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 determinations. 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

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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. 1). This body intruded a mafic- = 37.7. Average total Pb blank was 0.7 ng. Calculations were carried dominated volcanic sequence with older subduction- and out using the constants of Steiger & Jfiger (1977) and the equations of younger rift-related affinities (Reinsbakken 1986) and is affec- Ludwig (1980). The analytical data are listed in Table 1. Uncertainties ted by the planar grain-shape fabric which is present in the in the intercept ages were calculated as propagated internal errors and supracrustal rocks. The remaining two samples are from the are given as 95% confidence limits.

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Table 1. U-Pb isotopic data for zircon fractions from granitoids in some outboard terranes, central Scandinavian Caledonides

Sample Conc. Ratios Age Weight Size U Pbrad 2°6pb/2°4pb* 2°6pb/238Ut 2°7pb/235Ut Correl. 2°7pb/2°6pb "~ 2o8pb/2O6pbt 2o7pb/ZO6pb (mg) (lim) (ppm) (ppm) Coeff.~: (Ma)

Sample 1." Gjersvik (map-sheet M711/1924IV Royrvik, VM2110/9555), Gjersvik terrane 1 1.83 <45 1438 121.7 11200 0.07686 (15) 0.6033 (25) 0.581 0.05692 (19) 0.2226 489 2 3.70 45-60 1203 101.2 5100 0.07611 (19) 0.5982 (37) 0.525 0.05700 (30) 0.2278 492 3 4.65 60-74 ABR 1039 85.0 11300 0.07551 (18) 0.5912 (16) 0.890 0.05678 (07) 0.2019 483 4 9.93 74-90 1066 81.2 15400 0.07025 (34) 0.5500 (37) 0.748 0.05679 (26) 0.2032 483 5 6.13 90-106 ABR 1043 83.1 8200 0.07255 (12) 0.5676 (31) 0.491 0.05675 (27) 0.2194 482 6 4.10 106-150 1042 127.9 5300 0.07803 (18) 0.6085 (28) 0.579 0.05656 (21) 0.3015 475 7 1.29 > 150 1896 185.6 1000 0.07832 (27) 0.617 (10) 0.496 0.05711 (88) 0.3944 496 Sample 2. M~artotjakke (map-sheet 24F Tdrna, 145700/727890), StorJ)Mlet terrane 1 496 <45 1392 122.0 4900 0.07733 (17) 0.6079 (29) 0.560 0.05702 (23) 0.2590 492 2 8.22 45-74 1349 102.1 35000 0.07427 (13) 0.5823 (24) 0.545 0.05685 (20) 0.1273 486 3 7.39 45-74 ABR 1736 132.6 4050 0.07493 (20) 0.5878 (18) 0.892 0.05689 (08) 0.1286 487 4 7.27 74-106 ABR 1265 I00.0 13500 0.07734 (17) 0.6065 (19) 0.738 0.05688 (12) 0.1303 487 5 3.97 106-150 1236 99.9 4300 0.07768 (17) 0.6105 (62) 0.486 0.05700 (53) 0.1523 492 Sample 3." Vilasund (map-sheet 25F Umfors, 146535/732680), Storfjiillet terrane 1 4.38 <45 2015 132.5 12300 0.06921 (12) 0.5367 (20) 0.559 0.05624 (18) 0.0518 462 2 4.24 45-74 2486 163.4 23600 0.06992 (37) 0.5396 (37) 0.795 0.05597 (24) 0.0405 451 3 1.95 45-74 ABR 2429 161.2 4300 0.06989 (16) 0.5391 (14) 0.907 0.05594 (06) 0.0488 450 4 5.52 74-106 2645 172.9 16400 0.06976 (12) 0.5397 (16) 0.652 0.05610 (12) 0.0341 457 5 5.62 74-106 ABR 2676 172.1 18100 0.06833 (11) 0.5268 (28) 0.482 0.05592 (27) 0.0389 449 6 5.04 45-74 M 2825 187.4 24900 0.06953 (24) 0.5346 (25) 0.802 0.05577 (15) 0.0538 443

