3-D Lithospheric Profiles for the Axial Volcanoes of the Central Kenya Peralkaline Province, East Africa Rift: insights into Magma Generation and Storage at Compositional Density Boundaries
Abstract
We report a synthesis of remote sensing, chemical, chronological, and geophysical studies that provide a high-resolution image of 3-D lithospheric structure and depths of magma generation and ponding for central Kenya. Volcanoes include salic caldera and dome fields and adjacent fissure-fed mafic lava and cinder cone complexes. InSAR (e.g. Biggs et al., 2009, Geology 37, 979-982) images current shallow magma systems that underlie individual calderas. Geochemistry includes major- and trace-element data. QUILF and MELTS modeling (Ren et al., 2006, Lithos 91, 109-124; White et al., 2012, Lithos 152, 84-104) provide P-T and oxygen fugacity estimates for the magmas. The geophysical data combine active and passive seismic, magnetotelluric, and gravity. Taken together, these studies provide an image with the following characteristics: Oxidation state (QFM or lower) and elemental compositions require alkaline, mafic magmas that are parental to the evolved peralkaline trachytes, rhyolites, and phonolites. The mafic magmas erupt along fissures adjacent to the salic calderas and frequently mix with the evolved magmas. Seismic profiles image a low-velocity immediately below the Moho. At the latitude of the CKPP, Moho depth is 35 km below the axis of the structural graben and seismic velocity is 7.6 km/sec vs. 8.1 km/sec on the flanks. Mantle magma may well modify the lower crustal ganulite-facies mafic gneisses. Magmas then pond and undergo extensive (90 percent or more) at two depths that correspond to intracrustal boundaries (Mooney and Christensen, 1994, Tectonophysics 236, 391-408). The first ponding, at 400 MPa (14 km), is closed-system fractional crystallization at the boundary between amphibolite grade, dioritic gneiss and greenschist/lower amphibolite facies felsic gneiss. The second ponding, at shallow depth (3 to 5 km), includes open-system processes, e.g. autolith assimilation, magma mixing, and vapor exsolution. 40Ar/39Ar and U-series ages establish that caldera eruptions began at less than 240 ka and continue to present-day.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V53B..03A
- Keywords:
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- 3615 Intra-plate processes;
- MINERALOGY AND PETROLOGYDE: 7218 Lithosphere;
- SEISMOLOGYDE: 8121 Dynamics: convection currents;
- and mantle plumes;
- TECTONOPHYSICSDE: 8415 Intra-plate processes;
- VOLCANOLOGY