High-precision dating of granitoids, which allows to determine the age of the main stages of crust formation and establishes the temporal relationship between the felsic magmatism and the ore-forming processes, is of key significance for interpretation of the history of formation of the Precambrian continental crust, which is considered as one of the most important issues in modern geology [1]. The Baikal–Muya accretionary terrane (BMT), located at the boundary of the Central Asian orogenic belt and the Siberian craton [1], is an area where granitoids are widespread and the age and origin of which are debatable. These are the Kedrovskii massif granitoids, analogs of which are distributed within the South Muya block of the Baikal–Muya accretionary terrane. The freshness and the lack of gneissity in rocks and, also, the stock shape of the Kedrovskii massif gave grounds to refer these to Late Paleozoic intrusive rocks of the Konkuder–Mamakan Complex [2]. The gold–quartz veins of the Kedrovskoye deposit, located both in contact-related and internal zones of the massif among granodiorites and diorites, are spatially linked to this complex [3, 4]. This is considered as evidence of the genetic relationship between the gold mineralization and Late Paleozoic granitoid magmatism in the BMT ([3, 5] and others).

In this paper, it has been shown for the first time that the Kedrovskii massif granitoids have formed at one of the earliest stages (about 800–780 Ma) of the Neoproterozoic history of the BMT long before the formation of the gold mineralization of the region.

The Kedrovskii granitoid massif covering an area of 10 km2 is confined to the regional strike–slip fault zone bordering the Early Precambrian South Muya block from the east. It breaks through Neoproterozoic gabbroids of the Kedrovskii complex and metasedimentary rocks of the Kedrovskaya Formation [6] (Fig. 1) and is transversed by biotite–quartz–feldspar veins and dolerite and lamprophyre dikes supposedly of Late Paleozoic age. Granodiorites dominate in the massif rock composition to which quartz diorites and diorites are subordinated [7].

Fig. 1.
figure 1

A sketch map of the geological structure of the Kedrovskii granitoid massif area (Eastern Siberia) (compiled using the materials of Geological Prospecting Surveys of the Limited Liability Company (LLC) Prospectors Artel Zapadnaya and [4]). (1)  Gneisses, crystalline schists, marbles, calcifirs of the Kindikan Formation (AR(?)-PR1); (2) metasedimentary rocks (metasandstones, biotite, and two mica schists, sericite–chlorite schists, limestones, carbonaceous schists) of the Kedrovskaya and Ust-Tuldun Formations (without division) (PR2); (3) ortho-schists, mica-quartz schists, metaeffusives, tuff conglomerates of the Ust-Kelyan Complex stratum (PR2) and molassoid (conglomerates, gravelites, sandstones, schists) deposits of the Amatkan Formation (PR2); (4) gabbroids (gabbro, olivine gabbro, gabbro–diorites, gabbro–norites) of the Kedrovskii complex (PR2); (5) granitoids (granites, granodiorites, plagiogranites) of the Bambukoisky and Ileirskii complexes (PR2); (6) granitoids of the Konkuder–Mamakan Complex (PZ2); (7) alluvial deposits (Q); (8) tectonic faulting, and (9) Kedrovskii massif.

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The granodiorites and biotite–quartz–feldspar vein rocks have been studied that were exposed by drilling boreholes in the southern part of the Kedrovskii massif. The biotite and amphibole–biotite granodiorites have a medium- to coarse-grained hypidiomorphograined structure and are composed of felsic plagioclase (up to 55%), K-feldspar (up to 10%), quartz (up to 25%), biotite, and subordinate amphibole (up to 10%), accessory apatite, magnetite, zircon, and titanite. Granites are metaaluminous (A/CNK = 0.73–0.74) with a rather high magnesiality (Mg# = 0.44–0.46), a considerable prevalence of Na over K (Na2O = 3.87–4.49 mass %, K2О = 1.86–2.05 mass %), and moderate contents of Ti (0.51–0.54 mass %, TiO2) and Р (0.17 mass %, P2O5). High contents of Ва (up to 1700 µg/g), moderate concentrations of Sr (600 µg/g), Rb (50–60 µg/g), and Zr (200 µg/g), and low concentrations of Nb (≤6 µg/g) were found in granites. The REE distribution is notable for moderate fractination ((La/Yb)n = 10) under a more considerable enrichment in light REEs ((La/Sm)n = 4) rather than heavy REEs ((Gd/Yb)n = 1.7–1.8). No Eu anomaly was detected. Based on the Nb–Y (Y/Nb = 0.3–0.5) and Ta–Yb–Rb (Yb/Ta = 0.3–0.8) ratios, and the high LILE/HFSE ratio (for example, Ва/Nb ratio up to 300) granodiorites belong to granitoids of the volcanic island arcs and (or) active continental margins [8].

The geochronological U–Pb (ID-TIMS) dating for zircon from granodiorites and 39Ar/40Ar dating for biotite from a small (3–4 cm) quartz–plagioclase vein in granodiorites have been applied. The morphology of the zircon grain structure was studied using an electron microscope under the cathode luminescence (CL) and backscattered electron (BSE) regimes. The chemical zircon sample preparation was conducted employing the technique described in [9]. The cumulative background level of the chemical procedure for U and Pb did not exceed 1  and 15 pg. Respectively to determine the U and Pb contents, a mixed isotopic spike 235U + 202Pb was used. The measurements were performed on a Triton TI multicollector mass-spectrometer. The determination error for the isotope ratios 206Pb/238U and 207Pb/235U did not exceed ±0.5%. The isotopic data were processed using the PbDAT [10] and ISOPLOT software [11].

