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Early Caledonian Tectonothermal Evolution in Outboard Terranes, Central Scandinavian Caledonides: New Constraints from U-Pb Zircon Dates

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Early Caledonian Tectonothermal Evolution in Outboard Terranes, Central Scandinavian Caledonides: New Constraints from U-Pb Zircon Dates Journal of the Geological Society, London, Vol. 150, 1993, pp. 51-56, 4 figs, 1 table. Printed in Northern Ireland Early Caledonian tectonothermal evolution in outboard terranes, central Scandinavian Caledonides: new constraints from U-Pb zircon dates M. B. STEPHENS l,K. KULLERUD 2'3&S.CLAESSON 2 ~Geological Survey of Sweden, Box 670, S-751 28 Uppsala, Sweden 2Museum of Natural History, Laboratory of Isotope Geology, Box 50007, S-104 05 Stockholm, Sweden 3present address: Department of Geology, University of Oslo, Box 1047, N-0316 Oslo 3, Norway Abstract: Early Caledonian deformation in an outboard terrane (Stort]fillet) in the central Scandinavian Caledonides is constrained to the time-period Arenig to Caradoc on the basis of U-Pb zircon age determin- ations. A trondhjemite clast in a conglomerate belonging to a supracrustal sequence that was affected by the early deformational event yields an age of 489_+10 5 Ma. A granite intruded after this event defines an age of 44 J-6~ + 24 Ma. The clast is similar in age to a trondhjemite intrusion in a contiguous terrane (Gjersvik) dated at 483+~Ma and the granite age is in agreement with a Rb-Sr whole-rock isochron of 438 5= 6 Ma determined in a previous study. The three new ages provide further evidence for the separation of igneous events in outboard terranes in the Scandinavian Caledonides into older (Tremadoc to Arenig) and younger (Late Ordovician to Early Silurian) episodes. More critically, they argue against simple correlation of early Caledonian deformational events in outboard terranes with the Late Cambrian to Early Ordovician tectonometamorphic history along the margin of the continent Baltica (the so-called Finnmarkian orogenic phase). This age disparity provides support for significant spatial separation of the Storfj/illet terrane from Baltica during the early-mid Ordovician and an influence of the Taconic orogenic event on this outboard terrane is proposed. The structure of the Scandinavian Caledonides is dominated by deformation and metamorphism along the margin of Baltica to a sequence of major thrust sheets with top-to-the-east sense of this accretionary event (model 1). A major belt-length, pre- movement (see overview in Roberts & Gee 1985), in part later Late Ordovician unconformity in the tectonostratigraphically detached in an extensional tectonic regime with top-to-the-west higher thrust sheets has been proposed and has been interpreted sense of movement (Hossack 1984; Norton 1986). Thrust sheets as documenting this early accretion (Sturt 1984). The alterna- at lower tectonostratigraphic levels represent the tectonically tive hypothesis (model 2), which takes into account faunal shortened margin of the early Palaeozoic continent Baltica. provincial evidence in the mountain belt (e.g. Neuman & Tectonostratigraphically higher sheets comprise a variety of Bruton 1974; Bruton & Bockelie 1980; Bruton & Harper 1981), terranes derived from outboard of Baltica that were accreted to argues that many outboard terranes remained separated from this continent during the early Palaeozoic. These terranes con- Baltica by the Iapetus oceanic tract during most of the Or- sist of oceanic arc-basin systems, including ophiolites, higher- dovician Period and were not accreted to Baltica until Silurian grade metamorphic complexes of uncertain tectonic affinity, and Early Devonian continent-continent collision (e.g., Gee and exotic continental segments (Stephens 1988). 1975; Bruton & Bockelie 1980; Stephens & Gee 1985, 1989; Development of a foreland basin and radiometric dating Pedersen et al. 1988). This hypothesis allows the early tec- studies in the central Swedish part of the mountain belt (e.g. tonometamorphic events in individual outboard terranes (see Claesson 1980; Dallmeyer et al. 1985, 1991; Dallmeyer & Gee summary in Stephens & Gee 1989) to be unrelated in time and 1986, 1988; Mork et al. 1988; Dallmeyer & Stephens 1991) space both to each other and to the early accretionary history demonstrate an early tectonometamorphic event along the along the margin of Baltica (Gee 1987). margin of Baltica during Late Cambrian to Early Ordovician New U-Pb zircon dates for three granitoids are presented in times. This event locally involved high-pressure metamor- this study. These samples occur in two separate outboard phism, with the development of eclogites and glaucophane- terranes in the central part of the orogen and display critical bearing metabasic rocks in the outermost part of the margin yet contrasting relationships to the internal deformation in the (van Roermund & Bakker 1984; Stephens & van Roermund respective terranes. This study aims to constrain more tightly 1984; Andr6asson et al. 1985; Santallier 1988; Kullerud et al. the igneous and tectonic evolution of these terranes and, more 1990). This early Caledonian deformation and metamorphism critically, to compare the tectonic evolution with that along has