ABR, abraded fraction; M, magnetic fraction. * Measured values. t Corrected for blank and initial Pb. ~: Correlation coefficient of 2°7pb/235U versus 2°6pb/238U. Numbers in parentheses show 2 o error in last significant digits.

with the Rb-Sr whole-rock age of 438 + 6 Ma (Gee & Wilson 1974). The weakly-magnetic fraction plots slightly above this line (Fig. 3c) and its inclusion in the calculation does not affect the upper-intercept age but increases the MSWD value to 5.4. We interpret the three zircon ages presented above to be the primary crystallization ages of the respective granitoids. In samples 1 and 3, the U concentration shows a positive rather than the typical inverse correlation with grain size. Further- more, abrasion does not appear to reduce the degree of discor- dancy in the analysed samples to any significant extent (Fig. 3b and c).

Discussion

Igneous evolution U-Pb dating of arc-basin complexes (including most of the Fig. 2. Apophysis from Vilasund Granite cross-cutting axial ophiolites) in the Scandinavian Caledonides has yielded a surfaces of early isoclinal folds (DI) in the Storfj/illet terrane. range of ages between 497 +2 and 470+_9Ma (Fig. 4). Using the time-scale of Snelling (1985), these ages are Tremadoc and Arenig. One or more phases of island-arc development, rifting and basinal development in an oceanic set- Results ting, possibly in widely separated palaeogeographic locations, Seven fractions from sample 1 define a discordia line with have been suggested (e.g. Stephens 1980; Dunning & Pedersen MSWD -- 1.8 and an upper-intercept age of 483+~Ma 1988). (Fig. 3a). Five fractions from sample 2 are up to 6% discordant U-Pb zircon dating of major plutons which intrude the and the fitted discordia line has a MSWD value of 0.5 and an supracrustal units in the outboard terranes defines a younger upper-intercept age of 489_+10 5 Ma (Fig. 3b). From suite of ages between 456+2 and 426+8Ma (Fig. 4). As sample 3, five non-magnetic and one weakly-magnetic in the case of the Vilasund Granite, several of these intrusions fractions were analysed. They are 2-5% discordant. The non- were emplaced after an early deformational phase. Most of magnetic fractions define a discordia line with MSWD = 3.9 these Late Ordovician to Early Silurian dates overlap in time and an upper-intercept age of 445_+24 6 Ma, in agreement with the age of rift-related mafic volcanic rocks intercalated

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5O0 5O0 0.080

2o6

0.076 u 0.076 U

/+60 3 Iu:483 +~Mo 2 Mo+10

0.072 0.072 4A0 4

0.068 0.088 4z0 AMPLE 2] 207 ,.,, // , Gjersvik , ~U u /~/ IM;~rt°tjakke-I 23SUb

0.064 / ./ I , I , l , l 0.064 / ./ I , I = I , I a- 0.52 0.56 0.60 0.64 0.52 0.56 0.60 0.64 b.

0.078 480 2o+Pb /

0.074

0.070 420~5 ld~O~

0.066

0.062

Fig. 3. Concordia diagrams for analysed zircon fractions from samples 1-3. Black symbols represent abraded fractions. I. is 0.058 , \ / , I , I , I , I , I ' , I H7 I 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 upper-intercept age. Uncertainties are given as 95% confidence C. limits.

