Role of pre-kinematic fluid-rock interactions on phase mixing, quartz recrystallization and strain localization in low-temperature granitic shear zones

Abstract

This study explores the relative importance of fluid availability, temperature and mineral assemblage variations on the development of macro and microstructures in greenschist-facies shear zones of two granitoids of the Axial Zone (central Pyrenees) emplaced at the end of Variscan orogeny, at similar structural levels. The investigated shear zones formed during alpine compression at comparable temperature. In the Bielsa granitoid, deformation was distributed in a dense network of shear zones. Extensive mineralogical transformations resulted in variations in major, minor and rare earth element contents pointing to pre-kinematic hydrothermal alteration (at Permian-Triassic) at 270–350 °C and further alteration in fluid-abundant conditions during Alpine shearing. Quartz was fractured, exhibits weak plastic deformation and was weakly reworked, mainly by dissolution-reprecipitation. Strain was accommodated in the mica-rich matrix (40–50% vol. of mica). Mica significantly grew during pre-kinematic hydrothermal alteration. In the Maladeta granite, deformation was localized in sparse shear zones. Hydrothermal alteration occurred at similar to slightly higher temperature (280–380 °C). Elemental variation related to alteration was weaker, pointing to fluid-deficient conditions before and during Alpine compression. Limited fluid resulted in lower mica content (20–28% vol.) than in Bielsa. Quartz exhibits stronger intragrain plastic deformation, dynamic recrystallization by subgrain rotation and reworking by dissolution-reprecipitation and nucleation. Strain was accommodated by both quartz aggregates and mica-rich matrix. The transition from dominant brittle behavior of quartz to dynamic recrystallization by subgrain rotation occurred in a narrow range of 350–380 °C, well below 450 °C as generally described. The lower temperature and less hydrated conditions in Maladeta are in apparent contradiction with stronger quartz recrystallization and strain localization than in Bielsa. By promoting phyllosilicate growth and phase mixing before shearing, the metamorphic pre-conditioning of the granitic protolith appears to be the main factor controlling strain distribution at sample scales and differences in tectonic styles between the two massifs.