Combined mantle plume-island arc model for the formation of the 2.9 Ga Sumozero-Kenozero greenstone belt, SE Baltic Shield:: Isotope and trace element constraints

The Sumozero-Kenozero greenstone belt in the SE Baltic Shield is similar to 400 km long and up to 50 km wide and includes two main units with a total thickness of similar to 5 km. The lower unit consists of oceanic plateau-type submarine mafic-ultramafic lavas. The upper unit is made up of island are-type volcanic BADR (basalt-andesite-dacite-rhyolite)-series rocks and adakite-series subvolcanic rhyolites. Both units are separated by major thrust zones from the 3.2 Ga TTG-gneisses of the Vodla Block microcontinent. Komatiites of the lower unit were derived from a liquid containing similar to 30% MgO that erupted at a temperature of similar to 1570 degrees C. The komatiite liquid began melting at depths of 300-400 km in a mantle plume. The plume was 250 degrees C hotter than the ambient mantle and had the thermal potential to produce oceanic crust with an average thickness of similar to 35 km, which was at least in part unsubductable. The lower unit mafic-ultramafic lavas have high epsilon Nd(T) of +2.7 +/- 0.3, relatively unradiogenic Pb isotope compositions (mu(1) = 8.73 +/- 0.20), are depleted in highly incompatible elements, and show Nb-maxima (Nb/Nb* = 1.2 +/- 0.2, Nb/U = 43 +/- 6). These parameters are similar to those found in a number of early Precambrian uncontaminated greenstones and in recent Pacific oceanic flood basalts (OFB). They are regarded as plume source characteristics. The BADR-series mafic-intermediate to felsic volcanic rocks and subvolcanic adakitic-series rhyolites from the upper unit have island are geochemical signatures (enrichment in highly incompatible elements, large negative HFSE-anomalies, Nb/Nb* = 0.32 +/- 0.10, Nb/U = 8.8 +/- 2.5), but are characterized by high positive epsilon Nd(T) values of +2.4 +/- 1.2, indistinguishable from the lower unit mafic-ultramafic sequences. They represent mantle wedge-derived and slab-derived melts, respectively, erupted in the inner and frontal parts of an intraoceanic island are. U-Pb zircon age of 2875 +/- 2 Ma for the upper unit BADR-series felsic volcanic rocks, and Pb-Pb and Sm-Nd ages of 2892 +/- 130 and 2916 +/- 117 h fa for the lower unit komatiites and basalts are in good agreement and correspond to the time of emplacement of both sequences. The Sumozero-Kenozero greenstone belt reveals the coexistence of fragments of unsubductable oceanic crust represented by the lower unit mafic-ultramafic volcanic sequences, and the products of subduction-related magmatism of the upper unit. This observation is reconciled within a single model implying that overthickened plume-generated oceanic crust was first intruded and overlain by mafic and intermediate-felsic melts coming from both subducting slab and overlying mantle wedge and, later on, accreted to and obducted onto the microcontinent of the Vodla Block. Copyright (C) 1999 Elsevier Science Ltd.