Oceanic Island Basalts within the West Junggar Ophiolitic Mélanges: Petrogenetic and Tectonic Implications

Recently, we focused on the Darbut and Karamay ophiolitic m41anges in West Junggar of the Central Asian Orogenic Belt （CAOB, SengOr et al., 1993; Windley et al., 2007; Xiao and Santosh, 2014）, and made much progress. This study was supported by the National Nature Science Foundation of China （No. 41303027） and Special Fund for Basic Scientific Research of Central Colleges Project （No. 2014G1271058）. The achievements are illustrated as follows.

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 Tectonic Implications of the Hongliuhe-Xichangjing Ophiolitic Mélanges Belt in the Central Region of the Beishan Orogen, NW China——Constrained by the U-Pb Ages of Detrital Zircons of the Metasandstones

The tectonic attributes of different blocks within orogenic belts are of great significance for the study of accretionary processes and the evolution of Earth. The Hongliuhe-Niujianzi-Baiyunshan-Xichangjing ophiolitic mélange belt(HXOMB) is distributed in the heart of the Beishan Orogen, the Shuangyingshan and Minshui-Hanshan blocks being distributed in the south and north of the HXOMB respectively, and a large number of Early Paleozoic geological units are exposed on the blocks. According to the zircon age populations of the metasandstones in the Baiyunshan area recovered in this paper, when compared with the zircon age populations of the Paleozoic metasandstones reported in the Niujuanzi and Hanshan areas, we found that the metasandstones of the Shuangyingshan Block have age peaks at c. 598 Ma, 742 Ma, 828 Ma, 941 Ma, 990 Ma, 1168 Ma, 1636 Ma, 2497 Ma with non-significant age populations of 1500–1300 Ma, showing a possible affinity with the Tarim Craton;the metasandstones of the Minshui-Hanshan Block have age peaks at c. 606 Ma, 758 Ma, 914 Ma, 1102 Ma, 1194 Ma, 1304 Ma, 1672 Ma with significant age populations of 1500-1300 Ma, showing a possible affinity with the Chinese Central Tianshan Block. Therefore, the HXOMB of the Beishan Orogen is of great significance in plate segmentation, which separates the Tarim Craton in the south and the Chinese Central Tianshan Block in the north. Based on the evolutionary process of the Hongliuhe-Xichangjing ocean in the Beishan Orogen, we believe that break-up and convergence can be recognized as having occurred twice between the Chinese Central Tianshan Block and the Tarim Craton since the Mesoproterozoic in the Beishan area. This was related firstly to the break-up of the Columbia Supercontinent and the convergence of the Rodinia Supercontinent, mainly during the Middle Mesoproterozoic to Early Neoproterozoic, and secondly to the opening and closing of the Hongliuhe-Xichangjing ocean, mainly during the Early Paleozoic.

A New Understanding on the Emplacement of Ophiolitic Mélanges and Its Tectonic Significance: Insights from the Structural Analysis of the Remnant Oceanic Basin-Type Ophiolitic Mélanges

Ophiolites represent on-land fragments of paleooceanic crust and have been recognized as one of the key markers of suture zones.Here,we provide new insights into the emplacement of ophiolitic mélanges based on detailed geological mapping and structural analysis in the West Junggar and Songpan-Ganzi-Bayan Har orogens(Fig.1 and Fig.2).The results show that some ophiolitic mélange belts cannot be regarded as suture zones.The distribution of these ophiolitic mélange belts are usually associated with the structural processes during the closure of remnant oceanic basins.After the remnant-oceanic basin is filled with thick clastic deposit,the oceanic lithosphere material as the base of the remnant basin can be injected into the overlying sedimentary strata through various faultings under the regional compressive stress,forming the remnant oceanic basintype ophiolitic mélange system with dispersive distribution characteristics.Combining with previous researches,the emplacement mechanism of ophiolitic mélanges can be divided into four categories:subduction type which oceanic lithosphere subducted beneath active continental margin,obduction type which oceanic lithosphere obducted over passive continental margin,collision type between two continental lithospheres,and closure type of remnant-oceanic basin(Fig.3).These different types of ophiolitic mélange belts will be superimposed and even re-emplacement by the tectonic processes of post-plate convergence,complicating their distribution.Therefore,identifying the emplacement mechanism type of ophiolitic mélange belts formed in different tectonic processes and backgrounds is of importance for understanding the process of ocean-continental transition and the evolution of orogenic belts.

