Provenance and Tectonic Implications of Sedimentary Rocks of the Paleozoic Chiron Basin, Eastern Transbaikalia, Russia, Based on Whole-Rock Geochemistry and Detrital Zircon U–Pb Age and Hf Isotopic Data

: The Chiron Basin extends along the southern periphery of the Siberian Craton and the western margin of the Mongol–Okhotsk Belt. Here, we present whole-rock geochemical data (major and trace elements and Sm–Nd isotopes) along with zircon U–Pb geochronology and Lu–Hf isotopic data from Paleozoic sedimentary rocks within the Chiron Basin to investigate their provenance and tectonic history. ε Nd(t) values of the siliciclastics rocks of the Khara–Shibir, Shazagaitui, and Zhipkhoshi formations vary from − 17.8 to − 6.6, with corresponding two-stage Nd model ages (t Nd(C) ) ranging from 2.56 to 1.65 Ga. Detrital zircon grains from these rocks are predominantly Archean, Paleoproterozoic, and Carboniferous–Devonian in age. The data suggest that the southern ﬂank of the Siberian Craton is the only viable source area for Archean and Paleoproterozoic zircon grains with Hf model ages (t Hf(C) ) of > 2.20 Ga. The majority of zircon grains from sandstones from the Khara–Shibir, Shazagaitui, and Zhipkhoshi formations are Devonian–Carboniferous in age. With respect to their Hf model ages, the zircon grains can be subdivided into two groups. The ﬁrst group of Devonian–Carboniferous zircon grains is characterized by relatively old (mainly Paleoproterozoic) t Hf(C) model ages of 2.25–1.70 Ga and the source was the southern margin of the Siberian Craton. The second group of Devonian–Carboniferous zircon grains is characterized by signiﬁcantly younger (mainly Neoproterozoic) t Hf(C) model ages of 1.35–0.36 Ga, which are consistent with a juvenile source, most likely eroded island arcs. Our data, show that sedimentary rocks of the Chiron Basin likely formed in a back-arc basin on the southern periphery of the Siberian Craton facing the Paleozoic Mongol–Okhotsk Ocean. The powdered samples of sedimentary rocks were homogenized for XRF analysis by fusing with a mixture of Li metaborate and tetraborate in a mu ﬄ e furnace at a temperature of 1050–1100 ◦ C. Measurements were carried out on a 4S Pioneer X-ray spectrometer. Intensities of analytical lines were corrected for the background and for the e ﬀ ects of absorption and secondary ﬂuorescence. For ICP-MS analyses, samples were decomposed with acid mixtures of HCl, HNO 3 , HF, and HClO 4 . Measurements were performed in a standard regime by Perkin Elmer / Sciex Elan 6100 DRC ICP-MS whose sensitivity over the whole mass scale was calibrated using standard reference solutions that contained all elements to be analyzed in the samples. The relative measurement error for major and minor elements was 3%–10%.

