Zone Ophiolites Research Articles

The San Marcos mélange, Cordillera Septentrional of the Dominican Republic. Nature, origin and age

Understanding the nature, origin, and age of a mélange is crucial to deciphering the evolution of certain orogenic belts. Located in the Cordillera Septentrional of the Dominican Republic, the San Marcos Mélange is characterized by a blocks-in-matrix internal structure. Apparently arranged chaotically, the blocks have centimeter to kilometer sizes, include exotic ophiolitic, volcaniclastic, and intrabasinal sedimentary lithologies, and are of Cretaceous to Miocene in age, all immersed in a scaly and variably sheared argillaceous matrix. In less-deformed coherent sections, the Mélange consists of alternating sandstones, siltstones, and clays, indicative of a distal turbiditic depositional environment. Syn-sedimentary deformation is pervasive and produces systems of conjugated extensional shear fractures, veins filled with calcite, cataclastic bands, and boudinage and pinch-and-swell, symmetrical and asymmetrical structures. They are consistent with a layer parallel extension, produced in a low-energy slope setting, and indicate a low-ductility contrast between few or no lithified lithologies. Paleostress analysis of fractures and veins establishes a NE-SW trend of sub-horizontal extension, suggesting an orientation of the basin along a WNW-ESE to NW-SE axis (current coordinates). The biochronological data of nannofossils present in the matrix gives a sedimentary age for the Mélange from, at least, the top of the lower Miocene to the base of the upper Miocene. The San Marcos Mélange has therefore an essentially gravitational origin. Its deformation was temporally syn-sedimentary and was produced by processes of gravitational sliding, probably triggered by both instability processes within the basin and tectonic (earthquake shaking) mechanisms. Blocks and megablocks of ophiolitic rocks derived from the underlying Puerto Plata complex and of well-lithified, volcaniclastic and sedimentary rocks of the Imbert Fm, represent "exotic" blocks caused by the gravitational collapse of the high topographically margins of the depositional basin, slide down, and emplaced into the unconsolidated slope and basin turbiditic deposits, at least in the lower and middle Miocene. The existence of a large-scale, syn-sedimentary shearing surface directed to-the-ONO at the basal contact suggests that the whole Mélange was displaced once or several times in that direction, contributing to the chaotic nature of the unit. The San Marcos Mélange is not a subduction-zone mélange, but was caused by the submarine landslide of turbiditic sediments in a syn-collisional, piggy-back basin, transported on top of a supra-subduction zone ophiolite of the Caribbean plate.

PETROLOGICAL, MINERALOGICAL AND GEOCHEMICAL DATA FROM THE EOHELLENIC OPHIOLITIC NAPPE IN THE ISLAND OF SKYROS, GREECE

The ophiolite of Skyros mainly consists of serpentinized harzburgites, gabbroic rocks, dolerites, tholeiitic basaltic lavas, rodingites, as well as ophicalcites. This ophiolitic sequence comprises of an ophiolitic mélange of pre-Upper Cretaceous age belonging to the Eohellenic nappe, has been affected by low to moderate metamorphic and metasomatic processes. Their mineral chemistry as well as their whole rock chemistry suggests that these ophiolitic rocks are associated with supra-subduction related processes. The presence of vesuvianite crystals in rodingites, as well as the occurrence of relict spinels within serpentinitic rocks, further confirms this assumption. Geochemical and petrological comparison between Skyros ophiolitic rocks and similar rocks of the Eohellenic nappe in East Thessaly and other N. Sporades islands, reveal that they all share many petrogenetical features. These eastcentral Greece Eohellenic ophiolites, and the ophiolitic rocks of the upper tectonic unit of the Attico-Cycladic Zone may possibly form an elongated ophiolitic zone associated to the hanging wall of the North Cycladic Detachment System

Shallow and deep lithosphere slabs beneath the Dinarides from teleseismic tomography as the result of the Adriatic lithosphere downwelling

