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Sedimentary-Volcanic Successions of the Alta–Kvænangen

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Sedimentary-Volcanic Successions of the Alta–Kvænangen NORWEGIAN JOURNAL OF GEOLOGY Vol 95 Nr. 3–4 (2015) http://dx.doi.org/10.17850/njg95-3-01 Sedimentary-volcanic successions of the Alta–Kvænangen Tectonic Window in the northern Norwegian Caledonides: Multiple constraints on deposition and correlation with complexes on the Fennoscandian Shield Victor A. Melezhik1, Bernard Bingen1, Jan Sverre Sandstad1, Boris. G. Pokrovsky2, Arne Solli1, Anthony E. Fallick3 1Geological Survey of Norway, Postbox 6315 Sluppen, 7491, Trondheim, Norway. 2Geological Institute, Russian Academy of Sciences, Pyzhevsky drive, 7, 109017, Moscow, Russia. 3Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, Scotland. G75 0QF. E-mail corresponding author (Victor A. Melezhik): [email protected] Airborne geophysical data, sedimentological and geochemical characteristics of carbonate rocks, geochemical features of igneous rocks, carbon isotope chemostratigraphy and radiometric dating form a multiple approach applied for the reconstruction of depositional environments and the age of the sedimentary-volcanic succession exposed in the Alta–Kvænangen Tectonic Window (AKTW) in the northern Norwegian Caledonides. Aeromagnetic geophysical data confirm that the AKTW succession continues beneath the Caledonian nappe complexes and connects with the Kautokeino Greenstone Belt in the main part of the Fennoscandian Shield. The carbonate rocks, mainly dolostones, of the Kvenvik formation are markedly enriched in 13C (d13C = +7.4 ± 0.7‰, n = 51) and record a global positive excursion of carbonate carbon isotopes in sedimentary carbonates known as the Lomagundi –Jatuli isotopic event whose duration was constrained in the Fennoscandian Shield between c. 2220 and 2060 Ma. A radiometric date of 2146 ± 5 Ma (U–Pb, zircon) obtained from a gabbro comagmatic with mafic lavas provides a minimum age for the deposition of the 13C-rich, Lower and Upper dolostones and the accumulation age of the 13C-rich Uppermost dolostone. The carbonate rocks of the structurally overlying Storviknes formation show near-zero d13C (+1.1 ± 1.2‰, n = 41). Carbon isotope chemostratigraphy suggests that their deposition post-dated 2060 Ma which, together with the above mentioned radiometric date, indicates a non-depositional break/hiatus of over 80 Myr separating accumulation of the two neighbouring formations. Sedimentological features of the carbonate rocks are consistent with deposition in a carbonate platform/shelf, whereas the depositional features of shales suggest accumulation in a shallow-water epeiric sea. The 13 C-rich dolostones of the Kvenvik formation contain plentiful halite casts and sulphate pseudomorphs reflecting formation of abundant CaSO4 and NaCl in time-equivalent successions across the Fennoscandian Shield, which may represent a source for Na and Cl metasomatism that affected Palaeoproterozoic rocks in the AKTW and across northern Fennoscandia. Keywords: Caledonides, Norway, carbon isotopes, carbonates, evaporites, Palaeoproterozoic Received 11. March 2015 / Accepted 1. May 2015 / Published online 20. June 2015 Introduction rocks associated with abundant mafic extrusive and intrusive rocks (Zwaan & Gautier, 1980; Pharaoh et al., 1983; Bergh & Torske, 1986, 1988). Several tectonic windows in the northern Norwegian Caledonides represent an apparent northwestern There have been several attempts to correlate continuation of the Precambian Fennoscandian Shield sedimentary and volcanic sequences of the AKTW beneath the Caledonian nappes. The Alta–Kvænangen with those occurring in other tectonic windows as Tectonic Window (AKTW) is one such window (Figs. 1 & well as in the main part of Fennoscandian Shield (e.g., 2). It contains several informally established formations Pharaoh et al., 1983; Siedlecka et al., 1985; Bergh & with diverse lithologies of metamorphosed sedimentary Torske, 1986, 1988). These correlations are largely based Melezhik, V.A., Bingen, B., Sandstad, J.S., Pokrovsky, B.G., Solli, A. & Fallick, A.E. 2015: Sedimentary-volcanic successions of the Alta–Kvænangen Tectonic Window in the Northern Norwegian Caledonides: Multiple constraints on deposition and correlation with the Fennoscandian Shield. Norwegian Journal of Geology 95, 245–284. http://dx.doi.org/10.17850/njg95-3-01. © Copyright the authors. This work is licensed under a Creative Commons Attribution 4.0 International License. 