Polymict conglomerates are widespread in the Karelian Province of the Fennoscandinavian Shield [1, 2]. They mark important geodynamic events at the Archean–Proterozoic boundary and start a new stage of sedimentation after a long break. The polymict conglomerates, which occupy the stratigraphic position between the Archean basement rocks and Early Proterozoic (Sumian) volcanics, reflect the onset of formation of the basin, while the composition of the conglomerate clastic material characterizes the rocks that came to the surface at that time. The emergence of conglomerates was caused by the destruction of mountains and subsequent filling of large depressions that emerged in the Archean crust. The formation of these depressions was probably associated with intraplate rifting, which occurred in various provinces of the Fennoscandinavian Shield and produced the fields of Sumian continental basalts 2505–2430 Ma ago [3, 4]. The study of the material composition of terrigenous rocks and the age of clastic zircon grains contained therein are the main source of information for reconstructing the history of the Early Precambrian continental crust. Clastic zircon from quartzites and cement of the Jatulian quartz conglomerates [5], the Archean quartzites of the Matkalakhti Greenstone Belt [6], and the Sumian quartzite of the Kumsinskaya structure in Central Karelia [7] were previously studied within the Russian part of the Karelian Province.

The present work is the first attempt to estimate the composition and age of the sources of material for polymictic conglomerates in two areas of the Central Karelia Domain of the Karelian Province: near Sukkozero and Vottomuks lakes (Fig. 1), which characte-rize the NeoArchean and Paleoproterozoic stages of evolution of the Early Precambrian crust of the Fennoscandinavian Shield.

Fig. 1.
figure 1

(a) Schematic structure map of the Karelian Province of the Fennoscandinavian Shield and (b) the geological map of the Sukkozero Lake Vottomuks area (based on the 1 : 200  000 geological map, data of K.I. Heiskanen, supplemented by the authors. Numbers in circles are sampling sites: (1) Segozero, (2) Ilomantsi, (3) Suoyarvi. (1) Archean granitoids; (2) metasediments of the Gimolskaya series; (3) Archean conglomerates; (4) Proterozoic metavolcanics of the Bergaulskaya Formation of the Sumian suprahorizon; (5) Paleoproterozoic conglomerates; (6–7) poorly defined intrusive rocks: (6) acidic, (7) basic; (8) metasediments of the Yangozero formation of the Jatulian suprahorizon.

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The Central Karelia Domain together with the West Karelian and Vodlozero domains form the Archean Karelian province [8] within the southwestern part of the Fennoscandinavian Shield (Fig. 1a). The Karelian province consists of gneiss–granite areas with rocks of the tonalite–trondyemite–granodiorite (TTG) association separated by greenstone belts composed mainly of supracrustal rocks. The Central Karelia Domain, composed of granitoids no older than 2.78 billion years [9], is the youngest fragment of the Neoarchean crust on the shield. The supracrustal rocks of the greenstone belts of this domain are dominated by medium- and acidic metavolcanics and terrigenous sedimentary rocks, while komatiites and basalts are of subordinate importance. In the central part of the domain, there is a narrow Gimolskaya structure extending in the submeridional direction. The polymict conglomerates, shales, and basic volcanics occur at the base of the Gimolskaya paleodepression, overlain by metagraywacke and metaaleurolites. The terrigenous rocks of this paleodepression are most completely represented near the settlement of Sukko-zero and Vottomuks Lake (Fig. 1b).

The polymict conglomerates near the Sukkozero settlement compose the lower part of the section of supracrustal rocks of the Gimolskaya Series, the stratotype of the Neoarchean Fennoscandinavian Shield [1]. The Neoarchean conglomerates are strongly deformed (Fig. 2a), and the pebbles have a lens-like shape, although they often retain earlier schistosity arranged obliquely with respect to the conglomerate deformation. The size of the clasts in the cross-slanted shale varies from 0.5 × 3 to 15 × 50 cm, with the predominant size ranging within 5–10 cm. The clastic material usually accounts for 30 to 60–70%, sometimes reaching almost 90%. The cement is represented by quartz–biotite–amphibole–plagioclase schist with plagioclase phenocrysts. Similar rocks are present as interbeds in conglomerates. The conglomerates of the Sukkozero area are dominated by pebbles of coarse-grained gneiss–tonalites (sample C2013c) and meso-cratic plagioporphyries (sample C2013b).

Fig. 2.
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(a) Photo of Archean conglomerates near Sukkozero settlement and (b) Paleoproterozoic conglomerates of Vottomuks Lake.

