Introduction

Asia, as the largest extant accretionary continent, was made by the successive amalgamation of numerous magmatic arcs, continental blocks, and accretionary complexes from the Paleozoic to Cenozoic1,2,3. The Altaids (Fig. 1a), extending from the Pacific Ocean in the east to the Ural Mountains in the west, witnessed the shaping of Gondwana and Pangea from the late Neoproterozoic to late Cenozoic4,5,6,7,8,9,10,11. The final suturing of the southern Altaids was proposed to occur from the late Devonian through the Carboniferous to the late Permian-mid Triassic7,12,13,14,15,16,17. However, the exact docking time of the Karakum-Tarim cratons with the southern Altaids is high of debate, which gives rise to uncertainties on the final closure of the Paleo Asian Ocean (PAO) and the timing of their integration into Pangea. The South Tianshan Orogenic Belt (STOB) in the southern Altaids (Fig. 1a) formed in response to the suturing of the Kazakhstan-Yili-Central Tianshan Continent (KYCTC) with the Karakum-Tarim cratons as the South Tianshan Ocean (STO) closed. In the last decade, numerous studies including igneous and metamorphic petrology, sedimentology, and geochronology have been conducted to investigate geodynamic evolution of the STOB. However, little consensus has been made on the most speculative debate for the timing of the final collision (i.e., Triassic16,18 versus late Carboniferous17,19,20,21,22). This is probably due to complicated structural and metamorphic overprinting during long-lasting accretionary histories.

Fig. 1: Tectonic framework and outlook of the studied area.
figure 1

a Map of the southern Altaids, modified after ref. 19. b Regional geological map of the AMC in Chinese STO, modified after refs. 27,30. c Synthetic and schematic lithological profile, along the cross-section from north (top, A) to south (bottom, A’). d Field view of the Muzetekexie valley, modified after98; the dotted part of the line represents a more diffuse nature of this contact, and the studied unmetamorphosed basin is unconformably overlying the greenschist-unit. e Climbing ripples observed in sandstone outcrops. f Mud-crack preserved in sandstone crops. g Several cent-meters scale clasts in conglomerate outcrop.

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Sediments formed in intra-oceanic fore-arc environment with basins overlying relatively mature crust would be less likely to deform due to their cold and stiff lithospheric roots, and are more likely to survive collisional orogenesis, even if juxtaposed against weaker accretionary complexes23. Fore-arc basins may contain a sedimentary record of much longer duration than the time the arc front has been active, as in the Mariana arc24, and serve as a sensitive archive of tectonic processes accompanied by orogenic cycle25. In this study, we focus on a newly discovered, undeformed fore-arc accretionary basin. It is closely located to the south with the Akeyazi LT-UHP metamorphic complex (500–600 °C, 2.0–3.0 GPa3,26,27,28,29, AMC), which bears plentiful findings of coesite26,29,30 in various lithologies. Previous studies mainly focused on the P-T-time condition of high-grade metamorphized lithologies3,31 and their exhumation histories, however, little attention had been paid on regional accretionary basins. In this study, we integrate field mapping, detrital zircon U-Pb, and Lu-Hf, and trace element analyses have been conducted to provide evidence for the depositional age and provenance of studied sandstone in the Muzitekexie fore-arc accretionary basin (MFAB). Additional multi-disciplinary data was also compiled from regional various lithologies to fingerprint the temporal-spatial variations of detrital deposition and mantle “signal” characteristics. Available data provide insight into the evolution of the MFAB, the final closure of the PAO (as witnessed by the western accessory ocean basin, the STO), and implications for subduction processes along with regional geodynamics.

