Geochemistry, Chronology and Tectonic Implications of the Hadayang Schists in the Northern Great Xing’an Range, Northeast China

: The Late Paleozoic tectonic evolution of the Xing’an block in the eastern Central Asian orogenic belt has long been the subject of debate. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes and whole-rock elemental analyses was carried out on Hadayang schists. Representative samples of the epidote-biotite-albite schist and biotite-albite schist yielded the weighted mean 206 Pb/ 238 U ages of 360 ± 2 Ma and 355 ± 3 Ma, respectively. This indicated the presence of Late Devonian–Early Carboniferous intermediate-basic rocks in the eastern Xing’an block. The Hadayang schists exhibited a Na-rich, tholeiitic and calc-alkaline afﬁnity in composition with low Mg# (35.2–53.0), Cr (23.7–86.5 ppm), Ni (21.1–40.0 ppm) and Co (12.1–30.6 ppm). They were characterized by enrichment of LILEs, depletion of HFSEs and highly positive zircon ε Hf ( t ) values (the average values were +8.93 and +9.29, respectively). The magma source of the Hadayang schists was a mantle that consisted of both spinel and garnet lherzolite, with a partial melting degree of 1%–5%, and it had undergone fractional crystallization of olivine, orthopyroxene and plagioclase. The Hadayang schists, together with other Late Devonian–Early Carboniferous intermediate-basic magmatic rocks in the eastern Xing’an block, were formed in an intracontinental extension tectonic setting similar to that of the North American Basin and Range basalt. Moreover, Late Devonian–Early Carboniferous ophiolite under a similar tectonic background in the western Xing’an block has been reported. We believe that the Xing’an block would have been in the stage of intracontinental extension during the Late Devonian–Early Carboniferous.


Introduction
The Central Asian orogenic belt (CAOB) is located between the Siberian craton and the Sino-Korean craton, and is composed of a series of micro-continental blocks and suture zones [1][2][3][4][5] (Figure 1a).The Xing'an-Mongolia orogenic belt (XMOB) is located in the eastern section of the CAOB, and is made up of several microcontinents, including the Erguna, Xing'an, Songnen and Jiamusi-Khanka blocks [6][7][8][9][10][11] (Figure 1b).Although much progress has been made in the study area of the tectonic pattern and evolutionary history of the XMOB in recent years, there are still many controversies, among which the dating of the integration of the Xing'an (XB) and Songnen blocks (SB) is the most prominent.For example, Xu et al. [6] argued that the XB and SB combined along the Angin Sum-Xilinhot-Heihe line before the Devonian [7,12]; Liu et al. [8] proposed that these two blocks closed along the Hegenshan-Moguqi-Heihe line during the late Early Carboniferous and early Late Carboniferous [9,[13][14][15]; Xu et al. [11] believed that the XB and SB combined along the Xilinhot-Ulanhot-Nenjiang-Heihe line, and the time limit was the late Early Carboniferous [16].In addition, the time limit of convergence is also dated to the Late Devonian to Early Carboniferous [17,18], to before the Permian [19,20] and to the Triassic [21,22].
Minerals 2023, 13, x FOR PEER REVIEW 2 of 26 Carboniferous [16].In addition, the time limit of convergence is also dated to the Late Devonian to Early Carboniferous [17,18], to before the Permian [19,20] and to the Triassic [21,22].Through the analysis of the above different views, it can be found that one of the focuses of the debate is whether there was subduction in the ocean basin between the XB and the SB in the late Paleozoic, and the study of the tectonic background of the late Paleozoic igneous rock in the XB can provide important evidence for solving the above problems.Compared with granitic rocks, mafic magmatic rocks originate directly from partial melting of mantle and the geochemistry is less affected by late transformation, so they become ideal objects for the derivation of mantle source characteristics and regional tectonic background [23].Therefore, we report zircon U-Pb geochronology, Hf isotopes and whole-rock geochemistry studies on late Devonian-early Carboniferous metamorphic intermediate-basic volcanic rocks in the southern XB, to determine their origins and tectonic setting.This study provides a more detailed understanding of the evolution history of the XB and SB in the late Paleozoic.Furthermore, it provides important constraints for the study of the tectonic evolution of the XMOB.

