韓国，済州火山島の岩石学 Ｉ 岩石記載および総化学組成

Abstract

Jeju Island is a shield volcano formed by successive central eruption, possibly from vents near Mt. Halla, during Upper Pliocene to Lower Pleistocene. It is mainly composed of large amounts of thick voluminous basaltic lava flows and minor pyroclastic rocks. Some hyaloclastites also occur along the coast. On the basis of topographical feature, which is in close connection with the volcanism, and geological evidence, the volcanism of this island can be classified into three stages; Lava plateau stage, Halla volcanic edifice stage and Volcanic scoria cones stage. Both Lava plateau stage and Halla volcanic edifice stage form a continuous series of the alkali basalt-trachyte association and of the hawaiite-trachyte one, respectively, and they also different from each other in the texture and constitutent minerals. The basic rocks in the former series are mostly microporphyritic and contain olivine phenocryst 10-15 percents, less than 2mm in size, while the basic rocks of the latter series are mostly macroporphyritic and contain plagioclase, pyroxene and olivine phenocrysts 25-50 percents, 5 to 10mm in size. Main constituent minerals of these rocks are plagioclase, olivine, augite, ferrosalite, anorthoclase and magnetite, and minor constitutent minerals are ilmenite, apatite, hedenbergite and kaersutite. They exhibit a restricted range of composition in each rock type. Chemical analyses of 63 volcanic rocks from both Lava plateau stage and Halla volcanic edifice stage are given. The analyses are plotted in several variation diagram; SiO2-total alkali, Na2O-K2O, normative feldspar, SI-oxide, A-F-M and K2O-oxide diagram. Rocks of both stages show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthene normative and then to quartz-normative composition, and these rocks can be classified into alkali basalt, hawaiite, mugearite and trachyte by solidification index, main constituent minerals and their composition; alkali basalt ranges in SI from 42 to 35, hawaiite 35 to 15, mugearite 15 to 10, and trachyte 10-0.9. K2O-Na2O ratio of the basic rocks is between 1:3 to 1:2, except for a few specimens, and that of intermediate and acidic rocks is between 1:2 to 1:1. Normative plagioclases fall in the field of sodic series. A-F-M diagram shows a gradual trend toward alkali enrichment, with the curve rising closer to the iron apex. Intermediate lavas have higher total iron contents than alkali rocks in other areas such as Gough, Iki and Dogo. Both Lava plateau stage and Halla volcanic edifice stage show a very similar differentiation trend in the several diagrams. The above evidences support the view that all of the rock types in the island were derived from alkali olivine basalt magma, dominantly by process of crystallization differentiation.