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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 overlain by, or in fault contact with, metasedimentary phyllites and . 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 facies episode is documented in the basement units. A second younger and lower pressure granulite faciesmetamorphism centres around an intrusiveanorthosite-norite complex. A third ranging from to upper 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 theChester 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 ) 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 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 . 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 theChester 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

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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.

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TABLE1: ages

Location Sample Technique Age (Ma) Reference Conshohocken, Pa. Basement gneiss 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. Rb-Sr Whole-rock 50:i20}Foland & Muessig 1978 Mineral separates Rb-Sr Safe Harbor, Pa. Wissahickon greywacke Biotite K-Ar 360 Fishing Creek, Pa. Peters Creek Schist Whole-rock K-Ar Peach Bottom Whole-rock K-Ar ii:} Lapham & Bassett 1964 Port Deposit, Md. Port Deposit granodiorite Biotite K-Ar 330

ChesterValley, the Peters Creek -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 () with minor amounts of gneis- truded by a norite-anorthosite complex (gl, Wan, Fig. ses (metabasalts) (gl, Fig. 3) and 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 anorthositeto 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 ) 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 aRb-Sr mineral isochron from a single charnockite 1977). Theoccurrence of the graphiticgneiss in sample.K-Ar dates on in theWissahickon boththe granulite and amphibolite facies terrains, Group cluster between 350 and 320 Ma (Lapham & coupled with the continuous northward increase in the Bassett 1964). This suggests the temperatures in the metamorphic grade in the eastern part of the Honey regionhad dropped below the blocking temperature Brook Upland, suggests that the whole Upland repres- for muscovite by 320 Ma. ents one metamorphic terrain (Crawford1979). Move- NE-striking faults cut the Honey Brook Upland and ment along the Brandywine Manor Fault, followed by the Glenarm Terrain (Figs 3 and 4). The faults in the erosion, resulted in the pzsent division of the western SW quarter of the map are poorly exposed. Mackin Upland into 2 distinct blocks.

7

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FIG. 3. Geologicalmap, SE PennsylvaniaPiedmont. Diagonal line pattern-Basement gneiss; Vertical line pattern-Wilmington Complex; No pattern-Glenarm Supergroup and Palaeozoic rocks; Black-Ultramafic rocks; Stipple-Sedime&ry cover. Q Quaternary Q alluvium WqfsWissahickon quartzofeldspathic schist Tr-J Triassic-Jurassicclastic sedimentary rocks WanWilmington anorthosite intruded by diabase dykes and sills ?Precambrian and Precambrian-Basement gneiss €4 Cambro-Ordoviciancarbonates and clastic amphibolite grade sedimentary rocks a undifferentiated gneiss € Cambrianquartzites ag graphitic gneiss ?Lower Palaeozoic? gr Lima gr granulite grade HBanHoney Brook anorthosite U Utramaficrocks gd Springfieldgranodiorite gg graphitic gneiss PCqsPeters Creek quartzitic schist gh undifferentiatedgneiss, high P WmsWissahickon schist g1 undifferentiatedgneiss, low P Thebasement gneiss of theGlenarm Terrain is 2. Amphibolitefacies gneisses form the southern subdividedinto 3 partsbased both on metamorphic gneiss belt (a, Fig. 3; Fig. 4) which crosses from the grade and on rock type: easternalmost to the western margin of themap. 1. Thegranulite facies gneisses S of theCream Though the western end of the southern gneiss belt is Valley Fault (gh, Fig. 3; Fig. 4) form a northern belt of poorlyexposed, the available evidence suggests that gneiss. They consist of with com- the gneiss underlying the Glenarm Supergroup is pre- positions which rangefrom diorite to , and dominantlygranitic. Further E a migmatiticlayered quartzo-feldspathic granulites which range from tona- sequence composed of alternating amphibolite, meta- lite to granite in composition(Wagner & Crawford greywacke, and intermediate to felsic layers occupies 1975). Aluminous quartzites are a minor but distinc- the southern partof this gneiss belt. Minorcalc-silicate tive rock type. The gneisses are cut by diabase dykes andmarble units suggest a sedimentaryorigin for which preserve ophitic textures and show chilled mar- some of the rocks. The contact between the amphibo- gins against the gneiss. litefacies (southern) and granulite facies (northern)

