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The Spatial variation of initial $$^{87}$$Sr/$$^{86}$$Sr ratios in the Toki granite, Central Japan; Implications for the intrusion and cooling processes of a granitic pluton

土岐花崗岩体のストロンチウム同位体比初生値の空間分布から推定する花崗岩体の貫入定置過程及び冷却固化過程

湯口 貴史; 鶴田 忠彦; 濱 克宏; 西山 忠男*

Yuguchi, Takashi; Tsuruta, Tadahiko; Hama, Katsuhiro; Nishiyama, Tadao*

本稿は、中部日本に位置する土岐花崗岩体からストロンチウム同位体比初生値の空間分布を示し、それを用いて岩体の貫入・定置過程及び冷却固化過程を提示する。7本のボーリングコアから採取された14試料に基づき、土岐花崗岩体の全岩Rb-Sr年代は71.04Maとの結果を得た。この年代は土岐花崗岩体が貫入・定置した時代を示す。また土岐花崗岩体のストロンチウム同位体比初生値は0.708507から0.709409までの幅(不均質性)を持つ。ストロンチウム同位体比初生値の不均質性と花崗岩体内の化学組成の不均質性との比較検討の結果、土岐花崗岩体は貫入・定置の際に、地殻の混成作用並びに分別結晶作用が複合的に生じて形成されたことを明らかにした。また地殻の混成の影響の大きい場所から冷却されたという冷却固化過程に関しても言及を行った。

The spatial variation in initial $$^{87}$$Sr/$$^{86}$$Sr ratios (SrI) in the Toki granite, Central Japan, shows heterogeneity ranging from 0.708942 to 0.710069, which provides information on formation processes of plutons. The Toki granite has three mineralogy-based rock facies: muscovite-biotite granite (MBG), hornblende-biotite granite (HBG) and biotite granite (BG). Large SrI values were found to be distributed at the western margin (west MBG) and the lithologically central region (central BG), while small SrI values were found at the northeast margin (northeast MBG). Regions with high and low Sr concentrations were also found in the Toki granite. In the Sr-rich samples, SrI (0.708942-0.709789) increases with 100/Sr (0.7-1.5). This geochemical trend extends towards the country sedimentary rocks of the Mino Terrane, which can be interpreted to result from assimilation and fractional crystallization (AFC) between the original granitic magma and the Mino sedimentary rocks. The SrI values in the Sr-rich regions show a correlation with the Alumina Saturation Index (ASI). In particular, the west MBG, with large SrI values, is classified as a peraluminous granitoid, suggesting that the western margin of the pluton was strongly affected by assimilation during the intrusion process. The Sr-poor samples are present both in the central BG, with large SrI values, and in the northeast MBG, with small SrI values. The Sr-poor samples have small ASI and large differentiation indices, indicating that the central BG and the northeast MBG were generated either by different AFC process with different amounts of contaminants or by the intrusion and fractionation of different source magma with different SrI values. Overall, the geochemical spatial variations found in the Toki granite can be explained by various degrees of assimilation and fractional crystallization in the magma chamber and/or multi-stage intrusions with different degrees of crystallization of plural source magmas.

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パーセンタイル:51.14

分野:Mineralogy

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