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.

New Zircon U-Pb Age of the Babu Ophiolites in Southeast Yunnan,China and Constrains of Plate Subduction Time

Objective The Babu ophiolite in Malipo County of southeastern Yunnan is interpreted as remanant ocean crust and represents a possible branch of Paleo-Tethyan Ocean in South China. It consists mainly of mafic and ultramafic rocks. These rocks are very important to understand the evolution of the Paleo-Tethyan Ocean. However, the Babu ophiolite is still disputed and the mafic and ultramafic rocks have been inferred to be part of the Emeishan large igneous province （LIP） by some researchers. In this paper, we present zircon U-Pb data on the metabasalts in Malipo to reveal the formation time of mafic and ultramafic rocks and their tectonic nature.

Cold plumes trigger contamination of oceanic mantle wedges with continental crust-derived sediments:Evidence from chromitite zircon grains of eastern Cuban ophiolites

The origin of zircon grains, and other exotic minerals of typical crustal origin, in mantle-hosted ophiolitic chromitites are hotly debated. We report a population of zircon grains with ages ranging from Cretaceous(99 Ma) to Neoarchean(2750 Ma), separated from massive chromitite bodies hosted in the mantle section of the supra-subduction(SSZ)-type Mayari-Baracoa Ophiolitic Belt in eastern Cuba. Most analyzed zircon grains(n = 20, 287 ± 3 Ma to 2750 ± 60 Ma) are older than the early Cretaceous age of the ophiolite body, show negativeε_(Hf)(t)(-26 to-0.6) and occasional inclusions of quartz, K-feldspar,biotite, and apatite that indicate derivation from a granitic continental crust. In contrast, 5 mainly rounded zircon grains(297±5 Ma to 2126±27 Ma) show positive εHf(t)(+0.7 to +13.5) and occasional apatite inclusions, suggesting their possible crystallization from melts derived from juvenile(mantle)sources. Interestingly, younger zircon grains are mainly euhedral to subhedral crystals, whereas older zircon grains are predominantly rounded grains. A comparison of the ages and Hf isotopic compositions of the zircon grains with those of nearby exposed crustal terranes suggest that chromitite zircon grains are similar to those reported from terranes of Mexico and northern South America. Hence, chromitite zircon grains are interpreted as sedimentary-derived xenocrystic grains that were delivered into the mantle wedge beneath the Greater Antilles intra-oceanic volcanic arc by metasomatic fluids/melts during subduction processes. Thus, continental crust recycling by subduction could explain all populations of old xenocrystic zircon in Cretaceous mantle-hosted chromitites from eastern Cuba ophiolite.We integrate the results of this study with petrological-thermomechanical modeling and existing geodynamic models to propose that ancient zircon xenocrysts, with a wide spectrum of ages and Hf isotopic compositions, can be transferred to the mantle wedge above subducting slabs by cold plumes.

YARLUNG ZANGBO OPHIOLITES,SOUTHERN TIBET REVISITED

Field work conducted in September 1998 and Summer 1999 aimed to reassess the ophiolitic segment of the Yarlung Zangbo suture zone (YZS) and shed new light on the preserved fragments of Neo\|Tethys ocean\|floor. This eastern ophiolitic segment was partly surveyed during the 1980 Sino\|French Cooperative Investigation of Himalayas, but little work has been done since that time. Progress in ophiolite research field and new developments in modern ocean crust guided us in the recent field work investigation. Mantle peridotites and associated minor crustal units are assumed Early Cretaceous in age, while diabase interbedded with phyllites and radiolarian sediments of presumed seamount origin are attributed to Late Jurassic—Early Cretaceous age. Six different massifs were visited that are from west to east: Jiding, Qunrang, Beimarang, Dazhuqu, Luobusa, and Zedang. Each massif presents specific characteristics summarized below. The Jiding massif is made of partly to totally serpentinized granular upper mantle harzburgites with orthopyroxenite banding, a transitional Moho zone, a thick diabase sill\|dike complex intruded into heterogeneous gabbro, and pillow lavas.. High\|temperature plastic foliation, although generally oriented NW—SE, and lineation show folding. Numerous gabbroic and diabasic intrusions are observed in peridotites. The orientations of the mafic rocks foliation and lineation do not fit the structure of the host peridotites. The 350m thick transition zone is a syntectonically intrusive sequence of mantle peridotites cut by abundant different types of gabbro and diabase. In one case intrusion of gabbro postdates serpentinization of peridotites and the outer margin of the xenolith enclosed in fine\|grained gabbro has reacted to form pegmatitic hornblende gabbro. The crustal unit is made of gabbro intruded by multiple fine\|grained dikes. Hydrothermal circulation was locally intense and Cu mineralization and epidosite are observed close to shear zones.The Qunrang massif shows no transition zone overlying upper mantle unit, no significant gabbroic crustal unit and thick diabase and volcanic units. The foliation and lineation in granular lherzolite, harzburgite, and dunite show extremely wide variations and affected by late tectonics. The orientation of the structures is similar to the Jiding massif.

