The syn-metamorphic emplacement of the Magerøy Nappe

The Silurian Rocks of eastern and central Magerøya have been subjected to Barrovian-type regional metamorphism which varies in grade from low greenschist to high amphibolite facies. These rocks constitute a major overthrust nappe - the Magerøy Nappe - and lie on a substrate of migmatic gneisses formed from the psammitic rocks of the Kalak Nappe. The study of mineral assemblages in the thick sequence of blastomylonitic rocks developed at the contact of these two units shows that the temperatures attained during thrusting reached into the upper part of the amphibolite facies. It is concluded that, during overthrusting, the water released by dehydration reactions in the overlying Magerøy Nappe reduced the effective stress near the thrustplane and acted as an effective lubricant. It is proposed that the widespread late Cambrian/early Ordovician deformation and metamorphism in the Kalak Nappe, and the rocks of its substrate, be allocated the status of the Finnmarkian Phase of the Caledonian orogeny.