40Ar/39Ar Mineral Dates from Retrogressed Eclogites Within the Baltoscandian Miogeocline: Implications for a Polyphase Caledonian Orogenic Evolution

Home , Miogeocline

40Ar/39Ar mineral dates from retrogressed eclogites within the Baltoscandian miogeocline: Implications for a polyphase Caledonian orogenic evolution

R. D. DALLMEYER Department of Geology, University of Georgia, Athens, Georgia 30602 D. G. GEE Geological Survey of Sweden, Box 670, S-751 28 Uppsala, Sweden

ABSTRACT Late Silurian to Early Devonian transport suggest that this tectonic activity occurred while onto the Baltoscandian Platform. Baltica and Laurentia were separated by a con- Late Proterozcic, rift-fades dolerite dikes siderable expanse of the Iapetus Ocean (Ilruton within Baltoscandian rocks of the Seve INTRODUCTION and Bockelie, 1980). Nappe Complex locally underwent eclogite A comprehensive geochronological program metamorphism during Caledonian orogen- Scandinavian portions of the Caledonian oro- is underway to more fully document the ;xtent esis. Hornblende from retrograde amphibo- gen are represented by a succession of far- and character of the pre-Scandian tectono- lite selvages developed around two eclogite traveled allochthons which were emplaced onto thermal record within the Scandinavian Cal- boudins exposed at Grapesvare, Norrbotten the Baltoscandian Platform during the early to edonides. This report presents new 40Ar/39Ar County, Sweden, record identical 40Ar/39Ar middle Paleozoic (Fig. 1). Medium- and high- data for hornblende in retrograde selvages plateau dates of 491 ± 8 Ma. Phengitic mus- grade Baltoscandian rocks of the Seve Nappe developed from eclogite assemblages within covite from host schists records plateau dates Complex constitute one of the major tectonic Seve rocks. These results bear directly ori both of 447 ± 7 Ma and 436 ± 7 Ma. Coexisting units in central parts of the orogen (Zachrisson, the chronology and nature of pre-Scandian oro- biotite yields plateau dates of 594 ± 10 Ma 1973). Seve structural units separate underlying genesis and also provide controls for understand- and 808 ± 13 Ma. allochthons of lower grade metasedimentary ing the early middle Paleozoic tectonothermal The biotite dates are interpreted to reflect rocks with Baltoscandian affinities from overly- evolution of the Baltoscandian miogeocline. the presence of extraneous argon compo- ing, variably metamorphosed tectonic units with nents. The hornblende and phengitic musco- oceanic and/or island arc affinities. The high- REGIONAL GEOLOGIC SETTING vite ages are interpreted to date times of grade metamorphism recorded within the Seve postmetamorphic cooling through argon re- Nappe Complex has been considered to be of Seve Nappe Complex tention temperatures. Together with previous Middle Silurian (early Scandian) age, and the 40Ar/39Ar mineral ages from Jamtland, result of attempted underthrusting of Laurentia The Seve Nappe Complex is a westward- Sweden, they confirm that a significant pre- by Baltica during elimination of the Iapetus thinning, internally imbricated allochthon which 40 39 Scandian tectonothermal event is recorded Ocean (Gee, 1975). Recent Ar/ Ar dating of is composed of amphibolite facies and higher regionally in allochthonous sequences which hornblende within the Seve Nappe Complex grade rocks (Gee and Zachrisson, 1979). Within originated within the Baltoscandian miogeo- (Dallmeyer and others, 1983, 1985) in western central portions of the orogen (Jamtland and cline. The eclogite assemblages are inter- Jamtland (Fig. 1), however, suggests that, at Vasterbotten Counties of Sweden), tectonic preted to have formed during westerly sub- least locally, Scandian metamorphic tempera- units of high-grade gneissic rocks are structurally duction of distal [tortions of the miogeocline tures did not exceed -500 °C. This indicates underlain and overlain by amphibolite-facies with attendant development of an accretion- that the high-grade Seve assemblages developed schists and amphibolites (Trouw, 1973; 2)wart, ary wedge. The latter was subsequently up- during an earlier tectonothermal event which af- 1974; Williams and Zwart, 1977). Contacts be- lifted and eroded, providing a source for fected outer portions of the Baltoscandian mio- tween these Seve Nappes are generally marked Middle Ordovician through Lower Silurian geocline. The thermal evolution reflected by by development of high-temperature, ductile 40 39 clastic successions which accumulated in both these Ar/ Ar dates suggests that ocean- shear zones (Trouw, 1973; Arnbom, 1980; Sjos- eastern and weste rn basins. These, together directed (westward) subduction was active out- trom, 1983). Both U-Pb zircon and Rb-Sr with previously metamorphosed older por- board of the Baltoscandian margin by the Late whole-rock dating suggest that some portions of tions of the miogeocline, were imbricated, Cambrian. Faunal characteristics of allochthons the gneissic terrane are of middle Proterozoic folded, and variably metamorphosed during structurally above the Seve Nappe Complex age (Reymer, 1979; Claesson, 1982).

Figure 1. Simplified terrane map of the Scandinavian Caledonides. Subdivision of the tectonostratigraphy into Autochthon, Parautochthon, and the Lower, Middle, Upper, and Uppermost Allochthons is based on the Tectonostratigraphic Map of the Scandinavian Caledonides in Gee and Sturt (1985).

Geological Society of America Bulletin, v. 97, p. 26-34, 4 figs., 2 tables, January 1986.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 SCANDINAVIAN CALEDONIDES

TERRANE MAP

. * \ - ' \ I

01 100 200 "kr-.,'

Scale in km

NASAFJÄLL AREA (Fig.2) f

PERMIAN

DEVONIAN (Neoautoch.)

LAURENTIAN MIOGEOCLINE (?) (Uppermost Allochthon)

Koli Nappes EUGEOCLINAL

(upper part of TCDDAMrc Upper Allochthon)

Seve Nappes BALTOSCANDIAN (outermost part of miogeocline,

lower units of Upper Allochthon) ulnrrnri imt Undifferentiated MIOGEOCLINE- (Middle and Lower Allochthon) PLATFORM

