Isotropic Gabbros Research Articles

Mineral Chemistry–thermobarometry and Petrography of Metamorphic Sole Rocks of Kömürhan Ophiolite (SE Turkey): Constraints to Evolution and Emplacement

This paper presents the generation of metamorphic sole rocks through the detailed geochemical and petrographical analysis of field work carried out on the Kömürhan ophiolite. The metamorphic sole rocks of Kömürhan ophiolite are defined as amphibo-lite (Pl+Mg–Hbl+Ttn±Ap) plagioclase–amphibole schist (Pl+Mg–Hbl+Cpx+Ttn±Zrn±Ap), plagioclase–clinopyroxene–amphibole schist (Pl+Di+Mg–Hbl+Ttn±Ap), and epidote–plagioclase amphibole schist (Ep+Pl+Mg–Hbl+Ttn±Ap±Qtz±Zrn). This research mainly reports comprehensive petrography and mineral chemistry analyses of metamorphic sole rocks of Kömürhan ophiolite of SAOB (Southeast Anatolian Orogenic Belt) together with a goal of presenting geothermobarometric examination and unravelling the mineral sys-tematics. The metamorphic sole rocks have been observed as a thin slice and these rocks are seen at the base of the tectonites, metamorphosed in amphibolites facies throughout the intra–oceanic supra-subduction geodynamic environment. The Kömürhan ophiolite includes from the top to bottom volcanics, sheeted dike complex, isotropic gabbros cumulates, and tectonites and shows a complete oceanic lithospheric fragments. Analyses of mineral chemistry and petrography of metamorphic sole rocks have been used to exhibit the metamorphic processes of these rocks. Mineral chemistry analyses of pyroxene phenocrysts in the metamorphic sole rocks of Kömürhan ophiolite present similarities island arc tholeiite (IAT), proposing that protolith of the sole rocks was related to the supra-subduction geodynamic environment. The amphibolites were occurred by metamorphism of island arc tholeiite–type volcanics that separated from the front of the obducted ophiolite (Kömürhan ophiolite) and after that underplated.

Radiolarian Biochronology, Detrital Zircon Geochronological and Geochemical Constraints on Provenance and Depositional Environment of Cherts in the Southern Belt of the Western Yarlung Zangbo Suture Zone, Tibet

AbstractOphiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the ...

Early Cretaceous (Albian) intra‐oceanic subduction in northern branch of Neotethys in NW Iran: Zircon U–Pb geochronology and geochemistry of ophiolitic metagabbros from the Chaldoran area

The Chaldoran ophiolite in NW Iran and at the Turkish border is a part of the larger Khoy ophiolite. Serpentinized peridotites, pillow basalt, cumulate and isotropic gabbro, pelagic limestone, and radiolarite along with volcano‐sedimentary units are the main rock types in the area. These rocks are mixed together in some places as tectonic and “coloured” mélanges. The gabbro consituent of this ophiolite is either fresh or is metamorphosed up to upper greenschist facies. These meta‐basic rocks contain relatively large crystals of plagioclase and clinopyroxene, partially changed to amphibole. Metamorphic epidote is formed in some samples. Whole‐rock chemistry of these rocks shows a basaltic magmatic composition. Low SiO2 contents along with relatively high MgO and FeOtotal contents testify for a mantle source for the parental magma of the studied samples, intermediate between tholeiitic and calc‐alkaline series. LREE and HFSE are slightly depleted and LILE show enrichment in the studied samples and Ta, Nb, Ti show negative anomalies with respect to MORBs. All chemical features are compatible with a supra‐subduction zone setting for the rocks. The studied rocks are similar to primitive arc and less evolved rocks, geochemically. Also the samples show both MORB‐like magmas, formed at the initial stage of a subduction and IAT chemical features. U–Pb dating of zircons on the gabbro sample gave 107.5 ± 1.3 Ma (Albian) crystallization age. The metabasites from the Chaldoran ophiolite are similar to intra‐oceanic ophiolite‐related igneous rocks from the Zagros suture zone in Iran and NE Iraq and equivalent rocks from the Neotethyan sutures in east Turkey and Armenia and basic rocks from the Late Cretaceous ophiolites of the outer Zagros and Taurus, which are related to the subduction initiation (SI) stage.

