Palinspastic reconstruction and geological evolution of Permian residual marine basins bordering China and Mongolia

Gen-Yao Wu

Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

1 lntroduction＊

In Mongolia, the Upper Carboniferous, Upper Permian,Lower-Middle Jurassic and Lower Cretaceous are coalbearing systems (Erdenetsogtet al., 2009; Wu, 2013a).The Upper Permian coalfields, with giant reserves, excellent quality and convenient mining conditions, are mainly situated in the South Gobi (Ömnögovi)Province.The Nariin Sukhait Coal Mine is at a distance of 50 km from Ceke Port of Inner Mongolia, which exploits the coal fields and exports to China.The Tavan Dolgoi coalfield, 180 km awayfrom the Ganqimaodao Port, is still undeveloped, with reserves of several billion tons.These coalfields attract the attention of many investors, especially, Chinese investors.

The eastern elongation of the Upper Permian coalfields is cut off by a magmatic arc, which means that the so-called South Gobi Basin must be a tectonic basin, instead of a primary one.Originally, the South Gobi Basin stretched northwestwards into the Bayanhongor Province, but some of the Upper Permian coal-seams there were mistaken as Jurassic in age for a long time.In addition, the relation between the coal-bearing marine basins and the Paleo-Asian Ocean is unknown up to now, so that the tectonic setting,characteristics and contributing factors of these basins are controversial.

This paper focuses on the following aspects: (1)Based on the reconstruction of the evolution of the southern branch of the Paleo-Asian Ocean, it is revealed that the coal-bearing basins are residual marine ones after subduction of the oceanic crust.(2)To combine a comparative study with simultaneous basins in northern China, a distribution map of the primary residual marine basins bordering China and Mongolia is drawn.(3)The basins’development can be divided into two main stages, and the coalfields with enormous economic values are created at a later stage.This study will also lay a foundation to explore the regularities of coalbed occurrences (Wu, 2013b).

2 Regional tectonic framework

Tectonically, the China-Mongolia border area is attached to the Central Asian Orogenic Belt (CAOB; Figure 1), which grew out of the closure of Paleo-Asian Oceanthat occurred during the Neoproterozoic and Paleozoic.Sandwiched in between the Siberian subcontinent to the north and the Sino-Korean subcontinent to the south, the Paleo-Asian Ocean was divided into a northern branch and a southern branch by several landmasses in between (Figure 1).Furthermore, the superimposed deformation by subduction of the Mongolia-Okhotsk Ocean and the following continent-continent collision (Zorin, 1999; Bussienet al.,2011)resulted in more complicated structures in the area.

Figure 1 Tectonic scheme of the CAOB (based on the present geographic position).Names of cratons: AN-Angara; AD-Aldan;TM-Tarim; SK-Sino-Korean (North China + Jiaoliao).Names of intermediate masses: 1-Southern Tuva; 2-Dzabkhan; 3-Baydrag;4-Tarvagatay; 5-Hara; 6-Middle Gobi; 7-Ereen Davaa; 8-Ergun; 9-Bureya; 10-Jiamusi; 11-Junggar-Turpan-Hami.Names of sub-order continental blocks: Dr-Dariv; BB-Baga Bogd; BO-Barga Ovoo; HU-Hutag Uul; Xh-Xilinhot; TU-Tsagaan Uul.Names of sinistral shear zones: F1-Hangay; F2-Gobi Altai; F3-Gobi-Tienshan; F4-Zuunbayan; F5-Yilan-Yitong; F6-Dunhua-Mishan.Names of oceans: I-Northern branch of the Paleo-Asian Ocean; II-Southern branch of the Paleo-Asian Ocean: II1-Western segment of the southern branch; II2-Central segment of the southern branch; II2A-Dundgovi oceanic basin, II2B-Dalandzadgad oceanic basin,II2C-Engger Us oceanic basin; II3-Eastern segment of the southern branch; II3A-East Inner Mongolian oceanic basin, II3B-Solonker oceanic basin.Names of Late Neoproterozoic-Early Cambrian ophiolites: Ac-Agardagh Tes-Chem; Oz-Ozemaya belt; Bn-Bayannur; Kt-Khantaishir; By-Bayanhongor.

The southern branch of the Paleo-Asian Ocean can be divided into the western, the central and the eastern segments according to the chronology of creation and disappearance of the oceanic crust and its evolutional features.Each segment may be partitioned further into several oceanic basins by some sub-order continental blocks (Figure 1).

In West Mongolia, there occurs the famous and best developed Bayanhongor ophiolite belt, which, striking North-West, separates the Baidrag Block to the south from the Tarvagatay Block to the north.Kovachet al.(2005)reported an age of 665±15 Ma for the Bayanhongor ophiolite.Jianet al.(2010a)thought that an anorthosite(655±4 Ma), a rodingite (647±6 Ma)and an amphibole gabbro (647±7 Ma)yielded the oldest zircon ages for the plutonic part of the ophiolite, and the syenite porphyry(523±2 Ma)recorded the end of the orogeny or the orogen’s collapse.Namely, the oceanic crust of the Bayanhongor oceanic basin was created in 636-655 Ma, and the ophiolitic melange was emplaced between 523-485 Ma (Kröneret al., 2011).To the north of the Tarvagatay Block occurred the Adaatsag ophiolite with a U-Pb age of 325.4±1.1 Ma (Tomurtogooet al., 2005), indicating a NW-striking oceanic basin which constituted the western sector of the Mongolia-Okhotsk suture zone.Following the closure of the Adaatsag Ocean, the violent granitic magmatism, especially the latest Permian-Middle Triassic (255-230 Ma; Orolmaaet al., 2008)intrusion caused the Hangay area to be uplifted and eroded for a long time.

