Introduction

Paleozoic evolution of the Paleo-Asian Ocean (PAO) involved the development of the Siberian accretionary margin and the addition of a range of blocks from the Gondwana northern margin, which were finally amalgamated in the Permo-Triassic to form the Central Asian Orogenic Belt (CAOB)1,2 (Fig. 1a) and thus shaped the main body of Pangea3,4,5. At this time, numerous peri-Siberian juvenile terranes (e.g., Junggar and Altai), in accompany with the Gondwana-derived blocks (e.g., Tarim), travelled north to progressively collide with the Siberian margin, leading to multiple-stage orogenic events6,7,8. The paleopositions of these juvenile terranes is poorly constrained due to lack available paleomagnetic data. The Altai terrane was likely located near the peri-Siberian accretionary margin during the Paleozoic6,9, but the Junggar terrane has been placed at the Tarim northern margin10 or within the PAO realm throughout the Paleozoic1, even on the Siberian margin in the mid-Paleozoic11. Therefore, understanding its Caledonian-age interaction with the Gondwana-derived block and the Siberian margin is vital for the reconstruction of central Asia.

Fig. 1
Fig. 1
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(a) Overview map showing relationship of the study area with the Central Asian Orogenic Belt (modified after Windley et al.1, Zhao et al.5). (b) Schematic geological map of Junggar-Altai terrane and adjacent regions (modified after Chen and Jahn63, Badarch et al.21, Li et al.23), showing the main tectonic units and the age and distribution of the Early-Late Paleozoic magmatic rocks. (c) Geological map in the Karamaili area with sample locations of the compiled data. HS—Hovd suture zone; ZIS—Irtysh suture zone; ZAS—Zhaheba–Aermantai suture zone; KS—Karamaili suture zone; NTS—North Tianshan suture zone; YB—Yili block; CTB—Central Tianshan block. This figure is generated using CorelDRAW X8 created by the CorelDRAW Team under an open license (http://www.coreldraw.com/cn/product/graphic-design-software/).

The Junggar terrane is widely considered as a conjoined terrane mainly consisting of two independent units until the late Paleozoic2 (Fig. 1b), whereas some scholars treated it as an integral block in the early Paleozoic11. Recent data indicate that the Paleozoic strata of the Junggar terrane contain abundant Precambrian detrital materials like the Altai terrane12,13. However, the provenance of these ancient detritus and associated tectonism remains enigmatic. The combined U–Pb and Lu–Hf analysis of detrital zircons is probably an effective approach to solve the tectonic-sedimentary coupling problem because it has been widely applied to trace sedimentary provenance and determine tectonic transition processes14.

In this study, we reveal derivation of the Precambrian detrital materials in the Junggar terrane from the Siberia craton and adjacent Tuva-Mongolian microcontinent, as well as middle Paleozoic switching accretionary tectonics of the Junggar terrane, based on new and compiled zircon U–Pb–Hf data. We link the source-to-sink system and the accretionary tectonics with the relations among Junggar terrane, Altai terrane and Siberian continent, disclosing a short-lived connection between the unified Junggar-Altai terrane and the Siberian margin during mid-Paleozoic time. Our results offer new constraints on the paleogeographic reconstruction of the southern CAOB, promoting the understanding of the Pangea assembly.

Geological background

The CAOB is situated between the Siberia and Baltica cratons to the north and the Tarim and North China cratons to the south (Fig. 1a), and it underwent prolonged subduction-accretion processes during the evolution of the PAO from the Neoproterozoic to the late Paleozoic1. Tectonically, this belt can be divided into the Mongolia and the Kazakhstan collage systems2. The Mongolia collage system is presently south of the Siberia craton and comprises juvenile terranes (e.g., Altai, Hovd and Lake, Fig. 1b) and some Precambrian continental blocks (e.g., Tuva-Mongolian, Zavkhan and Baydrag). Several episodes of magmatic activities from the Archean to the Paleozoic are recorded in the western Mongolia and/or the Siberia craton15. It is generally suggested that the amalgamation events from the Altai-Mongolian terrane to the south margin of Siberia craton have occurred in the Early–Middle Paleozoic, leading to the formation of the Carysh-Terekta-Ulagan-Sanyan suture zone in the Altai-Sayan region and Ol’Khon suture zone in the Baikal region16,17,18.

