Major and trace element variations within a transition zone between monzonitic (larvikite) and alkali syenitic (nordmarkite) rocks in the central pluton of the Sande Cauldron show that the compositional transition between the units is continuous. Field relations, petrography and geochemical data indicate that the transition was produced by the influence of intruding nordmarkite magma on a solidified larvikite. The major element pattern in the zone may be reproduced by simple mixing of these endmembers, but the trace elements show more complex variations, due to their heterogeneous distribution in both primary larvikite and nordmarkite magma.