Final Amalgamation Processes of the Southern Altaids: Insights from the Triassic Houhongquan Ophiolitic Mélange in the Beishan Orogen (NW China)

The Altaids (or Central Asian Orogenic Belt) is the largest accretionary orogenic collage in the Phanerozoic [1–3] (Figure 1(a)). It is formed by numerous different tectonic terranes, including intra-oceanic/island arcs, microcontinents, oceanic plateaus, and accretionary prisms in the Phanerozoic [3–6]. The Altaids was formed by consumption of the Paleo-Asian Ocean along the South Tianshan–Beishan–Solonker suture zone at the south Altaids [3, 6]. Nevertheless, the final accretionary and amalgamation processes are controversial, varying from the Devonian [7–11] to Triassic [6, 12–14] in different ophiolitic belts. Accordingly, the Beishan orogen, the middle segment of the Tianshan–Beishan–Solonker suture zone, is critical for manifesting the accretionary tectonics and addressing the final time of amalgamation [11, 13, 15, 16].

The Beishan orogen comprises of several continental arcs, intraoceanic arcs, and accretionary prisms [16]. Although previous studies have established several evolution models for the multiblock accretion and their amalgamation processes, the tectonic and evolution in space during the Devonian to Triassic are still debatable. Both subduction settings [16–19] and postcollision rifting [9, 10, 20, 21] have been proposed. In the southern Beishan orogen, voluminous Devonian to Triassic magmatism, sedimentary rocks developed in the Liuyuan accretionary complex belt, which is critical for constraining the tectonic of the Beishan orogen. In past decades, several studies have obtained the ages (~286 Ma to 270 Ma) of gabbros in this belt, but the closure of the Liuyuan-Houhongquan Ocean has not been constrained [9, 16, 22]. The age of the matrices of an accretionary complex is used for constraining the subduction processes, with the youngest matrices close to the final amalgamation time [23]. The spectra of the detrital zircon usefully constrain the sedimentary provenance [24, 25] and its maximum depositional age (MDA) [26–29]. Therefore, the MDA of matrices of the mélange is a valuable method to study the emplacement processes of accretionary complexes.

This study reports new field mapping of the Houhongquan ophiolitic mélange in the Beishan orogen. We also present new geochronological data of the sandstone matrices and geochemical data of mafic rocks from the ophiolitic mélange, aiming to reveal the accretion process of the Houhongquan ophiolitic mélange and put new constraints on the final amalgamation history of the south Altaids.

The Beishan orogen consists of several WE-trending tectonic units, which are bounded by four ophiolitic mélanges (or accretionary complex) as illustrated in Figures 1(b) and 1(b) [11, 16]. The Houhongquan ophiolitic mélange is situated at the eastern segment of the Liuyuan accretionary complex situated between the Huaniushan and Shibanshan arcs (Figures 1(b) and 1(c)).

The Huaniushan arc comprises a set of Precambrian to Permian gneisses, migmatites, schists, sedimentary rocks, and carbonates [11, 16, 30, 31] and Ordovician–Permian arc-related basalt, andesite, rhyolite, and pyroclastic rocks [8, 13, 16, 32]. The sandstones, schists, and mylonites have the maximum depositional ages (MDA) of 293 to 457 Ma [9, 33, 34]. Extensive intrusions are ages from the Ordovician to Triassic, including I-, S-, and A-type granites, adakitic granites, and (ultra-)mafic intrusions (Figure 1(c) and Supplementary Table 2) [8, 9, 18, 22, 35–37].

The Shibanshan arc is the southernmost terrane of the Beishan orogen rooted in the Dunhuang block (Figures 2(b) and 2(c)). Two units constitute the Shibanshan arc: late Paleozoic volcanic-sedimentary unit and the metamorphic Beishan complex unit. The late Paleozoic volcanic-sedimentary unit, which is located at north Shibanshan arc, consists of Devonian–Permian arc-related volcanic rocks, sedimentary rocks, and some carbonates [8, 11, 16]. The Beishan complex unit comprises migmatites, gneisses, (mylonitic) schists, and marbles, with MDAs of 1450–254 Ma and volumes of Precambrian zircons (Figure 1(c) and Supplementary Table 2) [19, 20, 38]. Some Precambrian schists have also been discovered in this unit [10, 20, 39]. The arc-related granites are extensive in the two units and dated from the Carboniferous to Triassic (Figure 1(c) and Supplementary Table 2) [8, 40, 41].

