Petrogenesis and tectonic significance of Late Paleozoic magmatism in the Xilinhot micro-continent, central Xingmeng orogenic belt

ABSTRACT The evolution history of the Hegenshan Ocean is an important part of studying the evolution of the Xingmeng orogenic belt, but it is still controversial. Here we present an integrated study of petrology, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd isotopes on a series of intrusive rocks from the Baiyinguole area, Xiwuqi, central Inner Mongolia, NE China. Chronological data indicate that the intrusive rocks can be divided into three stages: 326–317, 297–295, and 259–254 Ma. The 326–317-Ma gabbros and diorites belong to the calc-alkaline series, and are characterized by positive whole-rock εNd(t) values (+1.1–+6.7), relatively low La/Ba ratios (0.04–0.10), and high La/Nb ratios (1.82–3.18), indicating that the magma source is from a subduction-modified lithospheric mantle. These samples have arc features owing to the southward subduction of the Hegenshan oceanic crust. The 297–295-Ma granodiorites and monzogranites show positive εNd(t) values (+1.9–+2.4), low 87Sr/86Sri ratios (0.704185–0.705234), and young Nd two-stage model ages (TDM2), which is consistent with typical I-type granites. On various tectonic setting discrimination diagrams, the granodiorites and monzogranites are plotted in the syn-collisional field. The 259–254-Ma syenogranites have low abundances of mafic minerals, low contents of Zr/Hf (18.31–31.35) and Nb/Ta (6.80–11.80) values, high SiO2 (76.36–78.54 wt.%) content, A/CNK (1.08–1.76), and D.I. of 96.8–97.4 (average 97.1), and notable negative Eu anomalies (δEu* = 0.07–0.17) and weakly tetrad-effects in REE pattern, which is affinity to the characteristics of highly differentiated granites. Moreover, they have positive εNd(t) values (+2.0–+2.1) with scattered TDM2 of 730–1217 Ma and low Nb/U ratios. This study suggests that the syenogranites are formed in a post-collisional tectonic setting, which is related to the closure of the Hegenshan Ocean. In addition, based on a review of the pre-existing literature data, a model of southward ridge subduction is proposed to explain the tectonic evolution of the Hegenshan Ocean. Graphical Abstract


Introduction
The Xingmeng Orogenic Belt (XMOB) belongs to the southeastern part of the Central Asian Orogenic Belt (CAOB; Figure 1), which was characterized by the Neoproterozoic microcontinent (e.g. Sun et al. 2013aSun et al. , 2013bWang et al. 2014;Luan et al. 2017Luan et al. , 2019, Paleozoic ophiolite sequences (e.g. Liu et al. 2003;Miao et al. 2007;Song et al. 2015;Zhang et al. 2015b;Wang et al. 2019;Zhou et al. 2019), widespread Late Paleozoic magmatism (e.g. Chen et al. 2000;Xiao et al. 2003;Liu et al. 2013Liu et al. , 2014Liu et al. , 2017Liu et al. , 2018aLi et al. 2016;Wu et al. 2016aWu et al. , 2016bWang et al. 2018a), and high-temperature /low-pressure metamorphic rocks (Zhao et al. 2002;Yu et al. 2012;Zhang et al.al. 2018aZhang et al.al. , 2018b. The Hegenshan Ocean is located between the Uliastai Continental Margin (UCM) and the Xilinhot microcontinent (XLMC, also called the Baolidao arc belt). It is an important part of the Paleo-Asian Ocean. The formation mechanism and evolution characteristics of the Hegenshan Ocean are the keys to the study of the XMOB. However, the forming age of the Erenhot-Hegenshan ophiolite and the evolution of the Hegenshan Ocean are still controversial (e.g. Tang and Zhang 1991;Shao et al. 1991;Nozaka and Liu 2002;Robinson et al. 1999;Miao et al. 2008Xiao et al. 2009Ma et al. 2017Ma et al. , 2019Liu et al. 2009Liu et al. , 2013Liu et al. , 2018bLiu et al. , 2021. The formation time of ophiolite has different understandings, such as middle-late Devonian (Bao et al., 1994;Robinson et al. 1999) early Carboniferous (Zhang et al. 2015b;Huang et al., 2016) Late Carboniferous  early Cretaceous (Nozaka and Liu 2002;Jian et al., 2012), etc. The proposed models that illustrate the issue include the following: (1) Early studies suggested that the Hegenshan ocean was the main ocean basin of the paleo Asian Ocean, and Erenhot-Hegenshan ophiolite has MORB characteristics and was formed in a mid-ocean ridge environment (Bao et al., 1994;Robinson et al. 1999;Nozaka and Liu 2002), representing the final closure position of the Paleo Asian Ocean. (2) Most scholars believe that the Erenhot-Hegenshan ophiolite has the characteristics of a subduction zone and belongs to the back-arc ocean basin Zhang et al. 2015b;Huang Bo et al., 2016). (3) In addition, some scholars believe that the Erenhot-Hegenshan ophiolite is not a typical ophiolite, but a mafic-ultramafic rock mass formed by multi-stage asthenosphere upwelling, and the plate assembly may be completed through the intrusion of deep mantle-derived materials (Jian et al., 2012;Shao et al., 2020). Previous studies focused more on the Hegenshan ophiolite while ignoring the related intrusive rocks. A large number of intrusive rocks contemporaneous with Hegenshan ophiolite are distributed in XLMC and UCM Liu et al. 2017;Wang et al. 2018a). These rocks may be  Jian et al. (2008). The black box shows the location of the study area. related to the evolution of the Hegenshan Ocean. However, the existing research on these intrusive rocks is generally attributed to the subduction of the paleo Asian Ocean, and there is no further research on the relationship with the Hegenshan Ocean.
To further constrain the relationship between the Hegenshan Ocean and the magmatism of XLMC, here we conducted geological field surveys, petrology, chronology, geochemistry, and Sm-Nd isotope of intrusions in the Baiyinguole area (116°38′-116°53′E and 44°20′-44° 30′N), which is located in the XLMC and close to the Hegenshan Suture Zone ( Figure 1B). Various Late Paleozoic intrusive rocks were exposed in the study area, facilitating the study of the tectonic evolution of the Hegenshan Ocean during the Late Paleozoic. We aim at studying the petrogenesis, magma source, and tectonic setting of these intrusive rocks in the XLMC.

