Tethyan Oceanic Lithosphere Research Articles

When did India-Asia Collide and Make the Himalaya?

Critical evaluation and comparison of the available geological and geochronological data from the northern parts of the Himalaya and Trans-Himalaya mountains highlight that these mountains did not initially evolve by the collision of continents of the Indian and Asian plates. Instead, subducted Tethyan oceanic lithosphere in front of the Indian continent melted to produce the calc-alkaline suite of the Shyok-Dras volcanic arc and the Ladakh batholith. Hence, the Indian plate initially subducted beneath and started building up the then existing intra-oceanic island arc. Timing of the first impingement of the Indian and Asia plates has been better constrained at around 57.5 Ma by comparing (i) the bulk ages from the Ladakh batholith (product of partial melting of the Tethyan oceanic lithosphere) with (ii) the subducted continental lithospheric and UHP metamorphosed Indian crust in the Tso Morari, and (iii) biostratigraphy of the youngest marine sedimentation in Zanskar. Thus, the Himalaya witnessed its first rise and emergence from deeply exhumed terrain in the Tso Morari after around 53 Ma, followed by sequential imbrication of the Indian continental lithosphere and associated exhumation during rise of the Himalayan mountains from north to the south since 45 Ma.

The Eastern Black Sea-type volcanogenic massive sulfide deposits: Geochemistry, zircon U–Pb geochronology and an overview of the geodynamics of ore genesis

The Meso-Cenozoic geodynamic evolution of the eastern Pontides orogenic belt provides a key to evaluate the volcanogenic massive sulfide (VMS) deposits associated with convergent margin tectonics in a Cordilleran-type orogenic belt. Here we present new geological, geochemical and zircon U–Pb geochronological data, and attempt to characterize the metallogeny through a comprehensive overview of the important VMS mineralizations in the belt. The VMS deposits in the northern part of the eastern Pontides orogenic belt occur in two different stratigraphic horizons consisting mainly of felsic volcanic rocks within the late Cretaceous sequence. SHRIMP zircon U–Pb analyses from ore-bearing dacites yield weighted mean 206Pb/238U ages ranging between 91.1±1.3 and 82.6±1Ma. The felsic rocks of first and second horizons reveal geochemical characteristics of subduction-related calc-alkaline and shoshonitic magmas, respectively, in continental arcs and represent the immature and mature stages of a late Cretaceous magmatic arc. The nature of the late Cretaceous magmatism in the northern part of the eastern Pontides orogenic belt and the various lithological associations including volcaniclastics, mudstones and sedimentary facies indicate a rift-related environment where dacitic volcanism was predominant. The eastern Pontides VMS deposits are located within the caldera-like depressions and are closely associated with dome-like structures of felsic magmas, with their distribution controlled by fracture systems. Based on a detailed analyses of the geological, geophysical and geodynamic information, we propose that the VMS deposits were generated either in intra arc or near arc region of the eastern Pontides orogenic belt during the southward subduction of the Tethys oceanic lithosphere.

Evidence for an east–west regional gravity trend in northern Tunisia: Insight into the structural evolution of northern Tunisian Atlas

The Atlas orogeny in northern Algeria and Tunisia led to the destruction of Tethys oceanic lithosphere and cumulated in a collision of microplates rifted off the European margin with the North African continental margin. The location of the boundary between African plate and Kabylian microplate is expressed in northern Algeria by a crustal wedge with double vergence of thrust sheets, whereas in northern Tunisia the geologic environment is more complex and the location of the plate boundary is ambiguous. In this study, we analyzed gravity data to constrain the crustal structure along the northern margin of Tunisia. The analysis includes a separation of regional and residual gravity anomalies and the application of gradient operators to locate density contrast boundaries. The horizontal gradient magnitude and directional gradient highlight a prominent regional E–W gravity gradient in the northern Tunisian Atlas interpreted as a deep fault (active since at least the Early Mesozoic) having a variable kinematic activity depending on the tectonic regime in the region. The main E–W gravity gradient separates two blocks having different gravitational and seismic responses. The southern block has numerous gravity lineaments trending in different directions implying several density variations within the crust, whereas the northern block shows a long-wavelength negative gravity anomaly with a few lineaments. Taking into account the geologic context of the Western Mediterranean region, we consider the E–W prominent feature as the boundary between African plate and Kabylian microplate in northern Tunisia that rifted off Europe. This hypothesis fits most previous geological and geophysical studies and has an important impact on the petroleum and mineral resource prospection as these two blocks were separated by an ocean and they did not belong to the same margin.

