Laurentian and Amazonian Sediment Sources to Neoproterozoic– Lower Paleozoic Maryland Piedmont Rocks GEOSPHERE; V

Research Paper

GEOSPHERE Laurentian and Amazonian sediment sources to Neoproterozoic– lower Paleozoic Maryland Piedmont rocks GEOSPHERE; v. 11, no. 4 Aaron J. Martin1, Scott Southworth2, Jennifer C. Collins1, Steven W. Fisher1, and Edward R. Kingman III1 1Department of Geology, University of Maryland, College Park, Maryland 20742, USA doi:10.1130/GES01140.1 2U.S. Geological Survey, M.S. 926, National Center, Reston, Virginia 20192, USA

8 figures; 1 table; 4 supplemental files ABSTRACT Pangea through supercontinent-destroying rifting and subsequent passive CORRESPONDENCE: [email protected] margin development. These juxtaposed and superimposed archives make the Several terranes of variable tectonic affinity and history underlie the central Appalachians an ideal place to compare similar processes that affected CITATION: Martin, A.J., Southworth, S., Collins, J.C., Appalachian Piedmont Province (eastern United States). These terranes mostly the same portion of a continent at different times. For example, in this paper Fisher, S.W., and Kingman, E.R., III, 2015, Laurentian and Amazonian sediment sources to Neoprotero zoic– consist of widespread metasedimentary and lesser metavolcanic rocks. Intense we contrast the tempo of the transition between rifting cessation and subduc- lower Paleozoic Maryland Piedmont rocks: Geosphere, and pervasive deformation and metamorphism have made the depositional tion initiation in eastern Laurentia/North America following rifting in latest v. 11, no. 4, p. 1042–1061, doi:10 .1130 /GES01140.1. ages and provenance of sediment in these rocks difficult to determine. The lack Neoproterozoic versus Late Triassic–earliest Jurassic time. Such constraints on of tight constraints on such basic information led to a century-long debate about the rates of tectonic events inform our general understanding of the supercon- Received 15 October 2014 the tectonic significance of these rocks, particularly how they correlate to simi tinent cycle and plate tectonics (e.g., Korenaga, 2006; Bradley, 2008). Revision received 7 April 2015 Accepted 6 May 2015 lar rocks along and across strike in the Appalachian orogen. We address these Regionally, the portion of the Appalachian orogen between New York City Published online 10 June 2015 issues using U/Pb isotopic ages from single spots in 2433 zircon grains from and Virginia is of interest because it lies between Ganderia to the north and 18 metasedimentary rock samples distributed across the Maryland Piedmont. Carolinia to the south (Fig. 1; all directions are in present-day coordinates). The resulting age signatures indicate that the Marburg Formation and These blocks were peri-Gondwanan terranes that accreted to Laurentia in early Prettyboy Schist, heretofore assigned to the Westminster terrane, actually Paleozoic time (Hibbard et al., 2006; van Staal et al., 2009; Pollock et al., 2012). belong to the Potomac terrane, making the Hyattstown thrust the contact be- Accretion of these terranes caused orogeny on the eastern margin of Lau- tween the two terranes. Ediacaran Laurentia could have supplied all Potomac rentia, documented by deformation, metamorphism, magmatism, and basin terrane sediment except for the detritus in one sample from the northern part of formation in the northern and southern Appalachians. Piedmont rocks of the the terrane that likely came from Amazonia. This is one of the first recognitions central Appalachians also record early Paleozoic deformation, metamorphism, of a Gondwana-derived terrane between Carolinia to the south and Ganderia to and magmatism (Drake, 1985b, 1989; Aleinikoff et al., 2002; Kunk et al., 2005; the north. Maximum depositional ages for Potomac terrane suprasubduction Southworth et al., 2007; Horton et al., 2010; Wintsch et al., 2010). However, zone sedimentary rocks are latest Neoproterozoic or early Cambrian, and some current syntheses do not show Gondwanan terranes exposed in the central may have been deposited ca. 510 Ma. Continental rifting ended ca. 560 Ma at Appalachian Piedmont Province, raising the question, What was the tectonic the longitude of our study, so the transition from rifting to subduction at this cause of early Paleozoic orogeny in the central Appalachians? In this paper we location in eastern Laurentia may have lasted only 50 M.y. Lower Ordovician test the interpretation that Gondwanan terranes do not crop out in the central arc intrusions into these rocks demonstrate that the transition lasted no longer Appalachians using U/Pb isotopic ages of detrital zircon to establish the prove- than 90 M.y. The Iapetan margin of central-eastern Laurentia was one of the nance of the sediment that became the Piedmont rocks of Maryland, northern shortest lived passive margins that formed in Neoproterozoic time. Virginia, and Washington, D.C. We also use the detrital zircon ages for two other purposes. First, although most apparently are not exotic to Ediacaran Laurentia, multiple terranes with INTRODUCTION different histories compose the central Appalachian Piedmont. Detrital zircon ages allow us to probe which formations share depositional affinities, and thus The Appalachian-Caledonian orogen is the type locality of the Wilson cycle, where terrane boundaries lie. Second, because the depositional ages of these the first location where geologists recognized repeated creation and destruc- rocks are not well known, we use detrital zircon ages to provide constraints tion of ocean basins between continents (Wilson, 1966; Bird and Dewey, 1970). on their maximum possible depositional ages. Both the locations of terrane This recognition was critical for development of the supercontinent cycle con- boundaries and the depositional ages are important for regional correlations For permission to copy, contact Copyright cept. Different parts of the Appalachians record two complete supercontinent and for understanding the tectonic evolution of this portion of the Appalachian Permissions, GSA, or [email protected]. cycles, from magmatic arc growth and collision that produced Rodinia and orogen. For example, the Sams Creek Formation and surrounding rocks in the

GEOSPHERE | Volume 11 | Number 4 Martin et al. | Sources of sediment to Maryland Piedmont rocks Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1042/3332988/1042.pdf 1042 by guest on 30 September 2021 Research Paper

Laurentian Realm Iapetan Realm Peri-Gondwanan Realm Piedmont/Goochland Carolinia Avalonia

Peri-Laurentian arc/ Ganderia Meguma 40 °N Peri-Gondwana arc

50 °N

Figure 2

04km 00 80 °E 70 °E 60 °E

Figure 1. Generalized geologic map of the Appalachian orogen. The study area lies between recognized peri-Gondwanan terranes Carolinia to the south and Ganderia to the north. Modified from Hibbard et al. (2006).

western Piedmont of Maryland were thought to have been deposited in the in the Paleozoic Era underlie the westernmost province, the Valley and Ridge latest Neoproterozoic to early Cambrian (Southworth et al., 2007), but Graybill Province; these rocks were faulted, folded, and cleaved during the Carbonifer- et al. (2012) suggested deposition in the earliest Neoproterozoic. This deposi- ous–Permian Alleghanian orogeny (Hatcher et al., 1989; Hibbard et al., 2006). tional age revision changes the interpreted tectonic setting from a basin pro- These strata were juxtaposed against the rocks that underlie the Blue Ridge duced by well-known rifting of eastern Laurentia ca. 570–560 Ma (Southworth, Province to the east by faults; at the latitude of our study area the boundary 1999; Southworth et al., 2009; Burton and Southworth, 2010) to a basin caused fault is called the Keedysville Fault. The Blue Ridge Province exposes late by putative rifting ca. 960 Ma (Graybill et al., 2012). Mesoprotero zoic granitoids and related rocks, metamorphosed to conditions Similarly, the Sykesville and Laurel Formations in the eastern Piedmont were as high as granulite facies during latest Mesoproterozoic mountain building interpreted by most modern workers to be metamorphosed sedimentary rocks related to the Grenville orogeny (McLelland et al., 2010; Southworth et al., that were deposited in deep water at a convergent margin (Drake, 1985a; Muller 2010). Rifting following the Grenville orogeny is represented by an initial pulse et al., 1989; Pavlides, 1989; Drake and Froelich, 1997). Fleming and Self (2010) of felsic and mafic magmatism ca. 780–670 Ma (Tollo et al., 2004, 2012; Holm- instead suggested that these rocks were mostly a thick pile of metamorphosed Denoma et al., 2014; McClellan and Gazel, 2014) and the bimodal volcanism arc-related ignimbrites that erupted broadly coeval with intrusion of Lower of the ca. 570–560 Ma Catoctin Formation, as well as felsic dikes as young Ordovician arc granitoids into the Sykesville and Laurel Formations. If this re as 555 ± 4 Ma (Southworth et al., 2009; Burton and Southworth, 2010). Rift interpre ta tion is correct, the succession represents a major volcanic arc terrane, or passive margin sedimentary rocks enclose the Catoctin Formation. All Blue whereas the conventional interpretation indicates trench or forearc basin fill. In Ridge Province rocks in the vicinity of our study area were metamorphosed this paper we use detrital zircon ages combined with field and thin section ob- to lower greenschist facies during late Paleozoic time (e.g., Kunk and Burton, servations to assess the origin of the Sykesville and Laurel Formations. 1999; Bailey et al., 2006; Southworth et al., 2007, 2009). The Piedmont Province lies directly east of the Blue Ridge Province across the Bull Run Mountain fault (Fig. 2). We discuss Piedmont rocks in detail in the next subsection. The east- GEOLOGIC SETTING ernmost province is the Coastal Plain, which is underlain by lower Cretaceous to Quaternary sediment deposited on the passive margin created by rifting of Geologists classically divide the southern and central Appalachian orogen Africa from North America during opening of the Atlantic Ocean (Olsson et al., into four tectonic provinces (Hatcher, 1989). Foreland basin strata deposited 1988; Edwards et al., 2010). Late Triassic to Early Jurassic rift basins super

