Detrital Zircon Geochronology Across the Chopawamsic

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Geoscience Canada Journal of the Geological Association of Canada Journal de l’Association Géologique du Canada

Detrital Zircon Geochronology Across the Chopawamsic Fault, Western Piedmont of North-Central Virginia: Implications for the Main Iapetan Suture in the Southern Appalachian Orogen K. Stephen Hughes, James P. Hibbard, Jeffrey C. Pollock, David J. Lewis and Brent V. Miller

Volume 41, Number 4, 2014 Article abstract The Chopawamsic fault potentially represents the main Iapetan suture, URI: https://id.erudit.org/iderudit/1062257ar previously unidentified in the southern extent of the Appalachian orogen. The DOI: https://doi.org/10.12789/geocanj.2014.41.052 fault trends through the north-central portion of the western Piedmont of Virginia and separates the composite metaclastic Potomac terrane, commonly See table of contents interpreted to be of Laurentian affinity, from the Chopawamsic terrane, the remains of a Middle Ordovician volcanic arc of uncertain crustal affinity. To gain insight on the first-order orogenic significance of the Chopawamsic fault, Publisher(s) we report the results of LA–ICP–MS U–Pb analyses of 1,289 detrital zircons from 13 metasedimentary rock samples collected from both sides of the fault. The Geological Association of Canada The near exclusivity of Middle Ordovician zircon grains (ca. 470 – 460 Ma) in four sampled metasedimentary rocks of the Chopawamsic Formation likely ISSN represents the detrital recycling of syndepositional Chopawamsic volcanic rocks. A subset of Cambrian and older grains hint at one or more additional, 0315-0941 (print) older sources. Samples from the Potomac terrane include mostly 1911-4850 (digital) Mesoproterozoic zircon grains and these results are consistent with previous interpretations that the metaclastic rocks are Laurentian-derived. The Explore this journal youngest zircons (ca. 550 – 500 Ma) and the age of cross-cutting plutons indicate that at least some parts of the Potomac terrane are Late Cambrian – Early Ordovician. The results imply temporally discrete and geographically Cite this article isolated sedimentary systems during deposition of sedimentary rocks in the Chopawamsic and Potomac terranes. Metasedimentary rocks near Storck, Hughes, K., Hibbard, J., Pollock, J., Lewis, D. & Miller, B. (2014). Detrital Zircon Virginia, previously identified as a successor basin, contain detrital zircon Geochronology Across the Chopawamsic Fault, Western Piedmont of populations that indicate they are actually peri-Gondwanan derived North-Central Virginia: Implications for the Main Iapetan Suture in the metasedimentary rocks unrelated to a successor basin system; their geographic Southern Appalachian Orogen. Geoscience Canada, 41(4), 503–522. position between the Laurentian-derived Potomac terrane and the https://doi.org/10.12789/geocanj.2014.41.052 Chopawamsic terrane suggests a peri-Gondwanan affinity for the Chopawamsic arc and geographic separation of the Chopawamsic and Potomac terranes in the Middle Ordovician. Consequently, we tentatively support the hypothesis that the Chopawamsic fault system represents the main Iapetan suture in the southern Appalachian orogen. Most detrital zircons from samples of the Arvonia successor basin crystallized in the Ordovician—Silurian or Mesoproterozoic. These data suggest that the Arvonia basin was deposited in the latest Ordovician to Early Silurian only after the Late Ordovician accretion of the Chopawamsic arc to Laurentia.

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HAROLD WILLIAMS SERIES

Texas A&M University sedimentary systems during deposition College Station, Texas, 77843, USA of sedimentary rocks in the Chopawamsic and Potomac terranes. SUMMARY Metasedimentary rocks near The Chopawamsic fault potentially Storck, Virginia, previously identified represents the main Iapetan suture, as a successor basin, contain detrital previously unidentified in the southern zircon populations that indicate they extent of the Appalachian orogen. are actually peri-Gondwanan derived The fault trends through the north- metasedimentary rocks unrelated to a central portion of the western Pied- successor basin system; their geograph- mont of Virginia and separates the ic position between the Laurentian- Detrital Zircon composite metaclastic Potomac ter- derived Potomac terrane and the rane, commonly interpreted to be of Chopawamsic terrane suggests a peri- Geochronology Across the Laurentian affinity, from the Chopa- Gondwanan affinity for the Chopa- Chopawamsic Fault, wamsic terrane, the remains of a Mid- wamsic arc and geographic separation dle Ordovician volcanic arc of uncer- of the Chopawamsic and Potomac ter- Western Piedmont of tain crustal affinity. To gain insight on ranes in the Middle Ordovician. Con- North-Central Virginia: the first-order orogenic significance of sequently, we tentatively support the the Chopawamsic fault, we report the hypothesis that the Chopawamsic fault Implications for the Main results of LA–ICP–MS U–Pb analyses system represents the main Iapetan Iapetan Suture in the of 1,289 detrital zircons from 13 suture in the southern Appalachian Southern Appalachian metasedimentary rock samples collect- orogen. ed from both sides of the fault. Most detrital zircons from Orogen The near exclusivity of Middle samples of the Arvonia successor Ordovician zircon grains (ca. 470 – basin crystallized in the Ordovician – K. Stephen Hughes1*, James P. 460 Ma) in four sampled metasedimen- Silurian or Mesoproterozoic. These Hibbard1, Jeffrey C. Pollock2, David tary rocks of the Chopawamsic For- data suggest that the Arvonia basin J. Lewis3, and Brent V. Miller3 mation likely represents the detrital was deposited in the latest Ordovician recycling of syndepositional Chopa- to Early Silurian only after the Late 1Department of Marine, Earth, and wamsic volcanic rocks. A subset of Ordovician accretion of the Atmospheric Sciences Cambrian and older grains hint at one Chopawamsic arc to Laurentia. North Carolina State University or more additional, older sources. Raleigh, North Carolina, 27695, USA Samples from the Potomac SOMMAIRE terrane include mostly Mesoprotero- La faille de Chopawamsic représente *Department of Geology zoic zircon grains and these results are peut-être la principale suture japéti- University of Puerto Rico—Mayagüez consistent with previous interpreta- enne, non-reconnue dans prolonge- Mayagüez, Puerto Rico, 00681, USA tions that the metaclastic rocks are ment sud de l’orogène des Appalaches. E-mail: [email protected] Laurentian-derived. The youngest zir- La faille traverse la portion nord du cons (ca. 550 – 500 Ma) and the age of centre du piedmont ouest de Virginie 2Department of Earth Sciences cross-cutting plutons indicate that at et sépare le terrane métaclastique de Mount Royal University least some parts of the Potomac ter- Potomac, d’affinité laurentienne pen- Calgary, Alberta, T3E 6K6, Canada rane are Late Cambrian – Early Ordo- sait-on, du terrane de Chopawamsic, vician. The results imply temporally vestige d’un arc volcanique de l’Or- 3Department of Geology and Geophysics discrete and geographically isolated dovicien moyen d’affinité crustale

Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.052 © 2014 GAC/AGC® 504 incertain. Afin de mettre en lumière la géographiquement durant le dépôt des INTRODUCTION signification orogénique première de la roches sédimentaires dans les terranes The Chopawamsic fault is a Late faille de Chopawamsic, nous présen- de Chopawamsic et de Potomac. Ordovician structure that bisects the tons les résultats d’analyses U-Pb par Les roches métasédimentaires western Piedmont of Virginia into two ICP–MS par AL sur 1 289 zircons près de Storck en Virginie, jadis inter- distinct crustal tracts known as the détritiques provenant de 13 échantil- prétées comme bassin successeur, ren- Potomac and Chopawamsic terranes lons de roches métasédimentaires ferment des populations de zircons (Fig. 1; Pavlides 1989, 1990, 1995; Hor- prélevés de chaque côté de la faille. détritiques qui indiquent qu’ils provien- ton et al. 1989; Virginia Division of L’existence quasi-exclusive de nent en fait de roches métasédimen- Mineral Resources 1993; Pavlides et al. grains de zircon de l’Ordovicien moyen taires péri-gondwaniennes sans rapport 1994; Mixon et al. 2000, 2005; Hughes (env. 470 – 460 Ma) dans quatre roches avec un système de bassin successeur; et al. 2013a; Hibbard et al. 2014). métasédimentaires de la Formation de leur localisation géographique entre le Whereas the majority of previous Chopawamsic représente vraisem- terrane de Potomac issu des Lauren- research involving the Chopawamsic blablement le recyclage détritique des tides et le terrane de Chopawamsic fault has been focused on identifying roches volcaniques synsédimentaires de porte à penser que l’arc de Chopawam- the local feature and its timing Chopawamsic. Un sous-ensemble de sic est d’affinité péri-gondwanienne, et (Pavlides 1989, 1990, 1995; Mixon et grains cambriens et plus anciens, que les terranes de Chopawamsic et de al. 2000, 2005; Hughes et al. 2013a), lit- évoque l’existence d’une ou plusieurs Potomac à l’Ordovicien moyen étaient tle has been done to ascertain its sources plus anciennes additionnelles. séparés géographiquement. En con- regional significance within the Les échantillons du terrane de séquence il nous semble justifié de pro- Appalachian orogen. Potomac renferment principalement poser que le système de faille de The Chopawamsic fault is of des grains de zircon du Mésoprotéro- Chopawamsic représente la principale broad significance because it potential- zoïque, ce qui correspond avec les suture japétienne dans le sud de ly marks the orogen-scale main Iapetan interprétations antérieures voulant que l’orogène des Appalaches. suture, the fundamental Appalachian les roches métaclastiques soient d’orig- La plupart des zircons détri- boundary between native Laurentian ine laurentienne. Les zircons les plus tiques des échantillons du bassin suc- and exotic peri-Gondwanan elements jeunes (env. 550 – 500 Ma) ainsi que cesseur d’Arvonia ont cristallisés entre (Hibbard et al. 2007, 2014). It is gen- l’âge des plutons qui recoupe l’encais- l’Ordovicien et le Silurien ou au Méso- erally accepted that the early Paleozoic sant indiquent qu’au moins certaines protérozoïque. Ces données suggèrent Potomac terrane represents Laurent- parties du terrane de Potomac sont de que le bassin d’Arvonia s’est rempli de ian-derived metaclastic rocks (e.g. la fin du Cambrien ou du début de la fin entre l’Ordovicien et le début du Pavlides 1989; Hibbard et al. 2014), but l’Ordovicien. Les résultats impliquent Silurien, seulement après l’accrétion de the cratonic affinity of the Middle l’existence de systèmes sédimentaires l’arc de Chopawamsic à la Laurentie, à Ordovician Chopawamsic terrane has distincts au cours du temps, et isolés la fin de l’Ordovicien. not been determined. Most previous Virginia Western Piedmont Mesozoic Culpeper and Danville basins and related rocks Ordovician to Early Silurian intrusive bodies. Area of Ordovician rocks of the Chopawamsic Cf Figure 2 C M Terrane (C) and Milton Terrane (M). P Cambrian to Ordovician rocks of the Potomac

