Age and Geochemistry of Late Jurassic Mafic Volcanic Rocks in the Northwestern Erguna Block, Northeast China

The northwestern Erguna Block, where a wide range of volcanic rocks are present, provides one of the foremost locations to investigate Mesozoic Paleo-Pacific and Mongol-Okhotsk subduction. The identification and study of Late Jurassic mafic volcanic rocks in the Badaguan area of northwestern Erguna is of particular significance for the investigation of volcanic magma sources and their compositional evolution. Detailed petrological, geochemical, and zircon U-Pb dating suggests that the Late Jurassic mafic volcanic rocks formed at 157–161 Ma. Furthermore, the geochemical signatures of these mafic volcanic rocks indicate that they are calc-alkaline or transitional series with weak peraluminous characteristics. The rocks have a strong MgO, Al2O3, and total alkali content, and a SiO2 content of 53.55–63.68 wt %; they are enriched in Rb, Th, U, K, and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE), similar to igneous rocks in subduction zones. These characteristics indicate that the Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithospheric mantle as it was metasomatized by subduction-related fluid and the possible incorporation of some subducting sediments. Subsequently, the fractional crystallization of Fe and Ti oxides occurred during magmatic evolution. Combined with the regional geological data, it is inferred that the studied mafic volcanic rocks were formed by lithospheric extension after the closure of the Mongol-Okhotsk Ocean.


Introduction
The northwestern margin of the Erguna Block underwent a complex tectonic evolution during the Phanerozoic, including ancient Asia, the Mongol-Okhotsk subduction, and the westward subduction of the Paleo-Pacific Ocean [1][2][3][4][5]. Multiple magmatic events resulted in the widespread distribution of igneous rocks in the northwestern Erguna Block [6][7][8][9]. The widely exposed igneous rocks provide a useful tool for reconstructing the tectonic evolution of this region and its surrounding areas. In recent years, with the increasing focus on the Mesozoic volcanic rocks in the Da Xing'anling region, a large amount of high-precision zircon U-Pb and 40 Ar/ 39 Ar age and geochemical data have been obtained. These data indicate that most of the Mesozoic volcanic rocks in the Da Xing'anling region formed during the Early Cretaceous [10,11], with most of the volcanic rocks consisting of calc-alkaline and alkaline types in southern and northern Da Xing'anling, respectively. Both derived from an enriched mantle [12]. Acidic volcanic rocks are divided into high Sr rhyolite and low Sr rhyolite types; high Sr rhyolite types were formed by the differentiation of calc-alkaline basaltic magma, and low Sr rhyolite types were produced by the partial melting of lower crustal rocks [13]. The interpretation of Mesozoic volcanism in this area remains controversial, with most focus placed on the mantle plume model [14][15][16][17], which is associated with the subduction of the ancient Pacific Plate [18][19][20] and the evolution of the Mongol-Okhotsk Ocean [18,21]. However, existing studies mainly focused on subduction, At present, there is still controversy about the opening of the Mongol-Ocean, but the Late Paleozoic-Mesozoic evolutionary history is relatively clear this period, the Mongol-Okhotsk Ocean likely subducted both northward and so [4,34,35].
The Mongol-Okhotsk Ocean evolution can be roughly divided into seven s during the Devonian period, the Mongol-Okhotsk oceanic crust began to sub low angle beneath the Siberian plate, causing diffuse lithospheric extension and uting to the collapse of the Early Paleozoic orogenic belt; (2) during the Early-L boniferous, the subduction steepened, prompting a shift from an extensional trusion regime; (3) the Late Carboniferous-Early Permian subduction plate brok reversed, leading to the extension of the continental lithosphere and the upw mantle material; (4) Late Permian-Middle Triassic igneous rocks provide evid both northward and southward subduction polarity, a typical and active co margin environment; (5) in the Late Triassic, the western part of the Mongolsuture zone closed, but the central and eastern oceans of the Mongol-Okhots were still open; (6) during the Early-Middle Jurassic period, magmatic ac Transbaikal and North-central Mongolia weakened significantly, likely due to duction slowing down at its final stage, and the eventual closure of the oceans orogenic belts; (7) in the Late Jurassic-Early Cretaceous period, the eastern pa At present, there is still controversy about the opening of the Mongol-Okhotsk Ocean, but the Late Paleozoic-Mesozoic evolutionary history is relatively clear. During this period, the Mongol-Okhotsk Ocean likely subducted both northward and southward [4,34,35].
The Mongol-Okhotsk Ocean evolution can be roughly divided into seven stages: (1) during the Devonian period, the Mongol-Okhotsk oceanic crust began to subduct at a low angle beneath the Siberian plate, causing diffuse lithospheric extension and contributing to the collapse of the Early Paleozoic orogenic belt; (2) during the Early-Late Carboniferous, the subduction steepened, prompting a shift from an extensional to an extrusion regime; (3) the Late Carboniferous-Early Permian subduction plate broke off and reversed, leading to the extension of the continental lithosphere and the upwelling of mantle material; (4) Late Permian-Middle Triassic igneous rocks provide evidence for both northward and southward subduction polarity, a typical and active continental margin environment; (5) in the Late Triassic, the western part of the Mongol-Okhotsk suture zone closed, but the central and eastern oceans of the Mongol-Okhotsk Ocean were still open; (6) during the Early-Middle Jurassic period, magmatic activity in Transbaikal and North-central Mongolia weakened significantly, likely due to the subduction slowing down at its final stage, and the eventual closure of the oceans to form orogenic belts; (7) in the Late Jurassic-Early Cretaceous period, the eastern part of the Mongol-Okhotsk Ocean eventually closed, causing the granites of this period to have intraplate granite features [36][37][38]. The samples dated in this study were obtained from the northwestern Erguna Block. The Late Jurassic mafic volcanic rocks in the study area are exposed over an area of approximately 200 km 2 , broadly spreading in a northeast direction, and are dominated by andesites and trachyandesites.
The andesite samples are dark in color, show a porphyritic texture, and boast a massive structure with a phenocryst content of 1-3%, with sizes ranging from 0.2 to 6 mm. The phenocrysts are dominated by plagioclase with a small amount of hornblende. The matrix comprises plagioclase microcrystals with an interwoven arrangement ( Figure 2). The andesite samples are dark in color, show a porphyritic texture, and boast a massive structure with a phenocryst content of 1-3%, with sizes ranging from 0.2 to 6 mm. The phenocrysts are dominated by plagioclase with a small amount of hornblende. The matrix comprises plagioclase microcrystals with an interwoven arrangement ( Figure  2).

