Petrological and geochemical elucidation of magma genetic processes beneath the Quaternary volcanic front in Northeast Japan arc.

低碱性拉斑玄武岩岩浆与日本东北部火山前缘的钙碱性岩浆密切相关。在本研究中，主要，微量和稀土元素的组成分析拉斑玄武岩和钙碱性熔岩秋田驹岳，船形，藏王，Adatara和那须火山位于火山前，日本东北部。为了阐明拉斑玄武岩和钙碱性岩浆的演化过程及其成因联系，本文还对阿达塔拉火山两套岩浆的Sr同位素组成和矿物学资料进行了研究。和稀土元素成功地模拟了所观察到的斑晶相的分离结晶。镁铁质斑晶（斜长石）的Fe/Mg（Ca/Na）比值与全岩的Fe/Mg（Ca/Na）比值呈正相关，支持这一模型。相反，钙-a的成分变化 ...更多信息 碱性岩套的形成不仅是由于结晶分离作用，还可能是由于镁质岩浆的间歇混合作用、同化作用和（或）某些重稀土富集矿物分馏后的液体的混入作用。晶体分馏模型成功地预测了主要的和大多数的微量元素，但没有考虑到不规则变化的Ni和Cr，或预测轻稀土/重稀土比酸性熔岩。部分钙碱性岩样斑晶间存在反分带和不平衡矿物组合，推测拉斑质母岩浆和钙碱性母岩浆具有明显的SiO_2和不相容元素含量、轻/重稀土、Rb/Ba、Zr/Nb和<87>Sr/<86>Sr比值。斜长角闪岩成分的下地壳深熔作用以及随后并入幔源玄武质熔体中可能是解释钙碱性母岩浆中高Rb/Ba、Zr/Nb和低^ Sr/^ Sr值的一个可能过程<87><86>，因为：（1）日本东北部下方的下地壳被认为具有斜长角闪岩成分;（2）相对于幔源尖晶石橄榄岩，这种含斜长角闪岩和角闪石的源物质应该具有较高的体积D_<Ba>/D_<Rb>和D_<Nb>/D_<Zr>值。少

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 … More 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