P SRA Terrane (P) and Smith River Allochthon (SRA). C

R Valley and RidgeBlue Ridge

SRA M E. Piedmont Coastal Plain

Figure 1. Lithotectonic elements of the western Piedmont of Virginia. Map modified from Hibbard et al. 2006. Cf = Chopawamsic fault; R = Richmond. GEOSCIENCE CANADA Volume 41 2014 505 workers have coupled the Chopawam- wanan affinities in the southeast orite (444 ± 4 Ma, Hughes et al. 2013a) sic terrane arc rocks with the Potomac (Williams et al. 1999; Zagorevski et al. it can be deduced that the fault was terrane accretionary rocks as part of a 2008). Because first order latitudinal active between 465 and 444 Ma. peri-Laurentian system in tectonic differences in the evolution of the oro- Recognition of the Chopawamsic fault models due to their current geographic gen have been recognized (Sinha et al. as a latest Middle to Late Ordovician positions and the interpretation that 1996; Sinha and McLelland 1999; structure is important in order to dis- some sedimentary rocks and ‘exotic Loewy et al. 2003; Tohver et al. 2004; tinguish it from common younger blocks’ in the Potomac terrane were Hibbard et al. 2007, 2010; Fisher et al. faults in the region (e.g. Gates 1986, derived from the Chopawamsic terrane 2010; McLelland et al. 2010; Hibbard 1997; Pavlides et al. 1994; Pavlides (Drake and Morgan 1981; Pavlides and Karabinos 2014), identifying the 2000; Spears and Bailey 2002; Bailey et 1989; Hibbard et al. 2014). This inter- timing and style of the main Iapetan al. 2004; Spears et al. 2004; Carter et al. pretation implies that the main Iapetan suture zone in the southern Appalachi- 2006; Spears 2010; Spears and Gilmer suture likely lies to the east of the ans is important to further our under- 2012). Chopa-wamsic terrane. Other standing of the Iapetan cycle along the In the study area, the meta- researchers have interpreted the entire length of the orogen, rather clastic Potomac terrane is comprised of Chopawamsic terrane to be peri-Gond- than only in the Canadian Appalachi- the Mine Run Complex and the Lunga wanan, implying that the Chopawamsic ans (e.g. Williams et al. 1988; Zagorevs- Reservoir Formation (Fig. 2). These fault between the Potomac and ki et al. 2006, 2007a, b; Zagorevski and units, along with their correlatives Chopawamsic terranes is the main van Staal 2011). along strike, have been interpreted as a Iapetan suture (Hibbard et al. 2007). collection of metasedimentary rocks In order to garner information GEOLOGIC SETTING deposited offshore of Laurentia in the about the provenance and timing of The western Piedmont of Virginia early Paleozoic (Evans 1984; Drake sedimentary dispersal systems active (Fig. 2) is chiefly comprised of the 1989; Pavlides 1989; Horton et al. during deposition of the Potomac and Potomac and Chopawamsic terranes. 1989, 2010; Carter et al. 2006; Bailey et Chopawamsic terranes, we present the The two terranes have been intruded al. 2008; Hibbard et al. 2014). Detailed results of laser ablation—inductively by various igneous bodies and are also thermochronology in the terrane along coupled plasma—mass spectrometry overlain by younger sedimentary units. and near the Potomac River (Kunk et (LA–ICP–MS) U–Pb analyses of 1,289 The western Piedmont is bordered to al. 2005; Wintsch et al. 2010) has detrital zircon grains from 13 metasedi- the west by the Blue Ridge province emphasized the composite nature of mentary samples on both sides of the and Mesozoic Culpeper basin; it is bor- the terrane. The Mine Run Complex Chopawamsic fault, as well as purport- dered to the east by the eastern Pied- includes 4 greenschist facies sub-units ed younger successor basins. The mont and Atlantic coastal plain (I–IV) of mostly folded phyllite and results clarify the supra-crustal relation- provinces. lesser metagreywacke that are identified ships between sedimentary units in the The Chopawamsic fault is the on the basis of slight compositional early Paleozoic and augment our most important structure in the west- variation and airborne magnetic prop- understanding of the location of the ern Piedmont because it marks the erties (Pavlides 1989). Pavlides (1989) main Iapetan suture in the southern boundary between the two primary interpreted numerous granitoid, felsic Appalachians. crustal tracts in the domain (Hibbard volcanic, mafic, and ultra-mafic blocks This contribution is particular- et al. 2014). Studies focused specifical- within the Mine Run Complex to rep- ly relevant to this volume dedicated to ly on the Chopawamsic fault have been resent exotic debris that were derived Hank Williams. It has been 50 years limited, although it has long been from a nearby, syn-depositional since Williams recognized the two- known that the fault, marked by Chopawamsic arc system. The Lunga sided geologic nature of the “steeply dipping mylonite at a number Reservoir Formation, which was origi- Appalachian orogen (Williams 1964) of places,” (Pavlides 1989, 2000) sepa- nally mapped as a granite body (Lons- and 25 years since his identification of rates two distinct tracts of bedrock dale 1927; Milici et al. 1963), was later the fundamental boundary between (Pavlides 1981, 1989, 1990, 1995, recognized as a metadiamictite native Laurentian and exotic peri- 2000). The Chopawamsic fault has (Pavlides 1989). This metamorphosed Gondwanan elements in Newfound- also been identified as a crustal-scale immature sedimentary rock most com- land. This boundary, termed the Red structure in seismic profile (Harris et monly consists of poorly sorted peb- Indian Line, was defined on the al. 1982, 1986; Pratt et al. 1988; Glover ble- to cobble-size vein quartz ‘lumps,’ marked stratigraphic, structural, faunal, 1989; Pratt 2012), as well as in other dark schist ‘chips,’ and granitoid clasts and isotopic contrasts in Ordovician– regional mapping (Brown 1979; Duke in a fine-grained quartz-feldspar-mus- Silurian Iapetan realm rocks in central 1983; Evans 1984; Wehr and Glover covite-biotite matrix. Both the Mine Newfoundland (Williams et al. 1988). 1985; Brown 1986; Marr 1990; Hughes Run Complex and the Lunga Reservoir The Red Indian Line is accepted as the 2011). Based upon the youngest high- Formation were previously thought to main suture zone in the northern precision volcanic crystallization TIMS have been deposited between the Lau- Appalachian orogen in that it separates age of the fault-bounded Chopawam- rentian continent and an accreting, peri-Laurentian ophiolitic sequences in sic Formation (465 ± 1 Ma, Hughes et peri-Laurentian Chopawamsic arc the northwest from arc-related volcanic al. 2013b) and the relationship and age (Pavlides 1989; Hibbard et al. 2014). and sedimentary rocks with Gond- of the cross-cutting Ellisville granodi- The only age controls for the Mine 506

A) KSH-11-01, Mine Run Complex Unit III 77°45'W 77°30'W B) KSH-11-05, Mine Run Complex Unit I C) KSH-11-08, Mine Run Complex Unit IV OЄlr G D) P310-4, Mine Run Complex Unit I Rappahannock Quantico E) KSH-11-18, Lunga Reservoir Formation ± Og E 38°30'N F J F) KSH-11-16, Lunga Reservoir Formation Ogc OЄsg OЄup G) KSH-11-19, Chopawamsic Formation M H) KSH-11-28, Chopawamsic Formation Orr OЄsq I) KSH-11-39, Chopawamsic Formation A J) KSH-12-70, Chopawamsic Formation Rapidan faultOhr D Ssc K) BREMO, Bremo Mbr, Arvonia Formation fault L) KSH-11-BUF, Buffards Formation B Fredericksburg Run M) KSH-11-40, Storck micaceous quartzite Olg Oc Mountain

38°15'N OЄmIII fault OЄmI Spotsylvania Ol Branch OЄmII OЄmIV one North SOq z km Anna H 0 5 10 15 20 SOe Lake Anna Long Successor Basin System SOgs Louisa Mineral Sb SOq SOa 38°N C Oc

wamsic I fault Rivanna Chopawamsic terrane Chopa ? Oc Ogc Ohr Orr shear Palmyra Church Potomac terrane ? fault South Anna OЄlr OЄmI-IV ork F x te belt e ti

gma Compl SOc Little Pe fault OЄsm 37°45'N sedimentary rocks Columbia Hill SOa Elk es Sdm SOc SOe SOgs Ssc r K Spotsylvania Sho Og Olg Ol dm