Sedimentary Cover BALTOSCANDIAN (Autochthon - Parautochthon) Basement PLATFORM

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 28 DALLMEYER AND GEE

Underlying Tectonic Units Allochthon has been recognized from southern Ryan and Sturt, 1985). Structurally higher por- of the Baltoscandian Margin Norway to northern Sweden (southern Vaster- tions of the Köli Nappe Complex exposed near botten County). Although similar lithologies Trondheim (Storen Nappe), contain variably Seve rocks teci:onically overlie various struc- overlie Cambrian black shales as far north as fragmented ophiolitic associations (Gale and tural units of the Middle Allochthon (Gee and Akkajaure in Norrbotten County, Sweden Roberts, 1974, Grenne and others, 1980; Fumes Sturt, 1985). The latter is composed mainly of (Fig. 1), there are no direct paleontologic age and others, 1980) which are overlain by Early late Proterozoic metasedimentary rocks which controls. No overlying Upper Ordovician or Ordovician metasedimentary rocks containing are in ductile tectonic contact with variably de- Lower Silurian sedimentary successions have faunas with North American affinities (Bruton formed, middle Proterozoic crystalline basement been recognized within the Lower Allochthon and Bockelie, 1980; Spjeldnaes, 1985). Thus at rocks of Baltic Sh ield affinity (Fig. 1). Metased- north of Jamtland County. In the Finnmark least some protoliths of the Köli Nappe Com- imentary rocks within the Middle Allochthon area of northernmost Norway (Fig. 1), metased- plex may have been derived from eugeoclinal ter- are represented by low-grade, fluviatile sand- imentary successions beneath the Middle Al- ranes more closely related to Laurentia than to stones which are overlain by dolomites, tillites, lochthon only extend into the Tremadoc. On the Baltica. and sandstones of probable Vendian age (Kum- basis of Rb-Sr and K-Ar isotopic age-determina- The highest tectonic units within the Scandi- pulainen, 1980). Locally, Cambrian and early tions, Sturt and others (1975, 1978) suggested navian Caledonides (Uppermost Allochthon; Fig. Middle Ordovician metasediments are present. that deposition along northernmost portions of 1) overlie eugeoclinal rocks of the Köli Nappe These late Proterozoic and early Paleozoic suc- the Baltoscandian miogeocline was interrupted Complex. The structurally high units are domi- cessions have been correlated with sequences in in the Early Ordovician by emplacement of pre- nated by a variety of gneissic rocks together with the structurally underlying Lower Allochthon viously metamorphosed nappes. ^Ar/^Ar min- platformal, metasedimentary cover sequences (Gee, 1975; Kumpulainen and Nystuen, 1985) eral ages from central Jamtland (Dallmeyer and which, at least in part, are thought to represent and are therefore thought to have originated in others, 1985), together with regional strati- components of the Laurentian miogeocline outboard (western) portions of the Baltoscan- graphic relations, support this interpretation and (Stephens and Gee, 1985). dian miogeocline. Higher tectonic units of the indicate that initial emplacement of allochthons Middle Allochthon are intruded by extensive onto the Baltoscandian Platform occurred GEOLOGY OF THE SAMPLE AREA swarms of rift-feicies, tholeiitic, dolerite dikes earlier in Finnmark than in more southerly (Stromberg, 1969; Solyom and others, 1979a) portions of the orogen. Retrogressed eclogites and host, polydeform- which record late Proterozoic radiometric ages ed schists within the Seve Nappe Complex were (Claesson, 1977; Claesson and Roddick, 1983). Overlying Units of Uncertain collected in the vicinity of Grapesvare, -40 km In several areas of the Swedish Caledonides, Palinspastic Affinities northwest of Jäkkvik, Norrbotten County, greenschist-facies metadolerite dikes and host Sweden (Figs. 1 and 2). The regional geologic sandstones of the Middle Allochthon (Sarv Seve rocks are overlain by the Koli Nappe setting has been previously described by Kulling Nappes) are ovedain tectonically by amphibo- Complex (Fig. 1) which consists of an internally (1982), and Andreasson and others (19S5) have lite-facies metabitsic rocks and schistose psam- imbricated sequence of contrasting litho-tectonic outlined the geology of the Grapesvare area and mites of the Seve Nappe Complex. This suggests units (Zachrisson, 1969; Stephens, 1980). These discussed the mineralogy and petrochemistry of that at least some protoliths of the Seve Nappe were apparently derived from a variety of eu- the eclogites and host metasedimentary rocks. Complex likely developed along outermost por- geoclinal terranes located within the Iapetus The host rocks are dominated by psammitic and tions of the Baltoscandian margin (Gee, 1975; Ocean, and they were subsequently accreted to semipelitic schists with subordinate pelitic Solyom and others, 1979b). the Baltoscandian margin during the early and schists and marbles. More pelitic compositions Beneath the Middle Allochthon in Jamtland, middle Paleozoic. Stephens (1980) and Stephens contain phengitic muscovite, which is locally successions of the Lower Allochthon range in and Gee (1985) summarized stratigraphic rela- rimmed by greenish-brown biotite and is typi- age from late Proterozoic to Middle Silurian tions in Koli Nappes within central portions of cally overgrown by subidioblastic garnet. Kya- (Fig. 1). These include Vendian through Lower the orogen, and they proposed a variety of paleo- nite is present in some places. Typical host rock Cambrian sandstones which are overlain by late environmental protolith terranes from which assemblages are diagnostic of lower to middle Middle Cambrian and Late Cambrian (locally the allochthonous units could have been derived. amphibolite-facies metamorphism. also Tremadoc) black shales. The latter facies is Lower Koli Nappes, included in the Virisen Variably metamorphosed dolerite oxurs as usually overlain by a thin sequence of Arenig terrane of Stephens and Gee (1985), are boudins within the Grapesvare area (Fig. 2). shale and/or limestone which passes com- characterized by arc-related metavolcanic rocks These may be traced into eclogite through the formably upward into an extensive Late Arenig (locally recording -490 Ma, U-Pb zircon crys- progressive disappearance of plagioclase pheno- through Caradoc, westerly derived graywacke tallization ages; Claesson and others, 1983) crysts and growth of garnet. Eclogite assem- succession (Karis, in Gee and Kumpulainen, which pass conformably upward into a variety blages are most completely preserved, within 1980). These graywackes are dominated by re- of phyllites, metagraywackes, and fanglomerates central portions of larger boudins. These are cycled quartz grains; however, the subordinate which, at least in part, are of Middle Ordovician dominated by omphacite (-25% jadeite) and a lithic fragments appear to have been derived age. These are conformably overlain by low- Mg-poor, Ca-rich garnet. Andreasson ar.d others from granites and rhyolites similar to those in grade, Upper Ordovician sandstones and lime- (1985) suggested that the eclogite paragenesis the overlying Middle Allochthon. Erosion of this stones and Lower Silurian shales and gray- formed at temperatures of 550-750 °C and or a similar basement terrane apparently con- wackes. Higher tectonic units within the Koli pressures of —15 kb. The eclogite assemblages tributed significant detritus during deposition of Nappe Complex provide evidence of a major were variably retrogressed during formation of a the westerly derived, Ordovician clastic succes- Ordovician (pre-Ashgill) unconformity, suggest- regional foliation in the host metasedimentary sion (Gee, 1975; Hossack and others, 1985). ing regionally significant pre-Scandian poly- rocks. Retrogression may be progressively The Ordovician graywacke facies of the Lower deformation and metamorphism (Sturt, 1984; traced from initial breakdown of ompha cite into

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 DATES FROM ECLOGUES, BALTOSCANDIAN MIOGEOCLINE 29

UPPER ALLOCH.

MIDDLE ALLOCH. AND UNDERLYING COVER

PRECAMBRIAN CRYSTALLINE ROCKS IN AUTOCH., PARAUTOCH. AND LOWER ALLOCH.

