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Fission-Track and K-Ar Dating of Tectonic Activity in a Transect Across the Møre-Trøndelag Fault Zone, Central Norway

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Fission-Track and K-Ar Dating of Tectonic Activity in a Transect Across the Møre-Trøndelag Fault Zone, Central Norway Fission-track and K-Ar dating of tectonic activity in a transect across the Møre-Trøndelag Fault Zone, central Norway A. GRØNLIE, C. W. NAESER, N. D. NAESER, J. G. MITCHELL, B. A. ST URT & P. R. INESON Grønlie, A., Naeser, C. W., Naeser, N. D., Mitchell, J. G., Stur!, B. A. & Ineson, P. R.: Fission-track and K-Ar dating of tectonic activity in a transect across the Møre-Trøndelag Fault Zone, central Norway. Norsk Geologisk Tidsskrift. Vol. 74, pp. 24-34. Oslo 1994. ISSN 0029-196X. Fission-track dating of apatite, zircon and sphene, and K-Ar dating of K-feldspars in samples taken from a transect across the Møre-Trøndelag Fault Zone in Trøndelag confirm the long-lived nature claimed for this complex fault zone. Apatite, zircon and sphene, which have blocking temperatures of 125° ± 25°C, 200° ± 50°C and 250° ± 50°C, respectively, together indicate a complex history of post-Caledonian tectonism. Uplift and cooling are indicated in Early Palaeozoic (Late Ordovician-Early Carboniferous) and Triassic-Jurassic times, with erosion of at least 3 km of Early and possibly Late Palaeozoic supracrustal cover since the Late Palaeozoic. Late Jurassic activity is indicated along the Verran Fault. A. Grønlie • & B. A. Sturt, Norges Geologiske Undersøkelse, Postboks 3006, N- 7002 Trondheim, Norway; C. W. Naeser & N. D. Naeser, US Geologica/ Survey, P.O. Box 25046, Denver Federa/ Center, Denver, CO 80225, USA; J. G. Mitche/1, Department of Physics, The University, Newcastle upon Ty ne NEI 7R U, England; P. R. lneson, Earth Sciences Unit, University of Sheffield, Sheffield SJ 7HF, England. *Present address: Statoil Nord-Norge, Postboks 40, N-9401 Harstad, Norway. Introduction faulting includes a basal Precambrian crystalline complex of heterogeneous gneisses.This is tectonostratigraphically The Møre-Trøndelag Fault Zone (MTFZ) is commonly overlain by amphibolite facies psammites, schists and regarded as an important Caledonian and Mesozoic fault amphibolites. Higher up, low-grade metasediments and zone (Gabrielsen & Ramberg 1979; Aanstad et al. 1981). greenstones occur (Wolff 197 6).The cover rock sequences lts deformational history has been related to the struc­ represent fragmented slices of the Lower to Upper Al­ tural evolution of offshore areas adjacent to central lochthons of the Caledonide orogen ( Gee et al. 1985).The Norway (Gabrielsen et al. 1984; Dore & Gage 1987; Precambrian gneisses, forming part of the Western Gneiss Brekke & Riis 1987). Region, are themselves strongly Caledonized (Tucker & To furnish tempora! constraints on episodes of fault­ Krogh 1988). South of the Verran Fault, the bedrock ing in the MT FZ, a programme of palaeomagnetic and consists mostly of Caledonian Nappe rocks, mainly of the isotopic dating was undertaken. The programme com­ Gula and Støren Nappes and the underlying Precambrian prises fission-track dating of apatite, zircon and sphene basement rocks (Bryhni & Sturt 1985). from fault rocks and other less deformed rocks, as well An important element of MT FZ geology is that of the as potassium-argon (K-Ar) dating of K-feldspars from late-orogenic Old Red Sandstone sediments on Ørlandet, thorium-mineralized hydrothermal veins� Samples have Hitra and Smøla.These sandstones and conglomerates are been taken in a transect across the MTFZ from Åfjord, of Late Silurian to Middle Devonian age (Steel et al. 1985). northwest of the Hitra-Snåsa Fault to Verdal, southeast In Beitstadfjord, pebbles of coa1 and bou1ders of of the Trondheimsfjord (Fig. 1). Samples were collected sideritic ironstone of Midd1e Jurassic age have been found between O and l 00 m above sea level, except sample on the western and northern shores (Horn 193 1; Oftedah1 AG-2, which was collected at 460 m above sea level. 197 2, 1975) showing that important fault movements The purpose of the present paper is to present and postdated the deposition of these Mesozoic sediments. interpret the results of this geochronological investiga­ This evidence has been further supported by results of a tion. Results from palaeomagnetic dating of fault rocks shallow seismic profiling survey (Bøe & Bjerkli 1989). have been published earlier by Grønlie &Torsvik ( 1989). K-Ar dating of K-feldspars Geological setting Methods In Inner Trondheimsfjord the MT FZ comprises a com­ plex fault zone with a width of approximately 50 km Mineral separates of the potassium feldspar fractions of (Fig. 1). In the northwest, the bedrock involved in the four of the thorium-rich carbonate vein samples (T able NORSK GEOLOGISK TIDSSKRIFT 74 (1994) Dating the Møre-Trøndelag Fault Zone 25 LEGEN O FAULTS 1---1 BEITSTAOFJORO M.JURASSIC ROCKS • SAMPLE LOCATION YL = YTTERØY LAMPROPHYRE FOSEN o 25Km Fig. l. Fault pattem of the Møre-Trøndelag Fault Zone in the Fosen-Trondheimsfjord region. Fission-track and K/Ar sample locations, with results, are shown (see also Tables l & 2). Beitstadfjord fault pattern is simplified from Bøe & Bjerkli ( 1989). A, Z, S = apatite, zircon & sphene fission-track ages (in Ma), K/Ar = potassiumfargon age (in Ma). Table l. Potassium-argon analytical data. Sam p le K Atmospheric Age 20 num ber Mineral (wt.