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＜departmental bulletin paper＞
Metamorphic process and its correlation to the isotopic age of metamorphic rocks : Example of the Higo Metamorphic rocks
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| Abstract | The Higo metamorphic complex developed in the western part of middle Kyushu consists of schists and gneisses of pelitic and psamitic origin with subordinate amount of marbles, mafic and ultramafic met...amorphic rocks. The metamorphic type correspond to a low pressure intermediate group, andalusite-sillimanite type of regional metamorphism and has been presumed to represent the western end of the Ryoke metamorphic belt in Japan. K-Ar and Rb-Sr biotite ages reported for samples from the higher grade zone were about 100 Ma. Several rock samples collected from the second highest zone of this area corresponding to amphibolite-granulite transition were analysed for Rb, Sr and Sr isotope ratio by mass spectrometric method with the following results. 1) Whole rock samples of amphibolite do not make an isochron. 2) Greenish amphibolite, diopside-plagioclase rock, and zoisite-calcite-plagioclase-quartz rock forming layered sequences less than several centimeters in total thickness in brownish amphibolites make whole rock Rb-Sr isochron with the age of 235±20 Ma and of initial 87^Sr/86^Sr=0.70647±0.00012. 3) The diopside-plagioclase rock is regarded as the reaction product of the other two. The mineral assemblages of this reaction zone indicate recrystallization at temperature of 570℃ as the upper limits and fluid pressure of about 3. 5 Kb. 4) It is suggested that the isotopic homogenization were effective only in thickness of\ several centimeters in such high temperature metamorphic conditions. 5) Brownish biotite separated from amphibolite gives whole rock-biotite Rb-Sr isochron age of 99. 8±2.8 Ma and of initial 87^Sr/86^Sr=0.70795±0.00006. 6) The Higo metamorphic rocks are polymetamorphosed. The older metamorphism (235 Ma) may be correlated to Hida metamorphic rocks with respect to the type and age of metamorphism and the lithology of original rocks, while the younger (99. 8 Ma mineral age) may have resulted by thermal metamorphism caused by later intrusion of the granitic rocks in the south.show more |
| Table of Contents | I. 序 Ⅱ. 変成岩の年代決定の問題点 Ⅲ. 肥後変成岩類の地質概略 IV. 肥後変成岩のRb-Sr分析 V. 反応帯についての考察 VI. 鉱物年代 VII. 肥後変成岩類の変成過程と同位体年代との対応 |
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| Created Date | 2021.10.25 |
| Modified Date | 2022.06.20 |