Tectonic Transition from Passive to Active Continental Margin of Nenjiang Ocean: Insight from the Middle Devonian-Early Carboniferous Granitic Rocks in Northern Great Xing’an Range, NE China

: Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al 2 O 3 /(MgO+FeO T ) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian


Introduction
The Central Asian Orogenic Belt (CAOB), which is tripled by the European Craton to the west, the Siberia Craton to the north, and the Tarim-North China Craton to the

Introduction
The Central Asian Orogenic Belt (CAOB), which is tripled by the European Craton to the west, the Siberia Craton to the north, and the Tarim-North China Craton to the south, is a huge accretionary orogenic belt in the world [1][2][3][4][5][6][7][8]. The formation of CAOB is closely related to the evolution of the Paleo-Asian Ocean (PAO), which was a typical archipelagic ocean consisting of a series of tectonic elements such as microcontinental blocks, accretionary wedges, oceanic islands/plateaus, ophiolites, and accretionary complexes in most Paleozoic times [3,[9][10][11][12][13][14][15][16]. The complex evolutionary history of these units and the huge oil and resource potential within the belt have attracted plenty of scholars to explore the special geological development history of it [11,12,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. Northeast China (NE China), which occupies the majority parts of the eastern CAOB, mainly consists of Erguna Block (EB), Xing'an Accretionary Terrane (XAT), Songliao Accretionary Terrane (SAT), Jiamusi Block (JB), Khanka Block (KB), Nadanhada Accretionary Terrane (NAT), and Xinlin-Xiguitu Suture (XXS), Hegenshan-Heihe Suture (HHS), and Mudanjiang-Yilan Suture (MYS) from northwest to southeast ( Figure 1b) [8,[32][33][34][35][36][37][38]. The HHS records the amalgamation suture zone of XAT and SAT [8,[38][39][40]. This tectonic belt extends from Erenhot to Hegenshan in the south, Dashizhai-Moguqi in the middle, and Nenjiang-Heihe in the north ( Figure 1). It is one of the most important regional sutures between Siberia Craton and North China Craton, the study of which can help us understand the Paleozoic tectonics of NE China [34,[41][42][43][44]. However, the tectonic evolution of HHS has still been controversial because of the weak study in the northern part of the suture. The tectonic history of HHS is mainly attributed to the evolution of the Nenjiang Ocean (a branch ocean of eastern PAO) during the Early Paleozoic to early Late Paleozoic [8,45,46]. The Nenjiang Ocean is believed to have started subducting beneath XAT since the Early Paleozoic and closed during the early Late Carboniferous [25,26]. The Devonian was usually seen as a quiet period (420 Ma~360 Ma) of tectonomagmatism in the Nenjiang Ocean because of the lack of a magmatic record [8,34] and widely developed sediments such as the Niqiuhe Formation in central and northern The HHS records the amalgamation suture zone of XAT and SAT [8,[38][39][40]. This tectonic belt extends from Erenhot to Hegenshan in the south, Dashizhai-Moguqi in the middle, and Nenjiang-Heihe in the north ( Figure 1). It is one of the most important regional sutures between Siberia Craton and North China Craton, the study of which can help us understand the Paleozoic tectonics of NE China [34,[41][42][43][44]. However, the tectonic evolution of HHS has still been controversial because of the weak study in the northern part of the suture. The tectonic history of HHS is mainly attributed to the evolution of the Nenjiang Ocean (a branch ocean of eastern PAO) during the Early Paleozoic to early Late Paleozoic [8,45,46]. The Nenjiang Ocean is believed to have started subducting beneath XAT since the Early Paleozoic and closed during the early Late Carboniferous [25,26]. The Devonian was usually seen as a quiet period (420 Ma~360 Ma) of tectono-magmatism in the Nenjiang Ocean because of the lack of a magmatic record [8,34] and widely developed sediments such as the Niqiuhe Formation in central and northern Great Xing'an Range [47,48], which may indicate a passive continental marginal tectonic setting during the Devonian, and the northwestward subduction restarted at the beginning of the Early Carboniferous [35,49,50]. But the tectonic process of this transition from passive to active continental margin remains poorly understood due to a lack of direct information, especially from the magmatism aspect, which seriously blocks our understanding of the tectonic evolution of the Nenjiang Ocean.
During the field survey, we first discovered some Middle-to-Late Devonian and Early Carboniferous granitic rocks in the northern Great Xing'an Range, which just formed in and after the tectonic quiet period. Study of these granitic rocks can well uncover the Minerals 2023, 13, 1003 3 of 16 tectonic process of the transition from passive to active continental margin of the Nenjiang Ocean. In this study, we performed petrological, geochemical, and geochronological analyses on granitic rocks exposed in the Woluohe area to determine their crystallization age, petrogenesis, and tectonic setting to reveal the tectonic transition process from passive continental margin to initial subduction of the Nenjiang Ocean from the Devonian to Early Carboniferous periods.

