RELATIONSHIP BETWEEN THE ELLISVILLE PLUTON AND CHOPAWAMSIC FAULT: ESTABLISHMENT OF SIGNIFICANT LATE ORDOVICIAN FAULTING IN THE APPALACHIAN

RELATIONSHIP BETWEEN THE ELLISVILLE PLUTON AND CHOPAWAMSIC FAULT: ESTABLISHMENT OF SIGNIFICANT LATE ORDOVICIAN FAULTING IN THE APPALACHIAN PIEDMONT OF VIRGINIA

K. STEPHEN HUGHES* ,† , JAMES P. HIBBARD*, and BRENT V. MILLER**

ABSTRACT. The Chopawamsic fault is the most significant boundary in the western Piedmont of north central Virginia; it separates the metaclastic Early Ordovician or older Potomac terrane of Laurentian affinity from the dominantly metavolcanic Middle to Late Ordovician Chopawamsic terrane of unknown cratonic heritage. On regional maps, the Ellisville pluton had previously been depicted as stitching the Chopawamsic fault, although this relationship has never been documented. It has been hypothesized that the Chopawamsic fault marks the suture of the early Paleozoic Iapetus Ocean, which once separated Laurentian and Gondwanan crustal elements. Consequently, it is important to examine the stitching relationship in detail in order to place timing constraints on motion along this fault. We integrate detailed field mapping, kinematic analysis, petrography, major-oxide, trace, and rare earth element geochemistry, and U-Pb zircon geochronology in order to deduce the relationships between the Ellisville pluton, the Chopawamsic fault, and thus, the Potomac and Chopawamsic terranes in central Virginia.

Our study reveals local textural and minor geochronologic variations in the Ellisville pluton, whereas composition and geochemistry are mostly homogenous throughout the body. These data, along with 1:24,000 scale mapping, collectively confirm that the Ellisville pluton stitches the Potomac and Chopawamsic terranes across the Chopawamsic fault. New U-Pb zircon geochronological analyses yield ages of ca. 444 Ma and ca. 437 Ma, and indicate that the latest significant movement of the fault occurred before a 443.7 ⴞ 3.3 Ma main phase of magmatism present throughout the Ellisville pluton. These dates, with previously determined crystallization ages from the Chopawamsic terrane, constrain significant movement on the Chopawamsic fault to a ca. 10 million year interval in the Late Ordovician between ca. 453 to 444 Ma. Whether the accretion of the Chopawamsic terrane involved the closing of either a back-arc seaway or a global ocean has yet to be determined; however, based on its timing and kinematic nature, we suggest that the development of the Chopawamsic fault may be related to the Late Ordovician to Early Silurian Cherokee orogeny.

Key words: Appalachian Piedmont, stitching pluton, Chopawamsic fault, Ellisville pluton

introduction

The Chopawamsic fault separates the two principal crustal components within the western Piedmont of north central Virginia (fig. 1), including the Potomac terrane to the west and the Chopawamsic terrane to the east (for example: Pavlides, 1989, 1990; Horton and others, 1989; VDMR, 1993; Pavlides and others, 1994; Pavlides, 1995; Mixon and others, 2000; Mixon and others, 2005; Hibbard and others, 2013). It has been hypothesized that this significant fault could represent the main Iapetan suture in the southern Appalachians because it separates the metaclastic Potomac terrane, commonly interpreted to be an accretionary complex of Laurentian affinity from the

* Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8208, USA; jphibbar@ncsu.edu ** Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA; bvmiller@geo.tamu.edu † Corresponding author: kshughes@ncsu.edu; kstephenhughes@gmail.com

K. S. Hughes and others

Virginia Western Piedmont

Area of

Mesozoic Culpeper and Danville Basins and

related rocks (Tr). Ordovician to Early Silurian intrusive bodies.

Figure 2

Red = felsic. Purple = mafic. OP Ordovician rocks of the Chopawamsic

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

Cambrian to Ordovician rocks of the Potomac Terrane (P) and Smith River Allocthon (SRA).

Valley and Ridge Blue Ridge Bsz

Piedmon astal Plain Appalachian Plateau

SRA Tr

E. C o

Geology modified from Hibbard and others, 2006. R = Richmond, Bsz = Brookneal Shear zone, Cf = Chopawamsic fault, OP = Occoquan pluton.

Fig. 1. Regional geology of the western Piedmont of Virginia.

Chopawamsic terrane, a magmatic arc of unknown, but potentially peri-Gondwanan origin (Hibbard and others, 2007, 2013).

Regional studies depict the Ellisville granodiorite as a stitching pluton across the Chopawamsic fault near Louisa, Virginia (fig. 2), (Glover, 1989; Pavlides, 1989; Horton and others, 1991; VDMR, 1993; Sinha and others, 2012) although no previous detailed mapping has been undertaken to substantiate such interpretations. Evaluation of this purported cross-cutting relationship is the main focus of this study. If the Ellisville pluton cross-cuts the Chopawamsic fault, its crystallization age can be used to constrain the timing of latest movement along this regionally significant structure. To assess the stitching nature of the Ellisville pluton, the tail of the pluton was mapped in detail in the area where reconnaissance maps have previously shown it to cross the Chopawam- sic fault. We use our mapping, along with new petrographic, geochemical, and modern thermal ionization mass spectrometry (TIMS) geochronological analyses, and data from previous studies to assess the potential stitching nature of the Ellisville pluton.

In addition to constraining fault timing, our detailed mapping in the vicinity of the pluton tail enables us to evaluate sparsely exposed kinematic indicators along the Chopawamsic fault. Considering that the Chopawamsic fault separates the Laurentian Potomac terrane from the Chopawamsic terrane of unknown crustal affinity, constraining the timing and kinematics of the fault is a critical and necessary first step towards clarifying the tectonic history of the region and evaluating whether the fault represents the main Iapetan suture or merely structural telescoping of a Laurentian arc system.

regional setting

The Chopawamsic fault trends northeasterly for more than 100 km through central Virginia (figs. 1 and 2), where it has generally been depicted as a thrust on the basis of seismic data and limited field observations (Harris and others, 1982, 1986; Pavlides, 1989, 1990, 1995; Horton and others, 1989; Pavlides and others, 1994; Mixon and others, 2000, 2005). Available seismic data (I-64 line: Harris and others, 1982, 1986; Keller and others, 1985; and Pratt and others, 1988) indicate that the fault dips

