Several small ultramafites are distributed in the Setogawa zone. Some of them, particularly Ooka peridotites and Aobane olivine-clinopyroxenite, are characterized by the prophyritic texture. These ultramafites consist of large orthopyroxene and clinopyroxene containing opposite pyroxene lamellae and groundmass-like part composed of relatively small grains of which pyroxenes do not show exsolution phenomena. Additionally the chemical compositions of two types of orthopyroxene which are morphologically distinctive, large grain with lamellae and small grain without exsolution lamellae, are plotted on the individual field of pyroxene quadrilateral, and frequency diagrams of various oxides percent such as Al2O3, CaO and Cr2O3 of the two types of orthopyroxenes display distinctive distribution patterns. But clinopyroxene does not show such clear difference between large and small grains. The texture of these rocks, the intricated boundaries between small grains and of large pyroxenes, occurrence of large pyroxenes especially clinopyroxene showing interstitial occurrence and in a few cases corroded form of large clinopyroxene, suggests that this texture is igneous origin and large crystal is to be a kind of phenocryst and small one is to form matrix. If it is correct, it is inferred that these ultramafites crystallized at relatively shallow depth in the crust and chemical compositions of matrix of such rocks inherited that of liquid which formed matrix of peridotites and olivine-clinopyroxenite. Former is able to be confirmed from the estimation of pressure condition of matrix. Pressure condition obtained is about less than 5 kb at 1030-1040℃ and 1000-1010℃ for peridotites and olivineclinopyroxenite respectively. These P. T. conditions also considered it to be unprobable to accept the metamorphic origin on the ultramafites in the Setogawa zone. Later possibility has come arise from the texture of ultramafites and as first approximate estimation, liquid composition is obtained from modal analyses of matrix, although large crystals more or less enlarged own margin with small grain simultaneously formed at the final stage of magma solidification. Results display the plagioclase-bearing Iherzolitic composition.

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[in Japanese]

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The ratio between the viscosities of different types of rocks is one of the important parameters which characterize the structures of not only minor but also large one such as crustal deformation. In this paper, the viscosity ratio between the granitic rocks and the Paleozoic sedimentary rocks, both of which are the elementary rock bodies composing the basements of the Kinki area, are estimated on the basis of the analysis of the large tectonic deformation. The Quaternary crustal deformation of this area has been considered to have taken place in the state of horizontal tectonic compression (HUZITA, 1969; HUZITA et al., 1973). The lateral shortening (layer-parallel shortening) and consequent vertical thickening of the basement rock bodies might have continued due to the horizontal compression. Based on a simplified plane strain model, the rate of thickening, namely the rate of uplift, is in inverse proportion to the viscosity of the basement rocks. Therefore, the difference of the viscosity between the two rock types mentioned above is expected to appear as the difference of the rate of the uplift. The areal distribution of vertical displacement through the whole Quaternary Period, which may be equivalent to that of the average rate of uplift in the same time, was compiled in The Quaternary tectonic map (THE RESEARCH GROUP FOR QUATERNARY TECTONIC MAP, 1969). This map shows that the rate of uplift is larger in the Paleozoic regions rather than in the granitic regions. Inferred from the relationship between the rate of the uplift and the ratio of the two rock types composing the basements, it is derived that the viscosity of the granitic rock bodies is larger than that of the Paleozoic sedimentary ones, and the ratio between them is estimated to be in a range of 2 to 5. This range of the value is nearly coincide with that of the value derived from the deformation of the conglomerates (GAY, 1968).

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The age of the Hikami Granites in the Kitakami Massif had been assigned to the Late Permian or to the Triassic, because it was believed that the granites intruded the rocks older than the Permian in age. While, some authors emphasized the movements ranging from the Devonian to the Early Carboniferous pertaining to the plutonism and metamorphism, supported by the radiometric dating of the Hikami Granites (354 m. y.). The writer and his co-workers (MURATA et al., 1974, 1975; OKAMI & MURATA, 1975) pointed out that the Hikami Granites, at least a part of it, constitute one of the basement rocks of the Silurian System in the Kitakami Massif, on the basis of the basal arkose sandstone of the Silurian Kawauchi Formation that lies on the Hikami Granites, and to the development of a slumping bed of granite breccia in the Lower Devonian Ono Formation. Three important problems still remain with regard to the Hikami Granites as the pre-Silurian basement rocks, namely, the relationship between two types of granite in the Hikami Granites, the result of radiometric dating of the Hikami Granites (354 m. y.), granite breccia in the Lower Devonian Ono Formation (295 m. y.) and of the basal sandstone of the Silurian Kawauchi Formation (264 m. y.), and the horizon of the slate hornfels which is intruded by the Hikami Granites at the vicinity of Tsukizawa, Rikuzen-Takada City, Iwate Prefec feature. The slate hornfels of the third problem may be referred to the Tsubonosawa Metamorphics, just the same as the metamorphic rocks exposed at the north of Setamai, Sumita-cho, Kesen-gun, Iwate Prefeture. The existence of the pre-Silurian basement complex of granite takes a new look on the tectonic history of the sedimentary basin in the Kitakami Massif. Many unconformities have been pointed out from the Paleozoic and Mesozoic sequences in the Kitakami Massif. Among them, the lowest horizon of the basal conglomerate which yields the pebbles of granitic rocks is the base of the Permian Sakamotozawa and the Tochu formations, except for the Silurian Kawauchi Formation. Many pebbles of granitic roks are contained within the tuff breccia and slumping beds in the. Devonian formations. However, no distinct basal conglomerate is known from the unconformities of the Devonian and Carboniferous sequences. The crustal movemennts are represented by the several unconformities recognized at the base of the Permian formations to the Jurassic System, and it can be considered that the blocks of the basement rocks which are arranged along the western margin of the southern part of the Kitakami Massif and the Tono - Takada - Kinkazan Tectonic Zone, were uplifted technically.

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