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The Central Asian Orogenic Belt (CAOB) was formed by the aggregation and collage of numerous Paleozoic subduction‐accretion assemblages and Precambrian microcontinental blocks. However, the tectonic nature of the southeastern CAOB remains controversial, which complicates the reconstruction of the Paleo‐Asian Ocean. To address this issue, a deep seismic reflection survey was initiated across the southeastern CAOB and reveals broad gentle sub‐horizontal reflectors in the middle‐lower crust and a relatively transparent zone in the upper crust. Combining with the Precambrian geological outcrops and other geophysical features, we support a microcontinental block, the Xilinhot Block, existed in the Paleo‐Asian domain. Thus, the Paleo‐Asian Ocean was separated into two branches that underwent north‐dipping and double‐dipping oceanic plate subduction, respectively, to form the Hegenshan‐Heihe and Solonker sutures. Multiple relics beneath Hegenshan‐Heihe Suture indicate that multiple sets of unidirectional oceanic subduction‐accretion and magmatism were important mechanisms of continental growth. Plain Language Summary: During the consumption of the Paleo‐Asian Ocean, a great number of Paleozoic subduction‐related accretionary complexes were developed and combined with pre‐existing Precambrian continental fragments to form the Central Asian Orogenic Belt (CAOB). However, research on the tectonic evolution of this area has been limited, and the tectonic nature of the southeastern CAOB remains controversial. A deep seismic reflection survey along the southeastern CAOB shows the crustal architecture in detail. The broad gentle sub‐horizontal reflectors and a relatively transparent zone in the profile reveal a Precambrian continental fragment existed in the North Orogenic Belt of CAOB. The Paleo‐Asian Ocean was further separated into two parts, the fossil subduction zones of which show northward and bidirectional dipping characteristics beneath the Hegenshan‐Heihe and Solonker sutures, respectively. Several relics of the unidirectional subduction beneath the Hegenshan‐Heihe Suture indicate that multiple sets of unidirectional oceanic subduction‐accretion and magmatism were important mechanisms of continental growth. Key Points: A preserved microcontinental block has been revealed in the North Orogenic Belt of Central Asian Orogenic BeltThe Paleo‐Asian Ocean was separated into two branches, which were closed respectively by north‐dipping and double‐dipping subductionMultiple unidirectional subduction‐accretion and oceanic magmatism may contribute to continental growth [ABSTRACT FROM AUTHOR]

The petrographical, mineralogical, whole-rock geochemical, and zircon U-Pb geochronological and Lu-Hf isotopic studies were carried out for investigating the provenance and genetic mechanism of the high-grade coal-bearing kaolin within the coal seams of the Permo-Carboniferous Taiyuan Formation in the Xiaoyu mine of the Datong coalfield, Shanxi Province. The coal-bearing kaolin is composed mainly of well-ordered kaolinite with no detrital quartz, feldspars and muscovite. Some samples contain boehmite that were formed through the desilication of kaolinite. The zircons in the coal-bearing kaolin are typically igneous origin with only one age population of ca. 300 Ma, same with the depositional age of the coal, confirming that the coal-bearing kaolin was altered from volcanic ash. Besides, the coal-bearing kaolin commonly contains tabular/columnar and vermicular kaolinite that are also diagnostic feature of alteration from volcanic ash. These coarse-grained kaolinites are thought to be pseudomorphic kaolinite altered from crystal fragments in volcanic ash, such as plagioclase and biotite. The zircon εHf(t) is dominant by a wide range of negative values, indicative of their formation from magma generated by substantial remelting of ancient continental crust, coinciding with that of contemporaneous magmatic rocks formed from continental arc volcanism in the Inner Mongolia Paleo-Uplift at the northern margin of North China. This suggests their cognate relationship. The geochemical fingerprints of trace element of zircons also indicate that they were crystalized from continental arc magma. The northern margin of the North China Craton (NCC) is an Andean-type continental margin in the late Paleozoic. During the coal-accumulating period of the Late Carboniferous to Early Permian in North China, the Paleo-Asian Ocean (PAO) plate was continuously southward subducted beneath the NCC, resulted in a frequent continental arc volcanism. The emitted volcanic ash fell into peat swamps developed in a paralic environment in North China, which were subjected to an intensive leaching and altered to the coal-bearing kaolin.

[Display omitted] • Phase equilibrium modelling was used to recognize four distinct dT/dP facies series. • U–Pb dating was used to determine the age of paired metamorphic belts in Paleozoic. • Two occurrences of paired metamorphic belts may record the two-stage subduction. The paired metamorphic belts in orogenic areas record the dual thermal regimes associated with subduction. In this study, based on detailed petrographic observations, phase equilibrium modelling, and zircon and rutile U–Pb dating of samples from different metamorphic units, we report two occurrences of paired metamorphic belts in central Inner Mongolia. The first included a HP–LT and a HT–LP metamorphic belt in which garnet mica schist sample XL04 and garnet-sillimanite biotite gneiss sample XL16 yielded peak P–T conditions, respectively of ∼19 kbar/∼510℃ (∼9℃/km) and ∼ 6 kbar/∼676℃ (∼32℃/km). The second includes a MP–LT and a HT–LP metamorphic belt in which blueschist sample WD13 and amphibolite sample XL10 yielded peak P–T condition of 8.6–8.8 kbar/520–530℃ (∼19℃/km) and ∼ 4.5 kbar/870–910℃ (∼60℃/km), respectively. Zircon and rutile U–Pb results indicate that the first pair of metamorphic belts (∼9℃/km and ∼ 32℃/km) were formed at 455–442 Ma, while the second (∼19℃/km and ∼ 60℃/km) were formed at 383–306 Ma based on previous studies. These ages are consistent with those of two arc magmatic pulses in central Inner Mongolia. Consequently, we suppose that the two occurrences of paired metamorphic belts are attributed to a trench–continental margin in the Early Paleozoic and a trench–basin–arc system in the Late Paleozoic. It is likely that the two occurrences of paired metamorphic belts record the tectonic transition from a completely active continental margin setting in the Early Paleozoic to a fore-arc extension system in the Late Paleozoic, driven by slab rollback mechanisms with trench retreat of the Paleo-Asian Ocean. [ABSTRACT FROM AUTHOR]

The petrogenesis and geodynamic implications of the Early Paleozoic adakitic rocks in the east of Inner Mongolia remain topics of debate. This study presents new petrology data through zircon U-Pb age and Lu-Hf isotopic composition, whole-rock major-trace element geochemistry, and Sr-Nd isotopes from adakitic rocks. The zircon U-Pb dating results demonstrate that the formation age is 242.8 ± 1.0 Ma, which is the product of Early Triassic magmatism. Petrology and geochemical study have shown that the granodiorite have high SiO2 (66.93~69.40%), Al2O3 (15.37~15.43%), and MgO (1.35~1.55%), with LREE enrichment and HREE deficit, and they have high Sr, low Y, and high Sr/Y ratios, showing typical signatures of adakitic rocks. The εHf(t) values of zircon vary between +11.3 and +13.8, with low whole-rock (87Sr/86Sr)i (0.703382) and positive εNd(t) values, and the average Mg# of the rock is 56.14, suggesting that adakite derived from partial melting of MORB materials and magma interaction with the mantle. Comprehensive analysis suggests that during the Late Permian to Early Triassic, the subducted slab of the Paleo-Asian Ocean broke off, and the residual oceanic slab preserved in the mantle beneath the subduction zone underwent partial melting, generating adakitic magma. [ABSTRACT FROM AUTHOR]

[Display omitted] • A double subduction system on the northern margin of NCC. • "Andean-style" southward subduction of the PAO during the Early Palaeozoic. • The regional arc-continent collision occurred at the end of the Early Palaeozoic. • Culmination of orogeny in the Early Devonian. • Preservation of intra-oceanic arcs and crust growth during arc collision. The Early Palaeozoic arc-related formations in the northern margin of the North China Craton (NCC) belonging to the Bainaimiao arc belt is an important part of the eastern Central Asian Orogenic Belt (CAOB). The tectonic evolution of this belt remains controversial. Here we present detailed data on the geochronology and geochemistry on a newly identified Early Palaeozoic igneous suite from the Faku area in northern Liaoning. Zircon U–Pb dating of the Faku volcanic and intrusive rocks yielded weighted mean 206Pb/238U ages of ca. 458 Ma and 433–405 Ma, respectively, which represent their crystallization (eruption) times. Zircon Hf isotopic analyses show a low negative to positive ε Hf (t) values of − 0.33 to + 12.4, implying depleted characteristics of the source. The geochemical features of Laolingshan basaltic andesites display higher TiO 2 and lower P 2 O 5 contents, similar to that of mid-ocean ridge basalts, and relatively high Zr/Y and low Ta/Yb ratios, suggesting oceanic island arc basalt affinity. Therefore, we suggest that the Laolingshan basaltic andesites were formed in an intra-oceanic environment. The Bachagou granodiorites, with adakitic geochemical signatures, belong to the calc-alkaline and high-K calc-alkaline series, implying formation in an active continental margin tectonic setting. The Baerhushan granodiorites, which geochemically belong to S-type granites, were probably formed in a syn -collisional environment influenced by arc–continent collision. The Donggou granitoids, which have higher SiO 2 and alkali and lower MgO and P 2 O 5 , show A-type granite features. Geochemical characteristics of the Donggou A-type granite suggest formation in an extensional setting caused by orogenic intermittent extension. The Baerhushan gabbro diorites, which display properties of both mid-ocean ridge and continental rift basalts, were produced under post-orogenic extensional tectonic setting after the arc–continent collision. Our study traces the Early Palaeozoic tectonic evolution of the south-eastern segment of the CAOB and the related accretionary tectonics. [ABSTRACT FROM AUTHOR]

Abstract The Central Asian Orogenic Belt (CAOB) was formed by the aggregation and collage of numerous Paleozoic subduction‐accretion assemblages and Precambrian microcontinental blocks. However, the tectonic nature of the southeastern CAOB remains controversial, which complicates the reconstruction of the Paleo‐Asian Ocean. To address this issue, a deep seismic reflection survey was initiated across the southeastern CAOB and reveals broad gentle sub‐horizontal reflectors in the middle‐lower crust and a relatively transparent zone in the upper crust. Combining with the Precambrian geological outcrops and other geophysical features, we support a microcontinental block, the Xilinhot Block, existed in the Paleo‐Asian domain. Thus, the Paleo‐Asian Ocean was separated into two branches that underwent north‐dipping and double‐dipping oceanic plate subduction, respectively, to form the Hegenshan‐Heihe and Solonker sutures. Multiple relics beneath Hegenshan‐Heihe Suture indicate that multiple sets of unidirectional oceanic subduction‐accretion and magmatism were important mechanisms of continental growth.