Geological Setting
The Woluohe area is located in the eastern part of the Northern Greater Xing'an Range, adjacent to the northwest side of HHS ( Figure 1). Some Late Paleozoic granite outcrops exist in this area ( Figure 2). According to our recent research, these granites can be divided into Middle Devonian-Early Carboniferous granodiorite and monzogranite, and Early Permian monzogranite. Middle Devonian-Early Carboniferous granodiorite and monzogranite underwent dynamic metamorphism and became granitic mylonite. In addition, the Early Cretaceous monzogranite was distributed sporadically in the southwest of the study area. The outcrop strata consist of Early Cretaceous volcanic rocks, and the main lithology is rhyolite ( Figure 2). In this study, we focus on the Middle Devonian-Early Carboniferous granitic rocks.
setting during the Devonian, and the northwestward subduction restarted at the begin-ning of the Early Carboniferous [35,49,50]. But the tectonic process of this transition from passive to active continental margin remains poorly understood due to a lack of direct information, especially from the magmatism aspect, which seriously blocks our understanding of the tectonic evolution of the Nenjiang Ocean.
During the field survey, we first discovered some Middle-to-Late Devonian and Early Carboniferous granitic rocks in the northern Great Xing'an Range, which just formed in and after the tectonic quiet period. Study of these granitic rocks can well uncover the tectonic process of the transition from passive to active continental margin of the Nenjiang Ocean. In this study, we performed petrological, geochemical, and geochronological analyses on granitic rocks exposed in the Woluohe area to determine their crystallization age, petrogenesis, and tectonic setting to reveal the tectonic transition process from passive continental margin to initial subduction of the Nenjiang Ocean from the Devonian to Early Carboniferous periods.

Geological Setting
The Woluohe area is located in the eastern part of the Northern Greater Xing'an Range, adjacent to the northwest side of HHS ( Figure 1). Some Late Paleozoic granite outcrops exist in this area ( Figure 2). According to our recent research, these granites can be divided into Middle Devonian-Early Carboniferous granodiorite and monzogranite, and Early Permian monzogranite. Middle Devonian-Early Carboniferous granodiorite and monzogranite underwent dynamic metamorphism and became granitic mylonite. In addition, the Early Cretaceous monzogranite was distributed sporadically in the southwest of the study area. The outcrop strata consist of Early Cretaceous volcanic rocks, and the main lithology is rhyolite ( Figure 2). In this study, we focus on the Middle Devonian-Early Carboniferous granitic rocks.   Samples PM421TW1 and 16TK16TW1 are granitic mylonites collected from Fengshoucun. The sample exhibits a classical granitic mylonite texture, and the size of the mineral grain ranges from medium to fine. The mineral assemblage is plagioclase (~25%) with a grain size of 0.5~5 mm, alkali feldspar and quartz (~60%) with a grain size of 0.5~1.5 mm, fine particle felsic minerals (~15%) with a grain size of less than 0.1 mm (Figure 3d,e), and some accessory minerals consisting of magnetite and zircon.

Sample Collection
( Figure 3d,e), and some accessory minerals consisting of magnetite and zircon.
Sample 16TK17TW1 is granitic mylonite collected from northwest Shenglitun. The sample exhibits a classical granitic mylonite texture, and the size of the mineral grain ranges from medium to fine. The mineral assemblage presented is plagioclase (~40%) with a grain size of 0.5~3 mm, quartz (~20%) with a grain size of 0.2~1 mm, biotite (~10%) with a grain size of 0.5~1.5 mm, and fine particle felsic minerals (~30%) with a grain size of less than 0.05 mm (Figure 3f).

