Late Devonian A-Type Granites from the Beishan, Southern Central Asia Orogenic Belt: Implications for Closure of the Paleo-Asia Ocean

: The closing time of the Paleo-Asian Ocean (PAO) has long been in the focus of research as well as of controversial debates. The Paleozoic A-type granites distributed in the Beishan Orogenic Belt (BOB) at the southern margin of the Central Asian Orogenic Belt (CAOB) provide pivotal clues to constrain the closure of the PAO. In this paper, the newly recognized Duhongshan A-type granites from the middle Huaniushan arc in the BOB (zircon LA-ICP-MS U-Pb ages of ca. 376–374 Ma) are thoroughly studied. The rocks have high SiO 2 , K 2 O contents with peralkaline character, and display high Zr + Nb + Ce + Y contents (354–543 ppm), 10,000 × Ga/Al (4.1–4.9), Y/Nb (3.2–5.3), Rb/Nb ratios (8.5–14.1), and a zircon saturation temperature in the range of 877–950 ◦ C, indicative of A2-type granites afﬁnities. The Duhongshan granites display enriched in Th and U; depleted in Ba, Sr, and Ti; with slightly positive whole-rock ε Nd (t) values (+1.86 to +2.21), indicating an origin related to partial melting of lower crustal material in post-collision extension settings. Combined with previous reported results, we conclude that the granitoids in the middle Huaniushan arc were mostly formed around 424–367 Ma and can be divided into two types based on petrochemistry: (a) A-type granites, which generally have high SiO 2 and K 2 O, derived from the relatively shallow crustal source in post-collision tectonic settings; and (b) adakite and I-type granites, which display high Sr/Y ratios as well as Nb, Ta, and Ti depletion, likely generated from the melting of juvenile crust in active continental margin arcs. Integrating the previous regional investigations, we propose that the Hongliuhe–Niujuanzi–Xichangjing Ocean (HNXO) of the PAO was closed and transformed in the post-collision extensional tectonic stage during the Late Devonian and formed as post-collision magmatism, while the arc magmatism may be related to the subduction of the Liuyuan Ocean, which is located in the Southern HNXO.


