STRATIGRAPHY OF THE KOSTOMUKSHA BELT IN KARELIA (UPPER ARCHEAN) AS INFERRED FROM GEOCHRONOLOGICAL, GEOCHEMICAL, AND ISOTOPIC DATA

Abstract

A new version of stratigraphic scheme for the Kostomuksha greenstone belt in Karelia is suggested on the basis of lithological, geochemical, and isotopic data obtained for ultramafic, mafic, and acidic volcanic rocks of the Kontokki Group and for different rock varieties of the Gimola Group. Geochemical characteristics and results of Sm-Nd isotopic dating showed that basic volcanics of the Niemijarvi and Ruvinvaara formations are coeval, but older than the acidic volcanic rocks. Basal conglomerates and metasediments of the Gimola Group rest on the latter. According to isotopic and geochemical parameters, parental magmas of basalts in the lower part of the section are of the contaminated type and have been generated in environments of continental rifting. Lithological and geochemical analysis also demonstrated that some ultramafic schists of the Gimola Group correspond to peridotitic komatiites similar to those from the underlying Kontokki Group. Accordingly, episodes of ultrabasic magmatism typical of the latter continued during the deposition of terrigenous ferruginous-siliceous sediments of the Gimola Group. The estimated formation time is 2808 ± 95 Ma (this work) or 2843 ± 39 Ma (Puchtel et al., 1997) for the lower mafic part of the section, and 2795 ± 10 Ma (this work) or 2795 ± 29 Ma (Samsonov et al., 1995) for acidic volcanics. The time range of metasediments in the Gimola Group is limited by the date of 2795 ± 10 Ma characterizing the underlying acidic volcanics, which presumably constituted a part of provenance for siliciclastic metasediments. The upper age limit is defined by the date of 2707 ± 31 Ma obtained for plagoigranite porphyry intruding the schists.