7. The Goochland-Chopawamsic Terrane Boundary, Central Virginia Piedmont
1Virginia Department of Mines, Minerals and Energy, Charlottesville, VA 22903.
2College of William and Mary, Williamsburg, VA 23187.
Introduction
The southern Appalachian hinterland is composed of a number of terranes with distinctly different geologic histories. Some terranes, such as the central and northern Virginia Blue Ridge, are clearly of North American or Laurentian affinity (Rankin and others, 1989; Horton and others, 1989). Others, such as the Carolina slate belt, are demonstrably exotic with respect to Laurentia (Secor and others, 1983; Hibbard and others, 2002). Still others, such as the Goochland and Chopawamsic terranes, are, because of a lack of definitive evidence, of uncertain affinity and considered to be “suspect” with respect to Laurentia. While it is generally understood that these exotic and suspect terranes were accreted to the Laurentian margin during Paleozoic orogenesis, major unresolved questions remain, including (1) the origin and affinity of such terranes, (2) the timing of their accretion, and (3) the kinematics of deformation along terrane boundaries.
In central Virginia, at the northern end of the southern Appalachians, exposed terranes include, from west to east, the Blue Ridge, the Western Piedmont, the Chopawamsic, the Goochland, and the Southeastern Piedmont (fig. 1). They are separated from each other by major fault systems along which multiple generations of motion are recognized. The present state of these boundaries holds clues to the past relations of the adjacent terranes. Understanding the affinity of these terranes and the kinematics and timing of their juxtaposition is a prerequisite for accurately describing the tectonic history of the Virginia Piedmont.
The Goochland and Chopawamsic terranes have markedly different geologic histories (fig. 2). The Goochland terrane, in the eastern Piedmont, has been described as a Middle to Late Proterozoic basement massif with relict granulite-facies mineral assemblages (Glover and others, 1982; Farrar, 1984; Farrar and Owens, 2001). The Chopawamsic terrane, in the central Piedmont, is an Ordovician volcanic-plutonic arc complex (Pavlides, 1981; Coler and others, 2000). The boundary between the Goochland and Chopawamsic terranes coincides with a pronounced northeast-trending aeromagnetic and aeroradiometric feature known as the Spotsylvania lineament. Neuschel (1970) first recognized this geophysical boundary before mapping, geochronology, and tectonic models provided an adequate framework for understanding its significance. Later workers described this discontinuity as a brittle thrust fault (Pavlides and others, 1980a), a mylonite zone (Farrar, 1984; Brown, 1986), and a “major suture” (Marr, 1991). Recent workers have named this zone the Spotsylvania high-strain zone (Spears and Bailey, 2002; Bailey and others, in press).
The purpose of this field trip is to focus on new research within and along the boundary between the Goochland and Chopawamsic terranes in central Virginia. We present new evidence describing the kinematics of deformation in the Spotsylvania high-strain zone with indications of large-scale relative displacement between the terranes. Surprising new geochronology in the Goochland terrane challenges longstanding assumptions about its history. We also examine two enigmatic fault slices and a suite of unusual mafic to ultramafic igneous rocks and speculate as to their origin. This work provides new insights into the spatial and temporal relation of the terranes, with implications for the tectonic assembly of the Piedmont.
Geology
Goochland Terrane
The Goochland terrane is composed of multiply deformed and metamorphosed gneiss, amphibolite, granite, and anorthosite (fig. 2). The oldest and structurally lowest unit in the Goochland terrane is the State Farm Gneiss (Brown, 1937), a coarse-grained granitic gneiss that crops out in a series of domes (fig. 3) that are overlain by the Sabot Amphibolite and the heterogeneous Maidens Gneiss (Poland, 1976). Both the Sabot and the Maidens are intruded by the Montpelier Anorthosite (Aleinikoff and others, 1996). U-Pb zircon analyses of the State Farm Gneiss and Montpelier Anorthosite yield Mesoproterozoic ages of 1,050 to 1,020 Ma (Aleinikoff and others, 1996; Owens and Tucker, 1999). A suite of small A-type granitoid plutons with U-Pb zircon ages of ~630 Ma intrudes the State Farm Gneiss (Owens and Tucker, 2000). The Maidens, the most extensive map unit in the Goochland terrane, is dominantly pelitic (biotite-garnet and muscovite-sillimanite gneiss) with some granitic gneiss.
