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. 26 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,