HR: 1400h
AN: V53B-07    [Abstracts]
TI: Mixing Between Basaltic and Trachytic Magmas to Generate the Trachyte-Phonolite Suite at Suswa Volcano, Kenya Rift: Evidence from Mineral and Glass Compositions
AU: * Espejel-Garcia, V V
EM: vvespejel@utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Ren, M
EM: ren@geo.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Anthony, E Y
EM: eanthony@utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Macdonald, R
EM: r.macdonald@lancaster.ac.uk
AF: Lancaster University, Environment Centre, Lancaster, LA1 4YQ, United Kingdom
AB: Suswa volcano, which formed from 0.24 Ma to recent, is the southernmost central vent volcano in the Kenya Dome area. This area includes 5 trachytic volcano complexes, two of which have pantellerites and comendites as their most recent eruptions. Suswa is distinct in that pre- and syn-caldera rocks include dominantly trachytic flows and ignimbrites, whereas post-caldera flows are trachyte to phonolite. The trachyte and phonolite suites have subparallel vertical trends on a TAS diagram with the phonolites at approximately 4 weight percent lower SiO2. Basaltic (B) to basaltic trachyandesite (BTA) flows are found in the low ground between the central vent volcanoes. Based on phenocryst compositions and melt inclusion and groundmass glass analyses, we have found evidence that Suswa developed by magma mixing between B/BTA and trachytic end-members. The evidence is as follows: 1) syncaldera ignimbrite contains both plagioclase (An45Ab52Or3) and alkali feldspar (An2Ab62Or36) in the same samples. The plagioclase compositions are identical to those found in the B/BTA flows and the alkali feldspar resembles that found throughout the Suswa sequence as phenocrysts. 2) Matrix glass in the pre-caldera rocks is trachytic, similar to whole-rock compositions. Syncaldera glass compositions range from trachyandesite to trachyte. The post-caldera phonolites show a wide range in matrix glass compositions from trachyandesite to phonolite. 3) Melt inclusion glass reflects the origin of the crystals: the inclusion glass in the plagioclase from the syncaldera ignimbrite is basaltic, similar to the most mafic whole-rock compositions for the flows from the area, whereas the inclusion glass in both oxides and clinopyroxene is trachytic. In the post-caldera phonolites, the inclusion glass in the alkali feldspar is basaltic trachyandesite to phonolite, whereas in coexisting olivine, clinopyroxene, and oxides, it exhibits higher SiO2 contents and is trachyandesite to trachyte. Based on these data as well as whole-rock chemistry, we propose that injection of a B/BTA magma into the Suswa pre-caldera trachytic chamber triggered syn-caldera eruption and mingling of the two magmas. The least evolved (i.e. least Si undersaturated) post-caldera phonolites represent a more complete hybridization of the two end-members. In order to produce the more evolved phonolites, this mixing process must be followed by crystal fractionation, with alkali feldspar having a dominant role.
DE: 8400 VOLCANOLOGY
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8415 Intra-plate processes (1033, 3615)
DE: 8439 Physics and chemistry of magma bodies
DE: 8440 Calderas
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly