Oceanic Slab Research Articles

Paleozoic tectonic setting and evolution of the Central Tianshan Block: Insights from Devonian arc-related granitoids

• Distinct Devonian arc-related granitoids record three phases of magmatism . • Devonian granitoids were derived from remelting of Mesoproterozoic rocks with addition of mantle and juvenile crustal components. • The Central Tianshan Block was a continental arc during the Paleozoic bipolar subduction of the Kangguer oceanic slabs. The Central Tianshan Block of the Central Asian Orogenic Belt (CAOB) contains Precambrian metamorphic basement and voluminous Paleozoic granitoids, but its Paleozoic tectonic setting remains debated. This study presents new geochronological and geochemical data for the granitoids in the Shuangqing district in combination with published data from the Central Tianshan Block to constrain the tectonic setting and evolution of the Central Tianshan Block. Zircon U–Pb dating of the different granitoid plutons identified three phases of magmatism, a foliated granite (408.9 ± 2.6 Ma), a K-feldspar granite (397.7 ± 2.5 Ma), and a granodiorite (porphyry) (ca. 390–388 Ma). These Devonian granitoids show medium- to high-K calc-alkaline affinities, large ion lithophile elements (LILE; e.g., Rb and Ba) enrichment, and high field strength elements (HFSE; e.g., Nb, Ta, and Ti) and Eu depletion, consistent with subduction-related I-type granitoids. They have negative ε Nd (t) values (−5.2 to −3.4), old zircon T DM C ages (1623–1036 Ma), and variable zircon ε Hf (t) values (−3.54 to +5.70), indicating they were mainly sourced from remelting of Mesoproterozoic rocks with variable incorporation of mantle and juvenile crustal components in their magma sources. Combining with published data and this study, we suggest the Central Tianshan Block was a continental arc in the Ordovician to Carboniferous during the bipolar subduction of the Kangguer oceanic slabs. During the Devonian, the subduction of the Kangguer oceanic slabs formed the Central Tianshan continental arc to the south, accompanied by emplacement of multiphase arc-related granitoids in the Shuangqing district.

Petrogenesis and tectonic implications of Cenozoic mafic volcanic rocks in the Kahak area of central Urumieh–Dokhtar magmatic arc, Iran

• Mafic volcanic rocks in the Kahak area were erupted at ∼ 60–55 Ma and ∼ 21 Ma. • The Kahak basaltic rocks are divided into two groups of low and high Nb/Ta ratios. • The petrogenesis of the Kahak basaltic rocks involved slab dehydration and melting. • Slab rollback and mantle metasomatism occurred during the Paleocene–Eocene and Miocene . Cenozoic volcanic rocks are widely distributed in the Urumieh–Dokhtar magmatic arc (UDMA), Iran, but their petrogenesis and tectonic implications remain enigmatic. The volcanic rocks in the Kahak area of the central UDMA are mostly basalt and basaltic andesite. They show enrichment in light rare earth elements and large ion lithophile elements and depletion in the high-field-strength elements (Nb, Ta, and Ti), corroborating a subduction-related origin. U–Pb ages of zircon from these volcanic rocks demonstrate two episodes of magmatism during the Paleocene–Eocene (∼60–55 Ma) and Miocene (∼21.9–21.4 Ma). These rocks have 87 Sr/ 86 Sri values from 0.70485 to 0.70577, ɛ Nd (t) values from +2.0 to +5.0, and zircon Hf isotopes ε Hf (t) values from +8.3 to +10.2. These features with isotopic modeling support derivation from partial melting of an asthenospheric mantle wedge in an extensional intracontinental-arc basin. This mantle source was previously metasomatized by subducted slab components of the Neotethys Ocean. The mafic melts underwent fractional crystallization and minor crustal contamination during magma evolution. The Paleocene–Eocene basaltic rocks have subchondritic Nb/Ta ratios, indicating that the magma source was affected by fluids with low-Nb/Ta ratios, which were released during shallow metamorphism of a subducted slab. However, the Miocene basaltic rocks have unusual superchondritic Nb/Ta ratios, indicating slab melt involvement. Geochemical data and field relationship indicate that mafic volcanism in the Kahak area occurred in an extensional setting associated with the Paleogene shallowing of the northeastward-subducting Neotethyan oceanic lithosphere followed by Late Paleogene and Neogene slab rollback and eventual partial melting of a deeply subducted oceanic slab.

