GABBROIDS OF THE VEMA FRACTURE ZONE: TEXTURES, COMPOSITION, AND TECTONIC SETTING

Abstract

Gabbroids were dredged together with ultrabasic rocks from the lower portion of the section of a large transverse ridge in the southern flank of the Vema Fracture Zone valley in the Central Atlantic. Their petrographic features, chemical compositions, concentrations of trace elements including rare-earths, and compositions of minerals were investigated. It was established that the gabbroids were formed from depleted tholeiitic melts of the N-MORB type in transitional magma chambers of small lateral dimensions, which appeared periodically in ultrabasic rocks. The gabbroids were not significantly differentiated through crystal fractionation. Most of them experienced recrystallization under plastic deformation conditions. During the recrystallization, evolved fluid-saturated melts enriched in iron and titanium entered the solid rock. Interaction between these melts and rocks resulted in an increase in the albite content of primary plagioclase, while pyroxene gained iron and lost titanium and aluminum. The melt crystallized abundant ilmenite and minor amounts of hornblende, which cemented recrystallized grains. Diorites were eventually formed from the residual melts. These processes occurred in the areas where deep detachment faults penetrated the gabbroids. These faults were formed at the junction of the rift and fracture zone and promoted the emplacement of the gabbroids and peridotites into the upper crustal levels. During ascent, gneissic amphibolite was formed after the gabbroids in the zones of detachment faults under amphibolite-facies conditions. The subsequent metamorphism of the gabbroids occurred under the greenschist-facies. The compositions of magmatic and metamorphic amphiboles show significant regular differences, which record various stages of gabbroid transformation.