Geochronology and Geochemistry of Gabbros from Moa‐Baracoa Ophiolitic Massif, Eastern Cuba: Implication for Early Cretaceous SSZ Magmatism

AbstractA series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto‐Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates (Iturralde‐Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back‐arc basin, forearc basin or plume setting (e.g. Giunta et al., 2009; Marchesi et al., 2016; Kerr et al., 1999). The Moa‐Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa (20°35′ 17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence (CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U‐Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2 (47.22 wt%–49.25 wt%), TiO2 (0.13wt%–0.26 wt%) and K2O (0.04 wt%–0.08 wt%) and high MgO (8.22 wt%–16.72 wt%), Cr (1337–2065 μg/g) and Ni (192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt% to 21.98 wt%, and 0.71 wt% to 1.87 wt%, respectively. In the SiO2 versus Zr/TiO2∗0.0001 diagram, the samples plot in the sub‐alkaline basalt field but show calc‐alkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents (ΣREE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE (LREE, (La/Yb)N = 0.34–0.50) and flat heavy REE (HREE, (Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies (Eu/Eu∗ = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle‐normalized trace element diagram. Geochemical features, such as high values of Ba/La (30–38) and low Th/Yb (0.02–0.03), medium values of (Ta/La)PM (0.30–0.35) and (Hf/Sm)PM (0.58–0.80), indicate enrichment by slab‐derived fluids and subduction‐related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran (Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the Moa‐Baracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.

Petrogenesis of the Neoproterozoic Xinlin ophiolite, northern Great Xing’an Range, northeastern China: Implications for the evolution of the northeastern branch of the Paleo-Asian Ocean

The Xinlin ophiolite in northeastern China marks the Xinlin–Xiguitu branch of the Paleo-Asian Ocean (PAO), and provides an opportunity to reconstruct the evolution of the Xing’an–Mongolian orogenic belt (XMOB). The main ophiolitic body is composed of mantle peridotites, mafic and ultramafic cumulates, isotropic gabbros, diabase dykes, and massive basaltic lava flows, from base to top. The uppermost pelagic sediment cover is missing and was presumably eroded. The cumulate gabbro yields a concordant zircon U–Pb age of 669 ± 8 Ma and has positive zircon εHf(t) values (+10.7 to +15.6). The mafic rocks of the ophiolite have rare earth element (REE) patterns that plot between the compositions of normal (N-) and enriched (E-) mid-ocean ridge basalt (MORB). The monzogranite that intruded into the Xinlin ophiolite yields a narrow age range (574–571 Ma), and is enriched in light REE and depleted in high field strength elements (HFSE). Zircon grains from the monzogranite have negative εHf(t) values (−5.6 to −10.0), suggesting the monzogranite was formed by partial melting of the Erguna continental crust, rather than oceanic crust. The geochemical characteristics of the ophiolite and spatially associated younger arc granites show that the Xinlin ophiolite developed at a mid-ocean ridge setting, and was subsequently accreted onto the Erguna continental margin before amalgamation with the Xing’an Block.

Geochronological, Geochemical and Sr–Nd–Pb Isotope Characteristics of the Meydan Ophiolite, SE Turkey: Petrogenesis and Implications for Mesozoic Tectonic Evolution

Gabbros, sheeted dikes and volcanic rocks of the Meydan ophiolite in the South-east Anatolian orogenic belt were examined to determine their U–Pb zircon ages, whole-rock, mineral chemistry and Nd‒Sr–Pb isotope compositions. The new U–Pb dating of zircon separates yielded a 88.3 ± 1.3 Ma crystallization age for gabbroic rocks of Meydan ophiolite. The ophiolite includes tectonites, ultramafic-mafic cumulates, isotropic gabbros, isolated mafic dikes, sheeted dikes, plagiogranites and rare basalts. The sheeted dikes and volcanics of the Meydan ophiolite exhibit near-flat rare earth element (REE) patterns, flat high field strength element trends and negative Nb anomalies that are similar to other SSZ–type eastern Mediterranean ophiolites. The whole-rock and mineral chemistry of the Meydan ophiolite/volcanic rocks suggests that they formed a supra-subduction zone and the dismembered ophiolite metamorphic sole formed beneath the ophiolite in a subduction trench setting. Isotopic ratios, i.e., 87Sr/86Sr versus 206Pb/204Pb and 143Nd/144Nd versus 87Sr/86Sr, show only limited variations, whereas 143Nd/144Nd versus 206Pb/204Pb, 87Sr/86Sr ratios are much more variable. The isotopic ratios are indicative of mixed magmatic contributions from DMM/MORB and EMII reservoirs that probably reflect the overall recycling of mantle wedge, pelagic sediment, oceanic crust and continentally derived sediment. The Meydan ophiolite, inferred arc volcanics and accretionary melange are all intruded by Late Cretaceous (87.7 ± 0.47 to 80.1 ± 0.46 Ma; zircon U–Pb) granitic rocks which are evaluated as the product of continental margin arc magmatism. The available regional comparisons suggest that the rocks studied were derived from the Berit Ocean to the north of the Bitlis Puturge continent, in which the Komurhan, Ispendere and Goksun ophiolites also formed during the Late Cretaceous.