The study area covers the Dinarides and southwestern part of the Pannonian basin as the marginal zone between the Adriatic microplate (African plate) and the Pannonian tectonic segment (Eurasian plate). We created a three-dimensional seismic velocity model to 450km depth using teleseismic tomography. Our travel-time dataset was collected by means of 40 seismic stations from the ORFEUS database and Croatian Seismological Survey database. A set of 90 teleseismic earthquakes were selected in the time range 2014–2015, and relative P-wave travel-time residuals were calculated. For the first time the seismic P-wave velocity model of a relatively high resolution on the entire Dinaridic mountain belt was obtained. Based on this model, a more reliable insight in the relations of the lithosphere plates has been achieved.We imaged a fast velocity anomaly extending underneath the entire Dinaridic mountain belt which indicates cold, rigid materials. The anomaly is steeply sloping towards the northeast and directly indicates the sinking of the Adriatic microplate underneath the Pannonian tectonic segment. In the Northern Dinarides the anomaly extends to the depth of 250km, whereas in the Southern Dinarides it covers greater depths, up to 450km. The shallow Adriatic slab extends along the External Dinarides, while the deep Adriatic slab extends beneath the Internal Dinarides and ophiolite zones in the area of central and southern Dinarides. Different slab depths are interpreted as the faster convergence of the plate in the southern Dinarides than in the northern, or the convergence of the plates had started in the southern part and systematically developed to the north.

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite, West Albania

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite, West Albania

Comparison of Different‐sized Chromite Mineralizations in the Yarlung‐Zangbo Ophiolite Belt, Southern Tibet

Podiform chromitites are characteristically occurred in ophiolites (e.g., Thayer, 1964; Dickey, 1975). However, the metallogenic processes for podiform chromitites are still unclear. Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers (Dickey, 1975; Boudier and Coleman, 1981), but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites (Dick, 1977; Dick and Bullen, 1984). Recent studies by the majority of authors have suggested that melt‐rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites (Zhou and Robinson, 1994; Zhou et al., 1996, 2005, 2014; Robinson, 2008; Page and Barnes, 2009; Uysal et al., 2009, 2012; González‐Jiménez et al., 2011, 2015). Based on the occurrence of some ultrahigh pressure minerals (e.g. diamond and coesite) in chromitites, it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling (Yang et al., 2007; Yamamoto et al., 2013). Although geat progresses have been made towards understanding the genesis of podiform chromitites, some fundamental issues in remain unanswered. For examples, what are the major controls on the size of chromitites? And why some ophiolites contain large podiform chromitite bodies, whereas most ophiolitic massifs are essentially chromitite‐barren?The Yarlung‐Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world. It contains fresh peridotites as well as different‐sided podiform chromitites. The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China. In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small‐scale chromitite bodies. Such characteristics make the Yarlung‐Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites.The Luobusha chromitites are large lens and enclosed in dunite. In contrast, the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes. The closely spatial association of the chromitites and dunite envelopes, together with their textural features, support a petrogenetic model that the chromitites from the Luobusha, Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite. In terms of chemical composition of chromite, there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo. Chromite from the Luobusha chromitites has high Cr# (71–82), whereas Chromite in the Dazhuqu chromitites show relatively low Cr# (16–63), and chromite in the Dongbo chromitites includes low Cr# (11–47) and high Cr# (70–81) types. For the Dongbo and Dazhuqu massifs, linear trends of Cr# with MgO, FeOt, Ni, Ga, V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites, suggesting that the melt‐rock reaction may provide major budget of Cr for the chromitites. The similar compositions at a given Cr# in chromite from these rocks also demonstrate that the chromitites may have been formed by in‐situ crystallization of chromite under low melt/rock ratio. In contrast, the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites, implying that the formation of the chromitite bodies requires a continual replenishment of Cr‐rich melts from deeper mantle. Fractionation and accumulation of chromite from a large volume of Cr‐rich melt may play an important role on the formation of the Luobusha chromitites. MORB‐normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr‐rich boninitic melt at surpra‐subduction zone (SSZ) setting. However, the Dongbo and Dazhuqu chromitites have formed originally from a MORB‐affinity melt at a mid‐ocean ridge (MOR) environment.In summary, the Luobusha chromitites crystallized from a Cr‐rich melt in a dynamic conduit, where fractional crystallization and crystal settling play a key role in formation of the large chromitites. In contrast, the small‐scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts. Podiform chromitites can be formed in MOR environment, whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.

