南部北上帯シルル系奥火の土層の凝灰岩類 Volcaniclastic rocks of the Silurian Okuhinotsuchi Formation in the Southern Kitakami Belt, Northeast Japan
The Lower-Middle Silurian Okuhinotsuchi Formation (KAWAMURA, 1977, 1980) in the Setamai Subbelt of the Southern Kitakami Belt characteristically contains intermediate to acidic volcaniclastic rocks in its lower member. The volcaniclastic rocks are welded tuff, non-welded tuff and tuffaceous slate. Also the upper member of the Formation intercalates thin layers of "tuffaceous" sandstone and pebbly limestone, which include epiclastic materials derived from acidic volcanic rock. The welded tuff constitutes the basal part of the lower member together with the basal pelitic rock which unconformably overlies the Okuhinotsuchi granite mass. And between the two, "lithic or pebbly part" carrying fragments of acidic volcanic rocks and granite is observed. While, the non-welded tuff is overlain by pale green limestone which yields some marine fossils, and interpreted as an intercalation of the middle part of the lower member. The welded tuff shows eutaxitic welded texture composed of devitrined glassy matrix and deformed essential lenses. Crystal fragments are only of plagioclase. The non-welded tuff shows vitroclastic texture consisting of devitrified vesicular glass shards. Average modal compositions of welded and non-welded tuffs are as follows; glass+matrix+essential lens, 86% and 78%; rock fragment, 6% and 11%; crystral fragment, 8% and 11%, respectively. So these are classified as vitric tuff. Crystal and rock fragments are somewhat rich in the non-welded tuff. Welded tuff is considered to be subaerial pyroclastic flow deposit, and non-welded tuff is probably formed as ash fall deposit derived from land volcano. Bulk chemical compositions of the tuffs show the approximate contents of SiO_2 66-70%, FeO+Fe_2O_3 4%, MgO 1-1.5%, CaO 0.4-0.7% and K_2O+Na_2O 6-7%. The tuffs are dacitic to rhyodacitic in chemical composition. K_2O/Na_2O ratios are variable, but total amounts of K_2O + Na_2O are rather constant. This compensative relationship of alkalis is inferred to be the result of chemical exchange during the diagenetic or comparable processes. Low content of CaO is probably due to the same cause. The Okuhinotsuchi Formation represents the lowest part of the Paleozoic (Silurian to Permian) sequence in the Southern Kitakami Belt. Early Silurian intermediate to acidic land volcanism as indicated by the tuffs of the Okuhinotsuchi Formation is interpreted to be the forerunning activity for the Devonian to Early Carboniferous island arc volcanism of the Southern Kitakami Belt.