U-PB MINERAL AGE DETERMINATIONS FROM ARCHEAN ROCKS IN EASTERN LAPLAND

Abstract

The bedrock in eastern Lapland is divided into the Naruska, Ahmatunturi and Vintilänkaira-Kemihaara granitoid complexes and the supracrustal Tuntsa metasedimentary and Tulppio metavolcanic belts. Traditionally the bedrock in the area has been regarded as Archean, but previous radiometric age determinations are few. New U-Pb data from 16 samples taken mainly from granitoid rocks within eastern Lapland are presented in this paper. The Lomperovaara tonalite within the Vintilänkaira-Kemihaara complex and syenitic rocks cross-cutting the Tulppio metavolcanic belt have minimum zircon ages of c. 2.80 Ga, and a hornblende-gneiss inclusion within the Lomperovaara tonalite gives the same 2.83 Ga as a tonalite of the Ahmatunturi complex. The tonalites of the Naruska granitoid complex register an age of 2.70-2.75 Ga. The granitic rocks within the same area give ages at 2.70-2.72 Ga and are thus coeval with the tonalites and possibly represent their migmatized counterparts. The 2.57 Ga registered by zircons of a granitic vein cross-cutting the Lomperovaara tonalite cannot be considered a true age. A quartz-feldspar schist from the Tulppio metavolcanic belt, interpreted as a felsic volcanic rock, contains zircons of Archean origin, which may indicate an age in the 2.8-2.9 Ga bracket. The minimum ages of the Takatunturi mafic dike cutting the Naruska granitoid complex and the Sulkarinoja granite within the Vintilänkaira-Kemihaara granitoid complex are 1.94-1.90 Ga and demonstrate that magmatic activity occured in Paleoproterozoic times as well. However, as Archean titanites have been preserved in the granitic gneisses found at Kairijoki and Naruskajoki it seems that the Paleoproterozoic magmatism was of rather local nature. The results from eastern Lapland support the findings of earlier studies indicating that the continental crust in northeastern Fennoscandian Shield was formed mainly in late Archean times and consists of rock associations generated at slightly different times in different geotectonic environments.