Geochemistry and geochronology of dolerite dykes from the Daba and Dongbo peridotite massifs, SW Tibet: Insights into the style of mantle melting at the onset of Neo-Tethyan subduction

This study reports compositional and whole-rock Sr-Nd isotope data as well as zircon U-Pb geochronological data on dolerite dikes from the Daba and Dongbo ultramafic massifs, southwest Yarlung-Zangbo Suture Zone (YZSZ), Tibet. The 120.6 \ub1 1.6 Ma dolerite dikes from the Daba peridotite exhibit normal mid-ocean ridge basalt (N-MORB)-type normalized mutli-element patterns [(La/Yb)N = 0.43-0.72] with noticeable negative Nb and Th anomalies. They have high initial 87Sr/8624 Sr ratios (87Sr/8625 Sr(i) = 0.70720-0.70788) and high \u3b5Nd(t) values (+7.4 to +7.9). The 125.4 \ub1 1.8 Ma dolerite intrusions within the Dongbo peridotite show N-MORB\u2012type trace element profiles [(La/Yb)N = 0.65-0.84] characterized by apparent negative anomalies in Nb and Th, and mild negative anomalies in Ti (\ub1 Y). They also have high 87Sr/8629 Sr(i) ratios (0.70611-0.70679) and elevated \u3b5Nd(t) values (+7.8 to +8.2). Semi-quantitative La/Yb vs. Dy/Yb modeling demonstrates that the parental magmas of the investigated dolerite dikes derived from more than 20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing N-MORB\u2013like lherzolitic composition. Our geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor input of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs could be linked to upwelling of an asthenospheric source that caused continental rift and subsequent seafloor spreading, followed by subduction initiation adjacent to a passive margin during the early Cretaceous (~130-120 Ma). Our study provides a more detailed, and perhaps more elegant, hypothesis for the tectono-magmatic evolution of the southwestern YZSZ "ophiolitic" peridotites after their accretion beneath a Neo-Tethyan marginal basin

Coexistence of MORB- and OIB-like dolerite intrusions in the Purang ultramafic massif, SW Tibet: a paradigm of plume-influenced MOR-type magmatism prior to subduction initiation in the Neo-Tethyan lithospheric mantle

The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the Zhongba-Zhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 \ub1 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb)N = 13\u201316], high initial 87Sr/86Sr ratios (0.70598\u20130.70765), and low \u3b5Nd(t) values (\u20132.6 to \u20132.3). Zircons separated from this group of dolerites have slightly radiogenic \u3b5Hf(t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 \ub1 2.5 Ma and 124.4 \ub1 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)-type normalized multi-element patterns [(La/Yb)N = 0.6\u20130.9] with noticeable negative Nb and Th (\ub1Ti) anomalies, and have high 87Sr/86Sr(i) (0.70295\u20130.70618) and high \u3b5Nd(t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radiogenic \u3b5Hf(t) values (+3.5 to +17.0). Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5%\u201310% polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORBtype mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous

null: Platinum-group mineral and chromitite genesis associated with the petrogenetic evolution of the Vourinos and Pindos ophiolite complexes, NW Greece

