INTRODUCTION AND REGIONAL OVERVIEW

K. Stephen Hughes James P. Hibbard

Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695, USA

Brent V. Miller

Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA

Jeffrey C. Pollock

Department of Earth Sciences, Mount Royal University, Calgary, Alberta T3E 6K6, Canada

Alet A. Terblanche Dillon M. Nance

Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695, USA

David J. Lewis

Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA

ABSTRACT

The Appalachian orogen represents the Paleozoic amalgamation of Laurentian and Gondwanan terranes; however, the suture of the interstitial early Paleozoic Iapetus Ocean has not been identifi ed in the southern Appalachians. In the western Piedmont of Virginia, the Potomac and Chopawamsic terranes are separated by the Chopawam- sic fault, which has been hypothesized to represent the main Iapetan suture. We have conducted new mapping, geochemistry, and geochronology on rocks from these ter- ranes to gain insight into their origin and interaction. Detrital zircon geochronology across correlative units of the metaclastic Potomac terrane is consistent with the inter- pretation that they are chiefl y derived from Laurentian Mesoproterozoic rocks and they were deposited sometime between 500 and 470 Ma. Detrital zircon geochronol- ogy and plutonic and volcanic crystallization ages in the metavolcanic Chopawamsic

Hughes, K.S., Hibbard, J.P., Miller, B.V., Pollock, J.C., Terblanche, A.A., Nance, D.M., and Lewis, D.J., 2014, Does the Chopawamsic fault represent the main Iapetan suture in the southern Appalachians? Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia, in Bailey, C.M., and Coiner, L.V., eds., Elevating Geoscience in the Southeastern United States: New Ideas about Old Terranes: Field Guides for the GSA Southeastern Section Meet- ing, Blacksburg, Virginia, 2014: Geological Society of America Field Guide 35, p. 41–61, doi:10.1130/2014.0035(02). For permission to copy, contact editing@ geosociety.org. © 2014 The Geological Society of America. All rights reserved.

Hughes et al.

terrane show that the Chopawamsic arc was active between 474 and 465 Ma. Stops on this fi eld trip will highlight key outcrops that help further our understanding of the tectonic development of the Potomac and Chopawamsic terranes prior to their amalgamation in the Late Ordovician. Based on the data presented in this fi eld guide, it remains plausible that the Chopawamsic fault represents either the main Iapetan suture or the closure of a smaller seaway.

INTRODUCTION AND REGIONAL OVERVIEW

metamorphosed clastic units (Mine Run Complex, Lunga Reser- voir Formation, Sykesville Formation, among others) that have

The main Iapetan suture is the structure that demarcates the been interpreted to be shed from the Laurentian craton in the early boundary between crustal components that once existed on oppo- Paleozoic (Drake, 1989; Pavlides, 1989; Horton et al., 2010). site sides of the early Paleozoic Iapetus Ocean. This boundary The Potomac terrane is intruded by a host of Ordovician to Early between native Laurentian rocks and those peripheral to Gond- Silurian granitoid bodies, the oldest of which is the ca. 472 Ma wana has not been identifi ed in the southern Appalachians, but Occoquan pluton (Aleinikoff et al., 2002). Based upon this cross- it has been recognized in Newfoundland as the Red Indian Line cutting relationship, rocks of the Potomac terrane are interpreted (Williams et al., 1988; Zagorevkski et al., 2007). The Chopawam- to be Early Ordovician or older. Smaller granitoid bodies in the sic fault may represent a southern corollary to the Red Indian Line Mine Run Complex have previously been interpreted to represent because it separates metaclastic rocks of the Potomac terrane, exotic blocks in a pelitic mélange; these bodies were assumed to believed to be derived of the Laurentian craton, from magmatic

be shed from the proximal Chopawamsic arc terrane (Pavlides, rocks of the Chopawamsic arc system of unknown, potentially 1989). Glover et al. (1997) interpreted the various “mélange” peri-Gondwanan, crustal affi nity (Figs. 1 and 2, summarized in units throughout the Piedmont to represent the suturing of the Hibbard et al., 2014).

magmatic arc terranes to the east (Chopawamsic rocks and rocks Little is known about the signifi cance of the poorly exposed of Carolinia) to North America. Chopawamsic fault, which has generally been inferred to be a

The Chopawamsic terrane is a collection of metamorphosed steeply dipping thrust emplacing the Chopawamsic terrane over magmatic rocks and associated metaclastic rocks that formed as the Potomac terrane (e.g., Brown, 1986; Pavlides, 1989, 1990, part of the Chopawamsic arc in the Middle–Late Ordovician. The 1995; Pavlides et al., 1994; Mixon et al., 2000, 2005). In both terrane is comprised of the greenschist-facies Chopawamsic For- terranes, no known basement rocks are exposed. However, rocks mation and amphibolite-facies Ta River metamorphic suite. The of the metaclastic Potomac terrane have traditionally been inter- Ta River metamorphic suite is sometimes considered a part of preted to be mostly derived of the Laurentian craton (Evans, 1984; the Chopawamsic Formation (Spears et al., 2013; Hughes et al., Drake, 1989; Pavlides, 1989). Although apparently built upon in press) but until comprehensive work shows correlation of the Mesoproterozoic crust (Coler et al., 2000), the cratonic affi nity greenschist-grade Chopawamsic Formation and the amphibolite- of the magmatic Chopawamsic terrane has not been determined. facies Ta River suite, we show the two as separate units (Fig. 2) If the Potomac terrane can be confi rmed to be Laurentian and the similar to Pavlides et al., (1994). The Chopawamsic Formation Chopawamsic terrane was built upon peri-Gondwanan crust, the has been dated at 471.4 ± 1.3 Ma (Coler et al., 2000) and at 453 ± Chopawamsic fault would represent the main Iapetan suture in

4 Ma (Horton et al., 2010), while a sample of the Ta River suite has the southern Appalachians.

been dated at 470 +1.3/–1.5 (Coler et al., 2000). These geochro- In central and northern Virginia, the composite Potomac nological data have led to complications with twentieth- century terrane is a collection of heterogeneously deformed and weakly models of tectonic development related to the Chopawamsic arc,

Virginia Western Piedmont

Mesozoic Culpeper and Danville basins and related rocks Ordovician to Early Silurian intrusive bodies.

