Origin Of The Pozanti-karsanti Ophiolite And Related Chromitite In Southern Turkey And The Northern Sub-belt Ophiolite Of The Western Yarlung Zangbo Suture Zone,Tibet,China

Turkey,situated predominantly in south-westernmost Asia,is composed of several continental and oceanic fragments which were assembled together during the Late Cretaceous-Early Tertiary period,as a result of the closure of Tethyan Oceans.From the north to the South,Turkey can be divided into four major tectonic units,including:the Pontides,the Kirsehir Massif,the Anatolide-Tauride Block and the Arabian Platform.These different units are separated mainly by three ophiolitic belts,namely?1?the Northern Ophiolitic belt,?2?the Tauride Ophiolitic belt and?3?the Southern Ophiolitic belt.The Pozanti-Karsanti ophiolite?PKO?within the eastern Tauride belt has a relatively complete ophiolitic sequence,from the bottom to the top,containing mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.Most of the crustal rocks of the Pozanti-Karsanti ophiolite were eroded during the unroofing processes since later Cretaceous.The PKO is about 20 km in width and 85km in length,covering an area of approximately 1300 km².Peridotite rocks covered the largest portions of surface area of the PKO,and chromite deposits are being mined from numerous locations through the region.The PK mantle peridotites are dominated by spinel harzburgites along with minor dunites.Mafic dikes with gabbroic and doleritic textures intrude the PKO and the underlying metamorphic sole;no dikes have been observed in the Aladag mélange.The modal mineralogy,mineral chemistry and whole-rock geochemistry indicate that the PK mantle peridotites have experienced a high degree of partial melting.Pt and Pd enriched characteristics of the mantle peridotites,especially the mantle dunites,suggest that the melts which reacted with the peridotites were sulphur-saturated?S-saturated?.The mantle dunites formed by Si-undersaturated and S-saturated melts reacting with depleted peridotites at high melt/rock ratios.U-shaped or spoon-shaped REE patterns indicate that the PK peridotites may have also been metasomatized by the light rare earth elements?LREE?-enriched fluids released from a subducting slab in a suprasubduction zone.On the basis of mineral and whole-rock chemical compositions,the PK peridotites show affinities to forearc peridotites.Chromitites occur both in the mantle peridotites and the mantle-crust transition zone?MCTZ?horizon.Chromitites from the two different horizons have different textures but similar mineral compositions,consistent with typical high-Cr chromitites.Chromitites hosted by mantle harzburgites generally have higher total platinum-group element?PGE?contents than those of the MCTZ chromitites.However,both chromitites show similar chondrite-normalized PGE patterns characterized by clear IPGEs?Irdium-group:Os,Ir,and Ru?,Rh-enrichments relative to Pt and Pd.Such PGE patterns indicate no or only minor crystallization of Pt-andPd-enrichedsulphidesduringformationofchromititesfroma sulphur-undersaturated melt?e.g.boninitic or island arc tholeiitic melt?.Dunites that envelope the chromitite lenses in the hosting harzburgite resulted from melt-rock reactions.The PK mafic dikes have basaltic-andesitic compositions and LREE-depleted chondrite-normalized REE patterns.In the primitive mantle-normalized trace element diagram,the Pozanti-Karsanti dikes show negative anomalies in HFSE?high field strength element?such as Nb,Ta and Ti,and variable enrichments in LILE?large ion lithophile element?such as Rb,Ba and U.These dikes have less radiogenic Nd isotopes but more radiogenic Sr isotopes than the depleted MORB mantle?DMM?source.Based on melting models of mantle source of different facies using La,Sm and Yb,we conclude that these dikes originated from Sp-or Sp+Grt-facies?Sp>Grt?mantle.The LREE and HFSE depleted characteristic combined with positive?_Nd?t?values suggest that these dikes were derived from a depleted mantle source similar to mid-ocean-ridge basalt?MORB?.However,the relatively higher Th/Nb ratios than MORB imply the addition of a subduction component in the Pozanti-Karsanti dikes.The oxygen fugacities of the Pozanti-Karsanti dikes are around quartz-fayalite-magnetite?QFM?higher than those of MORB?QFM-1?but lower than island-arc basalt?IAB??QFM+1?.Through comparison studies of the Pozanti-Karsanti dikes with volcanic rocks from IBM arc-basin system,we propose that the Pozanti-Karsanti dikes formed in the forearc environment during intra-oceanic subduction.Zircon U-Pb ages point to the emplacement of these dikes occur at 86.9±3.1 Ma?1 s.d.?.We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites.So far,more than 200 grains of microdiamond and more than100 grains of moissanite?SiC?have been separated from podiform chromitites.These minerals have been identified by energy dispersive