Introduction
The Central Asian Orogenic Belt (CAOB) is a vast Phanerozoic accretive orogenic belt, which is located along the Southern Siberian Craton, Eastern Baltica Craton, Northern North China Craton and Tarim Craton (Figure 1a). When the Paleo-Asian Ocean (PAO) subducted and closed during the Paleozoic, the CAOB was formed by a collage of seamounts, island arcs, microcontinents, and ophiolites [1][2][3][4][5]. Although numerous studies over the past decade have suggested that the southern margin of the CAOB is the final location of the closure of the PAO [6][7][8][9][10][11][12][13][14], the timing of the final closure of the PAO has been one of the most controversial issues. Fortunately, there exists a large number of different types of granites (I-, S-and A-type granites, adakite, etc.) associated with accretive orogenesis and oceanic subduction in the Southern CAOB [14][15][16][17][18][19], which is valuable utility to study for the subduction-collision history of the PAO and the formation of the CAOB.  [20]. (b) Tectonic divisions map of the BOB [20]. (c) Distribution of Paleozoic granitoids in the middle Huaniushan arc (modified after [21]). Figure 1c shows early Paleozoic felsic magmatism in the middle of the Huaniushan arc. The post-collision magmatism has produced A-type granites, and the arc magmatism is mostly I-type granites and adakite (age data from [15,18,[22][23][24][25][26][27][28]).
The Beishan Orogenic Belt (BOB) is a major part of the middle Southern CAOB (Figure 1a), as well as an important branch ocean of the PAO [20,21,29]. Four major branch oceans (as seen in Figure 1b; their former existence is evidenced by the Hongshishan ophiolite, Mingshui-Xiaohuangshan ophiolite, Hongliuhe-Niujuanzi-Xichangjing ophiolite, and Liuyuan ophiolite) and relative island arcs (Figure 1b) developed in the BOB over a long period of ocean-continental tectonic evolution during the Paleozoic, demonstrating a multi-island arc-ocean tectonic evolution framework. Some scholars have proposed that the ocean began its subduction during the Early Paleozoic [30][31][32][33], and that all of the branch oceans had closed during the Permian-Triassic period [19][20][21]32,34], influenced by the later superimposed reformation. However, other investigators conclude that the different branch oceans of the BOB closed one after the other from the Devonian to the Triassic [13,30,[35][36][37][38]. Specifically, the Hongliuhe-Niujuanzi-Xichangjing Ocean (HNXO) (Figure 1b), the major branch ocean of the PAO in the BOB, is the earliest ocean that closed [29,30,32,39]. Thus, the closing time of the HNXO is crucial to understand the tectonic evolution of the BOB during the Paleozoic. At present, there are different opinions on the closure time of the HNXO, e.g., Middle-Late Silurian or Late Devonian [11,37]. The disputes are ascribed to incomplete knowledge on the tectonic events that happened during ocean accretion, closure, and post-orogenesis in the BOB [37,40]. As a result, there is not enough reliable geological evidence to constraint the closure time of the ocean.
Most of the A-type granites in the Southern CAOB were developed during the postcollision extensional tectonic setting after the closing of the PAO [19,41,42], and offer reliable petrological and geochronological information to effectively identify the closure time of the ocean. In this paper, we present zircon U-Pb isotope as well as whole-rock major-and trace-element and Nd isotope data for the Duhongshan A-type granite from the Huaniushan arc in the Southern BOB to reveal its origin and tectonic setting, combining this with regional geological events to define the HNXO's closure time. The new results are particularly important to further study the tectonic evolution of the CAOB.
This research focuses on the middle Huaniushan arc, situated between the Niujuanzi ophiolite belt to the south and the Liuyuan ophiolite belt to the north (Figure 1b). The Huaniushan arc was generated by the subduction of the HNXO in the Early Paleozoic, which may be similar for the Japanese-type island arc [15,20,47]. The Huaniushan arc most likely has a Precambrian basement as indicated by the granitic g neiss (902-870 Ma) found in Liuyuan [48][49][50][51]. The Paleozoic to the Mesozoic in the middle of the Huaniushan arc are felsic intrusive rocks, e.g., granites, monzogranites, syenogranites, and granodiorites ( Figure 1c). Additionally, a small amount of mafic igneous rocks are also distributed in the Huaniushan arc [52]. The Neoproterozoic marble and dolomite are found in Shuangyingshan. Most of the Cambrian-Ordovician strata are located in the Niujuanzi area. The Cambrian is mostly composed of thin layers of limestone and micritic limestone, suggesting a shallow to semi-deep-sea environment. The Ordovician is a flysch formation interbedded with feldspar-bearing sandstone. The Silurian is a volcanic flysch formation composed of volcanic, volcaniclastic, and sedimentary clastic rocks. The Devonian includes limestone, sandstone, and andesite, which are only found in the Huaniushan arc [29,31,53,54]. The Carboniferous-Permian is mainly composed of basaltic to felsic volcanic rocks, carbonate and clastic sedimentary rocks in the Shibanshan arc [20,48,[54][55][56].

Description of Samples
The Duhongshan pluton addressed in this paper is located on the Northern Huaniushan arc (Figure 1c), covering an area of around 30 km 2 . It is mostly composed of Minerals 2023, 13, 565 4 of 17 monzogranite and intrudes into the Ordovician siliceous rocks. Two samples (DSH11 and DSH15) were collected for geochronological studies and fourteen samples for geochemical investigations (Figure 2). The Duhongshan granite is of light flesh-red color, medium-and coarse-grained and has a massive structure (Figure 3a,b). The samples consist mainly of the quartz (45 vol%), K-feldspar (25 vol%), plagioclase (20 vol%), biotite (~5 vol%), and a small amount of amphibole (<5 vol%) (Figure 3c). The K-feldspar and plagioclase grains are euhedral clinopyroxene. The biotite and amphibole from irregular crystals, filling in the spaces between the grains of quartz and feldspar showing late recrystallization (Figure 3c,d).