The Junggar terrane is bounded to the south by the North Tianshan suture zone, marking the site of closure of a PAO branch (i.e., North Tianshan Ocean) and collision with the Tarim craton and neighboring Central Tianshan block (CTB) (Fig. 1b). Both the Tarim craton and the CTB preserve a crystalline Precambrian basement and the Neoproterozoic and Paleozoic subduction-related tectono-magmatic records19,20. By the Irtysh suture zone to its north, the Junggar terrane is separated from the Altai terrane at the western margin of the Tuva-Mongolian terrane21, and this study mainly focused on the eastern Junggar terrane because the western Junggar experienced different evolution in the Paleozoic2 (Fig. 1b). The Junggar terrane, as an accretionary orogen related to the evolution of the PAO1, was formed by the amalgamation of multiple linear tectonic units including the Junggar block, the Yemaquan block, the Dulate orogenic belt and their intervening Karamaili and Zhaheba–Aermantai suture zones. There are abundant Paleozoic ophiolitic rocks along four suture zones (NTS, KS, ZAS and ZIS) in the Junggar-Altai region (Fig. 1b). The ages of the North Tianshan ophiolites have an almost continuous span of 494–325 Ma relative to those in other zones that are concentrated in the two periods of 503–481 Ma and 409–364 Ma22. Numerous and widespread Paleozoic magmatism took place in the Junggar and Altai terranes (Fig. 1b). The intermediate-felsic magmatic rocks commonly have arc affinities, suggesting a long-lived accretionary history that may extend at least to the Late Carboniferous2,23.

Unlike the Tarim craton and its neighbor, the Junggar and Altai terranes were underlain mainly by juvenile crust and lack large Precambrian basement rocks24. These two terranes have comparable Paleozoic tectonostratigraphic sequences, characterized by the deformed/metamorphosed Early Paleozoic strata and unconformably overlying Devonian–Carboniferous strata. The Lower Paleozoic is represented by the Huangcaopo and Kubusu Groups of the Junggar terrane and the Habahe and Kulumudi Groups of the Chinese Altai terrane, consisting mainly of marine volcano-siliciclastic rocks13,25. The dating results suggest that they were probably deposited in the Silurian to early Devonian13,26. Abundant Precambrian detrital materials are involved in these sedimentary rocks13,26,27,28. Immediately above the unconformity are the Early Devonian rift successions23,29, accompanied by the development of rift magmatic rocks30,31,32. This unconformity is also recorded in the Tuva-Mongolian terrane and it interrupted the Cambrian–Ordovician carbonate-clastic deposition, forming the Silurian volcaniclastic flyschoid sequences with minor conglomerate layers21. Recent study shows that the late Ordovician–Silurian tectonic contraction may take place in the Junggar terrane, which induced ca. 450–420 Ma crustal thickening-related magmatism across the Karamaili suture zone22.

In the mid-Carboniferous, the Karamaili ocean that opened in the early Devonian was the first to close relative to others33 (Fig. 1b), resulting in the amalgamation of the Junggar and Yemaquan blocks to create the Junggar terrane. The Carboniferous sedimentary rocks recorded local Paleozoic magmatism in the terrane and earlier allochthonous Precambrian detritus. In this paper, we undertook detrital zircon U–Pb–Hf analyses for six Lower Carboniferous sandstone samples collected from the Shuangjingzi and Kushuiquan domains in the Junggar terrane (Figs. 1c and 2).

Fig. 2
Fig. 2
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Comprehensive stratigraphic column for lithology and sampling sites and photomicrographs of the Early Carboniferous sandstones from the northern margin of the Junggar block and the southern margin of the Yemaquan block. D2k = Middle Devonian Karamaili Formation; D2p = Middle Devonian Pingdingshan Formation; C1t = Lower Carboniferous Tamugang Formation; C1d = Lower Carboniferous Dishuiquan Formation; C1s = Lower Carboniferous Songkaersu Formation; C1n and C2n = Lower Carboniferous Nanmingshui Formation (lower and upper part); C2s = Upper Carboniferous Shuangjingzi Formation; C2q = Upper Carboniferous Qingshui Formation; C2b = Upper Carboniferous Batamayineishan Formation; Q = quartz; Pl = plagioclase; Ls = lithic shards. The locations of the samples with (*) are from Li et al.33.

