岩石鉱物鉱床学会誌

花崗岩類中の黒雲母に見られる2つの異なった進化経路

村上允英

1969 年 62 巻 4 号 p. 223-248
発行日: 1969/10/05
公開日: 2008/08/07

DOIhttps://doi.org/10.2465/ganko1941.62.223

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西南日本産花崗岩類中,それぞれ起源を共通する2時期のものについて全岩および含有黒雲母の化学組成を検討した結果,次のとおりいちじるしい差異が見いだされた。
(1) 後期中生代-初期第三紀花崗岩類:岩石の固化指数 (ST) の減少に伴って全岩・黒雲母ともにFe/Mg比が増加し,しかもその値の差が小さい。含有磁鉄鉱量も小さく通常1.5％以下である。
(2) 古第三紀田万川深成岩類:全岩・黒雲母のFe/Mg比は岩石が斑糲岩から花崗閃緑岩に進化する過程では増加する。しかし,花崗閃緑岩から花崗岩への進化過程では黒雲母のFe/Mg比はいちじるしく減少し,全岩の同比と逆の傾向を示す。含有磁鉄鉱量はいちじるしく多く(通常1～4%,ときに以上),この磁鉄鉱は黒雲母を交代し,さらに間隙充?を示すものが普通的に存在する。
以上の諸性状をもとにて考察した結果,(1) の花崗岩類は酸素分圧のやや減少する条件下で形成されたもので,(2) の深成岩類は酸素分圧がほとんど一定か,やや増加する条件下の性成と推定される。(2) のような深成岩類は一般に鍋状陥没地の深成岩類に特有で,酸素の少なくとも一部は大気中より何等かの経路を経て供給されたものであろう。このような深成岩類では黒雲母の分解に伴ってアルカリ長石の組成も急変し,いわゆる逆ラバキビ組織の形成をみる。したがって田万川深成岩類中の含有鉱物の反応系列(Fig. 17)は一般の花崗岩類におけるものと可成り異なっている。

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大阪府二上山地域におけるカルク・アルカリ岩系マグマの分化

山崎貞治, 大貫仁

1969 年 62 巻 4 号 p. 249-263
発行日: 1969/10/05
公開日: 2008/08/07

DOIhttps://doi.org/10.2465/ganko1941.62.249

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Many pyroclastic rocks and lava flows were erupted in Nijo-san district in late Miocene age. The greater part of these rocks are in calc-alkaline rock series, and a few in tholeiitic rock series. Chemical compositions of 14 nonporphyritic and porphyritic rocks, enclosing 11 new analyses, are given in this paper. The calc-alkaline rocks of this district are chemically divided into two groups, namely MgO-poor group and MgO-rich group. Rocks of the former group carry garnet phenocrysts and erupted in the first stage of the volcanic activity of this district. Phenocrystic minerals of this group are changed from pyroxenes to hornblende and biotite in the late stage of magmatic differentiation. Garnet is considered to crystallize directly from the late stage liquid of this group, affected by the addition of garnet xenocrysts through contamination with garnet bearing metamorphic rocks. The magmatic differentiation of this group is mainly due to the crystallization differentiation of pyroxenes, plagioclase and magnetite. The rocks of MgO-rich group. erupted in the second stage of the volcanic activity, carry dissociated hornblende phenocrysts. The chemical ccmpositions of these hornblende are considered to be pargasitic or tschermakitic hornblende from the dissociated products. Magmatic differentiation of this group is mainly due to the crystallization differentiation of these hornblende.

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九州大村半島,多良岳火山の岩石学

古賀真綱

1969 年 62 巻 4 号 p. 264-274
発行日: 1969/10/05
公開日: 2008/08/07

DOIhttps://doi.org/10.2465/ganko1941.62.264

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Tara-dake volcano is located in Omura Peninsula, Northwestern Kyushu. This volcano forms the western end of Daisen Volcanic Zone. The basement of this volcano is consisted of early Tertiary sediments and Pliocene volcanics. The volcanic activity of Tara-dake can be devided into four stages. In the first stage, this volcano extruded alkali basalt flows principally with a little of calc-alkali andesite flows. The second stage is represented by numerous andesitic pyroclastic rocks, which directly cover the extrusives of the first stage. Most of the pyroclastic rocks are fit for the pyroclastic flow of the intermediate type (Armakai, 1957). They were formed by several eruptions of many volcanoes, constructing Tara-dake volcano. After this stage, andesitic lava flows were erupted and lava domes were formed finally.
In reference to the genesis of calc-alkali rocks, it is the important fact that the alkali basalts and clac-alkali andesites were erupted successively. The successive eruption of calc-alkali rocks and tholeiite (or high-alumina basalt) is known in various volcanoes of Japan. But similar relation between alkali and calc-alkali rocks has not been found out up to this time. In this area, both series of rocks include many xenoliths and xenocrysts, and both were comparatively affected the contaimination effect. So the Tara-dake andesite might be derived from the alkali basalt which assimilated the crustal material.

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