MARGINAL BASIN MAGMATISM IN AN ANCIENT VOLCANIC ARC - PETROLOGY OF THE PALAEOPROTEROZOIC MALA-GROUP BASALTS, SKELLEFTE DISTRICT, NORTHERN SWEDEN

A new stratigraphic unit, informally called the Mall group, is tentati vely suggested for the interval between the are-related volcanic Skell efte and Arvidsjaur Groups in the western part of the Skellefte Distri ct. The Mala group comprises turbiditic greywackes and high-Mg basalts . The basalts vary from sills and lavas in the lower levels to mainly volcaniclastites at the top. The basalts contain characteristic actino lite pseudomorphs after pyroxene. Geochemically the basalts have high contents of Mg, Cr, and Ni and relatively high LIL elements. Tectonic discrimination using trace element geochemistry suggests a volcanic ar e setting. The geological setting is interpreted as a marginal basin f ormed at an advanced stage of the development of the Skellefte extensi onal continental margin are.