The Permian to Triassic period represents a pivotal phase in the evolution of the Paleo-Asian Ocean, marked by significant tectonic transitions from subduction, collision, and post-orogenic extension. The timing of closure of the Paleo-Asian Ocean in northeastern China has always been controversial. In this contribution, the petrology, zircon U-Pb geochronology and geochemistry are conducted on granite found in well HFD1, Songliao Basin, eastern part of Central Asian orogenic belt. Zircon U-Pb dating indicates that granite crystallized at 258.9 ± 2.2 Ma, as the product of magmatism occurred in the early Late Permian. The rocks have high SiO2, Al2O3, Na2O content, negative Eu anomaly, light enrichment of rare-earth elements, depletion of heavy rare-earth elements, high Sr (448.29–533.11 ppm, average 499.68 ppm), low Yb (0.49–0.59 ppm, average 0.54 ppm), Y (4.23–5.19 ppm, average 4.49 ppm), and high Sr/Y ratios (98–125, average 112) and can be classified as O-type adakite. This is the first discovery of late Paleozoic adakite in the Songliao Basin and the neighboring areas. The geochemistry of adakite indicates derivation by partial melting of MORB-type subducted oceanic crust, indicating that the subduction of the Paleo-Asian Oceanic lithosphere lasted until at least 258.9 Ma. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ediacaran–Silurian intra-oceanic arcs formed juvenile continental crust in SW Paleo-Asian Ocean. • Continental margin arcs developed on juvenile sialic basement since Middle Devonian. • Meandering rivers replaced braided rivers during Middle–Late Devonian boom of lycopsids. In accretionary orogeny, the intra-oceanic subduction-accretion could generate juvenile continental crustal materials and form new continental blocks by their lateral tectonic accretion, and the juvenile continental crust would provide a sialic basement for continental margin arc during the subsequent oceanic-continental subduction-accretion. These two distinct subduction-accretion processes occurred successively in the southwestern part of the Paleo-Asian Ocean during the Neoproterozoic–Late Paleozoic and created a multi-stage evolution of continental crusts in West Junggar. In this study, a Late Silurian landmass in West Junggar is revealed by the detailed geological mapping and comparison of the Late Silurian–Early Devonian angular unconformity in southern and northern West Junggar. This crucial unconformity confines the Middle Silurian termination of Early Paleozoic intra-oceanic subduction-accretion, as evidenced by the pre-Late Silurian emplacement of Cambrian–Ordovician ophiolitic mélanges, the Middle Silurian cessation of Early Paleozoic arc magmatism and the Late Silurian–Early Devonian sedimentary hiatus. The Late Silurian landmass was consequently constructed by amalgamations of island arcs, accretionary complexes and seamounts, and its break-up was marked by the Middle Devonian opening of the Karamay back-arc Ocean in central West Junggar. In contrast with the Early Paleozoic intra-oceanic arc systems, continental margin arcs on the sialic basement of rifted landmass emerged since the Middle Devonian, as suggested by our new chronological data and field observations. We further unravel that the increase in mudstone of Devonian successions on the active continental margin is contemporaneous with the emergence and boom of lycopsid forest in West Junggar, which is in agreement with a global trend in alteration of fluvial systems due to evolution of early land plants. In summary, this paper presents a typical record for two distinguished styles of subduction-accretion systems that developed successively in an accretionary orogeny, and sedimentary successions of the later active continental margin documented the interaction between terrestrial sedimentation and early land plants. [ABSTRACT FROM AUTHOR]

This document is an editorial from the journal Frontiers in Earth Science, discussing the research topic of the Paleo-Asian and Tethyan domains. The Paleo-Asian Ocean and Tethyan Ocean were ancient oceans that formed during different time periods. The Central Asian Orogenic Belt and Tethyan Tectonic domains are two large tectonic regions associated with the evolution and closure of these oceans. The editorial highlights various studies on the tectonic and magmatic evolution of these domains, including investigations into volcanic regions, granitoids, tectonic evolution, and oil reservoirs. These studies contribute to our understanding of the geodynamic processes and tectonic history of these regions. [Extracted from the article]

The Paleo-Asian Ocean (PAO) had multiple collage systems and closed along multiple sutures. The final timing of the closure of the PAO was closely related to the formation of Pangea. The final timing of closure of the eastern PAO is controversial and may have been from the Middle Permian to the Middle Triassic. This paper presents new zircon U–Pb ages and Hf isotope data for sedimentary and volcanic rocks layer in the Linxi Formation in the Xingmeng Orogenic Belt (XMOB). We also obtained geochemical data for the volcanic rocks layer to constrain the timing and mechanisms of the PAO closure. Detrital zircon U–Pb dating yielded the youngest ages peak of ca. 263 and 260 Ma for two sandstone samples. Combined with the crystallization age of the volcanic rocks layer (254.2 ± 1.3 Ma), this indicates the Linxi Formation was deposited during the Late Permian. Age spectra and Hf isotopic compositions of the detrital zircons from the Linxi Formation suggest that detrital zircons with ages of <1500 Ma were derived from the XMOB, whereas those with ages of 2100–1600 Ma were derived from both the XMOB and the North China Craton (NCC). The detrital zircons with ages of >2400 Ma were derived from the NCC. Geochemical features of the volcanic rocks layer are consistent with eruption in a compressional tectonic setting and derivation by partial melting of the lower crust. Based on our data and the Late Permian history of magmatism and sedimentation, the Linxi Formation was likely deposited in a tectonic setting that transitioned from convergence to collision. Finally, we suggest the PAO closed in a ‘scissor-like’ fashion in the eastern CAOB during the Late Permian, in a setting of tectonic compression. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Xilinhot 316–285 Ma granitoids were evolved from coeval calc-alkaline basaltic magma. • The Linxi 525–261 Ma granites were derived from remelting of supracrustal materials. • Bidirectional subduction of the Solonker Ocean occurred until 243 Ma. • Amalgamation along the Solonker suture occurred after 243 Ma and closed the Paleo-Asian Ocean. The closure of the Paleo-Asian Ocean resulted in formation of the Central Asian Orogenic Belt (CAOB) accompanied by considerable Phanerozoic juvenile crustal growth. As an international frontier topic, whether the Solonker suture zone is the site of final closure of the Paleo-Asian Ocean and records the terminal evolution of the CAOB in Inner Mongolia is still in controversial. This study presents the geochemistry and zircon U-Pb-Hf isotopes of granitoids along the Solonker suture zone to constrain the subduction and closure processes of the Paleo-Asian Ocean. Granitoids from the Xilinhot area in the northern accretionary zone comprise diorites and granodiorites with LA-ICPMS zircon U-Pb ages of 316 ± 2 Ma, 302 ± 2 Ma, 285 ± 2 Ma and 243 ± 4 Ma, positive εHf (t) values of +2.39 to +13.31 and T DM2 model ages of 481–1191 Ma. The ca. 316–285 Ma granitoids were formed by fractional crystallization of regional coeval calc-alkaline basaltic magma in an arc-related setting and the ca. 243 Ma granitoid was generated mainly from partial melting of dominant basaltic source rocks and minor metagraywackes in the northward subduction-related setting. The granitoids from the Linxi area in the southern accretionary zone include muscovite monzogranites and monzogranites with LA-ICPMS zircon U-Pb ages of 525 ± 6 Ma, 274 ± 2 Ma and 261 ± 4 Ma. The ca. 525 Ma muscovite monzogranites have negative zircon εHf (t) values of −19.36 to −13.61 with T DM2 model ages of 2348–2711 Ma. They were generated from partial melting of Neoarchean to Paleoproterozoic supracrustal materials. The ca. 274–261 Ma monzogranites show negative zircon εHf (t) values ranging from −10.48 to −0.66 with T DM2 model ages of 1335–1942 Ma. They were derived from remelting of Paleoproterozoic to Mesoproterozoic supracrustal materials in the southward subduction zone along the northern active margin of the North China Craton. Combining our results with existing data, we infer that the early Paleozoic southward subduction of the Paleo-Asian Ocean started before ca. 525 Ma, and the northward subduction zone lasted to ca. 243 Ma, which implies the bidirectional subduction of the ocean lasted at least until ca. 243 Ma. Our work suggests that along the Solonker suture zone, the regional amalgamation may have occurred after ca. 243 Ma, which closed the Paleo-Asian Ocean. [ABSTRACT FROM AUTHOR]

The petrogenesis and geodynamic implications of the Early Paleozoic adakitic rocks in the east of Inner Mongolia remain topics of debate. This study presents new petrology data through zircon U-Pb age and Lu-Hf isotopic composition, whole-rock major-trace element geochemistry, and Sr-Nd isotopes from adakitic rocks. The zircon U-Pb dating results demonstrate that the formation age is 242.8 ± 1.0 Ma, which is the product of Early Triassic magmatism. Petrology and geochemical study have shown that the granodiorite have high SiO2 (66.93~69.40%), Al2O3 (15.37~15.43%), and MgO (1.35~1.55%), with LREE enrichment and HREE deficit, and they have high Sr, low Y, and high Sr/Y ratios, showing typical signatures of adakitic rocks. The εHf(t) values of zircon vary between +11.3 and +13.8, with low whole-rock (87Sr/86Sr)i (0.703382) and positive εNd(t) values, and the average Mg# of the rock is 56.14, suggesting that adakite derived from partial melting of MORB materials and magma interaction with the mantle. Comprehensive analysis suggests that during the Late Permian to Early Triassic, the subducted slab of the Paleo-Asian Ocean broke off, and the residual oceanic slab preserved in the mantle beneath the subduction zone underwent partial melting, generating adakitic magma.

