U-Pb ages and Hf isotope data from detrital zircons in the metasedimentary rocks of the Mongolian Altai Group, Mongolian part of the Altai-Mongolian Terrane: implications on the provenance and tectonic setting

Abstract

In this paper, we present the new results of the U–Pb age dating and Lu-Hf isotopic analysis of detrital zircons of the four representative metasedimentary rock samples from the Mongol Altai Group, Mongolian part of the Altai-Mongolian terrane. Our new results indicate that the metasedimentary rocks of the Mongol Altai Group were formed after ∼497 Ma, Late Cambrian and deposited during the Early-Middle Ordovician. The detrital zircons of four samples yield a two major age peaks at 503–517 Ma, and 775–843 Ma, respectively, with minor involvement of older zircons. The nearby Lake Zone of Ikh-Mongol Arc most likely provided plenty of Early Paleozoic materials, the subdominant Neoproterozoic detrital zircons could be supplied by the felsic intrusions along the western margin of the Tuva-Mongol microcontinent, and the sparse older zircons may be derived from its basement. With combination of previous studies in the Chinese Altai, Russian Altai and Hovd terrane, our data suggest that the Altai–Mongolian terrane possibly represents a coherent continental arc-accretionary prism system built upon the active margin of the western Mongolia during the Cambrian to Ordovician. Moreover, the dominant Neoproterozoic to Early Paleozoic detrital zircons from the Mongol Altai sequence yield largely varied εHf(t) values from −17.4 to +12.0, indicating that input juvenile material and reworking of crustal components are both important in the accretionary orogenesis. A compilation of U–Pb and Hf isotope data of detrital zircons shows that the source area underwent two most extensive magmatic activities at ca. 470–574 Ma and 687–967 Ma, respectively.

1. Introduction

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens worldwide, formed from Neoproterozoic to Mesozoic times by continuous accretion of various terranes (Şengör et al. 1993; Windley et al. 2007). It can be subdivided into two main parts: the eastern Mongolian and western Kazakhstan collage systems, which consist of Precambrian ribbon continents and Late Proterozoic to Early Paleozoic arcs, back-arcs and accretionary complexes (Şengör & Natal’in, 1996; Xiao et al. 2018). These vast orogenic systems are limited in the south by a collage of amalgamated small continental blocks, Paleozoic arcs and accretionary complexes north of the Tarim and North China cratons (Fig. 1).

Figure 1. Tectonic map of the Mongolian collage system and the adjacent Kazakhstan collage constituting the central part of the CAOB (modified from Sengör et al., 1993; Kröner et al., 2010; Guy et al., 2020).

The Altai-Mongolian terrane, one of the representative tectonic units in the Mongolian collage, plays an important role in reconstructing the evolution history of the CAOB. This ∼1000 km long and up to 250 km wide terrane or microcontinent extends southwards from the Russian Altai in Siberia, via the Mongolian Altai in Mongolia, to the Chinese Altai in China (Windley et al. 2007; Chen et al. 2014).

The Chinese Altai comprises mainly Cambrian to Silurian turbiditic and pyroclastic/volcanic sequence (Habahe and Kulumuti groups), and Devonian to Carboniferous low-grade metasedimentary and metavolcanic rocks (Windley et al. 2002; Long et al. 2007, 2008; Sun et al. 2008, 2009; Cai et al. 2011a). The Russian Altai consists of mainly thick middle Cambrian to early Ordovician greenschist-facies metasandstones and meta-siltstones, and transgressively overlying by middle Ordovician to early Silurian marine sediments and Devonian volcaniclastic sedimentary rocks (Buslov et al. 2001; Daukeev et al. 2008; Chen et al. 2014a, 2014b, 2015, 2016). The intrusive rocks in the Russian and Chinese Altai are dominated by early–middle Paleozoic and minor late Paleozoic to Mesozoic granitic plutons (Windley et al. 2002; Wang et al. 2006; Yuan et al. 2007; Sun et al. 2008; Cai et al. 2011b; Glorie et al. 2011).

The Altai-Mongolian terrane was previously considered a Precambrian microcontinent based on some old whole-rock Sr-Nd isotopic data of supracrustal rocks exposed in the Russian and Chinese Altai (Mossakovsky et al. 1994; Dobretsov et al. 1995; Hu et al. 2000; Buslov et al. 2001; Windley et al. 2002; Li et al. 2006; Yang et al. 2011). However, some geochronological studies revealed that the gneissic granitic rocks in the Chinese Altai were actually formed in ca. 480–450 Ma (Wang et al. 2006; Briggs et al. 2007; Sun et al. 2008; Cai et al. 2011 b) and thus do not support a Precambrian origin.

The geochronological studies from Chinese Altai show that these low-grade flysch-like sedimentary sequences, which grouped into the Habahe Group and their high-grade metamorphic equivalent-paragneiss, were deposited in an active continental margin or a continental arc setting prior to the Middle Ordovician in the Eastern Chinese Altai and between the Early Silurian and Early Devonian in the north-western Chinese Altai (Long et al. 2007, 2008, 2010; Jiang et al. 2011). The provenance of the metasedimentary rocks in the Chinese Altai was dominated by Cambrian to Early Ordovician igneous rocks, with subordinate Neoproterozoic and minor Paleoproterozoic and Archean crustal materials (Long et al. 2007, 2008, 2010). Moreover, the recent study shows that the meta-sedimentary sequences in the southern part of Russian Altai were deposited in the Early–Middle Ordovician in an active continental margin, with sediment sources mainly from the Neoproterozoic to Early Paleozoic magmatic rocks in the Tuva-Mongolian block and adjacent island arcs in western Mongolia in the Early Paleozoic (Chen et al. 2014). A similar conclusion has also been made for the high-grade paragneisses in the Tseel terrane southeastern extension of the Chinese Altai in western Mongolia (Jiang et al. 2012). Synthesizing all data from the Russian and Chinese Altai and Tseel terranes, arc-accretionary prism was built upon the active margin of western Mongolia in the Early Paleozoic (Jiang et al. 2017).

In the Mongolian part of the Altai-Mongolian terrane, Early Paleozoic flysch deposits are widely distributed and grouped into the ‘Mongolian Altai Group’ (Tomurtogoo et al. 1998). Although the lithological characters of the Mongolian Altai Group are quite similar to those of Chinese Altai and Russian Altai, the depositional age, provenance and tectonic setting are poorly constrained and still under debate.