K. S. Hughes and others—Relationship between the Ellisville pluton

Explanation

38° 15’ N

Geochemical sample site Geochronological and Geochemical sample site

Previous works Geochronological sample site

MRF

Ri ve r

E Anna

GS LBF

Louisa Mineral

38° N

wamsic

Crossroads Zion

Chopa

Geology

Q, A Q = Quantico Formation, A = Arvonia Formation--

Phyllite, schist, and local quartzite E Ellisville pluton-- biotite granodiorite

Latest Ordovician

to Early Silurian

Poore Creek pluton-- quartz diorite to granite

Green Springs pluton-- diorite to hornblendite L Lahore pluton-- mostly amphibole-bearing shoshonitic

monzonite CP Columbia composite pluton-- granodiorite to granite Chopawamsic Terrane

Riv

iddle to Late M

Ordovician

C Chopawamsic Formation-- meta-volcanic and sedimentary rocks

anna

Ta Ta River Metamorphic Suite-- amphibolite and biotite gneiss

A Ri ve

Potomac Terrane

Mine Run Complex-- block in phyllite matrix.

LFCF

or older

SSZ

Dark inliers represent various map scale bodies 37° 45’ N

within the phyllite matrix.

KM 0 2 4 6 8

Early Ordovician

Fig. 2. Simplified geologic map of the Ellisville pluton and vicinity. Geochemical and geochronological sample sites in Ellisville pluton and Columbia pluton are shown. Rectangle south of Louisa outlines the north half of the Ferncliff 7.5⬘ quadrangle. Abbreviations not explained in the legend are: LBF ⫽ Long Branch Fault, LFCF ⫽ Little Fork Church fault, MRF ⫽ Mountain Run fault. Geology modified after Hopkins, ms, 1960; Smith and others, 1964; Glover and Tucker, 1979; Duke, ms, 1983; Pavlides, 1989; Rossman, 1991; VDMR, 1993; Pavlides and others, 1994; Mixon and others, 2000; Spears and Bailey, 2002; Bailey and others, 2005; Hughes, 2011; and our own reconnaissance. Zircons from previous geochronological sample sites for the Lahore, Ellisville, and Green Springs/Poore Creek bodies were combined to obtain an age for each body (Pavlides and others, 1994; Wilson, ms, 2001; Sinha and others, 2012).

attitude of the fault represents an original geometry or is a result of subsequent attitude of the fault represents an original geometry or is a result of subsequent

may have been multiply reactivated as part of the Alleghanian Brookneal ductile shear zone (fig. 1) (Gates, ms, 1986, 1997; Bailey and others, 2004). The presence of the Chopawamsic fault remains unclear to the north of the Occoquan River, but it is inferred to separate the Early-Middle Ordovician Occoquan granite (fig. 1), which intrudes the Potomac terrane, from the Chopawamsic terrane (Heimgartner, ms, 1995; Horton and others, 2010).

The metaclastic Potomac terrane (Drake, 1989; Horton and others, 1989) in north-central Virginia is comprised chiefly of the mainly metaclastic Mine Run Complex (Pavlides, 1989) that has been divided into four zones on the basis of geophysical characteristics and exotic block content, and is numbered from east to west as units I through IV (fig. 2). The complex has been thought to be correlative with the Hardware metagraywacke and Shores me´lange at the James River (Pavlides, 1979; Evans, ms, 1984; Brown, 1986; Hibbard and others, 2013), and the Lunga Reservoir and Sykesville meta-diamictite formations in northern Virginia and Maryland (Pav- lides, 1989). Age constraints for the Potomac terrane are derived from U-Pb geochronological analyses of cross-cutting plutonic bodies, regional correlation, assumed age relationships of metavolcanic blocks within the Mine Run Complex, and detrital zircon analyses. The oldest documented intrusion into the Mine Run Complex is the ca. 456 Ma Goldvein pluton (Aleinikoff and others, 2002), indicating that the complex is Late Ordovician or older. Rocks of the Lunga Reservoir metadiamictite, interpreted to be correlative to the Mine Run Complex, are intruded by the ca. 472 Ma Occoquan pluton (Aleinikoff and others, 2002), indicating that the Potomac terrane in northernmost Virginia is Early Ordovician or older. Pavlides (1989) interpreted volcanic blocks in the Mine Run Complex to be derived of the Chopawamsic Formation to the east, which suggests that the Mine Run Complex is either younger or coeval with some Middle– Late Ordovician rocks of the Chopawamsic Formation. In contrast, Horton and others (2010) found that the Sykesville Formation, a correlative of the Mine Run Complex, included an abundant population of Mesoproterozoic zircons, but lacked any Ordovi- cian detrital zircons potentially derived from the Chopawamsic Formation. Thus, it seems the correlation of blocks in the Mine Run Complex to the Chopawamsic Formation, although geographically convenient, may be inappropriate. Rocks of the Potomac terrane and their correlatives are interpreted to be of Laurentian affinity on the basis of spatial relations with Laurentian rocks immediately to the west (Drake, 1989; Pavlides, 1989), depositional inter-fingering with known Laurentian strata (Evans, ms, 1984), and detrital zircon data from equivalent rocks in south-central Virginia (Carter and others, 2006; Bailey and others, 2008) and Maryland (Horton and others, 2010). On the basis of existing information, the Potomac terrane has been interpreted to represent an early Paleozoic accretionary complex that formed between the Chopawamsic volcanic arc terrane and Laurentia (Drake, 1989; Pavlides, 1989; Horton and others, 1989; Hibbard and others, 2013).

The Chopawamsic terrane (Williams and Hatcher, 1982, 1983; Horton and others, 1989), also referred to as a portion of the central Virginia volcanic and plutonic belt (Pavlides, 1981), consists of greenschist facies metavolcanic and metavolcaniclastic rocks of the Chopawamsic Formation (Southwick and others, 1971) as well as higher grade amphibolite and gneiss of the coeval Ta River Metamorphic Suite (Pavlides, 1981). Rocks of the Chopawamsic Formation in central Virginia have previously been referred to as the Cohasset volcanics (Brown, 1976; Bland, ms, 1978; Bland and Blackburn, 1980) although this name has fallen out of use. The Chopawamsic Formation extends for more than 150 km along strike and has been mapped as far

