Ocean-continent Transition to Suprasubduction Zone Origin of the Western Yarlung Zangbo Ophiolites in SW Tibet, China: Multi-stage, Transient Evolution of the Neotethyan Oceanic Lithosphere

The ophiolites that crop out discontinuously along the;000 km Yarlung Zangbo Suture zone（YZSZ）between the Nanga Parbat and Namche Barwa syntaxes in southern Tibet represent the remnants of Neotethyan oceanic lithosphere（Fig.1a）.We have investigated the internal structure and the geochemical makeup of mafic-ultramafic rock assemblages that are exposed in the westernmost segment of the YZSZ where the suture zone architecture displays two distinct sub-belts of ophiolitic and mélange units separated by a continental Zhongba terrane（Fig.1b）.These two sub-belts include the Daba–Xiugugabu in the south（Southern sub-belt,SSB）and the Dajiweng–Saga in the north（Northern sub-belt,NSB）.We present new structural,geochemical,geochronological data from upper mantle peridotites and mafic dike intrusions occurring in these two sub-belts and discuss their tectonomagmatic origin.In-situ analysis of zircon grains obtained from mafic dikes within the Baer,Cuobuzha and Jianabeng massifs in the NSB,and within the Dongbo,Purang,Xiugugabu,Zhaga and Zhongba in the SSB have yielded crystallization ages ranging between130 and 122 Ma.Dike rocks in both sub-belts show N-MORB REE patterns and negative Nb,Ta and Ti anomalies,reminiscent of those documented from SSZ ophiolites.*Harzburgitic host rocks of the mafic dike intrusionsmainly display geochemical compositions of abyssal peridotites（Fig.2）,with the exception of the Dajiweng harzburgites,which show the geochemical signatures of forearc peridotites（Lian et al.,2016）.Extrusive rocks that are spatially associated with these peridotite massifs in both sub-belts also have varying compositional and geochemical features.Tithonian to Valanginian（150–135 Ma）basaltic rocks in the Dongbo massif have OIB-like geochemistry and 138 Ma basaltic lavas in the Purang massif have EMORB-like geochemistry（Liu et al.,2015）.Tuffaceous rocks in the Dajiweng massif are140 Ma in age and show OIB-like geochemistry.We interpret these age and geochemical data to reflect a rifted continental margin origin of the extrusive rock units in both sub-belts.These data and structural observations show that the western Yarluang Zangbo ophiolites represent fragments of an Ocean-Continent Transition（OCT）peridotites altered by fluids in an initial supersubduction setting.We infer that mafic-ultramafic rock assemblages exposed in the SSB and NSB initially formed in an ocean–continent transition zone（OCTZ）during the late Jurassic,and that they were subsequently emplaced in the forearc setting of an intraoceanic subduction zone within a Neotethyan seaway during 130 to 122 Ma.The NSB and SSB are hence part of a single,S-directed nappe sheet derived from a Neotethyan seaway located north of the Zhongba terrane.

Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime:Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite(NHO)belt in the Indo-Myanmar range(IMR)represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene.Here,we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO,northeastern India to address(i)the mantle processes and tectonic regimes involved in their genesis and(ii)their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle.The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki,Ziphu,Molen,Washelo and Lacham areas.The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine,orthopyroxene,clinopyroxene and plagioclase.The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber.Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting,while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE.These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions.Tectonically,studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone(SSZ)fore-arc regime coherent with the subduction initiation process.The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf,Zr/Sm,Nb/Ta,Zr/Nb,Nb/U,Ba/Nb,Ba/Th,Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process:(i)a depleted fore arc basalt(FAB)type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle;(ii)the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration.The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation,fore arc extension and arc-continent accretion.

Ocean-Continent Subduction within the Paleotethyan Archiopeligic Ocean from Mineralogical Evidence of Muztag Ophiolite, Eastern Kunlun Mountain, China

Former studies show that the Muztag ophiolite, outcropped in the East Kunlun area of Xinjiang, formed in a supra-subduction zone environment. This study is to gain more information about the type of subduetion zone. Through field work, thin section observation and microprobe analysis, petrological and mineralogical characteristics of the metamorphic peridotites of this ophiolite are obtained. Although the olivines of metamorphic peridotites appear in three textures of metamorphic relict, metamorphic recrystallizations and orthopyroxene-melting crystallizations by thin-section observations, they have stable and low Fo range of 87.8- 89.5 by microprobe analysis. The orthopyroxenes show metamorphic relict and melting relict textures, with a low En of 88-90 and a wide range of Al2O3 content, from 2.90 wt% to 5. 13 wt%. The spinels develop anhedral-subhedral textures, with Cr^# （=Cr/（Cr＋AI）） focusing on two ranges of 0. 508-0. 723 and 0. 100-0. 118, respectively. Based on these petrological and mineralogical observations, and combined with the era and tectonic setting for the Muztag ophiolite, it can be concluded that the ophiolite formed in a supra-subduction zone where the oceanic crust subducted down to the continental are with a thick continental crust, and resulted from ocean-continent subducion within the Paleotethyan arehiopelagic ocean in the East Kunlun area of Xinjiang.

