Laboratory measurement of P-wave velocity in crustal and upper mantle xenoliths from Ichino-megata, NE Japan: ultrabasic hydrous lower crust beneath the NE Honshu arc

Ultrasonic laboratory measurements of P-wave velocity (Vp) were carried out up to 1.0 GPa in a temperature range of 25-400 °C for crustal and mantle xenoliths of Ichino-megata, northeast Japan. The rocks used in the present study cover a nearly entire range of lithological variation of the Ichino-megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast (NE) Honshu arc. The Vp values measured at 25 °C and 1.0 GPa are 6.7-7.2 km/s for the hornblende gabbros (38.6-46.9 wt.% SiO2), 7.2 km/s for the hornblende-pyroxene gabbro (43.8 wt.% SiO2), 6.9-7.3 km/s for the amphibolites (36.1-44.3 wt.% SiO2), 8.0-8.1 km/s for the spinel lherzolites (46.2-47.2 wt.% SiO2) and 6.30 km/s for the biotite granite (72.1 wt.% SiO2). Combining the present data with the Vp profile of the NE Honshu arc [Iwasaki, T., Kato, W., Moriya, T., Hasemi, A., Umino, N., Okada, T., Miyashita, K., Mizogami, T., Takeda, T., Sekine, S., Matsushima, T., Tashiro, K., Miyamachi, H. 2001. Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling. Geophys. Res. Lett. 28 (12), 2329-2332], we infer that the 15 km thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition. The present study suggests that the Vp range of the lower crustal layer (6.6-7.0 km/s) in the NE Honshu arc, which is significantly lower than that obtained from various seismic measurements (e.g. the northern Izu-Bonin-Mariana arc: 7.1-7.3 km/s), is due to the thick hydrous lower crustal layer where hornblende, plagioclase and magnetite are dominant.