Songwe Syenite

The Songwe syenites occupy a broad zone that extends for over 30 km close to the northern border of Malawi. At the eastern end they disappear beneath recent sediments, and so may continue into Zambia. The Ilomba intrusion (No. 103-00-002) lies wholly within Songwe syenites and this intrusion, together with Ulindi (No. 103-00-003), are included by Ray (1975) within his ‘Songwe Syenite Complex’. Most of the syenites are coarse, leucocratic rocks with lenticular perthites up to 3 cm in length. Primary flow textures are common but there is a weak to moderate tectonic foliation and a weak schistosity in some finer grained rocks. These syenites consist of microcline perthite, a little albite, quartz, biotite and minor pyroxene, which is partly replaced by amphibole; accessories include zircon and garnet. Cutting the syenites are aegirine-bearing gneisses which appear to have been formed by intense shearing that produced fine- to medium-grained rocks with a strong shear fabric and conspicuous lineation. The pyroxene forms elongate, corroded crystals of aegirine and aegirine-augite which may be partly replaced by riebeckite; it also forms discrete, minute prisms. The rest of the rock comprises perthite, quartz and accessories.