Oceanic Subduction Research Articles

Subduction‐related mafic to felsic magmatism in the Xiangpishan concentric calc‐alkaline complex, Northeast Tibetan Plateau

The Xiangpishan complex in the Zongwulong‐Qinghainanshan Tectonic Belt, Northeast (NE) Tibetan Plateau, is a composite concentric pluton consisting of a felsic core (granodiorite) surrounded by quartz diorite in the middle to diorite and minor gabbro at the margin with locally less volume of monzogranite. Many dioritic enclaves are unevenly distributed within host granitoids. We evaluate the petrogenesis of the complex by geochemical, geochronological, and Hf isotopic data in tandem with regional data in the belt. The results show that felsic rocks were emplaced during the Late Permian (ca. 262–256 Ma); whilst gabbros yielded a younger age of ca. 249 Ma. As the hybrid phase from mixing between felsic and mafic magmas, enclaves and diorites have analogous ages (ca. 257–254 Ma) to both. Zircons from gabbro and enclaves are marked by higher εHf(t) value up to +1.82; whilst granodiorites have lower εHf(t) value of −5.48, consistent with hybrid diorites possessing intermediate εHf(t) values of −3.14 to 0.34. Furthermore, the quantitative calculation from Mass Balances Modelling suggests that the mass of mafic magma (ca. 67%–79%) is involved to achieve the hybridization. Geochemically, these rocks crystallized from calc‐alkaline magma with different sources, but demonstrated consistent arc‐like signatures, as they are enriched in large‐ion lithophile elements and light rare earth element (LREE), and depleted in high‐ field‐strength elements and heavy rare earth element (HREE). Besides, they present negative Nb–Ta anomalies together with significant P and Ti troughs. Finally, an evolutional model has been proposed that the asthenosphere‐lithosphere interaction played an important role during the emplacement of the complex, where the limited volumes of mantle‐derived melt act as the suppliers of heat and mass (mainly volatile components) to induce partial melting of the juvenile mafic lower crust and mixed (or mingled) with the produced crust‐derived magma during the oceanic subduction, which led to the generation of diorites as well as mafic microgranular enclaves.

Origin and Geodynamic Mechanism of the Tibetan Demingding Porphyry Mo (Cu) Deposit from Oceanic Subduction to Continental Collision

Demingding is a promising porphyry Mo-dominated deposit recently discovered in the eastern Gangdese metallogenic belt in Tibet, China. We present zircon U-Pb-Lu-Hf isotopic studies, as well as geochemical data of the late monzogranites and the prior rhyolites from the Demingding porphyry deposit to uncover their origin and geodynamic mechanism. Zircon U–Pb dating yielded precise crystallization ages of 17.3 ± 0.6 Ma (MSWD = 2.5) and 186.5 ± 3.0 Ma (MSWD = 2.0) for monzogranite and rhyolite, respectively. The monzogranite is characterized by high-K calc-alkaline, adakitic affinities, and positive zircon εHf(t) values (+0.9∼+5.6, avg.+3.1) with TDM2 (0.73–1.04 Ga), while the rhyolite has εHf(t) values of (+2.1∼+7.3, avg.+5.2) and TDM2 of (0.76–1.09 Ga) similar to the monzogranite. Our results suggest that the Demingding porphyry Mo (Cu) deposit is related to magma generated from the Neo-Tethyan oceanic subduction. The subsequent monzogranite porphyry was likely formed by the remelting of previously subduction-modified arc lithosphere, triggered by continental collision crustal thickening in Miocene. The lower positive εHf(t) values of monzogranites suggest minor inputs from the Mo-rich ancient crust, suggesting that Mo favors the silicate melt. Such magmatic events and special metallogenesis typify intracontinental processes and porphyry copper deposits, which are normally confined to oceanic subduction and Cu-dominated style, thereby making the continental setting and Mo-dominated style of Demingding exceptional and possibly unique.

