NORWEGIAN JOURNAl OF GEOLOGY U-Pb zircon ages from the Bymarka ophiolite 19

U-Pb zircon ages from the Bymarka ophiolite, near

Trondheim, Central Norwegian Caledonides, and regional implications

David Roberts , Nicholas Walker, Trond Slagstad, Arne Solli & Allan Krill

Roberts, D., Walker, N., Slagstad, T., Solli, A. & Krill, A. 2002. U-Pb zircon ages from the Bymarka ophiolite, near Trondheim, Central Norwegian Caledonides, and regional implications. Norsk Geologisk Tidsskrift, Vol. 82, pp. 19-30. Trondheim 2002. ISSN 029-196X.

Zircon fractions extracted from an oceanic, low-K rhyodacite and an arc-related trondhjemite body in the Bymarka ophiolite, dose to Trondheim, Central Norwegian Caledonides, have yielded a precise U-Pb age of 482 ± 5 and an interpreted age of c. 481 Ma, respectively. The dates are consid­ered to represent the crystallisation ages of these felsic rocks. Geochemical data indicate that the rhyodacite is comparable to plagiogranites occur­ring in the nearby Løkken ophiolite, and is considered to be cogenetic with the metabasaltic greenstones of the ophiolite. The trondhjemite, on the other hand, transects the greenstones and shows a clear, subduction-related, geochemical signature. In common with neighbouring, dismembered and tectonically disrupted ophiolites, the Bymarka ophiolite is considered to have been obducted upon a microcontinental block represented by Gula Complex rocks, in Early Arenig time. Just prior to this development, seaward subduction ofoceanic crust had provided appropriate condi­tions for the formation of blueschist-facies metamorphic parageneses, and led to the generation of late-stage are products. A plausible palaeogeo­graphic scenario in Late Cambrian to Early Ordovician time along and outboard of Bal ti ca involved dual subduction systems, with seaward polar­ities, in the eastern part of Iapetus, or the Ægir Sea. This involved an anticlockwise-rotating Baltican plate facing first Siberia, in Cambrian time, and then Laurentia in the Early Ordovician. Fairly rapid contraction of this oceanic tract occurred in a transpressive regime involving two, dis­crete, tectonothermal and obduction/accretion events in the time interval Late Cambrian to Early Arenig.

David Roberts & Arne Soili, Norges geologiske undersøkelse, 7491 Trondheim, Norway; Nicholas Walker, Department of Geological Sciences, Brown University, Providence, RI 02912, USA. (Present address- Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139, USA); Trond Slagstad, Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3/5; Allan Krill, Department of Geol­ogy and Mineral Resources Engineering, Norwegian University of Science and Technology, Høgskoleringen 6, 7491 Trondheim, Norway.

lntroduction

Tectonically fragmented and dismembered ophiolite complexes are now considered to form an integral com­ponent of the tectonostratigraphy of the Scandinavian Caledonides (Furnes et al. 1980, 1985, Pedersen et al. 1988, Sturt & Roberts 1991). Their importance to our understanding of the polyphase evolution of the mountain belt far outweighs their comparatively mod­est and restricted areal extent; yet their very existence, as ophiolites, was unknown barely thirty years ago. At that time, detailed field studies and geochemical inves­tigations were initiated on several of the thick, Early Palaeozoic, 'greenstone' units occurring in the nappes of the Central Norwegian Caledonides (Gale & Roberts 1972); and it was soon realised that we were dealing not with simple, homogeneous, metabasaltic greenstones, but rather with a spectrum of mostly mafic, magmatic rocks of diverse palaeogeographic settings, including fragmented ophiolitic assemblages (Gale & Roberts 1974, Prestvik 1974, 1980, Grenne et al. 1980, Roberts 1980, Heim et al. 1987, Grenne 1989, Slagstad 1998 and in prep.). Similar studies in other parts of Norway led to comparable discoveries (Furnes et al. 1976, 1985,

1988, Sturt et al. 1979, 199 1, Boyle 1980, Boyd 1983, Thon 1985, Fuller 1986, Pedersen et al. 1988).

While the lithological variations, pseudostratigra­phies and geochemical signatures of these ophiolite frag­ments are now fairly extensively documented, their crys­tallisation ages are comparatively poorly constrained, with a few exceptions. For obtaining precise isotopic ages, reliance has been placed on U-Pb dating of zircons from the few felsic rock types that occur within the ophi­olitic assemblages, namely plagiogranites and their effu­sive or near-surface counterparts. The Norwegian Cale­donide ophiolites were initially divided into two age groups based on geological and biostratigraphic rela­tionships (Furnes et al. 1985). Although this early subdi­vision is partly incorrect, U-Pb geochronology has, in fact, demonstrated the presence of two generations of ophiolites, i.e., a Late Cambrian-Early Ordovician grouping of ophiolites generated in a general suprasub­duction-zone oceanic setting; and a smaller group of Late Ordovician-Early Silurian age, of mainly marginal basin origin (Dunning & Pedersen 1988, Pedersen et al. 1988). The early ophiolites generally fall within the time interval 495-475 Ma, whereas the younger anes were generated at around 445-437 Ma.

