Late Triassic back-arc spreading and initial opening of the Neo-Tethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet

Journal Pre-proof Late triassic back-arc spreading and initial opening of the NeoTethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet

Chao Lin, Jinjiang Zhang, Xiaoxian Wang, Prinya Putthapiban, Bo Zhang, Kai Liu, Tianli Huang PII:

S0024-4937(20)30045-1

DOI:

https://doi.org/10.1016/j.lithos.2020.105408

Reference:

LITHOS 105408

To appear in:

LITHOS

Received date:

24 August 2019

Revised date:

30 January 2020

Accepted date:

30 January 2020

Please cite this article as: C. Lin, J. Zhang, X. Wang, et al., Late triassic back-arc spreading and initial opening of the Neo-Tethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet, LITHOS(2020), https://doi.org/10.1016/j.lithos.2020.105408

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Late Triassic back-arc spreading and initial opening of the Neo-Tethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet a

Chao Lin , Jinjiang Zhang

a,

b

c

Tianli Huang

d

a

Ministry of E ducation Key Laboratory of Orogenic Belts and Crustal E volution, School of E arth

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a

a

*, Xiaoxian Wang , Prinya Putthapiban , Bo Zhang , Kai Liu ,

Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake

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b

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and Space Sciences, Peking University, Beijing 100871, China

Geoscience Program, Kanchanaburi Campus, Mahidol University, Kanchanaburi 71150,

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c

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Administration, Beijing 100085, China

Thailand

State Key Laboratory of Lithospheric E volution, Institute of Geology and Geophysics, Chinese

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d

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Abstract

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Academy of Sciences, Beijing 100029, China

The tectonic evolution of the Neo-Tethyan oceanic plate between the Indian Plate and the Lhasa Terrane is a key issue for the evolution of Gondwana and formation of the Tibetan Plateau. In this paper we report on the geochemistry, Sr–Nd isotope compositions, and zircon U–Pb dating of Late Triassic basalts from the Nieru Formation (T3n) in the Kampa region, southern Tibet. The basalts have relatively low contents of MgO, TiO 2, and total alkalis (K2O + Na2O), and they have affinities to tholeiitic basalt. They exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in LREEs and slightly negative Eu anomalies on a chondrite-normalized diagram. On a primitive-mantle-normalized spider diagram, they are

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characterized by slight enrichments in large ion lithophile elements (LILEs) and relatively flat patterns of high field strength elements (HFSEs), except for depletions in B a, Nb, Ta, and Ti. Their initial

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Sr/ Sr ratios vary from 0.718133 to 0.738977 and εNd(t) values are relatively

depleted (4.09–5.22), similar to mid-ocean ridge basalts in the Yarlung Zangbo ophiolite. We propose the T3 n basalts were derived from a shallow and depleted mantle source by relatively

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high-degree partial melting in the spinel stability field. They underwent slight crustal

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contamination as well as the fractional crystallization of clinopyroxene. The T3 n basalts are

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similar to typical back-arc basin basalts (BABB) such as the Okinawa BABB. This indicates an

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extensional back-arc-basin setting along the nort hern margin of Gondwana during the Late

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Triassic. The initial opening of the Neo-Tethyan Ocean was related to the southwards subduction of the Paleo-Tethyan oceanic plate and back-arc-basin spreading during the Lat e Triassic.

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Magmatic activity along the passive continental margin records key information on continental

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break-up and incipient ocean development.

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Keywords: Back-arc basin bas alts (BABB); Neo-Tethyan Ocean; Late Triassic; Gondwana; Southern Tibet 1. Introduction

The Tibetan Plat eau is a unique natural laborat ory for plate tectonics, magmatism, metamorphism, and economic geology, and attracts scientists from all over the world. It is widely accepted that the Tibetan Plateau formed as a result of several continental -terrane collisions and ocean closures (e.g. Yin and Harrison, 2000; Pan et al., 2012; Zhu et al., 2013). Since the late Paleozoic, a series of northwards-drifting terranes that belonged originally to Gondwana moved towards the southern margin of the Eurasian Plat e with successive openings and closures of the

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Paleo-, Meso-, and Neo-Tethyan oceans (e.g. Yin and Harrison, 2000; Pullen et al., 2008; Pan et al., 2012; Zhu et al., 2013). The Indus–Tsangpo Suture Zone (IYS Z) lies between the Lhasa Terrane and the Tet hyan Himalayan S edimentary Sequence (THS) and expos es deep-marine sediment ary sequences and ophiolitic fragments that are regarded as the remnants of the Neo-Tet hyan Ocean (e.g. Pan et al., 2012; Zhu et al., 2013; Wu et al., 2014). The Cenoz oic

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collision of the Indian and Asian plates and the closure of the Neo-Tet hyan Ocean established

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Harrison, 2000; Zhang et al., 2012; Song et al., 2017).

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the pres ent structural framework of the Tibetan Plateau and Himalayan Orogen (Yin and

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E vidence over a wide area records the early evolution of the Neo-Tethyan Ocean in

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various places, including Iran, Oman, Sibumasu, and the Himalaya (Berberian and King, 1981; Gaetani and Garzanti, 1991; Garzanti and Sciunnac h, 1997; Maury et al., 2003; Zhu et al., 2013

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and references therein). However, compared with the closure of the Neo-Tethyan Oc ean, its

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opening is still hotly debated with respect to both timing and mec hanism (Metcalfe, 2002;

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Stampfli and Borel, 2002). Various opening times have been proposed: (1) the early Permian (Gaetani and Garzanti, 1991; Garzanti et al., 1999); (2) the middle–late P ermian (Stampfli and Borel, 2002); (3) the Late Triassic (Pan et al., 2012; Zhu et al., 2013); and (4) the Late Triassic–Early Jurassic (Golonka, 2007). These different viewpoints can be attributed to the lack of well-preserved geological records, becaus e most information on the initial Neo-Tethyan oceanic basin was destroyed by later subduction and collision events. Previous studies have been focused mainly on the southern margin of the Lhasa Terrane and the Indus–Ts angpo Suture Zone (e.g. Pan et al., 2012; Wu et al., 2014). However, the Yarlung Zangbo ophiolites in the suture zone are accepted as supra-subduction zone (SS Z) ophiolites, which are generated at

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a subduction zone rather than a mid-ocean ridge (Wu et al., 2014). It is difficult, therefore, to find evidence of the incipient Neo-Tethyan Ocean in the IYS Z. On the other hand, magmatic activity at the passive continental margin shows a close relationship to strong continental rifting and lithospheric extension (i.e., the early stage of opening of an oc ean; Mizusaki et al., 1992; Garzanti et al., 1999) and it is obvious that these magmatic activities were controlled by the

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nature of the mantle sources and the degree of continental break-up (Taylor and Martinez, 2003).

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Therefore, a new perspective is provided for ex ploring the early evolution of the ocean, because

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a mature oc ean often develops as a result of continent al rifting or back-arc-basin spreading

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(Zhang et al., 2008; Pan et al., 2012; Zhu et al., 2013).

