大兴安岭绰尔河流域玄武岩地球化学特征及地质意义

大兴安岭绰尔河流域沿河谷分布有大量的沿北西向展布的上更新统大黑沟组玄武岩,控制厚度大于20 m,本次工作针对该地区玄武岩进行岩石地球化学研究。结果表明,该地区的玄武岩主要为气孔橄榄玄武岩和橄榄玄武岩,w(SiO_2)为47.14%~49.84%,铝质量分数为10.64%~12.75%,钙、镁质量分数分别为8.91%~10.70%和9.23%~10.89%,全碱(ALK)含量平均为4.37,其中Na_2O/K_2O比值平均为2.35,显示为偏钠质的特点,A/CNK为0.41~0.57,显示为准铝质岩石,δ平均为3.61,大多数属于高钾钙碱性系列岩石。同时表现为富集大离子亲石元素Ba, K等,亏损Nb,特别亏损P,Ti等高场强元素。稀土元素ΣREE=127.09~171.85×10^-6轻重稀土分馏变化较大,(La/Yb)_N=7.77~12.61,LREE/HREE=7.19~9.16,铕异常不明显,δEu平均为1.06。根据微量元素比值等判别图解认为玄武岩的岩浆来源于地幔,但受到了不同程度的地壳混染作用,工作区内玄武岩为大陆板内拉张环境背景下,基性岩浆沿断裂平静溢流式喷发的产物。

THE GEOCHEMICAL CHARACTERISTICS AND SIGNIFICANCE OF THE BA-SALT FROM THE NAOER RIVER BASIN IN THE GREAT XING'AN RANGE

Lithogeochemical method is used to study the Upper Pleistocene Daheigou Formation basalts,which has a control thickness of more than 20 m and distributed along the NW direction in the Naoer River Basin of the Great Xing′an Range.Results show that the basalt in the region is mainly composed of stomatous olivine basalt and olivine basalt.Their w(SiO2)ranges from 47.14%to 49.84%,w(Al)from 10.64%to 12.75%,w(Ca)and w(Mg)from 8.91%to 10.70%and 9.23%to 10.89%respectively.Total alkali content(ALK)of the rock is 4.37 on average,in which the Na2O/K2O ratio is 2.35 on average,showing the characteristic of partial sodium,and A/CNK is in the range of 0.41-0.57,showing the characteristic of quasaluminous rock with the averageδvalue 3.61,and most of the rocks belong to the high-K calc-alkaline series.The basalt is enriched in large ion lithophile elements such as Ba and K,and depleted in Nb,especially depleted in high field strength elements such as P and Ti.Rare earth elementsΣREE=127.09-171.85×10^-6,light and heavy rare earth fractionation changes greatly,(La/Yb)N=7.77-12.61,LREE/HREE=7.19-9.16,with slight Eu anomaly and with an averageδEu of 1.06.According to the discrimination diagram of trace element ratios,it is considered that the magma of basalt originated from the mantle with different degrees of crustal mixing and the basalt in the study area is the product of the eruption of the basic magma along the fault with a background of continental extension environment.