Fission track dating of secondary quartz in red weathering crusts of carbonate rocks in Guizhou Province
WANG Shi-jie,LIU Xiu-ming,ZHANG Feng(State Key Laboratory of Environmental Geochemistry,Institute of Geochemistry ,Chinese Academy of Sciences,Guiyang 550002,China)
The Cenozoic evolution history of Guizhou Province, which is locat ed on the southeastern flank of the Qinghai-Xizang Plateau, is unclear because of the lack of sedimentation records. The red weathering crusts widespread on the Yunnan-Guizhou Plateau may bear critical information about their evolution hist ory. This work firstly determined the ages of four red weathering crusts in east ern, central and northern Guizhou. The material used in fission track dating is well-crystallized quartz occurring in many in situ weathering crusts of carbona te rocks. The results showed that the fission track ages of quartz vary over a w ide range from 1 Ma to 25 Ma in the four profiles, significantly younger than th e ages of Triassic and Cambrian parent rocks. In combination with the regionally geological evolution history during the period from 25 Ma to 1 Ma, the ages of quartz can exclude the possibility that the origin of quartz has something to do with primary clastic minerals in parent rocks, secondary origin during diagenes is and hydrothermal precipitation or replacement by volcanic activities. It is d educed that the well-crystallized quartz was precipitated from Si-rich weather ing fluids during weathering processes of carbonate rocks. The recorded ages of quartz from the four profiles are consistent with the episodes of planation surf aces on the Qinghai-Xizang Plateau, the stages of red soil in the tropics of So uth China, the tectonically stable periods in Guizhou, and the ages of weatherin g in other parts of the world during the Cenozoic era. That is to say, the ages of secondary quartz dated by the fission track method are well feasible and cred ible.