Geochemistry and geochronology of basic igneous rocks in Bairin Right banner, southeastern inner Mongolia, China: Implications for the final closure of the Paleo—Asian Ocean along the Xar Moron suture zone

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

The ophiolite belts distributed in central Inner Mongolia can be divided into the Ondorsum ophiolite belt, Solonker-Mandula ophiolite belt, Kedanshan-Jiujingzi ophiolite belt (Xar Moron suture zone), Jiaoqier ophiolite belt, Daqing pasture-Diyanmiao ophiolite belt, and Erenhot-Hegenshan ophiolite belt ( Figure 1B; Song et al., 2015;Li et al., 2019). In recent years, many researchers have carried out studies on geochronology, petrology, and petrogeochemistry of the ophiolites (Xiao et al., 2003;Miao et al., 2008;Jian et al., 2012;Liu et al., 2013;Song et al., 2015;Zhang et al., 2015;Zhou et al., 2015;Yang et al., 2017;Zhu et al., 2017), which have provided reliable geological data for the study of regional tectonic evolution. However, the situation of the Xar Moron suture zone is relatively conjectural due to limited research which restricts the comprehensive understanding of the time and location of the final      collision between the Siberian and the North China plates. In this study, based on regional geological surveys and a detailed laboratory comprehensive study, we for the first time identified newly discovered basic igneous rocks in Bairin Right Banner, Inner Mongolia, and carried out petrology, geochemistry, and LA-ICP-MS zircon U-Pb isotope geochronological studies with the aim to establish their genesis and geodynamic setting. Our results have robust implications for the evolution of the Paleo-Asian Ocean, the time and location of the final suturing, and the regional tectonic evolution of the Siberian and the North China plates.
2 Regional geology and petrographic features 2.1 Regional geology The Xar Moron suture zone is located in the Eastern part of the Ondor Sum subduction accretion complex belt, consisting mainly of Kedanshan (Jian et al., 2008;Zhang et al., 2015), Xingshuwa (Song et al., 2015), Banlashan (Miao et al., 2008), and Jiujingzi (Liu et al., 2016) ophiolite fragments, which are distributed along the northern edge of the Xar Moron River in an east-west direction. The main rock types include serpentine peridotite, pyroxenite, gabbro, chert, and metamorphosed basalt. Most of them are isolated blocks in structural contact with the surrounding rock (Wang and Fan, 1997;Li et al., 2007;Liu et al., 2016). The Southern part of the study area is the Bainaimiao arc belt, while the Northern part is the Ondor Sum subduction accretion complex belt ( Figure 1B; Xiao et al., 2003;Li et al., 2019). From the Paleozoic to the Mesozoic, a series of intense calc-alkaline magmatic activities and marine volcano-sedimentary strata occurred (Li et al., 2007;Zhang et al., 2009;Song et al., 2015;Liu et al., 2016;Li et al., 2019). The Middle Permian Dashizhai Formation (P 2 d) is mainly composed of intermediate-acid volcanic rocks and pyroclastic rocks. The Middle Permian Zhesi Formation (P 2 zs) is a combination of sandstone slate with limestone lenticular bodies in neritic and littoral facies, abundant in fossils such as brachiopods. The Upper Permian Linxi Formation (P 3 l) is mainly composed of

