隠岐・島後の銚子・有木川流域に分布する隠岐変成岩の岩相記載と変成履歴

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Clockwise P-T path was constructed using petrographic and mineralogic data of high-T/low-P metamorphic rocks from Oki-Dogo Island, southwest Japan. The studied area is composed predominantly of Cretaceous pelitic gneiss with minor mafic and calcareous gneisses in upper amphibolite to lower granulite grade. There occur several types of leucocratic veins or pods associated with pelitic gneiss. Sillimanite-bearing leucosome (L2) is identified as thin and folded bands in Al-rich pelitic gneiss. In the L2 Ieucosome, sillimanite is present as needle-like aggregates associated with K-feldspar and quartz. We inferred the following dehydration melting reaction [1] for the assemblage.
Ms + Qtz + Pl - Sil + Kfs + L [1]
Garnet-bearing leucosome (L3) , which is younger than L2 because of its less-deformed character, Is composed of quartz, K-feldspar, plagioclase, and garnet. Garnet typically includes quartz and biotite, suggesting a progress of the following melting reaction [2] ;
Bt + Sil (And) + Qtz + Pl - Grt + Kfs + L [2]
Reactions [1] and L2] are prograde reactions and both L2 and L3 are considered to be products of prograde metamorphism. Available experimental data suggest that prograde pressure was about >4 kbar. Lack of Opx in pelitic gneiss suggests that peak temperature weLS Iower than that of the beginning of biotite dehydration melting to Opx + K-feldspar (T<820℃).
Garnet in pelitic gneiss was subsequently affected by retrograde breakdown reaction to biotite+sillimanite (andalusite) (reaction [3])
Grt + Kfs + H2O - Bt + Sil (And) + Qtz [3]
Petrographic and geothermobarometric data indicate that the reaction [3] took place around the boundary between sillimanite and andalusite fields. Our data therefore suggest that pelitic gneiss from Oki-Dogo Island suffered crustal thickening toward mid-crustal level followed by rapid uplifting and subsequent cooling.
Keywords : Metamorphic rock, Petrography, P-T history, Geothermobarometry, Oki-Dogo