Description of Samples
The Duhongshan pluton addressed in this paper is located on the Northern Huaniushan arc (Figure 1c), covering an area of around 30 km 2 . It is mostly composed of monzogranite and intrudes into the Ordovician siliceous rocks. Two samples (DSH11 and DSH15) were collected for geochronological studies and fourteen samples for geochemical investigations (Figure 2). The Duhongshan granite is of light flesh-red color, medium-and coarse-grained and has a massive structure (Figure 3a,b). The samples consist mainly of the quartz (45 vol%), K-feldspar (25 vol%), plagioclase (20 vol%), biotite (~5 vol%), and a small amount of amphibole (<5 vol%) (Figure 3c). The K-feldspar and plagioclase grains are euhedral clinopyroxene. The biotite and amphibole from irregular crystals, filling in the spaces between the grains of quartz and feldspar showing late recrystallization ( Figure  3c,d).

Analytical Methods
Zircon U-Pb dating and whole-rock geochemistry were performed at the Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University. Whole-rock Nd isotope measurements were carried out at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd., Nanjing, China.

Analytical Methods
Zircon U-Pb dating and whole-rock geochemistry were performed at the Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University. Whole-rock Nd isotope measurements were carried out at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd., Nanjing, China.

Zircon U-Pb Dating
Heavy liquid and magnetic methods were employed to separate the zircon grains from the 5 kg samples. They were selected, placed on epoxy resin discs, polished to half their thickness, coated with carbon, and photographed under reflected and transmitted light. The Mono CL3 Cathodoluminescence (CL) System was used to acquire high-resolution CL pictures. The U-Pb isotope ratios of the selected zircons were measured using an Agilent 7500X ICP-MS instrument combined with a Geo-Las200M laser ablation (LA) system. During the test, 91,500 and NIST610 standard samples were used as the external standard and element content, respectively, and Si was used as the internal standard element. The GLITTER4.4 and ISOPLOT3.0 software were employed to process the data.

Whole-Rock Major and Trace Element Analyes
First, the studied samples rock were mechanically crushed and ground to 200 mesh. The BLi 3 O 3 alkali melting procedure was employed in this work to decompose the rock powders prior to the main element test. ICP optical emission spectroscopy (ICP-OES) was used to evaluate the compositions of the main elements. The analytical precision was better than 2%. About 0.5 g of dry sample powder was heated at 1000 • C for two hours to achieve loss on ignition (LOI). ICP-MS was used for trace elements analysis, and the Agilent 7700 instrument's analytical errors were less than 10%. The standards were based on the US Geological Survey reference documents AGV-2 and BCR-2.

Whole-Rock Nd Isotope Analyses
The Nd-bearing elution was gently evaporated to dryness and re-dissolved in 1.0 mL 2 wt.% HNO 3 . Small aliquots of each sample were analyzed for exact elements concentration by Agilent Technologies 7700 quadrupole ICP-MS (Hachioji, Tokyo, Japan). A diluted solution (50 ppb Nd) was introduced into Nu Instruments Nu Plasma II MC-ICP-MS (Wrexham, Wales, UK) through the Teledyne Cetac Technologies Aridus II Desolvating Nebulizer System (Omaha, NE, USA). Raw data of Nd isotopic ratios were internally corrected for mass fractionation by normalizing to 146 Nd/ 144 Nd = 0.7219 with the exponential law. The international isotopic standard JNdi-1 was periodically analyzed to correct instrumental drift. The geochemical reference materials of USGS RGM-2 were treated as quality control. These Nd isotopic results agree with previous publications within analytical uncertainty [50].

Analytical Results
The zircon U-Pb isotopes and trace elements data, whole-rock major and trace elements data and whole-rock Nd isotope data for the Duhongshan granites are listed in Supplementary Tables S1-S4.

