Neo-Tethyan Subduction Research Articles

Dynamic evolution of competing same-dip double subduction: New perspectives of the Neo-Tethyan plate tectonics

Same-dip double-subduction (SDDS) systems are widely reported from present as well as past complex convergent plate tectonic configurations. However, the dynamics of their evolution is poorly understood, which is crucial to conceptualize anomalous subducting slab kinematics and associated observed geological phenomena, such as irregular trench migration rates, high convergence velocities, and slab break-off. To bridge this gap, we develop dynamic thermo-mechanical subduction models and investigate the initiation and evolution of SDDS systems, considering three different initial plate configurations: oceanic, oceanic-continental and multiple continental settings, based on Neo-Tethyan paleo-reconstructions. Each model offers new insights into the complex tectonic history of the major Neo-Tethyan subduction zones, particularly the Indo–Eurasian and Andaman convergent systems. We evaluate the slab-slab interactions, trench and subduction kinematics, inter-plate reorganization, and temporally varying mantle flow patterns involved in the dynamic evolution of these SDDS systems. The oceanic SDDS model simulations reveal that a sizable oceanic plate can initiate two subduction zones synchronously, and they evolve unequally in a competing mode, leading to exceptionally high convergence rates (∼16–17 cm/year) for a prolonged duration (∼7–8 Myr). This finding explains the coeval activity of coupled subduction zones in the Indo-Eurasia convergence during the Cretaceous evolution of the Neo-Tethys. We further implement a corresponding single subduction model to assess the additional effects of competing slab kinematics in an oceanic SDDS setting. The ocean-continent SDDS model, on the other hand, localizes subduction preferentially at passive margins between the oceanic plate and the continental block, forming double subduction zones that grow almost equally to form a spreading centre between the two trenches. These model results allow to reconstruct the Cenozoic evolution of the eastern Neo-Tethyan region, which ultimately led to the development of the Andaman subduction zone. We also show the post-Cretaceous evolution of the Indo–Eurasian collision zone as a consequence of the SDDS dynamics in presence of multiple continental blocks. These dynamics facilitated slab break-off, transforming the SDDS into a single subduction system in a relatively short time frame (∼3 Myr). We finish with a synthesis of the paleo-reconstructions of the Neo-Tethys in the perspective of these SDDS models.

Eocene calc-alkaline volcanic rocks related to Cenozoic episodic magmatism during Neo-Tethyan subduction, Tarom–Hashtjin Belt, NW Iran

Eocene calc-alkaline volcanic rocks related to Cenozoic episodic magmatism during Neo-Tethyan subduction, Tarom–Hashtjin Belt, NW Iran

The spatial–temporal evolution of Neotethyan subduction in central to Southeast Iran: Constraints from geochemistry, zircon U–Pb–Hf, and Sr–Nd–Pb isotopes

Igneous rocks in central Iran preserve proof of several episodes of Late Mesozoic–Cenozoic Neotethyan subduction, and record temporal and spatial variations from subduction to collisional geodynamic regimes that remain a subject of debate. We revisit magmatic rocks from the central Urumieh–Dokhtar magmatic arc (UDMA; 32° 30′ N to 36° 00′ N) and report new major and trace element analyses, whole-rock Sr–Nd–Pb-isotopic data and zircon U–Pb–Hf ages on plutonic rocks from the Kajan region. The data reveal magmatic pulses between 32 and 22 Ma that are represented by igneous rocks with geochemical and isotopic features resulting from melting of a metasomatized, enriched mantle. Modeling indicates the samples represent a mixture of 97% batch melt (from a source with 98.5% mantle melt +1.5% terrigenous sediment) with 3% upper continental crust. The oldest yielded age (i.e., 32 Ma) rules out a lull in magmatism in the central UDMA between 37 and 26 Ma and speaks against distinct flare-up magmatic episodes at ∼54–37 Ma and ∼ 20–5 Ma in the frontal arc. We hypothesize that magmatism records down-going slab tearing into two pieces beneath the central to SE UDMA, which then changes the geometry and buoyancy of subducting Neotethyan lithosphere. The down-going slab remained involved in central UDMA (i.e., rollback) during Early Miocene whilst arc retreat beneath the SE UDMA proceeded with detachment of subducted oceanic lithosphere (i.e., break-off). We hypothesize a diachronous collision in the UDMA, first in the southeastern segments, and subsequently in the central UDMA. These findings bear important implications for the geodynamic evolution of the Zagros orogen and run counter to a model suggesting that the collision initiated in the northwest to central and propagated progressively to the southeast along the Zagros suture zone.

