U.S. Geological Survey: Science for a Changing World - USGS visual identifier and link to main Web site at http://www.usgs.gov/

Road Log and Stop Descriptions

The field trip begins at the McDonald's restaurant on U.S. 522/U.S. 340 directly south of Exit 6 off I-66 in Front Royal, Va.


NOTE: Throughout the road log, numbers in the left-hand column indicate cumulative mileage for the entire field trip; numbers in the right-hand column indicate cumulative mileage within each stop-to-stop segment of the trip.

Mileage
Trip
Cumulative
 
Stop-to-Stop
Cumulative
 
 
0.0
Proceed south on U.S. 522/U.S. 340.
0.5
 
0.5
Cross the North Fork of the Shenandoah River.
Relatively undeveloped flood plain is visible on both sides of the highway.
0.8
 
0.8
Proceed straight at intersection with Va. 55 toward Front Royal.
1.0
 
1.0
Cross the South Fork of the Shenandoah River.
Residential development bordering the flood plain is visible on the left.
Remain in left lane.
1.6
 
1.6
Turn left at traffic light to remain on U.S. 522/U.S. 340/Va. 55.
Remain in right lane.
2.2
 
2.2
Proceed straight through traffic light to remain on U.S. 340 S/ Va. 55 E.
Proceed straight through Front Royal on U.S. 340 S (Royal Avenue).
3.0
 
3.0
Pass United Methodist Church.
 
The church is constructed largely of limestone of the type commonly obtained from
carbonate valleys of the central Appalachians.
3.5
 
3.5
Proceed straight through traffic light to remain on U.S. 340 S.
Merge into left lane.
4.0
 
4.0
Pass entrance to Shenandoah National Park on the left.
4.2
 
4.2
Turn left onto Va. 649 S (Browntown Road).
8.2
 
8.2
Roadcut on left side exposes very fresh, medium- to coarse-grained,
garnet+biotite leucogranite that contains locally abundant,
disseminated graphite.
8.4   8.4 Stop 1.
Turn right onto Gooney Falls Lane.
Follow Trip Leaders
PRIVATE PROPERTY

Stop 1. Garnetiferous alkali feldspar granite to syenogranite (Ygg).

This outcrop is part of a series of exposures along Gooney Run located near the northern end of the garnetiferous alkali feldspar granite to syenogranite (Ygg) pluton, which extends south to the vicinity of Gravel Springs Gap Road where it is cut, but not truncated, by a northeast-southwest-striking fault (fig. 3). This lithologic unit is typically garnet-bearing, leucocratic, and characterized by alkali feldspar-to-plagioclase ratios >1. The rock at this outcrop is syenitic, and may represent a feldspar cumulate. The syenitic composition is in contrast to that of other exposures in the area which are generally granitic, especially in the southern part of the pluton. The foliation visible in this outcrop (strike 355°, dip 65° E.) is likely to have developed during regional Paleozoic orogenesis due to the absence of high-temperature textural and mineralogical characteristics. This lithologic unit is inferred to intrude the orthopyroxene+amphibole quartz diorite (Ypqd), on the basis of field relations observed at Boyd's Mill (Stop 3). The granitoid is intruded by north-south-striking metadiabase dikes at this outcrop, one of which is about 20 m (66 ft) wide. The chemical composition of a sample collected from the largest dike is presented in table 5 (sample SNP-01-159), and the significance of these data is discussed in the text devoted to Stop 6). Such dikes are very common throughout the Blue Ridge, but are especially abundant in the northern Blue Ridge (Burton and others, 1995).

Throughout the field trip area, the Ygg unit includes white, medium- to very coarsegrained, inequigranular, weakly to strongly foliated syenogranite and white, mediumgrained, equigranular, weakly foliated alkali feldspar granite to syenogranite composed of 35 to 40 percent orthoclase, 15 to 25 percent plagioclase, and 20 to 25 percent quartz with 2 to 10 percent garnet, 2 to 5 percent biotite, and 2 to 5 percent pseudomorphs after orthopyroxene. Accessory minerals include apatite, zircon, and ilmenite. Locally strong foliation within medium- to very coarse-grained syenogranite is defined by alignment of tabular feldspar megacrysts and elongate, polygranular garnet in garnet+biotite-rich domains. Weak foliation in the medium-grained, equigranular alkali feldspar granite to syenogranite is defined by alignment of polygranular garnet-rich domains. Meter- to decimeter-scale dikes of medium-grained, equigranular alkali feldspar granite to syenogranite locally intrude medium- to very coarse-grained syenogranite. Rare pegmatitic dikes, typically <10 cm (4 in) in width, locally intrude all lithologic phases of this unit. This lithologic unit is mineralogically and geochemically similar to medium- to very coarse-grained, inequigranular, massive to weakly foliated granite and medium-grained, equigranular to inequigranular, massive to weakly foliated leucocratic granite (Ygr) that occurs in the Old Rag Mountain 7.5-minute quadrangle (Hackley and Tollo, in press) and adjacent areas (fig. 3). However, these units can be distinguished in the field on the basis of quartz color: quartz is typically gray to white in garnet-bearing alkali feldspar granite to syenogranite (Ygg); whereas coarse-grained Old Rag Granite (Yor of Hackley and Tollo, in press) and associated medium-grained leucogranite (Ygr of Hackley and Tollo, in press) typically contains blue quartz.

Rocks within this lithologic unit typically have silica contents in excess of 69 weight percent and are mildly to moderately peraluminous (fig. 5). These leucocratic granitoids have high concentrations of Na2O+K2O, a compositional feature that is reflected in the characteristic modal abundance of alkali feldspar relative to plagioclase (fig. 4). K2O/Na2O molar ratios are characteristically high and similar to those of other leucocratic rocks in the field trip area. The high-silica, alkali-rich compositions are a distinctive geochemical characteristic of the post-tectonic leucocratic granitoids in the field trip area (fig. 5E). The low silica concentration characterizing the granitoid at this exposure (table 4, sample SNP-03-197) is compositionally atypical of the Ygg unit in the field trip area and appears to be representative of a relatively minor lithologic variant. For this reason, the composition of a more chemically typical sample of the pluton collected 0.67 km (0.42 mi) northwest of the field trip stop along Gooney Run is also included in table 4 (sample SNP-01-164).

Ygg is part of a group of leucocratic granitoids that were emplaced following the main episode of local Grenvillian orogenesis that occurred at 1,080 to 1,060 Ma (Tollo and others, in press a). In addition to Ygg, these postorogenic rocks in the field trip area include the typically garnet- and (or) biotite-bearing leucocratic granitoids of the Old Rag magmatic suite (Ygr) (Hackley, 1999), which is associated with the pluton at Old Rag Mountain (fig. 3), and alkali feldspar granite (Yaf) that occurs as two small bodies near Madison (Bailey and others, 2003) (fig. 3). Southworth (1994, 1995) and Southworth and Brezinski (1996) mapped possibly correlative, nonfoliated to weakly foliated, leucocratic rocks in the northern Blue Ridge of northernmost Virginia and Maryland as garnet monzogranite (unit Ygt on the geologic map of Virginia (Virginia Division of Mineral Resources, 1993)).

Detailed SHRIMP analysis indicates that zircons from this locality are compositionally and isotopically complex. SHRIMP data suggest that crystallization occurred at 1,064±7 Ma (table 3, sample SNP-02-197). This age is similar to isotopic data obtained by conventional isotope-dilution thermal ionization mass spectrometry (ID-TIMS) techniques obtained by Aleinikoff and others (2000) for possibly related garnetiferous metagranite (Ygt) and white leucocratic metagranite (Yg) from the northern Blue Ridge dated at 1,077±4 Ma and 1,060±2 Ma, respectively.

