Early Precambrian Crustal Evolution in the Irkut Block (Sharyzhalgai Uplift, Southwestern Siberian Craton): Synthesis of U–Pb, Lu–Hf and Sm–Nd Isotope Data Open Access

Precambrian cratons store a record of crustal growth and evolution in the early Earth. The Siberian craton is the largest and still poorly documented cratonic unit in Eurasia. It comprises the Tunguska, Magan, Anabar, Olenek, Aldan, and Stanovoy tectonic provinces distinguished from geophysical (magnetic and gravity) data and compositions of drill core samples and crustal xenoliths (Rosen et al., 1994; Rosen, 2003; Smelov and Timofeev, 2007). The tectonic provinces consist of Archean gneiss-granulite and granite-greenstone terranes separated by orogens or sutures that formed during Paleoproterozoic collision events and amalgamation of the Siberian Craton (Rosen et al., 1994; Rosen, 2003). The basement is buried under 2 to 5 km thick Mesoproterozoic to Lower Cretaceous sedimentary cover over the greatest part of the craton but crops out in the Aldan and Anabar shields, in a few uplifts in the southwestern craton magin, and in the Angara and Akitkan orogens (Fig. 1 A). Early Precambrian crustal formation in the northern and central parts of the craton within the Yakutian diamond province was inferred from isotopic signatures of zircons from lower crustal xenoliths and kimberlite-hosted zircon xenocrysts. Namely, U–Pb and Lu–Hf zircon isotope data from the Anabar province revealed a major stage of Paleoarchean crustal growth (3.6–3.4 Ga), as well as recycling during Neoarchean and Paleoproterozoic tectonic and magmatic events with inputs of juvenile material (Kostrovitsky et al., 2016; Shatsky et al., 2016, 2018; Moyen et al., 2017). This evidence generally correlates with zircon isotope data for granulites exposed in the Anabar shield (Gusev et al., 2017, 2019). Data from exposed basement complexes can shed more light on the mechanisms of crustal growth and recycling, and on the sequence of early Precambrian events.

The early Precambrian basement complexes of the largest Tungus province in the western Siberian craton crop out only in the southwestern Sharyzhalgai, Biryusa, and Angara–Kan basement uplifts (Fig. 1A). The largest Sharyzhalgai Uplift extends for 350 km from Lake Baikal to the Oka River in the northwest. The uplift is bordered by the Main Sayan Fault (shear zone) in the southwest and is overlain by Neoproterozoic–Phanerozoic sediment cover in the northeast. It consists of the Bulun and Onot granite-greenstone and Kitoy and Irkut gneiss-granulite blocks delineated by northwest- and north-trending faults and suture zones (Fig. 1B). The suture zones are marked by sheets and small blocks of upper mantle peridotite and mafic to felsic lower crustal granulites. The final amalgamation of four blocks in the Paleoproterozoic was accompanied by metamorphism and granitic and mafic magmatism at 1.86–1.84 Ga, as part of general assembly of the Siberian craton and its incorporation into the Columbia supercontinent (Rosen, 2003). Through the recent decade, a wealth of geological, petrological, and isotope data has been obtained for the Irkut gneiss-granulite block which occupies about a half of the Sharyzhalgai Uplift territory (Fig. 1). This paper presents new U–Pb and Lu–Hf zircon isotope data from Archean and Paleoproterozoic metamorphic and magmatic complexes, as well as a synthesis of all geological, geochemical, and isotope characteristics of Archean and Paleoproterozoic units in the Irkut Block formed during ~1.6 Ga of early Precambrian history. New results is used together with previously published data to (1) describe main processes of magmatism and sedimentation; (2) time the onset of crustal growth, (3) reveal main crustal growth and recycling events, and (4) compare the early Precambrian crustal evolution of the Tungus and Anabar provinces of the Siberian craton.

This study is based on sensitive high-resolution ion mass spectrometry (SHRIMP II) U–Pb zircon data obtained previously by the author at the Center for Isotopic Research of the Russian Geological Research Institute (St. Petersburg) and other published geochronological data. The results are summarized in a number of main (1, 2) and supplementary (S1–S4) tables: ages of metamorphic and magmatic rocks in the Irkut Block corresponding to main events of its early Precambrian history in Table 1; compositions of representative samples in Table S1; new U–Pb zircon ages for felsic granulite and charnockite samples in Table S2; new Nd isotope data which extend the previous results (Gladkochub et al., 2009; Turkina, 2010; Turkina et al., 2012, 2017; Turkina and Kapitonov, 2019) in Table S3; new and previous Hf isotope zircon data for metamorphic rocks and granitoids in Table S4; Sm–Nd and Lu–Hf isotope systematics of rocks and zircons, respectively, in Table 2. The Lu–Hf isotope compositions were determined in 77 magmatic and 45 detrital zircons (122 grains in total); about a half of dates were reported earlier (Turkina et al., 2012, 2016; Turkina and Kapitonov, 2019). The analytical procedures are detailed in Supplementary Materials S5.

