High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone, southeast Tibetan Plateau: P-T-t path and tectonic implication

The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone(CMSZ)provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys.An integrated petrologic,phase equilibria modeling and geochronological study of the garnet amphibolites,southeast Tibetan Plateau,shows that the garnet amphibolites have a peak mineral assemblage of garnet,glaucophane,lawsonite,chlorite,rutile,phengite and quartz,and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000–2100 MPa and 495–515℃,indicating a cold geothermal gradient of about 240–260℃/GPa.Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies.The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies,and therefore the obtained zircon U-Pb age of 231±1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence.The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded highpressure eclogite-facies blueschist,instead of the previously proposed eclogites,and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.

The Ophiolitic Mélanges in Strike-slip Fault Zones in West Junggar,Xinjiang,NW China

The West Junggar region of western China,located in the far eastern end of the Kazakhstan orocline,occupies the junction of the Siberia,Tarim and Kazakhstan blocks,which is crucial for palinspastic reconstruction of the CAOB.The principal rock assemblages in West Junggar include Paleozoic ophiolitic mélanges and a thick,undeformed Upper Devonian–Lower Carboniferous sedimentary succession as the boundary of the mélanges,both of which are intruded by sub-circular Upper Carboniferous granitoid plutons and intermediate-basic-mafic dykes.On the basis of the sedimentary structures like cross bedding and convolute bedding and the geochronology data,the Upper Devonian–Lower Carboniferous sedimentary successions were identified as the Tailegula,Baogutu,and Xibeikulasi formations from the bottom up,which is an apparent shallowing-upwards ocean basin fill succession,from radiolarian cherts through 2000 meters of flysch to a more neritic Baogutu Formation to a fluvial Xibeikulasi Formation.At the bottom of the Tailegula Formation there is a peperite-bearing unit:a succession of extrusive mafic rock,mainly basaltic lava,with interbeds or blocks of sedimentary rocks including carbonate,radiolarian chert,calcareous siltstone and minor fine-grained tuffaceous sandstone.Peperites in the Tailegula are thickest and best developed as the type section.Four types of peperites were identified based on of the volcanic clast shapes and sediment-matrix properties in Tailegula:(1)arbonatesediment-hosted fluidal peperites,(2)sandstone-hosted fluidal peperites,(3)tuff-hosted mixed fluidal and blocky peperites and(4)carbonate-sediment-hosted blocky peperites.Zircon LA-ICP-MS U-Pb dating of a tuff lens enclosed by lava showed that the peperites formed in the Late Devonian(ca.364 Ma).The widespread peperitebearing succession in the Tailegula Formation is of variablethickness at different sites in West Junggar,such as the Tailegula,Baijiantan,Kalaxiuka,Saertuohai,Dagun,west of the Akebastaw granite and Shinaizha areas.The peperite-bearing unit is generally undeformed in contrast to the highly deformed slices of ophiolite,and is continuously distributed as a stratigraphic section regionally on either side of the Darbut and Baijiantan ophiolitic belts.It can be taken as a mark layer to demonstrate the existence of a shallow remnant ocean basin from the end of Devonian in West Junggar,which is an important component of oceanic crust in the remnant ocean basin.Peperite,underlying Devonian or earlier oceanic crust developed in the spreading process of the ocean basin,and overlying Carboniferous remnant ocean basin-fill succession constitute the complete evolution sequence of the remnant ocean basin.The Darbut and Baijiantan ophiolitic belts should not be interpreted as significant plate boundaries and represent the underlying ocean crust uplifted along tectonic lineaments within a continuous shallow remnant ocean basin.The Baijiantan and Darbut ophiolites are both steep fault zones(>70°)of serpentinite mélange,in contact on either side with regionally distributed and undeformed Upper Devonian–Lower Carboniferous ocean-floor peperitic basalts and overlying sedimentary successions.Ultramafic rocks is serpentinized and foliated to form the matrix of mélange.Some small blocks of peridotite are mylonitic and strongly foliated.Blocks of gabbro generally underwent prehnitization,epidotization and chloritization and many