Metamorphic rocks from the north-eastern part of the Ereendavaa terrane ( Eastern Mongolia ) : an origin of the Permian back-arc basin rather than the Proterozoic basement

In this paper, we have conducted geochronological and geochemical studies on the metamorphic rocks of the Khaychingol and Ereendavaa Formations in the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas from the Ereendavaa terrane and these rocks have been considered to be Precambrian in age. However, new LA–ICP– MS zircon U–Pb dating results indicate that the protolith of the studied metamorphic rocks was formed in two stages: 1) during ~ 296 285 Ma, the protolith of mafic, felsic and black schists formed; 2) during ~276 271 Ma, the protolith of gneiss and psammitic schists began to deposit. The Early Permian bimodal association composed of low-K basalt and comagmatic high-Na, low-K dacite with high-K calcalkaline rhyolite, represent protolith of the mafic and felsic schists which were formed in back-arc basin environment. The Middle Permian gneiss, and psammitic schists with sedimentary protolith have geochemical signatures of island arc rocks, such as enrichment of LILE relative to HFSE, and markedly negative Nb, Ta and Ti anomalies, suggesting that they were formed in a continental arc environment. Considering a close spatial relationship of the Ereendavaa terrane with the MongolOkhotsk Belt in the north-west, we propose that accompanied with the emplacement of arc magmatic rocks, the arc rifting occurred and formed the Early Permian bimodal volcanic rocks. In the Late Permian, after the formation of the back-arc basin, deposition of the immature deposits as wacke, arkose and litharenite dominated sediments in a continental arc environment started.


Original article
Mongolian Geoscientist situated in the North-Eastern Mongolia consists, from north to south, of the Ereendavaa, Undur-Khaan (Herlen terrane after Badarch et al., 2002) and Idermeg terranes, all extending in NE -SW direction (Badarch et al., 2002;Tomurtogoo, 2002). The Ereendavaa terrane is one of the constituent terranes of Kherlen Massif (Fig. 1b), which represents the Mongolian part of the Argun-Idermeg superterrane (Parfenov et al., 2009;Kotov et al., 2013) or so called Central Mongolia-Erguna Belt (Wang et al., 2017). The terrane is bordered by the Mongol-Okhotsk Orogenic Belt to the northeast (by Mongol-Okhotsk suture) and by the Idermeg and Undurkhaan terranes (by Kherlen suture) to the south and southeast (Tomurtogoo, 2002). It is widely accepted that the Paleozoic tectonic evolution of the Ereendavaa terrane was dominated by the amalgamation of microcontinental massifs and closure of the Paleo-Asian Ocean, while the Mesozoic evolution was largely influenced by the Mongol -Okhotsk tectonic systems (Dash et al., 2015;Wang et al., 2015;Miao et al., 2016;Narantsetseg et al., 2019;Sheldrick et al., 2020). The Ereendavaa terrane was previously classified as a cratonic terrane (Badarch et al., 2002), but it was later defined as an active continental margin (Tomurtogoo, 2002;2012, 2014 with Paleoproterozoic to Mesoproterozoic basement and Neoproterozoic metasedimentary and volcanic rocks. The metamorphic rocks of the Khaychingol and Ereendavaa Formation scattered in the Ereendavaa terrane, have long been interpreted to represent the Precambrian basement (Marinov et al., 1973;Blagonravov et al., 1990;Dorjnamjaa and Bat-Ireedui, 1991;Baymba, 1991;Dorjnamjaa et al., 2012;Tomurtogoo, 2012;Tomurtogoo, 2014;Erdenechimeg et al., 2017). However, recent studies show that various schist, gneiss and amphibolite from the north-eastern part of the Ereendavaa terrane mainly have protolith ages from Paleozoic to Mesozoic rather than Precambrian (Daoudene et al., 2009;Miao et al., 2017). In contrast, Late  , c) Biotite-plagioclase gneiss (sample O35), d) Biotite-plagioclase gneiss (sample O38) and biotite-quartz-plagioclase schist (sample O38/1), and e) Two-mica quartz schist (sample O46), muscovite schist (sample O46/1) and biotite-plagioclase gneiss (sample O46/1) from Khaychin Gol area. f) Quartz-chlorite schist (sample E6) from Emgentiin Bulag area Mesoproterozoic (ca. 1.2-1.15 Ga;Miao et al., 2020) and Late Neoproterozoic (ca. 550 and 630 Ma; Narantsetseg et al., 2015; ages were obtained from the south-western part of the terrane. However, the possibility of presence of Precambrian rocks within the north-eastern part of the terrane still not excluded. Therefore, our study focused on the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas which were considered to be key areas in having Precambrian rocks. This study reports integrated petrography, whole-rock geochemistry and LA-ICP-MS zircon U-Pb ages of metamorphic rocks from the Khaychingol and Ereendavaa Formation in the north-eastern part of the Ereendavaa terrane.

