| Project/Area Number |
09640568
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Petrology/Mineralogy/Science of ore deposit
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| Research Institution | Nagoya University |
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Principal Investigator |
SUZUKI Kazuhiro Nagoya Univ., Graduate School of Science, Professor, 大学院・理学研究科, 教授 (90111624)
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| Co-Investigator(Kenkyū-buntansha) |
KATO Takenori Nagoya Univ., Graduate School of Science, Assistant, 大学院・理学研究科, 助手 (90293688)
ENAMI Masaki Nagoya Univ., Graduate School of Science, Associate Professor, 大学院・理学研究科, 助教授 (20168793)
ADACHI Mamoru Nagoya Univ., Graduate School of Science, Professor, 大学院・理学研究科, 教授 (10113094)
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| Project Period (FY) |
1997 – 1998
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| Project Status |
Completed (Fiscal Year 1998)
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| Budget Amount *help |
¥3,100,000 (Direct Cost: ¥3,100,000)
Fiscal Year 1998: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1997: ¥2,300,000 (Direct Cost: ¥2,300,000)
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| Keywords | CHIME age / Monazite / Zircon / Granitoid / Metamorphic rocks / Nohi Rhyolite / Ryoke metamorphic belt / Denudation history / CHIMEage / アラナイト / 黒雲母変質 / Pb-Pb年代 / イオンマイクロプローブ / Monazite / Zircon / Granitoid / Motamorphic rocks / Denudation histong / T-P path / Geochronology |
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| Research Abstract |
The evolutional history of basement rocks in the Japanese Islands was investigated on the basis of the CHIME dating and the XRF and EPMA analyses of igneous ; metamorphic and sedimentary rocks. The main results are as follows.
(1)The CHIME zircon age is 85*5 Ma and the CHIME allanite age is 86*7 Ma for the Agigawa welded tuff sheet in the lowest volcanic sequence of the Nohi Rhyolite in central Japan. The Agigawa welded tuft sheet is chronologically identical with the post-tectonic Shinshiro Tonalite and Mitsuhashi Granodiorite in the Ryoke metamorphic belt. Acidic magmatism started simultaneously and widely at about 85 Ma after the denudation of the high T/P Ryoke belt metamorphosed at about 100 Ma.
(2)Biotite from fresh rock shows a sharp 10 A basal reflection, but its Na-modification gives distinct 10 and 12.5 A reflections. This shows that biotite even from fresh rocks contains microdomains of K-poor biotite and vermiculite. The Fe2+/Fe3+ ratio is higher in K-rich biotite than in K-poor biotite and vermiculite. This suggests oxidation of iron in octahedral sheet causes the K-loss to maintain local electrostatic balance, and accounts for the deviation of K-Ar biotite ages from CHIME monazite ages.
(3)The CHIME monazite ages for the Ryoke gneisses are concentrated between 102 and 98 Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism is contemporaneous with the geologically oldest plutons of at about 95 Ma in both the eastern and western parts separated by about 550 km. Plutonism continued from 95 Ma to 68 Ma in the eastern part, whereas in the western part it ceased at 85 Ma. These new age data do not support a current view that plutonism in the Ryoke belt becomes younger towards the east. The CHIME ages coupled with available geological data show that the western part was eroded rapidly (1.5mm/y) than the eastern part (0.8mm/y) over the time span from 91 Ma to 85 Ma.
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