Explore the vast range of titles available.

In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic melange belt, the gabbros from new discovered Zhongcang ophiolitic melange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element （REE） patterns of most of these rocks show flat types with slightly light REE （LREE） depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements （HFSE） （Nb, Ta） and enrichment in large ion lithophile elements （LILE）. These gabbros have island arc and mid-ocean ridge basalt af- finities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive εNd（t） values show that these gabbros were derived from -30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Creta- ceous, resembling modern active intra-oceanic subduction systems in the western Pacific.

Fluid‐rock interactions play an important role in element mobilization, mass transfer, and formation of critical metals in subduction zones. However, tracking the multistage fluid‐rock interactions within subduction channels remains elusive. Here we conducted bulk‐rock major and trace element and magnesium (Mg) isotopic analyses on a suite of subducted ophiolitic mélange rocks from Wadi Al Barramiyah in the Arabian‐Nubian Shield of the Eastern Desert (ED) of Egypt. The rock suite includes serpentinites, talc rocks, talc‐dolomite rocks, tremolite‐dominated schists, and marbles. Talc rocks are characterized by low MgO contents and high δ26MgDSM‐3 values (0.03–0.13‰) relative to serpentinites (−0.18‰), indicating the release of isotopically light fluid during the metasomatic replacement of antigorite by talc. Tremolite‐dominated schists and talc‐dolomite rocks display higher CaO contents and lower δ26Mg (−0.25‰ to −0.03‰ and −1.04‰ to −0.18‰, respectively) than those of talc rocks and serpentinites. These signatures, along with high CaO/Al2O3 and low Rb/Sr ratios, indicate infiltration of low‐δ26Mg carbonate‐rich fluids, supported by extremely low δ26Mg (down to −2.38‰) observed in nearby marbles. Our findings demonstrate that antigorite dehydration liberates substantial numbers of H2O‐rich fluids, facilitating the dissolution of carbonate minerals in marbles. Subsequent carbonate metasomatism effectively sequesters carbon from aqueous carbon‐bearing fluids, transforming silicate minerals into carbonates. These new results highlight the significant role of mélange rocks in the multistage fluid‐rock interactions and carbon recycling in subduction zones, offering valuable insights into mantle Mg isotopic heterogeneity and crust‐mantle interactions. Plain Language Summary Subduction zones are geodynamic regions characterized by extensive fluid‐rock interactions that significantly influence the physics and chemistry of Earth's crust and mantle. We investigated Mg mobility and Mg isotope fractionation in subducted ophiolitic mélanges from Wadi Al Barramiyah, located in the Arabian‐Nubian Shield of the ED of Egypt, in order to constrain multistage fluid‐rock interactions in subduction zones. We found distinct Mg isotopic signatures across different rock types, reflecting diverse types and extents of fluid metasomatism. Such components, transported into the mantle via subduction, would cause Mg isotopic heterogeneity in the mantle and mantle‐derived magmas. These findings shed light on crust‐mantle interactions and carbon recycling in subduction zones, emphasizing the utility of Mg isotopes for tracing such processes. Key Points Talc rocks have δ26Mg higher than serpentinites, reflecting the release of 24Mg‐rich fluids Low δ26Mg of tremolite‐dominated schists and talc‐dolomite rocks document carbonate fluid infiltration Mg isotopes reveal multistage fluid‐rock interactions and carbon recycling in subduction zones