The Liuyuan accretionary complex comprises the Gubaoquan eclogites as well as the Huitongshan, Huaniushan, Liuyuan, Houhongquan, and Zhangfangshan ophiolitic mélange from west to east. The ophiolitic mélanges are composed of ultramafic rocks, gabbros, basalts, cherts, sedimentary rocks, and limestones [9, 13, 16, 42]. The age of the ophiolitic fragments and eclogites varies from 1071 Ma to 270 Ma (Supplementary Table 2). The Liuyuan-Houhongquan mafic-sedimentary rock belt is located along the Liuyuan accretionary complex from Liuyuan to Houhongquan area. The Houhongquan area is well exposed in the eastern segment of this belt (Figures 1(c) and 2). It consists of gabbro, pillow basalt, massive basalt, cleavage basalt, chert, sedimentary rock, and some andesite to dacite block [9, 13, 43]. The gabbros of the Liuyuan area and Yinaoxia area have ages ranging from 270 to 286 Ma [9, 13, 44]. The cherts are of biochemical origin (contain radiolarians and sponge spicules) and deposited in the pelagic environment near a continental margin [43]. The sedimentary rocks have the MDAs of 293 Ma to 234 Ma [9, 45]. To date, the genesis of these basalts and gabbros is strongly debated: one model suggests that they are the ophiolitic remnants [13, 16], whereas another model proposed that these magmatic and sedimentary rocks formed in the rift [8, 9, 46] and were thrusted southward during 230–227 Ma [9] or in a back-arc setting [34] during 300-230 Ma and were thrusted southward during 270-217 Ma [34].

The Houhongquan ophiolitic mélange was exposed within the Mesozoic-Cenozoic sediments. The Houhongquan ophiolitic mélanges have the typical block-in-matrix structure (Figures 2 and 3). Altered gabbros, massive and pillow basalts, and chert fragments (Figure 4) and cleaved conglomerate, sandstone, and siltstone blocks (Figure 5) were thrust-imbricated into NEE-trending dismembered slices (Figures 2(c) and 2(d)). Volumes of basaltic and gabbroic fragments embraced in the cleaved sandstones (Figures 4(b) and 4(c)). The basaltic fragments that are close to the fault are strongly cleaved (Figure 4(c)). Usually, chert fragments are imbricated into basalts or occurred between basalts and sediments (Figures 2(c), 2(d), and 4(d)). The EW-trending granite fragments are located along faults (Figure 2). Sedimentary blocks are in fault contact with basalts (Figures 2(c) and 2(d)). The southwestern part sedimentary rocks are EW trending with S-dipping beddings and cleavages (Figures 2(b), 2(f), and 2(g)). However, the sedimentary blocks in the northern parts of the complex are EW-trending with vertical N-dipping beddings and cleavages (Figures 2(b), 2(h), and 2(i)).

Some turbidites that consist of conglomerates, sandstones, siltstones, and mudstones can be well recognized in the outcrop (Figure 5). They are composed of conglomerate/gravel-bearing sandstones in the bottom (A layer, Figures 5(a) and 5(d)), followed by sandstones with a horizontal bedding (B layer, Figures 5(a) and 5(b)), local cross-bedding (C layer, Figure 5(b)), and finally tuff siltstones, mudstones, and siliceous mudstones (D layer; Figures 5(b) and 5(c)). In general, siltstones and mudstones are highly cleaved (Figure 5(a)).

Thirteen basalts and six gabbros were picked (Figure 2) for geochemical studies. Two basalts and two gabbros were selected for Sr and Nd isotope analyses. Gabbro samples are grey and slightly altered (Figures 4(c) and 4(e)). They consist of medium-grained plagioclase (~50%) and pyroxene (~25%), as well as minor olivine and Fe–Ti oxides (Figure 4(e)). The basalts show variable degrees of chloritization, epidotization, and/or carbonation (Figures 4(a) and 4(b)). Locally, they have circular to elliptical amygdaloidal structures. The basalts consist of plagioclase, pyroxene, and minor olivine and a small amount of pyroxene phenocryst in some samples (Figure 4(f)).