Kinematics of Late Cretaceous subduction initiation in the Neo‐Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria

AbstractFormation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo‐Tethys Ocean between Gondwana and Eurasia. Suprasubduction zone ophiolites (i.e., emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers) were generated during this subduction event and are today distributed in the eastern Mediterranean region along three ~E‐W trending ophiolitic belts. Several models have been proposed to explain the formation of these ophiolites and the evolution of the associated intra‐Neo‐Tethyan subduction zone. Here we present new paleospreading directions from six Upper Cretaceous ophiolites of Turkey, Cyprus, and Syria, calculated by using new and published paleomagnetic data from sheeted dyke complexes. Our results show that ~NNE‐SSW subduction zones were formed within the Neo‐Tethys during the Late Cretaceous, which we propose were part of a major step‐shaped subduction system composed of ~NNE‐SSW and ~WNW‐ESE segments. We infer that this subduction system developed within old (Triassic?) lithosphere, along fracture zones and perpendicular weakness zones, since the Neo‐Tethyan spreading ridge formed during Gondwana fragmentation would have already been subducted at the Pontides subduction zone by the Late Cretaceous. Our new results provide an alternative kinematic model of Cretaceous Neo‐Tethyan subduction initiation and call for future research on the mechanisms of subduction inception within old (and cold) lithosphere and the formation of metamorphic soles below suprasubduction zone ophiolites in the absence of nearby spreading ridges.

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.

Глубинный геологический разрез и тектоника Северного Урала

Geological cross section of the Northern Urals to a depth of 7 km is presented. The author offers an interpretation of the tectonic structures in accordance with plate tectonics theory. West Urals Folding Zone is interpreted as a Thrust Zone of the Russian Plate formed during Kungurian Age of the Lower Permian. Polyudovo-Kolchimskiy anticlinorium is termed as an allochthon klippe of the Russian Plate. Central Urals Arc is identified as an accretionary prism of the Russian Plate. The Tagil-Magnitogorsk Depression is referred to as an obduction ophiolite zone. Two major tectonic faults were identified. The Western Urals Thrust combines the series of the platform autochthone thrusts and coincides to the north with the Timan-Urals Fault. The General Urals Fault is the obduction plane.

A Paleoproterozoic ophiolitic mélange, Yangtze craton, South China: Evidence for Paleoproterozoic suturing and microcontinent amalgamation

We document for the first time the presence of a Paleoproterozoic ophiolitic mélange in the Archean–Paleoproterozoic Kongling Complex in the northern Huangling Dome, Yangtze craton, South China. Detailed field mapping, petrographic, geochronological and geochemical studies reveal that the mélange consists of a suite of amphibolite facies mafic and ultramafic tectonic blocks including serpentinized harzburgite, olivine pyroxenite, pyroxenite, diabase, gabbro and basalt dispersed in a strongly sheared metasedimentary matrix. The mélange displays polyphase deformation and northwest-verging nappe structures, and underwent amphibolite to granulite facies metamorphism in the Paleoproterozoic (2.0–1.95Ga). The metasedimentary rocks are composed mainly of garnet-biotite-plagioclase gneiss, mica schist, mica-graphite schist, marble, quartzite, and banded iron formation (BIF). The mafic–ultramafic rock association shares the lithological and geochemical characteristics of Phanerozoic suprasubduction zone ophiolites.LA-ICP-MS U–Pb dating of zircons from the diabase yield ages between 2142 and 2148Ma for the magmatic cores and between 2042–2048Ma for the metamorphic rims. Zircon cores have εHf (t) values ranging from +5.4 to +10.3 (average=+7.2) with corresponding TDM1 ages of 2.24Ga, indicating that these rocks are remnants of juvenile crust derived from the depleted mantle at ca. 2.2Ga. All rock types in the mélange underwent Paleoproterozoic (2.0–1.95Ga) amphibolite–granulite facies metamorphism and were intruded by syn-metamorphic granite (∼2.0Ga), post-collisional (ca.1.85Ga) granite and mafic dykes. Intrusion age of one quartz monzonitic dyke crosscutting the serpentinized harzburgite is 1999Ma, constraining the minimum emplacement age of the mafic–ultramafic complex. The mafic–ultramafic rocks within the metasedimentary matrix in the northern Huangling Dome are fragments of a ca. 2.15Ga suprasubduction zone ophiolite that was incorporated into the suture zone during the accretionary-collisional process. The mélange recognized in the northern Yangtze craton provides important evidence for the Paleoproterozoic subduction and accretion processes possibly associated with the amalgamation of the proposed Columbia supercontinent.

