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Metamorphic and Tectonic History of the Pennsylvania Piedmont

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Metamorphic and Tectonic History of the Pennsylvania Piedmont J. geol. Soc. London, Vol. 137, 1980, pp. 311-320,4 figs., 2 tables. Printed in Northern Ireland. Metamorphic and tectonic history of the Pennsylvania Piedmont M. L. Crawford & W. A. Crawford SUMMARY: The Piedmont Province of SE Pennsylvania consists of gneisses overlain by, or in fault contact with, metasedimentary phyllites and schists. Correlation of these crystalline rocks with similar units in Maryland and isotopic ages of the metamorphic events suggest ages from pre-Grenville (1000Ma) to Ordovician; Cambrian and Ordovician fossil-bearing units occupya narrow band across the central part of the region. A 1000 Ma regional granulite facies episode is documented in the basement gneiss units. A second younger and lower pressure granulite faciesmetamorphism centres around an intrusiveanorthosite-norite complex. A third metamorphism ranging from greenschist to upper amphibolite facies affects the entire terrain; this is probably close to 440 Ma old. Major folds and faults formed during or after the last metamorphic episode; earlier structural features are either obliterated or not exposed. The proposed tectonic model includesa rifting of continental basement forming a basin with a lower Palaeozoiccarbonate bank on the NWmargin and anisland arc to the SE. Compression produced major nappes and possible thrusts during the last regional metamorphism. Vertical movements associated with major block faulting marked the end of the orogenic activity. The Piedmont Province of Pennsylvania, which con- Stose(1938). The known Palaeozoic section uncon- sists of moderately to highly metamorphosedrocks formablyoverlies theHoney Brook Upland gneiss ranging in age from Precambrian to Ordovician, forms alongthe southern border of theUpland; the basal the northern end of the southern Appalachian Pied- Cambrian clastic unit also occurs as scattered patches mont (Fig. 1). The rocks of the Piedmont continue SW within the gneiss terrain. across the northern tip of Delaware (Thompson 1975) The formations of the Chester Valley Region dip andinto eastern Maryland (Higgins 1972; Crowley moderately to steeply S andare overlain without 1976). To the NE the Piedmont rocks are covered by apparentstructural discontinuity (Freedman etal. the Newark Triassic-Jurassic basin and the Cretaceous 1964) by greenschist facies Wissahickon Group phyl- and younger sediments of the coastal plain. Gneisses lites of the Glenarm Terrain. Approximately half of and schists similar to those in SE Pennsylvania (Ford- thearea of theGlenarm Terrain S of theChester ham Geiss, Manhattan Schist) reappear in the vicin- Valley is underlain by pelitic and semi-pelitic (Wms, ity of New York City (Hall 1976). Fig. 3) or quartzofeldspathic (Wqfs, Fig. 3) units of the Wissahickon Group. An ENE trending belt of gneiss dividesthe Glenarm Terrain (Fig. 3). Thenorthern Regional geology part of this belt consists of granulite facies gneisses; the southern part is predominantly in the amphibolite The Pennsylvania Piedmont is most conveniently di- facies (Fig. 4). Evidence from U-Pb data on zircons vided into 3 parts: theHoney Brook Upland, the (Table 1) reported by Grauert ef al. (1974) suggest the Chester Valley Region, and the Glenarm Terrain (Fig. GlenarmTerrain amphibolite facies gneisses are 2). TheHoney Brook Upland, to the N, and the younger than the granulite gneisses. Glenarm Terrain, to the S, are composed of moder- Thewestern end of thesouthern gneiss belt is ately to highly metamorphosedsediments, volcanic overlain by the basal part of the Glenarm Supergroup. rocks and plutonic bodies. Theseunits (the Setters meta-quartzite and meta- The gneiss of theHoney Brook Upland and a arkoseand the Cockeysvillemarble) are similar to portion of the gneissin theGlenarm Terrain were formations of the Chester Valley Region. The proxim- metamorphosed during the Grenville orogeny. Dates ity and lithologic resemblance of the Setters and Coc- between 1100 and980 Ma (Table 1) were obtained keysville Formations to the known Palaeozoic section fromthe granulite facies gneisses of the 2 areas by of theChester Valley Region suggest a correlation Tilton et al. (1960) and Grauert et al. (1973). betweenthese units. In Fig. 3 theSetters and Coc- The rocks of the Chester Valley Region S of the keysville Formationshave been labelled Cambro- Honey Brook Upland contain fossils: Scolifhus linearis Ordovicianto point out this correlation. Mackin in a basal Cambrian clastic unit andMichelia sp. in the (1962) suggested the two prongs of gneiss mantled by upper part of the overlying Cambro-Ordovician carbo- the Glenarm Supergroup represent recumbent folds or nate sequence. In addition the