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The Tethyan Himalayan S edimentary Sequence (THS) indicates a typical passive continent al margin setting on the northern margin of the Indian Plate since the Late Triassic (Yin

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and Harrison, 2000; Sciunnacha and Garzanti, 2012). A complex sequenc e of basaltic eruption

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events is represented in these sediments as a series of thin interlayers in thick sediments that

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range from Carboniferous to Cretaceous in age, and they record rich information on the tectonic background and plat e dynamics of the northern margin of Gondwana (Garzanti et al., 1999; Zhu et al., 2006, 2010; Liao et al., 2015). Nevertheless, most research has been focused on the Cret aceous Comei–Bunbury Igneous Province in the eastern THS, whereas the regional mafic magmatic events, and especially the Triassic basalts, have received little attention (Zhu et al., 2006; Zeng et al., 2012). In this paper, we describe a suite of Late Triassic basalts in the Nieru Formation (T3n) of the Kampa region, Sout hern Tibet (Fig. 1). We obtained geochronological and geochemical data for these basalts, including Sr–Nd isotope compositions, with the aim of determining the tectonic

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setting of the northern margin of Gondwana during the Late Triassic, to constrain the timing of the initiation of the Neo-Tethyan Ocean, and finally to reveal the mechanisms behind the initial opening of the Neo-Tethyan Ocean between the THS and the Lhasa Terrane. 2. Geological Setting As one of the largest orogens on Earth, the Himalayan Orogen is located at the

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convergent margin of the Eurasian and Indian plates (Fig. 1a; Yin and Harrison, 2000; Zhu et al.,

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2013), and is divided into four tectonic units: the Tethyan Himalayan Sedimentary Sequence

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(THS ), the Great Himalay an Crystalline Series (GHC), the Lesser Himalayan Sequence (LHS ),

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and the Sub-Himalaya (Siwalik Group) (Fig. 1a; Yin and Harrison, 2000; Zhang et al., 2012).

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Among these, the THS consists of a sequence of unmetamorphosed to low-grade metamorphosed clastic and carbonate rocks that were deposited from the Ordovician t o the

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Eocene along the northern passive margin of the Indian Plate (Sciunnach and Garzanti, 2012).

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The THS has undergone multiple periods of mafic magmatism that are recorded by a complex of

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volcanic rocks, intrusive rocks, and plutons (e.g. Garzanti et al., 1999; Zhu et al., 2006, 2010). Voluminous mafic rock is exposed as a series of mafic lavas, dikes, and other intrusions within the Permian to Early Cretaceous strata, accompanied by small amounts of ultramafic rock (Zhu et al., 2013). The Nort h Himalayan Gneiss Domes (NHGD) consist of several discontinuous domes in the central part of the THS (Fig. 1a). The Kampa Dome (Fig. 1b), which is studied here, is located in the central part of the NHGD, bet ween the K angmar Dome and the Mabja Dome (Quigley et al., 2006; Liu et al., 2016). Previous studies have shown that the Kampa Dome can be divided into three discrete sub-domes that are characterized by cores consisting of orthogneiss (the Kampa Granite), pelitic gneiss, and leucogranite int rusion (Quigley et al., 2006;

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Lin et al., 2020). Cores of these three sub-domes are overlain by a series of unmetamorphosed to medium-grade metamorphosed sediment ary rocks of Carboniferous to Triassic age (Fig. 1b; Quigley et al., 2006). The Triassic strata in the Kampa area consist of two formations: 1) the Lower–Middle Triassic Lvcun Formation (T1+2lc) and 2) the Upper Triassic Nieru Formation (T3n) (Liu et al.,

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1990; Xu et al., 2003). The Lvc un Formation consists of a series of carbonaceous slates,

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sericite-bearing silty slates, chloritoid-bearing phyllites, and limestones. It directly overlaps the

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limestone karst of the Baidingpu Formation (P 3b) in the Kampa region, and it displays a

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sequence of semi-deep marine deposits (Liu and Einsele, 1994; Shi, 2001). In the Tethyan

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Himalaya, there is an important regional unconformity bet ween the Nieru Formation and the underlying Lvc un Formation, but there is a conformable relationship bet ween the Nieru

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Formation and the overlying Jurassic strata (Liu and Einsele, 1994; Xu et al., 2003). The Nieru

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Formation consists of grayish-green silty and sericite-bearing calcareous slates interbedded with

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yellowish calcareous siltstones, and it contains a series of interlayers of mafic volcanic lava. The sediment ary facies display the characteristics of deposition in a setting of rapid subsidence in a semi-deep marine graben basin (Liu and Einsele, 1994; Shi et al., 1996; Sciunnac h and Garzanti, 2012). Previous U–P b dating has given an age of ca. 200–300 Ma for the wides pread arc-volcanic rocks, basaltic fragments, and detrit al zircons in the Nieru Formation (Cai et al., 2016; Liu et al., 2019). In addition, Li et al. (2011) described a bivalve fauna that included Chalmys sp., Entolium quotidianum, H. norica., H. comate Bittner, and H. paracicula, indicating a Carnian to Norian age for the Nieru Formation. Lamellibranchiate (Indopecten sp., Monotis cf. salinaria Bronn) and ammonoid (Indojuvarites angulatus and Anatomites sp.) fossils were also

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discovered in the formation. On the basis of these observations and dat a, the strata of the Nieru Formation are placed in the Norian Period (208.5–227 Ma) of the Late Triassic (Xu et al., 2003). Wang et al. (2000) were t he first to describe in t he Kampa region the widely distributed volcanic rocks that include basaltic lavas, clastic sediments, ash, tuff, and glass. However, there has been no systemic geochronological or geochemical study of these basalts. The outcrops of

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T3n basalts are located in the nort hern and eastern parts of the Kampa Dome, and we carried out

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detailed field investigations in the Kare area in the northwestern part of the dome and the

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Qusong area in the northeastern part of the dome. Seven mafic rock samples were collected

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from the Nieru Formation (T3 n) in the Kare area and six samples from the Nieru Formation in the

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Qusong area (Table 1). The regional stratigraphic column is plotted in Fig. 2 and sample locations are marked in the Norian strata.