Regional geology
The XMOB contains five E-W-trending units (Figure 1). The five units from north to south are as follows: (1) the UCM, which is characterized by the presence of Permian alkaline granitoids and lack of a Precambrian continental basement (Hong et al. 1994(Hong et al. , 1996(Hong et al. , 2004Zhang et al. 2009;Cheng et al. 2014;Tong et al. 2015); (2) the Erenhot-Hegenshan ophiolitic accretionary belt, also called the Northern Orogenic Belt Wang et al. 2019); (3) the Precambrian basement of the XLMC, intruded by 1390-1516-Ma plutonic rocks, unconformably overlain by Phanerozoic strata and developed from the Paleozoic to Mesozoic volcanic and intrusive rocks (Shi et al. 2003;Sun et al. 2013aSun et al. , 2013b; (4) the Ondor Sum subduction-accretionary complex/Solonker-Linxi ophiolites (also called Southern Orogenic Belt; e.g. Xiao et al. 2003;Xu et al. 2014); and (5) the northern margin of the North China Craton, which comprises a Precambrian basement (e.g. Zhao et al. 2001Zhao et al. , 2005Wan et al. 2006;Santosh et al. 2009;Zhai and Santosh 2011)  Erenhot-Hegenshan ophiolite accretionary belt is distributed in Erenhot-Chaokeshan-Hegenshan-Wusinihei area in a northeast direction, extending nearly 400 km. The middle-late Devonian Taerbagete Formation is a set of flysch formations with a small amount of volcanic rocks, which are distributed along the belt. The late-Devonian Angeyinwula Formation contains clastic rocks of marine continental interactive facies, which are exposed to the north of the Hegenshan area. The lower Carboniferous strata are missing, and the Late Carboniferous intrusive rocks and volcanic rocks are widely exposed.
The Precambrian basement of the XLMC is represented by the Airgin Sum Group and Xilin Gol Complex. The rock assemblages of the basement include strongly deformed and metamorphosed rocks, such as biotite gneiss and schist with amphibolite interlayers (Bureau of Geology and Mineral Resources of Inner Mongol Autonomous Region (INMG BGMR) 1993). Some research suggests that these metamorphic rocks are a Paleozoic accretionary complex rather than Precambrian metamorphic rocks, according to zircon ages from the gneisses (Shi et al. 2003;Chen et al. 2009;Xue et al. 2010). However, recent research show more evidence to support the occurrence of Precambrian basements in XLMC: (1) Few Mesoproterozoic gneissic granites (1516-1360 Ma) exposed in the XLMC (Han et al., 2017;Sun et al. 2013a), and some late Paleozoic intrusive rocks contain Proterozoic captured zircons Liu et al. 2021). (2) The Xilin Gol Complex with Proterozoic ages was subsequently intruded by ca. 740 Ma meta-gabbros (Ge et al. 2011); (3) Nd mapping results support the occurrence of Precambrian basements at depth in the Hutag Uul-Xilinhot area . The XLMC probably experienced significant growth and reworking of continental crust during the Late Paleozoic and was transformed into a new arc terrane . The covering of Phanerozoic volcanic-sedimentary strata are widespread in the region (Li et al. 2014aZhang et al. 2017), including Mesozoic continental volcanic-sedimentary rocks, and Cenozoic lacustrine sediments are exposed extensively (Guan et al. 2018;Tang et al. 2020). Previous researchers had reported that the magmatic activity in the XLMC during the Carboniferous-Permian occurred during two phases , with peaks of activity at 311 and 285 Ma Liu et al. 2018b).
The strata in the Baiyinguole area are exposed from the Late Paleozoic to Quaternary. The oldest is the Permian Dashizhai Formation, which mainly comprises intermediate-acidic volcanic rocks (P 1 ds 1 ), slates, sandstones, and limestones (P 1 ds 2 ). These Permian rocks are exposed mainly as a NE-SW-trending strip that is unconformably overlain in the central portion of the Baiyinguole area by Mesozoic volcanic rocks (J-K; Figure 2). Furthermore, Carboniferous to Early Permian gabbroic to granitic intrusive rocks occur in this region.