Analytical Methods
Major and trace element analyses were completed at the Northeast Mineral Resources and Supervision Testing Center, Ministry of Land and Resources, Shenyang, China. Separation of zircon grains was completed at the Institute of Regional Geology and Mineral Survey Laboratory, Langfang, China. Targetry and photography of zircon grains were completed at Beijing Zhongxingmeike Technology Co., Ltd, Beijing, China. Zircon LA-ICP-MS U-Pb dating and Lu-Hf isotope analysis were completed in the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources, Tianjin, China. The main instrument parameters, experimental procedures, and data analysis methods are shown in [51].

Geochemistry
Major (wt.%) and trace (×10 −6 ) element data for the granitic rocks in the Woluohe area are shown in Table S1 in the Supplementary Materials.

Analytical Methods
Major and trace element analyses were completed at the Northeast Mineral Resources and Supervision Testing Center, Ministry of Land and Resources, Shenyang, China. Separation of zircon grains was completed at the Institute of Regional Geology and Mineral Survey Laboratory, Langfang, China. Targetry and photography of zircon grains were completed at Beijing Zhongxingmeike Technology Co., Ltd, Beijing, China. Zircon LA-ICP-MS U-Pb dating and Lu-Hf isotope analysis were completed in the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources, Tianjin, China. The main instrument parameters, experimental procedures, and data analysis methods are shown in [51].

Geochemistry
Major (wt.%) and trace (×10 −6 ) element data for the granitic rocks in the Woluohe area are shown in Table S1

Zircon LA-ICP-MS U-Pb Dating
Zircon LA-ICP-MS U-Pb data for the granitic rocks in the Woluohe area are shown in Table S2.
The zircons in granitic mylonite (PM421TW1) are idiomorphic columnar, and the

Zircon LA-ICP-MS U-Pb Dating
Zircon LA-ICP-MS U-Pb data for the granitic rocks in the Woluohe area are shown in Table S2.
The zircons in granitic mylonite (PM421TW1) are idiomorphic columnar, and the zircon crystals are 120~150 µm long. Ratios of the length and width range from 1.5:1 to 2:1, which present obvious oscillatory zoning ( Figure 6). Moreover, the zircons have characteristics of typical magmatic zircons, with Th/U ratios of 0.32~2.36 (except 0.06). The isotope age of 24 zircon points falls on the concordant curve and its periphery (Figure 7a), and the 206 Pb/ 238 U age is between 364 ± 4 Ma and 376 ± 5 Ma. The weighted average is 368 ± 2 Ma (MSWD = 0.76).
The zircons in granitic mylonite (16TK16TW1) are idiomorphic columnar, and th zircon crystals are 100~120 μm long. Ratios of the length and width range from 1.2:1 t 1.5:1, which present obvious oscillatory zoning ( Figure 6). Moreover, the zircons hav characteristics of typical magmatic zircons, with Th/U ratios of 0.73~1.36. The isotop age of 20 zircon points falls on the concordant curve and its periphery (Figure 7a), an the 206 Pb/ 238 U age is between 381 ± 4 Ma and 395 ± 4 Ma. The weighted average is 389 ± Ma (MSWD = 0.58).
The zircons in granitic mylonite (16TK17TW1) are idiomorphic columnar, and th zircon crystals are 120~150 μm long. Ratios of the length and width range from 1.2:1 t 1.5:1, which present obvious oscillatory zoning ( Figure 6). Moreover, the zircons hav characteristics of typical magmatic zircons, with Th/U ratios of 0.70~1.56. The isotop age of 22 zircon points falls on the concordant curve and its periphery (Figure 7a), an the 206 Pb/ 238 U age is between 347 ± 4 Ma and 354 ± 4 Ma. The weighted average is 351 ± Ma (MSWD = 0.32).