The Oldest Paleo-Tethyan Ophiolitic Mélange in the Xizang(Tibetan) Plateau

An Early Paleozoic ophiolitic mélange has recently been documented in the W.Gangma Co area,north-central Tibetan Plateau.It is composed of serpentinite,isotropic and cumulate gabbros,basalt and plagiogranite.Whole-

Geochronology, Geochemistry and Tectonic Setting of the Bairiqiete Granodiorite Intrusion(Rock Mass) from the Buqingshan Tectonic Mélange Belt in the Southern Margin of East Kunlun

This study focuses on the zircon U-Pb geochronology and geochemistry of the Bairiqiete granodiorite intrusion （rock mass） from the Buqingshan tectonic mélange belt in the southern margin of East Kunlun.The results show that the zircons are characterized by internal oscillatory zoning and high Th/U （0.14-0.80）,indicative of an igneous origin.LA-ICP-MS U-Pb dating of zircons from the Bairiqiete granodiorite yielded an age of 439.0 ± 1.9 Ma （MSWD =0.34）,implying that the Bairiqiete granodiorite formed in the early Silurian.Geochemical analyses show that the rocks are medium-K calc-alkaline,relatively high in Al2O3 （14.57-18.34 wt％） and metaluminous to weakly peraluminous.Rare-earth elements have low concentrations （45.49-168.31 ppm） and incline rightward with weak negative to weak positive Eu anomalies （δEu =0.64-1.34）.Trace-element geochemistry is characterized by negative anomalies of Nb,Ta,Zr,Hf and Ti and positive anomalies of Rb,Th and Ba.Moreover,the rocks have similar geochemical features with adakites.The Bairiqiete granodiorite appears to have a continental crust source and formed in a subduction-related island-arc setting.The Bairiqiete granodiorite was formed due to partial melting of the lower crust and suggests subduction in the Buqingshan area of the Proto-Tethys Ocean.

Characteristics of Blueschist in Shuangjiang Tectonic Mélange Zone,West Yunnan Province

Glaucophane in Shuangjiang area, West Yunnan Province, supplies a chance for studying south segment of Lancangjiang tectonic zone. But people are at odds as to whether two stage glaucophane exists or not, glaucophane is the result of dynamic metamorphism later, or indicates a high P/T metamorphic belt when Paleozoic Tethys Sea closed. Authors discover in a recent research that there is only one stage glaucophane in Shuangjiang area, and three blueschist belts are distributed near N S tending, and glaucophane in Shuangjiang area is related to the eastward subduction of Changning Menglian basin.

An Early Paleozoic Tectonic Mélange at the Western Margin of West Cathaysia: Constraints from Organic-walled Microfossils

The Jiangshan-Shaoxing-Pingxiang Fault(JSP Fault) is traditionally considered as the boundary between the Yangtze and Cathaysia blocks in South China. Whether the previously defined Shenshan and Kuli formations located along the JSP fault and near the Xinyu City, Jiangxi Province, are continuous strata or parts of a tectonic mélange is important for understanding the geological history of South China. A carbonaceous phyllite from the area, previously considered as part of the Neoproterozoic Shenshan and Kuli formations, is analyzed palynologically in this study. The AsteridiumComasphaeridium acritarch assemblage found in the slate can be correlated with the basal Cambrian AsteridiumHeliosphaeridium-Comasphaeridium(AHC) acritarch assemblage in Tarim and the Yangtze Block. The early Cambrian biostratigraphical age assignment for the carbonaceous phyllite indicates the presence of both Neoproterozoic and Cambrian rocks in the sedimentary package, and supports that the package is a part of tectonic mélange rather than a continuous Neoproterozoic strata. The Cambrian slate is the youngest known lithology in the mélange at present.

