Magma dynamics inferred from feldspar zoning in post-caldera phonolites, Suswa Volcano, Kenya Rift, Africa.

摘要

Suswa Volcano (0.4 M.a.) is located in the Kenya Rift. It is composed of trachytes that culminated in a caldera, followed by phonolite flows. Analyzing petrographic textures, crystal morphologies, zoning, and phenocryst compositions, Suswa magma dynamics that occurred within the magma chamber can be determined. Trachyte and phonolite feldspars and other phenocrysts were analyzed with an electron microprobe. The trachytic feldspars are not zoned and have a K-rich composition (An1-3Ab50-60Or 35-45). The phonolitic feldspars fall into three groups defined by composition and texture. Group A includes K-rich feldspars with euhedral overgrowths. Group B contains anorthite-rich (An8-12Ab67-70Or 18-23) cores with euhedral overgrowths. Group C has feldspars with intermediate (between A and B) composition (An5-8Ab67-69Or22-26 ), with mineral and melt inclusions, and no euhedral overgrowths. Other phenocrysts include olivine (Fa90, Fa68), clinopyroxene (hedenbergite, diopside), and oxides (ulvospinel). The zoning and bimodal core compositions for phonolites imply a mixing process.