The Intra-Pontide ophiolites in Northern Turkey revisited:From birth to death of a Neotethyan oceanic domain

The Anatolian peninsula is a key location to study the central portion of the Neotethys Ocean(s)and to understand how its western and eastern branches were connected.One of the lesser known branches of the Mesozoic ocean(s)is preserved in the northern ophiolite suture zone exposed in Turkey,namely,the Intra-Pontide suture zone.It is located between the Sakarya terrane and the Eurasian margin(i.e.,Istanbul-Zonguldak terrane)and consists of several metamorphic and non-metamorphic units containing ophiolites produced in supra-subduction settings from the Late Triassic to the Early Cretaceous.Ophiolites preserved in the metamorphic units recorded pervasive deformations and peak metamorphic conditions ranging from blueschist to eclogite facies.In the nonmetamorphic units,the complete oceanic crust sequence is preserved in tectonic units or as olistoliths in sedimentary melanges.Geochemical,structural,metamorphic and geochronological investigations performed on ophiolite-bearing units allowed the formulation of a new geodynamic model of the entire"life"of the IntraPontide oceanic basin(s).The reconstruction starts with the opening of the Intra-Pontide oceanic basins during the Late Triassic between the Sakarya and Istanbul-Zonguldak continental microplates and ends with its closure caused by two different subductions events that occurred during the upper Early Jurassic and Middle Jurassic.The continental collision between the Sakarya continental microplate and the Eurasian margin developed from the upper Early Cretaceous to the Palaeocene.The presented reconstruction is an alternative model to explain the complex and articulate geodynamic evolution that characterizes the southern margin of Eurasia during the Mesozoic era.

Geological Occurrence of Diamond-bearing Ophiolites

Diamonds and other ultrahigh-pressure（UHP）minerals exist in ophiolitic mantle peridotites and podiform chromitites from different orogenic belts.Most ophiolitehosted diamonds are small（;00-500μm across）,and

Sheeted Dike Complexes in Contemporary Oceanic Crust: Implications for Spreading Processes and the Interpretation of Ophiolites

As anticipated from studies of ophiolite complexes,direct investigations of the oceanic crust confirm that basaltic dikes are an integral part of the upper 2 km of the oceanic crust.Currently available information suggests

The Significance of Sheeted Dike Complexes in Ophiolites

The modern‘Penrose’definition of ophiolites is based largely on the Troodos complex of Cyprus,which contains a spectacular and well-exposed sheeted dike complex in which dike intrudes dike without intermediate screens of

PGE and isotopic characteristics of Shergol and Suru Valley Ophiolites,Western Ladakh:Implications for supra-subduction tectonics along Indus Suture Zone

Present study reports the PGE-geochemistry of mantle peridotites and Nd-isotope geochemistry of arc related mafic rocks from the Indus Suture Zone(ISZ),western Ladakh.The total PGE concentration of the Shergol and Suru Valley peridotites(∑PGE=96-180 ppb)is much higher than that of the primitive mantle and global ophiolitic mantle peridotites.The studied peridotites show concave upward PGE-distribution patterns with higher palladium-group PGE/Iridium-group PGE ratios(i.e.,0.8-2.9)suggesting that the partial melting is not the sole factor responsible for the evolution of these peridotites.The observed PGE-distribution patterns are distinct from residual/refractory mantle peridotites,which have concave downward or flat PGE-distribution patterns.Relative enrichment of palladium-group PGE as well as other whole-rock incompatible elements(e.g.,LILE and LREE)and higher Pd/Ir ratio(1.1-5.9)reflects that these peridotites have experienced fluid/melt interaction in a supra-subduction zone(SSZ)tectonic setting.Also,the Shergol mafic intrusives and Dras mafic volcanics,associated with the studied peridotites,have high^(143)Nd/^(144)Nd ratios(i.e.,0.512908-0.513078 and 0.512901-0.512977,respectively)and positiveε_(Nd)(t)(calculated for t=140 Ma)values(i.e.,+5.3 to+8.6 and+5.1 to+6.6,respectively),indicating derivation from depleted mantle sources within an intra-oceanic arc setting,similar to Spongtang and Nidar ophiolites from other parts of Ladakh Himalaya.The transition from SSZ-type Shergol and Suru Valley peridotites to Early Cretaceous tholeiitic Shergol mafic intrusives followed by tholeiitic to calc-alkaline Dras mafic volcanics within the Neo-Tethys Ocean exhibit characteristics of subduction initiation mechanism analogous to the Izu-Bonin-Mariana arc system within western Pacific.

