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The tectonic history and time of closure of the Palaeo-Asian ocean of the Altaids are issues of lively current debate. To address these issues, this paper presents detailed geological, petrological and geochemical data of the Liuyuan complex (LC) in the Beishan region in NW China, located in the southernmost Altaids, in order to constrain its age, origin and tectonic setting. The LC mainly comprises massive basalts, pillow basalts, basaltic breccias, gabbros and ultramafic rocks together with cherts and tuffs. Most prominent are gabbros and large volumes of basaltic lavas. These mafic rocks have high TiO2 contents, flat rare earth element (REE) patterns and show high-field-strength elements (HFSEs) similar to those of mid-ocean ridge basalts (MORB). The mafic rocks exhibit positive εNd(t) (6.6–9.0) values, representing magmas derived from the mantle. But these basic rocks are also enriched in Th relative to REEs, and are systematically depleted in Nb–Ta–(Ti) relative to REEs. There is also a large range in initial 87Sr/86Sr (0.7037–0.7093). All these variables indicate that mantle-derived magma was contaminated by fluids and/or melts from a subducting lithospheric slab, and formed in a supra-subduction zone (SSZ) setting. A gabbro intruded in the complex was dated by LA-ICP-MS on 20 zircons that yielded a 206Pb–238U weighted average age of 286 ± 2 Ma. Considering the fact that all these basalts are imbricated against Permian tuffaceous sediments and limestone, we propose that the LC formed as an ophiolite in a fore-arc in Carboniferous–Permian time. This indicates that the Palaeo-Asian ocean still existed at 286 ± 2 Ma in early Permian time, and thus the time of closure of the Palaeo-Asian ocean was in or after the late Permian.

The interaction of the Palaeo-Pacific and Palaeo-Asian Oceans is an enigmatic issue as their temporal and spatial features are controversial. To address this issue, we present a systematic study of large volumes of early Permian volcanic rocks and intrusions developed in the East Tianshan. The represented samples of basaltic andesites and rhyolites yield zircon crystallization ages of 285.1 ± 5.9 Ma and 275.3 ± 1.8 Ma, respectively. The basalts have normal mid-ocean ridge basalt (N-MORB) and arc-related geochemical signatures with high TiO2 contents, negative Rb, Th, U, Nb and Ta anomalies and positive Eu anomalies. Basaltic andesites and andesites have arc-related geochemical characteristics with moderate TiO2 contents and relatively negative Nb, Ta and Ti anomalies, together with slightly negative to positive Eu anomalies. The rhyolites show an affinity with A2-type granite with high SiO2, K2O + Na2O, Fe/Mg, Ga, Zr, Nb, Y, HFSE, REE and Y/Nb levels (>1.2). These geochemical data suggest that the rocks formed in a supra-subduction zone. The presence of high ϵNd(t) values of +4.6 to +8.2 and low (87Sr/86Sr) i (0.70342–0.70591) values indicates that the volcanic rocks originated from a depleted mantle. We propose that oblique subduction with slabs breaking off gave rise to transtension and to the emplacement of large volumes of mantle-derived melts in the early Permian in the East Tianshan, serving as an important record of the subduction zone of the Palaeo-Pacific Ocean.

The timing of the final welding of the southern Altaids to construct Pangea is a matter of debate which has led to considerable uncertainty regarding the configuration of northeast Pangea and the reconstruction of proto-Asia. Here we investigate the provenance of sediments deposited within a fore-arc accretionary basin located close to the Akeyazi low temperature ultra-high pressure metamorphic complex in the South Tianshan Orogenic Belt. Detrital zircon U-Pb analysis of sandstones yields a maximum depositional age in the early Triassic (around 231.7 million years ago) with a major early Permian peak and few Precambrian records. Bulk-rock composition and zircon Hf isotopic analysis, combined with a compilation of existing data, indicate the early Triassic sediment received additional detritus from the distal juvenile East Tianshan Arc region, rather than from any proximate tectonic units or an intra-oceanic arc. Available data suggests that the South Tianshan Ocean, an accessory ocean basin to the Paleo-Asian Ocean, could close in the early Triassic.