Late Cretaceous adakitic intrusive rocks in the Laimailang area, Gangdese batholith: Implications for the Neo-Tethyan Ocean subduction

Abstract The Gangdese batholith, emplaced from the Cretaceous to the Eocene in the southern Lhasa terrane of Tibet, provides critical constraints on the tectonic-magmatic evolution of the Neo-Tethyan Ocean and the India-Asia continental collision. In this article, we report new data for the Laimailang monzogranite in the southern Gangdese, including major and trace element analyses, zircon U–Pb dating, and in situ Hf isotope analyses. In situ zircon U–Pb dating of sample yielded Late Cretaceous ages (ca. 81 Ma). The monzogranite is characterized by relatively high-silica (≥56 wt% SiO2), Na-rich, and high-Al granitoids that are characterized by high Sr, low Y and HREE contents and strongly fractionated REE patterns, with no significant Eu anomaly, indicating that they are consistent with the definition of adakite. These monzogranite have high K2O (2.92–6.5%) and negative ε Hf(t) (−2.1 to −5.4), suggesting that these rocks were likely derived from the partial melting of the lower continental crust. We conclude that the Laimailang rocks of adakitic affinity were derived due to the melting of the thickened lower continental crust in the late Cretaceous.

Syn-collisional I-type granitoids linked to lateral lithospheric heterogeneity: A case study from the North Qaidam orogen, NW China

Granitoids constitute the major theme in evaluating the growth and reworking of continental crust on Earth. Here we present U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes of the early Paleozoic granitoids emplaced during pre- and syn-collisional stages in the North Qaidam orogen to gain insights on the granitoid typology, genetic mechanism, as well as the implications for the evolution of continental crust. The pre- and syn-collisional granitoids in this region all belong to I-type granite and are derived from different continental crustal sources including late Mesoproterozoic to early Neoproterozoic metamorphic crystalline basement, and juvenile continental crust formed during early Paleozoic oceanic subduction. Granitic magmas derived from the two sources underwent a series of magmatic processes such as mixing or assimilation, which lead to the transitional geochemical and isotopic features, suggesting that besides source components, magmatic processes from melt extraction to granitoid emplacement also exerted an important influence on the formation of these granitoids and crustal maturation. Although S-type granitoids are commonly taken as the fingerprint for continental collision, our study emphasizes that the role of I-type granites formed in syn-collisional setting should not be underestimated. Besides subducted slab processes such as roll-back, retreat, or break-off, we propose a novel geodynamic model which envisages that the lateral inhomogeneity in lithospheric thickness triggered by continental collision within the overriding plate above subduction zone controlled the magmatism. Both growth and reworking of continental crust occur during oceanic subduction, whereas syn-collision setting is dominated by continental crust reworking as in the North Qaidam orogen.

Grenville-age basement and early Paleozoic reworking in the Qilian Orogen, NW China: Implications for paleotectonic reconstruction of Rodinia and Gondwana

• Grenville-age terrane is identified in the Qilian orogen. • The Precambrian basement of the Qilian orogen is divided into four distinct mega-sequences. • ∼475 Ma metamorphism was related to the subduction of the Proto-Tethys Ocean. The Qilian orogen at the northeastern margin of the Tibetan Plateau records a complete history from continental breakup to oceanic basin evolution and back to continental collision during the Neoproterozoic to Paleozoic interval. The Qilian Ocean, as a branch of the Proto-Tethys Ocean, was connected to the Iapetus Ocean but is lesser known than its two more famous counterparts. This paper presents zircon U-Pb and biotite-muscovite 40 Ar/ 39 Ar isotope data from the Yemananshan Complex in the western Central Qilian belt to assess the age and provenance of late Mesoproterozoic-early Paleozoic successions in the Qilian orogen. The results indicate the Central Qilian belt is characterized by Late Mesoproterozoic-early Neoproterozoic orogenesis with magmatism from 1200 Ma to 900 Ma and clastic metasedimentary rocks deposited between 1050 and 920 Ma. Statistical analysis of detrital zircon data and auxiliary temporal information from the Qilian orogen affirms the notion that sedimentation occurred in four distinct megasequences: (1) 1950–1850 Ma, the oldest known strata, (2) 1250–920 Ma, during Grenvillian-Sveconorwegian orogenesis, (3) 840–760 Ma, the disintegration of Rodinia, and (4) 650–550 Ma, which is typical of the orogenic belts that sutured the Gondwana continental blocks. Metamorphic zircon and 40 Ar/ 39 Ar isotope data indicate early Paleozoic metamorphism at ca. 475 Ma, which overlaps with magmatic activity associated with arc-back-arc basin development in the Central Qilian belt, which was associated with subduction of the Proto-Tethys in eastern Gondwana.

