Recent studies differ on whether the Al content of hornblende in granitic rocks is pres- sure dependent or has a temperature dependence which masks any pressrue effect. To test these hypotheses, hornblendes in 15 late Paleozoic granitoids from the southern Appalachian Piedmont have been analyzed by electron microprobe. All but one of these plutons contain the assemblage hornblende + biotite + quartz + two feldspars + titanite + magnetite in apparent textural equilibrium. In addition, plagioclase rim compositions are approximately uniform, and hornblende, plagioclase, and biotite exhibit consistent distribution coefficients. Observed chemical variations in the Piedmont granitic hornblendes are attributed to hastingsite, pargasite, and CANT substitutions, summarized by the ex- change vectors [A](Na,K)[6](Fe3+,Al)[4]Al2[A]−1[6]Mg−1[4]Si−2, [6](Fe2+,Mn)[6]Mg−1, and [M4]Ca[6]Al[M4]Na−1[6]Ti−1[4]. Calculated pressures of emplacement for Piedmont plutons range from 2.1 to 5.1 kbar; temperatures range from 711 to 788 °C and correlate with pressure. Pressures and temperatures are comparable with those estimated from contact metamorphic assemblages. The temperature range is within the uncertainty of the geothermometer calibration; however, if the temperature differences are real, these hornblendes show both pressure- and temperature-dependent substitutions. Coexisting phases are related through a complex magmatic reaction, whose odd-even nature may restrict the pressure-sensitive hornblende substitution to only a part of the plutonic crystallization history. Epidote exhibiting textural and compositional characteristics suggestive of magmatic origin occurs in plutons emplaced at pressures of 2.8 ± 0.5 kbar or greater. This observation suggests that the occlurence of magmatic epidote in calc-alkaline plutons does not always require high pressures.

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