Field occurrences and sample descriptions
The intrusive rocks in the study area include four plutons ( Figure 2) and can be divided into gabbro-diorite, granodiorite-monzogranite, and syenogranite. Forty fresh samples were collected.

Gabbro-diorite
The gabbros and diorites (Cδ and Cν) are exposed in the northwestern and southeastern parts of the Baiyinguole area, respectively, involving two plutons, covering an area of 7 km 2 . The gabbros and diorites were intruded by granodiorite-monzogranite (Pγδ and Pγ) and syenogranite (Pξγ; Figures 3A and 3B).
The gabbro sample WS1 displays prismatic crystals, granoblastic textures, and a massive structure ( Figure 3C) that comprises pyroxene (25%-30%), plagioclase (~70%), and hornblende (1%-5%). The scattered green pyroxene grains form anhedral prisms. Pyroxene usually has reaction edges of biotite and hornblende, and some of them are metasomatized by saponite as false images, and some particles are embedded with plagioclase. The hornblendes have a grain-size range of 0.3-2 mm and distribute along the edge of pyroxene and metasomatized pyroxene. The anhedral and granular plagioclase grains are gray and are 0.2-2 mm in size. They show some alterations to clays and zoisite.
The diorite sample JL01 varies from gray to green, has a medium grain size (2.0-4.0 mm) ( Figure 3D), and mainly comprises hornblende (25%-30%), plagioclase (~70%), and biotite (1%-5%). Hornblende usually has reaction edges of biotite and the biotites distributed along the edge of hornblende and metasomatized hornblende. The accessory minerals are magnetite and zircon. In the QAP diagram, the samples of the gabbros and diorites are plotted in the gabbro and diorite area ( Figure 4).

Syenogranite
A pluton of syenogranite crops out to the north of the Jielinmuchang Diorite ( Figure 2) and trends approximately along the NE-SW direction. The pluton intrudes the Dashizhai Formation and diorites ( Figures 3G and 3H). The pluton is composed of coarsemedium-grained syenogranite in the south and fine-grained syenogranite granite in the north. The contact relationship is a gradual transition. Syenogranite sample JL02 comprises biotite (5%), plagioclase (25%), quartz (25%), and K-feldspar (45%), with grain sizes from 0.3  to 4 mm. The early crystalline plagioclase is often wrapped with K-feldspar, which shows the characteristics of poikilitic texture ( Figure 3I). Plagioclase is commonly replaced by K-feldspar, and the polysynthetic twin is undeveloped. All syenogranite samples fall in the syenogranite field of the QAP diagram ( Figure 4).

Analytical methods
This paper reports the zircon U-Pb geochronology, major and trace elements, and Sr-Nd isotopic data of these samples.