Methods
We selected nine mafic volcanic rocks for whole-rock geochemical analysis. Among them, we selected rocks with almost no alteration and moderate SiO2 contents (samples Zr15 and ZRD17) for zircon U-Pb dating.
The whole-rock major and trace elements were analyzed at the No. 240 Institute of Nuclear Industry. The main elements were analyzed using the X-ray fluorescence (XRF) glass frit method with a relative error of <5%; trace and rare-earth elements were analyzed on an inductively coupled plasma mass spectrometer (ICP-MS) (Elan 6100DRC, Perkin Elmer, Waltham, MA, USA) alongside AVG-1 and BHVO-1 international standards, with a relative error of <5%. Zircon sorting was performed at the Langfang Regional Geological Survey Research Institute, Hebei Province, China. Zircon target making and microscopic image acquisition were conducted at the Tianjin Geological Survey Center. Laser ablation (LA) ICP-MS zircon U-Pb chronology testing was completed at the Northeast Asia Mineral Resources Evaluation Key Laboratory, Ministry of Land and Resources, Jilin University. Transmission, reflection, and cathodoluminescence images of the andesite samples were acquired to determine the type of internal zircon genesis and structural composition. For this, high-purity He gas was used as the carrier gas for the exfoliating material.
Zircon U and Pb were determined with a ComPex102 ArF excimer laser at 193 nm wavelength on an Agilent 7500a ICP-MS machine. Instrument optimization was undertaken using NIST610, a standard reference substance composed of synthetic, silicate glass developed by the American Institute of Standards and Technology. Harvard University International Standard zircon 91500 was used as an external standard. The laser beam spot diameter used for the zircon determinations was 30 μm. Analytical data were calculated using Glitter software (1999, GEMOC, Macquaric University, CSIRO, Sydney, Australia), and ordinary Pb correction was performed following [39]. For detailed experimental test procedure and instrument parameters, refer to [39].