Cenozoic high-K alkaline magmatism and associated Cu–Mo–Au mineralization in the Jinping–Fan Si Pan region, southeastern Ailao Shan–Red River shear zone, southwestern China–northwestern Vietnam

The Jinping–Fan Si Pan (JFP) Cenozoic magmatic and Cu–Mo–Au metallogenic belt in the southeastern part of the Ailao Shan shear zone host the Tongchang, Chang′an, Habo, and Chinh Sang Cu–Mo–Au deposits. These deposits form an integrated epithermal-porphyry regional mineralization system associated with 40–32Ma high-K alkaline magmatism. The magmatic rocks in the belt have relatively low TiO2 (<0.73wt%), P2O5 (<0.29wt%), and FeO* (<4.99wt%), and high Na2O (2.86–4.75wt%) and K2O (4.01–7.98wt%). They also have high contents of incompatible trace elements, and are enriched in LILE (Rb, Ba, K, Sr) and LREE. They have marked Nb, Ta, Ti and P depletion in primitive mantle-normalized spidergrams, and plot close to the EMII mantle field in the Sr–Nd isotopic diagram. These characteristics are similar to those of the Eocene high-K alkaline rocks along the northern Ailao Shan belt, eastern Tibet plateau. The sulfur and lead isotope analyses of sulfide minerals from both the ores and related magmatic rocks confirm the involvement of a magmatic ore fluid. The Cenozoic alkaline intrusions and Cu–Mo–Au mineralization in the JFP were formed prior to the initiation of left-lateral shearing along the Ailao Shan shear zone. The magmas appear to have been derived from enriched mantle, possibly with mixing of materials from the buried Tethyan oceanic lithosphere, and/or crust.

Geochemical make-up of oceanic peridotites from NW Turkey and the multi-stage melting history of the Tethyan upper mantle

We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmancik region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the Izmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al2O3 contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al2O3 contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmancik peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmancik peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmancik peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the Izmir-Ankara-Erzincan suture zone.

Geothermal structure of the eastern Black Sea basin and the eastern Pontides orogenic belt: Implications for subduction polarity of Tethys oceanic lithosphere

The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin. The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt (49.4 km) to Black Sea basin (152.2 km). The Moho temperature increases from 367 °C in the trench to 978 °C in the arc region. The heat flow values for the Moho surface change between 16.4 mW m−2 in the Black Sea basin and 56.9 mW m−2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast, the trench region has a relatively low geothermal potential with respect to the arc and back-arc region. The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic–Cenozoic.

Late Cretaceous subduction initiation and Palaeocene–Eocene slab breakoff magmatism in South-Central Anatolia, Turkey

The Late Cretaceous Alihoca ophiolite in the Inner Tauride suture zone (ITSZ) of South-Central Turkey represents part of a single ophiolitic thrust sheet that originated from the Inner Tauride ocean. The ophiolite contains upper mantle peridotites, cumulate wehrlites, layered-to-isotropic gabbros, and microgabbroic-to-doleritic dikes. An ophiolitic mélange beneath the Alihoca ophiolite includes blocks of limestone, peridotite, dolerite, basalt, and deep-sea sedimentary rocks (radiolarite, chert) in a matrix comprising sheared serpentinite and mudstone. Isotropic gabbro and dolerite dike rocks show enrichment in Sr, K, Rb, Ba, and Th (LILE) and depletion of Ta, Nb, Zr, Ti, and Y (HFSE), indicating an island arc tholeiite (IAT) affinity. Relatively younger dolerite rocks display low TiO2 (<0.5 wt.%) contents, concave REE profiles with low HREE concentrations, and high LREE values, typical of boninitic affinities. The Alihoca ophiolite, hence, displays an IAT to boninitic geochemical progression in its magmatic evolution, reminiscent of many other Tethyan ophiolites in the region. It represents the remnant of a forearc oceanic crust, which developed during the early stages of subduction within the Inner Tauride ocean. Volcanic, volcano-sedimentary, and sedimentary rocks of the Ulukışla–Çamardı basin north of the ITSZ disconformably overlie the mafic-ultramafic rocks of the Alihoca ophiolite. Pillowed and massive lavas of the latest Cretaceous–Palaeocene Ulukışla Formation have alkaline basalt-to-basaltic andesite compositions, displaying relatively enriched LILE and LREE patterns with negative Nb and Ta anomalies. These geochemical features suggest that magmas of the Ulukışla–Çamardı volcanic rocks formed from partial melting of a metasomatized lithospheric mantle. This melting event was triggered by the influx of asthenospheric heat through a slab breakoff-induced window in the downgoing Tethyan oceanic lithosphere.

