クロムスピネルの化学組成から推察する東北地方の小規模超苦鉄質岩体のテクトニクス

抄録

  Petrography and relic spinel chemistry of small ultramafic bodies disseminated in the northeastern Honshu Island, Japan are studied at Wakasennin, Yakeishidake, Motai, Kawatabi, Kinkasan, Marumori and Surikami from north to south, in comparison with the large, well studied large ultramafic bodies at Hayachine and Miyamori in the South Kitakami terrain. Ultramafic rocks from Kinkasan contain olivine, and those from Marumori and Surikami bear both olivine and orthopyroxene, but others are completely serpentinized. However, chromian spinels commonly survive in their cores. Chromian spinels from the northern areas (Wakasennin, Yakeishidake, Motai, Kinkasan) are highly chromian (Cr#0.65-0.90), implying their origin in arc-related depleted mantle. However, chromian spinels from Marumori and Surikami show relatively low Cr#(0.35-0.65), implying their origin in an arc-related, relatively fertile mantle. These Cr-rich and -poor spinels are both present in the Hayachine and Miyamori bodies. However, chromian spinels from Kawatabi show a unique, higher-Mg# trend in the Cr#-Mg# diagram, and this provides the first identification of provenance for the high Mg# detrital chromian spinels reported from the Lower Jurassic Shizugawa Group in the South Kitakami terrain. Analogous high-Mg# spinels are known from anhydrous peridotites from the Mariana forearc. Comparison with the reported spinels from the Carboniferous conglomerate of the Nedamo zone indicates that the erosion of these ultramafic bodies started as early as the Carboniferous time. The difference in spinel chemistry between the Abukuma (Surikami) and Kitakami (other bodies) terrains is not significant, indicating pervasive exposure of the Early Paleozoic Hayachine-Miyamori forearc ophiolites in the Late Paleozoic-Mesozoic forearc areas of Northeast Japan, including the Abukuma terrain.