Genesis of the nishiasahi basic intrusive complex in the asahi mountains prior to the cretaceous felsic magmatism

Shin Ichi Kagashima, Koji Watanabe, Takeshi Murase, Rikako Nohara-Imanaka, Yuka Hirahara, Ryoko Senda

Research output: Contribution to journalArticlepeer-review


The Asahi Mountains, located along the border between Yamagata and Niigata Prefectures, lies to the northwest of the Tanagura Shear Zone, which represents the boundary of pre- Neogene basement rocks. The Asahi Mountains are comprised of Cretaceous plutonic rocks of various lithologies, of which the Nishiasahi basic intrusive complex composed of quartz dioritic rocks and gabbroic rocks represents the first phase of intrusive activity. Geochemical modeling suggests that the diŠerentiation of the quartz dioritic rocks resulted from the fractional crystallization of 25.4% amphibole, 23.0% plagioclase, 8.2% biotite, and 1.1% magnetite, whereas the gabbroic rocks were formed by the accumulation of these fractionated minerals. The gabbroic rocks have slight geochemical variations, which can be explained by the selective accumulation of the minerals. The initial Sr and Nd isotopic ratios of the Nishiasahi basic intrusive complex, corrected using an age of 120 Ma, are in the range of 0.70697-0.70797 and 0.51223-0.51229, respectively. This suggests the existence of an enriched source material, resembling the case of the Ryoke basic intrusive rocks in the Southwest Japan arc. The primitive magma of the quartz dioritic rocks geochemically corresponds to the experimental melt composition obtained by Rapp and Watson (1995) for amphibolite under the conditions of 1075 °C and 0.8GPa. Therefore, an enriched mafic lower crust (i.e., enriched amphibolite) would be appropriate for the source material of the Nishiasahi basic intrusive complex.

Original languageEnglish
Pages (from-to)189-204
Number of pages16
JournalJapanese Magazine of Mineralogical and Petrological Sciences
Issue number4
Publication statusPublished - 2015
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology
  • Economic Geology


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