GEOLOGICAL SETTING AND STRATIGRAPHIC SEQUENCES
The Ereendavaa terrane contains metamorphic rocks of Paleoproterozoic and Mesoproterozoic Khaychingol and Ereendavaa Formation overlying by Late Neoproterozoic-Early Cambrian terrigenous-carbonate and volcanoterrigenous formations, which were intruded by Upper Neoproterozoic to Mesozoic granitoid plutons (Fig. 1c). The Khaychingol Formation mainly exposed as sparse small blocks in the south-eastern and north-eastern part of the Ereendavaa terrane, especially the Ikh Khaychin Gol, Baga Khaychin Gol, Bituu Gol and Mogoitiin Gol areas. The Khaychingol Formation consists of gneiss, quartz-biotite schist, hornblende schist, two-mica schist, chlorite-sericite schist and minor amphibolite and marble, with an overall thickness of 1200 -1500 m. The Ereendavaa Formation is mainly distributed in the Nomon Gol, Nariin Gol, Zurug Gol, Onon Gol, Ulaan Zoogiin Bulag and Emgentiin Bulag areas and consists of black schist interlayered with thin metaconglomerate, metasandstone, metasiltstone and crystalline limestone in lower part and quartz-sericite, quartz-chlorite-sericite, sericitechlorite and quartz-chlorite-carbonateamphibole schists and minor calcareous sandstone and crystalline limestone in upper part. Total thickness is estimated about to be 3000 m (Blagonravov et al., 1990;Dorjnamjaa and Bat-Ireedui, 1991). The Khaychingol Formation shows mainly fault contact with the Ereendavaa Formation.

Stratigraphic sequence and petrography
During the field trip, we collected 32 typical gneiss and schists samples in the Khaychingol and Ereendavaa Formation from the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas, north-eastern part of the Ereendavaa terrane. The sampling sites are shown in Fig.1d, e and f and samples are described in the following paragraphs.

Khaychin Gol area
The Proterozoic sequences in the Khaychin Gol area are referred to the Kaychingol Formation with Paleoproterozoic (Blagonravov et al., 1968;Dorjnamjaa and Batireedui, 1991;Dorjnamjaa et al., 2012) or Mesoproterozoic (Erdenechimeg et al., 2017) ages. The studied section is located in the north-eastern part of the Western Khaychin Gol (49 o 22' 59.4'';112 o 51' 0.2''). The Khaychingol Formation in this area mainly comprises a sequence of yellowish gray garnet, rare tourmaline bearing biotite-plagioclase gneiss, light to grey mica schist and minor darkgreen hornblende schist with different thicknesses (Fig. 2b-e). Total thickness of the studied section is 1000 m. Hornblende schist gener ally fine-to medium grained (sample O40, O42) and lepidogranoblastic in texture. Mineral assemblage is variable and consists of mainly yellowish-green hornblende (Hbl, 60 -85%) and plagioclase (Pl, 15 -30%), clinopyroxene (Cpx, 10 -15%, only in sample O40 occur as relics) with minor amount of quartz and opaque minerals (Fig. 4a). The schistosity is well defined by oriented hornblende (1 -2 mm) grains alternating with plagioclase and quartz aggregates. The nature of the protolith is interpreted to be mafic. Biotite-plagioclase gneiss (samples O35, O36, O37, O38, O42/1) is fine to medium grained and consists of recrystallized quartz grains (Q, 60 -65 %) with serrated margins, plagioclase (Pl, 15 -20%), biotite (Bt, 5 -10 %) and trace amounts of garnet (Grt), tourmaline (Tur) and opaque minerals (Fig. 4b). The rock is slightly foliated, defined by aligned biotite grains. The biotite is generally subidiomorphic and occur at grain boundaries and ranges from 0.5 to 1 mm in length. Quartz together with plagioclase ranges from 0.1 to 2.0 mm in size. Tourmaline is euhedral and 0.6 to 2.4 mm in diameter. Garnet is euhedral to subhedral, and to 2.2 mm in size. The typical mineral assemblage is Grt+Bt+Pl+Qtz+Tur. The gneiss was deformed, with the polycrystalline quarts. Its protolith is interpreted to be sedimentary rocks.

ANALYTICAL METHODS U-Pb zircon geochronology
Zircon crystals were obtained from crushed rock using a combination of heavy liquid and magnetic separation techniques at Institute of Geology of Mongolian Academy of Sciences. Individual crystals were handpicked, mounted on adhesive tape, then enclosed in epoxy resin and polished to about half of their thickness. Cathodoluminescence (CL) images were obtained using a JXA-8100 Electron Probe Microanalyzer with Mono CL4 Cathodoluminescence System (Gatan) for highresolution imaging and spectroscopy in the Institute of Mineral Resource, Chinese Academy of Geological Sciences and Institute of Geochemistry of Chinese Academy of Sciences. U-Pb dating of zircons from 6 representative samples gneiss, schist and schistose sandstone was performed for LA-ICP-MS dating at Institute of Geology, Chinese Academy of Geological Sciences and Guangzhou Institute of Geochemistry of Chinese Academy of Sciences.
Sample mounts were placed in a sample cell designed by Laurin Technic Pty. Ltd, flushed with Ar and He. Laser ablation was accomplished using a pulsed Resonetic 193 nm ArF excimer laser, operated at a constant energy of 80 mJ, with a repetition rate of 8 Hz and a spot diameter of 31 μm. The ablated aerosol was carried to an Agilent 7500a ICP-MS by He gas via a Squid system to smooth signals. Data were acquired for 30 s with the laser off, and 40 s with the laser on, giving approximate 100 mass scans. Zircon TEMORA was used as the external standard to correct elemental fractionation, while zircon Plešovice (PL) was also used for quality control. Data reduction was performed off-line by ICPMSDataCal 7.2 (Liu et al., 2010). U-Pb ages of zircons were calculated using the U decay constants of 238 U=1.55125 x 10 -10 year -1 , 235 U=9.8454 x 10 -10 year -1 and the Isoplot 3 software (Ludwig, 2003).