S Arvonia L James OЄsg OЄsm OЄsq OЄup

Lakeside Sb 78°W 78°30'W 78°15'W

Figure 2. Geologic map of the study area. Red units are felsic intrusive bodies. Purple units are mafic intrusive bodies. Detrital zircon samples are listed from A) to M) and their locations are shown on map. Geology modified from Pavlides 1990, 1995; Virginia Division of Mineral Resources 1993; Mixon et al. 2000; Spears and Bailey 2002; Bailey et al. 2005; Hughes 2011; Spears et al. 2013; Terblanche 2013; and our reconnaissance mapping. Map unit abbreviations: Arvonia/Quantico successor basin system: Sb—Buffards Formation, SOa—Arvonia Formation, SOq—Quantico Formation. Rocks not related to a terrane: Og— Goldvein pluton, Ol—Lahore pluton, Olg—Locust Grove pluton, OЄup—unassigned phyllite, Sdm—Diana Mills body, SOc— Columbia pluton, SOe—Ellisville pluton, SOgs—Green Springs intrusive suite, Ssc—Salem Church complex, OЄsg—Storck granitoid, OЄsm—Shores mélange, OЄsq—Storck quartzite. Chopawamsic arc terrane: Oc—Chopawamsic Formation, Ogc— Garrisonville mafic complex, Ohr—Hunting Run pluton, Orr—Richland Run pluton; Potomac metasedimentary terrane: OЄlr— Lunga Reservoir Formation, OЄmI–IV—Mine Run complex units I–IV. GEOSCIENCE CANADA Volume 41 2014 507

Run Complex and Lunga Reservoir general paleontological consensus is western Piedmont of Virginia have Formation are provided by U–Pb zir- that they are Late Ordovician deposits been limited to the Arvonia/Quantico con data from plutonic bodies that (Darton 1892; Dale 1906; Watson and successor basin system and rocks cor- cross-cut its metaclastic units. Among Powell 1911; Stose and Stose 1948; relative to the Potomac terrane with no others, these include the ca. 472 Ma Smith et al. 1964; Brown 1969; Tillman samples taken from the Chopawamsic Occoquan pluton, the ca. 456 Ma 1970; Pavlides 1980; Pavlides et al. terrane. These previous studies have Goldvein pluton, and the ca. 444 Ma 1980; Kolata and Pavlides 1986; Hib- not focused on the specific significance Ellisville pluton (Wilson 2001; bard et al. 2014). The Late Ordovician of the Chopawamsic fault; however, Aleinikoff et al. 2002; Hughes et al. interpretation of these fossils appears some results from these investigations 2013a). to be at odds with the unconformable are pertinent to this study. The Chopawamsic Formation relationship of the Arvonia basin over Metasedimentary samples (Southwick et al. 1971) is the primary the ca. 444 Ma (latest Ordovician) within the Potomac terrane, to the component of the Chopawamsic ter- Carysbrook phase (U–Pb SIMS zircon: north of our focus area, yielded mostly rane. Named after exposures along Sinha et al. 2012) of the Columbia plu- Mesoproterozoic detrital ages (Horton Chopawamsic Creek in northern Vir- ton, which has been shown to be geo- et al. 2010; Martin et al. 2013; Bos- ginia, greenschist-facies metavolcanic graphically and geochemically linked to byshell et al. 2013) and indicate that and metavolcaniclastic rocks of the the ca. 444 Ma Ellisville pluton (Hop- these parts of the Potomac terrane Chopawamsic Formation have been kins 1960; Milici et al. 1963; Smith et were deposited adjacent to an older shown to extend into central Virginia al. 1964; Good et al. 1977; Duke 1983; continental margin. Most of these (Pavlides et al. 1974; Marr 1980a, b; Spears and Bailey 2002; Hughes et al. data, when considered with the deposi- Pavlides 1990; Bailey et al. 2005). Mul- 2013a). Because the Ellisville pluton tional interlayering of Potomac terrane tiple samples of Chopawamsic mag- stitches the Potomac and Chopawam- rocks and those of the Blue Ridge matic rocks have been dated to have sic terranes, the Arvonia basin could province (Evans 1984), are consistent crystallized between 474 and 465 Ma have only been deposited after the jux- with a Laurentian source for the (U–Pb TIMS on zircon: Coler et al. taposition of the Chopawamsic and Potomac terrane. In contrast, detrital 2000; Hughes et al. 2013b). On the Potomac terranes. With all data con- zircons from the Shores mélange (Fig. basis of these zircon ages, xenocryst sidered, it appears that the 2), which some geologists consider a ages, and an evolved isotopic signature Arvonia/Quantico system was deposit- part of the Potomac terrane, include a (Coler et al. 2000), the Chopawamsic ed in the latest Ordovician (fossil ages) population of early Mesoproterozoic terrane has been interpreted to repre- to earliest Silurian. Using major ele- (1.55 – 1.50 Ga) zircons that may be sent a Middle–Late Ordovician supra- ment and isotope geochemistry, Owens indicative of an Amazonian source subduction magmatic arc that devel- et al. (2013) showed that the Arvonia exotic to Laurentia (Bailey et al. 2008). oped on Mesoproterozoic continental basin was similar to post-450 Ma The Smith River allochthon (see Fig. crust (Pavlides 1981; Coler et al. 2000; deposits elsewhere in the orogen and 1), part of the overall metaclastic tract Hibbard et al. 2014). could have been derived from either in the western Piedmont that includes The Arvonia/Quantico suc- Laurentian, Chopawamsic terrane, or the Potomac terrane (Hibbard et al. cessor basin system metasedimentary mixed source areas. 2014), has been determined to be of rocks consist of slate, phyllite, Rocks to the west of the Rich- similar Laurentian, rift-related paleo- quartzite, and local metaconglomerate land Run pluton were previously geographic crustal affinity as the and metavolcanic layers. In this study, mapped as outliers of the greater suc- metasedimentary Lynchburg Group in we focus only on the Arvonia basin. cessor basin system that were deposit- the Blue Ridge province (Carter et al. Stratigraphy within the Arvonia succes- ed over the Chopawamsic fault (e.g. 2006; Merschat et al. 2010). Rocks of sor basin has been debated throughout Pavlides 1990, 1995; Mixon et al. the Smith River allochthon appear to the 20th century; however, it is accepted 2000). Detailed mapping in these areas be coeval with metaclastic rocks of the to include the Arvonia Formation has shown that rocks near Wilderness, Potomac terrane; their Laurentian phyllite, slate and schist, the Bremo Virginia, previously interpreted to be affinity is consistent with the interpre- Member quartzite of the Arvonia For- related to the successor basin system, tation that the Potomac terrane along mation and the Buffards Formation are actually part of the Chopawamsic strike was deposited peripherally to the metaconglomerate, quartzite, and phyl- Formation (Terblanche 2013) and Laurentian continent after the breakup lite. Some workers have favoured the rocks near Storck, Virginia, which were of Rodinia. Buffards Formation as the basal unit to targeted in this study, are known to be Samples interpreted to be the basin (Stose and Stose 1948) and distinct from the Potomac terrane and from basal units of the Arvonia and others have interpreted it as the highest Chopawamsic Formation (Hughes et Quantico formations have yielded a exposed unit in the basin, lying uncon- al. 2012). Their connection to the dominant population of Middle–Late formably over the Arvonia Formation greater Arvonia/Quantico system has Ordovician detrital zircon grains and (Brown 1969). The Arvonia and Quan- not been conclusively established. reportedly lack considerable Mesopro- tico formations are the only known terozoic zircon (Bailey et al. 2008); this fossiliferous Paleozoic rocks in the PREVIOUS DETRITAL ZIRCON distribution of ages led to the conclu- western Piedmont of Virginia; similar STUDIES sion that the Arvonia/Quantico system fauna are found in both units and the Previous detrital zircon studies in the and the underlying Chopawamsic ter- 508 rane were not derived in any part from not hand-picked with optical age determination. This methodology Laurentian crust (Bailey et al. 2008). microscopy. Zircon grains in the heavy for reporting detrital zircon ages is By sampling new target out- mineral fraction were subsequently similar to previous studies (e.g. Pollock crops in the terranes and successor identified and imaged with an automat- et al. 2007, 2009). All age uncertainties basin system of the western Piedmont, ed MLA–SEM and then analyzed using are reported at the 2σ confidence level. we aim to assess the supra-crustal rela- laser ablation–inductively coupled plas- Histograms and cumulative probability tionships within and between con- ma–mass spectrometry. All analyses plots were prepared with the Isoplot stituent rock units. Specifically, we were performed with a 10 μm beam software (Ludwig 2012) in Microsoft seek to: (1) explore the inferred depo- that scanned over a 40x40 μm area on Excel. sitional relationship between the each grain. Laboratory zircon stan- Chopawamsic and Potomac terranes, dards—Plešovice (206Pb/238U age of SAMPLES AND RESULTS (2) refine our understanding of the 337.13 ± 0.37 Ma; Sláma et al. 2008) Of 13 samples, six are from the cratonic affinity and depositional age and Harvard 91500 (206Pb/238U age of Potomac terrane, four are from the of units in the Potomac terrane, (3) 1062.4 ± 0.4 Ma; Wiedenbeck et al. Chopawamsic terrane, two are from assess and connect any older, non- 1995)—were analyzed after sets of 8 the Arvonia successor basin, and one Chopawamsic volcanic zircon in the unknowns were analyzed. The aggre- sample is from a package of metasedi- Chopawamsic terrane to a Proterozoic gate age of 180 analyses of the mentary rocks near Storck, Virginia, cratonic or micro-continental source, Plešovice standard in this study is formerly interpreted to be part of the and (4) gain insight into the deposi- 335.1 ± 1.2 Ma; the aggregate age of successor basin system. Sample loca- tional age and source of sediments in 186 analyses of Harvard 91500 stan- tions are shown in Figure 2 and photo- the Arvonia successor basin system. dard in this study is 1062.2 ± 4.4 Ma graphs are shown in Figure 3. His- (both ages are reported at the 2σ confi- tograms of the resultant data from METHODS dence level and also include decay con- each individual sample are shown in Thirteen metasedimentary samples stant errors). Analyses and concordia Figure 4. The results of all analyses, were selected from the western Pied- plots for the reference material analy- including discordant data not used in mont of Virginia for LA–ICP–MS ses can be found in Appendix 1 (avail- histograms, are reported in Table 2.1 detrital zircon U–Pb analysis. Crush- able at GAC’s open source GC Data of Appendix 2. A detailed explanation ing, disc-milling, Wilfley table separa- Repository at http://www.gac.ca/ of results from individual samples is tion, magnetic separation, and methyl- wp/?page_id=306). More detailed also included in Appendix 2 (see GC ene-iodide separation were carried out information on the methodology and Data Repository website). on nine samples at the Department of laser ablation system can be found in Geological Sciences at the University Pollock et al. (2007) and references AGE AND PROVENANCE OF of North Carolina at Chapel Hill. therein. LITHOTECTONIC COMPONENTS IN Samples BREMO and P310-04 were Signal processing and data THE WESTERN PIEDMONT OF processed at Memorial University, analysis were performed with ‘in- VIRGINIA Newfoundland. Samples KSH-11-16 house’ software at Memorial Universi- From the 1,289 detrital zircon analyses and KSH-12-70 were processed at ty. In most instances, the preferred age conducted, we can make interpreta- Texas A&M University in College Sta- is the concordia age of Ludwig (2012); tions on the provenance of the select- tion, Texas. With the exception of however, if the concordia age for any ed samples and begin to understand sample KSH-11-16, which was ana- given grain younger than 1.5 Ga has a any supra-crustal interactions during lyzed at Washington State University probability of fit value of <50%, the the time of deposition of these following the procedure described by 206Pb/238U age is reported, but only if it metasedimentary rocks. These inter- Chang et al. (2006), heavy mineral frac- is between 85 and 110% concordant pretations are made with consideration tions were processed at the Micro- with the 207Pb/206Pb age. For zircons of individual samples, combined data Analysis Facility at Memorial Universi- older than 1.5 Ga, when concordia for each terrane (Fig. 5A, 5B), and ty. A portion of the heavy mineral ages have a low probability of fit value, regional geological relationships and fraction from each sample was mount- the 207Pb/206Pb age was reported rock compositions. ed in epoxy and polished. To avoid because su fficient 207Pb is present in any potential bias, zircon grains were these older zircon grains for a precise

