Garnet and Rutile Mineral Chemistry and Zircon U-Pb Ages of Modern River Sand along the Western East African Rift (Albertine Rift, Uganda)

Abstract

Sand composition of large African rivers has been increasingly studied, but identification of source rocks often remains hypothetical, and mineral signatures of the Archean to Proterozoic basement are weakly constrained. We analyzed heavy minerals of headwater streams on the East African Plateau draining characteristic lithotectonic units with respect to modern single-grain techniques in provenance research. Despite strong tropical weathering, the target minerals are well conserved and widely distributed. We could distinguish four lithotectonic units from river sand. (1) The high-grade metamorphic rocks of the Neoarchean North Uganda Terrane deliver high-almandine, low-spessartine garnet with two groups of varying grossular contents. Rutile is mainly of metapelitic origin and exhibits amphibolite/eclogite facies to granulite facies. The Neoarchean age is confirmed by zircon U-Pb ages. (2) Rivers that mainly erode metasedimentary rocks of the Paleoproterozoic Rwenzori Fold Belt are identified by almandine-spessartine garnet, mainly amphibolite/eclogite facies rutile, and detrital zircon ages that correspond to the Eburnian orogenic cycle. (3) The Neoproterozoic Bunyoro Group in Uganda delivers mixed detritus with rutile assemblages similar to the underlying North Uganda Terrane and a wide range of zircon ages. (4) Although the basement block of the extremely uplifted Rwenzori Mountains contains rocks of both the Rwenzori Fold Belt and the North Uganda Terrane, some distinct provenance indicators could be identified: extremely high proportions of amphibole, exclusive dominance of spessartine-rich garnet, strong dominance of amphibolite to granulite facies rutile, a pronounced population of rutile with high Nb concentration, and pinkish zircon grains. Zircon age, however, was not distinctive. We show that single-grain analysis of heavy minerals in river sand can characterize different basement units under tropical climatic conditions. Detrital zircon age spectra confirmed known orogenic cycles in East Africa but could not distinguish specific Cenozoic fault blocks of the East African Rift. This could be solved by additional single-grain techniques.