Subduction Of Oceanic Crust Research Articles

Petrogenesis of supracrustal rocks from the Maxianshan and Xinglongshan Groups in the eastern Central Qilian block: Constraints on the construction of Rodinia

Controversy has long surrounded the reconstruction of the East Asian blocks in the Rodinia supercontinent, which was a coherent large landmass during 900–750 Ma and is now dispersed over all current major continents. The Central Qilian block is a Precambrian microcontinent in the early Paleozoic Qilian orogenic belt, which marks the junction of the North China, South China and Tarim cratons. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of supracrustal rocks from the Maxianshan Group and the Xinglongshan Group in the easternmost part of the block. The metasedimentary rocks from both groups overlie a gneissic granite, which has a zircon U-Pb age of 970 ± 6 Ma with εHf(t) values of −3.5 to + 2.5 and is an I-type granite formed in a back-arc setting. Paragneisses from the Maxianshan Group and micaschists from the lower formation of the Xinglongshan Group have detrital zircon U-Pb ages of 2465–876 Ma that peak at ca. 950 Ma. They show strongly decreasing zircon εHf(t) values of + 0.8 to −11.3 and ages from 1174 Ma to 876 Ma. Their protoliths constituted a sedimentary sequence with a long history of deposition during 1174–911 Ma in a continental arc-related basin. Metabasaltic tuffs from the middle formation of the Xinglongshan Group are tholeiitic with a zircon U-Pb age of 958 ± 9 Ma and indicate a back-arc setting. Metasandstones from the upper formation of the Xinglongshan Group have detrital zircon ages of 2668–732 Ma that peak at 810 Ma and 984 Ma and indicate a passive margin setting. Combining our results with existing data, we propose that the Maxianshan Group and the Xinglongshan Group make up an early Neoproterozoic trench-arc-basin system at a continental margin of Rodinia. Oceanic crust subduction underneath the continent at 1174–896 Ma with formation of a mature continental arc, and continuous subduction from 824 to 735 Ma with opening of the Proto-Tethys Ocean as a back-arc basin are suggested for the formation of the Central Qilian block.

Hydrothermal activity near the Permian–Triassic transition in the south‐western Ordos Basin, China: Evidence from carbonate cementation in Upper Permian sandstones

ABSTRACTCarbonate cementation in the Upper Permian sandstones informs the timing and temperature of hydrothermal activity in the south‐western Ordos Basin. This study presents a detailed examination of these hydrothermally influenced carbonate cements, constraining their age, carbonate diagenesis and relationship with regional geodynamic evolution. Sedimentological analyses demonstrate the development of deltaic plain and front sand bodies in the study area, which resulted in interbeds of volcanic matrix‐rich sandstones with matrix‐free sandstones. Petrography and electron microprobe analysis reveal four carbonate mineral growth phases of matrix‐free sandstones in the following sequence; scarce pure siderite, scarce Mg‐rich siderite, abundant blocky calcite and moderately abundant grain‐replacing calcite. The fluid inclusion data show anomalies of homogenization temperature of blocky carbonate cements during early diagenesis, over a wide range of ca 148 to 228°C. In addition, blocky carbonate cements show low δ13C (−5.9 to −13.1‰ Vienna PeeDee Belemnite) and δ18O (clustered tightly from −12.4 to −14.6‰ Vienna PeeDee Belemnite) values, interpreted to result from elevated temperatures during cementation, associated with activation of basement faults and concomitant hydrothermal fluid intrusion triggered by oceanic crust subduction in the south‐west margin of the Ordos Basin. Using in situ calcite U–Pb geochronology, the timing of hydrothermal activity was constrained to ca 247.0 ± 11 to 248.2 ± 4.7 Ma. This work provides a case study for applying intergranular calcite U–Pb dating to determine the absolute timing of fluid flow in sedimentary basins, offering tremendous potential to capture snapshots of various diagenetic evolution stages in sediments. The proposed diagenetic model can also provide new insights and understanding regarding hydrothermally influenced sediments. More importantly, hydrothermal activity may have commenced earlier than previously thought. The North Qinling Orogen uplift and associated Mianlue oceanic crust subduction may have begun at the Permian–Triassic transition with a protracted hydrothermal event in the south‐western Ordos Basin.

