Mixed volcanogenic–lithogenic sources for Permian bauxite deposits in southwestern Youjiang Basin, South China, and their metallogenic significance

    Mixed volcanogenic-lithogenic sources for Permian bauxite deposits in southwestern Youjiang Basin, South China, and their metallogenic significance Wenchao Yu, Thomas J. Algeo, Yuansheng Du, Qilian Zhang, Yuping Liang PII: DOI: Reference:

S0037-0738(16)30046-X doi: 10.1016/j.sedgeo.2016.04.016 SEDGEO 5050

To appear in:

Sedimentary Geology

Received date: Revised date: Accepted date:

27 March 2016 27 April 2016 28 April 2016

Please cite this article as: Yu, Wenchao, Algeo, Thomas J., Du, Yuansheng, Zhang, Qilian, Liang, Yuping, Mixed volcanogenic-lithogenic sources for Permian bauxite deposits in southwestern Youjiang Basin, South China, and their metallogenic significance, Sedimentary Geology (2016), doi: 10.1016/j.sedgeo.2016.04.016

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ACCEPTED MANUSCRIPT Mixed volcanogenic-lithogenic sources for Permian bauxite deposits in southwestern Youjiang Basin, South China, and their metallogenic

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significance

Wenchao Yua,b,c, Thomas J. Algeoa,b,c, Yuansheng Dua,b*, Qilian Zhangd, Yuping

State Key Laboratory of Biogeology and Environmental Geology, China University of

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a

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Liangd

Geosciences-Wuhan, Wuhan 430074, China b

State Key Laboratory of Geological Processes and Mineral Resources, China University of

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Geosciences-Wuhan, Wuhan 430074, China Department of Geology, University of Cincinnati, Cincinnati OH 45221-0013, U.S.A.

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General Academy of Geological Survey of Guangxi，Nanning 530023，China

*

Corresponding author: [email protected];Tel: +86 13971241916, Fax: +86 27

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87481365.

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c

ABSTRACT Bauxite deposits at the base of the Upper Permian Heshan Formation in the Youjiang Basin, South China, contain zircons with dominant age peaks at 263-262 Ma. During the Middle to Late Permian, the Youjiang Basin consisted of a number of isolated and attached carbonate platforms separated by inter-platform troughs. The bauxite deposits are limited to the isolated carbonate platform facies and are not present on attached carbonate platforms and inter-platform troughs. Discriminant plots based on the trace-element composition of the zircons indicate a combination of within-plate/anorogenic and arc-related/orogenic sources. Geochemical and isotopic 1

ACCEPTED MANUSCRIPT data suggest that the metallogenic materials of the bauxite deposit came from felsic volcanic rocks of the Emeishan Large Igneous Provence (ELIP) in South China and from the Truong Son volcanic arc located between the South China and Indochina

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cratons. The northwestern and southeastern parts of the Youjiang Basin received larger amounts of ELIP detritus and volcanic-arc detritus, respectively. Coarser

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siliciclastic material in proximal attached carbonate platform and inter-platform trough settings was delivered by rivers, but finer siliciclastics that accumulated on distally located carbonate platforms in isolated deep-water areas was probably

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transported by wind.

Keywords: zircon; sediment geochemistry; sediment provenance; discriminant plots;

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1. Introduction

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Emeishan; Heshan Formation

Bauxites are products of subaerial chemical weathering with residual enrichment of Al, Fe and Ti (D’Argenio and Mindszenty, 1995). Bauxitization usually entails

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disintegration of clay minerals in laterite soils and release of Al ions and silicic acid into solution (Eq. 1), followed by reprecipitation of the aluminum as Al-oxyhydroxides (Eq. 2): Al2Si2O5(OH)4 (kaolinite) + 6H+ = 2Al3+ + 2H4SiO4(aq) + H2O

(1)

Al3+ + 2H2O = AlO(OH) (boehmite) + 3H+

(2)

Prolonged chemical weathering of this type can enrich the Al-oxyhydroxides to ore grade. Climate has long been considered a major factor in the formation of bauxites, with warm and humid conditions favoring bauxitization (Price et al., 1997). The composition of parent rocks is also an important factor, with aluminum-rich parent rocks (Al >10%) favoring bauxite formation (Bogatyrev et al., 2009). Tectonic factors may also play a role in bauxitization, i.e., most large bauxite deposits are located on 2

ACCEPTED MANUSCRIPT stable platforms because a lack of epeirogenic motions provides sufficient time for deep chemical weathering (Bárdossy, 1982; Bogatyrev et al., 2009). As intensive weathering is integral to bauxite formation, the original chemistry of the sediments is

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commonly strongly altered by the weathering process, causing challenges for geochemically based provenance studies. However, heavy minerals such as zircon are

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relatively resistant to weathering alteration and, thus, are especially useful for bauxite provenance studies (Bárdossy, 1982). A number of recent studies have made use of detrital zircon U-Pb ages as a fundamental tool in determining sediment provenance

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(Deng et al., 2010; Boni et al., 2012; Mongelli et al., 2016).

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Two kinds of bauxite deposits exist in western Guangxi Province, South China. A paleo-karst bauxite deposit (Mediterranean-type) dated to 262-256 Ma is present at the base of the Upper Permian Heshan Formation (Deng et al., 2010; Hou et al., 2014),

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and an epigenetic (Salento-type) bauxite deposit of Quaternary age developed from

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the Permian deposit (Yu et al., 2014). Detailed mineralogical (Liu et al., 2012; Yu et al., 2014) and geochemical studies (Wang et al., 2010; Wei et al., 2013; Yu et al., 2014)

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of the deposits have shown that the bauxite ore is composed of aluminum minerals (boehmite and diaspore), iron minerals (hematite or pyrite), clay minerals (kaolinite, illite, and chlorite) and some minor minerals (e.g., zircon, anatase, and rutile) with

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average Al, Si, and Fe contents of 30-50%, 5-25%, 10-20%, respectively. The provenance of the western Guangxi bauxite deposits remains in dispute. Based on petrographic and geochemical similarities, Wei et al. (2013) proposed that the Heshan Formation bauxite has an affinity with carbonates of the underlying Maokou Formation. However, numerous zircons in the bauxite deposit have yielded an age cluster at 263-256 Ma, suggesting an origin related to the Emeishan Large Igneous Provence (ELIP) (Deng et al., 2010), which is known to have been active at 260±3 Ma (Shellnutt, 2014). In this case, the likely source of the material would have been pyroclastic ash fall following ELIP eruptions. An alternative hypothesis was advanced by Hou et al. (2014), who inferred an arc-related origin on the basis of zircon U-Pb dating and geochemical discriminant diagrams. 3

ACCEPTED MANUSCRIPT In this paper, we present new whole-rock and zircon geochemical and U–Pb geochronological data on the Upper Permian bauxite deposit of the Heshan Formation in western Guangxi Province, South China, supplemented with previously published

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elemental and zircon U-Pb ages and geochemical data from lower Upper Permian strata within the Youjiang Basin. The results of our study document a mixed

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provenance for the bauxite deposit, with metallogenic material derived both from felsic rocks of the ELIP to the north and from the magmatic arc between the South China and the Indochina cratons to the south (note: all coordinates given as Late

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Permian paleo-coordinates). The northwestern and southeastern parts of the Youjiang Basin were dominated by ELIP and volcanic-arc inputs, respectively, with a zone of

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2. Geological background

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approximately equal mixing through the center of the basin.