To the west of the Southern Tuva and Dzavhan Blocks,the western segment of the southern branch of the Paleo-Asian Ocean opened during Late Neoproterozoic-Early Cambrian, which is confirmed by the Agardagh Tes-Chem ophiolite (569±2 Ma; Pfänder and Kröner, 2004), the Ozemaya zone (527-522 Ma; Kovalenkoet al., 1996), the Bayannur ophiolite (571±4 Ma; Kozakovet al., 2002)and the Khantaishir ophiolite (568±4 Ma; Khainet al.,2003).All these oceanic basins closed during the Caledonian orogeny.During the Late Paleozoic, there developed the oceanic basins in southwest Mongolia to the south of the Baga Bogd Block, which disappeared before the latest Carboniferous-earliest Permian (Kröneret al., 2010).In short, the wide area of West Mongolia became a landmass in the Permian (details will be discussed below).

The northern part of Middle Mongolia housed the Zuunmod (Dzüünmod)oceanic basin, sandwiched in between the Hara Block to the northwest and the Middle Gobi Block to the southeast.The opening of the Zuunmod ocean can be traced back to the Ludlow-Pridoli Epoch of the Late Silurian (Kuriharaet al., 2009).The oceanic crust experienced three episodes of northward subduction, namely in the Middle Devonian-Early Carboniferous, Late Carboniferous-Early Permian and Late Permian-Middle Triassic respectively (Donskayaet al., 2013).The ocean finally closed in the latest Early Jurassic-early Middle Jurassic(Zorin, 1999), and constituted the eastern sector of the Mongolia-Okhotsk suture zone in the Mongolian territory,with an extant tectonic line being NE-oriented.

To the south of the Middle Gobi Block, there was the central segment of the southern branch of the Paleo-Asian Ocean.The linear Baga Bogd Block (Demouxet al., 2009a)divided the oceanic domain in the Mongolian Gobi area into the two sub-oceanic basins: the Dundgovi oceanic basin in the middle Gobi area and the Dalandzadgad oceanic basin in south Gobi area (Figure 1).The latter was bounded, to the south, by the Tsagaan Uul Block,whose existence was confirmed by dates from Demouxet al.(2009a).Wu (2013c)thought the block once being a westward elongation of the Hutag Uul Block in southeast Mongolia (Badarchet al., 2002), only was dislocated by the sinistral strike-slip of the Zuunbayan fault.To the south of the Tsagaan Uul Block, sandwiched between the block and the North China Craton, there developed another Late Paleozoic ocean named the Engger Us oceanic basin (Wuet al., 2009).

In addition, there occurred a NNE-orientated Erdenedalai trough in Middle Mongolia (Figure 2).It might be an aulacogen, created by the Zuunmod oceanic basin stretching into the young orogenic belts formed in the Hercynian orogeny.The trough might play two roles during the Permian.On one hand, it was a boundary between the upwelling region in West Mongolia and the marine depositional areas in Middle-East Mongolia; on the other hand,it was a channel-way connecting the residual marine basins with the open sea (the Zuunmod oceanic basin).

The eastern segment of the southern branch of the Paleo-Asian Ocean was situated in East Mongolia and northern China, to the south of the Ereen Davaa Block(Badarchet al., 2002)in Mongolia and the Ergun Block bordering China and Russia (Wuet al., 2008), and to thenorth of the enlarged North China Craton.The oceanic domain could be divided by the Xilinhot Block (Wu, 1998)in Inner Mongolia and the Hutag Uul Block (Badarchet al.,2002)in southeast Mongolia, into two sub-oceanic basins:the East Inner Mongolian oceanic basin in the north and the Solonker oceanic basin in the south (Figure 1).

The paper will focus on the tectono-palaeogeographic evolution of the Dalandzadgad as well as the Solonker oceanic basins during Late Paleozoic, reconstruct the original distribution of the Middle-Late Permian residual marine basins, and then discuss the geological evolution of the residual marine basins stage by stage.

3 Evolution of the Dalandzadgad ocean

3.1 Main tectonic units

Based on the study of Heloet al.(2006), an intra-oceanic island arc, a forearc basin, and a backarc basin can be recognized in South Gobi area.Owing to re-activation of dextral strike-slip activity by a group of WNW-striking faults during the Cenozoic (Cunningham, 2005), these tectonic units are, nowadays, WNW-orientateden echelon,parallel to the Gobi Altai mountains.

The Gurvan Sayhan and Zoolen ranges have previously been considered as an island arc and accretionary prism (Badarchet al., 2002).The samples from these two areas both have a predominately intermediate, calc-alkaline composition; they display LREE enrichment, excess abundance of fluid-soluble elements, and low concentrations of high-field strength elements.Both of them were part of a simple intra-oceanic island arc-forearc.In Gurvan Sayhan, adakites and high-Mg andesite were discovered,with a typical tectonic setting of a forearc basin.The abundant serpentinite there might be interpreted as remnants of uplifted mantle in a tectonically unstable forearc.Fur-thermore, the high initial εNd-values are consistent with an origin in a juvenile island arc-forearc setting (Heloet al., 2006).The igneous rocks from the Zoolen range have a similar geochemical signature and initial εNd-values as those from Gurvan Sayhan (Heloet al., 2006).Hence this paper names these two ranges the Gurvan Sayhan forearc basin and the Zoolen island arc respectively (Figure 3).