Early Jurassic tectonism occurred within the Basu metamorphic complex, eastern central Tibet: Implications for an archipelago-accretion orogenic model

The Basu metamorphic complex, surrounded by ophiolitic melanges and intruded by a large volume of undeformed granitoid rocks along the eastern segment of the Bangong-Nujiang suture, holds one of the keys to understanding the pre-Cenozoic tectonic evolution of central Tibet. Zircon U-Pb dating of rocks from the Basu metamorphic complex reveals that meta-igneous rocks yield Early Paleozoic crystallization ages of 500–492Ma and an Early Jurassic metamorphic age of ~173Ma, and that undeformed granitoid rocks yield crystallization ages of approximately 186–174Ma. Whole rock geochemical and zircon Lu-Hf isotopic data indicate that the undeformed granitoid rocks originated mainly from partial melting of ancient crustal sources, which may reflect a collisional orogenic setting. 40Ar/39Ar dating of biotite from a sillimanite-garnet-biotite paragneiss shows cooling to 300±50°C at ~165Ma. These data indicate significant Early Jurassic tectonism, during which most of the Basu metamorphic complex was formed. Furthermore, the age data resemble those of the Amdo metamorphic complex located approximately 500km to the west along the Bangong-Nujiang suture. Together, these complexes may represent a “destroyed or unrecognized” block, i.e., the Amdo-Tongka block, which may be the eastern extension of the South Qiangtang terrane. Based on the tectonic outlines of the multiple ophiolitic zones and magmatic belts, we suggest a new archipelago-accretion model that attributes the Early Jurassic tectonism to an arc-continent/micro-continent collision. This model further enables the reconstruction of the eastern Tethyan Ocean and the orogenic processes of central Tibet during the Mesozoic.

Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

The plate tectonic setting in which proto-ophiolite ‘oceanic’ lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling. Rapid deep upper-mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto-ophiolite from as deep as the mantle transition zone, thereby explaining diamonds and other 10–15 GPa UHP phases in Tibetan ophiolites.

Effects of geodynamic setting on the redox state of fluids released by subducted mantle lithosphere

Magnetite breakdown during subduction of serpentinised ultramafic rocks may produce oxidised fluids that oxidise the deep Earth and/or the sub-arc mantle, either via direct transport of ferric iron, or via redox reactions between ferric iron and other elements, such as sulfur. However, so far, there is no consensus on the oxidation state of fluids released during subduction of ultramafic rocks, or the factors that control this oxidation state.Subducted samples from a magma-poor rifted margin and a supra-subduction zone geodynamic setting were compared to examine evidence of changes in opaque phase assemblage and ferric iron content as a consequence of subduction, and as a function of geodynamic setting. Thermodynamic calculations in the system Fe–Ni–O–H–S and Fe–Ni–O–S at the pressures and temperatures of interest were used to constrain oxygen activities and fluid compositions.Samples from New Caledonia, which exemplify supra-subduction zone mantle, contain awaruite (FeNi3) and equilibrated with hydrogen-bearing fluids at oxygen activity less than the FMQ (fayalite–magnetite–quartz) buffer. In contrast, samples from the Zermatt Saas Zone ophiolite, Western Alps, which are thought to represent mantle from a subducted magma-poor rifted margin, contain magnetite plus sulfur-rich phases such as pyrite (FeS2), and are inferred to have equilibrated with hydrogen-poor fluids at oxygen activity greater than FMQ. This major difference is independent of differences in subduction pressure–temperature conditions, variation in peridotite protolith composition, or the nature of adjacent units. We propose that the Zermatt Saas Zone samples would have undergone more complete serpentinisation prior to subduction than the supra-subduction zone (SSZ) New Caledonian samples. This difference explains the different fluid compositions, because incompletely serpentinised rocks containing olivine and brucite retain or evolve awaruite-bearing assemblages that buffer fluid compositions to high hydrogen activity (aH2).Ultramafic rocks are associated with two distinctly different fluid compositions during pre-subduction and subduction serpentinisation. Initially, while olivine is in equilibrium with infiltrating fluid, mineral assemblages that include awaruite in the rocks buffer fluids to H2-bearing, low aO2 compositions. Deserpentinisation of incompletely serpentinised rocks in which awaruite is present also produces H2-bearing fluids. Once awaruite is exhausted, H2-poor, high aO2 fluids co-exist with awaruite-absent assemblages, and deserpentinisation of such rocks would produce H2O-rich fluids.Thus, deserpentinisation of ultramafic rocks could produce either hydrogen-bearing fluids that could infiltrate and reduce the sub-arc mantle, or more oxidised fluids, which could transfer redox budget to other geochemical reservoirs such as the sub-arc mantle. Therefore, the redox contribution of subducted ultramafic rocks to the deep Earth and sub-arc mantle depends on the extent of protolith serpentinisation. Pre-subduction settings that promote extensive serpentinisation by oxidised fluids at high fluid:rock ratios in open systems, such as slow and ultraslow spreading ridges, transform faults, oceanic core complexes, and exhumed mantle at rifted continental margins, may produce more oxidised fluids than those associated with less pervasive serpentinisation and fluids that may be rock-buffered to a reduced state.