From onset of Cadomian subduction to forearc-arc-continent collision: Insights from the Neoproterozoic Calzadilla Metaophiolite (SW Iberia)

ABSTRACT The Cadomian basement of SW Iberian Massif appears in the so-called Ossa-Morena Complex, which contains several allochthonous terranes that are key for deciphering the evolution of the Gondwana margin during the Neoproterozoic. In this context, the Calzadilla Ophiolite represents the remnants of an oceanic Ediacaran lithosphere formed by a boninitic mafic crust that overlies a serpentinized ultramafic section, the latter being the biggest outcrop of Cadomian serpentinites in Europe. In this work, the petrological and geochemical nature of the mantle section is analysed, as well as the tectonothermal evolution of both, the mafic crust and the ultramafic section. The algebraic analysis of whole rock geochemistry of the serpentinites indicates that the protolith corresponds to harzburgite. The geochemical fingerprint of the serpentinites, as well as the primary spinel crystals composition, shows that peridotites underwent high degrees of partial melting (up to 16%) in a forearc setting. Thermodynamic modelling and average PT methods were applied to serpentinites and mafic dykes from the ultramafic section as well as to amphibolites from the mafic section, both of which experienced progradation up to 4–5 kbar and 525–575°C. According to the data, the protoliths of the Calzadilla Ophiolite were formed in a peri-Gondwanan arc system developed during the subduction of oceanic crust below Gondwana. The roll-back of the subducting slab would induce the opening of the forearc basin at c. 602 Ma, protolith age of amphibolites from crustal section, with an extensive partial melting of the mantle wedge and the formation of boninitic-affinity magmas that constitute the mafic section of the Calzadilla Ophiolite. Metasomatism and serpentinization of the mantle wedge would occur via melt- and fluid–rock interactions. A compression-dominated stage at c. 540 led to the emplacement of the Calzadilla Ophiolite onto the Cadomian magmatic arc, with the development of the amphibolite-facies metamorphic imprint.

Provenance of cretaceous sediments in the West Kunlun piedmont belt and implications for tectonic evolutionary events

The West Kunlun orogenic belt located in the northwestern margin of the Qinghai-Tibet Plateau is an important record of the formation and northward extension of the plateau, but the current research mainly focuses on the tectonic activities of the Cenozoic era, and there is still considerable controversy regarding the formation and evolutionary history of pre-Cenozoic orogenic belts. This study focuses on Cretaceous sandstone samples from the Kedong region in the piedmont belt of the West Kunlun orogenic belt. U-Pb geochronological analysis was performed on 200 detrital zircon grains from the core samples. Combined with stratigraphic data and previous research, the main provenance direction was investigated to constrain the tectonic evolutionary history of the orogenic belt’s peripheral regions. The results show that the detrital zircons are aged from 290 to 208 Ma, 520–310 Ma, 810–580 Ma, 1,400–880 Ma and 2,548–1,730 Ma, reflecting the complexity of provenance in this area. Based on a comprehensive analysis of the characteristics of igneous rocks, zircon age composition and stratigraphic conditions in potential source areas, it is concluded that the primary source regions include the East Kunlun orogenic belt and the North and South Kunlun terranes, with a low likelihood of contributions from within the Tarim Basin. The evolution of the West Kunlun orogenic belt can generally be divided into two opening and two closing phases. The detrital zircon ages predominantly exhibit two peak values at 259 Ma and 459 Ma, respectively representing the ages of transition from oceanic crust subduction to continent-continent collision for the Paleo-Tethys Ocean and the Proto-Tethys Ocean. Additionally, there is a temporal gap between the evolution of the Proto-Tethys Ocean and the Paleo-Tethys Ocean. The Triassic period marks a transitional phase in tectonic evolution, shifting into an intracontinental evolutionary stage. This study provides new geochronological evidence for the early developmental history of the West Kunlun orogenic belt.

Propagating Neotethys slab break-off beneath Iran following Arabia-Eurasia collision

The closure of the Paleo- and Neotethys resulted in a long history of subduction of oceanic crust and production of a large variety of Phanerozoic magmatic rocks in the region occupied by present day Iran. Adakitic rocks of varying ages are common in this area and have distinctive geochemical signatures that have been variably linked to slab break-off or melting of the lower continental crust. The geographic distribution and age of the adakitic rocks indicates a potential younging trend from northwest to southeast Iran, but this trend was interrupted by an older outlier in the area of Tafresh, in the central part of the Neotethys arc. We obtained new geochemical and U-Pb geochronological data for Eocene volcanic and Miocene intrusive rocks from this anomalous locality. Our results show that adakitic signatures are only present in younger Miocene intrusive porphyritic bodies and not in the main calc-alkaline Eocene volcanic succession as previously thought. The Tafresh adakitic porphyritic bodies have an age of 15.7 ± 0.1 Ma (n = 183, 2σ SE), which fits well with a regional younging of the adakites towards the southeast of Iran. The Tafresh adakitic rocks are classified as the high-silica variety, and show trace element signatures and isotopic values that are consistent with melting of the lower continental crust. We hypothesise that progressive slab break-off provided a mechanism for the formation of high-silica adakitic rocks along the former Neotethys arc.