Mileage
Trip
Cumulative
Stop-to-Stop
Cumulative
    0.0 Proceed to intersection of Gooney Falls Lane and Va. 649.
8.7   0.3 Turn right (south) onto Va. 649.
      Buck Mountain, visible on the right, is underlain by typically
      coarse-grained leucocratic granitoid of the Ygg lithologic unit.
9.1   0.7 Prominent roadcut on the left exposes relatively fresh foliated
      pyroxene+biotite quartz diorite (Ypqd).
9.8   1.4 Stop 2.
      Outcrop on left (east) side of Va. 649.

Stop 2. Pyroxene+biotite quartz diorite (Ypqd).

This fresh outcrop of pyroxene+biotite quartz diorite (Ypqd) is typical of roadcuts along Va. 649. The unit is mapped as a small, elongate body that is intruded by and enclosed within garnetiferous alkali feldspar granite to syenogranite (Ygg). The body is interpreted to form an inlier within the larger leucocratic Ygg pluton. Gneissic layering defined by alternating feldspar- and pyroxene+biotite-rich domains is characteristic of this unit, and is best seen on weathered surfaces. Isoclinal folds developed locally within the compositional layering exhibit considerable thickening of the layers in the hinge zones, and are interpreted to have formed under ductile conditions during Grenvillian orogenesis.

Samples of this lithologic unit are typically dark-gray, medium-grained, equigranular, moderately to strongly foliated quartz diorite to quartz monzodiorite (fig. 4) composed of 20 to 25 percent orthoclase, 60 to 65 percent plagioclase, and 12 to 15 percent quartz with 5 to 7 percent orthopyroxene and 2 to 4 percent biotite. Accessory minerals include apatite, zircon, and Fe-Ti oxide. Alignment of biotite and orthopyroxene grains typically defines foliation that is parallel to centimeter-scale gneissic layering.

The Ypqd lithologic unit is one of the most chemically distinctive lithologic units documented within the field trip area. Two samples have similar low-silica compositions (fig. 5), with SiO2 contents ranging from 55 to 57 weight percent. These compositions rank among the most chemically primitive observed within the field trip area. Although major-element abundances are similar to the most silica-poor variants of the low-silica charnockite (Yfqj), trace-element concentrations differ significantly, indicating likely separate petrologic lineages. Lower concentrations of high-field-strength elements in the Ypqd samples suggest an origin involving sources with a significant volcanic-arc component, in contrast to the more typical crustal-type sources from which the low-silica charnockite was likely derived. Compositionally, the Ypqd unit corresponds closely to typical I-type granites, whereas the low-silica charnockite (Yfqj), with high Nb+Y concentrations and elevated 104×Ga/Al and FeOt/MgO ratios, exhibits close chemical affinity to A-type granitoids (fig. 7).

Field relations observed at Boyd's Mill (Stop 3) indicate that the Ypqd unit is older than, and likely intruded by, the surrounding Ygg leucogranitoids, suggesting that the former is older than 1,062 Ma. The available field and petrologic data are insufficient to suggest correlation between the Ypqd lithologic unit and older, similarly charnockitic units of known age that occur to the south.

Mileage
Trip
Cumulative
Stop-to-Stop
Cumulative
 
0.0
Proceed south on Va. 649.
10.2
0.4
Stop 3.
Intersection of Va. 622 and Va. 649 at Boyd's Mill.
Outcrop on left side of Va. 649.
Park on right side beyond intersection.

Stop 3. Garnetiferous alkali feldspar granite to syenogranite (Ygg) intruding pyroxene+biotite quartz diorite (Ypqd).

Field relations observed at this roadcut indicate intrusion of the Ygg leucocratic granitoid into medium-grained, equigranular, charnockitic rocks of the Ypqd inlier. Coarse-grained, garnet+biotite-bearing leucocratic granite is interlayered on a scale ranging from decimeters to meters with medium- to dark-gray, medium- to coarse-grained, foliated pyroxene-bearing gneissic granitoid that locally contains garnet. Geologically, this roadcut is located along the mapped north-south-trending contact separating leucocratic granitoid rocks of the Ygg pluton from the western border of the Ypqd inlier (fig. 3), and thus the exposure is interpreted as part of the contact zone formed by intrusion of leucogranitoid into the melanocratic gneissic rocks. The leucocratic granitoid in this roadcut is composed of abundant perthitic orthoclase+plagioclase+quartz+garnet+biotite+Fe-Ti oxide+chlorite. Plagioclase is typically altered to sericite; primary garnet is partly replaced by chlorite+minor titanite. Garnet in the melanocratic granitoid occurs in proximity to leucocratic granite and is interpreted as metamorphic in origin.

Geochemical data for a sample of leucocratic granitoid from this roadcut (table 4, sample SNP-01-174) indicate a high-silica, peraluminous composition with high K2O/Na2O molar ratio, corresponding closely to the average composition of rocks constituting the Ygg pluton. The composition of a representative sample of the melanocratic granitoid (table 4, sample SNP-01-155) bears close chemical similarity to rocks collected elsewhere from the inlier except for distinctly higher silica contents, lower concentrations of CaO, K2O, and Na2O, and correspondingly high values of aluminum saturation index (molar Al2O3/(Na2O+K2O+CaO). Such differences in composition are interpreted to reflect chemical exchange that occurred as a result of metamorphic reactions induced by intrusion of the leucogranite.

Mileage
Trip
Cumulative
Stop-to-Stop
Cumulative
    0.0 Turn around and proceed north on Va. 649.
15.9   5.7 Turn right (northeast) onto U.S. 340.
16.1   5.9 Turn right into the entrance to Shenandoah National Park and Skyline Drive South.
16.7   6.5 Pass through entrance station to Shenandoah National Park.
18.9   8.7 Shenandoah Valley Overlook.
      The view from the overlook on the right includes the abandoned former
      Avtek factory located along the Shenandoah River in Front Royal. This
      factory and the surrounding environmentally sensitive area constitute a
      designated Superfund site slated for remediation.
20.8   10.6 Dickey Ridge Visitor Center.
21.9   11.7 Signal Knob Overlook.
      On a day with high visibility, the view from this
      overlook provides a rich perspective on the geology of the central
      Appalachians. The double-topped linear ridges in the middle distance
      define Massanutten Mountain, a complex syncline whose topographic
      expression is defined by resistant Silurian sandstone. Most of the floor
      of the surrounding Shenandoah Valley is underlain by the Ordovician
      Martinsburg Formation and Cambrian and Ordovician carbonate rocks
      exposed in the footwall block beneath the Blue Ridge thrust that runs
      along the base of the foothills in the foreground. North Mountain, which
      forms the long linear ridge on the horizon, is capped by Devonian
      sandstone and is a typical landform of the Appalachian Valley and
      Ridge province.
      The roadcut on the east side of Skyline Drive exposes
      two lava flows of the Neoproterozoic Catoctin Formation separated by
      a 1- to 1.5-m (3.3–4.9 ft)-thick stratum of terrestrial sedimentary deposits.
      Jasper veins developed in the metasedimentary layer are typical of the contact
      zone beneath and between lava flows in this area.
23.1   12.9 Gooney Run Overlook.
      View to the west overlooks areas of the Chester Gap 7.5-minute
      quadrangle visited in Stops 1–3. Buck Mountain, underlain by
      leucocratic granitoids of the Ygg pluton, is located directly west
      of the overlook. Stop 1 is located at the northern end of the mountain;
      Stops 2 and 3 are located along the road extending along its
      eastern flank.
25.6   15.4 Stop 4.
      Outcrop on left (east) side of Skyline Drive.

