Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude

The petrology of the mid-Atlantic ridge between 22° and 23°N latitude may be typical of those portions of the ridge characterized by a linear topography parallel to the axis, a well-developed median valley, and an absence of volcanic cones. Submarine basalt lavas dredged at fifteen stations on the crest of the ridge are of three eruptive facies, all derived from essentially identical magmas; (1) pillow lavas, (2) sideromelane-rich tuffs, and (3) massive, mainly holocrystalline basalts. This association is well known from continental exposures of ancient submarine lavas. Chemically, the lavas are oceanic tholeiites and thus support the view that these low-potassium olivine basalts are by far the dominant eruptive on the deep-sea floor. In the 22° area, they are probably the product of voluminous fissure eruptions. The oceanic tholeiite is evidently the counterpart of the continental flood basalts, but it differs compositionally from these, especially in a lower potassium content. As a further characterization of the basalts, seven new analyses of major, minor, and trace elements are presented. Post-cooling hydrothermal metamorphism under some overburden has transformed some of these basalts into greenschists and lower-grade metamorphic rocks. New data indicate that faulting and shearing along the median valley combined with the introduction of hot, probably saline solutions were major agents in the metamorphism.