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Determining subduction‐related processes is crucial for understanding lithological heterogeneity, as substantial quantities of slabs are recycled into the mantle. Molybdenum isotopes are valuable for distinguishing sources materials due to the significant isotope differences between the crust and the mantle. In this study, we systematically investigate Mo isotopes in a suite of well‐characterized continental basalts from Keluo and Halaha‐Chaoer, located in Northeast China. The δ98Mo values of Keluo range from −0.41 to −0.23‰, with an average of −0.34‰, while Halaha‐Chaoer samples ranged from −0.18 to −0.12‰, with an average of −0.15‰. The δ98Mo values of Keluo basalts are lighter than the mean value of fresh oceanic basalts (−0.21‰), whereas those of Halaha‐Chaoer basalts are similar to oceanic basalts. Combined with other geochemical indications (LOI, Ce/Pb, La/Yb and so on), the Mo isotopic variations cannot be attributed to chemical weathering, continental crust contamination or magmatic processes. Instead, the δ98Mo variations in this study are explained by the incorporation of different oceanic crustal materials into the magma sources. Correlations of δ98Mo with Ba/Th, Th/U, 143Nd/144Nd indicated that both sediment and altered oceanic crust have significantly influenced these variations. This study demonstrates the potential of Mo isotopes to distinguish different types of recycled oceanic crust materials. [ABSTRACT FROM AUTHOR]

The subducting Pacific slab stagnates in the mantle transition zone and creates a big mantle wedge (BMW) system in East Asia. A similar BMW structure may have already existed since the Early Cretaceous (>120 Ma), but how such a structure evolved from Early Cretaceous to the present day remains unclear. We address this issue by comparing compositions and source heterogeneity of the 106–58 Ma basalts from Liaodong Peninsula and its adjacent areas (LPAA) in eastern China, with those formed in the modern BMW setting. The LPAA basalts display oceanic island basalts–like trace element patterns. Elemental and isotopic compositions of these basalts and their olivine phenocrysts point to peridotite and two recycled components in their source. One recycled component is altered lower oceanic crust given the low δ18Oolivine (2.8–5.2‰) of the ~99 Ma Liaoyuan alkali basalts. The second component consists of altered upper oceanic crust and pelagic sediments indicated by high δ18Oolivine (>6.0‰), represented by the ~58 Ma Luanshishanzi alkali basalts. The depleted mantle-like isotopes of these two components suggest derivation from a young HIMU source with characteristics of the Izanagi plate (e.g. Indian Ocean-type Sr-Nd-Pb-Hf isotopes), which may have resided in the mantle transition zone at that time. Our results reveal strong similarities between chemical and source characteristics of the mantle sampled by the 106–58 Ma LPAA basalts and those derived from the modern BMW. This implies that the BMW structure has been present since the Early Cretaceous, probably having lasted more than 120 Myr, and modulating the chemical properties of the upper mantle and influencing a variety of geological processes. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogenic Belt (EKOB), northwestern China, recording long-term and multiple accretionary and collisional events of the Tethyan Ocean, belongs to a high-pressure to ultra-high-pressure (HP-UHP) metamorphic belt that underwent complex metamorphic overprinting in the early Paleozoic. In this contribution, we carry out an integrated study, including field investigations, petrographic observations, whole-rock analyses, zircon U-Pb dating, and P-T condition modeling using THERMOCALC in the NCKFMASHTO system for the eclogites, especially for the newly discovered UHP eclogite in the eastern part of EKOB. The eclogites exhibit geochemistry ranging from normal mid-ocean ridge basalt (N-MORB) to enriched mid-ocean ridge basalt (E-MORB). Zircons from the eclogites yield metamorphic ages of 416–413 Ma, indicating the eclogite facies metamorphism. Coesite inclusions in garnet and omphacite and quartz exsolution in omphacite and pseudosection calculation suggest that some eclogites experienced UHP eclogite facies metamorphism. The eclogites from the eastern part of EKOB record peak conditions of 29–33 kbar/705–760 °C, first retrograde conditions of 10 kbar at 9.5–12.5 kbar/610–680 °C, and second retrograde conditions at ~6 kbar/<600 °C. New evidence of the early Paleozoic UHP metamorphism in East Kunlun is identified in our study. Thus, we suggest that these eclogites were produced by the oceanic crust subducting to the depth of 100 km and exhumation. The presence of East Gouli and Gazhima eclogites in this study and other eclogites (430–414 Ma) in East Kunlun record the final closure of the local branch ocean of the Proto-Tethys and the evolution from subduction to collision. [ABSTRACT FROM AUTHOR]

Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE-striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via fieldbased, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (~500-550 °C and ~5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep-dominated deformation. A strain rate of 10-12 s-1 and a differential stress of 25-39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47-0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence. [ABSTRACT FROM AUTHOR]

Understanding the subduction-driven recycling of economically important chalcophile metals (e.g., Cu, Au, Ag, and platinum-group elements, PGEs) and volatiles (e.g., sulfur) is critical for assessing models of ore formation. However, whether the sulfide remains stable in recycled altered oceanic crust (AOC) in the mantle is unclear, and the influence of AOC-derived metasomatic agents on the extraction efficiency of chalcophile elements from the deep mantle remains poorly understood. Here we report whole-rock Cu-Ag-Au-Re-PGEs of Cenozoic alkaline basalts at Hannuoba, eastern China, which were derived from the interaction of carbonated eclogite (recycled AOC)-derived melts with lithospheric peridotites. Volatile-rich primitive melts from carbonated eclogite are poor in strongly chalcophile metals (e.g., ∑PGE < 1 ng/g, Au < 1 ng/g, Cu < 30 μg/g). Remarkably, primitive melts show significant fractionation of elements with variable chalcophile affinities, such as lower Cu/Ag ratios than those in primitive mantle and MORBs. These features reflect the sequestration of these metals in residual mono-sulfide solid solution (MSS) during low-degree melting of the AOC in the deep mantle. Given the retention of sulfides in the AOC, partial melts derived from it are rich in sulfur but poor in strongly chalcophile metals. The Hannuoba alkaline melts scavenged some strongly chalcophile metals from peridotites (especially PGEs) during their ascent. However, this did not necessarily lead to the formation of chalcophile metal-enriched melts (e.g., Au < 1 ng/g and Cu < 60 μg/g). These processes appear to be common in eastern China, as evidenced by the low abundances of Cu and Au found in many primitive intraplate alkaline basalts. We propose that strongly chalcophile elements in subducted AOC are efficiently transported into, and stored in, the deep mantle, with a small proportion returning to the exosphere through volcanism. Moreover, the chalcophile metal-poor and volatile-rich melts of subducted AOC could also explain the addition of abundant sulfur (secondary sulfides), but limited chalcophile metals to the metasomatized mantle. [ABSTRACT FROM AUTHOR]

The Aqishan-Caixiashan polymetallic ore cluster, located in the eastern Tianshan Orogenic Belt, is part of the southern Central Asian Orogenic Belt (CAOB) and experienced extensive tectono-magmatic events with polymetallic mineralization during the Carboniferous. However, the Carboniferous tectonic affinity and magmatic evolution of the ore cluster are still controversial, which limits further understanding of the tectonic evolution and regional metallogeny. With the aim of solving these problems, we have carried out geochronological and geochemical studies on Carboniferous intrusive rocks and volcanic rocks from the ore cluster. The samples of the Carboniferous magmatic rocks are enriched in LREEs and LILEs but depleted in HFSEs, which attest to the I-type affinity and are similar to those formed in a continental arc. The early Carboniferous granodiorites and granites (350–343 Ma) in the Central Tianshan Block (CTB), with high Mg# values of 37–66, low Y values of 2.3–17.2 ppm, and high Sr/Y ratios (34–85), show characteristics of adakites that are formed by partial melting of delaminated oceanic crust and/or lower crust. Additionally, they have a wide range of εHf(t) values (5.4–15.9), high εNd (t) values (1.78–6.31) and low ISr values (0.705–0.706), indicating a mixed source of depleted mantle and lower crust for its origin. The Late Carboniferous volcanic rocks (314–306 Ma) in the Aqishan-Yamansu Tectonic Belt (AYTB) north of the CTB have obviously negative Nb–Ta-Ti anomalies, as well as high Y contents (12–20 ppm) and low Sr/Y ratios (5.6–29.9) compared to primitive mantle, which has a relatively flat chondrite-normalized REE pattern, resembling arc-related magmas. Additionaly, they have positive values of εHf(t) values (8.7–14.7), high εNd(t) values (3.04–4.45) and low ISr ratios (0.704–0.707), showing a depleted mantle source, which is weakly affected by the contamination of ancient crustal material. Combining this study and previous works, we suggest that the continuous southwards subduction of the North Tianshan Ocean during the Carboniferous generated the continental arc and forearc basin in the northern part of the CTB. The expansion of the Aqishan-Yamansu forearc basin formed intrusions and volcanic formations along the arc belt. The early Carboniferous intermediate-felsic mantle-derived adakitic magmas in the CTB are probably related to regional Zn-Pb enrichment and mineralization, and the Late Carboniferous intrusions and volcanic formations in the AYTB are related to large-scale Fe-Cu-Zn-Pb-Ag mineralization. [ABSTRACT FROM AUTHOR]

Genesis of the large-volume alkaline crust at active continental margin is still enigmatic for geologists worldwide. The point at issue is whether or not subducted oceanic crusts get involved and how they interact with the mantle source of the alkaline crust. Late Mesozoic juvenile alkaline crusts with high εNd(t)-εHf(t) values are widely distributed in the Great Xing'an Range of NE China, as parts of an arc magmatic belt related to the Mongol-Okhotsk Ocean closure. We carried out multiisotope analyses and 2-D high-resolution numerical modeling to trace the mantle source nature of the alkaline crust. The alkaline rocks show similar trace elements with the I-type enriched mantle and are originated from an upwelling oceanic-island basalt-like mantle. Their high field strength element depleted arc features indicate the crustal material addition in the source region. Low δ18O, mantle-like Sr-Nd-Hf and light Mg isotope compositions, limited δ7Li variations, no Nd-Hf decoupling, and our mixing calculation preclude continental crustal assimilation, marine-sediment melt and/or altered oceanic crust (AOC)-fluid metasomatism, and bulk marine sediment involvement, and provide evidence of the bulk AOC addition in the mantle source. Lower δ18O values than the mantle and relatively low δ7Li values further confirmed the involved AOC to be a high-temperature (high-T) AOC. Our multiisotope tracing successfully fingerprints the recycled high-T AOC into the source region of the alkaline juvenile crust. Then, our 2-D high-resolution numerical modeling reconstructs the high-T AOC recycling processes driven by mélange melting. [ABSTRACT FROM AUTHOR]