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.

Oligocene–late Miocene basin evolution in the Yengejeh syncline in the Central Iranian Basin in response to the Arabia–Eurasia orogeny

Abstract The Central Iranian Basin has developed during a multi-episodic collision between the Arabian and Eurasian continents since the late Eocene–early Oligocene, following the subduction of the Neo-Tethys Ocean. Herein, we present detailed sedimentological and provenance data of the Oligocene–upper Miocene synorogenic strata, including the unconformity-bounded Lower Red, Qom, and Upper Red formations, in the Yengejeh syncline in the NW termination of Central Iran, to analyze stratigraphy, depositional environments, and provenance. Our results indicate that the sedimentary system has evolved in five stages coeval with regional deformational and volcanic events: a) deposition of the Lower Red Formation in an alluvial fan containing the first appearance of Sanandaj–Sirjan metamorphic clasts sourced from the proximal southwestern Takab Complex, exhumed by the onset of Arabian–Eurasian soft collision in late Eocene–early Oligocene; b) Burdigalian transgression of the Qom Sea and shallow-water carbonate sedimentation influenced by continuous pyroclastic inputs and lava flows from an active volcanic center; c) deposition of the M1 unit of the Upper Red Formation in a continental sabkha in arid climate conditions during Burdigalian–Langhian and the first appearance of internal clasts derived from the folded Qom Formation layers due to the Arabian–Eurasian hard collision; d) fluvial deposition of the M2 unit during the Langhian–Tortonian with sediments derived from the Qom Formation and Takab Complex; and e) deposition of the uppermost siliciclastics of the M2 unit at the edge of an alluvial fan during the late Miocene, after a period of pyroclastic fallout in the Tortonian. In general, the source-to-sink relationship is controlled by the development of tectono-topographic relief in the crystalline core of the Zagros Mountains that configured the source areas in the Sanandaj–Sirjan metamorphic belt supplying the NW termination of Central Iran through a well-developed drainage system towards the Caspian Sea. Coeval with the deformational events, magmatic phases supplied a large volume of volcaniclastic inputs both before the Neo-Tethys slab break-off and after the hard continental collision. The depositional environments and provenance of the studied sedimentary record provide an analog for the development of synorogenic hinterland basins worldwide along with the well-known Altiplano Basin of the Andes and Hoh Xil Basin of Tibet.

SIMS Zircon U-Pb Ages and O Isotope Compositions of Gabbro from the Laguoco Ophiolite from Western Shiquanhe–Yonzhu Suture Zone (Tibet) and Their Geological Significance

The Shiquanhe–Laguoco–Yongzhu–Jiali ophiolite mélange belt in Tibet includes the Laguoco Tso ophiolite as part of its western segment. Researchers studying the evolution of the Tibetan Plateau and the Tethys have taken a keen interest in the debated relationship between this ophiolite belt and the Bangong–Nujiang ophiolite belt. However, there is little research in this field. This study reports the rock geochemistry, SIMS (the secondary ion mass spectrometry) zircon U-Pb ages, and O isotope compositions of gabbro from the Laguoco ophiolite. Gabbro has substantial depletion in HFSEs (the high field strength elements) such as Th, Nb, Zr, and Hf, as well as exhibits comparatively high Mg# (80.6 on average), and low K, N, P, and Ti contents. Therefore, it was possibly formed in an intra oceanic subduction island arc environment originating from ~30% partial melting of the depleted spinel lherzolite. The SIMS zircon U-Pb ages of the gabbro samples are 187 ± 1.2 Ma and 189 ± 2.1 Ma, and the weighted averages of δ18O are 5.24‰ ± 0.12‰ and 5.34‰ ± 0.13‰, which are close to mantle-derived zircon δ18O (5.3‰ ± 0.3‰). These results suggest that during the Early Jurassic, the Laguoco ophiolite recorded the intra oceanic subduction of the northern branch of the Neo-Tethys.

