Abstract:

Mineral compositions and palaeoclimatic indicator of palygorskite of the Late Oligocene sediments in Xunhua basin， Qinghai,were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. The results show that mineral compositions of the sediments are smectite， illite， palygorskite and chlorite， and the fibrous or acicular palygorskites are typically authigenic，which grew along the edges or the surfaces of platy smectite, respectively. Palygorskite is a characteristic clay mineral under semiarid and arid climate conditions, the occurrence of palygorskite in the Late Oligocene sediments in Xunhua basin is indicative of an arid palaeoclimate condition, which is consistent with the clay mineral assemblage and illite crystallinity. The relative content and crystallinity of palygorskite show an continuous increase from the lower to the upper portion, reflecting the increasing aridity. The arid palaeoclimate probably resulted from the uplift of Qinghai-Tibet Plateau or the global cooling event.

Key words: palygorskite, crystallinity；degree of order；palaeoclimate；Xunhua basin；Late Oligocene