Although carbonates are the primary form of carbon subducted into the mantle, their fate during recycling is debated. Here we report the first coupled high‐precision Zn and Mo isotope data for Cenozoic intraplate basalts from western China. The exceptionally high δ66Zn values (+0.39 to +0.50‰) of these lavas require involvement of recycled carbonates in the mantle source. Variable δ98Mo compositions (−0.39 to +0.27‰) are positively correlated with Mo/Ce, best interpreted as mixing between isotopically light Mo from dehydrated oceanic crust and heavy Mo from recycled carbonates, which is also supported by positive coupling between δ66Zn and δ98Mo. Modeling reveals that involvement of ≤5% carbonate‐bearing oceanic crust fully resolves the observed δ66Zn–δ98Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined δ66Zn–δ98Mo data sets for mantle‐derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science. Plain Language Summary: Carbon is an element of life and studying its geological cycle is crucial for understanding Earth's evolution including formation of a life‐supporting atmosphere. Here we report the first combined high‐precision Zn and Mo isotope data for Cenozoic intraplate lavas from western China, showing that the basalts record ≤5% carbonate‐bearing oceanic crust components in their mantle source. Our results provide new evidence for surficial carbonates being delivered into the deep upper mantle, which adds to the debate about the deepest extent of the terrestrial carbon cycle. Key Points: First combined zinc (Zn) and molybdenum (Mo) isotope data for mantle‐derived magmas to track the fate of subducted carbonatesZn−Mo isotopic compositions of Cenozoic Tarim basalts suggest surficial carbonates being delivered into the deep upper mantleWe highlight the utility of combined Zn−Mo isotope data as a powerful tool in deep carbon science [ABSTRACT FROM AUTHOR]

The newly identified Neoproterozoic Yuanmou high-Nb mafic and adakitic rocks at the western margin of the Yangtze Block in South China are investigated for petrogenesis and geodynamic implications. Zircon U-Pb ages show the adakitic granodiorite host and mafic microgranular enclaves (MMEs) were synchronously emplaced at 811-806 Ma, and were cut by an 802 ± 11.5 Ma mafic dike. The medium-grained granodiorites consist mainly of plagioclase, quartz, K-felspar, biotite, and amphibole. High SiO2 (62.8-67.5 wt%), Al2O3 (13.1-16.8 wt%) contents, high Sr/Y (17.4-49.0), and (La/Yb)N (16.3-52.6) but low Y (6.83-16.1 ppm) and Yb (0.74-1.82 ppm) concentrations of the granodiorites point toward their calc-alkaline and adakitic nature. Further, their high Dy/Yb (1.52-2.31) and Gd/Y (2.35-4.43) and low K2O/Na2O (0.22-0.48) and Th/La (0.14-0.20) ratios, constant bulk rock εNd(t) (-0.5 to -1.5) and zircon εHf(t) (0.0 to +2.3) values, suggest that the granodiorites were derived from partial melting of a mafic lower crustal source. The MMEs in the granodiorites and mafic dike intrusions are composed of amphibole, plagioclase, clinopyroxene, and olivine, and have similar chemical compositions, such as high Nb (15.7-41.9 ppm) and TiO2 (2.13-3.39 wt%) contents and high Nb/U (18.7-30.9) values, akin to alkaline high-Nb basalts (HNB). Their variable Ba/Nb (7.68-79.2), Nb/Zr (0.09-0.16), high Nb/Th (5.47-6.68), and low Th/Zr (0.01-0.03) and La/Nb (1.03-1.37) ratios indicate a mantle source modified by melts derived from subducted oceanic crust and sediments. The Yuanmou HNB with high εNd(t) (+4.8 to +6.9) were probably generated during back-arc extension by decompression melting of a mantle source metasomatized by adakitic slab melts formed during earlier stages of the subduction. This implies that the location of the back-arc was far from the subducted oceanic slab, possibly due to a slab rollback process. In this scenario, HNB intrusions provided the heat for partial melting and adakitic melt formation in the mafic lower crust. The model suggests that the Yuanmou and other similar associations of HNBs and adakitic rocks emplaced along the western margin of the Yangtze Block are products of a widespread Neoproterozoic back-arc extension. [ABSTRACT FROM AUTHOR]

This paper presents a study of the late Cenozoic Chifeng basalts (CBs) of NE China, including their olivine He isotopic compositions, whole‐rock major‐ and trace‐element contents, and whole‐rock Sr–Nd–Hf–Pb–Mo isotopic compositions, with the aim of constraining their mantle source. Results show that the basalts have high MgO, low CaO contents, and high FeOT/MnO values, which indicate that their mantle lithology was most likely pyroxenite. The CBs also exhibit ocean‐island‐basalt‐like trace‐element patterns (e.g., enrichment in light rare earth elements and high‐field‐strength elements) and have depleted Sr–Nd–Hf and relatively radiogenic Pb isotopic compositions, requiring both depleted and enriched components in their mantle. The low olivine He (3He/4He = 0.8–5.5 Ra) and whole‐rock Mo (δ98/95Mo = −0.71‰ to −0.18‰) isotopic values of the CBs, together with geophysical evidence, indicate that the rocks were derived from a depleted MORB mantle (DMM) enriched by recycled oceanic crust that was sourced from the mantle transition zone (MTZ). During the late Cenozoic, ascending wet mantle plumes triggered by dehydration of a stagnant Pacific oceanic slab are inferred to have transported preexisting recycled Pacific oceanic crust from the MTZ into the overlying asthenosphere mantle. The upwelling Pacific oceanic crust reacted with asthenospheric mantle peridotite (i.e., DMM) to produce mantle pyroxenite, whose partial melting at shallow depths generated the CBs. Considering the low δ98/95Mo values of both the CBs and coeval potassic basalts from NE China, we speculate that there may be a low δ98/95Mo reservoir in the MTZ beneath NE China. Key Points: The Chifeng basalts (CBs) exhibit relatively low 3He/4He ratios ranging from 0.8 to 5.5 RaThe mantle source end‐members of CBs were the depleted mid‐ocean‐ridge basalt mantle and recycled oceanic crustA low δ98/95Mo reservoir is inferred in the mantle transition zone beneath NE China [ABSTRACT FROM AUTHOR]

Lamprophyre is one of the most important probes to understand the enrichment and depletion processes of the lithospheric evolution. Determining the age and petrogenesis of lamprophyres could provide critical data for deducing the timing and process of lithospheric revolution of the Sulu Orogenic Belt (SOB) and North China Craton (NCC). In this study, the Taolin lamprophyre in SOB was delineated by zircon U–Pb dating with the ages of 126 Ma. This lamprophyre is characterized by high contents of SiO2 and total alkali contents (Na2O + K2O), low Mg# value, low contents of total Fe2O3, MgO, and compatible elements, enriched in LREE and LILE but strong depleted in HFSE, as well as enriched Sr–Nd and depleted Pb isotope compositions (87Sr/86Sr(i) = 0.707757 to 0.708048, εNd(t) = – 13.0 to – 12.7, 206Pb/204Pb(t) ratios of 16.934 to 17.000, 207Pb/204Pb(t) of 15.435 to 15.442, and 208Pb/204Pb(t) of 37.682 to 37.706). Interpretations of whole-rock elemental data suggest the Taolin lamprophyre was derived from 1% to 5% partial melting of spinel peridotite; and MELTS modelling indicates fractional crystallization of ~16.7% orthopyroxene and ~16.6% clinopyroxene under 1.5 Gbar. Based on the analyses of Neoproterozoic relic zircons and Sr–Nd–Pb isotopic components of the Taolin lamprophyre and adjacent lamprophyres, we put forwards that there are several subducted crustal materials were trapped into the SOB lithospheric mantle, and the magma source of these lamprophyres might have experienced metasomatic processes not only by the hydrous fluids derived from the subducting Paleo–Pacific oceanic crust in the Jurassic, but also by melts derived from partial melting of the subducting South China continental crust in the Triassic. A geodynamic trigger for generation of the Taolin lamprophyre is proposed to be related to lithospheric thinning coupled with asthenospheric upwelling beneath the North China Craton. [ABSTRACT FROM AUTHOR]

The Daliuhang gold deposit in the Qipengfu (Qixia–Penglai–Fushan) ore concentration area is a typical gold deposit of medium-low temperature hydrothermal veins. Uncertainties regarding the primary sources of ore-forming fluids, as well as whether host rocks contribute materials to the mineralization of the gold deposits in the Jiaodong Peninsula, are still subject to intense debate. Hydrogen–oxygen isotope results show that atmospheric water is involved in ore-forming fluids. According to the results of the helium–argon isotopes of pyrite, it is hypothesized that the initial fluid source was located in the oceanic crust or upper mantle lithosphere above the Early Cretaceous Paleo-Pacific Plate, as it was subducted into the eastern part of the eastern North China Craton. In situ sulfur isotope results show that high δ34S values characterize the pyrite in the main mineralization period. It is inferred that during the thinning and melting process of the lithospheric mantle, the volatile components enriched in pyrite contributed to the release of δ34S. At the same time, when the fluids ascended to the weak zones, such as fissures of ore-endowed peripheral rocks, the δ34S in the peripheral rocks were extracted, and the two processes acted together to cause high δ34S values to occur. Similarly, the lead and strontium isotopic compositions indicate a crust–mantle mixing attribute of the mineralized material source. The zircon U–Pb age of the ore-hosting granodiorite was 130.35 ± 0.55 Ma, and the Rb–Sr isochron age of the pyrite from the main mineralization period was 117.60 ± 0.10 Ma, which represents the timing of felsic magmatism and gold mineralization, respectively, with at least 10 Ma between the magmatism and mineralization. The magma gradually cooled over time after its formation, and when the granodiorite cooled down to 300 ± 50 °C, the temperature and pressure conditions were most conducive to the precipitation of gold. It is inferred that gold-rich initial mantle fluids with volatile components, rising along tectonically weak zones, such as fractures, underwent fluid phase separation in the fractured position of the granite and extracted the gold from the granodiorite, forming gold deposits. [ABSTRACT FROM AUTHOR]

