Tethyan Oceanic Crust Research Articles

Black Sea Morphotectonic Nature

Abstract The presence of oceanic crust on the Black Sea bottom indicates that this sea basin was a section - a Pontic Ocean gulf at the northern margin of the Phanerozoic Tethys Ocean. The late Phanerozoic subduction of the Tethys oceanic crust under the European continental massif in this area took place in the subduction zone, whose modern traces are found today south of the Crimean Peninsula. On the surface of the subducted oceanic crust were numerous fragments of the destroyed continental crust on the northern margin of the Gondwana continent. These relatively lighter than oceanic crust crustal fragments (or a significant part of them) were not subducted. They accumulate in the immediate vicinity of the subduction zone on the unsub ducted remnants of oceanic crust. Such is the case with the Black Sea. It is not an imposed subcontinental basin, but a last modern relict of the northern margin of the Late Phanerozoic Tethys Ocean.

Petrogenesis of the Late Jurassic Granodiorite and Its Implications for Tectonomagmatic Evolution in the Nuocang District, Western Gangdeses

The Gangdese magmatic rocks of the southern Lhasa terrane, are generally thought to be an important window to witness the formation and evolution of the Neo-Tethys oceanic opening, subduction, and closure, and India-Eurasian continental collision. We investigated a new occurrence of granodiorite in the Nuocang district of western Gangdese, southern Lhasa terrane, and conducted a series of analyses on their petrology, chronology, and geochemistry. The Nuocang granodiorites have the zircon U-Pb ages of 151–154 Ma, which suggest that Late Jurassic granitoids are present in the western Gangdese of southern Lhasa terrane. They are relatively high in SiO2, Al2O3, low K2O, Na2O, and Sr/Y ratios, enrichments of LILE and LREE, and depletion of HFSE, with the positive correlation between Rb and Th, and negative correlations between SiO2 and P2O5, Rb, and Y, showing the features of I-type granites. The relatively high (87Sr/86Sr)i values from 0.712231 to 0.712619, low εNd(t) values from −9.56 to −8.99, together with the negative εHf(t) values from −10.8 to −5.0 (mean value −8.9) suggested that the Nuocang granodiorites probably sourced from the partial melting of the ancient Lhasa terrane, with parts of mantle materials involving in. Combined with the previous geochronology and geochemical data of Mesozoic magmas in the Gangdese belt, as well as the Late Jurassic granodiorite, in this paper, we propose that the Nuocang granodiorites formed in a continental margin arc environment triggered by the northward subduction of Neo−Tethys oceanic crust.

Geodynamic evolution of the Tethyan lithosphere as recorded in the Spontang Ophiolite, South Ladakh ophiolites (NW Himalaya, India)

The Spontang Ophiolite complex represents the most complete ophiolite sequence amongst the South Ladakh ophiolites and comprises mantle rocks (depleted harzburgites, dunites and minor lherzolites) as well as crustal rocks (basalt, isotropic gabbros, layered gabbros etc.). In the present study, detailed geochemistry (whole rock as well as mineral chemistry) and Sr-Nd isotopic analyses of thirty-six ultramafic- mafic samples have been attempted to constraint the evolution and petrogenetic history of the Tethyan oceanic crust. Major, trace-element and REE patterns of the peridotites and their minerals indicate that the lherzolites experienced lower degrees of partial melting resembling abyssal peridotites (at higher temperatures, TREE = ∼1216 °C) than the harzburgites (6%–8% versus 15%–17%). Elevated εNd(t) and variable 87Sr/86Sr(t) ratios along with REE patterns suggest that the Spontang mafic rocks display N-MORB affinity with negligible participation of oceanic sediments in their genesis are originated from a depleted upper mantle with little contribution from subduction-related fluids. MORB-type Neotethyan oceanic crust is associated with the earliest phase of subduction (of older Jurassic age) through which a younger intra-oceanic island arc (Spong arc) subsequently developed. Harzburgites REE display typical U-shaped patterns, suggesting that these rocks have been metasomatized by LREE-enriched fluids. On the other side, mafic rocks are characterized by heterogeneous (Nb/La)PM and (Hf/Sm)PM and relatively homogeneous εNd(t), indicating interaction of subduction-related melts with the upper mantle during the initiation of subduction, in Early Cretaceous times.