K. S. Hughes and others—Relationship between the Ellisville pluton

geophysical characteristics (Pavlides and others, 1974). Metavolcanic rocks of the Chopawamsic Formation in central Virginia are dated at 471.4 ⫾ 1.3 Ma (U-Pb zircon, Coler and others, 2000) and in northern Virginia, a volcanic unit interpreted to be near the top of the Chopawamsic Formation yields an age of 453 ⫾ 4 Ma (U-Pb zircon, Horton and others, 2010). The apparent range in Chopawamsic volcanic ages indicates either a 15 to 20 Ma duration of volcanism, or perhaps at least two discrete phases of Chopawamsic volcanism (Horton and others, 2010; Sinha and others, 2012; Hib- bard and others, 2013). The Chopawamsic terrane has been interpreted to represent a suprasubduction magmatic island arc that developed over some form of Mesoprotero- zoic continental crust (Pavlides, 1981; Coler and others, 2000). The paleogeographic affinity of the Chopawamsic terrane has not been identified and the probable presence of Mesoproterozoic basement to the Chopawamsic terrane (Coler and others, 2000) is ambiguous in terms of provenance; however, two arguments may indicate a peri- Gondwanan origin (Hibbard and others, 2007). First, the Chopawamsic arc is overlain by black slate and phyllite of the Latest Ordovician–Silurian Arvonia and Quantico successor basins, a common feature of the peri-Gondwanan Iapetan arcs in the northern Appalachians (Williams and others, 1988). Secondly, Pb isotopic compositions of volcanic-hosted massive sulfide deposits in the Chopawamsic terrane are relatively radiogenic (Pavlides and others, 1982a; Swinden and others, 1988) and identical to those of peri-Gondwanan volcanic-hosted massive sulfide deposits of the northern Appalachians. In addition, unpublished detrital zircon data from one sample suggest that the successor basins, in some places interpreted to be interlayered with the Chopawamsic Formation (Southwick and others, 1971; Horton and others, 2010), have

a Gondwanan provenance (Bailey and others, 2008). However, these data are not sufficient to rule out the possibility of Laurentian provenance, and the origin of the Chopawamsic terrane remains suspect (Hibbard and others, 2013).

Both the Potomac and Chopawamsic terranes are intruded by various Ordovician- Early Silurian plutons (for example: Wilson, 2001; Aleinikoff and others, 2002; Horton and others, 2010; Sinha and others, 2012) that can be divided into three broad classes (Hibbard and others, 2013). A collection of peraluminous granites mostly intrudes the Potomac terrane in northern Virginia while an assortment of arc-related granite-tonalite intrudes only the Chopawamsic terrane. The ca. 444 Ma (U-Pb zircon, this study) Ellisville granodiorite, has been assigned to a third set—the gabbroic- granitic complexes— based upon its spatial and temporal relationship with the ca. 451/446 Ma (Wilson, ms, 2001; Sinha and others, 2012) shoshonitic Lahore monzonite (fig. 2). The nearby Green Springs diorite and Poore Creek Granite (Hopkins, ms, 1960; Rossman, 1991; fig. 2), which have a combined age of 448 ⫾ 3 Ma (Wilson, ms, 2001) and/or 432 ⫾ 7 Ma (Sinha and others, 2012), are part of this gabbroic-granitic group as well.

The southern portion of the Chopawamsic fault lies within an area of intermittent moderate earthquake activity known as the Central Virginia Seismic Zone (for example: Bollinger and Sibol, 1985; Kim and Chapman, 2005; Tarr and Wheeler, 2006). Rupture of the Mw5.8, August 23, 2011 Mineral, Virginia earthquake occurred along a structure that cuts through the Chopawamsic terrane (Bailey and Owens, 2012; Horton and others, 2012a, 2012b; Pratt, 2012; Spears, 2012; Spears and Gilmer, 2012). This structure does not correspond to the Chopawamsic fault, but may have been facilitated by relict Paleozoic features internal to the Chopawamsic terrane (Hughes and Hibbard, 2012a, 2012b).

previous work

In the past, geologists have mapped the Potomac-Chopawamsic terrane boundary In the past, geologists have mapped the Potomac-Chopawamsic terrane boundary

and others, 1982a). However, the presence of the Chopawamsic fault has been supported by the following evidence: aeromagnetic and aeroradioactive data (Neuschel, 1970), seismic profiles (Harris and others, 1982, 1986; Keller and others, 1985; Pratt and others, 1988), reported shear fabric at multiple locations along the contact (Brown, 1979; Pavlides, 1989, 2000), regional mapping (Duke, ms, 1983; Evans, ms, 1984; Wehr and Glover, 1985; Brown, 1986; Glover, 1989; Marr, 1990), disparate detrital zircon suites from the terranes astride the fault (Hughes and others, 2012a), and the juxtaposition of compositionally and structurally distinct lithotectonic masses of rock. Most workers who have depicted the interface as a fault generally infer that it is

a thrust, although this interpretation has not been confirmed by kinematic studies.

The Ellisville pluton (Hopkins, ms, 1960) outcrops over an area of ⬃210 km 2 and consists mostly of medium-grained biotite-granodiorite which commonly retains a primary porphyritic texture with euhedral orthoclase and microcline feldspar crystals up to 3 cm long. The main body is semi-tabular in three dimensions, and lies mostly within rocks of the Mine Run Complex, although a contact metamorphosed thermal aureole of biotite, muscovite, margarite, chloritoid, staurolite, kyanite, and fibrolite reportedly extends eastward across the Chopawamsic fault and into the Chopawamsic Formation (Pavlides, 1989; Pavlides and others, 1994). Until this study, the report of contact metamorphism across the Chopawamsic fault provided the strongest support- ing evidence for the stitching nature of the Ellisville pluton. Pavlides and Cranford (1982) recognized that the main body of the Ellisville pluton is a compositionally homogenous yet texturally composite (variations in crystal size) intrusion composed of two pulses of magmatism including: (1) a dominant, main phase of medium-grained, porphyritic biotite granodiorite, and (2) a subsidiary, younger fine-grained biotite granodiorite. The younger phase has only been observed at outcrop scale and thus, has not been depicted upon previous regional or local maps. The porphyritic phase of the pluton makes up the vast majority of the pluton. Both phases of the intrusion are heterogeneously deformed, ranging from massive to moderately foliated. No systematic distribution of deformation has been recognized or reported in the pluton. Geochemical analyses from the main body indicate that the pluton is calc-alkaline and weakly peraluminous (Pavlides and others, 1994). Previous geochronological data for the main body of the pluton include dates of 444 ⫾ 6 Ma (U-Pb zircon, Wilson, ms, 2001), 441 ⫾ 3 Ma (U-Pb zircon, Sinha and others, 2012), 438 ⫾ 10 Ma (Pb-Pb multiple zircon populations, Pavlides and others, 1994) and 441 ⫾ 8 Ma (Rb-Sr whole rock, Pavlides and others, 1982b), but it is unclear how these dates relate to the two observed magmatic phases of the pluton. Seven strontium isotope analyses from the Ellisville

pluton reported by Pavlides and others (1994) have an average initial 87 Sr/ 86 Sr value of .70613 when calculated at 444 Ma. This ratio is similar to the 87 Sr/ 86 Sr value of .70614 for the nearby Poore Creek granite (at 448 Ma; Wilson, ms, 2001); both values indicate some evolved crustal material was included in the magma for each body. The Ellisville pluton has been estimated to have been emplaced at 0.6 to 0.45 GPa, a depth of 18 to

