Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

1.   Introduction

The Central Asian Orogenic Belt (CAOB), which situated between the Siberia and Baltica cratons to the north and the Tarim and North China cratons to the south, is one of the largest and longest-lived accretionary orogenic belts on the Earth (Şengör AMC et al., 1993, 2018; Xiao WJ et al., 2003, 2009, 2015; Xu B et al., 2015; Liu YJ et al., 2017, 2019, 2021; Windley BF et al., 2002, 2007; Wu FY et al., 2011; Zhang Y et al., 2022). This immense system of Central and East Asia was firstly defined by Şengör AMC et al. (1993) under the name of “Altaids”, which is similar but not accurately the same as today’s CAOB. The broad scope of CAOB usually composed by the range of previously “Altaids” plus the Uralide and Baykalide orogens, which are typical accretionary belts suggested by majority of international researchers on series of subsequent works (Şengör AMC et al., 2018; Xiao WJ et al., 2015). So, nowadays more widely accepted CAOB extends from the Ural Mountains in the west through Kazakhstan, NW China, Mongolia, and NE China to the Pacific Ocean in the east, covering most area of central Asian (Fig. 1; Şengör AMC et al., 1993, 2018; Xiao WJ et al., 2015; Windley BF et al., 2002, 2007; Su MR et al., 2020; Song B et al., 2021).

Figure  1.  Fundamental paleotectonic units of Eurasia (modified from Liu YJ et al., 2017).

DownLoad: Full-Size Img PowerPoint

The tectonic evolutionary history of the CAOB is closely related to the micro-continental blocks or fragments, massifs, terranes, sea mountains, accretionary complex and volcanic arcs in the Paleo-Asian Ocean (PAO) (Kröner A et al., 2007; Liu YJ et al., 2017, 2021; Li JY et al., 2019; Li SZ et al., 2018; Xu WL et al., 2019; Dang ZC et al., 2022). To explore the nature or composition, and dynamic setting of these tectonic units are key points to constrain the tectonic mechanism of the CAOB. From 1250 Ma to 250 Ma, the entire CAOB has experienced considerable crustal growth dominated by the formation and amalgamation of these tectonic units in the PAO (Şengör AMC et al., 2018). Two tectonic evolutionary models of (1) oroclinal bending and slicing of a single arc (Sengör AMC et al., 1993, 2018) and (2) accretion and collision of multiple small island arcs, microcontinents, and terranes (Kröner A et al., 2007; Wakita K et al., 2013; Wilhem C et al., 2012; Windley BF et al., 2007; Yakubchuk A, 2004; Zonenshain LP et al., 1990) have been suggested to describe the tectonic evolution of the orogen. However, due to its complexity tectonic history, neither of these two models has been met the need of the tectonics reality of the CAOB, lots of questions still open debated now, and especially to the eastern segment of the CAOB, for the lateral Okhotsk and western Pacific tectonic regime influence and thick coving, series of basic scientific questions such as the basement nature of tectonic units within eastern PAO and the tectonic history of ocean basins between them are controversially, which seriously block the understanding of the entire CAOB. So, much work still needed.

The NE China composes the main body of the eastern CAOB (Figs. 1, 2), its Paleozoic tectonic framework was usually recognized as a typical archipelagic ocean, with several micro-blocks or continental fragments scattered in the eastern PAO, and then amalgamated together along various suture zone belts to form an entire block (Xu B et al., 2015; Liu YJ et al., 2017, 2019, 2021; Wu FY et al., 2002, 2011; Li JY et al, 2019; Xu WL et al., 2019). Usually, these micro-blocks were thought with Precambrian basement and lateral cover. However, plenty of recent works have proved that most previously reported Precambrian basement were Paleozoic or Early Mesozoic in age. Thus, they were actually lateral accretionary geological units (Wu FY et al., 2011; Liu YJ et al., 2019; Li JY et al, 2019). Additionally, the spatial distribution and formation time of the suture zones between the micro-blocks of eastern CAOB were still controversial by most peoples (Liu YJ et al., 2019; Li JY et al, 2019; Xu B et al., 2015). So, a more comprehensive work is necessary. Besides, plenty of new geological materials have been discovered and reported in recent years, which need timely update and renewal to improve our understanding of the region.

Figure  2.  Tectonic division of the NE China, showing the major blocks, sutures, faults and topographical units (modified from Liu YJ et al., 2017; Xiao WJ et al., 2015; Wu FY et al., 2011). Tectonic location shown in Fig. 1.

DownLoad: Full-Size Img PowerPoint

2.   Tectonic division of the eastern CAOB in NE China

The tectonic division of the eastern CAOB in NE China has been updated from time to time, and was different from person to person. However, none suggestion was accepted publicly. Wang HZ (1981) originally proposed the rough tectonic framework of NE China, dividing the region into Erguna fold belt, Xing’an fold belt, Songliao middle block, Khanka-Changbai fold belt and Nadanhada from west to east. In 1990s, the geosection program from Manzhouli to Suifenhe has provided foundmental profiles for establishing the basic tectonic framework of NE China. By which, researchers divided the region into Erguna-Xing’an, Songliao-Zhangguangcai and Jiamusi blocks from west to east (Ye M et al., 1994; Wu FY et al., 1995), and the blocks were separated by the Nenjiang and Mudanjiang faults, respectively.

After that, this model was developed by decades of scholars based on increasing evidences, and a more acceptable framework has been constructed, which divided the NE China region into Erguna Bolck (EB), Xing’an Accretionary Terrane (XAT), Songliao-Xilinhot Accretionary Terrane (SXAT) and Jiamusi Block (JB) from northwest to southeast, and they separated by Xinlin-Xiguitu Suture (XXS), Heihe-Hegenshan Suture (HHS) and Mudanjiang-Yilan Suture (MYS), respectively (Fig. 2; Xu B et al., 2015; Xiao WJ et al., 2003; Liu YJ et al., 2017, 2019, 2021; Wu FY et al., 2002, 2011; Li JY et al, 2019; Xu WL et al., 2019). Generally, the region was bounded by Mongolia-Okhotsk Suture (MOS) to the north by Siberia Craton and Solonker-Linxi Suture (SLS, Xiao WJ et al., 2003) and Changchun-Yanji Suture (CYS, Liu YJ et al., 2017) to the south by North China Craton (NCC; Fig. 2).

3.   Tectonic units of eastern CAOB in NE China

3.1   Erguna Block (EB)

The EB is located at northwestern part of the eastern CAOB, and extended to Mongolia to the southwest and Far East Russia to the northeast, bordered by the XXS zone to the south and MOS to the north (Figs. 1, 2). It’s an ancient micro-block with Precambrian basement and Paleozoic to Mesozoic cover. The Precambrian basement mainly consists of Paleo- to Neoproterozoic granitoids (700–1800 Ma) (Gou J et al., 2013; Tang J et al., 2013; Zhang L et al., 2013), Neoproterozoic meta-sediments ( Xinhuadukou, Ergunahe, and Jiageda groups) and complex (Mohe and Xinghuadukou complex) (IMBGMR, 1991; HBGMR, 1993). The Paleozoic to Mesozoic cover is dominated by granitic and associated volcanic rocks, and series of sedimentary sequence, some subordinated mediated to basic igneous and metamorphic rocks are included. Spatially, the Precambrian basements were mainly distributed at northern part of the EB (Mohe, Tahe and Xinlin regions), whereas the central and southern parts were seldom (Fig. 2; Wu G et al., 2012; Sun LX et al., 2012, 2013a, 2013b; Ge WC et al., 2015; IMBGMR, 1991; HBGMR, 1993). However, several Precambrian ages reported at central (western margin of Hailar basin; Li G et al., 2020) and southern EB suggested that it exist Precambrian basement there.

Traditionally, the Pricambrian basements of the EB were thought Paleo- to Mesoproterzoic products for their high degree metamorphism facies. However, recent geochronology studies show that they were acturally Neoproterozic in age (793 Ma, 843 Ma, and 817–729 Ma, Wu FY et al., 2011; 797–764 Ma, Zhao S et al., 2016a; 851–737 Ma, Tang J et al., 2013). These ages were similar with the basement of Siberian Craton (Han GQ et al., 2011; Wu FY et al., 2012; Ge WC et al., 2015), indicating the EB have Siberia affinity. Meanwhile, as the major component of Precambrian basement of the EB, the Xinghuadukou Complex is featured by a deep water sequence of mudstone, fine sandstone, and interbeded mediate to basic volcanic and carbonate layers (IMBGMR, 1991). The typical strata also developed at southern margin of Siberia Craton (Han GQ et al., 2011), confirmed the Siberia affinity of the EB.