Geochemistry and tectonic significance of the Fannuj-Maskutan SSZ-type ophiolite (Inner Makran, SE Iran)

ABSTRACT The nature and geodynamic setting of Inner Makran ophiolites (SE Iran) are controversial and are the focus of a growing debate. The Fannuj-Maskutan ophiolite complex (central Inner Makran) is made up of lherzolites, harzburgites, layered and isotropic gabbros and diabase dikes and basaltic lavas. Here we present new textural description, mineral and whole-rock chemistry of peridotites together with bulk geochemistry of magmatic mafic rocks and U-Pb geochronology and Lu-Hf isotopes of inherited zircons in gabbros, with the aim to reconstruct the petrogenesis of the Fannuj-Maskutan ophiolite. The results indicate to a subduction-metasomatized depleted mantle source experienced 10–20% of partial melting. A transition from IAT-gabbros to E-MORB-basalts is also outlined. We interpret the obtained results as evidence for the Fannuj-Maskutan genesis in a supra-subduction zone (SSZ) basin developed in the Iranian cadomian crust as indicated by the presence of inherited Cadomian zircons (ca. 515 Ma, εHf(t): −13.6 – +11.2). Comparing the Fannuj-Maskutan ophiolites to well-known SSZ-systems such as the South Sandwich SSZ, and integrating with the existing literature on Inner Makran ophiolite belt we interpret the Fannuj-Maskutan ophiolite as a part of a major SSZ-basin opened in the Iranian cadomian crust and evolved during the Neotethys subduction under Central Iran.

Siliceous–volcanic associations of the Northern Balkhash ophiolite Zone (Central Kazakhstan): Biostratigraphy, sedimentation and tectonic evolution in the Middle-Late Ordovician

The Northern Balkhash Zone is the largest and the most complex ophiolite zone of the Junggar-Balkhash region in the west of the Central Asian Orogenic Belt. The zone encompasses serpentinite mélange, containing fragments of dunite-harzburgite, dunite-verlite-pyroxenite complexes, layered and isotropic gabbro, sheeted dikes and plagiogranites, along with stacks of tectonic slivers composed of siliceous, siliceous-basaltic, and siliceous-volcanic-tuffaceous units. Conodont finds suggest the middle-late Darriwilian age of the siliceous-basaltic Itmurundy Fm., while the siliceous-basalt Tiuretai, siliceous Kazyk, and the siliceous-tuffaceous Uzuntobek formations are close in age, extend from the top of Darriwilian to the base of Katian.The siliceous-clastic-tuffaceous sediments of the Obaly Fm. are the Sandbian and Katian in age and the volcanic-sedimentary Dzhamanshuruk Fm. is broadly Upper Ordovician.Dating of these units has also permitted recognition of the units with intricate thrust-and-fold structure. Species of the genera Periodon, Pygodus, and Scabbardella dominate in the conodont assemblages in cherts. An uneven distribution of the conodonts in sections is caused by variable siliceous sedimentation rate that depends on productivity of radiolarians.The petrochemistry of the volcanics in the Itmurundy Fm. suggests that they correspond to OIB and E-MORB, being formed within seamounts or oceanic plateaus, while the Tiuretai basalts are close to N-MORB, erupted within a spreading center. The felsic volcanics in the Dzhamanshuruk Fm. have demonstrated an island arc affinity. The proposed evolution model suggests that during the Middle-Late Ordovician tectonic settings changed successively from the spreading center to island arc within the Northern Balkhash Zone of the Junggar-Balkhash region.

Late paleozoic to early mesozoic paleo-tethys tectonic evolution of central NE Tibetan Plateau: Insights from the Zhiduo mafic-ultramafic complex

