# Paleozoic tectonothermal history of the amalgamation of the Tarim–North China and Mongolian collages

Jérémie Soldner, Karel Schulmann, Pavla Štípská, Yingde Jiang

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Jérémie Soldner, Karel Schulmann, Pavla Štípská, Yingde Jiang. Paleozoic tectonothermal history of the amalgamation of the Tarim–North China and Mongolian collages. Journal of the Geological Society, 2025, 182, 10.1144/jgs2024-100. insu-05082251

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# Paleozoic tectonothermal history of the amalgamation of the Tarim–North China and Mongolian collages

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Jérémie Soldner<fnref n="1" />*, Karel Schulmann<fnref n="2" />,<fnref n="3" />, Pavla Štípská<fnref n="2" /> and Yingde Jiang<fnref n="4" />

**Abstract:** In the Mongolian Collage, metamorphic pressure–temperature (P–T) and timing reveal a one-stage evolution defined by a duality of late Neoproterozoic–Ordovician subduction-related low T/P metamorphism and suprasubduction high T/P metamorphism recorded in the Mongolia–Manchuria and Baikal–Sayan belts. This was followed by gradual prevalence of suprasubduction high T/P metamorphism towards the late Paleozoic corresponding to the Altai and South Altai cycles. In the Tarim–North China Collage, metamorphic P–T and timing reveal a two-stage evolution, from dominant intermediate T/P metamorphism possibly resulting from Ordovician–Devonian amalgamation and Andean-type evolution of the collage, to dual low and high T/P metamorphism in the Carboniferous–Permian reflecting subduction–collision processes along the South Tianshan suture in the west and a suprasubduction evolution along the Solonker suture in the east. Altogether, the Paleozoic tectonometamorphic evolution of the two collages in the Central Asian Orogenic Belt shows remarkable differences, with the Mongolian Collage displaying features typical of peripheral accretionary style reflecting recurrent tectonic switches that can be regarded as a single orogenic system, and a two-stage evolution of the Tarim–North China Collage with features of both peripheral–accretionary and interior–collisional orogenic cycles, but mostly related to recurrent subductions of interior oceans.

**Supplementary material:** A compilation of P–T–t data for Paleozoic metamorphic rocks from the Mongolian and Tarim–North China collages is available at https://doi.org/10.6084/m9.figshare.c.7476664

**Thematic collection:** This article is part of the Processes of Pangea construction collection available at: https://www.lyellcollection.org/topic/collections/processes-of-pangea-construction

**Received** 15 May 2024; **revised** 19 August 2024; **accepted** 13 September 2024

Plate tectonics on the modern Earth is characterized by the development of two main orogenic systems: (1) Pacific-type subduction–accretionary orogenic belts that form owing to subduction of ‘exterior’ oceanic lithosphere, best exemplified by Franciscan-type mélange consisting of a sedimentary matrix enclosing pieces of subducted oceanic crust (Cawood et al. 2009); (2) Alpine-type subduction–collisional orogenic belts formed by closure of short-lived ‘interior’ oceanic basins during continent assembly (Ernst 2005; Collins et al. 2011). The subduction–collisional Alpine orogenic type can be associated with burial of continental crust to high-pressure (HP) and ultrahigh-pressure (UHP) conditions and its exhumation in the form of coherent units, as described in typical interior orogens such as the Norwegian Caledonides (Andersen et al. 1991), the Western Alps (Chopin et al. 1991) or the Himalayas (Lombardo and Rolfo 2000). The development of two contrasting orogenic types at the interior and exterior parts of large continental blocks such as Gondwana is linked to global plate kinematic reorganization related to the supercontinent cycle (Cawood and Buchan 2007; Murphy et al. 2009; Collins et al. 2011). The assembly of Gondwana in the Cambrian (e.g. Collins and Pisarevsky 2005) and of Pangaea in the late Paleozoic–early Mesozoic (Stampfli and Borel 2002; Stampfli et al. 2013) were shaped by competition of accretionary and collisional orogens (Cawood and Buchan 2007). These orogenic systems were generally characterized by contrasting metamorphic evolutions that can be evaluated in terms of pressure–temperature–time (P–T–t) paths (Brown 2009).

Xiao *et al.* (2015) interpreted the Altaids (Şengör *et al.* 1993), or the Central Asian Orogenic Belt (CAOB) (Zonenshain 1990; Windley *et al.* 2007), as a supercollage formed by the Kazakhstan Collage in the west, the Mongolian Collage (MC) in the east and the Tarim–North China Collage (TNCC) in the south (Fig. 1). The Kazakhstan Collage and MC form giant oroclinal curvatures, which mainly developed in Carboniferous–Permian times (Xiao *et al.* 2018; Schulmann *et al.* 2023), whereas the TNCC is represented by an east–west-trending belt of Precambrian blocks with a long Paleozoic amalgamation history (Soldner *et al.* 2022b, 2024). Recent palaeogeographical reconstructions place the TNCC in the interior oceanic domains south of the Baltica and Siberian blocks and in the vicinity of the northeastern margin of Gondwana (Domeier 2018; Zhao *et al.* 2018; Merdith *et al.* 2021). There, the collision between the Tarim craton and the Chu–Yili arc of the southern flank of the Kazakhstan Collage during the late Paleozoic (e.g. Gao *et al.* 2011) marked the beginning of the CAOB assembly (Wang *et al.* 2018). However, the timing and mechanisms related to the progressive construction of the TNCC itself (Li *et al.* 2018; Zhao *et al.* 2018) and its position with regard to the Gondwana megablock in the south and the MC in the north remain ambiguous (Wang *et al.* 2021; Zhao *et al.* 2021).