The tectonics of the Proto-Tethys Ocean during the Early Devonian are still heavily debated in the North Qilian orogen. In order to further constrain this issue, we explore geology, chronology, geochemistry, and isotopes of three newly discovered Early Devonian adakitic granitoids of the Jiayuguan complex in the North Qilian orogen, NW China. The granitoids exhibit typical adakitic geochemical signatures with high SiO2 (>56%), Na, Al, and Sr contents, depleted in Yb and Y, and high Sr/Y (82–277) and (La/Yb)N (7.65–15.16) values. Additionally, their high Mg# (62–68) and εHf(t) (+6.5–+11.9) values indicate partial melting of slab genesis. However, they have comparatively low εNd(t) (−2.3–+0.13). Their incompatible Hf and Nd isotopes could be caused by contamination between the source magma and continental crust during the emplacement processes. The three adakitic granitoids yield zircon U–Pb ages of 415–403 Ma, implying that the northern Qilian Ocean was subducting until the Early Devonian (403 Ma) and that young/hot/ridge subduction formed the Jiayuguan adakitic granitoids. Combined with regional data, we propose that the Proto-Tethys Ocean was subducting until 403 Ma in the Qilian area.

The Paleo–Tethyan Ocean in Southeast (SE) Asia is generally considered to be primarily represented by the Jinshajiang–Ailaoshan suture between the South China and Indochina Blocks. However, the distribution of the Jinshajiang–Ailaoshan suture zone, particularly its southeastern continuation, remains poorly constrained. This study presents a comprehensive investigation into the detailed field study, new zircon U–Pb dating, and whole-rock geochemistry of the Babu ophiolite near the China–Vietnam border. Field observations and geochemical characteristics define two distinct types of basaltic rocks within the Babu ophiolite. Type 1 basaltic rocks, primarily metamorphosed basalts and diabase dykes that intrude into gabbro and serpentinite, represent the lower and central oceanic crust. These rocks were subjected to metasomatism by subduction-related fluids, leading to Zr–Hf depletion. Type 2 basaltic rocks are copper mineralized and likely represent the upper and marginal oceanic crust, which were influenced by crustal components, resulting in Zr–Hf enrichment. Both basalt types exhibit N-MORB geochemical affinities and lack characteristics associated with the Emeishan large igneous province (LIP), indicating that the Babu ophiolite is of a MORB-type origin, with its volcanic rocks derived predominantly from N-MORB tholeiitic magmas in a back-arc setting. Zircon U–Pb dating of two gabbros yields concordant ages of 272 ± 3 Ma and 264 ± 1 Ma. Integrating these results with previous data suggests that the Babu ophiolite formed during the Early Permian and was emplaced during the Late Triassic (ca. 230 Ma). The Babu ophiolite may correlate with the Cao Bang ophiolite in the Song Hien belt of Vietnam, which marks the western segment of the Dian–Qiong suture, a remnant of the Paleo–Tethys Ocean. These findings contribute to a better understanding of the spatial and temporal evolution, as well as the final closure mechanism, of the Paleo–Tethys Ocean in SE Asia.

The interplay between continental motions during the assembly of Pangaea and late Palaeozoic climate change, including severe glaciation and global aridification, remains enigmatic. Here we identify the provenance of Permian–Early Triassic sediments that recorded climate change of North China and estimate palaeoelevation to constrain tectonic-climate interaction during the assembly of northeast Pangaea. Detrital zircon U-Pb-Hf analysis indicates the sediments were locally sourced from the ancient basement and associated with a late Palaeozoic (410–260 million years ago) continental arc, devoid of input from juvenile arcs of the Altaids. These sediments were interpreted as deposited in a retroarc foreland basin ascribed to subduction of the Palaeo-Asian Ocean. Crustal thickness estimated from whole-rock La/Yb yields an average value of 58 ± 11 km, which corresponds to a palaeoelevation of 3.8 ± 0.7 km. The results reveal the existence in North China of an orogenic plateau comparable to the Altiplano of the Andes that blocked moisture transport from the ocean and served as an important orographic barrier to trigger Permian aridification.

