Shear zone nucleation by fluid-assisted heterogeneous nucleation recorded in texturally homogeneous eclogitized mafic granulites

Shear zone nucleation in massive rocks commonly exploits pre-existing planar structures, whose presence and type control fluid availability and redistribution. While fluids are widely recognized as key triggers for metamorphic reactions and mineralogical transformations that influence rock rheology, the exact feedback processes between fluid-rock interactions, metamorphism and deformation remain enigmatic. In specific cases, traditional softening mechanisms (e.g. reaction-induced grainsize reduction, crystallization of weak minerals) do not apply or are insufficient to explain shear zone nucleation and strain localization, implying the existence of alternative processes.

Paired shear zones developed at the selvages of hydration haoles are a common product of fluid infiltration along hydrofractures, and are a source of information to investigate the rheological effects of the different reaction extents occurring during fluid percolation. Here, we investigate a suite of samples containing eclogitic clinozoisite-filled veins surrounded by omphacite-rich haloes. The sample set includes haloes (a) weakly affected by ductile deformation, preserving pristine metasomatic textures, and (b) displaying paired shear zones at their selvages (Pennacchioni, 1996). The eclogitic host rock foliation, consisting of garnet, clinozoisite, amphibole and omphacite, is only partially obliterated in the hydration halo by the metasomatic overprint, dominated by replacement of clinozoisite by omphacite. EDS major element and in-situ LA-ICP-MS trace element analysis suggests that fluid propagation caused recrystallization, changes in mineral proportions and (re)distribution of major and trace elements, forming a compositional gradient across the halo. Garnet and clinozoisite rims record the gradient with a progressive decrease in the Fe²⁺ content and a progressive increase in LREE and Fe³⁺ concentrations from the vein selvage towards the reaction front, respectively.

Electron backscatter diffraction (EBSD) maps provide evidence for (i) a constant omphacite grainsize across the haloes and at their boundaries in samples weakly affected by ductile deformation, suggesting that metasomatism does not produce textural gradients, (ii) development of very fine-grained monomineralic ribbons of omphacite along the shear zones, suggesting that omphacite is responsible for weakening and localized shearing, (iii) local orientation of these ribbons at 20-30° to the shear zone trace, defining C' bands, and (iii) random orientation of the fine grains. We interpret these observations as evidence for diffusion-assisted grain boundary sliding (GBS) and creep cavitation as the main deformation mechanism active along the shear zones, and for heterogeneous nucleation of very fine-grained omphacite within fluid-filled cavities formed during GBS.

We conclude that, when metasomatic reactions do not directly result in textural gradients (e.g. grainsize decrease) traditionally considered responsible for softening at the propagation front (i.e. halo boundary), shear zones may develop by heterogeneous nucleation of fine grains during fluid-assisted GBS, which further fosters grainsize-sensitive deformation sustaining strain localization within fluid-rich domains.

[1] Pennacchioni, 1996. Journal of Structural Geology, 18, 549-561