Zircon Lu-Hf Isotopic
We analyzed the zircon Lu-Hf isotope after U-Pb isotopic dating, detailed data are shown in Table S3. The zircons in granitic mylonite (16TK16TW1) are idiomorphic columnar, and the zircon crystals are 100~120 µm long. Ratios of the length and width range from 1.2:1 to 1.5:1, which present obvious oscillatory zoning ( Figure 6). Moreover, the zircons have characteristics of typical magmatic zircons, with Th/U ratios of 0.73~1. 36. The isotope age of 20 zircon points falls on the concordant curve and its periphery (Figure 7a), and the 206 Pb/ 238 U age is between 381 ± 4 Ma and 395 ± 4 Ma. The weighted average is 389 ± 2 Ma (MSWD = 0.58).
The zircons in granitic mylonite (16TK17TW1) are idiomorphic columnar, and the zircon crystals are 120~150 µm long. Ratios of the length and width range from 1.2:1 to 1.5:1, which present obvious oscillatory zoning ( Figure 6). Moreover, the zircons have characteristics of typical magmatic zircons, with Th/U ratios of 0.70~1.56. The isotope age of 22 zircon points falls on the concordant curve and its periphery (Figure 7a), and the 206 Pb/ 238 U age is between 347 ± 4 Ma and 354 ± 4 Ma. The weighted average is 351 ± 2 Ma (MSWD = 0.32).

Zircon Lu-Hf Isotopic
We analyzed the zircon Lu-Hf isotope after U-Pb isotopic dating, detailed data are shown in Table S3. 22 zircon Lu-Hf analytical points are performed on granitic mylonite samples (PM421TW1 and 16TK17TW1). Most points 176 Lu/ 177 Hf ratios are from 0.0007 to 0.0042 (less than 0.002), and the total average is 0.0013, and this expresses that the zircons have a lower accumulation of radiogenic Hf in the process of rock formation, so we could explore genetic information of the granites by zircon 176 Hf/ 177 Hf ratios [56,57]. All points f Lu/Hf values are between −0.98 and −0.87, which is lower than those of the mafic crust (−0.34) and felsic crust (−0.72) [58]. Therefore, the two-stage model better reflects the timing of the extraction of source material from the depleted mantle or the residence time of source material in the crust.
The sample PM421TW1 has relatively high 176 Hf/ 177 Hf values. The ε Hf (t) values vary from 7.7 to 14.0. The two-stage model age of granite samples is 523 Ma~1087 Ma, representing the Early Paleozoic-Mesoproterozoic magmatic source. The sample 16CT17TW1 has relatively high 176 Hf/ 177 Hf values. The ε Hf (t) values vary from 6.2 to 15.0. The twostage model age of granite samples is 417 Ma~1211 Ma, representing the Late Paleozoic-Mesoproterozoic magmatic source. In summary, the zircon Lu-Hf isotopic data indicate that they are characterized by positive ε Hf (t) values and a relatively young two-stage model age.

A Paleozoic Tectono-Magmatism Quiet Period
Liu et al. [50] carried out a systematical analysis of the Paleozoic magmatic events of each tectonic unit of NE China in the eastern CAOB. The statistics show that all tectonic units have generally similar magmatic event peaks of the Early and Late Paleozoic magmatic stages with age populations of 510~420 Ma and 360~250 Ma, respectively, and with a relatively quiet period almost without magmatism from 420 to 360 Ma, during which shallow marine sequence (e.g., Niqiuhe Formation) was developed. This~60 Myr quiet period of the Devonian has now been accepted by most researchers [8,[24][25][26]33,35,37,38]. This phenomenon is clearly reflected in the XAT and SAT (Figure 8).
However, we know that this quiet period just had a sharp magmatism declines rather than no magmatism at all. Some minor magmatic peaks can still be seen (Figure 8), but there were no magmatic events previously reported close to the HHS zone. Recently, we found small scale granitic plutons on the northwest side of HHS in the Nenjiang region, Inner Mongolia (China). Geochronological data shows that they formed from the Middle Devonian to Early Carboniferous with crystal ages of 389 Ma, 368 Ma, and 351 Ma, which were just in the time of the quiet period and provide perfect proof for the study of the tectonic transition from the quiet period of the Devonian to the reactive period of the Early Carboniferous. with a relatively quiet period almost without magmatism from 420 to 360 Ma, during which shallow marine sequence (e.g., Niqiuhe Formation) was developed. This ~60 Myr quiet period of the Devonian has now been accepted by most researchers [8,[24][25][26]33,35,37,38]. This phenomenon is clearly reflected in the XAT and SAT (Figure 8). However, we know that this quiet period just had a sharp magmatism declines rather than no magmatism at all. Some minor magmatic peaks can still be seen (Figure 8), but there were no magmatic events previously reported close to the HHS zone. Recently, we found small scale granitic plutons on the northwest side of HHS in the Nenjiang region, Inner Mongolia (China). Geochronological data shows that they formed from the Middle Devonian to Early Carboniferous with crystal ages of 389 Ma, 368 Ma, and 351 Ma, which were just in the time of the quiet period and provide perfect proof for the study of the tectonic transition from the quiet period of the Devonian to the reactive period of the Early Carboniferous.

