秩父盆地尾田蒔丘陵にみられる中期更新世テフラの記載岩石学的特徴

概要

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Petrographic properties of Middle Pleistocene tephras on the Odamaki Hills, Chichibu, Saitama Prefecture, central Japan are described. Examined tephras are similar to each other in containing abundant hornblende phenocrysts, but represent a variety of heavy mineral composition including cummingtonite, oxyhornblende, and biotite. Of them, biotite-rich tephras are correlatable with the APms known as Middle Pleistocene widespread marker tephras in central Japan. Refractive indices (n 2 ) of cummingtonite in pumiceous tephras generally represent low values ranging from 1.657 to 1.661. Hornblende phenocrysts in most pumiceous tephras range in refractive index (n 2 ) from 1.665 to 1.672. Those of the lowermost tephra layer (OD1), however, exhibit a characteristically higher value of 1.672-1.681.
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