Geochemical and Isotopic Evidence of Archaean Mantle Plume Action on the Baltic Shield

A. B. Vrevsky Institute of Precambrian Geology & Geochronology, Makarova 2, St.-Petersburg, 199034, Russia

R. S. Krimsky Institute of Precambrian Geology & Geochronology, Makarova 2, St.-Petersburg, 199034, Russia

V. A. Matrenichev Institute of Precambrian Geology & Geochronology, Makarova 2, St.-Petersburg, 199034, Russia

In order to understand the geochemical and isotopic evolution of the Archaean upper mantle of the Baltic Shield, the geochemistry of incompatible and rare earth elements, oxygen and noble gas isotopic composition, Sm-Nd and
Rb-Sr systematics of ST-komatiites from 11 greenstone belts of Kola peninsula, Karelia and E.Finland were investigated.

The komatiites from the Archaean greenstone belts of the Eastern Baltic Shield can be divided based on their REE abundances in four geochemically distinct types with: 1) flat chondrite-normalized REE patterns (0.4-1.5, 1.5-3.0, 3.0-5.0 x chondrite abundances); 2) LREE depleted and flat HREE (La/Sm)N = 0.6-0.7; 3) light LREE enrichment (La/Sm)N = 1.4-1.6; 4) strong negative Eu anomaly Eu/Eu* = 0.04-0.1.

Established variations of C/36Ar (109-1010), 143Nd/144Nd (0.5122-0.5136), 87Sr/86Sr (0.7010-0.7035) ratios, dO18(+4 - +7) and overall REE (0.4-5.0x chond.) were correlated by computer simulations with such petrological and geological limitations as depth and degree of partial melting, age
of volcanism, possible contamination of crust material, migration of elements and isotopes during alteration and metamorphism.

The observance of all this limitation indicates that there were 3 geochemicaly and isotopicaly distinguished mantle reservoirs with which komatiite melts reequilibrated. This lateral mantle heterogeneity crossed the boundaries of supposed for the Baltic Shield Archaean tectonic terranes and could be explained by the concept of a starting mantle plume (Campbell and Griffiths, 1993; Lasher and Arndt, 1995; Maruyama, 1994). We assume that komatiites of the Baltic Shield are the products of the different stages of tilted assymmetric mantle plume evolution: 1) Early stage (3.05 - 2.90 G a) - the melting of a mixture of heated surroundings (moderate depleted upper mantle) and cooled source material (undepleted) in the head of a mantle plume produce the komatiites of East and Central Karelia, characterized by: high AL2O3 = 6-10%, moderately low FeO (6 -13%), low TiO2/Zr and Y/Zr ratios, slightly depleted (Ce/Sm)N = 0.66-0.73, "normal" overall HRRE content = 2.0 - 3.0 x chondrite and with high eNd values (+4.2). Some of the komatiite melts ( Palaj-Lamba and Hautavaara) were slightly contaminated: SiO2 = 48-50 %; high initial 87Sr/86Sr = 0.706 -0.707 and low eNd values (+1.7 - +1.3). 2) Middle stage (2.9 - 2.8 G a) of plume latterall spreading produces the komatiites of Kola, North Karelia and Kostamuksha with high MgO = 36-28%, FeO = 10-20% , chondritic Y/Zr ratios, flat REE patterns (Ce/Sm)N =0.92 -1.09, "chondritic" overall HREE content (0.3 -2.1 x chondrite), moderate high eNd values (+2.6 - +3.7), and moderate to low 87Sr/86Sr = 0.704 -0.701. This komatiites were probably derived from the nearly chondritic source in the hot axial zone of a mantle plume. 3) Late stage (2.8 - 2.7 G a) - the melting of a heated high depleted mantle surroundings in a tilted plume head produces the E. Finland komatiites: (Ce/Sm)N = 0.27 -0.57; overall HREE content = 3.3 -5.0 x chondrite; low TiO2/Zr and Y/Zr ratios.


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Maruyama, Sh., J. Geol. Soc. Japan 100, 24-42 (1994).