On the gneisses from a part of the north-western gneiss area of Southern Norway.

Abstract

The north-western part of Southern Norway is an area mainly of gneisses, formerly regarded as entirely of Archean age. In later years a number of writers have maintained that parts of the gneisses are Caledonian migmatites and that the parts of the gneisses of Archean origin have been influenced by tectonical movements and by migmatisation during the Caledonian orogeny. The scope of the present paper is to describe the gneisses occurring in the regions of Lesja - Romsdalen and Otta valley - Grotli (sketch map fig. 1). The rocks are quartzdioritic gneisses very poor in potash feldspar, with subordinate bands of amphibolite. Schistose anorthosites occur at two localities. Closely associated with the plagioclase gneisses are gneisses with considerable amounts of potash feldspar. Microscopical study of these rocks gives strong indications of the secondary instroduction of the potash feldspar by corrosive replacement processes. During these processes potash feldspar is added to the rock, to a great extent at the expense of the original plagioclase. There is also an increase in the amount of quartz and a strong decrease in the amount of ferro-magnesian minerals. The plagioclase gneisses, thus the motler rocks of the granodioritic gneisses. are regarded for a great part as of Archean age.