imposed on the older Piedmont rocks record initial rifting of these continents B l u e R i d g e P i e d m o n t within Pangea (Weems and Olsen, 1997; Southworth et al., 2007), and Coastal JTr CZ Gettysburg basin Plain province strata likewise unconformably overlie Piedmont rocks (Olsson Westminster P o t o m a c t e r r a n e Baltimore terrane terrane et al., 1988). OC 39.5° Y 77.5° OC Martic thrust4 Y Piedmont Province Tectonostratigraphy Frederick sd 11 CZ 7 um y 14 16 .. Y Hyattstown thrust The Piedmont Province consists of a collection of fault-bounded terranes CY Valle 6 of variable tectonic affinity. In Maryland, pre-Triassic rocks of the Piedmont 5 Potomac Rive ederick Fr synclinorium CZ MA Province are divided from west to east into the Frederick Valley synclinorium, r Sugarloaf 1 8 Baltimore 3 9 Sugarloaf Mountain anticlinorium, Westminster terrane, Potomac terrane, and fault 2 ove um Baltimore terrane (Figs. 2 and 3). The metamorphic grade in these rocks in- t Y CnzK creases from lower greenschist facies in the west to amphibolite facies in the Pleasant Gr Maryland um O east. In the following paragraphs, we focus on the protoliths of the rocks of Vi 10 rgini um C o a s t a l P l a i n these terranes. a Plummers Island faul um The lower and middle Cambrian Araby Formation is the stratigraphically 17 12 lowest unit in the Frederick Valley synclinorium (Reinhardt, 1974, 1977). This Bull Run Mountain fault Reston 13 O O 18 metasiltstone and metashale formation is depositionally overlain by an upper Rock Creek shear zone 39°N 76.5°W Cambrian to lower Ordovician succession dominated by carbonate rocks 15 Washington, D.C. S (Mathews and Grasty, 1909; Reinhardt, 1974, 1977). JTr Culpepper basin N The Sugarloaf Mountain anticlinorium is composed of two units. The lower um CnzK S 10 km Cambrian Sugarloaf Mountain Quartzite protolith was dominantly quartz arenite and the conformably overlying Urbana Formation mostly consisted of Paleozoic granitoid Depositional or intrusive contact muddy sandstone and siltstone (Jonas and Stose, 1938; Southworth, 1999). um Ultrama c bodies in Potomac terrane Fault The Urbana Formation also contains discontinuous map-scale pods of marble sd: Soldiers Delight Ultrama te surrounded by the siliciclastic lithologies. Sample location The Martic thrust placed the Westminster terrane on top of the Frederick Map Sample Unit Map Sample Unit Valley synclinorium and the Sugarloaf Mountain anticlinorium. Westminster number number name number number name terrane strata may be the deeper water equivalents of contemporaneous 1908004Ijamsville10909005 Marburg-S supracrustal rocks of the Blue Ridge Province (Rodgers, 1970; Smoot and 2908002Urbana 11 910002 Prettyboy Southworth, 2014). The Westminster terrane includes the Ijamsville Phyllite, 3908001 Sugarloaf 12 810001 Blockhouse Point Sams Creek Formation, and Wakefield Marble (Mathews and Grasty, 1909; 41010001 Sams Creek-N 13 310001 Bear Island-VA Rodgers, 1970; Southworth et al., 2007). The protoliths for the Ijamsville Phyl- 5908003 Sams Creek-S 14 1010002 Mather Gorge-N 6809002 Setters-W15909003 Sykesville-S lite were dominantly shale, siltstone, and sandy mudstone, some of which was 7909007 Setters-E16909001 Sykesville-N tuffaceous (Mathews and Grasty, 1909; Jonas and Stose, 1938). The Ijamsville 8910004Marburg-NW 17 909004 Northwest Branch Phyllite also contains other metamorphosed supracrustal lithologies such as 9910005Marburg-NE 18 808003 Laurel sandstone and basalt (Mathews and Grasty, 1909; Southworth et al., 2007). The Sams Creek Formation protoliths included a large variety of supracrustal Figure 2. Geologic map of the study area showing sample locations. Symbols for geologic time periods denote age of crystallization for intrusive rocks or deposition for supracrustal rocks. lithologies, including the metasandstone sampled for this study (Mathews and Sugarloaf MA—Sugarloaf Mountain anticlinorium. VA—Virginia. Grasty, 1909; Southworth et al., 2007). However, the Sams Creek Formation is dominated by mostly mafic metavolcanic rocks. The metabasalt flows contain pillows and are chemically similar to metabasalt of the Catoctin Formation in are not well known, but usually are assumed to be late Neoproterozoic to Cam- the Blue Ridge Province (Southworth, 1999). The Wakefield Marble was prob- brian (Southworth et al., 2007). ably deposited near Sams Creek Formation volcanic islands, and consists of A thrust fault placed the Potomac terrane on the Westminster terrane. Previ- both calcitic and dolomitic marble as well as sparse interbedded metamor- ously, the Pleasant Grove fault was identified as the terrane boundary because phosed sandstone and shale (Mathews and Grasty, 1909; Jonas and Stose, the Marburg Formation and Prettyboy Schist were included in the Westmin- 1938; Fisher, 1978). Depositional ages for the Westminster terrane formations ster terrane (Drake, 1985b, 1989; Horton et al., 1989; Southworth et al., 2007).

W E Frederick Sugarloaf Valley Mountain Westminster Baltimore Synclinorium Anticlinorium Terrane Potomac Terrane Terrane

450 t

age? ills fault fault tic thrust Grove faults ge (Ma) eek shear zone attstown thrust Frederick Mar Burnt M Hy

500 Pleasant Grove fault Plummers Island fault Rock Cr

Cash Smith ed depositional contac Laurel rr fe

Araby In Sykesville NW Branch ystallization A Urbana Blockhouse Pt. Bear Island Sugarloaf MQ 550 Marburg Wake eld Prettyboy Sams Creek Cockeysville Ijamsville Setters Deposition or Cr

600 ~~~~Scale shift~~~~ ca. 1075 Ma Baltimore Gneiss (ca. 1250 Ma)

Figure 3. Stratigraphic columns for the Piedmont Dominant protolith lithology Province directly north of the Potomac River, plus the Baltimore terrane. All these rocks have been Diamictite Limestone/Dolostone metamorphosed; the columns depict the protolith lithologies. Constraints on depositional ages are in Sandstone Felsic Igneous Table 1. The northern Mather Gorge Formation is not included in this diagram. Sugarloaf MQ—Sugarloaf Mudstone Ma c Igneous Mountain Quartzite; Blockhouse Pt.—Blockhouse Point domain of the Mather Gorge Formation; Bear Volcanic rocks shown extending across entire column. Island—Bear Island domain of the Mather Gorge For- Intrusive rocks shown only on right side of column. mation; NW Branch—Northwest Branch Formation. Detrital zircon sample, this study

However, based on our new detrital zircon ages, we include these two units in mation also contains meter-long to several kilometer–long metamorphosed the Potomac terrane; this new assignment makes the Hyattstown thrust the mafic and ultramafic blocks that do not appear to be bounded by major faults western boundary of the terrane. Metasedimentary rocks of the Potomac ter- (Drake, 1989; Southworth et al., 2007). Kunk et al. (2005) divided the Mather rane are divided into the Marburg, Mather Gorge, Northwest Branch, Oella, Gorge Formation into three domains with different 40Ar/39Ar cooling ages; from Laurel, and Sykesville Formations and the Prettyboy and Loch Raven Schists west to east these are the Blockhouse Point, Bear Island, and Stubblefield Falls (Southworth et al., 2007). Marburg Formation protoliths were dominantly domains. The Northwest Branch Formation protoliths were dominantly arenite siltstone and subordinate muddy sandstone (Jonas and Stose, 1938; Drake, (Drake, 1998). The Oella Formation protoliths likewise were dominated by 1994) whereas Prettyboy Schist protoliths were finer grained (Crowley, 1976). arenite, but they were interbedded with mudstone at the centimeter to meter Mather Gorge Formation protoliths were dominated by feldspathic arenite and scale (Crowley, 1976). The protolith for the Loch Raven Schist was dominantly wacke as well as sandy mudstone (Drake and Froelich, 1997); these rocks are shale (Crowley, 1976). Protoliths of the Laurel and Sykesville Formations were interpreted to be turbidites (Southworth et al., 2007). The Mather Gorge For- dominated by diamictite consisting of pebbles to boulders with a wide range