are metasomatized to rodingite.Pods of medium to fine grained amphibolites are encased in serpentinite and display relict gabbroic textures and amphibolite-facies assemblages.The Baijiantan ophiolitic mélange also includes amphibolite brecciasconsistingofcentimeter-sizedmylonitic amphibolite clasts embedded within a serpentinite matrix.Basalt lavas cropping out in the Baijiantan ophiolitic mélange are of two types:type 1 and type 2 lavas.The type1 lavas occur within the fault zones as small blocks withinthe matrix of ultramafic rocks,tectonically juxtaposed against other rocks.The type 2 basalt lava came from the peperite-bearing unit.Besides the ultramafic rocks,gabbros,and basalt lavas,the other supracrustal rocks in the ophiolitic mélange include sandstone,chert,tuff,and very rare limestone.Sandstones predominate and most of them are tuffaceous;their characteristics are consistent with the sandstones from surrounding Lower Carboniferous sedimentary formations.Sandstone blocks within the mélanges also have detrital zircon age distributions(300-400 Ma)and characteristics similar to surrounding Carboniferous sediments.The rock assemblages in the mélanges indicate the ophiolitic mélanges consist of locally derived rocks,in contrast to conventional ophiolitic mélanges.The ophiolitic mélanges show classic structural features of strike-slip shearing regimes,including subhorizontal slickenside lineations(<20°),consistent steeply dipping foliation(>75°)in the matrix,and elongated shapes of blocks aligned parallel to the shear zone.Consistent shear-sense indicators including slip-fiber lineations,Riedel shears,asymmetric blocks,shear band cleavages and veins indicate a horizontal sinistral sense of movement.The occurrence of the amphibolite and ultramafic mylonite in the mélanges probably record early,deep-seated strike slip,indicating that the fault zones extended downward through the oceanic crust.The amphibolite-facies metamorphism then was superimposed by brittle deformation at a shallow level to form fault breccias during the mélange formation.So the ophiolitic mélanges originated from crustal-scale sinistral strike-slip fault zones,not as major plate boundaries or subduction-suture zones.The youngest units of the mélanges are the deformed blocks of Lower Carboniferous basin-fill sedimentary rocks,indicating that the ultimate formation of the mélanges was after deposition of the Lower Carboniferous strata(detrital zircon age modes:320-330 Ma),but before the age of the intruding granite and the dike cutting the mélanges(~310 Ma).Based on above discussions and taking into consideration of the previous studies,a tectonic evolution scenario is proposed for the Devonian to Carboniferous in the West Junggar region.In the middle Devonian or earlier(>390Ma),a paleo-ocean basin existed,stretching across North Xinjiang from Darbut-Baijiantan area in West Junggar to the Kalamaili area in East Junggar.This basin was most likelyaback-arcbasinrelatedtothe Boshchekule–Chengiz–Yemaquan arc.Subduction ended in thepaleo-oceanbasinrepresentedbythe Hongguleleng-Kujibai-Armantai ophiolite belt by late Devonian(375-360 Ma),leading to slab break-off and upwelling of asthenosphere under the remnant ocean basin,which induced The OIB-like basalts in West Junggar.The oceanic basin started to receive sufficient sediment deposition into which OIB-like basalts flows could bulldoze to form the regional distributed peperites(~360 Ma).A little later,in the early Carboniferous(~340 Ma),continent-continent collision took place between the Junggar block and the Yemaquan arc,and Kalamaili ophiolite obduction occurred in the eastern part of Junggar block.The remnant ocean basin was preserved in the western part of the Junggar Block.Accompanying the relative motion between Junggar block and ocean basin in West Junggar during collision,a series of NW trending sinistral strike-slip faults were triggered and activated parallel to the western boundary of the Junggar block.During the late stage of the Early Carboniferous(~320 Ma),the remnant ocean basin was almost filled with sediments.The collision between the Yili and Junggar blocks at the beginning of the late Carboniferous reactivated the strike-slip faults,which disrupted the oceanic crust and basin-fill successions and caused diapirs of serpentinite to form the Baijiantan and Darbut ophiolitic mélanges.The emplacement of Upper Carboniferous(~310 Ma)stitching A-type granitoid plutons indicates the evolutionary history of the remnant ocean basin and strike-slip fault zone ophiolitic mélanges terminated by that time.

