[S-CG73] [JJ] 岩石・鉱物・資源

[SCG73-P21] 愛媛県南部、中新世御内花崗岩質岩体にみられる苦鉄質火成包有岩についての岩石学的研究

塩田 恵美²、山崎 楓真²、*齊藤 哲¹ (1.愛媛大学大学院理工学研究科、2.愛媛大学理学部)

キーワード：御内岩体、花崗岩、苦鉄質火成包有岩

Mafic igneous enclaves with sub-angular shape are commonly occurred in the Miocene Miuchi granitoid pluton, southern Ehime Prefecture. The mafic enclaves (61-64 wt.% SiO₂, 0.5-2.6 wt.% K₂O) are composed mainly of plagioclase, biotite, quartz and opaque minerals. Acicular apatites are common in the mafic enclaves. Intergrowth texture of biotite and quartz/plagioclase are locally observed in the biotite-rich mafic enclaves. K₂O contents of the mafic enclaves increase with increasing SiO₂. Orthopyroxene, locally surronded by biotite, occurs in the mafic enclave with the lowest SiO₂ and K₂O contents. Biotite-rich reaction rim is observed between the orthopyroxene-bearing mafic enclave and host granite. The host granite samples surrounding the mafic enclave are relatively depleted in the K-feldspar (granodiorite in composition) compared to the dominant rock type of the Miuchi granitoid pluton (monzogranite - granite in composition) (67-78 wt.% SiO₂, 3.3-6.5 wt.% K₂O). The petrographic and geochemical characteristics of the mafic enclaves are suggestive of interaction with host granite magmas through hydration crystallization reaction: hydrous granitic melt + pyroxene + Fe-Ti oxides ± calcic plagioclase = biotite + quartz ± sodic plagioclase (e.g. Bard et al. 2005, J. Geol.).