Stop 4. Orthopyroxene+amphibole layered granodiorite gneiss (Ylgn).

The large roadcut at Lands Run Gap on Skyline Drive exhibits characteristics typical of the orthopyroxene+amphibole layered granodiorite gneiss (Ylgn) and is designated as the lithologic type locality for this unit (Tollo and others, in press b). This stop is located about 0.5 km (0.3 mi) east of an unnamed fault that juxtaposes basement units and greenstone against basement (fig. 3). The Ylgn lithologic unit underlies a large area in the central part of the Chester Gap quadrangle where it is both overlain by and in fault contact with greenstones of the Catoctin Formation. Compton Peak, visible to the south, represents an isolated erosional remnant of the cover sequence that preserves a succession of rocks of the Swift Run and Catoctin Formations nonconformably overlying basement (Gathright, 1976) (fig. 3).

The Ylgn lithologic unit consists of dark-gray, medium- to coarse-grained, inequigranular, strongly foliated granodioritic (opdalite) gneiss composed of 35 to 40 percent alkali feldspar (microcline or orthoclase), 30 to 40 percent plagioclase, and 20 percent quartz with 3 to 5 percent amphibole and 3 to 5 percent orthopyroxene. Accessory minerals include apatite, zircon, and ilmenite. Characteristic gneissosity is defined by alternating centimeter-scale quartz+feldspar-rich (light) and pyroxene+amphibole-rich (dark) layers that form intrafolial folds within a foliation plane defined by discontinuous polygranular domains of amphibole and orthopyroxene. Interlayered and crosscutting, decimeter-scale sheets of light-gray to white, medium- to coarse-grained, inequigranular, massive leucogranite composed mostly of white feldspar and gray quartz are characteristic, and likely represent dikes of younger granitoid that may be related to the circa-1,060-Ma postorogenic type that includes the Old Rag magmatic suite and other similar rocks. The development of gneissosity, leucogranitoid boudins, and isoclinal intrafolial folds are interpreted to reflect deformation of probable Grenvillian age.

Geochemical data indicate that the Ylgn lithologic unit is mildly metaluminous and intermediate in composition with SiO2 contents of 64 to 67 weight percent (fig. 5D). The overall chemical correspondence of this unit to typical granodioritic compositions is interpreted as evidence for an igneous protolith. Ylgn is the least chemically evolved of the orthopyroxene-bearing granitic units characterized by silica contents >60 weight percent in the field area. Like the other charnockites, Ylgn exhibits compositional features that are transitional between I- and A-type granitoids (fig. 7) and was likely derived from sources of mixed composition, including both volcanic-arc and crustal components (fig. 6).

The characteristic gneissic fabric of this lithologic unit indicates that Ylgn is part of the older group of deformed rocks in the area. This group includes a compositionally diverse array of charnockitic and non-charnockitic granitoids that display mineralogic and textural evidence of deformation that occurred at high-grade conditions. Dated rocks within this group, which include the Ycf (1,147±16 Ma) and Ylg (1,183±11 Ma) lithologic units, represent the oldest recognized lithologies in the field area. Two tabular bodies of dark-colored, foliated to massive, fine-grained, biotite-rich granitoid, both <1.3 m (4.3 ft) in width, occur at a slight angle to foliation and gneissosity in a nearby roadcut, and are interpreted to be dikes cutting the Ylgn lithologic unit. Mineralogic and geochemical similarity of these dike rocks (table 4, sample SNP-01-149) to the pyroxene quartz diorite (Ypqd) lithologic unit (table 4, sample SNP-01-175) suggest that Ypqd is younger than Ylgn.

Mileage
Trip
Cumulative
 
Stop-to-Stop
Cumulative
 
0.0
Proceed south on Skyline Drive.
26.9
1.3
Indian Run Overlook.
      Rocks exposed in the long roadcut display one of the finest examples of
      columnar jointing in metabasalt of the Catoctin Formation within
      Shenandoah National Park. Such columns, which are common
      throughout the Catoctin Formation on the west limb of the Blue Ridge
      anticlinorium, are evidence of the subaerial eruptive origin of the
      lava flows.
28.5
2.9
Jenkins Gap Overlook.
33.7   8.1 Gravel Springs Gap.
35.1
9.5
Mount Marshall Overlook.
36.1
10.5
Stop 5.
      Little Devil Stairs Overlook. Outcrop on west side of road.

Stop 5. Orthopyroxene+amphibole syeno- and monzogranite (Ycf).

The large roadcut at Little Devil Stairs Overlook is located about 200 ft (61 m) north of the unconformable contact separating basement from the overlying Catoctin Formation, exposed in the hillslope to the south (Gathright, 1976). This stop is located about 1.1 km (0.7 mi) west of a fault that intersects the Stanley fault near Keyser Mountain to the southeast (Gathright, 1976). Despite their appearance in the roadcut, basement rocks at this locality are intensely weathered and preserve only partial evidence of the primary ferromagnesian mineral assemblage. Throughout the exposure, fine-grained charnockite forms meter-scale dikes cutting coarser grained charnockite. The rocks exhibit weak to moderately developed, steeply dipping foliation striking 075° to 090°. The predominantly east-west strike of the foliation is unusual in an area that is otherwise dominated by northeast-southwest fabrics associated with Paleozoic orogenesis (Mitra and Lukert, 1982), and is likely to have been developed during the Grenvillian orogeny. Both the fine- and coarse-grained varieties of charnockite are composed of alkali feldspar microperthite (dominantly microcline in the coarse-grained variety, mostly orthoclase in the fine-grained variety), plagioclase and quartz, with rare amphibole and abundant pseudomorphs that likely formed after orthopyroxene. This primary mineral assemblage is also characteristic of the foliated pyroxene granite (Yfpg) (table 2, fig. 3).

Chemical compositions of samples of both fine- and coarse-grained charnockite collected from this outcrop (table 4, samples SNP-01-142 and SNP-01-143, respectively) display geochemical similarities to samples of both the high-silica charnockite (Ycf) and foliated pyroxene granite (Yfpg) (table 4, samples SNP-96-10 and SNP-02-177, respectively). The Ycf unit and the foliated pyroxene granite (Yfpg), which have crystallization ages that differ by 45 m.y., are both foliated, amphibole+orthopyroxene-bearing, siliceous charnockites, and thus the mineral assemblage and chemical composition of the rocks at this roadcut do not represent sufficient evidence on which to establish lithodemic correlation. However, the more homogeneous (albeit foliated) fabric and more disseminated nature of the orthopyroxene in Ycf are features that are similar to the rocks at this exposure, contrasting with the characteristically very coarse and clustered nature of orthopyroxene in Yfpg. Thus, on the basis of this fabric, the rocks at Little Devil Stairs Overlook are considered part of the Ycf lithologic unit, pending detailed mapping of basement lithologies in the Bentonville 7.5-minute quadrangle.

