Abstract
The Ishizuchi Group in northwestern Shikoku was formed by a part of the Middle Miocene igneous activities associated with the Sea of Japan opening and the onset of the Shikoku Basin subduction. Thus, study of the group is expected to reveal information on these important tectonic events in the Southwest Japan arc. Most previous studies on the Ishizuchi Group have focused on the later stages of the igneous activity that formed the group. Detailed stratigraphic and petrological study of the lower part of the group is essential to understanding the complete picture of this igneous activity and its tectonic implications. In this study, we investigate the Takano Formation, the lowermost part of the group, along the Namekawa Gorge in Toon, Ehime Prefecture.
The Takano Formation in the study area comprises garnet-bearing volcaniclastic rocks and a basal conglomerate. The lower part of the formation consists of welded lapilli tuff, and the upper part of crystalline tuff characterized by phenocrysts of β-quartz, with a transitional zone between the two parts. Bulk-rock and garnet geochemical analyses reveal that the Takano Formation was formed by the eruption of at least three magmas with slightly different compositions. The lower part of the formation was deposited in association with a rhyolitic magma with a large Eu negative anomaly and Mg-poor almandine. Subsequently, the upper part of the formation was deposited in association with a rhyolitic magma with a weaker Eu anomaly and both Mg-poor and Mn-poor almandine. During the deposition of the upper part of the formation, the chemical composition of the magma gradually became more dacitic due to mixing with a more mafic component. After a short pause in igneous activity, the andesitic Kuromoritoge Formation was deposited. Zircon U–Pb ages indicate that all of these occurred within a short period around ~14.5 Ma.
