early cretaceous sedimentary evolution of the northern

Gondwana Research 73 (2019) 136–152

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Early Cretaceous sedimentary evolution of the northern Lhasa terraneand the timing of initial Lhasa-Qiangtang collision

Wen Lai a, Xiumian Hu a,⁎, Eduardo Garzanti b, Yiwei Xu a, Anlin Ma a, Wei Li a

a State Key Laboratory of Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, Chinab Department of Earth and Environmental Sciences, Università di Milano-Bicocca, Milano 20126, Italy

⁎ Corresponding author.E-mail address: [email protected] (X. Hu).

https://doi.org/10.1016/j.gr.2019.03.0161342-937X/© 2019 International Association for Gondwa

a b s t r a c t
a r t i c l e i n f o
Article history:Received 23 February 2018Received in revised form 20 December 2018Accepted 6 January 2019Available online 29 April 2019

Lower Cretaceous strata in the Baingoin basin of the northern Lhasa terrane record initial collision between theLhasa and Qiangtang blocks, followed by the early uplift of central Tibet. North-south traverses across theBaingoin basin highlight major differences between the Duba Formation in the north and the quasi-coevalDuoni Formation in the south. The Duba Formation documents upward transition from shallow shelf and deltaicenvironments to coarse-grained siliciclastic fluvial sedimentation. Abundance of detrital zircons yieldingJurassic-Cretaceous ageswith εHf(t) valuesmainly between−2 and+10, occurrence of chert, Cr-spinel, and py-roxene grains, together with southward paleocurrent directions indicate that the Duba Formation was sourcedfrom the southern Qiangtang terrane and Bangong-Nujiang suture zone to the north. The Duoni Formation inthe south was deposited in shelfal to fan-delta and fluvial environments. Abundant volcanic clasts, detrital zir-cons yielding Cretaceous ages with mainly negative εHf(t) values, and northward paleocurrents indicate an ac-tive volcanic source located in the central Lhasa terrane to the south, with minor input from the northernLhasa terrane. Only the northern part of the Baingoin basin was directly controlled by the Lhasa-Qiangtang col-lision andmay thus be considered a peripheral foreland basin, whereas the southern part wasmainly influencedby tectonic processes related to the northward subduction of Neotethyan lithosphere, and may thus be compa-rable to a retroarc foreland basin. But these sedimentary features and the 139–79Ma Baingoin plutonic intrusiondo not fit well with classical foreland-basin models. Zircon chronostratigraphy constrains the final consumptionof Bangong-Nujiang oceanic lithosphere and initial collision between the Lhasa and Qiangtang microcontinentsto have taken place by 122 Ma, which has major implications for paleotectonic reconstructions of the TibetanPlateau.

© 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords:Lhasa blockEarly CretaceousProvenance analysisBangong-Nujiang suture zoneLhasa-Qiangtang collision

1. Introduction

The Lhasa block, the last microcontinent accreted to Asia before finalcollision with India in the Paleocene, is delimited by the Indus-Yarlungsuture in the south and the Bangong-Nujiang suture in the north(Allègre et al., 1984; Yin and Harrison, 2000; Fig. 1a). Its complex geo-logical evolution, begun with northward drift away from Gondwanaand terminated by collision with the Qiangtang block in the northfollowed by collision with India in the south (Zhu et al., 2011a), is pin-pointed by tectonic and magmatic events recorded in sedimentary ar-chives. During the Cretaceous, the Lhasa block was influenced by bothongoing Lhasa-Qiangtang collision in the north and northward subduc-tion of Neotethyan lithosphere in the south (England and Searle, 1986;Pan et al., 2006; Kapp et al., 2007; Leier et al., 2007b; Zhu et al., 2016).This complex paleotectonic scenario would imply the development ofa peripheral foreland basin in the northern part of the Lhasa block

na Research. Published by Elsevier B.

south of the Bangong-Nujiang suture zone (Leeder et al., 1988;Murphy et al., 1997; Kapp et al., 2003a; Kapp et al., 2005; DeCelleset al., 2007) and of a distinct sedimentary basin in the rear of theGangdese arc, either an extensional back-arc basin (Zhang et al., 2004)or a compressional retroarc foreland basin (Zhang et al., 2011). Withinthis broad general canvas, all details remain to be painted. When wasthe Bangong-Nujiang suture zone closed, uplifted, and exhumed? Dur-ing the Early Cretaceous (Kapp et al., 2005), before the Early Eocene(Wang et al., 2008), or after the India-Asia collision onset (Rowley andCurrie, 2006)? Is there any direct sedimentary evidence in the Lhasablock that may help constraining the controversial timing of initialLhasa-Qiangtang collision, rangingwidely from as early asMiddle Juras-sic to as late as Late Cretaceous (Pan et al., 1983; Leier et al., 2007b;Baxter et al., 2009; Fan et al., 2014; Yan et al., 2016; Zhu et al., 2016;Bian et al., 2017; Li et al., 2017b; Ma et al., 2017)?

The aim of this article is to investigate the Early Cretaceoussedimentary record of the northern Lhasa terrane in order to providean answer to these questions and clarify the paleogeographic andpaleogeodynamic evolution of the Lhasa block and Lhasa-Qiangtang

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Fig. 1. (a) Tectonic outline of the Tibetan Plateau (Zhu et al., 2011b) showing major tectonic subdivisions and location of the Lhasa block. (b) Sketch geological map of the Lhasa block(modified from Pan et al. (2004)). (c) Simplified geological map of the Baingoin Basin (modified from Qu et al. (2011) and Chen et al. (2003)). BNSZ, Bangong-Nujiang suture zone;SNMZ, Shiquan River-Nam Tso Mélange Zone; LMF, Luobadui-Milashan Fault; IYZSZ, Indus-Yarlung Zangbo Suture Zone; NL, northern Lhasa terrane; CL, central Lhasa terrane; SL,southern Lhasa terrane; SGAT, Shiquan-Gaize-Amdo thrust; GST, Gaize-Selin Co thrust; ET, Emei La thrust; GLT, Gugu La thrust; GJT, Geren Co-Jiali thrust.

137W. Lai et al. / Gondwana Research 73 (2019) 136–152

collision zone. Sedimentary basins provide an essential key to constrainthe age ofmajor tectonic events and evaluate their effects on surface up-lift and topographic growth (Garzanti et al., 1987; Decelles et al., 1991;Sun et al., 2005; Najman, 2006; Wang et al., 2010). Early Cretaceousstrata are widely exposed in the northern Lhasa terrane, thus providingcrucial information to constrain the age of the Lhasa-Qiangtang collisiononset and of initial uplift of the central Tibetan Plateau.

In this study we have measured six stratigraphic sections along anorth-south traverse across the Baingoin basin in the northern Lhasaterrane. New data were collected from Lower Cretaceous strata on sed-imentary facies, sandstone petrology, paleocurrent directions, detritalCr-spinel chemistry, detrital zircon U-Pb ages and Hf isotopes in orderto determine provenance of detritus, evolution of the Baingoin basin,

and timing of the initial collision between the Lhasa and Qiangtangblocks.

2. Geological background

The Lhasa block (Fig. 1a) is subdivided into three terranes by theShiquanhe-Nam Co Mélange Zone (SMNZ) and Luobadui-MilashanFault (LMF) (Zhu et al., 2009a). The southern Lhasa terrane includesthe Upper Triassic to Cenozoic Gangdese granitoid arc batholith, themostly Paleogene Linzizong volcanic succession (Chu et al., 2006; Jiet al., 2009; Zhu et al., 2011b; Dong et al., 2014), and the CretaceousXigaze forearc basin (Dürr, 1996; Ding and Lai, 2003; Wang et al.,2012; An et al., 2014; Wang et al., 2017a). The central Lhasa terrane

138 W. Lai et al. / Gondwana Research 73 (2019) 136–152

includes a Precambrian crystalline basement (Dewey and Burke, 1973;Allègre et al., 1984), very-low-grade Carboniferous metasediments,Permian limestones, and Jurassic siliciclastic rocks (Leeder et al., 1988;Yin et al., 1988). Younger rocks include the widespread CretaceousZenong volcanics (Zhu et al., 2009a), the Late Cretaceous non-marineDaxiong Formation, and Cenozoic continental strata (XZBGM, 1993; G.Sun et al., 2015). The northern Lhasa terrane includes thick Late Jurassicmetasediments (XZBGM, 1993), Early Cretaceous marginal-marine anddeltatic clastics (Zhang et al., 2004; Leier et al., 2007a; Leier et al., 2007b;Volkmer et al., 2007; Zhang et al., 2011), Aptian-Cenomanian lime-stones of the Langshan Formation (Leier et al., 2007b; Scott et al.,2010; Rao et al., 2015; BouDagher-Fadel et al., 2017), and Late Creta-ceous to Cenozoic continental sandstone and conglomerate (DeCelleset al., 2007; Kapp et al., 2007; Zhang et al., 2011). Mid- to Late Creta-ceous plutonic rocks and volcanic rocks of the Qushenla Formationalso occur (Zhu et al., 2011b; G.Y. Sun et al., 2015).

