･ 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 tectonism.
･ 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 with relatively constant trace element ratios.
･ 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 end members.
･ 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.
･ Unzen volcano is located ~100km west from the volcanic front
of the Ryukyu Arc. No deep earthquakes occur beneath it. Many
E-W trending normal faults run in the middle of the Shimabara
Peninsula and displaced the central edifice of Unzen volcano constantly
(~2mm/y). The earliest products of Unzen volcano and the pre-Unzen
andesites, both are ~0.5 Ma, appear ~1000 m below sea level in
the central part of Unzen graben.
･ In SW Kyushu, Neogene-Quaternary OIB-type alkalic basalts spread
sporadically along with calc-alkalic andesites and rhyolites.
Volcanism of Shimabara Peninsula is among such volcanic activities,
and is different from that along the volcanic front.
･ Volcanism started ca. 6 Ma, and spread in the whole peninsula
until 0.5 Ma when Unzen volcano started to grow in the center
of the peninsula.
･ Pre-Unzen volcanics are olivine basalts, pyroxene andesites
and pyroxene-hornblende andesites. All but earliest Unzen eruptives
are characterized by large hornblende and plagioclase phenocrysts.
･ Pre-Unzen basalts (diamonds) and andesites (circles) form
a concave variation trend with decreasing MgO content, suggesting
that fractional crystallization is a governing process for the
differentiation of magmas.
･ Unzen andesites and dacites (triangles), on the other hand,
plot on a straight line between basic andesite and dacite. This
suggest that magma mixing is a dominant process for the variation
of Unzen products.
･ Pre-Unzen andesites to the north of Unzen (squares) form a different
variation trend from that of andesites to the south of Unzen (circles)
･Unzen andesites and dacites (triangles) have higher Cr contents
than Pre-Unzen andesites, suggesting that they are mixed with
Ni-rich basalt magmas.
･ Pre-Unzen basalts (diamonds) and andesites (circles and squares)
have similar Zr/Y and Ba/Sr ratios suggesting the common source
materials, while Unzen volcanics (triangles) have higher ratios
indicating the contamination of crustal materials with higher
Ba/Sr and Zr/Y ratios.
･ Younger Unzen eruptives (yellow triangles), particularly those
with higher MgO, have lower Ba/Sr and Zr/Y ratios than older Unzen
products (green and pink triangles) suggesting more input of mantle-derived
components.
･ Spidergram of pre-Unzen basalts suggests that these basalts
have similar geochemical characteristic with surrounding basalts
in NW Kyushu (JB-1), but with a little Ta depletion.
･ Sr and Pb isotopic ratios of Unzen and Pre-Unzen volcanics
are offset from Quaternary volcanic rocks along the volcanic front
of the Ryukyu Arc, but within the range of the Miocene-Quaternary
alkalic basalts in NE Kyushu.
･ Pre-Unzen basalts and andesites are on the mixing line between
EM2 and N-MORB mantle components. Unzen andesites are within the
range of Pre-Unzen volcanics suggesting that all volcanics in
Shimabara Peninsula have the common source materials.
･ Volcanism drastically changed at 0.5
Ma in Shimabara peninsula.
･ Monogenetic volcanoes scattered in the whole Shimabara Peninsula,
and eruptives are on a fractional crystallization trend. Volcanism
was centralized in the middle of the peninsula where an active
graben was started to be formed, and Unzen stratovolcano has grown.
A clear magma-mixing trend is observed among Unzen products.
･ Hornblende-bearing pyroxene andesites erupted at 0.5 Ma have
transitional petrographic and geochemical characteristics between
Unzen and Pre-Unzen volcanics.
 |
Pre-Unzen
stage 0.5-2 Ma ･ Monogenetic volcanism of olivine basalt & pyroxene andesite across the whole Shimabara Peninsula ･ Weak development of magma chamber ･ Fractional crystallization dominant differentiation |
| Older
Unzen stage 0.15-0.5 Ma ･ Birth of Unzen startovolcano(hornblende andesite) in the middle of the Shimabara Peninsula ･ Development of Unzen graben (crustal thinning and the rise of Moho) ･ Formation of felsic magma by crust melting ･ Development of crustal magma chamber and effective magma mixing of mafic and felsic magmas ･ Explosive pumice eruption |
|
| Younger
Unzen stage 0-0.15 Ma ･ Conversion of volcanic and fault activities in the central part of the Shimabara Peninsula ･ Lava dome growth and dome-collapse pyroclastic flows ･ Repeated edifice failure |