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Volume 99, Issue 3
Displaying 1-12 of 12 articles from this issue
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Koichiro TAKAHASHI1990 Volume 99 Issue 3 Pages 209-216
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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Akiho MIYASHIRO1990 Volume 99 Issue 3 Pages 217-229
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
JOURNAL FREE ACCESSThe chemical compositions of solid-solution minerals in metamorphic rocks are controlled by equilibria not only of exchange reactions but also of net-transfer reactions among coexisting minerals. In rocks with a variance of 2, the compositions of solid-solution minerals are functions of temperature and pressure alone as is illustrated in Figs. 2 and 3, and so can be used as a measure of the P-T condition under which the rocks were recrystallized. In rocks with a variance of 3 or greater, on the other hand, the compositions of solid-solution minerals vary not only with temperature and pressure but also with the bulk-rock composition (Fig. 4). The growth of zoned crystals causes fractional crystallization, resulting in a change of composition of the reacting system. If the variance of the system is 3 or greater, the change of the composition of the reacting system causes a change in the composition of the crystallizing mineral.
Pelitic metamorphic rocks are commonly simplified as belonging to the 6-component system : Al2O3-FeO-MgO-K2O-SiO2-H2O. In this case the mineral assemblages may be shown on the THOMPSON AFM diagram. Any 3-AFM phase assemblage in the diagram has a variance of 2, whereas any 2-AFM phase assemblage has a variance of 3.
In the paragenetic relations of garnet, MnO commonly plays an essential role, and so garnet-bearing metapelites are commonly treated as belonging to the 7-component system : Al2O3-FeO-MgO-MnO-K2O-SiO2-H2O. Most metapelites with MnO-rich garnet have a variance of 3 or greater. Garnets in low- and middle-grade metamorphic rocks are usually strongly zoned. Hence, the composition (or the MnO content) of garnet is strongly influenced by the initial bulk-rock composition and fractional crystallization. SPEAR (1988) has calculated the progressive compositional change of garnet that grows not only under complete equilibrium condition but also with fractional crystallization for garnet-bearing 3-AFM phase assemblages that are observed in the Barrovian sequence.
In the Sanbagawa high P/T ratio metamorphic belt, progressive metamorphism is represented by a sequence of the chlorite zone, garnet zone (with no biotite), and biotite zone (with garnet) for metapelites. In other words, garnet begins to occur at a considerably lower temperature than biotite. In the garnet zone, garnet probably forms by reaction involving Tschermak exchange component like (4), whereas in the biotite zone, garnet forms by the same reactions as in the garnet zone of the Barrovian sequence. Such a change in the garnet-producing reactions between the two zones may cause a break in the MnO and FeO curves in the composition profile of zoned garnets. Some of the observed features of the compositional trends of garnets could be ascribed to the effect of a variation in bulk-rock composition.
The average composition of zoned garnets in metapelite shows a progressive decrease of the MnO content with increasing metamorphic grade. This is probably related mainly to a progressive increase of the amount of garnet. Higher pressure tends to increase the amount of garnet, and to decrease its MnO content.View full abstractDownload PDF (2352K) -
Sumio SAKAGAMI, Akio HATTA1990 Volume 99 Issue 3 Pages 230-246
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
JOURNAL FREE ACCESSRecently we had chance to visit Palau Islands two times in 1986 and 1988. The aim of the present article is to introduce the geology including some new evidences investigated by us, and the human history of the islands is also briefly described.
The Aimeliik volcaniclastic rocks, which have been recognized as in the Eocene Aimeliik (=Aimiriki) Formation by the former scientists, from the quarry at Ngeruluobel located at about 1km west of the Palau International Airport terminal at Airai in southern end of Babelthuap Island, are discussed in Cepter V. Many foraminifers, calcareous algae and some corals were found as individuals in the matrix or in the limestone gravels of the volcaniclastic rocks. Among the determinable species of foraminifers, Biplanispira absurda, B, inflata, B. mirabilis, Pellatispira rutteni, Fabiania saipanensis and Discocyclina dispansa were reported from the Matansa Limestone (Upper Eocene) of Saipan, and Gypsina globulus, Eulepidina formosa from the Tagpochau Limestone (Lower Miocene) of Saipan by HANZAWA (1957). It seems there are at least two groups of foraminiferal fauna showing different ages, namely, the Upper Eocene and Lower Miocene (Aquitanian to Burdegalian). Accordingly, the depositional age of the Aimeliik volcaniclastic rocks should be Late Miocene or later.
In Chapter VI, we introduce our study (SAKAGAMI et al., 1987) on the environmental change in some period of the Miocene to Pliocene by the pollen and spore analyses of the Airai lignite-bearing beds in Babelthuap Island.