Recognition of Neogene tin mineralization in the Southeast Asian tin belt

The Southeast (SE) Asian tin belt is a major tin producer globally, with a prolonged mineralization history from the Permian to Paleogene (ca. 285−45 Ma). Tin deposits in this region are typically associated with tectonic settings that involve subduction and collision of the Paleo-, Meso-, and Neo-Tethys slabs. Ca. 40 Ma, a notable transition occurred in the tectonic regime of SE Tibet, with the Neo-Tethys subduction giving way to lateral extrusion of the Indochina block along major strike-slip faults. Previously, it was believed that this shift had brought tin mineralization to a halt. In this study, we present in situ laser ablation−inductively coupled plasma−mass spectrometry U-Pb cassiterite ages of 21−19 Ma from the Yunling tin deposit located in western Yunnan, China. Yunling produces gem-quality cassiterite that is transparent but contains low U contents, which renders usual U-Pb dating techniques unusable. To address this, a customized dating protocol involving cathodoluminescence (CL) imaging and testing of U distribution on crystallographically well-defined cassiterite crystals was applied. The study revealed two types of primary microstructures in cassiterite: volumetrically dominant concentric oscillatory growth zones and subordinate CL-dark sector zones. The U content shows a preferential enrichment in the CL-dark sector zones, typically tens of parts per million (ppm), which is two orders of magnitude greater than the U content in the concentric zone. This is significant, as the dating results (21−19 Ma) obtained through the targeted selection of CL-dark sector zones represent the youngest tin mineralization event in the SE Asian tin belt. Additionally, these results challenge the established belief that the ore-hosting Indosinian granite, dated to ca. 215 Ma, was related to Sn mineralization in the Yunling area. We suggest that emplacement of the early Neogene Sn mineralization at Yunling may be genetically connected to extensive delamination of the lithosphere in southeastern Tibet. The lithospheric delamination led to the upwelling and decompression partial melting of the asthenosphere, which provides a plausible explanation for the high temperature required for the release of Sn from biotite-dehydration melting of sedimentary protolith. The study also highlights the potential of Oligocene−Miocene-aged granites situated in the Sanjiang Tethys and adjacent areas of western Yunnan as prospective exploration targets for tin mineralization.

Petrogenesis and Metallogenesis of Late Cretaceous Adakites in the Nuri Large Cu-W-Mo Deposit, Tibet, China: Constraints from Geochronology, Geochemistry, and Hf Isotopes

The Gangdese metallogenic belt in Tibet is an important polymetallic metallogenic belt formed during the subduction of the Neo-Tethys Ocean and subsequent India–Asia collision. Adakitic rocks are widely distributed in this belt and are considered to be closely related to porphyry–skarn Cu-Mo polymetallic mineralization. However, the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt remain controversial. In this study, we focus on the quartz diorite in the Nuri Cu-W-Mo deposit along the southern margin of the eastern Gangdese belt. LA-ICP-MS zircon U-Pb dating yields a Late Cretaceous age of 93.6 ± 0.4 Ma for the quartz diorite. Whole-rock geochemistry shows that the quartz diorite possesses typical adakitic signatures, with high SiO2, Al2O3, and Sr contents, but low Y and Yb contents. The relatively low K2O content and high MgO, Cr, and Ni contents, as well as the positive zircon εHf(t) values (+6.58 to +14.52), suggest that the adakites were derived from the partial melting of the subducted Neo-Tethys oceanic slab, with subsequent interaction with the overlying mantle wedge. The Late Cretaceous magmatic flare-up and coeval high-temperature granulite-facies metamorphism in the Gangdese belt were likely triggered by Neo-Tethys mid-ocean ridge subduction. The widespread occurrence of Late Cretaceous adakitic intrusions and associated Cu mineralization in the Nuri ore district indicate a strong tectono-magmatic-metallogenic event related to the Neo-Tethys subduction during this period. This study provides new insights into the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt, and has important implications for Cu polymetallic deposit exploration in this region.

