TECTONIC IMPLICATIONS OF U-PB DETRITAL ZIRCON ANALYSIS OF METASEDIMENTARY COVER SEQUENCES IN THE LOFOTEN AND WEST-TROMS BASEMENT COMPLEXES, NORTH NORWAY

The West Troms (WT) and Lofoten-Vesteralen (LV) terranes compose a huge complex of (15,000 km²) Archaean to Paleoproterozoic plutonic continental basement rocks exposed along the west coast of northern Norway (lat. 69° N). This basement complex underlies a stack of east-vergent thrust nappes that overrode it during the Caledonian cont-cont collision, wherein the Baltic continental margin was subducted beneath Laurentia. Structurally isolated bodies of deformed metasedimentary rocks have strongly transposed and obscured contacts with surrounding WT-LV plutonic rocks, leaving debatable their significance for tectonic models involving the positions of possible pre-Caledonian microcontinental sutures, the Caledonian A-type subduction boundary, and the Iapetan suture between Laurentia and Baltica. We collected siliciclastics from three different sedimentary packages tectonically sandwiched by WT-LV basement granite and here report U-Pb LA-ICPMS ages on their detrital zircon populations. The first sample is a quartzite interlayered with schist and marble from the island of Andoya, having a solitary 2.7 Ga age population, indicating it was sourced from Archean-Paleoproterozoic basement (WT) directly underlying it. Sample two is a quartzite from the allochthonous Leknes Group from near the town of Leknes. This sample contains an age population of 1.0 to 1.2 Ga zircons, interpreted to reflect a Laurentian heritage, much younger than the LV basement (2.3-1.8 Ga) that lies above and below it. The third sample is a quartzite from the island of Engeloya, near Fort Dietel, which lies along the contact between the base of the Caledonian nappe stack and the top of the WT-LV basement. A solitary age peak at 488 Ma indicates that this unit contains detritus from an Iapetan (Gampian-Taconic) magmatic arc terrane, common to the Uppermost Caledonian allochthon. In conclusion, the Andoya metasedimentary package appears to be the depositional cover to Archean-Paleoproterozoic WT basement, narrowing down its pre-Baltican suture with the Svecofinninan craton to the south of our study area. The Leknes Group and Fort Dietel quartzites are Laurentian and Iapetan terranes, respectively, and thus we interpret their basal contacts to relate to the Laurentian (above) and Baltican (below) suture.