Printed in Northern Ireland. Geochronological evidence from discordant plutons for a late Proterozoic orogen in the Caledonides of Finnmark, northern Norway.
JournaI of the Geological Society, London, Vol. 148, 1991, pp. 29-40, 8 figs, 3 tables. Printed in Northern Ireland
Geochronological evidence from discordant plutons for a late Proterozoic orogen in the Caledonides of Finnmark, northern Norway J . S . DALY’,*, S . J . AITCHESON’,R.A. CLIFF’, R . A . G A Y E R 3 & A. H . N . RICE4 ’Department of Geology, University College Dublin, Belfeld, Dublin 4, Ireland ’Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, U K 3Department of Geology, University of Wales, Cardiff, PO Box 914, Cardiff CFl3YE, UK Geologisch-Palaontologisches Institut, Ruprecht-Karls-Universitat,INF 234, 0-4900 Heidelberg, F.R. Germany Abstrad. Rb-Sr whole-rock, Sm-Nd mineral and U-Pb zircon dates
are reported for minor granites and Seiland Igneous Province gabbros in the Kalak Nappe complex (Middle Allochthon), northern Norwegian Caledonides. The Litlefjord (Rb-Sr WR 813 f 62 Ma, MSWD 2.7; U-Pb zircon 804 f 19 Ma, MSWD 5.5) and Repvig (Rb-Sr WR851 f 130Ma, MSWD 5.1) granites and the Hasvik gabbro (Sm-Nd WR/minerals 700 f 33 Ma, MSWD 0.7) cut previously-deformed metasediments of the Scbrcby Succession while the Kvalfjord (Sm-Nd WR/minerals 612 f 33 Ma,MSWD 0.1) and Storvik (Sm-Nd WR/minerals 604 f 44 Ma, MSWD 0.2) gabbros cut ‘basement’ paragneisses. The main (Dl-D2) deformationaffectingthe Sbrcby Succession in theKalakNappecomplex, of the Caledonian orogeny, which was previously attributedto the early Ordovician Finnmarkian phase took place before c. 800 Ma in the pre-Caledonian ‘Porsanger Orogeny’. Early ‘Scbrcby’ plutons of the SeilandIgneousProvincewereapparentlyemplacedduringthisevent.However,the later, more voluminous ‘Stjerncby’ intrusions are c. 700-520Ma old and post-date the Porsanger orogeny. These pre-Caledonianintrusionsmustbeinter-,ratherthansyn-orogenicaspreviouslyclaimed.This obviates the problem of accomodating the voluminous Seiland magmatism during orogenic compression and is consistent with an early Caledonian rift origin for these rocks, probably related to the opening of Iapetus.
theycut, was of Middle Cambrian age (Holland & Stud 1970). Recent developments demand reappraisal a of this chronology. The palaeontological evidence for the depositional age of theS0r0ySuccessionhasbeendismissed by Debrenne (19M) and there is now no stratigraphical constraintonthetiming of deformation within the Kalak Nappe complex. Based on precise U-Pb zircon ages, Pedersen et al. (1989) haveshown thattheFinnmarkian orogeny was terminated by c. 520-530 Ma (MiddleCambrian) when nepheline syenite pegmatites, which post-date the major regional deformation, were emplaced at a high crustal level marking the end of Seiland Province magmatism. The syn-orogenic setting of the Seiland Provincehasalsobeen questioned (Krill & Zwann 1987) although much of the evidence presented has been criticized (Sturt & Ramsay 1988). Nonetheless the supposedly syntectonic emplacement of the very lage volumes of mafic magma involved in the Seiland Igneous Province presents a major mechanical problem. Thispaperreportstheresults of Rb-Srwhole-rock (WR), Sm-Nd mineral and U-Pb zircon investigations of the deformation chronology and updates results previously reported only in abstracts (e.g.Daly et al. 1987, 1988; of Aitcheson et al. 1989) which havebeenthesubject widespreaddiscussion(e.g. Roberts 1988; Townsend & Gayer 1989). Twosets of timemarkers with clear field relations were selected for dating. Minor granite sheets on Porsangerhalv0ya(Figs 1 & 2) appearedonstructural grounds to be synorogenic i.e. emplaced between D2 and
