| 其他摘要 | In southwestern China, karst region centered on Guizhou Province is the vastest one in the world and is overlain by non-isopachous red earth. Since carbonate rock is easy to solve, acidic insoluble residua is very low (generally less than 5%), pedogenesis process is very slow, weathering crust volume is intensively reduced during weathering, mother rock texture and semi-weathering layer cannot be retained, contact relation of rock-earth interface shows abrupt change, transitional layer is absent, and macroscopically there isn't direct field geological evidence to support that inheritable characteristic exists between basement rock and overlying weathering crust, the provenance of overlying weathering crusts in karst regions has been on the debate for a long time. Because different viewpoints exist about the genesis of weathering crusts in karst regions, which becomes the obstacle to use weathering crusts to undertake a series of scientic researches, to precisely understand the provenance and genesis of overlying weathering crusts in karst regions is very pressing and important. On the basis of results obtained, this paper selects karst terrains of mosaic occurrence among limestone, dolostone and clastic rock in Guizhou Province (including Jishou profile in western Hunan Province) as research area, attempts to use the grain size analysis method to discuss the grain size distribution characteristic of different basement rocks and overlying weathering crusts in detail. Based on the grain size analysis, merging with magnetic susceptibility, pH value, geochemistry and mineralogy data, Author discusses the provenance and profile development of overlying weathering crusts in karst region of Guizhou Province, and obtains several significant innovations as follows: 1. The grain size analysis is a valid and intuitionistic method during the research on the genesis of weathering crusts underlying carbonatites. Sedimentary rocks derived from different sedimentary backgrounds possess different grain size distribution features, and weathering crusts developed on the basis of them inherit the grain size distribution characteristic of protolith rocks, their grain size frequency distribution curves show consistency and gradual transition with protolith rocks. Under the condition of moderate weathering intensity, weathering crusts still retain the information of the provenance. Weathering crusts developed on the carbonatites inherit the grain size distribution features of acidic insoluble residua of basement rocks. Weathering crusts developed on the sedimentary rock containing different grain size compositions take on obvious discrepancy in this aspect too. 2. The grain size analysis results of different basement rocks and overlying weathering crusts show that the grain size distribution feature of every weathering profile obviously inherits that of its underlying basement rock. Among weathering profiles the grain size distribution features show remarkable diversities, which accounts for that there isn't the same provenance about weathering crusts in karst region of Guizhou Province. Weathering crusts underlying carbonatites are the offspring of basement rock weathering action through the dissolution of carbonates and long-term accumulation of acidic insoluble residua. During non-isometric weathering, weathering front (i.e. interface between rock and earth) is a important place of geochemical action. On this narrow interface, carbonate is dissolved wealthily, acidic insoluble residua is decomposed as well. Furthermore, weathering gradient on the weathering front obviously excesses that of weathering profile formed during subsequent weathering, i.e. from acidic insoluble residua of basement rock to the bottom of weathering crust, weathering intensity abrupt increases, and subsequent development of weathering crust is a slow process. Owing to plentiful water and heat condition, the phenomenon of eluviation and illuviation widely occurs in weathering profiles. Clay content shows gradual reduction from down to up, i.e. inverted phenomenon. In limestone soil profile which development intensity' is weak and which thickness is thin, illuvium layer may directly exist at the bottom of weathering profile. Though development intensity of limestone soil is low and it may be referred to as the early stage of weathering process of carbonatite, various weathering indices have shown significant change during the formation of limestone soil and possess the weathering feature of red weathering crust. In a strict sense, limestone soil cannot be looked as "transition layer" from basement rock to red weathering crust. Limestone soil profiles widely have typical in situ weathering features, and their grain size and geochemistry indices show monotonous change trend. Though the grain size parameters of red weathering crusts also have normal weathering features of weathering crusts on the whole, they show intensive fluctuation in the profiles. Geochemistry indices and mineral compositions show corresponding change in the profiles too. The cause resulting in this phenomenon may be explained as follows: On the one hand, owing to change of weathering condition during downward progress of weathering front; on the other hand, owing to reconstruction of paleo groundwater table fluctuation on weathering profile. Formation time of limestone soil profiles are short, during development of weathering crusts, weathering condition and hydrologic status are steady, development of weathering crusts progresses completely from shallow to deep under the condition of atmospheric precipitation and isn't altered by subsequent groundwater. As for thick red weathering crusts, by virtue of long formation time, variation of weathering condition and fluctuation of paleo groundwater table will occur frequently. Particularly, the interface between rock and earth is an important geochemistry interface, during progress of weathering front downward, variation of weathering condition inevitably influences the discrepancy of weathering degree of corresponding layers, so it induces the flucuation of weathering indices. On the weathering front, the discrepancy of weathering condition results in the fluctuation of grain size and geochemistry indices. During this process, the variation of grain size and geochemistry indices may be predicted, i.e. intensive weathering condition may result in occurrence of thinner grain, clay grain content and CIA increase, vice versa. Weathering process shows net loss of active elements. But reconstruction action of paleo groundwater table fluctuation on weathering profiles is changeable and cannot be predicted, it may not only import some matter into the profiles, but also export some matter out of the profiles, e.g. K-metasomatism or Na- metasomatism. Meanwhile, the effect of groundwater on the grain size composition of weathering profiles is very complex, import of allochthonous matter uncertainly implies that grain size becomes coarse and clay content reduces, export of profile matter also uncertainly implies that grain size becomes thin and clay content increases. Through common effect of two sorts of mechanism, development trend of weathering crusts tends to be more complex. In the profile, between clay content and CIA, some layers show good positive correlation, some layers show intensive negative correlation, other layers show non-correlation. Jishou profile is a typical of isometric weathering profile, its weathering process shows development feature of clastic rock and its clay content gradually increases from bottom to top. If there is reconstruction action of paleo groundwater on Jishou profile, its effect on grain size is weak. As for red weathering crusts, whether weathering indices fluctuation in the profiles are due to the discrepancy of weathering condition or the reconstruction of subsequent groundwater on profiles, profiles show that the trend of weathering intensifying upward on the whole. Both gradual fall of pH value and increase of magnetic susceptibility upward indicate strengthening weathering degree, and this is accordant with weathering time of profile undergone. Compared formation process of weathering crust developed on carbonatite with crystalline rock, weathering jumping-off point of former mother rock is high, i.e. maturity degree of mother rock is high. Weathering mother matter of carbonatite weathering crust is acidic insoluble residua of basement rock, and acidic insoluble residua itself is the result through supergenic weathering condition, so may say that weathering mother rock is weathering crust and basically don't contain easy weathering minerals such as plagioclase. On the ternary diagram of weathering trend of A-CN-K, has shown second development stage of weathering crust, i.e. along with A-K joint line and tending to A apex. 9. Some conservative elements which have been proved during weathering of crystalline and clastic rock, their ratios produce obvious variation during non-isometric weathering of carbonatite, this is a characteristic during weathering and pedogenesis of carbonatite and may cause some so-called conservative elements to present geochemistry fractionation. But its geochemistry mechanism is not clear yet. In the weathering profile, the ratios of conservative elements are constant and show weathering features of other types of rocks. |
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