Lithological Analysis of Nain Ophiolitic Zone Using ASTER Data

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

Palaeozoic-Recent geological development and uplift of the Amanos Mountains (S Turkey) in the critically located northwesternmost corner of the Arabian continent

We have carried out a several-year-long study of the Amanos Mountains, on the basis of which we present new sedimentary and structural evidence, which we combine with existing data, to produce the first comprehensive synthesis in the regional geological setting. The ca. N-S-trending Amanos Mountains are located at the northwesternmost edge of the Arabian plate, near the intersection of the African and Eurasian plates. Mixed siliciclastic-carbonate sediments accumulated on the north-Gondwana margin during the Palaeozoic. Triassic rift-related sedimentation was followed by platform carbonate deposition during Jurassic-Cretaceous. Late Cretaceous was characterised by platform collapse and southward emplacement of melanges and a supra-subduction zone ophiolite. Latest Cretaceous transgressive shallow-water carbonates gave way to deeper-water deposits during Palaeocene-Eocene. Eocene southward compression, reflecting initial collision, resulted in open folding, reverse faulting and duplexing. Fluvial, lagoonal and shallow-marine carbonates accumulated during Late Oligocene(?)-Early Miocene, associated with basaltic magmatism. Intensifying collision during Mid-Miocene initiated a foreland basin that then infilled with deep-water siliciclastic gravity flows. Late Miocene-Early Pliocene compression created mountain-sized folds and thrusts, verging E in the north but SE in the south. The resulting surface uplift triggered deposition of huge alluvial outwash fans in the west. Smaller alluvial fans formed along both mountain flanks during the Pleistocene after major surface uplift ended. Pliocene-Pleistocene alluvium was tilted towards the mountain front in the west. Strike-slip/transtension along the East Anatolian Transform Fault and localised sub-horizontal Quaternary basaltic volcanism in the region reflect regional transtension during Late Pliocene-Pleistocene (<4 Ma).

Quartz fabric variations across the greenschist facies shear zone separating the Zermatt-Saas and combin ophiolitic zones, Upper Val Gressoney, Western Alps

The Gressoney Shear Zone (GSZ) consists of a ca. 500 m thick, intensely deformed rock panel located at the top of the high-pressure ophiolitic rocks of the Zermatt-Saas Zone in the Western Alps. This greenschist-facies shear zone accommodated multiple non-coaxial deformation events with contrasting kinematics. In this study, detailed field mapping and structural analysis were integrated with the study of crystallographic preferred orientation (CPO) from mylonites associated with the GSZ. Quartz CPO displays a systematic variation across the shear zone: moving from the basal shear zone boundary, the c-axes pattern changes from type II cross-girdle distribution, to an asymmetric pattern characterized by clustering of c-axes on one side of the Z-direction (inclined single girdle), to a central cluster in the Y-direction. The observed CPO patterns are consistent with increasing shear strain toward the basal contact, which probably controls the transition from broad peripheral maxima indicative of basal slip to an inclined single girdle with no maxima, which is indicative of prism slip, and finally an elongate single maximum at the girdle centre produced by a combination of prism and rhomb slip. Our results further indicate that basal slip is dominant in pure quartz domains, whereas with increasing proportion of second phases, prism slip is activated. These features confirm that CPOs obtained from the almost pure quartzites analysed in most published studies, and generally associated with the activation of distinct slip systems controlled by temperature, cannot be straightforwardly applied to the analysis of heterogeneous shear zones and/or polymineralic mylonites. From a regional point of view, the structures observed in the field and the fabric analyses are consistent with top-to-the-SE extension post-dating subduction-related high-pressure metamorphism and collisional nappe stacking in the studied sector of the Western Alps.