The process of continental growth and closure of the Paleo‐Asian Ocean are controversial. To address this problem, we report new geochemical and geochronological data of granitic rocks along a N‐S corridor of the Dananhu arc. These rocks yield 401–270 Ma U‐Pb zircon ages. The arc‐related granites are enriched in the light rare earth elements (LREE), Rb, Ba, Th, Sr and Pb, but depleted in Nb and Ta, and have high zircons values of εHf(t) (+7.7−+19.6) and εNd(t) (+5.4−+7.2); they were possibly generated by partial melting of juvenile intermediate or mafic arc crust. A 356 Ma high‐K calc‐alkaline granite has high Ga/Al (3.28–3.37), low Mg# (14–16), and high zircon εHf(t) (+8.9−+12.0); it can be classified as A2‐type‐granite, suggesting it was a product of substantial fractional crystallization of a mantle‐derived arc‐related magma in an intra‐arc extensional setting. The youngest granite (254 Ma) is an adakite‐like granite which is characterized by high Sr/Y (26–38), depletion in heavy rare earth elements (HREE), and high εNd(t) (+7.0) and εHf (t) (+10.4 to +15.4) values, most likely it was derived from a subducted oceanic slab. Therefore, all the granitic rocks formed in subduction‐related settings. The granitic rocks are generally young southwards from the Kezier area (ca. 401–335 Ma) to the Tuwu area (334–315 Ma), and to the Kanguer area (ca. 280–254 Ma), indicating that the arc grew southwards, and the Paleo‐Asian Ocean closed possibly after 254 Ma. Therefore, our data provide a robust constraint on the prolonged growth of the Altaids. Plain Language Summary The continental growth mechanism and closure time of the Paleo‐Asian Ocean in the Altaids are global research hotspots. The arcs (especially the intra‐oceanic type) in the Paleo‐Asian Ocean are the essential accretionary component in the Altaids, however, detailed knowledge of the growth processes of the arcs that generated the different types of magmatism is still insufficient. The voluminous, continuous, and well‐exposed granitic rocks in the Dananhu arc provide an ideal opportunity to study the complicated processes of subduction and evolution of the southern Altaids. We report geochemistry and geochronology of granitic rocks along a N‐S corridor of the Dananhu intra‐oceanic arc in the Eastern Tianshan (NW China), southern Altaids. These I‐type, A2‐type and adakite‐like subduction related granitic rocks were emplaced from Devonian (ca. 401 Ma) to Latest Permian (ca. 254 Ma) with a general younging southward, indicating a complex episodic switch of subduction processes from the Devonian to Late Permian. Our results suggest that the arc grew southwards by retreat and rollback of the Kanguer oceanic plates where overall compressional and extensional events alternated, and the closure time of the Paleo‐Asian Ocean was possibly after 254 Ma. Key Points The subduction related granites are a general southward younging southward from Devonian (ca. 401 Ma) to Latest Permian (ca. 254 Ma) The arc grew southwards by retreat and rollback of the Kanguer oceanic plates where overall compressional and extensional events alternated The continental growth accompanied with volumes of granitic rocks and Paleo‐Asian Ocean possibly closed after 254 Ma in the southern Altaids

