The Tanzawa-garnet pumice:

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  • Tamura Itoko
    Department of Geography, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University
  • Takagi Hideo
    Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University
  • Yamazaki Haruo
    Department of Geography, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University

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Other Title
  • 南関東に分布する2.5 Maの広域テフラ：
  • 南関東に分布する2.5Maの広域テフラ--丹沢-ざくろ石軽石層
  • ミナミカントウ ニ ブンプ スル 2 5Ma ノ コウイキ テフラ タンザワ ザクロイシ カルイシソウ
  • TheTanzawa-garnetpumice: widely distributed 2.5 Ma tephra in the southern Kanto area, Japan
  • widely distributed 2.5 Ma tephra in the southern Kanto area, Japan
  • 丹沢－ざくろ石軽石層

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Abstract

We report a distinct garnet-bearing pumiceous tephra in Lower Pleistocene (following the geological time scale in ISC chart 2009) sediments (Kazusa Group) from outcrops in Choshi (Na-G), Kamakura (KGP), and Aikawa-cho (Kanagawa, Mk19), and from a spot core sample (depth: 1217 m) at Koto (Tokyo, KT1217) in the Southern Kanto region. To correlate these garnet-bearing tephras and discuss their possible source, we examined their petrography and performed EPMA and grain-size analyses of constituent garnets, as well as considering their stratigraphic context.<br>The garnet-bearing tephras contain sponge- and fiber-shaped glass shards, abundant garnet grains that contain ilmenite inclusions, minor amphibole (orthoamphibole and hornblende), minor biotite, and rare ortho- and clinopyroxenes and cordierite. They also contain rock fragments of green schist and minor granitic rocks. The range of chemical compositions of garnet is XFe=0.50−0.59, XMn=0.18−0.26, XMg=0.14−0.17, and XCa=0.07−0.11, similar to that of garnet phenocrysts from rhyolite (c. 2.5 Ma) in the Nakagawa area, southern Tanzawa Mountains. This garnet composition is markedly different from that of garnet phenocrysts from other Neogene-Quaternary tephras and volcanic rocks in central Japan. The age of the studied garnet tephras is estimated at about 2.5 Ma, based on the stratigraphic horizons at the CN12b-CN12c zone (Okada and Bukry, 1980), the NPD9 zone (Yanagisawa and Akiba, 1998), and the N.21 zone (Yanagisawa, 2006), as well as the magnetostratigraphy of the lower part of the Matuyama Chron.<br>The results indicate that the garnet tephras at the four locations are clearly correlatives, representing the newly defined Tanzawa garnet pumice (Tn-GP). The likely source of Tn-GP is located west of Aikawa-cho, in the Tanzawa Mountains, given the westward increase in garnet grain size, the existence of green schist fragments that correlate with the Tanzawa Group, and the existence of ∼2.5 Ma garnet-bearing rhyolite that intrudes into the Tanzawa Group. Tn-GP is likely to play an important role in terms of constraining the P/P boundary in the new geological time scale, the age of the Kurotaki Unconformity, and the subsurface geology of the Kanto Plain.

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