Sedimentary Geology 315 (2015) 1–13

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Sedimentary Geology

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Early Eocene sedimentary recycling in the Kailas area, southwesternTibet: Implications for the initial India–Asia collision

Jian-Gang Wang a,b,⁎, Xiu-Mian Hu c, Marcelle BouDagher-Fadel d, Fu-Yuan Wu a, Gao-Yuan Sun c

a State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinab State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, Chinac State Key Laboratory of Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, Chinad Department of Earth Sciences, University College London, London WC1E 6BT, UK

⁎ Corresponding author at: State Key Laboratory of LithGeology and Geophysics, Chinese Academy of Sciences, B

E-mail address: wangjiangang@mail.iggcas.ac.cn (J.-G.

http://dx.doi.org/10.1016/j.sedgeo.2014.10.0090037-0738/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 July 2014Received in revised form 19 October 2014Accepted 21 October 2014Available online 1 November 2014

Keywords:India–Asia collisionEarly EoceneSedimentary recyclingWedge-top basinSouthwestern Tibet

Syncollisional sedimentary rocks in the Himalayan orogen record important information about the early historyof the India–Asia collision. This article presents a combined stratigraphic, sedimentologic, micropaleontologic,and provenance data for the Early Eocene clastic strata (Dajin Formation) in the Kailas area, southwesternTibet. The Dajin Formation comprises ungraded and normally graded, matrix- and clast-supported conglomer-ates, ungraded and cross-stratified sandstones, and massive to poorly laminated mudstones. Deposition of therocks is characterized by subaerial and subaqueous debris flows, waning gravity flows and suspension fallout.The larger benthic foraminifera and the youngest ages of the detrital zircons constrain the depositional age tothe earliest Eocene (56–54Ma). The Dajin Formation contains abundant sandstone andmudstone clasts, indicat-ing significant sedimentary recycling in the source area. U–Pb ages of detrital zircons are mostly clustered at~120–54 Ma and have positive εHf(t) values, suggesting a Gangdese affinity. Clasts of the Dajin Formation areinterpreted as having recycled from the forearc strata that were originally derived from the Gangdese magmaticarc. Highly immature texture and recycled provenance lead us to propose that deposition of the Dajin Formationwas a result of the development of fold-thrusts in the Gangdese forearc. Our new results and published data fromthe coeval strata in the Himalayan orogen indicate that an underfilled foreland basin was initiated soon after theinitial India–Asia collision, and the Dajin Formation records the wedge-top depozone of the basin system.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The India–Asia collision created the Himalayan orogen and contrib-uted significantly to the uplift of the Tibetan Plateau. A variety of dataindicate that the initial India–Asia collision occurred during the EarlyPaleogene (e.g., Garzanti et al., 1987; Ding et al., 2005; Leech et al.,2005; Najman et al., 2010; Wang et al., 2011; Yi et al., 2011; Hu et al.,2012; Zhang et al., 2012; DeCelles et al., 2014; Wu et al., 2014), butour knowledge regarding the early history of the collision is poor. Thelargest obstacle for our studies is thatmost of the geological informationabout the early stage of the India–Asia collision has been obscured orremoved by later tectonism or erosion.

Studies of the residual syncollisional sedimentary records in theHimalayan orogen have proven to be an effective approach for under-standing the early history of the India–Asia collision (e.g., DeCelleset al., 2004, 2014; Najman et al., 2010; Wang et al., 2011; Hu et al.,2012; Zhang et al., 2012). In this article, we present an integratedstudy of the stratigraphy, sedimentology, micropaleontology, and

ospheric Evolution, Institute ofeijing 100029, China.Wang).

provenance of the Paleogene clastic strata, called the Dajin Formation,exposed in the Kailas area, southwestern Tibet. Our objective is toreconstruct the depositional process of the Dajin Formation and toprovide new details for the basin evolution immediately after the initialIndia–Asia collision.

2. Geological setting

The Tibetan Plateau and its southward-flanking Himalayan ramparthave developed in the context of northward subduction of the IndianNeo-Tethyan lithosphere beneath the Asian plate, climaxing with thecollision between the two continental landmasses (Dewey et al., 1988;Hodges, 2000). The Indus–Yarlung-Zangbo suture zone is the tectonicboundary between the India and Asia continents (Fig. 1A). It comprisesthe discontinuous east–west-trending mafic–ultramafic complexes(e.g., Hébert et al., 2012; Dai et al., 2013), radiolarian cherts (Ziabrevet al., 2003), serpentinite- and mud-matrix mélanges (Liu and Einsele,1996), and postcollisional molasse deposits (Wang et al., 2010, 2013;DeCelles et al., 2011).

North of the Yarlung-Zangbo suture zone, the trans-Himalayan(Kohistan–Ladakh–Gangdese–Myanmar) arc is a complex of calc-alkalinebatholiths and related volcanic rocks that formed as an Andean-style

Fig. 1. A) Simplified tectonic map and cross section (section A-A′) of the Himalayan orogen, modified after Yin (2006). Early Paleogene syncollisional strata of the India–Asia collision insouthern Tibet are shown, including: 1, limestones and clastic rocks of the Cuojiangding Group (Quxia and Jialazi formations) near Zhongba; 2, turbidites of the Sangdanlin and Zheyaformations near Saga; and 3, carbonate rocks of the Zongpu Formation in Tingri and Gamba areas. The dash linemarks the Indus-Yarlung-Zangbo suture zone. GB, Gangdese batholithes;GCT, Great Counter Thrust; GH, Greater Himalaya; LH, Lesser Himalaya; MBT, Main Boundary Thrust; MCT, Main Central Thrust; MFT, Main Frontal Thrust; Oph, ophilte; TH, TethyanHimalaya, YZSZ, Yarlung-Zangbu suture zone. STDZ, South TibetDetachment Zone; B)Geological sketchmapof southwestern Tibet and adjacent portion ofHimalayan thrust belt,modifiedafter a scale 1: 1 500 000 geologicmap (Pan et al., 2004). C)Geologicalmap and cross section (section B-B′) of theKailas area (modified after Yin et al., 1999), showing location ofmeasuredsections.