The Bangong-Nujiang suture zone (BNSZ), separating the Lhasa andQiangtang blocks, includes Jurassic deep-sea turbidites (Girardeau et al.,1984; Dewey et al., 1988; Li et al., 2017a), olistostrome (Y. Zeng et al.,2016; M. Zeng et al., 2016; Lai et al., 2017), and ophiolite remnantswith supra-subduction-zone (SSZ) and oceanic-island-basalt (OIB) geo-chemical affinities (190–161Ma;Wang et al., 2016). An angular uncon-formity separates ophiolites and the mélange from the overlying LateJurassic-Early Cretaceous shallow-marine clastic rocks (Li et al.,2017a). Mid-Cretaceous OIB-type basalts were interpreted as sea-mounts (Zhu et al., 2006; Fan et al., 2014) or intraplate volcanism.Late Cretaceous to Cenozoic fluvial or lacustrine sequences with inter-bedded volcanic rocks characterize the northern Nima and Lunpola ba-sins (XZBGM, 1993; Kapp et al., 2005; DeCelles et al., 2007; Kapp et al.,2007).

To the north of the BNSZ, the Qiangtang block is divided into north-ern and southern terranes either by the Longmu–Shuanghu suture zone(Li, 1987; Li et al., 1995; Wang et al., 2015) or by a blueschist-bearing,Triassic metamorphic mélange belt (Cheng and Xu, 1987; Kapp et al.,2000; Kapp et al., 2003b; Pullen et al., 2008; Zhang and Tang, 2009).In the southern Qiangtang terrane, Cambrian metasedimentary rocksintruded by Ordovician granites lie in tectonic contact withCarboniferous–Jurassic strata (Pullen et al., 2011). Widespread marineJurassic sandstone and limestone (Kapp et al., 2003b; Zhang et al.,2012; Ma et al., 2017) were intruded by arc-related intermediate tofelsic rocks in the Jurassic (150–170 Ma) and Cretaceous(100–130 Ma) (J.X. Li et al., 2014; S.M. Li et al., 2014; Liu et al., 2017).Minor Cretaceous strata and Cenozoic nonmarine deposits also occur(XZBGM, 1993; Kapp et al., 2005). Paleozoic strata and Triassic lime-stone are widely exposed in the central and northern Qiangtang block(Li et al., 1995; Kapp et al., 2000).

2.1. The Baingoin basin

The Baingoin basin, located between theNamCo and the BNSZ, is di-vided in two parts by a ~60 km-long and up to 10 km-wide east-westbelt of biotite-bearing granites of Cretaceous age (139–79 Ma;Volkmer et al., 2014; Fig. 1c). These igneous rocks experienced rapidto moderate cooling and exhumation by the latest Cretaceous-early Pa-leogene based on low-temperature thermochronology (70–55 Ma;Hetzel et al., 2011). The basin is characterized by widely exposedLower Cretaceous marginal-marine and deltatic clastic sediments inter-bedded with volcanic tuffs (Zhang et al., 2004; Leier et al., 2007a; Leieret al., 2007b; Volkmer et al., 2007; Zhang et al., 2011). Because of signif-icant stratigraphic differences between the northern and southern partsof the basin, they were defined as Duba Formation in the north (Leederet al., 1988; Leier et al., 2007b) and Duoni Formation in the Nam Co areato the south (XZBGM, 1993; Zhang et al., 2011; Sun et al., 2017). Bothunits are overlain byOrbitolina-bearing limestones of the Langshan For-mation (Leier et al., 2007b; Scott et al., 2010; Rao et al., 2015;BouDagher-Fadel et al., 2017) (Fig. 1c). Lower Cretaceous strata are in

fault contact with continental deposits of the Upper CretaceousJingzhushan Formation (Fig. 4a).

During the Early Cretaceous, the Baingoin basinwas bounded by thenorth-dipping and south-vergent Shiquanhe-Gaize-Amdo thrust in thenorth (SGAT in Fig. 1b), and by the south-dipping and north-vergentGeren Co-Jiali thrust in the south (GJT in Fig. 1b) (Ding and Lai, 2003;Kapp et al., 2005). As part of the Shiquanhe-Gaize-Amdo thrust (YinandHarrison, 2000; Kapp et al., 2005), the north-dippingMuggar thrustin the Nima area was activated around 108 Ma (Kapp et al., 2007).Shiquanhe-Gaize-Amdo thrust activity caused deformation and upliftabove sea level of Jurassic marine rocks and Lower Cretaceous graniteswithin the BNSZ (Yin and Harrison, 2000; Kapp et al., 2005; Kappet al., 2007). The NW–SE-trending Geren Co-Jiali thrust, extendingfrom Geren Co in the west to the Nam Co and Jiali areas in the east,placed Carboniferous strata and Early Cretaceous Zenong volcanicrocks in its hanging wall onto Early Cretaceous strata of the Baingoinand Selin Co basins in its footwall around 130 Ma (Ding and Lai, 2003).

3. Methods

3.1. Sedimentology and petrography

Detailed stratigraphic sections were measured at 6 localities (sitesshown in Fig. 1c). Sedimentary structures and lithofacies associationsfollowing criteria defined in Miall (1978, 1996) were identified.Paleocurrent directions were measured in the field from oblique lami-nation in sandstone beds and clast imbrication in conglomerate beds.Results were corrected to the horizontal by standard stereonet tech-niques, and the average trough-axis orientation of each pointwas deter-mined statistically on a stereographic plot of 15–20 trough limbs(method I of DeCelles et al., 1983).

The petrographic composition of 24 sandstone samples from theDuba Formation exposed in the Duba area and of 20 samples from theDuoni Formation exposed in the Nam Co area was determined bycounting over 400 sand grains per thin section following the Gazzi-Dickinson method (Ingersoll et al., 1984).

3.2. Zircon dating and Hf isotopes

Detrital zircons were separated from medium-grained sandstones.U-Pb dating was conducted by LA-ICP-MS at the State Key Laboratoryof Mineral Deposits Research, Nanjing University, China, followingJackson et al. (2004). The results were calculated by GLITTER 4.4 (VanAchterbergh et al., 2001), and common Pb correction (Andersen,2002) was made. We considered 206Pb/238U ages for grains youngerthan 1000 Ma and 207Pb/206Pb ages for grains older than 1000 Ma(Griffin et al., 2004) provided discordance was b10%. Age calculationsand concordia diagramswere created using Isoplot 3.23 (Ludwig, 2001).

In-situHf isotopic analyseswere carried out on detrital zircons yield-ing U-Pb ages younger than 250 Ma. Hf isotopic compositions were ob-tained by Thermo Scientific Neptune Plus (MC-ICP-MS) coupled with aNewWave UP193 solid-state laser ablation system at the State Key Lab-oratory of Mineral Deposits Research, Nanjing University. Zircon grainswere ablated with a beam diameter of 35 μmwith an 8-Hz laser repeti-tion rate, and with an energy of 15.5 J/cm2. Results were calculated as-suming 1.865 × 10−11 a−1 for the decay constant of 176Lu (Schereret al., 2001). The εHf(t) values and Hf crustmodel age (TCDM) were calcu-lated following Bouvier et al. (2008) and Griffin et al. (2002),respectively.

Overall, we dated 500 detrital zircons in 6 sandstone samples formthe Duba Formation and 20 igneous zircons in tuff sample 16BG17from the Duba area, and 150 detrital zircons in 2 samples from theDuoni Formation and 20 igneous zircons in tuff sample 17BJ01 fromthe Nam Co area (sampling sites shown in Figs. 2 and 3); 637 concor-dant ages and 265 Hf isotopic data from zircons younger than 250 Mawere obtained (Figs. 8, 9; Appendix Tables A.2 and A.3).

