抄録
The GSJ B326 borehole was drilled in the Joban coastal region, east of the Abukuma Plateau, reached a depth of ∼ 1005 m. Although most of the sequence is composed of Tertiary sediments, plutonic rocks occur from 815.1 m to 1005 m in depth. The lithology of the plutonic rocks is tonalite and subordinate aplite veins. Radiometric ages of zircons in three granitoids were obtained from the 238U/206Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe (SHRIMP II). The weighted mean of the zircon ages of a sample from the shallower part of the borehole is 293 ± 1.8 Ma, whereas that for two samples from the deeper part of the borehole is 300.3 ± 1.5 Ma and 304.3 ± 1.7 Ma. The previously reported U-Th-Pb chemical age of uraninite from the deeper part of the borehole is 285 ± 4 Ma. These data indicate that a melt intruded into the ∼ 300 Ma rock body at ∼ 290 Ma. Granitoid with 300 Ma are scarce in the Japanese Islands, and this is the first report of such granitoids in northeast Japan.