Constraining the exhumation history of the northwestern margin of Tibet with a comparison to the adjacent Pamir

Regional variations in the evolution of the Tibetan Plateau have important implications for our understanding of crustal deformation processes. There have been few studies of the evolution of the NW margin of the plateau and its transition to the Pamir Mountains to the west. We focus on this region with a multi-technique detrital study of two sedimentary sections in the Tarim Basin. Our provenance data show that an appreciable component of the detrital material in the sedimentary sections was derived from the Songpan-Ganzi–Tianshuihai composite terrane, with some contribution from the Karakoram and/or West Qiangtang. Given the proximity of the West Kunlun terrane to the sedimentary sections under study, and its long history of exhumation, this terrane in all likelihood also contributed to the studied successions. Our thermochronological data record phases of exhumation in the hinterland in the Triassic, Early Cretaceous and Oligo-Miocene. Similar to the Pamir Mountains, the Triassic and Oligo-Miocene periods of exhumation are attributed to the Cimmerian and Himalayan orogenies, respectively. The Early Cretaceous signal may reflect the distal effects of the Lhasa–Qiangtang collision. Coevality with deformation in the Pamir Mountains suggests a coupled geodynamic system, with retro-arc deformation associated with Neotethyan subduction in the west and terrane accretion in the east. Supplementary material: Detailed analytical method, sample information, petrographic, geochronological and low-temperature thermochronological data are available at https://doi.org/10.6084/m9.figshare.c.7040686

Comment on ‘The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–18

Abstract Azizi, H., Daneshvar, N., Asahara, Y., Minami, M. & Anma, R. (2023). The generation of Eocene mafic dike swarms during the exhumation of a Core complex, Biarjmand area, NE Iran. Journal of Petrology64, 1–18 have attributed the E–W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic–Early Cretaceous age. They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this article provides to shed light on specific aspects of the Late Mesozoic–Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s publication. Specifically, the assignment of an Eocene age to the emplacement of the E–W-oriented dike swarm, even though purportedly supported by U–Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the article contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari, A., Axen, G. J., Heizler, M. T. & Hassanzadeh, J. (2017). Large-magnitude continental extension in the northeastern Iranian plateau: insight from K-feldspar 40Ar/39Ar thermochronology from the Shotor Kuh-Biarjmand metamorphic core complex. Geosphere13, 1207–1233 previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s paper. In this comment, I outline the problems with the structural and regional geology, the zircon U–Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s work.

New U-Pb zircon geochronological data for Takestan magmatic rocks (Western Alborz) and their significance for the interpretation of Paleogene magmatism in Iran

ABSTRACT The Takestan area is in the western part of the Alborz magmatic arc. Magmatic rocks of the area consist of plutonic rocks (e.g. granitoids), effusive volcanic, and pyroclastic rocks. Using U-Pb zircon LA ICP-MS dating, we conclude that major parts of Takestan plutonic rocks were emplaced at 41–39 Ma (Late Eocene, Bartonian), but a small part of these rocks have ages of ~37 Ma (Late Eocene, Priabonian). The dacitic rocks have an age of ~39 Ma (Late Eocene, Bartonian) and the rhyolitic rocks are the youngest part of the magmatic rocks of the region with ages of 37–35 Ma (Late Eocene, Priabonian). Old zircons are present in all of granitoid and volcanic samples, except for a dacitic sample. They are interpreted both as earlier components in a long-lived magma chamber and inherited zircons from older continental crust. The age of magmatic rocks in the western part of the Alborz magmatic arc decreases from east to west, but the ages of the majority of them are limited to Palaeogene. The studied rocks like other Palaeogene magmatic rocks of Iran were possibly formed in a subduction related tectonic environment. Indeed, the Palaeogene magmatism of Iran is akin to geodynamic events related to Neotethyan subduction beneath Iranian micro-continent at the southern part of Eurasia.