Rocks of the Goochland terrane experienced an early granulite-facies metamorphic event that was overprinted by a later amphibolite-facies event (Farrar, 1984; Farrar and Owens, 2001). Farrar (1984) interprets the early granulite-facies metamorphism as Mesoproterozoic and the amphibolite-facies event as Alleghanian (~300–250 Ma). The origin of the Goochland terrane is unclear; the Mesoproterozoic rocks and A-type Neoproterozoic granitoids are similar to Laurentian basement in the Blue Ridge (Glover and others, 1978; Farrar, 1984; Glover and others, 1989; Aleinikoff and others, 1996). New Nd-isotopic results reported by Owens and Samson (2001, in press) for the State Farm Gneiss and Montpelier Anorthosite show that these units are isotopically similar to other blocks of Mesoproterozoic crust along the eastern and southern margins of Laurentia (for example, Adirondacks, Blue Ridge, Llano uplift); however, other workers have suggested that the Goochland terrane may be of peri-Gondwanan affinity (Rankin and others, 1989; Hibbard and Samson, 1995).
Previously, the entire Goochland terrane was considered to be a coherent block of Mesoproterozoic crust. Mesoproterozoic crystallization ages based on modern U-Pb zircon methods have been confirmed for both the Montpelier Anorthosite (1,045±10 Ma; Aleinikoff and others, 1996) and the State Farm Gneiss (~1,046–1,023 Ma; Owens and Tucker, 2003). In addition, Horton and others (1995) reported a U-Pb zircon age of 1,035±5 Ma for a granitic gneiss within the Maidens Gneiss near Amelia Courthouse, possibly indicating that the Maidens is also Mesoproterozoic. However, new results based on electron microprobe dating of monazite in more typical Maidens lithologies (metapelites, and so forth) have thus far revealed no ages older than about 420 Ma (R.J. Tracy, B.E. Owens, and C.R. Shirvell, unpub. data). If these monazite ages reflect the timing of granulite-facies metamorphism (a plausible interpretation), the long-held assumption that the high-grade event was Grenvillian is clearly incorrect (see Burton and Armstrong, 1997). Furthermore, new U-Pb zircon results for a probable metaigneous variety of Maidens Gneiss (Stop 3) indicate a Paleozoic age, suggesting that at least some portions of the Maidens Gneiss are younger than Mesoproterozoic. An interesting additional point in this regard is that Neoproterozoic granitoids have thus far not been recognized within the Maidens Gneiss; in other words, they appear to be restricted to the State Farm Gneiss (Owens and Tucker, 2003). These points suggest the possibility of a previously unrecognized unconformity or structural discontinuity between at least the western part of the Maidens Gneiss and the more easterly (State Farm, Sabot, and Montpelier) portion of the Goochland terrane.
Chopawamsic Terrane
The Chopawamsic terrane is composed of metamorphosed volcanic and sedimentary rocks with a suite of associated granitoid plutons, all of Middle to Late Ordovician age (fig. 2). The most widespread unit is the Chopawamsic Formation, a suite of mafic and felsic metavolcanic rocks dated at ~470 Ma (Horton and others, 1998; Coler and others, 2000). In central Virginia, the Chopawamsic is intruded by the Columbia pluton (fig. 3), a granite to granodiorite body that yielded a U-Pb SIMS (secondary ion mass spectrometry) zircon age of 457±7 Ma (Wilson, 2001). In the western part of the Chopawamsic terrane, both the Chopawamsic Formation and the Columbia pluton are unconformably overlain by the Arvonia Formation (figs. 2, 3), a metasedimentary package that contains Late Ordovician fossils (Darton, 1892; Watson and Powell, 1911; Tillman, 1970). The northeastern part of the terrane contains a similar metasedimentary unit, the Quantico Formation, which may be partly interlayered with the Chopawamsic Formation. Late Ordovician fossils also are present in the Quantico Formation (Pavlides and others, 1980b).
Rocks of the Chopawamsic terrane preserve evidence of one regional metamorphic event. Metamorphic mineral assemblages indicate that greenschist-facies conditions were reached along the northwest side of the terrane; these grade into amphibolite-facies assemblages in the southeast part of the terrane. Metamorphic hornblende from the Chopawamsic Formation dated by 40Ar/39Ar methods yielded ages of 318 to ~284 Ma (Burton and others, 2000). The Chopawamsic terrane is interpreted to be an Ordovician volcanic arc complex developed on continental crust outboard of Laurentia (Coler and others, 2000) and later accreted during the Late Ordovician Taconic orogeny (Glover and others, 1989).
