南部北上帯の石炭系(その2) : 構成岩類の形成環境

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The Carboniferous tectonic and sedimentary conditions of the South Kitakami Terrane are discussed on the basis of the petrochemistry of the volcanic rocks and carbonate sedimentology. The Carboniferous deposits of the terrane can be grouped into lower and upper facies from the lithological viewpoint. The lower facies is represented by siliciclastic rocks including an amount of volcanic rocks, while the upper is characterized by the dominance of carbonates. The volcanic rocks of the lower facies are composed of acid tuff, basaltic volcaniclastics and lava. Petrochemically, the basaltic rocks are totally subalkaline and include high-TiO_2 tholeiites. The volcanic rocks apparently show the bimodal nature on SiO_2 contents. The Early Carboniferous volcanism took place presumably in a volcanic arc to back-arc region. The carbonate rocks of the upper facies are classified into seventeen lithofacies types. They are grouped into five lithofacies assemblages, each of which individually reflects a depositional facies. The vertical and lateral distributions of these assemblages show a evolution of the sedimentary body from ramp type to rimmed-shelf type. During the Carboniferous time, the South Kitakami Terrane was situated in an active margin. The bimodal volcanism associated with differentiated basins apparent in the lower facies suggests a somewhat tentional tectonism in a volcanic arc to back-arc region during Tournaisian to Middle-Late Visean. The formation of carbonates at the Late Visean was initiated by the declining of the volcanism and global sea-level rise. Gradual subsidence of the basement led to the development of vast carbonate shelf with slope facies during Namurian to Early Westphalian.
地学団体研究会の論文
1989-05-25

南部北上帯の石炭系(その2) : 構成岩類の形成環境

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