tters 256 (2007) 389–402www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Neogene weathering and supergene manganese enrichment insubtropical South China: An 40Ar/39Ar approach

and paleoclimatic significance

Jian-Wei Li a,b,⁎, P. Vasconcelos c, N. Duzgoren-Aydin d, Dai-Rong Yan e,Wei Zhang f, Xiao-Dong Deng b, Xin-Fu Zhao d,

Zhong-Ping Zeng g, Ming-An Hu b

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,Wuhan, Hubei Province 430074, China

b Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, Chinac Department of Earth Sciences, The University of Queensland, Brisbane 4072, Australia

d Department of Earth Sciences, The University of Hong Kong, Hong Kong, Chinae Geological Survey of Hubei Province, Wuhan 430030, China

f Department of Mineral Resources Assessment, China Geological Survey, Beijing 100011, Chinag Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China

Received 4 September 2006; received in revised form 17 January 2007; accepted 17 January 2007

Available onlin

Editor: R.W. Carlsone 26 January 2007

Abstract

Deep weathering profiles and associated supergene Mn-oxides deposits are widely distributed in South China. However, data on theirages are not available untilmost recently. 40Ar/39Ar laser incremental heating analysis of potassium-bearing supergeneMn-oxides collectedfrom four weathering profiles in the Qinzhou–FangchengMn belt, Guangxi Province, South China, provides, for the first time, numericalconstraints on timing of weathering and supergene Mn enrichment. Thirty-eight of 42 cryptomelane grains yield well-defined plateau (36grains) or pseudo plateau (2 grains) ages ranging from 17.6±0.5 Ma to 4.71±0.08 Ma (2σ). The results, when combined with data fromMn-oxides deposits from the adjacent Guangdong and Hunan Provinces, document a protracted history of weathering and supergene Mnenrichment, spanning from the earliest Miocene to the latest Pliocene in South China. This indicates that warm and humid climaticconditions conducive to intense weathering and secondary mineralization prevailed across South China during the whole Neogene. Theclimatic conditions inferred from weathering geochronology are consistent with sedimentary records and fossil floral and faunaassociations from Cenozoic inland and marginal sea basins of South China. The weathering geochronology data also provide temporalconstraints on the initiation and intensification of the East Asian monsoon that has been the cause for the South China's humid climate.© 2007 Elsevier B.V. All rights reserved.

Keywords: weathering; 40Ar/39Ar; cryptomelane; Miocene; East Asian monsoon; South China

⁎ Corresponding author. Faculty of Earth Resources, ChinaUniversity of Geosciences, Wuchang, Wuhan, Hubei Province,430074, China. Tel.: +86 27 67883061; fax: +86 27 67885096.

E-mail address: jwli@cug.edu.cn (J.-W. Li).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.01.021

1. Introduction

Deepweathering profiles result from intense and long-term chemical weathering facilitated by an optimum

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combination of factors such as climate, tectonism, lith-ology, and biota [1]. Of these, climate (temperature,precipitation, and seasonality) and tectonics are fre-quently considered as the major controls [2,3]. Undercircumstances where weathering overwhelms physicalerosion, ancient, thick and mineralogically and geo-chemically stratified weathering profiles may developand be continuously exposed to weathering fluidsthroughout their history [4–6]. The complex evolutionand longevity of weathering profiles indicate that theymay bear valuable information on climatic and tectonicconditions prevailing in the geological past [7–10].However, a major limitation in using weathering profilesas a paleoclimatic indicator has been the lack of reliablenumerical ages for these profiles [3].

Weathering processes also promote supergene enrich-ment of ore deposits, as exemplified by some largetonnage lateritic Mn deposits [11,12]. 40Ar/39Ar dating ofsupergene Mn-oxides is a useful tool to directly andprecisely date weathering and related secondary Mnmineralization [13–18]. Several studies, mainly fromAustralia, Brazil, and West Africa, suggest that the super-geneMn-oxides inweathering profiles record a prolongedhistory of mineral dissolution and re-precipitation lastingfor millions of years, and their ages can be reliably inter-preted as time periods more propitious to chemicalweathering [19–23]. Unfortunately, direct dating ofweathering and supergene mineralization in Asia is notavailable.

South China is characterized by wet subtropical totropical climate, which has been commonly linked to theEast Asian monsoon that brings moisture to southwestand southeast China, resulting in abundant precipitationannually [24]. Timing regarding initiation and longevityof the wet climate and the East Asian monsoon is stillnot clear. Paleomagnetic dating of aeolian sedimentsfrom the Chinese Loess plateau (NW China) yieldedresults suggesting that the monsoonal climate started atca. 2.6 Ma [25], 8–7 Ma [26], or 22 Ma [27]. A recentstudy of radiolarians recovered from the ODP Site 1143,South China Sea, led to a proposal that the East Asianmonsoon commenced at around 12–11 Ma andstrengthened at ca. 8 Ma [28]. By contrast, lithologicaland paleobotanical records from Cenozoic basins ofSouth China indicate humid climate possibly datingback to late Oligocene or early Miocene [24,29,30]. Theapparent discrepancy on the timing of the South Chinaclimate and East Asian monsoon results largely from alack of precise age resolution for the climatic proxiesadopted in these studies.