As described above, the sedimentary blocks are composed of conglomerates, sandstones, siltstones, and mudstones. The conglomerates mainly consist of angular basalts, rhyolites, cherts, and minor quartz fragments (Figure 5(d)). The coarse-grained sandstones comprise angular plagioclase and basalt lithic fragments (e.g., sample 21T51, Figure 5(e)). The sandstones comprise of angular quartz, plagioclase, and minor lithic fragments (e.g., samples 21T50 and 21T55). The siltstones consist of quartz and tuff/clay (sample 21T56, Figure 5(f)).

The analysis results are presented in Table 1. The samples 21T48-7 and 21T57-5 are too altered (⁠$LOI>5$%), and we have excluded them from the dataset. These rocks plot tholeiitic basalt as shown in Figure 6. The compositions of these mafic samples are slightly correlated (Supplementary figure 2).

Eleven basalt samples have a relatively wide content of SiO₂ (46.5–54.3 wt. %), Al₂O₃ (12.8–15.1 wt. %), CaO (8.2–15.9 wt. %), MgO (3.1–6.9 wt. %), and Mg^# values of 43–61. These basalts exhibit relatively moderate to high contents of TiO₂ (1.6–3.3 wt. %). They have a wide range of Cr (10–280 ppm) and Ni (6.6–62.3 ppm) contents. The basalts are slightly enriched in LREEs (⁠$La/YbN=1.2–2.2$⁠) and have slightly negative Eu anomalies (⁠$Eu∗=0.8–1.0$⁠) ([47], Figure 7(a)). The basalts can be divided into two distinct groups: one group has slight depletions of Nb and Ta (Th/Nb $PM=1.08−2.38$⁠) (black lines in Figure 7(b)) whereas another group shows depletions of Th relative to Nb and Ta (⁠$Th/NbPM=0.86−0.96$⁠) (red lines in Figure 7(b)).

Six gabbro samples have SiO₂ contents of 46.9 to 52.4 wt. %, and TiO₂ contents of 2.0 to 2.6 wt. %. They are characterized by Al₂O₃ ranging from 14.2 to 16.4 wt. %, CaO ranging from 7.3 to 9.3 wt. %, and MgO ranging from 4.3 to 4.5 wt. % (⁠$Mg#=46−63$⁠), and $K2O+Na2O$ ranging from 3.4 to 5.3 wt. %. The gabbro samples have Cr and Ni contents of 70–210 ppm and 32–64 ppm. They show slight enrichment in LREEs (⁠$La/YbN=1.7–2.7$⁠) and weak Eu anomalies (⁠$Eu∗=0.9–1.0$⁠) in the chondrite-normalized REE diagram ([48]; Figure 7(c)). The gabbro samples also exhibit two distinct groups [46]: one group shows slight depletions of Nb and Ta (⁠$Th/NbPM=1.45−1.61$⁠) (black lines in Figure 7(d)), whereas another group shows depletion patterns (⁠$Th/NbPM=0.85−0.96$⁠) (red lines in Figure 7(d)).

The results of Sr and Nd isotopic analyses are summarized in Table 2 and shown in Figure 8. Two basalt samples have (⁸⁷Sr/⁸⁶Sr)_i values of 0.704748 and 0.704859 and ε_Nd (t) values of +6.2 and +6.3, respectively. The two gabbro samples have (⁸⁷Sr/⁸⁶Sr)_i values of 0.705602 and 0.705863 and ε_Nd (t) values of +5.8 and +6.0. Because the samples are slightly altered by the seawater (e.g., chloritization, epidotization), the Sr isotopes moved along the trend of the seawater alteration (Figure 8) [49].

Overall, 393 analyses of zircons from four sedimentary samples yielded 374 concordant ages (⁠$concordance%>90%$ or <110%). Different methods are suggested to constrain the MDA of sedimentary rocks in international community [26, 27], including the youngest one, the three youngest grains, and the youngest peak. Here, we adopt the weight mean age of the three youngest grains if they overlap at a 2$σ$ uncertainty or the youngest one if the weight mean age of the three youngest zircons far exceed the 2$σ$ uncertainty (MWSD is poor).