of compositions surrounded by a matrix of feldspathic muddy sandstone support the interpretation that the depositional setting for all these rocks was (Hopson, 1964; Muller et al., 1989; Pavlides, 1989). Gravel-sized clasts in both shallow water covering a continental shelf and slope (Reinhardt, 1977; South- formations include quartzite, schist, and phyllite; the Sykesville Formation worth et al., 2007). The prevalence of mafic volcanic and volcaniclastic rocks in also contains clasts of milky quartz, granite, and metamorphosed mafic and the Westminster terrane suggests deposition in a rift setting, likely related to the ultramafic rocks. Depositional ages of the Potomac terrane metasedimentary rifting of eastern Laurentia recorded by the 570–560 Ma Catoctin Formation in rocks are not well known but are usually considered to be latest Neoprotero- the Blue Ridge Province directly to the west (Southworth, 1999). zoic to Cambrian (Southworth et al., 2007). Unlike the more western Piedmont In contrast, rocks of the Potomac terrane contain nearly no carbonate strata terranes, the Potomac terrane contains Paleozoic plutonic rocks that intruded and the Mather Gorge Formation contains turbidites, both pointing to depo- all formations except the Marburg and Prettyboy. These felsic, arc-related sition in deep water outboard of the continental shelf. The wide range of clast magmas mostly crystallized in the Ordovician Period, with some additional types in the Sykesville Formation suggests deposition near a tectonically ac- Late Devonian granitic plutonism (Aleinikoff et al., 2002; Horton et al., 2010). tive continental margin; furthermore, if the crystallization age of the igneous The Potomac terrane also includes the Soldiers Delight Ultramafite, a thrust- clasts was near the depositional age of the diamictite, then the active margin bounded serpentinite body that separates the Sykesville Formation to the west had magmatic activity. The scarcity of bimodal volcanic or volcaniclastic strata from the Laurel Formation to the east (Drake, 1994). argues against a rift setting for deposition of any part of the Potomac terrane The Baltimore terrane (Williams and Hatcher, 1982) sits structurally beneath (cf. Alvaro et al., 2008; Ayalew and Gibson, 2009; Corti, 2009; Zhou et al., the Potomac terrane, juxtaposed by a series of thrusts. The oldest rocks ex- 2009). Deposition in a trench or forearc basin outboard of the continental mar- posed in the Baltimore terrane are felsic and minor mafic gneisses collectively gin would explain the wide range of clast types, the paucity of volcanic or vol called the Baltimore Gneiss (Williams, 1892); the volcanic protoliths of some caniclastic strata, the presence of the turbidites, and the nearly complete lack of the felsic rocks crystallized ca. 1250 Ma (Aleinikoff et al., 2004). This metavol- of bedded carbonate. Map-scale mafic and ultramafic blocks occur within the A: 211001 (Loch Raven) B: 310001 (Bear Island) canic and metasedimentary succession was intruded by several late Meso- Mather Gorge Formation, and the Laurel and Sykesville Formations surround a proterozoic granitic plutons, including foliated biotite granite that crystallized fault-bounded ultramafic body, the Soldiers Delight Ultramafite (Fig. 2). These ca. 1075 Ma (Aleinikoff et al., 2004). Seven antiformal domes on the north- mafic and ultramafic blocks could have been tectonically emplaced at depth in ern and western sides of the city of Baltimore expose the Mesoproterozoic a subduction channel (e.g., Cloos, 1982; Hernaiz Huerta et al., 2012; Aoya et al., rocks in their cores (Mathews, 1907; Hopson, 1964; Fisher and Olsen, 2004). 2013). We argue that the most likely tectonic scenario for both deposition of the 0.5 mm The Setters Formation was deposited nonconformably on these metaigneous Potomac terrane sedimentary rocks and emplacement of the kilometer-scale C: 808003 (Laurel) ms D: 809002 (Setters-W)

grt rocks and partially or completely rings each dome in map view (Williams, 1891; mafic and ultramafic blocks is an ocean-continent subduction zone. Other Knopf and Jonas, 1923; Hopson, 1964). The Setters Formation is dominated by workers likewise argued for deposition of the Mather Gorge, Sykesville, and grt metamorphosed feldspathic muddy sandstone, siltstone, and shale (Hopson, Laurel Formations at an ocean-continent convergent margin (Drake, 1985a; ms ms q 1964; Fisher, 1971) and is depositionally overlain by the Cockeysville Marble Muller et al., 1989; Pavlides, 1989; Drake and Froelich, 1997). The formation of

q ms (Williams, 1892; Mathews and Grasty, 1909; Choquette, 1960). The Setters For- the subduction zone involving oceanic lithosphere east of Laurentia did not end

E: 908001 (Sugarloaf) F: 908002 (Urbana) mation and the underlying Mesoproterozoic rocks were intruded by granitic sedimentation in the Frederick Valley synclinorium, Blue Ridge Province, and

q plutons in early to middle Paleozoic time (Fisher and Olsen, 2004). The depo Valley and Ridge Province during the Cambrian and into the Ordovician (Fig. 3; q q sitional ages of the Setters Formation and Cockeysville Marble are not well Reinhardt, 1974, 1977; Southworth et al., 2007; Smoot and Southworth, 2014). known but usually are inferred to be latest Neoprotero zoic or Cambrian (Wil- q q liams, 1891; Higgins, 1972; Fisher and Olsen, 2004). On the southeastern side the metaigneous rocks and their metasedimentary carapace were overthrust METHODS by a metamorphosed Cambrian mafic-ultramafic complex (Sinha et al., 1997). 1Supplemental Figure 1. Photomicrographs of thin We collected samples from pre-Triassic Piedmont Province metasand- sections from each sample except 810001 from the stone-bearing formations exposed in Maryland (Figs. 2 and 3; Table 1). In Blockhouse Point Domain of the Mather Gorge For- Summary of Depositional Settings for Piedmont Province Rocks mation. Samples 211001 of the Loch Raven Formation addition, we collected two samples (810001 and 310001) in Virginia and one and 909006 of the Araby Formation did not yield zir- The Frederick Valley synclinorium and the Westminster terrane each contain (909003) in Washington, D.C. Each sample came from the coarsest grained, con. Thin sections were cut perpendicular to foliation carbonate successions at least several hundred meters thick, and the Urbana least micaceous sandstone we identified in each outcrop. Photomicrographs and, when present, parallel to lineation. All images were acquired using transmitted, cross-polarized Formation within the Sugarloaf Mountain anticlinorium also contains calcareous of thin sections from these sandstones are shown in Figure 4 and Supplemen- light. The scale is the same for all images. Mineral rocks and beds of marble. The Sugarloaf Mountain Quartzite is predominantly tal Figure 11. In some outcrops in the eastern Piedmont, granitoids intruded abbreviations: grt—garnet, hem—hematite, ms— metamorphosed quartz arenite and conglomerate. Similarly, the Setters Forma- the metasedimentary rocks; in these cases we collected samples far from the muscovite, q—quartz. Please visit http://dx.doi.org /10.1130/GES01140.S1 or the full-text article on www tion is depositionally overlain by the Cockeysville Marble, which had a deposi- intrusions and avoided rock pieces with visible veins. We did not sample the .gsapubs.org to view Supplemental Figure 1. tional thickness of as much as 120 m (Knopf and Jonas, 1923). These lithologies Oella Formation of the Potomac terrane because we were unable to find an

TABLE 1. SAMPLE SUMMARY Ages of individual zircon Number of Maximum analyses used to determine Minimum Minimum Sample Lat Long Year analyses depositional age maximum depositional age depositional age depositional age; number Formation name (°N) (°W) analyzed† kept (this study;Ma) (Ma) (literature;Ma) reference Western group (Westminster terrane and Sugarloaf Mountain anticlinorium) 908004 Ijamsville 39.26951 77.33960 2009 127 1040 1005, 1007, 1012, 1013, 1013 430Wintsch et.al. (2010) 908002 Urbana 39.25285 77.38721 2009 153 1020 975, 988, 997, 999, 1000 430Wintsch et.al. (2010) 908001 Sugarloaf 39.26016 77.39168 2009 141 1040 1001, 1003, 1005 430? adjacent to Urbana Formation 1010001 Sams Creek north 39.46638 77.24840 2011 148 580554, 563 430Wintsch et.al. (2010) 908003 Sams Creek south 39.32987 77.32055 2009 151 1040 1003, 1007, 1009 430Wintsch et.al. (2010) Total: 720 analyses from 5 samples Setters Formation 809002 Setters west 39.37354 76.86314 2010 134 1000 954, 960, 985, 985340 Wasserburg et.al. (1957) 909007 Setters east 39.41682 76.54097 2010 136 910881, 882 340Wasserburg et.al. (1957) Total: 270 analyses from 2 samples Potomac terrane 910004 Marburg northwest 39.27559 77.31171 2011 123 1020 995, 1000430 Wintsch et.al. (2010) 910005 Marburg northeast 39.30576 77.25564 201180990 976, 986, 988, 990430 Wintsch et. al. (2010) 909005 Marburg south 39.12738 77.31362 2010 139 1020 989, 994, 998430 Wintsch et. al. (2010) 910002 Prettyboy 39.35935 77.06067 2011 130 1030 995, 1022370 Wintsch et.al. (2010) 810001 Blockhouse Point§ 39.05819 77.33440 2009 145 980921, 931 457? adjacent to Bear Island domain 310001 Bear Island VA§ 38.99641 77.25367 2010 105 990972, 987 457Kunk et.al. (2005) 1010002 Mather Gorge north 39.35307 77.01630 2011 130 540527, 527, 531, 532none 909003 Sykesville south 38.93209 77.11639 2010 175 1020 971, 979, 981472 Aleinikoff et.al. (2002) 909001 Sykesville north 39.36272 76.96720 2010 154 550529, 531 472 Aleinikoff et.al. (2002) 909004 Northwest Branch 39.04799 77.01083 2010 129 1000 971, 973, 975472?adjacent to Sykesville Formation 808003 Laurel 39.02996 77.00382 2009 133 530511, 518400 Kunk et.al. (2005) Total: 1443 analyses from 11 samples Grand total: 2433 analyses from 18 samples Note: Question marks indicate uncertainty because the minimum depositional age came from an adjacent formation. †Samples analyzed in 2009 using Isoprobe; in 2010 and 2011 using Nu Plasma. §The Blockhouse Point and Bear Island domains are part of the Mather Gorge Formation. VA—Virginia.