13 km (Pavlides and others, 1994). At regional scale, the southern tail of the Elllisville pluton has been interpreted to cross-cut the Chopawamsic fault near Louisa, Virginia (Pavlides, 1979; Duke, ms, 1983; Evans, ms, 1984; Pavlides, 1989; Horton and others, 1991; VDMR, 1993; Pavlides and others, 1994; Sinha and others, 2012). The two-dimensional map pattern of the Ellisville pluton may at first resemble some macro-porphyroclast feature, however, the noticeable magnetic and gravity trace of the pluton tail continues northward into the main body, suggesting that the tail of the pluton may represent a feeder dike to the

K. S. Hughes and others—Relationship between the Ellisville pluton

the main body and by Duke (ms, 1983) as the “southern sill” portion of the Ellisville granodiorite. Specific to the tail of the pluton, Hopkins (ms, 1960) mapped small bodies of hornblende meta-gabbro at and near the eastern margin of the tail of the pluton; due to their position within the tail, these small bodies may be the result of localized assimilation of mafic schists from the adjacent Chopawamsic Formation country rock.

Many geologists have linked the tail of the Ellisville pluton to the nearby Columbia composite pluton (fig. 2). Multiple configurations of the Ellisville-Columbia relationship have been proposed, including the following: (1) connection of the two bodies via the Ellisville tail (VDMR, 1963; Good and others, 1977; Duke, ms, 1983; Spears and Bailey, 2002); (2) referring to parts of the Columbia pluton as the Ellisville pluton (Conley and Johnson, 1975; Conley, 1978; Duke, ms, 1983); and (3) mapping a northern extension of the Columbia pluton towards the tail of the Ellisville pluton (Taber, 1913; Jonas, 1932; Smith and others, 1964; Bailey and others, 2005). The northwestern porphyritic granodiorite phase of the Columbia pluton (Goodman and others, 2001; Koteas and others, 2002; Bailey and others, 2005), formerly called the Carysbrook pluton and mapped by some geologists to be independent from the Columbia pluton (Stose and Stose, 1948; VDMR, 1993), is compositionally similar to the bulk of the Ellisville pluton. The Columbia pluton, dated at 457 ⫾ 7 Ma at its type locality (U-Pb zircon, Wilson, ms, 2001), intrudes the Chopawamsic terrane (Bourland and Glover, 1979; Goodman and others, 2001; Bailey and others, 2005). The northwestern (Carysbrook) phase of the pluton has a poorly constrained age of 444 ⫾ 11 Ma (U-Pb zircon, Sinha and others, 2012) and may indicate that this area crystallized coevally with the Ellisville pluton. The probable link between the Columbia pluton (intrusive to the Chopawamsic terrane) and the Ellisville pluton provides further support for the terrane-stitching nature of the Ellisville granodiorite.

At the outset of our study, the relationship between the Chopawamsic fault and the Ellisville pluton required assessment through detailed investigation. Only after this relationship is evaluated can the age of the Ellisville body be used to obtain timing constraints for this important terrane boundary within the western Piedmont of Virginia. Furthermore, kinematics along the Chopawamsic fault and the possible connection between the Ellisville pluton and the Columbia pluton remained little- explored issues.

ellisville pluton Geologic Field Mapping: Ellisville Pluton Tail

In order to assess the relationship of the Ellisville pluton to the Chopawamsic fault, we undertook detailed mapping along the southern tail of the pluton in the Ferncliff, VA 7.5⬘ USGS quadrangle, where it has previously been shown to intersect the Chopawamsic fault (Pavlides, 1989; VDMR, 1993). Our mapping and investigation into this area complement work already conducted on the main body of the Ellisville pluton to the north (Pavlides and others, 1994).

Our mapping verifies the presence of the ⬃1.5 km wide southern tail of the Ellisville pluton (Hughes, 2011). Ample outcrop exists along the Ellisville tail and it is continuous northward into the main body of the intrusion, north of 38° N latitude. The granodiorite is more resistant to weathering than the surrounding metasedimen- tary and metavolcanic rock, and thus outcrops more abundantly. The common porphyritic texture throughout the granodiorite (fig. 3A) further facilitates mapping in areas lacking direct exposure because the resultant saprolite and regolith retain a distinct coarse, rubbly texture.

establishment of significant Late Ordovician faulting in the Appalachian Piedmont 591

Fig. 3. The Ellisville pluton. (A) Biotite-granodiorite representative of the main phase of the pluton. Field notebook is 12 cm wide. Location in Mineral quadrangle, 38.03596 N, 77.97637 W. (B) Example of magmatic flow foliation seen in the Ellisville pluton. Compass is 7 cm wide. Location in Lahore quadrangle, 38.1405 N, 77.8895 W. (C) Intrusive contact of a dike of the finer grained c. 437 Ma phase into the medium-grained c. 444 Ma phase in the tail of the Ellisville pluton. Location in Ferncliff quadrangle, 37.96548 N, 78.03728 W. (D) Wisp-like assimilation of the medium-grained granodiorite phase into the younger finer-grained phase in the tail of the Ellisville pluton. Assimilated material directly above the hammer. Location in Ferncliff quadrangle, 37.95390 N, 78.04521 W. (E) Intrusive contact zone between the Ellisville pluton tail and surrounding Chopawamsic Formation country rock. Photo taken of a loose boulder. Similar, although less photogenic, in-place outcrop exists ⬃2 meters away. Inset: A slabbed surface from the upper right portion of the boulder in figure 3E. Location in Ferncliff quadrangle, 37.9955 N, 78.0197 W.

porphyritic with feldspar crystals up to 1 cm long. Locally, near its margins, are small xenoliths of gray, fine-grained metasandstone and small areas of massive pegmatite. Granodiorite in the tail almost universally has a weak foliation; similar weak foliations elsewhere in the pluton have been identified as magmatic flow foliations (fig. 3B;

K. S. Hughes and others—Relationship between the Ellisville pluton

to that of the Ellisville tail; however, the porphyritic texture is more developed (larger K-feldspar crystals up to 3 cm) in some places in the main body. Similar to observations made in the main body of the pluton (Pavlides and Cranford, 1982), we have also observed small areas of the younger, finer grained biotite granodiorite phase at outcrop scale that cannot be accurately resolved during mapping due to its limited distribution in addition to the lack of outcrop. This finer grained phase of biotite granodiorite appears to intrude the dominant medium-grained, commonly porphyritic biotite granodiorite phase of the pluton (figs. 3C and 3D).

