Provenance analysis of Cretaceous peripheral foreland basin in central Tibet: Implications to precise timing on the initial Lhasa-Qiangtang collision Yaofei Chen, Lin Ding, Zhenyu Li, Andrew K. Laskowski, Jinxiang Li, Upendra Baral, Muhammad Qasim, Yahui Yue Chen, Y., Ding, L., Li, Z., Laskowski, A. K., Li, J., Baral, U., ... & Yue, Y. (2020). Provenance analysis of Cretaceous peripheral foreland basin in central Tibet: Implications to precise timing on the initial Lhasa-Qiangtang collision. Tectonophysics, 775, 228311. 10.1016/j.tecto.2019.228311 © This manuscript version is made available under the CC-BY-NC-ND 4.0 license https:// creativecommons.org/licenses/by-nc-nd/4.0/ Made available through Montana State University’s ScholarWorks scholarworks.montana.edu 1 Provenance Analysis of Cretaceous Peripheral Foreland 2 Basin in Central Tibet: Implications to Precise Timing on 3 the Initial Lhasa-Qiangtang Collision 4 Yaofei Chen a,b*, Lin Ding a,c, Zhenyu Li a,c, Andrew K. Laskowski d, Jinxiang Li a,c, Upendra Baral a, 5 Muhammad Qasim a,e, Yahui Yue a 6 7 a Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, 8 Chinese Academy of Sciences, Beijing 100101, China 9 b University of Chinese Academy of Sciences, Beijing 100049, China 10 c Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 11 100101, China 12 d Department of Earth Sciences, Montana State University, Bozeman, Montana 59717 U.S.A. 13 e Department of Earth Sciences, COMSATS University Islamabad, Abbottabad Campus, Abbottabad, 14 Pakistan 15 16 Abstract 17 Mesozoic strata along northern margin of the Lhasa terrane near Dingqing, Tibet provide a semi- 18 continuous record of the Bangong-Nujiang Ocean closure and the subsequent Lhasa-Qiangtang 19 collision. In this study, we present results of sandstone petrographic, detrital zircon U-Pb, and Cr- 20 spinel geochemical data to determine the provenance of the Mesozoic strata (from the Triassic 21 Quehala Group and the Mid-Jurassic to Lower Cretaceous Xihu, Lagongtang, and Duoni 22 Formations, which are young from ~220 Ma to ~100 Ma) in this region, thereby allowing for 23 interpretation of their tectonic setting. The similar 1200-900 age cluster from the lower Xihu 24 Formation to that of the Triassic Quehala Group and the distinct age peaks at ~200 Ma and ~146 25 Ma from the upper Xihu Formation suggest a Lhasa terrane provenance to the south. Distinctive age 26 clusters of 300-210 Ma and ~1800 Ma and Cr-spinel composition analysis of the Lagongtang 27 Formation indicate a provenance shift from the Lhasa terrane to the Qiangtang terrane and the 28 Bangong-Nujiang suture zone to the north. The Lagongtang Formation was deposited in a bathyal- 29 abyssal to shallow-slope environment from north to south, that we interpret as the foredeep 30 depozone and distal foredeep of a peripheral foreland basin system that developed due to flexural 31 subsidence related to the Lhasa-Qiangtang collision and terrane accretion. The age of the Lhasa- 32 Qiangtang collision is constrained by a ~140 Ma tuffite U-Pb age at the base of the foreland basin 33 strata. An angular unconformity between the Xihu and Lagongtang Formation, which we interpret 34 as the result of the Lhasa-Qiangtang collision. Our results indicate that the Lhasa-Qiangtang 35 collision initiated around ~140 Ma in the Dingqing region, simultaneously with previous 36 determinations 1200 km to the west near Gaize area. Therefore, we prefer quasi-simultaneous onset 37 of collision along-strike to zippering collision models wherein the east collision age is older than 38 the west. 39 Keywords：Tibetan Plateau; Bangong-Nujiang Suture Zone; Lhasa-Qiangtang Collision; Foreland 40 Basin; Provenance Analysis; Collisional tectonics. 41 1. Introduction 42 Long before the Cenozoic India-Asia collision and formation of the Tibetan Plateau and 43 Himalayan orogen, sequential amalgamation of several continental terranes occurred beginning in 44 Paleozoic time. These successive accretion events pre-conditioned the Eurasian margin by forming 45 the Proto-Tibetan Plateau (Kapp et al., 2005; 2007; Wang et al., 2014; Pan et al., 2012; Ding et al., 46 2014; Kapp and DeCelles, 2019). Determining the timing of terrane accretion and attributing the 47 regional stratigraphic units and deformation events to these events aids our broader understanding 48 of the development of the Tibetan Plateau, the prototypical example of an active collisional orogen. 49 The Lhasa and Qiangtang terranes, formerly separated by the Bangong-Nujiang Ocean (BNO) (also 50 refer as Meso-Tethyan ocean), are the most prominent crustal fragments by area in the central 51 Tibetan Plateau. The Mesozoic BNO formerly separated the Qiangtang terrane to the north from the 52 Lhasa terrane to the south, persisting for approximately 100 Myr (Yin and Harrison, 2000; Liu et 53 al., 2018) and its final closure resulted in the Lhasa-Qiangtang collision. This was the final terrane 54 accretion event prior to the onset of Himalayan-Tibetan orogenesis. The precise timing of the initial 55 Lhasa-Qiangtang collision is still debated, with inferred ages ranging from Middle Jurassic (Pan et 56 al., 1983; Xu et al., 1985; Ma et al., 2017), to Late Jurassic-Early Cretaceous (Yin and Harrison, 57 2000; Zhang et al., 2014; Zhu et al., 2016), and even as late as Late Cretaceous (Li et al., 2014; 58 Zhang et al., 2012; Fan et al., 2018). These previous estimates were based on the Middle Jurassic 59 low-grade metamorphism, Late Jurassic unconformity above the accretionary complex or magmatic 60 rocks, and Early Cretaceous Ocean Island Basalt, respectively. The precise determination of the 61 collision age along the Bangong-Nujiang suture zone (BNSZ) will determine whether the Lhasa- 62 Qiangtang collision initiated synchronously (Zhang et al., 2012, 2014; Wang et al., 2016) or 63 diachronously in a scissor-like fashion wherein the onset age becomes younger to the west (Yin and 64 Harrison, 2000; Pan et al., 2004; Zhu et al., 2013; Yan et al., 2016). 