Diamonds and Other Exotic Minerals Recovered from Peridotites of the Dangqiong Ophiolite, Western Yarlung-Zangbo Suture Zone, Tibet

Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.

Lower Cretaceous turbidites in the Shiquanhe–Namco Ophiolite Mélange Zone, Asa area, Tibet: Constraints on the evolution of the Meso-Tethys Ocean

Turbidites fromthe Shiquanhe–Namco OphioliteMélange Zone(SNMZ)record critical information about the tectonic affinity of the SNMZand the evolutionary history of theMeso-Tethys Ocean in Tibet.This paper reports sedimentologic,sandstone petrographic,zircon U-Pb geochronologic,and clastic rocks geochemical data of newly identified turbidites(Asa Formation)in the Asa Ophiolite Mélange.The youngest ages of detrital zircon from the turbiditic sandstone samples,together with~115 Ma U-Pb concordant age from the tuff intercalation within the Asa Formation indicate an Early Cretaceous age.The sandstone mineral modal composition data show that the main component is quartz grains and the minor components are sedimentary and volcanic fragments,suggesting that the turbidites were mainly derived froma recycled orogen provenancewith a minor addition of volcanic arc materials.The detrital U-Pb zircon ages of turbiditic sandstones yield main age populations of 170–120 Ma,300–220 Ma,600–500 Ma,1000–700 Ma,1900–1500 Ma,and~2500 Ma,similar to the ages of the Qiangtang Terrane(age peak of 600–500 Ma,1000–900 Ma,~1850 Ma and~2500 Ma)and the accretionary complex in the Bangong–Nujiang Ophiolite Zone(BNMZ)rather than the age of the Central Lhasa Terrane(age peak of~300 Ma,~550 Ma and~1150 Ma).The mineral modal compositions,detrital U-Pb zircon ages,and geochemical data of clastic rocks suggest that the Asa Formation is composed of sediments primarily recycled from the Jurassic accretionary complex within the BNMZ with the secondary addition of intermediate-felsic island arc materials from the South Qiangtang Terrane.Based on our new results and previous studies,we infer that the SNMZ represents a part of the Meso-Tethys Suture Zone,rather than a southward tectonic klippe of the BNMZ or an isolated ophiolitic mélange zone within the Lhasa Terrane.The Meso-Tethys Suture Zone records the continuous evolutionary history of the northward subduction,accretion,arc-Lhasa collision,and Lhasa-Qiangtang collision of the Meso-Tethys Ocean from the Early Jurassic to the Early Cretaceous.

Petrology and Geochemistry of the Dangqiong Ophiolite,Western Yarlung-Zangbo Suture Zone,Tibet,China

The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(>20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.

