Melt Evolution of Upper Mantle Peridotites and Mafic Dikes in the Northern Ophiolite Belt of the Western Yarlung Zangbo Suture Zone (Southern Tibet)

THEMED ISSUE: Ophiolites, Diamonds, and UHP Minerals: New Discoveries and Concepts on Upper Mantle Petrogenesis

Melt evolution of upper mantle peridotites and mafic dikes in the northern ophiolite belt of the western Yarlung Zangbo suture zone (southern Tibet)

Fei Liu1,2,*, Yildirim Dilek2, Yanxue Xie2, Jingsui Yang1, and Dongyang Lian1,3 1CARMA, INSTITUTE OF GEOLOGY, CHINESE ACADEMY OF GEOLOGICAL SCIENCES, 26 BAIWANZHUANG ROAD, BEIJING 100037, CHINA 2DEPARTMENT OF GEOLOGY AND ENVIRONMENTAL EARTH SCIENCE, MIAMI UNIVERSITY, 208 SHIDELER HALL, 250 S. PATTERSON AVENUE, OXFORD, OHIO 45056, USA 3FACULTY OF EARTH SCIENCES, CHINA UNIVERSITY OF GEOSCIENCES, 388 LUMO ROAD, WUHAN 430071, CHINA

ABSTRACT

The Yarlung Zangbo suture zone (YZSZ) in southern Tibet is divided by the Zhongba terrane into two subparallel belts in its western end. The northern belt (NB) is tectonically juxtaposed against an accretionary prism complex and the Gangdese magmatic arc of Eurasia along dextral oblique-slip faults. Peridotite massifs in this belt are intruded by mafic dikes, providing critical geochemical, geochronological, and isotopic information about the melting-melt extraction history of the Tethyan mantle. Peridotites consist of harzburgite and clinopyroxene harzburgite with minor Iherzolite and dunite-chromitite. Dolerite and microgabbro dikes crosscutting these peridotites display U-Pb zircon ages of 128–122 Ma, and show normal mid-oceanic ridge basalt (N-MORB) like rare earth element patterns with negative Nb, Ta, and Ti anomalies, 143 144 87 86 high εNd(t) values (+8.39 to +9.28), and ( Nd / Nd)t = 128 Ma ratios of 0.51290–0.51295. They display high ( Sr/ Sr)t ratios of 0.70433–0.70489, and 206Pb/204Pb of 17.546–17.670, 207Pb/204Pb of 15.432–15.581, and 208Pb/204Pb of 37.724–37.845, suggesting that their N-MORB–like mantle source was modified by island arc melts. Slab rollback–induced extension in an arc-trench system along the Eurasian continental margin led to ~7%–12% partial melting of subduction-influenced, spinel lherzolite peridotites, producing dike magmas. The NB peridotite massifs and ophiolites thus represent a suprasubduction zone oceanic lithosphere formed in close proximity to the late Mesozoic active continental margin of Eurasia.

LITHOSPHERE; v. 10; no. 1; p. 109–132; GSA Data Repository Item 2017398 | Published online 5 January 2018 https://doi .org /10 .1130 /L689 .1

INTRODUCTION in their crustal derivatives in the oceanic lithosphere, which developed in different tectonic settings within the Neotethys Ocean basin; and (3) major The Yarlung Zangbo suture zone (YZSZ) in southern Tibet separates changes in intraocean basin tectonics and continental margin evolution of India and its northern passive margin units in the south from Eurasia and the Neotethys Ocean along its >3000 km length from the west to the east. its active continental margin units (e.g., Gangdese magmatic arc of the In this study we have investigated an ophiolitic mélange, upper mantle Lhasa block) to the north (Fig. 1). It has been traditionally interpreted peridotites, and their mafic dike intrusions, which occur discontinuously in as one of the major suture zones in the Earth that formed during the an east-west zone, making up the northern belt (NB) near the western end India-Asia continental collision, following the terminal closure of the of the YZSZ (Fig. 1), where they are tectonically sandwiched between the Neotethys (Aitchison et al., 2011; Hu et al., 2016). Mafic-ultramafic rock Gangdese magmatic arc (north) and the Zhongba terrane (south). The NB assemblages in different ophiolite massifs and mélanges along the YZSZ ophiolites are tectonically juxtaposed against an accretionary prism com- have been studied extensively during the past 30 years (i.e., Miller et al., plex of the late Mesozoic active continental margin of Eurasia, providing 2003; Liu et al., 2010; Bezard et al., 2011; Dai et al., 2011; Guilmette a critical spatial-temporal link between the magmatic development of the et al., 2012; Hébert et al., 2012; Guo et al., 2015; Li et al., 2015; Niu ophiolites and the Eurasian active margin tectonics in the Early Cretaceous. et al., 2015; Lian et al., 2016, 2017; Feng et al., 2017). Geodynamic We present new geochemical, geochronological (U-Pb zircon ages), and models explaining the geochemical evolution of the YZSZ ophiolites, isotopic data from mafic dike intrusions in two peridotite massifs, and the their tectonic origin of magmatic construction within a broad Neotethys results of our non-modal batch and aggregated fractional partial melting oceanic realm, and the paleography of this Mesozoic–Cenozoic ocean modeling of these peridotites and mafic dikes in order to constrain their basin vary significantly (Hébert et al., 2012; Dai et al., 2013; C. Liu et al., melt evolution and melt-residua genetic relationships. Our data and inter- 2014; Liu et al., 2015a; Gong et al., 2016; Lian et al., 2016; Xiong et al., pretations, together with a new tectonic model, provide important insights 2016). These variations stem largely from (1) widespread, contractional, into the geodynamic development of the western YZSZ. strike-slip and extensional deformation along and across the YZSZ that occurred during and after the initial India-Asia collision that significantly REGIONAL GEOLOGY AND STRUCTURE OF THE WESTERN YZSZ modified the primary structures and the original distribution of lithological units; (2) original heterogeneities in upper mantle compositions and The YZSZ is divided into three structurally different segments along its nearly 2000-km-long east-west trend (Fig. 1). The eastern segment *[email protected]; [email protected] extends from Xigaze to the Eastern Himalaya syntaxis (or Namche Barwa

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K SL KOHIST thyan SH Te N 30° rocks along the (HP) and high-pressure mélanges, ophiolitic ophiolites, Tethyan the distribution of the showing and southern Tibet, the Himalaya map of geological Simplified 1. Figure MBT—Main Boundary thrust; fault; KKF—Karakorum GT—Gangdese thrust; thrust; Counter GCT—Great therein). references and 2015, Xu et al., (modified after zone Zangbo suture Yarlung CBZ— BR—Baer; Ophiolitic massifs: Tibet detachment. STD—South syntaxis; Parbat NPS—Nanga syntaxis; NBS—Namche Barwa thrust; MFT—Main Front thrust; MCT—Main Central SP—Spongtang; SL—Shangla; SG—Saga; PR—Purang; ND—Nidar; LBS—Luobusa; KZ—Kazhan; JD—Jiding; GZ—Gongzhu; DQ—Dangqiong; DJW—Dajiweng; DB—Dongbo; Cuobuzha; ZL—Zhalai. ZG—Zhaga; ZD—Zedang; ZB—Zhongba; XGZ—Xigaze; XGGB—Xiugugabu; SS—Sangsang;

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syntaxis, NBS) in the east, and the western segment extends from Saga the products of partial melting of garnet-bearing spinel lherzolites in the to the Ladakh batholith in the northwest (Fig. 1). The eastern YZSZ subcontinental lithospheric mantle below northern India or beneath the is strongly affected by north-south–oriented contractional deformation, Zhongba terrane (Liu et al., 2015a). and the primary contacts between the Gangdese magmatic belt (Lhasa block), the YZSZ, and the Tethyan Himalaya sequence in and across this TECTONIC ARCHITECTURE ACROSS THE NORTHERN BELT segment have been largely modified by backthrusting and backfolding that developed during and after the India-Asia collision (Xu et al., 2015). We examined the petrological architecture and the internal structure South-vergent thrust faults within the central YZSZ are superimposed by of the NB and the tectonic entities adjacent to the western YZSZ along nearly east-west–extending normal faults and northeast-southwest– and several north-south profiles between longitude 80°E and 81°E. We sum- northwest-southeast–oriented oblique-slip fault systems that continue marize here the three major tectonic units that are pertinent to the geology into the Gangdese magmatic belt in the north and the Tethyan Himalaya of the western YZSZ. sequence in the south. The central segment of the suture zone is also juxtaposed against the Xigaze forearc group sequence in the north that Ophiolite Massifs does not exist in the other two segments to the east and the west (Fig. 1). Previously formed contractional structures in the western YZSZ have We have studied two of the most important ophiolite massifs (Baer and been significantly modified and deformed by the dextral Karakorum fault Cuobuzha) in the NB of the western YZSZ (Fig. 2). These two massifs and its splays (Fig. 1). The suture zone tectonic units here are juxtaposed are separated by different strands of the northwest-southeast–extending against the continental Zhongba terrane. Karakorum fault zone. The Baer massif is juxtaposed against carbona- The northwest-southeast–oriented, ~900-km-long and ~100-km-wide ceous shale-siltstone units of the Zhongba terrane in the south and Triassic Zhongba terrane divides the western YZSZ into the NB and southern belt slate and metasandstone units of the terrane in the north along strike-slip (SB) (Fig. 1). The Zhongba terrane consists of metamorphosed, early faults (Fig. 3A). Lower Cretaceous silicic tuffaceous rocks of the Zhongba Paleozoic–Mesozoic pelitic and carbonate rocks (Fig. 2), and is dissected terrane occur in a >300-m-wide, fault-bounded sliver within the Baer by northwest-southeast–extending dextral, oblique-slip fault systems. The massif. We define the peridotite exposures to the north and to the south NB is juxtaposed against the Gangdese magmatic belt along the north- of this sliver as the North Baer and South Baer submassifs, respectively. west-southeast–extending Karakorum fault zone and its local splays. The Intensely altered quartz-listwanite commonly occurs as an ~10-m-thick SB and its peridotite massifs tectonically overlie the Tethyan Himalaya metasomatic cover at the surface of the serpentinized peridotites. The sequence in the south. peridotites in both submassifs are intruded by microgabbro and dolerite Ophiolites within the NB are made mainly of large peridotite massifs, dikes. These dikes are commonly 0.5–1.5 m in thickness and have well- which occur discontinuously as lensoidal bodies in a serpentinite matrix developed chilled margins against their ultramafic host rocks. Previous mélange. Major peridotite massifs within the NB include the Dajiweng, in situ zircon dating of several of these dolerite and microgabbro dikes Kazhan, Baer, Cuobuzha, Zhalai, Gongzhu, and Saga ophiolite massifs in the North Baer submassif yielded crystallization ages ranging from (Fig. 1), which are commonly ~0.5–2 km in width and ~5–20 km in 128 Ma to 126 Ma (Liu et al., 2015b; Zheng et al., 2017). However, Liu length. Ophiolitic subunits in these massifs include upper mantle peri- et al. (2015b) reported the zircon U-Pb age of 128.1 ± 2.1 Ma yielded by dotites, cumulate to layered gabbros, and massive lavas that are locally merely 6 zircon grains with the SHRIMP (sensitive high-resolution ion interlayered with submarine shale, radiolarian chert, and siliceous lime- microprobe) method; Zheng et al. (2017) reported 2 laser ablation–induc- stone. Pillow lavas and sheeted dikes have not been observed in any of tively coupled plasma–mass spectroscopy (LA-ICP-MS) zircon ages of the massifs within the NB along the western YZSZ. The upper mantle 126.3 ± 2.4 and 125.6 ± 2.4 Ma, but their zircon ages span a large range, peridotites are composed predominantly of serpentinized lherzolite and including 145, 139, 135–119, 117 Ma (Zheng et al., 2017). Moreover, harzburgite in the Baer, Gongzhu, and Zhalai massifs (Lian et al., 2016, the whole-rock geochemical data from Zheng et al.(2017), especially 2017; Zheng et al., 2017), and clinopyroxene harzburgite and harzburgite the heavy rare earth element (HREE) contents, are not only much lower with minor dunite-chromitite occurrences in the Cuobuzha and Kazhan than those of samples from the same location in North Baer (12YL61, massifs (Liu et al., 2015c; Feng et al., 2015; Lian et al., 2016, 2017). this study), but all the mafic dikes from Dajiweng (Zhang et al., 2005), Northwest-southeast–striking dolerite, clinopyroxenite, and microgabbro North Baer (Liu et al., 2015b), South Baer (this study), Cuobuzha (Liu et dikes crosscut these peridotites in almost all ophiolite massifs in the NB. al., 2015c; this study) in the NB, and from Purang (Liu et al., 2010; Liu Ophiolites in the SB occur as much larger peridotite massifs (i.e., et al., 2011; Liu et al., 2013; Miller et al., 2003) and Xiugugabu (Bezard Dongbo, 400 km2; Purang, 650 km2; Xiugugabu, 700 km2; Dangqiong, et al., 2011) in the SB. The age and geochemical data from Zheng et al. 300 km2) that are intruded by mafic dike swarms and overlain by volcanic- (2017), therefore, are not 100% reliable. In this study we collected addi- sedimentary rock sequences. These peridotite massifs are locally thrust tional dike rocks from both the North and South Baer submassifs for new northward onto the Zhongba terrane, and southward over an ophiolitic geochemical and isotopic analyses and for U-Pb zircon dating. mélange and the Tethyan Himalaya Sequence by bivergent thrust fault The Cuobuzha massif, ~20 km southeast of the Baer massif, is 0.4– kinematics (Figs. 1 and 2). Peridotites are composed of lherzolite and 1.3 km wide and ~6 km long in the NB, and consists mainly of peridotites harzburgite in the Xiugugabu (Bezard et al., 2011), Zhaga (also called intruded by 1–3-m-wide dolerite and microgabbro dike swarms. Micro South Gongzhucuo; L. Zhang et al., 2016), and eastern Purang massifs gabbroic dikes are locally to 10 m in width and have coarser grained centers. (Miller et al., 2003; Zhou et al., 2014; Li et al., 2015), and harzburgite Peridotites are composed of clinopyroxene harzbugite, depleted harzburgite, with minor lherzolite and dunite in the Dongbo, Zhongba, and western and minor dunite. Previous in situ U-Pb zircon dating of 1–3-m-wide doler- Purang massifs (Fig. 2; Dai et al., 2011; Xu et al., 2011; C. Liu et al., ite and microgabbro dike rocks from the Cuobuzha massif yielded crystal- 2014; Niu et al., 2015). Disseminated and massive chromitite deposits lization ages of 126 Ma and 127 Ma, respectively (Liu et al., 2015c). In this are commonly associated with dunite occurrences within the harzburgites study we present new isotope analyses for the 1–3-m-wide microgabbro in the SB massifs. Geochemical and petrogenetic modeling of the melt dike rocks, and for U-Pb zircon dating of a 5–10-m-wide microgabbro dike evolution of the Purang and Dongbo peridotites has shown that they were (31°22′52.88″N, 80°02′40.48″E, 4986 m) intruding the Cuobuzha peridotite.