The charnockite is cut by five large metabasalt (greenstone) dikes of meter-scale thickness at the northern end of the exposure. Numerous fine- to medium-grained mafic dikes intrude basement rocks throughout the field trip area and across the Shenandoah massif. Such dikes exhibit diverse textures, metamorphic grade, and mineralogic compositions ranging from basalt to diabase to greenstone. Although largely undated by modern isotopic techniques, most mafic dikes have been mapped as either Mesozoic or Neoproterozoic in age on the basis of crosscutting relations, mineralogic composition, and the presence or absence of metamorphic fabrics (for example, Burton and others, 1995; Southworth, 1995; Southworth and Brezinski, 1996; Bailey and others, 2003). In general, dikes of late Neoproterozoic age contain mineralogic evidence of recrystallization at greenschist-facies metamorphic conditions, including development of assemblages containing actinolite+biotite+serpentine, which are commonly absent in Mesozoic dikes (Wilson and Tollo, 2001). Many Neoproterozoic dikes display tectonic cleavage as a result of deformation and retrograde metamorphic recrystallization; such cleavage is absent in Mesozoic dikes. Samples of the dikes cutting basement at this stop contain abundant chlorite+epidote±serpentine, and are typical of the most abundant type documented by studies within the Blue Ridge which generally have weakly quartz-normative, tholeiitic compositions. Nevertheless, dikes throughout the area show major- and trace-element characteristics that are largely bimodal and may be indicative of two ages of emplacement. As illustrated in figure 8, the bimodal major-element compositions of Blue Ridge dikes correspond closely to either the Mesozoic Mt. Zion Church Basalt in the nearby Culpeper basin or relatively unaltered lava flows and associated dikes of likely Neoproterozoic age (Badger, 1989). Chemical analyses of two of the dikes at this locality (table 5, samples SNP-01-144 and SNP-01-145) indicate high TiO2 contents and low values of 100(MgO/(MgO+FeOt) (fig. 8), suggesting close compositional affinity to dikes and lava flows associated with the Catoctin Formation (Badger, 1989). Such dikes may have served as conduits for magma moving toward the surface during eruption, an interpretation that is consistent with the physical proximity of these dikes to the overlying Catoctin greenstones. The usefulness of the chemical discriminants plotted in figure 8 is underscored by the similarity in compositions of two samples of thoroughly retrograded greenstone (squares with diagonal crosses) to those of less retrograded metabasalt and metadiabase (filled squares), indicating that, in many cases, Paleozoic greenschist-facies metamorphism did not result in mass transfer of chemical components. Dikes formed during Mesozoic rifting are apparently much less abundant in the Blue Ridge, and can be distinguished chemically from dikes of inferred Neoproterozoic age on the basis of lower TiO2 contents and higher 100(MgO/(MgO+FeOt) ratios (Wilson and Tollo, 2001) (fig. 8). Mesozoic dikes also contain higher SiO2 contents, are relatively enriched in compatible elements such as Ni and Cr, and show lower concentrations of high-field-strength elements, including Zr, Nb, and Y (Wilson and Tollo, 2001). In figure 8, only 4 dikes (from the total population of 29 dikes sampled as part of this study) correspond closely in composition to the tholeiitic Mount Zion Church Basalt, which is the oldest and most primitive basalt type in the nearby early Mesozoic Culpeper basin (fig. 2). This composition also corresponds to the most common type of diabase sheets that intruded sedimentary strata within the basin, and is the only Mesozoic diabase magma type that is comparable to any of the dikes in the Blue Ridge (Froelich and Gottfried, 1988).

Mileage
Trip
Cumulative
Stop-to-Stop
Cumulative
 
    0.0 Proceed south on Skyline Drive.
40.0   3.9 Pass Elkwallow Store.
47.3   11.2 Pass intersection with U.S. 211 at Thornton Gap.
47.4   11.3 Pass Panorama on right of Skyline Drive.
48.0   11.9 North portal of Mary's Rock Tunnel.
      A large, northeast-striking mafic dike displaying prominent
      columnar jointing cuts high-silica charnockite (Ycf) on the west side of
      the tunnel portal. The fine-grained diabase contains microphenocrysts
      of clinopyroxene+pigeonite+plagioclase and shows relatively little
      evidence of retrograde metamorphism. Nevertheless, the chemical
      composition of the dike (table 5, sample SNPD-99-1) indicates
      high-TiO2 content and trace-element characteristics that are typical of
      dikes of Neoproterozoic age (Wilson and Tollo, 2001). The overall
      lack of development of retrograde mineral assemblage and tectonic
      cleavage in the dike is a likely consequence of its occurrence within the
      rigid, relatively anhydrous charnockite pluton.
48.2   12.1 South portal of Mary's Rock Tunnel.
      Stop 6.
      Parking on left (east) side of Skyline Drive.

Stop 6. Orthopyroxene+amphibole syeno- and monzogranite (Ycf).

The view toward the east from the tunnel parking lot provides an informative perspective on the physiographic Piedmont province of the central Appalachians and geological framework of the Blue Ridge anticlinorium. The Blue Ridge foothills located in the foreground directly to the east are underlain mostly by Mesoproterozoic basement rocks, including some lithologic units that also occur along Skyline Drive. These foothills give way eastward to the typical low topography of the core of the Blue Ridge anticlinorium, most of which is included in the Piedmont physiographic province (Fenneman, 1938). The core of the anticlinorium is underlain primarily by Mesoproterozoic basement rocks and, locally, by igneous and metasedimentary rocks formed during Neoproterozoic rifting. The two symmetric hills in the middle distance (Little Battle Mountain and Battle Mountain, north and south peaks, respectively) are underlain by the Battle Mountain volcanic complex that includes both volcanic and subvolcanic felsic rocks associated with the extensionrelated, 730- to 702-Ma Robertson River batholith (Tollo and Aleinikoff, 1996; Tollo and Hutson, 1996). The elongate batholith continues both north and south of these hills, ultimately stretching nearly 110 km (70 mi) across the anticlinorium from the west limb in the north to the east limb in the south (fig. 2); however, the intrusion only locally underlies steep topography. The flat-topped ridge of the Bull Run Mountains occupies the horizon and is underlain by resistant quartzite of the upper Neoproterozic to Lower Cambrian Weverton Formation, which is stratigraphically underlain by the Catoctin Formation for much of the length of the ridge. These rocks constitute part of the Blue Ridge cover-rock sequence, thus locally defining the eastern limb of the Blue Ridge anticlinorium.

This stop is located in the central part of a pluton consisting of moderately to strongly foliated, coarse- to very coarse-grained, orthopyroxene±amphibole-bearing syeno- and monzogranite (charnockite and farsundite, Ycf). This lithologic unit occurs across most of the Thornton Gap quadrangle (fig. 3) and is a member of the oldest group of plutonic rocks in the region. In this area, the Ycf pluton includes map-scale inliers of coarse-grained, orthopyroxene+garnet granite gneiss (farsundite gneiss, Yg), which constitute probable screens of older rocks. Mapping also indicates that Ycf is intruded by elongate bodies of massive to weakly foliated granite of the Old Rag magmatic suite (Ygr) and massive to weakly foliated, low-silica charnockite (Yfqj) (fig. 3). Collectively, charnockitic rocks of the Ycf, Yg, and much younger Yfqj lithologic units constitute part of the classic orthopyroxene- bearing Pedlar Formation that was mapped throughout much of this part of the Blue Ridge (Gathright, 1976) before the advent of modern analytical methods made geochemical and age-based differentiation of mineralogically similar units possible.