In the present thesis the processes of the genesis of the chomitite occurrences of the Vounnos and Pindos ophiolite complexes combined with their content in platinum group elements are studied Special attention is given to the petrogenetic evolution of the mantle units of both ophiolitesequences in an attempt to understand better the origin and the formation of the ophiolites of the NW Greek area In geotectonic terms the ophiolite complex of Vounnos belongs to the Ypopelagonic isopic zone while the Pindos ophiolites are tectonically thrusted onto the Eocenic flysch of the Pindos zone Both complexes are considered as remnants of a certain ocean which today are found tectonically thrusted over the west margin of the Pelagonic micro plate Beyond the fact that both complexes show characteristics of intense tectonic deformation they do preserve all the types of the petrologie sequences that compose a complete ophiolite complex All the formations that have been studied in the present thesis are coming from the mantle units of both complexes The main part of the Vounnos ophiolite complex is composed of mantle tectonites while the mantle formations in thePindos ophiolite complex compose the so called Dramala unit In the Vounnos complex the samplingcovered the most important ore hosting areas of its northern and southern part while in the Pindoscomplex several samples were taken from the central southern parts of the Dramala unit and fromexotic blocks of mantle origin of the Avdella melange unit The field investigation in both complexes has led to the following conlusions a) The northern part of the Vounnos ophiolite complex shows clearly more characteristics of plastic deformation compared to its southern part in which characteristics of brittle to semi plastic deformation prevail A similar but not so well expressed discrimination can be made as far as concerns the structure of the areas of Miha and Pefki in the Pindos complex b) The statistical analysis of several motional indicators shows a NE thrust direction for the northern sector of the Vounnos complex and the area of Miha and a SE direction of motion for southern Vounnos and Pefki c) The chromitites are hosted in dunites and in the majority of the chromitite occurrences the accompanying rock type is thingrained dunite (serpentinised or not) In contrast to coarse grained dunite the thin-grained variety of dunite is not foliated d) The chromitites of northern Vounnos and Miha are usually of podiform type discordantly to sub-concordantly developed to their neighbouring thick-grained dunitic to harzburgitic pendotites The chromite ores of southern Vounnos and Pefki are stratiform having a concordant to sub concordant structure e) Chromitites of both complexes show a great variety of chromite textures although in each deposit a certain type of texture predominates In northern Vounnos and Miha the dominant textural type is massive and leopard chromitite while in southern Vounnos and Pefki the most common texture in chromitite is schlieren f) In southern Vounnos and Pefki are commonly observed interlayered bodies of harzburgite and dunite which are indicative of a stratified structure of the residual mantle superimposed on these areas The number of the pyroxenite dykes (rodingitised or not) is higher in these areas compared to those in northern Vounnos and Miha In their vast majority these dykes cut chromitite bodies which indicates that they are not genetically related to the ores except for one case of a chromitite that is hosted in a pyroxenite dyke in the northern part of the area of Pefki g) Always in the vicinity of the chromitites the lithology follows the sequence ±Clinopyroxene bearing harzburgite normal harzburgite transitional harzburgite coarse grained dunite thin-grained dunite chromitite A different lithological transition is quite unusual h) The common entrapment of harzburgite blocks in dunites indicates that the latter comes from the former although replacements of foliated harzburgites from thin grained dunite that show no mark of foliation supports the fact that some rocks may not be simply of residual origin I) The presence of clinopyroxene bearing harzburgites is more common in the Pindos ophiolite complex while in the Vounnos ophiolite complex bodies of thin-grained dunite in the form of dykes layers or lenses are more commonly observed The pétrographie investigation of the mantle pendotites has shown that the clinopyroxenebeanng harzburgites and the normal harzburgites are characterized by protogranular texture However the transition from normal to transitional harzburgites is marked by the appearance and the dominance of the porphyroclastic texure Porphyroclastic to mylonitic is the texture of the coarsegrained dunites while their thin grained variety shows (equi )granular texture The most important pétrographie feature of all the investigated rock types except for the thin grained dunites is the replacement of the orthopyroxene porphyroclasts by olivine and generally their enrichment in olivine Sparsely neo-formation of pyroxene has been observed and especially replacement of