Metastability and Nondislocation-Based Deformation Mechanisms of the Flem Eclogite in the Western Gneiss Region, Norway

Abstract

The eclogite in the Flem Gabbro from Flemsøya Island of the Western Gneiss Region in Norway contains atypical eclogitic minerals, such as olivine and orthopyroxene, and can be texturally divided into weakly deformed massive eclogite (MEC) and strongly deformed foliated eclogite (FEC). Based on phase equilibria modeling, peak metamorphic pressure-temperature conditions of ~600–750 °C and ~1.0–2.5 GPa and ~700–820 °C and ~2.7–3.7 GPa are recorded in MEC and FEC, respectively. These different pressure-temperature conditions between MEC (high-pressure, HP) and FEC (ultrahigh-pressure, UHP) in the same outcrop reflect deformation-enhanced eclogitization metamorphism (HP to UHP transition) via the addition of external water during subduction/burial to early exhumation stages and metastable preservation of the HP MEC assemblage at UHP condition due to the local lack of deformation and fluid access at the deep subduction interface or around Moho beneath continental collision zone. Based on the mineral microstructures, nondislocation-based creep mechanisms—such as diffusion creep, grain and phase boundary sliding, and rigid-body-like rotation—play dominant roles in governing the deformation features of FEC and developing the crystal preferred orientations of its major constituent minerals. These deformation mechanisms could considerably affect the interplate coupling at the subduction interface or the rheological strength of Moho beneath continental collision zones. Therefore, the effects of metastability-based (i.e., preservation of low-pressure assemblages at HP conditions) and contributions of nondislocation-based creep mechanisms should be included in the future geodynamic and petrological simulations of subduction and collision processes.