H AROLD W ILLIAMS S ERIES

Arvonia Basin samples =2 analyses = 163

Cat Square Basin

samples =5 analyses = 113 (Bream et al. 2004 and

Merschat et al. 2010)

n = 55

Age (Ma)

Figure 7. Histogram comparison of 206 Pb/ 238 U zircon ages from samples of the Arvonia and Cat Square basins. Data from this study and those of Bream et al. (2004) and Merschat et al. (2010) are only included for analyses with Th/U > 0.1 and when the 206 Pb/ 238 U age is ± 15% concordant with a corresponding 207 Pb/ 206 Pb age.

terozoic and Mesoproterozoic grains in samples of the Chopawamsic metased- imentary rocks hint at a possible peri- Gondwanan source area; they are also consistent with the possibility that there is an older basement to the Chopawamsic arc, as concluded by pre- vious workers (e.g. Pavlides et al. 1994; Coler et al. 2000). The Storck meta- sedimentary unit lies between the Chopawamsic and Potomac terranes for at least 10 km along strike and includes a peri-Gondwanan detrital zir- con signature. The position of the Storck rocks, considered in conjunc- tion with older detrital zircon in the Chopawamsic Formation, provides cir- cumstantial evidence to suggest that the Chopawamsic terrane is also peri- Gondwanan. The Storck rocks and other peculiar units along strike, including the Shores mélange, highlight the potential tectonic telescoping of intra-Iapetan terranes that may have occurred between the Chopawamsic and Potomac terranes. Similar to the Storck rocks, the Moretown Formation in Vermont and Massachusetts sam- pled a peri-Gondwanan source area and is associated with an Ordovician volcanic arc (Shelburne Falls arc) that accreted to Laurentia in the Ordovician (MacDonald et al. 2014). Further- more, both the Moretown Formation and Chopawamsic arc sutures with Laurentian rocks were intruded by post-accretion plutons (Middlefield and

Ellisville bodies) at 445 – 444 Ma (Hughes et al. 2013a; MacDonald et al. 2014).

New data from the Arvonia successor basin suggests that it was deposited with access to both recycled Potomac terrane and Chopawamsic terrane detritus after the Late Ordovi- cian accretion of the Chopawamsic ter- rane to Laurentia (Fig. 6). This conclu- sion is supported by the uncon- formable relationship of the Arvonia basin over granodiorite related to the latest Ordovician Ellisville pluton, which stitches the Potomac and Chopawamsic terranes. Also in sup- port of this conclusion, Nd isotope analyses by Owens et al. (2013) showed that rocks of the Arvonia Formation are most like sedimentary rocks in the Appalachians deposited after ca. 450 Ma (Late Ordovician). The youngest zircon grains in the Buffards Forma- tion of the Arvonia basin indicate that sedimentation in the Arvonia basin continued until at least ca. 430 Ma and may reflect syndepositional magmatic activity.

SUMMARY AND CONCLUSION Our new data lead to the following main conclusions concerning the tec- tonic development for Cambrian – Sil- urian metasedimentary rocks of the western Piedmont of north-central Virginia:

1. The youngest sampled units of the

composite Potomac terrane meta- sedimentary rocks were most likely deposited along the margin of Laurentia sometime between ca. 500 – 470 Ma and most impor- tantly were not derived from the Middle Ordovician Chopawamsic Formation.

2. The Storck metasedimentary rocks were deposited sometime after ca. 500 Ma and tapped a peri-Gond- wanan source area. Their geo- graphic position suggests a peri- Gondwanan affinity for the Chopawamsic arc.

3. Chopawamsic Formation sedimen- tary rocks were mostly derived from Chopawamsic terrane mag- matic rocks during the Middle Ordovician (ca. 467 Ma). The old- est grains in these samples are not suitable for asserting Mesoprotero- zoic cratonic affinity, but the pres- ence of Cryogenian – Cambrian grains and the general dearth of Mesoproterozoic zircon are consis- tent with a peri-Gondwanan source.

4. The Arvonia basin was only deposited after the Chopawamsic terrane accreted to Laurentia in the Late Ordovician and it derived sediment from both the Potomac and Chopawamsic terranes in addi- tion to ca. 430 Ma magmatic rocks.

The Chopawamsic fault marks the main boundary between the Potomac and Chopawamsic terranes. We favour a peri-Gondwanan affinity for the Chopawamsic arc based upon the lack of data to tie it to Laurentia, its structural position above and out- board of peri-Gondwanan derived metasedimentary rocks (the Storck rocks), and Neoproterozoic – Cambri- an zircons recovered from metasedi- mentary samples of the Chopawamsic Formation and Storck quartzite that are potentially derived from older peri- Gondwanan rocks. Because the Chopawamsic arc is interpreted as peri- Gondwanan, we advocate that the Late Ordovician Chopawamsic fault system demarcates the main Iapetan suture in the southern Appalachian orogen. A latest Middle to Late Ordovician Iapetan closure in the southern Appalachian orogen discussed here and illustrated by Hughes et al. (2014) is analogous to models proposed in the

516

500 1000

1500

2000

2500

15

10

15

10

Storck KSH-11-40

n=107

Shores n=88

Age (Ma)

WP2 (Bailey et al. 2008)

Figure 8. Histogram comparison of concordant detrital zircon ages from the Storck rocks and the Shores mélange (Bailey et al. 2008).

G EOSCIENCE C ANADA

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Virginia: Virginia Department of

Mines, Minerals, and Energy Division

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Allochthon and Lynchburg Group, Shaffer, and the rest at the Memorial

of Mineral Resources, Open File

Southern Appalachians: Implications University MicroAnalysis Facility are

Report 05-02, scale 1:24,000.

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Bailey, C., Eriksson, K., Allen, C., and

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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.