Finding of neoarchean (2.71 ga) conglomerates in the Kostomuksha iron ore province: on the youngest archean stratotectonic association in the Karelian craton

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Abstract

A lens of earlier unknown polymict conglomerates was found in the axis of the Kostomuksha greenstone belt (KGB), Karelian Craton. Their clastic portion consists of poorly graded, largely angular pebbles. They are comparable in composition to rocks from the surrounding greenstone complex, such as: 1) amphibolites similar to KGB’s Mesoarchean gabbroic rocks and basalts; 2) magnetite-amphibole quartzites similar to Mesoarchean magnetite-biotite-amphibole quartzites associated with banded iron formation; 3) Neoarchean (2.75 Ga) quartz metagraywacke. The conglomerates are highly deformed (γ> 10) by simple shear. The conglomerates were deposited about 2.71 Ga ago, as indicated by analysis of zircons from the matrix. Thus, a new Neoarchean stratotectonic association, the youngest in the greenstone complex, was found in KGB. It seems to have been formed in a pull-apart basin at the final stage of KGB formation, in which shearing played an important part. Volcanogenic and sedimentary associations of similar age are known in the Khedozero-Bolshozero, Kuhmo and Takanen greenstone belts of the Karelian Craton.

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About the authors

А. I. Slabunov

Karelian Research Centre of the Russian Academy of Sciences

Author for correspondence.
Email: slabunov@krc.karelia.ru
Russian Federation, Karelia, Petrozavodsk

N. S. Nesterova

Karelian Research Centre of the Russian Academy of Sciences

Email: slabunov@krc.karelia.ru
Russian Federation, Karelia, Petrozavodsk

S. V. Mudruk

Karelian Research Centre of the Russian Academy of Sciences; Kola Science Centre of the Russian Academy of Sciences

Email: slabunov@krc.karelia.ru

Geological Institute

Russian Federation, Karelia, Petrozavodsk; Apatity

О. А. Maksimov

Karelian Research Centre of the Russian Academy of Sciences

Email: slabunov@krc.karelia.ru
Russian Federation, Karelia, Petrozavodsk

A. V. Kervinen

Karelian Research Centre of the Russian Academy of Sciences

Email: slabunov@krc.karelia.ru
Russian Federation, Karelia, Petrozavodsk

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2. Fig. 1. Scheme of the geological structure of (a) the Kostomuksha greenstone belt ([11, 15], with the author's additions) and (b) a detailed area with a lens of Neoarchean (2.71 Ga) conglomerates. (a): 1 - Neoproterozoic (1.2 Ga) lamproites and kimberlites (see Table 1), 2 - Paleoproterozoic (2.40 and 2.14 Ga) dolerites (see Table 1); 3-9 - Neoarchean (see Table 1): 3 - 2.68 Ga granites; 4 - 2.71 Ga polymictic conglomerates; 5 - 2.71 Ga sanukitoids; 6 - 2.72 Ga granites; 7 - 2.78 Ga granitoids of the TTG association; 8–10 – rocks of the Gimolskaya Series (see Table 1): 8 – 2.76–2.74 Ga metagraywackes with BIF-3 interlayers; 9 – 2.76–2.74 Ga sills and dikes of metarhyolites (helleflints); 10 – metasediments of the Surlampinskaya Formation with BIF interlayers: a – weakly and b – strongly migmatized; 11–14 – Mesoarchean (2.87–2.78 Ga) (see Table 1): 11 – tuffs, tuffites of rhyolites (Shurlovar Formation); 12 – BIF-2 interlayers; 13 – komatiite-basalt with dacites complex (Ruvinvaara Formation); 14 – BIF-1 interlayers; 15 – basalts and komatiites (Niemijärvi Formation); 16 – faults; 17 – thrust; 18–19 – bedding elements: 18 – banding and gneissosity, 19 – mineral lineation. (b): 1 – 2.71 Ga polymictic conglomerates; 2–4 – Mesoarchean (2.80–2.78 Ga) rocks of the Šurlovar STA: 2 – felsic volcanics, 3 – BIF-2 interlayers; 4 – shale interlayers; 5–7 – Mesoarchean (2.87–2.81 Ga): 5 – komatiite-basalt with dacites complex (Ruvinvaara Formation), 6 – BIF-1 interlayers, 7 – barren quartzite and shale interlayers; 8 – faults; 9–10 – bedding elements: 9 – mineral linearity, 10 – banding and gneissicity

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3. Fig. 2. Neoarchean (2.71 Ga) conglomerates of the Kostomuksha greenstone belt in outcrop: (a) general view of the outcrop (scale – yellow tape measure 1 m), dotted line – structural lines, arrow – average orientation of the lineation plunge (NE 55 at an angle of 23); (b) conglomerates with ellipsoid fragments of metabasites, banded quartzites – quartz graywacke, banding is visible in the rocks, the plane of the photo is suborthogonal to the shear surface (scale – compass 10 cm); (c) conglomerates with angular and ellipsoid fragments of amphibolites (metabasalts), metabasalts with plagioclase porphyry, quartz metagraywacke (scale – compass 10 cm); (d) large fragment of deformed quartz metagrauwaaki (scale – compass 10 cm); (d) conglomerates in a plane orthogonal to the shear surface and parallel to the lineation, arrow – orientation of the lineation dip (NE 55° at an angle of 23°)

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4. Fig. 3. Figurative points of the compositions of amphibolites (blue stars) (a, b), quartzites (pink symbols) – quartz graywacke (red symbols) (c, d) from conglomerate fragments on the SiO2–Na2+K2O (a), SiO2–Y (c) diagrams and graphs of chondrite-normalized REE contents (b, d), green fields – composition of Mesoarchean basalts of the KZP, yellow fields – compositions of Mesoarchean quartzites associated with BIF [15]

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5. Fig. 4. Grain structure (a, c) and isotopic age (determined by LA-ICP-MS, n is the number of analytical points, D* is the discordance in %) of zircon from the conglomerate cement (b) and a fragment of quartz graywacke (d): (a, c) cathodic luminescence image of zircons from the cement (a) and a fragment (c) of the conglomerate, circles are the location of analytical dating points and the TPb–Pb values ​​in them in Ma (1σ), with the Th/U ratio in brackets; (b, d) histograms of 207Pb–206Pb ages (TPb–Pb) of zircon from the cement, TMDAPb–Pb is the 207Pb–206Pb age of the three youngest grains (MDA – maximum depositional age). * D=100×(T(206Pb/238U) / T(207Pb/206Pb)–1)

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