新疆克拉玛依百口泉蛇绿混杂岩中辉长岩岩石学和地球化学研究

Petrology and geochemistry of gabbro in Baikouquan,Keramay(Xinjiang,NW China):Implication of magmatic evolution

克拉玛依蛇绿混杂岩带百口泉剖面由尖晶石蛇纹岩、辉长岩、玄武岩和硅质岩组成。岩相学研究表明,百口泉辉长岩分为堆晶岩(具堆晶结构)和辉长岩(具辉长结构)两类,且均经历了低-中级变质改造。微量元素地球化学显示其岩浆起源于亏损地幔,强烈的Sr异常和Eu异常表明强烈的斜长石堆晶过程。根据其稀土配分模式推测其源区为尖晶石相的地幔橄榄岩。结合野外地质关系和地球化学特征,白碱滩尖晶石二辉橄榄岩能够代表其源区成分,利用微量元素模拟其岩浆演化过程显示:尖晶石二辉橄榄岩发生2.5%部分熔融所形成的熔体,通过10%～20%分离结晶可以形成堆晶辉长岩,经过80%～90%分离结晶则可以形成具辉长结构的辉长岩。因此,蛇绿混杂岩中零星分布的堆晶岩和辉长岩团块是同源岩浆演化的产物。对分离结晶过程中Nb元素地球化学行为的研究表明,岩浆的结晶分异能够导致辉长岩明显亏损Nb。

Baikouquan ophiolitic melanges consist of spinel serpentinite,gabbro,basalt,and silicalite.There are two types of gabbro:One has cumulate texture and the other shows typical gabbroic texture.Both of them have undergone metamorphism at low-middle metamorphic grade.Their strongly positive Sr and Eu anomalies imply the fractional crystallization of plagioclase during magma evolution.The magma source of gabbro was a depleted mantle based on the trace element geochemistry study.Calculation shows that the magma formed by 2.5% partial melting of spinel lherzolite could account for the cumulate gabbro by 10% ～ 20% fractional crystallization,and the gabbro with gabbroic texture by 80% ～ 90% factional crystallization.The obvious Nb depletion in the Baikouquan gabbro was caused by the melt fractional crystallization.