P-T CONDITIONS OF METAMORPHISM OF LITTLE PINE GARNET MINE ROCKS, WESTERN BLUE RIDGE, AS CONSTRAINED BY MINERAL ASSEMBLAGE DIAGRAMS

The Little Pine Garnet Mine (LPGM) in the Western Blue Ridge Province of the Southern Appalachians is located on a large shear zone between the Mars Hill Terrane and the Grenville Basement. Three groups of rocks occur in the LPGM : Group 1 are schists with garnet (Grt) + gedrite (Ged) + anthophyllite (Ath) + chlorite (Chl) + biotite (Bt) + staurolite (St) + plagioclase (Pl) ± sillimanite (Sil) + quartz (Qz) + ilmenite (Ilm) + rutile (Rt). Group 2 are gneisses contain hornblende (Hbl) + Bt + Pl + Ilm + Qz ± Grt ± St ± aluminosilicate (Als). Group 3 granofelses are predominated by either Gd/ Ath or Chl with minor Ilm, Pl ± Sil. The protoliths of these rocks were hydrothermally altered E-MORBs (Groups 1 and 3) and greywackes (Group 2) that likely represented part of an oceanic basin between Mars Hill and Grenville Basement Terranes.

Previous attempts at constraining the P-T conditions using petrogenetic grids, conventional thermobarometry, and multi-equilibrium calculations have resulted in estimates ranging from 450°C to 600°C and 4 to 9 kbar. New mineral assemblage diagrams (MAD) were calculated using Theriak-Domino to help better constrain the P-T history of samples from the LPGM. MA for a Group 1 sample constrains its peak metamorphic temperature between 650°C-700°C and pressures in excess of 6.5 kbar for the assemblage of Grt + St + Bt + Ged + Rt. Retrograde metamorphism between 575°C - 650°C and 3 - 4.5 kbar produced the assemblage Ath + Sil + Ilm + Pl + Chl. MAD for a Group 2 sample shows peak metamorphic conditions of 650°C-750°C and 5.5 kbar - 6.0 kbar based on the peak mineral assemblage of Grt + Hbl + Sil + Bt + Pl + Ilm + Rt. The new mineral assemblage diagrams for the LPGM samples therefore show that peak metamorphism for the samples from the Little Pine Garnet Mine occurred between temperatures of 650°-750°C and pressures in excess of 5.5 kbar whereas retrograde metamorphism occurred between 575°C - 650°C and 3.0 - 4.5 kbar.