300

350

400

450

500

550

c s m c g b

14MD14

14MD15

14MD1614MD17

14MD18

0

50

100

150

200

250

300

c s m c g b300

350

400

450

500

550

c s m c g b

N 31 25 13.62E 89 32 57.65

15MD01

15MD02

15MD07

15MD08

15MD03

15MD09

15MD10

15MD11

15MD13

15MD04

15MD05

15MD06

15MD12

FlSh

Fl

Fl

FlFl

Fl

Fl

Fl

FlFl

Fl

FlFl

Fl

Fl

FlFl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

FlFl

FlFlFl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

FlFl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Gch

Fl

Fl

Fl

Fl

Fl,Sh

Fl,Sh

Fl,Sh

Fl,Sh

Fl,Sh

Gch

Gch

Gch

Sh

Sh

Sp

Sp

Sh

ShSh

Sp

SmSh

Sh

Sh

Sp

SpGmm

Gmm

Gmm

Gmm

Gmm

GmmGmm

Sh

Sh

Sh,Sl

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sl

Sh

Sh

Sh

ShSh

Sh

ShSh

Sh,Sl

Sh

Sh

Sh

Sh

Sh Sh

Sh

Sh

Sh

Sh

ShSh

Sl

Sh

Sh

Sh

Sh

Sl

ShSh

SmSm

ShSh

Sh

Sm

Gmg

ShSh

Sh

Sh

ShSh

Sh

St

St

St

St

St

St

ShSh

Sh

Sh

ShSh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

ShSpSp

Sp

Sp

Sp

0

50

100

150

200

250

c s m c g b

N 31 23 5.38E 89 32 3.68

N 31 24 24.83E 89 42 01.25

16ZR01

16ZR02

16ZR03

16ZR04

16ZR06

16ZR07

16ZR08

16ZR09

16ZR10

16ZR11

16ZR12

16ZR13

16ZR05

16ZR14

N 31 22 58.91E 89 31 58.54

N 31 24 44.30E 89 42 21.93

N 31 24 54.88E 89 32 41.11

FlFl

Fl

Fl

Fl

Fl

Fl

FlFl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

FlFl

Fl

Fl,Sh

Fl,Sh

Gch

Gmg

Gmg

Gmg

Gmg

Sh

Sp

Sh ShSpSh

ShSp

St

ShSh

ShSp

St

St

St

ShSh

ShSh

Sh

St

Sh

Sh

Sh

Sh

Sp

Sp

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

Sh

ShSh

Sh

ShSh

Sh

SpSp

Sp

Sp

Sp

SlSh

Sp

SpSpSpSh

Sp

Sp

St

Sh

Sp

0

50

100

150

200

250

300

c s m c g b

F14MD13

14MD12

14MD11

14MD10

14MD09

14MD08

14MD07

14MD06

14MD05m m m m

DB01 section

DB02 section

DB02 section

DB03 section

DB03 section

m

?mL

angs

han

Fm.

n=20

n=21

n=10

n=22

n=7

121 MaDZ

122 MaDZ

114 MaDZ

122 MaDZ

128 MaDZ DCr

118 MaDZ DCr

Fig. 2. Stratigraphic sections of theDuba Formation in the northern Baingoin Basin showing sample locations, lithofacies, ages, and paleocurrent data. DZ, YC1σ(2+) age of detrital zircons;DCr, detrital Cr-spinel samples, legend is shown in Fig. 3.


0

50

100

150

c s m c g b N 30 57 43.94E 90 18 39.06

N 30 57 43.86E 90 18 45.80

Sh17BJ0217BJ01

SpSpSm 17BJ03

17BJ04Fr

Fr

Gch17BJ05Gch

SpSp

Sm 17BJ0617BJ07

GchGcm

Sp,Sh 17BJ08

Gt

Gt

GtGch

SpSp

17BJ09

17BJ10Sm

StSt

StStStStSt

St

Fl

17BJ11

250

300

350

400

450

c s m c g b

N 31 00 54.64E 90 22 36.32

NC01 section

16SC16Sp

Sl

Fl

Fl

Fl

Fl

Fr

Fl

Fl

Fl

Fl

Fl

Fl

Fl

FlFl

Sp

Sh

Sh

Sh

ShSmSm

ShShSh

ShStSt

ShSh

Sh

Sh

Sh

?Sh

16SC17

16SC21

16SC22

16SC23

16SC24

16SC25

N 31 3 44.23E 90 8 54.42

0

50

100

150

c s m c g b N 31 3 30.67E 90 8 60.0

17BJ12EGch

Gmm

FlFlFl

FlFl

Fl

Fl

Fl

Fl

FlFlFlFl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

FrFr

FrFl

Fr

Gmm

SpSp

SpSp

Sh

Sh

Sh

Sl

Sh

Sr

SrSp

SpSpSp

Sh

Sr

Sr

ShShSh

Sr 17BJ18

17BJ17

17BJ16

17BJ15

17BJ14

17BJ13

17BJ12A

Sp

Sp

SpSp

Sp

SpSp

Sp

Sp

Sp

16SC01

16SC0516SC02

16SC06

16SC07

16SC08

16SC09

16SC10

16SC12

16SC13

16SC14

16SC15

16SC11

Fl

Fl

Fl

Fl

Fl

Fl

Fl

Fl

FlFlFlFl

Fr

Fr

FlGmm

Fl

Fl

Fl

Fl

Gch

Sh

Sh

Sh

Sm

Sl

Sh

Gch

Gch

Gch

Gcm

Gcm

Sh

Sh

Sh

Sh

Sm

Gch

Gt

Gmm

16SC09A

Fr

Tuff

NC02 section NC03 section

Fl

Fl

Fr

Gt

Sh

SmSm

Sh

ShSh

Sm

16SC18

16SC19

16SC20

NC01 section

N 31 0 56.98E 90 23 2.68

0

50

100

150

200

250

c s m c g bm

m

m m

?m

Lang

shan

Fm

.

?m

Lang

shan

Fm

.

Clay,sand,gravel,

egend

No

Red/grey/green conglomerateswith clast-supported

Conglomerate lens or sandstone lens

Red/black/green

Red/green sandstone

Red/green shale with nodules

Red/grey/green conglomerateswith matrix-supported

? m Thickness is unknown

South

Lan

gsha

n Fm

.

NC

02 sectio nN

C01 se ction

NC

03 section

DB

0 2 se ctionD

B03 sect ion

DB

0 1 sec ti on

NorthDuoni Fm. Duba Fm.

n=20

n=20

114 MaDZ

DCr n=22

108 MaDZ 114 Ma

Tuff

no DCr

n=10

n=12

n=6

n=12

n=20

Palecurrents of Duba Fm.Palecurrents of Duoni Fm.

Fig. 3. Stratigraphic sections of the Duoni Formation in the southern Baingoin Basin, showing sample locations, lithofacies ages, and paleocurrent data. The composite column is shown inthe lower-right panel. DZ, YC1σ (2+) age of detrital zircons; DCr, detrital Cr-spinel samples.


3.3. Cr-spinel geochemistry

Two sandstone samples from the Duba Formation and two sand-stones samples from the Duoni Formation were treated, although

Fig. 4. Field photographs. Duba Formation: a) panorama of section DB02, showing stratigraphicFormation; b) green massive conglomerate lens in black shales (lower member, section DB01(middle member, section DB02); e) crude tabular oblique lamination in coarse-grained sandstsection DB03; g) clast-supported red conglomerate with clast imbrication indicating southwathe upper Duoni Formation and overlying Langshan Formation; i) clast-supported conglome(upper member, section NC01). (For interpretation of the references to color in this figure lege

detrital Cr-spinels could be obtained only from samples 16ZR14 and15MD09 from the lower and middle members of the Duba Formation.All samples were powdered to b250 μm size and dense minerals wereseparated by elutriation and magnetic methods. Cr-spinel grains were

contact with the overlying Langshan Formation and tectonic contact with the Jingzhushan); c) fining-upward sequence (middle member, section DB02); d) lenticular sandstonesones (middle member, section DB02); f) panorama of the middle and upper members inrd paleocurrent (section DB03). Duoni Formation: h) panorama of section NC01 showingrate (section NC01); j) wave ripples (section NC03); k) red shales with caliche nodulesnd, the reader is referred to the web version of this article.)

0.33

m

1.65

m

25m m

d e

4.89 m

0.33

m

f g

Langshan Fm.

upper Duoni Fm.

h i

kj

0.33

m

0.33

m

b c

middle Duoni Fm.

Langshan Fm.

GSTDuba Fm. Jingzhushan Fm.

a

bottom of Duba Fm. lower Duba Fm.

lower Duba Fm. lower Duba Fm.

upper Duba Fm.

middle Duba Fm.

upper Duba Fm.

middle Duoni Fm. upper Duoni Fm.

NNE

NNW

NE


Table 1Lithofacies distinguished in the measured sections (modified from Miall, 1996).