Large areas of South China are mantled byweathering profiles of great thickness, which host the

most important supergene Mn-oxides and bauxitedeposits in the country (Fig. 1a) [31,32]. It is unclearwhether these profiles and supergene deposits wereformed recently or relate to past climatic conditions. Themodern humid subtropical climate has led to specula-tions that deep weathering profiles and Mn-oxidesdeposits in South China are products of Quaternaryweathering [32]. Other authors, however, postulated thatsupergene Mn enrichment in South China started in theJurassic or Cretaceous [31]. The abundance of Mn-oxides hosted in deep weathering profiles and accessi-bility to vertical sections through these profiles at minesites provide an excellent opportunity to date weatheringprocesses and the formation of Mn-oxides deposits bythe 40Ar/39Ar method. In this paper, we report high-resolution 40Ar/39Ar ages of Mn-oxides sampled fromfour weathering profiles in the Qinzhou–Fangcheng(QF) Mn belt, Guangxi Province, to pose preciseconstraints on the timing and duration of weatheringand supergene Mn enrichment in South China. Theresults are further used to infer the paleoclimaticconditions that led to the formation of the South ChinaMn-oxides deposits.

2. Geological setting

The QF Mn belt (E108°00′–109°00′, N21°30′–22°10′) lies in the coastal plain of Guangxi Province,South China (Fig. 1). The area is characterized by humidsubtropical climate, with an average yearly precipitationof 2000 mm (up to 2200 mm) and mean annualtemperature of 21–22 °C [32]. Exposed bedrocksinclude Paleozoic marine sediments and Mesozoiccontinental deposits (Fig. 1b). The Silurian stratarepresent the earliest sedimentary cover in the area,consisting mainly of fine-grained sandstones, siltstonesand black shales. Conformably overlying the Siluriandeposits, the Upper Devonian Liujiang Formation isdominated by siliceous and carbonaceous shales, cherts,marles, and bioclastic limestones, with a total thicknessof 1900 m. These include four Mn-rich beds of cherts,shales, and limestones, individually 20 to 100 m thick,which have been the most important sources forsupergene Mn-oxides deposits throughout the belt[31–33]. Unconformably overlying the Devonianrocks, the Upper Permian consist of mudstones,sandstones and conglomerates, with thin interlayers ofcherts and coals. The Mesozoic strata are continentaland consist of red conglomerates, sandstones, and lesscommonly, siltstones and mudstones. Limited to somesmall inland basins, the Paleogene deposits consist ofconglomerates and sandstones, whereas the Neogene

Fig. 1. (a) Map of South China showing the distribution of major Mn-oxides deposits in South China and the location of the study area. X and Ddenote the Xinrong (Guangdong Province) and Dongxiangqiao (Hunan Province) Mn-oxides deposits, respectively. (b) Geological map of the QFarea. Also shown are the sampling sites from the Tanying (1), Liaolian (2), Dikuang (3), and Dadong (4) deposits (profiles). (c) A digital elevationmodel illustrating the geomorphic feature of the QF area.

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Fig. 2. (a) Typical landscape in the QF area composed of flat-top hills and low-lying valleys mantled by thick weathering profiles. (b) Thick, saprolite-dominated, and Mn-oxides enriched weathering profiles. The open pit mining provided excellent access to sample Mn-oxides throughout the profile.(c) Botryoidal Mn-oxides present as cavity infillings. (d) Well-crystallized, extremely pure, and cryptomelane-dominated Mn-oxides veins hosted inthe strongly weathered cherts (white) of the Upper Devonian Liujiang Formation. The veins comprise consecutive bands possibly precipitated over aprolonged time span.

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sediments, ranging in thickness from 100 to 1650 m, aremainly siltstones, grey mudstones, lignites, and oilshales [33]. Alluvial and fluvial deposits characterizethe Quaternary.

Separated by the NE-striking Nasu–Dazhi Fault, thenorthwestern part of the QF area is characterized by aseries of high and steep mountains, ranging in elevationfrom 400 to 1300 m above sea level (Fig. 1c). East of theNasu–Dazhi fault, landforms are dominated by flatplains and valleys, generally 20–50 m above sea level,with scattered flat top hills of 30–120 m and occa-sionally high elevation ridges of 200–420 m (Fig. 1c).Both the plains and the flat top hills are mantled byweathering profiles of variable thicknesses (Fig. 2a).They develop preferentially over the Paleozoic rocksand are typically 50–100 m deep, as revealed from ex-ploration drill holes [32] and mining open pits (Fig. 2b).In most cases, weathering profiles are dominated byferruginous and clayey saprolite. Profiles derivedfrom the Devonian rocks are significantly enriched in

Mn-oxides, whose concentrations are frequently highenough to allow commercial mining (Fig. 2b). Field andtextural relationships suggest that Mn-oxides formedmainly via direct precipitation of manganese from wea-thering solutions after lateral and/or vertical transport(Fig. 2c–d). A total of 12 Mn-oxide deposits have beendiscovered in the QF area, forming a 50-km long Mnbelt (Fig. 1b) [32,33]. These deposits are characterizedby high grade (28–45 wt.% Mn), high Mn/Fe ratios (6–10), low Si and P concentrations, and variable en-richment in Co, Ni, Cu, Ag, and Li (Li et al., un-published data).