Major and trace element analysis
Major element oxides (wt.%) of 11 samples were analyzed at the Acme Analytical Laboratories Ltd., Vancouver (Canada). Major element compositions and Sc, Ba, and Ni abundances were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The remainder of the trace elements and rare earth elements (REEs) was determined using inductively coupled plasma-mass spectrometry (ICP-MS) (for Acme codes 4A-4B and 1DX analytical procedures see http:// acmelab.com). A total of 21 samples were analyzed for major element at SGS Mongolia (invested by Switzerland), using standard procedure (https://www.sgs.mn). Major element concentrations were analyzed by X-ray Fluorescence Spectrometry (XRF). For the major element analysis, glass beads were prepared by fusing mixtures of 0.7 g of powdered sample with 6.0 g of lithium tetraborate.
Analytical uncertainties are generally better than 1%. Trace elements, including rare earth elements (REEs) of 21 samples were analyzed by a Perkin-Elmer Sciex ELAN 6000 ICP-MS, in the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry. Powdered samples (50 mg) were digested with mixed HNO 3 +HF acid in steel-bomb coated Teflon beakers for 2 days in order to assure complete dissolution of the refractory minerals. An internal standard solution containing the single element Rh was used to monitor signal drift. The USGS rock standards G-2, W-2, MRG-1 and AGV-1 and the Chinese national rock standards GSD-12, GSR-1, GSR-2 and GSR-3 were used to calibrate the elemental concentrations of the measured samples. Analytical precision was generally better than 5%. Sample preparation techniques and other details of procedures are described in the references Long et al., 2010).

Zircon U-Pb geochronology
Six representative samples of gneiss, schist and schistose sandstone considered typical for Khaychingol and Ereendavaa Formations were selected for LA-ICP-MS U-Pb zircon dating. They were collected along three sections, which are generally perpendicular strike of the strata and sample location are illustrated in Fig. 1d, e and f, and the analytical results are presented in Supplementary data, Table 1 and Figs. 6 and 7.