The Mongol-Okhotsk Belt is generally considered ( [2,3,[11][12][13] and references therein) to represent a remnant of the Mongol-Okhotsk Ocean, which closed owing to collision between the Siberian Craton and the Amuria Block (superterrane or microcontinent). However, many aspects of the evolution of this structure from ocean to suture zone remain unresolved.
Most controversial is the geodynamic framework of sedimentary formations and intrusive complexes within the Mongol-Okhotsk Belt, as well as the timing and nature of the accretion and collision. Constraints on the age and composition of sedimentary sequences within the Mongol-Okhotsk Belt and neighboring basins are required to better understand the evolution of the region. In this respect, U-Pb and Lu-Hf isotopic data from detrital zircon grains from a variety of different stratigraphic ages sedimentary sequences coupled with whole-rock Nd isotopic data are of crucial importance.
Recent geochronological studies have provided tighter constraints on the ages of Paleozoic sedimentary sequences in the western [14][15][16][17] and eastern [18][19][20][21] parts of the Mongol-Okhotsk Belt. In those studies, subduction zones of different age and polarity have been invoked to explain the evolution of the belt. Furthermore, detailed studies of Jurassic sedimentary rocks of the Mesozoic basins north [22][23][24] and south [25,26] of the Mongol-Okhotsk Belt have suggested that the Mongol-Okhotsk Ocean closed at the Early-Middle Jurassic boundary.
We performed whole-rock geochemical analyses together with U-Pb and Lu-Hf isotopic analyses of detrital zircon grains from late Paleozoic sedimentary rocks within the Chiron Basin to provide much-needed information regarding the evolution of the Mongol-Okhotsk Belt. The Chiron Basin is bordered by the southern edge of the Siberian Craton and the Transbaikal segment of the Mongol-Okhotsk Belt. Sedimentary formations within this basin carry important information regarding the geological events that occurred at the periphery of the Siberian Craton during the late Paleozoic associated with the evolution of the Mongol-Okhotsk Ocean.
Geological studies of Eastern Transbaikalia region have led to differing interpretations of the geodynamic nature of the Chiron Basin. Initially, Nagibina [27] identified this basin as a narrow synorogenic basin located at the north of the Mongol-Okhotsk Belt that was confined to the marginal fault zone. This interpretation was supported by Amantov [28], who termed the structures related to the Chiron Basin as "residual orogenic depressions" or "superimposed marginal basins".
Consistent with the current conceptualization of the Mongol-Okhotsk Fold Belt, Parfenov et al. [2,29] interpreted the Chiron Group to represent accumulation of sediments in a forearc basin. However, they did not indicate a magmatic arc adjacent to this forearc basin. Ruzhentsev and Nekrasov [7] ascribed the formation of the Chiron Basin to instability caused by convergence of the Siberian Craton and Argun Continental Massif. A similar interpretation was proposed by Shivokhin et al. [9].

Geological Background
The Chiron Basin ( Figure 1) is located in the northeastern Aga Zone [7,30] and occupies an area of >500 km 2 on the left bank of the lower reaches of the Aga River and within the Chiron River basin. The late Paleozoic Chiron Basin is filled with about 3500-4000 m of unmetamorphosed shallow-marine siliciclastic rocks of the Chiron Group (Figures 2 and 3). Extensive tectonic slices of the Chiron Group also occur to the west and north of the Chiron Basin, as well as in the southern spurs of the Mogoitui Range. Until the mid−1960s, the entire Chiron Group was assigned to the Permian [31]. Later, its age was reassigned to the late Visean to early Permian based on fossil content [32]. The Chiron Group lies unconformably upon metamorphic schists [33] known formerly as the Onon Formation, which was originally assigned a Silurian age by Tolukhonov [31]. However, Amantov [28] and Anashkina et al. [30] suggested a Proterozoic age for these rocks. Recently, metamorphic rocks of the Onon Formation have been regarded as late Paleozoic (middle Paleozoic, according to the Russian stratigraphic nomenclature) and are now termed the Aga-Borshchovochnyi metamorphic complex [9,34] (Figure 3).
The Chiron Group is a polycyclic stratigraphic sequence showing a gradual upward transition from coarse-to fine-grained siliciclastic rocks. Tutkhaltui, Khara-Shibir, Shazagaitui, and Zhipkhoshi formations [33] are assigned to late Visean through to the early Permian [32].
The Tutkhaltui Formation is separated by a sharp angular unconformity from the Aga-Borshchovochnyi metamorphic complex. From bottom to top, this formation consists of the following members [33]:

1.
Conglomerates and sedimentary breccias interbedded with coarse-grained sandstones. Clastic material consists of angular fragments of the metamorphic schists and quartz. Pebbles of felsic volcanic rocks and of granite are rare. The clasts of the sandstones are angular. This member varies from a few meters to 40 m in thickness.

2.
Massive medium-to fine-grained sandstones interbedded with calcareous sandstones and sandy siltstones 1-2 m thick, and lenses of bioclastic limestone. This member is 45 m thick.

3.
Sandy siltstones gradually passing upwards into massive siltstone. This member is 43 m thick.

4.
Massive medium-to coarse-grained sandstones interbedded with gray siltstones and sedimentary breccias similar in composition to Member (1). This member varies in thickness from 2 to 20 m.