Recent research has identified abundant Early Permian to Late Triassic ultramafic-mafic rocks along the west Jinsha to Garzê-Litang suture, central NE Tibetan Plateau. However, the petrogenesis and tectonic setting of these rocks are hotly debated. This paper presents a combination of zircon UPb ages, major and trace element concentrations, and SrNd isotopic data for mafic rocks from the Zhiduo Complex in the west Jinsha suture. Three geochemical subgroups are recognized among these rocks; metagabbros showing arc-type affinity (Group 1; 286–268 Ma); isotropic gabbros with an oceanic-island basalt-like signature (Group 2; 252 ± 2 Ma); and pillow/massive basalts (Group 3; ca. 235 Ma) having back-arc basin basalt affinity. Group 1 rocks exhibit moderate-high MgO (5.98–8.07 wt%) and Al2O3 (13.75–17.40 wt%) contents and are characterized by subduction-type trace element patterns and high initial 87Sr/86Sr ratios (0.71155–0.71261) and low εNd(t) values (−7.0 to −6.0), indicating that they originated from a spinel lithospheric mantle metasomatized by slab-derived fluids. Group 2 rocks have high contents of TiO2 (2.28–4.96 wt%), total Fe2O3 (11.67–14.80 wt%), Nb (18.3–22.8 ppm), and Ta (1.52–1.81 ppm) and more radiogenic Nd isotopes [εNd(t) = +0.3 to +0.9]. These data show that they were derived from a spinel + garnet mantle source enriched by plume-type components. Group 3 rocks possess high MgO (7.29–9.29 wt%), Cr (427–488 ppm), and Ni (215–238 ppm) contents but low Nb (5.00–5.60 ppm) and Ta (0.36–0.84 ppm), coupled with low Sr (initial 87Sr/86Sr = 0.70523–0.70567) and high Nd isotopes [εNd(t) = +3.1 to +3.5]. These values suggest that they were sourced from a garnet + spinel mantle affected by slight plume contamination. Combined with previous structural and geochemical studies, the ultramafic-mafic rocks of the Zhiduo Complex and arc magmatic rocks from nearby regions indicate an integrated Early Permian to Late Triassic oceanic basin–arc–back-arc system developed in the central NE Tibetan Plateau. Thus, the west Jinsha to Garzê-Litang represents a mature Paleo-Tethys Ocean rather than small back-arc basins. Southward subduction and associated back-arc spreading led to the genesis of widespread late Paleozoic to early Mesozoic magmatic rocks in the hinterland and along the northern margin of the North Qiangtang Block.

First mid-ocean ridge-type ophiolite from the Meso-Tethys suture zone in the north-central Tibetan plateau

Abstract The Meso-Tethys was a late Paleozoic to Mesozoic ocean basin between the Cimmerian continent and Gondwana. Part of its relicts is exposed in the Bangong–Nujiang suture zone, in the north-central Tibetan Plateau, that played a key role in the evolution of the Tibetan plateau before the India-Asia collision. A Penrose-type ophiolitic sequence was newly discovered in the Ren Co area in the middle of the Bangong–Nujiang suture zone, which comprises serpentinized peridotites, layered and isotropic gabbros, sheeted dikes, pillow and massive basalts, and red cherts. Zircon U-Pb dating of gabbros and plagiogranites yielded 206Pb/238U ages of 169–147 Ma, constraining the timing of formation of the Ren Co ophiolite. The mafic rocks (i.e., basalt, diabase, and gabbro) in the ophiolite have uniform geochemical compositions, coupled with normal mid-ocean ridge basalt-type trace element patterns. Moreover, the samples have positive whole-rock εNd(t) [+9.2 to +8.3], zircon εHf(t) [+17 to +13], and mantle-like δ18O (5.8–4.3‰) values. These features suggest that the Ren Co ophiolite is typical of mid-ocean ridge-type ophiolite that is identified for the first time in the Bangong–Nujiang suture zone. We argue that the Ren Co ophiolite is the relic of a fast-spreading ridge that occurred in the main oceanic basin of the Bangong–Nujiang segment of Meso-Tethys. Here the Meso-Tethyan orogeny involves a continuous history of oceanic subduction, accretion, and continental assembly from the Early Jurassic to Early Cretaceous.

Petrological characteristics of the Neoproterozoic Ess ophiolite mantle section, Arabian Shield, Saudi Arabia: a mineral chemistry perspective

The Ess ophiolite, one of the most important ophiolitic massifs of the Arabian Shield, consists of a lower succession of serpentinized mantle rocks overlain by an ultramafic cumulate sequence, layered and isotropic gabbros, sheeted dykes, pillow lavas and pelagic sediments. The Ess mantle section is composed mainly of serpentinized peridotites with chromitite pods, dunite, wehrlite and pyroxenite. Extensive metasomatism and alteration has transformed the ultramafic rocks to talc-carbonates, magnesite deposits and listvenite, especially along shear zones and fault planes. Nevertheless, relics of primary chromian spinel, olivine and pyroxenes are observed. Both primary and metamorphic olivines can be recognized in dunite; the latter is marked by very high forsterite content (97–98), low NiO content ( 0.6) and low TiO2 content (< 0.14 wt%) of fresh Cr-spinel and the high forsterite (0.90–0.93) and NiO contents (0.4–0.5 wt%) of fresh olivine are all consistent with residual mantle rocks that experienced high degrees of partial melt extraction. Orthopyroxene and clinopyroxene in the Ess peridotites have low CaO, Al2O3 and TiO2 contents resembling those typically found in depleted harzburgites from fore-arcs. Consequently, we propose that the Ess mantle peridotites formed in a forearc setting during subduction initiation that developed as a result of northwest subduction due to the convergence between East and West Gondwana, leading eventually to closure of the Mozambique Ocean during the Pan-African orogeny.