The Harihada–Chegendalai ophiolitic mélange, which is located between the Bainaimiao arc and the North China Craton, holds significant clues regarding the tectonic setting of the southern margin of the Central Asian Orogenic Belt. The ophiolitic mélange is mainly composed of gabbroic and serpentinized ultramafic rocks. Here, zircon U–Pb dating, in situ zircon Hf isotopic, whole-rock geochemical and in situ mineral chemical data from the ophiolitic mélange are reported. The zircons in the gabbroic rocks yielded concordia U–Pb ages of 450–448 Ma and exhibited slightly positive ϵ Hf(t) values (0.87–4.34). The geochemical characteristics of the gabbroic rocks indicate that they were generated from a mantle wedge metasomatized by subduction-derived melts from sediments with continental crust contamination, in a fore-arc tectonic setting. These rocks also experienced the accumulation of plagioclase. The geochemical characteristics of the ultramafic rocks and their Cr-spinels indicate that they may constitute part of residual mantle that has experienced a high degree of partial melting and has interacted with fluids/melts released from the subducted slab in the same fore-arc tectonic setting. The ophiolitic mélange may therefore have formed in this fore-arc tectonic setting, resulting from the northward subduction of the South Bainaimiao Ocean beneath the Bainaimiao arc during Late Ordovician time, prior to the collision between the Bainaimiao arc and the North China Craton during the Silurian to Carboniferous periods.

According to detrital zircons ages spectra and Hf isotopes in the matrix of the ophiolitic mélange, the Ailaoshan–Song Ma–Song Chay Indosinian suture zone can be divided into four units, namely M1, M2, M3, and M4. Different deposition ages (310–270 Ma, 260–255 Ma, ∼ 245 Ma, and > 255 Ma) demonstrate temporal heterogeneity. The M1 unit is in the middle and south segments, and inner part of the NW segment of the Ailaoshan ophiolitic mélange, and the southernmost part of Song Ma ophiolitic mélange. The Silurian and Devonian sedimentary rocks of the Indochina block are the major provenance of the M1 unit. The M2 unit is located in the southwest part of the NW segment of the Ailaoshan ophiolitic mélange. At least half of the 270–250 Ma detrital zircons might come from the South China block. The M3 unit is mainly located in the Ailaoshan–Song Ma ophiolitic mélange and sourced from Indochina block. M4 is mainly located in the Song Chay ophiolitic mélange, and its detrital zircons are mainly sourced from the South China block. The strike-parallel heterogeneity refers to the different distributions along the orogenic belt with various provenances. The strike-perpendicular heterogeneity characterises the geometry of M1, M2, and M3 units and is attributed to a Cenozoic positive-flower structure of the Red River Fault showing sinistral strike-slip tectonic event with SW-ward thrusting. These temporal and spatial heterogeneities correspond well to the different evolutionary stages of the eastern Paleo-Tethys.

Ophiolitic mélanges in Anatolia represent Mesozoic subduction-accretion complexes, which are unusually poor in land-derived coarse-clastic rocks. A segment of the ophiolitic mélange in the Beynam region south of Ankara was studied. The ophiolitic mélange consists of three accretionary units (AUs), which are distinguished by lithology, structure, age and geochemistry. At the base there is a serpentinite mélange, which is overlain by a semi-intact Upper Jurassic ophiolite with boninite geochemistry. The topmost AU consists of ocean-island-like alkali basalts with seamount-derived Triassic shallow-marine limestones and Jurassic radiolarian cherts, which are stratigraphically overlain by Upper Cretaceous fore-arc turbidites. The base of the fore-arc sequence is palaeontologically and isotopically dated to the early to middle Campanian (c. 81 Ma). Detrital zircons from the fore-arc sequence indicate a Late Cretaceous (87–81 Ma) magmatic arc as a source. The formation of the subduction-accretion complex was a two-stage process. The first stage took place during the Late Jurassic – Early Cretaceous, when supra-subduction type oceanic crust was generated, and subduction accretion was intra-oceanic. In the second stage during the Late Cretaceous the subduction jumped inboard, creating an Andean-type convergent margin, and the Jurassic oceanic crust was incorporated in the subduction-accretion complex. The lack of land-derived sandstones in the ophiolitic mélange can be attributed to the intra-oceanic subduction and to the limestone deposition in the upper plate during the main phase of subduction accretion in the Late Jurassic – Early Cretaceous.