Dextral transpression and late Eocene magmatism in the trans-Himalayan Ladakh Batholith (North India): implications for tectono-magmatic evolution of the Indo-Eurasian collisional arc

The trans-Himalayan Ladakh batholith is a result of arc magmatism caused by the northward subduction of the Tethyan oceanic lithosphere below the edge of the Eurasian plate. The batholith dominantly consists of calc-alkaline I-type granitoids which are ferromagnetic in nature with the presence of magnetite as the principal carrier of magnetic susceptibility. The mesoscopic and magnetic fabric are concordant and generally vary from WNW–ESE to ENE–WSW for different intrusions of ferromagnetic granites in different parts of the batholith. Strike of magnetic fabric is roughly parallel with the regional trend of the Ladakh batholith in the present study area and is orthogonal to the direction of India-Eurasia collision. In Khardungla and Changla section, the magnetic fabric is distributed in a sigmoidal manner. It is inferred that this sigmoidal pattern is caused by shearing due to transpression induced by oblique convergence between the two plates. U–Pb zircon geochronology of a rhyolite from the southern parts of the batholith gives a crystallization age of 71.7 ± 0.6 Ma, coeval with ~68 Ma magmatism in the northern parts of the batholith. The central part of the batholith is characterized by S-type two-mica granites, which gives much younger age of magmatism at 35.5 ± 0.5 Ma. The magnetic fabric of these two-mica granites is at a high angle to the regional trend of the batholith. It is proposed that these two-mica granites were emplaced well after the cessation of subduction and arc magmatism, along fractures that developed perpendicular to the regional strike of the batholith due to shearing.

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

Early-crystallizing chromian spinel (Cr-spinel) in the Nagqu ophiolite has high Os and low Re contents, and it is resistant to alteration during serpentinization, weathering and metamorphism. The chemical composition of primitive magma is preserved in Cr-spinel, which makes it suitable for determining the initial Os-isotope composition of the mantle source. This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro, respectively, in the same cumulate section of the ophiolite at Nagqu in Tibet. The results shed light on the formation and evolution of lithospheric mantle. The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone. It is a remnant of the Neotethyan oceanic crust, and contains cumulate dunite and gabbro. Zircon from the gabbro yielded a weighted mean 206Pb/238U age of 183.7±1 Ma. Cr-spinel exhibits γ Os values of −0.2 to −0.3, suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites. Thus, the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere, which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic. This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.

Discovery of Miocene adakitic dacite from the Eastern Pontides Belt (NE Turkey) and a revised geodynamic model for the late Cenozoic evolution of the Eastern Mediterranean region