Results

Detrital zircon U–Pb geochronology and Lu–Hf isotopic compositions

Detrital zircons from the Lower Carboniferous samples are colorless, transparent and prismatic with variable length/width ratios (1.0–3.0). Zircons with Paleozoic ages commonly show more euhedral/subhedral shapes and larger aspect ratios relative to Precambrian zircons. Almost all analyzed zircons have high Th/U ratios (> 0.1, Fig. 3f) and most of them possess oscillatory zoning under the cathodoluminescence (CL) images (Fig. S1), indicating a magmatic origin. Two Tamugang Formation (C1t) samples (KM-4 and KM-5) from the northern margin of the Junggar block yield 139 concordant detrital zircon U–Pb ages ranging from 348 ± 3 Ma to 3539 ± 30 Ma (Fig. 3a, b; Table S1). Their spectrum exhibits a major Paleozoic age population at ~ 350–520 Ma and some Precambrian age clusters around 740–910 Ma, 1150–1490 Ma, 2.3 Ga, 2.9 Ga and 3.5 Ga, almost identical to the Lower Carboniferous and Middle Devonian samples (Fig. 3c, d and S2). One Nanmingshui Formation (C1n) sample (KM-6) from the southern margin of the Yemaquan block give consistent prominent Paleozoic age population, and a few Precambrian ages at ~ 760 Ma and ~ 2.6 Ga (Fig. 3e; Table S1). A total of 1004 (new and compiled) detrital zircon U–Pb dating results of the Middle Devonian–Lower Carboniferous strata from the two block margins (i.e., across the Karamaili suture zone) are dominated by Paleozoic ages clustering mainly at ~ 365 Ma and 450–520 Ma (Fig. S2). Subordinate Precambrian zircons show a major age population around 700–1000 Ma and several peaks between 1.3 Ga and 3.4 Ga (Fig. S2). The Paleozoic zircons from the Lower Carboniferous samples have εHf(t) values varying from − 15.9 to + 15.7, with the Late Paleozoic zircons showing higher εHf(t) values (-5.4 to + 13.5). In contrast, the Precambrian ones are characterized by a larger spread of εHf(t) values between − 21.7 to + 18.2 (Fig. S3; Table S2).

Fig. 3
Fig. 3
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Detrital zircon U–Pb concordant diagrams of the Lower Carboniferous sandstone samples from the northern margin of the Junggar block and the southern margin of the Yemaquan block. The compared detrital zircon U–Pb age distribution for the Lower Carboniferous Tamugang Formation sandstone (SJ-1) is from Li et al.33.

Discussion

Precambrian detritus in the Junggar and Altai terranes from the siberian continent

In the Junggar terrane, the middle Devonian–Carboniferous sedimentary rocks show similar U–Pb age spectrum of detrital zircons with the older, Silurian–early Devonian strata, suggesting prolonged sedimentary recycling (Fig. 4a). The detrital zircons for the Silurian–Carboniferous rocks span a nearly continuous Archean–Paleozoic age range from 311 to 3539 Ma, clustering around four main Precambrian populations at 700–1000 Ma, 1.2–1.4 Ga, 1.8–2.1 Ga and 2.3–2.9 Ga (Fig. 4a). These Precambrian zircons have little possibility to derive from the Junggar terrane because it lacks ancient basement rocks24. In particular, abundant ~ 820 and ~ 930 Ma detrital zircons occur in the strata (Fig. 4a) but contemporaneous magmatic rocks are absent in this region. High psephicity of the Precambrian zircons (Fig. S1) indicates that the old detritus in the Silurian–Carboniferous strata more likely represent allochthonous components that shed onto the Junggar terrane via a long-distance transportation during Silurian–early Devonian time.

