Coexisting sodic augite and omphacite in a Sanbagawa metamorphic rock,Japan |
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Authors: | Masaki Enami Masayasu Tokonami |
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Institution: | (1) Department of Earth Sciences, Nagoya University, 464 Nagoya, Japan;(2) Mineralogical Institute, University of Tokyo, Hongo, 113 Tokyo, Japan |
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Abstract: | Coexisting sodic augite and omphacite were found in a zoisite amphibolite from the Iratsu epidote amphibolite mass in the
Sanbagawa metamorphic terrain of central Shikoku, Japan. The occurrences of the sodic augite-omphacite pairs are classified
into four types by texture: independent, composite, intergrowth and exsolution types. Sodic augite and omphacite of the independent
and composite types (pair A) have X
Na (=Na/(Na + Ca)) = 0.15 and 0.35, respectively, and were stable in the epidote amphibolite facies during the Sanbagawa progressive
metamorphism. On the other hand, X
Na values of sodic augite and omphacite of the intergrowth and exsolution types (pair B) are 0.10 and 0.44, respectively. The
Na-poor augite and Na-rich omphacite of the pair B were formed by re-equilibration of the pair A at lower temperature. The
pair A of the Iratsu sample suggests that a compositional gap lies between sodic augite and C2/c omphacite under epidote amphibolite
facies conditions, and is in marked contrast to the coexistence of sodic augite and P2/n omphacite reported from some low-grade,
high-pressure metamorphic terrains. A possible phase diagram to explain the chemistry and mode of occurrence of the coexisting
sodic pyroxenes is proposed. |
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Keywords: | |
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