The timing of formation of the Hegenshan suture is crucial to understanding the evolution of the Central Asian Orogenic Belt (CAOB). Here we present a well‐dated paleomagnetic pole (103 samples from 15 sites, 36.7°N/29.2°E, A95 = 3.3°) that passed a positive fold test, a reversal test, and a conglomerate test from the ∼250 Ma upper Gunbayn Formation andesites in the southeastern Mongolia Block (MOB). The high‐quality paleomagnetic database may demonstrate that the MOB drifted rapidly southward and collided with the Xilinhot–Songliao Block between 256 and 250 Ma, resulting in the closure of the Hegenshan Ocean at ∼250 Ma. The Hegenshan suture and the Solonker suture were formed almost simultaneously and marked the final closure of the Paleo‐Asian Ocean. Plain Language Summary: The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogenic belt on earth since begin of the Phanerozoic. Since the Late Paleozoic, two prominent sutures, namely the Hegenshan and Solonker sutures, have played crucial roles in the tectonic evolution of the eastern CAOB, as they were associated with the closure of the Paleo‐Asian Ocean (PAO). The timing of the formation of the Hegenshan suture, however, remains ambiguous due to the lack of reliable paleomagnetic data. In this study, we present accurately dated paleomagnetic constraints on the paleogeographic position of the Mongolia Block (MOB), located on the northern side of the Hegenshan suture, at ∼250 Ma. By combining high‐quality Late Permian–Early Triassic paleomagnetic data with geological evidence, we propose a new reconstruction that suggests the near‐simultaneous closure of both the Hegenshan and Solonker oceans around 250 Ma. This significant event marks the final disappearance of the PAO and represents a fundamental stage in the establishment of the tectonic framework of the eastern CAOB at that time. Key Points: We report a well‐dated ∼250 Ma paleomagnetic pole from the Mongolia BlockThe closure of both the Hegenshan and Solonker oceans at ∼250 Ma marked the final disappearance of the Paleo‐Asian Ocean [ABSTRACT FROM AUTHOR]

Various magmatisms during the subduction-collision process are crucial to reveal the long-term tectonic evolution of the eastern Central Asian Orogenic Belt. In this paper, we present major and trace elements of whole-rock, zircon U-Pb dating and Hf isotope of the Shanmen pluton. Results imply that the Shanmen pluton consists of quartz diorite and mylonitic granite, with zircon U-Pb ages of 263.7–259.6 Ma. The studied quartz diorite contains high Sr/Y (51.19–90.87) and (La/Yb)N (7.82–13.62) ratios, and belongs to adakitic rocks. Coupled with the positive εHf(t) values of +5.71 to +12.8 with no obvious Eu anomaly, we propose that quartz diorite is the product of the interaction between different degrees of slab melt and the overlying mantle wedge. In contrast, the mylonitic granite has lower MgO (0.28 wt% – 0.47 wt%) contents and positive εHf(t) values of +7.79 to +10.15, indicating an affinity with I-type granite originated by partial melting of the intermediate-basic lower crust. The geochemical characteristics and lithological assemblages, along with the Permian magmatic rocks in the Changchun-Kaiyuan area displaying arc rocks affinity, propose their formation is related to the southward subduction of the Paleo-Asian Ocean (PAO). Based on this study and previous evidence, we lean towards adopting a middle-late Permian slab break-off model, wherein the PAO did not close until the late Permian. [ABSTRACT FROM AUTHOR]

[Display omitted] • Borehole, seismic profile, palynology, and geochronology constrain sedimentary age of Linxi Formation precisly. • Geochemistry of 60 clastic borehole cores in a continental arc basin indicate a source of continental arc. • Continuous 600 m borehole cores from marine and transitional facies to continental facies records subduction to collision. The process and timing of the closure of the Paleo-Asian Ocean remain uncertain. The upper Permian Linxi Formation in the Songliao–Xilinhot block provides a key sedimentary record of the closure of the eastern Paleo-Asian Ocean and the associated orogenesis. Here we present a seismic reflection profile and geochronological, paleontological, and geochemical data from 600 m of borehole cores (2022–1394 m depth) from the Songliao Basin to constrain the timing, provenance, and tectonic background of the Linxi Formation. Detrital zircons from the Linxi Formation yield a maximum depositional age of 257.4 ± 4.1 Ma. The formation was intruded by a syenite porphyry dike that crystallized at 245.6 ± 2.7 Ma. Furthermore, the paleontological assemblage indicates an upper Permian stratigraphic age. Borehole samples yield fingerprints of major oxides that are similar to clastic rocks in continental arc basins. Their Rb contents are relatively high (92 ppm on average), Rb/Sr (0.02–2.86) and Th/U (1.12–6.30) ratios are low, and (La/Yb) N and Eu/Eu* ratios are 3.85–8.13 and 0.54–0.98, respectively. These geochemical characteristics indicate a source of continental magma arc. Aided by discrimination diagrams, we propose that the Linxi Formation was deposited within a continental arc or on a continental margin during the closure of the Paleo-Asian Ocean. The Linxi Formation was deposited in a water body that experienced a gradual reduction in salinity, transitioning from a marine environment at the bottom to a transitional marine-terrestrial environment at the top. The water body also became gradually shallower and deposited sediment in the tectonic setting of continental arc. These changes indicate that there was a transition from subduction to collision during the Late Permian. [ABSTRACT FROM AUTHOR]

The Permian to Triassic period represents a pivotal phase in the evolution of the Paleo-Asian Ocean, marked by significant tectonic transitions from subduction, collision, and post-orogenic extension. The timing of closure of the Paleo-Asian Ocean in northeastern China has always been controversial. In this contribution, the petrology, zircon U-Pb geochronology and geochemistry are conducted on granite found in well HFD1, Songliao Basin, eastern part of Central Asian orogenic belt. Zircon U-Pb dating indicates that granite crystallized at 258.9 ± 2.2 Ma, as the product of magmatism occurred in the early Late Permian. The rocks have high SiO2, Al2O3, Na2O content, negative Eu anomaly, light enrichment of rare-earth elements, depletion of heavy rare-earth elements, high Sr (448.29–533.11 ppm, average 499.68 ppm), low Yb (0.49–0.59 ppm, average 0.54 ppm), Y (4.23–5.19 ppm, average 4.49 ppm), and high Sr/Y ratios (98–125, average 112) and can be classified as O-type adakite. This is the first discovery of late Paleozoic adakite in the Songliao Basin and the neighboring areas. The geochemistry of adakite indicates derivation by partial melting of MORB-type subducted oceanic crust, indicating that the subduction of the Paleo-Asian Oceanic lithosphere lasted until at least 258.9 Ma.

Abstract The timing of formation of the Hegenshan suture is crucial to understanding the evolution of the Central Asian Orogenic Belt (CAOB). Here we present a well‐dated paleomagnetic pole (103 samples from 15 sites, 36.7°N/29.2°E, A95 = 3.3°) that passed a positive fold test, a reversal test, and a conglomerate test from the ∼250 Ma upper Gunbayn Formation andesites in the southeastern Mongolia Block (MOB). The high‐quality paleomagnetic database may demonstrate that the MOB drifted rapidly southward and collided with the Xilinhot–Songliao Block between 256 and 250 Ma, resulting in the closure of the Hegenshan Ocean at ∼250 Ma. The Hegenshan suture and the Solonker suture were formed almost simultaneously and marked the final closure of the Paleo‐Asian Ocean.

The crystallization age of the Sunjiafen Formation volcanic rocks is 253.9 ± 1.1 Ma. Geochemical data show that these volcanic rocks belong to the shoshonitic and metaluminous series and have high Sr and low Y contents (461–582 and 5.60–6.74 ppm, respectively), MgO = 1.32–2.65 wt%, and Na2O/K2O = 0.81–1.40. In MgO−SiO2, Cr−SiO2, Sr/Y−Y and (La/Yb)N−YbN diagrams, data for the samples plot in the adakite and "adakite related to thickened lower crust" fields. As such, we suggest that the Sunjiafen Formation C−type adakitic andesites formed in a compressional tectonic setting. The youngest age (youngest peak age) of two sandstone samples from the Linxi Formation are 251 Ma (257 Ma) and 237 Ma (254 Ma). Detrital zircons with ages of >1500 Ma have εHf(t) values of −5.87 to +6.31. A comparison of the age spectra and εHf(t) values of the detrital zircons in the Linxi Formation with those of igneous zircons from the Central Asian Orogenic Belt and North China Craton indicates the formation was derived from both the belt and the craton. We identified "Jura–type Folds" that formed in peripheral foreland basin in the Linxi Formation, and along with plant fossil fragments (Paracalamites frigidus). These and previously published results indicate the Paleo–Asian Ocean closed along the Xar Moron–Changchun Suture zone during the Late Permian. The study area was in a continuous compressional setting during the Late Permian and formed a peripheral foreland basin that was related to continent–continent collision. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB) is the world's largest accretionary orogenic belt, and its formation is related to the closure of the Paleo-Asian Ocean (PAO). However, the closure time and style of the PAO remain controversial. To address these issues, this paper presents zircon U-Pb dating, whole-rock geochemistry and zircon Lu-Hf isotope analyses of the volcanic rocks in the Faku-Kaiyuan area on the northern margin of the North China Craton. The results show that the Bachagou andesites formed in the Early Permian (287 ± 2 Ma), while the Chaijialing andesites and dacites formed in the Late Permian (253.3 ± 3.7 Ma) and Middle Triassic (244.3 ± 1.3 Ma), respectively. The Bachagou andesites and Chaijialing andesites are enriched in LILEs and LREEs and depleted in HFSEs and HREEs, indicating that they formed in the active continental margins. The Chaijialing dacites show similar geochemical signatures to adakite and formed in a syn-collisional setting. Geochemistry and isotopic analysis indicates that the Bachagou andesites were derived from a partial melting of the mantle wedge that was metasomatized by subduction fluids. The Chaijialing andesites were generated from a metasomatized mantle by slab-derived and sediment fluids. The Chaijialing dacites formed by a partial melting of thickened lower crust. Combined with previous research results, we can conclude that the Eastern PAO closed by a scissor-like movement from west to east during the Late Permian–Middle Triassic. [ABSTRACT FROM AUTHOR]