OP

Area of

Ordovician rocks of the Chopawamsic Cf CM Figure 2

Terrane (C) and Milton Terrane (M). P

Figure 1. Regional geology in the western Piedmont of Virgin-

P SRA Cambrian to Ordovician rocks of the Potomac

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

ia. R—Richmond; Cf—Chopawamsic fault; OP— Occoquan

Ridge

idge pluton. Geology modifi ed from Hibbard et al. (2006). in

Valley and Ridge Vall Valley and Ridge B Blue Ridge Blue Ridge

E. Piedmont E. Piedmont

Coastal Plain Coas Coastal Plain

Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia

once thought to be Early Cambrian or older (Pavlides, 1981, Richland Run pluton, Garrisonville mafi c complex) and some of and references therein) and directly related to volcanic rocks which are interpreted to be a result of post-Chopawamsic mag- of the crustal block of Carolinia (e.g., Glover et al., 1997). The matic activity (e.g., Latest Ordovician Ellisville pluton, Carbon- Chopawamsic terrane also includes several intrusive bodies— iferous Falmouth intrusive suite, Salem Church allochthon). The some interpreted to be related to Chopawamsic arc activity (e.g., Chopawamsic arc is interpreted to lie above older continental

Field Trip stops = Lunga

Res.

Rappahannock River

LR LR LR 2-2

GV GV GV 38°30′ N 2-6 Smith CF

G G 2-1

Lake

2-5

Culpeper

2-4 RR RR RR

MRF

Storck

2-3

Abel Lake

River

1-7

Rapidan

Lake of

SC SC SC

the W oods

1-5 Hunting

1-8

IV III

Run Res.

Wilderne

1-6 ss

Fredericksburg

LG LG

Orange

38°15′ N

1-4

Ta Ta Ta

LBF

II I

1-3 SSZ

L Spotsylvania L

CF C.H.

1-1

1-2

F Richmond F

Figure 2. Simplifi ed geologic map of the fi eld trip area, north-central Virginia Piedmont. Geologic unit abbreviations are: C—Chopawamsic Formation; E—Ellisville granodiorite pluton; F—Falmouth intrusive suite; G—Garrisonville mafi c complex; GV—Goldvein granite pluton; I, II, III, IV—Units I–IV of the Mine Run complex; L—Lahore pluton; LG— Locust Grove tonalite body; LR—Lunga Reservoir Formation; Q—Quantico Formation; RR—Richland Run compos- ite pluton; SC—Salem Church allochthon; Ta—Ta River metamorphic suite. Geologic contact abbreviations are: CF— Chopawamsic fault; LBF—Long Branch fault; MRF—Mountain Run fault; SSZ—Spotsylvania shear zone. Geology modifi ed after Pavlides (1990, 1995), Mixon et al. (2000, 2005), Terblanche (2013), and our own reconnaissance mapping.

Hughes et al.

crust because Ordovician Chopawamsic arc rocks yield inherited Mesoproterozoic zircons and an evolved ε Nd signature (Coler et al., 2000). The crustal affi nity of this Mesoproterozoic basement remains “suspect” (Williams and Hatcher, 1982; Hibbard et al., 2007, 2014).

The Quantico and Arvonia successor basin system was deposited over the Chopawamsic terrane after substantial ero- sion. The current boundary between schists and slates of the suc- cessor basins and the underlying Chopawamsic terrane varies. The clearly unconformable relationship of the Arvonia basin over intrusive rocks of the Chopawamsic terrane (Spears and Bailey, 2002, and references therein) indicates that uplift and erosion occurred between deposition of the Chopawamsic terrane and the successor basin rocks. On the basis of less direct observations, in other locales the contact between the Chopawamsic terrane and the overlying successor basin has been interpreted to be interlay- ered (Horton et al., 2010, and references therein) and in yet other places tectonic (Pavlides, 1990, 1995; Mixon et al., 2000).

NEW INSIGHTS

The purpose of this trip is to showcase new geochemical and geochronological data in the western Piedmont of Virginia. From these data, we are revising and refocusing our understanding of the origin and interaction of the Potomac and Chopawamsic terranes and how they may relate to the closing of Iapetus, as recorded in the southern Appalachians. Ongoing studies and new data presented in this guide are a product of collaboration among geologists from North Carolina State University, Mount Royal University, and Texas A&M University. Additional laboratory analyses have been performed at Memorial University in New- foundland and Washington State University. Thus far, this NSF (National Science Foundation)- and U.S. Geological Survey– funded research has resulted in an EdMap deliverable (Hughes, 2011), one completed master’s thesis (Terblanche, 2013), meet- ing abstracts (Hughes and Hibbard, 2011; Hibbard et al., 2012a; Hughes and Hibbard, 2012a, 2012b; Hughes et al., 2012a, 2012b, 2013b), fi eld trips (Hughes, 2010; Terblanche and Nance, 2012), one book chapter (Hibbard et al., 2014), one paper (Hughes et al., 2013a), and others in preparation.

Recently published results have shown that the Potomac and Chopawamsic terranes were joined along the Chopawamsic fault prior to emplacement of the ca. 444 Ma (Latest Ordovician) Ellisville pluton; thus, considering the Middle to Late Ordovi- cian age of the Chopawamsic arc, the Chopawamsic fault is a Late Ordovician structure (Hughes et al., 2013a). This research focused on evaluating the stitching nature of the Ellisville pluton by exploring its contacts with the surrounding country rock and examining its internal geochemistry and geochronology. Scant kinematic data for the poorly exposed Chopawamsic fault in the fi eld trip area suggest it formed in a sinistral transpressive envi- ronment (Hughes et al., 2013a); these data match kinematic indi- cators we have found at the western boundary of the Chopawam- sic terrane at the James River.

Detrital zircon geochronology of six samples (608 analy- ses) from the Potomac terrane reveals mostly Mesoproterozoic detritus with the youngest zircons being Cambrian (Hughes et al., 2012a). The youngest zircons present provide age control on the Potomac terrane, which is intruded by the ca. 472 Ma Occoquan granite (Aleinikoff et al., 2002). The lack of Ordovician detritus in samples of the Potomac terrane also indicates that the Potomac terrane was not linked to Middle Ordovician magmatic rocks of Chopawamsic terrane via sedimentary dispersal paths (see Stops 1-6, 1-7, and 2-2). Three small (0.1–15 km2) granitoid bodies, once interpreted to be detrital blocks in the Potomac terrane derived of the Chopawamsic arc (Pavlides, 1989), all crystallized at ca. 470 Ma (Hughes et al., 2013b); these bodies are now inter- preted to be either intrusive or tectonic additions to the Potomac terrane based upon the lack of detritus of corresponding age in the metasediments of the terrane (Stops 1-3, 1-4, and 1-5).

