P– T– t development of Archaean granulites in Varpaisjärvi, Central Finland : II. Dating of high-grade metamorphism with the U–Pb and Sm–Nd methods

Abstract

Exposed blocks of lower crustal Archaean granulites separated by Palaeoproterozoic faults occur near the western boundary of the Karelian craton in central Finland. The southwesternmost granulite block in the study area, the Jonsa block, differs from other granulites by having younger zircon U–Pb and Sm–Nd T DM model ages. The U–Pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the Jonsa block are ca. 2.63 Ga, which is interpreted as the age of granulite metamorphism. Igneous enderbites, which make up a considerable part of the bedrock in granulite blocks, are ca. 50 Ma older. Outside the Jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3.2–3.1 Ga, which are the protolith ages. Sm–Nd T DM model ages of the mafic granulites are 2.7–2.9 Ga in the Jonsa block but ca. 3.2 Ga in the other granulites. The age differences are interpreted to represent terrane accretion that juxtaposed 3.2 Ga rocks next to younger rocks. All granulites were metamorphosed in similar PT conditions at 2.63 Ga; granulite metamorphism affected the whole area but was not able to reset zircons in older rocks. In all blocks the Sm–Nd garnet-whole rock ages are younger, ranging from 2.48–2.59 Ga. This reflects relatively low closure temperature of the Sm–Nd system in garnet in these rocks. The U–Pb age of a Palaeoproterozoic dolerite which cuts the granulites is 2.3 Ga, close to the K–Ar age of amphibole in a retrogressed fracture.