K-Ar and 40Ar/39Ar whole-rock ages of slate/phyllite from allochthonous basement and cover in the tectonic windows of Finnmark, Norway: Evaluating the extent and timing of Caledonian tectonothermal activity

摘要

Slate/phyllite samples collected within allochthonous basement rocks (Raipas Super-group) exposed in the Komagfjord and Alfa-Kvaenangen tectonic windows in Finnmark, northern Norway, display variably discordant 40Ar/39Ar age spectra in which apparent ages systematically increase from ca. 400-425 Ma (recorded at low experimental temperatures) to dates as old as ca. 1750-1800 Ma (in some high-temperature increments). The apparent age variations are not matched by fluctuations in apparent K/Ca ratios and are therefore interpreted to reflect intracrystalline gradients in the concentration of 40Ar within constituent fine-grained white mica. The gradients are suggested to have developed as a result of a variable Caledonian thermal rejuvenation of white-mica argon systems which had initially cooled through argon retention temperatures at ca. 1700-1800 Ma following the Sveco-karelian orogeny. The extent of whole-rock spectra discordancy is related to the proximity of the basal thrust of the structurally overlying Kalak Nappe Complex. Whole-rock sample suites were also collected structurally downward (0.6-20 m) from the Kalak Thrust within penetratively cleaved, allochthonous cover sequences (Lomvatn Formation) exposed along the northern edge of the Komagfjord window. These display discordant 40Ar/39Ar age spectra with most intermediate-temperature gas fractions recording mutually similar apparent 40Ar/39Ar ages of 410-430 m.y. Observed variations in K-Ar and 40Ar/39Ar ages suggest that a late Caledonian tectonothermal event (ca. 420-425 Ma) variably affected the allochthonous basement and cover units exposed in the two Finnmark windows. This thermal overprint is interpreted to have developed during emplacement of the structurally overlying Kalak Nappe Complex while it was maintained at post-Scandian metamorphic temperatures in excess of at least 350 °C. Although the effects of a Late Cambrian-Early Ordovician orogenic event have been described from portions of the Kalak Nappe Complex, no clear record of correlative tectonothermal activity has been documented within structurally underlying tectonic units. It is therefore likely that most of the Paleozoic metamorphism and deformation observed along the eastern margin of the Caledonide orogen in northern Scandinavia developed during eastward transport of previously assembled, composite Scandian nappes into their present structural position on the Baltoscandian platform between ca. 400 and 425 Ma.