outcrop that was not pervasively intruded by granite. Samples from all for- multiple analyses with ages that overlap within uncertainty. Accordingly, we mations except the Araby Formation (Frederick Valley synclinorium) and Loch take the maximum possible depositional age to be the weighted mean of the 2 Raven Schist (Potomac terrane) yielded abundant zircon. Details of the zircon or more youngest analyses that overlap at the 1s level plus the 2s error on the separation and analysis procedure are given in Appendix 1. All uncertainties weighted mean, rounded up to the nearest 10 M.y. interval to avoid spurious are given at the two standard deviation level. significant figures. Determination of the maximum possible depositional age of a sedimentary Laurentia consisted of an Archean core surrounded by younger terranes, rock based on its youngest detrital zircon grains requires careful interpreta- some of which may have originated far from Laurentia (Whitmeyer and Karl- tion because of the confounding effects of lead loss, multiple age zones in a strom, 2007). Regardless of their origin, after accretion to the continent these single grain, growth of metamorphic zircon, analytical uncertainty, and dating terranes became part of Laurentia; therefore, we consider a Laurentian sedi a very small fraction of the zircon population present in a rock unit. Dickinson ment source to be a source in Laurentia as the continent existed at the time of and Gehrels (2009a) showed that using the youngest single age from a suite deposition. Because the sedimentary rocks considered here were deposited of detrital zircon grains can yield a maximum depositional age determination during or after the Ediacaran Period, we refer to the continent as Ediacaran that is younger than the true depositional age, so they recommended using Laurentia when discussing sediment provenance.

RESULTS $

Supplemental Figure 22 shows histograms and relative probability plots for individual samples; Figure 5 summarizes the dating results from all samples. We combine the Sugarloaf Mountain anticlinorium and Westminster terrane into a western group of rocks because their detrital zircon age distributions are nearly identical. The interpretation that some Westminster terrane rocks are deeper water equivalents of Sugarloaf Mountain anticlinorium strata supports this combination (Rodgers, 1970). Most Westminster terrane and Sugarloaf Mountain anticlinorium samples contain zircon with ages only between 1350 and 900 Ma, including uncertainties; these ages define a peak on the individual relative probability plots at 1100–1050 Ma. The Sugarloaf Mountain Quartzite sample (908001) also yielded two 1700–1600 Ma grains and the northern Sams Creek Formation sample (1010001) yielded one ca. 680 Ma and two ca. 560 Ma zircon grains. The probability density plot for the combined ages from the five western group samples displays a single spike ca. 1070 Ma (Fig. 5C). Although the Setters Formation crops out geographically east of all the other formations, its zircon age distribution is similar, but not identical, to those from the Westminster terrane and Sugarloaf Mountain anticlinorium rocks. Zircon from the western Setters Formation sample (809002) has ages only between 1300 and 900 Ma and produces a prominent peak at 1020 Ma % and a subordinate peak at 1170 Ma. The distribution of ages from the eastern 40 D: Setters – E (909007) n=136 35 peaks at 1010 and Setters Formation sample (909007) is nearly identical except for the addition of 30 1170 Ma q 25 two ca. 1350–1300 Ma grains and one ca. 700 Ma grain. Differences between 20 15 Setters Formation and western group age distributions include the following. 10 5 (1) Both Setters Formation samples produced two age peaks, at 1170 and 1020– 0 400600 800 1000 1200 1400160018002000 2200 2400260028003000

80 50 A: Sugarloaf E: Setters – W 1010 Ma, whereas all western group samples yielded only one main peak, at (908001) n=141 70 peaks at (809002) n=134 peak at 1020 and 40 1100 Ma 60 1170 Ma 1100–1050 Ma. (2) Sugarloaf Mountain Quartzite sample 908001 yielded two 50 30 40 1700–1600 Ma grains, but zircon of this age was not found in either Setters ms mag 20 30 Relative probabilit 20 Formation sample. (3) The eastern Setters sample yielded zircon with moder- ms 10 10 206 207 0 0 ately discordant Pb/ Pb ages of 882 ± 26, 881 ± 44, and 725 ± 104 Ma, but 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 400600 800 1000 1200 1400160018002000 2200 2400260028003000 100 chl 80 B: Urbana F: Sams Creek – S 90 the western Setters Formation sample and all western group rocks did not 70 peak at (908002) n=153 (908003) n=151 80 Number of ages

1070 Ma peak at y 60 70 1070 Ma produce similar ages. 50 60 q 50 q 40

40 Potomac terrane rocks contain zircon with a greater range of ages. All sam- 30 30 20 20 ples of these rocks have zircon in the age range 1350–900 Ma, like the western 10 10

0 0 400 600 800 1000 1200 1400 1600 180020002200 2400 2600 28003000 400600 800 10001200140016001800200022002400260028003000 group and Setters Formation samples. However, in all Potomac terrane sam- 80

50 C: Ijamsville G: Sams Creek – N 70 1 mm (908004) n=127 (1010001) n=148 ples the range of ages extends beyond 1350 Ma, to 1700 or 1800 Ma in most

40 peak at 60 peak at 1080 Ma 1050 Ma 50 samples and as old as 2100 Ma in the Laurel Formation sample, including un- 30 40

20 30 certainties. All Potomac terrane samples except the Blockhouse Point Domain Figure 4. (A) Photograph of a dark colored, foliated, pebble-sized clast with a large aspect ratio

20 10 sample (810001) additionally contain populations of (1) upper Archean zircon in the Sykesville Formation exposed along the Potomac River in western Washington, D.C. Flem- 10

0 0 ing and Self (2010) interpreted this type of clast as flattened pumice lapillus. (B) Photomicro- 400 600 800 1000 1200 1400 1600 180020002200 2400 2600 28003000 400600 800 1000 1200 1400160018002000 2200 2400260028003000 at 2850–2600 Ma, and (2) 750–500 Ma zircon. The zircon ages from most Poto- Best age (Ma) Best age (Ma) graph of a thin section through one of the clasts. The abundance of muscovite in the clot in the mac terrane samples produce peaks on individual relative probability plots at center of the image reveals that this feature is a flattened, metamorphosed mudstone clast, 2Supplemental Figure 2. Plots showing the spectrum 1480–1440 Ma and 1200–1030 Ma; on the compilation plot there are prominent not a flattened pumice clast. All other such features that we examined likewise are composed of detrital zircon U/Pb ages from each sample. Please peaks ca. 1470 and 1160 Ma and a minor peak ca. 1650 Ma (Fig. 5D). of ~90% muscovite, supporting the interpretation that the Sykesville Formation is primarily a visit http://dx.doi.org/10.1130/GES01140.S2 or the metasedimentary succession, not metavolcanic. Transmitted light, crossed polars. The micro- full-text article on www.gsapubs.org to view Supple- The detrital zircon age distribution in the northern Mather Gorge Forma- scope’s objective lens was not strain free, so interference colors are not standard. Abbreviations: mental Figure 2. tion sample (1010002) is quite different from the ages in all the other Potomac chl—chlorite, mag—magnetite, ms—muscovite, q—quartz.

B: Setters Formation 2 samples, n = 270 100 peaks at 1020 and 1160 Ma 80

20 Western Group Potomac terrane A 5 samples, 720 dates 10 samples, 1313 dates 0 0 C: Western group: c Westminster terrane 300 plus Sugarloaf MA peak at 250

hanerozoi 5 samples, n = 720 1070 Ma 500 500 1010001: 554 ± 22 Ma 200 cP 563 ± 12 Ma 681 ± 16 Ma 150 750 Figure 5. Summary of detrital zircon ages. (A) Each checkmark indicates that zircon grains with ages in the designated time 808003: 888 ± 78 Ma 900 100 interval typically are present in samples from the rock package Neoproterozoi 1000 shown in each column. (B–E) Ages are shown as relative prob-

50 Number of ages ability plots. The only age population in most western group rocks is 1350–900 Ma. This is a population similar to that found 0 in the Setters Formation, although the Setters samples have 1350 Potomac terrane 1160 Ma 1470 Ma D: two main age peaks whereas the western group rocks have only 1500 esoproterozoic excluding northern one. Most Potomac terrane rocks contain this age population Mather Gorge Fm. 200 Age (Ma ) 908001: 1638 ± 66 Ma plus others at 750–500, 2100–1350, and 2850–2600 Ma. All of 1691 ± 44 Ma 10 samples, n = 1313 these age populations could have been derived from Ediacaran Laurentia. In contrast, sample 1010002 from the northern Mather cM 150 Gorge Formation contains very different age populations, likely 2000 indicating derivation from Amazonia or a peri-Amazonian block. 2100 The Potomac terrane compilation does not include sample 1010002. Best age is 206Pb/207Pb age for zircon older than 700 Ma

aleoproterozoi 100 and 206Pb/238U age for younger grains. Sugarloaf MA—Sugarloaf 910002: 2367 ± 74 Ma Mountain anticlinorium. Histogram bins are 50 M.y. wide.

