Chemostratigraphic Insight Into Deposition of the Melkedalen Marble 35

NORWEGIAN JOURNAL OF GEOLOGY Chemostratigraphic insight into deposition of the Melkedalen Marble 35 Chemostratigraphic insight into deposition of the Melkedalen Marble, Narvik Nappe Complex, North–Central Norwegian Caledonides Victor A. Melezhik, Anton B. Kuznetsov, Boris G. Pokrovsky, Arne Solli, Igor M. Gorokhov, Anthony E. Fallick, Ingvar Lindahl, Galina V. Konstantinova, & Nikolai N. Melnikov Melezhik, V.A., Kuznetsov, A.B, Pokrovsky, B.G., Solli, A., Gorokhov, I.M, Fallick, A.E., Lindahl, I., Konstantinova, G.V. & Melnikov, N.N.: Chemostratigraphic insight into deposition of the Melkedalen Marble, Narvik Nappe Complex, North–Central Norwegian Caledonides. Norwegian Journal of Geology, Vol 94, pp. 35–52. Trondheim 2014, ISSN 029-196X. 13 18 87 86 Carbon and strontium isotope chemostratigraphy (57 δ Ccarb and δ O, and 37 Sr/ Sr analyses of carbonate components in whole-rock samples) was applied to constrain an apparent depositional age of high-temperature, amphibolite-facies, calcite marbles constituting a 20–30 m thick formation (the Melkedalen Marble) in the Narvik Nappe Complex, Nordland, Norway. The least altered 87Sr/86Sr (0.70772) and δ13C (V–PDB) (+3.0 to +5.0‰) values obtained from the Melkedalen marbles are consistent with a seawater composition in the time interval 610–590 Ma, i.e., Mid Ediacaran. The age obtained provides insight into the time of deposition of the sedimentary rock column of the Narvik Nappe Complex in the Upper Allochthon of the North–Central Norwegian Caledonides. Victor A. Melezhik, , Arne Solli, Ingvar Lindahl, Geological Survey of Norway, Postbox 6315 Sluppen, 7491 Trondheim, Norway. Anton B. Kuznetsov, Igor M. Gorokhov, Galina V. Konstantinova, Nikolai N. Melnikov, Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova 2, 199034 St. Petersburg, Russia. Boris G. Pokrovsky, Geological Institute, Russian Academy of Sciences, Pyzhevsky per.7, 109017 Moscow, Russia. Anthony E. Fallick, Scottish Universities Environmental Research Centre, Rankine Avenue, G75 0QF East Kilbride, Scotland. E-mail corresponding author (Victor A. Melezhik): [email protected] Published October 20. 2014. Introduction deposition of the youngest banded iron formations of commercial interest (Cox et al., 2013). The studied area represents the southeastern part of the 1:50,000 Evenes map-sheet 1331 IV (Boyd et al., The timing of magmatic and metamorphic events 1986) and is situated in the northern part of the county imprinted in the rocks of the Scandinavian Caledonides of Nordland, northern Norway, south of Ofotfjorden has now been reasonably well established. In contrast, (Fig. 1). In terms of Caledonide tectonostratigraphy, there are only very few, robust, time constraints on the Narvik Nappe Complex is considered to be part of sedimentary and volcanic processes, and consequently the Upper Allochthon (Roberts & Gee, 1985; Fig. 1B). the depositional history still remains not fully The region covered by the Narvik Nappe Complex is understood. Establishing the timing of various surface characterised by an unusually high concentration of processes and basinal, biological and climatic events is mineral occurrences and deposits, and is considered as seriously hampered by a scarcity of fossils and a lack of a Cu–Pb–Zn ‘metallogenetic province’ in the Norwegian material suitable for isotopic dating. In barren marble Caledonides. successions of high-grade metamorphic terranes, the concept of ‘carbon and strontium isotope stratigraphy’ In general, the depositional history of the diverse currently represents the only possibility for providing sedimentary and volcanic rocks which now constitute the apparent age constraints for carbonate sedimentation in Norwegian Caledonian orogenic belt spans c. 850–370 marine environments. Over the last decade, this approach Ma. This period of time is associated with major changes has been employed for indirect dating of sedimentation in climate, tectonics and biogeochemistry of the oceans of the carbonate protoliths of marbles in high-grade including several inferred Snowball episodes between c. metamorphic terranes in the Scandinavian Caledonides 760 and 560 Ma (Hoffman & Schrag, 2002), and by the (Melezhik et al., 2000b, 2001a, b, 2002a, b, 2003, 2005, 36 V.A. Melezhik et al. NORWEGIAN JOURNAL OF GEOLOGY (A) Location of the study area, and (B) Geological map of the Ofotfjorden area (modified after Gustavson, 1974b; Boyd & Søvegjarto, 1983; Boyd et al., 1986; Steltenpohl et al., 1990; Melezhik et et Melezhik 1990; al., et Steltenpohl 1986; al., et Boyd 1983; & Søvegjarto, Boyd 1974b; Gustavson, after (modified area of Ofotfjorden the map (B) Geological and area, of study the (A) Location Figure 1. Figure 2003). al., NORWEGIAN JOURNAL