The ε_Nd(T) values were calculated using CHUR ratios of ¹⁴³Nd/¹⁴⁴Nd = 0.512638 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.1967 (Jacobsen and Wasserburg, 1984) for the age established for magmatic or estimated for sedimentary rock. Single-stage model ages T_Nd(DM) were estimated relative to depleted mantle (DM) with reference to ¹⁴³Nd/¹⁴⁴Nd = 0.51315 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.2136 (Goldstein and Jacobsen, 1988). A two-stage model age was used in a few cases, at ¹⁴⁷Sm/¹⁴⁴Nd > 0.12. The initial ε_Hf(T) values were calculated for ages according to the ²⁰⁶Pb/²⁰⁷Pb ratio or for upper intercept disconcordia ages in the case of > 10% discordance, with reference to chondritic ratios of ¹⁷⁶Lu/¹⁷⁷Hf =0.0332 and ¹⁷⁶Hf/¹⁷⁷Hf = 0.282772 (Blichert-Toft and Albarède, 1997). Two-stage model ages $THfCDM$ were calculated based on ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 and ¹⁷⁶Hf/¹⁷⁷Hf = 0.28325 ratios for depleted mantle (Bouvier et al., 2008), by projecting the initial ¹⁷⁶Hf/¹⁷⁷Hf value of zircon onto the DM line, with the use of the average crustal value of ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 (Griffin et al., 2000).

The crustal growth processes are recorded in rocks with juvenile isotope signatures, including positive ε_Nd(T) and ε_Hf(T) ratios similar to those in depleted mantle of the respective age, as well as the model ages T_Nd(DM) and $THfCDM$ approaching the isotope ages of rocks and zircons (± 200 Ma). The oldest Nd and Hf model ages over a dataset are attributed to the earliest crustal growth event, while later crustal growth events are inferred either from the ages of mantle-related rocks and their derivates or from the model ages of rocks/zircons with juvenile isotope signatures.

Rock complexes produced by crustal recycling have lower ε_Nd(T) and ε_Hf(T) values than those with juvenile characteristics, and their model ages are at least 200 my younger than the U–Pb zircon ages. Recycling processes mainly occur during intra-crustal melting or by interaction of mantle-related magmas with sialic crust. The ages of (meta)magmatic rocks and near coeval metamorphism mark collision-related recycling events. The greatest difficulty is the estimation of the age of the crust involved in recycling processes. Its minimum ages are inferred from the maximum T_Nd(DM) and $THfCDM$ values for rock associations and the coeval populations of zircons. The Archean division used in this study is into the Eo- Paleo-, Meso-, and Neoarchean stages of ≥3.6 Ga, 3.6 to 3.2 Ga, 3.2 to 2.8 Ga, and 2.8 to 2.5 Ga, respectively.

The Irkut Block has a complex fold-thrust structure resulting from Paleoproterozoic collision (Grabkin and Melnikov, 1980; Hopgood and Bowes, 1990; Aftalion et al., 1991), including two events revealed in an exhaustively documented outcrops on the shore of Lake Baikal (Hopgood and Bowes, 1990). The early stage of compressional deformation resulted in tight isoclinal and then asymmetrical and quasi-vertical folds and was accompanied by vein injections of granitoids. The later event during post-collisional extension led to the formation of dome uplifts with granitic cores. The wings of the dome uplifts are composed of mafic and felsic granulites, and wide inter-dome zones consist of paragneisses and less abundant marbles and calciphyres (Grabkin and Melnikov, 1980). The structure of the western block part in the Toysuk–Kitoy interfluve comprises low-angle thrust sheets composed of predominant felsic and mafic orthogranulites and lesser amounts of high-Al paragneisses injected by granitoids (Turkina and Sukhorukov, 2015).

The metamorphic complex of the Irkut Block includes four rock associations (Tables 1 and S1). Predominant association 1: two-pyroxene (±amphibole) mafic and most abundant orthopyroxene-biotite felsic granulites in the wing domes and in zones of nearly isoclinal folds which alternate in the Baikal shore or form separate sheets in the western block part. Fragments of mafic granulites also occur as inclusions among dome granitoids. The magmatic protoliths of mafic and felsic granulites (Table S1) formed in the late Neoarchean, at 2.66–2.70 Ga (Poller et al., 2005; Turkina et al., 2012), possibly, coevally with metababbro sills containing ~2.65 Ga zircons (Sal’nikova et al., 2007). The compositions of mafic granulites correspond to tholeiite basalts (Mg#=45–67) with weakly fractionated REE spectra ((La/Yb)_n=0.8–4.1) (Fig. 2c), which bear typical subduction signatures with depletion in TiO₂ (0.7–1.5%) and Nb (2–8 ppm) and LILE enrichment (Fig. 3a). Felsic granulites show moderate contents of K₂O (1.0–3.3%) and fractionated REE spectra ((La/Yb)_n=10–78) with a weak negative Eu anomaly (Eu/Eu*=1.0–0.7) (Fig. 2b), which makes them different from Archean tonalite–trondhjemite–granodiorite (AR TTG) rocks. Like the mafic granulites, the felsic varieties have negative Ti and Nb anomalies in the multi-element spectra (Fig. 3b). Judging by the geochemical features and ε_Nd(T) variations from +3.9 to –4.8 and T_Nd(DM) = 2.9–3.3 Ga, the magmatic protoliths of the felsic granulites formed on the active margin of a Paleoarchean continental crustal block (Turkina et al., 2012).

Association 2: intermediate orthopyroxene and two-pyroxene granulites found as few inclusions among the rocks of association 1 in the eastern part of the Baikal shore outcrop. They differ from Neoarchean felsic granulites in relatively low SiO₂ contents and weakly fractionated REE spectra with (La/Yb)_n = 6.3–12.9 (Table S1, Fig. 2a). The granulites contain ~3.3–3.4 Ga magmatic zircon cores and Mesoarchean (~3.0 Ga) metamorphic zircons (Poller et al., 2005; Turkina et al., 2011) and are fragments of Paleoarchean crust (Table 1).