with clastic and carbonate rocks within several of the outboard Tectonic evolution terranes (Fig. 4). They also lie close to the U-Pb zircon age The volcanosedimentary sequence in the Stort]~illet terrane (443 + 3 Ma; Fig. 4) of an ophiolite complex (Solund) in an was deformed after 4 89_+10 5 and prior to 445_+24 6 Ma, outboard terrane along the southwestern coast of Norway i.e. during the time-period Arenig to Caradoc. This (Dunning & Pedersen 1988). An important development of rift- early deformational episode is younger than the late Cam- and, locally, spreading-related magmatism occurred during brian to early Ordovician, high-pressure tectonometamorphic Late Ordovician to Early Silurian times (Tucker et al. 1990; event along the outermost margin of Baltica (Fig. 4). Indeed, Pedersen et al. 1991) prior to (or during?) the initial stages of the protolith age of the trondhjemite clast (sample 2) over- continent-continent collision between Laurentia and Baltica. laps in time with and is even younger than the ages of uplift It is apparent that many outboard terranes in the Scandinavian of part of this margin through the crustal level necessary for Caledonides shared a similar igneous evolution at this time. intracrystalline retention of argon in amphibole (510- The U-Pb zircon dates for the pretectonic trondhjemitic 490Ma; 4°Ar/39Ar method; Dallmeyer & Gee 1986; Dal- (samples 1 and 2) and syntectonic granitic (sample 3) intrusions lmeyer et al. 1991), following the high-pressure metamor- reported here lie within the ranges of ages defined by the older phism. This important disparity in age supports the and younger igneous suites, respectively. Furthermore, the suggestion that the Storfj~illet terrane was not spatially data establish that Tremadoc to Arenig trondhjemite in- coupled to Baltica during the early-mid Ordovician. A Laur- trusions formed a source rock for the intraformational con- entian rather than Baltican palaeogeographic scenario has glomerates common in the Storfjfillet terrane. These con- previously been argued for this terrane at this time (Stephens glomerates were deposited during the interval Arenig to & Gee 1985). An influence of the Taconic orogenic phase on Caradoc. the Storfj/illet terrane is proposed here.

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Ma OUTBOARD TERRANES OUTERMOST MARGIN OF BALTICA (SNUA) Terrane(s) with Oceanic arc-basin systems and metamorphic complexes exotic continental of uncertain tectonic affinity 400~ crust (UmA) ~=(s) with ophioites S~°dfdl~lAl~t8 GJKel~sdiA~ ~KriNs~, southwest Norway (UA) "t 4 2 450- 3 !1

I ~ O? , 5 JT

5(X)~

O Igneous rock protolith age (U/Pb zircon/titanite) O High-P metamorphism age (Sm-Nd) I Age of mafic volcanic rocks (fossil evidence) V Uplift age (4°Ar/39Arhornblende) s50-d [] Constraintson age of early Caledonian deformationin outboard terranes

Fig. 4. Constraints on the timing of early Caledonian deformation in some outboard terranes, Scandinavian Caledonides and comparison with the early Caledonian deformation along the outermost margin of Baltica. From left to right within each vertical segment in the figure (e.g. Stort~/illet), the age data are arranged in a north to south direction along the mountain belt. UreA, UA, KNUA and SNUA as in Fig. I. Time-scale after Shelling (1985). Radiometric age data from: l, Dallmeyer et al. (1985); 2, Dallmeyer & Gee (1988); 3, Dallmeyer & Gee (1986) and Dallmeyer & Stephens (1991); (for 4°Ar/39Ar dating) and Mork et aL (1988) (for Sm-Nd dating); 4, Dallmeyer et al. (1991); 5, Claesson et al. (1983); 6, Claesson et al. (1988); 7, Roberts & Tucker (1991); 8, Wilson et al. (1983); 9, Senior & Andriessen (1990); 10, Pedersen et al. (1991); 11-14 (Karmoy, Gullfjellet, Solund and Leka, respectively), Dunning & Pedersen (1988); 15, Nordgulen & Schouenborg (1990); 16, Tucker et al. (1990).