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The studied bas alts form layers of volcanic lava around 0.5–1.5 m thick in the upper part

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of the Nieru Formation (Fig. 3a, b). The basalts are interbedded with dark gray mudstones, slates,

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siltstones, and limestones. The basalts are dark gray in color and have a fine-grained texture and massive structure (Fig. 3c–e). They show a slight mylonitic fabric and the effects of dynamic recrystallization in thin section. The mineral assemblage consists of pyroxene (20 vol.%), hornblende (20 vol.%), plagioclase (40 vol.%), and accessory chlorite (10 vol.%), epidote (6 vol.%), and hematite (4 vol.%) (Fig. 3d–f). Some of the pyroxenes and hornblendes have been altered to chlorite and epidote along their outer surfaces and they are poikilitic and filled with tiny plagioclase crystals. Plagioclase phenocrysts, a few small grains of plagioclase and pyroxene still preserve their original igneous textures (Fig. 3e, f). Plagioclase crystals are euhedral and fine-grained in the matrix. The studied basalts have experienced the greenschist - low

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amphibolite facies by the previous study (Quigley et al., 2006) and observation of radial and fibrous chlorite and actinolite (Fig. 2e). 3. Results 3.1. Bulk-rock geochemistry Eleven samples of the T3 n basalts were collected from t he Nieru Formation in the Kampa

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region (Fig. 2). Detailed analytic methods can be found in the Supplementary Text. The results of

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the bulk-rock geochemical analyses are listed in Table 1. They show low values of LOI (loss on

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ignition) of 0.73–1.09 wt.%, which suggests that the T3n basalts samples have undergone only a

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slight low-temperature metamorphism and alteration during post-magmatic events. The T3n

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basalts samples show slight variations in their major element contents with SiO2 = 47.17–50.57 wt.%, MgO = 6.84–7. 38 wt.%, Al2O3 = 13.17–16.67 wt.%, TFe2O3 = 12.17–12.97 wt.%, TiO2 =

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1.42–1.53 wt.%, MnO = 0.20–0.24 wt.%, CaO = 9.74–10.89 wt.%, and total alkalis (K 2O + Na2O) #

#

T

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= 2.71–3.69 wt.%. Moreover, the values of Mg (Mg = 100*MgO/(MgO + FeO )) in the samples

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range from 55.4 to 58. 7. The samples have low contents of total alkalis and show subalkaline basaltic affinities on the Zr/ TiO2*0.0001 vs. Nb/Y diagram (Fig. 4a). All samples plot in the field of T

the tholeiitic series and can be defined as tholeiitic basalt on the FeO –Alkalis–MgO ternary T

diagram (Fig. 4b) and the FeO /MgO vs. SiO2 diagram (Fig. 4c). The T3n basalt samples basically display flat rare earth element (REE) patterns on the chondrite-normalized diagram (Fig. 5a). Furthermore, they exhibit low total REE contents (46.48–55.75 ppm), weakly fractionated patterns with (La/Yb)N values ranging from 1.02 to 1.33, and slight enrichments in the MREEs ((La/Sm)

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= 0.76–1.04 and (Gd/ Lu)

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= 1.29–1.51).

Meanwhile, they are characterized by slightly negative Eu (E u/Eu* = 0.83–0.99) and Ce (Ce/ Ce*

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= 0.89–0.99) anomalies. Similarly, all the T3 n basalt samples have flat patterns on the primitive-mantle-normalized spider diagram (Fig. 5b). However, they are characterized by slight enrichments in the large ion lithophile elements (LILEs) and flat patterns of the high field strength elements (HFSEs), except for obvious depletions in Ba, Nb, Ta, and Ti and enrichments in Rb and S r (Fig. 5b). Their Sc, V,

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Cr, and Ni contents are slightly variable with values of 42.1–45.9, 310–358, 146–335, and

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146–335 ppm, respectively.

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3.2. Sr–Nd isotope composition

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Sr and Nd isotopic data for four T3n bas alts samples are listed in Table 1, and Fig. 6

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shows their Sr–Nd isotope compositions on the εNd(t) versus

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Sr/ Sr(t) diagram. All the points

plotted on Fig. 6 have symbol sizes greater than the analytical uncertainties. The initial

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Sr/ Sr

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values are corrected to the Late Triassic age of 224.7 Ma (youngest age in this zircon U-Pb

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dating), which show consistence with the previous detrital zircon chronology and paleontological

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data (e.g. Xu et al., 2013; Li et al., 2011; Cai et al., 2016). The samples show variable initial Sr/ Sr values that range from 0.718133 to 0.738977, while t hey have relatively constant εNd(t)

values of 4.09–5.22.

3.3. Zircon U–Pb dating Zircon grains from two T3n basalt samples (17KPA-7 and 17KPA-13) were dated and the results are listed in Supplementary Table. Concordia diagrams and weighted average age calculations were obtained using Isoplot (Ludwig, 2003). Errors given on individual analyses are reported at the 1σ level, but the weighted mean ages are reported at the 2σ level. We only consider ages with >90% concordance. Sixty-eight zircon grains from sample 17KPA-7 were

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obtained, for which 38 analytical spots were concordant. Forty zircon grains were separated from sample 17KPA-13, for which 28 analytical spots were concordant. The zircon crystals extracted from sample 17KPA-7 are mostly rounded or sub-rounded, colorless and trans parent, 40 to 120 µm in length, and with length/ width ratios of 2:1–4:1 (Fig. 7a). Most of these grains show core–rim structures and oscillatory zoning (Fig. 7a), indicative of

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magmatic growt h (Hoskin and Schaltegger, 2003). Some grains have bright rims and unzoned

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Pb/

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U ages that ranged from 2592 to 226 Ma (Fig. 7b),

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Zircons from sample 17KPA-7 yielded

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cores in the CL images (Fig. 7a), but no distinctive metamorphic overgrowth rims were obs erved.

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and the corresponding Th and U conc entrations and Th/U ratios ranged from 53 to 1629 ppm,

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529 to 10145 ppm, and 0.04 to 1.43, respectively. The CL characteristics and the variable U–Pb ages imply that the zircons in basaltic sample 17KPA-7 might have been captured by the mafic

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magma during processes of assimilation and cont amination. A major age population was

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identified with an age range of 226–263 Ma and a weight ed mean

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Pb/

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U age of 243.3 ± 6.4 206

Pb/

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U ages

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Ma (MSWD = 8.1, N = 15). The youngest three zircons in sample 17KPA-7 gave

of 227.9 ± 3.1, 228.7 ± 3.9, and 231.7 ± 6.1 Ma with weight mean age of 228. 7 ± 4.5 Ma (MSWD = 0.15), indicating sample 17KPA-7 formed after the Carnian Period. In addition, these zircons show bright hue, long columnar and oscillatory zoning in the CL images, indicating they are derived from magmatism (Hoskin and Schaltegger, 2003). Similarly, the selected zircon grains from sample 17KPA-13 are short to elongated prisms, colorless and transparent, and characterized by broad oscillatory growth zoning (Fig. 7c ). The grains are 40–100 µm long and have length/ width ratios of 2:1–4:1. These zircon grains have low to intermediate cont ents of Th (36–3625 ppm) and U (156–4680 ppm) and low to intermediate

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values of Th/U (0.01–2.32). Twenty-eight analyses yielded concordant

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Pb/

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Pb ages with a

wide scatter ranging from 224. 7 ± 2.5 to 2306.3 ± 21 Ma (Fig. 7d). A major age population of this sample yield the apparent age of 224.7 ± 2.5 to 261.4 ± 5.6 Ma with weight ed mean age of 253.6 ± 4.0 Ma (MSWD = 3.2, N = 15). However, the youngest zircon grain (17KP-13-28) shows clear oscillatory zoning in the CL images and yields a

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Pb/

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U age of 224.7 ± 2.5 Ma, which we take

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as defining the lower age limit of the T3n basalts.

