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40Ar39Ar mineral age record of a polyorogenic evolution within the Seve and Köli nappes, Trøndelag, Norway
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Tectonophysics, 179 (1990) 199-226
Elsevier Science Publishers B.V., Amsterdam - printed in The Netherlands
40Ar/39Armineral age record of a polyorogenic evolution within the Seve and Kijli nappes, Trmdelag, Norway R.D. Dallmeyer Department of Geology, University of Georgia, Athens, GA 30602 (U.S.A.)
(Received November 9,1989; accepted January 10.1990)
ABSTRACT Dallmeyer, R.D., 1990. ‘?Ar/“Ar mineral age record of a polyorogenic evolution within the Seve and Koli nappes, Trandelag, Norway. Tectonophysics, 179: 199-226. In the central Scandinavian Caledonides, eugeoclinal terranes represented in the KU Nappes were thrust > 500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe Complex are suspected to have been derived from the outer margin of continent Baltica. Hornblende within ampbibolite at one locality within westernmost exposures of Seve Nappe units in Trandelag, Norway, displays an internally discordant %r/39Ar age spectrum which suggests a polyphase thermal evolution including: (1) initial post-metamorphic cooling through argon retention temperatures at c. 475-480 Ma; and, (2) partial rejuvenation during a Silurian (c. 400-425 Ma) thermal overprint. Muscovite within the exposure was more extensively rejuvenated during the Silurian overprint. At twelve other Trondelag sample locations, hornblende, muscovite and biotite within both the Seve and Kirli Nappe Complexes record post-metamorphic cooling ages between 420 and 435 Ma and display no direct evidence of a pre-Silurian thermal record. However, the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre-Silurian tectonothermal history. Comparison with %r/39Ar mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenie evolution consisting of: (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the Late Cambrian-Early Ordovician over a west-dipping subduction zone; related arc volcanic activity occurred in more outboard, western sectors; (2) ductile imbrication of high- and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltica continental margin; and, (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian. Crystalline Baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high-temperature eclogites. Previously reported crystallization ages of c. 425 Ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures. Following eastward nappe transport, the basement experienced rapid uplift and resultant adiabatic decompression. This was followed by rapid cooling through argon retention temperatures at 390-400 Ma.
Introduction The structure of the Scandinavian Caledonides is dominated by a variety of major nappe complexes which were emplaced onto the Baltoscandian platform during early to middle Paleozoic closure of the Iapetus Ocean (e.g., Harland and Gayer, 1972; Gee, 1975a; Roberts and Gale, 1978; Stephens and Gee, 1985). The nappes 0040-1951/90/%03.50 Q 1990 - Elsevier Science Publishers B.V.
are of varying metamorphic grade and display marked differences in the complexity of internal deformation. As a result, correlation of specific metamorphic, deformational, and thrusting events along the length of the orogen has been uncertain and controversial (e.g., Gee and Roberts, 1983; Roberts and Gee, 1985). Lower tectonic units within central and southem parts of the Scandinavian Caledonides are
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represented by low-grade, metasedimentary successions of late Proterozoic to Middle Silurian age. The older successions originated along the Baltoscandian continental margin during separation of Baltica and Laurentia. Younger sequences were deposited during closure of the Iapetus oceanic tract. Si~fic~t thicknesses of wester& derived elastic material of Middle Ordovician and EariyMiddle Silurian age in these successions provide evidence of two periods of outboard instability along the Baltoscandian margin (e.g., Gee, 1975b). Early attempts to describe the tectonic evolution of central parts of the Caledonian orogen noted this instability (e.g., Gee, 1975a), however most of the internal deformation, metamorphism, and nappe translation has been considered to be of Middle Silurian through Early Devonian (Scandian) age (e.g., Roberts, 1971; Nicholson, 1974; Gee and Wilson, 1974; Gee, 1975b). In more northerly parts of the orogen, sedimentary sequences within lower tectonic units only range up into the Tremadoc and therefore provide little c~onolo~c control for translation of overlying nappes (e.g., Sturt et al., 1975). However, deformed and variably metamorphosed sequences within several of the higher allochthons are locally intruded by syn-tectonic plutons which record Rb-Sr whole-rock isochron dates of 530-490 Ma * (Sturt et al., 1975, 1978). Host metasedimentary sequences are thought to be correlative with others developed elsewhere within outer parts of the Baltoscandian miogeocline, and the radiometric controls were therefore interpreted to indicate that a major period of late Cambrian-Early Ordovician polyphase folding, metamorphism, and initial nappe imbrication occurred along this northern segment of the continental margin of Baltica. This event has been termed the Finnmarkian Orogeny (e.g., Sturt et al., 1975, 1978). The suggested diachronism of tectonothermal activity (Finnmarkian to Scandian) along the early to middle Paleozoic continental margin of Baltoscandia has been difficult to reconcile with
* All radiometric ages listed in this report are based on the
decay constants and isotopic abundance ratios presented by Steiger and J&ger (1977).
R.D.
DALLMEYER
otherwise nearly continuous sedimentological, strati~ap~c, and tectonic trends. Geochronological results in widely separated parts of the central Scandinavian Caledonides suggest that preScandian tectonothermal activity may have influenced parts of the tectonostratigraphy (see review in Dallmeyer, 1988). The evidence includes: (1) Ordovician Rb-Sr whole-rock dates reported for various high-grade metamorphic units within several nappe complexes (Reymer, 1979; Claesson, 1979); (2) an Early Ordovician, Rb-Sr thin-slab isochron date obtained from mylonitic rocks developed along an internal thrust within one of the major nappe complexes (Claesson, 1980); and, (3) 40!/39Ar hornblende ages reported from several areas (e.g., Dalhneyer et al., 1985; Dallmeyer and Gee, 1986, 1988). Although the suggested preScandian erogenic activity is consistent with the record of outboard instability provided by westerly derived, Ordovician elastic sequences within the Baltoscandian miogeocline, the regional extent and intensity of Scandian vs. Finnmarkian tectonothe~~ activity has been uncertain. Results of 40Ar/39Ar mineral dating studies in central J%mtland (Dallmeyer et al., 1985; Dallmeyer and Gee, 1988) and Norrbotten (Dallmeyer and Gee, 1986) Counties of Sweden suggest that Scandian metamorphic temperatures in some of the higher grade units did not exceed c. 500°C. This indicates that the observed high-grade assemblages must have developed during an earlier tectonothermal event which affected both outer parts of the Baltoscandian miogeocline and some of the more outboard (western) eugeoclinal terranes. The thermal evolution reflected by these ‘%r/39Ar mineral dates suggests that ocean-directed (westward) subduction was active outboard of the Baltoscandian margin by at least the Late Cambrian. Fauna1 ch~acte~stics of structurally high ~lochthons suggest that this tectonic activity occurred while Baltica and Laurentia were separated by a considerable expanse of the Iapetus Ocean (e.g., Bruton and Bockelie, 1980). A comprehensive ge~~onolo~~al research program is underway to more fully document the extent and character of the tectonothermal record within the central Scandinavian Caledonides. This report presents new 40Ar/39Ar mineral dates for
POLYOROGENIC
EVOLUTION
WlTHlN
SEVE
. . . q
UPPER
Metoredimcn+ory
Sevt Noppe Complex
ALLOCHTHON
Precombrlon
Basement
ALLOCHTHON
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MIDDLE
Sedimentary
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PARAUTOCHTHON-
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ALLOCHTHON
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AND KijLI NAPPES, NORWAY
q
Nopp
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Cover
Basement
LOWER
AUTOCHTHON
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Fig. 1. Regional geologic setting of the central Scandinavian Caledonides (adapted from Gee et al., 1985). Study area (Fig. 2) is outlined.
~~hthonous rocks of both B~tosc~~an and eugeoclinal affinities exposed in the area between Trondheimsfjorden and the international border, west-central Trsndelag County, Norway (Fig. 1). These results bear directly on both the chronology and nature of praScandian orogenesis, and also provide controls for understanding the early to middle Paleozoic t~tonothe~~ evolution of the Baltoscandian miogeocline. Regional geologic setting The Scandinavian Caledonides may be described in terms of several major tectonic units (Kulling in Strand and Kulhng, 1972; Gee and Zachrisson, 1979; Roberts and Gee, 1985), including: (1) autochthonous and parautochthonous sequences; (2) Lower, Middle, Upper, and Uppermost Allochthons; and, (3) ueoautochthonous, molasse deposits of Early to Middle Devonian
age. The dist~bution of these units witbin central parts of the orogen is shown in Fig. 1. Although the various allochthons generally thin westward, most may be traced across the entire orogen to the Norwegian coast. The entire nappe sequence was affected by regional north-south folding following structural emplacement onto the Baltoscandian platform. The overall t~tonostratigraphy of the central Scandinavian Caledomdes has been treated at length elsewhere (e.g., Gee, 1975b; Dyrehus et al., 1980; Roberts and Gee, 1985) and will not be reviewed here. Only those relationships particularly relevant to interpretation of the present geechronological results from the Upper Allochthon will be discussed. The Upper Allochthon is an extremely heterogeneous, internally imbricated structural complex. Basal tectonic units are composed of a westward thinning, up to 5 km thick assemblage of thrust sheets referred to as the Seve Nappe Complex
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(e.g., Zachrisson, 1969, 1973; Trouw, 1973; Zwart, 1974). This complex consist largely of amphibolite and higher grade metasedimentary and metaigneous units which are locally in tectonic contact with penetratively deformed and severely retrogressed middle to late Proterozoic crystalline basement rocks (Reymer et al., 1980; Claesson, 1981). Tectonic contacts between various structural units within lower parts of the Upper Allochthon are frequently marked by development of high-temperature ductile shear zones (e.g., Trouw, 1973; Ambom, 1980; Sjestrom, 1983). Variably retrogressed eclogites have been described from several structural units of the Seve Nappe Complex (e.g., Van Roermund and Bakker, 1984; Stephens and Roermund, 1984; Van Roermund, 1985; Andreasson et al., 1985). In Wsterbotten County, Sweden, tectonic units of high-grade gneissic rocks are structurally underlain and overlain by lower grade schists and amphibolites. The resultant three subdivisions have been informally termed the eastern, central, and western Seve belts (Trouw, 1973). The overall chemical character of basic rocks within lower parts of the Seve nappes is similar to that of dolerites in the S&-v Nappes of the underlying Middle Allochthon (Solyom et al., 1979a, 1979b). Metasedimentary protoliths in the two tectonic units are also comparable. It is therefore likely that most of the rocks now constituting the Seve Nappe Complex originated in distal, latest Proterozoic to Early Cambrian environments along the continental margin of Baltoscandia (Gee, 1975b; Dallmeyer and Gee, 1986). Overlying the Seve Nappe Complex is a succession of predominantly greenschist or lower amphibolite grade, metavolcanic and metasedimentary units of the Kiili Nappe Complex (Zachrisson, 1969; Stephens, 1980). Fossil controls suggest an Early Ordovician to Early Silurian age for some protoliths. Several Middle Ordovician units within lower parts of the Kiili Nappe Complex are characterized by lithic successions that can be directly correlated with the Baltoscandian margin (Gee, 1975b). The Early Ordovician successions appear to be unrelated to Baltica and were apparently derived from a variety of eugeoclinal terranes located within the Iapetus Oceanic tract. These exotic units were accreted to the Bal-
R.D. DALLMEYER
toscandian margin during the early through middle ‘Paleozoic (Stephens and Gee, 1985). The Koli Nappe Complex in the central Scandinavian Caledonides may be divided into three groups of related thrust sheets referred to as the Lower, Middle, and Upper Koli Nappes (Stephens, 1980). Lower Koli Nappes are dominated by arc-related volcanic rocks (Stephens, 1980) which record U-Pb zircon crystallization dates of c. 490 Ma (Claesson et al., 1983). These volcanic sequences pass upward into Middle Ordovician graywackes and fanglomerates which are conformably overlain by Upper Ordovician sandstones and limestones and Lower Silurian shales and graywackes (Kulling, 1933). The Middle Koli is represented by a sequence of nappes in which related successions are typically inverted. Arc volcanites, at least locally of Early Ordovician age, are overlain by volcaniclastic conglomerates and/or graywackes and calcareous phyllites. Eastern exposures of the metasedimentary rocks are intruded by metamorphosed gabbro plutons with MORB affinities; likewise the arc-volcanites contain a late (but pre-erogenic) intrusive suite of MORB basic rocks (Stephens, 1982). Post-arc sedimentary units in the rifted-arc association are probably of Middle to Late Ordovician age; correlations southward into eastern Trondelag County, Norway, suggest that they pass up into Llandovery black shales and then graywackes and fanglomerates. The Upper Kbli Nappes are locally represented by variably fragmented ophiolitic associations (Fumes et al., 1980; Grenne et al., 1980; Sturt, 1984) which are unconformably overlain by Early Ordovician sedimentary successions containing faunas of North American affinity (Bruton and Bockelie, 1980; Spjeldnaes, 1985). Thus, at least some protoliths within higher tectonic units of the Upper Allochthon appear to have been derived from euogeoclinal terranes which were more closely related to Laurentia than to Baltica. Locally within higher structural units of the Upper Allochthon, a major pre-Ashgill unconformity reflects significant pre-Scandian polydeformation and metamorphsim (Sturt, 1984; Ryan and Sturt, 1985). In addition, syn- to late-tectonic trondhjemites in Middle and Upper Koli Nappe
POLYOROOENIC EVOLUTION WITHIN SEVE AND KiiLI NAPPES, NORWAY
units have yielded 450-510 Ma U-Pb zircon and/or Rb-Sr whole-rock isochron ages (Klingspor and Gee, 1981). Geology of the sample area Introduction ““hr/39Ar incremental-release ages have been determined for minerals separated from rocks within the Seve and Kiili Nappe Complexes exposed in the area between Trondheimsfjorden and the international border in west-central Trsndelag County, Norway (Fig. 1). Summaries of the overall geologic character of the central Scandinavian Caledonides have been presented by many workers, including Roberts (1978), Wolff (1980), Gee and Kumpulainen (1980), Gee and Wolf (1981), and Gee et al. (1985). Therefore, only a brief summary of geologic relationships within the present study area (and adjacent portions of west-central JZlmtland County, Sweden) is present here. Baltoscandian Nappe Complexes Representatives of the Lower, Middle, and Upper Allochthons are found along the transect (Fig. 1). These are deformed into a series of upright, generally north-south trending folds which have gently undulating axes. As a result of this folding, higher portions of the tectonostratigraphy are preserved along doubly-plunging synforms (Are, Helags-Tlforsen, and Trsndelag structures). Lower parts of the tectonostratigraphy are exposed within antiformal culminations, including the Mullfj%llet, Skardora, Tomme&, and GrongOlden structures. A variety of Precambrian crystalline rocks constitute an autochthonous foundation for the central Scandinavian Caledonides. Tbis terrane is nonconformably overlain by a thin sequence of autochthonous Cambrian shales and Ordovician limestones. Precambrian porphyries and granitic rocks are exposed in several windows in western I&inland and eastern Trondelag. Along the culmination of the Grong-Olden structure the Precambrian terrane may be traced continuously
203
for c. 200 km from western Jtitland to the Norwegian coast. This terrane is overlain by successions of quart&e, radioactive black shales, and limestones which find direct correlatives in autochthonous successions exposed east of the thrust front. Rocks of the Lower Allochthon exposed along the eastern margin of the orogen are dominated by imbricated, low-grade metasedimentary formations of the Jtitland Supergroup which range up to Early Silurian (Llandovery) age. The amount of basement incorporated in the Lower Allochthon increases westward from the Are synform, and overlying metasedimentary successions are restricted to minor quart&es and local exposures of radioactive black phyllites and limestones. The Middle Allochthon is intermittently exposed in the study area. It is a composite nappe complex composed of various late Proterozoic elastic sequences which are imbricated with tectonic units of variably deformed and retrogressed Middle Proterozoic crystalline basement rocks. Upper composite units (S&v Nappes) include basal basement slices which are cut by dolerite dikes. These are tectonically overlain by feldspathic metasandstones which are intruded by similar dolerite dikes. The various units of the Middle Allochthon are all in variably ductile fault contact. Claesson (1980) reported a Rb-Sr thinslab isochron age of 485 f 50 Ma for mylonitic rocks developed along one of these internal Middle Allochthon thrusts in southern Jlltland, suggesting that initial assembly of the nappe complex may have begun in the Ordovician rather than in the Silurian as previously accepted. However, basal tectonic units of the Middle Allochthon locally overrode Silurian metasedimentary successions of the Lower Allochthon during emplacement into its present position on the Baltoscandian platform. The Middle Allochthon markedly thins westward. This is accompanied by an increase in the grade of metamorphism and the extent of internal deformation. Gee (1978) and Kautsky (1978) demonstrated that Middle Allochthon units (Offerdal and S&v Nappes) may be traced into Norway, although they undergo marked local changes in thickness, often being represented by only isolated tectonic lenses. In eastern Trondelag, Middle Al-
204
lochthon rocks are included in the Remsklepp Nappe of Wolff (1976, 1980). In addition, in the Gppdal area of southwestern Tmndelag, KrilI (1980) has defined the Risberget and Saetre Nappes, which he correlates with lithic sequences of the Middle Allochthon exposed in Sweden. Krill has demonstrated that following their emplacement onto the Baltoscandian margin, these western parts of the Middle Allochthon were complexly interfolded within underlying parautochthonous crystalline basement. Hornblende and muscovite within both basement and cover structural units exposed northwest of Trondheimsforden record similar 40Ar/39Ar plateau ages ranging between c. 390 and 400 Ma suggesting relatively rapid post-Scandian metamorphic cooling (Dallmeyer et al., 1990). Upper Allochthon Seve Nappe Complex
Exposures of lower parts of the Upper Allochthon are represented by the Seve Nappe Complex which is comprised by three contrasting tectonic units. The lowermost structural unit consists largely of polydeformed psammitic schists and amphibolites. These record upper greenschist to upper ~p~bo~te facies in regional rnet~o~~c assemblages, and locally complex textures suggest a polymetamorphic evolution. The middle unit is dominated by a polydeformed, granulite facies complex and minor metabasic rocks (Areskutan and Snasahijgarna Nappes). The contact between this terrane and underlying Seve met~ed~ent~ units is tectonic and marked by development of high-temperature mylonitic rocks (e.g., Ambom 1980; SjiistriSm 1983a, 1983b). The uppermost tectonic unit of the Seve Complex is only preserved in the Hand51 area, and consists of polydeformed amphibolite and kyanite schists. Geochronologic work by Claesson (1981) indicates a middle Proterozoic history for at least a part of the gneissic terrane within the Areskutan Nappe. Rb-Sr and U-Pb zircon studies by Claesson in this area further suggest that the Seve rocks underwent high-grade Scan&an metamorphism between c. 415 and 390 Ma. Stepwise zircon dissolution experiments indicate the possibility an
R.D.
DALLMEYER
earlier Caledonian thermal event. Dallmeyer et al. (1985) presented 40Ar/39Ar results from the Seve Nappe Complex exposed in J%mtland. Hornblende from various structural units of the Seve Nappe Complex exposed in the Are synform (Fig. 1) typically display internally discordant age spectra as a result of the incorporation of extraneous argon. However, variably well-defined plateau dates of 455 f 10 Ma to 458 rt 9 Ma are generally recorded. Biotite and muscovite from Seve Nappes exposed along the international border in the T&mforsen synform (Fig. 1) yield plateau dates of 408 f 6 Ma and 415 f 7 Ma respectively. Kiili Nappe Complex
Upper parts of the Upper Allochthon are represented by the Kijli Nappe Complex. Several distinct structural units have been defined along the Helags-Tannforsen synform (e.g., Beckhohnen, 1978, 1982; Sjiistriim, 1983a, 1983b). The lowest tectonic unit (Duved Nappe) is separated from underlying Seve Nappes by a major mylonite zone and consists of a variety of low-grade, polydeformed metasedimentary and minor metavolcanic rocks. Although diagnostic fossils are lacking, the overall lithic character and stratigraphic succession is similar to that of fossil-bearing, Upper Ordovician-Lower Silurian Kiili sequences farther north in tisterbotten County, Sweden (e.g., Zachrisson, 1969). The lower sequence of the Tglnnforsen Kiili is in tectonic contact with overlying structural units which consist largely of polydeformed, generally low-grade phyllite, calcareous mica schist, and hornblende g~bensc~effer. Several different nappe units have been mapped with intervening tectonic contacts marked by development of superposed ductile and brittle fabrics which suggest a polygenetic internal imbrication history. The TZnnforsen Koli Nappes thin westward and are tectonically overlain by the Trondheim Nappe Complex along the western limb of the Skardora antiform. However, lowgrade, polydeformed talc-phyllites and talc-schists of ~~forsen-tie reappear in western Trondelag near Levanger (Figs. 1 and 2). Rocks of the Trondheim Nappe Complex compose the highest tectonic units exposed within the present study area and represent upper structural parts of the
205
POLYOROGENIC EVOLUTION WITHIN SEVE AND KijLl NAPPES, NORWAY
Stffren Nappa
% 2
Gulo Noppe 3 P Mer(tker Nappe c TInnfors
ond
Levooger Noppes Seve
Noppe
Complex MIDDLE ALLDCHTNON LOWER ALLOCHTHON * . I!!!!
Sedimentury Cover Precombrion Basement
AUTOCHTHONPARAUTOCHTHON
Fig. 2. Simplified geologic map of east-central ‘lixmdelag (adapted from Gee et al., 1985). ‘%r/39Ar sample localities are numbered (Z-13).