Fig. 4
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(a-f) Age distributions and Hf isotopic compositions of the detrital zircons from the Silurian–Carboniferous strata in the Junggar terrane, the Tarim craton and neighbors, and the Altai terrane, respectively. The age distribution curves of the Precambrian (> 600 Ma) detrital zircons are shown for comparison, including those from the Neoproterozoic and the Silurian-Carboniferous strata in the Tarim craton, the Ordovician–Devonian strata in the Hovd terrane, and the Neoproterozoic strata in the Siberia craton. See Tables S6 for the U–Pb–Hf data sources of the detrital zircons. The compiled crystallization ages of the magmatic rocks in the Tarim craton and neighbors and in the Lake terrane and the Tuva-Mongolian microcontinent are from Han et al.20 and Soejono et al.15. , respectively.

Precambrian magmatic rocks are widely exposed in the Tarim craton and adjacent CTB and YB, but the 500–600 Ma, 1.2–1.4 Ga and 2.6–2.9 Ga magmatic events that are prevailed in detrital zircons from the Junggar terrane are essentially lacking or rather weak (Fig. 4b). The lack of these magmatic episodes is manifested by detrital zircon records in both Neoproterozoic and Silurian–Carboniferous strata (Fig. 4b). Furthermore, the 700–1000 Ma zircons in the Tarim and neighbors show εHf(t) values ranging from − 34.6 to 11.2 (81% negative values) (Fig. 4e), different from the range of -21.7 to 14.4 of those coeval zircons (24% negative values) in the Junggar terrane (Fig. 4d). In addition, numerous studies endorse the existence of a Paleozoic oceanic basin between the Junggar terrane and the CTB34, preventing the sediment transportation. These features suggest that the Tarim craton may not be the provenance of the Precambrian detritus in the Junggar terrane.

Instead, the Silurian–early Devonian strata in the Altai terrane show a comparable detrital zircon age spectrum with those in the Junggar terrane (Fig. 4a, c). The spectrum displays the 400–600 Ma, 700–1000 Ma, 1.2–1.4 Ga, 1.8–2.1 Ga and 2.3–2.6 Ga populations and the ~ 440 Ma, ~ 500 Ma, ~ 820 Ma and ~ 910 Ma peaks (Fig. 4c). Furthermore, the zircon εHf(t) values and positive/negative values proportions of the Precambrian age populations in the Altai terrane are broadly identical to the coeval ones in the Junggar terrane. Both the Altai terrane and the Junggar terrane are characterized by predominant positive εHf(t) values for the 1.2–1.4 Ga zircons (Fig. 4d, f). More importantly, the distinctive 500–600 Ma zircons in the Altai and Junggar terrane show an overlapped range of εHf(t) values (-25.0 to 16.5 and − 7.8 to 15.0), and a consistent negative value proportion (62% and 61%) (Fig. 4d, f). The above similarities, together with same storage horizon (i.e., Silurian–early Devonian strata), indicate that the Precambrian detrital materials in the Junggar and Altai terranes share a common provenance. It has been documented that the Precambrian magmatic events on the Siberia craton occurred spanning from the Archean to the Neoproterozoic and cluster in three major periods: 540–1000 Ma, 1.7–2.2 Ga, and 2.4–2.9 Ga35,36 (Fig. 4c). These three periods of magmatism define a εHf(t) value range from − 22.6 to 13.3 for the 700–1000 Ma zircons, from − 20 to 7 for the 1.8–2.1 Ga zircons, and from − 15 to 5 for the 2.3–2.6 Ga zircons37,38. Such age and Hf isotope brackets correspond well with those of the Precambrian detrital zircons from the Junggar and Altai terranes, indicating that the Siberia craton was an important source. In addition, both the Junggar and Altai terranes exhibit a similar age distribution pattern with the Hovd terrane (Fig. 4a, c), especially for the Neoproterozoic zircons, suggesting that the Mongolian collage system might also contribute the old detritus39. Firstly, the ca. 460–570 Ma and ca. 750–950 Ma magmatic activities have been reported in the Lake terrane and the Tuva-Mongolian, Zavkhan and Baydrag blocks15 (Fig. 4c), and these rocks have a capability of providing some Neoproterozoic clastic materials17. More importantly, minor 1.0–1.7 Ga zircons in two terranes, which are obviously absent in the Siberia craton (Fig. 4c), could be derived from the Precambrian basement of the Mongolian continental blocks40,41. These correlations enable us to argue that the Precambrian detrital materials in the Junggar and Altai terranes most likely came from the Siberia craton, with contributions from the Tuva-Mongolian microcontinent.

