Chris L. Kirkland, J. S. Daly, E. A. Eide, and M. J. Whitehouse
Tectonic evolution of the Arctic Norwegian Caledonides from a texturally- and structurally-constrained multi-isotopic (Ar-Ar, Rb-Sr, Sm-Nd, U-Pb) study (in John Rodgers memorial issue; Part II, Wintsch,)
American Journal of Science (February 2007), 307(2):459-526

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 Mageroy 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, approximately 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, approximately 400-428 Ma). A foliated granitic dike within the Mageroy Nappe yields a U-Pb zircon age of 438+ or -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+ or -2 Ma and garnet from a folded dike, also in the KNC, yields a Sm-Nd crystallization age of 436+ or -21 Ma. Thus early Silurian metamorphism within the KNC occurred at the same time as deformation and magmatism within the overlying Mageroy Nappe of the Upper/Uppermost Allochthon. The Scandian continent-continent collision is recorded both within the Mageroy Nappe and within the KNC. U-Pb dating of hydrothermal zircon in the KNC at 428+ or -4 Ma is interpreted to date high temperature conditions during Scandian metamorphism. Ar-Ar muscovite ages of 422+ or -2 Ma, associated with overprinting textures, and Rb-Sr biotite ages of 413+ or -3 Ma record cooling after this event. The Engesfjellet Granite in the Mageroy Nappe yields an Ar-Ar amphibole age of 423+ or -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+ or -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+ or -6 Ma. In one sample from the Mageroy Nappe, two apparent ages, one at 410+ or -8 Ma and another at 404+ or -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.