Methods
We selected nine mafic volcanic rocks for whole-rock geochemical analysis. Among them, we selected rocks with almost no alteration and moderate SiO 2 contents (samples Zr15 and ZRD17) for zircon U-Pb dating.
The whole-rock major and trace elements were analyzed at the No. 240 Institute of Nuclear Industry. The main elements were analyzed using the X-ray fluorescence (XRF) glass frit method with a relative error of <5%; trace and rare-earth elements were analyzed on an inductively coupled plasma mass spectrometer (ICP-MS) (Elan 6100DRC, Perkin Elmer, Waltham, MA, USA) alongside AVG-1 and BHVO-1 international standards, with a relative error of <5%. Zircon sorting was performed at the Langfang Regional Geological Survey Research Institute, Hebei Province, China. Zircon target making and microscopic image acquisition were conducted at the Tianjin Geological Survey Center. Laser ablation (LA) ICP-MS zircon U-Pb chronology testing was completed at the Northeast Asia Mineral Resources Evaluation Key Laboratory, Ministry of Land and Resources, Jilin University. Transmission, reflection, and cathodoluminescence images of the andesite samples were acquired to determine the type of internal zircon genesis and structural composition. For this, high-purity He gas was used as the carrier gas for the exfoliating material.
Zircon U and Pb were determined with a ComPex102 ArF excimer laser at 193 nm wavelength on an Agilent 7500a ICP-MS machine. Instrument optimization was undertaken using NIST610, a standard reference substance composed of synthetic, silicate glass developed by the American Institute of Standards and Technology. Harvard University International Standard zircon 91500 was used as an external standard. The laser beam spot diameter used for the zircon determinations was 30 µm. Analytical data were calculated using Glitter software (1999, GEMOC, Macquaric University, CSIRO, Sydney, Australia), and ordinary Pb correction was performed following [39]. For detailed experimental test procedure and instrument parameters, refer to [39].

Zircon U-Pb Ages
Two representative andesite samples from the Late Jurassic in the Badaguan region were analyzed using zircon LA-ICP-MS dating. The measured isotope ratios and the ages calculated from the zircon single-point analysis of the two samples are listed in Table S1. Weighted mean age and concordance plots are also displayed. The zircons were wellcrystallized and columnar in shape ( Figure 3), with oscillating growth rings and high Th/U ratios (0.48-2.76) characteristic of their magmatic origin [40].
Sample ZRD17 (Figure 4a), an andesite, was collected in the southeast of Badaguan. A total of 20 zircons were analyzed from this sample, 13 of which were located on or near the U-Pb concordia. 206 Pb/ 238 U ages ranged from 145 to 159 Ma with a weighted average age of 157 ± 3.4 Ma, and a mean squared weighted deviation (MSWD) of 4.5. Except for seven older zircons dating from 218-377 Ma (captured age), these results indicated that the granites formed during the Late Jurassic. For sample Zr15 (Figure 4b), an andesite also collected from southeast of Badaguan, 20 zircons were analyzed, 15 of which were located on or near the U-Pb concordia. Except for five older zircons dating from 190 to 205 Ma (captured age), the 206 Pb/ 238 U ages for this sample ranged from 152 to 168 Ma with a weighted average age of 161 ± 4.4 Ma and an MSWD of 3.5.

Zircon U-Pb Ages
Two representative andesite samples from the Late Jurassic in the Badaguan regi were analyzed using zircon LA-ICP-MS dating. The measured isotope ratios and the a es calculated from the zircon single-point analysis of the two samples are listed in Tab S1. Weighted mean age and concordance plots are also displayed. The zircons we well-crystallized and columnar in shape ( Figure 3), with oscillating growth rings a high Th/U ratios (0.48-2.76) characteristic of their magmatic origin. [40].
Sample ZRD17 (Figure 4a), an andesite, was collected in the southeast of Badagua A total of 20 zircons were analyzed from this sample, 13 of which were located on or ne the U-Pb concordia. 206 Pb/ 238 U ages ranged from 145 to 159 Ma with a weighted avera age of 157 ± 3.4 Ma, and a mean squared weighted deviation (MSWD) of 4.5. Except seven older zircons dating from 218-377 Ma (captured age), these results indicated th the granites formed during the Late Jurassic. For sample Zr15 (Figure 4b), an andes also collected from southeast of Badaguan, 20 zircons were analyzed, 15 of which we located on or near the U-Pb concordia. Except for five older zircons dating from 190 205 Ma (captured age), the 206 Pb/ 238 U ages for this sample ranged from 152 to 168 Ma w a weighted average age of 161 ± 4.4 Ma and an MSWD of 3.5.    (Figure 6a). The primitive-mantle-normaliz

