2021 Volume 127 Issue 9 Pages 563-574
The lower-middle Miocene Myojin Formation (Kuma Group) distributed in northwestern Shikoku contains gravels of granitoids and hypabyssal rocks. Based mainly on the southward paleocurrent directions of the Kuma Group, these gravels are considered to have originated from the Ryoke and San-yo igneous rocks. The Myojin Formation, therefore, provides information on the Inner Zone of this area at that time, although the Kuma Group is now distributed mainly in the Outer Zone. Examination of these igneous rock gravels through petrological studies and zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry provides more certainty on their origin. The U-Pb ages of 99-95 Ma for the granitic gravels and ca. 90 Ma for the hypabyssal gravels are consistent with previously reported ages of the Ryoke (and San-yo) granitoids, especially those in the Takanawa Peninsula and Yanai area, which are located north and northwest of the Kuma Group distribution. Petrological characteristics of these igneous rock gravels are also comparable with the Ryoke (and San-yo) granitoids and hypabyssal rocks in the Chugoku and Shikoku regions. These results suggest-and further support the conventional view-that igneous rock gravels clasts of the Myojin Formation were derived mainly from the Ryoke igneous rocks.