Figure 3. (next page) Field photos of selected detrital zircon samples and backscatter images of example detrital zircon grains. A. Site of sample KSH-11-01, hammer for scale is 30 cm long; B. Site of sample KSH-11-05, mechanical pencil for scale is 14 cm long; C. Site of sample KSH-11-08, hammer for scale is 30 cm long; D. Site of sample P310-4, pen for scale is 15 cm long; E. Site of sample KSH-11-18, hammer for scale in centre of photo is 30 cm long; G. Site of sample KSH-11-19, tip of boot for scale is 10 cm wide; H. Site of sample KSH-11-28 being extracted, manual jackhammer for scale is 4 cm across; I. Site of sample KSH-11-39 taken from upper beds in photo, ferns in upper right for scale; J. Site of sample KSH-12-70, hand lens for scale measures 2 cm across; K. Cross-bedding near sample BREMO, handle of geo-tool for scale measures 4 cm across; L. Bed- ding near sample KSH-11-BUF, head of hammer for scale measures 18 cm across; M. Site of sample KSH-11-40, yellow field notebook in centre-right for scale measures 12x19 cm. See Figure 2 for sample locations. GEOSCIENCE CANADA Volume 41 2014 509 A B C

jl23a75 jl23a15 jl24a24 jl25a37 jl24b28 jl24b47 jl26b32 jl25b35 jl25b96 jl25b32 1364 1160 1155 ± 39 Ma 1028 ± 32 Ma 1196 ± 58 Ma ± 39 Ma ± 46 Ma 3354 ± 17 Ma 1166 1070 1294 1663 ± 38 Ma ± 21 Ma ± 55 Ma ± 89 Ma D E G

jl29a49jl29a49 jl29a108 jl30a07 jl30a42 jl30a21 11031103 1450 ± 27 Ma ± 27 Ma ± 47 Ma

478 478 466 ± 26 Ma ± 26 Ma ± 21 Ma H I J

jl30b33 jl30b39 jl30b57 au01b46 au01b127 1006 743 au01b115 ± 35 Ma ± 53 Ma 521 457 ± 20 Ma ± 20 Ma

1170 ± 70 Ma 462 ± 39 Ma K L M

au06a91

1531 ± 26 Ma

au07a19 jl31a25 au01a11 au06a64 au06a27 au06a109 441 1177 433 1905 ± 17 Ma ± 38 Ma ± 18 Ma ± 43 Ma 1318 ± 68 Ma 615 ± 36 Ma 510

0.35 error ellipses are 1 18 1113 Figure 4. (opposite and following page) A KSH 11 01 KSH-11-01 16 1010 n=111 1600 Histogram and concordia plots for

206 238 14 Pb/ Uage=15 Concordia age = 96 detrital zircon samples. Thick black 0.25 12 Mine Run Complex Unit III, 1200 line in histogram plots represents the Rapidan River, U

10 238 1290 Potomac Terrane, cumulative probability of detrital ages 0.60 Pb/ 3000 8

(38.38958, -77.76225) 206 0.56 2800 and was calculated using 2σ uncertain- 6 0.15 0.52

800 U

1424 238 2600

Pb/ ties for each analysis. Note that the x- 4 548 206 0.48

2400

0.44 2 axis in the histogram plots is the same

0.40 7 9 11 13 15 17 19 0 0.05 207Pb/235U but the y-axis scale varies. Some con- 0 500 1000 1500 2000 2500 3000 3500 0 2 4 6 207Pb/235U 0.35 cordia plots include small inset plots 30 error ellipses are 1 B KSH 11 05 KSH-11-05 to show analysis ellipses for older zir- 1211 n= 108 1600 25 con grains analyzed. Concordia 206Pb/238Uage=26 Concordia age = 82 0.25 20 ellipses are drawn to the 1σ confidence 1141 Mine Run Complex Unit I, U 1200 level. See Figure 2 for sample loca-

Wilderness Run, 238 15 Potomac Terrane, Pb/ tions. 988 (38.30957, -77.73648) 206 10 0.15 800

5 499

0 0.05 0 500 1000 1500 2000 2500 3000 3500 0 2 4 6 Potomac Terrane 207Pb/235U 0.35 error ellipses are 1 The majority of detrital zircon grains KSH 11 08 25 KSH-11-08 C 1134 n= 113 1600 in samples from the Potomac terrane

207Pb/206Pb age = 1 are Mesoproterozoic with peak modes 20 206Pb/238Uage=28 1010 Concordia age = 84 0.25 at 1.015 Ga and 1.120 – 1.150 Ga. A 1200 Mine Run Complex Unit IV, U 15 238 cumulative histogram of statistically South Anna River, 3600 0.74 Potomac Terrane, Pb/ 206 0.70 3400 Number Number Number viable analyses (n=607) from the 10 (37.99697, -78.11893) 0.15 U 0.66 238 800 3200 Pb/ Potomac terrane (Fig. 5B) indicates 1420 206 0.62

5 3000 0.58 that the ages present in these samples

0.54 14 18 22 26 30 34 207Pb/235U are consistent with Grenvillian (ca. 0 0.05 0 500 1000 1500 2000 2500 3000 3500 0 2 4 6 207Pb/235U 1.08 – 1.0 Ga), Adirondian (ca. 1.18 – 10 0.35 P310-4 error ellipses are 1 1.08 Ga), and, to a lesser extent, Elzev- 9 D 1040 1154 P310-4 8 n=53 1600 erian (ca. 1.23 – 1.18 Ga) and Elsonian

206Pb/238U age = 38 7 Concordia age = 15 (ca. 1.46 – 1.23 Ga) events in Laurentia 0.25 6

Mine Run Complex Unit I, U 1200 (Gower and Krogh 2002). Some Ton- 5 Eley’s Ford, Rapidan River, 238

Pb/ ian zircon grains present may be Potomac Terrane,

4 206

Number (38.35749, -77.68502) 0.15 derived from Laurentian rift-related 3 800

2 rocks (Karabinos and Aleinikoff 1990;

1 Graybill 2012). Some of these ages are

0 0.05 also consistent with ages reported 0 500 1000 1500 2000 2500 3000 3500 0123456 207Pb/235U from the Sunsas belt (ca. 1.25 – 0.9 Age (Ma) Ga) of the Amazonian craton (Sad- 14 0.35 1101 error ellipses are 1 owski and Bettencourt 1996) but the KSH 11 18 KSH-11-18 12 E n= 100 1600 distribution of detrital zircon ages can 206Pb/238U age = 10 10 Concordia age = 90 be considered with other geologic fac- 1325 0.25 Lunga Reservoir Fm, 8 U 1200 tors to deduce cratonic affinity.

