Mongolian micro‐continental blocks in Columbia/Nuna: Zircon U–Pb–Hf isotopic evidence for long‐lasting Mongolia–Western Siberia connection

The Mongolian micro‐continental fragments play an important geodynamic role in the Palaeozoic evolution of the Central Asian Orogenic Belt. However, the original provenance of individual blocks within the Nuna/Columbia and Rodinia supercontinents is not well constrained. In this work, we present zircon U–Pb and Lu–Hf isotopic data from the Mesoproterozoic to Neoproterozoic metamorphosed cover of the Baidrag Block in central Mongolia. These data reveal important maxima in the Neoarchean, Palaeoproterozoic and locally Neoproterozoic. Although the majority of the new data can be attributed to local sources within the basement of the Mongolian micro‐continental fragments, an important contribution of juvenile 2.0 Ga zircons is linked to the western margin of the Siberian Craton, and 1.5 Ga zircons are either directly derived from North Australia or indirectly recycled from sediments in NW Laurentia.

micro-continental blocks are more enigmatic and subject to debate.Two generalized concepts have been proposed: either a long-lasting spatial relationship with Siberia (Şengör et al., 1993) or a Gondwanan (typically Tarim Craton) derivation (Kröner et al., 2014;Mossakovsky, 1994;Zhou et al., 2018).The latter is supported by palaeomagnetic data (Levashova et al., 2010) and previous detrital zircon studies of Neoproterozoic-Palaeozoic strata, which have emphasized the importance of a Tonian tectonic event that is observed in the detrital zircon record of Tarim but only insignificantly in Siberia (Bold et al., 2016;Rojas-Agramonte et al., 2011).Although this absence in Siberia is somewhat overstated in light of more recent geochronological work (Gillespie et al., 2018;Kochnev et al., 2022;Priyatkina et al., 2018).Herein, we present the first comprehensive detrital zircon geochronology study from Mesoproterozoic strata belonging to the Baidrag Block of the Mongolian Collage; these data shed new insight into this problem by providing constraints on the pre-Rodinian position of the Monoglian micro-terranes.

| G EOLOG IC AL S E T TING
The Baidrag Block belongs to a string of Precambrian micro-continents exposed in the Mongolian Collage (Figure 1a), the presentday distribution of which defines the Mongolian orocline.The basement of the Baidrag Block consists of high-grade gneisses and metasediments.The oldest igneous protoliths were emplaced at 2890-2650 Ma, followed by intrusions at c. 2400-2310 Ma and 1850-1820 Ma associated with c. 1850 Ma granulite facies metamorphism (Kozakov et al., 2007).

Statement of significance
The manuscript presents the first comprehensive detrital The studied samples are from the Burd Gol Zone, a NW-SE trending belt of metamorphosed volcano-sedimentary rocks intruded by Ediacaran-Cambrian granite (Figure 1b).The base of the Burd Gol Zone consists of a ∼250 m-thick sequence of quartzite and conglomerate followed by early Mesoproterozoic stromatolite-bearing limestone (Tomurtogoo & Gerel, 1999).These are overlain by a mélange of phyllites and graphite-rich chlorite-biotite schists that contain lenses of meta-igneous and -sedimentary rocks (Buchan et al., 2001).The Mesoproterozoic to Neoproterozoic age of the sediments was previously determined on the basis of microfossils (Mitrofanov et al., 1981) and a detrital zircon study using the single-zircon evaporation technique (Demoux, Kröner, Badarch, et al., 2009).The sequence was affected by Ediacaranearly Cambrian Barrovian metamorphism with a general increase towards the Bayankhongor Zone in the NE (Kozakov et al., 2006).

| ME THODS AND RE SULTS
Zircons from seven metasedimentary samples (Table 1) of the Burd Gol Zone have been analysed for U-Pb isotopic composition by LA-ICP-MS, and zircons from five of those samples have additionally been analysed for Lu-Hf isotopic compositions by MC-ICP-MS.The U-Pb isotopic data from two samples, PV256 and PV274, were previously reported in Štípská et al. (2023).Details of the analytical procedure are given in Appendix S1; U-Pb and Lu-Hf isotopic data are given in Appendix S2; cathodoluminescence images of analysed zircons and spot localizations are given in Appendix S3.

