PARAGENESIS OF SODIC PYROXENE-BEARING QUARTZ SCHISTS - IMPLICATIONS FOR THE P-T HISTORY OF THE SAMBAGAWA BELT

Sodic pyroxene (jadeite content X(Jd)=0.1-0.3) occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene + subcalcic (or sodic) amphibole + phengitic mica + albite + quartz + hematite + titanite +/- epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X(Acm)=0.26-0.50) than that in the epidote-free samples (X(Acm) = 0.45 -0.47). X(Jd) shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X(Jd) of the Saruta-gawa samples (0.21-0.29) is higher than that of the Asemi-gawa (0.13-0.17) and Bessi (0.14-0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5-6.5 kbar, < 360-degrees-C in the chlorite zone, 7-8.5 kbar, 440 +/- 15-degrees-C in the garnet zone and 89. 5 kbar, 520 +/- 25-degrees-C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1-1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3-5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central Shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X(Jd) from core to rim, the other type by decreasing X(Jd) from core to rim. Both types of zoned pyroxenes show an increase in X(Fe2+) [=Fe2+/(Fe2+ + Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100-degrees-C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100-degrees-C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.