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 Neo protero 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 Pretty boy 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 © 2015 Geological Society of America 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 N 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). Meso protero 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 inter pre 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