65 We interpret that the broad range of previously published initial Lhasa-Qiangtang collision 66 ages can be ascribed to the adoption of different approaches or different comprehensions of the term 67 “initial continental collision”. The timing of subduction-related magmatism (Li et al., 2014; Liu et 68 al., 2018), metamorphism (Xu et al., 1985), paleomagnetic closure (Yan et al., 2016; Ma et al., 2018b; 69 Cao et al., 2019), crustal shortening (Murphy et al., 1997; Kapp et al., 2007), and BNSZ ophiolite 70 emplacement ages (Zhu et al., 2006; Wang et al., 2016) have all been used to determine the timing 71 of the initial Lhasa-Qiangtang collision. Comparatively few studies have focused on sedimentary 72 records deposited along the northernmost margin of the Lhasa terrane (Leier, 2005, 2007; Kapp et 73 al., 2007; Zhang et al., 2014), and this method is widely used an acceptable in the context of 74 constraining the India-Asia collision time (e.g. DeCelles et al., 2014; Ding et al., 2016; Baral et al., 75 2019). Here we define the age of collision onset at the oldest age of sedimentary overlap between 76 strata derived in part from the Qiangtang terrane and the northern Lhasa terrane. 77 In the Dingqing area, eastern segment of the BNSZ, a semi-continuous Mesozoic sedimentary 78 succession exposed along the northernmost margin of the Lhasa terrane. In this study, we present a 79 new dataset of petrography, detrital zircon U-Pb geochronology and Cr-spinel compositions from 80 the Mesozoic sedimentary sequences (the Late Triassic Quehala Group, the Middle-Late Jurassic 81 Xihu Formation, the Early Cretaceous Lagongtang Formation, and the Early-Late Cretaceous Duoni 82 Formation (see Supplementary Tables S1-S3 for details)). This contribution allows us to reconstruct 83 the precise timing of final closure of the BNO and the subsequent initial Lhasa-Qiangtang collision. 84 The data are mainly from the newly-ascribed peripheral foreland basin strata (Lagongtang and 85 Duoni Formations) that unconformably overlie passive margin strata of the northern Lhasa terrane 86 (the Quehala Group and Xihu Formation). In addition, this is the first determination of the collision 87 age from the eastern segment of the BNSZ (Fig. 1), and a complete comparison with previous results 88 from central and western parts of the BNSZ (e.g. Ma et al., 2017, 2018a; Li et al., 2017a, 2017b). 89 2. Geological setting 90 The Tibetan Plateau, from south to north, is composed of five main continental terranes: The 91 Tibetan Himalaya, Lhasa, Qiangtang, Songpan-Ganzi and Kunlun, which are separated by the 92 Indus-Yarlung Zangbo suture zone, the BNSZ, the Jinsha suture zone, and the Kunlun suture zone, 93 respectively (Yin and Harrison, 2000; Kapp and DeCelles, 2019) (Fig. 1a). Our study area is located 94 in the Dingqing county along eastern segment of the BNSZ that includes from north to south the 95 following lithotectonic units: Qiangtang terrane, BNSZ and Lhasa terrane (Fig. 1b). Tectonic and 96 stratigraphic divisions are specifically described as below. 97 2.1. Qiangtang Terrane 98 The Qiangtang terrane is further subdivided into the southern and northern Qiangtang terranes 99 by the Triassic Longmo Co-Shuanghu suture, which is marked by the central Qiangtang 100 metamorphic belt (BGMRXAR, 1993; Pan et al., 2004; Li et al., 2009). The southern Qiangtang 101 terrane consists of the Neoproterozoic highly deformed metasedimentary crystalline basement rocks 102 (Jitang and Gemuri Group), intruded by 476-471 Ma orthogenesis (Pullen et al., 2011). The 103 Ordovician-Devonian limestone sequence, deposited on a stable carbonate platform in a continental 104 margin setting, was covered by the Upper Carboniferous to the Middle Permian sedimentary rocks 105 that containing abundant glacio-marine diamictites and basalt interlayers (Li et al., 2009). The Upper 106 Paleozoic strata are conformably overlain by shallow shelf facies of Triassic strata (Riganpeicuo 107 Formation) (BGMRXAR, 1993; Hou et al., 2018). Subsequent Jurassic to Early Cretaceous units 108 include Upper Jurassic sandstones and limestone (Amdo Formation) intruded by 180-150 Ma 109 granitoids (BGMRXAR, 1993), Middle Jurassic shales and sandstones intercalated with limestone 110 (Sewa, Shaqiaomu and Jiebuqu Formation) (BGMRXAR, 1993). These sedimentary sequences rest 111 on the Cretaceous terrestrial molasses deposits (Abushan Formation) by an angular unconformably 112 (BGMRXAR, 1993). 113 2.2. Bangong-Nujiang Suture Zone 114 The Bangong-Nujiang suture zone is an important tectonic unit dividing the Qiangtang terrane 115 to the north and the Lhasa terrane to the south in central Tibet, and is further subdivided into the 116 western (Bangong Lake-Gaize), the middle (Dongqiao-Amdo), and the eastern (Dingqing-Nujiang) 117 segments (Pan et al., 2012). Moreover, the BNSZ extends westwards for over 2000 km, represents 118 remnants of the BNO, and therefore records crucial information about evolution of the BNO 119 (Allègre et al., 1984; Yin and Harrison, 2000; Kapp et al., 2007; Zhu et al., 2016; Fig. 1a, b). 