| U-Pb zircon dating
Samples PV304, PV249, IS137, PV256 and PV251 are all characterized by the dominance of Palaeoproterozoic to Neoarchean zircon (Figure 2a).These data form maxima at c. 2050-1800 Ma and c. 2650-2500 Ma in each sample; however, the relative magnitude of these maxima varies (Figure 2b-h).Some scattered data are observed on either side of these maxima, and a small additional peak at c. 1500 Ma can be observed in each of these samples.Some younger dates at c. 1000 Ma, 550 Ma and 250 Ma are recorded in PV304, which may represent an artefact of metamorphic resetting as they correlate to the age of known thermal events in the region (Jahn et al., 2004;Khain et al., 2002).
Samples PV274 and IS192 differ from the others by the presence of a dominant Neoproterozoic maxima.For sample PV274, these Neoproterozoic dates were recorded in structure-less CLdark zircon rims characterized by low Th/U ratios (<0.11; n = 44, Figure 3a); as previously discussed in Štípská et al. (2023), these zircon rims likely reflect growth during a Tonian metamorphic event.
Considering only detrital zircon, this sample is characterized by a TA B L E 1 Location, petrography and summary of the main age peaks of the studied samples from the Burd Gol Zone.

| DISCUSS ION
The Burd Gol Zone represents the Mesoproterozoic to Neoproterozoic metamorphosed cover of the Archean and Palaeoproterozoic basements of the Baidrag Block, and our new U-Pb and Lu-Hf zircon data record important tectonic events in their source region(s).However, it is also evident from our data that the mapped zone does not represent a continuous (meta-)sedimentary sequence but is composed of strata deposited either side of a Tonian tectono-metamorphic event.
zircon U-Pb and Lu-Hf isotopic study of Mesoproterozoic to Neoproterozoic strata from a micro-continental fragment involved in the Mongolian Collage of the Central Asian Orogenic Belt (CAOB).A conventional view developed in the last 15-20 years is that these blocks were originally derived from Gondwana, specifically the Tarim Craton.However, our data imply a long-lasting and/or repeated spatial association with the Siberian Craton, specifically its present-day western margin, and that these Mongolian micro-continental blocks may well represent an important link between the Laurentian and Siberian cratons in Columbia/Nuna reconstructions.These data and their interpretation are of broad interest to anyone working on Proterozoic supercontinent reconstructions and specific significance to anyone working on the Proterozoic-Mesozoic accretionary processes in the CAOB.F I G U R E 1 (a) distribution of micro-continental fragments in the Mongolian Collage (after Jiang et al., 2017).Ba, Barguzin; Bd, Baidrag; Ere, Ereendavaa; Erg, Erguna; Ol, Olkhon; Ta, Tarbagatay; TM, Tuva-Mongol; Zv, Zavkhan.(b) Simplified geological map of the NE margin of the Baidrag Block (after Štípská et al., 2023) with the location of the studied samples.[Colour figure can be viewed at wileyonlinelibrary.com] Ma, 2300-2650 Ma, (c.250 Ma*, c. 550 Ma*, c. 1050 Ma*) Note: Asterisk (*) by age population indicates a possible spurious age population; (met.)indicates a metamorphic age population determined from a low Th/U (<0.1) ratio, zircon morphology and internal structure.A note on the calculation of maximum depositional ages is given in Appendix S1.Abbreviations: MDA, maximum depositional age; Y3G, youngest three grains with overlapping 2 s error; YSG, youngest single grain.few Neoarchean dates (2778-2469 Ma) and maxima at c. 1850 and 1500 Ma.These detrital peaks overlap with the maxima observed in the previous five samples, but scattered dates to c. 1100 Ma are also recorded.For sample IS192, Neoproterozoic dates are recorded in both zircon core and rims with clear oscillatory zoning and exhibit higher and more varied Th/U ratios (0.11-1.30); therefore, Neoproterozoic zircon is treated as detrital in this sample.In addition, this sample also exhibits detrital peaks at c. 2500, 1900 and 1500 Ma and scattered late Mesoproterozoic dates (1133-1030 Ma; n = 4); these peaks overlap with maxima observed in the other samples.The Neoproterozoic maxima is represented by a small shoulder at c. 900 Ma and the principle maxima at c. 800 Ma, as well as scattered younger analyses down to c. 640 Ma.F I G U R E 2 (a) U-Pb concordia diagrams of the zircon from the studied metasedimentary samples.