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-alkaline andesite.This andesite is characterizedby presenceof hornblende and biotite phenocrysts.
Three representative basalts and five andesites have been analyzed for 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 andesite suite follow a typical calc-alkaline trend with increasing fractionation. 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 element variations and strontium isotope ratios. However, REE and Ba concentrationsand patterns are not consistentwith this hypothesis.Accordingly,it is more probablethat 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 depthsof 40 to 60 km.
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-alkaline andesite.This andesite is characterizedby presenceof hornblende and biotite phenocrysts.Three representative basalts and five andesites have been analyzed for 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 andesite suite follow a typical calc-alkaline trend with increasing fractionation. 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 element variations and strontium isotope ratios. However, REE and Ba concentrationsand patterns are not consistentwith this hypothesis.Accordingly,it is more probablethat 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 depthsof 40 to 60 km.
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