TIMING OF MAGMATIC CRYSTALLIZATION AND METAMORPHIC TRANSFORMATIONS IN THE PYRSHIN AND ABVAR AUTONOMOUS ANORTHOSITE MASSIFS, LAPLAND GRANULITE BELT

Abstract

Four types of accessory zircons with U-Pb ages of 2452 ± 7, 2417 ± 3, 1943 ± 3, and 1906 ± 4 Ma were detected in the polymetamorphic rocks of the anorthosite association from the Pyrshin and Abvar massifs of the Lapland granulite belt. Crystals of zircon I were separated from grains of magmatic plagioclase, and their dating of 2452 ± 7 Ma is considered, therefore, as the age of igneous rock crystallization. The age sequence of the other three zircon types is correlated with the stages of metamorphic rock alterations established from the sequential appearance of respective assemblages: early granulite stage and corona formation, peak of the Lap-land granulite metamorphisrn, and postgranulite retrograde transformations. It was shown for the first time that such multiple metamorphic transformations under deep conditions lasted more than 550 Myr. Methodic difficulties and perspectives in the dating of polymetamorphic rocks from the granulite-gneiss regions of shields are discussed.