Figure 2 Middle-Late Permian basins in Mongolia and neighboring China (based on the present geographic position).a-The Zuunmod oceanic basin; b-Middle-Late Permian marine basin; c-Middle Permian marine basin changed into a non-marine basin during the Late Permian; d-Middle Permian-early Late Permian marine basin uplifted during the middle-late stage of the Late Permian; e-Transgressive basin during the middle-late stage of the Late Permian; f-Middle-Late Permian nonmarine basin; g-Coal mine.Basin names: I-Erdenedalai; II-Ondorsht; III-South Gobi; IV-Guaizihu; V-Hulun Buir; VI-Jalaid; VII-Linxi; VIII-Zhesi; IX-Jushitan;X-Chandmani-Bayanleg.Coal mines names: 1-Tsakhiurt Urt; 2-Khurren Gol; 3-Zeegt; 4-Nariin Sukhait; 5-Tavan Dolgoi.

The samples from Nemegt Uul represent a calc-alkaline, LREE-enriched island arc as well as tholeiitic LREE-depleted backarc basin signatures.The LREE-depleted metaplagiogranite probably originated at a spreading center of a backarc basin (Heloet al., 2006).Northwestwards,samples from the Bayanleg-Hatuu range reveal LREE-enriched calc-alkaline arc as well as tholeiitic LREE-depleted to slightly enriched backarc basin signatures.Nd isotopic data for Bayanleg-Hatuu and Tseel suggest a variably rejuvenated margin of a Paleoproterozoic continental block with an associated backarc basin (Heloet al., 2006).This paper names it the Nemegt backarc basin, which might be wide during that time (Figure 3).

The South Nemegt backarc basin, in the frontier region of northern Alxa, Inner Mongolia, occurred a continent marginal arc named the Yagan-Suoguozhuo arc, related to the Tsagaan Uul Block mentioned above (Figure 3).The Devonian was well developed to the south of the arc since the extension in the backarc region, and consist of, from old to young, the Zhusileng Formation (D1z), the Ih Us Formation (D2y), the Wotuoshan Formation (D2w)and the Xipingshan Formation (D3x), respectively.There occurs andesitic lava and breccia with a thickness of more than 300 m in the upper member of the Xipingshan Formation.

3.2 Geochronology of the trench-arc-basin system

Heloet al.(2006)reported the following dating results.Zircons in two samples from the metavolcanic rocks of the Zoolen locality yield mean SHRIMP U-Pb ages of 421±3.0 Ma and 417±2.2 Ma.Evaporation zircon ages of two metagranite samples from the Bayanleg locality yield 425.5±1.1 Ma and 433.5±1.1 Ma.The granitoid gneisses from Tseel yielded a Pb-evaporation zircon age of 360.5±1.1 Ma, in agreement with a U-Pb zircon age of 357Ma from a metagranitoid sample from Tsogt which is at the east of Tseel (Kozakovet al., 2002).Further SHRIMP dating of zircons from the granitic gneiss, granodioritic gneiss and metagranodiorite in Tseel yielded 397.0±3.2 Ma, 396.3±2.9 Ma and 289.2±2.3 Ma respectively (Heloet al., 2006).The zircon from the granite near Shinejinst yielded a206Pb/238U age of 410.8±20 Ma (Kröneret al.,2010).These datings show that the main active period for the trench-arc-basin system was the Silurian-Early Devonian, and the system was reactivated during the Late Devonian and Early Permian.

Figure 3 Trench-arc-basin systems of the southern belt, CAOB bordering Mongolia and China (based on the present geographic position).1-Cratons: Br-Baydrag, NC-North China; 2-Continental blocks: BB-Baga Bogd, MG-Middle Gobi, BO-Barga Ovoo,HU-Hutag Uul, Xh-Xilinhot, TU-Tsagaan Uul; 3-Ophiolite/Late Paleozoic spreading centers: By-Bayanhongor, Kt-Khantaishir,Ed-Edrengin; 4-Accretionary wedges (passive continental margin): Zg-Zag, Id-Idermeg; 5-Passive continental margins of the Mongolia-Okhotsk ocean; 6-Carboniferous magmatic arcs: Md-Mandalovoo, GT-Gobi-Tienshan; 7-(Silurian-)Devonian arcs:Zl-Zoolen, Ml-Mandal, YS-Yagan-Suoguozhuo; 8-Early-Middle Permian arcs: Ul-Ulanhot; 9-Middle Gobi Permian-Triassic volcano-plutonic belt; 10-Forearc basins: GS-Guvan Sayhan; 11-Backarc basins: Nm-Nemegt, GA-Gobi Altai, Gz-Guaizihu; 12-Coal mine; 13-Porphyry Cu-Au-Mo ore deposit; 14-Shear fractures: F1-Gobi Altai fault, F2-Zuunbayan fault; 15-Present boundary between the South Gobi Basin and the allochthonous magmatic arc.

