Deformation Mechanisms and Strain Localization in Ultramafic Rocks: Insights from the Balmuccia and Finero Peridotites, Ivrea-Verbano Zone

This study investigates the deformation mechanisms of olivine in ultramafic rocks from the Balmuccia and Finero massifs in the Ivrea-Verbano Zone, emphasizing the influence of dry and hydrous conditions on deformation behavior, microstructural features, and geochemical compositions. Advanced techniques, including EBSD, HR-EBSD, and EPMA, were utilized to provide detailed insights into the processes shaping these regions.

Our findings reveal significant chemical and microstructural distinctions between the Balmuccia and Finero peridotites. Olivine grains in the Balmuccia massif are smaller (~67 µm on average) and exhibit higher internal distortion. They display an A-type CPO pattern characterized by the alignment of the [100] axes with the stretching direction, the [010] axes perpendicular to the foliation plane, and the [001] axes within the foliation plane but normal to the lineation direction. The distribution of misorientation axes along the [010] direction and the alignment of Weighted Burgers Vectors (WBVs) along both [100] and [001] directions suggest the activation of both (001)<100> and (100)<001> slip systems, with the latter being more prominent in olivine grains in contact with orthopyroxene grains.

Conversely, olivine grains in the Finero massif are larger (~137 µm on average) and exhibit less internal deformation. These grains show a complex deformation history, with grain-size-dependent variations in CPO patterns. Larger grains predominantly display A-type CPO, while smaller grains exhibit a mixed B-type and A-type CPO pattern. The clustering of misorientation axes along the [001] direction in fine grains suggests the activation of the (010)<100> slip system in fine-grained olivine from Finero.

Geochemical analyses indicate that Balmuccia retains primary mantle characteristics with minimal metasomatic alteration. Spinels in this region have low Cr# (10–30) and high Al, indicative of a refractory mantle origin. In contrast, Finero samples exhibit strong evidence of metasomatism, with spinels enriched in Cr (Cr# 60–80) and TiO₂, reflecting interactions with subduction-related melts and fluids. Chemical profiles of Finero spinels show Cr enrichment and Al depletion along grain boundaries, pointing to chemical redistribution during deformation.

HR-EBSD analysis reveals that the maximum GND density in Balmuccia samples is two orders of magnitude higher than in Finero samples. In Balmuccia, areas with olivine grains in contact with orthopyroxene and clinopyroxene grains exhibit a higher frequency of subgrains with GND densities exceeding 10¹⁴ m⁻². Finero samples exhibit a relatively homogeneous stress distribution, with an average stress of approximately 24 MPa. Balmuccia samples show a more heterogeneous stress distribution. Stress maps align with GND density distribution patterns, and stress magnitudes in regions where olivine grains contact opx grains range from 3 to 4 GPa at subgrain boundaries.

We propose that Finero and Balmuccia initially experienced similar conditions at the onset of rifting. However, subsequent detachment faults amplified their divergence, displacing Finero into a foredeep position analogous to the Banda Sea foredeep and Balmuccia into a magmatic arc environment similar to the Banda Sea magmatic arc.