Faciescode

Description Interpretation

Gt Pebble conglomerate, well sorted,clast-supported, troughcross-stratified

Migration of gravelly 3-D dunesunder traction flows in fluvialchannels

Gch Pebble to cobble with few thick ofset-packages conglomerate, wellsorted and clastic-supported,sub-angular to subround, slightlynormal grading, horizontally stratifiedand basal erosive boundary

Deposition under clast-supporteddebris flows, shallow tractioncurrents, or gravelly low relief bars

Gcm Pebble to cobble size conglomerate,clastic-supported with a sandymatrix, subround gravel, poorly tomoderately sorted, crudelyhorizontal bedding, weak gradingwith imbricated clasts, poorlyorganized with few thick layers andbasal erosive boundary

Deposition by clast-rich debrisflows or traction currents underrelatively rapid accumulated rates

Gmm Massive, matrix-supported pebbleto boulder conglomerate, poorlysorted, disorganized, unstratified

Deposition by matrix-rich debrisflow under the braided riverchannel

Sp Fine- to very coarse-grainedsandstone with wedge shaped,moderately-poorly sorted, planarcross-stratification with sandylayers, can be pebbly

Deposition by migration of largedunes under shallow unidirectionalflows

Sr Fine- to medium-grained andmoderately sorted sandstone withsmall current ripples or symmetricwave ripples

Deposition by shallowunidirectional flows regime,migration of ripples

Sh Fine- to medium-grained sandstonewith horizontal lamination

Upper plane bed conditions underunidirectional flows, either strong(N100 cm/s) or very shallow

St Medium- to very coarse-grainedsandstone with troughcross-stratification, can be pebbly

Migration of large 3D ripples (dunes)under moderately powerful (40–100cm/s), unidirectional flows in largechannels

Sm Massive medium- to fine-grainedsandstone; bioturbated

Bioturbated or pedoturbated sand,penecontemporaneousdeformation

Fl Reddish/greenish mud and siltyshale beds, small current ripples,horizontally-laminated bed

Deposition by flood plain,continental shelf or abandonedchannel deposits

Fr Massive reddish or yellow laminatedred, green, or gray siltstone bedsbounded at the top by erosivesurfaces, horizontally laminated, lensor wedge shaped interbeddedsiltstone and fine sandstone,carbonate nodules common

Deposition of flood plain, distalalluvial plain, calcic or verticpaleosols


selected by hand-picking, mounted in epoxy, and polished for analysis.Geochemical composition was determined at the State Key Laboratoryof Mineral Deposits Research, Nanjing University, using a JEOL JXA-8100M electron microprobe following Hu et al. (2010). Only analysesclosing between 98 and 102%were considered valid. The 111 results ob-tained are listed in Table A.4.

4. Duba Formation

4.1. Sedimentology and stratigraphy

The stratigraphic succession of the northern Baingoin basin is wellexposed in the Duba area, although affected by faulting (Volkmeret al., 2014). A composite stratigraphic section had to be reconstructedfrom three measured partial sections (locations shown in Fig. 1c).Three members can be recognized.

4.1.1. Lower member: shallow shelf depositsThis member consists of N250 m-thick black laminated shale and

massive mudrocks with intercalated planar to lenticular gray-greensandstone beds mostly 10–30 cm-thick - but varying in thickness from1 cm to over 2 m - showing graded-bedding and parallel lamination(lithofacies Fl, Sh, Sm; section DB01, Figs. 2 and 4b). Three matrix-supported and poorly sorted green conglomerate beds containing rip-up mudrock and siltstone clasts occur at the top of section DB01(lithofacies Gmm; Figs. 2 and 4b).

4.1.1.1. This member accumulated on a shallow continental shelf epi-sodically affected by tides and storms. Mudrocks were depositedbelow fair-weather wave base, whereas sandstones and conglomeratesdocument high-energy events such as storm-surges or hyperpycnalflows (Kreisa, 1981; Dott Jr and Bourgeois, 1982; Duke et al., 1991).

4.1.2. Middle member: delta depositsThis member, N550 m-thick in sections DB02 and DB03, consists of

green sandstone and shale with minor pebble conglomerate. Fine- tocoarse-grained sandstone beds are generally ~3 m-thick and displaymainly planar horizontal lamination (lithofacies Sh) or tabular (Sp)and more rarely trough (St) oblique lamination, current ripples, andsymmetrical ripples (lithofacies Sr). Sandstone beds showing gradedbedding and containing mudclasts and burrows may be arranged incoarsening- and thickening-upward sequences. Main sandstone bodiesare underlain by conglomerate or pebbly sandstone layers b 1 m-thick.Lenticular sandstones, 2–3 m thick and displaying erosive base, alsooccur. Greenish and reddish shales are intercalated with green silt-stones and sandstones in the upper part of the member (Figs. 2 and4d), where sandstones showing trough and planar oblique laminationpointing to roughly southward paleocurrent direction may be arrangedin fining-upward sequences (Table 1).

4.1.2.1. This member was deposited in deltaic environments. Thick lentic-ular sandstone beds suggest high-energy fluvial outflow close to a rivermouth. Coarsening- and thickening-upward sequences of burrowedsandstones point to distributary-channel or mouth-bar environments(Orton and Reading, 1993). In the upper part, alternating sandstone, silt-stone, and varicolored shales document a subaerial delta-plain with dis-tributary channels, with (An et al., 2014) thin rippled sandstonesrepresenting crevasse-splay deposits. Sandstones displaying trough andtabular oblique laminationwere formedasmigrating3-Ddunesdepositedwhile the paleochannels were filling andmigrating laterally (Miall, 1996).

4.1.3. Upper member: fluvial channels and floodplainThis member, N90 m-thick in sections DB02 and DB03, consists of

reddish shale, siltstone, sandstone, and conglomerate. Section DB03documents a red sandstone-conglomerate fining-upward sequence.Clast-supported conglomerates display imbrication, crude horizontal

and weakly developed trough oblique lamination with basal scour sur-faces (Gch and Gcm; Fig. 4g). Clasts are mainly poorly sorted and wellrounded quartzose sandstones, ~10 cm in diameter on average andreaching up to ~30 cm (Fig. 4g). Conglomerate beds usually fine upwardinto pebbly sandstone with tabular oblique or horizontal lamination(Lithofacies Sp, Sh). Fine- to coarse-grained red sandstones also displaytabular oblique or horizontal lamination. Section DB02 to the south, in-stead, is dominated by red shales with root traces and vertical burrows(Fl), intercalated with fine-grained sandstone displaying parallel lami-nation (Sh) (Fig. 2). Tabular oblique lamination and clast imbrication in-dicate SW-directed paleocurrents, confirming previous observations(Leier et al., 2007b; Zhang et al., 2011) (Fig. 2).

4.1.3.1. Interpretation. This member was deposited by braided andmeandering rivers. Fining-upward sequences reflect sand dunes mi-gratingwithin paleochannels (Miall, 1996). Red shaleswith intercalatedfine-grained sandstone are interpreted as paleosols with occasionalcrevasse-splay deposits formed in floodplain to delta-plain environ-ments (Orton and Reading, 1993; Miall, 1996).


4.2. Sandstone petrography

Twenty-four, moderately sorted and subangular to subroundedsandstone samples are mainly feldspatho-litho-quartzose (averagemodal composition Q47F10L43; Table A.1; Figs. 4a, b, 7a). N95% of quartzgrains aremonocrystalline. Over 70% of feldspars are plagioclases. Lithicgrains (28–55%QFL) are mostly felsitic and basaltic volcanic, subordi-nately metamorphic (phyllite, schist) and locally sedimentary (chert)(Table A.1; Fig. 7b). Biotite, zircon, tourmaline, Cr-spinel, pyroxene,and serpentine are rare.

4.3. Detrital zircon U-Pb ages and Hf isotopes

Sample 16ZR14 (section DB01; Figs. 1c and 2) yielded 98 concordantU-Pb zircon ages, 40 of which between 117 and 160 Ma (peak at~130Ma). Thirty-three Mesozoic zircons yielded εHf(t) values between−12.7 and +12.2, with TCDM model ages between 0.47 and 1.98 Ga.Older U-Pb ages cluster at 250–350, 450–700, 750–900, 1000–1350,1600–1900 Ma, and 2400–2700 Ma.

Samples 14MD05, 14MD09, and 14MD15 (section DB02) yielded244 concordant U-Pb zircon ages, 187 of which between 115 and170 Ma (peak at ~143 Ma). Ninety-four Mesozoic zircons yielded εHf(t) values between −19.9 and +13.5, with TCDM model ages between0.41 and 2.48 Ga. Older U-Pb ages cluster at 250–350, 450–500,550–650, 700–900, 1100–1350, 1550–1650, and 1700–2000 Ma(Figs. 8, 9; Tables A.2 and A.3).

Sample 15MD01 and 15MD13 (section DB03) yielded 145 concor-dant U-Pb zircon ages, 86 of which between 120 and 170 Ma (peak at~130 Ma), Forty-six Mesozoic zircons yielded εHf(t) values between−5.0 and +6.9, with TCDM model ages between 0.75 and 1.52 Ga. OlderU-Pb ages cluster at 250–350, 450–600, 700–900, 950–1250, and1800–1950 Ma, and 2300–2500 Ma.

4.4. Detrital Cr-spinel

Cr-spinel is the only durable heavy mineral ultimately shed frommafic and ultramafic rocks, and as such it represents a potentially veryuseful indicator of erosion of ophiolitic complexes exposed within su-ture zones. Detrital Cr-spinels derived from different tectonic settingscan be differentiated by their chemical composition using binary plots(e.g., TiO2 vs. Al2O3 or TiO2 vs. Cr# [Cr/(Cr + Al)]; Arai, 1992;Kamenetsky et al., 2001; Hu et al., 2014).