Figure 2. Geological map of the Nasafjallet area (adapted from Andreasson and others, 1985). 40Ar/39Ar sample localities are indicated.

a plagioclase-clinopyroxene symplectite into a slightly discordant 40Ar/39Ar release spectra in concentrates are comparable with the plateau coarser-grained, plagioclase-amphibole symplec- which anomalously older dates are recorded in dates, but again extremely large 40Ar/36Ar and tite. Smaller mafic boudins are frequently sur- initial, low-temperature gas fractions (Table 1; 39Ar/36Ar ratios were measured. rounded or totally replaced by a retrograde Fig. 3). Identical plateau dates of 491 ±8 Ma amphibolite selvage which includes hornblende, are defined by >95% of the gas released from INTERPRETATION epidote, plagioclase, biotite, and sphene. each sample, however. An 40Ar/39Ar versus 39Ar/36Ar isochron age of 490 ± 8 Ma is de- The two hornblende concentrates display 40Ar/39Ar ANALYTICAL fined by the plateau increments of sample 1A slightly discordant 40Ar/39Ar age spectra, but TECHNIQUES (SUMS/n-2 = 1.21) with an 40Ar/36Ar inter- both record identical, well-defined plateau dates cept of 309 ± 26. Sample 2A yields a plateau of 491 ±8 Ma. These ages are interpreted to The techniques used during analysis of the isochron age of 492 ± 9 Ma (SUMS/n-2 = date postmetamorphic cooling through tempera- Seve samples generally followed those described 1.44), with an associated ordinate intercept of tures required for argon retention in hornblende in detail by Dallmeyer and Rivers (1983). Vari- 291 ± 13. (-500 ± 25 °C; Harrison, 1981). These horn- ations in the flux of neutrons along the length of Coexisting phengitic muscovite and biotite blendes are part of a retrograde alteration as- the irradiation assembly were monitored with were separated from samples of host Seve schists semblage; therefore, formation of the protolith several minerals, including MMhb-1 (Alexander collected immediately adjacent to both eclogite eclogite paragenesis must have occurred prior to and others, 1978). Measured isotopic ratios localities. The analytical data are listed in Table —490 Ma. Anomalously older dates are re- were corrected for the effects of interfering iso- 2 and are portrayed as age spectra in Figure 3. corded in the low-temperature gas fractions topes produced during irradiation, using the fac- Plateau dates of 436 ± 7 Ma and 447 ± 7 Ma evolved from both hornblende concentrates. tors reported by Dairymple and others (1981) are recorded by phengitic muscovite from sam- Similar patterns of discordance in hornblende for the reactor used in the present study. Appar- ples IB and 2B. Although similar ages are de- age spectra have been interpreted by Dallmeyer ent 40Ar/39Ar ages were calculated from the fined by plateau 40Ar/36Ar versus 39Ar/36Ar (1975), Harrison and McDougall (1981), Dall- corrected isotopic ratios, using the decay con- isochrons, precise determination of ordinate meyer and Rivers (1983), and Dallmeyer and stants and isotopic abundance ratios listed by 40Ar/36Ar intercepts is precluded by the exceed- others (1985) to reflect low-temperature, exper- Steiger and Jäger (1977). Total uncertainties in ing large measured isotopic ratios which require imental liberation of gas with large components each apparent age are quoted at two sigma and long ordinate projections. Biotite from sample of extraneous (excess) 40Ar relative to intracrys- have been calculated following the methods out- IB displays a slightly discordant spectrum in talline radiogenic 40Ar. This interpretation also lined by Dalrymple and Lanphere (1971) and which the initial, low-temperature gas fractions likely applies to the Grapesvare Seve results. Dallmeyer and Rivers (1983). record anomalously young apparent ages com- Phengitic muscovite within host Seve schists pared to the remainder of the gas liberated; displays internally concordant 40Ar/39Ar spec- RESULTS however, >90% of the gas evolved records a tra which define mutually overlapping plateau plateau date of 594 ± 10 Ma. Biotite from sam- dates of 436 ± 7 Ma and 444 ± 7 Ma. These are Hornblende was separated from two samples ple 2B shows a similarly discordant spectrum interpreted to closely date cooling through argon of retrogressed eclogite within Seve metasedi- with >85% of the gas corresponding to a plateau retention temperatures (-400 ± 25 °C; Jäger, 40 36 mentary rocks collected at Grapesvare, Norrbot- date of 808 ± 13 Ma. Ar/ Ar versus 1979). Coexisting biotite from the two samples 39 36 ten County, Sweden (Fig. 2). Both display Ar/ Ar plateau isochron ages for both biotite displays variably discordant age spectra which

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 30 DALLMEYER AND GEE

TABLE 1. ^At/^Ar ANALYTICAL DATA FOR INCREMENTAL HEATING EXPERIMENTS ON HORNBLENDE FROM RETROGRESSED ECLOGITES WITHIN THE SEVE NAPPE, GRAPESVARE, NORRBOTTEN COUNTY, SWEDEN

Release («WAO* (36Ar/39Ar)> (37Ar/39Ar)+ 3«Ar Apparent temp (°C) % of total age (Ma)*'

Sample 1: J =0.010702

600 159.38 0.17982 2.990 0.64 66.81 0.45 1,374 ± 2i 675 54.12 0.08513 3.533 1.06 54.03 1.13 492 ± 14 700 32.35 0.01239 6.781 7.43 90.35 14.88 493 ± 9 725 30.87 0.00673 6.608 13.48 95.26 26.72 494 ± 8 775 30.74 0.00675 6.313 12.13 95.14 25.43 493 ± 8 785 30.69 0.00762 6.104 9.12 94.25 21.80 488 ± 9 800 30.61 0.00750 6.372 8.78 94.41 23.10 488 t 9 825 30.19 0.00576 6.453 17.54 96.06 30.47 489 ± 8 850 30.55 0.00645 5.837 14.18 95.28 24.62 491 ± 8 Fusion 30.72 0.00705 5.965 15.60 94.76 23.03 491 i 8

Total 31.92 0.00957 6.187 100.00 93.81 24.06 496 ± 9

Total without 600 °C 99.36 491 ± 8

Sample 2: J = 0.010577

600 248.44 0.37545 3.572 0.49 55.46 0.26 1,624 ± 5-1 700 59.61 0.08637 5.891 1.50 57.97 1.86 564 T 32 750 39.87 0.03343 6.581 3.91 76.54 5.36 506 * 1 800 32.57 0.01098 6.390 15.64 91.60 15.84 497 ± 9 850 31.47 0.00806 3.213 19.78 94.00 20.97 493 ± 8 875 31.48 0.00880 6.467 14.97 93.37 19.99 490 I 8 885 32.03 0.01085 6.444 13.09 91.59 16.16 489 ± 8 900 32.91 0.01419 6.084 8.18 88.73 11.67 487 ± 9 925 31.22 0.00804 6.096 16.74 93.94 20.63 489 ± 8 Fusion 38.13 0.03125 6.398 5.72 77.12 5.57 490 ± 9

Total 33.98 0.01477 6.286 100.00 90.40 16.69 498 T 9

Total without 600, 700, and 750 °C 94.11 491 ± 8

•Measured. ^Corrected for postirradiat on decay of 37 Ar (35.1-day half-life). § 4 [ °Artol-(«AW)(;95.5)]/'«>Arwt. ••Calculated using correction factors of Dalrymple and others (1981); 2a error estimates.

n yield markedly different plateau dates of 594 ± 1 A HORNBLENDE 2A HORNBLENDE 550 550 10 Ma and 808 ± 13 Ma. An explanation for the • 491 ±8 491 18 spectra discordancy is not readily apparent. The 500 I L 500 biotites show no unusual petrographic character-