The results for the Storl~fillet terrane are consistent with References constraints on the timing of accretion of oceanic crustal frag- ANDRI~ASSON, P.-G., GEE, D. G. & SUKOTJO, S. 1985. Seve eclogites in the ments with a continental margin as documented in the Norrbotten Caledonides, Sweden. In: GEE, D. G. & STURT, B. A. (eds) The ophiolite-bearing terrane(s) in the southwesternmost part of Caledonide Orogen--Scandinavia and Related Areas. Chichester, John Wiley the Scandinavian Caledonides (Dunning & Pedersen 1988). and Sons, 887-901. Such a collision apparently occurred after 470+_9 Ma and prior BRUTON, D. L. & BOCKELIE, J. F. 1980. Geology and paleontology of the Hol- onda area, western Norway--a fragment of North America? In: WONES, D. to deposition of carbonates and clastic sedimentary rocks of R. (ed.) The Caledonides in the USA. Virginia Polytechnic Institute and State Ashgill age (Fig. 4). The timing of formation of the ophiolites University, Department of Geological Sciences Memoir 2, 41-47. in this terrane(s) and their subsequent collisional history has -- & HARPER, D. A. T. 1981. Brachiopods and trilobites of the early Ordovician also inspired a comparison with the early Caledonian tectonic serpentine Otta Conglomerate, south central Norway. Norsk Geologisk Tidsskrift, 61, 153-181. evolution along the Laurentian margin of Iapetus (Dunning & CLAESSOrq, S. 1980. A Rb-Sr isotope study of granitoids and related mylonites in Pedersen 1988). the T/innfis Augen Gneiss Nappe, southern Swedish Caledonides. Geo- logiska F6reningens i Stockholm F6rhandlingar, 102, 403420. , KL1NGSPOR,I. & STEPHENS,M. B. 1983. U-Pb and Rb-Sr isotopic data on an Summary Ordovician volcanic/subvolcanic complex from the Tjopasi Group, K61i Nappes, Swedish Caledonides. Geologiska F6reningens i Stockholm We suggest that simple correlation of early deformational F6handlingar, 105, 9-15. events in outboard terranes with the early tectonometamorphic , STEPHENS, M. I. & KLINGSPOR, I. 1988. U-Pb zircon dating of felsic in- evolution along the margin of Baltica (model 1) is not accept- trusions, Middle K61i Nappes, central Scandinavian Caledonides. Norsk able. Furthermore, models that merely divide the tec- Geologisk Tidsskrift, 68, 89-97. DALLMEYER,R. D. & GEE, D. G. 1986.40Ar/39Ar mineral dates from retrogressed tonometamorphic history of the Scandinavian Caledonides into eclogites within the Baltoscandian miogeocline: Implications for a poly- two events, one during the Late Cambrian to Early Ordovician, phase Caledonian orogenic evolution. Geological Society of America Bull- commonly referred to as Finnmarkian, and one during the etin, 97, 26-34. Silurian to Early Devonian and referred to as Scandian, are no -- & -- 1988. Polyorogenic 40Ar/39Ar mineral age record in the Seve and K61i Nappes of the Gfiddede area, northwestern Jfimtland, central longer tenable. The complexities emerging from U-Pb zircon Scandinavian Caledonides. Journal of Geology, 96, 181-198. age determination studies in outboard terranes necessitate care- & STEPHENS, M. B. 1991. Chronology of eclogite retrogression within the ful definition of orogenic-phase terms prior to continued usage. Seve Nappe Complex, Rfivvejaure, Sweden: evidence from 40Ar/39Ar mineral ages. Geologische Rundschau, 80, 729-743. , ANDI~ASSON, P. G. • SVENNINGSEN,O. 1991. Early tectonothermal evolu- This work was financed by a Swedish Natural Science Research Grant tion within the Paleozoic miogeocline of Baltica: Evidence from 40Ar/39Ar (No.4901-112) and funding by the Geological Survey of Sweden. We mineral ages from high-pressure units in the Sarek Mtns., Sweden, Journal of thank R. Boyd, D. Gee, A. Krill, M. Ripa and E. Zachrisson for Metamorphic Geology, 9, 203-218. helpful comments. --, GEE, D. G. & BECKHOLMEN,M. 1985.40Ar/39Ar mineral age record of early

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Received 11 February 1992; revised typescript accepted 6 May 1992.

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