U–Pb ages of granitoids around the Kofu basin: Implications for the Neogene geotectonic evolution of the South Fossa Magna region, central Japan

Abstract

The Japanese archipelago underwent two arc-arc collisions during the Neogene. Southwest Honshu arc collided with the Izu-Bonin-Mariana arc and the northeast Honshu arc collided with the Chishima arc. The complicated geological structure of the South Fossa Magna region has been attributed to the collision between the Izu-Bonin-Mariana arc and the southwest Honshu arc. Understanding the geotectonic evolution of this tectonically active region is crucial for delineating the Neogene tectonics of the Japanese archipelago. Many intrusive granitoids occur around the Kofu basin, in the South Fossa Magna region. Although the igneous ages of these granitoids have been mainly estimated through biotite and hornblende K-Ar dating, here, we perform U-Pb dating of zircon to determine the igneous ages more precisely. In most cases, the secondary post-magmatic overprint on the zircon U-Pb system was minor. Based on our results, we identify four groups of U-Pb ages: ca 15.5 Ma, ca 13 Ma, ca 10.5 Ma, and ca 4 Ma. The Tsuburai pluton belongs to the first group, and its age suggests that the granite formation within the Izu-Bonin-Mariana arc dates back to at least 15.5 Ma. The granitoids of the second group intruded into the boundary between the Honshu arc and the ancient Izu-Bonin-Mariana arc, suggesting that the arc-arc collision started by ca 13 Ma. As in the case of the Kaikomagatake pluton, the Chino pluton likely corresponds to a granodiorite formed in a rear-arc setting in parallel with the other granodiorites of the third group. The U-Pb age of the Kogarasu pluton, which belongs to the fourth group, is the same as those of the Tanzawa tonalitic plutons. This might support a syncollisional rapid granitic magma formation in the South Fossa Magna region.

Trace‐element composition of zircon in Kofu and Tanzawa granitoids, Japan: Quantitative indicator of sediment incorporated in parent magma

“…Cathodoluminescence images of representative zircons from each pluton. White and blue dotted circles mean analytical spots of Sawaki et al (2020) and Suzuki et al (2015), respectively.…”

Rock pillars shaped by columnar joints in granite at Mt. Mizugaki, Central Japan

“…Mizugaki, there is a roof pendant of the Cretaceous accretionary complex of the Shimanto Group (Figure 2). The Mizugaki granite is mainly composed of biotite granite (Sato & Ishihara, 1983;Sawaki et al, 2020). The zircon U-Pb age of the pluton yields 15.28 ± 0.33 Ma (Sawaki et al, 2020).…”

“…The Mizugaki granite is mainly composed of biotite granite (Sato & Ishihara, 1983;Sawaki et al, 2020). The zircon U-Pb age of the pluton yields 15.28 ± 0.33 Ma (Sawaki et al, 2020). The latest ideas on geotectonic evolution of the Kofu plutonic body are referred to Sawaki et al (2020).…”

“…The zircon U-Pb age of the pluton yields 15.28 ± 0.33 Ma (Sawaki et al, 2020). The latest ideas on geotectonic evolution of the Kofu plutonic body are referred to Sawaki et al (2020).…”

An obelisk‐shaped granitoid tower at Mt. Jizogadake in the Southern Alps of Japan: A 3‐D morphological study

“…The Kaikomagatake pluton intruded into the Shimanto accretionary complex of Cretaceous–Paleogene age (Figure 3), forming a contact metamorphic aureole of biotite and cordierite zones in the accretionary complex on its west as far as 5 km from the contact (Sawaki et al, 2020; Yuasa, 1976). The eastern part of the pluton is cut by the west‐dipping fault of the Itoigawa‐Shizuoka Tectonic Line, of which reverse displacement uplifted the mountains, including Mt.…”