This paper reports two types of basalt that have different ages (Carboniferous and Permian) but occur next to each other in the northern part of the Changning–Menglian accretionary wedge, southwestern China. We use the geochronology and geochemical data to evaluate the tectonic evolution of the Palaeo‐Tethys during this period. Zircon grains from the mafic‐ultramafic rocks associated with the Permian basalts in the Xiaomengtai area yield a U–Pb age of 281 Ma and εHf(t) values from +9.2 to +12.8. The Permian mafic‐ultramafic rocks and the associated basalts are all characterized by normal mid‐ocean ridge basalt (N‐MORB)‐like chondrite‐normalized rare earth elements patterns, moderately negative Nb‐Ta anomalies in the mantle‐normalized immobile incompatible trace element patterns, and positive εNd(t) values from +4.2 to +6.5, which are consistent with the geochemical characteristics of mafic‐ultramafic rocks in supra‐subduction zone (SSZ)‐type ophiolite. On the contrary, the Carboniferous basalts, which are associated with marine carbonates, are characterized by light REE enrichments, slightly positive Nb‐Ta anomalies, and εNd(t) values from +2.8 to +4.0. These features are similar to those of typical oceanic island basalts (OIBs) worldwide. Modelling results using REEs show that the parental magmas for the Carboniferous OIBs and the Permian basalts were likely derived from mantle peridotites at the depths of garnet and spinel stability, respectively, consistent with the formation depth of these two different types of basalt globally. The occurrence of these two different types of mafic‐ultramafic rocks with significantly different ages in the same area supports the view that they are the remnants of the accreted Palaeo‐Tethys oceanic crust. The Carboniferous OIBs are considered to be parts of an OIB‐carbonate seamount chain that originally formed in the southern part of the Palaeo‐Tethys. The Permian mafic‐ultramafic rocks are regarded as fragments of SSZ‐type ophiolites that were present in the northern part of the Palaeo‐Tethys. These different pieces of oceanic crust were accreted to the Simao–Indochina continental Block by subduction between the Late Permian and the Triassic. [ABSTRACT FROM AUTHOR]

To ascertain the Early-to-Middle Jurassic tectonic setting in the central Great Xing'an Range, this study investigated the Early and Middle Jurassic granitoids exposed in the Chaihe area in the central Great Xing'an Range based on isotopic chronology and petrogeochemistry. The results of this study show that the Early and Middle Jurassic granitoids have emplacement ages of 179–172 Ma. Moreover, the Early and Middle Jurassic granitoids are high-K calc-alkaline unfractionated I-type granitoids and high-K calc-alkaline fractionated I-type granitoids, respectively. The magma sources of the Early and Middle Jurassic granitoids both originated from the partial melting of newly accreted lower crustal basaltic rocks. Meanwhile, the Middle Jurassic magma sources were mixed with mantle-derived materials or ocean-floor sediments formed by the dehydration and metasomatism of subducted slabs. The Early and Middle Jurassic granitoids in the study area were formed in the subduction environment of the oceanic crust, in which the Mongol-Okhotsk oceanic plate was subducted southward beneath the Eerguna and Xing'an blocks. Moreover, the Siberian plate began to collide and converge with northeast China during the Middle Jurassic. [ABSTRACT FROM AUTHOR]

Recycled oceanic crust is believed to have played an important role in the formation of continental flood basalts, whereas the involvement of large amounts of CO2 derived from recycled marine carbonate in their mantle sources is highly debated. Zn isotopes have great potential to trace recycled carbonate due to the distinctly different δ66Zn values between marine carbonates and the mantle. Representative continental flood basalt samples from Keping (Group1) and Yingmai and Shengli (Group 2) in Tarim, NW China, were collected to investigate their mantle sources, and their Zn isotopes were studied systematically for the first time. The Zn isotope values of Keping basalts (between 0.29‰ ± 0.03‰ and 0.32‰ ± 0.05‰) are higher than values in the primitive mantle (δ66Zn = 0.16‰ ± 0.06‰) but similar to those of mid-ocean-ridge basalt (MORB; δ66Zn = 0.24‰-0.31‰). Considering their high fractionation (e.g., Mg# = 0.28-0.37) and potential involvement of crustal contamination (87Sr/86Sri between 0.70720 and 0.70779; εNd between -3.2 and -1.9), their Zn isotopes may not conclusively point to a carbonated mantle source. In contrast, Yingmai and Shengli basalts show heavy Zn isotope values (between 0.32‰ ± 0.03‰ and 0.39‰ ± 0.03‰), nonradiogenic 87Sr/86Sri (0.70459-0.70518) and εNd(t) (between -1.3 and 0.1) signatures, and less fractionation (Mg# = 0.46-0.71), implying the involvement of recycled carbonate components in their mantle source. Nonetheless, the lack of negative Zr, Hf, and Ti anomalies, low CaO/ Al2O3 ratios, and high SiO2 contents preclude direct melting of carbonate-bearing mantle. Alternatively, these features may suggest the melting of decarbonated subducted eclogite and variable interaction with subsolidus peridotite. This assumption is consistent with the positive correlations of δ66Zn with Gd/Yb and Sm/Yb ratios and Zn content as a result of mixing between eclogite-derived high-δ66Zn melt and peridotite-derived mantle-like δ66Zn melt. Our study provides a new model to reconcile the geochemical features of the Tarim continental flood basalts and highlights the potential role of decarbonated eclogite in the formation of continental flood basalts. [ABSTRACT FROM AUTHOR]

The zircon-bearing Liushuzhuang Peridotite in the Tongbai Orogen provides insights into the composition and evolution of Paleozoic sub-arc lithospheric mantle between the North China and Yangtze cratons. This orogenic peridotite is dominated by clinopyroxene-free spinel facies harzburgite to dunite. The refractory nature is indicated by high olivine Mg# (up to 92.4), elevated spinel Cr# (74–87), and low orthopyroxene Al2O3 (mostly < 1.0 wt.%). The occurrence of zircon, amphibole, phlogopite, apatite, and LREE-enriched orthopyroxene, with Raman probe analyses showing amphibole and phlogopite inclusions in zircon, testifies to extensive fluid-driven metasomatism of the mantle at ca. 420 Ma (U–Pb zircon). The high spinel TiO2 contents (0.4–1.4 wt.%), the Cl-enriched apatite, and the hydrous secondary phases indicate that the metasomatic agent was volatile- (CO2 + H2O) and TiO2-rich. The elevated oxygen fugacity (∆logfO2 (FMQ) > 2.5) coupled with high spinel Cr# of peridotites suggest that they formed within highly oxidizing supra-subduction zone environment. In-situ87Sr/86Sri amphibole and apatite data indicate that the metasomatic fluids were slightly to moderately radiogenic (0.7032–0.7090) and therefore likely partially derived from the subducted Shangdan oceanic crust that separated the North China and Yangtze cratons. The very low 176Lu/177Hf (mostly < 0.0002) of metasomatic zircon and similar εHf(t) to Tongbai Orogen arc-related magmas implies that zircon grew during modification of sub-arc mantle. Juxtaposition with the host gneisses occurred after 410 Ma and before U–Pb closure during apatite cooling at ~ 340 Ma. The Liushuzhuang Peridotite therefore records intense metasomatism of the Paleozoic sub-arc mantle wedge prior to collision of the North China and Yangtze cratons. [ABSTRACT FROM AUTHOR]

Determining subducted processes is very important for understanding lithological heterogeneity because substantial quantities of slabs are recycled into the mantle. Molybdenum isotopes have been used to distinguish sources materials because of the significant Mo isotopic differences between the crust and the mantle. Here, we undertake a systematic Mo isotopic investigation on a suite of well-characterized continental basalts from Keluo and HalahaChaoer, located in Northeast China. δ98Mo of Keluo varied from -0.41 to -0.23‰ with an average of -0.34‰, and samples from Halaha-Chaoer ranged in δ98Mo from -0.18 to -0.12‰ with an average of -0.15‰. The former is lighter than the mean value of fresh oceanic basalts (-0.21‰), while the latter is similar to oceanic basalts. Combined with other geochemical indications (LOI, Ce/Pb, La/Yb and so on), the Mo isotopic variations cannot be interrupted by chemical weathering, continental crust contamination, or magmatic processes. The δ98Mo variation in this study is explained by adding various oceanic crustal materials in magma sources. The relationships of δ98Mo with Ba/Th, Th/U, 143Nd/ 144Nd for the samples indicated that sediment and altered oceanic crust have contributed to the significant variations in these samples. This study indicates the great potential of Mo isotopes to distinguish different types of recycled oceanic crust materials in the mantle. [ABSTRACT FROM AUTHOR]

Serpentinites of the mantle wedge were identified from a composite oceanic–continental subduction zone that occurs as the Hong'an orogen in east–central China. They were comprehensively investigated by an integrated approach of whole-rock major and trace elements, mineral major and trace elements, whole-rock and olivine O isotopes, and phase equilibrium modeling as well as zircon U–Pb ages, trace elements, and Hf-O isotopes. These serpentinites show high Mg# and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents, resembling those from the mantle wedge rather than the subducting oceanic slab. Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge. Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic. The enrichments of Th, U, light rare earth elements and high field strength elements in the whole rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids. The ca. 310 Ma zircon domains exhibit positive ε Hf(t) values of 4.5 to 19.1 and variable δ18O values similar to those of oceanic-type eclogites in the Hong'an orogen but different trace element compositions, indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust. The ca. 430 Ma zircons exhibit U–Pb ages, trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites. The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high- to ultrahigh-pressure metamorphic rocks in the Hong'an orogen. Thus, these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust. High Th/U ratios for the ca. 310 Ma zircons imply their formation through the metasomatic reaction rather than direct precipitation from the subducting oceanic crust-derived fluids. The metasomatic zircons of Triassic age show negative ε Hf(t) values of −16.0 to −2.7, indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during the Triassic continental collision. In this context, the mantle wedge peridotites were first metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids. Afterwards, part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel, where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths. The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there. This gave rise to metamorphic olivine with extremely high Fo values of 96.7 to 97.6 and higher MnO but lower NiO contents than common mantle olivine. The coexistence of lizardite, brucite and magnesite in the serpentinites indicates their retrograde processes of serpentinization and carbonation on the subsurface. Therefore, the present observations and interpretations of orogenic serpentinites provide insights into the multistage fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision. [ABSTRACT FROM AUTHOR]