Neotethyan Ophiolites and Their Geodynamic Evolution During the Mesozoic: A Global Overview

Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of these seaways were diachronous events, resulting in E–W variations in the timing of oceanic crust production and ophiolite emplacement. The Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, isotopic fingerprints, and sedimentary cover types, pointing to major differences in their mantle melt sources and tectonic and paleogeographic settings of magmatic construction(Dilek and Furnes, 2019). The Jurassic Western Alpine and Ligurian ophiolites in Europe and their counterparts in southern and northern Iberia formed in a narrow basin(Western Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle, and the ophiolites display a Hess–type oceanic crustal architecture with MORB geochemical signatures(Dilek and Furnes, 2011). All these ophiolites were incorporated into continental margins from the downgoing oceanic lithosphere of the Western Tethys. Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways(Dilek et al., 1990), and their rift–drift, seafloor spreading and suprasubduction zone(SSZ) magmatic construction involved multiple episodes of melting, depletion and refertilization of previously or actively subduction metasomatized mantle sources. Deep mantle recycling processes through subduction zone tectonics and/or plume activities played a major role in their melt evolution, and in the incorporation of mantle transition zone(MTZ) materials into their peridotites(Fig. 1;Dilek and Yang, 2018;Xiong et al., 2019). Tectonic mélanges structurally beneath these ophiolites include Permo–Triassic, OIB–type extrusive rocks, indicating that the initial dismantling of the Pangea supercontinent that led to the opening of the Triassic and Jurassic ocean basins within the Neotethyan realm was associated with plume magmatism(Dilek, 2003 a;Yang and Dilek, 2015). This plume signature is absent in the Permo–Triassic magmatic record of the Western Tethys to the west. The Cretaceous ophiolites around the Arabia(Dilek et al., 1990;Dilek and Delaloye, 1992;Dilek and Eddy,1992) and India sub-continents(Fareeduddin and Dilek, 2015) occur discontinuously along a ~9000-km-long belt from SW Anatolia to SE Tibet and Indo-China. The majority of these ophiolites have a Penrose–type oceanic crustal architecture(Dilek, 2003 b) and display SSZ geochemical affinities, complete with a MORB–IAT–BON progression of their chemo-stratigraphy(Fig. 1;Dilek and Thy, 1998;Dilek et al., 1999;Dilek and Furnes, 2014;Saccani et al., 2018). They evolved above a N–dipping, Trans–Tethyan subduction–accretion system that was situated in sub-tropical latitudes within the Neotethyan realm. The Trans–Tethyan subduction–accretion system was segmented into two major domains(Western and Eastern domains) by the NNE–SSW–oriented, sinistral Chaman–Omach–Nal transform fault plate boundary. This Cretaceous intraoceanic arc–trench system was analogous to the modern Izu–Bonin–Mariana(IBM) and Tonga arc–trench systems in the western Pacific in terms of its size. Diachronous collisions of the Arabia and India sub-continents with this segmented Trans-Tethyan arc–trench system resulted in the southward emplacement of the SSZ Neotethyan ophiolites onto their passive margins in the latest Mesozoic(Dilek and Furnes, 2019). A separate N–dipping subduction system, dipping beneath Eurasia to the north during much of the Jurassic and Cretaceous, was consuming the Neotethyan oceanic lithosphere and was responsible for the construction of a composite magmatic arc belt extending discontinuously from Southern Tibet to Northern Iran. Slab rollback along this northern subduction system produced locally developed forearc–backarc oceanic lithosphere that was subsequently collapsed into the southern margin of Eurasia. The existence of these two contemporaneous, Ndipping subduction systems within Neotethys led to its rapid contraction and the fast convergence of India towards Eurasia during the late Mesozoic–early Cenozoic(Dilek and Furnes, 2019). It was the collision with Eurasia of the India sub-continent with the accreted ophiolites around its periphery in the Late Paleogene that produced the Himalayan orogeny.