Mogoitiin Gol area
Sample 320 fr om the black schist was collected from the Mogoitiin gol area, in the southern part of Khaliun Mountain (location: 49 о 29'40.5''; 112 o 55'49.5''). Zircons from Sample 320 are colorless, with euhedral to subhedral shape. They range in length from ca. 50 to 130 μm, with length to width ratios of 1:1 to 2:1 (rare to 3:1). CL images reveal that most of grains are oscillatory zoned and few have a core-rim structure (Fig. 6a). Total of 75 analyses were made on 73 zircons (Fig. 6b). Th/U ratio of zircons is range from 0.15 to 2.20, except two low values of the rim. The detrital zircons have concordant ages ranging between 291 ± 4 Ma and 1892 ± 27 Ma with three age populations with conspicuous peaks at 525 Ma, 414 Ma and 297 Ma, respectively (Fig. 6c). Other concordant analyses define ages from 743 ± 9 Ma to 1892 ± 27 Ma without obvious peaks. Two analyses on the rim have concordant 206 Pb/ 238 U ages of 465 ± 5 and 522 ± 5 Ma. Their low Th/U ratios (0.004 and 0.03) may indicate that their ages record the time of high-grade metamorphism. The youngest coeval zircon grains between 291 ± 4 and 299 ± 3 Ma yield a weighted mean age of 296 ± 2 Ma (MSWD = 0.59, n = 7) which constrains the youngest depositional age of protolith to be after 296±2 Ma (Fig. 6b). Sample 321 fr om quar tz-sericite schist was collected from the Mogoitiin Gol area, in the south of Khaliun Mountain (location: 49 о 29'45.9"; 112 o 55'46.6"). The zircon grains from the felsic schist sample mostly range from 60 to 160 µm in length with euhedral morphology (Fig. 6d). All the zircon grains display oscillatory growth zoning, consistent with an igneous origin (Koschek, 1993). Total of 72 analyses were made on 65 zircons and 49 concordant analyses on zircon grains yield a weighted mean 206 Pb/ 238 U age of 286 ± 3 Ma (MSWD = 1.3, n = 49), which likely represents the crystallization age of the protolith (Table 1; Fig. 6e-f).
Khaychin Gol area Sample O35 fr om biotite-plagioclase gneiss was collected from the Khaychin Gol area (location: 49 о 22'57.0''; 112 o 51'3.0''). Zircons from Sample O35 are colorless, with euhedral to subhedral shape. They range in length from ca. 50 to 150 μm, with length to width ratios of 1:1 Fig. 7. CL images of typical zircons, zir con 207 Pb/ 235 U -206 Pb/ 238 U concordia diagrams and age probability histograms of detrital zircons of Sample 035, 036, 037 (Biotite-plagioclase gneiss) from Khaychin Gol area and Sample E3 (Psammitic schist) from Emgentiin Bulag area. The notation for each spot (a, d, g, j) consists of spot number and the 206 Pb/ 238 U age. Inserted small figures are weighted average ages of youngest zircon population of each detrital zircon samples. to 2:1 (rare to 3:1). CL images reveal that most of zircon grains are oscillatory zoned and few have a core-rim structure with very thin rims (Fig. 7a). Totally, 53 zircons were analyzed. The Th/U ratios of zircons vary between 0.22 and 1.51. The high Th/U ratios of zircons (>0.1) and observed oscillatory growth zoning in CL images, suggest that most of the detrital zircons have a magmatic origin (Koschek, 1993). However, few zircons (see point 035-42 in the Fig. 7a) show white color in CL images, implying some loss of Pb. The detrital zircons have concordant ages range between 263 ± 2 Ma to 1864 ± 18 Ma with a single large population (72%) between 263 and 299 Ma, with prominent peak at 298 Ma. Other concordant analyses define ages from 300 ± 2 Ma to 1864 ± 18 Ma without obvious peaks. The youngest coeval zircon grains between 263 ± 2 and 279 ± 2 Ma yield a weighted mean age of 271 ± 4 Ma (MSWD = 7.8, n = 11) which constrains the youngest depositional age of protolith to be later than 271 ± 4 Ma ( Fig. 7b-c). Sample O36 fr om biotite-plagioclase gneiss was collected from the Khaychin Gol area (location: 49 о 23'04.7"; 112 o 50'57.0"). Zircons from Sample O36 are colorless, with euhedral to subhedral shape. They range in length from ca. 80 to 180 μm, with length to width ratios of 1:1 to 2:1 (rare to 3:1). CL images reveal that almost all of zircon grains are oscillatory zoned (Fig. 7d). Total of 96 analyses were made on 96 zircons. The detrital zircons have 78 concordant ages ranging between 265 ± 5 and 558 ± 8 Ma with a single large population between 258 and 299 Ma (79%) with a prominent peak at 283 Ma. Other concordant analyses (21%) define ages from 300 ± 2 Ma to 558 ± 8 Ma without obvious peaks. The youngest coeval zircon grains between 258 ± 6 and 279 ± 5 Ma yield a weighted mean age of 274 ± 2 Ma (MSWD = 0.78, n = 25) which constrains the youngest depositional age of protolith to be later than 274 ± 2 Ma ( Fig. 7e-f). Sample O37 fr om biotite-plagioclase gneiss was collected from the Khaychin Gol area (location: 49 о 23'03.0''; 112 o 50'56.9''). Zircons from Sample O37 are colorless, with euhedral to subhedral shape. They range in length from ca. 70 to 190 μm, with length to width ratios of 1:1 to 2:1. CL images reveal that all of zircon grains are oscillatory zoned (Fig. 7g). Total of 90 analyses were made on 90 zircons. The detrital zircons have 55 concordant ages ranging from 285 ± 7 to 325 ± 9 Ma with single large population with prominent peak at 281 Ma. The youngest coeval zircon grains between 268 ± 7 and 280 ± 4 Ma yield a weighted mean age of 275 ± 3 Ma (MSWD = 0.35, n = 15) which constrains the youngest depositional age of protolith to be later than 276±2 Ma (Fig. 7h-i).
Emgentiin Bulag area Sample E3 fr om schistose sandstone was collected from the Emgentiin Bulag area (location: 49 о 01'49.5''; 112 o 06'34.1''). Zircons from Sample E3 are colorless, with euhedral to subhedral shape. They range in length from ca. 50 to 160 μm, with length to width ratios of 2:1 to 3:1. CL images reveal that all of zircon grains are oscillatory zoned (Fig. 7j). Total of 67 analyses were made on 67 zircons. The Th/U ratios of zircons vary between 0.11 and 2.2. The high Th/U ratios of zircons (>0.1) and observed oscillatory growth zoning in CL images, suggest that the most detrital zircons were derived from a magmatic provenance (Koschek, 1993). The detrital zircons have 58 concordant ages ranging from 265 ± 3 to 968 ± 7 Ma. Most of them show concordance more than 90% and yield ages mainly clustering at ca. 265 -299 Ma and ca. 448 -500 Ma, with the prominent age peak at ca. 276 Ma and minor peaks at ca. 294 Ma and 459 Ma, respectively. The youngest coeval zircon grains between 265 ± 3 and 279 ± 4 Ma yield a weighted mean age of 273 ± 3 Ma (MSWD = 3.9, n=10) which constrains the youngest depositional age of protolith to be later than 273 ± 3 Ma (Fig. 7k-l).

Whole-rock geochemistry
The results of major and trace element analysis of the 32 schist and gneiss samples from the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas are listed in Supplementary data, Table 2 and 3.