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The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing ’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260 –240 Ma), respectively, forming the Xinlin-Xiguitu, Mudanjiang-Yilan, Hegenshan-Heihe, Solonker-Linxi, and Changchun-Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary orogenic belts in central Asia.©2022 China Geology Editorial Office.

The Paleo-Asian Ocean, as the processor of the Altaids, was a long-lived ocean, and its subduction gave rise to substantial continental growth in the Central Asian continent. However, the location of the major oceanic basin and its evolution history are debatable, impeding our better comprehension of the early accretionary tectonics of the Paleo-Asian Ocean and its link to the reconstruction of the Rodinia. Here, we report our newly discovered Neoproterozoic adakitic rock (Xingxingxia gneissic granite) in the Eastern Tianshan of the southern Altaids to address the above issues. The Xingxingxia gneissic granite displays high Sr/Y ratios (47–114) and strongly fractionated rare earth element (REE) patterns with low concentrations of Y (3.00–6.36 ppm) and Yb (0.23–0.62 ppm), indicative of the adakite affinity. The adakitic rock is depleted in Nb, Ta, P and Ti, and possesses low Mg# (37–42), comparatively high K2O/Na2O ratios (0.75–1.08). Zircon U–Pb dating reveal that the adakitic rock was formed at 869 ± 9 Ma, which has relatively depleted Hf isotopic compositions (εHf(t) = + 1.9 to + 4.7). Their geochemical features are consistent with those of an origin from a thickened lower arc crust. Our work suggests that subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan Arc occurred at least in the Neoproterozoic. Therefore, our study provides a solid and key evidence to show that the tectonic position of the major Paleo-Asian Ocean was located in the South Tianshan, where the paleo-ocean evolved into its mature stage following the subduction beneath the Central Tianshan at ca. 870 Ma. This work also shed light on the reconstruction of Rodinia and the interaction of the Paleo-Asian Ocean and the Jiangnan–North Tarim Ocean. [ABSTRACT FROM AUTHOR]

This paper reviews published and presents new data on U-Pb detrital zircon ages, and petrographic, geochemical and isotope (Sm-Nd, Lu-Hf) compositions obtained from greywacke sandstones of Kazakhstan in order to reconstruct fossil intra-oceanic arcs that once existed at Pacific-type convergent margins of the Paleo-Asian Ocean (PAO) in Paleozoic time. We focus on orogenic belts of central Kazakhstan (Itmurundy and Tekturmas) and eastern Kazakhstan (Zharma and Char) in the western Central Asian Orogenic belt. These orogenic belts host accretionary complexes with greywacke sandstones of early Paleozoic (central Kazakhstan) and middle-late Paleozoic (eastern Kazakhstan) ages. First, we evaluate general perspectives for studying sandstones to reconstruct survived and disappeared magmatic arcs, taking into account episodes of subduction erosion. Then we discuss the analytical data from sandstones to make conclusions about the ages and formation settings of their igneous protoliths and define maximum deposition ages. Finally, we discuss the role of serpentinite mélanges in tectonic reconstructions. We argue that sandstones hosted by accretionary complexes are typically greywackes deposited close to their igneous sources and buried rapidly. The provenances of the studied greywacke sandstones of central and eastern Kazakhstan were dominated by mafic to andesitic igneous protoliths derived from juvenile mantle sources. The igneous rocks in the provenances were emplaced in an intra-oceanic arc setting. The sandstones were deposited in fore-arc/trench basins or, to a lesser degree, in back-arc basins. The data from both sandstones and serpentinite mélanges reconstruct middle-late-Cambrian, Ordovician, late-Devonian and Carboniferous arcs of the western PAO. The middle-late Cambrian arcs were fully destroyed by subduction erosion, whereas the Ordovician and Carboniferous arcs survived. The late-Devonian arcs were also eroded, but partly. Both the early and late Paleozoic active margins of the PAO were characterized by alternating periods of accretionary growth and subduction erosion. [ABSTRACT FROM AUTHOR]

Late Paleozoic tectonic evolution of northeastern China is complicated by the interaction of the Central Asian Orogenic belt development and the initiation of subduction from the Pacific Ocean plate along the eastern margin of Asia. In this study, we elucidate this problem by collecting new geochemical and geochronological data from late Paleozoic bimodal igneous rocks from the eastern margin of the Jiamusi block. By LA–ICP–MS zircon U–Pb dating, the 206Pb/238U age of the granitoid sample is 313–279 Ma, and the diabase 206Pb/238U age is 298–276 Ma. Rock geochemical studies show that diabase and granitoid have obvious chemical differentiation characteristics, indicating that they are the products of non-homologous magmatic evolution. Zircon Lu–Hf isotopes indicate that diabase magma originates from the metasomatism of the mantle wedge by subducting oceanic crust melts, granitoid magma mainly comes from the lower crust and the new crust. Our results together with a synthesis of existing data show that bimodal magmatism occurred in the study area from the Late Carboniferous to Early Permian when the Jiamusi block was an active continental margin and in a stretching background. Our synthesis of the existing data places our new data into a regional tectonic framework, and our proposed tectonic model provides the following new insights: (1) the northward expansion of the Paleo-Tethys Ocean from the Late Carboniferous to the Early Triassic led to the overall northward drift of the Paleo-Asian Ocean and the Xing'an-Mongolian Orogenic belt, which marks the easternmost segment of the Central Asian Orogenic belt, (2) the Paleo-Asian Ocean continued Late Permian–Early Triassic subduction caused back extension that rifted the Jiamusi and Songnen blocks apart and formed the Mudanjiang Ocean. [ABSTRACT FROM AUTHOR]

This paper aims to analyze the deformation characteristics and constrain the deformation time of the Gudonghe ductile shear belt in the Yanbian area, NE China, the eastern segment of the Central Asian Orogenic Belt. Two stages of ductile deformation have been identified according to the macro- and micro-fabric characteristics. The NWW-trending D1 deformation, with pervasive dextral shearing and extensional features as indicated by shear folds and δ-porphyroclasts, occurred in the Early–Middle Triassic Yongxin pluton based on zircon U–Pb age of 246–236 Ma and previously published geochronological data, and the foliation was cut by an undeformed igneous rock of ~ 226 Ma; therefore, the D1 deformation time in the Gudonghe ductile shear belt is limited to the 236–226 Ma. The NE-trending D2 deformation event occurred in the Early Jurassic Baijin pluton. The S–C fabrics and σ-porphyroclasts in the D2 deformation stage show a pervasive sinistral shearing and were formed in a compression setting. Two deformed granites in the D2 deformation stage have a weighted mean age of 197 Ma, while the syn-tectonic granitic vein during the D2 deformation stage has a weighted mean age of 184 Ma. The undeformed granites, which intruded into the Early Jurassic Baijin pluton, have a weighted mean age of 179 Ma. Therefore, the D2 deformation of the Gudonghe ductile shear belt occurred during 184–179 Ma. Considering the regional tectonic evolution and the structural features of the Gudonghe ductile shear belt, the D1 deformation event could be related to the Middle–Late Triassic post-orogenic collapse after the closure of the Paleo-Asian Ocean, while the D2 deformation was formed in a compressional setting and was related to the onset of westward-directed subduction and accretion of the Paleo-Pacific Ocean in the Early Jurassic. [ABSTRACT FROM AUTHOR]

This paper reports the first discovery of basic igneous rocks in Bairin Right Banner in southeastern Inner Mongolia, China. To understand the genesis and tectonic evolution of these rocks, we have carried out geochronology and geochemistry research on them. The results show that LA-ICP-MS zircon U-Pb concordant mean ages of massive basalt, diabase, and pillow basalt are 272.0 ± 2.8 Ma, 261.5 ± 3.2 Ma, and 256.5 ± 2.5 Ma, respectively, suggesting their formation during the Late Permian. The massive basalt, diabase, and pillow basalt are tholeiitic with low Mg# values in the ranges of 46.2–66.8, 54.4–71.0, and 53.5–58.2, respectively. They have similar chondrite-normalized rare earth element (REE) patterns. The intensity of REE differentiation is similar to that of typical enriched mid-ocean ridge basalts (E-MORB). The pillow basalts are relatively enriched in Ba and U, while slightly depleted in Rb and Th, and show geochemical affinity to E-MORB. The massive basalts and diabases have the same primitive mantle-normalized patterns and are relatively enriched in Rb, Ba, and U, while depleted in Nb and Ta, similar to the typical island arc magmas. Our new findings indicate that the intra-oceanic subduction of the Paleo-Asian Oceanic slab was still ongoing during the late Permian in the Bairin Right Banner area and, combined with previous studies, it might have been closed during the Late Permian to Early Triassic along the Xar Moron suture zone.