The Chromitites Associated with the Pan-African Ophiolites in Egypt

Ophiolites components occur in Pan-African belt in Central Eastern Desert(CED)and South Eastern Desert(SED.The ultramafic components are severely serpentinized and in some areas occur as small fresh

Newly Discovered Ca. 163 Ma OIB-Type Diabase Dike from the Shiquanhe Ophiolites, Western Tibet

Objective The Shiquanhe ophiolite is an important tectonic belt in western Tibet. It has been debated whether the Shiquanhe ophiolite represents an allochthonous nappe derived from the Bangong-Nujiang suture zone to the north or the

Diamonds, Super-Reduced and Crustal Minerals in Chromitites of the Hegenshan and Sartohay Ophiolites, Central Asian Orogenic Belt, China

The Central Asian Orogenic Belt(CAOB)is a huge tectonic mélange that lies between the North China Craton and the Siberian Block.It is composed of multiple orogenic belts,continental fragments,magmatic and metamorphic rocks,suture zones and discontinuous ophiolite belts.Although the Hegenshan and Sartohay ophiolites are separated by nearly 3000 km and lie in completely different parts of the CAOB,they are remarkably similar in many respects.Both are composed mainly of serpentinized peridotite and dunite,with minor gabbro and sparse basalt.They both host significant podiform chromitites that consist of high-Al,refractory magnesiochromite with Cr#s[100Cr/(Cr+Al)]averaging<60.The Sartohay ophiolite has a zircon U-Pb age of ca.300 Ma and has been intruded by granitic plutons of similar age,resulting in intense hydrothermal activity and the formation of gold-bearing listwanites.The age of the Hegenshan is not firmly established but is thought to have formed in the Carboniferous.Like many other ophiolites that we have investigated in other orogenic belts,the chromitites in these two bodieshave abundant diamonds,as well as numerous super-reduced and crustal minerals.The diamonds are mostly,colorless to pale yellow,200-300μm across and have euhedral to anhedral shapes.They all have low carbon isotopes(δ14C=-18 to-29)and some have visible inclusions.These are accompanied by numerous super-reduced minerals such as moissanite,native elements(Fe,Cr,Si,Al,Mn),and alloys(e.g.,Ni-Mn-Fe,Ni-Fe-Al,Ni-Mn-Co,Cr-Ni-Fe,Cr-Fe,Cr-Fe-Mn),as well as a wide range of oxides,sulfides and silicates.Grains of zircon are abundant in the chromitites of both ophiolites and range in age from Precambrian to Cretaceous,reflecting both incorporation of old zircons and modification of grains by hydrothermal alteration.Our investigation confirms that high-Al,refractory chromitites in these two ophiolites have the same range of exotic minerals as high-Cr metallurgical chromitites such as those in the Luobusa ophiolite of Tibet.These collections of exotic minerals in ophiolitic chromitites indicate complex,multi-stage recycling of oceanic and continental crustal material at least to the mantle transition zone,followed by uprise and emplacement of the peridotites into relatively shallow ophiolites.

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.

