,如此高程度的部分熔融是否与地幔柱有关还有待进一步研究。鉴于橄榄石中的Ni含量未发现有与硫化物发生交换反应的证据,因此对于其铜镍硫化物成矿条件是不利因素,其他控制硫化物熔离的因素还需要进一步探讨,但铬尖晶石充分结晶分异为发育铬铁矿矿化提供了有利的条件。

Xiadong intrusion in Xinjiang is an ultramafic complex mainly containing dunites. Whole rock major element and electronic microprobe analysis show a very high MgO content (36% similar to 39%), and Fo of olivine ranging from 90 to 96. According to the data, a modeled composition of primitive magma is MgO = 28%, FeO = 9%, which shows the komatiitic composition. By modeling of the magma evolution, dunites and gabbros are products of crystal fractionation of komatiitic magma. Through MELTS calculation, the initial temperature of liquid magma is 1703 degrees C, and spinel, olivine and clinopyroxene yeild 1649 degrees C, 1555 degrees C and 1093 degrees C,respectively, which are in accordance with mineral assemblage. It is supposed that the primitive magma was derived from 24% bulk partial melting of primitive mantle. Such a high degree of partial melting need further studies about the possibility relating to mantle plume. For absence of evidence of interaction between olivine and sulfide, the ore-forming potential is somewhat gloomy, and other factors controlling the sulfide segregation is needed to discuss further. However the sufficient crystal fractionation of spinel is favorable For Cr mineralization.