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U

FIG.4. Metamorphic map, SE Pennsylvania Piedmont. Open stipple-Granulite facies, moderate to high pressure; Close stipple-Granulite facies, low to moderate pressure; Vertical lines-Amphibolite facies; Horizontal lines- Greenschist facies; No pattern-Cover rocks.

belts is obscured by poor exposure and by amphibolite the Wilmington Complex and that of the lower grade facies retrograde metamorphism of the granulites. At southern gneiss belt. 2 localitieswhere the amphibolite facies and the granulitefacies terrains are adjacent, a - rich amphibolite unconformably overliesfelsci granulite Glenarm Supergoup facies gneiss. In the centre of the map area, patchesof TheGlenarm Supergroup (Knopf & Jonas1923; granulitefacies gneiss occur within the amphibolite Southwick & Fisher 1967; Higgins1972) has a well facies terrain; the largest of these (gl) is shown in the defined lower sequence exposed in the western part of centre of Fig. 3. thearea. The basal Setters Formation is animpure 3. The granulite facies Wilmington Complex occurs quartzite and characterized by the presence of in the centre of the southern part of the map area and microclineand abundant . The overlying extends S into northern Delaware (gl, Fig. 3). It con- Cockeysvillemarble was a siliceouslimestone and sists of a group of massivetonalitic gneisses which . It now contains phlogopite, , quartz locally contain mafic layers up to a metre thick (Ward and locally . 1959). The gneiss is intruded by a norite-anorthosite Poorexposure restricts detailed mapping of the mass(Wan, Fig. 3)(Thompson 1975; Mark 1977). Wissahickon Group, so the stratigraphic description is, The northernmost rocks of the Wilmington Complex of necessity,very general. Theportion of the Wis- arejuxtaposed against the margin of thesouthern sahickon Group which overlies the Cockeysville For- gneiss belt. The rocks near the contact are extensively mation and the Ordovician carbonates of the Chester crushedand cut by minorfaults. Thepresence of Valley consists of an aluminous pelitic unit (Wms, Fig. bandedintermediate and mafic gneisses in both the 3) which grades upward into the quartz-rich, overlying WilmingtonComplex and the southern gneiss belt Peter Creek Formation (PCqs, Fig. 3). The aluminous suggests a possible connection between the gneiss of also lies W of the end of the southern gneiss belt