The study area is located in the West Guangxi Province, SW China.

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Paleogeographically, it belongs to the southwestern quadrant of the Youjiang Basin, which is also known as the Nanpanjiang Basin in some studies (e.g., Lehrmann et al., 2006). As a passive continental margin basin, the Youjiang Basin extended across the

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modern southwestern part of the South China Craton (Fig. 1A-B). The Youjiang Basin evolved as a distinct feature in the Devonian Period during rifting of the South China Craton away from the northeastern margin of Gondwana. Permian strata are well-developed in the study area, with thicknesses to 2000 m (Fig. 1C). By the Permian, the Youjiang Basin had deepened, producing three kinds of paleogeographic facies: (1) isolated carbonate platforms within the basin (e.g., Leye, Fusui, Pingguo, Debao and Jingxi platforms), (2) attached carbonate platforms around the margins of the basin (e.g., Laibin Platform), and (3) inter-platform troughs (e.g., Sidazhai, Napo, and Banai troughs). On the carbonate platforms, the Middle Permian Maokou Formation is represented by massive cherty limestone and the Upper Permian Heshan Formation by bauxite and black shale in the lower part shifting to bioclastic limestone upward. The correlative units in the inter-platform troughs are the Middle Permian 4

ACCEPTED MANUSCRIPT Sidazhai Formation, which consist of thin bioclastic limestone with chert nodules and thin chert layers, and the Upper Permian Linghao and Wuchiaping formations, which consist of radiolarian chert and siliciclastic sediments with interbedded volcaniclastic

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layers (Fig.2).

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A regional unconformity exists between Middle and Upper Permian strata in the Youjiang Basin and adjacent areas (Fig. 2). This unconformity reflects a major global sea-level drop at the Guadalupian–Lopingian boundary (GLB) (Kofukuda et al., 2014),

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which was reinforced in South China by regional crustal doming linked to ELIP emplacement (He et al., 2010a). On the isolated carbonate platforms, the base of the

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Upper Permian Heshan Formation consists of a 0.5- to 10-m-thick gray or red bauxite deposit that formed in part through weathering of the underlying Middle Permian Maokou Formation. This deposit accumulated on a paleokarst surface that developed

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on the top of the Maokou Formation, the relief on which controls the thickness of the

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bauxite deposit (Deng et al., 2010; Wei et al., 2013; Yu et al., 2014). On the attached carbonate platforms, 0.2- to 4-m-thick claystone layers were deposited on the

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paleokarst surface of the Maokou Formation carbonates and were covered by Heshan (or Wuchiaping) Formation (He et al., 2010b; Zhong et al., 2013). In the inter-platform troughs, the Middle-Upper Permian boundary is marked by a lithologic

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transition, e.g., from limestone of the Middle Permian Sidazhai Formation to basalt and siliciclastic sediments of the Upper Permian Linghao Formation at Napo (Huang et al., 2014), or from brecciated limestone of the Middle Permian Sidazhai Formation to mudstone of the Upper Permian Linghao (=Wuchiaping) Formation at Banai and Sidazhai (Yang et al., 2012). The Emeishan Large Igneous Province (ELIP) is centered about 150 km to the northwest of the Youjiang Basin (Fig. 1B). The flood basalt succession ranges in thickness from ~5 km in the inner zone to several hundred meters in the outer zone and consists mostly of picrites in the lower half and basalts and andesites in the upper half with pyroclastic rocks and rhyolitic tuffs intercalated throughout the succession (Ali et al., 2005; Shellnutt, 2014). The ELIP was active from ~263 Ma to ~257 Ma 5

ACCEPTED MANUSCRIPT with a main eruption phase at 259.6±0.5 Ma (He et al., 2007; Shellnutt et al., 2012). Erosion of mafic-ultramafic rocks of the ELIP succession provided detritus to the northwestern Youjiang Basin, as shown by geochemical fingerprinting of siliciclastic

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sediments of the Upper Permian Linghao Formation (Huang et al., 2014; Yang et al., 2012). In attached carbonate platform facies close to the ELIP, fine-grained sediments

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at the Middle-Upper Permian boundary were derived via erosion of both felsic and mafic volcanic rocks (He et al., 2010b).

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The southeastern margin of the Youjiang Basin is defined by the E-W-trending Dian-Qiong suture, which extends from Yunnan (also known as Dian in Chinese)

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Province to Hainan (also known as Qiong in Chinese) Province, and which separates the South China Craton from the North Vietnam Terrane (Cai and Zhang, 2009). The North Vietnam Terrane includes the northeastern part of present-day Vietnam, and its

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southwestern boundary (separating it from the Indochina Craton) is the Song Ma

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suture zone (Halpin et al., 2015; Żelaźniewicz et al., 2013) (Fig. 1B). Detrital zircon U-Pb ages in sedimentary strata from northern Vietnam reveal a crustal affinity

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between the South China Craton and North Vietnam Terrane (Halpin et al., 2015). Suturing of the South China Craton and North Vietnam Terrane was part of the larger collision between the South China and Indochina cratons. Although the subduction

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polarity of this collision zone was disputed in earlier studies (Lepvrier et al., 2004; Zhong et al., 2013), recent studies favor subduction of the southern margin of the South China Craton beneath the northern margin of the Indochina Craton beginning in the Middle Permian, an event accounting for rapid contemporaneous subsidence of the Youjiang Basin (Faure et al., 2014; Halpin et al., 2015). Subduction led to development of the Truong Son volcanic arc between the Indochina and South China cratons (Halpin et al., 2015), whose magmatic activity is recorded in detrital zircons with minimum U-Pb ages of 277-252 Ma that accumulated in Middle and Upper Permian clastic strata of the North Vietnam Terrane (Halpin et al., 2015). Along the Song Ma suture zone, several isolated bodies of serpentinite are considered to be slivers of oceanic crust obducted in a forearc setting during the collision between 6

ACCEPTED MANUSCRIPT Indochina and South China (Ngo et al., 2011, 2015). In the Ha Lang area of northeastern Vietnam, 274-262-Ma ophiolites have been interpreted as evidence of limited back-arc spreading on the southern margin of the South China Craton (Halpin

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et al., 2015). Within the Youjiang Basin, subduction activity was recorded by arc-derived zircons with a 269-260-Ma age peak in claystone layers of the Laibin

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Platform (Zhong et al., 2013) and a 262-Ma age peak in bauxite deposits of the basal

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Heshan Formation of the Pingguo Platform (Hou et al., 2014).

