| Research Abstract |
Low-alkali tholeiitic magmas are closely associated with calc-alkaline magmas at the volcanic front of Northeast Japan. In the present study, major-, trace- and rare earth-element compositions were analyzed for tholeiitic and calc-alkaline lavas from Akita-Komagatake, Funagata, Zao, Adatara and Nasu volcanoes located at volcanic front, Northeast Japan. In order to elucidate the evolutionary processes of the tholeiitic and calc-alkaline magmas and also genetic relationship between the two magmas, investigated were also the Sr-isotopic compositions and mineralogical data of the two series of lavas at Adatara volcano, as well as the above mentioned data.
For tholeiitic suite, variations of major-, trace-, and rare earth- elements are successfully modelled by fractional crystallization of observed phenocrystic phases. Positive correlations of Fe/Mg (Ca/Na) ratios of mafic phenocrysts (plagioclase) with those of whole rocks support this model. In contrast, compositional variations for calc-a
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lkaline suite are not solely explained by fractional crystallization, but also by other processes such as (1) intermittent mixing of magnesian magmas, (2) assimilation, and/or (3) incorporation of liquid which has suffered fractionation of some heavy-REE-enriched mineral(s). The crystal fractionation model successfully predicts major- and most of the trace-elements but failed to account for irregular variations of Ni and Cr, or predict light-REE/heavy-REE ratios for acidic lavas. Also reverse zoning and disequilibrium mineral assemblages are observed among phenocrysts in some of the calc-alkaline samples.
The tholeiitic parental magma and calc-alkaline parental magma are inferred to have distinct SiO_2 and incompatible elements contents, light-REE/heavy-REE, Rb/Ba, Zr/Nb, and ^<87>Sr/^<86>Sr ratios. Anatexis of lower crust of amphibolite composition and subsequent incorporation into mantle-derived basaltic melt may be a possible process to explain high Rb/Ba, Zr/Nb and low ^<87>Sr/^<86>Sr values in calc-alkaline parental magma, because: (1) lower crust beneath Northeast Japan is believed to have amphibolitic composition; and (2) such plagioclase- and hornblende-bearing source materials ought to have high bulk D_<Ba>/D_<Rb> and D_<Nb>/D_<Zr> values relative to mantle-derived spinel peridotites. Less
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