Upper ~~hthon. aloud the internal structure of the nappe complex has long been controversial, most workers agree that a generally north-trending, pr~o~~tly rnet~~~ent~ unit (Gula Nappe) is bordered on the west and the east by metasedimentary and metavolcanic units of generally lower grade (Fig. 2). The western sequences constitute the Stncren Nappe, and these locally contain fossil-bearing rocks of Arenig-Ashgill age. The eastern succession is represented by the Mer&ker Nappe, with uppermost units containing graptolite-bearing, Llandovery phylhtes. Many units of the Gula, Steren, and Mer&er Nappes record polydeformational fabrics and locally complex metamorphic textures; therefore, details of the t~tono~e~~ evolution of the Trondheim Nappe Complex are uncertain. Clearly some of the metamorphism and deformation must be of Scandian age because fo~~-~a~g sequences of Early Silurian age are affected. However, numerous internal tectonic contacts preclude assigning alI the t~tonothe~ record to this organic event, and, in fact, preliminary Rb-Sr whole rock and U-Pb zircon analyses of trondhjemite dikes cutting poiydeformed and previously rnet~o~hos~ rocks of the Gula nappe suggest Ordovician crystallization ages (Rlingspor and Gee, 1981). Biotite from these dies yield conventional K-Ar dates of
c. 400 Ma (Wilson et al., 1973) su~~~g a prolonged or polyphase thermal evolution for the Gula Nappe. In addition, Wilson et al. (1973) reported inventions K-Ar dates for minerals from the Meraker Nappe. Hornblende dates range between 426 and 550 Ma whereas muscovite and biotite ages range between 411 and 444 Ma. Because of possible incorporation of extraneous argon components during Scandian metamorphism, unequivocal ~t~retation of these data has not been possible. However, the older homblende dates may be suggestive of a significant pm-Scandian t~tono~e~~ record. DalImeyer et al. (1985) reported 4oAr/39Ar ages for minerals separated from several of the lower structural units of the Kijli Nappe Complex exposed along the T&mforsen synform (Fig. 1). Hornblende from the Middagsfj~let Nappe yielded variably discordant age spectra as a result of widespread incorporation of extraneous argon components. Ages systematically decreased t~o~~out most analysis to yield variably well-defined plateau ages which range from 475 -fr 10 Ma to 510 f 11 Ma. Coexisting biotite shows widely variable plateau dates (413 of:6 Ma to 524 f 9 Ma) as a result of incorporation of extraneous argon components. Muscovite from these units records mutually concordant plateau dates between 409 f
206
7 Ma to 425 f 7 Ma. Hornblende and biotite from an exposure within a structurally lower portion of the Middagsfjallet Nappe yielded mutually similar, well-defined plateau dates of 414 + 9 Ma and 418 + 6 Ma respectively. Biotites within a lower tectonic unit of the Tginnforsen Kijli (Gevsjisn Nappe) yielded plateau dates of 415 f 6 Ma to 427 f 7 Ma. Tectonothermal evolution Field relationships and the geochronology previously reported for the Upper Allochthon exposed in eastern segments of the central Scandinavian Caledonides have helped to constrain the tectonothermal evolution. 40Ar/3gAr hornblende ages from lower (Seve) and upper (Kiili) parts of the Upper Allochthon suggest a complex, polyphase pre-Scandian thermal history. The 455-458 Ma hornblende dates from the Seve nappes clearly indicate a younger period of cooling through hornblende closure temperatures than the 475-510 Ma dates in the Kiili Nappes. The thermal conditions of Caledonian rne~o~~srn in both areas do not appear to have been greatly in excess of the temperatures required for intracrystalline retention of argon in hornblende, and it is unlikely that the difference in dates reflects diachronous cooling following a single tectonotbermal event. It is more likely that separate pre-Scandian tectonothermal events are recorded. The effects of Scandian orogenesis were regionally of sufficient magnitude to reset all mica argon systems and locally modify or totally reset hornblende argon systems. There appears to have been a significant diachronism between post-Scandian metamorphic cooling in eastern parts of the area (427-429 Ma biotite dates from the Seve nappes at Are) and coastal areas (390-400 Ma hornblende and muscovite dates). This is consistent with the east to west younging trend in post-Scandian thermal decay outlined for more northerly parts of the orogen. In the central Scandinavian Caledonides, post-Scandian cooling although apparently diachronous, was everywhere relatively rapid because similar dates are recorded by minerals with widely different argon closure temperatures (e.g., hornblende and muscovite in the Tginnforsen Koli and in the coastal sections).
RD.
DALLMEYER
The 40Ar/3gAr plateau ages recorded by homblende within lower portions of the Upper Allochthon exposed in the Ttinforsen Synform reflect significant erogenic activity during and/or just prior to 475-510 Ma. A 475-510 Ma range corresponds to an Early Ordovician interval on most time-scale calibrations, perhaps extending into the latest Cambrian. By contrast, lower tectonic units of the Ttinnforsen Kijli Complex (Duved Nappe) contain metasedimentary successions which appear to be direct lithostratigraphic correlatives with Ashes-Ll~dove~ Koli sequences exposed at similar tectonic levels elsewhere in the Scandinavian Caledonides. These Tannforsen sequences must therefore have been tectonically imbricated with the older metamorphic terrane during a late- to post-Llandovery phase of assembly of the Koli Nappe Complex. All Koli Nappes exposed in the nnnforsen Synform record a 410425 Ma overprint, although it is of variable grade. According to most time-scale calibrations this interval corresponds to the Middle-Late Silurian (We~~k-Proof), and suggests that this thermal event was likely associated with late phases of tectonic imbrication of the Kbli Nappe Complex. This is consistent with development of mylonitic zones along internal thrust contacts within the K&i Nappe Complex which indicate that at least some components of thrust translation occurred under elevated temperatures. However, tectonic movement appears to have outlasted maintenance of elevated metamorphic conditions because most internal thrust contacts display evidence of later brittle reactivation (Betoken, 1982). This is consistent with the rapid Late Silurian post-metamorphic cooling suggested by the “OAr/3gAr mineral dates, and likely reflects final phases of assembly of the Koli Nappe Complex during its translation to higher structural levels within the succession of Caledonian allochthons. The tectonothermal evolution of the Seve Nappe Complex in eastern sectors of the central Scandinavian Caledonides also appears to have been polyphase. High-grade, early Caledonian metamorphic crystallization must have occurred prior to cooling through hornblende retention temperatures between 455 and 460 Ma. Imbrication of basement (gram&e terrane) and cover compo-
POLYOROGENIC
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NAPPFS,
NORWAY
nents of the Seve Nappe Complex must have occurred earlier because Arnbom (1980) provided evidence that high temperatures (> 55OOC) were maintained during development of mylonitic rocks along ~te~e~g tectonic contacts. These results combine to suggest that cooling through the temperatures appropriate for intracrystalline retention of argon in hornblende likely occurred when the contrasting metamorphic terranes were translated to higher crustal levels during initial tectonic assembly of basement and cover components of the Seve Nappe Complex. Both nappe units appear to have experienced a similar thermal evolution following their initial imbrication. Previously reported geochronologic results have not permitted resolution of the effects of Scandian vs. earlier tectonothermal events in western exposures of the Seve Nappe Complex or structurally higher of the Koli Nappe Complex exposed in Trondelag. %r/39Ar mineral ages from this area are presented herein and provide important new constraints for the tectonothermal chronology of orogenesis recorded within the central Scandinavian Caledonides. Analytical methods The techniques used during ““Ar/39Ar analysis of the Trondelag samples generally followed those described in detail by Dalhneyer and Keppie (1987). Mineral concentrates were wrapped in aluminum-foil packets, encapsulated in sealed quartz vials, and irradiated for 40 hrs at 1000 Kw in the central thimble position of the U.S. Geological Survey TRIGA reactor in Denver, Colorado. Variations in the flwr of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including MMhb-1 (Alexander et al., 1978). The samples were incrementally heated until fused with an RF generator. Each heating step was maintained for 30 min. Measured isotopic ratios were corrected for the effects of mass discrimination, total-system blank levels, and interfering isotopes produced during irradiation (with factors reported by Dalrymple and others, 1981 for the reactor used in the pres-
207
ent study). Apparent %r/39Ar ages were calculated from the corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and 3ager (1977). Two categories of unce~~nti~ are en~~tered in ‘%r/ 39Arincremental-release dating. One group involves intralaboratory uncertainties related to measurement of the isotopic ratios used in the age equation. The other group considers interlaboratory ~ce~~nties in the other parameters used in the age equation (monitor age, J-value determination, etc.), and are the same for each gas increment evolved from a particular sample. Therefore, to evaluate the significance of potential incremental age variations within a sample, only intralaboratory uncertainties should be considered. These are reported here and have been calculated by statistical propagation of uncertainties associated with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are c. 1.25-1.5% of the quoted age. Total-gas ages have been computed for each sample by appropriate weighting of the age and percent 39Ar released witbin each temperature increment. A “plateau” is considered to be defined if the following criteria are met: (1) increments have similar apparent K/Ca ratios; and (2) ages recorded by two or more contiguous gas fractions each representing > 4% of the total 39Arevolved (and together constitu~g > 50% of the total quantity of 39Ar evolved) are mutually similar within a f 1% intralaboratory uncertainty. Analysis of the MMhb-1 monitor indicate that apparent K/Ca ratios may be calculated through the relationship of 0.518 ( f 0.~5) X 39At/ 37Ar corrected. The analysis of the amphibole concentrates have been plotted on 36Ar/40Ar vs. 39h/40Ar isotope correlation diagrams (Roddick et al., 1980; Radicati de Brozolo et al., 1981). Regression techniques followed the methods of York (1969). The mean square of the weighted deviated (MSWD) has been used to evaluate isotopic correlations. Roddick (1978) suggests that an MSWD > 2.5 indicates scatter about a correlation line greater than that which can be explained only by experimental errors.
208
Results
Twelve hornblende, six muscovite, and four biotite concentrates have been prepared from various structural units of the Upper Allochthon (Seve and Koli Nappe Complexes) exposed in eastcentral Trondelag (sample locations shown in Fig. 2 and described in the Appendix). These have been analyzed using ‘?Ar/ 39Arincremental-release techniques. The analytical data are listed in Tables 1, 3 and 4 and are portrayed as age spectra in Figs. 3-10. Isotope correlation calculations from the amphibole data are listed in Table 2. Apparent K/Ca ratios are relatively small and display considerable intrasample variations in the hornblende analyses (Figs. 3, 5, 6 and 8). Apparent K/Ca ratios are large and show no significant or systematic variations throughout the mica analyses. Therefore they are not presented with the muscovite or biotite age spectra.
Seve Nappe Complex (1) Hornblende: Amphibole concentrates were prepared from samples of foliated amphibolite collected within exposures of the Seve Nappe Complex along the southeastern limb of the Grong-Olden antiform (location 1, Fig. 2) and along the western limb of the Tommerbs antiform (locations 2-4, Fig. 2. These display variably discordant age spectra (Table 1, Fig. 3) which define total-gas ages ranging between 461.8 + 3.6 Ma (I ) and 525.6 f 5.2 Ma (2). Low-temperature gas fractions display considerable variation in apparent ages. These are mathed by intrasample fluctuations in apparent K/Ca ratios which suggests experimental evolution of argon from compositionalIy distinct, relatively non-retentive phases. These could be represented by very minor, optically undetectable mineralogic contaminants in the hornblende concentrates and/or petrographically unresolvable exsolution or compositional zonation within constituent amphibole grains. In general, most gas fractions evolved from the Seve homblende concentrates at intermediate and high experimental temperatures are characterized by similar intrasample apparent K/Ca ratios, indicating
R.D. DALLMEY
ER
evolution of gas occurred from compositionally uniform amphibole phases. However, markedly different apparent age relationships are observed in these portions of the four experiments. The 775 “-850 o C increments evolved from sample 1 (comprising c. 79% of the total gas evolution) record similar apparent ages which define a plateau of 439.9 f 2.4 Ma. The plateau data yield a well-defined isotope correlation (MSWD < 2.0; Table 2) which defines an inverse ordinate intercept of 395.2 &-5.1. This is larger than the 295.5 ‘?Ar/36Ar ratio in the present-day atmosphere, and suggests that minor extraneous ((‘excess”) argon is present within constituent homblende grains. Using the inverse abscissa intercept (40Ar/39Ar) in the age equation yields a plateau isotope correlation age of 429.3 f 0.9 Ma. Because this calculation does not depend upon assumption of a modem-day “OAr/36Ar ratio it is considered more reliable than the plateau age directly calculated from the analytical data. The 429 Ma age is interpreted to date the last cooling through those temperatures required for intracrystalline retention of argon within constituent hornblende grains. Harrison (1981) indicated that closure temperatures for argon systems within igneous hornblende are not significantly affected by compositional variations. He suggested that values of 500 f 25°C are appropriate in the range of cooling rates likely to be encountered in most geologic settings. Samples 2 and 3 display similarly discordant spectra in which apparent ages systematically decrease throughout intermediate-temperature parts of the analyses to a high-temperature minimum and then increase slightly in the fusion increments. Similar types of “saddle-shaped” discordance in hornblende 40Ar/39Ar age spectra have been interpreted by Dallmeyer (1975), Harrison and McDougall (1981), Dallmeyer and Rivers (1983) and Dallmeyer et al. (1985) to reflect experimental liberation of gas with large and variable components of extraneous (excess) 40Ar relative to intracrystalline radiogenic ‘?Ar. The 740 “-875°C increments evolved from sample 2 (constituting c. 78% of the total) yield a well-defined isotope correlation corresponding to an age of 435.4 + 4.1 Ma (inverse ordinate intercept of 545.9 zt 12.2). The 815”C-fusion increments of sample 3 (c. 66%
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
KijLl
NAPPES,
209
NORWAY
TABLE 1 ‘OAr/39Ar analytical data for incremental heating experiments on hornblende
concentrates
from the Seve and Koli Nappe
Complexes, central Scandinavian Caledonides, Trondelag, Norway 39Ar
!%@Ar
36Arc,
temp.
(W of
non-
(W)
(“C)
total)
atmos. ’
Release
( @Ar/ 39Ar) a
( 36Ar/ 39Ar) a
( “Ar/
39Ar) b
Apparent age d (Me)
Seve Nappe Complex
Sample I: J = 0.008708 500
745.01
1.60154
36.340
0.26
36.87
0.62
2233.0 f 201.9
600
726.99
1.95306
52.042
0.33
21.19
0.72
1569.6 f 144.3
700
495.11
1.49331
49.552
0.70
11.68
0.90
755.1 f
81.3
750
90.08
0.20865
48.464
2.96
35.87
6.32
459.8 f
10.1
775
47.25
0.06943
48.248
10.95
64.71
18.90
438.1 f
2.6
800
45.38
0.05937
45.611
10.30
69.41
20.90
448.8 f
2.9
825
40.51
0.04671
45.079
11.34
74.86
26.25
433.8 f
1.8
850
34.31
0.02432
46.051
46.10
89.93
51.51
440.3 f
1.5
875
41.24
0.03391
45.989
11.04
84.65
36.88
491.5 f
2.8
900
51.04
0.06189
44.470
3.19
71.15
19.54
508.2 f
3.8
107.21
0.23933
28.534
2.82
36.17
3.24
533.2 f
4.7
49.92
0.06987
45.674
100.00
78.09
35.93
461.8 f
3.6
439.9 f
2.4
2371.9 f
64.2
Fusion Total
Total without 500 “-750 O, 875 O, 900 o C and fusion
78.88
Sample 2A: J = 0.008985 550
436.21
0.45316
3.236
0.75
69.36
0.19
600
244.55
0.58793
3.710
0.62
29.08
0.17
892.9 * 110.2
625
175.02
0.41680
4.793
0.38
29.84
0.31
695.9 f 107.3
675
110.32
0.17274
14.987
0.88
54.82
2.36
788.9 k 31.8
700
75.88
0.12128
16.047
1.81
54.46
3.60
574.6 f
20.1
720
58.30
0.06143
16.955
3.59
71.19
7.51
577.1 f
12.2
740
44.19
0.03054
17.427
10.91
82.73
15.52
517.4 f
3.4
760
41.98
0.02522
17.345
12.27
85.56
18.71
509.5 f
3.1
780
42.76
0.02779
17.309
10.52
84.04
16.94
509.6 f
3.3
800
40.63
0.02394
17.356
13.41
86.01
19.72
497.4 *
3.4
825
37.92
0.01861
17.364
20.47
89.17
25.38
483.3 f
3.7
875
39.09
0.01963
17.203
10.90
88.68
23.83
493.9 f
4.1
900
46.16
0.05222
17.049
4.47
69.53
8.88
461.5 f
5.2
Fusion
67.99
0.10401
16.795
9.02
56.77
4.39
542.0 f
4.8
Total
50.04
0.04479
16.980
lOQ.00
80.88
17.36
525.6 f
5.2
9.3
Sample 3: J = 0.008702 625
94.06
0.10859
2.670
3.69
66.11
0.67
781.2 f
675
50.24
0.03462
7.639
5.40
80.85
6.00
548.2 f
5.5
715
44.27
0.02561
10.018
5.46
84.71
10.64
512.2 f
6.4
725
43.84
0.02792
9.821
4.33
82.97
9.57
498.7 f
6.2
735
44.27
0.03192
10.456
3.26
80.58
a.91
490.5 f
a.5
765
40.71
0.02408
11.405
4.55
84.76
12.88
476.6 +
5.7
785
38.88
0.01899
11.675
7.11
87.96
16.72
412.9 f
5.8
815
37.00
0.01584
11.858
9.40
89.91
20.37
461.6 f
7.2
825
36.45
0.01490
12.094
10.77
90.57
22.08
458.6 f
5.4
835
35.78
0.01316
12.210
9.13
91.86
25.24
456.8 +
4.9
850
35.31
0.01315
12.378
8.04
91.81
25.61
452.0 *
4.7
865
35.85
0.01546
12.302
5.55
90.00
21.65
449.4 f
5.5
885
36.09
0.01717
12.319
3.66
88.66
19.51
446.1 f
6.9
Fusion
36.17
0.01444
12.408
19.63
90.94
23.36
457.2 f
4.1
Total
40.49
0.02165
11.272
100.00
87.84
18.57
481.2 f
5.6
210
R.D. DALLMEYER
TABLE 1 (continued) 39Ar
%J4Ar
36ArCa
Apparent
temp.
(W of
non-
(W)
age d
(“C)
total)
atmos. ’
Release
(%r/“%r)
a
(Y&/Q)
B
( 3’Ar/%)
b
(Ma)
Sample 4A: J = 0.008333
625
73.50
0.06502
3.719
6.02
74.26
1.56
617.4 f
3.5
675
46.40
0.04759
5.349
3.24
70.61
3.06
436.6 &
4.4
700
54.18
0.07218
8.445
1.52
61.87
3.18
446.4 *
9.1
715
48.85
0.05500
10.662
2.07
68.47
5.21
446.1 *
6.7
125
43.90
0.03299
11.977
3.47
79.91
9.88
465.9 +
4.7
735
42.99
0.02777
12.037
3.16
83.15
11.79
473.4 &
7.5
150
44.16
0.03385
12.157
2.89
79.55
9.77
466.2 +
6.1
765
43.38
0.02913
12.416
3.20
82.44
11.59
413.7 f
7.5
775
45.21
0.03332
12.607
3.40
80.45
10.29
480.8 *
8.3
785
41.62
0.02218
12.786
5.02
86.71
15.68
477.5 +
8.2
800
42.45
0.02624
12.978
4.86
84.18
13.45
473.5 *
6.1
825
40.01
0.01812
12.876
6.76
89.19
19.33
472.9 +
6.7
850
37.26
0.00862
13.384
17.08
96.04
42.25
474.1 +
5.1
885
37.68
0.00661
13.217
20.35
97.62
54.36
485.7 i
5.6
Fusion
39.81
0.01493
13.030
16.96
91.53
23.74
481.6 *
4.9
Total
42.49
0.02172
12.048
100.00
88.45
27.12
487.5 +
6.8
478.6 f
5.8
Total without 625”-715OC
87.15
K&Ii Nappe Complex Merdker Nappe
Sample 6: J = 0.009302 550
184.16
0.57279
9.986
0.47
8.52
0.47
247.3 * 156.2
600
151.66
0.40990
9.538
0.43
20.64
0.63
463.3 i 161.3
650
66.29
0.12051
12.458
1.78
47.78
2.81
700
33.19
0.02046
14.051
10.68
85.17
18.68
424.5 +
3.4
720
33.15
0.01877
15.451
8.10
86.99
22.38
432.4 k
5.7
740
33.64
0.02118
15.808
8.38
85.15
20.30
429.9 *
2.2
160
35.16
0.02706
16.148
5.22
80.94
16.23
427.5 +
4.6
780
35.93
0.03014
15.899
4.71
78.14
14.35
421.8 i
4.9
800
30.94
0.01196
16.010
16.31
92.72
36.41
430.5 *
4.3
825
29.98
0.00966
16.159
27.35
94.79
45.51
426.9 i
1.5 2.9
469.0 k 26.4
850
31.02
0.01408
16.143
11.09
90.75
31.19
423.4 *
875
37.62
0.04276
16.127
3.53
69.84
10.26
398.0 k
8.6
Fusion
84.22
0.20139
15.241
1.94
30.79
2.06
393.2 f
34.2
Total
34.88
0.02650
15.666
100.00
86.34
29.33
425.2 i
5.3
426.9 it:
3.3
Total without 550 “-650 O, 895 o C and fusion
91.84
POLYOROGENIC
EVOLUTION
WITHIN
SEVE AND
KiiLl
NAPPES,
211
NORWAY
TABLE 1 (continued) 39Ar
w%
xAb
temp.
(W of
non-
@)
(“C)
total)
atmos. ’
Release
(?4r/%r)
a
(%r/%r)
a
(%r/%r)
b
Apparent age d (Ma)
Sample 73: J = ~.~9~15 550
51.04
0.05587
1.021
5.79
67.80
0.50
490.1 +
5.6
600
40.47
0.03646
1.137
4.39
73.59
0.85
429.3 f
7.3
625
44.52
0.05620
1.580
2.56
62.91
0.76
406.8 *
12.0
665
48.35
0.05991
2.735
3.08
63.83
1.24
443.5 +
6.7
700
47.21
0.05320
6.977
4.31
67.87
3.57
459.5 f
8.4
725
47.94
0.05867
15.545
4.22
66.43
7.21
459.3 f
5.3
750
45.18
0.05332
21.130
4.29
68.87
10.78
451.4 f
10.8
775
40.48
0.04249
21.723
4.87
73.28
13.90
432.8 f
7.5
800
38.36
0.03615
21.708
3.70
76.69
16.34
429.7 f
7.1
825
38.39
0.02916
21.967
3.82
82.13
20.49
456.9 + 10.3
850
38.24
0.03469
21.980
4.33
77.79
17.23
433.9 + 11.6
875
33.09
0.01463
20.972
39.33
92.01
38.98
442.7 f
900
35.84
0.02042
23.145
11.63
88.33
30.82
458.9 f
3.6
104.52
0.25223
22.863
3.68
30.44
2.47
460.8 f
12.1
40.90
0.03860
17.530
100.00
79.91
22.86
447.5 f
5.4
Fusion Total
2.2
Sample 10: J = ~.~~5~I 600
1143.02
3.80095
23.764
0.12
1.90
0.17
700
1041.03
3.15781
43.689
0.60
10.70
0.38
1118.8 f
750
520.60
1.53646
46.949
0.10
13.51
0.83
783.3 f
62.4
775
213.88
0.54913
53.205
0.47
26.13
2.64
649.9 f
54.3
800
72.14
0.14089
30.710
2.95
45.70
5.93
405.7 k 22.3
825
45.54
28.848
21.21
74.76
16.79
417.1 f
4.1
840
46.99
0.05179
29.279
11.78
72.43
15.38
417.0 f
4.6
860
38.93
0.02275
29.045
26.73
88.72
34.73
422.5 +
2.9
885
39.06
0.02320
28.607
35.72
88.32
33.54
422.0 f
2.6
910 Fusion Total
276.4 f 106.3 81.3
412.85
1.28868
28.445
0.27
8.31
0.60
420.0 rt 82.1
1993.51
6.60164
0.660
0.05
2.15
0.00
502.1 i- 126.3
57.53
0.08610
29.129
100.00
81.14
26.83
Total without 600°-800”,
910°C
and fusion
95.43
418.9 f
3.4
420.1 f
2.7
Sample II: J = 0.007603 600
401.06
1.20944
15.773
0.40
11.20
0.35
534.8 * 209.1
615
93.05
0.19126
10.622
0.91
40.17
1.51
453.9 f
56.1
680
95.95
0.20350
11.593
0.96
38.29
1.55
447.4 f
34.3
705
68.20
0.10930
13.161
1.02
54.18
3.28
450.0 f
32.1
730
41.34
0.02641
14.572
11.69
83.94
15.01
425.9 f
4.4
750
39.91
0.02191
14.647
16.99
86.71
18.18
424.9 f
3.1
760
44.08
0.03449
14.716
7.39
79.55
11.60
429.9 f
3.8
770
43.32
0.03474
14.361
6.19
78.95
11.24
420.4 f
4.6
780
39.74
0.02180
14.356
8.86
86.68
17.91
423.1 f
3.9
800
36.44
0.00986
14.277
32.55
95.14
39.40
425.4 f
4.1
820
38.60
0.02197
14.651
5.79
86.21
18.14
410.3 +
4.3
845
43.48
0.03425
14.390
3.71
79.37
11.43
423.8 f
7.3
875
45.40
0.04722
14.377
2.96
71.80
8.28
402.7 f
10.1
573.30
1.86066
14.070
0.56
4.29
0.21
311.8 k 62.3
o.cMO73
14.382
100.00
85.38
22.59
Fusion Total
45>43
Total without 6009-705 O, 820 O-875 o C and fusion
83.67
424.1 f
6.4
425.2 f
2.8
212
R.D. DALLMEYER
TABLE 1 (continued) Release
3pAr
%@Ar
36Ar,-a
Apparent
temp.
( 40Ar/ 39Ar) =
( 36Ar/ 39Ar) a
( 37Ar/39Ar) b
(% of
non-
(8)
age d
(“C)
total)
atmos. ’
(Ma)
Kjolhaugen Group Sampje 9: .I = ~.#7344 550
1112.70
3.50532
31.048
0.42
7.13
0.24
841.4 rt 631.4
600
365.49
1.09023
26.650
0.46
12.44
0.66
527.2 i 207.3
635
192.23
0.55689
28.548
1.05
15.59
1.39
364.6 rt 150.1
675
251.94
0.74376
36.281
0.67
13.92
1.33
421.8 f 163.4
125
60.50
0.09104
31.638
8.44
59.73
9.45
775
46.20
0.04634
30.437
22.03
75.64
17.87
419.1 f
3.1
800
69.04
0.13115
34.157
2.94
47.83
7.08
399.3 +
6.9
820
48.16
0.06029
36.379
8.48
69.08
16.41
402.4 +
5.1
840
38.18
0.02231
32.814
36.81
89.79
40.01
418.3 Lt
2.4
860
41.02
0.02568
30.353
15.89
87.43
32.15
428.9 I
2.8
Fusion
61.89
0.09790
30.685
2.82
57.23
8.53
424.3 I
18.3
Total
54.29
0.07385
32.005
100.00
77.69
26.40
419‘9 *
6.3
432.1 rt: 6.0
Gula Nappe Sample 5: .I = 0.008922 550
72.26
0.12821
2.996
0.80
47.89
0.64
486.1 rt 28.8
600
52.13
0.08548
2.563
0.60
51.93
0.82
390.8 rt 41.3
625
67.06
0.13881
3.244
0.47
39.21
0.64
380.8 i: 34.7
665
51.34
0.08351
5.352
0.80
52.76
1.74
391.6 f
700
34.89
0.01700
6.106
5.10
86.99
9.77
433.7 rt
3.8
125
32.32
0.00812
5.911
13.53
94.03
19.79
434.2 sr
1.3
750
31.49
0.00578
5.927
25.56
96.07
27.92
432.4 rt
1.2
775
31.41
0.00590
5.906
20.64
95.94
27.23
420.9 jr
2.2
25.7
800
31.79
0.00673
5.812
13.77
95.20
23.50
432.5 _C 2.7
825
31.28
0.00584
5.778
8.14
95.95
26.93
429.4 5
850
31.64
0.00692
5.811
3.82
95.00
22.86
429.9 rt
8.1
Fusion
38.58
0.03030
5.915
6.71
78.01
5.31
430.4 *
5.1
Total
33.04
0.01124
5.836
100.00
92.65
22.62
431.5 i
3.4
432.0 rt
2.6
Total without 550 “-665 o C
97.33
4.7
Sample 12: J = 0.007568 525
216.39
0.27432
12.894
1.06
63.01
1.28
1286.6 f
19.4
575
18.39
0.11044
22.347
1.09
60.65
5.50
561.3 f
18.6
625
89.88
0.16604
5.997
1.01
45.94
0.98
492.2 rt 24.3
660
59.48
0.08150
6.853
2.18
60.43
2.29
435.8 F
8.8
loo
55.01
0.06379
7.221
3.03
66.78
3.08
444.4 I:
7.2
725
51.97
0.05663
8.394
2.70
69.09
4.03
435.7 It
6.1
745
50.38
0.05528
8.901
2.53
68.98
4.38
423.4 r?c 5.8
765
41.48
0.02426
10.163
8.56
84.67
11.39
427.7 f
2.7
790
37.75
0.01485
9.955
18.49
90.48
18.23
417.1 zt
1.9
810
35.81
0.00558
10.166
26.56
97.66
49.53
426.1 f
1.8
935
35.52
0.00503
10.145
26.28
98.09
54.84
424.6 i
2.8
860
38.69
0.02464
10.092
3.23
83.26
11.14
395.8 + 10.1
890
42.38
0.03405
9.981
1.81
78.14
7.97
405.7 f
Fusion
98.30
0.22158
10.022
1.48
34.21
1.23
411.3 It 14.3
Totaf
42.55
0.02475
9.992
100.00
89.04
32.88
Total without 525 O-745 *, 860 O, 890 o C and fusion
79.88
9.3
434.9 f
4.3
423.6 f
2.5
FOLYOROGENIC
EVOLUTION
WITHIN
SEVE
213
AND ICiiLt NAPPES, NORWAY
TABLE 1 (continued) Release temp.
( %r/
39Ar) a
( %/
%r) a
( %r/
3%) b
(“C)
39Ar (46of total)
w?4r nonatmos. ’
36%a (W)
Apparent age d (Ma)
Sampie 13: J 52.5 575 625 660 700 725 740 760 780 800 820 845 Fusion
= ~.~~~~~ 71.45 42.42 40.80 44.15 43.02 42.83 42.94 41.62 39.73 36.31 35.87 37.64 42.11
0.11391 0.02241 0.02058 0.03136 0.02936 0.03026 0.03176 0.02765 0.02056 0.00831 0.00733 0.01230 0.02671
2.134 0.575 1.392 3.221 7.442 9.720 10.448 11.261 12.184 11.958 11.635 11.981 12.149
2.91 4.01 11.75 4.44 4.94 4.94 4.64 8.40 9.95 21.22 14.77 6.47 1.57
53.12 84.48 85.35 79.58 81.21 80.93 80.08 82.52 87.16 95.86 96.55 92.89 83.56
0.51 0.70 1.84 2.79 6.90 8.74 8.95 11.08 16.12 39.12 43.18 26.50 12.37
458.8 f 5.5 435.7 f 6.3 425.0 f 2.3 428.8 f 2.8 427.6 f 3.0 425.2 f 4.6 422.3 f 3.8 422.0 f 2.9 425.3 f 3.1 427.3 f 1.7 425.3 f 2.8 429.0 k 3.8 431.5 f 18.6
Total
40.31
0.02087
9.103
100.00
87.74
20.69
427.3 f
3.5
425.8 f
2.9
Total without 525 “-625 o C and fusion
79.81
a Measured. b Corrected for post-irradiation
decay of 37& (35.1 day half-life).
’ [40Ar,,l.- ( 36Ar,,,,. X29~.5N/40~t,,, d Calculated using correction factors of Dalrymple et al. (1981); two r&ma, intralaboratory
errors.
(481.22
0
20
CUMULATIVE
40
% 3gAr
80 60 RELEASED
5.6)
100
Fig. 3. 4oAr/39Ar incremental-release age and apparent K/Ca spectra of hornblende from foliated amphibolite of the Seve Nappe Complex, east-central Trendelag Locations shown in Fig. 2. Analytical uncertainties {two sigma, intraiaboratory) are represented by vertical width of bars. Experimental temperatures increase. from left to right. Plateau or total-gas (parentheses) ages are listed on each spectrum. Plateau increments delineated with arrows. All spectra have coordinates shown for sample 2.
214
R.D. WALLMEYER
TABLE 2 36Ar/40Ar vs. 39Ar/40Ar isotope correlations from ~crement~-hating experiments on hornblende concentrates from the Seve and KGIi Nappe Complexes, centrai Scandinavian Caledonides, Trondelag, Not-way Sample
Isotope correlation age a
%r/ 36Ar intercept b
MSWD
Increments included ’
Total 39Ar (V
(Ma)
Calculated 40Ar/ “Ar age (Ma) *
Seve Nappe CompIex 1 429.3 * 0.9 2A 435.4 rlr4.1 3 423.9 f 5.2 4A 476.5 * 2.7 Kiili Nappe CompIex
5.1 545.9 f 12.2 514.8 f 23.8 286.1 f 9.2
0.37 0.10 0.28 0.26
775-850 740-875 815-F 725-F
78.88 78.48 66.19 87.15
439.9 +_2.4 499.5 + 3.6 456.1 f 4.9 478.6 jI 5.8
299.1 f 321.7 f 273.7 5 296.1 +
9.4 16.3 9.3 10.3
0.18 0.35 0.06 0.08
700-850 725-900 825-885 730-800
91.84 78.19 95.43 83.67
426.9 f 446.2 rt 420.1 + 425.2 k
299.9 + 12.6
0.99
725-860
94.37
419.5 f 4.3
336.2 f 19.6 270.5 f 12.8 293.3 f 8.3
0.18 1.61 0.07
700-F 765-835 600-845
97.33 79.88 79.81
432.0 f 2.6 423.6 f 2.5 425.8 rt 2.9
392.5 *
Mer&ker Nappe Fumfsjss Group
6 422.8 & 1.2 7B 434.8 + 2.1 10 418.9 rf 1.6 11 421.7 4 2.1 Kj~ihffugen Group 9 421.1 & 3.1
3.3 4.4 2.7 2.8
Gula Nappe 5
12 13 a b ’ *
428.7 10.9 422.6 + 1.3 424.2 rf: 1.5
Calcuiated using the inverse abscissa intercept (40Ar/3pAr ratio) in the age equation. Inverse ordinate intercept. Table 1. o C (F = fusion).
of the total) yield an isotope correlation age of 423.9 + 5.2 Ma (inverse ordinate intercept of 514.8 t_ 23.8). The isotope correlation ages are considered geologically reliable and interpreted to date post-rnet~o~~c cooling through argon closure temperatures. Sample 4 displays a discordant spectrum in which apparent ages systematically increase in the 675 O-725OC increments to define an intermediate and high-temperature plateau age of 478.6 + 5.8 Ma (735*C-fusion increments). The plateau data yield a similar isotope correlation age (476.5 + 2.7 Ma) and an inverse ordinate intercept of 286.1. Because these correlations do not suggest extraneous argon contamination, the 476 Ma date is considered a reliable post-metamo~~c cooling age. The nature of age discordancy displayed in the 675°-7250C increments is similar to that predicted by Turner (1968) to develop as a result of partial, intracrystalline diffusive loss of radiogenic
“OAr during a superimposed thermal event. This behavior has been documented for thermally overprinted hornblende in other geologic settings by several workers, including Turner (1970), Dallmeyer et al. (1981), and Harrison (1981). Comparison of the Seve spectrum with the theoretical spectra of Turner (1968) suggests that the Seve hornblende experienced only slight (c. < 25%) loss of argon during a low-grade thermal overprint following initial post-metamorphic cooling through argon closure temperatures at c. 476 Ma. Experimental evolution of gas from a non-retentive, relatively high K phase at low temperatures precludes accurate determination of the age of the geologic overprint. However, it must have been younger than the 436.6 + 4.4 Ma apparent age recorded by the 675°C evolved from the Seve hornblende. (2) Muscooife: Muscovite concentrates have been prepared from two samples of schist inter-
POLYOROGENIC
EVOLUTION
WITHIN
SEVE AND
KijLI
NAPPES,
215
NORWAY
TABLE 3 %r/39Ar
analytical data for incremental-heating experiments on muscovite concentrates from the Seve and Kisli Nappe Complexes,
central Scandinavian Caledonides, Tremdelag, Norway Release
(%r/%r)
a
( %r/?4r)
temp.
a
39Ar
WaAr
Apparent
(W of total)
non-atmos. ’
age d (Ma)
(“C) Seve Nappe Complex Sample 38: J = 0.007595 525
41.18
0.01389
2.28
90.02
441.5 f
1.6
515
39.70
0.00334
3.97
97.50
464.9 f
2.0
625
37.33
0.00670
6.94
94.68
428.9 f
4.0
700
34.98
0.00289
13.50
97.54
415.1 f
2.2
800
34.83
0.00165
28.68
98.59
418.1 f
1.2
850
35.12
0.00176
8.54
98.51
420.9 -I: 2.4
900
35.57
0.00224
8.03
98.14
424.2 f
2.6
Fusion
36.76
0.00599
28.05
95.19
425.2 f
2.5
Total
35.99
0.00379
100.00
96.94
423.8 f
2.2
420.8 f
1.9
8.2
Total without 525 O-625 o C
86.80
Sample 48: J = 0.008862 500
63.65
0.11701
4.70
45.70
413.7 f
550
40.86
0.03745
16.05
72.97
422.9 f
2.3
600
41.52
0.03657
35.14
74.03
434.6 f
2.4
615
45.64
0.04943
18.19
68.05
438.6 f
2.5
750
51.55
0.06768
9.12
61.99
444.8 f
3.2
Fusion
46.11
0.04861
16.81
68.91
447.6 f
2.3
Total
47.20
0.05506
100.00
69.15
431.3 f
3.7
Sample 8: J = 0.009368 500
31.64
0.00433
2.01
86.61
412.2 f
3.1
550
28.91
0.00329
5.94
96.62
419.3 f
1.7
600
29.09
0.00299
10.99
96.94
422.9 k 1.2
615
29.03
0.00326
27.36
96.66
421.1 f
0.7
750
28.89
0.00273
26.13
97.19
421.3 f
0.9
825
28.66
0.00178
26.85
98.15
421.9 f
0.7
Fusion
43.93
0.04720
0.66
68.25
446.6 f
8.2
Total
29.05
0.00321
100.00
96.83
421.4*
0.9
421.6 f
0.8
Total without 500 o C and fusion
97.27
K(LliNappe Complex Gula Nappe Sample 5: J = 0.009378 500
33.61
0.02031
1.39
82.14
415.3 f
5.9
550
29.86
0.00491
3.19
95.12
426.1 f
2.5
600
29.29
0.00330
6.09
96.65
424.7 f
1.7
675
28.60
0.00270
30.91
97.19
417.9 f
0.9 1.6
750
28.80
0.00241
19.30
97.50
421.6 f
825
28.20
0.00133
33.32
98.59
418.Ozt 0.7
Fusion
30.08
0.00564
5.80
94.45
426.1 f
1.1
Total
28.14
0.00271
100.00
91.25
419.7 f
1.1
418.8 f
0.9
Total without 500 O-600 o C and fusion
83.50
216
R.D. DALLMEYER
TABLE 3 (continued) Release
( @Ar/ ‘9Ar) ’
( %r/%r)
a
temp.
39.4r
sNAr
(S of total)
non-atmos. ’
(“C)
Apparent age d (Ma)
Sample 12: J = 0.00761 I 500
46.25
0.05035
0.79
68.01
387.4 + 32.5
550
37.42
0.00822
5.24
93.94
428.1 + 3.2
600
35.97
0.00420
28.56
96.54
423.0 *
650
35.51
0.00190
16.45
98.41
425.1 + 1.3
1.1
725
35.75
0.00303
13.69
97.48
424.4 f
0.7
825
35.28
0.00201
30.63
98.31
422.6 f
0.7
Fusion
38.17
0.01188
4.65
90.84
422.5 f
2.0
Total
35.91
0.00392
100.00
96.89
423.4 f
1.4
423.5 i
1.0
Total without 500 o and 550 o C
93.98
Sample 13: J = 0.007502 550
37.22
0.00874
4.12
93.09
416.8 f
1.0
600
36.47
0.00471
5.49
96.17
421.3 f
0.9
635
36.30
0.00452
4.76
96.31
420.1 & 1.0
675
36.09
0.00349
4.98
97.13
421.1 + 0.7
725
35.83
0.00302
9.78
97.49
419.8 f
0.6
750
36.07
0.00362
5.09
97.02
420.4 f
1.8
775
36.27
0.00362
4.21
97.04
422.6 f
1.5
800
35.90
0.00299
3.12
97.52
420.6 1
1.3
825
35.82
0.00267
4.26
97.78
420.8 + 1.5
850
35.97
0.00270
3.62
97.77
422.3 f
1.7
875
35.87
0.00297
4.23
97.54
420.3 f
1.2
925
35.23
0.00202
18.89
98.29
416.5 + 0.6
975
35.10
0.00242
27.16
97.95
413.9 *
Fusion
91.59
0.19110
0.29
38.35
421.9 f 12.7
Total
35.85
0.00366
100.00
97.25
417.9 f
0.9
418.0 f
0.8
Total without 550 o C and fusion
95.59
0.4
a Measured. b ?‘%,,.(36Ar,,,,.)(295.5)1/40Ar,ol~ ’ Calculated using correction factors of Dahymple et al. (1981); two sigma, intrafaboratory errors; 37Ar/39Ar corrected ratio < 0.020 in all analyses.
layered
with
amphibolite
in westernmost
ex-
posures of the Seve Nappe Complex (locations 2 and 4, Fig. 2). These display variably discordant 40Ar/39Ar age spectra (Table 3, Fig. 4). That from sample 3 yields a well-defined 420.8 4 1.9 Ma plateau in the 7OO”C-fusion increments (corresponding to c. 87% of the total gas evolved). This is interpreted to date the last cooling through temperatures required for intracrystalline argon retention. Although not fully calibrated experimentally, using the preliminary data of Robbins (1972) in the diffusion equations of Dodson (1973)
indicates muscovite closure temperatures of c. 400°C. This is similar to those suggested for muscovite on the basis of empirical comparisons with other mineral isotopic systems (e.g., Wagner et al., 1977; tiger, 1979). The muscovite concentrate from sample 2 displays an internally discordant age spectrum in which apparent ages systematically increase from 413.7 f 8.2 Ma in the 500°C increment to 447.6 k 2.3 Ma in the fusion increment. This type of spectra discordance is suggestive of a partially rejuvenated intracrystalline argon system (e.g., Turner, 1968). Such pat-
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
KiiLl
NAPPES,
420 8 t 1.9
550
8
-I
500 450 4001
421.6 + 0.8
i
0
20 CUMULATIVE
40
-I
60 60 % 3gAr RELEASED
217
NORWAY
100
Fig. 4. ‘%r/39Ar incremental-release age spectra of muscovite from schist interlayered with amphibolite of the Seve Nappe Complex, east-central Trendelag. Data plotted as in Fig. 3.
at c. 400-425 Ma. Because of the extensive rejuvenation it is not possible to estimate the initial post-metamorphic age. However, it must have been older than the 448 Ma recorded by the fusion increment of the Seve muscovite. Muscovite has also been separated from a sample of schist collected within the Seve Nappe Complex exposed along the western limb of the Skadora antiform at the international border (location 8, Fig. 1). The concentrate displays an internally concordant age spectrum with the 550”-825°C increments yielding a plateau age of 421.6 f 0.8 Ma (c. 98% of the total evolved gas). This is interpreted to be a reliable post-metamorphic cooling age. Kb’li Nappe Complex Meraker Nappe
terns have been documented for thermally overprinted muscovite in other areas by several workers, including Dallmeyer and LRcorche (1989) and Snee et al. (1988). Comparison of the Seve spectrum with the theoretical spectra of Turner (1968) suggests extensive diffusive loss of radiogenie argon occurring during a thermal overprint
c
0
Fig. 5. %r/39~
20 CUMULATI”?%
(1) Hornblende: Hornblende concentrates have been prepared from four samples of foliated amphibolite collected within the Fundsjo Group of the MerAker Nappe (locations 6, 7, 10 and 11, Fig. 2). Samples 6, 10 and 11 display slightly discordant age spectra (Table 1, Fig. 5) in which anomalous apparent ages are recorded in both
6
100 “:‘RELEiiED
incremental-release age and apparent K/Ca spectra of hornblende from foliated amphibolite of the KiAi Nappe Complex (Fundsje Group, MerAkerNappe), east-central Tmndelag. Data plotted as in Fig. 3.
218
R.D. DALLMEYER
low- and high-temperature parts of the analyses. These are matched by significant ~uctuations in apparent K/Ca ratios, and likely reflect the effects of optically undetectable mineral contaminants in the concentrates and/or exsolution or zoning within constituent hornblende grains. Inte~~iate-temperat~e increments display negligible intrasample variations in apparent K/Ca ratios suggesting experimental evolution of gas occurred from populations of compositionally uniform intracrystalline sites. These increments (comprising between 84 and 94% of the total gas evolved) record mutually similar intrasample apparent ages corresponding to plateau ages of 426.9 4 3.3 Ma (6), 420.1 f 2.7 Ma (10) and 425.1 f 2.8 Ma. The plateau data from each analysis yields well-defined isotope correlations (MSWD < 2.0: Table 2) which define ages comparable to those directly calculated from the analytical data. Inverse ordinate intercepts range between 273.7 f 9.3 and 299.1 f 9.4, and do not suggest the presence of extraneous argon components. Therefore, the plateau ages are considered geologically significant and are interpreted to date the last cooling through appropriate closure temperatures. Hornblende from sample 7 displays a more complex age spectrum (Fig. 5). Apparent K/Ca ratios systematic~ly decrease throughout the 550 o 700°C increments suggesting a significant compositional inhomogenity in the concentrate. Although apparent K/Ca ratios are constant in the 725 “-900°C increments, there are significant variations in apparent ages (429.7 f 7.1 Ma to 459.3
f 5.3 Ma), and a plateau is not defined. These increments correspond to an isotope correlation (MSWD = 0.35) yielding an age of 434.8 f 2.1 Ma and an ordinate intercept of 321.7 _+16.3 (Table 2). This is interpreted to be a reliable post-metamorphic cooling age. A hornblende concentrate was also prepared from a sample of metagabbro collected within the Kjolhaugen Group of the Meraker Nappe (location 9, Fig. 2). The concentrate displays an internally discordant, saddle-shaped *Ar/39Ar age spectrum corresponding to a total-gas age of 419.9 + 6.3 Ma (Table 1, Fig. 6). Apparent K/Ca ratios are similar throughout the analysis. The 725860°C increments (comprising c. 94% of the total gas evolved) yield an isotope correlation age (MSWD = 0.99) of 421.1 + 3.1 Ma and an inverse ordinate intercept of 299.9 f 9.9. This is considered a reliable post-metamorphic cooling age. (2) Biotite: A biotite concentrate was prepared from a sample of schist interlayered with ~p~bo~te at location 7 in the Mer&ker Nappe. The concentrate displays an internally concordant age spectrum (Table 4, Fig. 7) with the 500850 o C increments (comprising c. 96% of the total gas evolved) yielding a plateau age of 435.7 + 1.4 Ma. This is interpreted to data the last cooling through temperatures required for intracrystalline retention of argon. Harrison et al. (1985) have demonstrated that closure temperatures are dependent on Fe/Mg compositional ratios, and suggested a range of c. 275-325°C is appropriate for the range of cooling rates likely to be encountered in most geologic settings. Gula Nappe Samples of hornblende garbenschiefer schist were collected at locations 5, 12 and 13 within the Gula Nappe (Fig. 2). Concentrates of hornblende, muscovite and biotite were prepared from each sample.
