CHARACTERISTICS OF CRETACEOUS IGNEOUS ACTIVITY IN JAPAN ARC

1)研究了日本白垩纪前白垩纪的岩性、地层、年龄和地质构造，阐明了日本白垩纪火成岩活动的背景。2)进行了岩石描述、成分分带和某些深成岩体的锶同位素测定。3)IV区深成岩脉中的一些脉岩是钠长花岗岩和高钾安山岩，其Nb含量高于岛弧拉斑玄武岩，因此具有不同的矿物组成。这些岩石的产出对北上山脉白垩纪的构造背景具有重要意义。4)北上山脉二区深成岩体的高锶系列岩石具有高SiO_2>61wt.%，高Na_2O，低K_2O，Al_2O_3略高，FeO~*和Mg偏低，Sr、Ga较丰富，Rb、Pb和Y较差的特征，这些特征在成分上与高Al_2O_3奥长花岗岩、英安岩和英安岩(High-TTD)相似。5)结合日本西南部地区白垩纪至中新世的特点，阐明了本区本源活动的产物是活动大陆边缘弧前发育的外隆起带的双峰式活动产物。

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.