The mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the North Arm Mountain massif, Bay of Islands ophiolite complex, is not consistent with crystal-liquid fractionation of a primitive mid-ocean ridge basalt at low pressures. The appearance of clinopyroxene and orthopyroxene having very high Mg numbers <100¿Mg(Mg+Fe)=85 to 92> and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine (Fo86-92) suggest that moderate- to high-pressure phase relationships are applicable. We propose on the basis of mineral chemistry, crystallization sequences, and structural relationships that the basal ultramafic cumulates on the North Arm massif represent the products of high-pressure (>10 kbar) polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid-ocean spreading center. These basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from crustal level magma chamber to mantle depths that exceed 30 km. After their formation, the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid-ocean ridge.