120 The accretionary complex (Mugagangri Group) mainly consists of deformed deep-sea 121 turbidites containing dismembered ophiolitic fragments (Kapp et al., 2005; Pan et al., 2004). Those 122 ophiolite fragments include mid-ocean ridge basalts (MORB-type), supra-subduction zone (SSZ- 123 type) ophiolites (Wang et al., 2016), oceanic island-seamount basalts (Zhu et al., 2006, 2013). In 124 addition, Wang et al (2016) suggests that Early-Middle Jurassic intra-oceanic subduction-related 125 eruptions of SSZ-related lavas (ca. 190-164 Ma), followed by OIB-type (Ocean island basalts) mafic 126 magmatism during the Early Cretaceous (ca. 132-108 Ma). A regional unconformity exists between 127 the accretionary complex and the overlying strata (Girardeau, 1984; Yin and Harrison, 2000; Deng 128 et al., 2017; Li et al., 2017a; Ma et al., 2017; Fig. 1b). 129 2.3. Lhasa Terrane 130 The Lhasa terrane is divided into northern, central and southern subterranes by the Shiquan 131 River-Nam Co Mélange Zone and the Luobadui-Milashan Fault (Zhu et al., 2013). The northern 132 Lhasa terrane is composed of chemically juvenile crust (Zhu et al., 2011a) covered by Middle 133 Triassic-Cretaceous sedimentary sequences with abundant Early Cretaceous volcanic rocks and 134 associated granitoids (Pan et al., 2004; Zhu et al., 2016). The central Lhasa terrane mainly consists 135 of a Precambrian basement (Allègre et al., 1984), a Carboniferous-Permian metasedimentary 136 sequence and the volcano-sedimentary sequence (Zhu et al., 2016). The southern Lhasa terrane is 137 characterized by the Mesozoic-Tertiary Gangdese batholith (BGMRXAR, 1993) and associated 138 Jurassic-Paleogene volcanic sequences (Zhu et al., 2008, 2016). 139 3. Stratigraphy of the northern Lhasa terrane and Sampling 140 The Mesozoic sequence exposed along the BNSZ in Dingqing area consists of upper Triassic- 141 upper Cretaceous marine to terrestrial deposits (Fig. 1b, 2). The BNSZ is bounded by the 142 Qiuzongma-Xuelashan-Zhuajinzha Fault to the north and the Quehala-Suruka Fault to the south 143 (BGMRXAR, 2014b; Fig. 1e). The Mesozoic sequence in Lhasa terrane composed of, from oldest 144 to youngest, the Quehala Group, the Xihu Formation, the Lagongtang Formation, and the Duoni 145 Formation (Fig. 1c-f, 2). 146 The Quehala Group is juxtaposed against the coarse-clastic wedge-top strata of the 147 Jingzhushan Formation across a thrust fault. In addition, it is in unconformable contact with the 148 Xihu Formation to the south (Fig. 1c, e, 2, 3a). In stratigraphic section Ms-1 near Shagong town 149 (Fig. 1e, Fig. 5): the lower Quehala Group is dominated by grey dolomitic limestone intercalated 150 with thin bedded fine sandstone; the middle part is dominated by alternating beds of limestone and 151 grey-black medium-fine quartz-sandstone or siliceous mudstone, some fossils were found in the 152 limestone; the upper part is mainly composed of massive black mudstones with sandstone or 153 limestone, which are lenticular in shape and of varying scales. The total thickness of the Quehala 154 Group is ~320 m based on our measured sections. Limestone with terrestrial clastic rocks was 155 deposited in subtidal low-energy zone, and alternating beds of limestone and fine quartz-sandstone 156 was formed in mixed siliciclastic-carbonate system (Flügel, 2010; BGMRXAR, 2014b), while 157 mudstone intercalated lenticular sandstone or limestone represents the continental shelf slope 158 environment (Mutti, 1992; BGMRXAR, 2014b). These strata contain madrepore (Thamnasteria 159 rectilamellosa Winker) and bivalve fossils (Myophpria elegans and Halobiidae) that suggest a Late 160 Triassic depositional age (BGMRXAR, 2014b). Sandstone samples 2019TFD23 (n=1) and 161 2017TF55-49 (n=7) were collected from the lower and upper part of this group, respectively (Fig. 162 2, 5). 163 The Xihu Formation is in fault contact with Mugagangri Group to the north (Fig. 1c) and sits 164 unconformably atop Triassic Quehala Group strata (Fig. 1c, 2, 3a). In the Ms-1 section near Shagong 165 town (Fig. 1e, 5), Xihu Formation is composed of gray-black carbonaceous mud shale and lenticular 166 medium-fine sandstone (Fig. 3b). Lenticular sandstone beds are common, with convex-down 167 geometries, small scale, laterally extension of 3-8 m, maximum thickness of 5-20 cm, and shaly 168 beds were found to contains plant debris and rich in organic matter. Massive organic-rich mud shale 169 with minor small lenticular sandstones from the section Ms-1 were deposited in a prodelta setting 170 (Pulham, 1989; Zhou and Zhang, 2009). At the base of the MS-1 section a ~20 m thick basal 171 conglomerate is exposed, consisting of gravel-sized limestone clasts interbedded with a minor 172 sandstone (Fig. 3a). Total thickness of the lower Xihu Formation in section Ms-1 is ~100 m. The 173 upper Xihu Formation was studied in section Ms-2 near Gayouka town, consisting mainly of 174 interbedded gray, medium-fine-silt sandstones of varying bed thicknesses with common parallel 175 lamination and occasional ripple marks (Fig.6). Those fined-grained sandstones with ripple marks 176 and parallel lamination were formed in shallow marine sedimentary environment (Zhou and Zhang, 177 2009). The upper Xihu Formation in section Ms-2 is ~50 m. Sporopollen (Perishinctidae) and 178 ammonoid (Virgatosphinctes sp.) fossils suggests a Middle to Late Jurassic age for this formation 179 (BGMRXAR, 2014b), and our detrital zircon data (Fig.4; Table 1; Supplementary Tables S1) from 180 the upper part yield a well-constrained maximum depositional age of ~146 Ma based on YC1σ (3+) 181 [weighted mean age (±1σ incorporating both internal analytical error and external systematic error) 182 of youngest cluster of three or more grain ages] technique (Dickinson and Gehrels, 2009; Table 1). 183 Three (2019TFD25/26/28) and four (2019TFD53/54/55/56) sandstone samples were collected from 184 the lower (Ms-1) and upper part (Ms-2), respectively. 