Recent metamorphic and petrochronological studies of the Paleozoic evolution of the MC along the southern margin of Siberia allowed constraint of the mechanisms and time scales of polycyclic formation of this collage (Broussolle *et al.* 2018; Jiang

<footnote n="1">¹ Institute of Geological Sciences, Polish Academy of Sciences, Senacka 1, 31-002 Kraków, Poland</footnote>

<footnote n="2">² Czech Geological Survey, Centre for Lithospheric Research, Klárov 3, 118 21 Praha 1, Czech Republic</footnote>

<footnote n="3">³ Université de Strasbourg, CNRS, ITES UMR 7063, 5 rue René Descartes, 67084 Strasbourg Cedex, France</footnote>

<footnote n="4">⁴ State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China</footnote>

<footnote>ID JS, 0000-0002-4096-431X; KS, 0000-0002-5814-9161; PŠ, 0000-0001-9782-9568</footnote>

<footnote>* Correspondence: j.soldner@ingpan.krakow.pl</footnote>

<footnote>Present addresses: JS, Institute of Geological Sciences, Polish Academy of Sciences, Senacka 1, 31-002 Kraków, Poland</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Tectonic map of the Tarim–North China, Mongolia and Kazakhstan collages constituting the Central Asian Orogenic Belt (CAOB). Source: modified after Guy *et al.* (2020).</figcaption></figure>

et al. 2019; Kong et al. 2022; Štípská et al. 2023; Zhu et al. 2023). Similar studies of the TNCC allowed the proposal of new temporal and geodynamic scenarios of its construction (Wang et al. 2017; Soldner et al. 2022a, 2024). However, the correlation of metamorphic evolutions of these two collages and processes related to their amalgamation remain enigmatic. To fill the gap, we use a wide database of pressure (P), temperature (T) and petrochronological data from late Neoproterozoic to early Mesozoic metamorphic rocks together with a review of compressive and extensional tectonic cycles to evaluate and correlate the tectono-thermal and temporal evolutions of these two orogenic systems. The review of P–T and temporal trends allows us to propose a modified geodynamic model for the early Paleozoic evolution of two contrasting types of convergent plate boundaries governing the amalgamation of East Pangaea during the late Paleozoic.

## Geological setting and Paleozoic metamorphism

The MC is separated from the Siberian craton in the north by the Cambrian granulite- to amphibolite-facies Sayan–Baikal belt (Donskaya et al. 2000; Gladkochub et al. 2008), and from the North China and Tarim cratons to the south and SW by the late Permian to early Triassic South Tianshan–Solonker suture zone (Xiao et al. 2003; Eizenhöfer and Zhao 2018). Although less clearly defined, the far east limit of the MC includes the Erguna, Xing’an, Songliao and Jiamusi–Khanka blocks in NE China (Fig. 1) (Zhou et al. 2018), which preserve relics of the Cambrian granulite-facies ‘Khondalite belt’ (Zhou et al. 2011). The Jiamusi and Songliao blocks are separated by the Permian to Jurassic HP metamorphic Heilongjiang complex, which is considered to reflect the interaction between the Mongol–Okhotsk subduction system to the north and the Pacific subduction to the east (Aouizerat et al. 2019, 2020).

The MC consists of four major lithological units. From north to south these are as follows:

(1) Mongolian Precambrian blocks (Tuva–Mongol, Zavkhan, Baidrag and Erguna blocks).

(2) A late Proterozoic–early Cambrian accretionary belt consisting of oceanic rocks and arcs (Khukhuudei et al. 2022) and an early Cambrian eclogite belt rimming the southern and eastern margins of the Mongolian Precambrian blocks collectively assigned to the Lake Zone (Štípská et al. 2010; Javkhlan et al. 2019; Zhu et al. 2023). Both the Precambrian and accretionary units were intruded by igneous and

volcanic rocks of the Cambrian–Ordovician (c. 520–470 Ma) Ikh–Mongol arc (Janoušek et al. 2018).

(3) The Cambrian to Devonian volcano-sedimentary successions of the Altai accretionary wedge (Jiang et al. 2017; Soejono et al. 2018) that were affected by middle to late Devonian (c. 410–380 Ma) and Permian (c. 290–260 Ma) magmatic, metamorphic and deformational events mostly connected with Buchan-type metamorphism (Broussolle et al. 2018; Jiang et al. 2022).

(4) Volcano-sedimentary sequences of Devonian to Carboniferous volcanic arcs and back-arcs forming the Trans-Altai and East Junggar terranes to the south (Xiao 2004; Xiao et al. 2009; Nguyen et al. 2018). The Altai accretionary wedge is separated from the East Junggar–Trans-Altai terranes by a major Permian collisional zone marked by presence of (U)HT granulites and Buchan-type metamorphism coinciding with the Erqis zone in NW China (Jiang et al. 2019, 2022) (Fig. 1).

The TNCC is formed by the Tarim craton in the west and the North China craton in the east and includes in its central parts the Precambrian Dunhuang and Alxa micro-blocks and the Beishan Orogen (Fig. 1). The northern edge of the TNCC is bounded by the late Carboniferous to early Triassic South Tianshan and Solonker suture zones (Xiao et al. 2015; Eizenhöfer and Zhao 2018) (Fig. 1). In the west, Carboniferous ophiolitic mélanges and late Ordovician to Carboniferous clastic–carbonate and volcanic successions of the South Tianshan accretionary belt were metamorphosed at HP–UHP conditions. These rock assemblages record the closure of the Turkestan or South Tianshan ocean and late Paleozoic collision of the Tarim craton with the Kazakhstan Collage (e.g. Gao and Klemd 2003; Soldner et al. 2017; Wang et al. 2018). Along the South Tianshan suture zone, the South Tianshan accretionary belt is juxtaposed to the Precambrian basement of the Central Tianshan block, which was extensively intruded by Ordovician to Devonian granitoids (Ge et al. 2012; Ma et al. 2014) and associated oceanic arc (Gao et al. 2023). In the east, the Cambrian–Devonian Bainaimiao arc complex delimits the northern margin of the North China craton and Alxa block (Zhang et al. 2014; Chen et al. 2020) and is juxtaposed to the late Ordovician–early Silurian greenschist- to blueschist-facies Ondor Sum tectonic mélange (de Jong et al. 2006). The latter was reactivated during early Triassic amphibolite-facies metamorphism (J. Zhang et al. 2018b) affecting the neighbouring Erdaojing subduction mélange formed during the
final closure of the Palaeo-Asian Ocean (Xiao et al. 2003; Eizenhöfer and Zhao 2018). To the south, the TNCC is bounded by the Kunlun–Qilian–Qinling orogens, which record the evolution of the Palaeo-Tethys Ocean (Liu et al. 2015) and mark the northernmost extent of the Tethysides (Fig. 1).