The 126 Cr-spinels analyzed can be divided into two groups accord-ing to their TiO2 content (Fig. 10; Table A.4). Group A spinels (n = 96,

Table 2Summarized characteristics of detrital zircon U-Pb ages of sandstone samples in the Duoni and

Section Sample Analyzed numbers ofzircon grain

Percentage of theMesozoic ages

Maximum depositionalage (Ma)

DB01 16ZR14 98 51% 117.8 ± 3.0DB02 14MD05 75 84% 121.7 ± 3.7

14MD09 72 69% 128.0 ± 2.414MD15 97 76% 114.2 ± 5.3

DB03 15MD01 73 37% 122.1 ± 1.915MD13 72 82% 120.6 ± 4.0

NC01 16SC01 75 89% 108.1 ± 2.116SC15 75 75% 113.5 ± 1.7

a YDZ = age calculated by the “Youngest Detrital Zircon” routine of Isoplot (Ludwig, 2001).b YSG = youngest single detrital zircon age with 1δ uncertainty.c YPP = youngest graphical detrital zircon age peak on an age-probability plot or age-distribd YC1σ(2+)=weightedmean age (±1σ incorporating both internal analytical error and ex

1σ.e YC2σ(3+)=weightedmean age (±1σ incorporatingboth internal analytical error and ext

2σ (Dickinson and Gehrels, 2009).

86%), having low TiO2 (b0.9%; 71% of grains b0.2%) and Al2O3, Cr2O3,and Cr# ranging respectively 4.7–27.7 wt%, 34.9–64.8 wt%, and0.57–0.90, plot in the “supra-subduction-zone peridotite” and MORBfields of Kamenetsky et al. (2001) (Fig. 10a). Group B spinels (n = 15,14%), mainly extracted from sample 14MD09 of the middle memberand having instead relatively high TiO2 (0.9–3.5%) and Al2O3, Cr2O3,and Cr# ranging respectively 12.8–19.8 wt%, 40.8–46.9 wt%, and0.58–0.71, plot in the oceanic-island basalts field of Kamenetsky et al.(2001) (Fig. 10a) and in the “intra-plate basalts” field of Arai (1992)(Fig. 10b).

4.5. Age constraints

The maximum depositional age of siliciclastic sediments containingdetrital zircons inferred to be generated during penecontemporaneousvolcanic eruptions is best constrained by the youngest cluster of atleast two zircon ages (YC1δ (2+) of Dickinson and Gehrels (2009);Table 2). A YC1σ (2+) zircon age of 117.8 ± 3.0 Ma was obtainedfrom the sample 16ZR14 collected at the top of section DB01. Samples14MD05, 14MD09, and 14MD15 from section DB02 yielded YC1σ (2+) zircon ages of 121.7± 3.7, 128.0± 2.4, and 114.2± 5.3 Ma, respec-tively. Samples 15MD01 and 15MD13 from section DB03 yielded YC1σ(2+) zircon ages of 122.1 ± 1.9 and 120.6 ± 4.0 Ma, respectively.

Nineteen zircons extracted fromvolcanic tuffs intercalated at the topof the upper member yielded two populations with weighted averageages of 125.7 ± 1.3 Ma (n = 14, MSWD = 0.01) and 109.6 ± 1.9 Ma(n = 5, MSWD = 0.21) (Figs. 5 and 6d, sample 16BG17). These agesare interpreted to represent the age of older source rocks and the ageof penecontemporaneous volcanic eruptions, respectively.

Our results indicate that deposition of the Duba Formation began by122 Ma and continued to ~110 Ma (Aptian–early Albian), which isbroadly compatible but somewhat younger than the maximum deposi-tional age of 125 ± 2 Ma obtained from detrital zircons by Leier et al.(2007b), and ages of 138 ± 2 and 119 ± 5 Ma obtained for volcanictuffs by Volkmer et al. (2014).

4.6. Provenance interpretation

Southward to southwestward paleocurrents indicate sedimenttransport from the north. Although no ophiolitic detritus was detectedin sandstone thin sections, minor chert and presence of Cr-spinel, py-roxene may be indicative of detritus from the Bangong-Nujiang suturezone. In fact, geochemical signatures of detrital Cr-spinel match well

Duba formations.

YDZa YSGb YPPc YC1σ(2+)d YC2σ(3+)e

115.7 + 3.4–5.9 117 ± 2 128 117.8 ± 3.0 (n = 3) 119.3 ± 2.0 (n = 5)117.5 + 4.8–15 119 ± 7 132 121.7 ± 3.7 (n = 4) 121.7 ± 3.7 (n = 4)123.1 + 4.3–12 125 ± 6 144 128.0 ± 2.4 (n = 7) 131.6 ± 3.0 (n = 11)110.6 + 6.2–13 111 ± 6 148 114.2 ± 5.3 (n = 2) 117.0 ± 3.8 (n = 5)119.0 + 2.9–9.5 119 ± 4 123 122.1 ± 1.9 (n = 5) 122.4 ± 1.7 (n = 5)117.0 + 4.9–6.2 119 ± 3 129 120.6 ± 4.0 (n = 3) 121.1 ± 3.6 (n = 4)105.7 + 3–4.5 107 ± 2 120 108.1 ± 2.1 (n = 4) 108.1 ± 2.1 (n = 4)110.3 + 2.6–5.3 112 ± 2 121 113.5 ± 1.7 (n = 6) 114.0 ± 1.4 (n = 9)

ution curve.ternal systematic error) of youngest cluster of two ormore grain ages overlapping in age at

ernal systematic error) of youngest cluster of three ormore grain ages overlapping in age at

Fig. 5. U-Pb zircon weighted mean ages of tuff beds from the (a) Duba and (b) Duoniformations.


the occurrence of both supra-subduction-zone and MORB ophiolitesand OIB-type basalts in the BNSZ (Wang et al., 2016).

The abundance of volcanic detritus including N40% of detrital zircongrains with U-Pb ages between 110 and 170Ma points to an active vol-canic source. Mesozoic detrital zircons never yielded εHf(t) values N 10,indicating that provenance was not from the Gangdese arc character-ized by high positive εHf(t) (Fig. 9; Chu et al., 2006; Ji et al., 2009; Zhuet al., 2011b). Detrital zircons dated between 150 and 170 Ma withεHf(t) values between −12 and +6.6 match well those granitoids andvolcanic rocks from east of Daru Co in the BNSZ (Y. Zeng et al., 2016;M. Zeng et al., 2016; Zhu et al., 2016) and the magmatism with150–170 Ma in southern Qiangtang terrane (J.X. Li et al., 2014; S.M. Liet al., 2014; Liu et al., 2017). However, between 130 and 150 Mamagmatism was nearly absent in both BNSZ and southern Qiangtang(Zhu et al., 2016), and many detrital zircons in the Duba Formationyielded ages in this range. These zircons yielded positive εHf(t) values,which are different from those in central Lhasa subterrane, character-ized by negative εHf(t) values (Zhu et al., 2011b). The Early Cretaceousdetrital zircons yielding εHf(t) values between −12 and +8 can betraced within the Baingoin Batholith itself (Zhu et al., 2016). The mostlikely source for these detrital zircons is the northern Lhasa terrane(Zhu et al., 2011b; Hou et al., 2015).

Because detritus from the Lhasa block have reached the mé-lange in trench of BNSZ since early Jurassic (Lai et al., 2017),these pre-Mesozoic detrital zircons, which have similar age spec-tra as those in the central Lhasa terrane, might be also recycledfrom the BNSZ.

Combined petrographic, geochemical and geochronological evi-dence suggests that the Duba Formation was fed partly from the BNSZand southern Qiangtang terrane in the north, where supra-

subduction-zone ophiolites, a Jurassic magmatic arc, and pre-Cretaceous strata were exposed to erosion, and partly from Lower Cre-taceous magmatic rocks of the northern Lhasa terrane itself.

5. Duoni Formation

5.1. Sedimentology and stratigraphy

The Duoni Formation of the southern Baingoin basin can alsobe subdivided into three members. The lower member, consistingof black laminated shales and massive mudrocks is poorly ex-posed and could not be measured accurately anywhere. The mid-dle and upper members, instead, were carefully studied in threesections in the Nam Co area, from the greenish beds of NC03 sec-tion in lower part to red terrestrial beds of NC01 and NC02 sec-tions in the upper part (Chen et al., 2003; locations shown inFigs. 1c and 3).

5.1.1. Middle member: Gilbert-type fan-delta depositsThis member, 70 to 250-m-thick, consists of at least 9 fining-

upward sequences of red or greenish conglomerate (Gt, Gch,Gcm, Gmm), sandstone (Sp, Sh, Sm, Sr), and shale (Fl, Fr) (Fig. 3).Mainly clast-supported and poorly sorted conglomerate beds(~0.5–10 m-thick) display subhorizontal stratification, troughoblique lamination, and clast imbrication (Fig. 4i). Subroundedclasts, ~5–10 cm in diameter on average but reaching up to 30 cm(Fig. 4i), are mainly granite, volcanic rocks, quartzose sandstone,and metasiltstone (Fig. 6g–j). Conglomerates pass upwards to peb-bly sandstones with planar oblique or horizontal lamination, andnext to 1- to 3-m-thick, fine-grained, commonly lenticular, red orgreenish sandstones with horizontal lamination or wave ripples(Sh and Sr; Fig. 4j), followed in turn by laminated mudrocks locallycontaining abundant pedogenetic nodules (Fr; Fig. 4k). Northwardpaleocurrent directions are indicated by imbricated clasts and pla-nar oblique lamination (Fig. 3).

