The Askvoll group, in the westernmost part of the Western Gneiss Region, Norway, is a sequence of mylonitic and phyllonitic schists that houses metagabbroic to granitic rocks, of which the former predominate. It comprises three tectonostratigraphic units. The highest Sandvika Unit consists of phyllonites, quartz schists, marble and fragments of tholeiitic, MORB-type metagabbro. The Vikanes Unit is dominated by epidote-actinolite mylonites and minor amounts of felsic schists and phyllonites, hosting abundant lenses of calc-alkaline metagabbros and subordinate 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 epidote-actinolite mylonites is very similar to that of the calc-alkaline metagabbros. The Kumble Unit at the bottom contains felsic schists and garnet-amphibole mica schists with lenses of garnet-amphibolite. The calc-alkaline metagabbros and associated differentiates in the Vikanes Unit represent are magmatism. The epidote-actinolite mylonites are genetically related to the gabbros. On the basis of the geochemical characteristics, it is proposed that the heterogeneous Askvoll group was formed by subduction-related are magmatism. The igneous rocks of the Askvoll group correlate well with time coeval rocks in southwestern Sweden and southern Norway. These were formed during the Gothian/Kongsbergian/Labradorian orogeny ( 1700-1500 Ma) when subduction-related magmatism occurred along the margins of present Fennoscandia and northeastern Laurentia.

Ø. Skår & H. Furnes, Geologisk Institutt, Allégt. 41, N -5007 Bergen, Norway;
S. Claesson, Laboratoriet for isotopgeologi, Naturhistoriska riksmuseet, Box 50007, S-10405 Stockholm, Sweden.