Birnin Kudu

This poorly exposed complex consists of a north-south-trending group of isolated exposures of granite porphyry, the arc-like form of which suggests they may be part of a ring-dyke with a diameter of 7 km, and an east-west line of hills of volcanic rocks covering 8 km2. The granite porphyry is strongly altered with no fresh mafic minerals remaining, these being pseudomorphed by Fe-Ti oxides and fluorite. In the south a fine-grained facies forms a marginal zone to the dyke which is a vertically banded, eutaxitic, crystal-poor ignimbrite that was probably a feeder dyke for the volcanic rocks. In fresher rocks in this area hedenbergite and scarce fayalite are preserved. The volcanic rocks are ignimbrites with pronounced eutaxitic textures. Quartz and alkali feldspar microphenocrysts are sparse, biotite and Fe-Ti oxides form aggregates, possibly pseudomorphing pyroxene or fayalite, with some discrete biotite and calcite; fluorite and biotite are present within fiamme. Some facies are crystal-rich and a little unaltered hedenbergite has been found.