PETROLOGY OF PHONOLITES AND TRACHYTES FROM SUSWA VOLCANO, AFRICA

The Southern Kenyan Rift Valley is located in the East African Rift Zone. Various volcanic complexes are present in this valley. We focused on the Suswa Volcano. The general petrology of the study area consists of alkali rich rock packages. The Suswa volcano consists of a series of magma depositional events defined by phonolitic and trachytic suites. Both post caldera phonolites and pre/syn emplacement trachytes are found in association. The young phonolites contained higher concentrations of Na2O, at 8.41 weight percent, compared with the older phonolitic packages, which had a concentration value of around 7.43 weight percent. The syn-caldera trachytes also contained a higher concentration of Na2O than the pre-emplacement trachytes with nearly the same ratio as the younger and older phonolites. In relation to this, the pre-Suswa suite contains higher amounts of SiO2 and lower concentrations of Na2O. Textural relationships can also be observed between these different rocks. Feldspars, in both the pre and syn emplacement trachytes are similar and are orthoclase rich. In observing the phonolites two distinct textural relationships among the feldspars were discovered. The first generation formed in clots and expressed dominant twinning and is similar to the trachytes. The second generation has pronounced wedge shaped terminations similar to other phonolites. The pyroxenes in the trachytes were subhedral augites, whereas the pyroxenes in the phonolites were expressed by anhedral clots of clinoferrosilite and rims of augite with cores of ferrosilite. Looking at this evidence, magma mixing would be the most probable cause for this rock assemblage at the Suswa Caldera.