Tectonic evolution of the Arctic Norwegian Caledonides from a texturally- and structurally-constrained multi-isotopic (Ar-Ar, Rb-Sr, Sm-Nd, U-Pb) study

Abstract

This study presents the results of an integrated Ar-Ar, Sm-Nd, U-Pb, and Rb-Sr isotopic and structural investigation from the Kalak Nappe Complex (KNC) and overlying Magerøy Nappe in Finnmark, Arctic Norway. The results indicate a Paleozoic tectonometamorphic history dissimilar to that previously inferred for the region. The Kalak Nappe Complex (KNC) has been classically regarded as a major component of the allochthonous margin of Baltica variably affected by two Paleozoic tectonometamorphic events separated widely in time -an earlier Finnmarkian event (late Cambrian, ∼490 -- 510 Ma) first defined from field relationships of country rocks to gabbroic intrusions, and a later over printing Scandian event (Silurian to early Devonian, ∼400 --428 Ma). A foliated granitic dike within the Magerøy Nappe yields a U-Pb zircon age of 438 ± 3 Ma, an age identical to intrusive bodies that cross cut early deformation structures within this nappe. An early generation of muscovite within the KNC produces a weighted mean Ar-Ar cooling age of 438 ± 2 Ma and garnet from a folded dike, also in the KNC, yields a Sm-Nd crystallization age of 436 ± 21 Ma. Thus early Silurian metamorphism within the KNC occurred at the same time as deformation and magmatism within the overlying Magerøy Nappe of the Upper / Uppermost Allochthon. The Scandian continent-continent collision is recorded both within the Magerøy Nappe and within the KNC. U-Pb dating of hydrothermal zircon in the KNC at 428 ± 4 Ma is interpreted to date high temperature conditions during Scandian metamorphism. Ar-Ar muscovite ages of 422 ± 2 Ma, associated with overprinting textures, and Rb-Sr biotite ages of 413 ± 3 Ma record cooling after this event. The Engesfjellet Granite in the Magerøy Nappe yields an Ar-Ar amphibole age of 423 ± 8 Ma, ascribed to cooling after the Scandian collision and thus provides a minimum age for the time of emplacement of this unit onto the KNC. A late, undeformed cross-cutting pegmatite within the KNC yields a U-Pb zircon age of 425 ± 5 Ma and indicates that the Scandian collision was completed prior to this time. Biotite Ar-Ar furnace step-heating analyses have pervasive excess radiogenic argon contamination. Ar-Ar UV laser-probe biotite analyses are also contaminated with excess argon but reveal unaffected portions of grains with ages of 401 ± 6 Ma. In one sample from the Magerøy Nappe, two apparent ages, one at 410 ± 8 Ma and another at 404 ± 4 Ma, correspond to different fabrics with distinctive trapped argon components. The older ages are affected by excess argon. However analyses of the younger biotite fabric reveal ^40^Ar/^36^Ar values of atmospheric composition and are interpreted as dating cooling in biotite. This is consistent with the pervasive excess argon flux recorded throughout northern Norway having developed during a short-lived event in the Scandian Orogeny. These results have significant implications for understanding the development of the Caledonian orogenic belt in North Norway, but do not support the premise of a Finnmarkian Orogeny (in the original sense of the term) within its type locality. The results show that the KNC was affected by a tectonometamorphic event some 10 Ma prior to the climactic Scandian collision, likely related to slab roll back in the subducting Iapetus Ocean.