Schist sequence of the Mogoitiin Gol area
According to the field investigation and thin section observation, felsic schists (quartzsericite and chlorite-quartz schists) with minor intercalation of mafic (hornblende schist) and black schists are the most common varieties of metamorphic rocks within the studied section in the Mogoitiin Gol area.   Sun and McDonough (1989). OIB and N-MORB data from Sun and McDonough (1989) are plotted for comparison, respectively. OIB, Oceanic island basalt; N-MORB, Normal mid-ocean ridge basalt.
Mg# value (50) ( Table 2). On a total alkali versus silica (TAS) diagram, the rock is plotted into the basaltic andesite field (Fig. 8a). Geochemically, such rock belongs to the transitional tholeiitic series (Fig. 8b). The A/ CNK and A/NK ratios are 0.91 and 2.93, respectively, corresponding to a metaluminous type (Table 2). Trace elements. The mafic schist is characterized by enrichment of LREE relative to HREE (La n /Yb n = 4.13, La n /Sm n = 2.44) (Fig.  9a). On primitive mantle normalized plots, the rock exhibits slight enrichment of Cs, Ba, U, K and Pb, and depletion of Nb, Ta, Sr and Ti relative to adjacent elements (Fig. 9b).

Gneiss and schist sequence of Khaychin Gol area
The metamorphic rocks in this area mainly comprise a sequence of garnet and tourmaline bearing gneiss (biotite-plagioclase gneiss) and psammitic schist (mica schists) with sedimentary protolith and contain minor mafic (hornblende schist) schist with igneous protolith.

Schist sequence of the Emgentiin Bulag area
The metamorphic rocks in this area mainly comprise a sequence of psammitic schist (quartz -sericite schist and schistose sandstone) with sedimentary protolith and contain lenses of mafic (quartz-chlorite and amphibole-chlorite schist) schist with igneous protolith.

DISCUSSION Formation/Deposition time of studied
metamorphic rocks Zircon U-Pb data for the felsic schist from the Mogoitiin Gol area yield a crystallization age of 286 ± 3 Ma (Table 1; Fig. 6e). CL images of zircons reveal euhedral shapes and typical oscillatory growth zoning, consistent with an igneous origin (Koschek, 1993). In addition, the youngest age population of the black schist from this area is 296 ± 2 Ma and these zircons have oscillatory growth zoning and a relatively high Th/U ratio of 0.15 -2.20, indicating a magmatic origin (Table 1, Fig. 6a-c). Therefore, we regard 296 Ma as the best estimate for the maximum deposition age of the black schist and 285 Ma as the protolith age of the felsic schist. A total of 224 analyzes with discordance less than 10% from four samples of gneiss and psammitic schists from the Khaychin Gol and Emgentiin Bulag areas yield apparent ages from 1809 to 263 Ma (Table 1). Zircons from these samples exhibit oscillatory growth zoning and high Th/U ratios (0.22 -1.51), indicative of a magmatic origin, suggesting that the obtained U -Pb ages represent the timing of crystallization of the zircons (Koschek, 1993). Therefore, the youngest age populations of these zircon samples, namely 271 ± 4 Ma for sample 035, 274 ± 2 Ma for sample 036, 276 ± 2 Ma for sample 037 and 273 ± 3 Ma for sample E3, imply that their maximum deposition ages are ~276 -271 Ma (Table 1; Fig. 7). A granitic intrusion of Middle Permian Ulz complex (270 -230 Ma, Erdenechimeg et al., 2017) which are intruded into the Ereendavaa Formation can be represent an upper age limit of gneisses and psammitic schists. Taken together, LA-ICP-MS zircon U-Pb dating results indicate that the protolith of the studied metamorphic rocks from the northeastern part of the Ereendavaa terrane were formed in two stages: 1) ~ 296 -285 Ma, for the formation of protolith of felsic and black schists, which were previously believed to be Mesoproterozoic or Neoproterozoic in age; 2) ~276 -271 Ma for the deposition of protolith of gneiss and psammitic schists, which were believed before to be Paleoproterozoic and Mesoproterozoic in age. For the mafic schists, Fig. 12. a) Comparison of incompatible element patterns of the High-Na dacites from the Mogoitiin Gol area, Ereendavaa terrane and Troodos ocean ridge granite (ORG) and Jean Charcoat Trough, Southwest Pacific (Nakada et a., 1994). Normalizing values of ORG is from Pearce et al (1984); b) Y/15 -La/10 -Nb/8 (Cabanis and Lecolle, 1989) and c) Hf/3 -Th -Ta (Wood 1980) tectonic discrimination diagrams; d) Tectonic interpretation diagram of ophiolitic basaltic types based on Th N -Nb N systematics (Saccani, 2015) for the mafic and felsic schists. N-MORB, normal mid-ocean ridge basalt; E-MORB, enriched mid-ocean ridge basalt.
we assume that these rocks do not belong to the Middle Permian strata and can be represent an Early Permian bimodal association together with felsic schists. Presence of Early Permian bimodal association is supported by the new ages recently obtained from the magmatic rocks in the north-eastern part of the Ereendavaa terrane. For example, mylonitic granites (sample MO93 and MO98), augen gneiss (sample MO99) and amphibolite (sample MO89) which are located in the north-east of the Mogoitiin Gol area have protolith ages of 296 ± 5 Ma, 289 ± 5 Ma, 295 ± 5 Ma and 295 ± 3 Ma, respectively (Miao et a., 2017). Early Permian magmatic activity is also supported by the occurrence of 283 ± 5 Ma and 282 ± 7 Ma orthogneises from the central part of the Ereendavaa Range (Dauodene et al., 2013). Considering a close spatial relationship of the Ereendavaa terrane with the Mongol-Okhotsk Belt in the north-west, we propose that the Early Permian bimodal magmatism and deposition of the protolith of the Early Permian black schist and Middle Permian gneiss and psammitic schist, are likely related to the evolution of the Mongol-Okhotsk Ocean.