sectromerter?EDS?and Laser Raman analyses.The diamonds and moissanite are accompanied by large amounts of rutile.Additionally,zircon,monazite and sulphides are also common phases within the heavy mineral separates.Both diamond and SiC have been analyzed for carbon and nitrogen isotopic composition using the CAMECA 1280-HR large geometry Secondary Ion Mass Spectrometer?SIMS?at the Helmholtz Zentrum Potsdam.Zircons have also been analyzed for the U-Pb and oxygen isotopic composition by SIMS.Zircons in the Pozanti-Karsanti chromitites show different colors,including colorless,slightly brownish and dark brownish.The crystal shapes of the zircons vary from prismatic to subhedral and rounded.The cathodoluminescence?CL?,second electron?SE?and back-scatter electron?BSE?images of polished zircons revealed a great variety and complexity of internal structures.SIMS age dating on these zircons yielded ²⁰⁶Pb/²³⁸U or ²⁰⁷Pb/²⁰⁶U ages ranging from 123-3476 Ma.?¹⁸O_VSMOW values of these zircons from the chromitites range from 4.87 to 8.60‰.These zircons may have been detrially deposited within oceanic sediments and subducted with oceanic slab.The parental melts of the chromitites may have assimilated these subducted crustal materials.Zircons,together with other crustal minerals such as rutile,monazite and quartz,were incorporated into the chromitites during the crystallization of the chromites from the crustal mineral-bearing parental melts.In total,sixty-one?¹³C_PDB results for diamond were acquired,exhibiting a range from-28.4‰to-18.8‰.SIMS results reveal a relatively large variation in diamond nitrogen isotopic composition with forty?¹⁵N_AIR results ranging from-19.1‰to 16.6‰.The ¹³C-depleted isotopic characteristic suggests a carbon source of subducted organic material.The nitrogen contents and nitrogen isotopic compositions of the PK diamonds are consistent with those of peridotitic and eclogitic diamonds.Metallic alloys,silicate minerals and fluid inclusions have been found in the diamonds.Metal alloy inclusions in the PK diamond suite imply a formation depth of>²50 km.The coexistence of alloy+silicate+fluids testifies to the natural origin of the PK diamonds and implies a metal-rich diamond formation media.Thirty-one?¹³C_PDB results for SiC vary between-30.5‰to-27.2‰,with a mean value of-29.0‰.SiC grains are dominated by6H-SiC polytypes,with minor 3C,4H and 15R polytypes.Native Si?Si⁰?inclusions have been observed in some SiC grains.SiC and Si⁰ inclusions from the Pozanti-Karsanti chromitite suggest an ultra-highly reducing environment during formation of SiC.Recycled organic carbon generally has a strongly ¹³C-depleted isotopic composition,making itself an ideal candidate carbon source for the SiC in the chromitites.We concluded that the SiC in the Pozanti-Karsanti chromitite crystallized from a highly-evolved fluid in an ultra-highly reduced micro-environment.The discovery of diamond,SiC and the other unusual minerals from podiform chromitites of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions.Considering the unusual minerals,the high Mg#silicate inclusions,and the needle-shaped exsolutions in the PK chromitites,the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment.We suggest melt-rock reactions,magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PK chromitites.The northern sub-belt situated in the western segment of the Yarlung Zangbo Suture Zone,mainly includes the Dajiweng,Kazhan,Baer,Cuobuzha,Jianabeng and Zhalai ophiolitic massifs.These ophiolites are strongly dismembered,typically 1-2 km wide and 10-20 km long,composed chiefly of peridotites with minor volcanic and siliceous sedimentary rocks.No cumulates have been observed in the Northern Sub-belt ophiolites.Mineralogical and geochemical studies indicate that peridotites from the Northern Sub-belt ophiolites have experienced varied degree of partial melting and also melt-rock reaction processes.We propose that the ophiolites in the northern belt of the western segments have formed in an intra-oceanic forearc-arc-backarc system of a subduction zone.According to the zircon U-Pb age of mafic intrusions,geochemical characteristics of both mafic and ultramafic rocks,detrital zircon study of Zhongba“terrane”,and klippen structure of ophiolitic massif in the southern belt,we conclude that the northern and southern ophiolitic belts were developed in a same intra-oceanic subduction system.By reviewing characteristics of the unusual mineral-bearing ophiolite,we found out that all of these ophiolites carry suprasubduction zone?SSZ?characteristics,which may indicate that the occurrence of these unusual minerals is related to subduction process.The PKO and the NSO are both remants of Neo-Tethyan Ocean and show many similarities with each other.The NSO is also SSZ-type ophiolite and is promising in carrying diamond,SiC and other unusual minerals.