Tectonic history of granitic bodies in the South Fossa Magna region, central Japan: new evidence from fission-track analysis of zircon

Abstract

A fission-track (FT) analysis of zircons has been performed to clarify tectonic history of the Tertiary granitic bodies in the northern part of the Izu—Bonin arc and the South Fossa Magna region of central Japan. A total of 30 FT ages was obtained from the Kaikoma, Tsuburai, Ashigawa, Tokuwa and Tanzawa granitic bodies. Confined FT lengths were also measured on 11 zircon samples from the granitic rocks to obtain more information on the thermal history of the region. The FT ages and confined FT length data from the Kaikoma indicate that this granitic body cooled below the isotherm of zircon (240 °C) at 9.4± 0.4 Ma (2 σ). The Tsuburai, Ashigawa, Tokuwa and Tanzawa granitic bodies, which have a similar lithology and tectonic setting, lie from northwest to southeast, respectively. The FT ages for these bodies are: 15.0 ± 0.6Ma for the Tsuburai; 11.9 ± 0.4 Ma for the Ashigawa; 12.5 ± 1.4 Ma for the Tokuwa (which is comparable with the FT age of the Ashigawa); 7.1 ± 2.0 Ma for the Tanzawa (which is the youngest). The data indicate a differential uplift of the granitic bodies in the South Fossa Magna region. In general, the FT ages become younger as one proceeds southeastwards approaching the Izu-Bonin arc. The FT age data provide constraints on the tectonic effects of the collision of the Izu-Bonin arc with the central Honshu of the Japan arc.