PETROLOGY AND GEOCHEMISTRY OF THE LITTLE PINE GARNET MINE SCHISTS, WESTERN BLUE RIDGE

The Little Pine Garnet Mine (LPGM) is located between the Mars Hill and Grenville basement terranes in the Western Blue Ridge province of the Southern Appalachians, and was mapped as part of the garnet zone of the Taconic orogeny (e.g. Hatcher & Merschat, 2005). It exposes anthophyllite (Anth) – gedrite (Gd) - chlorite (Chl) – biotite (Bt) - staurolite (St) ± coarse garnet (Gt) - ilmenite (Ilm) – rutile (Rt) – sillimanite (Sill) schists (group I), Gt (fine-grained) – Bt – hornblende (Hb) – quartz ± kyanite (Ky) ± clinozoisite (Czo) ± Sill gneisses (group II), and Chl – Gd - Anth - Ilm schists (group III), interbedded with quartzofeldspathic gneisses. Both the fine and coarse grained garnets are almandine – rich (X_alm = 0.59 – 0.67; X_prp = 0.22 – 0.3; X_grs = 0.05 – 0.09), with the fine – grained variety weakly zoned with higher Mn and lower Mg in core. Gedrite has Al^iv of 1.41-1.5 for cores and 1.6-1.74 for rims. Textural and mineral chemical relations suggest that Gd + St + Bt + Gt + Rt were replaced by Anth + sillimanite + Ilm. Conventional thermobarometry, coupled with petrogenetic grid considerations suggest that peak P-T conditions for LPGM were 480 - 530°C, 5.5 – 6.5 bar, whereas multiequilibrium calculations yield higher P (6 – 9 kbar) and lower T (450 – 550°C), which are inconsistent with the occurrence of Sill.

Bulk chemical analysis reveals that group I & III schists were E-MORBs affected by hydrothermal alteration prior to metamorphism, whereas group II rocks were likely greywackes. None of the rocks analyzed have any subduction zone signatures. Isocon diagrams show that group I schists were depleted in Ca, Sr, Ba, Na, K, Ni, and Sc, but enriched in Mg, Fe, Zr, Hf, Ti, La, and Ce relative to E-MORBs. Group III schists show similar trends with unusual enrichments in HREE and Zr, and depletion in Ti. These results suggest that the parent E-MORBs were affected by hydrothermal alteration on the seafloor at T < 350°C, and variable water : rock ratios, indicating the existence of a small ocean basin between the Mars Hill terrane and Laurentia prior to Paleozoic orogenies and metamorphism.