(b-h) Kernel density estimates of concordant zircon age populations.Concordance is assessed by the relative % difference (−10−+10) between the 206 Pb/ 238 U and 207 Pb/ 206 Pb ages for analyses with 207 Pb/ 206 Pb age >1000 Ma; for analyses with 207 Pb/ 206 Pb age < 1000 Ma, concordance is assessed by the relative % difference (−10−+10) between the 206 Pb/ 238 U and 207 Pb/ 235 U ages.The thick solid line indicates the estimated maximum depositional age according to the youngest single detrital grain method.The dashed line represents the weighted mean of the three youngest detrital grains with an overlapping 2σ error.A note on the calculation of maximum depositional ages is given in Appendix S1. [Colour figure can be viewed at wileyonlinelibrary.com]Lu-Hf zircon isotopes The Lu-Hf analyses were targeted at zircon grains/domains, which yielded concordant U-Pb data and were of sufficient size to accommodate the larger (40 μm diameter) laser beam size.Due to the small size or convoluted zoning of many of the zircon grains, this limited the total number of Hf isotopic analyses to 105 from five samples.However, comparing KDE's for all U-Pb spots and just the Lu-Hf analyses, there is broad similarity except for an overrepresentation of the Neoproterozoic data in the Lu-Hf spot analyses (Figure 3b).Neoarchean and early Palaeoproterozoic (2800-2410 Ma) zircons (n = 31) are characterized by a significant spread of the εHf (t) values on either side of chondritic values (−8.57 to +7.09, mean: +0.49).These zircons are henceforth referred to as population 1 (Figure 3c; it unclear how widely distributed late Neoproterozoic strata are within the Burd Gol Zone.It is therefore possible that the Burd Gol Zone could be composed of either two or more distinct units separated by a hidden unconformity or a tectonic mélange splicing together strata of different stratigraphic ages.Nonetheless, outside of the Neoproterozoic zircons, sample IS192 contains zircons of similar age and composition as the older samples (Figure 3); thus, there has likely not been a major shift in the source rocks of the studied strata or a more exotic derivation for this sample.Therefore, detrital zircon from this sample is treated together with the other samples in the following correlations between the Baidrag Block and associated micro-continents and cratonic blocks.Considering the known ages and isotopic composition of the basement of the Baidrag Block and other micro-continental blocks in the Mongolian Collage, local sources are permissible for zircon populations 1 and 3, but not population 2 (Figure 4).Population 4 may have a cryptic local source; a sample of granite gneiss from the Gurvan Bogd mountains (SE of the Baidrag Block) yielded a 1.52 Ga emplacement age Population 6 has potential local sources from the Tonian magmatic rocks of the neighbouring Tuva, Tarvatagay and Zavkhan Blocks.Therefore, it is important to determine the original provenance of populations 2 and 4, which do not have clear local sources.To address this problem, data has been compiled from similar age strata on the margins of major cratonic blocks and micro-continents associated with the CAOB (Figure5).A correlation with the CAOB micro-continental fragments of the Kazakh Collage or Beishan Block can be immediately disregarded owing to the high contribution of TA B L E 2 Summary of Hf in zircon populations and their primary interpretation.in brackets have only a single data point in this population.F I G U R E 4 Compilation of published geochronological and whole-rock Nd isotopic data from Precambrian magmatic rocks of the Mongolian Collage.Data sources(Bold et al., 2019; Demoux, [Colour figure can be viewed at wileyonlinelibrary.com]Mesoproterozoic zircon in these datasets.The Tarim Craton can also be disregarded as there is significantly more scatter in the Neoarchean and early Palaeoproterozoic data from Tarim, and more importantly, c. 2.0 Ga juvenile zircon is essentially absent.More promising is the correlation to Siberia, where zircons comparable to our population 2 are present in strata deposited on both the western and eastern margins of the craton.Of these, the western margin is preferred owing to a remarkable similarity in populations 1 and 3 of our dataset, whereas Eastern Siberia strata exhibit significant dissimilarity in both of these populations.Nonetheless, population 4 zircons are absent in Western Siberia.Data