At the present depth of exposure, the tail of the pluton lies wholly within metavolcanic and metavolcaniclastic rocks of the Chopawamsic Formation. This interpretation is based upon the direct observation of the intrusive contact of the Ellisville pluton into the Chopawamsic Formation (fig. 3E), the consistent distribution of Chopawamsic Formation rocks around the tail, and the presence of small mafic zones (Hopkins, ms, 1960) that may reflect assimilation of mafic Chopawamsic Formation country rock. Our interpretation contradicts previous depictions (Pavlides, 1989; VDMR, 1993), which show the Ellisville tail to be intrusive into rocks of both the Mine Run Complex and Chopawamsic Formation, across a tightly folded Chopawam- sic fault; this geometry of the fault may be a mapping artifact created by the compilation of regional maps that met at 38° N latitude.

Observations of: (1) continuous outcrop from the tail to the main body, (2) the intrusive relationship between the Ellisville tail and the Chopawamsic Formation (fig. 3E), (3) previous observations of the main body being intrusive into the Mine Run Complex (Pavlides and others, 1994), and (4) the presence of the Ellisville thermal aureole into both the Mine Run Complex and the Chopawamsic Formation (Pavlides, 1989; Pavlides and others, 1994), collectively support the deduction that the Ellisville pluton intruded across the terrane-bounding Chopawamsic fault. The available mapping data indicate that the Ellisville pluton cross-cuts the Chopawamsic fault just north of 38° N latitude near or in Louisa, Virginia. Outcrop in this vicinity is sparse and no contact between the intrusive Ellisville pluton and fabrics related to the Chopawamsic fault has been discovered. The most thorough previous local mapping (Hopkins, ms, 1960) and aeromagnetic data (for example: Neuschel, 1970; VDMR, 1971; Snyder, 2005) reinforce the conclusion that the tail lies within and truncates features within the Chopawamsic Formation: the aeromagnetic signature of an iron-rich layer in the Chopawamsic formation is present to each side of the Ellisville pluton tail, but the magnetic signature is absent where the tail has been mapped and Duke (ms, 1983) also interpreted the iron-rich layer in addition to surrounding rocks of the Chopawamsic Formation to be “sharply crosscut by the Ellisville granodiorite.”

In reconnaissance mapping, we have encountered rock similar to the Ellisville tail granodiorite in the approximately 15 km between the Ferncliff quadrangle and the northern extension of the Columbia pluton mapped by Smith and others (1964) and Bailey and others (2005). In light of our collective field observations, we subscribe to the previously established mapping interpretation that the Ellisville pluton and at least some part of the Columbia pluton form a continuous body linked through the Ellisville tail (VDMR, 1963; Smith and others, 1964; Duke, ms, 1983; Spears and Bailey, 2002).

Petrography

Mineralogy throughout the pluton includes quartz, plagioclase feldspar, microcline, and biotite. Subordinate minerals include muscovite, zircon, and epidote, as well as rare myrmekite intergrowths. Microcline crystals range up to 3 cm long and in some places show oscillatory zonation. Petrographically, this mineral composition is homogenous throughout the Ellisville main body and tail.

establishment of significant Late Ordovician faulting in the Appalachian Piedmont 593

Fig. 4. Examples of magmatic epidote in the Ellisville pluton. Sample numbers correspond to samples and locations listed in table 1. Sample KSH-85 is from the tail of the pluton. Sample KSH-68 is from the main body of the pluton. Each large figure is in cross polarized light and includes an inset figure of the same location in plane light. Diameter of field of view in each photo measures ⬃2.5 mm. Abbreviations are b ⫽ biotite, e ⫽ epidote, f ⫽ feldspar, and q ⫽ quartz.

of magmatic epidote include euhedral crystals that have sharp crystal faces in contact with biotite crystals and/or contain cores of primary igneous minerals such as allanite (Zen and Hammarstrom, 1984; Schmidt and Poli, 2004). On the basis of the established criteria, we have identified examples of magmatic epidote in both the main body as well as the tail of the pluton, where it has not been previously recognized (fig. 4). Furthermore, we find magmatic epidote to exist in samples from both textural phases of the pluton. Analysis of a cored, magmatic epidote crystal in the Ellisville tail (sample KSH-85 in table 1), using an ARL-SEMQ electron microprobe yields a composition

with a Pistacite (Ps) value of Ps [Ps value calculated as: (Fe ⫹3 28 /(Fe ⫹3 ⫹ Al)) ⴱ 100], when calculated on the basis of 13 anions. Values from magmatic epidote in the main body of the pluton (Pavlides and others, 1994) have similar values of Ps 27-31 when re-calculated on the basis of 13 anions.

Among several early Paleozoic intrusions in the region, the only other plutons that reportedly display this feature are the Late Ordovician–Early Silurian Leatherwood pluton, and the Early–Middle Ordovician Lake Jackson pluton (Hibbard and others, 2013). Thus, the recognition of magmatic epidote is not common in early Paleozoic intrusive bodies and serves to provide an additional petrogenetic link between not only the tail and main body of the Ellisville pluton, but also between both textural phases of the pluton—an observation that indicates they were emplaced under similar thermody- namic and barometric conditions in the crust. These links are further supported by the nearly identical composition of the magmatic epidote, on the basis of Pistacite values, analyzed throughout the pluton. The presence of magmatic epidote, which may crystallize at 0.7 to 0.3 GPa in intermediate composition melts (Schmidt and Poli, 2004), in the Ellisville granodiorite is compatible with previous geobarometric esti- mates of 0.6 to 0.45 GPa for the intrusive body (Pavlides and others, 1994).