3. Materials and methods

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At all study sites, weathered karst deposits directly overlie the Middle Permian Maokou Formation. At Jingxi, the contact between the Maokou Formation and the

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overlying bauxite deposit shows irregular relief, consistent with development on a

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paleokarst surface (Fig. 3B-C). At Leye, the 3-m-thick bauxite deposit consists of a lower 0.5-m Al-claystone layer (with high Al2O3 content of 20-30%), a middle 2-m

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massive gray bauxite layer, and an upper 0.5-m Al-claystone layer (Fig. 3A). Bauxite deposits in the Jingxi, Fusui, and Debao section lack the lower Al-claystone layer but exhibit the upper Al-claystone layer. In these three sections, the thickness of bauxite

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layer ranges from 4 to 10 m, and that of the upper Al-claystone layer ranges from 0.5 to 4 m. The upper Al-claystone layer transitions gradationally to mudstone and marlstone upward. Thin (~0.2-0.5 m) coal seams are observed in the upper Al-claystone layer at Debao (Fig. 3B). Bauxite and Al-claystone samples at the base of the Heshan Formation were collected from the Leye, Jingxi, Debao, and Fusui carbonate platforms for the purpose of detrital zircon U-Pb age analysis (see Fig. 1C for sampling sites). The samples for detrital zircon analysis were collected from the bauxite layer: LY-1 (Leye), DJ-1 (Debao), JX-1 (Jingxi), and FS-1 (Fusui) (Fig. 2B). Another six samples were collected at Leye for general geochemical analysis, including two (C-1, C-2) from the lower Al-claystone layer, two (C-3, C-4) from the middle bauxite layer, and two (C-5, 7

ACCEPTED MANUSCRIPT C-6) from the upper Al-claystone layer (Fig. 2). Previously published geochemical and zircon U-Pb age data from the lower

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Upper Permian strata within the Youjiang Basin and from the adjacent area were

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compiled (Table 1). The samples include claystone or mudstone between the Maokou

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and Heshan (=Wuchiaping) formations on attached carbonate platforms (i.e., the Laibin and Chaotian sections), bauxite at the bottom of the Heshan Formation on isolated carbonate platforms (i.e., the Pingguo section), and sandstone and mudstone

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between the Sidazhai and Linghao (=Wuchiaping) formations from inter-platform trough locales (i.e., the Sidazhai, Napo and Banai sections) (Figs. 1C, 2).

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Elemental analyses were carried out by ALS Chemex Laboratory (Guangzhou, China) on the six whole-rock samples collected at Leye for geochemical analysis. The whole-rock samples were crushed in a corundum jaw crusher (to 60 mesh) and then

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powdered in an agate ring mill to finer than 200 mesh. Major-element measurements

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were made using a Shimadzu 1800X X-Ray fluorescence (XRF) unit, with

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calibrations based on the Chinese national bauxite standard GSB 04-2606-2010; the detection uncertainty was <3%. Trace elements and rare earth elements (REEs) were analyzed by inductively coupled plasma-mass spectrometer (ICP-MS). Powdered

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samples were digested by HF + HNO3 in Teflon bombs per standard techniques (e.g., Zhou et al., 2016). The detection limit for individual trace elements and REEs ranges from 2 to 8 ppm, all elemental concentrations have uncertainties <10% except for La and Pr (10-20%) (Liu et al., 2008, 2010). Enrichment factors (EF), which reflect enrichments of individual elements in a sample, were calculated as: EFX = (X/Al)sample / (X/Al)standard, where X is the element of interest. If EFX is greater than 1, then element X is enriched relative to a chosen standard (Tribovillard et al., 2006), for which we adopted average upper continental crust (AUCC) or average bulk continental crust (ABCC; McLennan, 2001). Zircons were separated by standard techniques prior to casting in epoxy mounts, polishing and imaging in transmitted light. Carbon-coated mounts were further imaged by cathodoluminescence (CL) to observe the internal structure of the grains 8

ACCEPTED MANUSCRIPT and to guide in situ analysis. U–Pb geochronology and trace element concentrations of zircon grains were analyzed by laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) using operating conditions as given in Liu et al. (2008).

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Both CL and LA-ICP-MS analysis were carried out at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of

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Geosciences, Wuhan. Laser sampling was performed using a GeoLas 2005. An Agilent 7500a ICP-MS was used to acquire ion-signal intensities, and Agilent Chemstation was used for the acquisition of each individual analyses. Off-line

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selection and integration of background and analytical signals, time-drift correction, U-Pb dating and zircon trace-element corrections were performed using

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ICPMS-DataCal. Zircon 91500 was used as external standard for U-Pb dating, and was analyzed twice every five analyses. Time-dependent drifts of U-Th-Pb isotopic

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ratios were corrected using a linear interpolation with time for every five analyses of

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the zircon 91500 standard, the preferred U-Th-Pb isotopic ratios of which are from Wiedenbeck et al. (1995). Concordia diagrams and weighted mean calculations were

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made using Isoplot/Ex ver. 3.75 (Ludwig, 2008). Zircon trace element concentrations were calibrated against multiple-reference materials (BCR-2G, BIR-1G and

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BHVO-2G) without applying internal standardization.