Accretionary complexes and their hosted Ocean Plate Stratigraphy (OPS) units record a full story of ancient oceans from their birth at mid-oceanic ridges to their death at subduction zones. However, their reconstruction is difficult because accretionary complexes possess very complicated structures and therefore require careful and detailed in-field geological and structural studies and up-to-date analytical data. In this paper, we present results of detailed geological and structural studies of OPS sediments and new U–Pb detrital zircon data from sandstones of the Gorki Formation, a major part of the Ulaanbaatar accretionary prism/complex. The structure of the Gorki Fm. consists of five units separated from each other by parallel NE-striking regional faults and deformed by intra-formational detachments. It formed by offscraping and accretion of OPS sediments and subsequent thrusting and duplexing. Each unit is characterized by a specific association of OPS rocks, type and degree of deformation, style of folding and bedding geometry suggesting dychronous deposition and accretion of marine sediments. The ages and lithologies of the five units of the Gorki Fm. allowed us to trace the whole story of OPS sedimentation and further deformation—from oceanic floor (pre-accretion), to young and growing accretionary prism (syn-accretion) to accretionary prism stabilization (post-accretion). The dominantly southward vergence of the Gorki OPS thrust sheets suggests a generally northward subduction polarity. The greywacke sandstones of Unit I yielded U–Pb zircon ages ranging from 389 to 317 Ma with two main peaks at 346 and 367 Ma suggesting a provenance dominated by Famennian to Visean igneous complexes. The absence of the Famennian peak in the U–Pb detrital zircon spectra of sandstones from younger Unit II suggests tectonic erosion of a late Devonian arc. In general, the repeated layer-parallel thrusts inside lithologically uniform sediment beds indicate a regime of strong compression along the Benioff plane during subduction at a Pacific-type convergent plate boundary. [ABSTRACT FROM AUTHOR]

This paper presents first U–Pb detrital zircon ages, major and trace element geochemical data and whole-rock Nd and Hf-in-zircon isotope data from sandstones of the Char and Zharma zones of eastern Kazakhstan. Petrographically the sandstones represent greywackes. The U–Pb ages of detrital zircons show unimodal distributions peaked at 345–340 and 330–325 Ma indicating post-Early Carboniferous deposition. The sandstones have high CIA values (60–68), suggesting moderate to weak chemical weathering. Their high ICV values (1.1–1.3) and relatively high MgO and low Y, Nb indicate an immature probably mafic–intermediate igneous source (low La/Th, medium Hf). Positive whole-rock εNd(t) and zircon εHf(t) indicate dominantly juvenile character of igneous rocks in the provenance. In total, the unimodal character of U–Pb detrital zircon age patterns, the chemical composition of sandstones similar to that of associated volcanic rocks and the positive values of εNd(t) and εHf(t) suggest their derivation from one or more intra-oceanic arcs. The data from eastern Kazakhstan fit those obtained from late Paleozoic greywacke sandstones of NW China. The new geochemical and geochronological data from sandstones and published data from oceanic and supra-subduction ophiolites of eastern Kazakhstan suggest an extended Pacific-type subduction system in the western Paleo-Asian Ocean in Devonian–early Carboniferous time. [ABSTRACT FROM AUTHOR]

[Display omitted] • A complete subduction initiation record exposed in the ophiolitic complex. • The ocean spreading reached a peak age during the Cambrian–Early Ordovician. • The SW Paleo-Asian Ocean started to subduct in the Early Ordovician. • Slab-derived sediments have a significant contribution to oceanic arc evolution. As a key step of the Wilson cycle, subduction initiation is regarded as a start hallmark of the ocean-continent transformation. However, the crust-mantle interaction and crustal growth processes related to subduction initiation remain enigmatic. Here we report a newly recognized complete magmatic sequence containing three rock assemblages in the Hongguleleng ophiolitic complex (HOC) in West Junggar, NW China. Group I contains typical Mid-Oceanic Ridge (MOR)-derived rocks including abyssal peridotite, gabbro, plagiogranite, and basalt. Group II consists of forearc basalt (FAB) and boninitic gabbro and has a high continentality index (CI > 130), similar to the rock assemblages in the Mariana proto-arc. Group III composed of calc-alkaline andesite and arc tholeiite basaltic andesite exhibits middle CI values (50 < CI < 100), suggesting that they form in a transitional oceanic arc. Zircon U-Pb ages show that the N-MORB gabbro and plagiogranite samples are emplaced at 483–481 Ma, and the boninitic rocks, calc-alkaline andesites, and arc tholeiite basaltic andesites have the crystalline ages of 472–467 Ma, 456 Ma, and 452 Ma, respectively. An increasing trend of Ba/Th and V/Ti ratios from Group I to Group II suggests the addition of aqueous fluid and elevated oxidation in the mantle source during subduction initiation. However, it exhibits an uptrend in Th/Nb and (La/Sm)cn ratios but a downtrend in Ba/Th ratios from Group II to Group III, indicating a significant contribution of slab-derived sediments to the mantle during oceanic arc evolution. Combined with regional data, we consider that there developed an E-W–trending sub-ocean basin of the Paleo-Asian Ocean spanning northern West Junggar and East Kazakhstan during the Middle Cambrian–Ordovician. The MOR-type and forearc ophiolites in northern West Junggar as well as our results suggest that the ocean happened mid-ocean ridge spreading at ca. 512–478 Ma and subduction initiation at ca. 476–467 Ma, and developed a transitional oceanic arc at ca. 456–452 Ma. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Itmurundy accretionary complex hosts two groups of immature greywacke sandstones. • Group 1 sandstones were derived from juvenile arc igneous rocks and deposited in fore-arc basin/trench. • Group 2 sandstones were derived from mixed arc igneous rocks and deposited in back-arc basin. • Sandstones of both groups formed by erosion of an Ordovician intra-oceanic arc. Sandstones hosted by Pacific-type orogenic belts and related accretionary complexes carry records of their source intra-oceanic or continental magmatic arcs. Study of arc-derived sandstones is important for reconstructing tectonic evolution of Pacific-type convergent margins in case such arcs get partly or completely eroded during subduction. In this paper, we present first U-Pb detrital zircon ages and Hf-in-zircon isotope ratios and whole-rock geochemical and Nd data from Ordovician to early Silurian clastic sediments (sandstones) of the Itmurundy Pacific-type orogenic belt and its hosted accretionary complex (AC). The Itmurundy AC is located in the northern Balkhash area of central Kazakhstan and includes sandstones of the Itmurundy and Kazyk fms. (central segment) and Tyuretai Fm. (eastern segment). The U-Pb ages of detrital zircons from Group 1 sandstones show unimodal distributions with major peaks at ca. 467–456 Ma and the youngest clusters of ages from 442 to 439 Ma Ma suggesting post-Llandovery deposition. The distributions of the U-Pb ages of detrital zircons from Group 2 sandstones are polymodal with major peaks at ca. 462–461 Ma and subordinate peaks at 788, 992, 2472 Ma. The Group 2 zircons yielded two youngest clusters of U-Pb ages: 458 Ma and 424 Ma suggesting post-middle Ordovician and post-Ludlow maximum depositional ages. According to petrographic classifications, both Group 1 and 2 sandstones are poorly-sorted and poorly-rounded greywacke or litharenite. The chemical compositions imply that these sandstones are immature and weakly weathered sediments. The Group 1 greywackes are characterized by relatively low SiO 2 (63 wt%) compared to Group 2 greywackes (SiO 2 = 66 wt% in average). The major and trace element compositions of both groups of sandstones resemble those of supra-subduction andesitic to dacitic volcanic rocks. The positive values of whole-rock ɛNd(t) (+1 to + 5.3) and zircon ɛHf(t) (+9.8 to +17.5) obtained from the Group 1 greywackes suggest juvenile igneous rocks in the source area. On the contrary, the negative values of ɛNd(t) (−7.2) and variable values of zircon ɛHf(t) (−24.5 to +11.5) from the Group 2 sandstones suggest that their parental igneous rocks were derived from mantle sources possessing both juvenile and recycled characteristics. Conclusively, the greywacke sandstones of both groups formed by erosion of an Ordovician intra-oceanic volcanic arc once existed at an active margin of the Paleo-Asian Ocean, however those of Group 1 deposited in a fore-arc basin and trench, while those of Group 2 deposited in a back-arc basin. [ABSTRACT FROM AUTHOR]

Permian granitoids are widely distributed throughout the northern margin of the North China Craton (NCC). However, their origin (petrogenesis) and the tectonic settings in which they formed remain highly debated. The present study focuses on a granitic complex, the Xiari complex located in the western segment of Bainaimiao arc terrane that is composed of granodiorite porphyry, monzogranite, and granite porphyry. Zircon U Pb dating yields an emplacement age range of ca. 263–254 Ma. Geochemically, all the granites show characteristics of adakitic rocks, with high K 2 O contents and enriched Nd isotopic compositions ranging from −14.3 to −6.0. These compositions are distinct from those of typical slab-derived adakitic lavas. Furthermore, the granites exhibit significantly higher SiO 2 and lower Fe 2 O 3 contents, yet show similar MgO, Cr, Co, Ni, and Mg# values compared to the coeval mafic enclaves indicate that these granites are unlikely to have formed by fractional crystallization of basaltic lavas. Instead, we infer that the Xiari complex most likely originated from partial melting of a thickened lower crust within the garnet stability field. Combining this data with information from previously reported A-type granites and volcanic rocks in adjacent regions, the age and petrogenesis of the Xiari complex suggest that the Bainaimiao arc terrane documents a collision-related crustal thickened event. This event was most likely linked to the far-field response of the amalgamation between the Mongolia block and the North China Craton. [Display omitted] • Middle to Late Permian high-K adakitic granites were identified in the Xiari area. • High-K adakitic granites were derived from overthickened lower continental crust. • The closure of the eastern PAO occurred before ca. 263 Ma in the western Bainaimiao arc. [ABSTRACT FROM AUTHOR]