Zircon U-Pb Ages
Two representative andesite samples from the Late Jurassic in the Badaguan r were analyzed using zircon LA-ICP-MS dating. The measured isotope ratios and th es calculated from the zircon single-point analysis of the two samples are listed in S1. Weighted mean age and concordance plots are also displayed. The zircons well-crystallized and columnar in shape (Figure 3), with oscillating growth rings high Th/U ratios (0.48-2.76) characteristic of their magmatic origin. [40].
Sample ZRD17 (Figure 4a), an andesite, was collected in the southeast of Bada A total of 20 zircons were analyzed from this sample, 13 of which were located on or the U-Pb concordia. 206 Pb/ 238 U ages ranged from 145 to 159 Ma with a weighted av age of 157 ± 3.4 Ma, and a mean squared weighted deviation (MSWD) of 4.5. Excep seven older zircons dating from 218-377 Ma (captured age), these results indicated the granites formed during the Late Jurassic. For sample Zr15 (Figure 4b), an and also collected from southeast of Badaguan, 20 zircons were analyzed, 15 of which located on or near the U-Pb concordia. Except for five older zircons dating from 1 205 Ma (captured age), the 206 Pb/ 238 U ages for this sample ranged from 152 to 168 Ma a weighted average age of 161 ± 4.4 Ma and an MSWD of 3.5.      Figure 5).  (Figure 6a). The primitive-mantle-normalized trace element spider diagram (Figure 6b) further indicates that these rocks contain high Rb, Th, U, K, and LREEs, and are depleted in Nb, which is similar to subduction zone igneous rocks.

Whole-Rock Major and Trace Element Composition
The Late Jurassic mafic volcanic rock samples from the Badaguan area show an obvious linear trend in the Harker diagram (Figure 7), suggesting that segregation and fractional crystallization may have occurred during the evolution of the magma. The negative correlation between SiO 2 and TiO 2 suggests that Fe and Ti oxides also separated and crystallized. Rb, Th, U, K, and LREEs, and are depleted in Nb, which is similar to subduction zone igneous rocks. The Late Jurassic mafic volcanic rock samples from the Badaguan area show an obvious linear trend in the Harker diagram (Figure 7), suggesting that segregation and fractional crystallization may have occurred during the evolution of the magma. The negative correlation between SiO2 and TiO2 suggests that Fe and Ti oxides also separated and crystallized.   Rb, Th, U, K, and LREEs, and are depleted in Nb, which is similar to subduction zone igneous rocks. The Late Jurassic mafic volcanic rock samples from the Badaguan area show an obvious linear trend in the Harker diagram (Figure 7), suggesting that segregation and fractional crystallization may have occurred during the evolution of the magma. The negative correlation between SiO2 and TiO2 suggests that Fe and Ti oxides also separated and crystallized.

Ages of Mafic Volcanic Rocks
Due to the lack of accurate age and biostratigraphic information, the age of the v canic-bearing strata in the Da Xing'anling area was mainly determined based on ro

Ages of Mafic Volcanic Rocks
Due to the lack of accurate age and biostratigraphic information, the age of the volcanic-bearing strata in the Da Xing'anling area was mainly determined based on rock assemblage features and regional stratigraphic correlations. The Late Jurassic mafic volcanic rocks in the Badaguan area are similar to those of the Tamurangou Formation, which lack isotopic age data. Some researchers consider these rocks as belonging to the Middle Jurassic Tamurangou Formation [30]. However, our new U-Pb zircon data indicate otherwise. Our derived ages (157-161 Ma) correspond to the Late Jurassic, which is younger than the previously assumed Middle Jurassic age of these rocks.