The origin of EM1 (Enriched Mantle 1) reservoir, initially defined by the ocean island basalts (OIBs) with extremely low 143Nd/144Nd and 206Pb/204Pb, has been long debated, because melting of the ambient refractory peridotite along with the EM1 component will dilute the "EM1 fingerprints" recorded in these rocks. Comparing to the OIBs, Cenozoic potassic basalts from northeast China, the typical EM1‐type basalts in continental region, are formed at a lower‐degree melting, and therefore have the chance to preserve more information of the EM1 component. Here high‐precision Fe isotopes of these potassic basalts are reported to constrain the source lithology. Their δ57Fe (0.15–0.28‰) are positively correlated with the K2O, SiO2, K/U, and Rb/Y, and negatively correlated with the εNd and δ26Mg, forming binary mixing arrays. One endmember is the inferred EM1 reservoir, whereas the other is the local lithospheric mantle. Major elemental compositions of the melts released from the EM1 component resemble those sediment‐derived experimental melts. Combining with their heavier Fe isotopes and higher Zn/Fe ratios relative to those mid‐ocean ridge basalts (MORBs), an eclogitic source of these potassic basalts is therefore proposed to account for these features. Differing from the most conventional thinking of the metasomatized, phlogopite‐bearing lithospheric mantle, we argue that the EM1 component in the source of continental potassic basalts are composed of ancient subducted crustal materials (i.e., recycled sediment ± oceanic crust). This deep EM1 component will transform into eclogite and release high‐SiO2 potassic melts when ascending to the shallow asthenosphere. Plain Language Summary: Because of the compositional differences between mantle and crustal materials (e.g., oceanic crust, sediment), the subduction of crustal materials into the mantle will cause the chemical heterogeneity of mantle, recording by mantle‐derived basalts. The enigmatic EM1 (Enriched Mantle 1) component found in the mantle source of some oceanic island basalts (e.g., Pitcairn basalts) is frequently speculated to be formed by this process. Such component is also identified in the source of the potassic basalts from northeast China, providing us the ideal candidates for exploring its genesis in the continental setting. Here we use iron isotopes as the new tracer to recognize the mantle source of these potassic basalts. We find that they have high δ57Fe values with low MgO, high K2O and SiO2, and high Zn/Fe. Moreover, the olivines crystallized from these basalts show low Mn/Fe and moderate Ni/(Mg/Fe). All these features suggest that the potassic basalts are most likely to originate from the subducted crustal materials (sediment ± oceanic crust) in the form of eclogite. This finding indicates the similar origin of the EM1 component in the source of both continental and oceanic basalts, and highlights the important role of subducted sediment in formation of the EM1 component. Key Points: The continental potassic basalts from northeast China show EM1 (Enriched Mantle 1) isotopic signature with heavy Fe isotopic compositionsLithology of the EM1 component in the source of the potassic basalts are eclogite derived from the recycled crustal materialsThe interaction between initial eclogite‐derived melt and lithospheric peridotite ultimately forms the EM1‐type potassic basalts [ABSTRACT FROM AUTHOR]

Incompatible element ultra‐depleted melt (UDM) has occasionally been found as olivine‐hosted inclusion in plume‐related oceanic island basalts and mid‐oceanic ridge basalts and is genetically linked with the involvement of lower oceanic gabbroic crust. However, it has rarely been reported in intracontinental magmatism, and the contribution of such a recycled component to the origin of intracontinental magmas remains unclear. Here, we first report UDM inclusions hosted in olivine from Late Cretaceous intracontinental basaltic andesites in South China. These UDM inclusions are characterized by low SiO2 (46.4–47.9 wt%) and extremely high Al2O3 (18.47–21.79 wt%) contents, positive Sr‐Eu anomalies, and low 207Pb/206Pb and 208Pb/206Pb ratios. Such geochemical features resemble those of high‐pressure melts derived from young lower oceanic gabbro. These results, in combination with geochemical modeling, plate reconstruction, and seismic topography, suggest that tearing or fragmentation of the subducted paleo‐Pacific slab during Late Cretaceous may trigger melting of the refractory lower oceanic cumulates. Our results demonstrate that lower oceanic gabbroic crust could also play an important role in the generation of intracontinental magmatism, which may previously have been underestimated. Plain Language Summary: The diversity of recycled oceanic slabs, including marine sediments, upper oceanic crust, lower oceanic gabbro, and oceanic mantle, plays an important role in producing chemical heterogeneity of Earth's mantle. Melt inclusions trapped in olivine crystals are isolated from external melt and have the potential to preserve information on the heterogeneous composition of mantle‐derived magmas, which is completely or partially homogenized in bulk rocks. Incompatible element ultra‐depleted melts (UDMs) have been occasionally found as olivine‐hosting inclusions in plume‐related oceanic island basalts and mid‐oceanic ridge basalts and are considered as evidence for the involvement of lower oceanic gabbroic crust in origin. Here, we are the first to document olivine‐hosted UDM inclusions from Late Cretaceous intracontinental basaltic andesites in South China. Their geochemical features resemble those of high‐pressure melts by partial melting of a young lower oceanic gabbroic crust. Partial melting of the refractory lower oceanic gabbro was likely attributed to slab tearing or fragmentation, as revealed by seismic tomographic images that the torn paleo‐Pacific slabs are still stagnated between 1,000 and 1,500 km in the lower mantle. Our results reveal the role of lower oceanic crust in producing the geochemically heterogeneous mantle even beneath the intracontinental regions. Key Points: This study is the first to document ultra‐depleted melt (UDM) inclusion in intracontinental basaltic andesites from South ChinaThe geochemistry of UDM inclusions resembles those of high‐pressure melts of lower oceanic gabbroic crustThe newly identified UDM inclusions may reflect the contribution of recycled paleo‐Pacific slab to the mantle source of intracontinental magmas [ABSTRACT FROM AUTHOR]

In the Mesozoic, the age of Paleo‐Pacific plate that flatly subducted beneath the South China was ∼80 Ma, resulting in a cold and dense oceanic slab. The transition process from flat to steep subduction of this old slab and factors that affect it are still unclear. Using 2‐D thermomechanical numerical models, we investigate the dependence of the transition mode on the size of TOC (length and thickness) and the subduction rate. The results indicate that under low subduction rates (e.g., 1 cm/yr), even a 100‐km long eclogitized TOC can induce slab delamination beneath continental lithosphere. The maximum subduction rate for slab delamination to occur increases with an increasing length or thickness of TOC. However, when the subduction rate is over a certain critical value (e.g., ≥4 cm/yr), oceanic slab rolls back. The presence of the TOC delays the initial rollback time, as compared to cases with no TOC. In addition, with a TOC of small size (e.g., 100‐km length and 20‐km thickness) under a certain range of subduction rate, an 80‐Ma old slab can maintain a long‐lasting flat subduction. The comparisons between the numerical results and global plate motion models indicate that the northwestward motion of South China Block since ca. 195 ± 5 Ma induced the delamination of oceanic slab. The arrival of the TOC in the Paleo‐Pacific Plate to the continental arc should be later than the previously estimated ca. 250 Ma. Determination of the TOC size requires three‐dimensional simulations and more detailed geological records. Key Points: The maximum subduction rate for slab delamination to occur increases with an increasing length or thickness of the thickened oceanic crustThe arrival time of the thickened oceanic crust to the continental arc of the South China Block is expected to be later than ca. 250 MaThe size of the thickened oceanic crust can be estimated from the arrival time of the thickened oceanic crust and the subduction rate [ABSTRACT FROM AUTHOR]

SUMMARY: Oceanic plateaus (or aseismic ridges) can be either subducted into the deep mantle, or accreted onto the overriding plate. Furthermore, some oceanic plateaus can change subduction mode from steep to flat-slab subduction. What factors control the fate of oceanic plateaus during subduction remain enigmatic. Here, we investigate the controls on these modes and their respective geological effects using 2-D thermomechanical simulations. We systematically examine the characteristics of an oceanic plateau (including crustal thickness and length), plateau-trench distance, convergence rate and eclogitization of the oceanic crust. Our models confirm that the size of the plateau and eclogitization are the main factors controlling the subduction characteristics. For the eclogite models, a relatively thin oceanic plateau (≤20 km thick) undergoes steep subduction, a moderate-scale plateau (25–30 km thick) favours flat-slab subduction and large-scale plateaus (≥35 km thick) are more susceptible to collide and accrete to the overriding upper plate. Eclogitization significantly reduces the formation chance and duration of flat-slab subduction. The switch from flat-slab to steep subduction occurs rapidly (<5 Ma), and the steepening occurs twice as fast as the flattening. The plateau-trench distance determines the location of the break-off, and shallow break-off (<300 km) of the frontal slab will significantly change the subduction pattern. Either fast convergence rates (≥8 cm yr−1) or overthrusting of the overriding plate promotes the formation of flat-slab subduction. The mode changed from flat-slab to steep subduction explains the landward migration of magmatism followed by a trenchward migration in Eastern China since the Mesozoic. [ABSTRACT FROM AUTHOR]

Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a >500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist-to amphibolite-facies metamorphism at 460-400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450-440 Ma and addition of juvenile mantle-derived melts at 420-410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking. [ABSTRACT FROM AUTHOR]

Whether auriferous hydrothermal fluid is generated during crustal metamorphism or by subcrustal processes to form orogenic gold deposits is hotly debated. Noble gas isotope and halogen abundance ratios can provide important constraints to help resolve this controversy. The Danba gold deposit is a rare example of a Jurassic age hypozonal orogenic gold deposit on the western margin of the Yangtze Craton, China. It is characterized by thick barren quartz veins with fractures infilled by thin auriferous quartz veinlets. The syn -gold quartz veinlets are associated with plagioclase and pyrrhotite and are distinguished from the barren quartz vein hosts on the basis of cathodoluminescence, trace-element content and Raman spectroscopy and microthermometry of fluid inclusions. The dominant fluid inclusions in the gold-bearing quartz and plagioclase are H 2 O–CH 4 with vapor fill of ∼ 15–45 vol% and high pressure-corrected trapping temperatures of ∼ 500 °C, low salinities (<4 wt% NaCl equiv.) and high Mg/Fe ratios. Crushing syn -gold quartz and plagioclase released fluid inclusions with 40Ar/36Ar ratios of between 490 and 710, molar Br/Cl ratios of (0.3–0.7) × 10−3 and molar I/Cl ratios of (18–60) × 10−6. Crushing syn -gold pyrrhotite released fluid inclusions with 3He/4He ratios of 0.21–0.38 Ra (Ra = air value, 1.39 × 10−6) that are higher than typical crustal values of < 0.1 Ra, and 40Ar/36Ar of 330–640 that are similar to the gold-related quartz-hosted inclusions. The combined noble gas and halogen data do not favor an auriferous fluid derived from K- and U-rich Proterozoic granitic basement, which is expected to have radiogenic 40Ar/36Ar and 3He/4He ratios. Instead, the data are more consistent with a fluid source from underlying mantle lithosphere containing a high proportion of altered ocean crust (AOC). The auriferous fluid that formed the Danba gold deposit is thus considered to have been derived from a source comprising both metasomatized mantle lithosphere that had been fertilized in a Neoproterozoic subduction event and a portion of relict AOC beneath the western margin of the Yangtze Craton. In contrast with syn -gold fluid inclusions, those in pre-gold barren quartz are H 2 O–CO 2 dominated. Crushing pre-gold quartz released fluid inclusions with 40Ar/36Ar ratios of 2410–5450, Br/Cl ratios of (1.2–1.5) × 10−3 and I/Cl ratios of (60–80) × 10−6 that are distinctly different to the gold-ore stage. The higher 40Ar/36Ar ratios of the pre-gold fluids are consistent with a source for the barren host quartz veins from prograde greenschist- to amphibolite-facies metamorphism of the Proterozoic granitic basement. [ABSTRACT FROM AUTHOR]

