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

Abstract

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 13C-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.