Spotsylvania High-Strain Zone
The Spotsylvania high-strain zone (SHSZ) forms the boundary between the early Paleozoic Chopawamsic terrane and the Mesoproterozoic-Paleozoic(?) Goochland terrane in the central Virginia Piedmont (fig. 3). This boundary was originally recognized as a sharp geophysical (aeromagnetic and aeroradiometric) lineament (Neuschel, 1970) and interpreted as a brittle fault. In the Piedmont of southern Virginia, the SHSZ appears to connect with the Hyco shear zone, a component of the Alleghanian-age central Piedmont shear zone, a major boundary traceable for over 500 km (kilometers; 300 mi (miles)) in the southern Appalachians (Hibbard and others, 1998; Wortman and others, 1998). Hibbard and others (1998) interpreted the Hyco zone in southern Virginia to be a ductile thrust that emplaced the Carolina terrane over the Chopawamsic terrane. Farrar (1984), Pratt and others (1988), and Glover and others (1989) interpreted the Spotsylvania zone as a significant thrust fault (not a suture) along which granulite/amphibolite-facies rocks of the Goochland belt were emplaced to the northwest in the late Paleozoic. In north-central Virginia, Pavlides and others (1980a) interpreted the Spotsylvania zone to be a 2- to 3-km (1–2 mi)-wide zone of predominantly brittle en-echelon faults. In central Virginia, Marr (1991) reported the presence of a tectonic mélange zone within the SHSZ and suggested it may represent a suture. Bourland (1976) and Spears and Bailey (2002) recognized brittle fault rocks in the SHSZ and interpreted these to have formed during Mesozoic reactivation of the Paleozoic high-strain zone. The Spotsylvania zone is located within the central Virginia seismic zone (Bollinger and others, 1986; Çoruh and others, 1988) and as recently as 2003, small earthquakes have occurred at depth along this structure.
We define the SHSZ as a ~15-km (~9-mi)-wide belt of heterogeneously deformed mylonitic rocks that typically lacks distinct boundaries (fig. 3). Its northwestern boundary is defined by the geophysical lineament at the contact between amphibole-rich gneisses of the Elk Hill Complex to the northwest and mylonitic rocks derived from more granitic to pelitic protoliths to the southeast. Gneissic rocks to the southeast of the Spotsylvania lineament are strongly deformed well into the Goochland terrane. Mylonitic biotite schist, granitic mylonite, biotite-rich ultramylonite, amphibolite, and protomylonitic pegmatite are the most common rock types in the SHSZ. Foliation in the SHSZ strikes to the northeast and generally dips moderately to gently to the southeast. At some locations, where SHSZ is located in the hanging wall of listric Mesozoic normal faults, dips of mylonitic foliations flattened out due to horizontal axis rotations associated with normal faulting. Lineations (both elongation and mineral) plunge shallowly to the northeast and southwest in the plane of the foliation. Asymmetric porphyroclast tails and boudins from surfaces normal to foliation and parallel to lineation consistently exhibit a strike-parallel dextral asymmetry across the SHSZ. Pegmatite dikes are commonly folded and boudinaged in a geometry consistent with bulk constrictional strain (K>1) (Bailey and others, in press). Folded dikes are asymmetric; the folds generally verge to the northwest. The geometry of asymmetric structures, both parallel and normal to the elongation lineation, is consistent with a modest triclinic deformation symmetry (Bailey and others, in press).
Minimum sectional strains, estimated from boudinaged and folded dikes on lineation-parallel surfaces, range from 8:1 to >20:1. Feldspar porphyroclasts, pegmatitic boudins, and amphibolite boudins are superficially similar to clasts or blocks in a mélange, but exhibit consistent dextral asymmetries and at many locations occur as tabular bodies with a pinch-and-swell character (Stops 3, 4, 8). Backward-rotated porphyroclasts are common in SHSZ ultramylonites and vorticity analysis yields Wn-values between 0.8 and 0.4, indicating general shear deformation that significantly deviated from simple shear (Bailey and others, in press).
Quartz grains in mylonitic rocks from the SHSZ are completely recrystallized, straight extinction is common, and strong crystallographic preferred orientations are well developed. Feldspar porphyroclasts display core and mantle structures and strong undulose extinction. In mylonites and ultramylonites, myrmekite and flame perthite are localized along high-strain grain boundaries. Synkinematic metamorphic minerals include biotite, garnet, epidote, and staurolite. Microstructures preserved in mylonitic rocks from the SHSZ are consistent with deformation conditions in the upper greenschist to lower amphibolite facies (450–500°C).
