Granitoids with 300 Ma in the Joban coastal region, east of Abukuma Plateau, northeast Japan

Abstract

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.