A Model of Plate Bending at the Transition Zone From Subduction to Collision in Northernmost Manila Trench

AbstractIt is not rare to observe subduction zones transferring into collision zones. However, it is poorly understood the lateral variation in flexural deformation. Constrained by newly published seismic reflection data, we investigate the mechanical behavior of lithosphere from oceanic subduction to collision at the northern Manila trench using the 3‐D flexural modeling. Results show that both plate rigidity and the loadings increase from the Manila subduction zone to the Taiwan orogenic belt. In order to fit all observations, an upward loading was required at the transition zone from subduction to collision, which may be related to the variation of buoyancy due to density contrast of plate from south to north. The resultant maximum bending stress is consistent with the concentrated seismic activity at the southwest of Taiwan, suggesting that huge loading from the orogeny and lateral density change of subducting plate play important roles in bending and seismicity.

Permian arc magmatism in southern Tibet: Implications for the subduction and accretion of the Zhikong–Sumdo Paleo-Tethys Ocean

The tectonic evolution of the Zhikong–Sumdo Paleo-Tethys Ocean (Hereafter referred to as Zhikong–Sumdo Ocean) in southern Tibet is controversial due to the limited records of arc magmas. Here, the newly discovered Zhangda complex in the Zhikong area of Tibet is selected as the research object. Zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry and Sr-Nd isotope analyses performed based on field geological surveys show that the Zhangda complex is composed of Early Permian hornblende gabbro (278.3 ± 1.1 Ma), cumulate gabbro (277.4 ± 1.7 Ma), tonalite dike (281.1 ± 1.9 Ma), Middle–Late Permian gabbro diorite (267.8 ± 2.9 Ma, 269.0 ± 3.1 Ma), and tonalite (258.0 ± 2.7 Ma). The Zhangda mafic rocks show weak fractionation of light and heavy rare earth elements, and they are enriched in large ion lithophile elements and depleted in high field strength elements. Meanwhile, the Zhangda tonalites show the characteristics of metaluminous calc-alkaline I-type granite. They are enriched in light rare earth and large ion lithophile elements and are depleted in heavy rare earth and high field strength elements. The ɛHf(t), ɛNd(t) and the initial 87Sr/86Sr values of the mafic rocks are + 9.3–+14.4, +4.2–+5.9 and 0.702803–0.704264, respectively, while the ɛHf(t), ɛNd(t) and initial 87Sr/86Sr values of the tonalites are + 9.8–+16.3, +5.5–+6.1 and 0.703105–0.704071, respectively. Combined with the regional geological survey results, the Permian Zhangda complex was found to be formed in a tectonic setting of subduction north of the Zhikong–Sumdo Ocean. The Zhangda mafic rocks were formed by partial melting of the depleted mantle modified by subducted metasomatic fluids, while the Zhangda tonalites were the products of partial melting of the juvenile crust. During the Zhikong–Sumdo Ocean subduction, mantle-derived magma underplating led to the growth of juvenile crust in southern Tibet.

Deformation and uplift at the transition from oceanic to continental subduction, Sumba Island, Indonesia