Petrogenesis
The three samples from the Woluohe area show very narrow variations of major elements (SiO2, Al2O3, K2O, and Na2O) and low LOI contents, indicating they have not experienced or have experienced very minor contamination during the magmatic evolution process. However, the negative Nb and Ta contents suggest a significant crustal magmatic contribution to the composition of all three plutons. Meanwhile, we notice that the Middle Devonian granitic rocks are without Eu anomalies, and the Late Devonian and Early Carboniferous granitic rocks have positive Eu anomalies; these features are distinctively distinguished from the highly fractionated regional rocks, which are clearly characterized by the negative Eu anomaly. So, the formation of the Middle Devonian to Early Carboniferous granitic rocks has no or very minor contamination affection and without clear crystallization and fractionation during their magma evolution process; thus, we can use their geochemical features to discuss their petrogenesis.

Petrogenesis
The three samples from the Woluohe area show very narrow variations of major elements (SiO 2 , Al 2 O 3 , K 2 O, and Na 2 O) and low LOI contents, indicating they have not experienced or have experienced very minor contamination during the magmatic evolution process. However, the negative Nb and Ta contents suggest a significant crustal magmatic contribution to the composition of all three plutons. Meanwhile, we notice that the Middle Devonian granitic rocks are without Eu anomalies, and the Late Devonian and Early Carboniferous granitic rocks have positive Eu anomalies; these features are distinctively distinguished from the highly fractionated regional rocks, which are clearly characterized by the negative Eu anomaly. So, the formation of the Middle Devonian to Early Carboniferous granitic rocks has no or very minor contamination affection and without clear crystallization and fractionation during their magma evolution process; thus, we can use their geochemical features to discuss their petrogenesis.
All samples are calc-alkaline rocks with low (Na 2 O+K 2 O)/CaO ratios and Zr+Nb+Ce+Y total contents, displaying typical I-type granite features ( Figure 9a); meanwhile, they are all with high Si, Al, K, and Na contents but with low Mg, Fe, and Ti contents, which suggests they come from partial melting of crustal materials rather than mantle source regions. This is consistent with their enriched trace element features and negative Nb and Ta contents.
For crustal source rocks, their source material composition can alternatively be varied from metasediments (including both metapelites and metagreywackes) and igneous rocks, or both. Usually, the metasediments have high Al contents versus their igneous counterparts; metapelites have lower Ca contents than metagreywackes [59]. Thus, it can use these indexes of C/MF and A/MF to discriminate the source compositions of the granitic rocks from the Nenjiang area. The analysis results show that all three samples had low Al contents, but sample PM421TW1 was plotted in metagreywacke areas, and samples 16TK16TW1 and 16TK17TW1 were mainly plotted in meta basaltic to meta tonalitic source areas (Figure 9b). So, the magmatic source of Middle Devonian granitic rocks may be the partial melting of igneous rocks contaminated by metagreywackes. Whereas Late Devonian and Early Carboniferous granitic rocks were only formed from partial melting of igneous rocks. In respect to the Hf isotopic, granitic rocks from the West Nenjiang area were shown positive ε Hf (t) futures (Figure 10), suggesting that the source of those igneous rocks was derived from partial melting of young lower crust materials that fractionated shortly from the lithospheric mantle.
use these indexes of C/MF and A/MF to discriminate the source compositions of the granitic rocks from the Nenjiang area. The analysis results show that all three samples had low Al contents, but sample PM421TW1 was plotted in metagreywacke areas, and samples 16TK16TW1 and 16TK17TW1 were mainly plotted in meta basaltic to meta tonalitic source areas (Figure 9b). So, the magmatic source of Middle Devonian granitic rocks may be the partial melting of igneous rocks contaminated by metagreywackes. Whereas Late Devonian and Early Carboniferous granitic rocks were only formed from partial melting of igneous rocks. In respect to the Hf isotopic, granitic rocks from the West Nenjiang area were shown positive εHf(t) futures (Figure 10), suggesting that the source of those igneous rocks was derived from partial melting of young lower crust materials that fractionated shortly from the lithospheric mantle.   nitic rocks from the Nenjiang area. The analysis results show that all three samples had low Al contents, but sample PM421TW1 was plotted in metagreywacke areas, and samples 16TK16TW1 and 16TK17TW1 were mainly plotted in meta basaltic to meta tonalitic source areas (Figure 9b). So, the magmatic source of Middle Devonian granitic rocks may be the partial melting of igneous rocks contaminated by metagreywackes. Whereas Late Devonian and Early Carboniferous granitic rocks were only formed from partial melting of igneous rocks. In respect to the Hf isotopic, granitic rocks from the West Nenjiang area were shown positive εHf(t) futures (Figure 10), suggesting that the source of those igneous rocks was derived from partial melting of young lower crust materials that fractionated shortly from the lithospheric mantle.

