NGT36-3/4-04
1956
The geology of the Ørsdalen district, Rogaland, S. Norway.
36
3, 4
167-212

The tungsten and molybdenite deposit in the area has been dealt with earlier by the author; this paper deals with the general geology, petrology and rock metamorphism of the region.
After the regional geology has been briefly described, the paper deals with the petrographical classification of the rocks exposed. These are amphibolites and two types of quartzo-feldspathic gneisses.
Then follows a short description of the rock structures. It is shown how a heavy falding of orogenic nature has affected all the rocks and it is concluded that they must have reacted to the falding in a plastic way.
A chapter where the metasomatic processes are discussed follows next and the conclusions reached are that the ane kind of the gneisses may have originated through metasomatism of anorthositic rocks while strong evidence is that the other has been formed by replacement of amphibolites and related rocks. The metasomatism is believed to be synkinematic.
The metamorphism is dealt with at same length and it is shown that all the rocks must once have belonged to granulite facies, which is also the PT conditions during which the metasomatism took place. Later a retrograde metamorphism has affected the rocks within certain zones which are believed to represent tectonical zones of weakness. A map showing the metamorphic situation of the area is presented and here a division of the rocks into three groups according to differences in metamorphism is undertaken. These are: high amphibolite facies (including gran u lite facies), low amphibolite facies, epidote amphibolite-and greenschist facies. It is shown how a division of the rocks into ane of these three groups can be based upon variations in the minerals of the quartzo-feldspathic gneisses alone.
The paper ends with same considerations upon the relative age of the Telemark and Egersund formations of pre-Cambrian rocks. Based on the data known today the author considers a contemporaneous origin as being most pro bable.