Two major orogenic episodes have affected the Caledonides of N Norway and have been interpreted as separate phases of the Caledonian orogeny. Most of the deformation and nappe emplacement took place during the younger ‘Scandian’ event (Gee 1975) at c. 400-425 Ma (Dallmeyer et al. 1988). Finnmark, In the Scandian assembled a tectonostratigraphycomprisingthreemajortectonicunits: theUpper, MiddleandLower Allochthons, which are widely recognized throughout Scandinavia. Theearlier‘Finnmarkian’event was claimed to have involved large-scale translations of nappes belonging to the LowerandMiddle Allochthons in anearlyphase of the Caledonian orogeny (Ramsay & Sturt 1976). In the Middle as the Kalak Nappe Allochthon in Finnmark, known complex (Fig. l), amphibolite-facies metamorphism and two majordeformationphases(D1andD2)affectedathick marine sedimentary sequence, the S 0 r ~ y Succession (Ramsay 1971),anditsbasementgneisses. Theseevents were dated by Sturt et al. (1978) at c. 540-490 Ma, thereby establishingtheFinnmarkianasanearlyCaledonian i.e. mid-Cambrian to early Ordovician event. This was based on Rb-Sr ages of granitic veins cutting the mafic plutons of the Seiland Igneous Province their oraureoles. These voluminous mafic, ultramafic and alkaline intrusions make up a major proportionof the uppermost nappe of the Kalak Nappe complex. Critical to the interpretation of Sturt et al. (1978) weretheassumptions:(1)thatthegranitesformed during intrusion of the Seiland plutons; (2) that the Seiland IgneousProvinceplutonsweresyn-orogenic(Robins & Gardner 1974, 1975); (3) that the S0r0y Succession, which 29
30
J . S . DALY E T AL.
km
100
Autochthon
cover sediments
Fig. 1. Outline geological mapof the main tectonic units of Finnmark, showing the area of the map in Fig. 2. Note that the location of the area on the inset in Fig.2. in northern Scandinavia is shown D3. Both events were then (1984) regarded as Finnmarkian and coevalwithtectonic transportalongboundingthrust faults (Chapman et al. 1985). Secondly, several mafic intrusions belonging to the Seiland Igneous Province, which or the cuteitherthe SdrGy Succession metasediments paragneiss ‘basement’, were selected after establishing that they postdated D2. The results show that deposition of the S@my Succession andmajororogenicdeformationtookplaceintheKalak Nappe complex before c. 800 Ma. Aitcheson (1990) further constrainsthe timing of events usingSm-Ndmodelages. of a late Proterozoic Theseresultsestablishtheexistence orogen in the Kalak Nappe complex.
Regional setting From the top downwards, the regional tectonostratigraphy in Finnmark (Fig. 1) consists of the Magerey Nappe (Upper Allochthon),theKalakNappecomplex(Middle Allochthon), the Laksefjord Nappe complex and the Gaissa Thrust Belt. The lowertwounits togethercomprisetheLower Allochthon (Rice et al. 1989). The basal Gaissa thrust cuts autochthonous sediments, in part of Cambrian age, which rest unconformably on the Svecokarelian rocks of the Baltic Shield. Regionalcorrelation of theindividualnappesforming the KalakNappecomplexhasresulted in onlypartial
et al. agreement (cf. Gayer et al. 1985,1987;Ramsay 1985a,b). For the purposes of this paper, four allochthonous units have been outlined in the Sdrdy-SeilandPorsangerhalvGya areas (Fig. 2 ) . From the top downwards, these are: S~rdy-Seiland Nappe, Hawatnet imbricate Stack,OlderfjordNappeandKolvikNappe. The Kolvik Nappe rests directly on the Gaissa Thrust Belt where the Laksefjord Nappe complex is missing, south of Porsangerhalvdya (Fig. 1). The individualallochthonsformingtheKalakNappe complex arestructurallycomplexbutgenerallycomprise imbricate stacks (Gayer et al. 1985) in which metasediments of the SdrdySuccession(Ramsay1971) and astructurally underlyingbasement of Archaeanorthogneiss(Aitcheson 1989; Akselsen 1982) or paragneiss (Ramsay & Sturt 1986) have been transported and strained. Ductile structures within the nappes have been described on Sdrdy e.g. by Roberts (1968) and on Porsangerhalv~ya by Gayer et al. (1985) who also attempted to rationalize the previously published deformation chronologies. D1 and D2 structures affecting the S B ~ Succession B~ on S d r ~ ymay be broadly correlated regionally although application of thin-skinned deformation models has tended to emphasize diachroneity (e.g. Rice 1984, Chapman et al. 1985). In this paper the major D2 events on Sdrdy (e.g. Sturt & Ramsay 1988) have been correlated with F2 folds developed in the Serey Succession metasediments on Porsangerhalvdya.