20 D. Roberts et al.

Legend a upper Hovin and Horg Groups � ':='ns��·

Ophlolltlc greenslione compleXes Gula Complex, wi1h local IZ2I m6tange un� in southeast

N

i

Fig. 1. Simplified geological map showing the locations of the princi­pal fragmented ophiolites of the region south and southwest of Trondheim (modified from Heim et al. 1987). A secondary thrust or slide, corresponding to the Horg Fault of Walsh (1986) and marked by a line with single tags, dissects the Hovin and Horg Groups and parts of the ophiolitic assemblages.

In this contribution we present the results of a U-Pb dating study on zircons separated from two felsic mag­matic rocks - a gneissic trondhjemite and a low-K rhy­odacite - which form a small part of the Bymarka ophi­olite (Slagstad 1998, in prep.), in the environs of Trond­heim (Fig. 1). We also discuss briefly the palaeogeo­graphic scenario favoured by us for subduction and obduction in the context of a contracting seaway between the palaeocontinents Baltica, Siberia and Lau­rentia, involving a microcontinent just outboard of Baltica.

Geological setting The dismembered and fragmented ophiolites of central Norway occur within the exotic terranes of the Koli Nappes, which form the higher parts of the Upper Allochthon of Scandinavian Caledonide tectonostratig­raphy. In the western Trondheim Region, and in the case of the Bymarka ophiolite, we are dealing specifi­cally with the Støren Nappe (Fig. 1), at the base of which lies the fragmented Støren ophiolite (Gale & Roberts 1974). There is good evidence to suggest that the Støren ophiolite and the tectonically subjacent rocks of the Gula Complex (Gula Nappe) were initially deformed and metamorphosed in Early Ordovician time (Guezou 1978, Lagerblad 1983, Grenne & Lagerblad 1985) (the 'Trondheim event'), broadly coevally with obduction (Furnes et al. 1980), prior to uplift, erosion and accumulation of the Mid Arenig and younger, volcanosedimentary successions of the Hovin

NORWEGIAN JOURNAL OF GEOLOGY

and Horg Groups (Vogt 1945, Bruton & Bockelie 1980) (Fig. 1). The Lower and Upper Hovin Groups, in par­ticular, are richly fossiliferous, especially in the Hølonda-Løkken district where the Arenig-Llanvirn faunas are largely of Laurentian affinity (Bruton & Bockelie 1980, Neuman & Bruton 1989). In southeast­ern parts of the Lower Hovin Gro up, however, fossils of this age are less common and of Baltican aspect (Spjeldnæs 1985, pers. comm. 2002). The Ordovician Hovin and Horg Group successions were subsequently deformed and metamorphosed during the Late Sil­urian-Early Devonian, Scandian orogeny. Along the eastern side of the Gula Complex, black phyllites inter­calated with pillowed basalts of the Fundsjø Group contain the European dendroid graptolite Rhabdino­pora flabelliforme, of Tremadoc age. These U- and V­rich, black phyllites are a characteristic feature of the epicontinental, Baltoscandian margin of Baltica ( Gee 1981, Andersson et al. 1985).

Ophiolites, or fragmented ophiolites, have been described from areas northwest of the Støren ophiolite, at Vassfjellet (Grenne et al. 1980), Løkken (Grenne 1989), Resfjell (Heim et al. 1987) and Grefstadfjell (Ryan et al. 1980) (Fig. 1). In reality, and judging from the map pattern (Fig. l), these all appear to belong to one and the same ophiolitic complex. The Bymarka ophiolite of the present study has, indeed, been corre­lated with the Vassfjellet and Løkken ophiolites (Grenne et al. 1980). In his memoir on the geology of the Trondheim Region, Carstens ( 1920) introduced the term 'Bymark Group' (named after the forested and moorland area 'Bymarka' W and SW of Trondheim) for the major part of the thick, volcanic, 'greenstone' units occurring in the western Trondheim district. In this definition, the Bymark Group embraced not only the magmatic rocks but also the overlying polymict 'green­stone conglomerate' which occurs in many places in the region (Figs. l & 2). Interestingly, Carstens (1920) con­sidered that all the 'greenstone' units mentioned above (now described as fragmented ophiolites) formed part of his Bymark Group. Of the aforementioned ophio­lites, only those at Løkken and Vassfjellet have hitherto been dated (U-Pb, zircons) (Dunning 1987, Dunning & Grenne, in prep; also Dunning, pers. comm., cited in Sturt & Roberts 1991, p. 751), yielding ages for pla­giogranite dykes of 493 ± 10 and 487 ± 5 Ma for Løkken, and 480 ± 4 Ma for Vassfjellet.