A Paleoproterozoic(Orosirian) Ophiolitic Mélange, North Yangzte Craton

Ophiolites represent fragments of ancient oceanic lithosphere,tectonically incorporated into continental margins during plate subduction or remained in the subduction–collisional orogenic belt.They provide

Discovery of Ophiolitic Mélange in the North Region of Geji County, Northwest Tibet and Its Zircon U-Pb Age

Objective The Bangong Co–Nujiang River suture zone is the great boundary between the Lhasa and Qiangtang terranes in the Tibetan Plateau.A series of major scientific issues are still controversial at present such as the subduction polarity and evolutionary process of this suture（Wei Shaogang et al.,2017）.

DEEP Neoarchean Subduction Indicated by Ultra-high Pressure TiO_(2)(Ⅱ) Inclusion in Ophiolitic Podiform Chromitite Block in Mélange, Central(Taihang) Orogenic Belt, North China

Understanding crust-mantle recycling through time,especially into the Archean,is crucial for understanding how the crust has grown from the mantle,and in turn how crustal material returned to the mantle alters the mantle reservoir.Recycling of crustal material,such as C-H-O-NS-P are especially important for understanding how processes such as subduction,melting,volcanism,and the release of volatiles have regulated climate and life on Earth over billions of years.

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 ).

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.

Chromian Spinels in Listwaenite and Related Rocks in the Sartohay Ophiolitic Mélange, Xinjiang, NW China

Listwaenite,carbonate-talc schist,and serpentinite of Sartohay ophioliticmélange,Xinjiang,northwestern China,contain variably altered chromian spinels.During the hydrothermal alteration from serpentinite to listwaenite

Petrogenesis of Triassic Mafic Complexes with MORB/OIB Affinities from the Western Garzê-Litang Ophiolitic Mélange, Central Tibetan Plateau

There is a general consensus that most ophiolites formed above subduction zones(Pearce,2003),particularly during forearc extension at subduction initiation(Shervais,2001;Stern,2004;Whattam and Stern,2011).'Supra-Subduction zone'(SSZ)ophiolites such as the well-studied Tethyan ophiolites,generally display a characteristic sequential evolution from mid-oceanic ridge basalts(MORBs)to island arc tholeiities(IATs)or bonites(BONs)(Pearce,2003;Dilek and Furnes,2009,2011),which were generated in sequence from the decompression melting of asthenospheric mantle and partial melting of subduction-metasomatized depleted mantle(Stern and Bloomer,1992;Dilek and Furnes,2009;Whattam and Stern,2011).However,ophiolites with MORB and/or oceanic-island basalt(OIB)affinities are rare,and their origin and tectonic nature are poorly understood(Boedo et al.,2013;Saccani et al.,2013).It is interesting that the composition of these ophiolites from the central Tibetan Plateau(CTP)is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs,which is inconsistent with most ophiolites worldwide(Robinson and Zhou,2008;Zhang et al.,2008).But the generation and tectonic nature of these ophiolites are still controversial.*In this study,we present new geochronological,mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzê-Litang suture zone(GLS),a typical Paleo-Tethyan suture crossing the CTP(Fig.1).The Triassic ophiolite in the western GLS has been described by Li et al.(2009),who foundthat it mainly consists of gabbros,diabases,pillow basalts and a few metamorphic peridotites.The ophiolite has been tectonically dismembered and crops out in Triassic clastic rocks and limestones as tectonic blocks.The Chayong and Xiewu mafic complexes are generally regarded as important fragments of the Triassic ophiolites(e.g.,Jin,2006;Li et al.,2009).Zircon LA-ICP-MS U-Pb ages of234±3 Ma and 236±2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes,respectively.The basalts and gabbros of the Chayong complexexhibitenrichedMORB(E-MORB)compositional affinities except for a weak depletion of Nb,Ta and Ti relative to the primitive mantle,whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities.The geochemical features suggest a probable fractionation of olivine±clinopyroxene±plagioclase as well as insignificant crustal contamination.The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithosphericmantlemetasomatizedbyOIB-like components.The ratios of Zn/Fet,La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet+minor spinel-bearing peridotite or spinel±minor garnet-bearing peridotite.We propose thatback-arcbasinspreadingassociated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times,and the decompression melting of the depleted MORB-type asthenospheremantleandpartialmeltingof sub-continental lithosphere were metasomatized by plume-related melts,such as OIBs,which led to the generation of the Chayong and Xiewu mafic melts.