3.2   Xing’an Accretionary Terrane (XAT)

The nature of the so called “Xing’an Block” has long been debated. Some researchers argued that it has union Precambrian metamorphic basement, represented by rock associations including Xinghuadukou, Xinkailing, Luomahu, Wolegen groups, and Egunahe and Jiageda Fms. (1∶250000 Geological Survey of Zhalantun area, Inner Mongolia; 1∶250000 Geological Survey of Arongqi Banner, Inner Mogolia; IMBGMR, 1991; HBGMR, 1993). Whereas others suggested that it was an accretionary terrane composed by series of accretionary complex, island and/or magmatic arcs, without Precambrian basement (Li JY, 2006; Wu FY et al., 2011, Sun LX et al., 2014; Feng ZQ et al., 2018a, 2018b), usually named as Xing’an arc (Su YZ, 1996; Li YC et al., 2013), south Mongolia-Great Xing’an Range orogenic belt (Li JY, 2006) or Xing’an Accretionary Terrane (Wu FY et al., 2011) in different publications.

Previously, the former opinion was widely accepted for the lack of isotopic ages. However, as the development of geochronology, especially the widely used zircon U-Pb method, the previously recognized “Precambrian rocks” were dated to Paleozoic to early Mesozoic in age (Liu YJ et al., 2019, 2021). For example, Na FC et al. (2018) reported 520 Ma zircon U-Pb age of Chlorite Muscovite schist at Zhalantun area from Xinghuadukou Group. Sun W et al. (2014) got 308–485 Ma crystal ages from Xinkailing Group at Nenjiang and Duobaoshan regions, and got 410–420 Ma maximum depositional ages of Luomahu Group at Galashan and Taerqi regions. Qin T et al. (2019) got 505–447 Ma ages of ryholite and gabbro from Jiageda Formation at Zhalantun area. These facts denied the existence of Precambrian basement of “Xing’an Block”.

Liu YJ et al. (2019) summarized the composition of the “Xing’an Block”, and found that it was an accretionary terrane mainly consists of Paleozoic accretionary complex, magmatic arcs and ridge-arc-basin systems (Liu YJ et al., 2019; Zhang Y et al., 2018), named as Xing’an Accretionary Terrane (XAT, Liu YJ et al., 2019, 2021). The formation of which dominated by the northward subduction of Nenjiang oceanic plate beneath the EB. Retreat subduction of the oceanic plate produced series of progressively southward younger accretionary complexes or magmatic belts such as Xinlin-Yakeshi accretionary belt (700–480 Ma), Nenjiang-Dashizhai (502–420 Ma) and Yakeshi-Zhalantun magmatic belts (390–330 Ma), Yakeshi back-arc basin (420–330 Ma), and Moguqi-Dashizhai-Xilinhot post-collisional alkaline magmatic belt (292–260 Ma). They contribute significantly to the growth of the terrane (Liu YJ et al., 2019; Ma YF et al., 2019b).

3.3   Songliao-Xilinhot Accretionary Terrane (SXAT)

The vast region, including Songliao basin and Xilinhot areas banded by HHS to the north, MYS to the east, and SLS and CYS to the south, was usually treated as Songnen (or Songliao-Xilinhot) Block (Liu YJ et al., 2017; Wu XW et al., 2018). High grade metamorphic rock associations such as Zhangguangcailing, Dongfengshan, Yimianpo and Fengshuigou groups in and around the region were taken as the Precambrian basement of the block previously (Zhang MS et al., 1998). However, recent geochronological studies show they were Paleozoic to Mesozoic assembles (Wang F et al., 2012), rather than Precambrian basements.

However, at Xilinhot and eastern and western margin of Songliao Basin, there reported Precambrian ages, which were seen as Precambrian basements of the block (Pei FP et al., 2006; Zhang C et al., 2018; Wu XW et al., 2018; Xu B et al., 2015). Liu YJ et al. (2021) carried out a systematically study of these Precambrian ages, found that they were concentrated at Xilinhot ( Sun LX et al., 2013a, 2018, 2020), Longjiang (Zhang C et al., 2018; Wu XW et al., 2018) and Yichun ( Gao FH et al., 2013; Luan JP et al., 2017; Wu CL et al., 2010) areas (Fig. 3). Li JY et al. (2019) argued that these local Precambrian ages can only represent old fragments there, rather than union basement of the entire block. Wu FY et al. (2000) collected basement samples of Songliao basin form more than 200 oil drill cores, and found that they were Paleozoic and Mesozoic in age, at odd with widespread Precambrian basement of the region. Thus, there only exist ancient micro-continental fragments at three above regions, named as Xilinhot, Longjiang and Yichun mini-blocks, respectively (Fig. 3; Liu YJ et al., 2021). Other places probably have Precambrian rocks, but very rare (Pei FP et al., 2006; Wang Y et al., 2006; Gao FH et al., 2016). Therefore, in fact, the previously “Songnen Block” was a Paleozoic to Mesozoic accretionary terrane, named as Songliao-Xilinhot Accretionary Terrane (SXAT; Liu YJ et al., 2021).

Figure  3.  Precambrian basements distribution of the eastern CAOB in NE China, showing the location of Xilinhot, Longjiang and Yichun mini-blocks in SXAT (modified from Liu YJ et al., 2021). Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

For Xilinhot mini-block, Precambrian rocks were mainly Mesoproterzoic in age (1516–1390 Ma, 1373–1399 Ma and 1377–1385 Ma, Sun LX et al., 2018a, 2020; 1045 Ma and 1248 Ma, Xu B et al., 1996; Hao X and Xu B, 1997), whereas the Paleo- to Neoproterzoic ages were seldom (739 Ma, Zhou WX and Ge MC, 2013; ca. 2500, Pan SK et al., 2015). For Longjiang mini-block, Precambrian rocks were with plenty of Paleoproterzoic (1808–1879 Ma, Zhang C et al., 2018; 1964 Ma, Cheng ZX et al., 2018) and few Neoarchean ages (2579 Ma, Qian C et al., 2018; 2699 Ma, Wu XW et al., 2018). For Yichun mini-block, the Precambrian components were Neoproterzoic in age (757 Ma, Quan JY et al., 2013; 747–561 Ma and 805–561 Ma, Gao FH et al., 2016; 929–871 Ma, Luan JP et al., 2019; and 821 Ma, 752 Ma and 551 Ma, Wang Y et al., 2014) except minor Paleoproterzoic ones (1821 Ma, Wu CL et al., 2010; 2336 Ma, Cao X et al., 1992). Generally, the Precambrian ages of the Xilinhot and Longjiang mini-blocks were Paleo- and Mesoproterzoic (1.4 Ga and 1.8 Ga, respectively), which were commonly documented in neighboring NCC (Zhao GC and Cawood PA, 2012). This similarity indicates the NCC-affinity of the two mini-blocks. The truth was further strengthened by the facts that the two mini-blocks shared similar Hf and Sr-Nd isotopic signatures with NCC (Wu FY et al., 2000, 2011; Zhang C et al., 2018). Whereas the Precambrian ages of Yichun mini-block were Neoproterzoic, similar with basement ages of the JB, indicate its JB-affinity, which also can be confirmed by simultaneously metamorphic events occurred at both units.

3.4   Jiamusi Block (JB)

The JB is located at the easternmost of the NE China, adjacent to SXAB to the west by MYS, and Nadanhada Accretionary Terrane (NT) to the east by Yuejinshan Fault, and connected with Bureya block to the north and Khanka block to the south, respectively (Fig.2; Liu YJ et al., 2017, 2021).