• The Australian margin boundary collision produces forearc dextral transpression . • Interplate transfer of shear stress and forearc block escape induce trench migration. • Northeastward folding has been propagating across Sumba Island since the Pliocene . • One-third of Sumba Island was inundated by flexure and gravitational collapse . The transition along the strike of the Sunda subduction zone, from oceanic subduction in the west to subduction of continental Australian lithosphere in the east is envisioned as one of the canonical examples of the structural changes that take place within an overriding plate when a continental lithosphere wedge enters a subduction zone. Yet, the along-strike offset of the trench toward the Australian margin represents a structural response opposite to the predictions of numerical models. To understand this paradox, we analyse the morphotectonic evolution of Sumba island located at the transition from oceanic to continental Indo-Australian lithosphere subduction. Drainage evolution allows us to constrain the topographic evolution of the island since the Pliocene. Flights of uplifted coral reef terraces document Quaternary deformation. Focal mechanisms of shallow crustal earthquakes constrain the current stress field. Together, these data reveal that the island is affected by dextral en-échelon folding. Offshore, west of the island, reverse and strike-slip focal mechanisms evidence an active dextral transpressional zone. The emergence of the island and dextral shearing of the accretionary prism were triggered by subduction of the western lateral boundary of the Australian continental margin. We contend that the Plio-Quaternary tectonic evolution of the region, with transpression and migration of the trench toward the Australian margin is primarily dictated by shear stress transfer from the lower plate to the overriding plate, favored by strong interplate coupling, and by southwestward escape of the Savu-Sumba block following the impingement of the Australian continental margin against Timor island.

Triassic granites in the West Qinling Orogen, China: implications for the Early Mesozoic tectonic evolution of the Paleo-Tethys ocean

ABSTRACT The Western Qinling Orogen is marked by secular Triassic magmatism, a thorough understanding of the mechanism of which could provide critical insights into the reconstruction of the Palaeo-Tethys tectono-magmatic activity. Here, we present new geochemical, geochronological and Lu-Hf isotopic data for three batholiths and review regional data. These batholiths have similar rock assemblages (mainly granodiorite and monzogranite), which are characterized by dominantly plagioclase, quartz, and K-feldspar, subsequently amphibole and biotite. Zircon U-Pb data reveal that they represent the long-duration magmatism during the Triassic (ca. 251–223 Ma). Mineralogical characteristics and geochemical affinities manifest that these rocks in the Zeku district can be explicitly delineated as high-K, calc-alkaline, weakly fractionated I-type granites. They yield significantly negative εHf(t) values ranging from −12.81 to −1.26 with old two-stage mantle depleted model ages between 2054 and 1319 Ma. In tandem with Th/Nb (0.98), Th/La (0.39) and La/Nb (2.40) ratios and moderate pressure conditions (ca. 7–10 kbar), the studied granites were derived from partial melting of the Middle Paleoproterozoic to Middle Mesoproterozoic lower crust source region with minor mafic hydrous magma addition. Our new understandings, in conjunction with the temporal-spatial distribution characteristics of magmatism in the West Qinling Orogen, as well as the regional tectonic evolution, suggest that the superimposed orogeny evolved from the northward subduction of the Palaeo-Tethys Ocean (264–225 Ma) through syn-collision (225–215 Ma) to post-collision (beginning at ca.215 Ma) between the North China Craton and South China Block. In this scenario, at the convergent continental margin, the oceanic plate may have undergone melting producing the melts parental to the Zeku granites.

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.

