NEW EVIDENCE FOR LOADING DURING METAMORPHISM OF THE EMUCKFAW GROUP, EASTERN BLUE RIDGE, ALABAMA

Garnet-bearing high-Al pelite from the Josie Leg Member of the Emuckfaw Group, within the Alexander City Fault Zone, provides new evidence for loading in the Eastern Blue Ridge. Metamorphic minerals include: garnet, staurolite, biotite, chlorite, muscovite, quartz, and rutile. Garnet ranges from ~0.1 to ~2.5 cm in diameter and contains inclusions of staurolite, chlorite, rutile, ilmenite and quartz confirming a high-Al pelite composition. The peak mineral assemblage is inferred to be garnet, biotite, muscovite, quartz and rutile. Garnet displays simple concentric zoning in Mn (decreasing from core to rim), Fe (slightly decreasing from core to rim), and Mg (increasing from core to rim). Ca zoning varies from simple concentric in a larger grain to oscillatory in a smaller grain. We interpret a single metamorphic event based on Mn zoning. The Ca variations of ~0.03 mole fraction likely reflect growth in equilibrium with minerals that varied during garnet growth and/or fluid flow.

MnNaCaKFeMgAlSiHTi P-T pseudosections and garnet compositions provide P-T estimates for core and rim growth. The core (Mn=0.05, Fe=0.74, Mg=0.05, & Ca=0.17) of a large garnet grain (1.75 cm) from the first sample indicates initial growth at ~540°C at ~6 kbar. The rim (Mn=0.02, Fe=0.72, Mg=0.13, & Ca=0.15) indicates an increase in P-T to ~650°C at ~10.5 kbar. Modal garnet predictions from ~2% to ~10% at termination of growth are compatible with growth along the predicted P-T path. The high Mn center of a smaller garnet (0.45 cm) from a second sample does not align with the geometric center of the grain. The garnet compositional isopleths do not intersect in P-T space and we infer post-peak metamorphic resorption and disequilibrium.

The increase in P-T from 6 to 10.5 kbar during regional metamorphism and similar results for the Wedowee Group to the northeast is inferred to result from Neoacadian crustal thickening in the Eastern Blue Ridge.