Recycling of Paleo-Tethyan oceanic crust: Geochemical record from Early–Middle Triassic igneous rocks in the East Kunlun Orogen in western China

Abstract Deciphering the contribution of crustal materials to generation of mafic arc igneous rocks at different subduction stages is of great significance to unravel the fate of the subducted paleo-oceanic crust. Here we present an integrated geochemical study on two types of early Mesozoic mafic arc igneous rocks from the East Kunlun Orogen. Zircon U-Pb isotopic analyses yield ages of 252–248 Ma for lamprophyres and 239–238 Ma for diorite porphyries. All the samples display arc-like trace element distribution patterns, high zircon δ18O values, and variably low zircon εHf(t) values. However, significant geochemical distinctions exist in terms of trace element concentrations, radiogenic isotopes, and other geochemical variations between them. The Early Triassic lamprophyres are characterized by significant enrichment in fluid-mobile trace elements and weakly enriched whole-rock Sr-Nd-Hf isotopes, whereas the Middle Triassic diorite porphyries show high contents of light rare earth elements, Th, Zr, and Hf, and more enriched Sr-Nd-Hf isotopes. Furthermore, the lamprophyres exhibit remarkably higher ratios of Ba/Th, Ba/La, K/La, and Sr/Nd and slightly higher ratios of La/Sm, Th/Yb, and Th/La than mid-oceanic-ridge basalt (MORB), while the diorite porphyries display higher La/Sm, Th/Yb, Th/Nd, and Th/La ratios compared to normal MORB but closer to those of seafloor sediments. Taken together, these differences can be attributed to the incorporation of two distinct slab liquids into their mantle sources, including oceanic slab-derived aqueous solutions and minor sediment-derived hydrous melts for the formation of the lamprophyres, and sediment-derived hydrous melts for the formation of the diorite porphyries. As a result, we suggest the lamprophyres were generated during the Early Triassic subduction of the Paleo-Tethyan oceanic crust, while the diorite porphyries may be generated due to rollback of the subducting oceanic slab in response to the closure of the Paleo-Tethyan Ocean basin. Therefore, the studied Early–Middle Triassic mafic igneous rocks provide important evidence for the recycling of the Paleo-Tethyan oceanic slab at different stages.

Mineral precipitation sequence from multi-stage fluids released by eclogite during high-pressure metamorphism

Abstract Arc magmas above subduction zones hold abundant fluid-mobile elements attributed to fluids released from the dehydrating subducted oceanic crust. However, the quantity of trace elements in the fluids and their evolution with the metamorphic processes during subduction and exhumation are still unclear. The precipitation sequence of vein minerals preserves the nature of multi-stage high-pressure (HP) metamorphic fluids and the fingerprint of mass exchange in deep subduction zones. In this contribution, we conducted detailed petrological studies and phase equilibria modeling on a unique HP omphacite-rich vein and its host eclogite from the Chinese southwestern Tianshan. The host eclogite consists mainly of garnet, omphacite, epidote, glaucophane, phengite, quartz, and rutile. Garnet in the eclogite records prograde subduction and early exhumation characterized by decompression heating at P-T conditions of ~2.4–2.6 GPa and 460–540 °C. The embedded omphacite-rich vein has similar mineral assemblage to the host eclogite. Garnet grains in this vein are predominantly distributed along or intersect the vein wall, which record similar eclogite-facies metamorphic conditions to that of the host eclogite. Omphacite is dominant in the vein, while epidote and glaucophane occur interstitially. Phase equilibria modeling reveals a sequential growth of garnet-dominated, omphacite-dominated, and epidote-dominated assemblages from fluids originating from the breakdown of different hydrous minerals. These lines of evidence suggest that the formation of multi-stage HP fluids are a continuous long-term process with a spontaneously short-distance transport and sequential mineral precipitation. Calculated fluid compositions demonstrate that the fluids released by lawsonite breakdown during exhumation have great potential to modify the trace element systematics of arc magmas. Our findings reveal the nature and evolution of multi-stage HP metamorphic fluids from internal sources during subduction and exhumation of oceanic crust, providing valuable insights into the chemical compositions of arc magmas.