The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system

Supercritical fluids are ideal media for mass transfer from the subducting slab into the mantle wedge. However, little is known about the role of natural supercritical fluids in subduction zones. A combined study of petrology, geochemistry and zirconology was carried out for a coesite-bearing eclogitic vein and its surrounding eclogites from the Dabie orogen, one of the typical continental subduction zones on Earth. The results are used to reveal the composition and source of supercritical fluids, the P-T conditions and timing of their formation and their geochemical effects in subduction zones. The eclogitic vein is composed of garnet, omphacite, quartz, amphibole, rutile and apatite, with minor amounts of epidote, plagioclase and zircon. Coesite was identified in omphacite in the vein, demonstrating its formation under ultrahigh-pressure (UHP) metamorphic conditions that correspond to subarc depths in oceanic subduction zones. Zircons from the vein give concordant U-Pb ages of 225 ± 5 Ma and flat HREE patterns for newly grown rims, consistent with their crystallization at the UHP eclogite facies. The vein shows similar Hf-O isotope compositions to the host UHP eclogites, indicating that the UHP vein-forming fluid is internally buffered within the UHP eclogites. Minerals in the vein contain not only higher contents of Cr, Ni, Sr, REE and HFSE, but also multiphase crystal inclusions (such as omphacite, quartz, epidote, apatite, amphibole, plagioclase, mica, rutile, calcite, and anhydrite) as well as liquid and gas phases of H2O. The calculated fluid composition of major elements is 31 wt% SiO2, 20 wt% CaO, 10 wt% (SO4)2−, 8 wt% (CO3)2−, 8 wt% FeO, 7 wt% Al2O3, and 9 wt% H2O, with traces of Na2O, K2O and TiO2. This composition is responsible for the UHP veining from supercritical silicate-rich fluids in equilibrium with peak UHP minerals. The P-T-t path of the UHP eclogite-vein system indicates that the second critical endpoint of the basalt-H2O system is located close to 3.4 GPa and 770°C. The high contents of sulfate and carbonate in the vein suggest that supercritical fluids were oxidized and likely a predominant agent to transfer sulfur and carbon at subarc depths. High Nb/Ta ratios of rutile crystals in the vein and quantitative modelling indicate that dehydration of subducting eclogites at subarc depths can produce supercritical fluids with suprachondritic Nb/Ta ratios. The UHP eclogites and mantle wedge peridotites metasomatized by supercritical fluids can acquire suprachondritic Nb/Ta ratios and thus provide a complementary reservoir to balance the subchondritic reservoirs on Earth. Based on a statistics of Nb-Ta data, two indices are proposed to identify the existence of supercritical fluids in fossil subduction zones: (1) rutile crystallized from supercritical fluids shows lower Nb contents than that from aqueous solutions and hydrous melts; (2) UHP eclogites associated with supercritical fluids exhibit a distance >0.1 in their Nb-Ta compositions to the line defined by basalts in the plot of log[Nb] vs log[Ta]. Therefore, the existence of supercritical fluids during UHP metamorphism at subarc depths can be recognized from their effect on the mobility of fluid-immobile trace elements in UHP metamorphic rocks.

U–Pb Zircon Ages and Geochemistry of the Wuguan Complex and Liuling Group: Implications for the Late Paleozoic Tectonic Evolution of the Qinling Orogenic Belt, Central China

The tectonic evolution of the Qinling orogen is key to understanding the process of convergence between the North China Block (NCB) and the South China Block (SCB). The Wuguan Complex and Liuling Group, situated along the southern margin of the Shangnan–Danfeng suture zone (SDSZ) between the North Qinling Terrane (NQT) and the South Qinling Terrane (SQT), are important indicators of the late Paleozoic tectonic evolution of the Qinling orogen. In this paper, the detrital zircon U–Pb geochronology and geochemical analysis of the Wuguan Complex and Liuling Group are carried out. Detrital zircons from two metasedimentary rock samples of the Liuling Group yield a major age peak at 460 Ma and two subordinate peaks at 804 Ma and 920 Ma, with a few older grains having formed between 1000–2549 Ma. One metasedimentary rock sample of the Wuguan Complex has a similar age spectrum as that of the Liuling Group, which shows the main age peak at 440 Ma and two subordinate peaks at 786 and 927 Ma, indicating all detrital zircon age results have the same source area. Geochemical analyses suggest that the sedimentary rocks of the Liuling Group and part of the Wuguan Complex were deposited in the tectonic setting of the continental island arc (CIA), while the geochemical characteristics of the other group of sedimentary rocks of the Wuguan Complex indicate the mixing of basic rock sources. The protolith of garnet amphibolite and hornblende schist, which were collected from the Wuguan Complex, were classified as andesite and basalt, with the nature of arc andesite and oceanic island basalt, respectively. In combination with regional data, we suggest that the Liuling Group and the Wuguan Complex were deposited in a fore-arc basin. Additionally, the Wuguan Complex was subsequently incorporated into the tectonic mélange by the northward subduction of the Paleo-Qinling Ocean. Zircons from the subduction-related metamorphic igneous rocks in the Wuguan Complex yielded a weighted mean age of 365 ± 19 Ma, indicating that the Paleo-Qinling Ocean between the SQT and NQT was still subducted at the end of Devonian.