The northward subduction of the Paleo-Tethys oceanic crust in the Paleozoic to Mesozoic is critical for the tectonic evolution of the Qinling-Tongbai-Hong'an-Dabie-Sulu-Imjingang-Gyeonggi orogenic belt. However, the Paleozoic geological fingerprint of the Paleo-Tethys oceanic crust subduction in the Dabie-Sulu orogenic belt remains obscure. In the present study, apatite from the Muniushan monzogranite in the Houkuang area was analyzed to constrain the age of metamorphism in the Jiaobei Terrane and is regarded as the response to the Paleo-Tethys oceanic crust subduction in the Early Permian. Muniushan apatite with obvious negative Eu anomaly is enriched in LREE and depleted in HREE. The chondrite-normalized REE patterns of apatite correspond with I-type granitoids and mafic igneous rocks, implying a magmatic origin. Igneous apatite grains have reset compositional zonation in the cathodoluminescence image. Apatites plotted on a support vector machine apatite classification biplot and Eu/Y-Ce discrimination diagram shows a tendency from the region of "mafic igneous rocks and I-type granitoids" to "low and middle metamorphic". This evidence consistently suggests that the Muniushan apatite suffered metamorphism at a later stage. The twenty-six apatite grains from the Muniushan monzogranite yield a metamorphic age of 297 ± 8 Ma (by LA-ICP-MS U-Pb), which is different from the Muniushan zircon SHRIMP U-Pb results of 2110 ± 4 Ma, indicating the metamorphism occurred in the Early Permian and reset the U-Pb system of apatite. The Early Permian metamorphism that occurred in the Jiaobei Terrane is synchronous to the subduction of the Paleo-Tethys oceanic crust and is the response to the Paleo-Tethys oceanic crust subduction. [ABSTRACT FROM AUTHOR]

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve the understanding of plate tectonic processes and the history of continental growth. Here, we report on newly recognized trondhjemite in the north of Diyanmiao ophiolite belt in North China. The trondhjemite was found along the Erenhot-Hegenshan suture zone. U-Pb zircon dating revealed that the trondhjemite crystallized at 309 ± 2.1 Ma. The trondhjemite had a high amount of SiO2 (68.94–76.45 wt %), Al2O3 (13.37–15.90 wt %), and Sr (232–601 ppm); and a low amount of K2O (1.57–2.70 wt %), Y (6.91–9.39 ppm), Ni (1.10–4.19 ppm), and Cr (1.55–13.50 ppm). The Na2O/K2O ratios were 1.90–4.37. There was a lack of negative Eu anomalies. It was relatively enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, K, and Sr; was depleted in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P; and had low total rare-earth element (REE) contents (27.73–49.63 ppm) with distinct REE fractionation (chondrite-normalized (La/Yb)N of 5.76–10.52), which was similar to adakitic rocks formed by partial melting of subducted oceanic crust. The trondhjemite, together with Diyanmiao ophiolite (335.6 Ma), may have formed during the stages of intra-oceanic subduction, suggesting that in the Early Carboniferous–Late Carboniferous, the southern Paleo-Asian Ocean was in its subduction stage. [ABSTRACT FROM AUTHOR]

As a portion of the Central Asian Orogenic Belt (CAOB), the Beishan Orogen is key to the evolution of the Paleo-Asian Ocean (PAO). The Mazongshan ophiolitic mélange in central Beishan Orogen comprises peridotite, gabbro, dolerite, anorthosite, massive and pillow basalt, and chert and turbiditic mudstone. Its age and tectonic environment remain controversial, which seriously affects the understanding of the tectonic evolution of the CAOB and PAO. Geochemical results demonstrated that gabbros could be divided into two distinct groups. Group A was enriched in both large ion lithophile elements (LILE) and high field strength elements (HFSE) and had high total rare earth element (REE) concentrations with the light REE (LREE) distinctly enriched relative to heavy REE (HREE), showing geochemical characteristics of oceanic island basalt (OIB). Group B has enriched LILE but flat HFSE patterns with LREEs slightly depleted relative to HREEs, which was similar to the patterns of normal mid-ocean ridge basalt (N-MORB). Basalts and peridotites displayed consistent geochemistry of MORB. Negative εSr(t) (− 44.89 to − 33.45) and positive εNd(t) (2.21 to 3.29) values of Group A indicated mantle origin. Gabbro and dolerite from Group A yielded a U–Pb age of 420 ± 2 Ma and 414 ± 3 Ma, respectively, but gabbro from Group B and tuff yielded a U–Pb age of 486 ± 15 Ma and 425 ± 6 Ma. The anorthosite dykes were formed approximately during 370 ± 5 Ma. These results demonstrate that the Mazongshan ophiolite represents relicts of the Ordovician–Devonian Paleo-Asian oceanic crust formed in the mid-ocean ridge and oceanic plateau environments. [ABSTRACT FROM AUTHOR]

Archean K-rich intermediate rocks hold important information of crust-mantle geodynamic evolution of the early Earth. However, the nature of their magma sources and thermodynamic origin remain enigmatic. Two distinct groups of late Neoarchean (∼2551–2498 Ma) K-rich intermediate rocks were identified in the North China Craton. The high-Mg group is compositionally similar to low-Ti sanukitoids, and includes mostly quartz diorites and monzonites, which show high MgO (2.5–6.7 wt%) and Mg# (38.6–65.1), but low TiO 2 (0.4–0.9 wt%), with zircon Ɛ Hf (t) of +2.3 to +7.4 and δ18O of 5.2‰ to 6.5‰. In comparison, the high-Fe-Ti group rocks are titanite-bearing monzonites and compositionally similar to the very rare high-Ti sanukitoids that were previously only found in the Kaapvaal and Superior cratons. They show higher TiO 2 (1.1–1.6 wt%) and Fe* (FeO T /(FeO T + MgO), 0.75–0.86), lower Mg# (22.9–37.5), with lower zircon Ɛ Hf (t) (+1.3 to +5.5) but heavier δ18O (5.6‰ to 7.1‰). Zircon Hf–O isotopic and thermodynamic modeling suggest that the two groups of intermediate rocks were derived from variably metasomatized mantle sources. In detail, the high-Mg rocks originated from a shallow mantle source (∼2 GPa) metasomatized by ∼15% sediment melts and ∼25% oceanic crust melts. In contrast, the high-Fe-Ti rocks were differentiated from a deeper (∼3–4 GPa) mantle source metasomatized by ∼30% sediment melts. Importantly, our modeling data reveal a critical control of melting pressures on the TiO 2 contents of intermediate melts from metasomatized mantle sources. Therefore, the scarcity of high-Ti sanukitoids may thus indicate very rare deep subduction in the Archean. Combined with the coeval increase of K 2 O/Na 2 O, Sr + Ba, TiO 2 , and Fe* of global late Archean intermediate rocks, we propose that it is the enhanced mantle recycling of supracrustal materials that may have triggered late Archean voluminous and diverse intermediate magmatism, likely evolved under a gradually stabilized plate tectonic system. • Late Neoarchean high-Mg and high-Fe-Ti varieties of K-rich intermediate rocks. • Origin from mantle sources variably metasomatized by supracrustal materials. • First documentation of Archean high-Ti sanukitoids in the North China Craton. • High-Ti sanukitoids indicate very rare deep subduction in the Archean. • Late Archean enhanced supracrustal recycling possibly under a plate tectonic system. [ABSTRACT FROM AUTHOR]

Flat-slab subduction is a distinctive phenomenon that leads to the development of a wide orogenic zone (≥ 800 km) along former continental margins, triggering extensive intraplate magmatism and deformation. However, occurrences of flat-slab subduction on ancient convergent margins are exceptionally rare, possibly owing to multiple episodes of structural destruction and/or poor preservation. Furthermore, the impact of flat-slab subduction on mantle heterogeneity remains ambiguous. In this study, we present comprehensive whole-rock geochemical analyses as well as in-situ zircon U Pb dating and Hf O isotopic data for Early Jurassic A-type granites from the Pitou and Shibixia plutons in the interior of the South China Block to investigate the potential influence of flat-slab subduction on intraplate magmatism. The Pitou granites exhibit characteristics consistent with those of A 2 -type granite, displaying zircon ε Hf (t) values ranging from −4.9 to +0.3 and elevated δ18O values of 6.2–7.6‰. These features suggest that the granites formed through the differentiation of a basaltic parental magma, which underwent varying degrees of crustal contamination. In contrast, the Shibixia granites show A 1 -type geochemical features and were derived from basaltic parental magma without significant crustal contamination. The zircons in these granites exhibit positive ε Hf (t) values ranging from +6.6 to +10.6 and anomalously low δ18O values of 3.3–4.0‰, suggesting the involvement of recycled altered lower oceanic crust components in their mantle source. This provides strong evidence for the occurrence of flat-slab subduction beneath the interior of the South China Block during the early Mesozoic, which was located at least 800 km away from the trench. Combined with the presence of post-delamination ocean island-like intraplate magmatic rocks in the South China interior, it is suggested that the flat-slab delamination at approximately 190 Ma facilitated late Mesozoic intraplate OIB-like magmatism. This process, associated with flat-slab subduction, is expected to exert a significant impact on mantle isotopic heterogeneity within continental back-arc regions. • The interior of South China Block hosts early Jurassic A 1 - and A 2 -type granites. • The low-δ18O A 1 -type granites contain recycled lower oceanic crustal component. • Flat-slab subduction reached ∼800 km inland of SCB from the trench at ∼190 Ma. • Flat-slab delamination at ∼190 Ma resulted in mantle heterogeneity beneath the SCB. [ABSTRACT FROM AUTHOR]

An ophiolite suite, predominantly composed of residual mantle peridotites, mid-ocean-ridge basalt (MORB)--like ultramafic rocks, and oceanic plagiogranites, has been identified in the Zhenghe-Dapu fault zone, Cathaysia block, South China. The peridotites experienced strong serpentinization and are characterized by low 187Os/188Os ratios of 0.11621-0.12008 and very low 187Re/188Os values of 0.031-0.129, similar to those from highly refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites, can be classified into two groups. Group I is characterized by high Ni and Cr and low K2O contents. Their spoon-like rare earth element (REE) patterns, along with lower concentrations of highly incompatible elements, indicate that the protolith was of cumulate origin. Group II displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which are geochemically similar to normal (N) MORB. Both groups exhibit positive εNd(t) values (1.2-4.1) and relatively high (87Sr/86Sr)i ratios (0.7046-0.7096), suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration. The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O (0.09-0.21 wt%) and total REE contents along with low K2O/Na2O and Rb/Sr ratios, which are consistent with those of typical oceanic plagiogranite. They exhibit strongly positive zircon εHf(t) values (+9.5 to +15.1) and positive whole-rock εNd(t) values (+2.8 to +3.6). Their extremely low MgO (0.6-1.65 wt%), Cr (0.22-6.26 ppm), and Ni (0.77-4.74 ppm) compositions and low Mg# (22.4-31.9) preclude their origination from mantle-derived primary magma but favor oceanic crust. Low zircon δ18O values (4.02‰-5.4‰) and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region. The enriched light rare earth element (LREE) patterns with strongly positive Eu anomalies, similar to the East Karmøy--type plagiogranite in western Norway, imply that the plagiogranites were derived from anatexis of amphibolite in an active shear zone near the mid-ocean ridge. The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The simultaneous magmatism and metamorphism also signify an active high-temperature shear zone, where the plagiogranites were formed slightly later than the oceanic crust. The age of ca. 520 Ma represents the formation age of the oceanic crust rather than its emplacement age. The identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian, and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca. 520 Ma, most likely during the late early Paleozoic. [ABSTRACT FROM AUTHOR]

Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured 187Os/188Os ratios are moderately to highly radiogenic (0.186–0.313) with age-corrected γOs values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (87Sr/86Sr (i) = 0.70349–0.70384; εNd (i) = +1.3 to +4.9) and fresh perovskite grains (87Sr/86Sr (i) = 0.70340–0.70390; εNd (i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (εNd (i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (87Sr/86Sr (i) = 0.70362–0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume. [ABSTRACT FROM AUTHOR]

Ocean island basalts (OIBs) with Zn isotopic ratios higher than the normal mantle (δ66Zn = 0.17 ± 0.08‰) or mid-ocean ridge basalts (MORBs; δ66Zn = 0.27 ± 0.06‰) generally also have an enriched Sr-Nd isotopic signature, suggesting carbonate-bearing eclogites, whose protolith is inferred to be subducting altered oceanic crust, in their mantle source. On the contrary, continental intraplate basalts with high δ66Zn usually show depleted Sr-Nd isotopic signatures (i.e., decoupled Zn-Sr-Nd isotopic composition). To elucidate the origin of the decoupled Zn-Sr-Nd isotopic composition in continental intraplate basalts, we report the discovery of both coupled and decoupled Zn-Sr-Nd isotopic data for a suite of Cenozoic continental intraplate basalts from the Zhejiang province, Southeast China. These basalts display clear spatial and temporal geochemical variations, with early-stage inland low-silica samples presenting moderately enriched Sr-Nd isotopic signatures and high δ66Zn (coupled Zn-Sr-Nd isotopic composition, similar to OIBs), and later-stage coastal high-silica samples that display a pronounced δ66Zn decrease with increasing SiO 2 and 87Sr/86Sr and with decreasing alkali contents and 143Nd/144Nd (decoupled Zn-Sr-Nd isotopic composition). The early-stage basalts with coupled high Zn-Sr-Nd isotopic signatures are also more enriched in incompatible elements than any other basalts from eastern China reported so far. We explain the spatial and temporal geochemical variations of these basalts as the result of two main melting events: 1) the low-silica early-stage magmatism mostly occurs inland and results from high-pressure partial melting of a carbonated eclogite-bearing asthenospheric mantle. Because of the presence of a thick lithosphere limits the melting of the depleted mantle component, the signature of the Zn-Sr-Nd isotopically enriched, and more fusible carbonated eclogite is preserved. 2) At the later stage, magmatism mostly occurs on the coast where the subcontinental lithosphere is thinner. Hence, decompression melting progresses to shallower depth, resulting in an increase of the contribution from the depleted peridotite matrix and a dilution of the signal from the isotopically enriched fusible component. Further upwelling and in-situ melting at the base of the subduction-modified sub-continental lithospheric mantle (SCLM) explains both the decoupled Zn-Sr-Nd isotopic signature of the coastal basalts and their major and trace element variability. We further propose that decompression melting is driven by small-scale convection resulting from variations of lithospheric thickness. Our data highlight the importance of dynamic melting of carbonated eclogite-bearing asthenosphere and subsequent lithospheric melting in preservation and destruction of the coupled enriched Zn-Sr-Nd isotopic signature of carbonated eclogite component and generation of the apparent decoupled Zn-Sr-Nd isotopic signal commonly observed in continental intraplate basalts. [ABSTRACT FROM AUTHOR]

The Weishan REE deposit is located in the southwest of the Luxi Terrane of the North China Craton (NCC), where a large number of lamprophyre dikes are spatially exposed with the deposit. Here, we report petrology, geochemistry and zircon U-Pb geochronology data for the lamprophyre of the Weishan REE deposit in order to develop constraints for the determination of the petrogenesis, magma source and evolution of the lamprophyre and the tectonic environment. LA-LCP-MS zircon U-Pb dating shows that the crystallization age of the lamprophyre is 125 ± 0.86 Ma. The geochemical data suggest that these lamprophyres have high levels of Al2O3, K2O, MgO and alkalis, moderate level of Na2O and low levels of SiO2, Fe2O3 and TiO2, and that they are enriched with LREEs (La, Ce) and LILEs (Rb, Ba) and depleted with regard to HREEs and HFSEs (Nb, Ta, Ti). They displayed negative εHf(t) values of −14.98 to −9.03, TDM1 ages of 1.1–1.4 Ga and TDM2 ages of 1.7–2.1 Ga, which suggest that the magma source originates from an enriched mantle. Low Rb/Sr and high Dy/Yb ratios suggest that the enriched mantle source was partially melted at the amphibole-bearing lherzolite garnet-facies. The high Ba/Th and Sr/Th ratios indicate that the enriched source was derived from subduction dehydration fluids of the oceanic crust. We propose that the mafic dike intrusions are consistent with an Early Cretaceous alkaline magma emplacement in an extensional setting, in which the magma was not contaminated by crustal material during its emplacement. [ABSTRACT FROM AUTHOR]

Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and ε Nd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al 2 O 3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle. [ABSTRACT FROM AUTHOR]

The NE–SW-trending graphite belt found in the Xinrong Region, Datong City is one the richest graphite reserves in China, extending for more than 22 km. The ore-bearing layer of the Huangtuyao Formation is a graphite-bearing gneiss. Based on major and trace element analysis data, we determined from three diagrams that the graphite-bearing gneiss is a parametamorphic rock, and this gneiss was formed by the regional metamorphism of carbonaceous argillite. The detritus of this ore deposit originated from an arc region as seen from the K2O/Na2O–SiO2 diagram. According to the Ta/Yb, Sr/Yb and K2O/Na2O ratios as well as the composition of Gehuyao gneiss, we concluded that this gneiss has typical low-potassium adakitic rock characteristics. Combined with the MgO/SiO2 diagram and the characteristics of low K2O and high Al2O3, the genetic model of Gehuyao gneiss is melted subducted oceanic crust. Based on regional geological data and comparison of samples, we found an inherent relationship in the main composition content between the graphite-bearing gneiss and the Gehuyao gneiss. This indicates that the ore deposit accumulated the weathering products of Gehuyao gneiss during the sedimentary period. This research provides evidence for ore body evolution and makes it possible to establish the sedimentary–metamorphic model of the graphite ore in this region. In light of the range of the ore body, we predict that the potential area for graphite mineralization in the North China Craton was 2 ± 1.5 km away from the boundary. Thematic collection: This article is part of the Applications of Innovations in Geochemical Data Analysis collection available at: https://www.lyellcollection.org/cc/applications-of-innovations-in-geochemical-data-analysis [ABSTRACT FROM AUTHOR]

Establishing when modern-style plate tectonics with deep subduction began on Earth is one of the biggest questions in geosciences today. A lack of Archean age (>2.5 billion y ago [Ga]) eclogites or eclogite-facies crustal rocks (the high-pressure equivalent of basalt or gabbro) has led to an assertion that modern plate tectonics did not operate in the Archean. Here, we report eclogite-facies garnet clinopyroxenite associated with metagabbro in 2.52-to 2.53-billion-y-old ophiolitic mélange in the northern Central Orogenic Belt (COB) within the North China Craton. The garnet clinopyroxenites with normal mid-ocean ridge basalt (N-MORB) geochemical signatures are relicts of oceanic crust, recording peak eclogite-facies metamorphic assemblages indicating conditions of 792 to 890 °C/19.8 to 24.5 kbar, supported by abundant exsolution microstructures in garnet and clinopyroxene. Zircon U-Pb dating of the metagabbros and a granitic dike cross-cutting the metamorphic layering of the metagabbro constrain deformation and eclogite-facies metamorphism to >2.47 Ga. This finding implies that Archean oceanic crust was subducted to at least 65 to 70 km at the end of the Archean. Together with other asymmetric subduction records in the COB, it is inferred that modern-style plate tectonics evidenced by deep and asymmetric subduction along the circa 1,600-km-long orogen was operating at least by the end of the Archean era, when the planet was making a transition to the Proterozoic, witnessing the Great Oxidation Event, widespread emergence of continents, and development of crown node eukaryotic species on a more habitable planet. [ABSTRACT FROM AUTHOR]

The Tianyu Cu‐Ni sulfide deposit occurs in the north margin of the Central Tianshan Arc in East Tianshan orogenic belt, Xinjiang, NW China. The intrusions consist of gabbro, peridotite, and olivine pyroxenite. The peridotite and pyroxenite are the main host rock for the Cu‐Ni ores. Rhenium and osmium isotopic analyses of Ni‐ and Cu‐bearing sulfide minerals from the deposit have been used to determine the source of osmium, and by inference, the sources of ore metals. Sulfide ore samples have Os and Re concentrations varying in the ranges 1.85 to 4.58 ppb and 93.56 to 146.00 ppb, respectively. An initial 187Os/188Os ratio ranges from 0.86 to 1.23 for the ores and the γOs values from 592 to 2227. Osmium isotopic data suggest that the Tianyu intrusion and associated Cu‐Ni mineralization has derived from crustal‐contaminated mantle melts. The intrusions early show island‐arc geochemical signatures, which indicate that the Hulu mafic–ultramafic intrusions, along with the Cu‐Ni deposit, formed as a result of subduction of oceanic crust in the Early Permian. [ABSTRACT FROM AUTHOR]

• O isotopes can effectively identify the altered oceanic crust in the TTG source. • Subduction failure led to partial melting of the oceanic crust under medium pressure. • Subduction success gave rise to the mantle source of mafic igneous rocks. • Archean subduction experienced a series of processes from failure to success. When and how plate tectonics started and evolved to the style as we know it today is a fundamental yet highly controversial question. Numerical geodynamic modelling predicts that the transition into plate tectonics in the Archean was episodic with possible alternation between success and failure of subduction. However, direct geological evidence for failed subduction is scarce. While this possibility has been suggested by numerical modelling, it is still lack of geological evidence. Here we present a combined study of zircon U-Pb ages and Hf-O isotopes, as well as whole-rock major and trace elements, for meta-igneous rocks from the Alxa Block in the westmost North China Craton (NCC). Two periods of Late Archean magmatism ca. 2.75 Ga and 2.5 Ga are identified to occur surrounding a pre-3.0 Ga continental nucleus. Geochemical analyses of the ca. 2.75 Ga meta-mafic and meta-felsic igneous rocks show two different modes of tectonic regime. The felsic rocks resemble typical Archean TTG (tonalite-trondhjemite-granodiorite) in composition, with variable ε Hf (t) values from -2.8 to 11.0 and δ18O values from 4.3 ‰ to 7.9 ‰. Petrogenetic modelling suggests that their magmatic source was the ca. 3.1–2.75 Ga oceanic crust that was hydrothermally altered at different temperatures and then mixed with the older continental crust and partially melted in lower crust in the garnet stability field. This requires tectonic accretion of the oceanic crust to the continental nucleus, signifying an attempted or failed subduction. On the contrary, the meta-mafic rocks exhibit arc-like trace element patterns and calc-alkaline evolution trend, indicating a metasomatic mantle source due to successful subduction of the oceanic slab to subarc depths. Taken together, the present results provide robust constraints on the behavior of oceanic slab at the Archean convergent margin, where successful oceanic subduction would be achieved after several failed attempts. Such failure-success processes of oceanic subduction may be applicable to the whole NCC and other cratons elsewhere in the world, reflecting the gradual maturation of plate tectonics in the Archean, consistent with predictions of numerical modelling. [ABSTRACT FROM AUTHOR]

