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Abstract
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.
Highlights
Localization of ductile strain within rocks arises from weakening associated with grain-size refinement processes by dynamic recrystallization, metamorphic reactions, and microfracturing (e.g. Platt, 2015, and reference therein)
Metamorphic reactions contributed importantly to strain weakening within the Rieserferner granitoid mylonites
Topotactic replacement has been inferred from the coincidence between myrmekitic Plg and parent Kfs grain crystal lattices in pristine myrmekite
Summary
Localization of ductile strain within rocks arises from weakening associated with grain-size refinement processes by dynamic recrystallization, metamorphic reactions, and microfracturing (e.g. Platt, 2015, and reference therein). Myrmekite replacement is either related to Kfs chemical instability (Cesare et al, 2002), in some cases involving local metasomatic fluids (Phillips, 1980), or triggered by stress concentration and intra-crystalline strain in Kfs during deformation (Simpson and Wintsch, 1989; Menegon et al, 2006). This replacement is acknowledged as a weakening mechanism during ductile deformation of granitoid rocks (LaTour and Barnett, 1987; Simpson and Wintsch, 1989; MacCaffrey, 1994; O’Hara et al, 1997; Tsurumi et al, 2003; Pennacchioni, 2005; Menegon et al, 2006; Pennacchioni and Zucchi, 2013; De Toni et al, 2016).
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