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4. Discussion

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4.1. Crustal contamination and fractional crystallization

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Crustal cont amination modifies the elemental compositions of igneous rocks and has an

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effect on the geochemic al indicators of tectonic setting (Zhu et al., 2012). Mantle-derived magmas are often contaminated by continent al crust during their formation, ascent, and

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emplacement. The continental crust in general is enriched in LILEs and total alkalis but depleted

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in TiO2, P2O5, Nb, and Ta (S un and McDonough, 1989; Rudnick and Gao, 2003). Therefore,

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magmatic rocks that are contaminated by continent al crust often display weak negative Nb, Ta, and Ti anomalies but positive Zr and Hf anomalies (e.g. Zhu et al., 2010). Our samples of the T3n basalts are slightly enriched in LILEs, and they have low contents of TiO2 (1. 34–1.53 wt.%) and P2O5 (0.09–0.13 wt.%). They contain numerous captured zircons, an indication that the magmas were contaminated by continental crust. The values of Nb/U ratios in MORB and OIB are uniform, and they show a unique relationship to the sourc e material. In MORB and OIB, these values have not been affected by magmatic processes such as differentiation, assimilation, and contamination (Hofmann et al., 1986). The values of Nb/ U therefore provide useful indicators when crustal contamination does

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occur. The Nb/U ratios (Nb/ U = 13.2–19. 0) of the T3n bas alts generally fall between those of average continental crust (Nb/U = 10) and average primitive mantle (Nb/ U = 30) (Hofmann et al., 1986), which suggests some slight crustal contamination. Similarly, the Th/ Ta ratios of the T3n basalts range from 2. 69 to 4.87, which is close to the primitive mantle value (Th/ Ta = 2.3), but very different from the continental crust value (Th/ Ta = 10) (Sun and McDonough, 1989), which

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again indicates only slight crustal contamination.

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The double-ratio diagrams of the HFSEs can be used to discriminate crustal

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contamination. On the (La/ Nb)PM–(Th/Nb)PM diagram, for example, the T3n basalts display

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relatively low ratios near the mantle field and plot near the starting point of the evolution line from

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mantle to continental crust (Fig. 8a). On the Nb/U–Nb/Th diagram, our samples lie along the evolution line from mantle to continental crust (Fig. 8b), indicating they underwent a certain

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degree of crustal contamination. In addition, all our samples show relatively low values of Th/ Nb

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(0.19–0.31), Ce/Nb (3. 90–5.48), Th/Yb (0.20–0.29), and Zr/Hf (31.8–36.6) as well as low Th

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concentrations (0.53–0.79), indicating little metasomatic overprinting in the magma source (Pearce, 2014). On the diagram of εNd(t) versus MgO (Fig. 8), there are no obvious linear relationships bet ween Nd isotope and MgO cont ents, suggesting that contamination by the continent al crust had only a small effect on the petrogenesis of the T3 n basalts. The initial 87

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Sr/ Sr ratios are increasing, whereas the εNd(t) values remain fairly constant, indicating that the

T3n basalts samples underwent seawater alteration after their formation (Fig. 6; Zhu et al., 2010). Fractional crystallization of a magma plays an important role in the evolution of the magma and petrogenesis of the resultant rocks. The T3n basalt samples have low contents of MgO #

(6.84–7.38 wt.%) and values of Mg (55.4–58. 7), indicating an evolved composition. Furthermore,

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they have low contents of compatible elements such as Cr (180–335 ppm) and Ni (64. 9–104 ppm) compared to those in a primitive mantle-derived magma (Cr = 300–500 ppm and Ni = 300–400 ppm ) (Frey et al., 1978). These geochemical features indicate that the magmas underwent a certain degree of fractional crystallization during their evolution. Our s amples show a positive correlation of Cr and Ni contents against values of Mg#, implying the likelihood of the

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fractional crystallization of olivine and clinopyroxene in the mafic magma (Fig. 8e, f). Because of

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the differences in partition coefficients, the Ni vs. Cr and V vs. Cr diagrams can be used to

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distinguish between the fractional crystallization of olivine, clinopyroxene, and hornblende (Fig.

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8f, g; Rollision et al., 2014), and in the case of the T3n basalts it is clear that fractional

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crystallization of clinopyrox ene rather than olivine or hornblende played a major role during their magmatic evolution (Fig. 8g, h). This is consistent with the relatively low pyroxene contents

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observed in thin sections. In the T3 n basalts, the slightly negative Eu anomalies (Eu/Eu* =

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0.83–0.99) indicate a weak fractional crystallization of plagioclase. We conclude, therefore, that

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the T3 n basaltic magmas underwent slight fractional crystallization involving clinopyroxene and to a lesser extent plagioclase during their magmatic evolution. 4.2. Tectonic setting

The T3 n basalts in the Kampa region generally have flat REE distribution patterns on the chondrite-normalized diagram and show similarities to typical N-MORB (Sun and McDonough, 1989) and N-MORB in the Indus–Tsangpo Sut ure Zone (Zhu et al., 2007). Their geochemical characteristics include slight enrichments in the LREEs (e.g. Cs, Rb, and Th), depletion in the HFSEs, and slight Nb, Ta, and Ti anomalies, and these suggest that the magmas were similar to those of typical island-arc bas alts (Sun and McDonough, 1989) and produced in a

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subduction-related tectonic setting. However, they show relatively high values of (Nb/La)P M, which range from 0.47 to 0.67, slightly negative Ti anomalies, and slight deviations from typical arc basalts, which are characterized by very clear depletions in Nb and Ti and (Nb/La)PM values of 0.27 (Hawkes worth et al., 1991). Moreover, all our samples plot in the IAB and MORB fields on the Zr/Y vs. Zr diagram (Fig. 9a; Pearce and Norry, 1979), the La/ Nb vs. La diagram (Fig. 9b;

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Pearce and Cann, 1973), and the Nb*2–Zr/4–Y diagram (Fig. 9c; Meschede, 1986), indicating

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that the T3 n basalts in the Kampa region have a geochemic al signature that is unique, lying

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between IAB and MORB. Usually, the development of a back-arc basin can induc e the

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production of widespread volcanic rocks such as island-arc basalts (IAB), back-arc basin basalts

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(BABB), and MORB-like basalts (e.g. Taylor and Martinez, 2003). Moreover, the BABB have mixed geochemical features that differ from those of IAB and MORB (S hinjo et al., 1999). The

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fact that all our T3n basalt samples plot in the BABB field on the Zr/Y vs. Y (Fig. 9a) and

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affinity.