Petrographic and geochemical characterization of the crustal section of the Pujada Ophiolite, southeastern Mindanao, Philippines: Insights to the tectonic evolution of the northern Molucca Sea Collision Complex

The Molucca Sea Collision Complex (MSCC) preserves the complex interaction between the Eurasian, Philippine Sea and the completely subducted Molucca Sea Plates. Petrological studies of obducted arc and ophiolitic materials in this region thus provide a unique opportunity to elucidate the tectono-magmatic evolution of the area. In this study, we present new petrographic and geochemical data on the crustal section of the Pujada Ophiolite which is extensively exposed in southeastern Mindanao, Philippines. Our work reveals that this ophiolite is a remnant of an oceanic lithosphere with backarc affinity. U-Pb dating of zircons from the isotropic gabbros further constrains the age of the Pujada Ophiolite to Late Cretaceous (90 Ma). This age is consistent with the inferred age of the proto-Molucca Sea Plate based on tectonic reconstructions and tomography. Exposed to the north of the Pujada Ophiolite and separated by a NE-SW trending thrust fault is the Iba Formation. This unit originally purported to be the carapace of the Pujada Ophiolite records an IA (island arc) affinity. The geochemical signature of the Iba Formation suggests derivation from an enriched mantle source affected by slab-derived components related to the East Mindanao-Halmahera Arc. We therefore interpret the Pujada Ophiolite as a trapped fragment of the proto-Molucca Sea Plate that was thrust onto the Halmahera Arc possibly during the waning stages of the collision between the Sangihe and Halmahera Arcs.

Formation of chromitites and ferrogabbros in ultramafic and mafic members of the Variscan Ślęża ophiolite (SW Poland)

Ultramafic and mafic rocks of the Variscan Ślęża ophiolite (Bohemian Massif, Sudetes, SW Poland) contain abundant Cr and Fe-Ti mineralizations. The occurrence of chromitites is related with serpentinites of refractory harzburgitic protoliths. Chromite I from chromitites has variable Cr and Mg numbers (Cr# = 0.41–0.58, Mg# = 0.62–0.83) and low TiO2 contents (<0.20 wt%). Chromite II surrounds chromite I or fills fissures in it and is enriched in Cr and depleted in Al relative to chromite I (Cr# = 0.54–0.68). Chromite I grains contain spherical grains of diopside, phlogopite, pargasite, pyrite and chalcopyrite, occurring as mono- or polyphase solid inclusions. Metagabbros with isotropic fabric contain locally up to 14.6 vol% of magnetite and ilmenite. Magnetite has variable TiO2, Al2O3 and V2O3 contents (0.19–9.82 wt%, 0.13–1.67 wt% and 1.08–1.61 wt%, respectively). It forms xenomorphic grains with trellis or sandwich exsolutions of ilmenite, as well as aggregates with individual ilmenite grains.The moderate Cr# of chromite I points to its origin from a MORB-type melt. The occurrence of hydrous inclusions and nodular textures indicates chromitite formation and alteration under low pressure conditions. Primary sulfide, phlogopite and pargasite inclusions suggest that the parental melt was moderately hydrous and sulphur-rich. Water in the magmatic system originated supposedly from interaction of primary MORB melt with hydrothermal fluids and a significant influence of fluids on chromite composition is expressed by its very low Ti content.Metagabbros containing abundant Fe-Ti oxide mineralization may be classified as ferrogabbros. High V and Ti contents in magnetite indicate a magmatic origin. Its xenomorphic and intragrain mode of occurrence suggest crystallization between pre-existing gabbroic minerals like pyroxene and plagioclase. Ilmenite exsolutions indicate its growth at the expense of magnetite under oxidizing conditions. Occurrence of chromitites in harzburgites and ferrogabbros related to isotropic gabbros may suggest various spreading conditions during formation of the mafic and ultramafic members of the Ślęża ophiolite.