4. Results

4.1. Major elements The bauxite ore samples (C-3, C-4) and Al-claystone samples (C-1, C-2, C-5, C-6) from the Leye section exhibit distinctive geochemical characteristics (Table 2). The bauxite ore samples contain high Al2O3 (42.0-42.9% with a mean of 42.5%), low SiO2 (14.0-15.6% with a mean of 14.8%), and moderate Fe2O3 (18.3-24.7% with a mean of 21.5%). The Al-claystone samples contain moderate Al2O3 (24.9-30.8% with a mean of 28.7%), moderate to high SiO2 (33.8-49.8% with a mean of 43.2%), and low to moderate Fe2O3 (3.7-21.5% with a mean of 11.4%). Alkali and alkaline earth 9

ACCEPTED MANUSCRIPT elements exhibit low concentrations in all samples, e.g., Na2O (mean 0.31%, max. 0.59%), K2O (mean 0.72%, max. 1.98%), CaO (mean 0.27%, max. 0.45%), and MgO (mean 0.72%, max. 2.29%) because of their extreme mobility during the weathering

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process. TiO2 content ranges from 2.4% to 5.6% with a mean of 4.0%.

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Al/Si ratios (element ratios) are higher in the bauxite ore samples (3.03-3.30) than in the Al-claystone samples (0.68-0.81). SiO2 exhibits a strong negative correlation with Al2O3 (r = ‒ 0.81; p(α) <0.01; n = 73), indicating major silica loss

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during chemical weathering. TiO2 exhibits a strong positive correlation with Al2O3 (r = +0.71; p(α) <0.01; n = 73) because of the immobility of titanium during the

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weathering process. Compared with the projected weathering lines of average bulk continental crust (ABCC) and average upper continental crust (AUCC) (McLennan,

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2001), Al and Ti exhibit pronounced enrichments (Fig. 4A-B).

4.2. Trace and rare earth elements

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Some trace elements show moderate to strong depletion in the bauxite ore samples (C-3, C-4) relative to AUCC, including Be (EF = 0.41-0.94), Rb (EF = 0.19-0.94), Sr (EF = 0.022-0.031), and Ba (EF = 0.022-0.028) (Table 2). Other

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elements show enrichment, especially in the lower Al-claystone samples (C-1, C-2), including Li (EF = 19.4-29.8) and Cu (EF = 0.13-8.11). Enrichment of these elements was probably due to vertical mobility (translocation) within the paleokarst weathering profile (Wang et al., 2013). Some high-field-strength elements (HFSEs) such as Zr, Sc, Th, and Ta exhibit strong positive correlations with Al2O3, reflecting their stability during the weathering process (Fig. 4). The REE abundances of Leye samples range from 235 to 921 ppm, with a mean of 520 ppm. Bauxite ore and Al-claystone samples from the Heshan Formation show LREE-depleted and HREE-enriched patterns (LaN/YbN = 0.31-0.81 with a mean of 0.50) (Table 2; Fig. 5A). Both positive and negative Ce anomalies are seen in these samples, and some samples have unusually high Ce contents. Positive Ce anomalies 10

ACCEPTED MANUSCRIPT and Ce-bearing minerals are widespread in the Guangxi Permian bauxite deposits (Wang et al., 2010; Yu et al., 2014). Positive Ce anomalies reflect oxidation of Ce to Ce4+ and precipitation as cerianite (CeO2) in the weathering profile. Ce3+ can also

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form Ce-minerals, e.g., parisite (Ce2Ca(CO3)3F2) (Mongelli, 1997), as seen in the

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Guangxi Permian bauxite ore deposits (Wang et al., 2010; Yu et al., 2014). For PAAS-normalized REE distributions, three patterns can be distinguished in the study samples (Fig. 5). Most bauxite and Al-claystone samples at the base of the

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Heshan Formation show large positive Ce anomalies with enrichments of HREEs (Fig. 5A-B). The Laibin claystone samples show relatively flat REE patterns with moderate

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to large negative Eu anomalies (Fig. 5C). Clastic rocks samples from Chaotian, Banai, and Napo show similar REE patterns with MREE enrichment and positive Eu

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anomalies (Fig. 5D-F).

4.3. Zircon morphology, chronology and geochemical compositions

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The sizes (long axes) of zircon grains in the bauxite samples, which were measured using a petrographic microscope, range from 20 to 180 μm. Cathodoluminescence imaging of the zircon grains revealed several different internal

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growth patterns (Fig. 6). Most zircons exhibit well-developed oscillatory zoning, suggesting direct crystallization from a magma reservoir. Other zircons exhibit more complex growth patterns, e.g., some crystals contain a homogeneous or oscillatory zoned core surrounded by a mantle consisting of multiple bright and dark zones; these crystals have or have not a bright rim lacking internal structure (Fig. 6, see photos A, C, H, J, K, Z). A third group of grains display homogeneous low-luminosity zones or bright zones that lack internal structure and are inferred to have a metamorphic origin (Fig. 6, see photo O) (Corfu et al., 2003). A total of 265 zircon U-Pb ages were generated from four samples. Discussions of the age data are based on 206

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Pb/206Pb ages for grains older than 1000 Ma and

Pb/238U ages for younger grains (cf. Compston et al., 1992). All analyses are shown 11

ACCEPTED MANUSCRIPT on concordia plots (Fig. 7). Of the 265 analyses, 195 of them fall on or near the concordia trend (discordance ≤10%) and are displayed on relative probability plots and age histograms (Figs. 7 and S1). Age uncertainties for individual analyses are

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given as 1σ values in the data table and concordia plots. Zircon U–Pb isotopic

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compositions are presented in the supplemental data file.

A dominant age peak at 263-262 Ma is evident in all four samples (Fig. 7). In sample FS-1, 44 out of 46 zircon grains yield a main

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Pb/238U age peak at 262.6±

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1.2 Ma with mean square weighted deviation (MSWD) of 1.30. In sample JX-1, 43 out of 50 zircons define a tight age cluster with a mean

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Pb/238U age of 263.0±1.2

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Ma (MSWD = 0.85). In sample DJ-1, 35 out of 45 zircons have a mean 206Pb/238U age of 263.4±1.0 Ma (MSWD = 1.04). In sample LY-2, 40 out of 50 zircons have a mean age of 262.5±1.1 Ma (MSWD = 1.30). In addition, a subset of zircons showing

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any obvious age peak.