A new and exciting aspect of the Caledonian geol­ogy of this region, and indeed of the Bymarka district, has been the discovery of relics of a blueschist-facies assemblage (primarily glaucophane, zoisite, rutile and albite) within the highly deformed, very basal part of the Bymarka ophiolite (Eide & Lardeaux 2002) (Fig. 2). The high-pressure paragenesis is strongly retrogressed, firstly at amphibolite facies and subsequently under greenschist-facies conditions. We will come back briefly to the consequences of this new development in the discussion.

NORWEGIAN JOURNAL OF GEOLOG Y

!·>"·::: . .-.;] Lower Hovin Group with ' ' • basal 'greenstone conglomerate'

� Klementsaunet quartz keratophyre

IIIliiilTIJ Fagervika granitoid

� Byneset trondhjemite

c:::J Bymarka ophiolite, undifferentiated mafic rocks (gabbro, mafic dykes, metabasalt)

Tectonic conJact

[-=-=-J Undifferentiated schists, mostly rocks of the Gula Co!nplex

u-Pb zircon ages from the Bymarka ophiolite 21

Trondheimsfjorden

Fig. 2. General map of the Bymarka ophiolite showing the localities of the samples taken for U-Pb zircon dating. The blueschist locality (Eide & Lardeaux 2002) is als o indicated.

Bymarka ophiolite

The constituent lithologies of the Bymarka ophiolite, and their petrography and geochemistry, have been described in detail by Slagstad (1998 and in prep.). Accordingly, we here present just a summary, concen­trating on the geochemical features and in particular on the felsic rocks which are the subject of this isotopic dat­ing investigation.

Although the rocks were moderately to strongly deformed during the Early Ordovician and later, Scan­dian orogenic events, large parts of an original ophiolite pseudostratigraphy appear to be preserved. In western areas, gabbroic rocks are predominant, but these give way eastwards first to a unit dominated by mafic dykes and then to a thick sequence of pillowed basalts with sporadic hyaloclastites, tuffites, quartz keratophyres (metarhyodacites) and layers of jasper and rare coticules. Overlying the pillow lavas is a unit dominated by felsic and mafic agglomerates with thinner layers of mag­netite-rich metachert and felsic volcanites. Locally, the agglomerates grade into greenstones and pillow lavas. This unit is interpreted to represent ocean-floor sedi­ments and exhalatives.

Geochemically, the mafic dykes and la vas have ocean­floor affinities, similar to correlatable ophiolitic rocks in the Løkken area (e.g., Gale & Roberts 1974, Grenne 1989). Three main types of felsic intrusive or extrusive

rocks have been identified in the Bymarka ophiolite. These are termed the Klemetsaunet rhyodacite, Fagervika trondhjemite and Byneset trondhjemite (Fig. 2). The rocks are, in fact, metarhyodacites and meta­trondhjemites, but the prefix 'meta' has been omitted for simplicity. Of these, only the first two have been the sub­ject of isotopic dating and are described briefly below (for details, see Slagstad 1998 and in prep.).

The Klemetsaunet rhyodacite (sample KLI; Figs. 2 and 3a) occurs as sheets and dykes ranging from a few dm to several tens of metres in thickness, and up to a few hundred metres in length. Similar rhyodacites are associ­ated with the sedimentary unit described above, suggest­ing that it formed in an ocean-floor setting, coevally with the ophiolite. It is composed of microcrystalline quartz and albite, and is characterised by abundant, euhedral garnet and amphibole porphyroblasts interpreted to be metamorphic based on field observations such as bleached haloes and transection of foliation. The Fagervika trondhjemite (sample TR97K; Figs. 2 and 3b) is a medium- to coarse-grained rock of transitional trond­hjemitic to granitic composition and gneissic character, forming an intrusive body ca. 10 km in length and up to hundreds of metres thick (Fig. 2). Along its margins and in thin dyke apophyses, the Fagervika trondhjemite is chilled against the host greenstones. In addition to quartz, plagioclase and K-feldspar, muscovite is locally important, constituting up to 15-20 vol.% of the mode.

22 D. Roberts et al. NORWEGIAN JOURNAl OF GEOlOGY

Fig. 3. Field photographs of the sampling localities. (a) The Klemetsaunet rhyodacite, looking approximately east. P ho to, D.R. (b) The Fagervika trondhjemite, at the western end of a smal� abandon ed quarry, looking west. P ho to, A.K.