3. Sampling and analytical procedures

We investigated four weathering profiles (Tanying,Liaolian, Dadong, and Dikuang) that host industrial Mn-oxides deposits (Fig. 1b). These profiles are 10–20 kmapart from each other, but display similar geomorphicfeatures characterized by flat top hills ranging from 30 to

Fig. 3. (a) Densely botryoidal texture suggesting direct precipitation of cryptomelane from weathering solutions into open spaces. (b) Microscopicgrowth bands of cryptomelane. The black arrows indicate the direction of cryptomelane growth. (c) Acicular crystallities of cryptomelane usuallyb1 μm in diameter. (d) Multiple generations of cryptomelane (Cm) indicating dissolution and re-precipitation of Mn-oxides.

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100 m in elevation. During the course of sampling, wecollected Mn-oxides exhibiting botryoidal and bandedtextures (Fig. 2c–d), which are generally devoid ofcontaminants and thus ideal for 40Ar/39Ar geochronology.Active open pit mining at Tanying and Liaolian (Fig. 2b)permitted systematic sampling from the top to the bottomof the profiles. At Dikuang, samples were collected fromdiscrete mining pits that span 40 m depth. At the Dadongsite, Mn-oxides were obtained only on flat hilltops 50–64 m in elevation and sampling at depth was not possibledue to inaccessibility. A total of 21 hand specimens werecollected. Vasconcelos [15] provided instrumental detailson how to obtain Mn-oxides samples suitable forweathering geochronology, which applied to this study.

Polished thin sections of each hand specimen werestudied by reflected- and transmitted-light microscopyto determine the mineralogy and paragenesis of thesupergene phases, the number of generations of Mn-oxide minerals, and potential contamination of primaryminerals. After petrographic examination, suitable partsof each sample were crushed, washed in distilled waterin an ultrasonic bath for 40–50 min, and dried. For each

sample, 30–50 grains of visually pure Mn oxidesranging from 1 to 2 mm in diameter were hand pickedunder a binocular microscope. The purity, composition,and crystallinity of these grains were further investigat-ed by a combination of X-ray diffractometry (XRD),electron microprobe analysis (EMP), and scanningelectron microscopy (SEM).

Powder XRD analysis was performed using a PhilipsPW3040 series diffractometer equipped with a copper-target tube, operating at 40 kV and 40 mA. The XRDoriented patterns were recorded using a step size of 0.02°2θ and a scan speed of 1° 2θ/min.After XRDexamination,three to four grains from each sample were mounted in anepoxy container and polished for SEM and EMP analysis.The mounted grains were carbon-coated and investigatedwith a Quanta200 environmental scanning electronmicro-scope equipped with an energy dispersive spectrometer,which allowed combined analysis of morphology andcomposition. EMP analyses were made using a JEOL 733Superprobe and a JEOL 8800R Superprobe.

On the basis of XRD, EMP, and SEM investigations,five to ten grains from each sample were loaded into

Fig. 4. Histogram of K concentrations (a) and K–Ba correlation(b) suggest that the samples used for 40Ar/39Ar dating are K-rich andcomposed mainly of cryptomelane, with minor amounts of hollandite.

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aluminum irradiation disks along with the Fish Canyonstandards (28.02±0.08Ma) [34]. The disks werewrappedin Al-foil, vacuum-sealed in silica glass tubes andirradiated for 14 h at the B-1 CLICIT facility at theRadiation Center, Oregon State University. Sample andflux monitor irradiation geometry followed those in [15].After a 2–3-month cooling period, we selected 1–3 grainsfrom each sample for 40Ar/39Ar laser heating analysis.Heating with a laser beam (Innova 300 Ar-ion laser)defocused to 2 mm diameter permitted each grain to beheated nearly homogeneously, as confirmed by visualobservation of grains through a video camera. Argongases were extracted at consecutively higher laser powerslasting for 54–62 s at each designated power. The gasesreleased during stepped heating were purified by exposedto a cryocooled trap (−136 °C) and two C-50 Fe–Ti–ZrSAES getter pumps. The cleaned gases were thenadmitted into an MAP 215-50 mass spectrometer,equipped with a third getter pump, for argon isotopicanalysis. During the argon analysis, a total of 86 full

system blanks and 43 air pipettes were analyzed (a blank-air-blank series before and after each grain), yielding40Ar/36Ar discrimination values ranging from 1.0022±0.0034 to 0.9957±0.0033 (Table S1). Full system blanksfor 40Ar range from 0.00534±0.00025 to 0.00979±0.00013nA during the analysis, values considerablybelow the 40Ar signals obtained for most gas fractionsanalyzed. All dates are reported using 5.543×10−10 a−1

as the total decay constant for 40K [35], and the values forthe reactor correction factors are: (2.64±0.02)×10−4 for(36Ar/37Ar)Ca, (7.04±0.06)×10

−4 for (39Ar/37Ar)Ca and(8.00±0.3)×10−4 for (40Ar/39Ar)K. The J factors foreach irradiation disk were obtained through theanalyses of at least 15 individual grains of the FCsanidine standard geometrically positioned as in [15],ranging from 0.003633±0.000024 to 0.003584±0.000015.

