並列タイトル等インドネシア中部ジャワ、スンドロ火山の岩石学的および地球化学的研究 : 単一火山の成長におけるマグマ発生源プロセスおよび分化作用の時間変遷
一般注記Subduction zone resembles site of arc crustal building through complex magmatic processes involving subduction components such as mantle wedge, subduct ing slab, and overriding crustal in which all of them might chemically and/or physically change through time. Thorough understanding of volcanic arc must include careful examin ations of the magmatism of individual volcanoes including reconstruction of its magma supply systems through recorded past activities. D uring their lifetime, many individual volcanoes experienced magmatic compositional change including their isotopic compositions and incompatible elements related to the slab contribution. This compositional change might occur within the same magma series , with similar mineral assemblages , through time reflecting various processes both in the intra -crustal level and deep mag ma source level. However temporal variations of these processes are not well understood yet, mainly due to lack of combination of detailed petrological and geochemical study with thorough field observation and dense resolution of the precise age determinat ion data . This is mainly the case in Sunda arc despite of the well -known strong signature of sediments and wide range isotopic compositions of the volcanic products. This study reported new Sr-Nd -Pb radiogenic isotope ratios in addition to the complemen ting whole rock geochemistry and trace elements combined with mineral chemistry of representative rocks of Sundoro volcano in the central Java sector of Sunda arc . Collected s amples cover stratigraphically well -constrained volcanic products from the past 34 ka activities ensuring useful information to our understanding in the temporal variations of differentiation and source process of magmatism in an arc volcano. The rocks of the volcano span from basalt (SiO 2 51. 5 wt. %) to andesite (62.9 wt.%) and are d ominated by basaltic andesite. Least evolved samples contains MgO less than 6 wt.% and is considered as evolved basalt. Samples can be divided into three magma types A, B, and C on the basis of isotopic compositions. Relatives to each other, A -type has low 87 Sr/ 86 Sr and Pb- isotopic ratios. B -type has medium 87 Sr/ 86 Sr and Pb- isotopic ratios. C -type has high 87 Sr/ 86 Sr and Pb- isotopic compositions . Open system differentiation processes were recorded in the Pb isotopic compositions change through progressive differentiation indices. Shifting of wide range bimodal plagioclase core compositions from mafic rich in the basaltic samples to felsic rich in the andesitic one suggests for magma mixing process. Distinct evolution trends of the isotopic compositions of the three magma types are reflected as well in the ratios of incompatible elements such as Ba/Zr, La/Yb, Th/Zr, and Ba/La . The three discrete evolution trends suggest for major role of three discrete couple of mafic and felsic end -member magmas. Felsic end -member magmas cannot be produced by simple fractional crystallization of respected mafic end -member magmas and probably are derived from different source . However, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotopic ratios that remain unchanged through differenti ation process and discrete variations of each magma types in the isotopic compositions and ratio of incompatible elements allow us to further discuss the significance of differentiation in the deep magma source to the genesis of Sundoro magma. 87 Sr/ 86 Sr and Pb -isotopic ratios show positive correlation to the proxies of subducted slab contributions and suggest for significance of subducted slab materials to the petrological and geochemical variation of Sundoro rocks . W e have identified the existence of three distinct slab -derived materials containing various sediment contributions approximately 50%, 55%, and 60% which correspond to generation of magma A -, B -, and C -type, respectively. These slab -derived materials were added to mantle wedge in various rates wi thin the range of 1- 2% . Temporal change of magma type shows the existence of A -type in 20- 9 ka and remarks co -existence of A - and B -type in the period of 14- 17 ka before the abrupt change to C -type since 9 ka. S upply of mafic magma show s decreasing -trend in the active period of A -type magma and relatively constant for C -type magma. Genesis of the three magma types in the source level is controlled by continuous dehydration and partial melting generating three distinct slab -derived fluxes. Time interval bet ween these three fluxes is about 3 -8 ky and shows increasing portion of sediment contribution and slab flux rate through time. Further application of these approaches to dataset of Merapi volcano revealed sequentially increasing rate of slab -derived flux to the magma genesis which might characterize the unusual arc magmatism beneath volcanic front region of central Java sector of Sunda arc.
(主査) 教授 中川 光弘, 教授 橋本 武志, 准教授 栗谷 豪, 助教 吉村 俊平
理学院（自然史科学専攻）
コレクション（個別）国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
受理日（W3CDTF）2017-07-03T04:10:06+09:00
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