The Yagan-Suoguozhuo arc was created during the Early Devonian.In addition to the palaeontological evidence from simultaneous sediments in the backarc basin,the eruption of arc volcanic rocks was accompanied by intrusions of the Wuzhuergashun quartz diorite and the Beishan granite in Ejin Qi, Alxa district.The former intrudes the Silurian and itself is intruded by middle Hercynian plagiogranites; the latter yields an age of 409.4 Ma, being covered unconformably by the Upper Carboniferous (Bureau of Geology and Mineral Resources of Nei Mongol Autonomous Region, 1991).

Arc magmatism occurred again in Yagan-Suoguozhuo during the Late Devonian, supported by the andesitic rocks within the Xipingshan Formation (D3x).The Early Devonian arc enlarged and migrated southwards since the oceanic crust subducted southwards.The arc reactivated in the “Taiyuan Stage” recorded by andesitic lava and breccia near Yagan, northern Ejin Qi, with a thickness of about 2500 m.According to the new stratigraphic chart of China,the lower part of the “Taiyuan Stage” belongs to the Late Carboniferous, and the middle-upper part is part of the Early Permian.The andesite is intruded by a plagiogranite in age of 278 Ma (based on the regional survey report),which confirms that the andesitic rocks belong to the Late Carboniferous-Early Permian.

3.3 Tectono‑palaeogeographic evolution

3.3.1Pre-Silurian

Zircons in a granitic gneiss sample from the Baga Bogd range (about 101°-102° east longitude)yield a SHRIMP age of 1519±11 Ma, and the zircon xenocrysts yield a SHRIMP age of 1701 Ma (Demouxet al., 2009a), which formed part of a continental crust occurring there during the Paleoproetrozoic and Mesoproterozoic.At an eastward elongation, the metamorphic rocks of the basement are present to 109° east longitude, where the basement rocks are intercepted by the Late Carboniferous Oyut Ulaan volcanic group of the Mandalovoo arc (see fig.3 of Blightet al.,2008), which is resulted from the northward subduction of the Dundgovi Ocean.Towards the west, the block extended to 96°E.Some localities in this region, such as Bayanleg,Tsogt and Tseel, yielded information of melting and rejuvenation of old continent crustal material with mean residence times up to 1.5 Ga, and were once named the Tseel Block and Tsogt Block (Heloet al., 2006).

The calc-alkaline granites with ages of 502 Ma and 498 Ma in the Baga Bogd range (Demouxet al., 2009a)marked a subduction event during the Middle-Late Cambrian in the South Gobi area, affected by which the Baga Bogd Block expanded.During the Ordovician, a relatively stable development of the Dalandzadgad oceanic basin probably took place, and the Ordovician sediments became widely distributed, including shallow-sea facies deposits on oceanic islands and bathyal deposits on slopes of oceanic islands.

3.3.2Silurian-Devonian subduction

The Dalandzadgad ocean subducted southwards during the Silurian-Early Devonian, when the Zoolen arc and the Gurvan Sayhan forearc basin were created.The eastern sector of the arc was an intra-oceanic arc, while the development of the western sector of the arc might be influenced by the continental block (Heloet al., 2006).So far, no Silurian-Devonian strata of nonmarine facies have been discovered in southern Mongolia, which confirms the eastern sector of the Zoolen arc being a submarine one.To the south of the arc, there occurred a wide backarc basin,namely the Nemegt basin (Figure 4a).The Ovoo Dolgoi,Baruun Naran and Tavan Dolgoi coal mines were situated in this backarc basin at that time, where Silurian-Early Devonian basalt, spilite, andesite, siltstone, sandstone and argillite were formed; the sediments became metamorphic and schistose.

A sedimentary differentiation along the Zoolen arc started in the Early Devonian, when sandstone was deposited in the western sector in a fluvial facies (Kröneret al., 2010).A still larger differentiation took place during the Middle-Late Devonian, which might be related to the local magmatism.The subduction and the related arc igneous activities in the Middle-Late Devonian mainly occurred in the western sector of the Zoolen arc in southwestern Mongolia, such as the Tseel range.Besides the dates mentioned above, ages of 384±2 Ma (Kozakovet al., 2002)and 361±1 Ma (Kröneret al., 2007)for the metagranite in the Tseel-Tsogt region were reported.The zircons from a rhyolite in the Gichgene range of East Tsogt(near 98°E)yielded SHRIMP U-Pb ages of 397.0±3.3 Ma and 397.0±3.2 Ma (Demouxet al., 2009b), and the zircons from a tonalitic gneiss in Gichgene were dated as 363.9±3.9 Ma (Kröneret al., 2010).

As a response to the southward subduction of the Dalandzadgad ocean, a continent marginal arc, named the Yagan-Suoguozhuo arc, was created during the Early Devonian.The main body of the arc might occur along the national boundary of Mongolia and China, where the andesitic succession consists of tufflava and ignimbrite, with few intercalations of sedimentary rocks.The early stage of the arc was a submarine one, but plant fossils occur in the upper layers of the Lower Devonian which means that the arc (or part of the arc)rose above the sea-level in a later stage.Correspondingly, the backarc region depressed further and transgressed during the Middle Devonian.Andesites erupted again in the Late Devonian, indicating the Yagan-Suoguozhuo arc was enlarged and became more mature (Figure 4b).

