Polycyclic Deformation, Reactions and Disequlibrium in Mylonites from An Evolving Fault Zone

Rocks from the Norumbega ductile fault zone (NFZ) of southern Maine show that they evolved from gneissic through mylonitic to bands of ultramylonitic textures indicative of progressive strain localization. Amphiboles from mafic lithologies show compositional zoning and textural variability that indicate that the NFZ deformed episodically in time and space during retrogression from upper amphibolite to greenschist facies conditions. We use amphibole composition and thermometry to monitor these retrograding metamorphic conditions from ~700˚C to < 400˚C and from ~6.3 to ~7.8 Si/f.u. respectively. High-grade amphibole grains from amphibolite boudins within the 4 km wide footwall of the NFZ crystallized above 700˚C. During cooler deformation (~650˚C) lozenge shaped amphiboles from the edges of meter scale boudins adjacent to mylonitic schists preserve continuous compositional zoning from ~6.3 to ~7.2 Si/f.u. This suggests deformation by dissolution and precipitation creep at ~650˚C. In contrast, amphiboles from the interior of the meter-scale boudins were fractured and were healed by new amphibole precipitation of ~7.4 Si/f.u. also at ~650˚C. Amphibole porphyroblasts in ultramylonites from local high-strain zones crystallized around clasts of high-temperature relic hornblende. Abrupt compositional changes from porphyroblast cores to rims in the ultramylonites suggests new amphibole precipitation from ~575˚C to <400˚C. Amphibole grain size decreases from ~1000 μm to ~100 μm long with decreasing temperature, but actinolitic amphiboles are strongly embayed by quartz and feldspars. Together these results show that amphibole grain size reduction in the NFZ between ~650˚C and <400˚C is dominated by dissolution, reprecipitation and mineral replacement rather than by fracture mechanisms.