Fingerprinting fluid sources in Troodos ophiolite complex orbicular glasses using high spatial resolution isotope and trace element geochemistry

The Troodos igneous complex (Cyprus) is a ca. 90Ma old, well preserved supra-subduction zone ophiolite. Troodos is unique in that it shows evidence of fluid-saturation throughout the complex, from its base (i.e. podiform chromitites) to its uppermost units – the upper pillow lavas (UPL). However, it is unclear what the source of dissolved water in UPL tholeiites is, with possibilities including shallow seawater infiltration, assimilation of altered Troodos oceanic crust, recycled serpentinized oceanic crust, or subducted pelagic sediments. In order to identify and characterize these components we have carried out a detailed high-resolution study on tholeiitic lavas on orbicular structures and glasses from the UPL in Troodos. Basaltic orbicules were measured for their Sr–Nd–Hf–Pb isotope compositions, and in situ for their B isotopes using LA-MC-ICP-MS. UPL orbicules display a very narrow range in ∊Nd and ∊Hf (+7 to +8 and +13 to +15, respectively) indicating melting of a depleted mantle source. Lead isotopes, specifically 207Pb/204Pb vs. 206Pb/204Pb, form a mixing array with pelagic sediments. Furthermore, high-resolution characterization of individual orbicules revealed that UPL tholeiites display strong variability in 87Sr/86Sr (0.7039–0.7060) at the outcrop scale. Samples display δ11B between −8.2 (±0.5)‰ and +5.9 (±1.1)‰ with an average B content of ca. 5μg/g. Contrary to expectation, altered orbicules and their associated hyaloclastite matrixes display lower δ11B (down to −10‰) and higher B contents (max. 200μg/g) when compared to fresh glass. Furthermore, the orbicules studied here show little or no evidence of interaction with seawater, which is supported by their trace element contents and isotope compositions. When all isotope systems are taken into account, UPL lavas reflect melting of a depleted mantle source that was overprinted by hydrous sediment melts, and potentially, fluid-like subduction components that in part originate from serpentinized oceanic crust. Subsequent low-temperature alteration then drove δ11B to lower values coupled with increased B uptake due to its adsorption into palagonite.

Lithological, structural, and geochemical characteristics of the Mesoarchean Târtoq greenstone belt, southern West Greenland, and the Chugach – Prince William accretionary complex, southern Alaska: evidence for uniformitarian plate-tectonic processes

The Mesoarchean Târtoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) with well-preserved duplex structures, representing a relict accretionary prism. The Târtoq greenstone belt is a remnant of a supra-subduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite–trondhjemite–granodiorite suites. Field observations, along with geochemical and zircon U–Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012 ± 4 and 2993 ± 6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach – Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and (or) accreted oceanic crust during slab window magmatism. Archean granitoid–greenstone terrains share many geological characteristics of Phanerozoic subduction–accretion complexes such as the Alaskan and Altaid subduction–accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of tonalite–trondhjemite–granodiorite generation in subduction–accretion complexes.

Mineralogy and composition of chromitites and host peridotites from the Colkahan ultramafic complex (Nazdasht mine), Kerman, southeastern Iran

Several chromitite pods of variable sizes are hosted in dunite-harzburgite of the about 14 km2 mining district of Nazdasht, in the Colkahan ultramafic complex, located in the Haji Abad-Esfandagheh district in the Kerman province, southeastern Iran. Olivine, orthopyroxene, and to a lesser extent clinopyroxene of a highly refractory nature, are the primary silicates found in the dunites and harzburgites. The matrix minerals between chromian spinel grains are serpentinized olivine and pyroxene, Cr-rich chlorite (kaemmererite), uvarovite, and veins of magnesite. Chromian spinel mainly occurs as massive chromitite pods and as thin massive chromitite bands together with minor disseminations in dunites and harzburgites. Chromian spinels in massive chromitites show relatively high Cr-numbers (0.60–0.82), Mg-numbers (0.54–0.74) and very low TiO2 contents (averaging 0.16 wt.%) which may reflect the crystallization of chromite from a boninitic magma. The Fe3+-number is very low, down to <0.17 wt.%, in the chromian spinel of chromitites and associated peridotites of the Colkahan ultramafic complex. The harzburgite and dunite contain accessory chromian spinel with Cr-numbers of about 0.66 and 0.69, respectively. The Colkahan dunites and harzburgites are high depleted with low CaO (0.01–0.47 wt.%) and Al2O3 (0.29–0.64 wt.%) compared to primitive mantle. The chemical and mineralogical characteristics of these chromitites have a strong affinity to podiform chromitites in the mantle section of supra-subduction zone ophiolites. Calculated parental melts of the chromitites and geochemical discrimination diagrams based on mineral chemistry of peridotites are consistent with the differentiation of arc related magmas with boninitic affinity and support the supra-subduction paleogeodynamic setting of Haji Abad-Esfandagheh district previously proposed by some authors.