185 The Lagongtang Formation is in fault contact with the Mugagangri Group to the north (Fig. 1c, 186 3f). The Lagongtang Formation from Ms-3 section near Dongrema town (Fig. 1f, 3f, 3g) and Ms-4 187 section near the Selazeren town (Fig. 1f, 1d) are of similar composition and are interpreted as 188 turbidites (Fig. 1c, 1d, 1f), consisting of ness rhythmic interbedded medium-fine sandstone, siltstone, 189 and dark mudstone with some lenticular coarse sand beds (Fig. 7). Sandstone beds, commonly 190 arranged in thinning-upward cycles (Fig. 3g), are laterally continuous and mostly vary from 5 to 50 191 cm thick, and occasionally have erosional bases and fine upward (Fig. 3g). Flute casts were found 192 on the baseside of sandstone layer (Fig. 3f). Lenticular sandstone, with convex-down geometries, 193 extends ~5 m in laterally and ~ 0.5 m in vertically. The rhythmic interbedded sandstones and dark 194 mudstone, fine upward and flute casts were deposited by low-density gravity flows, while the 195 lenticular sandstones were formed in the channels of a middle-distal submarine fan (Miall, 1997; 196 Zhu and Pan, 1986; BGMRXAR, 2014b). A ~4 m width granite dike intrudes the Lagongtang 197 Formation in the Ms-3 section near Dongrema town (Fig. 3g). The thickness of this formation in 198 Ms-3 and Ms-4 is ~130 m and ~50 m, respectively. In the Ms-2 section near Gayouka town (Fig. 199 1f), the Lagongtang Formation overlies the Xihu Formation above an angular unconformity (Fig. 200 3c), indicated by nearly horizontal interbeds of siltstone and marlite overlying the strongly deformed 201 argillaceous siltstone with an inclination of 43° . In the Ms-2, the lower part of Lagongtang 202 Formation mainly consists of dark gray-black thin interbedded fine-siltstone and marlite with plane- 203 parallel-lamination (Fig. 6), a ~30 cm of light-gray volcanic ash layer is exposed about 5 m from 204 the bottom (Fig. 6, 3e). The middle part is composed of medium-thin layered gray-black fine 205 sandstone with grayish white lenticular limestone or vice versa (Fig. 3d). Layered sandstone or 206 limestone is rich in fossils debris such as bivalves, corals and ammonites. Lenticular sandstone or 207 limestone is thick in the middle and pinches out gradually toward both ends, ranging in size 208 from >10 m to ~1 m. The upper part is dominated by interbedded gray medium-fine calcareous 209 sandstone and siltstone, which commonly display parallel-lamination. The thin interbedded fine- 210 siltstone and marlite to fine sandstone with lenticular limestone were deposited in marginal reefs to 211 upper slope (Flügel, 2010), while the interbedded calcareous sandstone and siltstone with parallel- 212 lamination were formed in high-energy shallow marine sedimentary (Flügel, 2010). U-Pb ages of 213 the tuffite layer constrains a 140 Ma age for the Lagongtang Formation (Fig. 8; Table 1; 214 Supplementary Tables S1). Seven (2017TF/22/21/66-70) sandstone samples were collected from 215 the Ms-4 section, two sandstones (2019TFD16/17) and two granitic rocks (2019TFD14/15) were 216 sampled from section Ms-3. Three sandstones (2019TFD47/49/50) and one tuffite (2019TFD48) 217 were collected from the lower part and two sandstones (2019TFD44/45) from the upper part of this 218 formation in section Ms-2. 219 Duoni Formation is variably juxtaposed again the Lagongtang Formation across fault or 220 unconformable contacts (Li et al., 2017b; BGMRXAR, 2014b). In the Ms-2 section (Fig. 1f), the 221 Duoni Formation mainly consists of light grey fine-siltstone with lenticular sandstone and thin 222 layers of coal (Fig. 3h, 6). The lenticulars are large-scale, consisting of coarse-medium sandstones 223 with a large amount of plant debris. Fossils of bivalves and ferns from this formation suggest an 224 Early-Late Cretaceous age (BGMRXAR, 2014b). Lenticulars with plant debris and the coal layer 225 the were formed in channel and marsh of a delta plain. Sandstone samples 2019TFD42/43 were 226 collected from the base of the Duoni Formation in section Ms-2 (Fig. 6). 227 4. Methods 228 Thirty-six total samples were collected from the localities summarized above (Fig. 1, 2; 229 Supplementary Table S1) for analysis at the Key Laboratory of Continental Collision and Plateau 230 Uplift in the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. Thin-sections 231 were prepared for all samples and the twelve least-altered and coarsest-grained sandstones were 232 selected for petrographic modal analysis (Supplementary Table S3). Thin-sections from the 233 Lagongtang Formation that contained Cr-spinel grains were selected for Cr-spinel geochemical 234 analysis (Supplementary Table S2). 235 Detrital zircon geochronology followed the approach of Ding et al (2013) and Li et al (2017a), 236 utilizing an Agilent 7500a Quadruple ICP-MS attached to a New Wave UP 193 nm ArF excimer 237 laser-ablation system. The ages reported in the text are 206 Pb/ 238 U ages for grains < 1000 Ma 238 and 207 Pb/ 206 Pb ages for grains >1000 Ma. For petrographic analysis, at least 300 points were 239 counted per slide and crystals larger than 62.5 μm within lithic fragments were counted as mono- 240 crystalline grains following the Gazzi-Dickinson method (Ingersoll et al., 1984). Compositions of 241 spinels were determined using a JXA-8230 Electron Microprobe Analyzer (EMPA). The 242 accelerating voltage was 15 kV, the sample current was 20 nA, and the beam diameter was 5 μm. 243 5. Results 244 5.1 Sandstone petrography 245 Six sandstones from the Quehala Group are moderate to well sorted with mostly subrounded 246 monocrystalline quartz grains (85.6-93.5 %) with less polycrystalline quartz grains (0.9-8.0 %) and 247 feldspars (1.4-5.5 %). Sedimentary lithic fragments (~1.5 %) include sandstone and shale. These 248 samples plot in craton interior provenance field (Fig. 9a, 9d, 9e). 