Ophiolites as Archives of Recycled Crustal Material Residing in the Deep Mantle

Deeply subducted lithospheric slabs may reach to the mantle transition zone(MTZ,410-660 km depth)or even to the core–mantle boundary(CMB)at depths of^2900km.Our knowledge of the fate of subducted surface material at the MTZ or near the CMB is poor and based mainly on the tomography data and laboratory experiments through indirect methods.Limited data come from the samples of deep mantle diamonds and their mineral inclusions obtained from kimberlites and associated rock assemblages in old cratons.We report in this presentation new data and observations from diamonds and other UHP minerals recovered from ophiolites that we consider as a new window into the life cycle of deeply subducted oceanic and continental crust.Ophiolites are fragments of ancient oceanic lithosphere tectonically accreted into continental margins,and many contain significant podiform chromitites.Our research team has investigated over the last 10 years ultrahigh-pressure and super-reducing mineral groups discovered in peridotites and/or chromitites of ophiolites around the world,including the Luobusa(Tibet),Ray-Iz(Polar Urals-Russia),and 12 other ophiolites from 8orogenic belts in 5 different countries(Albania,China,Myanmar,Russia,and Turkey).High-pressure minerals include diamond,coesite,pseudomorphic stishovite,qingsongite(BN)and Ca-Si perovskite,and the most important native and highly reduced minerals recovered to date include moissanite(Si C),Ni-Mn-Co alloys,Fe-Si and Fe-C phases.These mineral groups collectively confirm extremely high?pressures(300 km to≥660 km)and super-reducing conditions in their environment of formation in the mantle.All of the analyzed diamonds have unusually light carbon isotope compositions(δ13C=-28.7 to-18.3‰)and variable trace element contents that*d i stinguish them from most kimberlitic and UHPmetamorphic varieties.The presence of exsolution lamellae of diopside and coesite in some chromite grains suggests chromite crystallization depths around>380 km,near the mantle transition zone.The carbon isotopes and other features of the high-pressure and super-reduced mineral groups point to previously subducted surface material as their source of origin.Recycling of subducted crust in the deep mantle may proceed in three stages:Stage 1–Carbon-bearing fluids and melts may have been formed in the MTZ,in the lower mantle or even near the CMB.Stage 2–Fluids or melts may rise along with deep plumes through the lower mantle and reach the MTZ.Some minerals,such as diamond,stishovite,qingsongite and Ca-silicate perovskite can precipitate from these fluids or melts in the lower mantle during their ascent.Material transported to the MTZ would be mixed with highly reduced and UHP phases,presumably derived from zones with extremely low f O2,as required for the formation of moissanite and other native elements.Stage 3–Continued ascent above the transition of peridotites containing chromite and ultrahigh-pressure minerals transports them to shallow mantle depths,where they participate in decompressional partial melting and oceanic lithosphere formation.The widespread occurrence of ophiolite-hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in-situ oceanic mantle.Because mid-ocean ridge spreading environments are plate boundaries widely distributed around the globe,and because the magmatic accretion of oceanic plates occurs mainly along these ridges,the on-land remnants of ancient oceanic lithosphere produced at former mid-ocean ridges provide an important window into the Earth’s recycling system and a great opportunity to probe the nature of deeply recycled crustal material residing in the deep mantle

Fluidizate-Explosive Occurrences in Ophiolites as Indicator of the Subduction Zone Activity: The Urals Example