5.
Predominantly siltstones with rare thin interlayers of massive medium-grained sandstones. This member is 102 m thick.  The Tutkhaltui Formation contains numerous fossilized remains of bryozoans, brachiopods, bivalves, and crinoids [32,33]. Most brachiopod species occur widely in the Kuzbass, Mongolia, Verkhoyansk (Siberia), and Kolyma-Omolon regions of NE Russia [35]. In Mongolia [36] and the Verkhoyansk region [37], ammonoids of the genera Epicanites and Pachylyroceras also occur. These ammonoids are common in late Visean to Serpukhovian sedimentary rocks in Western Europe, the Urals, and North America [38]. On this basis, the Tutkhaltui Formation has been assigned a late Visean to Serpukhovian age.
The Khara-Shibir Formation conformably overlies the Tutkhaltui Formation and comprises two first-order cycles that have the same lithological composition and faunal assemblages. The first-order cycles are considered as Lower and Upper subformations. From bottom to top, the Lower Subformation consists of the following members [33]:
Sandstones contain angular and poorly sorted clasts. Individual cycles vary in thickness from 20 to 40 m.

2.
Alternating massive siltstones and sandy siltstones with flow structures, with gradual transitions between units. This member is 120 m thick.

3.
Alternating fine-to medium-grained massive and bedded sandstones and massive siltstones. Individual layers are 5-20 m thick, the overall thickness of this member is up to 250 m thick.
Predominantly coarse-grained rocks including second-order cycles consisting (from bottom to top) of medium and fine pebble conglomerates, coarse-to medium-grained sandstones, and sandy siltstones grading into massive siltstones. Individual cycles vary in thickness from 20 to 30 m. Pebbles are dominated by felsic volcanic rocks, together with granites, quartz, quartzites, and gray siltstones. This member is 90 m thick.

2.
Alternating beds of massive medium-to fine-grained sandstones, massive siltstones, and sandy siltstones with gradual transitions between units. Siltstone and sandy siltstone layers are 6-27 m thick and sandstones 1-12 m thick. Single pebbles of felsic volcanic rocks are present at the base of sandy siltstone layers. This member is about 200 m thick.

3.
Mainly siltstones with intercalations of sandstones as thick as 0.2-3 m irregularly distributed within vertical thickness. The member is up to 280 m thick.

4.
Alternating beds of massive coarse-grained sandstones, medium-grained sandstones, and sandy siltstones with flow texture. Layers vary in thickness from 14 to 40 m. Coarse-grained sandstones contain flattened fragments of siltstone 2-3 cm in size. The member is 190 m thick.
The Khara-Shibir Formation is characterized by an extensive assemblage of bryozoans, brachiopods, bivalves, and crinoids [32,33]. Brachiopods in the Khara-Shibir Formation and the underlying Tutkhaltui Formation share a large number of common species. The Khara-Shibir brachiopods are most similar to brachiopods from the Waagenoconcha gigantea Zone (upper part of the Magarsky super-stratohorizon) in the Kolyma-Omolon region [39] and the Sajakella formosa Zone (upper part of the Khatynakh stratohorizon) in the South Verkhoyansk region [35]. Considering the stratigraphic position of the aforementioned analogs of the Khara-Shibir Formation, which overlie layers containing late Visean-Serpukhovian ammonoids and lie below sediments containing late Bashkirian ammonoids, an early Bashkirian age for the Khara-Shibir Formation is implied.

1.
The Shazagaitui Formation conformably overlies the Khara-Shibir Formation. From bottom to top, the succession consists of the following members [33]: 2.

3.
Alternating beds of massive siltstones and sandy siltstones with flow structures, with gradual transitions between units. Interbeds of fine-to medium-grained sandstones 0.5−2 m in thickness with distinct planar boundaries show no regular pattern. This member is >500 m thick.

4.
Massive coarse-grained sandstones passing upward into medium-and fine-grained rocks with rare interbeds of dark-gray massive siltstones 1-2 m thick. The thickness of this member exceeds 125 m.

