10:45 〜 12:15
[SCG45-P14] Isotopic variation of calcite veins formed in tectonic stress fields-implication of geofluids evolution with seismic cycle in Mugi mélange
キーワード：calcite, Mugi melange, carbon-oxygen isotope, geofluid, seismic cycle
The geofluids properties evolution in seismic cycle process is significant for better understanding of the motivation of variation tectonic stress and events. In seismic cycle, the stress state changes with geofluids evolution under the variation of P-T condition, geofluid sources participation, elements composition or physical-chemical properties, etc.
The Mugi mélange is one of the most well studied on-land tectonic mélanges, of the variety that form near the traditional updip limit of seismogenic zone along the interface of a subducting plate. Previous studies of Mugi mélanges geofluid such as Matsumura et al., (2003) and Ujiie et al., (2008) demonstrated multiple stages of calcite-quartz veins were formed at pressures constrained by fluid inclusion analysis. Hosokawa and Hashimoto, (2022) firstly revealed different tectonic stress backgrounds and the relationship between fluid pressure ratio (λ) and differential stress (Δσ) between normal fault stress regime and reverse fault stress regime. They also showed the close relationship between tectonic event in seismic cycle and the transformation of reverse-normal stress regime where multiple calcite-quartz veins are located in.
In this study, we analyzed the carbon-oxygen isotope of calcite of the veins collected from stress regimes of normal, reverse and no-Andersonian (from the method shown in Hosokawa and Hashimoto, 2022) near the boundary between mélange and altered basalt of Mugi mélange area. The result shows carbon isotope range of those three type veins are virtually identical (-14.5‰ - -17‰, V-PDB), but obvious difference of oxygen isotope value especially between reverse stress vein (16‰-17.5‰, SMOW, λ~1.1) and no-Andersonian stress vein (17.5‰-19‰, SMOW, λ is unknown). Moreover, carbon-oxygen isotopic values of the three type veins all show trends similar to each other from light to heavy. After quantitative calculation of isotope fractionation in mineral-fluid reaction and temperature effect, we believe that the external geofluids should join in the stress evolution process between low λ to high λ, it may be the factor which caused overpressure in the result and tectonic events in seismic cycles in Mugi mélange. Although geofluids source and evolution trends of the three types of veins are still in discussion, our further result such as the Δ47 of calcite and oxygen isotope value of contemporaneous quartz can be effective for the interpretation of geofluid evolution in seismic cycle with tectonic stress change.
The Mugi mélange is one of the most well studied on-land tectonic mélanges, of the variety that form near the traditional updip limit of seismogenic zone along the interface of a subducting plate. Previous studies of Mugi mélanges geofluid such as Matsumura et al., (2003) and Ujiie et al., (2008) demonstrated multiple stages of calcite-quartz veins were formed at pressures constrained by fluid inclusion analysis. Hosokawa and Hashimoto, (2022) firstly revealed different tectonic stress backgrounds and the relationship between fluid pressure ratio (λ) and differential stress (Δσ) between normal fault stress regime and reverse fault stress regime. They also showed the close relationship between tectonic event in seismic cycle and the transformation of reverse-normal stress regime where multiple calcite-quartz veins are located in.
In this study, we analyzed the carbon-oxygen isotope of calcite of the veins collected from stress regimes of normal, reverse and no-Andersonian (from the method shown in Hosokawa and Hashimoto, 2022) near the boundary between mélange and altered basalt of Mugi mélange area. The result shows carbon isotope range of those three type veins are virtually identical (-14.5‰ - -17‰, V-PDB), but obvious difference of oxygen isotope value especially between reverse stress vein (16‰-17.5‰, SMOW, λ~1.1) and no-Andersonian stress vein (17.5‰-19‰, SMOW, λ is unknown). Moreover, carbon-oxygen isotopic values of the three type veins all show trends similar to each other from light to heavy. After quantitative calculation of isotope fractionation in mineral-fluid reaction and temperature effect, we believe that the external geofluids should join in the stress evolution process between low λ to high λ, it may be the factor which caused overpressure in the result and tectonic events in seismic cycles in Mugi mélange. Although geofluids source and evolution trends of the three types of veins are still in discussion, our further result such as the Δ47 of calcite and oxygen isotope value of contemporaneous quartz can be effective for the interpretation of geofluid evolution in seismic cycle with tectonic stress change.