Based on the geochemical data, the two rock types are clearly distinguished (Table 1). The Klemetsaunet rhyo­dacite has a flat rare earth element (REE) pattern with a slight light REE (LREE) depletion and a negative Eu anomaly, similar to that of plagiogranites in the correlat­able Løkken ophiolite (Grenne 1989) (Fig. 4). A small, negative Nb-Ta anomaly may suggest formation in a suprasubduction-zone setting (Elthon 1991), as at Løkken (Grenne 1989). Alternatively, this anomaly may derive from fractionation. In contrast, the Fagervika trondhjemite has a strongly enriched LREE and slightly enriched heavy REE (HREE) pattern, also with a nega­tive Eu anomaly (Fig. 4), features which are comparable to those of quartz-porphyritic rhyolites in the upper

200

100 Q) ...... L.. "C c o ..c. () -a, 10 o. E c?3

La Pr Sm Gd Dy Er Yb Ce Nd Eu Tb Ho Tm Lu

Fig. 4. Chondrite-normalised REE patterns for the Klemetsaunet rhyodacite (vertical lines) and Fagervika trondhjemite (horizontal lines); modified from Slagstad (in prep.).

member of the Løkken ophiolite (Grenne 1989). In diverse, tectonic discrimination diagrams (Wood 1980, Pearce et al. 1984), the Klemetsaunet rhyodacite plots in the field of ocean ridge plagiogranite; and its oceanic ori­gin is also shown by high Yb/ A1203 ratios. The geochem­istry is consistent with the field relationships suggesting that the Klemetsaunet rhyodacite formed in an ocean­floer setting, coeval with generation of the ophiolite. The Fagervika trondhjemite, on the other hand, shows clear subduction-related signatures indicative of an increasing are influence at the time of its generation.

Finally, it should be noted that overlying the pillow lavas in the east (Fig. 2), above a strongly sheared, inferred unconformable contact, is a polymict conglom­erate dominated by clasts of metabasalt, gabbro, trond­hjemite and rhyodacite with minor jasper, set in a green­schist matrix. This constitutes the basal conglomerate of the Lower Hovin Group.

U-Pb analytical methods and results

Analytical methods The samples were crushed and separated into zircon con­centrates at the Geological Survey of Norway, Trond­heim. The chemical separations and zircon analyses were carried out at the Department of Geological Sciences, Brown University, Rhode Island, USA (in 1998 for sam­ple TR97-K and in 2000 for sample KL-l). Zircon con­centrates were cleaned in successive solutions of 2N HN03, 2N HCl and distilled H20, and then split by mag­netic character using a Frantz Isodynamic Barrier separa­tor. The zircons were then sieved and/or hand sorted into representative subpopulations on the basis of morpho-

NORWEGIAN JOURNAL OF GEOLOGY

Table 1. -Averaia composition of investigated rocks, from Slag-stod (in prep.). 'n=number of samples. Average REE, Th,·Nb, Hf, and Ta based on three representative samples from eaeh rock type.

Klemetsaunet Fagervika rhyodacite trondhjemite

n 19 15 Si02 76.47 74.56 Ti02 0.13 0.22

Al203 12.12 12.91 Fe203 1.84 1.65 Mn O 0.03 0.05 MgO 0.24 0.60 Ca O 1.07 0.93

Na20 6.21 4.94 K20 0.22 1.96 P20s 0.02 0.04 LOI 0.57 1.42 Total 98.88 98.86

Ba 39 734 Rb 5 54 Sr 51 80 y 100 35 Zr 273 136 Nb 7 5 Th 1.46 17.68 Hf 9.55 4.06 Ta 0.46 0.36

La 12.13 34.28 Ce 35.97 63.64 Pr 5.87 6.93 Nd 30.56 24.37 S m 11.45 5.32 Eu 2.40 0.92 Gd 13.70 4.56 Tb 2.40 0.75 Dy 16.62 5.14 Ho 3.48 1.09 Er 11.67 3.79 Tm 1.67 0.56 Yb 11.11 3.86 L u 1.90 0.68

(Eu/Eu*)N 0.58 0.55 (La/Yb)N 0.79 6,28

logical similarity, employing visual discriminants such as size, form, colour, clarity and aspect ratio. Zircons from sarnple KL-l were characterised by bulbous, epitaxial overgrowths of inferred metarnorphic zircon up to 20 flm thick that armoured the entire grain. When viewed in cross-section parallel to the c-crystallographic axis, it is apparent that the armouring zircon has grown on other­wise euhedral, zircon crystals. These grains were there­fore subjected to a vigorous, 24-hour mechanical abra­sion to remove the overgrowths and the outermost few microns of the underlying euhedral zircon.

U-Pb zircon ages from the Bymarka ophiolite 23

Zircon dissolution and ion-exchange procedures were comparable to those described by Krogh ( 1973) and Parrish et al. (1987). A mixed 205pb.233U_235U tracer was employed. Lead was loaded to W filarnents and U to Re filaments and analysed on the Finnigan MAT 261 multicollector mass spectrometer at Brown University. Pb was analysed in static multicollector mode employing Faraday cup collection of masses 208, 207, 206 and 205 while simultaneously collecting mass 204 in a secondary electron multiplier (SEM). Uranium was analysed by peak hopping into a SEM. Additional analytical details are given in the footnotes to Table 2.

