Prehnite- and Pumpellyite-Bearing Mineral Assemblages, West Side of the Appalachian Metamorphic Belt, Pennsylvania to Newfoundland |
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| Authors: | ZEN E-AN |
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| Institution: | U.S. Geological Survey Washington, D.C. 20244 |
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| Abstract: | Prehnite- and/or pumpellyite-bearing meta-igneous rocks arefound on the west side of the Appalachian metamorphic belt (1)near Jonestown, south-eastern Pennsylvania; (2) on RensselaerPlateau, eastern New York; (3) near Quebec City, Quebec; and(4) at Little Port, Humber Arm, western Newfoundland. The assemblagescritical 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 showsa later vein of analcime-calcite. All the assemblages also includequartz, sphene, calcite, K-mica, and albite. Analysis of the mineral assemblages by the Schreinemakers methodfor the phases actinolite-chlorite-epidote-hematite-prehnite-pumpellyite-stilpnomelaneshows that the different localities can be assigned differentmetamorphic grades. Though the detailed results of the Schreinemakersanalysis depend on the assumed source of ferric iron in epidote,the major conclusions are not affected. The thermodynamic roleof calcite is more problematic, but it appears that CO2 didnot behave as a boundary-value component during metamorphism.If calcite is treated as an excess phase, the Schreinemakersbundle decomposes to a net in a multisystem. Plotted on sucha net, the various localities again occupy different parts signifyingdifferent metamorphic grades. The occurrence of pumpellyite-bearing assemblages on the westflank of the northern Appalachian metamorphic belt might suggestthat these assemblages, contrary to the ideas of Miyashiro andof others, do not indicate high-pressure and low-temperaturetype of metamorphism. These assemblages, however, are compatiblewith an alternative interpretation as remnants of a high-pressure,low-temperature Taconic metamorphic regime, whose imprint withinmost of the Appalachian metamorphic belt has been obliteratedby later events. Such a reconstruction is compatible with thesuggestion that this zone, lying near the margin of the earlyPaleozoic craton, was an active subduction zone during the Taconicorogeny. |
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