Geochemistry of red residua underlying dolomites in karst terrains of Yunnan-Guizhou Plateau: I. The formation of the Pingba profile

• Published in: Chemical geology Vol. 203; no. 1; pp. 1 - 27
• Main Authors: Ji, Hongbing, Wang, Shijie, Ouyang, Ziyuan, Zhang, Shen, Sun, Chenxing, Liu, Xiuming, Zhou, Dequan
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.01.2004
• Subjects: Elements; Karst terrain; Material sources; Red residua; Weathering; Clay-rich deposit; Dolomites; Karst terrain; Weathering; Red residua; Elements; Guizhou; Material sources
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2003.08.012

We present a set of bulk chemical and mineralogical compositions of the Pingba red residua on karst terrains of the Yunnan-Guizhou Plateau that show evidence pertaining to the source and to evolution of the profile. The known potential sources of clay-rich deposits on the plateau include: (1) Triassic dolomites, (2) Quaternary sediments, (3) Chinese loess, (4) Early Pliocene–Late Holocene volcanic ash (although the existence of volcanic ash is not supported by geological or mineralogical data), or a mixture of the four sources. The mineralogical compositions of the profile have shown an obvious inheritance relationship between the regolith and insoluble residues from the underlying dolomites, without allochthonous components involved. In the regolith, the contents of Al 2O 3 and Zr are fairly constant and both the TiO 2/Al 2O 3 and Fe 2O 3T/Al 2O 3 ratios are consistent with those of bedrock below in the profile. The chemical index of alteration (CIA) for the profile varies from 84 to 94. An obvious variation is only found in the lower regolith. In the Al 2O 3-CaO*+Na 2O-K 2O (A-CN-K) triangular diagram, almost all the samples are concentrated, and cannot be distinctly distinguished, close to the A-K joint, and they tend to the A end. The SiO 2/Al 2O 3 ratio in the samples from the regolith is less variable and low (≈1), but the K 2O/Na 2O ratio is very variable (2–18). The above two ratios are quite different from those of PAAS, upper continental crust (UCC) and Chinese loess. Average UCC-normalized spiderdiagrams for the samples from the regolith and the insoluble residues in the dolomites share much in common with respect to element distribution pattern. The ratios of mobile to immobile elements (Ca/Al, Mg/Al, Na/Al, K/Al and Mn/Al) and between the immobile elements (Al/Zr, Ti/Zr and Ti/Nb) reflect the characteristics of in situ weathering rather than a sedimentary origin. The above lends great support to a cognate relation between the dolomite and its overlying regolith. The chemical weathering of dolomite can be divided into two stages in the major element and high field strength element (HFSE) diagrams—namely for “leaching-accumulating trend” and “weathering trend”. The research results of this paper demonstrate that the Pingba red residua is a typical in situ chemical weathering crust and material sources of the profile are derived from underlying Triassic dolomites. The previously postulated two-stage development model for red residua in the karst terrain can be discriminated clearly, among them, the change of chemical composition occurs notably in the transition periods of the two stages. Moreover, it is the leaching processes that have an important meaning in the formation of horizons of the soil and ferruginous crust from the profile.