A tectonic megalens, part of the Nordmannvik Nappe of the Upper Allochthon, is located at Takvatnet in Central Troms. The nappe unit is dominated by upper amphibolite- to granulite-facies gneisses. A Rb-Sr whole-rock dating of a metadiorite within this unit gives an isochron age of 492 ± 5 Ma with an initial Sr-isotopic ratio of 0. 70814 ± 0.00008. The isochron age is interpreted to represent the high-grade metamorphic event associated with a complete resetting of the isotope system. The obtained age thus represents evidence of a pre-Scandian metamorphic event for at !east one of the high-grade units within the Upper Allochthon in the Troms Caledonides.

Mona Lindstrøm, Department of Geology, University of Trondheim-NTH, N-7034 Trondheim, Norway;
A. Andresen, Department of Geology, University of Oslo, P. O. Box 1947 Blindern, 0316 Oslo 3, Norway.