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79°30′E 79°45′E 80°00′E 80°15′E 80°30′E 80°45′E 81°00′E 81°15′E 81°30′E 81°45′E 82°00′E 82°15′E 82°30′E 32° 00′N Dajiweng

N 31° G a n g d e s 45′N

020 40 Fig.3- Baer Kazhan Profile 1 31° km 30′N Zhada Cuobuzha Z h o n g b a T e r r a n e Lhasa Fig.3- e Profile 2 Bloc Daba M a g m a t i c k 31° 15′N Dongbo Menshi Kailash

Qulong 31° T e t h y a 00′N Baga B e l Zhalai t 30° n Lake Gongzhu 45′N Purang Yangbw a Manasarovar H i m a l a y Mayumu

Zhaga Tunla 30° 30′N

Profile location a Thrust fault 30° Fault 15′N

Strike-slip fault 30° 00′N Zhongba Terrane Tethyan Oceanic Lithophere Units Slate interbedded with metasandstone, metasandstone and bioclast limestone and micrite interbedded with siltstone Ophiolitic mélange (Mesozoic) (Jurassic - Cretaceous) Micrite, slate interbedded with micrite, metalithic/feldspathic quartz sandstone, metaconglomerate, dolomite interbedded Ophiolite massifs with limestone and shale (upper Paleozoic) (Jurassic - Cretaceous) Slate interbedded with micrite, metaquartz sandstone, dolomite, limestone, metasandstone interbedded with dolomite (lower Gangdese Magmatic Belt Paleozoic) Granitoids - volcanic rocks (Cretaceous - Tertiary) Tethyan Himalaya Units Alluvium and undifferentiated rocks (Paleogene - Quaternary) Sparite, bioclast limestone, micrite interbedded with shale and Greater Himalaya Units siltstone, lithic/feldspathic quartz sandstone, shale (Mesozoic) Schist, gneiss, biotite Slate, phyllite, dolomite, metaquartz sandstone interbedded with (hornblende) granulite slate and phyllite, marble, minor siltstone, shale (upper Paleozoic) (Precambrian) Metafeldspathic quartz sandstone, metasiltstone, slate, limestone, micrite, dolomite (lower Paleozoic) Granitoids (Unknown) Two - mica granite (lower Paleozoic)

Figure 2. Simplified geological map of the western part of the Yarlung Zangbo suture zone, showing the locations of two geological profiles (Fig. 3) and main tectonic units from north to south, consisting of Gangdese arc, northern ophiolite belt, Zhongba terrane, southern ophiolite belt, and Tethyan Himalaya. The eastern and western Purang submassifs are divided by the western margin of Lake Yungbwa.

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NE 0 100 m A m BAER PROFILE 4950 4950 Cr

4850 4850

195° 215° 50° 50° 49° 34° 55° Dip/dip angle 55° 4750 4750 0 100 m NE m CUOBUZHA PROFILE B

4950 Si Si Cr 4950 Si Si Si Si Si Si Si Si Si 4850 4850

4750 4750 SW N24°E C

Gangdese Peridotite Volcanic magmatic Seamount sequence Andesite Volcaniclastic breccia belt Listwanite silty shale rocks

Ophiolite Zhongba Terrane Accretionary Complex Gangdese Magmatic Belt (Jurassic-Cretaceous) (Paleozoic-Mesozoic) (Cretaceous) (Mesozoic-Cenozoic) Volcaniclastic Si Listwanite Lherzolite Mylonitic rocks rock Si Siliceous shale Granodiorite Cr Chromitite Gabbro Silty shale Andesite Silty shale Granitic aplite

Harzburgite Dolerite Carbonaceous Si Siliceous Massive alkaline Gabbro shale Si limestone basatic lava Radiolarian Dunite Massive tuff Si Si chert Magmatic Breccia

Figure 3. (A) Geological profile of the Baer massif. (B) Geological profile of the Cuobuzha massif. (C) Panoramic view of profile B across the northern belt of the western Yarlung Zangbo suture zone.

Accretionary Prism Complex lava–sedimentary rock associations are analogous to those of seamounts, which occur widely in the modern Pacific Rim accretionary prism com- The Cuobuzha massif is bound to the north by an ~1-km-wide chaotic plexes (Cawood et al., 2009) and in ancient, exhumed accretionary prism turbiditic assemblage, composed of pelitic and carbonate rocks, tectoni- complexes within the Tethyan orogenic belt (i.e., Codegone et al., 2012; cally interleaved with andesitic volcanic-volcaniclastic rocks and massive Sarifakioglu et al., 2017). alkaline basaltic lava flows, which are locally stratigraphically overlain by The structural anatomy and the lithological units in the turbiditic cha- radiolarian chert, siliceous limestone, and siliceous and silty shale (Figs. otic assemblage to the north of the Cuobuzha ophiolite massif are remi- 3B, 3C). These rock assemblages are repeated in southwest-vergent thrust niscent of active continental margin accretionary prisms (Anma et al., sheets and commonly show tight, asymmetric, and southwest-overturned 2011). We thus interpret this assemblage as a Cretaceous accretionary folds. Sandstone, siltstone, and shale make up a turbiditic sequence, also prism of the Gangdese magmatic arc and the associated trench system containing blocks of andesitic volcanic rocks, reminiscent of External of the Eurasian active continental margin. Ligurian mélanges from the northern Apennines and of modern chaotic deposits documented from active continental margins (Remitti et al., 2011; Gangdese Magmatic Arc Units Festa et al., 2010, 2012). Alkaline basaltic rocks occur within the turbiditic chaotic assemblage as blocks or thrust sheets of 2–3-m-thick, vesicular The accretionary prism complex is juxtaposed against the plutonic- massive lava flows directly overlain by hyaloclastites and chert. The com- volcanic rocks of the Gangdese magmatic arc along a network of west- positions, chemostratigraphy, and lithological makeup of these alkaline northwest–east-southeast–oriented, subparallel dextral faults (Figs. 1 and

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2). Medium- to coarse-grained diorite-granodiorite rocks close to these H2O and CO2 is 0.01 wt%. The major and trace element compositions faults show a strong mineral fabric defined by the lineation (112°) of are given in Table 2. magmatic biotite and hornblende. Lineation-parallel, fine-grained gabbro dikes intrude the diorite-granodiorite rocks and are in turn crosscut by Sr, Nd, and Pb Isotope Analyses subparallel, 2–3-m-wide aplite dikes (Fig. 3B). These structural relationships in the outcrop suggest tectonically controlled emplacement of the Sr, Nd, and Pb isotope analyses of five microgabbro samples (12YL60– mafic and felsic intrusions in the southern edge of the Gangdese mag- 7–12) from the North Baer submassif and five microgabbro samples matic arc. Gabbros show compositional zoning, defined by their increased (12YL74–2–6) from the Cuobuzha massif were performed on a VG 354 clinopyroxene contents toward the south. mass spectrometer with five collectors at the Center of Modern Analysis, These intrusive rocks are overlain by a magmatic breccia along a Nanjing University, China. Rb, Sr, Sm, Nd, U, Th, and Pb were separated gently south-dipping normal fault (Fig. 3B). Highly angular and poorly and purified through standard ion exchange techniques. The Sr and Nd sorted clasts in this breccia range in size from 5 to 20 cm, and are made isotopic ratios were normalized against 86Sr/88Sr = 0.1194 and 146Nd/144Nd of diorite, granodiorite, granite, gabbro, aplite, and andesitic rocks, all = 0.7219, respectively. Sr standard NBS-987 yielded 87Sr/86Sr = 0.710233 derived from the magmatic arc. The matrix of the breccia is mainly lithic ± 0.000006 (2σ) and Nd standard La Jolla gave 143Nd/144Nd = 0.511863 ± sandstone-siltstone. The age of this breccia is unknown, but its tectonic 0.000006 (2σ). Pb isotopic ratios were corrected by Pb standard NBS981: occurrence on top of the magmatic arc plutonic rocks suggests that the 206Pb/204Pb = 16.941 ± 0.008, 207Pb/204Pb = 15.487 ± 0.0011, 208Pb/204Pb timing of its formation may signal an important episode of extensional = 36.715 ± 0.009. The analytical procedures for Nd, Pb, and Sr isotopic unroofing along the southern edge of the Gangdese magmatic arc. measurements were described in detail by Wang et al. (2007). The Sr- Nd-Pb isotopes data are listed in Table 3. ANALYTICAL METHODS RESULTS OF ZIRCON GEOCHRONOLOGY U-Pb Zircon Dating CL images of zircon grains from dolerite dike samples from the South Zircon grains are separated from a dolerite dike sample (12YL54–9) and North Baer submassifs and a microgabbro dike sample from the in the South Baer submassif, from a dolerite dike sample (12YL61–9) in Cuobuzha massif (Fig. 4) show subhedral habits with long prismatic to the North Baer submassif, and from a microgabbro dike sample (13YL30– tabular and faceted edges. Grain sizes range from 60 to 150 µm in length, 30) in the Cuobuzha massif for U-Pb dating. Zircons were extracted by and the analyzed zircons exhibit weak oscillatory zoning and no mineral heavy liquid and magnetic methods, and were then hand-picked under a inclusions. Some of the Cuobuzha zircons are rimmed by metasomatic binocular microscope. The separated zircons were mounted in a rounded margins that are <2–10 µm wide. Zircons from the South Baer submas- epoxy resin. Cathodoluminescence (CL) reflected and transmitted light sif are relatively fresh, and their Th/U ratios vary between 1.55 and 5.31 images were taken to detect the microstructures of each zircon grain, and with an average of 2.90; 0.66–2.04 with a mean of 1.49 for Cuobuzha and to select the best positions for analysis. In situ zircon U-Pb dating was 0.41–1.77 with an average of 0.67 for North Baer zircon grains (Table 1). carried out by a Thermo Finnigan Neptune multicollector-ICP-MS at These CL images and Th/U ratios are comparable to those of magmatic zir- the Institute of Mineral Resources, Chinese Academy of Geological Sci- cons (Rubatto, 2002; Grimes et al., 2009). In a 207Pb/235U against 206Pb/238U ences (Beijing). Laser sampling was performed by a New Wave UP213 concordia diagram (Fig. 5), in situ zircon grains from samples 12YL54–9 LA system. Laser spot size was set to 35 µm. The ablated material was (South Baer), 12YL61–9 (North Baer), and 13YL30–30 (Cuobuzha) show carried into the ICP-MS by helium and argon gas streams. Time-depen- weighted average 206Pb/238U ages of 122.1 ± 0.5 Ma, 124.8 ± 1.4 Ma, and dent drifts of U-Th-Pb isotopic ratios were corrected by reference zircon 128.5 ± 1.6 Ma, respectively. GJ-1 as external standards (Hou et al., 2007). The Plešovice zircon was calibrated as another standard for unknown samples (Sláma et al., 2008). GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF MAFIC Isotopic ratios and element concentrations of zircons, concordia ages, DIKES and diagrams were calculated by Isoplot/Ex (3.00; Ludwig, 2003). The results are listed in Table 1. Major and Trace Element Geochemistry

