Rb-Sr geochronology of the Hikami granite, Kitakami mountains, Japan

Whole-rock samples of the Hikami granite from the southern Kitakami mountains in Northeast Japan give a Rb-Sr age of 339+-12 m.y with an initial 87Sr/86Sr ratio of 0.7079+-0.0003, indicating that the granite intruded in the late Devonian to early Carboniferous time. A slightly high initial 87Sr/86Sr ratio for the Hikami granite suggests that the granite assimilated some amount of older crust. The Tsubonosawa gneiss, which is contained in the Hikami granite as xenolithic masses, gives an isochron age of 334 m.y with an initial 87Sr/86Sr ratio of 0.7129. The results show that the Sr isotopic homogenization occurred among the whole masses of the gneiss at the time of granite intrusion. A higher initial 87Sr/86Sr ratio suggests that the gneiss originated in the Precambrian time. Granite boulders from the Devonian Ono formation do not define any isochron, whereas the sandstone from the Silurian Kawauchi formation gives an apparent age of 264 m.y. The whole-rock systems of boulders and sandstone may have remained open with respect to Rb and Sr, probably because of their involvement in sedimentary process. The Rb-Sr isotopic data for the Cretaceous Tono and Kesengawa granites confirm the evidence that almost all igneous rocks in Northeast Japan, irrespective of age, have low initial 87Sr/86Sr ratios, reflecting a distinctive nature of crust in Northeast Japan. (auth.)