0Major-element, Rb, Sr, Y and Zr composition and origin of volcanic rocks from the Kikai Caldera, south of Kyushu

Abstract

X-ray fluorescence analysis was carried out for major elements on 13 volcanic rocks and for Rb, Sr, Y and Zr on 24 volcanic rocks from the Kikai Caldera. The 30 available major-element analyses indicate that the rocks constructing the land portions of Kikai Caldera are tholeiitic and calc-alkalic and their SiO2 contents show two separate ranges, 50-60% and 69-73%. No clear correlation is observed between the stage of caldera evolution to which the samples belong and the chemical composition of the samples. Most of all the samples seem to have been more or less affected by mixing of magmas with mafic and felsic compositions. All the analyses are in limited ranges of ratios between highly incompatible elements ; K/Rb=270-340 and Zr/Rb=2.2-2.9. This implies that all the Kikai magmas were ultimately derived from a single source. The rhyolites are likely to be derivatives by partial melting of materials formed from a batch (or batches) of basalt magma emplaced within the crust ahead of others. The Rb/Y-Zr/Y systematics strongly suggests that the basalt source region was enriched in Rb by mantle metasomatism.