K-Ar ages of metamorphic clasts from the Miocene Aoiwa Conglomerate, Hiki Hills, Central Japan: Implications for exhumation history of source rocks

  • Arai Hiroyoshi
    Department of Earth Sciences, School of Education, Waseda University
  • Takagi Hideo
    Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University
  • Iwasaki Ichiro
    Department of Earth Sciences, School of Education, Waseda University
  • Asahara Yoshihiro
    Geochemistry and Cosmochemistry Laboratory, Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University
  • Takahashi Masaki
    Paleogeodynamics Research Group, Institute of Geology and Geoinformation, Geological Survey of Japan, AIST

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Other Title
  • 関東山地東縁部,比企丘陵中新統青岩礫岩中の変成岩礫のK-Ar年代
  • カントウ サンチ トウエンブ ヒキキュウリョウ チュウ シントウアオイワレキガン チュウ ノ ヘンセイガンレキ ノ K Ar ネンダイ

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Abstract

The exhumation of high-P/T metamorphic rocks plays a key role in the rearrangement of the geotectonic framework in arc-trench systems. The deposition of clasts of exhumed rocks constrains the timing of exhumation and the denudation history. We obtained the K-Ar ages of clasts from the Miocene Aoiwa Conglomerate, Hiki Hills, eastern Kanto Mountains. The Aoiwa Conglomerate is an unsorted breccia (maximum clast diameter up to ca. 3 m) composed of clasts of high-P/T metamorphic rocks (greenstones and crystalline schists), granitic rocks, low-P/T metamorphic rocks (mafic to felsic plutono-metamorphic rocks), and sedimentary rocks.<br>The clasts of high-P/T metamorphic rocks were previously considered to have been derived from the Sanbagawa metamorphic rocks, which have been exposed nearby since the Miocene. However, schist clasts yield K-Ar phengite ages of 127.2-121.0 and 98.4-80.7 Ma, significantly older than ages obtained for the Sanbagawa metamorphic rocks in the Kanto Mountains, although the mineral assemblage and metamorphic grade of clasts are similar.<br>Clasts of granitic rocks and low-P/T metamorphic rocks yield K-Ar hornblende ages of 131.5-114.0 Ma and phengite ages of 97.8-88.9 Ma, significantly older than ages obtained for Ryoke granites in the Kanto Mountains. In addition, a tonalite mylonite clast yields a lower 87Sr/86Sr initial ratio (0.70510-0.70520 or 0.70498-0.70505) than that of the Ryoke granites.<br>Based on the results of K-Ar dating and a detailed petrologic study of the clasts, we conclude that the schist clasts were derived from an older unit within the Sanbagawa metamorphic rocks and that granitic and mafic metamorphic clasts were derived from the paleo-Ryoke terrane, which previously overlaid the Sanbagawa metamorphic rocks and today occurs only as sporadic allochthonous rocks. The Aoiwa Conglomerate was probably deposited in association with denudation of the paleo-Ryoke and uppermost Sanbagawa terranes. Deposition was accelerated during exhumation of Sanbagawa metamorphic rocks during the Middle Miocene.

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