Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya

Two suites of felsic eruptives and intrusives are represented in a set of samples from the summit region of the Plio-Pleistocene volcano, Mt. Kenya. Most of the samples are moderately or strongly undersaturated and have ⁸⁷Sr/⁸⁶Sr initial ratios in the range 0.70360–0.70368 (mean=0.70362). Members of this phonolitic suite are phonolites, nepheline syenites or kenytes and as a group they show a wide variation in TiO₂, FeO, P₂O₅, Sr, Ba, Zr and Nb. The minor and trace element geochemistry reflect variation in the nature of the parental basaltic magmas from which the phonolitic rocks evolved and variation in the crystal fractionation process in individual cases. Crystal fractionation involving plagioclase, alkali feldspar, clinopyroxene, olivine and magnetite is the process by which most of the phonolitic rocks evolved and variation in the relative proportions of these phases in individual cases has led to a broad spectrum of trace and minor element behaviour. The second suite of felsic samples is critically saturated and consists of trachytes showing either slight oversaturation or slight undersaturation with respect to SiO₂. This trachyte suite has lower initial ⁸⁷Sr/⁸⁶Sr ratios (mean=0.70355) and is derived from transitional alkalic basalts by low pressure (crustal) crystal fractionation involving feldspar, clinopyroxene, magnetite and olivine. The range in minor and trace element chemistry observed among the felsic rocks is a consequence of variation in the parental basalts which is related to mantle source variation and to the specific nature of the crystal fractionation process.