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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 ∼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...
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| Published in: | Geochimica et cosmochimica acta 1999-11, Vol.63 (21), p.3579-3595 |
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| Main Authors: | , , , , , |
| Format: | Article |
| Language: | English |
| Citations: | Items that this one cites Items that cite this one |
| Online Access: | Get full text |
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| Summary: | 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 εNd(T) of +2.7 ± 0.3, relatively unradiogenic Pb isotope compositions (μ
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 εNd(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 cr |
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| ISSN: | 0016-7037 1872-9533 |
| DOI: | 10.1016/S0016-7037(99)00111-8 |