Nepheline Gabbro from the Mid-Atlantic Ridge

Abstract

IN the course of a long range programme aimed at the study of the petrology of the Equatorial Mid-Atlantic Ridge, a series of dredgings were made by Miami's Institute of Marine Sciences on the walls of the Romanche trench. The Romanche is a major tectonic fracture which intersects the Ridge close to the Equator, and offsets its axis, perpendicular to its elongation. At one locality on the north wall of the trench (station P6707-25; 00° 22′ S, 20° 09′ W) about 120 kg of rock fragments was recovered from 5,100–5,300 m below sea level. The dredged material consists of about 95 per cent (by weight) of basalt and glassy basaltic breccias; 2.5 per cent of gabbros; 2 per cent of cataclastic breccias; 0.5 per cent of serpentinized peridotite. Among the gabbros norites, troctolites and gabbros with alkali affinities were observed; among the latter, some contain modal nepheline and are similar to theralites. These alkali gabbros are of special interest, particularly because rocks with modal nepheline have not been reported in the past from oceanic ridges. We present here a description of them and a brief discussion of their origin.