LATERAL CONNECTIVITY OF SHALLOW MAGMA SYSTEMS IN A QUATERNARY TRACHYTE – COMENDITE – PANTELLERITE VOLCANIC SUITE, THE CENTRAL KENYA PERALKALINE PROVINCE (CKPP), EAST AFRICAN RIFT

The Central Kenya Peralkaline Province (CKPP) coincides with the apical region of the Kenya Dome, an area of high elevation, modified lithospheric structure (shallow depth to Moho and lithosphere – asthenosphere boundary), and volcanoes in a state of unrest that record numerous eruptions from ca. 500 ka to present. Unrest is recorded by composition of fumarolic gases and present day uplift and subsidence centered on the volcanoes. Felsic volcanism includes five caldera complexes: Menengai (comenditic to pantelleritic trachyte), the Eburru complex (trachyte and pantellerite), the Greater Olkaria Volcanic Complex (comendite with minor trachyte), Longonot (pantelleritic trachyte and mixed trachyte/hawaiite), and Suswa (trachyte and phonolite that contain immiscible globules of magmatic carbonate). Modeling of the geodetic data requires that the top of the felsic magma systems lie within 2 to 5 km of the surface. Mafic lava fields (alkali basalt, basaltic trachyandesite, and trachyandesite) lie in the rift floor adjacent to the caldera complexes. Magma genesis is via prolonged crystal fractionation of mafic parental material and magma mixing. Patterns of reverse zoning in the alkali feldspars imply that the mixing results from recharge and convective overturn prior to volcano eruption. Given that each caldera complex retains distinctive eruptive histories, there must exist limited lateral connectivity of the magma chambers, even though the eruptions are sometimes essentially synchronous and the volcanic centers lie within 25 km of each other. The combined volcanologic and petrological data provide a compelling 4D (time and space) image of this peralkaline volcanic field. The image serves as a model for other peralkaline systems in the geologic record.