Occurrence of calcite in Sanbagawa pelitic schists: implications for the formation of garnet, rutile, oligoclase, biotite and hornblende

Abstract

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¯uid 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 de®ne 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¯uid 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.

Ca-Al hydrous silicates in the chlorite-grade pelitic schists in Sanbagawa metamorphic belt and a petrogenetic analysis in the model mixed-volatile system

“…Occasionally tiny epidote crystals form aggregates. Pumpellyite and lawsonite occur only in the chlorite zone but not in the garnet zone, whereas epidote occurs throughout the Sanbagawa belt ubiquitously (Goto et al, 2002).…”

“…Chlorite, epidote, titanite, tourmaline, apatite, pyrite, pyrrhotite and carbonaceous materials are ubiquitous accessory minerals throughout the Sanbagawa belt. Calcite was also present as a minor but ubiquitous phase during the Sanbagawa metamorphism (Goto et al, 2002). The average CaO content of 35 pelitic schists in the chlorite zone is 0.64 wt% (Goto et al, 1996).…”

“…Index minerals defining mineral zones should be more commonly present. For example, the frequency of occurrence of garnet is as high as 90% in the garnet zone pelitic schist, and that of biotite exceeds 60% in the biotite zone pelitic schist (Goto et al, 2002). The low modal abundance and sporadic occurrence of three Ca -Al silicate minerals might be due to the low CaO content of pelitic schists.…”

“…Nevertheless, Goto et al (2002) revealed that most Sanbagawa pelitic schists ubiquitously contain calcite and epidote as Cabearing phases, on the basis of the systematic and extensive microscopic observation of 1876 samples from central Shikoku. Occurrence of lawsonite was reported by Watanabe and Kobayashi (1984) and that of pumpellyite by Banno (1998) referring to Sakai (1985) in pelitic schists from the chlorite zone of the Sanbagawa belt in central Shikoku.…”

“…Based on the ubiquitous presence of calcite, Goto et al (2002) described most of the mineral formation reactions including isograd -defining ones for the Sanbagawa pelitic schists as mixed volatile equilibria containing both CO 2 and H 2 O instead of pure dehydration equilibria. Their suggestion brought a marked progress in understanding metamorphic reactions forming the Ca -bearing phase in the pelitic schists.…”