The Bay of Islands ophiolite may be divided into a lower series of ultramafic tectonites representing mantle material and a higher series of cumulate and extrusive rocks and sediments which may be correlated with oceanic crust. The tectonite series consists of a lower spinel-lherzolite member overlain by harzburgites. Both are cut by numerous olivine-pyroxene veins which represent early crystallization products from a picritic tholeiite magma derived at 18-22 kbar by approximately 23% partial melting of the spinel lherzolite. The remainder of this magma crystallized as differentiated cumulate and extrusive rocks under crustal conditions. The petrogenetic model derived implies a rising diapiric body beneath an accreting centre and allows for the production of tholeiitic, transitional and mildly alkaline basalts from a single parent. The nature of the basalt erupted depends upon the rate of upwelling and crystallization in the diapir. The model suggests that crystallization takes place in the diapir at depths of less than 60 km.