Results

The MFAB in the Akeyazi metamorphic complex

The Altaids are a long-lasting composite collage between the East European craton, Siberian craton, Karakum-Tarim Craton, and the North China craton (Fig. 1a)8,9,32. The 2500 km-long STOB (Check Supplementary Note 1 for detailed geological background in Supplementary Information), is located in the southern Altaids, extends from the western deserts of Uzbekistan, Tajikistan, Kyrgyzstan, and Kazakhstan to the northern Xinjiang in China, and formed in response to the final amalgamation between the Tarim and Siberian cratons1,6,12,18,33,34. The Chinese STOB is comprised of the KYCTC in the north and South Tianshan accretionary complex (AC19,35, including the South Tianshan Belt, STB) in the south, and is bounded to the south by the North Tarim Fault (NTF) with the North Tarim Craton (NTC, check the Supplementary Information for the detail of regional geological setting). To East, the geographical South Tianshan is separated, by Toksun-Kumishi High Road36 (along longitude ~89 °E), as the Central Tianshan (CTS) and East Tianshan (ETS). The AMC constitutes the eastern part of the Kazakhstan collage system within the Altaids and extends for about 200 km along the South Central Tianshan Fault (SCTF, Fig. 1b), and correlates with the Atbashi37,38,39, Chatkal40,41 and FanKarategin (U)HP metamorphic complexes42, which crop out in the Kyrgyz and Tajik segments of the STOB, respectively. Our study area lies to the south of the AMC, as a ~1.5 km wide across, fore-arc accretionary basin (located at the branch of Akeyazi river, named Muzitekexie valley, Fig. 1b–d) resting unconformably on top of the greenschist-facies unit and clamped between the SCTF and the local greenschist-UHP detachment fault. To south, it is closely associated with the regional LT-UHP unit, which hosts plentiful findings of coesite26,29 and predominantly composed of strongly schistosed meta-volcanoclastics hosting mafic metavolcanics as pods, lenses, boudins, thin layers or massive blocks43 and marble horizons26. In contrast, studied MFAB is ~1500 m in thickness and uniformly constituted of undeformed sandstones with minor cobble conglomerate blocks and local carbonaceous mudstone seams. Obvious sedimentary bedding and climbing ripples (Fig. 1e) are well-developed in sandstone. The conglomerate blocks wrapped within the sandstones have sizes ranging from tens of centimeters to several meters, and are matrix-supported and polymictic with poorly sorted, angular, and clastic quartzites (Fig. 2b).

Fig. 2: Petrographic features of the MFAB sandstone.
figure 2

a Triassic sandstones and b conglomerates in the MFAB. c, d, f BSE images and e SEM-EDS mapping show the preservation of high-pressure mineral relics (i.e., Rt, Ph, and Omp) in detritus; mineral abbreviation is following ref. 128. g Sandstone petrographic data for studied samples including Q-F-L129 (quartz, feldspar, and lithic clasts). h, i Mineral composition of phengite and omphacite relics found in studied sandstones.

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Sampling strategy

To determine the stratigraphic age of the MFAB and the provenance information of the sedimentary rocks, several sandstone and conglomerate samples were collected for petrographic, mineralogical, and geochemical analysis. Of these, representative sandstone samples of the MFAB (AKMZ05-09, Fig. 1d) were selected for bulk-rock major and trace elements composition analysis (Table S1) and in-situ LA-ICP-MS U-Pb geochronology and Lu-Hf isotopic measurements of detrital zircon (Tables S2–4).

Compilation of regional multi-disciplinary geochronological and geochemical data

To elucidate the crustal response and the geodynamic evolution of STOB-KYCTC-NTC subduction-collision system as witnessed by the formation of the MFAB, we fingerprint the spatial and temporal variations of regional detrital deposition and magma source characteristics, based on systematic geochemical and chronological data collection of various lithologies (i.e., metamorphic, magmatic and sedimentary rocks; relevant references are listed in Tables S6–S9) from studies among the KYCTC, AC and ETS regions (data from NTC region is also provided for comparison). This includes: (1) 503 intermediate rocks with complete major and trace element compositions, radiometric ages, and bulk-rock Sr-Nd isotopic data (Table S6); (2) 2591 zircon Lu-Hf isotopic data, together with ages, from igneous and sedimentary rocks (Table S7); (3) 7350 detrital zircon U-Pb ages from sedimentary rocks (Table S8). We utilized zircon εHf(t), together with bulk-rock Sr/Y, εNd(t), and Nb/Yb elemental ratios, and detrital zircon age spectrum as proxies to differentiate magmatism with distinct source signatures and sedimentary detritus provenance. Furthermore, a compilation (Table S9) of 140 age data of regional metamorphism, depositing, and main strike-slip faulting & thrusting events was also conducted from chronological comparison.