Doming along the Zagros transpression zone, SW Iran: insights from microstructural analysis of heterogeneous deformation

Detailed structural and kinematic analyses of deformed structures in the Sanandaj-Sirjan Metamorphic Belt in the Ghouri area revealed the occurrence of three gneissic sub-domes with semi-elliptical shapes with their major axes parallel to the Zagros Transpression Zone boundary (NW-SE direction). These sub-domes are cored by sheared quartzo-feldspathic gneisses which surrounded by sequences consisting of garnet amphibolites, phyllites, phyllonites, muscovite schists, and deformed conglomerates. The Kuh-e-Zard, Chah-Sabz, and Chah-Goni sub-domes resulted from the interaction of the lower to middle continental crust and hot subducting Tethyan oceanic crust during subduction/continental collision. These sub-domes were squeezed between thrusts in the Zagros dextral transpression zone. The variations in the kinematic vorticity and strain estimates revealed that sub-simple shear transpressional deformation is highly heterogeneous in each sub-domes.

Micropalaeontological and Geochemical Evidence of the Late Jurassic Radiolarians Cherts of Naga Ophiolite Hill, Nagaland, Northeast-India

We analyzed the radiolarian assemblages (pelagic sediments) and geochemical studies of 15 samples collected from Meluri district Nagaland, Naga Hills Ophiolite belt (NHO) and it consists of the Flysch Naga-Arakan-Yoma trough beyond the Indo-Myanmar border. It reflects one of the many components of the Himalayan Orogenic systems of Tethyan oceanic crust discovered in Nagaland at Meluri district of India and they have very well-preserved studies and can be identified to Upper Jurassic (Kimmeridgian-mid-Tithonian). As well as a continuous sequence of Kimmeridgian-mid-Tithonian, five radiolarian assemblages have been identified in this study. This is so far the first recovery of pelagic sediment assemblages (Radiolarians) from the Naga Hills Ophiolite complex. Samples of Naga Hills Ophiolite belts fell in the Sublitharenite, Litharenite, and Fe-shale Fe-sand area, and the A-CN-K diagram indicates that these sediments in the source region were exposed deep to moderate weathering conditions. In the discriminant plot, the Naga Hill-Ophiolite-Chert samples occupy the field of origin of the sediment quartzose, and the log (K2O/Na2O) versus SiO2 samples fall into the active continental marginal field, and only one sample falls into an island arch, suggesting that the tectonic framework of the Naga Hills-Chert samples was deposited in the active continental margin and the diagrams K2O/Al2O3 and MgO/Al2O3 illustrated the tectonic setting of the marine environment from Naga Hill Ophiolite Chert. Such findings have been consistent with the current geology of Naga Hills in the province of Nagaland.

The copper polymetallic deposits and resource potential in the Tibet Plateau

Many large and super-large copper deposits have been discovered and explored in the Tibet Plateau, which makes it the most important copper resource reserve and development base in China. Based on the work of the research team, the paper summarizes the geological characteristics of the main copper deposits in Tibet and puts forward a further prospecting direction. A series of large accumulated metal deposits or ore districts from subduction of Tethys oceanic crust to India-Asia collisionhave been discovered, such as Duolong Cu (Au) ore district and Jiama copper polymetallic deposit. The ore deposits in the Duolong ore district are located in the lowstand domain, the top of lowstand domain, and the highstand domain of the same magmatic-hydrothermal metallogenic system, and their relative positions are the indicators for related deposits in the Bangong Co-Nujiang metallogenic belt. The polycentric metallogenic model of the Jiama copper polymetallic deposit is an important inspiration for the exploration of the porphyry mineralization related to collision orogeny. Further mineral exploration in the Tibet Plateau should be focused on the continental volcanic rocks related to porphyry-epithermal deposits, orogenic gold deposits, hydrothermal Pb-Zn deposits related to nappe structures, skarn Cu (Au) and polymetallic deposits, and the Miocene W-Sn polymetallic deposits.

Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite (NHO) belt in the Indo-Myanmar range (IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene. Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO, northeastern India to address (i) the mantle processes and tectonic regimes involved in their genesis and (ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle. The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Washelo and Lacham areas. The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase. The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber. Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE. These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions. Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone (SSZ) fore-arc regime coherent with the subduction initiation process. The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process: (i) a depleted fore arc basalt (FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle; (ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration. The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.