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(Kuhlman1975). S of thegneiss belt, however, the Each of the 3 metamorphic events can be traced from Wissahickon is apredominantly quartzo-feldspathic rocks in whichthey representthe dominant or only schist (Wqfs, Fig. 3) interlayered with amphibolite. In eventto areas in whichthey aremodified by the the vicinity of Philadelphia, E of theWilmington effects of the youngest (M3) metamorphism. However, Complex,argillaceous and quartzo-feldspathic facies no overlap occurs between M1 and M2. of the Wissahickon Group are interbedded. The east- The gneisses of thenorthern belt of theGlenarm ern portion of the Wissahickon Group is also charac- Terrainpreserve evidence of a high pressureM1 terized by interlayeredamphibolite layers and granulitefacies metamorphic event (8-9 kb,750- ultramafic lenses and pods, up to 5 km long, concen- 8OO0C, Table2) according to Wagner &c Crawford tratednear the contact with the basement gneisses. (1975). is abundant, and Ultramaficrocks are also common along the Cream mesoperthite are ubiquitous, garnet-clinopyroxene as- ValleyFault N of thenorthern gneiss belt, along a semblages occur in mafic rocks, and grew in zoneseparating the northern and southern gneiss the few Al,O,-rich rocks. Theabundance of garnet, belts,and within the Wissahickon schist terrain en- the distinctively brown colour of the hornblende, and closed between the western ends of the northern and the absence of coexisting K- and as southerngneiss belts (Fig. 3).The mineralogy and separate grains, all serve to distinguish this high pres- structures of theultramafic rocks suggest they were sure granulite facies metamorphism from that of the emplaced prior to the regional metamorphism. A large Honey Brook Upland. In the Upland, mafic granulites granodiorite mass and several smaller bodies intrude containhypersthene, , hornblende and plagio- the Wissahickon Group schists W of Philadelphia. The clase,while felsic granulites consist of hypersthene, largest granodiorite body (gd, Fig. 3) is associated with augite,mesoperthite, plagioclase quartz.and aK-feldspar augen gneiss in thesurrounding schist. Graphite-bearinggranulites havehypersthene, Muscovite and microcline commonly con- plagioclase,microcline perthite, quartz and graphite. tainingberyl are also abundant in theregion im- The M1 granulite facies metamorphism appears to be mediately surrounding Philadelphia. alower pressure type in theHoney Brook Upland than in theGlenarm Terrain, although useful Metamorphic history geobarometer assemblages are absent. Allthe Upland gneisses show evidence of M3 The metamorphic history of the region is summarized greenschistretrogradefacies metamorphism in Table 2. As isotopicage data in the region are (Huntsman1975; Demmon 1977). Pyroxene is sparse,theinterpretation of thesequence of uralitized,blue-green rims surround metamorphic events relies on evidence of superposi- hornblendegrains, and calcic plagioclase is saus- tion of 2metamorphic episodes in portions of the suritized. Many of the felsic gneisses contain separate region.In broad terms the metamorphism can be grains of microclineand plagioclase in additionto characterized as beginning with the 1100-980 Ma reg- mesoperthite. ional high gradeepisode (Ml) which affectedthe The Glenarm Terrain granulites also show evidence basement gneisses, and a separate and younger granu- of the later M3 metamorphic event which overprints lite facies metamorphism (M2) within and surrounding thegranulite facies assemblages with amphibolite the WilmingtonComplex. Both of theseare over- faciesminerals. Garnet coronas are present around printed by a Lower Palaeozoic regional metamorphic ferromagnesianminerals, clinopyroxene-garnet rims event (M3) which increases in intensity from N to S. form along orthopyroxene-plagioclase boundaries, and

TABLE2: Metamorphic events in SE Pennsylvania

MetamorphismAge (Ma) Grade Map symbol Part of Piedmont

M1 1000 Granulite, High P Glenarm Terrain Granulite, Low P Honey Brook Upland upper Amphibolite Honey Brook Upland M2 ?500-440 Granulite, Low P Wan gl, Wilmington Complex, Glenarm Terrain M 3 440-320 Greenschist gl, gg, HBan, a, ag, 440-320HBan,a,GreenschistM3 gg, gl, € Honey Brook Upland GO Chester Valley Region Wms, PCqs Glenarm Terrain, N of gneiss belt upperAmphibolite gh, U, gl,gd, gr,Wan, Wgfs, Glenarm Terrain, gneiss belt and Wms, GO S through Wilmington Complex