Detrital zircon U–Pb geochronology and geochemistry of sandstones in the Siziwang Banner, Central Inner Mongolia: Implication for tectonic evolution

The central Inner Mongolia, located at the intersection of the northern margin of the North China Craton (NCC) and the Central Asian Orogenic Belt, is crucial for deciphering the Late Palaeozoic tectonic evolution associated with the subduction and closure of the Palaeo‐Asian Ocean (PAO). Our study focused on petrology, detrital zircon LA–ICP–MS U–Pb geochronology and whole‐rock geochemistry for the Late Carboniferous to Permian sandstones within the Shuanmazhuang, Dahongshan, Naobaogou, and Laowopu formations in Siziwang Banner, central Inner Mongolia. This comprehensive analysis shed light on the dynamic interplay between the NCC and the South Mongolia Block. Detrital zircon U–Pb ages in investigated samples mainly cluster between 250 and 2650 Ma, with significant peaks at 2.4–2.5 Ga, 1.8–2.0 Ga, 400–430 Ma, and 250–320 Ma, respectively. The geochemistry data are characterized by SiO2 contents (56.29–77.95 wt. %), Na2O / K2O ratios (0.45–1.58) and SiO2/Al2O3 ratios between 4.33 and 7.44. Moreover, they exhibit the slight enrichment in large ion lithophile elements (Rb and Ba) and the depletion in high field strength elements (Nb, Ta, Th, and U). These facts indicate that the sedimentary detritus predominantly originates from felsic sources, probably deriving from the Late Carboniferous–Permian continental island arc‐related intermediate‐acid igneous rocks, the Late Ordovician‐Silurian magmatic rocks in the Bainaimiao arc and the basements of the NCC. Furthermore, our present results also suggest that during the Early–Middle Permian, accelerating oceanic crust subduction triggered significant magmatic events in Siziwang Banner, leading to rapid uplift and the erosion of arc magmatic rocks, as well as the abundant corresponding sediments. Subsequently, the gradual convergence and eventual collision between the NCC and the Southern Mongolian Block took place at the end of the Permian, representing final closure of the PAO.

Mesoarchean–Neoarchean coupled crust–mantle differentiation followed by gravity-driven lithospheric delamination and subduction initiation in the North China Craton

The geodynamic regime that formed Archean silicic continental crust is unclear. The episodic Archean TTGs (tonalite–trondhjemite–granodiorite rocks) and associated granitoids, with emplacement ages of ca. 2.9, 2.7, and 2.5 Ga, occurred in the Jiaobei Terrane, North China Craton (NCC), provide a continuous record of the formation of the Mesoarchean–Neoarchean silicic continental crust. To investigate the crust–mantle differentiation, crustal reworking and recycling, and geodynamic regime associated with formation of the Mesoarchean–Neoarchean silicic continental crust, we undertook zircon U–Pb dating and Hf–O isotope, and whole-rock major and trace element analysis of Archean TTGs and associated granitoids. The episodic Archean TTGs and juvenile crust coexisted spatially–temporally in the Jiaobei Terrane, indicating Mesoarchean-Neoarchean synchronous differentiation of crust and mantle. In addition, the ca. 2.9 and 2.7 Ga TTGs have an identical Mesoarchean juvenile crustal source and similar mantle-like zircon δ18O values, whereas the ca. 2.5 Ga TTGs were derived mainly from younger Neoarchean juvenile crust, indicating removal and replacement of the Mesoarchean juvenile lower crust. Some ca. 2.5 Ga TTGs, granitic gneisses, and sanukitoids have markedly higher zircon δ18O values compared with mantle δ18O values, which are indicative of Neoarchean supracrustal recycling. Consequently, we can constrain the geodynamic processes responsible for the formation of the Mesoarchean–Neoarchean silicic continental crust in the Jiaobei Terrane, NCC. Episodic hot upwelling mantle (or plume)–lithosphere interactions at ca. 2.9, 2.7, and 2.5 Ga resulted in coupled crust–mantle differentiation, which produced the TTGs, juvenile crust, and dense lower crustal restites. Subsequently, lithospheric delamination triggered by gravitational instability occurred followed by continental uplift, subduction of altered oceanic crust, and upwelling of asthenosphere and mantle-derived mafic melts. This resulted in extensive anatexis and metamorphism with anticlockwise P–T paths in the middle to lower crust at ca. 2.5 Ga. Furthermore, following the large-scale melting and cooling of the mantle, a thick, stable, cratonic lithosphere had likely developed by the end of the Neoarchean. The geodynamic processes during the Mesoarchean–Neoarchean were diverse, and episodic hot upwelling mantle (or plume)–lithosphere interactions could have been responsible for the formation of Archean silicic continental crust in the Jiaobei Terrane, NCC.