1450 Beaver Dam Creek, 238 936 0.60 3000

Pb/ Among other regional relationships, 6 Potomac Terrane, 0.56 2800 206

Number (38.50487, -77.43166) 0.15 0.52 interlayering between rocks of the Lau- U 2600 4 800 238 Pb/ 206 0.48 rentian Blue Ridge province and those 567 2400 2 0.44 considered part of the Potomac ter- 0.40 7 9 11 13 15 17 19 207Pb/235U 0 0.05 rane (Evans 1984) supports the Lau- 0 500 1000 1500 2000 2500 3000 3500 0 2 4 6 207Pb/235U 0.35 rentian affinity for at least part of the data-point error ellipses are 1ı KSH-11-16 1800 KSH-11-16n= 122 Potomac terrane. The absence of zir- 207Pb/206Pb age = 5 KSH-11-16 F 1120 206Pb/238U age = 95 20 1060 n=Concordia 122 age = 22 con populations potentially derived 1015 207Pb/206Pb age = 5 206Pb/238U age = 95 Concordia age = 22 0.25 1400 from the Ventuari-Tapajos orogen (ca. 15 1310 Lunga Reservoir Fm, U 2.10 – 1.87 Ga) in Amazonia (Tassinari 0.60 1430 Aquia Creek, 238 2900 Pb/ 1000 0.56 et al. 2000; Juliani et al. 2002) and the

10 Potomac Terrane 206

Number 0.52 2700 0.15 U (38.48978, -77.46853) 238 Brasiliano/Pan-African orogen (ca. 660 Pb/

206 0.48 2500 5 600 525 0.44 – 600 Ma) in the detrital record also 2300

0.40 791113151719 207 235 favour a Laurentian, rather than a 500 Pb/ U 0.05 0 0123456 0 500 1000 1500 2000 2500 3000 3500 Gondwanan source for the Potomac 207Pb/235U Age (Ma) terrane. GEOSCIENCE CANADA Volume 41 2014 511

25 error ellipses are 1 470 KSH 11 19 G 0.20 KSH-11-19 Figure 4. (continued). n= 25 1100 20 206 238 Pb/ U age = 1 0.16 Concordia age = 24 900 Chopawamsic Fm,

15 U Chopawamsic Creek, 0.12 700 238 Chopawamsic Terrane, Pb/ 206

Number 10 (38.53449, -77.38839) 500 0.08 With the caveat that the

300 youngest detrital zircon grains in each 5 0.04 sample may be significantly older than

0 0.00 0 500 1000 1500 2000 2500 3000 3500 0.0 0.4 0.8 1.2 1.6 2.0 2.4 the depositional age (e.g. Moecher and 207Pb/235U error ellipses are 1 Samson 2006), they do provide control 45 KSH 11 28 0.20 H KSH-11-28 1100 40 457 n= 68 for the depositional age of units within

35 206Pb/238U age = 2 0.16 Concordia age = 66 900 the Potomac terrane. The three 30 Chopawamsic Fm,

U youngest zircon grains in the Mine Run Lake Anna, 0.12 700

25 238 Chopawamsic Terrane, 500 Pb/ 20 Complex have concordia ages (2σ) of 206

Number (38.12930, -77.85023) 0.08 15 499 ± 15 Ma, 551 ± 19 Ma, and 554 ± 300 10 0.04 34 Ma. The youngest zircon grains 5 present in the Lunga Reservoir Forma- 0 0.00 0 500 1000 1500 2000 2500 3000 3500 0.0 0.4 0.8 1.2 1.6 2.0 2.4 207Pb/235U tion have concordia ages (2σ) of 502 ± error ellipses are 1 80 458 KSH 11 39 0.20 KSH-11-39 18 Ma, 527 ± 11 Ma, and 569 ± 21 I n= 97 1100 70 206Pb/238U age = 0 Ma. The ca. 500 Ma grains from both 0.16 Concordia age = 97 60 900 Chopawamsic Fm, units indicate that deposition in some

50 U South Anna River, 0.12 700 238 Chopawamsic Terrane, sub-units of the Mine Run Complex

40 Pb/

(37.95398, -78.00282) 206 500 Number 0.08 30 and in the Lunga Reservoir Formation

20 300 occurred after the Middle Cambrian. 0.04 10 Cross-cutting Ordovician intrusions

0 0.00 0 500 1000 1500 2000 2500 3000 3500 0.0 0.4 0.8 1.2 1.6 2.0 2.4 such as the Goldvein pluton (456 ± 9 207Pb/235U 40 KSH-12-70 error ellipses are 1 KSH 12 70n= 38 Ma, Aleinikoff et al. 2002) and the 206 238 0.20 476 Pb/ U age = 2 KSH-12-70 J Concordia age = 36 35 n= 38 1100 Occoquan pluton (472 ± 4 Ma, 206 238 30 Pb/ U age = 3 0.16 Concordia age = 35 900 Aleinikoff et al. 2002) place limits on 25 Chopawamsic Fm, U Smith Lake, 0.12 700 238 the minimum possible age of deposi- 20

Chopawamsic Terrane, Pb/

206 tion for the Potomac terrane. These (38.48899, -77.419183) 500 Number 15 0.08 data indicate that the youngest sampled 10 300 0.04 5 units of the composite Potomac ter-

0 0.00 rane were deposited in the Late Cam- 0 500 1000 1500 2000 2500 3000 3500 0.0 0.4 0.8 1.2 1.6 2.0 2.4 207 235 Age (Ma) Pb/ U brian to Early Ordovician.

18 error ellipses are 1 The Mine Run Complex and BREMO 0.32 16 1010 BREMO 1700 K 452 n= 84 Lunga Reservoir portions of the 0.28 14 1500 206Pb/238U age = 40 Concordia age = 44 Potomac terrane contain various sized 12 0.24 1300

Bremo Mbr, U 10 blocks of debris that were formerly 0.20 James River, 238 1100

8 Pb/ interpreted to be shed from the

206 0.16 (37.71290, -78.30061) 900 6 Chopawamsic volcanic arc (Pavlides 0.12 700 4 500 1989). Our samples, including those 0.08 2 300 proximal to purported Chopawamsic- 0 0.04 0 500 1000 1500 2000 2500 3000 3500 012345 207Pb/235U derived blocks, are all devoid of any 80 error ellipses are 1 KSH 11 BUF 0.32 430 KSH-11-BUF 1700 zircon that would be consistent with a 70 L n= 94 0.28 1500 60 206 238 derivation from the Middle Ordovician Pb/ U age = 4 Concordia age = 90 0.24 1300 50 volcanogenic Chopawamsic terrane.

Buīards Mbr, U 0.20 238 1100 40 Volcano Ln., Thus the origin of these clasts, frag- Pb/

206 0.16 30 (37.65385, -78.36009) 900 ments, and map-scale bodies must be

0.12 700 20 some previously unidentified source. 500 10 0.08 A similar scenario exists in the Shores 300 0 0.04 0 500 1000 1500 2000 2500 3000 3500 012345 207Pb/235U mélange complex at the James River;

error ellipses are 1 KSH 11 40 0.28 many of the blocks were considered to 10 M KSH-11-40 1500 1152 n= 107 be derived from the Chopawamsic ter- 0.24 1300 207Pb/206Pb age = 1 8 607 Concordia age = 106 rane (Bland and Blackburn 1979; Storck Rocks, 0.20 1100 U 6 978 near Deep Run, 238 Evans 1984; Brown 1986), however, no 533 0.48 0.16 2400 (38.41893, -77.63665) Pb/ 900 1333 0.44 206 such statistically valid Ordovician zir- Number4 Number Number 2200 700 0.40 0.12 U 2000 238 0.36 Pb/ con grains were identified in a 206 1800 2 0.32 0.08 500 0.28 metasedimentary sample from the

0.24 4.0 5.0 6.0 7.0 8.0 9.0 10.0 207 235 0 0.04 Pb/ U Shores complex (Bailey et al. 2008). 0 500 1000 1500 2000 2500 3000 3500 0.5 1.5 2.5 3.5 207 235 Age (Ma) Pb/ U These observations and the range in 512

and 5B.2). The Sykesville Formation 200 Chopawamsic Forma on metadiamictite, a correlative to the 467 Chopawamsic Terranen= 228 100 B 1120-1150 All Potomac terrane A 206 238 Pb/ U age = 6 Concordia age = 222 Lunga Reservoir Formation in north- 150 n= 228 n= 607 206 238 80 1015 ern Virginia and Maryland, has a detri- Pb/ U age = 6 207Pb/206Pb = 7 Concordia age = 222 206Pb/238U age = 211 tal zircon signature that is remarkably 60 Concordia age = 389 100 similar to the Lunga Reservoir Forma- 1310 tion (Horton et al. 2010; Fig. 5B.2). 40 Number 1430 The likely Laurentian affinity 50 20 for the Mine Run Complex, based upon its detrital signature and correla-

0 0 tion to rocks interlayered with Laurent- 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 ian strata is fully supported by our 70 Mine RunMine Complex Run Complex Lunga ReservoirLunga Reservoir Forma Fm.on data. The Lunga Reservoir Formation B.1 n= 385 35 B.2 n= 222 1150 207Pb/206Pb = 1 1120 207Pb/206Pb = 6 Potomac Terrane206 238 Potomac Terrane206Pb/238U age = 104 60 Pb/ U age = 107 1055 also appears to be Laurentian-derived Concordia age = 277 Concordia age = 112 30 and we interpret that the slight differ- 50 n= 385 1015 n= 222 1015 207 206 207 206 Pb/ Pb age = 1 25 1310 Pb/ Pb age = 6 ences between the detrital signatures of 40 206Pb/238U age = 107 206Pb/238U age = 104 the two units may be a result of the Concordia age = 277 20 1430 Concordia age = 112 30 Lunga Reservoir Formation being a 15 Number more proximal sedimentary depocentre 20 10 500 to its source area than the mature, well- 10 499 5 sorted sediment of the Mine Run

