| 其他摘要 | The subtropical karst in southwest of China covers 500,000 km2.Guizhou province, as the center, has largest continuous the karst area in the world. Guizhou province, the important zone of west-high and east-low of Chinese topography is located in the southeast of Tibet Plateau and the east of Yunnan-Guizhou Plateau. This resurch not only give enough useful apply of Cenozoic geology and landform of Guizhou Plateau but also give the information of the direct effect on Guizhou Plateau by the rise of Tibet Plateau. Weathering crusts as the important component of planation surface haven’t been pay more attentation in the past time. As more and more successful cases continuously appearing, it is gradually beginning to be realized that weathering crusts are not only the impotant evidence of identifying and rebuilding planation surface, but also the crucial carrier of environmental information about planation surface. Guizhou province as the central of the karst area, weathering crusts profiles of carbonate rocks are broadly distributed here, especially in the center and north of Guizhou, all red weathering crusts developed on the planation surface of each rank. Weathering crusts of carbonate rocks could provide reliable evidence of identifying planation surface and determining the formation age. Especially, determining the formation age of weathering crusts has the positive significance to the establishment of time scale of planation surface. Many scholars have researched a lot on determining the formation age of red weathering crusts, but there exists a lot of controversy about how to determine the formation age of red weathering crusts in Guizhou area .Although there are possibility of other reasons, the essential reason causing this kind of controversy lies in that the formation age of red weathering crusts is not obtained according to direct dating datum but speculated by other materials(such as planation surface, climatic cases etc.) .Therefore, direct dating of weathering crusts is just a crucial key to solve these questions.
Depending on former research we use SEM scanning system to analyze the surface texture of quartz grains of red weathering crusts and obtain new evidences about the origin of weathering crusts overlying carbonate rocks. We also find and separate authigenic quartz grains from the weathering crusts by the study of geochemical and mineral analysis techniques. So we use F.T. dating to date the age of authigenic quartz grains and finally study the formation eras of weathering crusts overlying carbonate rocks in the area of Guizhou province. Some significant innovations have been obtained.
(1) The optimal selection of quartz grains etched conditions
The length and quantity of quartz grains’ fission track are related to the quartz grains’ crystal framework, the content of Uranium, and the age and etc; so different kinds of quartz grains have different optimal etched conditions. At present, there is not uniform international standard about etched conditions of quartz grains because of the application cases about quartz grains’ F.T. dating are less. Although many kinds of etchants that can be used it is lacking of comparison and evaluation of advantages and disadvantages among these etchants. Thus we need to compare etching efficiency among all kinds of etchants to select the best one. The results show that the 40% HF is the best etchant that is easily control and highly security. The optimal time of 40%HF etching takes 40min in the winter (about 4℃) and 30min in the summer (about 29℃).
(2) The shape and surface mechanical texture of quartz grains
SEM analysis show that there are tow kinds shape of quartz grains: the angular and angular-liked quartz grains、the round and round-liked quartz grains. In the angular and angular-liked quartz grains there are some crystal quartz grains.
SEM analysis show three kinds origins of quartz grains by the surface mechanical texture and related geological imformation. First, the original detritus quartz grains of carbonate rocks have the shape of round and round-liked. Second, the chert piece quartz grains that take on the fresh surfaces with uneven shape of angular and angular-liked. Third, crystal quartz grains with the shape of angular and sharp edges and better crystal that have the secondary micro-crystal on the surface of quartz grains. These results indicate that three kinds of quartz grains all have the characteristics of carbonate rock. The first and the second kinds of quartz grains direct inherit from the indiscerptible residues of carbonate rocks. The third one is the authigenic quartz grains in the weathering crusts of carbonate rocks. This results obtain the new evidence distinguishing the origin of the weathering crusts and inherited relationship with underlying carbonate rocks in Guizhou Province.
(3) The surface chemical texture of quartz grains and the weathering intensity
SEM analysis show that Daxing and Xinpu and Guanba three profiles’ chemical dissolution and deposition are all very strong，which demonstrates three profiles are all in the humid and hot environment and all at the period of strong weathering. Daxing profile is the strongest weathering one. The sequence of weathering from strong to weak is Daxing, Xinpu, Guanba. This sequence of weathering is consistent with the analytic result of mineral, geochemistry, Si-Al ratio, Al-Si-Fe ratio, and relative weathering intensity. This result also testifies the feasibility and dependability of SEM analysis of quartz grains.
(4) The F.T.ages of quartz grains
The dating results of these three kinds of quartz grains show different kinds of quartz grains have different ages. The F.T. age of round and round-liked quartz grains is older than the F.T. age of angular and angular-liked quartz grains that is older than the F.T. age of crystal quartz grains. The F.T. age of round and round-liked quartz grains of different profiles form in the same era. The F.T. age of angular and angular-liked quartz grains of different profiles form in the same era. These tow kinds of quartz grains are not suitable to the dating of weathering crusts. The crystal quartz grains of different profiles and deepth form in different eras. That kind of quartz grains form at the beginning of weathering process has the same age with weathering crusts. The result is consistency with the result of SEM analysis.
(5) The new evidences about the origin of weathering crusts
The surface texture analysis of quartz grains formation show the origin of weathering crusts is the indiscerptible residues of carbonate rocks, and an obvious inherited relationship with the weathering crusts of carbonate rocks. This analytic result shows the consistency with the result of distilling the indiscerptible residues of carbonate rocks, geochemical technique, mineral technique, grain size analysis and geology made by Wang Shi-Jie et al. So we obtain the new and detailed evidences about the origin of weathering crusts overlying carbonate rocks.
(6) The initial building of the history of Cenozoic geology-environment evolvement Guizhou province
At present, in view of relatively lacking of study of weathering-climate period for Guizhou and peripheral regions, less datum information, and disintegrated system, we only can build the initial weathering-climate period sequence of Guizhou Cenozoic now because the study that is on the base of authigenic quartz grains’ F.T. dating study isn’t intensive. According to the study for distributing F.T age of authigenic quartz grains in profile soil and the study results of predecessors, we simply and rudimentally rebuild the history of Cenozoic geology-environment evolvement Guizhou province since 25Ma. There are sever conformation-weathering periods after the Miocene epoch: 25-19Ma、16-13Ma、10-6.5Ma、5-2Ma、1.7-1Ma. |
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