5.
Alternating beds of sandy siltstones with massive and flow structures and fine-to medium-grained sandstones, the latter in some places preserving cross-bedding. Intercalations of massive calcareous sandstones and sandy limestones also occur. The thickness of this member varies between 170 and 250 m.
The Shazagaitui Formation contains numerous remains of bryozoans, brachiopods, and crinoids [33]. Most brachiopod species are distributed throughout the Verkhoyansk region, (Siberia), and are associated with late Bashkirian ammonoids of the Braneroceras-Gastrioceras and Diabaloceras-Axinolobus zones [38,40]. On this basis, the Shazagaitui Formation has been assigned a late Bashkirian age [32]. Overall, the composition, texture, and structure of sedimentary rocks and benthic fauna suggest that the Shazagaitui Formation, as well as the underlying Tutkhaltui and Khara-Shibir formations, formed in a shallow-to moderately deep-marine environment close to the sedimentary source.
The Zhipkhoshi Formation in the central part of the Chiron Basin lies with a stratigraphic hiatus and visible conformity on the Shazagaitui Formation, and angular unconformity the Aga-Borshchovochnyi metamorphic complex in the marginal parts of the basin [33]. The Zhipkhoshi Formation is limited in extent, being restricted to the central part of the Chiron Basin, where the lower portions of the section are exposed. Stratigraphically higher horizons are truncated by a fault that bounds the Chiron Group to the northwest.
From the base upward, the Zhipkhoshi Formation consists of the following members: [33]: 1.
Coarse-grained sandstones with intercalations of conglomerate. Pebble-to-boulder conglomerates comprising clasts composed mainly of rhyolite and quartzite and less commonly granitoid. This member varies irregularly in thickness between 90 to 170 m.

2.
Siltstones containing plant detritus, fossil wood fragments, megaspores, and nodules formed around the remains of conularia and pelecipods. The thickness is about 300 m.

3.
Medium-to fine-grained massive and bedded sandstones with scarce thin siltstone intercalations. The thickness of this member is 400 m.

4.
Intercalated fine-grained massive and bedded sandstones and sandy siltstones. Sandstone horizons show cross-bedding and ripple marks. This member is 70 m thick.
On the basis of the assemblage of brachiopods and bryozoans, the Zhipkhoshi Formation is assigned a Sakmarian to early Artinskian age by analogy with the Verkhoyansk units, (Siberia), which, in addition to the brachiopods similar to the Zhipkhoshi brachiopods, contain ammonoids of the Uraloceras and Paragastrioceras genera [32,35]. The bivalve assemblage suggests a late Asselian to early Artinskian age for the Zhipkhoshi Formation [41]. Thus, the stratigraphic gap between the Shazagaitui and Zhipkhoshi formations spans the Moskovian, Kasimovian, and Gzhelian stages, and probably part of the Asselian stage, a total time span of about 18 Myr. Considering the stratigraphic unconformity between these formations, the inclusion of the Zhipkhoshi Formation within the Chiron Group is controversial [42,43].
The texture of sediments, in combination with the presence of plant detritus, megaspores, fossil wood, and benthic fauna, suggest that rocks of the Zhipkhoshi Formation accumulated in shallow-marine environments, whereas the angular clastic materials provide evidence for a close sedimentary source.
We investigated siliciclastic rocks of the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations, as well as schists of the Aga-Borshchovochnyi metamorphic complex, representing the basement rocks of the Chiron Basin. Sample locations are listed in Table 1 and are shown in Figure 2.

Major and Trace Element Analyses
Chemical compositions of siliciclastic rocks were studied using X-ray fluorescence analysis (XRF) (major elements, Zr) at the Institute of Geology and Nature Management, Far East Branch of the Russian Academy of Sciences, Blagoveshchensk, Russia, and inductively coupled plasma mass spectrometry (ICP-MS) (Cs, Ga, Rb, Sr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Nb, Hf, Ta, Th, U, Pb) at the Institute of Tectonics and Geophysics, Far East Branch of the Russian Academy of Sciences, Khabarovsk, Russia.
The powdered samples of sedimentary rocks were homogenized for XRF analysis by fusing with a mixture of Li metaborate and tetraborate in a muffle furnace at a temperature of 1050-1100 • C. Measurements were carried out on a 4S Pioneer X-ray spectrometer. Intensities of analytical lines were corrected for the background and for the effects of absorption and secondary fluorescence. For ICP-MS analyses, samples were decomposed with acid mixtures of HCl, HNO 3 , HF, and HClO 4 . Measurements were performed in a standard regime by Perkin Elmer/Sciex Elan 6100 DRC ICP-MS whose sensitivity over the whole mass scale was calibrated using standard reference solutions that contained all elements to be analyzed in the samples. The relative measurement error for major and minor elements was 3%-10%.

