Elsevier

Sedimentary Geology

Volume 373, 1 October 2018, Pages 292-306
Sedimentary Geology

Terrestrial deposition processes of Quaternary gibbsite nodules in the Yongjiang Basin, southeastern margin of Tibet, and implication for the genesis of ancient karst bauxite

https://doi.org/10.1016/j.sedgeo.2018.06.010Get rights and content

Abstract

The terrestrial deposition process for karst bauxite is difficult to constrain due to its allochthonous origin, multiple controls and the difficulty in geodynamic reconstruction. This ambiguity has been clarified from investigation of the provenance and geologic-geographic conditions for the formation of Quaternary gibbsite nodules in the Yongjiang Basin. This karst basin, located at the southeastern propagation front of the Tibet Plateau, was affected by the SE Asia monsoon climate developed since the Late Oligocene and a later Quaternary glaciation. A thick blanket of Quaternary laterite is located on Paleozoic carbonate bedrock, and covers an area about 1000 km2 along the east-flowing river systems in the basin. The laterite profiles are well layered, with a layer of iron-rich gibbsite nodules overlying on a basal clay zone and irregularly covered by a thin clayey topsoil. Gibbsite nodules from most localities yield detrital zircons with age peaks at ca. 1000 Ma, 450 Ma and 250 Ma, corresponding to those of the Mesozoic clastic and magmatic rocks exposed on the western and southern surrounding highlands; nevertheless, those from the Pingtianshan area produce an exclusive spike at ca. 100 Ma, consistent with the emplacement age of nearby granites. Apatite fission-track modeling for the Mesozoic clastic rocks and Darongshan granites shows a rapid cooling since the early Miocene, and that for the Pingtianshan Cretaceous granite reveals fast exhumation since the late Miocene. These data indicate differential erosion reshaped the topography and consequently a low-relief karst basin was developed, allowing a diverse material supply from the surrounding and local highlands via sheet flow and east-flowing rivers. Under a monsoon-influenced subtropical climate, aluminum and iron oxides/hydroxides were accumulated as nodules or concretions, which were cemented by and coated with Fe oxides due to variations in redox potential and alternating consolidation and swelling, while silica was leached. At greater depth, groundwater solutions preferentially removed and brought upward Fe under a reducing environment during the wet seasons, while an underlying clay layer occurred due to the reduced solubility of silica. Later, this bauxitization process was probably hindered by the Quaternary glaciation, which could have promoted the formation of the top clay layer. This terrestrial deposition process of Quaternary gibbsite nodules serves as one analogue for ancient karst bauxite, in which the following submergence would improve the quality via further precipitation of alumina hydroxide and leaching of extra Fe.

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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