Fig. 6. “OAr/39Ar incremental-release spectra of hornblende Complex
(Fundsjer
age and apparent
from metagabbro Group,
Trendelag.
Mersker
K/Ca
of the Ktili Nappe Nappe),
Data plotted as in Fig. 3.
east-central
(1) Ho~b~ende: The three hornblende concentrates display slightly discordant ?Ar/39Ar spectra in which anomalously old apparent ages are recorded at low experimental temperatures (Table 1, Fig. 8). These are matched by marked
219
POLYOROGENIC EVOLUTION WITHIN SEVE AND I&L1 NAPPES. NORWAY
fluctuations in apparent K/Ca ratios suggesting that they reflect the affects of relatively non-retentive, compositionally distinct mineralogical cont~ants in the concentrates. Most of the intermediate- and high-temperature gas fractions (com-
prising between c. 80% and 97% of the total gas evolved from the samples) are characterized by mutually similar intrasample apparent K/Ca ratios su~esting expe~ment~ of gas from compositionally uniform populations of intracrystal-
TABLE 4 40Ar/39Ar analytical data for incremental-heating experiments on biotite concentrates from the Koli Nappe Complex, central Scandinavian Cakdonides, Trendelag, Norway Release
(?q?kr)
*
( 36Ar/39Ar) a
temp.
39Ar
W@Ar
Apparent
(a6 of total)
non-atmos. b
age ’ (Ma)
(“C) Merdker Nappe Sample 7A: .I = 0.009115 475
39.54
0.05728
3.07
57.18
338.1 + 2.5
500
31.08
0.~
12.39
96.14
434.5 f
1.9
550
30.44
0.00158
18.85
98.44
435.6i
1.2
675
30.74
o.cO211
20.38
97.95
431.5 f
1.6
750
30.46
0.00144
24.64
98.58
436.4 f
1.4
850
30.31
0.00155
19.52
98.48
434.1 f
1.0
Fusion
46.89
0.05662
1.15
64.31
438.1 f
8.4
Total
31.03
0.80430
100.00
96.44
432.7 f
1.5
435.7 f
1.4
Total without 475 * C and fusion
95.78
Gula Nappe Sample 5: J = 0.009234 475
29.90
0.02401
15.64
76.25
344.5 f
1.9
500
29.41
0.00210
14.84
91.87
425.1 f
1.2
550
29.39
0.00127
15.73
98.70
428.0 f
1.8
675
29.68
0.00197
19.23
98.03
429.1 + 1.5
760
29.13
0.00081
27.41
99.16
426.3 f
0.8
850
29.68
0.00219
6.29
97.80
428.3 f
2.1
Fusion
75.42
0.15572
0.87
38.98
433.1 f
9.8
Total
29.88
0.00636
100.08
94.48
414.3 f
1.4
427.1 f
1.3
1.7
Total without 475’ C and fusion
83.49
Sample I2: f = 0.~7~88 475
36.46
0.01210
3.70
90.19
401.8 f
500
36.63
0.00256
9.50
97.92
434.3 f
1.1
550
36.38
0.00182
12.51
98.51
434.0 f
0.8
675
36.41
0.00195
16.84
98.40
433.8 f
1.0
750
35.83
0.00093
30.28
99.22
430.8 f
1.0
850
36.10
0.00105
26.77
99.12
433.4 f
1.0
Fusion
89.02
0.18195
0.39
39.62
427.9 f 17.2
Total
36.38
0.00252
100.00
98.27
431.7*
Total without 475 ’ C and fusion
95.91
1.1
432.8 f 0.9
220
RD.
DALLMEYER
TABLE 4 (continued) 39Ar (Se,of total)
Release temp.
W‘=Ar non-atmos. b
Apparent age ’ (Ma)
(“C) Sample
475 500 550 650 750 850 Fusion
13: J = O.&I7415 39.38 37.52 36.92 37.38 37.12 36.95 45.72
Total
37.30
0.01880 0.00297 0.00181 0.00295 0.00155 0.00172 0.02933
2.78 10.83 16.26 11.25 40.76 16.92 1.20
85.87 97.64 98.53 97.65 98.75 98.61 81.06
403.6 * 433.5 i: 430.8 + 432.1 + 433.7 It 431.4* 438.0 +
0.00274
100.00
97.88
431.8 i: 0.8
Total without 475’ C and fusion
96.02
1.9 0.6 0.6 0.6 0.9 1.0 3.1
432.6 It 0.8
a Measured. ’ ?Ar,,,(36Arat,,,oS. )(2P5.5)1/40Ar,,,, ’ Calculated using correction factors of Dahympie et al. (1981); two sigma, intralaboratory errors; 37Ar/39Ar corrected ratio < 0.020 in all analyses.
line sites. These increments record similar intrasample ages corresponding to plateaux of 432.0 + 2.6 Ma (5), 423.6 & 2.5 Ma (12) and 425.8 f 2.9 Ma (13). Plateau data from each sample yields well-defined isotope correlations (MSWD < 2.0) corresponding to ages ranging between 422.6 + 1.3 Ma (12) and 428.7 + 0.9 Ma (5). Inverse ordinate intercepts range between 270.5 f 12.8 and 336.2 f 19.6 and do not suggest extraneous argon contamination. Therefore, the plateau ages are interpreted to be geologically significant post-metamorphic cooling dates. (2) Murcovite: The muscovite concentrates display internally concordant age spectra (Table 3, Fig. 9) corresponding to plateau ages of 418.8 f 0.9 Ma (5), 423.5 rt: 1.0 Ma (12) and 418.0 + 0.8 Ma
~~~~~~
0
20 40 60 80 CUMULATIVE % 39Ar RELEASED
100
Fig. 7. WAr/39Ar incremental-release age of biotite from schist interlayered with amphibolite of the Ktili Nappe Complex (Fundsje Group, Mer&ker Nappe), east-central Trnndelag. Data plotted as in Fig. 3.
(13). These are interpreted to date post-metamorphic cooling through c. 400°C. (3) Biotite: The three biotite concentrates also display generally concordant age spectra (Table 4, Fig. 10) which yield plateau ages of 427.1 + 1.3 Ma (5), 432.8 f 0.9 Ma (12) and 432.6 rf: 0.8 Ma (13). These are considered to represent reliable post-metamorphic cooling ages. Geologic significance Interpretating the geologic significance of the *Ar/39Ar results depends on calibration of the Ordovician and Silurian time-scales (e.g., Harland et al., 1982; Palmer 1983). Snelling (1985) and Kunk et al. (1985) suggest that the OrdovicianSilurian boundary (base of the Llandovery) is c. 435-440 Ma. This together with a 420 Ma calibration of the Ludlow (Wyborn et al. 1982) is used for interpretation of the Seve and Koli 40Ar/39Ar results from Trerndelag. 40Ar/39Ar results from structural units of the Seve Nappe Complex exposed in Trsndelag suggest significant local differences in t~tonotherm~ evolution. An initial post-metamorphic cooling age of c. 475-480 Ma is locally recorded by homblende (location 4), suggesting that significant orogenie activity occurred during and/or shortly prior
POLYOROOENIC
EVOLUTION
WITHIN
SEVE AND
KijLI
NAPPFB,
NORWAY
t
I.
452.0* 2.6
J 1
I
4007 0
20
40
CUMULATIVE
60
00
100
% 3gAr RELEASED
Fig. 9. %r/39Ar incremental-release age spectra of muscovite from garbenschiefer schist of the K6li Nappe Complex (Gula Nappe), east-central Trondelag. Data plotted as in Fig. 3.
i-
5501--
Bgsc&
0
.,_
“56’”
20
40
CUMULATIVE
60
% 3gAr
60
100
RELEASED
Fig. 8. ‘%r/39Ar incremental-release age and apparent K/Ca spectra of hornblende from garbenschiefer schist of the Koli Nappe Complex (Gula Nappe), east-central Trondelag. Data plotted as in Fig. 3.
ord of a pre-Silurian thermal evolution may be directly detected in 40Ar/39Ar results from the Koli Nappe units examined. However, the widespread presence of extraneous argon components does point to a pre-Scandian history. Hornblende, muscovite, and biotite within both the MerSlker and Gula nappes record generally similar post-metamorphic cooling ages between c. 420 and 435 Ma, again suggesting relatively rapid post-metamorphic cooling following culmination 550
5
4
500
427.1 +c1.3 450
to Arenig. This hornblende appears to have been slightly rejuvenated during a Silurian (c. 400-425 Ma) metamorphic overprint. Muscovite from this locality was more extensively rejuvenated. At three sample locations within the Seve Nappe Complex both hornblende and muscovite were totally rejuvenated during the overprint, and record postScandian metamorphic cooling ages of 425-435 Ma and 420-425 Ma respectively. The near concordance of hornblende and muscovite cooling ages suggests relatively rapid post-metamorphic cooling. through the contrasting temperatures required for intracrystalline argon retention. No rec-
4001 550500-
432.8
t 0.9
450400
13 $yyiq-$
0
20
CUMULATIVE
40
60
100
% 3gAr REL&ED
Fig. 10. ‘%r/39Ar incremental-release age spectra of biotite from garbenschiefer schist of the Koli Nappe Complex (Gula Nappe), east-central Trondelag. Data plotted as in Fig. 3.
LLL
of Scandian metamorphism in the Early Silurian. Sequences within the lower Koli nappes include units which appear to be correlative with AshgillLlandovery successions exposed at similar structural levels in Vbterbotten County, Sweden. Comparison of protolith ages and the c. 430 Ma estimate for attainment of peak metamorphic conditions suggest that tectonic activity must have commenced shortly after deposition of the youngest (Late Llandovery) sequences. Regional tectonic significance
The complex tectonic history suggested by the 40Ar/39Ar mineral dates recorded in structural units of the Kbli and Seve Nappe Complexes exposed in Trsndelag is generally similar to the polyorogenic evolution previously outlined for the Upper Allochthon in several other areas of the central Scandinavian Caledonides; however, there appear to be significant differences in the relative timing of individual tectonothermal events. For example, in central Norrbotten County, Sweden, c. 490 Ma “Ar/39Ar plateau ages were reported by Dallmeyer and Gee (1986) for hornblende within non-foliated, retrograde amphibolite selvages developed from eclogite in miogeoclinal successions of the Seve Nappe Complex. By contrast, in the Gaddede area of northern Jamtland and in the Are area of west-central Jg;mtland hornblende within Seve amphibolite has yielded %r/39Ar cooling ages of c. 460-475 Ma (Dallmeyer et al., 1985; Dallmeyer and Gee, 1988). Thus a complex and apparently diachronous early Caledonian evolution is apparent for this part of the Baltoscandian margin. A similarly contrasting tectonothermal record is also present in the exotic lithologic elements which comprise the Kiili Nappe Complex. Hornblende in garbenschiefer of the lower Kbli Nappe exposed in the Tglnnforsen area (Middagasfallet Nappe) locally yield @Ar/ 39Ar plateau ages between 475 and 510 Ma (Dallmeyer et al., 1985). Elsewhere in the same nappe unit hornblende records c. 415 Ma post-Scandian cooling ages. In the Gaddade area, Scandian metamorphism appears to have resulted in complete reuvenation of intracrystalline argon systems throughout both the Lower and Middle KBli
R.D. DALLMEYER
Nappe units, and hornblende, muscovite, and biotite record “OAr/39Ar plateau ages of 420-435 Ma (Dallmeyer and Gee, 1988). Dallmeyer and Gee (1986, 1988) suggested that the early Paleozoic evolution of central parts of the Baltoscandian margin included: (1) Tremadoc and Late Cambrian high-pressure metamorphism during entrainment of distal, Baltoscandian miogeoclinal sequences and associated volcanic successions (Seve Nappe Complex protoliths) within an evolving accretionary wedge; (2) ductile translation of high-pressure units to shallower crustal levels (with attendant retrogression of high-pressure assemblages) and imbrication with miogeoclinal sequences maintained at intermediate-pressure, middle amphibolite facies metamorphic conditions; and (3) consolidation of the accretionary wedge prior to cooling below homblende argon closure temperatures in the Arenig to Caradoc. Development of the Seve accretionary prism in the Early Ordovician was apparently contemportaneous with construction of an outboard (western) volcanic arc now represented by some successions comprising lower units of the Kijli Nappe Complex (e.g., Ankarede Formation in the Ggiddede area). Younger successions in the Lower Kiili Nappes were deposited at least into the Late Llandovery. Shortly thereafter, Scandian deformation commenced, and Middle Kbli Nappe units were inverted and thrust onto Lower Koli sequences. The resultant composite allochthon was structurally emplaced onto previously imbricated and variably metamorphosed miogeoclinal sequences. Assembly of the nappe units resulted in crustal depression and variable metamorphism. Cooling associated with subsequent rapid uplift to shallower crustal levels is reflected in the c. 420435 Ma “OAr/39Ar plateau ages recorded by hornblende, muscovite, and biotite throughout the Upper Allochthon. Emplacement of this composite Scandian allochthon onto crystalline, middle Proterozoic basement of western Baltica apparently resulted in its depression to depths appropriate for formation of high-temperature eclogite assemblages. These record c. 425 Ma ages (U-Pb zircon and Sm-Nd mineral: Krogh et al., 1974; Griffin and Brueckner, 1980), indicating that they formed at approximately the same time
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
K6LI
NAPPFiS,
NORWAY
that minerals within the Upper Allochthon were cooling through argon retention temperatures at much shallower, superstructural crustal levels. Following eastward translation of the Scandian nappes, the basement experienced nearly isothermal depressurization followed by very rapid cooling through hornblende and muscovite argon closure temperatures at c. 390400 Ma (Dallmeyer et al., 1990). Acknowledgments This work was supported by a grant from the Crustal Structure and Tectonics Program of the U.S. National Science Foundation (EAR8407027). Drs. D. Gee, H. Kish, and D. Roberts collaborated in the field during sample collection. Freight was, in part, paid by the Geological Survey of Sweden, the Geological Survey of Norway, and the University of Lund. Appendix: Sample locations 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Road Road Road Road Road Road Road Road Road Road Road Road Road
exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure
along E-6, 1.5 km north of Heia. along E-6, 0.5 km south of Steinkjer. along E-6, at Kollset. along E-6, 1 km south of Koabjerea. along Rt. 72 at Feren. along Rt. 757, 1.5 km northwest of Julnes. along Rt. 72,1.0 km northwest of Stormoen. along Rt. 72 at international border. along E-75 at Turefors bridge. along E-75 at Gudl bridge. along R-75, 0.1 km west of Gudl bridge. along E-75, 1.5 km east of Floraholmen. along E-75 at Floraholmen.
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R.D. DALLMEYER
Dalimeyer, RD., Odom, A.L., G’Driscoll, C.F. and Hussey, E.M., 1981. Geochronology of the Swift Current granite and host volcanic rocks of the Love Cove Group, southwestern Avalon Zone, Newfoundland: evidence of a late Proterozoic volcanic-subvolcanic association. Can. J. Earth Sci., 18: 699-707. Dallmeyer, R.D., Gee, D.G. and Beckholmen, M., 1985. 4oAr/39Ar mineral age record of early Caledonian t~tonothe~~ activity in the Baltoscandian miogeocline. Am. J. Sci., 285: 532-568. Dallmeyer, R.D., Moller, C. and Johansson, L., 1990. Tectonothermal evolution of high-pressure rocks in the Western Gneiss Terrane, Vestranden, Norway: 40Ar/39Ar mineral age control. Geol. Sot. Am. Bull., in press. Dah-ymple, G.B., Alexander, E.C., Lanphere, M.A. and Kraker, G.P., 1981. Irradiation of samples for “OAr/39Ar dating using the Geological Survey TRIGA reactor. U.S. Geol. Surv., Profess. Pap., 1176: 55 pp. Dodson, M.H., 1973. Closure temperature in cooling geochronological and petrological systems. Contrib. Mineral. Petrol., 40: 259-274. Dyrelius, D., Gee, D.G., Gorbatschev, R., Ramberg, H. and Zachrisson, E., 1980. A profile through the central Scandinavian Caledonides. Tectonophysics, 69: 247-284. Fumes, H., Roberts, D., Sturt, B.A., Tbon, A. and Gale, G.H., 1980. Gphiolite fragments in the Scandinavian Caledonides. In: A. Panayiotou (Editor}, Proc. International Gphiolite Symposium, Cyprus, 1979. Geological Survey of Cyprus, Nicosia, pp. 582-600. Gee, D.G., 1975a. A geotraverse through the Scandinavian Caledonides&tersund to Trondheim. Sver. Geol. Unders., Ser. C, 717: 66 pp. Gee, D.G., 1975b. A tectonic model for the central part of the Scandinavian Caledonides. Am. J. Sci., 27514: 468-515. Gee, D.G., 1978. Nappe displacement in the Scandinavian Caledonides. Tectonophysics, 47: 393-419. Gee, D.G. and Kumpulainen, R., 1980. An excursion through the Caledonian moutain chain in central Sweden from &.tersund to Storlien. Sver. Geol. Under&, Ser. C, 774: 66 PP. Gee, D.G. and Roberts, D., 1983. Timing of deformation in the Scandinavian Caledonides. In: P. Schenk (Editor), Regional Trends in the Geology of the Appalachian-Caledonian-Hercynian-Mauritanide Orogen. Reidel, New York, N.Y., pp. 279-292. Gee, D.G. and Sturt, B.A., 1985. The Caledonide orogenScandiiavia and Related Areas. Wiley, Chichester. Vol. 1, 266 pp. Gee, D.G. and Wilson, M.R., 1974. The age of erogenic deformation in the Swedish Caledonides. Am. J. Sci., 274: l-19. Gee, D.G. and Wolff, F., 1981. The Central Scandinavian Caledonides-Ostersund to Trondheim. Uppsala Caledonide Symp., Excursion Guide A2, IGCP Proj. 27, Meet., 85 PP. Gee, D.G. and Zachrisson,
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in
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EVOLUTION
WITHlN
SEVE
AND
KOLI
NAPPES,
NORWAY
Wasserburg, J., 1981. %r/3gAr and Rb-Sr age determinations on Quarternary volcanic rocks. Earth Planet. Sci. Lett., 53: 233-244. Reymer, A.P.S., 1979. Investigations into the metamorphic nappes of the central Scandinavian Caledonides on the basis of Rb-Sr and K-Ar age det~ati~s. Thesis, University of Leiden, L&den, 123 pp. Reymer, A.P.S., Boelrijk, N.A.I.M., Heveda, E.H., Priem, H.N.A., Verdumen, E.A. and Verschure, R.H., 1980. A note on the Rb-Sr whole-rock ages in the Seve Nappe of the central Scandinavian Cdledonides: additional evidence for the northward extension of the Sv#no~~~ “front” along the coast of Norway. Norsk Geol. Tidsskr., 60: 139-147. Robbins, C.S., 1972. Radiogenic argon diffusion in muscovite under hydrothermal conditions. Ms. Thesis, Brown University, Providence, RI., 88 pp. (unpublished). Roberts, D., 1971. Timing of Caledonian erogenic activity in the Scandinavian Caledonides. Nature (London), Phys. Sci., 232: 22-23. Roberts, D., 1978. Caledonides of south-central Norway. In: Caledonian-Appalachian orogen of the North Atlantic region. Can. Geol. Surv., IGCP Proj. 27, pp. 31-38. Roberts, D. and Gale, G.H., 1978. The Caledonian-Appalachian Iapetus Ocean. In: D.H. Tarling (Editor), The Evolution of the Earth’s Crust. Academic Press, London, pp. 255-341. Roberts, D. and Gee, D.G., 1985. An introduction to the structure of the Scandinavian Caledonides. In: D.G. Gee and B.A. Sturt (Editors), The Caledonian OrogenScandinavia and Related Areas. Wiley, Chichester, pp. 485-497. Roddick, J.C., 1978, The application of isochron diagrams in ‘%r/3QAr dating: a discussion. Earth Planet. Sci. Lett., 41: 233-244. Roddick, J.C., Cliff, R.A. and Rex, D.C., 1980. The evolution of excess argon in alpine biotites-a “Ar/3QAr analysis. Earth Planet. Sci. Lett., 48: 185-208. Ryan, P.D. and Sturt, B.A., 1985. Early Caledonian orogenesis in northwestern Europe. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide Orogen-Scandinavia and Related Areas. Wiley, Chichester, pp. 1227-1239. Sjostram, H., 1983a. The Seve-Kati nappe complex of the Handel-Storlien-Essandsjoen area, Scandinavian Caledonides. Geol. Foren. Stockholm Fiirh., 105: l-24, Sjostrom, H., 1983b. Structure and metamorphism of the Seve-Koli nappe complex in the Handal-Storlien area, Scandinavian Caledonides. Thesis, Univ. Uppsala, Uppsala, 17 pp. Snee, L.W., Sutter, J.F. and Kelly, EC., 1988. Thermocbronology of economic mineral deposits: dating stages of mineralixation at Panasqueira, Portugal, by high-precision 4oAr/3gAr age-spectrum techniques on muscovite. Econ. Geol., 83: 335-354. Snelling, N.J., 1985. The Chronology of the Geological Record. Geol. Sot. London, Mem., 10: 162 pp. Solyom, Z., Gorbatschev, R. and Johansson, I., 1979a. The
225 Ottfjallet Dolerites. Geochemistry of the dyke swarm in relation to the geodynamics of the Caledonide Grogen in central Scandinavia. Sver. Geol. Unders., Ser. C, 751: 38 PP. Solyom, Z., Andreasson, P.G., and Johansson, I., 1979b, Geochemistry of ~p~~~t~ from Mt. Sylarna, central Scandinavian Caledonides. Geol. F&en. Stockholm F&h., 101: 17-25. Spjeldnaes, N., 1985. Biostratigraphy of the Scandinavian Caledonides. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide Orogen-Scandinavia and Related Areas. Wiley, Chichester, 317-330. Steiger, R.H. and Jager, E., 1977. Subcommission on geochro nology convention on the use of decay constants in geoand cosmochronology. Earth Planet. Sci. Lett., 36: 359-362. Stephens, M.B., 1980. Occurrence, nature, and tectonic significance of volcanic and high-level instrusive rocks within the Swedish Caledonides. In: D.R. Worms (Editor), The Caledonides in the USA. Blacksburg, Va. Poiytech. Inst. and State Univ., Mem., 2: 289-298. Stephens, M.B., 1982. Field relationships, petrochemistry and petrogenesis of the Stekenjokk volcanites, central Swedish Caledonides. Sver. Geol. Unders., Ser. C, 786: 111 pp. Stephens, M.B. and Gee, D.G., 1985. A tectonic model for the evolution of the eugeoclinal terranes in the central Scandinavian Caledonides. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide Orogen-Scandinavia and Related Areas. Wiley, Chichester, pp. 953-978. Stephens, M.B. and Van Roermund, H.L.M., 1984. Occurrence of glaucophane and crossite in eclogites of the Seve Nappes, southern Norrbotten Caledonides, Sweden. Norsk Geol. Tidsskr., 69: 155-163. Strand, T. and Kulling, O., 1972. The Scandinavian Caledonides. Wiley-Interscience, London, 302 pp. Sturt, B.A., 1984. The accretion of ophiolitic terrains in the Scandinavian Caledonides. Geol. Mijnbouw, 63: 201-212. Sturt, B.A., Pringle, I.R. and Roberts, D., 1975. Caledonian nappe sequence of Finnmark, northern Norway, and the timing of orgenic deformation and metamorphism. Geol. Sot. Am. Bull., 86: 710-718. Sturt, B.A., Pringle, I.R. and Ramsay, D.M., 1978. The Finnmarkian phase of the Caledonian orogeny. J. Geol. Sot. London, 135: 597-610. Trouw, R.A.J., 1973. Structural geology of the Marsfjallen area, Caledonides of Vasterbotten, Sweden. Sver. Geol. Unders., Ser. C, 689: 115 pp. Turner, G., 1968. The distribution of potassium and argon in chrondrites. In: L.H. Ahrens (Editor), origin and Distribution of the Elements. Pergamon. New York, pp. 378-398. Turner, G., 1970. Thermal histories of meteorites. In: SK. Runcom (Editors), Paleogeophysics. Academic Press, London, pp. 491-502. Van Roermund, H.L.M., 1985. Eclogites of the Seve nappe, central Scandinavian Caledonides. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide Orogen-Scandinavia and Related Areas. Wiley, Chichester, pp. 873-886. Van Roermund, H.L.M. and Bakker, E., 1984. Structure and
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Tectonophysics, 179 (1990) 199-226
Elsevier Science Publishers B.V., Amsterdam - printed in The Netherlands
40Ar/39Armineral age record of a polyorogenic evolution within the Seve and Kijli nappes, Trmdelag, Norway R.D. Dallmeyer Department of Geology, University of Georgia, Athens, GA 30602 (U.S.A.)