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Petrographic features of the basic igneous rocks
The newly discovered basic igneous rocks are located about 12 km east of Bairin Right Banner, Inner Mongolia ( Figure 2A). These rocks are in fault contact with the Late Paleozoic sedimentary formation and the Middle Permian Dashizhai Formation, which are covered by Quaternary sediments. These basic igneous rocks were intruded by a few diorite porphyrite dikes and quartz dikes. The rock types mainly include pillow basalt, diabase, and massive basalt ( Figure 2B). The strike of the fault varies from 300°-360°, with a dip angle ranging from 40°-55°.
The massive basalt is gray-black to black-green and, due to weathering, shows a gray-brown color, with a porphyritic massive structure ( Figure 3A). Plagioclase phenocryst (varying from 5% to 12%) is euhedral, narrow platy surrounded by a vitreous microcrystalline plagioclase matrix. The particle size of plagioclase phenocryst varies from 0.1 mm to 0.5 mm. Mafic minerals are mostly altered into epidote and chlorite ( Figure 3D).
The diabase is dark gray and, due to weathering, shows a greyblack color, with massive structures ( Figure 3B). The rock is composed of plagioclase (varying from 62% to 68%) and pyroxene (varying from 32% to 38%). The plagioclase is subhedral platy columnar, with grain size varying from 0.3 mm to 1.2 mm. Pyroxene is granular, columnar, light brown, shows high protrusion, fills the gaps in plagioclase ( Figure 3E), and has second-order interference colors with particle sizes varying from 0.4 mm to 1.1 mm.
The pillow basalt appears gray-green but its color when weathered is mostly black-green and gray-brown. Weathering-induced small carbonate cracks are observable and the rocks show pillow structures ( Figure 3C). The pillow structures are relatively wellpreserved, displaying elliptic, spherical, and renal shapes with varying sizes ranging from 25 cm to 65 cm wide and 100 cm to 150 cm long. The basic plagioclase (varying from 3% to 10%) phenocrysts are euhedral platy, showing twining, and cleavages, with vitreous microcrystalline plagioclase matrix. The plagioclase microcrystals are elongated and show a hollow skeleton crystal structure. Particle size ranges from 0.1 mm to 0.5 mm, and the microcrystalline spaces are mostly filled by vitreous materials ( Figure 3F).

Samples and methods
For whole-rock geochemical analysis, four representative samples (K2-1-K2-4) of massive basalt, three representative samples (L1-1 to L1-3) of diabase, and nine representative samples (S3-1 to S3-9) of pillow basalt were collected from the Bairin Right Banner area ( Figure 2B). All the samples were used to conduct different analyses. Among them, a sample (BL0002) of massive basalt (BL0002), a sample (BL0001) of diabase, and a sample (BL0003) of pillow basalt were selected for LA-ICP-MS zircon U-Pb dating analysis. The sampling locations are shown in Figure 2B

LA-ICP-MS zircon U-Pb analysis
LA-ICP-MS zircon U-Pb geochronological analysis was done in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences. The instrument used for the analyses was an Analyte Excite ICP-MS of Teledyne Cetac Technologies Co., Ltd. with a matching New Wave UP 193 laser ablation system. The diameter of the laser ablation spot beam was 32 μm, with 20 μm-40 μm laser ablation depth. The internationally accepted standard zircon GJ-1 was used as the external standard for age calculation, and the element content was corrected by NIST SRM610, a silicate glass synthesized by the National Bureau of Standards and Materials, and 29 Si was used as the internal standard element (Liu et al., 2010a;Hu et al., 2011). The data processing was done by ICPMSDataCal 8.4 program (Liu et al., 2010a, b), while age calculation and concordant diagrams were done using ISOPLOT ver. 3.0 (Ludwig, 2003;Yuan et al., 2004). U-Pb dating results are shown in Table 1.

Major and trace element analyses
The major elements, rare earth elements, and trace elements of the rocks were analyzed in the Analysis and Testing Research Center at the Beijing Institute of Geology, Nuclear Industry. The major element analysis instrument was a Philips PW2404 X-ray fluorescence spectrometer (XRF), with an analysis deviation of 1%-5%. The analytical instrument for trace elements and rare earth elements was a Finnigan MAT Element I high-resolution plasma mass spectrometer (HR-ICP-MS), with an analytical deviation of 1%-3%. The test basis and method refer to DZ/T0223-2001 inductively coupled plasma mass spectrometry (ICP-MS), with a room temperature of 20°C and relative humidity of 30% during the test. The international standards AGV-1, BHVO-1, and BCR-1 were used to calibrate the element concentration of the samples (Li et al., 2002). The results of the analyses of major and trace elements are shown in Supplementary Table S1.

