SUMIAN BASALTS AND BASALTIC ANDESITES OF THE KARELIAN PROVINCE OF THE FENNOSCANDIAN SHIELD: GEOLOGIC POSITION, COMPOSITION, AND FORMATION CONDITIONS
N.A. Arestova, V.P. Chekulaev, Yu.S. Egorova, G.A. Kucherovskii
Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia
Keywords: Fennoscandian (Baltic) Shield, Karelian province, Paleoproterozoic, Sumian, basalts, geochemistry, Sm-Nd isotope systematics
Abstract
We performed a petrochemical study and determined the Sm-Nd isotope composition of the Sumian (early Paleoproterozoic) mafic metavolcanic rocks in the Vodlozero domain (Semch’, Kumsa, and Kamennye Ozera structures) and the Central Karelian domain (Lake Vottomuks and Lake Kogu structures, the western flank of the Lekhta structure) of the Karelian province of the Fennoscandian Shield and compared them with the coeval volcanic rocks of the Kola-Norwegian province and the intrusive rocks of the Karelian and Belomorian provinces. The Sumian mafic rocks intruding the rocks of the Archean domains with different crust ages (>3.1 and 2.7-2.8 Ga) have similar geochemical characteristics (SiO2 = 44-55 wt.%, MgO = 5-14 wt.%; LREE enrichment, (La/Yb)N = 5-17) and similar values of εNd( T ) (from -2.7 to -0.5) and TNd(DM) (from 3.0 to 2.7 Ga), which distinguishes them from their Archean analogues and indicates a single large magma source. The crustal contamination of the primary melts did not exceed 15% and could not determine the isotope-geochemical specifics of the Sumian mafic rocks. Their specific composition is better explained by the model of the formation of primary melts through the low-pressure (2.5-3.0 GPa) melting of the Neoarchean metasomatized zones of the subcontinental lithospheric mantle, with the addition of the primitive-mantle melts. In the period 2.41-2.45 Ga, the early Paleoproterozoic mafic rocks of the Fennoscandian Shield formed in the intracontinental magma generation setting.
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