Whole-Rock Major and Trace Element Geochemistry Doleritic dike rocks analyzed for geochemistry commonly display porphyric or ophitic textures. The majority of all these dike rocks consist We analyzed six dolerite dike samples (12YL54–9–30) from the mainly of subhedral to anhedral grains of subhedral to anhedral tabu- South Baer submassif and seven dolerite dike samples (12YL61–2–8) lar plagioclase (modal ~45%–50%), clinopyroxene (~5%), and green to from the North Baer submassif for major and trace element chemistry. greenish-brown altered hornblende (modal ~40%–45%) with magnetite, Samples were prepared by removing the oxidized surface and visible ilmenite, and chlorite as accessory phases (~5%). secondary materials, then by crushing these cleaned samples to ~200 Dolerite dike samples (12YL61) from the North Baer submassif are

mesh. Major elements were analyzed by X-ray fluorescence spectrometry characterized by higher SiO2 (48.82–51.65 wt%), MgO (8.76–12.58

at the Chinese Academy of Geological Sciences (Beijing). The analyti- wt%), and total alkaline (3.84–5.24 wt%) contents and lower FeOtotal

cal accuracy is estimated to be 1% relative for SiO2 and 2% relative for (6.96–7.40 wt%) and CaO (4.67–8.90 wt%) contents in comparison to the other oxides. Trace elements, including REEs, were determined the dolerite dike rocks (12YL54) from the South Baer submassif. Average

by ICP-MS. Analytical uncertainties are estimated to be 10% for trace values of the TiO2, K2O, and Al2O3 contents in all dike rocks are 0.83–1.12

elements with abundances <10 ppm, and ~5% for those >10 ppm. Loss wt%, 0.72–0.87 wt%, and 15.44–15.58 wt%, respectively. These TiO2 on ignition was determined by gravimetric techniques in which the contents are lower than those of typical ocean island basalt (OIB; 2.87 sample is heated in a closed container and the water vapor is collected wt%) and normal mid-oceanic ridge basalt (N-MORB; 1.27 wt%) values # in a separate tube, condensed, and then weighed. The detection limit for (Sun and McDonough, 1989). Mg numbers [Mg = 100 Mg/(Mg + Fetotal)]

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TABLE 1. LASER ABLATION–INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY ZIRCON U-Pb DATING FOR DOLERITE DIKES FROM THE SOUTH BAER (12YL54-9) AND NORTH BAER (12YL61-9) SUBMASSIFS, AND FOR A MICROGABBRO DIKE (13YL30-30) FROM THE CUOBUZHA MASSIF IN THE NORTHERN BELT OF THE WESTERN YARLUNG ZANGBO SUTURE ZONE Spot Th U Th/U 207Pb/235U ± 1σ 206Pb/238U ± 1σ 206Pb/238U ± 1σ (ppm) (ppm) (Ma) 12YL54-9-1111.31 67.46 1.65 0.13121 ± 0.002960.01894 ± 0.00017 120.94 ± 1. 09 12YL54-9-2315.47 184.56 1.71 0.12901 ± 0.002680.01891 ± 0.00019 120.78 ± 1. 21 12YL54-9-3 85.02 94.82 0.90 0.12401 ± 0.004390.01892 ± 0.00036 120.84 ± 2.28 12YL54-9-4123.81 64.58 1.92 0.13426 ± 0.00635 0.01920 ± 0.00026 122.63 ± 1. 62 12YL54-9-6 93.02 61.361.52 0.13107 ± 0.006080.01925 ± 0.00030 122.94 ± 1. 89 12YL54-9-7172.60 85.01 2.03 0.12919 ± 0.001870.01894 ± 0.00014 120.94 ± 0.87 12YL54-9-8 43.44 65.34 0.66 0.13046 ± 0.00314 0.01906 ± 0.00020 121. 69 ± 1. 28 12YL54-9-9131.45 64.31 2.04 0.13291 ± 0.00229 0.01891 ± 0.00015 120.75 ± 0.94 12YL54-9-10 123.71 68.85 1.80 0.13369 ± 0.00302 0.01920 ± 0.00019122.59 ± 1. 20 12YL54-9-12 130.87 73.54 1.78 0.13339 ± 0.001640.01924 ± 0.00017122.87 ± 1. 10 12YL54-9-14 308.46 161.23 1.91 0.12581 ± 0.001650.01903 ± 0.00014121.52 ± 0.90 12YL54-9-15 111.06 57.181.94 0.13461 ± 0.00225 0.01911 ± 0.00014122.02 ± 0.91 12YL54-9-16 84.44 112.66 0.75 0.12785 ± 0.00209 0.01922 ± 0.00016 122.71 ± 1. 02 12YL54-9-17 158.24 96.90 1.63 0.13100 ± 0.001770.01923 ± 0.00016 122.77 ± 1. 02 12YL54-9-18 132.79 117. 37 1. 13 0.13364 ± 0.00201 0.01933 ± 0.00022 123.45 ± 1. 42 12YL54-9-19 160.70 93.99 1.71 0.13778 ± 0.00286 0.01954 ± 0.00029 124.72 ± 1. 84 12YL54-9-20 198.93 126.62 1.57 0.13563 ± 0.00216 0.01935 ± 0.00024 123.54 ± 1. 52 12YL54-9-21 91.35 60.36 1.51 0.13595 ± 0.00294 0.01928 ± 0.00027 123.11 ± 1. 68 12YL54-9-22 87.41 104.21 0.84 0.14022 ± 0.00255 0.01987 ± 0.00027 126.81 ± 1. 73 12YL54-9-23 128.38 75.64 1.70 0.13024 ± 0.00219 0.01898 ± 0.00018121.22 ± 1. 15 12YL54-9-24 86.85 64.97 1.34 0.13143 ± 0.002390.01894 ± 0.00017 120.95 ± 1. 07 12YL54-9-25 72.0171. 16 1. 01 0.13684 ± 0.002180.01940 ± 0.00017 123.87 ± 1. 05 12YL54-9-26 120.80 66.84 1.81 0.13093 ± 0.00253 0.01921 ± 0.00022 122.68 ± 1. 38 12YL54-9-27 99.03 63.63 1.56 0.12815 ± 0.00267 0.01896 ± 0.00024 121. 10 ± 1. 52 12YL54-9-28 83.30 48.29 1.72 0.13784 ± 0.00673 0.01954 ± 0.00054 124.73 ± 3.41 12YL54-9-29 183.65 181.39 1. 01 0.12765 ± 0.001640.01904 ± 0.00021 121. 59 ± 1. 32 12YL54-9-30 94.77 86.90 1.09 0.13019 ± 0.00302 0.01921 ± 0.00028 122.68 ± 1. 76 13YL-30-30-2 765.56 274.14 2.79 0.19824 ± 0.02785 0.02037 ± 0.00069 130 ± 4 13YL-30-30-6 2128.62 400.96 5.31 0.15437 ± 0.012690.01999 ± 0.00049 128 ± 3 13YL-30-30-9 1186.51 569.55 2.08 0.14214 ± 0.008800.02023 ± 0.00045 129 ± 3 13YL-30-30-101478.13414.80 3.56 0.13220 ± 0.010680.01989 ± 0.00047 127 ± 3 13YL-30-30-12 824.82 298.61 2.76 0.13335 ± 0.0119 0.02063 ± 0.00050 132 ± 3 13YL-30-30-141064.07 405.67 2.62 0.14657 ± 0.011270.02047 ± 0.00049 131 ± 3 13YL-30-30-151622.53 587.64 2.76 0.11211 ± 0.007650.02000 ± 0.00046 128 ± 3 13YL-30-30-16915.34 316.60 2.89 0.13362 ± 0.012050.02027 ± 0.00051 129 ± 3 13YL-30-30-171661.65 541.08 3.07 0.15529 ± 0.011240.02045 ± 0.00047 130 ± 3 13YL-30-30-191348.30 364.72 3.70 0.15710 ± 0.013270.02039 ± 0.00048 130 ± 3 13YL-30-30-20 598.85 284.30 2.11 0.12896 ± 0.014650.01977 ± 0.00049 126 ± 3 13YL-30-30-22 1427.52 539.46 2.65 0.23575 ± 0.019730.02038 ± 0.00048 130 ± 3 13YL-30-30-24 616.86 399.011.55 0.19512 ± 0.015240.01962 ± 0.00046 125 ± 3 13YL-30-30-25 852.96 308.76 2.76 0.17975 ± 0.019690.01964 ± 0.00052 125 ± 3 12YL61-9-1 224.24 339.82 0.66 0.12129 ± 0.008540.01967 ± 0.00052 126 ± 3 12YL61-9-2 99.55 159.59 0.62 0.14988 ± 0.013770.01991 ± 0.00057 127 ± 4 12YL61-9-3 99.61 144.75 0.69 0.12173 ± 0.012480.02023 ± 0.00060 129 ± 4 12YL61-9-4 170.67 218.15 0.78 0.15091 ± 0.011060.01881 ± 0.00051 120 ± 3 12YL61-9-5 83.02 153.32 0.54 0.14493 ± 0.013330.01874 ± 0.00054 120 ± 3 12YL61-9-6 330.47 187. 10 1.77 0.15110 ± 0.011430.01980 ± 0.00055 126 ± 3 12YL61-9-7 135.91 251.34 0.54 0.13154 ± 0.010340.01953 ± 0.00051 125 ± 3 12YL61-9-8 65.00 138.34 0.47 0.11332 ± 0.011430.01941 ± 0.00058 124 ± 4 12YL61-9-9 53.85 126.18 0.43 0.12717 ± 0.013100.02088 ± 0.00063 133 ± 4 12YL61-9-10153.26 325.29 0.47 0.11647 ± 0.007760.01916 ± 0.00047122 ± 3 12YL61-9-1177.47 164.66 0.47 0.13791 ± 0.011740.02004 ± 0.00056 128 ± 4 12YL61-9-12 53.12101.02 0.53 0.14947 ± 0.01485 0.01949 ± 0.00061 124 ± 4 12YL61-9-14 46.62 85.47 0.55 0.15239 ± 0.017530.01984 ± 0.00065 127 ± 4 12YL61-9-15 64.35 156.91 0.41 0.14173 ± 0.012490.01963 ± 0.00058 125 ± 4 12YL61-9-16172.54 252.38 0.68 0.13864 ± 0.011080.01921 ± 0.00052 123 ± 3 12YL61-9-17198.23 313.57 0.63 0.13047 ± 0.00861 0.02007 ± 0.00053 128 ± 3 12YL61-9-18109.87 107.02 1.03 0.12072 ± 0.013120.01995 ± 0.00073 127 ± 5 12YL61-9-20 172.07 209.85 0.82 0.14520 ± 0.01091 0.01960 ± 0.00053 125 ± 3 12YL61-9-21 120.97 199.80 0.61 0.15425 ± 0.012140.01999 ± 0.00055 128 ± 3 12YL61-9-22 149.77 208.55 0.72 0.15822 ± 0.012160.01912 ± 0.00056122 ± 4 12YL61-9-23 44.63 74.95 0.60 0.12826 ± 0.019080.02012 ± 0.00079 128 ± 5 12YL61-9-25 144.83 232.67 0.62 0.12456 ± 0.00909 0.01912 ± 0.00052122 ± 3 12YL61-9-26 190.64 279.72 0.68 0.13023 ± 0.00877 0.01858 ± 0.00047 119 ± 3 12YL61-9-27 134.29 205.94 0.65 0.13678 ± 0.00985 0.01990 ± 0.00051 127 ± 3

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TABLE 2. MAJOR AND TRACE ELEMENT COMPOSITIONS OF DOLERITE DIKES FROM THE SOUTH BAER (12YL54) AND NORTH BAER (12YL61) SUBMASSIFS IN THE NORTHERN BELT OF THE WESTERN YARLUNG ZANGBO SUTURE ZONE Samples 12YL54-912YL54-25 12YL54-26 12YL54-27 12YL54-29 12YL54-30 12YL61-2 12YL61-3 12YL61-4 12YL61-5 12YL61-6 12YL61-7 12YL61-8 South Baer North Baer

SiO2 48.65 45.99 44.90 44.95 49.22 48.57 50.1249.75 51.6551. 47 50.9050.63 48.82

TiO2 1.31 1.28 1.30 1.27 0.92 0.63 0.86 0.79 0.83 0.79 0.85 0.86 0.85

Al2O3 14.98 14.92 15.37 14.81 15.54 17.00 15.9115.27 16.2714.83 15.4215.69 15.64

Fe2O3 1.56 1.82 2.121.90 1.99 1.52 1. 41 1. 43 1. 46 1. 32 1. 31 1. 44 1. 48 FeO 8.177.90 7.63 7.45 5.93 4.78 6.05 5.78 6.09 5.77 5.98 6.04 5.84 MnO 0.17 0.17 0.17 0.17 0.20 0.140.150.160.150.150.160.170.16 MgO 6.30 6.37 6.32 6.42 10.29 12.28 11.6612.17 8.76 9.41 9.57 10.2812.58 CaO12.57 16.63 18.22 18.76 8.50 6.31 4.67 5.75 5.53 8.90 7. 49 6.49 6.64

