Equatorial mid-atlantic ridge: petrologic and Sr isotopic evidence for an alpine-type rock assemblage

Abstract

A preliminary petrological description is presented of rocks obtained where the Mid-Atlantic Ridge is intersected by the Chain, Romanche and St. Paul fracture zones and from unfractured sections of the Ridge between 6° and 8°N. Ultramafic rocks are predominant at the base of the sections; harzburgites prevail, but lherzolites, dunites and plagioclase-peridotites are also present. Gabbros, including norites, olivine gabbros and nepheline gabbros are found at intermediate levels, together with minor quantities of quartz-diorite. Dolerites and basalts, including some with alkali affinities, are recovered at the upper levels of the sections. Members of a metamorphic series ranging from greenschist to amphibolite facies are found throughout the sections. TheSr 87/Sr 86 ratio of the basalts and gabbros ranges from 0.702 to 0.704, within the values previously obtained from oceanic basalts. The peridotites have values ranging from 0.706 and 0.723 and lowRb/Sr ratios. These values are similar to those found previously in alpine-type peridotites from various parts of the world. It is concluded that the peridotites from the Mid-Atlantic Ridge (excluding St. Peter-Paul rocks) are neither the parent material of modern oceanic basalts nor the residual product after basalt extraction. These peridotites are residual and were depleted of lithophile elements at some early stage of their history, before the opening of the Atlantic rift. The abundance of intrusive, alpine-type peridotites suggest that in the upper mantle below the Equatorial Atlantic a zone exists of residual, alpine-type peridotitic material, probably left over since the differentiation of a sialic crust. The data indicate strong similarities between the Mid-Atlantic Ridge and alpine-complexes from various parts of the world.