Trace element model studies of Nyanzian greenstone belts, western Kenya

The Archean greenstone belts of the Nyanzian System in western Kenya are composed principally of andesite with minor tholeiitic basalt and siliceous volcanics. The Nyanzian tholeiite is an intermediate-K tholeiite with a flat REE pattern. There are two chemically-distinct andesites: a low-K andesite (Andesite I) and a high-K andesite (Andesite II). The REE pattern of the Andesite II is enriched in light REE and depleted in heavy REE relative to Andesite I. Major and trace element calculations indicate an origin for the Nyanzian tholeiite by 35-40% equilibrium melting of a lherzolite source followed by 10% shallow fractional crystallization. Similar calculations best explain Andesite I and Andesite II by 20 and 5% melting, respectively, of an ecologite or garnet amphibolite source of Nyanzian tholeiite composition. The rhyolite may have formed either by 20-30% partial melting of a siliceous granulite or by 20-30% fractional crystallization of a granodiorite parent magma. With respect to total exposure areas, the Nyanzian volcanics have significantly less tholeiite and more Andesite and siliceous volcanics than other Archean greenstone belts. If these abundances are representative, two models are proposed to explain the anomalous abundances of Andesite and siliceous volcanics. The first model involves an Archaen upper mantle with a relatively low geothermal gradient beneath Kenya, while the second model involves a relatively cool mantle plume. Both models inhibit ascent of a significant amount of primary tholeiite to the surface and prevent formation of secondary tholeiite. Other Archean greenstone terranes with higher mantle geotherms or hotter mantle plumes would receive higher proportions of mafic and ultramafic magmas.