Na2O 2.40 1.00 0.34 0.29 2.26 1. 13 3.71 3.51 4.40 3.45 3.51 3.56 2.77

K2O 0.37 0.14 0.05 0.05 1.46 3.150.960.630.840.390.670.770.79

P2O5 0.11 0.10 0.11 0.11 0.08 0.06 0.08 0.07 0.08 0.07 0.08 0.08 0.07 LOI 2.31 2.84 3.34 3.19 3.33 4.58 4.46 4.73 3.65 3.42 3.81 3.92 4.65 Total 98.90 99.16 99.87 99.37 99.72 100.15100.04100.0499.71 99.9799.75 99.93100.29 FeOT 9.57 9.54 9.54 9.167.72 6.157.327.077.406.967.167.347.17 Mg# 54.23 54.58 54.39 55.78 70.58 78.23 74.1475.668.06 70.8870.64 71.6 75.95 M 58.12 59.21 59.85 60.80 75.75 82.22 77.6279.1272.1474.59 74.2375.39 79.50 Sc 35.30 36.70 34.40 34.30 37.30 35.60 31.8030.1031. 30 30.8032.70 31.4030.60 Ti 7890.00 7840.00 7383.00 7893.00 5587.00 3880.004922.00 4623.004791. 00 4689.005136.00 5185.004869.00 Cr 67.20 69.1061. 50 66.10218.00 260.00 69.3070.40 59.8081. 20 76.9069.00 66.50 V 266.00 272.00 247.00 256.00 212.00 163.00 194.00 185.00 194.00 188.00 201. 00 198.00 194.00 Ni 26.20 27.80 27.40 28.10 62.80 77.2037. 10 42.1038.40 35.0036.60 37.9035.40 Co 36.1037.40 33.80 36.70 34.80 33.50 30.2028.70 29.8029.70 32.7031. 10 29.80 Cu 30.80 30.30 28.60 38.80 21.80214.00 87.9013.10 14.7010.80 23.2075.80 100.00 Zn 75.80 74.70 70.70 72.80 99.90 45.60 46.0044.30 45.3040.90 45.5045.90 45.00 Ga 15.90 16.40 16.30 16.20 14.1013.00 14.8013.20 14.3013.90 14.0013.80 13.80 Mn 1406.001445.001339.001435.001577.00 1178.00 1185.00 1249.001195.00 1200.001306.00 1343.001221. 00 Mo 0.22 0.25 0.23 0.30 0.34 0.120.110.310.120.130.080.130.13 Pb 0.90 0.54 0.62 0.52 0.51 0.39 0.36 0.40 0.39 0.30 0.34 0.39 0.45 Rb 5.41 2.00 0.71 0.72 38.80 86.50 20.4013.90 16.207.0213.60 16.2016.80 Sr 167.00 81.00 30.30 37.80 136.00 144.00 183.00 195.00 317. 00 218.00 359.00 318.00 195.00 Y31. 90 32.00 30.60 30.90 24.00 19.1022.30 20.1022.40 21.2022.20 21.7021. 00 Zr 92.50 72.30 89.80 72.1057.30 28.60 59.0055.30 59.8054.40 59.0060.60 54.10 Nb 1.27 0.96 1.02 1.25 0.58 0.38 0.54 0.62 0.59 0.48 0.62 0.58 0.54 Ba 170.00 66.00 12.1010.90 472.00 238.00 112.00 86.40126.0044.80 87.0087. 30 82.40 La 3.22 2.90 2.98 3.10 2.03 1.29 1. 94 1. 89 2.10 2.29 2.24 2.22 2.03 Ce 9.40 8.71 8.87 8.81 6.31 4.24 5.90 5.46 6.42 6.25 6.32 6.50 6.11 Pr 1.87 1. 76 1.79 1.75 1.27 0.89 1. 19 1. 08 1. 25 1. 20 1. 21 1. 25 1. 18 Nd 9.62 8.79 8.97 8.80 6.61 4.72 6.15 5.77 6.35 5.97 6.15 6.11 6.10 Sm 3.58 3.22 3.28 3.24 2.60 1.91 2.34 2.11 2.37 2.20 2.36 2.34 2.22 Eu 1.28 1.23 1.28 1. 19 0.98 0.75 0.90 0.80 0.88 0.81 0.85 0.84 0.86 Gd 4.76 4.48 4.58 4.50 3.56 2.67 3.26 2.95 3.34 3.04 3.11 3.14 3.13 Tb 0.82 0.80 0.80 0.78 0.62 0.48 0.57 0.52 0.57 0.52 0.57 0.55 0.54 Dy 5.56 5.28 5.40 5.19 4.19 3.35 3.82 3.42 3.80 3.52 3.84 3.64 3.74 Ho 1. 14 1.08 1. 11 1.06 0.88 0.67 0.78 0.71 0.78 0.74 0.78 0.77 0.77 Er 3.61 3.39 3.46 3.24 2.72 2.05 2.47 2.29 2.45 2.32 2.42 2.39 2.38 Tm 0.48 0.44 0.45 0.45 0.37 0.27 0.34 0.31 0.33 0.31 0.34 0.32 0.32 Yb 3.17 2.97 2.97 2.89 2.29 1.77 2.17 1. 98 2.16 2.00 2.18 2.15 2.09 Lu 0.47 0.45 0.45 0.43 0.35 0.26 0.33 0.30 0.33 0.31 0.34 0.32 0.32 Hf 2.47 1.99 2.43 1.95 1.64 0.99 1. 65 1. 54 1. 66 1. 50 1. 71 1. 61 1. 57 Ta 0.10 0.08 0.08 0.09 0.05 <0.05 <0.050.06<0.05 <0.05<0.05 <0.05<0.05 Th 0.14 0.10 0.13 0.13 0.08 <0.05 0.10 0.14 0.11 0.08 0.10 0.09 0.08 U 0.08 0.06 0.08 0.12 0.10 0.08 0.32 0.04 0.19 0.06 0.08 0.07 0.08 Th/La 0.04 0.03 0.04 0.04 0.04 0.03 0.05 0.07 0.05 0.03 0.04 0.04 0.04 Note: Major elements are in weight percent; trace elements are in parts per million. Mg# = 100 Mg/(Mg + ∑Fe); M = 100 Mg/(Mg + Fe2+).

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TABLE 3. Sr, Nd, AND Pb ISOTOPIC COMPOSITIONS OF THE MAFIC DIKES FROM THE NORTH BAER AND CUOBUZHA MASSIFS IN THE NORTHERN BELT OF THE WESTERN YARLUNG ZANGBO SUTURE ZONE Sample 12YL60-7 12YL60-9 12YL60-1012YL60-1112YL60-1212YL74-212YL74-312YL74-412YL74-512YL74-6C9-1* C9-4* Purang dolerite in North Baer dolerite in northern belt Cuobuzha microgabbro in northern belt southern belt Rb 5.39 4.016.74 9.03 8.95 5.436.855.437.034.891.701.20 Sr 192.10 265.40 276.20 213.50 216.30 235.70 379.20 271.50 380.30 142.60 681.00 404.00 Sm 2.42 1.99 2.63 2.93 2.44 2.011.971.872.121.921.181.49 Nd 6.81 5.38 7.64 8.06 6.84 5.24 5.165.045.575.733.404.30 U 0.05 0.04 0.05 0.08 0.06 0.070.060.130.090.06<0.05 <0.05 Th 0.10 0.09 0.11 0.14 0.13 0.090.090.090.090.09<0.05 0.05 Pb 0.04 0.05 0.56 7123.00 0.50 0.500.490.470.460.46<5<5 87Rb/86Sr 0.08490 0.04510 0.07230 0.09240 0.11870 0.06780 0.05370 0.05740 0.05460 0.101800.00722 0.00859 87Sr/86Sr 0.70448 0.70445 0.70446 0.70447 0.70450 0.70460 0.70457 0.70460 0.70459 0.70461 0.70532 0.70491 1σ 979866798919 7 87 86 ( Sr/ Sr)t 0.70433 0.70437 0.70433 0.70430 0.70428 0.70448 0.70447 0.70449 0.70449 0.70443 0.70531 0.70489 147Sm/144Nd 0.21040 0.22350 0.20480 0.21490 0.21020 0.22190 0.22470 0.20880 0.22040 0.21320 0.21000 0.21000 143Nd/144Nd 0.513106 0.513099 0.513112 0.513121 0.513079 0.513099 0.513102 0.513085 0.513119 0.513127 0.51309 0.51310 1σ 679878697617 7 143 144 ( Nd/ Nd)t 0.512930 0.512912 0.512941 0.512941 0.512903 0.51291 0.5129140.51291 0.512934 0.512949 0.51290 0.51291

εNd(0) 9.13 8.99 9.25 9.42 8.6 8.999.058.729.389.548.909.00

εNd(t) 8.91 8.56 9.13 9.13 8.39 8.588.608.528.999.288.608.70 206Pb/204Pb 17.591 17.563 17.546 17.634 17.679 17.984 17.826 17.769 17.752 17.978 17.948 17.905 1σ 30 32 23 29 26 25 31 30 32 29 16 2 207Pb/204Pb 15.496 15.473 15.432 15.542 15.581 15.78415.753 15.691 15.667 15.779 15.4969 15.4885 1σ 25 28 24 29 31 30 25 27 28 31 94 208Pb/204Pb 37.741 37.724 37.769 37.826 37.845 37.85137. 873 37.91637. 938 37.952 37.8794 37.8482 1σ 30 28 31 28 30 30 32 25 29 30 31 2 206 204 ( Pb/ Pb)t 16.111 16.524 17.429 17.634 17.525 17.81217. 675 17.4317. 503 17.819 207 204 ( Pb/ Pb)t 15.424 15.422 15.426 15.542 15.57415.776 15.746 15.675 15.655 15.771 208 204 ( Pb/ Pb)t 36.782 36.941 37.69 37.826 37.736 37.781 37.796 37.837 37.8637. 869

87 86 87 86 −1 –1 143 144 Note: ( Sr/ Sr)CHUR = 0.7045 (CHUR—chondrite uniform reservoir), ( Rb/ Sr) CHUR = 0.0827, λSr = 0.0000142 Ma ,λNd = 0.00000654 Ma , Nd/ Nd = 0.512638, 147Sm/144Nd = 0.1967, the corrected age 128 Ma. *After Liu et al. (2013).

and M values [M = 100 Mg/(Mg + Fe2+)] for the North Baer samples those displayed by many other mafic dike rocks documented from the range from 68.06 to 75.95 (72.41 on average) and 72.14 to 79.50 (76.09 NB of the western YZSZ (Figs. 8C, 8D). on average), and for the South Baer samples range from 54.28 to 78.23 (61.30 on average) and 58.12 to 82.22 (65.99 on average), respectively Sr-Nd-Pb Isotopes

(Table 2). The SiO2, TiO2, and FeOtotal contents of all dike samples cor-

relate negatively, and Al2O3 and Ni correlate positively against the MgO We have calculated the initial Sr, Nd, and Pb isotope ratios of our values (Fig. 6). dike samples at 128 Ma, which represents the crystallization ages of a

In the Nb/Y-Zr/TiO2 diagram, all analyzed dike samples plot within the microgabbro dike from the North Baer submassif (128.1 ± 2.1 Ma, sample basalt field (Fig. 7A). In a Co-Th diagram the analyzed samples plot in 12YL60; Liu et al., 2015b), and microgabbro and dolerite dikes from the the basalt and basaltic andesite–andesite fields of the island arc tholeiite Cuobuzha massif (128.5 ± 1.6 Ma, 13YL30–30, and 128.2 ± 1.4 Ma, (IAT) series (Fig. 7B). These geochemical features are similar to those sample 12YL74; Liu et al., 2015c). The North Baer microgabbro dike 87 86 of microgabbro and dolerite dike rocks from the North Baer (Liu et al., samples (12YL60) exhibit slightly lower age-corrected ( Sr/ Sr)t ratios 2015b; Zheng et al., 2017) and Cuobuzha massif (Liu et al., 2015c) in of 0.70428–0.70437 in comparison to the age-corrected 87Sr/86Sr ratios the NB of the western YZSZ (Fig. 7). (0.70443–0.70449) obtained from the Cuobuzha microgabbro dikes. How- 143 144 Chondrite-normalized REE patterns of the South Baer dolerite dike ever, the North Baer microgabbro dikes have relatively high ( Nd/ Nd)t