The Mongolian Altai Group was originally assigned a Middle Cambrian-Early Ordovician (Dergunov et al., 1980; 2001; Tomurtogoo et al. 1998), but some workers favoured Cambrian or Middle-Upper Cambrian ages (Badarch et al. 2002; Tovuudorj & Sumya, 2008; Erdenechimeg et al. 2018) without precise geochronological data. High-resolution geochronological data were reported by Long et al. (2019) Sukhbaatar et al. (2022) and Soejono et al. (2018) only from the southern and western parts of the Mongolian Altai. Based on the detrital zircon U-Pb age dating of five metasedimentary rocks, these authors proposed that the sedimentary sequence in the southern part of Mongolian Altai was deposited during the Late Silurian and Early Devonian on an active continental margin, which is consistent with the Habahe group in the western Chinese Altai (Long et al. 2007, 2010, 2012; Dong et al. 2018). However, this conclusion cannot be used for the whole Mongolian Altai without studying the northern part, where almost all key sections of the Mongolian Altai Group were reported.

Therefore, in order to fill this gap, we have conducted detailed field investigations and sampling for geochronological and geochemical analyses of the metasedimentary sequences of the Mongolian Altai Group exposed in the northern part of the Mongolian Altai. In this paper, we present the new results of the U-Pb age dating and Lu-Hf isotopic analysis of detrital zircons from the metasedimentary rocks of the Mongolian Altai Group with the aim of constraining the depositional ages, provenances and tectonic setting in order to reconstruct the Early Paleozoic evolution of the orogenic belt.

2. Geology of the study area

The Mongolian Altai or so-called Mongolian Altai Fold System (Tomurtogoo, 2014, 2017), located in western Mongolia, extends to the west and north into China, Kazakhstan and Russia (Dergunov et al. 1980; Dobretsov et al. 1995; Badarch et al. 2002; Tomurtogoo, 2002; Tomurtogoo, 2012, 2014; 2017). According to the latest tectonic subdivision of Mongolia, the Mongolian Altai is structurally subdivided into five major tectonic units, namely the Southern Altai, Ulgii, Khovd, Tsagaanshiveet and Bodonch terranes, which converged probably in the Early Ordovician (Tomurtogoo, 2014). The Tolbonuur Fault defines the boundary between Southern Altai and Ulgii terranes, while the Khovd and Bairam Faults constitute the boundary between Ulgii/Khovd and Khovd/Tsagaanshiveet terranes, respectively (Fig. 2a). The Southern Altai Terrane (Altai Terrane by Badarch et al. 2002) makes up the most extreme southwest high-altitude part of the Mongolian Altai Range. According to the State Geological Map with 1:200000 scale, the terrane consists of basalt, mafic tuff, limestone of the Upper Neoproterozoic–Lower Cambrian Zamt Formation (NP₃–Ɛ₁zm), quartzite, metasandstone and metasiltstone of the Lower Cambrian Uzuurtolgoi Formation (Ɛ₁uz), calcareous and siliceous metasandstone, metasiltstone and minor metaconglomerate of the Lower-Middle Cambrian Khovdgol Formation (Ɛ_1-2kb), and thick monotonous terrigenous sandstone-siltstone of the Middle-Upper Cambrian Mongolian Altai Group (Tovuudorj et al. 2008) (Fig. 2b).

Figure 2. (a) The tectonic map of Mongolia (Tomurtogoo, 2014), showing the location of the Mongolian Altai Orogenic System. (b) Simplified geological map of the Southern Altai terrane (modified after Tovuudorj & Sumya, 2008).

The terrane is overlain by Silurian, Devonian and minor Lower Carboniferous shallow-marine sedimentary and volcanic rocks and intruded by Upper Ordovician, Early Devonian and Upper Carboniferous granite plutons (Fig. 2b). Among them, sandstone-siltstone series rocks of the Mongolian Altai Group are characterized by wide distribution and are usually compared with the ‘Gorny-Altai’ series, determined in the adjacent area of the Russian Altai (Dergunov et al. 1980; Chen et al. 2014, 2016) and with the Habahe Group in the Chinese Altai (Sun et al. 2008; Long et al. 2007, 2008, 2010; Jiang et al. 2012).

The Mongolian Altai Group extensively outcrops in the northern part of the Mongolian Altai and is stratigraphically divided into the Maikhant (Ɛ_2-3mh), Borburgas (Ɛ_2-3br), Tsengel (Ɛ_2-3cn) and Jivert-Uul (Ɛ_2-3zv) Formations from bottom to top based on geological correlation (Fig. 2b).

The Maikhant Formation is well exposed along the left and right banks of the River Khovd and Bor Us, Khoton Khargai, Serge Zurkh, Khovd and Bayanzurkh Mountains. According to the field observation, the Maikhant Formation is composed mainly of intercalation of greenish grey and greenish fine- to medium-grained metasandstone, metasiltstone, schist and minor calcareous sandstone, and liliaceous metasiltstone (Figs. 3a and b). The sediments are scrutinized by numerous discontinuous faults, generally dipping 30–50° to the northeast, and occur as monoclinal structures.

Figure 3. Field photographs showing constituent metasedimentary rocks from the Mongolian Altai Group, northern part of the Altai terrane, Western Mongolia. (a, b) fine- to medium-grained quartz metasandstone, metasiltstone (Maikhant Formation). (c, d) fine to medium-grained feldspar-quartz sandstone and metasiltstone (Borburgas Formation). (e, f) liliaceous siltstone with interlayers of sandstone (Tsengel Formation). (g, h) medium- to coarse-grained sandstone, and siltstone (Jivert-Uul Formation).

The Borburgas Formation is characterized by lesser distribution and studied along the right bank of the River Khovd, the southern part of the Ikh-Khar Lake, Gurvan Khairkhant and the Tashint Mountains area. The formation consists of an alternation of greenish and greenish-grey fine to medium-grained feldspar-quartz sandstone, quartz sandstone and metasiltstone, minor schist and lens of metaconglomerate (Figs. 3c and d). The formation is bounded by a northwest-oriented tectonic fault with sedimentary rocks from the Maikhant and Tsengel formations. The sediment is dipping 35–40° to the east and southeast.

The Tsengel Formation is studied along the right and left banks of the Rivers Khargant, Khatuu, Tsagaan and Nuraat Mountain. The formation in these areas consists of greenish to liliaceous siltstone with interlayers of sandstone and a minor conglomerate lens (Figs. 3e and f). The siltstone has a rhythmic-bedded structure and is micro-folded. The sediment is dipping 40–60° to the northwest and is cut by granites of the Late Devonian Altai Complex.

The uppermost Jivert-Uul Formation is studied around the Balgant Mountain area. In this area, about 300 m-thick greenish grey and greenish medium- to coarse-grained sandstone interbedded with siltstone belong to the Mongolian Altai Group (Figs. 3g and h). Lithology of succession is quite identical to those of the Tsengel and other Formations from Mongolian Altai Group. They have undergone intense horizontal compression and folding, and have been crosscut by 0.1–0.5 cm late-stage quartz veins, with displacement along faults.

A total of seventy metasedimentary samples were collected from a Paleozoic sequence in the northern part of the Altai terrane, Western Mongolia (Fig. 2b). Among these samples, four were prepared for detrital zircon U-Pb dating and Lu-Hf isotopic analysis.

