Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications

    Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications Chao Wang, Lin Ding, Zhi-Chao Liu, Li-Yun Zhang, Ya-Hui Yue PII: DOI: Reference:

S0024-4937(16)30412-1 doi:10.1016/j.lithos.2016.11.016 LITHOS 4150

To appear in:

LITHOS

Received date: Accepted date:

29 June 2016 11 November 2016

Please cite this article as: Wang, Chao, Ding, Lin, Liu, Zhi-Chao, Zhang, LiYun, Yue, Ya-Hui, Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications, LITHOS (2016), doi:10.1016/j.lithos.2016.11.016

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Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south

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Tibet: Age, petrogenesis and tectonic implications

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research,

Chinese Academy of Sciences, Beijing 100101, China

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a

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Chao Wanga,b*, Lin Dinga,c, Zhi-Chao Liud, Li-Yun Zhanga,c, Ya-Hui Yuea,c

University of Chinese Academy of Sciences, Beijing 100049, China

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Chinese Academy of Sciences Center for Excellence in Tibetan Plateau Earth Sciences, Beijing

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b

100101, China

School of Earth Science and Geological Engineering, Sun Yat-Sen University, Guangzhou

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d

510275, China

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*Corresponding author at: Key Laboratory of Continental Collision and Plateau Uplift, Institute of

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Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. Tel: +86 10 84097104; Fax: +86 10 84097079.

Abstract

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E-mail address: [email protected] (C. Wang).

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Limited geochronological and geochemical data from Early Cretaceous igneous rocks of the Gangdese Belt have resulted in a dispute regarding the subduction history of Neo-Tethyan Ocean. To approach this issue, we performed detailed in-situ zircon U–Pb and Hf isotopic, whole-rock elemental and Sr–Nd isotopic analyses on Late Mesozoic volcanic rocks exposed in the Liqiongda area, southern Lhasa terrane. These volcanic rocks are calc–alkaline series, dominated by basalts, basaltic andesites, and subordinate rhyolites, with a bimodal suite. The LA–ICPMS zircon U–Pb dating results of the basaltic andesites and rhyolites indicate that these volcanic rocks erupted during the Early Cretaceous (137–130 Ma). The basaltic rocks are high-alumina (average >17 wt.%), enriched in large ion lithosphile elements (LILEs) and light rare earth elements (LREEs), 1

ACCEPTED MANUSCRIPT and depleted in high field strength elements (HFSEs), showing subduction-related characteristics. They display highly positive zircon εHf(t) values (+10.0 to +16.3) and whole-rock εNd(t) values

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(+5.38 to +7.47). The silicic suite is characterized by low Al2O3 (<15.4 wt.%), Mg# (< 40), and

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TiO2 (< 0.3 wt.%) abundances; enriched and variable concentrations of LILEs and REEs; and strongly negative Eu anomalies (Eu/Eu* = 0.08–0.19), as well as depleted Hf isotopic compositions (εHf(t) = +4.9 to +16.4) and Nd isotopic compositions (εNd(t) = +5.26 to +6.71).

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Consequently, we envision a process of basaltic magmas similar to that of MORB extracted from a

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source metasomatized by slab-derived components for the petrogenesis of mafic rocks, whereas the subsequent mafic magma underplating triggered partial melting of the juvenile crust to

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generate acidic magma. Our results confirm the presence of Early Cretaceous volcanism in the

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southern Lhasa terrane. Combined with the distribution of the contemporary magmatism,

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deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo-Tethyan oceanic lithosphere was flat-lying beneath the Lhasa terrane during the Early

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Cretaceous.

Keywords: Gangdese belt, Bimodal suite, Early Cretaceous, Zircon U–Pb geochronology and Hf isotopes, Sr–Nd isotopes, Southern Lhasa terrane.

1. Introduction

Subduction zones are the only places on Earth where superficial materials sink into the deep mantle. The most spectacular product of this recycling process is arc magmatism, which represents the second largest type of magma formation after ocean ridges, and the main source of continental crust growth (Tatsumi and Kogiso, 2003). Therefore, an understanding of arc magmatism in the

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ACCEPTED MANUSCRIPT subduction zone is of key importance to unravel the past geodynamic evolution of subduction systems, from both petrological and tectonic point of views. The main geological expression of

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subduction-related magmatism is present in numerous plutonic bodies that interfinger to form the

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famous Gangdese batholiths in the southern Lhasa terrane (Chung et al., 2005; Chu et al., 2006; Ji et al., 2009 a, b; Mo et al., 2005; Schärer et al., 1984; Wen et al., 2008; L.-Y. Zhang et al., 2014 and references therein). Furthermore, studies have repeatedly shown that the Gangdese batholiths

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mainly consist of Late Triassic–Early Eocene intermediate-felsic intrusive rocks (mainly granites)

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with highly positive εNd(t) (+2.4 to +8.5) and εHf(t) values (+10.2 to +17.6) (Chu et al., 2006; Chung et al., 2005; Ji et al., 2009a, b; Ma et al., 2013a, b; Mo et al., 2005; Wen et al., 2008; Zhu et

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al., 2008 and references therein). The temporal-spatial distribution of magmatism in the Gangdese

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Belt has been well established; however, the history of the arc migration and exhumation of the

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Gangdese arc during the Mesozoic remains controversial. The underlying cause of these disputes arises because the Early Cretaceous magmatic rocks (~137–100 Ma) are ubiquitous in the northern

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parts of the Lhasa terrane, whereas the contemporary magmatic records along the southern margin are scarce. Therefore, different geodynamic models have been proposed to reconstruct the geological setting of the southern Lhasa subterrane during the Early Cretaceous: (1) low-angle to flat subduction of the Neo-Tethyan oceanic crust (Coulon et al., 1986; Ding and Lai, 2003; Kapp et al., 2005; 2007); (2) intra-oceanic subduction (Aitchison et al., 2000; McDermid et al., 2002); and (3) high-angle subduction, slab roll-back and lithospheric delamination (Zhang et al., 2012; Zhu et al., 2009). These hypotheses have been intensely debated, and there is little consensus in the literature as to which one best explains the regional observations in southern Tibet. This article presents zircon U–Pb geochronology for the newly identified bimodal volcanic 3

ACCEPTED MANUSCRIPT rocks from the Liqiongda area on the southern margin of the Gangdese Belt. Coupled with whole rock geochemical data and in-situ zircon Hf isotopic data, our goal is to gain a better

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understanding of the petrogenesis of the Early Cretaceous volcanic rocks as well as the subduction

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history of the Neo-Tethyan oceanic crust. Our data provide valuable constraints on the geodynamic processes involved in the generation of the Early Cretaceous magmas in the

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Gangdese Belt.

