Anatomy of a transitional brittle–ductile shear zone developed in a low-T meta-andesite tuff: A microstructural, petrological and geochronological case study from the Bükk Mts. (NE Hungary)

Microstructures of the low-T metavolcanites deriving from the transitional brittle–ductile Lillafüred Shear Zone (LSZ, Eastern Bükk Mts., NE Hungary) reveal that the prominent mylonitic rock-structure observed at hand-specimen scale was predominantly achieved at the grain scale by (1) intense microfracturing of rigid minerals (feldspar, opaque phases), and (2) intergranular sliding associated with solution mass transfer within the prevailing, fine-grained, phyllosilicate-rich matrix. Fluid migration during shearing—indicated by widespread growth of syntectonic, secondary calcite and pervasive retrograde alteration of feldspar into “weak”, fine-grained mica aggregates—led to substantial weakening and localization of deformation into narrow zones. This is accompanied microstructurally by an evolution from a cataclasis-dominated regime towards a more ductile, mylonitic deformation. Various metamorphic petrological and geochronological parameters (illite Kübler index, chlorite “crystallinity” indices, crystallite thickness, mean lattice strain, chlorite-AlIV thermometry, K-Ar ages on <2 μm white K-mica) indicate uniformly peak metamorphic conditions around the boundary of the anchi/epizone (300–350 °C) and subsequent cooling about 76 Ma both in the heavily sheared rocks of the LSZ and the enclosing, weakly deformed rock mass. These relationships suggest that the LSZ represents a late-stage, relatively “cold” (≤250–270 °C), fluid-assisted, dextral strike-slip zone formed after 76 Ma on the retrograde path of the Eoalpine tectonometamorphic cycle.