RF/PHI ANALYSIS OF FELDSPAR IN DEFORMED ORTHOGNEISS: COMPARING MACRO- AND MICRO-ANALYTICAL TECHNIQUES

Rf/phi analysis is a widely used technique to quantify strain in deformed rock. Software applications (i.e. Ellipsefit) have simplified the Rf/phi process by integrating grain boundary tracing with an automated process for identifying best-fit ellipses, including for mineral grains with irregular boundaries. Although more commonly used for strain analysis on quartz-bearing sedimentary and metasedimentary rocks, some geologists have had moderate success using Rf/phi to quantify the strain recorded by feldspars in granitoids and metagranitoids. In highly deformed orthogneiss, feldspars may be more useful than quartz as they are less likely to have suffered the intense ductile deformation that can dismember primary quartz grains. There are, however, a number of potential pitfalls associated with Rf/phi analysis of feldspars in deformed orthogneiss. First, it is often difficult to visually distinguish similarly-colored quartz and feldspar grains on smooth-faced hand samples cut for Rf/phi analysis. Secondly, grain boundaries between similarly-colored quartz and feldspar grains may be challenging to identify and subsequently trace for the purposes of Rf-phi analysis. Collectively, this has the potential to result in significant inconsistencies between different workers or across different samples during the grain tracing process. Third, it is possible that strain may be partitioned differently across feldspars of various sizes in the same sample, therefore complicating the selection of hand samples vs. thin-sections for analysis. In an attempt to identify pitfalls and determine best practices in Rf/phi analysis of orthogneiss, we compare and contrast the results from granitoids with no apparent deformation vs. deformed orthogneiss samples from the Kowaliga Gneiss in the eastern Blue Ridge of Alabama. Rf/phi analysis will be conducted using hand samples, thin sections, and chemical mapping of grains utilizing the EDS capabilities of a scanning electron microscope. By comparing Rf/phi results using all three techniques, and across feldspars of differing sizes, we will be able to assess interpretation of bulk strain as recorded by feldspar in these rocks. Results of this project will help determine best practices and pitfalls for conducting Rf/phi analysis using feldspar in deformed granitoids.