## Methods and metamorphic data analysis

The thermal history of the Earth's crust is stored in metamorphic rock records (Brown and Johnson 2018) characterized by distinct metamorphic facies reflecting transformations at variable *P*–*T* conditions. Principal metamorphic facies series (high, intermediate and low *T/P*) represent the products of contrasting heat flow in different tectonic settings along convergent plate boundaries (Miyashiro 1961). In this study, we compiled the dataset of *T*, *P*, thermobaric ratio (*T/P*) and age (*t*) of late Neoproterozoic to early Mesozoic (overall 94% of Paleozoic ages) metamorphic records to encompass a total of 180 localities in the TNCC (mainly from a review of Soldner 2023) and the MC. Available distinct peak-*P* and peak-*T* conditions have been compiled and the values of metamorphic gradients and thermobaric ratios (*T/P*) have been calculated and projected in Figure 2 together with their corresponding age constraints. The thermobaric *T/P* ratios are more useful parameters than *T* or *P* alone, because they better reflect particular plate-tectonic settings (Brown and Johnson 2019). Based on these values, metamorphic rocks have been classified into three groups by Brown (2007): high *T/P* type (>775°C GPa⁻¹), including common high- to ultrahigh-temperature (HT to UHT) granulites; intermediate *T/P* type (775–375°C GPa⁻¹), including HP granulites and medium- to high-temperature (MT to HT) eclogites; and low *T/P* type (<375°C GPa⁻¹), including blueschists, low-temperature (LT) eclogites and ultrahigh-pressure (UHP) metamorphic rocks. This classification is adopted in this study and the compiled data for the TNCC and MC are available in Table S1. Palaeogeographical reconstructions presented in this study have been made using the

GPlates open-source computer software designed to work with full-plate reconstructions (Boyden et al. 2011). GPlates-compatible global plate models of Domeier (2018) combined with those of Domeier and Torsvik (2014) were used with specific focus on the TNCC and MC. Relative density plots of metamorphic ages for each of the orogenic collages were produced using Isoplot/Ex (Ludwig 2003).

In the MC, metamorphic age records range from 548 to 213 Ma (Figs 2a and 3a; Table S1). Based on these data, two metamorphic age groups can be distinguished: the latest Ediacaran to Silurian (548–436 Ma) and Devonian to Triassic (406–213 Ma). With the exception of six outliers, the former group yields metamorphic gradients of 7–39°C km⁻¹ corresponding to *T/P* ratios of 248–1446°C GPa⁻¹ (Figs 2b, 3c and 4a), and is characterized by a bimodal distribution dominated by both cold (<10°C km⁻¹) and hot (>27°C km⁻¹) metamorphic gradients (Fig. 3e). With the exception of six outliers, the second group yields metamorphic gradients of 11–46°C km⁻¹ corresponding to *T/P* ratios of 410–2000°C GPa⁻¹ (Figs 2b, 3c and 4a), and is characterized by a normal distribution dominated by hot metamorphic gradients of *c.* 27°C km⁻¹ (Fig. 3e). In the TNCC, metamorphic ages range from 467 to 237 Ma (Figs 2a and 3b; Table S1). Two metamorphic age groups can be distinguished: the Ordovician to early Carboniferous (467–340 Ma) and the late Carboniferous to early Triassic (325–237 Ma). With the exception of three outliers, the first group yields metamorphic gradients ranging from 6 to 27°C km⁻¹ corresponding to *T/P* ratios of 224–1000°C GPa⁻¹ (Figs 2b, 3d and 4b), and is characterized by a normal distribution dominated by warm metamorphic gradients of *c.* 16°C km⁻¹ (Fig. 3f). With the exception of three outliers, the second group yields metamorphic gradients ranging from 4 to 37°C km⁻¹, corresponding to *T/P* ratios of 157–1367°C GPa⁻¹ (Figs 2b, 3d and 4b), and is characterized by a bimodal distribution dominated by both cold (<10°C km⁻¹) and hot metamorphic gradients (>24°C km⁻¹) (Fig. 3f; Table S1).

## Paleozoic metamorphic records

### Metamorphic evolution of the MC

The semi-continuous metamorphic records of the MC range from the late Neoproterozoic to Triassic with a period of metamorphic quiescence in the late Silurian–early Devonian (Fig. 3a). The early Paleozoic metamorphic records are mainly characterized by clockwise *P*–*T* paths and penecontemporaneous low and high *T/P* metamorphism, although high *T/P* metamorphism is clearly dominant (Fig. 3c and e). In the Mongolian–Manchurian belts rimming the southern border of the MC it is exemplified by the *c.* 550–510 Ma *LT* eclogites and associated *HP* metamorphic rocks (Fig. 3c) formed during oceanic and continental subduction of the extended margin of the Mongolian Precambrian blocks (Štípská et al. 2010; Javkhlan et al. 2019; Zhu et al. 2023) represented by the *c.* 536 Ma age peak (Fig. 3a). In contrast, *c.* 530–500 Ma *HP* granulite-facies metamorphism of the Olkhon terrane in the Sayan–Baikal belt in the north (Donskaya et al. 2000; Gladkochub et al. 2008; Sklyarov et al. 2020; Li et al. 2023a) was followed by the *c.* 470–440 Ma *HT/LP* syn-extensional overprint (Fig. 3e) (Li et al. 2023b). Similar *HT/LP* conditions dated at *c.* 540–495 Ma were reported in the central part of the MC (Kozakov et al. 2008, 2021; Kozakov and Azimov 2017). Metamorphic conditions of eclogite and granulite from the MC correlate in both age and *P*–*T* conditions with those of the Khondalite belt described in NE China (Zhou et al. 2011, 2015, 2018). The *HT* metamorphic conditions in both domains culminated at *c.* 496 Ma (Fig. 3a) and are interpreted as a paroxysmal stage related to the collision of the Siberian craton with the MC (Gladkochub et al. 2008) or NE China blocks (Wilde 2015) (Fig. 2a). These metamorphic events resulted from a late Ediacaran to early Ordovician orogeny (Caledonian orogeny in Russian

