COMPARISON: Webster-Addie Amphibolites to Buck Creek and Carroll Knob Amphibolites

1. Typical Analyses and CIPW Norms of WA-Wi Amphibolites COMPARISON: Webster-Addie Amphibolites to Buck Creek and Carroll Knob Amphibolites Whole-Rock Geochemical Variations of WA-WI Amphibolites Amphibolites from the Webster-Addie and Willets areas differ markedly in their compositions from the Buck Creek, Carroll Knob, and other amphibolite exposures in the southwestern North Carolina Blue Ridge. All the WA-WI amphibolites are relatively high in SiO2 (52-60% wt), and are generally strongly quartz normative. On classification diagrams such as Na+K vs. SiO2 or K2O vs. SiO2, the WA-WI samples plot as more evolved igneous rocks (i.e, andesite and dacite), and are distinctly less mafic than like rocks from either Buck Creek or Carroll Knob. On plots of MgO vs. Al2O3, and CaO vs. Al2O3, it is clear that the WA-WI rocks cannot be explained as simple mixtures of mafic minerals - if these rocks had igneous protoliths, those protoliths were most likely intermediate composition lavas, and not mafic cumulate rocks. Perhaps the most telling difference between WA-WI amphibolites and others of the SW Blue Ridge is evident in the AFM diagram, where these rocks follow a distinct calc-alkaline trajectory, suggesting that unlike the rocks of Buck Creek and/or Carroll Knob, they are probably related to subduction zone processes. Constraints on the origin of Webster-Addie/Willets amphibolites Sample Preparation and Analytical Techniques On most tectonic classification diagrams, the WA-WI amphibolites do not plot coherently. On the V vs. Ti classification plot of Shervais (1982) and the TiO2 vs. Fe# plot of Serri (1981), the WI-WA rocks plot largely in the oceanic basalt fields. On the V vs. Fe/Mg classification diagram of Desmos (1980), WA-WI rocks fall largely between the calc-alkaline and tholeiitic fields, and on the V vs. Cr plot of Pearce (1973), most of the WA-WI samples fall outside the field for MORBs. The WA-WI amphibolites differ from calc-alkaline lavas in that they have high concentrations of TiO2, evidenced in the samples as abundant titanite, ilmenite, and rutile. The process of partial melting during migmatite formation sequesters Fe-Mg-Ti bearing minerals such that on the scale of hand samples, anomalously high (and low) Fe and Ti concentrations are possible, depending on whether the sample included more leucosome vs. melansome. Interestingly, even if one takes the mean values of our amphibolite samples, they still plot anomalously on classification diagrams. Options for explaining these patterns include the extraction of anatectic melt, which would shift the compositions of the rocks (and would imply their protoliths were even more felsic), or the WA-WI amphibolites may not, in fact, represent igneous protoliths. Summary of Observations and Inferences: WA-WI amphibolites occur as lenticular bodies in the field, on a variety of scales, and may or may not be in close association with ultramafic rocks. Amphibolite fabrics and textures range from strongly foliated to migmatitic. "Block in Matrix" structures are commonly observed in the field.  Mineral assemblages are Hbld + Plag + Qz + Bio ± Gt + Titanite ± ilmenite ± rutile, with chlorite, clinozoisite, and carbontes as secondary metamorphic phases.   Major-element chemistry of WA-WI amphibolites points to probable igneous rock protoliths of intermediate, calc-alkaline compostions. The processes of metamorphism, migmatization, and melt extraction have extensively redistributed even "immobile" trace elements, preventing more detailed assessments of tectonic affinities. In terms of composition and probable protolith, the WA-WI amphibolites are profoundly different from those associated with the Buck Creek and Carroll Knob complexes to the SW. The differences in the amphibolites may indicate different origins and histories for the rocks in these regions of the Blue Ridge.