Orthoamphibole – Chlorite schists from the Little Pine Garnet Mine, Southern Appalachians: Chemical controls, P-T evolution and tectonic implications

Abstract

The Little Pine Garnet Mine area in the Western part of the Blue Ridge, southern Appalachian Mountains has outcrops of gedrite - anthophyllite – chlorite ± garnet ± biotite ± staurolite schists with minor quartz, ilmenite, rutile ± plagioclase ± sillimanite, interbedded with hornblende gneisses and muscovite – bearing quartzofeldspathic gneisses. Major and trace element analysis suggests that the gedrite - chlorite schists represent metamorphosed E-MORBs, whereas the hornblende gneisses represent metamorphosed greywackes. Isocon diagrams and cluster of slopes analysis shows that the E-MORB protoliths were enriched in Mg, Fe, Zn and HREE, and depleted in Ca, Na, and Si ± Sr ± Ba by hydrothermal alteration on the seafloor at T = 200–300 °C and high water: rock ratios >10 before metamorphism. Application of conventional thermobarometry and multiequilibrium calculations leads to the conclusion that the peak mineral assemblages Grt – Ged – St – Bt – Rt and Grt – Hbl – Pl – Bt – Rt formed at 8–9 kbar, 700 °C, before equilibrating isothermally at P = 7–8 kbar. Mineral assemblage diagrams calculated for a Ged – Chl schist and a Grt – Hbl gneiss confirm these conditions and show that the Ged schists retrogressed into the field of Ged – Ath – St – Chl – Ilm ± Pl at ∼4–5 kbar, 600–630 °C. Regional metamorphism most likely occurred during the Taconic orogeny.