Mid-paleozoic tectonic link between the Junggar and Altai terranes

Oceanic subduction at convergent margins controls magmatic generation and evolution in the upper plate, and the major change of the accretionary regime can be revealed by the variation of zircon Hf isotopic compositions42,43. Compiled U–Pb–Hf isotopic data for the Paleozoic zircons from the sedimentary and magmatic rocks in the Junggar terrane show a marked change of εHf(t) values at ca. 420–410 Ma, with a wide range (-17 to + 16) between 520 and 420 Ma, contrasting with the predominantly positive εHf(t) values (+ 2 to + 18) for ca. 410–350 Ma zircons (Fig. 5b). We interpret such a zircon Hf isotopic change at ca. 420–410 Ma as an accretionary tectonic transition from an advancing to retreating mode. This is because the advancing accretion could induce crustal compression and thickening and thus greater contribution of crustal materials into magmas with a decreasing trend of zircon εHf(t) values42. In contrast, the retreating accretionary process would enhance the input of mantle materials by the resultant crustal extension and cause a gradual increase of zircon εHf(t) values43,44. This tectonic switching event at ca. 420–410 Ma in the Junggar terrane is manifested by the generation of extension-related magmatic rocks including the Early Devonian Well LC1 basaltic rocks within the Junggar Basin23 (Fig. 1b) and granitic intrusions in the Yemaquan block31, which was synchronous with those occurred in the Chinese Altai terrane45.

Fig. 5
Fig. 5
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(a-b) Plot of εHf(t) values versus U–Pb ages of detrital and magmatic zircons from the Altai terrane (compiled data by Li et al.22) and the Junggar terrane (this study; Tables S4,S5). (c) Summary of mid-Paleozoic key geological records in the Junggar and Altai terranes concerning tectonic regime22,47, metamorphic event51,52 and magmatic distribution (see Table S3 for data sources), as well as the ages of the ophiolitic rocks along the Karamaili, Zhaheba-Aermantai and Irtysh suture zones22.

Both the Junggar terrane and the northern Tarim craton were in the advancing accretionary stage since the Late Cambrian, but the former experienced an accretionary tectonic shift markedly earlier than the latter that happened at ca. 400 Ma46 (Fig. 5b). Furthermore, the Junggar terrane is dominated by the Early Paleozoic volcano-sedimentary successions, with the development of numerous Ordovician–Silurian arc magmatic rocks constituting the Dananhu arc along the southern margin of the Junggar block34. This contrasts with the Cambrian–Ordovician carbonate-clastic deposition on the northern Tarim craton and its neighbors. These discrepancies demonstrate the existence of a northward oceanic subduction beneath the Junggar terrane during early Paleozoic time, similar to the case of the Altai terrane27. However, in view of the inconsistent oldest arc magmatic record (Fig. 5c), the two regions were likely related to different subducting branches of the PAO at that time (Fig. 6a). The advancing subduction in the Junggar region may result in the closure of the Karamaili Ocean and amalgamation of the Junggar and Yemaquan blocks during the late Ordovician–Silurian to form the unified Junggar terrane22. Given that the Junggar terrane received the Precambrian detritus from the Siberia continent coevally with the Altai terrane, and the depositional ages of the Silurian–early Devonian rocks in both terranes that preserves these old materials have been constrained to be ca. 440–410 Ma13,26, we propose that the Junggar terrane had collided with the Altai terrane at that time. This consideration is corroborated by the concurrence of tectonic contraction and related crustal thickening in the two terranes22,47, as well as a dramatic decrease of zircon εHf(t) values in the Altai terrane that more likely corresponds to a collision event48 (Fig. 5a). Geological responses to this collisional event are expressed by the deformation/metamorphism of the Early Paleozoic strata spanning from the Junggar to Altai terrane and a regional unconformity on top of the strata26,49,50. Ca. 430–420 Ma metamorphic event has been determined in the Chinese Altai51,52. All of the above evidence supports a mid-Paleozoic collisional event between the Junggar terrane and the Altai terrane during the advancing accretionary orogenesis (Fig. 6b). Following the tectonic transition at ca. 420–410 Ma, the Junggar-Altai region underwent an intense and rapid extension. This tectonic regime is expressed by the early Devonian crustal thinning in the Junggar terrane22 and extension-related sub-horizontal foliation structure in the Altai terrane53. Voluminous Early Devonian rift magmatism occurred in the broad region of Junggar to western Mongolia, with the generation of ca. 398 Ma basaltic rock in the Junggar Basin23, ca. 413 Ma leucogranite in the Yemaquan arc31, and ca. 415–390 Ma bimodal volcanic suites in the Altai and Sayan terranes54,55. Moreover, the Lower Devonian transgressive depositional successions in the Junggar terrane also favor the extensional setting29. Such an extensional event may lead to the opening of a series of oceanic basins in the Junggar-Altai region, as proved by 409–403 Ma ophiolitic record in the Zhaheba–Aermantai/Irtysh and Karamaili suture zones (Fig. 5c). A notable feature is that the Middle Paleozoic arc magmatism of the Junggar block migrated to its southern margin (i.e., Dananhu) in the late Silurian, indicates that the ca. 420–410 Ma accretionary tectonic switch was likely driven by the rollback of the subducting PAO plate (Fig. 6c). These results demonstrate that, following the dock of Tuva-Mongolian microcontinent to the Siberian margin18, the Junggar terrane had been accreted to the Altai-Mongolian terrane in the mid-Paleozoic, which is temporally consistent with the amalgamation among the Gorny Altai, Altai-Mongolian and West Sayan terranes17,18.

