INTENSIVE PARAMETERS FOR THE TRACHYTE-PANTELLERITE ASSOCIATION DEDUCED FROM MINERAL CHEMISTRY: THE EBURRU VOLCANIC COMPLEX, KENYA RIFT

The Eburru volcanic complex is located in the Kenya Dome in the middle portion of the Kenya rift. Volcanism in the complex spanned from 1.2 Ma to approximately 100 years ago, and the area is an active geothermal field. The complex consists of trachyte and pantellerite lava and ash-flow tuffs. Mineral phases from the trachyte and the youngest pantellerite have been analyzed by electron microprobe. Samples contain potassium feldspar and minor quartz. The early trachyte has a ferromagnesian mineral assemblage of fayalite + ferrohedenbergite + ilmenite + magnetite + aenigmatite. The younger trachyte ferromagnesian assemblage is fayalite + ferrohedenbergite + ilmenite + aenigmatite. The pantellerites show three different ferromagnesian mineral assemblages, and all have amphibole phenocrysts. The three assemblages are: 1) fayalite + ferrohedenbergite + ilmenite + aenigmatite + amphibole; 2) ferrohedenbergite + ilmenite + aenigmatite + amphibole; and 3) amphibole alone.

Ferrohedenbergite compositions vary from early trachyte to pantellerite, with increasing SiO2 and Na2O, and decreasing CaO. This trend correlates with bulk rock silica values and peralkalinity. QUILF95 geothermometry has been used to estimate the temperature and oxygen fugacity for the samples. Trachytes show higher temperature and oxygen fugacity than pantellerites. The appearance of amphibole (ferro-richterite) in pantellerite agrees with this observation. Pyrrhotite has been found in all samples; its appearance suggests reduced and high sulfur fugacity conditions during crystallization of these rocks. Low oxidation state is a characteristic of other lavas in the Kenya Dome and implies that the magmas were either derived directly from the mantle or represent melting of recent underplated material. In either case, whole-rock and mineral chemistry indicate extensive crystal fractionation as a dominant process for forming these peralkaline rocks.