Data and age interpretation Four fractions were analysed from the sample of trond­hjemite from Fagervika (TR97-K) and three from the Klemetsaunet rhyodacite (KL-l). The data are presented in Tab le 2 and graphically displayed in Figs. 5 and 6.

Zircons from sample TR97-K are all moderately dis­cordant and the data plot in a duster, despite having analysed different morphological crystal types. We inter­pret this discordance to result from Pb-loss rather than from inheritance of older radiogenic Pb. This interpreta­tion seems justified by the very narrow range of 207Pbf206pb ages of these zircon fractions. If inheritance was the mechanism of discordance, we would expect a much greater range of 207pbf206pb ages. A discordia tra­jectory anchored at O Ma through the data defines an upper intercept of 481.4 ± 4.7 Ma (inclusive of decay constant uncertainties). Although we have no independ­ent data regarding when these zircons lost Pb, they are not high-U zircons and the initial, Arenig, amphibolite­facies metamorphism is believed to have taken place shortly after zircon crystallisation. Hence, there would have been very little radiation damage in these grains prior to metamorphism; and the Arenig metamorphism is therefore unlikely to have resulted in significant loss of radiogenic Pb. Another way of assessing the age of this sample is by recourse to the mean weighted 207pb_206pb age of the data set, which is 481.3 ± 2.6 Ma. We therefore interpret the crystallisation age of this rock to be c. 481 Ma, but acknowledge that additional zircon analyses are required to refine the age and uncertainty of this sample.

As described above, zircons from sample KL-l are composite grains comprising nearly euhedral crystals armoured with an epitaxial, bulbous overgrowth of new zircon that is almost certainly of metamorphic origin. The overgrowths probably developed during the time of garnet and amphibole porphyroblastesis. In order to determine the igneous emplacement age of this rock, we therefore analysed 3 fractions of strongly abraded zir­cons. The crystals were examined several times during the abrasion cyde and extracted only after the over­growths had been eradicated and the outermost few microns of the pre-existing crystal removed (to l essen the likelihood of analysing those parts of the igneous

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NORWEGIAN jOURNAl OF GEOLOG Y

crystal that had undergone Ph-loss during the event that produced the overgrowths). The data define a precise concordia age (Ludwig 1998) of 481.5 ± 4.5 Ma (inclu­sive of decay constant uncertainties) that we interpret as the time of emplacement of these sheets and dykes. For purposes of convenience in the discussion, the age of sample KL- l is hereafter cited as 482 Ma and, likewise, the age for TK97-K is given as c. 481 Ma.

Discussion Age

The crystallisation ages presented here - 482 Ma for the Klemetsaunet rhyodacite and c.481 Ma for the Fagervika trondhjemite - are mutually indistinguishahle, even though the two rock-types show different geochemical sig­natures. The field criteria clearly denote that the rhyodacite and trondhjemite form part of the deformed and meta­morphosed Bymarka ophiolite, although the latter proha­hly intruded at a comparatively late stage. Their present distrihutions within the disrupted and sheared mafic rocks of the ophiolite prohahly owe much to tectonic imhrica­tion, firstly during the Early Ordovician ohduction event and later at the time of polyphase Scandian (Late Silurian­Early Devonian) orogenic deformation.

Based on the current time-scale for the Early Palaeo­zoic (Tucker & McKerrow 1995, Davidek et al. 1998, Landing et al. 2000), the dated felsic rocks from Fagervika and .Klemetsaunet range from Late Tremadoc to Early Arenig, taking into account the error hars (Fig. 7). Comparing these ages with the precise U-Ph dates generated from zircons extracted from plagiogranite dykes in the Løkken and Vassfjellet ophiolites (Dunning 1987, Dunning & Grenne, in prep.) (Fig. 7), we see that there is a complete overlap in the data. The trondhjemite

0.0766 TR97-K

0.0762 ca. 481 Ma

::J 0.0758 :11 'li.... ..c a.. 0.0754 �

0.0750

0.0746 0.580 0.584 0.588 0.592 0.596

207Pbt=u

Fig. 5. U-Pb concordia diagram for the 4 zircon fractions from the Fagervika trondhjemite, Bymarka ophiolite. The data are presented in Table2.

::J l_ ..c a.. �

0.080

0.079

0.078

0.077

0.076

U-Pb zircon ages from the Bymarka ophiolite 25

KL-1 482+/-5 Ma

0.075 +-�--+L-�-+--�--+--�___,f--� 0.56 0.58 0.60 0.62 0.64

Fig. 6. U-Pb concordia diagram for the 3 abraded, overgrown zircon fractions from the Klemetsaunet rhyodacite. MSWD (of concordance)

= 0.078. Probability (of concordance) = 0.78. The data are presented in Table2.

from Vassfjellet ( 480 ± 4 Ma) is closest in age to the rocks from Bymarka, whereas the felsic rocks from the Løkken ophiolite are largely Tremadoc, extending into the Late Camhrian. Here, we accept that the revised houndary hetween the Camhrian and the Ordovician is now placed at circa 490 Ma (Davidek et al. 1998).