2500 600– 50 1650 Ma 2720 Ma 2600 510 Ma

0 Archea nP 2850 peak at E: Mather Gorge – N 3000 630 Ma (1010002) n = 130 30

2070 Ma 10

0 400 600 1000 1400 1800 2200 2600 3000 Best age (Ma)

terrane samples, and it is unlike those in our other Piedmont samples (Fig. 5). Creek Formation sample (1010001) produced an exception to the late Meso This sample produced a nearly continuous distribution of ages within the proterozoic maximum depositional ages for the other Westminster terrane rocks range 2650–520 Ma (including uncertainties), with wide gaps only between ca. because this sample yielded grains with 206Pb/238U ages of 681 ± 16, 563 ± 12, 2450–2210 Ma and 1700–1550 Ma. The age distribution does not exhibit the and 554 ± 22 Ma (Supplemental Table 1 [see footnote 3]). A metamorphic ori- major Mesoproterozoic age peaks found in all other Piedmont samples, but gin for the zircon in the Sams Creek Formation is unlikely because maximum instead has a single prominent spike in age probability ca. 630 Ma. metamorphic conditions reached only lower greenschist facies (Wintsch et al., 2010). Although some Sams Creek Formation zircon grains have thin rims visible in cathodoluminescence images, we did not intersect these rims with the INTERPRETATIONS AND IMPLICATIONS laser beam during analysis; all analyses targeted the cores of the grains. We interpret these three young grains to be detrital. The two youngest analyses Terrane Affinity of the Marburg Formation and Prettyboy Schist give a maximum possible depositional age of 580 Ma. These analyses are con- cordant and show no evidence for lead loss or mixing of multiple age zones Our Marburg Formation samples (910004, 910005, 909005) and our Pretty during laser ablation (although the 206Pb/238U age and especially the 206Pb/207Pb boy Schist sample (910002) yielded detrital zircon with age ranges that are age are imprecisely determined for the youngest analysis because of low U nearly identical to those found from the other Potomac terrane samples and concentration). The ages of the two youngest grains are consistent with partial are unlike the Westminster terrane rocks (Fig. 5; Supplemental Figure 2 [see derivation of Sams Creek Formation sediment from the 570–560 Ma Catoctin footnote 2]). We therefore include the Marburg Formation and Prettyboy Schist Formation and related rift volcanic rocks (Southworth et al., 2009; Burton and in the Potomac terrane, making the Hyattstown thrust the boundary between Southworth, 2010). Southworth (1999) tied deposition of the Sams Creek and the Potomac and Westminster terranes (Fig. 2). This assignation defines the Catoctin Formations by showing matching major and trace element chemistry Piedmont terranes based on shared characteristics that were set at the time of of metabasalt from each formation. initial rock formation, meaning deposition for sedimentary rocks or crystalliza- Our interpretation that the Sams Creek Formation was deposited after tion for igneous rocks. We do not use attributes of the rocks that formed later, 580 Ma conflicts with the depositional age proposed by Graybill (2012) and during deformation or metamorphism, to group rock units into terranes. Graybill et al. (2012), who dated 1 spot in each of 18 zircon grains from a phyllite within the Wakefield Marble adjacent to the Sams Creek Formation and found 206Pb/207Pb ages ranging from 1321 ± 20 to 956 ± 32 Ma. Graybill (2012) Depositional Ages and Graybill et al. (2012) interpreted the phyllite to be a metamorphosed tuff and used the three youngest ages (964 ± 28, 961 ± 26, and 956 ± 32 Ma) to Potomac Terrane infer a depositional age of 970–950 Ma for the tuff. Graybill (2012) and Graybill et al. (2012) applied this depositional age to the entire Sams Creek Formation Maximum possible depositional ages for most Potomac terrane rocks range and Wakefield Marble. Deposition of the Sams Creek Formation at 970–950 Ma from 1030 to 980 Ma (Table 1). Although all Potomac terrane samples except cannot be reconciled with the new detrital zircon results from our northern sample 810001 from the Blockhouse Point domain of the Mather Gorge Forma- Sams Creek Formation sample. The most likely cause of the discrepancy is that tion yielded at least 1 analysis younger than 600 Ma, only 3 samples produced the phyllite collected by Graybill (2012) was actually a metasedimentary rock, at least 2 such grains with ages that overlap at the 1s level (Table 1; Supple- and the zircon grains in the rock were detrital. Nearly all well-studied tuffs con- mental Table 13). The Laurel (sample 808003) and northern Sykesville (sample tain little or no xenocrystic zircon (Brown and Fletcher, 1999; Reid and Coath, 909001) Formations accordingly have maximum possible depositional ages of 2000; Brown and Smith, 2004; Charlier et al., 2005; Simon and Reid, 2005; 530 and 550 Ma, respectively. The northern Mather Gorge Formation (sample Zhang et al., 2007; Simon et al., 2008; Zou et al., 2010; Tollo et al., 2012; but 1010002) has a maximum possible depositional age of 540 Ma. see Page and Laing, 1992), but 15 of 18 zircon grains from the Sams Creek putative metatuff crystallized tens or hundreds of millions of years before the inferred eruption age. Although such a large fraction of older zircon grains is Setters Formation, Westminster Terrane, and Sugarloaf Mountain rare for tuffs, it is common for sedimentary rocks to contain a large proportion Anticlinorium of detrital zircon grains that crystallized hundreds of millions of years before 3Supplemental Table 1. Uranium and lead isotopic deposition. The 1320–950 Ma age spectrum from the alleged metatuff closely data for the zircon grains from each sample. Stan- The maximum depositional ages obtained from the Setters Formation sam- matches the relative probability distributions of detrital zircon ages found in dard analyses are appended to the end of the analy ples are 1000 and 910 Ma, slightly younger than the 1040–1020 Ma maximum our Sugarloaf Mountain anticlinorium and Westminster terrane metasand- ses from each sample. Please visit http://dx.doi.org /10.1130/GES01140.S3 or the full-text article on www depositional ages for most of the formations in the Sugarloaf Mountain anti- stone samples, including the two Sams Creek metasandstones, as well as the .gsapubs.org to view Supplemental Table 1. clinorium and Westminster terrane (Table 1). Analyses from the northern Sams distribution of detrital zircon ages in a metasandstone within the Wakefield