Late Jurassic oceanic plateau subduction in the Bangong–Nujiang Tethyan Ocean of northern Tibet

Oceanic plateau accretion and subsequent flat-slab subduction in modern convergent settings have profoundly influenced the nature of subduction and mantle dynamics. However, evaluating similar impacts in ancient convergent settings, where oceanic plateaus have been subducted but geological records are limited, remains challenging. In this study, we present geochronological and geochemical data for a suite of ore-associated plutonic rocks from the Gaobaoyue area of northern Tibet. These rocks have zircon U–Pb ages of 152–146 Ma, with high Sr contents and Sr/Y and La/Yb ratios, low MgO, Yb, and Y contents, and depleted Sr–Nd–Hf isotopic compositions, consistent with an adakitic affinity that was generated by the partial melting of subducting oceanic crust. We compare the Late Jurassic adakitic magmatism with the spatiotemporal evolution of magmatism in northern Tibet to infer oceanic plateau subduction and subsequent flat-slab subduction in the Bangong–Nujiang Tethyan Ocean. This tectonic model explains (i) slab-derived adakitic magmatism, (ii) the observed lull in magmatic activity, (iii) intraplate compression and uplift, and (iv) subduction jump and initiation. We also propose that the subduction of heterogeneous oceanic crust (i.e., buoyant oceanic plateau subduction) provided favorable conditions for tectonic exhumation, vertical slab tearing, and the formation of Cu–Au deposits. Our findings not only have implications for establishing the fundamental process of oceanic plateau accretion in ancient subduction zones but also provide an alternative explanation for Late Jurassic complex tectonomagmatic activity in northern Tibet.

Neoproterozoic-Early Cambrian Northern Chah-Farsakh Cu-Zn VMS-type deposit, Iran: Constraints from geology, geochemistry, fluid inclusions and geodynamic

Northern Chah-Farsakh is the first seafloor VMS described in the Torud-Chahshirin metallogenic belt of Iran. The deposit is hosted by Neoproterozoic-Early Cambrian mafic (basaltic andesitic)-siliciclastic rocks in volcano-sedimentary basin. The evolution of the ore-bearing basin is governed by the Proto-Tethys Oceanic crust subduction beneath the Central Iranian microcontinent. Three ore facies can be distinguished based on the ore textures studies: stringer zone ore facies, massive-replacement ore facies, bedded ore and distal facies. Pyrite, chalcopyrite, magnetite and sphalerite are the main minerals, together with minor amounts of pyrrhotite, bornite, tetrahedrite and tennantite. Textures include laminated, banded, replacement, massive, and vein-veinlet texture. There is a distinct metal zonation in the massive sulfide orebody; Gold, Ag, and Zn contents increase vertically from the stringer to the bedded ore facies; Cu and Co increase in the massive ore facies, and Pb is highest in the stringer zone. Ore mineralization in the Northern Chah-Farsakh deposit has emplaced in two stages. The fine-grained ore bands (stage 1) which are intricately interlayered with host rock beds, exhibit sedimentary structure such as laminae and bedding are consistent with a syn-sedimentary origin. Coarser-grained stage 2 ores have massive and vein-veinlet textures that are considered to form by replacement during sub-seafloor fluid flow. Based on SEM-EDS analysis, significant compositional variations were observed in the Py-1b grains. Py-1b is enriched in Pb compared to Py-1a. The elemental distribution map shows that the chalcopyrite (Cpy-2) minerals in Northern Chah-Farsakh deposit do not have variable composition and are homogeneous. Fluid inclusions in quartz minerals rom the footwall stringer zone have trapping temperatures between 203 and 500 °C with salinities (3.39–16 wt% NaCl equiv.) higher than modern seawater. Fluid inclusion studies suggest that the ore-forming fluids were derived from seawater. High level of Se in the massive ore facies of the Northern Chah-Farsakh supports the formation of mineralization from a high-temperature hydrothermal fluid. The Northern Chah-Farsakh Cu-Zn VMS deposit typify the mafic-siliciclastic type (Besshi-type), where the majority of the host stratigraphy is composed of a mafic flow-volcaniclastics and sedimentary rocks.

Eclogite with biotite porphyroblasts—Which conditions are responsible for their formation? An example from the northern Fleur‐de‐Lys Supergroup, Newfoundland, Canada

AbstractAn eclogite from the Early Palaeozoic Fleur‐de‐Lys Supergroup in Newfoundland was studied because of its biotite porphyroblasts, which very rarely occur in this rock type. Thermodynamic modelling suggests that eclogitic biotite in common metabasite (former basalt–gabbro) is limited to (1) bulk‐rock compositions, which are relatively rich in Fe2+ and K and poor in Fe3+, and (2) the low‐pressure range of the eclogite facies. The latter reason is supported by the determination of the pressure–temperature (P–T) path of the Newfoundland eclogite. Chemical zonation of garnet, presence of phengite with Si contents of ~3.4 per formula unit, Zr contents in rutile and petrographic observations resulted in a P–T trajectory starting at medium‐pressure conditions. Nearly isothermal burial led to a peak pressure of 18–19 kbar at ~575°C, followed by exhumation and slight heating. Deformation occurred at or close to the peak pressure. Subsequent introduction of hydrous fluids caused the formation of porphyroblasts of biotite and Ca–amphibole in the pressure range of 12–17 kbar at peak temperatures of 625–640°C. Retrogression led to very fine‐grained symplectites around omphacite and phengite and marginal replacement of biotite porphyroblasts by plagioclase and titanite. Geodynamic scenarios invoking either a flat subduction of oceanic crust followed by continent–continent collision or intracontinental subduction along a transpressional fault system might best explain the formation of eclogite with biotite porphyroblasts in general. For the Newfoundland eclogite, the latter scenario is preferred.

