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Subdivision of the Sanbagawa pumpellyite–actinolite facies region in central Shikoku, southwest Japan
Author(s) -
Sakaguchi Masumi,
Ishizuka Hideo
Publication year - 2008
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/j.1440-1738.2008.00620.x
Subject(s) - epidote , actinolite , chlorite , geology , metamorphic rock , geochemistry , schist , pelite , mineralogy , metamorphic facies , facies , paleontology , quartz , structural basin
The mineral assemblages of the pumpellyite–actinolite facies such as pumpellyite + actinolite + epidote + chlorite or actinolite + epidote + hematite + chlorite occur in the Sanbagawa low‐grade metamorphic region, central Shikoku, southwest Japan. Chemical compositions of these minerals from the eight newly studied areas were analyzed in order to evaluate the areal extent and thermal structure of the region. In the buffered assemblage of pumpellyite + actinolite + epidote + chlorite, the Fe 3+ /(Fe 3+ + Al) values of epidote decrease slightly with decreasing Fe 2+ /(Fe 2+ + Mg) values for chlorite. The changes in these values show a general correlation with temperature. The presence of this relationship implies that the Fe 3+ /(Fe 3+ + Al) values of epidote can be used to divide the Sanbagawa low‐grade metamorphic region into low‐, medium‐ and high‐grade subzones. The areal distribution of these subzones indicates that: (i) the temperature seems to decrease in the same sense as envisaged by the zonal mapping of the higher‐grade pelitic schists; and (ii) there is no significant gap of metamorphic conditions through the boundary between the two structural units (Besshi and Oboke units). It follows that the Sanbagawa low‐grade metamorphic region decreases in temperature going up the structural section, and tectonic discontinuities have not affected the thermal structure.