Petrography

Studied sandstones are relative sandy silt and mainly composed of medium- to fine-grained (100–400 µm across) sub-angular to moderate-abrased quartz, cemented by calcite (Fig. 2a, c), implying comparatively moderate to long-distance transportation of detritus prior to deposition. Compositionally, sandstones of the MFAB are mainly composed of quartz and lithic grains but lack feldspar (Fig. 2g). The dominant lithic fragments are intermediate to acid volcanic rocks with microlithic and lathwork textures (Fig. 2a–c). In addition, high-pressure mineral relics (i.e., omphacite, retrograded phengite with Si content of 3.25–3.35 a.p.f.u. and rutile, although rutile sometimes could be of magmatic origin), which has the similar solid-solution composition to those from the adjacent southern HP-UHP metamorphic complex (Fig. 2h, i), were found either as inclusions in sub-angular quartz grains (Fig. 2d) or as fragments around them (Fig. 2f).

Bulk-rock composition

The results of bulk-rock major and trace element composition and detrital zircon U-Pb age, rare earth elements, and Lu-Hf isotopic composition of analyzed sandstone samples are respectively presented in Tables S1–4. Bulk-rock major elements data (Fig. 3a, b) suggest a composition ranged between litharenite and graywacke, with relatively lower CIA (chemical index of alteration)44 and higher ICV (index of compositional variability)45 values compared to those of PAAS (post-Archean Australian shales46). Compositions of trace elements (Fig. 3c–f) of studied sandstone show obviously higher abundance of La and Th and lower concentration of Hf with respect to poorly sorted sediments from oceanic island arc but reflecting similarity with those from the active continental margin.

Fig. 3: Geochemical classification and discrimination of the MFAB sandstones.
figure 3

a log(Na2O/K2O) vs log(SiO2/Al2O3) and b ICV vs CIA for studied sandstones; the index of chemical alteration, CIA = [Al2O3/(Al2O3 + CaO*+Na2O + K2O)] ×100, where CaO* is the content of CaO in silicate minerals only, is given after ref. 44 and the Index of Compositional Variability, ICV = (CaO+K2O + Na2O + Fe2O3 + MgO + MnO + TiO2)/Al2O3 is given after ref. 45. c La vs Th; d La/Th vs Hf after ref. 130; sources: Mesoproterozoic crust. 131, PAAS46, ref. for average graywacke compositions132. e, f spider and REEs pattern diagrams of studied sandstones; values of primitive mantle133, chondrite133, and PAAS46 are used for normalization and comparison.

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Detrital zircon U-Pb and Lu-Hf isotopes

The age data of detrital zircons are plotted as KDEs and histograms (Figs. 4a, b, 5, 6c) using isoplotR47. CL images of representative detrital zircon grains with age, U concentration, and Th/U ratio are shown in Supplementary Fig. 1. In addition, diagrams of ages versus detrital zircon rare earth elements characters (denoted by δEu, (Lu/Tb)N and REEs pattern, Fig. 4c–g) are also made to investigate the detritus provenance. A total of 213 spots of detrital zircon U-Pb ages (Fig. 4a, b) were analyzed for sandstone samples in the MFAB. Most zircon grains are subhedral to euhedral characterized by obvious oscillatory zoning (Supplementary Fig. 1). They display comparable Precambrian and Paleozoic-late Triassic U-Pb age spectrum (~227–1765 Ma) of detrital zircon on the KDE plot (Figs. 4b, 6c). About three-quarters and less than one-tenth of zircons yield late and early Paleozoic ages, respectively, far more than the amount (~14%) of ages of Precambrian. In addition, one major age peak of all zircons in sandstones is concentrated at ~290–296 Ma (~74%), with minor Devonian-Precambrian ages at ~410, ~711, ~930, ~1199, and ~1765 Ma. Thereinto, ~4% of Triassic ages are reported with weighted average at 231.7 ± 1.1 Ma (YC2σ(3+)48, MSWD = 6.3, n = 4, Figs. 4b, 6c). The trace elements geochemistry of analyzed detrital zircons is complicated, with various Th/U ratio (0.184–2.394) likely indicating assorted detritus source. Most of them (including four youngest Triassic zircon grains) show REEs pattern with obvious inclined LREEs and HREEs feature ((Lu/Tb)N > 4) and strong negative Eu anomaly (δEu ~0.4, Fig. 4c, d), suggesting a possible inherited magmatic origin49. In addition, a part of ~300 Ma and ~315 Ma detrital zircon grains, respectively, display strong negative Eu anomaly (δEu ~0.3) and depleted HREEs (((Lu/Tb)N ~3) and inconspicuous negative Eu anomaly (δEu ~1.0) and HREEs plateau ((Lu/Tb)N ~1–2, Fig. 4c–g), implying metamorphic derivations identical to regional zircons overgrew under greenschist- and blueschist/eclogite-facies conditions29. The Hf isotopic compositions of most of detrital zircon (~67%) in sandstones yield contrasting εHf(t) values from −18.4 to +18.2 (Fig. 7a). Thereinto, Triassic-Permian detrital zircons have broadly positive εHf(t) values from −1 to +10. Further, typical HP metamorphic (omphacite and rutile) and magmatic (needle-like apatite) mineral inclusions (Supplementary Fig. 2) were identified in euhedral and rounded zircon grains, respectively.

