Quaternary volcanoes of the Lake Rudolf region: II. The lavas of North Island, South Island and the Barrier

Abstract

North Island, South Island, and the Barrier are Quaternary volcanoes in the Lake Rudolf portion of the East African Rift Valley. They have produced lavas ranging in composition from olivine basalt through trachybasalt to trachyte. Magnesian olivine, diopsidic augite, and calcic plagioclase appear as phenocrysts in the more basic lavas; anorthoclase is the only phenocryst in the trachyte of the Barrier. Xenocrysts of calcic plagioclase occur in the trachyte of North Island. The groundmasses are composed of clinopyroxene, plagioclase, magnetite and/or ilmenite, and interstitial glass. The amount of potassium and iron in the plagioclase is systematically related to the anorthite content, and although the general relations are the same for feldspars from the three areas, the absolute amount of these elements is different in North Island feldspars from that of the South Island and Barrier feldspars.