東北日本,安達太良火山におけるソレアイト,カルクアルカリマグマ系列 : その進化メカニズムと成因関係(<特集>火山のモデル(II))

抄録

A petrologic model concerning generation and evolution processes of coexisting low-alkali tholeiitic and calc-alkaline magmas at Adatara volcano is proposed. For tholeiitic suite, major-element variations are reproduced by addition-subtraction models of phenocrystic phases. Also, the variations of trace- and rare earth-elements and Sr isotopic compositions are consistent with the fractionation hypothesis. There are mineralogical observations supporting this hypothesis including : (1) positive correlations of Fe/Mg (Ca/Na) ratios of mafic phenocrysts (plagioclase) with those of the host lavas ; (2) increase of Usp content in the core of magnetite phenocrysts with increasing SiO₂ of host lavas ; and (3) olivine disappearance in the lavas containing > 55% SiO₂. In contrast, similar fractionation models to those used for the tholeiitic suite revealed that compositional variations for calc-alkaline suite are not explained solely by fractional crystallization ; 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) may have operated as additional processes. Also, mineralogical data are compatible with this view. Tholeiitic parental magma is distinctively poor in SiO₂ and incompatible elements, low in light-REE/heavy-REE, Rb/Ba, Zr/Nb and high in ⁸⁷Sr/⁸⁶Sr ratios, relative to the calc-alkaline parental magma. Further, the low-alkali tholeiitic magma has evolved under conditions of higher temperature and lower oxygen fugacity than the calc-alkaline magma. Different degrees of melting in a common peridotitic source can successfully predict the differences in incompatible-element concentration levels and light-REE/heavy-REE ratios between the two types of parental magmas, but it is inconsistent with the differences in Rb/Ba, Zr/Nb, or ⁸⁷Sr/⁸⁶Sr. Since lower crust beneath Northeast Japan is believed to have amphibolitic or gabbroic composition with ⁸⁷Sr/⁸⁶Sr as low as 0.7030, anatexis of the lower crust and subsequent incorporation of partial melt into mantle-derived basaltic magma may be a possible process to explain high Rb/Ba, Zr/Nb and, possively, low ⁸⁷Sr/⁸⁶Sr in the calc-alkaline parental magma.