0 0 Complex. These coeval units appear 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Age (Ma) Age (Ma) to demonstrate the effect that hydraulic sorting, zircon fertility, and sedimenta- Figure 5. Compiled detrital zircon histogram plots. A. Data from 4 samples of ry dispersal paths can have on detrital the Chopawamsic formation. B. Data from 6 samples (including the Mine Run zircon signatures (e.g. Moecher and Complex and the Lunga Reservoir Formation) of the Potomac terrane. B.1. Data Samson 2006; Thomas 2011) in dissim- for only 4 samples of the Mine Run Complex. B.2. Data for only two samples of ilar sedimentary rocks derived from the Lunga Reservoir Formation. Dashed line is the probability curve from the similar source areas. Sykesville Formation (Horton et al. 2010). Note variations in the y-axis scales of Chopawamsic Terrane the plots. The 228 detrital zircon grains analyzed depositional age discussed above con- mapping (Pavlides 1989, 1990, 1995; from samples of the Chopawamsic tradict the long-standing interpretation Mixon et al. 2000; Hughes 2011) and Formation are strongly unimodal. The that parts of the Potomac terrane were our reconnaissance mapping suggests peak mode occurs at ca. 467 Ma (Fig. deposited concurrent with Chopawam- that the four sub-units are composi- 5A). Most analyses are identical to this sic arc accretion to Laurentia (Pavlides tionally similar and, in some places, value within 2σ analytical uncertainty. 1989; Drake 1989; Pavlides et al. 1994; separated by gradational and conform- A population of seven Mesoprotero- Hibbard et al. 2014). If any part of able contacts (Hopkins 1960; Hughes zoic zircons (ca. 1.17 – 1.01 Ga) is the Middle Ordovician Chopawamsic 2011), rather than faults. Because Edi- ambiguous in terms of cratonic affini- arc was a source for the Potomac ter- acaran – Cambrian zircon is not pres- ty. U–Pb zircon TIMS ages from mag- rane, such an influence was not recog- ent in all samples of the Mine Run matic rocks in the Chopawamsic arc nized in any of the Potomac terrane Complex analyzed, it remains possible span 474 – 465 Ma (Coler et al. 2000; samples analyzed in this study. It that some portions were deposited ear- Hughes et al. 2013b). The concurrence seems that the Chopawamsic arc did lier than others; however, due to the of coeval volcanic and detrital ages not feed any part of the Potomac ter- shared characteristics of the four sub- indicates that interlayered sedimentary rane, parts of which were buried and units, we tentatively apply the youngest lenses of the Chopawamsic Formation intruded (e.g. at ca. 472 Ma by the detrital zircon ages present to the were derived almost exclusively from Occoquan pluton) by the time of whole complex (Fig. 6). Pavlides contemporaneous volcanic activity. In Chopawamsic arc activity (474–465 (1989) considered slightly different support of this conclusion, most detri- Ma). source areas for the Mine Run Com- tal zircon grains in Chopawamsic sam- Although secondary to the plex and Lunga Reservoir Formation ples retain an angular, unmodified crys- investigation of the relationship based upon differences in sedimentary tal shape, which is indicative of short- between the Chopawamsic and facies. Our detrital zircon data may lived sedimentary transport. Detrital Potomac terranes, the composite reflect the compositional dissimilarity systems dominated by syndepositional nature of the Potomac terrane (see that could arise from distinct source volcanism have been documented as Kunk et al. 2005; Wintsch et al. 2010) areas. In particular, the Lunga Reser- the typical result of deposition within must be addressed when assessing cra- voir Formation has a larger proportion and along the margin of an active vol- tonic affinity. The Mine Run Complex of 1.25 – 1.60 Ga zircons relative to canic arc (Pollock et al. 2007; Cawood consists of four sub-units; previous the Mine Run Complex (see Fig. 5B.1 et al. 2012). GEOSCIENCE CANADA Volume 41 2014 513

Western Piedmont of Virginia detrital zircon dispersal paths The 28 Cambrian and older detrital zircon grains represent sedi- Potomac terrane Chopawamsic terrane mentary sources other than Chopa- 420 wamsic magmatism, but little can be garnered in terms of cratonic affinity Buffards Middle due to the overwhelming prominence Formation of Ordovician ages. Isotopic studies deposition 430 c. 430 have led other workers to propose that Silurian the Chopawamsic volcanics were built Arvonia E ? upon some form of Mesoproterozoic Formation crust (Pavlides 1981; Coler et al 2000) 440 deposition but no known basement is exposed; 444 for this reason, these Cambrian and 444 444 older grains are important in character- izing such basement. Well constrained L Chopawamsic (concordia, 2σ uncertainties) Cambrian, 456 fault Ediacaran, and Cryogenian ages 459 460 include, among others, 510 ± 26 Ma, 512 ± 35 Ma, 521 ± 20 Ma, 537 ± 20 465 Ma, 575 ± 34 Ma, and 640 ± 43 Ma M Chopa- grains. Considered alone, 21 Cryogen- ian – Cambrian zircons are ambiguous 472 472 wamsic arc Ordovician 474 in terms of cratonic affinity as they ? E could potentially be attributed to peri- 480 Gondwanan arc activity or Laurentian Potomac rift magmatism (e.g. ca. 570 Ma Catoctin and ca. 760 Ma Mount Rogers terrane formations, Aleinikoff et al. 1995) deposition related to the rifting of Rodinia, L Storck although many of the youngest Cam- quartzite brian zircons in Chopawamsic samples are younger than the majority of 500 ca. 500 deposition M ca. 504 Rodinian rift rocks and only coeval

Iapetus Ocean with the youngest rift-related rocks, which are geographically and volumet- rically limited (ca. 532 Ma Mt. Rigaud and Chatham-Grenville stocks, E ? McCausland et al. 2007). However, the

Cambrian Neoproterozoic – Cambrian popula- 520 tion can be evaluated in conjunction with a general dearth of zircon potentially derived from the 1.08 – 1.0 Ga Laurentia Carolinia and Amazonia Grenville orogen (Gower and Krogh (or recycled Amazonia) 2002) and the complete lack of any zircons related to post-Grenville Tonian rift rocks (Graybill 2012). The few Figure 6. Timeline and potential sediment dispersal path figure as discussed in the Mesoproterozoic zircons present may text. Timescale is in millions of years. The Laurentian-derived Potomac terrane be inherited from a source craton or sedimentary rocks and peri-Gondwanan Storck and Chopawamsic rocks had no recycled from metasedimentary rocks supra-crustal interaction until the accretion of the Chopawamsic arc, effectively older than Chopawamsic magmatism, closing the Iapetus Ocean sometime in the Late Ordovician. The Arvonia Forma- such as the Storck rocks (see discus- tion was deposited unconformably over ca. 444 Ma granodiorite that is intrusive to sion of Storck rocks below and Fig. 6). the Chopawamsic terrane. The Arvonia and Buffards formations had access to Given the remarkable zircon both Chopawamsic and Potomac terrane debris when being deposited. Thickness fertility of some Mesoproterozoic of sedimentary dispersal arrows indicates the relative amount of detrital zircon magmatic provinces (Moecher and input but isn’t necessarily correlative with overall sedimentary input. Gradient Samson 2006), the shortage of Meso- shading for Storck quartzite and Buffards Formation indicates unknown minimum proterozoic zircon indicates that the age of deposition. Gradient shading for the Potomac terrane indicates the possi- Chopawamsic arc had no direct depo- bility of some older, but still Laurentian, components within the composite sitional or recycled access to Laurent- Potomac terrane. ian Mesoproterozoic and early Neo- 514 proterozoic source areas (as the similar Laurentian source areas to the mation, but not in the Bremo Member Potomac terrane had). When consid- Arvonia basin (Fig. 6). The Paleozoic of the Arvonia Formation, suggests ered with the shortage of Mesopro- component of the Bremo Member (but doesn’t prove) that the Buffards terozoic zircon and the youngest detrital signature potentially represents Formation is the younger of the two. known Rodinian rift-related magmatic a mixture of zircons from Chopawam- The detrital zircon data presented here rocks, the presence of the population sic arc activity (474 – 465 Ma) and supports the model of Brown (1969), of Cambrian, Ediacaran, and Cryogen- post-Chopawamsic accretion magmatic who proposed that the Buffards For- ian zircon suggests that they are poten- activity (ca. 450 – 435 Ma) represented mation is younger than and lies uncon- tially derived from a source area that by intrusions such as the Ellisville plu- formably above the Arvonia Forma- contains Cambrian, Ediacaran, Cryo- ton, Lahore pluton, Goldvein pluton, tion, rather than the alternate interpre- genian, and Stenian rocks or one that and Green Springs intrusive suite (Fig. tation of Stose and Stose (1948). consists of mostly Cambrian – Edi- 6). The bimodality of the detrital sig- In a regional perspective, the acaran units. Metasedimentary rocks in nature in the Bremo Member leads us results of our analyses from the Arvo- the peri-Gondwanan microcontinents to interpret that it was deposited with nia basin are somewhat similar to of Carolinia (Pollock et al. 2010; Den- access to both Potomac and reported detrital ages from the nis et al. 2012) and Ganderia (Fyffe et Chopawamsic sources, only after the migmatitic Cat Square terrane in Geor- al. 2009), and Ganderian-derived sedi- Chopawamsic terrane was accreted to gia and North and South Carolinas mentary rocks (Pollock et al. 2007) Laurentia in the Late Ordovician. This (Fig. 7; Bream et al. 2004; Merschat et contain similar Cambrian, Ediacaran, interpretation is consistent with the al. 2010). Like the Arvonia metasedi- and Cryogenian detrital zircons and unconformable relationship of the mentary rocks, the Cat Square basin generally lack ca. 1.2 – 1.0 Ga zircon. Arvonia basin over granodiorite related includes populations of Mesoprotero- Cryogenian – Cambrian ages are also to the ca. 444 Ma Ellisville pluton, zoic – Tonian debris with supplemen- present in volcanic rocks of the Victo- which intruded both the Potomac and tary Ordovician – Silurian zircon that ria Lake Supergroup of the peri-Gond- Chopawamsic terranes. are interpreted to be detrital, rather wanan Penobscot Arc and its basement The Mesoproterozoic compo- than metamorphic. The purported (Rogers et al. 2006; McNicoll et al. nent of debris in the detrital signature Paleozoic detrital zircon from the Cat 2008; Zagorevski et al. 2010). of the Buffards Formation (Fig. 4L) of Square system indicates that it and the the Arvonia basin is muted compared Arvonia/Quantico system could have Arvonia Successor Basin and to that in the Bremo Member, howev- been deposited coevally. However, in Storck Rocks er, we interpret it to represent a similar contrast to those studies, the Arvonia In contrast to previous sampling (Bai- recycling of debris from the metasedi- system data include considerably more ley et al. 2008), we found considerable mentary Potomac terrane and/or direct Ordovician – Silurian zircon and do Mesoproterozoic zircon within rocks contribution from Laurentia with pos- not include any zircon that is Edi- of the Arvonia successor basin. This sible recycling from the underlying acaran or any older than 1.7 Ga. With observation may be due to the strati- Arvonia Formation below (Fig. 6). the limited data on hand, it is possible graphic position within the basin from The unique ca. 430 Ma mode in the that the Arvonia/Quantico and the Cat which we sampled. While Bailey et al. Buffards Formation data (Fig. 4L) Square systems may have had some (2008) sampled from basal units of the appears to include analyses from some shared source areas, but it seems they Arvonia and equivalent Quantico for- of the youngest zircon contributed to were not derived from identical mations, we collected from stratigraph- sampled metasedimentary rocks in the regions. ically higher portions in the Arvonia Arvonia basin and likely reflects a The detrital zircon results basin. It seems that basal units near an younger time of deposition than the from the Storck micaceous quartzite unconformable contact would likely Bremo Member sample. The Buffards (Fig. 4M) are the most intriguing of contain considerable, if not complete, Formation contains many volcanic this study, for these rocks contain a detrital contribution from the directly clasts and this ca. 430 Ma age may rep- detrital signature dissimilar from the underlying Chopawamsic terrane. resent the crystallization age of some Potomac terrane, the Chopawamsic Because our samples are higher in the of the volcanic rocks that produced Formation, and the Arvonia successor Arvonia section, they may well be bet- these clasts. No volcanic rocks basin system. The Storck sample con- ter suited to evaluate any broader sedi- younger than ca. 450 Ma exist in the tains considerable Cryogenian – Cam- mentary source area for the successor area but there are ca. 430 – 450 Ma brian (ca. 800 – 500 Ma) material that basin system. intrusive rocks to the northwest in the is common in peri-Gondwanan ter- The Mesoproterozoic zircon Potomac terrane (Buckingham com- ranes; the sample also contains a signif- grains present in the Bremo Member plex, Diana Mills body, Green Springs icant population of Paleoproterozoic sample (Fig. 4K) of the Arvonia For- intrusive suite; Wilson 2001). Consis- (2.1 – 1.7 Ga) zircon not seen in any of mation are similar in age to those in tent with the Silurian plutonic bodies the Laurentian-derived Potomac ter- the Mine Run Complex metasedimen- to the northwest, Brown (1969) inter- rane metasedimentary rocks sampled in tary rocks (Fig. 5B). We interpret this preted the sedimentary source area for this study. Furthermore, the universal similarity to reflect the recycling of the Buffards Formation to be to the Stenian (1.1 – 1.0 Ga) zircon grains detrital zircon from the Mine Run northwest. The overwhelming presence found in Grenville-related Laurentian Complex and/or contribution from of Silurian grains in the Buffards For- sedimentary rocks are not prominent GEOSCIENCE CANADA Volume 41 2014 515