Mesozoic tectonic transition from the Tethyan to Paleo-Pacific domains in South China: New insights from Mesozoic igneous rocks in the western Nanling Range

• The studied Mesozoic igneous in the Nanling Rang include post-orogenic ultramafic–mafic rock, subduction-related mafic rocks and other calc-alkaline intermediate-felsic rocks. • Those igneous rock assemblages in the Nanling Rang reflect the tectonic transition of the SCB. • The Nanling Rang was a Tethyan domain before ca. 170 Ma but a Paleo-Pacific domain after that. • The Mesozoic evolution of the SCB involved four stages: post-orogenic extension, slab foundering, and tectonic transition from subduction to rollback, followed by slab rollback. Mesozoic tectono-magmatic evolution of the Nanling Range in the South China Block (SCB) remains controversial, especially about the role of the Tethyan and Paleo-Pacific dynamics in the generation of Mesozoic igneous rocks. The Mesozoic magmatic events have produced a diverse variety of rocks, dominantly granites and spatially associated mafic–ultramafic rocks, lamprophyres and diorites. We present new zircon U-Pb ages and in-situ Hf isotope data, along with whole-rock elemental and Sr-Nd isotopic data for the Mesozoic igneous rocks from the western Nanling Range. The two ca. 200 Ma ultramafic–mafic rocks have low SiO 2 contents (40.00–44.94 wt%) and typical ocean island basalt (OIB)-like trace elemental affinities with positive Nb and Ta anomalies, positive ε Nd (t) (4.47 to 4.88) and variable ε Hf (t) values (–3.65 to 11.91), suggesting that they were generated by partial melting of an asthenospheric mantle source. The ca. 165 Ma Jiangyong lamprophyre dyke has relatively high SiO 2 content (51.70 wt%) and arc-like trace elemental affinities with significantly negative Nb and Ta anomalies and variable ε Hf (t) values (–9.10 to 2.65). They were derived from a lithospheric mantle source previously modified by subducted component, along with a contribution from the asthenospheric mantle. Granites from the ca. 146–137 Ma Taibao granitic pluton have high SiO 2 contents (63.41–66.66 wt%), variable (La/Yb) N and Sr/Y values, and negative to positive ε Hf (t) values (−4.16 to 0.86). These granites have compositions transitional from adakite to arc-type rocks and were likely generated by partial melting of Mesoproterozoic meta -igneous rocks, with an addition of a mantle-derived component. Compared with those granites, diorites of the ca. 99 Ma Eryue pluton exhibit moderate SiO 2 contents (59.90–60.30 wt%), a wide range of zircon ε Hf (t) values (−12.04 to 0.50) and negative ε Nd (t) values (–8.61 to –8.57), and they were probably formed by magma mixing of crust-mantle materials associated with back-arc extension. Our new data, together with other geological evidence, provide further insights for the evolution of the Tethyan and Paleo-Pacific tectonic domains during the Late Triassic to Late Cretaceous in the SCB. In the early stage, the Tethyan tectonic domain, ultramafic–mafic rocks from the Ludong and Fuchuan plutons in the Nanling Range was related to the post-orogenic extension. This domain was gradually changed to the Paleo-Pacific domain during the period from Late Triassic to Early Jurassic and the tectonic transition was accomplished at ∼ 170 ± 5 Ma. In the later stage, the Paleo-Pacific tectonic domain, the Jiangyong lamprophyre, Taibao granites and Eryue diorites were produced via the crustal thickening, localized extension, and large-scale lithospheric extension due to the advancing, slab-foundering, and rollback of the oceanic slab during the period from Middle Jurassic to Late Cretaceous.

Modern-style plate tectonics manifested by the late Neoarchean TTG-sanukitoid suite from the Datong-Huai'an Complex, Trans-North China Orogen

When modern-style plate tectonics started and operated on a specific old craton has been a key point involving the early Earth's evolution. In order to address this issue on the evolution of the Neoarchean North China Craton (NCC), we investigated a representative successive magmatic rock suite of tonalite-trondhjemite-granodiorite (TTG) and sanukitoid in the Datong-Huai'an Complex. Geochemically, TTG gneisses in the Datong-Huai'an Complex can be divided into high-pressure (HP) and low-pressure (LP) TTG rocks. The HP TTGs emplaced at ∼2538–2497 Ma and 2441 Ma, respectively. Geochemical data shows that they are characterized by steep rare earth element (REE) patterns, obvious negative Nb, Ta, Ti anomalies and positive δEu anomalies, high Sr/Y, Nb/Ta and (La/Yb) N ratios, and positive ɛ Hf (t) (+2.1 to +9.1) and δ 18 O values (avg. 5.5‰ to 6.0‰). Their protolith is interpreted as the consequence of partial melting of a subducted oceanic slab with garnet and minor rutile as residual phases. In contrast, the LP TTGs emplaced at ∼2518 Ma. They display flat REE patterns, slightly negative Nb, Ta, Ti anomalies and negative δEu anomalies, low Sr/Y, Nb/Ta and (La/Yb) N ratios, positive ɛ Hf (t) (+3.4 to +5.9) and high δ 18 O values (avg. 5.9‰ to 6.1‰). They were most likely generated by partial melting of the mafic lower crust with residual plagioclase and amphibolite in the source region. The sanukitoid rocks emplaced at 2517–2485 Ma and show high MgO, Cr and Ni concentrations but relatively low (La/Yb) N values, positive ɛ Hf ( t ) values (+1.5 to +5.4), and higher δ 18 O values (avg. 6.8‰ to 7.5‰), suggesting that they were originated from partial melting of the mantle peridotite that was previously modified by slab-derived or sediment-derived melts. Combined with previous petrological, geochronological and geochemical data, we propose that the Datong-Huai'an Complex recorded the prolonged magmatism through ca. 2.56 Ga to 2.44 Ga, which was characterized by the high-angle subduction of oceanic lithosphere, leading to the opening of a mantle wedge, slab rollback and back-arc extension. All these lines of evidence manifested that a modern-style plate tectonics regime started along the western margin of the NCC Eastern Block during the late Neoarchean, though the interior Eastern Block was still dominated by the mantle plume-induced crustal growth and vertical tectonism. • Coeval TTGs and sanukitoids were recognized in the Datong-Huai’an Complex. • They recorded the prolonged high-angle subduction with rollback and back-arc extension. • Modern-style plate tectonics started along the margin of NCC Eastern Block in late Neoarchean.

