瀬戸内地域における高マグネシア安山岩の形成  [in Japanese] Generation of magnesian andesites in the Setouchi region  [in Japanese]

Rocks from the Middle Miocene Setouchi volcanic belt, of which a glassy variety was referred to as "sanukite" by Weinschenk (1891), are characterized by abundant bronzite crystals. The Setouchi volcanic rocks, ranging in composition from basalt to rhyolite, which can be termed the sanukite series, appear to have derived mainly from a parental high-magnesian andesite (HMA) magma. The Setouchi (sanukitic) HMAs differ from boninitic HMA in high contents of in compatible elements. The normalized in compatible element pattern for the Setouchi HMAs is similar to that for the tholeiites in the same region, except for Rb and K markedly enriched by addition of hydrous fluids, suggesting that the Setouchi HMAs were formed from a mantle source with the same in compatib1e element characteristics as that of the tholeiites. Cenozoic sanukitic HMAs are widespread in the Japanese islands; they occur in the Oligocene volcanics of Okushiri Island located in the Japan Sea off Hokkaido, and at westernmost Kyushu an andesite with orthopyroxene up to Mg# 90 and Cr_2O_3 1.25% is found in the late Miocene to early Pliocene volcanics. However, HMAs are most abundant in the Setouchi volcanic rocks of about 13 Ma. Generation of the Cenozoic sanukitic HMAs may be related to backarc spreading in the Japan Sea. The mantle wedge heated by the rising asthenosphere to the spreading center could generate sanukitic HMA gaining sufficient water from the subducted slab, whereas subduction immediately beneath a spreading axis might form high-temperature boninitic HMA.