Genesis of Late Cretaceous-Paleogene Granitoids with Contrasting Chemical Trends in the Chubu District, Central Japan

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  • 日本の中部地方の対照的に異る化学的性質を持つ白亜紀後期ー古第三紀花崗岩類の成因

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Abstract

Late Cretaceous to Paleogene rhyolites (11 samples) and granitoids (Shirakawa 29, Toki 7, Naegi 9, Ryoke 10; total 55 samples), which have high intial Sr ratios, were analyzed by XRF and ICP-MS for 11 major elements and 32 trace elements. Granitoids of the Shirakawa area intruding the Nohi Rhyolites in the Hida metamorphic terrane, belong to I-type magnetite series and contain commonly mafic enclaves. They are composed of a high Na2O group of monzodiorite to granodiorite and a low-Na2O group of biotite granites. The former is rich in mafic and calcic components, and was generated in mafic igneous source rocks of the Hida metamorphic terrane. The latter is felsic and leucogranitic, yet its Rb/Sr is not high enough to be a fractionated magma. The leucogranite, typically of the Hirase body, could be a minimum melt generated from an intermediate igneous source. The Shirakawa granitoids are depleted in Y and HREE, implying the existence of garnet and hornblende in the source region. Late Cretaceous-Paleogene granitoids occurring in the Mino sedimentary terrane belong to I -type ilmenite-series composed of high-level plutons of the Toki and Naegi areas and intermediate-level plutons of the Ryoke granitoids. These granitoids are reduced and per-aluminous implying genetic connection with sedimentary and less igenous sources. The Toki and Naegi bodies are composed mostly of biotite granite, intruding discordantly, and are rich in lithophile elements. The Naegi granite is especially high in Rb, Y, Th, and U and its Rb/Sr ratio is the highest among the studied rocks. Its REE pattern is flat with high HREE and a strong Eu anomaly, which could be considered as fractionated I type. The Ryoke granitoids are mostly biotite granite but horblende-biotite granodiorite is also common. They are less fractionated and have own geochemical characters compared with the Toki and Naegi granites. The studied granitoids have chemical compositions reflecting basically their basement characteristics, and also partly their degree of the magmatic fractionation.

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