Generation of magmatism under active continental margins: A thermodynamic study of subduction and translithospheric diapirs

A recent model of continental arc magmatism states that subducted sediments mix physically with the oceanic slab basalts in the subduction channel and produce diapirs that ascend through the mantle wedge, undergo melting and relaminate to the base of the continental crust. From here, melt or magma batches ascend through the crust, producing the Cordilleran batholiths and associated volcanism ranging in composition from 0 to 25 wt% maficity (FeO t + MgO) and 44–79 wt% SiO 2 , with a gap around 11.5 wt% maficity and 57–60 wt% SiO 2 . This model has been predicted by thermomechanical numerical studies and later supported by phase equilibrium experiments; however, thermodynamic modelling to verify whether the composition of the Cordilleran batholiths is reproduced by this model has not yet been carried out to complement the experimental approach. In this article, the evolution of the mélange along the subduction path and during the ascent of the diapirs is investigated, with focus on the conditions of generation of the diapirs, the composition and proportion of melt at the P-T conditions of relamination of the diapirs, the relationship between the basalt:sediment proportion (the composition) of the diapir to this melt composition and proportion and whether the composition of the melts match the Cordilleran batholiths. The key point to test is the process or processes that produce the compositional variability observed in these granitoids. Our findings indicate that (1) parental melts and/or magmas range in maficity from ∼1 to ∼11.5 wt%, with the compositional gap of the Cordilleran trend representing the most mafic composition possible of the parent, (2) the diapirs are restricted to having 50–60% of sediment component: diapirs with lower sediment contents may be too dense to detach from the slab and mélanges with higher sediment contents may be too less dense to be subducted to the depth of formation of diapirs, (3) melts at 1100 °C and 1.5 GPa from these diapirs have the bulk granodioritic composition of the batholiths, (4) restite unmixing occurs when magma batches segregate from the relaminated diapirs, (5) the lower the temperature of the region of the diapir from which the batch segregates, the higher the restite content, (6) the temperature, by controlling the restite content, is the ultimate factor controlling the compositional trends of Cordilleran batholiths, and (7) fractional crystallization or cotectic evolution is still needed to account for the full compositional range of these batholiths, from the parental 1–11.5 wt% to the observed 0–25 wt% in maficity. • Cordilleran granitoids can be formed by melting of subducted basalt and sediment. • Parental magmas of Cordilleran granitoids derive from diapirs relaminated to the crust. • The main process controlling the composition of granitoids is restite entrainment. • The higher the temperature of the diapir, the lower the amount of restites entrained. • Composition of diapirs is restricted to having 50–60% sediment component.

Early Jurassic high-temperature granites along eastern Bangong-Nujiang suture in Tibet and its geodynamic implications

The varying geochemical compositions of granites are generally considered as the proxy record of different geodynamic processes. The granites collected in the Kaqiong microblock along the eastern Bangong-Nujiang suture in Tibet show A-type granite affinity, but do not strictly follow the definition of A-type granite in some aspects. Zircon U Pb dating shows that the Kaqiong granite pluton was emplaced in the Early Jurassic (186–176 Ma), bracketing in the timing of the lithospheric and asthenospheric mantle-derived magmatism (197–175 Ma) in the Kaqiong microblock. These granites are metaluminous to peraluminous and high-K calc-alkaline to shoshonitic rocks. They have high contents of alkalis (K 2 O + Na 2 O), high field strength elements (e.g., Zr, Nb), and rare earth elements (except Eu), with high magmatic temperatures; these features are typical of A-type granite. However, these granites show slightly lower 10,000 × Ga/Al ratios of 2.3–3.0 and lower FeO tot /(FeO tot + MgO) values of 0.58–0.68 than the typical A-type granite. Their whole-rock Sr Nd isotopic compositions ( I Sr = 0.72996–0.73481, ε Nd (t) = −12.4 to −10.8, T DM C = ~1.9 Ga) and zircon Hf isotopic compositions (ε Hf (t) = −18.9 to −6.1, T DM2 = ~2.0 Ga) indicate that they mainly originated from pre-existing crustal materials. We suggest that the Kaqiong granites were produced by protracted partial melting of the basement rocks at relatively high temperature and high oxygen fugacity conditions. The Early Jurassic high-temperature granites were formed due to a significant mantle heat addition into the crust in an extensional continental arc, caused by the rollback of the subducted Bangong-Nujiang Tethyan oceanic slab. The study is an example of how high-temperature granites could shed light on the geodynamic processes. • Geochronology, geochemistry, and Sr-Nd-Hf isotope compositions of the Kaqiong granites • The Kaqiong granites are the high-temperature granites. • The Kaqiong granites formed in an extensional continental arc • The type of granites provides insight into the geodynamic processes of a subducting slab.

