44 Reidar G. Tronnes & Bjorn Sundvoll NGU . BULL 427. 1995

Isotopic composition, deposition ages and environmentsof Central Norwegian Caledonian marbles

REIDAR G. TR0NNES & BJ0RN SUNDVOLL

Reidar Tronnes, Min.-Geol. Museum, University of Oslo, Sars gate 1, N-0562 Oslo, Norway.

Bjorn Sundvoll, Geologica l Survey of Norway. Address: Min,-Geol. Museum, Sars gate 1, N-0562 Oslo, Norway.

Tectonostratigraphy of marblesThe CO, 0 -, and Sr-isotopic chemistry of 48 sam­ples of Central Norwegian Caledonian marblesand metalimestones are used to constrain theextent of post-depositional fluid-carbonate andsilicate-carbonate exchange and the sedimenta­ry ages and environments . The marbles occurmainly at three tectonostratigraphical levels: (1)the Helgeland Nappe Complex (HNC) of theUppermost Allochthon; (2) the Kbli-equivalentnappes (KN); and (3) the Seve-equivalent nap­pes (SN) of the Upper Allochthon. The carbona­tes in the KN and SN are closely associated with,and are generally deposited on, mafic metavol­canic rocks. The KN-rocks are generally in thegreenschist facies and have undergone compa­ratively little deformation . The carbonates arepartly fossiliferous and were deposited onOrdovician ophiolite and island arc fragments tal­ling within two main age groups: 500-470 Maand 443-437 Ma (Roberts et al. 1984, Pedersenet al. 1992) .

The SN-units in the Vestranden gneiss com­plex are metamorphosed in the amphibolite togranulite facies and have undergone extensiveand complex deformation (Solli 1989) . Most ofthe sampled SN-units occur as strongly flattenedand isoclinally infolded bands in the basementgneisses . At several locations the marble unitsare tectonically thickened and thinned by plasticflow. Based on observations further east (inSweden) the SN originated in the outermost partof the early passive margin of Baltica, and expe­rienced metamorph ism up to eclogite facies justprior to 500 Ma (Mark et al. 1985, Dallmeyer &Gee 1986, Andreasson & Gee 1989) .

The HNC includes two lithologically distinctgroups : (1) nappes composed of a basement ofhigh-grade gneisses and overlying calcareousmetasedimentary rocks, largely pelites and mar­bles; (2) nappes with mafic and ultramafic rocksof ophiolite affinity overlain by marbles and clas­tic metasediments (e.g. Thorsnes & l.eseth199 1). In terms of age and depositional environ­ment, the ophiol itic sequences of the HNC areequivalent to the Ordovician Kbli ophiolites inTrandelag.

C-, 0- and Sr-isotope relationsTrennes (1994) provides details of the locationsof the samples used in this study, as well as theanalytica l methods and numerical results. Theonly sample of the high-grade marbles from theEide area at Nordmare is classified here as a SNmarble. The supracrustals in the Eide may con­ceivably belong to strongly deformed and meta­morphosed KN (P. Robinson, pers.comm. 1993 ,Tronnes 1994). For the sake of consistency interms of age and depositional environment, twomarble samples associated with Ordovicianophiolite fragments within the HNC are groupedwith the Kbli units.

The Co, 0 -, and Sr-isotope relations areshown in Fig. 1. Forty of the 48 analysed sam­ples have more than 95 wt% carbonate and 44 ofthe samples have more than 90 wt% carbonate(major element data in Trennes, 1994 ). Thesamples are calcite marbles , except for 5 dolorni­tes. In Fig. 1 the samples contain ing more than54 .8 wt% CaO from marble deposits exceeding200 m in thickness are separated from the rest ofthe samples by large symbols. The carbonatefraction in these samples has presumably experi­enced a minimum of isotopic exchange with sili­cate (and oxide) minerals within the marble andwith the wall rocks during metamorph ic (andmetasomatic) processes following carbonatedeposition.

