Crystallization and metasomatism of nepheline syenite xenoliths in quartz-bearing intrusive rocks in the Permian Oslo Rift, SE Norway

Small bodies of metasomatized nepheline syenite occur as xenoliths in syenitic and granitic intrusions in the Mykle area, ca. 30 km N of the Larvik pluton in the Vestfold Graben of the late Paleozoic Oslo rift of SE Norway. The nepheline syenite has a metaluminous major element composition. The xenoliths were metasomatized as a result of interaction between nepheline syenite and younger silica-saturated to oversaturated magmas and their associated fluids. Early, pervasive metasomatism led to breakdown of nepheline, replacement of pyroxene by biotite ± garnet and crystallization of quartz. Recrystallization took place at solidus-near temperatures (700-725°C), and was controlled by an increase in silica activity and oxygen fugacity. The xenoliths indicate the former presence of larger bodies of nepheline syenite in an area where no such rocks were known previously. These intrusions were almost completely disgested by later intrusions. -from Authors