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Tectonic maps of distribution of metamorphic rocks (coloured circles and squares) from the Mongolian and Tarim–North China collages in the CAOB (n = 180) showing (a) their peak metamorphic ages (Ma) and (b) thermobaric *T/P* ratios (°C GPa⁻¹). Image Google Earth and Landsat/Copernicus.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Relative probability density plots of the late Neoproterozoic to Triassic metamorphic rocks from (a) the Mongolian (n = 95) and (b) the Tarim-North China (n = 85) collages. The pie chart indicates the proportion of different geochronological methods used to calculate the relative probability density of metamorphic ages. Dating methods in the pie charts: 1, zircon U-Pb; 2, monazite U-Pb; 3, garnet Lu-Hf and Sm-Nd; 4, mica or amphibole (K)Ar-Ar; 5, whole-rock Rb-Sr; 6, titanite U-Pb. P-T diagrams of the late Neoproterozoic to Triassic metamorphic rocks from (c) the Mongolian and (d) the Tarim-North China collages. The colour coding represents peak metamorphic ages (Ma). Z, zeolite; P-P, prehnite-pumpellyite; BS, blueschist; E, eclogite; UHP, ultrahigh-pressure; GS, greenschist; A, amphibolite; E-A, epidote amphibolite; HPG, high-pressure granulite; GRA, granulite; UHT, ultrahigh-temperature. Metamorphic gradient (°C km⁻¹) boxplots and histograms of metamorphic rocks for different groups from (e) the Mongolian and (f) the Tarim-North China collages. For the box plot diagrams, the line across the box represents the median value of the data and ranges are the interquartile ranges (differences between third and first quartile, Q3-Q1). Source: four representative thermal gradients and fields for the standard metamorphic facies are modified after (Brown 2009).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Metamorphic age v. T/P ratio for metamorphic rocks from (a) the Mongolian (n = 95) and (b) the Tarim-North China (n = 85) collages. Both plots are contoured for kernel density.</figcaption></figure>

literature, Yarmolyuk *et al.* 2006; Kozakov *et al.* 2008) and will be referred to as Sayan-Baikal-Mongolia-Manchuria cycle hereafter.

In the Altai accretionary wedge of western Mongolia, c. 455 Ma MP/MT (Barrovian-type) metamorphism was followed by c. 400–370 Ma LP/HT (Buchan-type) metamorphic overprint (Soejono *et al.* 2021). In the eastern part of the Altai accretionary wedge, a metamorphic cycle was marked by Barrovian-type MP/MT supposedly of early Silurian age (Burenjargal *et al.* 2014) followed by late Devonian Buchan-type LP/HT metamorphism (Sukhbaatar *et al.* 2022). The Devonian HT metamorphic event was associated with regional magmatism and it is considered as related to massive melting of the Altai accretionary wedge (Hanžl *et al.* 2016; Jiang *et al.* 2016, 2019). This trend is interpreted as related to the tectonic switch from late Ordovician–early Silurian shortening and crustal thickening to important heating and Devonian thinning of the accretionary wedge above the retreating subduction zone (e.g. Kong *et al.* 2022). The syn-extensional metamorphic fabrics were later affected by early Carboniferous syn-compressional doming and heat advection to supracrustal levels related to a second phase of Buchan metamorphism (Broussolle *et al.* 2015; Jiang *et al.* 2022). Moreover, Nakano *et al.* (2015) suggested that this event was related to local Barrovian overprints. The latter two metamorphic events are well exemplified by the c. 381 and c. 356 Ma age peaks (Fig. 3a). Altogether, the c. 450–440 Ma Barrovian, c. 410–370 Ma Buchan and c. 350–340 Ma Barrovian–Buchan overprints (Broussolle *et al.* 2015; Aguilar *et al.* 2024) reflect a single orogenic cycle (Hercynian orogeny in Russian literature; Kozakov *et al.* 2002, 2019), referred to as the Altai cycle hereafter.

The late Paleozoic metamorphic records are characterized by clockwise and anticlockwise P-T paths both corresponding to high
T/P metamorphism (Fig. 3c and e). Both metamorphism and associated extensive magmatism are spatially constrained to the southern margin of the Altai accretionary wedge (Fig. 2a and b). This event is constrained by (1) poorly dated (c. 320 Ma) HP granulites (Li et al. 2022a) located at the southernmost edge of the Chinese Altai, (2) a broad c. 290–260 Ma HT–UHT granulite belt (Fig. 3a) (Tong et al. 2014; Liu et al. 2020a, b; Jiang et al. 2022) and (3) Buchan-type (cordierite–andalusite) overprints of previous metamorphic fabrics (Jiang et al. 2022; Aguilar et al. 2024). The (U)HT and Buchan-type metamorphism are interpreted to result from rapid switches between extensional and compressional episodes (Jiang et al. 2022; Sukhbaatar et al. 2022), both associated with massive alkaline magmatism (e.g. Broussolle et al. 2019). The compressional cycle was interpreted as reflecting the collision of the Altai accretionary wedge with the East Junggar and Trans-Altai terranes (Broussolle et al. 2018; Xiao et al. 2018; Li et al. 2021). In the eastern part of the MC, high T/P metamorphism recorded at c. 350–285 Ma in the Xilingol accretionary complex was interpreted as reflecting either oceanic subduction beneath the Mongolian terranes or post-orogenic extension (Li et al. 2017; J. Zhang et al. 2018a; Cao et al. 2022). This late Paleozoic event is reflected by several age peaks in the range of c. 293–259 Ma (Fig. 3a) and will be referred to as the South Altai cycle hereafter.

