Early Jurassic volcanic rocks in the Xiongcun district, southern Lhasa subterrane, Tibet: Implications for the tectono-magmatic events associated with the early evolution of the Neo-Tethys Ocean

Lithos 340–341 (2019) 166–180

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Early Jurassic volcanic rocks in the Xiongcun district, southern Lhasa subterrane, Tibet: Implications for the tectono-magmatic events associated with the early evolution of the Neo-Tethys Ocean Xinghai Lang a,b,⁎, Xuhui Wang a,⁎, Yulin Deng a, Juxing Tang c, Fuwei Xie a, You Zhou a, Yong Huang d, Zhuang Li c, Qing Yin a, Kai Jiang e a

College of Earth Science and MLR Key Laboratory of Tectonic Controls on Mineralization and Hydrocarbon Accumulation, Chengdu University of Technology, Chengdu 610059, China State Key Laboratory of Continental Dynamics, Northwest University, Xi'an 710069, China c Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China d Chengdu Center of China Geological Survey, Chengdu 610081, China e Tibet Tianyuan Mineral Exploration Co. Ltd., Shigatse 857000, China b

a r t i c l e

i n f o

Article history: Received 6 December 2018 Accepted 10 May 2019 Available online 17 May 2019 Keywords: Xiongcun Formation Intra-oceanic island arc Early Jurassic Neo-Tethys Lhasa terrane

a b s t r a c t Early Mesozoic (Late Triassic to Middle Jurassic) igneous rocks in the southern Lhasa subterrane, Tibet, record important information on the early tectono-magmatic evolution of the Neo-Tethys Ocean. This paper presents petrological, geochronological, and geochemical data for the Xiongcun Formation ignimbrites, to constrain the petrogenesis and tectonic setting of these rocks in the southern Lhasa subterrane. The Xiongcun Formation is dominated by andesitic and dacitic rocks. These rocks yield Early Jurassic (195–176 Ma) zircon U\\Pb ages and display typical arc-like geochemical signatures, being enriched in large-ion lithophile elements (e.g., Rb, Ba, and U) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). Sr–Nd–Pb–Hf isotopic compositions are highly depleted. These chemical and isotopic data suggest that the Xiongcun Formation ignimbrites probably formed in an intra-oceanic island arc setting related to the northward subduction of the Neo-Tethys oceanic slab. The parental magmas were derived from a depleted mantle source modified by fluids released from the NeoTethys oceanic slab. Combined with previously reported data, we suggest that the volcanic rocks in the Early Jurassic Xiongcun and Yeba formations of the southern Lhasa subterrane were generated by a single subduction system, but developed on oceanic and continental crust, respectively. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The southern Lhasa subterrane consists mainly of intermediate– silicic intrusions and associated volcanic–sedimentary rocks, which have long been considered to be related to northward subduction in the Neo-Tethys Ocean and India–Asia collision. The geodynamic setting and petrogenesis of Cretaceous and Cenozoic igneous rocks in this subterrane are well documented (e.g., Chu et al., 2006, 2011). However, the tectonic setting of the early Mesozoic igneous rocks within this subterrane remains controversial, with previous studies proposing a continental arc (e.g., Ma et al., 2017a; Meng et al., 2016a, 2016b; Wang et al., 2016; Zhu et al., 2008) or an intra-oceanic island arc (e.g., Aitchison et al., 2000, 2007; Lang et al., 2019; Ma et al., 2017b, 2018; McDermid et al., 2002; Tang et al., 2015). This uncertainty limits ⁎ Corresponding authors at: College of Earth Science, Chengdu University of Technology, No. 1, East 3rd Road, Erxianqiao, Chenghua District, Chengdu 610059, China. E-mail addresses: [email protected] (X. Lang), [email protected] (X. Wang).

https://doi.org/10.1016/j.lithos.2019.05.014 0024-4937/© 2019 Elsevier B.V. All rights reserved.

our understanding of the early Mesozoic tectonic evolution of this subterrane. Two isolated Early–Middle Jurassic volcanic–sedimentary sequences have been identified in the southern Lhasa subterrane: the Yeba Formation (YF) (e.g., Liu et al., 2018; Wei et al., 2017; Zhu et al., 2008) and the Xiongcun Formation (XF) (Lang et al., 2019; Tang et al., 2007). These formations are located in the northeast and at the southwestern margin of the southern Lhasa subterrane, respectively (Fig. 1c). The YF volcanic rocks are considered to have formed in arc (Early Jurassic) or initial back-arc rifting (Middle Jurassic) settings associated with northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane (e.g., Liu et al., 2018; Wei et al., 2017; Zhu et al., 2008). Lang et al. (2019) proposed that the XF sandstones were deposited in an intraoceanic island arc setting during the Early–Middle Jurassic, although the petrogenesis of the XF volcanic rocks has not yet been studied in detail. The relationships and tectonic links between the two volcanic– sedimentary sequences are unclear. In this paper, we present precise zircon U\\Pb ages, whole-rock geochemical and Sr–Nd–Pb isotopic data, and in situ zircon Hf isotopic