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.

Munabulake ophiolitic mélange:Implication for the evolution history of the north branch of the Proto-Tethys ocean

One of the ophiolites that record the Proto-Tethys Ocean’s episodic closure is the Munabulake ophiolitic mélange,which is situated in the middle of the Kunlun-Qaidam and Altun-Qilian blocks.Detailed field mapping revealed that the Munabulake ophiolitic mélange comprises local(ultramafic rocks,basalts,andesites,gabbros,diorites,and plagiogranites)and exotic(marble,gneiss,schist,and amphibolite)blocks,many of which are in the schist matrix(Qimantage Group).Based on geochronological,geochemical,and petrological observations,the mafic rocks in the Munabulake ophiolitic mélange can be categorized into three categories:498-Ma OIB-like gabbros,468-Ma Hawaiian alkaline basalt-like dolerite and pillow basaltic slices,and 428 Ma massive SSZ-like ultramafic rocks.The 501-452 Ma I-type granites exhibit arc affinities due to the oceanic crust subduction.These findings,along with spatial relationships,suggest that the Early Paleozoic ophiolite complex,island arc rocks,and accretionary complex generated as an intra-oceanic arc system as a result of obduction of the south Altun Ocean’s onto the Central Altun block within a north-directed subduction event.A dextral strike-slip was evident throughout the Early Paleozoic oceanic crust subduction based on the whole set of planar and linear structural data,and the subduction polarity was likely to the north.According to the ophiolitic mélange’s youngest rocks and the existence of 413 Ma granite dykes that are widely exposed in the Munabulake ophiolitic mélange,the Munabulake ophiolitic mélange was most likely emplaced during the Middle Silurian.This Munabulake ophiolitic mélange is similar in age and petrochemical characteristics to the other ophiolites in the South Altun subduction-collision belt,indicating that the Manabulak ophiolite mélange is a westward extension of the Apa-Mangya subduction-collision belt,which formed during the northward subduction of the South Altun Ocean slab during the Early Paleozoic.Thus,the final closing time of the South Altun Ocean is between 413 and 428 Ma.

Lithological Analysis of Nain Ophiolitic Zone Using ASTER Data

The Mesozoic ophiolitic Mélange, north of Nain in the Central-East Iran Microplate (CEIM) comprises serpentinized ultramafic rocks, harzburgites, dunite, gabbro, peridotite, pelagic limestone and other carbonate rocks. The excellent and vast exposure of this desert region is well suited for geologic mapping of this rock suite using remote sensing, especially using data from the satellite-borne advanced Space borne Thermal Emission and Reflection Radiometer (ASTER) imaging system which was designed for mapping mineral information. In this study, data processing methods like Method Minimum noise fraction (MNF), Feature Oriented Principal Components Selection (FPCS), Band Ratios (BR) and Optimum Index Factor (OIF) were used to process ASTER data to optimize the mapping of ophiolite rock types. For example, a simple color composites of OIF (Red: B3, Green: B4, and Blue: B8) and Band ratios (e.g. Red: (B2 + B4)/B3, Green: (B5 + B7)/B6, Blue: (B7 + B9)/B8) were useful for discriminating serpentinite, meta-basalt and granite rock types. It is concluded here that proposed ASTER data has the potential for mapping similar ophiolites elsewhere using the global archive of ASTER imagery.

