Rb/Sr isotopic and compositional retentivity of muscovite during deformation

Area of Science:

• Geochemistry
• Isotope Geology
• Metamorphic Petrology

Background:

• Investigating mineral retentivity under metamorphic conditions is crucial for geochronology.
• Muscovite is a common accessory mineral in pegmatites and metamorphic rocks.
• The Rb/Sr system in muscovite is sensitive to alteration and deformation.

Purpose of the Study:

• To assess the compositional and Rb/Sr isotopic retentivity of Permian muscovite during Cretaceous metamorphism.
• To understand the influence of deformation on the Rb/Sr geochronometer in muscovite.
• To differentiate primary compositional variations from deformation-induced changes.

Main Methods:

• Analysis of major and trace element compositions of muscovite.
• Rb/Sr and Sr isotopic ratio measurements (⁸⁷Rb/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr).
• Microstructural analysis to identify relic magmatic muscovite.

Main Results:

• Permian muscovite largely preserved major and trace element compositions during Cretaceous deformation.
• Significant loss of Sr from muscovite occurred, leading to higher ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios.
• Lower ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios in specific fractions yielded Permo-Triassic apparent ages, interpreted as pre-metamorphic.
• Higher ratios showed a linear array, indicating deformation-related Sr loss and progressively younger apparent ages.

Conclusions:

• Deformation is the dominant factor controlling the Rb/Sr geochronometer in muscovite at temperatures below 500°C.
• Sr loss during deformation significantly resets the Rb/Sr system, irrespective of primary compositional variations.
• The study highlights the importance of considering deformation effects when interpreting Rb/Sr ages in metamorphic terrains.