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Fig. 1. (a) Simplified map show the location of the Lhasa terrane; (b) simplified regional geological map of the Lhasa terrane, showing the location of the study region (modified from Zhu et al., 2011); and (c) geological map of the study region, showing the distribution of Mesozoic volcanic rocks (modified from Lang et al., 2019). Age data are from Chen et al. (2009); Geng et al. (2006); Liu et al. (2018); Wei et al. (2017); Zhu et al. (2008).

compositions of the XF ignimbrites (XFI). Combined with previous results, these new data enhance our understanding of the petrogenesis and tectonic setting of the XF volcanism, and of the early Mesozoic tectono-magmatic events associated with the early evolution of the Neo-Tethys Ocean in the southern Lhasa subterrane. 2. Geological background The Lhasa terrane is bounded to the north by the Bangong–Nujiang Suture Zone (BNSZ) and to the south by the Yarlung–Zangbo Suture Zone (YZSZ) (Fig. 1b; Yin and Harrison, 2000). It is divided into northern, central, and southern subterranes by the Shiquan River–Nam Tso Mélange Zone (SNMZ) and the Luobadui–Milashan Fault (LMF) (Fig. 1b; Zhu et al., 2011). The present study is focused on the southern Lhasa subterrane, which comprises juvenile crust that records the tectonic evolution from the Mesozoic accretionary orogeny (Hou et al., 2015). This orogeny was related to subduction of the Neo-Tethys oceanic slab since the Late Triassic and India–Asia collision during the early Cenozoic (Chu et al., 2006, 2011; Ji et al., 2009; Yin and Harrison, 2000). South of the southern Lhasa subterrane, the YZSZ is a subduction complex that preserves a record of the events in the Neo-Tethys. From north to south, the YZSZ comprises three main lithotectonic units: (1) the Xigaze forearc basin; (2) the Yarlung–Zangbo ophiolitic belt and associated mélange; and (3) an accretionary prism (Dai et al., 2011). Magmatism in the southern Lhasa subterrane occurred mainly between the Cretaceous and Paleogene, resulting from northward subduction or possible break-off of the Neo-Tethys oceanic slab beneath the Lhasa terrane (Ji et al., 2009; Mo et al., 2008). Early Mesozoic igneous rocks are relatively rare in this subterrane and comprise mainly the YF (e.g., Liu et al., 2018;Wei et al., 2017; Zhu et al., 2008), Sangri Group (Chen et al., 2019; Kang et al., 2014, 2015; Wang et al., 2016), XF (Lang et al., 2019; Tang et al., 2007), and sporadically distributed

intermediate–silicic calc-alkaline intrusive rocks (Guo et al., 2013; Ji et al., 2009; Lang et al., 2014; Ma et al., 2017a; Meng et al., 2016a; Xie et al., 2018), which form a 600-km-long magmatic arc between 88°E and 94°E (Fig. 1c; Hou et al., 2015). The YF crops out in the northeast of the southern Lhasa subterrane, extending ~250 km in an E–W direction from Dazi to Milin County (Fig. 1c). The YF volcanic sequence is bimodal, being dominated by basaltic and silicic volcanic rocks with rare intermediate rocks. The sedimentary units that occur mainly in the uppermost section of the YF comprise fine-grained sandstone, calcareous slate, and bioclastic limestone interbedded with siliceous rocks (Wei et al., 2017; Zhu et al., 2008). Zircon U\\Pb age data show that the YF volcanic rocks are Early–Middle Jurassic in age (192–168 Ma; Geng et al., 2006; Zhu et al., 2008; Chen et al., 2009; Wei et al., 2017; Liu et al., 2018). YF volcanism has been interpreted as related to northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane. The basaltic and silicic volcanic rocks of the YF were generated by partial melting of the mantle and juvenile basaltic lower crust, respectively (Liu et al., 2018; Wei et al., 2017; Zhu et al., 2008). The Sangri Group volcanic rocks are discontinuously distributed between Dazhuka and Sangri County along the southern margin of the Lhasa terrane (Fig. 1c). This group comprises the lower Mamuxia Formation and upper Bima Formation. The Mamuxia and Bima formations were considered to be Late Jurassic and Early Cretaceous in age, respectively (Zhu et al., 2009). However, recent zircon U\\Pb dating indicates that the upper Bima Formation formed in the Early–Middle Jurassic (Chen et al., 2019; Kang et al., 2014, 2015), whereas the lower Mamuxia Formation formed during the Early Cretaceous (Kang et al., 2015; Zhu et al., 2009). These dating results are inconsistent with field relationships (i.e., the Mamuxia Formation is conformably overlain by the Bima Formation) (Zhu et al., 2009). In addition, Wang et al. (2016) identified Middle–Late Triassic (237–212 Ma) volcanic rocks of the Sangri Group in Changguo village. Therefore, more field work and

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Fig. 2. (a) Geological map of the Xiongcun district, showing the sampling locations (modified from Lang et al., 2014); and (b) A–B cross-section in the Xiongcun district, showing the sampling locations (XC-5, XC-6 and XC-7). The location of the A–B cross-section is shown in Fig. 2a.

