Presentation Information
[R6-02]Dynamic redox evolution of volcanic arc granite magmas recorded in zircon: Insights from integrated oxybarometry and hygrothermobarometry
*Satoshi SAITO1, Chiho IHIRA, Taichi KAWASHIMA, Yuka TANIWAKI, Kazuya Shimooka2, Mayuko Fukuyama3 (1. Ehime Univ. Sci., 2. Kwansei Gakuin Univ. , 3. Akita Univ. Sci.)
Keywords:
Zircon,Melt inclusion,Redox state,Hygrothermobarometry
Granitoids are globally classified into magnetite- and ilmenite-series (Ishihara, 1977), reflecting the contrasting redox states (oxidized vs. reduced) of their petrogenetic environments and tectonomagmatic settings. In Southwest Japan, this distinction is reflected in their zonal distribution: the Cretaceous-Paleogene San’in Belt is dominated by magnetite-series rocks, whereas the Cretaceous Ryoke-San’yo Belt and Miocene Outer Zone predominantly comprise ilmenite-series rocks. Although these series are traditionally thought to reflect the physicochemical conditions of silicic magmas from their source to solidification, whole-rock characteristics such as magnetic susceptibility reflect only the homogenized, bulk signatures of the magma, missing localized evolutionary stages.
To decipher these high-resolution trans-crustal magmatic processes, this study integrates zircon trace-element oxybarometry (ΔFMQ) with homogenized melt-inclusion hygrothermobarometry (P–T–H2O). We investigated three representative intrusions encompassing distinct ages, emplacement depths, and redox frameworks: the Cretaceous Gamano granodiorite (Ryoke Belt), which occurs concordantly with high-grade metamorphic rocks indicating deep-crustal emplacement; the Paleogene Daito granodiorite (San’in Belt), which intrudes country-rock volcanic welded tuffs indicating shallow-crustal emplacement; and the Miocene Miuchi granite (Outer Zone), which discordantly intrudes the Shimanto accretionary complex to form a distinct contact metamorphic aureole (metasedimentary hornfels) and locally exhibits miarolitic cavities, indicating shallow-crustal emplacement. Petrographic examinations demonstrate that zircons in all intrusions predominantly occur along grain boundaries or as inclusions within the rims of major rock-forming minerals, indicating late-stage magmatic crystallization.
Zircon oxybarometry (Loucks et al., 2020) yields distinct redox ranges: the Gamano granodiorite and Miuchi granite exhibit consistently reduced signatures (ΔFMQ = -2.2 – -0.2 and -2.8 – -0.3, respectively). Conversely, the Daito granodiorite shows oxidized values (ΔFMQ = +0.1 – +1.7), except for a single heterogeneous zircon grain that records remarkably reduced values (ΔFMQ = -0.2 – -1.7). Homogenized melt-inclusion hygrothermobarometry (Taniwaki et al., 2025; Saito et al., 2026) constrains the zircon crystallization conditions for the Gamano (751–707 ℃, 563–266 MPa, 6.4–11.3 wt% H2O), Daito (763–705 ℃, 265–161 MPa, 4.1–8.1 wt% H2O), and Miuchi (785–733 ℃, 235–92 MPa, 2.4–6.0 wt% H2O) magmas.
The constrained H2O contents and pressures from the melt inclusions closely follow the H2O solubility curve for haplogranitic melts (Johannes and Holtz, 1996), demonstrating that zircon crystallized from nearly H2O-saturated melts. Furthermore, the constrained P–T relationships for all three intrusions exhibit temperatures well above the H2O-saturated haplogranite solidus (Johannes and Holtz, 1996), consistent with the interpretation that zircon crystallized from high-temperature magmas. While the zircon-recorded redox states are generally comparable to the magnetite-ilmenite-series classification (Wones, 1981), the pressure–ΔFMQ relationships reveal dynamic, divergent intra-crustal paths with decreasing pressure during magma ascent: the Gamano magma probably underwent progressive oxidation, whereas the Daito and Miuchi magmas experienced reduction. In particular, the anomalously reduced zircon domains from the Daito granodiorite capture a localized, heterogeneous assimilation event or transient fluid/melt interaction. These findings underscore that the integrated zircon-melt inclusion approach provides an exceptionally powerful window into the complex, dynamic physicochemical evolution of felsic magma plumbing systems in volcanic arcs.
