Zircon U–Pb dating from the mafic enclaves in the Tanzawa Tonalitic Pluton, Japan: Implications for arc history and formation age of the lower-crust

Highlight
• We performed zircon U–Pb dating from mafic enclaves in the Tanzawa Tonalitic Pluton (TTP).
• We found older zircons (10–43 Ma) in mafic enclaves than in host tonalite.
• Formation of lower-crust beneath the TTP would have begun before ca. 43 Ma.
• The age of continental lower-crust can be obtained using zircons in mafic enclaves.

The petro-chemical characteristics of the arc lower-crust, important for understanding continental growth, have been rarely obtained because of their scarcity at the surface of the Earth. To constrain the formation age of the arc lower-crust, U–Pb zircon dating was applied to mafic enclaves in tonalites of the Tanzawa Tonalitic Pluton (TTP), which is regarded as the exposed middle crust of the former Izu–Bonin–Mariana arc, using a laser ablation-inductively coupled plasma–mass spectrometer (LA-ICP-MS).The texture and shape of mafic enclaves indicate an injection of mafic magma into tonalitic magma at the mid-crustal level. While 44 zircon grains from a host tonalite show a narrow-age distribution with a mean age of 4.6 ± 0.2 Ma, 301 zircon grains from 9 mafic enclaves show wide-age distributions from ca. 5 to 43 Ma. This study is the first to reveal a U–Pb age older than previously reported for the rock materials that compose the TTP, now identified to be 18 Ma compared with an age range from 4 Ma to 9 Ma. Because there are no other components of the TTP yet identified to be older than 17 Ma, the zircons separated from the TTP in this study which dated to be 18–43 Ma are interpreted to be xenocrysts derived from the arc lower-crust beneath the TTP. The oldest zircon age obtained from the mafic enclaves indicates that the formation of the arc lower-crust beneath TTP took place before 42.9 ± 8.6 Ma, consistent with the history of the Izu–Bonin–Mariana arc. This is the first time that the formation age of the lower crust has been estimated using zircons from mafic enclaves. This study shows that the zircon U–Pb dating from mafic enclaves in granites can yield significant information about the age of the continental lower crust.