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Occurrence of calcite in Sanbagawa pelitic schists: implications for the formation of garnet, rutile, oligoclase, biotite and hornblende
Author(s) -
Goto A.,
Banno S.,
Higashino T.,
Sakai C.
Publication year - 2002
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.0263-4929.2001.00365.x
Subject(s) - geology , schist , geochemistry , calcite , biotite , phengite , pelite , hornblende , albite , metamorphic rock , metamorphism , mineral , chlorite , mineralogy , quartz , eclogite , chemistry , paleontology , organic chemistry , subduction , tectonics
The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite‐biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H 2 O, CO 2 and CH 4 . In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca‐bearing phases must be taken into account to analyze the phase relations of calcite‐bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.