Solidification pressures and ages recorded in mafic microgranular enclaves in the Kurobegawa Granite

Suzuki, Kota*; Kawakami, Tetsuo*; Sueoka, Shigeru   ; Yamazaki, Ayu*; Kagami, Saya   ; Yokoyama, Tatsunori   ; Tagami, Takahiro*

The Kurobegawa Granite is the youngest exposed pluton in the world and its exhumation mechanism is highly debated. In reconstructing exhumation histories of young plutons, combining solidification pressure estimate by Al-in-hornblende geobarometers with age determination by U-Pb zircon dating is an effective method. In the case of the Kurobegawa pluton, however, rare hornblende occurrence in the host granite makes it difficult to estimate the solidification pressure by this method. Alternatively, we focus on mafic microgranular enclaves (MMEs) and evaluate whether the solidification pressures and ages of MMEs coincide with those of the host granite. In each sample, the estimated solidification pressures of the MMEs and the host granite overlap well. The ages for MMEs tend to be younger than the host granites, although they slightly overlap within uncertainty. The age variations may reflect the difference in timing of zircon crystallization between the MMEs and the host granites. This is supported by the relatively lower zircon saturation temperatures of the MMEs compared to the lower Kurobegawa Granites. Therefore, the U-Pb zircon ages of the MMEs may better represent the final solidification timing of the host granites. Assuming rock density of 2700 kg/m and lithostatic pressure, solidification depth of the MME from the middle unit is estimated as 6.9 1.1 km to 9.2 1.5 km. Dividing the solidification depth by the U-Pb zircon age obtained from the same MME gives average exhumation rate of 7.1-14.5 mm/yr. Similarly, the MME from the lower unit gives average exhumation rate of 5.5-14.4 mm/yr. Further petrological and geochronological studies using MMEs throughout the Kurobegawa pluton may help us to understand the exhumation mechanism of the world's youngest exposed granitic pluton.