References
Ishihara (1977) Min. Geol., 27, 193-305; Johannes and Holtz (1996) Petrogenesis and Experimental Petrology of Granitic Rocks. Springer, Berlin, 335 pp; Loucks et al. (2020) J. Petrol., 61, egaa034; Saito et al. (2026) Japanese Magazine of Mineralogical and Petrological Sciences, 55, gkk.260331; Taniwaki et al. (2025) Lithos, 504-505, 108029; Wones (1981) Mining Geol., 31(4), 191-212.
To decipher these high-resolution trans-crustal magmatic processes, this study integrates zircon trace-element oxybarometry (ΔFMQ) with homogenized melt-inclusion hygrothermobarometry (P–T–H2O). We investigated three representative intrusions encompassing distinct ages, emplacement depths, and redox frameworks: the Cretaceous Gamano granodiorite (Ryoke Belt), which occurs concordantly with high-grade metamorphic rocks indicating deep-crustal emplacement; the Paleogene Daito granodiorite (San’in Belt), which intrudes country-rock volcanic welded tuffs indicating shallow-crustal emplacement; and the Miocene Miuchi granite (Outer Zone), which discordantly intrudes the Shimanto accretionary complex to form a distinct contact metamorphic aureole (metasedimentary hornfels) and locally exhibits miarolitic cavities, indicating shallow-crustal emplacement. Petrographic examinations demonstrate that zircons in all intrusions predominantly occur along grain boundaries or as inclusions within the rims of major rock-forming minerals, indicating late-stage magmatic crystallization.
Zircon oxybarometry (Loucks et al., 2020) yields distinct redox ranges: the Gamano granodiorite and Miuchi granite exhibit consistently reduced signatures (ΔFMQ = -2.2 – -0.2 and -2.8 – -0.3, respectively). Conversely, the Daito granodiorite shows oxidized values (ΔFMQ = +0.1 – +1.7), except for a single heterogeneous zircon grain that records remarkably reduced values (ΔFMQ = -0.2 – -1.7). Homogenized melt-inclusion hygrothermobarometry (Taniwaki et al., 2025; Saito et al., 2026) constrains the zircon crystallization conditions for the Gamano (751–707 ℃, 563–266 MPa, 6.4–11.3 wt% H2O), Daito (763–705 ℃, 265–161 MPa, 4.1–8.1 wt% H2O), and Miuchi (785–733 ℃, 235–92 MPa, 2.4–6.0 wt% H2O) magmas.
The constrained H2O contents and pressures from the melt inclusions closely follow the H2O solubility curve for haplogranitic melts (Johannes and Holtz, 1996), demonstrating that zircon crystallized from nearly H2O-saturated melts. Furthermore, the constrained P–T relationships for all three intrusions exhibit temperatures well above the H2O-saturated haplogranite solidus (Johannes and Holtz, 1996), consistent with the interpretation that zircon crystallized from high-temperature magmas. While the zircon-recorded redox states are generally comparable to the magnetite-ilmenite-series classification (Wones, 1981), the pressure–ΔFMQ relationships reveal dynamic, divergent intra-crustal paths with decreasing pressure during magma ascent: the Gamano magma probably underwent progressive oxidation, whereas the Daito and Miuchi magmas experienced reduction. In particular, the anomalously reduced zircon domains from the Daito granodiorite capture a localized, heterogeneous assimilation event or transient fluid/melt interaction. These findings underscore that the integrated zircon-melt inclusion approach provides an exceptionally powerful window into the complex, dynamic physicochemical evolution of felsic magma plumbing systems in volcanic arcs.
References
Ishihara (1977) Min. Geol., 27, 193-305; Johannes and Holtz (1996) Petrogenesis and Experimental Petrology of Granitic Rocks. Springer, Berlin, 335 pp; Loucks et al. (2020) J. Petrol., 61, egaa034; Saito et al. (2026) Japanese Magazine of Mineralogical and Petrological Sciences, 55, gkk.260331; Taniwaki et al. (2025) Lithos, 504-505, 108029; Wones (1981) Mining Geol., 31(4), 191-212.