The zircons of the sample 21T50 are euhedral, and some are slightly rounded. They range in length of 50–120 μm with length/width ratios of 1.2–2.0, with oscillatory zones (Figure 9(a)). They have Th/U values of 0.18–2.49. One hundred grains were analysed, and ninety grains yield concordant ages with a multipeak spectrum and peaks ca. 268 Ma (~43.3%) and ca. 390 Ma (~40.1%) and some Precambrian ages scattered at 615 Ma, 878 Ma, 1246 Ma, and 2144 Ma (Figure 9(b)). The youngest three zircons (⁠$222±5$ Ma, $235±3$ Ma, and $235±5$ Ma) yield a weight mean age of $232±16$ Ma (Figure 9(a)); however, the MWSD is poor (⁠$MWSD=10$⁠), so we suggest that the MDA of the sandstone is the youngest zircon gain (222 Ma).

Zircons from sample 21T55 in the eastern part of the Houhongquan ophiolitic mélange are euhedral and are 50–150 μm long. They display oscillatory zonation (Figure 9(c)) and Th/U ratios of 0.09–2.61. Ninety-nine grains of 100 analysed grains yielded concordant ages with a multipeak spectrum. The two major peaks are at ca. 262 Ma (~63.3% of the total) and ca. 398 Ma (~29.3%) (Figures 9(c) and 9(d)). There are eight Precambrian zircon ages ranging from 902 Ma to 2484 Ma (~7.4%). The three youngest zircons yield $233.8±2.3$ Ma (⁠$MSWD=0.39$⁠) (Figure 9(c)), indicating the MDA of the sandstone.

Zircons from sample 21T51 have irregular lengths of 50–120 μm and clear oscillatory zoning (Figure 9(e)). The analysed zircons show Th/U ratios of 0.36–1.49.87 concordant grains (~96%) displaying two major peaks at ca. 270 Ma (~86.7%) and ca. 402 Ma (~11.1%). Only two Precambrian zircons have 740 Ma and 1040 Ma, respectively (Figures 9(e) and 9(f)). The youngest three zircon ages yield $263.5±2.8$ Ma (⁠$MSWD=0.001$⁠) (Figure 9(e)), which represent the MDA for the sandstone.

Zircons from sample 21T56 are euhedral which are 80–120 μm long (⁠$length/width=1–1.5$⁠), with sharp oscillatory zones (Figure 9(g)). Their Th/U values vary from 0.60 to 1.70. Ninety-eight concordant analysed grains (98%) have a single peak at ca. 270 Ma (~99% of the total) (Figure 9(h)). The three youngest zircons yield $263.4±2.5$ Ma (⁠$MSWD=0.06$⁠) (Figure 9(g)), which is the MDA of the sandstone matrix.

The basalt and gabbro are tholeiitic magma with relatively high ε_Nd (t) (+5.8–+6.3 and low (⁸⁷Sr/⁸⁶Sr)_i (0.704748–0.70563) (Figure 8) ([45, 50], this study), suggesting that they were sourced from the depleted mantle. However, their geochemical data also indicate that these mafic blocks are derived from the different types or ages of oceanic crust. The compositions of the samples are uncorrelated, suggesting they have different sources (supplementary figure 2). Their slight enrichment of LREE (Figures 7(a) and 7(c)) is like that of E-MOR- and SSZ-type ophiolites. They also display back-arc basin basalts (BABB) and N-MORB geochemical signatures with slight depletions of Nb-Ta compared to the Th and La (⁠$Th/NbPM=1.08−2.38$⁠) and depletions of Th relative to Nb and Ta (⁠$Th/NbPM=0.86−0.96$⁠) (Figures 7(b) and 7(d)) [51, 52]. They are also plotted in the N-MORB/BAB MORB realm on the V–Ti/1000 diagram ([53], Figure 10(a)). On the Hf-Th-Ta diagram ([54], Figure 10(b)), they are plotted in the N-MORB field and the arc-related basalt field, which coincide with the Lau Basin and Mariana Trough. All of these suggest that mafic fragments are composed of N-MOR- and SSZ-type ophiolites.