geochronological data are needed to constrain the nature and age of the Sangri Group. The XF is mainly exposed near Xiongcun Village in Xietongmen County, within the western section of the Jurassic arc in the southern Lhasa subterrane (Fig. 1c). The XF is divided into three volcanic– sedimentary sections (Figs. 2 and 3) (Lang et al., 2019; Tang et al., 2007). The lower section consists of volcanic agglomerate and breccia, with minor lavas, ignimbrites, sandstones, and siltstones interbedded with argillites (Figs. 3 and 4a–c). The middle section comprises ignimbrites and minor lavas, and siltstones interlayered with argillites (Figs. 3 and 4d). The upper section comprises sandstones and siltstones interbedded with argillites, with minor ignimbrites, conglomerates, slates, and limestones (Figs. 3 and 4e–h). The siltstone contains wellpreserved plant fossils (Ptilophyllum spp.) (Fig. 4i). The XF volcanic– sedimentary rocks have been intruded by intrusive rocks (Figs. 2 and 4c) of Jurassic and Eocene age (Fig. 2a; Lang et al., 2014, 2017, 2018). The Jurassic intrusions include Early–Middle Jurassic quartz dioritic porphyry and diabase dikes (Lang et al., 2014, 2018). The Eocene intrusions include biotite granodiorite, quartz diorite, granitic aplite dikes, and

lamprophyre dikes (Lang et al., 2017). A super-large porphyry copper–gold district (i.e., the Xiongcun district) has been discovered in the XF and associated porphyries (Fig. 2a; Lang et al., 2014; Tang et al., 2015). 3. Sample descriptions and analytical methods All samples analyzed in this study were collected from outcrops and drill holes in the Xiongcun district (Fig. 2a). In the outer peripheral (or distal) zone, ~500 m from the deposits, the XF volcanic rocks are unaltered or weakly sericitized with carbonate alteration (Fig. 5b). In the inner (or proximal) zone, the rocks near the deposits show strong pervasive hydrothermal alteration, such as chloritization and sericitization (Fig. 5a). Hand specimens of XF volcanic rocks are gray–black in color, with a welded tuffaceous texture and flow-like structures. Phenocrysts of feldspar and quartz are visible in hand specimen (Fig. 5b). These samples contain phenocrysts of plagioclase, quartz, and minor hornblende, in addition to accessory minerals such as Fe\\Ti oxides, zircon, and apatite,

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Fig. 3. Stratigraphic column of the XF (modified from Lang et al., 2019).

set in a fine- to very fine-grained groundmass (Fig. 5c–d). Therefore, these samples are inferred to be ignimbrites. All samples were unaltered or weakly altered, and weathered surfaces were removed before analysis. The samples contain almost no exotic lithic fragments and represent the original magma composition. After petrographic examination, samples XC-13 and -14 were selected for zircon U\\Pb dating by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) and zircon Hf isotopic analysis. Twelve samples (XC-5 to -10 and XC-15 to -20) were selected for major and trace element analyses. Samples XC-6 and XC-8 to -10 were

also used for Sr–Nd–Pb isotopic analyses. The analytical methods and results are provided in the supplementary material. 4. Results 4.1. Zircon U\\Pb age data We determined the ages of zircons separated from two ignimbrite samples (XC-13 and -14). Cathodoluminescence (CL) images of the zircon grains are shown in Fig. 6. Most of the zircons are transparent,

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Fig. 4. Field photos of the XF volcano–sedimentary rocks. (a) volcanic agglomerate, (b) volcanic breccia, (c) lava, (d) diabase dikes intruded into the ignimbrite, (e) sandstone, (f) sandstone and siltstone interlayered with argillite, (g) slate, (h) limestone, and (i) plant fossil (Ptilophyllum) preserved in siltstone.

colorless to slightly brown, and display oscillatory zoning typical of magmatic growth. They are mainly euhedral with long to short prismatic shapes, average lengths of 80–150 μm, and length-to-width ratios of up to 3:1. Zircon Th/U ratios vary from 0.50 to 2.33 in sample XC-13 and from 0.57 to 1.41 in sample XC-14 (Supplementary Table 1), again indicating a magmatic origin (Hoskin and Black, 2000). Twenty-one zircon U\\Pb analyses from sample XC-13 yielded concordant 206Pb/238U ages of 178.4–187.6 Ma with a weighted-mean 206 Pb/238U age of 183.0 ± 1.1 Ma (2σ) (mean square weighted deviation (MSWD) = 0.7) (Fig. 7b). Fourteen zircon U\\Pb analyses from sample XC-14 yielded concordant 206Pb/238U ages of 178.5–187.2 Ma with a weighted-mean 206Pb/238U age of 183.4 ± 1.7 Ma (2σ) (MSWD = 1.4) (Fig. 7d). The data fall on the concordia in a 206Pb/238U versus 207 Pb/235U diagram (Fig. 7a and c), indicating the zircons have not been affected by the loss of common Pb. Therefore, these ages are interpreted as the best estimates of the crystallization ages of these samples. 4.2. In situ zircon Hf isotopic data Zircons from samples XC-13 and -14 were subjected to in situ Hf isotopic analyses (15 and 11 analyses, respectively; Supplementary Table 2; Fig. 6). Sample XC-13 is characterized by positive initial zircon εHf(t) values ranging from +11.6 to +15.1, with an average of +13.3. Corresponding one- and two-stage Hf model ages (TDM1 and TDM2, respectively) vary from 231 to 376 Ma (average = 304 Ma) and 259 to 486 Ma (average = 373 Ma), respectively. Sample XC-14 yielded similar zircon εHf(t) values, ranging from +10.5 to +14.6, with an average of +12.7. Corresponding TDM1 and TDM2 ages vary from 258 to 424 Ma