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2. Geological background

The Tibetan Plateau is the youngest and most spectacular of all continent–continent collision

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belts on Earth, consisting of the Songpan–Ganzi flysch complex, Qiangtang terrane, and Lhasa

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terrane from north to south (Fig. 1a, b; Allègre et al., 1984; Tapponnier et al., 2001; Yin and

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Harrison, 2000). The Lhasa terrane in southern Tibet is separated from the Qiangtang terrane to the north by the Bangong–Nujiang suture, which closed in the Late Jurassic–Early Cretaceous (Kapp et al., 2005, 2007; Yin and Harrison, 2000), and from the Himalaya Belt to the south by the

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Indus-Yarlung Zangbo suture zone, which formed in the Indo–Asian collision during the Early Paleocene to Early Eocene (Fig. 1a, b; Cai et al., 2011; DeCelles et al., 2014; Ding et al., 2005; Wu et al., 2010; XBGMR, 1992). The Lhasa terrane is a huge tectonic–magmatic unit mainly composed of Paleozoic–Paleogene sedimentary strata and associated igneous rocks (Chang et al., 1986; Dewey et al., 1988; Yin and Harrison, 2000). The basement of the Lhasa terrane, which formed along the peri–Gondwana margin, is composed of Paleozoic and Proterozoic rocks near Amdo, Dongjiu, and Bomi (Lin et al., 2013; Xu et al., 2013). The northern–central Lhasa terrane contains scattered outcrops of Paleozoic shelf sediments, middle Triassic–Cretaceous sedimentary rocks, abundant Early Cretaceous volcanic rocks, and lower Cretaceous volcano-sedimentary 4

ACCEPTED MANUSCRIPT sequence as well as Cretaceous granitoid batholiths (Kapp et al., 2005, 2007; Yin and Harrison, 2000; and references therein). An active continental margin at the southern part of the Lhasa

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terrane may have formed due to the northward subduction of the Neo-Tethyan oceanic lithosphere

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before the collision between India and Asia (Ding and Lai, 2003; Ding et al.,2014; Murphy et al., 1997), now exposed as the voluminous Andean-type calc-alkaline magmatism of the Gangdese batholith and coeval volcanic rocks (e.g., Chu et al., 2006; Guo et al., 2013; Ji et al., 2009a, b;

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Kang et al., 2014; Wen et al., 2008; Zhu et al., 2008, 2009). These pre-collisional igneous rocks

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can be divided into two magmatic episodes that occurred 205–152 Ma and 109–80 Ma, with magmatic flare–ups at ca. 180 and 90 Ma (Ji et al., 2009a, b; Ma et al., 2013a, b; Wen et al., 2008).

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By contrast, the northern Lhasa terrane was covered in places by shallow marine

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limestone-bearing fluvial sedimentary rocks during the Aptian–Albian (Kapp et al., 2005; Leier et

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al., 2007; Zhang et al., 2012), together with widespread Mesozoic igneous rocks (e.g., Early Jurassic Amdo granitoids, Guynn et al., 2006; Cretaceous volcanic rocks of the Zenong Group and

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Duoni Formation, Kang, 2009). The latest Cretaceous–early Tertiary rocks in the Lhasa terrane are dominated by volcanic sequences of the Linzizong Formation (Fig. 1), which have been dated to 65–45 Ma (Mo et al., 2008 and references therein). Throughout much of the Lhasa terrane, the conspicuous regional unconformity between gently folded Linzizong volcanic rocks and strongly deformed Cretaceous and older rocks demonstrates that major upper crustal shortening predates the India–Asia collision (Ding and Lai, 2003; Mo et al., 2008; Kapp et al., 2005; Yin and Harrison, 2000).

3. Petrology and Sampling

Cretaceous bimodal volcanic rocks are present near Liqiongda in the southern Lhasa terrane 5

ACCEPTED MANUSCRIPT (Fig. 1c), which were mapped as limestone and marble of the Late Jurassic Duodigou Formation (J3d). In the sampled sections, the volcanic rocks are mainly composed of interbedded green–black

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basalts and basaltic andesites, as well as mafic volcaniclastic rocks (e.g., breccias, agglomerates)

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and purple rhyolites. Mafic and felsic lavas in the same area have similar ages (see below), thus indicating a bimodal sequence. The total thickness of the bimodal suite section is approximately 1500 m thick, and the basaltic rocks are commonly usually thicker than the rhyolites, with the

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proportion of mafic to felsic lavas at a ratio of approximately 5:1 (Fig. 1d). The thick, poorly

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sorted polymictic breccia layer in the upper volcanic sequence indicates a subaerial debris flow with a deposited distal component. Neither mafic xenoliths nor any features of magma mixing

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have been observed during our investigations. The section and sampling locations are shown in

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Fig. 1 c, d. The mafic rocks have intergranular textures, ranging from aphyric to coarsely

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porphyritic rocks. Some of the basaltic samples have a fumarole–amygdaloidal structure, filled with late-stage carbonates or alteration products, such as chlorite and kaolin. Phenocrysts in the

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examined rocks are among the most common types in the basalts, including plagioclase, clinopyroxene, orthopyroxene, amphibole, and opaque minerals. The matrix is mainly composed of fine-grained clinopyroxene and plagioclase, with minor oxides. Some of the basaltic samples have a fumarole–amygdaloidal structure that is filled with limestone. The rhyolites are fresh or very weakly altered, and have well-preserved magmatic texture, structure, and mineral associations. The silicic rocks are commonly glassy or aphanitic, with a few phenocrysts of alkali feldspar, quartz, and fine quartzofeldspathic minerals as groundmass. The rhyolitic ignimbrite has a welded tuff texture and pearl-shaped cracks, and consists of quartz and K-feldspar crystal fragments, glass shards, and aphanitic volcanic ash. Some of the glass shards and volcanic ash 6

ACCEPTED MANUSCRIPT have devitrified to very fine-grained quartzofeldspathic minerals (Fig. 2).

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4. Analytical techniques

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Zircon U–Pb dating, Hf isotopes, whole-rock trace element and Sr–Nd isotope analyses were

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carried out at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), whole-rock major element analyses were conducted at the Institute of Geology and Geophysics,

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Chinese Academy of Sciences (IGGCAS). Fresh rock chips were powdered to < 200 μm for major

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and trace elemental concentrations and radiogenic isotope analyses. Sample disks were determined by a Phillips PW X–ray fluorescence spectrometer (XRF–2400) for major element compositions.

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Rare earth element (REE) and trace element concentrations were measured by an Inductively

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Coupled Plasma–Mass Spectrometer (ICP-MS). Zircons U–Pb dating was performed by using a

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New Wave UP193FX Excimer laser (New Wave Instruments, USA) coupled with an Agilent 7500a ICP-MS, while zircon in-situ Lu–Hf isotope analysis was obtained on a Nu Plasma II multi–collector inductively coupled plasma mass spectrometer (MC-ICP-MS，Nu Instruments Ltd.,

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UK), which is coupled to a New Wave Research UP193FX Excimer laser. Whole-rock Sr and Nd isotope analyses were conducted by using the Nu Plasma II MC-ICP-MS, Sr isotopes were normalized to 86Sr/88Sr =0.1194 while Nd isotopes were normalized to 146Nd/144Nd =0.7219. More information on the analytical details pertaining to the data obtained during the course of this study is present in the Appendix Analytical Methods.