Zircon U-Pb Geochronology
The samples DSH11 and DSH15 were collected from medium-grained and mediumto-fine grained monzogranite of the Duhongshan pluton, respectively. Concordia diagrams and representative CL images of the analyzed zircon grains are shown in Figure 4. The zircon grains from the sample DSH11 are short prismatic with a length ranging from 50 to 180 µm and a width ranging from 50 to 100 µm. They displayed a relative dark color in the CL image with a high content of U (Table S2), which indicates a U-rich zircon [57]. As seen in Figure 5, they display positive Ce and large negative Eu anomalies [58,59]. In addition, the zircon grains have high Th/U ratios (0.52-1.79). These features indicate a zircon of magmatic origin. Sixteen zircon grains yielded concordant 206 Pb/ 238 U ratios with a weighted mean 206 Pb/ 238 U age of 376 ± 2.1 Ma (MSWD = 1.06; n = 15), interpreted as the crystallization age of the DSH11 sample ( Figure 4). The zircon grains from the DSH15 sample are columnar with a length ranging from 100 to 200 µm and a width ranging from 60 to 80 µm. They are also U-rich (Table S2) and displayed characteristics of magmatic origin, namely high Th/U ratios (0.31-0.79) with positive Ce and large negative Eu anomalies ( Figure 5). Fourteen zircon grains have concordant 206 Pb/ 238 U ratios with a weighted mean 206 Pb/ 238 U age of 374.7 ± 2.4 Ma (MSWD = 1.08; n = 14), interpreted as the crystallization age of the DSH15 sample ( Figure 4). This age, together with the age of the DHS11 sample, suggested that the Duhongshan pluton was formed in the Late Devonian.

Whole-Rock Nd Isotope
Initial Nd isotope ratios were calculated using the zircon U-Pb ages (374 Ma). The samples from the Duhongshan pluton have uniform measured 143 Nd/ 144 Nd ratios (0.512732-0.512753), which correspond to relatively constant initial 143 Nd/ 144 Nd ratios (0.512248-0.512267). They have positive εNd(t) values from +1.9 to +2.2 with two-stage Nd model ages (TDM2) between 1.23 and 1.27 Ga (Table S4), which are older than zircon U-Pb crystallization ages.

Early-Late Paleozoic Magmatism of Middle Huaniushan Arc
Intense magmatic activities occurred during the Late Paleozoic in the middle of the Huaniushan arc, with the reported LA-ICP-MS zircon U-Pb ages being centered in the range of 424-367 Ma (Figure 1c). The majority of the rocks are granitoids (SiO2 > 63 wt.%), which include A-and I-type granites as well as adakites ( Table 1). The rocks were divided into two groups based on their tectonic setting: post-collision [18,[22][23][24][25][26] and active continental margin arc [28][29][30]. For A-type granites ( Figure 8) belonging to the post-collision group, the zircon U-Pb ages range from 415 to 371 Ma. They exhibit the characteristics of high SiO2 (63.4-80.2 wt.%) and K2O (3.7-5.3 wt.%) [15,27,28], and all the samples plot into the granite field in the TAS diagram ( Figure 6a) as well as into the high-K calc-alkaline series in the SiO2-versus-K2O diagram (Figure 6b), with are peraluminous to metaluminous (Figure 6c). These rocks are enriched in Rb, Th, K, and Pb and depleted in Ba, Sr, and Ti (Figure 7b), with significant Eu anomalies (Figure 7a), indicating that they were generated from partial melting of a relatively shallow crustal source. Furthermore, in the tectonic setting discrimination diagram (Figure 9), the samples plot into the plate granites zone. In contrast, the granites of the active continental margin arc are mostly I-type

Whole-Rock Nd Isotope
Initial Nd isotope ratios were calculated using the zircon U-Pb ages (374 Ma). The samples from the Duhongshan pluton have uniform measured 143 Nd/ 144 Nd ratios (0.512732-0.512753), which correspond to relatively constant initial 143 Nd/ 144 Nd ratios (0.512248-0.512267). They have positive ε Nd (t) values from +1.9 to +2.2 with two-stage Nd model ages (T DM2 ) between 1.23 and 1.27 Ga (Table S4), which are older than zircon U-Pb crystallization ages.

