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M:ETAM()RPHIC MAP ()F the APPALACHI;\NS by :C II DEPART1v1ENT OF THE INTERIOR UNITED STA1~Ef GEOLOGICAL SURVEY M:ETAM()RPHIC MAP ()F THE APPALACHI;\NS By Benjamin A. Mor~an MISCELLANEOUS GEOLOGIC INVESTIGATIONS MAP 1-724 3: 0 A) 0 :z:> I 3: trl ---1 )> 3: 0 ::0 :c'U n > 'U 'U )> l' )> () :r:.......... );> z (/) ~- t\J CJ1 0 p 0 0 0 PUBLISHED BY THE U.S. GEOLOGICAL SURVEY WASHINGTON. D.C. 20242 1972 DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-724 UNITED STATES GEOLOGICAL SURVEY METAMORPHIC MAP OF THE APPALACHIANS By Benjamin A. Morgan INTRODUCTION and Gulf Coastal Plain. In areas where a continuous This map depicts the major metamorphic belts of gradation between greenschist and unmetamorphosed the Appalachian mountain system within the United rocks exists, a subgreenschist or transitional facies States. Degree of metamorphism is expressed in of metamorphism has been recognized. The two terms of isograds, mineral facies, and mineral facies principal areas designated as subgreenschist facies series. are northern Maine and Cambrian-Ordovician units The isograds on the accompanying map are defined within the Great Valley and the eastern part of the by the first prograde appearance of an index mineral Ridge and V ~lley provinces in the central and southern without regard to a specific chemical reaction and Appalachians. · The subgreenschist facies in northern without rigor as to the chemical composition of the Maine is the area designated as a subchlorite zone by index mineral or chemistry of the total system. Hussey and others (1967). Coombs, Horodyski, and Wherever possible, isograds are shown in rocks of Naylor (1970) have reported the occurrence of a approximately similar total composition.. A more prehnite-pumpellyite facies within a part of this area. rigorous definition and use of an isograd was not Formations within the Great Valley and the Valley and practicable, owing to the necessity of compiling Ridge provinces may or may not be regarded as meta­ information from a wide variety of reports and broad morphosed (see for example Ray and Gault, 1961; extrapolation in many areas. The isograds used for Maxwell, 1962; Drake, 1969) depending on the defini­ rocks of pelitic composition are biotite, garnet, tions used. Although A. A. Drake, Jr. (ora] commun., andalusite, kyanite, sillimanite, and sillimanite­ 1970) has evidence that much of the Martinsburg For­ orthoclase. The complete set of isograds is used mation in the Great Valley province of eastern Penn­ only in New England and New York; insufficient sylvania is metamorphosed to the greenschist facies, information for the occurrence of biotite and kyanite the western boundary of the subgreenschist facies is prevented depiction of these isograds throughout the arbitrarily taken as the western contact of the Martins­ southeastern Appalachians. burg Formation within eastern Pennsylvania and the Mineral facies and mineral facies series, as used here, western boundary of Cambrian-Ordovician. rocks are defined on the basis of the isograd data and are at east of the Pulaski fault in Virginia and Tennessee and some variance with definitions proposed. by several southeast of the Rome fault in Georgia. !he occur­ writers (Fyfe and Turner, 1966; Winkler, 1967). rence of pumpellyite at Bunker Hills, Pa. (Z€m, 1971), The facies of metamorphism are mineral facies based and prehnite in the greenstones of the Catoctin For­ on key minerals from pelitic rocks rather than en­ mation near Luray, Va. (W. G. Melson, oral commun., .sembles of mineral assemblages from specific localities 1969), indicates a possibility of a widespread prehnite­ (Thompson, 1957). The granulite facies is defined pumpellyite metamorphic facies within these rocks by the assemblage quartz-sillimanite-orthoclase in heretofore undetected because of the limited number rocks of pelitic composition. The epidote-amphibolite of investigations and the limited occurrence of rocks and almandine amphibolite facies have been combined of the prerequisite chemical composition. as the amphibolite facies and are delimited by the Laumontite-quartz-albite assemblages have been garnet and sillimanite-orthoclase isograds. The described in arkoses in the Connecticut Triassic basin greenschist facies includes all masses of rock of (Heald, 1956). These rocks may be in a zeolite pelitic composition that do not contain garnet. facies, but this metamorphism is clearly post-Paleozoic The lower limit of greenschist metamorphism and and not related to other metamorphic episodes shown the boundary between metamorphism and diagenesis on the map. The laumontite-quartz-albite assemblages are nut defined. This poses no problems in map are not transitional into areas of prehnite-pumpellyite compilation along the entire eastern and southern or greenschist