Petrogenesis of Paleozoic high Ba Sr granitoids from the Qinling orogen, Central China: Implications to the formation of Archean sanukitoids and TTGs

Modern analogues of Archean granitoids are particularly important, for they offer an opportunity to investigate processes of early crustal formation and tectonic transition. High Ba Sr granitoids are usually considered as the Phanerozoic equivalents of Archean sanukitoids, however, their petrogenesis is still equivocal. Here, we collected a suite of high Ba Sr granitoids from the Banshanping pluton in the Qinling orogen, central China, and carried out an elaborated study including mineral chemistry, whole rock elemental and Sr Nd isotopic compositions, as well as zircon U Pb, Lu Hf and O isotopes, to constrain its petrogenesis. Our new secondary ion mass spectroscopy (SIMS) zircon U Pb geochronology suggests the emplacement age of the Banshanping pluton at 435 ± 2 Ma (MSWD = 0.37). The rocks have variable SiO 2 , elevated Mg# (50–53), Cr, and Ni concentrations, as well as strong enrichment of Ba and Sr (Ba + Sr = 1310–3040 ppm), but depletion of high field strength elements (HFSE). Some trace element ratios are positively (e.g., La/Yb, Sr/Y) or negatively (e.g., Dy/Yb) correlated with their SiO 2 concentrations, indicating that amphibole fractionation might play a crucial role in their formation. They also exhibit restricted and relatively high initial 87 Sr/ 86 Sr (0.70437–0.70515) and chondritic to slightly positive ε Nd (t) values (0.29–1.32), but high zircon ε Hf (t) (8.0–9.9) and normal mantle zircon δ 18 O (5.45 ± 0.13 ‰) values. Isotopic modeling constrains incorporation of about 4% oceanic crust-derived fluid and 0.3% sediment melt into their mantle source. Combined with regional geological evolution, the Banshanping pluton is inferred to originate from a two-stage process: its mantle source was metasomatized by the subducted proto-Tethyan oceanic slab during the early stage and partial melting of the metasomatites generated the high Ba Sr granitoids induced by slab roll-back. The high Ba Sr granitoids geochemical signatures overlap with Archean sanukitoids in many aspects, thus offering essential constraints on their formation. Compared with Archean sanukitoids, our results suggest that the sanukitoids derived from fluid-dominated metasomatic mantle might have the potential to evolve into TTG through amphibole-dominated fractional crystallization. • The Banshanping pluton represents a suite of high Ba-Sr rocks geochemically similar to Archean sanukitoids. • They were produced by partial melting of a fluid metasomatic mantle and undergo amphibole-dominated fractional crystallization. • Fluid-dominated metasomatic mantle-derived Archean sanukitoids may have the potential to evolve into TTGs.

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

Molybdenum isotopes in mafic igneous rocks record slabmantle interactions from subarc to postarc depths

Abstract Arc volcanic rocks erupted above oceanic subduction zones generally have heavy molybdenum (Mo) isotopic compositions, which are attributed to contributions from oceanic slab–derived fluids with isotopically heavy Mo at subarc depths. Given that mafic igneous rocks with light Mo isotopic compositions have been rarely identified, it is unclear where light Mo isotope reservoirs reside, as these must exist due to oceanic subduction through geologic time. We present Mo isotope data from Mesozoic–Cenozoic mafic igneous rocks from the eastern North China craton, which were affected by continuous subduction of the oceanic slab. The Mesozoic mafic igneous rocks have island-arc basalt (IAB)–like features and high δ98Mo values of –0.15‰ to +0.09‰. In contrast, the Cenozoic mafic igneous rocks have ocean-island basalt (OIB)–like features and low δ98Mo values of –0.53‰ to –0.19‰. In addition, these two suites of rocks exhibit systematic differences in Sr-Nd isotopic compositions and fluid- and melt-mobile element contents, indicating that their mantle sources contained crustal components derived from different depths. The heavy Mo and weakly enriched Sr-Nd isotopic compositions and IAB-like trace-element signatures were inherited from slab fluids (including oceanic crust–derived aqueous solutions and seafloor sediment–derived hydrous melts) liberated at subarc depths. The light Mo and depleted Sr-Nd isotopic compositions and OIB-like trace-element signatures were inherited from dehydrated slab–derived melts (with rutile breakdown) at postarc depths. Therefore, Mo isotope systematics of mafic igneous rocks are a powerful means with which to trace the geochemical fluxes from subducting oceanic slabs to the mantle wedge at different depths.

