Monazite behaviour and time-scale of metamorphic processes along a low-pressure/ high-temperature field gradient (Ryoke belt, SW Japan)

E. Skrzypek, T. Kato, T. Kawakami, S. Sakata, K. Hattori, T. Hirata, Takeshi Ikeda

研究成果: ジャーナルへの寄稿記事

2 引用 (Scopus)

抄録

Low-pressure/high-temperature metamorphic rocks exposed in the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan) include a section with increasing temperature conditions from ~425 to 880°C. We use this setting to explore the evolution of monazite grain size, texture and composition, and variations in the whole-rock composition of 11 metapelite, metapsammite or metachert samples collected along the metamorphic field gradient. Monazite grain size increases with rising metamorphic grade, regardless of the whole-rock composition. From low- to high-grade conditions we infer: (1) the initial nucleation of monazite aggregates after allanite (~425°C); (2) monazite coarsening and coalescence driven by incipient monazite recycling; that is, dissolution of small grains to grow larger ones by Ostwald ripening (500-600°C); (3) a first major recycling stage enhanced by fluid liberation owing to muscovite breakdown (600-630°C); (4) a second recycling stage assisted by an increase in the proportion of anatectic melt owing to biotite breakdown (> 850°C). A succession of four compositional domains is recognized in monazite. We emphasize the usefulness of comparing their Ce/ThMnz, Ce/YMnz and Th/UMnz molar ratios with those derived from whole-rock analyses to constrain the origin of each domain. Domain I, with variable ratios, reflects the progressive transfer of Th ± U from allanite to monazite at low-grade conditions. Domain II, with Ce/ThMnz matching the whole-rock values, indicates growth under rock(decimetre)- scale equilibrium conditions. Domains II and III, with Th/UMnz and Ce/YMnz departing from the whole-rock values, record the competition with zircon (for U) and garnet (for Y) during growth at peak P-T conditions. Domain IV points to Y supply by garnet resorption during retrograde chloritization (< 550°C). In the highest-grade sample, zircon grains included in garnet or cordierite show metamorphic rims with sillimanite and Si-rich inclusions. These rims formed at suprasolidus conditions (650-880°C) and yield 206Pb/238U ages of 103-97Ma (±5 Ma), which bracket the timing of high-temperature metamorphism. Monazite dating by electron microprobe and laser ablation inductively coupled plasma mass spectrometry reveals two age groups. For domains I-III, some relatively old 206Pb/238U ages (99-95±3-5 Ma) represent minimum estimates for the timing of prograde to peak metamorphism, whereas the similar oldest 206Pb/238U age for domain IV (93 ± 7 Ma) points to rapid cooling after the temperature peak. A more dominant population of younger Th-U-Pb and 206Pb/238U dates is ascribed to age resetting by heterogeneous annealing of the monazite crystal lattice. The youngest concordant 206Pb/238U age of 86 ± 4Ma is correlated with the end of intermediate-temperature conditions (< 200-300°C) below which monazite, in metamorphic rocks as well as in the surrounding granitoids, eventually behaved as a closed isotopic system. This contribution is dedicated to the memory of Professor Kazuhiro Suzuki, for his contribution to geochronology and Japanese geology.

元の言語英語
ページ(範囲)1109-1144
ページ数36
ジャーナルJournal of Petrology
59
発行部数6
DOI
出版物ステータス出版済み - 6 1 2018

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monazite
low pressure
Japan
Temperature distribution
temperature distribution
timescale
gradients
rocks
Rocks
grade
recycling
garnets
Garnets
metamorphic rocks
Metamorphic rocks
rock
Recycling
rims
allanite
garnet

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

これを引用

Monazite behaviour and time-scale of metamorphic processes along a low-pressure/ high-temperature field gradient (Ryoke belt, SW Japan). / Skrzypek, E.; Kato, T.; Kawakami, T.; Sakata, S.; Hattori, K.; Hirata, T.; Ikeda, Takeshi.

:: Journal of Petrology, 巻 59, 番号 6, 01.06.2018, p. 1109-1144.

研究成果: ジャーナルへの寄稿記事

Skrzypek, E. ; Kato, T. ; Kawakami, T. ; Sakata, S. ; Hattori, K. ; Hirata, T. ; Ikeda, Takeshi. / Monazite behaviour and time-scale of metamorphic processes along a low-pressure/ high-temperature field gradient (Ryoke belt, SW Japan). :: Journal of Petrology. 2018 ; 巻 59, 番号 6. pp. 1109-1144.
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T1 - Monazite behaviour and time-scale of metamorphic processes along a low-pressure/ high-temperature field gradient (Ryoke belt, SW Japan)

AU - Skrzypek, E.

AU - Kato, T.

AU - Kawakami, T.

AU - Sakata, S.

AU - Hattori, K.

AU - Hirata, T.

AU - Ikeda, Takeshi

PY - 2018/6/1

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N2 - Low-pressure/high-temperature metamorphic rocks exposed in the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan) include a section with increasing temperature conditions from ~425 to 880°C. We use this setting to explore the evolution of monazite grain size, texture and composition, and variations in the whole-rock composition of 11 metapelite, metapsammite or metachert samples collected along the metamorphic field gradient. Monazite grain size increases with rising metamorphic grade, regardless of the whole-rock composition. From low- to high-grade conditions we infer: (1) the initial nucleation of monazite aggregates after allanite (~425°C); (2) monazite coarsening and coalescence driven by incipient monazite recycling; that is, dissolution of small grains to grow larger ones by Ostwald ripening (500-600°C); (3) a first major recycling stage enhanced by fluid liberation owing to muscovite breakdown (600-630°C); (4) a second recycling stage assisted by an increase in the proportion of anatectic melt owing to biotite breakdown (> 850°C). A succession of four compositional domains is recognized in monazite. We emphasize the usefulness of comparing their Ce/ThMnz, Ce/YMnz and Th/UMnz molar ratios with those derived from whole-rock analyses to constrain the origin of each domain. Domain I, with variable ratios, reflects the progressive transfer of Th ± U from allanite to monazite at low-grade conditions. Domain II, with Ce/ThMnz matching the whole-rock values, indicates growth under rock(decimetre)- scale equilibrium conditions. Domains II and III, with Th/UMnz and Ce/YMnz departing from the whole-rock values, record the competition with zircon (for U) and garnet (for Y) during growth at peak P-T conditions. Domain IV points to Y supply by garnet resorption during retrograde chloritization (< 550°C). In the highest-grade sample, zircon grains included in garnet or cordierite show metamorphic rims with sillimanite and Si-rich inclusions. These rims formed at suprasolidus conditions (650-880°C) and yield 206Pb/238U ages of 103-97Ma (±5 Ma), which bracket the timing of high-temperature metamorphism. Monazite dating by electron microprobe and laser ablation inductively coupled plasma mass spectrometry reveals two age groups. For domains I-III, some relatively old 206Pb/238U ages (99-95±3-5 Ma) represent minimum estimates for the timing of prograde to peak metamorphism, whereas the similar oldest 206Pb/238U age for domain IV (93 ± 7 Ma) points to rapid cooling after the temperature peak. A more dominant population of younger Th-U-Pb and 206Pb/238U dates is ascribed to age resetting by heterogeneous annealing of the monazite crystal lattice. The youngest concordant 206Pb/238U age of 86 ± 4Ma is correlated with the end of intermediate-temperature conditions (< 200-300°C) below which monazite, in metamorphic rocks as well as in the surrounding granitoids, eventually behaved as a closed isotopic system. This contribution is dedicated to the memory of Professor Kazuhiro Suzuki, for his contribution to geochronology and Japanese geology.

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