%) Radiogenic 40Ar contamination (%) (Ma± lu) FT-2 Microcline 16.71± 0.55 ( 1.179 ± 0.008) X 10-l 3.8 206±7 PR-07 Microcline 15.14± 0.05 ( 1.245± 0.008) X 10-1 4.4 238±2 FT-1 Adularia 15.13 ± 0.18 ( 1.204± 0.008) X 10-l 4.1 231±8 FT-5 Adularia 16.32± 0.30 ( 1.263 ± 0.()()9) X 10- l 7.7 225±4 l) were obtained by conventional mineral separation FT-5 shows splitting of both. FT-1 may, therefore, be techniques. The resulting separates werc of >99 % purity. regarded as a monoclinic adularia while the FT-5 struc­ X-ray diffraction analyses, using a Phillips PW 1450 ture incorporates an unquantified proportion of triclinic diffractometer at 35 kV with a copper target and 1/8° per material. The consequences of structural state variations min. scan rate, showed samples FT-2 and PR-07 to be in adularias for potassium-argon age interpretations microcline. Detailed interpretation of the diffraction data have been discussed by Halliday & Mitchell (1976). for samples FT-1 and FT-5 indicates that they are both Conventional potassium-argon ages were determined adularia. The powder pattem for FT- 1 shows no evi­ for the four feldspar separates. Potassium was deter­ dence of splitting of the ( 130) and ( 131) peaks, whereas mined in triplicate by fiame photometry using a Coming 26 A. Grønlie et al. NORSK GEOLOGISK TIDSSKRIFT 74 (I994) EEL 450 instrument with a Li internat standard. Argon mineral containing fission tracks allows ions displaced was extracted from 300 mg aliquots of the powders by along the damage zone of the track to move back into fusing them in an RF fumace followed by gas purifica­ normal crystallographic positions in the mineral. Repair tion using Ti sponge and Al-Zr alloy getters. The mea­ of the damage zone leads to shortening and ultimately to surement of argon was performed in duplicate on a the total disappearance of the fission track. The anneal­ VG 1200C mass spectrometer by isotope dilution using a ing of fission tracks is a time-temperature function. 38 Ar spike. Interlaboratory calibrations using standard Short heating times at high temperatures can have the minerals suggest an accuracy of these procedures of same effect on fission tracks as long heating times at hetter tban 2%. Data from these analyses are presented lower temperatures. Over the periods of time that are 5 in Table l. usually required for geological processes ( > 10 yr), total Table l gives unweighted mean values of the potas­ fission-track annealing will take place in apatite (ftuorap­ sium and argon contents. Ages were calculated using the atite) at temperatures between 100° and 150°C (Naeser decay constants of Steiger & Jager ( 1977), with the 1979, 1981). The shorter the heating time, the higher the uncertainty specified by ±la. The atmospheric contami­ temperature required for total annealing. nation value is the higher of the values obtained in the Annealing temperatures of apatite are also affected by duplicated argon analyses. chemical composition (Green et al. 1989; Crowley et al. Interpretation of the potassium-argon ages of K­ 1990). F-, Sr-F and OH-apatite anneal at very similar feldspars must be approached with caution. Conven­ temperatures, but chlorine-rich apatite is significantly tional K-Ar ages of K-feldspars (induding microdine) more resistant to annealing; at fixed heating times in the from plutonic rocks are generally regarded as unreliable laboratory, Cl-apatites anneal at temperatures up to ( see, for example, Hart 1964, Dalrymple & Lanphere 30°C higher than other apati te varieties ( Crowley et al. 1969). Interpreting such ages within the context of their 1990). Similar variation in annealing temperatures re­ geological setting and the ages of dated coexisting miner­ lated to Cl-content has been observed in a drill hole in als demonstrates that the apparent age is invariably the Otway Basin, Australia, where apatite grains low in young. A mechanism of argon loss is now generally chlorine are totally annealed at a present temperature of accepted as the explanation for these young ages. 92°C, but chlorine-rich apatite grains are not totally Recent work (Berger & York 1981; Harrison & Mc­ annealed until a temperature of � 125°C is reached Dougall 1982; Zeitler & Fitz Gerald 1986), using the (Green et al. 1989). Fortunately, apatite suites are typi­ 40Arj39Ar method, has shown that K-feldspars can be a cally so dominated by ftuorapatite that most can be useful low-temperature thermochronometer ( � 200°C). interpreted using ftuorapatite annealing data. As a prac­ Zeitler & Fitz Gerald ( 1986) showed from stepwise heat­ tical matter, evidence suggests that the presence or ab­ ing that thermally disturbed K-feldspars have a complex sence of significant concentrations of chlorine can be argon release pattem.
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