Traditionally, the Heilongjiang and Mashan complexes were taken as Precambrian basement of the JB (Guo XZ et al., 2014; Fig. 4). Previously studies determine the Heilongjiang Complex as a Meso- to Neoproterzoic metasedimentary sequence (Dang YS and Li DR, 1993). However, recent researches show it was a suit of ophiolite mélange, consists of blue schist, green schist, mica schist, felsic mylonite, ultrabasic rock, metamorphic siliceous rocks and marble (Zhang XZ and Sklyarov EV, 1992; Ye HW et al., 1994; Wu FY et al., 2007). The formation and deformation of which were related to subduction and subsequent exhumation process between the SXAT and JB (Li WM et al., 2020). Therefore, the Heilongjiang Complex was the oceanic remnants mark the suture zone locations of SXAT and JB, rather than basement of the JB. The formation time of the complex was suggested to Neoproterozoic to Ordovician (Liou JG et al., 1989; Cao et al., 1992; Zhang XZ, 1992), Late-Paleozoic (Li XP et al., 2009), and Jurassic (Wu FY et al., 2007; Li WM et al., 2009, 2020) in different publications. Recent detrital zircon studies limited the maximum depositional ages of the complex to end-Permian to Early Triassic, with age peak at ca. 250 Ma (Zhou JB et al., 2009; Zhao LL et al., 2011; Li WM et al., 2011). The oceanic crust remnants ( pillow basalt and metamorphic gabbro) of the Heilongjiang Complex yield crystal ages of 250–220 Ma (Lv CL et al., 2016), indicating the formation of the complex probably occurred at Early to Middle Triassic, or even later.

Figure  4.  Geological sketch map of the Jiamusi Block and its surrounding areas (after Wu FY et al., 2007). Tectonic location shown in Fig. 2. Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

The Mashan Complex is a high degree metamorphic rock association (granulite face) mainly composed by a set of khondalitic rocks, including granitic rocks, felsic granulite, marble and graphitic schist (Li WM et al., 2020), and widely distributed at Liumao, Mashan, Luobei, Huanan and Hutou areas of the JB (Fig. 4). The ages of the complex was dominated by Neoproterzoic (755–898 Ma, Yang H et al., 2017, 2018), and with few Cambrian ones (530–510 Ma, Yang H et al., 2014), and thus can be taken as Precambrian basement of the JB (Li WM et al., 2020; Yang H et al., 2017, 2018).

Unlike the basement of EB, which characterized by low metamorphism and deep-water sequence, the basement of JB was a suit of khondalitic rocks with high metamorphism and shallow marine signatures (Li WM et al., 2020), similar associations were discovered at Antarctic, Western Australia, India and Sri Lanka of super-Gondwana (Wilde SA et al., 1997, 1999, 2000), indicating the JB have Gondwana affinity. The opinion was further confirmed by Pan-Africa metamorphic events which traced both in Mashan Complex of JB and continents of Gondwana system (Li WM et al., 2020).

North of JB, there was Bureya Block in southern Far East Russia. It has similar Neoproterzoic basement with JB (Sorokina AA et al., 2016; Ovchinnikov RO et al., 2018), and experienced coeval Pan-Africa metamorphic events (Ovchinnikov RO., 2019), suggesting they were union micro-block named Jiamusi-Bureya Block (JBB). Plate reconstruction shows that the JBB was connected with the South China Block (SCB) in the Early Paleozoic (Li SZ et al., 2018), which can be insighted form similar fossils and metamorphic events at both blocks. Li SZ et al. (2018) proposed a Greater SCB concept to describe the united JBB and SCB. This Greater SCB located at NE Gondwana in the early Paleozoic period, during the breakup of and split of Gondwana super continent in late Early Paleozoic to Late Paleozoic, the JBB escaped northward to the PAO system from the super Gondwana (Li SZ et al., 2018).

3.5   A short summary of the tectonic units of eastern CAOB in NE China

Summarily, there are four major tectonic units of eastern CAOB in NE China, EB, XAT, SXAT and JB. Each unit has unique composition and with different affinities, and formed through special tectonic evolutionary history. The EB and JB were having Precambrian basements but with different affinities, the former was Siberia-derived while the later was Gondwana-derived. The XAT and SXAT were Paleozoic accrationary terranes without union Precambrian basements. They formed during the oceanic plate subduction and closure of branch ocean basins of the eastern PAO, and mainly composed by progressively accreted magmatic arcs, ophiolite mélanges, subduction complex and prisms between these oceans, and usually compose trench-arc-basin system (Zhang Y et al., 2018; Liu B et al., 2021).

However, there were still Precambrian rocks in the accrationary terranes (Xu B et al., 2015; Xu WL et al., 2019). They were fragments of neighboring Cratons, which splitted and drifted away during the breakup of Supercontinent and opening of PAO, then involved into younger accreted terrane. So, these Precambrian fragments cannot be treated as basement of the blocks, but elements of accretionary complex. Similar situations were commonly seen in other cases worldwide (the Iceland Microcontinent of NE Atlantic Realm; Foulger GR et al., 2020).

4.   Tectonic evolution of the branch ocean basins of eastern PAO

4.1   Opening of the PAO

The opening of an ocean is usually related to the breakup of supercontinent under extension or rift setting (Hoffman PF, 1991; Evans DAD and Mitchell RN, 2011). Precambrian paleogeography research shows that the opening of PAO is dominated by the relatively position of Siberia and Laurentia cratons (Zhao GC et al., 2018). Paleomagnetic data indicate that the two cratons were probably in a fixed position to each other since ca. 1500 Ma until at least ca. 700 Ma with southern Siberia facing northern Laurentia (Wingate MTD et al., 2009). Till Late Neoproterzoic, the split of the two cratons as the breakup of Rodinia (750–600 Ma) (Hoffman PF, 1999; Cawood PA et al., 2007; Li ZX et al., 2008 and references therein) has led to the opening of PAO (Zhao GC et al., 2018). The primitively PAO was surrounded by Siberia and eastern Europe cratons to the north and west, and several East Asian blocks (North China, Alex and Tarim) to the south (Fig. 1; Xiao WJ et al., 2003, 2015; Zhao GC et al., 2018), and a lot of craton or block derived fragments within. As the surrounding cratons and blocks were drifted away with rapid expansion of PAO, these fragments were left in the ocean and forced to different movement paths and rotation manner during lateral tectonic process, shaping the PAO as a typical archipelagic ocean composed of numerous subordinate branch ocean basins and geological units between them (Fig. 5). More importantly, they (1) greatly initiated and facilitated the oceanic-oceanic or oceanic-continental subduction for the PAO system, and (2) provided as originally nuclei for the crustal growth and accretion of the CAOB.

Figure  5.  Simplified geographic map show the branch ocean basins of the eastern PAO. By today’s latitude and longitude. Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

Tectonically, the eastern PAO lies between the Siberia Craton to the north and NCC to the south, composed of Xinlin-Xiguitu, Heilongjiang (reopened Mudanjiang), Nenjiang, and Solonker branch ocean basins from north to south (Fig. 5; Liu YJ et al., 2017).

4.2   Xinlin-Xiguitu Ocean

The precise opening time of the Xinlin-Xiguitu Ocean is difficult to determine for the lack of geological records. However, the ophiolite fragments including Gabbros in Jifeng area (647 ± 5 Ma), pyroxenite (628 ± 10 Ma) and diabase (668 ± 10 Ma) in Gaxian area, and Gabbros (697 ± 5 Ma) in Huanerku area along the XXS indicate the formation of Xinlin-Xiguitu Ocean before Neoproterozoic (Liu YJ et al., 2019; Miao LC et al., 2015). These rocks have depleted mantle magmatic source and were spreading ocean ridge products (Liu YJ et al., 2019), indicating that the ocean was at early expanding stage. To the middle Cambrian, the basalts (511 ± 5 Ma) in the Toudaoqiao area have ocean island signature, implying the occurring of oceanic-oceanic subduction (Miao LC et al., 2015). Thus, the ocean has matured and probably entered shrink stage (Feng ZQ et al., 2016; 2018a). In sedimentary aspect, the Neoproterzoic Xinghuadukou Complex in eastern EB characterized by deep water face, whereas the Upper Paleozoic Suzhong and Lower Paleozoic Niqiuhe formations in the region were featured by shallow marine facies (IMBGMR, 1991; HBGMR, 1993), reflecting the retreat of the ocean at early and late Paleozoic.

The Late Cambrian post-orogenic A-type granites of 494–480 Ma in Tahe area (Ge WC et al., 2005) indicate the closure of the Xinlin-Xiguitu Ocean has finished before. Besides, the Toudaoqiao blueschists, which is a subduction/collision-related high-pressure series formed at 510 Ma, has suffered peak metamorphisim at 490 Ma, the Xinghuadukou Complex at Hanjiayuanzi area has experienced high-grade metamorphism at ca. 500 Ma (Liu YJ et al., 2017, 2019; Feng ZQ et al., 2016; 2018a, 2018b). These metamorphic events were related to the collision and amalgamation of EB and XAT, and thus indicate the closure of Xinlin-Xiguitu Ocean happened at ca. 500 Ma (Liu YJ et al., 2017). Meanwhile, the provenance study of late Paleozoic strata (Hongshuiquan and Niqiuhe formations) in northern Great Xing’an Range area were with bi-directional signature, supporting the amalgamation of EB and XAT had finished before that time (Liu YJ et al., 2017).