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characteristics of inheritance yields older ages ranging from 2500 to 400 Ma without

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Multiple zircon grains with 206Pb/238U ages in the range of 270-255 Ma (32 from LY-2, 32 from JX-1, 24 from DJ-1, and 28 from FS-1) were selected for detailed REE analyses. Two groups of chondrite-normalized REE patterns were distinguished

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among these zircon grains. Group 1 zircons show flat LREEs and an increasing HREE pattern, with relatively high La contents and small positive Ce anomalies. Group 2 zircons have steeply increasing REE patterns from La to Lu, with strong positive Ce anomalies and negative Eu anomalies (Fig. S2). The Th/U ratios of these zircon grains range from 0.08 to 1.7 (Fig. 8A). Their Ti contents are higher in LY-2 (2.2-763 ppm, avg. 61 ppm) than in other samples (JX-1: 1.3-154 ppm, avg. 17 ppm; DJ-1: 1.7-215 ppm, avg. 24 ppm; FS-1: 2.0-24 ppm, avg. 6.1 ppm).

5. Discussion 5.1. Zircon genesis

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ACCEPTED MANUSCRIPT Zircons from Upper Permian bauxite deposits in the Youjian Basin mainly have a magmatic origin (Fig. 6). Although some pre-Permian (inherited) zircons have homogeneous or core-mantle-rim internal structures, almost all Permian-age zircons

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exhibit internal oscillatory zoning, and all except one (FS-1-26) have Th/U ratios ranging from 0.16 to 1.7 (Fig. 8A). Both oscillatory zoning (Corfu et al., 2003) and

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high Th/U ratios (>0.10; Rubatto, 2002) are indicative of magmatic genesis of zircons. In contrast, hydrothermal genesis has been inferred on the basis of flat chondrite-normalized LREE patterns (Hoskin, 2005; Pelleter et al., 2007), although

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some researchers have argued that zircons from granitoids can yield a similar pattern (Belousova et al., 2002). For this reason, the inference of hydrothermal genesis

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requires additional evidence, e.g., homogenous internal structure or inclusions (Corfu et al., 2003) or trace-element enrichment (e.g., Nb, Ta, Th, U, Hf, Y, and HREEs)

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(Hoskin, 2005). In this study, the Group 1 zircons have a flat chondrite-normalized

they show similar

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LREE pattern but lack the typical internal structures of hydrothermal zircons, and Pb/238U ages, trace element concentrations, and Th/U ratios to

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Group 2 zircons. Previous studies in the Youjiang Basin have linked the flat LREE pattern to ELIP igneous rocks (Huang et al., 2014; Yang et al., 2012). All these lines

origin.

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of evidence support a magmatic origin for these zircons rather than a hydrothermal

The particular tectonic setting of magmatic zircons can be distinguished by their trace-element compositions (Hawkesworth and Kemp, 2006). Nb/Hf-Th/U and Hf/Th-Th/Nb discriminant plots have been successfully utilized to distinguish the within-plate/anorogenic and arc-related/orogenic origins for detrital zircons within middle and upper Permian strata of the Youjiang Basin (Huang et al., 2014; Yang et al., 2012, 2015; Zhong et al., 2013; Hou et al., 2014). In this study, zircons from the bauxite deposits with Middle to Late Permian ages can be separated into two types: zircons that fall within the within-plate/anorogenic field mostly come from sample LY-2, whereas zircons that fall within the arc-related/orogenic field come from samples JX-1, FS-1, and DJ-1 (Fig. 8C-D). Thus, zircons from the bauxite ore deposit 13

ACCEPTED MANUSCRIPT have both within-plate/anorogenic and arc-related/orogenic characteristics.

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5.2. Mixed provenance for Upper Permian bauxite deposits in western Guangxi Previous studies of Upper Permian strata in the Youjiang Basin revealed two

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main sources of material: ELIP-related volcanic rocks and arc-related volcanic ash. On the attached carbonate platforms on the northern margin of the basin (e.g., Chaotian), the mudstone from the “Heshan Bed” and “Wangpo Bed” between the

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Maokou and Wuchiaping formations shows affinity to ELIP felsic volcanic rocks (He et al., 2010b). In deep shelf facies within the basin (e.g., Napo and Banai), the age

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clustering of the detrital zircons at ~260 Ma and the geochemical signatures of the sandstone and mudstone from the lower part of the Upper Permian Linghao

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(=Wuchiaping) Formation indicate a sedimentary source dominated by ELIP mafic

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rocks (Huang et al., 2014; Yang et al., 2012), but at Sidazhai the coeval clastic sediments have a source in ELIP felsic rocks (Yang et al., 2015). On the other hand,

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on attached carbonate platforms on the northeastern side of the basin (e.g., Laibin), zircons in the claystone layers between the Maokou and Heshan formations indicate a mixed provenance: the claystone layer below the Middle-Upper Permian boundary

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shows affinity with the ELIP-related mafic rocks, and the claystone above the boundary shows affinity with the arc-related volcanic ash. The main age peak of detrital zircons appears at 269-260 Ma, but minor age peaks are also present at 850-820 Ma, indicating an extra source of detritus from the basement of the South China Craton, probably from the Jiangnan Orogenic Belt (Zhong et al., 2013). The bauxite deposit on the Pingguo Platform in the central part of the basin is considered to derive from weathering products of arc-related volcanic ash (Hou et al., 2014). Sediments at Pingguo and Laibin likely have the same provenance, i.e., arc volcanism between the South China and Indochina cratons (i.e., the Truong Son arc). Although Wei et al. (2013) proposed that the bauxite deposits in western Guangxi were derived through weathering of the underlying Maokou Formation, this 14

ACCEPTED MANUSCRIPT nearly pure limestone unit does not contain sufficient aluminum for bauxitization (Bárdossy, 1982). The Maokou Formation in the Youjiang Basin is composed of medium-bedded to massive bioclastic and micritic limestones with chert nodules, and

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marlstones are not present (BGMRGZAR, 1985). The original thickness of the Maokou limestone ranges from 250 m to 500 m with an average of 350 m (He et al.,

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2010a). The Al2O3 content of the Maokou Formation limestone ranges from 0.1% to 1.5% with a mean of 0.7% and the SiO2 content ranges from 0.05% to 6.7% with a mean of 1.5% (Wei et al., 2013; Yu et al., 2014). The thickness of the bauxite deposits

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ranges from 4 to 10 m, so if the Maokou Formation were the only Al source for the bauxite deposit, dissolution of ~240 to 600 m of limestone would have been necessary

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to provide enough aluminum, i.e., roughly the entire thickness of the Maokou Formation. A more reasonable interpretation is that the Maokou Formation provided

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only a fraction of the weathered materials for bauxite genesis.