It is known that the formation of oceanic crust occurs in different geodynamic settings,accompanying by the emergence of mantle-magmatic ophiolite complexes having a distinctive properties.In the process of mantle-crustal evolution of the ophiolites are undergoing significant changes with the formation of peculiar(on structure and composition)rocks,sometimes with unusual mineral paragenesis.The presence of such rocks in mélange tectonic zones greatly complicates to determine their origin.In the Ural folded belt(length more than 2,000 km)separating the East European Platform and the West Siberian sedimentary basin,ophiolites are widespread forming a chain of mafic-ultramafic massifs(Fig.1)located in the allochthonous position with mélange at the bottom(Puchkov,2013).With the Urals ophiolites are associated occurrences of eclogites,jadeites,ruby and other rocks of unclear nature,sometimes regarded as potentially diamondiferous.Such formations of unclear genesis include the associating with ophiolites metabasites of higher alkalinity composing the body in the mantle peridotites of the mélange Main Uralian Fault(MUF)zone(Shmelev,2005).By this time they are determined in different parts of the fault zone,but most completely are known in the Sub Polar Urals,where are distinguished under the name of Sertynya alkaline-ultramafic complex,which is located just 25 km east of Hartes kimberlitic complex(Fig.1).Formally,its affiliation to diamond-bearing associations is confirmed by finding of grains and fragments of natural diamond in the weathering crust.A detailed study of the rock complexes shows that in reality they have a polygenic nature,combine theelements of proper magmatic and fluidizate-explosive formations,the appearance of which was interfaced with the processes at the slab-mantle wedge boundary in subduction zones.Polygenic nature of the rocks is reflected in the existence of three interrelated structural-geological units:(1)bodies and dikes of uniformmetadiabasesanddensefine-grainedmetadolerites(lamprophyres),(2)fluidal-brecciated dolerites('tuff breccias')and(3)structural weathering crust with angular or rounded fragments(blocks)of metadolerites and serpentinites.The rocks have experienced rodingitization and permeated with net of veins a vesuvianite composition.The host peridotite matrix(harzburgites and dunites)has undergone serpentinizationandchloritization.Structural relationships give grounds for distinguishing in the history of the complex formation the magmatic proper(dolerite dyke and lamprophyre intrusion)and infiltration fluidizate-explosive(metasomatic transformation of dolerite)stages.Peculiarities of petrography and mineralogy of rock complexes does not allow to compare them with lamproites and kimberlites.Metadiabases demonstrate relics of ophitic structure,as primary paragenesis is completely replaced by aggregate of chlorite,zoisite and leucoxene.Dolerites(lamprophyres)have a uniform fine-grained or porphyry structure with phenocrysts of clinopyroxene,brown amphibole and leucoxene(sphene),which are immersed in a fine-scaly aggregate of light green mica.In the rocks amphibole,garnet and vesuvian are present.Clinopyroxene corresponds to augite with moderate content of titanium and alumina(up to 3.5wt.%),showing a normal magmatic zonation in composition.Mica previously wrongly called as phlogopite,actually has an extremely ferrous composition and corresponds to biotite(annite).Amphibole is presented by magmatic titaniferous tschermakite hornblende and metamorphic(bluish)variety of sodium-calcium composition(taramite).Garnet is presented by exceptionally grossular of rodingite type.Mineralogy of weathering crust reveals similar features,but in the samples it is marked the presence of muscovite,orthoclase and weakly ferrous diopside.An important feature of the weathering crust is the presence of shear surfaces on minerals,resulting in fracturing due to internal stress,confirming the explosive nature of protolith.The bulk chemical composition of rocks is characterized by significant variations in the content of silica(30-46 wt.%)and alkalis(0-6.5 wt.%).These metabasites have consistently a low magnesia number and high titanium oxide content(1.5-3.0 wt.%).Side by side with these are been established the uniform slope REE distribution trends similar to the trend of oceanic basalts N-MORB type(Fig.2).The level of trace element compositions does not depend on variations in the alkalinity of the rocks,but clearly correlates with the titanium content.Unlike them the Hartes kimberlites demonstrate the distribution with deficit of HREE,andthe level of the elements content is correlated with the alkalinity of rocks(Mahotkin et al.,1998).Another important geochemical feature of the Sertynya complex rocks is a regular behavior of the mobile LILE elements(Cs,Rb,Ba,K).In the varieties of rocks with mica enriched by alkalis,it is recorded extremely high level of LILE,exceeding the level of contents in N-MORB basalts at 10-10000 times!In the metabasites varieties with low level of alkalinity,LILE content is sharply(except Cs)reduced to minimum values(Fig.2).The observed pattern of the element distribution is undoubtedly the result of postmagmatic fluid-metasomatic alteration of the original rocks.Tectonic position and the primary composition characteristics of the metadolerites give reason to consider them as fragments of the ophiolite sheeted dike complex(Shmelev,2005).The famous dike complexes in the ophiolite massifs of the MUF zone(east of mélange)belong to suprasubduction formations of Paleozoic age.However the obtained mainly ancient U-Pb zircon dating(up to Archean inclusive)for metadolerites of the Sertynya complex,make it possible to assume its Vendian-Early Cambrian(530-617 Ma)age.It permits to compare the Sertynya metabasites with the Vendian metaophiolites of the MUF zone in the Middle Urals(Petrov et al.,2010).It is noteworthy that similar age datings(520-550 Ma)are also established for kimberlites of the Hartes complex located to the west of ophiolites.Therefore,thepresenceofthe Vendian-Cambrian ophiolite of MOR-type in the MUF mélange zone,'changing'to the east of Ordovician ophiolites SSZ-type,seems quite possible.The obtained data allow to suggest an original interpretation of nature of the Urals fluidizate-explosive formations considering the process specifics in the subduction zones(Bebout and Barton,2002).Accordingto this model,the pre-Ordovician(?)oceanic crust has undergone transformations and deformations on the slab-mantle wedge boundary during the subduction.As a result of slab dehydration it occurred a flow of aqueous fluids,which were enriched with the extracted from sedimentary rocks the LILE elements and percolated through the mantle substrate with dolerite dyke complex.Interaction with them led to the formation of chlorite-zoisite and/or mica(biotite-bearing)fluidizates and in the presence of a gas phase-fluidizate-explosive breccias with subsequent development of weathering crust.In the surrounding peridotites an explosive process is marked by the formation of pseudokimberlite breccias.Fluidized-explosive occurrences in mantle peridotites of mélange zones should be considered as indicators of the subduction slab-mantle interaction at relatively shallow levels involving enriched LILE fluids(without melts participation),rising as the front from the subduction zone.In this interpretation,there is no need toappealtothealkaline-ultramaficor lamproit-kimberlite hypothesis of the genesis of these formations,however,the question of their potential diamondiferous remains to be open.The proposed interpretation of the fluidizate-explosive occurrences makes it possible to comprehendthat in reality the mélange is a complex formation with signs of not onlycollisional(as usually is considered),but also of earlier subduction events.