The Cenozoic magmatic record within the ca. 500km long eastern Pontides orogen, located within the Alpine metallogenic belt, is critical to evaluate the tectonic history and geodynamic evolution of the eastern Mediterranean region. In this paper we report for the first time late Miocene adakitic rocks from the southeastern part of the eastern Pontides belt and present results from geochemical and Sr–Nd isotopic studies as well as zircon U–Pb geochronology. The Tavdagi dacite that we investigate in this study is exposed as round or ellipsoidal shaped bodies, sills, and dikes in the southeastern part of the belt. Zircons in the dacite show euhedral crystal morphology with oscillatory zoning and high Th/U values (up to 1.69) typical of magmatic origin. Zircon LA–ICPMS analysis yielded a weighted mean 206Pb/238U age of 7.86±0.15Ma. SHRIMP analyses of zircons with typical magmatic zoning from another sample yielded a weighted mean 206Pb/238U age of 8.79±0.19Ma. Both ages are identical and constrain the timing of dacitic magmatism as late Miocene. The Miocene Tavdagi dacite shows adakitic affinity with high SiO2 (68.95–71.41wt.%), Al2O3 (14.88–16.02wt.%), Na2O (3.27–4.12wt.%), Sr (331.4–462.1ppm), Sr/Y (85–103.7), LaN/YbN (34.3–50.9) and low Y (3.2–5ppm) values. Their initial 143Nd/144Nd (0.512723–0.512736) and 87Sr/86Sr (0.70484–0.70494) ratios are, respectively, lower and higher than those of normal oceanic crust. The geological, geochemical and isotopic data suggest that the adakitic magmatism was generated by partial melting of the mafic lower crust in the southeastern part of the eastern Pontide belt during the late Miocene. Based on the results presented in this study and a synthesis of the geological and tectonic information on the region, we propose that the entire northern edge of the eastern Pontides–Lesser Caucasus–Elbruz magmatic arc was an active continental margin during the Cenozoic. We identify a migration of the Cenozoic magmatism towards north over time resulting from the roll-back of the southward subducted Tethys oceanic lithosphere. Slab break-off during Pliocene is proposed to have triggered asthenospheric upwelling and partial melting of the subduction-modified mantle wedge which generated the alkaline magmatic rocks exposed in the northern part of the magmatic arc.

The origin and pre-Cenozoic evolution of the Tibetan Plateau

Different hypotheses have been proposed for the origin and pre-Cenozoic evolution of the Tibetan Plateau as a result of several collision events between a series of Gondwana-derived terranes (e.g., Qiangtang, Lhasa and India) and Asian continent since the early Paleozoic. This paper reviews and reevaluates these hypotheses in light of new data from Tibet including (1) the distribution of major tectonic boundaries and suture zones, (2) basement rocks and their sedimentary covers, (3) magmatic suites, and (4) detrital zircon constraints from Paleozoic metasedimentary rocks. The Western Qiangtang, Amdo, and Tethyan Himalaya terranes have the Indian Gondwana origin, whereas the Lhasa Terrane shows an Australian Gondwana affinity. The Cambrian magmatic record in the Lhasa Terrane resulted from the subduction of the proto-Tethyan Ocean lithosphere beneath the Australian Gondwana. The newly identified late Devonian granitoids in the southern margin of the Lhasa Terrane may represent an extensional magmatic event associated with its rifting, which ultimately resulted in the opening of the Songdo Tethyan Ocean. The Lhasa−northern Australia collision at ~263Ma was likely responsible for the initiation of a southward-dipping subduction of the Bangong-Nujiang Tethyan Oceanic lithosphere. The Yarlung-Zangbo Tethyan Ocean opened as a back-arc basin in the late Triassic, leading to the separation of the Lhasa Terrane from northern Australia. The subsequent northward subduction of the Yarlung-Zangbo Tethyan Ocean lithosphere beneath the Lhasa Terrane may have been triggered by the Qiangtang–Lhasa collision in the earliest Cretaceous. The mafic dike swarms (ca. 284Ma) in the Western Qiangtang originated from the Panjal plume activity that resulted in continental rifting and its separation from the northern Indian continent. The subsequent collision of the Western Qiangtang with the Eastern Qiangtang in the middle Triassic was followed by slab breakoff that led to the exhumation of the Qiangtang metamorphic rocks. This collision may have caused the northward subduction initiation of the Bangong-Nujiang Ocean lithosphere beneath the Western Qiangtang. Collision-related coeval igneous rocks occurring on both sides of the suture zone and the within-plate basalt affinity of associated mafic lithologies suggest slab breakoff-induced magmatism in a continent−continent collision zone. This zone may be the site of net continental crust growth, as exemplified by the Tibetan Plateau.

