Microscale Geochemical Analysis of Primitive Lavas from Bufumbira, Uganda: Implications for Rift Magmatism
Abstract
Magmatism in the Western Branch of the East African Rift System is characterized by low volume, alkaline melts, which differ from much of the rest of the rift system and provide an opportunity to explore tectonic and geochemical controls on rift-related magmatism. In the Western Branch, magma compositions lack evidence for asthenospheric mantle plume input—rather, most show evidence for metasomatized lithospheric mantle sources. We focus on the Bufumbira (Uganda) cinder cones in the Virunga Volcanic Province, where bulk geochemistry indicates that primitive mafic lavas are derived from garnet ± phlogopite ± amphibole ± zircon-bearing pyroxenite, indicating a deep metasomatized SCLM source. Isotopic and trace element data suggest that extent of melting increased with depth of melting, a signature of lithospheric drip magmatism, which may occur in response to local and far field tectonic stresses. Source isotopic and mineralogical compositions vary on a short spatial scale in all portions of the Western Rift, suggesting heterogeneity driven by metasomatism in the regional lithospheric mantle. We explore the mechanisms that produce this heterogeneity and the implications it has for promoting rifting and magmatism in the EARS Western Rift.
Mineralogical composition data for olivine and clinopyroxene provide additional constraints on melting depth and magma evolution. The majority of olivine phenocrysts are highly magnesian (Fo87-91); crystals in equilibrium with the host lava indicate crystallization temperatures of 1200-1400 °C. Multiple populations of clinopyroxene are commonly present, and petrographic observations and thermobarometry indicate that some are xenocrysts. Clinopyroxene-liquid thermobarometry suggests crystallization pressures of 4-12 kbar; some grains crystallized at multiple depths. Trace element analyses of zoned crystals place constraints on metasomatism in the source of the Bufumbira magmas. Variations in composition and textures are used to constrain conditions of petrogenesis, source compositions, and the extent and variability of metasomatism in the region. Mineral/glass composition data from a suite of associated pyroxenite xenoliths provide additional constraints on magmatic sources and evolution.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V24A..02P
- Keywords:
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- 1207 Transient deformation;
- GEODESY AND GRAVITYDE: 9305 Africa;
- GEOGRAPHIC LOCATIONDE: 9604 Cenozoic;
- INFORMATION RELATED TO GEOLOGIC TIMEDE: 8178 Tectonics and magmatism;
- TECTONOPHYSICSDE: 8185 Volcanic arcs;
- TECTONOPHYSICSDE: 8415 Intra-plate processes;
- VOLCANOLOGYDE: 8488 Volcanic hazards and risks;
- VOLCANOLOGYDE: 8499 General or miscellaneous;
- VOLCANOLOGY