Volcanotectonic history of Shimabara Peninsula and the evolution of Unzen volcano in Southwest Japan
Abstract
Geologic, geochronologic and geochemical studies are performed on Pliocene-Quaternary volcanic rocks in the back-arc region of SW Japan in order to verify the relationship between back-arc volcanism and regional tecotnism. Shimabara Peninsula is located in the western Kyushu Island, and Pliocene-Quaternary volcanic rocks cover most of its surface. Active Unzen volcano sits in the middle of the peninsula, and is displaced by an E-W trending active Unzen graben. Volcanic products of Unzen subsided more than 1000 m beneath the sea level inside the graben. Two drillings at the northeastern and eastern flank of Unzen volcano and associated field research have revealed the detailed volcanic history in the Shimabara Peninsula. Even though the oldest volcanic rocks are ca. 6 Ma, volcanism widely occurred in the whole peninsula between 2.5 Ma and 0.5 Ma. Monogenetic volcanoes of olivine basalt and pyroxene andesite are major components. Some andesite lavas carry small (<1.5mm) hornblende phenocrysts. Major element chemical variations of these volcanic rocks are clearly on a simple fractional crystallization trend even though andesites have lower Nb contents than basalts. At 0.5 Ma, monogenetic volcanism stopped and Unzen volcano started to grow in the central part of the peninsula. Except for the earliest eruptives, all Unzen products are hornblende andesites and dacites, and are characterized by abundant large (>3mm) phenocrysts of plagioclase and hornblende. Major elements variations are on a straight mixing line between basaltic and rhyolitic endmembers. Unzen volcanics have higher K2O but lower FeO*/MgO ratios than pre-Unzen volcanics at given SiO2 contents. Formation of Unzen graben became significant after the birth of Unzen volcano. Crustal thinning and the consequent mantle uplift are synchronous with the graben formation, and this may have urged the magma formation and ascent to be centralized in the middle of the Unzen graben at 0.5 Ma. A steady-state magma chamber was formed to mix mafic and felsic magmas, and then have fed mixed magma to the surface and have formed the composite Unzen volcano.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.V52A1035U
- Keywords:
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- 1010 Chemical evolution;
- 1035 Geochronology;
- 8400 VOLCANOLOGY;
- 9320 Asia