5. Results

5.1. Geochemistry All of the samples in this study were collected far from any calcite vein. They do not show 7

ACCEPTED MANUSCRIPT large (>4 wt %) losses on ignition nor evidence (e.g., Ce anomalies) that might suggest any significant mobility of the LREE. Appendix Table 1 lists the whole-rock major element data

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obtained for representative samples collected from Liqiongda bimodal volcanic rocks. The

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samples are classified as basalts, basaltic andesites, and rhyolites, which show a clear Daly gap on the TAS diagram (Fig. 5a). The basaltic samples contain 48.4–55.7 wt.% SiO2, relatively high Al2O3 (generally>16 wt.%, with an average of 17.5 wt.%) and high CaO (averaging ~7.4 wt.%)

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abundances. They also show calc–alkaline affinities in terms of the AFM binary diagram (Fig. 5b).

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MgO abundances are 2.50 to 7.94 wt.% (mean = 4.71 wt.%), TiO2= 0.48–1.19 wt.% (mean = 0.86 wt.%) and Mg# (calculated as 100*Mg2+/(Mg2+ + Fe2+), Fe2+ considered as 0.8998 *Fe3+) ranges

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from 37 to 59, indicating a variably fractionated suite. Compared with the mafic rocks, the

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rhyolites are mainly characterized by higher SiO2 (74.8–79.5 wt.%), lower Al2O3 (11.7–14.3 wt.%),

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CaO (0.08–1.60 wt.%), MgO (0.20–0.28 wt.%) and Mg# values (28–40). They also show calc–alkaline affinities in the AFM diagram (Fig. 5b). On the SiO2–variation diagrams (Fig. 6 a–d),

SiO2.

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these rocks define liquid lines of descent evolving from basaltic rocks to rhyolites with increasing

The trace element compositions of basaltic rocks are characterized by variable REE concentrations (37–98 ppm; mean = 62 ppm), light REE enrichment ((La/Yb)CN= 2.60–7.65; the subscript CN refers to the chondrite normalized value, chondrite values are from Sun and McDonough (1989)), and high abundances of ferromagnesian trace elements (Sc, 14.4–56.7 ppm; Co, 9.07–42.3 ppm; Cr, 1.85–158 ppm; Ni, 5.47–42.6 ppm). Most of the samples lack a Eu anomaly or have a faint positive Eu anomaly (Eu/Eu* = 0.72–1.12) and show apparent depletion in Nb and Ta relative to La. All samples have much higher Th/Nb ratios (0.25–0.88) than the very 8

ACCEPTED MANUSCRIPT narrow range exhibited by N–MORB, E–MORB, and OIB (0.06–0.07, Sun and McDonough, 1989), suggesting features of subduction enrichment. The rhyolites have very low Sc = 0.88–1.77

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ppm, Cr = 1.14–2.54 ppm, Co = 0.24–0.42 ppm, Ni = 0.43–0.94 ppm, and enriched and variable

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concentrations of LILE (e.g. Ba = 43.4–92.7 ppm, Sr = 53.1–174 ppm). The total REE abundance of rhyolites varies from 102 to 158 ppm (mean = 126 ppm), with obvious enrichment in LREEs and highly refracted heavy REEs (HREEs) ((La/Yb)CN = 9.11–21.2, mean = 11.5), as well as deep

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Eu, Sr, Nb, and Ti anomalies. The strong negative Eu (Eu/Eu*=0.08–0.19) and Sr anomalies are

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consistent with plagioclase fractionation, either in the solid residuum left by partial melting or through fractional crystallization (Rollinson, 1993). The high, unfractionated HREE abundances

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preclude equilibration with either garnet or any other HREE–bearing phase. Although the rhyolites

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differ from the basalts in terms of their depletion in Sr, Eu, and Ti and enrichment in Th, their

7).

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primitive mantle–normalized patterns still show some similarities with those of the basalts (Fig.

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Whole rock Sr–Nd data for 10 intermediate–basic rocks and rhyolites from the Liqiongda bimodal suites are provided in Appendix Table A3 and are plotted in Fig. 8. The initial 87Sr/86Sr ratios, and εNd(t) values were calculated based on the U–Pb zircon ages (ca. 137 Ma) determined in this study. In general, all of the bimodal volcanic rocks in the study area have relatively homogeneous isotopic Sr and Nd compositions (Appendix Table A3). As shown in a plot of εNd(t) vs. (87Sr/86Sr)i (Fig. 8), the mafic rocks have initial

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Sr/86Sr ratios from 0.7048 to 0.7066 and

positive εNd(t) values (+5.38 – +7.47). The rhyolite samples possess a similar isotopic composition with initial 87Sr/86Sr ratios of 0.7017–0.7054 and a εNd(t) value of +5.26 – +6.71. However, LILEs (Rb and Sr) are mobile during alteration, and this will lead to the affected Sr isotopes (e.g. sample 9

ACCEPTED MANUSCRIPT TS1-24). Hence, the following discussion will focus on the Nd isotopes.

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5.2. Zircon U–Pb ages and Hf isotopes

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The zircons separated from the rhyolites and basaltic andesites were analyzed in order to 207

Pb analyses in

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obtain precise ages of the bimodal volcanism. Following the imprecise

Phanerozoic zircons, the more reliable weighted mean 206Pb/238U ages of the analyzed zircons are

weighted mean of

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adopted here. For each group of U–Pb isotope data for single samples (Appendix Table A1), a Pb/238 U age was calculated by means of the ISOPLOT program of Ludwig

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(2003). The results are placed on the Wetherill-type Concordia diagram with 1σ error (Fig. 3). The zircons from the basaltic andesite samples (TS3–08, TS3–09, TS3–11 and TS3–12) are

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mainly euhedral and transparent, with typical oscillatory zoning CL textures, and ranging from

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100 to 250 μm in size, with length/width ratios varying from 1:1 to 3:1. The zircons have high

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Th/U values (mostly>0.4, see Appendix Table A1) and yield weighted mean

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Pb/238U ages of

133.7 ± 0.6 Ma, 134.1 ± 1.3 Ma, 135.4 ± 1.5 Ma, and 137.9 ± 1.4 Ma. The zircons from six

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rhyolites samples (TS1–03, TS1–04, TS1–06, TS1–09, TS1–13 and TS1–15) are mostly clear, euhedral to subhedral, stubby to elongate prisms, with average crystal lengths of ~ 80–150 μm and length/width ratios up to 2:1. The zircons have high Th/U values (mostly>0.4, see Appendix Table A1) and yield weighted mean 206Pb/238U ages of 137.1 ± 1.2 Ma, 134.4 ± 0.8 Ma, 134.9 ± 0.7 Ma, 129.5 ± 0.7 Ma, 130.1 ± 0.6 Ma, and 130.3 ± 0.8 Ma. Zircons from 4 felsic rocks and 4 mafic rocks were chosen for Hf isotope analysis, and the results are listed in Appendix Table A2. Zircon analyses from the rhyolites have consistent initial 176

Hf /177Hf ratios, ranging from 0.282829 to 0.283156, with εHf(t) values from +4.9 to +16.4, and

model ages TDMC of 0.13 to 0.87 Ga (Fig. 4a and b). Zircon analyses from the basaltic andesites 10

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Hf/177Hf ratios of 0.282968–0.283151, with εHf(t) values of +10.0 to

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+16.3, and model ages TDMC of 0.14 to 0.56 Ga (Fig. 4a and b).