Association 3: orthopyroxene–sillimanite–garnet–cordierite–biotite high-Al paragneisses in thin sheets alternating with mafic and felsic granulites in the western block part. The paragneisses were derived from mudstone protoliths, their source provinces are dominated by felsic rocks and minor mafic varieties (Turkina and Sukhorukov, 2015). Moderately fractionated REE spectra of the paragneisses ((La/Yb)_n = 5–9) without a negative Eu anomaly (Table S1, Fig. 2d) are similar to those of Archean shales (Taylor and MacLennon, 1985). Detrital zircons from paragneisses show a large range of ages (Table 1) with two predominant populations of ~3.2 and 3.0 Ga. The youngest detrital zircons defines the time of sedimentation near ~2.77 Ga which is slightly older than age of the Neoarchean orthogranulite protoliths (Turkina et al., 2017). Judging by the age of metamorphic monazite (2557 ± 11 Ma), the protoliths of high-Al paragneisses found in the western part of Baikal shore outcrops likewise formed in the Archean (Levchenkov et al., 2012).

Association 4: biotite and garnet-biotite (±orthopyroxene, ±cordierite) paragneisses interlayering with marbles and calciphyres in large inter dome zones in the eastern part of the Baikal shore outcrops (Turkina and Urmantseva, 2009). Migmatization of paragneisses results in formation banded migmatites and diatexites and injection of granitic veins. The paragneisses derived from graywacke to mudrock protoliths. They differ markedly from the Archean paragneisses in relatively high Th contents and a distinct negative Eu anomaly (Table S1), which record a potassic granitic component in the source province (Fig. 4a). The ages of detrital zircons from paragneisses range from ~2.75 to 1.94 Ga with two peaks at ~2.2–2.3 and 1.95–2.0 Ga; the ages of youngest detrital zircons and metamorphic zircon rims constrain the deposition time between 1.94 and 1.86 Ga (Turkina et al., 2010). The upper age bound of the paragneiss protoliths corresponds to the 1.85 Ga age of magmatic zircons from migmatites (Turkina and Sukhorukov, 2017).

Within the Irkut block there were Neoarchean and Paleoproterozoic events of high-temperature metamorphism and related granitic magmatism (Table 1). An older Mesoarchean (~3.0 Ga) metamorphism is record only in zircons from relict Paleoarchean granulites and occurred at 700–750 °C according to Ti-in-zircon thermometer. On the other hand, oscillatory zoning and high (Lu/Gd)_n ratios of 17 to 42 in the ~3.0 Ga detrital zircons from Archean paragneisses indicate their magmatic source (Turkina et al., 2017). Thus, metamorphism about 3.0 Ga was nearly coeval with magmatism.

The mafic and felsic granulites (association 1) and the Archean paragneisses record the Neoarchean high-temperature metamorphism at ~2.54–2.57 Ga at 790 to 830 °C (according to Ti-in-zircon thermometer for orthogranulites) (Poller et al., 2005; Levchenkov et al., 2012; Turkina et al., 2012, 2017). The Neoarchean metamorphism was near coeval with emplacement of numerous veins and small intrusions of gneissic granitoids at 2.53–2.56 Ga (Gladkochub et al., 2005; Sal’nikova et al., 2007; Turkina et al., 2012). The vein granitoids have I-type potassic comositions with negative anomalies in Eu, Nb, Ti, and Sr in multi-element patterns (Table S1; Fig. 2d, sample 110-06). Zircons from these rocks were altered during Paleoproterozoic collisional events at ~1.86 Ga (Turkina et al., 2012).

The Paleoproterozoic stage of granulite metamorphism (~1.85–1.86 Ga) involved all rock associations, including the Paleoproterozoic paragneisses (Table 1). The parameters of the peak of metamorphism are T = 850–870 °C and P of 7–8 kbar. The Paleoproterozoic metamorphism is characterized by a P–T path close to isothermal decompression indicating an extension setting (Sukhorukov, 2013; Sukhorukov and Turkina, 2018). The coeval magmatism (1.84–1.87 Ga) produced large granitic intrusions in the northwestern part of the Irkut Block and small charnockite intrusions in the southeast, as well as numerous postcollisional vein granitoids (Sal’nikova et al., 2007; Turkina and Kapitonov, 2019). Paleoproterozoic intrusive granitoids include high-Fe monzodiorites, granodiorites, and granites enriched in HFSE, which are compositionally similar to A-type granites (Table S1; Fig. 4b). High- and low-K charnockites have I-type granite compositions and fractionated REE spectra with HREE depletion and weak Eu anomalies, comparable to those in felsic granulites (Table S1; Fig. 4c). Mafic magmatism during the Paleoproterozoic stage (~1.86 Ga) produced gabbro-dolerite dikes and small gabbro and mafic-ultramafic intrusions (Gladkochub et al., 2013; Mekhonoshin et al., 2016), as well as subalkaline basitic veins (Ivanov et al., 2019).

Thus, the crustal growth in the Irkut gneiss granulite block began in the Paleoarchean (~3.4–3.3 Ga). The ages of predominant detrital zircons from Archean paragneisses mark two events of Mesoarchean presumably felsic magmatism at ~3.2 and 3.0 Ga. The lateral extent of Paleo-Mesoarchean crust is traceable in isotopic signatures of Neoarchean granulites and granites (T_Nd(DM)=2.9–3.3 Ga) and Paleoproterozoic charnockites (T_Nd(DM)=2.7–3.2 Ga) (Table S4), as well as confirmed by finds of ~3.7 to ~3.0 Ga detrital zircons in Archean paragneisses (Turkina et al., 2017).