Zircon U-Pb ages
Twenty-five zircon grains with grain sizes ranging from 50 μm-100 μm long and length-to-width ratios ranging from 1:1 to 2:1 from the massive basalt were analyzed ( Figure 4A). The zircon U-Pb ages yielded two groups of age suits ranging from 2306.3 ± 22.8 Ma to 271.5 ± 2.9 Ma. The first group contains twenty-three zircons grains, and ages were traddled on or near the concordant curve ( Figure 5A) on the concordant diagram. Zircon grains varied in size, and most of them were rounded-sub-rounded. The cathodoluminescence images were translucent-transparent, while some were black. Some zircon grains showed inherited cores, and some of them had zonal structures, showing the characteristics of captured zircon, with Th/U ratios ranging from 0.07 to 1.01. Except for Frontiers in Earth Science frontiersin.org one zircon grain (No. 12), all the other Th/U ratios were >0.1, which showed the characteristics of magmatic zircon (Corfu et al., 2003;Wu and Zheng, 2004). There were thirteen 207 Pb/ 206 Pb zircons older than 1Ga, the oldest 207 Pb/ 206 Pb age was 2519.1 ± 18.8 Ma, and the youngest 206 Pb/ 238 U age was 600.2 ± 6.2 Ma. Two obvious peak intervals existed in this age group. The zircons yielding the concordant mean ages of 961.9 ± 7.2 Ma (MSWD = 0.20, n=8) and 1084 ± 20 Ma (MSWD = 0.105, n = 4) were the captured zircons ( Figures 5B, C). Zircons No. 15 and No. 23 were transparent-translucent, short-columnar-longcolumnar, had relatively perfect crystallization, and wide magmatic oscillatory zoning, with Th/U ratios between 0.63 and 0.64, showing the characteristics of magmatic zircon (Corfu et al., 2003;Wu and Zheng, 2004). The concordant mean age of 272.0 ± 2.8 Ma (MSWD = 0.30, n = 2) represented the formation age of massive basalt ( Figure 5A).
Sixteen zircons with grain sizes ranging from 60 μm to 110 μm long and length-to-width ratios ranging from 1:1 to 3:1 from diabase were analyzed ( Figure 4B). The grains varied in size, were rounded, columnar, and angular, and most of them were relatively developed. The U-Pb ages ranged from 2629.9 ± 25.6 Ma to 235.7 ± 3.9 Ma, and could be divided into three groups according to the crystal morphology and internal structure of zircon ( Figure 5D). The first group contained seven zircon grains, of which 2629.9 ± 25.6 Ma, 2517.5 ± 36.0 Ma, and 2510.9 ± 27.4 Ma were the oldest ages, while the other 206 Pb /238 U ages ranged from 472.3 ± 5.0 Ma to 342.6 ± 3.6 Ma. The zircon grains were rounded and irregular in morphology, and the cathodoluminescence images were transparent and translucent in color. Some of them had inherited cores and weak bands. Most of the zircon grains were irregularly fractured residues, which were characteristic of captured zircon. The second group contained four zircon grains, the 206 Pb/ 238 U ages ranged from 305.1 ± 3.5 Ma to 303.3 ± 3.1 Ma, and the Th/U ratios varied from 0.82 to 1.10, indicating the characteristics of magmatic zircon (Corfu et al., 2003;Wu and Zheng, 2004). The cathodoluminescence images were transparent and translucent, while some images were dark. Some zircons had inherited cores, some had weak zones, and most were irregularly fractured residues; the zircon grains were relatively small, which indicated they might have been captured by magma during an eruption. These zircons in the second group yielded a concordant mean age of 302.3 ± 3.3 Ma (MSWD = 0.21, n = 4)