The onset of Neo-Tethys subduction in the Early Jurassic: evidence from the eclogites of the North Shahrekord Metamorphic Complex (Sanandaj-Sirjan Zone, W Iran)

AbstractGeodynamic models implying subduction of continental crust either consider this process happening during collision, when the continental margin of the lower plate attempts subduction, or in pre-collisional stages, when tectonic erosion of the upper plate or subduction of continental extensional allochthons drag continental crust in the subduction channel. In the Zagros orogen (W Iran), high-pressure rocks are known only from the Sanandaj-Sirjan Zone, NE of the Main Zagros Thrust. Here, eclogites of the North Shahrekord Metamorphic Complex suggest subduction of continental crust slices derived from the upper plate (Central Iran) during the onset of the Neo-Tethys subduction along the southern margin of Iran. Eclogites record a clockwise pressure-temperature-time path, with pre-eclogitic epidote-amphibolites-facies phase assemblages preserved in garnet cores, a high-pressure stage, and a subsequent retrogression at amphibolite-facies conditions. By means of forward thermodynamic modelling and 40Ar/39Ar geochronology, the peak metamorphism has been constrained at 1.9-2.1 GPa and 550-600 °C, in the 191-194 Ma time span. The following retrogression during exhumation lasted at least until 144 Ma. Our data suggest that the onset of the Neo-Tethys subduction traces back prior to 190 Ma, involving together with the Neo-Tethys oceanic lithosphere also slices of the upper plate continental crust scraped off by means of tectonic erosion processes.

Impact of Kerguelen, Marion, and Reunion plumes on the ophiolitic obduction at the northern margin of Indian plate

Geochronological data compiled for 29 major ophiolitic occurrences from the northern margin of the Indian plate precisely coincide with the three major mantle plume interactions over Indian plate (viz., Kerguelen at 130–110 Ma, Marion at 90–84 Ma, and Reunion at 65–70 Ma). We analysed i) the geological juxtapositions of these ophiolitic belts, ii) seismic tomographic imaging of the slab breakoffs, iii) equivalent deflections in the marine magnetic anomalies (MMA), iv) stratigraphic records of the Cretaceous formations and v) the paleomagnetic inclination anomalies; and propose significant influence of plume-driven forces over the subduction and obduction processes in Himalayas. The impingement of Kerguelen plume strongly influenced the ∼ 2500 km long subduction of Neo-Tethys, leading to Indus-Tsangpo-Suture-Zone (ITSZ) ophiolites with their metamorphic soles. The impact of Marion plume further led to repeated tilting of the Indian plate reinvigorating Tethyan subduction at 90 ± 5 Ma. The Reunion event at ∼ 65 Ma, led to the mega-obduction along the Tethyan margin of the Indian subcontinent. The Kerguelen plume thus played a key role in initiating the intraoceanic subductions in equatorial Neo-Tethys, and the combined slab pull and plume induced rifting separated Greater India microcontinent from the northern margin of Indian subcontinent at ∼ 120 Ma. The impact of the Reunion plume resulted in the obduction of ITSZ ophiolites at the northern margin and the Pakistan ophiolites at the northwestern margin. The mantle plume impingements caused plate tilting and triggered the subduction/obduction that is well preserved by sharp inflexions in MMA and the spreading rates. We therefore infer that the three mantle plumes have a greater role in the subduction, obduction and tilting of the Indian plate throughout the Late Mesozoic.

