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(<特集>紀伊半島における中新世火成作用とテクトニクス)  [in Japanese] Regional variation of whole rock chemistry of the Miocene felsic igneous rocks in the Kii Peninsula, southwest Japan(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

紀伊半島の中新世珪長質火成岩について全岩主成分,微量元素組成を報告し,その広域的特徴について検討した.その結果,外帯花崗岩質岩には重希土類元素の枯渇が見られず瀬戸内火山岩類の流紋岩は重希土類元素に極めて枯渇すること,外帯域のSタイプ,Iタイプ花崗岩の分布境界は北東・南西方向であり,高マグネシア安山岩の分布および現在の和達ベニオフ面の等深度線と沿うことを見出した.さらに外帯Sタイプ珪長質火成岩および瀬戸内火山岩類の流紋岩の希土類元素組成について,堆積岩の部分融解実験に基づくモデル計算を行い,融解が起こった深度の違いにより,前者は斜長石に富む溶融残渣を,後者はざくろ石に富む溶融残渣を持つことで,説明できることを示した.外帯Iタイプ花崗岩の希土類元素組成についても玄武岩質岩の部分融解実験に基づくモデル計算を行い,やや軽希土類元素に富む起源物質の部分融解で説明できる事を示した.

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 SiO_2 and K_2O 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 Patino 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.