北上市東方に産する稲瀬火山岩類のK-Ar年代と岩石学的特徴

抄録

K-Ar dating and petrologic description were carried out on the Inase valcanic rocks from eastern part of the Kitakami district of Iwate Prefecture, Northeast Japan. We obtained K-Ar whole rock ages of 15.1±0.4 Ma and 15.5±0.4 Ma for two andesites. The results of the present and previous K-Ar age detamination indicate that the volcanic activities in the Inase district took place in the range of 15.5-13.2 Ma. The chemical compositions of representative 32 andesitic samples were determined by XRF technique. The upper Inase volcanics composed of tholeiitic andesites with SiO₂ of 53.61-60.61% have chemical characters similar broadly to those of the Quaternary andesites from the NE Japan arc, whereas the lower Inase ones composed also of tholeiitic andesites with SiO₂ of 53.86-55.28% are characterized by high contents of HFS elements such as P, Ti, Nb, Y and Zr, suggesting that the latter are different in chemical compositions from the typical island arc andesites. We estimated the trace element composition of the primary magmas of the lower and upper Inase volcanics using the method of Sakuyama and Nesbitt (1986) and Yoshida and Aoki (1988). The process identification diagram for the primary magma compositions of the Inase volcanics and the Quaternary volcanics in the NE Japan arc and the surrounding areas suggests that the upper Inase primary magma may be derived from the magma source chemically similar to the present sub-continental mantle beneath the NE Japan arc, while the lower Inase primary magma may be derived from the magma source which is more enriched in HFS element contents than the sub-continental mantle.