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6. Discussion

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6.1. Alteration effects

The loss-on-ignition values for basaltic and rhyolitic rocks range from 0.72–4.18 wt.% and

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1.10–2.32 wt.%, respectively. Thus, the effect of post-deposition alteration on elemental mobility

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needs to be considered prior to the processing and interpretation of geochemical datasets in the Liqiongda area. Alteration intensity was assessed by petrography and by applying several

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geochemical filters, such as indices estimating the degree of rare earth and major element mobility.

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Binary diagrams versus LOI, used as a proxy for alteration intensity, had no correlations of

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elevated LOI with mobility in major or trace element data. A number of elements with different geochemical behaviors, including TiO2, Nb, Th, Ce, V, Sr, Rb, Ba and Pb for these volcanic rocks, are plotted against Zr to evaluate their mobility during alteration. With the exception of the large

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ion lithophile elements (LILEs, such as Rb, Sr, Ba), the other elements correlate tightly with Zr, indicating that these elements are immobile during alteration (not shown). Therefore, the immobile elements (including the high field strength elements: Nb, Ta, Zr, Hf, Y, Th; the REEs; some transitional elements: Cr, Ni, Co, V, Sc; and some major elements: Ti and Al) are used for geochemical classification and petrogenetic discussion. 6.2. Petrogenesis The main issues of the Liqiongda volcanic rocks include explaining the petrogenesis of both mafic and silicic rocks, their relationships, and the reasons for the presence of compositional gap

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ACCEPTED MANUSCRIPT in the volcanic sequence. The data obtained in this study will be discussed with the aim of providing constraints on the genesis of the Liqiongda bimodal volcanic rocks and to explore the

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implications for Early Cretaceous volcanism in the southern Lhasa terrane.

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6.2.1. Origin of the mafic rocks

The high Nd–Hf isotopes of the Liqiongda basaltic rocks indicate that their parental magma

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was derived from the depleted mantle. However, the slightly variable MgO and Fe2O3 contents, low and the variable Cr and Ni concentrations imply that the mafic magmas of the Liqiongda area

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possibly experienced variable degrees of fractionation of olivine and clinopyroxene, from parental magmas either in magma chambers or during transport to the surface (Fig. 6; Appendix Table A3).

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The Liqiongda basaltic rocks have low MgO abundances, with Mg# ranging from 37 to 58, indicating evolved compositional features. In the SiO2 vs. MgO diagram (Fig. 6a), the Liqiongda

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basaltic samples are negatively correlated, suggesting fractional crystallization of olivine and clinopyroxene. The Al2O3/CaO ratios have an evident positive correlation with MgO abundance

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(Fig. 6e), suggesting that clinopyroxene fractionation was important. Amphibole fractionation was insignificant due to the absence of MREE depletion (Fig. 7c). The high Al2O3 contents and absence of Eu anomalies (Eu/Eu* = 0.72–1.12) in the basaltic rocks (Fig. 7c) argue against significant fractionation of plagioclase in the parent magma. In addition, there is not an obvious correlation between MgO and Ti or V (not shown), which argues against significant Fe-Ti oxide fractional crystallization. Mantle-derived melts are more or less contaminated by continental crust during their emplacement or storage in the magma chamber, as seen in the majority of cases of mantle-derived magmas erupted in a continent-related setting. Compared with some possible mantle reservoirs, 12

ACCEPTED MANUSCRIPT the Liqiongda basaltic rocks show higher incompatible element ratios, which are more close to those of continental crust. These observations imply that crustal materials were significantly their

origination,

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contamination

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or

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involved

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subduction-related enrichment. The most compelling evidence to distinguish them would be a correlation between isotopic compositions (Nd) and fractionation index (SiO2, MgO), since such a correlation would imply that changes in isotopic composition were produced during differentiation,

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while the magma was ascending and emplaced in the crust. However, there is no correlation

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between SiO2 and the isotopic compositions in this study (Fig. 8b). In this regard, significant crustal contamination can be ruled out, the signature of the basaltic group should be mainly

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inherited from their melting source and the crustal material might have been involved in the

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mantle by source mixing during the Cretaceous oceanic subduction.

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The basaltic rocks in this study have relatively high and homogeneous εHf(t) values (+10.0 to +16.3) and εNd(t) values (+5.38 to +7.47), indicating that the magmas largely originated from a

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depleted mantle source. HFSE and REE have similar partition coefficients for most mafic minerals, and their ratios generally remain stable during partial melting and fractional crystallization and thus can be used to reflect the nature of the mantle source (Weaver, 1991). In the Th/Yb versus Nb/Yb (Fig. 9a) diagram, the apparent sub-parallel trend of the data array to mantle array is due to melting of the wedge and is supportive of a subduction setting (Pearce, 2008). The Liqiongda basaltic rocks are displaced from the MORB array to higher Th values, indicating some input from the subducted slab and/or continental crust. Such characteristics are generally related to a supra–subduction setting where the residual depleted mantle was metasomatized by slab–derived fluids or silicate melts (Pearce and Peate, 1995; Turner et al., 1996). Moreover, the Liqiongda 13

ACCEPTED MANUSCRIPT basaltic rocks are enriched in LILEs (although altered) and LREE and depleted in HFSEs (e.g., Nb, Ta, and Ti), strongly favoring a fluid metasomatism in the mantle source, which was related to the

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deep subduction of the oceanic crust (Jahn et al., 1999; Pearce, 1983). The Nb–Ta negative

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anomalies can be explained either by the presence of rutile/amphibole during melting/dehydration processes in the subducted slab or the selectively low solubility of Nb and Ta in subduction fluids (McCulloch et al, 1991; Ringwood et al., 1990). Aqueous fluids released from the altered oceanic

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crust (AOC) are considered to be the main carriers of LILEs and H2O into the mantle wedge in

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most modern arc settings, and they are abundant in arc environments as a result of dehydration reactions in the down-going oceanic crust (Pearce, 1983; Pearce and Peate, 1995; Stolper and

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Newman, 1994). The high εNd(t) values (+5.38–+7.47), low Nb/Zr (0.01–0.04) and Th/Yb

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(0.29–1.49) ratios indicate that their source regions were MORB-like mantle enriched by

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AOC-derived fluids (Woodhead et al., 2001). Subducted sediment melts can also contribute to arc magmas as well as contribute the distinctive trace element signatures and isotopic features to arc

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systems (Plank and Langmuir, 1998). The linear trend of the Liqiongda volcanics in the Th/Yb vs. Th/Sm plot could be interpreted as mixing between N-MORB (or a partial melt thereof) and a partial melt of subducted sediment (Fig. 9b). The modeling result of two-component mixing indicates that the origin of the Liqiongda basaltic rocks can be explained by mixing with contributions of 5–10 % of global subducting sediments to attain the measured Sr–Nd isotopic composition (Fig. 8). Importantly, negative Nb, Ta, and Ti anomalies (Fig. 7), high Al2O3 contents (average >17 wt.%), and geochemical signatures similar to those of the high–alumina basalts erupted at Central-South Chile high Andes and Aleutian Island characterize these mafic rocks (Brophy et al., 14

