北滿洲五大連池火山産アルカリ玄武岩類の成因に關する1考察(其の2)
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概要
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Wutalienchih volcano(五大連池火山) of North Manchuria is composed of a group of alkali-basalts, which contain about 5% of K_2O., The essential constituents of these effusives are olivine, augite, anorthoclase, leucite, magnetite, titan-iron ore, and glass, plagioclase being almost excluded., Their average chemical composition is very similar to that of the world average minette, given by R.,A., DALY., In these effusives, xenocrysts from granitic rocks, though not abundant, are universally distributed and in Shihlung lava (石龍熔岩) which occupies the central part of this district, xenoliths of the same origin are occasionally found., While alkali-feldspars of these xenoliths and xenocrysts are largely resolved, and hornblende and biotite are also completely opacitized (and slightly resolved), quartz and plagioclase are only partly attacked., The materials assimilated in these effusives must be, therefore, in the main SiO_2, Al_2O_3, Na_2O, K_2O., The ratio among CaO, MgO, and FeO+Fe_2O_3 in the average composition of these alkali-basalts, that are believed to be practically unaffected by assimilation, are compared with those of the Cenozoic basic effusives from・East Asiatic Province, which are considered to be not contaminated., It was found that the ratio is very similar to that of the average basalt of Chihsingshan volcano (七星山火山) in Central Manchuria., If we take, therefore, the average composition of Chihsingshan volcano as the composition of magmatic origin in the North Manchurian alkali-basalts under consideration, we can estimate the composition assimilated in the latter by constructing the variation diagram from the Chihsingshan average and the North Manchurian average., The graphical construction shows that the material assimilated has a trachytic composition rich in K_2O., According to the observations recorded on the selective resorption of granitic rocks in basic magma and the melting phenomena of igneous rocks in the laboratory, the formation of trachytic liquid rich in K_2O is possible in the comparatively earlier stage of resorption., The North Manchurian alkali-basalts are considered, therefore, to be formed by selective resorption of granitic rocks by basaltic magma which will be slightly more basic than the average Chihsingshan basalts., Lastly the geographical distribution of some effusives which are especially rich in K_2O relative to Na_2O is examined, as the result of which they are found to distribute only in the continental regions, but are absent from the oceanic regions.,
- 日本地質学会の論文
- 1940-12-20