Nature and tectonic setting of the Early to
Middle Permian metamorphic rocks The schist and gneiss samples from the northeastern part of the Ereendavaa terrane have low LOI (mostly <3.5 wt.%) values, except few samples which show high LOI values ranging from 3.92 to 11.4 (Table 2). In the chondritenormalized REE and primitive mantlenormalized trace element spider diagrams, the studied metamorphic rocks exhibit coherent pattern for REE and HFSE, indicating that these elements are relatively immobile during the metamorphism. Besides, LILEs also exhibit similar patterns (Figs. 9 and 11), suggesting that the LILEs are not considerably affected by the metamorphism. In addition, the (Th/La) PM and (Nb/La) PM ratios of these rocks mostly remain constant and do not show significant correlations with the loss on ignition, also supporting the low mobility of HFSE and REE (except Eu). Thus, our discussion on the affinity and tectonic setting of the Early Permian bimodal magma association and source-area weathering, provenance characteristics and tectonic setting of the Early and Middle Permian gneiss and schist are based mainly on the immobile HFSE and REE and some LILE.
As mentioned above, the Early Permian felsic schists have rhyolitic and dacitic compositions. The high SiO 2 (75.1 -80.4 wt.%) and high-K calc-alkaline geochemical characteristics of the rhyolites together with LILE and LREE enrichment and HFSE depletion in primitive mantle-normalized spider diagrams ( Fig. 8a and  b, Fig. 9c and d) imply that the primary magma was derived by partial melting of crustal materials. In addition, obvious negative Eu anomalies (Eu/Eu* = 0.21 -0.59) suggest that plagioclase was a residual phase in the magma source. However, rhyolites are depleted in Na 2 O (1.11 -1.70 wt.%, mean 1.11 wt.%), while dacites are (67.8 -68.5 wt.% SiO 2 ) enriched in Na 2 O (5.25 -7.20 wt.%, mean 6.22 wt.%). Also, the dacites are characterized by flat chondrite-normalized REE pattern with slight enrichment of LREE, parallel to those of mafic schists from the Mogoitiin Gol and Khaychin Gol areas (Fig. 9a). Moreover, the dacites show enrichment of fluid-mobile elements of Cs, Rb, Ba, U, Pb and Sr, and depletion of Nb, Ta and Ti and have similar pattern to those of mafic schist (Fig. 9b). Almost identical incompatible element distribution patterns of the high Nadacites (felsic schist) and low-K basalts (mafic schist) suggest that a petrogenetic relationship link between these magma types. High-Na dacites compositionally resemble trondhjemite or oceanic plagiogranite (Barker, 1979). Incompatible element patterns of high-Na dacites form the Mogoitiin Gol area normalized to Ocean ridge granite (ORG) of Pearce et al (1984) are given in Fig. 12a. Except slight enrichment in LILE (K, Rb, Ba and Th), the high Na-dacites are close to the Troodos Ocean Ridge plagiogranite and comparable to the High-Na dacites from the Jean Charcoat Trough (Vanuatu), Southwest Pacific (Nakada et a., 1994). As the ORG magma type is interpreted to have evolved via simple fractional crystallization of N-MORB (Pearce et al., 1984), our data suggest that the high-Na dacites from the Mogoitiin Gol area could be generated by extensive fractionation of a slightly LILE-enriched BABB or transitional-MORB. In addition, the observed SiO 2 bimodality with a gap between 67.8 and 53.5 wt.% for the felsic and mafic schists also might be explained by partial melting of basaltic crustal materials. However, strong trace element similarities between mafic schist (basalt) and high-Na dacites indicative that the fractionation of low-K basalts had played a main role (Crawford et al., 1988). The Na 2 O contents of the felsic (rhyolite) and mafic (basalt) schists are not high compared with dacites (Table 2). This implies that high-Na characteristics of dacites must be acquired during the differentiation process of the basaltic magmas and dacites can be derived from the low -K basalts via crystal fractionation (Nakada et al., 1994). Thus, the geochemistry data of felsic and mafic schists from the Mogoitiin Gol area indicative that an Early Permian bimodal association may formed in a back-arc basin environment. This conclusion is also supported by the various immobile trace elements discriminant diagrams for tectonic setting. For example, the mafic schists from the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas together with high-Na dacites are plotted in the calk-alkaline and BABB, N-MORB basalt fields in the Y/15 -La/10 -Nb/8 (Cabanis and Lecolle, 1989) and Hf/3-Th-Ta (Wood, 1980) ternary diagrams ( Fig. 12b and c), which are compatible with the fields for typical BABB ((e.g. Mariana Trough (Pearce et al., 2005)) and basalts from extensional rift basins behind the Izu-Bonin arc (Hochstaedter et al., 2001). Backarc basin basalts (BABB) may form in both oceanic and continental backarc basins as a result of seafloor spreading in ensimatic and ensialic settings, respectively (Dilek and Furnes, 2011). Therefore, we have used Th N versus Nb N systematics of Sassani (2015) to infer the nature of the Early Permian mafic schists from the north-eastern part of the Ereendavaa terrane. As shown in the Fig. 12d, mafic schists and high-Na dacites from the Mogoitiin Gol area and the mafic schists from the Khaychin Gol areas plot in the 'Backarc-A' field, while mafic schists from the Emgentiin Bulag area plot in the 'Backarc-B field'. As suggested by Sassani (2015), 'Backarc A' indicates backarc basin basalts characterized by input of subduction or crustal components (e.g., immature intra-oceanic or ensialic backarcs), whereas 'Backarc B' indicates BABBs showing no input of subduction or crustal components (e.g., mature intra-oceanic backarcs). Thus, we can propose that, the back-arc bimodal magmatism in the north-eastern part of the Ereendavaa terrane, probably happened in response to upwelling of asthenospheric mantle and partial melting of the overlying mantle enriched by crustal materials, as BABBs commonly derived from partial melting of ascending N-MORB source-like asthenosphere, as a consequence of the back-arc rifting (Taylor and Martinez, 2003;Pearce and Stern, 2006).

