Ascent of migmatites of a high-temperature metamorphic complex due to buoyancy beneath a volcanic arc: a mid-Cretaceous example from the eastern margin of Eurasia

摘要

Mixtures of melt and residue in a high-T metamorphic complex have a lower density and viscosity than the surrounding host crust, and the mixtures should ascend due to buoyancy. The mixtures are recognized as migmatites in the high-T metamorphic complex. To confirm ascent of migmatites, we conducted numerical simulations of ascent of a model migmatite (buoyant viscous fluid). The numerical simulations show that the model migmatite could rise to shallow levels of a model crust so long as it is continuously produced at the bottom of the model crust. Otherwise it ceases to rise at depth due to loss of buoyancy by cooling. The numerical simulations also show that the model migmatite experiences vertical thinning during the ascent. The ascent mechanism proposed in this paper requires the continuous production of partially melted rocks at the base of the crust, which is provided by a continuous input of energy into the crust from the mantle. Given that high-T metamorphic complexes are associated with igneous activity beneath a volcanic arc, the igneous activity reflects the energy input into the lower crust from the mantle. A high-grade part (migmatites) of a high-T metamorphic complex in the Omuta district of northern Kyushu, southwest Japan, experienced thinning during ascent. Large amount of igneous rocks, such as plutonic and volcanic rocks, are also distributed in northern Kyushu. Zircon U-Pb ages of igneous rocks from northern Kyushu reveal that igneous activity continued from 115 to 93 Ma, and that peak igneous activity at 110-100 Ma was synchronous with the ascent of migmatites of the high-T metamorphic complex in northern Kyushu. Therefore, the numerical simulations may provide an appropriate model of the ascent of migmatites of the high-T metamorphic complex beneath a volcanic arc, at the eastern margin of Eurasia in the mid-Cretaceous.