3.3.3Carboniferous

The Early Carboniferous-early Late Carboniferous was a relatively stable developing period for South Gobi area in southern Mongolia, with volcano-sedimentary rocks being widely distributed (Figure 4c).A similar stable development occured in the eastern segment of the southern branch of the Paleo-Asian Ocean in East Mongolia and northern China (to be discussed below), but it was distinctly different for the Dundgovi Ocean which subducted and closed during the Early Carboniferous (Blightet al.,2008, 2010).

The Carboniferous evolution in southwestern Mongolia showed distinctive features.Owing to the violent subduction in the Middle-Late Devonian, the forearc basin closed and the collision followed between the western sector of the Zoolen arc and the Baga Bogd Block during the Early Carboniferous.The representative igneous rocks for the tectono-thermal event were as follows: gneissic granites from the north slope of Gichgene of which the age is 340.9±2.5 Ma-350.4±1.7 Ma (Kröneret al., 2010),the andesite near Shinejinst of which the age is 357.8±3.2 Ma (Kröneret al., 2010), and the granitic complex near Chandmani of which the age is 345 Ma (Hrdličkováet al.,2008).During the Late Carboniferous and Early Permian,the northern part of southwestern Mongolia was uplifted and eroded.

Figure 4 Tectonic evolution of the Dalandzadgad oceanic basin in the central segment of southern belt, CAOB.1-Continental block; 2-Oceanic crust; 3-Passive continent margin; 4-Oceanic island; 5-Continent marginal arc; 6-Intra-oceanic arc;7-Inactive arc; 8-Orogenic belt; 9-Molasse; 10-Alkali granite;11-Residual marine basin.Name of tectonic unit: TU-Tsagaan Uul Block; YS-Yagan-Suoguozhuo arc; Nm-Nemegt backarc basin; Zl-Zoolen arc; GS-Guvan Sayhan forearc basin; BBBaga Bogd Block; GT-Gobi-Tienshan alkali granite belt; MLMain Mongolia Lineament alkali granite belt; Gz-Guaizihu Basin; SG-South Gobi Basin.a-Silurian-Early Devonian; b-Late Devonian; c-Early Carboniferous; d-Latest Carboniferousearly Early Permian; e-Middle-late stage of Early Permian;f-Middle Permian-early Late Permian; g-Middle-late stage of Late Permian.

The Devonian strata in the Trans-Altai range which is located to the South Gobi Altai mountains are very similar to the Devonian in the Nemegt range, which could illustrate that the Trans-Altai range once extended to west of the Nemegt backarc basin.The Silurian-Devonian subduction of the Edrengin ocean caused the sea-floor spreading and creation of oceanic crust in the Trans-Altai region (Kröneret al., 2010).Influenced by the Late Devonian subduction, the area was uplifted during the latest Devonian and earliest Carboniferous, then subsided and filled with sediments from the Visean Stage to the Late Carboniferous.The ocean subducted southwards during the Late Carboniferous, and the Gobi-Tienshan magmatic arc was created, accompanied by calc-alkaline intrusions yielding ages of 302±3 Ma (Yarmolyuket al., 2008),301.4±1.2 Ma, 301.1±1.6 Ma and 299.9±1.6 Ma (Kröneret al., 2010).The closure of the ocean and its subsequent collision resulted in the uplift and erosion of the area during the Permian.

3.3.4The latest Carboniferous-Permian

The oceanic crust in the South Gobi area disappeared in the latest Carboniferous-early Early Permian, accompanied by violent volcanic eruptions in the Nemegt backarc basin (Figure 4d), and the oceanic subduction caused a sedimentary differentiation in the backarc region.Some localities, like Noyon Uul (Hendrixet al., 1996), were once uplifted and were eroded and covered unconformably by Permian nonmarine strata.The major areas of the former backarc region deposited varicoloured marine conglomerates and coarse sandstones, with a thickness of 300-1200 m, during the middle-late stage of the Early Permian.Afterwards, a relatively stable residual marine basin occurred during the Middle-Late Permian, named the South Gobi Basin by Erdenetsogtet al.(2009).

Similarly, a southward subduction, recorded by the andesite, occurred in the Mongolia-China border area during the late Late Carboniferous-early Early Permian.The subductional orogeny created a molasse basin in the middle-late stage of the Early Permian, where the conglomerate was deposited unconformably on the andesitic rocks.The conglomerate is interbedded with limestone and sandstone, with abundant brachiopod and crinoid fossils, which confirmed that a relatively stable residual marine basin had been created in the former backarc region starting from the Middle Permian and is named the Guaizihu Basin here(Figure 4e).

Closely following the closure of the oceanic basin, an arc-arc (continent)collision occurred during the middlelate stage of the Early Permian.A northern orogenic belt was created by the collision between the Zoolen arc and the Baga Bogd Block, and a southern one was caused by the collision between the Zoolen arc and the Yagan-Suoguozhuo arc.Accompanied by the arc-arc (continent)collision, molasse was deposited along the forelands of the newly formed mountain chains (Figure 4f, 4g).

Furthermore, the collapse of the orogenic belts resulted in intrusions of peralkaline rocks and alkalic granitoids and created two alkalic magmatic belts with lengths of more than 1000 km.The northern alkalic magmatic belt is named the Main Mongolia Lineament and is related to the collapse of the Zoolen arc-Baga Bogd Block collisional belt, which are dated as 294 Ma and 292 Ma (Johnet al.,2009).The southern one is named the Gobi-Tienshan belt and is related to the collapse of the Zoolen arc-Yagan-Suoguozhuo arc collisional belt, which were dated as 290 Ma, 276 Ma, 286 Ma, 284 Ma and 277 Ma (Johnet al.,2009).