Early-Late Paleozoic Magmatism of Middle Huaniushan Arc
Intense magmatic activities occurred during the Late Paleozoic in the middle of the Huaniushan arc, with the reported LA-ICP-MS zircon U-Pb ages being centered in the range of 424-367 Ma (Figure 1c). The majority of the rocks are granitoids (SiO 2 > 63 wt.%), which include A-and I-type granites as well as adakites ( Table 1). The rocks were divided into two groups based on their tectonic setting: post-collision [18,[22][23][24][25][26] and active continental margin arc [28][29][30]. For A-type granites (Figure 8) belonging to the post-collision group, the zircon U-Pb ages range from 415 to 371 Ma. They exhibit the characteristics of high SiO 2 (63.4-80.2 wt.%) and K 2 O (3.7-5.3 wt.%) [15,27,28], and all the samples plot into the granite field in the TAS diagram (Figure 6a) as well as into the high-K calc-alkaline series in the SiO 2 -versus-K 2 O diagram (Figure 6b), with are peraluminous to metaluminous (Figure 6c). These rocks are enriched in Rb, Th, K, and Pb and depleted in Ba, Sr, and Ti (Figure 7b), with significant Eu anomalies (Figure 7a), indicating that they were generated from partial melting of a relatively shallow crustal source. Furthermore, in the tectonic setting discrimination diagram (Figure 9), the samples plot into the plate granites zone. In contrast, the granites of the active continental margin arc are mostly I-type granites ( Figure 8) and adakites, and their mineral compositions are rich in aluminum minerals [15,27,28]. They display high SiO 2 contents (Figure 6a), mainly fall into the high K and calc-alkaline (Figure 6b) series, and were from metaluminous to peraluminous (Figure 6c). Moreover, these rocks exhibit high Sr/Y ratios (6.5 to 176) with no Sr or Eu anomalies (Figure 7a) and are depleted in Nb, Ta, and Ti (Figure 7b), suggesting that they were formed by juvenile crust melting in an active continental margin arc setting, falling in the volcanic arc granite category in Figure 9. K and calc-alkaline (Figure 6b) series, and were from metaluminous to p ure 6c). Moreover, these rocks exhibit high Sr/Y ratios (6.5 to 176) with n alies (Figure 7a) and are depleted in Nb, Ta, and Ti (Figure 7b), suggesti formed by juvenile crust melting in an active continental margin arc sett volcanic arc granite category in Figure 9.

Petrogenesis of the Duhongshan Granites
Granites can be divided into S-, I-, M-, and A-types based on the geochemical and mineralogical features [66][67][68][69]. The Duhongshan granites have high K 2 O (6.6-7.2 wt.%) contents, unlike M-type granites with low K 2 O (<1 wt.%)., The Duhongshan granites have low A/CNK values (0.75-0.86) with contain no Al-rich minerals (e.g., garnet and andalusite), indicating the Duhongshan granites are not S-type granites. In addition, zircon saturation temperatures of the studied samples rang of 877-950 • C are higher than those of fractionated granites (average = 780 • C) [70], and majority samples plot in the A-type granite field rather than the field of fractionated felsic granites on the (K 2 O + Na 2 O)/CaO vs. (Zr + Nb + Ce + Y) diagram ( Figure 8). Thus, the Duhongshan granites are not highly fractionated granites.
However, remarkably, The Duhongshan granites have geochemical characteristics typical of A-type granites [64,68]. The studied sample rocks exhibit high SiO 2 , low CaO and MgO contents, a K 2 O/Na 2 O ratio of 1.1-1.6, typical of the shoshonite features (Figure 6b), a 10,000 × Ga/Al value of 4.1-4.9 and a (Zr + Nb + Ce + Y) value of 354-543 ppm. It also exhibits a high Rb (341-397 ppm), and a high zircon saturation temperature [67] between 877 and 950 • C, much higher than the I-and S-type granites and consistent with A-type granites [68]. In addition, on the 10,000 × Ga/Al vs. (Zr + Nb + Ce + Y) diagram (Figure 10), the samples plot display a evolution trend of A-type granites, which are distinct from those of other granites. Therefore, these features indicate that the Duhongshan granites are belong to A-type granites.
The Duhongshan granites display the same geochemical evolution collision magmatism from the middle Huaniushan arc, such as Al2O3, Ca Ba, and Nb being negatively correlated with SiO2 ( Figure 11). They have Na2O than other granitoids with belong the shoshonite series (Figure 6b) are a late product of post-collision magmatism. The Mg # values for partial is less than 45, whereas partial melts involving mantle material exceeds 45 that the Duhongshan granites (Mg # values of 5-26) were derived from the of crustal material. Moreover, the Duhongshan granites are enriched in Th ure 7b), and the Rb/Nb ratios (8.5-14.1) are close to the crustal value [60]. T The Duhongshan granites display the same geochemical evolution trends as postcollision magmatism from the middle Huaniushan arc, such as Al 2 O 3 , CaO, Fe 2 O 3 , Na 2 O, Ba, and Nb being negatively correlated with SiO 2 (Figure 11). They have higher K 2 O and Na 2 O than other granitoids with belong the shoshonite series (Figure 6b), indicating they are a late product of post-collision magmatism. The Mg # values for partial melting magma is less than 45, whereas partial melts involving mantle material exceeds 45 [77], indicating that the Duhongshan granites (Mg # values of 5-26) were derived from the partial melting of crustal material. Moreover, the Duhongshan granites are enriched in Th, U, and K (Figure 7b), and the Rb/Nb ratios (8.5-14.1) are close to the crustal value [60]. The Nb/Ta ratios are close to the lower crust value (8.3) [78], a good proxy that indicates that the studied samples are formed by the partial melting of crust-derived material. However otherwise, the studied samples have positive whole-rock ε Nd (t) values (1.87 to 2.20), indicating the source of the Duhongshan A-type granites add the mantle material. The two-stage Nd isotopic model ages (T DM2 ) of the Duhongshan granites are in the range of 1226-1270 Ma. Therefore, these characteristics lead us to suggest that the Duhongshan A-type granites originated from the partial melting of the Mesoproterozoic lower crust material, with minor part from the mantle.
In conclusion, the Late Devonian A-type granites from the Duhongshan were generated from the heating by asthenosphere underplating subsequently resulted partial melting of the lower crust, and the underwent fractional crystallization. Minerals 2023, 13, x FOR PEER REVIEW 13 of 19 Figure 11. Harker diagrams for granitoids from the middle Huaniushan arc. Geochemical data of post−collision from [18,[22][23][24][25][26].

