U-Pb dating of zircons and monazites from Archean granulites in Varpaisjarvi, Central Finland:: Evidence for multiple metamorphism and Neoarchean terrane accretion

U-Pb age data from Archean lower crustal granulites in the Varpaisjarvi area, central Finland, indicate several metamorphic events that took place in Mesoarchean-Neoarchean and Paleoproterozoic time. Zircons. separated from the granulites fall into two distinct age groups. In the older group, the protolith age is 3.2 Ga and many zircons indicate metamorphism at ca. 3.1 Ga. In contrast, the oldest zircons in the younger group are ca. 2.73 Ga. Both terranes were migmatized at ca. 2.70 Ga with the emplacement and crystallization of igneous enderbites. The enderbites and the leucosomes of migmatites in both granulite terranes have 2.7 Ga zircons. High grade metamorphic conditions prevailed from 2.70 to 2.63 Ga which is the age range of most zircons in the eastern terrane. Also the enderbites show metamorphic zircon growth at ca. 2.64 Ga. Two distinct age groups are interpreted to be the result of juxtaposition of two terranes of different ages. This indicates that collisional and accretionary processes operated during Neoarchean time. The area east from the eastern granulite terrane underwent almost pervasive deformation and amphibolite facies metamorphism during the Svecofennian orogeny at ca. 1.9 Ga. However, the U-Pb ages of zircons in the retrograde area are similar to those of the eastern terrane, thus indicating that these rocks were metamorphosed under granulite facies conditions (2.70-2.63 Ga). Although the ca. 1.9 Ga Svecofennian metamorphism is not evident in the ion microprobe results, some Paleoproterozoic Pb-loss is indicated by TIMS U-Pb analyses of zircons. In addition, monazite age of 1.89 Ga from the retrograde area reflects Svecofennian metamorphism. (C) 2002 Elsevier Science B.V. All rights reserved.