4:30 PM - 4:45 PM
[SMP26-10] Evaluation of thermal-material budget of the Ryoke complex using structured geological map and magma advection mode
Keywords:Metamorphic rocks, Plutonic rocks, Ryoke
The thermal-material budget of the Ryoke complex was evaluated using a structured geological map and magma advection model.
Structured geologic map analysis was used to estimate the components of the Cretaceous crust of the inner zone of Southwest Japan (iSWJp). The iSWJp is divided into a deep part of the crust (deep part) consisting of Cretaceous felsic plutonic rocks (Cfp) and Cretaceous high-T metamorphic rocks (CTm), namely the Ryoke complex, and a shallow part of the crust (shallow part) in which Cretaceous felsic volcanic rocks (Cfv) are distributed. Cfv are associated with the caldera and caldera cluster. Using the Seamless Geological Map of Japan V2 (GSJ, AIST, 2022), each polygon was structured by breaking down its attributes into elements such as age, chemical composition, depositional environment, and metamorphic conditions. A statistical analysis of geologic units was performed using the structured geologic map shapefile and QGIS.
The results of the QGIS analysis indicate that the geological units comprising the deep part are 80% Cfp and 20% CTm. In the shallow part, Cfv is 30%, Cfp is 30%, and old crustal remnants is 40%. These results indicate that the older crust of iSWJp was replaced by Cfp, i.e., Cretaceous granites (Cgr), on a larger scale at depth.
To reproduce the replacement of the older crust by Cgr and the P-T conditions recorded in the Ryoke metamorphic rocks (Rmr) derived from the older crust, we used a 1D magma advection model (Miyazaki, 2004) and calculate granitic magma fluxes for the Rmrs in the Mikawa (Miyazaki, 2010) and Yanai (Ikeda, 2004) areas. The results are further compared with magma fluxes for the Cretaceous caldera cluster (Ccc) in the Ako area (Sato et al., 2016). Based on recent results (e.g., Miyazaki et al., 2023), we assume that magma advection at depth occurs in pulses with intervals of a few to ten Myr. Pulses with intervals ranging from a few to 10 Myr also exist for Ccc (Sato et al., 2016). On the other hand, shallow magma chamber, which are essential for caldera formation, are assumed to be formed by magma pulses consisting of thin sheet intrusions (Annen, 2009). The intervals of sheet intrusions are much shorter than those of deep magma advection and caldera formation within caldera cluster. However, short-interval pulses may also exist at the surface and at depth. In that case, formations of caldera cluster, large and shallow magma chamber, and deep high-T metamorphic complex would be multi-timescale processes.
Magma advection model results indicate that the magma flux for the Rmr is about 10 times greater than that for the Ccc. Magma fluxes for the Rmr are within the following lower and upper limits: a lower limit is the long-term average magma accretion rate of the pluton (Annen, 2009) and an upper limit is the lower magma accretion rate of shallow magma chamber (Annen, 2009). The model results predict that magma advection causes a uniformly high-T conditions in the deep crust, while a very high geothermal gradient is realized in the shallow crust. Large-scale felsic volcanism with caldera requires the formation of a large and shallow magma chamber. Formation of large and shallow magma chamber requires a high geothermal gradient of > 30°C/km (Annen, 2009). The higher the geothermal gradient, the more favorable it is for the large and shallow magma chamber. The present results show that high geothermal gradients of > 30°C/km can be maintained for very long duration in the shallow crust. Less than 10 Myr interval magma pulses deep in the crust contribute to maintaining high T conditions for longer duration, and higher magma fluxes at depth contribute to higher geothermal gradients in the shallow crust. Therefore, it is possible that the formation of high-T metamorphic complexes deep in the crust and large-scale felsic volcanism at the surface are inextricably linked.
Structured geologic map analysis was used to estimate the components of the Cretaceous crust of the inner zone of Southwest Japan (iSWJp). The iSWJp is divided into a deep part of the crust (deep part) consisting of Cretaceous felsic plutonic rocks (Cfp) and Cretaceous high-T metamorphic rocks (CTm), namely the Ryoke complex, and a shallow part of the crust (shallow part) in which Cretaceous felsic volcanic rocks (Cfv) are distributed. Cfv are associated with the caldera and caldera cluster. Using the Seamless Geological Map of Japan V2 (GSJ, AIST, 2022), each polygon was structured by breaking down its attributes into elements such as age, chemical composition, depositional environment, and metamorphic conditions. A statistical analysis of geologic units was performed using the structured geologic map shapefile and QGIS.
The results of the QGIS analysis indicate that the geological units comprising the deep part are 80% Cfp and 20% CTm. In the shallow part, Cfv is 30%, Cfp is 30%, and old crustal remnants is 40%. These results indicate that the older crust of iSWJp was replaced by Cfp, i.e., Cretaceous granites (Cgr), on a larger scale at depth.
To reproduce the replacement of the older crust by Cgr and the P-T conditions recorded in the Ryoke metamorphic rocks (Rmr) derived from the older crust, we used a 1D magma advection model (Miyazaki, 2004) and calculate granitic magma fluxes for the Rmrs in the Mikawa (Miyazaki, 2010) and Yanai (Ikeda, 2004) areas. The results are further compared with magma fluxes for the Cretaceous caldera cluster (Ccc) in the Ako area (Sato et al., 2016). Based on recent results (e.g., Miyazaki et al., 2023), we assume that magma advection at depth occurs in pulses with intervals of a few to ten Myr. Pulses with intervals ranging from a few to 10 Myr also exist for Ccc (Sato et al., 2016). On the other hand, shallow magma chamber, which are essential for caldera formation, are assumed to be formed by magma pulses consisting of thin sheet intrusions (Annen, 2009). The intervals of sheet intrusions are much shorter than those of deep magma advection and caldera formation within caldera cluster. However, short-interval pulses may also exist at the surface and at depth. In that case, formations of caldera cluster, large and shallow magma chamber, and deep high-T metamorphic complex would be multi-timescale processes.
Magma advection model results indicate that the magma flux for the Rmr is about 10 times greater than that for the Ccc. Magma fluxes for the Rmr are within the following lower and upper limits: a lower limit is the long-term average magma accretion rate of the pluton (Annen, 2009) and an upper limit is the lower magma accretion rate of shallow magma chamber (Annen, 2009). The model results predict that magma advection causes a uniformly high-T conditions in the deep crust, while a very high geothermal gradient is realized in the shallow crust. Large-scale felsic volcanism with caldera requires the formation of a large and shallow magma chamber. Formation of large and shallow magma chamber requires a high geothermal gradient of > 30°C/km (Annen, 2009). The higher the geothermal gradient, the more favorable it is for the large and shallow magma chamber. The present results show that high geothermal gradients of > 30°C/km can be maintained for very long duration in the shallow crust. Less than 10 Myr interval magma pulses deep in the crust contribute to maintaining high T conditions for longer duration, and higher magma fluxes at depth contribute to higher geothermal gradients in the shallow crust. Therefore, it is possible that the formation of high-T metamorphic complexes deep in the crust and large-scale felsic volcanism at the surface are inextricably linked.