Investigation of intra-oceanic subduction can improve our understanding of plate tectonic processes and the history of continental growth. Evidence for intra-oceanic subduction in the Paleo-Asian Ocean has recently become an important focus of research, including the Diyanmiao ophiolite in central Inner Mongolia, North China. Here, we report a newly discovered occurrence of early Permian gabbro in the Diyanmiao ophiolite zone. The gabbro yields a weighted mean zircon U–Pb age of 294.4 ± 2.2 Ma. The gabbro samples are characterized by moderate SiO2 (47.32–50.51 wt%), low TiO2 (0.26–0.54 wt%) and K2O (0.04–0.75 wt%), and high Na2O (1.84–4.52 wt%) contents, high Na2O/K2O ratios (2.92–58.29), and depleted chondrite-normalized light rare Earth element patterns that are similar to N-MORB. The gabbros show slightly lower contents of high-field-strength elements (e.g., Nb, Ta, and Ti) and slightly higher contents of large-ion lithophile elements (e.g., K, Rb, Ba, and U) relative to N-MORB. In addition, the gabbros show high εNd (t) values (8.0–9.8) that are similar to those of forearc basalt in the Diyanmiao ophiolite and N-MORB. Integrating these new data with available results for ophiolite and arc-magmatic rocks of central Inner Mongolia, we propose that the studied gabbro was formed during the initial stage of intra-oceanic subduction and that the Paleo-Asian Ocean was still in a subduction setting during the early Permian.

The paper provides an overview of the geological structure and presents new data on petrography and geochemistry of igneous rocks of the Tekturmas accretionary complex (TAC). Consideration is being given to the age, petrogenesis and probable tectonic settings of formation of igneous rocks. It is shown that the TAC and adjacent ophiolite belt are parts of the Late Cambrian – Ordovician Pacific-type orogenic belt formed at an active margin of the Paleo-Asian Ocean. The TAC is dominated mainly by tholeiitic mafic igneous rocks (basalt, dolerite, gabbro). There are recognized three groups of rocks: high-Ti, medium-Ti and low-Ti. Geological and geochemical features of TAC igneous rocks suggest their formation in three tectonic settings: mid-ocean ridge (medium-Ti), oceanic island/seamount (high-Ti) and over subduction zones (low-Ti).

The paleogeographic position of the Xilinhot–Songliao Block (XSB) is crucial to understand the spatiotemporal relationship between the Paleo‐Asian Ocean (PAO) and East Paleo‐Tethys Ocean (EPTO) tectonic domains. However, the lack of robust early Permian paleomagnetic data in the XSB has led to uncertainty in its paleogeographic position. Here, we report a well‐dated (286.1–281.25 Ma) paleomagnetic pole (38.4°N/17.6°E, A95 = 4.0°) from andesites of the Dashizhai Formation in the XSB. We combined our paleomagnetic data with available geological and paleontological data to reconstruct the paleogeographic relationship between the PAO and EPTO. The XSB was located on the northern margin of the EPTO at ∼285 Ma, suggesting that the northern boundary of the EPTO tectonic domain extended to the southern margin of the XSB, the Hegenshan Ocean (between the XSB and the Mongolia Block) corresponds to the eastern PAO, and the XSB separated the eastern PAO and EPTO at ∼285 Ma. Plain Language Summary: The evolution of the Paleo‐Asian Ocean (PAO) and the East Paleo‐Tethys Ocean (EPTO) tectonic domains constructed the late Paleozoic paleogeographic pattern of Central and East Asia. Due to contradictions between the tectonic paleogeography and paleobiogeography of the Xilinhot–Songliao Block (XSB) during the early Permian, the spatiotemporal relationship between the two tectonic domains has been unclear. In this study, we present well‐dated paleomagnetic constraints on the paleogeographic position of the XSB at ca. 285 Ma. We provide a new reconstruction for the Northeast Asian block, which suggests that the XSB was located between the eastern PAO and the EPTO during that time and was the intermediate unit that separated the two tectonic domains. Key Points: The Xilinhot–Songliao Block (XSB) was located at ∼19.3°N on the northern margin of the East Paleo‐Tethys Ocean (EPTO) at 285 MaThe northern boundary of the EPTO tectonic domain extended to the southern margin of the XSBThe Hegenshan Ocean between the XSB and Mongolia Block was the main ocean basin of the eastern Paleo‐Asian Ocean at 285 Ma [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]

The Changchun–Yanji suture zone (CYSZ) in NE China is considered as the suture between the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB). The geochronology, geochemistry and Sr‐Nd‐Hf isotopes of Early–Middle Triassic adakitic plutions from the CYSZ, are presented in this paper to discuss their petrogenesis and tectonic setting, as well as to constrain the timing and style of the Paleo‐Asian Ocean's final closure. In Early Triassic, the Dayushan pluton (ca. 250 Ma) from western CYSZ has negative εNd(t) values, bidirectional provenances (NCC and CAOB) of εHf(t), which are formed in a collision tectonic setting. In contrast, in eastern CYSZ, the early Triassic samples in Liangshan (ca. 242 Ma) were high Mg# values, positive εNd(t), single provenances (CAOB) of εHf(t) resulting from a subduction setting. In the Middle Triassic, the A‐type granites in western CYSZ are found in previous studies representing a post‐collisional extensional environment, whereas syn‐collisional Lianyanfeng granites (ca. 237 Ma) in eastern CYSZ with low ISr and large scale εNd(t) and εHf(t) values from bidirectional provenances (NCC and CAOB), represent a collisional setting. The Paleo‐Asian Ocean's occurred in a scissor‐like fashion along the CYSZ during the Triassic period. [ABSTRACT FROM AUTHOR]

[Display omitted] • Early Paleozoic meta-igneous rocks in central Jilin Province subdivided into two types. • MORB-type rocks record the initial subduction of the Paleo-Asian oceanic plate. • Arc-type rocks record the evolution of a supra-subduction zone tectonic setting. To constrain the evolution of an Early Paleozoic island arc belt abutting the North China Craton (NCC), this study presents new geochronological, geochemical, and Sr–Nd–Hf–O–Pb isotopic data for igneous rocks from the central part of Jilin Province within the southeastern part of the Central Asian Orogenic Belt. The igneous complex in this area hosts both mid-ocean ridge basalt (MORB)- and arc-type rocks that yield zircon 206Pb/238U ages of ca. 492 and 488–459 Ma, respectively. The MORB-type units in this area include metacumulate gabbros, metadiabases, and metabasalts that have normal MORB (N-MORB)-like compositions with (La/Yb) N values of 0.29–0.99, Th/Nb values of 0.07–0.52, whole rock ε Nd (t) values from +4.32 to +7.29, and initial 208Pb/204Pb values of 38.12–38.20. These compositions are indicative of derivation from the depleted asthenospheric mantle. The arc-type igneous rocks consist of amphibolite, epidiorite, and metagabbro with major- and trace-element and isotopic compositions that are similar to island arc tholeiite to calc-alkaline rocks, with (La/Yb) N values of 1.11–4.26, Th/Nb values of 0.11–2.25, zircon ε Hf (t) values from +6.03 to +15.30, zircon δ18O values from +4.32 to +5.1, whole-rock ε Nd (t) values from +2.09 to +7.25, and initial 208Pb/204Pb values of 38.09–38.11. These compositions are indicative of derivation from partial melting of the hydrous depleted mantle wedge material within an intra-oceanic arc setting. The MORB-type rocks formed within a forearc region of the SSZ ophiolite fragment, whereas the arc-type rocks record the evolution of this region from immature to mature island arc tectono-magmatic settings. Combining these new data with the results of previous research suggests that initial subduction of the Paleo-Asian oceanic plate abutting the northern NCC began during the latest Cambrian. [ABSTRACT FROM AUTHOR]

The evolution of the eastern Paleo-Asian Ocean (PAO) has controlled the formation of the southeastern Central Asian Orogenic Belt (CAOB). However, the evolution history and final closure time of the eastern PAO still remain controversial, which greatly restricts understanding of the formation process of the CAOB. To address these issues, we provide detailed zircon chronology and Hf isotope and geochemical data of Paleozoic to Triassic magmatic rocks in the southeastern CAOB. We have identified four periods of magmatism as evidenced by: Early Silurian quartz diorites (434.7 Ma), Early Devonian monzogranites (394.2 Ma), Middle Permian granites (260.2–264.5 Ma) and Late Permian-Early Triassic syenogranite (250.8–253.6 Ma). These rocks have features of low MgO and mantle-compatible elements, are enriched in Th, U, K, Pb, Sr, Zr and Hf and depleted in Nb, Ta, La, Ce, P, and Ti. The quartz diorites belong to the medium-K calc-alkaline series with εHf(t) values of −0.76 to 2.21, indicating that they may be derived from partial melting of mafic lower crust with minor contribution of mantle magma. The monzogranites and syenogranite have high Zr + Nb + Ce + Y (260–390; 261–461 ppm, respectively), total alkali contents (9.98–10.80; 8.46–9.29 wt.%, respectively), and high zircon saturation temperature (807–840; 810–885 °C). They can be classified as A-type granites. Monzogranites have εHf(t) values (between −1.20 and +3.34); hence, we believe that they were derived from the crust modified by mantle-derived fluids or melts. Syenogranite have high εHf(t) values (5.49–11.36), and we suggest that they were derived from the juvenile lower crust that originated from the depleted mantle. The granites have high Sr/Y ratios (118–257), low Y (1.42–2.82 ppm), and Yb (0.31–0.41 ppm), consistent with the features of adakite. Considering the εHf(t) isotopic values (2.99–8.50), we suggest that they originated from thickened juvenile lower crust. Combining the results from our own and previous studies, we propose a new evolution model of the eastern PAO from Paleozoic to Triassic. It can be divided into two stages: (1) Late Cambrian to Early Devonian; (2) Early Permian to Triassic. The first stage is the evolution of the Bainaimiao ocean (secondary ocean basin of the PAO), which closed in the Late Silurian and led to the Bainaimiao arc accretion to the North China Cratons. The second stage is the final closure of the eastern PAO during the Late Permian (~254 Ma). [ABSTRACT FROM AUTHOR]

