Quaternary plutonic magma activities in the southern Hachimantai geothermal area (Japan) inferred from zircon LA-ICP-MS U–Th–Pb dating method

Abstract

In order to constrain the timing of plutonic magma emplacements relevant to current geothermal activity in the Hachimantai geothermal area, northeast Japan, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Th–Pb analyses were performed on zircons from two different origins. One is from the Quaternary Kakkonda Granite in the Kakkonda geothermal field, whose present temperature at ~3700m depth is 500°C, and the other is from a clastic dike in the Matsukawa geothermal field. Zircons from the Kakkonda Granite were obtained at a depth of 1998m and their 238U–206Pb and U–Th ages showed ~0.1Ma, which probably indicates that the Kakkonda Granite is the world's youngest emplaced granitic magma body ever confirmed. The clastic dike in the Matsukawa geothermal field contains dioritic to tonalitic xenoliths; therefore zircons extracted from the dike should also be of plutonic origin. 238U–206Pb age of 1.08±0.08Ma (after correction for initial 230Th disequilibrium; error shown as 95% confidence level) was obtained from the zircons. Therefore it was found that there were at least two Quaternary plutonic magma activities (~1.1Ma and ~0.1Ma) in the Hachimantai geothermal area. Through this study, it was also found that U–Th–Pb dating using a quadrupole type ICP-MS coupled with excimer laser is applicable to Quaternary zircons as young as 0.1Ma and Fish Canyon Tuff zircon is usable as a standard material for U–Th–Pb dating method.