The Songwe scarp carbonatite and associated feldspathization in the Mbeya Range, Tanganyika

Abstract

The carbonatite and associated rocks described occur on the Mbeya mountain range, which is at the junction of the eastern and western rift valleys in south-west Tanganyika. The main occurrence of carbonatite is a dyke-like body, possibly in discontinuous lenses, with an approximately linear outcrop trending nw-se over a distance of some 12 miles at the base of the Songwe scarp. This scarp bounds the Mbeya Range and delimits the southern end of the Rukwa trough. The carbonatite was emplaced in a zone of structural weakness along which there was probably rift-valley faulting before the introduction of the carbonatite and along which there has since been considerable faultmovement. The rift-faulting here follows the north-westerly trend of the crystalline basement rocks. Typically the carbonatite is ankeritic, fine-grained, and not flow-banded. Commonly there is a fragmentary or brecciated texture, the fragmentary material consi sting largely of feldspathized and carbonated country-rock. Minor amounts of niobium, uranium, and thorium are present but the location of these elements is uncertain and rare accessory minerals have not been detected. Feldspathization by the introduction of potassium is strongly developed in the immediate vicinity of the carbonatite and also sporadically elsewhere on the Mbeya Range; both quartzofeldspathic cataclastic schists and coarsegrained mica-garnet schists have been converted to aggregates of potassium feldspar. Such feldspathization is particularly well displayed in the vicinity of Itete on the north-east flank of the Mbeya Range, where elongated belts of feldspathized country-rock illustrate the strong control exercised over their formation by the regional schistosity. Injection and brecciation of these feldspathized schists by carbonatite veins is common. It is suggested that the Songwe scarp carbonatite is a high-level representative of the crystallization of a parent carbonatite at depth; the unusual form is due to the strong structural grain of the basement schists of the area.