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magmatic arc along the southern flank of the Lhasa terrane in responseto the northward subduction of the Tethyan oceanic lithosphere fromLate Mesozoic to Paleogene time (Chu et al., 2006; Mo et al., 2008;Wen et al., 2008; Ji et al., 2009). The Xigaze forearc basin, filled withCretaceous to Paleogene clastic rocks (mainly turbidites) and minor

carbonates, developed immediately south of the arc (Wang et al.,2012; An et al., 2014) (Fig. 1A).

South of the Yarlung-Zangbo suture zone, the Himalayan beltincludes four major tectonic domains (Yin, 2006; Fig. 1A): (1) TheTethyan Himalaya, consists of Lower Paleozoic to Eocene clastic and

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carbonate rocks deposited on the northern Indian passive margin;(2) The Greater Himalaya, delimited by the South Tibetan DetachmentSystem in the north and by the Main Central Thrust in the south,includes upper amphibolite-facies gneisses and leucogranites; (3) TheLesser Himalaya, delimited at the base by the Main Boundary Thrust,consists of various subunits, including Proterozoic to Eocene sedimenta-ry rocks metamorphosed up to lower amphibolite facies and intrusivebodies of granitic augen gneiss; and (4) The Sub-Himalaya consists ofNeogene alluvial sedimentary rocks that represent an overfilled stageof the Himalayan foreland basin.

In the Kailas area, the Great Counter Thrust (referred to as the SouthKailas thrust by Yin et al., 1999) is amajor structure (Heim and Gansser,1939; Fig. 1B, C). This northward-verging thrust system consists ofseveral thrusts that juxtaposed Tethyan Himalayan strata, ophioliticrocks and accretionary mélange against the trans-Himalayan magmaticrocks and the Kailas Conglomerate in the footwall (Yin et al., 1999;Murphy and Yin, 2003). The Dajin Formation is tectonically a thrustslice within the Great Counter Thrust system and is in fault contactwith the mélange belt in the south and with the Upper Oligocene–Lower Miocene Kailas Conglomerate in the north.

Fig. 2. Logs of theDajin Formation from threemeasured sections, signedwith lithofacies and locRose diagrams show paleocurrent directions measured from section A, and pie charts show cla

3. Stratigraphy and sedimentary facies

The Dajin Formation was first mapped by the Tibetan Bureau ofGeology andMineral Resources (1987) during the 1:1 000 000 regionalgeological mapping. The Dajin Formation is a suite of coarse-grainedclastic rocks and consists of conglomerates, sandstones andmudstones.The Dajin Formationwas noted because of the presence of lager benthicforaminifera (Yan et al., 2005, 2006). The total thickness of the strata isunknown as a result of structural deformation. In the three measuredsections, its thickness is 40 m, 17 m and 32 m, respectively (Fig. 2).

Rocks of the Dajin Formation are divided into ten facies, which aredescribed and interpreted in Table 1. Lithofacies are grouped intothree associations that are interpreted as having been deposited ondifferent positions of a fan-delta system.

3.1. Facies association 1—proximal fan-delta

3.1.1. DescriptionThis association occurs in the uppermost part of section A and

section B. It comprises predominantly greenish gray- and red-colored,

ations of samples for detrital zircon and Cr-spinel analyses. See Table 1 for lithofacies codes.stic compositions of conglomerate from the Dajin Formation.

Table 1Lithofacies found in the Dajin Formation and interpretations of their depositional procedure.

Faciescode

Description Interpretation

Gcm Several tens of centimeters to several meters thick, pebble to boulder conglomerate, subround to angular gravel,moderately sorted, clastic supported, massive, poorly organized, beds extend laterally for tens of meters

Clast-rich debris flow deposits or channeldeposits

Gmm 10 mm to 1 m thick, pebble to cobble conglomerate, subround to subangular gravel, poorly sorted, matrixsupported, massive, disorganized, beds extend laterally for tens of meters

Plastic debris flow deposits

Gmn Several tens of mm thick, lense shape, pebble conglomerate, angular to subangular gravel, poorly sorted, matrixsupported, normal grading, beds are laterally continuous for several to tens of meters

Pseudoplastic debris flow deposits

Gmi About 1 m thick, pebble to cobble conglomerate, angular to subangular gravel, poorly sorted, matrix or clasticsupported, poor inverse grading, beds extend laterally for tens of meters

Plastic debris flow deposits; hyperconcentratedflow deposits

Gt Several tens of mm thick, lense shape, fine pebble conglomerate, angular to subround gravel, poorly to moderatelysorted, matrix or clastic supported, trough cross stratified, sometimes with poor normal grading, beds extendlaterally for tens of meters

Channel fills

Sm 10 mm to 1 m thick, massive fine- to coarsen-grained sandstone, can be pebbly and have poor normal grading,beds are discontinuous