(average = 331 Ma) and 292 to 553 Ma (average = 413 Ma), respectively.

4.3. Whole-rock geochemistry The chemical compositions of the XFI are listed in Supplementary Table 3. The loss-on-ignition (LOI) values of the samples vary from 2.20 to 4.54 wt%, implying slight to moderate hydrothermal alteration. Therefore, the major element contents were recalculated on a volatile-free basis. SiO2, MgO, and Al2O3 contents of the XFI vary from 57.78 to 71.83 wt% (average = 65.33 wt%), 0.99 to 3.53 wt% (average = 2.10 wt%), and 15.55 to 20.44 wt% (average = 18.36 wt%), respectively. In the SiO2 versus Zr/TiO2 diagram (Fig. 8a), data for all samples plot in the andesite or dacite field. In the Y versus Zr diagram (Fig. 8b), data for almost all samples fall in the calc-alkaline and transitional fields, reflecting transitional tholeiitic to calc-alkaline compositions. The rare earth element (REE) compositions of the XFI samples are similar in all cases, with total REE contents of 38.9–166.9 ppm (average = 88.9 ppm). (La/Yb)N ratios of 2.81–11.65 indicate marked fractionation between the heavy REEs (HREEs) and light REEs (LREEs). Chondrite-normalized REE patterns (Fig. 9a) are LREE enriched and show small variable Eu anomalies (δEu = 0.71–1.45; average = 0.99). Primitive-mantle-normalized trace element patterns (Fig. 9b) exhibit large-ion lithophile elements (LILEs) enrichment (e.g., Rb, Ba, and U) and high-field-strength elements (HFSEs) depletion (e.g., Nb, Ta, and Ti). In addition, the samples have negative to positive Sr anomalies and two samples (XX-29 and -30) are depleted in Zr.

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Fig. 5. Hand specimen photos (a, b) and thin-section photomicrographs (c, d) of the XFI. Py = Pyrite, Q = Quartz, Pl = Plagioclase, Hbl = Hornblende.

4.4. Sr–Nd–Pb isotopic data Whole-rock Sr–Nd–Pb isotopic data for the XFI samples are listed in Supplementary Table 4. The initial Sr, Nd, and Pb isotopic ratios of the XFI samples were calculated at 183 Ma. The XFI samples have uniform (87Sr/86Sr)t ratios of 0.70405–0.70521 and εNd(t) values of +5.51 to +7.07. Their Pb isotopic compositions are also uniform: (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t ratios vary from 18.112 to 18.261, 15.525 to 15.573, and 38.107 to 38.239, respectively.

of ignimbrite samples from the lower and upper sections of the XF were determined to be 195.0 ± 4.6 and 176.0 ± 5.0 Ma, respectively (Fig. 3; Qu et al., 2007; Tang et al., 2010). These age data indicate that the volcanism in the XF occurred during the Early Jurassic (195–176 Ma). In addition, Lang et al. (2018) reported a zircon U\\Pb age of 165.3 ± 1.0 Ma for the Middle Jurassic diabase dikes that intrude the XF (Fig. 4c). This age and the field relationships further demonstrate the Early Jurassic age of volcanism in the XF. Based on our U\\Pb zircon ages and previous studies of the XF, we propose that the XFI formed during the Early Jurassic, at the same time as the YF volcanic rocks (Fig. 1c; Zhu et al., 2008; Wei et al., 2017; Liu et al., 2018).

5. Discussion 5.2. Petrogenesis 5.1. Timing of XF volcanism The ages of two ignimbrite samples from the middle section of the XF (Fig. 3) are 180.0 ± 1.1 and 183.4 ± 1.7 Ma. The zircon U\\Pb ages

5.2.1. Effects of hydrothermal alteration on elemental mobility Since the XFI have been variably altered, it is necessary to evaluate the effect of hydrothermal alteration on element mobility for

Fig. 6. Representative cathodoluminescence (CL) images of zircons from the XFI. The red and white circles mark U\ \Pb dating and Lu\ \Hf isotopic analyses sites, respectively. All scale bars are 100 μm in length.

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Fig. 7. Zircon U\ \Pb concordia diagrams (a, c) and weighted average age diagrams (b, d) of the XFI.