249 Two sandstone samples from the Xihu Formation are moderate with mostly subrounded- 250 rounded monocrystalline quartz grains (82.4-87.5 %), with feldspars (6.3-9.8 %), and less 251 polycrystalline quartz grains (0.9-1.8 %). These samples plot close to the craton interior provenance 252 field (Fig. 9b, 9d, 9e). 253 Four sandstone samples from the Lagongtang Formation are dominated by quartz grains (62.7- 254 71.7 %) and lithic fragments (20.2-32.1 %), mostly poorly sorted and angular to subangular in shape 255 (Fig. 9c). Quartz grains are mainly monocrystalline (42.7-55.5 %) and show undulating extinction. 256 Lithic fragments contain sedimentary lithic and volcanic lithic fragments. While, feldspar (4.2- 257 8.4 %) is a minor phase and shows twinning. These samples plot within the recycled orogen 258 provenance field (Fig. 9c, 9d, 9e). 259 5.2. Cr-spinel compositions 260 Because of the stability of the physical and chemical properties of spinel, it is a proxy to 261 identify tectonic setting in clastic rocks (Lenaz et al., 2000; Kamenetsky et al., 200l). Chromium- 262 spinel compositions of sample from Lagongtang Formation are characterized by low TiO2 (generally 263 < 0.1 %) and the Cr# numbers are in the range of 51-89, respectively (Fig. 9f, 9g). 264 5.3 U-Pb geochronology 265 5.3.1 Igneous rock zircon U-Pb geochronology 266 The two granitic vein samples (2019TFD14, 2019TFD15) intruding the Lagongtang 267 Formation near Dongrema town (Ms-3; Fig. 1d, 3f) yielded 37 concordant ages from samples 268 2019TFD14 and 2019TFD15, producing weighted mean ages of 114.3±1.1 Ma and 113.7±0.7 Ma, 269 respectively (Fig. 8; Supplementary Tables S1). The granitic veins intruded after the sediment, thus 270 constricting the upper limit of the formation age for the Lagongtang Formation. 271 Sample 2019TFD48 from the tuffite layer in the basal Lagongtang Formation near Gayouka 272 town (Ms-2; Fig.3e) yielded 17 concordant ages between 141-138 Ma, producing a weighted mean 273 age of 139.4±0.6 Ma (Fig. 8). This age constrains the timing of deposition of the Lagongtang 274 Formation to 140 Ma. 275 5.3.2 Detrital zircon U-Pb geochronology 276 Zircon crystals extracted from the Quehala Group near Shagong town (Ms-1, Fig. 1e) are 277 mainly subhedral and yield U/Th ratios ranging from 0.37 to 21.27, consistent with typical 278 compositions of igneous zircon. 880 analyses yielded 729 usable ages after application of 279 concordance filters (Fig. 5; Supplementary Tables S1). Major age populations were identified in the 280 ranges of 1200-900 Ma. Additional ages scattered between 900-600 Ma and 1800-1500 Ma. 281 Zircon crystals extracted from the lower part of Xihu Formation near Shagong town (Ms-1, 282 Fig. 1e) are rounded with U/Th ratios ranging from 0.56 to 14.92, also consistent with igneous zircon. 283 220 analyses yield 160 ages (Fig. 5; Supplementary Tables S1). Most of the ages cluster between 284 1200-900 Ma. Additional ages are scattered between 900-700 Ma and 1700-1600 Ma. Age 285 distribution of the upper part of the Xihu Formation from section Ms-2 near Gayouka town are 286 distinct from the lower part. In samples from the upper part, 440 analyses yielded 340 usable ages 287 and the zircon crystals are euhedral to subhedral with U/Th ratios ranging from 0.53 to 13.03. The 288 U-Pb ages are characterized by age-probability peaks at ~146 Ma and ~195 Ma, with minor 289 components scattered between 600-400 Ma. 290 The crystal characteristics and age distribution of detritus zircon in all samples of Lagongtang 291 Formation are similar. Zircon crystals are mainly euhedral to subhedral with U/Th ratios ranging 292 from 0.1 to 19.08. 1540 zircon grains were analyzed from the Lagongtang Formation, of which 1268 293 passed the concordance filters (Fig. 6, 7; Supplementary Tables S1). Major age populations lie in 294 the ranges of 300-210 Ma, 1900-1700 Ma and 145-128 Ma with peaks at ~250 Ma, ~1800 Ma and 295 ~140 Ma. Subordinate populations are between 180-160 Ma, 500-400 Ma and 210-185 Ma. 296 Zircon crystals collected from the Duoni Formation near Gayouka town (Ms-2, Fig. 1f) are 297 rounded-subrounded with U/Th ratios ranging from 0.58 to 9.92. 220 analyses yield 158 usable ages 298 (Fig. 6; Supplementary Tables S1). Samples of the Duoni Formation show the similar age spectrum 299 with the Lagongtang Formation. The most significant age populations are between 300-210 Ma, 300 200-180 Ma and 140-120 Ma. Additional ages are scattered between 500-400 Ma. 301 6. Discussion 302 6.1 Provenance analyses 303 6.1.1. Potential sources 304 The Mesozoic strata were deposited within or near the BNSZ, so the most proximal potential 305 sedimentary sources include the Lhasa terrane and the Qiangtang terrane, or a mixed provenance 306 when the BNO was nearly closed. The Songpan-Ganzi terrane and the Qiangtang terrane had already 307 sutured following closure of the Paleo-Tethys Ocean in Early Jurassic time (Ding et al., 2013), while 308 most of the post-orogenic exhumation in the Songpan-Ganzi terrane did not occur prior to 30 Ma 309 (Roger et al., 2010). Therefore, the Songpan-Ganzi terrane can be excluded as a possible source as 310 it was not exposed. Those terranes south of Lhasa could also be ruled out as possible sources because 311 the Neo-Tethys Ocean separating the Lhasa terrane from the Greater India/northern Australia was 312 not closed until Late Cretaceous time (Ding et al., 2005). In order to distinguish the sources of 313 Mesozoic strata in our study area, we compare the age spectra of these formations with published 314 U-Pb age data of detrital and magmatic zircon from the Lhasa and Qiangtang terrane (Fig. 10). 