Source-area weathering, provenance and tectonic setting of the Early Permian black schist and Middle Permian gneiss and psammitic schists
The chemical index of alteration (CIA) and index of compositional variation (ICV) can be used for quantitative evaluation of weathering degree of rocks (Nesbitt and Young, 1982;Cox et al., 1995). CIA and ICV for the Early Permian black schists from the Mogoitiin Gol area have an average value of 75 and 0.73, respectively, indicating intense weathering at the source area. In contrast, CIA and ICV values of the Middle Permian gneiss and psammitic schist from the Khaychin Gol and Emgentiin Bulag areas ranging between 51 and 68 and between 0.88 and 1.42, respectively indicating low degrees of weathering at the source (Fig. 10b, Table 3). Exceptions are two carbonate schist samples showing extremely low CIA (19 -22) and high ICV (4.84 -5.31) values (Fig. 10b, Table 3). According to the Al 2 O 3 -(CaO* + Na 2 O) -K 2 O (A-CN-K) diagram, Middle Permian gneiss and psammitic schist mainly fall in the granodiorite -  (Bhatia and Crook, 1986) diagrams for the studied Early and Middle Permian black schist, gneiss and psammitic schists. granite trend, suggesting that the Middle Permian schists were derived from a source dominated by felsic igneous rocks (Fig. 13a). However, Early Permian black schist plotted within the 'Muscovite' region, an indication of high weathering at the source. Thus, Middle Permian psammitic schist represent mainly immature deposits, while Early Permian black schist represent more mature source. From the high alteration indexes, it can be inferred that black schist from the Mogoitiin Gol area are geochemically and texturally mature. In addition, relatively high Rb/Sr ratios of the Early Permian black schist (average 7.7) indicate strong weathering and sedimentary recycling, because weathering and diagenetic processes can lead to a significant increase in Rb/Sr ratios (McLennan et al., 1993). In contrast, the relatively low Rb/Sr ratios of Middle Permian gneiss, psammitic schist and metasandstone (mean 0.81) suggest simple recycling process for such sediments. Also, the Th/Sc ratios was used for provenance study, because Th is enriched in silicic rocks, while Sc is more enriched in basic rocks, and the ratio does not vary significant during sedimentary recycling (McLennan et al., 1993;Cullers, 1994). In contrast, Zr/Sc ratio will increase significantly during sediment recycling with zircon enrichment, and can be considered as a useful indicator of heavy mineral concentration (McLennan, 1989). In this respect, the Early and Middle Permian gneiss and schists show variable Th/Sc (0.43 -6.85) and Zr/Sc (5.32 -50.5) values, and form a good positive correlation on the Th/Sc vs Zr/Sc diagram, indicating that the provenance was dominated by felsic source rocks rather than sediment recycling with zircon enrichment (Fig.  13b). This point is supported by the LREE enrichment and strong negative Eu-anomalies (Eu/Eu* = 0.29 -0.86) in the chondritenormalized REE pattern of these samples (Fig.  11a), reflecting their derivation from materials having feldspar fractionation and suggesting a provenance dominated by felsic magmatic rocks. The Early and Middle Permian black schist and gneiss, psammitic schists have geochemical signatures of island arc rocks, such as enrichment of LILE relative to HFSE, and markedly negative Nb, Ta and Ti anomalies (Fig. 11b). They plot in the continental arc field, in the La-Th-Sc (Bhatia and Crook, 1986) and Ti/Zr vs La/Sc diagrams, suggesting that they were formed in a continental arc environment ( Fig. 13c and d). Only few samples plotting near the active continental margin field. In addition, based on the detrital zircon population, the source area of the Early Permian schists contained Early Devonian, Early Cambrian rocks, with minor presence of Neoproterozoic materials. In contrast, the source area of the Middle Permian schists is limited and dominated only by Early Permian rocks.