Geochronological results from the Chopawamsic terrane indicate that magmatism occurred between 474 and 465 Ma (Hughes et al., 2013b). Preliminary data show that plutonic rocks in the Chopawamsic arc record crystallization ages between 474 and 468 Ma (see Stops 2-1 and 2-4). Newly dated Chopawam- sic volcanic rocks record crystallization ages between 468– 465 Ma (see Hughes et al., 2013b, and Stop 2-3). Detrital samples in the Chopawamsic Formation include mostly recycled debris from their coeval volcanic units; most detrital zircons have ages between 470 and 460 Ma (see Stop 1-2). The detrital samples also include a small population of Cambrian–Mesoproterozoic zircons, refl ecting the infl uence of an additional older source area for the Chopawamsic metasedimentary rocks.

Metasedimentary rocks in the vicinity of Wilderness and Storck were previously mapped as successor basins unconform- ably deposited on rocks of the Potomac and Chopawamsic ter- ranes as well as the Chopawamsic fault (Pavlides, 1990, 1995; Mixon et al., 2000). These rocks were interpreted to be two small outlier basins related to the larger Early Silurian Quantico and Arvonia depositional basins that overlie the Chopawamsic ter- rane (Taber, 1913; Southwick et al., 1971; Mixon et al., 2000). Focused mapping in the Wilderness and Storck areas by North Carolina State University masters’ students has shown that no such successor basin exists in the Wilderness area (Terblanche, 2013), and that the metasedimentary rocks near Storck lie against, rather than above, the Chopawamsic fault (Dillon Nance, 2013, personal commun.). A sample of the metasediments near Storck reveals a detrital signature unlike either the Potomac or Chopawamsic terranes or the Quantico and Arvonia system (see Stop 2-4). We interpret the detrital signature to indicate that the Storck rocks were derived from a peri-Gondwanan or mixed peri-Gondwanan and Laurentian source area. The position of the Storck rocks inboard of the Chopawamsic terrane favors peri- Gondwanan affi nity for the Chopawamsic arc.

The Chopawamsic fault remains a viable candidate as the main Iapetan suture in the southern Appalachians (Fig. 3). The mapping, geochemistry, and geochronology we have undertaken in the western Piedmont can lead to partial revision of tectonic

Downloaded from

Taconic Suture

A Geolo

Potomac accretionary Potomac terrane

c. 470 Ma

complex

?Taconic Arc?

Chopawamsic arc

c. 470-460 Ma

Chopawamsic arc

(early phase)

(one phase)

fieldguides.gsapubs.org

Taconic Seaway

gy

,g Laurentia

lithospheric mantle

lithospheric mantle

hemistry

Fragment of Gondwana

, and g

B Chopawamsic fault

c. 460 Ma

ii

c. 455-450 Ma

(Main Iapetan Suture)

Main Iapetan

on April 1, 2014

2 Carolinia Seaway

Carolinia

hr onolo

gy of the western Piedmont of northern

Incipient slab

Slab break off (x2)

breakoff Successor basin

C Ellisville Pluton c. 450 Ma

Chopawamsic arc

(late phase)

iii

c. 444-430 Ma

Successor basin

Slab breakoff

/ablation

Hibbard et al., 2014 (devised in 2007)

Modified from Hibbard et al. (2014) with consideration of our new data. ir V ginia

Figure 3. Tectonic models for the accretion of the Chopawamsic arc. Model from Hibbard et al. (2014) and our working model are shown.

Hughes et al.

models for the accretion of the Chopawamsic arc to Laurentia. is provided. The total mileage from Stop 1-1 through all interme- Figure 3 shows a model devised in 2007 (Hibbard et al., 2014) diate stops to Stop 2-6 is ~120 miles (193 km). alongside our modifi cations. In the Hibbard et al. (2014) model

In the interest of time, some fi eld trip stops listed as optional (Figs. 3A, 3B, and 3C), the accretion of the Chopawamsic arc may be cut from the day’s itinerary. The fi rst day of the fi eld trip and Potomac accretionary complex to Laurentia occurred when will focus on sites to the south of the Rappahannock River while the Taconic seaway closed. This model supported two phases of the second day will focus on sites to the north of the river. Chopawamsic volcanism and a Laurentian affi nity for continen- tal basement beneath the Chopawamsic arc.

■ DESCRIPTION OF STOPS

All stops on the trip, other than roadcuts, are on private represent the “Taconic arc” in the southern Appalachians because land. Proper permission should be obtained to access each stop.

Our new data suggest that the Chopawamsic arc may not

it was active from 474–465 Ma (with the bulk of magmatism at

470–469 Ma); this span of magmatic ages is during one of the Day 1

noted magmatic gaps recorded in peri-Laurentian arcs related to

the Taconic orogeny in Newfoundland (van Staal et al., 2007). Stop 1-1. Ellisville Granodiorite Pluton at Lake Anna

Also, the position of the Storck rocks, with a peri-Gondwanan 38.1404ºN, 77.8897°W; Lahore Quadrangle (Fig. 4) detrital signature, inboard and to the west of the Chopawamsic terrane suggests that the Chopawamsic arc is peri-Gondwanan.

This outcrop on the edge of Lake Anna is referred to as “fi sh- In our modifi ed model (Figs. 3i, 3ii, and 3iii), we hypothesize ing rock” by the property owners. This layered granodiorite is that in the Middle Ordovician, the Potomac terrane was proximal located on the Woodside portion of the ca. 1805 Prospect Hill to Laurentia, was buried, lithifi ed, intruded, and underwent a fi rst plantation. The foliation feature present here (Fig. 5A) is charac- episode of deformation when a Taconic arc (not presently exposed) terized by euhedral, aligned feldspar crystals and it is interpreted was accreted to Laurentia. The Chopawamsic arc formed as one to be a magmatic, rather than metamorphic, fabric. This mag- continuous phase of volcanism mostly in the Middle Ordovician matic fl ow foliation also occurs elsewhere in the Ellisville pluton (474–465 Ma) above a Mesoproterozoic block of Gondwanan (Pavlides et al., 1994). The rock here is part of the main phase of crust. The Chopawamsic arc was accreted to Laurentia in the Late medium-grained granodiorite found throughout the pluton. This Ordovician as Iapetus closed. This event superimposed a second phase of the pluton has been dated further south in the pluton at episode of deformation upon some parts of the Potomac terrane. 443.7 ± 4.4 Ma and a less voluminous fi ne-grained granodiorite After accretion, the fault was stitched by the Ellisville pluton at ca. phase of the pluton (not present here) has been dated at 436.8 ± 444 Ma; ensuing uplift related to the accretion of Carolinia (see Hibbard et al., 2012b) after the closure of a Cherokee seaway, and subsequent erosion was followed by unconformable deposition of successor basins over the Chopawamsic terrane in the Early

Kilometers

0 0.25 0.5 0.75 Silurian. Our fi gure does not show the presence of allochthonous 1 tectonic slivers along the trace of the Chopawamsic fault; these

include the Shores complex (Brown, 1986), the Storck rocks, and potentially other small packages of metasedimentary and meta- mafi c rock that were mobilized along the Chopawamsic fault interface as Iapetus was closing.