PRELIMINARY STUDY ON THE OPHIOLITIC MELANGE IN THE YALU TSANGPU GRAND CANYON AREA

Since 1980’s Chinese geologists have organized several scientific expeditions with foreign colleagues to enter the Canyon to study its geology and geography.1\ Major petrologic units\;There are three major petrologic units in this region: (1)Namche barwa formation(Pt nj ). It is situated in the center of great bend area ,representing the lower part of Indian plate wedged into Eurasia plate, which is composed of plagioclase gneiss, amphibole\|containing high\|pressure granulite lenses. (2)Gandise formation(Pt gd ).It is exposed in the outside of Namche barwa edge(Pt nj ), and is considered to be the lower part of Eurasia plate which is composed of plagioclase\|gneiss, amphibolite, marble, quartzite, etc., and were intruded by large granite batholiths (γ 2 5 to γ 6). (3)Great bend formation(Mz). It stretches along Yarlung Zangbo river and is constrained between Pt nj and Pt gd . It is composed of metamorphic basic igneous rocks , ultramafic rocks and quartzite. The whole rock assemblage is characterized by ophiolite suite. Nice ophiolitic mélange exposures exist in Pangxin, Jiaresa and between Pailong and Zhaqu.2\ Basic rock assemblage\;The present ophiolitic components are mostly metamorphic rocks after structural movements and metamorphism for long period. (1) Meta\|intermediate rocks. Amphibolite is a dominant component of ophiolitic mélange in this region . Chemical analysis and protolith recognition shows that amphibolite originated from basalt, diabase and gabbro. (2) Ultramafic rocks. This kind of rocks distributes along the northeast edge of the wedge from Pangxin to Jiaresa. Original pyroxene\|olivinite and serpentine exposures have been found . Electronic\|probe and bulk\|rock analysis suggest that original olivinite is strongly MgO\|enriched and can be named magnesian olivinite. (3) Quartzite. It is exposed along the northeast and southeast margin of the wedge forming steep cliffs and high peaks . In some places it consists of pure fine\|grained quartz and may contain muscovite, biotite, plagioclase or epidote grains in different amount, which suggest that quartzite in this area was formed in small sea basin and not deep oceanic environment as traditionally considered for standard ophiolite suite. (4) Marble blocks. Pure white marble blocks have been found near the village of Bayu and Daduka which were surrounded by amphibolite , quartzite (Mz) or plagioclase\|gneiss(Pt nj ).

Tectonic Evolution of Neotethys Ocean: Evidence of Ophiolites and Ocean Plate Stratigraphy from the Northern and Southern belts in the Western Yarlung Zangbo Suture Zone, Tibet

The Yarlung Zangbo suture zone(YZSZ)separates Indian plate and its northern passive margin units to the south from Eurasian plate and its active continental margin units of Xigaze forearc basin and Gangdese batholith to the north(Xu et al.,2015;Yang et al.,2015).The western YZSZ in southern Tibet is divided by the Zhongba terrane into the northern belt(NB)and southern belt(SB).Ophiolites in the NB are dismembered as ophiolitic mélanges.Peridotite,cumulated gabbro,ocean plate stratigraphy(OPS)of seamount remnants and pelagichemipelagic sequence as blocks in serpentinite matrix are mainly observed,from west to east,in Dajiweng,Baer,Kazhan,Cuobuzha,Zhalai,Gongzhu.Ophiolites in the SB are absent ophiolitic units of sheeted dikes and MORB-like pillow lavas,occur as much larger peridotite massifs(i.e.,Dongbo,400 km^2;Purang,650 km^2;Xiugugabu,700 km^2;Dangqiong,300 km^2)which are intruded by mafic dike swarms and overlain by volcanic sedimentary OPS(Liu et al.,2018).We propose that the SB mafic–ultramafic rocks and volcanic sedimentary OPS represent fragments of an early Cretaceous continental margin ophiolite whose magmatic evolution was influenced by 140–137 Ma plume magmatism(Liu et al.,2015;Zheng et al.,2019).Relics of Late Paleocene to very Early Eocene deep-marine basin were developed in Saga and Gyirong(Ding,2003;Li et al.,2018).In contract,the NB ophiolitic mélanges report a travel log of an oceanic plate ranging from Middle Triassic to Early Cretaceous.

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.

ACCRETION OF AN EARLY CRETACEOUS INTRA- OCEANIC ISLAND ARC TO INDIA: EVIDENCE FROM THE YARLUNG ZANGBO SUTURE ZONE

A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the Early Cenozoic. Should we assume that the entire Tethyan Ocean basin that lay between these two continental blocks was oceanic or might it have been more complex? Fragments of any terranes that developed within this once extensive ocean potentially lie within the YTSZ. Detailed investigations over the past three field seasons reveal the presence of several terranes distributed along this zone. Work is currently underway to analyze the nature of individual terranes and the timing of any inter\|relationships.

Remnants of a Middle Triassic Oceanic Lithosphere in the Western Yarlung Zangbo Suture Zone, Southern Tibet

Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.

