Nobert Opiyo-Akech

The petrology of the Chyulu Hills volcanic province, southern Kenya

Abstract The Quaternary Chyulu Hills Volcanic Province is located more than 100 km east of the Kenya Rift Valley. It consists of a large number of free-standing and coalesced volcanoes and cinder cones and numerous lava flows ranging in composition from nepheline-normative nephelinites, basanites, alkali basalts and hawaiites to orthopyroxene-normative subalkali basalts. In this paper, the authors briefly outline the geological setting of the Chyulu Hills Volcanic Province, present a classification scheme for its lavas and describe their petrography. Mineral chemistry data for selected olivine and clinopyroxene phenocrysts are presented together with the bulk rock major element compositions of selected samples. The petrography, phenocryst chemistry and bulk rock composition of the typically primitive Chyulu Hills lavas are consistent with a differentiation history dominated by olivine control. A process of delayed olivine fractionation, combined with limited mantle olivine accumulation, is proposed to explain the considerable compositional variability observed among olivine phenocryst cores. A trend of decreasing degree of silica-undersaturation from the oldest lavas, erupted in the northern Chyulu Hills, to progressively younger lavas in the southern part of the province is explained as a result of an age progressive decrease in the depth of melt generation and a coincident increase in the degree of melting.

Geochemistry and Tectonomagmatic affinities of the Mozambique belt intrusive rocks in Matuu-Masinga area, central Kenya

Abstract The Matuu-Masinga study area, located about 70 km north-east of Nairobi, is predominantly underlain by rocks of the Neoproterozoic Mozambique Belt (MB) of Kenya. The rocks vary from medium to high grade gneisses and granulites, that are intruded by granites, anorthosites, diorites and gabbros. Important high-grade tectonothermal events in the belt took place between about 845 and 715 Ma B.P. The subsequent cooling and uplift of the basement has been traced by K-Ar dates on biotites, which range between 438 and 528 Ma. The belt has been inferred to mark the sites for several superimposed Proterozoic subduction zones and collisional sutures (Muhongo, 1998, and references therein). The general structural trend in the survey area varies from NNW-SSE to NW-SE direction, with westerly dips and localized concentric trends around granitoid intrusions. Competent mafic lensoidal layers that have undergone ductile deformation and associated with rotated boudins and displaced micro-faults define a sinistral sense of shear. The several shear zones, cleavage patterns, boudins and thinning suggest severe strains. The mafic and granitic rocks are interpreted to be mainly metaluminous to slightly peraluminous. Geochemical data and field studies suggest that the granites are of calc-alkaline origin. The discriminant diagrams suggest a dominantly island arc-tectonic setting with subordinate within plate environment. Rb-Sr whole-rock age of 558 ± 16 Ma is given on the granite from Mavoloni hills, Matuu area. From the low initial 87 Sr/ 86 Sr ratio of 0.70398, the age is interpreted to indicate the time of emplacement for granite magma from a deep crustal or upper mantle material. The gabbroic and granitic rocks in Matuu-Masinga area, with high Ba (av. 1331 ppm) and Zr (av. 370 ppm) contents, are interpreted to have contained hornblende and biotite mineral assemblage phases that had a relatively high distribution coefficients for Ba and Z. The relatively high concentration of Cu (188–5810 ppm, av. 1960 ppm) and Zn (88–264 ppm, av. 155 ppm) in the mafic rocks of the study area invokes further exploration of their ore minerals.

Earth Science Education in Kenya

Abstract The University of Nairobi is currently the only University in Kenya which offers degree level courses in the earth sciences. The Department was founded in 1961, became part of an autonomous University in 1970, awarded its first degrees in 1972 and awarded its first Geology degrees in 1978. The Department currently offers a four-year B.Sc. course - delivered as part of a course unit system - and an M.Sc. programme in Geology. Students are admitted to the University after eight years of primary and four years of secondary schooling. Graduates find employment either in government departments or in the private sector. The Department currently has 15 academic staff who are involved in a variety of research projects. Currently, there are collaborative links with Universities in Germany, Sweden and South Africa.