Fig. 4: Detrital zircon U-Pb isotopic and rare earth elements (REEs) results.
figure 4

a, b Detrital zircon U-Pb concordia and KDE plots of the MFAB sandstones. c–g Diagrams of ages versus detrital zircon rare earth elements characters (denoted by δEu, (Lu/Tb)N, and REEs pattern) to deduce the multiple detritus provenance.

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Fig. 5: Geological maps with the highlighted localities of compiled multi-disciplinary data.
figure 5

a collected detrital zircon U-Pb ages of regional sediments (corresponding to data in Table S7), b compiled zircon U-Pb ages of regional intermediate rocks (corresponding to data in Table S6). c Hf isotope contour map after ref. 110, based on Paleozoic granitoid rocks and felsic volcanic rocks among the STOB, showing the spatial variation of the ancient crustal basement (fragmented craton or micro-continent) with relative old Hf model ages.

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Fig. 6: KDE plots of detrital zircon U-Pb ages of sediments in adjacent key tectonic units.
figure 6

a Yili Basin, b Central Tianshan Arc, c Triassic MAFB (this study), d Fore-arc accretionary Complex, e South Tianshan Belt, and f North Tarim Craton. The proportion of Triassic, late Paleozoic, early Paleozoic, and Precambrian detrital zircons, as well as the maximum depositional ages (MDA, blue solid-line) of Triassic MAFB, are highlighted for comparison. Relevant data are listed in Table S2 and S7.

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Fig. 7: Plots of compiled multi-disciplinary evidence.
figure 7

It shows the spatial-temporal evolution of a inherited & detrital zircon εHf(t), b bulk-rock Sr/Y; data distribution of scatter plot is evaluated by Voronoi density134. c εNd(t) and d Nb/Yb of arc intermediate rocks among the MFAB, CTS, ETS region, and NTC; crustal thickness is derived from the method of ref. 135; yellow and red arrows indicate the evolution of mantle “signal” respectively beneath ETS and CTS Arc regions. e KDE plot of ages of regional arc intermediate rocks; ages marked by numbers 1–4 with black star are respectively cited from refs. 68,135,136,137,138. f KDE plot of ages of regional metamorphism, depositing, and main strike-slip faulting & thrusting events; ages marked by numbers 5–10 with black, colored stars are respectively cited from refs. 3,139. The widespread rapid cooling signal85,86,87,88,89 from low-temperature thermochronology across the STOB, in response to the final amalgamation of southern Altaids, is highlighted by cyan column. g Bulk-rock εNd(t)-zircon εHf(t) isotope diagram for ~260–330 Ma arc or plume-related intermediate rocks from regional tectonic untis; the error bars are derived from the variation of zircon εHf(t) values of individual intermediate rock in region. Relevant data are presented in Tables S2–S4. Compiled regional data are listed in Tables S6–S7.

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Discussion

Deposition timing and provenance analyses

“The law of detrital zircons50” is widely proved to be effective in sedimentary basins among numerous tectonic settings48,51,52,53,54. The maximum depositional ages is consistent with the true depositional age of sediments if their depositional sites are proximal to magmatic arcs48. Our study shows that sandstones from different outcrops in the MFAB exclusively yield Paleozoic-late Triassic ages with only a few Precambrian records. Thereinto, the prominent early Permian (~290–296 Ma) age peak could be related to the tectono-magmatic events in response to plates convergence and intracontinental shortening in southern Altaids55,56, and a substantial early Triassic population (~5%, Figs. 4a, b, 6c) is identified (231.7 ± 1.1 Ma, YC2σ (3 + ), MSWD = 6.3, n = 4). This implies that the timing of deposition of studied sedimentary strata extended, at least, to the early Triassic.