## Metamorphic evolution of the TNCC

In the TNCC, metamorphic records range from Middle Ordovician to Triassic (Fig. 3b). The early Paleozoic orogenic events are characterized by clockwise P–T paths corresponding to low to intermediate T/P metamorphism (Fig. 3d and f). Middle Ordovician to early Carboniferous metamorphism initiated with c. 462 Ma HT eclogite-facies metamorphism in the Beishan Orogen (e.g. Qu et al. 2011; Soldner et al. 2020) (Fig. 3b) and c. 453–449 Ma LP blueschist-facies metamorphism in the northern margin of the North China craton (de Jong et al. 2006). From the Silurian to Devonian, HP/HT amphibolite- to granulite-facies metamorphism affected the Dunhuang and Alxa blocks (e.g. Chen et al. 2015; Q. W. L. Zhang et al. 2020a; Soldner et al. 2022a) and the northeastern Tarim craton (Zhou 2004; L. Zhang et al. 2018c), whereas HP/LT eclogite formed in the Dunhuang block (Wang et al. 2017). Altogether, these metamorphic events culminated at c. 415 and c. 392 Ma (Fig. 3b). Soldner et al. (2022a) showed that the high T/P metamorphism was associated with the development of horizontal to steep orogenic fabrics that recorded a stage of moderate crustal thickening of a previously thinned active margin. Importantly, in the Dunhuang block and in the northern margin of the TNCC, intermediate to high T/P metamorphism operated over a period of c. 60 Myr from c. 385 to 330 Ma (Zhao et al. 2020; Li et al. 2022b; Si et al. 2022; X. Wang et al. 2022). This metamorphic event was associated with orthogonal structural reworking of all the previous metamorphic fabrics, and reflected a new and independent tectonometamorphic cycle (Soldner et al. 2022a, 2024). Altogether, the TNCC was affected by two principal orogenic compressional cycles marked by the prevalence of HP–MP and HT metamorphism: an early Paleozoic cycle from c. 465 to 390 Ma and a middle Paleozoic cycle from c. 385 to 330 Ma, corresponding to the amalgamation of the Dunhuang block, Alxa block and possibly the Tarim craton, and subsequent Andean cycle of the TNCC, respectively.

The late Carboniferous to Triassic metamorphism was mainly recorded along the western South Tianshan and eastern Solonker segments, with minor records in the southern Dunhuang block (Fig. 2a). They are characterized by penecontemporaneous low and high T/P metamorphism (Figs 3d, f and 4b), marked by clockwise P–T paths (Table S1). High T/P metamorphism represented by upper amphibolite- to granulite-facies conditions (Fig. 3b) mainly

occurred along the northern margin of the Tarim and North China craton in the late Carboniferous–Permian (Xia et al. 2014; Worthington et al. 2017; Y. Zhang et al. 2020b) and early Triassic (Zhang et al. 2016) (Fig. 2a and b). Low T/P metamorphism is best exemplified by the LT eclogite-facies rocks from the South Tianshan that are represented by the c. 315 Ma age peak (Fig. 3b) (Gao and Klemd 2003), although eclogite- and blueschist-facies metamorphism was also recorded in the northernmost North China craton in the late Carboniferous and early Triassic (Ni et al. 2006; Zhang et al. 2015). Late Paleozoic orogenic events have been attributed to the closure of the Palaeo-Asian Ocean, which led to the collision of the TNCC with the Kazakhstan and Mongolian collages in the west and east, respectively (Xiao et al. 2015). This dual metamorphic cycle will be referred to as Tianshan–Solonker cycle.

## Summary

In summary, three main features are revealed by the compilation of metamorphic P–T–t data in this study. First, new data support the contrasting nature of the metamorphic records in the TNCC as previously proposed by Soldner (2023): (1) intermediate to high T/P metamorphism in the early Paleozoic to the Carboniferous associated with two orthogonal orogenic cycles and (2) dual low and high T/P metamorphism characterizing the late Paleozoic (Figs 3d, f and 4b). In contrast to the TNCC, the compiled metamorphic data for the MC show dual low and high T/P metamorphism mainly from c. 550 to 495 Ma (Figs 3c, e and 4a). The metamorphic activity then migrated further south (in current coordinates) and was dominated by HT/LT Buchan-type metamorphism, with less important incursions of intermediate T/P metamorphism in the Ordovician and Carboniferous. However, the high T/P metamorph- ism remained the characteristic feature for the whole Paleozoic evolution of the MC (Figs 3c, e and 4a). The data compilation clearly reveals the contrasting spatial–temporal nature of the Paleozoic metamorphic trends of the TNCC and MC. For instance, the distribution of metamorphic rocks in the MC outlines a southwestward younging metamorphic age trend (in current coordinates) from the southern Siberian flank towards the Erqis fault (Fig. 2a). In contrast, the metamorphic records of the TNCC are mainly distributed along the roughly east–west-striking South Tianshan and Solonker suture zones of the northernmost Tarim and North China cratons, and Beishan and Alxa blocks, which have been active throughout the Paleozoic (Fig. 2a and b).

## Tectonic settings of Paleozoic T/P metamorphic evolution of the MC

An attempt will be made to attribute geodynamic and palaeogeо- graphical significance to individual metamorphic cycles described in this paper. This will be done via correlation of P–T and temporal trends with palaeogeographical reconstructions of Domeier and Torsvik (2014) and Domeier (2018).

