AMPHIBOLITE DEFORMATION VIA SOLUTION-PRECIPITATION CREEP AT LOWER CRUSTAL LEVELS: EVIDENCE FROM GEOTHERMOMETRY AND CHEMICAL ZONING

Chemical zoning in individual amphibole grains from lineated amphibolites of the eastern Blue Ridge, North Carolina, provides evidence that the deformation mechanism for these amphibolites was solution-precipitation creep. Understanding deformation mechanisms in rocks under high grade conditions from the thin section to the orogenic scales is critical to understanding the rheological behavior of the middle and lower crust. Most believe that rocks deformed in the greenschist facies conditions undergo deformation through congruent and incongruent pressure solution, but at higher grades, crystal plastic processes dominate (e.g. Snoke et al. 1998). However, where an aqueous fluid is present, the plastic deformation maybe replaced by solution transfer via stress induced chemical potential gradients along the grain boundary. At amphibolite facies conditions, this solution transfer can be the process for fabric development via mechanically induced dissolution of mineral grains orthogonal to the principal shortening direction and precipitation of new mineral on the host grain in the instantaneous shortening direction.

Amphibolites studied were sampled from a NW to SE transect perpendicular to the regional trend in order to capture the regional metamorphic gradient. Mineral assemblages suggest that the metamorphic conditions reached upper amphibolites conditions; conditions thought to be sufficient enough to homogenize chemical variations via lattice diffusion (Passchier et al., 2005). However, we have identified chemical zoning in the dominant amphibole grains as well as in plagioclase and clinozoisite-epidote solid solutions. Temperatures were quantified using the amphibole-plagioclase geothermometer, and we observed temperature ranges from 450°C to 750°C, which correspond to compositional zoning from actinolite to ferrotschermakite within single grains. Using pressures and temperatures calculated from grt+amph+plag cores and rims as well as Ti-bearing mineral textures observed via SEM, we can constrain a semi-quantitative pressure temperature path. Not only can the temperature variations within amphiboles constrain loading and exhumation paths, but we also observe that in the presence of a metamorphic fluid amphibolites deform predominately by dissolution-precipitation creep.