In order to better constrain the kinematics and tectonic significance of the SHSZ, Bailey and others (in press) used estimated values for vorticity and three-dimensional strain to restore the Goochland terrane to its paleogeographic position prior to dextral transpression. Deformation in the SHSZ produced significant thinning (~40–70 percent) normal to the zone and up to 500 percent stretching parallel to the zone boundaries. With the Chopawamsic terrane fixed in position, the Goochland terrane is retrodeformed to a predeformation position 80 to 300 km (50–186 mi) northeast of its present location. These displacement estimates are minimum values because strains were calculated from boudinaged and folded dikes that are, in themselves, minimum strain indicators. Furthermore, the Brookneal/Shores high-strain zone and the Mountain Run fault zone, more westerly structural discontinuities in the Virginia Piedmont (fig. 1), also exhibit dextral motion (Gates and others, 1986; Bobyarchick, 1999). Thus, the Goochland terrane, relative to the more western elements in the Virginia Piedmont, experienced significant southwestern translation during the Alleghanian orogeny.
Fault Slices of Uncertain Affinity Associated with the Terrane Boundary
Two narrow belts of rocks originally described by Taber (1913) are now recognized to be fault-bounded blocks of unknown affinity (Spears and Bailey, 2002). Although well documented by Taber (1913) and Brown (1937), the pegmatite belt and Elk Hill Complex were excluded from map compilations in the second half of the twentieth century (for example, Virginia Division of Mineral Resources, 1993). We find that both blocks exist as mappable fault-bounded units spatially associated with the terrane boundary. No published geochronology exists for any of the rocks in these two fault blocks. Comparison of these rocks to the Goochland and Chopawamsic terranes does not yield obvious correlations to units in either of the adjacent terranes.
Pegmatite Belt
Taber (1913) used the term “pegmatite belt” to describe an area underlain by pegmatite and granite in western Goochland and northern Cumberland Counties. Some later workers (Jonas, 1932; Brown, 1937) honored Taber’s nomenclature and included a similar area of pegmatite on their geologic maps. However, the 1963 geologic map of Virginia (Virginia Division of Mineral Resources, 1963) depicts this area as an extension of the Columbia granite. Farrar (1984) recognized pegmatite in this area and interpreted it to be associated with the intrusion of the Columbia pluton. However, on the 1993 geologic map of Virginia (Virginia Division of Mineral Resources, 1993), this area was mapped as biotite gneiss with small intrusions of biotite granite, all within the Chopawamsic terrane.
We find that these rocks are lithologically distinct and separated by faults from the Chopawamsic Formation and the Elk Hill Complex (fig. 3). The recently described Little Fork Church fault, mappable by a lithologic discontinuity coincident with both ductile and brittle fault rocks (Spears and Bailey, 2002) forms the western boundary of the pegmatite belt (fig. 3). The eastern boundary is defined by the Lakeside fault, which separates the pegmatite belt from the Elk Hill Complex (Stops 6, 10) (fig. 3). The belt can be separated into three distinct lithologic packages from west (structurally lowest) to east (structurally highest). The structurally lowest unit (pgg) is composed of light-gray, fine-grained, weakly layered micaceous granitic gneiss with abundant white to pink potassium feldspar-quartz-muscovite pegmatite (Stop 8; fig. 4). The pegmatite is concordant with the foliation in the surrounding gneiss and is commonly deformed into lens-shaped domains containing potassium feldspar porphyroclasts in a fine-grained matrix of muscovite, quartz, and microcline. Large feldspars are commonly kaolinized and display throughgoing brittle fractures. The middle unit (pga, fig. 4) is composed of weakly layered granitic gneiss similar to the lower unit, with less pegmatite, and generally concordant bodies of fine-grained amphibolite that are commonly deformed into boudins (Stop 10C). The upper unit (pgl) is strongly compositionally layered amphibolite, biotite gneiss, and minor pegmatite (Stop 10B, fig. 4).
Elk Hill Complex
The Elk Hill Complex was named by Taber (1913) for exposures in cuts along the railroad southeast of Elk Hill plantation in western Goochland County. At its type locality, the Elk Hill is dominated by strongly compositionally layered hornblende gneiss with lesser amounts of biotite gneiss and pegmatite. We find that, in addition to these lithologies, the Elk Hill contains gneissic diorite, talc-chlorite soapstone, and, especially south of the James River, distinctive phenocrystic felsic rocks resembling pinkish, fine-grained granite in outcrop.
