Pegmatie veins and the surrounding rocks IV. Genesis of a discordant pegmatite, St. Hansholmen, Risør, Norway
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A detailed petrographic study was made of the banded gneiss surrounding a pink, microcline rich, discordant pegmatite vein crossing the western end of the island St. Hansholmen near Risør, southern Norway. The banded gneiss is part of the pre-Cambrian Kongsberg-Bamble formation and consists of alternating, quite regular bands of amphibolite and granodioritic to quartz dioritic gneiss.
Modal analyses of five series' of samples of the gneiss and one of the amphibolite reveal a K metasomatism of the gneiss adjacent to the pegmatite vein,the intensity of which diminishes with distance from the vein. This is rendered even clearer by the calculated cation percent compositions of the samples. The amphibolite has been very little if at all affected by the emplacement of the vein. Volume change calculations indicate that the material which constitutes the vein entered into a crack across the banded gneiss. It is concluded that the material which filled the crack was derived from the nearby granulite facies rocks.
Temperature determinations using the feldspar thermometer, both of the pegmatite and the gneiss, indicated that the pegmatite formed at a higher temperature than the surrounding gneiss (ca. 570° C vs. 460° C ), although lack of equilibrium between co-existing feldspars in the gneiss near the vein makes it impossible to determine temperature differences in the gneiss as related to distance from the vein, if such existed.
The nature of the pegmatite forming fluid is considered. Four possibilities are discussed: 1) magma; 2) hydrothermal solution; 3) fluidized gas-solid system; and 4) disperse phase of relatively high energy particles. Possibility 2) is discarded because of the large quantity of water which would be required and because 10 km3 of rock would have had to have been <> to yield solution sufficient to form a vein 500 m x 2m X 500 m. Possibly 3) is not advocated because none of the characteristic features of veins formed by such a system have been found. None of the observations which were made are found to constitute either decisive proof for or evidence fatal to possibilities 1) and 4); no conclusion can be drawn except that the vein formed from a fluid (magma or disperse phase ) <> into a crack in the banded gneiss.