Chevkinite-group minerals from salic volcanic rocks of the East African Rift

Electron microprobe analyses are presented of chevkinite-group minerals occurring as microphenocrysts in peralkaline rhyolites of the Greater Olkaria Volcanic Complex (Kenya) and as a groundmass phase in a peralkaline quartz trachyte lava from the Tarosero volcano (Tanzania), both in the East African Rift Valley. Their compositions conform closely to the formula: (REE, Ca, Th)4 Fe2+ (Fe2+, Al, Ti, Zr, Nb)2 Ti2 (Si4O22). Compared to published analyses of chevkinite-group minerals, the Olkaria phases are relatively enriched in Nb and the LREE; the Tarosero phase is more calcic and relatively Zr- and Nb-rich. The main substitution in the A site at Olkaria is Ca ⇌ Ce. The overall charge-balancing substitution seems to be (McDowell, 1979):Phenocryst/glass ratios are presented for Nb, REE, Sr, Th, U and Y in two, and Ba, Zr and Hf in one, Kenyan samples. Partition coefficients are lower in the more peralkaline rock, with the exception of Sr, which is higher. The lower values are consistent with a lower degree of polymerization of more peralkaline melts. The higher Sr value may be a function of Sr partitioning into phenocryst phases coexisting with chevkinite.

Rb–Sr dating of the Bokan Mountain granite complex and its country rocks

Seven out of 11 whole-rock samples of the Bokan granitic complex give a Rb–Sr isochron date of 151 ± 5 Ma (Late Jurassic) as a minimum age for the complex. This result is discordant with published Early to Middle Jurassic K–Ar and U–Pb dates for the same unique peralkaline rock suite. A Late Jurassic age is tenable if excess radiogenic Ar and a trace of xenocrystic zircon are postulated to explain the previously reported results and the U–Pb data are reinterpreted to give an age between 156 ± 6 and [Formula: see text]. Granitic country rock for the complex is dated by Rb–Sr at 432 ± 19 Ma (Early Silurian), in agreement with published K–Ar dates.