The rocks near the tunnel expose strongly foliated, coarse- to very coarse-grained, orthopyroxene-amphibole, inequigranular to megacrystic syeno- and monzogranite (charnockite and farsundite) that is typical of the Ycf pluton. The rock is composed of 22 to 57 percent alkali feldspar microperthite (chiefly microcline), 10 to 37 percent plagioclase, and 11 to 49 percent quartz with <1 to 14 percent orthopyroxene, 0 to 11 percent amphibole, and rare clinopyroxene. Accessory minerals include apatite, ilmenite, magnetite, zircon, epidote, and actinolite. Locally prominent gneissic layering is defined by interlayered quartz+feldspar- and ferromagnesian mineral-rich domains ranging from less than 1 cm (0.4 in) to greater than 13 cm (5 in) and is especially visible on weathered surfaces. Foliation is defined locally by planar alignment of ferromagnesian minerals and is parallel to gneissic layering. Subhedral to euhedral, monocrystalline alkali feldspar megacrysts range up to 13 cm (5 in) in length and are best observed on weathered surfaces. The strongly developed fabric is characteristic of the older group of plutonic rocks in the field trip area.

Electron microprobe analyses indicate that pyroxenes in the Ycf lithologic unit display strong Quad (Ca-Fe-Mg) compositional features, according to the criteria of Morimoto (1988). Pyroxene compositions show very little variation both within individual grains and between grains in single thin sections. Clinopyroxenes in the Ycf charnockite are relatively Fe-rich (average Wo43En22Fs35); however, application of the charge-balance criteria of Lindsley (1983) indicate that ferric iron contents are negligible. Orthopyroxenes in Ycf are very Ca-poor and Fe-rich (average Wo2En27Fs71). Visible exsolution is rare in either pyroxene type. Compositions of coexisting pyroxenes in Ycf indicate equilibration temperatures of less than 500°C, as calculated by the QUILF program of Andersen and others (1993) for pressures consistent with observed granulite-facies mineral assemblages. Because such temperatures fall hundreds of degrees below likely crystallization temperatures for igneous charnockites (Kilpatrick and Ellis, 1992), and because the pyroxenes exhibit evidence of thorough compositional homogenization, the rocks are inferred to have undergone an extended interval of subsolidus re-equilibration at temperatures that remained elevated but below peak granulite-facies conditions.

Amphiboles are typically brown and closely associated with orthopyroxene, locally separated by a narrow optical transition zone. Compositions of the amphiboles correspond to hornblende (sensu lato) according to the criteria of Deer and others (1992) and to the calcic group of Leake and others (1997). Data from electron microprobe traverses of individual grains indicate little evidence of compositional zoning, and repeated analyses of multiple grains within single samples indicate restricted compositional variation. Amphiboles in Ycf (and Yfqj) display compositional similarities to amphiboles observed in charnockites from the calc-alkaline Louis Lake batholith in the Wind River Range of Wyoming, which were interpreted by Frost and others (2000) to represent primary phases that crystallized from igneous melts. Amphiboles of likely igneous origin occur in charnockitic rocks of tholeiitic affinity from many other locations (Malm and Ormaasen, 1978; Duchesne and Wilmart, 1997; Sheraton and others, 1992), suggesting that the occurrence of igneous amphiboles in charnockitic rocks is not uncommon. For this reason, and because of their association with pyroxene that displays textural features consistent with original igneous crystallization, the Blue Ridge charnockitic amphiboles are also interpreted as primary magmatic minerals.

Chemical analyses indicate that Ycf rocks show only restricted compositional variation thoughout the field trip area. The granitoids are mildly metaluminous, with SiO2 contents in the range of 68 to 72 weight percent (table 4; fig. 5). The high-silica Ycf pluton is one of several siliceous rocks of charnockitic affinity in the northern part of the study area. Others include the garnetiferous granite gneiss (Yg), foliated pyroxene granite (Yfpg) (both units shown on fig. 3) and a small, typically medium-grained, equigranular charnockitic pluton documented by recent mapping in the Big Meadows quadrangle (not shown in fig. 3). However, of these four orthopyroxene-bearing units, only the charnockite from Big Meadows and the distinctive Ycf unit are characterized on fresh surfaces by the dark greenish-gray appearance that is typical of charnockites worldwide. Trace-element analyses of Ycf rocks suggest derivation from mixed sources of possible volcanic-arc and crustal affinity (fig. 6) and indicate that the pluton displays chemical characteristics that are transitional between I- and A-type granitoids (fig. 7).

U-Pb isotopic analyses obtained using SHRIMP techniques indicate that zircons in the Ycf lithologic unit are compositionally complex. Two populations are present, based on crystal morphology: (1) euhedral, elongate prisms displaying dipyramidal terminations and (2) smaller, nearly equant grains (Tollo and others, in press a). Examination of the prismatic zircons in cathodoluminescence (CL) indicates the presence of cores characterized by concentric oscillatory zoning surrounded by narrow unzoned rims. Multiple analyses indicate a weighted average 206Pb/238U age of 1,159±14 Ma for the zoned cores, which is interpretedas the time of igneous crystallization. Analyses of the equant grains and overgrowths on the prismatic grains indicate a composite age of 1,052±14 Ma, which likely corresponds to a period of regional thermal activity. Results from ID-TIMS dating by Aleinikoff and others (2000) indicate that meta-igneous rocks of similar age, possibly including a charnockite, occur in the Blue Ridge north of the field trip area.

Mileage
Trip
Cumulative
 
Stop-to-Stop
Cumulative
 
    0.0 Proceed south on Skyline Drive.
48.7   0.5 Stop 7.
      Buck Hollow Overlook.
      Parking on left (east) side of Skyline Drive.

Stop 7. Orthopyroxene+garnet granite gneiss (Yg) and orthopyroxene+amphibole syeno- and monzogranite (Ycf).

The roadcut opposite the overlook exposes one of the freshest examples of orthopyroxene+garnet granite gneiss in Shenandoah National Park. Field relations indicate that this exposure is part of a large screen of garnetiferous gneiss mapped as lithologic unit Yg that is enclosed within high-silica charnockite (Ycf) (fig. 3), suggesting that the latter pluton intruded and enveloped the former (Tollo and others, in press a,b). Direct crosscutting relations have not been observed in outcrop, however. This field relation and the ubiquitous, strongly developed fabric of the granitic gneiss, which is characteristic of rocks that predate the main episode of Grenvillian orogenesis in the area, suggest that Yg predates the high-silica charnockite (Ycf) and therefore is older than 1,159 Ma. The curved contacts of the screen and adjacent rocks of the Old Rag magmatic suite suggest the presence of map-scale folds developed in the basement rocks (fig. 3). The locally parallel alignment of gneissic layering defined by primary minerals and the trend of the folded contacts further suggest that such deformation may be Grenvillian in age.

Rocks at this locality include light-gray to gray, medium- to coarse-grained, inequigranular, strongly foliated syeno- to monzogranite gneiss (farsundite gneiss) composed of 28 to 49 percent alkali feldspar microperthite (chiefly microcline), 14 to 30 percent plagioclase, and 28 to 35 percent quartz with 4 to 6 percent orthopyroxene, <1 to 7 percent garnet, <1 to 3 percent biotite, and rare clinopyroxene. Accessory minerals include apatite, ilmenite, magnetite, zircon, and chlorite. Typically prominent gneissic layering is defined by interlayered quartz+feldspar- and ferromagnesian mineral-rich domains ranging from less than 1 cm (0.4 in) to greater than 70 cm (28 in) in thickness. Foliation is defined locally by planar alignment of ferromagnesian minerals and is parallel to gneissic layering. A locally well-developed mineral lineation is defined by elongate domains of garnet and biotite located within the foliation plane.

