HR: 1330h
AN: V22D-0614    [PDF]
TI: Thermal structure of pumpellyite-actinolite facies regions in the Sanbagawa belt, Shikoku, SW Japan
AU: * Sakaguchi, M
EM: masumi-s@cc.kochi-u.ac.jp
AB: On the basis of the mineral assemblages of pelitic rocks, the Sanbagawa belt in Shikoku, SW Japan, has been divided, from low- to high-grade parts, into the chlorite, garnet, albite-biotite and oligoclase-biotite zones (Higashino, 1990). Also, the mineral assemblage of pumpellyite + actinolite + epidote + chlorite or epidote + actinolite + hematite + chlorite, which defines the pumpellyite-actinolite (PA) facies (e.g., Banno, 1998), is widely recognized in metabasites in the chlorite zone (e.g. Banno \& Sakai, 1989). However, the detailed study on the PA facies regions has been done only in the Omoiji-Nagasawa area (Nakajima et al., 1977) and Asemigawa-Shirataki area (Nakajima, 1982) in central Shikoku, and thus, it is still hard to solve the regional thermal structure of the PA facies region. This study is aimed to reveal the thermal structure of the PA facies region of the Sanbagawa belt in Shikoku by analyzing the mineral assemblages and mineral chemistries of metabasites from the nine newly studied areas. The studied areas studied belong to the chlorite zone in the Oboke and Besshi units; the Oboke unit structurally underlay the Besshi unit. The mineral assemblages include pumpellyite + epidote + actinolite, epidote + actinolite _ hematite and epidote + Na-amphibole + actinolite + hematite. The metabasites from some areas involve Na-pyroxene-bearing assemblages, but the analyses of the Schreinemakers bundle of Tagiri et al. (1992) show that these assemblages do not define the Na-pyroxene-chlorite subfacies. As the low-grade metamorphic rocks do not have the hematite + pumpellyite paragenesis, its metamorphic temperature is estimated to be higher than the discontinuous reaction temperature of pumpellyite + hematite + quartz = epidote + actinolite + H2O, as shown by Nakajima et al. (1977). It is difficult to detect the difference in temperature in the PA facies regions by analyzing mineral assemblages. To detect the difference in temperature, and then to reveal thermal structure, I attempt here to analyze the Fe3+-values of epidote and pumpellyite in the pumpellyite + actinolite + epidote + chlorite assemblage, because these values, if coexisting chlorite has the fixed value of Fe*, have a tendency to decrease with increasing metamorphic temperature (Nakajima et al., 1977). The results show that, if coexisting chlorite compositions are fixed within XFe* = 0.45 - 0.56, Fe3+ values of epidote have the systematic variation as compared each areas, and at least three zones can be divided such as high (minimum XFe3+=0.13-0.19), medium ( = 0.21), and low (= 0.25) temperature zones. The lowest temperature zone in the Sanbagawa belt is located at the Omoiji-Nagasawa area. The metamorphic temperature in the Besshi unite is decreasing away from the high grade zone, but it is not clear whether the temperature decreases continuously or discontinuously. The structural evidence indicated that the Oboke unit is the lowest horizon in the Sanbagawa belt, but the metamorphic temperature of the Oboke unit is higher than those of same areas belonging to the Besshi unite. The Oboke and Besshi units are separated by ductile thrust, and radiometric ages of thermal peaks are different between these units as reported by Takasu \& Dallmeyer (1990). Therefore, although the chlorite zones of the Besshi unit and Oboke unit belong to the similar PA facies regions, it is not contradictory that the thermal structure from the Oboke to Besshi units is not continuous. These results could be important to adjust the Sanbagawa metamorphic history.
DE: 1519 Magnetic mineralogy and petrology
DE: 9320 Asia
DE: 9609 Mesozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting