PROTEROZOIC OROGENIC MAGMATISM WITHIN THE WESTERN GNEISS REGION, SUNNFJORD, NORWAY

The Askvoll group, in the westernmost part of the Western Gneiss Regio n, Norway, is a sequence of mylonitic and phyllonitic schists that hou ses metagabbroic to granitic rocks, of which the former predominate. I t comprises three tectonostratigraphic units. The highest Sandvika Uni t consists of phyllonites, quartz schists, marble and fragments of tho leiitic, MORB-type metagabbro. The Vikanes Unit is dominated by epidot e-actinolite mylonites and minor amounts of felsic schists and phyllon ites, hosting abundant lenses of calc-alkaline metagabbros and subordi nate granodiorites, quartz diorites and granites. A quartz diorite has yielded a U/Pb-zircon age of 1640.5 +/- 2.3 Ma, and is interpreted to date the calc-alkaline magmatism. The chemical composition of the epi dote-actinolite mylonites is very similar to that of the calc-alkaline metagabbros. The Kumble Unit at the bottom contains felsic schists an d garnet-amphibole mica schists with lenses of garnet-amphibolite. The calc-alkaline metagabbros and associated differentiates in the Vikane s Unit represent are magmatism. The epidote-actinolite mylonites are g enetically related to the gabbros. On the basis of the geochemical cha racteristics, it is proposed that the heterogeneous Askvoll group was formed by subduction-related are magmatism. The igneous rocks of the A skvoll group correlate well with time coeval rocks in southwestern Swe den and southern Norway. These were formed during the Gothian/Kongsber gian/Labradorian orogeny (1700-1500 Ma) when subduction-related magmat ism occurred along the margins of present Fennoscandia and northeaster n Laurentia.