Petrology of the Laurel Creek amphibolite, Georgia Blue Ridge

Uncommon amphibolite mineral assemblages of the Laurel Creek Mafic-Ultramafic Complex, Rabun County, Georgia, are believed to be the result of unusual external physical conditions. These assemblages provide a fairly complete view of an amphibolite facies type for this grade of metamorphism. Important low variance assemblages include horn blende + kyanite + staurolite + anorthite + gedrite + chlorite, horn blende + staurolite + garnet + anorthite + chlorite, and hornblende + garnet + anorthite + gedrite + chlorite.

Chemographic treatment of these assemblages in the system FeO-MgOA1₂O₃-CaO-Na₂O-SiO₂-TiO₂-H₂O indicates that equilibrium was approached. It is graphically and algebraically demonstrated that assemblages con taining either hornblende + kyanite + staurolite + anorthite, hornblende + gedrite + staurolite + kyanite, or gedrite + anorthite are related by reaction to the common amphibolite assemblage, hornblende + epidote + chlorite + garnet + oligoclase. The common assemblage is prevalent in several medium pressure metamorphic terrains. Higher variance assem blages such hornblende + epidote + oligoclase + chlorite and hornblende + epidote + plagioclase + garnet can coexist at the same conditions that hornblende coexists with kyanite, staurolite, or anorthite. These are common assemblages in medium pressure terrains at upper amphibolite facies conditions. It has been suggested that the presence of horn blende + kyanite + staurolite presents relatively high pressure meta morphism at moderate temperatures.

Garnet-biotite geothermometry indicates that the final temperature of equilibration was 600 ± 50°C. The results of geobarometry are ambiguous. The garnet + plagioclase + kyanite + quartz geobarometer indicates a pressure of 5.6 ± 1 kbar, whereas the garnet + kyanite + ilmenite + rutile + quartz geobarometer indicates a minimum pressure of 7-9 kbar.