Zircon U-Pb dating
Samples were first crushed using conventional crushing methods and then separated using heavy liquids and a magnetic separator at the Langfang Mineral Separation Laboratory of the Bureau of Geology and Mineral Resources of Hebei Province. Zircon grains were separated using conventional magnetic and density techniques. The grains were hand-picked under a binocular microscope. The internal structures of the zircon grains were examined using a transmitted electron microscope and imaged by backscattered electron (BSE) and cathode luminescence (CL) prior to U-Pb isotopic analyses. The BSE and CL imaging was carried out using a LEO1450VP scanning electron microscope with a MiniCL detector at the Institute of Geology, Chinese Academy of Geological Sciences. Zircon analyses were performed on the Neptune multiple-collector inductively coupled plasma mass spectrometer (Thermo Fisher Ltd.) with a 193 nm-FX ArF excimer laserablation system (ESI Ltd.) at the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources. NIST610 glass was used as an external standard to calculate U, Th, and Pb concentrations in the zircons. The common Pb correction used was the 208 Pb method (Stacey and Kramers 1975), and the Temora zircon was used as an external standard to normalize isotopic fractionation during the analysis. Uncertainties of the individual analyses are reported with 1σ errors; weightedmean ages are reported at the 2σ confidence level. The age calculations and concordia diagram plots were completed using Isoplot (Ludwing. 2003). The detailed analytical technique is described in Li et al. (2009).

Major and trace elements
Major elements were analyzed by X-ray fluorescence analysis (XRF; PHILIPS PW1480) using fused glass disks at the Langfang Regional Geological Survey, Hebei Province, China. The content of oxide was analyzed using wet chemical methods, and loss-onignition was determined by the gravity method. Analysis accuracy was better than 1%. Trace element and rare earth element (REE) abundances were measured using inductively coupled plasma-mass spectrometry (ICP-MS) following the techniques of Liang et al. (2000) and Gao et al. (2008) with analytical precision for Zr, Hf, Nb, and Ta better than 5%; most elements had values within 10% of certified values.

Sm-Nd isotope
Samples were analyzed on a Finnigan MAT262 multireceiver mass spectrometer in the Analytical Testing Research Center, Beijing Research Institute of Uranium Geology, following the analytical procedures described by Zhao et al. (2018b).

Zircon laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating
Eight samples were collected for geochronology ( Figure 2; Table S1). The zircons are transparent crystals with oscillatory zoning (not shown) and possess a Th/U ratio range of 0.28-2.79, suggesting a magmatic origin (Koschek 1993).

Gabbro-diorite
Twenty-five grains were analyzed for sample WS01 (gabbro), and the results are presented in Table S1; Figure 5. All the analyses were concordant, and the 206 Pb/ 238 U ages were between 318 and 333 Ma, and the weighted mean 206 Pb/ 238 U age was 326 ± 2 Ma (MSWD = 1.2). The weighted mean 206 Pb/ 238 U age of the WS02 sample (gabbro) was 322 ± 1 Ma (MSWD = 0.6), which represents the crystallization age of the gabbro. A diorite sample (JL01) was collected from the Jielinmuchang area. Twenty zircons from JL01 were analyzed. The 206 Pb/ 238 U ages ranged from 330 to 307 Ma (Table S1), and the weighted average age was 317 ± 3 Ma (MSWD = 4.1), representing the crystallization age.

Granodiorite-monzogranite
The BY01 sample is a granodiorite collected from the Wusuhundi area. The analytical results form two populations on the 206 Pb/ 238 U-207 Pb/ 235 U concordia diagram ( Figure 5) with concordant ages of 316 ± 3 Ma (MSWD = 0.3, n = 7) and 297 ± 1 Ma (MSWD = 1.0, n = 15). The 297 Ma ages are interpreted as the timing of emplacement of the granodiorite, and the 306 Ma ages represent the zircons captured from the gabbro collected from the Dashizhai Formation. The BY02 sample is a monzogranite collected from the Wusuhundi area. The analysis of 21 spots yielded a weighted mean 206 Pb/ 238 U age of 297 ± 2 Ma (MSWD = 1.9, n = 21), indicating the crystallization age of the monzogranite.

