Igneous activity forming hybrid rocks and leucogranites in the Obara area, San'in zone, Southwest Japan

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  • 山陰帯島根県尾原に分布する混成岩および優白質花崗岩の火成活動
  • サンインタイ シマネケン ビ ゲン ニ ブンプ スル コンセイガン オヨビ ユウ ハクシツ ハナオカガン ノ カセイカツドウ

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Abstract

Cretaceous to Paleogene plutonic rocks are widely distributed in the San'in zone of the southwestern Japan arc. They are mainly classified as magnetite series granitoids and are associated with Mo mineralization. A unique plutonic association of heterogeneous mingled rocks and leucogranites is present throughout the San'in zone. The mingled rock is locally named “hybrid rock” and is often regarded as an important host rock for Mo and/or Fe ore deposits.<br>One of these associations is present throughout the Obara area, eastern Shimane prefecture. Here, plutonic rocks are divided into three types: quartz-diorite, tonalite, and leucogranite. Mingled quartz-diorite and tonalite assemblages are present over a wide area (ca. 12 km2) and are intruded by the leucogranites. Rb–Sr whole-rock isochron dating of the leucogranite gives an emplacement age of 61.2 ± 1.8 Ma, indicating that main igneous activity in the Obara area ceased in the early Tertiary. The tonalite contains higher Na concentrations (average Na2O of 4.35 wt.%) than the quartz-diorite and leucogranite rocks; in addition, the leucogranite is characterized by a wide range in Rb/Sr ratios and by large negative Eu anomalies in chondrite-normalized rare earth element (REE) spidergrams. Geochemical modeling indicates that both the quartz-diorite and tonalite magmas fractionated hornblende and plagioclase, whereas the leucogranite magma underwent significant feldspar differentiation during crystallization. These differences led to the variations in Rb/Sr ratio and Eu anomalies between the various plutonic suites. The higher Na contents of the tonalite may suggest that the magma was formed by partial melting of mafic crust, leaving a plagioclase-poor restite. The area hosts significant Fe mineralization but not Mo, suggesting that oxidation occurred during cooling of the plutonic rocks, leading to oxygen fugacity conditions that enabled the precipitation of Fe minerals, but not preferred to Mo mineralization.

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