3. Sample locations and descriptions

Based on detailed geological field work, we selected four representative samples from the metasedimentary strata, which are mapped as the Middle-Upper Cambrian Maikhant, Borburgas, Tsengel and Jivert-Uul Formations of the Mongolian Altai Group, respectively, for detrital zircon U-Pb dating and Lu-Hf isotopic analysis. Sample locations are shown in Figure 2b, and a short description of the rocks is given below.

3.a. Metasandstone sample M21-1079/1

Sample M21-1079/1 was collected from the north-western part of the Gurvan Khairgant Mountain, right bank of the Khovd River (GPS location: 48° 50′ 49.7″ N; 88° 59′ 42″ E). It is a greenish-grey, medium-grained metasandstone with a psammitic structure, belonging to the Maikhant Formation. The rock consists of quartz 30–35 vol.%, feldspar (10–15 vol.%), epidote (5 vol.%), biotite and muscovite (5 vol.%) and lithic fragments (10–15 vol.%) in a fine-grained matrix of quartz, chlorite and sericite (30–35 vol.%). The grains tend to be moderately well-rounded and moderately sorted. Lithic fragments are quartzite and schists in composition (Fig. 4a).

Figure 4. Representative photomicrographs of the samples used for detrital zircon U-Pb geochronology. (a) Medium-grained metasandstone sample M21-1079/1; (b) Medium-grained metasandstone (sample M21-2062); (c) Medium-grained schistose sandstone (sample M21-1061); (d) Medium-coarse-grained metasandstone (sample M21-1063/1). Legend: Q-quartz, Pl-plagioclase, Epi-epidote, Tur-tourmaline, Py-pyrite, Bi-biotite, Ser-sericite, rock fragments: Qrtz-quartzite.

3.b. Metasandstone sample M21-2062

Sample M21-2062 was collected from the south-eastern part of the Ust Uzuur Mountain (GPS location: 48° 51′ 7.7″ N; 89° 16′ 45.4″ E). It is a medium-grained metasandstone belonging to the Borburgas Formation. The metasandstone is psammitic in structure and moderately sorted and consists of sub-rounded and rounded clasts of quartz 30–35 vol.%, feldspar (15–20 vol.%), epidote (5–10 vol.%), opaque mineral (5–10 vol.%), minor biotite and lithic fragment (10–15 vol.%) in a fine-grained matrix of quartz, chlorite, sericite and clayey materials (15–20 vol.%) (Fig. 4b).

3.c. Metasandstone sample M21-1061

Sample M21-1061 is a medium-grained metasandstone from the Tsengel Formation, collected north-eastern part of the Jivert Mountain (GPS location: 48° 46′ 0.2″ N; 89° 18′ 43.8″ E). The metasandstone is moderately sorted and slightly metamorphosed, which is represented by schistose structure. The sample consists of moderately rounded clasts of quartz 10–15 vol.%, feldspar (25–30 vol.%), carbonate (5 vol.%), minor amphibole (hornblende) and lithic fragments (15–20 vol.%) in a fine-grained matrix of quartz, chlorite and sericite (35–40 vol.%). Accessory mineral is represented by zircon grains (Fig. 4c).

3.d. Metasandstone sample M21-1063/1

Sample M21-1063/1 is a metasandstone from the strata, which was mapped as the Jivert-Uul Formation. A sample was taken from the south-eastern part of the Balgant Mountain (GPS location: 48° 46′ 43.3″ N; 89° 17′ 8.4″ E). The metasandstone is psammitic in structure and moderately sorted and consists of moderately well-rounded quartz 40–45 vol.%, feldspar (10–15 vol.%), epidote (5–10 vol.%), minor tourmaline, opaque minerals and lithic fragment (15–20 vol.%) in a fine-grained matrix of quartz, chlorite, sericite and clayey materials (15–20 vol.%) (Fig. 4d).

4. Analytical methods

Rock samples were crushed in a steel jaw crusher. A shaking bed and heavy liquid techniques were used to separate heavy minerals. Zircon grains from the heavy mineral fraction by hand-picking under a binocular microscope. About 300 grains were selected from each sample and mounted on adhesive tape, then enclosed in epoxy resin and polished to about half of their diameter. After being photographed under reflected and transmitted light, the samples were prepared for cathodoluminescence (CL) imaging, U-Pb dating and Hf isotope analysis.

4.a. Zircon CL imaging and U-Pb dating

Prior to analysis, CL imaging of the zircons was performed using an FEI PHILIPS XL30 SFEG instrument with a 2-min scanning time at 15 kV and 120 nA at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences (CAGS), Beijing, China.

U-Pb dating of zircon was conducted by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Sincere spectrum Detection Technology (Langfang) Co., Ltd., China. Experiments were carried out on the Analytik Jena AG PQMS030 elite ICP-MS instrument (Germany) in combination with an excimer 193 nm laser ablation system (NewWave, NWR193). All LA-ICP-MS measurements were carried out using time-resolved analysis in fast, peak jumping mode. Each spot analysis consisted of approximately 30s background acquisition followed by 30s data acquisition from the sample. The excimer laser system is equipped with apertures, which image the laser beam onto the sample surface. This optical configuration allows the selection of a constant fluence that is independent of crater diameters. Helium was used as a carrier gas. Details of the instrumental operating conditions and measurement parameters are reported in Tables 1 to 4.

Table 1. U–Pb data for zircons from metasandstone in the Maikhant Formation (Sample M21-1079/1)

Disc.(%) = 100* (207Pb/235U age)/(206Pb/238U age)) -100.

Table 2. U–Pb data for zircons from metasandstone in the Borburgas Formation (Sample M21-2062)

Disc.(%) = 100* (207Pb/235U age)/(206Pb/238U age)) -100.

Table 3. U–Pb data for zircons from metasandstone in the Tsengel Formation (Sample M21-1061)

Disc.(%) = 100* (207Pb/235U age)/(206Pb/238U age)) -100.

Table 4. U–Pb data for zircons from metasandstone in the Jivertuul Formation (Sample M21-1063/1)

Disc.(%) = 100* (207Pb/235U age)/(206Pb/238U age)) -100.

The spot size and frequency of the laser were set to 35 µm and 13Hz, respectively, in this study. NIST SRM610 was used to yield the highest sensitivity, the lowest oxidation and background and a stable signal to achieve optimal conditions. Setting a 15-ms dwell time for ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb ²⁰⁸Pb, ²³²Th and ²³⁸U. Zircon Plesovice (Sláma et al. 2008) and Qinghu (Li et al. 2013) were used as external standards for U-Pb dating and blind samples for monitoring the instrument condition, respectively, and were measured twice every five samples analyzed. Approximately 30 seconds of background data acquisition were conducted for each analysis. Data reduction was made using GLITTER 4.0 software (Macquarie University) and plotted by Isoplot 4.15 (Ludwig, 2003). Concentration values of NIST SRM 610 used for external calibration were taken from (Pearce, 1997).

