The Ieshima Islands, in the eastern Seto Inland Sea of southwest Japan, are dominated by sedimentary complex in the Tamba Belt, Late Cretaceous volcanic and plutonic rocks, and Quaternary sediments. The Late Cretaceous volcanic rocks consist mainly of dacite pumiceous lapilli tuff, rhyolite welded crystal tuff, and rhyolite tuff breccia. Here, they are described for the first time as the Ieshima Formation. Rhyolite welded crystal tuff overlies the dacite pumiceous lapilli tuff, and both are intruded by plutonic rocks. I present laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and fission-track (FT) ages of the rhyolite welded crystal tuff. U-Pb ages of individual zircons range from 86.0 to 104.5 Ma, and show a bimodal distribution with peaks at ca. 90 and 100 Ma. The younger cluster (89.8 ± 0.9 Ma, n = 19) corresponds the eruption age of the felsic volcanic rocks of the Ieshima Formation, while the older grains may be inherited from the basement rocks. The FT age (76.1 ± 2.5 Ma) is significantly younger than the U-Pb dating age, and represents thermal resetting by concealed plutonic rocks.
Special Issue “Development of researches on rock-water interaction and Professor Naotatsu Shikazono” Part 2
Many pottery stone deposits are distributed in the western region of Amakusa-Shimoshima Island, Kumamoto Prefecture. They extend 30 km in the north-south direction and 5 km in the east-west direction. More than two million tons of pottery stone has been produced from these deposits and used as raw material for tableware and insulators. Nagasaki metamorphic rocks and Cretaceous and Paleogene sedimentary rocks that have been intruded by a Miocene granodiorite stock and rhyolite dikes are distributed in this area. The pottery stone deposits were formed by the alteration of the rhyolite dikes and they resemble a vein deposit in their form. Major deposits such as the Sarayama vein and the Kaigan vein extend 4-5 km. The width of the veins is generally about 10 m but reaches a maximum of 25 m. Previous studies on the pottery stones revealed the mineral assemblages, the geochemical features of major and minor elements, and the K-Ar ages. The mineral zonation at Denbeikoba quarry, the Sarayama vein, was also clarified. The remarkable characteristics of the Amakusa pottery stone deposits include their widespread distribution, long and narrow form of the ore bodies, and similarity in mode of alteration and mineral assemblage in the high-grade ore. The thermal history and thermal structure of the area surrounding the pottery stone deposits was proposed based on a close examination of the clay minerals in the pottery stone and the vitrinite reflectance of the carbonaceous matter in the surrounding sedimentary rocks. Recently, some data on the microthermometry of fluid inclusions in the pottery stone and radiometric dating by various methods have been reported. These results suggest that the genesis of the Amakusa pottery stone deposits has a close relation with the high geothermal gradient in this area. The genetic process of the Amakusa pottery stone deposits can be better understood by confirming the physicochemical properties, such as the temperature, salinity, and fugacity of carbon dioxide of the alteration fluids.
Information collected from geological newsmagazines in 2015 is reviewed. First topic is mineral resources such as mineral commodity summary of 2014 in USA and mineral resource information in Cuba. Second topic is rocks and minerals such as heritage stone proposal in Europe and boron minerals in Antarctica. Third topic is regional geology and planetary geology. This topic includes geological tour sites in Uganda and overview on planetary geological mapping. Fourth topic is academic evaluation on subjects of earth science and the way to prepare poster presentation.
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April 11, 2015
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