Abstract

More than 20 layered basic intrusions are known among the crystalline rocks forming an isolated part of the late Pre-Cambrian and Lower Palaeozoic fold-belt in north-east Africa and the Red Sea region.

This paper discusses metamorphic and structural changes in and around five intrusions in an area between 20 and 40 miles north of Hargeisa and some 200 miles south-south-west of Aden: the Dudub, Rakdasafaka, Hamar, Ubali, and Gul Sakar gabbros. These consist largely of melanocratic olivine-gabbro with subordinate pyroxenite, peridotite, troctolite, and anorthositic leucogabbro. They were intruded into rocks that had already reached the amphibolite or granulite metamorphic facies during an early period of regional metamorphism and migmatization. Parts of these gabbros and country-rocks were later metamorphosed and deformed in a period of regional metamorphism that followed the intrusion of the basic rocks and was accompanied by the intrusion of granite. This late metamorphic event was only patchily developed and some gabbros retain many primary features, including rhythmic and cryptic layering, irregular layering, orbicular textures, poikilitic pyroxene, and flow-oriented olivine and plagioclase. The Hamar gabbro retains an incomplete contact-aureole and contains large country-rock inclusions believed to represent projections from a side wall.

The primary layering in all five intrusions now dips steeply and in three intrusions has been folded into structures a mile or two across. During the late movements, parts of the intrusions were converted to metagabbro or amphibolite and developed secondary structures which include shear-belts, minor folds and lineation, and oblique schistosity. Many of these structures have their equivalents in the country-rocks, which were affected by the later metamorphism and in which pre-gabbro high-grade mineral assemblages were partially replaced by lower-grade assemblages. The deformation of country-rock and gabbro is compared and three stages in the conversion of layered gabbro to concordant lineated and foliated metagabbro are recognized. The Somali layered intrusions are compared with deformed basic intrusions in other fold-belts and the deformation of such intrusions is briefly considered.