Tectonic Evolution of the Dongbo Ophiolite in Western Yarlung Zangbo Suture Zone, Xizang(Tibet)

The Dongbo ophiolite in the western part of the Yarlung-Zangbo suture zone in southern Tibet rests tectonically on the middle-late Triassic and Cretaceous flysch units,and consist mainly of peridotites,mafic dikes,

Timing and mechanism of opening the Neo-Tethys Ocean:Constraints from mélanges in the Yarlung Zangbo suture zone

The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not.In this study,we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean.Based on field observations,the YZSZ mélanges can be divided into three segments.The western(west of Zhongba)and eastern(Sangsang-Renbu)segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent.Mélanges in the central segment(Zhongba-eastern Saga)typically have a siliciclastic matrix,and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent.Based on our and previously published geochemical data,the mafic rocks in the YZSZ mélanges are ocean island basalt(OIB)-like,with ages in the Late Permian-Middle Triassic,the Middle-Late Jurassic,and the Early Cretaceous,respectively.An OIB-like block with an age of ca.253 Ma is identified from the Zhongba mélanges in the western segment,and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean.Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading.In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts,radiolarian cherts,and normal mid-ocean ridge basalt-like oceanic crust,we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at~250–243 Ma in the Early Triassic,not later than the early phase of Middle Triassic.In addition,a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean.On the other hand,we have also identified a suite of ca.160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment,which is the same age as the OIB lithologies previously reported in the Zhongba mélanges.Based on the sill-like occurrence and absence of plume-related rock associations in this region,the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana.Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata,but its tectonic significance requires further investigation.Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites,Gangdese belt igneous rocks,and sedimentary rocks,we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean,mainly involving continental rifting and ocean opening,subduction initiation,ultraslow-spreading ridge-trench conversion,subduction re-initiation,and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites.These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction.Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics.

Reconsideration of Neo-Tethys Evolution Constrained from the Nature of the Dazhuqu Ophiolitic Mantle, Southern Tibet

The nature(i.e., sub-oceanic, sub-arc or subcontinental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re-Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr~# [Cr/(Cr + Al), ~ 0.3–0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr~#(~0.3–0.7) overlapping with the ranges of both abyssal and fore-arc peridotites(Day et al., 2017;Parkinson and Pearce, 1998);two samples have spinel Cr~# higher than 0.6, which is probably ascribed to intense melt-rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0–6% garnetfacies melting followed by 10% –18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites(Hellebrand et al., 2002) and implies deep initial melting(> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites(commonly <80 km in origin). The Dazhuqu peridotites have unradiogenic 187 Os/188 Os of 0.11836–0.12922, which are commonly lower than the recommended value of primitive upper mantle(PUM)(Meisel et al., 2001). All but one samples yield relatively younger Re depletion ages(TRD = 0.06–0.81 Ga) with respect to the only one sample having an older TRD age of 1.66 Ga. Re-Os isotopes and highly siderophile element(HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites(Day et al., 2017) and the Oman southern massifs(Hangh?j et al., 2010) but are distinct from noncratonic sub-continental lithospheric mantle(SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge–trench collision.

Genesis of Pyroxenite Veins in the Zedang Ophiolite,Southern Tibetan Plateau

Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro,diabase,and basalt.Veins of two pyroxenite varieties cut the southern part of the Zedang massif.These pyroxenite rocks have different geochemical characteristics,where the first variety(type-I)has relatively higher contents of SiO_(2)(51.82–53.08 wt%),MgO(20.08–23.23 wt%),andΣPGE(3.42–13.97 ppb),and lower Al_(2)O_(3)(1.59–2.28 wt%)andΣREE(1.63–2.94 ppm).The second pyroxenite variety(type-II)is characterized by SiO_(2)(45.44–49.61 wt%),Mg O(16.68–19.78 wt%),Al_(2)O_(3)(4.24–8.77 wt%),ΣPGE(14.46–322.06 ppb),andΣREE(5.82–7.44 ppm).Pyroxenite type-I shows N-MORB-like chondritenormalized REE patterns.Zircon U-Pb ages of pyroxenite type-I(194±10 Ma),associated ophiolitic gabbro(135.3±2.0 Ma),and plagiogranite(124.2±2.3 Ma)evidently imply episodic evolution of the Zedang ophiolites.The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction-initiated hydrous melting of metasomatized sub-arc mantle,later overprinted by sub-slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times.The geochemical variations in pyroxenite vein composition,coupled with age differences amongst the other ophiolite units,may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.