Sand-rich debris flow deposits

St Ten to several tens of mm thick, generally several tens of mm thick, lense shape, medium- to very coarsen-grainedsandstone with through cross-stratification

Migration of 3-D ripples (dunes) undermoderately powerful unidirectional flows inchannels

Sh 10 to 30 cm bed thick, fine- to medium-grained sandstone with parallel lamination Upper plane bed conditions under strong or veryshallow unidirectional flows

Fl Siltstone or mudstone, horizontal stratified Distal fan; suspension falloutFm Massive siltstone or mudstone Waning gravity flow deposits; suspension fallout

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poorly sorted, clast- and matrix supported conglomerates (facies Gcmand Gmm). Clasts are angular to subangular and of pebble to bouldergrade (up to 50 cm in diameter) (Fig. 3E, F). Beds are massive, disorga-nized, and are interbedded with subordinate sandstone units (facies

Fig. 3. Field photos of typical lithofacies from the Dajin Formation: A) trough cross-stratifiedbetween conglomerates (Gcmand Gmm), from the upper part of section A; C)massivematrix-sA; D)massive mudstone (Fm) from the lower part of section A; E) massive cast-supported congerate with sandstone and d limestone clasts, from section B.

Sm). Facies Gcm commonly have erosional bases. Beds are 1 to 3-m-thick and extend laterally for tens ofmeters. The gravels are unstratifiedand lack clast imbrication. A muddy sand matrix (20%–30%) surroundsthe clast-supported framework. Sections (up to 10-m-thick) composed

conglomerate (Gt) from the upper part of section A; B) sandstone bed (Sm) intercalatedupported conglomerate (Gmm) rest on a concave-up base, from themiddle part of sectionlomerate with bouler-size clasts, from section B; and F) massive cast-supported conglom-

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of a stack of beds of Facies Gcm are present. Facies Gmm consists ofpebbles (1–5 cm in diameter) within a mud or sandy mud matrix.Beds are 0.3 to 1-meter-thick and can be traced laterally for tens ofmeters. Clasts are poorly organized and lack internal structure. Massiveor poorly trough crossed-stratified sandstone units (facies Sm and St)are a subordinate component of this facies association (Fig. 3B). Theyconsist of green, poorly sorted sand and are interbedded betweenboth Facies Gcm and Gmm units. Sandstone beds are discontinuous.

3.2. Interpretation

This association is attributed to deposition by sediment gravity flows(e.g., Miall, 1977; Horton and Schmitt, 1996). The deposition wascharacterized by clast-rich (facies Gcm), plastic debris flows (Gmm)and minor hyperconcentrated flows (Sm) (Table 1). The presence ofred-colored rocks and the absence of mudstones and marine fossilsindicate deposition in a subaerial environment. Facies Gcm units witherosional bases indicate channelized debris flows, whereas faciesGmm units were most likely deposited from unchannelised debrisflows (Lee and Chough, 1999).

3.3. Facies association 2—middle fan-delta

3.3.1. DescriptionThis association occurs in themiddle part of section A and section C.

It contains ungraded (facies Gmm), normally graded (facies Gmn) andminor inverse graded (facies Gmi) conglomerates, ungraded (faciesSm) and trough cross-bedded (facies St) sandstones, and massive topoorly laminated mudstones (facies Fm). Normally graded, matrix-supported conglomerates (Gmn) are the primary facies of this associa-tion. Beds are 0.1 to 1-meter-thick and are laterally continuous forseveral to tens of meters. In general, Gmn units have erosional basesand rest on concave-up bases (Fig. 3C). Clasts are mostly angular tosubangular in shape, 1 to 5 cm in diameter. Trough cross-stratifiedconglomerates (Gt) and sandstones (St) are also common in this associ-ation (Fig. 3A). Paleocurrent measurements from trough cross lamina-tions following the method described by DeCelles et al. (1983)indicate that sediment transport directions were mostly to the south-south-west (Fig. 2). Rocks of this association are green and are finegrained relative to facies association 1. Larger benthic foraminifera arepresent in this facies.

3.3.2. InterpretationThe presence of relatively well-preserved, uniform-aged (see

Section 6.1) larger benthic foraminiferal fossils in this facies associationsuggests a marine depositional environment. Subaqueous sedimentgravity flows dominated the depositional environment of this associa-tion, and are characterized by plastic and pseudoplastic debris flowsandwaning debris flows. The laterally continuous, ungraded or normal-ly graded, poorly sorted conglomerates (facies Gmm and Gmn) areproducts of episodic deposition of subaqueous debris flows (Mulderand Alexander, 2001), whereas the trough cross-stratified conglomer-ate and sandstone may represent underwater channel-fill depositsdeposited on the subaqueous surface of a fan-delta (e.g., Horton andSchmitt, 1996). The massive and poorly laminated sandstone (Sm)and mudstone are the result of the draping of sand or mud underwaning flow conditions. The larger benthic foraminifera, often presentin facies Gmm units, were drawn into or buried by the debris flowsduring sedimentary transport and deposition.

3.4. Facies association 3—distal fan-delta

3.4.1. DescriptionThis association occurs in the lower part of section A. Green-colored,

massive and poorly graded sandstone (facies Sm) andmassive to poorlylaminated siltstone and mudstone (facies Fm and Fl) compose this

facies association (Fig. 3D). The sandstone beds are 5 to 20-cm-thick,laterally continuous for tens of meters, and have planar, nonerosionalbases. Sedimentary structures in sandstone and mudstone are rare.