Geological evolution of the Karakoram Ranges

The Karakoram terrane in Northern Pakistan is geologically equivalent to the Qiangtang terrane of central Tibet but unlike Tibet shows extreme topographic relief, high uplift-exhumation and erosion rates and exposes deep crustal metamorphic and igneous rocks of the lower crust. The Karakoram terrane has been above sea-level since Early to Mid-Cretaceous, the age of the youngest marine sedimentary rocks. During the Late Jurassic to Mid-Cretaceous northdipping subduction of Tethyan oceanic lithosphere beneath the Karakoram resulted in widespread Andean I-type granite-granodiorite magmatism (Hunza, Hushe plutonic units) with possible concomitantlow-pressure andalusite-sillimanite grade Buchan-type metamorphism.The Kohistan intra-oceanic island arc developed above a second north-dipping subduction zone to the south during the Aptian- Eocene (~120-45 Ma) and was accreted to the southern margin of Asia concomitantly with obduction onto India during the Late Cretaceous.Final closure of Neo-Tethys and collision of India with Asia occurred at ca. 50 Ma. Continuous crustal thickening occurred across the southern Karakoram resulting in widespread regional Barrovian facies kyanite- and sillimanite-grade metamorphism with U-Pb zircon and monazite ages ranging from Latest Cretaceous (~63 Ma) to Pliocene times (4-3 Ma). Crustal melting resulted in intrusion of several sets of leucogranite dykes (Hunza dykes) at ~50 Ma and 35 Ma. The major phase of crustal melting in the Baltoro plutonic unit spanned at least 13 million years from 26-13 Ma (Late Oligocene to Mid-Miocene), and resulted in intrusion of a large-scale biotite monzogranite to garnet two-mica leucogranite intrusion of batholithic proportion. Some extra heat contribution from the upper mantle may have been required to account for the large size of the granite batholith and the lamprophyric dykes intruded around the margins. The youngest metamorphism and partial melting is known from the deep crustal metamorphic-migmatite gneiss domes (Dassu dome) of Pliocene age in the Baltoro region and the Late Miocene Sumayar leucogranite in Hunza. Geological and U-Pb geochronological data show that the Karakoram Range was a dynamic active mountain range with semi-continuous crustal thickening, deformation, metamorphism and partial melting for at least the last 63 million years and continues to this day.

One-Dimensional Thermal Modeling of the Eastern Pontides Orogenic Belt (NE Turkey)

The eastern Pontides orogenic belt is one of the most complex geodynamic settings in the Alpine–Himalayan belt due to the lack of systematical geological and geophysical data. In this study, 1D crustal structure and P-wave velocity distribution obtained from gravity modeling and seismological data in the area has been used for the development of the thermal model of the eastern Pontides orogenic belt. The computed temperature-depth profiles suggest a temperature of 590 ± 60°C at a Moho depth of 35 km indicates the presence of a brittle-ductile transition zone. This temperature value might be related to water in the subducted crust of the Tethys oceanic lithosphere. The Curie temperature depth value of 29 km, which may correspond to the crustal magma chambers, is found 5–7 km below the Moho depth. Surface heat flow density values vary from 66.5 and 104.7 mW m−2. High mantle heat flow density value of 48 mW m−2 is obtained for the area should be related to melting of the lithospheric mantle caused by upwelling of asthenosphere.

Subsidence and uplift by slab-related mantle dynamics: a driving mechanism for the Late Cretaceous and Cenozoic evolution of continental SE Asia?

Abstract Continental SE Asia is the site of an extensive Cretaceous–Paleocene regional unconformity that extends from Indochina to Java, covering an area of c . 5 600 000 km 2 . The unconformity has previously been related to microcontinental collision at the Java margin that halted subduction of Tethyan oceanic lithosphere in the Late Cretaceous. However, given the disparity in size between the accreted continental fragments and area of the unconformity, together with lack of evidence for requisite crustal shortening and thickening, the unconformity is unlikely to have resulted from collisional tectonics alone. Instead, mapping of the spatial extent of the mid–Late Cretaceous subduction zone and the Cretaceous–Paleocene unconformity suggests that the unconformity could be a consequence of subduction-driven mantle processes. Cessation of subduction, descent of a northward dipping slab into the mantle, and consequent uplift and denudation of a sediment-filled Late Jurassic and Early Cretaceous dynamic topographic low help explain the extent and timing of the unconformity. Sediments started to accumulate above the unconformity from the Middle Eocene when subduction recommenced beneath Sundaland.

Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel?

We herein investigate the extent to which extensive hydration of the oceanic lithosphere influences the preservation and exhumation of large-scale ophiolite bodies from subduction zones. The Zermatt–Saas ophiolite (ZS, W. Alps), which was subducted during the late stages of oceanic subduction, preserves a complete section of Mesozoic Tethys oceanic lithosphere and particularly fresh eclogites, and represents, so far, the largest and deepest known portion of exhumed oceanic lithosphere. Pervasive hydrothermal processes and seafloor alteration led to the incorporation of large amounts of fluid bound in the hydrated upper layers of the oceanic crust (now as lawsonite eclogites, glaucophanites, and chloritoschists) and in associated ultramafic rocks. Internally, the ZS ophiolite is made up of a series of tectonic slices of oceanic crust (150–300 m thick) which are systematically separated by a 5 to 100 m thick layer of serpentinite. This stack of slices is separated from the underlying eclogitized continental crust (e.g., Monte Rosa) by a thick (~ 500 m) serpentinite sole. Field observations, textural relationships and pseudosection modelling reveal that lawsonite was abundant and widespread in mafic eclogites when the ophiolite detached from the slab at around 550 °C and 24 kbar. Comparison between fresh eclogitic samples and pseudosection modelling shows that (i) water remained in excess from burial to eclogitic peak conditions, (ii) the lightest eclogitized metabasalts correspond to the portions of oceanic crust where metasomatism was the strongest, (iii) crystallization of widespread hydrated parageneses (such as lawsonite, glaucophane and phengite) instead of garnet and omphacite decreased by 5 to 10% the rock density and subsequently enhanced its buoyancy. We propose that this density decrease acted as a ‘float’ which prevented the slices from an irreversible sinking in the mantle. These slices were subsequently detached from the downgoing slab and stacked in the serpentinized subduction channel at pressures between 15 and 20 kbar, in the epidote blueschist facies. Exhumation of the underlying, positively buoyant continental crust dragged this “frozen” nappe-stack from the subduction channel towards the surface.

PETROLOGY OF MANTLE PERIDOTITES AND INTRUSIVE MAFIC ROCKS FROM THE KERMANSHAH OPHIOLITIC COMPLEX (ZAGROS BELT, IRAN): IMPLICATIONS FOR THE GEODYNAMIC EVOLUTION OF THE NEO-TETHYAN OCEANIC BRANCH BETWEEN ARABIA AND IRAN

The Kermanshah ophiolitic complex consists of a melange formation, which includes dismembered ophiolitic sequences. These ophiolites are located along the Main Zagros Reverse Fault, which marks the ophiolitic suture zone between the Zagros belt and the Sanandaj-Sirjan zone. They represent the Neo- Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Kermanshah ophiolites were emplaced onto platform carbonate rocks, which represented the northeastern Arabian margin. The Kermanshah ophiolitic complex is composed of various partial sequences, which are represented by: (1) mantle tectonites consisting of depleted lherzolites and both clinopyroxene- (cpx-) rich and cpx-free harzburgites; (2) a troctolite-cumulate gabbro-isotropic gabbro sequence mainly showing pegmatoid texture; (3) a wehrlite-cumulate gabbro-isotropic gabbro sequence showing foliated texture; (4) a dyke complex; (5) very scarce pillow basalts. Mantle tectonites are volumetrically predominant and tectonically overlay the gabbroic sequences. A number of conclusions may be drawn based on petrographic observations, mineral chemistry, whole-rock chemistry, and rare earth elements (REE) modelling carried out on both mantle tectonites and intrusive rock associations. (1) The Foliated Gabbro Unit has an N-MORB chemical signature and represents a portion of oceanic crust generated in a mid-ocean ridge setting from an N-MORB-type sub-oceanic mantle. (2) The Pegmatoid Gabbro Unit displays an E-MORB signature and represents a portion of oceanic crust most likely generated from a sub-oceanic mantle source enriched in light REE (LREE). A comparison with the well-studied Oman ophiolites suggests that this sequence may have formed during the early stage of oceanic spreading. (3) The depleted lherzolites present mild depletions in heavy REE (HREE) and variable depletion in LREE. REE modelling shows that they may represent a residual mantle after 15-20% removal of N-MORB melts. Some lherzolites show a moderate enrichment in La and Ce with respect to Sm, suggesting that this residual MORB mantle was subsequently trapped in a supra-subduction zone (SSZ) mantle wedge and enriched in LREE by subduction-derived fluids. (4) The depleted harzburgites present a significant depletion in incompatible elements and REE, coupled with a marked LREE enrichment with respect to medium REE. REE modelling shows that they may represent a residual mantle after 25-30% removal of boninitic-type melts in an intra-oceanic arc setting.