Intra−Neo-Tethyan subduction initiation inferred from the Indawgyi mafic rocks in the Central Ophiolite Belt, Myanmar

Geological evidence has demonstrated the presence of an intra−Neo-Tethyan subduction system during the Cretaceous. However, when and how this intra-oceanic subduction was initiated, especially for the eastern Neo-Tethys, are still not well constrained. Here we present geochemical and geochronological analyses of the Indawgyi mafic rocks from the Central Ophiolite Belt in the West Burma Block (Myanmar), which record early forearc spreading during the intra−Neo-Tethyan subduction initiation. Zircon U-Pb ages of gabbros indicate the ophiolitic crust formation at ca. 120 Ma. Gabbros show mid-oceanic-ridge basalt−like rare earth element patterns and depleted Sr-Nd-Pb isotopic compositions with negative anomalies of high field strength elements (e.g., Nb, Ta, Zr, and Hf), similar to forearc basalt characteristics. Basalts show more slab-derived component signatures than the gabbros and represent mantle wedge magmas most likely formed between forearc spreading and arc maturation. These data, together with regional geological records and geophysical observations, suggest that the Indawgyi gabbros were derived from an intra−Neo-Tethyan forearc setting during the early stage of subduction initiation. Considering the timing of supra-subduction zone ophiolites and metamorphic sole in the Indo-Burma Range, we propose that spontaneous subduction initiation and sinking of the eastern Neo-Tethyan lithosphere during the Early Cretaceous (ca. 120 Ma) led to formation of the Indawgyi forearc crust, whereas subsequent mature subduction resulted in the Middle Cretaceous (ca. 108‒90 Ma) arc magmatism in the West Burma Block. These findings confirm the double-subduction model of the Neo-Tethys Ocean and shed new light on the intra−Neo-Tethyan subduction initiation.

Mid-Cretaceous to early Eocene Neo-Tethyan subduction records in West Sulawesi, Indonesia

Situated in the heart of the Indonesian archipelago, Sulawesi records well-developed Cenozoic magmatism, yet its Cretaceous magmatic evolution remains enigmatic. Here, we report new U-Pb-Hf isotopic data of detrital zircons from West Sulawesi, Indonesia to constrain its Cretaceous to Eocene magmatic tempo. Detrital zircons aged at ca. 105−80 Ma and ca. 70−45 Ma occur as the most dominant age populations and show high positive εHf(t) values, indicating derivation from juvenile sources with limited continental crustal contamination. Our new data, combined with available results, support the existence of an Andean-type continental margin in West Sulawesi during mid-Cretaceous to early Eocene times. Importantly, the magmatic tempo of West Sulawesi is also consistent with those of southern Lhasa (Tibet) and Sumatra (Indonesia), but contrasts with those of Paleo-Pacific subduction-related arcs in SE China, SE Vietnam, East Malaysia, and NW Borneo. Therefore, we put forward that West Sulawesi may be the southeasternmost component of the Neo-Tethyan arc system that spreads over 7500 km, from southern Tibet to SE Sundaland. Such a huge arc system with concurrent magmatic flare-ups and lulls in South Asia may have played a significant role in global-scale plate reorganization.

The geochemical characteristics of Cretaceous volcanics in southern Hainan Island and implications for tectonic evolution in the South China Sea

