Beauty and complexity of metamorphism: case studies from the frontal part of the Adria microplate

Cesare, Bernardo; Mazzoli, Claudio; Sassi, Raffaele; Spiess, Richard; Sassi, Francesco

doi:10.1007/s12210-010-0078-0

Cited by 6 publications

(2 citation statements)

References 47 publications

(57 reference statements)

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“…The tonalitic-granodioritic Rieserferner pluton (Eastern Alps; Bellieni, 1978) was emplaced at ∼ 15 km depth (0.4 GPa; Cesare et al, 2010) into the Austroalpine nappe system at 32 Ma (Romer and Siegesmund, 2003). During post-magmatic cooling, a main set of ductile shear zones exploited shallowly ESE-dipping joints, and the joint-filling Qtz and Ep veins (set 2 of Ceccato and Pennacchioni, 2018).…”

Section: Geological Setting and Field Descriptionmentioning

confidence: 99%

Myrmekite and strain weakening in granitoid mylonites

et al. 2018

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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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Section: Geological Setting and Field Descriptionmentioning

confidence: 99%

Myrmekite and strain weakening in granitoid mylonites

et al. 2018

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“…The tonalitic-granodioritic Rieserferner pluton (Eastern Alps) (Bellieni, 1978) emplaced at ~15 km depth (0.4 GPa; Cesare et al, 2010) into the Austroalpine nappe system at 32 Ma (Romer and Siegesmund, 2003). During post-magmatic cooling, a main set of ductile shear zones developed exploiting precursor shallowly ESE-dipping joints and the locally associated quartz and epidote veins 5 filling the joints (Ceccato, 2018;Ceccato and Pennacchioni, 2018).…”

Section: Geological Setting and Field Descriptionmentioning

confidence: 99%

Myrmekite and strain weakening in granitoid mylonites

Ceccato¹,

Menegon²,

Pennacchioni³

et al. 2018

Preprint

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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.

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The Fibrolite Problem: Implications for Reaction Overstep Studies

Sharma

Pant

Saikia

2022

Journal of the Geological Society of India

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Despite several experimental studies the phase relations in aluminosilicates, especially involving the high-temperature polymorph, are not well understood. The widespread occurrence of fibrolite before sillimanite in rocks is described as the ‘fibrolite problem’. Fibrolite is considered to have higher free energy than sillimanite because of the excess surface energy contribution due to the higher surface/volume ratio. Equilibrium thermodynamics fails to reconcile the stability of fibrolite, with higher free energy, at lower temperatures than sillimanite. The existing literature invokes reaction kinetics to suggest that the low energy of {110} faces leads to acicular fibrolite growth rather than prismatic sillimanite. Here the ‘fibrolite problem’ is discussed from a perspective of high nucleation rates and hindered growth rates. An analog to fibrolite-sillimanite crystallization is suggested in the crystallization of mullite, a member of the Al2O3-SiO2 system. The crystallization of fibrolite in rocks shifts the position of the sillimanite producing reactions from the equilibrium PT depending upon the ΔS and ΔV of the reaction. Thermodynamic datasets calibrated using such natural samples tend to produce accurate predictions for sillimanite stability in geological terranes but inherently possess a reaction overstep and hence should be used carefully while quantifying reaction oversteps.

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Beauty and complexity of metamorphism: case studies from the frontal part of the Adria microplate

Cited by 6 publications

References 47 publications

Myrmekite and strain weakening in granitoid mylonites

Myrmekite and strain weakening in granitoid mylonites

Myrmekite and strain weakening in granitoid mylonites

The Fibrolite Problem: Implications for Reaction Overstep Studies

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