450 450 istics which could explain the discordant, and perhaps it may reflect a certain degree of isotopic redistribution within biotite interlayers a z: during neutron irradiation. Several workers 1 B MUSCOVITE have shown that the temperatures required for intracrystalline retention of radiogenic 4()Ar are markedly lower for biotite (-300 °C) than for muscovite or hornblende (Steiger, 1966; Jäger, 436 *1 1979; Berger and York, 1981). Therefore, biotite should record younger or at least similar postmetamorphic cooling dates (depending on - 1B BIOTITE 2B BIOTITE cooling rate). Biotite in the Seve schists records 600 - l=F=t 850 plateau dates older than both coexisting phen- -594 ±10- gitic muscovite or hornblende within retro- 550 - J 1 £ 800 graded eclogite assemblages, and therefore it 500 - 750 must contain extraneous (excess) argon tximpo- nents. As well-defined plateaus characterize 20 40 60 100 40 CUMULATIVE % "Ar RELEASED these samples, both excess and radiogenic Ar components must have been liberated simul- Figure 3. 40Ar/39Ar incremental-release spectra of hornblende, muscovite, and biotite from taneously during incremental heating. Similar the Seve rocks (in Grapesvare in the Nasafjall area. Sample numbers refer to locations shown inabilities to separate radiogenic and excess '"'Ar on Figure 2. Abscissa coordinates are indicated at lower left. Analytical uncertainties (2 a) are within biotite have been described from other represented by the vertical width of bars. Experimental temperatures increase from left to orogenic terranes by Pankhurst and others right. Plateau ages are listed on each spectrum. (1973), Roddick and others (1980), Dallmeyer

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 DATES FROM ECLOGUES, BALTOSCANDIAN MIOGEOCLINE 31

TABLE 2. *>/u/xfu ANALYTICAL DATA FOR INCREMENTAL HEATING EXPERIMENTS ON MUSCOVITE AND BIOTITE and Rivers (1983), and Dallmeyer and others FROM SCHISTS WITHIN THE SEVE NAPPE, GRAPESVARE, NORRBOTTEN COUNTY, SWEDEN (1985). It is also significant to note the apparent

inability of muscovite within the Seve samples Release («Ar/^Ar)' (36Ar/39Ar)- »Ar «"«Ar Apparent to accommodate extraneous 40Ar components temp (°C) % of total nonatmos^ age (Ma)®

relative to biotite and hornblende. This appears Muscovite to confirm the suggestion of Dallmeyer and oth- Sample IB: J = 0.010687 ers (1985) that biotite is inherently more "per- 475 31.03 0.02542 0.96 75.78 404 ± 11 meable" to inward diffusion of extraneous argon 525 27.70 0.00549 3.14 94.12 443 ± 9 components than either muscovite and horn- 600 26.73 0.00397 24.82 95.59 436 ± 7 650 26.53 0.00317 23.54 96.45 436 ± 7 blende. 700 26.39 0.00342 9.15 96.14 433 ± 8 750 26.31 0.00290 9.97 96.72 434 ± 8 800 26.22 0.00202 22.81 97.70 437 ± 7 SIGNIFICANCE 825 26.75 0.00392 3.63 95.64 436 ± 10 850 29.11 0.01244 1.41 87.35 434 ± 11 40 39 Ar/ Ar mineral ages from retrogressed Fusion 50.30 0.09023 0.57 46.98 406 ± 16

eclogites and host Seve schists exposed on Total 26.76 0.00405 100.00 95.81 435 ± 8

Grapesvare provide controls for the timing of Total without 475 and 850 °C and fusion 97.05 436 ± 7 pre-Scandian tectonothermal activity within the

Baltoscandian miogeocline. Although there is Sample 2B: J = 0.010775 debate regarding radiometric calibration of the 475 35.63 0.04585 0.42 61.95 385 ± 9 Paleozoic time scale (McKerrow and others, 525 30.38 0.00747 1.72 92.72 478 ± 9 575 27.88 0.00495 6.21 94.73 452 ± 8 1980; Gale and others, 1980; Harland and oth- 600 27.01 0.00367 13.82 95.96 444 ± 7 ers, 1982; Palmer, 1983), the 490 Ma plateau 625 26.62 0.00226 15.31 97.47 445 ± 7 650 26.44 0.00216 8.17 97.56 442 ± 8 dates recorded by retrograde hornblende require 700 26.73 0.00268 13.31 97.01 444 ± 7 750 26.54 0.00178 20.77 98.02 446 ± 7 conversion of rift-facies dolerite dikes into eclo- 775 26.39 0.00184 14.13 97.91 443 t 7 gitic assemblages prior to at least the late Early 800 26.49 0.00252 4.16 97.16 442 ± 9 Fusion 29.80 0.01371 1.98 86.38 442 ± 10 Ordovician (Arenig). The 436-444 Ma dates 0.00302 96.75 445 ± 8 recorded by phengitic muscovite within host Total 26.86 100.00 Seve schists indicate postmetamorphic cooling Total without 475 and 525 °C 97.85 444 ± 7 through argon-retention temperatures during the Biotite

Late Ordovician or earliest Silurian (Ashgill- Sample IB: J = 0.010320 Llandovery). By comparison, hornblende from 475 39.00 0.05888 1.99 55.38 363 ± 7 amphibolites within the Seve Nappe Complex 500 36.86 0.01101 6.19 91.16 537 ± 10 525 38.50 0.00433 17.57 96.66 586 ± 10 exposed near Are in western Jâmtland, Sweden 550 38.73 0.00243 24.31 98.13 597 ± 10 (Fig. 1), record plateau 40Ar/39Ar ages of 600 38.28 0.00255 7.65 98.01 590 ± 10 700 39.22 0.00317 16.21 97.60 601 ± 10 455-465 Ma, whereas micas from both Seve 750 38.46 0.00299 19.01 97.69 591 ± 10 Fusion 38.96 0.00316 7.07 97.59 597 ± 11 and Kôli units in Jâmtland yield Scandian plateau dates (400-425 Ma; Dallmeyer and others, Total 38.59 0.00471 100.00 96.37 586 ± 11 1983, 1985). The hornblende and muscovite Total without 475 and 500 °C 91.86 594 ± 10 ages from Grapesvare are significantly older than results from western Jâmtland, suggesting Sample 2B: J = 0.010470 that although both portions of the Seve Nappe 450 125.50 0.37917 0.59 10.72 238 ± 13 475 48.82 0.04431 1.83 73.17 573 ± 12 Complex record pre-Scandian orogenesis, it 500 54.09 0.01313 12.26 92.81 762 i 13 may have developed diachronously southwest- 525 54.77 0.00426 17.30 97.69 802 ± 13 550 54.99 0.00223 16.77 98.79 812 ± 13 ward along the orogen. This is in harmony with 625 55.37 0.00253 13.28 98.64 816 ± 13 700 54.63 0.00276 17.14 98.50 806 ± 13 radiometric age-determinations reported from 725 54.54 0.00240 11.29 98.69 806 ± 13 northernmost Scandinavia which suggest that 750 54.17 0.00247 4.88 98.64 802 ± 14 Fusion 56.14 0.00670 4.66 96.47 810 ± 14 early Caledonian (Finnmarkian) tectonothermal 55.09 0.00727 100.00 96.68 795 ± 14 activity occurred between ca 540 and 480 Ma Total (Sturt and others, 1975, 1978). The 40Ar/39Ar Total without 450 and 500 °C 85.32 808 ± 13

