西昆仑山前陆盆地粘土矿物特征及其地质意义

Clay Mineralogy and Its Geological Significance of Sediments in the Foreland Basin of West Kunlun Mountains

为重建西昆仑山地区晚新生代的地质构造及环境演变过程,本文利用X射线衍射(XRD)及扫描电子显微分析(SEM)方法,对西昆仑山前陆盆地(叶城盆地)该时期沉积的一套磨拉石建造中的粘土矿物进行了研究。结果表明,在整个晚中新世到早更新世的沉积物地层中,粘土矿物的种类始终以伊利石和蒙脱石为主,含少量的绿泥石和高岭石。但伊利石、蒙脱石、高岭石的相对含量,以及伊利石结晶度值等均显示出明显的变化。在8～3.5 Ma B.P.期间,伊利石相对含量值和结晶度值均比较高,而蒙脱石相对含量低,并有少量的高岭石产出;在3.5～3.3 Ma B.P.期间,伊利石相对含量值和伊利石结晶度值均急剧降低,而相反,蒙脱石相对含量值急剧升高,高岭石基本消失;3.3～2.8 Ma B.P.期间沉积物的粘土矿物参数与8～3.5 Ma B.P.时期的粘土矿物参数非常接近;而2.8～1.8 Ma B.P.期间沉积物的粘土矿物参数则与3.5～3.3 Ma B.P.期间的粘土矿物参数相类似。根据沉积物中的粘土矿物特征变化规律可以推断,物源区母岩成分的变化是粘土矿物特征变化的主要原因,古气候古环境变化则是次要影响因素。粘土矿物特征的变化规律显示西昆仑山体在晚新生代具有不均匀的阶段性隆升的特点,在3.5 Ma B.P.、3.3 Ma B.P.、2.8 Ma B.P.左右分别发生了三次明显的构造隆升事件,而且总伴随着风化母岩成分的变化。物源区在早更新世中期(12.5 Ma B.P.)很可能处于干旱的古气候古环境中。

To reconstruct the evolution process of tectonics and paleoenvironment during the late-Cenozoic time in West Kunlun area,clay mineralogy of the molasse sediments of the foreland basin of West Kunlun mountains were investigated using X-ray diffraction（XRD） and scanning electron microscopy（SEM） methods.The results show that clay minerals in sediments of 8-1.8Ma B.P.are mainly illite and smectite,with minor chlorite and kaolinite,while their relative contents and illite crystallinity change apparently along the profile.Illite content and its crystallinity are both notably high,and the content of smectite is low,with minor kaolinite occurring throughout the sediments of 8 ～ 3.5 Ma B.P.The content of smectite increases sharply while the content of illite,as well as the illite crystallinity,decreases,and no kaolinite is present in the period of 3.5 ～ 3.3Ma B.P.Clay mineral indices of the period of 3.3 ～ 2.8 Ma B.P.are similar to those of 8 ～ 3.5 Ma B.P.,and which of the period of 2.8 ～ 1.8 Ma B.P.are similar to those of 3.5 ～ 3.3 Ma B.P.It is concluded that changes in clay mineralogy of the Yecheng sediments was dominantly controlled by the changes of source rocks and,to some extent,by changes in climate around the area.The uplift of the West Kunlun is uneven and stepwise according to clay mineralogy of sediments.There were three times structural uplift events in the west Kunlun area,which had occurred around 3.5 Ma B.P.,3.3 Ma B.P.and 2.8 Ma B.P.,respectively.In addition,an arid episode occurred around 2.5 Ma B.P.in the region.