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

日本东北部弧第四纪火山锋下方岩浆成因过程的岩石学和地球化学阐明。

基本信息

批准号：
63540643
负责人：
FUJINAWA Akihiko
金额：
$ 1.15万
依托单位：
Ibaraki University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1988
资助国家：
日本
起止时间：
1988 至 1989
项目状态：
已结题

项目摘要

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

低碱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。本研究对秋田驹岳的拉斑玄武岩和钙碱性熔岩进行了主量、微量和稀土元素成分分析。位于日本东北部火山前缘的船形火山、藏王火山、安达太拉火山和那须火山，旨在阐明火山的演化过程。拉斑岩岩浆和钙碱性岩浆以及两种岩浆之间的成因关系，还研究了阿达塔拉火山两个系列熔岩的锶同位素组成和矿物学数据，以及上述数据。对于拉斑斑岩岩套，变化为通过观察到的 Fe/Mg (Ca/Na) 比率的分步结晶，成功地模拟了主量、痕量和稀土元素。镁铁质斑晶（斜长石）与整个岩石的分布支持了这一模型，相比之下，钙碱性套件的成分变化不仅可以通过分步结晶来解释，还可以通过其他过程来解释，例如（1）镁质的间歇混合。岩浆，(2) 同化，和/或 (3) 掺入一些富含重稀土矿物的液体。晶体分馏模型成功地预测了这一点。主要和大多数微量元素，但未能解释镍和铬的不规则变化，或预测酸性熔岩的轻稀土元素/重稀土元素比率，并且在一些斑晶中观察到反向分区和不平衡矿物组合。推断拉斑斑岩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，轻稀土元素/重稀土元素、Rb/Ba、Zr/Nb和^ 87 Sr/^ 86 Sr比率可能是角闪岩成分下地壳的深熔以及随后并入地幔衍生的玄武岩熔体中。解释钙碱性母岩浆中高Rb/Ba、Zr/Nb和低^<87>Sr/^<86>Sr值的过程，因为：（1）日本东北部下方的下地壳被认为具有角闪石成分；(2) 这种含有斜长石和角闪石的源材料应该具有高体积 D_<Ba>/D_<Rb> 和 D_<Nb>/D_<Zr> 值相对于幔源尖晶石橄榄岩较少。