3.4.2. InterpretationThe facies association is dominated by suspension sedimentation

that was occasionally influenced by subaqueous debris flows.Well-defined, thin-bedded mudstone (facies Fm and Fl) was depositedduring suspension fallout, whereas massive, fine-grained sandstone(Sm) and siltstone were deposition by distal subaqueous debris flows(Ghibaudo, 1992).

4. Analytical methods

Conglomerate clast counts were performed using a grid meth-od. Approximately 100 clasts were counted at each site, using a10 × 10 cm, 5 × 5 cm or 3 × 3 cm grid depending on the gravel size.Detrital modal analyses of sandstones were performed in thin sectionsfollowing the Gazzi-Dickinson point-counting method (Ingersoll et al.,1984). For consistency and accuracy, ~400 points were counted ineach sample. Heavy minerals were separated using a combination ofelutriation, heavy liquid andmagnetic separation techniques. Individualzircon and Cr-spinel grains were handpicked randomly, mounted inepoxy resin and then polished to produce a smooth, flat surface,which exposed the interiors of the grain.

U–Pb dating of detrital zircon was performed via laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) atthe State Key Laboratory of Mineral Deposit Research, NanjingUniversity, China (for samples 06QD06 and 06QD13) and at theState Key Laboratory of Lithosphere Evolution, Institute of Geologyof Geophysics, Chinese Academy of Sciences (for samples 12DJ08and 12DJ-B09). Details of the instrumental conditions and dataacquisition can be found in He et al. (2010) and Xie et al. (2008),respectively. Various spot diameters (25, 30, 44 μm) were usedduring the analysis depending on the grain size of zircon. The rawcount rates for 204Pb, 206Pb, 207Pb, 208Pb, 232Th and 238U were collectedfor age determination. The 207Pb/206Pb and 206Pb/238U ratios werecalculated using the GLITTER program (GEMOC, Macquarie University;Griffin et al., 2008). Common Pb corrections were performed usingthe method described by Andersen (2002). Age calculations andplotting of concordia diagrams were performed using Isoplot 3.0(Ludwig, 2003). The results described in this article exclude analyseswith N20% discordance. Zircon age interpretations are based on the206Pb/238U ages for grains younger than 1000 Ma and the 207Pb/206Pbages for grains older than 1000Ma because 206Pb/238U ages are generallyprecise for younger ages, whereas 207Pb/206Pb ages are more precise forolder ages.

Zircon Hf isotope analyses were conducted at the State KeyLaboratory of Lithosphere Evolution, Institute of Geology of Geophysics,Chinese Academy of Sciences, using a Neptune Multi-Collector ICP–MSequipped with the Geolas 193 laser-ablation system. Details of theinstrumental conditions and data acquisition can be found in Wu et al.(2006). Hf isotope analyses were performed on the same sites as theU–Pb analyses using a slightly larger (d = 60 μm) laser beam size.During the analysis, the average 176Hf/177Hf ratio of the standard zirconMud Tank was 0.282499 ± 20, which is consistent with the previouslyreported values (Woodhead and Hergt, 2005); thus, further externaladjustment was not applied to the analytical results. To calculate theεHf(t) value and the ‘crustal’ model age (TC DM), we adopt a depletedmantle model with 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384(Griffin et al., 2000) and chondrite values for 176Hf/177Hf = 0.282772and 176Lu/177Hf = 0.0332 (BlichertToft and Albarede, 1997). Weassumed that the protolith of the zircon's host magma has the averagecontinental crustal 176Lu/177Hf ratio of 0.015 (Griffin et al., 2002) andused a decay constant of 176Lu–176Hf of 1.867 × 10−11 per year(Söderlund et al., 2004).

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Cr-spinel geochemical composition was determined using a JEOLJXA-8100M electronmicroprobe at the State Key Laboratory of MineralDeposit Research, Nanjing University. Analyses were performed underthe following conditions: accelerating voltage 15 kV, beam current20 nA, beamdiameter of 1 μm, and the ZAF correctionmodel. The detec-tion time was 10 s for Al, Fe, and Mg, and 20 s for Ti, Mn, Cr, V, and Ni,and 30 s for Zn. The detection limits were ~200 ppm for all elements.All Fe was expressed as FeO, and the ferric iron content of each analysiswas calculated following Barnes and Roeder (2001). All analytical dataare presented in the Appendix.

Fig. 4. Photomicrographs of foraminiferal fossils from the Dajin Formation. A) a, Discocyclin(Finlay), ×20. C) Alveolina elliptica (Sowerby), ×18. D) Alveolina ellipsoidalis Schwager, ×14. Emamillatus (Fichtel & Moll), ×10. G) Alveolina pasticillata Schwager, ×18. H) Nummulites rotula

5. Analytical results

5.1. Larger benthic foraminifera

Larger benthic foraminifera were observed in 10 thin sections fromsection A. The larger benthic foraminifera fossils include Alveolinapasticillata, Nummulites minervensis, Nummulites mamillatus, Nummulitesrotularius, Alveolina elliptica, Alveolina sp., Assilina laminosa, Assilina sp.,Assilina granulosa, Alveolina aragonensis, Alveolina ellipsoidalis, Alveolinaregularis, Acarinina soldadoensis,Nummulites neglectus, Lockhartia conditii,

a ranikotensis Davies and Pinfold; b, Assilina laminosa (Gill), ×13. B) Acarinina primitiva) a, Nummulites minervensis Schaub; b, Assilina granulosa (d′Archiac), ×12. F) Nummulitesrius Deshayes, ×42. I) Assilina sublaminosa Gill, x10.

