Volcanostratigraphy and eruptive products of the Jonsvatn greenstone formation, central Norwegian Caledonides.

Abstract

The predominantly volcanic, Ordovician, Jonsvatn greenstone formation comprises appr. 500 m of tholeiitic basalts and associated volcanic products, with interbedded sediments totalling some 100-200 m in thickness. In the Nidelv river profile at least 25 seperate phases of lava effusion can be distinguished with flows ranging in thickness from 5 m to more than 40 m. Although the majority of the flows are pillowed, some massive flows characterize the middle portion of the formation. Pillowed flows show a wide range of pillowed form, but elongate pillows are predominate. Linear zones of hollow elongate pillows probably indicate the location of master feeding tubes near the top of a pillowed flow. Some hollow pillows were partly filled by new lava after initial drainage. Multiple lava shelves within hollow layered pillows are common in some flows. This shelving, which is considered to represent the palaeohorizontal plane, shows a fairly consistent SE dip throughout the profile even though the attitude of pillows and their elongation may vary appreciably. Data indicate that the main lava flow direction was towards ca. WSW, but there is a good deal of directional variation. Pillow breccias occur throughout the basalt pile; these are of differing origin, although tha majority are assumed to be of flow-foot rubble type. The Jonsvatn metabasalts, which are predominantly of ocean floor tholeiite chemistry, are interpreted as products of local volcanic centres situated along rift zones in a back-arc marginal basin environment.