NGT36-3/4-05
1956
Pegmatite veins and the surrounding rocks. I. Petrography and structure.
36
3, 4
213-240

A very detailed map of the island St. Hansholmen, Risør, southern Norway, is presented. The rock type on the island is the banded gneiss common along the south coast. It consists of alternating bands of gneiss and amphibolite with pegmatite veins and inter-boudin fillings confined to the amphibolite bands.
The amphibolite usually consists almost wholly of hornblende and plagioclase. The gneiss consists of plagioclase, quartz, sometimes microcline, with small amounts of biotite and magnetite. The plagioclase in both the amphibolite and gneiss lies in the range An36 to An27, average An31. The pegmatites are of three types. On the west end of the island there is a pink, microcline rich pegmatite vein which cross-cuts both the amphibolite and gneiss. Confined to the amphibolite bands there occur white pegmatites containing plagioclase An30 and, as fillings between boundins, there occur white pegmatites containing plagioclase An23.

The structural elements of the island, contacts and foliation of the gneiss, are nearly vertical and strike ca. EW to WSW. A sharp, almost isoclinal fold is revealed on the island.
The pegmatite veins, confined to the amphibolite bands, tend to follow two directions with one of these dominant on both limbs of the fold. Postulating that the fold revealed on St. Hansholmen is a minor fold on the limb of a larger fold, this fact, along with the development of boudinage structure in the amphibolite and the .occurence of pegmatite fillings between boudins, is explained as being due to differences in rigidity between the amphibolite and the gneiss during the dynamo-thermal metamorphism of the area. Three posssible modes of origin for the pegmatites are discussed. They can be characterized as; (1) magmatic, (2) hydrothermal, and (3) by metamorphic differentiation. The conclusion is reached that the plagioclase pegmatite veins and pegmatite fillings between boudins can best be explained as the result of crystallization from a dispersed phase which diffused intergranularly towards the low pressure areas which arose during the deformation of the area. The alkali content of the feldspars in the surrounding gneiss indicates that the tempera ture of the environment in which these pegmatites formed was well below the range of magmatic temperatures. The suggestions offered account for the location, composition variations and relative grain size of the plagioclase pegmatites.
Very little can be said about the genesis of the pink microcline rich pegmatite which crosses the western tip of St. Hansholmen, on the basis of the present study.
The spacial variation in the microcline content of the gneiss may be interpreted such that it supports the genesis by metamorphic differentiation wbich has been suggested for the banded gneiss.