Coeruption of contrasting magmas and temporal variations in magma chemistry at Longonot volcano,central Kenya

The Quaternary central volcano Longonot is situated on the floor of the Gregory Rift Valley, Kenya, at 0° 55 S, 36° 25 E. Although the majority of its products are lavas and pyroclastics of pantelleritic trachyte composition, small volumes of alkali basalt magma have been coerupted with pantelleritic trachyte magma to produce mixed lavas. These lavas were the first products following each of three caldera collapses and mark the start of three successive cycles of whole-rock chemical variation with time. For the first two mixed-lava eruptions identified, field, petrographic and mineralogical evidence suggests that the contrasting magmas comingled, and in places hybridized, during eruption. Whole-rock geochemistry requires the alkali basalt component to have been contaminated prior to coeruption with trachyte. Syenite is suggested as a possible contaminant of the basalt component in the last two mixed-lava eruptions. Field and whole-rock chemical evidence points to the trachyte magma chamber being underlain by a basalt magma root zone. Inputs of fresh basalt magma into the root zone may have initiated each pre-caldera pyroclastic event and subsequent caldera formation and may have also caused the trachyte magma to overturn and commence a fresh cycle of chemical evolution. Some of the hot, buoyant basalt magma was able to leak towards the surface up peripheral fractures where it was coerupted with the initial trachyte magma of each cycle.