Geological Settings and Sampling
The study area was located in the Hadayang area of Molidawa Banner in the eastern part of the Inner Mongolia Autonomous Region and it tectonically belongs to the southern part of the XB (Figure 2).As the oldest block in the XMOB, the Precambrian record of the EB is well developed, and the oldest rock exposed is a set of Neoarchean-Paleoproterozoic granitic gneiss [24][25][26][27].The Xinghuadukou Group, which was previously considered to Through the analysis of the above different views, it can be found that one of the focuses of the debate is whether there was subduction in the ocean basin between the XB and the SB in the late Paleozoic, and the study of the tectonic background of the late Paleozoic igneous rock in the XB can provide important evidence for solving the above problems.Compared with granitic rocks, mafic magmatic rocks originate directly from partial melting of mantle and the geochemistry is less affected by late transformation, so they become ideal objects for the derivation of mantle source characteristics and regional tectonic background [23].Therefore, we report zircon U-Pb geochronology, Hf isotopes and whole-rock geochemistry studies on late Devonian-early Carboniferous metamorphic intermediate-basic volcanic rocks in the southern XB, to determine their origins and tectonic setting.This study provides a more detailed understanding of the evolution history of the XB and SB in the late Paleozoic.Furthermore, it provides important constraints for the study of the tectonic evolution of the XMOB.

Geological Settings and Sampling
The study area was located in the Hadayang area of Molidawa Banner in the eastern part of the Inner Mongolia Autonomous Region and it tectonically belongs to the southern part of the XB (Figure 2).As the oldest block in the XMOB, the Precambrian record of the EB is well developed, and the oldest rock exposed is a set of Neoarchean-Paleoproterozoic granitic gneiss [24][25][26][27].The Xinghuadukou Group, which was previously considered to be of Paleo-Mesoproterozoic age, may have formed in the Neoproterozoic [27][28][29][30][31].
be of Paleo-Mesoproterozoic age, may have formed in the Neoproterozoic [27][28][29][30][31].In addition, Neoproterozoic magmatism is also well developed in the EB [31][32][33][34].The XB is traditionally based on Precambrian strata such as the Xinghuadukou Group, the Jiageda Formation and the Wolegen Group, but recent research shows that these strata were formed in the Paleozoic [35][36][37].In recent years, a series of Neoarchean-Paleoproterozoic magmatic rocks [38][39][40][41][42] and metamorphic rock series [43] have been identified in the Ulanhot-Longjiang area, suggesting the existence of Precambrian basement in the XB.After the convergence of the EB and XB along the XXS in the early Paleozoic [44][45][46][47][48][49] (Figure 1b), the Erguna-Xing'an block (EXB) was covered by the Early Paleozoic-Lower Permian marine and continental alternative deposition, the Upper Permian continental sedimentary strata and the Mesozoic volcaniclastic rocks, and developed multistage Paleozoic-Mesozoic magmatic intrusion.The SB is located in the southeast of the EXB, which is bounded by the XHS (Figure 1b).The Dongfengshan Group and Tadong Group in the eastern SB, which are traditionally considered as "Paleoproterozoic", were formed in the Neoproterozoic [50][51][52], accompanied by contemporaneous magmatic intrusions [53][54][55][56].The western SB is mostly covered by late Mesozoic-Cenozoic volcaniclastic sediments.The late Paleozoic strata in the study area include the Lower Devonian Huolongmen Formation and the Lower Carboniferous Morgenhe Formation (Figure 2).The Lower Devonian Huolongmen Formation is mainly composed of clastic sedimentary rocks with a small amount of intermediate volcanic rocks, and is affected by a poorly developed ductile deformation.The formation age of chlorite muscovite schist (protolith is volcanic rock) is 414 Ma [57].The Lower Carboniferous Morgenhe Formation is mainly composed of weakly metamorphic intermediate-acid volcanic rocks, which are generally mylonitic.The The late Paleozoic strata in the study area include the Lower Devonian Huolongmen Formation and the Lower Carboniferous Morgenhe Formation (Figure 2).The Lower Devonian Huolongmen Formation is mainly composed of clastic sedimentary rocks with a small amount of intermediate volcanic rocks, and is affected by a poorly developed ductile deformation.The formation age of chlorite muscovite schist (protolith is volcanic rock) is 414 Ma [57].The Lower Carboniferous Morgenhe Formation is mainly composed of Minerals 2023, 13, 1379 4 of 24 weakly metamorphic intermediate-acid volcanic rocks, which are generally mylonitic.The zircon U-Pb age of the volcanic rocks is 353 to 352 Ma [58,59].Late Paleozoic intrusive rocks include Late Devonian mylonitic syenite granite [60], Late Carboniferous-Early Permian granodiorite, syenogranite and alkali feldspar granite.The above are mostly covered by Mesozoic volcanic rocks and Cenozoic detrital sedimentary rocks.In addition to the above features, there is also a Hadayang melange developed in the study area [61,62].The matrix in the melange is mainly composed of schist, while blocks consist of schist, mafic-ultramafic rocks and meta-acidic volcanic rocks [62].