Marble analyzed by Graybill et al. (2012). These comparisons with other tuffs Amazonia (plus Arequipa-Antofalla), Rio de la Plata, and West Africa cratons and sandstones support our suggestion that the zircon grains from the puta- (Pollock et al., 2012). Accreted peri-Gondwanan blocks along strike to the tive metatuff sample were actually detrital grains in a metasedimentary rock. north and south of the Maryland Piedmont were derived from the margins of Amazonia (Ganderia, Avalonia, and Carolinia) and West Africa (Meguma and Suwanee) (van Staal et al., 2009; Pollock et al., 2012). Thus it is possible Sykesville Formation Protolith that these Gondwanan cratons and/or peri-Gondwanan terranes, in addition to Ediacaran Laurentia, provided sediment to the Maryland Piedmont basins in Fleming and Self (2010) argued that the Sykesville and Laurel Formations, latest Neoproterozoic and early Paleozoic time. as well as the correlative Indian Run Formation in northern Virginia, are domi- Figure 6 shows that Ediacaran Laurentia contained igneous rocks with a wide nated by metamorphosed ignimbrites and that deposition of these formations range of crystallization ages, and zircon growth during metamorphism overlaps occurred adjacent to major silicic volcanic calderas. One of the main pieces and extends beyond the igneous crystallization ages. Three post-Archean gaps of evidence presented by Fleming and Self (2010) is their reinterpretation of stand out: 2500–2000 Ma was a period of little metamorphism or felsic magma- foliated, pebble-sized clasts with large aspect ratios as metamorphosed flat- tism in Laurentia except for the Wopmay orogen on the northwestern corner of tened pumice lapilli with flame structures (Fig. 4A). All of these clasts that we the continent (Hildebrand et al., 2010; Hoffman et al., 2011); 950–780 and 670– examined are dominantly muscovite (Fig. 4B). Features so rich in aluminum 580 Ma also were times of little metamorphism or felsic magmatism in Lau- and potassium are more consistent with an origin as mudstone clasts than as rentia. The only well-known Laurentian exceptions are two Iapetus rift-related dacitic or rhyolitic pyroclasts (e.g., Hess, 1989), supporting the conventional plutons in western Newfoundland that crystallized at 617 ± 8 and 602 ± 10 Ma interpretation that the Sykesville Formation is dominated by metasedimen- (Williams et al., 1985; van Berkel and Currie, 1988). The Long Range dikes of tary rocks. Furthermore, only 1 of 133 zircon grains from the Laurel Formation western Newfoundland and eastern Labrador also crystallized ca. 615 Ma, but sample (808003) and 1 of 329 zircon grains from the 2 Sykesville Formation they are dominantly mafic and contain little zircon (Kamo et al., 1989). Smith samples (909003, 909001) yielded U/Pb ages within 20 M.y. of the inferred (2003) reported a crystallization age of 602 ± 2 Ma for a felsite dike from the ignimbrite eruption age of 475–450 Ma, but nearly all well-studied tuffs are Reading Prong of eastern Pennsylvania. However, this age determination came dominated by zircon that crystallized within a few million years prior to erup- from thermal ionization mass spectrometry of a single mechanically abraded tion (references in previous subsection). In contrast, the distributions of zircon zircon grain, and multiple grains are required to ensure repeatability and thus ages in the Laurel and Sykesville Formations closely match the distributions a robust age. Furthermore, the volume of lithologically similar felsite in this re- in other samples from the Potomac terrane that are interpreted to be meta gion is small. Although the Goochland terrane in the central Virginia Piedmont sedimentary rocks, such as the Marburg and Mather Gorge Formations and contains several small granitic plutons that crystallized ca. 660–580 Ma (Owens the Prettyboy Schist (Supplemental Figure 2 [see footnote 2]; also see Horton and Tucker, 2003), it is unknown whether this tiny continental fragment was part et al., 2010). These comparisons with both tuffs and other metasedimentary of Laurentia by the beginning of the Cambrian Period. rocks in the Potomac terrane likewise support the conventional interpretation The Amazonia, Rio de la Plata, and West Africa cratons likewise contain that the protolith of the Sykesville Formation was sedimentary, not volcanic. igneous and metamorphic rocks with a wide range of Archean and Proterozoic crystallization ages, but important differences in their geologic history allow recognition of distinctive potential sediment sources. Unlike Laurentia, the Sources of Sediment Amazonia and Rio de la Plata cratons contain abundant felsic igneous rocks with crystallization ages between ca. 2250 and 2020 Ma (Rapela et al., 2007; Possible Source Continents: Laurentia, Amazonia, Rio de la Plata, Buenano Macambira et al., 2009; Brito Neves, 2011). The Goias magmatic and West Africa arc lies in the early Cambrian collision zone between Amazonia and the Sao Francisco craton to the southeast. Igneous crystallization ages in the Goias According to Li et al. (2008), the Amazonia and Rio de la Plata cratons arc range from ca. 930 to 610 Ma, with important peaks in tectonic activity were positioned against eastern Laurentia following the middle Neoprotero near 900 and 630 Ma (Laux et al., 2005; Moura et al., 2008; Matteini et al., zoic breakup of the supercontinent Rodinia. Because we do not address other 2010), although the Sao Francisco craton and superjacent Goias arc did not join blocks within Pannotia (Powell, 1995; Nance et al., 2014), we refer to this with Amazonia until ca. 550–510 Ma (Trindade et al., 2006; Moura et al., 2008; joined Laurentia–Amazonia–Rio de la Plata group of continental blocks as the McGee et al., 2012; Tohver et al., 2012). The Rio de la Plata craton is bounded by LAR group. Alternatively, only a sliver of the Amazonia craton, the Arequipa- rocks that record 850–750 Ma rifting and 650–600 Ma metamorphism (Rapela Antofalla block, may have remained adjacent to Laurentia after middle Neo- et al., 2011; Tohver et al., 2012). Post-Mesoarchean West Africa craton rocks proterozoic time (Escayola et al., 2011). Following latest Neoproterozoic to crystallized throughout the periods 2750–1750 and 760–550 Ma, with a notable earliest Cambrian rifting, the Iapetus Ocean separated Laurentia from the gap between ca. 1700 and 1000 Ma (summarized in Abati et al., 2010).

60°N 110°W ca. 550–500 Ma

Continental rift boundary 525–510 Ma granitoids ? (New Mexico, Colorado only)

Eastern rift deposits (780–670 and 580–530 Ma) N 1.3–1.0 Ga collisional orogens 42° and 1.3–0.95 Ga granitoids (Grenville and Llano Prov)

1.45–1.35 Ga granitoids Setters basin Western group 1.55–1.35 Ga juvenile crust (Granite-Rhyolite Prov)

1.65–1.60 Ga granitoids

1.72–1.68 Ga granitoids Potomac terrane

1.80–1.65 Ga juvenile crust 5 2 0 – 5 1 0 Ma d e t r i t u s? (Yavapai + Mazatzal Prov)

1.9–1.8 Ga reworked 500 km Archean crust

2.0–1.8 Ga juvenile crust Potomac sources Western group sources (except northern (Westminster terrane and Setters sources Mather Gorge Fm.) Sugarloaf Mountain anticlinorium) (symbol size indicates relative importance) >2.5 Ga crust

Figure 6. Map of major geologic provinces in Laurentia ca. 550–500 Ma showing possible ultimate source areas for the sediment of the western group, Potomac terrane, and the Setters basin on the Baltimore terrane. Ediacaran Laurentia could have supplied zircon with all of the age populations found in the metasedimentary rocks of these basins; there is no need to call on transport from another continent to explain the presence of any of the age groups. The arrows depict possible ultimate sources for the zircon in the Piedmont terrane metasedimentary rocks, not actual sediment distribution pathways at the time of deposition of the Piedmont rocks. For ease of illustration, only one symbol is shown for each possible source; detritus could have come from anywhere within the source region, indicated by color. The position of the Potomac terrane and sources for the Setters basin are both shown schematically for ease of illustration; they do not indicate preference for any of the models in Figure 8. The position of the rift-related rocks now found in the Blue Ridge Province is shown schematically to the east of their present-day location to account for westward transport during Paleozoic orogeny. Question marks associated with 525–510 Ma sources indicate uncertainty that these rocks provided sediment to the Potomac terrane because we found only two detrital zircon grains of this age, both in the Laurel Formation. Base map simplified from Whitmeyer and Karlstrom (2007). Rifting at 539–530 Ma in Oklahoma taken from Hanson et al. (2013); 525–510 Ma granitoids in New Mexico from Amato and Mack (2012) and in Colorado from Schoene and Bowring (2006). Map projection: spherical Transverse Mercator centered on the 100°W meridian. Fm—Formation; Prov—province.

Sediment Sources for Maryland Piedmont Rocks Setters Formation The discussion in the following subsections concerns the ultimate sources 2 samples, n = 270 of zircon in Piedmont metaclastic rocks, not actual sediment transport pathways at the time of deposition. That is, some detrital zircon grains in the Pied- Western group: Westminster mont rocks could have been stored as clasts in sedimentary rocks or xeno terrane plus Sugarloaf MA crysts in igneous rocks for a period between initial zircon crystallization and 5 samples, n = 720 subsequent deposition in Piedmont basins. All units except the northern Mather Gorge Formation. Figure 6 shows that Remainder of Potomac terrane 10 samples, n = 1313 known sources in southern and central Ediacaran Laurentia could have produced all 2303 detrital zircon grains analyzed from our 17 samples (not including the northern Mather Gorge Formation sample). The main age peak in all y Central Blue Ridge these samples is 1250–950 Ma. The ca. 1250–1020 Ma part of this range closely n = 912 matches the crystallization ages of the abundant felsic plutons that compose Southern Blue Ridge the Grenville Province now exposed in inliers directly to the west and the ca. n = 747 1050–950 Ma part corresponds to the age of the final Grenvillian metamorphism of these rocks (McLelland et al., 2010; Southworth et al., 2010; Tollo Northern Mather Gorge Formation et al., 2010, 2012). The northern Appalachian portion of the Grenville Province (1010002) n = 130 also experienced granitic intrusion between ca. 1050 and 950 Ma (Rivers, 1997; McLelland et al., 2010). In Potomac terrane rocks, there is another important age group between 1500 and 1300 Ma with a peak ca. 1470 Ma, but few analy Normalized relative probabilit Arequipa-Antofalla: Puncoviscana ses between 1500 and 1600 Ma. These ages closely match the crystallization n = 53 ages of both the Granite-Rhyolite Province rocks and the 1450–1350 Ma gran itoids that intruded much of central Laurentia through eastern Canada. The main age peaks in our Piedmont Province samples are similar to age signa- Carolinia: Aaron Formation tures from upper Neoproterozoic and lower Cambrian metasandstone depos- n = 47 ited on Blue Ridge Province Proterozoic rocks (Fig. 7; Carter et al., 2006; Hebert et al., 2010; Satkoski et al., 2012; Satkoski, 2013), supporting our interpretation Meguma: Upper Goldenville (Tancook) n = 64 that Ediacaran Laurentia supplied the Piedmont sediment (not including the northern Mather Gorge Formation sample). Avalonia: Redmans Formation Laurel Formation sample 808003 contained two grains that yielded 206Pb/238U n = 55 dates of 518 ± 10 and 511 ± 10 Ma. The uncertainties from these two low U/Th analyses overlap at the 1s level, suggesting that the dates could reflect predepo 400 600 1000 1400 1800 2200 2600 3000 sition crystallization ages rather than lead loss or metamorphic zircon growth Detrital zircon age (Ma) after deposition of the Laurel Formation (cf. Dickinson and Gehrels, 2009a). If Figure 7. Relative probability plots for ages from our samples compared to similar so, the possible sources for these zircon grains may be limited to central New plots for one sample from each of the peri-Gondwanan terranes Avalonia (Pollock Mexico or south-central Colorado (western United States). Felsic igneous rocks et al., 2009), Meguma (Waldron et al., 2009), Carolinia (Pollock et al., 2010), and with crystallization ages between 525 and 510 Ma are rare in southern Lauren- Arequipa-Antofalla (Escayola et al., 2011). Relative probability plots for a compilation of ages from basal Cambrian sandstone of the southern and central Blue tia, but granite and syenite of this age are exposed in central New Mexico and Ridge are also shown (Satkoski, 2013). The northern Mather Gorge Formation south-central Colorado (Schoene and Bowring, 2006; Amato and Mack, 2012). sample has ages that are unlike the remainder of the samples from the Maryland Spencer et al. (2014) found abundant ca. 520 Ma detrital zircon in the upper part Piedmont but that share many similarities with ages from the peri-Gondwanan terranes. Each curve was normalized for the number of ages to give an equal area of the Cambrian Van Horn Sandstone in nearby western Texas, supporting the under each curve using an Excel macro available from the Arizona LaserChron interpretation that the ca. 518 and 510 Ma grains in the Laurel Formation came Center. The published data were processed for concordance and use of 206Pb/207Pb from this part of Laurentia. Alternatively, these two young grains could have or 206Pb/238U ages to provide consistency with the data from our samples. MA— been derived from the ca. 580–530 Ma rift-related magmatic rocks in eastern Sugarloaf Mountain anticlinorium. The central Blue Ridge curve includes ages from Satkoski (2013) samples 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, and 25. Ediacaran Laurentia; the detrital grain ages could be slightly younger than the The southern Blue Ridge curve includes ages from Satkoski (2013) samples 56, 33, magmatic ages due to a small amount of lead loss from the detrital zircon. 23, 24, 35, 34, 39, 38, 37, 42, and 43.