Petrogenesis and the Nature of the Magma Source
Calc-alkaline, or transitional mafic volcanic, rocks are an essential part of orogenic belts, and understanding their genesis is of great importance for revealing the formation, growth, and crust-mantle interaction of the Earth's crust [43]. Based on previous research, the following processes have been proposed regarding the genesis of calc-alkaline or transitional mafic volcanic rocks: (1) partial melting of the lithospheric mantle, metasomatized by subduction-related fluids [4,24,44,45]; (2) separation and crystallization of mantle-derived basaltic magma [46]; (3) mixing of crust-sourced feldspathic magma with mantle-sourced basaltic magma [47]; and (4) the partial melting of subcrustal material due to the intrusion of mantle-derived basaltic magma at the base [48,49].
The studied mafic volcanic rocks are relatively low in SiO 2 content (53.55-63.68 wt %), but high in Al 2 O 3 (15.49-19.86 wt %) and MgO (0.53-2.37 wt %), and have Mg # values of 14.93-47.06. Furthermore, the crustal source magma has relatively high Lu/Yb (0.16-0.18) and Rb/Sr (>0.5) ratios. In contrast, the Lu/Yb and Rb/Sr ratios of the Late Jurassic mafic volcanic rock samples in the study area range from 0.20 to 0.29 and from 0.01 to 0.08, respectively, which are significantly lower than those of crustal-sourced magmas. Indeed, the Lu/Yb (0.14-0.15) and Rb/Sr (0.03-0.047) ratios are similar to those of mantle-derived magma [50]. Therefore, our results indicate that the Late Jurassic mafic volcanic rocks in the Badaguan area were not the product of the partial melting of the mafic lower crust.
In addition, the Late Jurassic volcanic rocks in the northern Da Xing'anling region are dominated by andesites and rhyolites, with some inclusions, but contain no large-scale basaltic magmatic rocks [5]. The characteristics of mafic volcanic rocks in the Badaguan area imply that they are not the product of the differentiation of basaltic magma. The zircon Lu-Hf isotopic system has a high confinement temperature, and when magma mixing occurs, previously crystallized zircon can effectively record the Hf isotopic signature of the mixed end elements [4]. Both Lu and Hf are incompatible trace elements and relatively immobile; however, Hf is more incompatible than Lu and is relatively enriched in the crust and in silicate melts. U-Pb and Lu-Hf isotope systems in zircon record crust-mantle differentiation through time, i.e., fluid-assisted melt extraction from the mantle to form juvenile crust. Negative epsHf values reflect enrichment with respect to the bulk Earth, while positive epsHf values (0 to +15) reflect an origin from a source intermediate between depleted mantle (DM) and bulk Earth. Therefore, when the value of εHf(t) is negative, it usually represents the remelting of ancient crust or the mixing of ancient crust during the formation of magma. When the value of εHf(t) is both positive and negative, it may mean that the granite is of mixed crust-mantle origin. With positive εHf(t) values, the magma is derived from the partial melting of new basal crustal material added from the depleted mantle [4,5].
Previously published data from the Erguna Block have also shown that the zircons have a relatively homogeneous Hf isotopic composition (Figure 8). From these data, it seems that there is a rough evolution through time toward more depleted mantle sources, with εHf(t) values ranging between +0.7 and +11.0, and the corresponding second-stage model (TDM 2 ) ages ranging from 441 to 1156 Ma (Table S3 [ [51][52][53][54][55]). This implies that magma mixing was not the main pathway of the mafic magma [5]. Slab melts and lower crust melts can also react with the mantle to form mafic magmas, which are characterized by adakites with high Sr/Y ratios and low Y and Yb contents. The mafic volcanic rocks from the Badaguan area have relatively low Sr/Y ratios and high Y and Yb contents (Figure 9a), which differs from the adakites of subduction slab or subcrustal detachment but is similar to typical arc volcanic rocks. These rocks are distinctly different from the magmas produced by the partial melting of basal lower crustal material [49].
tism, involving fluids formed by dewatering of the oceanic crust, and melt accounting fo oceanic sediments. The influence of subduction-related fluids and sediment melt can b determined based on trace element ratios. Fluid-mobile elements (e.g., Ba, Rb, and Sr) ar easily transported in aqueous fluids, while melt-active elements (e.g., Th and La) easi enter the sediment melt. Accordingly, the ratio of fluid, sedimentary melt activity el ments, and weakly active HFSEs or heavy rare-earth elements (HRESs) indicates th contribution of subduction fluids and sediment melts to the source region. The Late Ju rassic mafic volcanic rocks from the study region generally correspond to subdu tion-related fluid, while individual samples have high Th/Yb ratios (Figure 9d) indicatin the possible addition of subducting sediments. Taken together, these results indicate th the Late Jurassic mafic volcanic rocks in the Badaguan region are derived from the parti melting of the lithosphere, metasomatized by subduction-related fluid, with the possib addition of subducting sediments.  Table S3.  Table S3.
Mantle-derived magma in the process of upward intrusion would have inevitably experience crustal material mixing. Small amounts of captured zircon suggest that crustal material mixing may have occurred.
Although some of the mafic volcanic rocks have high K 2 O/P 2 O 5 ratios, and the K 2 O/P 2 O 5 ratios are positively correlated with their SiO 2 content, the K 2 O/P 2 O 5 ratios of most of the samples show little variation, and do not increase with increasing SiO 2 content (Figure 9b). Combined with the references in Table S3 for the Erguna Block [51][52][53][54][55], these observations imply that the mafic magma in this region did not undergo significant crustal contamination during its ascent through the continental crust.
Nb/La-Ba/Rb diagrams can distinguish between crustal mixing and a single mantle source. The evolution of mantle-derived magma via crustal mixing results in a wide range of Nb/La values and a narrow range of Ba/Rb values; in contrast, the Ba/Rb values of enriched mantle are variable, and Nb/La values are highly consistent [56][57][58]. The studied mafic volcanic rocks show a parallel trend in the Nb/La-Ba/Rb diagram (Figure 9c), indicating that the magma is not mixed with crustal material. Furthermore, the geochemical characteristics mainly reflect the magma source area. However, mafic magma mixed with crustal material has high 87 Sr/ 86 Sr ratios positively correlated with SiO 2 content, whereas the 87 Sr/ 86 Sr ratios of the mafic volcanic rocks in the Manzhouli area of the Erguna Massif show a low degree of variation, and do not increase with increasing SiO 2 content [58]. Thus, the degree of crustal material contamination in the study region is likely low.