Major and Trace Element Analysis
The major and trace element analyses were carried out at the Shenyang Mineral Resources Supervision and Inspection Center, Shenyang, China.Whole-rock major element concentrations were determined by X-ray fluorescence spectrometry, yielding an analytical precision better than 2%.Trace element and rare earth element (REE) concentrations

Major and Trace Element Analysis
The major and trace element analyses were carried out at the Shenyang Mineral Resources Supervision and Inspection Center, Shenyang, China.Whole-rock major element concentrations were determined by X-ray fluorescence spectrometry, yielding an analytical precision better than 2%.Trace element and rare earth element (REE) concentrations were determined by ICP-MS, yielding an analytical precision better than 5% for elements with concentrations of >10 ppm, <8% for elements with concentrations of <10 ppm and <10% for the transition metals.

Zircon U-Pb Dating
U-Pb dating analyses were conducted by LA-ICP-MS at Beijing Createch Testing Technology Co., Ltd.(Beijing, China).The detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction were the same as described by Hou et al. [63].Laser sampling was performed using an ESI NWR 193 nm laser ablation system.An AnalytikJena PQMS Elite ICP-MS (Analytik Jena AG, Jena, Germany) instrument was used to acquire ion-signal intensities.Helium was applied as a carrier gas.Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP.Each analysis incorporated a background acquisition of approximately 15-20 s (gas blank) followed by 45 s of data acquisition from the sample.Off-line raw data selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for U-Pb dating, were performed by ICPMSDataCal [64].Zircon GJ-1 was used as the external standard for U-Pb dating, and was analyzed twice every 5-10 analyses.Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a linear interpolation (with time) for every 5-10 analyses according to the variations of GJ-1 (i.e., 2 zircon GJ-1 + 5-10 samples + 2 zircon GJ-1) [64].Uncertainty of preferred values for the external standard GJ-1 was propagated to the ultimate results of the samples.In all analyzed zircon grains, the common Pb correction was not necessary due to the low signal of common 204 Pb and high 206 Pb/ 204 Pb.U, Th and Pb concentration was calibrated by NIST 610.Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3.The zircon Plesovice was dated as unknown samples and yielded a weighted mean 206 Pb/ 238 U age of 337.5 ± 2.4 Ma (2SD, n = 5), which was in good agreement with the recommended 206 Pb/ 238 U age of 337.13 ± 0.37 Ma (2SD) [65].

Lu-Hf Isotope Analysis
Zircon in situ Hf isotope analysis was carried out using a RESOlution SE 193 laserablation system attached to a Thermo Fisher Scientific Neptune Plus MC-ICP-MS at Beijing Createch Testing Technology Co., Ltd (Beijing, China).Instrumental conditions and data acquisition protocols were as described by Hou et al. [66].A stationary spot used a beam diameter of ~55 µm.Helium was used as the carrier gas to transport ablated material from the laser-ablation cell after which it was mixed with Ar prior to entering the ICP-MS torch. 176Lu/ 175 Lu = 0.02658 and 176 Yb/ 173 Yb = 0.796218 ratios were determined to correct for the isobaric interferences of 176 Lu and 176 Yb on 176 Hf.For instrumental mass bias correction, Yb isotope ratios were normalized to 172 Yb/ 173 Yb = 1.35274 and Hf isotope ratios to 179 Hf/ 177 Hf = 0.7325 using an exponential law.The mass bias behavior of Lu was assumed to follow that of Yb; the mass bias correction protocol was as described by Hou et al. [66].

