I. Ultramafic rocks - Peridotite—gabbro—basalt complex from the equatorial Mid-Atlantic Ridge

Abstract

Rocks were dredged where the Mid-Atlantic Ridge is intersected by the Chain, Romanche, St Paul and Vema Fracture Zones, and from unfractured portions of the Ridge between 6 and 8° N. Peridotites are common at the fracture zones, but were found also in unfractured sections of the Ridge; harzburgites prevail, but lherzolites, dunites and plagioclase peridotites are also present. A variety of gabbros was recovered, generally above the peridotites, including norites, troctolites, quartz gabbros and theralites. The chemistry of these gabbros indicates a marked crystal-liquid fractionation, following both a ‘tholeiitic’ and an ‘alkali’ trend. The basalts show also both trends, but less markedly. Metamorphic rocks ranging from ‘greenschist’ to ‘amphibolite’ facies are found throughout the sections. Strontium isotopic data suggest that the peridotites (excluding St Peter-Paul rocks) are not related genetically to the associated gabbro-basalt, in a situation similar to that of alpine complexes on the continents. The peridotites are probably residual and were depleted of lithophile elements at some early stage of their history, before the opening of the Atlantic rift. It is postulated that in the upper mantle below the equatorial Atlantic a zone exists of residual, alpine-type peridotite, while the lower crust consists of a mixture of ultramafics and intrusive gabbros. The data indicate strong similarities between the Mid-Atlantic Ridge and alpine complexes from various parts of the world.