The rocks of the Neoacrchean stage include the 2.7–2.62 Ga magmatic protoliths of mafic and felsic granulites and the ca. 2.77 Ga sedimentary protoliths of high-Al paragneisses. Neoarchean history ended with high-temperature metamorphism and granitic magmatism at ca. 2.5 Ga. Another sedimentation event in the Paleoproterozoic (1.9–41.86 Ga) ended by collisional granulite metamorphism and granitic and mafic magmatism at 1.88–1.84 Ga.

Sample 11-08 was collected from orthopyroxene-biotite granulite (association 1) intruded by branching veins of pegmatitic granites in the outcrop on the shore of Lake Baikal (51°44′816″ N; 103°57′198″ E). The sample has a rhyodacitic composition and a strongly fractionated REE pattern with (La/Yb)_n = 79) common to felsic granulites of association 1 (Turkina et al., 2012). Zircons occur as 150–350 μm long prismatic (aspect ratios of 1:2 to 1:3) crystals with oscillatory zoned rims (Fig. 5b). They contain 341 to 3252 ppm U and 54 to 566 ppm Th, and their Th/U ratios (0.13–0.67) most often correspond to magmatic zircons, except for low Th/U of 0.03 to 0.07 in some grains with the highest U and lowest Th contents (Table S2). A concordant age of 2709.7 ± 6.7 Ma (MSWD= 2.0), obtained for thirteen zircon grains (Fig. 6 a) provides reliable timing for the formation of zircons and the protolith of felsic granulite, due to the lack of correlation between the ²⁰⁷Pb/²⁰⁶Pb age and the U contents. The age 2744 ± 13 Ma (MSWD=0.02) for two zircons (Nos. 3 and 7) may represent an older radiogenic Pb component in the source.

Sample 76-16 of Paleoproterozoic charnockite was collected from a large body, with a visible size of 200 m, on the Baikal shore (51°48′42.7″ N; 104°31′23.8″ E). It has a plagiogranitic composition and fractionated REE spectra with (La/Yb)_n = 55, which is typical of these rocks (Table S1; Fig. 4c). The sample contains 300–500 μm euhedral prismatic zircons with aspect ratios of 2–3. Most of the zircons have dark or weakly zoned cores occupying up to half grain (Fig. 5,a) and CL light, oscillatory zoned magmatic rims. The magmatic zircon rims contain 99 to 256 ppm Th and 50 to 147 ppm U resulting to high Th/U ratios (1.1–2.8), whereas the dark cores have Th contents lower than U (70–217 ppm against 105–678 ppm, respectively), and Th/U ratios (0.3–1.1) within the range for magmatic zircons (Table S2). The eleven magmatic zircon rims yielded a concordant age of 1848 ± 5 Ma (MSWD = 0.07) (Fig. 6b). The weakly zoned cores are of different ages: three Neoarchean ones with an ²⁰⁷Pb/²⁰⁶Pb ages of 2529 to 2589 Ma (D = 1–3%) and two others cores with ²⁰⁷Pb/²⁰⁶Pb ages of 2384 and 2306 Ma (D = 1 and 8%) may belong to the same ~2.4 Ga population. The 1855 ± 7 Ma age obtained for the dark unzoned core of one zircon grain (point 2.1) is the same as for its magmatic rim (point 2.2), i.e., the core probably lost Pb due to interaction with melt. Assuming partial loss of Pb in other cores as well, their age may be estimated as ~2.6 and 2.4 Ga.

Magmatic zircons (~2.7 Ga) from sample 11-08 of orthopyroxene-biotite felsic granulite (association 1) have little variable ε_Hf and $THfCDM$ values of +2.6 to +0.2 and 3.0–3.1 Ga, respectively (Table S3; Fig. 7), within the previously constrained (Turkina et al., 2012) ranges for Neoarchean felsic granulites: ε_Hf +2.9 to –0.5 and T^C_Hf(DM) 3.0–3.2 Ga (Table 2). Magmatic zircons from Neoarchean gneissic granites have lower ε_Hf (–2.8 to –5.3) and higher $THfCDM$ of 3.2–3.4 Ga.

The oldest ~3.3–3.2 Ga cores of zircons from sample 77-84 of Paleoarchean orthogranulite have mainly positive ε_Hf(T) (+2.9 to –0.4) and $THfCDM$ 3.4–3.6 Ga (Table S3). The same Hf model age (3.4–3.6 Ga) was obtained for metamorphic (~3.0 Ga) zircons that inherited the isotope composition of the magmatic cores, i.e., formed in a closed isotopic system.

Paleoarchean and predominant Mesoarchean zircons were analyzed from Archean paragneiss sample 16-13 (association 3). A few oldest (≥3.3 Ga) zircon grains showed e_Hf values from +1.3 to –4.1 and $THfCDM$ = 4.1–3.8 Ga and thus had an Eoarchean crustal source (Table S3). The predominant detrital zircons with ages of ~3.2 and 3.0 Ga and rare younger (~2.8–2.9 Ga) grains show progressively decreasing ε_Hf values and Hf model ages in range from 3.6–3.9 to 3.3–3.9 Ga, respectively (Table 2).

Magmatic zircons from charnockite (sample 76-16) have the least radiogenic compositions (ε_Hf from –12.1 to –16.9; $THfCDM$ =3.2–3.5 Ga) compared to zircons from Paleoproterozoic granitoids in the northern Irkut Block (Table S3). Their Hf isotopic signatures correspond to evolution trend of inherited zircon cores with ages of ~2.5–2.6 Ga (ε_Hf –3.2 to –8.0; $THfCDM$ = 3.3–3.5 Ga) and ~2.3–2.4 Ga (ε_Hf from –11.2 to –11.8; $THfCDM$⁠) =3.6 Ga).

