Itinome-gata and Sannome-gata volcanoes,northeastern Japan,are characterized by the eruption of mafic to salic andesiteof 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 seemslikely that this is the first discovery of upper mantle peridotite xenoliths in calc-alkalineandesite.This andesite is characterizedby presenceof hornblendeand biotite phenocrysts.
Three representativebasaltsand five andesiteshave been analyzedfor major elements by a conventional method, and for rare earth element sand Ba by the isotope dilution method.
Major element variations of the basalt and andesitesuite follow a typical calc-alkaline trend with increasingfractionation. In addition, there are no essentialdiferences of Ba and REE concentrations and chondrite-normalized patterns between them. Furthermore, rations in basalt and the most salic andesite are nearly the same(0.7030and0.7033, respectively).
It is possible to conclude that the andesite magmas are produced by fractional crystallization of basalt magmabasedon mineral assemblages,major elementvariations and strontium isotope ratios. However, REE and Ba concentrationsand patterns are not consistentwith this hypothesis.Accordingly,it is more probablethat andesiteand basalt magmasare formed independentlyby nearly the samedegreeof partial melting of hydrous and less hydrous parts of upper mantle peridotite, with increasingtemperature, at the depthsof 40 to 60 km.