Terrestrial deposition processes of Quaternary gibbsite nodules in the Yongjiang Basin, southeastern margin of Tibet, and implication for the genesis of ancient karst bauxite
Introduction
Bauxite deposits in karst terrain are categorized as a karst type, and those derived from substrate aluminosilicate rocks are called a laterite type. The majority of karst bauxites are considered to be allochthonous, or at least parallochthonous and parautochthonous, thus much more enigmatic and less understood compared to the autochthonous laterite type (Bardossy and Combes, 1999; Wang et al., 2016). Intensive studies have tried to clarify the genesis of karst bauxite, such as the source rocks, the sedimentology of bauxite materials, and the controlling geodynamic factors (Bardossy and Combes, 1999; Zarasvandi et al., 2008; Mongelli et al., 2014; Wang et al., 2016; Ahmadnejad et al., 2017; Hou et al., 2017; Kelemen et al., 2017). Although various lithologies, including igneous rocks, volcanic ashes and even underlying carbonate rocks, have been proposed to be protoliths for karst bauxite, the relative contributions of source rocks and the evaluation of parental relationships are difficult to determine (Comber, 1974; Petrascheck, 1989; D'Argenio and Mindszenty, 1995; Zarasvandi et al., 2008; Boni et al., 2013; Mongelli et al., 2014, Mongelli et al., 2015; Wang et al., 2016; Hou et al., 2017; Liu et al., 2017). As such, the terrestrial deposition process, including provenance exhumation, transportation and deposition of weathered remnants, bauxitization (formation of alumina hydroxide), and later geochemical and mechanical alteration, is still ambiguous due to the fact of that the paleotopographic and geodynamic backgrounds for ancient karst bauxites are not well understood.
In the Yongjiang Basin, located on the southwestern propagation front of the Tibet Plateau, a thick blanket of Quaternary laterite containing abundant gibbsite nodules is developed on karst terrain. This bauxitic layer provides us a unique chance to clarify the terrestrial deposition process of karst bauxite deposit, since the bauxitization is ongoing and the paleogeographic and geologic circumstances can be reconstructed. Multidisciplinary approaches were adopted in this study, including: (1) investigating the climatic, hydrological, and topographic background of the study area, (2) examining the geological occurrence, mineralogical and geochemical compositions of the gibbsite nodules, (3) analyzing the geochronologic, isotopic, and trace element data of detrital zircons separated from the nodules and regional strata to determine the potential parent rocks, and (4) measuring apatite fission-tracks (AFT) of potential protoliths to constrain landform evolution and deposition age of the gibbsite nodules.
Section snippets
Geological setting
The Yongjiang Basin, in the southwestern part of South China block, is situated at the distal propagation front of the Tibet Plateau (Fig. 1a). The South China block has experienced widespread magmatism at ~1000 Ma and extensive magmatism and metamorphism at ~450 Ma, which make up the diagnostic age peaks in the Paleozoic sedimentary rocks (Lin et al., 2008; Wang et al., 2011). Revealed by 40Ar/39Ar, apatite or zircon fission track and (U-Th)/He data (Leloup et al., 1993; Kirby et al., 2002;
Occurrence of gibbsite nodules
The Quaternary laterite covers a large area about 1000 km2 and displays a W-E trending extension along the Yongjiang river systems. The laterite profiles are well layered, with a thick layer comprised of iron-rich gibbsite nodules in the middle, overlying on a basal clay layer and irregularly covered by a clayey topsoil layer, as shown in the YB and RZ profiles (Fig. 4, Fig. 5). The topsoil layer consists chiefly of clay minerals, with a small amount of gibbsite nodules, and usually disappears
Sampling
Fifty-seven gibbsite nodule samples were collected from seven laterite profiles (marked as DX-E, MG-E, ML-E, XY-E, QT-E, ST-E and WL-E, Fig. 3a) for whole-rock geochemical analysis. The analyzed gibbsite nodules were classified into four groups with respect to their longest dimensions, namely <0.1 cm, 0.1–1 cm, 1–3 cm and >3 cm. U-Pb age determinations, trace element and in-situ Hf isotope analysis were performed for detrital zircons from six gibbsite nodule samples >10 cm (marked as DX, YB,
Mineralogical and geochemical compositions of gibbsite nodules
The mineralogy of the gibbsite nodules is ubiquitously dominated by gibbsite, hematite, goethite, and kaolinite, occasionally with minor amount of Mn-oxide minerals (Fig. 6, Fig. 7). Small amount of detrital minerals, zircons and quartzs, are also occasionally detected. Photomicrographs show that sub-spheroidal structures (ooids of gibbsite-clay) are grain-supported or clayey matrix-supported, and the gibbsite-clay ooids are coated with Fe-oxides, and irregularly cemented together with
Potential provenance: high diversity of parentage
Bauxitization is a complex process that acts on a wide range of source rocks, during which fractionation of major, trace and rare earth elements takes place, making it difficult to recognize the protoliths from which the bauxite derives. Alkali and alkaline earth elements are thought to be the most susceptible to weathering, recycling and diagenesis, while Al2O3, TiO2, HFSE trace elements concentrations, REE patterns, as well as Sm/Nd ratios and Eu-anomalies, are usually used for provenance
Conclusions
Provenance analysis of the Quaternary gibbsite nodules in the Yongjiang Basin based on multidisciplinary methods found two major groups of protoliths from surrounding and local highlands: (1) the widespread Middle Triassic turbidites from the Youjiang Basin, the Cretaceous sandstones from Shiwandashan Mountain and local areas, as well as Darongshan granites, might have served as the major source; and (2) the Cretaceous granites provided a dominant contribution locally.
The paleogeographic
Acknowledgments
Thanks to Claudia C. Johnson for valuable comments about the manuscript, the Editor-in-Chief Jasper Knight and two anonymous reviewers for constructive suggestions. This research is jointly supported by the National Natural Science Foundation of China (No. 41672089), the Key Project of the Resource Exploration Bureau in Guangxi Province (No. 201649), the National Basic Research Program (No. 2015CB452600), and the Fundamental Research Funds for the Central Universities (No. 2652015056).
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2020, Earth-Science ReviewsCitation Excerpt :Intensified Cenozoic cooling and exhumation along the southeastern margin of the Tibetan Plateau are indicated by both 40Ar/39Ar, apatite or zircon fission track and (UTh)/He data (Clark et al., 2005; Wang et al., 2012a; Deng et al., 2013; Tian et al., 2013), and geological data (Li et al., 2014a; Wang et al., 2014a). The first-stage of uplift began in the late Oligocene to early Miocene, with more recent and rapid exhumation from the late Miocene to Pliocene (Fig. 1; Li et al., 2015b; Yang et al., 2018b). The uplift of the Tibetan Plateau facilitated the development of an Asian Monsoon climate in the Youjiang Basin (An et al., 2001; Sun and Wang, 2005), with its intensity increasing from the early Miocene onwards as accelerated uplift caused erosion and weathering on the margins of the Tibetan Plateau (Guo et al., 2002; Sun and Wang, 2005; Li et al., 2007a).