METAGABBROS IN THE MODUM COMPLEX, SOUTHERN NORWAY - AN IMPORTANT HEAT-SOURCE FOR SVECONORWEGIAN METAMORPHISM

The Precambrian Modum Complex in southern Norway is characterized by supracrustal rocks penetrated by numerous gabbro intrusions, occurring as elongated bodies subparallel with the regional foliation. These gabbroic bodies show a partial alteration to amphibolites. The best preserved parts of the metagabbros show a subophitic texture with olivine and plagioclase as the major cumulus phases and clinopyroxene, +/- orthopyroxene and ilmenite as the major intercumulus phases. Subsequent alteration of these igneous phases has led to formation of coronas around olivine in contact with plagioclase and recrystallization of clinopyroxene. The igneous phases indicate intrusion temperatures in the range of 1100-1000-degrees-C and pressures below 5-8 kbar. Formation of coronas and recrystallization of clinopyroxene occurred in the pressure-temperature range of 7-10 kbar and 800-600-degrees-C. A 1224 +/- 15 Ma Sm-Nd mineral isochron obtained from igneous phases indicates emplacement at the start of the Sveconorwegian metamorphism. A complex evolution is suggested for the Sveconorwegian metamorphic event, involving both magmatism and high-grade metamorphism.