LATEPROTEROZOICOROGEN,FINNMARK
31
J . S. DALY E T A L .
32
These folds are tight to isoclinal structures whichfold an earlier S1 foliation,mainly defined by biotite. Correlation of deformationaffectingtheSdrdySuccession with that in theallochthonous‘basement’rocks is problematical. An earlier history in the basement is proven angular in rare places by the preservation of an unconformity between the Sdrdy Succession and the et al. 1979; Sturt & Archaean orthogneiss (Ramsay Austrheim 1985). In contrast, the relationship between the paragneiss and the Sdrdy Succession is uncertain but Akselsen (1982) presents evidence for some structures inthe paragneisses being older than those in the Sdrdy Succession. or before the D1 The twounitswerejuxtaposedduring deformation in the Sdrdy Succession, but no unconformity has been recognized between them. Mafic, ultramafic and alkaline intrusions of the Seiland IgneousProvince (Robins & Gardner 1975) makeup a major proportion of the Sdrdy-Seiland nappe. On Sdrdy, of the these rocksoccur asintrusionsinmetasediments SdrdySuccession,while to the south on Seiland, Stjerndy and south of Stjernsund (Fig. 2), the paragneisses form the on Sdr0y and N. Seiland host rocks. Foliated metagabbros are the oldest intrusions and were emplaced before D2. The later (post-D2) intrusions are generally little deformed apart from minor shear zones. Layered gabbros are volumetrically the most important of the post-D2 intrusions and include the Kvalfjord andStorvik olivine gabbrosandthe Hasvik gabbro (Fig. 2). The gabbros are cut by ultramafic central complexes (Bennett et al. 1986), thought to have originated ascumulatesfrom picritic magmas.Alkalineintrusions comprising pyroxenite, syenite, nepheline syenite and carbonatite arethe youngest component,some of which a shallowerlevelthan the preceeding wereemplacedat mafic and ultramafic intrusions (Pedersen et al. 1989). Minor granite bodies, generally adamellitic in composition, occur widely in the Kalap Nappe complex. The larger bodies are shown on Fig. 2. The graniteshavenotbeen subject to systematic investigation and their tectonic significance and relationship to the Seiland Igneous Province is notknown.Neither is it clearwhether or notthey all belong to a single group. The Litlefjord body (L, Fig. 2) has clearlybeenemplaced after D2 (see below) butinmany
cases the contactswith the country rocks are concordant and the time relations are only poorly known.
