Petrology of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high‐pressure intermediate type of metamorphic belt

Abstract

The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa = 0.10-0.28)-bearing assemblages in pelitic schists, and represents the highestgrade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet +biotite +chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming P H ,~ = Psol,d suggests that metamorphic conditions of this zone are about 610+2S"C and 10 +_ 1 kbar.Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclasebiotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized 'upper staurolite zone' as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as: omphacite zone garnet + glaucophane + omphacite +muscovite = oligoclase-biotite zone biotite + hornblende + plagioclase +quartz + H20 oligoclase-biotite zone garnet + hornblende + paragonite + muscovite = upper staurolite zone staurolite + biotite + plagioclase -I-quartz + H 2 0 0263-4929/83/0/0600-0141%02.00 0 1983 Blackwell Scientific Publications dP/dT values of these equations calculated using natural compositions are 3-6 bar/deg. and 16-19 bar/deg., respectively, and their gradients are gentle. This implies that the contrast in mineral parageneses of pelitic schists related by the above equations result mainly from the difference in pressure during progressive metamorphism.The mineral assemblages observed in the Sanbagawa oligoclase-biotite zone are characteristic of pelitic schists in the high-grade part of a high-pressure intermediate type of metamorphism.

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

“…In central Shikoku region, the Sanbagawa belt is divided into the chlorite, garnet, albite -biotite and oligoclase -biotite zones on the basis of the mineral assemblages of the pelitic schists in ascending order of peak metamorphic temperature (Enami, 1983;Higashino, 1990). The chlorite zone occupies the lowest structural horizon and thin layers of the higher -temperature zones overlie it.…”

“…Since pumpellyite + actinolite + chlorite + epidote is stable in hematite -free basic schists (Nakajima, 1982), our sampling areas belong to the pumpellyite -actinolite facies. According to Enami (1983) and Enami et al (1994), the maximum pressure and temperature conditions of each mineral zone are as follows, garnet zone, 0.7 -0.8 GPa, 440 ± 15 °C; albite -biotite zone, 0.8 -0.9 GPa, 520 ± 25 °C; oligoclase -biotite zone, 0.9 -1.1 GPa, 610 ± 25 °C. Goto et al (2007) estimated the maximum pressure and temperature conditions of the chlorite zone as less than 0.8 GPa and less than 360°C with XCO 2 ranging from 0.0001 to 0.0005.…”

Peak conditions of kyanite-bearing quartz eclogites in the Sanbagawa metamorphic belt, central Shikoku, Japan

“…The Sanbagawa belt in central Shikoku is subdivided into four metamorphic zones: chlorite, garnet, albite biotite and oligoclase biotite zones in ascending order of metamorphic grade (e.g., Higashino, 1975;Enami, 1983;Higashino, 1990). The higher grade albite biotite and oligoclase biotite zones are widely distributed in the Besshi region of central Shikoku (Fig.…”

Compositional zoning and inclusions of garnet in Sanbagawa metapelites from the Asemi-gawa route, central Shikoku, Japan

“…Based on geochronological, tectonical, and petrological studies, it is considered that the Sanbagawa belt in central Shikoku comprises two major tectonic units: the structurally lower and higher units, Oboke and Besshi, respectively (e.g., Takasu and Dallmeyer, 1990;Wallis, 1998). The regional thermal structure is usually described in terms of four mineral zones, based mainly on the occurrence of index minerals in metapelites: the chlorite, garnet, albite -biotite, and oligoclase -biotite zones, in ascending order of metamorphic grade (Enami, 1983;Higashino, 1990) (Fig. 1a).…”