Mineral chemistry of isotropic gabbros from the Kermanshah ophiolite: Evidence for it's tectonic setting

Mineral chemistry of isotropic gabbros from the Kermanshah ophiolite: Evidence for it's tectonic setting

The Eastern Makran Ophiolite (SE Iran): evidence for a Late Cretaceous fore-arc oceanic crust

ABSTRACTThe nature, magmatic evolution, and geodynamic setting of both inner and outer Makran ophiolites, in SE Iran, are enigmatic. Here, we report mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope composition of mantle peridotites and igneous rocks from the Eastern Makran Ophiolite (EMO) to assess the origin and tectono-magmatic evolution of the Makran oceanic realm. The EMO includes mantle peridotites (both harzburgites and impregnated lherzolites), isotropic gabbros, diabase dikes, and basaltic to andesitic pillow and massive lava flows. The Late Cretaceous pelagic limestones are found as covers of lava flows and/or interlayers between them. All ophiolite components are somehow sheared and fragmented, probably in Cenozoic time, during the emplacement of ophiolite. This event has produced a considerable extent of tectonic melange. Tectonic slices of trachy-basaltic lavas with oceanic island basalt (OIB)-like signature seal the tectonic melange. Our new geochemical data indicate a magmatic evolution from fore-arc basalt (FAB) to island-arc tholeiite (IAT)-like signatures for the Late Cretaceous EMO lavas. EMO extrusive rocks have high εNd(t) (+8 to +8.9) and isotopically are similar to the Oman lavas. This isotopic signature indicates a depleted mid-ocean ridge basalt (MORB) mantle source for the genesis of these rocks, except isotopic gabbros containing lower εNd(t) (+5.1 to +5.7) and thus show higher contribution of subducted slab components in their mantle source. High 207Pb/204Pb and 208Pb/204Pb isotopic ratios for the EMO igneous rocks also suggest considerable involvement of slab-derived components into the mantle source of these rocks. The variable geochemical signatures of the EMO lavas are mostly similar to Zagros and Oman ophiolite magmatic rocks, although the Pb isotopic composition shows similarity to the isotopic characteristic of inner Zagros ophiolite belt. This study postulates that the EMO formed during the early stages of Neo-Tethyan subduction initiation beneath the Lut block in a proto-forearc basin. We suggest subduction initiation caused asthenospheric upwelling and thereafter melting to generate the MORB-like melts. This event left the harzburgitic residues and the MORB-like melts interacted with the surrounding peridotites to generate the impregnated lherzolites, which are quite abundant in the EMO. Therefore, these lherzolites formed due to the refertilization of mantle rocks through porous flows of MORB-like melts. The inception of subduction caused mantle wedge to be enriched slightly by the slab components. Melting of these metasomatized mantle generated isotropic gabbros and basaltic to andesitic lavas with FAB-like signature. At the later stage, higher contribution of the slab-derived components into the overlying mantle wedge causes formation of diabase dikes with supra-subduction zone – or IAT-like signatures. Trachy-basalts were probably the result of late-stage magmatism fed by the melts originated from an OIB source asthenospheric mantle due to slab break-off. This occurred after emplacement of EMO and the formation of tectonic melange.

Petrology, geochemistry and geochronology of the Zhongcang ophiolite, northern Tibet: implications for the evolution of the Bangong-Nujiang Ocean

Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP U–Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ɛHf(t) values (+15.0 to +20.2) and mantle-like δ18O values (5.29 ± 0.21)‰ indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt (N-MORB) like immobile element patterns with high MgO, low TiO2 and moderate rare earth element (REE) abundances, and negative Nb, Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt (OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Early–Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous (ca. 110 Ma).

Supra-subduction zone ophiolites from Inner Mongolia, North China: Implications for the tectonic history of the southeastern Central Asian Orogenic Belt

The recently discovered Diyanmiao ophiolite in the eastern part of the Erenhot-Hegenshan ophiolite belt forms part of the southeastern domain of the Central Asian Orogenic Belt (CAOB). The ophiolite suite is composed of the northern Baiyinbulage ophiolite and the southern Naolaiketu ophiolite, each extending for nearly 100km, and a width of around 6km. The suite preserves a well-exposed and a nearly-complete ophiolite stratigraphy, and includes, from bottom to top, harzburgite, bedded gabbro, isotropic gabbros, anorthosite, spilite, pillow basalt, keratophyre, baschtauite, with overlying chert, and plagiogranite veins locally intruding the pillow basalt and gabbro. The pillow basalts possess the geochemical characteristics of Hahajima island arc tholeiite (IAT), indicating that the Diyanmiao ophiolite formed in the incipient arc setting of the supra-subduction zone. LA-ICP-MS U-Pb dating of zircon grains from three gabbro samples yield ages of 361±13Ma, 356.7±3.8Ma and 340±14Ma, indicating an early-Carboniferous age for the Diyanmiao ophiolite. We suggest that the intra-oceanic subduction initiation of the Erenhot-Hegenshan paleo-ocean during the Early Carboniferous resulted in the formation of an incipient arc oceanic crust, and that the Diyanmiao ophiolite represents an important north-dipping subduction event of the paleo Asian oceanic crust beneath the southern margin of the Siberian Plate after Early Carboniferous and Early Permian. Our study provides further insights into the evolution of the Paleo-Asian Ocean and eventual construction of CAOB, the globe's largest Phanerozoic orogen.