The Nd isotope composition was characterized in 49 samples. The newly determined 2.48–2.50 Ga Nd model ages of Paleoproterozoic paragneisses and migmatites (Table S4; Fig. 8a) fit the lower values of the previously estimated range between 2.46 and 3.10 Ga for these rocks (Table 2). Low- and high-K charnockites form small intrusions in dome cores have extremely negative ε_Nd(T) of –9.3 to –11.7 and T_Nd(DM)=2.7–3.2 Ga indicating a long-living crustal source (Table S4). The model age range overlaps that of Neoarchean felsic granulites with T_Nd(DM) from 2.9 to 3.3 Ga (Table 2; Fig. 8 a).

The model age T_Nd(DM) = 3.55 Ga of Paleoarchean granulites with ε_Nd(T) = +0.7 places time constraints on the earliest crustal growth event (Fig. 7). The ca.2.7 Ga mafic granulites with ε_Nd(T) from +3.9 to +1.6, which were derived from depleted mantle source, indicate the Neoarchean crustal growth. Neoarchean (~2.7–2.55 Ga) felsic orthogranulites, paragneisses, and granitoids show a large ε_Nd(T) range from +1.2 to –4.8 and an Nd model age of 2.9–3.3 Ga (Table 2). These isotopic signatures can be interpreted as either resulting from recycling of Mesoarchean crust or being a record of Paleoarchean crust recycling with a input of juvenile mantle component. The latter interpretation is reasonable for felsic orthogranulites with ε_Nd varying from the field of Paleoarchean crust evolution to values of mafic granulites (Fig 8). The Neoarchean paragneisses and granitoids show a narrower T_Nd(DM) range of 3.1–3.3 Ga, which impedes interpretation of their sources (Table 2 and S4).

Paleoproterozoic paragneisses and migmatites have large ranges of ε_Nd(T) and model ages (–1.6 to –8.6 and 2.4 to 3.1 Ga, respectively) indicating inputs of both Archean and juvenile Paleoproterozoic crust in source province (Table 2; Fig. 8). Paleoproterozoic granitoids and charnockites show the variable negative ε_Nd(T) values and T_Nd(DM) model ages (2.5–3.3 Ga) as evidence of Archean crust material prevalent in the melting region (Table 2; Fig. 8). Negative ε_Nd(T) values as low as –12.3 were obtained for charnockites and vein granites from the southeastern part of the block. Judging by their Nd model ages (2.7–3.3 Ga) and εNd values (Fig. 8), the melts had mainly Paleo- and Mesoarchean crustal sources. On the contrary, granitoids of large intrusions in the northern block part have more radiogenic compositions (ε_Nd(T) = –4.0 to –5.5; T_Nd(DM) = 2.5–2.6 Ga) indicating Neoarchean crust inputs. Compared to these granitoids, melanocratic monzodiorites have lower ε_Nd(T) values ranging from –7.2 to –10.2 (Fig. 8), which may represent melting of enriched lithospheric mantle (Turkina and Kapitonov, 2019). Paleoproterozoic crustal growth with inputs of magma from mantle sources can be inferred from mafic–ultramafic magmatism nearly synchronous with collisional granitoid magmatism (Gladkochub et al., 2013; Ivanov et al., 2019; Mekhonoshin et al., 2016).

Thus, according to the constraints from Sm–Nd isotope systematics in samples from the Irkut Block, the continental crustal growth began in the Paleoarchean, and recycling was its predominant process till the late Paleoproterozoic (Fig. 8b). Inputs of juvenile material in the Neoarchean and late Paleoproterozoic were associated with subduction and collisional magmatism, respectively.

Rock associations of the Irkut Block show polymodal distribution of Hf model ages of zircons ranging from Eoacrhean to Paleoproterozoic (4.1–2.0 Ga). Five main peaks at 3.9–3.8, 3.6–3.5, 3.4–3.3, 3.2–3.1, and 3.0–2.9 Ga (Fig. 9) fall within the range from Paleoarchean to Measoarchean. The $THfCDM$ = 4.1–3.6 Ga ages of Paleo- and Mesoarchean detrital zircons from paragneisses suggest that the crustal growth began in the Eoarchean One Paleoarchean growth event is recorded in melanocratic orthogranulite (sample 77-84) with $THfCDM$ = 3.4–3.6 Ga which contains ~3.4–3.3 Ga magmatic zircons and 3.0 Ga metamorphic zircons with the respective inherited isotope signatures. Mesoarchean zircons predominant in paragneisses record two magmatic events at ~3.2 and ~3.0 Ga, and the $THfCDM$ values of 3.3–3.9 Ga of zircons represent recycling of Eoarchean to Paleoarchean crust in their sources (Table 2; Fig. 7b). On the other hand, the lack of the ε_Hf(T) decreasing trend in Mesoarchean zircons younging from 3.2 to 2.8 Ga indicates inputs of juvenile material. Thus, the Irkut Block underwent Eoarchean to latest Mesoarchean crustal growth and recycling of older material. Main peaks of juvenile crust correspond to ~3.9–3.8 and ~3.4–3.6 Ga events.