Basalt Mo isotope evidence for crustal recycling in continental subduction zone

Highly variable molybdenum (Mo) isotope compositions are common in mafic arc magmas above oceanic subduction zones, but Mo isotopes of mafic magmas related to continental deep subduction have not yet been documented. Here, we report for the first time the Mo isotope composition of mafic igneous rocks above a typical continental subduction zone in east-central China, where the North China Block was subducted by the South China Block in the Triassic. Continental basalts of Early Cretaceous age show negative δ98Mo values of −0.98 to −0.16 ‰, significantly lower than the normal mantle value of −0.20 ± 0.01 ‰. These basalts also exhibit arc-like trace element compositions, high (87Sr/86Sr)i ratios of 0.7050 to 0.7058, negative εNd(t) values of −15.2 to −10.4, and negative εHf(t) values of −18.7 to −7.9. The light Mo isotope signatures are associated with enrichments in not only radiogenic Sr-Nd-Hf isotopes but also melt-mobile incompatible elements in the basalts. This indicates that the mantle source of these basalts would be generated by metasomatic reaction of the mantle wedge peridotite with felsic melts derived from partial melting of the deeply subducted continental crust. Given that continental crust usually exhibits heavy Mo isotope compositions, it would be dehydrated during its subduction to subarc depths. While the dehydration would have released isotopically heavy Mo fluids from the subducting crust, it leaves isotopically light Mo in the residual crust. The dehydrated continental crust underwent partial melting at subarc depths to produce felsic melts with low δ98Mo values, transferring the light Mo isotope signature into the mantle source of the basalts. Therefore, Mo isotopes in mafic igneous rocks are a powerful means to decipher the recycling of crustal components at convergent plate margins. Our study provides the first insight into Mo isotope recycling in continental subduction zones.

Batholith recorded mesozoic multistage tectonic evolution of the South china block: A case study of the guandimiao intrusions

South China is a well-known grand felsic igneous rocks province. However, it is still controversial and not well understood whether the Mesozoic tectono-magmatic pattern is dominated by the subduction of the paleo-Pacific oceanic plate. In this study, we address this question by concentrating on the long-term evolutionary Guandimiao batholith, which has complex lithofacies with different formation ages and can be a superb record of the Mesozoic tectonic evolution in South China. Geochronologically, four stages of magmatism can be identified combined with previous reports: granodiorite (G1, 239 Ma), biotite monzogranite (G2-1) and two-mica monzogranite (G2-2) (230–203 Ma), granite porphyry (G3, 211–190 Ma), and lamprophyre (L4, 121 Ma). G1 and G2-1 have an affinity with I-type granite and were derived from metabasaltic to metatonalitic sources, whereas G2 and G3 show S-type granite characteristics and were derived from the para-metamorphic basement of the Cathaysia block. The L4 was derived from partial melting of garnet and spinel lherzolite and underwent mixing between Mesoproterozoic pelagic and/or terrigenous sediments and the subcontinental lithosphere mantle (SCLM) of South China. The granitoids of the Guandimiao batholith underwent intensely fractional crystallization of feldspar, Ti-bearing minerals, allanite and monazite. The zircon U–Pb dating of L4 in the Guandimiao batholith completely records the six stages of pre-Mesozoic tectonic events in the SCB. During the Mesozoic, the main body of the Guandimiao batholith (G1, G2-1 and G2-2) recorded the closure of the paleo-Tethys Ocean in the Triassic and the subsequent regional extension of the postcollision. G-3 and L4 of the Guandimiao batholith documented the transition of tectonic and dynamic regimes in the early Yanshanian and the rollback and steep subduction of the paleo-Pacific Ocean in the late Yanshanian.

