Aurdal. Beskrivelse til det geologiske gradteigskart.

Abstract

The gneisses of the area can be divided into two distinct groups, plagioclase gneisses of a quartzdioritic composition and granodioritic gneisses. In the stripe of Archean rocks along the valley of the Begna there is a distinct boundary between plagioclase gneisses in the north and granodioritic gneisses in the south. The plagioclase gneisses, earlier described by the writer (1943), are grey, often banded rocks consisting mainly of quartz, plagioclase about oligoclase and biotite, with bands or larger bodies of amphibolite. On the whole these gneisses are fine-grained and of a low mineral facies in the south, while they are more coarse-grained and of higher mineral facies in the north, where they may pass locally into massive pyroxene quartz diorite. The granodioritic gneisses are commonly reddish streaked rocks, varying in texture from highly schistose to nearly massive. The mineral composition varies between rather wide limits and they may contain intercalated bands of plagioclase gneiss. In comparison to the plagioclase gneisses the granodioritic gneisses are more homogeneous and contain less of amphibolite bands, the amphibolites, when present, are often biotitized.