Fig. 8. (a) SiO2 versus Zr/TiO2 (Winchester and Floyd, 1977) and (b) Y versus Zr (Barrett and MacLean, 1994) diagrams of the XFI.

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Fig. 9. (a) Chondrite-normalized rare earth element patterns and (b) primitive mantle-normalized trace-element patterns of the XFI. Chondrite and primitive mantle data are from Sun and McDonough (1989); and Early Jurassic YF volcanic rocks are from Liu et al. (2018).

preventing the misinterpretation of the data. In this study, Zr was chosen as an immobile element to examine the mobility of other elements (Cann, 1970; Hastie et al., 2007; Hill and Worden, 2000). This showed that, apart for some LILEs (e.g., Rb, Ba, and Sr), most other trace elements can be used to investigate their petrogenesis. In addition, since Sr is mobile during alteration, it may cause randomly variable and elevated (87Sr/86Sr)t ratios. However, in (87Sr/86Sr)t versus SiO2 and εNd (t) versus (87Sr/86Sr)t diagrams (Figs. 10a and 11a), the (87Sr/86Sr)t ratios are relatively constant. The initial Nd and Pb isotopic values are more uniform than initial Sr isotopic ratios (Figs. 10b, 11c and d). These isotopic data suggest that the effects of hydrothermal alteration on the Sr, Nd and Pb isotopic system of XFI were minimal. 5.2.2. Crustal contamination Magmas usually undergo open-system magmatic evolution processes such as crustal contamination and fractional crystallization during their ascent. Volcanic rocks contaminated by continental crust typically show more positive Zr\\Hf anomalies, higher (87Sr/86Sr)t ratios, and lower εNd(t) values than corresponding uncontaminated volcanic rocks. The studied ignimbrites do not exhibit positive Zr\\Hf anomalies (Fig. 9b) and are characterized by low (87Sr/86Sr)t ratios (0.70405–0.70521) and high positive εNd(t) values (+5.51 to +7.07) (Supplementary Table 4), precluding significant crustal contamination. Crustal contamination would also cause significant variations in the Sr\\Nd isotopic compositions, negative correlations between SiO2 contents and εNd(t) values, and positive correlations between SiO2 contents and (87Sr/86Sr)t ratios. These features are not observed in the studied ignimbrites (Fig. 10a–b). Moreover, the Pb isotopic compositions of the XFI are extremely uniform (Fig. 11c–d), which is also inconsistent with crustal contamination. These features suggest that crustal contamination played an insignificant role during magma evolution and ascent. 5.2.3. Fractionation crystallization Fractional crystallization, batch partial melting, and magma mixing can be distinguished in plots of incompatible trace elements with bulk solid/melt partition coefficients D (Schiano et al., 2010). For two highly incompatible elements H (typically Rb, Ba, Th, or U) and moderately incompatible elements M (typically Nd, Sm, Sr, Hf, or Zr), the fractional crystallization equation gives the following (Allègre and Minster, 1978; Treuil and Joron, 1975): C H ¼ C H0 = f

and C M ¼ C M0 =f

where CH (CH0) and CM (CM0) are the concentrations of elements H and M in the liquid (source), respectively, and f is the weight fraction of the

residual liquid. Therefore, (CH/CM) = (CH0/CM0) = a, where a is a constant. In a CH/CM versus CH plot (Fig. 10f), partial melting and mixing processes result in linear and hyperbolic arrays, respectively. Crystal fractionation produces an exponential array approximating a horizontal straight line (Hofmann and Feigenson, 1983; Schiano et al., 2010). In La/Sm versus La, Th/Nd versus Th, and Ce/Sm versus Ce diagrams (Fig. 10c–e), data for the XFI exhibit fractional crystallization trends. The decreasing MgO, FeOT, and Cr contents with increasing SiO2 indicate fractional crystallization of olivine and clinopyroxene (Fig. 12a, b, d, and e). However, the nearly constant Al2O3 contents with increasing SiO2 imply that fractional crystallization of plagioclase was negligible (Fig. 12c). The negative to positive Eu anomalies, but no obvious linear relationship between SiO2 and δEu (Fig. 12f), indicate plagioclase accumulation in local areas (Zhao et al., 2019). Therefore, we infer that the XFI compositional variability was affected primarily by fractional crystallization of a parental magma, with olivine and clinopyroxene being the major fractionating phases. 5.2.4. Nature of the magma sources The most commonly invoked models for the origins of intermediate–silicic magmas in subduction zones are: (1) extensive fractional crystallization of mantle-derived basaltic magma; and (2) anatexis of a crustal protolith. In predominantly andesitic systems, the formation of volumetrically subordinate dacite–low-Si rhyolite magmas is often attributed to coupled fractional crystallization and crustal assimilation (e.g., Feeley and Davidson, 1994), whereas voluminous rhyolitic magmas associated with large-scale calderas are typically ascribed to crustal anatexis (e.g., Graham et al., 1995). Given the XFI is dominated by andesitic and dacitic rocks, rather than rhyolitic rocks, these magmas were probably generated by mechanism (1) above. In addition, the relatively low (87Sr/86Sr)t (0.70405–0.70521), (206Pb/204Pb)t (18.112–18.261), (207Pb/204Pb)t (15.525–15.573), and (208Pb/204Pb)t (38.107–38.239) ratios, and relatively high εNd(t) values (+5.51 to +7.07) are similar to those of the Yarlung–Zangbo ophiolites (Fig. 11c–d; Mahoney et al., 1998; Xu and Castillo, 2004; Zhang et al., 2005; Guilmette et al., 2009), indicating derivation by partial melting of a depleted mantle source. The high positive εHf(t) values (+10.5 to +15.1) are also consistent with a depleted mantle source (Fig. 11b). The enrichment of LILEs relative to HFSEs is consistent with a mantle wedge source with added slab-derived components (Elliott, 2003; Pearce and Peate, 1995). The slab-derived components can include aqueous fluids from hydrated oceanic crust or overlying sediments. Ba, Pb, and Sr enrichment is generally attributed to aqueous fluids, whereas Nb, Th, La, Ce, and Nd are thought to be derived by partial