Late Silurian-Devonian Magmatism Responses to Closure of PAO
The BOB comprises a complete record of the subduction, closure, and post-collision evolution history of the PAO during the Paleozoic-Mesozoic [11,33,34,37,79]. The 424-367 Ma magmatic activities are most widely distributed in the middle Huaniushan arc in the BOB and linked with ocean subduction and the continental collision of branch oceans of the PAO, and they are able to provide credible proof of the closure of the ocean [24,28]. The HNXO (Hongliuhe-Niujuanzi-Xichangjing ophiolite) is believed to be the earliestsubducted ocean (536-464 Ma) [32,39,40,80], and the ocean subduction was initiated in the Early Paleozoic. The subduction-related island arc-type magmatism developed in the Huaniushan arc and the Gongpoquan arc, which are believed to be the result of the southward subduction of the HNXO [17,28,31,40], accompanied by a series of brittle-ductile shearing deformation [6]. Additionally, this ocean continued to subduct during the Silurian, also supported by the Gongpoquan groups, resulting in a series of volcanic-sedimentary formations with felsic volcanic rocks in the island arc environments [54].
However, there is an abundance of geological evidence to support the theory that the HNXO began to close and collide during the Early Devonian. (a) SHRIMP dating of the cumulate gabbro, an important part of the Hongliuhe ophiolite, have provided a crystallization age from 520 Ma to 516 Ma, while the age of the undeformed biotite granites invading the Hongliuhe ophiolite is 404 Ma, and the SHRIMP zircon age of the undeformed granite overlying it is 413 Ma [30,80]. (b) The bedded limestone and silica-rich deposits in the central part of the BOB, which were produced in deep-sea environments during the Middle-Late Cambrian, are unconformably underlined by Devonian sedimentary rocks [57]. (c) Thick Devonian molassic deposits [53][54][55][56] and the large post-orogenic granite crop out in the Huaniushan arc (Figure 1c). These features suggest that the HNXO was closed in the Early Devonian. According to geochemical data (Y/Nb and Rb/Nb ratios) as well as tectonic information, the A-type granites can be further divided into A1-type in non-orogenic and A2-type in syn-collision or post-collision tectonic environments [66]. The Duhongshan granites exhibit high Y/Nb (3.2-5.3) and Rb/Nb (8.5-14.1) ratios and samples plot into the WPG field in Figure 9a,b, indicating they are A2-type granites formed in postcollision tectonic settings. They exhibit homogeneous geochemical characteristics for the granitoids in the middle Huaniushan arc, which were formed in post-collisional settings during the Silurian-Devonian (Figures 7a,b, 8 and 9). Beyond that, the lower Devonian

