Some 30 specimens from five profiles through the Skålvær greenstone, south Helgeland, have been analysed for major and trace elements. Variations in the chemical compositions have been found to relate mostly to primary fractionation processes, but variations in Sr content are probably due to secondary alteration. Both major and trace element chemistry indicate that the Skålvær greenstone is an ocean-floor type basalt, in most respects similar to the abyssal tholeiites of mid-oceanic ridges. Consideration of the volcanic and sedimentary characteristics and the regional geology, however, has led to the conclusion that the basalt was most probably formed in a marginal, back-are basin. Based on this conclusion and on the regional geological development, a geotectonic model for the area is brietly outlined.