Clinopyroxene in the garnetiferous granite gneiss (Yg) is generally too altered for meaningful chemical analysis. However, orthopyroxene (Wo1En28Fs71) is similar in composition to orthopyroxene in the high-silica charnockite (Ycf), but consistently indicates slightly lower temperatures of equilibration. Like orthopyroxene, garnet exhibits very little compositional variation both within individual grains and among multiple grains. Garnet in Yg is Fe-rich (Grs: 9.3, Alm: 79.5, Prp: 11.2), reflecting the elevated FeOt/MgO wholerock ratio (fig. 5F).

Samples of the Yg lithologic unit are borderline metaluminous to moderately peraluminous with SiO2 contents of 70 to 75 weight percent (fig. 5D). The Yg rocks display overall compositional features that are similar to other siliceous charnockites in the region, including relatively low TiO2, FeOt, and CaO contents (table 4). Compositionally, Yg differs from the Ycf unit that encloses it in having generally higher SiO2 contents, higher FeOt/MgO ratios, and distinctly peraluminous bulk compositions (table 4; fig. 5D, F). These geochemical characteristics and normative compositions of granitic affinity suggest that Yg is meta-igneous in origin.

Map relations and the characteristic deformed fabric of the garnetiferous granite gneiss suggest that the lithologic unit is one of the oldest rocks in the area. Like the leucogranite gneiss (Ylgg) lithologic unit that occurs in the Fletcher 7.5-minute quadrangle (fig. 3), Yg rocks occur only as inliers that are isolated within igneous plutons of likely younger age. These field relations suggest that the present erosion surface in the Blue Ridge coincides with a paleodepth interval at which Grenvillian plutons were successively emplaced. This zone of emplacement is dominated by plutons and is characterized by an apparent paucity of preexisting country rocks. Whether the missing country rock was removed by subsequent erosion of higher levels or remains as roof pendants concealed at depth is not presently known.

Large outcrops and roadcuts of medium-grained, gray-green, orthopyroxene+amphibole syeno- and monzogranite (Ycf) occur nearby, adjacent to Skyline Drive about 200 ft (61 m) north of the Buck Hollow Overlook exposure. The contact separating Ycf from the Yg inlier is not exposed, but likely is located in the east-trending stream valley that crosses Skyline Drive south of Skinner Ridge. The atypical finer grain size of the Ycf rocks in this area is interpreted to result from contact-related cooling against the Yg screen during magmatic emplacement.

Mileage
Trip
Cumulative
Stop-to-Stop
Cumulative
0.0
Proceed south along Skyline Drive.
49.3
0.6
Meadow Springs Overlook and Parking Area.
      Exposures of low-silica charnockite (Yfqj) along the west side of
      Skyline Drive opposite the overlook are part of a small dike that
      intruded along the contact separating high-silica orthopyroxene+
      amphibole syeno- and monzogranite (charnockite+farsundite,Ycf) from
      garnetiferous granite gneiss (farsundite gneiss, Yg) (fig. 3). Sample
      SNP-96-17 (table 4) was collected from the roadcut and is
      mineralogically and geochemically typical of the Yfqj pluton, but
      generally finer grained.
52.3   3.6 Jewel Hollow Overlook.
      The view to the west includes the Massanutten Mountain synclinorium
      in the middle distance and North Mountain on the horizon. Outcrops at
      the overlook expose high-silica charnockite of the Ycf lithologic unit.
53.5   4.8 A large northeast-striking mafic dike cuts garnetiferous granite gneiss
      (Yg) in the roadcut on the west side of Skyline Drive. The chemical
      composition of the dike (table 5, sample SNPD-99-3) is typical of
      Catoctin-related mafic rocks in the area. This geochemical affinity and
      close spatial association with extrusive metabasalt of the Catoctin
      Formation occurring to the south suggest that the dike is part of the
      feeder complex that provided magma to the rift-related basalt flows.
54.4
5.7
Stony Man Mountain, a well-known landmark in Shenandoah National
      Park, is visible directly to the south. The mountain is underlain by
      metavolcanic and metasedimentary rocks of the Catoctin Formation
      (Gathright, 1976) that define five major eruptive flow units and at least
      two layers of intercalated metasedimentary rocks and volcanic breccia,
      all of which dip gently toward the southeast (Badger, 1999). The view to
      the west includes a broad expanse of the eastern Shenandoah Valley. The
      South Fork of the Shenandoah River is visible east of Massanutten
      Mountain, where it has a low-gradient path characterized by numerous
      meanders.
55.1   6.4 Skyline Drive ascends uphill through roadcuts of Catoctin Formation
      greenstone.
57.5   8.8 Entrance to Skyland on the right, highest point on Skyline Drive
      at 3,680 ft (1,122 m). Like the eastern part of Skyland, most of the
      higher elevations in the Park are underlain by greenstones of the
      Catoctin Formation. Examples include Hawksbill Mountain at 4,050 ft
      (1,234 m), Stony Man Mountain at 4,011 ft (1,223 m), and Hazeltop
      Mountain at 3,812 ft (1,162 m)
59.0   10.3 Stop 8.
      Timber Hollow Overlook.

Stop 8. Unakite developed in nonfoliated low-silica amphibole-bearing charnockite (Yfqj).

This overlook provides views to the west of the eastern Shenandoah Valley and Massanutten Mountain in the middle distance. On a clear day, talus slopes composed of variably sized blocks of white Silurian quartz arenite of the Massanutten Formation can be observed on the flanks of the ridges. Such talus deposits are a characteristic feature of the hillslopes, which are typically capped by resistant quartz-rich sandstone. New Market Gap, where U.S. 211 crosses Massanutten Mountain, can also be observed west of Luray. The north-striking Stanley fault, part of the thrust that transported Blue Ridge basement rocks northwestward over lower Paleozoic sedimentary strata during late Paleozoic Alleghanian orogenesis, is located about 4.0 km (2.5 mi) west of the overlook in the valley below.

Timber Hollow Overlook is underlain by low-silica charnockite that is correlated mineralogically and geochemically with a larger pluton composed of amphibole-bearing farsundite and quartz jotunite (Yfqj) mapped to the south (fig. 3). Gathright (1976) mapped stratified rocks of the Catoctin Formation (dominantly greenstones) and underlying Swift Run Formation (dominantly phyllites) as unconformably overlying charnockite in this area, with a contact located about 1,000 ft (305 m) to the east of the overlook. As a result of the low dip of this contact, charnockite outcrops located below the overlook display abundant mineralogic evidence of contact metamorphism related to thermal effects caused by extrusion of overlying lava flows. The normally dark-greenish-gray charnockite is typically bleached light gray due to retrograde recrystallization of abundant primary plagioclase and lesser alkali feldspar. Ferromagnesian minerals are completely pseudomorphed by hydrous, low-temperature assemblages. Parts of the charnockite located adjacent to major joints are locally converted to unakite, in which plagioclase is mostly replaced by green epidotegroup minerals, alkali feldspar is replaced largely by a pink hematite-bearing assemblage, and small amounts of originally blue quartz remain relatively unaltered. The localization of well-developed unakite adjacent to fractures is a likely consequence of hydrothermal fluid migration induced by heat from the overlying lavas during Neoproterozoic extension. The effects of thermal metamorphism decrease rapidly downhill away from the contact. Unakite is a visually distinctive, but rare, rock type that is typically developed in Blue Ridge basement rocks through retrograde recrystallization associated with (1) extrusion of the overlying Catoctin Formation, (2) enhanced fluid flow associated with fault zones, or (3) local effects of regional Paleozoic metamorphism (Tollo and others, in press b; Wadman and others, 1998).