This study presents new data from zircon U–Pb dating and Hf isotope analysis, as well as whole-rock major- and trace-element compositions of the Hongtaiping high-Mg diorite in the Wangqing area of Yanbian, NE China. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb dating gives an eruption age of ca. 267 Ma for the high-Mg diorite. These samples have MgO contents of 13.30% to 16.58% and high transition metal element concentrations, classified as sanukite. Their rare earth element (REE) contents range from 45.2 to 68.4 ppm and are characterized by slightly positive Eu anomalies (Eu/Eu* = 1.08–1.17). They show enrichment in light REEs (LREEs) and depletion in heavy REEs (HREEs), with LREE/HREE ratios = 6.54–6.97 and (La/Yb)N values = 7.24–8.08. The Hongtaiping high-Mg diorite is enriched in Rb, U, K, and Sr, but depleted in Th, Nb, and Ta. High MgO contents, Mg# values, and transition metal element concentrations imply that the magma experienced insignificant crystallization fractionation and crustal contamination. Relatively homogenous positive Hf isotopic values indicate that the original magma was generated by the partial melting of a depleted mantle wedge that was metasomatized by subducting slab fluids. The magma was generated by the moderate degree partial melting (20%–30%) of a garnet lherzolite source. Combined with previous studies, this shows that the high-Mg diorite was formed by the northward subduction of the Paleo-Asian oceanic plate during the Middle Permian. [ABSTRACT FROM AUTHOR]

The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth, as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins, which are longstanding conundrums for understanding the tectonics of the Paleo‐Asian Ocean (PAO). As the processor of the Altaids, the PAO was a long‐lived ocean, and its multiple subductions led to substantial continental growth in the Central Asian continent. Here we report our newly discovered Paleozoic adakitic rocks (granite and dacite) developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues. The Jijitaizi granite and dacite contain high concentrations of Sr (203–343 ppm) and low concentrations of Y (3.00–6.36 ppm) and Yb (0.23–0.62 ppm), indicative of the adakitic affinity. Zircon U‐Pb dating results show that the adakitic rocks were formed in three episodes at 372.26 ± 1.51 Ma, 356.02 ± 1.11 Ma and 334.51 ± 0.68 Ma from the Late Devonian to Carboniferous. The rocks are depleted in Nb, P and Ti, and possess variable Mg# values (28.51–57.22), high Na2O/K2O ratios (1.39–4.94), as well as relatively depleted Hf isotopic compositions, all of which point to a subduction origin from slab melting. Our new finding with the date from the Jijitaizi complex (ca. 321 Ma) suggests continuing subduction of the Paleo‐Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous. Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca. 372 to 321 Ma. [ABSTRACT FROM AUTHOR]

[Display omitted] During Carboniferous time, tremendous juvenile arc crust was formed in the southern Central Asian Orogenic Belt (CAOB), although its origin remains unclear. Herein, we presented zircon U-Pb-Hf and whole-rock geochemical and Sr-Nd isotopic data for a suite of volcanic and pyroclastic rocks from the Khan-Bogd area in southern Mongolia. These Carboniferous pyroclastic rocks generally have some early Paleozoic zircons, probably derived from the granitic and sedimentary rocks of the Lake Zone and the Gobi-Altai Zone to the north, indicative of a continental arc nature. In addition, they have a main zircon U-Pb age of ca. 370–330 Ma, positive Hf and Nd isotopes, and mafic-intermediate arc affinity, similar to the coeval arc magmatism. Moreover, the pyroclastic rocks of the northern area have more mafic and older volcanic components with depositional time (ca. 350–370 Ma; Visean and Bashkirian stages) earlier than that in the southern area (mainly ca. 350–315 Ma; Serpukhovian and Bashkirian stages). Combining a preexisting northward subduction supported by the available magnetotelluric data with a slab rollback model of the main oceanic basin of the Paleo-Asian Ocean (PAO) during Carboniferous and Triassic times, we infer that the Carboniferous arc magmatism was probably derived from a backarc ocean triggered by slab rollback. Thus, the juvenile arc volcanism of Mongolia, together with other areas (e.g., Junggar) in the southern CAOB, represented a significant lateral accretion that terminated after the Carboniferous due to a significant contraction of the PAO. [ABSTRACT FROM AUTHOR]

The closing time of the Paleo-Asian Ocean and the tectonic evolution of the northern margin of the North China Craton are still controversial. The geochronology and geochemistry of the Zhaojinggou monzogranite pluton provide new constraints on the late Paleozoic tectonic evolution and the closure time of the Paleo-Asian Ocean in the southern Central Asian Orogenic Belt (CAOB). The monzogranite yielded a zircon U-Pb age of 286.7 ± 1.2 Ma. Due to the characteristics of low–moderate Mg# values (25.87–39.21), low Fe2O3T values (1.13–1.72), and A/CNK > 1, we show that the pluton is weak peraluminous, high in potassium calc–alkaline series, and displays the feature of S-type granite. The total REE content is low, the distribution curve is right dipping, and the LREE is enriched; the δEu average value is 1.32 (1.11–1.54). The granite presents relatively high (87Sr/86Sr)i values of 0.712345–0.713723, low εNd(t) values of −8.89–−8.21 (an average value of −8.56), and a TDM2 of 1718–1773 Ma. Furthermore, the zircon in situ Hf isotopic analysis shows 176Hf/177Hf ratios of 0.282342 to 0.282614, low εHf(t) values of −9.27–0.38 (mean −4.74), and a TDM2 of 1275–1887 Ma. Additionally, high field strength elements such as Nb, Ta, and Ti are depleted, and large ion lithophile elements, e.g., Rb, Ba, K, and Sr, are enriched. The above features of the Zhaojinggou monzogranite indicate that the pluton was derived from late Paleoproterozoic to Mesoproterozoic lower crustal mafic materials. By discussing the genesis and tectonic implications of the pluton massif, we propose that the Zhaojinggou monzogranite represents a magmatic event caused by the crustal–mantle interaction during the southward subduction of the eastern Paleo-Asian Ocean in the northern margin of the North China Craton during the Early Permian. [ABSTRACT FROM AUTHOR]

The final closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of mafic igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40Ar/39Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40Ar/39Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2σ, MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 ℃ in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon εHf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived fluids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

[Display omitted] • Detailed maps and sections of ophiolites and their oceanic cover (OPS). • New tectonic model for the Lake and Mongolian Altay zones and their relationships to the CMM. • Ophiolites and OPS formed between two converging microcontinents. Following the breakup of Rodinia and the opening of the Paleo-Asian ocean the relative positions of geological units (now in Mongolia) remain uncertain, and in consequence relevant paleogeographic reconstructions are poorly construed. In this paper, we apply an integrated approach to the reconstruction of the Paleo-Asian ocean using preserved ophiolites and ocean plate stratigraphy (OPS) in the Mongolian Lake and Altay zones in western Mongolia, which surround the Precambrian Central Mongolian Microcontinent (CMM) in the center of Central Asia. The Lake Zone of Mongolia contains many sediments of Ocean Plate Stratigraphy (OPS) and their underlying basalt-dominated ophiolites, which occur in three main zones along the western margin of the Precambrian crystalline basement of the CMM. The ophiolites in all zones have similar isotopic ages of ca. 570 Ma. The sections of OPS, together with serpentinite mélanges and other volcanogenic-sedimentary assemblages, have been thrust in nappe sheets over the Zavkhan block of the CMM. In the Neoproterozoic microcontinents derived from Rodinia drifted towards Siberia, and a passive margin developed on the Zavkhan block on the eastern margin of the Paleo-Asian ocean. In mid-upper Neoproterozoic times the eastern side of the Paleo-Asian oceanic plate was thrust over the CMM, and in the early Cambrian the Khasagt and Tas Khairkhan ophiolites were obducted and a mid-ocean ridge (?) reached the CMM, preventing further obduction. In the Neoproterozoic the western margin of the Paleo-Asian ocean was subducted under the Rodinia-derived microcontinent and evolved into an active continental margin. In mid-Cambrian–early Ordovician times thick turbidites were deposited in the Altay. Marine sediments, which had accumulated in the Devonian to Carboniferous in the Paleo-Asian ocean are preserved in a remnant ocean in the Lake Zone. Locally in the Devonian, terrigenous – volcanogenic sequences were deposited in the Ulgey extensional superimposed basin, and later in Meso-Cenozoic time (?) some ophiolitic and OPS fragments (Tsagaan Nuur, Ulaankhus and Bodonch) were exposed along fault escarpments that were possibly re-activated during transpression related to the India-Eurasia collision. Our analysis of the Neoproterozoic–Middle Paleozoic tectonic development of western Mongolia leads to a new holistic model for the evolution of the western Altaids, according to which the ophiolites and OPS formed in the Mongolian Lake and Altay zones between two microcontinents within a new tectonic framework of the western Altaids. [ABSTRACT FROM AUTHOR]

