Hornblende chemistry in southern Appalachian granitoids; implications for aluminum hornblende thermobarometry and magmatic epidote stability

Recent studies differ on whether the Al content of hornblende in granitic rocks is P dependent or has a T dependence which masks any P effect. To test these hypotheses, hornblendes in 15 late Palaeozoic granitic rocks from the southern Appalachian Piedmont have been analysed by electron microprobe. Epidote exhibiting textural and compositional characteristics suggestive of magmatic origin occurs in plutons emplaced at P of 2.8 + or - 0.5 kbar or greater. This suggests that the occurrence of magmatic epidote in calc-alkaline plutons does not always require high P.