data presented by Dallmeyer and others (1983, •Measured. t 40 1985) for Seve nappes at Are also indicate that . [ ArIol-(36AW)(295.S)J/«Arlot. ^Calculated using correction factors of Dalrymple and others (1981); Ar/ Ar corrected ratio less than 0.2 in all analyses; 2a error high-temperature (>500 °C) imbrication of su- estimates. pracrustal rocks (amphibolites and schists) and middle Proterozoic crystalline basement (gneisses) occurred during early Caledonian TECTONOTHERMAL EVOLUTION within the Autochthon and Lower Allochthon orogenesis. Similar pre-Scandian ductile imbri- OF CENTRAL PORTIONS OF THE contain no igneous rocks. Similarly, both the cation of basement and cover is suggested by a BALTOSCANDIAN MIOGEOCLINE crystalline basement and Late Proterozoic meta- —485 Ma, thin-slab, Rb-Sr isochron age re- sedimentary cover sequences within the Middle ported by Claesson (1980) for a mylonite zone Subduction Polarity Allochthon display no evidence of Caledonian developed along an internal tectonic contact Except for a few, thin, bentonite horizons, igneous activity except in the Seiland pétro- within the Middle Allochthon in Jâmtland. Cambrian-Silurian sedimentary successions graphie province in the type area for Finnmark-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 32 DALLMEYER AND GEE

ian orogeny in northern Norway (Barth, 1953; developed in response to westward-dipping sub- ious tectonic units and their palinspastic restora- Robins and Gard ner, 1975). These relationships duction. Consumption of the lapetus oceanic tion have been discussed elsewhere (for exam- suggest that pre-Scandian subduction along the lithosphere situated east of this system was likely ple, see Gee, 1975, 1978; Hossack and others, Baltoscandian continental margin was likely di- accompanied by formation of an accretionary 1985) and are not reviewed here. Tectonic rela- rected westward and away from the margin. complex analogous to those actively developing tionships within the higher structural units have This is consistent with development of various along many western Pacific arc complexes been outlined by Stephens and Gee (1985), who volcanic/volcaniclastic sequences within lower (Karig, 1974; Hamilton, 1979). suggested that volcanic arcs initially developed tectonic units of the Koli Nappe Complex outboard of both Laurentia and Balticii during (Zachrisson, 1969; Stephens, 1980). Both Rb-Sr Development of the Accretionary Wedge lapetus closure. Final ocean closure led to colli- whole-rock and U-Pb zircon radiometric ages of sion of Laurentia and Baltica with associated -490 Ma have recently been reported by Claes- The following tectonic model (Fig. 4) is pro- eastward translation of the various arc-se- son and others (1983) for one of these units, posed for the evolution of the Baltoscandian quences during Scandian orogenesis. Contrast- suggesting that an outboard arc was active dur- margin on the basis of the 40Ar/39Ar mineral ing tectonic models for arc development have ing the Early Ordovician and that it may have cooling ages discussed previously and their rela- been presented by Gale and Roberts (1972, persisted into the Middle Ordovician. It seems tionship to the sedimentary and igneous record 1974), Roberts and Gale (1978), and Roberts likely therefore that the pre-Scandian orogenesis in underlying and overlying allochthons. Evi- and others (1985). Ramsay (1973), Robins and recorded within the Baltoscandian succession dence for distances of displacement of the var- Gardner (1975), and Gayer and others, (1985)

EARLY CALEDONIAN EVOLUTION OF THE BALTOSCANDIAN MARGIN

EARLY CAMBRIAN LEGEND SLOPE RISE PLATFORM

Late Ordovician shales (in east) and limestones (in west)

I:;v.-J Early-Mid Ordovician graywackes Fand conglomerates EARLY ORDOVICIAN Early-Mid Ordovician limestones VIRISEN Cambrian black shales ARC ACCRETIONARY PRISM CLASTIC «WEDGE

[ I Accretionary prism

Passive margin Vendían and Riphaean sst and tillites Basic volcanites and high IT, level intrusions (rifting-related) MID ORDOVICIAN VIR5SEN Volcanic arc

\ Ultromafites • Oceanic crust Continental crust LATE ORDOVICIAN

Horizontal stole 0 100 200 KM 1 I I I I 0 50 100 KM Vertical scale

Figure 4. Suggested tectonothermal evolution of the Baltoscandian margin during the Cambrian and Ordovician.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 DATES FROM ECLOGUES, BALTOSCANDIAN MIOGEOCLINE 33

