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Volume 62 (1969)
- Issue 6 Pages 311-
- Issue 5 Pages 275-
- Issue 4 Pages 223-
- Issue 3 Pages 177-
- Issue 2 Pages 55-
- Issue 1 Pages 1-
Volume 62, Issue 4
Displaying 1-3 of 3 articles from this issue
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NOBUHIDE MURAKAMI1969 Volume 62 Issue 4 Pages 223-248
Published: October 05, 1969
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSTwo markedly different trends of evolution of biotites were found in the granitic rocks of Southwest Japan.
(1) Late Mesozoic to early Tertiary granites: The iron/magnesium ratios in both rocks and biotites increase with decreasing solidification index (SI). The difference of the ratio between rocks and biotites is small. The magnetite content in the granities is below 1.5 per cent usually.
(2) Palaeogene Tamagawa plutonic rocks: As the differentiation progresses, the iron/magnesium ratio has a similar trend as that in (1) from gabbro to granodiorite in both biotites and rocks, whereas from granodiorite to granite the ratio becomes reversed in the biotites due to their turning into magnesium rich compositions. The content of magnetites is high, amounting to 4 per centor more. They replace biotites and form interstitial fillings.
On the basis of physico-chemical aspects, it is assigned that the oxygen partial pressure during the evolution of biotites has been under slightly decreasing c on dition in the late Mesozoic to early Tertiary granites, while almost constant or somewhat in creased in the Tamagawa plutonic rocks in which at leasta part of the oxygen are presumed to have been derived from atmosphere, although its invading process is quitely questionable. The latter case is common in the plutonics of the cauldron subsidence area.
The reaction series of the constituent minerals for the Tamagawa plutonic rocks is conspicuously differed from ordinary types of granite belonging to calc-alkaline rock series as shown in Fig. 17.View full abstractDownload PDF (2762K) -
Teiji Yamazaki, Hitoshi Onuki1969 Volume 62 Issue 4 Pages 249-263
Published: October 05, 1969
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSMany 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.View full abstractDownload PDF (1520K) -
Masatsuna Koga1969 Volume 62 Issue 4 Pages 264-274
Published: October 05, 1969
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSTara-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.View full abstractDownload PDF (648K)
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