Crystallization history of a Permian composite monzonite-alkali syenite pluton in the Sande cauldron, Oslo rift, southern Norway

Monzonitic rocks (larvikite) occur as megarafts embedded in younger alkali syenite in the Sande central pluton. Mineral equilibrium data are presented, in order to establish a crystallization history for the members of the complex. The larvikites (in part orthopyroxene-bearing) have followed cooling paths close to the QFM buffer. During intrusion of the alkali syenite (nordmarkite) the older intrusives were reheated to the solidus, with sencondary, buffered cooling taking place at higher oxygen fugacity levels (roughly equivalent to the NNO buffer), which are within the f{hook}_O2 range estimated for the nordmarkite magma. Gradual transitions between larvikite and nordmarkite are results of the secondary reheating.