Basalts from the Northern Part of the Rift Zone of the Mid-Atlantic Ridge

Descriptions are given of a large collection of very fresh basic igneous rocks obtained in dredge hauls along with colonial branching corals and foraminifera from the central part of the Mid-Atlantic Ridge at approximately 45° 50′ N; the rocks come from two localities 5 miles apart and were collected during the autumn cruise of R.R.S. Discovery II in 1960. All prove to be olivine-bearing basalts or dolerites carrying a single pyroxene phase and most have transitional or slightly alkaline affinities, although types approaching typical tholeiitic basalts are also represented. Many of the specimens have chilled selvedges and can be shown to be pillow lavas. Important features of the mineralogy include the development of cristobalite and tridymite in vugs and in the residuum of the dolerites, and the frequent presence of xenocrysts or xenocrystal aggregates composed of highly magnesian olivine, picotite, and very calcic plagioclase. The significance of these features is discussed in relation to the tectonic setting of the rocks. Potassium-argon age data are presented.