Kinematics of a former oceanic plate of the Neotethys revealed by deformation in the Ulukışla basin (Turkey)

AbstractKinematic reconstruction of modern ocean basins shows that since Pangea breakup a vast area in the Neotethyan realm was lost to subduction. Here we develop a first‐order methodology to reconstruct the kinematic history of the lost plates of the Neotethys, using records of subducted plates accreted to (former) overriding plates, combined with the kinematic analysis of overriding plate extension and shortening. In Cretaceous‐Paleogene times, most of Anatolia formed a separate tectonic plate—here termed “Anadolu Plate”—that floored part of the Neotethyan oceanic realm, separated from Eurasia and Africa by subduction zones. We study the sedimentary and structural history of the Ulukışla basin (Turkey); overlying relics of this plate to reconstruct the tectonic history of the oceanic plate and its surrounding trenches, relative to Africa and Eurasia. Our results show that Upper Cretaceous‐Oligocene sediments were deposited on the newly dated suprasubduction zone ophiolites (~92 Ma), which are underlain by mélanges, metamorphosed and nonmetamorphosed oceanic and continental rocks derived from the African Plate. The Ulukışla basin underwent latest Cretaceous‐Paleocene N‐S and E‐W extension until ~56 Ma. Following a short period of tectonic quiescence, Eo‐Oligocene N‐S contraction formed the folded structure of the Bolkar Mountains, as well as subordinate contractional structures within the basin. We conceptually explain the transition from extension, to quiescence, to shortening as slowdown of the Anadolu Plate relative to the northward advancing Africa‐Anadolu trench resulting from collision of continental rocks accreted to Anadolu with Eurasia, until the gradual demise of the Anadolu‐Eurasia subduction zone.

Petrogenesis and geodynamics of plagiogranites from Central Turkey (Ekecikdağ/Aksaray): new geochemical and isotopic data for generation in an arc basin system within the northern branch of Neotethys

In the Late Cretaceous, throughout the closure of the Neotethys Ocean, ophiolitic rocks from the Izmir–Ankara–Erzincan ocean branch were overthrusted the northern margin of the Tauride-Anatolide Platform. The ophiolitic rocks in the Ekecikdag (Aksaray/Central Turkey) region typify the oceanic crust of the Izmir–Ankara–Erzincan branch of Neotethys. The gabbros in the area are cut by copious plagiogranite dykes, and both rock units are intruded by mafic dykes. The plagiogranites are leucocratic, fine- to medium-grained calc-alkaline rocks characterized mainly by plagioclase and quartz, with minor amounts of biotite, hornblende and clinopyroxene, and accessory phases of zircon, titanite, apatite and opaque minerals. They are tonalite and trondhjemite in composition with high SiO2 (69.9–75.9 wt%) and exceptionally low K2O (<0.5 wt%) contents. The plagiogranites in common with gabbros and mafic dykes show high large-ion lithophile elements/high-field strength element ratios with depletion in Nb, Ti and light rare-earth elements with respect to N-MORB. The plagiogranites together with gabbros and mafic dykes show low initial 87Sr/86Sr ratios (0.70419–0.70647), high ƐNd(T) (6.0–7.5) values with 206Pb/204Pb (18.199–18.581), 207Pb/204Pb (15.571–15.639) and 208Pb/204Pb (38.292–38.605) ratios indicating a depleted mantle source modified with a subduction component. They show similar isotopic characteristics to the other supra-subduction zone (SSZ) ophiolites in the Eastern Mediterranean to East Anatolian–Lesser Caucasus and Iran regions. It is suggested that the Ekecikdag plagiogranite was generated in a short time interval from a depleted mantle source in a SSZ/fore-arc basin setting, and its nature was further modified by a subduction component during intra-oceanic subduction.