orthopyroxene by clinopyroxene and of olivine by orthopyroxene The changes in the morphology of accessory Cr spinels with the type of the pendotite is another significant feature of the investigated rocks The most fertile rocks preserve anhedral brown Cr spinels while the most depleted euhedral dark Cr rich spinels In very few cases anhedral pyroxene spinel pairs which are interpreted as break down product of a garnet precursor have been observed Serpentinisation effects in various extents are common as well In the Pindos ophiolite complex was noticed the occurrence of serpentinites that show clear features of re crystallisation like hour glass and interpenetrating texture high contents of magnetite and chlorite In the assemblage of the chromitites participates olivine and rarely other mineral phases as pyroxenes The serpentinisation effect leads to the transformation of the silicate matrix of the chromitites into serpentine tremolite and chlorite In both chromitites and pendotites primary hydrous minerals were not found except for one case of Mg rich horneblende in a harzburgite from the Pindos massif The inclusions that were found in the chromite crystals of the chromitites are both primary and secondary in origin Except for olivine pyroxenes serpentine chlorite actinolite tremolite rare base metal sulfide (BMS) and alloy crystals in the Vounnos chromitites edenitic amphibole was also found as inclusion In the Pindos chromitites edenitic amphibole phlogopite and hydrogamet also take part as inclusions The presence of hydrous minerals in the form of inclusions in the chromitites of both complexes indicates the hydrous nature of the magma that lead to their formation Additionally the absence of phlogopite in the Vounnos chromite ore deposits is indicative of a high ratio of fluid/rock The majority of the inclusions was found in chromitites of massive and leopard texture The general absence of BMS as inclusions in the mantle phases and their restncted occurrence in interstitial places in the pendotites of both complexes indicates a) Depletion of the pendotites in sulfides because they have been affected by high degrees of melting b) later processes of impregnation of the pendotites by melts that were efficient at crystallising interstitially grains of Fe Ν ι sulfides As for the chromitites only these of Korydallos do clearly present syncrystallisation phenomenae of chromite with sulfides The sulfides occur both as inclusions in chromite and in interstitial places as well which means that the magma could generate immiscible sulfide droplets even at the time of chromite crystallisation However the common assemblge Awaruite pentlandite miliente that was described in the mantle pendotites of Vounnos and Pindos can not be formed as product of the fractionation process of a sulfide melt that is exsolved from a silicate magma This assemblage represents the final product of two combined processes a) The local reduction of primary sulfide grains and b) the neo formation of part of them via hydrothermal fluids The accessory Cr-beanng spinels of the ultrabasic rocks from Vounnos and Pindos are classified as magnesiochromites as do the chromite crystals of the ores The same phases of the pendotites and a part of those in the chromitite from the area of Korydallos are classified as spinels due to their more ΑΙ-rich composition The magnesiochromites of the clinopyroxene bearing and normal harzburgites have Cr# smaller than 0 60 and are usually depleted in Ti02 The magnesiochromites of the other formations are richer in Ti02 and have Cr# greater than 0 60 The spinels of the Korydallos pendotites have Cr# that ranges between 0 36-0 42 The magnesiochromites of the Vounnos chromitites have Cr# that range from 0 58 up to 0 83 while those from the Pindos ores have Cr# between 0 74 0 84 The spinels of the chromitite from Korydallos have Cr# that range from 0 43 to 0 69 The compositions of the accessory magnesiochromites from the clinopyroxene bearing and the majority of the normal harzburgites reflect the residual origin of these pendotite types which have been formed in a mid-ocean ridge (MOR) The magnesiochromites of higher Cr# indicate that the pendotites that host them constituted part of a mantle wedge above a supra subduction zone (SSZ) These magnesiochromites are similar in compositional terms to spinels in equilibrium with boninite melts The compositions of the mantle spinels from the area of Korydallos are similar to those from Ν MOR basalts Mantle pyroxenes of both complexes show compositional characteristics representative ofthose from SSZ and especially fore arc pendotites Additionally mantle clinopyroxenes presentanomalous compositional core-nm zoning The Vounnos mantle clinopyroxenes show an increase inSi02 Ti02 and CaO contents from core to rim while mantle clinopyroxenes from Pindos show anincrease in Ti02 and CaO contents towards their rims The fact that mantle clinopyroxenes of bothcomplexes exhibit high Mg# (>0 94) (higher than that of the other mantle phases) is indicative of an episode of cryptic metasomatism by which the pendotites have been affected Another