4.b. Hf isotope analysis

Zircon Lu-Hf isotope analyses were performed using a Newwave UP213 laser ablation microprobe attached to a Neptune multicollector ICP-MS at the Institute of Mineral Resources, CAGS, Beijing. The instrumental conditions and data acquisition follow those described by Hou et al. (2007). A stationary spot with a beam diameter of either 40 μm or 55 μm was used, depending on the size of the ablated domains. The gas, in combination with Ar, was used as the carrier gas to transport the ablated sample from the laser ablation cell to the ICP-MS torch via a mixing chamber. To correct for the isobaric interferences of ¹⁷⁶Lu and ¹⁷⁶Yb with ¹⁷⁶Hf, ¹⁷⁶Lu/¹⁷⁵Lu = 0.02658 and ¹⁷⁶Yb/¹⁷³Yb = 0.796218 ratios were determined (Chu et al. 2002). For instrumental mass bias corrections, Yb isotope ratios were normalized to a ¹⁷²Yb/¹⁷³Yb ratio of 1.35274 (Chu et al. 2002) and Hf isotope ratios to a ¹⁷⁹Hf/¹⁷⁷Hf ratio of 0.7325 using an exponential law. The mass-bias behaviour of Lu was assumed to follow that of Yb. The mass bias correction protocols were performed as described by Hou et al. (2007). The zircon GJ1 was used as the reference standard, with a weighted mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282008 ± 27 (2σ) during our routine analyses. This ratio is not distinguishable from a weighted mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282013 ± 19 (2σ), according to in situ analysis by Elhlou et al. (2006). The calculation of the Hf model age (single-stage model age; T_DM) is based on a depleted-mantle source with a modern ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.28325 and the ¹⁷⁶Lu decay constant of 1.865 × 10⁻¹¹ year⁻¹ (Scherer et al. 2001). The calculation of the ‘crust’ (two-stage) Hf model age (T_DM ^C) is based on the assumption of a mean ¹⁷⁶Lu/¹⁷⁷Hf value of 0.011 for average continental crust (Wedepohl, 1995). The calculation of εHf(t) values was based on zircon U-Pb ages and chondritic values (¹⁷⁶Hf/¹⁷⁷Hf = 0.282772, ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332; Blichert-Toft & Albarede, 1997).

5. Results

The U-Pb ages and Hf isotope compositions of the zircons from four analyzed metasandstone samples are given in Tables 1 to 5, respectively.

5.a. Metasandstone from the Maikhant formation (Sample M21-1079/1)

A total of one hundred and ten zircon grains were analyzed. Zircons from sample M21-1079/1 are colourless, with euhedral to subhedral shapes. They range in length from ca. 80 to 150 μm, with length-to-width ratios of 1.5:1 to 3:1. In CL images, most grains display oscillatory growth zoning, indicating a magmatic origin, whereas the few rims are overgrowth formed during the migmatization (Hanchar & Hoskin, 2003) (Fig. 5a).

Figure 5. Representative cathodoluminescence (CL) images of zircons from the Mongolian Altai Group. Scale bar = 100µm.

The Th/U ratios of zircons vary between 0.1 and 1.2 (except for M21-1079_77, 36, 90 < 0.1). The high Th/U ratios of zircons (>0.1) and observed oscillatory growth zoning in CL images suggest that the most detrital zircons were derived from a magmatic provenance (Koschek, 1993).

From one hundred and ten analyses performed in the metasandstone M21-1079/1, ninety-seven yield concordant ages ranging from 470 ± 10 to 1970 ± 39 Ma (Fig. 6a). The obtained ages are mainly clustering at 470–530 Ma, 744–967 Ma and 1944–1970 Ma, with the prominent age peak at 509 Ma (n = 74; 76 %), 775 Ma (n = 17; 18 %), and minor peaks at 1589 Ma (n = 4; 4 %), and 1959 Ma (n = 2; 2 %), respectively (Fig. 6b). The youngest coeval zircon grains between 470 ± 10 and 485 ± 11 Ma yield a weighted mean age of 474.0 ± 6.1 Ma (MSWD = 1.49, n = 10) (Fig. 6c).

Figure 6. U-Pb concordia and distribution diagrams for detrital zircons of metasandstone from the Mongolian Altai Group. The 206Pb/238U and 207Pb/206Pb ages are used for zircons younger than 1000 Ma and older than 1000 Ma, respectively. Each sample is shown on a separate diagram. See Tables 1–4 for a list of zircon ages.

Sixteen representative zircons were analyzed for Hf isotope compositions. Twelve Cambrian and Ordovician zircons give ¹⁷⁶Hf/¹⁷⁷Hf values varying from 0.281975 to 0.282671. The εHf(t) value and T_DM ^C model ages range from −17.4 to +4.9 and from 1.0–2.5 Ga, respectively. Other two Neoproterozoic zircons have ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of 0.282093–0.282113 and negative Ɛ_Hf(t) values ranging from −7.3 to −3.8. Their T_DM ^C model ages vary from 2.0 to 2.1 Ga. The rest two Mesoproterozoic and Paleoproterozoic zircons have ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of 0.280947–0.281848 and Ɛ_Hf(t) values ranging from −30.8 to +1.7 (Fig. 7a). The T_DM ^C model ages range from 2.2 to 4.1 (Table 5).

Figure 7. Diagram of ƐHf(t) values versus crystallizing ages for zircons from the Mongolian Altai Group metasandsones. The 206Pb/238U ages are used for zircons younger than 1000 Ma, and 207Pb/206Pb ages are used for zircons older than 1000 Ma.

Table 5. Lu-Hf data for zircons from the metasandstone in the Mongol Altai Group

^aT ^C_DM = t + (1/λ) * ln[1 + ((¹⁷⁶Hf/¹⁷⁷Hf)_S,t - (¹⁷⁶Hf/¹⁷⁷Hf)_DM,t)/((¹⁷⁶Lu/¹⁷⁷Hf)_UC - (¹⁷⁶Lu/¹⁷⁷Hf)_DM)], where UC, S and DM are the upper continental crust, the sample and the depleted mantle, respectively. The ²⁰⁶Pb/²³⁸U ages are used for zircons younger than 1000 Ma, and ²⁰⁷Pb/²⁰⁶Pb ages are used for zircons older than 1000 Ma.

5.b. Metasandstone from the Borburgas Formation (Sample M21-2062)

A total of one hundred and ten zircon grains were analyzed and twenty-nine analyses for this sample plot below the concordia curve, suggesting a loss of Pb. In the CL images of sample M21-2062, most zircon cores have oscillatory zoning and rounded shapes edge, and 60 – 140 μm, with length-to-width ratios of 1.5:1 to 2.5:1 (Fig. 5b). The zircons display typical oscillatory zoning, and Th/U ratios range from 0.1 to 0.9, indicating a magmatic origin. However, a few zircon grains have Th/U ratios ranging from 0.01 to 0.09 (Table 2), indicating a metamorphic origin.

