THE EFFECTS OF GRAIN SIZE ON STRAIN PARTITIONING IN COARSE-GRAINED METAMORPHIC ROCKS:EXAMPLES FROM THE SOUTHERN APPALACHIANS (USA)

Rf/Phi analysis is one of several techniques used to quantify strain in deformed rocks. As such, it has been widely used in kinematic studies ranging from microstructural to regional in scale. Although Rf/Phi analysis is most commonly utilized in studies of sedimentary rocks and their metamorphic equivalents (e.g. metaconglomerate), some workers have applied the technique to deformed granitoids (i.e. orthogneiss) with some success. Previous workers have noted that the bulk strain (Rs) recorded by large feldspar grains (e.g. cm-scale) in feldspathic orthogneiss differed significantly from bulk strain recorded by smaller feldspars (e.g. mm-scale) in the same rock. This observation suggests that strain partitioning between feldspars of differing size might affect the results of Rf/Phi analysis. In order to determine if quartz was similarly affected by size-dependent strain partitioning, Rf/Phi analysis was conducted on a metaconglomeratic sample of the Devonian Cheaha Quartzite from the Talladega belt of Alabama. The Cheaha Quartzite, which experienced plastic deformation during Mississippian, lower greenschist facies metamorphism, commonly presents the effects of peak metamorphic deformation as a stretched pebble metaconglomerate, and is thus an excellent candidate for Rf/Phi analysis. Over 500 primary quartz clasts observed in the S1-S3 plane, ranging in size from approximately 0.01 millimeters to 4.0 millimeters, were traced as part of this work and analyzed using EllipseFit, a widely used software package created for geological strain analysis. Traced quartz clasts were then sorted according to the length of their long axes and grouped into overlapping size populations of n=150. Bulk strain calculations were then conducted separately on each size population. Initial results suggest significant variation in calculated bulk strain (Rs = 1.5 to 2.4) as a function of grain size, with larger grains systematically recording larger Rs values than smaller grains. These results are similar to earlier studies recognizing size dependent strain partitioning in feldspar grains in deformed orthogneiss. Additional quartz-dominant metamorphic rocks from Alabama, Georgia, and New Mexico will be similarly analyzed as a follow up to this work.