新疆东天山土墩基性-超基性杂岩体橄榄石成分特征及其成因意义

橄榄石通常是玄武质岩浆最早结晶出的矿物之一,其化学成分可以很好地反演母岩浆成分、岩浆结晶分异、硫化物熔离等成岩及成矿信息。本文以土墩镁铁质-超镁铁质杂岩体为研究对象,采用电子探针对岩体中的橄榄石矿物颗粒进行化学成分测试。利用橄榄石的Fo值和其中Ni含量,计算得到土墩杂岩体母岩浆中Mg O含量约为12.95%,是一种富镁的玄武质岩浆。同时,定量模拟结果表明,土墩杂岩体母岩浆中硫化物熔离几乎与橄榄石结晶作用同时进行,早阶段由橄榄石结晶(分离结晶程度约2%)而导致硫化物的熔离程度为0.2%。随后,橄榄石分离结晶程度在6%~7%时,硫化物熔体的熔离程度仅为0.01%。这些表明土墩杂岩体发生过一定程度的硫化物熔离,但成矿前景不是很好。此外,部分数据显示出Ni-Fo的负相关性,表明少许富铁橄榄石和晶间硫化物熔浆发生了Fe-Ni物质交换反应,这对橄榄石的成分有重要影响。

Composition features and genetic significance of olivine from the Tudun mafic-ultramafic complex, East Tianshan Mountains, Xinjiang

Abstract: Olivine is one of the earliest crystallization minerals from basaltic magma, and its composition can directly give the metallogenesis information on parent magma composition, processes of magmatic fractional crystallization, and sulfide segregation degree. In this study, the authors selected the Tudun mafic-ultramafic complex to conduct electron microprobe analysis of olivine minerals. According to the Fo and Ni values of olivine, he MgO content of parent magma, which contains the highest Fo olivine, was estimated to be 12.95%, indicating that the parent magma of the Tudun complex is a high-MgO basalt. The modeling results show that sulfide segregation and fractional crystallization almost took place at the same time. A slight sulfide segregation (about 0.2%) occurred during the early stage (0-2%) of olivine fractional crystallization process, and less sulfide segregation (about 0.01%) occurred during the later stage (6%-7%), which suggests that the metallogenic potential of the Tudun complex is limited. In addition, some data implying the negative relationship between Fo and Ni indicate that Fe-Ni exchange reaction happened between Fe-rich olivine and trapped sulfide liquid, which caused remarkable differences in olivine composition.