5.1.1.1. This member accumulated on a fan delta (Dart et al., 1994;Jordan et al., 2001). Poorly sorted conglomerates indicate rapid deposi-tion by debris flows. Lithofacies (Sp, Sh, Sm, and Sr) suggest depositionin distributary channels, with commonwave ripples indicating a coastalenvironment (Raaf et al., 1977). Mudrocks sedimented during waningflow stages.

5.1.2. Upper member: meandering river depositsThis member, over 240 m-thick and mainly represented by red or

greenish mudrocks (Fr, Fl; Fig. 4k), is overlain by limestones of theLangshan Formation. Fining-upward sequences are well developed, lo-cally beginning with 10–40 cm-thick pebbly-sandstones followed by1–5 m-thick, laterally extensive sandstones with planar and obliquelamination (Sp, St) or massive (Sm). Wave ripples occur in sectionNC03 (Sr; Fig. 4j). Fine-grained red sandstones locally showing horizon-tal lamination or climbing ripples (Sh, Sr) occur in the upper part, over-lain by commonly laminated shales locally containing abundant calichenodules. Oblique lamination indicates northward paleocurrent direc-tions (Fig. 3).

5.1.2.1. This member was deposited bymeandering rivers, with coarser-grained sandstones indicating channel deposits and red mudrocks withcaliche nodules suggesting soil formation in the adjacent floodplain(Cant and Walker, 1978).

5.2. Sandstone petrography

Twenty, moderately well sorted and subangular to subroundedsandstone samples are mainly feldspatho-litho-quartzose (averagemodal composition Q48F12L40 (Table A.1; Figs. 6f and 7a). Quartz is

Fig. 6. Sandstone petrography. Duba Formation: a) feldspatho-litho-quartzose sandstone (16ZR06, lower member, section DB01), showing Cr-spinel and chlorite grains; b) feldspatho-quartzo-lithic volcaniclastic sandstone with a few low-rank metasedimentary lithics (14MD11, middle member, section DB02); c) quartzose-sandstone pebble (15MD10, uppermember, section DB03); d) felsic tuff from the top of the Duba Formation (16BG17). Duoni Formation: e) felsic tuff (17BJ01, middle member, section NC02); f) feldspatho-quartzo-lithic sandstone (17BJ16, upper member, section NC03); g) epidotization granodiorite pebble (16SC02, middle member, section NC01); h) siltstone pebble (16SC04, middle member,section NC01); i) andesite pebble (16SC05, middle member, section NC01); j) quartzose-sandstone pebble (17BJ12A, middle member, section NC03). Quartz (Qm, monocrystalline;Qp, polycrystalline); Pl, plagioclase; Kf, K-feldspar; lithic fragments (Lv, volcanic; Ls, sedimentary; Lm, metamorphic); Chl, chlorite; Ep, epidote.


mainlymonocrystalline. 6–19% of QFL are feldspars with plagioclase: K-feldspar= 4:1. Lithic grains (16–51%QFL) are dominantly intermediateto felsic volcanic, with minor sedimentary (chert), basaltic and meta-morphic grains (Table A.1; Fig. 7b). Biotite, zircon, tourmaline, andmag-netite are also seen in thin sections.

5.3. Detrital zircon U-Pb ages and Hf isotopes

Samples 16SC01 and 16SC15 from section NC01 (Figs. 1b and 3)yielded 150 concordant U-Pb zircon ages, 123 of which between 110and 140 Ma (peak at ~121 Ma). Ninety-two Mesozoic zircons yielded


εHf(t) values between−11.4 and+10.6, with TCDMmodel ages between0.54 and 1.91 Ga. Older U-Pb ages cluster at 450–800, 950–1200 (peakat ~1020 and ~1150 Ma), and 2400–2550 Ma.

5.4. Age constraints

A tuff bed at the base of section NC02 (sample 17BJ01, Fig. 3)yielded a U-Pb zircon age of 114.4 ± 2.1 Ma (n = 18, MSWD =4.0) (Fig. 5, Table A.2), implying that deposition of the Duoni For-mation had begun by 114 Ma. Two samples 16SC01 and 16SC15from section NC01 yielded YC1σ (2+) zircon ages of 108.1 ± 2.1and 113.5 ± 1.7 Ma (Table 2), indicating that sedimentation mayhave continued even after 108 Ma, seemingly slight later than theDuba Formation.

5.5. Provenance interpretation

Sedimentary structures, together with the regional distribu-tion of lithofacies pointing to northward deepening of the basin,indicate sediment transport toward the northeast (Fig. 3). More-over, the lack of detrital zircons in the 150–190 Ma age range(Fig. 8), which is characteristic of magmatic activity in the BNSZand southern Qiangtang terrane, rule out these two potentialnorthern sources.

Dominant volcanic rock fragments indicate a volcanic source forDuoni sandstones. Most zircons yielded Early Cretaceous ages andnegative εHf(t) values, which matches very well the age populationsranging between 143 and 107 Ma with negative εHf(t) values char-acterizing Zenong volcanic rocks of the central Lhasa terrane (Chuet al., 2006; Zhu et al., 2009a; Zhu et al., 2011b). Furthermore, detri-tal zircons in the 950–1200 Ma age range characterize Upper Paleo-zoic strata of the central Lhasa terrane (Leier et al., 2007b; Leieret al., 2007c; Gehrels et al., 2011; Zhang et al., 2011; Zhu et al.,2011a; G. Li et al., 2014). Detrital zircons in the Duoni Formationcompare poorly also with zircons with extremely positive εHf(t) values in the Gangdese magmatic arc and Xigaze forearc basinof the southern Lhasa terrane (Chu et al., 2006; Ji et al., 2009; Wuet al., 2010; Zhu et al., 2011b; An et al., 2014), whereas some Creta-ceous zircons with mildly positive (b8.0) εHf(t) values resemblethose of magmatic zircons from the northern Lhasa terrane (Zhuet al., 2011b; Hou et al., 2015). Lower Cretaceous Zenong volcanicsand Upper Paleozoic strata of the central Lhasa terrane were thusthe predominant source of detritus for the Duoni Formation, possiblywith minor contributions from the northern Lhasa terrane.

F

Qm

Lta

14MD from section D

16ZR from section D

16SC from section N

15MD from section D

17BJ from section N

Fig. 7. Petrography of Duba and Duoni sandstones. Q, quartz; F, feldspa

6. Discussion

6.1. Comparison between the Duba and Duoni formations

Despite their broad similarities in clastic lithology, depositional envi-ronments, and age, the Duba and Duoni formations differ markedly inthe following aspects:

(1) sedimentary structures indicate southward paleocurrents in theDuba Formation, in stark contrast with NE-NNE paleoflow direc-tions in the Duoni Formation;

(2) conglomerates are far more common in the southern NC01 andNC02 sections of the middle member of the Duoni Formationthan in theNC03 section to the north, or than in theDuba Forma-tion;

(3) the Duba Formation was overlain by limestones of the LangshanFormation by 110 Ma, whereas sedimentation of the Duoni For-mation has continued until at least 108Ma, as indicated by zirconchronostratigraphy;

(4) gravel is mainly composed of quartzose sandstone and volcanicrocks in the Duba Formation, and by granite, andesite, limestone,quartzose sandstone, and siltstones in the Duoni Formation;

(5) Duoni sandstones are dominated by volcanic grains, whereasDuba sandstones also contain sedimentary and metamorphicrock fragments;

(6) Cr-spinel, and pyroxene, hinting at provenance from the BNSZ,were found in the Duba Formation but not in the Duoni Forma-tion;

(7) age spectra and εHf(t) values of detrital zircons in the Duba For-mation are mainly 115 and 170 Ma (peaks at ~143 or ~130 Ma)with εHf(t) values between−19.9 and +13.5, including a larger150–170 Ma cluster, whereas the Duoni Formation is character-ized by 110–140 Ma (peak at ~121 Ma) with εHf(t) values be-tween −11.4 and +10.6.

Sedimentary units comparable to the Duoni Formation - depositedbetween 114 and ~108 Ma in the southern Baingoin basin and mostlysourced from the central Lhasa terrane to the south - are found in theCoqen basin (Sun et al., 2017) and in the Selin Co basin (Zhang et al.,2011) along the southern part of the northern Lhasa terrane. Instead,units comparable to the Duba Formation - deposited at between 122and 110 Ma in the northern Baingoin basin and mostly sourced fromthe BNSZ and southern Qiangtang terrane to the south – occur in theNima basin (Kv unit, DeCelles et al., 2007) and in the Xiagangjiang(Volkmer et al., 2007) and Lunpola areas (Volkmer et al., 2014).

