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Volume 73, Issue 10
Displaying 1-3 of 3 articles from this issue
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MAKOTO WATANABE, AKIRA SOEDA, TOSHIYUKI OHTANI1978 Volume 73 Issue 10 Pages 283-299
Published: October 05, 1978
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESS -
SHIGENORI MARUYAMA, YOSHIO UEDA, SHOHEI BANNO1978 Volume 73 Issue 10 Pages 300-310
Published: October 05, 1978
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSA new member of the Kurosegawa tectonic zone was found in the serpentinite near Kochi city. They are high P and low T schists derived from basalt and chert.
Three metamorphic events can be deciphered in the high-pressure schists, based upon the texture and mineral paragenesis: first, low P metamorphism at intermediate- to highgrade, second, high P and low T metamorphism of the jadeite-glaucophane facies and the thrid, retrograde crystallization of the second stage high P schists within the stability field of lawsonite+pumpellyite+glaucophane. Further, the formation of analcime replacing jadeite took place. The second and third metamorphism can be distinguished on pyroxene mineralogy that jadeite+quartz was stable in the second, but albite+quartz+aegirinejadeite in the third stage of metamorphism.
Not all of the high P and low T schists had suffered low P metamorphism before they were metamorphosed by the high P one. Some basaltic rocks directly changed to high P and low T schists.
Two muscovites in the schists give K-Ar ages of 208-240 m.y., and a relic igneous biotite, being partly replaced by chlorite, gives 225 m.y. of K-Ar age. These values are different not only from those of the Sanbagawa schists, but also from the other members of the Kurosegawa zone.View full abstractDownload PDF (1777K) -
1. PETROCHEMISTRY AND ROCK-FORMING MINERALOGYHitoshi Onuki, Takeyoshi Yoshida, Takashi Suzuki1978 Volume 73 Issue 10 Pages 311-322
Published: October 05, 1978
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Fujiwara plutonic complex emplaced as largely solid state at the present position in the Sanbagawa metamorphic belt of central Shikoku, about 400m long and 200m wide, consists of a mafic unit and an ultramafic unit. The former appears to be in contact with the latter also tectonically. In the mafic unit the main rock-type is a Mg-rich medium-grained gabbro which is penetrated by iron-rich fine-grained gabbro. The ultramafic unit comprises serpentinites derived from dunite and wehrlite. The Sanbagawa metamorphism in epidote amphibolite facies has intensely superimposed on the whole complex. The metamorphic overprinting is characterized by associations with hornblende, zoisite, albite, garnet and chlorite. Not only wehrlitic serpentinite but also most gabbroic rocks still contain some relic clinopyroxenes.
18 whole rock chemical analyses were performed. The chemical compositions of olivines, clinopyroxenes, hornblendes, garnet and albite were determined by means of EPMA and the classic wet method. The whole rock chemistry suggests that the complex is mainly composed of meta-cumulates and the minor fine-grained gabbro represents a part of residual liquid. Probably some gabbroic rocks primarily contained a considerable amount of olivine. The ultramafics are relatively rich in iron and Al2O3. A significant gap in Mg/(Mg+Fe) ratio between olivine and its host rock is notable in the serpentinite and this fact could be indicative of the recrystallization of this mineral.
The analysed iron-rich hornblendes are relatively rich in Na in M4, but they are also rich in (Na+K) in A. This peculiarity may be a reflex an iron-rich environment and a low oxidation state during metamorphism.View full abstractDownload PDF (1651K)
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