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Whole-rock geochemical analyses document a close relationship between Upper Permian bauxite deposits and coeval sedimentary rocks in the Youjiang Basin. A

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strong negative correlation of Al2O3 with SiO2 and moderate to strong positive correlations of Al2O3 versus TiO2, Zr, Sc, Th, and Ta are present in both the bauxite layer samples and the siliciclastic sediments, indicating that the bauxite deposits were

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the product of weathering of the latter (Fig. 4). In La-Th-Sc and Zr-Th-Sc ternary discriminant plots, bauxite samples from the Napo section show a close relationship with Emeishan basalts, and samples from all other sections fall along a mixing trend between mafic and felsic endmembers (Fig. 9), in which the mafic endmember represents ELIP basalt and the felsic endmember represents a combination of felsic volcaniclastics from ELIP and the Truong Son arc. PAAS-normalized REE patterns of bauxite samples show features of chemical weathering products with HREE-enriched and positive Ce anomalies patterns (Fig. 5A, B). The REE patterns of Laibin claystone samples (Fig. 5C) show an affinity with those of magmatic arc rocks (Zhong et al., 2013). The REE patterns of clastic rock samples from the Chaotian, Banai, and Napo sections have an affinity with those of Emeishan igneous rocks (Fig. 15

ACCEPTED MANUSCRIPT 5D-F). The zircon U/Pb and elemental data indicate that the Upper Permian Heshan

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Formation bauxite deposits have two main sources, which are shared with coeval

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mudstones and sandstones of the Wuchiaping and Linghao formations in the Youjiang

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Basin.. Zircons from bauxite samples all give similar U-Pb age peaks of 263-256 Ma (Deng et al., 2010; Hou et al., 2014; this study). These ages are close to those for zircons from sandy-silty detrital sediments at Napo (262-261 Ma), Banai (261-260

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Ma), and Sidazhai (262-257 Ma), and from the claystone at Laibin (269-260 Ma). In view of the ages of Emeishan magmatism (263-257 Ma; He et al., 2010b, Yang et al.,

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2012) and of volcanic arc magmatism between the South China and Indochina cratons (~300-260 Ma; Halpin et al., 2015), either volcanic source could potentially have provided clastic materials to the Youjiang Basin. Based on Nb/Hf-Th/U and

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Hf/Th-Th/Nb discriminant plots, multiple origins for the zircon crystals can be

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recognized (Fig. 8C-D). The Upper Permian bauxite deposit at Leye contains a high proportion of zircon grains from within-plate/anorogenic sources, whereas bauxite

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deposits at Fusui, Jingxi, and Debao contain mainly zircons from arc-related/orogenic sources (Fig. 8D). Further evidence is provided by differences in the Ti content of the zircons. Zircons in the LY-2 sample (Leye) contain a higher concentration of Ti than

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zircons in the other three samples. The Ti content of zircons, which is related to rutile inclusions, has a strong temperature dependence but only a weak or absent relationship to pressure (Ferry and Watson, 2007). The Ti contents of zircons in sample LY-2 indicate a crystallization temperature of 700-800℃ (Fig. 8B), a result that is close to the temperature range of Emeishan felsic magmas (~750℃ - ~1000℃; Shellnutt and Jahn, 2010; Shellnutt and Iizuka 2011, 2012; Shellnutt et al., 2011a, b; Usuki et al., 2015) as well as to the estimated temperatures of zircons from the Upper Permian Wuchiaping Formation, which were sourced from erosion of Emeishan felsic volcanics (Yang et al., 2015). In contrast, the Ti concentrations of zircons in samples DJ-1, JX-1, and FS-1 indicate lower crystallization temperatures, from 600℃ to 700℃ (Fig. 8B). If these zircons are arc-derived (as inferred in this study), then the higher 16

ACCEPTED MANUSCRIPT water content of subduction-related arc magmas would account for the lower temperatures of these zircons (Miller et al., 2003).

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Based on the analysis above, a NE-trending boundary line can be drawn through

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the Youjiang Basin that separates platforms and deep-shelf areas to the north that

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received volcaniclastic material mainly from weathering of ELIP rocks from carbonate platforms to the south that received volcaniclastic material mainly from the

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Truong Son arc between the South China and Indochina cratons (Fig. 10A).

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5.3. Eolian influences on bauxite formation

In the Youjiang Basin, lower Upper Permian bauxite deposits are present only in isolated carbonate platform facies. Between these platforms, the inter-platform

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troughs (as represented by the Napo, Banai, and Sidazhai sections) accumulated

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greater thicknesses (~200-300 m) of Upper Permian deposits consisting of mineralogically immature (including volcanic and carbonate rock fragments) and

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poorly sorted (0.1-1 mm) grains, indicating a proximal detrital source (Huang et al., 2014; Yang et al., 2015, 2012). These troughs trapped coarse (i.e., sand-sized) fluvial detritus, but mud-sized material (i.e., clay and silt) may have been swept in

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suspension across the troughs and deposited on the isolated platforms. However, the most likely mechanism for transportation of fine (silt-sized) zircons to the isolated carbonate platforms is wind (Fig. 10B). Quaternary analogs provide examples of the influence of eolian processes on the geochemical composition of carbonate-platform soils. In northern Jamaica, Quaternary terra rossa soils accumulated on carbonate reef terraces. Results from major and trace element geochemistry show that siliciclastics in the soils were derived not from local limestones or igneous rocks but, rather, from airborne dust wafted from Africa and volcanic ash from the Lesser Antilles volcanic arc (Muhs and Budahn, 2009). Quaternary bauxite deposits on Jamaica are also considered to be primarily eolian in origin, representing a mixture of volcanic ash and airborne dust (Comer et al., 17

ACCEPTED MANUSCRIPT 1980). Similarly, a study of the mineralogical, geochemical and Sr–Nd–Hf–Pb isotope compositions of Neogene sediments from the eastern Mediterranean Sea reveals substantial inputs of Aegean volcanic arc material to the Herodotus Trench (Klaver et

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al., 2015). As zircon is resistant to weathering and eolian transport minimizes grain erosion, the zircon crystals in eolian sediments commonly preserve pristine shapes

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despite long-distance transport (Brimhall et al., 1988; Seranne, et al., 2008). Volcanic ash is a good source for Al in bauxite deposits (Bárdossy, 1982; Comer,

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1974, 1980). Its fine granularity and composition make volcanic ash prone to rapid weathering under warm and humid climate conditions, forming first a lateritic soil and,

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with continued weathering, a bauxitic soil. Upper Permian claystone and mudstone layers at Chaotian and Laibin represent the original sediments prior to bauxitization. Wind-derived volcanic ash can be transported over large distances. In the Caribbean,

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the Lesser Antilles volcanic arc is ~1000 km away from Jamaica. In the eastern

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Mediterranean Sea, the distance of the Aegean volcanic arc from the Herodotus Trench is ~800 km (Klaver et al., 2015; Muhs and Budahn, 2009). New

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paleomagnetic data from Upper Permian volcanic sequences in northwestern Vietnam yield a paleolatitude of ~15°S (Chi et al., 2014), whereas the study sites on the southwestern margin of the Youjiang Basin were located at a paleolatitude of ~5°S

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(Domeier and Torsvik, 2014; Jerram et al., 2016), a distance of >1000 km from the arc sources.