Analyses on the zircons with oscillatory zoning give concordant U–Pb ages between 456 ± 10 and 3230 ± 33 Ma (Fig. 6d). The ages are mainly clustering at 456–574 Ma, 718–882 Ma, 1012–1335 Ma and 2380–3230 Ma, with the prominent age peak at 517 Ma (n = 44; 54 %), 781 Ma (n = 18; 22 %), and minor peaks at 1299 Ma (n = 7; 9 %), 1921 Ma (n = 4; 5 %) and 2887 Ma (n = 8; 10 %), respectively (Fig. 6e). The youngest coeval zircon grains between 456 ± 10 and 514 ± 11 Ma yield a weighted mean age of 496.6 ± 6.8 Ma (MSWD = 2.53, n = 21) (Fig. 6f).

Fourteen representative zircons were analyzed for Hf isotope compositions. Eight Cambrian and Ordovician zircons have ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of 0.282260–0.282379 and their Ɛ_Hf(t) values ranging from −7.6 to −2.9 (except for M21-2062_74 = −54.9). Their T_DM ^C model ages range from 1.7 to –1.9 Ga. Four Neoproterozoic zircons give ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios varying from 0.282135 to 0.282511. The Ɛ_Hf(t) values and T_DM ^C model ages range from −5.4 to +3.1 and 1.4 to 2.0 Ga. One Mesoproterozoic zircon has a high ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratio of 0.282131 and a positive Ɛ_Hf(t) value of +5.23 (Fig. 7b). T_DM ^C model age is 1.7 Ga (Table 5).

5.c. Metasandstone from the Tsengel Formation (Sample M21-1061)

One hundred and ten zircon grains were dated in the metasandstone M21-1061, and ninety-six of them are concordant within error. Zircon grains from a metasandstone sample M21-1061 are typically short, columnar in shape, rounded and 70–140 μm, with length-to-width ratios of 1.5:1 to 2:1. In CL images, most zircon cores have oscillatory zoning and rounded edges and form two distinct groups: one group consists of euhedral to subhedral grains with concentric oscillatory zoning and a sharp edge (87 %), whereas the other consists of rounded anhedral, homogeneous grains or ones with nebulous zoning (13 %) (Fig. 5c). Most of the zircons with high Th/U ratios range from 0.1 to 1.7, indicating a magmatic origin. The Th/U ratio of a few zircon grains ranges from 0.02 to 0.08, indicating a metamorphic origin (Table 3).

Analyses on the zircons with oscillatory zoning give concordant U–Pb ages ranging from 458 ± 11 to 2175 ± 36 Ma (Fig. 6g). The ages are mainly clustering at 458–529 Ma, 687–972 Ma, 1293–1339 Ma and 1605–2175 Ma, with the prominent age peak at 513 Ma (n = 57, 60 %), 795 Ma (n = 30, 31 %), and minor peaks at 1319 Ma (n = 4, 4 %), and 1995 Ma (n = 5, 5 %), respectively (Fig. 6h). The youngest coeval zircon grains between 458 ± 11 and 506 ± 12 Ma yield a weighted mean age of 481.5 ± 6.8 Ma (MSWD = 1.27, n = 13) (Fig. 6i).

Fifteen representative zircons from sample M21-1061 were analyzed for Hf- isotope compositions. Ten Cambrian and Ordovician zircons have high ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of 0.282246–0.282799. They have varied εHf(t) values and T_DM ^C model ages ranging from −7.9 to +12.0 and 0.7 to 1.9 Ga, respectively. Three Neoproterozoic zircons give ¹⁷⁶Hf/¹⁷⁷Hf values varying from 0.282116 to 0.282437. The εHf(t) and T_DM ^C model ages range from −7.8 to +7.6 and from 1.3 to 2.1 Ga, respectively. The other two zircons with Mesoproterozoic to Paleoproterozoic ages yield largely varied initial Hf isotopic compositions, with εHf(t) and T_DM ^C model ages varying from −6.2 to +8.6 and 1.5 to 2.8 Ga, respectively (Table 5).

5.d. Metasandstone from the Jivert-Uul Formation (Sample M21-1063/1)

A total of one hundred and six zircon grains were analyzed and eighty-one of them are concordant within error. In the CL images of sample M21-1063/1, most are subhedral to rounded with oscillatory zoning or nebulous zoning and shapes edge to rounded, and 60–140 μm, with length-to-width ratios of 1:1 to 2:1. A slightly rounded grain in samples 15, 97 and 1 shows a complex internal structure in which a rounded core is surrounded by several low-luminescence layers with variable recrystallized features, and analyses on the core and mantle portions give concordant ages of 730 Ma, 862 Ma and 1.75 Ga, respectively (Fig. 5d). Most of the zircons with high Th/U ratios range from 0.1 to 1.7 (except for M21-1063/1-58 = 0.02), indicating a magmatic origin (Table 4).

Analyses on the zircons with oscillatory zoning give concordant U–Pb ages ranging from 488 ± 12 to 2730 ± 34 Ma (Fig. 6j). The ages are mainly clustering at 488–554 Ma, 702–879 Ma, 1755–1775 Ma, 2035–2165 Ma and 2710–2730 Ma, with the prominent age peak at 517 Ma (n = 54, 66 %), 843 Ma (n = 16, 19 %), and minor peaks at 1767 Ma (n = 2, 3 %), 2097 Ma (n = 7, 9 %), and 2721 Ma (n = 2, 3 %), respectively (Fig. 6k). The youngest coeval zircon grains between 488 ± 12 and 512 ± 12 Ma yield a weighted mean age of 491.7 ± 7 Ma (MSWD = 0.32, n = 25) (Fig. 6l).

Fifteen representative zircons were analyzed for Hf isotope compositions. Ten Cambrian and Ordovician zircons have varied ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of 0.282234–0.282805 and Ɛ_Hf(t) values ranging from −8.1 to +11.7. Their T_DM ^C model ages varying from 0.7 to 2.0 Ga. Two Neoproterozoic zircons give ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios varying from 0.281768 to 0.282214. The εHf(t) and T_DM ^C model ages range from −17.2 to −1.2 and from 1.8 to 2.8 Ga, respectively. The other two zircons with Paleoproterozoic and Neoarchean ages all yield low initial Hf isotopic compositions, and εHf(t) values vary from 0.280996 to 0.281496 and from −3.7 to −1.0, respectively (Fig. 7d). Their T_DM ^C model ages range from 2.7 to 3.4 Ga (Table 5).