Whole-Rock Geochemistry
Results of the major element, trace element and REE analyses of the samples from the Hadayang schists are listed in Table 1.4a-b), all of these metavolcanic rocks plot in the basalt and basaltic andesite field.On a plot of SiO 2 vs. T FeO/MgO (Figure 4c), the samples plot in the tholeiitic field.25.The Na2O/K2O ratios ranged from 1.56~2.98,indicating that the samples belonged to the sodium series.As can be seen from the plot of SiO2 vs. K2O + Na2O and Nb/Y vs. Zr/TiO2*0.0001diagrams (Figure 4a-b), all of these metavolcanic rocks plot in the basalt and basaltic andesite field.On a plot of SiO2 vs. T FeO/MgO (Figure 4c), the samples plot in the tholeiitic field.The epidote-biotite-albite schists had total rare earth element (REE) concentrations from 194.53 to 202.99 ppm, with an average value of 197.56 ppm.On the chondrite-normalized REE diagram (Figure 5a), the light rare earth elements (LREEs) are moderately enriched relative to the heavy rare earth elements (HREEs) with (La/Yb)N ratios ranging from 8.97 to10.78, while the HREEs are relatively flat.These metavolcanic rocks had slightly negative Eu anomalies (δEu = 0.74-0.83).The primitive-mantle-normalized incompatible element diagrams (Figure 5b) show that the epidote-biotite-albite schists were relatively enriched in large ion lithophile elements (LILEs) (such as Rb, Ba, Th and U) relative to high-field-strength elements (HFSEs) (such as Nb, P and Ti).The epidote-biotite-albite schists had total rare earth element (REE) concentrations from 194.53 to 202.99 ppm, with an average value of 197.56 ppm.On the chondritenormalized REE diagram (Figure 5a), the light rare earth elements (LREEs) are moderately enriched relative to the heavy rare earth elements (HREEs) with (La/Yb) N ratios ranging from 8.97 to10.78, while the HREEs are relatively flat.These metavolcanic rocks had slightly negative Eu anomalies (δEu = 0.74-0.83).The primitive-mantle-normalized incompatible element diagrams (Figure 5b) show that the epidote-biotite-albite schists were relatively enriched in large ion lithophile elements (LILEs) (such as Rb, Ba, Th and U) relative to high-field-strength elements (HFSEs) (such as Nb, P and Ti).

The Biotite-Albite Schists
The biotite-albite schists had SiO2 of 57.59-60.69wt% and high TiO2 (1.05-1.62 wt%), ∑FeO (6.63-7.81wt%) and Al2O3 (16.47-17.21wt%).Their alkalis (K2O + Na2O) ranged from 4.26 to 5.77 wt%, and the low K2O/Na2O ratio (0.16 to 0.74) indicated that the samples belonged to the sodium series.All of the samples plot in the andesite field on a SiO2 vs. Na2O + K2O diagram (Figure 4a).As can be seen from the plot of Zr/TiO2×0.0001vs. Nb/Y (Figure 4b), most of the samples plot in the subalkaline basalt and andesite fields.On the other hand, on the plot of FeO T /MgO vs. SiO2, except for two samples that fall into the  4a).As can be seen from the plot of Zr/TiO 2 × 0.0001 vs. Nb/Y (Figure 4b), most of the samples plot in the subalkaline basalt and andesite fields.On the other hand, on the plot of FeO T /MgO vs. SiO 2 , except for two samples that fall into the tholeiitic field, the biotite-albite schist samples plot in the calc-alkaline field.