LOGISTICS

Some outcrops in this trip have been described by previous workers (Pavlides, 1976, 1989; Horton et al., 2010) but most have not been previously written up in fi eld trip guides. The choice to sample these sites for geochronological analyses and visit them in this trip is twofold: they are easy targets, well described and easily accessed; and many serve as reference locations for the units across the western Piedmont.

Geographic coordinates (WGS-84 coordinate system) and quadrangle locations are listed for each stop. All stops are shown on the geologic map in Figure 2 and a small inset map is shown for each stop as well. Topography in the small inset maps is from 7.5′ USGS quadrangle maps and geology is modifi ed from Figure 4. Location of Stop 1-1. Lahore and Belmont 7.5′ quadrangles. Mixon et al. (2000, 2005) and our mapping. No mileage road log SOe—Ellisville granodiorite; O CmII—Mine Run Complex Unit II.

Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia

Figure 5. (A) Magmatic fl ow banding at Stop 1-1 in the Ellisville granodiorite. Compass points north and is 7 cm wide. (B) Ash layer within the Chopawamsic Formation at Stop 1-2. Black marker top points north. Marker measures 14 cm long. (C) Volcanic conglomerate within the Chopawamsic Formation at Stop 1-2. Black marker top points north. Marker measures 14 cm long. (D) Rip-up-clast of quartzo-feldspathic graywacke of the Mine Run Complex at Stop 1-6. (E) Con- tact between pebbly and phyllitic bedding in the Mine Run Complex near Stop 1-7. Bedding (048°, vert.) fi nes upward to the southeast and is oblique to foliation (202°, 74° NW). End of rock hammer pick for scale. Location along Wilderness Run at 38.3587°N, 77.7045°W. (F) Fist-sized, fi ne-grained mafi c xenoliths in plagiogranite of the Richland Run pluton near Stop 1-8. Location is on Rapidan River at 38.3658°N, 77.6299°W.

Hughes et al.

4.2 Ma (Hughes et al., 2013a). The contact of these two phases (Coler et al., 2000). The limited number of older zircons may be in only rarely seen in outcrop, but limited fi eld observations also due to rapid and voluminous Chopawamsic arc volcanism fol- confi rm that the fi ne-grained phase is younger (Pavlides and lowed by subsequent infl ux into and domination of sedimentary Cranford, 1982; Hughes et al., 2013a).

systems. The model envisioned for deposition of the sampled The Ellisville pluton intrudes both the Potomac and metasediments in the Chopawamsic terrane is similar to the intra- Chopawamsic terranes. In this area it is intrusive to the Mine arc depositional setting proposed by Cawood et al. (2012). Run Complex of the Potomac terrane but further south it intrudes

rocks of the Middle Ordovician Chopawamsic Formation. Thus Stop 1-3. Plentiful Creek Trondhjemite at Sullivan Road

the Ellisville pluton effectively stitches the Chopawamsic fault 38.1996°N, 77.8201°W; Belmont Quadrangle (Fig. 8) and its crystallization age (ca. 444 Ma) can be used as a date of latest possible motion on the fault (Hughes et al., 2013a). Motion

The rock here was mapped as a block of mylonitic tonalite- on the fault occurred sometime between 465 and 444 Ma.

granodiorite gneiss covering ~0.45 km 2 on the Fredericksburg 30′ The epicenter of the M5.8 “Mineral” earthquake that hap- × 60′ quadrangle geologic map (Mixon et al., 2000). Surrounded pened on 23 August 2011 is ~25 km to the SSW. This event has by phyllite and metasandstone of the Mine Run Complex, this resulted in a resurgence of focus upon the Ellisville pluton and block and others were interpreted to be shed from an approach- some workers (Harrison, 2012) have reported brittle features to ing Chopawamsic arc as it was being accreted to North America exist along the southeast margin of the “tail” of the pluton and (Pavlides, 1989). Importantly, the contact between the country interpret them as potential reactivation surfaces. Other work has rock and this gneissic block has not been observed. Its gneissic shown that Paleozoic ductile strain recorded in Chopawamsic character made it an appealing geochronological target to check Formation rocks to the east of the pluton is spatially coincident for exposed basement in the Potomac terrane. with the modern seismogenic surface (Hughes et al., in press).

Geochemical analysis indicates that this rock contains The main house of the Prospect Hill plantation, just 2 km to 68.8% SiO 2 , 14.5% Al 2 O 3 , 0.2% CaO, 5.6% Na 2 O, and 1.8% the southeast of here, was built in 1812 and now functions as the K 2 O. Because of the lack of CaO, this composition falls in the Littlepage Inn bed and breakfast. Creation of the lake in the early alkali feldspar granite fi eld of the Q-ANOR [Q= 100*(Qtz/ 1970s for the nuclear power plant fl ooded the North Anna River (Qtz+Ab+Or+An)) and ANOR= (100*An/(An+Or))]diagram and its local tributaries. At other locations on the property, there (Streckeisen and LeMaitre, 1979). It lies in the trondhjemite fi eld remain partially completed mill stones, carved from the local out- of the Ab-An-Or ternary diagram of Barker (1979, Fig. 9A). The crops of Ellisville Granodiorite.

