中文摘要：

采用X射线衍射、扫描电子显微镜分析方法,对青海循化盆地晚渐新世-早中新世沉积物中黏土矿物的微观形貌、体积分数、结晶度及其古气候指示作用进行了深入的研究。分析结果表明,晚渐新世盆地沉积物中的黏土矿物主要为蒙脱石、伊利石、坡缕石和绿泥石,以蒙脱石为主,指示循化地区总体为相对温暖潮湿的气候条件,蒙脱石、伊利石和绿泥石相对体积分数及伊利石、蒙脱石结晶度值均出现明显的周期性波动,表明循化地区气候经历了干旱-温暖潮湿交替的变化;早中新世盆地沉积物中的黏土矿物主要为伊利石、蒙脱石和绿泥石,以伊利石为主,各黏土矿物体积分数及伊利石、蒙脱石结晶度值的变化范围不大,表明循化地区气候以相对持续冷干为特征。从晚渐新世到早中新世,气候条件由相对温暖潮湿到相对冷干的转变,揭示其间（约21.3Ma）出现了一次极端的降温事件,可能与青藏高原隆升及亚洲沙漠化密切相关。

英文摘要：

Micro-morphology and relative content and crystallinity of clay minerals of the Late Oligocene and Early Miocene sediments in Xunhua Basin,Qinghai,were investigated using X-ray diffraction（XRD） and scanning electron microscopy（SEM）.The results show that the Late Oligocene sediments are composed of smectite,illite,palygorskite,and chlorite,dominated by smectite.This suggests that hot and humid climate conditions prevailed over the period in Xunhua Basin.Fluctuations in the crystallinity of illite and smectite and the relative contents of clay minerals indicate that several arid to hot and humid cycles occurred in the Late Oligocene in this basin.While the clay mineral compositions of the Early Miocene sediments are illite,smectite and chlorite with illite in domination.No significant change was observed in the relative contents of clay minerals and the crystallinity of illite and smectite.This suggests the cold and dry palaeoclimate of Early Miocene time in Xunhua Basin.An abrupt change clay indices reflects a rapid change in climate from hot and humid to cool and dry during the transition period of Late Oligocene to Early Miocene（～21.3 Ma）in this area.This may have resulted from the uplift of Qinghai-Tibet Plateau and the desertification of Asia.