Geochronology of the Ukrainian Precambrian

Abstract

Geochronological investigations of the Ukrainian Shield have been carried out for all regions by means of the U–Th–Pb method for accessory minerals and by the K–Ar method for hornblende and mica minerals. The absolute ages of hornblende and accessory minerals are the basis for the geochronological division of the geologic history, whereas the absolute ages of mica reflect only the time of epigenetic processees.The Precambrian history of the Ukrainian Shield is divided into five megacycles as follows: (1) Precambrian I—from > 3500 to 2600 ± m.y.; (2) Precambrian II—from 2600 ± 100 to 2000 ± 100 m.y.; (3) Precambrian III—from 2000 ± 100 to 1700 ± 100 m.y.; (4) Precambrian IV—from 1700 ± 100 to 1100 ± 100 m.y.; and (5) Precambrian V—from 1100 ± 100 to 570 m.y.;Precambrian I is a stable platform formed by ancient, folded, mineralized geosynclinal formations. The Konkian orogenic cycle, operative within the period 3500 to 3100 m.y. and the Aulian cycle, operative within the period 3100 to 2700 m.y. have been recognized.The folded structures formed within the second Precambrian megacycle are developed over most of the Ukrainian Shield. Folded structural stages are known which formed within the period of the Rosinsky or Basavlukian orogenic cycle, corresponding to the period of 2700 to 2300 m.y. Later structural stages formed within the period of the Bug–Podolian orogenic cycle, whose absolute age is shown to be from 2300 to 2000 m.y.The folded structural stages of the third Precambrian megacycle are developed in the central part of the Ukrainian Shield, in the Ingulo–Ingulets dividing area. These stages formed within the period of Krivoi–Rog (Ingulets) cycle during the period from 2000 to 1700 m.y. The granitic intrusions of this megacycle also developed in the western part of the shield, in the basin of the Teterev River. The main part of the Ukrainian Shield became stable after the Krivoi–Rog cycle of folding, magmatism, and metamorphism.The structural stages formed in the area of mobile zones during the period of the fourth Precambrian megacycle developed in the northwest part of the Ukrainian Shield. There we recognized formations formed within the Volynian orogenic cycle, whose absolute age is stated to range from 1700 to 1500 m.y.; and formations formed during the Ovruch orogenic cycle, with ages ranging from 1500 to 1200 m.y. The stabilization of the Ukrainian Shield and East European platform ended with Ovruchian folding.The folded stages formed within the fifth Precambrian megacycle occur in the Galitsian folded belt, which frames the Ukrainian Shield to the south and west. These stages are found in the pre-Mesozoic basement of the Soviet Carpathians and in the basement of the Crimea mega-anticlinorium. There we recognize formations of the Black Sea cycle from 1000 to 800 m.y., and the Rakhovian cycle from 700 to 570 m.y. The Galitsian folding is synchronous with the Baikal folding. The mobile zone was also deformed in Early Paleozoic times. The folded belts of the fifth Precambrian megacycle frame the East European platform, the Timanian belt to the northeast, the Dalsland belt to the northwest, and the Galitsian belt to the southwest.The Ovruchian and Volynian cycles are synchronous with the Gothian cycles of the Baltic Shield. According to our ideas, the mobile folded zones of the fourth Precambrian megacycle are widely developed in the Russian block of the East European platform. They frame the Ukrainian Shield, Baltic Shield, and Voronezh massif which had become stable at the end of the third Precambrian megacycle.