Gabbro-diorite
The gabbros and diorites have positive whole-rock ε Nd (t) values (+1.1-+6.7; Figure 8) and T DM2 from 572 to 1096 Ma (Table S3), indicating an obvious contribution from an isotopically depleted source. The gabbro-diorite samples are characterized by enrichments in LILEs but depletions in Nb, Ta, and Ti, possibly arc signatures, or the compositions are affected by subducted sediments and/or assimilation of continental crustal material. Ba/ Nb, La/Nb, and Nb/Th can be differentiated obviously during oceanic subduction, but the ratio remains stable during partial melting and separation crystallization. The samples have relatively low La/Ba (0.04-0.10) and high La/Nb (1.82-3.18) ratios ( Figure 9A; Saunders et al. 1992) related to a subduction-modified continental-lithospheric mantle source. The gabbro-diorite samples show variable Th/Yb ratios with constant Ba/La ratios, indicating the contribution of sediments ( Figure 9B). Moreover, the samples have high Th/La (0.44-0.63) and Th/Ce ratios (0.18-0.27) and positive Zr-Hf anomalies ( Figure 7B), suggesting that crustal contamination was involved in the magma evolution process (Taylor and a b  Figure 10A). In the Sm/Yb -La/Sm diagram, the samples show the melting degree of the mantle source from 5% to 15%, suggesting a quite shallow mantle melting zone ( Figure 10B). These rocks were derived from a highly fractionated magma, as evidenced by the low to medium Mg# values (44-55) as well as V content (168-282 ppm) and high SiO 2 (52.06-59.40 wt.%) content. This finding is supported by the presence of low Cr (22.7-34.0 ppm), Co (18.0-28.7 ppm), and Ni (11.2-24.2 ppm) content. A Harker diagram shows a fractional crystallization trend for the gabbros and diorites (supplementary Fig. 1s), and the other variation diagrams show negative correlations between Mg# and CaO, P 2 O 5 , Ni, as well as Cr. The plots on these diagrams indicate the fractional crystallization of Fe-Ti oxides, plagioclase, and apatite. Clinopyroxene has experienced fractionation because of the negative correlation between CaO and CaO/Al 2 O 3 .

Granodiorite-monzogranite
The granodiorites and monzogranites have positive ε Nd (t) values (1.9-2.4, average 2.2), low initial 87 Sr/ 86 Sr ratios a b  Figure 7. The samples have low values of Nb/Ta (7.10-11.66) and Zr/Hf (26.35-33.53), which are similar to the melted crustal materials (11.4 and 33, respectively; Rudnick & Gao, 2004), rather than the primitive mantle (17.8 and 37, respectively). These features suggest that the granodiorites and monzogranites were derived from juvenile crust controlled by depleted mantle-derived material. The granodiorite and monzogranite samples have low initial 87 Sr/ 86 Sr ratios and are weakly peraluminous while Th concentrations increase with increasing Rb (Figure 11). These observations are consistent with typical I-type granites (Chappell and White 1992). Wu et al. (2017) suggested that crystallization differentiation is an important mechanism of compositional variations of granitic magma and summarized a series of geological characteristics and petrochemical indicators of highly differentiated granites: low content of mafic mineral, high U or Th content of zircon, the high value of Aluminous Saturation Index, low Zr/Hf and Nb/Ta values, low rare earth element content, usually showing a tetrad-effects in REE pattern and related to mineralization of W, Sn, Nb, Ta, Li, be, Sb and REE. Although the lack of typical field evidence, petrological characteristics show that syenogranite of this study contains minerals of different generations ( Figure 3I), indicating that it has experienced crystallization differentiation. The low abundances of mafic minerals, low contents of Zr/Hf (18.31-31.35) and Nb/Ta (6.80-11.80) values, high SiO 2 (76.36-78.54 wt.%) content, A/CNK (1.08-1.76), and D.I. (D.I. = Q+ Or+Ab, C.I.P.W norms) of 96.8-97.4 (average 97.1), and notable negative Eu anomalies (δEu* = 0.07-0.17) and weakly tetrad-effects in REE pattern, which is affinity to the characteristics of highly differentiated granites (Wu et al., 2017). Furthermore, on various discrimination diagrams, all the samples fall in the highly fractionated I-type field ( Figures 12A and 12B).