Detrital zircon U–Pb and Hf isotopic data from the Xigaze fore-arc basin: Constraints on Transhimalayan magmatic evolution in southern Tibet

Before the continental collision of India with Asia, northward subduction of the Tethyan oceanic lithosphere beneath South Asia resulted in widespread arc magmatism in the Lhasa terrane of southern Tibet from Early Jurassic to Eocene time. The detailed magmatic history of this Transhimalayan arc system, however, remains unclear. Here we report in- situ detrital zircon U–Pb and Hf isotopic analyses of sedimentary rocks from the Xigaze fore-arc basin, which developed along the southern margin of the Lhasa terrane. The zircon U–Pb results allow us to better constrain the depositional age of the Xigaze Group, which consists from bottom to top of the Chongdui, Ngamring, Padana and Qubeiya formations deposited between ca. 116 and 65 Ma, with the main stage (i.e., the Ngamring Formation) being deposited between ca. 107 and 84 Ma. The majority of these zircons are characterized by high 176Hf/ 177Hf isotopic ratios and positive ε Hf( t) values that are similar to those of magmatic zircons from the Gangdese batholith, suggesting the latter was a predominant source provenance of the fore-arc sediments. In the younger sequences, i.e., the upper Ngamring and Padana/Qubeiya formations, zircons that record either pre-Mesozoic U–Pb ages or negative ε Hf( t) values become more abundant, implying additional sources from the northern Lhasa and/or Qiangtang terranes due to northward development of the fluvial systems. The overall zircon U–Pb and Hf isotopic data furthermore suggest that during fore-arc deposition, the exposed Gangdese arc was dominated by igneous rocks formed between 130 and 80 Ma, associated with lesser amounts zircon with ages between 190 and 150 Ma, thus recording two important stages of arc magmatism that are only sporadically preserved in modern outcrops as a result of extensive erosion related to uplift during Mesozoic–Cenozoic time.

The Eclogite-facies Allalin Gabbro of the Zermatt-Saas Ophiolite, Western Alps: a Record of Subduction Zone Hydration

The Allalin gabbro is a 2 km × 0·5 km block of layered olivine-gabbro and troctolite included in the Zermatt–Saas ophiolite nappe of the Western Alps. Comprehensive texture, mineral and rock composition data together with a thermodynamic analysis of the complex phase associations permit a detailed reconstruction of the igneous and metamorphic reaction history recorded by the gabbroic rocks. Based on rock and mineral composition data, the Allalin gabbro represents part of a Middle Jurassic underplate of mafic magma at the base of the continental crust of the Apulian plate (Dent Blanche–Sesia Lanzo system). Granulite-facies recrystallization during cooling at ∼825°C and 1·0 GPa, involving the formation of Opx–Grt coronas between Ol–Pl, can be related to crustal thickening. Eocene subduction of the Tethys oceanic lithosphere under the Apulian plate detached the gabbro from the base of the continental crust and incorporated it into the ophiolite. Increasing pressure in the descending slab had little effect on parts of the gabbro, which still locally contain unaltered igneous Ol, Aug and Pl and well-preserved magmatic textures. With increasing subduction depth an increasing amount of aqueous fluid accessed the gabbro and transformed Pl to Zo–Jd–Ky–Qtz. At about 2·5 GPa (93 km) at c. 610°C, a dramatic hydration process converted most of the rocks (>90 vol. %) into a fully hydrated eclogite-facies assemblage of Omp + Zo + Tlc + Cld ± Grt ± Ky + Rt. The full hydration under water-present conditions occurred at the greatest depth reached by the gabbro. After detachment from the downgoing slab (i.e. along the ascent path), Gln, Pg and Mrg formed as additional hydrates. This last phase of hydration desiccated the metagabbro at a depth of c. 78 km and from then on the rocks were essentially devoid of a free fluid phase. The Allalin gabbro provides strong evidence that the fundamental high-pressure transformation of mafic rocks involves the hydration of gabbro to form eclogite.

