Characterizing weathering intensity and trends of geological materials in the Gilgel Gibe catchment, southwestern Ethiopia

Detailed geological and geochemical characterization is crucial to support soil studies in such geologically and topographically complex systems as the Gilgel Gibe catchment in southwestern Ethiopia. Field studies, as well as mineralogical, petrological and geochemical analyses on selected rock samples and their weathering products revealed that the catchment is dominantly underlain by rhyolites and trachytes, which occur as both lava flows and pyroclastic associations. Most of the lavas have a trachytic texture, while few others are massive or show spherulitic or perlitic texture. The rocks have a SiO2-content ranging from about 62 to 73 wt% (intermediate to felsic composition, on an anhydrous base) and a relatively high Na2O + K2O content ranging from about 9 to 12 wt% (anhydrous base). The dominant phenocrysts present in the rocks are plagioclase, sanidine and Fe-Ti oxide minerals. Alkali-rich amphiboles and quartz occur in most of them, while hornblende, titanite and clinopyroxene are rare. The amount of phenocrysts varies from less than 1 vol.% to about 30 vol.%. The pyroclastic associations are discontinuously scattered within the study area. They all have a glassy matrix (vitrophyric texture) and are composed of a mixture of lithics, crystals and glass. In comparison with the lava samples, the pyroclastic samples exhibit a more variable chemistry. In contrast, the X-ray diffractograms of the pyroclastic deposits and the lavas show little difference. The Chemical Index of Alteration values for the studied samples vary from 53 to 99 indicating moderate to high intensity of weathering. Samples from lava flows have shown less degree of weathering than samples of the pyroclastic associations.