The alkaline ultrabasic rooks (damkjernite) of the Fen alkaline complex contain abundant rounded inclusions of spinel lherzolite, closely similar in composition and mineralogy to the nodules found in many undersaturated basaltic rocks. Clinopyroxene compositions indicate crystallization at P=10-13 KB, T=1200-1250° C. A model involving partial melting in a rising diapir of mantle peridotite allows interpretation of the nodules either as mantle fragments or as cognate cumulates, and suggests that crustal thinning in the Oslo area possibly began as early as 500-600 m.y. ago. The presence of the nodules indicates that the damkjernite did not pause for intracrustal differentiation during its rapid ascent to the surface. The crustal contamination suggested by recent Sr-isotope studies may have occurred during differentiation at the base of the continental crust.