Whether or not there is any real difference in general age hetween the Løkken ophiolite on the one hand, and the Bymarka and Vassfjellet ophiolites, is difficult to say, even though one might expect a hroad contemporaneity, judging from the geological map (Fig. 1). It is not uncom­mon, however, for oceanic magmatic rocks in ophiolite assemhlages to have heen generated over a time interval of l 0-15 million years. In the case of the ophiolites in the Karmøy-Bømlo region of SW Norway, a period of some 25 million years separates the oldest and youngest mem­hers of the fragmented assemhlages (Pedersen 1992).

On the island of Ytterøya, some 50 km northeast of Bymarka, an oceanic metatrondhjemite interhanded with metahasalts, has yielded a U-Pb zircon crystallisa­tion age of 495 ± 3 Ma (Roberts & Tucker 1998). On Leka, in the Leka Ophiolite Complex, a trondhjemite in the intermediate-level gahhro/dolerite dyke unit has heen dated (U-Ph zircon) to 497 ± 2 Ma (Dunning & Pedersen 1988). Although these two datings point to similar, Late Camhrian ages, they derive from felsic rocks that are considered to coexist with different parts of dif­ferent mafic, oceanic assemhlages. Moreover, they over­lap in time with at least one of the dated samples from Løkken (Fig. 7) - and hoth of the Løkken samples over­lap in age with our two samples dated from the Bymarka ophiolite. The logical inference from these comparisons, therefore, is that although we cannot easily separate these various ophiolite fragments in terms of age, the data do suggest that the Vassfjellet and Bymarka rocks

26 D. Roberts et al.

Ma 440

.I::. 1/) <(

450 o o

"C (.)

460 c: >

-l

470

480

-· 490

500

510

Base of Lower Hovin Group

�

Fig. 7. Ages and age ranges of zircons extracted from felsic rocks from fragmented ophiolite assemblages in the Trondheim Region (includ­ing Leka). References given in the text. Also shown is the approximate base of the post-obduction Hovin Group succession, and the inferred age range of the shoshonitic Hølonda Porphyrites (HP) (Grenne & Roberts 1998). Time-scale based on Tucker & McKerrow (1995) with revisions by Davidek et al. (1998) and Landing et al. (2000).

are younger than those on Leka and Ytterøya. The dated zircons are, in all cases, extracted from felsic rocks inferred to be broadly coeval with different parts of ocean floor magmatic complexes, spanning a time inter­val of some 15-20 million years, from the Late, or possi­bly late Mid Cambrian up to earliest Arenig time. This also accords with the upper constraint on their age, imposed by the Mid to Late Arenig faunas in the sedi­mentary successions of the area southwest of Trondheim - in a basin accumulating debris shedded from the rap­idly eroding, obducted and metamorphosed ophiolite.

Tectonic model

As noted earlier, the early Caledonian phase of deforma­tion and metamorphism in this region involved obduc­tion of the Støren ophiolite upon its Gula Complex sub­strate. The weight of isotopic and biostratigraphic evi­dence points to an Early Arenig tempora! slot for this major tectonothermal event, which also evidently involved the other, neighbouring ophiolite fragments, including that of Bymarka. While the timing of this Early

NORWEGIAN jOURNAl OF GEOLOGY

Ordovician 'Trondheim event' seems secure, the palaeo­geographic and plate-kinematic scenario is more specula­tive (cf., Gale & Roberts 1974, Gee 1975, Roberts et al . 1985, Stephens & Gee 1985, Grenne & Roberts 1998, Grenne et al. 1999). Models relying specifically on faunal province affiliation in Early Ordovician time and the inference that faunas of, e.g., largely North American affinity occur only in sedimentary rocks deposited on or along the margins of Laurentia, have naturally favoured reconstructions involving the early Caledonian obduc­tion of most of the Norwegian ophiolites upon the mar­gin of the Laurentian palaeocontinent ( Gee 1975, Bruton & Bockelie 1980, Stephens & Gee 1985). Reliance on other criteria, such as general geological and structural relation­ships and geochemistry, led some workers (Roberts & Gale 1978, Ryan et al. 1980, Sturt & Roberts 1991) to question the too simplistic view of faunal provincialism. In this case, palaeoreconstructions favoured initial ophio­lite emplacement upon either the margin of Baltica or an arc-related microcontinent (Roberts 1980, Roberts et al. 1985). This view has received support from the discovery of early Caledonian, HP eclogite-facies, metamorphic parageneses in the Baltica-Iapetus transition zone mafic rocks of the Seve Nappes (lower part of the Upper Allochthon) in northern Sweden (Stephens & van Roer­mund 1984, Andreasson et al. 1985, Mørk et al. 1988)­edogites generated in a subduction zone facing away from the Baltican margin - and some support has also come from lead isotope data collected from the nappes in Central Norway (Bjørlykke et al. 1993).

