PHENOCRYSTIC FLUORITES IN PERALKALINE RHYOLITES, OLKARIA, KENYA RIFT-VALLEY

The first occurrence of phenocrystic fluorite in a peralkaline rhyolit e is reported from Quaternary lavas (agpaitic index 1.11-1.37) erupted from the Greater Olkaria Volcanic Complex, Kenya. Fluorite compositio ns are almost stoichiometric, with 0.65-1.49 Sigma REE + Y oxides. Rel ative to coexisting glass (melt), the fluorites are enriched in REE, Y and Sr, are strongly depleted in all other trace elements and concent rate MREE relative to LREE and HREE. Y partitions into fluorite more s trongly than the HREE. Fractionation of fluorite in its modal abundanc e would not significantly affect the REE abundances in the residual li quids, nor result in stronger Sr depletion. The P, T and composition c onditions which stabilize fluorite in rhyolitic magmas are still obscu re.