EARLY PROTEROZOIC IGNEOUS PROVINCE OF SILICEOUS HIGH-MG BONINITE-LIKEROCKS IN THE EASTERN BALTIC SHIELD

A large Paleoproterozoic igneous province of siliceous high-Mg (bonini te-like) rocks approximately 0.8 x 10(6) km(2) in area is located with in three main structural domains of the eastern Baltic Shield: the Arc hean Kola and Karelian cratons and the Belomorian mobile belt between them. The province is composed of volcanogenic-sedimentary complexes i n grabenlike structures, dike swarms, and large layered intrusions. Th e Belomorian belt contains small synkinematic intrusions of the drusit e complex. The province was formed at two stages: Sumian (similar to 2 .55-2.4 Ga) and Sariolian (2.4-2.3 Ga). The geodynamic setting of the Baltic province is similar to Phanerozoic intraplate environments, whi le the province is the most ancient continental rift zone. However, un like reference rift areas, it contains boninite-like rocks typical of active Phanerozoic continent-ocean margins. During the Sumian-Sariolia n, the Kola and Karelian cratons were extensional areas above the spre ading heads of mantle superplumes composed of an ultramafic depleted m aterial. The genesis of the series is related to large-scale assimilat ion of crustal material by high-temperature mantle melts during their ascent to the surface. Early Proterozoic magmatic processes display in herited evolution within the same belts during, at least, 1 Ga, from t he Late Archean through the Early Proterozoic. This may be related wit h the dominant plume-tectonic style of geodynamic evolution of this ar ea in the Early Precambrian. Geological evidence of the change from pl ume tectonics to plate tectonics began to occur at approximately 2 Ga, practically simultaneously with the appearance of Fe-Ti picrites and basalts.