ACCEPTED MANUSCRIPT 1999; Lopez-Escobar et al., 1977). The chemical features of HAB magma are a composite of both the source compositions and history of melting, extraction, ascent, and subsequent modification

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(Brophy and Marsh, 1986). Explanations for the origin of high-alumina magmas include partial

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melting of subducted oceanic crust ± pelagic/terrigenous sediment (Brophy and Marsh, 1986), extensive mafic phenocryst fractionation from a more primitive (MgO–rich, Al2O3–poor) mantle–derived melt (Brophy et al., 1999; Lopez-Escobar et al., 1977; Nicholls and Ringwood,

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1973; Perfit et al., 1980), and accumulation of plagioclase phenocrysts in more primitive

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calc–alkaline basalt (Crawford et al., 1987). Regardless of the controversial interpretations, the key factor for HAB genesis may relate to plagioclase accumulation. In many cases, the

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high-alumina basalts are rich in plagioclase crystals and commonly have a good correlation

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between wt.% Al2O3 and plagioclase abundance, whereas the aphyric basalts in magmatic arcs

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rarely contain more than 17 wt.% (Crawford et al., 1987; Perfit et al., 1980). The synchronous crystallization and convection in the basaltic magma chamber result in crystal sorting along both

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size and mineralogic lines, which lead to the preferential retention of low density plagioclase along the margins of a convecting basalt chamber (Marsh and Maxey, 1985). More plagioclase in the rocks would yield higher Al2O3, while the bulk rocks have low Mg#. The magmatic arc environments offer unique thermal characteristics that provide advantages to form the appropriate velocities of the convection and styles of crystallization to generate the high-alumina magmas (Brophy, 1989). Thus, high–alumina basalts prevail in the arc volcanic system. The mantle sources vary from garnet lherzolite, through spinel lherzolite, to plagioclase lherzolite, with a gradual reduction in depth (Gurenko and Chaussidon, 1995). Strongly fractionated La/Yb and Dy/Yb ratios are expected in the case of low-degree partial melting (<10%) of garnet peridotite (Xu et al., 15

ACCEPTED MANUSCRIPT 2005). In contrast, there is little change in the La/Yb ratios and the Dy/Yb ratios remain almost constant during low degrees of partial melting in the spinel stability field (Ding et al., 2003; Miller

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et al., 1999). The Liqiongda basalts mainly consist of low La/Yb ratios (3.62–10.67) and relatively

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constant Dy/Yb ratios (1.68–2.23), indicating various degrees of spinel peridotite facies melts rather than garnet-bearing facies melts. In addition, the TiO2/Yb ratios in mafic rocks can be affected by the presence of garnet in the melt residue, leading to extensive fractionation of the

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TiO2/Yb ratios derived from the deeper mantle in the garnet stability field (Pearce and Peate, 1995;

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Pearce, 2008). The studied basaltic samples have relatively constant TiO2/Yb ratios (0.34–0.87), consistent with the inference that their residual phase was garnet-poor. On the whole, although the

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limited geochemical data for the Liqiongda area are insufficient to develop detailed, realistic

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models for these basaltic rocks, our data suggest that the Liqiongda basaltic rocks are interpreted

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to have been derived from partial melting of the garnet-poor peridotite in the mantle wedge (MORB + sediment + fluid) and the subsequent fractional crystallization of olivine and pyroxene.

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6.2.2. Petrogenesis of the felsic rocks

The common age, similar geochemical composition, as well as close association in space, suggests that the basaltic and rhyolitic volcanic rocks in the Liqiongda area belong to a bimodal suite. The origin of the felsic rocks in a bimodal suite has remained a classic petrogenetic problem (Yoder, 1973). Two hypotheses have been proposed to account for the generation of silicic end-members. One hypothesis suggests that felsic rocks and associated basalts represent two genetically independent melts, with the basalts coming from the mantle and the silicic liquids being generated in the crust, by melting of either old crust or young underplated basalts (e.g., Bohrson and Reid, 1997; Davies and Macdonald, 1987). In this case, the mafic and felsic rocks 16

ACCEPTED MANUSCRIPT usually have different geochemical signatures and Sr–Nd isotopic ratios, with large scales of felsic rocks (Doe et al., 1982; Hildreth et al., 1991). The other hypothesis proposes continuous fractional

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crystallization, starting from the basaltic parents, possibly with a role for crustal contamination

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(Geist et al., 1995; Mungall and Martin, 1995). In this case, the mafic and felsic rocks share a common mantle–derived parental magma and have similar geochemical signatures and Sr–Nd isotopic ratios, with mafic rocks dominating the volume (e.g., Bonin, 2004; Geist et al., 1995).

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Because the Liqiongda basalts and rhyolites are interlayered in the same volcanic bodies, it is

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quite conceivable that they could have a common origin, i.e., it is probable that they originated within the same magma chamber and that the felsic volcanic rocks were derived from crystal

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fractionation of the primary basalt melt. The felsic rocks show trace element spider-diagrams and

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chondrite-normalized REE patterns (Fig. 7) similar to those of the mafic rocks, except for the

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negative Eu and Sr anomalies. The large negative Eu and Sr anomalies require major plagioclase fractionation (Rollinson, 1993). Fractionation of clinopyroxene and/or olivine could account for

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the positive correlations in the Cr vs. Ni diagrams (Fig. 6f). However, the fractional crystallization of mafic magmas alone may not be able to explain the centralized geochemical features of the high-silica Liqiongda rhyolites. Furthermore, it is hard to explain the fractionation of basaltic melts that generated these rhyolites without producing intermediate compositions (e.g., Luhr and Carmichael, 1985). An alternative hypothesis is that the partial melting of intermediate and mafic plutonic rocks related to Mesozoic magmatism could produce silicic melts in that had an appropriate chemical and isotopic composition to be parental to the Liqiongda felsic magma. Numerical modeling indicates that the felsic continental arc magmas can be generated by the interaction between hot, 17

ACCEPTED MANUSCRIPT hydrous, mantle-wedge-derived basaltic magmas and lower crustal lithologies in DCH zones (deep crustal hot zones; Annen et al., 2006) or MASH zones (melting, assimilation, storage, and

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hybridization; Hildreth and Moorbath, 1988) when mafic magmas repeatedly invade the lower

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crust. Under relatively high temperatures (~ 950 °C) and pressures (~ 1 GPa), dehydration melting of the mafic lower crust (hornblende) caused by the underplating of mafic magma could generate high-silica melts (Rapp and Watson, 1995). Furthermore, previous studies have revealed that low

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degree partial melting of gabbroic rocks can generate Fe-rich melts, leading to high Fe#

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[FeOT/(FeOT + MgO)] values (Frost et al., 2001). The high Fe# values (0.75–0.83) of the rhyolitic samples agree well with the model. The Liqiongda rhyolites have Sm/Nd ratios of 0.14–0.18,

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which are significantly lower than those of common mantle, whereas the Ce/Yb ratios are

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remarkably higher (>20), similar to those of the continental crust. The calculated zircon saturation