Fluid flowrates and compositions and water–rock interaction in the Hikurangi margin forearc, New Zealand

The decrease in interseismic coupling and slip rate deficit from south to north, from locked to slipping, of the Hikurangi margin is attributed from interpretation of geophysical data to more fluid-saturated sediments in the north. To assess this interpretation, fluid flowrates were measured in springs of the subaerial forearc, structures correlated with flowrates and the fluids geochemically and isotopically evaluated to understand their origins and source depths. Extrapolations from on-ground measurements show that ∼95% of aqueous fluids and ∼98% of gas are expelled to the surface in the slipping zone, within the northern subaerial forearc. The relative fluid residence time, from the equilibration of CH4 and CO2 and measured ascent rates, is 3x shorter in the slipping zone suggesting less constricted migration pathways than in the locked zone. Fluids are expelled not only from the sediments that produce Na–Cl fluids and thermogenic gas, but also from the oceanic igneous mafic slab that releases high-Cl fluids and abiotic CH4 from serpentinization, giving rise to Na–Ca–Cl fluids. Chloride-rich surface emissions consist of ∼70% by volume Na–Cl and ∼30% Na–Ca–Cl fluids. Higher rates of fluid expulsion in the north coincide with higher permeability and greater production of aqueous fluids from clay dehydration, serpentinization and a higher degree of diagenesis in sedimentary rocks. Mantle gas, subduction interface fluids and porewaters have the most direct access to the surface in the slipping Center, where the degree of water–rock interaction and homogeneity of water compositions suggest more recent fluid generation than other sectors in the subaerial forearc. Porewaters and thermogenic gas equilibrate at 110° + 20 °C with a slight increase in temperature from south to north. Overall, ∼120 Tg/a water is subducted in the Hikurangi margin and <0.5 Tg/a CH4 is being emitted to the atmosphere. This study shows the added impact of waters of serpentinization on the overall fluid compositions, aqueous fluid subduction budget, extent of fluid distribution in the oceanic slab and, indirectly, the temperature estimate of 185° + 25 °C for fluids ascending from the subduction interface in the Hikurangi margin.

Forced Cenozoic continental subduction of Tarim craton-like lithosphere below the Tianshan revealed by ambient noise tomography

AbstractThe possibility that craton-like lithosphere may undergo subduction during convergence of major tectonic plates is still poorly investigated. We addressed this issue using ambient noise tomography of the Tarim Basin and the Tianshan (Central Asia). Our S-wave velocity model reveals a flat-lying high-velocity anomaly beneath the Tarim Basin in the 45–60 km depth range, consistent with intrusion of mafic rocks at the base of the lower crust above a depleted lithospheric mantle, as expected after interaction of the lithosphere with a mantle plume. This high-velocity anomaly can be followed northward, steeply dipping (∼45°) beneath the Tianshan, which indicates that the Tarim craton-like lithosphere was subducted to mantle depths. It is connected with a fast P-wavespeed anomaly in the upper mantle, interpreted as a relict of the South Tianshan Ocean. A long period of tectonic quiescence, after the closure of the South Tianshan Ocean and before the Cenozoic tectonic rejuvenation of the Tianshan, suggests a minor role of oceanic slab pull in controlling continental subduction. The major player is instead the northward push of India within the framework of Cenozoic India-Asia convergence. We conclude that forced subduction can be experienced not only by thinned continental crust but also by a strong craton-like lithosphere.

Recycling of carbonates into the deep mantle beneath central Balkan Peninsula: Mg-Zn isotope evidence