Syenogranite
The notable depletions in Sr and Eu suggest the fractionation of plagioclase, which is confirmed by the highly variable Sr concentrations and restricted Ba/Sr ( Figure 13A). The variation trend of Sr vs. Ba suggests that K-feldspar and plagioclase are the major crystallizing phases ( Figure 13B). Biotite is an Al-bearing mineral which has low partition coefficients for Yb, but extremely high value for Sc. The negative correlation between SiO 2 /Al 2 O 3 and Sc/Yb in these I-type granitoids suggests that biotite is a major fractionating mineral ( Figure 13C). On the (La/Yb) N vs. La diagram( Figure 13D), the variations in REE concentrations during the evolution of these magmatic rocks were restricted by the separation of allanite and monazite.
Two possible models can be hypothesized for highly fractionated I-type granites: (1) fractional crystallization of mafic magmas derived from the mantle (Wyborn et al. 2001) and (2) melting of crustal source rocks affected by juvenile mantle genesis and experienced fractional crystallization (Qiu et al. 2008;Zhu et al. 2009;Liao et al. 2019). However, the magma cannot have originated from the fractional crystallization of mafic magma because this would have been recorded by large-scale contemporaneous mafic magmatism in the study area. The syenogranite samples have scattered Nd model ages (T DM2 ) of 730-1217 Ma and positive ε Nd (t) values (+2.0-+2.1). The samples have low Nb/U ratios (6.12-11.62, average 7.73), which are far below those of melted low crustal materials (Nb/U = 25; Rudnick et al. 2003). This evidence suggests that the syenogranites were likely formed by partial melting of crustal materials influenced by mantle-derived materials.
Overall, the syenogranites are highly fractionated I-type granites that originated from the partial melting of a Proterozoic infracrustal metabasaltic source, with subsequent extensive fractional crystallization. The main crystallizing phases contain K-feldspar, plagioclase, biotite, monazite, and allanite.

Geodynamic process
The XLMC is generally regarded as a typical magmatic arc belt and is one of the prominent features of the XMOB. The Late Paleozoic magmatism in the northern XMOB is marked by two peak episodes of activity at 330-  et al. 2018b). These two epochs are generally ascribed to the normal northwards subduction of the Paleo-Asian oceanic plate for the older stage (Chen et al. 2001;Liu et al. 2009Liu et al. , 2013Liu et al. , 2014 and post-collisional to the intraplate setting for the younger stage (Li et al. 2014a;Liu et al. 2018a). However, some researchers have recently suggested that some of the Late Paleozoic magmatism may have been induced by the subduction of a midocean ridge system Li et al. 2022;Yang et al., 2022).
Statistics of the collected data from the Late Paleozoic magmatism in the XLMC (Figure 1) and the new age data in this study (n = 77; Table 1) demonstrate three major peaks of activity at 322, 310, and 281 Ma (Figure 14), and can be classified into three stages: Early Carboniferousearly Late Carboniferous (346-305 Ma), Late Carboniferous-Early Permian (305-290 Ma), and Middle-Late Permian (290-254 Ma). The three groups of intrusive rocks of the Baiyinguole area is just corresponding to the three-stage magmatism in the XLMC. They have different geochemical and isotope compositions, suggesting different tectonic settings.

