Preliminary investigations of some Ordovician volcanics from Stord, West Norway.

Abstract

Later Ordovician volcanics on Stord, W. Norway, comprise alkaline rhyolite flows and ignimbrites, and ferro-basalt (icelandite) and basaltic andesite lavas. The latter are hy-normative, \"evolved\" rocks rich in residual trace elements and low in Ni and Cr. The rhyolites are highly fractionated rocks. They contrast with later intrusions of granite porphyry which are clac-alkaline and poorer in Zr, Nb, Y, and REE, and richer in Ca, Mg and Sr than rhyolites. Subaerial volcanism of this type suggests an intra-continental, possibly rifted, environment in this part of southwestern Norway in the late Ordovician. Younger dolerite sills and basalt dykes in the area are continental tholeiites and transitional basalts.