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Ten kilometers east of the settlement of Sukkozero, in the area of Vottomuks Lake, the Archean rocks are overlain by basaltic andesites (Fig. 1) and Paleoproterozoic polymict conglomerates, which form a sequence with an apparent thickness of about 190 m, in which, along with the prevailing boulder–pebble conglomerates, there are conglomerate–breccia and gravel–pebble conglomerates [2]. The conglomerates of Vottomuks Lake (Fig. 2b, C2012 in Fig. 1b) are less deformed and have a more diverse composition of clasts. They are dominated by rounded pebbles (often angular, but with smoothed corners) of dense fine-grained dark gray metavolcanics (Fig. 122g), often with amygdaloidal or nodular structure, which is typical of the Sumian basaltic andesites. Among the clasts, there are pebbles of gabbroid rocks, rarely of ultrabasic composition, gneissogranites, metavolcanites of medium acid composition, leucogranites, and quartz. The predominant size of pebbles is 3–5 cm, but their size may vary from 0.5 cm up to 15–20 cm. Pebbles constitute more than 70% of the rock volume. The cement is coarse-grained metasandstone, mostly quartz–biotite, with amphibole, tourmaline, and garnet.

Analysis of the chemical composition (Table 1) showed that both conglomerates are dominated by heterogeneous magmatic material, which is confirmed by the negative values of the DF(x) function (Fig. 3a). The DF(x) function is calculated from petrochemical data using the formula DF(x) = 26.64 – 0.24SiO2 – 0.16TiO2 – 0.25Al2O3 – 0.28FeO* – 0.30MgO – 0.48CaO – 0.79Na2O – 0.46K2O – 0.10P2O5, where FeO* = 0.9Fe2O3 + FeO, the major element contents (wt %) are given without conversion to a water-free base [10]. This function allows us to distinguish between the sedimentary or magmatic protolith of the metamorphic complex with a high degree of probability. The low (less than 55) values of the CIA maturity index (CIA = 100Al2O3/(Al2O3 + CaO + Na2O + K2O), [11]) indicate the absence of significant weathering of most of the rocks studied.

Table 1. Chemical composition of pebbles and cement of Archean and Proterozoic conglomerates and their assumed sources in the Central Karelia Domain
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Fig. 3.
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Comparison of the chemical composition of pebbles and cement of the Archean conglomerates of the Sukkozero settlement and the Proterozoic conglomerates of Vottomuks Lake with potential sources of the Central Karelia Domain: Neoarchean metadacites and granitoids [9] and Sumian basalt andesites of Vottomuks Lake (new data), as well as with Neoarchean metase-dimentary rocks: metagraywackes of greenstone belts [12] and metaterrigenous rocks of the Gimolskaya series (newly published data).

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In spite of the certain similarity in the content of major elements, the cement of the Neoarchean conglomerate (Sample C2013) differs significantly in higher contents of Sr, Ba, LREEs, and the Sr/Y, Zr/Y, (La/Yb)n ratios (Fig. 3b). The similar geochemical features were found in the pebbles of gneiss–tonalite (Sample C2013c) and plagioporphyry (Sample C2013b), which dominate in the composition of Archean conglomerates. Their chemical compositions correspond to the Neoarchean (2.78–2.73 Ga) TTG-rocks and medium to acidic volcanics (plagioporphyries and metadicites) of the Central Karelia Domain (CKD), which is a fragment of the youngest Archean crust of the Karelian Province [8] (Table 1, Fig. 3). The TTG rocks represent the most likely source of terrigenous material for the Neoarchean conglomerates near Sukkozero (Fig. 1). The sanukitoid massifs of the Karelian province are also of similar age (2.72–2.75 Ga); their contribution could provide elevated concentrations of Ba, Sr, and LREEs. Since the cement composition of Neoarchean conglomerates is close to that of pebbles (Table 1, Fig. 3), they are probably poorly sorted small fragments of the same rocks transformed into quartz–biotite–amphibole–plagioclase shale as a result of late deformation and metamorphism. The Paleoproterozoic conglomerates are dominated by pebbles similar in composition to the 2.45 Ga–old Sumian basaltic andesites, which are widely developed in this region (Table 1, Fig. 3).

To estimate more accurately the age of the terrigenous material sources involved in the formation of NeooArchean and Paleoproterozoic conglomerates, zircon was sampled from cement of the samples C2013 and C2012, respectively. The U–Th–Pb isotopic dating (LA-ICP-MS) of the zircon was conducted at Institute of Precambrian Geology and Geochrono-logy, Russian Academy of Sciences with an ICP MS ELEMENT XR instrument equipped with a NWR-213 laser ablation system, following the procedure described in [14]. The isotopic analysis quality was controlled using standard zircons Harvard 91 500 and Plešovice, which gave concordant age values of 1071 ± 10 and 329 ± 8 for sample C2012 and 1070 ± 8 and 338 ± 4 for sample C2013. The results of the measurements are given in Table 2.