### Sayan–Baikal–Mongol–Manchuria cycle (530–450 Ma)

Paleogeographical reconstructions at c. 500 Ma of Domeier (2018) indicate that the southern margins of the Amuria block and the eastern part of the MC (Baydrag, Zavkhan and Tuva–Mongol blocks) interacted with the Panthalassa Ocean to the north (Fig. 5a). This interaction started with continental subduction of the Mongolian basement blocks beneath marginal basins of the peripheral Panthalassa Ocean (e.g. Jiang et al. 2017) (Fig. 6). This event accounted for the formation of low T/P metamorphic rocks in the c. 1000 km long discontinuous subduction eclogite- bearing accretionary wedge along the southern margin of Mongolian Precambrian blocks (Štípská et al. 2010; Zhou et al.
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Position of the Mongolian and Tarim–North China collages at (a) c. 500 Ma, (b) c. 390 Ma, (c) c. 320 Ma and (d) c. 250 Ma. Simplified plate boundaries are shown as follows: red, convergent; blue, divergent; purple, transcurrent. Plate and terrane abbreviations: A, Alxa; Al-Qi-Q, Altyn–Qilian–Qaidam; Am, Amuria; An, Annamia; B, Beishan; Ba, Baltica; D, Dunhuang; KC, Kazakhstan Collage; MC, Mongolian Collage; NC, North China; SC, South China; Sib, Siberia; T, Tarim. Ocean abbreviations: Ju, Junggar Ocean; M-O, Mongol–Okhotsk Ocean; RAN, NE extension of the Rheic Ocean; Tu, Turkestan Ocean. Exterior and interior oceans are indicated in light and dark blue, respectively. Others: NP, North Pole; SP, South Pole. (e) Schematic tectonic map of the Mongolian and Tarim–North China collages and neighbouring continental blocks showing the location of representative metamorphic rocks and domains corresponding to distinct orogenic cycles. Source: positions of the collages are from the full-plate models of Domeier (2018) (DOM18) for the early Paleozoic and of Domeier and Torsvik (2014) (DT14) for the late Paleozoic, reconstructed using GPlates software of Boyden *et al.* (2011); the position of the Dunhuang block is from Soldner *et al.* (2020).</figcaption></figure>

2015; Zhu *et al.* 2023). In the rear part of the MC, the closure of small interior basins led to the collision of island arcs and microcontinents with the Siberian craton (Gladkochub *et al.* 2010; Donskaya *et al.* 2017) and to the formation of the HT/MP to LP Sayan–Baikal belt (Donskaya *et al.* 2000; Gladkochub *et al.* 2008; Li *et al.* 2023a). The duality of low and high T/P metamorphic rocks from the late Neoproterozoic to early Cambrian is a main feature of the amalgamation of the MC and reflects a major advancing stage of the Panthalassa oceanic subduction at the time (Figs 4a and 6). The second c. 500–450 Ma high T/P metamorphic event is typified by LP migmatization and Buchan-type overprinting of previous mineral assemblages in the Olkhon terrane and HT metamorphism

along the Khondalite belt in NE China (Kozakov *et al.* 2008, 2012; Zhou *et al.* 2018, 2011; Li *et al.* 2023b). Here, we propose that it reflects a major period of crustal extension and exhumation of partially molten lower crust (e.g. Yang *et al.* 2023) interpreted here as a result of retreat of the Panthalassa Ocean (Figs 4a and 6). This evolution is compatible with a model of a large mantle wedge at c. 500–440 Ma, defining a broad region of upper mantle above a stagnant slab in the mantle transition zone (Cui *et al.* 2024). In this model, the slab roll-back operated at that time east of the MC (in palaeo-coordinates in the study by Domeier (2018); Fig. 5a) and generated a large extensional and hot region affecting the whole collage in the terminal stages of this cycle (Fig. 6).

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Schematic tectonic cross-sections for the Mongolian (A–A') and Tarim–North China (B–B') collages from c. 550 to c. 250 Ma based on the palaeogeographical reconstructions of Figure 5. Distributions of representative metamorphic rocks are given. Plate and terrane abbreviations: KC, Kazakhstan Collage; MC, Mongolian Collage; NC, North China; Sib, Siberia; TNCC, Tarim–North China Collage. Ocean abbreviations: M-O, Mongol–Okhotsk Ocean; PAN, Panthalassa; PAO, Palaeo-Asian Ocean; Proto-TO, Proto-Tethys Ocean; PTO, Palaeo-Tethys Ocean; RAN, NE extension of the Rheic Ocean.</figcaption></figure>
## Altai cycle (450–340 Ma)

The 450–440 Ma period is characterized by incursion of low to intermediate *T/P* metamorphism affecting the Altai accretionary wedge (Soejono et al. 2021). This event is interpreted to reflect a transient crustal thickening related to the renewed advancing mode of the Panthalassa Ocean (Figs 4a and 6). From c. 400 Ma, the Altai accretionary wedge entered a c. 150 Myr period dominated by high *T/P* metamorphism and less importantly intermediate *T/P* metamorphism. These metamorphic records reflect short-lived cycles of extension (400–370 Ma) (Jiang et al. 2022; Kong et al. 2022) and compression (450–440 and 360–340 Ma) (Broussolle et al. 2015; Nakano et al. 2015; Aguilar et al. 2024), indicating that this wedge was either in a back-arc or forearc position at the time (Buriánek et al. 2022) (Figs 4a and 6). Altogether, the tectonometamorphic evolution of the Altai accretionary wedge culminating with Devonian magmatic flare-up reflected cycles of compression and extension that are attributed to tectonic switches typical for peripheral accretionary orogens (Collins 2002a; Collins and Richards 2008). For the Altai accretionary wedge, these events are interpreted as related to dynamics of retreating and advancing modes of the Panthalassa Ocean, the origin of which remains unknown (Figs 5b, c and 6). In this context, the predominantly high *T/P* metamorphism distributed between c. 800 and c. 1500°C GPa⁻¹ summarized in this study is a characteristic feature of the whole Altai cycle and compatible with such suprasubduction hot systems (Collins 2002b).