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frontiersin.org ( Figure 5E). The zircons in the third group were transparenttranslucent, with euhedral platy morphology and relatively perfect crystallization, exhibiting well-developed wide oscillatory zoning. The 206 Pb/ 238 U ages varied from 262.7 ± 2.5 Ma to 259.8 ± 2.9Ma, The Th/ U ratios were in the ranges of 0.33-1.25, showing the characteristics of magmatic zircon (Corfu et al., 2003;Wu and Zheng, 2004). These zircons yielded a concordant mean age of 261.5 ± 3.2 Ma (MSWD = 0.12, n = 6), which represented the formation age of diabase ( Figure 5F).
Sixteen zircon grains with grain sizes ranging from 50 μm-100 μm long and length-to-width ratios ranging from 1:1 to 2:1 from pillow basalt were analyzed ( Figure 4C). The zircon grains varied in size, and most of them showed rounded and broadly platy morphology. The zircon U-Pb ages ranged from 1830.6 ± 19.0 Ma to 255.9 ± 2.8 Ma, and could be divided into three groups according to the crystal morphology and internal structure of zircon ( Figure 5G). In the first group, the zircon ages were 1830.6 ± 19.0 Ma, 825.2 ± 7.5 Ma, and 540.2 ± 6.6 Ma, and the zircon grains were relatively small, rounded, and irregular, with inherited cores. Some weak bands and dark cathodoluminescence images showed the characteristics of captured zircon. In the second group, the ages of 206 Pb/ 238 U varied from 322.3 ± 3 Ma to 303.7 ± 3.5 Ma, and the Th/U ratios varied from 0.48 to 0.92, indicating the characteristics of magmatic zircon (Corfu et al., 2003;Wu and Zheng, 2004). These zircons mainly occurred as euhedral-subhedral grains, with transparent and translucent cathodoluminescence images, and some of them showed inherited cores and zoning. Most of the zircon grains were relatively small and irregularly fractured, which indicated they might have been captured by magma during an eruption. The zircons in the second group produced a concordant mean 206 Pb/ 238 U age of 320.6 ± 2.4 Ma (MSWD = 0.036, n = 7) ( Figure 5H). The zircons in the third group were transparent-translucent with euhedral platy morphology, had relatively perfect crystallization, and exhibited well-developed wide oscillatory zoning. The Th/U ratios were between 0.46 and 0.92, which showed the characteristics of magmatic zircon (Corfu et al., 2003;Wu and Zheng, 2004). These zircons in the third group yielded a concordant mean 206 Pb/ 238 U age of 256.5 ± 2.5 Ma (MSWD = 0.18, n = 5), which represented the formation age of pillow basalt ( Figure 5I).

Trace element geochemistry
The sum of the total contents of REEs (ΣREE) of the massive basalt, diabase, and pillow basalt samples ranged from 50.50-100.56 × 10 −6 (average 79.49 × 10 −6 ). Their ΣREE were in between those of oceanic island basalt (OIB,198.96 × 10 −6 ) and enriched mid-ocean ridge basalt (E-MORB, 49.09 × 10 −6 ), and close to E-MORB. Their (La/ Yb) N values and δEu values ranged from 1.80-3.25 and 0.95-1.14, respectively. In the chondrite normalized rare earth element (REE) diagrams (Figure 7), their REEs patterns were characterized by slightly LREE-enrichment and nearly flat HREE like that of E-MORB, with the slightly positive Eu anomalies. In the primitive mantle-normalized incompatible element diagrams (Figure 7), the massive basalt and diabase samples were relatively enriched in large ion lithophile elements (LILEs) (e.g., Rb, Ba, and U), while depleted in high field strength elements (HFSEs) (e.g., Nb, Ta). The pillow basalt samples were relatively depleted in Rb and Th and enriched in Ba and U.