4 Evolution of the Solonker ocean

The eastern segment of the southern branch of the Paleo-Asian Ocean, separated by the Zuunbayan fracture which was possibly a transform fault during the Paleozoic,from the Dalandzadgad and Dundgovi oceans, is situated in eastern Mongolia and neighbouring northern China.The Hutag Uul Block (Badarchet al., 2002)and the Xilinhot Block (Wu, 1998)divided the eastern segment into the Solonker ocean to the south and the East Inner Mongolia ocean to the north.

4.1 Pre‑Permian

The Solonker ocean experienced a long evolutional history.The Ondor Sum-Kedanshan ophiolite yielded Sm-Nd isochron ages of 881±29 Ma and 665±46 Ma (Chenet al., 1991), documenting that the oceanic crust was brough forth during the Neoproterozoic.The whole rock Sm-Nd isochron age of 446 Ma, and the apatite U-Pb ages of 455 Ma and 422 Ma from the plagiogranite in Ondor Sum (Tang and Shao, 1996)indicate that the oceanic crust was subducted during the Late Ordovician-Silurian.The ocean might have been subducted southwards, and the related magmatic arcs migrated northwards progressively,because the back-retreat style subduction is characterized by oceanward arc migration (Wu, 1998).

During the Late Paleozoic the Solonker ocean was subducted southwards, and the magmatic arcs migrated northwards once more.Its earlier subduction occurred during the Devonian, as evidenced by the Hegenshan ophiolite with a Sm-Nd isochron age of 403 Ma (Baoet al., 1994), and by the Erdaojing-Qagan Nur ophiolite which is unconformably covered by Famennian sediments of the late Late Devonian (Tang and Shao, 1996).

The Carboniferous was a period of sea-floor spreading and oceanic crust development.Some oceanic islands were scattered within the basin and formed the outline of an archipelago.Neritic limestones with fossils were deposited on the oceanic islands, which are named the Sulinheer terrane, indicating ages from the Carboniferous to the Asselian stage of the Early Permian (Badarchet al., 2002).

4.2 Permian-Triassic

4.2.1The early Early Permian

Oceanic subduction started once again in 294-280 Ma(Jianet al., 2010b), and the Solonker forearc ophiolite was formed at this time, which was preceded by a phase of ex-tension in the forearc region occurring from 299-290 Ma(Jianet al., 2010b).The oceanic islands developed until the Sakmarian since the oceanic basin became narrower and the stress field changed.In addition, the subduction rejuvenated the northern boundary fracture of the North China Craton (the Bayan Obo-Chifeng-Kaiyuan fault)along which continental arc magmatism took place (Jianet al., 2010b).

4.2.2The late Early Permian-early Middle Permian

An active spreading ridge reached the trench and then a ridge-trench collision occurred from 281-273 Ma,which led to N-MORB magmatism and MORB-sediment interaction (Jianet al., 2010b).A forearc melange was formed at this time, and the arc rose to above the sealevel.The collision-related molasse was deposited along the foreland of the orogenic belt.Continental arc magmatism continued to develop along the northern margin of the North China Craton.The Xilinhot-Hutag Uul Block drifted towards the trench, accompanied by bimodal volcanism and A-type granitic intrusion, which was once regarded as evidence that the oceanic crust disappeared during the Devonian and that the continental rifting took place during the Permo-Carboniferous (Baoet al., 2007).

4.2.3Middle Permian

The oceanic crust was subducted again as indicated by the eruption of andesitic rocks named the Dashizhai Formation of early Middle Permian.The Solonker ocean finally disappeared and arc-continent collision took place.The Xilinhot-Hutag Uul Block was uplifted and underwent erosion.The datings of Jianet al.(2010b)indicate that these events occurred during the Middle Permian(271-260 Ma).

It should be pointed out that the residual marine basins were shaped during the late Middle Permian.The residual marine basin in the former backarc region was named the Zhesi Basin after the famous stratigraphic Zhesi Formation.The residual marine basin in the former forearc region was named the Wujiatun Basin, which joined the residual marine basins that appeared after closure of the East Inner Mongolia ocean, so as to result in a vast expanse of northern China’s territory.

4.2.4Late Permian-Early Triassic

A phase of collision and collapse of the orogenic belt occurred from 255-248 Ma ago (Jianet al., 2010b).The subducted plate beneath the continent delaminated, and upwelling of deep mantle material resulted in alkaline magmatism, and a flux of asthenosphere through a slabgap induced sanukitoid magmatism (Jianet al., 2010b).Regional uplifting occurred during this phase, when the Zhesi Basin was reversed, and the Wujiatun Basin disintegrated and split into two narrow lacustrine basins during the Late Permian, respectively as the Linxi Basin in the south and the Jalaid Basin in the north, and both of the lacustrine basins were reversed by the end of Permian.

5 Evolution of the residual marine ba‑sins

A characteristic element of the southern branch of the Paleo-Asian Ocean is marine molasse, followed by littoralneritic fine clastic sediments as well as limestones, which indicates that residual marine basins occurred in a wide area after the disappearance of oceanic crust.Based on two unconformities within stratigraphic records, the evolution of these residual marine basins can be divided into two main stages.Two upward-fining sedimentary cycles can be recognized, composed of clastic sediments of the Lower-Middle Permian and the Upper Permian respectively.We will illustrate the geological evolution on the basis of examples from the Guaizihu Basin in western Inner Mongolia, the South Gobi Basin in southern Mongolia and the Zhesi Basin in eastern Inner Mongolia (Figure 5).