Compositions & Melt Evolution of Upper Mantle Peridotites in the Tethyan Ophiolites

The Jurassic–Cretaceous ophiolites in the Alpine–Himalayan orogenic belt represent fragments of oceanic lithosphere,developed in different seaways separated by Gondwana–derived ribbon continents within a broad

Fabrics and Seismic Properties of Ophiolites: Implications for Mantle Flow

Based on ophiolite sequences,seismic velocities of rocks and seismic profiles of ocean basins,the oceanic crust-mantle boundary can be defined as the contact between the solid,ductile deformed mantle and the

Crustal Architecture,Geochemistry and Geochronology of the Ophiolites and Ophiolitic Mélanges in Southeastern Anatolia

The Neotethyan ophiolites exposed in SE Anatolia–Syria occur along two,ENE-WSW-trending,sub-parallel belts(Fig.1).The ophiolites and ophiolitic mélanges in the southern belt include the Cretaceous K?z?lda?(Hatay),

Chemical Variation of Chromian Spinel Compositions in a Serpentinized Peridotites: Implications for Evolution of the Neoproterozoic Ophiolites

Tectonic setting of the Neoproterozoic ophiolites is poorly understood.Because of extensive serpentinization/metamorphism in the mantle section,accessory chromian spinel has been used as an important geotectonic indicator.

Origin of Some Neo-Proterozoic Ophiolites from Eastern Desert of Egypt:Geochemical Constraints

The Neo-Proterozoic ophiolites occur in the central and southern Eastern Desert along suture zone as dismemberedmassesinvolcano-sedimentary assemblages.The ophiolite component includes ultramafic rocks mainly serpentinites,mafic rocks,minor bodies of trondhjemite,sheeted dykes,metabasalts,and pillow lavas.The Present studies include two ophiolites in Central and southern eastern Desert named Mubarak–El Mayet and Ghadir respectively(Fig.1).The first one named after wadi Mubarak and wadi El Mayet area(55 km north Marsa Alam city)and the second named after Wadi Ghadir(30 km south Marsa Alam city).The dismembered ophiolite components in MubarakEl Mayet mainly composed mainly of serpentinites,ophiolitic metagabbros,sheeted dykes and Pillow lavas.All components occur as thrusted blocks and sheets in metavolcano-sedimentary assemblages(matrix).Gabbros sometimes occur as coarse grained gabbros(appenites)whereas pillows range in size from 30 cm to 1 m(Fig.2a).The second ophiolite sequence expose in Wadi Ghadir and its tributary.It consists of serpentinized peridotites,layered gabbro,massive isotropic gabbro,1fine grained gabbro,sheeted diabase dykes,pillowed basaltic lavas and minor plagiogranites.The serpentinizedperidotites,metapyroxenitesand serpentinites occur as allochthonous dismembered blocks,fragments and sheets in highly sheered metasediments and metavolcanics(mélange)(Fig.2b,c).The massive and layered gabbros occur in the main wadi(Fig.2d,f)and in many places contain some veins and pockets from plagiogranites.The present gabbros intruded by syenogranites from the east.Pillow lavas occur between metasedimentary mélange and ophiolitic gabbros in wadi El Beda as tributary of Wadi Ghadir and sometimes occur as fragments in mélange.The pillows range in size from 40 cm to 1 m(Fig.2f)and are mainly amygdaloidal and porphyritic basalt and spilite.Sheeted dykes cut the gabbros and pillow lavas in the Wadi El Beda and composed mainly from diabase(Fig.2g,h).In the present study,47 samples for major,trace and REE elements from different rock types in Mubarak-El Mayet(18 samples)and Ghadir ophiolite(29 samples)were analyzed.The field work and the geochemical data will discussed in the present work to evaluate the tectonic setting,origin and mantle source for two ophiolite suites.