from Meso-/ Neoproterozoic strata of eastern Baltica and North China Craton are also depicted; however, both datasets include populations exotic to the Baidrag Block and are excluded as possible correlatives.Since the source of the early Mesoproterozoic zircon (population 4) remains unclear; the position of the western part of the Siberian Craton is considered in Mesoproterozoic palaeogeographic reconstructions.There is general agreement on a long-lasting connection between southern Siberia and northern Laurentia (e.g.Evans et al., 2016); however, debate persists regarding a tight-(Ernst et al., 2016) or loose-fit(Pisarevsky et al., 2008) connection during the Mesoproterozoic.Nonetheless, the relative orientations of Siberia and Laurentia results in the present-day western margin of Siberia aligning with NW Laurentia (Figure6).Magmatic rocks with a 1.5-Ga emplacement age are also exotic in NW Laurentia; this time period (1.61-1.49Ga) is referred to as the North American Magmatic Gap(Ross & Villeneuve, 2003).Nonetheless, some Laurentian Mesoproterozoic strata do contain significant, almost unimodal, components of c. 1.5 Ga zircon (PR1 basin,Medig et al., 2014).These zircons are thought to be derived from accretionary belts in North Australia, where juvenile c.1.5 Ga zircons are abundant(Griffin et al., 2006) and are a linchpin in Columbia/Nuna reconstructions.However, since the Mesoproterozoic basins of North Australia contain further zircons exotic to the Baidrag Block (Figure 5), re-deposition of detritus from the unimodal PR1 basins of NW Laurentia may be a more satisfactory explanation.Samples PV274 and IS192 additionally record an important Tonian tectono-metamorphic event; although this event is not recorded in the compilation of zircon Hf data from Western Siberia, we note recent U-Pb zircon studies have documented Tonian zircons in mid-to late-Neoproterozoic strata (e.g.Kochnev et al., 2022), and there is a strong correlation in the Hf zircon data to the Central Taymyr Belt of Northern Siberia (Priyatkina et al., 2017; Figure 5).Nonetheless, this Tonian tectono-metamorphic event is often presented as diagnostic of a Tarim-Mongolia connection in the Neoproterozoic (Bold et al., 2016; Rojas-Agramonte et al., 2011), and our new data and a Mesoproterozoic position proximal to Western Siberia do not exclude this possibility.However, any model supporting a Neoproterozoic link to Tarim would thus have to break the proposed Mesoproterozoic link to western Siberia and resolve the re-assembly of the Mongolian Collage to the southern margin of Siberia by the early Palaeozoic (e.g.Li, Jiang, Collett, Štípská, Schulmann, Wang, Sukhorukov, Bai, & Zhang, 2023).ACK N O WLE D G E M ENTSThis work was supported by the Czech Science Foundation (grant number 19-27682X to K.S.).P. Hanžl is thanked for providing the geological maps.Zircon geochronological data from sample PV256 was collected by N. Cathelin from the University of Strasbourg during internships at the Czech Geological Survey.The manuscript has been improved thanks to comments by Viktor Kovach (Russian Academy of Sciences), two anonymous reviewers and the associate editor Tomas Capaldi., J., & Albarède, F.(1997).The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system.Earth and Planetary ScienceLetters, 148,[243][244][245][246][247][248][249][250][251][252][253][254][255][256][257][258].https:// doi.org/ 10. 1016/ S0012 -821X(97) 00040 -X Bold, U.,Isozaki, Y., Aoki, S., Sakata, S., Ishikawa, A., Sawaki, Y., & Sawada,  H. (2019).Precambrian basement, provenance implication, and tectonic evolution of the Gargan block of the Tuva-Mongolia terranes, F I G U R E 6 Palaeogeography of the Nuna supercontinent at c.1.3Ga after Kirscher et al. (2021).The hypothetical position of the Baidrag Block is depicted by a black star, and proposed detrital transport is indicated by arrows.Small symbols indicate the approximate location of basins included in the data compilation; symbols and colours are the same as in Figure 5; * indicates deposition post-dates presented in the reconstruction.The Gulf of Nuna (after Mitchell, Kirscher, et al., 2021) represents a hypothesized Mesoproterozoic oceanic euxinic event, the identification of which in the graphite-rich shales of the Burd Gol Zone may represent a test of the proposed palaeogeographic position.[Colour figure can be viewed at wileyonlinelibrary.com] Table 2).