formations in the Ches- nappes plunging SW. ter Valleycan be traced westward into thelower The Wissahickon Group of theGlenarm Super- Palaeozoic rocks of the Lancaster Valley. The stratig- group stratigraphically overlies the Cockeysville For- raphy of these units is given in detail by Bascom & mation.Between the gneiss-cored nappes and the OOlf~7649/80/0500-0311$02.00@ 1980 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/3/311/4886324/gsjgs.137.3.0311.pdf by guest on 28 September 2021 3 12 M. L. Crawford B W. A. Crawford PA. FIG. 1. Mid-Atlantic seaboard, USA Piedmont is delineated by vertical ruled pattern; the Honey Brook Upland, Chester Valley Region, and Glenarm Terrain (Fig. 2) lie in the stippled rectangle. FIG. 2. Geographical relationship between the Honey Brook Upland, Chester Valley Region, and Glenarm Terrain of the SE Pennsylvania Piedmont. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/3/311/4886324/gsjgs.137.3.0311.pdf by guest on 28 September 2021 Metamorphicand tectonic history of the PennsylvaniaPiedmont 313 TABLE1: Rock ages Location Sample Technique Age (Ma) Reference Conshohocken, Pa. Basement gneiss Zircon 206pb/Z38u 207pb/23Su t:::} Tilton et al. 1960 207Pb/206Pb 1120 West Chester Prong, Pa. Basement gneiss Zircon 206pb/238u 1050-980 Grauert et al. 1973 Glen Mills, Pa. Gneiss Zircon 206pb/238u Discordant Grauert et al. 1974 Wilmington, Del. Charnockite Rb-Sr Whole-rock 50:i20}Foland & Muessig 1978 Mineral separates Rb-Sr Safe Harbor, Pa. Wissahickon greywacke Biotite Muscovite K-Ar 360 Fishing Creek, Pa. Peters Creek Schist Whole-rock K-Ar Peach Bottom Slate Whole-rock K-Ar ii:} Lapham & Bassett 1964 Port Deposit, Md. Port Deposit granodiorite Biotite K-Ar 330 ChesterValley, the Peters Creek quartz-rich schist (1962) suggested these are low angle thrusts formed (PCqs,Fig. 3) andone or more graphitic units are on the sheared lower limbsof the gneiss-cored nappes. interlayered with theWissahickon Group pelitic The Cream Valley Fault in the Glenarm Terrain and schists. The exact relationship between these forma- the BrandywineManor Fault of theHoney Brook tions is notwell established; the structure has been Upland (Fig. 4) are steep or vertical with a postulated interpreted as a syncline cored by the graphitic schist dominant dip-slip movement. These two faults juxta- (Knopf & Jonas 1923) oras a monoclinal sequence pose granulite facies gneisses on the upthrown block dippingNW (Hopson 1964; Southwick & Fisher againstlower grade schists. The southern portion of 1967). Higgins (1972) andHiggins et al. (1977) the gneiss belt in the Glenarm Terrain consists of a suggestedthat the Glenarm Supergroup is mainly complex of shear zones locally marked by ultramafic Cambrianand Ordovician, younger than 650 Ma. lenses. Thesouthern boundary of thegneiss belt Seiders et al. (1975), on the other hand, proposed that against the Wissahickonschist, locally known as the this Supergroup, in Virginia, is older than 560 Ma. Rosemont Fault, is part of this fault complex. In the central and eastern part of the area, the lower Glenarm units are missing and the Wissahickon Group Stratigrapby occurs adjacent to the gneiss. S of the gneiss belt, and Gneissic basement W of Philadelphia, an eastern portion of the Wissahic- kon Group (Wms/Wqfs, Fig. 3) is separated from the In the Honey Brook Upland, N of the Brandywine centraland western portions of theWissahickon by Manor Fault, the granulite facies rocks arefelsic gneis- rheWilmington Complex, a terrain of gneissesin- ses (charnockites) with minor amounts of mafic gneis- truded by a norite-anorthosite complex (gl, Wan, Fig. ses (metabasalts) (gl, Fig. 3) and felsic graphitic gneiss 3). The WilmingtonComplex extends into northern (metagreywackesand metavolcanics) (gg, Fig. 3) Delaware(Ward 1959; Thompson 1975). Twoages (Thomann 1977; Demmon 1977). These gneisses are have been determined from the rocks of the Wilming- intruded by an elliptically shapedpluton ranging in ton Complex. Charnockite, associated with the norite- compositionfrom a centralcore of anorthosite to a anorthositeintrusive rocks in theeastern portion of dioritic border (HBan, Fig. 3) (Crawford et al. 1971; the Wilmington Complex, has been dated at 502Ma Organist 1978). The gneisses are also cut by diabase (Table 1, Foland & Muessig 1978). Grauert & Wagner dykes. (1975) reported a date of 440Ma obtained from the The amphibolite facies terrain of the Upland, pre- Wilmington Complex gneisses. This nearly concordant dominantly felsic and intermediate gneiss (f graphite) zircon date is thoughtto represent the age of the with mafic units (a, ag, Fig. 3), may represent a meta- granulite facies metamorphism of this complex. Foland volcanicand metagreywacke sequence (Huntsman & Muessig (1978) obtained a similar 440 Ma age from 1975; Crawford & Huntsman 1976; Demmon
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