Late Silurian-Devonian Magmatism Responses to Closure of PAO
The BOB comprises a complete record of the subduction, closure, and post-collision evolution history of the PAO during the Paleozoic-Mesozoic [11,33,34,37,79]. The 424-367 Ma magmatic activities are most widely distributed in the middle Huaniushan arc in the BOB and linked with ocean subduction and the continental collision of branch oceans of the PAO, and they are able to provide credible proof of the closure of the ocean [24,28]. The HNXO (Hongliuhe-Niujuanzi-Xichangjing ophiolite) is believed to be the earliest-subducted ocean (536-464 Ma) [32,39,40,80], and the ocean subduction was initiated in the Early Paleozoic. The subduction-related island arc-type magmatism developed in the Huaniushan arc and the Gongpoquan arc, which are believed to be the result of the southward subduction of the HNXO [17,28,31,40], accompanied by a series of brittle-ductile shearing deformation [6]. Additionally, this ocean continued to subduct during the Silurian, also supported by the Gongpoquan groups, resulting in a series of volcanic-sedimentary formations with felsic volcanic rocks in the island arc environments [54].
However, there is an abundance of geological evidence to support the theory that the HNXO began to close and collide during the Early Devonian. (a) SHRIMP dating of the cumulate gabbro, an important part of the Hongliuhe ophiolite, have provided a crystallization age from 520 Ma to 516 Ma, while the age of the undeformed biotite granites invading the Hongliuhe ophiolite is 404 Ma, and the SHRIMP zircon age of the undeformed granite overlying it is 413 Ma [30,80]. (b) The bedded limestone and silica-rich deposits in the central part of the BOB, which were produced in deep-sea environments during the Middle-Late Cambrian, are unconformably underlined by Devonian sedimentary rocks [57]. (c) Thick Devonian molassic deposits [53][54][55][56] and the large post-orogenic granite crop out in the Huaniushan arc (Figure 1c). These features suggest that the HNXO was closed in the Early Devonian. According to geochemical data (Y/Nb and Rb/Nb ratios) as well as tectonic information, the A-type granites can be further divided into A1-type in non-orogenic and A2-type in syn-collision or post-collision tectonic environments [66]. The Duhongshan granites exhibit high Y/Nb (3.2-5.3) and Rb/Nb (8.5-14.1) ratios and samples plot into the WPG field in Figure 9a,b, indicating they are A2-type granites formed in post-collision tectonic settings. They exhibit homogeneous geochemical characteristics for the granitoids in the middle Huaniushan arc, which were formed in post-collisional settings during the Silurian-Devonian (Figure 7a,b, Figures 8 and 9). Beyond that, the lower Devonian stratigraphic strata of volcanic-sedimentary sequence in the southern part of the BOB indicate an extensional tectonic setting, distinct from the molassic deposits [11,38]. This indicates that the oceans closed during the Ordovician-Silurian, and that the region was an unstable environment. In the Late Devonian, this region entered the post-collisional extensional tectonic stage. During that period, the crust was thickened continuously due to the subduction and closure of the HNXO with occurred slab breakoff, and the lower crust was heated by the upwelling of asthenosphere magma, which resulted in the partial melting of the lower crust and developed the large-scale post-collisional magmatism in the Huaniushan arc.
In addition, the Late Silurian-Devonian magmatism of active continental margin arc is also wide distribution in the middle Huaniushan arc (Figures 1c and 9), and it is mainly present as calc-alkaline I-type granites and adakites formed by the melting of the subducting plate [17,28,37]. Middle Silurian to Devonian Liuyuan turbidite and adakite were formed by the subduction of the Liuyuan Ocean in the Huaniushan region [37,81]. The early Carboniferous Hongliuyuan formation in the south of the BOB is a main part of the volcanic arc, deposited in a sedimentary basin adjacent to the island arc [82]. This indicates that the Liuyuan Ocean continued northward subduction during the Early Carboniferous [31] and closure in the Late Permian to Early Triassic [46]. In conclusion, the magmatic activities of the active continental margin arc are the product of the northward subduction of the Liuyuan Ocean on the southern margin of the Paleo-Asian Ocean during the Late Paleozoic.

