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

Exposed blocks of lower crustal Archaean granulites separated by Palaeoprot erozoic faults occur near the western boundary of the Karelian craton in ce ntral Finland. The southwesternmost granulite block in the study area, the Jonsa block, differs from other granulites by having younger zircon U-Pb an d Sm-Nd T-DM model ages. The U-Pb ages on zircons and monazites from leucos omes and mesosomes of mafic and pelitic granulites in the Jonsa block are c a. 2.63 Ga, which is interpreted as the age of granulite metamorphism. Igne ous enderbites, which make up a considerable part of the bedrock in granuli te blocks, are ca. 50 Ma older. Outside the Jonsa block both conventional a nd 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 gr anulites. The age differences are interpreted to represent terrane accretio n that juxtaposed 3.2 Ga rocks next to younger rocks. All granulites were m etamorphosed in similar PT conditions at 2.63 Ga; granulite metamorphism af fected 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 syst em in garnet in these rocks. The U-Pb age of a Palaeoproterozoic dolerite w hich cuts the granulites is 2.3 Ga, close to the K-Ar age of amphibole in a retrogressed fracture. (C) 2000 Elsevier Science B.V. All rights reserved.