4. Results

4.1. Mineralogy

Petrographic observations and XRD and EMPanalyses indicate that our samples are composedprimarily of cryptomelane, with minor amounts ofpyrolusite, lithiophorite, and hollandite. Goethite andhematite are present in some samples, intergrown withor replaced by Mn-oxides. The XRD data also suggestpresence of trace amounts of quartz in a few samples,which, however, was not detected during EMP analysis.Cryptomelane typically displays botryoidal textureformed by micrometric bands (Fig. 3a–b), sometimesalternating with hollandite and lithiophorite. Suchmicroscopic bands resulted from incremental precipita-tion of cryptomelane needles, usually b1 μm in diameter(Fig. 3c), on the surface of a cavity or a previouslyprecipitated Mn-oxide layers. Multiple generations ofcryptomelane, as expressed by crosscutting relation-ships (Fig. 3d), suggest several events of dissolution andre-precipitation of Mn-oxides. The potassium concen-trations of cryptomelane, based on 264 EMP spotanalyses, range from 0.45 to 4.92 wt.%, with an averageof 2.79 wt.% (Fig. 4a), values sufficiently high to yieldreliable 40Ar/39Ar ages [17]. The K–Ba correlations(Fig. 4b) suggest the presence of hollandite andcompetition between K and Ba for the tunnel sites inthe hollandite-group mineral structure [36].

4.2. 40Ar/39Ar geochronology

All 40Ar/39Ar results, corrected for interfering isotopesand mass discrimination, are included in Table S1 and

Table 1Summary of the 40Ar/39Ar ages of Mn-oxides from the Qinzhou–Fangcheng area, South China

Sample Grain Run ID# Plateau age(Ma)±2σ

Integrated age(Ma)±2σ

Ideogramage (Ma)

Isochron age(Ma)±2σ

Intercept40Ar/36Ar

±

TanyingTY-02 TY-02-1 3458-01 6.95±0.05 6.95±0.05 6.96 6.94±0.06 301 8

TY-02-2 3458-02 6.46±0.06 6.47±0.06 6.46 6.48±0.07 292 4TY-02-3 3458-03 6.77±0.13 6.55±0.11 6.69 6.47±0.19 299 4

TY-06 TY-06-1 3461-01 7.65±0.08 7.52±0.07 7.65 7.68±0.08 281 13TY-06-2 3461-02 7.42±0.08 7.35±0.06 7.42 7.82±0.17 281 12TY-06-3 3461-03 *6.60±0.80 6.50±0.60 6.60 6.80±1.40 283 15

TY-07 Inner TY-07 Inner-1 3464-01 5.79±0.05 5.77±0.05 5.77 5.83±0.06 291 3TY-07 Inner-2 3464-02 5.82±0.06 5.85±0.06 5.92 5.85±0.09 294 3TY-07 Inner-3 3464-03 5.95±0.06 5.97±0.05 5.94 5.89±0.08 295 6

TY-07 Mid TY-07 Mid-1 3466-01 4.97±0.11 5.20±0.18 5.00 4.99±0.18 300 5TY-07 Mid-2 3466-02 4.99±0.10 5.01±0.19 5.10 5.0±0.16 296 5TY-07 Mid-3 3466-03 5.10±0.13 5.10±0.18 5.10 5.01±0.18 304 7

TY07 Outer TY-07 Outer-1 3467-01 4.91±0.09 4.93±0.09 4.89 4.63±0.17 305 4TY-07 Outer-2 3467-02 4.80±0.04 4.80±0.04 4.90 4.77±0.06 312 9TY-07 Outer-3 3467-03 4.71±0.08 4.75±0.08 4.73 4.42±0.20 306 5

TY10 TY-10-1 3470-01 5.85±0.07 5.83±0.06 5.42 5.40±0.16 301 6TY-10-2 3470-02 5.45±0.10 5.55±0.09 5.81 5.73±0.15 310 14

TY-13 TY-13-1 3472-01 8.41±0.05 8.41±0.05 8.42 8.80±0.17 290 4TY-13-2 3472-02 8.09±0.07 7.99±0.06 8.01 7.50±0.40 296 3TY-13-3 3472-03 7.83±0.06 7.84±0.06 7.85 7.40±0.20 289 10

LiaolianLL-02 LL-02-1 3553-01 5.79±0.08 5.79±0.08 5.89 5.85±0.14 291 8LL-04 LL-04-1 3555-01 *7.9±0.4 8.8±0.3 7.26 7.6±0.7 300 5LL-06 LL-06-1 3557-01 6.50±0.07 6.52±0.07 6.50 6.49±0.07 303 4LL-07 LL-07-1 3558-01 N.A. 9.42±0.10 N.A 9.97±0.15 269 9LL-10 LL-10-1 3565-01 5.69±0.07 5.70±0.07 5.70 5.58±0.10 314 10LL-11 LL-11-1 3566-01 6.83±0.09 6.84±0.09 6.80 6.77±0.10 303 3LL-13 LL-13-1 3568-01 10.3±0.3 10.5±0.3 10.9 10.6±0.3 300 4

DadongDD-01 DD-01-1 3578-01 12.2±0.3 12.8±0.4 12.2 12.4±0.4 307 14

DD-01-2 3578-02 17.6±0.5 16.2±0.8 16.6 17.0±0.6 302 6DD-06 DD-06-1 3580-01 15.6±0.4 15.20±0.40 14.9 14.2±0.6 305 9

DD-06-2 3580-02 15.5±0.2 14.68±0.20 15.5 15.5±0.2 309 10DD-07 DD-07-1 3583-01 11.70±0.20 11.90±1.00 11.5 12.2±0.3 284 5

DD-07-2 3583-02 10.54±0.15 10.44±0.14 10.5 9.3±0.4 300 10

DikuangDK-01 DK-01-1 3571-01 11.79±0.12 11.57±0.12 11.80 11.77±0.13 284 9

DK-01-2 3571-02 12.02±0.14 11.90±0.14 11.90 11.63±0.19 310 10DK-04 DK-04-1 3573-01 7.85±0.09 7.79±0.09 7.90 7.81±0.1 300 6