Specifically, both the Storck and Shores 30 Arvonia Basin metasedimentary rock units lie just n = 55 west of the main Chopawamsic fault samples =2 20 and may represent semi-continuous, analyses = 163 poorly exposed fragments of peri- Gondwanan metasedimentary rock that 10 lie beneath the Chopawamsic arc or once existed between the Chopawam- sic and Potomac terranes. Regardless 0 of the connection to the Shores 30 Cat Square Basin mélange or any other units, a peri- Gondwanan source for the Storck samples =5 rocks is important for determining the 20 analyses = 113 affinity of the Chopawamsic terrane. (Bream et al. 2004 and Because they are bounded by the Late 10 Ordovician Chopawamsic fault system, Merschat et al. 2010) it is unlikely that the Storck rocks have been tectonically shuffled along the 0 Chopawamsic–Potomac terrane inter- 500 1000 1500 2000 2500 face during later Paleozoic deforma- tional events; therefore, the current rel- Age (Ma) ative position of the Storck rocks to the Chopawamsic arc likely reflects Figure 7. Histogram comparison of 206Pb/238U zircon ages from samples of the their original paleogeographic configu- Arvonia and Cat Square basins. Data from this study and those of Bream et al. ration prior to the Late Ordovician. (2004) and Merschat et al. (2010) are only included for analyses with Th/U > 0.1 206 238 207 206 and when the Pb/ U age is ± 15% concordant with a corresponding Pb/ Pb DISCUSSION age. The detrital zircon data presented here in the Storck sample. These important detrital zircon sources of the Virgilina and regional geologic relationships observations all suggest a peri-Gond- (630 – 610 Ma; Samson et al. 1995; indicate that the sedimentary packages wanan source for the Storck rocks. Wortman et al. 2000) and Albemarle of the Potomac terrane were most like- Because Middle Ordovician zircon (575 – 532 Ma; Hibbard et al. 2002) ly derived from a Laurentian source potentially derived from the magmatic sequences. Furthermore, the area. Contrary to previous models for Chopawamsic arc (474 – 465 Ma) is Neoproterozoic to Early Cambrian the Virginia Piedmont (e.g. Pavlides also absent, we consider the Storck population present in the Storck sam- 1989; Pavlides et al. 1994), the detrital rocks to represent a tectonic sliver not ple is similar to the detrital signature zircon data show that Chopawamsic derived from the Chopawamsic mag- observed in metasedimentary samples arc volcanic rocks were not a source matic rocks. This observation may from Carolinia (Pollock et al. 2010). for the Potomac terrane metasedimen- indicate that considerable tectonic tele- Paleoproterozoic zircon in the Storck tary rocks. The lack of any Ordovician scoping of potential intra-Iapetan ter- sample may have originally formed in detrital zircon in the sampled metasedi- ranes occurred during the Chopawam- magmatic events related to Amazonia mentary rocks of the Potomac terrane sic accretion to Laurentia; alternatively, in the Ventuari–Tapajos (ca. 2.10 – and the presence of Middle Ordovi- the Storck rocks could be an older, 1.87 Ga; Tassinari et al. 2000; Juliani et cian intrusive bodies in the Potomac deeper part of the Chopawamsic ter- al. 2002) and parts of the Rio terrane are consistent with a model rane that was deposited before Middle Negro–Juruena (ca. 1.80 – 1.75 Ga; wherein the Potomac terrane sediment Ordovician Chopawamsic arc magma- Geraldes et al. 2001) orogens. was already deposited, buried, and tism initiated. The lack of Ordovician – Sil- intruded by the time the Chopawamsic On the basis of the youngest urian zircon and absence of a domi- arc was active (474 – 465 Ma; Fig. 6). zircon grains present in the sample (ca. nant population of Stenian grains, With these observations in mind, we 504 Ma), it appears that the Storck among lithological differences, indi- propose that the Potomac terrane is metasedimentary rocks may have been cates that the Storck rocks are not unrelated to the Middle Ordovician deposited coevally with the sampled related to the Arvonia/Quantico sys- Chopawamsic arc. Additionally, there Potomac terrane metasedimentary tem. Potential correlatives of the Stor- is no direct evidence for any older arc rocks, but were deriving sediment from ck rocks include the Shores mélange that could be related to the Potomac a non-Laurentian source (Fig. 6). Neo- (Fig. 8; Bailey et al. 2008), along strike terrane metasedimentary rocks. proterozoic to Early Cambrian detrital to the south at the James River and Detrital zircon data for the zircon grains in the Storck sample are other enigmatic peri-Gondwanan Chopawamsic Formation show that the consistent with derivation from the derived metasedimentary rocks to the main source for Chopawamsic sedi- peri-Gondwanan microcontinent of north (Bosbyshell et al. 2013; Martin et mentary rocks was coeval Chopawam- Carolinia which includes potential al. 2013; MacDonald et al. 2014). sic magmatic rocks. Limited Neopro- 516