The closure location of Xinlin-Xiguitu Ocean was marked by suture zone between the EB and XAT. Plenty of ophiolite suits and/or fragments outcropped at Xinlin (Li RS, 1991), Tayuan (Zhong H and Fu JY, 2006; Wu FY et al., 2011), Jifeng (Hu DG et al., 2001; Feng ZQ et al., 2016), and Gaxian (She HQ et al., 2012) regions marked the spatial position of the suture, together with subduction related high-pressure blueschists at Toudaoqiao area (Miao LC et al., 2015), a NE trend tectonic belt goes along the Toudaoqiao, via the Jifeng, Xinlin, to the Huma was constructed, under the name of Xinlin-Xiguitu Suture (XXS; Fig. 2).

4.3   Nenjiang Ocean

The Nenjiang Ocean was a middle branch ocean basin of the eastern PAO between XAT and SXAT (Fig. 5; Liu YJ et al., 2017; Ma YF, 2020a; Feng ZQ et al., 2018b). Several other names such as Hegenshan ( Xiao WJ et al., 2015; Zhang DH et al., 2018b), Hegenshan-Heihe (Ma YF et al., 2019a), and Heihe-Nenjiang oceans (Zhang JM et al., 2020) were used in different publications. The existence of the ocean was evidenced by the ophiolite mélanges (Jian P et al., 2012; Miao LC et al., 2008) and/or ophiolite fragments (Liu YJ et al., 2019), and Ocean Plate Stratigraphy (OPS) (Liu YJ et al., 2010; Feng ZQ et al., 2018b) outcropped at Hegenshan, Dashizhai, Moguqi and Nenjiang areas (Fig. 6).

Figure  6.  Schematic geological map of Great Xing’an Range area, showing the main tectonic units and the distributions of two subduction related Palaeozoic magmatic arc belts (480–420 Ma and 360–330 Ma). Modified from Liu YJ et al., 2021. Tectonic location shown in Fig. 2. Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

The primitive stage of the ocean was still remains poorly understood for the limited geological proofs. The Neoproterzoic to Early Cambrian (620–510 Ma) basalts of Toudaoqiao island arc was the earliest records of the northward subduction of the Nenjiang oceanic plate (Liu YJ et al., 2019), indicating the existence of the ocean. Since then, the ocean has greatly enlarged via long-term southward retreat subduction process from early to early Late Paleozoic (Liu YJ et al., 2019). Widespread middle and late Early Paleozoic OPS, including the Ordovician slates, marble, and limestone sequences in Woduhe area, the Silurian slate, siltstone, and greywacke sequences of the in Heihe area, and the slate and mudstone sequences associated with andesites in Chaihe area (Feng ZQ et al., 2018b), were formed in central and northern Great Xing’an Range areas, indicating the mature of the ocean (Feng ZQ et al., 2018b and references therein; Liu YJ et al., 2019).

In Late Paleozoic, the Nenjiang Ocean entered retreat stage as the continuous convergence of XAT and SXAT at each side, and ultimately closured at early Late Carboniferous (ca. 320 Ma) along HHS (Liu YJ et al., 2017, 2019; Ma YF et al., 2019a, 2020a, 2020b). The closure of the ocean marked by the collision event of EB and XAT, and can be detected from both sedimentation and magmatism aspects. In sedimentation perspective, the central Great Xing’an Range area was dominated by marine facies strata (Lower Devonian Niuqiuhe and Daminshan formations) before Late Carboniferous. While it transferred to terrestrial face in Late Carboniferous and Early Permian ( marked by Baoligaomiao and Gegenaobao formations). This obviously paleogeographical transition limited the closure time of the Nenjiang Ocean before Late Carboniferous (Liu YJ et al., 2010; Zheng CQ et al., 2013b). To the magmatism view, the syn-collisional S-type granite of 320 Ma in middle part of the HHS implies the occurring of XAT and SXAT collision happed at early Late Carboniferous (Ma YF et al., 2019a). Meanwhile, the subduction related arc magmatism along HHS were occurred in Early Carboniferous, whereas the Late Carboniferous and Permian ones were post collisional A-type and bimodal magmatism (Ma YF et al., 2019a, 2019b, 2020a, 2020b).

The closure mechanism of the Nenjiang Ocean has seldom concerned. Several studies favor a single NW subduction model ( Liu YJ et al., 2017; Li Y et al., 2014), related proofs include the Early (480–420 Ma) and Late（360–330 Ma） Paleozoic magmatic arcs (Fig. 6; Liu YJ et al., 2017) and the ridge-arc-basin system in the XAT (Zhang Y et al., 2018). Recently, a Late Paleozoic magmatic arc along western margin of the SXAT extended from Wulanhot, via Jalaid Banner, to Longjiang areas has been discovered (Fig. 6; Ma YF et al., 2020a), indicating the eastward subduction of the oceanic plate beneath SXAT. Therefore, this study recommends a bi-directional subduction model of sea floor subduction of the Nenjiang Ocean ( Liu YJ et al., 2019; Ma YF., 2020c).

The closure position of the Nenjiang Ocean was marked by the HHS between XAT and SXAT. Spatially, the suture stretch from Erenhot, via Hegenshan, Dashixhai, and Moguqi, to Nenjiang and Heihe areas (Liu YJ et al., 2017), traced by the ophiolites in Erenhot and Hegenshan areas (Liu YJ et al., 2019; Song SG et al., 2015; Jian P et al., 2010, 2012; Miao LC et al., 2008), tectonic mélanges and OPS in Dashizhai area (Feng ZQ et al., 2018a), Sodic amphibole-bearing mylonites in Nenjiang area, gabbros in Moguqi area (Feng ZQ et al., 2018c), and A-type granites in Heihe area (Fig. 6; Wu FY et al., 2002).

4.4   Heilongjiang and Mudanjiang oceans

The Heilongjiang Ocean is an ancient ocean basin between SXAT and JB. The formation of which can be traced back to the rift of JB from NE Gondwana Super Continent and drift to the PAO system at ca. 500 Ma (Liu YJ et al., 2021). Much development information of the ocean could get from the Heilongjiang Complex, a suit of ophiolite mélange formed at Heilongjiang oceanic circle in early Paleozoic and seriously affected and reinvented by Mudanjiang oceanic circle in latest Paleozoic to Mesozoic, and suffered various degree metamorphism at both circles (Li WM et al., 2020). The discovery of Ordovician Chitinozoans from chert of Heilongjiang Complex in Mudanjiang area suggested the existence of the ocean, which can be confirmed by the Early Paleozoic ophiolite suits and fragments identified from the Heilongjiang Complex from Jiayin-Mudanjiang ophiolite belt (Wu FY et al., 2007). Meanwhile, the Early Paleozoic adakite rocks (510–482 Ma) at Tadong and Shuguang areas indicate the subducting of the Heiongjiang oceanic plate (Liu YJ et al., 2019).

The closure of the Heilongjiang Ocean occurred progressively from north to south in the Early Paleozoic, related proofs can be obtained from both sedimentation and magmatism aspects (Liu YJ et al., 2017). In sedimentation aspect, the Devonian Heilonggong and Baoquan formations with maximum depositional ages of 386Ma and 403 Ma, respectively, at eastern SXAT were have detrital zircons derived from basement of JB, indicating that they amalgamated together prior to Early Devonian (Liu YJ et al., 2010; Meng E et al., 2010, 2011; Xu WL et al., 2012). In magmatism aspect, plenty of Ordovician to Silurian syn-and post-orogenic granitic rocks (485–425 Ma) had been widely reported along Mudanjiang fault from north to south (Liu JF et al., 2008; Xu WL et al., 2012), implying that the closure of the ocean and collision of the units at each side had finished at least before Silurian (ca. 420 Ma; Liu YJ et al., 2021). This opinion was further confirmed by the fact that the SXAT and JB were experienced a long quiet period (ca. 90 Ma) after 420 Ma, during which similar Devonian strata (Heilonggong, Baoquan and Heitai formations) were accumulated at both units.