Stop 1-2. Weathered Chopawamsic Formation along Power Line Cut, Brookridge Estates Road

Kilometers

38.1437°N, 77.7955°W; Belmont Quadrangle (Fig. 6)

Although saprolitic, this outcrop (Figs. 5B and 5C) show- cases the heterogeneity of the Chopawamsic Formation. At least four rock types are present within this small outcrop. These

fault

include a silty phyllite, a garnetiferous biotite schist, a volca- nogenic conglomerate, and a metamorphosed fi ne ash layer. Graded, vertical bedding strikes 030° and shows the stratigraphic younging direction at this location is to the WNW. Layers such as these in the Chopawamsic “pile” are diffi cult to trace from

outcrop to outcrop. !

A nearby outcrop of Chopawamsic Formation metasandstone (~5 km to the SW) was sampled for detrital zircon analysis. The collective data from this sample and three other metasedimen- tary samples in the Chopawamsic Formation show a peak mode

Chopawamsic

population at 467 Ma with a small population of 1.0–1.15 Ga zircons (Fig. 7A). The ca. 467 Ma zircons are interpreted to be can- nibalized debris derived from simultaneous volcanism at the time of deposition of the sediments. Therefore, the ca. 467 Ma peak matches known ages for the Chopawamsic Formation (471 Ma,

Long Branch fault

Coler et al., 2000; 474–465 Ma, Hughes et al., 2013b). The Figure 6. Location of Stop 1-2. Belmont 7.5′ quadrangle. O CmII— Mesoproterozoic zircons are consistent with the Chopawamsic Mine Run Complex Unit II; Oc—Chopawamsic Formation; Sq— arc being built on top of some form of older continental crust Quantico Formation.

Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia

467 70 Chopawamsic Terrane

Chopawamsic Forma on

B Mine Run Complex A Mine Run Com lex Concordia age = 222 n= 385 206 Pb/ 238 U age = 6

n= 228

Potomac Terrane 206 Pb/ 207 238 Pb/ U age = 107 206 Pb = 1 Concordia age = 277

n = 228 206 Pb/ 238 U age = 6

50 n = 385

Concordia age = 222

207 Pb/ 206 Pb age = 1 40 206 Pb/ 238 U age = 107

Stop 1-2

Concordia age = 277

Number

Stop 1-6

35 C Lunga Reservoir Fm. Lunga Reservoir Forma on n= 222 10 D 1120 Storck Rocks 1055

Potomac Terrane Potomac Terrane

1015 n = 107

n = 222

Pb/ 978 Pb age = 1

Concordia age = 106

20 1430 Concordia age = 112

206 Pb/ 238 U age = 104

Stop 2-4

Number 15

Stop 2-2

Age (Ma)

Age (Ma)

Figure 7. Detrital zircon histogram and probability plots. (A) Chopawamsic Formation. (B) Mine Run Com- plex. (C) Lunga Reservoir Formation. (D) Storck quartzite. Note vertical scales are different in each plot.

difference between these two plots is that the Ab-An-Or ternary diagram incorporates Na 2 O (via Ab) into its calculation while the Q-ANOR diagram only uses An and Or values derived from

y the major oxide chemistry. It is possible that element mobility

through metasomatism has altered the original wt% of the major elements, however, the lack of exposure of this rock type makes identifying any potential unaltered compositions diffi cult. On the igneous spectrum of Hughes (1973), this rock plots just outside of the unaltered envelope, near the spillite fi eld. Although a spillite

is technically basalt altered by the replacement of CaO by Na 2 O, its position on the Hughes plot may indicate that similar altera- tion affected this rock if it was originally intrusive into oceanic crust. In support of a case for alteration, the rock contains enough

Na 2 O to be considered Na-altered on a Na 2 O versus Al 2 O 3 /Na 2 O plot. A sample from this outcrop yields an εNd (470) value of −1.4, which is a value that could support formation in a setting with near-CHUR (chondritic uniform reservoir) values. Contradictory to a potential oceanic crust origin, on Pearce et al. (1984) trace-

element discrimination diagrams, this sample consistently plots

Kilometers

0 0.3 0.6 0.9 1.2 in the volcanic arc granitoid fi eld (Fig. 9B). Trace elements for this sample have been normalized to primitive mantle values of

Figure 8. Location of Stops 1-3 and 1-4. Belmont 7.5′ quadrangle. Sun and McDonough (1989) and are shown in Figure 9C. Nega- O CmI—Mine Run Complex Unit I; O CmII—Mine Run Complex tive niobium and titanium anomalies are consistent with the vol- Unit II; Opc—Plentiful Creek trondhjemite; Opy—Paytes granite.

canic arc origin as suggested by Figure 9B.

Hughes et al.

A sample from this outcrop (KSH-11-17) was selected for

A An

U-Pb single-crystal zircon TIMS (thermal ionization mass spec- trometry) analysis. After crushing, separating, and picking, fi ve analyzed zircons have a concordant weighted mean age of 469.5 ±

0.8 Ma (Fig. 10A, 2σ uncertainties include uranium decay constant uncertainty). The age of this sample alone is compatible with it being shed from the Middle–Late Ordovician Chopawamsic ter- rane but supporting detrital zircon data from the Mine Run Com- plex and correlative Lunga Reservoir Formation (see Stops 1-6,

LG

ite

1-7, and 2-2) indicate no Ordovician or younger debris is present in the Potomac terrane. If a map-scale block were to be shed into a

Tonalite

anodior

depositional basin, smaller coeval detritus would certainly be pres-

Gr

ent as well. With these data in mind, we now interpret this body as either an intrusive body or a fault-bounded tectonic sliver.

PY Granite

Ab Trondhjemite PC

Or

Stop 1-4 (Optional). Paytes Granite at Craigs Church Road

B 38.2281°N, 77.8231°W; Belmont Quadrangle (Fig. 8)

Syn-Collisional The rock here, like the Plentiful Creek body, is also mapped (S-type)

Within-plate

as a block of mylonitic tonalite-granodiorite on the Fredericks-

(A-type)

burg 30′ × 60′ quadrangle geologic map (Mixon et al., 2000). It

has been mapped to cover ~0.10 km 2 and is the site of a spring in 100 the headwaters of Robertson Run. Similar to the rock at Stop 1-3, no exposed contact between this granitoid body and the surround- Rb

PY

ing Mine Run Complex has been discovered. It was also targeted Volcanic Arc PC

to test for basement exposure and to test the hypothesis that these (I-type) bodies represent detrital blocks within the pelitic mélange of the

Mine Run Complex (Pavlides, 1989).