Cold subduction zone in northern Calabria (Italy) revealed by lawsonite–clinopyroxene blueschists

AbstractLawsonite blueschists are important markers of cold subduction zones, subjected to intense fluid circulation. This is because lawsonite preservation in exhumed blueschists and eclogites is typically linked to cold exhumation paths, accompanied by hydration. In the Catena Costiera (Calabria, southern Italy), lawsonite–clinopyroxene blueschists of the Diamante–Terranova Unit, affected by ductile shearing and retrogression, are exposed. Blueschists contain zoned clinopyroxene crystals, showing core–rim compositional variation from diopside to omphacite and hosting primary inclusions of lawsonite and titanite. Thermodynamic modelling of phase equilibria in the NCKFMASHTO system revealed peak metamorphic conditions of 2.0–2.1 GPa and 475–490°C for the Alpine subduction in Calabria. The subsequent post‐peak metamorphic evolution mainly proceeded along a decompression and cooling path up to ~1.1 GPa and ~380°C. The final exhumation stages are recorded in the sheared blueschists where a mylonitic to ultramylonitic foliation developed at ~0.7 GPa and 290–315°C. Therefore, the P–T evolution of the Diamante–Terranova blueschists mostly occurred in the stability field of lawsonite, sustained by H2O‐saturated conditions during the exhumation path. The results of this study indicate that the blueschists underwent peak metamorphic conditions higher than previously thought, reaching a maximum depth of ~70 km under a very cold geothermal gradient (~6.6°C/km), during the Eocene subduction of the Ligurian Tethys oceanic crust in Calabria.

The Age, Origin, and Emplacement of the Tsiknias Ophiolite, Tinos, Greece

AbstractThe Tsiknias Ophiolite, exposed at the highest structural levels of Tinos, Greece, represents a thrust sheet of Tethyan oceanic crust and upper mantle emplaced onto the Attic‐Cycladic Massif. We present new field observations and a new geological map of Tinos, integrated with petrology, THERMOCALC phase diagram modeling, U–Pb geochronology and whole rock geochemistry, resulting in a tectonothermal model that describes the formation and emplacement of the Tsiknias Ophiolite and newly identified underlying metamorphic sole. The ophiolite comprises a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks that represent the Moho Transition Zone. A plagiogranite dated by U‐Pb zircon at 161.9 ± 2.8 Ma, reveals that the Tsiknias Ophiolite formed in a suprasubduction zone setting, comparable to the “East‐Vardar Ophiolites,” and was intruded by gabbros at 144.4 ± 5.6 Ma. Strongly sheared metamorphic sole rocks show a condensed and inverted metamorphic gradient, from partially anatectic amphibolites at P‐T conditions of ~8.5 kbar 850–600 °C, downstructural section to greenschist‐facies oceanic metasediments over ~250 m. Leucosomes generated by partial melting of the uppermost sole amphibolite, yielded a U‐Pb zircon protolith age of ~190 Ma and a high‐grade metamorphic‐anatectic age of 74.0 ± 3.5 Ma associated with ophiolite emplacement. The Tsiknias Ophiolite was therefore obducted ~90 Myr after it formed during initiation of a NE dipping intraoceanic subduction zone to the northeast of the Cyclades that coincides with Africa's plate motion changing from transcurrent to convergent. Continued subduction resulted in high‐pressure metamorphism of the Cycladic continental margin ~25 Myr later.

Geochemistry and origin of Mn deposits in the Bela ophiolite complex, Balochistan, Pakistan