6. Conclusions Geochronologic dating and trace-element analysis of whole-rock and detrital zircon samples from bauxite deposits at the base of the Upper Permian Heshan Formation indicate two sources of volcaniclastic material to the Late Permian Youjiang Basin of the South China Craton. On the north side of the basin, bauxite deposits on isolated carbonate platforms and siliciclastic sediments in inter-platform troughs (as preserved in the Napo, Leye, and Banai sections) were derived from 18

ACCEPTED MANUSCRIPT volcanic ash sourced in the Emeishan Large Igneous Province (ELIP), whereas on the south side of the basin, volcanic ash was derived mainly from the Truong Son arc between the South China and Indochina cratons. Although coarser clastic material was

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deposited in deep-water troughs proximal to the ELIP, the fine-grained material transported to isolated carbonate platforms within the Youjiang Basin was delivered

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mainly through eolian processes. Thus, both igneous rocks from ELIP and volcanic ash from the volcanic arc between the South China and Indochina cratons were

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important sources of detritus to the Late Permian Youjiang Basin.

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Acknowledgments

We thank Brian Jones for editorial handling and Greg Shellnutt and an

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anonymous reviewer for constructive reviews of the manuscript. This work was

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supported by China Geological Survey (CGS) “Formation and Enrichment Regularities of Bauxite Deposit in Integrated Exploration Area of West Guangxi”

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Zhou, L., Algeo, T.J., Feng, L., Zhu, R., Pan, Y., Gao, S., Zhao, L., Wu, Y., 2016. Relationship of pyroclastic volcanism and lake-water acidification to Jehol Biota mass mortality events (Early Cretaceous, northeastern China). Chemical Geology 428, 59-76.

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IP

T

Figure captions

Fig. 1. (A) Paleogeography of Tethys Ocean region during the late Middle Permian

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(~260 Ma). Base map courtesy of R. Blakey (http://jan.ucc.nau.edu/rcb6/). (B) Generalized tectonic domains of southwestern South China Craton and middle-late Permian paleogeography of the Youjiang Basin. IP = isolated carbonate platform;

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NVT = North Vietnam Terrane; ELIP = Emeishan Large Igneous Province. The age of ELIP rocks are from Shellnutt (2014), ages of Permian arc-related rocks are from

MA

Halpin et al. (2015) and Li et al. (2006). (C) Permian strata in northwestern Guangxi (modified from BGMRGR, 1985) and ages of bauxite deposits and clastic rocks

D

above the Middle-Upper Permian boundary. Data sources: Debao, Fusui, Jingxi and

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Leye isolated carbonate platform sections (this study), Pingguo isolated carbonate platform section (Deng et al., 2010; Hou et al., 2014), Chaotian (He et al., 2010),

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Napo (Huang et al., 2014), Banai (Yang et al., 2012), and Laibin (Zhong et al., 2013).

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Fig. 2. (A) Stratigraphic correlations of Middle-Upper Permian carbonate platform facies and inter-platform trough facies in the Youjiang Basin, Guangxi, South China. (B) Expanded view of the Middle-Upper Permian transition and sampling locations. Sources as in Figure 1.

Fig. 3. Field pictures of (A) Leye section, (B) Debao section, and (C-D) Jingxi section. Note the bauxite deposits in karst pits of the underlying Maokou Formation in (D).

Fig. 4. Al2O3 versus (A) SiO2, (B) TiO2, (C) Zr, (D) Sc, (E) Th, and (F) Ta. Yellow area in (B) indicates high-Ti basalt. Blue and red dashed lines indicate the projected 28

ACCEPTED MANUSCRIPT weathering line of average upper continental crust (AUCC) and average bulk continental crust (ABCC), respectively (McLennan, 2001). Data sources as in Figure 1 plus Emeishan basalt (Xiao et al., 2004) and bauxite samples (Wei et al., 2013; Yu et

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IP

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al., 2014; Hou et al., 2014).

Fig. 5. PAAS-normalized REE distributions. (A) Bauxite, Heshan Formation, Leye isolated carbonate platform. (B) Bauxite, Heshan Formation, Jinxi, Pingguo, and

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Fusui isolated carbonate platforms. (C) Claystone, Heshan Formation, Laibin attached carbonate platform. (D) Mudstone, Maokou Formation, Chaotian. (E) Tuffs and

MA

clastic rocks, Banai. (F) Emeishan basalt and clastic rocks, Napo. Data sources as in Figures 1 and 4. Post-Archaean average Australian sedimentary rock (PAAS) data are

TE

D

from McLennan (1989).

Fig. 6. Cathodoluminescence (CL) images showing internal structures of

CE P

representative zircons. A-G come from sample FS-1, H-N come from sample JX-1,

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O-U come from sample O-U, V-Z come from sample DJ-1.

Fig. 7. Detrital zircon U-Pb concordia diagrams. (A) FS-1, (B) JX-1, (C) DJ-1 and (D) LY-2.

Fig. 8. (A) Zircon Th/U age spectrum; all zircons have a magmatic source. (B) Histogram of zircon Ti concentrations; temperature estimates based on Ferry and Watson (2007). (C) Th/U versus Nb/Hf and (D) Th/Nb versus Hf/Th for 270-260-Ma zircons. Data sources as in Figures 1 and 4. The discrimination fields in (C) and (D) are after Yang et al. (2012).

29

ACCEPTED MANUSCRIPT Fig. 9. Discriminant diagrams for bauxite samples and clastic rock samples above the Middle-Upper Permian boundary. (A) La–Th–Sc ternary plot; (B) Zr-Th-Sc ternary

IP

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plot (after Taylor and McLennan, 1985). Data sources as in Figures 1 and 4.

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Fig. 10. (A) Zircon sources to the Middle-Late Permian Youjiang Basin of South China. Source areas of ELIP detritus are shown in yellow. (B) Cross-section of Middle-Late Permian Youjiang Basin showing Truong Son volcanic arc on the left

AC

CE P

TE

D

MA

NU

(southeast) and the Emeishan LIP on the right (northwest).

