Mineral Chemistry and Microstructural Evolution of Plagioharzburgite and Troctolite from the Honningsvåg Igneous Complex (Norway): Insights from EPMA, EBSD, and Single-Crystal XRD

The Honningsvåg Igneous Complex (Magerøya, N Norway) provides a unique window into the syn-tectonic crystallization and deformation of mafic-ultramafic cumulates within the Caledonian orogenic belt. This study integrates High-Resolution Electron Backscatter Diffraction (EBSD) mapping, Single-Crystal X-ray Diffraction (SC-XRD) analysis and refinement of the crystal structure, and Electron Probe Microanalysis (EPMA) to decipher the microstructural and chemical evolution of plagioharzburgites and troctolites. Our EBSD results reveal intense intra-crystalline plastic deformation in olivine, characterized by well-developed subgrain boundaries and translational deformation lamellae resulting from high-temperature dislocation creep. SC-XRD structural refinements substantiate this behavior, demonstrating that tectonic strain was accommodated by structural distortion restricted to the highly anisotropic M2 octahedral site. In contrast, clinopyroxene and plagioclase exhibit strong chemical and structural disequilibrium relative to the olivine framework (apparent olivine–orthopyroxene KDFe−Mg ≈ 1.08 vs. discordant olivine–clinopyroxene pairs), recording late-stage intercumulus crystallization from fractionated residual melts or episodic magma replenishment in an open system. Furthermore, plagioclase documents a continuous poly-rheological transition, shifting from magmatic alignment and dynamic recrystallization within a dense crystal mush to pervasive brittle micro-fracturing during late-stage tectonic exhumation. Finally, uniform crystallographic orientations of post-magmatic serpentine meshes reveal a strict epitaxial growth mechanism onto the parent olivine, proving that regional ductile deformation had entirely ceased prior to low-temperature H2O–CO2 hydrothermal fluid influx. These findings underscore the role of the Honningsvåg intrusion as a highly dynamic, open magmatic system evolving within an active orogenic environment, effectively bridging the gap between deep-seated magmatic processes and crustal-level tectonic deformation.