10 LG This rock is characterized by 61.5% SiO 2 , 15.3% Al 2 O 3 ,

Ocean Ridge

1.3% CaO, 4.4% Na 2 O, and 3.61% K 2 O. From these composi-

(OR-type)

tions, fi eld observations, and thin-section analysis, it is evident that this rock is not identical to the rock seen at Stop 1-3; further-

more the foliation here is much weaker than that in the Plentiful Creek trondhjemite. The Paytes body falls in the quartz syenite

Y+Nb

C fi eld of the Q-ANOR diagram (Streckeisen and LeMaitre, 1979)

Potomac terrane

and in the granite fi eld of the Ab-An-Or Barker (1979) diagram (Fig. 9A). The rock lies well within the igneous spectrum of

Hughes (1973) and shows no signs of Na 2 O loss or alteration. On Pearce et al. (1984) trace-element discrimination plots,

e Mantle

PY

this rock consistently plots in the within-plate-granite fi eld (Fig.

imitiv 10

9B). On a plot of 10,000*Ga/Al versus Zr (Whalen et al., 1987),

PC

the Paytes granite plots as an A-type (anorogenic/anhydrous)

ed to Pr

LG

granite and can be more specifi cally recognized as an A2 type when plotted on the Y-Nb-Ce ternary diagram of Eby (1992).

maliz 1

A2 granites are interpreted to be derived of the lower crust and to form in postcollisional settings. A sample from this outcrop

Sample Nor Th Nb La Ce Nd Zr Sm Eu Gd Ti Dy Y Er Yb Lu

yields an εNd (470) value of −3.3, slightly more evolved than the Plentiful Creek granitoid. Trace-element concentrations of this sample are normalized to primitive mantle values (Sun and

Figure 9. Geochemical plots. (A) Ab-An-Or ternary diagram af- McDonough, 1989) and shown in Figure 9C. This sample shows ter Barker (1979). (B) Trace-element discrimination diagram after

no niobium anomaly, but does show a negative titanium anomaly Pearce et al. (1984). (C) Trace-element plot comparing Potomac

terrane granitoid samples to primitive mantle values after Sun and

and a positive zirconium anomaly.

McDonough (1989). Abbreviations: LG—Locust Grove tonalite;

A sample of the Paytes granite (KSH-12-05) was selected PC—Plentiful Creek trondhjemite; PY—Paytes Granite.

from this outcrop for U-Pb single-crystal zircon TIMS analysis.

Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia

Preliminary results from seven dissolved zircons provide an upper intercept age of 470.1 ± 1.0 Ma (Fig. 10B, 2σ uncertain-

KSH-11-17

error ellipses are 2σ

A 480 ties include uranium decay constant uncertainty). This age is very

Plentiful Creek Trondhjemite

similar to the ca. 470 Ma age of the Plentiful Creek trondhjemite

n=5

seen at Stop 1-3. For reasons explained in the description of Stop Stop 1-3

1-3, the Paytes body is also interpreted to represent either a small

intrusive body or small tectonic sliver. Three zircon analyses from this body were interpreted to be from inherited zircons; these

are all discordant analyses and have 207 Pb/ 206 Pb ages of 590 Ma,

1005 Ma, and 1170 Ma. A chord forced through the three anal-

yses has a lower concordia intercept of 447 ± 100 Ma and an

Pb/

upper concordia intercept of 1206 ± 93 Ma. The signifi cance

of the upper intercept ages depends on the unfounded assump-

469.5 ± 0.8 Ma

tion that these three zircons originally crystallized coevally.

(2σ, decay-const. errs included)

At minimum, the discordant inherited zircons indicate that the

MSWD (of concordance) = 0.54,

melt that produced this granite body assimilated some form(s)

Pb/ Probability (of concordance) = 0.46 235 U

of Neoproterozoic–Mesoproterozoic crust. Mesoproterozoic zir-

cons are common in the metasediments of the Mine Run Com-

plex (Stops 1-6 and 1-7) that surround the Paytes granite body.

KSH-12-05

error ellipses are 2σ

The close proximity (~2.5 km) of two small, coeval gran-

B Paytes Granite

itoid bodies (stops 1-3 and 1-4) that have potentially different

n=7

modes of formation is puzzling. Heterogeneities among the bod- Stop 1-4

ies in the Mine Run Complex formerly interpreted to be detrital

blocks may be circumstantial evidence for tectonic, rather than intrusive, emplacement.

Stop 1-5. Locust Grove Tonalite at Flat Run Farm

38.3249°N, 77.7834°W; Mine Run Quadrangle (Fig. 11)

The rock here is the third and fi nal stop at a body previously

Pb/

interpreted to represent a depositional block in the metaclastic

470.1 ± 1.0 Ma

Mine Run Complex (site P-78-11 of Pavlides, 1989). However,

(2σ, decay-const. errs included) MSWD (of concordance) = 5.2,

this body is much larger than the other interpreted “blocks” and

Probability (of concordance) = 0.023

covers ~15 km 2 . Originally mapped as the Locust Grove granite

Pb/ U

by Lonsdale (1927), it was called an altered tonalite/granodiorite

on the Fredericksburg 30′ × 60′ quadrangle map (Mixon et al., 2000) and is mostly weakly to moderately foliated throughout.

KSH-12-04

C error ellipses are 2σ 480

Locust Grove tonalite

The rock here contains 64.0% SiO 2 , 15.5% Al 2 O 3 , 4.6%

n=8

CaO, 3.9% Na 2 O, and 0.3% K 2 O. The Locust Grove tonalite

Stop 1-5

at this outcrop plots outside of the igneous spectrum and in the spillite fi eld on the Hughes (1973) diagram; this supports the previous interpretation of alteration. The present composition of

the rock falls in the tonalite fi elds of both the Q-ANOR and Ab- An-Or diagrams (Streckeisen and LeMaitre, 1979; Barker, 1979; Fig. 9A). The potential “spillitization” of this rock may indicate

it formed in a similar setting as the Plentiful Creek trondhjemite

Upper intercept at

Figure 10. Geochronological plots for the Plentiful Creek trondhjemite

(A), Paytes granite (B), and Locust Grove tonalite (C) at Stops 1-3, 207 Pb/ 235 U

472.6 ± 4.8 Ma

MSWD = 1.5

1-4, and 1-5. Black ellipse represents weighted concordia age. Each

ellipse represents the data from one dissolved zircon crystal. White el- 0.52 0.54 0.56 0.58 0.60 lipse with dashed border was not used in the regression calculation for

the Locust Grove tonalite.

Hughes et al.