(Received November 9,1989; accepted January 10.1990)
ABSTRACT Dallmeyer, R.D., 1990. ‘?Ar/“Ar mineral age record of a polyorogenic evolution within the Seve and Koli nappes, Trandelag, Norway. Tectonophysics, 179: 199-226. In the central Scandinavian Caledonides, eugeoclinal terranes represented in the KU Nappes were thrust > 500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe Complex are suspected to have been derived from the outer margin of continent Baltica. Hornblende within ampbibolite at one locality within westernmost exposures of Seve Nappe units in Trandelag, Norway, displays an internally discordant %r/39Ar age spectrum which suggests a polyphase thermal evolution including: (1) initial post-metamorphic cooling through argon retention temperatures at c. 475-480 Ma; and, (2) partial rejuvenation during a Silurian (c. 400-425 Ma) thermal overprint. Muscovite within the exposure was more extensively rejuvenated during the Silurian overprint. At twelve other Trondelag sample locations, hornblende, muscovite and biotite within both the Seve and Kirli Nappe Complexes record post-metamorphic cooling ages between 420 and 435 Ma and display no direct evidence of a pre-Silurian thermal record. However, the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre-Silurian tectonothermal history. Comparison with %r/39Ar mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenie evolution consisting of: (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the Late Cambrian-Early Ordovician over a west-dipping subduction zone; related arc volcanic activity occurred in more outboard, western sectors; (2) ductile imbrication of high- and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltica continental margin; and, (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian. Crystalline Baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high-temperature eclogites. Previously reported crystallization ages of c. 425 Ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures. Following eastward nappe transport, the basement experienced rapid uplift and resultant adiabatic decompression. This was followed by rapid cooling through argon retention temperatures at 390-400 Ma.
Introduction The structure of the Scandinavian Caledonides is dominated by a variety of major nappe complexes which were emplaced onto the Baltoscandian platform during early to middle Paleozoic closure of the Iapetus Ocean (e.g., Harland and Gayer, 1972; Gee, 1975a; Roberts and Gale, 1978; Stephens and Gee, 1985). The nappes 0040-1951/90/%03.50 Q 1990 - Elsevier Science Publishers B.V.
are of varying metamorphic grade and display marked differences in the complexity of internal deformation. As a result, correlation of specific metamorphic, deformational, and thrusting events along the length of the orogen has been uncertain and controversial (e.g., Gee and Roberts, 1983; Roberts and Gee, 1985). Lower tectonic units within central and southem parts of the Scandinavian Caledonides are
200
represented by low-grade, metasedimentary successions of late Proterozoic to Middle Silurian age. The older successions originated along the Baltoscandian continental margin during separation of Baltica and Laurentia. Younger sequences were deposited during closure of the Iapetus oceanic tract. Si~fic~t thicknesses of wester& derived elastic material of Middle Ordovician and EariyMiddle Silurian age in these successions provide evidence of two periods of outboard instability along the Baltoscandian margin (e.g., Gee, 1975b). Early attempts to describe the tectonic evolution of central parts of the Caledonian orogen noted this instability (e.g., Gee, 1975a), however most of the internal deformation, metamorphism, and nappe translation has been considered to be of Middle Silurian through Early Devonian (Scandian) age (e.g., Roberts, 1971; Nicholson, 1974; Gee and Wilson, 1974; Gee, 1975b). In more northerly parts of the orogen, sedimentary sequences within lower tectonic units only range up into the Tremadoc and therefore provide little c~onolo~c control for translation of overlying nappes (e.g., Sturt et al., 1975). However, deformed and variably metamorphosed sequences within several of the higher allochthons are locally intruded by syn-tectonic plutons which record Rb-Sr whole-rock isochron dates of 530-490 Ma * (Sturt et al., 1975, 1978). Host metasedimentary sequences are thought to be correlative with others developed elsewhere within outer parts of the Baltoscandian miogeocline, and the radiometric controls were therefore interpreted to indicate that a major period of late Cambrian-Early Ordovician polyphase folding, metamorphism, and initial nappe imbrication occurred along this northern segment of the continental margin of Baltica. This event has been termed the Finnmarkian Orogeny (e.g., Sturt et al., 1975, 1978). The suggested diachronism of tectonothermal activity (Finnmarkian to Scandian) along the early to middle Paleozoic continental margin of Baltoscandia has been difficult to reconcile with
* All radiometric ages listed in this report are based on the
decay constants and isotopic abundance ratios presented by Steiger and J&ger (1977).
R.D.
DALLMEYER
otherwise nearly continuous sedimentological, strati~ap~c, and tectonic trends. Geochronological results in widely separated parts of the central Scandinavian Caledonides suggest that preScandian tectonothermal activity may have influenced parts of the tectonostratigraphy (see review in Dallmeyer, 1988). The evidence includes: (1) Ordovician Rb-Sr whole-rock dates reported for various high-grade metamorphic units within several nappe complexes (Reymer, 1979; Claesson, 1979); (2) an Early Ordovician, Rb-Sr thin-slab isochron date obtained from mylonitic rocks developed along an internal thrust within one of the major nappe complexes (Claesson, 1980); and, (3) 40!/39Ar hornblende ages reported from several areas (e.g., Dalhneyer et al., 1985; Dallmeyer and Gee, 1986, 1988). Although the suggested preScandian erogenic activity is consistent with the record of outboard instability provided by westerly derived, Ordovician elastic sequences within the Baltoscandian miogeocline, the regional extent and intensity of Scandian vs. Finnmarkian tectonothe~~ activity has been uncertain. Results of 40Ar/39Ar mineral dating studies in central J%mtland (Dallmeyer et al., 1985; Dallmeyer and Gee, 1988) and Norrbotten (Dallmeyer and Gee, 1986) Counties of Sweden suggest that Scandian metamorphic temperatures in some of the higher grade units did not exceed c. 500°C. This indicates that the observed high-grade assemblages must have developed during an earlier tectonothermal event which affected both outer parts of the Baltoscandian miogeocline and some of the more outboard (western) eugeoclinal terranes. The thermal evolution reflected by these ‘%r/39Ar mineral dates suggests that ocean-directed (westward) subduction was active outboard of the Baltoscandian margin by at least the Late Cambrian. Fauna1 ch~acte~stics of structurally high ~lochthons suggest that this tectonic activity occurred while Baltica and Laurentia were separated by a considerable expanse of the Iapetus Ocean (e.g., Bruton and Bockelie, 1980). A comprehensive ge~~onolo~~al research program is underway to more fully document the extent and character of the tectonothermal record within the central Scandinavian Caledonides. This report presents new 40Ar/39Ar mineral dates for
POLYOROGENIC
EVOLUTION
WlTHlN
SEVE
. . . q
UPPER
Metoredimcn+ory
Sevt Noppe Complex
ALLOCHTHON
Precombrlon
Basement
ALLOCHTHON
:::*
~~~~~~~~~~+~~~~~~“+
MIDDLE
Sedimentary
Cover
I::
PARAUTOCHTHON-
And cove, Sequences
ALLOCHTHON
Precombrton
.:.:.
Kati
:,p:.;.:; OeVOlllLln $:;j:.X.:
Helgatond
UPPERMOST
NWW Complex ALLOCHTHDN
-
201
AND KijLI NAPPES, NORWAY
q
Nopp
Comptex
Cover
Basement
LOWER
AUTOCHTHON
-
Fig. 1. Regional geologic setting of the central Scandinavian Caledonides (adapted from Gee et al., 1985). Study area (Fig. 2) is outlined.
~~hthonous rocks of both B~tosc~~an and eugeoclinal affinities exposed in the area between Trondheimsfjorden and the international border, west-central Trsndelag County, Norway (Fig. 1). These results bear directly on both the chronology and nature of praScandian orogenesis, and also provide controls for understanding the early to middle Paleozoic t~tonothe~~ evolution of the Baltoscandian miogeocline. Regional geologic setting The Scandinavian Caledonides may be described in terms of several major tectonic units (Kulling in Strand and Kulhng, 1972; Gee and Zachrisson, 1979; Roberts and Gee, 1985), including: (1) autochthonous and parautochthonous sequences; (2) Lower, Middle, Upper, and Uppermost Allochthons; and, (3) ueoautochthonous, molasse deposits of Early to Middle Devonian
age. The dist~bution of these units witbin central parts of the orogen is shown in Fig. 1. Although the various allochthons generally thin westward, most may be traced across the entire orogen to the Norwegian coast. The entire nappe sequence was affected by regional north-south folding following structural emplacement onto the Baltoscandian platform. The overall t~tonostratigraphy of the central Scandinavian Caledomdes has been treated at length elsewhere (e.g., Gee, 1975b; Dyrehus et al., 1980; Roberts and Gee, 1985) and will not be reviewed here. Only those relationships particularly relevant to interpretation of the present geechronological results from the Upper Allochthon will be discussed. The Upper Allochthon is an extremely heterogeneous, internally imbricated structural complex. Basal tectonic units are composed of a westward thinning, up to 5 km thick assemblage of thrust sheets referred to as the Seve Nappe Complex
202
(e.g., Zachrisson, 1969, 1973; Trouw, 1973; Zwart, 1974). This complex consist largely of amphibolite and higher grade metasedimentary and metaigneous units which are locally in tectonic contact with penetratively deformed and severely retrogressed middle to late Proterozoic crystalline basement rocks (Reymer et al., 1980; Claesson, 1981). Tectonic contacts between various structural units within lower parts of the Upper Allochthon are frequently marked by development of high-temperature ductile shear zones (e.g., Trouw, 1973; Ambom, 1980; Sjestrom, 1983). Variably retrogressed eclogites have been described from several structural units of the Seve Nappe Complex (e.g., Van Roermund and Bakker, 1984; Stephens and Roermund, 1984; Van Roermund, 1985; Andreasson et al., 1985). In Wsterbotten County, Sweden, tectonic units of high-grade gneissic rocks are structurally underlain and overlain by lower grade schists and amphibolites. The resultant three subdivisions have been informally termed the eastern, central, and western Seve belts (Trouw, 1973). The overall chemical character of basic rocks within lower parts of the Seve nappes is similar to that of dolerites in the S&-v Nappes of the underlying Middle Allochthon (Solyom et al., 1979a, 1979b). Metasedimentary protoliths in the two tectonic units are also comparable. It is therefore likely that most of the rocks now constituting the Seve Nappe Complex originated in distal, latest Proterozoic to Early Cambrian environments along the continental margin of Baltoscandia (Gee, 1975b; Dallmeyer and Gee, 1986). Overlying the Seve Nappe Complex is a succession of predominantly greenschist or lower amphibolite grade, metavolcanic and metasedimentary units of the Kiili Nappe Complex (Zachrisson, 1969; Stephens, 1980). Fossil controls suggest an Early Ordovician to Early Silurian age for some protoliths. Several Middle Ordovician units within lower parts of the Kiili Nappe Complex are characterized by lithic successions that can be directly correlated with the Baltoscandian margin (Gee, 1975b). The Early Ordovician successions appear to be unrelated to Baltica and were apparently derived from a variety of eugeoclinal terranes located within the Iapetus Oceanic tract. These exotic units were accreted to the Bal-
R.D. DALLMEYER
toscandian margin during the early through middle ‘Paleozoic (Stephens and Gee, 1985). The Koli Nappe Complex in the central Scandinavian Caledonides may be divided into three groups of related thrust sheets referred to as the Lower, Middle, and Upper Koli Nappes (Stephens, 1980). Lower Koli Nappes are dominated by arc-related volcanic rocks (Stephens, 1980) which record U-Pb zircon crystallization dates of c. 490 Ma (Claesson et al., 1983). These volcanic sequences pass upward into Middle Ordovician graywackes and fanglomerates which are conformably overlain by Upper Ordovician sandstones and limestones and Lower Silurian shales and graywackes (Kulling, 1933). The Middle Koli is represented by a sequence of nappes in which related successions are typically inverted. Arc volcanites, at least locally of Early Ordovician age, are overlain by volcaniclastic conglomerates and/or graywackes and calcareous phyllites. Eastern exposures of the metasedimentary rocks are intruded by metamorphosed gabbro plutons with MORB affinities; likewise the arc-volcanites contain a late (but pre-erogenic) intrusive suite of MORB basic rocks (Stephens, 1982). Post-arc sedimentary units in the rifted-arc association are probably of Middle to Late Ordovician age; correlations southward into eastern Trondelag County, Norway, suggest that they pass up into Llandovery black shales and then graywackes and fanglomerates. The Upper Kbli Nappes are locally represented by variably fragmented ophiolitic associations (Fumes et al., 1980; Grenne et al., 1980; Sturt, 1984) which are unconformably overlain by Early Ordovician sedimentary successions containing faunas of North American affinity (Bruton and Bockelie, 1980; Spjeldnaes, 1985). Thus, at least some protoliths within higher tectonic units of the Upper Allochthon appear to have been derived from euogeoclinal terranes which were more closely related to Laurentia than to Baltica. Locally within higher structural units of the Upper Allochthon, a major pre-Ashgill unconformity reflects significant pre-Scandian polydeformation and metamorphsim (Sturt, 1984; Ryan and Sturt, 1985). In addition, syn- to late-tectonic trondhjemites in Middle and Upper Koli Nappe
POLYOROOENIC EVOLUTION WITHIN SEVE AND KiiLI NAPPES, NORWAY
units have yielded 450-510 Ma U-Pb zircon and/or Rb-Sr whole-rock isochron ages (Klingspor and Gee, 1981). Geology of the sample area Introduction ““hr/39Ar incremental-release ages have been determined for minerals separated from rocks within the Seve and Kiili Nappe Complexes exposed in the area between Trondheimsfjorden and the international border in west-central Trsndelag County, Norway (Fig. 1). Summaries of the overall geologic character of the central Scandinavian Caledonides have been presented by many workers, including Roberts (1978), Wolff (1980), Gee and Kumpulainen (1980), Gee and Wolf (1981), and Gee et al. (1985). Therefore, only a brief summary of geologic relationships within the present study area (and adjacent portions of west-central JZlmtland County, Sweden) is present here. Baltoscandian Nappe Complexes Representatives of the Lower, Middle, and Upper Allochthons are found along the transect (Fig. 1). These are deformed into a series of upright, generally north-south trending folds which have gently undulating axes. As a result of this folding, higher portions of the tectonostratigraphy are preserved along doubly-plunging synforms (Are, Helags-Tlforsen, and Trsndelag structures). Lower parts of the tectonostratigraphy are exposed within antiformal culminations, including the Mullfj%llet, Skardora, Tomme&, and GrongOlden structures. A variety of Precambrian crystalline rocks constitute an autochthonous foundation for the central Scandinavian Caledonides. Tbis terrane is nonconformably overlain by a thin sequence of autochthonous Cambrian shales and Ordovician limestones. Precambrian porphyries and granitic rocks are exposed in several windows in western I&inland and eastern Trondelag. Along the culmination of the Grong-Olden structure the Precambrian terrane may be traced continuously
203
for c. 200 km from western Jtitland to the Norwegian coast. This terrane is overlain by successions of quart&e, radioactive black shales, and limestones which find direct correlatives in autochthonous successions exposed east of the thrust front. Rocks of the Lower Allochthon exposed along the eastern margin of the orogen are dominated by imbricated, low-grade metasedimentary formations of the Jtitland Supergroup which range up to Early Silurian (Llandovery) age. The amount of basement incorporated in the Lower Allochthon increases westward from the Are synform, and overlying metasedimentary successions are restricted to minor quart&es and local exposures of radioactive black phyllites and limestones. The Middle Allochthon is intermittently exposed in the study area. It is a composite nappe complex composed of various late Proterozoic elastic sequences which are imbricated with tectonic units of variably deformed and retrogressed Middle Proterozoic crystalline basement rocks. Upper composite units (S&v Nappes) include basal basement slices which are cut by dolerite dikes. These are tectonically overlain by feldspathic metasandstones which are intruded by similar dolerite dikes. The various units of the Middle Allochthon are all in variably ductile fault contact. Claesson (1980) reported a Rb-Sr thinslab isochron age of 485 f 50 Ma for mylonitic rocks developed along one of these internal Middle Allochthon thrusts in southern Jlltland, suggesting that initial assembly of the nappe complex may have begun in the Ordovician rather than in the Silurian as previously accepted. However, basal tectonic units of the Middle Allochthon locally overrode Silurian metasedimentary successions of the Lower Allochthon during emplacement into its present position on the Baltoscandian platform. The Middle Allochthon markedly thins westward. This is accompanied by an increase in the grade of metamorphism and the extent of internal deformation. Gee (1978) and Kautsky (1978) demonstrated that Middle Allochthon units (Offerdal and S&v Nappes) may be traced into Norway, although they undergo marked local changes in thickness, often being represented by only isolated tectonic lenses. In eastern Trondelag, Middle Al-
204
lochthon rocks are included in the Remsklepp Nappe of Wolff (1976, 1980). In addition, in the Gppdal area of southwestern Tmndelag, KrilI (1980) has defined the Risberget and Saetre Nappes, which he correlates with lithic sequences of the Middle Allochthon exposed in Sweden. Krill has demonstrated that following their emplacement onto the Baltoscandian margin, these western parts of the Middle Allochthon were complexly interfolded within underlying parautochthonous crystalline basement. Hornblende and muscovite within both basement and cover structural units exposed northwest of Trondheimsforden record similar 40Ar/39Ar plateau ages ranging between c. 390 and 400 Ma suggesting relatively rapid post-Scandian metamorphic cooling (Dallmeyer et al., 1990). Upper Allochthon Seve Nappe Complex
Exposures of lower parts of the Upper Allochthon are represented by the Seve Nappe Complex which is comprised by three contrasting tectonic units. The lowermost structural unit consists largely of polydeformed psammitic schists and amphibolites. These record upper greenschist to upper ~p~bo~te facies in regional rnet~o~~c assemblages, and locally complex textures suggest a polymetamorphic evolution. The middle unit is dominated by a polydeformed, granulite facies complex and minor metabasic rocks (Areskutan and Snasahijgarna Nappes). The contact between this terrane and underlying Seve met~ed~ent~ units is tectonic and marked by development of high-temperature mylonitic rocks (e.g., Ambom 1980; SjiistriSm 1983a, 1983b). The uppermost tectonic unit of the Seve Complex is only preserved in the Hand51 area, and consists of polydeformed amphibolite and kyanite schists. Geochronologic work by Claesson (1981) indicates a middle Proterozoic history for at least a part of the gneissic terrane within the Areskutan Nappe. Rb-Sr and U-Pb zircon studies by Claesson in this area further suggest that the Seve rocks underwent high-grade Scan&an metamorphism between c. 415 and 390 Ma. Stepwise zircon dissolution experiments indicate the possibility an
R.D.
DALLMEYER
earlier Caledonian thermal event. Dallmeyer et al. (1985) presented 40Ar/39Ar results from the Seve Nappe Complex exposed in J%mtland. Hornblende from various structural units of the Seve Nappe Complex exposed in the Are synform (Fig. 1) typically display internally discordant age spectra as a result of the incorporation of extraneous argon. However, variably well-defined plateau dates of 455 f 10 Ma to 458 rt 9 Ma are generally recorded. Biotite and muscovite from Seve Nappes exposed along the international border in the T&mforsen synform (Fig. 1) yield plateau dates of 408 f 6 Ma and 415 f 7 Ma respectively. Kiili Nappe Complex
Upper parts of the Upper Allochthon are represented by the Kijli Nappe Complex. Several distinct structural units have been defined along the Helags-Tannforsen synform (e.g., Beckhohnen, 1978, 1982; Sjiistriim, 1983a, 1983b). The lowest tectonic unit (Duved Nappe) is separated from underlying Seve Nappes by a major mylonite zone and consists of a variety of low-grade, polydeformed metasedimentary and minor metavolcanic rocks. Although diagnostic fossils are lacking, the overall lithic character and stratigraphic succession is similar to that of fossil-bearing, Upper Ordovician-Lower Silurian Kiili sequences farther north in tisterbotten County, Sweden (e.g., Zachrisson, 1969). The lower sequence of the Tglnnforsen Kiili is in tectonic contact with overlying structural units which consist largely of polydeformed, generally low-grade phyllite, calcareous mica schist, and hornblende g~bensc~effer. Several different nappe units have been mapped with intervening tectonic contacts marked by development of superposed ductile and brittle fabrics which suggest a polygenetic internal imbrication history. The TZnnforsen Koli Nappes thin westward and are tectonically overlain by the Trondheim Nappe Complex along the western limb of the Skardora antiform. However, lowgrade, polydeformed talc-phyllites and talc-schists of ~~forsen-tie reappear in western Trondelag near Levanger (Figs. 1 and 2). Rocks of the Trondheim Nappe Complex compose the highest tectonic units exposed within the present study area and represent upper structural parts of the
205
POLYOROGENIC EVOLUTION WITHIN SEVE AND KijLl NAPPES, NORWAY
Stffren Nappa
% 2
Gulo Noppe 3 P Mer(tker Nappe c TInnfors
ond
Levooger Noppes Seve
Noppe
Complex MIDDLE ALLDCHTNON LOWER ALLOCHTHON * . I!!!!