Zircon Hf Isotopies
The zircons from the epidote-biotite-albite schist (sample G1631-1) had homogeneous 176 Hf/ 177 Hf ratios of 0.282749 to 0.282872, with εHf(t) values of +6.5 to +11.0 (Figure 8), one-stage Hf model (T DM1 ) ages from 552 to 746 Ma and two-stage Hf model (T DM2 ) ages from 661 to 902 Ma.Similarly, the zircons from the biotite-albite schist sample (sample G1631-4) also possessed relatively homogeneous Hf isotopic compositions of 0.282783 to 0.282870, εHf(t) values of +7.9 to +10.9 (Figure 8), T DM1 ages from 547 to 649 Ma and T DM2 ages from 660 to 859 Ma (Table 3).All of these characteristics indicate that the parent magmas of the Hadayang schists were produced from a relatively depleted mantle source.

Protolith Restoration
The trace elements of the Hadayang schists were generally enriched in LILEs and depleted of HFSEs, Sr/Ba > 1, Cr/Ni > 1.Furthermore, the samples retained the crystalloblasts, which indicates that Hadayang schists should be a metaigneous rock.On a plot of A-C-FM diagram (Figure 9a), the epidote-biotite-albite schist samples plot in the calc-silicate rock and quartzite and basic volcanic rocks and ferrous-dolomitic tuff fields, while the biotite-albite schist samples plot in the intermediate-acid volcanic rock field.As can be seen from the plot of K vs.A (Figure 9b), all of the samples were igneous rock.On the other hand, on the (al + fm) − (c + alk) vs. Si diagram (Figure 9c), all the samples were volcanic rocks.According to the comprehensive analysis of geochemical and petrographic characteristics, the protolith of Hadayang schist was a set of intermediate-basic volcanic rocks, among which the protolith of the epidotite-biotite-albite schist was a basalt, and the protolith of the biotite-albite schist was andesite.

Crustal Contamination
Before using the elemental and isotopic compositions of the samples to discuss the petrogenesis of the schists, we first assess the effects of post-magmatic alteration.The protolith of the Hadayang schists was an intermediate-basic volcanic rock, which had undergone greenschist facies metamorphism under the influence of regional metamorphism in a later period, which is supported by their LOI content of 2.23-4.05wt% and the fact that some of the minerals had been partially replaced by chlorite and epidote.After rock metamorphism, high-field-strength elements (HFSEs), such as REEs, Y, Th, U, Zr, Hf, Ti, Nb, According to the comprehensive analysis of geochemical and petrographic characteristics, the protolith of Hadayang schist was a set of intermediate-basic volcanic rocks, among which the protolith of the epidotite-biotite-albite schist was a basalt, and the protolith of the biotite-albite schist was andesite.

Crustal Contamination
Before using the elemental and isotopic compositions of the samples to discuss the petrogenesis of the schists, we first assess the effects of post-magmatic alteration.The protolith of the Hadayang schists was an intermediate-basic volcanic rock, which had undergone greenschist facies metamorphism under the influence of regional metamorphism in a later period, which is supported by their LOI content of 2.23-4.05wt% and the fact that some of the minerals had been partially replaced by chlorite and epidote.After rock metamorphism, high-field-strength elements (HFSEs), such as REEs, Y, Th, U, Zr, Hf, Ti, Nb, Ta, etc., tend to exhibit inert migration characteristics without significant migration [89], so the characteristics of HFSEs can better reflect the protolith characteristics of metamorphic rocks.
Mantle-derived magma will inevitably be subjected to crustal contamination during the ascent process [90].The crustal material is characterized by low Nb/La, Sm/Nd, Nb/Ta and Mg#, and high Th/La, La/Sm and SiO 2 .Therefore, Mg# is positively correlated with the ratios of Nb/La, Nb/Ta and Sm/Nd, and negatively correlated with Th/La, La/Sm and SiO 2 , which can indicate crustal contamination [91].As can be seen from Figure 10, Mg# was positively correlated with Nb/La, Nb/Ta, Th/La, La/Sm and SiO 2 , and negatively correlated with Sm/Nd, which is significantly different from the characteristics of crustal contamination.In addition, the Hadayang schists had significantly depleted Zr and Hf elements (Figure 5b), excluding inherited zircons, and had positive εHf(t) values of +5.8 to +9.3, suggesting that crustal contamination was not a significant factor during the magmatic evolution of the Hadayang schists.Ta, etc., tend to exhibit inert migration characteristics without significant migration [89], so the characteristics of HFSEs can better reflect the protolith characteristics of metamorphic rocks.Mantle-derived magma will inevitably be subjected to crustal contamination during the ascent process [90].The crustal material is characterized by low Nb/La, Sm/Nd, Nb/Ta and Mg#, and high Th/La, La/Sm and SiO2.Therefore, Mg# is positively correlated with the ratios of Nb/La, Nb/Ta and Sm/Nd, and negatively correlated with Th/La, La/Sm and SiO2, which can indicate crustal contamination [91].As can be seen from Figure 10, Mg# was positively correlated with Nb/La, Nb/Ta, Th/La, La/Sm and SiO2, and negatively correlated with Sm/Nd, which is significantly different from the characteristics of crustal contamination.In addition, the Hadayang schists had significantly depleted Zr and Hf elements (Figure 5b), excluding inherited zircons, and had positive εHf(t) values of +5.8 to +9.3, suggesting that crustal contamination was not a significant factor during the magmatic evolution of the Hadayang schists.6 ppm) contents relative to primitive-mantle-derived magmas, suggesting that our samples were evolved magmas rather than primitive magmas.The negative correlation between Mg# and CaO and FeO T , and the fact that Mg# was positively correlated with Cr, Ni, SiO2 and Na2O, indicated olivine and clinopyroxene fractionation (Figure 11).Based on the negative Eu anomalies, plagioclase fractionation also must have played an important role in the evolution of these magmas.

