Itinome-gataa nd Sannome-gatav olcanoes,n ortheasternJ apan, are characterizedb y 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 characterizedb y presenceo f hornblendea nd 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 diferences of Ba and
REE concentrations and chondrite-normalized patterns between them. Furthermore,
5tE775.8ra0t iosi n basalta nd the most salica ndesitea re nearlyt he same( 0.7030a nd 0.7033,
respectively).
It is possible to conclude that the andesite magmas are produced by fractional
crystallization of basalt magma based on mineral assemblages,m ajor 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.