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

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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 harzbur- gite 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 lithologi- cal 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 LITHOSPHERE© 2018 The Authors. | Volume Gold 10Open | Number Access: 1 This | www.gsapubs.org paper is published under the terms of the CC-BY-NC license. 109 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/1/109/4048634/109.pdf by guest on 24 September 2021 FEI LIU ET AL. on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/1/109/4048634/109.pdf 110 75°E 80°E 85°E 90°E 95°E 70°E 80°E 90°E 100°E 30°N NPS 35°N KOHISTAN Lhasa NBS Kohista n NPS EURASIA KK N E U R A S I A LADAKLA F D Lad aAkh SL H Delhi 0 500 20°N SP km INDIA Karachi SH MB T ND GH Fig.2 Indian Ocean DJW BR KZ CBZ Lhasa Block 30°N ITSITSZZL Z GZC DB NBS 30°N LH DQ SG PR ZG Gangdese ZD XGGB GT ITSZITSZ ZB GT SS GCT XGZ LBS Thrust fault TH JD STD MFT MCT Detachment fault GH LH MBT MCT Strike-slip fault I N D I A SH Nappe/overthrust Tectonic Window Western Segment Central Segment Eastern Segment www.gsapubs.org 75°E 80°E 85°E 90°E 95°E Tethyan Realm India Affinity Eurasia Affinity Mélange (Jurassic-Cretaceous) Forearcbasin sequence SH Sub-Himalaya TH Tethyan Himalaya (Cretaceous - Eocene) | Tethyan ophiolite (Jurassic-Cretaceous) Zhongba Terrane Kohistan - Ladakh- Gangdese Volume 10 Volume LH Lesser Himalaya magmatic belt HP metamorphic rocks occurrence GH Greater Himalaya | Figure 1. Simplified geological map of the Himalaya and southern Tibet, showing the distribution of the Tethyan ophiolites, ophiolitic mélanges, and high-pressure (HP) rocks along the Number 1 Yarlung Zangbo suture zone (modified after Xu et al., 2015, and references therein). GCT—Great Counter thrust; GT—Gangdese thrust; KKF—Karakorum fault; MBT—Main Boundary thrust; MCT—Main Central thrust; MFT—Main Front thrust; NBS—Namche Barwa syntaxis; NPS—Nanga Parbat syntaxis; STD—South Tibet detachment. Ophiolitic massifs: BR—Baer; CBZ— Cuobuzha; DB—Dongbo; DJW—Dajiweng; DQ—Dangqiong; GZ—Gongzhu; JD—Jiding; KZ—Kazhan; LBS—Luobusa; ND—Nidar; PR—Purang; SG—Saga; SL—Shangla; SP—Spongtang; | SS—Sangsang; XGGB—Xiugugabu; XGZ—Xigaze; ZB—Zhongba; ZD—Zedang; ZG—Zhaga; ZL—Zhalai. LITHOSPHERE Melt evolution of upper mantle in the western Yarlung Zangbo ophiolite | THEMED ISSUE 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.
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