Stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the Nagu-Korpo area,SW Finland

The volcanic rocks of the Nagu-Korpo area in SW Finland form a thin (< 1000 m) formation that is folded into narrow synforms separated by migmatite-filled antiforms. In Nagu the volcanic formation comprises three units: a lower unit of subvolcanic banded gabbro sills, a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer. The volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses. Geochemically the metavolcanic rocks are tholeiitic and plots based on Ti, Y, Zr and other trace elements show that they resemble recent within-plate lavas. MORB- and mantle-normalised trace element plots show patterns strongly differing from those of recent arc lavas, indicating a tectonic environment of (plate boundary related?) initial rifting. The upper ultramafic lavas are enriched in LREE relative to the lavas of the much thicker amphibolitic middle unit below. A model for the initial formation of the volcanic rocks of the Svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc-like environment.