Fig. 6
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Sketch illustrations (not to scale) showing accretionary tectonic evolution of the Junggar terrane, and its amalgamation and dispersion with the Altai terrane in the global plate reconstruction during mid-Paleozoic time. See text for details. ZAO—Zhaheba-Aermantai Ocean; KO—Karamaili Ocean. This figure is generated using CorelDRAW X8 created by the CorelDRAW Team under an open license (http://www.coreldraw.com/cn/product/graphic-design-software/).

Implications for tectono-paleogeographic reconstruction in central Asia

Our work links the mid-Paleozoic source-to-sink sedimentary system with accretionary tectonic events in the Junggar-Altai region. Their spatio-temporal relationships unravel a short-lived connection between the Junggar and Altai terranes, possibly during the late Ordovician–Silurian, which provides new insights into Paleozoic tectonic reconstruction in central Asia. The Junggar terrane is a small but important tectonic component of the CAOB2, and its tectono-paleogeographic framework, especially during the Early Paleozoic, always lacks available constraints. Numerous studies have shown that the Junggar terrane is underlain predominately by juvenile crust, possibly with a very limited ancient basement22,24. Our results suggest that it did not receive the detrital materials from the Tarim craton that was located on the Gondwana northern margin in the Early Paleozoic46. Furthermore, the characteristic 500–600 Ma detrital zircons in the Junggar terrane have predominant positive εHf(t) values contrasting with those more evolved Hf isotopic compositions from the Neoproterozoic–Early Paleozoic strata in the North Indian, Australian and North African terranes56. These indicate that the Junggar terrane was not derived from the Gondwana and unconnected with the Tarim craton prior to their final amalgamation in the late Paleozoic. Instead, it was more likely to reside in the northern hemisphere all the time, and lie in the PAO realm adjacent to the peri-Siberian accretionary margin during the Early Paleozoic (Fig. 6). The determination of advancing subduction-induced terrane accretion event not only reveals the mid-Paleozoic tectonic affinity of the Junggar terrane and the peri-Siberian accretionary system (Fig. 6b), but also defines an Early Paleozoic paleogeographic boundary for the CAOB roughly between the Junggar block and the CTB (i.e., North Tianshan suture zone; Fig. 1b) that can extend to the Solonker suture zone. Such a division scheme is supported by the distribution of Silurian Tuvaella brachiopod fauna57, different from that for Late Paleozoic one along the South Tianshan–Solonker suture zone2. Divergent double subduction of the PAO plate has been shown to govern the accretionary evolution of the Siberian margin to the north and the Gondwana continent to the south during the Early Paleozoic5. The middle Paleozoic switching accretionary tectonics that is recognized in the Junggar-Altai region commonly took place in the peri-Siberian terranes16,18, the eastern Kazakhstan block58 and the Gondwana marginal blocks such as northern Tarim and northern North China46. These together constitute an accretionary orogenic girdle above the PAO subduction zone at that time, which is reconciled with the global plate reconstruction by Merdith et al.59. The advancing accretion brings a kinematic constraint that the Siberia and Gondwana may proceed a northward drift during the Early Paleozoic, at least after the final assembly of Gondwana at ~ 500 Ma60,61. The subsequent retreating accretion since the early Devonian is more likely to induce the breakup of Gondwana6,46 and the building of the Late Paleozoic archipelago geographic framework in the PAO realm2. Additionally, we reveal the Caledonian-age tectonic evolution in the southwestern CAOB connecting the Neoproterozoic and Late Paleozoic accretionary history at the periphery of Siberia and link it with those concurrently along the circum-PAO accretionary margin. The Paleozoic accretionary tectonics have been documented to spatially overlap the Neoproterozoic ones along the Kazakhstan eastern margin and the Gondwana northern margin18,19. These support that the PAO was not a newborn ocean but represent a remnant of the Ran-Rodinia Ocean5. Therefore, its long-lived subduction during the Neoproterozoic to Paleozoic and final closure possibly governed the evolution from Rodinia to Gondwana62 and the development of the CAOB1.