245 246 V.A. Melezhik et al. on lithological characteristics of sedimentary rocks and volcanic rocks of the Raipas Supergroup are and geochemical signatures of volcanic units. They tectonically deformed and metamorphosed at greenschist demonstrated some similarities of the AKTW rock facies. The Raipas Supergroup is unconformably overlain complex with those occurring across northern Norway, by the Bossekop Group, comprising stromatolitic and consequently resulted in the general acceptance of dolostone and siltstone, followed by the Borras Group their Palaeoproterozoic age. However, to date, there are which consists of sandstone, siltstone, conglomerate and no actual radiometric dates available which can provide tillite of inferred Late Neoproterozoic age (Fig. 2; Zwaan robust constraints on the age of deposition of the AKTW & Gautier, 1980). rock successions. Several minor copper deposits, partly mined in the past, Here, we use sedimentological constraints, carbon are known in the Raipas Supergroup. In the lower part isotope chemostratigraphy, U–Pb radiometric dating of a of the Kvenvik formation, copper sulphide occurs as gabbro, and airborne geophysical data to provide the first disseminations in albite felsites and as hydrothermal insight into the depositional history of the AKTW rock sulphide-quartz-carbonate veins in volcanic rocks successions and their correlation with the sedimentary- throughout the formation. Sediment-hosted copper volcanic formations across the Fennoscandian deposits are also widespread in the Storviknes formation Shield. These correlations are used for placing the (e.g., Vik, 1985). Here, copper sulphides occur as cement palaeogeographic and palaeotectonic positions of the in breccias and disseminations in dolostone and shale AKTW rocks within the framework of the evolution of lithologies (Fig. 3). the shield in northern Fennoscandia. Analytical methods Geological background Major and trace elements were analysed at the Geological Sedimentary and volcanic rocks occurring in the AKTW Survey of Norway (NGU), Trondheim, by a PANalytical constitute the Raipas Supergroup whose base remains Axios at 4 kW X-ray spectrometer. The precision (1σ) unknown. The supergroup has been subdivided into four is typically around 2% of the major oxide present. informal formations (Zwaan & Gautier, 1980). It starts For marble samples, elemental concentrations were with the Kvenvik formation which has a cumulative determined on acidified extracts of whole-rock (cold 10% thickness of over 2200 m (Fig. 3). Based on lithological HCl) by inductively coupled plasma-atomic emission composition, the formation can be further subdivided spectrometry (ICP–AES) using a Thermo Jarell Ash ICP into two units. The lower unit (>1000 m), defined by 61. Detection limits for Fe, Mg, Ca and Mn are 5, 100, 200 Vik (1985) as the Lower Kvenvik greenstone formation and 0.2 µg/g, respectively. The total analytical uncertainty (Member A hereafter), is composed mainly of gabbro including element extraction (1σ) is ±10% rel. and subordinate dolostone, albite felsites, shale, albite- carbonate-magnetite rock, and mafic tuff and tuffite. The Stable carbon and oxygen isotope analyses were performed upper unit (c. 1200 m), defined by Vik (1985) as the Upper at the Scottish Universities Environmental Research Kvenvik greenstone formation (Member B hereafter), Centre (SUERC) in Glasgow and at the Geological consists mainly of mafic tuff, massive and pillowed Institute of the Russian Academy of Sciences in Moscow. tholeiitic basalt, subordinate dolostone, limestone and Approximately 1 mg powder was reacted overnight with black shale (Bergh & Torske, 1988). phosphoric acid at 70˚C. Ratios were measured on PRISM III, AP2003 and DELTA V mass spectrometers. Repeat The Kvenvik formation is overlain by the Storviknes analyses of NBS–19 and internal calcite standards are formation (300–600 m), composed of dolostones with generally better than ± 0.2‰ for carbon and ± 0.3‰ for stromatolites, dolostone breccias, and purple and grey oxygen, and interlaboratory differences are within these siltstone (Bergh & Torske, 1988). The major dolostone ranges. Carbon and oxygen isotopic values are reported unit occurring at Raipas and Borras hosts copper deposits, and discussed in the conventional delta notation relative and several copper showings also occur in the upper and to V–PDB and V–SMOW, respectively. thinner dolostone unit interbedded with siltstone (Fig. 3). Zircons for U–Pb geochronology were extracted from The Storviknes formation is succeeded by the Skoađđovárri one crushed sample, using a water table followed by formation, a c. 1700 m-thick unit composed mainly of separation in heavy liquids and magnetic field. Zircon sandstone containing interbeds of conglomerate, pebbly crystals selected for radiometric dating were mounted in sandstone and shale (Zwaan & Gautier, 1980; Bergh & epoxy together with the reference material, and polished Torske, 1986). to approximately half thickness. The grains were imaged individually with a panchromatic cathodoluminescence The youngest unit in the Raipas Supergroup is the (CL) detector in a variable pressure
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