The amphibolite-to-granulite facies transition in the Franklin and Corbin Knob quadrangles, North Carolina Blue Ridge

Metamorphism in the southern Appalachians reached the granulite facies in the vicinity of Franklin, NC. Rocks from within the field area have been correlated with the Otto Formation, Coweeta Group, and Tallulah Falls Formation. The deformational history is similar within the amphibolite and granulite facies terranes suggesting that the metamorphism that affected this area was associated with the Mid-Paleozoic regional event.

The amphibolite-granulite facies transition in the Blue Ridge of North Carolina is characterized by a series of prograde zones: ky + ms, sil + ms, sil + kfs in rocks of politic composition, and grt + hbl, grt + hbl + opx + cpx in amphibolites. The transition from amphibolite to granulite facies has been placed at the boundary between the sil + ms and sil + kfs zones. The close spatial association of the second sil isograd and partial melting suggests that the breakdown of muscovite was the driving force behind anatexis. The most likely reaction for producing melts is ms + ab + qtz = kfs + als + melt. This suggests that metamorphism occurred in the absence of a free aqueous fluid phase.

Estimated conditions of metamorphism are 580-680 deg C, 5.5-6.8 kbar, aH₂O = 0.8 in the amphibolite facies, and 680-780 deg C, 6.5-8.0 kbar, aH₂O = 0.25 in the granulite fades. The drop in aH₂O can be attributed to the breakdown of ms and the development of in-situ partial melts, which scavenged water.

The increase in temperature with pressure indicates that increasing depth of burial was an important part of the development of this terrane. The presence of ky in what are interpreted to be in-situ melts suggests that the granulite terrane underwent nearly isobaric cooling. A model employing both crustal thickening and regional magmatic activity appears necessary to explain both a prograde path of increasing pressure, and an isobaric cooling path.