The Boninite-like Dolerites in the Xigaze Ophiolites, Tibet: Similar to the MORB-like Dolerites

The Early Cretaceous Xigaze ophiolites(XO)exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet,preserved the structure of the upper mantle and oceanic crust,is interpreted as a definite record of Neo-Tethys oceanic lithosphere.Many evolution models of the XO have been proposed since the 1980s(Nicolas et al.,1981;Hébert et al.,2012;Maffione et al.,2015).However,the geodynamic environment of the XO whether at a MOR or SSZ remains controversial.As key evidence for the identification of the SSZ ophiolite,the boninite-like dolerites from the XO are still controversial and poorly constraint(Chen et al.,2003;Bao et al.,2013;Dai et al.,2013).According to previous reports,dolerites were subdivided to MORB-like type and boninite-like type(Chen et al.,2003;Dai et al.,2013).Apart from high-SiO2(>52 wt%),high MgO(>8 wt%)and low-TiO2(<0.5 wt%),we found the boninite-like and MORB-like dolerites are indistinguishable in outcrop,mineral and chemical.They are sills invaded into mantle sequence of the XO,with 50.35–56.80 wt%SiO2,0.32–1.19 wt%TiO2,7.39–8.89 wt%Fe2O3T,5.50–10.42 wt%MgO and Mg#of 0.49–0.74.MORB-like dolerites display trace element and REE patterns similar to those of most fore-arc or/and back-arc basalts from Izu-Bonin-Mariana(IBM),i.e.enrichment in large ion lithophile elements(LILEs,e.g.Cs,Rb,Ba,Pb,U)and depletion in high-field strength elements(HFSE,Nb,Ta)(Fig.1).Boninite-like dolerites resemble the MORB-like dolerites in trace elements and REE patterns,excepting for lower concentrations in REE,but distinct from the boninites discovered from the IBM or Troodos ophiolite(Fig.1c,d).Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs.Nb/Yb diagram(Fig.2a),suggesting constant subduction component added to the mantle source(Pearce et al.,1995).We chose depleted MORB source Mantle(DMM,Workman and Hart,2005)as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean(2012).The results show that MORB-like and boninitelike dolerites can be explained by non-batch fractional melting(≥15%)of a depleted spinel peridotite with few subductionderived components(Fig.2b).Therefore,the boninite-like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab-derived fluid and the XO cannot be used as analogs for the subduction initiation.

