Petrographic and chemical characteristics of the Panda Hill carbonatite complex, Tanzania

Abstract

The Panda Hill carbonatite complex, situated near the south-western flank of the Rukwa Trough in south-western Tanzania, comprises a carbonatite body, a fenite aureole and feldspathic agglomerate and breccia. Petrographic study, supported by some analyses of both rocks and minerals, indicates that the fenite zone and the agglomerate and breccia are very rich in potash. Fenites consist mainly of monomineralic orthoclase rock grading to basement gneiss away from the carbonate body. Fragments of agglomerates range from feldspathic fenites to altered basement gneiss with some fragments carbonatized. The central plug and most of the carbonatite body consist of sövite. Beforsite occurs as dyke-like bodies often cutting across flow-lines of sövite. Beforsite forms the outermost boundary of the carbonatite body on the south-east. Ferrocarbonatite forms the outermost boundary of the main body in the eastern foothills. A number of pseudo-phoscorite layers are associated with sövite often imparting a flow structure to the rock. Several mafic dykes intrude into the main carbonatite body. All these dykes are more or less carbonatized. Two types of mafic dykes are distinguished: carbonatized lamprophyre dykes; dolerite dykes. The primary magma from which the Panda Hill carbonatite and its associates originated appears to have had a potash-rich fraction. Explosiv eruption of alkali-rich magma and influx of alkali-rich solution led to extensive fenitization. The central plug indicates forceful injection of the body simultaneously with the explosive eruption. The distribution of tuffs and volcanic breccia around three hill-tops shows existence of volcanic vents. Intrusion of minor dykes of mafic and ultramafic rocks took place along with the intrusion of the carbonatite plug. During falling temperature, permeation of the country rocks surrounding the carbonatite body by carbonate fluids caused partial replacement of all the minerals by carbonates.