The Ryoke granitic and metamorphic rocks are found in the northern half of Awaji-shima. The Ryoke granitic rocks can be divided into three rock-masses by their petrological characters: they are called the Shizuki granodiorite, the Myojin granite and the Sumoto granodiorite in order of emplacement from older to younger. The Shizuki granodiorite is surrounded by the Myojin granite that is also surrounded by the Suraoto granodiorite, as shown in the geological map. The larger part of the Shizuki granodiorite mass is characterized by gneissose structure due to parallel arrangement of biotite and hornblende. The gneissose structure is distinct in the southern part of the gradiorite mass, but it becomes indiscernible in the northern part. Gneisses derived from sediments are found enclosed only in the southern part of the mass, where the other metamorphic rocks are also enclosed. The mineral assemblage of these metamorphic rocks found in the granodiorite mass may correspond to the middle to high-grade part of the amphibolite facies. The Shizuki granodiorite suffered contact metamorphism genetically related to the Myojin granite. The Myojin granite shows also gneissose structure characterized by parallel arrangement of porphyroblasts of K-feldspar, which is distinctive in the southern part of the granite mass. The fieldevidences suggest the successive emplacement of the Sumoto granodiorite after that of the Myojin granite. Metamorphic rocks or xenoliths in the two granitic rock-masses may show the mineral assemblage corresponding to that of low (to middle)-grade part of the amphibolite facies. Through correlation of petrological characters of the Ryoke granitic rocks in Awaji-shima with those in the adjacent areas such as the Yamato plateau, Shodo-shima (Shodo Island) and the Yanai district, it was concluded that the Shizuki granodiorite and the Myojin granite-Sumoto granodiorite belong to the older and the younger granite in the Ryoke zone, respectively.

抄録全体を表示

For the purpose of determinating the history of volcanic activities during the Cenozoic period, in Hokkaido, the authors undertook a palaeomagnetic studies of various volcanic rocks, developed in Central Hokkaido. In this paper, the results of palaeomagnetic observations of the Cenozoic welded tuffs and lava flows, widely developed in near Mt. Sahoro, will be briefly given. The stability of rock magnetism is throughly tested. The NRM of the samples used in the present study are all normal. This suggests that the age of these welded tuffs and lava flows might be the lowermost Pleistocene or the uppermost Pliocene.

抄録全体を表示

This paper is written to critisize the concept of high-alumina basalt recently proposed by H. KUNO (1960; 1965). He has considered that the highaim ina basalt is of intermediate character between the tholeiitic and alkali basalts. Many tholeiitic basalts have been hitherto dredged from such oceanic ridges as East Pacific Rise, Mid-Atlantic Ridge and Mid-Indian Ridge, as shown in Table 1. Detailed examination on the chemistry of these tholeiitic basalts led the present writters to the following conclusions: 1. The tholeiitic basalts dredged from the oceanic ridges differ from those of the oceanic islands. The former has much higher alumina and lower titania than the latter. This relation is clearly shown in Fig. 1. 2. The high-alumina basalts are not of intermediate character between the tholeiitic and alkali basalts, but are rather of tholeiitic affinity as evident from Figs. 4, 5, and 6. The writer thinks that KUNO made a mistake by his erroneous generalization of the results in the Izu-Hakone province in central Japan to the other provinces. 3. Regardless of KUNO'S concept on the high-alumina basalt, the existence of high-alumina basalts is evident in oceanic and other regions. The researches on the physico-chemical conditions under which the high-alumina basalts are built must explain the predominance of these basalts on the oceanic ridges.

抄録全体を表示

1. The Neogene system of the Motojuku district can be divided as shown in Table. 1. The Motojuku formation and the Kabutoiwa formation, which are the object of the present report, are both referred to upper Miocene (Funakawa stage). The formations are unconformable with the older formations of the Neogene system, as well as with the Mesozoic and Paleozoic formations. 2. The greater part of the Motojuku formation and the lower part of the Kabutoiwa formation abut the basement in an unconformable relation. At their base, a poorly sorted basal conglomerate, containing angular rock fragments of boulder size, is developed. In the geologic map the line of this unconformity is generally straight to arcuate. Near the unconformity line a fault-fracture zone is recognized. These facts indicate that the sedimentary basin, in which the Motojuku and Kabutoiwa formations were deposited, is a result of depression caused by normal faults dipping at high angles which took place just before the sedimentation. We have denned the depression as the primary depression structure. The structure is equilateral triangle in shape, each side being 13 km long. 3. The upper parts of the Motojuku and Kabutoiwa formations overlap against the basement, and are more widely distributed than the lower parts. 4. In the central part of the distribution area of the Motojuku formation, the beds dip 30° to 70° towards the center of the basin, whereas the dip of the surrounding parts is almost horizontal. Sheets of basaltic andesite are developed in the part where the dip changes. Such a structure can be attributed to a depression that was formed after the deposition of the Motojuku and Kabutoiwa formations. We have denned this depression as the secondary depression structure. The structure is 8 km in longer diameter and 4 km in shorter diameter. 5. The Motojuku and Kabutoiwa formations are altered throughout. The degree of alteration is higher around plutonic rocks, and several zones of alteration can be distinguished (Yoshimura, 1967). However, since the alteration becomes stronger towards lower horizons, the relation between the intrusion of the plutonic rocks and the alteration may not be necessarily reciprocal. Further study of the relation is in progress. 6. It has been generally believed that alteration of the Neogene system in Japan is limited to lower to middle Miocene, but in the present case the upper Miocene beds are found to have undergone alteration. Similar phenomena are known in Miyagi and Shizuoka Prefectures, so that altered upper Miocene beds may occur in some other areas, too. 7. The scope of the primary depression structure is roughly equal to the size of the unit of Neogene sedimentary basins, as exemplified by several basins which have been clarified in Fukushima and Miyagi Prefectures. In other words, the fact that the depression is 8 to 15 km in longer diameter can lead to a proposition that a basin, 8-15 km in longer diameter, may have been the minimum unit of the Neogene crustal movement (Green Tuff movement) of the Japanese Islands. 8. The primary depression structure of the Motojuku district was formed just before the commencement of volcanic activity. Quaternary calderas, on the other hand, are generally believed to have caved in after the respective volcanic activities. It is not known as yet whether the depression-forming mechanism was different between the two cases, or whether there is a possibility of detecting the same mechanism in formation of the Quaternary calderas as that in this district. To elucidate these points, studies by volcanologists are required. 9. In the middle part of the Motojuku formation, reversal of geomagnetism, is noticed at two horizons (Nomura, 1967). Plutonic rocks are composite bodies of quartz diorite, gabbro and porphyrite, constituting the so-called volcano-plutonic complex (Oide, 1966). The intrusive relations of these rocks are being studied.

抄録全体を表示