The Reinfjord ultramafic complex, an orogenic peridotite, was emplaced near the margin of an extensive sheet of two pyroxene gabbro during an early phase of the Caledonian orogeny. The complex is cylindrically zoned and consists of a dunite core with high Mg/Fe ratios which is separated from lherzolitic marginal zones with relatively low Mg/Fe ratios by a series of layered wehrlites with intermediate Mg/Fe ratios. Igneous textures are developed throughout, and the layering is shown to be of magmatic origin. Contact relationships confirm the high temperature of the intrusion and show that melting and assimilation of both metasediment and gabbro occurred. The composition of hybrids so formed, the formation of sills consisting of layered peridotite and the development of coeval ultrabasic veins all suggest that the rocks crystallized from melts of ultrabasic composition. Multiple intrusion of ultrabasic magmas and reaction with wall rocks explain the evolution of the complex. Confining pressures of 6–8 kb (20–30 km depth) were operative during intrusion and the magmas involved could have existed at temperatures as low as 1300°C. The implications of this for the evolution of the Seiland province are discussed.