Paleoproterozoic (2.45 Ga) polymictic conglomerates in Central Karelia, Russia: age, sources and sedimentation environment
DOI:
https://doi.org/10.17308/geology/1609-0691/2026/1/18-32Keywords:
conglomerates, Paleoproterozoic, Karelian cratonAbstract
Introduction: paleoproterozoic sedimentary rocks are a key element of Precambrian continental crust, so their study provides information on the age and composition of denudation areas that are often concealed by younger deposits or subjected to deep erosion. Additionally, sedimentary rocks formed at the Archean-Proterozoic boundary offer insights in the changes in atmosphere composition and climate, including the Huronian glaciation. The article focuses on the paleoproterozoic conglomerates of the Karelian Craton, the southeastern part of the Fennoscandian Shield.
Methodology: clasts and matrix of the polymictic conglomerates were studied using petrographic and geochemical methods. U-Pb ages of zircons derived from the conglomerate matrix were determined laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS).
Results and Discussion: paleoproterozoic polymictic conglomerates of the Koikary and Elmus areas in the Karelian Craton are poorly sorted boulder-pebble conglomerates with matrix represented by lithitic and quartz-feldspar graywacke. The age of the conglomerates was determined to be younger than 2.41 Ga.
The Koikary conglomerates contain predominantly granite and granodiorite clasts, similar in composition to Neoarchean (2.86 Ga) granites and rhyolite-dacites of the Vodlozero domain, as well as the clast of Neoarchean amphibolites, dolerites, diorites, and siltstones. In the Elmus conglomerates, the clasts consist mainly of Paleoproterozoic andesitic basalts, as well as Neoarchean siltstones and diorites. Additionally, fragments of plagioporphyrites with an estimated age of 2.62 Ga have been identified in the clastic material of these conglomerates. The 2.62 Ga felsic volcanic rocks have not yet been found on the present-day erosional surface within the Vodlozero domain.
Conclusions: the studied Paleoproterozoic conglomerates of the Karelian Craton belong to proximal facies deposited in tectonic depressions. Clastic material has local sources of erosion. Differences in zircon age distribution and clast composition between the conglomerates from the Koikary and Elmus areas are due to differences in the depth of the primary erosional surface.
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