Weathering on the alkaline rocks of the mbozi gabbro syenite complex sw–tanzanian, east africa

M.K.D. MUTAKYAHWA, Department of Geology, University of Dar es Salaam, TanzaniaFollow

Section

Physical Sciences

Abstract

The Mbozi gabbro syenite complex is about 800 Ma and is associated with the Bukoban tectonic episode. Tectonically it follows the regional strike direction of the Ubendian System, which is a part of the polycyclic Mozambique mobile belt. The complex is a typical ring structure with the central part down-faulted. Deep weathering profiles on the ring structure were developed on a peneplain and as a result of uplift intersected hilly landscape of early tertiary times. The truncated 5-15m thick weathering profiles occur on three rock types, olivine gabbro, pyroxenite and nepheline perthite syenite (pethoste). The chemical and mineralogical processes of supergene transformation have been studied. On the olivine gabbro the saprolite is composed mainly of kaolinite and smectite. Gibbsite occurs as a major alluminium hydroxide mineral in the goethitic bauxitic horizon making the Box-horizon. Within lower parts of the box-horizon, epigenetic accumulations of manganese oxides together with al-phosphates took place, which suggests a reducing environment. On the pyroxenite, very thick profile contains only the lower smectitic-kaolinitic saprolite with preserved relic textures and relic minerals like magnetite, spinels and ilmenite. Chromium is high in content due to the presence of Cr-spinel. Secondary silica occurs as quartz box work or as lenticular layers 10 to 20 cm long. Calcite formed later in these quartz layers as the pH conditions changes intermittently form acid to alkaline. On the nepheline perthite syenite, about 4 m thick weathering profile with a good vertical zonation and well preserved relic textures was observed. Sphene is preserved as fresh relic mineral or leucoxene. Feldspars and feldspathoids become mainly replaced by kaolinite, halloysite and gibbsite. In some case they have been dissoloved completely leaving behind pore spaces. A layer of compact red earth (soil) on top of the overlying ferromanganiferous horizon may be reworked from higher altitudes. Distribution of the main and trace elements with depth in these profiles depends on the mineralogy of the parent rocks and the conditions of weathering (oxidizing or reducing).

Recommended Citation

MUTAKYAHWA, M.K.D. (1995) "Weathering on the alkaline rocks of the mbozi gabbro syenite complex sw–tanzanian, east africa," Tanzania Journal of Science: Vol. 21: Iss. 1, Article 5.
Available at:https://doi.org/10.65085/2507-7961.2423