suggested that the early Caledonian (Finnmark- with initiation of pre-Ordovician outboard sub- sisted through the Llandovery and into the mid- ian) tectonic evolution in the type area involved duction along the Baltoscandian margin. dle Wenlock before passing through an estua- east-dipping subduction and development of a Early Ordovician (Fig. 4B). Progressive de- rine facies into continental sandstones which related syntectonic igneous complex. This his- velopment of an accretionary prism along outer were derived largely from Scandian nappes ad- tory is markedly different from that proposed portions of the Baltoscandian margin appears to vancing from the northwest. here for other segments of the Baltoscandian have been underway in the Early Ordovician, margin. with construction of an associated, outboard Scandian History Vendian-Early Cambrian (Fig. 4A). Depo- volcanic arc (Virisen terrane of Stephens and sition of sandstones and conglomerates occurred Gee, 1985). Host sedimentary sequences within Nearly complete closure of the Iapetus Ocean within basins which had developed during initial distal portions of the Baltoscandian miogeocline, appears to have been accomplished by the Late attenuation and rifting of Baltica from Lauren- together with rift-facies dolerite dikes, had al- Ordovician, when eugeoclinal terranes which tia. These largely fluviatile successions were ready been carried to significant crustal depths had developed along the Laurentian margin overlain by Vendian tillites related to the wide- within the subduction zone. Eclogite assem- were juxtaposed with volcanic arc sequences of spread Varanger glaciation. The tillites are blages had formed, undergone variable retro- the Virisen terrane. After relative tectonic quies- generally succeeded by shallow-marine sand- gression, and cooled through hornblende argon cence throughout the Late Ordovician and Early stones and subordinate shales which thin east- retention temperatures by the Arenig. Initial, Silurian, Scandian orogenesis commenced in the ward and overlap onto crystalline rocks of the high-temperature ductile imbrication of crystal- late Llandovery with thrusting of Laurentian eu- Baltoscandian Platform. Outboard portions of line Baltica basement and Late Proterozoic geoclinal sequences onto the Virisen terrane. the Baltoscandian miogeocline were pervasively cover was at least locally accomplished. The These were subsequently imbricated and folded intruded by a system of rift-facies, dolerite dikes. Cambrian black shale facies, however, now rep- with the Baltoscandian accretionary prism. The In more distal portions of the miogeocline, these resented in the Lower Allochthon, persisted into resultant composite allochthon was thrust east- may have been related to extrusion of basaltic the early Tremadoc, suggesting that the western ward over the Ordovician clastic wedge and sequences which are now represented by am- belt of tectonic instability did not provide detri- emplaced onto Silurian sedimentary successions. phibolite and metadolerite within portions of the tus to the inboard Jàmtland basin until later in Sequential transport of the nappe complex onto Seve Nappe Complex. Protrusion of a few ul- the Early Ordovician. the Baltoscandian Platform appears to have oc- tramafic bodies into portions of the miogeocline Middle Ordovician (Fig. 4C). Continued curred throughout the Middle and Late Silurian; may have occurred at this time; however, it is development of the accretionary prism resulted in more southerly areas, it probably continued likely that the majority of these bodies were tec- in Middle Ordovician uplift. This probably was into the Early Devonian. Details of this tectonic tonically emplaced during formation of an ac- the erosional source for westerly derived turbid- evolution have been previously discussed by cretionary wedge. ites which were deposited along the western Gee (1975) and Hossack and others (1985). Late Cambrian. The 490 Ma 40Ar/39Ar ages margin of the Baltoscandian Platform (Jamtland Emplacement of the composite Scandian al- recorded by hornblende within retrograded ec- basin) and the turbidites and fanglomerates (Gil- lochthons onto crystalline, middle Proterozoic logite assemblages in the Seve Nappe Complex liks facies) which accumulated within the out- basement of western Scandinavia apparently re- date uplift and associated cooling following a board Virisen terrane. sulted in its depression to depths appropriate for high-pressure metamorphism. On the basis of formation of eclogite assemblages which record phase equilibria within Seve eclogite parage- The Late Ordovician-Early Silurian Tectonic U-Pb zircon and Sm-Nd mineral ages of -425 neses, van Roermund (1985), Andreasson and Interlude Ma (Krogh and others, 1974; Griffin and others (1985), and Stephens and van Roermund Brueckner, 1980). Caledonian eclogites in (1984) suggest depression of these miogeoclinal Extensive erosion of both the accretionary Scandinavia thus appear to have been generated rocks to depths of at least 50 km. At present day prism and associated island-arc terrane con- both during a Late Cambrian-Early Ordovician, subduction-rates, this could have been accom- tinued during the Late Ordovician (Fig. 4D). It continent-arc collision and the subsequent plished within 1 to 2 Ma. Consideration of the was probably caused by uplift which resulted Scandian continent-continent collision. character of conductive thermal relaxation from subduction movement of Baltica crust of processes (Oxburgh and Turcotte, 1974; Toksoz transitional and/or continental character. This ACKNOWLEDGMENTS and Bird, 1977; England, 1978), however, indi- ultimately terminated subduction, and the Ash- cates that at least 20 Ma probably would have gill-Llandovery interval was apparently a rela- This work was supported in part by a grant been required to allow attainment of appro- tively quiescent tectonic interval during which from the National Science Foundation of the priate eclogite-forming conditions within sub- shallow-marine sandstone, shale, and coral lime- United States (EAR-8407027) and the Swedish ducted portions of the outboard, miogeoclinal stone were deposited both west and east of the Natural Science Research Council. Sample col- sedimentary succession. It is thus likely that eroding accretionary prism. These facies proba- lection was facilitated by Dr. P.-G. Andreasson westward-directed subduction along the Balto- bly extended unconformably across eroded, de- and L. Johansson. The manuscript has benefited scandian margin was initiated at least by the formed, and metamorphosed older portions of greatly from discussion with Dr. M. B. Stephens. Late Cambrian. This would imply that deposi- the Baltoscandian miogeocline. Deepening of tion of Late Cambrian black shale on the plat- this depositional basin occurred during the mid- REFERENCES CITED form could have occurred in response to tectonic dle Llandovery with a rapid transition into a Alexander. E. C., Jr., Michelson, G. M., and Lanphere, M. A., 1978, A new loading along outer portions of the miogeocline. black shale facies. In the late Llandovery and ^Ar/^Ar dating standard, in Zartman, R. E., ed.. Short papers of the Documentation of island-arc-related, Late International Conference on Geochronology, 4th, Cosmochronology early Wenlock, a thick sequence of graywacke and Isotope Geology: U.S. Geological Survey Open-File Report, Cambrian volcanic activity in units of the Koli was deposited, suggesting outboard tectonic in- v. 78-701, p. 6-8. Andrtasson, P.-G., Gee, D. G., and Sukotjo, S„ 1985, Seve eclogites in the Nappe Complex exposed in southwesternmost stability. In southernmost (more distal?) areas Norrbotten Caledonides, Sweden, in Gee, D. G., and Sturt, B. A., eds.. The Caledonide Orogen — Scandinavia and related areas: Chichester, Norway (Furnes and others, 1983) is consistent such as Oslo, the carbonate and shale facies per- England, Wiley & Sons, p. 887-901.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021 34 DALLMEYER AND GEE