Implications of zircon U-Pb ages
The analyzed zircons from the samples of the massive basalt, diabase, and pillow basalt exhibited a wide range of ages, i.e., 2629.9 ± 25.6 Ma to 255.9 ± 2.8 Ma. Consequently, we collected a set of recently published zircon ages to compare with the ages obtained in this study for better interpretation (Figure 8). Based on the age data, we argued that the zircon grains might have been derived from various sources. The zircons yielding ages greater than 1 Ga might have been sourced from the North China Craton (NCC) (Zhao et al., 2000;Lu et al., 2020), while zircon grains of ages from 1 Ga to 600 Ma might have been sourced from Precambrian micro-massif in the adjacent area, which is considered as an important tectonic feature of the Xingmeng orogenic belt to the north of the NCC (Rojas-Agramonte et al., 2011;Xu et al., 2013;Zhang et al., 2013;Song et al., 2015). In addition, the zircons yielding 600 Ma-472 Ma ages have probably been sourced from the adjacent early Paleozoic Island arc magmatic rocks (Jian et al., 2008;Li et al., 2012;Zhou et al., 2012).
The U-Pb age data of the adjacent areas ( Figure 8A) showed three distinct intervals: 490-405 Ma, 360-270 Ma, and 250-200 Ma (Yang et al., 2018;Lu et al., 2020). The age range of 490-405 Ma and 360-270 Ma represents the early-late Paleozoic Island arc magmatic events in the region (Lu et al., 2020;Yang et al., 2022). These events produced the volcanic rocks of the Dashizhai Formation , basic-ultrabasic rocks of the Xilinhot massif (Lu et al., 2020), and the early Paleozoic Island arc-related rocks of Sonidzuoqi-Xiwuqi (Jian et al., 2008), which are widely distributed in the north of the study area. The concordant mean ages of 302.3 ± 3.3 Ma and 320.6 ± 2.4 Ma yielded by diabase and pillow basalt samples, respectively, reflect largescale magmatic events during the Carboniferous-Permian Frontiers in Earth Science frontiersin.org  in the region (Song et al., 2015;Liu et al., 2016;Zhang et al., 2016;Liu et al., 2022). The concordant mean ages of the samples of massive basalt, diabase, and pillow basalt were 272.0 ± 2.8 Ma, 261.5 ± 3.2 Ma, and 256.5 ± 2.5 Ma, respectively; these ages, in conjunction with the ages of the gabbro in the Banlashan area (256 Ma, Miao et al., 2008) and the O-type adakitic andesite in the Bairin Right Banner area (257 Ma, Yang et al., 2018), represent intensive Late Permian magmatic activity. The age range of 250-200 Ma can be interpreted as the regional syn-to postcollisional magmatic events, i.e., Shuangjingzi granites (Li et al., 2007), Jianshetun granites (Liu et al., 2016), and Triassic granitic intrusions in the northern margin of the NCC Jing et al., 2022).

Magma source
