NJG 83 209-242.Pdf

NORWEGIAN JOURNAL OF GEOLOGY Isotope chemostratigraphy and detailed mapping of high-grade marble sequences 209 New insights into the geology of high-grade Caledonian marbles based on isotope chemostratigraphy Victor A. Melezhik, Bouke K. Zwaan, Gedeminas Motuza, David Roberts, Arne Solli, Anthony E. Fallick, Igor M. Gorokhov & Anton B. Kusnetzov Melezhik, V.A., Zwaan, B.K, Motuza, G., Roberts, D., Solli, A., Fallick, A.E., Gorokhov, I.M. & Kusnetzov, A.B. 2003: New insights into the geology of high-grade Caledonian marbles based on isotope chemostratigraphy. Norwegian Journal of Geology,Vol. 83, pp. 209-242. Trondheim 2003. ISSN 029-196X. A first attempt to employ carbon and strontium isotope stratigraphy together with 1.20,000 scale mapping for chronstratigraphic subdivision and geological correlation of non-fossiliferous sedimentary successions in the polydeformed, high-grade, marble-dominated terrane has been success- δ13 δ18 87 86 ful. The isotope stratigraphy was based on 315 analyses of major and trace elements, as well as on 231 Ccarb and O, and 104 Sr/ Sr whole- rock analyses of calcite and dolomite marbles, representing all major marble units of the Narvik, Evenes and Niingen nappe complexes, and the Bogen Group in the north-central Norwegian Caledonides. A thick succession of calcite and dolomite marbles in the Ofoten Synform, previously considered to be stratigraphically homogeneous and of Late Ordovician-Early Silurian age, is shown to be a complex assemblage of Neoproterozoic, Cambrian and Early Silurian carbonate formations which were tectonically imbricated and emplaced in a non-chronostratigraphic order. The real breakthrough with the new geological mapping has been the establishment and use of the Cambrian and Early Silurian chemo/chronostratigraphic markers that reveal stratigraphic heterogeneity and tectonic repetition, provide a reliable stratigraphic subdivision, and facilitate the compilation of a chronologically-based geological map. Melezhik, V.A., Zwaan, B.K, Roberts, D. & Solli, A., Norges geologiske undersøkelse, 7491 Trondheim, Norway; Motuza, G., University of Vilnius, Chi- urlionio 21, Vilnius, Lithuania; Fallick, A.E., Scottish Universities Environmental Research Centre, G75 0QF East Kilbride, Glasgow, Scotland; Gorok- hov, I.M. & Kusnetzov, A.B., Institute of Precambrian Geology and Geochronology, nab. Makarova, 2, 199034 St. Petersburg, Russia Introduction leading to the formation of carbonate deposits of different kinds. The Ofotenfjord area (Fig. 1) has been cho- Despite abundant petrological and structural studies, sen as a first-priority target because of the presence as well as geological mapping, performed over several there of a vast volume of marbles constituting part of decades in the North-Central Norwegian Caledonides the Uppermost Allochthon in that region. The main by both Norwegian and international research teams objectives of this contribution are (i) to illustrate the (Foslie 1946, 1949; Gustavson 1966, 1972, 1974a, use of isotope stratigraphy for indirect dating, subdivi- 1974b; Bartley 1981; Boyd 1983; Boyd & Søvegjarto sion and correlation of high-grade marble sequences; 1983; Boyd et al. 1986; Tull et al. 1985; Steltenpohl and (ii) to demonstrate progress towards the produc- 1987; Steltenpohl & Bartley 1987; Steltenpohl et al. tion of a new generation of geological maps in high- 1990; Tucker et al. 1990; Oliver & Krogh 1995; Andre- grade metamorphic terranes, as based on detailed map- sen & Steltenpohl 1994; Coker et al. 1995; Northrup ping combined with strontium and carbon isotope 1997), little research and detailed geological mapping stratigraphy. has been done on marble formations in the Norwegian Caledonides. At the Geological Survey of Norway, a special project entitled ‘Carbonate formations of Nor- Analytical techniques way: from basic research to industry’ has been launched to study depositional environments, metamorphism, Analytical techniques are described in Appendix 1. stratigraphic correlation and ages of marble formations in selected regions of the Norwegian Caledonides. With the main goal of the project being to develop an explo- Regional geology, metamorphism, ration strategy for new marble deposits, a significant amount of basic research has been required in order to deformation and isotopic ages establish a reliable tectonostratigraphic base that aims The study area is located north of Ofotfjorden and Her- at discriminating between the local and regional factors jangsfjorden and south of Astafjorden (Figs. 1 & 2) and 210 NORWEGIAN JOURNAL OF GEOLOGY V. A. Melezhik et al. In the area north of Ofotfjorden, lithologies within the Narvik nappe complex include garnet-mica schists, cal- careous schists, monotonous biotite schists and subor- dinate bituminous