Arnbom, J. O., 1980, Metan orphism of the Seve rocks on Areskutan, southern 1978, Nappe displacement in the Scandinavian Caledonides: Tectono- Roermund, H.L.M. van, 1982, On eclogites from the Seve Nappe. Jamtland, Swedish Caledonide«: Geologiska Foreningens i Stockholm Forhand- physics, v. 47, p. 393-419. central Scandinavian Caledonides [Ph.D. thesis]: Utrecht, Netherlands, lingar, v. 102, p. 359 -372. Gee, D. G., and Kumpulainen, R., 1980, An excursion through the Caledonian University of Utrecht, 99 p. Barth, T.F.W., 1953, The la/ered gabbro series at Seiland, northern Norway: mountain chain in central Sweden from Östersund to Storlien: Sveriges 1985, Eclogites of the Seve Nappe, central Scandinavian Ciledonides, Norges geologiske undersokelse, v. 184, p. 191-200. geologiska Undersökning, ser. C, no. 774,66 p. in Gee, D. G., and Sturt, B. A., eds., The Caledonide orogen — Berger, G. W., and York, D., 1981, Geothermometry from Âr/Âr dating Gee, D. G., and Sturt, B. A., 1985, The Caledonide orogen — Scandinavia and Scandinavia and related areas: Chichester, England, Will y & Sons, experiments: Geochinica et Cosmochimica Acta, v. 45, p. 795-811. related areas: Chichester, England, Wiley & Sons, 1,266 p. p. 873-886. Bergstrom, J., 1980, Middle s nd Upper Cambrian biostratigraphy and sedimen- Gee, D. G., and Zachrisson, E., 1979, The Caledonides in Sweden: Sveriges Ryan, P. D., and Sturt, B. A., 1985, Early Caledonian orogenesis in northwest- tation in south central Jamtland, Sweden: Geologiska Foreningens i geologiska Undersökning, ser. C, no. 769,48 p. ern Europe, in Gee, D. G., and Sturt, B. A., eds., The Caledo lide orogen Stockholm Forhandlingar, v. 102, p. 373-376. Grenne, T., Grammeltvedt, G., and Vokes, F. M., 1980, Cyprus-type sulphide — Scandinavia and related areas: Chichester, England, Wi ey & Sons, Bruton, D. L., and Bockelie, J. F., 1980, Geology and palaeontology of the deposits in the western Trondheim district, central Norwegian Cal- p. 1227-1239. Helonda area, western Norway — A fragment of North America? in edonides, in Panayiotou, G. C , ed., Ophiolites: International Ophiolite Sjöström, H„ 1983, The Seve-Köli Nappe Complex of the Hancöl-Storlien- Wones, D. R., ed„ The Caledonides in the USA: Blacksburg Virginia Symposium, Proceedings, Cyprus 1979: Nicosia, Geological Survey of Essandsjoen area, Scandinavian Caledonides: Geologiska Foreningens i Polytechnic Institute and State University, Department of Geological Cyprus, p. 727-743. Stockholm Forhandlingar, v. 105, p. 1-24. Sciences, Mem., 2, p. 41-47. Griffin, W. L., and Brueckner, H. K., 1980, Caledonian Sm-Nd ages and a Solyom, Z., Gorbatschev, R., and Johansson, I., 1979a, The Ottfjäll :t Dolerites. Claesson, S., 1977, The ag; of the Ottfjallet dolerites of the Sarv Nappe, crustal origin for Norwegian eclogites: London, Nature, v. 285, Geochemistry of the dyke swarm in relation to the geodynnmics of the Swedish Caledonides: A reply: Geologiska Fdreningens i Stockholm p. 319-321. Caledonide orogen in central Scandinavia: Sveriges geologiska Under- Forhandlingar, v. 99, p. 405-408. Hamilton, W., 1979, Tectonics of the Indonesian region: U.S. Geological Sur- sökning, ser. C, no. 751, 38 p. 1980, A Rb-Sr is0t0))e study of granitoids and related mylonites in the vey Professional Paper 1078, 345 p. Solyom, Z., Andreàasson, P. G., and Johansson, I., 1979b, Geochemistry of Tannas Augen Gneiis Nappe, southern Swedish Caledonides: Geolo- Harland, W. B„ Cox, A. V., Llewellyn, P. G., Picton, C.A.G., Smith, A. G„ and amphibolites from Mt. Sylarna, central Scandinavian Caledonides: Geo- giska Foreningens i Stockholm Forhandlingar, v. 102, p. 403-420. Walters. R., 1982, A geological time scale: Cambridge, England. Cam- logiska Foreningens i Stockholm Forhandlingar, v. 101, p. 17-25. 1981, Caledonian metamorphism of Proterozoic Seve rocks on Mt. bridge University Press, 131 p. Spjeldnaes, N., 1985, Biostratigraphy of the Scandinavian Caledon des, in Gee, Areskutan, southern Swedish Caledonides: Geologiska Fdreningens i Harrison, T. M., 1981, Diffusion of *°Ar in hornblende: Contributions to D. G., and Sturt, B. A., eds., The Caledonide Orogen — Scandinavia Stockholm Forhandlingar, v. 103, p. 291-304. Mineralogy and Petrology, v. 78, p. 324-331. and related areas: Chichester, England, Wiley & Sons, p. 317-329. Claesson, S., and Roddick, .1. C., 1983, Âr/Âr data on the age and meta- Harrison, T. M., and McDougall, I., 1981, Excess Ar40 in metamorphic rocks Steiger, R. H„ 1964, Dating of orogenic phases in the central Alps by K-Ar ages morphism of the Ottfjallet dolerites, Sarv Nappe, Swedish Caledonides: from Broken Hill, New South Wales: Implications for Âr/Âr age of hornblende: Journal of Geophysical Research, v. 69, p. .'>407-5421. Lithos, v. 16, p. 61-13. spectra and the thermal history of the region: Earth and Planetary Steiger, R. H., and Jäger, E., 1977. Subcommission on geochronology conven- Claesson, S„ Klingspor, I., and Stephens, M. B., 1983, U-Pb and Rb-Sr isotopic Science Letters, v. 55, p. 123-149. tion on the use of decay constants in geo- and cosmochron Mogy: Earth data on an Ordovicit n volcanic/sub volcanic complex from the Tjopasi Hossack, J. R„ Garton, M. R., and Nickelsen, R. P., 1985, The geological and Planetary Science Letters, v. 36, p. 359-362. Group, Koli Nappes. Swedish Caledonides: Geologiska Foreningens i section from the foreland up to the Jotun thrust sheet in the Valdres Stephens, M. B., 1980, Occurrence, nature, and tectonic significance of volcanic Stockholm Forhandl ngar, v. 105, p. 9-15. area. South Norway, in Gee, D. G., and Sturt, B. A., eds.. The Cal- and high-level intrusive rocks within the Swedish Caledonides, in Dallmeyer, R. D., 1975, Âr/Âr release spectra of biotite and hornblende edonide orogen — Scandinavia and related areas: Chichester, England, Wones, D. R., ed., The Caledonides in the USA: Blacksburg. Virginia from the Cortlandt ind Rosetown Plutons, New York, and their re- Wiley & Sons, p. 443-456. Polytechnic Institute and State University, Department or Geological gional implications: Journal of Geology, v. 83, p. 629-643. Jäger, E., 1979, Introduction to geochronology, in Jäger, E., and Hunziker, Sciences, Memoir 2, p. 289-298. Dallmeyer, R. D., and Ri»ers, T., 1983, Recognition of extraneous argon J. C., eds., Lectures in isotope geology: Berlin, Springer Verlag, p. 1-12. Stephens, M. B., and Roermund, H.L.M. van, 1984, Occurrencî of glauco- components through incremental-release Âr/Âr analysis of biotite Karig, D. E., 1974, Evolution of arc systems in the western Pacific: Annual phane and crossite in eclogites of the Seve Nappes, southern Norrbotten and hornblende across the Grenvillian metamorphic gradient in Review of Earth and Planetary Science, v. 2, p. 51-75. Caledonides, Sweden: Norsk Geologisk Tidsskrift, v. 69, p. 155-163. southwestern Labrador: Geochimica et Cosmochimica Acta, v. 47, Krogh, T. E., Mysen, B. O., and Davies, G. L., 1974, A Palaeozoic age for the Stephens, M. B., and Gee, D. G., 1985, A tectonic model for the evolution of p. 413-428. primary minerals of a Norwegian eclogite: Annual report of