Worth to notice, there were suggestions argued that the ancient ocean between SXAT and JB was Mudanjiang Ocean, which lasted to Early Mesozoic before its final close (Wu FY et al., 2007, 2011; Zhou JB et al., 2009). The most reliable proofs were Late Permian-Early Triassic OIB and E-MORB-affinity basaltic of the Heilongjiang Complex (Zhou JB et al., 2009). However, these rocks were formed in rift tectonic setting related to the split of an ocean. So, the Mudanjiang Ocean should be a reopened ocean basin after the closure of Heilongjiang Ocean in Early Paleozoic (Liu YJ et al., 2017, 2019). Li SZ et al. (2018) proposed that the JB was moved from great South China Bolck (SCB), and the Mudanjiang Ocean was the NE extension of the Shangdan or Mianlue oceans, belongs to Paleo-Tethys Ocean system. Therefore, further research is needed for the evolution of Heilongjiang and Mudanjiang oceans.

Closure of the Mudanjiang Ocean caused a remarkable metamorphic event which was clearly recorded in the Heilongjiang Complex (Zhao LL and Zhang XZ, 2011). Li WM et al. (2009, 2011) selected newly formed metamorphic minerals of silica muscovite and calcareous amphibole from Heilongjiang Complex, through ⁴⁰Ar/³⁹Ar method to obtain its ages of 145–185 Ma (Li WM et al., 2009, 2011), indicating the ocean closed at Jurassic.

The closure location of Heilongjiang and Mudanjiang oceans can be traced by suture zone between SXAT and JB. This suture was marked by the Heilongjiang complex outcropped at Luobei, Yilan and Mudanjiang regions from north to south, spatially, composing an N-S trending suture zone under the named of Mudanjiang-Yilan Suture (MYS) (Liu YJ et al., 2017, 2019; Li WM et al., 2020).

4.5   Solonker Ocean

The Solonker Ocean was a Paleozoic to early Mesozoic ocean basin between the united eastern CAOB and NCC, usually taken as the main ocean basin of the eastern PAO (Liu YJ et al., 2017, 2019; Xiao WJ et al., 2015; Xu B et al., 2014, 2015; Wilde SA, 2015). It formed probably as early as the breakup of Rodinia Supper Continent and split of PAO in Neoproterzoic (ca. 750 Ma, Zhao GC et al., 2018). Till now, most studies were focused on the closure time and location of the ocean (Liu YJ et al., 2017, 2019; Xiao et al., 2015; Xu B et al., 2014, 2015; Li JY et al., 2019; Wilde SA, 2015). However, as the thick cover of the Songliao Basin, it can only get information from western (western Solonker Ocean) and eastern (eastern Solonker Ocean) parts, respectively.

To the western Solonker Ocean, the Sonid Zuoqi magmatic arc in the north and Bainaimiao arc in the south indicated the bi-directional subduction of the oceanic plate has been started at least from Late Cambrian (Liu YJ et al., 2019), and lasted to Ordovian and Silurian marked by ophiolite in Erdaojing-Honger (ca. 483 Ma) and Ondor Sum (467–429 Ma) areas (Liu DY et al., 2003). Tectonic study shows that the bi-directional subduction developed under extensional setting (Xu B et al., 2015), indicating the expansion and growth of the Solonker Ocean in early Paleozoic.

In Late Paleozoic, the bi-directional subduction was continued, but transferred into convergent tectonic setting (Xu B et al., 2015), indicating the retreat of the ocean. Xiao WJ et al. (2003) carried out detail study of the Ulan subduction-accretion complex and suggested that the Solonker Ocean closed at end Permian-Early Triassic. This opinion was supported by ophiolite studies at Solonker (279 Ma, Miao LC et al., 2008), Mandula (274–253 Ma, Jian P et al., 2010; Chen C et al., 2012), Ondor Sum (250–260 Ma, Miao LC et al., 2008; Chu H et al., 2013), Wudaoshimen (277 Ma, Wang YY et al., 2014), Xingshuwa (280 Ma, Song SG et al., 2015), Banlashan (256 Ma, Miao LC et al., 2008), and Jiujingzi (275 Ma, Liu JF et al., 2016) areas of SE Inner Mongolia (Fig. 7). In sedimentation aspect, the Middle Permian Zhesi and Yujiabeigou formations in SE Inner Mongolia were marine facies strata. Whereas the Upper Permian Linxi Formation was terrestrial sedimentation (Zhang YJ et al., 2019). This sharply transition indicates the significant retreat of the west Solonker Ocean from Middle to Late Permian. The typical red layers of Lower Triassic Laolongtou Formation marked the final closure of the western Solonker Ocean at Early Triassic (Zhang YJ et al., 2019). Detrital zircon provenance analysis (Han GQ et al., 2012) also obtain similar conclusion.

Figure  7.  Simplified tectonic map of the SE Inner Mongolia, showing its structures and tectonic belts. Tectonic elements modified from Xiao WJ et al., 2015. Tectonic location shown in Fig. 2. XAT–Xing’an Accretionary Terrane; HHS–Hegenshan-Heihe Suture; SXAT–Songliao-Xilinhot Accretionary Terrane; SLS–Solonker-Linxi Suture; OSC–Ondor Sum Complex; BA–Bainaimiao Arc; NCC–North China Craton.

DownLoad: Full-Size Img PowerPoint

The closure location of the western Solonker Ocean marked by accretionary complex (Xiao WJ et al., 2003, 2015) and ophiolite suits (Jian P et al., 2010; Miao LC et al., 2007) alongside Solonker-Linxi area, named as Solonker-Linxi Suture (SLS), which divided Ondor Sum Complex (OSC) and Bainaimiao Arc (BA) to the south and SXAT to the north (Fig. 7).

To the eastern Solonker Ocean, the Late Ordovician (443 Ma) subduction-related magmatisms at southern SXAT (Pei FP et al., 2014) and north margin of NCC indicate the bi-directional subduction and expansion of the ocean. This process lasted to end-Early Paleozoic as the regional tectonic setting transferred from extension to convergence (Xu B et al., 2015). The ophiolite belts of Shitoukoumen-Kaishantun (277–260 Ma), Jifanggou-Shuiquliu (ca. 290 Ma) and Xiaosuihe-Xinhuacun (250–208 Ma) suggest that the closure of eastern Solonker Ocean happened at Early Permian to Middle and Late Triassic (Liu YJ et al., 2019). This opinion was further confirmed by the Early to Middle Triassic metamorphic events of ca. 230 Ma (Zhou JB et al., 2013) and ca. 250 Ma (Wu FY et al., 2007) led by the collision between eastern CAOB and NCC. Besides, plenty of Early to Middle Triassic syn-collisional magmatic rocks (248–216 Ma granites, Sun DY et al., 2004, 2005; 242–243 Ma monzogranites, Xin YL et al., 2011; and 245–248 Ma porphyry monzogranites, Zhang YB et al., 2004) in the region were indicators of the vanish of eastern Solonker Ocean. In the field, the Permian Molasse was found overlay unconformity on the Early/Middle Triassic strata in Shuangyang, Baishuitan and Shiren basins (Xin YL et al., 2011), implying the orogenic events in these regions at Early/Middle Triassic. Based on above facts, the authors argue that the eastern Solonker Ocean closed at Late Permian to Middle Triassic.

The original suture zone of the eastern Solonker Ocean was strongly destroyed by Mesozoic northward thrusting (Tang KD et al., 2004) and strike-slipping movement (Sun XM et al., 2008). However, there still some subduction-accretion complexes at Changchun-Panshi-Dunhua and Yanji areas to trace the spatial final closure location of the ocean (Liu YJ et al., 2017). Here we name it as Changchun-Yanji Suture (CYS). Combined with SLS in the west, we constructed Solonker-Linxi-Changchun-Yanji Suture (SLCYS) as the amalgamation and collision suture of eastern CAOB and NCC.

The eastward scissor closure mechanism of the Solonker Ocean has been widely accepted, proofs of which can be insight from following aspects. First, ophiolite suits alongside SLCYS were become younger eastward (Liu YJ et al., 2019). Second, the final closure time of the western Solonker Ocean (Early Triassic) was slightly earlier than the eastern counterpart (Middle Triassic) (Xiao WJ et al., 2003, 2015; Wilde SA, 2015). Third, paleomagnetic data provided that the NCC (anti-clockwise) and united eastern CAOB (clockwise) had obviously face to face rotation during Late Permian to Early/Middle Triassic (Zhang DH et al., 2018a; Ren Q et al., 2020; Zhao P et al., 2020), which led to the eastward scissor closure of the Solonker Ocean.