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Coskinolina sp., Lockhartia sp., Ranikothalia sp.,Operculina sp.,Discocyclinaranikotensis, Discocyclina sella, Globigerina sp., Miscellanea cf. miscella,Miscellanea miscella, Discocyclina ranikotensis, and Discocyclina new sp.B. (Fig. 4). Moreover, a few planktonic foraminifera, such as Acarininaprimitiva (Fig. 4B) and Acarinina soldadensis were present sporadicallyat the top of the section. Larger benthic foraminifera indicate a shallowwater forereef to reef depositional environment.

Most of the foraminiferal fossils from the Dajin Formation are wellpreserved and did not experience diagenesis before deposition. Thus,we suggest that the age of the foraminiferal assemblages representsthe depositional age of the Dajin Formation. A few grains that werebroken might be subjected to the depositional process of debris flows.The co-occurrence of the larger benthic foraminiferawith the planktonicforaminifera at some levels was useful for correlation and refining thebiostratigraphic ages of some of the long-ranging benthic species.

5.2. Clastic compositions of conglomerates and sandstones

Clasts of the conglomerates in the Dajin Formation are of polymicticcomposition. In the middle part of section A, the conglomerates arepebble size, and the clasts are dominantly shale/slate (~60%), withsubordinate sandstone (~25%) and limestone (~15%) gravels. In theupper part of section A and sections B and C, the conglomerates arecobble to boulder size, and consist of predominant sandstone gravels(N75%) and minor shale (b10%), limestone (b5%), volcanics (b5%) andquartzite (b5%) (Fig. 2). Occurrence of abundant angular clasts,boulder-size clasts and carbonate clasts, which are susceptible to chem-ical weathering, suggests that sediment underwent limited weatheringand transport prior to deposition.

Sandstones from the Dajin Formation are litharenites, and have anaverage modal composition of Q:F:L = 18:4:78. Quartz grains areangular to subangular and mostly monocrystalline, clear, and withuniform extinction; some grains display embayments (Fig. 5C, D)indicative of a volcanic origin. Lithic fragments are the most importantdetrital component of the sandstones and constitute 60% to 90% of thetotal framework grains. Sedimentary rock fragments are the principallithic component, and are composed of shale, siltstone and carbonate

Fig. 5. Photomicrographs of typical detritus from sandstones in the Dajin Formation: A) abundquartz, low grade metamorphic and volcanic clasts; and D) serpentinite clast. Lc, carbonatserpentinite fragment; Pl, plagioclase; Q, quartz.

(Fig. 5A–D). Volcanic fragments are the subordinate lithic componentand contain rhyolite, andesite and basalt (Fig. 5C). Minor low-grademetamorphic fragments are all slates (Fig. 5C). Presence of a fewserpentinite fragments in some samples is notable and indicates mate-rial input fromultramafic rocks (Fig. 5D). Feldspar is aminor constituentand all are of plagioclase (Fig. 5C). On the Q-F-L and Lm-Lv-Ls diagrams,sandstones from the Dajin Formation are plotted into a field similar tothe clasts from a ‘clastic-wedge provenance’ (Fig. 6).

5.3. U–Pb and Hf isotopes of detrital zircons

Detrital zircons from four samples (Fig. 2)were selected for U–Pb andHf isotopic analysis. Zircons are mostly euhedral and have elongated tostout prismatic habits, with length-to-width ratios from 2:1 to 3:1, andonly a few grains are rounded in shape and/or brown in color. A totalof 317 zircon grains were dated using the U–Pb method, of which 287ages are of sufficient concordance and precision for provenance interpre-tation (Fig. 7). Zircon ages from all of the samples are mostly clusteredaround ~120–54 Ma, and the ages form a notable peak at ~90 Ma(Fig. 8). Older (N200 Ma) zircons are rare, and are mostly dated to theMesoproterozoic to Early Paleozoic ages (Fig. 8).

The youngest age cluster of detrital zircons from clastic rocks areimportant because they can provide a maximum depositional age ofthe strata (i.e., depositional age should be younger than the youngestcluster of detrital zircon ages; Dickinson and Gehrels, 2009). Theyoungest three ages from each sample are 49 ± 2, 54 ± 1, and 55 ±2 Ma for sample 06QD13 (bottom of section A), 54 ± 2, 55 ± 2, and57 ± 3 Ma for sample 12DJ08 (middle of section A), 51 ± 1, 52 ± 1,and 53 ± 1 Ma for sample 06QD06 (top of section A), and 60 ± 2,62 ± 2, and 65 ± 1 Ma for sample 12DJ-B09 (middle of section C),respectively. These young ages form the youngest age peak at ~54 Ma,except for sample 12DJ-B09, which has its youngest age peakat ~62 Ma (Fig. 8).