The Hf isotope composition of magmatic zircons from mafic granulite (ε_Hf(T) +7.2 to +2.3; $THfCDM$ =2.6–2.7 Ga) derived from a depleted mantle source (Fig. 7) records the Neoarchean event of crustal growth (2.7–2.66 Ga). Additional evidence of crustal growth is provided by positive ε_Hf(T) values of zircons from felsic orthogranulites, while the large range of ε_Hf(T) from +2.9 to –0.5 and maximum $THfCDM$ of 3.2–3.1 Ga indicates the presence of older components in the sources, which agrees with the Nd isotope signatures of the samples (T_Nd(DM) = 2.9–3.3 Ga). The ranges of –2.8 to –5.3 ε_Hf(T) and 3.2–3.4 Ga $THfCDM$ for zircons from Neoarchean granites, as well as the respective values for the Archean cores of zircons from Paleoproterozoic leucogranites of the Toysuk intrusion (ε_Hf(T) = +1.0 to –5.3 and $THfCDM$ =3.2–3.4 Ga) and for charnockite samples (ε_Hf(T) = –3.2 to –8.0 and $THfCDM$ =3.3–3.5 Ga), likewise result from recycling of older crust in the Neoarchean (Table 2; Fig. 7). The Neoarchean zircons from felsic granulites and granitoids mostly plot above the field of Eoarchean-Paleoarchean crustal evolution (Fig. 7 a), i.e., recycling of ancient crust was accompanied by inputs of subduction-related juvenile material.

Paleoproterozoic crustal growth and recycling can be inferred from large ranges of isotope parameters obtained for magmatic and detrital zircons from granitoids and paragneisses (Table 2; Fig. 7). Recycling of crust formed towards the end of the Mesoarchean produced charnockites contain inherited Archean zircon cores and Paleoproterozoic magmatic zircons with extremely low ε_Hf(T) of –12.1 to –16.9 and model ages $THfCDM$ of 3.2–3.6 Ga which suggest melting of Paleoarchean crust. On the other hand, predominant positive ε_Hf(T) values (+10 to –3.1) of detrital zircons from paragneisses record Paleoproterozoic crustal growth with two major events at ~2.3–2.4 and ~2.0 Ga recorded in zircons with highest ε_Hf(T) values. The full range of isotope parameters in detrital zircons trace contribution of Archean crust with $THfCDM$ up to 2.8 Ga in their source province.

Isotopic signatures in most of zircons from Paleoproterozoic granites bear evidence of both recycled Archean crust and juvenile Paleoproterozoic components (Fig. 7,b). As noted before, the $THfCDM$ = 3.2–3.4 Ga model ages of inherited zircon cores from leucogranites constrain the minimum crust age in the melting region. An input of juvenile material is further confirmed by more radiogenic Hf isotope compositions (ε_Hf(T) from –5.0 to –10.1) of magmatic zircons from the Toysuk and Lower Kitoy granites (Turkina and Kapitonov, 2019), which plot above the field of Paleo-Mesoarchean crust evolution (Fig. 8 a). Additional inputs from enriched lithospheric mantle synchronous with recycling can be inferred from negative ε_Hf values of –6.0 to –10.7 in zircons from monzodiorites produced by fractionation of mafic magma (Turkina and Kapitonov, 2019). The involvement of enriched subcontinental lithospheric mantle into magma generation is recorded also by negative ε_Nd from 0 to –18 in subalkaline mafic rocks from Paleoproterozoic dike complexes (Ivanov et al., 2019). In general, the isotope systematics of zircons from Paleoproterozoic paragneisses and granitoids suggest limited crustal growth at the account of both depleted and enriched mantle sources and concurrent large-scale recycling of Archean crust during granitic magmatism under postcollisional extension.

Thus, recycling of Archean crust with ≥3.5 Ga ages shown by maximum $THfCDM$ of zircons (Fig. 7b) was the main process throughout the Precambrian history of the Irkut Block. Most of continental crustal growth occurred in the Eoarchean and Paleoarchean periods, while several later events at ~2.7, ~2.4, ~2.0, and ~1.85 Ga were associated with mantle magmatism and formation of juvenile crust that became the source of detrital zircons.

Paleo- and Mesoarchean. The analyzed collection of metamorphic rocks includes two grains of ~3.7 and ~3.6 Ga detrital zircon from Archean paragneisses with magmatic trace-element signatures (Turkina et al., 2017), which represent unknown events of Eoarchean magmatism. The –1.7 and +1.3 ε_Hf(T) values of the zircons indicate a contribution of weakly depleted/enriched mantle to their magma sources. Based on analysis of a large database on the Hf isotope composition of zircons from TTG rocks Guitreau and co-authors (2012) concluded that the Archean continental crust derived by magmatic processes related with melting of non-depleted mantle.

Remnants of Paleoarchean crust found at two sites in the southeastern Irkut Block are intermediate (SiO₂ = 58–59 wt.%) mesocratic orthopyroxene or two-pyroxene granulites with mafic granulite layers. Compared to TTG rocks prevalent in the Archean record, mesocratic granulites have similar low contents of K₂O (0.67–0.77%) and most incompatible trace elements but differ in low Sr (250–290 ppm), high FeO (8–9%), and enrichment in Y and HREE, with the respective low ratios of (La/Yb)_n = 6–13 and Sr/Y = 7–14 (Table S1; Fig. 2a). These compositions may result from low-pressure melting/fractionation of a mafic source equilibrated with a garnet-free residue. Similar rocks, which differ markedly from typical TTG in the absence of HREE depletion, were found among Paleoarchean volcanics in the Pilbara craton (Smithies et al., 2019) and oldest Acasta gneisses in the Canadian shield (Reimink et al., 2016), which formed prior to the common TTG complexes. These old rocks with high FeO contents share similarity with felsic volcanics from Iceland (Reimink et al., 2016). They may result from low-pressure fractionation of tholeiite-basaltic melts (Smithies et al., 2019), more likely in a setting of plume magmatism rather than subduction inferred for Archean TTG (Martin, 1994). Unlike the Eoarchean detrital zircons, the isotope composition of the Irkut Paleoarchean mesocratic granulites show a contribution from a weakly depleted mantle source (ε_Hf(T) +2.9 to +1.3).

