Petrology and geochemistry of calc-alkaline andesite of presumed upper mantle origin from Itinome-gata, Japan

Itinome-gata and Sannome-gata volcanoes, northeastern Japan, are characterized by the eruption of mafic to salic andesite of the calc-alkaline series and high-alumina basalt, both of which contain small amounts of high-pressure diopside and forsterite megacrysts, lherzolite and websterite derived from the upper mantle, and gabbro and amphibolite xenoliths from the lower crust (about 20 to 30 km in depth). It seems likely that this is the first discovery of upper mantle peridotite xenoliths in calc-alkaline andesite. This andesite is characterized by presence of hornblende and biotite phenocrysts.

Three representative basalts and five andesites have been analyzed for major elements by a conventional method, and for rare earth elements and Ba by the isotope dilution method.

Major element variations of the basalt and andesite suite follow a typical calc-alkaline trend with increasing fractionation. In addition, there are no essential differences of Ba and REE concentrations and chondrite-normalized patterns between them. Furthermore, Sr⁸⁷/Sr⁸⁶ ratios in basalt and the most salic andesite are nearly the same (0.7030 and 0.7033, respectively).

It is possible to conclude that the andesite magmas are produced by fractional crystallization of basalt magma based on mineral assemblages, major element variations and strontium isotope ratios. However, REE and Ba concentrations and patterns are not consistent with this hypothesis. Accordingly, it is more probable that andesite and basalt magmas are formed independently by nearly the same degree of partial melting of hydrous and less hydrous parts of upper mantle peridotite, with increasing temperature, at the depths of 40 to 60 km.