We have not observed magmatic epidote in petrographic analysis of the Columbia pluton, although fewer samples from this body were collected. Epidote has been noted as an accessory mineral in the Columbia pluton (for example: Smith and others, 1964; Bailey and others, 2005), but is not reported as a primary igneous mineral.

Geochemistry

Samples.—In order to support our field and petrographic interpretations that the

XRF major oxide data and locations of geochemical samples

and

Sample Pluton/Area

KSH-18 Ellivsille Tail

61.44 17.00 3.96 3.20 4.01 3.56 2.85 0.48 0.18 0.06 1.04 others—Relationship

KSH-47 Ellivsille Tail

69.04 18.56 3.33 2.66 3.89 4.16 2.26 0.46 0.07 0.04 0.73 KSH-68 Ellisville Main Body Mineral

Ellivsille Tail

56.57 17.82 5.16 3.95 4.15 3.24 3.02 0.59 0.20 0.07 1.28 KSH-96 Ellisville Main Body Louisa

Ellivsille Tail

2.18 3.85 3.59 2.76 0.44 0.16 0.06 1.14 KSH-103 Ellisville Main Body Mineral

Columbia

Palmyra 37.75769 -78.25643 60.56 16.45 3.67

3.09 1.53 2.87 2.99 3.16 0.45 0.20 0.05 1.25 KSH-105 Ellisville Main Body Mineral

KSH-104 Ellisville Main Body Mineral

3.86 2.55 3.31 2.74 3.20 0.44 0.16 0.06 0.51 KSH-115 Ellisville Tail Ferncliff 37.94918 -78.06857 -- - - ------ 0.62 KSH-116

KSH-117 Columbia

KSH-126 Columbia

Ellisville Tail

Ferncliff

pluton

XRF data of trace elements in ppm

Sample S Cl Sc V Cr Ni Cu Zn Ga As Rb Sr Y Zr Nb Ba Ce Pb Th U

KSH-18 249.45 400.50 <LD 71.68 32.88 9.94 7.45 13.14 17.61 15.64 76.86 546.87 6.82 132.17 10.58 1126.29

93.59 18.97 9.92 <LD

KSH-47 141.62 196.87 10.06 103.31

28.65 13.40 9.38 16.36 18.01 <LD 65.76 600.99 10.89 164.67 12.65 757.35 68.44 13.69 <LD <LD

Late

KSH-66 131.63 267.82 12.54 71.29 24.00 5.58 13.80 3.95 18.89 <LD 63.90 503.18 4.95 174.17 9.97 516.82 <LD 19.23 6.29 <LD KSH-68 198.13 97.78 <LD 40.31 6.88 <LD 10.98 10.28 17.50 <LD 105.10 358.38 3.86 191.51 10.83 1097.21 127.92 20.19 15.69 4.99 KSH-85 146.41 264.97 <LD 70.07 28.31 6.94 <LD 11.90 16.99 <LD 72.39 573.64 6.49 120.04 9.58 1405.78

Ordovician

93.04 11.03 8.95 <LD

KSH-86 133.82 154.40 9.36 61.14 20.47 8.09 <LD <LD 17.63 16.74 87.97 458.02 10.77 122.10 15.19 812.44 68.83 15.74 12.00 <LD KSH-94 178.98 297.12 <LD 96.96 45.11 13.36 12.68 21.32 21.23 <LD 92.75 536.71 7.48 157.13 10.81 899.72 118.70

17.00 9.00 <LD KSH-96 125.89

75.43 7.86 51.86 19.34 <LD 7.59 4.51 17.05 <LD 102.76 453.22 8.10 139.91 13.57 944.30 101.09 22.32 13.68 <LD KSH-102 133.82 90.17 <LD 66.81 37.35 8.40 5.42 5.65 16.83 <LD 86.40 455.53 9.29 126.35 13.61 1005.42 <LD 16.69 11.17 <LD KSH-103 128.08 67.15 <LD 36.26 10.77 <LD 32.39 <LD 16.29 19.20 115.40 352.11 4.54 111.19 13.27 576.26 <LD 26.25 11.88 5.37

faulting

KSH-104 177.20 99.63 <LD 52.55 43.46 <LD 8.17 9.56 18.46 <LD 104.34 403.98 5.40 155.48 13.12 932.14 134.06 20.92 17.30 <LD KSH-105 268.46 99.67 <LD 61.22 33.52 10.27 9.72 11.11 16.63 <LD 96.19 504.13 6.11 162.74 10.80 992.87 124.39 19.09 11.78 <LD KSH-115 133.69 108.68 <LD 19.02 9.65 <LD <LD 3.29 21.74 <LD 155.15 221.51 8.82 125.04 17.21 680.29 81.09 26.97 11.36 <LD KSH-116 129.72 57.97 11.47 70.21 34.66 9.06 16.85 11.92 17.76 <LD 73.95 247.17 7.77 113.64 12.82 923.17 62.57 7.02 8.97 <LD KSH-117 339.21 81.67 14.50 88.30 22.30 9.66 <LD 13.62 17.52 <LD 88.53 258.86 14.31 134.41 18.39 828.89 46.53 16.86 12.80 <LD

the

KSH-126 119.87 30.09 <LD 56.09 32.64 10.49 5.98 10.50 15.87 <LD 107.84 428.32 9.16 107.52 14.87 999.28 87.01 23.21 13.39 4.73

KSH-127 138.47 47.74 <LD 26.05 <LD <LD <LD 6.63 19.79 <LD 154.73 239.06 9.26 123.89 15.31 758.13 69.46 27.37 13.79 6.84 KSH-128 139.70 247.24 <LD 79.25 40.95 9.27 <LD 7.31 19.09 <LD 92.77 442.68 8.83 109.02 13.63 801.03 96.92 19.10 11.06 <LD

Appalachian

Detection 17.45 33.25 8.18 6.47 7.32 4.34 4.09 2.19 3.43 15.14 0.83 1.40 0.74 1.35 0.84 22.14 42.09 4.56 3.74 4.35 Limit (LD)

Piedmont

K. S. Hughes and others—Relationship between the Ellisville pluton

the main body for geochemical analyses. These samples collected are mostly from the main, medium-grained phase of the pluton. We also collected four samples from the northwestern phase of the nearby Columbia composite pluton to compare with the data from the Ellisville pluton. Samples with xenoliths were avoided and no unusual mineral assemblages or compositions were found in hand specimen or in thin section analysis. Geochemical analytical techniques are outlined in Appendix 1 and the results are shown in tables 1, 2 and 3.