Kilometers

which he describes as a “fi ne-grained, weakly foliated, poorly

0 0.25 0.5 0.75 1 sorted, feldspathic graywacke.” This location also has been more recently described by Terblanche and Nance (2012). The rock

consists of fi ne–medium-sized grains of quartz, plagioclase,

Lake of the

muscovite, biotite, and chlorite. Most areas of the Mine Run

Woods

Complex matrix rocks have considerably less feldspar than this location. In this outcrop, and a few nearby, there are small (up to fi st-sized) rip-up clasts of identical composition material within the sedimentary matrix (Fig. 5D).

A sample of this rock (KSH-11-05) was collected for detrital

zircon analysis. Three hundred and eighty-fi ve acceptable analy- ses from this and three other samples of the Mine Run complex (Fig. 7B) show that this rock was dominantly derived from a Mesoproterozoic source area. These results are consistent with

a regional, Laurentian source area of crystalline rocks—some of which would have formed during the Grenville orogeny. A concordant 499 ± 15 Ma (2σ uncertainty) zircon represents the youngest zircon analyzed from this outcrop and is the youngest zircon analyzed from all samples of the Potomac terrane in our study (Hughes et al., 2012a). A ca. 500 Ma zircon in a sample of the correlative Lunga Reservoir Formation (Stop 2-2) provides

Figure 11. Location of Stop 1-5. Mine Run 7.5′ quadrangle. Olg— additional credibility to the lone 499 Ma zircon in this sample. Locust Grove tonalite; O CmII—Mine Run Complex Unit II; O CmIII These ca. 500 Ma and other Cambrian–Ediacaran zircons help to —Mine Run Complex Unit III.

delimit the age of deposition of the components of the Potomac terrane. Taken into account with the 472 Ma Occoquan granite (Aleinikoff et al., 2002) that intrudes the Potomac terrane to

(Stop 1-3). However, also like the Plentiful Creek trondhjemite, the north, it appears that the Mine Run Complex was deposited on Pearce et al. (1984) trace-element discrimination diagrams, the Locust Grove tonalite consistently plots in the volcanic arc gran- ite fi eld (Fig. 9B). Trace elements normalized to primitive mantle values (Sun and McDonough, 1989; Fig. 9C) display prominent

niobium and titanium anomalies, suggesting a supra-subduction Kilometers

zone, or recycled supra-subduction zone, setting (Pearce, 1982; Baier et al., 2008).

This site was sampled (KSH-12-04) for U-Pb single-crystal zircon TIMS analysis. Out of nine dissolved zircons, seven data ellipses defi ne a chord with an upper intercept value of 472.6 ±

fault

4.8 Ma (Fig. 10C, 2σ uncertainties include uranium decay con- stant uncertainty). If U-decay uncertainties are ignored, the error reduces to ± 2.4 Ma. The two analyses not included in the inter- cept calculation are one analysis that lies slightly off the line and

a very discordant zircon with a 206 Pb/ 238 U age of ca. 890 Ma, a 207 Pb/ 235 U age of ca. 1120 Ma, and a 207 Pb/ 206 Pb age of ca. 1590 Ma.

The ca. 473 Ma crystallization age for the Locust Grove body shows that all three granitoid bodies sampled in the Mine Run Complex have identical crystallization ages, within analytical uncertainties. However, it remains unclear if they formed in simi- lar tectonic settings.

Stop 1-6. Mine Run Complex Unit I at Wilderness Dam

Chopawamsic

38.3096°N, 77.7365°W; Chancellorsville Quadrangle (Fig. 12)

Figure 12. Location of Stop 1-6. Mine Run and Chancellorsville This site was used by Pavlides (1989) as one of the reference 7.5′ quadrangles. O CmI—Mine Run Complex Unit I; Oc—

locales for Mine Run Complex Unit I. This is his site P-77-34, Chopawamsic Formation.

Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia

offshore of Laurentia sometime between 500–470 Ma, during Late Cambrian to Early Ordovician.

Kilometers

The lack of detritus in any samples (n = 6 samples, n = 607

analyses) of the Potomac terrane that is similar in age to the Mid- dle Ordovician Chopawamsic terrane metavolcanics to the east suggests that the Mine Run Complex was not derived from the Chopawamsic arc (Hughes et al., 2013b). It seems unlikely that all potential zircon from the Chopawamsic arc could have been

selectively removed based upon hydraulic sorting in the hydro- logic system feeding the Potomac terrane depositional basin. Pavlides (1989) interpreted the Mine Run Complex rocks to have

fault

been partially shed from, and deposited ahead of, the Chopawam- sic arc as it approached North America. However he noted that “it is striking that volcanic metagraywacke has not been found in any [part of the Mine Run complex], considering the potential detrital source area of the [Chopawamsic] volcanic-arc terrane that lies immediately east.” The detrital zircon results presented here indi- cate that there are not volcaniclastic strata in the Mine Run Com- plex because its sediment was not derived of the Chopawamsic arc, rather it was already deposited, buried, intruded, and poten- tially metamorphosed by the time of Chopawamsic arc volca- nism (ca. 474–465 Ma).

Figure 13. Location of Stop 1-7. Chancellorsville 7.5′ quadrangle.

Stop 1-7 (Optional). Mine Run Complex Unit I at Eley’s

O CmI—Mine Run Complex Unit I; Oc—Chopawamsic Formation;

Ford, Rapidan River

Orr—Richland Run pluton.

38.3575°N, 77.6850°W; Chancellorsville Quadrangle (Fig. 13) This outcrop in the east bank of Eley’s Ford Road is another

example of Unit I of the Mine Run Complex. It was also a stop in a recent fi eld trip led by Terblanche and Nance (2012). This

Kilometers

outcrop is ~170 m from another reference outcrop for Mine Run

Complex I, which is down on the Rapidan River (Pavlides, 1989, site P-70-152). The rock here is more typical of the rest of the Mine Run Complex than the relatively feldspar-rich composition seen at Stop 1-6. While it is mostly well-foliated phyllite, there are also pebbly layers present. These pebbly layers are common in this area of the Mine Run Complex and have been best recog-

nized along the lower reaches of Wilderness Run, ~2 km to the west (Fig. 5E). The rounded, fi ne–medium-grained clasts include quartz with minor feldspar. In some areas the quartz includes dis-

Hunting

tinct blue quartz.