The massive basalt, diabase, and pillow basalt samples showed low SiO 2 (up to 51.56 wt%), high MgO (up to 11.95 wt%), and Fe 2 O 3 T (up to 17.85 wt%) (Supplementary Table S1), suggesting that their precursor magma was sourced from the mantle. However, their Mg # values were lower than those of mantle-derived magma (68-72, Frey et al., 1978), indicating the fractionation of their precursor magma during the evolution process. All the rock suits displayed slightly positive Eu anomalies (δEu = 0.95-1.14), indicating absence or minor plagioclase fractionation. In addition, the precursor melts of the studied rocks were contaminated by crustal materials during upwelling, as indicated by the presence of xenocrystic zircons. But the δCe values ranged from 0.88 to 1.01 (δCe=Ce N /(La N × Pr N ) 1/2 ), which were consistent with the range of δCe values of the rocks lacking crustal contamination (0.9-1.1, Polat and Hofmann, 2003). Also, the negative Th anomalies in the primitive mantle-normalized incompatible element diagrams (Figure 7) did not support significant crustal contamination. However, combined with the zircon ages obtained in this study, we argue that crustal contamination occurred to a certain extent during the formation of these rocks. The massive basalt and diabase displayed similar trace element patterns, i.e., enriched in Rb, Ba, and U, and depleted in Nb and Ta (Figure 7), suggesting the same magma source with the addition of subduction material (Pearce, 1982;Xie et al., 2016).
High field strength elements, such as Nb, Ta, Zr, Hf, Th, and HREE, were not easily affected by late-stage hydrothermal alteration and metamorphism during the magma evolution process (Pearce and Cann, 1973); consequently, these elements could distinguish the magma source and tectonic environment of the rocks. The Sm/Yb-Sm diagram ( Figure 9A) shows that the massive basalt, diabase, and pillow basalt might have been produced as a result of a 10%-20% partial melting of spinel lherzolite mantle. Their partial melting curves deviated from DMM (depleted mantle) and were close to WAM (western Anatolia mantle), indicating that there was no single source of the depleted mantle (Aldanmaz et al., 2000). The U/Th-Th/Nb diagram ( Figure 9B), shows that the massive basalts, diabases, and pillow basalts have similar magmatic sources as Izu-Bonin backarc basalts (Xie et al., 2016), which are located near the source of the depleted mantle and contaminated by minor crustal contamination. The low U/Th values of some samples of the massive basalt, diabase, and pillow basalt suggest that they interacted with subduction plate fluids as well as the melt formed from subducting sediments. Combined with previous studies on the Nd and Sr isotopes (εNd (t) = 2.5-5) of intermediate-basic volcanic rocks in the Xilinhot and Xingshuwa areas (Miao et al., 2008;Song et al., 2015) and high-Mg andesite in the Linxi area (249 Ma, Zhang et al., 2009), we argue that the formation of massive basalt, diabase, and pillow basalt is closely related to the subduction-related depleted mantle. However, the genesis of massive basalt, diabase, and pillow basalt in this study is not supported by effective Sr, Nd, and Hf isotopic data, which must be focused on in future studies.

