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Fractional Differentiation and Liquation (in Situ) in Komatiitic Lavas
Upper Archean komatite-tholeittic series were studied in detail both geologically and geochemically in Central Karelia. Magma differentiation processes, which took place in the course of fractional crystallization and as the result of liquation, were shown to be the main processes responsible for the evolution of melt after its ejection on to the earth surfase, the former being more common. To understand differentiation processes, layered lava flows of peridotite and pyroxenite komatiites from the Koikary and Palaselga struktures of the Vedlozero-Segozero greenstone belt, Central Karelia, and komatiites from the Siivikkovaara structure of the Kuhmo greenstone belt, East Finland, were studied. Two to ten zones showing various spinifex (A1-A..) and cumulate (B1-B..) structures are differentiated in the above units. The petrogeochemical study of compositional variations in flows from one zone to another suggests that their formation can be explained most correctly using a model of melt fractionation in the course of crystallization and gravitational settling of Ol+Crt in the ratio 0.98-0.02 or 0.97-0.03 from a melt compositionally corresponding to peridotite komatiite. The formation of various spinifex structures depends on the saturation of a melt with crystallization centres, the rote at which the crustallization front moves and convectional motion in a flow. Differentiation, which takes plase in the course of liquation divizion, is well-defined in the variolitic lavas of pyroxenite komatiites in the Koykary structure. Variolites , in which the globules compositionally correspond to low-K andesidacites, were studied there. Geochemical calculations show that variolites are formed in flows in the course of low temperature metastable liquation which follows the ejection of lava. Such a process gives rise not only to compositionally contrasting globules, but also to bigger forms such as lenses and patches in lava flows and pillows.