09:30 〜 09:45
[R6-03] The use of contact metamorphism to identify plutons associated with major contemporaneous eruption and estimate magma flux
キーワード：Magma flux, Pluton, Contact metamorphism
Magmatic flux is an important factor controlling the heat and material transport in the continental crust; it is also thought to be a major control on the potential for any mamga to erupt, and duration of eruption and time scale between major events. Plutons are a clear record of magma input into the earth’s crust and volume estimates combined with geochronological studies have been used to estimate past magma flux rates. However, areas suitable for such studies are limited and this approach cannot asses possible loss of magma through associated volcanic eruption. Contact metamorphic aureoles developed around plutons record the thermal effects of magma input into the crust and their study provides a potential way to use the geological record to estimate ancient magma flux rates even if material has been lost from the system by eruption.
A compilation of aureoles surrounding silicic plutons around the world shows that there is a wide variation in normalized thicknesses. Comparison with thermal modelling shows some aureoles are compatible with rapid input of the entire volume of magma recorded in the pluton whereas other aureoles are very narrow and imply slow rates of magma input. There are also numerous examples of aureoles broader than can be explained easily by either instantaneous intrusion models or incremental growth of plutons. Such broad aureoles may be explained by 1) unusually high crustal temperatures just before intrusion and 2) the contribution of magma that has subsequently been lost from the pluton system due to eruption.
The Shinshiro tonalite and the Busetsu granite are two plutons formed in the same Hongusan area and have similar zircon U–Pb ages. Although the Busetsu granite has a larger volume, it is associated with a much narrower aureole than the Shinshiro tonalite. Because the intrusions formed at the same time in the same area, the difference in aureole widths cannot be explained by contrasting background crustal temperatures. Thermal modeling shows that only eruptive loss of magma can adequately account for the characteristics of the Shinshiro tonalite aureole. Combining petrological studies of metamorphic aureoles with thermal modeling can identify plutons that developed in association with major volcanic eruptions and constrain ancient magma flux.
A compilation of aureoles surrounding silicic plutons around the world shows that there is a wide variation in normalized thicknesses. Comparison with thermal modelling shows some aureoles are compatible with rapid input of the entire volume of magma recorded in the pluton whereas other aureoles are very narrow and imply slow rates of magma input. There are also numerous examples of aureoles broader than can be explained easily by either instantaneous intrusion models or incremental growth of plutons. Such broad aureoles may be explained by 1) unusually high crustal temperatures just before intrusion and 2) the contribution of magma that has subsequently been lost from the pluton system due to eruption.
The Shinshiro tonalite and the Busetsu granite are two plutons formed in the same Hongusan area and have similar zircon U–Pb ages. Although the Busetsu granite has a larger volume, it is associated with a much narrower aureole than the Shinshiro tonalite. Because the intrusions formed at the same time in the same area, the difference in aureole widths cannot be explained by contrasting background crustal temperatures. Thermal modeling shows that only eruptive loss of magma can adequately account for the characteristics of the Shinshiro tonalite aureole. Combining petrological studies of metamorphic aureoles with thermal modeling can identify plutons that developed in association with major volcanic eruptions and constrain ancient magma flux.