ORIGIN CONDITIONS OF THE MID-ATLANTIC RIDGE PLUTONIC COMPLEX AT 13-DEGREES-17-DEGREES-N

Diverse plutonic rocks and their metamorphic derivatives are widesprea d in the intersection area between the Mid-Atlantic Ridge (MAR) crest zone and the Fifteen Twenty (Cape Verde) Fracture Zone (Fifteen Twenty FZ). These rocks are exposed at the seafloor in rift valley walls, tr ansform faults, and inside and outside corner highs. The plutonic rock s of the Fifteen Twenty FZ can be classified into four major petrograp hic groups: (1) gabbroids with well-preserved primary textures, (2) gn eissic gabbro, (3) acid rocks similar to trondhjemite, and (4) amphibo lites without relies of primary magmatic textures. Petrological and ge ochemical data suggest the following scenario of the development of th e Fifteen Twenty FZ plutonic complex. (1) The origin of olivine-plagio clase, olivine-pyroxene-plagioclase, and plagioclase-pyroxene gabbroid s during the fractional crystallization of normal tholeiitic melts. Th e gabbronorites of the northern MAR segment are the derivatives of a m agmatic melt, whose composition differed from that of normal MORB melt s. (2) Gneissic gabbro originated simultaneously with ductile deformat ions. The parental magmas of some of the rocks involved melts that had separated from geochemically heterogeneous sources. (3) Intrusion of trondhjemite into gabbro, with the latter rocks undergoing contact tra nsformations and giving rise to hybrid gabbroid varieties. The metamor phic transformations of plutonic rocks in the MAR crest zone at its in tersection with the Fifteen Twenty FZ were closely related to the intr acrustal evolution of magmatic chambers, on the one hand, and depended on the emplacement dynamics of mantle restites at the upper crustal l evels, on the other.