Volcanic and geochemical stratigraphy of the Nussir Group of Arctic Norway – an early Proterozoic greenstone suite

SUMMARY: The Nussir Group of West Finnmark is a suite of metabasaltic lavas and tuffs about 1.8 km thick, associated with continental and shallow marine sediments. This assemblage is typical of Karelian (Early Proterozoic) supracrustal sequences throughout northern Norway and northernmost Sweden. Primary textures are well preserved and permit the establishment of a volcanic stratigraphy for the Nussir Group. The Krokvatn Formation is dominated by mafic aquagene tuffs while the overlying Svartfjell Formation is composed mainly of submarine pillow basalt. Petrographic evidence for the petrogenesis of the metabasalts is poor because of greenschist facies metamorphism during the Svecokarelian Orogeny c. 1840 Ma. The earliest lavas have the most primitive compositions, but subsequent flows exhibit strong enrichment in Fe, Ti and incompatible lements as a consequence of extensive crystal fractionation. The tholeiitic nature of the suite is confirmed by the trace element data. The later lavas are more uniform and less evolved: and are considered to represent steady-state compositions produced by a periodically replenishedkapped open-system magma chamber. The field relationships, lithological and geochemical evidence suggest that the Nussir Group lavas were erupted in a submarine rift basin within continental crust. They form part of an extensive tholeiitic magmatic province which occupied much of northern Norway, Sweden and Finland in the arly Proterozoic. The tectonic environment of this province is poorly constrained by the geochemical data. Continental plateau basalt provinces such as the Columbia River and Karoo provide the closest Phanerozoic geochemical analogues. Metabasaltic lithologies (‘greenstones’) make up a significant proportion (c. 50%) of Karelian (early Proterozoic) supracrustal sequences in northern Nor