composite Potomac terrane meta- 15 sedimentary rocks were most likely Storck deposited along the margin of 10 KSH-11-40 Laurentia sometime between ca. 500 – 470 Ma and most impor- n=107 5 tantly were not derived from the Middle Ordovician Chopawamsic 0 Formation. 2. The Storck metasedimentary rocks 15 were deposited sometime after ca. Shores 500 Ma and tapped a peri-Gond- 10 WP2 (Bailey et al. 2008) wanan source area. Their geo- n=88 graphic position suggests a peri- 5 Gondwanan affinity for the Chopawamsic arc. 3. Chopawamsic Formation sedimen- 0 tary rocks were mostly derived 500 1000 1500 2000 2500 from Chopawamsic terrane mag- Age (Ma) matic rocks during the Middle Ordovician (ca. 467 Ma). The old- Figure 8. Histogram comparison of concordant detrital zircon ages from the est grains in these samples are not Storck rocks and the Shores mélange (Bailey et al. 2008). suitable for asserting Mesoprotero- zoic cratonic affinity, but the pres- terozoic and Mesoproterozoic grains in Ellisville bodies) at 445 – 444 Ma ence of Cryogenian – Cambrian samples of the Chopawamsic metased- (Hughes et al. 2013a; MacDonald et al. grains and the general dearth of imentary rocks hint at a possible peri- 2014). Mesoproterozoic zircon are consis- Gondwanan source area; they are also New data from the Arvonia tent with a peri-Gondwanan consistent with the possibility that successor basin suggests that it was source. there is an older basement to the deposited with access to both recycled 4. The Arvonia basin was only Chopawamsic arc, as concluded by pre- Potomac terrane and Chopawamsic deposited after the Chopawamsic vious workers (e.g. Pavlides et al. 1994; terrane detritus after the Late Ordovi- terrane accreted to Laurentia in the Coler et al. 2000). The Storck meta- cian accretion of the Chopawamsic ter- Late Ordovician and it derived sedimentary unit lies between the rane to Laurentia (Fig. 6). This conclu- sediment from both the Potomac Chopawamsic and Potomac terranes sion is supported by the uncon- and Chopawamsic terranes in addi- for at least 10 km along strike and formable relationship of the Arvonia tion to ca. 430 Ma magmatic rocks. includes a peri-Gondwanan detrital zir- basin over granodiorite related to the The Chopawamsic fault marks con signature. The position of the latest Ordovician Ellisville pluton, the main boundary between the Storck rocks, considered in conjunc- which stitches the Potomac and Potomac and Chopawamsic terranes. tion with older detrital zircon in the Chopawamsic terranes. Also in sup- We favour a peri-Gondwanan affinity Chopawamsic Formation, provides cir- port of this conclusion, Nd isotope for the Chopawamsic arc based upon cumstantial evidence to suggest that analyses by Owens et al. (2013) showed the lack of data to tie it to Laurentia, the Chopawamsic terrane is also peri- that rocks of the Arvonia Formation its structural position above and out- Gondwanan. The Storck rocks and are most like sedimentary rocks in the board of peri-Gondwanan derived other peculiar units along strike, Appalachians deposited after ca. 450 metasedimentary rocks (the Storck including the Shores mélange, highlight Ma (Late Ordovician). The youngest rocks), and Neoproterozoic – Cambri- the potential tectonic telescoping of zircon grains in the Buffards Forma- an zircons recovered from metasedi- intra-Iapetan terranes that may have tion of the Arvonia basin indicate that mentary samples of the Chopawamsic occurred between the Chopawamsic sedimentation in the Arvonia basin Formation and Storck quartzite that and Potomac terranes. Similar to the continued until at least ca. 430 Ma and are potentially derived from older peri- Storck rocks, the Moretown Formation may reflect syndepositional magmatic Gondwanan rocks. Because the in Vermont and Massachusetts sam- activity. Chopawamsic arc is interpreted as peri- pled a peri-Gondwanan source area Gondwanan, we advocate that the Late and is associated with an Ordovician SUMMARY AND CONCLUSION Ordovician Chopawamsic fault system volcanic arc (Shelburne Falls arc) that Our new data lead to the following demarcates the main Iapetan suture in accreted to Laurentia in the Ordovician main conclusions concerning the tec- the southern Appalachian orogen. A (MacDonald et al. 2014). Further- tonic development for Cambrian – Sil- latest Middle to Late Ordovician more, both the Moretown Formation urian metasedimentary rocks of the Iapetan closure in the southern and Chopawamsic arc sutures with western Piedmont of north-central Appalachian orogen discussed here and Laurentian rocks were intruded by Virginia: illustrated by Hughes et al. (2014) is post-accretion plutons (Middlefield and 1. The youngest sampled units of the analogous to models proposed in the GEOSCIENCE CANADA Volume 41 2014 517 northern Appalachians (e.g. Zagorevski lites of the Catoctin and Mount central Appalachian Piedmont of and van Staal 2011, and references Rogers Formations, central and south- southeastern Pennsylvania and north- therein; MacDonald et al. 2014). The ern Appalachians: Evidence for two ern Delaware (abstract): Geological results of the current detrital zircon pulses of Iapetan rifting: American Society of America Abstracts with study are limited to supra-crustal inter- Journal of Science, v. 295, p. 428–454, Programs, v. 45, no. 7, p. 810. actions that can be subject to the vari- http://dx.doi.org/10.2475/ajs.295.4.4 Bream, B.R., Hatcher, R.D., Jr., Miller, C.F., 28. and Fullagar, P.D., 2004, Detrital zir- ability of sedimentary dispersal system Aleinikoff, J.N., Horton, J.W., Jr., Drake, con ages and Nd isotopic data from dynamics. To fully evaluate the A.A., Jr., and Fanning, C.M., 2002, the southern Appalachian crystalline Chopawamsic fault as the main Iapetan SHRIMP and conventional U–Pb ages core, Georgia, South Carolina, North suture in the southern Appalachians, of Ordovician granites and tonalities Carolina, and Tennessee: New prove- future research will focus upon assess- in the Central Appalachian Piedmont: nance constraints for part of the Lau- ing and refining any intra-crustal rela- Implications for Paleozoic tectonic rentian margin, in Tollo, R.P., Cor- tionships among terranes in the west- events: American Journal of Science, riveau, L., McLelland, J., and ern Piedmont that can be deduced v. 302, p. 50–75, http://dx.doi.org/ Bartholomew, M.J., eds., Proterozoic with Nd, Pb, and Hf isotopic analyses 10.2475/ajs.302.1.50. tectonic evolution of the Grenville as well as whole rock geochemistry and Bailey, C.M., and Owens, B.E., 2012, Tra- orogen in North America: Geological supporting high-precision zircon crys- versing suspect terranes in the central Society of America Memoirs, v. 197, Virginia Piedmont: From Proterozoic p. 459–475, http://dx.doi.org/ tallization ages. anorthosites to modern earthquakes, 10.1130/0-8137-1197-5.459. in Eppes, M.C., and Bartholomew, Brown, W.R., 1969, Geology of the Dill- ACKNOWLEDGEMENTS M.J., eds., From the Blue Ridge to the wyn Quadrangle Virginia, Report of This work was supported by funding Coastal Plain: Field Excursions in the Investigations 10, Virginia Division of from the following: a grant from the Southeastern United States: Geologi- Mineral Resources, 77 p. USGS EDMAP program cal Society of America Field Guide Brown, W.R., 1979, Field guide to the (G10AC00265) to JPH and National 29, p. 327–344. Arvonia-Schuyler district, in Glover, Science Foundation grants to JPH Bailey, C.M., Francis, B.E., and Fahrney, L., III, and Tucker, R.D., eds., Field (EAR-1048476) and BVM (EAR- E.E., 2004, Strain and vorticity of Trip No. 1, Virginia Piedmont geology 1048472). Thanks to Marine Corps transpressional high-strain zones from along the James River from Richmond Base Quantico for land access and base the Virginia Piedmont, USA, in Alsop, to the Blue Ridge: Guides to field G.I., Holdsworth, R.E., McCaffrey, trips 1–3 for Southeastern Section archaeologist John Haynes for his time. K.J.W., and Hand, M., eds., Flow meeting, Geological Society of Ameri- We also thank the many other private Processes in Faults and Shear Zones: ca: VPI-SU, p. 24–41. land owners that allowed access and Geological Society, London, Special Brown, W.R., 1986, Shores complex and sampling on their properties. Invalu- Publications, v. 224, p. 249–264, mélange in the central Virginia Pied- able field assistance was provided by http://dx.doi.org/10.1144/GSL.SP.20 mont, in Neathery, T.L., ed., Southeast- Adam Hughes, Alet Terblanche, Dillon 04.224.01.16. ern Section of the Geological Society Nance, and Megan Rumble. We are Bailey, C.M., Loteas, G.C., Relyea, J.A., of America Centennial Field Guide very grateful to Drew Coleman, Josh Weikel, E.O., Dubose, J., and Good- Volume 6: Geological Society of Rosera, and Katie Wooten at UNC- man, M.C., 2005, Geologic map of America, Boulder, CO, p. 209–214. Chapel Hill for their hospitality during the Columbia 7.5 minute quadrangle, Carter, B.T., Hibbard, J.P., Tubrett, M., and sample processing. Wilfredo Diegor, Virginia: Virginia Department of Sylvester, P., 2006, Detrital zircon Rebecca Lam, David Grant, Michael Mines, Minerals, and Energy Division geochronology of the Smith River of Mineral Resources, Open File Allochthon and Lynchburg Group, Shaffer, and the rest at the Memorial Report 05-02, scale 1:24,000. Southern Appalachians: Implications University MicroAnalysis Facility are Bailey, C., Eriksson, K., Allen, C., and for Neoproterozoic–Early Cambrian thanked for their assistance while run- Campbell, I., 2008, Detrital zircon paleogeography: Precambrian ning samples and processing data. Jeff geochronology of the Chopawamsic Research, v. 147, p. 279–304, Vervoort, Charles Knaack, and Luz terrane, Virginia Piedmont: Evidence http://dx.doi.org/10.1016/j.precam- Romero provided assistance with for a non-Laurentian provenance res.2006.01.024. analysis of sample KSH-11-16 at (abstract): Geological Society of Cawood, P.A., Hawkesworth, C.J., and Washington State University. Thanks America Abstracts with Programs, v. Dhuime, B., 2012, Detrital zircon to Chuck Bailey for sharing detrital zir- 40, no. 6, p. 449. record and tectonic setting: Geology, con information for his Shores Bland, A.E., and Blackburn, W.H., 1979, v. 40, p. 875–878, mélange sample. Thanks to Bill Bur- Geochemical studies on the green- http://dx.doi.org/10.1130/G32945.1. stones of the Atlantic seaboard vol- Chang, Z., Vervoort, J.D., McClelland, ton for suggestions concerning the fig- canic province, south-central W.C., and Knaack, C., 2006, U–Pb ure organization. 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Received February 2014 Accepted as revised July 2014 First published on the web September 2014

For access to Hughes et al. (2014) sup- plementary material (Appendices 1 and 2), please visit GAC’s open source GC Data Repository (Harold Williams Series folder) at http://www.gac.ca/wp/?page_id=306.