Petrology of a Georgia Blue Ridge amphibolite unit with hornblende + gedrite + kyanite + staurolite Available to Purchase

The Laurel Creek amphibolite is composed of subsets of the assemblage hornblende + chlorite + garnet + kyanite + staurolite + orthoamphibole + plagioclase (An₄₇-An₉₇) + quartz + ilmenite + rutile + pyrrhotite. Low-variance assemblages provide a relatively complete view of a rare amphibolite fades type. These rare assemblages are believed to be the result of high-pressure metamorphism at moderate temperatures, not of unusual bulk compositions. Laurel Creek amphibolites containing kyanite, staurolite, and gedrite have Mg-rich, olivine-normative basaltic compositions. Mass-balance calculations indicate that the bulk compositions of these rocks can produce common amphibolite assemblages under appropriate conditions. The assemblage hornblende + staurolite (or + kyanite) has previously been demonstrated to be in reaction relation with the assemblage hornblende + plagioclase + epidote + chlorite (and/or garnet), the common amphibolite assemblage in medium-pressure metamorphic terranes. The assemblages hornblende + chlorite + epidote + plagioclase and hornblende + garnet + epidote + plagioclase are graphically demonstrated to be compatible at the same conditions that hornblende coexists with aluminous phases for Mg-rich and Fe-rich bulk compositions, respectively. However, the common low-variance assemblage hornblende + chlorite + garnet + epidote + plagioclase is precluded by the coexistence of hornblende + kyanite. We also demonstrate that the latter assemblage is in reaction relation with gedrite + anorthite, hornblende + gedrite + kyanite, hornblende + corundum, and hornblende + anorthite. Gamet-biotite geothermometry indicates a temperature of 540-625 °C for both adjacent pelitic schists and the Laurel Creek amphibolite. Garnet + kyanite + quartz + ilmenite + rutile equilibrium in the Laurel Creek amphibolite indicates a minimum pressure of 7.7 kbar. The coexistence of hornblende + kyanite + staurolite + gedrite + plagioclase (An₈₈-An₉₇) in mafic rocks indicates unusual metamorphic conditions. Geothermobarometry from Laurel Creek, as well as other field and experimental data, indicates that amphibolites bearing kyanite-staurolite + hornblende result from a pressure higher than that appropriate for the common amphibolite assemblage.