Sm-Nd Isotopic Analyses
Sm-Nd isotopic analyses were performed at the Collective Enjoyment Center of Isotope Geochemical Investigations, Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia. About 100 mg of the whole rock powder was dissolved in a mixture of HF, HNO 3 , and HClO 4 . A 149 Sm-150 Nd spike solution was added to all samples before dissolution. REE were separated on BioRad AGW50-X8 200-400 mesh resin using conventional cation exchange techniques. Sm and Nd were separated by extraction chromatography with LN-Spec (100-150 mesh) resin [44]. Total blanks during the measurement were 0.1-0. The εNd(t) values were calculated using the present-day values for a chondritic uniform reservoir (CHUR) 143 Nd/ 144 Nd = 0.512638 and 147 Sm/ 144 Nd = 0.1967 [46]. Whole-rock t Nd(DM) mean crustal residence ages were calculated using the model of Goldstein and Jacobsen [47] according to which the Nd isotopic composition of the depleted mantle evolved linearly since 4.56 Ga ago and has a present-day ε Nd(0) value of +10 ( 143 Nd/ 144 Nd = 0.513151 and 147 Sm/ 144 Nd = 0.21365). To account for changes in the 147 Sm/ 144 Nd ratios due to fractionation of accessory minerals during granitoid melt crystallization, two-stage Nd model ages t Nd(DM2-st) [48] were calculated from the mean crustal ratio 147 Sm/ 144 Nd = 0.12 [49].

Zircon U-Pb Dating
Zircons were isolated using conventional heavy liquid and magnetic techniques before handpicking under a binocular microscope at the Institute of Geology and Nature Management, Far East Branch of the Russian Academy of Sciences, Blagoveshchensk, Russia. Zircon grains were handpicked from concentrates and mounted in a 1" epoxy mount along with multiple fragments of zircon standards (FC, SL2, and R33). The mounts were imaged by BSE using a Hitachi S−3400N scanning electron microscope (SEM) equipped with a Gatan Chroma CL2 detector.
For geochronology, U-Th-Pb analyses of individual zircon grains were performed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Arizona LaserChron Center, Department of Geosciences, University of Arizona, Tucson, AZ, USA. For details of the analytical procedures, see [50]. The isotopic analyses involved the ablation of zircon using a Photon Machines Analyte G2 excimer laser coupled to a Thermo Element 2 single-collector-ICP-MS, using a drill rate of~1 µm/s, resulting in a final ablation pit depth of~15 µm. Grain selection and analysis sites were selected based on CL and BSE images with care taken to avoid inclusions and cracks. Calibration was performed using the FC zircon standard, with a well-established age of 1099.3 ± 0.3 Ma [51]. The Sri Lanka (SL2) and R33 [52] reference zircons were used as secondary standards for data quality control. Our 206 Pb/ 238 U and 207 Pb/ 206 Pb ages for SL2 zircon were 563.2 ± 4.8 Ma and 568 ± 16 Ma (2σ), respectively, which were in good agreement with the ID-TIMS ages reported by Gehrels et al. [53]. Our results yielded weighted mean 206 Pb/ 238 U and 207 Pb/ 206 Pb ages for the R33 zircon of 417 ± 7 Ma and 415 ± 8 Ma, respectively, which were consistent with the ages of Black et al. [52] and Mattinson [54].
Systematic errors were 0.9% for 206 Pb/ 238 U ratios and 0.8% for 206 Pb/ 207 Pb ratios (2σ). U concentrations and U/Th ratios were calibrated relative to the SL2 zircon standard and were accurate to~20%. The concordia U-Pb ages were calculated using Isoplot v. 4.15 [55]. The following data were excluded from the final age calculations: data for which it was not possible to calculate a concordia age and 206 Pb/ 238 U and 207 Pb/ 235 U ratios with errors of >3%, as these exceeded the precision of the LA-ICP-MS method. The peak age was derived from the AgePick program developed by George Gehrels [56].