important compositional characteristic of the pyroxenes is their depleted nature in REE+Y The depletion is extreme for the LREE-MREE which is supportive of their residual rather than of their magmatic origin The olivine crystals of the pendotites of both complexes exhibit high Mg# which is characteristic of olivines from pendotites of SSZ origin Their high NiO content does not follow thenormal compositional trend of mantle olivines a feature that supports the addition of olivine in therocks by metasomatic processes The olivine crystals that are hosted in the chromitites of sourthernVounnos chromitites tend to be poorer in NiO and richer in MnO compared to those of the chromitites of the northern part of the complex The plots of the compositions of chromite crystals of all the mantle formations in the projections Cr#Mg^hr Fooi indicate that the magnesiochromites of the chromitites and of some pendotites of the Vounnos massif do not follow the expected compositional trend of mantle spinelolivine pairs In particular their plots follow the fractionation line of boninites which means that they have been crystallised from a bonmite melt that had the opportunity to react with the hosting depleted pendotites because of a lack of thermo chemical equilibrium For the Pindos massif the same plot shows that some magnesiochromite olivine pairs correspond to unexpectedly high degrees of partial melting The positive correlations of Cr# with DR# the relatively high Ti02 contents and the high Fe+3# of magnesiochromites of both complexes do not agree with their residual origin but indicate that metasomatic processes have also played role in their formation The morphological change from vermicular and anhedral to euhedral magnesiochromites with the hthological transition from chnopyroxene bearing harzburgites and normal harzburgites to dunites indicate significant differences in the process of their genesis The high Cr# Ti02 Fe+3# and DR# of the dunitic magnesiochromites means that they are crystallisation products of a boninitic melt The magnesiochromite crystals of the transitional harzburgites from the Vounnos complex that have high Cr# and DR# are of magmatic origin as well In contrast the magnesiochromites of the same pendotite type from the Pindos ophiohtes are not well shaped (DR# 13 ppm) is attributed to the exsolution of immiscible sulfide droplets from the magma contemporaneously with the crystallisation of spinel The sulfide droplets scavenged the highly chalcophile PGE from the magma The decrease of the PGE content of the southern Vounnos chromitites with the Cr# of their magnesiochromite grains indicates crystal fractionation processes The high Cr# of the magnesiochromites of the chromite ores from the Vounnos massif and the areas of Milia and Pefki from the Pindos massif in combination with their high contents in IPGE are indicative of their formation from boninitic melts The Al rich character of the chromitite of Korydallos and it s high content in PPGE and BMS are supportive of their formation from a tholeitic magma These two different types of melts can be produced in a SSZ environment In particular the boninites can be formed straight under the arc and in the fore arc area of a SSZ while the MOR basalts can be produced in the opening back arc basin (BAB basalts) The PGM study was based on both the in situ and m-concentrates investigation The poor nature of the studied chromitites in PGE made the application of the recovery methods (hydroseparation and superpanners) necessary for obtaining representative results about the real PGM assemblage of each chromitite body The Vounnos chromitites host PGM assemblages typical of ophiolitic chromitites These are characterised by the predominance of members of the launte erhchmanite series and alloys of Os Ir composition The launte crystals of the southern Vounnos chromitites represent a wider range of OsRu substitution and are more Os enriched compared to those from the northern Vounnos chromitites This is indicative of the fact that the former have been formed under conditions of higher fS2 and/or lower Τ Phenomena of transformation of bi-sulfides into Ru-Os Ir Fe Ni bearing oxides are common due to hydrothermal alteration and/or weathering effects Some of these oxides exhibit internally strange and unknown until now in such minerals uncommon rosette textures Rare composite crystals consisting of erhchmanite Os Ir alloys and sulfasrenides which have been recovered from the chromitites of Voidolakkos and Xerolivado areas are indicative of the rapid rise of the fS2 locally in the magma during the formation of these chromite ore deposits The PGM assemblage of the chromitites from the area of Miha is quite similar to that of theVounnos chromitites The PGM assemblage of the area of Pefki is different with bi-sulfidessulfarsenides bi arsenides and PPGM The restricted number of Os Ir alloys in the Pefki schromitites and the high amount of sulfarsenides and bi-arsenides indicate conditions of higher fS2fAs and/or lower Τ than those prevailing during the formation of the chromitites of the area of MiliaDe arsenicatio

Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece: Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites

Several small chromium (Cr) ore bodies are hosted within a unit of tectonically thinned dunite in the retired Ayios Stefanos mine of the western Othris ophiolite complex in Greece. Chromium ores consist of tectonically imprinted bodies of semi-massive to massive, podiform and lenticular chromitites composed of chromian spinel [Cr-spinel] with high Cr# [Cr/(Cr + Al) = 0.51–0.66] and Mg# [Mg/(Mg + Fe2+) = 0.58–0.76], low Fe3+# [Fe3+/(Fe3+ + Fe2+) ≤ 0.43] and low TiO2 (≤0.21 wt %) content. This composition is characteristic of Cr-spinels in equilibrium with melts of intermediate affinity between island-arc tholeiites (IATs) and mid-ocean ridge basalts (MORBs). Several Cr-spinel crystals in these ores exhibit imperfect zones made up of spinel hosting oriented lamellae of Mg-silicates (mostly chlorite) locally overgrown by porous domains along grain boundaries and fractures. From the Cr-spinel core to the lamellae-rich rim Cr#, Mg# and Fe3+# generally increase (0.68–0.87, 0.78–0.88 and 0.55–0.80, respectively), whereas from the core or the spinel zones with oriented lamellae to the porous domains Mg# and Fe3+# generally decrease (0.45–0.74 and ≤0.51, correspondingly). The lamellae-rich rims formed at oxidizing conditions, whereas the porous rims resulted from a later reducing event. Several tiny (≤30 μm), subhedral to anhedral and elongated Zr-bearing silicate mineral grains were discovered mainly along open and healed fractures cutting Cr-spinel. Most of the Zr-bearing silicate minerals (30 out of 35 grains) were found in a chromitite boulder vastly intruded by a complex network of gabbroic dykes. The dominant Zr-bearing silicate phase is by far zircon displaying a homogeneous internal texture in cathodoluminescence (CL) images. Raman spectroscopy data indicate that zircons have experienced structural damage due to self-irradiation. Their trace-element contents suggest derivation from a plagioclase-bearing, low-SiO2 intermediate to mafic source. Combined micro-textural and minerochemical data repeat the possibility of zircon derivation from limited volumes of high-T fluids emanating from the gabbroic intrusions. Once zircon is precipitated in cracks, it may be altered to Ca-rich Zr-bearing silicate phases (i.e., armstrongite, calciocatapleiite). Almost all zircons in these samples show evidence of gains in solvent compounds (CaO, Al2O3 and FeO) possibly due to re-equilibration with late deuteric fluids