The marbles associated with the gneiss­metasediment nappe units of the HNC have Sr­and C-isotope compositions distinctly differentfrom those of the SN and the KN calcite marbles(Fig. 1). The O-isotope compositions overlapcomplete ly between the three groups, but the6180 values are roughly correlated with the CaOcontents (corresponding to total carbonate con­tent) of the calcite mables. All of the sampleswith more than 54.8 wt% CaO from marblesexceeding 200 m in thickness have6180 (SMOW) > 22%0. Only 4 of the 30 remainingsamples have 6180 values exceeding 22 '1'00.Because no sample has 6180 exceeding 24'1'00and 8 of the 10 large-symbol samples have 6180values of 23-24%0, it is likely that this is the origi-

NGU - BULL 427. 1995

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Fig. 1. Sr-O-C-isotopic relations (upper two diagrams) and1)'8D-CaO relations of carbonates from the CentralScandinavian Caledonides. Large symbols are for sampleswith CaO>54.8 wt% from deposits exceeding 200 m in thick­ness. The 87SrJ86Sr initial ratiosfortheKN, SN, and HNCmar­bles are calculated at 470, 560, and 600 Ma, respectively. Inmost of the samples the Rb/Sr ratios are so low that thediffe­rences between theinitialratiosat 470 Maand 600 Ma arelessthan 0.00003. The isotopic analyses were performed on thecarbonate fractionof the samples after extraction using 100 %phosphoric acid at 50 ·C for the calcite-dominated samples,and at 80·C tor the dolomite-dominated ones. The analyticalprecision, in the form of 20, is 0.0001 , 0.2%0, and 0.4%0 for87SrJ86Sr, 1)'3C, and 1)180 , respectively.

Reidar G. Tronnes & Bjam Sundvoll 45

nal isotope signature of the carbonate fraction inall of the sampled marbles.

Metamorphic and/or metasomatic reactionswill affe ct the O-isotope values more than the C­isotope values because C is much less ava ilablethan 0 from the silicate wa ll rocks and silicateminerals in the marbles. The 1)180 value of aninfiltrating fluid wi ll generally not reflect the fluidsource, but will rather be buffered towards that ofthe local wa ll rock lithology. Metamorphic andmagm atic rocks and meteoric and ocean watershave 1)180 (SMOW) values between - 50%0 and+150/00, and metamorphic and metasomat ic ove r­print will therefore lower the I)180 of the marbles.Whereas the local silicate rocks have a large buf­fer ing capacity for 1)180 , changes in 1)13C willmore likely reflect external fluid sou rces , e.g .CO2 release dur ing the crystall isat ion of a mant­le-derived magma in the lower crust (1)13C(PDB)=- 5 to -10 %0). All other sources of C (exceptcarbonates and dolomitisation fluids) are alsocharacterised by negative 1)13C(PDB).Equilibrium fract ionation factors for oxygen bet­ween calcite and var ious silicate minerals atmetamorphic temperatures are small (1000 In acalcite/silicate is in the - 1.1 to +0.6 range for thefract ionation between calcite and quartz, alkalifeldspar , plag ioclase and muscovite at 450°C,data compiled by Faure 1986). Decarbonatio nreactions lead ing to ca lc-s ilicate formation wi llproduce coupled 180 and 13C depletions of theremaining carbonate (see Valley 1986 , Fig. 3).Several studies have indicated that marbles arecommonly relative ly impermeable to fluids, evenunder high -grade metamorphism (Rye et al.1976, Nab lek et al. 1984 , Valley 1986) .

In Fig . 1 (upper diagram) the samples with >54.8 wt% CaO from >200 m thick marble unitsplot in the uppermost right end of elongate 'tails 'towards decreasing 1)13C and 1)180 values.These 'tails' with larger 1)180 than 1)13C compo­nents, probab ly result from the interaction of theanalysed carbonate fract ion with the silica teminerals in the marbles and with poss ible exter­nal fluids. The dolomite marbles in the SN inVestranden are characterised by cons iderablyheavier C than the average of the SN calcitemarbles. The I)13C in the carbonate fraction maybe increased by dolomitisation fluids contain ingcarbon dioxide and/or bicarbonate with heavycarbon equilibrated with (or residual fro m) met­hane with extremely light carbon (Murata et al.1969 , Hudson 1977).

Two calcite marble samp les (Fig. 1) from thenorthernmost part of Ves tranden have elevated1)13C values at the same level as the HNC mar­bles (see Tronnes 1994). Several of the marbles

46 Reidar G. Tronnes & Bjorn Sundvall GU - BULL 427, 1995

Fig. 2. Global time variations in Sr- and C-isotope ratios in sea­water and marine carbonates from 850 to 400 Ma. The smoothcurves represent best estimate values summarised fromVeizer & Hoefs (1976). Faure (1986), Derry et al. (1992) andKaufmann et al. (1993). The isotopic composition of the carbo­nates from the HNC, SN and KN least affected (or unaffected)by post-depositional processes are shown by horizontal, thinlines.

