南部石狩低地帯の後期更新世のテフラ : 斜方輝石の屈折率とMg-Fe比との比較研究

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In this paper, a score of stratigraphic units of tephras erupted mainly from Shikotsu Caldera are studied on the distribution of the tephras, the assemblage of color minerals, the Mg-Fe ratios of orthopyroxenes by EPMA analysis and the refractive indices of minerals and glass in the tephras. These tephras are composed in ascending order of 1) four units of the Azuma Ash-fall Deposits (Aafa 4-1), 2) four units of the Mukawa Pumice-fall Deposits (Mpfa 3, 2 b, 2 a, 1), 3) half a score of units of the Shikotsu Pumice-fall Deposits (Spfa 10-1) intercalating a unit of the Hidaka Pumice-fall Deposits (Hpfa) and 4) a unit of the Eniwa-a Pumice-fall Deposits (En-a). Each of the upper three tephras, namely En-a, Spfa 1 and Spfa 2 has been traced to the Tokachi Plain about 150 km east-ward. In the Southern Tokachi Plain, downward sequence of Orange Pumice-fall Deposits-1 (Op1), Orange Pumice-fall Deposits-3 (Op-3), Taiki Pumice-fall Deposits (Tpfa) and Kamisatsunai Pumice-fall Deposits (Kpfa) is found in peaty deposits of the Biraotori Formation. Kpfa is found in the upper part of the peaty Horokayantoh Formation, too. In the Southern Ishikari Low-land, Aafa 1-4 are bedded in peaty deposits of the Shiomi Formation. Spfa 3-5, Mpfa 3, Aafa 1 and Aafa 2 can be correlated to Op-1, Op-3, Tpfa and Kpfa, respectively, based on the investigation of the stratigraphic sequence, the mineral assemblage, the chemical composition of orthopyroxene and the refractive indices of orthopyroxene, hornblende and glass in the tephras. Such a correlation of the marker tephras makes it possible to compare the above two areas from the viewpoint of paleoclimatology and paleogeography. Table 3 shows the chronological sequence between the tephras and their ^<14>C ages. It is also confirmed that the frequency distribution of the refractive indices of orthopyroxenes is similar to that of Mg-Fe ratios. It has been clarified that the refractive index (γ) of the orthopyroxene which contains less than 0.07 atoms of Al (per 6 atoms of oxygen) decreases by approximately 0.00125 for each atomic per cent of Mg/(Mg+Fe^<+2>+Fe^<+3>+Mn), but in the Al-rich orthopyroxene which contains 0.07〜0.14 atoms of Al, the index (γ) is higher by 0.005 for the composition En_<100>, and the effect of Al weakens with decrease of En component. Therefore, in order to estimate the refractive index (γ) of orthopyroxene from the analysed Mg-Fe ratio, it is indispensable to analyse not only Mg, Fe and Ca but Al and Mn.
1980-01-25

南部石狩低地帯の後期更新世のテフラ : 斜方輝石の屈折率とMg-Fe比との比較研究

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