Alkaline igneous rocks of the coastal belt, south of Luderitz, South West Africa : a petrological study

The Luderitz Alkaline Province, as it is at present known, comprises the subvolcanic central complexes of Drachenberg, Pomona, and Granitberg. An attendant dyke swarm strikes NE-SW and crops out between the latitudes of 27°00' and 27°30' S. Stratigraphic indications (now confirmed by a K/Ar age from Granitberg) are that the Luderitz Province is early-Cretaceous in age and therefore older than the Klinghardt phonolites (Eocene) as well as the smaller melilitite and nephelinite intrusions. Granitberg is a circular foyaite complex, in the centre of which is preserved a large fragment of sedimentary rocks that originally formed the roof of the intrusion. The foyaites have been emplaced into the feldspathic sandstones and dolomites of the Bogenfels Formation, and three major intrusive phases can be recognised. The first phase produced chilled nepheline syenites beneath the roof of the intrusion. These chilled rocks grade downwards into coarse-grained foyaites. The second phase was the emplacement of the Inner Foyaite which crystallized as a cylindrically zoned plug, capped by a zone of layered, laminated, and xenolithrich foyaites. The third phase was the emplacement of the Outer Foyaite, into which the Roof Zone and the Inner Foyaite foundered. The Outer Foyaite is zoned with a miaskitic core, and an agpaitic outer zone

Aenigmatite stability in silica-undersaturated rocks

Aenigmatite is common in many trachytes, phonolites and agpaitic nepheline syenites. Petrographic evidence suggests that the aenigmatite in these rocks arises by the reaction of Ti-magnetite with a peralkaline silica-undersaturated liquid, and it is postulated that a no-oxide field, where aenigmatite is stable, exists in alkaline undersaturated magmas