Combined mantle plume-island arc model for the formation of the 2.9 Ga Sumozero-Kenozero greenstone belt, SE Baltic Shield: Isotope and trace elementconstraints
Is. Puchtel et al., Combined mantle plume-island arc model for the formation of the 2.9 Ga Sumozero-Kenozero greenstone belt, SE Baltic Shield: Isotope and trace elementconstraints, GEOCH COS A, 63(21), 1999, pp. 3579-3595
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-ty
pe submarine mafic-ultramafic lavas. The upper unit is made up of island ar
e-type volcanic BADR (basalt-andesite-dacite-rhyolite)-series rocks and ada
kite-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. T
he plume was 250 degrees C hotter than the ambient mantle and had the therm
al 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 foun
d in a number of early Precambrian uncontaminated greenstones and in recent
Pacific oceanic flood basalts (OFB). They are regarded as plume source cha
racteristics.
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 characteri
zed by high positive epsilon Nd(T) values of +2.4 +/- 1.2, indistinguishabl
e from the lower unit mafic-ultramafic sequences. They represent mantle wed
ge-derived and slab-derived melts, respectively, erupted in the inner and f
rontal parts of an intraoceanic island are.
U-Pb zircon age of 2875 +/- 2 Ma for the upper unit BADR-series felsic volc
anic 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 corresp
ond 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-ultramaf
ic volcanic sequences, and the products of subduction-related magmatism of
the upper unit. This observation is reconciled within a single model implyi
ng 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 on
to the microcontinent of the Vodla Block. Copyright (C) 1999 Elsevier Scien
ce Ltd.