Conclusion

This study uses detrital zircon U–Pb–Hf data to investigate the provenance of the Precambrian sedimentary materials in the Junggar terrane and related tectonic factor. Our results manifest that the Precambrian detritus in the Junggar terrene shares a single provenance with those in the Altai terrane and they are derived from the Siberian continent. The Junggar terrene underwent a mid-Paleozoic accretionary tectonic switch from an advancing to retreating mode. The advancing subduction induced the amalgamation of the Junggar terrene with the Altai terrane during the Silurian–early Devonian, allowing the transportation of old detritus from the Siberia craton and adjacent Tuva-Mongolian microcontinent. These findings help to delineate an Early Paleozoic tectono-paleogeographic boundary of the CAOB along the North Tianshan–Solonker suture zone. In combination with regional tectonic correlation, we suggest that the PAO is a long-lived ocean representing a remnant of the Ran-Rodinia Ocean, with its protracted subduction governing the Neoproterozoic-Paleozoic accretionary orogenic evolution throughout the Siberia southern margin (present coordinates), the Kazakhstan eastern margin and the Gondwana northern margin.

Sample and analytical methods

This study established the Paleozoic connection among the Junggar terrane, Altai terrane and Siberian craton by examining their source-to-sink sedimentary and accretionary tectonic correlation, based on new and compiled zircon U–Pb–Hf data from the three regions. They included 2289 radiometric ages and 1776 zircon εHf(t) isotopic analyses from the Junggar and Altai terranes, as well as 2498 zircon U–Pb dating results with 871 εHf(t) values from the Tarim and Siberian cratons and their neighbors. See the supplementary information for sample description and (our and compiled) analytical data used in this study.