(12YL 54) and the North Baer dolerite dike samples (12YL 61) display ratios of 0.51290–0.51294 and εNd(t)values of +8.39 to +9.13, just as the typical N-MORB–like affinities, with a light (L) REE-depleted pattern Cuobuzha dolerite dike samples have (0.51291–0.51295 and +8.58 to

but slightly higher ratios of SmN/YbN (average 1.23 and 1.20, respectively) +9.28) (Table 3). These isotope ratios and values we have obtained from in comparison to N-MORB values (0.96; Sun and McDonough, 1989). the North Baer and Cuobuzha dike intrusions are slightly higher than those There are no Eu anomalies observed, and the average Eu* = 2Eu/(Sm + documented from dolerite dikes in the Purang ophiolite exposed along the 87 86 Gd) values are 0.98 for South Baer and 0.97 for North Baer (Fig. 8A). SB of the western YZSZ, showing ( Sr/ Sr)t ratios of 0.704893–0.705310, 143 144 These features are comparable to those of mafic dike rocks documented ( Nd/ Nd)t ratios of 0.51290–0.51291, and εNd(t) values of +8.6 to +8.8 from the Cuobuzha and North Bear massifs in the NB of the western (Miller et al., 2003; Liu et al., 2013). YZSZ (Figs. 8A, 8B; Liu et al., 2015b, 2015c; Zheng et al., 2017). In the The North Baer microgabbro dike rocks have initial ratios of 206Pb/204Pb N-MORB–normalized multielement diagrams, the majority of the dike = 17.546–17.679, 207Pb/204Pb = 15.432–15.581, and 208Pb/204Pb = 37.724– samples exhibit enrichment in large ion lithophile elements (LILE) and 37.845, which are slightly lower than those of the Cuobuzha microgab- in LREEs in comparison to high field strength elements (HFSE; i.e., Nb, bro dikes (17.752–17.984, 15.667–15.784, and 37.851–38.952, respec- Ta, Th, Zr, Hf). Highly negative Nb, Ta, and Ti anomalies are similar to tively). All these dikes plot within the Indian Ocean N-MORB field with

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A, sample 12YL54-9 100 μm

123 4 6 7 8 9 10

120.9±1.1 120.7±1.2 120.8±2.2 122.6±1.6122.9±1.9 120.9±0.87 121.7±1.3 120.8±0.93 122.6±1.2

12 14 15 16 17 18 19 20 21

122.9±1.1 121.5±0.89 122.0±0.91122.7±1.0 122.8±1.0 123.4±1.4124.7±1.8 123.5±1.5123.1±1.7

22 23 24 25 26 27 28 29 30

126.8±1.7 121.2±1.2121.0±1.1 123.9±1.0122.7±1.4121.1±1.5 124.7±3.4 121.6±1.3 122.7±1.8

B, sample 13YL30-30 26910121415 50 μm

130±4 128±3 129±3 127±3 132±3 131±3128±3

16 17 19 20 22 24 25

C, sample 12YL61-9 129±3 130±3 130±3126±3 130±3 125±3 125±3 100 μm

123 4 5 6 7 8 9 10 11 12

126±3 127±4129±4 120±3 120±3126±3 125±3 124±4 133±4 122±3 128±4 124±4

14 15 16 17 18 20 21 22 23 25 26 27

127±4125±4 123±3 128±3 127±5 125±3 128±31122±4 128±5 122±3119±3 27±4

Figure 4. Cathodoluminescence images of representative zircons. (A) From a dolerite dike (sample 12YL54–9) in South Baer. (B) From a microgabbro dike (sample 13YL30–30) in the Cuobuzha massif. (C) From a dolerite dike (sample 12YL61–9) in the North Baer ophiolite in the northern belt. The circles show spots of laser ablation–inductively coupled plasma–mass spectroscopy dating; spot numbers and U-Pb ages are listed near the individual zircon grain.

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0.0215 South Baer 129 Sample: 12YL54-9

127 0.0205 130

38 125 /U 0.0195 Pb 123 20 62

120 121 0.0185

119 A Mean = 122.1 ± 0.5 (n = 27) 0.0175 B 0.1050.115 0.125 0.1350.145 0.155 117 MSWD= 1.01, probability = 0.44 2072Pb/U35 0.0225 Cuobuzha 136 140 Sample: 13YL30-30 134 0.0215

132

0.0205 130 130

38 128 /U 0.0195

Pb 126 20 62 120 0.0185 124 C 122 Mean = 128.2 ± 1.4 (n = 17) 0.0175 MSWD= 0.46, probability = 0.97 D 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 120 2072Pb/U35 0.024 North Baer 134 Sample: 12YL61-9

140 0.022 130 8 3 /U 126

Pb 0.020 62 0 2

120 122

0.018

118 Mean = 124.8 ± 1.4 (n = 22) 2072Pb/U35 E 0.016 MSWD= 0.66, probability = 0.87 F 0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21 114

Figure 5. Zircon U-Pb concordia diagrams. (A, B) Dolerite from South Baer. MSWD—mean square of weighted deviates. (C, D) Microgabbro from the Cuobuzha massif. (E, F) Dolerite from the North Baer ophiolite in the western Yarlung Zangbo suture zone, southern Tibet.

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54 18 A B

) 16 .%

t 50 w 2 O( O (wt.%) 23 Si

48 Al Sout h Baer dolerite (This study) 14 Nort h Baer dolerite (This study) Nort h Baer microgabbro (Liu et al., 2015b) 46 Nort h Baer dolerite (Zheng et al.,2017) Cuobuzh a dolerite (Liu et al., 2015c) Cuobuzha microgabbro (Liu et al., 2015c) 44 12 6810 12 14 6810 12 14 C 20 D 1.6

) 16 1.2 2 O (wt.% O (wt.%) 12 Ti 0.8 Ca

0.4 8

0.0 4 6810 12 14 6810 12 14 14 160 E F 140

) 12 120 .% t m) (w T pp 100 i (

eO 10 N F 80

8 60

6 20 6810 12 14 6810 12 14 MgO (wt.%) MgO (wt.%)

Figure 6. Plots of major elements and Ni against MgO for the mafic dike samples in the northern belt.

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S outh Baer dolerite (This study) Cuobuzha dolerite (Liu et al., 2015c) ABNort h Baer dolerite (This study) Cuobuzha microgabbro (Liu et al., 2015c) North Baer microgabbro (Liu et al., 2015b) North Baer dolerite (Zheng et al., 2017) 1 10 Alkali Rhyolite High-K calc-alkaline Phonolite Shoshonite

1 Rhyolite+Dacite Trachyte m) p

p 1 0.1 nolite Calc-alkaline i Pho *0.000 h ( 2 chy Tephr Tra T Basaltic andesite iO Andesite Andesite desite te An Island Arc Zr/T Basalt-Andesi 0.1 Tholeiite 0.01 Alkali Foidite Basalt

Basalt Basalt Dacite/Rhyolite 0.01 0.001 70 60 50 40 30 20 10 0 0.01 0.11 10 100 Nb/Y Co (ppm)

Figure 7. (A) The Zr/TiO2 versus Nb/Y (after Winchester and Floyd, 1977) classification diagrams for mafic dikes in the northern belt. (B) Co versus Th (after Hastie et al., 2007).

the initial radiogenic Pb isotope ratios of 17.31–18.5, 15.43–15.56, and Magmas originated from partial melting of a spinel lherzolite source dis- 37.1–38.7, respectively (Mahoney et al., 1998; Xu and Castillo, 2004). play flat REE patterns (Aldanmaz et al., 2000; Workman and Hart, 2005). The age-corrected Pb values of the North Baer microgabbro samples All our analyzed dike samples display flat N-MORB–type REE patterns 206 204 are characterized by ratios of ( Pb/ Pb)t = 16.111–17.634 (average in the chondrite-normalized diagrams, indicating a depleted mantle origin 207 204 208 204 17.045), ( Pb/ Pb)t = 15.424–15.574 (average 15.478), and ( Pb/ Pb)t (Figs. 8A, 8B). However, they also show a conspicuous HREE fractionation = 36.782–37.826 (average 37.395). These values are slightly lower than pattern in comparison to middle REEs. We interpret this geochemical fea-

those of the Cuobuzha microgabbro dikes with initial ratios of 17.503– ture as a garnet signature with high (Sm/Yb)N ratios, as documented from 17.819, 15.655–15.776, and 37.781–38.869, respectively. basaltic lavas of the Alpine Corsica ophiolites (1.1–2.6, most >1.5; Saccani et al., 2008), and from aphyric basaltic lavas of the Mid-Atlantic Ridge MANTLE MELT SOURCE AND MAGMA EVOLUTION OF MAFIC (0.98–1.72; Niu et al., 2001). These signatures may be a manifestation of DIKES either the initiation of partial melting in the garnet-peridotite stability field deep in the mantle, or partial melting of a lithospheric mantle source that Mantle Melt Source contains networks of garnet-pyroxenite veins (Saccani et al., 2008). The

average (Sm/Yb)N ratio is 1.23 and 1.20 for the mafic dike rocks from the The Mg#s of the analyzed mafic dike rocks (average 61.47, 72.26 for South and North Baer submassifs, respectively. We have also calculated the the South and North Baer, respectively, and 64.56 for the Cuobuzha mas- mean ratio for the Cuobuzha microgabbro dikes (1.25; Liu et al., 2015c) sifs; Liu et al., 2015c) are slightly higher than those of primitive MORBs, and for the dolerite dikes from the North Baer submassif (1.20; Liu et al.,

with average Mg#s of 52.8–59.7, from the Atlantic, Pacific, and Indian 2015b). These (Sm/Yb)N ratios are markedly lower than those of the gabbro ocean spreading centers (Wilkinson, 1982). However, the observed nega- intrusions in the Corsica ophiolites. Therefore, this comparative analysis

tive correlations of the SiO2, TiO2, and FeOtotal contents and the positive and evaluation of the (Sm/Yb)N ratios indicate that primitive magmas of the

correlations of the Al2O3 and Ni values against the MgO contents in the mafic dike intrusions in the NB reflect very little contribution from partial dike rocks (Fig. 6) are comparable to the worldwide modern MORB pat- melting of garnet peridotites. The main mantle source for their magmas terns, which show decreasing Ti, Mn, Na, P, and increasing A1, Ca, Ni, Cr therefore appears to be composed mainly of spinel lherzolite. values with increasing Mg#s (Wilkinson, 1982). N-MORB extrusive rocks The Sm/Yb ratios would not be changed during partial melting of a in the External Liguride ophiolites in the Apennines (Italy) (Montanini lherzolitic mantle source because both Sm and Yb have similar partition et al., 2008) and in the Alpine ophiolites in Corsica (France) display cor- coefficients, but La/Sm ratios and Sm contents of melts produced from relations similar to those of our dike samples from the South and North such a lherzolitic mantle source may decrease (Aldanmaz et al., 2000; Baer and Cuobuzha massifs. The External Liguride and Corsican ophio- Pearce, 2008). In the Sm/Yb versus La/Sm diagram, all the mafic dike lites represent a mid-ocean ridge–generated Jurassic oceanic lithosphere rocks from the Cuobuzha and North and South Baer massifs along the (Saccani et al., 2008; Dilek and Furnes, 2011, 2014). Our samples and NB of the western YZSZ plot slightly above the spinel lherzolite melting these modern and ancient N-MORB upper crustal rocks are interpreted curve and close to N-MORB, indicating that their magmas may have been to have formed by various degrees of partial melting of depleted and produced by ~7%–15% partial melting of a spinel lherzolite source with heterogeneous mantle sources (Wilkinson, 1982, and references therein). an N-MORB affinity (Fig. 9). We thus infer that mafic dike intrusions In chondrite-normalized REE diagrams, magmas generated from par- within the peridotite massifs in the NB were originated from magmas, tial melting of a garnet-lherzolite mantle source generally exhibit rela- derived predominantly from partial melting of a depleted N-MORB spinel tively high fractionation of HREEs due to the presence of residual garnet. lherzolite mantle source with minor garnet-bearing relics.

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100 100 A D S outh Baer (this study) Nort h Baer (this study) Nort h Baer (Liu et al., 2015b) Cuobuzha (Liu et al., 2015c)

RB 10

O N-MORB BAB Mariana FAB-D Lau IAT -M

10 N k/ c

Ro 1 Rock/Chondrite

1 0.1 MB La Pr Nd Eu Tb Dy Ho Er Tm Yb Lu Ce Sm Gd Rb Th Nb La Pr Nd Hf Eu Ti Dy Ho Tm Lu Ba U Ta Ce Sr Zr Sm Gd Tb Y Er Yb 100 B E e B R 1

10 MO k/N- c Rock/Chondrit Ro

1 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Nb La Ce Pr Nd Hf Sm Ti Gd Tb Dy Ho Er Tm Yb

100 C F

10 RB O 1 M - N ck/ o

1 R Rock/N-MORB

0.1 0.1 Rb Th Nb La Pr Nd Hf Eu Ti Dy Ho Tm Lu Th Nb La Ce Pr Nd Hf Sm Ti Gd Tb Dy Ho Er Tm Yb Ba U Ta Ce Sr Zr Sm Gd Tb Y Er Yb

Figure 8. Chondrite-normalized rare earth element patterns and normal mid-oceanic ridge basalt (N-MORB; Sun and McDonough, 1989) normalized rare element diagrams for mafic dikes from the northern belt in the western Yarlung Zangbo suture zone, Tibet; backarc basalts (BAB; Gale et al., 2013), forearc diabases, and boninites from the Mariana arc (Reagan et al., 2010); Lau island arc tholeiite (IAT) (Hergt and Woodhead, 2007). FAB-D—forearc basalt-diabase; MB—Mariana boninite.