Porsangerhalveya granites Field relations The Litlefjordgraniteintrudesinvertedamphibolite-facies metasediments of the Klubben Psammite Group (the lowest of theSdrdy Succession) within the stratigraphic unit Hawatnet Imbricate Stack (Gayer et al. 1985) on the west (Fig. 2). The granite is a coast of Porsangerhalvdya strongly-foliatedadamellitecontaining quartz,K-feldspar, plagioclase and biotite. Itforms a thin.sheet(generally about 500m thick) broadly concordant with northwesterly dippingbeddingandfoliation in the enclosingmigmatitic psammites and semi-pelites. Notethat while Gayer et al. (1985, plate 1 and 1987, fig. 2) showed the hanging wall of the Litlefjord body in contact with basement gneisses, our present view is that although presumed basement rocks are imbricated with the cover, the hanging wall of the granite lies in the Sdrdy Succession. Independently, Ramsay et al. (1985a, Fig. 1) showed the Litlefjord granite cutting KlubbenPsammiteGrouprocks in agreement withthis interpretation. The granite can be followed along strike for about 2 km and in several places, the hanging wall contact cuts across the foliation in the country rocks. At Gorbasfjellet (L, Fig. 2) the granite cuts F2 folds (Fig. 3a) which deform an earlierfoliation.Similarrelationswere seen at one locality in the footwall of the body. The granite itself has a strong S3 foliation whichis parallel to the earlier S2foliation and F2 axialplanesinthecountryrock, accounting foritsgenerallyconcordantcontacts. In the hanging wall, discordant, post42 sheets of the granite are folded by F3 folds whichare coaxial with the earlier F2 folds developed in the psammite (Fig. 3b). The Repvig granite, which is exposed for a strike length of c. 10 km northwards from Repvig along the east coast of Porsangerhalvdya (Fig. 2), cutstheKlubbenPsammite Group in the lower part of the Olderfjord Nappe (Gayer er al. 1985). Thisbody is petrographicallysimilar to the Litlefjord granite, although it is somewhat finer-grained and contains signficant amounts of garnet and sphene. Its time
a)
Fig.3. Outcrops of the Litlefjord graniteat Gorbasfjellet (L, Fig. 2) showing (a) the granite cutting an F2 fold in the Klubben Psammite
by biotite; and (a) an apophyse ofthe granite also cutting the Klubben Group metasediment. Thefold deforms an earlier foliation defined Psammite, folded by later, M folds, presumably Caledonian in age. Such folds are coaxial with the earlier F2 fold developed in the psammite.
FINNMARK PROTEROZOIC OROGEN, LATE
33 relations are less clear than those of the Litlefjord body. It was notpossible todemonstrate unambiguously that F2 foldswerecut by thegranite.Howeverthe first fabric N-S lineation, affecting the granite, predominantly a appears to be the same as that associated with F3 folds in the country rocks.
Table 1. Rb-Sr isotopic data
87Rb/86Sr 87Sr/86Sr Rb/Sr Sr Rb Litleford granite, W Porsangerhalv@ya
5.757 51.2 295.0 7184-4a 5.537 53.8 297.0 7184-4b 76 56.6 259.17184-5 4.245 68.6 291.3 7184-6a 7184-6b 7184-6c 7/84-7a 4.249 67.9 288.6 7184-7b
0.90890
Rb-Sr geochronology 295.2 281.5 285.1
67.6 67.8 68.0
4.364 4,151 4.196
Repvdg granite, E Porsangerhalv@ya 262.0 99.8 2.626 7184-8 261.1 96.0 2.723 7184-9.1
271.4 256.1 269.1 272.2 271.4 281.4
7184-9.2 7184-9.3 7/84-11 7/84-12a 7184-12~ 7/84-13
96.5 97.1 92.3 92.9 93.7 81.7
2.813 2.638 2.916 2.931 2.8% 3.446
12.82 12.19 12.32
0.86083 0.85865
7.68 7.96 8.23 7.72 8.53 8.58 8.47 10.10
0.81422 0.81991 0.82161 0.81826 0.82364 0.82739 0.82349 0.84502