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temperatures reflect the temperature of the melt that produced rhyolites at the time of extraction from restite and may be considered to be the emplacement temperature of the magma so long as

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there is not significant cooling as the magma rises to the surface (Barrie, 1995). In this study, the Zr saturation temperatures indicate that the Liqiongda rhyolites have partial melting temperatures between 789 °C and 847 °C, suggesting that partial melting can be triggered by the underplating of high–temperature mantle-derived magma. The wide range of εHf(t) values (12 εHf units in individual samples) of zircons from the rhyolites can be explained by magma mixing between mantle-derived magma and a range of crustal components; hence, the underlying metasomatized mantle not only have provided heat for the crustal melting but may also have provided materials to the rhyolites (e.g., Shaw and Flood, 2009). The low Rb/Sr (0.17–1.20) and Rb/Ba ratios (0.32–1.47) indicate little clay-rich sedimentary contribution to the magma (Sylvester, 1998). 18

ACCEPTED MANUSCRIPT Therefore, the geochemical and isotopic compositions of the Liqiongda rhyolites argue for an origin from the partial melting of the juvenile mafic lower crust, and during their ascent to the

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surface, the melts experienced extensive fractional crystallization.

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6.2. Tectonic setting and implications

The early Cretaceous (ca. 137–130 Ma) bimodal volcanic rocks in the Liqiongda area are

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characterized by enrichment in LILEs (although altered) and LREEs, depletion in HFSEs (e.g. Nb,

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Ta and Ti), and εHf(t) and εNd(t) values, suggesting that they are the products of arc-related magmatism, and that the magma source have undergone modification by subducted slab–derived

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fluids and sediment–related melts. The isotopic and trace element variations of the basalts could

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be interpreted in terms of MORB-like end-member and subsequently contaminated by a small

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amount of crust–derived component.

Mafic rocks in mature island arcs and/or active continental margins are generally calc–alkaline in character; enriched in LILE, Th, and LREE; depleted in Nb and Ti; and have

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variable εNd(t) values (Pin and Paquette 1997). Bimodal volcanic suites occur in both separating plate environments with–plate and destructive plate margin environments and can genetically be related to (1) continental spreading, (2) crustal areas undergoing extension, (3) crustal areas above mantle hot spots, and (4) continental terranes above subduction zones. Corresponding to the different tectonic settings, the bimodal magmatic suites exhibit distinct geochemical characteristics and diagnostic relationships between the mafic and felsic end–members (Bonin, 2004; Pin and Paquette, 1997). A combined, immobile trace element and isotopic approach can place useful constraints on the petrogenesis of these associations and contribute to the

19

ACCEPTED MANUSCRIPT interpretation of their initial tectonic settings (Pin and Paquette, 1997). In the case of the Liqiongda area, the geochemical characteristics of Early Cretaceous (ca. 137–130 Ma) bimodal

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volcanic rocks suggest that they are commonly related to a supra–subduction setting, where the

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residual depleted mantle was metasomatized by slab–derived fluids and sediment–derived melts. In this study, the Ti/V ratios of the mafic members are between 12 and 20, consistent with the Ti/V ratios of IAB between 12 and 20 (Fig. 10a, Shervais, 1982). This observation is confirmed by

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other tectonic environment discrimination diagrams, such as the Hf/3–Th–Ta diagram, in which

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the Liqiongda basalts fall in the field of continental island–arc calc–alkaline basalts (Fig. 10b, Wood, 1980). This observation is also supported by the Th/Ta versus Yb discrimination diagram,

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in which the Liqiongda rhyolites indicate an active continental margin affinity (Fig. 10c, Gorton

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and Schandl, 2000). Nb and Y were selected as efficient discriminants between most types of

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ocean–ridge granite, within–plate granite, volcanic–arc granite, and syn–collisional granite (Pearce et al., 1984). In this diagram, the Liqiongda felsic volcanic rocks plot in the ‘volcanic–arc

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granite or syn-collisional granite’ setting field (Fig. 10d). Sedimentary research shows that the lower Cretaceous strata in the southern Lhasa terrane consist of mudstone, quartzose sandstone, and subordinate quartzite-pebble conglomerate beds that were deposited in shallow marine and fluvial environments (Leier et al., 2007). Hence, it appears that the Liqiongda volcanic rocks are most likely produced in an active continental margin setting. To date, the tectonic evolution of the southern Lhasa terrane during the Late Jurassic–Early Cretaceous remains controversial (Coulon et al., 1986; Dai et al., 2015; Ji et al., 2009b; Kapp et al., 2007; Ma et al., 2013 a, b; Wen et al., 2008; Zhang et al., 2012; Zhu et al., 2009; 2013). An intra-oceanic subduction system to which the Zedong terrane belonged was thought to be active 20

ACCEPTED MANUSCRIPT during the Late Jurassic to Early to mid Cretaceous (Aitchison et al., 2000; McDermid et al., 2002). However, the Liqiongda volcanic rocks have clear calc-alkaline features and could have

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been formed in an active continental margin arc rather than in an intra-oceanic arc setting.

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Furthermore, through a detailed geochemical and geological comparison, L.-L.Zhang et al. (2014) considered the Zedong terrane to be a part of the Gangdese arc as a response to the northward subduction of the Neo-Tethyan ocean slab rather than the remnant of an intra-oceanic arc. The

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double subduction model explains the magmatic data from the Lhasa-Qiangtang collision zone

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(Zhu et al., 2013). However, the southward subduction of the Bangong-Nujiang oceanic slab may have limited its impact on the tectonic evolution of the southern Lhasa terrane. Many previous

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studies have identified the Late Triassic–Early Jurassic arc-related magmatic rocks along the south

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margin of the Lhasa terrane (Chu et al., 2006; Ji et al., 2009 a, b; Kang et al., 2014; Wang et al.,

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2016; Zhu et al., 2008). The previously reported Cretaceous Sangri Group (Bima Formation) volcanics actually erupted during the Early Jurassic (~195–177 Ma) and exhibit similar arc-related

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calc-alkaline volcanic rocks as the Yeba Formation (Kang et al., 2014). The widespread Early Jurassic magmatic activities can not be reasonably explained by the southward subduction of the Bangong-Nujiang oceanic slab. Therefore, the northward subduction of the Neo-Tethyan oceanic slab may have developed since the Late Triassic. Because of the long duration of subduction (and therefore abrasion), the subduction segment may be near the critical dip beyond which pressure forces cannot be balanced and near-horizontal subduction occurs (Jarrard, 1986; Tovish et al., 1978). Combined with the moderate convergence rate of the Indian plate relative to Eurasia (~ 6 cm/yr) during the Early Cretaceous (van Hinsbergen et al., 2011), a high-angle subduction model seems questionable. Different subduction styles will lead to distinct tectonic consequences in 21

ACCEPTED MANUSCRIPT terms of the magmatic arc evolution and deformation in the upper plate. For example, flat-slab subduction will cause abnormal magmatic arc evolution (landward migration, magmatic lull,

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composition changes, etc.) and strong lateral deformation in the upper plate (fold/thrust belt,

