Age and sources of the anorthosites of the Neoarchean Kolmozero-Voron’ya greenstone belt (Fennoscandian Shield)

Abstract

New data are reported on U-Pb (SHRIMP-II) age (2662 ± 7 Ma), isotope (Sm-Nd) and geochemical compositions of the anorthosites of the Patchemvarek Massif and “ovoidal” anorthosite sills of the Neoarchean Kolmozero-Voron’ya greenstone belt. Mesoarchean (2938 ± 8 Ma) zircons found in the Patchemvarek anorthosite have low Th/U ratio, are overgrown by a thin rim, and may be interpreted as xenogenic crystals assimilated by primary melts of the gabbro-anorthosite massifs from host Mesoarchean tonalites during crystallization in a magmatic chamber. The “ovoidal” anorthosite sills are dated at 2730–2740 Ma on the basis of U-Pb local zircon isotope analysis. The sills of the “ovoidal” anorthosites in the Kolmozero-Voron’ya GSB represent the older (2730–2740 Ma) rock group, which differs from the Patchemvarek anorthosites in strongly depleted Nd isotope composition and some geochemical features. In terms of age and Sm-Nd isotope characteristics, the “ovoidal” anorthosites are close to the komatiites of the lower volcanogenic sequence (eNd(Т) + 3.0–3.2), and metaandesites (2778 ± 5.4 Ma, U-Pb TIMS, eNdТ + 3.5) and metatholeiites of the upper volcanogenic sequence (eNd(Т) + 3.5–3.7) of the supracrustal complex of the Kolmozero-Voron’ya GSB.