been related to collision of Baltica with a cryptic volcanic the margin of Baltica. A simple, two-stage tectonometa- arc complex (Dallmeyer & Gee 1986) and preceded ultimate morphic model for the Scandinavian Caledonides, implicit in collision of Baltica with the continent Laurentia during the model l, can thus be tested. Silurian and Early Devonian. There is, however, a notable lack of consensus concerning the timing of accretion of the Geological scenario outboard terranes to Baltica. Two main models dominate the literature. Between latitudes 64 ° and 66°N, in the central part of the Some authors (e.g. Gale & Roberts 1974; Sturt 1984; Scandinavian Caledonides, outboard terranes occur within the Roberts et al. 1985; Rast et al. 1988; Sturt et al. 1991; Sturt & packet of thrust sheets collectively referred to as the K61i Nap- Roberts 1991) tectonically link the outboard terranes to Bal- pes (in the Upper Allochthon) and in the Uppermost AI- tica in the Early Ordovician and relate the early Caledonian lochthon (Fig. 1). The K61i Nappes contain three separate 51 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/1/51/4891958/gsjgs.150.1.0051.pdf by guest on 27 September 2021 52 M. B. STEPHENS ET AL. I UTBOARD TERRANES Caledonian intrusion in outboard terrane ~] Exoticcontir~ntal crust (UmA) Metamorphic complex of [~] uncertain tectonic affinity in Storfj<~llet terrane (KNUA) Oceanic arc-basin [~ systems in A=Virisen and B=Gjersvik terranes (KNUA) BALTICA ~ Tectonically shortened margin (SNUA, MA, LA) Proterozoic shielcl ~and Paleozoic cover rocks l Ou~rd east of the Caledonides Thrust 50Kin ? ! Fig. 1. Terrane map of the central part of the Scandinavian Caledonides showing sample sites. UmA, Uppermost Allochthon; KNUA, K61i Nappes, Upper Allochthon; SNUA, Seve d Nappes, Upper Allochthon; MA, Middle Allochthon; LA, Lower Allochthon. terranes which, in ascending tectonostratigraphic order, are Storfj/illet terrane (Fig. l). Sample 2 is a trondhjemite clast in referred to here as the Virisen, Gjersvik and Storfj/illet terranes a conglomerate. This conglomerate was affected by all the (terranes 4, 5 and 11, respectively, of Stephens & Gee 1989). phases of deformation as well as the medium-grade metamor- The Virisen and Gjersvik terranes contain volcano- phism in this terrane. Sample 3 is from the Vilasund Granite sedimentary sequences metamorphosed under low-grade con- which intruded after an early phase of isoclinal folding (DI ditions and related to early Palaeozoic oceanic arc-basin phase of Senior & Otten 1985; Fig. 2) but prior to thrust em- systems. Fossil evidence and U-Pb zircon radiometric age placement onto underlying tectonic units. Samples 1 and 2 are, determinations (Claesson et al. 1983, 1988; Roberts & Tucker thus, pretectonic in character while sample 3 may be referred to 1991) provide Early Ordovician (Tremadoc to Arenig), Late as syntectonic. Ordovician and Early Silurian ages for some of the protoliths. The Storl]fillet terrane consists of a volcanosedimentary sequence generally metamorphosed under higher-grade con- Sample descriptions and analytical procedure ditions (H/iggbom 1978) and with a more uncertain age and Sample 1 is a foliated trondhjemite with a planar grain-shape fabric tectonic affinity. Conglomerates containing clasts of felsic vol- defined by seams of sericite enveloping elongate albite-quartz canic and intrusive rocks as well as limestones (locally with aggregates. Secondary chlorite, carbonate and opaque minerals occur pelmatozoa) are a conspicuous component. This terrane also in distinct zones. The zircons are pale brown with length/width ratios displays an early deformational history prior to intrusion of up to 2, have well-developed crystal faces and are optically homo- felsic (Vilasund Granite) and mafic plutons, dated at geneous without any zoning. Sample 2, the trondhjemite clast, is an 438 +6Ma (Rb-Sr whole-rock, 287Rb = 1.42 × 10-=]a-I; albite-quartz rock with a weak planar grain-shape fabric defined by 95% confidence limits, Gee & Wilson 1974) and 434 + 5 Ma oriented biotite, muscovite and chlorite. The albite is partly sericitized. (U-Pb zircon; confidence limits not given Senior & Andriessen Zircons are optically similar to those in sample 1. Sample 3 is a 1990), respectively. medium-grained, non-foliated granite. The zircons are pale brown with The terrane(s) in the Uppermost Allochthon (terrane 13 of length/width ratios up to 7. No zoning has been observed. Stephens & Gee 1989) is(are) composite in character with dom- The least magnetic zircons were divided into size fractions and two inant continental and subordinate oceanic components. These fractions from each sample were abraded (Krogh 1982). Chemical separation followed standard techniques (Krogh 1973). U was deter- were sealed together prior to emplacement of a granite pluton mined on an AVCO and Pb on a Finnegan MAT 261 multicollector dated at 444 + 11 Ma (U-Pb zircon; Nordgulen & Schouen- mass spectrometer. Pb and U ratios were corrected for 0.12 and borg 1990). 0.1% /ainu mass fractionation, respectively, and initial Pb correction Sample 1 in this study is from a trondhjemite pluton in the was made using 2°6pb/2°4pb = 18.0, 2°Tpb/2°4pb = 15.5 and 2°8pb/2°4pb Gjersvik terrane (Fig.
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