The southeastern Eurasian plate, where the South China Sea (SCS) is located, lies in a complex tectonic setting between the Pacific and Tethys tectonic belts. It is widely accepted that the tectonics of the SCS area were influenced by subduction in the late Mesozoic, but there is still controversy over whether it was paleo-Pacific subduction or Tethyan subduction. Volcanic activity in the south of Hainan Island was intense during the Cretaceous, and geochemical analysis of the collected basaltic andesite, andesite and rhyolite samples in this study indicate those intermediate-acid series igneous rocks are high-K calc-alkaline or calc-alkaline. Some andesites have high MgO contents and Mg# values (2.04–5.34 wt% and 36.83–55.29; Mg# = 100× Mg2+/(Mg2+ + TFe2+). Light rare earth elements (LREEs) and large ion lithophile elements (LILEs) are enriched in all the samples, but high field strength elements (HFSEs) are depleted. The negative Eu anomalies are more obvious in the rhyolites than andesites. The geochemical characteristics of the volcanic arc igneous rocks show that the mid-Cretaceous tectonic setting of Hainan Island can be classified as an Andean active continental margin. During the mid-Cretaceous, intermediate volcanism occurred in Hainan Island and its adjacent areas. The zircon-saturation temperatures of the acid volcanic rocks in study area exhibit relatively low values (ranging from 746°C–790°C). Unlike igneous rocks forming in the coastal area east of the South China Block at the same time, no A-type granitoids with alkaline dark minerals appear in Hainan Island. During the late Mesozoic, the western SCS, where Hainan Island was located, may not have been affected by the subduction of the paleo-Pacific Plate, but rather Neotethyan subduction which dominated the Cretaceous magmatic and tectonic activities along the western margin of the SCS. This finding helps to understand the late Mesozoic tectonic evolution of the southeastern edge of the Eurasian plate.

The Middle Jurassic Cheshmeh-Ghasaban High-Mg gabbro (Hamedan, NW Iran): A plume-influenced intracontinental rifting event?

The Jurassic mafic to felsic magmatism affecting the older Ediacaran-to-Cambrian basement of the Sanandaj-Sirjan Zone of Iran has been traditionally interpreted as the product of arc and/or back-arc magmatism related to the early stages of Neo-Tethys subduction beneath Iran in the early Jurassic. Recent works and new compositional and geochronological data have started challenging this commonly accepted model in favor of scenarios involving continental rifting, mantle plume activity, and/or passive margin formation. In the Hamedan area of the central sector of the Sanandaj-Sirjan Zone, the Jurassic Cheshmeh-Ghasaban gabbro (ca. 165 Ma) is a key formation to better understand the tectono-magmatic framework of the whole area. Our new data, combined with the existing literature, suggest a transitional to alkaline OIB-like compositional character for this gabbro similar to the nearby but slightly younger (ca. 145 Ma) Panjeh and Ghalaylan basaltic complexes (in the Songhor-Ghorveh area). When integrated with the existing geochemical data of Jurassic mafic rocks from the central Sanandaj-Sirjan Zone, our results point to a scenario of intracontinental rifting, possibly involving the upwelling old metasomatized (by Proto-Tethys subduction?) mantle or mantle-plume activity.

From Tethyan subduction to Arabia-Eurasia continental collision: Multiple geo-thermochronological signals from granitoids in NW Iran

NW Iran, situated between the Arabian and Eurasian plates, carries a record of both Paleo-Tethyan and Neo-Tethyan tectonic evolution. During the Wilson Cycle of Tethys Ocean opening and closing, several episodes of magmatism from Late Paleozoic to latest Cenozoic generated a massive volume of intrusive rocks. These intrusives, which record cooling histories from high-temperatures to final exhumation, are ideal for multiple geo-thermochronological studies. Our new zircon and apatite U-Pb results suggest three regional magmatic events in the Late Carboniferous, mid-Cretaceous and middle Eocene, which could be related to Paleo-Tethys subduction, Neo-Tethys subduction and Neo-Tethyan ridge subduction, respectively. New apatite fission track and (U-Th)/He data reveal post-magmatic cooling and differential exhumation related to subduction and collision. By integrating published regional thermochronological data from northeastern part of the Middle East, a broader tectono-thermal framework is outlined as follows: 1) Cretaceous cooling signals are most pronounced in the Alborz and Caucasus, and reflect back-arc extension during Neo-Tethyan subduction; 2) diachronous Neo-Tethyan evolution led to the Paleocene assemblage of Anatolia in the west, and subduction associated with a late Paleocene-Eocene magmatic flare-up in Iran. Subsequent westward escape of Anatolia and extensive surface uplift were driven by propagation of Arabia-Eurasia collision in the Miocene; 3) Continuous northward indentation of Arabia into Eurasia triggered late Miocene-Pliocene fast exhumation of the Zagros, Alborz and the Caucasus.