To determine the provenance of the sedimentary rocks, we compiled 2591 zircon Lu-Hf isotopic data, together with ages, from igneous and sedimentary rocks and 7350 detrital zircon U-Pb ages from sedimentary rocks for comparison. The detrital zircon age spectrums of regional sedimentary rocks sampled on the assumed upper-plate (KYCTC), the accretionary wedge (AC, including the STB), and the passive lower continental plate (NTC) are presented in Fig. 6. Sediments from these tectonic units (Fig. 5a) display a substantial (~36–77%) Precambrian detritus population, in contrast to those (<15%) from Yili Basin and the MFAB in this study. For early Mesozoic-Paleozoic detrital records: (1) sediments from the STB and NTC show consistently binary age peaks, respectively, at 270–310 and 400–500 Ma, interpreted as the reflection of melt derived from the Tarim Permian mantle plume57,58,59 and the active continental arc which formed along the North Tarim margin during the Silurian-Ordovician57,59,60,61,62; (2) broadly continuous early Mesozoic-Paleozoic detrital records characterize the age spectrum of sediments from the AC, KYCTC and Yili Basin, serving as the response of continuous arc magmatism during the northward subduction of Paleo STO19,63. However, detrital zircon age spectra from these tectonic units (Fig. 5a) do not match with that observed in the MFAB (Fig. 6). Besides, one study16 reports Triassic zircon ages but with eclogite-facies metamorphic origin. This implies that a local deposition for the detritus provenance of studied sandstones in the MFAB could be unlikely, unless a missed Triassic intra-oceanic arc in fact existed, but has not been found yet. However, the relatively high ICV values (Fig. 3b) of studied sandstone, which imply an immature source typical of active continental margin setting45, and the Th-La-Hf concentrations suggest that the detritus in sandstone was not sourced from an oceanic island arc64 (Fig. 3c, d). We emphasize that the identification of minor high-pressure mineral relics-bearing detritus (Fig. 2c–f) and corresponding ~315 Ma detrital zircons with blueschist- and/or eclogite-facies metamorphic origin (Fig. 4c–g, S2) in studied sandstones indicate that at least a few of detrital materials (but not the majority) are deposed from an additional proximal provenance.

Based on the geochemical and geochronological data presented in studied sandstones, we suggest that sediment which filled the MFAB could be derived from a distal source. The western part of ETS, which extends to the eastern side of geographic Chinese South Tianshan until the Xingxingxia Fault (Fig. 1a), is also clamped between the KYCTC and the NTC35. A considerable quantity of Triassic igneous and sedimentary rocks65,66 had been reported in the ETS region, especially those with adakitic geochemical affinity67,68. According to our data compilation for intermediate rocks among the Central Tianshan-Yamansu arc, two major age peaks, respectively at ~230–240 and ~280–290 Ma (Fig. 6e) characterize the ETS arc magmatism, and it is completely distinct from those in the KYCTC and the NTC. Such an age pattern, as well as relatively positive εHf(t) value, of the ETS arc intermediate rocks (Fig. 6e) broadly resembles that of detrital zircon from sandstones in the MFAB (Figs. 4a, b, 6c), implying a potential distal provenance from the ETS region. Additional evidence comes from the sub-angular to moderate-abrased shape (although not all, Fig. 2a, c) of most of detritus and these Triassic detrital zircon grains (Supplementary Fig. 1) in sandstones, which suggests a relatively moderate to long-distance of transport, for example via contour current69, prior to their deposition. This speculation is also supported by the presence of climbing ripples on sandstone outcrop (Fig. 1e) which advocate a deposing environment with high suspension load current70. Moreover, the long-lasting (late Carboniferous-early Triassic) regional nearly W-E trend strike-slip movements (as we compiled in Fig. 7f), mainly along the SCTF (Fig. 1a), could also facilitate the trench-paralleled transportation of detritus drive by contour current. In short, studied sandstones represent reworked sediment of contourite facies in which bottom currents strongly contributed to the reworking and redistribution of turbidite fine sands derived from basin margins (thereby generating mixed turbidite/contourite depositional basin)70,71.