Geo- the eugeoclinal terranes in the central Scandinavian Caledonides, in Dallmeyer, R. D„ Gee, D. C., and Beckholmen, M„ 1983, Âr/Âr evidence physical Laboratory, Carnegie Institute, Washington, D.C., v. 73, Gee, D. G., and Sturt, B. A., eds., The Caledonide orogen — of superposed metamorphism in the central Scandinavian Caledonides, p. 575-576. Scandinavia and related areas: Chichester, England, Wiley & Sons, Jamtland, Sweden: Geological Society of America Abstracts with Pro- Kulling, O., 1982, Översikt öer södra Norrbottensfjällens kaledonberggrund: p. 953-978. grams, v. 15, p. 552. Sveriges geologiska Undersökning, ser. Ba, no. 26, 295 p. Strömberg, A., 1969, Initial Caledonian magmatism in Jamtland a:ea, Sweden, Dallmeyer, R. D„ Gee, D. «3., and Beckholmen, M„ 1985, Âr/Âr mineral Kumpulainen, R., 1980, Upper Proterozoic stratigraphy and depositional en- in Kay, M.. ed., North Atlantic — Geology and continental drift: Amer- age record of early Caledonian tectonothermal activity in the Balto- vironments of the Tossisfjället Group, Särv Nappe, southern Swedish ican Association of Petroleum Geologists Memoir 12, p. 3 '5-387. scandian miogeocl ne: American Journal of Science, v. 285, Caledonides: Geologiska Foreningens i Stockholm Forhandlingar, Sturt, B. A., 1984, The accretion of ophiolitic terrains in the Scandinavian p. 532-568. v. 102, p. 531-550. Caledonides: Geologie en Mijnbouw, v. 63, p. 201-212. Dalrymple, G. B.. and Lanjhere, M. A., 1971, Âr/Âr technique of K-Ar Kumpulainen, R., and Nystuen, J. P.. 1985, Late Proterozoic basin evolution Sturt, B. A., Pringle, I. R„ and Roberts, D., 1975, Caledonian nappe sequence dating: A comparison with the conventional technique: Earth and and Sedimentation in the westernmost part of Baltoscandia, in Gee, of Finnmark, northern Norway, and the timing of orogenic deformation Planetary Science U tters, v. 18, p. 300-308. D. G,, and Sturt, B. A., eds., The Caledonide orogen — Scandinavia and metamorphism: Geological Society of America Bu.letin, v. 86, Dalrymple, G. B„ Alexander, E. C., Lanphere, M. A., and Kraker, G. P., 1981, and related areas: Chichester, England, Wiley & Sons, p. 213-232. p. 710-718. Irradiation of samples for Âr/Âr dating using the Geological Survey McKerrow, W. S., Lambert, R. St.J., and Chamberlain, V. E., 1980, The Sturt, B. A., Pringle, I. R„ and Ramsay, D. M„ 1978, The Finnmarkian phase TRIGA reactor: U.S. Geological Survey Professional Paper 1176, 55 p. Ordovician, Silurian and Devonian time scales: Earth and Planetary of the Caledonian orogeny: Geological Society of London Journal, England, P. C., 1978, Some thermal considerations of the Alpine meta- Science Letters, v. 51, p. 1-8. v. 135, p. 597-610. morphism — Past, present and future: Tectonophysics, v. 40, p. 21-40. Oxburgh, E. R., and Turcotte, D. L., 1974, Thermal gradients and regional Toksöz, M. N.. and Bird, P., 1977, Modelling of temperatures in continental Furnes, H., Roberts, D., Sun, B. A., Thon, A., and Gale, G. H„ 1980, Ophio- metamorphism in overthrust terrains with special reference to the east- convergence zones: Tectonophysics, v. 41, p. 181-193. lite fragments in the Scandinavian Caledonides, in Panayiotou, A., ed., ern Alps: Schweizerische Mineralogische und Petrographische Mittei- Trouw, R. A.J., 1973, Structural geology of the Marsfjällen area, C Uedonides of Ophiolites: lnternat.onal Ophiolite Symposium, Proceedings, Cyprus lungen, v. 54, p. 641-662. Västerbotten, Sweden: Sveriges geologiska Undersokring, ser. C, 1979: Nicosia, Geological Survey of Cyprus, p. 582-600. Palmer, A. R„ compiler, 1983, The decade of North American geology: 1983 no. 689, 115 p. Furnes, H., Austerheim, H., Amaliksen, K. G., and Nordis, J., 1983, Evidence Geologic time scale: Geology, v. 11, p. $03-504. Williams, P. F„ and Zwart, H. J., 1977, A model for the development of the of an incipient early Caledonian (Cambrian) orogenic phase in south- Pankhurst, R. J., Moorbath, S., Rex, D. C., and Turner, G., 1973, Mineral age Seve-Köli Caledonian Nappe Complex, in Saxena, S. K. and Bhatta- western Norway: Gi »logical Magazine, v. 120, p. 607-612. patterns in ca 3.700 my old rock from West Greenland: Earth and charju, S., eds., Energetics of geological processes: Berlin, Springer- Gale, G. H., and Roberts, [)., 1972, Palaeogeographical implications of green- Planetary Science Letters, v. 20, p. 157-170. Verlag, p. 169-187. stone geochemistry in the southern Norwegian Caledonides: Nature Ramsay, D. M., 1973, Possible existence of a stillborn marginal ocean in the Zachrisson, E., 1969, Caledonian geology of northern Jämtlnnd-southem Physical Sciences, v. 238, p. 60-61. Caledonian orogenic belt of northwest Norway: Nature Physical Västerbotten: Sveriges geologiska Undersökning, ser. C, no. 644, 33 p. 1974, Trace element geochemistry of Norwegian Lower Palaeozoic Sciences, v. 245, p. 107-109. 1973, The westerly extension of Seve rocks within the Sev>:-Köli Nappe basic volcanics and its tectonic implications: Earth and Planetary Reymer, A.P.S., 1979, Investigations into the metamorphic nappes of the cen- Complex in the Scandinavian Caledonides: Geologiska Foreningens i Science Letters, v. 22, p. 380-390. tral Scandinavian Caledonides on the basis of Rb-Sr and K-Ar age Stockholm Förhandlingar, v. 95, p. 243-251. Gale, N. H., Beckinsale, R. D„ and Wadge, A. J., 1980, Discussion of a paper determinations [Ph.D. thesis]: Leiden. Netherlands, University of Zwart, H. J.. 1974, Structure and metamorphism in the Seve-Köli nappe com- by McKerrow, Lanbert and Chamberlain on the Ordovician, Silurian Leiden, 123 p. plex (Scandinavian Caledonides) and its implications concerning the and Devonian time scales: Earth and Planetary Science Letters, v. 51, Roddick, J. C., Cliff, R. A., and Rex, D. C., 1980, The evolution of excess formation of metamorphic nappes, in Belliere, J., and others, eds., p. 9-17. argon in alpine biotites — A 40Ar/39Ar analysis: Earth and Planetary Géologie des domaines crystallina Liège, Belgium, Societc Géologique, Gayer, R. A., Humphreys, U. J., Binns, R. E., and Chapman, T., 1985, Tectonic Science Letters, v. 48, p. 185-208. p. 129-144. modelling of the Fiimmark and Troms Caledonides based on high level Robins, B.. and Gardner, P. M., 1975, The magmatic evolution of the Seiland igneous rock geochtmistry, in Gee, D. G., and Sum, B. A., eds.. The Province, and Caledonian plate boundaries in northern Norway: Earth Caledonide orogen — Scandinavia and related areas: Chichester, Eng- and Planetary Science Letters, v. 26, p. 167-178. land, Wiley & Sons, p. 931-951. Roberts, D., and Gale, G. H., 1978, The Caledonian Appalachian lapetus MANUSCRIPT RECEIVED BY TOESOCIET Y FEBRUARY II, 1985 Gee, D. G., 1975, A tectoiic model for the central part of the Scandinavian Ocean, in Tarling, D. H., ed„ Evolution of the Earth's crust: London, REVISED MANUSCRIPT RECEIVED JUNE 24, 1985 Caledonides: American Journal of Science, v. 275A, p. 468-515. Academic Press, p. 255-342 MANUSCRIPT ACCEPTED JUNE 27, 1985

Prin.ed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/26/3445033/i0016-7606-97-1-26.pdf by guest on 30 September 2021