Petrogenesis of Permian to Triassic granitoids from the East Kunlun orogenic belt: implications for crustal evolution during oceanic subduction and continental collision

ABSTRACT The study of Permian-Triassic granitoids, which widely occur within the East Kunlun Orogenic Belt (EKOB), is relevant for a better understanding of the evolution of the Paleo-Tethys Ocean and regional crustal evolution. This study focuses on the Permian-Triassic granitoids from the Nagengkangqieer region, east segment of EKOB. The Late Permian granodiorite (ca. 252 Ma) shows high-K calc-alkaline and metaluminous features with moderate SiO2 (63.31–67.04 wt.%), relatively high Mg# (48–52), and low Sr/Y ratios. The granodiorite is enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) and shows enriched isotopic compositions (εNd (t) = −5.53 to −5.14, εHf (t) = −5.6 to −1.1). These geochemical characteristics imply that the Late Permian granodiorite was derived from a mixing source of mafic lower crust and mantle materials (<30%). The Middle Triassic porphyritic monzogranite (ca. 239 Ma) and monzogranite (ca. 239 Ma) are high-K calc-alkaline to shoshonitic and weak peraluminous with high SiO2 (70.58–76.54 wt.%) contents and low Mg# (19–36). They display relatively higher FeOT/MgO ratios, lower P2O5 contents, and more enriched isotopic compositions (εNd (t) = −7.87 to −6.71, εHf (t) = −13.7 to −1.2) than those of granodiorites. These findings indicate that the Triassic granites belong to fractionated granites derived from partial melting of the lower continental crust. In combination with the results of this study and the spatio-temporal distribution of regional magmatic, metamorphic, and sedimentary events, we conclude that Late Permian granodiorite formed in a subduction setting, whereas the Middle Triassic granites were formed in a syn-collision setting. Closing of the Paleo-Tethys Ocean occurred in the Middle Triassic (ca. 240 Ma). Additionally, only a small proportion of juvenile materials (<30%) was involved in magmatism and contributed to continental crustal growth during Late Permian subduction, whereas crustal reworking dominated regional magmatism and resulted in a higher crustal maturity.

Cretaceous basalt-andesite sequence in the Southern Pamir: arc—back-arc architecture at the Pamir Plateau genetically related to the northward flat subductions of the Neo-Tethys Ocean

The Pamir-Karakorum is one of the most important regions for our understanding the Tethys evolution. Although it is commonly known that the northward subduction of the Neo-Tethys Ocean induced the formation of the magmatic arc in Southern Pamir, the temporal-spatial configuration of the arc—back-arc during the Neo-Tethys evolution remains unclear. In this study, we report sedimentary feature, petrography, zircon U Pb ages and Hf O isotope compositions as well as bulk-rock and mineral geochemistry of the well-preserved Cretaceous volcanic sequence in Chinese Wahan Corridor at South Pamir. The volcanic sequence is mainly composed of phenocrysts- bearing basalt- andesites with minor rhyolite and volcaniclastic rock. Zircon U Pb ages reveal the basalt-andesite sequence erupted at 100–98 Ma. Bulk-rock geochemistry defines their potassic calc-alkaline signature and both their rock association and elemental geochemistry share most features of the continental arc volcanic sequences. On the other hand, they have variably enriched Nd isotope compositions with ε Nd (t) varying from −5.9 to −9.6, coupled with their enriched zircon Hf isotope signature with ε Hf (t) between −13 to −8. Whole-rock geochemistry and zircon oxygen isotope compositions (average δ 18 O = 7.74 ± 0.14) demonstrate that the primitive magma of the basalt-andesites were derived from partial melting of a metasomatized sub-continental lithospheric mantle and underwent variable assimilation fractional crystallization. Late Jurassic to Cretaceous volcanic-sedimentary sequences and coeval voluminous arc-like granodiorite and granites are widely distributed along the Southern Pamir. In the Central Pamir and further north, late Jurassic to Cretaceous sequences resemble features of back-arc basins. Taken together, we suggest that a northward low-angle or flat subduction of the Neo-Tethys Ocean along the Shyok zone was geodynamically related to the formation of the broad magmatic arc in South Pamir-Karakorum and the back-arc basins north of the South Pamir. • Arc-like Cretaceous basalt-andesite sequence in Southern Pamir (SP) erupted at ~100 Ma. • Architecture of the late Jurassic-Cretaceous arc-back-arc at Pamir was constructed. • Flat subduction of the Neo-Tethys Ocean produced the Cretaceous igneous rocks in SP.