Abstract
A careful petrologic analysis of mylonites’ mineral assemblages is crucial for a thorough comprehension of the rheologic behaviour of ductile shear zones active during an orogenesis. In this view, understanding the way new minerals form in rocks sheared in a ductile manner and why relict porphyroblasts are preserved in zones where mineral reactions are generally supposed to be deformation-assisted, is essential. To this goal, the role of chemical potential gradients, particularly that of H2O (µH2O), was examined here through phase equilibrium modelling of syn-kinematic mineral assemblages developed in three distinct mylonites from the Calabria polymetamorphic terrane. Results revealed that gradients in chemical potentials have effects on the mineral assemblages of the studied mylonites, and that new syn-kinematic minerals formed in higher-µH2O conditions than the surroundings. In each case study, the banded fabric of the mylonites is related to the fluid availability in the system, with the fluid that was internally generated by the breakdown of OH-bearing minerals. The gradients in µH2O favoured the origin of bands enriched in hydrated minerals alternated with bands where anhydrous minerals were preserved even during exhumation. Thermodynamic modelling highlights that during the prograde stage of metamorphism, high-µH2O was necessary to form new minerals while relict, anhydrous porphyroblasts remained stable in condition of low-µH2O even during exhumation. Hence, the approach used in this contribution is an in-depth investigation of the fluid-present/-deficient conditions that affected mylonites during their activity, and provides a more robust interpretation of their microstructures, finally helping to explain the rheologic behaviour of ductile shear zones.
Highlights
Ductile shear zones are the locus of the Earth’s lithosphere where stress is largely accommodated, representing domains that tend to remain weaker than the host rocks even after their formation
This contribution is focussed on the petrologic nature of syn-kinematic minerals in mylonites, resulting from chemical processes related to strain weakening in already formed ductile shear zones, both under compressive- and extensional-dominating tectonics
As deformation favours diffusion of elements along grain boundaries and permeability is enhanced during ductile shearing (Etheridge et al 1983), chemical potential gradients can be either eliminated or favoured in the rock (Powell et al 2019), producing equilibrium or disequilibrium mineral assemblages, respectively (Regenauer-Lieb et al 2009; Hobbs and Ord 2017)
Summary
Ductile shear zones are the locus of the Earth’s lithosphere where stress is largely accommodated, representing domains that tend to remain weaker than the host rocks even after their formation This contribution is focussed on the petrologic nature of syn-kinematic minerals in mylonites, resulting from chemical processes related to strain weakening in already formed ductile shear zones, both under compressive- and extensional-dominating tectonics. Pennacchioni and Cesare 1997; Hentschel et al 2020; Tursi et al 2020a, b) This is the central issue concerned to this paper, a topic still debated amongst metamorphic petrologists (Powell et al 2019), which here is explored in rocks naturally deformed through simple shear. Chemical potential grids were calculated in the present research to detect the control of the chemical potential of some mobile major components, like major elements or H 2O, on the mineral assemblages of three mylonites from the polymetamorphic terrane of Calabria, that experienced shearing under overall fluid-deficient conditions either under compressive- or extensional-dominating tectonics
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