Southward continuation and slab rollback of the Neotethyan arc−back-arc system: Insights from Eocene mafic intrusions from North Sumatra, SE Asia

The Neotethyan subduction and collision with the Asian, Indian, and Australian plates is evidenced by a giant Andean-type arc system and a remarkable Neotethyan igneous belt along the southwestern Eurasian margin. However, the southward extent, and cause, of Neotethyan subduction is poorly understood. Here, a zircon U-Pb dating and whole-rock geochemical study of two groups of Eocene (50−48 Ma) mafic intrusions from Tangse, North Sumatra, helps to unravel the Neotethyan subduction story. Group 1 consists of diabase samples, which have mid-oceanic-ridge− and arc-like geochemical affinities, with low chondrite-normalized La/Yb(cn) (∼0.6) ratios and a high measured εNd(t) value of +8.7. Such geochemical signatures indicate that the magmatic rocks were derived from a refractory depleted mantle source that was metasomatized by slab melts. Group 2 consists of gabbro-diorites, which are enriched in the large-ion lithophile elements and depleted in some high field strength elements. These samples have high La/Yb(cn) (2.9−6.6), low Nb/La(cn) (0.2−0.3), εNd(t) values of +1.1 to +5.0, and εHf(t) values of +8.8 to +15.3. Group 2 samples were produced by the melting of a depleted mantle wedge metasomatized by slab fluids. The magmatism provides evidence for the southern continuation of the Neotethyan igneous belt and tectonic system, whereby slab rollback triggered melting and upwelling in an arc−back-arc geodynamic system from South Tibet to SE Asia.

Zircon U–Pb geochronology and Hf isotopic compositions of igneous rocks from Sumatra: implications for the Cenozoic magmatic evolution of the western Sunda Arc

Abstract Sumatra is located at the western end of the Sunda Arc, which resulted from the subduction of the Indo-Australian Plate beneath the Eurasian Plate. In this study, we report detailed zircon U–Pb and Hf isotope data for Cenozoic igneous rocks from the entire island of Sumatra to better constrain the temporal and spatial distribution of arc magmatism. The new dataset, combined with literature information, identifies the following two magmatic stages: (1) Paleocene to Early Eocene (66–48 Ma) and (2) Early Miocene to Recent (23–0 Ma), with a 25 myr-long period of magmatic quiescence in between. The magmatic zircons show predominantly positive and high ε Hf ( t ) values, ranging from +19.4 to +7.1 in western Sumatra, +17.1 to +1.6 in central Sumatra and +18.0 to +7.0 in eastern Sumatra, indicating an isotopically juvenile magma source in the mantle wedge along the western Sunda Arc. We explain the negative and low ε Hf ( t ) values (+0.5 to −13.1) of young samples around the supervolcano Toba as evidence for the subduction of sediment. We argue for a change in the subduction processes, where the first magmatic stage ceased owing to the termination of the Neo-Tethyan subduction and the following stage corresponded to the modern Sunda subduction.

Palynological evidence for the Middle Jurassic environmental evolution in the eastern part of the Alborz Mountains (northern Iran)

The palynological assemblages of the Dalichai Formation were studied to reconstruct the paleoenvironmental evolution of the Middle Jurassic interval in the eastern part of the Alborz Mountains (N Iran). Dinoflagellate cyst biozones, planktonic foraminifera, and ammonites suggest a Bajocian-Callovian age for the Dalichai Formation. The palynodebris, mostly of terrestrial origin, are dominated by black opaque phytoclasts. Dinocysts, spores, pollen grains, and amorphous organic matters (AOM) are minor components in most samples. Terrestrial palynoflora are mainly represented by coastal gymnosperms (Araucariaceae, Cheirolepidiaceae), lowland simple monosulcate pollen producers (Cycadales/Bennettitales), ferns, upland seed ferns and conifers (Corystospermales and other bisaccate pollen producers). These assemblages indicate a warm climate with seasonally-dry conditions. However, an increase in Cheirolepidiaceae (Classopollis) in Bathonian assemblages reflects higher temperature during this interval. The dominance of the land-derived phytoclasts and presence of brackish water Botryococcus colonies with cells arranged irregularly suggest a proximal setting with high terrestrial and freshwater input during the Bajocian. The palynofacies analysis suggests a sea-level rise during the Bathonian, reflected by higher abundance and diversity of dinocysts as well as the presence of degraded small phytoclasts. This transgression phase is related to the increased subsidence of the Alborz region due to ongoing Neotethys subduction and creation of a large extensional back arc basin in the Eurasian margin. The syn-rift deposits are characterized by ammonitico rosso facies as well as hemipelagic carbonates containing planktonic foraminifera and pelagic bivalves.