DK-04-2 3573-02 7.01±0.08 7.01±0.08 6.90 7.0±0.08 305 12DK-11 DK-11-1 3577-01 7.94±0.13 7.94±0.13 7.96 7.74±0.18 309 5

DK-11-2 3577-02 10.17±0.20 11.05±0.19 10.12 10.2±0.2 296 3

Note: (a) uncertainties in reported ages are at the two sigma level; (b) * denotes pseudo plateau age; (c) N.A.=not applicable.

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graphically illustrated in Fig. S1 as Supplementary Data.A summary of these results is presented in Table 1. In thisstudy, an age plateau is defined as a sequence of three ormore continuous steps that contain more than 50% of thetotal amount of 39Ar released, and yield apparent agesreproducible at the 95% confidence level. A pseudoplateau age is defined in two cases: (1) two or more

continuous steps containing more than 50% of the totalamount of 39Ar released yield apparent ages reproduc-ible at the 90% confidence level, or (2) the steps containslightly less than 50% of the total amount of 39Ar re-leased but the apparent ages are reproducible at the 95%confidence level. Uncertainties of all the results werereported at the 2σ level.

Fig. 5. 40Ar/39Ar age spectra, age probability density plots, and isochrons of selected samples from Tanying and Liaolian profiles show extremeconsistency between the plateau, ideogram, and isochron ages, attesting to the reliability of the results.

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4.2.1. The Tanying profileThree grains from sample TY-01 yield complex

spectra (Supplementary Fig. S2) that clearly indicatepresence of old minerals intergrown with the supergenephases. In this case, meaningful age information cannotbe retrieved and these results are simply discarded andwill not be considered in the subsequent discussion.

Twenty grains from other seven samples yield very well-defined plateau ages ranging from 8.41±0.05 Ma to4.71±0.08 Ma (Fig. S1a–g), indicating that the mineralshost their radiogenic and nucleogenic gas fractions intight crystallographic reservoirs that have been closedduring the history of the samples [16,36]. Moreover,characteristics of the spectra suggest that 39Ar loss by

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recoil and contamination of old phases did not occur inthese samples. In most cases, the plateau ages fordifferent grains from the same sample are reproduciblewithin errors, demonstrating that the grains represent asingle generation and that their plateau ages are trueprecipitation ages of the Mn-oxides analyzed [17]. Thespectra of different grains from samples TY-10 and TY-13 are not reproducible (Fig. S1f–g), and the slightdifference (0.4–0.6 m.y.) in the plateau ages betweengrains reflect real time span of their precipitation [37].Presence of incremental growth bands of Mn-oxides(Fig. 3a–b) is consistent with this interpretation. Thesamples also yield isochron ages and ideogram agesinternally consistent with their plateau ages (e.g.,Fig. 5a–c; Table 1). The isochrones have initial40Ar/36Ar ratios ranging from 314±9 to 281±13,indistinguishable from the present-day atmosphericvalue (295.5) and indicating that there was no excessargon associated with the decomposition of parentalminerals incorporated in the samples.

Dating of distinct bands of botryoidal Mn-oxides isuseful in providing information on the duration and ratesof mineral precipitation [14,21,22,37]. These data, whencombined with detailed geochemical information, maypermit evaluation of compositional variation of weath-ering solutions from which Mn-oxides were precipitated[15]. Three sub-samples extracted from the inner, mid-dle, and outer bands of sample TY-07, a 1-cm widecryptomelane concretion, yield progressively youngerplateau ages, ranging from 5.95±0.06 Ma in the core,5.10±0.13 Ma in the middle, to 4.71±0.08 Ma at theouter band (Fig. S1c–e), results consistent with therelative timing of the mineral precipitation inferred frompetrographic study. The results indicate that a 1-cm thickMn-oxide bands may host minerals precipitated duringmore than 1.55 m.y., yielding an average precipitationrate of 6.7×10−3 mm Kyr−1.

4.2.2. The Liaolian profileFive of seven samples from the Liaolian profile yield

well-developed plateau ages, ranging from 10.3±0.3Mato 5.69±0.07 Ma (Fig. S1h–l). Despite the fact that onlyone grain from each sample was analyzed, the excellentconsistency between the plateau, ideogram, and isochronages for each sample (Fig. 5d–f; Table 1) suggests thatthe results provide reliable constraints on the formationages of the supergene Mn-oxides. The spectra alsoindicate that the samples contained no excess argon anddid not undergo argon loss after their formation.

Sample LL-04 yields a relatively disturbed spectrumwith large errors, which did not reach a plateau(Fig. S1m). The large uncertainties of the apparent

ages are most likely due to significant incorporation ofatmospheric argon in the tunnel sites of the cryptome-lane mineral [15] and the low radiogenic argon gasfraction (generally b25%; Table S1). However, the hightemperature steps (steps J and K), accounting for ca.60% of the total 39Ar released, yield a pseudo plateauage at 7.9±0.4 Ma (Fig. S1m). Nine of the 11 heatingsteps (excluding steps G and I) yield a well-definedisochron, with an isochron age of 7.6±0.7 Ma and an40Ar/36Ar ratio of 300±5 (insert of Fig. S1m). Theisochron age is consistent with the pseudo plateau age,and thus can be taken as the best estimate for theprecipitation age of this sample.