## South Altai cycle (300–260 Ma)

The South Altai reveals exclusively high *T/P* metamorphism. It was interpreted as reflecting a (U)HT event related to large-scale c. 300–290 Ma extension and associated crustal melting (Tong et al. 2014; Sukhbaatar et al. 2022). This event was related to the opening of a Carboniferous back-arc basin and break-up of the Ordovician crust of the Palaeo-Asian Ocean (Jiang et al. 2024) (Fig. 6). The subsequent c. 270–260 Ma compressional deformation led to the closure of the back-arc basin and extrusion of partially molten rocks and (U)HT granulites along narrow NW–SE-trending tabular zones (Broussolle et al. 2018; Guy et al. 2020), and development of wide cleavage fronts intruded by pegmatite dyke swarms responsible for a generalized high *T/P* metamorphic overprint (Jiang et al. 2019, 2022; Aguilar et al. 2024). Altogether, the South Altai cycle resulted from collision of the Altai accretionary wedge with the southerly Carboniferous East Junggar oceanic arc at HT conditions, indicating major perturbation of the mantle at these times (Hu et al. 2023). This cycle represented terminal stages of orogenic evolution related to the interaction of the MC with remnants of the Palaeo-Asian Ocean and its final closure during Permian times (Figs 5d and 6).

## Tectonic settings of *T/P* metamorphic evolution of the TNCC

In contrast to the MC, the region of the future TNCC was characterized by the existence of several interior oceans such as the NE extension of the Rheic Ocean and Proto-Tethys in the late Proterozoic to Cambrian as exemplified by palaeogeographical reconstructions of Domeier (2018) (Fig. 5a). This period was marked by the initiation of oceanic subduction, possibly of the Proto-Tethys Ocean, along the northern margins of the Beishan Orogen and Dunhuang block (e.g. Shi et al. 2018; Gan et al. 2020), while the Panthalassa Ocean subducted beneath the North China craton (Figs 5a and 6). However, the characteristic feature of the c. 530–500 Ma period is the absence of metamorphic records (Fig. 3d). Altogether, this is compatible with the suprasubduction

Cordilleran-type position of the continental blocks constituting the future TNCC continental assemblage, mostly located in the interior oceanic domains and along the margin of the exterior Panthalassa Ocean to the west of Gondwana (Fig. 5a).

## TNCC amalgamation cycle (465–390 Ma)

Between c. 465 and 390 Ma, the Beishan Orogen, the Dunhuang and Alxa blocks and the North China craton were affected by intermediate to high *T/P* metamorphism (Fig. 4b) that typifies the first orogenic cycle of the TNCC (Soldner 2023; Tang et al. 2024). The corresponding tectonic processes may encompass subduction to collision events similar to those operating along the eastern Gondwana margin (Foster et al. 2005), or a suprasubduction switch from thinning to thickening of thermally softened crust in an active margin setting (Soldner et al. 2022b). Recent tectonic and palaeogeographical reconstructions together with the absence of UHP records are in favour of progressive closure of small interior oceanic basins among the Gondwana-derived blocks (Li et al. 2018; Zhao et al. 2018), similar to the present-day SE Asia archipelago (Pubellier and Meresse 2013; Wakita et al. 2013). These processes affected the northern margins of all the constituent blocks of the TNCC until the Devonian (Han et al. 2016; Cope 2017), as exemplified by the northern active margins of the Tarim and North China cratons (Fig. 6) (Ma et al. 2019; Ning et al. 2023). Hence, the first metamorphic cycle can be interpreted as a result of amalgamation of the TNCC by a series of closures of interior oceanic domains and docking of continental blocks (Figs 5b and 6). Hot and previously stretched active continental margins were progressively thickened, starting with the Beishan Orogen at c. 460–440 Ma (Soldner et al. 2020), followed by the thickening of the Dunhuang block crust at c. 430–410 Ma (Soldner et al. 2022a) and of the northern Tarim at c. 410–400 Ma (Ning et al. 2023), eventually resulting in the final amalgamation of the TNCC (Fig. 5a and b).

## TNCC Andean cycle (385–330 Ma)

The late Devonian–Carboniferous tectonometamorphic cycle recorded by the amalgamated TNCC was characterized by the development of intermediate *P/T* metamorphism and ubiquitous vertical metamorphic fabrics associated with horizontal shortening and exhumation of previously buried rocks (Soldner et al. 2022a, 2024). This tectonometamorphic event is orthogonal to the one that characterized the TNCC amalgamation cycle and started in the Beishan Orogen at c. 438–429 Ma (Soldner et al. 2020; Li et al. 2023c) and continued in the Dunhuang block at c. 385–356 Ma (Soldner et al. 2022a, 2024) and in the northern Tarim at c. 370–360 Ma (Ning et al. 2023). However, recent studies of the Dunhuang block by Si et al. (2022) and Soldner et al. (2024) indicate that this tectonometamorphic event could have lasted at least until the late Carboniferous. The palaeogeographical reconstructions indicate that the TNCC Andean cycle was related to a major change of plate configurations at the time and possibly resulted from the establishment of two opposite Palaeo-Tethys and Panthalassa oceanic subduction zones surrounding the previously amalgamated ribbon-like TNCC in late Silurian to Devonian times (Fig. 5b). In the Carboniferous, this configuration was replaced by the two opposite Palaeo-Tethys and Palaeo-Asian Ocean subduction zones beneath the TNCC (Fig. 5c). As a result, the whole TNCC remained continuously in suprasubduction position implying hot metamorphic gradients (Figs 3d, f and 4b), with the opposite oceanic subductions probably exerting dominant horizontal compressive stress and contractional deformation from the Devonian to Carboniferous (Fig. 6).
## Tianshan–Solonker cycle (325–260 Ma)