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(Niu, 1997); (3) C (trace element concentrations in the original mantle 10 O Sout h Baer dolerite (This study) Garnet lherzolite 1 rock) as an N-MORB–like mantle source (depleted MORB mantle source; North Baer dolerite (This study) North Baer microgabbro (Liu et al., 2015b) Ewart et al., 1994); (4) partition coefficients of minerals in spinel lherzolite 5 Cuobuzh a dolerite (Liu et al., 2015c) Spinel+Garnet (Table 4) are taken from Stracke et al. (2003); (5) initial partial melting Cuobuzha microgabbro (Liu et al., 2015c) lherzolite occurred in a spinel lherzolite stability field because the analyzed mafic

10 SpinelGarnet and Figure 10. We also calculated the REE contents of melts created by 20 5 1 non-modal instantaneous fractional melting (equation after Shaw, 1970) 5 10 1 1 from a depleted MORB mantle source (see GSA Data Repository Item ). 10 E-MORB Spinel lherzolite 20 N-MORB PM C/LOC1=+/[DFO (1 –P)]. (1) Depletion Enrichment

1 10 . (2) La/Sm C/SOC[=×(DOO–PF)/(1–F)][1/(D + F(1–P)] Figure 9. La/Sm versus Sm/Yb diagram of mafic dikes in the northern belt, Tibet. Garnet and spinel melting curves are after Aldanmaz et al. (2000, and 1/P . (3) references therein); numbers along lines represent the degree of the partial C/LOC1=×/F [1 –(1–PF /DO )] melting. Primitive mantle (PM), normal mid-oceanic ridge basalt (N-MORB), and enriched (E) MORB compositions are from Sun and McDonough (1989). 1/P C/SOC1= /(1–F)/(1–PF /DO ) . (4)

Modeling of Non-Modal Batch and Fractional Partial Melting DxO1=+kd12xkdx23++kd3 … (5)

Scientists have applied non-modal batch melting equations to evalu-

ate and to model variations in REE concentrations during partial melt- Py=+11kd yk22dy++33kd … (6) ing of depleted MORB mantle, primitive mantle, and inferred enriched

mantle sources (Aldanmaz et al., 2000; Stracke et al., 2003; Liang and Liu, CL, CO, and CS are trace element concentrations in the melt, in the

2016). The upper mantle peridotites in the Cuobuzha massif are composed original mantle rock, and in the residue mantle rock, respectively. DO mainly of depleted harzburgite, clinopyroxene harzburgite, and minor represents the bulk distribution coefficient, and is calculated by using

dunite, the first two of which are intruded by microgabbro and dolerite Equation 5. The term x1 represents a starting mode, the weighted mean

dikes (Liu et al., 2015c; Feng et al., 2015, 2017). Harzburgites appear of the solid partition coefficient of mineral 1, and y1 means the propor-

to have undergone various degrees of partial melting, melt-rock interaction of phase 1 in melt; kd1 represents the Nernst partition coefficient of tions, or infiltration (Feng et al., 2017). However, the nature and origin of the inferred fluids or melts and their association with the generation TABLE 4. MINERAL/MELT PARTITION COEFFICIENTS USED FOR PARTIAL of mafic dike magmas are unclear. To approach this problem, we have MELTING MODELS modeled and compared the REE contents of partial melts with Cuobuzha Di (partition coefficients) mafic dikes, and residual solid harzburgites with Cuobuzha peridotites. DMM Cuobuzha and other mafic dikes in almost all ophiolitic massifs are OlivineOrthopyroxene Clinopyroxene Spinel northwest-southeast striking in the NB, they have very similar chronologi- La 0.2060.0002 0.0031 0.0800 0.0006 cal, geochemical features (Figs. 5, 7–9) and Sr-Nd-Pb isotopic contents Ce 0.7220.000070.0031 0.15000.0006 Nd 0.8150.000420.000520.2600 0.0006 (see below). These uniform field occurrences and compositional properties Sm 0.2990.00110.0016 0.4900 0.0010 likely show that a finite amount of melt remained in equilibrium with the Eu 0.1150.0005 0.0090 0.5500 0.0009 residual solid at all times, or buoyant melts segregated from the residues Gd 0.4190.00110.0065 0.6000 0.0006 as soon as they formed, but pooled all the fractional melts together in Dy 0.5250.00170.01100.6500 0.0015 Er 0.3470.01090.02100.7200 0.0030 a magma chamber, then at some point these melts were released and Yb 0.3470.0240 0.0380 0.7100 0.0045 moved outward to produce the dike magmas. This process is in line with Lu 0.0540.0200 0.0400 0.7200 0.0045 an equilibrium non-modal batch partial melting and aggregated fractional Note: DMM—depleted mid-oceanic ridge basalt mantle after Ewart et al. melting, because the mode proportions of olivine, pyroxene, and spinel (1994); partition coefficients after Stracke et al. (2003). are likely to change during partial melting in the mantle source. Some assumptions about the composition and mineralogy of the mantle are: (1) the mantle source of spinel lherzolite is composed of 58% oliv- 1 ine, 27% orthopyroxene, 12% clinopyroxene, and 3% spinel (McKenzie GSA Data Repository Item 2017398, modeling results of the REE contents (ppm) of melts created by non-modal instantaneous fractional melting from a depleted and O’Nions, 1991); (2) these minerals enter the melts in the proportions MORB mantle source, is available at http://www.geosociety.org/datarepository/2017, of −17% olivine, 65% orthopyroxene, 47% clinopyroxene, and 5% spinel or on request from [email protected].

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TABLE 5. MODELING RESULTS OF THE REE CONTENTS OF MELTS CREATED BY NON-MODAL BATCH AND AGGREGATED FRACTIONATION PARTIAL MELTING FROM A DEPLETED MORB MANTLE SOURCE

12Y 12YL Melts created by non-modal batch partial melting with various degrees (F) REE DM L74 78 0.05 0.06 0.07 0.08 0.090.100.110.120.130.140.150.160.17 La 1.89 2.45 0.206 3.52 3.02 2.65 2.36 2.121.931.771.641.521.421.331.251.19 Ce 5.47 7. 16 0.722 11.06 9.69 8.61 7. 76 7. 05 6.47 5.97 5.55 5.18 4.85 4.57 4.32 4.09 Nd 5.19 6.87 0.815 10.8 9.67 8.76 8.01 7. 37 6.83 6.36 5.95 5.59 5.28 4.99 4.74 4.51 Sm 1.88 2.45 0.299 3.04 2.82 2.63 2.46 2.322.192.071.971.871.781.711.631.57 Eu 0.72 0.96 0.115 1.09 1.02 0.96 0.9 0.850.810.770.730.7 0.67 0.64 0.62 0.59 Gd 2.64 3.32 0.419 3.81 3.57 3.37 3.19 3.022.872.742.622.512.4 2.31 2.22 2.14 Dy 3.10 3.87 0.525 4.51 4.26 4.03 3.83 3.653.483.333.193.062.952.842.732.64 Er 2.00 2.47 0.347 2.65 2.52 2.41 2.31 2.21 2.122.041.971.9 1. 83 1. 77 1. 71 1. 66 Yb 1. 76 2.17 0.347 2.45 2.34 2.24 2.15 2.071.991.921.861.791.741.681.631.58 Lu 0.27 0.33 0.054 0.38 0.37 0.35 0.34 0.320.310.3 0.29 0.28 0.27 0.26 0.26 0.25 Residues created by non-modal batch partial melting with various degrees (F) CH DH DM 0.05 0.06 0.07 0.08 0.090.100.110.150.160.170.180.190.20 La 0.03 0.02 0.206 0.03 0.03 0.02 0.02 0.020.010.010.010.010.01 000 Ce 0.07 0.05 0.722 0.18 0.15 0.130.11 0.1 0.08 0.07 0.04 0.04 0.03 0.03 0.02 0.02 Nd 0.07 0.04 0.815 0.29 0.25 0.22 0.19 0.170.150.130.080.070.060.050.040.03 Sm 0.06 0.03 0.299 0.15 0.14 0.12 0.11 0.1 0.09 0.08 0.05 0.04 0.04 0.03 0.03 0.02 Eu 0.03 0.01 0.115 0.06 0.06 0.05 0.05 0.040.040.030.020.020.020.010.010.01 Gd 0.18 0.07 0.419 0.24 0.22 0.2 0.180.160.150.130.090.080.070.060.050.04 Dy 0.26 0.09 0.525 0.32 0.29 0.26 0.24 0.220.2 0.18 0.12 0.10.090.080.070.06 Er 0.2 0.06 0.347 0.23 0.21 0.19 0.18 0.160.150.140.1 0.09 0.08 0.07 0.06 0.05 Yb 0.24 0.07 0.347 0.24 0.22 0.2 0.19 0.180.160.150.110.1 0.09 0.09 0.08 0.07 Lu 0.04 0.01 0.054 0.04 0.04 0.03 0.03 0.030.030.030.020.020.020.010.010.01 Note: REE (rare earth elements) are in parts per million. MORB—mid-oceanic ridge basalt. DM—deleted mantle (after Ewart et al., 1994). 12YL74 and 12YL78 represent the mean compositions of Cuobuzha microgabbro dike samples (n = 7) and dolerite dike samples (n = 7), respectively (after Liu et al., 2015c). CH and DH represent the mean compositions of Cuobuzha clinopyroxene harzburgites (n = 8) and depleted harzburgites (n = 5), respectively (after Feng et al., 2017).

TABLE 6. MODELING RESULTS OF THE REE CONTENTS OF MELTS AND MANTLE RESIDUES CREATED BY AGGREGATED FRACTIONATION PARTIAL MELTING FROM A DEPLETED MORB MANTLE SOURCE

12Y 12Y Melts created by non-modal aggregated fractionation partial melting with various degrees (F) REE DM L74 L78 0.06 0.07 0.08 0.09 0.10 0.110.120.130.140.15 0.16 0.170.180.190.20 La 1.89 2.45 0.206 3.43 2.94 2.57 2.29 2.06 1. 87 1. 72 1. 58 1. 47 1. 37 1. 29 1. 21 1. 14 1. 08 1. 03 Ce 5.47 7. 16 0.722 11.71 10.18 8.97 8.00 7.21 6.566.025.555.164.814.514.254.013.803.61 Nd 5.19 6.87 0.81512.02 10.76 9.70 8.78 8.00 7.33 6.75 6.25 5.81 5.43 5.09 4.79 4.53 4.29 4.07 Sm 1.88 2.45 0.299 3.39 3.18 2.98 2.80 2.63 2.47 2.32 2.192.061.951.841.741.651.571.49 Eu 0.72 0.96 0.115 1.21 1. 14 1.08 1.02 0.97 0.91 0.87 0.82 0.78 0.74 0.70 0.66 0.63 0.60.57 Gd 2.64 3.32 0.419 4.19 3.98 3.79 3.6 3.42 3.25 3.09 2.94 2.79 2.65 2.53 2.42.292.182.08 Dy 3.10 3.87 0.525 4.94 4.72 4.51 4.31 4.12 3.93 3.763.593.423.273.122.982.842.722.60 Er 2.00 2.47 0.347 2.87 2.77 2.67 2.57 2.48 2.392.302.222.132.051.971.901.821.751.69 Yb 1. 76 2.17 0.347 2.64 2.56 2.48 2.40 2.32 2.24 2.172.1 2.03 1. 96 1. 89 1. 83 1. 76 1. 70 1. 64 Lu 0.27 0.33 0.054 0.41 0.40 0.39 0.38 0.36 0.350.340.330.320.310.300.290.280.270.26 Residues created by non-modal aggregated fractionation partial melting with various degrees (F) CH DH DM 0.05 0.06 0.07 0.08 0.09 0.100.110.130.14 0.15 0.16 La 0.03 0.02 0.206 00000000000 Ce 0.07 0.05 0.722 0.04 0.02 0.01 00000000 Nd 0.07 0.04 0.815 0.15 0.1 0.07 0.04 0.03 0.02 0.01 0000 Sm 0.06 0.03 0.299 0.12 0.1 0.08 0.07 0.05 0.04 0.03 0.02 0.01 0.01 0.01 Eu 0.03 0.01 0.115 0.05 0.05 0.04 0.03 0.03 0.02 0.02 0.01 0.01 0.01 0 Gd 0.18 0.07 0.419 0.21 0.18 0.15 0.13 0.1 0.090.070.040.030.020.02 Dy 0.26 0.09 0.525 0.28 0.24 0.21 0.18 0.15 0.130.1 0.07 0.05 0.04 0.03 Er 0.2 0.06 0.347 0.21 0.19 0.160.150.13 0.110.090.070.060.050.04 Yb 0.24 0.07 0.347 0.22 0.2 0.18 0.16 0.14 0.130.110.090.070.060.05 Lu 0.04 0.01 0.054 0.04 0.04 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.02 0.01 Note: REE (rare earth elements) are in parts per million. MORB—mid-oceanic ridge basalt. DM—deleted mantle (after Ewart et al., 1994). 12YL74 and 12YL78 represent the mean compositions of Cuobuzha microgabbro dike samples (n = 7) and dolerite dike samples (n = 7), respectively (after Liu et al., 2015c). CH and DH represent the mean compositions of Cuobuzha clinopyroxene harzburgites (n = 8) and depleted harzburgites (n = 5), respectively (after Feng et al., 2017).