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Y/15–La/10–Nb/8 (Fig. 9d; Cabanis and Lecolle, 1989) diagrams suggests they have a BABB

The T3n basalts show REE and trace element distribution patterns that are similar to those of well-studied BABB such as the Okinawa BABB (Fig. 5; Shinjo et al., 1999). For mafic magma, Ti is an incompatible element and decreases in proportion to increasing degrees of partial melting, but V shows the opposite kind of geochemical behavior. Therefore, the V vs. Ti*0.001 diagram can be used to evaluate the influence of subduction-related melts (Fig. 10a; Shervais, 1982; Pearc e, 2014). IABs have relatively low Ti/V ratios, whereas OIBs have higher Ti/V ratios. MORB-like basalts, which have intermediate Ti/V ratios, show a reduction in these Ti/V ratios and exhibit an arc-type basalt trend when they are affected by subduction-related fluids

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(Shervais, 1982). Our T3n basalt samples have moderate Ti/V ratios (24. 5–28.6), show a slight affinity to arc-type basalts, and fall into the MORB and BABB fields, all of which is consistent with the Okinawa BABB. Similarly, the Ti/ Zr vs. Zr (Fig. 10b; Woodhead et al., 1993) and La/Nb vs. Y (Fig. 10c; Winchester and Floyd, 1977) diagrams provide further support for a BABB-like tectonic setting for the T3n basalts.

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The Th–Nb–Y b system is considered to be one of the most efficient proxies for estimating

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the effects of subduction-related material (Pearce, 2008) and different geochemical behaviors in

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the subduction zone. In general, Nb and Yb contents remain stable in the subduction zone, while

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Th can be mobilized by subduction components such as slab fluids and sediment-derived melts.

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Henc e, supra subduction-related magmas are characterized by high Th/Nb ratios (Pearce, 2014). Our T3n basalt samples show moderately high Th/Yb ratios (Th/Yb = 0.15–0. 29) and fall above

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the MORB–OIB array on the Th/Y b vs. Nb/Yb diagram (Fig. 10d), implying the presence of a

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subduction-related component. However, the T3n basalt samples still belong to tholeiitic basalt

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series and are far from the volcanic arc array when it comes to their Nb/Y b (0.79–0.99) and Th/Yb (0. 15–0.29) ratios (Fig. 10d). Furt hermore, all our samples clearly fall in the Okinawa BABB field, which suggests they are related to a back-arc basin setting. The mafic magmatism in the THS since the Permian provides abundant information on mantle sources in the deep dynamic processes that are closely related to t he evolution of the Neo-Tet hyan Ocean (Table 2; Garzanti et al., 1999; Zhu et al., 2006; Chauvet et al., 2008; Zeng et al., 2012). E ven so, there have been only a few studies of the Triassic basalts in the THS. In previous studies, the Early Triassic basalts in the Rema Formation (T1 r) and the Middle Triassic basalts in the Sangkang Formation (T2s) near the Gyangze and Kangmar regions were

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interpreted as a series of alkaline basalts with OIB-type features and with clear evidence of crustal contamination (Table 2; Zhu et al., 2006). Moreover, He et al. (2016) argued that the Early Triassic basalts in the Zhongba region are MORB- and OIB -type continental rifting basalts that record t he early evolution of the Neo-Tethys Ocean. However, the Late Triassic basalts we describe here have distinctive geochemical and isotopic features, so that in the Lhozhag and

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Kangmar regions, the layers of basalt in the Nieru Formation show tholeiitic basaltic affinities,

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E-MORB -type geochemical features, and slight crustal contamination in a tectonic setting of

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continent al margin rifting (Zhu et al., 2006). Recently, Huang et al. (2018) reported on the

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Triassic bimodal magmatism in the Qiongduojiang area. Their gabbro samples of Late Triassic

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age are similar to typical OIB types, and they were derived from garnet lherzolites that underwent only small amounts of partial melting. Furthermore, their Lat e Triassic diabase samples display

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geochemical and isotopic features similar to E-MORB derived from an enriched mantle source in

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the spinel facies stability field and with relatively high degrees of partial melting (Huang et al.,

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2018).

Our T3n basalt samples are, to some extent, similar to these other reported Late Triassic basalts, including their values of Mg#, the mantle sourc es, the degrees of partial melting, and the slight amounts of crustal contamination. However, the T3n basalts are characterized by slight enrichments in LILEs, depletions in HFSEs, flat REE distribution patterns, slightly negative Nb–Ta–Ti anomalies, and depleted Nd isotopes, indicating formation in a back-arc basin similar to the Quaternary Ryukyu Arc–Okinawa Trough (Shinjo et al., 1999). 4.3. Mantle source Geochemically, the T3n basalt samples show flat HREE and HFSE distribution patterns

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with slight depletions in LREEs, implying a mantle source similar to the mantle source of N-MORB. Incompatible trace elements such as Nb, Y, Yb, and Zr are often used for examining the nature of the mantle source because they are largely unaffected by crustal contamination and slab-derived metasomatic processes (Pearce, 2014). The T3n samples have relatively high values of Zr/Nb (26.3–33.1), Y/Nb (10.5–13.1), and Yb/Nb (1.01–1.21) that are similar to the

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values in N-MORB (Zr/ Nb = 31.8, Y/Nb = 12.0, and Yb/Nb = 1.31) (S un and McDonough, 1989).

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Moreover, all the T3 n bas alt samples fall into the BABB field above the N -MORB point on the

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MORB–OIB array on the Th/Yb vs. Nb/Yb diagram (Fig. 10d), indicating that they may have been

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derived from a depleted mantle s ourc e similar to t he source of typical N-MORB. Furthermore,

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they have relatively high values of εNd(t) (4.08–5. 24) and show a good consistency with IYSZ MORB on the Sr–Nd isotopic diagram (Fig. 6). Considering the subduction-related geochemical

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features such as slight enrichments in fluid-mobile elements (e.g. Rb, Th, and U) and depletions

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in HFSEs (e.g. Nb, Ta, and Ti), our sample of T3n basalt may have been derived from a depleted

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mantle-wedge source, similar to the mantle source of N-MORB. Diagrams such as the Y b vs. La/Yb and Sm/Yb vs. La/Sm diagrams, which are based on the nonmodal batch melting equation with REE simulations , are efficient approaches to constraining the characteristics of the mantle source and the degree of partial melting (Aldanmaz et al., 2000; Chen et al., 2018). The HREEs (e.g. Yb, Lu, and Y) show a strongly compatible affinity to garnet, but they are incompatible with spinel, clinopyroxene, and olivine (McKenzie and O'Nions, 1991; Aldanmaz et al., 2000). Obviously, therefore, the thermody namic characteristics of garnet during partial melting cont rol the Yb contents of the melt (Pearce, 2008). The melts that are derived from the garnet facies stability field in the mantle source have higher La/Yb ratios

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than melts derived from the spinel facies stability field (Peters et al., 2008). All our T3n bas alt samples fall on the spinel lherzolite melting curve and form a straight line nearly parallel to the Y-axis (Yb content) on the Yb vs. La/Yb diagram (Fig. 11a), suggesting they may have formed from spinel lherzolites in the mantle with a degree of partial melting that was approximat ely 13%–17%. In the spinel facies stability field, the ratios of La/Sm and La/Yb in the melt gradually