Middle Devonian
A large area of Devonian strata is exposed in the Great Xing'an Range [47]. The Early Devonian Niqiuhe Formation is a terrigenous clastic-carbonate rock assemblage with brachiopods, bivalves, graptolites, and corals [62,63]. The studies on lithofacies characteristics, fossil assemblage, and storm sedimentary sequence indicate that it formed in a shore-shallow Marine sedimentary environment [48,[63][64][65]. The late Middle and Late Devonian Daminshan Formation is composed of volcanic (clastic) rock, terrigenous clastic rock, carbonate rock, and siliceous rock assemblages with brachiopods, corals, and ammonoids [47], formed in the continental slope and marginal sea environment near the continent [66]. Zhu and Liu [67] reported that the basic and intermediate-basic rocks at the Lower Daminshan Formation formed in the continental margin rift environment, while the basic-acid volcanic rocks at the Upper Daminshan Formation were island arc calc-alkaline rocks formed in the continental margin volcanic arc environment. Liu [68] identified a set of mafic rocks, intermediate rocks, and acid arc volcanic rocks assemblages at the top of the Daminshan Formation. Sun [69] obtained the zircon U-Pb isotopic age of the rhyolite from the top of the Daminshan Formation as 360 ± 2 Ma. This evidence indicates that in the Early Devonian, the Great Xing'an Range should have been in a stable continental margin environment and deposited shallow Marine sedimentary sequences. In the late Middle and Late Devonian, the emergence of arc volcanic rocks indicated that the tectonic environment changed and the subduction of the Nenjiang Ocean started.
Previously, studies have seldom referred to the tectonic evolution of the Nenjiang Ocean in the Devonian because of the lack of magmatic rocks. They usually suggest a quiet tectonic setting for the entire NE China region during the Devonian [50]. However, our new finding of a few Devonian granitic rocks in this study shows there was stilllimited magmatism in the region during the Devonian. Our analysis shows that the Middle Devonian granitic rocks have calc-alkali magmatic series and rightward fractionated geochemistry features, indicating I-type granites formed on a subduction background. The characteristics of low Sr (≤400 ppm), high Y (18 ppm), and Yb (>2 ppm) contents are different from typical adakitic rocks [70], indicating that the Middle Devonian granitic rocks are classical island arc rock series that formed related to oceanic lithosphere subduction. In the granite rock background discrimination diagram (Figure 11a,b), all the Middle Devonian granitic rock samples are plotted in the classical island arc rock region, further confirming the subduction setting of these rocks. Therefore, there was some subduction-related magmatism in the Middle Devonian. However, the pluton we reported in this study was of very limited scale, and there were very minor coeval rocks across the Great Xing'an Range region ( Figure 12) [71][72][73]. These facts indicate there was only limited local subduction and minor related magmatism during the Middle Devonian in NE China [50]. Our study area is located on the northwest side of the northern HHS, and combined with previous studies [46], we suggest that during the Middle Devonian, the northwestward subduction of the Nenjiang Ocean had just initiated.