Of the 2303 detrital zircon grains analyzed, only one yielded an age be- detritus in the northern Mather Gorge Formation but only 1% in the other tween 2600 and 2100 Ma: a grain from Prettyboy Schist sample 910002 gave a Maryland Piedmont rocks. (2) Unlike the rest of the Maryland Piedmont units, 206Pb/207Pb age of 2367 ± 74 Ma. Three other grains yielded ages in or near the northern Mather Gorge Formation detritus was not derived from Ediacaran range 2100–2000 Ma: 2025 ± 62 Ma from Laurel Formation sample 808003, Laurentia. The most likely source continent in this scenario is Amazonia. The 2005 ± 14 Ma from northern Sykesville Formation sample 909001, and 1994 ± 1700–1000 Ma zircon in the northern Mather Gorge Formation rules out West 20 Ma from southern Sykesville Formation sample 909003. Similarly, the 17 Africa as a sediment source, and the Rio de la Plata and other Gondwanan samples yielded only 12 of 2303 zircon grains that crystallized between 900 cratons were within or on another side of the Gondwana supercontinent, not and 600 Ma. The paucity of detrital zircon that crystallized in the periods 2600– on the margin bordering the Iapetus Ocean (Pollock et al., 2012). Amazonia 2000 and 900–600 Ma is compatible with derivation from Ediacaran Laurentia (Li et al., 2008) and Arequipa-Antofalla (Escayola et al., 2011) had separated but inconsistent with derivation of abundant sediment from Amazonia, Rio de from Laurentia by the time of deposition, so sediment transport alone cannot la Plata, or West Africa. The predominance of Mesoproterozoic zircon in all of explain the unusual detrital zircon age signature of the northern Mather Gorge our Piedmont samples rules out West Africa as a major source of sediment. Formation. Instead, if option 2 is correct, the northern Mather Gorge Formation Although the sediment for all the Piedmont rocks except the northern would be a fragment of an exotic terrane that originated near Amazonia and Mather Gorge Formation could have been derived from Ediacaran Lauren- was tectonically emplaced on Laurentia in the Paleozoic. tia, Potomac terrane sources within Ediacaran Laurentia were different than We favor scenario 2 for the following reasons. (1) The northern Mather the sediment sources to the Westminster terrane, Sugarloaf Mountain anti Gorge Formation zircon ages are very similar to detrital zircon age distribu- clinorium, and Setters Formation basins. The sources of detritus to these latter tions from some rocks from the known peri-Gondwanan terranes Avalonia, basins were nearly completely restricted to the Grenville Province and its over- Meguma, Carolinia, and Arequipa-Antofalla (Fig. 7). The distinctive features lying rift deposits. The only exceptions are two grains from Sugarloaf Moun- of the age distributions from all these samples are a dominant peak at 630– tain Quartzite sample 908001 that yielded 206Pb/207Pb ages of 1691 ± 44 and 600 Ma and the absence of a major peak at 1200–1000 Ma. (2) In southern and 1638 ± 66 Ma. In contrast, the ultimate sources for Potomac terrane sediment eastern Laurentia, 670–580 Ma felsic igneous rocks were rare (Cawood et al., were nearly every pre-Ordovician rock suite in southern and central Lauren- 2001; Whitmeyer and Karlstrom, 2007; Burton and Southworth, 2010), but they tia (Fig. 6), plus possibly the Wopmay orogen in northwestern Laurentia. The were common in West Gondwana (Laux et al., 2005; Moura et al., 2008; Abati detrital zircon ages from the Piedmont rocks also differ sharply from the age et al., 2010; Matteini et al., 2010; Rapela et al., 2011; Tohver et al., 2012). (3) The populations in Ediacaran and Cambrian sandstone deposited on the northern small-volume Laurentian felsic rocks that crystallized ca. 617 and 602 Ma can- margin of Laurentia, which commonly contain a prominent detrital zircon age not have been the source for the abundant older detrital zircon that crystallized peak ca. 1850 Ma (Beranek et al., 2013). at 680–630 Ma. Furthermore, rift metasandstones in western Newfoundland Northern Mather Gorge Formation. The distribution of detrital zircon ages with sediment sources partially in the 617–602 Ma granite bodies have their acquired from northern Mather Gorge Formation sample 1010002 is unlike the most prominent detrital zircon age peak at or before ca. 1000 Ma (Cawood and detrital zircon ages from all of our other Piedmont Province samples (Figs. 5 Nemchin, 2001), not ca. 630 Ma as for our northern Mather Gorge Formation and 7). Unlike the other samples, the northern Mather Gorge sample produced sample (Fig. 5). One Newfoundland rift deposit, the South Brook formation, a prominent peak in ages ca. 630 Ma and a nearly continuous distribution of was deposited unconformably on the 602 ± 10 Ma Round Pond granite, yet ages within the range 2650–520 Ma (including uncertainties), with wide gaps only 1 of 52 zircon grains from a metasandstone from the South Brook for- only between ca. 2450–2210 and 1700–1550 Ma. 52% of the grains from this mation yielded an age younger than 900 Ma (Cawood and Nemchin, 2001). sample crystallized in the range 800–500 Ma, whereas in our other Piedmont Because 617–602 Ma granite contributed only a small fraction to the total samples only 1% crystallized during this interval. These age disparities indi- zircon population in the proximal metasandstone samples, it is unlikely that cate very different sediment sources for the northern Mather Gorge Forma- these plutons would produce a major age peak in a sample 1500 km along tion compared to the remainder of the Maryland Piedmont. Two end-member strike. (4) For similar reasons, the Goochland terrane seems an unlikely source scenarios can explain the provenance differences. (1) Northern Mather Gorge for the ca. 630 Ma detrital zircon grains. Ca. 1050 Ma intrusions are the vol- Formation detritus was derived from Ediacaran Laurentia, but from different umetrically dominant Precambrian rocks in the Goochland terrane, and the parts of the continent than the sediment in the remainder of the Maryland areally most extensive Precambrian rock type, the ca. 1050 State Farm Gneiss, Piedmont rocks. The 2500–2000 Ma zircon in the northern Mather Gorge For- contains abundant zircon (Owens and Tucker, 2003). Therefore, we expect a mation came from the Wopmay orogen and the 800–500 Ma grains came from major age peak ca. 1050 Ma in zircon derived from the Goochland terrane, as rift-related rocks. The many detrital zircon ages that are outside the ranges of found in sediment largely derived from Blue Ridge Province Proterozoic rocks crystallization ages of rift-related igneous rocks reflect small magnitude lead (Fig. 7; Carter et al., 2006; Satkoski et al., 2012; Satkoski, 2013). loss from grains that actually crystallized at 780–670 or 580–530 Ma, or pos- If scenario 2 is correct, the northern Mather Gorge Formation is one of the sibly ca. 617–602 Ma. In this scenario, the rift rocks contributed ~50% of the first units in the mid-Atlantic sector of the Appalachian Piedmont identified as

having received sediment from a Gondwanan source (see also Bailey et al., that the northern Mather Gorge Formation is part of an exotic terrane. These 2008; Bosbyshell et al., 2012; Hughes et al., 2014). As such, it is one of the first provenance differences indicate that the sampled units in the Mather Gorge candidates for an exotic terrane in this region. Further data on the depositional Formation likely were not depositionally correlative and thus should not be age, provenance, and structural setting of this unit will aid understanding of its combined into a single formation. Kunk et al. (2005) reached the same conclu- tectonic significance. sion using muscovite 40Ar/39Ar ages from these rocks.