A large number of Late Silurian–Early Devonian intrusive rocks are distributed in the central Beishan orogenic belt (BOB). Tectonic setting of these intrusive rocks is of great significance to the study of the subduction and accretion of the Paleo-Asian Ocean. Previous studies show that most of the intrusive rocks in this region are S-type or A-type granitoids. In this study, we firstly reported the Late Silurian–Early Devoniandia bases, granodiorites on the southside of the Baiyunshan ophiolitic mélanges belt, as a part of Hongliuhe-Xichangjing ophiolitic mélanges belt (HXOMB). Zircon LA-ICP-MS U-Pb dating yields emplacement ages between 418 and 397 Ma, REE distribution patterns exhibit enriched LREE and flat HREE in the diabases, the discriminant diagrams show that the diabases have geochemical characteristics of intraplate basalt. The granodiorites in this paper present more like S- and A-type granitoids reported, showing the geochemical characteristics of syn/post-collision granites. Actually, the bimodal magmatic rocks are developed during Late Silurian–Early Devonian on both sides of the HXOMB, which are related to the tectonic background of the post orogeny extension. The diabases are tholeiitic with relative strong depleted ε Hf ( t ) (+8.1 to +13.0), which are mainly from relative depleted mantle. The granodiorites are calc-alkaline with relative slightly depleted ε Hf ( t ) (+0.7 to +5.6) and the lower Mg # and MgO contents (34.6–36.9, 0.50 wt.%–1.19 wt.% respectively), reflecting the source characteristics of meta-basalt. Therefore, the remelting of juvenile crust may be the main way of continental crust accretion during Late Silurian–Early Devonian in the central BOB.

The Jinshajiang ophiolitic mélange zone in southwest China represents a remnant of the eastern Paleo‐Tethys Ocean. Field, geochronological and geochemical studies have identified two distinct suites of plagiogranites within the mélange, the Dongzhulin trondhjemite and Jiyidu tonalite, which represent rift and subduction settings, respectively, related to opening and closing of the ocean. SHRIMP U‐Pb analysis on zircons extracted from the Dongzhulin trondhjemite yields a mean 206Pb/238U age of 347 ± 7 Ma. REE and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith. Highly variable Hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components. This, together with geologic relations, suggests formation near an embryonic spreading center in a continent‐ocean transition setting. The Jiyidu tonalite has a U‐Pb zircon age of 283 ± 3 Ma, and geochemical data indicates high Sr/Y, (La/Yb)N, Nb/Ta and low Y, and marked heavy REE depletion. These signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile. A slab‐melt origin is also supported by in situ Hf and O data for zircon that show isotopic compositions comparable with typical altered oceanic crust. Thus, the crystallization age of the Jiyidu high Sr/Y tonalite provides a constraint for the subduction of the Jinshajiang ocean floor. The rift‐related Dongzhulin trondhjemite and subduction‐related Jiyidu high‐Sr/Y tonalite constrain the timing and setting of opening and closing of this segment of the Paleo‐Tethys Ocean. Key Points We identify two suites of plagiogranite intrusions with distinct age and origin Temporal and genetic model relating to Paleo‐Tethys is established Constrain opening and subduction history of Paleo‐Tethys at Jinshajiang

Jifeng ophiolitic mélange (ultramafic rocks, meta-basalts and gabbros) crops out in the northern segment of the Great Xing’an Range, the eastern segment of the Central Asian Orogenic Belt, which marks the closure of the Xinlin–Xiguitu Ocean associated with the collision between the Erguna block and Xing’an block. In order to investigate the formation age and magma source of the Jifeng ophiolitic mélange, the gabbros from newly discovered the Jifeng ophiolitic mélange are studied with zircon U–Pb ages, whole-rock geochemistry and zircon Hf isotopes. Zircon U–Pb dating from the ophiolitic gabbros yields U–Pb age of 647 ± 5.3 Ma, which may represent the formation age of the ophiolitic mélange. The gabbros display low SiO 2 , TiO 2 , K 2 O contents, high Na 2 O, LREE contents and indistinctive REE fractionation [(La/Yb) N  = 1.97–2.98]. It shows an E-MORB-like affinity, while the element concentrations of the Jifeng samples are lower than that of E-MORB. More importantly, Nb displays negative anomaly in comparison with Th, which shows a transitional SSZ-type ophiolite signature. Moreover, the ε Hf ( t ) values of ~647 Ma zircons in the gabbros range from +8.4 to +13.4, and the corresponding Hf single-stage ages ( T DM1 ) are between 687 and 902 Ma, which is obviously older than the crystallization age of 647 Ma. These geochemical features can be explained as melts from the partial melting of a depleted mantle source meta-somatized by fluids derived from a subducted slab. Accordingly, we conclude that the Jifeng ophiolitic mélange is probably related to transitional SSZ-type ophiolite and developed in an intra-oceanic subduction, which indicates that an ocean (the Xinlin–Xiguitu Ocean) existed between the Erguna block and Xing’an block. The Ocean’s formation might be no later than the Neoproterozoic (647 Ma), and it was closed in the Late Cambrian because of the collision between the Erguna block and Xing’an block.