5.1 Early-Middle Permian

5.1.1Creation phase

An orogeny occurred during the early Early Permian,which was recorded by a magmatic arc or ophiolite which was dated as 299-280 Ma (Jianet al., 2010b),i.e., during the Asselian and early Sakmarian.The orogeny was closely followed by molasse formation and collapse of the orogenic belt, and the latter was indicated by peralkaline rocks and alkalic granitoids with ages of 290-276 Ma (Johnet al., 2009), namely, the late Sakmarian and Artinskian.

Conglomerates or coarse clastic rocks were deposited in all of the three residual marine basins during the middle-late stage of the Early Permian.In the Guaizihu Basin, the basal conglomerate of the Maihanhada Formation became 100 m or more thick, and unconformably covered the andesite.The Lower-Middle Permian in the South Gobi Basin was once called the Toгooтxop Formation,consisting of volcanic rocks in its lower part and clastic rocks in its upper part; the basal conglomerate of the latter is 300-1200 m thick.The simultaneous clastic sedimentsin the Zhesi Basin, named the Qingfengshan Formation, is over 800 m thick.

5.1.2Stable developing phase

The Maihanhada Formation in the Guaizihu Basin is dominated upwards by sandstones with interlayers of limestone, and abundant benthos fossils, like brachiopods and crinoids, which indicate that a stable shallow marine basin was created during the early Middle Permian.The simultaneous sediments in the South Gobi Basin are argillite and siltstone, indicating a very stable environment.Furthermore, it should be pointed out that there was neither volcanism nor magmatism during this phase in either of the basins.

Subduction took place during the early Middle Permian which is recorded by andesite rocks named the Xilimiao Formation in the Solonker ocean (Bureau of Geology and Mineral Resources of Nei Mongol Autonomous Region,1991)and the Dashizhai Formation in the East Inner Mongolia ocean (Baoet al., 2005).An accompanying diorite intruded into the Lower Permian (Bureau of Geology and Mineral Resources of Nei Mongol Autonomous Region,1991), confirming the age of the volcanism as early Middle Permian.The episode of subduction was the last evolutionary phase of both oceanic basins, and afterwards, the residual marine basins occurred in the former forearc and backarc regions, one of which is the Zhesi Basin.

5.1.3Full blossom phase

The Guaizihu and South Gobi Basins continued to develop during the late Middle Permian, and the lithologic characters were similar to the early Middle Permian sediments.The Zhesi Basin developed continuously, and the succession was named the Zhesi Formation, after the correlative strata of the Jisu Honguer Formation in Mongolia(Bureau of Geology and Mineral Resources of Nei Mongol Autonomous Region, 1991), which suggests that the basin once stretched westwards into Mongolia and joined the South Gobi Basin.The later sinistral strike-slip movement along the Zuunbayan Fault caused the eastern sector of the Mandalovoo arc to move southwestwards, so that the eastward stretches of the South Gobi Basin, and also the Upper Permian coal fields, were intercepted by the magmatic arc with Cu-Au-Mo ore deposits of porphyry type nowadays (Figure 3).Fossils of the Zhesi Basin are famous for its Brachiopod-coral-Fusulinid assemblage.It should be emphasized that few fossil Fusulinids have been discovered in other residual marine basins in northern China, which might mean that the Zhesi Basin was not connected with other basins during that time.

Figure 5 Permian stratigraphy in the Guaizihu, South Gobi and Zhesi Basins.

The late Middle Permian was a full blossom phase for the development of residual marine basins.In addition to the three basins mentioned above, the Wujiatun Basin north of the Zhesi Basin, which extended over a vast area,including eastern and northern Inner Mongolia, as well as the region of the extant Songliao Basin, which covered the western Heilongjiang and Jilin Provinces.To the southeast of the Guaizihu Basin was the Jushitan Basin, which was named after the Jushitan Formation and was characterized by the representative fossil of ammonites, instead of Brachiopods and corals.Furthermore, after closure of the Dundgovi oceanic basin, a residual marine basin, named the Ondorsht Basin, formed in the former forearc region(Figure 2).

Andesitic or basaltic volcanics erupted during the late stage of the phase, which might be related to the colli-sion of the accreted Baga Bogd Block, the Tsagaan Uul Block and the North China Craton.The majority of the upper part of the Aqide Formation in the Guaizihu Basin is andesite, intercalated with sandstone and limestone,unconformably covered by the Late Permian conglomerate.In the Jushitan Basin, basalt layers are intercalated between the rocks of the middle-upper part of the Jushitan Formation.

5.2 Late Permian

5.2.1Regional differentiation phase

Differentiation of the residual marine basins’ evolution started in the earliest Late Permian, with a general trend of uplift in the east and subsidence in the west.

The coarse clastic rocks, such as molasse resulting from the collision orogeny, were deposited in the Guaizihu and South Gobi Basins, accompanied by eruptions of basaltic and acidic lavas, indicating the collapse of the orogenic belt.The coal-bearing rocks of the Upper Permian in the South Gobi Basin are named the Tavan Dolgoi Formation,of which the lower member, namely, the basal conglomerate intercalated with coal seams, has a thickness of 240-260 m.The basalt kept flowing out till the Triassic in the South Gobi Basin.In contrast, only interlayers of limestone, instead of coal seams, are present in the Guaizihu Basin, and both dacite and basalt erupted during the early Late Permian.

