K‒Ar ages of the Mikabu Greenstones in the northwestern part of Wakayama Prefecture, Southwest Japan

Abstract

    In the northwestern part of the Kii Peninsula, strata of the Sambagawa (Sambagawa Belt in strict sense and Mikabu Belt), Chichibu and Shimanto Belts are distributed from north to south based on metamorphic grade, geologic age and lithologic characters. The Mikabu Belt is occupied by the Oishi, Numata and Saga Formations from north to south.
    K‒Ar ages were determined on white mica grains for 23 samples from the Mikabu Belt, 22 samples of which are pelitic schists and one sample is a greenschist.
    K‒Ar ages of the Oishi, Numata and Saga Formations range over 81.7‒98.2 Ma, 96.7‒117.0 Ma and 113‒118 Ma, respectively. The Numata Formation ranges over 96.7‒125.1 Ma, including the data of Kurimoto (1995). Judging from metamorphic grade and closure temperature of the K‒Ar system in muscovite, it is believed that K‒Ar ages from these Mikabu samples show the time of metamorphism.
    The K‒Ar age of the Oishi Formation is roughly concordant with that of the Kebara Formation (Kurimoto, 1993) which is considered to belong to the Mikabu Belt. On the other hand, the K‒Ar ages of the Saga and the Numata Formations are older than those of the Oishi and the Kebara Formations.