We have studied geochemical characteristics, mineralogy and origin of the manganese deposits in Bela ophiolitic complex. Geochemical investigation was conducted in order to discuss the elemental correlations and to infer the probable origin of manganese deposits in Bela ophiolites. Ore microscopy was conducted to identify different manganese minerals and their paragenesis. Psilomelane and braunite were found to be the major ore minerals. Mineral paragenesis sequence as observed was braunite forming first. Psilomelane formed in later stages because it has been found that psilomelane is altering the braunite. Magnetite was observed as a secondary mineral as vein filling and cutting through both the braunite and psilomelane, therefore, younger than the psilomelane and braunite. The gangue minerals observed in the studied samples were quartz, cryptocrystalline silica and calcite. The correlations among different major and trace elements showed diversity of relations. MnO showed negative correlation with Fe2O3 (− 0.73), Si2O (− 0.27), positive correlation with Al2O3 (0.54), TiO2 (0.36), MgO (0.22), Pb (0.23), Ni (0.07), Cr (0.12), and no correlation was established with Zn, Cu, Co. Binary diagram of Si versus Al and ternary discrimination diagrams of Fe–(Ni + Co + Cu) × 10–Mn and Ni–Zn–Co showed hydrothermal-diagenetic-type deposits. Furthermore, the present study suggests that the enriched manganiferous fluid during its upward movement within the Tethys oceanic crust near spreading center started precipitating Mn along with Fe on the sea floor. Later on, these manganese minerals were obducted on land between Indian plate and Helmond block of Eurasian plate in existing position accompanying pillow basalt and pelagic sediments.

An appraisal of crustal structure of the Indo-Burmese subduction region

We analyzed teleseismic receiver functions of 9 sites to determine the crustal structure beneath the Indo-Burmese subduction region (IBR), which is seismically one of the most active subduction zones of the Indian subcontinent. The receiver function analysis of the primary and reverberated phases allowed us to image the down going Moho beneath the Burmese micro plate parallel to the subducting slab. We observed that this subduction zone is characterized by sharp dip of a thick (˜45 km) crust in the Indian Territory, which steeply bends down to ˜57 km in the Burmese Territory separated by the Churachandpur Mao Fault (CMF). The Imphal valley, located in the central part of the IBR, has a sedimentary thickness of 2.5 km. The Poisson’s ratio values correlate well with the tectonic units comprises the subduction region. We observed high Poisson’s ratio values throughout the region, which reflects bulk mafic to ultramafic crustal composition. Our observations of Poisson’s ratio beneath the IBR reveal that the Indian crust along the IBR is oceanic in nature, which is possibly the Tethyan oceanic crust of Southeast Asia.

Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq

One of the major debated aspects of the Zagros orogenic system in the Middle East is the timing of onset of continental collision between Arabia and Eurasia. The Zagros hinterland in the Kurdistan region of Iraq contains an ~2-km-thick clastic depositional sequence of the Red Bed Series (RBS) that rests unconformably on the Arabian foreland and structurally below the Main Zagros fault, which carries the allochthonous volcaniclastic rocks of the Walash and Naopurdan groups in its hanging wall. Detrital zircon (DZ) U-Pb geochronol- ogy constrains both the depositional age and the provenance of the RBS and pinpoints the timing of initial arrival of Eurasian sediment on the Arabian plate. The youngest DZ U-Pb ages for the laterally extensive (~150 km) basal RBS (Suwais unit) imply a middle Oligocene (ca. 26 Ma) maximum depositional age. The provenance data reveal dominant DZ U-Pb age modes of late Paleocene (ca. 55–60 Ma) and middle Eocene (ca. 37–44 Ma) and, importantly, the presence of ~10%–15% DZ grains that are unequivocally derived from Eurasia, includ- ing of Jurassic (150–200 Ma) and late Paleozoic (270–380 Ma) DZ age modes. These data suggest that the RBS deposits were mainly sourced from forearc- and/or arc-related terranes along the southwest margin and hinterland of Eurasia. We advocate that by ca. 26 Ma, Neo- Tethys oceanic crust had been consumed and that Arabia-Eurasia continental collision well was underway as indicated by deposition of strata with Eurasian provenance on the Arabian margin. These DZ U-Pb data from the RBS highlight the significance of provenance data from synorogenic deposits in revealing the timing of initial continent collision by document- ing the earliest arrival of upper-plate sediment on the lower plate.

Blueschist facies overprint of late Triassic Tethyan oceanic crust in a subduction–accretion complex in north-central Anatolia, Turkey