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basement-core faulting), whereas high-angle subduction will most likely lead to the opposite tectonic consequences. From the temporal and spatial distribution of the magmatic rocks in the Lhasa terrane, the Late Triassic–Early Cretaceous (205–137 Ma) magmatic rocks are mainly

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distributed across the southern Lhasa terrane, but are absent in the northern Lhasa terrane; Early

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Cretaceous (130–110 Ma) magmatic rocks are prevalent in the northern Lhasa terrane and rare in the southern Lhasa terrane (Ji et al., 2009 a, b; Kang, 2009; Ma et al., 2013a; Zhang et al., 2012;

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Zhu et al., 2013 and references therein). Overall, the distribution of Mesozoic arc magmatism of

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Lhasa terrane shows that the magmatism became younger from south to north (landward migration)

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and that the center of magmatic movement was to the north and intensified during the Early Cretaceous in the northern Lhasa terrane. Moreover, numerous Early Cretaceous leucogranites

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were mapped in the Chazi, Wenbu, and Nyainqentanglha areas of the central Gangdese block, indicating that the Gangdese block underwent crustal thickening associated with subduction of the Neo-Tethyan Ocean and followed exhumation during ~140–130 Ma (Ding and Lai, 2003). Combined with extensive marine transgression, extensional deformation, and post-orogenic magmatism in the central Lhasa terrane, Zhang et al. (2012) suggested that the Andean-type orogen broke down due to extension resulting from lithospheric delamination and asthenospheric upwelling during the Early Cretaceous. However, many lines of evidence suggest that the northern part of the Lhasa terrane during the Early Cretaceous was in a compressional environment and characterized by active thrust faults (Ding and Lai, 2003; Kapp et al., 2005, 2007; Murphy et al., 22

ACCEPTED MANUSCRIPT 1997). The sedimentary characteristics of Lower Cretaceous strata in the Lhasa terrane are most consistent with a peripheral foreland basin setting (DeCelles et al., 2007; Leier et al., 2007). Hence,

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this evidence favors a flat-slab/low-angle subduction model. It is beyond the scope of this paper to

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review the timing of each geological transition and how these might have occurred. However, it is worth considering whether these transitions might correspond with specific geological events. In the Late Triassic (Norian), the Neo-Tethyan oceanic slab began to subduct northward at a normal

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angle beneath the south Lhasa terrane, and a relatively mature active continental margin developed

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with widespread arc-related magmatism along the southern margin of the Lhasa terrane during the Early Jurassic. Due to the long duration of subduction and the possible landward mantle flow, the

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subduction angle became gentler and caused the magmatic front to move farther north in the Early

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Cretaceous (approximately 150–140 Ma). Then, the subsequent collision between the Lhasa and

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Qiangtang terrane may have accelerated the rate of subduction of the Neo-Tethyan oceanic slab beneath the Lhasa terrane and finally culminated in the early Late Cretaceous magmatic flare-up

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ca. 100–80 Ma (DeCelles et al., 2007; Ma et al., 2013a). The low-angle subduction model seems contradictory with the presence of Liqiongda volcanics and Mamen adakites. Many geophysical and geochemical observations have already provided insights into the presence of slab tearing and the subsequent development of the slab window (e.g., Ferlito, 2011; Hu and Liu, 2016; Miller et al., 2006; Shi et al., 2016; L.-Y. Zhang et al., 2014). The process of slab tearing creates gaps in the lithosphere that localize asthenospheric upwelling, thus triggering magmatic activity that could potentially deviate from the geochemistry of typical subduction-related magmas (e.g., Guivel et al., 2006). Ferlito (2011) proposed that along the Aleutian-Kamchatka subduction zone, the Pacific Plate tears apart and produces adak-type 23

ACCEPTED MANUSCRIPT andesites and high-Al basaltic rocks. By simulating the two flat slabs in Peru and Chile using data-orientated geodynamic models, Hu and Liu (2016) concluded that these slabs are internally

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torn and that the small-scale presence of adakitic magmatism is closely related to the slab tearing.

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L.-Y. Zhang et al. (2014) considered the temporal and spatial distribution of Oligo-Miocene adakitic magmatism in southern Tibet to be the result of the progressive tearing of the Indian Plate. Receiver-function imaging also indicates a transition from low-angle to steep subduction of the

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Indian Plate at 91.5 °E (Shi et al., 2016). Similar processes may have occurred in the Neo-Tethyan

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slab during the Cretaceous, and generated Liqiongda bimodal volcanic rocks and Mamen adakite-like rocks. The formation of the tear in the Neo-Tethyan slab and the subsequent

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development of the slab window can be attributed to the presence of a transform fault or a

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mechanical weakness, such as a fracture zone. The upwelling asthenosphere beneath the tearing

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triggered melting of the subducting Neo-Tethyan oceanic crust and metasomatized the lithospheric mantle. The interaction between the subducted Neo-Tethyan oceanic slab and peridotite in the

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mantle wedge generated the adakitic melts (Kang, 2009; Zhu et al., 2009). Moreover, partial melting of the metasomatized lithospheric mantle and the subsequent fractional crystallization produced the Liqiongda basaltic rocks. Meanwhile, part of the mafic magma underplated the lower crust, triggering the melting of the juvenile lower crust underneath the southern Lhasa terrane and resulting in the eruption of the rhyolitic rocks.

7. Conclusions

According to LA-ICP-MS zircon U–Pb dating and whole-rock geochemical and isotope data presented, the Early Cretaceous volcanism that crops out in the Liqiongda area is characterized by a cal–alkaline bimodal suite: basaltic rocks and rhyolites (137–130 Ma). The mafic magmas, with 24

ACCEPTED MANUSCRIPT high aluminum (average >17 wt.%), flat REE patterns, presence of prominent negative Nb and Ta anomalies, (87Sr/86Sr)i in the range 0.7048–0.7066, and positive εNd(t) values (+ 5.38–+ 7.47), are

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related to low pressure melting of a mantle wedge in an active continental margin setting. Weak

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contamination by a crustal component is apparent in the genesis of these basic lavas. The felsic rocks show similar geochemical features with the basic lavas at their origin, except for more evolvement of crustal components (εHf(t) = +4.3–+16.4; εNd(t) = +5.26–+6.71). The rhyolites show

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a slight depletion of MREE relative to HREE and large negative Eu anomalies (Eu/Eu* =

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0.08–0.19). Based on the outcrop field relationships, the bimodality of the suite, and the geochemical and isotopic evidence, we propose that the petrogenesis of the Liqiongda Early

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Cretaceous bimodal basalt–rhyolite association was initiated by partial melting of the

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metasomatized upper mantle wedge, which produced a primitive mafic melt that was probably

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influenced by slab–derived fluids and sediment–derived melts. The primary mafic melt intruded into a higher level of the juvenile continental crust similar in composition to the associated basalts,

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whose partial melting produced these calc–alkaline rhyolites. The projection of different elements characterizing the geochemistry of this Early Cretaceous magmatism into active continental margin tectonomagmatic diagrams indicates a subducted setting for the genesis of magmatism. Our results confirm the presence of Early Cretaceous volcanism in the southern Gangdese Belt. Combined with the distribution of the contemporary magmatism, deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo-Tethyan oceanic lithosphere was flat-lying beneath the Lhasa terrane during the Early Cretaceous.

