[SGL31-P01] Stratigraphy and age of the Harachiyama Formation in Hachinohe - Tanesashi Coast area
Keywords:Northeast Japan arc, Cretaceous subduction zone, Arc volcanic rock, Zircon U-Pb age
Stratigraphy: We newly divided middle to upper parts of the Harachiyama Formation in the study area into H1 to H6 lithological units in stratigraphically ascending order. H1 and H4 are composed mainly of tuff, H2 and H5 of andesite tuff breccia, H3 of rhyolite tuff breccia, and H6 of rhyolite lava. Andesite dikes, andesite and basaltic sheet, and granophyre intruded in H5to H6. Bedding planes strikes NW-SE and dip northeast.
Zircon U-Pb age: Zircon U-Pb ages of 124 - 126 Ma were obtained from the three samples collected from H 3, H 5, and H 6. Zircons from H6 rhyolite occasionally contain inherited cores of Proterozoic and Triassic - Jurassic ages. Age distribution of the inherited cores is similar to that of detrital zircons in sandstone of the northern Kitakami accretionary complex as the basement. It is thus considered that the inherited cores originated from detrital zircons in the basement sandstone partly melted or assimilated during evolution of the rhyolitic magma.
Conclusion: The obtained zircon U-Pb ages of the Harachiyama Formation are almost contemporaneous with the ages of the Kitakami granites in the vicinity. The granites seems to have intruded immediately after the Harachiyama volcanic activity. As suggested by the inherited cores, the rhyolitic magma developed in the basement accretionary complex, whose youngest ages is as short as 5 m.y before the magmatic activity. In addition, the harachiyama volcanic activity was contemporaneous with the transition from the oceanic Sorachi Group to terrigeneous Yezo Group in central Hokkaido, implying that the emplacement of the Sorachi oceanic basin triggered the Harachiyama volcanic activity as the trench was rearranged.