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

Research Article| September 01, 1988 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 R. D. DALLMEYER; R. D. DALLMEYER 1Department of Geology, University of Georgia, Athens, Georgia 30602 Search for other works by this author on: GSW Google Scholar J. G. MITCHELL; J. G. MITCHELL 2Department of Geophysics and Planetary Physics, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, United Kingdom Search for other works by this author on: GSW Google Scholar T. C. PHARAOH; T. C. PHARAOH 3Department of Geology, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, United Kingdom Search for other works by this author on: GSW Google Scholar A. REUTER; A. REUTER 1Department of Geology, University of Georgia, Athens, Georgia 30602 Search for other works by this author on: GSW Google Scholar A. ANDRESEN A. ANDRESEN 4Institute of Geology and Biology, University of Tromsø, 9001 Tromsø Norway Search for other works by this author on: GSW Google Scholar GSA Bulletin (1988) 100 (9): 1493–1501. https://doi.org/10.1130/0016-7606(1988)100<1493:KAAAAW>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. D. DALLMEYER, J. G. MITCHELL, T. C. PHARAOH, A. REUTER, A. ANDRESEN; 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. GSA Bulletin 1988;; 100 (9): 1493–1501. doi: https://doi.org/10.1130/0016-7606(1988)100<1493:KAAAAW>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.