Devonian Tectonic Evolution of the Southern BOB
Combined with the reported data, we developed a tectonic evolution model for the Southern BOB, shown in Figure 12. At this time, the main tectonic units in the south of the BOB are the Gongpoquan arc, the Huaniushan arc (microcontinental block), the HNXO (Niujuanzi ophiolitic mélange), and the Liuyuan Ocean (Liuyuan accretive wedge). From the end of the Silurian to the Early Devonian, the HNXO underwent southward subduction and closed, and then evolved into the post-collision extensional tectonic stage, accompanied by magmatic activities of the post-collision of this area [18,[22][23][24][25][26], then, the Liuyuan Ocean underwent northward subduction to the Huaniushan arc, forming a large amount of calc-alkaline felsic rocks in the active continental margin arc [15,27,28]. stratigraphic strata of volcanic-sedimentary sequence in the southern part of the BOB indicate an extensional tectonic setting, distinct from the molassic deposits [11,38]. This indicates that the oceans closed during the Ordovician-Silurian, and that the region was an unstable environment. In the Late Devonian, this region entered the post-collisional extensional tectonic stage. During that period, the crust was thickened continuously due to the subduction and closure of the HNXO with occurred slab breakoff, and the lower crust was heated by the upwelling of asthenosphere magma, which resulted in the partial melting of the lower crust and developed the large-scale post-collisional magmatism in the Huaniushan arc.
In addition, the Late Silurian-Devonian magmatism of active continental margin arc is also wide distribution in the middle Huaniushan arc (Figures 1c and 9), and it is mainly present as calc-alkaline I-type granites and adakites formed by the melting of the subducting plate [17,28,37]. Middle Silurian to Devonian Liuyuan turbidite and adakite were formed by the subduction of the Liuyuan Ocean in the Huaniushan region [37,81]. The early Carboniferous Hongliuyuan formation in the south of the BOB is a main part of the volcanic arc, deposited in a sedimentary basin adjacent to the island arc [82]. This indicates that the Liuyuan Ocean continued northward subduction during the Early Carboniferous [31] and closure in the Late Permian to Early Triassic [46]. In conclusion, the magmatic activities of the active continental margin arc are the product of the northward subduction of the Liuyuan Ocean on the southern margin of the Paleo-Asian Ocean during the Late Paleozoic.

Devonian Tectonic Evolution of the Southern BOB
Combined with the reported data, we developed a tectonic evolution model for the Southern BOB, shown in Figure 12. At this time, the main tectonic units in the south of the BOB are the Gongpoquan arc, the Huaniushan arc (microcontinental block), the HNXO (Niujuanzi ophiolitic mélange), and the Liuyuan Ocean (Liuyuan accretive wedge). From the end of the Silurian to the Early Devonian, the HNXO underwent southward subduction and closed, and then evolved into the post-collision extensional tectonic stage, accompanied by magmatic activities of the post-collision of this area [18,[22][23][24][25][26], then, the Liuyuan Ocean underwent northward subduction to the Huaniushan arc, forming a large amount of calc-alkaline felsic rocks in the active continental margin arc [15,27,28].

Conclusions
A petrographic, geochronological, whole-rock major and trace element, and Nd isotopic study of Late Devonian granites in the BOB has led to the following conclusions.

Conclusions
A petrographic, geochronological, whole-rock major and trace element, and Nd isotopic study of Late Devonian granites in the BOB has led to the following conclusions.
(1) The LA-ICP-MS zircon U-Pb dating places the Duhongshan granites in the range of 376-374 Ma. (2) The Duhongshan granites exhibit features of A2-type granites, suggesting that they were formed by partial melting of lower crustal material in post-collision extension settings. (3) Combined with our new studies and previous regional investigations, we propose that the HNXO of the branch PAO closed during the Devonian.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/min13040565/s1, Table S1: Results of U-Pb dating by of zircons from the Duhongshan granites; Table S2: Zircon trace elements (ppm) analysis result of the Duhongshan granites; Table S3: Whole-rock major (wt.%) and trace element (ppm) analyses of the Duhongshan granites; Table S4: Whole-rock Nd isotopic data for the Duhongshan granites.