6. Discussion

6.a. Timing of deposition of metasedimentary rocks

The time of sedimentary deposition must be later than the formation of detrital zircons; the age of the youngest detrital zircon can be used to constrain the maximum age of deposition with the proviso that there was no disturbance in the U-Pb isotopic system (Nelson, 2001; Williams, 2001; Fedo et al. 2003). This approach has been successfully applied to sedimentary systems, especially to Precambrian successions where biostratigraphy cannot be used (Bingen et al. 2001; Guan et al. 2002; Griffin et al. 2004; Luo et al. 2004; Andersen, 2005; Payne et al. 2006; Moecher & Samson, 2006; Xia et al. 2006 a). The Mongolian Altai Group was originally assigned a Middle Cambrian–Lower Ordovician age, but some researchers have suggested a Cambrian or Middle to Upper Cambrian age (Badarch et al. 2002; Tovuudorj & Sumya, 2008; Erdenechimeg et al. 2018).

Our new study shows that, for the Maikhant and Tsengel Formations, Mongolian Altai Group, the youngest zircon population ages are 474.0 ± 6.1 Ma (sample M21-1079/1) and 481.5 ± 6.8 Ma (sample M21-1061), respectively, indicating their depositional ages must be younger than Lower Ordovician (Figs. 6c, i). While, for the Jivert-Uul and Borburgas Formations in the Mongolian Altai Group, the youngest zircon population ages are 491.7 ± 7 Ma (sample M21-1063/1) and 496.6 ± 6.8 Ma (sample M21-2062), respectively, indicating their depositional ages to be younger than the Guzhangian stage of the Cambrian (Figs. 6f, l). The upper limit of the depositional age of metasedimentary rocks can be constrained by the 450.9 ± 7.8 Ma crystallization age of the Tsagaangol complex intruded into the Mongolian Altai Group. Moreover, metasedimentary rocks of the Mongolian Altai Group are covered by the marine fossil-bearing Tsagaangol Formation, which has a maximum depositional age of 442.4 ± 4 Ma (Narantsetseg et al. 2024, unpublished data). All these data indicate that metasedimentary rocks of the Mongolian Altai Group were formed after the Guzhangian stage of the Cambrian and continued to accumulate during the Early-Middle Ordovician in the northern part of the Mongolian Altai.

All samples display a uniform age spectrum that consists of the highest Early Paleozoic age peak at Cambrian, a subdominant peak at Tonian and rare minor peaks with older ages (Fig. 6). Such similar patterns demonstrate that these samples share analogous clastic sources, and the tectonic setting of Mongolian Altai during the Early Paleozoic had not changed remarkably.

6. b. Sedimentary provenance

Based on the U-Pb age data, detrital zircons in the Mongolian Altai flysch sequence can be divided into three populations: an Early Paleozoic, Neoproterozoic and Pre-Neoproterozoic. The detrital zircons from each of the four metasedimentary samples from the Mongolian Altai Group all have a predominant population between 544 and 470 Ma (with main peaks at 517–509 Ma). Additionally, there is a subordinate population with ages between 972 and 687 Ma, showing main peaks at 843–775 Ma, and sparsely distributed between 3.2 and 1.0 Ga (Figs. 6b, e, h and k). These zircon age populations clearly indicate that the provenance of the Mongolian Altai sequence was dominated by rocks from the Neoproterozoic to Early Paleozoic periods. The detrital zircons generally show concentric zoning and high Th/U ratios, which are consistent with an igneous origin. Zircon Hf isotopic compositions indicate that the provenance contained a significant amount of juvenile materials (Fig. 7). Their euhedral shapes suggest that these detrital zircons experienced relatively short sedimentary transport and are probably related to proximal magmatism. On the basis of these lines of evidence, we propose that a Cambrian-Early Ordovician continental arc was the main source for the Mongolian Altai Group sedimentary rocks. More importantly, both the Precambrian blocks and the Lake Zone were intruded by numerous subduction-related granitoids (Kröner et al. 2010; Rudnev et al. 2012; Soejono et al. 2016) of a giant, >1,800 km-long Cambrian Ikh-Mongol arc system (Janoušek et al. 2018). These rocks and their eruptive equivalents yielded formation ages between 460 and 520 Ma with a peak around 510 Ma (see summary in Glorie et al. 2011; Jiang et al. 2011; Rudnev et al. 2012; Janoušek et al. 2018), exactly matching the predominant zircon population in the studied sedimentary succession.

The Neoproterozoic age populations and the rare older zircons, however, cannot be ignored. These Precambrian grains are well-rounded in shape and have complex internal structures (some of them have a metamorphic rim), suggesting they were likely derived from a more distant source that contained Precambrian materials and possibly that they experienced later metamorphism. Because the Precambrian rocks are absent within the Mongolian Altai sequence and the well-rounded shape of most Precambrian zircons indicates long-distance transportation or recycling, it is unlikely that the Precambrian sediments were derived from nearby sources. The Neoproterozoic zircon population of the Mongolian Altai sequence shows several age peaks at 775, 781, 795 and 843 Ma (Figs. 6b, e, h and k). An analogous Neoproterozoic age distribution was found in the Tuva-Mongol Massif, since new U-Pb dating of detrital zircons from meta-sediments in this massif revealed Neoproterozoic age peaks at 572, 584, 605 and 876 Ma (Kelty et al. 2008). Previous geochronological studies demonstrated that widespread Neoproterozoic magmatism occurred in the Tuva-Mongol Massif and along the southern margin of the Siberian craton (Badarch et al. 2002; Tomurtogoo, 2006; Windley et al. 2007; Kelty et al. 2008). Reasonably, we suggest the areas along the western margin of the Tuva-Mongol microcontinent could be the potential derivation of the subdominant Tonian zircon group. Moreover, older detrital zircon grains (with main peaks at 1.3, 1.9, 2 and 2.8 Ga) were likely derived from coeval felsic magmatism in the Tuva-Mongolian, Zavkhan and Baydrag blocks (Kuzmichev et al. 2001; Badarch et al. 2002; Kuzmichev et al. 2007; Demoux et al. 2009; Kuzmichev and Larionov, 2013; Zhang et al. 2015e; Bold et al., 2016).

6. c. Implications for the tectonic evolution

The Southern Altai terrane is overlain by Silurian, Devonian and Mississippian volcanic and shallow-marine sedimentary rocks and intruded by Upper Ordovician, Upper Devonian, Carboniferous and Permian granite plutons (Dergunov et al. 1980; Gavrilova, 1975).