Sedimentury Cover Precombrion Basement
AUTOCHTHONPARAUTOCHTHON
Fig. 2. Simplified geologic map of east-central ‘lixmdelag (adapted from Gee et al., 1985). ‘%r/39Ar sample localities are numbered (Z-13).
Upper ~~hthon. aloud the internal structure of the nappe complex has long been controversial, most workers agree that a generally north-trending, pr~o~~tly rnet~~~ent~ unit (Gula Nappe) is bordered on the west and the east by metasedimentary and metavolcanic units of generally lower grade (Fig. 2). The western sequences constitute the Stncren Nappe, and these locally contain fossil-bearing rocks of Arenig-Ashgill age. The eastern succession is represented by the Mer&ker Nappe, with uppermost units containing graptolite-bearing, Llandovery phylhtes. Many units of the Gula, Steren, and Mer&er Nappes record polydeformational fabrics and locally complex metamorphic textures; therefore, details of the t~tono~e~~ evolution of the Trondheim Nappe Complex are uncertain. Clearly some of the metamorphism and deformation must be of Scandian age because fo~~-~a~g sequences of Early Silurian age are affected. However, numerous internal tectonic contacts preclude assigning alI the t~tonothe~ record to this organic event, and, in fact, preliminary Rb-Sr whole rock and U-Pb zircon analyses of trondhjemite dikes cutting poiydeformed and previously rnet~o~hos~ rocks of the Gula nappe suggest Ordovician crystallization ages (Rlingspor and Gee, 1981). Biotite from these dies yield conventional K-Ar dates of
c. 400 Ma (Wilson et al., 1973) su~~~g a prolonged or polyphase thermal evolution for the Gula Nappe. In addition, Wilson et al. (1973) reported inventions K-Ar dates for minerals from the Meraker Nappe. Hornblende dates range between 426 and 550 Ma whereas muscovite and biotite ages range between 411 and 444 Ma. Because of possible incorporation of extraneous argon components during Scandian metamorphism, unequivocal ~t~retation of these data has not been possible. However, the older homblende dates may be suggestive of a significant pm-Scandian t~tono~e~~ record. DalImeyer et al. (1985) reported 4oAr/39Ar ages for minerals separated from several of the lower structural units of the Kijli Nappe Complex exposed along the T&mforsen synform (Fig. 1). Hornblende from the Middagsfj~let Nappe yielded variably discordant age spectra as a result of widespread incorporation of extraneous argon components. Ages systematically decreased t~o~~out most analysis to yield variably well-defined plateau ages which range from 475 -fr 10 Ma to 510 f 11 Ma. Coexisting biotite shows widely variable plateau dates (413 of:6 Ma to 524 f 9 Ma) as a result of incorporation of extraneous argon components. Muscovite from these units records mutually concordant plateau dates between 409 f
206
7 Ma to 425 f 7 Ma. Hornblende and biotite from an exposure within a structurally lower portion of the Middagsfjallet Nappe yielded mutually similar, well-defined plateau dates of 414 + 9 Ma and 418 + 6 Ma respectively. Biotites within a lower tectonic unit of the Tginnforsen Kijli (Gevsjisn Nappe) yielded plateau dates of 415 f 6 Ma to 427 f 7 Ma. Tectonothermal evolution Field relationships and the geochronology previously reported for the Upper Allochthon exposed in eastern segments of the central Scandinavian Caledonides have helped to constrain the tectonothermal evolution. 40Ar/3gAr hornblende ages from lower (Seve) and upper (Kiili) parts of the Upper Allochthon suggest a complex, polyphase pre-Scandian thermal history. The 455-458 Ma hornblende dates from the Seve nappes clearly indicate a younger period of cooling through hornblende closure temperatures than the 475-510 Ma dates in the Kiili Nappes. The thermal conditions of Caledonian rne~o~~srn in both areas do not appear to have been greatly in excess of the temperatures required for intracrystalline retention of argon in hornblende, and it is unlikely that the difference in dates reflects diachronous cooling following a single tectonotbermal event. It is more likely that separate pre-Scandian tectonothermal events are recorded. The effects of Scandian orogenesis were regionally of sufficient magnitude to reset all mica argon systems and locally modify or totally reset hornblende argon systems. There appears to have been a significant diachronism between post-Scandian metamorphic cooling in eastern parts of the area (427-429 Ma biotite dates from the Seve nappes at Are) and coastal areas (390-400 Ma hornblende and muscovite dates). This is consistent with the east to west younging trend in post-Scandian thermal decay outlined for more northerly parts of the orogen. In the central Scandinavian Caledonides, post-Scandian cooling although apparently diachronous, was everywhere relatively rapid because similar dates are recorded by minerals with widely different argon closure temperatures (e.g., hornblende and muscovite in the Tginnforsen Koli and in the coastal sections).
RD.
DALLMEYER
The 40Ar/3gAr plateau ages recorded by homblende within lower portions of the Upper Allochthon exposed in the Ttinforsen Synform reflect significant erogenic activity during and/or just prior to 475-510 Ma. A 475-510 Ma range corresponds to an Early Ordovician interval on most time-scale calibrations, perhaps extending into the latest Cambrian. By contrast, lower tectonic units of the Ttinnforsen Kijli Complex (Duved Nappe) contain metasedimentary successions which appear to be direct lithostratigraphic correlatives with Ashes-Ll~dove~ Koli sequences exposed at similar tectonic levels elsewhere in the Scandinavian Caledonides. These Tannforsen sequences must therefore have been tectonically imbricated with the older metamorphic terrane during a late- to post-Llandovery phase of assembly of the Koli Nappe Complex. All Koli Nappes exposed in the nnnforsen Synform record a 410425 Ma overprint, although it is of variable grade. According to most time-scale calibrations this interval corresponds to the Middle-Late Silurian (We~~k-Proof), and suggests that this thermal event was likely associated with late phases of tectonic imbrication of the Kbli Nappe Complex. This is consistent with development of mylonitic zones along internal thrust contacts within the K&i Nappe Complex which indicate that at least some components of thrust translation occurred under elevated temperatures. However, tectonic movement appears to have outlasted maintenance of elevated metamorphic conditions because most internal thrust contacts display evidence of later brittle reactivation (Betoken, 1982). This is consistent with the rapid Late Silurian post-metamorphic cooling suggested by the “OAr/3gAr mineral dates, and likely reflects final phases of assembly of the Koli Nappe Complex during its translation to higher structural levels within the succession of Caledonian allochthons. The tectonothermal evolution of the Seve Nappe Complex in eastern sectors of the central Scandinavian Caledonides also appears to have been polyphase. High-grade, early Caledonian metamorphic crystallization must have occurred prior to cooling through hornblende retention temperatures between 455 and 460 Ma. Imbrication of basement (gram&e terrane) and cover compo-
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
KijLI
NAPPFS,
NORWAY
nents of the Seve Nappe Complex must have occurred earlier because Arnbom (1980) provided evidence that high temperatures (> 55OOC) were maintained during development of mylonitic rocks along ~te~e~g tectonic contacts. These results combine to suggest that cooling through the temperatures appropriate for intracrystalline retention of argon in hornblende likely occurred when the contrasting metamorphic terranes were translated to higher crustal levels during initial tectonic assembly of basement and cover components of the Seve Nappe Complex. Both nappe units appear to have experienced a similar thermal evolution following their initial imbrication. Previously reported geochronologic results have not permitted resolution of the effects of Scandian vs. earlier tectonothermal events in western exposures of the Seve Nappe Complex or structurally higher of the Koli Nappe Complex exposed in Trondelag. %r/39Ar mineral ages from this area are presented herein and provide important new constraints for the tectonothermal chronology of orogenesis recorded within the central Scandinavian Caledonides. Analytical methods The techniques used during ““Ar/39Ar analysis of the Trondelag samples generally followed those described in detail by Dalhneyer and Keppie (1987). Mineral concentrates were wrapped in aluminum-foil packets, encapsulated in sealed quartz vials, and irradiated for 40 hrs at 1000 Kw in the central thimble position of the U.S. Geological Survey TRIGA reactor in Denver, Colorado. Variations in the flwr of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including MMhb-1 (Alexander et al., 1978). The samples were incrementally heated until fused with an RF generator. Each heating step was maintained for 30 min. Measured isotopic ratios were corrected for the effects of mass discrimination, total-system blank levels, and interfering isotopes produced during irradiation (with factors reported by Dalrymple and others, 1981 for the reactor used in the pres-
207
ent study). Apparent %r/39Ar ages were calculated from the corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and 3ager (1977). Two categories of unce~~nti~ are en~~tered in ‘%r/ 39Arincremental-release dating. One group involves intralaboratory uncertainties related to measurement of the isotopic ratios used in the age equation. The other group considers interlaboratory ~ce~~nties in the other parameters used in the age equation (monitor age, J-value determination, etc.), and are the same for each gas increment evolved from a particular sample. Therefore, to evaluate the significance of potential incremental age variations within a sample, only intralaboratory uncertainties should be considered. These are reported here and have been calculated by statistical propagation of uncertainties associated with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are c. 1.25-1.5% of the quoted age. Total-gas ages have been computed for each sample by appropriate weighting of the age and percent 39Ar released witbin each temperature increment. A “plateau” is considered to be defined if the following criteria are met: (1) increments have similar apparent K/Ca ratios; and (2) ages recorded by two or more contiguous gas fractions each representing > 4% of the total 39Arevolved (and together constitu~g > 50% of the total quantity of 39Ar evolved) are mutually similar within a f 1% intralaboratory uncertainty. Analysis of the MMhb-1 monitor indicate that apparent K/Ca ratios may be calculated through the relationship of 0.518 ( f 0.~5) X 39At/ 37Ar corrected. The analysis of the amphibole concentrates have been plotted on 36Ar/40Ar vs. 39h/40Ar isotope correlation diagrams (Roddick et al., 1980; Radicati de Brozolo et al., 1981). Regression techniques followed the methods of York (1969). The mean square of the weighted deviated (MSWD) has been used to evaluate isotopic correlations. Roddick (1978) suggests that an MSWD > 2.5 indicates scatter about a correlation line greater than that which can be explained only by experimental errors.
208
Results
Twelve hornblende, six muscovite, and four biotite concentrates have been prepared from various structural units of the Upper Allochthon (Seve and Koli Nappe Complexes) exposed in eastcentral Trondelag (sample locations shown in Fig. 2 and described in the Appendix). These have been analyzed using ‘?Ar/ 39Arincremental-release techniques. The analytical data are listed in Tables 1, 3 and 4 and are portrayed as age spectra in Figs. 3-10. Isotope correlation calculations from the amphibole data are listed in Table 2. Apparent K/Ca ratios are relatively small and display considerable intrasample variations in the hornblende analyses (Figs. 3, 5, 6 and 8). Apparent K/Ca ratios are large and show no significant or systematic variations throughout the mica analyses. Therefore they are not presented with the muscovite or biotite age spectra.
Seve Nappe Complex (1) Hornblende: Amphibole concentrates were prepared from samples of foliated amphibolite collected within exposures of the Seve Nappe Complex along the southeastern limb of the Grong-Olden antiform (location 1, Fig. 2) and along the western limb of the Tommerbs antiform (locations 2-4, Fig. 2. These display variably discordant age spectra (Table 1, Fig. 3) which define total-gas ages ranging between 461.8 + 3.6 Ma (I ) and 525.6 f 5.2 Ma (2). Low-temperature gas fractions display considerable variation in apparent ages. These are mathed by intrasample fluctuations in apparent K/Ca ratios which suggests experimental evolution of argon from compositionalIy distinct, relatively non-retentive phases. These could be represented by very minor, optically undetectable mineralogic contaminants in the hornblende concentrates and/or petrographically unresolvable exsolution or compositional zonation within constituent amphibole grains. In general, most gas fractions evolved from the Seve homblende concentrates at intermediate and high experimental temperatures are characterized by similar intrasample apparent K/Ca ratios, indicating
R.D. DALLMEY
ER
evolution of gas occurred from compositionally uniform amphibole phases. However, markedly different apparent age relationships are observed in these portions of the four experiments. The 775 “-850 o C increments evolved from sample 1 (comprising c. 79% of the total gas evolution) record similar apparent ages which define a plateau of 439.9 f 2.4 Ma. The plateau data yield a well-defined isotope correlation (MSWD < 2.0; Table 2) which defines an inverse ordinate intercept of 395.2 &-5.1. This is larger than the 295.5 ‘?Ar/36Ar ratio in the present-day atmosphere, and suggests that minor extraneous ((‘excess”) argon is present within constituent homblende grains. Using the inverse abscissa intercept (40Ar/39Ar) in the age equation yields a plateau isotope correlation age of 429.3 f 0.9 Ma. Because this calculation does not depend upon assumption of a modem-day “OAr/36Ar ratio it is considered more reliable than the plateau age directly calculated from the analytical data. The 429 Ma age is interpreted to date the last cooling through those temperatures required for intracrystalline retention of argon within constituent hornblende grains. Harrison (1981) indicated that closure temperatures for argon systems within igneous hornblende are not significantly affected by compositional variations. He suggested that values of 500 f 25°C are appropriate in the range of cooling rates likely to be encountered in most geologic settings. Samples 2 and 3 display similarly discordant spectra in which apparent ages systematically decrease throughout intermediate-temperature parts of the analyses to a high-temperature minimum and then increase slightly in the fusion increments. Similar types of “saddle-shaped” discordance in hornblende 40Ar/39Ar age spectra have been interpreted by Dallmeyer (1975), Harrison and McDougall (1981), Dallmeyer and Rivers (1983) and Dallmeyer et al. (1985) to reflect experimental liberation of gas with large and variable components of extraneous (excess) 40Ar relative to intracrystalline radiogenic ‘?Ar. The 740 “-875°C increments evolved from sample 2 (constituting c. 78% of the total) yield a well-defined isotope correlation corresponding to an age of 435.4 + 4.1 Ma (inverse ordinate intercept of 545.9 zt 12.2). The 815”C-fusion increments of sample 3 (c. 66%
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
KijLl
NAPPES,
209
NORWAY
TABLE 1 ‘OAr/39Ar analytical data for incremental heating experiments on hornblende
concentrates
from the Seve and Koli Nappe
Complexes, central Scandinavian Caledonides, Trondelag, Norway 39Ar
!%@Ar
36Arc,
temp.
(W of
non-
(W)
(“C)
total)
atmos. ’
Release
( @Ar/ 39Ar) a
( 36Ar/ 39Ar) a
( “Ar/
39Ar) b
Apparent age d (Me)
Seve Nappe Complex
Sample I: J = 0.008708 500
745.01
1.60154
36.340
0.26
36.87
0.62
2233.0 f 201.9
600
726.99
1.95306
52.042
0.33
21.19
0.72
1569.6 f 144.3
700
495.11
1.49331
49.552
0.70
11.68
0.90
755.1 f
81.3
750
90.08
0.20865
48.464
2.96
35.87
6.32
459.8 f
10.1
775
47.25
0.06943
48.248
10.95
64.71
18.90
438.1 f
2.6
800
45.38
0.05937
45.611
10.30
69.41
20.90
448.8 f
2.9
825
40.51
0.04671
45.079
11.34
74.86
26.25
433.8 f
1.8
850
34.31
0.02432
46.051
46.10
89.93
51.51
440.3 f
1.5
875
41.24
0.03391
45.989
11.04
84.65
36.88
491.5 f
2.8
900
51.04
0.06189
44.470
3.19
71.15
19.54
508.2 f
3.8
107.21
0.23933
28.534
2.82
36.17
3.24
533.2 f
4.7
49.92
0.06987
45.674
100.00
78.09
35.93
461.8 f
3.6
439.9 f
2.4
2371.9 f
64.2
Fusion Total
Total without 500 “-750 O, 875 O, 900 o C and fusion
78.88
Sample 2A: J = 0.008985 550
436.21
0.45316
3.236
0.75
69.36
0.19
600
244.55
0.58793
3.710
0.62
29.08
0.17
892.9 * 110.2
625
175.02
0.41680
4.793
0.38
29.84
0.31
695.9 f 107.3
675
110.32
0.17274
14.987
0.88
54.82
2.36
788.9 k 31.8
700
75.88
0.12128
16.047
1.81
54.46
3.60
574.6 f
20.1
720
58.30
0.06143
16.955
3.59
71.19
7.51
577.1 f
12.2
740
44.19
0.03054
17.427
10.91
82.73
15.52
517.4 f
3.4
760
41.98
0.02522
17.345
12.27
85.56
18.71
509.5 f
3.1
780
42.76
0.02779
17.309
10.52
84.04
16.94
509.6 f
3.3
800
40.63
0.02394
17.356
13.41
86.01
19.72
497.4 *
3.4
825
37.92
0.01861
17.364
20.47
89.17
25.38
483.3 f
3.7
875
39.09
0.01963
17.203
10.90
88.68
23.83
493.9 f
4.1
900
46.16
0.05222
17.049
4.47
69.53
8.88
461.5 f
5.2
Fusion
67.99
0.10401
16.795
9.02
56.77
4.39
542.0 f
4.8
Total
50.04
0.04479
16.980
lOQ.00
80.88
17.36
525.6 f
5.2
9.3
Sample 3: J = 0.008702 625
94.06
0.10859
2.670
3.69
66.11
0.67
781.2 f
675
50.24
0.03462
7.639
5.40
80.85
6.00
548.2 f
5.5
715
44.27
0.02561
10.018
5.46
84.71
10.64
512.2 f
6.4
725
43.84
0.02792
9.821
4.33
82.97
9.57
498.7 f
6.2
735
44.27
0.03192
10.456
3.26
80.58
a.91
490.5 f
a.5
765
40.71
0.02408
11.405
4.55
84.76
12.88
476.6 +
5.7
785
38.88
0.01899
11.675
7.11
87.96
16.72
412.9 f
5.8
815
37.00
0.01584
11.858
9.40
89.91
20.37
461.6 f
7.2
825
36.45
0.01490
12.094
10.77
90.57
22.08
458.6 f
5.4
835
35.78
0.01316
12.210
9.13
91.86
25.24
456.8 +
4.9
850
35.31
0.01315
12.378
8.04
91.81
25.61
452.0 *
4.7
865
35.85
0.01546
12.302
5.55
90.00
21.65
449.4 f
5.5
885
36.09
0.01717
12.319
3.66
88.66
19.51
446.1 f
6.9
Fusion
36.17
0.01444
12.408
19.63
90.94
23.36
457.2 f
4.1
Total
40.49
0.02165
11.272
100.00
87.84
18.57
481.2 f
5.6
210
R.D. DALLMEYER
TABLE 1 (continued) 39Ar
%J4Ar
36ArCa
Apparent
temp.
(W of
non-
(W)
age d
(“C)
total)
atmos. ’
Release
(%r/“%r)
a
(Y&/Q)
B
( 3’Ar/%)
b
(Ma)
Sample 4A: J = 0.008333
625
73.50
0.06502
3.719
6.02
74.26
1.56
617.4 f
3.5
675
46.40
0.04759
5.349
3.24
70.61
3.06
436.6 &
4.4
700
54.18
0.07218
8.445
1.52
61.87
3.18
446.4 *
9.1
715
48.85
0.05500
10.662
2.07
68.47
5.21
446.1 *
6.7
125
43.90
0.03299
11.977
3.47
79.91
9.88
465.9 +
4.7
735
42.99
0.02777
12.037
3.16
83.15
11.79
473.4 &
7.5
150
44.16
0.03385
12.157
2.89
79.55
9.77
466.2 +
6.1
765
43.38
0.02913
12.416
3.20
82.44
11.59
413.7 f
7.5
775
45.21
0.03332
12.607
3.40
80.45
10.29
480.8 *
8.3
785
41.62
0.02218
12.786
5.02
86.71
15.68
477.5 +
8.2
800
42.45
0.02624
12.978
4.86
84.18
13.45
473.5 *
6.1
825
40.01
0.01812
12.876
6.76
89.19
19.33
472.9 +
6.7
850
37.26
0.00862
13.384
17.08
96.04
42.25
474.1 +
5.1
885
37.68
0.00661
13.217
20.35
97.62
54.36
485.7 i
5.6
Fusion
39.81
0.01493
13.030
16.96
91.53
23.74
481.6 *
4.9
Total
42.49
0.02172
12.048
100.00
88.45
27.12
487.5 +
6.8
478.6 f
5.8
Total without 625”-715OC
87.15
K&Ii Nappe Complex Merdker Nappe
Sample 6: J = 0.009302 550
184.16
0.57279
9.986
0.47
8.52
0.47
247.3 * 156.2
600
151.66
0.40990
9.538
0.43
20.64
0.63
463.3 i 161.3
650
66.29
0.12051
12.458
1.78
47.78
2.81
700
33.19
0.02046
14.051
10.68
85.17
18.68
424.5 +
3.4
720
33.15
0.01877
15.451
8.10
86.99
22.38
432.4 k
5.7
740
33.64
0.02118
15.808
8.38
85.15
20.30
429.9 *
2.2
160
35.16
0.02706
16.148
5.22
80.94
16.23
427.5 +
4.6
780
35.93
0.03014
15.899
4.71
78.14
14.35
421.8 i
4.9
800
30.94
0.01196
16.010
16.31
92.72
36.41
430.5 *
4.3
825
29.98
0.00966
16.159
27.35
94.79
45.51
426.9 i
1.5 2.9
469.0 k 26.4
850
31.02
0.01408
16.143
11.09
90.75
31.19
423.4 *
875
37.62
0.04276
16.127
3.53
69.84
10.26
398.0 k
8.6
Fusion
84.22
0.20139
15.241
1.94
30.79
2.06
393.2 f
34.2
Total
34.88
0.02650
15.666
100.00
86.34
29.33
425.2 i
5.3
426.9 it:
3.3
Total without 550 “-650 O, 895 o C and fusion
91.84
POLYOROGENIC
EVOLUTION
WITHIN
SEVE AND
KiiLl
NAPPES,
211
NORWAY
TABLE 1 (continued) 39Ar
w%
xAb
temp.