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sillimanite is replaced by . The diabase dykesin evidence of the M3 metamorphic eventonly. The argil- this terrain are also affected by the M3 metamorph- laceous phyllites immediately S of the Chester Valley ism;they show well developedgarnet coronas on Region are in thechlorite and biotite zones of the ferromagnesianminerals. Conditions during the M3 greenschist facies. Biotite is rare due to the aluminous metamorphism of the Glenarm gneisses were approxi- composition of therocks, but chloritoid and small mately 650-7OO0C, 7 or 8 kb(Wagner & Crawford scatteredgarnets occur. The quartzites, feldspathic 1975). quartzites, and carbonates in the Chester Valley Reg- Themetamorphism of theWilmington Complex ion are not well suited to an analysis of metamorphic (M2)differs in P-Tconditions and in agefrom the conditions.Examination of some of theargillaceous moderateto high pressuregranulite facies M1 quartziteunits show that biotite has not formed. metamorphism in theGlenarm Terrain northern Phlogopite, muscovite and occur in an argillace- gneiss belt. Hypersthene is ubiquitous in the Wilming- ousdolomite. These few observationssuggest the tonComplex gneisses. It occurs in mafic rockswith ChesterValley Region rocks are also in thelower clinopyroxene, plagioclase and green-brown to brown greenschist facies. hornblende and in more aluminous rocks with plagio- Within and S of the basement gneiss belt, the Wis- claseand biotite. In a charnockite, hypersthene and sahickon Group rocks are of highermetamorphic cordieriteare found. Garnet is absent.Olivine, rim- grade. Except along the very western margin of the med by hypersthene,crystallized in thenorite, sug- region the metamorphic grade increases abruptly, over gesting moderate to low pressure and high tempera- a distance of less than 1km, to kyanite-bearing schists tureconditions of formation.Metamorphism of the just S of the Peters Creek Formation and sillimanite- norite-anorthosite intrusive rocks did not destroy their bearing rocks within andS of the basement gneiss belt. igneous texture; their mineralogy conforms to that of Inthe eastern part of theregion, W and N of thesurrounding gneisses of equivalentcomposition. Philadelphia,staurolite, kyanite and then sillimanite Thepresence of sillimanite,cordierite, garnet and are found in succession S of the basement gneiss. In in the Wissahickon Group schists adjacent the high grade portion of the terrain affected by this tothe Wilmington Complex suggests that M2 high regionalmetamorphism (M3, Table 1) conditions temperatureand low pressuremetamorphism also reached 7-8 kbat temperatures of 650-700°C. The affected these schists. The mineral assemblages formed ultramaficbodies emplaced in the schists nearthe duringthe M2 granulite facies metamorphism were borders of the gneiss belts in the Glenarm Terrain are overprinted by a later metamorphic event (M3) which metamorphosed to the same degree as the surrounding resulted in kyanite replacing sillimanite; garnet, biot- schists, exceptthat higher temperature assemblages ite, staurolite, and muscovite growing over sillimanite; includingorthopyroxenite and norite persist in the biotite and sillimanite pseudomorphing cordierite; and cores of somebodies probably due to lack of water staurolite, muscovite, biotite and chlorite assemblages (Roberts 1969). formingalong seams and veins in therocks. The distribution of thoserocks which preserve traces of Regional synthesis the early assemblage (M2) suggests that deformation during event M3, accompanied by the introduction of The gneisses of the Honey Brook Upland and thoseof water,favoured recrystallization and replacement of the Glenarm Terrain differ in rock type. The granulite theearlier minerals. W of thegranodiorite body and amphibolite facies gneisses of the Honey Brook (gd,Fig. 3),coarse square prisms, completely Upland are predominantlyfelsic to intermediate, while pseudomorphed by kyanite, are thought to have been those in the Glenarm Terrain are mainly intermediate andalusite prisms, subjected to the overprinting effects to mafic. The distinctive graphite gneiss found in the of event M3. The occurrence of cordierite, of cordier- Upland is missing in the Glenarm Terrain. Thus, de- ite and hypersthene, and of andalusite, suggest low to spiteasimilar age of metamorphism,the Honey moderatemetamorphic pressures (-4 kb)at high Brook Upland rocks are quite distinct from the gneis- temperatures (700-750°C) for the early metamorph- ses of thenorthern belt of theGlenarm Terrain, ism (M2, Table 1) in schists adjacent to the Wilming- exceptthat both are cut by diabase dykes which tonComplex (Mark 1977; Crawford &L Mark1977). post-date the granulite facies M1 metamorphism. The These conditions agree with those predicted from the southern belt of the Glenarm Terrain and the Wil- Wilmington Complex granulite facies rocks. Crystalli- mington Complex gneisses are similar in grade to the zation of kyanite,replacement of cordierite by Honey Brook Upland rocks. Despite the resemblance sillimanite-biotiteaggregates and the occurrence of in grade, however, the isotopic age dates, the general kyanite-orthoclaseassemblages adjoining the structuralrelations, and the lack of diabasedykes granodiorite,point to a significantly higher pressure suggest thatthe southern gneisses may be younger during the M3 metamorphic episode. than the M1 metamorphism and hence can neither be Elsewhere in the Glenarm Terrain, the Wissahickon equated with the northern Glenarm Terrain gneisses Groupand other Glenarm Supergroup rocks show nor the Honey Brook Upland rocks.

