新疆北山罗东岩体橄榄石特征对岩浆演化和硫化物熔离的指示

Olivine Characteristics' Implication for Magmatic Evolution and Sulfide Segregation of Luodong Complex in Beishan Area,Xinjiang

罗东岩体是新疆北山地区沿北山裂谷带的一个典型镁铁质—超镁铁质岩体,研究表明坡北杂岩带与东天山杂岩带的产出时代具有时间上的耦合性,并且其大地构造产出背景与东天山镁铁质—超镁铁质杂岩也具有一定的关系,可能与碰撞背景下幔源物质的上侵有关,并且受到俯冲板片的影响。该岩体具有较完整的岩相分异,包括橄榄岩、辉石岩、辉长岩。镁橄榄石指数Fo值为81～88,属于贵橄榄石,橄榄石具有环带结构,从橄榄石边缘到中心,Mg逐渐升高,说明橄榄石在岩浆演化过程中发生了与残余岩浆的交换反应。通过主量元素反演,得到原始岩浆中w(MgO)=8%、w(FeO)=6%,属于拉斑玄武岩。通过橄榄石的定量模拟计算,显示罗东岩体的橄榄石为正常岩浆演化的产物,橄榄石中Ni的质量分数没有发生明显亏损,这些证据表明岩浆可能没有经历过显著的硫化物熔离作用。

The Luodong complex is one of the typical mafic-ultramafic intrusions along Beishan Fault in Beishan area of Xinjiang and its generation shows a temporal consistency with the mafic-ultramafic complex belt in Eastern Tianshan,another typical mafic-ultramafic complex belt in Xinjiang well known for its rich deposits.Both the belts may share a similar tectonic background,related to intrusions of mantle melts during stretching after collision and the origion is affected by diving plates.Luodong intrusion consists of peridotite,lherzolite and gabbro.Olivines mainly consist of forsterites,with Fo from 81 to 88.Halo rings are characteristic of olivines,showing increasing trend of Mg moving from the marginal to central parts,which represents a reaction between the crystalized olivines and silicate melts.Based on calculations,the main composition of the primitive magma is simulated to obtain w（MgO）=8%,w（FeO）=6%,with a tholeiitic basalt composition.Quantitative simulated calculation implies that olivines are the products of magmatic evolution,without markable evidence of massive sulfide segregation.