315 The Permian-Triassic Angjie and Duoburi Formation from the south of Selincuo in northern Lhasa 316 terrane contains 1200-950 Ma, 600-500 Ma and 1700-1500 Ma age populations, and with peaks at 317 ~990 Ma, ~1100 Ma and ~550 Ma (Fan et al., 2017). Age populations of 1150-900 Ma and 600-500 318 Ma, with peaks at ~ 1100 Ma and ~ 550 Ma (Gehrels et al., 2011) dominate the Permian strata from 319 the south of Amdo in northern Lhasa terrane. The Early Jurassic (210-185 Ma) igneous rocks are 320 widely distributed in the central and southern Lhasa terrane with a dominant age-probability peak 321 at ~195 Ma (Chu et al., 2006; Zhang et al., 2007a, 2007b; Yang et al., 2008; Ji et al., 2009; Zhu et 322 al., 2008, 2011a, 2013). Late Jurassic to Early Cretaceous igneous rocks in the northern and central 323 Lhasa terrane are characterized by 154-129 Ma high relative probability (Volkmer et al., 2007; Jiang 324 et al., 2010; Du et al., 2011; Zhu et al., 2009, 2011b; Chao et al., 2016). 325 The Triassic sandstones from the Nima area in southern Qiangtang terrane are characterized 326 by 300-210 Ma and 2000-1800 Ma ages (Gehrels et al., 2011). Similarly, the Triassic Riganpeicuo 327 Formation, near the Rongma town in southern Qiangtang terrane, contains major age clusters of 328 300-210 Ma, 470-400 Ma, and 1950-1800 Ma (Hou et al., 2018). Age distribution of the Jurassic 329 Bilucuo and Shamuluo formations sandstone, which got from Duoma area, contains major age 330 groups between 300-200 Ma, 470-400 Ma, 1950-1800 Ma and 180-160 Ma (Ma et al., 2017, 2018a). 331 Two intense magmatism peaks of ~162 Ma and ~118 Ma were discovered in the southern Qiangtang 332 terrane, and there is a magmatic gap between 140-130 Ma (Guynn et al., 2006; Li et al., 2014; Zhang 333 et al., 2017; Li et al., 2017; Liu et al., 2018; Li et al., 2018). 334 6.1.2. Provenance of the Quehala Group and Xihu Formation in passive continental margin 335 of northern Lhasa terrane 336 Sandstone compositions of samples in the Quehala Formation are monocrystalline quartz-rich 337 and lithic-poor showing signatures of the continental terrane provenance (Fig. 9d, 9e). Detrital 338 zircons from these samples are dominated by 1200-900 Ma ages, similar to the Permian-Triassic 339 strata, however the 600-500 Ma age cluster is absent (Fig. 10). The age spectrum is very similar 340 with the Triassic strata from the northeast Indian continent which is also characterized by age cluster 341 1200-900 Ma (Veevers and Saeed, 2009), which possibly implies that the Lhasa terrane was close 342 to India (Cai et al., 2016) at this time. 343 Sandstone compositions of samples in the Xihu Formation are monocrystalline quartz-rich and 344 lithic-poor showing signatures of the continental terrane provenance (Fig. 9d, 9e). Combined with 345 the geochemical characteristics of sandstone (Zhou and Zhang, 2009), a passive continental margin 346 setting for the Xihu Formation is likely. In the lower part of the Xihu Formation, major age cluster 347 of 1200-900 Ma is significantly resembling the age pattern of the Triassic Quehala Group (Fig. 10) 348 and thus indicating its provenance of Lhasa terrane. 349 The detrital zircon age pattern of upper part of the Xihu Formation is different from that of the 350 lower part. It is lacking of age group of around 1200-900 Ma; however, it has two distinct age peaks 351 at ~195 Ma and ~146 Ma respectively. This suggests that the former most likely derived from the 352 Early Jurassic igneous rocks in the southern or/and center Lhasa terrane, as the later came from the 353 Late Jurassic to Early Cretaceous magmatic rocks in the center or/and northern (Chu et al., 2006; 354 Zhang et al., 2007a, b; Yang et al., 2008; Ji et al., 2009; Zhu et al., 2008, 2009, 2011a, 2011b 2013). 355 It is also worth noting that the absence of age clusters of 300-200 Ma and 1950-1750 Ma indicate 356 that the southern Qiangtang terrane is an unlikely source for the upper part of the Xihu Formation. 357 Besides, geological evidence of northward paleocurrent direction also corroborates the provenance 358 of Lhasa terrane for the upper part of the Xihu Formation (Qin et al., 1999; Zhang et al., 2012). 359 In summary, considering the combined sandstone compositions and their sedimentology, it is 360 clearly that the Quehala Group and Xihu Formation were deposited on the passive continental 361 margin of the northern Lhasa terrane, comprised of sediments from the Lhasa to the south, which 362 are also candidates for the basement of the peripheral foreland basin along the BNSZ. 363 6.1.3. Provenance of the Lagongtang and Duoni formations in the peripheral foreland basin 364 Sandstone compositions of samples from the Lagongtang Formation are dominated by quartz 365 grains and lithic fragments plotting within the recycled orogen provenance field (Fig. 9d, 9e). 366 Compared with the Xihu Formation, samples from the Lagongtang Formation show quite different 367 detrital zircon age patterns, characterized by conspicuous 300-210 Ma and 1950-1750 Ma clusters 368 (Fig. 10). The major age groups in the range of 300-210 Ma, 1950-1750 Ma, and minor group of 369 180-160 Ma are matched well with the Triassic-Jurassic strata of southern Qiangtang terrane (Fig. 370 10). The cluster of 180-160 Ma may also represent the Middle Jurassic magmatism in southern 371 Qiangtang terrane (Liu et al., 2018). Meanwhile, the minor age clusters of 145-128 Ma and 210-185 372 Ma may represent the magmatism of Lhasa terrane. The southward and northward bidirectional 373 paleocurrent also indicates the existence of a mixed source for the Lagongtang Formation (Qin et 374 al., 1999; Zhang et al., 2012). 