Age and epsilon(Nd) constraints on the Palaeoproterozoic tectonic evolution in the Baltic-Sea region

The metamorphic Precambrian basement in the western part of the East Europe an Craton (EEC) has previously been considered to be Archaean in age. Recen t U-Pb and Sm-Nd studies in Estonia, Poland and Belarus have, however, reve aled a vast area of Palaeoproterozoic crust. In order to survey the crustal ages in Latvia and Lithuania, and to establish the relationships of these territories with adjacent regions of the EEC, particularly those exposed in the Baltic and Ukrainian Shields, three new U-Pb zircon age determinations and 21 epsilon (Nd) whole-rock analyses were performed. In Latvia, a granodiorite yielded an intrusion age of 1816 +/-5 Ma and a fe lsic metavolcanic rock an age of metamorphism at 1810 +/-2 Ma, with indicat ions of a minimum protolith age of ca. 1840 Ma. In Lithuania, a metasedimen t yielded a multigrain U-Pb zircon age of 2302(-100)(+200) Ma, very similar to multigrain zircon ages of Svecofennian metasediments in Scandinavia. The epsilon (Nd) survey of various rock types demonstrated relatively littl e variation, with depleted-mantle model ages between 2.4 and 1.9 Ga for the igneous rocks and between 2.5 and 2.0 Ga for the metasedimentary ones. Thi s indicates only minor influence of Archaean crust but close isotopic simil arity to rocks of the same lithologies in the Svecofennian Domain of Sweden and Finland. Together with recent U-Pb and Sm-Nd results from other parts of the EEC, th e present study suggests a close relationship between the Palaeoproterozoic crust in Estonia, western Latvia and western Lithuania, and the southern p art of the Svecofennian Domain within the Baltic Shield. All these regions have U-Pb zircon ages of rock formation between 1.90 and 1.82 Ga, and were subjected to a major metamorphic event between 1.83 and 1.81 Ga. These ages contrast with ages between 2.0 and 1.9 Ga for similar rocks farther cast a nd southeast, i.e. closer to the Ukrainian Shield. Altogether, the current results reaffirm an accretionary character of the Palaeoproterozoic crust i n the western part of the EEC, as previously concluded by other investigato rs. In addition, there is basis to suggest that the crust in western Latvia and Lithuania was formed during the late stages of the Svecofennian orogen y and thus can be correlated with juvenile late Svecofennian crust in south eastern Sweden. (C) 2001 Elsevier Science B.V. All rights reserved.