We investigated the petrology and geochronology of Pre-Neogene basement core samples obtained at Kukizaki (southern Tsukuba observatory well) in 2006 by the National Research Institute for Earth Science and Disaster Prevention (NIED). The depths of the excavated core samples of the basement rocks were 801-804 m, 944-947 m, 1,111-1,114 m, and 1,114-1,117 m. The basement rocks consisted mainly of mylonitized hornblende biotite tonalite (shallower cores) and hornblende gabbro (deeper cores). In the 944-947-m core, the mylonitic foliation dips northward at low to high angles (average dip, ~40°), as determined from an oriented sample. The thickness of the mylonite zone is at least 200 m. Kinematic indicators within the oriented mylonite sample indicate a top-to-the-southwest thrust/sinistral shear sense. The K-Ar hornblende age obtained on the gabbro (depth, 1,115 m)is 66.1±1.1 Ma, and the SHRIMP U-Pb zircon age of tonalite (depth, 803 m) is 86.3±0.6 Ma. Initial
87Sr/
86Sr ratios (SrI), calculated using the U-Pb ages of whole-rock samples of both tonalite (depth, 803 m) and gabbro (depth, 1,115 m), are 0.7067 and 0.7073, respectively. These isotopic ages and SrI values of rocks from the Kukizaki core are concordant with those of the Ryoke granitic rocks, including the Iwatsuki mylonite obtained at a depth of 3506.6 m in the Kanto Plain, whereas the ages are discordant with the Tsukuba granitic rocks, which are younger in age (ca. 60 Ma) and show higher SrIs (>0.711). However, zircon fission-track age dating of the tonalite sample (depth, 803 m) yielded an age of 16.9±0.6 Ma (Miocene), which is discordant with the U-Pb age but is similar to the K-Ar biotite age of 17.65±0.29 Ma of the Iwatsuki quartz porphyry (depth, 3346-2864 m), suggesting that thermal effects of the acidic intrusion at Iwatsuki extend eastward to Kukizaki. Considering the isotopic data and the sinistral/thrust shear sense in the Kukizaki core, we infer that the mylonite zone in the Kukizaki core is the northeastern extension of the mylonite zone observed in the Ryoke belt along the Median Tectonic Line in the Kanto region.
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