上野玄武岩類および地蔵峠火山岩類のK-Ar年代と化学組成の時間変化

抄録

K-Ar age determinations along with major- and trace-element analyses were carried out on the Ueno Basaltic Rocks and the Jizo-toge Volcanic Rocks, distributed around Quaternary Ontake and Norikura Volcanoes in central Japan. The Ueno Basaltic Rocks consist of olivine-dominant basalt and basaltic andesite lava flows, intrusives and associated pyroclastics. They are distributed sporadically in an area of 60 by 60 kilometers square and are products of independent monogenetic volcanoes. Most of the volcanism occurred during 2.8 and 1.4 Ma and an exceptionally younger lava erupted at 0.9 Ma. The Jizo-toge Volcanic Rocks are distributed in an area of 20 by 30 kilometers square and are composed of lavas and volcaniclastics of plagioclase-dominant andesite with a minor amount of dacite and basalt. The volcanism started at 3.5 Ma and terminated at 1.6Ma. Most of them are inferred to be originated from a compound volcano. The earliest lavas of the Ueno Basaltic Rocks have low LILE/HFSE (large-ion-lithophile element/high-field-strength element) ratios, while there is a clear trend of the increasing LILE/HFSE ratios with time. The MORB-normalized incompatible-element pattern of the earliest lava shows its affinity with intra-plate basalts characterized by absence of Nb and Zr negative anomalies. In contrast, the incompatible-element pattern of a basalt belonging to the Jizo-toge Volcanic Rocks shows high LILE/HFSE ratios with Nb depletion. These characteristics are similar with those of basalts in Quaternary Ontake Volcano and are common in island-arc basalts.