Acknowledgements

We would like to thank Prof. Sun-Lin Chung, Dr. Jin-Gen Dai and one anonymous reviewer 25

ACCEPTED MANUSCRIPT for their constructive and thoughtful comments. We are grateful to Jing Xie, Shou-qian Zhao, Yu-qiong Wang, and Ya-li Sun for their assistance with CL imaging, trace elements and Sr–Nd

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isotope analysis. We thank Ren-Deng Shi, Fu-Long Cai, Houqi Wang for useful discussion on this

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early manuscript. We also acknowledge Qiang Xu, Shun Li and Suo-Ya Fan participated in the geologic field survey in the southern Tibet. This study was financially supported by the Chinese Academy of Sciences (XDB03010401 to Ding), the National Natural Science Foundation of China

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(41490610 to Ding).

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Figure captions

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Fig. 1. (a) Tectonic outline of the Tibetan Plateau; (b) Tectonic framework of the Lhasa Terrane showing major tectonic subdivisions and distribution of Early Cretaceous magmatism

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(modified after Ding and Lai, 2003; Kapp et al., 2005; data source are from reference: Ding

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and Lai, 2003; Kang, 2009; Kapp et al., 2005; 2007; Zhu et al., 2009; 2011; this paper); (c) Simplified geological map of the Liqiongda area (modified from 1:250000 geologic map); (d)

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Schematic cross section across section A shown in Fig. 1. (c).

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Fig. 2. Representative field photos and photomicrographs of rocks from bimodal volcanic rocks in

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the Liqiongda area: (a) photos of part of section A; (b) volcaniclastic rock; (c–d) hand specimen and photomicrographs of rhyolite; (e–f) hand specimen and photomicrographs of basalt. R means rhyolitic rocks while B means basaltic rocks. Scale bar are equal to 500 μm.

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Fig. 3. LA-ICP-MS zircon U–Pb concordia diagrams of the Liqiongda area, southern Tibet. Red circles mean ages without being calculated in the weighted average age. Representative cathodoluminescence (CL) images of zircons are also shown with U–Pb ages and εHf(t) values (scale bar =100 μm). U–Pb analysis spots are shown by solid circles, and Hf isotope spots are shown by dashed circles. Fig. 4. Zircon Hf isotopic compositions of studied volcanic rocks in the Liqiongda area. All these zircons are characterized by high positive εHf(t) values and young Hf model ages. (a) Zircon U–Pb age–εHf(t) variations; (b) histograms of the TDMC ages. The green solid squares represent basaltic rocks and orange solid circles represent rhyolitic rocks. 40

ACCEPTED MANUSCRIPT Fig. 5. Total alkalis (K2O+Na2O) vs. SiO2 (a), and AFM (b) diagram for volcanic rocks. Data from the Early Cretaceous adakite-like rocks of the southern Gangdese Belt are cited from Zhu et

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al. (2009).

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Fig. 6. Selected chemical variation diagrams to illustrate fractional crystallization of the Liqiongda volcanics. Data from the Early Cretaceous adakite-like rocks of the southern Gangdese Belt are cited from Zhu et al. (2009).

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Fig. 7. Chondrite-normalized REE patterns (a and c) and primitive mantle-normalized trace

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elements spider diagrams (b and d) for the rhyolite and basalt, respectively. The chondrite values, primitive mantle values, N-MORB and E-MORB values are from Sun and

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McDonough (1989). Data from the Early Cretaceous adakite-like rocks of the southern

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Gangdese Belt are cited from Zhu et al. (2009).

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Fig. 8. (a) εNd(t) vs. (87Sr/86Sr)i and (b) εNd(t) vs. SiO2 diagrams for the felsic and mafic rocks from the Liqiongda area. Data sources are as follows: Neo-Tethyan ophiolites (Zhang et al., 2005)

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and references therein, marine sediments (Plank and Langmuir, 1998; Ben Othman et al., 1989), MORB from the Yarlung Zangbo suture zone (DZ98-12D, Nd = 7.65 ppm, εNd(t) = +8.9, Sr = 128.0 ppm, 87Sr/86Sr = 0.7045,143Nd/144Nd=0.5131; Dai et al., 2013; Zhang et al., 2005), Global Subducting Sediment (GLOSS; Sr=327 ppm, Nd=27 ppm,

87

Sr/86Sr =0.7173,

143

Nd/144Nd =0.5122 , Plank and Langmuir, 1998), the lower crust (Sr=300 ppm, Nd=26 ppm,

87

Sr/86Sr=0.7100,143Nd/144Nd=0.5115, Miller et al., 1999), the Amdo orthogneiss ( Nd=34

ppm,

87

Sr/86Sr=0.7326,143Nd/144Nd=0.5121, Harris et al., 1988). Data from the Early

Cretaceous adakite-like rocks of the southern Gangdese Belt are cited from Zhu et al. (2009). Fig. 9. (a) Th/Yb versus Nb/Yb. (b) Th/Yb versus Th/Sm. Field for Kamchatka lavas is from 41

ACCEPTED MANUSCRIPT Kepezhinskas et al. (1997). Data for N-MORB and GLOSS are taken from Sun and McDonough (1989) and Plank and Langmuir (1998). Field for Linzizong volcanics &

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Gangdese batholith is from Mo et al. (2008), Ji et al. (2009a) and references therein. Field for

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basalts in YZS ophiolite and Xigaze peridotites are from Zhang et al. (2005) and Dai et al. (2013).

Fig. 10. Tectonic discrimination diagrams for bimodal volcanic rocks in the Liqiongda area. (a)

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Ti–V diagram for mafic-member rocks (Shervais, 1982); (b) Hf/3–Th–Nb/16 tectonic settings

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discrimination diagram for the basalts (after Wood, 1980); (c) Yb versus Th/Ta diagram (after Gorton and Schandal, 2000) for Liqiongda rhyolites; (d) Nb–Y diagram (after Pearce et al.,

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Supplementary Figure captions

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1984).

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Appendix Fig. A1. Field photo of section A showing in the Fig. 1d.

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Supplementary Table captions

Appendix Table A1. Zircon LA-ICP-MS U–Pb in-situ analyzing results for the Liqiongda volcanic rocks.

Appendix Table A2. LA-MC-ICPMS zircon Hf isotopes of volcanic rocks from Liqiongda area. Appendix Table A3. Whole-rock major, trace element and Sr–Nd isotope data of the volcanic rocks from the Liqiongda area.

Supplementary Analytical Methods

Detailed methods for zircon in-situ U–Pb–Hf isotopic analysis and whole-rock major, trace element ad Sr–Nd anglysis. 42

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Figure 10

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Highlights

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An Early Cretaceous (137–130 Ma) bimodal volcanic event was identified in the

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southernmost margin of the Lhasa terrane.

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Basaltic rocks were derived from a depleted mantle metasomatized by slab-derived components.

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Rhyolites were produced by partial melting of juvenile lower crust.

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The bimodal volcanic suites were likely attributed to the tearing of subducted

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Neo-Tethyan slab.

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