Magnesium (Mg) and zinc (Zn) isotopes have been applied to trace whether surface carbonates have been recycled into the deep mantle beneath Balkan Peninsula, where a stagnant slab exists in the mantle transition zone. Here we investigate a suite of Cenozoic sodic alkaline basaltic rocks (Na2O/K2O > 1), called the East Serbian Mafic Alkaline Rocks (ESMAR) from East Serbia in central Balkan Peninsula, which is an important segment of the Tethyan orogenic belt. The ESMAR have lower δ26MgDSM-3 (−0.49‰ to −0.39‰; n = 21) and higher δ66ZnJMC 3-0749L (0.35‰ to 0.45‰; n = 21) than those of normal mantle-derived melts (e.g., mid-oceanic ridge basalts; MORBs; δ26Mg = −0.25 ± 0.06‰ and δ66Zn = 0.27 ± 0.06‰). After excluding the effects of low-T and magmatic processes, the Mg and Zn isotopic anomalies of the ESMAR are proposed to have been caused by recycled carbonates in their mantle sources. The significantly low Zn contents, Zn/Fe and Fe/Mn ratios of the ESMAR indicate a highly depleted peridotitic mantle source. Their major element compositions (e.g., low Ti2O and high CaO/Al2O3) are consistent with those of experimental partial melts of “peridotite + CO2”. The incremental batch melting and end-member mixing models suggest that ∼5–10% of Mg-rich carbonates (dolomite and magnesite) have been recycled into the mantle sources of the ESMAR. Our results provide evidence for deep recycling of carbonates related to subduction of a Tethyan oceanic slab, most likely into the mantle transition zone beneath the Balkan Peninsula. This contributes a significant part of global deep carbon cycling during the Cenozoic.

Adakite genesis and plate convergent process: Constraints from whole rock and mineral chemistry, Sr, Nd, Pb isotopic compositions and U-Pb ages of the Lakhshak magmatic suite, East Iran

The genesis of Eocene to Early Oligocene plutons and dykes emplaced in the suture zone between two colliding continental blocks in southeastern Iran has been the subject of debate concerning the processes that formed magmas and influenced their intrusive history. We report geological, geochemical, mineralogical and isotope evidence that constrains the origin of the Lakhshak plutonic complex. SiO 2 contents in the granodioritic pluton and dacite dykes average 67 wt%, and in the monzodiorite dykes 53 wt%. Based on (La/Yb) N vs. Yb N the rocks correspond to adakites, originating from 10 to 25% melting of garnet amphibolite. The high A1 2 O 3 (14.95–17.15 wt%), Sr (442–1385 ppm) and Ba (601–1935 ppm) contents and relatively high La/Yb (25–45) and Sr/Y (35–57), but low Yb (mostly <1.6 ppm) and Y (mostly <18.2 ppm) are also features typical of adakitic rocks. Zircon and titanite from two samples of the pluton yield U-Pb ages of 29.98 ± 0.08 and 29.90 ± 0.05 Ma, a monzodiorite dyke yields 29.02 ± 0.10 Ma, and a dacite dyke 29.39 to 27.96 Ma. These ages confirm that the magmatism was post-collisional. The monzodiorite and dacite dykes have ( 87 Sr/ 86 Sr) i = 0.7053–0.7056 and εNd i = −1.1 to 2.2 (except one sample with εNd i = −8.6), which limit the extent of crustal assimilation and plot along a mixing array between Sistan ophiolites and the country rocks of the pluton. This is consistent with an origin from oceanic slab melts variously mixed with melts from flysch sediments. Isotope compositions of Sr, Nd and Pb, xenocrystic zircons, and magma mixing and mingling textures support such a process. Thermobarometry on various minerals indicates a continuum of crystallization stages during magma ascent, with pyroxene formed at 8 kbar, followed by amphiboles, which record conditions decreasing from 1166 °C / 7.8 kbar to 933 °C / 4.0 kbar. Tectonically, the Lakhshak adakites were most likely the consequence of post-collisional delamination that led to melting of Sistan oceanic lithosphere and its interaction with melts from pelagic sediments. • Lakhshak suite includes a granodiorite pluton and dacitic and monzodioritic dykes. • Intruded at 30 to 28 Ma after closing of Sistan Ocean and continental collision • It corresponds to adakites, originating from 10 to 25% melting of garnet amphibolite. • Caused by delamination, melting of oceanic lithosphere and some melted sediments

New geochronological and geochemical data for the Eocene shoshonitic trachyandesites in NW Iran: Constraints on their petrogenesis and tectonic setting