Lv/10

Lm

Lsb

B02, Duba Fm.

B01, Duba Fm.

C01, Duoni Fm.

B03, Duba Fm.

C02&03, Duoni Fm.

r; L, lithic grains (Lv, volcanic; Ls, sedimentary; Lm, metamorphic).

300

600 Southern Lhasa (Gangdese arc)Igneous zircons n=412

100

200

50

100

Central LhasaIgneous zircons n=904

Central LhasaDetrial zircons, n=1032

30

60

2

Section NC01, Duoni Fm.Age ≤ 250 Ma, n=123

Section NC01, Duoni Fm.Age > 250 Ma, n=27

8

16

3

Section DB01, Duba Fm.Age ≤ 250 Ma, n=40

Section DB03, Duba Fm.Age > 250 Ma, n=58

15

303

Section DB03, Duba Fm.Age ≤ 250 Ma, n=86


40

80

4

8Section DB02, Duba Fm.Age ≤ 250 Ma, n=187

40

80


500 1000 1500 2000 2500 3000 350050 100 150 200 250

200

400

50

100Central QiangtangIgneous zircons n=240

150

300BNSZ & Southern QiangtangDetrial zircons, n=2807

BNSZ & Southern QiangtangIgneous zircons n=1046

Central QiangtangDetrial zircons n=998

U-Pb Age (Ma)

(a)

(b)

(d)

(c)

(e)

(f)

(g)

(h)

Num

ber

Num

ber

Fig. 8. Relative U-Pb age probability for detrital zircons from the Duba (c–e; sections DB01 to 03) and Duoni formations (f; section NC01). Results are compared with: a) detrital zirconsfrom central Qiangtang (Kapp et al., 2007; Pullen et al., 2008; Dong et al., 2011; Fan et al., 2011; Gehrels et al., 2011; Zhu et al., 2011a; Ma et al., 2017) and igneous zircons from centralQiangtang (Yang et al., 2011; Liu et al., 2013; Li et al., 2015); b) detrital zircons from southern Qiangtang and BNSZ (Kapp et al., 2007; Pullen et al., 2008; Dong et al., 2011; Fan et al., 2011;Gehrels et al., 2011; Zhu et al., 2011a; Ma et al., 2017) and igneous zircons from southern Qiangtang and BNSZ (J.X. Li et al., 2014; S.M. Li et al., 2014; Hao et al., 2016; Liu et al., 2017);g) detrital zircons from central Lhasa (Leier et al., 2007b; Leier et al., 2007c; Gehrels et al., 2011; Zhang et al., 2011; Zhu et al., 2011a; G. Li et al., 2014) and igneous zircons fromcentral Lhasa (Chu et al., 2006; Zhu et al., 2009a; Zhu et al., 2011b); and h) igneous zircons from southern Lhasa (Chu et al., 2006; Ji et al., 2009; Zhu et al., 2011b).


6.2. Aptian-Albian paleogeography of the Lhasa block

During the latest Early Cretaceous (Aptian-Albian), the articulatedpaleogeography of the Lhasa block included a residual seaway in thenorth, separated by central Lhasa highlands from a shallow seaway in

the middle, separated in turn by the Gangdese arc from the Neotethysocean to the south (Fig. 11).

Provenance of Duba Formation clastics from the BNSZ and southernQiangtang implies that these northern domains were being uplifted(Fig. 11), which is consistent with geological evidence. The residual

Fig. 9.U-Pb age vs. Hf isotope plot of detrital zircons from the Duba and Duoni formations.Results are comparedwith data from central Qiangtang (Yang et al., 2011; Liu et al., 2013;Li et al., 2015), southern Qiangtang and BNSZ (J.X. Li et al., 2014; S.M. Li et al., 2014; Haoet al., 2016; Liu et al., 2017), northern Lhasa (Zhu et al., 2011b; Zhu et al., 2016), centralLhasa (Chu et al., 2006; Zhu et al., 2009a; Zhu et al., 2011b), and southern Lhasa (Chuet al., 2006; Ji et al., 2009; Zhu et al., 2011b).


sea in the southern Qiangtang basin should have retreated before theAptian (Ma et al., 2018), because by 118 Ma continental redbedssourced from Qiangtang highlands were deposited in the northernNima basin ~300 km west of our studied area (DeCelles et al., 2007;Kapp et al., 2007).

In the Aptian to early Albian, the Duoni and Duba formations are ex-posed in the southern and northern region Baingoin Basin (Fig. 1c), re-spectively. These zircon ages demonstrate that Early Cretaceous clasticrocks began accumulating before ~122 Ma; continued to accumulateuntil ~110–109 Ma. The large benthic foraminifera and rudist of theLangshan limestone yielded an Aptian-Cenomanian age (Leier et al.,2007b; Scott et al., 2010; Rao et al., 2015; BouDagher-Fadel et al.,2017), thus, implying that these three formations were diachronousphenomenons in the Baingoin Basin during the Aptian to early Albian.Sun et al. (2017) also found the Early Cretaceous diachronous phenom-enons of clastic rocks and limestones in the Coqen Basin on the west ofnorthern Lhasa terrane. Similar to the Selin Co basin located to the west(Zhang et al., 2011), the Baingoin basin was a progressively shrinking

Fig. 10. Geochemistry of Cr-spinel grains from the Duba Formation compared with data from thArai (1992).MORB,mid-ocean-ridge basalt; CFB, continental-flood basalt; OIB, oceanic-island batholeiitic basalt; AB, alkaline basalt; IAB, island-arc basalt; Bon, boninite.

shallow seaway bounded by two shorelines in the southeast and north-west, defined by the prograding delta and fan-delta of the Duba andDuoni formations, fed from opposite sides (Fig. 11). The N500-m-thickLangshan limestones deposited within a shallow sea, is supported tobe deposited between Duoni and Duba formations in the centralBaingoin Basin on the northern Lhasa terrane (Zhang et al., 2004; Leieret al., 2007b; Zhang et al., 2011; Sun et al., 2017) during the sameperiod.Due to the global sea-level rising (Sun et al., 2017) or subsidence (Leieret al., 2007b) during Albian to Cenmanian, the Duba and Duoni forma-tions were replaced by the Langshan marine carbonate environmentson the northern Lhasa terrane.

Provenance of the Duoni Formation from the central Lhasa terraneimplies that central Lhasawas being uplifted aswell, which is supportedby the lack of Early Cretaceous strata (Pan et al., 2004) and bydepositionaround 111Ma of the alluvial-fan Damxung Conglomerate southeast ofNam Co (Wang et al., 2017b) (Fig. 11).

Farther south in the Linzhou basin, b200-m-thickOrbitolinid-bearinglimestones (Penbo Member of the Takena Formation) with terrestrialdebriswere being deposited in a shallow seaway separating the upliftedcentral Lhasa terrane from the Gandese arc, and replaced by northward-prograding fluvio-deltaic deposits not earlier than the late Albian (Leieret al., 2007a). The simultaneous limestones were also deposited in thewest of the central Lhasa terrane (G. Sun et al., 2015). The presence ofthis southern seaway prevented detritus from central Lhasa to reachthe Xigaze forearc basin at that time (Wu et al., 2010; An et al., 2014;Orme and Laskowski, 2016). Detritus from the Gangdese arc wereprevented to be transported farther into northern Lhasa basins as well(this study and Sun et al., 2017).

The Linzhou basin and the Xigaze forearc basin were both fed fromthe Gangdese arc, that thus was above sea-level (Leier et al., 2007a;Wu et al., 2010; An et al., 2014; Orme and Laskowski, 2016). Duringthe Aptian-Albian, instead, radiolarian cherts and deep-sea turbiditeswere being deposited in the underfilled Xigaze forearc basin flooredby the Yarlung-Zangbo ophiolites (An et al., 2014; Orme andLaskowski, 2016; Wang et al., 2017a), implying that the southernmostLhasa terrane lay at abyssal depths at the northern edge of the vastNeotethyan oceanic realm (Fig. 11).