Late Devonian to Early Carboniferous
Similar to Middle Devonian granitic rocks, the Late Devonian to Early Carboniferous granitic rocks also have subduction-related geochemical features such as calcalkaline magmatic series and rightward fractionated REE and trace element signatures. However, they have relatively high Sr (>400 ppm) and low Y (≤18 ppm) contents and are adakitic rocks [70], and the Late Devonian samples plot in the transition region between classical island arc rocks and adakitic rocks, while the Early Carboniferous samples plot in the typical adakitic rocks region (Figure 11a,b). Different from the classical island arc rocks, adakitic magmatism is mainly situated in environments where a young (T<20 Ma) and hot oceanic lithosphere is subducted [77], which is associated with the accelerated  [74], (b) modified after [75], (c,d) modified after [76]).

Late Devonian to Early Carboniferous
Similar to Middle Devonian granitic rocks, the Late Devonian to Early Carboniferous granitic rocks also have subduction-related geochemical features such as calc-alkaline magmatic series and rightward fractionated REE and trace element signatures. However, they have relatively high Sr (>400 ppm) and low Y (≤18 ppm) contents and are adakitic rocks [70], and the Late Devonian samples plot in the transition region between classical island arc rocks and adakitic rocks, while the Early Carboniferous samples plot in the typical adakitic rocks region (Figure 11a,b). Different from the classical island arc rocks, adakitic magmatism is mainly situated in environments where a young (T < 20 Ma) and hot oceanic lithosphere is subducted [77], which is associated with the accelerated expansion of the mid-ocean ridge. Meanwhile, they also plot volcanic arc regions in the tectonic discriminate diagrams (Figure 11c,d). Therefore, we suggest that the Nenjiang Ocean was widely subducted northwestward under the XAT from the Late Devonian to Early Carboniferous; different from the Middle Devonian period, the subduction of this stage accelerated continuously from the Late Devonian to Early Carboniferous, leading to large scale magmatism in the Great Xing'an Range region. This is confirmed by the widely distributed Early Carboniferous magmatic rocks in the region ( Figure 12) [46,72,73,[78][79][80][81]. According to the age and scale of volcanic rocks distributed in XAT, Qian [82] also proposed a gradual process of arc magmatism from weak to strong in the Middle Devonian-Early Carboniferous. Based on the magmatism scale and the geochemical features in the region, we suggest that the northwestward subduction of the Nenjiang Ocean plate initiated locally in the Middle Devonian and became widespread in the Late Devonian and Early Carboniferous with time.
Recently, some researchers reported there was an Early Carboniferous m related to the subduction along the west margin of SAT to the east of HHS cating there also exists southeastward subduction of the Nenjiang Ocean. combination with the previous research and our studies, we believe that th ward subduction of the Nenjiang Ocean started in the Middle Devonian, an old ocean crust was subducted slowly. In the Late Devonian-Early Carbo oceanic crust subduction accelerated, and finally, bidirectional subduc Nenjiang Ocean might have started in the Early Carboniferous ( Figure 13). Recently, some researchers reported there was an Early Carboniferous magmatic arc related to the subduction along the west margin of SAT to the east of HHS [25,81], indicating there also exists southeastward subduction of the Nenjiang Ocean. Therefore, in combination with the previous research and our studies, we believe that the northwestward subduction of the Nenjiang Ocean started in the Middle Devonian, and the colder old ocean crust was subducted slowly. In the Late Devonian-Early Carboniferous, the oceanic crust subduction accelerated, and finally, bidirectional subduction of the Nenjiang Ocean might have started in the Early Carboniferous ( Figure 13). cating there also exists southeastward subduction of the Nenjiang Ocean. Therefore, combination with the previous research and our studies, we believe that the northwe ward subduction of the Nenjiang Ocean started in the Middle Devonian, and the cold old ocean crust was subducted slowly. In the Late Devonian-Early Carboniferous, oceanic crust subduction accelerated, and finally, bidirectional subduction of Nenjiang Ocean might have started in the Early Carboniferous ( Figure 13).    Table S1: Major (wt.%) and trace (×10 − ) elements for the granitic rocks in the Woluohe area; Table S2: Zircon LA-ICP-MS U-Pb data for the granitic rocks in the Woluohe area; Table S3: Zircon Lu-Hf isotope data for the granitic rocks in the Woluohe area.