つくば南観測井(茎崎コア)の基盤深成岩の統合放射年代とその帰属  [in Japanese] Integrated isotopic ages and correlation of the basement plutonic rocks of the borehole (Kukizaki core) at southern Tsukuba City, Japan  [in Japanese]

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Abstract

2006年に防災科学技術研究所が掘削したつくば南観測井の深度801-1,117 mの基盤岩コア(以下茎崎コア)について,岩石学的,年代学的検討を実施した.コアはマイロナイト化した角閃石トーナル岩~斑れい岩であり,その一部では北傾斜の面構造,左ずれ逆断層の剪断のセンスが確認された.斑れい岩中の角閃石K-Ar年代は66.1 Ma,トーナル岩中のSHRIMPジルコンU-Pb年代は86.3 Ma, Sr同位体比初生値は0.7067,0.7073となった.これらの年代およびSr同位体比初生値から,茎崎コアの帰属は筑波花崗岩類には求められず,中部地方や関東の領家花崗岩類に対比できる.したがって,コアに認められた厚さ200 mを越えるマイロナイト帯は,関東平野下の中央構造線の延長部である可能性が高い.その南側に位置し,中央構造線を切断するとされていた利根川構造線の位置の再検討が必要である.

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 <sup>87</sup>Sr/<sup>86</sup>Sr 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.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 121(9), 325-337, 2015

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130005113548
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    026809764
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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