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20 ABF=7 20 F=9 F=6 F=10

e F=11 F=6 F=13 F=12 F=15 e

10 F=17 F=16 9 Rock/Chondrit 10 F=20 9 Rock/Chondrit 8 8 7 7 Cuobuzha microgabbro dike (12YL74, n=7) 6 Cuobuzha dolerite dike (12YL78, n=7 ) 6

5 5 La Ce Nd Sm Eu Gd Dy Er Yb Lu La Ce Nd Sm Eu Gd Dy Er Yb Lu

CD F=5 F=6 1.0 F=10 1.0 F=5

e F=6

e F=11 F=16 F=8 F=18 F=15 F=10 F=11

F=20

Rock/Chondrit F=13 Rock/Chondrit Cuobuzha depleted harzburgite, n=8 F=14 Cuobuzha clinopyroxene harzburgite n=5 F=15 F=16 0.1 0.1

La Ce Nd Sm Eu Gd Dy Er Yb Lu La Ce Nd Sm Eu Gd Dy Er Yb Lu

Figure 10. (A, B) Results of the rare earth element abundances created by non-modal batch and aggregated fractional partial melting modeling. Results of the modeling for melt concentrations calculated by Equation 1 show non-modal batch melting, and those calculated by Equation 2 show non-modal aggregated fractional melting compared with mean bulk-rock composition of the Cuobuzha microgabbro and dolerite dikes (Liu et al., 2015c). (C, D) Results of non-modal batch melting created by Equation 3 and non-modal aggregated fractional melting calculated by Equation 4 for residual solid peridotite concentrations compared with the mean bulk-rock composition of

the Cuobuzha clinopyroxene harzburgite and depleted harzburgites (Feng et al., 2017). CO (trace element concentrations in the original mantle rock) is assumed to be depleted mid-oceanic ridge basalt (MORB) mantle; initial partial melting is presumed to have taken place in a spinel lherzolite stability field because mafic dikes have a predominantly spinel peridotite origin (Fig. 9). Depleted MORB mantle values are from Ewart et al. (1994). Mineral modes of spinel lherzolite (olivine = 0.58, orthopyroxene = 0.27, clinopyroxene = 0.12, spinel = 0.03) are from McKenzie and O’Nions (1991). Minerals modes enter melts in the proportions olivine = −0.17, orthopyroxene = 0.65, clinopyroxene = 0.47, spinel = 0.5 (after Niu, 1997). Equations 1–4 are after Shaw (1970).

element 1 between mineral 1 and the melt. F shows the degree of partial reflect ~7%–8% degree of non-modal batch partial melting (Fig. 10A), melting, and P represents the bulk distribution coefficient for the melt- compatible with ~7%–12% degree partial melting as also observed in ing assemblage of minerals weighted by the proportion that each mineral the La/Sm versus Sm/Yb diagram (Fig. 9), which is slightly lower than contributes to the melt, and is calculated by Equation 6. 10%–13% degree created by modeling of aggregated fractional melting The modeling results, especially the HREE contents, of non-modal (Fig. 10B). batch partial melting show that magmas of the Cuobuzha microgabbro The Cuobuzha depleted harzburgites are compositionally similar to dike rocks were produced from ~12%–14% degree of partial melting of residual mantle rocks after 17%–20% degree calculated by non-modal a spinel lherzolite mantle source (Fig. 10A), and are nearly consistent batch partial melting of a depleted spinel lherzolite source (Fig. 10C), with our observations (~9%–12%) in the La/Sm versus Sm/Yb diagram which is basically consistent with 10%–17% degree partial melting of (Fig. 9), which is much lower than those of ~16%–19% degree modeled the Cuobuzha depleted harzburgites as proposed by Feng et al. (2017). by aggregated fractional melting (Fig. 10B). Cuobuzha dolerite dike rocks This result, however, is obviously higher than those of modeling melts

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with 11%–13% degree of aggregated fractional melting (Fig. 10D). The 2016). The εNd(t) values of the Luobusa basalts and dolerite dikes from 87 86 Cuobuzha clinopyroxene harzburgites represent the residues after 5%–8% the eastern YZSZ range from +5.0 to +10.5 and the ( Sr/ Sr)t ratios range degree modeled by non-modal batch partial melting (Fig. 10C), which is from 0.70349 to 0.70672 (Fig. 11A; Zhong et al., 2006; C. Zhang et al., 206 204 208 204 similar to the modeling results of 5%–9% degree created by aggregated 2016). In the ( Pb/ Pb)t versus ( Pb/ Pb)t diagram, the Northern fractional melting (Fig. 10D), and consistent with the results of 5%–8% Baer microgabbro and the Cuobuzha microgabbro dike rocks, Deji and degree modeled by the spinel and olivine compositions (Feng et al., 2017). Angren basalts and dolerite dikes in the central YZSZ (Niu et al., 2006), Our partial melting modeling results confirm that Cuobuzha clinopyrox- and the basaltic lavas in the eastern YZSZ (Zhong et al., 2006) all plot in ene harzburgites represent residues after a low degree (5%–8%) of non- the field of the Indian Ocean MORB (Fig. 11B). However, the Pb and Nd modal batch partial melting from a spinel lherzolite source; this process isotopic values of the analyzed mafic dikes from the NB and of similar mainly dominated the primitive magma formation of Cuobuzha dolerite dikes and basalts from the rest of the YZSZ deviate significantly from the

dikes. Depleted harzburgites extracted ~17%–20% degree melts from trends of subducted sediments (Fig. 11). The positive εNd(t) values and the same mantle source. The Cuobuzha microgabbro magmas formation Pb isotopes showing Indian Ocean MORB-type isotopic signatures indi- may not pertain to the simple end-member melting models, and likely cate that the Baer and Cuobuzha mafic dikes originated from a depleted 87 86 derived from the hybrid mantle sources of spinel lherzolite and remelting mantle source. However, their slightly high ( Sr/ Sr)t ratios and enrich- clinopyroxene harzburgite. ment of fluid-mobile elements suggest that this inferred mantle source was modified by hydrous fluids derived from subducted-altered oceanic Isotopic Fingerprinting of Mafic Dike Magmas crust (Fig. 11A; Escrig et al., 2009).

The North Baer and Cuobuzha microgabbro dikes have εNd(t) values of Subduction Influence in Melt Evolution 87 86 +8.39 to +9.28 and ( Sr/ Sr)t ratios of 0.70433–0.70489 that are compa- 87 86 rable to those of dolerite dikes [εNd(t) values of +8.6 to +8.8 and ( Sr/ Sr) Arc magmas mostly originate from the subarc mantle wedge, which is

t ratios of 0.70300–0.70531] in the Purang ophiolite in the SB (Fig. 11A; commonly modified by fluids and melts from subducted oceanic crust and

Miller et al., 2003; Liu et al., 2013). The εNd(t) values of mafic extrusive overlying pelagic sediments (Münker et al., 2004). HFSEs such as Nb, Ta, rocks and dolerite dikes from the Angren, Deji, Jiding, and Pengcang Zr, Hf, and Ti relative to LILEs such as Rb, Sr, Ba, Pb, Th, and LREEs 87 86 massifs in the central YZSZ range from +8.3 to +9.9, and the ( Sr/ Sr)t are not commonly transported into aqueous fluids (Pearce and Norry, ratios range from 0.70304 to 0.70484 (Niu et al., 2006; L.L. Zhang et al., 1979), so they exhibit conservative behavior during elements transfer

12 39.5 Indian MORB (0 Ma) 10 Indian MORB 39.0 8 Sea water alteration Subducted 6 Semail sediments 38.5 t

(150 Ma) )

4 b ) Lau basin 04

2 /P

Nd(t 38.0 b ε 0 20 82

Subducted sediments (P -2 37.5

-4 37.0 -6 A NHRL B -8 36.5 0.7020.703 0.7040.705 0.7060.707 0.708 0.709 16 17 18 19 20 87 86 206204 (Sr/ Sr) (Pb/ Pb)t t W estern segment Central segment Eastern segment N orth Baer in NB (this study) Deji, Rangren (Niu et al., 2006) Luobusa (C. Zhang et al., 2016) N orth Baer in NB (Zheng et al., 2017) Luobusa (Zhong et al., 2006) Cuobuzha in NB (this study) Deji, Pengcang (L.L. Zhang et al., 2016) Purang in SB (Miller et al., 2003;Liu et al., 2013)

Figure 11. Age (128 Ma) corrected Sr-Nd and Pb isotopic data for mafic dikes intruding peridotites in the northern belt (NB), Tibet. SB—southern belt. Mafic rocks are from Semail in Oman (Godard et al., 2006). Indian mid-oceanic ridge basalt (MORB) values are after X. Liu et al. (2014); Lau Basin basalts are after Escrig et al. (2009); subducted sediments are after Plank and Langmuir (1998) and X. Liu et al. (2014). NHRL—Northern Hemisphere reference line.

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at the slab–mantle wedge interface, and the abundances of HFSE in arc SB and NB plot in the field of BAB-like Lau Basin basalts in the La/Sm rocks reflect those in the mantle wedge. In contrast, LILEs and LREEs versus Ba/Nb diagram (Fig. 12). They display depletion in incompatible have nonconservative properties in subduction zones due to their higher elements, indicated by their low La/Sm ratios, but exhibit enrichment in mobility in slab-derived fluids (Keppler, 1996). When the subduction- the fluid-mobile elements, indicated by their high ratios of Ba/Nb (Escrig related fluids metasomatised the mantle wedge, island arc basalts would et al., 2009). Thus, these geochemical signatures indicate that magmas inherit the properties of being depleted in HFSEs and enriched in LILEs of the analyzed mafic dikes in the NB peridotite massifs were influenced with regard to MORB. by subduction-derived fluids. HFSE and HREE patterns, with the exceptions of mobile elements such as Rb, Ba, Cs, U, K, Sr, P and Pb, of mafic dike rocks can be used effec- Tectonic Setting of Magmatism tively to decipher possible subduction influence in their melt evolution (Pearce and Cann, 1973; Dilek and Furnes, 2011, 2014; Pearce, 2014). In HFSE and HREE values of mafic rocks in ophiolites are helpful to dis- Figures 8E and 8F, we have placed HFSEs and HREEs in an order by their criminate possible tectonic settings of their melt and magmatic evolution, average incompatibility during mantle melting. All the microgabbro and because they are relatively immobile during medium to high degrees of dolerite dike samples from the NB display negative Nb, Ti, and slightly mantle melting, low- to high-grade metamorphism, and moderate hydro- negative Hf anomalies, which resemble those of subduction zone–related thermal alteration (Pearce, 2008, 2014; Dilek and Furnes, 2009, 2011, basalts in suprasubduction zone settings (Figs. 8E, 8F). 2014). We have made several tectonic discrimination diagrams based on In order to evaluate the nature and the specific setting of subduction the HFSE concentrations of the analyzed mafic dike rocks and similar influence in the magmatic evolution of the dike rocks, we have plotted the dike intrusions in the NB as reported by other researchers (Fig. 13). In multielement diagrams (Fig. 8) of the general trends and fields of backarc a Ti-Zr-Y diagram, almost all dike samples plot in the field of ocean- basin basalts (BAB), Lau Basin IATs, Mariana forearc basalt-diabase, and floor basalts (Fig. 13A), suggesting that they represent typical ocean Mariana boninites. The BAB trend in these plots represents an average tholeiitic basalt. However, when plotted on a Zr-Zr/Y proxy diagram, composition of backarc basin basalts recovered from the Lau, Manus, the same samples are between the overlapping fields of island arc basalts Mariana, Scotia, and Woodlark basins (Gale et al., 2013). BABs are gener- and MORBs (Fig. 13B). Fluids or melts derived from a subducted slab

ally characterized by slightly high SiO2 and Al2O3, and low FeO and TiO2 and its thin sediment veneer are commonly enriched in Th, leading to

contents. Their TiO2 contents decrease and Al2O3 values increase with high Th/Yb ratios in produced magmas (Pearce, 2014). In a Th/Yb ver- increasing MgO contents as a result of their moderately high magmatic sus Nb/Yb diagram (Fig. 13C), the investigated mafic dike rocks scatter water contents, which lead to suppression of plagioclase crystallization above the MORB-OIB mantle array and plot close to the oceanic arc relative to olivine and clinopyroxene (Langmuir et al., 2006; Eason and field. This inference is further supported by the V/Ti ratios between 20 Dunn, 2015). The investigated mafic dikes in the NB of the western YZSZ and 30 (Fig. 13D), typical of mixed MORB and IAT melts. Based on all display that these geochemical features are significantly different from these geochemical and isotopic features of the analyzed mafic dikes and those of BAB and their magmatic derivatives. The FeO contents of the their host peridotites, we infer that magmas of these dikes evolved from

mafic dike rocks are much higher than Fe2O3 contents; their LREE, Th, partial melting of a spinel lherzolite mantle source that produced tholeiitic and Nb contents are remarkably lower than those of BAB (Figs. 8A, 8C). basaltic melts beneath an embryonic island arc–forearc spreading center. The differences are further substantiated by the former showing negative Nb and Ti anomalies in the REE patterns (Fig. 8E) and suggest that the mafic dikes did not form in a backarc basin environment. 7 The REE and trace element abundances and patterns of the investi- Northern Belt Southern Belt gated mafic dikes are similar, however, to those of IAT lavas from the Lau S outh Baer dolerite (This study) Purang island arc (Figs. 8B, 8C, 8E; Hergt and Woodhead, 2007), but different 6 Nort h Baer dolerite (This study) Xiugugabu from those of diabase, basalt, and boninite rocks recovered from the Izu- Nort h Baer dolerite (Zheng et al., 2017) Bonin-Mariana (IBM) forearc setting (Reagan et al., 2010). Extrusive Nort h Baer microgabbro (Liu et al., 2015b) 5 rocks recovered from the IBM forearc environment are enriched in fluid- Cuobuzha dolerite (Liu et al., 2015c) Cuobuzha microgabbro (Liu et al., 2015c) mobile elements and LILEs, are distinctly depleted in HFSEs and HREEs, and show stronger negative Nb anomalies in comparison to Th (Figs. 8D, 4 8F) on N-MORB–normalized multielement diagrams. Boninitic lavas or m Enriched +melt dikes, which represent the latest phase of subduction initiation magma- Mantle