The final closure of the STO

It is challenging to constrain the timing of the final closure of a long-lived accretionary subduction system. Methods such as the timing of a regional unconformity, bimodal magmatism, stitching plutons, and extensional deformation were used to constrain the time of final closure of the STO, which consequently led to different conclusions (as reviewed by ref. 19). Some authors argued that late Devonian to early Carboniferous times12,15,72,73 should be attributed to the timing of the final assembly of southern Altaids occurred along the North Tianshan Suture Zone in the late Carboniferous subsequent to the collision of the KYCTC and NTC. This model is based on the following assumptions that: (1) the late Devonian-early Carboniferous regional angular unconformity was related to the collision of the STB and the KYCTC, (2) eclogite-facies peak metamorphism of the UHP terranes happened at ~350–345 Ma, and (3) top-to-north thrusting of the UHP rocks over the KYCTC is formed consequence of south-ward oceanic subduction. Alternatively, authors19 suggested the final suturing of southern Altaids was achieved by northward subduction of the STO and subsequent collision of the KYCTC and the Karakum-Tarim cratons at ~320 Ma, based on: (1) ~320–330 Ma ophiolite/ophiolitic mélange had been identified as the youngest Mid-ocean ridge (MOR)-type ophiolite along the SCTF (in Guluogou and Wuwamen areas19,74), broadly consistent with the timing of LT-UHP eclogite-facies metamorphism in Akeyazi and Atbashi areas3,29,40; (2) the latest Carboniferous molasse-type conglomerate overlying the Atbashi (U)HP rocks37, and the ~285 Ma post-orogenic S-type leucogranite dike crosscutting the Akeyazi UHP complex17 and (3) the resumption of widespread magmatism in the STB and the NTC at ~270–310 Ma75,76,77,78.

It is worth noting that a substantially younger Permian (~265 Ma) MOR-type ophiolitic mélange79 from the Bindaban area in the eastern Central Tianshan along the SCTF, the late Carboniferous radiolarian80 in chert from Kyrgyzstan Atbashi range, and the late Permian radiolarian81 in clastic rocks from Chinese South Tianshan have been recently identified, further advocating the possible existence of a much younger ocean basin in the southern Altaids. This speculation is echoed by the find of ~300 Ma glaucophane-bearing blueschist to greenschist transitional facies meta-volcanoclastic units in Akeyazi metamorphic complex which suggests the subduction of the Paleo STO was probably still active during the late Carboniferous3. In addition, the suggestion that “the metamorphic age of deeply recovered high-grade protolith formation constrains the timing of oceanic closure and/or continental collision16,21,82” only works in place where late exhumation associated with oceanic closure and continental subduction occurred (Yet, only found in the western Alps, New Caledonia, and Central Cuba)83. This implies that most of metamorphic age of deeply recovered high-grade rock essentially has no direct link with the timing of oceanic closure. Recent work3 has also highlighted that the final juxtaposition, at crustal level, of the diverse sub-units with various metamorphic grades making the metamorphic dome in the AMC could have occurred at around ~280–290 Ma, broadly consistent with the regional ~285 Ma crosscut post-orogenic S-type leucogranite dike17, prior to the STO closure. Further evidence, as constrained by low-temperature thermochronology84, come from the widespread Triassic cooling signals interpreted in response to the final amalgamation of southern Altaids in the Uzbekistan-Tajikistan (Chatkal-Kurama85, Kyzylkum-Nurata86 and Muruntau87 terranes), Kazakhstan-Kyrgyzstan (Karatau-Talas range88) and ETS89 segments of the STOB, echoed with recent studies55,65,66,90 which also reveals abundant sedimentary basins with Triassic MDAs. Moreover, the preservation of climbing ripples (Fig. 1e), petrographic characters of Triassic zircon population (and most of detritus, Figs. 2a, c, S2), and the detrital zircon age pattern of studied sandstones in the MFAB, jointly call for the existence of a relatively board ocean basin (Fig. 8), until the early Triassic, to facilitate relatively moderate to long distance of detritus transport, likely via contour current91 with high suspension load current70, from a potential distal provenance in the ETS region.

Fig. 8: An updated geodynamic model for the Permo-Triassic (~250 Ma) evolution of the South Tianshan Ocean (STO).
figure 8

The trench-paralleled slab geometry “anisotropy” (i.e., temporary flat-slab subduction beneath the CTS region in the KYCTC), during which witnesses its fate from the Triassic to Permian, is highlighted. AC accretionary complex, KYCTC Kazakhstan-Yili-Central Tianshan Continent, NTC North Tarim Craton, NTF North Tarim Fault. The schematic diagrams of various slab geometry are cited from the results of numerical modeling by refs. 103,140. Temperature is indicated by non-dimensional value, and Tm = 1 is considered the dimensionless temperature of the mantle.