In the palaeogeographical reconstructions of Domeier and Torsvik (2014), the early Carboniferous is marked by the interaction of the Palaeo-Asian Ocean to the north of the TNCC with the southern branch of the MC (Fig. 5c). This event coincided with rapid expansion of the Mongol–Okhotsk Ocean, which had previously opened between Siberia and Amuria in the Silurian–Devonian (Fig. 5c and d) (Domeier 2018; Zhu et al. 2024). During this period, the Tarim craton entered in collision with the Kazakhstan Collage (Han et al. 2011). The late Carboniferous UHP metamorphic rocks from the South Tianshan testify to this deep subduction and progressive exhumation of HP/UHP tectonic units typical of the interior Alpine–Himalayan systems (Zhang et al. 2019), and are related to the subduction of remnants of oceanic lithosphere of the Turkestan Ocean (Fig. 5b–d). Together with the contemporaneous high *T/P* metamorphism characterizing the late Paleozoic metamorphism of the South Altai cycle, these data show that the collision between the TNCC and the Kazakhstan Collage operated simultaneously with the subduction of the Palaeo-Asian Ocean beneath the MC. By the late Permian, interior oceanic domains of the western CAOB were closed and the Tarim craton, the Kazakhstan Collage and the westernmost MC were fully amalgamated and included in the Pangaea supercontinent (Fig. 5d). However, some palaeogeographical and palaeomagnetic reconstructions suggest that the Palaeo-Asian and Mongol–Okhotsk oceans remained open at the time (Fig. 5d) (Domeier and Torsvik 2014; Huang et al. 2018). In the Permian to Triassic, the Palaeo-Asian Ocean was gradually closed eastwards between the easternmost TNCC and Amuria (Eizenhöfer and Zhao 2018; T. Wang et al. 2022) (Figs 5d and 6). This explains the early Permian to Triassic intermediate to high *T/P* metamorphism and highlights the similarities between the tectonothermal evolutions of the opposite active margins of these continental blocks (Figs 2, 5e and 6).

## Contrasting Paleozoic tectonometamorphic evolutions of the Mongolian and Tarim–North China collages

The compiled metamorphic *P–T–t* data reveal two contrasting tectonic modes of construction for the TNCC and the MC in the Paleozoic. As proposed by Soldner (2023), the data support a two-stage evolution of the TNCC with Ordovician to Carboniferous *HT*–*HP* suprasubduction metamorphic events corresponding to the construction and Andean-type evolution of the collage and late Paleozoic UHP metamorphism related to the closure of the Palaeo-Asian Ocean followed by the collision of the TNCC and Kazakhstan Collage in the west and concomitant *HT* metamorphic events related to the continuous subduction of the Palaeo-Asian Ocean in the east (Figs 5e and 6). The linear spatial distribution of Paleozoic metamorphic records along the northern margin of the TNCC highlights the fact that these margins were subject to continuous oceanic subduction and crustal reworking throughout the Paleozoic. Based on this fundamental distinction, it can be argued that the tectonothermal evolution of the TNCC displays features of suprasubduction (TNCC amalgamation and Andean cycles) and interior-collisional (Tianshan–Solonker cycle) orogens, which correspond to three different and successive orogenic phases (Fig. 5e). In contrast, the data reveal a one-stage evolution of the MC defined by ubiquitous *HT* to *UHT* suprasubduction metamorphism associated with subduction-related low *T/P* metamorphism outboard of the collage during the Baikal–Sayan–Mongolia–Manchuria cycle (Figs 5e and 6a–d). The Altai cycle reflects tectonic switching related to alternating Barrovian- and Buchan-type overprints whereas the South Altai cycle is generally related to (U)*HT* conditions for both extensional and compressional

events (Fig. 5e). The latter cycle may represent a spectacular far-field response to the Tianshan–Solonker cycle (Figs 5c–e and 6).

Altogether, the Paleozoic tectonometamorphic evolution shows remarkable differences between the two collages. Whereas the MC reflected recurrent tectonic switches related to the subduction of the Panthalassa Ocean, the TNCC showed tectonic–metamorphic cycles mostly related to recurrent subductions of interior oceans beneath active margins of continental blocks until the late Paleozoic. The latest Tianshan–Solonker and South Altai cycles are penecontemporaneous but very different in *T/P* types of metamorphism, thereby expressing the complex role of relics of the Palaeo-Asian lithosphere landlocked between the TNCC and MC and Kazakhstan Collage.

## Conclusions

(1) The TNCC is characterized by intermediate to high *T/P* metamorphism related to the amalgamation and Andean suprasubduction cycles in the early Paleozoic. In the late Paleozoic, low and high *T/P* metamorphism reflected a subduction–collisional cycle related to subduction and closure of the Palaeo-Asian Ocean.

(2) The MC is characterized by dual low and high *T/P* metamorphism related to the Baikal–Sayan–Mongolia–Manchuria cycle in the early Paleozoic. The Altai and South Altai cycles were marked by the prevalence of high *T/P* metamorphism in the late Paleozoic. The MC displays features typical of peripheral accretionary style reflecting recurrent tectonic switches.

(3) The progressive northward migration of terranes towards the Siberian landmass responsible for the assembly of eastern Pangaea was completed via contrasting tectonometamorphic low *T/P* Tianshan–Solonker and high *T/P* South Altai cycles. These coeval but thermally contrasting events point to the complex thermomechanical role of relics of the Palaeo-Asian lithosphere landlocked between the TNCC, MC and Kazakhstan Collage.

Scientific editing by José Ramón Martinez Catalan

**Acknowledgements** Constructive comments from Dmitrii Gladkochub and Min Sun to the earlier version of this manuscript are gratefully acknowledged. We thank José Ramón Martinez Catalan for his editorial work.

**Author contributions** *JS:* conceptualization (equal), data curation (lead), formal analysis (equal), funding acquisition (equal), investigation (equal), writing – original draft (equal); *KS:* conceptualization (equal), funding acquisition (equal), investigation (equal), writing – original draft (equal); *PS:* conceptualization (supporting), data curation (equal), formal analysis (equal), investigation (equal), writing – original draft (equal); *YJ:* investigation (supporting).

**Funding** This work was financially supported by project No. 2021/43/P/ ST10/02996 co-funded by the National Science Centre and the EU H2020 research and innovation programme under MSCA GA No. 945339. For the purpose of open access, the author has applied a CC-BY public copyright licence to any author accepted manuscript (AAM) version arising from this paper. K.S. and P.S. acknowledge the support of the Czech Science Foundation (grant number 19-27682X to K.S.) and of an internal grant of the Czech Geological Survey (number 329805 to K.S.).

**Competing interests** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Data availability** The datasets generated during and/or analysed during the current study are available in the supplementary material of this article.
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