AGE AND SIGNIFICANCE OF MESOPROTEROZOIC CFB MAGMATISM, LAGE LADOGA REGION, NW RUSSIA

The south-central part of the Fennoscandian (or Baltic) Shield in Sweden, Finland, and adjacent Russia experienced widespread CFB-type basaltic magmatism and red-bed sedimentation in the Mesoproterozoic. In the Lake Ladoga region, Russian Karelia, this led to the formation of a faulted-bounded basin, ca. 150 km in diameter, with intercalated sandstone, doleritic sills and dikes, and mafic lavas. The Lake Ladoga dolerites and lavas are alkaline, relatively evolved (< 6 wt. % MgO, 43 - 57 wt. % SiO2), and strongly enriched in the LREE. One concordant and one nearly concordant baddeleyite fraction from a dolerite on the Valaam Island in the northwestern part of Lake Ladoga yielded 207Pb-206Pb ages of 1459 +/- 3 and 1457 +/- 2 Ma, respectively. This shows that the Mesoproterozoic CFB magmatism in the south-central Fennoscandian Shield commenced ca. 200 Ma earlier than previously anticipated and very shortly after the emplacement of the classic, 1670 - 1470 Ma, rapakivi granites. The basin formation probably started immediately after the emplacement of the extension-related rapakivi intrusions due to thermal contraction of the rift zones and the relatively thin crust hosting the rapakivi granites permitted ascent of the mantle-derived basaltic magmas. The basaltic magmatism in Lake Ladoga region has initial epsilon-Nd (at 1460 Ma) values of -9.2 to -8.6 and initial Sr isotopic ratios of 0.7041 to 0.7055. These point to a Neoarchean, metasomatically enriched lithospheric mantle source.