Tectonic environment
In the Hf/3-Th-Nb/16 diagram ( Figure 10A), massive basalts are plotted in the transition field between enriched mid-oceanic ridge basalt (E-MORB) and calk-alkaline basalt (CAB), while the diabase samples (except one sample) are plotted in the island arc tholeiite (IAT) field, and pillow basalts are straddled across the E-MORB field. In the V-Ti diagram ( Figure 10B), all of these rock samples are plotted  (Winchester and Floyd, 1977) and (B) SiO 2 -FeO T /MgO diagram (Miyashiro, 1974)   Frontiers in Earth Science frontiersin.org 13 characteristics of tholeiite-calc-alkaline transition. In addition, they were relatively enriched in large ion lithophile elements (e.g., Rb, Ba and U) and depleted in high field strength elements (e.g., Nb and Ta), which indicates that the formation of these rocks was influenced by subduction fluid (Condie, 1989;Song et al., 2015). The pillow basalts are plotted near E-MORB, close to the oceanic island arc field.  Miao et al., 2008;Song et al., 2015;Lu et al., 2020); (B) Age spectrums of the study area; (C) Age spectrums of the North China Craton (after Lu et al., 2020); (D) Age spectrums of the study area.

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frontiersin.org 14 FIGURE 9 (A) Sm/Yb-Sm diagram (Aldanmaz et al., 2000) and (B) U/Th-Th/Nb diagram (Xie et al., 2016) (Wood, 1980); (B) V-Ti diagram (Shervais, 1982); (C) Zr-Ti diagram (Pearce and Cann, 1973); and (D) Ta/Yb-Th/Yb diagram (Pearce, 1982)  In the primitive mantle-normalized incompatible element diagram (Figure 7), the distribution curves of massive basalt, diabase, and pillow basalt are similar, and the trace elements contents decrease with the decrease of element incompatibility, which are different from the "hump" type of the typical continental within-plate basalt and "big uplift" type of the oceanic island basalt. In general, the La/Ta ratios of island arc basalts and some depleted midocean ridge basalts were greater than 15, and their Nb/La ratios were less than 1, while the La/Ta ratios of within-plate basalts and transitional mid-ocean ridge basalts were less than 15, and the Nb/ La ratios were greater than 1 (Condie, 1989). The La/Ta values (32-51) and Nb/La values (0.59-0.84) of the massive basalts and diabases, respectively, were different from those of within-plate basalts and transitional mid-ocean ridge basalts. The element ratios of island arc basalts (Th/Yb > 0.1, Th/Nb > 0.07 and Hf/Th < 8) were opposite to those of the depleted mid-ocean ridge basalts (Condie, 1989). The Th/ Yb values (0.19-0.56), Th/Nb values (0.10-0.23), and Hf/Th values (1.74-7.33) in both the massive basalts and diabases showed the characteristics of island arc basalt; similar findings can be observed in the Hf/3-Th-Nb/16, V-Ti, Ti-Zr, and Th/Yb-Ta/Yb diagrams, and they also show a certain affinity to enriched mid-ocean ridge basalt (Figure 7). The Nb contents of pillow basalt samples (8.4 × 10 -6 −9.8 × 10 −6 , average 9.2 × 10 −6 ) were similar to those of the enriched midocean ridge basalts (8.3 × 10 −6 ), and Sr contents (259 × 10 −6 −449×10 −6 ) were significantly higher than those of the enriched mid-ocean ridge basalts, which were in between those of E-MORB and ocean island basalt (OIB). Pillow basalts were relatively enriched in Ba and U, while slightly depleted in Rb and Th, which were similar to E-MORB, suggesting that they might have been formed in a similar setting. Although the trace elements composition and rare earth element (REE) distribution patterns of the pillow basalts were similar to E-MOBR, there were some differences, i.e., TiO 2 contents were similar to those of the back-arc basin basalts (Xu et al., 2003).
In the Th N -Nb N ( Figure 11A) diagram, the massive basalt and diabase samples are plotted in the transition field between CAB and IAT, and close to the IAT field, while the pillow basalt samples fall in the E-MORB field, and the evolution trends of all the samples are consistent with those of the supra-subduction zone enrichment (SSZ-E). In the Th/Nb-Ce/Nb diagram ( Figure 11B), the samples of massive basalts, diabases, and pillow basalts are plotted in the transition field of the pillow basalt in Bompoka and OIB.
To sum up, the formation of massive basalts and diabases was consistent with the arc magmatism of the Late Paleozoic in the region, which is extensively distributed in southeast Inner Mongolia. Pillow basalt showed geochemical affinity to E-MORB (Pearce, 2008;Furnes et al., 2012), such as pillow basalts in the Solund-Stavfjord ophiolite belt in Norway and pillow basalts in Bompoka, Andaman Islands, India (Furnes et al., 2012;Jafri and Sheikh, 2013). The composition of the basic igneous rocks in the Bairin Right Banner area had both the characteristics of MORB and island arc magmatic rocks, and rocks with such characteristics are usually produced above the subduction zone (Xu et al., 2003). The rare earth elements (REEs) and trace element characteristics of massive basalts, diabases, and pillow basalts were more similar to those of Okinawa Trough basalts (Shinjo et al., 1999). Therefore, we conclude that the massive basalt and diabase with island arc characteristics and the pillow basalt with E-MORB characteristics in the basic igneous rocks of Bairin Right Banner might have been formed by the subduction of oceanic crust in the depleted upper mantle and then reacted to mantle peridotite during the subduction of the Paleo-Asian Oceanic slab (Shinjo et al., 1999;Belousov et al., 2021).