Early Carboniferous-early Late Carboniferous (346-305 Ma)
Zircon grains from the gabbros and diorites yielded 206 Pb/ 238 U ages of 317-326 Ma. Furthermore, the gabbros and diorites show obvious enrichments in LILEs, and depletions in HFSEs, Nb, Ta, and Ti anomalies (Figure 7). This means that the magma source has been changed by fluids derived from the subduction plates (Turner et al. 1992(Turner et al. , 1996Zhang et al. 2008). Moreover, on the Hf-Th-Nb (Wood 1980) and Ti-Zr-Y (Jahn et al. 1999) diagrams, all samples were plotted in the typical arc field (Figure 15), which is related to the subduction of the oceanic crust.
However, we cannot simply assume that the study area is in the tectonic setting of oceanic crust subduction during this period. Recently, the Early Carboniferousmiddle Late Carboniferous magmatism of the XMOB  Whalen et al. (1987), (B) is modified from Sylvester (1989), (C) is after Pearce et al. (1984), (D) is modified from Pearce (1996) and (E) is modified from Batchelor and Bowden (1985). The data are from the references in Table 1 has been shown to be diverse in its characteristics . The Early Carboniferous-early Late Carboniferous magmatic activity in the XLMC produced mainly ophiolites (complexes) that were retained as thrust slices in the strongly deformed Permian strata Wang et al. 2019 Hanson (1978); Rollinson (1993) and Yang et al. (2012) .   Stern and Kilian 1996;Benoit et al. 2002;Gorring et al. 2003. Guivel et al. 2003Thorkelson et al. 2011;Osozawa et al. 2012;Wang et al. 2020b;Liu et al. 2021). (2) The Early Carboniferous strongly peraluminous granites belt in the XLMC (Ma et al. 2018;Liu et al. 2021) has the geochemical characteristics of S-type granite and arc affinity, and may have been developed in supracrustal environments with relatively higher temperature and lower pressure; (3) coeval (317-319 Ma) adakitic granodiorites occurring in the XLMC, exhibit relatively low Nb/ Ta (9.2-15.4, average = 12.0) and high Zr/Sm ratios (28.8-66.8, average = 39.9) (Dong et al., 2020), similar to the composition of typical adakites formed during ridge subduction (Nb/Ta = 10.7-18, average = 13.4; Zr/ Sm = 16.9-75.7, average = 40.2) (Aguillon- Robles et al., 2001;Tang et al., 2010); (4) The basalts in the northern of the Erenhot-Hegenshan-Xi-Ujimqi ophiolite belt exhibit characteristics of normal-type to enriched-type midocean ridge basalt affinities, comparable to modern Eastern Pacific mid-ocean ridge basalts (Song et al. 2015). (5) Although we have no method to obtain the temperature of the gabbros and diorites, we have estimated the temperature of granodiorite-monzogranite and syenogranites. The zircon saturation temperatures (T Zr ) (Table S2) vary from 775 to 831°C (with an average value 801°C) and 751 to 829°C (with an average value 790°C) for granodiorite-monzogranite and syenogranites, respectively (Watson & Harrison, 1983; T Zr = 12,900/ (2.95 + 0.85*M + lnD Zr, zircon/melt), M = cation ratio (Na + K + 2*Ca)/(Al*Si)).The low  Table 1 concentrations of Zr and the zircons lack of inherited cores indicate that the temperature represents the minimum temperature of the magma (Ferreira et al., 2003). The appearance of these relatively high-temperature granites indicates that after the subduction of the midocean ridge, the overriding plate still retains a high temperature. These lines support the occurrence of midocean ridge subduction during the Carboniferous period. 2022) proposed a northwards subduction of the paleo-Asian oceanic ridge subduction accompanied by a slab rollback model for the diverse magmatism. However, this model cannot explain the following two problems: (1) the hightemperature and low-pressure metamorphism only occurred on the XLMC(P-T estimates of 760-790°C or >760°C at 5-6 kbar; Zhang et al. 2018aZhang et al. , 2018b; (2) Three ophiolite belts that become younger from north to South are distributed in XMOB (Xiao et al. 2003;Miao et al. 2007;Jian et al. 2010;Li et al. 2012Liu et al. 2013;Song et al. 2015;Zhang et al. 2015b;Wang et al. 2018aWang et al. , 2019. Thus, we coclude that the southward subduction model of the Hegenshan mid-ocean ridge can explain these phenomena more reasonably ( Figure 16A). The oldest arc magmatism is 346 Ma Xiao et al. 2015), indicating that the subduction of the Hegenshan oceanic crust was earlier than 346 Ma. As the southward subduction of the Hegenshan Oceanic crust continued, the spreading ocean ridge was dragged into the subduction zone. Owing to hot asthenosphere upwelling, the 'slab window' has formed under the overlying XLMC, subsequently forming the MORB-like to OIB-like magmatism ( Figure 16A). Considering the geochemical characteristics of the gabbros and diorites, we propose that the gabbros and diorites are belong to the 'normal' arc, which originated from slab fluid-metasomatized mantle due to the subduction of normal oceanic plates on two sides of the subducted oceanic ridge or slab window. The gabbros and diorites, with other magmatic rocks together form the magmatic rock suite related to the subduction of the mid-ocean ridge (Wang et al. 2020b). Furthermore, hightemperature/low-pressure metamorphic rocks formed along with the 'slab window'. Previous studies and the new age data showed that the ages of the metamorphism and magmatism of the XLMC are between 305 and 348 Ma with a peak of 310-322 Ma (Table 1, Figure 14), indicating that the southward subduction of the Hegenshan oceanic ridge peaked at 310-322 Ma and ceased at ca. 305 Ma.