6.3. Tectonic setting of Early Cretaceous sedimentary basins in northernLhasa

The early tectonic evolution of the Lhasa block during the Creta-ceous, the origin of sedimentary basins, and the subduction polarity ofBangong oceanic lithosphere remain controversial (Leeder et al., 1988;Murphy et al., 1997; Yin and Harrison, 2000; Kapp et al., 2003a; Kapp

e BNSZ (Zhang, 2007). TiO2–Al2O3 plot after Kamenetsky et al. (2001), TiO2-Cr# plot aftersalt; ARC, arc; SSZ, supra-subduction zone; AP, abyssal peridotite. IPB, intraplate basalt; TB,


et al., 2005; DeCelles et al., 2007; Zhu et al., 2009a; Zhu et al., 2016). Ithas been proposed that, following closure of the Bangong Ocean in thenorth, a broad Cretaceous peripheral foreland basin formed in thenorthern part of the Lhasa block under the weight of the overthrustingQiangtang terrane and Shiquanhe-Gaize-Amdo thrust system (Leederet al., 1988; Murphy et al., 1997; Kapp et al., 2003a; Kapp et al., 2005;DeCelles et al., 2007). Considering instead the Gangdese arc in thesouth, the northern Lhasa terrane was proposed to have hosted aback-arc extensional (Zhang et al., 2004) or retroarc foreland basin(Zhang et al., 2011). The geochemistry of widespread Cretaceous mag-matic rocks in the northern Lhasa terrane suggest an orogenic ratherthan extensional setting (Zhu et al., 2009a; Zhu et al., 2011b; Chenet al., 2014; Hao et al., 2016), as independently indicated bymajor com-pressional tectonic structures bordering the Baingoin basin, includingthe Geren Co-Jiali thrust, Baingoin thrust, and the Shiquanhe-Gaize-Amdo thrust (Yin and Harrison, 2000; Ding and Lai, 2003; Kapp et al.,2007; Volkmer et al., 2014).

Our new stratigraphic observations indicate that the origin of theBaingoin basin is more complex than previously thought. The rapidprogradation of fluvio-deltaic systems fed from the north - as docu-mented in the Duba Formation by paleocurrent data, sandstone petrog-raphy, age spectra and isotopic fingerprints of detrital zircons, andappearance of SSZ- and OIB-type Cr-spinels (Fig. 10) – suggests thatthe northern part of Baingoin basin was indeed a peripheral forelandbasin (Leier et al., 2007b). The southern part of the Baingoin basin,

Fig. 11. Envisaged paleogeography of the Lhasa block during the Aptian-Albian, showing thestratigraphic sections: S1 in northern Nima basin (DeCelles et al., 2007; Kapp et al., 2007)(BouDagher-Fadel et al., 2017; Sun et al., 2017); S4 in southern Selin Co basin (Zhang et al., 20et al., 2017b), S9 (G. Sun et al., 2015), S10 in Linzhou basin (Leier et al., 2007a; BouDagher-Fadand S12 in eastern Xigaze forearc basin (An et al., 2014; Orme and Laskowski, 2016; Wang etCo-Jiali thrust; SGAT, Shiquanhe-Gaize-Amdo thrust; BG, Baingoin; DMC, Damxung Conglomer

however, had a different origin. The Duoni Formation, as well as analo-gous units in the Coqen (Sun et al., 2017) and Selin Co basins (Zhanget al., 2011), were in fact chiefly derived from the south. Isotopic signa-tures of detrital zircons in both the southern Baingoin basin (this study)and the Coqen basin (Sun et al., 2017), however, are distinct from thoseof the Gangdesemagmatic arc (Fig. 9; Chu et al., 2006; Ji et al., 2009; Zhuet al., 2009b; Zhu et al., 2011b; Dong et al., 2014) and point instead toprovenance from the central Lhasa terrane. Therefore, a retroarc fore-land basin model is more suitable to the southern Baingoin basin.

It is worth noting that neither the peripheral-foreland-basin modelnor the retroarc-foreland-basin model can explain the intrusion of theBaingoin 139–79 Ma plutonic complex into the basement of theBaingoin basin penecontemporaneous with sedimentation of the Dubaand Duoni formations (Qu et al., 2011; Volkmer et al., 2014). Besides,stacking patterns of coarse clastic deposits on both sides indicate thatthe depocenterwas located in the center of basin,which is incompatiblewith the distribution of isopachs in classical foreland basins (DeCellesand Giles, 1996).

Though the northern side of the basin may be considered a periph-eral foreland basin, whereas the southern part is more comparable toa retroarc foreland basin, the peculiar features of the Aptian-AlbianBaingoin basin thus donotfit wellwith any textbookmodel. This specialbasin architecture was plausibly controlled by the Lhasa-Qiangtang col-lision in the northern part but mainly influenced by tectonic processesrelated to the northward subduction of Neotethyan lithosphere in the

residual northern sea, the southern seaway, and the Neotethys Ocean. Data sources for, S2 in northern Baingoin basin (this study and Leier et al., 2007b), S3 in Coqen basin11), S5 in southern Baingoin basin (this study), S6 and S7 (Zhu et al., 2011b), S8 (Wangel et al., 2017), S11 in western Xigaze forearc basin (An et al., 2014; Wang et al., 2017a),al., 2017a). Locations shown in Fig. 1b. BNSZ, Bangong-Nujiang suture zone; GJT, Gerenate; XFB, Xigaze Forearc Basin; CD-NR, Chongdui-Ngamring; Fm., Formation.


southern part (England and Searle, 1986; Yin and Harrison, 2000; Panet al., 2006; Zhu et al., 2009a).

6.4. Timing of Lhasa-Qiangtang collision and BNSZ exhumation

Our findings suggest that lots of detrital zircons from Duba Forma-tion, yielded 170–150Ma with negative εHf(t) values, are all likelihoodderived from the southern Qiangtang arc (Guynn et al., 2006; J.X. Liet al., 2014; S.M. Li et al., 2014; Liu et al., 2017). Therefore, the preven-tion of detrital exchange between Lhasa and Qiangtang (e.g. oceanicbasin) should have disappeared before the deposition of Duba Forma-tion. In addition, chert, SSZ-,MORB-, and OIB-type Cr-spinel, and pyrox-ene grains presumably derived from the Bangong-Nujiang suture zone,constrain that strata in BNSZ should be exhumated prior to deposition ofDuba Formation (e.g. N122 Ma), which is in accordance with sedimen-tary environments changing from marine to nonmarine during125–118 Ma in the Northern Nima Basin of BNSZ to the west (Kappet al., 2007).

The age of provenance changing from one plate to another plate inthe continuous strata, has been proved as a direct way to constrain thetiming of initial continental collision (Hu et al., 2017). Despite no accu-rate age, this study can clearly constrain the upper limit of the time ofLhasa-Qiangtang collision (e.g. N122 Ma) confirmed by the above evi-dence. This result is comparable with the 140–130 Ma collision basedon magmatic gap in southern Qiangtang block (Zhu et al., 2016), EarlyCretaceous (N130 Ma) collision determined by the paleolatitude (Bianet al., 2017 and reference therein), 166 Ma collision based on theuncomformity in southern Qiangtang basin (Ma et al., 2017), and theangular unconformities between the ophiolites and overlying DongqiaoFormation conglomerate at 143–131Ma in BNSZ (Girardeau et al., 1984;Sun, 2005).

7. Conclusions

The Duba andDuoni formationswere deposited, in the northern andsouthern parts of the Baingoin basin respectively, during the Aptian andearly Albian. They similarly document progradation of fluvio-deltaicsystems onto a shallow shelf, and were both overlain by rudist- andOrbitolina-bearing limestones of the Langshan Formation in the mid-Albian. Their provenance and tectonic significance are however radi-cally different. Paleocurrent data, sandstone petrography, U-Pb agesand in-situ Hf isotope signatures of detrital zircons, and geochemistryof detrital Cr-spinel indicate that the Duba Formation was sourcedfrom the Bangong-Nujiang suture zone and southern Qiangtang terraneto the north,whereas the Duoni Formationwas derived from the centralLhasa terrane in the south. Age spectra and isotopicfingerprints of detri-tal zircons suggest that both units may have received detritus from anactive volcanic arc located in the northern part of the Lhasa block,whereas provenance from the Gangdese arc is ruled out.

The northern part, chiefly evolved in response to the Lhasa-Qiangtang collision in thenorth,maybe assimilated to a peripheral fore-land basin, whereas the southern part, influenced by tectonic processesrelated to northward subduction of Neotethyan lithosphere moreclosely compare to a retroarc foreland basin. However, because of its pe-culiar sedimentary architecture, and because subduction polarity of theBangong oceanic lithosphere remains controversial, the origin of theBaingoin basin is difficult to interpret in terms of classical foreland-basin models.

Provenance data from the Duba Formation, and specifically age-spectra of detrital zircons, indicate that collision between the Lhasaand Qiangtang terranes, together with the exhumation of Bangongophiolitic remnants, began before 122 Ma.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dummy.2019.01.002.

Acknowledgments

We appreciate that Bin Wu, Pan Sun, Xiong Yan, and Shijie Zhangprovided much help in analyzing the zircon U-Pb ages and Hf isotopes.We thank Jiangang Wang, Xiao Yu, Bo Zhou, Jiapeng Ye, and GaoyuanSun for their assistance in the field. We are grateful to helpful adviceand constructive suggestions from two anonymous reviewers, to theeditors Shoujie Liu and M. Santosh for careful handling. This study wasfinancially supported by the National Natural Science Foundation ofChina Project (91755209, 41472081). Data for this paper can beaccessed from the Supporting information.

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