The Ordovician Grøndalsfjell Intrusive Complex: Central Scandinavian Caledonides: field relations, petrography and emplacement

Abstract

The Grøndalsfjell Intrusive Complex (GIC) is part of a major plutonic province located in the Gjersvik Nappe of the Upper Allochthon in the Caledonides of Central Norway. Field relationships and petrography suggest that the magmatic evolution of the pre-456 Ma complex took place in three main stages. It commenced with the intrusion of relatively primitive mafic magma and crystal- lisation of coarse-grained olivine gabbro, presumably in a macro-dyke or smaller intrusive body. This was followed by intrusion of more evolved basaltic melts to form a larger magma chamber, now preserved as the Layered Series, the emplacement of which was characterised by active stoping and incorporation of large number of xenoliths. Continuous influx of magma led to deposition of a thick pile of layered olivine gabbro cumulates which now form the Lower Zone. As the influx of magma decreased, fractional crystallisation caused an evolution towards gabbronorite in the Middle Zone and quartz diorite in the Upper Zone.