Previous works have tried to place a constraint on the age of the Houhongquan ophiolitic mélange, but none of them have been successful. Moreover, these rocks were considered coherent strata instead of being part of the tectonic mélange, and their ages were constrained by the felsic rocks around the Houhongquan ophiolitic mélange according to stratigraphic correlations of the regional sedimentary rocks [45, 50, 55]. For example, the rhyolite and the dacite south to the mélange have zircon ages of $291.1±2.6$ Ma and $289.5±2.3$ Ma [50], and the calcareous sandstone has an age of $275.8±1.4$ Ma in the Houhongquan ophiolitic mélange [45].

The detrital zircon age of the sedimentary rocks can provide a vital constraint on the MDA [27, 28]. The interval time between MDA and the true depositional age can be very short in the arc-related basins and accretionary prisms [26]. The clasts of conglomerates are mainly the angular chert, rhyolite, and basalt (Figure 5(d)), the coarse-grained sandstone contains the angular/broken basalt and rhyolite fragments (Figure 5(e)), and most of the zircons are euhedral and weak/not rounded (Figure 9), indicating they have proximal sources and deposited in the subduction zone [26, 56]. Therefore, the MDA of detrital zircons can represent the deposition time of these rocks [26]. The sandstone samples (21T50 and 21T55) yielded Late Triassic MDAs of $222±5$ Ma and $233.8±2.3$ Ma, respectively (Figures 9(a) and 9(b)). The sandstone samples (21T51 and 21T56) yielded Middle Permian MDAs of $263.5±2.8$ Ma and $263.4±2.5$ Ma, respectively (Figures 8(c) and 8(d)). In addition, Guo et al. [45] reported a calcareous sandstone in the Houhongquan ophiolitic mélange, which has a MDA of $275.8±1.4$ Ma. All the results for each sample indicate that the ages of sedimentary blocks range from Middle Permian to Late Triassic. Previous geochronological studies for the gabbro of the Liuyuan-Houhongquan mafic-sedimentary rock belt revealed that they have ages of 270 Ma to 286 Ma [9, 13, 44] (Supplementary Table 2, Figure 11(a)). Some sedimentary blocks with MDAs of 234 Ma, 268 Ma, and 285 Ma (Figure 11(a)) were also reported in the Liuyuan-Houhongquan mafic-sedimentary rock belt [9]. Thus, the Liuyuan–Houhongquan Ocean still existed at 222 Ma. This conclusion is consistent with the rift model in which the Liuyuan basin closed during 230–227 Ma [9] and the back-arc basin model in which the Liuyuan back-arc basin closed until 217 Ma [34]. Therefore, no matter the Liuyuan–Houhongquan oceanic basin is a rift basin, a back-arc basin, or the Paleo-Asian Ocean, it did not closed until ca. 222 Ma.

As discussed above, the components and petrology of the sandstones suggest they were sourced from the nearby arc [26, 56]. The Houhongquan ophiolitic mélange is just positioned between the Huaniushan arc in the north and the Shibanshan arc in the south (Figure 1(b)). Previous studies revealed that the Huaniushan arc is a Japan-type arc during Early Ordovician-Permian, and the Shibanshan arc is an Early Devonian-Permian continental margin arc on the Dunhuang block [16, 35, 41]. They mainly comprise Triassic–Ordovician arc-related basalts, andesites, rhyolites, and granitoids (Figures 2, 11(b) and 11(c); Supplementary Table 2) [18, 19, 22, 30, 41] and some Precambrian gneisses and schists [10, 20, 39]. Usually, the sedimentary rocks of the two arcs contain voluminous Precambrian zircon grains [19, 20, 31, 38, 57] (Figures 12(d), 12(f)–12(h)).

