Regional variation of whole rock chemistry of the Miocene felsic igneous rocks in the Kii Peninsula, southwest Japan

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  • 紀伊半島中新世珪長質火成岩類の全岩組成の広域的変化
  • キイ ハントウ チュウシンセイ ケイチョウシツ カセイガンルイ ノ ゼンガン ソセイ ノ コウイキテキ ヘンカ

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We present newly obtained whole rock major and trace element compositions of the Miocene felsic igneous rocks in the Kii Peninsula. The felsic igneous rocks include I-and S-type granitic rocks in the Outer Zone and felsic member of the Setouchi Volcanic Rocks. Regional variation of whole rock composition discussed for data set including previously reported one suggests following observations: (1) Granitic rocks in the Outer Zone are characterized by relatively constant chondrite-normalized middle to heavy REE abundance, whereas rhyolitic rocks of the Setouchi Volcanic Rocks show strongly HREE-fractionated patterns. (2) Distribution of S-type and I-type felsic rocks forms trench parallel zones and runs SW-NE direction, which is almost parallel to those of the high-Mg andesites in the Outer Zone. This direction coincides with the strikes of the contour lines of Wadati-Benioff Zone beneath eastern Kii peninsula. S-type felsic rocks in the Outer Zone and rhyolitic rocks of Setouchi Volcanic Rocks are peraluminous with high SiO2 and K2O contents, which are expected to be produced by melting of sedimentary rocks. REE abundances of sediment melt were modeled based on the melt fractions and residual mineralogies of Pati–o Douce and Beard's (1995) melting experiments. Differences in the REE compositions of these two rock types are explained by simply the difference of the melting depth where S-type felsic rocks were formed at shallower level with plagioclase-rich residue, and rhyolitic rocks of the Setouchi Volcanic Rocks at deeper level with garnet-rich residue. REE abundance model based on the melt fraction and residual mineralogy of Beard and Lofgren's (1990) melting experiments of basaltic rocks suggests that source rock of the I-type granitic magma should be enriched in light REE.<br>

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