Multiple episodes of partial melting, depletion, metasomatism and enrichment processes recorded in the heterogeneous upper mantle sequence of the Neotethyan Eldivan ophiolite, Turkey

The Eldivan ophiolite along the Izmir-Ankara-Erzincan suture zone in north-central Anatolia represents a remnant of the Neotethyan oceanic lithosphere. Its upper mantle peridotites include three lithologically and compositionally distinct units: clinopyroxene (cpx)-harzburgite and Iherzolite (Group-1), depleted harzburgite (Group-2), and dunite (Group-3). Relics of primary olivine and pyroxene occur in the less refractory harzburgites, and fresh chromian spinel (Cr-spinel) is ubiquitous in all peridotites. The Eldivan peridotites reflect a petrogenetic history evolving from relatively fertile (lherzolite and cpx-harzburgite) toward more depleted (dunite) compositions through time, as indicated by (i) a progressive decrease in the modal cpx distribution, (ii) a progressive increase in the Cr#s [Cr / (Cr + Al)] of Cr-spinel (0.15-0.78), and (iii) an increased depletion in the whole-rock abundances of some magmaphile major oxides (Al2O3, CaO, SiO2 and TiO2) and incompatible trace elements (Zn, Sc, V and Y). The primitive mantle-normalized REE patterns of the Group-1 and some of the Group-2 peridotites display LREE depletions. Higher Yb-N and lower Sm-N/Yb-N ratios of these rocks are compatible with their formation after relatively low degrees (9-25%) of open-system dynamic melting (OSDM) of a Depleted Mid-ocean ridge Mantle (DMM) source, which was then fluxed with small volumes of oceanic mantle-derived melt [fluxing ratio (beta): 0.7-12%1. Accessory Cr-spinel compositions (Cr# = 015-0.53) of these rocks are consistent with their origin as residual peridotites beneath a mid-ocean ridge axis. Part of the Group-2 harzburgites exhibit lower YbN and higher SmN/YbN ratios, LREE-enriched REE patterns, and higher Cr-spinel Cr#s ranging between 0.54 and 0.61. Trace element compositions of these peridotites can be modeled by approximately 15% OSDM of a previously 17% depleted DMM, which was then fluxed (beta: 0.4%) with subduction-influenced melt. The Group-3 dunite samples contain Cr-spinel with elevated Cr#s (0.73-0.78) and low-TiO2 contents (<0.13 wt.%), implying higher degrees of melting (21-24%) of an already depleted DMM that was triggered by infiltration of low-Ti boninite melt with fluxing rates of 0.4-4.0%. The existence of interstitial, idiomorphic Cr-spinel (high Cr# and low Ti) in the Group-3 dunites is consistent with this interpretation. The occurrence of both MOR- and SSZ-type peridotites in the Eldivan ophiolite suggests that its heterogeneous upper mantle was produced as a result of different partial melting and melt -rock reaction processes in different tectonic settings within the Neotethyan realm. (C) 2016 Elsevier B.V. All rights reserved

Petrogenesis of lherzolites from the Purang ophiolite, Yarlung-Zangbo Suture Zone, Tibet: origin and significance of ultra-high pressure and other 'unusual' minerals in the Neo-Tethyan lithospheric mantle

The Purang ultramafic massif, located in the Yarlung-Zangbo Suture Zone (YZSZ) of the Tibetan Plateau, consists mainly of harzburgites and minor lherzolites. The spinel-bearing lherzolites of the NW part of the massif display a granular texture, consisting of large olivine and pyroxene crystals with curvilinear grain boundaries. These lherzolites contain chromian spinel (Cr-spinel) of low Cr# [100 7 Cr/(Cr +Al) = 24.7\u201330.2], enstatite with high Mg# [100 7 Mg/(Mg + Fe2+) = 90.0\u201391.2] and relatively high Al2O3 content (3.3\u20134.1 wt%), and diopside with high Mg# (90.2\u201393.3) and Al2O3 content (4.6\u20135.0 wt%). These compositions are analogous to those of spinel and pyroxenes from residual peridotites. However, the Purang lherzolites show U-shaped primitive mantle (PM)- normalized rare earth element (REE)-profiles, which are not consistent with a potential origin as melting residues. The high LREE contents and positive Ti anomalies shown by the investigated lherzolites coupled with the low TiO2 content of their mineral constituents imply that these rocks possibly stored LREE- and Ti-bearing arc-related melts/fluids in their groundmass. A mineral assemblage composed of diamond, super-reduced [(SuR) moissanite, native Cr] and crustal-derived minerals (zircon, corundum, rutile), has been separated from the Purang lherzolites. Uranium-Pb geochronological dating of zircons yielded an age range between 1718 and 465 Ma, indicating that they represent ancient crustal material delivered into the upper mantle via previous subduction events. Diamonds and old zircons (\ub1 crustal minerals) were carried to shallow mantle levels by asthenospheric magmas produced during a slab rollback-induced decompression melting process. The recovery of SuR minerals is consistent with fluid percolation and crystallization of alteration-related minerals in the lithospheric parts of a (hydrated) mantle wedge, resulting in the formation of highly reduced micro-environments