Hf isotope analyses were performed on the same grains as the U–Pbanalyses. As shown on the εHf(t) vs. zircon age diagram (Fig. 9), the larg-est group of zircons occurs at Cretaceous–Eocene, and they have slightlynegative to positive εHf(t) values (ca. –5 and+15), with corresponding

ant sedimentary clasts; B) well-preserved larger benthic foraminiferal fossils; C) embayede rock fragment; Lm, metamorphic rock fragment; Ls, sedimentary rock fragment; Lu,

Fig. 6. Triplots of detrital framework compositions of sandstones from the Dajin Formation (red square), and samples from the Xigaze forearc basin (gray diamond, data from An et al.,2014) are shown for comparison. Provenance ofmodernmagmatic arc, clastic wedge and their exhumation trends (Garzanti et al., 2007) are shown for comparison. Qm, monocrystallinequartz; Q, total quartz grains=Qm+Qp (polycrystalline quzrtz); F, feldspar; L, lithic fragment; Lt, total lithic fragment= L+Qp; Lch p chert fragment; Lm,metamorphic rock fragment;Ls, sedimentary rock fragment; Lv, volcanic rock fragment. Provenance field: CB, continental block provenance; RO, recycled orogen provenance; MA, magmatic arc provenance(Dickinson, 1985). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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TDMC ages varying from 0.2 to 1.4 Ga. The other two groups of zircons in

the b200 Ma age range occur at the Early Jurassic and the Late Jurassic–Early Cretaceous, and their εHf(t) values are 7 to 14 and −5 to −12,respectively. The old-aged (N200 Ma) zircons show a large variability ofεHf(t) values between −17 and +10, and the corresponding TDM

C agesranging from 1.5 to 3.4 Ga.

5.4. Detrital Cr-spinel geochemistry

Cr-spinel is a common accessory mineral in the Dajin Formation.They are dark in color, euhedral to subhedral in shape. The spinels arecharacterized by extremely low TiO2 contents (commonly b0.2 wt.%;Fig. 10A) and relatively high Fe2+/Fe3+ ratios (commonly N3, and6.09 on average). Al2O3 contents show a wide range from 5.28 wt.% to40.36 wt.%, with a corresponding Cr# varying from 0.31 to 0.66. TheMg# are between 0.38 and 0.68 (Fig. 10B). MnO, NiO, V2O5 and ZnOare present in low abundances (generally less than 0.5 wt.%).

6. Discussion

6.1. Depositional age of the Dajin Formation

The biostratigraphic age interpretation was based upon identifica-tion of the Tethyan larger benthic foraminiferal Shallow Benthic Zones(SBZ) of Serra-Kiel et al. (1998) calibrated with the planktonic

Fig. 7.U–Pb concordia diagrams for detrital zircons. Ages are inMa and ellipses show 1σ errors.interpretations.

foraminiferal zonal scheme of BouDagher-Fadel (2013), which is tiedto the time scale of Gradstein et al. (2012).

The presence of the larger benthic foraminifera Alveolina pasticillata(Fig. 4G) and Nummulites minervensis (Fig. 4E-a) identify SBZ6. This isconfirmed by the occurrence of the planktonic foraminifera Acarininaprimitiva (Fig. 4B) and Acarinina soldadensis which indicate a latestPaleocene to Early Eocene age (P5–P6a) (BouDagher-Fadel, 2013).However, the presence of Alveolina elliptica (Fig. 4C) proves an age notolder than the Early Eocene (BouDagher-Fadel, 2008). This gives anage of around, 56.0–54.9 Ma. The youngest age peak of detrital zirconis at ~54Ma, nearly synchronouswith the fossil age, providing additionalconstraint on the depositional age of the Dajin Formation.

6.2. Provenance

The north India continent and the south Asianmargin have differentmagmatic histories. Zircons from the south Asianmargin (the Gangdesearc) are dominated by Jurassic to Early Paleogene-aged grains, andtypically, have positive Hf values (Ji et al., 2009; Wu et al., 2010; Zhuet al., 2011). Whereas, zircons from the north Indian margin arecomposed of Proterozoic to Early Paleozoic grains (DeCelles et al.,2004; Zhu et al., 2011) and Early Cretaceous grains (Hu et al., 2010),and most of these zircons have negative Hf values (Hu et al., 2010).

Detrital zircons from the Dajin Formation are mostly clusteredbetween 120 and 54 Ma, and have positive Hf isotopes. These isotopicfeatures indicate an Asian affinity (Ji et al., 2009; Zhu et al., 2011).

Analyses filled with gray are highly discordant, and hence are excluded during provenance

Fig. 8. Relative U–Pb age probability for detrital zircons from the Dajin Formation. Data from the Jialazi Formation (data from Hu et al., in press; Orme et al., 2014) and the Sangdanlin andZheya formations (data from Wang et al., 2011; DeCelles et al., 2014; Wu et al., 2014) are shown for comparison.

9J.-G. Wang et al. / Sedimentary Geology 315 (2015) 1–13

However, clasts of the Dajin Formation could not have been directlyderived from the Gangdese arc, as indicated by the presence ofabundant recycled clasts (Fig. 5). We infer that the clasts were recycledfrom a sedimentary source, of which the sediments had been originally

Fig. 9.U–Pb age vs. εHf(t) plots of detrital zircons from the Dajin Formation. Field for zircons fro2011) is shown for comparison.

derived from the Gangdese arc. Therefore, the forearc basin strata,which were derived from the Gangdese arc and persevered in theforearc setting, are the most likely source. The similarity of the detritalzircon age spectra between theDajin Formation and the youngest strata

m the Gangdese magmatic arc (data mainly from Chu et al., 2006; Ji et al., 2009; Zhu et al.,

Fig. 10. Source discrimination plots for detrital Cr-spinels from the Dajin Formation. A) TiO2 versus Al2O3 diagram; B) TiO2 versus Cr# diagram. Field for spinels from the Yarlung-Zangboophiolite, the Tethyan Himalayan and the Xigaze forearc strata are shown for comparison (data are from Hu et al., 2014 and reference therein).