Hf-in-Zircon Isotopes
Hf isotope analyses were performed using a Nu Instruments high-resolution (HR)-ICP-MS and attached Photon Machines Analyte G2 Excimer laser at the Arizona LaserChron Center. Instrument settings were established first by analysis of 10 ppb solutions of JMC475 and a Spex Hf solution followed by analysis of 10 ppb solutions containing Spex Hf, Yb, and Lu. When all solutions yielded 176 Hf/ 177 Hf ratios of~0.28216, instrument settings were considered optimized for laser ablation analyses and seven standard zircons (Mud Tank (MT), 91500, Temora (TEM), R33, FC52 (FC), Plesovice (PLES), and SL or (SL-F)) were analyzed. When precision and accuracy were considered acceptable, the unknowns were analyzed using the same acquisition parameters as for the standards.
Laser ablation analyses were conducted using a laser beam diameter of 40 µm, with the ablation pits located on the U-Pb analysis pits. Each acquisition consisted of one 40-s integration on backgrounds (on peaks with no laser firing) followed by 60 one-second integrations with the laser firing. Analyses were performed using a typical laser fluence of~5 J/cm 2 , a pulse rate of 7 Hz, and an ablation rate of 0.8 µm/s. Each standard was analyzed once for every~20 unknowns. For details of the analytical procedures, see [50].
The 176 Hf/ 177 Hf ratio at the time of crystallization is calculated from measurement of present-day 176 Hf/ 177 Hf and 176 Lu/ 177 Hf ratios, using the decay constant of 176 Lu (λ = 1.867 × 10 −11 ) from Scherer et al. [57] and Söderlund et al. [58]. Calculation of ε Hf(t) values is based on the chondritic ratios of 176 Hf/ 177 Hf (0.282785) and 176 Lu/ 177 Hf (0.0336) given by Bouvier et al. [59]. The mantle model age t Hf(DM) was calculated using the measured 176 Lu/ 177 Hf ratio of the zircon, although this only provides a minimum age for the source material of magma from which the zircon crystallized. Therefore, we also calculated the crustal model age t Hf(C) , which assumes that the parental magma of the zircons was produced from average continental crust. 176 Lu/ 177 Hf ratios for average continental crust vary significantly from~0.001 to 0.023 [60]. Vervoort and Patchett [60], Amelin and Davis [61] proposed a 176 Lu/ 177 Hf ratio of 0.0093 for the average continental crust, while Griffin et al. [62] recommended a value of 0.015. However, we considered a value of 0.093 to more realistically reflect the 176 Lu/ 177 Hf ratio of average continental crust, because in some cases the use of the higher ratio results in Hf crustal model ages that are unrealistically higher than the corresponding whole-rock Nd model ages [63][64][65]. The t Hf(C) values calculated for 176 Lu/ 177 Hf ratio of 0.015 are given in Supplementary Materials Table  S3 for comparison. The depleted mantle line was defined by present-day 176 Hf/ 177 Hf = 0.28325 and 176 Lu/ 177 Hf = 0.0384 [62].

Major and Trace Element Geochemistry
Chemical composition of siliciclastic rocks of the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations are given in Supplementary Materials Table S1.
On a log(SiO 2 /Al 2 O 3 ) versus log(Na 2 O/K 2 O) classification diagram, [66], the siliciclastic rocks from all formations plot as greywacke (Figure 4a). On a log(SiO 2 /Al 2 O 3 ) versus log(Fe 2 O 3 /K 2 O) classification diagram, [67], the samples plot mostly as arkose and wacke, and rarely as litharenite  Table S1), indicating that rocks in the source area recorded low degrees of weathering. A CIA versus WIP diagram [68] shows that the siliciclastic rocks plot as fresh rocks (Figure 4c).

In Situ Zircon Lu-Hf Isotopic Analyses
The results of Lu-Hf isotope analysis are shown in Figure 8 and listed in Table S3. As follows from Section 4.2 and Figures 7 and 8, major zircon age populations in the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations occurred in the Carboniferous-Devonian, Cambrian, Neoproterozoic, Paleoproterozoic, and Archean, and displayed a wide range of Hf model ages. The Paleoproterozoic and Archean zircons had Hf model ages (t Hf(C) ) ranging from 2.98 to 2.21 Ga. A group of Cambrian and Neoproterozoic zircons had younger Hf model ages (t Hf(C) ) of 1.53-1.00 Ga. A more complicated pattern was observed for the Hf isotopic composition of Carboniferous-Devonian zircon grains ( Figure 8), which had t Hf(C) values ranging from 1.88 to 0.46 Ga (Table S3). Similar Hf model ages values characterized zircon grains from the Aga-Borshchovochnyi metamorphic complex (Figure 8).

Whole-Rock Nd Isotopic Analyses
The results of Sm-Nd isotope analysis are presented in Figure 9 and Table 2 Table 2), in comparison with whole-rock compositions of the magmatic and metamorphic rocks of the Dzhugdzhur-Stanovoy Superterrane (grey field, data from [70]). * Age of the youngest zircon grains. ** The errors (2σ) refer to the last digits.

