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| Titel |
Microstructural record vs chemical and geochronological preservation in muscovite: implications for P-T-t estimates in deformed metapelites |
| VerfasserIn |
Laura Airaghi, Pierre Lanari, Clare J. Warren, Julia de Sigoyer, Stéphane Guillot |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250140308
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| Publikation (Nr.) |
 EGU/EGU2017-3677.pdf |
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| Zusammenfassung |
| Pressure-temperature-deformation (P-T-ɛ) paths for metamorphic rocks commonly relies on
the link between successive metamorphic assemblages and the microstructures. However,
with increasing P-T conditions, metamorphic minerals in an early microstructure can
re-equilibrate by changing their chemical composition. The direct link between deformation
phases and mineral compositions for thermobarometry purposes can therefore be distorted.
This study focuses on a series of garnet-biotite metapelites from the Longmen Shan (Sichuan,
China) that preserve muscovite of different chemistry in distinct microstructures. To quantify
the degree of re-equilibration of muscovite, a microstructural study was coupled with
high-resolution chemical mapping. Then, the chemical evolution of muscovite (Si4+
and XMg) was modeled using Gibbs free energy minimization along a P-T loop
previously constrained by phase equilibria calculations, semi-empirical and empirical
thermobarometry.
Our results show that the studied metapelites experienced a “typical” three stages
metamorphic history: (1) heating and burial up to 11 kbar, 530˚ C, (2) minor decompression
and heating up to 6 kbar, 580˚ C and (3) decompression and cooling down to 4-5 kbar,
380-450˚ C. However, muscovite has been partially or completely re-equilibrated during the
three stages by idiomorphic replacement, although it is mainly observed in prograde
microstructures preceding the pressure peak. The main factors controlling the degree of
re-equilibration are the intensity of the deformation and the fluid availability during
metamorphism. The P-T conditions of metamorphic assemblages thus reflect pulses of fluids
release that enhanced mineral resorption and local replacement. The metamorphic
peak (2) was dated by in situ 40Ar/39Ar on biotite porphyroblasts and by in situ
(U-Pb)/Th laser ablation on allanite (REE-rich epidote) at 185±15 Ma. Muscovite
grains preserved in prograde microstructures and partially re-equilibrated during
stages (2) and (3) yield younger ages, at 150±10 Ma. These results, in apparent
contradiction with the microstructural observations, suggest a key role of grains size and
deformation and re-equilibration in the Ar recording. The micro-chemical behavior
unrevealed in this study is probably relatively common in metapelites and questions
the reliability of the P-T-t predictions based on relict phase chemistry, apparently
preserved in microstructures that might have been affected by later re-equilibration. |
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