Mafic and ultramafic inclusions from the initial (Cambrian?) volcanism in the central Trondheim region, Norway.

A few horizons of metavolcanics occur within the Cambrian Gula Schist Group of the Caledonides of the central Trondheim Region. In spite of a moderate vertical thickness they have a wide lateral extension, being more or less stratabound and associated with bituminous pelitic schists. The metavolcanics consist mostly of mafic assemblages and include clusters or small bodies of alpine-type ultramafics which in places grade into gabbroic varieties. A polyphase deformation and metamorphism has affected the original mineral assemblages and the ultramafics appear mostly as hornblendites, actinolit-chlorite rocks, tale-chlorite schists and serpentinites. Petrochemical investigations reveal a fractionation pattem following a progressive Fennertrend. The Cr/Ni ratio is nearly constant throughout the series and the Cr + Ni-content is closely related to the magnesia content of the rocks. A cognate, cumulate origin of the inclusions is suggested.