Cambrian ages for metavolcanic rocks in the Lower K?li Nappes, Swedish Caledonides: implications for the status of the Virisen arc terrane

The K & ouml;li Nappe Complex (KNC) of the Scandinavian Caledonide orogen originated as oceanic terranes within the Iapetus Ocean. These terranes have characteristics of magmatic arcs and associated forearc or back-arc basins and underwent several periods of rifting and magmatism prior to their accretion to the Baltican margin. We present new U-Pb zircon ages from the Lower K & ouml;li Ankarede Volcanite Formation in V & auml;sterbotten, Sweden. U-Pb ages of magmatic zircon grains from metamorphosed dacitic to andesitic rocks show ages of 512 +/- 3.5, 497 +/- 2, 491 +/- 1 and 488 +/- 4 Ma. The three younger ages fit with previous ages for Lower K & ouml;li volcanic rocks, but the 512 Ma age is older than any previous age for this unit. These dates constrain the age of magmatism in an ensimatic arc system within Iapetus. We compare this evolution with published information from the other K & ouml;li nappes. Magmatic ages within the KNC overlap with ages for an early episode of ultrahigh-pressure (UHP) metamorphism within the underlying Seve Nappe Complex (SNC), supporting the hypothesis that attributes UHP metamorphism within the SNC to subduction beneath the island arc now preserved within the Lower K & ouml;li Nappes.