Stratigraphy of the Kostomuksha belt in Karelia (upper Archean) as inferred from geochronological, geochemical, and isotopic data

A new version of stratigraphic scheme for the Kostomuksha greenstone belt i n Karelia is suggested on the basis of lithological, geochemical, and isoto pic data obtained for ultramafic, mafic, and acidic volcanic rocks of the K ontokki Group and for different rock varieties of the Gimola Group. Geochem ical characteristics and results of Sm-Nd isotopic dating showed that basic volcanics of the Niemijarvi and Ruvinvaara formations are coeval, but olde r 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 contin ental rifting. Lithological and geochemical analysis also demonstrated that some ultramafic schists of the Gimola Group correspond to peridotitic koma tiites similar to those from the underlying Kontokki Group. Accordingly, ep isodes of ultrabasic magmatism typical of the latter continued during the d eposition of terrigenous ferruginous-siliceous sediments of the Gimola Grou p. The estimated formation time is 2808 +/- 95 Ma (this work) or 2843 +/- 3 9 Ma (Puchtel et at, 1997) for the lower mafic part of the section, and 279 5 +/- 10 Ma (this work) or 2795 +/- 29 Ma (Samsonov et al., 1995) for acidi c volcanics. The time range of metasediments in the Gimola Group is limited by the date of 2795 +/- 10 Ma characterizing the underlying acidic volcani cs, which presumably constituted a part of provenance for siliciclastic met asediments. The upper age limit is defined by the date of 2707 +/- 31 Ma ob tained for plagoigranite porphyry intruding the schists.