Depositional ages and environmentsThe global time variations in Sr- and C-isotoperatios in seawater and marine carbonates havebeen summarised by Faure (1986) and Veizer &Hoefs (1976), respectively. New data for theProterozoic-Phanerozoic transition is presentedin Derry et al. (1992) and Kaufmann et al. (1993).

within assumed SN in the northernmost part ofthe Vestranden area are assoc iated with micaschists and calc-silicate gneisses (calcareousmetasediments?) rather than with mafic metavol­canites. These marbles may have a depositionalage and environment similar to the HNC mar­bles. Work in progress (Tronnes et al. in prep.)seeks to clarify whether the carbonate depositsin the Indre Folia and Kongsmoen areas are ofHNC affinity.

The 87Sr/86Sr ratios of the carbonate fract ionmay be changed by interaction with the silicateminerals in the marbles and externa l fluids.Diagenetic and metamorphic processes maycause increasing 87Sr/86Sr ratios in marb lesinter layered or mixed with alkali feldspars andmicas. This tendency is expected for the HNCcarbonates associated with continental clasticmaterial. In the SN and KN carbonates whichare interlayered and mixed with mafic volcanic orvolcanoclastic material, isotope exchange withthe silicate fraction and the wall rocks will lowerthe 87Sr/86Sr ratios of the carbonate fraction.This tendency is observed in Fig. 1. In particular,a sample from the Holonda carbonate-andesitebreccia has the lowest 87Sr/86Sr-ratio (0.7074) ofall the KN samples in Fig. 1.

Fig. 2 is a simplified diagram based on thesecompilations.

A comparison of the Sr- and C-isotope ratiosin the least altered carbonates within the diffe­rent tecton ic and stratigraphic groups of Fig. 1with the global evolution curves of Fig. 2 resultsin estimated depositional ages of 600-750 Mafor the non-ophiolitic marbles of the HNC. Theleast altered SN- and KN-marbles are isotopical­ly overlapping and intersect the seawater evoluti­on curve for Sr at 470 and 560 Ma. The indepen­dent age constraints on these nappe units(Roberts et al. 1984, Dallmeyer & Gee 1986,Mark et al. 1988, Pedersen et al. 1992) estab lishthe depositional ages as Ordovician and Vendianor Early Cambrian for the KN- and SN-marbles,respectively.

The overlapping isotopic compositions of theSeve and K61 i marbles renders such data use­less for correlation and discrimination betweenUpper Allochthon marbles in the centralNorwegian Caledonides. However, the Sr-C-iso­tope composition is potentially a useful comple­ment to geological field relations for the discrimi­nation between late Proterozoic and Ordovicianmarbles within the Uppermost Allochthon inNordland and Troms.

The environments of the Late ProterozoicHNC carbonates differ markedly from those ofthe Ordovician (KN) and the Vendian or EarlyCambrian (SN) marbles. Whereas theProterozoic carbonates are interlayered withthick epiclastic sedimentary units, the youngercarbonates were almost invariably depos iteddirectly on top of mafic lavas or pyroclastics. TheSN carbonates were probab ly deposited alongthe early passive margin of Baltica (Andreasson& Gee 1989). Palaeomagnetic reconstructions ofthe movement of Baltica during Vendian andCambrian times (Torsvik et al. 1992) indicatethat the SN carbonates were deposited at latitu­des of about 30·S. Based on faunal evolution,the deposition of the KN ophiolite-related carbo­nates may have occurred on the Laurentian sideof the lapetus Ocean before , during and afterobduction of the ophiolites (Pedersen et al.1992), although other data favour a relationshipto Baltica (Sturt & Roberts 1991, Bjarlykke et al.1993). In the Early to Mid Ordovician the lapetus­facing parts of Baltica and Laurentia were loca­ted at latitudes of about 40·S and 10·S, respecti­vely (Torsvik et al. 1992).

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NGU - BULL 427,1 995 Reidar G. Trennes & Bjern Sundvoll 47

AcknowledgementsThe C- and O-isotope analyses were performed at theUniversity of Bergen. and we thank Eystein Jansen for this ser­vice. Our study was fundedby the coordinated geological pro­grams of the Nord-Trondelag and Nordland counties, carriedout at the Geological Survey of Norway. Tom Andersen andAnneBirkeland provided helpful reviews.

References

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