Mileage
Trip
Cumulative
Stop-to-Stop
Cumulative
    0.0 Proceed south along Skyline Drive.
63.5   4.5 Stop 9.
      Fisher's Gap.
      Proceed to Dark Hollow Falls.

Stop 9. Erosional window exposing coarse-grained, inequigranular, amphibole-bearing charnockite (Yfqj).

Fisher's Gap and the surrounding area are located within the thrust sheet that carried basement and overlying rocks northwest over Paleozoic strata during Alleghanian orogenesis. Drainage from the nearby Big Meadows area flows northeast along Hogcamp Branch, which is partly responsible for developing an erosional window through cover rocks north of the falls (Gathright, 1976). Dark Hollow Falls is located within stratified rocks of the Catoctin Formation, which locally include massive and phyllitic greenstone, amygdaloidal greenstone, and metaconglomerate containing cobble-sized clasts of basement rocks (Badger, 1999). The Catoctin Formation nonconformably overlies basement rocks throughout the area, but has been removed by erosion in the vicinity of Hogcamp Branch, leading to exposure of the underlying basement rocks. The contact separating greenstone from charnockite is located in the stream valley below the intersection of the access road and Hogcamp Branch (Gathright, 1976). Charnockite is well exposed in ledges located within and adjacent to the stream, from an elevation of about 2,600 ft (790 m) down to near the intersection of Hogcamp Branch and the small stream that descends from Rose River Falls. The basement rocks are composed of massive to weakly foliated, coarse-grained, inequigranular, amphibole-bearing charnockite that is correlated with the large Yfqj pluton located about 3.2 km (2 mi) to the southeast (fig. 3). This low-silica charnockite is distinguished in the field by typically massive to weakly foliated fabric, relatively low quartz content, abundance of pyroxene, and local presence of magnetite. This widespread lithologic unit underlies a significant part of the Big Meadows 7.5-minute quadrangle (geology not shown in fig. 3), as well as most of the Fletcher 7.5-minute quadrangle (fig. 3).

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Proceed north along Skyline Drive.
68.0   4.5 Timber Hollow Overlook.
78.3   14.8 Buck Hollow Overlook.
78.8   15.3 Mary's Rock Tunnel.
79.8   16.3 Turn right off Skyline Drive toward U.S. 211.
79.9   16.4 Turn left onto U.S. 211 and proceed east.
80.6   17.1 Earth flows are continually developed in loose soils underlying
      the steep slope adjacent to the road on the left.
81.3   17.8 A small quarry in greenstone of the Catoctin Formation is located
      on the left.
81.7   18.2 Outcrops located along the road expose garnetiferous granite gneiss (Yg)
      near the northern end of a large screen within foliated amphibole-
      bearing high-silica charnockite (Ycf) of 1,159-Ma age.
85.4   21.9 Stop 10.
      Outcrop on left side of U.S. 211.

Stop 10. Old Rag magmatic suite (Ygr).

This outcrop is located near the eastern border of a map-scale dike of garnet-bearing leucogranite that cuts foliated amphibole-bearing high-silica charnockite (Ycf). The large dike is part of the Old Rag magmatic suite and is composed of both medium- and coarse-grained leucogranites that, in this area, cannot be mapped separately at a scale of 1:24,000. The lithologically similar, large pluton that underlies Old Rag Mountain and vicinity is the most likely source of this magma but may be separated from the dike by the Sperryville high-strain zone (SHSZ, fig. 3) originally mapped by Gathright (1976).

Rocks of the Old Rag magmatic suite include massive to moderately foliated, fine- to coarse-grained, inequigranular granite and leucogranite that locally contains biotite+orthopyroxene+garnet (Hackley, 1999). This composite unit was mapped as Old Rag Granite by previous workers (Allen, 1963; Gathright, 1976). In this study, Old Rag Granite is defined only in the vicinity of Old Rag Mountain, where it forms a triangular-shaped plutonic body composed of homogeneous, massive to weakly foliated, coarse- to very coarse-grained, inequigranular, garnet+orthopyroxene granite and leucogranite. Using mineralogic, geochemical, and field criteria, Hackley (1999) demonstrated that the fine- to locally coarse-grained, inequigranular granite and leucogranite that is the dominant lithology of the map-scale dike (and other smaller bodies) and the coarse- to very coarse-grained granite at Old Rag Mountain are petrologically consanguineous, and considered both lithologic varieties to constitute the Old Rag magmatic suite. All granitoids within the suite are composed of variable amounts of alkali feldspar+plagioclase+quartz (typically blue). Orthopyroxene, biotite, and garnet occur nonsystematically as primary ferromagnesian phases. The rocks are typically massive to weakly foliated; alignment of ductilely deformed quartz locally defines a crude lineation.

Rocks of the Old Rag magmatic suite are characterized by high SiO2 contents (table 4) and peraluminous compositions (fig. 5D). The suite includes some of the most chemically evolved rocks in the area; however, similar FeOt/MgO ratios suggest that the Old Rag rocks are not petrologic derivatives of the penecontemporaneous low-silica Yfqj charnockite (fig. 5F). Two representative chemical analyses of rocks of the suite are included in table 4: OR-97-35, coarse-grained garnetiferous leucogranite; and OR-97-51, fine-grained biotite leucogranite. Both samples were collected from Old Rag Mountain; zircons from OR-97-35 were analyzed isotopically for U-Pb geochronology, as described below. These leucogranites display trace-element features that suggest derivation from sources of mixed composition and are typical of post-tectonic granitoids (Förster and others, 1997) (fig. 6). As such, rocks of the Old Rag magmatic suite display geochemical characteristcs that are transitional between I- and A-type granitoids (fig. 7).

Mineralogically and geochemically similar, massive to weakly foliated, leucocratic granitoids constitute a significant lithologic component of Blue Ridge Mesoproterozoic basement, including the Old Rag magmatic suite, garnetiferous alkali feldspar and syenogranite (Ygg), and alkali feldspar granite (Yaf). Gathright (1976) was among the first to recognize that the nonfoliated to weakly foliated fabric and crosscutting field relations of the Old Rag granitoids indicated a relatively late emplacement age among basement rocks. Results from high-precision SHRIMP analyses of zircons from a sample of coarse-grained Old Rag Granite collected from Old Rag Mountain demonstrate a complex geochronological history for this unit. CL imaging indicates that nearly all of the zircons have oscillatory zoned cores and dark overgrowths. Thirteen analyses of the oscillatory zoned portions of zircons from the Old Rag Granite yield a weighted average of the 207Pb/206Pb ages of 1,060±5 Ma, which we interpret as the time of emplacement of the Old Rag Granite and, by extension, the Old Rag magmatic suite. Ages of overgrowths indicate two episodes of post-magmatic crystallization occurring at 1,019±15 Ma and 979±11 Ma, which we interpret as indicating times of thermal disturbance. Monazite from the Old Rag Granite was also dated by SHRIMP methods. Five analyses yield an age of 1,059±8 Ma, identical within uncertainty to the emplacement age interpreted from zircon. Six other monazite analyses yield an age of 1,027±9 Ma and one grain is 953±30 Ma, both within uncertainty of the zircon overgrowth ages of 1,019±15 and 979±11 Ma, respectively. Thus, these data appear to indicate that a significant episode of post-tectonic igneous activity occurred at about 1,060 Ma, following a period of deformation that must have begun after 1,080 Ma (Tollo and others, in press a). This magmatism was, in turn, followed by one or two periods of regional thermal disturbance.