(W of
non-
@)
(“C)
total)
atmos. ’
Release
(?4r/%r)
a
(%r/%r)
a
(%r/%r)
b
Apparent age d (Ma)
Sample 73: J = ~.~9~15 550
51.04
0.05587
1.021
5.79
67.80
0.50
490.1 +
5.6
600
40.47
0.03646
1.137
4.39
73.59
0.85
429.3 f
7.3
625
44.52
0.05620
1.580
2.56
62.91
0.76
406.8 *
12.0
665
48.35
0.05991
2.735
3.08
63.83
1.24
443.5 +
6.7
700
47.21
0.05320
6.977
4.31
67.87
3.57
459.5 f
8.4
725
47.94
0.05867
15.545
4.22
66.43
7.21
459.3 f
5.3
750
45.18
0.05332
21.130
4.29
68.87
10.78
451.4 f
10.8
775
40.48
0.04249
21.723
4.87
73.28
13.90
432.8 f
7.5
800
38.36
0.03615
21.708
3.70
76.69
16.34
429.7 f
7.1
825
38.39
0.02916
21.967
3.82
82.13
20.49
456.9 + 10.3
850
38.24
0.03469
21.980
4.33
77.79
17.23
433.9 + 11.6
875
33.09
0.01463
20.972
39.33
92.01
38.98
442.7 f
900
35.84
0.02042
23.145
11.63
88.33
30.82
458.9 f
3.6
104.52
0.25223
22.863
3.68
30.44
2.47
460.8 f
12.1
40.90
0.03860
17.530
100.00
79.91
22.86
447.5 f
5.4
Fusion Total
2.2
Sample 10: J = ~.~~5~I 600
1143.02
3.80095
23.764
0.12
1.90
0.17
700
1041.03
3.15781
43.689
0.60
10.70
0.38
1118.8 f
750
520.60
1.53646
46.949
0.10
13.51
0.83
783.3 f
62.4
775
213.88
0.54913
53.205
0.47
26.13
2.64
649.9 f
54.3
800
72.14
0.14089
30.710
2.95
45.70
5.93
405.7 k 22.3
825
45.54
28.848
21.21
74.76
16.79
417.1 f
4.1
840
46.99
0.05179
29.279
11.78
72.43
15.38
417.0 f
4.6
860
38.93
0.02275
29.045
26.73
88.72
34.73
422.5 +
2.9
885
39.06
0.02320
28.607
35.72
88.32
33.54
422.0 f
2.6
910 Fusion Total
276.4 f 106.3 81.3
412.85
1.28868
28.445
0.27
8.31
0.60
420.0 rt 82.1
1993.51
6.60164
0.660
0.05
2.15
0.00
502.1 i- 126.3
57.53
0.08610
29.129
100.00
81.14
26.83
Total without 600°-800”,
910°C
and fusion
95.43
418.9 f
3.4
420.1 f
2.7
Sample II: J = 0.007603 600
401.06
1.20944
15.773
0.40
11.20
0.35
534.8 * 209.1
615
93.05
0.19126
10.622
0.91
40.17
1.51
453.9 f
56.1
680
95.95
0.20350
11.593
0.96
38.29
1.55
447.4 f
34.3
705
68.20
0.10930
13.161
1.02
54.18
3.28
450.0 f
32.1
730
41.34
0.02641
14.572
11.69
83.94
15.01
425.9 f
4.4
750
39.91
0.02191
14.647
16.99
86.71
18.18
424.9 f
3.1
760
44.08
0.03449
14.716
7.39
79.55
11.60
429.9 f
3.8
770
43.32
0.03474
14.361
6.19
78.95
11.24
420.4 f
4.6
780
39.74
0.02180
14.356
8.86
86.68
17.91
423.1 f
3.9
800
36.44
0.00986
14.277
32.55
95.14
39.40
425.4 f
4.1
820
38.60
0.02197
14.651
5.79
86.21
18.14
410.3 +
4.3
845
43.48
0.03425
14.390
3.71
79.37
11.43
423.8 f
7.3
875
45.40
0.04722
14.377
2.96
71.80
8.28
402.7 f
10.1
573.30
1.86066
14.070
0.56
4.29
0.21
311.8 k 62.3
o.cMO73
14.382
100.00
85.38
22.59
Fusion Total
45>43
Total without 6009-705 O, 820 O-875 o C and fusion
83.67
424.1 f
6.4
425.2 f
2.8
212
R.D. DALLMEYER
TABLE 1 (continued) Release
3pAr
%@Ar
36Ar,-a
Apparent
temp.
( 40Ar/ 39Ar) =
( 36Ar/ 39Ar) a
( 37Ar/39Ar) b
(% of
non-
(8)
age d
(“C)
total)
atmos. ’
(Ma)
Kjolhaugen Group Sampje 9: .I = ~.#7344 550
1112.70
3.50532
31.048
0.42
7.13
0.24
841.4 rt 631.4
600
365.49
1.09023
26.650
0.46
12.44
0.66
527.2 i 207.3
635
192.23
0.55689
28.548
1.05
15.59
1.39
364.6 rt 150.1
675
251.94
0.74376
36.281
0.67
13.92
1.33
421.8 f 163.4
125
60.50
0.09104
31.638
8.44
59.73
9.45
775
46.20
0.04634
30.437
22.03
75.64
17.87
419.1 f
3.1
800
69.04
0.13115
34.157
2.94
47.83
7.08
399.3 +
6.9
820
48.16
0.06029
36.379
8.48
69.08
16.41
402.4 +
5.1
840
38.18
0.02231
32.814
36.81
89.79
40.01
418.3 Lt
2.4
860
41.02
0.02568
30.353
15.89
87.43
32.15
428.9 I
2.8
Fusion
61.89
0.09790
30.685
2.82
57.23
8.53
424.3 I
18.3
Total
54.29
0.07385
32.005
100.00
77.69
26.40
419‘9 *
6.3
432.1 rt: 6.0
Gula Nappe Sample 5: .I = 0.008922 550
72.26
0.12821
2.996
0.80
47.89
0.64
486.1 rt 28.8
600
52.13
0.08548
2.563
0.60
51.93
0.82
390.8 rt 41.3
625
67.06
0.13881
3.244
0.47
39.21
0.64
380.8 i: 34.7
665
51.34
0.08351
5.352
0.80
52.76
1.74
391.6 f
700
34.89
0.01700
6.106
5.10
86.99
9.77
433.7 rt
3.8
125
32.32
0.00812
5.911
13.53
94.03
19.79
434.2 sr
1.3
750
31.49
0.00578
5.927
25.56
96.07
27.92
432.4 rt
1.2
775
31.41
0.00590
5.906
20.64
95.94
27.23
420.9 jr
2.2
25.7
800
31.79
0.00673
5.812
13.77
95.20
23.50
432.5 _C 2.7
825
31.28
0.00584
5.778
8.14
95.95
26.93
429.4 5
850
31.64
0.00692
5.811
3.82
95.00
22.86
429.9 rt
8.1
Fusion
38.58
0.03030
5.915
6.71
78.01
5.31
430.4 *
5.1
Total
33.04
0.01124
5.836
100.00
92.65
22.62
431.5 i
3.4
432.0 rt
2.6
Total without 550 “-665 o C
97.33
4.7
Sample 12: J = 0.007568 525
216.39
0.27432
12.894
1.06
63.01
1.28
1286.6 f
19.4
575
18.39
0.11044
22.347
1.09
60.65
5.50
561.3 f
18.6
625
89.88
0.16604
5.997
1.01
45.94
0.98
492.2 rt 24.3
660
59.48
0.08150
6.853
2.18
60.43
2.29
435.8 F
8.8
loo
55.01
0.06379
7.221
3.03
66.78
3.08
444.4 I:
7.2
725
51.97
0.05663
8.394
2.70
69.09
4.03
435.7 It
6.1
745
50.38
0.05528
8.901
2.53
68.98
4.38
423.4 r?c 5.8
765
41.48
0.02426
10.163
8.56
84.67
11.39
427.7 f
2.7
790
37.75
0.01485
9.955
18.49
90.48
18.23
417.1 zt
1.9
810
35.81
0.00558
10.166
26.56
97.66
49.53
426.1 f
1.8
935
35.52
0.00503
10.145
26.28
98.09
54.84
424.6 i
2.8
860
38.69
0.02464
10.092
3.23
83.26
11.14
395.8 + 10.1
890
42.38
0.03405
9.981
1.81
78.14
7.97
405.7 f
Fusion
98.30
0.22158
10.022
1.48
34.21
1.23
411.3 It 14.3
Totaf
42.55
0.02475
9.992
100.00
89.04
32.88
Total without 525 O-745 *, 860 O, 890 o C and fusion
79.88
9.3
434.9 f
4.3
423.6 f
2.5
FOLYOROGENIC
EVOLUTION
WITHIN
SEVE
213
AND ICiiLt NAPPES, NORWAY
TABLE 1 (continued) Release temp.
( %r/
39Ar) a
( %/
%r) a
( %r/
3%) b
(“C)
39Ar (46of total)
w?4r nonatmos. ’
36%a (W)
Apparent age d (Ma)
Sampie 13: J 52.5 575 625 660 700 725 740 760 780 800 820 845 Fusion
= ~.~~~~~ 71.45 42.42 40.80 44.15 43.02 42.83 42.94 41.62 39.73 36.31 35.87 37.64 42.11
0.11391 0.02241 0.02058 0.03136 0.02936 0.03026 0.03176 0.02765 0.02056 0.00831 0.00733 0.01230 0.02671
2.134 0.575 1.392 3.221 7.442 9.720 10.448 11.261 12.184 11.958 11.635 11.981 12.149
2.91 4.01 11.75 4.44 4.94 4.94 4.64 8.40 9.95 21.22 14.77 6.47 1.57
53.12 84.48 85.35 79.58 81.21 80.93 80.08 82.52 87.16 95.86 96.55 92.89 83.56
0.51 0.70 1.84 2.79 6.90 8.74 8.95 11.08 16.12 39.12 43.18 26.50 12.37
458.8 f 5.5 435.7 f 6.3 425.0 f 2.3 428.8 f 2.8 427.6 f 3.0 425.2 f 4.6 422.3 f 3.8 422.0 f 2.9 425.3 f 3.1 427.3 f 1.7 425.3 f 2.8 429.0 k 3.8 431.5 f 18.6
Total
40.31
0.02087
9.103
100.00
87.74
20.69
427.3 f
3.5
425.8 f
2.9
Total without 525 “-625 o C and fusion
79.81
a Measured. b Corrected for post-irradiation
decay of 37& (35.1 day half-life).
’ [40Ar,,l.- ( 36Ar,,,,. X29~.5N/40~t,,, d Calculated using correction factors of Dalrymple et al. (1981); two r&ma, intralaboratory
errors.
(481.22
0
20
CUMULATIVE
40
% 3gAr
80 60 RELEASED
5.6)
100
Fig. 3. 4oAr/39Ar incremental-release age and apparent K/Ca spectra of hornblende from foliated amphibolite of the Seve Nappe Complex, east-central Trendelag Locations shown in Fig. 2. Analytical uncertainties {two sigma, intraiaboratory) are represented by vertical width of bars. Experimental temperatures increase. from left to right. Plateau or total-gas (parentheses) ages are listed on each spectrum. Plateau increments delineated with arrows. All spectra have coordinates shown for sample 2.
214
R.D. WALLMEYER
TABLE 2 36Ar/40Ar vs. 39Ar/40Ar isotope correlations from ~crement~-hating experiments on hornblende concentrates from the Seve and KGIi Nappe Complexes, centrai Scandinavian Caledonides, Trondelag, Not-way Sample
Isotope correlation age a
%r/ 36Ar intercept b
MSWD
Increments included ’
Total 39Ar (V
(Ma)
Calculated 40Ar/ “Ar age (Ma) *
Seve Nappe CompIex 1 429.3 * 0.9 2A 435.4 rlr4.1 3 423.9 f 5.2 4A 476.5 * 2.7 Kiili Nappe CompIex
5.1 545.9 f 12.2 514.8 f 23.8 286.1 f 9.2
0.37 0.10 0.28 0.26
775-850 740-875 815-F 725-F
78.88 78.48 66.19 87.15
439.9 +_2.4 499.5 + 3.6 456.1 f 4.9 478.6 jI 5.8
299.1 f 321.7 f 273.7 5 296.1 +
9.4 16.3 9.3 10.3
0.18 0.35 0.06 0.08
700-850 725-900 825-885 730-800
91.84 78.19 95.43 83.67
426.9 f 446.2 rt 420.1 + 425.2 k
299.9 + 12.6
0.99
725-860
94.37
419.5 f 4.3
336.2 f 19.6 270.5 f 12.8 293.3 f 8.3
0.18 1.61 0.07
700-F 765-835 600-845
97.33 79.88 79.81
432.0 f 2.6 423.6 f 2.5 425.8 rt 2.9
392.5 *
Mer&ker Nappe Fumfsjss Group
6 422.8 & 1.2 7B 434.8 + 2.1 10 418.9 rf 1.6 11 421.7 4 2.1 Kj~ihffugen Group 9 421.1 & 3.1
3.3 4.4 2.7 2.8
Gula Nappe 5
12 13 a b ’ *
428.7 10.9 422.6 + 1.3 424.2 rf: 1.5
Calcuiated using the inverse abscissa intercept (40Ar/3pAr ratio) in the age equation. Inverse ordinate intercept. Table 1. o C (F = fusion).
of the total) yield an isotope correlation age of 423.9 + 5.2 Ma (inverse ordinate intercept of 514.8 t_ 23.8). The isotope correlation ages are considered geologically reliable and interpreted to date post-rnet~o~~c cooling through argon closure temperatures. Sample 4 displays a discordant spectrum in which apparent ages systematically increase in the 675 O-725OC increments to define an intermediate and high-temperature plateau age of 478.6 + 5.8 Ma (735*C-fusion increments). The plateau data yield a similar isotope correlation age (476.5 + 2.7 Ma) and an inverse ordinate intercept of 286.1. Because these correlations do not suggest extraneous argon contamination, the 476 Ma date is considered a reliable post-metamo~~c cooling age. The nature of age discordancy displayed in the 675°-7250C increments is similar to that predicted by Turner (1968) to develop as a result of partial, intracrystalline diffusive loss of radiogenic
“OAr during a superimposed thermal event. This behavior has been documented for thermally overprinted hornblende in other geologic settings by several workers, including Turner (1970), Dallmeyer et al. (1981), and Harrison (1981). Comparison of the Seve spectrum with the theoretical spectra of Turner (1968) suggests that the Seve hornblende experienced only slight (c. < 25%) loss of argon during a low-grade thermal overprint following initial post-metamorphic cooling through argon closure temperatures at c. 476 Ma. Experimental evolution of gas from a non-retentive, relatively high K phase at low temperatures precludes accurate determination of the age of the geologic overprint. However, it must have been younger than the 436.6 + 4.4 Ma apparent age recorded by the 675°C evolved from the Seve hornblende. (2) Muscooife: Muscovite concentrates have been prepared from two samples of schist inter-
POLYOROGENIC
EVOLUTION
WITHIN
SEVE AND
KijLI
NAPPES,
215
NORWAY
TABLE 3 %r/39Ar
analytical data for incremental-heating experiments on muscovite concentrates from the Seve and Kisli Nappe Complexes,
central Scandinavian Caledonides, Tremdelag, Norway Release
(%r/%r)
a
( %r/?4r)
temp.
a
39Ar
WaAr
Apparent
(W of total)
non-atmos. ’
age d (Ma)
(“C) Seve Nappe Complex Sample 38: J = 0.007595 525
41.18
0.01389
2.28
90.02
441.5 f
1.6
515
39.70
0.00334
3.97
97.50
464.9 f
2.0
625
37.33
0.00670
6.94
94.68
428.9 f
4.0
700
34.98
0.00289
13.50
97.54
415.1 f
2.2
800
34.83
0.00165
28.68
98.59
418.1 f
1.2
850
35.12
0.00176
8.54
98.51
420.9 -I: 2.4
900
35.57
0.00224
8.03
98.14
424.2 f
2.6
Fusion
36.76
0.00599
28.05
95.19
425.2 f
2.5
Total
35.99
0.00379
100.00
96.94
423.8 f
2.2
420.8 f
1.9
8.2
Total without 525 O-625 o C
86.80
Sample 48: J = 0.008862 500
63.65
0.11701
4.70
45.70
413.7 f
550
40.86
0.03745
16.05
72.97
422.9 f
2.3
600
41.52
0.03657
35.14
74.03
434.6 f
2.4
615
45.64
0.04943
18.19
68.05
438.6 f
2.5
750
51.55
0.06768
9.12
61.99
444.8 f
3.2
Fusion
46.11
0.04861
16.81
68.91
447.6 f
2.3
Total
47.20
0.05506
100.00
69.15
431.3 f
3.7
Sample 8: J = 0.009368 500
31.64
0.00433
2.01
86.61
412.2 f
3.1
550
28.91
0.00329
5.94
96.62
419.3 f
1.7
600
29.09
0.00299
10.99
96.94
422.9 k 1.2
615
29.03
0.00326
27.36
96.66
421.1 f
0.7
750
28.89
0.00273
26.13
97.19
421.3 f
0.9
825
28.66
0.00178
26.85
98.15
421.9 f
0.7
Fusion
43.93
0.04720
0.66
68.25
446.6 f
8.2
Total
29.05
0.00321
100.00
96.83
421.4*
0.9
421.6 f
0.8
Total without 500 o C and fusion
97.27
K(LliNappe Complex Gula Nappe Sample 5: J = 0.009378 500
33.61
0.02031
1.39
82.14
415.3 f
5.9
550
29.86
0.00491
3.19
95.12
426.1 f
2.5
600
29.29
0.00330
6.09
96.65
424.7 f
1.7
675
28.60
0.00270
30.91
97.19
417.9 f
0.9 1.6
750
28.80
0.00241
19.30
97.50
421.6 f
825
28.20
0.00133
33.32
98.59
418.Ozt 0.7
Fusion
30.08
0.00564
5.80
94.45
426.1 f
1.1
Total
28.14
0.00271
100.00
91.25
419.7 f
1.1
418.8 f
0.9
Total without 500 O-600 o C and fusion
83.50
216
R.D. DALLMEYER
TABLE 3 (continued) Release
( @Ar/ ‘9Ar) ’
( %r/%r)
a
temp.
39.4r
sNAr
(S of total)
non-atmos. ’
(“C)
Apparent age d (Ma)
Sample 12: J = 0.00761 I 500
46.25
0.05035
0.79
68.01
387.4 + 32.5
550
37.42
0.00822
5.24
93.94
428.1 + 3.2
600
35.97
0.00420
28.56
96.54
423.0 *
650
35.51
0.00190
16.45
98.41
425.1 + 1.3
1.1
725
35.75
0.00303
13.69
97.48
424.4 f
0.7
825
35.28
0.00201
30.63
98.31
422.6 f
0.7
Fusion
38.17
0.01188
4.65
90.84
422.5 f
2.0
Total
35.91
0.00392
100.00
96.89
423.4 f
1.4
423.5 i
1.0
Total without 500 o and 550 o C
93.98
Sample 13: J = 0.007502 550
37.22
0.00874
4.12
93.09
416.8 f
1.0
600
36.47
0.00471
5.49
96.17
421.3 f
0.9
635
36.30
0.00452
4.76
96.31
420.1 & 1.0
675
36.09
0.00349
4.98
97.13
421.1 + 0.7
725
35.83
0.00302
9.78
97.49
419.8 f
0.6
750
36.07
0.00362
5.09
97.02
420.4 f
1.8
775
36.27
0.00362
4.21
97.04
422.6 f
1.5
800
35.90
0.00299
3.12
97.52
420.6 1
1.3
825
35.82
0.00267
4.26
97.78
420.8 + 1.5
850
35.97
0.00270
3.62
97.77
422.3 f
1.7
875
35.87
0.00297
4.23
97.54
420.3 f
1.2
925
35.23
0.00202
18.89
98.29
416.5 + 0.6
975
35.10
0.00242
27.16
97.95
413.9 *
Fusion
91.59
0.19110
0.29
38.35
421.9 f 12.7
Total
35.85
0.00366
100.00
97.25
417.9 f
0.9
418.0 f
0.8
Total without 550 o C and fusion
95.59
0.4
a Measured. b ?‘%,,.(36Ar,,,,.)(295.5)1/40Ar,ol~ ’ Calculated using correction factors of Dahymple et al. (1981); two sigma, intrafaboratory errors; 37Ar/39Ar corrected ratio < 0.020 in all analyses.
layered
with
amphibolite
in westernmost
ex-
posures of the Seve Nappe Complex (locations 2 and 4, Fig. 2). These display variably discordant 40Ar/39Ar age spectra (Table 3, Fig. 4). That from sample 3 yields a well-defined 420.8 4 1.9 Ma plateau in the 7OO”C-fusion increments (corresponding to c. 87% of the total gas evolved). This is interpreted to date the last cooling through temperatures required for intracrystalline argon retention. Although not fully calibrated experimentally, using the preliminary data of Robbins (1972) in the diffusion equations of Dodson (1973)
indicates muscovite closure temperatures of c. 400°C. This is similar to those suggested for muscovite on the basis of empirical comparisons with other mineral isotopic systems (e.g., Wagner et al., 1977; tiger, 1979). The muscovite concentrate from sample 2 displays an internally discordant age spectrum in which apparent ages systematically increase from 413.7 f 8.2 Ma in the 500°C increment to 447.6 k 2.3 Ma in the fusion increment. This type of spectra discordance is suggestive of a partially rejuvenated intracrystalline argon system (e.g., Turner, 1968). Such pat-
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
KiiLl
NAPPES,
420 8 t 1.9
550
8
-I
500 450 4001
421.6 + 0.8
i
0
20 CUMULATIVE
40
-I
60 60 % 3gAr RELEASED
217
NORWAY
100
Fig. 4. ‘%r/39Ar incremental-release age spectra of muscovite from schist interlayered with amphibolite of the Seve Nappe Complex, east-central Trendelag. Data plotted as in Fig. 3.
at c. 400-425 Ma. Because of the extensive rejuvenation it is not possible to estimate the initial post-metamorphic age. However, it must have been older than the 448 Ma recorded by the fusion increment of the Seve muscovite. Muscovite has also been separated from a sample of schist collected within the Seve Nappe Complex exposed along the western limb of the Skadora antiform at the international border (location 8, Fig. 1). The concentrate displays an internally concordant age spectrum with the 550”-825°C increments yielding a plateau age of 421.6 f 0.8 Ma (c. 98% of the total evolved gas). This is interpreted to be a reliable post-metamorphic cooling age. Kb’li Nappe Complex Meraker Nappe
terns have been documented for thermally overprinted muscovite in other areas by several workers, including Dallmeyer and LRcorche (1989) and Snee et al. (1988). Comparison of the Seve spectrum with the theoretical spectra of Turner (1968) suggests extensive diffusive loss of radiogenie argon occurring during a thermal overprint
c
0
Fig. 5. %r/39~
20 CUMULATI”?%
(1) Hornblende: Hornblende concentrates have been prepared from four samples of foliated amphibolite collected within the Fundsjo Group of the MerAker Nappe (locations 6, 7, 10 and 11, Fig. 2). Samples 6, 10 and 11 display slightly discordant age spectra (Table 1, Fig. 5) in which anomalous apparent ages are recorded in both
6
100 “:‘RELEiiED
incremental-release age and apparent K/Ca spectra of hornblende from foliated amphibolite of the KiAi Nappe Complex (Fundsje Group, MerAkerNappe), east-central Tmndelag. Data plotted as in Fig. 3.