Brown (1937) recognized the Elk Hill Complex on his geologic map of Goochland County, but the name was excluded from the literature for the rest of the 20th century. A hornblende gneiss unit was indicated in this area on the 1963 geologic map of Virginia (Virginia Division of Mineral Resources, 1963); however, on the 1993 geologic map of Virginia (Virginia Division of Mineral Resources, 1993), rocks in this area were mapped as “biotite gneiss” contiguous with the Central Virginia volcanic-plutonic belt, which at that time was a synonym for the Chopawamsic terrane.
Our work demonstrates that the Elk Hill Complex is distinct and separated by faults from both the Chopawamsic and the Goochland terranes. The Lakeside fault, previously mapped from the early Mesozoic Farmville basin northeastward to a point just south of the James River (Virginia Division of Mineral Resources, 1993), in fact extends farther northeastward across the James River and across western Goochland County at least as far north as I-64. This fault separates the Elk Hill Complex from the pegmatite belt and the Chopawamsic Formation throughout the area mapped. The eastern boundary of the Elk Hill is marked by the strongly mylonitic rocks of the Spotsylvania high-strain zone.
Although the Elk Hill Complex and the Chopawamsic Formation are superficially similar in that they are both dominated by mafic metavolcanic rocks, we note certain dissimilarities. The Elk Hill includes, particularly south of the James River, distinctive fine-grained felsic metavolcanic rocks interlayered with amphibolite. The felsic rocks contain concentrically zoned plagioclase phenocrysts, presumably of primary volcanic origin; such phenocrysts are not observed in the Chopawamsic Formation at this latitude. Geophysically, the Chopawamsic Formation is characterized by a high-amplitude, short-wavelength pattern on total intensity aeromagnetic maps; the pattern over the Elk Hill Complex is lower amplitude and longer wavelength. Furthermore, the Chopawamsic Formation contains substantial deposits of precious metals and massive sulfides. These well-known deposits were heavily exploited beginning in the early 19th century, to the point that a well-defined band of rocks now recognized as the Chopawamsic terrane was known as the “gold-pyrite belt” (Lonsdale, 1927; Spears and Upchurch, 1997). Despite intense prospecting by gold seekers, adjacent parts of the Piedmont remained largely unproductive. The Elk Hill (as well as the pegmatite belt and the Goochland terrane) is apparently barren of metallic mineralization, as demonstrated by the total absence of known mines.
These dissimilarities raise suspicions that the Elk Hill Complex and the Chopawamsic Formation, while both ostensibly of volcanic origin, may be of different ages and affinities. Additional work is needed to fully characterize the differences between these two blocks, and to establish the possible relation of the Elk Hill to other metavolcanic units in the Piedmont.
Conclusions
The Goochland and Chopawamsic terranes have markedly different histories that indicate that they developed independently and were not juxtaposed until post-Late Ordovician. Unpublished monazite ages in the Goochland terrane, referred to above, raise the intriguing possibility that they were separate until even later, post ~420 Ma (Silurian), and that the granulite-facies metamorphic event may be middle Paleozoic. While the basement rocks of the Goochland terrane, including the State Farm Gneiss, superficially resemble Laurentian rocks of the Virginia Blue Ridge, our work on the kinematics of its western boundary indicates that it originated far to the north. Quantitative understanding of the kinematics does not resolve whether the Goochland is a native Laurentian or an exotic terrane; however it does place meaningful limits on its pre-Alleghanian position in the Appalachian orogen. If the Goochland terrane is Laurentian, it originated somewhere between the Pennsylvania reentrant and the New York promontory, not outboard of the Virginia Blue Ridge.
The Elk Hill Complex and the pegmatite belt form two fault slices of uncertain affinity between the Goochland terrane and the Chopawamsic terrane proper. In addition, we recognize an unusual suite of mafic to ultramafic rocks associated with faults along the terrane boundary. Further work is needed to establish the significance of these units and their relation to adjacent terranes. These previously unrecognized crustal elements must be considered in future models for the tectonic assembly of the southern Appalachian Piedmont.
Acknowledgments
This manuscript benefited greatly from a review by Bill Burton. Amy Gilmer assisted with conversion and drafting of figures. We thank the many landowners who provided access to outcrops.
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