GEOCHRONOLOGICAL CONSTRAINTS ON THE EMPLACEMENT HISTORY OF AN ANORTHOSITE - RAPAKIVI GRANITE SUITE - U-PB ZIRCON AND BADDELEYITE STUDY OF THE KOROSTEN COMPLEX, UKRAINE

U-Pb zircon/baddeleyite ages obtained for the Korosten anorthosite-rapakivi granite complex, Ukrainian shield, suggest that different magmatic phases were emplaced during a period of ca. 30 million years as a series of distinct igneous episodes. The earliest 1789.1 +/- 2.0 Ma anorthosites were followed by 1781.3 +/- 3.2 Ma dykes of plagiogranite porphyries. The emplacement of a major rapakivi granite phase took place at 1767.4 +/- 2.2 Ma, and was followed by emplacement of layered intrusions of anorthosites, gabbronorites, diabases and ultrabasic rocks between 1761 and 1758 Ma. The minimum duration of magmatism of about 30 million years, the 6-15 million years interval between igneous pulses, and alternation of discrete episodes of basic and felsic magmatism are common features of major anorthosite-mangerite-charnockite-rapakivi granite complexes. Temporal distribution of igneous activity in the Korosten complex shows that the gabbro-anorthosites and the granites are not comagmatic, although they are possibly cogenetic, and that at least four portions of granitic and basic magmas were generated during a relatively long period of at least 30 million years. The time gap of about 20-25 million years between early basic and later and more voluminous granitic magmatism, characteristic of the Korosten pluton, Wiborg and Salmi batholiths, probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust.