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[SCG47-02] Formation of anorthite megacrysts in the Mt. Kayo Oguro Lava in Senboku City Akita Prefecture
Abstract
An anorthite megacrysts with a maximum length of about 3 cm is found in the Oguro
lava of Mt.Kayo in Senboku City Akita Prefecture. These megacrysts are characteristic
minerals of volcanic rocks from the Japanese Islands, but their formation and origin
remain largely unexplained. Outside Japan, only a few megacrysts smaller than 1 cm have
been reported from the Isle of Skye, Scotland (Donaldson 1975) and the Galapagos
Islands (Sinton et al. 1993). Although several hypotheses have been proposed as the
formation mechanism of anorthite megacrysts, there is no model that considers the
internal structure and chemical zoning of anorthite megacrysts. In this study, describe the
mineralogical characteristics of the anorthite metacysts from Mt.Kayo Based on the
results of the analyses, the causes of the formation of anorthite megacrysts.
One of the characteristics of the anorthite mafic crystals in the Oguro lava is that the
grain size of the anorthite mafic crystals is uniform in the upper part of the lava, and the
anorthite megacrysts are abundant in the significantly foamed part. Polarized light
microscopy revealed that the mottled minerals are normal pyroxene and anorthite, and
the lithic base is autoclase plagioclase. From SEM-EDS observations, it was found that
the feldspar mottle has a low An # (about 50%) rim and a high An # (83%-86%) core, and
the rim has a similar An component value to that of the base. From the samples collected
at the boundary between the Oguro lava and the Maego lava of the upper layer, an inverse
accumulation zone structure was found in the middle part of the core and rim sections. In
addition, some low-An# rims were found in the feldspar mottling of the foam part,
surrounding several high-An# anorthite cores and forming a single large mottling. The
whole rock composition analysis showed that the lava was classified as basalt, and there
was no significant change in chemical composition between the upper foam aand the lower
part.
In this paper, discuss the origin and formation environment of the anorthite megacrysts
of Mt.Kayo. The boundary between the core and the rim is not as sharply zoned as that
of the samples collected at other sites, and the extinction angles of the non-twinned
portions are not identical in polarized light microscopy. It is thought that the megacrysts
grew into megacrysts by coalescence of the feldspar base with relatively large nearby
mottle crystals due to magma mixing. In spite of the difference in crystal diameter and
lithology between the upper and lower part of the lava, the results of XRF analysis are
similar.This suggests that advanced magma mixing was occurring in the magma pool
before the eruption of the Oguro lava.It is considered that the magma mixing was caused
by the inflow of magma from the magma pool, and crystals were mixed during the mixing,
and the rim part was crystallized surrounding the core part during the cooling process.
An anorthite megacrysts with a maximum length of about 3 cm is found in the Oguro
lava of Mt.Kayo in Senboku City Akita Prefecture. These megacrysts are characteristic
minerals of volcanic rocks from the Japanese Islands, but their formation and origin
remain largely unexplained. Outside Japan, only a few megacrysts smaller than 1 cm have
been reported from the Isle of Skye, Scotland (Donaldson 1975) and the Galapagos
Islands (Sinton et al. 1993). Although several hypotheses have been proposed as the
formation mechanism of anorthite megacrysts, there is no model that considers the
internal structure and chemical zoning of anorthite megacrysts. In this study, describe the
mineralogical characteristics of the anorthite metacysts from Mt.Kayo Based on the
results of the analyses, the causes of the formation of anorthite megacrysts.
One of the characteristics of the anorthite mafic crystals in the Oguro lava is that the
grain size of the anorthite mafic crystals is uniform in the upper part of the lava, and the
anorthite megacrysts are abundant in the significantly foamed part. Polarized light
microscopy revealed that the mottled minerals are normal pyroxene and anorthite, and
the lithic base is autoclase plagioclase. From SEM-EDS observations, it was found that
the feldspar mottle has a low An # (about 50%) rim and a high An # (83%-86%) core, and
the rim has a similar An component value to that of the base. From the samples collected
at the boundary between the Oguro lava and the Maego lava of the upper layer, an inverse
accumulation zone structure was found in the middle part of the core and rim sections. In
addition, some low-An# rims were found in the feldspar mottling of the foam part,
surrounding several high-An# anorthite cores and forming a single large mottling. The
whole rock composition analysis showed that the lava was classified as basalt, and there
was no significant change in chemical composition between the upper foam aand the lower
part.
In this paper, discuss the origin and formation environment of the anorthite megacrysts
of Mt.Kayo. The boundary between the core and the rim is not as sharply zoned as that
of the samples collected at other sites, and the extinction angles of the non-twinned
portions are not identical in polarized light microscopy. It is thought that the megacrysts
grew into megacrysts by coalescence of the feldspar base with relatively large nearby
mottle crystals due to magma mixing. In spite of the difference in crystal diameter and
lithology between the upper and lower part of the lava, the results of XRF analysis are
similar.This suggests that advanced magma mixing was occurring in the magma pool
before the eruption of the Oguro lava.It is considered that the magma mixing was caused
by the inflow of magma from the magma pool, and crystals were mixed during the mixing,
and the rim part was crystallized surrounding the core part during the cooling process.