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Retrograde metamorphism affects all the gneisses to is the high angle Cream Valley Fault (Fig. 4) marked somedegree. In theUpland, where the Palaeozoic by extensive blastomylonite; shear or crush zones are sequencehas been metamorphosed only to the alsocommon within the gneiss belt and along the greenschist facies during M3, the retrograde alteration southernmargin (Rosemont Fault), separating the of the gneissesalso appears to indicategreenschist amphibolitefacies gneisses from the Wissahickon facies conditions (Huntsman 1975; Demmon 1977).In Group. Movement along the Cream Valley Fault fol- the Glenarm Terrain the granulite facies assemblageis lowed, at least in part, the last metamorphic episode, partiallyreplaced with amphibolite facies minerals. asstaurolite and kyanite schist of the Wissahickon This conforms to the grade of metamorphism of the Group is faultedagainst chlorite phyllite. Amenta surrounding schists. Most of the Glenarm Supergroup (1974) suggested that the Rosemont Fault developed and the Palaeozoic rocksof the Chester Valley Region early in the last metamorphic episode; however, this wereaffected only by this last M3 metamorphic faultcuts across the regional metamorphic episode. (Wyckoff 1952).Apparently the gneisses were emp- laced, at least in the eastern part of the terrain, by Deformation movements following the maximum stage of the last metamorphic event, but while the rocks werestill hot. Boththe eastern (Amenta 1974) and Western (MC The crush zones do not show the featuresof mylonites Kinstry 1961; Mackin 1962; Wise 1970) portionsof the associated with low temperature deformation. Faulting Glenarm Terrain are characterized by an early recum- has obscured the relations between the northern and bent folding. The western end of the southern gneiss southern gneiss belts in the Glenarm Terrain. belt is formed by large nappes, transportedto theNW. The age of the Brandywine Manor Fault within the The metamorphic isograds curve around the nappes, Honey Brook Upland is unknown except that it cuts at least on the northern side, suggesting that nappe theCambrian strata within the Upland and hence formation may have accompanied the last metamor- must at least be post-Cambrian. According to Freed- phic episode. In the E the recumbent folds apparently man et al. (1964), uplift of the westernmost end of the pre-date the culmination of the last metamorphism. HoneyBrook Upland (not shown on Fig. 3) arched The exact relation of the amphibolite facies gneiss thepre-existing major regional schistosity. Uplift of of the southern belt to the granulite facies gneiss of the northern gneiss block of the Glenarm Terrain may the northern belt in the Glenarm Terrainis unclear. In have folded the nose of the nappe at the western end the Honey Brook Upland, Crawford (1979) suggested of the southern gneiss belt of the Glenarm Terrain. All that high angle faulting had raised the block on the N these major faults described disrupt the metamorphic of theBrandywine Manor fault and juxtaposed the sequenceor deform the rocks without complete re- amphiboliteand granulite facies rocks. A similar crystallization,hence they probablyare post- model might also be proposed for the gneisses of the metamorphic.They may beassociated with thereg- Glenarm Terrain. Alternatively the amphibolitef acies ional movements accompanying the later uplift of the migmatiticgneisses may be ayounger unit, stratig- metamorphic terrain. raphically overlying previously metamorphosed granu- There is also considerable evidence of faulting be- lite facies rocks, or they might be tectonically emp- tweenthe gneisses of theWilmington Complex and laced over previously metamorphosed basement gneis- theadjacent schist, andwithin the schists showing ses,possibly along a lowangle thrust. In the latter relics of the M2 metamorphism. Some of this faulting case, the thrusting may be related to the formation of is associated with blastomylonite zones similar to those the nappes discussed above. of thegneiss belts. Elsewhere zones, interpreted as A crushzone along the NW border of the faults, contain minerals defining a new coarse fabric, granodiorite body emplaced in the eastern Wissahick- with a different orientation from that in the adjacent on Group W of Philadelphia separates the granodior- schists. Thesezones apparently represent fault or ite from theschist. Elsewhere the contact between the shear zones active early during the M3 metamorphism. granodiorite and the schist is apparently gradational, leadingPoste1 (1940) to suggest thatthe whole granodioritebody was formed by granitization. In Tectonic model places the schistnear the granodiorite has become feldspathized,as indicated by the development of Therelations outlined above suggest a preliminary of K-feldspar.Kyanite is abundantly modelfor the development of thePiedmont in SE developed near and within the crush zone in places Pennsylvaniawhich can serveas a basis for further andkyanite-orthoclase assemblages are found in investigation. Thesequence of eventsobserved sug- schists near the granodiorite. The emplacement of the gests volcanic activity (diabase dykes) associated with granodiorite,therefore, apparently occurred during thinningand rifting of the1000 Ma basement rep- the M3 metamorphism. resented by the granulite grade gneisses. The diabase The northern boundary of the Glenarm gneiss belt dykeswhich cut both the Honey Brook Upland and