Zircon U–Pb dating

Zircons were separated from samples processed by crushing, heavy-liquid, and magnetic methods and then were mounted in epoxy resin and polished to expose the interior. Cathodoluminescence images were used to examine the internal structures of zircons prior to isotopic analysis. In-situ zircon U–Pb dating was carried out at the Northwest University in Xi’an, China, using a laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) assembled with a GeoLas 200 M ArF-excimer 193 nm laser ablation system and an Agilent 7500a quadrupole ICP–MS. Laser frequency was 10 Hz and spot sizes were between 30 and 40 μm. A carrier gas (high-purity helium) was mixed with a make-up gas (high-purity argon) before entering the ICP–MS to achieve stable and optimum conditions, resulting in negligible contribution of 204Hg to 204Pb (count rate of the mass 204 for blank < 100 counts per second). The Harvard zircon 91,500 was used as a standard for isotopic ratio corrections. The GLITTER 4.0 program (Macquarie University) was used to process raw count data to obtain U–Th–Pb isotopic ratios and elemental concentrations. U, Th and Pb contents were calculated by using NIST SRM 610 as an external standard and29Si as an internal standard. Common Pb was corrected using the ComPbCorr program (ver. 3.16e)64. Zircon standard GJ-1 analyzed as an unknown yielded a weighted mean 206Pb/238U age of 601.0 ± 3.6 Ma (MSWD = 2.9, n = 45), which is consistent with the TIMS age of 600.4 ± 0.6Ma65. Detailed instrumental settings and analytical procedures also refer to Diwu et al.66. and Liu et al.67. The Excel macros “NORMALIZED PROB PLOT” 2/3 created by G. Gehrels at the University of Arizona was used for age data statistics and presentation (Ariona LaserChron Center: https://sites.google.com/a/laserchron.org/laserchron/).

Zircon Lu–Hf isotope analysis

Zircon Hf isotopes for the Carboniferous sandstones were analyzed at the Northwest University in Xi’an, China, and determined using a Nu Plasma II MC–ICP–MS and a RESOLution M-50 (ASI) excimer ArF laser ablation system all housed at the SKLCD, Northwest university in Xi’an, China. The laser ablation system (RESOlution M-50, ASI) consisted of an excimer laser (193 nm), a two-volume laser ablation cell (Laurin Technic S155, 155 mm × 105 mm), a Squid smoothing device, and a computer-controlled high-precision X-Y stage. The two-volume laser ablation cell was designed to avoid cross contamination and reduce background flushing time, while the Squid smoothing device could give a smooth signal with laser pulse rates down to 1 Hz. Sensitivity and fractionation were independent of the sampling position in the cell. Helium was used as a carrier gas for the laser ablation process, and it entered the cell body at its bottom to fill the big cell. Helium from both bottom and argon from the funnel cell were admixed downstream, in front of the squid signal smoothing device, into the MC–ICP–MS. The Nu Plasma multi-collector ICP–MS system (Nu plasma ΙΙ, Nu Instruments Wrexham, UK) used for Hf analysis represents the latest generation of double-focusing mass spectrometers. The new collector has sixteen Faraday detectors for greater flexibility, and its five full-size discrete dynode multipliers enable the determination of precise isotope ratios, even for isotopes with very low abundance. Its zoom optics system allows the observation of instant changes in dispersion and perfect peak overlap, without slow and potentially unreliable detector movement. In this study, the ion beams of 180Hf, 179Hf, 178Hf, 177Hf, 176Hf+176Yb+176Lu, 175Lu, 174Yb, 173Yb, 172Yb, 171Yb were collected in Faraday cups H4, H3, H2, H1, Ax, L1, L2, L3, L4, L5, respectively. Among the measured isotopes, the 179Hf/177Hf ratio was applied to calculate the mass fractionation of Hf (βHf), the 175Lu signal and 176Lu/175Lu=0.02656 were used to calculate the interference of 176Lu on 176Hf. 173Yb–171Yb was applied to calculate both βYb, and the 173Yb signal and 176Yb/173Yb=0.78696 were used to calculate the interference of 176Yb on 176Hf. The detail information of analysis strategy and data deduction can be found in published literature68.

Calculation of Hf parameters

Zircon initial 176Hf/177Hf ratios were calculated using measured 176Lu/177 Hf and 176Hf/177Hf ratios and a 176Lu decay constant of 1.867 × 10− 11 yr− 1 according to the method of Söderlund et al.69. To calculate εHf(t) values, we adopted a present-day 176Lu/177Hf value of 0.0336 and a 176Hf/177Hf value of 0.282785 for the chondritic uniform reservoir (CHUR)70. Depleted mantle model ages (TDM1) were calculated with reference to a depleted mantle reservoir having present-day 176Lu/177Hf value of 0.0384 and 176Hf/177Hf value of 0.2832571. Crustal model ages (TDM2) were calculated by assuming that the parental magma from which each zircon crystallized was originated from an average continental crust (176Lu/177Hf=0.015)71 which was derived from the depleted mantle.