Svecofennian metamorphic zones in the basement of Estonia

Abstract

Svecofennian (Palaeoproterozoic) orogenic, folded metamorphic rocks dominate in the basement structure in Estonia. In northern Estonia (Tallinn zone), supracrustal rock associations (metavolcanic and -sedimentary sequences), their structure and amphibolite metamorphic grade resemble the Svecofennian metamorphic island arc suites of southern Finland. In northeastern Estonia, the metapelitic sequences of the Alutaguse zone resemble those of the NE marginal metasedimentary basins of the Svecofennian orogen, as studied in the St. Petersburg District (NW Russia) and SE Finland. Local variations of the metamorphic grade, related to fault zones or local metamorphic domes (metamorphosed up to granulite assemblages) feature the amphibolite facies areas of northern Estonia. In southern Estonia, tectonically undefined, predominantly mafic metavolcanics are of granulite metamorphic grade. The granulite region of southern Estonia and northern Latvia is much larger than known in the 1.9-1.8 Ga Svecofennian metamorphic zones of southern Finland. The peak conditions of granulite metamorphism in Estonia at ca. 800 °C and 4-6 kbar resemble those of the Pielavesi granulites (Proterozoic), central Finland. However, the U-Pb age of 1778 ± 2 Ma for monazite and the Sm-Nd age of 1728 ± 24 Ma for garnet from the sample Kõnnu 3005150 are clearly younger than any comparable results from Finland, and suggest that the granulite facies metamorphism in southern Estonia is distinct from that recorded in southern and central Finland.