Petrogenesis of ultramafic rocks from the eastern Orhaneli ophiolite, NW Turkey: Hints on the initiation and evolution of melt-peridotite interaction processes within a heterogeneously depleted mantle section

Uysal, Ibrahim/0000-0001-7607-2893WOS: 000413384800004The eastern Orhaneli ophiolite in NW Anatolia (Turkey) consists of voluminous dunite and minor harzburgite intruded by clinopyroxenite veins. Harzburgite contains spinel of low Cr# [100 x Cr/(Cr + Al) = 40-45] and diopside of low Al2O3 and TiO2 contents, whereas dunite contains spinel of higher Cr# (62-82) and diopside (blebs) (even more) depleted in Al2O3 and TiO2 (than harzburgite). the concentrations of Heavy Rare Earth Elements (HREE) in harzburgite are consistent with derivation of this type of peridotite from 19% dry melting of a fertile mantle protolith at a MOR regime. Dunites have lower concentrations of HREE than harzburgite implying that dunites were generated by higher degrees (> 30%) of (cumulative) melting of the same protolith. Furthermore, the characteristic U-shapes of the chondrite-normalized REE-patterns of dunites indicate the involvement of hydrous melt-peridotite interaction processes in their genesis. Nevertheless, a set of mosaic-in texture dunite samples contain olivine that has lower Fo# [100 x Mg/(Mg + Fe2+)] and NiO contents than olivine in harzburgite. These dunites are probably of cumulate origin as it is also indicated by their enrichment in Pt and Pd (<= 17.92 ppb). Micro-textural and Re-Os isotopic data support that clinopyroxenite intrusions do not have a pure magmatic origin and their formation was partly controlled by metasomatic processes. Overall data indicate that the eastern Orhaneli ultramafic rocks have a complex petrological history including various stages of partial melting, metasomatism and magmatism in an evolving from MOR to SSZ geotectonic setting.Scientific Research Foundation of Karadeniz Technical University [2008.112.005.17]; National Natural Science Foundation of China (NNSFC)National Natural Science Foundation of China (NSFC) [41402065]This study was financially supported by the Scientific Research Foundation of Karadeniz Technical University (Project# 2008.112.005.17) and the National Natural Science Foundation of China (NNSFC; Youth Science Fund Project# 41402065). We would like to express our sincere thanks to P. Stutz for his help in the preparation of the polished-thin sections. Editor Dr. Derek Wyman and an anonymous reviewer are thanked for their critical and constructive comments

Genesis and geodynamic significance of chromitites from the Orhaneli and Harmancik ophiolites (Bursa, NW Turkey) as evidenced by mineralogical and compositional data

Chromitites from Orhaneli and Harmancik ophiolites (Bursa, NW Turkey) are mainly associated with dunites. They occur as banded and lenticular bodies showing massive and disseminated textures and are composed of magnesiochromite grains characterized by the following composition: Cr2O3 = 58.30-61.19 wt.%, Al2O3 = 8.32-10.03 wt.% and TiO2 = 0.14-0.21 wt.%. The Cr# [Cr/(Cr + Al)] values of magnesiochromite from both ophiolites range between 0.80 and 0.83 and their Mg# [Mg/(Mg + Fe2+)] values vary from 0.52 to 0.69. However, one chromitite sample (BO13B) from the Orhaneli ophiolite contains magnesiochromite with relatively low Cr2O3 (4838 wt.%), high Al2O3 (15.16 wt.%) and TiO2 (030 wt%) contents, and average Cr# and Mg# values of 0.68 and 056, respectively. Although the total content of platinum-group elements (PGE) in most chromitite samples varies between 83 and 169 ppb, two samples from the Orhaneli ophiolite show enrichments in total PGE (up to 500 ppb). Several platinum-group mineral (PGM) grains occur as minute inclusions in magnesiochromite grains (1-15 mu m across). Laurite is dominant PGM phase, followed by erlichmanite, unidentified Ru-Ni-Fe sulfide, irarsite, hollingworthite, native osmium and iridium. Millerite and pentlandite represent the most abundant primary base-metal mineral inclusions in magnesiochromite, whereas bornite, awaruite, heazlewoodite, violarite and orcelite are mostly observed in the secondary silicate groundmass. Olivine, clinopyroxene and amphibole are also identified as silicate inclusions in the magnesiochromite. The composition of the magnesiochromite and the parental melts in equilibrium indicate that these magnesiochromite grains were most likely crystallized from boninitic melts in an arc setting. However, BO13B chromitite sample from the Orhaneli chromitites may have crystallized from arc-related melt of probably an island arc tholeiite affinity. (C) 2014 Elsevier B.V. All rights reserved

