Puchtel, I. S., Hofmann, A. W., Amelin, Yu V., Garbe-Schönberg, C. D., Samsonov, A. V. and Shchipansky, A. A. (1999) 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. Geochimica et Cosmochimica Acta, 63 (21). pp. 3579-3595. DOI 10.1016/s0016-7037(99)00111-8.

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Abstract

The Sumozero-Kenozero greenstone belt in the SE Baltic Shield is ~400 km long and up to 50 km wide and includes two main units with a total thickness of ~5 km. The lower unit consists of oceanic plateau-type submarine mafic-ultramafic lavas. The upper unit is made up of island arc-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 ~30% MgO that erupted at a temperature of ~1570°C. The komatiite liquid began melting at depths of 300-400 km in a mantle plume. The plume was 250°C hotter than the ambient mantle and had the thermal potential to produce oceanic crust with an average thickness of ~35 km, which was at least in part unsubductable. The lower unit mafic-ultramafic lavas have high [var 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 arc 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 [var 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 arc. 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 Ma 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.

Document Type:	Article
Research affiliation:	Kiel University
Refereed:	Yes
Date Deposited:	02 Feb 2012 09:15
Last Modified:	11 Apr 2018 12:02
URI:	https://oceanrep.geomar.de/id/eprint/16370

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