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 tectonichistory of the PennsylvaniaPiedmont 319 thenorthern belt of basementgneisses in the (probable) deeper water facies now exposed W and S Glenarm Terrain may be feeders for the mafic units in of the basement gneiss belts. the southern gneiss belt. The southern gneiss belt may The volcanicunits and the volcanicrocks inter- consist of units of several ages. Comparison with the layered in the Wissahickon Group could be part of an Baltimore gneiss domes in Maryland, which also show island arc complex marking the eastern margin of the amphibolite facies gneisses overlain by the Setters and basin.High heat flow associatedwith the island arc, CockeysvilleFormations, suggests that thewestern- andeven possible subvolcanic plutons (norite- most gneisses may be 1000 Ma old. Other portions of anorthosite intrusive rocks and associated charnockite) this gneiss belt include which interfinger may have been responsible for the high temperature, with the Wissahickon Group schists tothe S and low pressure metamorphism of the Wilmington Com- banded gneisses which may correspond to parts of the plexgneisses and theadjacent Wissahickon Group Wilmington Complex. We suggest that the amphibo- schists. lite facies gneisses and the gneisses of the Wilmington Subsequentcompression generated gneiss-cored Complexare best interpreted as an assemblage of nappes which thickened the crust sufficiently to cause metavolcanic units and metamorphosed volcaniclastic the hightemperature and high pressure regional sediments. These may correlate with the James Run metamorphism of the tectonically thickened sequence. Formation in Maryland and the volcanic rocks of the Thismetamorphism is presumedto be Taconic or Carolina Slate Belt further S. younger, based on the 440 Ma zircon date from the Sediments and volcanic rocks accumulated in a late Wilmington Complexgneisses, and on the 350- Precambrian-early Palaeozoic basin which developed 320Ma K-Ar dates on muscovite andbiotite. The SE of the rifted edge of the continental margin. This approximately vertical movement of basement blocks basin filled with a deep-water clastic sequence to form which deformed the earlier recumbent structures and theWissahickon Group while, on the edge of the the isograds associated with the regional metamorph- continent, a thin basal clastic sequence formed over- ism, represent the initial part of the post-metamorphic lain by a carbonate bank (Rodgers 1968). The south- history of uplift of the area. ernand eastern portions of the Wissahickon Group containinterlayered amphibolites, most abundantly developed where the Wissahickon Group is least peli- tic. The Wissahickon Group clastic sedimentary sequ- ACKNOWLEDGMENTS. Support for this projectwas provided by the Ida H. Ogilvie Bequest to the Departmentof Geology, ence prograded onto the continental margin and over- BrynMawr College and NSF Grant EAR 76-84210. We lies the youngest (middle Ordovician) sedimentsin the would like to thankthe many students whose work on carbonatebank. The near-shore facies of the Wis- projects in the area have,contributed much of the data and sahickon is considerablymore argillaceous than the have helped in the interpretations presented here.

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Received 9 April 1979. MAMA LUISA CRAWORDand WILLIAM A. CRAWORD,Department of Geology, Bryn Mawr, Pennsylvania, 19010 USA.

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