10 J.-G. Wang et al. / Sedimentary Geology 315 (2015) 1–13

in the forearc basin (the Jialazi Formation, Fig. 8; Orme et al., 2014; Huet al., in press) supports this interpretation. The only issue regardingthis interpretation is that the present exposure of the Xigaze forearcstrata occurs mostly between the Zhongba and Xigaze areas, which isfar from the area in which the Dajin Formation was deposited(Fig. 1A). One possible explanation for this problem is that the Xigazeforearc basin was originally much larger than its present-day extentbut was partly eroded during tectonic shortening following the India–Asia collision. Alternatively, the recycled clasts might be derived fromthe suture zone or from the Tethyan Himalaya (e.g., strata analogousto the Sangdanlin Formation near Saga, Figs.1A and 8) rather thanfrom the Xigaze forearc basin. But this interpretation is highly unlikelybecause those strata situated in a deep marine environment and wereunexposed in the Early Eocene time, and additionally, the interpretationcontradicts the paleocurrent results.

Cr-spinels and a few serpentinite fragments in the Dajin Formationindicate detrital input from a mafic/ultramafic source. The spinels arecharacterized by extremely low TiO2 contents (commonly b0.2 wt.%)and relatively high Fe2+/Fe3+ ratios (commonly N3), suggesting anorigin from a residual mantle source (Kamenetsky et al., 2001). TheYarlung-Zangbo ophiolite adjacent to the Dajin strata is the most likelysource of these clasts. In this scenario, the Yarlung-Zangbo ophiolitesmust have been emplaced before the Early Eocene. An alternativesource for these mafic detritus is the Lhasa terrane, because Cr-spinelswith similar compositions are found in the Xigaze forearc strata derivedprimarily from the Lhasa Terrane (Hu et al., 2014).

6.3. Geodynamics and basin setting

Considering the location and depositional age of the Dajin Forma-tion, there are two potential interpretations for its depositional setting:(1) continuation of the Xigaze forearc basin, or (2) syn-collisional basinafter the Indian–Asian continental collision.

A forearc basin setting was proposed by some former researchersbecause 1) foraminiferal assemblages in the Dajin formation (TibetanBureau of Geology and Mineral Resources, 1987; Yan et al., 2005) aresimilar to those from the coeval forearc strata (e.g., the Jialazi Formationin the Cuojiangding area, Hu et al., in press), and 2) the geochemicalcompositions of the rocks show a magmatic arc affinity (Yan et al.,2006). However, our new provenance data show that the source ofthe Dajin Formation is in contrast with that of the Xigaze forearcrocks. Detrital frameworks of the rocks from the forearc basin aredominated by volcanic clasts and plagioclases and have been deriveddirectly from the Gangdese magmatic arc (An et al., 2014) (Fig. 6).However, theDajin Formation contains a large portion of recycled clasts,

indicating derivation from a ‘lithic recycled provenance’ (Dickinson,1985) that was most likely a ‘clastic-wedge’ (Fig. 4; Garzanti et al.,2007).

We suggest that the deposition of the Dajin Formation was inresponse to the development of fold-thrusts along the southernmostAsian margin induced by the India–Asia continental collision. Despiteremaining controversial, growing data indicate that the initialIndia–Asia collision event occurred during the Paleocene–Early Eocenetime (e.g., Garzanti et al., 1987; Ding et al., 2005; Leech et al., 2005;Najman et al., 2010; Wang et al., 2011; Yi et al., 2011; Hu et al., 2012;Zhang et al., 2012; Wu et al., 2014). After the collision, northwardsubduction of the buoyant Indian plate would result in the formationof a thrust wedge at the leading edge of the south Asian margin. Theavailable data suggest that such structures might have been initiatedas early as the Late Paleogene–Early Eocene (Ratschbacher et al., 1994;Ding et al., 2005). Folds and thrusts developed along the south Asianmargin could lead to uplift and erosion of the forearc strata, and inturn, resulted in sedimentary recycling of the forearc strata and deposi-tion of the Dajin Formation. Recent studies suggest that sedimentationin the forearc basin ceased at ~54 Ma (Orme et al., 2014; Hu et al., inpress), nearly simultaneous with the deposition of the Dajin Formation.In fact, small basins (piggyback basin) often occur at the top of thrustwedges (e.g., Garzanti et al., 1996; Horton, 1998). The sedimentaryrecord of such basins is rarely observed simply because most of thestrata are uplifted and eroded shortly after deposition as a result offorward propagation of the thrust belt.

6.4. Regional stratigraphic comparison and Early Eocene underfilled Hima-layan foreland basin system

Early Paleogene syncollisional strata of the India–Asia collision insouthern Tibet are shown in Fig. 1; they include: 1) the Quxia and Jialaziformations in the Cuojiangding area near Zhongba, 2) the Sangdanlinand Zheya formations in the Sangdanlin area near Saga, and 3) theZongpu Formation in the Tingri and Gamba (Fig. 11A).

