MAGMATIC PROCESSES OF SUSWA VOLCANO DEDUCED FROM FELDSPAR ZONING IN POST-CALDERA PHONOLITES, EAST AFRICAN RIFT, KENYA

Suswa Volcano is a Holocene volcano in the axis of the Kenya Rift. It is composed of trachytes that culminated in a caldera, followed by phonolites. For the purpose of this research, trachyte and phonolite feldspars, along with other phenocrystals, were studied.

The trachytic feldspars show no zoning and have a composition similar to other trachytes from Kenya Dome (An_1-3Ab_50-60Or_35-45). The phonolitic feldspars fall into three groups defined by composition and texture. Group A includes feldspars with trachytic core composition and euhedral overgrowths; group B contains a population of An-rich (An_8-12Ab_67-70Or_18-23) core feldspars also with euhedral ovegrowths; and, group C tends to be of intermediate composition between the other two groups (An_5-8Ab_67-69Or_22-26), presenting weak zoning, and melt inclusions with similar composition to the glass in matrix. Ternary diagrams of the core feldspar compositions show a bimodal distribution with the group C feldspars in between. Other phenocrystals consists of olivine and clinopyroxene, with Mg contents increasing from trachytes (Fa₉₀, hedenbergite) to phonolites (Fa₆₈, diopside). Oxides in all rocks are ulvöspinel, requiring low oxidation state.

The zoning and the bimodal distribution of core compositions for phonolites imply some type of mixing process that produced the euhedral overgrowths, reverse zoning and the group C feldspars.