The late Cretaceous accretionary complex along the İzmir–Ankara–Erzincan suture zone, northern Turkey, includes various types of metamorphic rock together with radiolarite, sandstone, mudstone, serpentinite, basalt and limestone. Meta-plagiogranite blocks (up to 5 m in diameter) and a meta-gabbro slice (800 × 500 m) cross-cut by meta-plagiogranite are observed in a matrix of serpentinite, mudstone and radiolarian chert. These meta-gabbros and meta-plagiogranites show subduction-related geochemical characteristics. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons from two meta-plagiogranites yielded 222.3 ± 1.5 and 227.2 ± 1.6 Ma crystallization ages. This study shows that the meta-gabbro and meta-plagiogranite from the accretionary complex are remnants of the metamorphosed equivalents of the late Triassic lower crustal rocks of the subducted Tethyan oceanic crust. They are the oldest lower crustal rocks observed in the İzmir–Ankara–Erzincan suture zone. Both the meta-gabbros and meta-plagiogranites have Na–Ca amphiboles, indicating blueschist facies metamorphism. However, phengites from a blueschist facies rock yielded an 40 Ar/ 39 Ar age of 104.3 ± 0.7 Ma, indicating that blueschist metamorphism occurred during the Albian. All these data suggest that the oceanic crust formed during the Norian, metamorphosed during the Jurassic to early Cretaceous and then dismembered during accretionary complex formation in the late Cretaceous. Supplementary material: A complete description of the analytical methods ( electron microprobe, LA-ICP-MS zircon U–Pb and 40 Ar/ 39 Ar age analyses, pseudo-section modelling and THERMOCALC) are available at https://doi.org/10.6084/m9.figshare.6509984 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

PALAEONTOLOGICAL (RADIOLARIAN) LATE JURASSIC AGE CONSTRAINT FOR THE STEPANAVAN OPHIOLITE (LESSER CAUCASUS, ARMENIA)

Micropalaeontological age evidence for the sedimentary cover of ophiolites is important to understand the palaeogeographic and geodynamic evolution of Tethyan realms. The Stepanavan ophiolitic suite of Northern Armenia consists of peridotites, gabbros, plagiogranite and lavas with a radiolarite sedimentary cover. It is regarded as the northern extension of the Sevan Akera ophiolitic zone and may be considered as the eastern extension of the Izmir-Ankara suture zone. It represents the relics of a slow-spreading mid oceanic ridge that was active between Eurasia and the South-Armenian Block of Gondwanian origin. Radiolaria extracted from radiolarites of the Stepanavan ophiolite provide for the first time a Late Jurassic (late Kimmeridgian to early Tithonian) age constraint for this part of Tethyan oceanic crust preserved in Lesser Caucasus.

Zircon LA-ICP-MS Dating and Geochemical Characteristics of I-type Granitoids from the Yanhu Area, West Segment of the Bangongco-Nujiang Suture (Western Tibet): Petrogenesis and Implications for the Southward Subduction of the Tethyan Ocean

ABSTRACT The Yanhu granitoids are located in the west segment of the Bangongco-Nujiang suture in the western Tibetan Plateau. The main rock types of the granitoids are diorite porphyry, quartz diorite, granodiorite, granite and granite porphyry. Here, their zircon LA-ICP-MS U-Pb ages and petrogeochemical data are reported. Three groups of magmatic events can be distinguished from the Yanhu area: group 1 includes samples AK01 and ZK01 of diorite porphyry, and sample D3658 of quartz diorite that yield mean zircon U-Pb ages of 121.0 ± 2.7 Ma, 116.6 ± 2.0 Ma and 116.0 ± 3.9 Ma, respectively; group 2 includes sample D0050 of diorite porphyry, samples D1393 and D3660 of granodiorite and sample D3065 of granite porphyry that yield mean zircon U-Pb ages of 104.9 ± 2.0 Ma, 105.4 ± 3.8 Ma, 104.2 ± 1.9 Ma and 104.2 ± 1.9 Ma, respectively; group 3 includes sample D3093 of granite that yields mean zircon U-Pb ages of 93.6 ± 1.5 Ma. The zircon LA-ICP-MS U-Pb ages suggest that the Yanhu granitoids were emplaced at 121.0 – 93.6 Ma, representing Cretaceous magmatism in the west segment of the Bangongco-Nujiang suture. The granitoids are composed of SiO2 (56.57 to 76.98 wt.%), Al2O3 (12.20 to 17.90 wt.%), Na2O (3.61 to 4.98 wt.%), K2O (2.06 to 4.71 wt.%) and CaO (0.27 to 5.74 wt.%). The Yanhu granitoids exhibit enrichment in LREE (light REE) and LILE (large ion lithophile elements) such as Rb, Th, U, Pb and K and depletion of HREE (heavy REE), P, Ti, Nb, Ta and Zr. Their A/CNK ratios of 0.85-1.06 are <1.1, implying that they are high-K, metaluminous-weakly peraluminous I-type granites. The Yanhu granitoids were generated mainly by partial melts of the meta-igneous lower crust and some arc-related materials. The Yanhu granitoids probably formed in VAG and syn-COLG tectonic settings related to the southward subduction of the Tethyan Ocean. Diorite porphyry and quartz diorite magmatism from 121.0 Ma to 116.0 Ma may be associated with the southward Bangongco–Nujiang Tethys oceanic crust subduction. Diorite porphyry, granodiorite, and granite porphyry magmatism from 105.4 Ma to 104.2 Ma may be associated with the rising asthenosphere induced by the slab breakoff. Granite magmatism from 93.6 Ma may be related to the crustal thickening induced by the final amalgamation of the Lhasa Terrane and the Qiangtang Terrane.