Depositional Age of the Formations
The Aga-Borshchovochnyi metamorphic complex is tentatively assigned a late Paleozoic age (middle Paleozoic, according to the Russian stratigraphic nomenclature) [34], but the youngest concordant zircon age in this complex is 492 ± 6 Ma. We consider that if the Aga-Borshchovochnyi metamorphic schists were indeed late Paleozoic in age, then they would have contained Devonian and, possibly, early Carboniferous zircons, as is the case with the other formations. On this basis, a Cambrian age for the Aga-Borshchovochnyi metamorphic complex cannot be excluded. Alternatively, the complex contains rocks of different ages.
As shown in Section 2, the stratigraphic position of the Khara-Shibir Formation corresponds to the lower Bashkirian stage [32], with the youngest concordant zircon age being 330 ± 4 Ma, corresponding to the Visean-Serpukhovian boundary. On the basis of faunal data, the Shazagaitui Formation is assigned an upper Bashkirian stage [32]. The youngest concordant zircon age found in the sandstone of the Shazagaitui Formation is 324 ± 4 Ma, which essentially corresponds to the Mississippian-Pennsylvanian boundary and does not contradict its biostratigraphic age.
There is an inconsistency between the Sakmarian-early Artinskian [32] or late Asselian-early Artinskian ages [41] proposed for the Zhipkhoshi Formation, as determined from faunal data, and the youngest zircon concordant age of (313 ± 3 Ma). It looks somewhat odd considering that intense magmatism is recognized both in the Carboniferous and Permian in the region (see Section 5.2 therein).

Provenance
Given the major age populations and variability in isotopic compositions of zircon grains such as ε Hf(t) (from −17.8 to −6.6) from the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations and whole-rocks two-stage Hf model ages t Hf(C) (from 2.56 to 1.65 Ga), we can assume that the sources of these zircons varied markedly. As structural analysis of the region attests [7,9,10,30,[71][72][73][74][75], the oldest zircon grains in sandstones of the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations, which have Hf model ages (t Hf(C) ) of >2.20 Ga, could be derived from the southern edge of the Siberian Craton ( Figure 1). Igneous and metamorphic complexes that represent fragments of the cratonic basement could be a source for these zircons. Suitable Archean rocks include c. 2.86 Ga tonalite gneiss [70] [97], could also have been a source for Paleoproterozoic zircon grains within sedimentary rock assemblages of the Chiron Basin.
Zircon grains with Cambrian ages are rare in sedimentary rocks of the Chiron Basin, and we report Hf isotope data from a single grain only ( Figure 8, Table S3). This grain has a t Hf(C) of 1.00 Ga and could be related to denudation of a differentiated andesite-dacite-rhyolite association assigned to the Caledonian stage of the Uda-Vitim island-arc system, which extends along the southern periphery of the Siberian Craton [100,102]. According to Gordienko [102], volcanic rocks have the following crystallization ages: 529 ± 3 Ma (plagio-rhyolites in the Eravna Area), 516 ± 5 Ma (quartz porphyry in the Eravna Area), 529 ± 3 and 534 ± 6 Ma (rhyolites in the Oldynda Area), and 530 ± 26 Ma (dacites in the Kydzhimit Area).
The majority of zircon grains from sandstones of the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations are Devonian-Carboniferous in age. In terms of Hf model ages, the zircon grains can be subdivided into two groups. The first group is characterized by relatively old (mainly Paleoproterozoic) model ages (t Hf(C) = 2.25-1.7 Ga) ( Figure 8, Table S3), the sources of which could be the southern fringe of the Siberian Craton, where granitoids of the Olekma Complex have an age of 358 ± 6 Ma [103]. In addition, granitoids of the Barguzin (or Vitimkan) Complex, which are dated at 330-310 Ma [104,105], could constitute a source of the early Carboniferous zircons. However, this is unlikely as the age of this complex is 297-279 Ma [106]. In addition, there is no certainty regarding the source of Late Devonian zircons. Conceivably, their sources could be intrusions of the Krestovsky and Kruchinin complexes [9], for which there are no reliable age data.
The second group of Devonian-Carboniferous zircons is characterized by significantly younger (mainly Neoproterozoic) model ages (t Hf(C) = 1.35-0.36 Ga) ( Figure 8, Table S3), suggesting derivation from a source with a significant juvenile component, likely to have been eroded island arcs. Considering the location of the Chiron Basin along the boundary between the southern margin of the Siberian Craton and the western segment of the Mongol-Okhotsk Fold Belt (Figure 1), the sources of such zircons could have been Paleozoic island arcs of the Mongol-Okhotsk Ocean, whose existence is inferred in most geodynamic models ( [2,29,107] and references therein). Conceivably, a fragment of such an arc is the Zun-Shiveya Complex, which consists of rhyolites, rhyodacites, and granite porphyry, which have tentatively been ascribed as Late Devonian to early Carboniferous in age [9].
In conclusion, it should be noted that like zircon grains from the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations, zircon grains from the Aga-Borshchovochnyi metamorphic complex display a similar range of ages (with the exception of those with Devonian-Carboniferous ages) and have identical isotopic characteristics ( Figure 8; Table 2, Table S3). This supports an interpretation in which Archean, Paleoproterozoic, Neoproterozoic, and Cambrian zircons of the Aga-Borshchovochnyi metamorphic complex and Khara-Shibir, Shazagaitui, and Zhipkhoshi formations were derived from the same source.
Sandstones of the Shazagaitui Formation have the oldest model t Nd(C) ages of 2.65 Ga (Table 2), consistent with the observation that the highest number of Paleoproterozoic and Archean zircons occur in these rocks (Figure 7c). On this basis, we assume that source materials for these rocks must have been derived from the Siberian Craton. Sandstones from the Khara-Shibir and Zhipkhoshi formations have younger model ages (t Nd(C) = 1.70-1.65 Ga) and contain fewer Paleoproterozoic and Archean zircons (Figure 7b,d). Considering the Hf isotope data, we suggest at least two sources, one within the Siberian Craton and another that does not represent reworked early Precambrian continental crust, such as the island arcs mentioned above.