OF GEOLOGY Chemostratigraphic insight into deposition of the Melkedalen Marble 37 2008b, 2013; Slagstad et al., 2006), the Scottish and Irish and sheared mafic and ultramafic rocks (e.g., Hodges, Caledonides (Thomas et al., 2004; Prave et al., 2009a, b), 1985). In the structurally upper part of the nappe in the Pan-African Mozambique Belt (Melezhik et al., complex, the lithologies become more diverse (Figs. 1B, 2006, 2008a), and also in East Antarctica (Satish-Kumar 2). Apart from mica schists, there is a calcite marble unit et al., 2008; Otsuji et al., 2013). (Melkedalen Marble), an ‘iron horizon’ (Sjåfjell iron ore horizon), a quartzite formation and several thin units The current contribution has the following objectives: (i) of graphite schists (Balsnes graphite schist) hosting the to constrain apparent depositional ages of the protolith Bjerkåsen massive sulphide deposit (cf., Foslie, 1946, of the Melkedalen Marble in the Narvik Nappe Complex 1949). by applying carbon and strontium isotope stratigraphy; (ii) to correlate marbles of this nappe complex with other Although the Narvik Nappe Complex is cut by marble formations known in the region; (iii) to provide Caledonian felsic intrusions belonging to at least three a lower age limit for the marble-hosted sulphide deposit. generations (Steltenpohl et al., 2003), Foslie (1946) reported a significant difference between the schists occurring below the iron ore horizon and those above Geological background it. The former are injected by granitic material in the form of dykes, veins and lenses, and hence represent The bedrock geology of the study area is dominated by a complex of migmatised rocks (e.g., Hodges, 1985), parautochthonous, Precambrian crystalline basement whereas the upper schists are not. Along the contacts of and overlying Caledonian nappes (Gustavson, 1974a, b; the Melkedalen Marble, the dykes, veins and lenses are Bartley, 1981; Tull et al., 1985; Steltenpohl, 1987). The more abundant, reaching several metres in thickness. latter include the lowermost unit of quartz-feldspathic gneisses and granitic gneisses, the Narvik, Evenes and Bogen groups (Fig. 2). The Narvik Group has Available age constraints, previously been redefined as the Narvik Nappe Complex metamorphism and deformation (Andresen & Steltenpohl, 1994). Similarly, the Evenes Group was redefined as the Evenes nappe complex for Exact depositional ages of the schist and marble units of reasons explained in Melezhik et al. (2002a, 2003). In the Narvik Nappe Complex remain unknown but they terms of the principal subdivisions of the Scandinavian predate the emplacement of both the pre-Scandian Råna Caledonides (Roberts & Gee, 1985), the Evenes, Bogen massif (Fig. 2) and felsic dykes constrained at 437 ± 1 and Niingen units are considered to form part of the Ma (U–Pb zircon, Tucker et al., 1990; Northrup, 1997). Uppermost Allochthon, with the Narvik Nappe Complex The ophiolite fragments of the Bjerkvik Nappe have representing the Upper Allochthon (Boyd & Søvegjarto, been correlated with the Lyngen Magmatic Complex 1983; Boyd et al., 1986). In the study area, the lowest (Andresen & Steltenpohl, 1994), a metatonalite which structural unit of the Caledonian nappe pile is the has been dated to 469 ± 5 Ma (Oliver & Krogh, 1995). Middle Allochthon (quartz-feldspathic gneisses and Metatonalites in the Bjerkvik Nappe in the Ofotfjorden granitic gneisses). The focus of our study is the Narvik region have yielded ages of 479 ± 1 Ma and 474 ± 1 Ma Nappe Complex of the Upper Allochthon. (U–Pb on zircon, Northrup, 1997 and Augland et al., 2014, respectively). The Narvik Nappe Complex has a tectonic contact with the lowermost unit of quartz-feldspathic gneisses All of the formations in the Narvik Nappe Complex and granitic gneisses (Fig. 2), and it is separated from have been affected by polyphase, Caledonian, orogenic the overlying Evenes nappe complex by a thin tectonic deformation (Gustavson, 1972; Tull et al., 1985; unit comprising amphibolites and tonalites. These were Steltenpohl, 1987). The nappe complex is deformed into described by Boyd (1983) as representing an ophiolite the Ofoten Synform, a major, late-Scandian structure in fragment which, together with the overlying Elvenes the western part of the study area (Fig. 1B) and farther to Conglomerate, was termed the Bjerkvik Nappe (Fig. 2). the northeast (Gustavson, 1972; Steltenpohl, 1987). This The isotope chemostratigraphy suggests that the latter NE–SW-trending, asymmetric synform plunges mainly is separated from both the Narvik and the Evenes nappe to the northeast, and deforms the regional schistosity complexes by tectonic contacts (Melezhik et al., 2002a). and all lithostratigraphic units and thrust contacts in the region. The asymmetry is reflected

Load more