Table 2. Results of U–Th–Pb LA-ICP-MS geochronological studies of detrital zircons from polymict conglomerates
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Zircon from sample C2013 is represented predominantly by idiomorphic slightly elongated grains often containing cores. The prevalent size of the grains is 150–200 µm. The morphology and internal structure of the grains indicate their magmatic origin. Zircons from samples C2012 and C2013 are morphologically similar (Fig. 4). The CL images often show oscillatory zoning and rare inclusions. The grain size varies within 100–200 µm. The results of the geochronological studies are shown in Fig. 4.

Fig. 4.
figure 4

Distribution of ages of detrital zircon in the cement of Archean (sample C2013, Sukkozero settlement) and Paleoproterozoic conglomerates (sample C2012, Vottomuks Lake).

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Ninety-four grains of sample C2013 were studied, 67 of which gave concordant age values.

As can be seen from Fig. 4, the dominant zircon population (more than 80% of the studied grains) is about 2750 Ma old. Figure 5 shows the relative age probability distribution for detrital zircon. The peak age for sample C2013 is 2757 Ma (44 grains), and the peak age for sample C2012 is 2750 Ma (21 grains). This age is comparable with the ages of the Neoarchean TTG rocks and complementary middle-acid volcanics and plagioporphyries of the dikes that participate in the structure of the Central Karelia Domain [9], including the Ilomantsi block in its southwestern part (eastern Finland) [15–17] (Fig. 1). The obtained age of zircon from the Neoarchean conglomerate cement is close to the average age of terrigenous complexes of the Tipasjärvi-Kuhmo greenstone belt (about 2750 Ma [16]) in the Karelian Province [8]. A similar age was obtained for conglomerate pebbles in eastern Finland [15, 16].

Fig. 5.
figure 5

Distribution of the relative age probability for detrital zircon from the Archean (sample C2013, Sukkozero settlement) and Paleoproterozoic conglomerates (sample C2012, Vottomuks Lake).

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Eighty-eight grains of sample C2012 were studied; concordant ages were obtained for 35 of them (Fig. 4). More than 80% of the zircon grains in this sample are also about 2750 Ma old. This suggests that the Neoarchean rocks of the intermediate to acid composition continued to be one of the key sources of terrigenous material for the Paleoproterozoic conglomerates. The absence of zircon of Proterozoic age in the cement seems to be related to its absence in the Sumian basalt andesites that are common in the area studied.

Isolated zircon grains with ages of 2.9–3.2 billion years found in the Archean and Proterozoic conglo-merates indicate a minor contribution of an older source of clasts, which could be rocks of the TTG-associated Mesoarchean basement of the Vodlozero domain [13]. These conglomerates do not contain any significant amount of clastic zircon grains with ages of 2705–2695 Ma [13, 17, etc.], corresponding to the ages of the plagiomicrocline granites [13] that are common in the Karelian Province. Most likely, these granitoids did not surface in this part of the province during the Paleoproterozoic.

Summarizing the above, one of the main sources for both Neoarchean (settlement of Sukkozero) and Paleoproterozoic (Vottomuk Lake) polymictic conglomerates was Neoarchean magmatic rocks of medium acid composition (TTG and volcanics) with an age of about 2.75 billion years, widely developed in the Central Karelia Domain of the Fennoscandinavian Shield. Since the Neoarchean rocks of the TTG series are considered to be the products of partial mel-ting of the lower crust basites, the presence of fragments of these rocks in conglomerates means that they were exhumed to the surface by the Early Proterozoic.

Since the rocks bearing 2.75-Ga clastic zircon were the predominant source for the Neoarchean conglomerates of the Central Karelia Domain, a small portion of the Mesoarchean zircon material could have been brought from the older Vodlozerskii domain. Thus, the age of about 2.75 Ga can be accepted as the lower age limit for the Neoarchean conglomerates (Sukkozero settlement) of the Central Karelia Domain. An additional source of terrigenous material for the Paleoproterozoic conglomerates was Sumian basaltic andesites, which prevailed among the pebbles and provided enrichment of the FeO, MnO, and MgO cement and depletion in SiO2 and K2O. Fragments of Sumian basalts found in these conglomerates allowed the upper age limit of the Paleoproterozoic conglomerates (Lake Vottomux) to be assessed at 2.45 billion years.

The absence of zircon populations in the cement of Archean and Paleoproterozoic conglomerates with ages of about 2.70 and 2.45 Ga, corresponding, respectively, to the Neoarchean granite magmatism and Paleoproterozoic basite volcanism in the Karelian Province, as well as the low maturity index of terrigenous material, indicates that the conglomerate accumulation had occurred in the vicinity of the provenance area. This fact suggests that there was an area with dissected topography within the Gimolskaya and Vottomuks structures of the Central Karelia Domain that might have served as limited local source areas.