隠岐島後における末期中新世，隠岐アルカリ火山岩類の地質とマグマ供給系

Abstract

A huge volume of lavas and pyroclastic rocks were erupted in Oki-Dogo island in the latest Miocene. They compose the Oki Alkaline Volcanic Group, the Hei Trachyte Group, the Tsuzurao Volcanic Group and the Tokage Trachyte Group. We examine the geology, petrography and geochemistry mainly for the Oki Alkaline Group, and present a magma plumbing story in the latest Miocene age. The rocks from the Oki Alkaline Volcanic Group are shoshonite, trachyandesite, trachyte and rhyolite, based on their chemical composition and stratigraphy. The trachytes are divided into Trachyte-1 (Nb 35-50 ppm) and -2 (Nb 49-75 ppm), and the rhyolites are divided into Lower Rhyolites (Moderate-SiO2: 71.4-73.6 wt%), Middle Rhyolites (High-SiO2: 74.7-75.7 wt%), and Upper Rhyolites (Low-SiO2: 70.7-71.6 wt%). Lower Rhyolites, Trachyte-1 were formed at the early volcanic stage. Middle Rhyolites, Trachyte-1 were formed at the middle stage, and Upper Rhyolites, Trachytes-1 and -2, trachyandesite and shoshonite were formed at the later stage. Rhyolitic lava domes with different compositions and feeder dikes of trachyte corresponding to individual volcanic vents occur at a number of places. This may indicate that the eruptions took place from several different vents aligned along a circular fracture. Lower Rhyolites were derived from the lower crust, and Middle Rhyolites were derived from middle crust by partial melting processes. While, Upper Rhyolites, Trachyte-2 and trachyandesite were generated by fractional crystallization from shoshonite magma. The heat source triggered melting of the crusts is inferred to be latent heat of crystallization in the mafic magma. These rhyolite to shoshonite magmas could have ascended through the ring fractures formed by the doming event, and have consolidated as the rocks composing the Oki Alkaline Volcanic Group. After the formation of this group, Cauldron collapse, and it is followed by the eruption of the Hei Trachyte Group and the Tsuzurao Volcanic Group. In the early Pliocene, The tectonic environment of the crust changed from a compressive to an extensional stress field. Then the eruption of the Omine Group and the Tokage Trachyte Group took place.