The 39Ar/40Ar dating of biotite was carried out employing the techniques described in [12]. The reference biotite sample MSA-11 (OSO no. 129-88), 311.0 ± 1.5 Ma in age, which was certified in compliance with the Bern-4M muscovite and LP-6 biotite international reference samples was used as a monitor. The measurement of the isotopic composition of Ar in irradiated samples was performed on a Noble Gas 5400 Micromass mass-spectrometer. Determination of the gas fractions and argon isotope ratio analysis were carried out in the temperature range from 500 to 1200°C. Based on the results of systematic analyses of the air Ar isotope ratios, a mass discrimination correction was introduced. The cumulative background for 40Ar did not exceed 5 × 10–10 ncm3.

Zircon from granodiorites is represented by transparent, prismatic, and long-prismatic light-pink crystals (Fig. 2, I–III). Their size varies from 50 to 350 µm, and the elongation coefficient attains 2.0–3.0. The grains of zircon exhibit a well-displayed zonality in their structure (Fig. 2, IV–VI), which indicates its magmatic origin. Cracked inherited irregularly shaped nuclei are found in certain zircon grains (Fig. 2, VI).

Fig. 2.
figure 2

Microphotographs of zircon crystals from granodiorites of the Kedrovskii massif (sample С-54-56-15) obtained using the scanning electron miscoscope ABT-55: I–III, under the backscattered electron regime; IV–VI, under the cathode luminescence regime.

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Three zircon microbatches (20–25 grains) were sampled from the size fractions <50 and 50–75 µm (see Table 1). The points of their isotope composition are located on the discordia (Fig. 3), the upper intersection of which with the concordia corresponds to 783 ± 8 Ma (MSWD = 0.56), whereas the lower intersection corresponds to 636 ± 1200 Ma. One of the points (no. 3, see Table 1) is located on a concordia (Fig. 3). The age of zircon from this microbatches attains 781 ± 3 Ma (MSWD = 1.0; the probability of concordance is 0.31). This age is assumed to be the age of the Kedrovskii massif granodiorites.

Table 1. Results of isotope U–Pb study of zircon from granodiorites (sample C-56-54-15) of the Kedrovskii massif (Eastern Siberia, Russia)
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Fig. 3.
figure 3

U–Pb diagram with the concordia for zircon from granodiorites of the the Kedrovskii massif (sample С 56-54-15). The point numbers on the diagram correspond to the sequence numbers of the samples shown in Table 1.

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The distribution of 39Ar/40Ar age values (Fig. 4) obtained from the data of the analysis of 12 argon fractions has the shape of an extended plateau (about 95% of the extracted 39Ar) to which the age value of 626 ± 11 Ma corresponds. This distribution pattern suggests that the 39Ar/40Ar age corresponds with the Ediacaran event when the K–Ar isotope system of biotite was cloused. This age may be interpreted as the age of a series of vein biotite–quartz–feldspar rocks that transverse the Kedrovskii massif granodiorites, or as the age of a later thermal event that occurred in the region after the formation of vein rocks.

Fig. 4.
figure 4

The distribution plot of the 39Ar/40Ar age values for biotite from the biotite–quartz–feldspar vein of the Kedrovskii massif.

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The obtained U–Pb age of zircon shows that the granitoids of the Kedrovskii massif (781 ± 3 Ma) formed in the Early Neoproterozoic and are supposed to be of the same age as gneiss–granites of the Ileirskii Complex (784 ± 6 and 786 ± 9 Ma [13]). The latter identify a metamorphic event that was related to the accretion of the Anamakit–Muya terrane of early Baikalids of the BMT with the southern margin of the Siberian craton [13, 14]. The geochemical and Sr–Nd-isotope ((87Sr/86Sr)0 = 0.704, εNd(T) = −1.3, and TDM = 1.6 Ga) characteristics of the Kedrovskii massif granodiorites are clear evidence that the Paleoproterozoic continental crust of the Anamakit–Muya terrane was the source of their parent melts. The 39Ar/40Ar age (626 ± 11 Ma) of the vein biotite correlates with the Late Baikalian metamorphic events took place in the BMT at about 640–610 Ma [13, 14].

The U–Pb age of granitoids of the Kedrovskii massif (781 ± 3 Ma) appeared, to a certain extent, to be older compared to the Sm–Nd isochron age (735 ± 26 Ma [6]) of the Kedrovskii complex gabbro-norites intruded by the former. This discrepancy may be caused by the following:

(1) Disturbance of the gabbro-norites Sm–Nd system closure due to later thermal and hydrothermal Ediacaran and Late Paleozoic events;

(2) Possible combination of the rocks in the Kedrovskii gabbro-anorthosite pluton, which are referred to basite rock series of various composition and age [15].

The above results suggest a new insight into the history of the processes occurring within the BMT. They indicate widely developing Tonian intrusions in its South Muya block thus supporting the occurrence of two thermal events in the history of the BMT. The formation of granitoids of the Kedrovskii massif and, presumably, of their petrographic analogs in the South Muya block is linked to an early event that was coincidental in time with the main stage (about 800–780 Ma) of the formation of the Proterozoic continental crust of the BMT. The age of biotite–quartz–felspar veins correlates with the Ediaracan (640–610 Ma) final stage of accretion of the BMT continental blocks, which was accompanied by high-pressure metamorphism [13, 14]. The results obtained considerably change the concept on the origin of the hydrothermal mineralization of the known Kedrovskoye gold deposit, which is associated with the studied Upper Proterozoic massif of the same name. The parental granitoid melts and their probable derivatives are not explicitly related to the formation of fluids that served as sources of gold-bearing mineralization of the Kedrovskoye deposit. According to quite reliable datings [4], its age attains 273 ± 4 Ma.