metamorphic facies. These rocks are margins of the Appalachians where the greenschist indicated on the map by the letter z. and amphibolite facies rocks are unconformably Two facies series have been recognized in the overlain by unmetamorphosed rocks of the Atlantic Appalachians: a low pressure/temperature (P/T) 1 facies series in the northern part of New England and North Carolina and Virginia which had not been .an intermediate PIT facies series in the remainder of previously described. the Appalachians. These facies series are defined by Although the degree of Precambrian metamorphism the polymorphic transitions of andalusite to sillimanite is variable within the polymetamorphic belt, the and kyanite to sillimanite, respectively. There are greenschist facies is absent throughout. Relict no occurrences of glaucophane, metamorphic orthopyroxene and sillimanite in quartzo-feldspathic aragonite, jadeite, or lawsonite in the eastern. United rocks are present i.n a \:vide area from the Hudson States to define a high PIT facies series. highlands of New York southward to about the North Carolina-Virginia boundary. Retrograde rocks simi­ PRECAMBRIAN METAMORPHIC. BELTS lar to charnockites have been described in the core of The areas shown in shades of brown represent the the Blue Ridge anticlinorium (Bloomer and Werner, oldest rocks known in the Appalachians. The 1955). Relict orthopyroxene (dotted overprint) Adirondack massif was metamorphosed during the survives only in the western part of the older Pre­ Precambrian but not by subsequent Paleozoic meta­ cambrian belt where the Paleozoic metamorphic morphic events. The Adirondack massif, flanked overprint is in the lower greenschist facies (an im­ by unmetamorphosed Paleozoic rocks, is a part of portant exception being the Wilmington complex the Precambrian basement of Grenville age that. may described by· Ward, 1959). Consequently ·it is underlie the unmetamorphosed fold belt along the difficult to depict . the earlier distribution of the west flank of the Appalachian mountain system. In granulite facies of metamorphism. The prevalence the area of the Adirondack. massif, this basement of a granulite mineral facies and the general radio­ has not been extensively modified by Paleozoic meta­ metric age bracket for these rocks suggest that they morphism except for cataclastic shearing and local are broadly correlative with rocks of the New Jersey retrogression along the eastern and southeastern highlands and the Adirondacks in New York. margins. The greater part of the Adirondacks is considered to be in the granulite facies (Buddington, PALEOZOIC METAMORPHIC ·BELTS 1963; Waard, 1965) defined by an orthopyroxene­ InN ew England and in the southeastern Appalach­ clinopyroxene-hornblende isograd (h) in rocks of •ans, three major longitudinal axes. of higher basaltic composition and the widespread occurrence grade metamorphism (amphibolite and granulite of the assemblage quartz-sillimanite-orthoclase in mineral facies) of Paleozoic age are separated by areas rocks of appropriate composition. In the nc;>rth­ of lower grade metamorphism (green,schist facies western Adirondacks, the regional metamorphism and garnet zone of amphibolite facies). In New is in the amphibolite facies; sillimanite is abundant, England, these three belts include an. O\]ter or north­ cordierite is present, kyanite is absent. The pro­ western belt extending from the Manhattan prong gressive metamorphism from amphibolite to granulite northward through the core of the Green Mountain facies described by Engel and Engel (1958, 1960) is anticlinorium, a central belt compnsmg central probably typical of a low PIT facies series. Connecticut and Massachusetts, New Hampshire, The darker brown areas represent the Precambrian and western Maine, and finally a .southeastern belt, infrastructure within the Appalachian system. These including Rhode Island, eastern Massachusetts, and areas are polymetamorphic and have been meta­ the southern tip of Maine. Analogous belts in the morphosed 1.1 to 0.8 b.y. B. P. and again during the southeastern Appalachians are a western belt in the PaleozoiC approximately 0.50 to 0.23 b.y. B. P. These Blue Ridge anticlinorium of North Carolina and rocks are unconformably overlain by sedimentary and Georgia, a central belt or Inner Piedmont extending volcanic rocks which have a metamorphic history from Alabama northeast through central Virginia to restricted to the Paleozoic. The distribution of older, southeastern Pennsylvania, and a poorly exposed polymetamorphic Precambrian rocks in New England southeastern belt extending from Macon, Ga., through and in the central Appalachians is moderately well Columbia, S. C., to eastern North Carolina and known. One problematic area is southeastern east-central Virginia. Massachusetts
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