CHARACTERISTICS OF CRETACEOUS IGNEOUS ACTIVITY IN JAPAN ARC

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1) The pre-Cretaceous lithology, stratigraphy, age and geolgic structures were investigated to clarify the background of the Cretaceous igneous activity in Japan.2) Petrographic description, compositional zonation and measurements of Sr-isotopic ratios of some plutons were carried out for the basic data.3) Some dyke rocks in Zone IV plutons are shoshonite and high-K andesite having higher Nb content than those in island-arc tholeiites, and have different mineral compositions, therefore, the occurrence of those rocks has a important significance for the tectonic setting of the Kitakami Mountains during the Cretaceous age.4) The high-Sr series rocks of Zone II plutons in the Kitakami Mountains are characterized by high SiO_2 greater than 61 wt. %, higher Na_2O and lower K_2O, slightly higher Al_2O_3 and lower FeO* and MgO.They are richer in Sr and Ga, and poorer in Rb, Pb and Y.These characteristics are similar in composition to those of the high-Al_2O_3 trondhjemite, tonalite and dacite (high-TTD). Slab-melting was playda greater role for the origin of high-Sr series rocks.5) On the characteristics of the Southwest Japan-are from thecretaceous to Miocene age, it is clarified that the products of the Shimanto-autochthonous activity is those of the bimodal ones at the outer-uprifted zone developed at the fore-arc in the active continental margin.