Geochemical evidence for the geotectonic setting of early Proterozoic metavolcanic sequences in Lapland

Abstract

Systematic geochemical study of Early Proterozoic metavolcanic rocks from the northern part of the Baltic shield supports the concept that a complex mosaic of continental masses, volcanic arcs and marginal basins were welded into one cratonic massif during the Svecokarelian orogeny (1900-1750 Ma). In the north, in Finnmark and northern Norrbotten, tholeiitic metabasalts, together with thick shallow-water clastic metasediments, form the main part of the Karelian supracrustal sequences, which in turn overlie Archaean basement. The trace element signatures of these metabasalts are comparable to those of post-Palaeozoic continental tholeiites which erupted in zones of lithosphere attenuation and which suffered contamination by sialic crust. It is not, however, possible to determine by geochemistry alone whether the continental rift-basin in question lay in an intraplate or back-arc setting. Metabasalts with a volcanic-arc geochemical signature become an increasingly important component of supracrustal sequences as the diffuse southern margin of the Karelian continent is approached. In the Skellefte field, an ensialic volcanic arc developed and fed considerable quantities of immature clastic sediment into the deep water ‘Norrland geosyncline’, which may have been fully oceanic in character. Subduction magmatism continued during the Svecokarelian orogeny, giving rise to synorogenic metavolcanic suites such as the Kiruna porphyries. The evidence for marked spatial variation of magmatic activity in the Early Proterozoic is in contrast to the rather monotonous pattern of greenstone belt development reported from the Archaean. We believe that this spatial variability is a consequence of geotectonic processes similar to those of modern day plate tectonics, and present a plate tectonic model for the Early Proterozoic development of the Baltic shield.