218
R.D. DALLMEYER
low- and high-temperature parts of the analyses. These are matched by significant ~uctuations in apparent K/Ca ratios, and likely reflect the effects of optically undetectable mineral contaminants in the concentrates and/or exsolution or zoning within constituent hornblende grains. Inte~~iate-temperat~e increments display negligible intrasample variations in apparent K/Ca ratios suggesting experimental evolution of gas occurred from populations of compositionally uniform intracrystalline sites. These increments (comprising between 84 and 94% of the total gas evolved) record mutually similar intrasample apparent ages corresponding to plateau ages of 426.9 4 3.3 Ma (6), 420.1 f 2.7 Ma (10) and 425.1 f 2.8 Ma. The plateau data from each analysis yields well-defined isotope correlations (MSWD < 2.0: Table 2) which define ages comparable to those directly calculated from the analytical data. Inverse ordinate intercepts range between 273.7 f 9.3 and 299.1 f 9.4, and do not suggest the presence of extraneous argon components. Therefore, the plateau ages are considered geologically significant and are interpreted to date the last cooling through appropriate closure temperatures. Hornblende from sample 7 displays a more complex age spectrum (Fig. 5). Apparent K/Ca ratios systematic~ly decrease throughout the 550 o 700°C increments suggesting a significant compositional inhomogenity in the concentrate. Although apparent K/Ca ratios are constant in the 725 “-900°C increments, there are significant variations in apparent ages (429.7 f 7.1 Ma to 459.3
f 5.3 Ma), and a plateau is not defined. These increments correspond to an isotope correlation (MSWD = 0.35) yielding an age of 434.8 f 2.1 Ma and an ordinate intercept of 321.7 _+16.3 (Table 2). This is interpreted to be a reliable post-metamorphic cooling age. A hornblende concentrate was also prepared from a sample of metagabbro collected within the Kjolhaugen Group of the Meraker Nappe (location 9, Fig. 2). The concentrate displays an internally discordant, saddle-shaped *Ar/39Ar age spectrum corresponding to a total-gas age of 419.9 + 6.3 Ma (Table 1, Fig. 6). Apparent K/Ca ratios are similar throughout the analysis. The 725860°C increments (comprising c. 94% of the total gas evolved) yield an isotope correlation age (MSWD = 0.99) of 421.1 + 3.1 Ma and an inverse ordinate intercept of 299.9 f 9.9. This is considered a reliable post-metamorphic cooling age. (2) Biotite: A biotite concentrate was prepared from a sample of schist interlayered with ~p~bo~te at location 7 in the Mer&ker Nappe. The concentrate displays an internally concordant age spectrum (Table 4, Fig. 7) with the 500850 o C increments (comprising c. 96% of the total gas evolved) yielding a plateau age of 435.7 + 1.4 Ma. This is interpreted to data the last cooling through temperatures required for intracrystalline retention of argon. Harrison et al. (1985) have demonstrated that closure temperatures are dependent on Fe/Mg compositional ratios, and suggested a range of c. 275-325°C is appropriate for the range of cooling rates likely to be encountered in most geologic settings. Gula Nappe Samples of hornblende garbenschiefer schist were collected at locations 5, 12 and 13 within the Gula Nappe (Fig. 2). Concentrates of hornblende, muscovite and biotite were prepared from each sample.
Fig. 6. “OAr/39Ar incremental-release spectra of hornblende Complex
(Fundsjer
age and apparent
from metagabbro Group,
Trendelag.
Mersker
K/Ca
of the Ktili Nappe Nappe),
Data plotted as in Fig. 3.
east-central
(1) Ho~b~ende: The three hornblende concentrates display slightly discordant ?Ar/39Ar spectra in which anomalously old apparent ages are recorded at low experimental temperatures (Table 1, Fig. 8). These are matched by marked
219
POLYOROGENIC EVOLUTION WITHIN SEVE AND I&L1 NAPPES. NORWAY
fluctuations in apparent K/Ca ratios suggesting that they reflect the affects of relatively non-retentive, compositionally distinct mineralogical cont~ants in the concentrates. Most of the intermediate- and high-temperature gas fractions (com-
prising between c. 80% and 97% of the total gas evolved from the samples) are characterized by mutually similar intrasample apparent K/Ca ratios su~esting expe~ment~ of gas from compositionally uniform populations of intracrystal-
TABLE 4 40Ar/39Ar analytical data for incremental-heating experiments on biotite concentrates from the Koli Nappe Complex, central Scandinavian Cakdonides, Trendelag, Norway Release
(?q?kr)
*
( 36Ar/39Ar) a
temp.
39Ar
W@Ar
Apparent
(a6 of total)
non-atmos. b
age ’ (Ma)
(“C) Merdker Nappe Sample 7A: .I = 0.009115 475
39.54
0.05728
3.07
57.18
338.1 + 2.5
500
31.08
0.~
12.39
96.14
434.5 f
1.9
550
30.44
0.00158
18.85
98.44
435.6i
1.2
675
30.74
o.cO211
20.38
97.95
431.5 f
1.6
750
30.46
0.00144
24.64
98.58
436.4 f
1.4
850
30.31
0.00155
19.52
98.48
434.1 f
1.0
Fusion
46.89
0.05662
1.15
64.31
438.1 f
8.4
Total
31.03
0.80430
100.00
96.44
432.7 f
1.5
435.7 f
1.4
Total without 475 * C and fusion
95.78
Gula Nappe Sample 5: J = 0.009234 475
29.90
0.02401
15.64
76.25
344.5 f
1.9
500
29.41
0.00210
14.84
91.87
425.1 f
1.2
550
29.39
0.00127
15.73
98.70
428.0 f
1.8
675
29.68
0.00197
19.23
98.03
429.1 + 1.5
760
29.13
0.00081
27.41
99.16
426.3 f
0.8
850
29.68
0.00219
6.29
97.80
428.3 f
2.1
Fusion
75.42
0.15572
0.87
38.98
433.1 f
9.8
Total
29.88
0.00636
100.08
94.48
414.3 f
1.4
427.1 f
1.3
1.7
Total without 475’ C and fusion
83.49
Sample I2: f = 0.~7~88 475
36.46
0.01210
3.70
90.19
401.8 f
500
36.63
0.00256
9.50
97.92
434.3 f
1.1
550
36.38
0.00182
12.51
98.51
434.0 f
0.8
675
36.41
0.00195
16.84
98.40
433.8 f
1.0
750
35.83
0.00093
30.28
99.22
430.8 f
1.0
850
36.10
0.00105
26.77
99.12
433.4 f
1.0
Fusion
89.02
0.18195
0.39
39.62
427.9 f 17.2
Total
36.38
0.00252
100.00
98.27
431.7*
Total without 475 ’ C and fusion
95.91
1.1
432.8 f 0.9
220
RD.
DALLMEYER
TABLE 4 (continued) 39Ar (Se,of total)
Release temp.
W‘=Ar non-atmos. b
Apparent age ’ (Ma)
(“C) Sample
475 500 550 650 750 850 Fusion
13: J = O.&I7415 39.38 37.52 36.92 37.38 37.12 36.95 45.72
Total
37.30
0.01880 0.00297 0.00181 0.00295 0.00155 0.00172 0.02933
2.78 10.83 16.26 11.25 40.76 16.92 1.20
85.87 97.64 98.53 97.65 98.75 98.61 81.06
403.6 * 433.5 i: 430.8 + 432.1 + 433.7 It 431.4* 438.0 +
0.00274
100.00
97.88
431.8 i: 0.8
Total without 475’ C and fusion
96.02
1.9 0.6 0.6 0.6 0.9 1.0 3.1
432.6 It 0.8
a Measured. ’ ?Ar,,,(36Arat,,,oS. )(2P5.5)1/40Ar,,,, ’ Calculated using correction factors of Dahympie et al. (1981); two sigma, intralaboratory errors; 37Ar/39Ar corrected ratio < 0.020 in all analyses.
line sites. These increments record similar intrasample ages corresponding to plateaux of 432.0 + 2.6 Ma (5), 423.6 & 2.5 Ma (12) and 425.8 f 2.9 Ma (13). Plateau data from each sample yields well-defined isotope correlations (MSWD < 2.0) corresponding to ages ranging between 422.6 + 1.3 Ma (12) and 428.7 + 0.9 Ma (5). Inverse ordinate intercepts range between 270.5 f 12.8 and 336.2 f 19.6 and do not suggest extraneous argon contamination. Therefore, the plateau ages are interpreted to be geologically significant post-metamorphic cooling dates. (2) Murcovite: The muscovite concentrates display internally concordant age spectra (Table 3, Fig. 9) corresponding to plateau ages of 418.8 f 0.9 Ma (5), 423.5 rt: 1.0 Ma (12) and 418.0 + 0.8 Ma
~~~~~~
0
20 40 60 80 CUMULATIVE % 39Ar RELEASED
100
Fig. 7. WAr/39Ar incremental-release age of biotite from schist interlayered with amphibolite of the Ktili Nappe Complex (Fundsje Group, Mer&ker Nappe), east-central Trnndelag. Data plotted as in Fig. 3.
(13). These are interpreted to date post-metamorphic cooling through c. 400°C. (3) Biotite: The three biotite concentrates also display generally concordant age spectra (Table 4, Fig. 10) which yield plateau ages of 427.1 + 1.3 Ma (5), 432.8 f 0.9 Ma (12) and 432.6 rf: 0.8 Ma (13). These are considered to represent reliable post-metamorphic cooling ages. Geologic significance Interpretating the geologic significance of the *Ar/39Ar results depends on calibration of the Ordovician and Silurian time-scales (e.g., Harland et al., 1982; Palmer 1983). Snelling (1985) and Kunk et al. (1985) suggest that the OrdovicianSilurian boundary (base of the Llandovery) is c. 435-440 Ma. This together with a 420 Ma calibration of the Ludlow (Wyborn et al. 1982) is used for interpretation of the Seve and Koli 40Ar/39Ar results from Trerndelag. 40Ar/39Ar results from structural units of the Seve Nappe Complex exposed in Trsndelag suggest significant local differences in t~tonotherm~ evolution. An initial post-metamorphic cooling age of c. 475-480 Ma is locally recorded by homblende (location 4), suggesting that significant orogenie activity occurred during and/or shortly prior
POLYOROOENIC
EVOLUTION
WITHIN
SEVE AND
KijLI
NAPPFB,
NORWAY
t
I.
452.0* 2.6
J 1
I
4007 0
20
40
CUMULATIVE
60
00
100
% 3gAr RELEASED
Fig. 9. %r/39Ar incremental-release age spectra of muscovite from garbenschiefer schist of the K6li Nappe Complex (Gula Nappe), east-central Trondelag. Data plotted as in Fig. 3.
i-
5501--
Bgsc&
0
.,_
“56’”
20
40
CUMULATIVE
60
% 3gAr
60
100
RELEASED
Fig. 8. ‘%r/39Ar incremental-release age and apparent K/Ca spectra of hornblende from garbenschiefer schist of the Koli Nappe Complex (Gula Nappe), east-central Trondelag. Data plotted as in Fig. 3.
ord of a pre-Silurian thermal evolution may be directly detected in 40Ar/39Ar results from the Koli Nappe units examined. However, the widespread presence of extraneous argon components does point to a pre-Scandian history. Hornblende, muscovite, and biotite within both the MerSlker and Gula nappes record generally similar post-metamorphic cooling ages between c. 420 and 435 Ma, again suggesting relatively rapid post-metamorphic cooling following culmination 550
5
4
500
427.1 +c1.3 450
to Arenig. This hornblende appears to have been slightly rejuvenated during a Silurian (c. 400-425 Ma) metamorphic overprint. Muscovite from this locality was more extensively rejuvenated. At three sample locations within the Seve Nappe Complex both hornblende and muscovite were totally rejuvenated during the overprint, and record postScandian metamorphic cooling ages of 425-435 Ma and 420-425 Ma respectively. The near concordance of hornblende and muscovite cooling ages suggests relatively rapid post-metamorphic cooling. through the contrasting temperatures required for intracrystalline argon retention. No rec-
4001 550500-
432.8
t 0.9
450400
13 $yyiq-$
0
20
CUMULATIVE
40
60
100
% 3gAr REL&ED
Fig. 10. ‘%r/39Ar incremental-release age spectra of biotite from garbenschiefer schist of the Koli Nappe Complex (Gula Nappe), east-central Trondelag. Data plotted as in Fig. 3.
LLL
of Scandian metamorphism in the Early Silurian. Sequences within the lower Koli nappes include units which appear to be correlative with AshgillLlandovery successions exposed at similar structural levels in Vbterbotten County, Sweden. Comparison of protolith ages and the c. 430 Ma estimate for attainment of peak metamorphic conditions suggest that tectonic activity must have commenced shortly after deposition of the youngest (Late Llandovery) sequences. Regional tectonic significance
The complex tectonic history suggested by the 40Ar/39Ar mineral dates recorded in structural units of the Kbli and Seve Nappe Complexes exposed in Trsndelag is generally similar to the polyorogenic evolution previously outlined for the Upper Allochthon in several other areas of the central Scandinavian Caledonides; however, there appear to be significant differences in the relative timing of individual tectonothermal events. For example, in central Norrbotten County, Sweden, c. 490 Ma “Ar/39Ar plateau ages were reported by Dallmeyer and Gee (1986) for hornblende within non-foliated, retrograde amphibolite selvages developed from eclogite in miogeoclinal successions of the Seve Nappe Complex. By contrast, in the Gaddede area of northern Jamtland and in the Are area of west-central Jg;mtland hornblende within Seve amphibolite has yielded %r/39Ar cooling ages of c. 460-475 Ma (Dallmeyer et al., 1985; Dallmeyer and Gee, 1988). Thus a complex and apparently diachronous early Caledonian evolution is apparent for this part of the Baltoscandian margin. A similarly contrasting tectonothermal record is also present in the exotic lithologic elements which comprise the Kiili Nappe Complex. Hornblende in garbenschiefer of the lower Kbli Nappe exposed in the Tglnnforsen area (Middagasfallet Nappe) locally yield @Ar/ 39Ar plateau ages between 475 and 510 Ma (Dallmeyer et al., 1985). Elsewhere in the same nappe unit hornblende records c. 415 Ma post-Scandian cooling ages. In the Gaddade area, Scandian metamorphism appears to have resulted in complete reuvenation of intracrystalline argon systems throughout both the Lower and Middle KBli
R.D. DALLMEYER
Nappe units, and hornblende, muscovite, and biotite record “OAr/39Ar plateau ages of 420-435 Ma (Dallmeyer and Gee, 1988). Dallmeyer and Gee (1986, 1988) suggested that the early Paleozoic evolution of central parts of the Baltoscandian margin included: (1) Tremadoc and Late Cambrian high-pressure metamorphism during entrainment of distal, Baltoscandian miogeoclinal sequences and associated volcanic successions (Seve Nappe Complex protoliths) within an evolving accretionary wedge; (2) ductile translation of high-pressure units to shallower crustal levels (with attendant retrogression of high-pressure assemblages) and imbrication with miogeoclinal sequences maintained at intermediate-pressure, middle amphibolite facies metamorphic conditions; and (3) consolidation of the accretionary wedge prior to cooling below homblende argon closure temperatures in the Arenig to Caradoc. Development of the Seve accretionary prism in the Early Ordovician was apparently contemportaneous with construction of an outboard (western) volcanic arc now represented by some successions comprising lower units of the Kijli Nappe Complex (e.g., Ankarede Formation in the Ggiddede area). Younger successions in the Lower Kiili Nappes were deposited at least into the Late Llandovery. Shortly thereafter, Scandian deformation commenced, and Middle Kbli Nappe units were inverted and thrust onto Lower Koli sequences. The resultant composite allochthon was structurally emplaced onto previously imbricated and variably metamorphosed miogeoclinal sequences. Assembly of the nappe units resulted in crustal depression and variable metamorphism. Cooling associated with subsequent rapid uplift to shallower crustal levels is reflected in the c. 420435 Ma “OAr/39Ar plateau ages recorded by hornblende, muscovite, and biotite throughout the Upper Allochthon. Emplacement of this composite Scandian allochthon onto crystalline, middle Proterozoic basement of western Baltica apparently resulted in its depression to depths appropriate for formation of high-temperature eclogite assemblages. These record c. 425 Ma ages (U-Pb zircon and Sm-Nd mineral: Krogh et al., 1974; Griffin and Brueckner, 1980), indicating that they formed at approximately the same time
POLYOROGENIC
EVOLUTION
WITHIN
SEVE
AND
K6LI
NAPPFiS,
NORWAY
that minerals within the Upper Allochthon were cooling through argon retention temperatures at much shallower, superstructural crustal levels. Following eastward translation of the Scandian nappes, the basement experienced nearly isothermal depressurization followed by very rapid cooling through hornblende and muscovite argon closure temperatures at c. 390400 Ma (Dallmeyer et al., 1990). Acknowledgments This work was supported by a grant from the Crustal Structure and Tectonics Program of the U.S. National Science Foundation (EAR8407027). Drs. D. Gee, H. Kish, and D. Roberts collaborated in the field during sample collection. Freight was, in part, paid by the Geological Survey of Sweden, the Geological Survey of Norway, and the University of Lund. Appendix: Sample locations 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Road Road Road Road Road Road Road Road Road Road Road Road Road
exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure exposure
along E-6, 1.5 km north of Heia. along E-6, 0.5 km south of Steinkjer. along E-6, at Kollset. along E-6, 1 km south of Koabjerea. along Rt. 72 at Feren. along Rt. 757, 1.5 km northwest of Julnes. along Rt. 72,1.0 km northwest of Stormoen. along Rt. 72 at international border. along E-75 at Turefors bridge. along E-75 at Gudl bridge. along R-75, 0.1 km west of Gudl bridge. along E-75, 1.5 km east of Floraholmen. along E-75 at Floraholmen.
References Alexander, E.C., Jr., Michelson, G.M. and Lanphere, M.A., 1978, A new %r/39Ar dating standard. In: R.E. Zartman (Editor), Short Papers of the Fourth International Conference on Geochronology, Cosmochronology and Isotope Geology. U.S. Geol. Surv., Open-File Rep. 78-701: 6-8. Andreasson, P.-G., Gee, D.G. and Sukotjo, S., 1985. Seve eclogites in the Norrbotten Caledonides, Sweden. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide OrogenScandinavia and Related Areas. Wiley, Chichester, pp. 887-901. Ambom, J.O., 1980. Metamorphism of the Seve rocks on Areskutan, southern Swedish Caledonides. Geol. F&en. Stockholm F&h. 102: 359-372.
223 Be&ho&n, M., 1978. Geology of the Nordhallen-DuvedGreningen area in J&rntland, central Swedish Caledonides. Geol. F&en. Stockholm Fijrh., 100: 335-347. Beckholmen, M., 1982. Mylonites and pseudotachylites associated with thrusting of the Koli Nappes, Tlforsfaltet, central Swedish Caledonides. Geol. Foren. Stockholm F&h., 104: 23-32. Bruton, D.L. and Bockelie, J.F., 1980. Geology and palaeontology of the Holonda area, western Norway-a fragment of North America? In: D.R. Wones (Editor), The Caledonides in the USA. Blacksburg Va. Polytech. Inst. and State Univ., Dep. Geol. Sci., Mem., 2: 41-47. Claesson, S., 1979, Pre-Silurian erogenic deformation in the north-central Scandinavian Caledonides. Geol. Foren. Stockholm Ftirh., 101: 353-356. Claesson, S., 1980. A Rb-Sr isotope study of granitoids and related mylonites in the T%nn%s Augen Gneiss nappe, southern Swedish Caledonides. Geol. F&en. Stockholm FBrh., 102: 403-420. Claesson, S., 1981. Caledonian metamorphism of Proterozoic Seve rocks on Mt. Areskutan, southern Swedish Caledonides. Geol. Foren. Stockholm F&h., 103: 291-304. Claesson, S., 1986. U-Pb ion microprobe studies of high-grade paragneisses from the Seve Nappes, Swedish Caledonides. Geol. F&en. Stockholm Forh., 108: 276. Claesson, S., Khngspor, I. and Stephens, M.B., 1983. U-Pb and Rb-Sr isotopic data on an Ordovician volcanic/ subvolcanic complex from the Tjopasi Group, Koli nappes, Swedish Caledonides. Geol. Fiiren. Stockholm Fbrh., 105: 9-15. Dallmeyer, R.D., 1975. ‘?Ar/39Ar release spectra of biotite and hornblende from the Cortlandt and Rosetown Plutons, New York, and their regional implications. J. Geol., 83: 629-643. Dallmeyer, R.D., 1988. Polyphase tectonothermal evolution of the Scandinavian Caledonides. In: A.L. Harris and D.J. Fettes (Editors), The Caledonian-Appalachian Oregen. Geol. Sot. London, Spec. Pub]., 38: 365-379. Dallmeyer, R.D. and Gee, D.G., 1986. %r/39Ar mineral dates from retrogressed eclogites within the Baltoscandian miogeocline: implications for a polyphase Caledonian orogenie evolution. Geol. Sot. Am. Bull., 97: 26-34. Dallmeyer, R.D. and Gee, D.G., 1988. Polyorogenic *Ar/“Ar mineral age record in the Seve and Koli Nappes of the Gaddede area, northwestern JZimtland, central Scandinavian Caledonides. J. Geol., 96: 181-198. Dallmeyer, R.D. and Keppie, J.D., 1987. Late Paleozoic tectonothermal evolution of the southwestern Meguma Terrane, Nova Scotia. Can. J. Earth Sci., 24: 1242-1245. Dallmeyer, R.D. and L&orche, J.P., 1989. 4oAr/39Ar polyorogenie mineral age record within the central Muaritanide orogen, West Africa. Geol. Sot. Am. Bull., 101: 55-70. Dallmeyer, R.D. and Rivers, T., 1983. Recognition of extraneous argon components through incremental-release “OAr/“Ar analysis of biotite and hornblende across the Grenvillian metamorphic gradient in southwestern Labrador. Geochim. Cosmochim. Acta, 47: 413-428.
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