The terrane contains a high proportion of thrust-imbricated metabasites, suggesting that the rocks were deposited in an arc-proximal setting and later deformed and metamorphosed to greenschist grade (Badarch et al., 2002). The Mongol Altai sedimentary rocks have long been suspected to be of passive continental margin affinity (Zonenshain, 1973); however, Watanabi et al. (1994), Mossakovsky, Dergunov (1985) and Byamba, Dejidmaa (1999) argued that the Mongolian Altai sequence was deposited in a forearc/back-arc basin or island arc tectonic settings based on their identification of volcaniclastic sedimentary units. However, some researchers argued that the Mongolian Altai sequence has been formed in a forearc/back-arc basin (Badarch et al. 2002; Yakubchuk, 2004; Long et al. 2007, 2010; Sun et al. 2008; Chen et al. 2016; Jiang et al. 2011, 2017) or an accretionary prism (Sengör et al. 1993; Sengör & Natal’in, 1996; Yuan et al. 2007; Sun et al. 2008; Long et al. 2007, 2008, 2010, Long et al., 2015). Moreover, the Altai-Mongolian terrane used to be interpreted as a remnant of oceanic island arcs that have Late Neoproterozoic–Early Paleozoic age (Buslov et al., 2004 ; Cai et al. 2015; Xiao et al. 2015, 2018) or a giant accretionary wedge of the Lake Zone of the Ikh-Mongol arc system (Xiao & Kusky, 2009; Soejono et al. 2017; Janoušek et al. 2018).

Determination of the depositional environment of the studied rocks is difficult due to their metamorphic overprint. However, zircon morphologies and internal structures, together with the distribution of supposed maximum depositional ages, allow for estimating the sedimentary tectonic setting (Cawood et al. 2012). All the studied samples are dominated by detrital zircon ages close to their possible age of sedimentation, which indicates an active margin setting in a plot by Cawood et al. (2012). This study shows that large amounts of detrital zircons were crystallized in the Early Paleozoic arc magmas derived from mantle or crustal sources. Therefore, our data favour a continental active margin history (Fig. 8).

Figure 8. Cumulative distribution curves of detrital zircons from the Early Paleozoic metasedimentary rocks in the Mongolian Altai. The depositional ages of individual units are inferred from the youngest detrital zircon ages. The color fields and reference dashed lines representing different tectonic settings of deposition are after Cawood et al., (2012) and the references therein.

The similarly supported sources of the Lower Ordovician metasandstones (M21-1079/1 and M21-1061) and the Middle-Upper Cambrian metasandstones (M21-2062 and M21-1063/1) are also supported by their εHf(t) in zircon data. The diagram of _ƐHf(t) versus crystallizing age clearly shows that two important crustal accretion events occurred in the Early Paleozoic and Neoproterozoic in the Mongolian Altai. As shown in the age-εHf(t) diagram (Fig. 9a), the oldest zircon (2.71 Ga, sample M21-1063/1-69) has a Hf crustal model age of ca. 3.14 Ga, which indicates that crustal material of the Altai-Mongolian terrane formed as early as the Mesoarchean. The metasandstone samples show Paleoproterozoic and Neoarchean zircons with mostly negative εHf(t) values, which is a feature typical of the Mongolian Altai zones. However, it is worthwhile to point out that the Mesoproterozoic zircons (1.58–1.29 Ga) mostly have positive εHf(t) values (Fig. 9a) and show that the crustal model ages are <200 Ma larger than their U-Pb ages (Fig. 9b).

Figure 9. (a) Diagram of ƐHf(t) values versus crystallizing ages for zircons from the Mongolian Altai Group metasandsones. (b) Crystallization age-Hf crustal model age plot of the detrital zircons from the Mongolian Altai Group metasandsones.

Therefore, their positive εHf(t) values represent the addition of juvenile crustal material, which indicates that the Mesoproterozoic is possibly an important period of continental crust growth. The Cambrian to Ordovician zircons reveal both negative and positive εHf(t) values, which is a feature typical for other parts of the Mongolian Altai zones. It is noticeable that ∼70% of detrital zircons have negative εHf(t) values, suggesting a synchronous crustal reworking. The Hf isotopic composition of these detrital zircons indicates two crust-forming events that happened in western Mongolia during the Neoproterozoic and Early Paleozoic, respectively. Therefore, it is reasonable to suggest that the subduction process of the Paleo-Asian Ocean in the nearby Ikh-Mongol Arc System caused the Early Paleozoic crustal growth and reworking (Long et al. 2019).

Based on recent Lu-Hf isotopic and U-Pb geochronological data for sedimentary rocks, the tectonic evolutionary history of the Mongolian Altai can be reconstructed as follows (Fig. 10). During the Neoproterozoic, a tract of the Paleo-Asian Ocean began to be subducted northward, and a continental arc evolved along the southern margin of the Siberian Craton, resulting in significant continental growth (Fig. 10a), as proposed by Long et al. (2010). With the above subduction process continuing, the Tuva-Mongol microcontinent docked with the Siberian Craton, probably during the Late Neoproterozoic to Early Paleozoic (Fig. 10b), as suggested by the high-grade metamorphic belt along the southern margin of the Siberian Craton (Khromykh et al. 2004; Dobretsov et al. 2006; Cai et al., 2014; Li et al. 2023a, b). Subsequently, during the Cambrian, a new arc system developed in western Mongolia and started an east-dipping subduction process. Although these tectonic complexes are exotic with respect to the Siberian Craton, they exhibit a close chronological correlation with the arc–back-arc systems surrounding microcontinental blocks in Mongolia (Burianek et al. 2017; Khain et al. 2003; Kozakov et al. 2012; Kröner et al. 2011; Lehmann et al. 2010; Rudnev et al. 2012; Li et al. 2023).

Figure 10. Geodynamic model showing the Neoproterozoic to Ordovician tectonic evolution of the Altai-Mongolian terrane.

Such subduction within the Paleo-Asian Ocean in the Late Cambrian not only significantly reworked the continental sliver but also led to the considerable formation of juvenile crustal material. Large volumes of newly formed arc material and a small number of clastic sediments from the microcontinent and the metamorphic belt were incorporated into the accretionary prism. With continued growth of the accretionary prism, that rollback of the trench caused upwelling of the hot asthenospheric mantle from depth and triggered partial melting of accreted juvenile material to generate widespread granitoids (Long et al. 2007, 2010; Jiang et al. 2016). In the Cambrian-Ordovician, the Ikh-Mongol arc system was built on the southern of the continental margin, and ocean crust subduction continued (Janoušek et al. 2018). In the Late Cambrian, a forearc basin was formed on the accretionary complex, and as a result, the sedimentary sequence of the Mongolian Altai Group began to accumulate (Fig. 10c).

In the Late Ordovician period, the area was intruded by granitoids of the Tsagaangol Complex. After that, it was covered by sedimentary sequences of the Tsagaangol formation, with a sediment accumulation age of 442.4 ± 4.4 Ma (Narantsetseg et al. 2024, unpublished data). By the Late Silurian, a regional tectonic uplift took place and was reflected by a regional unconformity recorded by the absence of strata underneath the Early Devonian volcanic rocks.

The Early Paleozoic sedimentary sequences of the Mongolian Altai Group, as a forearc basin, simultaneously received the sediments not only from a newly formed Ikh-Mongol Arc but also from the Neoproterozoic felsic rocks and basement materials of the Tuva-Mongol microcontinent.

