This paper is an attempt to estimate quantitatively the chemical composition and volume of magma, the reactivation of which resulted in the formation during the Paleozoic of the northeastern part of the Baltic Shield in the Kola alkaline province. In contrast to the known models, a gravity-based method of 3-D density modelling was used to evaluate the extent of alkaline magmatism in the province and, accordingly, to estimate the volume of mantle magma produced during the Paleozoic cycle of activity. During the early period of study, along with sample collection and geochemical analyzes, the deep structure of the province was investigated, and 3-D density models were derived for all alkaline intrusions of the province to a depth of 22.5km. The next step included the high-precision determinations of trace element concentrations in the rocks using an ICP-MS method as a basis for calculating the weighted mean concentrations of these elements in the rocks of the province, deriving models for mantle rock melting, and estimating the geodynamic consequences of these mantle processes. The calculations showed that the total volume of the Paleozoic magma produced in the NE part of Fennoscandia had been 15000 2700 sq. km. The calculated composition of the magma that might have been melted from the mantle of an intermediate composition revealed that a significant amount of incoherent elements must have been added to the primary mantle material. It is shown with a high degree of certainty that the primitive magma was produced in the Kola Province because of the low melting degree of the substrate 0.3-0.5% whose composition answered to a phlogopite amphibole bearing garnet lherzolite residing at a depth of a garnet mantle. The enrichment degree of this substrate was calculated to have been 3 times higher than the mean contents of incompatible elements in the primitive mantle. It is shown that the process of magma generation involved a significant portion of the NE Fennoscandian lithosphere ca. 400km across and 120km in depth, that is, covered the entire depth of the mantle garnet lherzolite. This areal estimate agrees with the area of Paleozoic igneous rocks in the region. The estimated depth correlates with the PT conditions of the mantle xenoliths found in the dikes and breccia pipes of the region.
Alkaline magmatism, mantle melting, Baltic shield.
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