PETROGENESIS OF TRACHYTE AND PHONOLITE FROM SUSWA VOLCANO, KENYA RIFT

Suswa is a Quaternary volcano in the graben floor of the Kenya Rift. It is built of pre- and syn-caldera trachyte and post-caldera phonolite. This association is intriguing from a petrologic stand, as we try to understand the genesis of highly alkaline rocks, and is important for understanding the geodynamics of the Kenya Rift, in that the other Quaternary central volcanoes are trachyte and rhyolite, not phonolite. Other characteristics of the volcano are 1) it is underlain by a shallow, high density body which we interpret to be the residuum of the chamber which fed the volcano, and 2) high heat flow makes it one of the best targets for geothermal exploration. Previous studies included mapping, whole-rock and mineral chemistry, and documentation of magmatic CO2 in syn-caldera trachytic ignimbrite. This study contributes additional mineral and glass chemistry and a complete suite of trace elements. Pre- and syn-caldera trachyte are more SiO2-rich, in both the whole-rock and glass, than the phonolites, and have feldspars less An-rich. Other indices correlate with these, and imply that the trachytes are more evolved than phonolites. Trace element concentrations for the pre-caldera trachyte span the entire range for the volcano and form their own separate array. The sun-caldera trachyte and the phonolite form a coherent array, and the trachyte has higher concentrations of incompatible elements. Our preferred model at this time is that the pre-caldera trachyte was an early discreet magma pulse, followed by a separate pulse in which a zoned magma chamber developed. More primitive phonolites were overlain by a volatile-rich, evolved trachyte cap. The cap vented to form the caldera, followed by eruption of phonolite from the main chamber.