Tectonic Background of Andesite Formation
Late Jurassic volcanic rocks are mainly distributed in the Da Xing'anling Mountains on the south side of the Mongol-Okhotsk suture zone; volcanism was largely absen during this period in the Songliao Basin and eastern Jihei [4,60,61]. As the subduction direction of the ancient Pacific Plate relative to Eurasia was northward during this peri od, the spatial and temporal distribution of the volcanic rocks suggests that their for mation was mainly related to the evolution of the Mongol-Okhotsk Ocean. In recen years, the detailed study of the Early Mesozoic granites and porphyry copper-molybdenum deposits in the Manzhouli-Erguna area [62,63] has confirmed the southward subduction of the Mongol-Okhotsk Ocean. The formation of the Late Triassic Taipingchuan and Badaguan porphyry copper-molybdenum ores and the Early Jurassic Unugtushan porphyry copper-molybdenum ores reflects the tectonic background of the active continental margin during this period. Furthermore, the coeval granites are mainly granodiorite diorite granite assemblages, which are similar to those forming at active continental margins, supporting the southward subduction of the Mongol-Okhotsk Ocean.
Based on our analyses of the Mesozoic igneous rocks of the Erguna Massif, the calc-alkaline or transitional igneous rock assemblages are Early-Late Jurassic in age. The development of bimodal volcanic rocks in the Early Cretaceous and the change in mag matism characteristics record the transition from the Early Jurassic oceanic subduction to The main mechanism of lithospheric mantle enrichment is subduction metasomatism, involving fluids formed by dewatering of the oceanic crust, and melt accounting for oceanic sediments. The influence of subduction-related fluids and sediment melt can be determined based on trace element ratios. Fluid-mobile elements (e.g., Ba, Rb, and Sr) are easily transported in aqueous fluids, while melt-active elements (e.g., Th and La) easily enter the sediment melt. Accordingly, the ratio of fluid, sedimentary melt activity elements, and weakly active HFSEs or heavy rare-earth elements (HRESs) indicates the contribution of subduction fluids and sediment melts to the source region. The Late Jurassic mafic volcanic rocks from the study region generally correspond to subduction-related fluid, while individual samples have high Th/Yb ratios (Figure 9d) indicating the possible addition of subducting sediments. Taken together, these results indicate that the Late Jurassic mafic volcanic rocks in the Badaguan region are derived from the partial melting of the lithosphere, metasomatized by subduction-related fluid, with the possible addition of subducting sediments.

