| Abstract |
Mount Suswa is a large (>700 km2) trachytic shield volcano with two nested summit calderas that erupted peralkaline trachyte and phonolite lavas and tuffs from approximately 240 to around 10 ka. Suswa is the southernmost volcanic center in the Central Kenya Peralkaline Province (CKPP), a region of intensive geothermal development. This development includes active production at Olkaria and drilling at Menengai as a collaboration between Geothermal Development Company, Ormat, and KenGen. This report focuses on Suswa, which represents the only salic center to have erupted significant volumes of silica-undersaturated lavas and tuffs. The eruptive products of Suswa can be clearly divided into two series, which correspond closely to the volcano’s eruptive history. The earlier series (C1) includes lavas and tuffs that built the initial shield volcano and erupted during the first caldera collapse; these rocks are dominated by peralkaline, silica-saturated to mildly under-saturated trachyte. The later series (C2) includes lavas and tuffs that erupted within the caldera structure following the initial collapse and during the creation of a second smaller, nested caldera and central “island block”; these rocks are dominated by peralkaline phonolite. Major-, trace-, and thermodynamic-modelling suggest that C1 is the result of protracted fractional crystallization of a fairly “dry” alkali basalt (less than 1 wt% H2O) under relatively high pressure (400 MPa) and low oxygen fugacity (FMQ to FMQ-1). Although C1 appears to be primarily the result of closed system processes, a variety of open system processes are responsible for C2. We propose that crystallization of C1 trachyte resulted in the formation of a syenitic residue, which was assimilated during a later stage of recharge and differentiation of alkali basalt to produce post-caldera ne-trachyte. Post-caldera phonolites were in turn the result of fractional crystallization of this ne-trachyte. Later-erupted (ring trench) phonolites, however, are the result of feldspar resorption prompted perhaps by magma recharge as evidenced by reverse zoning in alkali feldspar and linear compatible trace element patterns. This complexity reflects the open-system behavior of East African rift volcanoes, where mafic dikes periodically intrude compositionally evolved shallow magma chambers. Examples of this phenomenon occurred in 2005 in the Afar region of Ethiopia, where an episode of recharge/diking opened a 60 km rupture in the crust, and in 2007, when a magma-driven earthquake swarm struck northern Tanzania for a period of two months and triggered an eruption at Oldoinyo Lengai volcano. Similar recharge is thought to have happened recently at Suswa and a number of the adjacent volcanoes based on geodetic (InSAR). |