Light oxygen isotopic composition in deep mantle reveals oceanic crust subduction before 3.3 billion years ago

Compositional heterogeneity exists in Earth’s deep mantle, which can be caused by the subduction of oceanic slabs. How early this process started on Earth remains highly debated due to the scarcity of early Archean materials with pristine mantle compositional signatures. Here, using the oxygen isotope and elemental compositions of fresh olivine grains in the 3.27-Ga komatiites of the Weltevreden Formation in the Barberton Greenstone Belt in Southern Africa, we discovered two groups of samples with primitive olivine grains. Group I exhibits normal mantle-like δ18O values and high Fo contents (δ18O = 4.9–5.4‰; Fo = 93–95); Group II is characterized by lower δ18O values with slightly lower Fo contents (δ18O = 3.6–4.7‰; Fo = 91–93). These δ18O values correlate with other geochemical proxies of olivine-poor iron-rich pyroxenite sources, indicating that the Weltevreden komatiites were derived from two distinct mantle sources. The existence of the low-δ18O magmas can be best explained by recycling of the altered oceanic crust into deep mantle arguably by subduction, which started 3.3 billion years ago and is responsible for the deep mantle heterogeneity in early Earth.

Sedimentological Evidence for Pre‐Early Permian Continental Subduction in the Dabie Orogen, Central‐East China

AbstractHow to constrain the onset of continental subduction and prograde metamorphism in an orogen remains a fundamental question. The widespread Triassic ultrahigh‐pressure (UHP) metamorphic rocks in the Dabie orogen in central‐east China are generally attributed to continental subduction. The earliest Triassic peripheral basins (the Huangshi, Yueshan and Nanjing basins) around the Dabie orogen represent an ideal archive to reconstruct the early evolution of the orogen. Here we present a multidisciplinary provenance study on these Triassic strata, including framework petrography and heavy mineral analyses combined with U‐Pb age and trace‐element analyses of detrital zircon, rutile and apatite. The abundant metapelitic lithics and muscovite grains, a heavy mineral population dominated by metamorphic apatite, and a significant Permian (270–290 Ma) age peak from U‐Pb age spectra of detrital zircon, rutile and apatite imply an Early Permian medium‐high grade metamorphic source in the uppermost structural levels of the Dabie orogen. Our provenance data indicate that the onset of northward continental subduction of the South China Block commenced no later than the Early Permian (c. 290 Ma). This event is clearly earlier and distinct from the more widespread Triassic UHP metamorphism of the Dabie orogen, suggesting that continental subduction and mountain‐basin interaction were protracted processes (>55 Myr, c. 290–235 Ma). When combined with other geological evidence, our results show that the prolonged continental subduction is not always preceded by subduction of oceanic crust.

The significance of recycled oceanic mantle lithosphere beneath the Arctic Gakkel Ridge

Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. The oceanic lithospheric mantle, the largest volume of recycled plates, however, is often inferred but rarely observed. Here we report a collection of evidence suggesting that the Gakkel Ridge sampled recycled refractory ocean lithosphere. (1) A gradient in composition in the western volcanic zone (WVZ) approaching the sparsely magmatic zone (SMZ) suggests lower extents of melting of more depleted sources. (2) The SMZ itself contains substantial portions of sea floor formed by mantle emplaced at the surface. (3) Sparse enriched basalts from the SMZ are consistent with melting of refractory but metasomatized lithosphere. (4) Unique high-Ti basalts at the WVZ-SMZ boundary originated by deep melting of oxide-gabbro-bearing ocean lithosphere mantle. The volume of recycled ocean lithospheric mantle is enormous– it equals roughly 25 % of the entire mantle per gigayear. However, it is almost never observed as an isolated source. If the crustal portion of the lithosphere is isolated to form discrete sources in the mantle, ten to fifteen times as much depleted mantle lithospheric sources would be formed. The rarity of this source material thus suggests that ordinarily recycled ocean crust and mantle lithosphere become well mixed during mantle convection. The enclosed basin and ultra-slow spreading rate of the Gakkel Ridge may have provided inhibited mixing conditions where recycled oceanic mantle lithosphere was able to be preserved.