Comparison of two different ophiolite districts in terms of some soil physical properties of grounds

In this study, some physical characteristics of the soils formed on the metamorphic sole and the mantle section of the ophiolitic sequence which is represented by ultramafic cumulates and tectonites located in Karacasu district in Kahramanmaraş and formed on the crustal rocks which are located around Göksun-Elbistan towns to the north of Kahramanmaraş, were investigated. In order to correlate the soil properties with the bedrocks from the different parts of the ophiolite, rock samples were collected from the same locations with 18 surface soil samples. Field capacity, permanent wilting point, liquid limit, plastic limit, coefficient of linear extensibility and volumetric shrinkage tests were performed on the soil samples. The crustal section is represented by the three different rock groups such as: cumulate gabbro (amphibole gabbro, olivine gabbro and gabbro), isotropic gabbro (gabbro) and sheeted dike complex (diabase). According to independent “t test” results; the physical properties of two fields were different from each other, except for the linear extension coefficient, ( P <0.001). Statistically significant relationships were determined among the measured variables, organic matter content, and soil texture. This is attributed to the fact that the ophiolites in the Karacasu area are more altered than the ophiolites in the Göksun-Elbistan area, and this difference in the alteration affects the soil properties over the mineralogical composition.

Tectonic significance of the cumulate gabbros within Kuluncak Ophiolitic suite (Malatya, SE Turkey) inferred from geochemical data

The Kuluncak (Malatya) ophiolite in the Eastern Tauride, Turkey, consists of mantle tectonites, ultramafic-mafic cumulates, isotropic gabbros, a sheeted dike complex, plagiogranites, and a volcanic complex. The best exposure of the mafic cumulate rocks of the Kuluncak ophiolite is in the Hekimhan region, where they are represented by olivine gabbro and gabbro showing orthocumulate to mesocumulate textures. The cumulus and postcumulus minerals do not show significant zoning. The crystallization order of the cumulates is olivine, plagioclase, clinopyroxene and orthopyroxene. Whole-rock major- and trace-element geochemistry of the mafic cumulate rocks indicates that the primary magma generating the Kuluncak ophiolite is similar in composition to those observed in modern island-arc tectonic settings. Rare earth element (REE) concentrations of the mafic cumulates exhibit depleted light rare earth element (LREE) patterns, with CeN/SmN and CeN/YbN ratios ranging from 0.21 to 0.75 and from 0.17 to 0.66, respectively. The coexistence of anorthite-rich plagioclase (An 73.4-93.7 ), highly magnesian olivine (Fo 65.2-86.9 ), clinopyroxene (Mg# 75-92.1 ), and orthopyroxene (Mg# 77.6-84.3 ) in the cumulate gabbroic rocks is indicative of an intraoceanic subduction setting and suggests that the Kuluncak ophiolite was formed during the Late Cretaceous closure of the Inner Tauride Ocean.

Permo–Triassic and Liassic Tethyan Oceanic Tracts within the Pontide Belt Along the Southern Margin of Eurasia, Northern Anatolia