4.6   A simplified tectonic evolution of the eastern PAO

On the basis of the above analysis, here we reconstruct the tectonic evolution of branch ocean basins of the eastern PAO. Given their locations, we present the evolutions of the western and eastern parts of the eastern CAOB separately (section position shown by the white dotted line in Fig. 2). The western part includes the Xinlin-Xiguitu, Nenjiang, and western Solonker oceans, and the eastern part includes the Heilongjiang (and reopened Mudanjiang) and eastern Solonker oceans.

4.6.1 Western part of the eastern CAOB

(1) Neoproterozoic (700–620 Ma): This was the early rifting stage of the eastern PAO. The Xinlin-Xiguitu, Nenjiang, and western Solonker oceans were located from north to south between the EB, Pre-XAT (represented by the Toudaoqiao arc), Pre-SXAT (represented by the Xilinhot and Longjiang mini-blocks), and NCC. The Jifeng-Gaxian-Huanerku ophiolite suites and Toudaoqiao blueschist indicate northward subduction of the Xinlin-Xiguitu and Nenjiang oceanic plates, respectively (Fig. 8a).

Figure  8.  Tectonic model of the Xinlin-Xiguitu, Nenjiang, and western Solonker oceans of the eastern PAO. Section position shown in Fig. 2. Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

(2) Neoproterozoic-Early Ordovician (620–480 Ma): Northward subduction of the Xilin-Xiguitu oceanic plate finally closed the ocean at ca. 500 Ma, which led to the collision of the EB and XAT along the XXS. Northward subduction of the Nenjiang oceanic plate produced substantial ophiolite mélanges and magmatic arcs (the Duobaoshan-Aershan Ophiolite belt and Duobaoshan-Yiershi magmatic arc), with this continental growth forming the main body of the XAT. The Early Paleozoic Sonid Zuoqi Arc in the southern SXAT and the Bainaimiao Arc in the northern NCC represent the start of bi-directional subduction within the western Solonker Ocean (Fig. 8b).

(3) Early Ordovician-late Silurian (480–420 Ma): During this stage, the Nejiang Ocean experienced a period of quiescence, with only minor magmatic activity. In contrast, bi-directional subduction continued within the Solonker Ocean, forming a series of ophiolite suites on each side (the Erdaojing-Honger ophiolite in the north and the Ondor Sum ophiolite in the south), causing accretion of the Ondor Sum Complex and enlargement of the Bainaimaio Arc (Fig. 8c). These processes led to substantial continental growth of the eastern CAOB.

(4) Early Devonian-early Carboniferous (420–350 Ma): Southward subduction was initiated in the Nenjiang Ocean, marked by the development of an early Carboniferous arc in the northern margin of the SXAT. Bi-directional subduction within the western Solonker Ocean was still active but transitioned to a retreat stage (Fig. 8d).

(5) Early-early late Carboniferous (350–320 Ma): During bi-directional subduction within the Nejiang Ocean, the XAT and SXAT became greatly enlarged through continuing subduction-related accretion. These accretionary units finally amalgamated, causing the closure of the Nenjiang Ocean during the early Late Carboniferous (ca. 320 Ma) along the HHS. Meanwhile, subduction and retreat within the western Solonker Ocean continued (Fig. 8e).

(6) Late Carboniferous-late Permian (320–260 Ma): The western Solonker Ocean retreated substantially, and most of the eastern CAOB region transitioned to a terrestrial environment, leaving only a relict ocean basin between the amalgamated eastern CAOB (containing the EB, XAT, and SXAT) and the NCC. Meanwhile, abundant volcanic rocks were emplaced (the Dashizhai Fm.), and sedimentary strata were deposited (the Zhesi Fm.; Fig. 8f).

(7) Late Permian-Middle Triassic (260–240 Ma): After prolonged bi-directional subduction and accretion, the western Solonker Ocean finally closed during the late Permian-Middle Triassic along the Solonker-Linxi Suture (SLS), leading to the collision and amalgamation of the NCC and united eastern CAOB (Fig. 8g).

4.6.2 Eastern part of the eastern CAOB

(1) Neoproterozoic (700–620 Ma): As the JB drifted northward from NE Gondwana toward the PAO system, two branch ocean basins formed in the eastern part of the eastern CAOB, namely, the Heilongjiang Ocean in the north and the eastern Solonker Ocean in the south. These two ocean basins were bounded or divided by the JB, Yichun micro-block (pre-SXAT), and NCC from north to south, respectively (Fig. 9a).

Figure  9.  Tectonic model of the Heilongjiang and eastern Solonker oceans of the eastern PAO. Section position shown in Fig. 2. Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

(2) Neoproterozoic-Early Ordovician (620–480 Ma): Within the Heilongjiang Ocean, southward subduction of the Heilongjian oceanic plate was initiated beneath the SXAT, generating adakitic magmatism in the eastern Tadong and Shuguang areas. Southward subduction also commenced in the eastern Solonker Ocean, producing Early Paleozoic magmatism in the northern margin of the NCC that served as primitive materials of the Bainaimiao Arc (Fig. 9b).

(3) Early Ordovician-Late Silurian (480–420 Ma): With continuing collision and amalgamation of the SXAT and JB along the MYS, the Heilongjiang Ocean was finally closed at the end of the Silurian (ca. 420 Ma). South of the ocean, Late Ordovician magmatism in the southern SXAT marked the occurrence of northward subduction within the eastern Solonker Ocean, and the generation of a subduction complex in the northern NCC suggests that southward subduction within the ocean continued, implying a bi-directional subduction regime (Fig. 9c).

(4) Late Silurian-Middle Permian (420–260 Ma): Subduction retreat and roll-back of the Panthalassa Plate caused back-arc extension along the western margin of the JB, leading to rifting of the former MYS to form the Mudanjiang Ocean. The eastern Solonker Ocean completed its Early Paleozoic expansion and the tectonic regime transitioned to subduction retreat during the Late Paleozoic, with the generation of voluminous magmatism and accretionary complexes at both sides of the ocean (Fig. 9d).

(5) Middle Permian-Middle Triassic (260–240 Ma): After a prolonged period of subduction and accretion, the eastern Solonker Ocean finally closed during the Middle Triassic, causing collision of the NCC and SXAT along the CYS. The Mudanjiang Ocean expanded, with a series of subduction-related igneous rocks and OIB and E-MORB-affinity basalts being generated in the northern SXAT (Fig. 9e).

(6) Middle Triassic-Late Jurassic (240–145 Ma): The Mudanjiang Ocean narrowed during the Jurassic. The formation of high-pressure blueschist in the Heilongjiang Complex during the Jurassic (190–145 Ma) suggests the reoccurrence of subduction/collision of the SXAT and JB, leading to the final closure of the Mudanjiang Ocean at least before the Late Jurassic (ca. 145 Ma; Fig. 9f).

5.   Discussion

5.1   Lifespan and geological influence of branch ocean basins of the eastern PAO

Each branch ocean basin of the eastern PAO had a unique lifespan and scope of geological influence. During most of the Paleozoic, the Xilin-Xiguitu and Heilongjiang, Nenjiang, and Solonker oceans were located in the northern, central, and southern parts of the eastern CAOB, respectively. These oceanic regimes controlled the tectonic development of the eastern PAO from north to south.

The Xilin-Xiguitu Ocean was active from the Neoproterozoic to late Cambrian (700–500 Ma; Liu YJ et al., 2019, 2021). Northward subduction of the Xilin-Xiguitu oceanic plate beneath the EB caused a series of magmatic events and structural deformation, and led to various degrees of metamorphism in the region. During the existence of the Xilin-Xiguitu Ocean, various subduction-related accretionary complexes, magmatic arcs, and sedimentary prisms were formed and accreted to the southern margin of the EB (Liu YJ et al., 2017, 2019; Feng ZQ et al., 2018a, 2018b), leading to significant southward continental growth of the block. The scope of tectonic influence of the Xilin-Xiguitu oceanic regime was restricted mainly to the northwestern part of the eastern CAOB (northern Great Xing’an Range area), north of the XXS (Fig. 2).

