北海道北部, 利尻火山噴出物の岩石学的進化 Petrological evolution of Rishiri volcano, northern Hokkaido, Japan
Rishiri volcano is an isolated polygenetic volcano that lies at a distance of about 200 km from Quaternary volcanic fields in Hokkaido. The volcanic activity has started abruptly since <0.2 Ma after∼9 My hiatus of volcanism and might have ended since several thousand years ago. The volcanic activity can be divided into three, initial, climactic and final stages in terms of its eruption rate. The climactic stage can be also subdivided into three, C-1, C-2 and C-3 stages in the ascending order.<br> The eruptive products span from basaltic to rhyolitic rocks (SiO<sub>2</sub>=49.2-73.8%) which can be distinguished by phenocryst assemblages and whole-rock chemistry. Judging from incompatible trace elements, felsic rocks from each stage cannot be formed by fractional crystallization of the basaltic magma and may be produced by crustal melting. Phenocryst assemblage, Ti contents in hornblende, and geological thermometers indicate that magmatic temperature of the dacitic and rhyolitic rocks increases from ∼800°C in the initial stage to ∼1050°C in the C-2 stage, and then decreases to ∼700°C in the final stage. On the other hand, chemical compositions of olivine and spinel suggest that the basaltic magma has also changed its source mantle with the volcanic activity. It can be estimated that the source mantle for the basaltic magmas has changed from fertile peridotites in the initial stage to depleted ones in the climactic stage, and finally changed to fertile ones in the final stage.<br> The variation in magmatic temperature of the felsic magmas indicates that the volcanic activity with higher eruption rate has produced magma with higher temperature. This suggests that the production of these magma can be explained by ascending and cooling of a mantle diapir which plays as a heat source to melt crustal materials. It can be concluded that the whole-life of Rishiri volcano may be explained by evolution of a single mantle diapir. Our petrological analysis could provide important constraints on modeling magma generation of a polygenetic volcano, because effect of preceding and/or adjacent volcanism is negligible in the case of Rishiri volcano.
- 岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology
岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology 94(8), 279-294, 1999-08
Japan Association of Mineralogical Sciences