Zircon Ages of Metasedimentary Rocks in the Wuwamen Ophiolitic Mélange,Chinese South Tianshan:Implications for the Paleozoic Subduction-Accretion in the Southern Central Asian Orogenic Belt

High-temperature and high-pressure(high-grade)metamorphic complexes of variable ages are common in the Central Asian orogenic belt(CAOB),and their precise geochronology and origin are essential to unravel the orogenic architecture and crust-production rate of the CAOB.Hereby it is essential to differentiate between pre-orogenic Precambrian basement and Paleozoic arc-accretionary complexes.This study provides precise in situ zircon U-Pb ages for the metasedimentary rocks in the Wuwamen ophiolitic mélange,which is traditionally thought to represent the pre-orogenic basement of the southwestern CAOB.A meta-sandstone from the meta-flysch sequence revealed a widespread ca.1.8 Ga high-grade metamorphic overprint similar to that of the underlying orthogneisses and,thus,was interpreted to represent pre-orogenic basement fragments that occur as tectonic blocks in the Paleozoic ophiolitic mélange.In contrast,a schist from the mélange matrix is characterized by a ca.333 Ma high-grade metamorphic overprint,indicating that the northward subduction of the South Tianshan Ocean was still active at this time.The thrust-imbricated high-grade metamorphic rocks of contrasting origins in an accretionary complex have important implications for the understanding the accretionary history and crustal growth of the CAOB.Furthermore,the strongly deformed ophiolitic mélange was intruded by an undeformed granite dyke with an emplacement age younger than ca.294 Ma,thus providing a minimum age limit for the final closure of the South Tianshan Ocean.

Petrogenesis and Tectonic Implications of the Late Silurian–Early Devonian Bimodal Intrusive Rocks in the Central Beishan Orogenic Belt,NW China:Constraints by Petrology,Geochemistry and Hf Isotope

A large number of Late Silurian–Early Devonian intrusive rocks are distributed in the central Beishan orogenic belt(BOB).Tectonic setting of these intrusive rocks is of great significance to the study of the subduction and accretion of the Paleo-Asian Ocean.Previous studies show that most of the intrusive rocks in this region are S-type or A-type granitoids.In this study,we firstly reported the Late Silurian–Early Devoniandia bases,granodiorites on the southside of the Baiyunshan ophiolitic mélanges belt,as a part of Hongliuhe-Xichangjing ophiolitic mélanges belt(HXOMB).Zircon LA-ICP-MS U-Pb dating yields emplacement ages between 418 and 397 Ma,REE distribution patterns exhibit enriched LREE and flat HREE in the diabases,the discriminant diagrams show that the diabases have geochemical characteristics of intraplate basalt.The granodiorites in this paper present more like S-and A-type granitoids reported,showing the geochemical characteristics of syn/post-collision granites.Actually,the bimodal magmatic rocks are developed during Late Silurian–Early Devonian on both sides of the HXOMB,which are related to the tectonic background of the post orogeny extension.The diabases are tholeiitic with relative strong depletedεHf(t)(+8.1 to+13.0),which are mainly from relative depleted mantle.The granodiorites are calc-alkaline with relative slightly depletedεHf(t)(+0.7 to+5.6)and the lower Mg#and MgO contents(34.6–36.9,0.50 wt.%–1.19 wt.%respectively),reflecting the source characteristics of meta-basalt.Therefore,the remelting of juvenile crust may be the main way of continental crust accretion during Late Silurian–Early Devonian in the central BOB.