Late Carboniferous-Early Permian (305-290 Ma)
The Late Carboniferous-Early Permian magmatism produced mainly I-type granites and a few diorites, such as the Weilasituo Quartz Diorite (Xue et al. 2010), the Yuejin Quartz Diorite , the Xi Ujimqin Quartz Diorite (Ma et al. 2016), and the Houtoumiao Syenogranite (Liu et al. 2018b). As reported in this paper, the zircon grains from the granitoids yielded 206 Pb/ 238 U ages of 296-297 Ma. On various discrimination diagrams for tectonic settings (Figures 12C and 12D; Pearce et al. 1984), the samples were plotted in the boundary area between the volcanic arc granites and the syn-collisional fields. Moreover, the samples were plotted in the syncollisional field on the R1-R2 diagram ( Figure 12E; Batchelor and Bowden 1985), suggesting that the tectonic setting was syn-collisional. After the subduction of the mid-ocean ridge, the dragged oceanic crust continues to subduct, and the Hegenshan Ocean gradually closed . With the demise of the Hegenshan Ocean, the XLMC entered the syncollisional stage and formed syn-collisional granites, whereas the widespread late Carboniferous strata were deposited mainly in an inland sea (Zhao et al. 2016). Simultaneously, the Paleo-Asian Ocean branch (Hegenshan Ocean) in this region has closed and the Erenhot-Hegenshan ophiolites were emplaced, and a regional unconformity unconformity between the late Paleozoic strata and the ophiolites or Pre-Carboniferous strata was formed (Bao et al. 2005;Xu et al. 2015;Zhou et al. 2015). The A-type and S-type granites were emplaced at 290-276 Ma (Shi et al. 2003;Li et al. 2016;Wang et al. 2018a). Hence, the collision between the XLMC and the UCM occurred from 305 to 290 Ma ( Figure 16B).

Middle-late Permian (290-254 Ma)
The Middle-Late Permian magmatism mainly comprises A-and S-type granites and bimodal volcanic rocks (Dashizhai Formation) (Table 1), which are considered to be formed in an extensional geological setting (Shi et al., 2004;Bao et al., 2007;Zhang et al. 2008Zhang et al. , 2017. By contrast, the samples of syenogranite with ages of 254-259 Ma are highly fractionated I-type granitoids. On the tectonic discrimination diagrams (Figs. Figures 12C and  12D), the syenogranite samples straddle the boundaries among the within-plate, syn-collisional, and volcanic-arc granite fields. On the R1-R2 diagram ( Figure 12E), the samples were plotted in the post-orogenic area. It is worth noting that the Middle-Late Permian magmatism XLMC are different from the A-type granites of UCM(e.g. Jahn et al. 2009;Zhang et al. 2015b;Ganbat et al. 2021), showing significant depletion of Nb, Ta, Ti, and Ba, which is the characters of the arc-related intrusive rocks. Magmatic compositions are depends more importantly on their source rock nature and melting conditions than the tectonic setting. So we suggested that the subduction signature was inherited from the source rocks, and there is still residual oceanic crust in the lithospheric or c a b Figure 16. Simplified cartoon depicting the geodynamic scenario of the XMOB during the Late Paleozoic. See details in the text. asthenospheric mantle ( Figure 16C). Therefore, the evidence indicates that the XLMC was in a post-orogenic stage of development after ca. 290 Ma (Shi et al. 2003;Li et al. 2016), and the tectonic setting was post-collisional. The appearance of A-type (Shi et al. 2003), S-type Wang et al. 2018a), and highly fractionated I-type granites in the XLMC indicates a change in the tectonic environment to an extensional regime. Mantle upwelling and crustal melting yielded post-collisional magmatism in the XLMC ( Figure 16C). The tectonic environment of Carboniferous to Permian magmas show a trend of development from a VAG setting to post-orogenic ( Figure 12). The sedimentary facies in the XLMC transitioned from the Middle Permian marine facies to Late Permian lake facies Wang et al. 2020b).

Conclusions
Based on the new data for geochronology and geochemistry of the Late Paleozoic intrusive rocks from the Baiyingguole area, the following conclusions were drawn.
(2) The gabbro-diorite was related to a subductionmodified continental-lithospheric mantle source. They show arc characteristics, which may be related to the southward subduction of the Hegenshan oceanic crust. (3) The granodiorites and monzogranites have low initial 87 Sr/ 86 Sr ratios (0.704185-0.705234), positive ε Nd (t) values (+1.9-+2.4), and young Nd model ages, consistent with their typical I-type granitic compositions. They are situated in a syncollisional tectonic setting related to the amalgamation of the XLMC and the UCM. (4) The syenogranites are highly fractionated I-types, suggesting a post-collisional geodynamic environment, which is related to the closure of the Hegenshan Ocean.

Highlights
(1) This study documents Late Paleozoic magmatic activity in southeastern Inner Mongolia.
(3) The late-Permian syenogranite is of high-K calc-alkaline series with highly fractionated I-type affinity. (4) A southward ridge subduction occurred in the Carboniferous.