雅鲁藏布江中段日喀则地区却顶布—路曲地幔橄榄岩岩石地球化学特征、成因及其地质意义

日喀则蛇绿岩是雅鲁藏布江缝合带的重要组成部分,但是对该蛇绿岩的成因和构造背景还存在较大的争议。本文以日喀则地区却顶布—路曲蛇绿岩为研究对象,通过详细的野外地质、岩石学、矿物化学及岩石地球化学研究,显示该蛇绿岩主要由方辉橄榄岩组成,另有少量辉长岩、辉绿岩和玄武岩。与路曲方辉橄榄岩相比,却顶布方辉橄榄岩中橄榄石的Fo值较低,斜方辉石Al_(2)O_(3)和Cr_(2)O_(3)含量较高,单斜辉石Al_(2)O_(3)含量和Cr#值较高,尖晶石Cr#值和TiO_(2)含量较低。却顶布—路曲方辉橄榄岩具有较高的MgO含量和较低的Al_(2)O_(3)、CaO、TiO_(2)含量,稀土元素总量为(0.17~1.63)×10^(-6),低于原始地幔和亏损地幔。路曲方辉橄榄岩的LREE/HREE和(La/Yb)_(N)比值较高,分别为1.34~9.22和0.56~12.14;却顶布方辉橄榄岩与其相反,LREE/HREE和(La/Yb)_(N)比值分别为0.29~0.81和0.11~0.25。路曲方辉橄榄岩呈轻稀土富集型的U型稀土元素配分模式;却顶布方辉橄榄岩为轻稀土亏损型的左倾型稀土元素配分模式。路曲方辉橄榄岩和却顶布方辉橄榄岩具有近似的微量元素分布特征,但是又有一定区别,路曲方辉橄榄岩大离子亲石元素Rb、Ba高于却顶布方辉橄榄岩,但K、Sr低于却顶布方辉橄榄岩。却顶布—路曲方辉橄榄岩的源区可能来源于地幔中的尖晶石相二辉橄榄岩。却顶布—路曲方辉橄榄岩矿物化学和全岩地球化学特征与深海地幔橄榄岩相似。矿物学和全岩岩石地球化学特征显示却顶布方辉橄榄岩为经历了10%~15%部分熔融的残余,路曲方辉橄榄岩为经历了20%~25%部分熔融的残余。却顶布—路曲方辉橄榄岩形成于慢速超慢速拉张的洋中脊环境。日喀则蛇绿岩的多样性与源区部分熔融程度和后期流体交代密切相关。

西藏泽当蛇绿岩玄武岩SHRIMP锆石U-Pb年龄及其地质意义

雅鲁藏布江缝合带中各蛇绿岩体的准确定年对特提斯洋演化和青藏高原隆升的研究具有重要意义。泽当蛇绿岩是雅鲁藏布江缝合带东段最大的蛇绿岩块体,关于其形成年龄目前仍存在不同的认识。通过SHRIMP锆石U-Pb测年得到蛇绿岩中玄武岩的形成年龄为154.9Ma±2.0Ma(95%置信度,MSWD=0.98)。蛇绿岩中的玄武岩是洋脊扩张的产物,其形成年龄代表了扩张事件的时间,也代表了蛇绿岩的形成时代。结合已有的雅鲁藏布江缝合带蛇绿岩的形成年龄,该年龄进一步反映出雅鲁藏布江缝合带蛇绿岩形成时间具有东早西晚的特点。泽当蛇绿岩与含有埃达克质英云闪长岩的泽当岛弧火成岩基本为同期形成的。地球化学特征显示定年的玄武岩形成于俯冲带之上,且具有指示洋内俯冲环境的地球化学特征。因此,泽当SSZ型蛇绿岩可能形成于洋内俯冲机制。

日喀则夏鲁N-MORB型辉长岩与辉绿岩:雅鲁藏布江特提斯洋早白垩世初始俯冲记录

雅鲁藏布江结合带130~120 Ma蛇绿岩分布广泛,但其形成环境存在较大争论。以日喀则地区夏鲁辉长岩及辉绿岩锆石U-Pb定年、主量及微量元素数据为基础,结合大量的日喀则蛇绿岩研究数据,分析蛇绿岩的构造环境及其动力学意义。夏鲁辉长岩LA-ICP-MS锆石U-Pb年龄为123.8±1.1 Ma(MSWD=0.97),表明其是日喀则130~120 Ma蛇绿岩残片之一。主量、微量元素特征显示,部分辉长岩样品高CaO、低SiO_(2)及极低的K_(2)O、Na_(2)O含量,为异剥钙榴岩化所致,而辉绿岩无流体影响。夏鲁辉长岩与辉绿岩显示正常型大洋中脊玄武岩(N-MORB)特征,且轻稀土元素较N-MORB亏损,来自亏损地幔尖晶石二辉橄榄岩源区高程度部分熔融。日喀则130~120 Ma的蛇绿岩N-MORB型基性岩来自俯冲组分混入不均的地幔源区,表现为洋中脊玄武岩至火山弧玄武岩过渡特征,并且无陆壳物质混入,最有可能形成于洋内弧系统。综合区域地质资料,认为日喀则130~120 Ma的蛇绿岩在发育时限、岩石组合及地球化学特征上与伊豆小笠原-玻安岛-马里亚纳(IBM)弧前蛇绿岩类似,代表雅鲁藏布江新特提斯洋一次洋内俯冲的开始。