Our studies reveal that the sandstones (21T50, 21T51, 21T55, and 21T56) define two groups of age patterns. (1) The sedimentary samples (21T51 and 21T56) show Middle Permian MDAs with a single peak of ca. 274 Ma and a small amount of ages ranging from 362 Ma to 447 Ma (9 grains, ~5%) and minor Precambrian ages (2 grains, ~1%). These characteristics are like the detrital zircon age patterns of the sedimentary rock in intraoceanic arc setting, e.g., the Char and Zharma zones of eastern Kazakhstan [58], as well as the sediments of the Talkeetna arc in Alaska [59], of the Outer Melanesian Arc in the Fiji Islands behind the Tonga arc [60] and of the Izu-Bonin-Marianas arc [61]. These age compositions are different to the Permian sandstones in the Huaniushan arc and the Shibanshan arc (Figures 12(c), 12(e)–12(g)) but are consistent with two samples in the northern Shibanshan arc (Figure 12(d)). The detrital zircon age patterns of these sedimentary rocks suggest an intraoceanic arc in the Liuyuan–Houhongquan Ocean, which do not contain Precambrian zircon grains. Furthermore, the mafic rocks of the Houhongquan ophiolitic mélange plot in the same fields of Mariana arc and Lau Basin as shown in Figure 10(b), also suggesting that they deposited in the intraoceanic subduction zone. (2) Samples 21T50 and 21T55 have Triassic MDAs. Their detrital zircon age spectra are similar to the magmatic and sedimentary records from the Huaniushan arc and the Shibanshan arc with dominant age peaks from Triassic to Devonian and some scattered Precambrian ages (Figures 12(c), 12(e)–12(g)). These two sedimentary samples (21T50 and 21T55) were probably sourced from either the Huaniushan arc or the Shibanshan arc.

In summary, an independent intraoceanic arc was the primary source for the Permian matrix of the Houhongquan ophiolitic mélange. The Huaniushan arc or the Shibanshan arcs were the provenances for the Triassic sandstone matrixes.

Our geological mapping reveals that the mafic rocks are thrust-imbricated within the Houhongquan ophiolitic mélange complex. The basalts and gabbros demonstrate the N-MOR- and SSZ-type ophiolitic geochemical signatures. The chert blocks contain radiolarian and sponge ancient needles ([43], Supplement Figure 1). All the data suggest that they are the relic fragments of the ophiolite [62, 63]. In addition, zircon U–Pb ages reveal oceanic crustal blocks in the Liuyuan accretionary complex age ranging from 1071 Ma to 270 Ma, as previously described (Figure 11(a)). The sedimentary and metamorphic sedimentary blocks have the MDAs that range from 457 Ma to 222 Ma ([9, 64], this study).

In the Eastern Tianshan to Beishan orogen, no consensus regarding the closure timing of the Paleo-Asian Ocean emerged, to date, the proposed timing ranging from Devonian to Triassic. The main models proposed by different authors include (1) Devonian [10, 65], (2) Carboniferous [8, 9, 11], (3) Latest Carboniferous–Early Permian [66], (4) Late Permian [3, 16], and (5) Late Triassic [41, 67–70]. Our new geochronological results of the Houhongquan ophiolitic mélange indicate that the Liuyuan-Houhongquan Ocean was still subducting at ca. 222 Ma.

As discussed before, although geologists hold different viewpoints on oceanic basin type of the Liuyuan–Houhongquan Ocean during the Permian to late Triassic, their comprehensive studies also suggest that the Liuyuan–Houhongquan Ocean basin closed by the southward thrust and folded during the Late Triassic. For example, (1) Wang et al. [9] suggest that the Liuyuan rift basin closed during 230–227 Ma, (2) Tian & Xiao [34] suggest that the Liuyuan back-arc basin closed until 217 Ma, and (3) the branch of the Paleo-Asian Ocean closed posterior to the Late Triassic in the Eastern Tianshan–Beishan region [69, 71].

In recent years, some lines of evidence have reported the existence of the Paleo-Asian Ocean in the Late Triassic in the Tianshan–Beishan orogens: (1) Late Triassic (ca. 234 Ma) sedimentary matrix in the Kanguer mélange [67]; (2) multiple sources before the Permian–Triassic, as indicated by sedimentary rocks between the Dananhu and Yamansu arcs [68]; (3) Middle to Late Triassic adakite-like, arc-related granites and mafic plutons from eastern Tianshan to Beishan orogen [9, 22, 34, 72–74]; (4) Mid–Late Triassic eclogite facies metamorphic rocks and forearc sedimentary rocks in the western Tianshan [14, 75–77]; (5) tectonic SN-trending thrusting and folding occurred [9, 34]; and (6) tectonic relations also support that the oceanic basins closed after ca. 232 Ma [3, 15, 69, 71]. Combined with previous data, we propose a new model for the eastern Tianshan-Beishan orogen during Permian to Triassic.