Refinements to the Early Palaeozoic, palaeomagnetic database for Baltica, Laurentia and Siberia over the last few years have also had an important bearing on inter­pretations of the palaeotectonic settings for Norwegian Caledonide ophiolite generation, metamorphism and obduction. From Late Vendian time and through much of the Cambrian period, Baltica lay in an inverted posi­tion and reasonably dose to the Siberian plate rather than Laurentia (Smethurst 1992, Torsvik et al. 1992, 1995, Torsvik & Rehnstrom 2001), but by Late Cambrian time it had begun to rotate quite rapidly in an anticlock­wise sense. Collision of Baltica with a cryptic are (Dallmeyer & Gee 1986), or arc-affected microcontinent (Sturt & Roberts 1991), above the edogite-generating subduction zone (and, thus, broadly coeval with Late Cambrian-Early Ordovician, Finnmarkian deforma­tion), was then reinterpreted as relating to interaction between Baltica and Siberia and the intervening are (Smethurst 1992, Torsvik et al. 1995, 1996, Torsvik 1998), or arcs. This is in the oceanic tract which has been termed the 'Eastern Iapetus Ocean' (Pickering & Smith 1995) or Ægir Sea (Torsvik & Rehnstrom 2001) (Fig. Sa).

Farther south, into Central Norway, the Seve Nappes are succeeded by the more exotic Kali Nappes, including the Gula Complex and Støren ophiolite, and Fundsjø Group in the east, units which were initially deformed and metamorphosed in Early Ordovician time. The epi­continental Gula differs lithologically, in many ways,

NORWEGIAN jOURNAL OF GEOLOGY

a)

b) Late Cambrian N

Siberia

..____ _

C) Early Ordovician N

'Gula' s

"""" '-� � +--- - <;bf'::,

Blueschist

forma ti on

Fig. 8. Palaeogeographic reconstruction and suggested plate tec­tonic scenario for the Late Cambrian to Tremadoc time interval, c. 500-480 Ma. Prior to this, in Early Cambrian time, Baltica had been in a more inverted position and rotated fairly rapidly anti­clockwise in to comparative proximity with Siberia, across the East­ern Iapetus Ocean or Ægir Sea, by Late Cambrian time (Torsvik 1998, Torsvik & Rehnstriim 2001). (a) General palaeomagnetic reconstruction for this broad Late Cambrian-Tremadoc period, modified from Torsvik (1998}. This emphasises the Late Cambrian situation, with Baltica then facing Siberia. With an anticlockwise­rotating Baltica, the Ægir Sea rapidly contracted in a dextral trans­pressive scenario, leading to non-synchronous obduction!thrusting of ophiolites. G-- Gula microcontinental block; NCB - North China Block; SCB - South China Block; Encircled B - Tremadoc­Arenig, platformal and arc-fringing, Bathyurid trilobite occur­rences. Encircled P - Tremadoc-Arenig, platformal Ptychopy­gine!Megalaspid trilobite occurrences. (b) N-S schematic section at the early stages of oceanic contraction, in Late Cambrian time, with eclogites developed at depth in the earlier, Siberia-facing, sub­duction zone. These eclogite-facies rocks were accreted onto the Baltoscandian margin (as part of the Seve Nappes) in Late Cam­brian to earliest Tremadoc time (the Finnmarkian orogenic event). (c) Schematic section showing the ophiolite assemblages and the blueschist parageneses of the Trondheim Region originating from the suprasubduction-zone domain and actual subduction zone, respectively, to the north of the «Gula microcontinent», in a Lau­rentia-facing situation, following anticlockwise rotation of Baltica. 1n the early part of Arenig time, the ophiolites are believed to have been obducted onto the microcontinent during this Early Ordo­vician, transpressive, tectonothermal event.

U-Pb zircon ages from the Bymarka ophiolite 27

from the Seve, and along its western edge contains a tec­tonic melange in deep-marine pelites directly beneath the obducted, mylonitised Støren ophiolite (Home 1979). In the east, the Baltican graptolite Rhabdinopora occurs in phyllites intercalated with metabasalts of the Fundsjø Group. Accepting that the Støren, Løkken, Vass­fjellet and Bymarka ophiolite fragments did form part of a common, subduction-influenced, ocean floor in Late Cambrian to Tremadoc time, then the HP blueschist­facies parageneses described by Eide & Lardeaux (2002) from the base of the Bymarka ophiolite occupy a campa­rable position to that of the melange; effectively record­ing evidence of both subduction and obduction at roughly the same structural level.