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decrease with increasing degrees of partial melting, but their Sm/Yb and Dy/Yb ratios remain

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almost constant (Aldanmaz et al., 2000). These differences lead to similar Sm/Yb and Dy/Yb

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ratios in a melt that is derived from spinel lherzolite, but significantly higher Sm/Yb and Dy/Yb

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ratios in a melt that is derived from garnet lherz olite. Our samples fall mainly on the spinel

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lherzolite melting curve on the Sm/Yb vs. La/Sm diagram (Fig. 11b), indicating that they originated from about 7% –9% partial melting of a shallow and depleted MORB mantle (DMM)

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(McKenzie and O’Nions, 1991). This deduction is also supported by the relatively low Ce/Y ratios

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(0.37–0.42) on the Ce/Y vs. Zr/ Nb diagram (Fig. 11c) and the (Dy/Yb)N ratios of 1.17–1.36 on the

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(Dy/Yb)N vs. (Gd/Lu)N diagram (Fig. 11d). 4.4. Geodynamic implication It has been suggested that the mechanism of formation of a typical back-arc basin is controlled by the geometrical relationships of the subducted slab and the overriding plat e, as well as the effect of lateral mantle flow on the mantle wedge (Sdrolias and Müller, 2006). Subduction of an old and cold oceanic slab will lead to the development of flow circulation in the mantle wedge beneath the overriding plate, trigger decompression melting of the overriding lithosphere, and subsequently form a spreading center (Sdrolias and Müller, 2006). Therefore, the lithosphere in the back-arc will weaken in rheological strength and it will tend t o thin due to

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ongoing mantle flow migration and decompression melting ( Tayor and Martinez et al., 2003). Moreover, rollback of the subducted slab will further intensify these deep processes and provide sufficient space for spreading in t he developing back-arc basin (Nakakuki and Mura, 2013). It is accepted that the Paleo-Tethyan Ocean occupied a large area around the equator from the Devonian until the Triassic (Zhu et al., 2010; Stampfli et al., 2013). Subsequently, subduction of

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the Paleo-Tethyan oceanic plate led to the break-up of Gondwana and separation of the

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Cimmerian continent into an array that includes present-day Iran, Afghanistan, the Qiangtang

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Terrane, Lhasa Terrane, Sibumasu Terrane, and Malaya (Yin and Harrison, 2000; Stampfli and

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Borel, 2002; Pan et al., 2012; Stampfli et al., 2013; Zhu et al., 2013). The development of the

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Neo-Tet hyan Ocean was cont rolled by the southwards Paleo-Tethyan oceanic subduction system and initially opened as a back-arc basin between the Lhasa Terrane and THS during the

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al., 2013).

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Late Triassic (Yin and Harrison, 2000; Golonka, 2007; Dai et al., 2008; Pan et al., 2012; Zhu et

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The back-arc basin spreading model for the incipient opening of Neo-Tethys is very simple, and it is also an attractive model becaus e it is capable of being proved or disproved by many other geological observations. Dai et al. (2008) systematically compiled data for the Sr–Nd isotope compositions of the THS and Lhasa Terrane, and proved the homogeneity of their basement rocks, and they argued, therefore, that these blocks were close to one another during the Late Triassic but separated by a narrow marine trough. Furthermore, rec ent provenance analysis of the well-studied Langjiexue Group, located between the Lhasa Terrane and THS, and previously regarded as an isolated microterrane (Liu et al., 2019 and references therein ), has shown it to have detrital zircon distribution patterns, sedimentary facies, paleogeography, and

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tectonic setting similar to the Nieru Formation in the Kangmar, Kampa, and Gyangze regions (Cai et al., 2016). Trace element contents of zircons with ages of 300 to 200 Ma indicate that fragments of the Nieru Formation were derived from a continental magmatic arc ( Li et al., 2011; Cai et al., 2016; Liu et al., 2019 and references therein), which suggests the presence of a magmatic arc related to a system of southwards subduction during the Paleozoic and until the

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Early–Middle Triassic, and the potential existence of a back-arc basin during the Late Triassic.

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Moreover, the massive volume of graben deposits, including T1+2lc and T3 n strata in the THS, is

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often considered to be a res ult of the “Triassic Extension Movement”, related to the “Triassic

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Ocean B asin” north of the northern margin of the Indian Plate in the Middle to Late Triassic (Liu

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et al., 1990; Liu and Einsele, 1994; Shi et al., 1996; Shi, 2001; Xu et al., 2003). Taking petrographical characteristics and depositional environments into account, the Lvc un Formation

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in the Tethyan Himalaya was deposited in a setting of continental stretching–subsidence–rifting

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along wit h mafic volcanic activities, with the continental subsidence and rapid deepening of the

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seawater taking place in the Early to Middle Triassic, whereas the Nieru Formation was an abyssal–bat hyal depositional system throughout the Late Triassic (Liu and Einsele, 1994; Shi et al., 1996; Li et al., 2011; Cai et al., 2016). The results of recent studies on tectonic subsidence have indicat ed a rapid subsidenc e event (51.45 m/Myr) took place during the Late Triassic in the THS, providing further support for a regional extensional movement along the northern margin of Gondwana (e.g. Sciunnac h and Garzanti, 2012; Li et al., 2017; Cao et al., 2018). Furthermore, recent provenance studies have shown that the Lhasa Terrane has not provided fragmental material to the Tethyan Himalaya since the Late Triassic (Cao et al., 2018). This important transformation of provenance provides further evidenc e for the existence of a narrow back-arc

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basin and support for the hypothesis that the initial opening of the Neo-Tethyan Ocean took place during the Late Triassic. Widespread magmatic activities along the northern margin of Gondwana (Table 2; e.g. the Panjal traps in Zanskar, the Abor volcanics in the E astern Himalaya, the Nar Ts um spilites in Nepal, and the Jirong (P1j) and Selong (P2+3s) basalts in the Gyirong region) were related to the

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break-up of Gondwana and the initial opening of the Neo-Tethy an Ocean during the late

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Paleozoic to early Mesozoic (Bhat, 1990; Vannay and Spring, 1993; Garzanti et al., 1999; Zhu et

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al., 2010; Huang et al., 2018). The well-studied Panjal traps of the middle Permian are often

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regarded as continental flood basalts that were related t o Permian rifting of the Neo-Tethys

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system (Table 2). Garzanti et al. (1999) were the first to describe the tholeiitic Bhote Kosi basalts in the Gyirong region, and they regarded them as the products of continental rifting during the

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break-up of Gondwana and the incipient opening of Neo-Tethys during the early Permian. In

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addition, in the THS, the early Permian basalts (P 1j) and middle–late Permian basalts (P 2+3s) in

