前橋泥流の流下域に残存する安山岩塊の比重とSr同位体比.

The Maebashi volcanic mudflow deposits constitute major basement strata in the Maebashi-Takasaki area, central Japan. Sr isotope analysis of large volcanic blocks and 14C dating of buried wood blocks elucidated that the volcanic mudflow originated from a failure event of Kurofu volcano, an initial stage volcanic edifice of Asama volcano located about 50km west of these cities, which occurred ca.27 ka cal BP within the last glacial period. However, the cause of sector collapse and mechanism of long-distance movement of volcanic blocks are unknown. In order to get some clues for these problems, we studied the mode of occurrence of volcanic blocks and their specific gravity and Sr isotope ratio（87Sr/86Sr）, which are sparsely remained along the route of the Maebashi mudflow from the northern foot of Asama volcano to the Maebashi-Takasaki area through the Agatsuma and Tone Rivers. Here, we report the results for 13 volcanic rocks from a hummocky hill near Asama volcano, large blocks near the Agatsuma River and small blocks within the Maebashi mudflow deposit. All of them, except one sample, yielded narrow range Sr isotope ratios（0.70394-0.70416; av.：0.70406 ± 0.00001）close to the published data for Asama volcano. A large block transported by the Tenmei mudflow caused by the eruption of Asama volcano in 1783 was confirmed to have the same isotope ratio as above values. The two large blocks similar to Iwagami-no-Tobi-ishi, a representative large block related to the Maebashi mudflow, occur on the slopes 30-50m above present riverbed of the Agatsuma River, suggesting the devastating volume of the mudflow which transported these blocks. An exceptional case on the riverbed of the Tone River in the foot of Akagi volcano was interpreted as a block originated from the eroded hummocky hill from Akagi volcano. [ABSTRACT FROM AUTHOR]