In the Kokchetav area, a late Neoproterozoic to early Palaeozoic metamorphic, ophiolitic, and volcanic–sedimentary complex provides important insights into crustal accretion in the northwestern Altaids. We carried out a systematic study of the Northern Complex in the mélange within the Chaglinka, Zhanatalap, and Chelkar regions. The mélange includes fragments of mafic igneous rocks, pelagic cherts, volcaniclastic rocks, and turbidites. The mafic rocks have high Mg#, Cr, and Ni values, small negative Nb–Ta anomalies, juvenile ε Nd (t)–ε Hf (t) values, and low 87 Sr/ 86 Sr (i) ratios, suggesting they were derived from a depleted mantle source in a supra-subduction zone setting. The volcaniclastic rocks exhibit moderate light rare earth element enrichment, small negative Nb–Ta anomalies, juvenile ε Nd (t)–ε Hf (t) values, and low 87 Sr/ 86 Sr (i) ratios, indicating they were derived from arc-related igneous rocks. The granites have features consistent with high-Sr/Y or A-type granites and variable ε Nd (t) and ε Hf (t) values, implying they formed by melting of arc-related volcanogenic sediments in the mélange. Zircon U–Pb ages for the volcaniclastic rocks vary from 540 to 423 Ma (Early Cambrian to late Silurian). These ages are consistent with those of volcanic rocks exposed in the Stepnyak–Selety arc. Subduction of the oceanic lithosphere beneath the Stepnyak–Selety arc may have occurred during the Early Cambrian and was followed by roll-back of the subducting slab during the Early Ordovician and continued subduction in the early Silurian. The successive subduction and accretion resulted in gradual maturation of the Stepnyak–Selety arc, which produced the compositionally and isotopically variable igneous rocks in the Kokchetav mélange zone and generated the juvenile crust of present-day northern Kazakhstan. Graphical abstract

The Permian–Triassic tectonic setting is still controversial in the southern Altaids. The Beishan orogen is an ideal region to address the final tectonic of the Altaids. These systematic mapping, geochemistry, and geochronology studies on the Houhongquan ophiolitic mélange in the south Beishan are conducted to address this issue. New mapping reveals that the Houhongquan ophiolitic mélange consists of blocks of gabbro, basalt, chert, granite, and strongly deformed and cleaved sandstone in the southern Beishan. The studies reveal that the mafic fragments are relics of normal-mid-ocean ridge (N-MOR) and suprasubduction zone (SSZ) types of oceanic lithosphere. The four sandstone matrix samples yield the maximum depositional ages of 222±5 Ma, 233.8±2.3 Ma, 263.4±2.5 Ma, and 263.5±2.8 Ma, respectively, indicating that the youngest sandstones were tectonic emplaced in the Houhongquan ophiolitic mélange after ca. 222 Ma. The sandstone matrices display two types of age spectra. Early Permian sandstones have a single Devonian to Early Permian peak age patterns, indicating the existence of an independent Permian intraoceanic arc. In contrast, Late Triassic sandstones have multiple peaks with some Precambrian zircons, suggesting that they were sourced from a continental arc. Accordingly, we consider that the Houhongquan ophiolitic mélange tectonic was emplaced in the intraoceanic island arc during the Middle Permian and docked to a continental margin arc during the Late Triassic. Thus, we argue that the terminal amalgamation timing of the southern Altaids was probably during ca. 222-217 Ma.

Much debate exists concerning mechanisms of crustal material transfer from subducting slab to overlying mantle. Formation of mélange rocks by physical mixing of slab components within subduction plate interface is predicted to transfer their compositional signal to source of arc magmas by ascending as diapirs from slab-top. Despite being supported conceptually and through modeling, existence of these diapirs in global subduction architecture remains inconclusive. Here we use petrological observations, thermometry and thermodynamic modeling, combined with geochemical constraints and compilation of massive existing data, to investigate eclogites from a deeply buried mélange “package” in Kyrgyz Tianshan, southern Altaids. We find that various slab components physically mixed to form eclogitic mélange rocks at threshold depth of the subarc (i.e., ≥85 km). Index mineralogical and Pressure-Temperature records indicate a thermal history with substantial heating after peak burial to condition crossing wet solidus. Such translation, toward hot corner of mantle wedge, is short-lived around several hundred thousand to few million years, serving as first tangible evidence substantiating mélange diapirs propagate and dynamically mix with overlying mantle. Contemporaneous Late Carboniferous flare-up of regional arc magmatism with mélange diapir melting signal also advocates that non-negligible process of mantle wedge hybridization by buoyant mélange materials, to transfer volatile, generate arc lavas and regulate terrestrial geochemical cycles, stands.