Permian tectonic evolution and continental accretion in the eastern Central Asian Orogenic Belt: A perspective from the intrusive rocks

The tectonic evolution and history of continental accretion of the eastern Central Asian Orogenic Belt (CAOB) are not yet fully understood. In this study, we investigate Permian intrusive rocks from the Jiamusi Block of the eastern CAOB to constrain the tectonic evolution and continental accretion of this region during the late-stage evolution of the Paleo-Asian Ocean. Our new data show that Early Permian gabbro-diorites were derived from the partial melting of depleted mantle metasomatized by oceanic-slab-released fluids. Middle Permian adakitic granites have low Na2O and MgO and high K2O contents, indicating a thickened-lower-crust source. Late Permian S-type granites were derived from the partial melting of continental crust. A compilation of the available geochronological data for Permian intrusive rocks (including adakitic and A-, S-, and I-type granites and mafic rocks) from the eastern CAOB reveals that the A-type granites formed mainly during the Early–Middle Permian, S-type and adakitic granites mostly during the Middle–Late Permian, and I-type granites and mantle-derived mafic rocks throughout the Permian. The A-type granites, which are proposed to have been sourced from thinned continental crust, indicate an extensional setting in the eastern CAOB during the Early Permian. The Middle–Late Permian adakitic granites imply a thickened continental crust, which indicates a compressional setting. Therefore, the eastern CAOB underwent a transition from extension to compression during the Middle Permian, which was probably triggered by the late-stage subduction of Paleo-Asian oceanic crust. Considering the petrogenesis of the intrusive rocks and inferred regional tectonic evolution of the eastern CAOB, we propose that vertical underplating of mantle- and oceanic-slab-derived magmas contributed the materials for continental crust accretion.

Early Paleozoic back-arc basin in the East Kunlun Orogen, northern Tibetan Plateau: Insight from the Wutumeiren ophiolitic mélange

The Early Paleozoic tectonic architecture of East Kunlun Orogen in the northern Tibetan Plateau is crucial to deciphering the tectonic evolution of the Proto-Tethys ocean. The Wutumeiren ophiolitic mélange located in the Qimantagh-Xiangride suture between the North Qimantagh and Central Kunlun belts represents essential records of the Proto-Tethyan tectonic domain. It comprises serpentinite, dolerite, basalt and black chert with minor siltstone. The serpentinites possess high MgO, low SiO2 and ΣREE contents, and are characterized by slight depletion of mid-REE, showing ophiolitic ultramafic affinities. Both dolerites and basalts exhibit tholeiitic compositions with high Mg# numbers, low (Na2O + K2O)/SiO2 ratios and ΣREE contents. In comparison, the dolerites show flat patterns of REE, while the basalts display slight enrichment of LREE. All these features suggest they were generated from an E-MORB-type mantle source by different degrees of partial melting. The cherts exhibit distinctive high SiO2 and low ΣREE contents, and show depletion of LREE with negative Ce anomalies and high Y/Ho rations, indicating they are typical abyssal cherts. Together with the comparable basements on both sides of the suture, a back-arc basin setting is suggested for these ophiolitic components. The SHRIMP zircon U–Pb age of 416 ± 34 Ma for the basalt, and the youngest age of 490.6 ± 6.6 Ma for the black siltstones interbedded with cherts signal the back-arc basin from ca. 490–416 Ma. We thus infer a west-Pacific-type active continental margin in the East Kunlun Orogen associated with the northward subduction of the Proto-Tethys oceanic crust in the Early Paleozoic.

Differences in Geochemical Signatures and Petrogenesis between the Van Canh and Ben Giang-Que Son Granitic Rocks in the Southern Kontum Massif, Vietnam