Magma Source
The source region of basic volcanic rocks is generally asthenosphere or lithospheric mantle [92].The low silica and alkali contents of the Hadayang schists, and their high MgO, Ti, Co and Cr contents, all favor a mantle magmatic source.This conclusion is also supported by the positive εHf(t) values (the average values were +8.93 and +9.29, respectively) (Figure 8).The TDM1 ages (552 to 781 Ma and 547 to 679 Ma, respectively) indicated

Magma Source
The source region of basic volcanic rocks is generally asthenosphere or lithospheric mantle [92].The low silica and alkali contents of the Hadayang schists, and their high MgO, Ti, Co and Cr contents, all favor a mantle magmatic source.This conclusion is also supported by the positive ε Hf (t) values (the average values were +8.93 and +9.29, respectively) (Figure 8).The T DM1 ages (552 to 781 Ma and 547 to 679 Ma, respectively) indicated that their parental magma fractionated from mantle in the Neoproterozoic period.As mentioned above, the Hadayang schists have LILE enrichment and significant HFSE depletion with moderately fractionated REE patterns and relatively low HREE abundances, suggesting that the components of the magma source may have included aqueous fluids released by dehydration of the subducted oceanic plate and/or partial melts derived from the subducted oceanic crust/sediments.The Ba/Th ratio of the Hadayang schist samples varied significantly, while the (La/Sm) N ratio was relatively constant (Figure 12b), indicating that fluids played a key role in the enrichment process, and the influence of sediment was small or absent.The wide range of the Sr/Nd (10.1-30.7)ratio values further indicated that fluid action played a major role in the process of mantle-derived magma enrichment.The above characteristics are supported by the Nb/Zr vs. Th/Zr plot (Figure 12a).The abundances and ratios of REEs are widely used to distinguish the source regions of magma and the degree of partial melting [93].As can be seen in Sm/Yb vs. Sm and Sm/Yb vs. La/Yb plots (Figure 12c, d), the Hadayang schist samples plot mainly near or slightly deviating from the garnet + spinel lherzolite melting curves, and they fall between these two melting curves with starting compositions of primitive mantle, implying a mantle source consisting of spinel and garnet lherzolite, and that it took 1%-5% partial melting of this mantle source to produce the parent magma of the Hadayang schists (Figure 12c,d).