Geochemical and geochronological constraints on the origin of the Sabzevar ophiolites (NE Iran): forced far-field subduction initiation in the upper-plate of the Neo-Tethys subduction zone

The Sabzevar ophiolites, located at the northern margin of the Central-East Iranian microcontinent (CEIM), are part of the Mesozoic-Paleogene Neotethyan suture zone developed along the Alpine-Himalayan convergence zone. These ophiolites consist mostly of oceanic lithospheric remnants, covered by early Campanian-late Maastrichtian volcano-sedimentary successions. A distinctive characteristic of the Sabzevar ophiolites is the occurrence of mafic dike swarms (gabbros, gabbronorites and diorites) with forearc-arc-tholeiitic geochemical signature, intruding the mantle section. Occurrence of orthopyroxene, development of pegmatitic texture, crystallization of clinopyroxene prior to plagioclase, and the presence of anorthite-rich plagioclase imply relatively high H2O content in the magmatic plumbing system. Rare plagiogranites (tonalite and trondhjemite compositions) show geochemical features compatible with a supra-subduction setting, whereas late (hornblende-bearing) gabbro dikes show a within-plate signature. The bimodal geochemical affinity (subduction vs. intraplate) is also attested by clinopyroxene compositions. The gabbroic, plagiogranitic and gabbronoritic samples yield Early Cretaceous SHRIMP zircon U-Pb ages of 96.7 ± 1, 98 ± 1 and 94 ± 1 Ma, respectively. A progression from tholeiitic MORB-like to more depleted high-Mg andesite and eventually alkaline affinities is here proposed, framing the magma evolution as generated in an evolving forearc setting that post-dated (of at least 9 Myr) the formation of the metamorphic sole during the infant stages of subduction of the Sabzevar Ocean.A scenario of far-field forced subduction initiation of the Sabzevar Ocean is proposed as consequence of propagation of the residual stresses transmitted from the Arabia-Eurasia convergence zone across the CEIM during Cretaceous times (Albian-Campanian).

Dynamic link between Neo-Tethyan subduction and atmospheric CO2 changes: insights from seismic tomography reconstruction

Volcanic arc degassing contributes significantly to atmospheric CO2 levels and therefore has a pivotal impact on paleoclimate changes. The Neo-Tethyan decarbonation subduction is thought to have played a major role in Cenozoic climate changes, although there are still no quantifiable restrictions. Here we build past subduction scenarios using an improved seismic tomography reconstruction method and calculate the subducted slab flux in the India–Eurasia collision region. We find remarkable synchronicity between calculated slab flux and paleoclimate parameters in the Cenozoic, indicating a causal link between these processes. The closure of the Neo-Tethyan intra-oceanic subduction resulted in more carbon-rich sediments subducting along the Eurasia margin, as well as continental arc volcanoes, which further triggered global warming up to the Early Eocene Climatic Optimum. The abrupt termination of the Neo-Tethyan subduction due to the India–Eurasia collision could be the primary tectonic cause of the ∼50–40 Ma CO2 drop. The gradual decrease in atmospheric CO2 concentration after 40 Ma may be attributed to enhance continental weathering due to the growth of the Tibetan Plateau. Our results contribute to a better understanding of the dynamic implications of Neo-Tethyan Ocean evolution and may provide new constraints for future carbon cycle models.