Description of the geological maps "Tromsø" and "Målselv", Troms I. The Precambrian window of Mauken-Andsfjell.

Abstract

This paper is the first part of a description of the bedrock on the two 1:100 000 map sheets, \"Målselv\" and \"Tromsø\". The geological map \"Målselv\", accompanies the paper. This part deals with the \"window\" of Precambrian rocks exposed beneath the Caledonian nappes in Målselv, just N of latitude 69 N. The basement consists of mountainious areas that rise far above the surrounding Precambrian peneplane. These Precambrian rocks are composed of two main units, the Mauken amphibolite and the Andsfjellet granodiorite. The amphibolite is thought to be a meta-basalt. Chemical analyses indicate a \"normal\", relatively acid, basaltic composition. The analyses are compared to analyses of meta-basalts from other parts of the North Scandinavian Precambrian. The granodiorite is believed to be a product of a Precambrian granitisation. Chemical analyses of the granodiorite are given. It is shown to have an intrusive contact against the amphibolite. The junction between these two Precambiran units and the overlying schists is a pronounced thrust plane, which is a part of the Caledonian \"major thrust\". The question whether these basement outcrops formed actual mountains as early as in the Precambrian, ot whether they result from a Caledonian uplift is discussed, and the latter hypothesis is thought most likely.