Tectonic Evolution and Geological Significance
Although a lot of progress has been made in the study of Late Devonian-Early Carboniferous magmatic rocks in the XB, there are still major disputes about their tectonic setting, including the active continental margin or island arc [16,58,59,79,82,83,86,87], backarc basin [61,88], post-collision extensional [60] and subduction-related extension environment [78,81].Some authors also believe that there was a transition from passive continental margin to island arc environment during this period [84,94].In the following discussion, we suggest that the Late Devonian-Early Carboniferous magmatic rocks were formed in an intracontinental extension setting.
As mentioned in an earlier section, the Hadayang schists were characterized by LILE enrichment and significant HFSE depletion, and also exhibited moderately fractionated REE patterns and relatively low HREE abundances, which is also seen in the Late Devonian to Early Carboniferous basic magmatic rocks of the XB [61,78,86,88].These geochemical characteristics are similar to those arc-type magmatic rocks [98], but there is some uncertainty in judging whether basic igneous rocks are arc magmatic rocks [99][100][101].Previous studies have shown that the participation of fluids can change the geochemical characteristics of trace elements, and the precipitation of water-bearing fluid from subducting plates, or the release of water-bearing fluid from deep fluid circulation under the background of subducting plates, may lead to water-rich fluid metasomatism in the mantle source region, forming arc-like trace element characteristics [101][102][103].Due to the differences in the contents of Zr, Ti and Sr elements between arc-like basalts and arc basalts, it is very effective to distinguish the two types of basalts by using a discriminant diagram of the above three elements [101].On the plots of Ti/1000 vs. V, Zr vs. Zr/Y and Zr vs. Ti (Figure 13a,b,d), all of the Late Devonian to Early Carboniferous intermediate-basic magmatic rocks in the XB plot in the intraplate basalt field or deviating from the arc basalt area, similar to the geochemical characteristics of the North American Basin and Range basalt.As can be seen from the plot of Zr/Sm-Sr/Nd-Ti/V (Figure 13c), most of the samples plot in or near the intraplate magma and Basin and Range regions, which is significantly different from the nearly vertical characteristics of arc basalt.Based on the above analysis, we propose that the Late Devonian-Early Carboniferous intermediate-basic magmatic rocks in the eastern XB may have been formed in an intracontinental extension tectonic setting similar to the North American Basin and Range basalt.Coincidentally, the same tectonic background also appears in the western part of the XB.Previous studies have shown that the Erenhot-Hegenshan ophiolite in the XB was formed during the Late Devonian-Early Carboniferous [104][105][106][107][108][109][110] and was the product of the upwelling process of the asthenosphere caused by the extension of the lithosphere [111,112].Based on the identification of basement rocks and other evidence around the ophiolite [113], it was indicated that the ophiolite would have been formed in the continental crust extension zone [6,114].In summary, we believe that the XB would have been in the stage of intracontinental extension during the Late Devonian-Early Carboniferous.[95]), La/Sm vs. Sm/Yb ((b), after [96]), Sm vs. Sm/Yb ((c), after [97]) and La/Yb vs. Sm/Yb ((d), after [97]) for the Hadayang schists.
As mentioned in an earlier section, the Hadayang schists were characterized by LILE enrichment and significant HFSE depletion, and also exhibited moderately fractionated REE patterns and relatively low HREE abundances, which is also seen in the Late Devonian to Early Carboniferous basic magmatic rocks of the XB [61,78,86,88].These geochemical characteristics are similar to those arc-type magmatic rocks [98], but there is some uncertainty in judging whether basic igneous rocks are arc magmatic rocks [99][100][101].Previous studies have shown that the participation of fluids can change the geochemical characteristics of trace elements, and the precipitation of water-bearing fluid from subducting plates, or the release of water-bearing fluid from deep fluid circulation under the background of subducting plates, may lead to water-rich fluid metasomatism in the mantle source region, forming arc-like trace element characteristics [101][102][103].Due to the differences in the contents of Zr, Ti and Sr elements between arc-like basalts and arc basalts, it is very effective to distinguish the two types of basalts by using a discriminant diagram of the above three elements [101].On the plots of Ti/1000 vs. V, Zr vs. Zr/Y and Zr vs. Ti (Figure 13a-b, d), all of the Late Devonian to Early Carboniferous intermediate-basic magmatic rocks in the XB plot in the intraplate basalt field or deviating from the arc basalt area, similar to the geochemical characteristics of the North American Basin and Range basalt.As can be seen from the plot of Zr/Sm-Sr/Nd-Ti/V (Figure 13c), most of the samples plot in or near the intraplate magma and Basin and Range regions, which is significantly different from the nearly vertical characteristics of arc basalt.Based on the above analysis, we propose that the Late Devonian-Early Carboniferous intermediate-basic magmatic rocks in the eastern XB may have been formed in an intracontinental extension tectonic setting similar to the North American Basin and Range basalt.Coincidentally, the same tectonic background also appears in the western part of the XB.Previous studies have Figure 12.Plots of Nb/Zr vs. Th/Zr ((a), after [95]), La/Sm vs. Sm/Yb ((b), after [96]), Sm vs. Sm/Yb ((c), after [97]) and La/Yb vs. Sm/Yb ((d), after [97]) for the Hadayang schists.shown that the Erenhot-Hegenshan ophiolite in the XB was formed during the Late Devonian-Early Carboniferous [104][105][106][107][108][109][110] and was the product of the upwelling process of the asthenosphere caused by the extension of the lithosphere [111,112].Based on the identification of basement rocks and other evidence around the ophiolite [113], it was indicated that the ophiolite would have been formed in the continental crust extension zone [6,114].
In summary, we believe that the XB would have been in the stage of intracontinental extension during the Late Devonian-Early Carboniferous.