Lastly the literatures on geological sciences are comprehensively compiled for the convenience to the future study.View full abstractDownload PDF (4262K) -
Hisao ANDO1990 Volume 99 Issue 3 Pages 247-262
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
JOURNAL FREE ACCESSThe depositional sequence concept was established in newly developed sequence stratigraphy, as an unconformity-bounded stratigraphic unit formed during one complete sea-level cycle. This paper reviews general meanings of “sequence”, the definition of depositional sequences, their hierarchial patterns and recognition, and sequence boundaries problems, from a viewpoint of sedimentary geology based on outcrops and bore-hole samples.
Though the word, “sequence” has many meanings generally applied to successive geologic events and processes in chronologic order, a depositional sequence is defined in a special sense, as “a relatively conformable succession of genetically related strata bounded at its top and base by unconformities and their correlative conformities”. The depositional sequence as one of hierarchial transgressive and regressive units (T-R units), has the first- to forth-order operational units, that is, the megasequence, supersequence, sequence and parasequence in descending order. A sequence boundary with a significant hiatus (=unconformity) is formed by subaerial exposure, concurrent subaerial erosion and partly submarine erosion during eustatic falls or low-stand sea level.
The latter half of this paper emphasizes the difference between sequence boundaries and ravinement surfaces. The ravinement surface formed as one of diastems or “transgressive surfaces”, is an erosional surface by shoreface retreat during the following transgression after a sea-level fall. In general, it is lithologically more distinct than the underlying sequence boundary. The right recognition of the difference leads correct reconstructions of sedimentary history.View full abstractDownload PDF (2520K) -
Yukio HAGIWARA1990 Volume 99 Issue 3 Pages 263-269
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
JOURNAL FREE ACCESSIt has long been believed that the Newtonian gravitational inverse-square law holds good in both laboratory and planetary dimensions. In recent years, however, doubt about the equivalence of laboratory and planetary values of the Newtonian constant has arisen. A hypothesis suggests that a Yukawa-type potential of the fifth force acts in short mass separations in addition to the conventional Newtonian gravitational potential. In that case the Newtonian constant can be expressed as a function exponentially decreasing with the mass separation.
Many researchers have devoted themselves to finding the fifth force by accurate torsion-balance experiments or gravimetries down a deep mine shaft. Some of them concluded that the fifth force behaves as an attraction, while the others as a repulsion. Furthermore, from theoretical standpoint, the fifth force would depend on the chemical composition of materials. Some experimenters insisted that they detected possible evidence for the composition-dependency, but the others denied it. These facts may imply that the existence of the fifth force is now far from being considered a certainty.
Why does such an uncertainty still remain ? One of the reasons is that the strength of the fifth force is supposed much weaker than that of the Newtonian force, so that it is very difficult to identify the fifth force even by highly accurate experiments. The other reason consists in gravity anomalies varying from place to place on the earth's surface. The importance of gravity anomalies should be minded in interpreting torsion-balance or gravity data before concluding that tha data indicates possible evidence for the fifth force.View full abstractDownload PDF (1003K) -
Shinji NAGAOKA, Hideaki MAEMOKU, Yoshio TANAKA1990 Volume 99 Issue 3 Pages 270-278
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
JOURNAL FREE ACCESSLate Quaternary glacier variation of the Ngojumba Glacier in the Khumbu Himal are determined by detailed field mapping of glacial landforms, stratigraphy of fossil soil, S-1, S-2, and S-3 in desending order, and 14C-dates of fossil soil horizon.
The moraines of different ages indicate seven advance stages as follows ; the Gokyo stage, the Nah stage, the Machhermo stage, the Dole stage, the Phortse Drangka stage, the Phortse stage, and the Konar stage. The Gokyo and Nah stages, of which moraines are covered with very thin S-1 soil correspond to the period from the Little Ice Age of A.D. 19-16c. to the middle Neoglacial readvance. Since the moraines of the Machhermo stage are covered with S-2 soil of 600-5, 000 y. B. P. by 14C-dating, were probably formed during early Neoglaciation, 7-8 ka. The moraines, overlain by S-3 soil, of the Dole and Phortse Drangka stages are much larger than those of other stages in size, and two stages probably correspond to the Last Glacial Stage. The Ngojumba Glacier in the Phortse Drangka stage, the Last Glacial Maximum, extended 8-9km downstream (3, 500m a. s. 1.) from present glacial terminus (4, 700m a. s. 1.). The Phortse and Konar stages are included among the glacier advances before the Last Glacial Stage because their moraines are well-erod and covered with thick soil layers.View full abstractDownload PDF (1234K) -
Takeo ODA1990 Volume 99 Issue 3 Pages 279-280
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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Toshiya MIYAZAWA1990 Volume 99 Issue 3 Pages 281-288
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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Takashi NAKATA1990 Volume 99 Issue 3 Pages 289-298
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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[in Japanese]1990 Volume 99 Issue 3 Pages 299-300
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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[in Japanese]1990 Volume 99 Issue 3 Pages Plate1
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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Shinji NAGAOKA1990 Volume 99 Issue 3 Pages Plate2-Plate4
Published: June 25, 1990
Released on J-STAGE: November 12, 2009
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