The evidence of Measoarchean events is limited to the Hf isotope composition of zircons from Paleoarchean mesocratic orthogranulites and Neoarchean paragneisses. The trace-element signatures of ~3.2 and 3.0 Ga zircons from paragneisses correspond to magmatic origin (Turkina et al., 2017), while ~3.0 Ga zircons from orthogranulite samples are variably depleted in HREE as a result of metamorphism (Turkina et al., 2011). The features of magmatism represented by ~3.2 Ga zircons are unclear but the mostly negative ε_Hf(T) values indicate recycling of ancient crust. The synchronicity of magmatism and metamorphism about 3 Ga, as well as the negative ε_Hf(T) of zircons suggesting recycling of 3.2–3.8 Ga crust, provide evidence of intracrustal melting and metamorphism due to crustal thickening by magmatic over- and underplating or tectonic stacking. Therefore, the recycling processes indicate a formation of large volumes of continental crust by ~3.0 Ga, its differentiation during intracrustal melting and stabilization. Note that prevalence of recycling in the Mesoarchean inferred from isotopic data is inconsistent with subduction-related crustal formation and related inputs of juvenile material.

Neoarchean. The Neoarchean tectonic and magmatic activity produced mafic to felsic orthogranulites exposed in abundance in the present erosion surface of the area, as well as synfolded gneissic granitoids that occur as numerous veins or more rarely as small intrusions. Magmatic processes and geodynamic conditions of two stages of Neoarchean magmatism are discussed in detail in (Turkina et al., 2012). The protoliths of mafic to felsic granulites have formed during subduction on the active margin of a Paleo-Mesoarchean continent. Unlike the earlier events, crustal growth in ~2.7–2.66 Ga was maintained by inputs of mafic magma and their mixing with crustal melts to produce intermediate-felsic igneous rocks with large ε_Hf(T) and ε_Nd(T) ranges. Along with the Paleoarchean event, the Neoarchean stage was among most important crustal growth events in the Irkut Block history, which ended by synchronous metamorphism and granitic magmatism about 2.55 Ga with intracrustal melting recorded by negative ε_Hf(T) and ε_Nd(T) values in zircons and granitoids.

The onset of the Paleoproterozoic remained mute in the geological record of the Irkut Block and only left isotopic signatures in detrital zircons from Paleoproterozoic paragneisses. The ~2.2–2.3 and 1.95–2.0 Ga zircons derived from magmatic sources with a large range of mainly positive ε_Hf(T) values provide evidence of crustal growth and recycling but lack explicit indications of magmatic processes. We can implicitly judge these processes based on the compositions of paragneisses that show predominance of felsic rocks over mafic lithologies at the source province. Judging by positive ε_Hf(T) values of zircons, Paleoproterozoic juvenile crust may have subduction-related or within-plate origin. In the end of the Paleoproterozoic stage, the area underwent large-scale granitic and basite intrusive magmatism in a setting of postcollisional extension. The isotope signatures of most granitoids correspond to recycled Archean crust while monzodiorite and mafic rocks record limited crustal growth with contributions of enriched mantle sources. The latter inference is especially interesting as isotopic signatures do not allow discriminating enriched mantle-derived magmatic rocks from crust melting products without special studies (Couzinié et al., 2016)

In summary, continental crustal growth during the Paleoarchean and Neoarchean events were driven by different mechanisms: possibly plume magmatism in the former case and subduction in the latter. The two events were separated by a Measoarchean stage of intracrust melting and metamorphism that produced a compositionally differentiated (stratified) and gravitationally stable continental crust. The change of crustal growth mechanisms in the Neoarchean presumably resulted from mantle cooling, formation of rheologically rigid plates, and the onset of conditions for stable subduction (Condie, 2018). According to Re–Os isotope data, the subcontinental lithosphere of the central Siberian craton formed in the Paleo-Mesoarchean before 2.9 Ga (Griffin et al., 2002), and its growth continued through Neoarchean and Paleoproterozoic time (Pearson et al., 1995; Ionov et al., 2015).

The eastern half of the Siberian craton comprises the Magan, Anabar, and Olenek tectonic provinces and its basement is exposed, respectively, in the Magan, Daldyn, and Khapchan domains of the Anabar shield divided by the Kotuykan and Bilakh suture zones. The crustal history of northern and central parts of the Siberian craton which belongs to the Yakutsk kimberlite province, was reconstructed using zircons from lower- and middle-crust xenoliths (Shatsky et al., 2016, 2018; Moyen et al., 2017), zircon xenocrysts from kimberlites (Kostrovitsky et al., 2016), as well as zircons of rocks of the Anabar shield (Gusev et al., 2017, 2019), and alluvial sediments (Paquette et al., 2017).

