Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt |
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| Authors: | Masaki Enami Simon R Wallis Yasuyuki Banno |
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| Institution: | (1) Department of Earth and Planetary Sciences, School of Science, Nagoya University, 464-01 Nagoya, Japan;(2) Department of Geology and Mineralogy, Faculty of Science, Kyoto University, 606 Kyoto, Japan;(3) Present address: Geological Survey of Japan, 1-1-3 Higashi, 305 Tsukuba, Japan |
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| Abstract: | 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°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°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
Fe
2+=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°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°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. |
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