RECONCILING METAMORPHIC TIMING BETWEEN PRECAMBRIAN BASEMENT AND CALEDONIAN ALLOCHTHONOUS COVER IN THE LOFOTEN TERRANE, NORTH NORWAY

Previous 40Ar/39Ar dating of muscovite from allochthonous terranes across the Caledonian orogen at 68° N latitude, from the foreland in Sweden to the most internal exposures in the Lofoten archipelago of Norway, has revealed an E-to-W younging trend that spans the time interval from Early Silurian to Permian, which is interpreted to reflect progressive uplift of Baltica after its partial subduction beneath Laurentia. Archean and Proterozoic basement in Lofoten remains ‘suspect' WRT Baltica, however, because clear evidence for Caledonian involvement is not yet documented and its allochthonous cover, the Leknes Group, is not confidently correlated to any specific Caledonian allochthon overlying Baltic basement to the east. Muscovite ages from a Proterozoic metagranite on Andøya as well as 40Ar/39Ar cooling ages determined for basement gneiss on Senja and the Caledonian mainland represent the maximum single-crystal ages of c. 393 Ma. Further to the west, muscovite from a pegmatite on Moskenesøya was dated at c. 310 Ma. We report single-crystal 40Ar/39Ar ages for samples of undeformed muscovite from basement rocks of Lofoten. Muscovite phenocrysts from granitic pegmatite that cuts eclogitized gneiss on Flakstadøy yield ages ranging from 365-330 Ma, and post-kinematic porphyroblastic muscovite from an associated mylonitic gneiss has a similar age range of 356-327 Ma. These cooling ages are compatible with previous muscovite data from the allochthonous Leknes Group. Available data show that the Lofoten basement rocks and the allochthonous rocks of the Leknes Group experienced a similar, protracted Devonian-Carboniferous unroofing history through the muscovite closure interval for retention of radiogenic 40Ar.