The lifespan of the Heilongjiang Ocean extended from the Neoproterozoic to Late Devonian (700–420 Ma; Liu YJ et al., 2019, 2021). During this period, the tectonic evolution of the ocean strongly influenced the northeastern part of the eastern CAOB (Li WM et al., 2020). Southward subduction of the Heilongjiang oceanic plate beneath the SXAT produced various magmatic arcs and tectonic mélanges, and their accretion and ultimate amalgamation enlarged the terrane substantially (Li WM et al., 2020; Li SZ et al., 2018). During the late Paleozoic to early Mesozoic, the JB and SXAT moved/rotated from the northeast to east of the eastern CAOB, forming the western (right) limb of the NE China Orocline (Liu YJ et al., 2021). In a study of the Heilongjiang Complex, Liu YJ et al. (2019) proposed that the Mudanjiang Ocean was a reopened basin of the previously closed Heilongjiagn Ocean (Liu YJ et al., 2017, 2019). Rifting to reopen this ocean basin started during the early Permian (ca. 260 Ma), and the basin closed during the latest Jurassic (ca. 150 Ma). These two oceanic regime cycles strongly influenced the tectonic development of the easternmost CAOB (Lesser Xing’an Range and Zhangguangcai Range areas).

The Nenjiang Ocean had a longer lifespan than the Xinlin-Xiguitu and Heilongjiang oceans, approximately from the Neoproterozoic to late early Carboniferous (700–320 Ma). This long-lived oceanic regime was characterized by continuous bi-directional subduction of the Nenjiang oceanic plate beneath the EB and XAT to the north and the SXAT to the south (Ma YF et al., 2019a, 2020a). The northward subduction produced a series of magmatic arcs and accretionary complexes to form the main body of the XAT. The southward subduction formed a magmatic arc along the northern margin of the SXAT (Ma YF et al., 2020a). The evolution of the Nenjiang Ocean influenced mainly the central part of the eastern CAOB in NE China (central Great Xing’an Range area).

Of the various branch ocean basins of the eastern PAO, the Solonker Ocean had the longest lifespan, from the break-up of the Rodinia supercontinent during the Neoproterozoic (ca. 750 Ma; Zhao GC et al., 2018; Ma YF., 2020c) to the amalgamation of the CAOB and NCC during the latest Permian to Middle Triassic (ca. 240 Ma) (Xiao WJ et al., 2003, 2015; Jian P et al., 2010). The bi-directional subduction of the Solonker oceanic regime had a strong influence on the geological and tectonic development of the southern CAOB and northern NCC (Xiao WJ et al., 2015; Liu YJ et al., 2017; Karel S and Scott P, 2011; Wu FY et al., 2011). The northward subduction and accretion dominated the formation of the SXAT, whereas the southward subduction formed a series of accretionary complexes and magmatic arcs in the northern margin of the NCC (Fig. 7). Paleomagnetic data indicate a large difference in the paleolatitudes of the NCC and CAOB during most of Paleozoic (Zhang DH et al., 2018a, 2018b; Ren Q et al., 2020), suggesting that the Solonker Ocean measured a considerable width during that time.

Some previous studies have regarded the Solonker Ocean as the main ocean basin of the eastern PAO (Liu YJ et al., 2017; Ma YF et al., 2020a), as the ophiolite suites that cropout along the SLCYS (SLS and CYS) are younger than the corresponding suites of other suture zones, indicating that the Solonker Ocean was the last ocean to close (Liu YJ et al., 2017, 2019; Jian P et al., 2010; Xiao WJ et al., 2003; Li JY et al., 2019, 2021). However, a recent study has argued that the Nenjiang Ocean (or Hegenshan Ocean) was the main ocean basin, as it appears larger than the Solonker Ocean, as interpreted from paleomagnetic profiles. In fact, owing to the complex tectonic history of the eastern CAOB, it is unclear which ocean was the main ocean basin of the eastern PAO, and different branch ocean basins may have been the main basin at different times according to their activity. The Xinlin-Xiguitu and Heilongjiang oceans were active during the Early Paleozoic, whereas the Nejiang and Solonker oceans were active during the Late Paleozoic, meaning that two oceanic regime cycles of evolution affected the eastern CAOB.

5.2   Paleozoic east-west-trending tectonic patterns in the eastern CAOB

Liu YJ et al. (2021) proposed an orocline model in the eastern CAOB on the basis of a study of the movement/rotation of different tectonic units within the eastern PAO. In that model, the EB, JB, XAT, and SXAT were ribbon tectonic units and initially constituted straight chains in the eastern PAO. During the early and early Late Paleozoic, these chains had an east-west-trending tectonic pattern. The chains drifted and rotated northward to form an orocline (the NE China Orocline) during the Late Paleozoic and Early Mesozoic, with the EB and XAT as the west (left) limb, the JB and Zhangguangcai Range (ZGCR) as the (east) right limb, and the SXAT as the core (Liu YJ et al., 2021). This interpretation is consistent with paleomagnetic reconstructions of micro-blocks in the eastern CAOB (Zhao GC et al., 2018; Li SZ et al., 2018).

With respect to field structures, regional geological surveys and investigations have found that unlike the NE- or NNE-trending tectonic forms of the Mesozoic Okhotsk and Paleo-Pacific systems, the Paleozoic structural patterns commonly show an east–west orientation (Qin T et al., 2018; Zheng CQ et al., 2013a, 2013b), typically represented by east–west-trending mylonitic lineation and foliation in ductile shear zones. These east–west-trending structures occur widely across NE China, including the Great Xing’an Range in the west (Qin T et al., 2018) and the Zhangguagncai Range in the east (Li JY et al., 2019), and represent the main tectonic pattern of the Paleozoic PAO system.

Subduction accretionary complexes, magmatic arcs, faults, and suture zones of the SE Inner Mongolia region are oriented nearly east-west (Fig. 7). These structures were only weakly affected by the Okhotsk and Paleo-Pacific systems on account of their remoteness. Thus, the east–west-oriented structures are most likely products of the Paleozoic PAO system. Owing to the slightly clockwise rotation of the united eastern CAOB during the final closure of the PAO during the late Permian and Early Triassic (Ren Q et al., 2020), some originally east–west-trending structures were rotated to become oriented NEE (Fig. 7).

5.3   North-south collisional and accretionary younging of the eastern CAOB

During the Paleozoic to Early Mesozoic, the branch ocean basins of the eastern PAO closed from north to south and correspondingly caused southward-younging accretion and collision events. Specifically, the northern Xinlin-Xiguitu and Heilongjiang oceans closed during the late Cambrian (ca. 500 Ma; Liu YJ et al., 2017; Feng ZQ et al., 2018a, 2018b) and latest Silurian (ca. 420 Ma; Li WM et al., 2020), respectively. The central Nenjiang Ocean closed during the early late Carboniferous (ca. 320 Ma; Ma YF et al., 2019a, 2019b, 2020a, 2020b, 2020c), and the south Solonker Ocean closed during the Late Permian to Middle Triassic (260–240 Ma; Liu YJ et al., 2017; Xiao WJ et al., 2003, 2009, 2015; Jian P et al., 2010; Li JY et al., 2019). The collision and amalgamation of the EB and JB, XAT, SXAT, and the northern margin of the NCC occurred progressively from north to south (Figs. 10a–f), establishing a north to south collisional and accretionary trend within the eastern CAOB. This trend is further supported by the following lines of evidence.

Figure  10.  Paleozoic (and related Neoproterozoic and Early Mesozoic) paleogeographic reconstructions of Siberia, Gondwana, the NCC, and the eastern CAOB in NE China at (a) ca. 620 Ma, (b) ca. 500 Ma, (c) ca. 420 Ma, (d) ca. 320 Ma, (e) ca. 250 Ma, and (f) ca. 230 Ma. Modified from Zhao GC et al. (2018). Dashed black arrows indicate the general direction of continental drift at the given time periods. Abbreviations are the same as those of Fig. 2. Abbreviations refer to Fig. 2.

DownLoad: Full-Size Img PowerPoint

(1) The formation times of subduction-related accretionary complexes and magmatic arcs young from north to south. For example, the Toudaoqiao basalt island arc, Duobaoshan-Aershan ophiolite belt, and Duobaoshan-Yiershi magmatic arc are distributed continuously from north to south in the northern Great Xing’an Range area (Liu YJ et al., 2019; Zhang Y et al., 2018). These units were formed during the Neoproterozoic (620–510 Ma), early Paleozoic (510–480 Ma), and middle Paleozoic, respectively, revealing a southward-younging accretionary trend. The even younger Carboniferous magmatism took place in the central Great Xing’an Range, and the youngest (Permian) magmatic events occurred mainly in the southern Great Xing’an Range (Ma YF et al., 2019a, 2019b).