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In short, our new data, integrated with the compilation of regional multi-disciplinary evidence, suggest that oceanic subduction was ongoing until the early Triassic in the STOB. Triassic closure of accessory ocean basins, which paleo-geographically belongs to the PAO, were reported among the Altaids, e.g., the Kanguer accretionary mélange in East Tianshan65, the Alxa block51, central Inner Mongolia92, and Solongker suture zone55,56,93,94, implying that the PAO closed almost synchronously along the western, central and eastern parts of the Altaids.

Trench-paralleled slab geometry “anisotropy” and geodynamic implication

A relatively fast and young oceanic plate that subducts beneath the South American plate less steeply, including horizontal slab segments, is known as the Peru-Chile type subduction95, with symbolic features like marginal subduction erosion, inboard migration of upper-plate deformation and weak to absent arc magmatism96. Its rareness97 and remarkable influences on overlying continental plate96 and economically important ore deposits98 make it of great significance and diagnostic, especially for ancient orogens99,100.

To reconcile numerous controversies, we suggest the possibility of flat-slab subduction tectonics, providing an alternative hypothesis (Fig. 8) for the geodynamic model of the STOB. Common models, whatever advocating mainly north- or south-ward subduction beneath the KYCTC17,19 or the NTC15,101, all based on a priori assumption that the down-going STO slab is ultimately coherent object with almost no topographical and geometrical “anisotropy” (i.e., differences). In fact, substantial impacts on the CTS arc magmatism during the Carboniferous, by trench-perpendicular topographical “anisotropy” (i.e., seamount chain102), had been highlighted in region, calling for the possible existence of trench-paralleled slab geometry “anisotropy” during the northward subduction of the paleo STO.

As we presented above, studied early Triassic, HP mineral relics-bearing sandstones likely received detritus mainly from distal provenance-the ETS region whose arc magmatism characterize by similar age (broadly, also the εHf(t), Fig. 7a) pattern of Triassic population with a major early Permian peak (Figs. 4a, b, 6c). In this case, at least a part of geographical ETS region (i.e., the west side of Xingxingxia fault, Fig. 1a), which previously has not been considered yet, is non-negligible with respect to the geodynamic model of the STOB17,63,101. According to our compilation (Fig. 7), the age gap of ~220–280 Ma, between the continuous ETS (~220–330 Ma) and the intermittent CTS (~280–330 Ma, Figs. 5, 7e) arc magmatism, is speculated as the period of the absence of CTS arc magmatism, during which crustal thickening (Fig. 7b) and mantle “signal” soaring (Fig. 7a, c, d) in the ETS are accompanied. Such phenomenon is well observed in the present-day Chilean subduction zone where the area suffered from flat-slab subduction is characterized by to-some-extent broadening (in trench-paralleled and -perpendicular directions) of magmatic arc and subsequent cessation of magmatism97,103,104. In addition, Nd-Hf isotopic decoupling (Fig. 7g) is evident for Permian arc magmatic rocks in the CTS region, suggesting a “zircon effect” because of the addition of subducted terrigenous sediments into magma sources105 and could imply the Permian arc magmatic in the CTS region was derived from a mixed mantle source due to the interaction between arc inherited magmatism and upwelling magma of oceanic lithosphere induced by flat-slab subduction and potential subsequent slab rollback, although such a decoupling was interpreted as the inducing of the activity of Tarim Plume22. Flat-slab subduction could be also facilitated by strong discontinuities in the oceanic (i.e., oceanic plateau and seamount chain106) and overriding plates structure (craton and micro-continent with fairly deep Moho depth91). Similar processes have been identified in region, coincidentally supporting the speculation of temporary flat-slab subduction beneath the CTS region during the northward subduction of paleo STO: 1) Long-lasting Carboniferous-Devonian seamount chain subduction102; the relative thickened crust, which could maintain extra compositional buoyancy, of seamount, oceanic plateau and aseismic ridge effectively prohibit the slab from sinking into the mantle107,108; 2) Localized micro-continents with cratonic lithosphere were identified among the CTS region109,110, and their spatial distribution (as revealed by Hf isotope contour map; Fig. 5c) is broadly in coincidence with the area where the early Triassic to early Permian absence of CTS arc magmatism is confirmed with speculated flat-slab subduction; numerical modeling shows that a thick cratonic root can increase the magnitude of suction acting on the subducting plate due to the mantle wedge flow and this suction effect will vary along strike if craton has a finite width111,112. We do respect the possibility that our observations could not be exclusive to flat-slab subduction hypothesis since other features ideally associated with flat-slab subduction, for example, the crustal contractional deformation (e.g., in Mexico113), migration of arc volcanism (e.g., in Chile, Andes95), distinctive foreland basin record114 and low-angle normal faults as expected in the rolling back flat-slab system115, had not been observed yet in the STOB. These are probably overprinted by the magmatic products during almost simultaneous subduction (compared to that of STO), beneath the KYCTC, of North Tianshan oceanic plate116,117 and the pervasive nearly W-E trend strike-slip deformation3,118,119 along the SCTF (Figs. 1a, 6f), implying a feature of the subsequent poly-phase tectonic evolution of the STOB.

