No abstract

Prograde P-T paths recorded by the chemistry of minerals of subduction-related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P-T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south-western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca-rich normal zoning, Ca-poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally-zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz ± biotite. Pressure and temperature histories were inversely calculated from the normally-zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P-T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP ⁄ dT of 0.4-0.5 GPa ⁄ 100°C. Garnet started growing at around 470°C and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520°C, 0.9 GPa). The high-temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.

No abstract

The early Toarcian oceanic anoxic event (T-OAE; ~183 Ma) was a significant palaeoenvironmental perturbation associated with marked changes in oceanic redox conditions. However, the precise redox conditions and redox history of various water masses during the T-OAE, especially those from outside the Boreal and Tethyan realms, are unclear. To address this issue, we present pyrite framboid data from an upper Pliensbachian to lower Toarcian succession deposited on the NW Panthalassic margin in a shallow-water setting (Sakuraguchi-dani section, Toyora area, SW Japan). Izumi et al., 2017 Page 2 21/12/2017Available data on redox-sensitive trace elements from the same succession suggest that dysoxic bottom-water conditions generally prevailed, with intermittent short-term oxygenation events. Size-distribution analysis of pyrite framboids reveals that framboid size populations from the silty mudstones during the OAE were characterized by small mean diameters and standard deviations. This suggests that euxinic conditions at least intermittently occurred during the T-OAE interval. Most likely, this water-column euxinia was associated with the expansion of an oxygen minimum zone linked to increased primary productivity. This interpretation is consistent with a previously reported increase in fluvial discharge and thus nutrient flux caused by a strengthening of the hydrological cycle.

The mode of occurrence of garnet in pelitic schists of the Sanbagawa metamorphic belt along the Asemigara River in central Shikoku is examined. Two different types of garnets, A and B, are identified from the albite -biotite and the oligoclase -biotite zones of the Sanbagawa belt. Type -A garnets are larger than type -B garnets and show concentric normal chemical zoning. There is no evidence of disequilibrium crystal growth in type -A garnets. On the contrary, type -B garnets are small, and some of them occasionally show sector zoning, which is a strong evidence for a disequilibrium crystal growth. The chemical trend observed between the core and the rim of type -B garnets is different from that observed in the case of type -A garnets, although only some of the type -B garnets show chemical sector zoning. Type -B garnets are typically found in clusters in muscovite -rich layers or are included in albite porphyroblasts. These two types of garnets can be distinguished from each other on the basis of grain size, cluster formation, and chemical composition. Type -B grains are found only in the structurally lower part of the oligoclase -biotite zone and in the lower albite -biotite zone. These two types of garnets are thought to represent two stages of garnet growth. Further, in the case of type -A garnets (early garnet), chemical equilibrium is maintained during prograde metamorphism, and hence, these garnets are suitable for the use in thermobarometric studies.