In the Cuojiangding region, the late Paleocene–the earliest EoceneQuxia and Jialazi formations consist of conglomerate, sandstone andsandy foraminiferal limestone that were deposited in fluvial to alluvial,and shallow marine environments (Wang et al., 2012; Qrme et al.,2014). These strata uncomformably overlie the Late Cretaceous shallowmarine strata of the Xigaze forearc basin, and the clasts were deriveddominantly from magmatic rocks of the arc. The strata are considereda record of the latest stage of the Xigaze forearc basin (Orme et al.,2014) or syncollisional sedimentation after the initial India–Asia colli-sion (Ding et al., 2005; Hu et al., in press). The end of the sedimentation

Fig. 11.A) Stratigraphic comparison of Paleogene strata in theHimalayan orogen, southern Tibet. Arrows indicate arrival of Asia-derived clasts to the north IndianMargin. Dash areamarksthe depositional timing of the Dajin Formation (~56–54 Ma). Stratigraphic data are from: Tingri and Gamba (Najman et al., 2010; Hu et al., 2012; Zhang et al., 2012; Li et al., 2014);Sangdanlin (Wang et al., 2011; DeCelles et al., 2014); Cuojiangding (Wang et al.,2012; Hu et al., in press). B) A north–south cross section shows the Early Eocene underfilled peripheralforeland basin system that developed soon after the initial India–Asia continental collision. Numbers indicate Early Eocene syncollisional strata as shown in Fig. 1A.

11J.-G. Wang et al. / Sedimentary Geology 315 (2015) 1–13

in the Cuojingding area occurred at ~54Ma (Orme et al., 2014; Hu et al.,in press), nearly coeval with the deposition of the Dajin Formation(Fig. 11A).

In the Sangdanlin region, deep-marine turbidite deposits of theDenggang Formation (Late Campanian–Paleocene) are overlain by sub-marine litharenites and cherts of the Sangdanlin Formation (Paleocene)and deep-marine turbidites of the Zheya Formation (Paleocene–EarlyEocene) (Fig. 11A; Wang et al., 2011); these deposits are interpretedto represent foreland basin sedimentation (Ding et al., 2005; DeCelleset al., 2014). U–Pb detrital zircon age spectra of sandstones from theSangdanlin and Zheya formations are similar to those from the DajinFormation (Fig. 8), suggesting a similar source.

In the Tingri and Gamba areas, the Paleocene–Early Eocene stratacomprise carbonate rocks of the Zongpu Formation, deposited on atectonic-induced carbonate ramp (Hu et al., 2012; Zhang et al., 2012).The strata are overlain by Asian-derived clastic rocks of the Enba andZhaguo formations (Fig. 11A).

Deposition of the Early Eocene Dajin formation and the coeval stratain the Himalayan orogen could be interpreted under an underfilledperipheral foreland basin framework (DeCelles et al., 1998; Hu et al.,2012; Zhang et al., 2012; DeCelles et al., 2014) (Fig. 11B). According toDeCelles and Giles (1996), a foreland basin system consists of fourdiscrete depozones, referred to as the wedge-top, foredeep, forebulgeand back-bulge depozones. Immature textures and recycled provenanceof the Dajin Formation indicate the rocks were deposited proximal to afold and thrust belt; therefore, theDajin Formation is considered to havebeen deposited in thewedge-top depozone of the Early Eocene Himala-yan foreland basin. The Sangdanlin and Zheya formations turbidites, assuggested by former studies, represent the foredeep depozone (Wanget al., 2011; DeCelles et al., 2014). The Early Eocene carbonate rocks inthe Tingri and Gamba areas were deposited on a carbonate ramp

developed above the forebulge (Hu et al., 2012; Green et al., 2008).The Early Eocene strata in the Lesser Himalaya (e.g., the BhainskatiFormation) were likely deposited in the backbulge depozone of theforeland basin (DeCelles et al., 1998).

In the Himalayan orogen, the voluminous foreland basin sedimentswere preserved in the sub-Himalaya since Early Miocene and consistof nonmarine facies (Critelli and Garzanti, 1994; DeCelles et al., 1998;Najman et al., 2004). The lack of an underfilled stage for the Himalayanperipheral foreland basin has puzzled Himalayan geologists for a longtime (e.g., Burbank et al., 1996; Najman et al., 2004). Our results andrecently published data from southern Tibet suggest that a forelandbasin system was initiated at least as early as the Early Eocene; thus,the early history of the Himalayan orogen is comparable with othercollisional orogens (e.g., the Alps orogen) on earth.

7. Conclusions

The lithofacies of the Dajin Formation are dominated by ungradedand normally graded, matrix- and clast-supported conglomerates(facies Gcm, Gmm and Gmn), ungraded and cross-stratified sandstones(facies Sm and St), and massive to poorly laminated mudstones (faciesFm and Fl). The deposition was characterized by sediment gravityflows that developed on a fan-delta system. The larger benthic forami-nifera and the youngest ages of detrital zircons indicate a depositionalage of the earliest Eocene (56–54 Ma). Occurrence of angular clasts,boulder-size clasts and carbonate clasts susceptible to chemicalweathering suggests deposition proximal to source. Petrology, U–Pband Hf isotopes of detrital zircons indicate that the Dajin formationcontain abundant recycled Gangdese clasts that were most likelyderived from the Xigaze forearc basin. We propose that the depositionof the Dajin Formation was a result of development of a fold-thrust

12 J.-G. Wang et al. / Sedimentary Geology 315 (2015) 1–13

belt along the leading edge of the Asian continent. The Dajin Formationand the coeval strata in the Himalayan orogen were considered to havebeen deposited in an underfilled foreland basin induced by theIndia–Asia collision. The Dajin Formation records the wedge-topdepozone of the foreland basin system.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.sedgeo.2014.10.009.

Acknowledgments

We thank Bin Zhu and Yue-Heng Yang for help in the zircon U–Pband Hf isotopic analyses. This work was financially supported by aNational Natural Science Foundation of China grant (41202082), theMOST 973 Project (2012CB822001) and the Foundation of the StateKey Laboratory of PetroleumResources and Prospecting, China Universityof Petroleum, Beijing (grant no. PRP/open-1309).

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