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Fig. 10. (a) (87Sr/86Sr)t versus SiO2, (b) εNd(t) versus SiO2, (c) La/Sm versus La, (d) Th/Nd versus Th, and (e) Ce/Sm versus Ce diagrams of the XFI (c-f after Schiano et al., 2010). Also shown is a schematic diagram (CH/CM versus CH, where CH and CM represent the content of hypermagmatophile and magmatophile elements, respectively) with curves showing compositional trends of magmatic processes including partial melting, fractional crystallization, and mixing.

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Fig. 11. (a) εNd(t) versus (87Sr/86Sr)t, (b) εHf(t) versus t, (c) (207Pb/204Pb)t versus (206Pb/204Pb)t, and (d) (208Pb/204Pb)t versus (206Pb/204Pb)t diagrams of the XFI. Data sources: magmatic rocks in Marianas are from Lin et al. (1990); magmatic rocks in Andean arc are from Pankhurst et al. (1999); YF volcanic rocks are from Wei et al. (2017) and Liu et al. (2018); magmatic rocks in the Kohistan-Dras arc are from Ravikant et al. (2009); magmatic rocks in the Zedong terrane are from Wei et al. (2007), Wang et al. (2012), and Zhang et al. (2014); Quxu appinite are from Ma et al. (2018); quartz diorite porphyries in Xiongcun district are from Tang et al. (2015) and Yin et al. (2017); Yarlung Zangbo ophiolite are from Zhao et al. (2009).

melting of subducted sediments (Castillo and Newhall, 2004; Kelemen et al., 2003). LILEs are readily concentrated and transported by aqueous fluids from the slab, whereas the immobile HFSEs remain in the slab. In contrast, most of the other incompatible elements are transported in silicate melts (Keppler, 1996; Tatsumi et al., 1986). The XFI are strongly enriched in Pb (4–322ppm), reflecting typical fluid-related enrichment. In a U/Th versus Th/Nb diagram, data for the XFI samples plot parallel to the U/Th axis, also suggesting that the source was modified by slabrelated fluids (Fig. 13a). In addition, in a δCe versus Th diagram (Fig. 13b), data for most samples fall in the field of primitive arc rocks, such as those from Vanuatu, Palau, and New Britain, which implies an insignificant sediment input (Hawkins and Ishizuka, 2009). In summary, the XFI were likely derived from a depleted mantle source modified by fluids released from a subducted slab. Their parental magmas experienced extensive fractional crystallization with limited or no crustal contamination during magma evolution. 5.3. Geodynamic model The chondrite-normalized REE patterns of the XFI exhibit LREE enrichment relative to the HREEs (Fig. 9a). Negative Nb–Ta–Ti anomalies are evident in primitive-mantle-normalized trace element patterns (Fig. 9b). These characteristics are generally regarded as signatures of

arc-related rocks (Pearce, 1982). In fact, most previous studies concur that the northward subduction of the Neo-Tethys oceanic slab beneath the southern Lhasa subterrane began in the Late Triassic–Jurassic or earlier, and that the early Mesozoic igneous rocks within this subterrane formed in this arc setting (e.g., Chen et al., 2019; Chu et al., 2006, 2011; Guo et al., 2013; Ji et al., 2009; Kang et al., 2014; Lang et al., 2014; Liu et al., 2018; Ma et al., 2017a; Meng et al., 2016a, 2016b; Tang et al., 2015; Wang et al., 2019; Wei et al., 2017). The remaining question is whether the XFI formed in a continental margin or an intra-oceanic island arc setting. In the YZSZ, Aitchison et al. (2000) first identified a Late Jurassic intra-oceanic arc system recorded by Zedong terrane, whereas later Zhang et al. (2014) thought Zedong terrane formed in the active continental margin arc setting. Combining the previous data (Fig. 11a–b), we tend to believe that the Zedong terrane represents the remnants of a Late Jurassic intraoceanic arc system. In addition, Ma et al. (2018) recently identified a Late Triassic intra-oceanic arc system recorded by Quxu appinite within Neo-Tethys. With the Late Triassic and Late Jurassic intra-oceanic arc systems have been identified within Neo-Tethys, does the XF represent an Early–Middle Jurassic component of intra-oceanic arc system within Neo-Tethys? The YF has been widely considered to be developed at a continental margin (Liu et al., 2018; Wei et al., 2017; Zhu et al., 2008). Compared to