The fossil assemblage of Codonofusiella in the upper part of the Zhesi Formation indicates that the Zhesi Basin still existed during the early Late Permian, and that the fossil assemblage was in full bloom during the early Late Permian (Bureau of Geology and Mineral Resources of Nei Mongol Autonomous Region, 1991).Afterwards, the Zhesi Basin was reversed, uplifted and eroded during the middle-late stages of the Late Permian.

The Wujiatun Basin was reversed either by the end of Middle Permian, which was disintegrated and split into two nonmarine basins, named the Linxi Basin to the south and the Jalaid Basin to the north respectively.The strata were named the Linxi Formation in Inner Mongolia and the Sizhan Formation in the Songliao Basin.The rocks consist of dark gray slate and black platy phyllite, which were conformably deposited onto the Middle Permian marine strata (Bureau of Geology and Mineral Resources of Nei Mongol Autonomous Region, 1991).Similarly,the Jushitan Basin became a nonmarine basin during the Late Permian, and the sediments from the Fangshankou Formation.

5.2.2Transgressive phase

Fine clastic rocks with interbedded coal seams and thin limestones were deposited in the South Gobi Basin during the middle Late Permian.Simultaneously, neritic sediments developed in the Guaizihu Basin.What differs from the South Gobi Basin is that coal beds are absent in the Guaizihu Basin.

Constrained by the regional tectonic evolution, the marine transgressed towards the west and two narrow basins were created during this period along Chandmani-Bayanleg of southwestern Mongolia, where the coal-bearing strata of the Middle Member of the Tavan Dolgoi Formation were deposited, unconformably, on the Lower Carboniferous.

The South Gobi and Chandmani-Bayanleg marine basins continued developing during the late Late Permian and fine clastic rocks were deposited with interbedded coal seams.From the viewpoint of coal geology, the early stage of the late Late Permian was an extremely favorable phase for coal formation, with increasing thicknesses and extent of the coal beds and intercalated clastic sediments.Even in the Chandmani-Bayanleg Basin, some valuable coal fields were formed, for example, the Zeegt coal field near Chandmani.

5.2.3Shrinkage and reversal phase

The residual marine basins shrank during the latest Permian.Because the basins became narrower and more active, relatively coarse sediments, including conglomerates, were deposited along margin of the basin.Relevantly,the spatial distribution of coalbeds became unstable, and few coal fields with economic value have been discovered within the latest Permian horizon.All Middle-Late Permian basins were reversed at the end of Permian or the earliest Triassic; they became uplifted and underwent erosion.Generally, the Permian was unconformably covered by the Jurassic in areas of Mongolia and neighbouring China.

6 Conclusions

Two main conclusions can be drawn regarding the evolution of the southern branch of Paleo-Asian Ocean: a multiple-direction subduction (or multiple-polarity orogeny)and a multiple-episode of subduction and collision took place.

Subduction polarity of the Paleo-Asian Ocean has given rise to much controversy (Lamb and Badarch, 1997; and references therein).Wu (1998)thought that the Solonkerocean (the Solen Mountain-Erlian-Hegen Mountain suture zone)subducted southwards beneath the North China Craton, which was confirmed by later works (Xiaoet al.,2003; Jianet al., 2010b).The present contribution indicates that the Dalandzadgad ocean subducted southwards,while the Dundgovi ocean subducted northwards (Badarchet al., 2002), beneath the landmass in between.In addition,the most eastern part of the southern branch of the Paleo-Asian Ocean subducted eastwards beneath the Bureya-Jiamusi Block (Wuet al., 2008; Huanget al., 2009).In short, the southern branch of the Paleo-Asian Ocean once experienced multiple-direction subduction during its evolutional history.

Furthermore, the multiple-episode subduction-collision occurred before the creation of Central Asian Orogenic Belt.This paper shows that subduction occurred in the Silurian, Devonian, Carboniferous and Permian, with some intervals of relatively stable development of oceanic crust.Although the subduction has a multiple-polarity,the magmatic arc was migrating consistently oceanwards.Corresponding to the oceanward migration of the arc, the forearc trench removed oceanwards and a wide sedimentary wedge was created.It formed a new type of orogeny called the Altaid style or the Turkic-type one (Sengör,1992, 1993), and the China-Mongolia border area supports a representative example for this type of orogeny.

Another important feature of the closure of the southern branch of the Paleo-Asian Ocean was the development of a group of residual marine basins after the disappearance of oceanic crust in both of the forearc and backarc regions.The residual marine basins may have been created in the middle-late stage of the Early Permian or the early Middle Permian, and culminated during the late Middle Permian.Differentiation of the basin evolution occurred in the earliest Late Permian.The Zhesi Basin reversed at that time, and afterwards it became uplifted and eroded.The Guaizihu and South Gobi Basins continued developing as marine basins throughout the Late Permian.The Wujiatun Basin in northern China was disintegrated and split into two narrow nonmarine basins, and the Jushitan Basin became a nonmarine one too.It should be pointed out that the coal fields with huge economic values are located within the Upper Permian marine strata in the South Gobi Basin,so the study of residual marine basins has an important practical significance.

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