Results.—Chemical alteration and element mobility over geologic time can hinder attempts at interpreting geochemical data. Because the Ellisville pluton is an early Paleozoic body and situated near areas of known mineralization within the Chopawam- sic terrane (for example: Good and others, 1977; Gair, 1978; Sweet, 1980; Pavlides and others, 1982a; Duke, ms, 1983; Spears and Upchurch, 1997), several efforts have been made to assess the degree of geochemical alteration within our samples. After normalizing major oxide concentrations to an anhydrous 100 percent, all samples

contain 60 to 70 percent SiO 2 , indicating that only minimal, if any, hydrothermal silica loss or gain has occurred in these rocks of granodiorite composition. Our entire sample set falls within the igneous spectrum of Hughes (1973), suggesting that regional metamorphism has not affected the bulk concentrations of the easily mobi-

lized elements of Na and K (fig. 5A). Furthermore, in all of our samples, Na 2 O shows no significant sign of loss or alteration when compared to Al 2 O 3 (fig. 5A). The results from these measures of alteration indicate that the geochemical data can be used and interpreted as the original igneous composition for each sample.

The rocks of the Ellisville pluton can be chemically classified as calc-alkaline using both the A-F-M ternary diagram of Irvine and Baragar (1971) and the SiO 2 vs K 2 O plot of Pecerillo and Taylor (1976) (fig. 5B). These rocks also mostly plot within the granodiorite field of both the Ab-An-Or (after Barker, 1979) and Q-A-P (after Streck- eisen, 1974) ternary diagrams (fig. 5B). All samples plot in the I-type magma field of

Chappell and White’s (1974, 2001) Na 2 O vs SiO 2 diagram (fig. 5B). Primitive mantle-normalized (Sun and McDonough, 1989) trace element compositions are similar across all analyzed samples (fig. 5C). All eighteen of our samples show consistent patterns with prominent Nb anomalies, commonly interpreted to indicate a subduction-related origin (for example: Pearce, 1982; Baier and others, 2008). Trace element discrimination diagrams of Yb vs Ta and Y ⫹ Nb vs Rb (Pearce and others, 1984) suggest that the samples analyzed are representative of syn- collisional to volcanic-arc type granitoids (fig. 5C).

Chondrite-normalized (Sun and McDonough, 1989) REE data show enrichment in the light REE and relative depletion in the heavy REE (fig. 5C). As with major oxides and trace elements, the REE signatures are quite similar for all Ellisville pluton samples analyzed and display no systematic or obvious variations. Furthermore, our results correlate well with REE data obtained by Pavlides and others (1994) for the main body of the pluton.

The composition of all samples from the Ellisville pluton analyzed suggests it was emplaced as a result of subduction-related arc magmatism. This data is consistent with previous interpretations of the tectonic setting during the timing of Ellisville intrusion and support the model that the Ellisville pluton was emplaced when the oceanic crust outboard of the Chopawamsic arc subducted westward (modern geographic orienta- tion) beneath the modified Laurentian margin. The geochemical similarities throughout the Ellisville pluton indicate that the geochemical composition of granodiorite is consistent throughout the two major textural phases observed in the pluton. Most

ICPMS data for trace and rare earth elements in ppm

Sample Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd (160) Tb Dy Ho Er Tm Yb Lu Hf Ta Th KSH-18 7.24 192.16 7.94 1140.65

KSH-66 4.65 162.44 6.98 456.13 11.58 22.69 2.40 8.42 1.43 0.64 0.96 0.16 0.95 0.18 0.56 0.08 0.45 0.09 3.46 1.06 5.25 KSH-68 3.81 189.63 8.03 1086.68

Ordovician

KSH-85 7.58 174.87 8.51 1155.57 25.41 54.56 5.50 19.20 3.38 0.89 2.21 0.34 1.73 0.31 0.76 0.10 0.66 0.09 3.79 1.09 8.44 KSH-86 10.73 174.21

KSH-104 5.20 197.94 9.26 1024.25 29.28 63.56 6.37 21.74 3.43 0.84 1.51 0.24 1.32 0.22 0.58 0.08 0.60 0.08 4.45 1.03 15.50 KSH-105 7.09 215.70 9.41 1103.98

37.79 67.08 7.20 24.29 3.68 0.85 1.78 0.30 1.54 0.27 0.71 0.11 0.75 0.09 4.61 1.11 13.08 KSH-115 9.70 149.25

13.73 691.51 37.48 65.75 7.02 23.51 3.74 0.73 2.06 0.40 2.05 0.34 0.91 0.14 0.89 0.13 3.71 1.87 18.09 KSH-116 7.57 117.74 9.54 795.94 31.55 50.47 5.72 19.80 3.01 0.67 1.59 0.28 1.57 0.27 0.81 0.11 0.81 0.11 2.78 1.20 10.60 KSH-117 14.27 184.51

KSH-127 8.62 131.66 11.02 844.74 15.51 37.26 3.03 10.56 2.02 0.56 1.44 0.25 1.60 0.33 0.88 0.15 1.06 0.13 3.32 1.55 12.25 KSH-128 8.67 152.01

Appalachian

Detection 0.051 0.299 0.221 1.368 0.061 0.055 0.045 0.778 0.371 0.136 0.150 0.059 0.165 0.044 0.264 0.048 0.293 0.029 0.402 0.345 0.041 Limit

Piedmont

K. S. Hughes and others—Relationship between the Ellisville pluton

Igneous Spectrum

Weak to no alteration

(K 2 O/(K 2 O + Na 2 O)) * 100 Na 2 O

Shoshonitic

FeO + Fe 2 O 3

K O 2 ,C aline alc -Alk

Quartz-rich

A granitoid

High K Medium K, Transitional

Monzo- diorite

Grano-

Low K, Tholeiitic

To nalite

Tonalite Granodiorite

monzanite Quartz

monzodiorite/ Quartz

Ca lc-Alkaline

K 2 O + Na 2 O

C 100

Within Plate (A-type)

1000 Syn-Collisional

(S-type) Within Plate

100 (A-type) 1000

Collisional (S-type)

10 Syn-

10 1 a ppmT

R 10 b ppm

Volcanic Arc

Ocean Ridge

Volcanic Arc Ocean Ridge (OR-type) 1 (I-type)

(I-type)

(OR-type)

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sample/Chon.

Y+Nb ppm

Geochemical symbols Explanation

Solid symbols represent our data. Hollow symbols represent data from previous work (Pavlides et al, 1994; Wilson, 2001).