Comparison to Blue Ridge Cambrian Sandstone and Tectonic Setting Tempo of Transition from Rifting to Subduction During Deposition Zircon U/Pb dating over the past ~12 years has illuminated an important U/Pb dating of detrital zircon from late Neoproterozoic and early Cambrian difference in the timing of subduction initiation following breakup of Pangea sandstone throughout the Blue Ridge Province shows that zircon ages from versus the LAR group. In the mid-Atlantic region, rifting of Pangea began in central Blue Ridge basins in Maryland, West Virginia, and northern Virginia are the Late Triassic Epoch (ca. 220 Ma) and basalt erupted in latest Triassic and similar to our western group and Setters Formation spectra, whereas zircon earliest Jurassic time (ca. 200 Ma; Manspeizer et al., 1989; Weems and Olsen, ages from southern Blue Ridge basins are like our Potomac terrane ages (not 1997; Blackburn et al., 2013). Subduction has not yet started at this latitude, so including the northern Mather Gorge Formation; Fig. 7; Carter et al., 2006; the interval between the end of rifting and the beginning of subduction is at Hebert et al., 2010; Satkoski et al., 2012; Satkoski, 2013). Figure 8 shows three least 200 M.y. In contrast, flood basalt in the Catoctin Formation records final end-member models to explain the different sediment sources and deposi- rifting of this portion of the LAR group ca. 570–560 Ma (Southworth et al., 2009; tional settings for the latest Neoproterozoic to Cambrian basins of the Blue Burton and Southworth, 2010) and arc-related granitoids intruded the Sykes- Ridge and Maryland Piedmont. Figure 8A shows deposition of the Piedmont ville Formation at 478 ± 6 and 472 ± 4 Ma (Drake and Fleming, 1994; Aleinikoff terranes across strike of one another but at different times to allow different et al., 2002), giving a maximum interval of 90 M.y. between rifting cessation sediment sources. Figure 8B depicts deposition at approximately the same and subduction initiation. time but with the Potomac terrane along strike from the other basins, not di- The Potomac terrane metasedimentary rocks offer the opportunity to fur- rectly outboard of the western group. This option implies later along-strike ther refine constraints on the pace of this transition because the Mather Gorge, translation of the Potomac terrane to insert it between the Westminster and Sykesville, and Laurel Formations likely were sourced from Ediacaran Lauren- Baltimore terranes. A challenge to both models is the transport of sediment tia and deposited in an oceanic trench or forearc basin. If these rocks were into relatively deep water past the continental shelf and then into the shal- deposited in Laurentian suprasubduction zone basins, a tighter constraint on low water of the Setters basin on the Baltimore terrane. Figure 8C presents a the age of subduction initiation comes from their depositional ages than from solution to this problem by positing eastern sources for at least some Setters the crystallization ages of the granitoids that intruded them. Our Laurel Forma- detritus. This option also explains the minor differences between the Setters tion sample (808003) yielded one 518 ± 10 and one 511 ± 10 Ma zircon, and it is Formation and western group detrital zircon age spectra: these sets of detrital possible that these grains reflect detrital input into the Laurel Formation rather zircon ages are broadly similar but differ in detail because different blocks of than lead loss or metamorphic zircon growth. In many cases on convergent crust of similar age produced the sediment for the two basins. margins the crystallization age of the youngest detrital zircon is near the true depositional age (e.g., Grove et al., 2008; Dickinson and Gehrels, 2009a), suggesting that the period of transition from the end of rifting to the beginning of Status of the Mather Gorge Formation subduction actually may have lasted only 50 M.y. Whether or not this last speculation is correct, it is clear that the tempo of The three Mather Gorge Formation samples yielded disparate detrital zir- the transition from the cessation of rifting to subduction initiation at the study con age spectra (Supplemental Figure 2 [see footnote 2]). The Bear Island do- latitude was much faster following breakup of the LAR group than Pangea (see main ages are nearly identical to the ages from Potomac terrane rocks outside also Waldron et al., 2014). Furthermore, the post-LAR group transition in east- the Mather Gorge Formation. The Blockhouse Point domain age distribution is ern Laurentia at the study latitude is among the quickest known examples of similar to that from the Bear Island domain in that both contain an age peak conversion to subduction following rifting in the Neoproterozoic Era (Bradley, near 1450 Ma and a continuous distribution of ages between ca. 1700 and 900, 2008). The unusual celerity of the eastern Laurentia case suggests a tectonic but the spectra are dissimilar in that the Blockhouse Point domain sample did situation that was not present in most Neoproterozoic–early Paleozoic settings not yield ages outside this range. As described herein, detrital zircon from the around the globe. More rapid subduction initiation in eastern Laurentia fol- northern Mather Gorge Formation produced an age spectrum that is so unlike lowing Neoproterozoic compared to Late Triassic rifting also is opposite the the age distributions from all other Maryland Piedmont rocks that we conclude global trend of slower transitions to subduction in the Proterozoic compared

central Blue Ridge Time 1 A: Different deposition times central Blue Ridge B: Different deposition locations basins basins Western group Baltimore terrane Grenville basin (shallow) Province Grenville sediment Western group Province sources Baltimore terrane sediment basin (shallow) sources southern Blue Ridge basins CONTINENTAL Potomac terrane SHORELINE SHELF EDGE distally- sourced Time 2 detritus

Grenville Province southern Blue Ridge Potomac sediment basins terrane sources N

distally- sourced detritus

central Blue Ridge Time 1 C: Eastern sediment source basins

Grenville Province sediment Western group eastern sources Baltimore terrane basin (shallow) sediment source

CONTINENTAL SHORELINE SHELF EDGE Time 2

Grenville Province southern Blue Ridge Potomac sediment basins terrane sources

distally- sourced detritus

Figure 8. Three end-member explanations for the different zircon populations in the Blue Ridge and Piedmont terranes. These alternatives are not mutually exclusive. (A) The Potomac terrane was deposited spatially between the Westminster and Baltimore terranes, but at a different time. The different times of deposition allow different sediment sources to the basins. (B) The southern Blue Ridge rocks and the Potomac terrane were deposited along strike of and at approximately the same time as the central Blue Ridge rocks, the Western group, and the Setters Formation in the Baltimore terrane. The separate along-strike positions allow different sediment sources to the terranes. A challenge for both models is transport of sediment from the continent past the deep water of the continental slope and rise and then up into the shallow water of the Baltimore terrane. (C) To address this issue, the sources for the Baltimore terrane basin are located in the Baltimore terrane itself and in rifted Grenvillian crust to the east that is not currently exposed. The western group includes the Sugarloaf Mountain anticlinorium and the Westminster terrane. Options for Potomac terrane deposition do not consider the northern Mather Gorge Formation.

to the Phanerozoic eons (Bradley, 2008). Better timing constraints on Paleozoic and Kramers (1975). Uncertainties of 1.5 for 206Pb/204Pb and 0.3 for 207Pb/204Pb were applied to these events recorded in Piedmont rocks in Maryland and beyond are one avenue lead composition values based on the variation in lead isotopic composition in modern crustal rocks. These corrections and other data reduction were performed off-line using an Excel program that may lead toward a fuller understanding of these differences, and insights developed at the Arizona LaserChron Center. We removed from further consideration analyses on their causes will contribute to our knowledge of the processes involved in with: (1) high 204Pb, (2) low 206Pb/204Pb ratio, (3) >5% error on the 206Pb/207Pb date, (4) >10% error on 206 238 subduction initiation. the Pb/ U date, (5) >25% normal discordance or 5% reverse discordance, (6) high U concentration, or (7) high U/Th ratio. We use the remaining analyses for our interpretations (Supplemental Table 1 [see footnote 3]; Figs. 5 and 7; Supplemental Fig. 2 [see footnote 2]). We used Isoplot to calculate weighted means and to produce concordia plots (Ludwig, 2008). ACKNOWLEDGMENTS 206Pb/238U dates usually are more precise than 206Pb/207Pb dates for zircon younger than ca. Philip Piccoli enabled our use of the electron microprobe. We acknowledge the support of the 1.4 Ga, whereas the reverse is true for older grains. However, 206Pb/207Pb dates are only minimally Maryland NanoCenter and its NispLab (Nanoscale Imaging, Spectroscopy, and Properties Labora- affected by recent lead loss, so in most cases they more closely indicate the time of crystallization tory), which is supported in part by the National Science Foundation (NSF) as a MRSEC (Materials for zircon older than ca. 1 Ga. Thus during interpretation it generally is preferable to use 206Pb/238U Research Science and Engineering Center) shared experimental facility. We thank George Gehrels, dates for grains younger than 1 Ga and 206Pb/207Pb dates for older zircon. However, almost all of Victor Valencia, Mark Pecha, and the staff of the Arizona LaserChron Center for facilitating our zir- our samples contain zircon with a range of ages that spans this 1 Ga preferred cutoff value. Ac- con analyses in their laboratory. The LaserChron Center is supported by NSF grant EAR-1032156. cordingly, we used 700 Ma as the cutoff in order to avoid an artificial break in the interpreted ages. Irene Kadel-Harder and Rebecca Ohly provided able laboratory assistance. We thank Steven Whit- meyer for generously providing the digital files to produce Figure 6. 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