Lake Malombe Vents

Lying to the west of Lake Malombe, in a fault zone marking the western edge of the main rift valley, are the large, circular vents of Nsala and Kongwe to the south of which are half a dozen small vents that extend for about 1.5 km along a north-south line. Nsala and Kongwe are respectively 1.2 and 1 km in diameter, and form crater-like hollows whereas the small vents are generally elongate in plan and form steep-sided hills. The smaller vents are all filled with nephelinitic agglomerate consisting of angular fragments of country rock gneiss up to some 30 cm across in a fine-grained matrix with phenocrysts of brown amphibole, commonly rimmed by biotite, augite with green cores, biotite, apatite and small crystals of magnetite, perovskite and nepheline in a base of pyroxene and analcime. At the Achirundu Vent the matrix contains what Garson (1965d) considered to be apatite or melilite much replaced by carbonate. The Nsala and Kongwe vents contain feldspathic agglomerates and rings of fenitized rocks, while in the centre of the former are plugs of microfoyaite, phonolite and carbonate-rich feldspathoidal rock. The agglomerates consist of fragments of K-feldspar rocks in a comminuted feldspathic groundmass partly replaced by carbonate. These rocks are considered to be the products of feldspathization of fenitized gneisses. Fragments of nephelinitic agglomerate occur amongst the feldspathic agglomerates. Approximately two thirds of the Nsala Vent and one quarter of Kongwe are occupied by extensively shattered and fenitized rocks. The fenites include various types of gneiss which are veined by blue fibrous amphibole and aegirine. The circular plug in the centre of the Nsala Vent is about 450 m in diameter and comprises a rock of large nephelines, alkali feldspar, aegirine-augite zoned to aegirine, titanite, magnetite and a little sodalite. Analcime-bearing phonolites within the foyaite may be local variants or possibly form a later plug. The rock of a small intrusion just west of the foyaite consists largely of analcime much impregnated by carbonate, nepheline microphenocrysts, relicts of albite and chloritic pseudomorphs, possibly after aegirine. An analysis of this rock and the foyaite is given by Garson (1965d). Dykes of nephelinite, microfoyaite, phonolite, the most abundant type, and trachyte occur up to several kilometres from, and radial to, the Nsala and Kongwe vents. Garson (1965d) reports possible melilite and melanite in nephelinite and aenigmatite, låvenite, astrophyllite and possible lamprophyllite in phonolites.