Run

A mixed sandy and pebbly layer at this location was selected

Reservoir

for detrital zircon analysis (sample P310-4). Similar to the feld- spathic graywacke at Stop 1-6, and all other samples in the Mine Run Complex, most of the zircons in this sample are Mesopro- terozoic (Fig. 7B).

Stop 1-8 (Optional). Richland Run Pluton at Hunting Run Reservoir Dam, Spotswood Furnace Road

38.3537°N, 77.6388°W; Chancellorsville Quadrangle (Fig. 14) Figure 14. Location of Stop 1-8. Chancellorsville 7.5′ quadrangle.

The dam and most of the reservoir is underlain by the Rich- Oc—Chopawamsic Formation; Orr—Richland Run pluton; Sq— land Run pluton. There is good outcrop of the pluton just north Quantico Formation. of Spotswood Furnace Road in a small drainage to the west of

Hughes et al.

the main channel of Hunting Run (this area may be very over- crust that existed between the Chopawamsic arc and the Lau- grown!). The rock here, and in much of the Richland Run pluton, rentian craton. Contacts with surrounding Chopawamsic terrane contains common xenoliths of fi ne-grained mafi c material (Fig. rocks are not exposed, but it appears that intrusive rocks of the 5E), potentially derived of the Garrisonville mafi c complex.

Chopawamsic arc intrude the Garrisonville complex.

A sample of the rock here contains 74.6% SiO 2 , 14.8%

Phases of the Garrisonville complex include (from oldest to

Al 2 O 3 , 2.0% CaO, 5.6% Na 2 O, and 0.8% K 2 O. This composition youngest, excluding phase 6):

plots as a trondhjemite on the Ab-An-Or ternary plot of Barker (1) a fi ne-grained meta-basalt which seems to be the most (1979) and a tonalite on the Q-ANOR diagram of Streckeisen

voluminous and oldest of the phases, and LeMaitre (1979).

(2) a medium-grained meta-gabbro that commonly occurs in The 89-ft-tall Hunting Run dam was built in 2002 and the

association with the fi ne-grained meta-basalt, reservoir behind it covers 420 acres and contains 3.1 billion gal-

(3) a pervasive medium-grained granodiorite-tonalite that lons of water used to supply Spotsylvania County.

almost everywhere includes fragmental xenoliths of the fi ne- and/or medium-grained meta-gabbro phases,

Day 2

(4) a fi ne–medium-grained felsite that appears to be younger than the granodiorite-tonalite phase, but

Stop 2-1. Garrisonville Mafi c Complex at Vulcan

doesn’t contain xenoliths,

Stafford Quarry

(5) an aphanitic mafi c dike set that appears to be the young- 38.4844°N, 77.4483°W; Stafford Quadrangle (Fig. 15)

est and least voluminous phase present in the quarry, and (6) a dark green porphyritic ?actinolite-tremolite ultramafi c?

The Stafford Vulcan quarry is an active pit mine with large

rock that only has been observed in the western area of

machinery and dangerous quarry walls. Please stay on the

the complex, not in the vicinity of the Vulcan quarry. The

safe side of constructed berms. All other safety measures and

age relationship of this phase to other phases is unknown.

instructions from your hosts should be strictly followed.

Geochemical analyses from mafi c phases (excluding the This quarry is the best site to see exposures of almost all ?actinolite-tremolite ultramafi c? phase) of the Garrisonville com- phases of the Garrisonville mafi c complex. Most phases of the plex show a subalkaline basalt signature when plotted on SiO 2 complex are metamorphosed mafi c rocks and Pavlides (1990, versus alkali and trace-element plots (LeBas et al., 1986; Win- 1995) interpreted it to represent a preserved fragment of oceanic chester and Floyd, 1977; Pearce 1996; Fig. 16A). These phases

contain 41%–52% SiO 2 , 13%–17% Al 2 O 3 , 9%–21% Fe 2 O 3 (T), 5%–13% MgO, 10%–13% CaO, 0%–3% Na 2 O, 0.1%–0.5%

Kilometers

K 2 O, and 0%–4% TiO 2 . Trace-element concentrations normal-

0 0.25 0.5 0.75 1 ized to primitive mantle values (Sun and McDonough, 1989)

are shown in Figure 16B. The two felsic phases present are geo- chemically similar to felsic volcanics of the Chopawamsic For- mation (Fig. 16A).

A sample of the medium-grained granodiorite-tonalite (KSH-12-32) was taken from the quarry for geochronological analysis. Five single-grain analyses of dissolved zircons yield a concordant age of 467.7 ± 1.0 Ma (Fig. 17A, 2σ uncertainties

fault

S?g S?g

with uranium decay uncertainty included); no zircon analyses from this sample are indicative of xenocryst inheritance.

A sample of the fi ne–medium-grained felsite (KSH-12-33) was also taken for geochronological analysis. Out of seven analy- ses, fi ve cluster for a concordia age of 469.1 ± 0.6 Ma (Fig. 17B, 2σ uncertainties with uranium decay uncertainty included). The two remaining data ellipses are slightly discordant, likely a result of Pb loss from the crystals analyzed. Like the granodiorite sam-

ple, no inherited zircons were found in the felsite sample. The results from the geochemical and geochronological anal- yses from the felsic phases in the Garrisonville complex indicate it was intruded by magmatism similar in composition and age to the Chopawamsic volcanics. It seems that if the mafi c phases of the

Figure 15. Location of Stops 2-1 and 2-2. Stafford and Joplin 7.5′ quadrangles. Ogmc—Garrisonville mafi c complex; Orr— complex represent a preserved piece of oceanic crust, it was likely Richland Run pluton; O Clr—Lunga Reservoir Formation; S?g— obducted onto or otherwise amalgamated with the Chopawamsic ?Silurian? granite.

arc by the time of bulk Chopawamsic plutonism and volcanism

Geology, geochemistry, and geochronology of the western Piedmont of northern Virginia

A 480

KSH-12-32

error ellipses are 2σ

Garrisonville Granodiorite

n=5

75 SiO A

Rhyolite

Stop 2-1

470 65 Rhyodacite/Dacite

70 Comendite/Pantellerite

Chopawamsic felsic volcanic

55 Chopawamsic intrusive rocks

Garrisonville felsic dikes

50 Sub-Alk

Pb/

Garrisonville mafic phases

Basalt

467.7 ± 1.0 Ma

(2σ, decay- const. errs included) n=2

45 Bas/Trach/Neph

Zr/TiO 2 450

MSWD (of concordance) = 1.7,

Probability (of concordance) = 0.19