Frontiers in Earth Science
frontiersin.org Dong et al., 2022). Ophiolites play an important role in studying the development and evolution history of ancient oceans. Solonker-Mandula ophiolite in the west of the Xar Moron suture zone was formed in a fore-arc setting as a result of intra-oceanic subduction during the Early Permian (290-284 Ma, Jian et al., 2010;Chen et al., 2012). In addition, the ophiolite in the Sonidyouqi area was formed in the mature back-arc basin during the Early Permian (297 Ma, Song et al., 2015). These ophiolites, in conjunction with the Paleozoic Island arc magmatic rocks on either side (Eizenhöfer et al., 2014;Song et al., 2015), indicate that the subduction of the Paleo-Asian Ocean in the west of the Xar Moron suture zone continued until the Early Permian . During the Late Carboniferous to Early-Middle Permian, extensively magmatic events were witnessed in the Xilinhot to Bairin Right Banner area (Liu et al., 2022;Yang et al., 2022). The main rock types are calc-alkaline, high-potassium calc-alkaline, and alkaline rocks. These rock assemblages and their geochemical characteristics indicate that the rocks were formed in a subduction-induced island arc environment (Fan et al., 2012;Cheng et al., 2013;Zhang et al., 2018;Yang et al., 2022). In the Early Permian, rodingite (280 Ma) in the Xingshuwa ophiolite mélange was formed in the forearc-island arc environment (Song et al., 2015), and the Wudaoshimen pillow basalts (277 Ma) were formed in the normal mid-ocean ridge basalt (N-MORB) environment Song et al., 2015). The formation of these rocks was related to the subduction of the Paleo-Asian Oceanic slab, suggesting that the Paleo-Asian Ocean was not closed in this period . The marine clastic-sedimentary rock series of the Shoushangou Formation, Dashizhai Formation, and Zhesi Formation were developed in the Middle-Lower Permian (Li, 2006); also, the development of Permian radiolarian fossils in chert of Xingshuwa ophiolite (Wang and Fan, 1997) indicate that the Paleo-Asian Ocean was still open in the Middle Permian. On the north bank of the Xar Moron River, the Upper Permian Linxi Formation changed from marine facies to continental facies, which is consistent with the sedimentary characteristics of a remnant ocean basin, rather than an extensional rift environment (Li, 2006;Zheng et al., 2014). The massive basalt (272.0 ± 2.8 Ma) and diabase (261.5 ± 3.2 Ma) in this study showed characteristics of oceanic island arc magmatic rocks, and the pillow basalt (256.5 ± 2.5 Ma) showed geochemical affinity to E-MORB, which indicates that there was still the remnant ocean basin of the Paleo-Asian Ocean in this period.
In the Late Permian, Angara flora and Cathaysian flora appeared on both sides of the Xar Moron River, such as the bivalves and bryozoan fossils in the Linxi Formation section in the Linxi area and the sporopollen fossils of the Xingfuzhilu Formation in the Bairin Right Banner area Shi et al., 2016). The study of paleomagnetic data of volcanic-sedimentary strata showed that the Songliao accretionary terrane and the northern margin of the North China Craton had reached around 30°N in the Permian (Ren et al., 2021;Jiang et al., 2022). In the Triassic, a large syn-collision or postcollision magmatic belt was formed as a result of the collision of the North China and the Siberian plates (Li et al., 2007;Wu et al., 2011;Eizenhöfer et al., 2014;Liu et al., 2014;Zhao et al., 2019;Zhao et al., 2021). A large magmatic belt was formed of Shuangjingzi Bimica granites (237-229 Ma), Zhuanshanzi granites (246 Ma (Liu et al., 2022).
The formation ages of massive basalt, diabase, and pillow basalt in the study area were from 272 Ma to 256 Ma, which were similar to those of the supra-subduction zone (SSZ) type ophiolite Furnes, 2011, 2014), and were the products of the subduction of the Paleo-Asian Oceanic slab in the Late Permian. Based on the regional geological data and previous studies, the remnant ocean basin of the Paleo-Asian Ocean might have been closed after the Late Permian. On this basis, we propose that the closure of the remnant ocean basin of the Paleo-Asian Ocean occurred after the Late Permian, not during the Late Devonian-Early Carboniferous, which indicates that the remnant ocean basin of the Paleo-Asian Ocean was finally closed along the Xar Moron suture zone.

Conclusion
1. The ages of massive basalt, diabase, and pillow basal in the Bairin Right Banner area are 272.0 ± 2.8 Ma, 261.5 ± 3.2 Ma, and 256.5 ± 2.5 Ma, respectively, indicating their formation during the Late Permian. 2. The massive basalts and diabases show the geochemical characteristics of subduction-derived island arc magmatic rocks, while the pillow basalts show geochemical affinity to E-MORB, which is similar to the SSZ type ophiolite. They were formed when the oceanic crust was subducted in the depleted upper mantle and reacted to mantle peridotite during the subduction of the Paleo-Asian Oceanic slab. 3. The basic igneous rocks in the Bairin Right Banner area are the embodiment of the subduction of the Paleo-Asian Oceanic slab in the Late Permian. Combined with the regional data and previous studies, the Paleo-Asian Ocean was finally closed along the Xar Moron suture zone during the Late Permian-Early Triassic, and the collision between the North China and Siberia plates might have occurred during the Early Triassic.

Data availability statement
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation. Requests to access the datasets should be directed to orcidme@163.com.