Abstract

The Quaternary Ethiopian volcano Fantale is a central volcano with a summit caldera. The most recent flows are pantellerites characterized by high Rb/Sr ratios. These cover earlier weakly peralkaline rhyolites and trachytes.

Results of Sr87/Sr86 analyses on selected samples from the volcano are presented. The whole-rock results on a basalt, two trachytes, and nine more silicic rocks show a correlation of the Sr87/Sr86 ratio with Rb/Sr. Eleven of the rocks fall on an isochron giving an apparent age of 2.6 m.y. However, this does not correspond to the age of the volcano. Data from separated feldspar phenocrysts and host residual glassy silicic matrix show that the feldspars are enriched in radiogenic strontium relative to the matrix.

Chemical analyses of the lavas suggest that the composition of the upper part of the sub-volcanic magma chamber at Fantale changed progressively from that of a trachyte to that of a weakly peralkaline rhyolite, finally becoming pantelleritic. It is suggested that the isotopic composition also changed with increasing enrichment in Rb/Sr and Sr87/Sr86, thus accounting in a general way for the isochron relation noted above. Data from the Fantale tuff, a 2 km3 welded ash flow, indicates that the Fantale magma chamber was chemically zoned. The contrasting isotopic compositions of the feldspar phenocrysts and matrix support the concept of feldspar fractionation in this zoned magma chamber. It is suggested that the phenocrysts crystallized at a high level in the magma chamber and then fell into a less radiogenic, less fractionated environment.

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