Geologiske og petrografiske undersøkelser i Modumfeltet.
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Geological mapping of part of the Modum area, the northernmost part of the Kongsberg-Bamble area in the Precambrian of southern Norway, was carried out. The different rocks are described, and metasomatic processes which have taken place in some of them, are described and discussed. The tectonic features, such as faults, breccias and more of less hypothetical folds are treated. Finally an attempt is made to set up a stratigraphy of the rocks, which most likely are of sedimentary origin. The rocks in the area are amphibolites and gabbros, banded gneisses, pegmatites, different types of micaschists - among which some are sillimanite-bearing-sillimanite granites, quartzites, rocks consisting of magnesite and serpentine (often with a number of rare minerals), scapolite rocks and different sorts of albite rocks. The gabbros always change gradually into amphibolites towards their borders; most of the amphibolites in the area were most probably originally gabbroic rocks. It is shown how sillimanite micaschists with plagioclase gradually change into sillimanite granites by K-metasomatism, whereby the potassium reacts with sillimanite and quartz to give muscovite, and with the plagioclase to give microcline.