Heavy and light marker minerals around the East African Rift System (EARS) with special reference to the evolution of Zanzibar Island, Tanzania

Abstract

Mineral assemblages encompassing heavy (HM), light (LM) and clay minerals (CM) can be used as marker minerals for geodynamic and weathering processes. The highly diverse lithology of the Precambrian basement rocks around and the rift-related magmatic rocks of the East African Rift system (EARS) form an ideal platform to use sediment-mineralogical techniques to study the origin of the Island of Zanzibar, Tanzania, during the Quaternary. Four reference mineral associations have been defined: (1) Precambrian basement rocks (Tanzania Craton + Proterozoic rim), (2) Mesozoic rocks (magmatic rocks of the rift-and-graben system), (3) Cenozoic rocks (magmatic rocks of the rift system), (4) Quaternary placers, duricrusts and soils which form a transition zone between bedrock and the depocenter under study. Detrital grains of pyroxene and amphibole s.s.s. and autochthonous kaolinite group minerals in context with Fe–Al oxide/hydrates play a leading role for the geodynamic and geomorphological-mineralogical correlation of Zanzibar with mainland Tanzania. During the Holocene HM are likely to have been introduced into the beach sands from a magmatic submarine source and assigned Zanzibar to the marginal facies of the EARS, while a central island ridge blanketed by an argillaceous duricrust zone denotes a HM barrier towards mainland Tanzania. The difference between the mineral assemblage along the east coast of Zanzibar and the mineral assemblage of the mainland fed by the large fluvial drainage systems argues for a local magmatic HM source.