U-Pb isotope analyses of zircons from a Precambrian gneiss area in northern Sweden and their chronostratigraphic implications

Abstract

Abstract A zircon study has been made on folded, conformable gneisses and an intrusive igneous rock from the northeastern part of Sweden. U-Pb ages of size and colour fractions of zircon concentrates from the gneisses show complex distributions on the concordia plot. This is related to varying degrees of secondary overgrowth on zircon cores and the formation of new zircons during the Svecokarelian (early Proterozoic) orogeny. Detrital zircons from a very pure quartzite have a maximum age of about 2700 Ma. They almost completely lack overgrowth textures and indicate an age close to that of the Archaean, conceivably igneous, source material. Zircons from the mica and amphibole gneisses have subhedral and rounded cores which, together with the observed layering, imply a sedimentary origin. They have apparent ages of less than 2063 Ma and close to the main metamorphic event, about 1850 Ma ago. Because of the low overgrowth/core volume ratios observed, one may suspect that the zircon core material in the mica ...