COMPARISON OF DEFORMED FELDSPAR AT AMPHIBOLITE FACIES USING RF-PHI ANALYSIS: SIGNIFICANCE FOR REGIONAL STRAIN STUDIES

In the southernmost Appalachians of Alabama and Georgia, the Ashland-Wedowee-Emuckfaw belt (Eastern Blue Ridge) contains a number of Ordovician to Carboniferous, pre- to synkinematic orthogneiss bodies of varying lithology. These deformed metagranitoids were initially emplaced in a thick (>10 km) sequence of marginal basin sedimentary units, the Wedowee and Emuckfaw Groups, originating in a Laurentian plate, Ordovician back-arc basin during the southern Appalachian Taconic orogeny. Both the plutons and metasedimentary host rocks were metamorphosed under middle-upper amphibolite facies conditions during the earliest Alleghanian orogeny (ca. 330 Ma), prior to pervasive thrust faulting affecting the southern Appalachian margin during the final assembly of Pangea. In addition to Alleghanian terrane-bounding faults, some workers have proposed the existence of major pre- to synkinematic faults internal to these terranes. In an attempt to quantify localized and regional strain in the Alabama Eastern Blue Ridge, we utilize Rf-phi analysis of feldspar porphyroclasts as a mechanism for comparing strain gradients across proposed fault zones with strain gradients in non-faulted regions of the belt within the context of regional strain. Critical to this analysis is a comparison of the behavior of different types of igneous feldspar deformed under peak metamorphic conditions. In order to determine if contrasting mechanical behavior of k-feldspar vs. plagioclase affect Rf-phi analysis of middle-upper amphibolite facies, mineralogically-variable metagranitoids (e.g. granite vs. granodiorite), we separately analyzed k-feldspar and plagioclase in the same samples for comparison. Results indicate Rs values for individual plagioclase vs. k-feldspar populations vary less in individual samples (variance <0.01) than bulk Rs values for different samples analyzed from the same outcrop, suggesting differences in mechanical behavior of feldspar are not significant at middle-upper amphibolite facies in the context of Rf-phi analysis. This suggests that Rf-phi analysis of silicic orthogneiss bodies at these P-T conditions is not dependent on variations in feldspar composition, significantly expanding the range of suitable lithologies for regional strain studies using Rf-phi analysis.