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Solid Earth An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 6
Solid Earth, 9, 1399–1419, 2018
https://doi.org/10.5194/se-9-1399-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Solid Earth, 9, 1399–1419, 2018
https://doi.org/10.5194/se-9-1399-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Dec 2018

Research article | 10 Dec 2018

Myrmekite and strain weakening in granitoid mylonites

Alberto Ceccato1,a, Luca Menegon2, Giorgio Pennacchioni1, and Luiz Fernando Grafulha Morales3 Alberto Ceccato et al.
  • 1Department of Geosciences, University of Padova, 35131 Padova, Italy
  • 2School of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UK
  • 3Scientific Centre for Optical and Electron Microscopy (ScopeM) – ETH Zürich, Zurich, Switzerland
  • anow at: School of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UK

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accommodated by a combination of fracturing, dissolution–precipitation, and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by the formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we use electron backscattered diffraction to (i) investigate the microstructure of a granodiorite mylonite, developed at  ∼ 450 °C during cooling of the Rieserferner pluton (Eastern Alps); and (ii) assess the microstructural processes and the weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of plagioclase in pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain-size reduction and anti-clustered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain-size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws, calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite) during deformation at 450 °C, show that grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain-size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the felsic middle crust.

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Short summary
Metamorphic fine-grained reaction products make continental crust rocks weaker. Microstructural processes related to the transformation of strong K-feldspar into weak aggregates of plagioclase and quartz during crustal deformation have been investigated through electron microscopy. Rheological calculations show that the occurrence of even small amounts of weak aggregates, whose deformation is mainly diffusion-assisted, would lead to a decrease in rock viscosity of several orders of magnitude.
Metamorphic fine-grained reaction products make continental crust rocks weaker. Microstructural...
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