Permian Ben Giang-Que Son and Triassic Van Canh granitic rocks are widely distributed across the southern Kontum Massif, the basement of which consists mainly of metasedimentary rocks. The Ben Giang-Que Son granitic rocks are classified as I- to S-type and ilmenite-series granitic rocks, while the Van Canh granitic rocks are classified as I-type and magnetite-series granitic rocks. Both granitic rock suites exhibit more or less adakitic properties, suggesting that the subduction of the high-temperature Song Ma Ocean crust, part of the Paleo-Tethys Ocean, beneath the Indochina Block produced adakitic magma. It is hypothesized that the differences between the two granitic rock suites were caused by differences in the quantities of incorporated continental crustal materials and carbon or graphite in clastic sedimentary rocks when their adakitic magma intruded into the continental crust. Based on their high initial Sr isotope ratios, the Ben Giang-Que Son granitic rocks evidently incorporated a higher quantity of continental crustal materials compared to the Van Canh granitic rocks, resulting in the former showing the signatures of ilmenite-series and I- to S-type granitic rocks. Consequently, the Ben Giang-Que Son granitic rocks have relatively high A/CNK ratios and high total Al contents in their biotite, whereas the Van Canh granitic rocks have low A/CNK ratios and low total Al contents in their biotite. The intrusion of the Ben Giang-Que Son granitic rocks caused high-temperature metamorphism, which decomposed some of the carbon or graphite in the surrounding continental crustal materials, such as clastic sedimentary rocks. Meanwhile, the Van Canh granitic rocks, which intruded later than the Ben Giang-Que Son granitic rocks, incorporated smaller quantities of carbon or graphite in continental crustal materials, resulting in them retaining the chemical characteristics of adakitic, magnetite-series, and I-type granitic rocks, different from the Ben Giang-Que Son granitic rocks.

High-pressure melting in metapelites of a 2 Ga old subducted oceanic crust (Usagaran belt, Tanzania): implications from melt inclusions, fluid inclusions and thermodynamic modelling

Investigation of polymineralic melt inclusions preserved in garnet of eclogite-facies metapelites of the Usagaran belt, Tanzania, is of particular importance as these metapelites, intercalated in oceanic metabasites, document the rare case of partial melting at high temperatures in a subducted oceanic crust. With an age of 2 Ga the rocks represent one of the oldest oceanic crusts and confirm a subduction process already at Paleoproterozoic times. Partial melting probably was initiated by dehydration melting under the presence of a CO2-rich fluid phase. The melt is preserved in siliceous polymineralic inclusions, while CO2 locally reacted with the garnet host to form dolomite-quartz-kyanite inclusions. During this reaction, the REE spectrum of garnet is adopted by the dolomite. Furthermore, graphite inclusions in garnet must have precipitated from the CO2 fluid by reduction. The highly ordered graphite structure indicates a formation temperature of at least 700 °C. Rehomogenization experiments of the siliceous polymineralic inclusions yield a homogeneous melt of rhyolitic, peraluminous composition. Thermodynamic modelling enables to deduce a P–T path in accordance with high P–T conditions (minimum 2.0 GPa, 900 °C) where a partial melt formed due to phengite breakdown leading to the preserved peak mineral assemblage garnet, alkali feldspar, kyanite, quartz and rutile. A very fast uplift of the oceanic crustal rocks can be deduced from the occurrence of very finely exsolved metastable ternary feldspar and from the preserved prograde zoning in garnet.

Munabulake ophiolitic mélange: Implication for the evolution history of the north branch of the Proto-Tethys ocean

One of the ophiolites that record the Proto-Tethys Ocean’s episodic closure is the Munabulake ophiolitic mélange, which is situated in the middle of the Kunlun–Qaidam and Altun–Qilian blocks. Detailed field mapping revealed that the Munabulake ophiolitic mélange comprises local (ultramafic rocks, basalts, andesites, gabbros, diorites, and plagiogranites) and exotic (marble, gneiss, schist, and amphibolite) blocks, many of which are in the schist matrix (Qimantage Group). Based on geochronological, geochemical, and petrological observations, the mafic rocks in the Munabulake ophiolitic mélange can be categorized into three categories: 498-Ma OIB-like gabbros, 468-Ma Hawaiian alkaline basalt-like dolerite and pillow basaltic slices, and 428 Ma massive SSZ-like ultramafic rocks. The 501–452 Ma I-type granites exhibit arc affinities due to the oceanic crust subduction. These findings, along with spatial relationships, suggest that the Early Paleozoic ophiolite complex, island arc rocks, and accretionary complex generated as an intra-oceanic arc system as a result of obduction of the south Altun Ocean’s onto the Central Altun block within a north-directed subduction event. A dextral strike–slip was evident throughout the Early Paleozoic oceanic crust subduction based on the whole set of planar and linear structural data, and the subduction polarity was likely to the north. According to the ophiolitic mélange’s youngest rocks and the existence of 413 Ma granite dykes that are widely exposed in the Munabulake ophiolitic mélange, the Munabulake ophiolitic mélange was most likely emplaced during the Middle Silurian. This Munabulake ophiolitic mélange is similar in age and petrochemical characteristics to the other ophiolites in the South Altun subduction–collision belt, indicating that the Manabulak ophiolite mélange is a westward extension of the Apa–Mangya subduction–collision belt, which formed during the northward subduction of the South Altun Ocean slab during the Early Paleozoic. Thus, the final closing time of the South Altun Ocean is between 413 and 428 Ma.