Eight samples were collected from four sites across the western end of the Litlefjord granite at Gorbasfjellet (L, Fig. 2) over a distance of c. 600 m along a blasted road section. All samples were collected as single blocks weighing c. 10 kg. Where more than one sample was collected at anysite,thesamplesarelettered a, betc.Alleight samples(Table1)yieldaRb-Srisochronage of813 f 62Ma (MSWD = 2.7) with an initial 87Sr/86Sr ratio of 0.717 f 7 (Fig. 4a). The three samples from site form 6 linear a array barely distinguishablewithinanalyticalerrorcorresponding to an age of 395 f 167 Ma. six Eightsamples of the Repvig granite were prepared from blockscollectedfromfivesitesoveradistanceof200mfrom blasted roadside exposures at Langstrand (R, Fig. 2). Samples 12a and 12c were collected together at a single site. Sample 9 was spilt intothree bysawing off smallersubsamples 9.2and9.3,each as subsample 9.1. weighing about 3 kg, leaving approximately 10 kg All eight samples (Table 1) form a crude linear array corresponding to an errorchron ageof 851 f 130Ma (MSWD = 5.1) with an initial 87Sr/S6Sr ratio of 0.722f 7 (Fig. 4b). The three subsamples of sample form 9 linear a array corresponding to an age of 461 f 153 Ma and an initial 87Sr/86Sr ratio of 0.767 f 17. This line is also colinear with samples 12c and
0.86430
Isotopicmeasurementsweremade on a VG Micromass30 at University College Dublin on spiked samples as described by Menuge (1988). Uncertainties of 1.5% in 87Rb/%r and 0.01% in87Sr/MSrwereusedinisochroncalculations (2a errors). 87Sr/%rratioshavebeennormalizedto86Sr/88Sr=0.1194. During the period of this work the SRM 987 standard gave a value of 87Sr/%r = 0.71031 f 0.00006.
8 7 ~I r"Sr
/ /-
7 ~ l ar 6 S r
13
REPVAG
.e10
LITLEFJORD GRANITE
.eo0
813 f 62 M a 0.717 f 7 MSWD 2.71
.E90
GRANITE
I_
,840
(b)
.830 12a
851 2 130 Ma
0.722 f 7 MSWD 5.05
p
.880 5
,810
i
i-
.870
87Rb l "Sr
7
8
Q
10
I
I
l
I
Fig.4.
Rb-Sr isochrons, Litlefjord (a) and Repvhg (b) granites. Individualdata points are labelled
,850
1 87Ab f "Sr
11
12
17
13
1614
15
with abbreviated sample numbers (cf. Table1). Points lettered in sequence were collected within one metreof one another at the same site. Samples 9.1 etc. (diamond symbol) were sawn from a single block.
34
J . S . DALY E T A L .
11. When thesesamples are included,theresultingisochron corresponds to an age of 473 f 83 Ma (MSWD = 0.1) with an initial %r/=Sr ratio of 0.766 f 10.
U-Pb zircon geochronology In order to confirmthesurprisinglyoldRb-Srages,zirconswere separatedfrom one of theLitlefjordgranitesamples(7/84-4a). Non-magneticfractionsfromthefourlargergrain-sizeintervals (Table 2) werehand-picked to select the more euhedral elongate grains ( I : W = 4, Fig. Sa) from the more common stubby prisms with less-welldevelopedfaces ( I : W = 2.5,Fig.5b).Togetherwith I : w = l S ) , the latter rounded, partially corroded grains (Fig. 5e; madeupmostofthezirconpopulation.Occasionalgrainswith
"J
0.136C
~
c
804 k 19 Ma
ft
1
MSWD 5.5
0.124
1.34
1.26
1.18
1.10
1.42
1.50
Fig.6. U-Pb zircon concordia plot for the Litlefjord granite (sample 7/84-4a). Labelson the individual error ellipses indicate the sample type or grain sizein pm.
Fig.5. SEM photographsof zircons from sample 9/84-4a.(a) Pale ( I : W = 4); yellow euhedral grain with a good pyramidal termination (b) stubby prism( I : W = 2.5) with less-developed faces, generally darker in colour than(a); (c) and (a) distinctive colourless, generally well-faceted and euhedral 'gems' with a brilliant lustre; (e) rounded, partly corroded light-coloured grain ( I : W = 1.5). Apart from the l50 pm. Grains of type (b) and (e) make up most of the population.
distinctiveroundedcores,togetherwithlesscommonprismatic grainswitheuhedralovergrowths,werealsodiscarded.Asmall number of distinctive,generallywell-faceted,euhedralbrilliant grains('gems')wereselectedfromthelargergrain-sizeintervals (Fig. 5c, d). The
