Abstract <p>Problems of evolved gabbroic associations and local granitoid magmatism in an ocean spreading ridge system are of key importance for the understanding the planetary differentiation during the ocean crust formation. The paper presents new data on plutonic associations, which are exposed in two different types of tectonic “windows” of the Mid-Atlantic Ridge: the footwall of a large-offset detachment fault (Ashadze complex, 13° N) and an uplifted wall of a transform fault (Peyve Seamount, 8° N, Vernadsky Fault). Regular features of crystal differentiation with fractionation of a significant amount of Fe–Ti oxides up to the formation of silicic residual melt composition are shown on the basis of mineral compositional trends and crystallization modeling of MORB-type melts in COMAGMAT 3.75 program. The simulated decrease in oxygen fugacity to QFM level is a result of closed-system oxygen consumption by crystallization of a large volume of magnetite. This is indirectly confirmed by regular formation of reactive fayalite in most evolved gabbroids as a result of redox interaction of reduced residual melt with early magnetite. The earliest zircon crystallized in the apatite-bearing oxide microgabbro has a low-Hf composition (&lt;1.3 wt % HfO₂). Experimental study of melt inclusions in apatite and zircon yields the real major-element composition of the residual granitoid melts. Further differentiation of silicic residual melts in gabbro-hosted felsic injections occurs as zircon enrichment in HfO₂ up to 2.3 wt % (Ashadze complex) and up to 3.5 wt % (Peyve Seamount).</p>