Genesis and geodynamic significance of chromitites from the Orhaneli and Harmancik ophiolites (Bursa, NW Turkey) as evidenced by mineralogical and compositional data

Uysal, Ibrahim/0000-0001-7607-2893; AVCI, Erdi/0000-0002-3659-992X;WOS: 000348084400002Chromitites from Orhaneli and Harmancik ophiolites (Bursa, NW Turkey) are mainly associated with dunites. They occur as banded and lenticular bodies showing massive and disseminated textures and are composed of magnesiochromite grains characterized by the following composition: Cr2O3 = 58.30-61.19 wt.%, Al2O3 = 8.32-10.03 wt.% and TiO2 = 0.14-0.21 wt.%. the Cr# [Cr/(Cr + Al)] values of magnesiochromite from both ophiolites range between 0.80 and 0.83 and their Mg# [Mg/(Mg + Fe2+)] values vary from 0.52 to 0.69. However, one chromitite sample (BO13B) from the Orhaneli ophiolite contains magnesiochromite with relatively low Cr2O3 (4838 wt.%), high Al2O3 (15.16 wt.%) and TiO2 (030 wt%) contents, and average Cr# and Mg# values of 0.68 and 056, respectively. Although the total content of platinum-group elements (PGE) in most chromitite samples varies between 83 and 169 ppb, two samples from the Orhaneli ophiolite show enrichments in total PGE (up to 500 ppb). Several platinum-group mineral (PGM) grains occur as minute inclusions in magnesiochromite grains (1-15 mu m across). Laurite is dominant PGM phase, followed by erlichmanite, unidentified Ru-Ni-Fe sulfide, irarsite, hollingworthite, native osmium and iridium. Millerite and pentlandite represent the most abundant primary base-metal mineral inclusions in magnesiochromite, whereas bornite, awaruite, heazlewoodite, violarite and orcelite are mostly observed in the secondary silicate groundmass. Olivine, clinopyroxene and amphibole are also identified as silicate inclusions in the magnesiochromite. the composition of the magnesiochromite and the parental melts in equilibrium indicate that these magnesiochromite grains were most likely crystallized from boninitic melts in an arc setting. However, BO13B chromitite sample from the Orhaneli chromitites may have crystallized from arc-related melt of probably an island arc tholeiite affinity. (C) 2014 Elsevier B.V. All rights reserved.scientific projects - Karadeniz Technical UniversityKaradeniz Technical University [BAP 804]; TUBITAKTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [109Y219]This study was supported by scientific projects funded by Karadeniz Technical University (BAP 804) and TUBITAK (109Y219). Fernando Gervilla and Jose Maria Gonzalez-Jimenez are greatly acknowledged for their help with the SEM images and preliminary investigations of platinum-group and base-metal minerals. We thank Federica Zaccarini and Giorgio Garuti for the electron microprobe analyses and element mapping of platinum-group minerals. Karl Thomas Fehr, Rupert Hochleitner and Melanie Kaliwoda are thanked for their generous help with the electron microprobe analyses. Comments from Benxun Su and an anonymous reviewer as well as Editor Franco Pirajno helped improve this paper