30

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D

MA

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IP

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Fig. 1

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D

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IP

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Fig. 2 32

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Fig. 3

33

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D

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Fig. 4

34

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Fig. 5

CE P

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D

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IP

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35

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Fig. 6

CE P

TE

D

MA

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IP

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36

D

MA

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IP

T

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CE P

TE

Fig. 7

37

TE

D

MA

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IP

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CE P

Fig. 8

38

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IP

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CE P

TE

D

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Fig. 9

39

Fig. 10

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D

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40

IP

T

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Lithology

Facies

Data

Source

Geochemical and zircon

Yu et al., 2014 and this study

Fuisui

Bauxite

Isolated carbonnte platform

2

Pingguo

Bauxite

Isolated carbonnte platform

Geochemical and zircon

3

Debao

Bauxite

Isolated carbonnte platform

Zircon

4

Jingxi

Bauxite

Isolated carbonnte platform

Geochemical and zircon

5

Leye

Bauxite

Attached carbonnte platform

Geochemical and zircon

This study

6

Laibin

Claystone

Attached carbonnte platform

Geochemical and zircon

Zhong et al., 2013

7

Napo

sandstone and mudstone

Geochemical, zircon and

8

Banai

sandstone and mudstone

Inter-platfrom trough

Geochemical and zircon

Yang et al., 2012

9

Sidazhai

sandstone and mudstone

Inter-platfrom trough

zircon

Yang et al., 2012

CE P

TE D

MA N

US

1

AC

Number Location

CR

Table 1. Data source for sections with different facies

Inter-platfrom trough

Sr-Nd isoptpe

41

Wei et al., 2013; Deng et al., 2010 and Hou et al., 2014 This study Wei et al., 2013; Deng et al., 2010 and this study

Huang et al., 2014

ACCEPTED MANUSCRIPT Table 2. Elemental analysis results for samples in Upper Permian bauxite deposit ( Leye section ) C-2

C-3

C-4

C-5

C-6

SiO2

33.77

49.82

14.05

15.64

41.19

48.23

Al2O3

24.92

30.81

42.04

42.9

29.33

29.68

TFe2O3

21.47

3.68

24.68

18.27

14.95

5.48

MgO

0.19

0.11

CaO

0.45

0.32

Na2O

0.54

0.33

K2O

1.08

TiO2

3.04

P2O5 MnO

0.16

0.13

0.25

0.14

0.32

0.06

0.07

0.29

0.59

1.98

0.01

0.03

0.21

0.99

4.84

4.75

5.55

2.35

3.61

0.05

0.05

0.02

0.02

0.02

0.01

0.14

<0.01

0.02

0.01

0.01

<0.01

13

7.61

11.15

15.35

8.65

9.9

98.65

99.55

99.20

98.65

98.16

98.97

980

790

50.6

51.1

76.1

58.2

Rb

18.5

45.9

0.2

1.4

1.1

23.2

Be

3.2

1.2

7.8

3.5

1.8

1.5

Sr

131.5

105.0

20.2

29.8

75.3

170.5

Ba

311

552

33.7

43.9

115.0

391

Cu

333

6.4

23.5

87.5

89.1

106.0

Zn

149

6

214

114

154

48

Ga

52.7

37.7

50.1

38.4

39.4

55.7

Bi

1.24

1.67

2.45

3.56

1.25

1.71

Ni

82.6

3.4

100.0

55.8

80.6

49.2

Cr

460

320

820

730

260

850

V

345

370

420

479

211

428

Sc

11.2

23.5

32.1

36.4

16.2

15.6

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MA

CE P

Li

TE

Total

(ppm)

IP

1.02

LOI

Trace elements

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0.56

NU

2.29

D

Major oxides(%)

Samples

T

C-1

Elements

42

1465

1135

1505

1500

730

Hf

22.5

33.3

27.4

37.6

39.0

17.9

Nb

111.5

175.0

138.0

187.0

216

85.3

Ta

6.7

10.9

8.2

11.2

13.4

5.3

W

5

3

2

4

1

4

Th

29.8

35.4

39.7

46.0

29.0

U

11.30

7.38

6.62

6.91

8.83

15.05

La

91.3

95.8

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46.9

83.5

57.1

62.3

55.5

Ce

362

238

294

86.1

656

43.4

Pr

22.3

18.00

25.0

8.73

18.90

15.20

Nd

80.3

53.1

95.7

29.2

75.6

59.6

Sm

15.10

7.99

23.4

8.42

22.4

12.25

Eu

2.91

1.49

4.81

2.24

4.39

2.48

14.95

6.01

19.70

10.90

19.75

10.45

3.09

1.33

3.41

2.43

3.85

1.88

Dy

22.8

10.55

20.5

17.70

22.0

13.35

Ho

5.23

2.45

4.31

4.24

4.43

2.92

Er

15.65

7.67

11.20

12.90

13.15

7.60

Tm

2.38

1.22

1.59

1.98

2.01

1.14

Yb

15.35

8.69

10.35

13.80

14.25

7.78

Lu

2.32

1.44

1.60

2.09

2.16

1.16

Y

148.5

54.7

100.5

90.5

96.1

76.6

LREE

573.9

414.4

526.4

191.8

839.6

188.4

HREE

81.8

39.4

72.7

66.0

81.6

46.3

∑REE

655.7

453.7

599.1

257.8

921.2

234.7

L/H

7.0

10.5

7.2

2.9

10.3

4.1

LaN/YbN

0.44

0.81

0.60

0.31

0.32

0.53

Eu/Eu*

0.91

1.01

1.05

1.08

0.98

1.03

Rare earth elements

AC

CE P

Tb

TE

Gd

MA

(ppm)

43

IP

T

918

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Zr

D

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1.32

1.47

0.87

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CE P

TE

D

MA

NU

SC R

IP

T

Ce/Ce*

44

4.37

0.34

MA

NU

SC R

IP

T

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TE

D

Graphical abstract

45

ACCEPTED MANUSCRIPT Highlights

T

IP

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 

Upper Permian bauxite deposits accumulated on isolated carbonate platforms Bauxite Al was derived from two sources of wind-blown volcanic ash Mafic volcanic clasts was derived from the Emeishan Large Igneous Province to the SW Felsic volcanic ash was derived from the Truong Son volcanic arc to the SE These two ash sources exhibit a mixing gradient across the Youjiang Basin

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  

46