6. d. Regional correlation

The Mongolian Altai, located in Western Mongolia, extends to the west and north into China, Kazakhstan and Russia. Regional correlation involves studying the continuity of geological features, rock formations and tectonic structures across these different countries. The understanding of the Mongolian Altai requires collaboration between geologists and researchers from various regions to piece together a comprehensive picture of the orogenic history and its impact on the landscape.

Information from detrital zircons can provide additional palaeogeographic constraints (Cawood et al. 2003). High-resolution U-Pb dating of detrital zircons has been conducted for the Mongolian Altai and its adjacent Neoproterozoic-Paleozoic sedimentary rocks in recent years (Buchan et al. 2002; Dijkstra et al. 2006; Kelty et al. 2008; Rojas-Agramonte et al. 2011; Salnikova et al. 2001; Jiang et al. 2011, 2012, 2017; Chen et al. 2014, 2016, Long et al. 2007, 2008, 2010, 2012, 2019; Yang et al. 2011; Soejono et al. 2017, 2018; Dong et al. 2018; Sukhbaatar et al. 2022).

Published data from the Habahe flysch sequence and high-grade metasedimentary rocks from Chinese Altai show similar age spectra and detrital zircons yield a predominant Early Paleozoic age clustering at 460–540 Ma with a minor contribution of Precambrian zircons (Fig. 11). In addition, whole-rock geochemical studies show that the Habahe Group was deposited in an active continental margin or continental arc setting in the Late Ordovician to Silurian, with sources mainly from intermediate-felsic igneous rocks of a nearby magmatic arc (Long et al. 2007, 2008, 2010, 2012; Li et al., 2012, 2017, 2019; Chen et al. 2014, 2016; Windley et al. 2002; Wang et al. Wang et al., 2014; Yuan et al. 2007; Sun et al. 2008; Cai et al. 2011a; Broussolle et al. 2019). Precambrian zircon age patterns from these metasedimentary rocks broadly resemble those of the TM block in the northeast and its surrounding arc-related terranes (Jiang et al. 2011).

Figure 11. Relative probability plots (concordant data only) for detrital zircons from the (a) southern part of Mongolian Altai (Long et al., 2019), (b) Chinese Altai (Long et al., 2012; Wang et al., 2014; Dong et al., 2018), (c) Russian Altai (Chen et al., 2014; 2016), as well as the adjacent (d) Hovd Zone (Soejono et al., 2018) and (e) Mongolian Altai in this study. (modified from Long et al., 2019).

Detrital zircons from the metasedimentary sequences from the Russian Altai also show the most prominent zircon population with Cambrian to Early Ordovician (504–475 Ma) ages, with a subordinate Late Neoproterozoic age (Chen et al., 2015). These rocks were deposited in the early-middle Ordovician, with sediment sources mainly from the Neoproterozoic to early Paleozoic magmatic rocks in the TM and adjacent island arcs (Chen et al. 2014, 2015, 2016).

Our new results show that the detrital zircons in the Mongolian Altai flysch sequence have a predominant Cambrian to Early Ordovician population between 544 and 470 Ma (with main peaks at 517–509 Ma) with subordinate Neoproterozoic and Pre-Neoproterozoic age populations. All these zircon age populations clearly indicate that the provenance of the Mongolian Altai sequence was dominated by rocks from the Neoproterozoic to Early Paleozoic periods. Thus, detrital zircon ages from the Russian and Chinese Altai, southern (Jiang et al., 2019) and eastern proximal Hovd terrane (Soejono et al. 2017, 2018) parts of Mongolian Altai, closely resemble those observed in the Paleozoic samples from the northern part of Mongolian Altai in our study. For further comparison, detrital zircon age data were compiled from literature on these five regions, as shown in Figure 11.

Thus, our new data, in combination with published detrital zircon data from the Chinese and Russian Altai, as well as from the southern and eastern part of the Mongolian Altai, support point that the Cambrian-Ordovician detrital zircons from metasedimentary rocks from Altai-Mongolian terrane originated from the coeval arc magmatism from the so-called Ikh-Mongol Arc (Janoušek et al., 2018; Li et al. 2019; Guy et al. 2020; Sukhbaatar et al. 2022).

In addition, recent U-Pb dating of detrital zircons from Neoproterozoic to Paleozoic arc-related terranes adjacent to the Tuva-Mongol block revealed that the Tuva-Mongol block significantly contributed the old materials to these sedimentary basins (Kelty et al., 2008; Rojas-Agramonte et al., 2011). Therefore, we suggest that the depositional environment of the Altai-Mongolian terrane, as well as the proximal Hovd terrane, should be alike and uniform in a huge consolidated sedimentary basin that shares analogous provenance during the Early Paleozoic. Apparently, the Mongolian Altai zircons show a quite similar age pattern with those from the Chinese and Russian, i.e., all three of them show similar age peaks at 500–513 Ma, 806 Ma, 1.9 Ga and 2.5 Ga, suggesting a possible identical source of old materials for the three regions. Taken together, all the available data support the idea that the Tuva-Mongol block and the adjacent island arcs were possibly the important sources for the Early Paleozoic metasedimentary rocks in the Mongolian Altai.

According to our study and the results of other researchers, it is plausible that the lithology and depositional age similarities of the sedimentary sequence in Chinese Altai, Russian Altai and Mongolian Altai indicate they were deposited from similar sources in the same basin during the Middle Cambrian to Lower Ordovician periods. This suggests a geological connection between these regions during the Early Paleozoic, possibly indicating a shared depositional environment or geological processes (Fig. 12).

Figure 12. Simplified stratigraphic columns comparing the northern Altai-Mongolian terrane (AM) in the Russian Altai (based on the geological maps of Daukeev et al., 2008; Chen et al., 2014; 2016), the Chinese Altai in northwestern China (after Long et al., 2012; Broussolle et al., 2019), the Mongolian Altai in the southeastern part of Mongolia (after Long et al., 2019), and the Mongolian Altai in western Mongolia (this study).

7. Conclusions

On the basis of our U-Pb and Hf isotope data for the detrital zircons from the metasedimentary rocks of the Mongolian Altai, we have the following major conclusions:

1. 1. The metasedimentary rocks of the Mongolian Altai Group were formed after ∼497 Ma in the Guzhangian stage of the Cambrian and accumulated during the Early-Middle Ordovician in the northern part of the Altai terrane.

2. 2. The provenance of the metasedimentary rocks in the Mongolian Altai was dominated by Cambrian to Early Ordovician igneous rocks, with subordinate Neoproterozoic and minor Paleoproterozoic and Archean crustal materials. The Tuva-Mongol Massif and adjacent island arc and metamorphic belt may be alternate source regions for the sedimentary sequence in the eastern Mongolian Altai.

3. 3. The Early Paleozoic and Neoproterozoic were important accretionary periods for the Mongolian Altai Group, during which large volumes of juvenile materials were added to the crust. The interpretation of the tectonic setting of the Altai-Mongolian terrane as an active continental margin, possibly acting as a forearc basin, is supported by the geological evidence we have provided.