PALEOPROTEROZOIC GEOCHRONOLOGY OF THE PECHENGA-VARZUGA STRUCTURE, KOLA-PENINSULA

Geochronological data from volcanics and intrusions of the Pechenga-Va rzuga volcano-sedimentary structure indicate that mantle magmatism the re spanned about 700 m.y. (2540 - 1860 Ma), a value that rises to abou t 800 m.y. if the final stage of granite formation, silicic volcanism, and metamorphism is added. Thus, this study is the first to geochrono logically substantiate the geotectonic idea that Paleoproterozoic intr acratonic (protoriftogenic) systems may have an enormous lifetime in c ontrast to the marginal Archean, Proterozoic, and Phanerozoic systems with lifetimes of only 40 - 150 m.y. The periodicity of main magmatic pulses in the protorifts equals 60 - 120 m.y., their temporal limits c oinciding with stratigraphic boundaries. The magmatic stages temporall y correlate with the transformations occurring in the country rock inf racomplexes; this allows the compilation of a detailed geochronologica l scale reflecting the principal age boundaries in the Paleoproterozoi c of the Baltic Shield. Following the hypothesis of depleted upper man tle differentiation beneath the Archean cratons, the anomalous Nd and Sr isotopic characteristics of the Sumian mafic rocks are believed to have resulted from the melting of upper mantle areas enriched in litho phile elements.