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the Selong region are within-plate tholeiitic basalts that formed in a Permian extensional tectonic setting (Zhu et al., 2010). It is clear, therefore, that the widespread Permian basalts in the THS have similar geochemical features, and that they were associated with continental rifting and an extensional tectonic setting (Fig. 12; e.g. Garzanti et al., 1999; Chauvet et al., 2008; Zhu et al., 2010, 2013). Similarly, the Permian basalts in the Qiangtang Terrane (e. g. Zhai et al., 2013; Wang et al., 2014; Xu et al., 2016; Zhang and Zhang, 2017) and the Sibumasu Terrane (Liao et al., 2015) are thought to be related to the rifting and break-up of the Cimmerian continent along the northern margin of Gondwana. The contemporaneous basaltic magmatism in all these blocks show similar features, and they are OIB types, tholeiites, and continental flood basalts that

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formed during rifting. Considering the Permian–Triassic arc-type magmatism in the Lhasa Terrane (e.g. Zhu et al., 2013; Wang et al., 2016; Ma et al., 2018), the subduction of the Paleo-Tethyan oceanic plate might have been the main cause behind the eventual development of intens e back-arc extensional and continental rifting along the northern margin of Gondwana since the Permian

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(Fig. 12; Pan et al., 2012). As subduction continued, the rollback of the subducted slab would

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have led to further thinning of the lithosphere, the development of a back-arc basin (the future

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Neo-Tet hyan Ocean), and widespread BABB-type magmatism, and all this is consistent with the

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evolution of the Triassic basalts (Zhu et al., 2006). In general, the Late Triassic basalts in the

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THS indicate relatively low degrees of partial melting at shallow dept hs in a depleted mantle (Fig. 11b). Furthermore, our samples of the T3 n basalts have Nd isotope compositions that are similar

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to those previously reported for MORB in the IYS Z (Fig. 6), Lat e Triassic basalts in the

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Qiongduojiang area (Huang et al., 2018), and Late Triassic basalts in the Lhozhag area (Zhu et

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al., 2006). These similarities provide support for the idea that the mature Neo-Tethyan Ocean originated from this initial back-arc basin (Fig. 12). The high values for initial

87

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Sr/ Sr ratios in the

T3n basalts (Fig. 6) imply the presence of seawater alteration, and further support the idea that they formed beneat h the seawater in which the Nieru Formation was deposited (Fig. 2). The T3n basalts have BABB-type features, a relatively depleted mantle source, and a high degree of partial melting, which suggests that the back-arc basin was limited in scale. In addition, the slight crustal contamination in the T3n bas alts (Fig. 8a, b), the fractional crystallization (Fig. 7e–h), and the presence of captured zircons (Fig. 7) indicate that a thin slice of continent al crust still existed above the mantle source of the BABB. Altogether, these features indicate an important evolution

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took place from a narrow back-arc basin to a mature ocean, thus implying that the incipient opening of the Neo-Tethyan Ocean occurred along the northern margin of Gondwana during the Late Triassic. In general, our new data on fragments of arc magmatic rocks, back-arc basinal deposits, and interbedded Chile-type BABB in the Nieru Formation of the Tethyan Himalaya support our back-arc spreading model (Fig. 12).

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This hypothesis is also supported by paleomagnetic studies. The Indian Plate migrated

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northwards during the Early to Middle Triassic, the Himalayan Terrane moved 2 degrees

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southwards during the Late Triassic, and the Lhasa Terrane drifted rapidly nort hwards since

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around 220 Ma (Li et al., 2003), and these observations suggest these were discrete continental

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blocks at those times. Moreover, it has been confirmed from paleo-magnetic data that the initial separation of those blocks occurred in the Middle to Late Triassic and that complete separation

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5. Conclusions

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occurred after ca. 200 Ma (Song et al., 2017).

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(1) The T3n basalts in the Kampa region display geochemical and isotopic feat ures similar to typical back-arc basin basalts such as the Okinawa BABB. This basaltic magmas underwent slight crustal contamination and fractional crystallization of clinopyroxene during their evolution. (2) The T3n basalts were derived by a high degree of partial melting from a shallow and depleted mantle source in the spinel facies stability field. They have similar Nd compositions with MORB in t he Indus–Tsangpo Suture Zone, and record the early evolution of the Neo-Tethyan Ocean. (3) Sout hwards subduction of the Paleo-Tethyan oceanic plate led to back-arc spreading and the formation of BABB-type basalts in the Tethyan Himalayan Sediment ary Sequence ( THS).

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The Neo-Tethyan Ocean developed from these back-arc basins along the northern margin of Gondwana during the Late Triassic. Acknowledgments This study was supported by the National Key Research and Development Project of China (Grant No. 2016Y FC0600303) and the National Nature Science Foundation of China

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(Grant Nos 41402175 and 41602054). We thank the editor and t wo anonymous reviewers for

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their valuable suggestions and comments, which helped to improve this manuscript. We also

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appreciate the help given by Prof. Prinya Nutalaya, Prof. Han Baofu, Associate Prof. Huang

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Xuemeng, Dr. Ge Maohui, Dr. Qi Nan, and Dr. Gao Lei.

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Figure Captions

Fig. 1. (a) Regional tectonic map of the central Himalayan orogen including the North Himalayan

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gneiss domes, modified from Zhang et al. (2012). (b) Geologic al map of the Kampa Dome,

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modified from Quigley et al. (2006), Liu et al. (2016) and Lin et al. (2020).

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Fig. 2. Stratigraphic columns showing the geological succession in the study area, and the Nieru Formation in the Kampa, Lazi, and Lhozhag regions (modified from Xu et al., 2003, Quigley et al., 2006, and Cai et al., 2016). Fig. 3. Representative outcrops and photomicrographs of the T3n basalts. (a–c) Exposures of interbedded basalts in the Nieru Formation of the Kampa region. (d–f) Mineral assemblage and petrographic characteristics of the basalts under crossed polarized microscope. Chl = chlorite; Ep = epidote; Hb = hornblende; Pl = plagioclase; Py = pyroxene; Ttn = titanite. Fig. 4. Chemical classification diagrams. (a) Zr/TiO2*0.0001 vs. Nb/Y diagram (Winchester and T

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Floyd, 1977). (b) FeO –Alkalis–MgO ternary diagram (Irvine and B aragar, 1971). FeO =

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total Fe contents. (c) FeO /MgO vs. SiO2 diagram (Miyashiro, 1974). Fig. 5. Chondrite-normalized REE patterns and primitive-mantle-normalized trace element spider diagrams of the T3n basalts. OIB and N-MORB data and the normalization values are from Sun and McDonough (1989). Data for the Okinawa BABB come from Shinjo et al. (1999) and for the Permian basalts in the THS from Zhu et al. (2010). 87

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Sr/ Sr(t) diagram. IYS Z MORB represents the MORB-type ophiolite

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Fig. 6. εNd(t) versus initial

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in the Indus–Ts angpo S uture Zone (Wang et al., 2016). The green filled circles represent

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data for the T3n basalts. LB represents the T3 basalts in the Lhasa Terrane (Wang et al.,

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2016), QB the Triassic gabbros in the Qiongduojiang area (Huang et al., 2018), NB the

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Late Triassic basalts in the Lhozhag region (Zhu et al., 2006), and PB the Permian basalts in the THS including the P 1+2j (Jirong Formation) and P 2s basalts (Selong Formation) (Zhu

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et al., 2010). Data for the mantle array come from Zindler and Hart (1986).

