Metagabbros from the Mid-Atlantic Ridge at 06°N: Contact-Hydrothermal-Dynamic Metamorphism beneath the Axial Valley

Abstract

Metagabbros with a transitional greenschist-amphibolite-facies mineralogy are exposed on the lower slopes of the eastern wall of the axial valley of the Mid-Atlantic Ridge at 06°N. These metagabbros are associated with metabasalts in the greenschist facies and with only mildly or nonmetamorphic basalts. Among the metagabbros some are finely banded, consisting of alternating amphibole-rich and plagioclase-rich zones, each about 1 mm thick. Specimens showing gabbro-basalt contacts are interpreted as representing the margins of basaltic dikes originally crossing through the gabbroic lower oceanic crust. The chemical composition of the metagabbros appears to be nearly unchanged from that of their parent rocks, which probably were gabbros with intermediate Fe/Mg ratio, and with olivine tholeiite normative composition and low K content characteristic of abyssal tholeiites. One of the metagabbros was dated at between 2 and 3 m.y. by the fission-track method on zircon crystals. The metamorphic processes which gave rise to the metagabbros probably took place not deeper than about 1.5 km, and in some cases just a few hundred meters beneath the axial valley; given probable regional conductive heat gradients below the Ridge axis, they cannot be accounted for solely by "regional" burial metamorphism; in fact, the classical concept of "regional" burial metamorphism is considered not applicable within the axial zone of an actively spreading ridge. Contact, dynamic, and hydrothermal metamorphism beneath the axial zone of frequent basalt injection and of intense faulting can account for the formation of the metagabbros. The banding of some metagabbros is explained by analogy with observations in the Appennine ophiolites, where basaltic dikes crossing gabbros are exposed; fine banding, observed in the gabbros within narrow zones at each contact with a dike, probably developed within the lower oceanic crust in shear zones along faults and fractures, locus of dike injection.