A ROLE FOR SYENITE “RECYCLING” IN THE PETROGENESIS OF A TRACHYTE-PHONOLITE SUITE, MOUNT SUSWA, CENTRAL KENYA PERALKALINE PROVINCE, EAST AFRICA RIFT

Suswa, the southernmost volcanic center in the Central Kenya Peralkaline Province (CKPP), represents the only salic center in the province to have erupted significant volumes of silica-undersaturated lavas and tuffs. Suswa and the silica-oversaturated systems in the CKPP (Menengai, Longonot, Eburru, and Olkaria) are all basalt-driven, in that basaltic magma has been the fundamental source of heat and volatiles in these systems. These peralkaline systems remain in the volcanic stage as long as the basalt input is sufficiently high to inhibit crystallization, and enter the plutonic stage following a decline in heat input. Suswa will eventually consolidate to syenitic complexes similar to the adjacent but older (~2 Ma) Kilombe volcano. The eruptive products of Suswa can be clearly divided into two series, which correspond closely to the 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. C1 appears to be primarily the result of fractional crystallization of mafic magma with limited assimilation (White et al., 2012; Lithos, doi:10.1016/j.lithos.2012.01.023). A variety of open-system processes have been 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 a post-caldera ne-trachyte. The first post-caldera phonolites resulted from fractional crystallization of the ne-trachyte. Later-erupted phonolites, however, show clear evidence of extensive feldspar resorption, prompted perhaps by magma recharge and mixing, as seen in reverse zoning in alkali feldspar and linear compatible trace element patterns. This study demonstrates the importance of melting and assimilation of early crystallites and active open system processes in the modification of later eruptive stages.