15:30 〜 15:45
[SVC30-07] Estimating magma flux from contact metamorphic aureoles
キーワード：深成岩形成、熱モデリング、マグマ流量、カルデラ噴火、接触変成帯、領家花崗岩
The distribution of melt beneath volcanos and how it coalesces during large eruptions are major unresolved issues and better knowledge of these features would contribute to improving our ability to forecast destructive eruptions. Plutons are a clear record of magma emplacement into the crust and studies of these domains are an important source of information on sub-volcanic processes. However, in nearly all cases exposure of the pluton at the earth’s surface is associated with the erosion and removal of the overlying volcanic edifice and only very rarely can the relationship between plutonic and eruptive processes be examined in the same region. The magmatic flux into the pluton region is the most important controlling factor in determining whether the magma will be eruptible or not during incremental growth of a pluton. Previous estimates of this flux have mainly been based on zircon geochronology (e.g. de Saint-Blanquat et al., 2011). An alternative but relatively unexplored approach is to use thermal modeling to investigate whether a pluton has been associated with large amounts of a magmatic eruption (Schöpa & Annen, 2013).
In this study we use thermal modeling of an unusually broad contact metamorphic aureole around a pluton to estimate the associated magmatic flux. Although it is generally difficult to discuss magma that is no longer in the pluton, we suggest that broad contact metamorphic aureoles around plutons that imply enthalpy input disproportionately large compared to the volume of the pluton are good markers of magmatic centers associated with significant magma flux and volcanic eruption.
Recent zircon geochronological studies have established that numerous plutons exposed in central Japan are coeval with a series of extensive ignimbrite deposits associated with caldera-forming eruption formed in around 70 Ma and with a present volume reaching 5000–7000 km3 (Koido, 1991; Sonehara & Harayama, 2007; Hoshi et al., 2016; Takatsuka et al., 2018). The existence of this world-class volcano-plutonic complex provides a valuable opportunity to discuss the relationship between pluton characteristics and amount of associated volcanic ejecta. In this study, we focus on a contact metamorphic aureole around the Shinshiro tonalite that formed around 70 Ma at ~9 km depth. The thermal structure is well defined by changes in the crystallinity of carbonaceous material, Ti content in quartz, and mineral assemblages of metamudstone and the dimensions of the intrusion are well constrained by field observations and gravity anomaly data. Numerical modeling shows that the contact metamorphic aureole is too broad to be simply explained by the volume of the intrusive body. However, a model that incorporates continuous magma replenishment and tapping of magma can yield the observed broad contact metamorphic aureole with > 550 km3/myr as a long-term magma influx rate. The estimated minimum magma influx rate can maintain an eruptible magma chamber and suggest the Shinshiro tonalite acted as one of the magmatic cores to a large caldera-forming eruption during the ~70 Ma magmatic flare-up event of SW Japan.
A new compilation of the thickness of the contact metamorphic aureoles around plutons where there is information on the depth and shape of the intrusion shows a wide range of pluton thermal types. Several plutons are similar to the Shinshiro tonalite and show much broader contact metamorphic aureole than can be accounted for by intrusion models that do not incorporate fluxes of erupted magma irrespective. We suggest that for a wide range of different crustal conditions and magma types, the thickness of contact metamorphic aureole can be used to identify those associated with the passage of large amounts of erupted magma.
In this study we use thermal modeling of an unusually broad contact metamorphic aureole around a pluton to estimate the associated magmatic flux. Although it is generally difficult to discuss magma that is no longer in the pluton, we suggest that broad contact metamorphic aureoles around plutons that imply enthalpy input disproportionately large compared to the volume of the pluton are good markers of magmatic centers associated with significant magma flux and volcanic eruption.
Recent zircon geochronological studies have established that numerous plutons exposed in central Japan are coeval with a series of extensive ignimbrite deposits associated with caldera-forming eruption formed in around 70 Ma and with a present volume reaching 5000–7000 km3 (Koido, 1991; Sonehara & Harayama, 2007; Hoshi et al., 2016; Takatsuka et al., 2018). The existence of this world-class volcano-plutonic complex provides a valuable opportunity to discuss the relationship between pluton characteristics and amount of associated volcanic ejecta. In this study, we focus on a contact metamorphic aureole around the Shinshiro tonalite that formed around 70 Ma at ~9 km depth. The thermal structure is well defined by changes in the crystallinity of carbonaceous material, Ti content in quartz, and mineral assemblages of metamudstone and the dimensions of the intrusion are well constrained by field observations and gravity anomaly data. Numerical modeling shows that the contact metamorphic aureole is too broad to be simply explained by the volume of the intrusive body. However, a model that incorporates continuous magma replenishment and tapping of magma can yield the observed broad contact metamorphic aureole with > 550 km3/myr as a long-term magma influx rate. The estimated minimum magma influx rate can maintain an eruptible magma chamber and suggest the Shinshiro tonalite acted as one of the magmatic cores to a large caldera-forming eruption during the ~70 Ma magmatic flare-up event of SW Japan.
A new compilation of the thickness of the contact metamorphic aureoles around plutons where there is information on the depth and shape of the intrusion shows a wide range of pluton thermal types. Several plutons are similar to the Shinshiro tonalite and show much broader contact metamorphic aureole than can be accounted for by intrusion models that do not incorporate fluxes of erupted magma irrespective. We suggest that for a wide range of different crustal conditions and magma types, the thickness of contact metamorphic aureole can be used to identify those associated with the passage of large amounts of erupted magma.