La/S 3 tism in the IBM forearc setting and in many Tethyan ophiolites (Dilek et Subduction al., 2008; Dilek and Thy, 2009; Dilek and Furnes, 2014; Pearce, 2014; Input Saccani et al., 2017), have not been observed in the western part of the 2 YZSZ. These signatures together with the various Sr, Nd, and Pb isotopes +ﬂuid suggest that magmas of the mafic dikes intruding the Cuobuzha and Baer peridotites were influenced by island arc–type fluids. This interpretation 1 is consistent with the geochemical and Re-Os isotopic data of peridotites Lau Basin basalts from the Cuobuzha, Dongbo, and Purang massifs, suggesting that their 0 upper mantle peridotites had interacted with arc-related fluids and melts 110 100 1000 (Xu et al., 2011; Niu et al., 2015; Su et al., 2015; Feng et al., 2017). Ba/Nb Enrichment in fluid-mobile elements commonly results in high Ba/ Figure 12. La/Sm versus Ba/Nb diagram modified from Escrig et al. Nb ratios in melt compositions, whereas enrichment in melt-mobile ele- (2009) for mafic dikes in the northern belt and southern belt, Tibet. ments is characterized by high La/Sm values (Pearce and Stern, 2006; Mafic dikes are from Purang (Liu et al., 2011; Liu et al., 2013) and Escrig et al., 2009). The majority of the mafic dike samples from both the Xiugugabu (Bezard et al., 2011).

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100 A Ti/100 B Sout h Baer dolerite (This study) Nort h Baer dolerite (This study) Nort h Baer microgabbro (Liu et al., 2015b) Cuobuzha dolerite (Liu et al., 2015c) Cuobuzha microgabbro (Liu et al., 2015c)

Within plate 10 Within plate

Zr/Y basalts basalts Low-K tholeiites

Mid-ocean ridge basalts Ocean-floor basalts Low-K tholeiites Island arc basalts 1 Calc-alkali Calc-alkali basalts basalts 10 100 1000 Zr (ppm) Zr Y.3

500 10 20

B 10 C D R Continental B AB arcs IAT MO y Boninites & & a 400 T OIB r MO B ar d IA 30 R B

-OI ) 1 Oceanic Mixe

pm 50 arcs 300 MORB p V( Th/Yb E-MORB 200 0.1

N- MORB 100

0.01 0.1 1 10 100 0 015 10 5 Nb/Yb Ti/1000 (ppm) Figure 13. Discrimination diagrams for mafic dikes from the ophiolites in the northern belt (NB), Tibet. (A) After Pearce and Cann (1973). (B) After Pearce and Norry (1979). (C, D) After Dilek and Furnes (2011) and Pearce (2014). MORB—mid-oceanic ridge basalt; E—enriched; N—normal; OIB—ocean island basalt; IAT—island arc tholeiite; BAB—backarc basalts.

Subduction-derived fluids facilitated the partial melting of this relatively The Gangdese magmatic arc started developing along this active margin fertile mantle source and caused fluid-mobile element enrichment of the during the late Triassic–early Jurassic and included both plutonic and produced basaltic melts. volcanic suites in an ~1000-km-long, east-west–trending belt (Zhang et al., 2007; Kang et al., 2014). The accretionary prism developed at this TECTONIC MODEL continental margin contained detrital material largely derived from the Gangdese magmatic arc and the Lhasa continental block, as well as slivers In Figure 14 we present a geodynamic model depicting the tectono- of seamounts scraped from the downgoing Neotethys slab. Such seamount magmatic evolution of the western YZSZ with an emphasis on the devel- fragments and thrust sheets exist within the accretionary prism complex opment of the NB to the west of longitude 82°E. In the late Jurassic, the bounding the Cuobuzha peridotite massif in the study area. Neotethys Ocean basin to the south of Eurasia included two subbasins We infer that rapid rollback of the subducting Neotethys slab in the separated by the Zhongba terrane, a ribbon continent with a likely origin Early Cretaceous caused upper plate extension at the leading edge of the from the northern edge of East Gondwana (Fig. 14A). The ocean floor Lhasa block and exhumation of the continental lithospheric mantle to of the northern subbasin was strewn with seamounts and was subducting shallow depths. This process set up asthenospheric convection beneath northward beneath the Lhasa block along the southern edge of Eurasia. and partial melting of the upper mantle rocks of the southward-migrated,

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S N N E O T E T H Y S E U R A S I A Zhongba Accretionary Seamount Gangdese I N D I A TH Terrane prism arc Continental crust Lhasa Crust SCLM Asthenosphere A s t h e n o s p h e r e

150 Ma A

Figure 14. Tectonic model depict- ZT Seamount Ophiolite Gangdese arc I N D I A TH AP ing the evolution of the upper Continental crust Lhasa Crust mantle peridotites and the active margin of the Lhasa block during SCLM the late Mesozoic. See text for dis- Convecting Mantle wedge cussion. AP—accretionary prism; asthenosphere with rock – melt NB—northern belt; SB—south- interaction ern belt; SCLM—subcontinental A s t h e n o s p h e r e lithospheric mantle; TH—Tethyan Slab Himalaya; ZT—Zhongba terrane. rollback 130 – 120 Ma B NB Seamount Peridotite SB ZT massif APand I N D I A TH mélange Lhasa Crust Continental crust SCLM New subduction forming A s t h e n o s p h e r e

~ 115 Ma C

extended arc-forearc front (Fig. 14B). Asthenospheric upwelling and fluid-mobile elements, producing magmas with arc-like signatures, and injection of high-temperature asthenospheric material may have fertilized they were injected into the residual peridotites to form 122.1 ± 0.5 Ma the mantle wedge peridotites, as widely documented from the active conti- South Baer doleritic dike, 124.8 ± 1.4 Ma North Baer doleritic dike, and nental margins of the western Pacific Ocean, where lherzolitic peridotites 128.5 ± 1.6 Ma Cuobuzha microgabbro dike intrusions during their ascent predominate in mantle wedges (Arai et al., 2007; Ishikawa et al., 2007). (Fig. 14B). The oceanic slab subduction process lasted for at least 7 m.y., In the Timor-Tanimbar arc-trench system in the Melanesia region the slab and occurred before 129 Ma. These multiple partial melting events left rollback and the associated arc-forearc spreading took place ca. 8–5 Ma, depleted harzburgite and clinopyroxene harzburgite with minor dunite in nearly coeval with the arrival of the Australian continental margin at the the mantle wedge (Feng et al., 2017; this study). trench (Berry and McDougall, 1986). Some researchers have proposed The collision of the Zhongba terrane with the trench ca. 115 Ma and its that encroachment of the Australian continental margin on the subduction underplating beneath the accretionary prism and the forearc front caused zone likely induced an asthenospheric mantle flow and asthenospheric uplift, exhumation, erosion, and gravity sliding, producing a chaotic melt injection into the mantle wedge in the upper plate (Ishikawa et al., mélange formation (Fig. 14C). Combined with the shallowing slab angle 2007). This scenario is similar to the arrival of the Zhongba terrane at the of the subducting Neotethys lithosphere, this collision-driven uplift of the Gangdese subduction zone in the early Cretaceous (ca. 130 Ma; Fig. 14B). forearc-trench system resulted in the unroofing of the depleted harzburgi- Our partial melting models and geochemical investigations of the tes in the mantle wedge and in erosional removal of a thin ophiolitic crust. Cuobuzha peridotites in the NB ophiolites have demonstrated that clino- Exhumed peridotites were incorporated into the mélange as megablocks. pyroxene harzburgites were the residues of ~5%–8% partial melting of an Collison-induced shortening within the southern edge of the Lhasa block N-MORB spinel lherzolite with minor networks of garnet-bearing veins. produced backthrusts and large-scale folds. Zircon (U-Th)/He thermo- Slab rollback-induced extension in the arc-trench system led to partial chronology of some metamorphic rocks in the accretionary prism mélange melting of these peridotites and produced an oceanic crust, complete revealed the cooling ages of 107 Ma (Laskowski et al., 2017), consistent with lower and upper crustal components. With continued subduction and with the inferred timing of uplift exhumation in the trench–frontal forearc rollback, downgoing and altered Neotethys oceanic crust became dehy- system. The Zhongba terrane in the lower plate also underwent significant drated, and released fluids percolated through the overlying mantle wedge, shortening, which was taken up by south-directed thrust faulting and fold- triggering further partial melting. Basaltic melts were then enriched in ing (Fig. 14C). These processes collectively produced the initial tectonic

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architecture of the NB in the western YZSZ by the latest Cretaceous. The DD20160022–01), the Ministry of Science and Technology of China (201511022), the National Science Foundation of China (41303019, 41373029, 41573022), and the Basic Outlay of Sci- oblique collision of greater India with Eurasia starting in the early Paleo- entific Research Work from the Ministry of Science and Technology of China (J1321, J1618). gene superimposed further shortening and strike-slip deformation in and Prof. Dilek acknowledges research support from the Chinese Academy of Geological Sciences across the Zhongba terrane, NB, and the Gangdese magmatic arc–Lhasa (Beijing) for his field work in Tibet. block. 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Ewart, A., Bryan, W.B., Chappell, B.W., and Rudnick, R.L., 1994, Regional geochemistry of of the northern branch of Neotethys to the west of longitude 82°E (in the the Lau-Tonga arc and backarc systems, in Hawkins, J., et al., Proceedings of the Ocean present coordinate system). The results of our study in the NB suggest Drilling Program, Scientific Results, Volume 135: College Station, Texas, Ocean Drilling major variations in the nature and tectonic setting of oceanic lithosphere Program, p. 385–425, https://doi.org/10.2973/odp.proc.sr.135.141.1994. Feng, G.Y., Yang, J.S., Xiong, F.H., Liu, F., Niu, X.L., Lian, D.Y., Wang, Y.P., and Zhao, Y.J., formation during the late Jurassic–Cretaceous, as well as in the modes 2015, Petrology, geochemistry and genesis of the Cuobuzha peridotite in the western and mechanisms of ophiolite emplacement along the >3000-km-long Yarlung Zangbo suture zone: Geology in China, v. 42, p. 1337–1353 (in Chinese with east-west span of Neotethys. English abstract). Feng, G., Yang, J., Dilek, Y., Liu, F., and Xiong, F., 2017, Petrological and Re-Os isotopic constraints on the origin and tectonic setting of the Cuobuzha peridotite, Yarlung Zangbo ACKNOWLEDGMENTS suture zone, southwest Tibet, China: Lithosphere, https://doi.org/10.1130/L590.1. We acknowledge Prof. Zhiqin Xu, Dr. Xiaolu Niu, Dr. Guangying Feng, Dr. Weiwei Wu, Dr. Festa, A., Pini, G.A., Dilek, Y., and Codegone, G., 2010, Mélanges and mélange-forming Xuxuan Ma, and Dr. Bin Shi for their constructive discussions on the regional geology and processes: A historical overview and new concepts: International Geology Review, v. 52, laboratory assistance. We thank Hui Zhao, Li Zhang, Jian Huang, Lan Zhang, and Guanlong p. 1040–1105, https://doi.org/10.1080/00206810903557704. Li for their help with field work in remote western Tibet. We also thank three anonymous Festa, A., Dilek, Y., Pini, G.A., Codegone, G., and Ogata, K., 2012, Mechanisms and processes referees for their constructive and valuable comments and Science Editor Damian Nance for of stratal disruption and mixing in the development of mélanges and broken forma- editorial help and handling. This research was funded by grants from the Ministry of Science tions: Redefining and classifying mélanges: Tectonophysics, v. 568, p. 7–24, https://doi and Technology of China (2014DFR21270), the China Geological Survey (DD20160023–01 and .org/10.1016/j.tecto.2012.05.021.

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