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Abstract
Radiometric data for igneous rocks, intruding early Proterozoic supracrustals, from two separate areas in north-eastern Sweden are presented. U-Pb analyses on zircons from plutonic rocks of intermediate composition around Lake Paittasjärvi yield an age of 1880 ± 28 Ma. A whole-rock isochron age of 1915± 145 Ma and a plagioclase-hornblendemica age of 1550± 40 Ma are calculated for the same rocks with the Rb-Sr method. Correlated granitoids from the Juoloyanjarvet area, which in part have suffered replacement of plagioclase by scapolite and are intruded by gabbros and younger granites, give a whole-rock Rb-Sr reference line of 1705 Ma and mineral ages of 1535 and 1495 ± 30 Ma. The whole-rock age probably lacks direct geological significance but the mineral ages obtained from both of the above areas may reflect influences from younger plutonism on the dated rocks. One of the intruding gabbros has a Rb-Sr mineral age of 1617 ± 107 Ma. This figure is considered a minimum age of the rock. The impact of scapolitization on the calculated ages is discussed. Decay constants recommended by the IUGS Subcommission on Geochronology (1977) have been used in the calculation of the Rb-Sr and U-Pb ages, which are given at the 95% confidence level.