U-Pb zircon geochronology of the Paleogene – Neogene volcanism in the NW Anatolia: Its implications for the Late Mesozoic-Cenozoic geodynamic evolution of the Aegean

The northern Aegean region was shaped by subduction, obduction, collision, and post-collisional extension processes. Two areas in this region, the Rhodope-Thrace-Biga Peninsula to the west and Armutlu-Almacık-Nallıhan (the Central Sakarya) to the east, are characterized by extensive Eocene to Miocene post-collisional magmatic associations. We suggest that comparison of the Cenozoic magmatic events of these two regions may provide insights into the Late Mesozoic to Cenozoic tectonic evolution of the Aegean. With this aim, we present an improved Cenozoic stratigraphy of the Biga Peninsula derived from a new comprehensive set of U-Pb zircon age data obtained from the Eocene to Miocene volcanic units in the region.The compiled radiometric age data show that calc-alkaline volcanic activity occurred at ~43–15Ma in the Biga Peninsula, ~43–17Ma in the Rhodope and Thrace regions, and ~53–38Ma in the Armutlu-Almacık-Nallıhan region, which are slightly overlapping. We discuss the possible cause for the distinct Cenozoic geodynamic evolution of the eastern and western parts of the region, and propose that the Rhodope, Thrace and Biga regions in the north Aegean share the same Late Mesozoic to Cenozoic geodynamic evolution, which is consistent with continuous subduction, crustal accretion, southwestward trench migration and accompanying extension; all preceded by the Late Cretaceous – Paleocene collision along the Vardar suture zone. In contrast, the Armutlu-Almacık-Nallıhan region was shaped by slab break-off and related processes following the Late Cretaceous – Paleocene collision along the İzmir-Ankara suture zone. The eastern and western parts of the region are presently separated by a northeast-southwest trending transfer zone that was likely originally present as a transform fault in the subducted Tethys oceanic crust, and demonstrates that the regional geodynamic evolution can be strongly influenced by the geographical distribution of geologic features on the subducting plate.

Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite, India: Implications for petrogenesis

The Naga Hills Ophiolite (NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite, diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx. Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features. The basalts are characterized by 44.1–45.6 wt.% of SiO2 with 28–38 of Mg#, and the gabbros by 38.7–43.7 wt.% of SiO2, and 26–79 of Mg#. The ultramafic rocks are characterized by 37.4–52.2 wt.% of SiO2, and 80–88 of Mg#. In multi-element diagrams (spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr, Nb, Sr and a gentle negative anomaly of P. However, the rare earth element (REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE (LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite (IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340–4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97–1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources (fertile and refractory).

Recycling of Paleotethyan oceanic crust: Geochemical record from postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens

