塔里木盆地巴楚小海子二叠纪超基性脉岩的地球化学特征及其成因探讨

Geochemical characteristics and petrogenesis of Permian Xiaohaizi ultrabasic dyke in Bachu area,Tarim basin.

巴楚小海子水库南闸附近出露的橄榄二辉岩岩墙SiO2含量平均为42%,Na2O＋K2O＜1%,Mg#=0.67～0.69,Fe2O3为16.29%～19.37%,具高镁富铁型原生玄武质岩浆的地球化学特征.橄榄二辉岩稀土元素标准化值呈LREE富集的右倾平滑曲线,轻重稀土元素分馏明显,微量元素配分曲线呈大隆起型,指示板内拉斑玄武岩系列的地球化学特征.Eu异常很不明显,可能由于斜长石的弱分离结晶作用所致,Sr、K等大离子亲石元素的相对亏损缘于热液蚀变和流体交代.该超基性脉岩与其附近发现的基性脉岩具有相同的产状,相似的地球化学特征,指示它们来源于同一岩浆源区,并在相同的构造环境下产出.其Th/Ta、Nb/U、Nb/La和Zr/Ba等微量元素比值均显示岩浆源区可能为亏损的软流圈地幔,岩浆的侵位根源于深部地幔物质上涌,以及二叠纪板内区域拉张环境的有利配合.

The olivine-pyroxenite, outcropped near the Nanzha of Xiaohaizi reservoir in Bachu, with an average abundance of SiO2 around 42%, Na2O ＋ K2O 〈 1% , Mg^# =0.67 -0.69, FeO3 = 16.29% - 19.37%, was derived from primary basaltic magma rich in ferrum and magnesium. The olivine-pyroxenite is enriched in LREE forming a fight inclined curve, which indicates definite fractionation of REE. Its trace elements pattern looks like a big rise, the same as the geochemical features of within-plate basalts. There is even no Eu anomaly in the olivine-pyroxenite. It may be greatly related with the weak fractional crystallization of plagioclase. Depletion of LILE （ such as Sr, K） may be caused by erosion and metasomatism of thermal fluid rather than by fractional crystallization of plagioclase. The ultrabasic dyke have similar geochemical characteristics as well as the same attitude to the diabase and diabase- porphyrite found nearby, which indicates that they were derived from the same magma source and occurred in the same tectonic setting. Their trace elements ratios, such as Th/Ta,Nb/U,Nb/La and Zr/Ba, present as an evidence for the possible magma source of depleted asthenosphere mantle. It is quite possible that the magma came from partial melting of asthenospheric mantle, then intruded into the cracks of the wall rock with its continuous uplifting and regional extention in Permian.