Metamorphic Evolution of Eastern Blue Ridge Calc-Silicates in Southwestern North Carolina, Northeastern Georgia, and Northwestern South Carolina

Calc-silicate rocks in the eastern Blue Ridge of SW North Carolina, NE Georgia, and NW South Carolina are products of contact and regional metamorphism. Two types of calc-silicates occur in this area: Type A rocks occur as contact metamorphosed xenoliths in the ~336 Ma Rabun Granodiorite batholith, and Type B rocks occur as mappable thin, elongate, ribbon-like units in the Tallulah Falls Formation east of the Rabun batholith. Petrographic and geochemical data confirm similar composition for both types and mineral assemblages that includes amphibole, garnet, epidote, diopside, quartz, and plagioclase with accessory zircon, sphene, and opaque minerals. Calcite, however, is present only in Type A xenoliths. Despite similar compositions, they have markedly contrasting textures: Type A calc-silicates are megacrystic and well-foliated, while the Type B calc-silicates are weakly to non-foliated and have a glassy, quartzitic appearance. The objective of this study was to investigate the conditions of formation and metamorphic history of these calc-silicates to determine how they formed. Conclusions from analysis are: (1) Type A and Type B originated from the same protolith; (2) That protolith was interbedded calcareous Fe-rich shale and sandstone; (3) Upper amphibolite facies peak regional metamorphism was responsible for initial formation of the calc-silicates; and (4) Increased temperatures and metamorphic fluids associated with contact metamorphism caused remobilization and recrystallization, producing the observed assemblages and textures in Type A. Deposition of the protolith took place post-Rodinia rifting but prior to orogenic events culminating in the amalgamation of Pangea. Peak regional metamorphism is associated with terrane accretion to the Laurentian margin during the Taconic or Acadian orogenies. Successive collisional events caused crustal thickening, which led to melting in the lower crust and ultimately the formation of the Rabun Granodiorite. During emplacement of this batholith, calc-silicate xenoliths of the Tallulah Falls Formation were incorporated into the magma and subjected to another phase of metamorphism. Understanding the polymetamorphic evolution of these rocks has provided more insight into the tectonic history of the southern Appalachians.