Geology and geochemistry of the late Archaean greenstone associations, Maseno Area, Kenya.

The greenstone belt of Kenya is an extension of what is commonly referred to as the Tanganyika "Shield". The two supracrustal sequences recognized in Kenya are the Nyanzian and the Kavirondian. The rocks found in these sequences are diverse, with dominant volcanics in the Nyanzian, whereas the Kavirondian is predominantly sedimentary. The Nyanzian lavas represent a diverse range from basalts and basaltic andesites, through andesites and dacites to rhyolites. From geochemical studies the basalts and basaltic andesites are tholeiitic, whereas the andesites, dacites and rhyolites are calc-alkaline. The sedimentary sequence ranges from mudstone, through shales, sandstones and grits to conglomerates. The plutonic rocks range in composition from gabbro to true granites, but tonalite is the dominant rock type. The chemical differences between the tholeiitic basalts and the calc-alkaline andesitic to rhyolitic sequences suggests that these volcanic suites are derived from different sources and/or through different processes. The granitoids have close chemical similarities with the silicic volcanics. From the geochemical and field relationships, the Nyanzian and Kavirondian sequences are considered to have developed on a continental segment which had not yet attained full stability. The model employed for the generation of these volcanics considers the basalts to have been generated in a region undergoing extension, similar to that of a modern back-arc environment, whereas the calc-alkaline sequences, including the granitoids, are broadly comparable with those found in present day continental arc environments.