RHEOLOGICAL CONTROLS ON APATITE DEFORMATION MECHANISMS AND THE POSSIBILITIES OF STRUCTURAL PETROCHRONOLOGY

The recrystallization of chronometrically important accessory phase minerals, like apatite, can lead to decay-product loss and effective resetting of the decay system (e.g., U-Pb) thus recording the age of ductile deformation. There has not been a systematic study of the deformation mechanisms of apatite across different lithologic and P-T conditions, thus limiting our ability to leverage these observations to understand shear zone processes. Developing a systematic knowledge of the factors controlling apatite deformation mechanisms will broaden opportunities to date deformation and metamorphism in shear zones. In this study, we investigate the microstructure and geochronometry of apatite within a range of lithologic units deformed during the Devonian Acadian Orogeny at greenschist to amphibolite facies conditions within the Norumbega fault system (Maine, USA). Using electron backscatter diffraction (EBSD) analysis, we characterized the deformation mechanisms of apatite and the primary strain-accommodating phases that host it. Apatite hosted in amphibolite and quartzofeldspathic gneiss is characterized by the development of neoblasts via dynamic recrystallization and low-angle boundaries via dislocation glide. The matrix is comprised of recrystallized amphibole and plagioclase, respectively. Apatite within a mylonitic granite is restricted to bands of foliated biotite that wrap around plagioclase and K-feldspar deformed by dislocation creep and dissolution-precipitation creep. Apatite grains lack intracrystalline strain but exhibit zoning indicative of interface coupled dissolution-precipitation. Our results show apatite deformation mechanisms are primarily controlled by the rheology of surrounding phases. Fluid-mediated recrystallization via dissolution-precipitation reactions is similarly important. Initial results from LA-ICP-MS U-Pb analyses indicate that syn-kinematic dissolution-precipitation reactions can efficiently reset the U-Pb age of apatite. Dating dislocation creep accommodated dynamic recrystallization of apatite neoblasts is limited by the resolution of laser ablation, however dislocation glide accommodated intracrystalline strain may effectively reset the U-Pb system of apatite porphyroclasts.