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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 01 Aug 2018

Research article | 01 Aug 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Solid Earth (SE).

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, 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 accomplished by a combination of fracturing, dissolution/precipitation and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we investigate with EBSD the microstructure of a granodiorite mylonite, developed at 420–460°C during cooling of the Rieserferner pluton (Eastern Alps), to assess the microstructural processes and the role of weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of the plagioclase of pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain size reduction and ordered 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), show that during mylonitization at 450°C, 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 mid-upper continental (felsic) crust.

Alberto Ceccato et al.
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Alberto Ceccato et al.
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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...