Incorporating these various elements into a credible palaeotectonic setting for this segment of the Baltoscan­dian margin in Late Cambrian-Early Ordovician time, we build on the notion that the ophiolites of the Trond­heim district, including Bymarka, were generated in a suprasubduction-zone setting outboard of a microconti­nent (Roberts 1980, Ihlen et al. 1997, Grenne et al.1999), represented today by the Gula Complex (Fig. 8b ), in Late Cambrian to Tremadoc time. Their obduction, in approximately Early Arenig time, was a form of large­scale tectonic inversion, utilising the pre-existing, ocean­facing subduction zone, down which the HP blueschists were generated (Fig. Se). The 'Gula microcontinent' is envisaged as an elongate block or sliver of Baltican crust that rifted off and drifted away from the palaeocontinent Baltica in Vendian-Early Cambrian time, during the ini­tiation ofiapetus opening. Farther north, this microcon­tinent may have narrowed and possibly merged with the cryptic are system that is inferred to have been involved in the earlier, Finnmarkian, arc/continent collision, which peaked in Late Cambrian to Tremadoc time -again, a case of mega-inversion following the oceanward subduction that had generated eclogite-facies rocks (Dallmeyer & Gee 1986, Mørk et al. 1988) (Fig. 8b).

Our composite reconstruction thus involves two sep­arate subduction systems (Fig. 8), with obduction­related, accretionary deformation at different times, first (Finnmarkian; Fig. 8b) within the contracting Ægir oceanic tract between Baltica and Siberia (cf., Eide & Lardeaux 2002). As Baltica continued to rotate anticlock­wise, its then northern margin and outboard microcon­tinent passed from the Ægir Sea into a contracting Iape­tus Ocean, facing Laurentia, by Early Ordovician time; thus explaining the influx of North American faunas in basinal and arc-fringing successions which were later to become parts of the Støren Nappe. In this framework, the Bymarka and related ophiolite fragments of the Trondheim district are representative of the most out­board and younger, subduction/obduction couplet (Fig. Se). In central Norway, there are no eclogite-facies rocks involved in the inboard, older, 'Seve subduction' system. Either subduction died out southwards, or HP Finn­markian assemblages, if they existed, have been tectoni­cally excised during Scandian thrusting.

28 D. Roberts et al.

Conclusions Zircons extracted from an oceanic rhyodacite sheet and an arc-related trondhjemite body in the Bymarka ophio­lite, dose to Trondheim, Central Norwegian Cale­donides, have yielded U-Ph ages of 482 ± 5 and c. 481 Ma, respectively. The dates, straddling the Tremadoc­Arenig boundary on the currently accepted time-scale, are interpreted to represent the crystallisation ages of these felsic rocks. Geochemical data indicate that the rhyodacite is comparable to plagiogranite dykes occur­ring in the nearby Løkken ophiolite, and is considered to be cogenetic with the metabasaltic greenstones of the Bymarka ophiolitic assemblage. The trondhjemite, on the other hand, transects the greenstone layering and shows a clear, subduction-related, geochemical signa­ture.

In common with neighbouring, dismembered and tectonically disrupted ophiolites, the Bymarka ophiolite is considered to have been obducted upon a microconti­nent represented by rocks of the Gula Complex, in Early Arenig time. The oldest fossil-bearing rocks overlying the deformed, metamorphosed and uplifted ophiolites of this district are of Mid Arenig age. Prior to the obduc­tion event, seaward subduction of oceanic crust led to the generation of late-stage are products and provided the appropriate conditions for the formation of high­pressure blueschist-facies metamorphic assemblages (Eide & Lardeaux 2002). This outboard, latest Cam­brian-Early Ordovician, subduction/obduction system is considered to have been located in the easternmost part of the Iapetus Ocean, merging into the Ægir Sea. After facing Siberia in Cambrian time, an anticlockwise-rotat­ing Baltica passed into the contracting Iapetan oceanic realm in the Earl y Ordovician, repositioning to face La u­rentia. Plate rotation in a contracting ocean thus pro­vided ideal conditions for intra-oceanic subduction, are/back-are systems and transpressive accretionary events involving ophiolite/arc obduction.

Acknowledgements. We are grateful to Elizabeth Eide and William Size for their helpful comments on the manuscript, and for discussions on the geology and trondhjemite geochemistry of the Trondheim Region. The work has also benefitted from numerous discussions with Tor Grenne on Central Norwegian ophiolites in general; and from the help­ful comments of the reviewers, Rolf Birger Pedersen and Mike Stephens.

Nils Spjeldnæs kindly read the revised manuscript and provided perti­nent comments, and also suggested small additions to the text.

NORWEGIAN JOURNAL OF GEOLOGY

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