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Fig. 7. Zircon CL images and concordia diagrams for the T3n basalts. 87

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Sr/ Sr(t)

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Fig. 8. Plots of (a) Th/Nb) PM vs. (La/ Nb) PM ( Near et al., 2002 ), (b) Nb/U vs. Nb/Th, (c)

vs. MgO, (d) εNd(t) vs. MgO, (e) Ni vs. Mg#, (f) Cr vs. Mg#, (g) Ni vs. Cr, and (h) V vs. Cr after (Rollision et al., 2014). ACC = average continent composition, N-MORB = normal mid-ocean ridge bas alt, OIB = ocean island basalt. Data for OIB and N-MORB are from Sun and McDonough (1989) and for ACC they are from Rudnick and Gao (2003). The green filled circles represent data for the T3n basalts. Fig. 9. Tectonic setting of the T3 n basalts (plotted as green filled circles). (a) Zr/Y vs. Zr diagram (Pearce and Norry, 1979). (b) La/Nb vs. La diagram (Pearce and Cann, 1973). (c) Nb*2–Zr/4–Y triangular diagram (Meschede, 1986). (d) Y/15–La/10–Nb/8 triangular

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diagram (Cabanis and Lecolle, 1989). E-MORB = incompatible element-enriched mid-ocean ridge basalt, VAB = volcanic arc basalt, VAT = volcanic arc tholeiitic basalt, WP Alk = within-plate alkaline basalts, WP Th = within-plate tholeiitic basalt. Fig. 10. Tectonic discrimination diagrams for the T3n basalts (plotted as green filled circles). (a) V vs. Ti/1000 diagram after Shervais (1982). E-MORB = enriched MORB, IAB = island

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alkaline basalts, CFB = continental flood basalts. (b) Ti/ Zr vs. Zr diagram (Woodhead et al.,

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1993). (c) La/Nb vs. Y diagram (Winchester and Floyd, 1977). FAB = forearc basalts. (d)

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Th/Yb–Nb/Yb diagram (Pearce, 2008). CA = calc-alkaline, TH = tholeiitic.

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Fig. 11. REE discrimination diagrams for mantle sourc es. (a) Yb vs. La/Yb diagram (Chen et al.,

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2018). Data for the T1r and T3n basalts are from Zhu et al. (2006), data for the Qiongduojiang gabbro and diabase (Lat e Triassic) are from Huang et al. (2018). (b)

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Sm/Yb vs. La/Sm diagram (Aldanmaz et al., 2000). The compositions of t he depleted

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MORB mantle (DMM) are from McKenzie and O’Nions (1991). Dat a for the primitive

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mantle (PM), N-MORB, and E-MORB are from Sun and McDonough (1989). The partial melting curves are calculated by the batch melting model (Shaw, 1970) and partition coefficients (Mckenzie and O’Nions, 1991). (c ) Ce/Y vs. Zr/Nb diagram (Zhu et al., 2007). (d) (Dy/Yb) N vs. (Gd/Lu)

N diagram

modified after (Chen et al., 2018).

Fig. 12. Back-arc basin spreading model for the initiation of the Neo-Tethyan Oc ean along the northern margin of Gondwana during the late Paleozoic to early Mes ozoic, modified after Yin et al., 2006, Dai et al., 2008, and Zhu et al., 2010. Stage 1 (c ontinental rifting): wides pread P ermian continental rifting with contemporaneous continental flood basalts along the northern margin of Gondwana. Stage 2 (back-arc basin): southwards subduction

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of the P aleo-Tet hyan oceanic plate led to int ensive crustal extension, lithospheric thinning and initial continent break-up along the northern margin of Gondwana. Subsequently, an incipient back-arc basin formed and spread bet ween the THS and the Lhasa Terrane, creating the incipient opening of the Neo-Tethyan Ocean during the Late Triassic. The T3n basalts erupted as back-arc basin basalts, which were interbedded with the sedimentary

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sequence in the back-arc basin. Stage 3 (passive continental margin): a mature and wide

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Neo-Tet hyan Ocean developed on the basis of back -arc basin spreading. The THS

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represents a typical passive continental margin that was present during the Jurassic and

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Cretaceous.

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Table Captions

Table 1. Whole rock geochemistry and Sr-Nd isotope composition data.

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Table 2. Representative P-T mafic magmatic events in the northern Gondwana.

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Appendix A. Supplementary data

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Supplementary Text. Analytical methods. Supplementary Table. Zircon U-Pb dating analyses results of T3n basalts. References

Aldanmaz, E., Pearce, J. A., Thirlwall, M. F., Mitchell, J. G., 2000. Petrogenetic evolution of late Cenozoic, post-collision volcanism in western A natolia, Turkey. J Volcanol Geoth Res 102, 67-95. Ali, J. R., Aitchison, J. C., Chik, S. Y., Baxter, A. T., Bryan, S. E., 2012. Paleomagnetic data support Early Permian age for the Abor Volcanics in the lower Siang Valley, NE India: significance for Gondwana-related break-up models. J Asian Earth Sci 50, 105-115.

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Berberian, M., King, G., 1981. Towards a paleogeography and tectonic evolution of Iran. Can J Earth Sci 18, 210-265. Bhat, M. I., 1990. Petrogenesis and the mantle source characteristics of the Abor volcanic rocks, eastern Himalayas. J Geol Soc India 36, 227-246. Cabanis, B., Lecolle, M., 1989. The La/10-Y/15-Nb/8 diagram: A tool for distinguishing volcanic

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series and discovering c rustal mixing and/ or contamination: Comptes Rendus de

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l’Academie des Sciences, serie 2. Science de la Terre 309, 20.

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Cai, F., Ding, L., Laskowski, A. K., Kapp, P., Wang, H., Xu, Q., Zhang, L., 2016. Late Triassic

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Earth Planet Sc Lett 435, 105-114.

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paleogeographic reconstruction along the Neo–Tethyan Ocean margins, southern Tibet.

Chauvet, F., Lapierre, H., Bosch, D., Guillot, S., Mascle, G., Vannay, J., Cotten, J., Brunet, P.,

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Keller, F., 2008. Geoc hemistry of the Panjal Traps basalts (NW Himalaya): records of

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383-395.

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☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights： The T3n basalts are similar to typical back-arc basin basalts.

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Subduction of the Paleo-Tethyan Ocean led to break-up of Gondwana.

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The Neo-Tethyan Ocean open incipiently during the Late Triassic.

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