The Pontide belt in northern Turkey includes three major tectonic terranes, the Strandja Massif (SjM), and the Istanbul (ISZ) and Sakarya Zones (SZ) (Fig. 1). We present new age and geochemical data from ophiolites and ophiolitic mélanges within the Sakarya Zone and show that these mafic–ultramafic rocks are the remnants of Tethyan oceanic lithosphere formed in different tectonic settings. The main ophiolite occurrences investigated in this study along the Karakaya Suture (KS) are associated with the latest Triassic Cimmeride orogeny, and in the Küre–Yusufeli ophiolite belt are part of the Alpide orogeny.The Karakaya Suture Zone ophiolites in northern west Turkey are comprised mainly of the Denizgoren (Çanakkale) ophiolite, Boğreve;azköy (Bursa), Geyve (Sakarya), Almacık (Düzce) and Çele (Bolu) metaophiolites. The Denizgören ophiolite largely contains upper mantle peridotites, which are equivalents of the Permo–Triassic Lesvos peridotites and mélange units farther SW in the northern Aegean Sea. The Boğreve;azköy ophiolite includes serpentinite and metagabbro, and the Almacık and Geyve ophiolites display an almost complete Penrose–type sequence consisting of serpentinized upper mantle peridotites, cumulate ultramafic–mafic rocks, isotropic gabbros, dolerite and plagiogranite dikes, and extrusive rocks. U–Pb zircon dating of plagiogranite dikes from Çele has revealed an igneous age of 260 Ma, and 255, 235, 227 Ma from Almacık (Bozkurt et al., 2012a, b). Consistent with the previously published Permo–Triassic age, we obtained a 268.4±6.3 Ma U–Pb zircon age from a plagiogranite dike within the Almacik ophiolite to the west. This KS ophiolite belt containing the Çele, Almacık, Geyve ophiolites within the SZ extends westward into the Armutlu Peninsula and then into the Biga Peninsula (i.e. Denizgören ophiolite) and most likely connects with the remnants of the Triassic Meliata–Meliac ocean basin (Stampfli and Borel, 2002) in the Balkan Peninsula. The KS ophiolites also continue eastward within the Pontide Belt into the Elekdağreve; ophiolite (eastern Kastamonu) and then to the Refahiye ophiolite in NE Anatolia.Triassic granites in the SZ represent a magmatic arc that formed as a result of the northward subduction of the Izmir–Ankara–Erzincan oceanic lithosphere existing during the late Paleozoic through Cretaceous (Sarifakioglu et al., 2014) beneath the Pontides. We obtained a U–Pb zircon age of 231±2 Ma from a metagranitic intrusion into the Variscan basement of the SZ in the Kastamonu region of the central Pontides. This metagranite is enriched in LILE (Rb: 63 ppm; Ba:65 ppm; Sr: 200 ppm) and depleted in HFSE (Y: 12.58 ppm; Yb: 1.26 ppm; TiO2: 0.2 wt.%; Nb: 7.6 ppm; Hf: 3.9 ppm), characterizing it as subduction–related calc‐alkaline pluton. Lead (3.9 ppm), U (1.6 ppm) and Ce (59 ppm) contents are interpreted as evidence for contamination by continental crust. The Küre basin to the north opened during the late Triassic to Liassic, following a backarc rifting episode in the central Pontides. Metabasic dike intrusions in the Devrekani metamorphic massif represent the first magmatic stage of this backarc rifting event. Whole‐rock 40Ar‐39Ar dating of the metabasic dikes has yielded cooling ages of 160.5±1.2 Ma. We infer that this age was reset due to thermal heating during the emplacement of the Middle Jurassic granitoids as the Küre oceanic basin was closing. The Küre ophiolite contains upper mantle peridotites and oceanic crustal rocks composed mainly of pillow–massive–breccia basalts, dacitic and rhyolitic lavas–tuffs, diabase dyke swarms, massive gabbros and a limited extent mafic cumulates. We obtained 182.6±1.9 Ma as a whole‐rock 40Ar‐39Ar age from a pillow basalt and a U–Pb zircon age of 171±1 Ma from the granitic intrusion cross‐cutting the peridotites. The easternmost representatives of the Küre ophiolite occur in the Yusufeli (Artvin) area in the eastern part of the Pontide belt. Here, oceanic crustal rocks are tectonically related to metamorphic rocks of the Variscan basement of the SZ. The ophiolitic crustal rocks contain isotropic gabbro and mafic and felsic dikes. Serpentinized upper mantle peridotites are scarce. Pillow lava basalts are overlain by a thick metasandstone–metashale association with locally foliated meta–lava and some manganiferous chert and mudstone interlayers. We obtained a U–Pb zircon age of 172.5±1.4 Ma from the granitic intrusion cross‐cutting the Yusufeli ophiolite and of 181.9±0.9 Ma from a felsic dike (plagiogranite) in the Yusufeli ophiolite. The Middle Jurassic granites are related to the closure of the Küre‐Yusufeli marginal ocean basin. The Küre and Yusufeli ophiolites have been previously interpreted as the remnants of the Paleotethys or the Intra‐Pontide Ocean. However, we posit that these ophiolites represent a marginal, short‐lived (∼10 Ma) ocean basin, which opened during the late Triassic through Liassic, and then closed in Dogger. This oceanic lithosphere is similar to the Evros ophiolite in the northeastern Greece in terms of its ages and geochemical characteristics.