(2) The Early Paleozoic strata (the Wolegen and Xinghuadukou complexes and the Tongshan, Duobaoshan, and Egunahe formations) cropout mainly in the northern Great Xing’an Range. In comparison, the early late Paleozoic strata (the Devonian Niuqiuhe and Daminshan formations) cropout mostly in the central Great Xing’an Range. The latest Late Paleozoic strata (the Permian Dashizhai, Zhesi, and Linxi formations) are distributed predominantly in the central and southern Great Xing’an Range (IMBGMR, 1991; HBGMR, 1993). The southward-younging pattern of sedimentation reflects the north to south tectonic evolutionary trend of the eastern CAOB.

(3) The ophiolite suites are generally older in the north and younger in the south in the eastern CAOB. For example, the 443–432 Ma Xinglong ophiolite, 477 Ma Dawusu ophiolite, 506–484 Ma Duobaoshan ophiolite and 464 Ma Aershan ophiolite are found in the northern Great Xing’an Range (Liu YJ et al., 2019). In contrast, the 294–281 Ma Dashizhai ophiolite (Zhou ZG et al., 2018), 279 Ma Solonker ophiolite, 250 Ma Ondorsum ophiolite and 280 Ma Xingshuwa ophiolite (Song SG et al., 2015) occur in the central and southern Great Xing’an Range. This pattern again reflects the north to south tectonic evolutionary trend and corresponding collision and accretion.

5.4   West-east closure trend of the eastern PAO

The branch ocean basins of the eastern PAO typically underwent diachronous and unsynchronized tectonic processes during the Paleozoic. Generally, the basins show a west to east closure trend. The trend reflects the processes occurring in the northern Xinlin-Xiguitu and Heilongjiang oceans during the early Paleozoic oceanic evolutionary cycle and in the southern Solonker Ocean during the late Paleozoic cycle.

During the Early Paleozoic oceanic evolutionary cycle, the Xilin-Xiguitu Ocean was bounded by the EB to the north and the XAT to the south in the northwestern part of the eastern CAOB. Ophiolite ages and syncollisional magmatism show that the ocean was closed during the Cambrian at ca. 500 Ma (Feng ZQ et al., 2016). The Heilongjiang Ocean was located in the northeastern part of the eastern CAOB, bounded by the JB to the north and the SXAT to the south. The Heilongjiang Ocean closed during the late Silurian at ca. 420 Ma (Li WM et al., 2020), approximately 80 Myr later than the closure of the Xinlin-Xiguitu Ocean to its west. Thus, the oceans constitute an early west and late east closure trend in the northern part of the eastern CAOB (Figs. 10a–c).

During the late Paleozoic oceanic cycle, the SXAT and NCC drifted rapidly northward, as shown by paleomagnetic profiles. However, the NCC drifted more quickly than the SXAT (Ren Q et al., 2020), causing the continuous shrinking of the Solonker Ocean between them. Simultaneously, the NCC was undergoing large-scale anticlockwise rotation, whereas the SXAT was undergoing slight clockwise rotation, the result of which was the scissor-like closure of the Solonker Ocean from west to east from the Late Permian to the Early or even Middle Triassic (Figs. 10e, f; Liu YJ et al., 2019; Ren Q et al., 2020). Ophiolite suites and collision-related magmatic rocks young from west to east along the SXCYS (Liu YJ et al., 2017, 2019), further confirming the west to east closure of the eastern PAO.

5.5   The “soft collision” and accretionary orogenic type of the eastern CAOB

The planet earth has experienced numerous cycles of oceanic basin closure and collision of geological units at both sides, and the following orogenic process from the start of tectonic till today. According to current mountain belt formation process, two types of orogenic belts can be constructed (Karel S and Scott P, 2011; Xiao WJ et al., 2015). One is collisional orogenic belt, which formed as the result of continent-continent collision after the subducted ocean plate is totally consumed (Xiao WJ et al., 2018; Şengör AMC et al., 2018). The collision is largely a destructive process (Yin A and Harrison TM, 2000) called “hard collision” in numerous studies (Song SG et al., 2015). Represented example is the Alpine-Himalaya orogenic belt that runs through Europe and Asia (Yin A and Harrison TM, 2000). The formation of this orogenic belt usually needs both prior oceanic plate subduction and the“passive margin” connected continental lithosphere, which lead to intensive crustal shortening/thickening and huge scale mountain building, and the high-grade (UHP, HP to MP, and HT) metamorphism is widespread occurred (O’Brien, 2001; Song SG et al., 2015; Wang M et al., 2014).

The other is accretionary orogenic belt, which formed by the amalgamation or connection of the previously accretionary complex rather than directly continental-continental collision. This orogenic type usually occurred when the ocean lithosphere have bidirectional subduction signature (Song SG et al., 2015). Passive continental margin is absent under this case and thus no continental lithosphere can be dragged into subduction zones, which cause often called “soft collision” in relevant studies (Song SG et al., 2015). In this case, previously subduction usually brings together segments of crust of various types, compositions and origins to form a larger continent in a process known as accretion (Karel S and Scott P, 2011; Xiao WJ et al., 2018; Şengör AMC et al., 1993, 2018). The best example of this orogenic type is “Appalachian” or “CAOB”, in which the huge mountain builds process and high-grade metamorphism was rarely occurred (Xiao WJ et al., 2015; Şengör AMC et al., 1993, 2018).

To the convergent history of eastern CAOB, the Nenjiang and Solonker oceans of the eastern PAO were featured by bidirectional subduction during its tectonic circles (Liu YJ et al., 2019; Ma YF et al., 2019a, 2019b, 2020a). Series of magmatic arcs, subduction related accretionary prisms and complexs were formed at both sides of the ocean (Ma YF et al., 2020a). In this case, two opposite convergent geological units at both sides of the ocean were active continental margins. After the long-lived subduction and accretion, the primitive continental core was surrounded by significant lateral accretionary rim. As the ocean closured, the foreland accretionary rim of both convergent geological units were collided and amalgamated together under the “soft collision” condition, formed the largest accretionary orogenic belt in the world.

6.   Conclusions

(i) The eastern CAOB in NE China comprises the Erguna and Jiamusi blocks and Xing’an and Songliao-Xilinhot accretionary terranes from north to south, which are bounded or separated by the Xinlin-Xiguitu, Hegenshan-Heihe, Mudanjiang-Yilan, Solonker-Linxi, and Changchun-Yanji suture zones, respectively. The corresponding Paleo-Asian Ocean branch oceans were the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans. These oceans closed during the Late Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (270–240 Ma), respectively.

(ii) The evolution of the eastern PAO was characterized by the occurrence of two tectonic cycles. The northern Xinlin-Xiguitu and Heilongjiang oceans were involved in the Early Paleozoic cycle, and the central and southern Nenjiang and Solonker oceans in the Late Paleozoic cycle. These oceans strongly influenced the tectonic histories of the northern and southern parts of the eastern CAOB, respectively.

(iii) Tectonism in NE China during the Paleozoic featured the formation of east-west-oriented structural features. Collisional and accretionary events in the eastern CAOB younged from north to south. The closure of the eastern PAO took place from west to east, with a scissor-like closure mechanism.

(iv) Subduction in the eastern PAO was dominated by a bi-directional subduction regime, which produced significant crustal growth and “soft collision” of geological units on both sides of the ocean. The typical mountain type formed was an accretionary orogenic belt.

CRediT authorship contribution statement

Yong-jiang Liu and Yong-fei Ma designed and directed the project; Yong-fei Ma, Yan Wang, Wei-min Song, Yu-jin Zhang and Cheng Qian carried out the field work; Tong-jun Liu checked the references; Yong-fei Ma, Yong-jiang Liu and A.Yu. Peskov developed the theoretical framework; Yong-fei Ma and Yong-jiang Liu wrote the article.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

The authors are grateful to Li-dong Zhang, Xiao-pin Yang and Jun-yu Fu for their kindhearted discussion during the construction of this work. The authors also thank two anonymous reviewers for their thoughtful and constructive reviews of this manuscript. This study was financially supported by the National Natural Science Foundation of China (42130305 and 42002227), project of the China Geological Survey (DD20190039–04, DD20179402, DD20190360 and DD20221632), National Key R&D Program of China (2017YFC0601300 and 2013CB429802), Taishan Scholars (ts20190918), and Qingdao Leading Innovation Talents (19–3–2–19–zhc).