Petrochemical characterization of mafic rocks from the Ligurian ophiolites, southern Apennines

New mineralogical and chemical data for ophiolitic rocks from the southwesternmost Liguride Units are presented in order to constrain their ocean-floor origin and subsequent emplacement in an accretionary wedge. Their complete petrochemical evolution is particularly well preserved in the southern Apennine metabasites. Metadolerites show amphibolite and greenschist facies mineral assemblages of ocean-floor metamorphism. Metabasalts display greenschist facies ocean-floor metamorphism and spilitic alteration. Veins cutting the mafic rocks show mineral assemblage of the prehnite–pumpellyite metamorphic facies. HP/LT orogenic metamorphism, reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction of the western Tethys oceanic lithosphere produced a mineral assemblage typical of the lawsonite–glaucophane facies. Bulk-rock chemistry suggests that the mafic protoliths had a MORB-type affinity, and were affected by ocean-floor rodingitic and/or spilitic alteration. Hydrothermal alteration-induced LREE mobility and LREE enrichment may be correlated with the ocean-floor metamorphism.

Structure and tectonics of lower crustal and upper mantle rocks in the Jurassic Mirdita ophiolites, Albania

The Jurassic Mirdita ophiolite crops out in the western branch of the Hellenide‐Dinaride ophiolite belt in the Balkan Peninsula; it represents a remnant of the Tethyan oceanic lithosphere developed between the Apulian and Pelagonian microcontinents. Emplaced during the Cretaceous on the western margin of Pelagonia, the Mirdita ophiolite was involved in southwestward thrusting of the Hellenide‐Dinaride tectonic units during the Eocene alpine tectonics. It was little affected, however, by these alpine events as marked by nearly horizontal Cretaceous limestone deposits overlying the ophiolite. A continuous section of Middle Jurassic oceanic crust, including thin gabbros, a N‐S‐trending sheeted dike complex, and extrusive rocks, is exposed in the central part of the ophiolite and thickens eastward within a regional, N‐S‐trending synform. Peridotite massifs exposed along the western and eastern edges of this synform show major structural and petrological differences. The eastern ultramafic domain has a harzburgitic mantle exhibiting a high‐temperature asthenospheric foliation dipping steeply to moderately to the west. Major chromite deposits are restricted to these eastern ultramafic massifs. The western ultramafic domain has a zoned mantle, with asthenospheric harzburgite exposed at its western margin, progressively replaced eastward by plagioclase‐peridotites that were highly strained at low temperatures, and that are bounded by amphibole‐peridotites along with crustal rocks. The occurrence of plagioclase is ascribed to melt impregnation processes that occurred immediately before or during intense lithospheric deformation. The gabbros in the eastern massifs are thicker (1–2 km and layered, whereas they are highly thin and discontinuous in the western massifs. Mantle peridotites of the western massifs locally are in direct contact with the overlying diabase and volcanic rocks along ductile shear zones. Development of epidote‐amphibolite facies metamorphism in upper‐crustal rocks was produced by hydrothermal alteration in the oceanic realm, and was intense and widespread in the western domain. The Mirdita ophiolite likely originated in a short‐lived, narrow ocean basin, which was closed during the Middle Jurassic by eastward overthrusting of the western domain, and westward subduction of the eastern domain. This constrictional phase was followed by dextral oblique convergence between the Pelagonian and Apulian microcontinents. This model implies a parautochtonous origin of the Mirdita ophiolite between these two microcontinents.