ABSTRACT

A suite (V25-RD6) of gabbros showing a wide range of composition from the Mid-Atlantic Ridge is described in this paper. The gabbros may be classified into three groups representing stages of progressive fractionation: troctolite and olivine-pyroxene gabbros of the early stage of fractionation, hornblende-augite gabbros of the middle stage, and Fe-Ti oxide-rich hornblende-augite gabbros of the late stage. The major and trace element compositions and the mineralogy of gabbros in the three groups are consistent with the view that the gabbros are cumulates (adcumulates and mesocumulates) derived from abyssal tholeiite magma at various stages during advancing fractionation. Early-stage gabbros probably were formed by accumulation of crystals from abyssal tholeiite magmas that were in a normal range of composition, whereas middle- and late-stage gabbros were accumulated from magmas that were formed by extraordinarily advanced fractionation, presumably in a very large intrusive mass.