Zircon U–Pb dating of gabbro and diorite from the Bato pluton, northeast Japan

To constrain the timing of the tectonothermal events and formation process of a plutonic suite, U-Pb dating was carried out by laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence imaging on zircon grains extracted from the Bato pluton, northern Yamizo Mountains, Japan. The Bato pluton consists of gabbro and diorite. Zircon grains separated from a gabbro sample had a unimodal U-238-Pb-206 age (105.7 +/- 1.0 Ma). It was interpreted as the solidification age of the gabbro. Cathodoluminescence observation showed that the zircon grains from a diorite sample were characterized by anhedral cores, oscillatory zoned mantles, and dark rims. The U-238-Pb-206 age of the anhedral cores ranged from 2 165 Ma to 161 Ma, indicating the assimilation of surrounding sedimentary rocks. The U-238-Pb-206 ages of the oscillatory zoned mantles and dark rims are 109.0 +/- 1.3 Ma and 107.7 +/- 1.3 Ma, respectively. Observation under polarizing microscopy suggests that the anhedral cores occurred before plagioclase and hornblende, and the oscillatory zones around the anhedral cores had crystallized at the same time as the crystallization of biotite. Moreover, the dark rims formed at the same time as the crystallization of quartz and K-feldspar. The formation process of the gabbro-diorite complex in the Bato pluton was inferred as follows. (i) A mafic initial magma intruded into Mesozoic sedimentary rocks, and the assimilation of these sedimentary rocks led to geochemical variation yielding a dioritic composition. Subsequently, plagioclase and hornblende of the diorite were crystallized before 109.0 +/- 1.3 Ma. (ii) Biotite crystallized in the middle stage around 109.0 +/- 1.3 Ma. (iii) Quartz and K-feldspar of the diorite were crystallized at 107.7 +/- 1.3 Ma. The gabbroic magma solidified (105.7 +/- 1.0 Ma) after solidification of the diorite.