青藏高原东北缘早渐新世—早中新世干旱化的粘土矿物学证据——以兰州盆地为例

为揭示青藏高原东北缘早渐新世—早中新世气候环境演化及干旱化事件,采用X射线衍射(XRD)、扫描电子显微分析(SEM)等现代测试技术,对甘肃兰州盆地早渐新世—早中新世沉积物中粘土矿物的微观形貌、相对含量、V(Ill+Chl)/V(Sme)比值等参数进行了系统研究。结果表明,早渐新世—早中新世沉积物中粘土矿物主要有伊利石、绿泥石、蒙脱石以及少量的伊-蒙混层粘土矿物,指示该时间段气候总体以干旱为特征,局部出现相对温暖潮湿的气候。根据粘土矿物相对含量及V(Ill+Chl)/V(Sme)比值变化特征将兰州地区早渐新世—早中新世的气候环境演化划分为以下4个阶段:(Ⅰ)31.5~28.8 Ma相对温干偏湿的气候阶段;(Ⅱ)28.8~26.2 Ma相对冷干的气候阶段;(Ⅲ)26.2~22.1 Ma相对温暖潮湿的气候阶段;(Ⅳ)22.1~16.5 Ma相对温干偏湿的气候阶段。扫描电子显微分析表明,在相对温暖潮湿的时期,粘土矿物溶蚀较明显;在相对干旱的时期,粘土矿物表现显著的物理风化特征。以上粘土矿物学特征所揭示的阶段性干旱可能与青藏高原的阶段性隆升相对应。

Clay mineralogical evidence of Early Oligocene-Early Miocene aridification on the northeast margin of the Tibetan Plateau: a case study of Lanzhou Basin

To reveal palaeoclimatic evolution and aridification events of Early Oligocene-Early Miocene period on the northeast margin of Tibetan Plateau,the authors investigated micro-morphology,relative content of clay minerals and V(Ill+Chl)/V(Sme) ratios of sediments in Lanzhou Basin by means of X-ray diffraction(XRD) and scanning electron microscopy(SEM).The relative content of clay minerals shows that the sediments are composed of illite,smectite,chlorite and minor illite-smectite mixed-layer clay(Ill/Sme),indicating that aridification prevailed in Lanzhou in that period,with intervals of relatively warm and humid climate conditions.According to relative content and V(Ill+Chl)/Ill(Sme) ratios,the palaeoclimate evolution in Lanzhou could be divided into four stages,i.e.,(Ⅰ) relatively warm and dry and slightly humid period(31.5～28.8 Ma),(Ⅱ) relatively cold and dry period(28.8～26.2 Ma),(Ⅲ) relatively warm and humid period(26.2～22.1 Ma),and(Ⅳ) relatively warm and dry and slightly humid period(22.1～16.5 Ma).SEM results indicate that clay minerals were apparently corroded in the relatively warm and humid period and presented distinctly physical weathering characteristics in the relatively arid period.The aridification events probably resulted from the multi-stage uplifts of the Tibetan Plateau.