9:15 AM - 9:30 AM
[SMP28-02] Melt advection and heat storage in migmatites: their role in the formation of a plutono-metamorphic and volcanic arc
Keywords:thermal model, Ryoke, melt, migmatite
The pressure (P)-temperature (T) structure of the Ryoke metamorphic belt, a representative high-T metamorphic belt in Japan, is reproduced by a thermal modelling that incorporates partial melting of the crust. The modelling also includes advection of melt generated by partial melting of the crust in addition to the advection of melt from the base of the crust. Based on this model, we discuss the role of the crustal partial melted zone (migmatite zone) on the thermal evolution at depths of the crust and volcanic activity at the surface.
In the Yanai district, southwest Japan, migmatites are widely distributed in the Kfeldspar-cordierite (KC), sillimanite-Kfeldspar (SK) and garnet-cordierite (GC) zones of Ikeda (1998). Dehydration melting of muscovite + quartz (R1) occurs in KC and SK zones, and sillimanite + biotite + quartz (R2) occurs in GC zone. The P-T conditions at which R1 and R2 occur were determined using a bulk rock composition of meta-mudstone in the Yanai district and Perplex software. Based on the calculated pseudosection, we adopted linear approximation of R1 and R2 on P-T space. The base model of the melt advection was taken from Miyazaki et al. (2023). Since multiple pulses of metamorphic zircon growth (Miyazaki et al., 2023) and metamorphic garnet growth, were observed in the Yanai district, the influx of melt from the bottom of the crust was assumed to occur as multiple pulses.
The model calculations were performed for three cases: no partial melting of the crust (model-I), partial melting of the crust (model-III), partial melting of the crust and advection of the partial melted melt. In model-III, the amount of melt that can advect is limited to maximum of 5%. The results show that model-II and model-III have a more pronounced effect on reducing T-increase in the crust than model-I. This is because the partial melting of the crust consumes the heat transported from the lower crust. In model-III, thermal structure different from those of model-I and model-II appears, and the calculated P-T curve becomes aligned with reaction curves of R1 and R2. The thermal structure of model-III reproduces P-T conditions of SK zone, and that of model-I reproduces P-T conditions of GC zone. These differences may reflect differences in amount of partially melted metamudstone between SK and GC zones. The leucogranite abundant in migmatite zone is presumed to be an evidence of advection of melt produced by the partial melting of crust in model-III.
The heat storage in advective melt suppresses the T change in the crust. This effect is clearly pronounced in model-III. In all models, relatively uniform high-T conditions are maintained for a long period in deep to middle crust due to melt advection. This high-T results in a typical high-T and low-P conditions in the shallow crust. The calculation results show that the highest high-T conditions are achieved in the shallow crust by model-III. Such conditions favor the maintenance of magma chambers in the shallow crust. Therefore, they contribute volcanic activity at the surface.
GC and SK zones in the Yanai district can be recognized as area where partial melting of the crust has progressed (Ikeda et al., submitted). In this district, high-T anomaly, where T is clearly higher than surrounding area, has a diameter of several tens of kilometers (Ikeda et al. submitted). In such areas, P is also higher than in the surrounding areas. This structure can be considered to be a buoyant dome-like structure caused by the prolonged presence of melt at deeper parts of the metamorphic blet. The size of the high-T anomaly areas in the Ryoke metamorphic belt is roughly equal to size of Cretaceous ignimbrite flare-up with caldera clusters in southwest Japan. The similarity in distribution and size between the high-T anomaly and the caldera clusters suggests that the high-T anomaly in the Ryoke metamorphic belt may correspond to the root of the magma-plumbing system of the Cretaceous caldera clusters.
In the Yanai district, southwest Japan, migmatites are widely distributed in the Kfeldspar-cordierite (KC), sillimanite-Kfeldspar (SK) and garnet-cordierite (GC) zones of Ikeda (1998). Dehydration melting of muscovite + quartz (R1) occurs in KC and SK zones, and sillimanite + biotite + quartz (R2) occurs in GC zone. The P-T conditions at which R1 and R2 occur were determined using a bulk rock composition of meta-mudstone in the Yanai district and Perplex software. Based on the calculated pseudosection, we adopted linear approximation of R1 and R2 on P-T space. The base model of the melt advection was taken from Miyazaki et al. (2023). Since multiple pulses of metamorphic zircon growth (Miyazaki et al., 2023) and metamorphic garnet growth, were observed in the Yanai district, the influx of melt from the bottom of the crust was assumed to occur as multiple pulses.
The model calculations were performed for three cases: no partial melting of the crust (model-I), partial melting of the crust (model-III), partial melting of the crust and advection of the partial melted melt. In model-III, the amount of melt that can advect is limited to maximum of 5%. The results show that model-II and model-III have a more pronounced effect on reducing T-increase in the crust than model-I. This is because the partial melting of the crust consumes the heat transported from the lower crust. In model-III, thermal structure different from those of model-I and model-II appears, and the calculated P-T curve becomes aligned with reaction curves of R1 and R2. The thermal structure of model-III reproduces P-T conditions of SK zone, and that of model-I reproduces P-T conditions of GC zone. These differences may reflect differences in amount of partially melted metamudstone between SK and GC zones. The leucogranite abundant in migmatite zone is presumed to be an evidence of advection of melt produced by the partial melting of crust in model-III.
The heat storage in advective melt suppresses the T change in the crust. This effect is clearly pronounced in model-III. In all models, relatively uniform high-T conditions are maintained for a long period in deep to middle crust due to melt advection. This high-T results in a typical high-T and low-P conditions in the shallow crust. The calculation results show that the highest high-T conditions are achieved in the shallow crust by model-III. Such conditions favor the maintenance of magma chambers in the shallow crust. Therefore, they contribute volcanic activity at the surface.
GC and SK zones in the Yanai district can be recognized as area where partial melting of the crust has progressed (Ikeda et al., submitted). In this district, high-T anomaly, where T is clearly higher than surrounding area, has a diameter of several tens of kilometers (Ikeda et al. submitted). In such areas, P is also higher than in the surrounding areas. This structure can be considered to be a buoyant dome-like structure caused by the prolonged presence of melt at deeper parts of the metamorphic blet. The size of the high-T anomaly areas in the Ryoke metamorphic belt is roughly equal to size of Cretaceous ignimbrite flare-up with caldera clusters in southwest Japan. The similarity in distribution and size between the high-T anomaly and the caldera clusters suggests that the high-T anomaly in the Ryoke metamorphic belt may correspond to the root of the magma-plumbing system of the Cretaceous caldera clusters.