ND AND SR ISOTOPIC GEOCHEMISTRY OF MAFIC LAYERED INTRUSIONS IN THE EASTERN BALTIC SHIELD - IMPLICATIONS FOR THE EVOLUTION OF PALEOPROTEROZOIC CONTINENTAL MAFIC MAGMAS
Yv. Amelin et Vs. Semenov, ND AND SR ISOTOPIC GEOCHEMISTRY OF MAFIC LAYERED INTRUSIONS IN THE EASTERN BALTIC SHIELD - IMPLICATIONS FOR THE EVOLUTION OF PALEOPROTEROZOIC CONTINENTAL MAFIC MAGMAS, Contributions to Mineralogy and Petrology, 124(3-4), 1996, pp. 255-272
Nd and Sr isotopic data are presented for the 2449-2441 Ma Olanga and
Burakovka layered mafic complexes in the eastern Baltic Shield. These
complexes have similar tectonic position, but differ in two aspects: t
he age of the enclosing crust and the post-crystallization metamorphic
history. The Sm-Nd isotopic results for the Kivakka and Lukkulaisvaar
a intrusions, Olanga Complex, are consistent with the model of closed-
system crystallization of a single magma without significant wallrock
assimilation. The Rb Sr systems of minerals were disturbed by late Rb
addition during 1.75-1.50 Ga metamorphism. The Nd and Sr isotopic syst
eins in the Burakovka complex show no metamorphic disturbance and indi
cate mixing of at least four isotopically distinct components. Isotopi
c variations in the Burakovka Complex can be explained by a 4-20 per c
ent contamination of a primary komatiitic or picritic magma with a Mes
oarchean crust. similar to that exposed in the region. A similar model
, applied to the Olanga Complex using a Neoarchean crustal isotopic co
mposition, cannot reproduce the observed isotopic signature. The nearl
y uniform initial EN, values between -1 and -2.3, observed in the Kiva
kka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in
the other 2.50-2.44 Ga layered mafic intrusions throughout the easter
n Baltic Shield, are better explained by a mantle plume model with sma
ll amounts of crustal contamination and minor involvement of asthenosp
heric material. This model is also consistent with the geological obse
rvations and the temporal distribution of the Paleoproterozoic mafic m
agmatism in the eastern Baltic Shield. As an alternative, the enriched
isotopic characteristics may be explained by melting of a metasomatic
ally modified lithospheric mantle source.