Plate tectonics can develop from oceanic subduction to continental collision, with disappearance of presubducted oceanic crust in continental subduction zones. This is typical for collisional orogens where Tethyan oceanic crust was consumed during continental convergence, with recycling of the Tethyan oceanic crust into the mantle. However, it is difficult to trace the geochemical signature of the previously subducted oceanic crust in collisional orogens. Postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens provide a geochemical record of crust-mantle interaction during Paleotethyan oceanic subduction preceding the Triassic continental collision. These mafic igneous rocks were emplaced in the Cretaceous after continental collision and exhibit oceanic-island basalt (OIB)−like trace-element distribution patterns, with enrichment of large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but no depletion of high field strength elements (HFSEs). They also show relatively depleted whole-rock Sr-Nd-Hf isotope compositions, with initial 87 Sr/ 86 Sr ratios of 0.7043−0.7050, e Nd ( t ) values of −1.8−4.5, and e Hf ( t ) values of 4.7−10.3. Such geochemical features indicate their origin from a juvenile mantle source that contained subducted oceanic crust−derived material. Thus, we suggest that the mantle source was generated by a melt-peridotite reaction at the slab-mantle interface in the Paleotethyan oceanic subduction channel during continental collision. The original peridotite would have been characterized by depletion of melt-mobile incompatible trace elements and radiogenic Sr-Nd-Hf isotopes, corresponding to the juvenile subarc lithospheric mantle. In contrast, the subducted crust−derived material would have been characterized by enrichment of LILEs and LREEs but no depletion of HFSEs, corresponding to felsic melts that were derived from partial melting of the subducting Paleotethyan oceanic basalt and sediment outside the rutile stability field. As such, the melt-peridotite reaction would generate pyroxenite-rich ultramafic metasomatites in the lower part of the mantle wedge, with relative enrichment of LILEs and LREEs but no depletion of HFSEs. In this model, partial melting of the metasomatites gave rise to the postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens. Therefore, the postcollisional mafic igneous rocks record the recycling of Paleotethyan oceanic crust in the collisional orogen.

Mg, Sr, and O isotope geochemistry of syenites from northwest Xinjiang, China: Tracing carbonate recycling during Tethyan oceanic subduction

Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ26Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ26Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ26Mg values (−0.46 to −0.26‰ and −0.41 to −0.17‰, respectively) significantly lighter than the mantle (−0.25±0.07‰, 2SD). No correlation of δ26Mg with either SiO2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ26Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87Sr/86Sr ratios (0.70814 to 0.71105) and negative correlation between δ26Mg and δ18O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources.

Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern Oman mountains

The Cretaceous Semail ophiolite (northern Oman and the United Arab Emirates) includes an intact thrust slice of Tethyan oceanic crust and upper mantle formed above a northeast-dipping subduction zone that was the site of initiation of obduction. The normal metamorphic sole of the Semail ophiolite comprises a highly condensed sequence of hornblende + plagioclase ± garnet amphibolites with small enclaves of garnet + clinopyroxene granulites immediately beneath the mantle sequence peridotites, tectonically underlain by a series of epidote amphibolite and greenschist facies lithologies in a highly deformed ductile shear zone. Peak metamorphic conditions of 770–900 °C and 11–15 kbar indicate metamorphism at depths far greater than can be accounted for by the preserved thickness of the ophiolite (∼15 km). In the mountains of northern Oman, the 1.2-km-thick Bani Hamid thrust sheet is composed of intensely folded granulite and amphibolite facies rocks within mantle sequence peridotites, exhumed by late-stage out-of-sequence thrusting along the Bani Hamid thrust. The Bani Hamid thrust slice includes two-pyroxene quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + wollastonite + scapolite marbles and calc-silicates and amphibolites (hornblende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, intruded by hornblende pegmatites. The Bani Hamid granulites represent metamorphosed cherts and calcareous turbidites probably derived from the distal Haybi Complex and Oman Exotic limestones, which have an alkali basaltic substrate. Metamorphic modeling using the program THERMOCALC in the system NCKFMASHTO (Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O) gives peak pressure-temperature conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar, a pressure that is much lower than that of the metamorphic sole, suggesting a different origin. The 206 Pb/ 238 U zircon dates indicate that the gabbroic crust of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. The 206 Pb/ 238 U zircon dates from the metamorphic sole range from 95.7 to 94.5 Ma, and suggest that metamorphism and melting was either synchronous with or slightly postdated ridge magmatism. The Bani Hamid granulites are younger; zircon and titanite U-Pb dates span ca. 94.5–89.8 Ma. Peraluminous granitic dikes intruding the mantle sequence peridotites are as young as 91.4 Ma and likely reflect localized partial melting of crustal material during the late stage of the obduction process. A minimum of 130 km shortening is recorded by restoration of the major folds within the Bani Hamid thrust sheet, and more than 30 km offset has occurred along the west-directed breaching out-of-sequence Bani Hamid thrust. These rocks may be representative of deep-level duplexes imaged on recent seismic sections across the mountains of northern Oman–United Arab Emirates.