Methods

Bulk-rock major and trace elements analysis

Major elements compositions were analyzed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Major oxides were determined by a Phillips PW1480 X-ray fluorescence spectrometer on fused glass disks. Loss on ignition was measured after heating to 1000 °C. Uncertainties for most major oxides are ca. 2%, for MnO and P2O5 ca. 5%, and totals are within 100 ± 1 wt.%. Whole rock Fe2O3 content is constrained by potassium permanganate titration120. Trace element concentrations were analyzed by sector field inductively coupled plasma mass spectrometry (ICP-MS) using a Finnigan MAT ELEMENT spectrometer at the IGGCAS. Relative standard deviations are within ±10% for most trace elements but reach ±20% for V, Cr, Co, Ni, Th, and U according to analyses of rock standards. Detailed major and trace element analyses are presented in Table S1.

LA-ICP-MS zircon U-Pb dating and trace elements composition

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 (CL) imaging was conducted with a scanning electron microscope at the IGGCAS, to select suitable grains and optimal target sites for the U-Pb dating and Lu-Hf isotopic analyses. U-Pb dating and trace element composition of zircon were simultaneously conducted by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Co., Ltd., China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as in ref. 121. Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e quadrupole ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A “wave” signal smoothing device is included in this laser ablation system122. The spot size and frequency of the laser were set to 60 µm and 6 Hz, respectively, in this study. Zircon 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Each analysis incorporated a background acquisition of approximately 20–30 s followed by 50 s of data acquisition from the sample. An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction, and quantitative calibration for trace element analysis and U-Pb dating123. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3124. Relevant data are listed in Tables S2 and S3.

LA-ICP-MS zircon Lu-Hf isotopic analysis

In-situ zircon Hf isotopic analyses were conducted on the same spots where U-Pb analyses were made. Hf isotopic compositions were determined by a Finnigan Neptune multi-collector ICP-MS equipped with Geolas Plus 193 nm ArF excimer laser at the Wuhan SampleSolution Analytical Technology Co., Ltd., China. A laser spot size of 40 μm and a laser repetition of 8 Hz with an energy density of 15 J/cm2 were used during the analyses. The signal collection model was one block with 200 cycles, with an integration time of 0.131 s for 1 cycle and a total time of 26 s during each analysis. Zircon 91500 was used as an external standard for Hf isotopic analyses and was analyzed twice every five analyses. Replicate analyses of 91500 yielded a mean 176Hf/177Hf ratio of 0.282300 ± 24 (2σ, n = 82), which is concordant with the 176Hf/177H ratios, measured by ref. 125. The detailed analytical procedures are described in ref. 126. Relevant data are listed in Table S4.

Microprobe analysis

In-situ major element compositions of garnet and inclusion minerals were obtained from polished thin sections by electron microprobe analyses at the Wuhan SampleSolution Analytical Technology Co., Ltd., China, with the use of Jeol JXA-8100. Quantitative analyses were performed using wavelength dispersive spectrometers with an acceleration voltage of 15 kV, a beam current of 15 nA, a 3 μm beam size, and 30 s counting time. Natural minerals and synthetic oxides were used as standards, and a program based on the ZAF procedure was used for data correction. Results of representative microprobe analyses of identified HP minerals in studied sandstone samples are presented in Table S5.

Raman spectrum analysis

To identify mineral inclusions in detrital zircons, Raman spectroscopy was performed at IGGCAS using a Renishaw Raman MKI-1000 system equipped with a CCD detector and an Ar ion laser. The laser beam with a wavelength of 514.5 nm was focused on inclusions through ×50 and ×100 objectives of a light microscope. The laser spot size was focused at 1 μm. The reproduction of spectra for the same spot is better than 0.2 cm−1.