Sample LL-07 yields an ascending staircase spec-trum (Fig. S1n). In 40Ar/39Ar geochronology of Mn-oxides, staircase spectra commonly reflect presence ofmicrometric growth bands spanning a prolonged periodof precipitation [15]. If the older bands are morethermally stable, they release their radiogenic andnucleogenic argon contents at progressively highertemperature steps, resulting in ascending spectra, asexemplified by sample LL-07. When the thermalstabilities of distinct bands are only slightly different,a plateau or pseudo plateau age may still be obtained[34]. Otherwise, a plateau cannot be reached. In eithercase, nevertheless, meaningful age information of suchbanded samples is still retrievable: the age differencebetween the oldest and youngest bands may approxi-mate the time span of the mineral precipitation [15].Examination on the spectrum of sample LL-07 suggeststhat this 2-mm thick grain host minerals precipitatedduring a time interval of ca. 4.5 m.y., and thus record aprotracted history for Mn-oxides precipitation at anaverage rate of 0.44×10−3 mm Kyr−1. This value islower than that obtained for samples from the Tanyingprofile, but comparable with those previously reported[21,22,37].

4.2.3. The Dadong profileSix grains from three samples at Dadong yield

plateau ages ranging from 17.6±0.5 Ma to 10.5±0.2 Ma(Fig. S1o–q). While sample DD-06 yield plateau agesreproducible between distinct grains, a slight agedifference (1.24 Ma) exists between grains from sampleDD-07. Age differences of this magnitude are notuncommon in mm-scale botryoidal Mn-oxides grainsand may reflect true age variation in distinct bands ofsuch grains. However, the age difference between thetwo grains from sample DD-01 is up to 5.4 m.y., whichmay indicate presence of more than one generation ofcryptomelane in the sample, as indicated by petrograph-ic investigations (Fig. 3d).

Fig. 6. (a) 40Ar/39Ar age distribution of supergene Mn-oxides fromGuangxi (this study), Guangdong [38], and Hunan [39] Provinces,substantiating extensive weathering and supergene Mn enrichment ofNeogene time throughout South China. (b) Age probability densityplot of Mn-oxides in the QF area, showing the broad consistency ofweathering ages between profiles.

398 J.-W. Li et al. / Earth and Planetary Science Letters 256 (2007) 389–402

Mn-oxides analyzed in this study are characterized bybotryoidal texture consisting of micrometric bands, span-ning a prolonged period of precipitation (Fig. 3a–b).Incremental heating analyses of such banded samples willrelease gas fractions from distinct bands simultaneously,giving rise to descending or ascending staircase spectra[15]. If the younger phase is more thermally stable, thespectrum is descendent. When the older phase is morestable, the spectrum is ascendant. One grain (run 3578-02)from sample DD-01 yields a descending staircasespectrum (Fig. S1o), whereas one grain (run 3580-02)from sample DD-06 yields an ascending spectrum(Fig. S1p). Both grains yield well-defined plateau agesat high temperature steps and pseudo plateau ages at low-to-middle temperature steps. The difference between theplateau and pseudo plateau ages from each grain mayrepresent the period of mineral precipitation in the grain.

4.2.4. The Dikuang profileSix grains from three samples at the Dikuang profiles

yield well-defined plateau ages ranging from 12.0±0.1to 7.01±0.08 Ma (Fig. S1r–t). The plateau ages of eachgrain are also consistent with the respective integrated,isochron, and ideogram ages (Table 1), confirming theirreliability. Results of two grains from sample DK-01 areextremely reproducible, whereas different grains fromthe other two samples yield plateau ages showing agedifferences of 0.8 to 2.2 m.y., reflecting true variation inage of the sample.

5. Discussion

5.1. The Neogene weathering of South China

Thirty-eight Mn-oxide grains from four weatheringprofiles in the QF area, up to 50 km apart, yield well-defined plateau (36 grains) or pseudo plateau (2 grains)ages ranging from 17.6±0.5 Ma to 4.71±0.08 Ma,indicating a protracted history of weathering andsupergene Mn enrichment from the early Miocene to theearly Pliocene (Fig. 6). The fact that most weatheringprofiles in the study area are dominated by truncatedsaprolite zone and devoid of indurated hardcaps impliesactive erosion during the formation of weathering profiles[3,10]. This in turn suggests that the oldest supergeneminerals precipitated at the early stage of weatheringcould have been eroded, leading to incomplete records ofthe weathering history. Thus, the present geochronolog-ical results provide merely a minimum constraint on thetiming of weathering and supergene Mn enrichment. Anearliest Miocene or latest Oligocene weathering event isdocumented at the Xinrong Mn-oxides deposit, Guang-dong Province, 200 km east of the QF area (Fig. 1a),where cryptomelane samples from the uppermost level ofweathering profiles yielded 40Ar/39Ar plateau ages in the23.48–21.6 Ma range [38]. Likewise, the youngest ageof Mn-oxides at 4.71 Ma do not mean weatheringceased after the early Pliocene, as samples at greaterdepths in each profile were not available during thecourse of sampling. Younger results would be envis-aged for Mn-oxides approaching the active weatheringfronts, as demonstrated at the Xinrong deposit, where anactive mining open pit has exposed Mn-oxides from thelowest part of weathering profiles, some of which yielded40Ar/39Ar plateau ages of 2.46–2.06Ma [38], indicating alatest Plioceneweathering event. Immediately to the northof Guangxi Province, the Dongxiangqiao Mn-oxidesdeposit, southern Hunan Province (Fig. 1a), yielded40Ar/39Ar plateau ages between 13 and 4 Ma, revealingmiddle-late Miocene to early Pliocene weathering and

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supergene Mn enrichment [39]. Taken together, existingdata indicate that intense weathering may have operatedduring the whole Neogene in South China (Fig. 6a).