Implications for the Late Paleozoic evolution
of the Mongol-Okhotsk Ocean Based on the above discussion, we propose that the protolith of the Early Permian felsic and mafic schists in the north-eastern part of the Ereendavaa terrane, were formed during a back-arc extension in response to the southward subduction of the Mongol-Okhotsk Oceanic plate (slab). Recent studies show that the post-collisional extensional tectonic regime was dominated during the Devonian in the Ereendavaa terrane, the southern margin of the Mongol-Okhotsk Ocean (MOO) (Narantsetseg et al., 2019;Miao et al., 2020) and since the Early Carboniferous (ca. 325 Ma), southward subduction of the Mongol-Okhotsk oceanic plate was initiated (Fig. 14a), forming the Adaatsag and Huhu Davaa ophiolites (Tomurtogoo et al., 2005;Zhu et al., 2018). Additionally, magmatic arc zircons recorded in the composition of detrital zircons of the sandstone samples from the Ereendavaa terrane support this point (Bussein et al., 2011). In the Permian to Triassic, the subduction of the Mongol-Okhotsk oceanic plate was active, to form Andean-type active margin along the both sides of whole Mongol-Okhotsk Belt (MOB) (Badarch et al., 2002;Bussein et al., 2011;Sheldrick et al., 2020). The southern continental arc in the Ereendavaa terrane represented by the Middle Carboniferous -Triassic Middle Gobi volcanic plutonic Belt including Carboniferous to Jurassic granitoids in the Kherlen depression (Bussein et al., 2011). According to this scenario, we propose that accompanied with the emplacement with the arc magmatic rocks, the arc rifting occurred and formed the Early Permian bimodal volcanic rocks, a protolith of the mafic and felsic schists from the northeastern part of the Ereendavaa terrane (Fig.  14b). The black schist which recorded in the schists sequence may represent finer silty sediments of the back-arc basin. Also, the high concentration of the U, As and Sb suggest that these elements were accumulated from seawater on oxygenpoor environment or by significant hydrothermal activity. In addition, published data indicate that the dated Lower Permian mylonitic granites, augen gneiss, gabbro-amphibolite (296 ± 5 to 289 ± 5 Ma, Miao et a, 2017) and orthogneises (283 ± 5 Ma and 282 ± 7 Ma, Dauodene et al., 2013) from the Ereendavaa terrane are mainly felsic and mafic in composition, suggesting that these rocks may belong to the intrusive analogy of the bimodal volcanic association. In the Late Permian, after the formation of the back-arc basin, deposition of the immature deposits as wacke, arkose and litharenite dominated sediments started. The provenance characteristics indicate that these sedimentary rocks were deposited on a continental arc environment which received detritus from neighboring Early Permian felsic magmatic rocks (Fig. 14c). In the Triassic, a southward subduction of the Mongol-Okhotsk slab underwent partial melting during the closure of the Mongol-Okhotsk Ocean, forming a high-Si adakite (Sheldrick et al., 2020). Later, during Late Jurassic to Early Cretaceous, the region was affected by a large-scale NW-SE extensional tectonic event with formation of Ereendavaa metamorphic core complex (Daoudene et al., 2009;.

Based on new
geochronological and geochemical analysis of the studied metamorphic rocks, we can draw the following conclusions:  New LA-ICP-MS zircon U-Pb dating results indicate that the protolith of the metamorphic rocks from the north-eastern part of the Ereendavaa terrane were formed in two stages: 1) ~ 296 -285 Ma, forming of protoliths of mafic, felsic and black schists, which were previously believed to be Mesoproterozoic or Neoproterozoic in age; 2) ~276 -271 Ma, deposition of protolith of gneiss and psammitic schists, which were believed before to be Paleoproterozoic and Mesoproterozoic in age.  The Early Permian bimodal association with low-K basalt and comagmatic high-Na, low-K dacites together with high-K calc-alkaline rhyolites, a protolith of the mafic and felsic schists were formed in a back-arc environment as a result of southward subduction of the Mongol-Okhotsk oceanic plate beneath the Ereendavaa terrane and probably related to the roll-back of the subducted slab.  The Early Permian black schist and Middle Permian gneiss, psammitic schists have geochemical signatures of island arc rocks, such as enrichment of LILE relative to HFSE, and markedly negative Nb, Ta and Ti anomalies, suggesting that they were formed in a continental arc environment. The source area of the Early Permian schists contained Early Devonian, and Early Cambrian rocks, with minor presence of Neoproterozoic materials. In contrast, the source area of the Middle Permian schists is limited and dominated by only Early Permian rocks.  We conclude that accompanied with the emplacement with the arc magmatic rocks in the north-eastern part of the Ereendavaa terrane, the arc rifting was occurred and formed the Early Permian bimodal volcanic rocks and Late Permian immature deposits, all which probably metamorphosed during the later Mesozoic magmatic activity.