Geochemistry of Palaeoproterozoic porphyritic felsic volcanites from the olden and T⊘mmerås windows, central Norway

Abstract

Porphyritic felsic volcanites in the Olden Nappe and Formo‐foss Nappe Complex in the Grong‐Olden Culmination and T⊘mmerås Window of Central Norway can be classified as high‐K rhyolites and trachytes. Subordinate mafic volcanic rocks are mainly basaltic. The rhyolites and trachytes, which probably accumulated as ash‐flow tuffs or ignimbrites, are weakly peraluminous and are similar in chemical composition to the 1.65 Ga Olden Granite. Various incompatible element plots indicate that the felsic volcanites are of within‐plate character. The minor basaltic rocks also show a within‐plate signature, though possibly with a subduction‐related component involved. REE patterns and abundances show that the felsic rocks are highly evolved, with LREE enrichment and a marked negative Eu anomaly. The data favour a continental source region for the anorogenic rhyolite/trachyte‐basalt association, in an extensional regime. It is suggested that lower crustal necking and distension, and a low to moderate degree of partial melting of an evolved crustal source eventually culminated in abundant effusion of the mainly rhyolitic and trachytic pyroclastic products.

Roberts, D., 1997: Geochemistry of Palaeoproterozoic poiphyritic felsic volcanites from the Olden and T⊘mmerås Windows, Central Norway. GFF, Vol. 119 (Pt. 2, June), pp. 141–148. Stockholm. ISSN 1103–5897.

Keywords:

• Palaeoproterozoic
• geochemistry
• porphyritic volcanites
• rhyolite‐trachyte‐basalt
• Olden‐T⊘mmerås district
• Norway