Weathering and supergene mineral precipitation areassociated with regional factors (e.g., climate andtectonism), and dating of Mn-oxides from different pro-files at a regional scale should yield internally consistentresults [15]. This is supported by the compatibility ofweathering ages between Tanying, Liaolian, and Dikuang(Fig. 6b). However, the ages of Mn-oxides from theDadong profile are obviously older than from the otherthree profiles (Fig. 6a). Such an age discrepancy is afunction of sampling, because the Dadong samples werecollected solely at hilltops, and progressively youngerphases at depth of the profile were not sampled due toinaccessibility. Despite this sampling limitation, ages ofthe Dadong samples still partially overlap with those fromthe Dikuang and Liaolian profiles (Fig. 6a). On the otherhand, absence of old ages from Tanying and Liaolianresulted largely from the mining operations that haveremoved the upper parts of the weathering profiles, whereold supergene minerals were hosted. Collectively, thepresent geochronological data indicate that weathering inthe QF area is temporally consistent, compatible amongdifferent profiles, and can be interpreted as processesdriven by external factors (e.g., climate).

5.2. Implications for Neogene climates of South China

In weathering environments, tetravalent Mn-oxidesform by oxidative dissolution of primary Mn-bearingminerals under acidic conditions, or via direct precipitationof Mn4+ oxyhydroxides from weathering solutions as aresult of oxidation ofMn2+ [40]. In either case, abundanceof meteoric solutions would be required. Early-formedMn-oxides may also be dissolved by strongly acidicweathering solutions [41,42], as indicated by crosscuttingMn-oxide generations (Fig. 3d). Plant exudates anddecay products of organic matter, typical of humid sub-tropical to tropical zones, supply the dissolved organicacids necessary to create acid conditions in near surfaceenvironments [3,41]. The dissolved Mn2+ is generallytransported downward and re-precipitate as Mn-oxides indepth of the profile, where organic ligands are lessabundant. Thus, development of deep weathering profilesand massive accumulations of Mn-oxides related to theprofiles reflect intensive weathering driven by sufficientrainfall, relatively high ambient temperature, and abundantorganic acids present in weathering solutions [15,43],characteristic of humid and warm climate. A recent studyshows that the late Miocene intensification of weatheringand Mn-oxides precipitation in the Minas Gerais region,

eastern Brazil, correlated well with the δ18O decrease ofplanktonic foraminifera, kaolinite content increase of theoceanic sediments, and rise of sea-level in the AtlanticOcean [19], factors suggestive of warm and wet climaticconditions. Therefore, extensive weathering and precipi-tation of supergeneMn-oxides between 23.5 and 2.1Ma inSouth China indicate humid climatic conditions during theentire Neogene. This is consistent with the high Mn/Feratios (6–10) of the Mn deposits in the QF area (Li et al.,unpublished data), reflective of effective separationbetween Fe and Mn under wet subtropical to tropicalclimates [41,44].

South China had been dominated by very dryclimates from the Late Cretaceous to the Eocene,marked by widespread distribution of gypsum, halite,potash salt, mirabilite, and glauberite within a number ofred-beds basins [24,29]. During this period, pollenassemblages associated with the red beds and evaporitesare mainly Ephedripites, Beaupreaidites, and Proteaci-dites [30], which today grow in the arid to semi-aridregions of NW China. Origin of this longstandingaridity has been attributed to the control of a subtropicalhigh [29]. Although aridity may have still prevailedduring the Oligocene, wet climates emerged intermit-tently in southern South China, as reflected by thepresence of intercalated oil shales and lignite in somesmall Oligocene basins [45]. Profound climatic changeoccurred probably around the Oligocene/Mioceneboundary, when humid and warm climate with seasonaltemperature variations prevailed across large areas ofSouth China, with the arid zone retreating to NW China[24,29]. The present 40Ar/39Ar ages (Fig. 6a) confirm anearly Miocene age for the onset of wet climate in SouthChina and its prevalence during the whole Neogene,view consistent with sedimentary and paleontologicalrecords from Neogene basins (see below).

The Neogene sediments of South China are mostlylacustrine and alluvial and confined to inland basins [24],many ofwhich contain economic accumulations of ligniteand oil shales [33,45–48]. These document pervasiveswampy environments and thus very humid climate.Abundant and diversified fossils of flora and fauna arewell preserved in most coal-bearing basins. Commonfossil plants found are Cinnamomum bodinieri,Machiluschinensis, Cladrestis, Desmodium, Colocedrus, Ocotea,Sabalites, Quercus, Magnollaceae, Lauracea, andAnnonacea [30,47–52], which suggest that much ofSouth China was covered by evergreen broad-leavedforests spanning the entire Miocene. A large number ofmangrove pollen (mainly Rhizophora and Sonneratia),marine dinoflagelates and pollen of tropical andsubtropical plants (e.g., Castanopsis, Ilex, Palmae and

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Sapotaceae) preserved in the Miocene to Pliocene sed-iments of the continental shelf of South China Sea fur-ther support that the Neogene climate was warm andhumid [53].