Tectonic Background of Andesite Formation
Late Jurassic volcanic rocks are mainly distributed in the Da Xing'anling Mountains on the south side of the Mongol-Okhotsk suture zone; volcanism was largely absent during this period in the Songliao Basin and eastern Jihei [4,60,61]. As the subduction direction of the ancient Pacific Plate relative to Eurasia was northward during this period, the spatial and temporal distribution of the volcanic rocks suggests that their formation was mainly related to the evolution of the Mongol-Okhotsk Ocean. In recent years, the detailed study of the Early Mesozoic granites and porphyry copper-molybdenum deposits in the Manzhouli-Erguna area [62,63] has confirmed the southward subduction of the Mongol-Okhotsk Ocean. The formation of the Late Triassic Taipingchuan and Badaguan porphyry coppermolybdenum ores and the Early Jurassic Unugtushan porphyry copper-molybdenum ores reflects the tectonic background of the active continental margin during this period. Furthermore, the coeval granites are mainly granodiorite diorite granite assemblages, which are similar to those forming at active continental margins, supporting the southward subduction of the Mongol-Okhotsk Ocean.
Based on our analyses of the Mesozoic igneous rocks of the Erguna Massif, the calcalkaline or transitional igneous rock assemblages are Early-Late Jurassic in age. The development of bimodal volcanic rocks in the Early Cretaceous and the change in magmatism characteristics record the transition from the Early Jurassic oceanic subduction to the Late Jurassic post-orogenic extension of the Mongol-Okhotsk Ocean [2,64]. Most of the Late Mesozoic igneous rocks of the Erguna Massif are of type A and A 2 [64], formed by lithospheric extension after the closure of the Mongol-Okhotsk Ocean. At the same time, the igneous rocks from this period tend to change from adakites to calc-alkaline igneous rocks, indicating that the granite source rocks formed by partial melting during the extensional collapse and crustal thinning of the Mongol-Okhotsk Orogenic Belt [65,66].
The Late Jurassic mafic volcanic rocks in the Badaguan area of the northern section of the Da Xing'anling region have typical geochemical characteristics of subduction-related fluid, which are enriched in large ionic lithophile elements (e.g., Rb, Th, U, and K) and depleted in HFSEs and HREEs. These rocks also have high Nb contents. Our samples are plotted in the continental arc region of the Th/Yb-Nb/Yb diagram (Figure 10a), suggesting that they formed in an active continental margin environment. Indeed, the samples have high La/Yb and Th/Yb ratios similar to Andean-type active continental margin andesites ( Figure 10b). All studied rocks plotted within the field of subduction-related IAB series, forming a mixing trend toward more depleted mantle sources (Figure 10c).
The Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithosphere, metasomatized by subduction-related fluid, with the possible addition of subducting sediments. This tectonic background is closely related to the evolution of the Mongol-Okhotsk Ocean. This interpretation is consistent with the tectonic context displayed by the bimodal volcanic rocks at Tamurangou (160-164 Ma) and Manketourbo (159-162 Ma), as well as the A1 rhyolite of the Baiyingaolao Formation (139-142 Ma) [12,13,50]. This is broadly consistent with the timing of the formation of mafic volcanic rocks in the study region. Finally, the northwest part of the Erguna Massif is very close to the Mongol-Okhotsk suture zone; the Middle-Late Jurassic volcanic rocks only occur in the western part of the Songliao Basin, and tend to be younger and deeper in the source area from west to east [13]. These rocks likely derived from the partial melting of the lithosphere, previously metasomatized by subduction-related fluid, with some contribution of subducted sediments; these sediments belonged to a Late Jurassic-Early Cretaceous suite of magmatic rocks that evolved from a typical calc-alkaline "active margin" type toward a more depleted or, alternatively, more alkaline type during post-collisional crust delamination.
samples are plotted in the continental arc region of the Th/Yb-Nb/Yb diagram ( Figure  10a), suggesting that they formed in an active continental margin environment. Indeed, the samples have high La/Yb and Th/Yb ratios similar to Andean-type active continental margin andesites (Figure 10b). All studied rocks plotted within the field of subduction-related IAB series, forming a mixing trend toward more depleted mantle sources (Figure 10c). The Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithosphere, metasomatized by subduction-related fluid, with the possible addition of subducting sediments. This tectonic background is closely related to the evolution of the Mongol-Okhotsk Ocean. This interpretation is consistent with the tectonic context displayed by the bimodal volcanic rocks at Tamurangou (160-164 Ma) and Manketourbo (159-162 Ma), as well as the A1 rhyolite of the Baiyingaolao Formation (139-142 Ma) [12,13,50]. This is broadly consistent with the timing of the formation of mafic volcanic rocks in the study region. Finally, the northwest part of the Erguna Massif

Conclusions
Based on the chronology and geochemistry of the mafic volcanic rocks in the Badaguan area combined with existing research, we draw the following main conclusions:

Data Availability Statement:
The data presented in this study are available in this article.