ABSTRACT

U-Pb isotopic analyses of zircons from two granodiorites of the Abukuma Plateau indicate a period of plutonic activity in that area approximately 110m.y. ago. Zircons from two granodiorites of the Shidara area have anomalously high ²⁰⁷Pb/²⁰⁶Pb ratios, resulting from older radiogenic lead acquired during generation or emplacement of the magmas. The inherited component has a minimum age of about 600 m. y. and probably is considerably older. Two periods of emplacement are suggested for the Shidara granodiorites, one at 70-75 m.y. and the other at perhaps 80-90 m.y.