• (1)

  During Middle Permian, an intraoceanic arc developed in the Liuyuan–Houhongquan Ocean and deposited sandstone with Middle Permian MDAs and a single peak of detrital zircon age (Figure 13(a)). The intraoceanic subduction zone was amalgamated and welded to the Shibanshan continental arc during Middle Permian to Late Triassic and led to the emplacement of these intraoceanic arc-sourced sandstone blocks in the forearc mélange [33, 34]

• (2)

  During Late Triassic, the Ocean was not closed in the eastern Tianshan and Beishan orogen (Figure 13(b)). Moreover, at least two oceanic basins of the Paleo-Asian Ocean existed, e.g., the Liuyuan–Houhongquan Ocean and the Kanguer Ocean [67, 73]

The Liuyuan–Houhongquan Ocean may be a limited narrow ocean basin in the Late Triassic because the sandstone blocks in the Houhongquan ophiolitic mélange contain a certain proportion of Precambrian zircons compared to the Permian sandstones. The Kanguer Ocean is a limited narrow ocean basin that cannot prevent material exchange from the Dananhu and Yamansu arcs [67, 68, 70]. The Liuyuan–Houhongquan Ocean subducted bidirectionally below the Huaniushan arc and the Shibanshan arc, resulting in voluminous arc-related magmatism: (1) in the Huaniushan arc, the 240–238 Ma adakite-like granites [22], the 249–234 Ma arc-related granites [72, 74], the calc–alkaline 228 Ma mafic rocks [78], and the 225–217 Ma I-type granite-related A₂-type high-fractionation orogenic granites [22, 70]; (2) in the Shibanshan arc, the 241 Ma arc-related granite was also reported [19]. Combining with the Mid–Late Triassic eclogites and forearc sedimentary rocks in the Western Tianshan [14, 75, 77, 79], we conclude that the Tarim Craton and Dunhuang block finally docked at the Altaids at the Late Triassic.

• (1)

  Geological mapping and field relationships indicate that the Houhongquan ophiolitic mélange is characterized by top-to-south thrusting with block-in-matrix structure. The relics of the subducted oceanic plate contains the blocks of gabbro, pillow basalt, massive basalt, and radiolarian cherts

• (2)

  The basalts and gabbros in the Houhongquan ophiolitic mélange contain N-MOR- and SSZ-type ophiolite

• (3)

  The MDAs of the sandstone blocks from the Houhongquan ophiolitic mélange are $222±5$ Ma, $233.8±2.3$ Ma, $263.4±2.5$ Ma, and $263.5±2.8$ Ma, respectively. These dating results indicate that the Liuyuan–Houhongquan Ocean closed at ca. 222 Ma

• (4)

  The sandstone blocks from the Houhongquan ophiolitic mélange display two types of detrital age patterns. The Middle Permian MDA sandstones have a single Middle Permian with a Devonian peak, indicating an intraoceanic arc. However, the Late Triassic MDA sandstones have multiple peaks and contain Precambrian zircons, suggesting that they were sourced from a continental arc. These results indicated that the Liuyuan–Houhongquan Ocean was a multi-island ocean in the Middle Permian and that the intraoceanic arc docked on the Shibanshan arc in the Late Triassic and terminated the Paleo-Asian Ocean in this part of the southern Altaids

The data for this study are available in this manuscript and supplementary material.

The authors declare that they have no conflicts of interest.

This study was financially supported by the Third Xinjiang Scientific Expedition Program (2022xjkk1301), the National Natural Science Foundation of China (41888101, 41822204), One Hundred Talent Program of the Chinese Academy of Sciences (E2250403), and Science and Technology Major Project of Xinjiang Uygur Autonomous Region, China (2021A03001 and 2022A03010-1), and Youth Innovation Promotion Association of the Chinese Academy of Sciences (2022446). This contributes to IGCP 622, IGCP 669, and IGCP 710.