An independent study of Fe–Al-rich tridymite–hercynite xenoliths (32–34 wt.% Al2O3 and 20–22 wt.% Fe2O3) entrapped in the middle Miocene basalts(15.7 Ma) in southeastern China provided additionalevidence for weathering-prone climatic conditions ofearly Miocene age [54]. Mineralogical and chemicalcompositions, Sr–Nd–Pb–O isotopic data, and notablythe strong positive cerium anomaly demonstrate that thexenoliths represent aluminous lateritic paleosol formedby intensive tropical weathering prior to the 15.7-Mavolcanism. Calculation of chemical index of alterationindicates that the climatic conditions under which thepaleosol developed was characterized by an averageyearly temperature N19 °C and annual rainfall more than1650 mm [54].

From a global perspective, the late Miocene has beenthought to be a relatively cool and dry period, commonlylinked to the major east Antarctic ice sheet growth[55,56]. However, massive accumulation ofMn-oxides indeep weathering profiles during the late Miocene (Fig. 6)suggests that humid climate was still prevailing in SouthChina during that time interval. The late Miocene claymineralogy of the ODP site 1146 (South China Sea),which is dominated by a high ratio of kaolinite andsmectite to illite [57], also indicates that south Chinareceived abundant precipitation, leading to intenseweathering of this time. Moreover, oxygen isotopiccompositions of benthic foraminifera from ODP site1148 suggest that, during the late Miocene, the surfacetemperature of South China Sea was 5–6 °C higher thantoday [58]. Taken together, the marine and terrestrialproxies consistently suggest a warm and wet climateprevailed in South China throughout the Neogene.

5.3. Temporal link to the East Asian monsoon

It is widely accepted that the climate of China hasbeen largely controlled by the East Asian monsoon[24,29], and there also is a consensus that the monsoonsystem has resulted mainly from the uplift of the Tibetanplateau [26,27,29,59]. Therefore, precise age data fromindependent climatic proxies will provide informationon the evolutionary history of the East Asian monsoonand its relationship to Tibetan uplift. Previous studies onthe East Asian monsoon, focused on the aeolian depositsfrom the Chinese Loess plateau (NW China), yieldeddata suggesting start or intensification of the monsoon atca. 8–7 Ma, 3.6–2.6 Ma, and post 2.6 Ma, respectively

[25,26,59]. However, recent discovery of a Mioceneloess-paleosol profile located in Qinan, 160 km north-east of the Tibetan Plateau, has extended the monsoondevelopment back to around 22 Ma [27]. On the otherhand, marine sediments from South China Sea provideevidence suggesting establishment of the monsoon atearly [60] or late Miocene [28]. The existing results thusdemonstrate that the East Asian monsoon has a muchmore prolonged and complicated history than previous-ly thought, displaying great variability with multipleperiods of intensification [26,28,59,60]. Our 40Ar/39Arages for supergene Mn-oxides (Fig. 6a) support an earlyMiocene or late Oligocene age for the start of the EastAsian monsoon and significant uplift of the Tibetan pla-teau. Ourweathering age peak at ca. 8–5Ma (Fig. 6b) alsocorrelates well with the most significant intensification ofthe monsoon, inferred from independent records [26].Notwithstanding, we advocate further weathering geo-chronology studies on a more regional scale and moredetailed sampling strategy to address precise timing andevolution of the East Asian monsoon.

6. Conclusions

40Ar/39Ar dating of supergene Mn-oxides collectedfrom four weathering profiles in the Qinzhou–Fang-cheng Mn belt, South China, yields high-resolutionplateau ages ranging from 17.6±0.5 Ma to 4.71±0.08 Ma (2σ). When combined with results from otherprofiles of Guangdong and Hunan Provinces, these datadocument a prolonged history of weathering andsupergene Mn enrichment and thus prevalence ofhumid subtropical climate during the whole Neogenein South China. Climatic implications inferred fromweathering geochronology are consistent with sedimen-tary and paleontological records in Late Cenozoic basinsof South China. 40Ar/39Ar ages of the South China'sMn-oxides also support onset of the East Asian monsoon andsignificant uplift of the Tibetan Plateau in the earlyMiocene, and its intensification in late Miocene.

Acknowledgments

This study was supported by Natural ScienceFoundation of China (40202010, 40573021), the FokYing Tong Education Fund (101017), and the Programfor New Century Excellent Talents in University(NCET-05-0665). 40Ar/39Ar geochronology was fundedby the UQ-AGES. We thank colleagues from GuangxiGeological Survey for logistic supports, and Drs. DavidThiede and Isabela Carmo for help in sample analysis.We are grateful for the constructive reviews by Drs.

401J.-W. Li et al. / Earth and Planetary Science Letters 256 (2007) 389–402

Gilles Ruffet and Fred Jourdan, and for the editorialcomments by Richard W. Carlson, which help improvethe presentation of the paper.

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.epsl.2007.01.021.

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