【摘要】 为揭示临夏盆地晚渐新世沉积物的矿物组成对环境气候的指示意义,利用X射线衍射和扫描电子显微分析方法,对临夏盆地晚渐新世的沉积物的矿物组成以及坡缕石的矿物学特征进行了深入的研究.结果表明,沉积物中的坡缕石主要为自生成因,极少量具有碎屑成因.自生成因的坡缕石呈细长丝状的形态特征,往往沿片状的伊/蒙混层粘土矿物颗粒的边缘交代、生长;碎屑成因的坡缕石呈平直细纤维状的,以单颗粒形式产出,与细小的粘土矿物颗粒混杂,或叠合在片状粘土矿物颗粒的表面.在临夏盆地晚渐新世沉积物中,下段岩层中的粘土矿物组合为伊利石、伊/蒙混层粘土和高岭石;中段岩层的为伊利石、伊/蒙混层粘土;而上段岩层则为伊利石、伊/蒙混层粘土和绿泥石,反映在盆地形成之后,气候演化经历从湿润→冷干的变化过程.而沉积物中坡缕石的含量自下而上逐渐增多,其分布特征与粘土矿物组合所反映的气候环境变化趋势相吻合.伊利石和伊/蒙混层粘土普遍存在于临夏盆地晚渐新世沉积物中,伊利石、伊/蒙混层粘土和高岭石的共存表明这些粘土矿物具有不同的来源.坡缕石是干旱、半干旱地区泥灰岩风化以及风成沙漠尘土沉积物的特征矿物,临夏盆地晚渐新世的沉积物中碎屑成因坡缕石的发现,表明自~29Ma以来便出现风成黄土的沉积.更多还原

【Abstract】 Palygorskite in the late Oligocene sediments at Dongxiang, Linxia, northwestern China was investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that authigenic palygorskite occurs in the late Oligocene sediments, and trace amount of detrital palygorskite also can be found in the deposits. The authigenic palygorskite is present as silk-like aggregates with thin and long morphology, replacing the I/S mixed-layers along the plate edges, while the detrital palygorskite shows relatively straight fibers, occurring as a single particle in mixture with other fine-grained clay or covering on the surface of plated clay. Clay mineral assemblages of the lower section, the middle section, and the upper section of the late Oligocene sediments are illite, I/S mixed-layers, and kaolinite, illite and I/S mixed-layers, and illite, I/S mixed-layers, and chlorite, respectively, indicating the climatic evolution from humidity to aridity. The amount of palygorskite increasing from the lower to the upper section of the sediments reflects the similar evolution pattern with clay assemblage. Illite and I/S mixed-layers are ubiquitous throughout the late Oligocene sediments. However, the coexistence of illite, I/S mixed-layers, and kaolinite suggests different provenance of the clay. Palygorskite is a common clay mineral of desert soils and is characteristic of aeolian desert dust in the semi-arid and arid regions, and therefore, the occurrence of detrital palygorskite in the late Oligocene sediments implies the loess sedimentation from ~29 Ma in Linxia basin.更多还原