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Vol. 1 (1998 year), No. 3

Balagansky V.V., Timmerman M.Ya., Kislitsyn R.V., Dayly J.S., Balashov Yu.A., Gannibal L.F., Sherstenikova O.G., Ryunenen G.I.
The Isotope Age of the Kolvitsa Belt and the Umba Granulite Terrane (South-East of the Lapland Granulite Belt), the Kola Peninsula

The Lapland Granulite Belt (LGB) in the northern Baltic Shield is interpreted to have resulted from plate tectonic processes in the Palaeoproterozoic. The Kolvitsa Belt and the tectonically overlying metasedimentary Umba Granulite Terrane (UGT) are thought to be isolated fragments of the LGB in the Kola Peninsula. U-Pb (on zircon) and Rb-Sr (whole rock) ages, as well as Sm-Nd and Rb-Sr data presented in the paper indicate that the Kandalaksha metabasalt sequence, the Por’ya Guba mafic to intermediate granulite complex, the Kolvitsa massif of gabbro-anorthosites and its dyke complex are related to a single volcano-plutonic suite, which was developing 2.43-2.47 Ga ago at an initial stage of the earliest Palaeoproterozoic rifting of a Neoarchaean craton. Values of initial 143Nd/144Nd and 87Sr/86Sr ratios in rocks of this Kolvitsa suite coincide with those of 2.44-2.50 Ga old layered mafic intrusions in the Baltic Shield. In contrast with the Kolvitsa suite, the deposition of sedimentary protoliths of garnet-quartz-feldspar granulites and garnet, sillimanite and cordierite gneisses of the UGT was occurring ca. 2.0-2.1 Ga ago as suggested by model depleted mantle ages of 2.1-2.4 Ga and age data available on the granulite metamorphism (1.91 Ga). The sedimentary protoliths of the Umba paragranulites were derived largely from Palaeoproterozoic juvenile material, which permits to interprete the depositional environment as an island arc setting.

(in Russian, стр.14, fig. 5, tables. 3, ref 44, MS Word 95, MS Word 95 1420 Kb)

Vol. 1 (1998 year), No. 3

Liferovich R.P., Bayanova T.B., Gogol O.V., Sherstenikova O.G., Delenitsin O.A.
The Genesis of Postcarbonatite Phosphate Mineralization of the Kovdor Phoscorite - Carbonatite Complex

⁸⁷Sr/⁸⁶Sr isotopic ratios were studied in carbonates and phosphates from postcarbonatite hydrothermal mineral assemblages occurring in the phoscorite-carbonatite complex of the Kovdor alkaline-ultrabasic massif. The obtained results compared with those published earlier for the Kovdor carbonatites and phoscorites show that the hydrothermal phosphatic mineralization in cavities of dolomitic carbonatites and phoscorites was formed by juvenile solutions. The latter were not derived from drainaged rock (i.e. from dolomitic carbonatites), but were fluids produced by a magmatic chamber, which was generating phoscorite-carbonatite series during a calcitic stage of formation of the Kovdor complex. The following fact is of interest: mineral-forming solutions, despite being formed in the crust, have a mantle ⁸⁷Sr/⁸⁶Sr ratio, which can be explained by their relation to a mantle-derived carbonatite. Data obtained on Sr isotopic composition of carbonate-fluorapatite, which is a cement-forming mineral in apatite-francolite breccias of the central area of phoscorite-carbonatite complex, suggest an explosive origin for these rocks, followed by significant hydrothermal alteration. The role of exogenic processes in the formation of present-day apatite-francolite breccia composition rises toward the marginal area of the Kovdor phoscorite-carbonatite complex.

(in Russian, стр.8, fig. 1, tables. 1, ref 21, MS Word 95, MS Word 95 85 Kb)