The ~3.8 Ga ages of few zircons from two-pyroxene granulites (Gusev et al., 2019) and model ages $THfCDM$ = 3.8–3.9 Ga of zircons from plagiogneisses mark the onset of Eoarchean crustal growth in the Anabar shield (Gusev et al., 2017). The U–Pb ages of detrital zircons from recent alluvium in the central Daldyn and eastern Khapchan terranes have three main peaks at 3.4–3.0, 2.8–2.4, and 2.0–1.8 Ga (Paquette et al., 2017). The Paleoarchean zircons with positive ε_Hf presumably represent crustal growth while Neoarchean zircons with variable positive to negative ε_Hf values correspond to juvenile crustal material and recycled Paleoarchean crust at the magma sources respectively (Paquette et al., 2017). Paleoproterozoic zircons with negative ε_Hf may likewise record a predominant component of recycled crust. Zircon xenocrysts in kimberlites are mainly of Paleoproterozoic ages (2.1–1.8 Ga) while Paleo- and Neoarchean (3.62–3.53 and 2.97–2.5 Ga) grains are much fewer (Kostrovitsky et al., 2016). All Neoarchean zircons with negative ε_Hf record recycling of Paleoarchean crust (⁠$TDMC$= 3.26–3.50 Ga), while the variable Hf isotope compositions of Paleoproterozoic zircons bear traces of both recycled Archean crust and early Paleoproterozoic juvenile material (⁠$TDMC$=2.5–2.3 Ga). Paleoproterozoic crustal growth at 1.8–1.87 Ga was inferred from isotopic signatures of zircons from lower crust mafic granulites in the Udachnaya kimberlite (Daldyn-Markha domain), whereas Neoarchean upper crust tonalite xenoliths with close to chondritic ε_Hf represent the Archean stage in the crustal formation (Moyen et al., 2017). Data on zircons from lower crust xenoliths in the Anabar kimberlites (Daldyn and Markha terranes) reveal the main event of crustal growth in the Paleoarchean (⁠$TDMC$ = 3.1–3.65 Ga) (Shatsky et al., 2016, 2018). The Paleoarchean crust was recycled in several tectonothermal events at 2.9–2.85, 2.75–2.7, and 2.0–1.95 Ga, without significant juvenile inputs (Shatsky et al., 2016, 2018).

Thus, the available data on U–Pb ages and Hf isotope compositions of zircons from the Anabar province reveal the main Paleoarchean stage of crustal growth (3.6–3.4 Ga) followed by two major tectonothermal events. In the Neoarchean, the older crust was recycled (Kostrovitsky et al., 2016; Shatsky et al., 2016, 2018), and inputs of juvenile material maintained further crustal growth (Paquette et al., 2017). Large-scale crust recycling in the early Paleoproterozoic (2.0–1.8 Ga) was concurrent with juvenile inputs (Kostrovitsky et al., 2016), including the material of 1.84–1.87 Ga lower crustal mafic granulites (Moyen et al., 2017).

Geochronological estimates and isotope systematics show features of both similarity and difference in the history of crustal growth and recycling between the Irkut Block in the southwestern Siberian craton and the Anabar province in its central and northern parts. The Paleoarchean was the main stage of crustal growth in both regions, while most of recycling occurred during the Neoarchean and Paleoproterozoic events at moderate inputs of juvenile material. Unlike the Anabar province, the Irkut Block underwent prolonged crustal recycling in the Mesoarchean and large-scale subduction-related crustal growth in the Neoarchean.

Findings of few zircons with ≥3.6 Ga Hf model ages evidence that the crustal growth in the Irkut Block began in the Eoarchean. The continental crust evolution comprised two Paleoarchean (3.6–3.4 Ga) and Neoarchean (~2.7–2.66 Ga) crustal growth stages. The Paleoarchean event most likely was associated with plume magmatism and fractionation of mafic magmas. The extent of Paleoarchean crust is traceable in isotopic signatures of magmatic and detrital zircons from most of samples dated from Measoarchean to Paleoproterozoic. The Neoarchean crustal growth was maintained by subduction magmatism fed from depleted mantle on the active margin of a Paleo-Mesoarchean continent. Limited crustal growth occurred also in the Paleoproterozoic from 2.3 to 1.85 Ga. In 1.86–1.85 Ga, mafic magmas and products of their fractionation formed under postcollisional extension and had subcontinental lithospheric mantle sources enriched during Neoarchean subduction processes. The continental crust underwent three major recycling events in Mesoarchean (~3.0 Ga), Neoarchean (~2.55 Ga), and Paleoproterozoic (1.86–1.85 Ga) time. All three events included intracrustal melting and coeval metamorphism, and the ~2.55 Ga and 1.86–1.85 Ga recycling processes occurred in the collisional settings. The recycling involved ≥3.5 Ga ancient crust, which is traceable in isotopic signatures of rocks and zircons. In the Neoarchean, the Paleo-Mesoarchean crust underwent recycling through felsic subduction magmatism. Interpretation of geological and geochronological data from the Irkut Block allows modeling its history in the context of predominant vertical growth and recycling of continental crust for about two billion years.

The presented synthesis of U–Pb, Sm–Nd, and Lu–Hf isotope data has revealed features of similarity and difference in the crustal evolution in different parts of the Siberian craton: in southwestern, northern, and central provinces. Both regions are characterized by the main Paleoarchean crustal growth and combination recycling and growth at Neoarchean and Paleoproterozoic. The Irkut Block in the southwest was remarkable by a long recycling processes in the Mesoarchean and large-scale Neoarchean crustal growth.

I appreciate the contributions of I.N. Kapitonov, E. Adamskaya, and N.S. Priyatkina who determined the Lu–Hf isotope composition of zircons, as well as N.G. Berezhnaya, N.V. Rodionov, and E.N. Lepekhina from VSEGEI (St. Petersburg) who participated in U–Pb dating, as well as P.A. Serov and T.A. Bayanova from Geological Institute (Apatity) who determined Sm–Nd isotope composition of rocks. The study was supported by grant 20-05-00265 from the Russian Foundation for Basic Research. Data on Archean and Paleoproterozoic felsic magmatism were obtained as part of a basic research project carried out at the Institute of Geology and Mineralogy (Novosibirsk).