Understanding the processes involved in the transformation of juvenile basaltic oceanic arc crust into mature continental crust remains a key challenge in Earth sciences. In this contribution, we present a comprehensive synthesis of in situ zircon U Pb age and Hf O isotope data for Paleozoic intrusions within the West Junggar oceanic arc, NW China. Our study reveals four distinct pulses of magmatic activity: Early Cambrian to Early Ordovician (515 to 486 Ma); Late Ordovician to Middle Devonian (445 to 392 Ma); Early Carboniferous (343 to 310 Ma) and Late Carboniferous to Middle Permian (309 to 259 Ma). These pulses have varied spatial and temporal distributions. All magmatic rocks display consistently high zircon Hf and whole-rock Nd isotope values, but substantial variations in zircon O isotopes. There are two groups of intrusions: those with high zircon δ18O (>6.5‰) and those with mantle-like zircon δ18O (ca. 5.5‰). The high zircon δ18O intrusions are predominantly concentrated in the southern West Junggar and their Hf and Nd isotopes indicate the involvement of supracrustal material and juvenile basaltic crust in their petrogenesis. Binary mixing calculations indicate a contribution from the supracrustal rocks ranging from 10% to 50%. The intrusions with mantle-like zircon δ18O are found primarily in northern West Junggar with a small amount occurring in southern West Junggar. The intrusions record a variety of magma sources and processes as demonstrated by Hf O isotope and geochemical data. These data indicate partial melting of metasomatized depleted mantle, mixing of depleted mantle and juvenile crust, and partial melting of trapped juvenile oceanic crust or mafic lower crust. Hf model ages reveal significant crustal growth in the West Junggar, characterized by three distinct episodes of crust formation occurring at approximately 656–684 Ma, 524–536 Ma, and 441–471 Ma, involving periodic remelting of igneous material derived from a depleted mantle source. This newly-formed crust maintains a mantle-like oxygen isotope composition despite being repeatedly sampled by magmas for up to 0.26 Ga. Since the timing of crustal growth occurred independently of the major magmatic pulses, the latter reflect primarily reworking and remelting processes. Two significant episodes of magmatic activity, the late Silurian to early Devonian and the late Carboniferous to early Permian, preserve a signature of ocean ridge subduction. High-temperature magmatism during these periods promoted extensive melting of the mafic lower crust, oceanic crust, and supracrustal rocks, leading to the compositional transformation from basaltic to felsic continental crust. This comprehensive compilation provides valuable insights into granite petrogenesis, crustal evolution, and the diverse processes involved in the maturation of oceanic arc crust and its contribution to continental crust formation and evolution. [ABSTRACT FROM AUTHOR]

• Episodic upwelling mantle-lithosphere interactions drove coupled crust-mantle differentiations. • Mesoarchean lower crustal removal was caused by dense lithospheric delamination. • Gravity-driven subduction was responsible for the Neoarchean subduction initiation. 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. [ABSTRACT FROM AUTHOR]

Alkaline-silicate rocks represent an important worldwide reservoir of rare earth elements (REE). The weathering of these alkaline-silicate rocks has the potential to create regolith-hosted REE deposits. The factors contributing to the enrichment of REE in these alkaline-silicate rocks remain unclear. A giant regolith-hosted REE deposit, found in the Gejiu district on the western edge of the Youjiang Basin in Southwest China is linked to nepheline syenites. This study presents new geochemical and petrographic data for the Puxiong and Baiyunshan nepheline syenites, providing insights into the processes responsible for the enrichment of REE in the Gejiu alkaline-silicate system. The Gejiu nepheline syenites originated in the late Cretaceous period (84–80 Ma) and exhibit an enrichment of large-ion lithophile elements (LILEs), a depletion of high field strength elements (HFSEs), elevated (87Sr/86Sr) i and Rb/Sr ratios, low εNd(t) values, and elevated Ba/Rb ratios. The geochemical features of the Gejiu nepheline syenites suggest that they were formed from a metasomatic mantle with a high abundance of phlogopite. The zircon Hf-O isotopes, characterized by εHf(t) values ranging from −8.8 to −1.4 and δ18O values between +7.0 ‰ and + 8.9 ‰, in conjunction with elevated Ba and Rb contents, indicate that the origin of the alkaline rocks involved metasomatism by volatile-rich materials released from subducted marine sediments. The REE minerals present in these alkaline rocks are mainly britholite-(Ce) and tritomite-(Ce), indicating their magmatic and hydrothermal origins. The observed petrographic features indicate a higher concentration of REE minerals in the Gejiu alkaline rocks as a result of a self-hydrothermal process. The REEs and volatile phases present in these alkaline rocks are inferred to have originated primarily from the source, suggesting that the refertilization of REEs within the mantle enriches the Gejiu alkaline-silicate rocks with REEs. Moreover, the weathering crust derived from these alkaline-silicate rocks also demonstrates considerable potential as a suitable environment for REE deposits originating from regolith. [Display omitted] • The Gejiu nepheline syenites were emplaced at 84–80 Ma. • The Gejiu alkaline rocks originated from a phlogopite-bearing mantle. • The source was modified by volatile-rich materials from subducted oceanic crust. • The refertilized mantle is responsible for REE enrichment of Gejiu alkaline rocks. [ABSTRACT FROM AUTHOR]

The Ediacaran-Cambrian (E-C) succession in South China records remarkable oceanic, biological and geochemical variations, but it was not well defined geochronologically, which hinders the interpretation of the spatio-temporal seawater chemical architecture during the time E-C interval. This study presents two Re-Os isochron ages of 520.2 ± 6.1 and 561.7 ± 8.5 Ma for the barite-rich black shales from the top Liuchapo and Doushantuo formations respectively in Tianzhu County, Guizhou Province. In combination with existing age data, the two new Re-Os isochron ages suggest that the Liuchapo Formation was deposited between 550 and 520 Ma. Moreover, like the polymetallic Ni-Mo-PGE layers of shelf margin (or platform) facies and V-rich horizons of transitional (or shelf slope) to deep-water facies, the barite deposits were likely formed due to differential mineralization. The timing offset likely resulted from differential elemental concentration related to certain local factors (i.e., hydrothermal fluids, seawater redox and biological activity). The isochron-derived initial 187Os/188Os ratios of the top Liuchapo Formation (0.902 ± 0.048) and the Doushantuo Formation (0.740 ± 0.042) fall within the range of continental weathering flux (1.54) and oceanic crust (0.126), implying the involvement of marine hydrothermal fluids. Moreover, their difference of initial 187Os/188Os ratios may reflect variations of continental weathering intensity and uplift magnitude. [ABSTRACT FROM AUTHOR]

Magmatic Ni-Cu sulfide deposits in orogenic belts have distinct petrological and geochemical features compared with world-class intraplate magmatic deposits. One such feature is the presence of abundant primary hydrous minerals such as phlogopite and amphibole, indicating a H2O-rich parental magma. However, the origin and nature of the fluids have not been established. In the southern Central Asian Orogenic Belt, the Permian Baishiquan and Tianyu mafic–ultramafic intrusions host magmatic Ni-Cu sulfide deposits. Both intrusions are composed of lherzolite, olivine websterite, gabbronorite, gabbro, and diorite. The main ore-host rocks are lherzolite and websterite, with disseminated, net-textured, and massive ores comprising the dominant primary ore types. Low zircon εHf (− 6.9 to 7.9 ‰) and varying δ18O values (5.2 to 6.6 ‰), as well as both arc-type and MOR-type zircon Nb/Yb (0.0013–0.011) and U/Yb (0.33–10.22), identify an altered lower oceanic crust component in the mantle source. A combination of altered lower oceanic crust and upper crust can account for the zircon Hf–O isotopic characteristics. A mixing model further reveals that the Baishiquan and Tianyu magmatic sulfide deposits originated from a depleted mantle source that experienced two stages of subduction metasomatism with a 30% addition of altered oceanic crust in the first stage and 20% in the second. In the second stage, > 10% oceanic sediment with high δ18O must also have been added to result in olivine δ18O values higher than those in mantle. This staged evolution is reflected in both the olivine and zircon signatures with olivine from disseminated and net-textured ores at the Tianyu deposit having high δ18O (4.6 to 6.3 ‰) with a similar range to zircon and high δ7Li isotopic compositions (12.03 to 19.46 ‰). Contamination in the source by oceanic sediment with high δ18O values can account for the olivine δ18O values (4.8 to 5.7 ‰ at Baishiquan) being higher than those in the mantle, while the addition of brine (5% at the Baishiquan deposit) can explain the olivine δ7Li values (20.96 to 31.01 ‰). The results suggest that H2O content, Li content, and δ7Li and δ18O isotopic signatures in olivine and zircon are good indicators of mantle metasomatism and olivine fluid processes in magmatic Ni-Cu sulfide deposits. [ABSTRACT FROM AUTHOR]

The mechanisms and processes by which subducted slab interacted with mantle plume remain controversial, as direct observation of such interaction is difficult to impossible. Compositional heterogeneity of large igneous provinces (LIPs) additionally makes plume-slab interaction hard to detect. Oxygen isotopes are sensitive enough to trace the source of magmas. Here we provide evidence for plume-slab interaction mainly based on in situ zircon Hf-O isotope analyses, as well as whole-rock elemental and Sr-Nd-Hf isotope analyses, on the Late Permian and Early Triassic A-type granites on the margin of the Emeishan LIP in SW China. These granites show typical A-type geochemical characters, such as high total alkali (7.93-9.68 wt%) and field strength element (HFSE, e.g., Zr and Nb) contents, and high FeOT/(FeOT+MgO) (0.87-0.98) and Ga/Al (3.67-5.06) values. The Late Permian (ca. 259 Ma) and Early Triassic (ca. 248 Ma) granites show high Nb/Th (>3.0) and low Y/Nb (<1.2) and Yb/Ta (<2.0) ratios similar to the oceanic island basalts and have near-zero εNd(t) (-0.83 to -0.13 and -0.15 to +0.16, respectively) and depleted εHf(t) (+2.71 to +3.39 and +2.62 to +3.55, respectively). In situ zircon O-Hf analyses yielded anomalously low δ18O (0.2-2.0‰ and 3.2-4.8‰, respectively) and positive εHf(t) (1.6-7.0 and 3.9-8.8, respectively), suggesting varying proportions of hydrothermally altered oceanic crust in their source region. Our results imply that significant amounts of altered Paleotethyan oceanic crust have been subducted in the upper mantle beneath the western South China Block. The nearby rising Emeishan mantle plume may have rapidly entrained and incorporated these oceanic crustal materials to the shallow mantle so that their low-δ18O isotope feature was preserved. Subsequent decompression-related partial melting of this hybrid source formed parental rocks of the low-δ18O A-type granites. Our findings also suggest that LIPs could obtain their compositional (especially oxygen isotope) diversity through the interaction between the subducting slab and rising mantle plume. [ABSTRACT FROM AUTHOR]

