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Origin of garnet crystals in calc-alkaline volcanic rocks from the Setouchi volcanic belt, Japan

Hiroshi Kawabata and N. Takafuji
Origin of garnet crystals in calc-alkaline volcanic rocks from the Setouchi volcanic belt, Japan
Mineralogical Magazine (December 2005) 69 (6): 951-971

Abstract

Calc-alkaline dacites from the Setouchi volcanic belt contain garnet crystals that are classified petrographically and chemically into two types: type I and type M. Type-M garnets are characterized by acicular sillimanite inclusions or dissolution textures, and may be accompanied by xenolith fragments. They exhibit extensive compositional zoning with an increase in MgO/FeO and decrease in MnO content towards the margin. These petrographical and compositional features are identical to those of garnets from metamorphic xenoliths entrained in the Setouchi volcanic rocks, suggesting a xenocrystic origin for the type-M garnets. In contrast, type-I garnets lack sillimanite inclusions and have different rim compositions from the type-M garnets. Transmission electron microscope analysis has revealed the presence of minute glass inclusions in the type-I garnets, which indicate conclusively that these garnets grew in the presence of a melt. Type-I garnets have oscillatory zoning characterized by an antipathetic variation between FeO and MgO. This zoning was probably caused by magma heterogeneity within magma batch. Differences in rim compositions between the two types of garnets, and the presence of reaction rims indicate that the xenocrystic type-M garnets were incorporated into the magma after phenocrystic type-I garnet became unstable due to decompression during magma ascent.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 69
Serial Issue: 6
Title: Origin of garnet crystals in calc-alkaline volcanic rocks from the Setouchi volcanic belt, Japan
Affiliation: Japan Agency for Marine-Earth Science and Technology, Institute for Research on Earth Evolution, Yokosuka, Japan
Pages: 951-971
Published: 200512
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 75
Accession Number: 2006-034870
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, sketch map
N34°14'06" - N34°14'06", E133°45'00" - E133°45'00"
Secondary Affiliation: Hokkaido University, JPN, Japan
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200619
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