375 In addition, we also performed analysis of Cr-spinel compositions on samples from the 376 Lagongtang Formation (see the section of “5.2 Cr-spinel compositions”). As stated earlier, the 377 Lagongtang Formation is characterized by low content of TiO2 (generally <0.1%) and the #Cr 378 numbers are in the range of 51-89, which are inferred most likely to derive from peridotites (Lenaz 379 et al., 2000). In the plot of TiO2 vs. Al2O3, all fall in supra-subduction zone area (Fig. 9f). In the plot 380 of Cr# vs Mg#, according to the classifications of Dick and Bullen (1984), spinels correspond best 381 with Type III peridotites (Fig. 9g), and consistent with the compositions of spinel in the Dingqing 382 ophiolitic rock (Fig. 9g). In other words, sediments of the Lagongtang Formation were likely fed 383 from the BNSZ based on analysis of Cr-spinel compositions. The unconformable contact between 384 the Early Cretaceous Lagongtang Formation and Middle to Late Jurassic Xihu Formation highlights 385 the sedimentary interruption between the two formations (Zhu and Pan 1986; BGMRXAR, 2014a, 386 b; Li et al, 2017c). It is worth mentioning the notable apparent switch of source areas and 387 sedimentary environment between the Xihu Formation and Lagongtang Formation. The Duoni 388 Formation dominated by gray siltstone with thin coal bed. The major age cluster of U-Pb zircon 389 ages between 300-200 Ma suggests derivation from the Triassic-Jurassic strata of the southern 390 Qiangtang terrane. The 140-120 Ma age cluster may come from the magmatism of Lhasa terrane. 391 In summary, provenance of the Early Cretaceous Lagongtang Formation was from the BNSZ, the 392 southern Qiangtang terrane to the north, and the Lhasa terrane to the south in order of decreasing 393 importance based on our detrital zircon data and Cr-spinel composition data. Similarly, 394 provenance of the Duoni Formation was also from the southern Qiangtang terrane to the north and 395 the Lhasa terrane to the south, again in order of decreasing importance. Deposition atop passive 396 continental margin basement, dominantlynorthern provenance for both units, andsouthward- 397 decreasing thickness of the Lagongtang and Duoni Formations indicates that they were most likely 398 deposited in the foredeep and/or forebulge depozone of a peripheral foreland basin system that 399 formed due to initial Lhasa-Qiangtang collision. 400 Lagongtang Formation samples are relatively rich in quartz, and poor in lithic grains and 401 feldspar compared to typical sandstone compositions of peripheral foreland basin systems (Fig.9d, 402 9e; e.g. Najman et al., 2005; DeCelles et al., 2014; Baral et al., 2018). We interpret that this 403 discrepancy arises from the paucity of exposed intrusive igneous rocks in the southern Qiangtang 404 terrane. Furthermore, most of the exposures are concentrated in the west , far from our sample 405 localities (e.g. Pullen et al., 2008; Zhang et al., 2012; Zhai et al., 2013; Li et al., 2014; Liu et al., 406 2018). Sedimentary rocks of the Lagongtang Formation were mainly recycled from the Jurassic- 407 Triassic strata exposed in the BNSZ and the southern Qiangtang terrane rather than derived from 408 igneous exposures, consistent with the high relative age-probability of300-200 Ma detrital zircon 409 ages. During recycling, quartz is more resistant to weathering, resulting in enrichment of quartz 410 relative to feldspar and lithic grains. 411 6.2. Implications for the Initial Lhasa-Qiangtang Collision 412 The Lhasa-Qiangtang collision is considered as the initial contact of continental terranes after 413 disappearance of oceanic lithosphere and the first development of a peripheral foreland basin on the 414 subducting Lhasa terrane (Ding et al., 2017; Li et al., 2017a). This study yields a new maximum 415 depositional age and provenance of the Mesozoic strata provides further information for the initial 416 Lhasa-Qiangtang collision in Dingqing area, along the eastern segment of BNSZ. 417 The lower part of Xihu Formation is dominantly composed of carbonaceous shale and 418 sandstone with plant debris indicating a delta facies. The similarity of U-Pb age spectrum to the 419 underlying Quehala Formation supports the derivation from the Lhasa terrane source. The upper 420 Xihu Formation with a maximum depositional age ~146 Ma, consists of grayish sandstone, 421 mudstone and limestone, with the parallel bedding and ripple marks, which indicates a shallow sea 422 environment of the passive continental margin of Lhasa terrane (Fig.11a). The detrital zircon age 423 distribution and northward paleocurrent direction also indicate the Lhasa terrane source. A regional 424 unconformity has been reported between the Xihu Formation and the overlying Early Cretaceous 425 (140 Ma) Lagongtang Formation (Zhu and Pan, 1986; BGMRXAR 2014a, b; Li et al, 2017c), which 426 was also clearly observed in the field in this study. This distinct unconformity has long been thought 427 of forming in response to the initial Lhasa-Qiangtang collision and stands for a sedimentary 428 interruption between the two formations (Zhu and Pan, 1986; BGMRXAR 2014a, b; Li et al, 2017c). 