Recrystallization and intracrystalline crystal-plastic deformation of naturally deformed hornblende

Despite hornblende’s widespread occurrence in deformed rocks from exhumed crustal shear zones and metamorphic soles, its dominant deformation mechanism(s) and the respective microstructural fingerprints remain poorly constrained. Several deformation mechanisms have been documented in hornblende, including cataclastic flow, twinning, dissolution–precipitation, and dislocation-mediated deformation. Hornblende’s easy slip system, (100)[001], can be inferred from observations of intragrain misorientation axes (MOA) or crystallographic rotation about the [010] axis (Meher et al., 2026). Notably, even where some contribution from dislocation-mediated deformation is observed, hornblende is rarely deformed solely by dislocation creep. While crystallographic preferred orientation (CPO) and recrystallization suggest dislocation creep for most minerals (e.g., calcite, quartz, and olivine), in hornblende, these features seldom arise from alternative mechanisms.

We used electron backscatter diffraction (EBSD) to analyze microstructures in four natural hornblende-rich samples spanning a range of P-T conditions: (1) Mamonia complex, Cyprus (0.5 GPa, ~ 600 °C), comprising mm-scale conjugated kink bands. (2) Koralpe, Austrian Alps (~2.1 GPa, 750 °C), dominated by sigmoidal hornblende porphyroclasts surrounded by smaller, tabular grains. (3) Mayodiya, India (0.78–0.82 GPa, 770–820 °C), containing large grains with high intragrain misorientations and some twinning, and smaller needle-shaped grains with serrated boundaries between large grains. And (4) Koraput, India (0.76–0.84 GPa, 860–883 °C), which exhibits recrystallization of a centimeter-scale porphyroclast with smaller grains with lobate boundaries forming a core–mantle microstructure. By examining both CPO and MOA using detailed EBSD analysis, our goal is to (i) constrain the underlying deformation mechanism in these samples, and (ii) identify temperature-dependent transitions under natural conditions.

The Mamonia sample that experienced the lowest deformation temperatures exhibits deformation through fractures and kink bands, with no evidence of recrystallization. However, the MOA cluster is oriented toward [010], consistent with dislocation glide, suggesting semi-brittle deformation (e.g., Meher et al., 2026). The Koralpe sample exhibits a characteristic recrystallization microstructure, strain-free grains around large and highly strained porphyroclasts, and an MOA clustering around [101], which fits the orientation of (-101) twin planes and suggests twinning-driven recrystallization. The Mayodiya sample exhibits elongated recrystallized grains with MOA clustering around [001], while the porphyroclast exhibits MOA toward [010], again indicating twinning-driven recrystallization. The Koraput sample displays recrystallized grains that are slightly rotated compared to the parent porpyroclast with rotation around [010], consistent with hornblende’s easy slip system, (100)[001].

We infer that at low P-T conditions, hornblende deforms through semi-brittle deformation. At intermediate temperatures (Koralpe and Mayodiya), twinning-driven recrystallization dominates, activated via the (-101)[101] and (100)[001] twinning systems, respectively. At the highest temperatures (Koraput), hornblende undergoes grain-size reduction via dislocation-driven recrystallization. Together, those samples suggest a temperature-controlled transition from semi-brittle to dislocation creep mediated deformation between < 600 to > 850 °C.  

 

Meher, B., Incel, S., Renner, J. and Boneh, Y., 2026. Experimental deformation of textured amphibolites in the semi‐brittle regime: Microstructural signatures of dislocation‐mediated deformation. Journal of Geophysical Research: Solid Earth, 131(1), p.e2025JB031852.