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Fig. 12. (a–d) Selected major elements versus SiO2, (e) Cr versus SiO2, and (f) δEu versus SiO2 diagrams of the XFI.

the andesite and dacite of YF (Th = 3.10–11.0 ppm, mean = 6.34 ppm; Hf = 2.70–6.50 ppm, mean = 4.80 ppm; Hf/Yb = 1.18–2.24, mean = 1.73; Zhu et al., 2008; Wei et al., 2017), the andesite and dacite samples (excluding sample XC-5 and XC-7) in XF have lower Th and Hf contents and Hf/Yb values (Th = 2.13–5.25 ppm, mean = 3.63 ppm; Hf = 1.92–3.45 ppm, mean = 2.76 ppm; Hf/Yb = 0.69–2.15; mean =

1.58). The characteristic differences of these trace elements indicate that the XF volcanic rocks are unlikely to be formed in a continental arc setting same as the YF but possibly in an intra-oceanic island arc setting (Bailey, 1981). In addition, in the εNd(t) versus (87Sr/86Sr)t diagram (Fig. 11a), the εNd(t) values of the XFI samples are higher than those of rocks from a typical continental margin arc (e.g., Andean arc) and YF,

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177

Fig. 13. (a) U/Th versus Th/Nb and (b) δCe versus Th (after Hawkins and Ishizuka, 2009) diagrams of the XFI.

but are similar to those of rocks from the Mariana island arc as well as Zedang and Quxu. Furthermore, in the εHf(t) versus t diagram (Fig. 11b), the εHf(t) values of the XFI are consistent with those of rocks from the Kohistan–Dras arc, which is an Early Cretaceous intraoceanic island arc system within the western YZSZ (e.g., Bignold et al., 2006; Dhuime et al., 2007; Mahéo et al., 2004). Moreover, the εHf(t) values of the XFI are consistent with those of rocks from the Zedong and Quxu (Fig. 11b). These results show that the XFI probably formed in an intra-oceanic island arc setting. On the other hand, sedimentary rocks can constrain the tectonic setting, perhaps more reliably than the compositions of volcanic rocks (Li et al., 2015). The inference that the XFI formed in an intra-oceanic island arc setting is also supported by sedimentological evidence. Lang et al. (2019) and Ma et al. (2019) obtained detrital zircon ages from the XF and YF sandstones, respectively. The XF sandstones lack older zircons (Fig. 14a), whereas the YF sandstones, which were deposited in a continental marginal arc setting, contain a minor population of older zircons (Fig. 14b). In addition, geochemistry of the XF sandstones indicates that these sandstones were deposited in an intra-oceanic island arc setting (Lang et al., 2019). Therefore, based on the geochemical characteristics of the XFI, along with the sedimentological evidence, we conclude that the XF represents an Early–Middle Jurassic intra-oceanic arc system within Neo-Tethys. To explain the tectono-magmatic evolution of the southern Lhasa subterrane during the Early–Middle Jurassic, previous studies have proposed several tectonic models. First, the Neo-Tethys oceanic slab is proposed to have dipped beneath the continental crust whilst being subducted northward during the Early–Middle Jurassic (Liu et al., 2018; Ma et al., 2019; Wei et al., 2017; Zhu et al., 2008). Basaltic rocks of the YF were derived by partial melting of the mantle above the subducting slab, and silicic rocks were generated by partial melting of the basaltic lower crust (Liu et al., 2018; Wei et al., 2017; Zhu et al., 2008). This model can explain the tectonic setting and volcanism of the YF (Fig. 15a), but not of the XF. Second, Tang et al. (2015) and Lang et al. (2019) (Fig. 15b) proposed that the Neo-Tethys oceanic slab was subducted northward beneath oceanic crust during the Early–Middle Jurassic. This model can explain the tectonic setting and petrogenesis of the XF volcanic–sedimentary rocks, but does not account for the tectonic setting and volcanism in the YF. We propose a third model (Fig. 15c) whereby a single subduction zone existed in the northern Neo-Tethys, which was distal from the continental margin to the south and adjacent to continental crust to the north (southern Lhasa subterrane). This single north-dipping subduction zone generated the XF volcanic–sedimentary rocks above an intra-oceanic subduction

zone in the south, as well as the YF volcanic rocks on the active continental margin (southern Lhasa subterrane) to the north (Fig. 15c). According to this model, which is also supported by zircon U\\Pb age data, the XF formed closer to the subduction zone than the YF, implying that volcanism in the XF is slightly older than that in the YF. This is consistent with the oldest ages of the XF and YF volcanic rocks of 195 Ma (Qu et al., 2007) and 192 Ma (Fig. 1c; Chen et al.,

Fig. 14. Relative probability histograms of detrital zircon ages for the XF and YF sandstones. Data sources: YF sandstones (Lang et al., 2019); YF sandstones (Ma et al., 2019).

