Proterozoic tectonothermal history in the western part of the East European Craton: Ar-40/Ar-39 geochronological constraints

dTwenty-three new Ar-40/Ar-39 amphibole age determinations have been carrie d out to date major crustal boundaries in the sediment-covered crystalline basement of the western part of the East European Craton, in the area betwe en the Baltic (Fennoscandian) and the Ukrainian Shields. That crystalline b asement is characterised by a pronounced alternation of granulite-and amphi bolite -facies rock belts identified from geophysical surveys and deep dril l core samples. These belts strike NNE to NW and are considered to represen t accretionary stacking of different Palaeoproterozoic (2.0-1.85 Ga) terran es of juvenile crust along the margins of the Archaean protocontinents of F ennoscandia, Sarmatia and Volgo-Uralia, which make up the cores of the EEC. Amphiboles were selected from drill core samples taken from (1) rocks in th e internal parts of the granulite- and amphibolite-facies belts, (2) amphib olite-facies mylonites along the boundaries between the belts and along fau lt zones within the belts, and (3) metamorphosed mafic/intermediate dykes a nd mylonites along E-W-trending zones of ductile faulting superimposed upon the older beltiform structures. The oldest Ar-40/Ar-39 ages (1778-1740 Ma) are recorded by granulites of th e Belarus-Baltic Granulite Belt, and are interpreted to date the time of co oling after peak metamorphism around 1800 Ma. Rocks from mylonites of the s econd group yielded Ar-40/Ar-39 ages in the range of 1710-1660 Ma, regardle ss of the previous histories of the hosting rock belts. Ar-40/Ar-39 ages of 1550-1450 Ma, in contrast, are obtained from the rocks of the third group. The first two of the dated events were associated with compressional tecto nics during accretion and collision of the Palaeoproterozoic terranes again st the Archaean nuclei of the East European Craton. The third event is clos ely related to anorogenic anorthosite-rapakivi granite magmatism controlled by a set of E-W-trending shear zones. (C) 2001 Elsevier Science B.V. All f ights reserved.