Tectonic Environments of Sedimentation
For establishing compositions of source rocks, we used well-known discrimination diagrams based on the values and ratios of major and trace elements. Some of these diagrams are shown in Figure 10. Plots of data from siliciclastic rocks within the Khara-Shibir, Shazagaitui, and Zhipkhoshi formations in Na 2 O-K 2 O-CaO (Figure 10a  On the whole, the geochemical characteristics of the siliciclastic rocks ( Figure 11), including their wacke/greywacke composition (Figure 4a,b), weak degree of source weathering (Figure 4c), and the presence of interbedded conglomerate and gravelstone (see Section 2), are evidence that sedimentation proceeded in an active tectonic regime. Considering that the clastic sedimentary rocks were sourced from igneous and metamorphic rocks of the Siberian Craton on the one hand, and island arcs on the other, we believe that sedimentation occurred in a backarc-basin setting of the southern periphery of the Siberian Craton facing the Paleozoic Mongol-Okhotsk Ocean.  [111] (e), and Co-Zr/10-Th after [112] (f) for siliciclastic rocks of the Chiron Basin. Fields for the various tectonic settings: A-Oceanic island arc; B-Continental island arc; C-Active continental margin; D-Passive margin.

Conclusions
On the basis of our investigation of detrital zircon U-Pb and Hf isotope data and whole-rock Nd isotope data for sedimentary rocks of the Chiron Basin, we draw the following conclusions.

1.
Geochemical characteristics of the siliciclastic rocks, as well as the presence of conglomerate and gravelstone interbeds, suggest that sedimentation occurred in an active tectonic regime.

2.
The main source of sedimentary rocks of the Chiron Basin was igneous and metamorphic rocks from the southern periphery of the Siberian Craton. Devonian-Carboniferous zircon grains with relatively young (mainly Neoproterozoic) Hf model ages were derived from island arcs.

3.
Combining the new data with regional geological data suggests that sedimentary rocks of the Chiron Basin likely formed in a back arc basin setting on the southern periphery of the Siberian Craton facing the Paleozoic Mongol-Okhotsk Ocean.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/3/279/s1, Table S1. Chemical composition of the sedimentary rocks of the Chiron Basin. Table S2. U-Th-Pb LA-ICP-MS data for zircons from metasedimentary rocks of the Chiron Basin. Table S3. Hf isotopic data for zircons from metasedimentary rocks of the Chiron Basin. References.