NEW DATA ON THE BASEMENT AGE OF THE WESTERN PART OF THE EAST EUROPEAN PLATFORM: U-PB ISOTOPIC ANALYSIS OF ZIRCON ON A NORDSIM ION MICROPROBE

Abstract

The crystalline basement in the west of the East European Platform is composed of arc-shaped tectonic belts and blocks metamorphosed to the amphibolite and granulite facies. Our recent geochronologic dating (U-Pb on zircon and Sm-Nd on whole-rock samples) dispelled the previously widely adopted notion of the Archean age of the basement and provided grounds to parallel most of these provinces with the Early Protero-zoic complexes of the Baltic Shield. However, the previously utilized geochronologic techniques left several issues uncertain, mostly concerning the evolution of the granulite belts and blocks. In this study, the U-Pb isotopic age of zircon was determined on a secondary-ion mass spectrometer, which made it possible to conduct the whole complex of isotopic analysis on a single mineral grain at a spot 30 μm in diameter. Taken collectively with earlier isotopic geochronologic data, our results on zircon, newly obtained with the use of an ion microprobe, testify to the absence of an Archean component even in the protolith of the metasediments and igneous rocks, except for the Braginskii granulite massif. This makes these rocks different from the Svecofennian metasedimentary rocks of the Baltic Shield and seem to provide evidence that the sedimentation area was remote from the Archean craton. Granulite-facies rocks become younger northwestward, from 2100 Ma in the Braginskii Massif, to 1950 Ma in the Vitebsk Massif, 1890 Ma in the Belorussian-Baltic Massif, and 1800 Ma in the Western Lithuanian Massif. This confirms the proposed accretionary-tectonic model with a series of episodes of the growth and reworking of the continental margin of Sarmatia.