The Eocene magmatic flare-up in NW Iran provides valuable information on the tectonic evolution of the region. This study presents detailed geochronological and geochemical data from Eocene trachyandesitic rocks from the Ahar region of NW Iran. 40 Ar/ 39 Ar ages indicate that the Ahar trachyandesites were produced during the Eocene (44 Ma). The Ahar trachyandesites have a limited range of SiO 2 contents (53.4–54.4 wt%) and low MgO contents (1.57–2.78 wt%), and they belong to the shoshonitic series with high Na 2 O + K 2 O contents (9.1–9.8 wt%) and K 2 O/Na 2 O ratios (0.4–1). The Ahar trachyandesites are characterized by arc-like trace element patterns, including enrichment in large-ion lithophile and light rare earth elements and depletion in high field strength elements. They have depleted 87 Sr/ 86 Sr (i) ratios (0.704616–0.704736) and εNd (t) (+2.0 to +3.5) and εHf (t) (+6.6 to +6.8) values and high Th/La and Ba/La ratios, similar to those of previously studied Eocene K-rich mafic–intermediate rocks in NW Iran. The geochemical characteristics of the Ahar trachyandesites suggest that they originated from an enriched, fertile mantle source that was produced by the reaction between mantle wedge peridotite and oceanic slab-derived components (sediment-derived melts and slab-derived fluids) during the subduction of the Neo-Tethys Oceanic slab along the Zagros segment. Taking the regional tectonic evolution into consideration, we propose that NW Iran was in an extensional setting caused by the rollback of the Neo-Tethys Oceanic slab during the Eocene. Partial melting of the enriched, fertile mantle source would have produced the Ahar trachyandesites and other contemporaneous K-rich mafic–intermediate rocks in NW Iran. Accordingly, the Ahar trachyandesites and contemporaneous K-rich mafic–intermediate rocks provide a geochemical record of interaction between the crust and mantle in an oceanic subduction channel. • The Ahar shoshonitic trachyandesites were emplaced during the Eocene (44 Ma). • The mantle source of Ahar trachyandesites was enriched by slab-derived components. • The extensional setting for Eocene magmatism was triggered by the slab rollback.

Fingerprinting secondary mantle plumes

Many vertical seismic velocity anomalies observed below different parts of the Eurasian plate are rooted in the transition zone between the upper and lower mantle (410–660 km), forming so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical numerical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth's upper mantle. Our objective is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the continental lithosphere, as often detected in seismo-tomographic studies. To this end, we examine the effect of the following parameters: (1) the compositional deficit of the plume density due to the presence of water and hydrous silicate melts, (2) the width of the weak zone in the overlying lithosphere formed because of plume-induced magmatic weakening and/or previous tectonic events, and (3) a tectonic regime varied from neutral to extensional. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to pass upwards through the lithospheric mantle to shallow depths near the Moho when (1) the compositional density contrast is ≥ 100 kg m−3 and (2) the width of the lithospheric weakness zone above the plume is ≥ 100 km. An extensional tectonic regime facilitates plume penetration into the lithosphere but is not mandatory. Our findings can explain observations that have long remained enigmatic, such as the “arrow”-shaped zone of low seismic velocities below the Tengchong volcano in south-western China and the columnar (“finger”-shaped) anomaly within the lithospheric mantle discovered more than two decades ago beneath the Eifel volcanic fields in north-western Germany. It appears that a chemical component is a characteristic feature not only of conventional hydrous plumes located over presently downgoing oceanic slabs, but also of upper mantle plumes in other tectonic settings.

Mantle plume-subducted oceanic slab interaction contributes to geochemical heterogeneity of the Emeishan large igneous province

Mantle-derived rocks in large igneous provinces yield large variations in their chemical and isotopic compositions as well as redox conditions; however, the origin of these heterogeneities remains unclear. We present new geochemical and Sr–Nd–Hf–O isotopic data for the Hongge mafic–ultramafic intrusion and associated syenite in the Emeishan large igneous province (ELIP), and combine them with previously published data from coeval intrusive and volcanic rocks in the ELIP, to constrain the generation of the ELIP and associated world-class Fe–Ti–V oxide deposits. The rocks from the marginal zone of the Hongge intrusion have high-Ti basaltic-like compositions with high initial 87Sr/86Sr ratios (0.7084–0.7096) and unradiogenic εNd (−7.7 to −5.9) and εHf (−6.0 to −2.4) values, and represent the most enriched endmember of the high-Ti basaltic series observed in the ELIP to date. The marginal zone rocks, temporally and spatially associated basalts, and syenites yield depleted Zr and Hf contents with Zr/Sm (7.1–38.3) and Hf/Nd (0.05–0.12) ratios between those of marine sediment and OIB. The NdHf isotopic compositions of the marginal zone rocks plot above the terrestrial array, suggesting decoupling between Nd and Hf isotopic compositions. Mixing between subducted oceanic crusts, marine sediments and mantle peridotite accounts for the enriched SrNd isotopic compositions, ZrHf depletion, and the decoupling between Nd and Hf isotopic compositions in these samples as well as in other mafic–felsic intrusions of the ELIP. Zircon grains from the syenites yield more radiogenic εHf values (+3.7 − +6.6) than the marginal zone rocks and have δ18O values of 3.9–5.7‰, most of which are much lower than mantle-zircon values, suggesting that oceanic crust altered at high temperature was involved in the mantle source. The integrated Sr–Nd–Hf–O isotopic compositions of these mafic–felsic intrusions can be explained by a model for the generation of the ELIP whereby a mantle plume interacted with the subducted Paleo-Tethyan oceanic lithospheric slab and overlying marine sediment. Our results suggest that the inclusion of the recycled altered oceanic slab in the mantle source of the ELIP led to its geochemical and, potentially, redox heterogeneity.