schists, amphibolites, keratophyres, and marbles metamorphosed at amphibolite-facies. The Narvik nappe complex is separated from the overlying Evenes nappe complex by a thin tectonic unit of amphibolites and tonalites. Similar rocks on the sout- hern side of Ofotfjorden were described by Boyd (1983) as representing an ophiolite fragment (Fig. 3), which together with the overlying Elvenes Conglomerate was termed the Bjerkvik Nappe. The latter is separated from both the Narvik and the Evenes nappe complexes by tectonic contacts (Melezhik et al. 2002a). The next tectonostratigraphic unit is the Evenes nappe complex, composed of several discrete units of different age which were tectonically emplaced in a non-stratigraphic order (Melezhik et al. 2002a). In ascending structural order, these are the Steinsland, Ramstad, Eve- nestangen and Tangen thrust sheets (Fig. 3). Also inclu- ded are the Langmark schists (Fig. 3) whose relative and absolute age and contact relationships with adja- cent units remain unknown. The Langmark schists could belong to either the Steinsland or the Ramstad thrust sheets (Figs. 2 & 3). The Steinsland, Ramstad and Evenestangen thrust sheets and the Langmark schists, collectively, correspond to the Evenes Marble of Gus- tavson (1966) and Steltenpohl et al. (1990). Fig. 1. Location of the study area and simplified tectonic subdivision of the Scandinavian Caledonides (modified after Roberts & Gee 1985). The contact of the Evenes nappe complex with the structurally overlying Bogen Group has been defined as represents the northeastern extension of the Ofoten a thrust (the Bogen thrust of Andresen & Steltenpohl Synform. The investigated area exceeds 800 km2,of 1994). However, the precise tectonostratigraphic posi- which 150 km2 has been covered by : 20.000 scale geo- tion of the thrust remains unclear. Originally, it was logical mapping undertaken between 1997 and 2000. placed structurally above the marble of the Tangen The bedrock geology is dominated by two principal Sequence (Steltenpohl & Bartley 1985; Steltenpohl subdivisions; parautochthonous Precambrian crystal- 1987). Later, the Tangen Sequence was extended up to line basement and overlying, allochthonous, Caledo- the base of the Bø Quartzite (Fig. 3) and the thrust was nian nappe complexes (Gustavson 1974a, b; Bartley placed at the structural base of this quartzite unit (Stel- 1981; Tull et al. 1985; Steltenpohl 1987). The allochtho- tenpohl et al. 1990; Andresen & Steltenpohl 1994). The nous complexes include the lowermost unit of quartz- detailed mapping performed by the present authors has feltspatic gneisses and granitic gneisses, and the Narvik, failed to confirm the presence of a major thrust at the Evenes, Bogen and Niingen Groups (Fig. 2, see page structural base of the Bø Quartzite. Instead, a thrust 213). The Narvik and Niingen Groups have previously and associated retrograde metamorphism have been been redefined as the Narvik and Niingen nappe com- documented structurally above the Tangen Sequence plexes (Andresen & Steltenpohl 1994). Similarly, the marble. Thus, we prefer the position of the thrust as it Evenes Group is re-defined as the Evenes nappe com- was originally defined by Steltenpohl & Bartley (1985) plex for reasons explained in Melezhik et al. (2002a) and Steltenpohl (1987). The current lithostratigraphy and later in this paper. In terms of the principal subdi- of the Bogen Group includes a variety of amphibolite- visions of the Scandinavian Caledonides (Roberts & facies mica schists (Lower, Middle and Upper Mica Gee 1985), the Evenes, Bogen, and Niingen units are Schists, Fig. 3), quartzites, iron-manganese ores and considered to form part of the Uppermost Allochthon, several marble formations (Foslie 1946; Gustavson with the Narvik nappe complex representing the Upper 1966; Steltenpohl 1987; Steltenpohl et al. 1990). Abun- Allochthon (Fig. 2). In the area of Ofotfjorden, the dant felsic dykes in the Bogen Group schists and mar- lowest structural unit of the Caledonian nappe pile is bles contrast strongly with the subjacent Evenes nappe the Middle Allochthon (Fig. 2) which does not concern complex, which lacks such intrusions (Steltenpohl us here. 1987; Steltenpohl & Bartley 1987). NORWEGIAN JOURNAL OF GEOLOGY Isotope chemostratigraphy and detailed mapping of high-grade marble sequences 211 Fig. 3. Tectonostratigraphic column of the Uppermost Allochthon in the Ofoten district (based on Gustavson 1966, 1972; Boyd et al. 1986; Stel- tenpohl et al. 1990; Andresen & Steltenpohl 1994; Melezhik et al. 2002). The ages of the carbonate formations are based on δ13C and 87Sr/86Sr values of carbonates with reference to the age calibration curves of δ13C and 87Sr/86Sr in seawater (this study). The age of the

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