1.
The protolith of the Hadayang schists was intermediate-basic volcanic rock.LA-ICP-MS U-Pb dating of zircons from the epidote-biotite-albite schist and biotite-albite schist yielded crystallization ages of 360 ± 2 Ma (MSWD = 1.4) and 355 ± 3 Ma (MSWD = 2.1).This, combined with the presence of contemporaneous magmatic rocks in the region, indicates an important magmatic event in the eastern XB during the Late Devonian-Early Carboniferous.

2.
The magma source of the Hadayang schists was a mantle that consisted of both spinel and garnet lherzolite, with a partial melting degree of 1%-5%, and which underwent fractional crystallization of olivine, orthopyroxene and plagioclase.

3.
The Late Devonian-Early Carboniferous intermediate-basic magmatic rocks in the eastern XB were formed in an intracontinental extension setting, which is consistent with the tectonic setting of contemporaneous ophiolites in the western section.The Xing'an block would have been in the stage of intracontinental extension during the Late Devonian-Early Carboniferous.

Figure 2 .
Figure 2. Simplified geological map of Hadayang area and locations of samples used in this study.

Figure 2 .
Figure 2. Simplified geological map of Hadayang area and locations of samples used in this study.

26 Figure 6 .
Figure 6.Cathodoluminescence images of zircon grains selected for analysis from the epidote-biotite-albite schist (a) and biotite-albite schist (b).The smaller white circles represent locations of LA-ICP-MS U-Pb dating, whereas the bigger yellow circles (dotted line) indicate locations of Hf isotopic analyses.

Figure 6 .
Figure 6.Cathodoluminescence images of zircon grains selected for analysis from the epidotebiotite-albite schist (a) and biotite-albite schist (b).The smaller white circles represent locations of LA-ICP-MS U-Pb dating, whereas the bigger yellow circles (dotted line) indicate locations of Hf isotopic analyses.

Figure 8 .
Figure 8. Diagrams of zircon Hf isotopic of the Hadayang schists.

Figure 8 .
Figure 8. Diagrams of zircon Hf isotopic of the Hadayang schists.Figure 8. Diagrams of zircon Hf isotopic of the Hadayang schists.

Figure 8 .
Figure 8. Diagrams of zircon Hf isotopic of the Hadayang schists.Figure 8. Diagrams of zircon Hf isotopic of the Hadayang schists.

6 . Conclusions 1 .
The protolith of the Hadayang schists was intermediate-basic volcanic rock.LA-ICP-MS U-Pb dating of zircons from the epidote-biotite-albite schist and biotite-albite schist yielded crystallization ages of 360 ± 2 Ma (MSWD = 1.4) and 355 ± 3 Ma (MSWD = 2.1).This, combined with the presence of contemporaneous magmatic rocks in the region, indicates an important magmatic event in the eastern XB during the Late Devonian-Early Carboniferous.

Table 2 .
LA-ICP-MS zircon U-Pb dating results for the Hadayang schists.

Table 3 .
In situ zircon Lu-Hf isotopic results for the Hadayang schists.