Prehnite- and pumpellyite-bearing mineral assemblages, west side of the Appalachian metamorphic belt, Pennsylvania to Newfoundland

Prehnite- and/or pumpellyite-bearing meta-igneous rocks are found on the west side of the Appalachian metamorphic belt (1) near Jonestown, south-eastern Pennsylvania; (2) on Rensselaer Plateau, eastern New York; (3) near Quebec City, Quebec; and (4) at Little Port, Humber Arm, western Newfoundland. The assemblages critical to determining the conditions of metamorphism are (1) chlorite-epidote-hematite-pumpellyite-prehnite; actinolite-chlorite-hematite-pumpellyite-stilpnomelane; (2) actinolite-chlorite-epidote-stilpnomelane; chlorite-epidote-pumpellyite-stilpnomelane; chlorite-epidote-hematite-pumpellyite; (3) chlorite-epidote-hematite-pumpellyite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite; and (4) chlorite-epidote-prehnite; chlorite-prehnite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite. One pumpellyite-bearing rock from western Newfoundland shows a later vein of analcime-calcite. All the assemblages also include quartz, sphene, calcite, K-mica, and albite.

Analysis of the mineral assemblages by the Schreinemakers method for the phases actinolite-chlorite-epidote-hematite-prehnite-pumpellyite-stilpnomelane shows that the different localities can be assigned different metamorphic grades. Though the detailed results of the Schreinemakers analysis depend on the assumed source of ferric iron in epidote, the major conclusions are not affected. The thermodynamic role of calcite is more problematic, but it appears that CO₂ did not behave as a boundary-value component during metamorphism. If calcite is treated as an excess phase, the Schreinemakers bundle decomposes to a net in a multisystem. Plotted on such a net, the various localities again occupy different parts signifying different metamorphic grades.

The occurrence of pumpellyite-bearing assemblages on the west flank of the northern Appalachian metamorphic belt might suggest that these assemblages, contrary to the ideas of Miyashiro and of others, do not indicate high-pressure and low-temperature type of metamorphism. These assemblages, however, are compatible with an alternative interpretation as remnants of a high-pressure, low-temperature Taconic metamorphic regime, whose imprint within most of the Appalachian metamorphic belt has been obliterated by later events. Such a reconstruction is compatible with the suggestion that this zone, lying near the margin of the early Paleozoic craton, was an active subduction zone during the Taconic orogeny.