Ellisville tail Columbia pluton Rb Ba Th Nb Ta

1 Sample/P

Ellisville main body

La Ce Pr Sr Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

n=18

n=10 n=7

Fig. 5. Geochemical diagrams. Explanation shown at bottom-right of figure. (A) Tests for element mobility. All data plot inside of the Igneous Spectrum of Hughes (1973). When comparing Na 2 O to Al 2 O 3 /Na 2 O, no analyzed samples show indications of sodium loss or alteration. (B) Major oxide geochemistry. Q-A-P (Streckeisen, 1974) and Ab-An-Or (Barker, 1979) ternary diagrams show the majority of samples plot in the granodiorite field. A-F-M (K 2 O ⫹ Na 2 O ⫺ FeO ⫹ Fe 2 O 3 ⫺ MgO) (Irvine and Baragar, 1971) and SiO 2 vs K 2 O (Pecerillo and Taylor, 1976) diagrams show that the samples analyzed are calc-alkaline in nature. All samples also plot in the I-type field of the K 2 O vs Na 2 O diagram of Chappell and White (1974). (C) Rare earth and trace element plots. When normalized to primitive mantle values of Sun and McDonough (1989), trace element data show similar patterns for all samples analyzed. Also, rare earth element data display consistent arrays when normalized to chondrite values (Sun and McDonough, 1989). The gray field in the rare earth element plot represents data from the Ellisville pluton determined by Pavlides and others (1994). Trace element discrimination diagrams of Pearce and others (1984) indicate that the Ellisville pluton has a volcanic-arc to syn-collisional signature.

pluton stitches the Chopawamsic fault and can be used as a reliable timing constraint for latest motion upon the structure.

Geochronology

Samples.—In order to constrain the timing of motion on the Chopawamsic fault, we dated two samples of the cross-cutting Ellisville pluton by U-Pb thermal ionization mass spectrometry (TIMS). We wrestled a sample from the previously undated tail of the pluton (sample KSH-86 in table 1) from an outcrop along the South Anna River in southern Louisa County; this rock represents the medium-grained, commonly porphyritic biotite granodiorite main phase of magmatism. We sampled the main body of the pluton (sample KSH-68 in table 1) from the same quarry in Louisa County that previously yielded samples with calculated ages of 444 ⫾ 6 Ma (U-Pb zircon, Wilson, ms, 2001), 438 ⫾ 10 Ma (Pb-Pb multiple zircon populations, Pavlides and others, 1994), and 441 ⫾ 8 Ma (Rb-Sr whole rock, Pavlides and others, 1982b); our sample was taken from the subsidiary fine-grained biotite granodiorite phase of the pluton. Re-sampling the main body of the pluton allows us to compare ages throughout the pluton and by dating two new samples simultaneously, we effectively minimize inter- laboratory and methodological biases that may mask the resolution of any, potentially small, age difference between any two areas or textural phases within the pluton. We took care to avoid any xenoliths and found no unusual or heterogeneous compositions of our samples in hand sample or thin section. Geochronological analytical techniques are outlined in Appendix 2. All errors are both plotted on diagrams and reported in tables at the 2␴ confidence level.

Results.—Seven fractions from medium-grained sample KSH-86, consisting of five single zircon grains and two pairs of grains, yield an upper intercept age of 443.7 ⫾ 4.4 Ma with a mean square of weighted deviates (MSWD) value of 2.1 (fig. 6). One relatively imprecise analysis with a large proportion of common Pb fell off the regression line, likely due to slight inaccuracies in the common Pb correction (dashed in fig. 6). Seven fractions, consisting of six single zircon grains and one pair of two small zircon grains were analyzed from fine-grained sample KSH-68 and yield an upper-intercept zircon age of 436.8 ⫾ 4.2 Ma with MSWD ⫽ 1.08 (fig. 6). One single-grain analysis yielded a nearly concordant age of 1090 Ma and is interpreted to indicate the presence of Mesoproterozoic inheritance or melt source contribution to the magma. All values from TIMS analyses are reported in table 4. Photographs of all zircons dissolved and analyzed are shown in figure 6. The lower intercept value for each regression was within error of the origin.

We interpret the upper intercept ages to reflect the times of crystallization of magmatic zircons from each sample. Our data are consistent with previously determined ages for the Ellisville pluton (Pavlides and others, 1994; Wilson, ms, 2001; Sinha and others, 2012) as well as a regionally extensive suite of Late Ordovician to Early Silurian magmatism recorded in the western Piedmont of Virginia (for example: Wilson, ms, 2001; Aleinikoff and others, 2002; Horton and others, 2010; Sinha and others, 2012; Hibbard and others, 2013). However, our analyses were conducted by the same analytical methods, in the same laboratory, under the same operating conditions; they therefore provide a consistent basis for comparison of the two phases of granodiorite present in the Ellisville pluton. Furthermore, the above stated age-errors include uncertainties in the U decay constant, which must be considered when comparing with other geochronologic methods, but cancel out when comparing U-Pb ages. By excluding U decay constants as a source of uncertainty, age-errors are reduced to 436.8 ⫾ 1.9 Ma (fine-grained phase, KSH-68) and 443.7 ⫾ 3.3 Ma (medium-grained phase, KSH-86), rendering resolvable the age difference of the two samples. These

establishment of significant Late Ordovician faulting in the Appalachian Piedmont 599

K. S. Hughes and others—Relationship between the Ellisville pluton

Fig. 6. Geochronology of the Ellisville pluton. A fine-grained granodiorite sample (light gray ellipses— sample KSH-68) from the main body of the pluton yields an upper intercept zircon age of 437 ⫾ 4 Ma. Errors include Uranium decay constant errors (shown as the band along the concordia curve). A medium-grained granodiorite sample (dark gray ellipses—sample KSH-86) from the tail of the pluton yields an upper intercept zircon age of 444 ⫾ 4 Ma. Error ellipses are displayed at the 2-sigma confidence interval. Photographs show images of zircons dissolved and analyzed. (A and B) are from sample KSH-68. (C and D) are from sample KSH-86. The “Z” numbers correspond to analyses listed in table 4. One of the zircons in fraction Z12 was lost during sample preparation; therefore the data for this sample only reflect the dissolution of one of the crystals shown in the photograph.

RELATIONSHIP BETWEEN THE ELLISVILLE PLUTON AND CHOPAWAMSIC FAULT: ESTABLISHMENT OF SIGNIFICANT LATE ORDOVICIAN FAULTING IN THE APPALACHIAN