The closure of the Paleo-Asian Ocean (PAO) is crucial for understanding the late Paleozoic–early Mesozoic paleogeography of Proto-Asia and the tectonic configuration of NE Pangea. However, the timing and mechanism for the PAO closure and final amalgamation of the Central Asian Orogenic Belt (CAOB) remain highly controversial. Available studies argue either for a pre-Carboniferous closure or eastward, scissor-style diachronous closure from late Carboniferous to late Permian. This ambiguity is mainly caused by the different criteria applied to evaluate the final closure of oceanic basins and termination of accretionary orogenesis. Here we summarize new evidence including the youngest mélange-bearing accretionary complex and provenance change of syn-orogenic sediments in order to trace the terminal closure of the PAO from Tianshan in the west, through Beishan and Alxa, to Solonker and Changchun-Yanji in the east, a total distance of ∼5000 km. The Tianshan marks the demise of the western segment of the PAO where the Tarim Craton collided with the Kazakhstan–Yili–Central Tianshan arc. Several ophiolitic mélanges from the Kyrgyz and Chinese Tianshan show the relicts of Permian–Triassic oceanic lithosphere that amalgamated during the Middle-Late Triassic. In Beishan, three ophiolitic mélanges contain Permian igneous rocks and Permian–Triassic sedimentary matrix, suggesting closure of the ocean between ∼245–222 Ma. In the Engger Us mélange of Alxa, a turbidite matrix overlying Permian ocean plate stratigraphy yields a ∼244 Ma maximum depositional age, constraining the time of emplacement of the accretionary complex. To the east, several ophiolitic mélanges in Inner Mongolia contain Permian–Early Triassic gabbro/basalt and middle-late Permian radiolarians, suggesting subduction-accretion continued into the Early Triassic. Along the Changchun–Yanji suture in NE China, several accretionary complexes yield emplacement ages of ∼246–234 Ma. Therefore, the youngest ophiolitic mélanges and accretionary complexes across the whole southern CAOB were formed during the Middle-Late Triassic. In addition, Middle-Late Triassic provenance changes have been identified in syn-orogenic sediments in Tianshan, Beishan, Alxa, Inner Mongolia and NE China. These data are in agreement with Triassic deformation and high-pressure metamorphism, which together indicate that the western, central, and eastern segments of the PAO were closed almost contemporaneously during the Middle-Late Triassic. To the south, multidisciplinary data suggest that the amalgamation between South China, Sibumasu, Indochina, Qiangtang, and North China took place between ∼230–225 Ma, which led to the closure of the Paleo-Tethys Ocean. Taken together, the Indosinian orogeny leading to the amalgamation of the East Asian blocks marks the final assembly of Pangea. The joining of East Asia in the Late Triassic maximized the land area of Pangea, which potentially fostered the dramatic climate change and formation of the megamonsoon. • Permian–Triassic ophiolitic mélanges and accretionary complexes in the southern Central Asian Orogenic Belt are reviewed. • Contemporaneous closure of the western, central and eastern segments of the Paleo-Asian Ocean in the Middle-Late Triassic. • Middle-Late Triassic orogenesis in East Asia marks the final assembly of Pangea. [ABSTRACT FROM AUTHOR]

We conducted broadband Magnetotelluric survey to obtain 2D lithospheric electrical structures of Alxa block through two NE-SE trending profiles, which revealed the electrical evidence of bidirectional subduction of Paleo-Asian Ocean with spatial difference beneath Enger Us fault belt. [Display omitted] • Lithospheric electrical structures of the Alxa block were imaged through two NW-SE BMT profiles. • There was bidirectional subduction of the Paleo Asian Ocean beneath the Enger Us fault belt. • The low resistivity bodies of the middle and upper crust in the northwest are related to large-scale horizontal nappe tectonics. • The high conductivity bodies that extend deep in the southeast are attributed to the partial melting. The Alxa Block, situated between the Tarim and North China cratons, is essential to understand the tectonics of northwest China. It has undergone several tectonic cycles, including rifting from northern Gondwana in Neoproterozoic, subduction-accretion of the Paleo-Asian Ocean during Paleozoic, and intracontinental deformation in Mesozoic. Broadband magnetotelluric data covering a frequency range of 320 Hz to ∼ 10000 s was collected along two NW-SE trending lines across major tectonic units of the Alxa region to investigate the deep structures of the Alxa Block. By applying the nonlinear conjugate gradient (NLCG) inversion approach, two-dimensional (2D) electrical resistivity models were obtained at a 100 km depth. These findings indicate that the Zhusileng-Hangwula and Zongnaishan-Shalazhashan tectonic zones have high resistivity in their lower crust and upper mantle, with an overall layered high-low–high resistivity pattern. The upper crust of the Nuru-Langshan tectonic zone is primarily characterized by high resistivity values, whereas the lower crust and upper mantle contain a large-scale low-resistivity (or conductive) zone that is likely the result of partial melting. The large-scale Early–Middle Jurassic nappe structures may be connected to sub-horizontal low-resistivity anomalies in the shallow strata, which are located up to 20 km below the Yagan and Zhusileng–Hangwula tectonic zones. Below the Engger Us fault, two low-resistivity bands are interpreted as the product of bidirectional subduction and final closure of the Paleo-Asian Ocean. The Alxa Block may have subducted southwards beneath the Ordos Block, as suggested by a clear low-resistivity anomaly between the two blocks. Furthermore, partial melting may have happened for high-conductivity bodies in the upper mantle based on the modified Archie formula's evaluation of their melting degree. [ABSTRACT FROM AUTHOR]

[Display omitted] • Collision of the Bainaimiao island arc and North China Craton took place in the late Silurian. • The Inner Mongolia Paleo-uplift underwent rapid uplift during the Carboniferous. • Initial subduction of the Paleo-Asian Ocean was triggered by an arc–continent collision. The driving mechanisms for initial oceanic subduction beneath a passive continental margin remain unclear. The Paleozoic convergence history between the North China Craton (NCC) and the Paleo-Asian Ocean provides an ideal opportunity to investigate the driving mechanisms of the onset of oceanic subduction. Our detrital zircon U–Pb dating results yield Middle Devonian to Permian ages (391–273 Ma) in Paleozoic strata within China's Aoji region, specifically in the Bainaimiao arc belt situated between the NCC and Central Asian Orogenic Belt (CAOB). Detrital zircon age spectra, ε Hf (t) values, and two-stage model ages of the Devonian to Permian sediments suggest zircon contributions from both the NCC and CAOB. Combined with data from various regions along the Bainaimiao arc belt, findings suggest that the Bainaimiao island arc was accreted to the northern margin of the NCC through arc–continent collision during the late Silurian, rendering the NCC margin passive until the end-Devonian. At this point, southward subduction in the Paleo-Asian Ocean beneath the NCC commenced and continued into the Permian, transforming the northern margin of the NCC from passive to active. This initial subduction (ca. 359 Ma) in the Paleo-Asian Ocean resulted from collision-induced magmatism and deformation due to the protracted convergence and also led to a reversal in subduction polarity. Subsequently, in response to Paleo-Asian Ocean subduction, the northern margin of the NCC underwent rapid uplift during the Carboniferous, which led to widespread exposure of the NCC basement rocks in the late Carboniferous. [ABSTRACT FROM AUTHOR]

The precise timing of subduction initiation of the Paleo-Asian Ocean (PAO) in the northern margin of the North China Craton (NCC) remains controversial. In this study, we have conducted zircon U Pb dating, whole-rock major and trace elements, and zircon Hf isotope analyses on metadacites and metamorphic greywackes from the Changde volcano-sedimentary rocks, feldspar quartz sandstones and argillaceous siltstones from the Luquantun Formation and the Sidao sedimentary rocks in central-eastern Jilin Province (NE China), to constrain the initial closure of the PAO. We obtained Early Carboniferous zircon U Pb ages (345 ± 2 Ma and 351 ± 7 Ma) from the Changde volcano-sedimentary rocks. The metadacites are characterized by the enrichment in light rare earth elements and large ion lithophile elements, depletion in heavy rare earth elements and high field strength elements, and rare earth element ratios similar to those of the continental crust, indicating that these rocks originated from crustal melting. The metadacites exhibit affinity with the peraluminous A-type granite, with high zircon ε Hf (t) values (+3.46 − +12.21) and T DM2 of 1132–571 Ma, suggesting that they formed in a post-collisional extensional setting. Detrital zircon U Pb ages from the Luquantun Formation and Sidao sedimentary rocks constrain their maximum depositional ages to the Late Carboniferous (319 ± 5 Ma) and Early Permian (284 ± 4 Ma), respectively. Most detrital zircons have age peaks at 1800 Ma with negative zircon ε Hf (t) values, and 2500 Ma with positive zircon ε Hf (t) values, suggesting that the Carboniferous–Early Permian sediments from these two formations were mainly sourced from the NCC. Moreover, the chemical index of alteration, index of compositional variability, and plagioclase index of alteration values suggest that the sediments of the Changde volcano-sedimentary rocks originated from a mafic igneous source with a low degree of weathering and a sedimentary recycling history, while the samples from the Luquantun Formation and Sidao sedimentary rocks were sourced from an intermediate-felsic igneous rock with a moderate to intense degree of weathering and mainly influenced by source composition. The crystallization age curves and summary plots of detrital zircons indicate that the Early Carboniferous metamorphic greywackes from the Changde Formation were mainly deposited in a passive margin, while the Carboniferous–Early Permian sediments of the Luquantun Formation and Sidao sedimentary rocks were mainly deposited in an active margin. We suggest that the tectonic transition from passive to active margin occurred at ∼333 Ma, marking subduction initiation in the PAO close to the northern margin of the NCC. [Display omitted] • The Carboniferous–Early Permian sediments in central-eastern Jilin Province (NE China) are mainly sourced from the NCC. • During the Carboniferous, the eastern part of the northern margin of the NCC transitioned from passive to active. • The precise subduction initiation of the PAO occurred at approximately 333 ± 5 Ma along the northern margin of the NCC. [ABSTRACT FROM AUTHOR]