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-rap akivi granite complex, Ukrainian shield, suggest that different magmat ic phases were emplaced during a period of ca. 30 million years as a s eries of distinct igneous episodes. The earliest 1789.1 +/- 2.0 Ma ano rthosites were followed by 1781.3 +/- 3.2 Ma dykes of plagiogranite po rphyries. The emplacement of a major rapakivi granite phase took place at 1767.4 +/- 2.2 Ma, and was followed by emplacement of layered intr usions of anorthosites, gabbronorites, diabases and ultrabasic rocks b etween 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 a re common features of major anorthosite-mangerite-charnockite-rapakivi granite complexes. Temporal distribution of igneous activity in the K orosten complex shows that the gabbro-anorthosites and the granites ar e not comagmatic, although they are possibly cogenetic, and that at le ast 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 volum inous granitic magmatism, characteristic of the Korosten pluton, Wibor g and Salmi batholiths, probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust.