Microstructural vs compositional preservation and pseudomorphic replacement of muscovite in deformed metapelites from the Longmen Shan (Sichuan, China)

Airaghi, Laura; Lanari, Pierre; de Sigoyer, Julia; Guillot, Stéphane (2017). Microstructural vs compositional preservation and pseudomorphic replacement of muscovite in deformed metapelites from the Longmen Shan (Sichuan, China). Lithos, 282-283, pp. 262-280. Elsevier 10.1016/j.lithos.2017.03.013

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Pressure-temperature-deformation (P-T-ε) paths for metamorphic rocks that experienced a complex deformation history commonly rely on the link between successive metamorphic assemblages and the different microstructures preserved at the sample scale. However, with changing P-T conditions, metamorphic minerals in a specific microstructure can chemically re-equilibrate. 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. The chemical evolution of muscovite was modeled along a P-T loop previously constrained with phase equilibria calculations, semi-empirical and empirical thermobarometry. Results show that metapelites experienced a three-stage metamorphic history: (1) heating and burial up to 11 ± 2 kbar, 530 ± 20 °C, (2) minor decompression and heating up to 6.5 ± 1 kbar, 575 ± 10 °C and (3) decompression and cooling down to 4 ± 1 kbar, 380–450 °C. Muscovite partially re-equilibrated by pseudomorphic replacement during the three metamorphic stages, although it is mainly observed in prograde microstructures preceding the pressure peak. The main factors controlling the 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. Such micro-chemical behavior is probably relatively common in metapelites and questions the reliability of the P-T predictions based on relict phase chemistry, apparently preserved in microstructures that might have been affected by later re-equilibration.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Petrology

UniBE Contributor:

Lanari, Pierre

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0024-4937

Publisher:

Elsevier

Language:

English

Submitter:

Pierre Lanari

Date Deposited:

19 Sep 2019 10:47

Last Modified:

04 Nov 2019 16:06

Publisher DOI:

10.1016/j.lithos.2017.03.013

BORIS DOI:

10.7892/boris.112210

URI:

https://boris.unibe.ch/id/eprint/112210

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