An alternative tectonic model for the Yarlung Zangbo suture zone

The new field investigations along the Yarlung Zangbo ophiolites zone show that these series underwent low green-schist metamorphism and were then fractured and occurred as slabs in tectonic melanges,without regional tectonic polarity.No large shear zone in north-south direction has been identified between ophiolite bodies and flysch layers on both side and a conformable contact relationship can be observed locally between them.A great mass of tectonic mélange has been substantiated as submarine olistolith bodies.The Mesozoic sedimentary facies and its evolution in both north and south of the ophiolite zone are corresponding in time.The ophiolite zone has often been divided into parallel branches,separated by narrow flysch slats.There is also a similarity of the Paleozoic and the basement of the High Himalaya,Lhasa and Qiangtang Terranes,and they are distinctly different from those of the Indian continent.The geologic information does not warrant a postulate that the Himalaya and Tibet were once separated by a great ocean;it is therefore consistent with an alternative tectonic model by back-arc basin collapse with its juvenile narrow oceanic crust.The real plate tectonic suture,the Neotethys might be covered under the Miocene Siwalik molasse in the southern slope of the High Himalaya range.Based on the new model,the Neotethyan ocean floor was subducted beneath the Asia since the Late Triassic.The outer continental margin of Eurasia was split from the Lhasa Terrane so that a back-arc basin came into existence.Hemi-pelagic and deep sea sediments were deposited before the Late Cretaceous flysch sedimentation,with the linear juvenile oceanic crust when back-arc volcanism occurred in the Gandese region.The Yarlung Zangbo back-arc basin was eventually eliminated when the High Himalayas were sutured onto Eurasia.The ocean floor lightly underthrusted to north and south sides,sediments of the basin were deformed as fold-thrusting.The Neotethys was eliminated during the Tertiary when India collided with Himalaya arc from the Oligocene.The crust of southern Tibet suffered penetrative shortening,the ophiolite zone became exposed during regional uplifting,and the South Tibet Detachment System,doming in southern Tibet and rifting in a north-south direction since the Middle Miocene Epoch.

西藏雅鲁藏布江缝合带的普兰地幔橄榄岩中发现金刚石：蛇绿岩型金刚石分类的提出

西藏罗布莎蛇绿岩的地幔橄榄岩和铬铁矿中发现金刚石和特殊矿物群引发了新的问题,罗布莎地幔橄榄岩含特殊地幔矿物是不是一个孤立的特殊现象,或这是一个普遍存在的规律?显然,这是一个至关重要的问题。本文报道在雅鲁藏布江缝合带西段,距离罗布莎1000km以远的普兰蛇绿岩的地幔橄榄岩中发现与罗布莎类似的金刚石和特殊地幔矿物群。普兰的地幔橄榄岩体主体为方辉橄榄岩,含少量的纯橄岩和二辉橄榄岩,研究表明,属典型MOR型亏损地幔橄榄岩。通过分选,在657kg的地幔橄榄岩大样中发现了金刚石和碳硅石等30余种矿物的特殊矿物群,包括自然铬、自然铁和自然锌等强还原条件下形成的单质元素矿物。该矿物群与罗布莎地幔橄榄岩和铬铁矿中发现的特殊矿物群十分相似,表明罗布莎的地幔橄榄岩不是雅鲁藏布江缝合带中的一个特例。结合在俄罗斯乌拉尔Ray-Iz铬铁矿中发现类似的矿物群,以及世界其他地区的有关阿尔卑斯型地幔橄榄岩中金刚石的报道,认为蛇绿岩地幔橄榄岩中可能普遍含有金刚石,并将蛇绿岩地幔橄榄岩中产出的金刚石归为一种新的金刚石产出类型,即蛇绿岩型金刚石,不同于金伯利岩型金刚石和超高压变质带中产出的变质金刚石类型。