Hectometric blocks of Middle Triassic mafic pyroclastic rocks, represented by volcanic agglomerates/breccias and lapilli tuffs, form part of the ophiolitic mélange of Mt. Medvednica, situated in the southwestern segment of the Zagorje-Mid-Transdanubian Zone. These rocks share petrochemical characteristics with pyroclastic derivatives of alkali, within-plate basaltic lavas of Mts. Medvednica, Samoborska Gora, and Kalnik, indicating the occurrence of explosive events preceding the dominant effusive submarine volcanism during the Middle Triassic (Illyrian-Fassanian?) stages. The formation of these pre-ophiolitic pyroclastics is associated with an intracontinental rift setting and reflects melts derived from an OIB-type enriched mantle plume source. These pyroclastics represent uncontaminated melts that erupted through a highly thinned continental crust. In geodynamic terms, the formation of pyroclastites occurred during the Late Anisian-Early Ladinian along the continental margin of Palaeotethys through the proto back-arc rifting of continental lithosphere (Adria Plate), leading to the formation of the Maliak/Balkan Neotethys Rift, in the emerging northwestern segment of Neotethys. The investigated pyroclastic rocks of Mt. Medvednica document the extension in an evolved intracontinental rift basin, which immediately preceded the generation of the initial Neotethyan oceanic lithosphere during the Upper Triassic.

We report two newly identified Ordovician ophiolite belts in west Junggar, NW China: Tajin–Tarbahatai–Kujibai–Honguleleng (TTKH) and Tangbale–Baijiantan–Baikouquan (TBB) ophiolitic belts. These two ophiolitic belts provide constraints for the Palaeozoic reconstruction of Central Asia and the geological evolution of this region. The TTKH and TBB ophiolitic belts are dismembered parts of different ophiolitic belts which represent relics of Ordovician oceanic floor; they subducted to the north under the Chingiz–Tarbahatai arc and to the south under the Junggar plate, respectively. The Baijiantan–Baikouquan ophiolite mélanges comprise the major part of the TBB. Flat rare Earth element (REE) patterns with positive Eu anomalies and insignificant depletion of high-field-strength elements (HFSE) relative to melts of primitive mantle suggest a mid-ocean-ridge basalt (MORB) origin for the metagabbro. Lherzolite samples define a Sm–Nd isotopic isochron with age of 474 Ma and ɛ Nd(t) of +8.9. Lherzolite samples with positive ɛ Nd(t) values of +8.8 to +9.1 and initial 87Sr/86Sr ratios of 0.7037–0.7040 are rather homogeneous in Sr–Nd isotopic composition, whereas metagabbro samples show wider Sr–Nd isotopic compositional ranges with ɛ Nd(t) of +5.9 to +11.0. The Sm–Nd isotopic isochron age (c. 380 Ma) for garnet amphibolite samples, consistent with a zircon U–Pb age (c. 385 Ma) for metagabbro, represents a magmatic event prior to subduction. Thermodynamic calculations for garnet amphibolite yield a clockwise pressure–temperature path with peak metamorphic condition of c. 15 kbar and 520–560°C at 342 Ma, indicating a subduction-channel setting. The Rb–Sr isochron ages (335 Ma, 333 Ma) for metagabbro represent a metamorphic event during exhumation.