Radiometric ages of plutonic and hypabyssal rocks from the Vittangi-Karesuando area, northern Sweden

Some of the intrusion and metamorphic episodes in the Svecokarelian (early Proterozoic) orogenic development of northern Norrbotten County are discussed on the basis of U-Pb zircon and Rb-Sr whole-rock analyses. Prior to the main Svecokarelian folding, the mafic volcanic rocks of the Vittangi-Soppero-Karesuando belt were penetrated by hypabyssal sills, of which an albite-diabase has yielded a zircon age of about 1874 Ma. According to zircon data, intermediate to acid plutonic rocks were emplaced in close connection with the hypabyssal sills and in an early-orogenic stage at about 1880–1860 Ma. Corresponding Rb-Sr whole-rock systems have been more or less disturbed, however, indicating metamorphic-metasomatic conditions until at least 1700 Ma ago. A large granite massif south of Vittangi was emplaced, according to zircon data, at about 1795 Ma, while the 40 Ma younger Rb-Sr whole-rock isochron age is due to uplift or a delayed cooling process. Another granite dome, outcropping to the west of the above, has migmatized members of the Porphyry Group. The calculated Rb-Sr age of 1688 Ma for this granite, not an isochron age sensu stricto, is considered to be a minimum age of rock intrusion. It is thus conceivable that these dome-like granite massifs are contemporaneous late-orogenic (about 1800 Ma old) intrusions, which would position the extrusion of the volcanic rocks of the Porphyry Group within an early stage of the Svecokarelian orogenic development.