The nature and source of magmatism associated with large igneous provinces (LIPs) remain disputed. Here we investigate the role of recycled materials that contributed to mantle heterogeneity in the Tarim Large Igneous Province (TLIP) in China through integrated Zn-Mg-Sr-Nd isotopes of a rare tephritic rock suite. The Sr--Nd isotopes [(87Sr/86Sr)i = 0.70368-0.70629; εNd(t) = -0.25-4.64] and δ26Mg values (-0.23‰ to -0.34‰) of the tephritic porphyries are consistent with a normal mantle origin. In contrast, the whole rock and pyroxene phenocrysts yield δ66Zn values of +0.28‰ to +0.46‰ and +0.30‰ to +0.39‰, which are slightly heavier than those of the terrestrial mantle (+0.16 ± 0.06‰) and mid-oceanic-ridge basalts (MORBs) (+0.27 ± 0.05‰). We exclude the possibility that the heavy Zn isotopes of the Wajilitag tephritic porphyries are caused by magmatic processes such as fractional crystallization and partial melting and correlate the isotopic features to the role of altered oceanic crust along with magnesite in the mantle source. The Wajilitag tephritic porphyry displays trace-element patterns similar to those of the melts from natural hornblendite, especially those of hornblendite + peridotite. Additionally, the geochemical characteristics such as enrichment in Nb and Ta, depletion in K, high TiO2, and constant Na2O/K2O ratios also suggest that the tephritic porphyries were derived from an amphibole-bearing source contributed by altered oceanic crust along with sedimentary carbonates. Our study provides insight into the contribution of subducted materials to the mantle heterogeneity beneath the TLIP and highlights the role of subduction in the deep carbon cycle and subducted slab-lithosphere-plume interaction in the generation of LIPs. [ABSTRACT FROM AUTHOR]

The Youjiang Basin in SW China is the second‐largest Carlin‐like goldfield in the world after Nevada, USA. However, the age and the hydrothermal origin of the gold mineralization are still in controversy, which leads to ambiguities in understanding the geodynamic setting and genetic mechanism of the gold deposition. Nibao is a large, stratabound, and fault‐controlled Carlin‐type gold deposit in the north of the Youjiang Basin. Three ore stages have been recognized in the evolution of the mineralizing hydrothermal fluid, with the stages being characterized by assemblages of the early quartz–porous pyrite, the main quartz–calcite‐apatite–sericite–pyrite–arsenopyrite‐gold, and the late quartz–calcite–fluorite–realgar–orpiment–stibnite. Calcite veins from the main and late ore stage at Nibao are enriched in middle rare earth elements, which is a distinctive characteristic of hydrothermal calcite veins associated with low‐temperature hydrothermal Au‐Sb deposits in the Youjiang Basin, and yield a Sm‐Nd isochron age of 138 ± 1 Ma. The vast majority of the ore stage calcite samples have narrow 87Sr/86Sr ratios and εNd(t) values ranging from 0.708119 to 0.708423 and − 1.1–−3.3, respectively, indicating that the gold‐bearing hydrothermal fluid could be derived from the mixing of mantle and crustal materials. Therefore, we proposed that the gold mineralization during the Early Cretaceous in the Youjiang Basin was triggered by the large‐scale lithosphere extension following the retreat of the subducted Paleo‐Pacific oceanic crust, and the hydrothermal fluid could be originated from the mantle and underwent the contamination of Yangtze upper continental crust. [ABSTRACT FROM AUTHOR]

Whether orogenic gold deposits formed from crustal or subcrustal sources is debated, and their link to orogenic processes is ambiguous. Gold mineralization in the Triassic East Kunlun-West Qinling Orogen, China, displays a spatial zonation in terms of its ages and stable isotope compositions. In the West Qinling segment, most gold deposits formed in a back-arc setting at 220~210 Ma during a collisional episode within late slab rollback. These deposits have dominant δ34S of 5-15‰ and δ18Ofluid of 10-14‰, whereas those formed in the suture zone at 210-170 Ma, during a post-collisional episode after slab break-off, have lower d34S of -5-+5‰ and d18Ofluid of 6-10‰. In the East Kunlun segment, those deposits that formed in a continental-arc setting and its related suture zone at 240-200 Ma, in collisional to post-collisional episodes associated with slab break-off, have δ34S and δ18Ofluid values that are essentially similar to those in the West Qinling suture. d34S values of ore sulfide separates and rims of zoned pyrites that have mantle-like signatures, in contrast with crustal signatures of host rocks, are indicative of subcrustal ore-fluid sources. The combined chronological and stable isotope shifts are consistent with a model in which ore fluids for gold mineralization in a back-arc setting were sourced from mantle lithosphere that was metasomatized by subducted oceanic sediment; whereas those in a continental-arc setting--including its suture zone--were sourced from fluid derived from altered oceanic crust. This study thus provides new insights into the complexity of orogenic gold systems in evolving orogens. [ABSTRACT FROM AUTHOR]

The mechanics of crustal growth in the Altaids during the Phanerozoic is still not well understood. This paper reports new geochronological, geochemical, and isotopic data from Paleozoic adakitic intrusions in the Dananhu arc in the Chinese Eastern Tianshan with the aim of better understanding the specific mechanisms of crustal growth. Our new LA-ICPMS U–Pb zircon age data show that four phases of adakitic intrusions formed at 410–413 Ma, 385–387 Ma, 335 Ma, and 313–311 Ma. Combined with previous data of ours and from the literature, there were seven phases of adakitic intrusions from the Silurian to early Permian. All these adakitic intrusions have distinctive geochemical-isotopic signatures with high Na and Al, high Sr, low Yb and Y, high Sr/Y and (La/Yb)N values, relatively high MgO contents and Mg numbers (Mg# = 41–70), high abundances of compatible elements (Cr = 1.66–46.30 ppm, Ni = 2.40–26.96 ppm), and positive high εHf (t) (+ 7.1 to + 15.6) and a TDM (Ma) range from 325 to 670 Ma. These adakitic intrusions were associated with essential Cu polymetallic mineralization in the Dananhu arc. The above chemical data and relations indicate that the processes of oceanic slab melting generated these adakitic rocks. Our new geochronological, geochemical and isotopic data, integrated with the geology of the Dananhu intra-oceanic arc, lead us to conclude that multiphase of hot/young oceanic crust subduction and melting generated the massive crustal growth in the Silurian–early Permian. Our results indicate that the magmas were derived by slab melting of hot/young oceanic crust, which was one of the most critical mechanisms of continental crustal growth in the Paleozoic accretionary orogen of the Altaids. [ABSTRACT FROM AUTHOR]

The Emeishan large igneous province (LIP) consists of Permian flood basalts and coeval intrusive rocks with compositions varying from mafic–ultramafic rocks to syenites and granites. The volatiles in the Emeishan LIP are dominated by H2O with high H2, indicating that the plume‐derived magma was generated under a reduced condition. Picrite has minor contents of H2S without detectable SO2, and the basalt samples show no detectable H2S and SO2, indicating that the basaltic magma may have undergone sulphur‐saturation during magma evolution. The δ13C values of CO2 and CH4 in picrites and basalts are between the values of mantle and crustal organic carbon. This, together with the distribution patterns among CH4 homologue, indicates contribution of recycled sediments to the Emeishan mantle‐derived magma. The 3He/4He ratios of volatiles extracted from olivine and pyroxene phenocrysts of picrites vary from 0.01 to 0.66 Ra, and those from pyroxene separates of olivine websterite samples in the Zhubu intrusion vary from 0.92 to 1.05 Ra. The low 3He/4He ratios of the picrites from Emeishan LIP are lower than those from the Siberian LIP, but within the values of continental crust and oceanic crust. The positive correlation between 4He/40Arc and 4He contents in Emeishan picrite suggests that the isotopic ratios were dramatically affected by crustal contamination rather than degassing. The He and Ar isotopes indicate that Emeishan picrite experienced addition of subducted oceanic crustal materials, and the Zhubu magma was contaminated by continental crustal materials. [ABSTRACT FROM AUTHOR]

Oceanic arc subduction systems are the loci of substantial recycling of oceanic crust and production of juvenile arc crust that differentiates to more evolved felsic crust. Inevitably, some juvenile sediments are subducted with the oceanic crust. However, distinguishing the incorporation of juvenile sediments in oceanic arcs is not always straightforward, because they may not measurably shift many geochemical signatures, such as Sr and Nd isotopes, of oceanic arcs. Nevertheless, combined zircon U-Pb, Hf, and O isotope data can provide a powerful tool to decipher sedimentary flux into oceanic arc magmas, and here we report a case study for the late Paleozoic A-type granites from the West Junggar oceanic arc in the southern Central Asian Orogenic Belt. These plutons contain hastingsite and iron biotite diagnostic minerals and have high alkali, FeOT/MgO, Zr, and Ga/Al, but possess low CaO contents, and strongly negative Eu, Sr, and Ba anomalies, demonstrating their close affinity with A-type granites. Zircon U-Pb analyses indicate that these A-type granites emplaced in the Late Carboniferous to Early Permian (ca. 307-298 Ma). Their high zircon εHf(t) values (+12.4 to +15.5), suggest that the magmas were derived from a mantle or juvenile crustal source. However, their δ18Ozrn (+7.2‰ to +11.9‰) values are significantly higher than that of the mantle, and modeling using Hf-O isotope and rare earth element data indicate the assimilation of sedimentary materials at a proportion of ~50%. Our data suggest that juvenile sediments (e.g., greywacke) played an important role in the formation of the studied A-type granites. The re-melting of sedimentary material induced by the late Carboniferous ridge subduction can promote the transition from an intra-oceanic arc to continental crust. Our results show that the subduction and re-melting of juvenile sediments in oceanic arc systems could be an important mechanism for the maturation of oceanic arc crust. [ABSTRACT FROM AUTHOR]