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Fig. 15. Alternative tectonic models for the southern Lhasa subterrane during the Early–Middle Jurassic period. See text for explanations.

2009), respectively. A similar model has been proposed to explain the tectonic setting and magmatism of the Dras arc and Ladakh batholith in the South Ladakh region to the west of the Lhasa terrane (Mahéo et al., 2004). The Dras arc was related to subduction of the Neo-Tethys oceanic slab beneath the oceanic margin of the Ladakh batholith. It formed an arc comprising three calc-alkaline volcanic suites emplaced

onto oceanic crust (Dietrich et al., 1983; Reuber, 1989). At the same time, in the northern part of the Dras arc the southern Ladakh batholith underwent Andean-type calc-alkaline magmatism. This example suggests that intra-oceanic island and Andean-type arcs (i.e., a continental margin arc) can form at a single subduction zone.

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Combining these previously published models (Lang et al., 2019; Tang et al., 2015; Zhu et al., 2008) with our new geochemical and geochronological data, we propose a subduction model for the tectonic setting and magmatism in the southern Lhasa subterrane (Fig. 15c). During the Early–Middle Jurassic, a north-directed subduction system formed in the Neo-Tethys, which generated the XF volcanic–sedimentary rocks on oceanic crust and the YF volcanic–sedimentary rocks on continental crust. 6. Conclusions Based on an integrated geochronological, geochemical, and isotopic study of the XFI, we reached the following conclusions. (1) Zircon U\\Pb dating of the XFI indicates that volcanism in the study area occurred during the Early Jurassic. (2) The XFI were formed by magma that underwent fractional crystallization and negligible crustal contamination. The parental magmas were derived mainly from a depleted mantle source modified by fluids released from the Neo-Tethys oceanic slab. (3) XF volcanism occurred on oceanic crust, whereas YF volcanism occurred on continental crust, but both suites of volcanic rocks were likely generated in the same subduction system. Acknowledgments This research was jointly supported by the Deep Resources Exploration and Mining, National Key R&D Program of China (grant numbers: 2018YFC0604105), the Opening Foundation of State Key Laboratory of Continental Dynamics, Northwest University (grant numbers: 18LCD04), the Opening Foundation of State Key Laboratory for Mineral Deposits Research (grant numbers: 2017-LAMD-K04), the Opening Foundation of State Key Laboratory of Ore Deposit Geochemistry (grant numbers: 201503), and the China Geological Survey Programs (grant numbers: DD20160346). We are grateful to anonymous reviewers and editors for their constructive comments, which have considerably improved the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.lithos.2019.05.014. References Aitchison, J.C., Zhu, B.D., Davis, A.M., Liu, J.B., Luo, H., Malpas, J.G., McDermid, I.R.C., Wu, H.Y., Ziabrev, S.V., Zhou, M.F., 2000. Remnants of a cretaceous intraoceanic subduction system within the Yarlung–Zangbo suture (southern Tibet). Earth Planet. Sci. Lett. 183, 231–244. Aitchison, J.C., McDermid, I.R.C., Ali, J.R., Davis, A.M., Zyabrev, S.V., 2007. Shoshonites in southern Tibet record late Jurassic rifting of a tethyan intraoceanic island arc. J. Geol. 115, 197–213. Allègre, C.J., Minster, J.F., 1978. Quantitative models of trace element behavior in magmatic processes. Earth Planet. Sci. Lett. 38, 1–25. Bailey, J.C., 1981. Geochemical criteria for a refined tectonic discrimination of orogenic andesites. Chem. Geol. 32, 139–154. Barrett, T.J., MacLean, W.H., 1994. Chemostratigraphy and hydrothermal alteration in exploration for VHMS deposits in greenstones and younger volcanic rocks. In: Lentz, D.R. (Ed.), Alteration and Alteration Processes Associated with Ore-Forming Systems. Geological Association of Canada, pp. 433–467 Short Course Notes 11. Bignold, S.M., Treloar, P.J., Petford, N., 2006. Changing sources of magma generation beneath intra-oceanic island arcs: an insight from the juvenile Kohistan island arc, Pakistan Himalaya. Chem. Geol. 233 (1), 46–74 2006. Cann, J.R., 1970. Rb, Sr, Y, Zr and Nb in some ocean floor basaltic rocks. Earth Planet. Sci. Lett. 10, 7–11. Castillo, P.R., Newhall, C.G., 2004. Geochemical constraints on possible subduction components in lavas of Mayon and Taal volcanoes, southern Luzon, Philippines. J. Petrol. 45, 1089–1108. Chen, W., Ma, C.Q., Bian, Q.J., Hu, Y.J., Long, T.C., Yu, S.L., Chen, D.M., Tu, J.H., 2009. Evidences from geochemistry and zircon U-Pb geochronology of volcanic rocks of Yeba Formation in Demingding area, the east of Middle Gangdise, Tibet. Geol. Sci. Technol. Inf. 28, 31–40 (in Chinese with English abstract).

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