Dynamics of oceanic slab tearing during transform-fault oblique subduction: Insights from 3D numerical modeling

Dynamics of oceanic slab tearing during transform-fault oblique subduction: Insights from 3D numerical modeling

Global Constraints on Intermediate‐Depth Intraslab Stresses From Slab Geometries and Mechanisms of Double Seismic Zone Earthquakes

AbstractDouble seismic zones (DSZs), parallel planes of intermediate‐depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may be a ubiquitous feature of downgoing oceanic plates. Focal mechanism observations from DSZ earthquakes sample the intraslab stress field at two distinct depth levels within the downgoing lithosphere. A pattern of downdip compressive over downdip extensive events was early on interpreted to indicate an unbending‐dominated intraslab stress field. In the present study, we show that the intraslab stress field in the depth range of DSZs is much more variable than previously thought. Compiling DSZ locations and mechanisms from literature, we observe that the “classical” pattern of compressive over extensive events is only observed at about half of the DSZ locations around the globe. The occurrence of extensional mechanisms across both planes accounts for most other regions. To obtain an independent estimate of the bending state of slabs at intermediate depths, we compute (un)bending estimates from slab geometries taken from the slab2 compilation of slab surface depths. We find no clear global prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab (un)bending. Focal mechanism observations are frequently inconsistent with (un)bending estimates from slab geometries, which may imply that bending stresses are not always prevalent, and that other stress types such as in‐plane tension due to slab pull or shallow compression due to friction along the plate interface may also play an important role.

Early Paleozoic Continental Arc Mafic Magmatism in the North Qaidam Tectonic Belt: Implications for Subduction of the Proto-Tethyan Oceanic Lithosphere

Abstract The North Qaidam tectonic belt is characterized by the development of high-pressure to ultrahigh-pressure eclogite formed during deep subduction of the continental lithosphere. However, the tectonic processes that occurred prior to continental collision/subduction are relatively poorly studied and this leads to controversy over the evolutionary history of the North Qaidam tectonic belt. In this contribution, we present an integrated study of field observations, petrography, geochronology, and geochemistry (whole-rock major elements, trace elements, and Sr–Nd isotopes as well as zircon Lu–Hf isotopes) of continental arc mafic rocks in the North Wulan metamorphic complex to track Proto-Tethyan oceanic subduction and the nature of metasomatism of the mantle wedge. Zircon U–Pb geochronology demonstrates that continental arc mafic rocks crystallized at ca. 483–472 Ma. Mafic intrusions are enriched in light rare earth elements and large ion lithophile elements but are depleted in high field strength elements; these are typical features of arc-related magmatic rocks. They have relatively radiogenic Sr–Nd–Hf isotope compositions, with relatively high initial 87Sr/86Sr ratios of 0.710363 to 0.719404, low εNdt values of -7.77 to -2.30, variable zircon εHft values ranging from -8 to +2.8, and the single-stage Hf model ages of ca. 1.4–1.0 Ga. These features suggest that the mafic intrusions were sourced from ancient subcontinental mantle that was modified by subducted oceanic slab-derived components. The modified mantle source was generated by the interaction between subcontinental lithospheric mantle peridotite in the overlying mantle wedge and hydrous fluid and felsic melt that were derived from a subducted oceanic slab and seafloor sediments with ancient terrestrial origin in the rutile stability field. Crust-mantle interaction transferred the subducted crustal geochemical signatures to the mantle source during subduction of the Proto-Tethyan oceanic lithosphere. Retreat and rollback of the Proto-Tethyan oceanic slab triggered asthenosphere upwelling and the partial melting of metasomatized and enriched fertile lithospheric mantle to form continental arc mafic magmas in the North Qaidam tectonic belt. Combined with the coeval LP-HT metamorphism, the North Wulan metamorphic complex records the early Paleozoic tectonic evolution of a continental arc-back-arc system, and these continental arc mafic rocks record the subduction of the Proto-Tethyan oceanic lithosphere.

Slab remnants beneath the Myanmar terrane evidencing double subduction of the Neo-Tethyan Ocean.

Closure of the Neo-Tethyan Ocean is one of the most significant tectonic events of the Cenozoic, forming the longest continental collision belt on Earth and influencing global climate and biodiversity. However, whether late Mesozoic subduction of the Neo-Tethyan Ocean occurred along one single or a double subduction system remains controversial. Here, upper mantle imaging from seismic tomography and waveform modeling in the Myanmar region reveals two prominent, parallel, slab-like structures with high seismic velocities that trend to the north-south and dip to the east. The western high-velocity zone has been observed previously and represents the modern subducting slab. The eastern zone has not been previously reported and exhibits high-velocity anomalies of 1.0 to 2.5% to a depth of ~300 km. This zone likely represents a remnant of another Neo-Tethyan oceanic slab that subducted ~40 million years ago. Double subduction of the Neo-Tethyan Ocean during the late Mesozoic to early Cenozoic requires reevaluation of previous tectonic models.