429 Based on the age of uppermost Xihu Formation and the age of lowermost Lagongtang Formation, 430 the unconformity we observed in the field spans an age interval of 146-140 Ma. The Lagongtang 431 Formation, is a bathyal-abyssal environment characterized by turbidite near the BNSZ, and is 432 shallow-slope facie deposits of limestone interbedded with mudstone or sandstone about 20 km 433 southward, that two parts may represent the distal foredeep and foredeep depozone, respectively 434 (Fig 11b). The firstly appearance of masses of Permian-Triassic and Paleoproterozoic detritus from 435 the detrital zircon age spectra of Lagongtang Formation indicating the Qiangtang source. This is the 436 first occurrence of the Qiangtang detrital components in the strata of Lhasa terrane, and it indicate 437 that the minimum age of Lhasa-Qiangtang collision was ~140 Ma (Early Cretaceous). Thus, 438 combining with the sharp provenance shift between the Xihu and Lagongtang formations, we 439 propose that the 146-140 Ma regional unconformity between those two formations most likely 440 formed in response to the initial Lhasa-Qiangtang collision occurred at sometime between 146 Ma 441 and 140 Ma at eastern segment of the BNSZ in Dingqing area. The Early-Late Cretaceous Duoni 442 Formation shows fault/unconformity contact with the Lagongtang Formation. Moreover, the Duoni 443 Formation was formed in the delta or marine-terrigenous depositional environments (BGMRXAR, 444 1993, 2014a, b; Li et al., 2017c) with a mixture source, suggesting that the Duoni Formation may 445 represents the forebulge of the peripheral foreland basin formed by collision between Lhasa and 446 Qiangtang terrane. 447 The 145-130 Ma magmatic activity gap of the southern Qiangtang indicate collision onset at 448 ~140 Ma that emplaced due to the termination of normal subduction (Zhu et al., 2016). The 449 paleomagnetic data from Shiquanhe area suggests onset of collision at ~151 Ma (Ma et al., 2018b). 450 The latest paleomagnetic analysis from south of Shuanghu county implies that the width of the BNO 451 was 2,600 ± 710 km during the Middle Jurassic (Cao et al., 2019). Our results, in combination with 452 previous work, suggest that initial Lhasa-Qiangtang collision occurred at 146-140 Ma in the eastern 453 part of BNSZ. 454 6.3 Implications for collision mode of the BNSZ 455 A scissor-like continental collision model that commenced from east to west for the Lhasa- 456 Qiangtang collision are widely quoted although this tectonic model has not been effectively tested 457 (Fig. 12a; Yin and Harrison, 2000; Pan et al., 2004; Zhu et al., 2013; Yan et al., 2016). However, our 458 data combined with predecessors, this paper is more in favor of the collision of the Lhasa and 459 Qiangtang terranes was synchronous (Fig. 12b; Zhang et al., 2012, 2014; Wang et al., 2016). The 460 evidence is as follows: 1) A mature peripheral foreland basin was identified in the Dingqing area, 461 that provide an accurate 146-140 Ma age for the initial Lhasa-Qiangtang collision in the eastern part 462 of BNSZ (this paper). 2) Southward-directed paleocurrent and the lithic-rich of the Lower 463 Cretaceous sandstone in Duba area (Leier, 2005, 2007) and Asa basin (Zhang et al., 2014) suggest 464 the BNSZ and southern Qiangtang terrane provenance, that indicating the existence of possible 465 foreland basin and narrowing the Lhasa-Qiangtang Collision occurred at the earliest Cretaceous in 466 the middle segment of BNSZ. 3) ~60% north-south shortening of the Lhasa crustal occurred during 467 the Early Cretaceous in Coqin area (Murphy et al, 1997) and major deformation and exhumation 468 occurred at Early Cretaceous in Nima, Gaize and Domar area (Paul et al., 2005, 2007; Raterman et 469 al., 2014), all these events can be attributed to the continuous compression after the initial Lhasa- 470 Qiangtang collision in the earliest Cretaceous. 4) Provenance analysis of the Late Jurassic to Early 471 Cretaceous Wuga Formation in Gaize county, western segment of BNSZ, shows a north provenance 472 from the BNSZ and Qiangtang terrane and a possible foreland basin setting, that suggesting ~150 473 Ma for the initial Lhasa-Qiangtang collision (Li et al., 2017). In conclusion, the initial collision 474 timing of Lhasa-Qiangtang is simultaneous from the western Gaize to the central Nima and Naqu 475 to the eastern Dingqing, and occurred in the earliest Cretaceous (Fig. 11b, 12b). 476 7. Conclusions 477 Field observations, U-Pb isotopic ages of detrital zircons, Cr-spinel analysis and petrographic 478 investigations of the Mesozoic sequence developed along the northernmost margin of the Lhasa and 479 within the BNSZ in and around Dingqing area of central Tibet have led us to draw following 480 concluding remarks: 481 1. Detritus of the Middle-Late Jurassic (~170-146 Ma) Xihu Formation were derived from the 482 Lhasa terrane in the south and deposited on the passive continental margin on the northern Lhasa 483 terrane. 484 2. The Lagongtang Formation was formed during Early Cretaceous (140 Ma) and developed in a 485 mature peripheral foreland basin in response to the initial Lhasa-Qiangtang collision with major 486 provenance of the Qiangtang terrane to the north. 487 3. A ~146-140 Ma angular unconformity between the Xihu Formation and Lagongtang Formation 488 is interpreted to mark the migration of flexural forebulge through this region. 489 4. The final closure of the BNO as well as the initial contact between the Lhasa and Qiangtang 490 terranes are most likely to occur at the time interval of 146-140 Ma indicated by abrupt provenance 491 and depositional environment transition which are identified in the Lagongtang Formation. 492 493 Acknowledgments 494 This study was financially supported by the Ministry of Science and Technology of China (Grant 495 No. 2016YFC0600303), Strategic Priority Research Program of Chinese Academy of Sciences 496 (Grant No. XDA20070301) and National Natural Science Foundation of China (Grant No. 497 41490615). 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