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Proceed east on U.S. 211.
91.5   6.1 Turn left onto U.S. Bus. 211 and proceed north toward Washington, Va.
92.7   7.3 Continue straight on Main Street at intersection with Va. 622.
      Va. 622 branches left from Va. 628; continue straight on Va. 628.
95.2   9.8 Stop 11.
      Turn right onto Horseshoe Hollow Lane.
      Follow Trip Leaders
      PRIVATE PROPERTY

Stop 11. Foliated pyroxene granite (Yfpg).

Foliated pyroxene granite (Yfpg) defines a circular pluton that appears to cut Flint Hill Gneiss in the southeastern part of the Chester Gap 7.5-minute quadrangle (fig. 3). The northwest border of the body is truncated by the Stanley fault, which juxtaposes Yfpg charnockite against greenstone of the Catoctin Formation. This meta-igneous intrusive body is part of a subset of charnockitic rocks characterized by high to moderately high silica contents, but its emplacement age is different from any other charnockites presently dated within the field trip area. The ages of silica-rich charnockitic rocks already dated and field characteristics of a recently discovered high-silica charnockite in the Big Meadows 7.5-minute quadrangle suggest that silica-rich charnockitic magmas were emplaced during each of the three main episodes of magmatic activity in the northern Blue Ridge.

The Yfpg pluton is composed of light-gray, medium- to coarse-grained, inequigranular, strongly foliated monzogranite (farsundite) composed of 30 to 40 percent microcline, 30 to 40 percent plagioclase, and 15 to 20 percent quartz with 5 to 7 percent orthopyroxene, 0 to 2 percent amphibole, 0 to 5 percent biotite, and rare garnet. Accessory minerals include apatite, zircon, and ilmenite. Foliation is defined by discontinuous, centimeter-scale domains composed of ferromagnesian minerals. Dikes composed of coarse-grained, nonfoliated alkali feldspar+blue quartz, that range up to 1 m (3.3 ft) in width, and coarse-grained, nonfoliated garnet-bearing leucogranite, that range up to 1.5 m (4.9 ft) in width, locally intrude monzogranite and are typically oriented parallel to foliation. The strong foliation defined by discontinuous clusters of minerals sets this lithologic unit apart from all other charnockites in the area. Overall grain size is greatly reduced, and mineralogic segregation becomes less pronounced where the pluton is affected by deformation along the Stanley fault.

Rocks analyzed from the Yfpg pluton exhibit a range in SiO2 content of 67 to 72 weight percent and are weakly peraluminous to borderline metaluminous (fig. 5D). The composition of a sample collected from this outcrop for U-Pb geochronologic analysis is included in table 4 (sample SNP-02-177). The Yfpg rocks are geochemically similar to other, older high-silica charnockites, including the garnetiferous gneiss (Yg) and amphibole-bearing charnockite and farsundite (Ycf) (fig. 5). Trace-element data indicate that, like these rocks, Yfpg was probably derived from sources of mixed composition.

Zircons analyzed from a sample collected from this outcrop are isotopically complex and provide considerable insight into the regional geologic history. Analyses of zircon cores provide a weighted average age of 1,178±14 Ma, which we interpret as indicative of inheritance. Analyses of zoned mantles indicate a likely crystallization age of 1,115±13 Ma, which is interpreted as the time of pluton emplacement. Analyses of rim overgrowths indicate ages of 1,050±13 Ma and ~1,010 Ma. The 1,115-Ma emplacement age indicates that the Yfpg pluton intruded nearly contemporaneously with the 1,111-Ma Marshall Metagranite, a lithologically complex noncharnockitic granitoid that occurs in the Blue Ridge approximately 32 km (20 mi) to the northeast, and which was dated by Aleinikoff and others (2000) using ID-TIMS techniques. The local significance of this magmatic episode is not yet understood; however, McLelland and others (1996) suggested that magmatism of similar age in the Adirondacks resulted from far-field effects of activity associated with development of the Midcontinent Rift. The 1,178-Ma inherited age of the Yfpg pluton corresponds closely to the age of megacrystic leucocratic granite gneiss (Ylg) that occurs in the southern part of the field trip area and suggests that crust produced during the earlier magmatic episode was partly recycled during subsequent magma genesis. Finally, the overgrowth ages of 1,050 and 1,010 Ma are each considered to preserve evidence of thermal disturbance. The former age corresponds to the timing of the final magmatic episode in the Blue Ridge, during which rocks such as the Old Rag magmatic suite (Ygr) and low-silica charnockite (Yfqj) were emplaced, and the 1,010-Ma date falls within the range of a possible heating event for which evidence is preserved as overgrowths on zircons from other basement rocks in the Blue Ridge (table 3) (Tollo and others, in press a).

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Proceed north on Va. 628.
96.6   1.4 Turn left to continue on Va. 628 at intersection with Va. 606.
97.4   2.2 Turn right onto Va. 628 at (poorly marked) four-way intersection.
99.3   4.1 Turn left onto Va. 630, which becomes unpaved.
101.2   6.0 Stop 12.
      Outcrop on left side of Va. 630 before bridge.

Stop 12. Flint Hill Gneiss (Yfh).

The Flint Hill Gneiss constitutes a distinct lithologic unit that underlies a large, elongate area of the Blue Ridge basement core east of the Sperryville high-strain zone in northern Virginia (Virginia Division of Mineral Resources, 1993). The rock is characterized by a strongly developed gneissic fabric and abundance of typically blue quartz. The unit has not been dated by modern isotopic techniques, but the strongly deformed fabric suggests that it is likely part of the oldest group of basement rocks in the region.

The Yfh lithologic unit includes dark-gray, medium- to coarse-grained, inequigranular, strongly foliated syeno- to monzogranitic gneiss composed of 35 percent microcline, 20 percent plagioclase, and 20 percent quartz with 2 percent biotite and 5 percent chlorite. Accessory and secondary minerals include apatite, leucoxene, zircon, and ilmenite. Gneissic layering is defined by alternating quartz+feldspar-rich and biotite+chlorite-rich domains that are typically 1 to 10 cm (0.4–4 in) in thickness. Foliation is defined by planar alignment of biotite and is parallel to gneissic layering. Granular quartz ranges from dark gray to blue; locally abundant quartz veins are typically blue. The characteristic compositional banding in the gneiss is commonly contorted and kinked (Clarke, 1984), preserving visible evidence of the high degree of deformation that has affected this unit. Veins composed primarily of blue quartz and dikes composed of fine- to medium-grained leucocratic granite are common within the Flint Hill Gneiss. Clarke (1984) reported that the widespread blue quartz veins are generally conformable to gneissic layering, whereas dikes typically crosscut banding.

The Flint Hill Gneiss is characterized by SiO2 contents that are generally in the range of 69 to 72 weight percent (table 4; fig. 5). Limited sampling of Flint Hill rocks suggests that a lower silica variant with SiO2 contents about 64 weight percent is present in nearby quadrangles. The rocks are generally mildly peraluminous, consistent with the ubiquitous presence of biotite. Normative compositions and trace-element characteristics and occurrence of this lithologic unit within a terrane that is otherwise dominated by rocks of intrusive magmatic origin suggest that the Flint Hill Gneiss is meta-igneous in origin. Results from previous geochronologic investigations involving U-Pb isotopic analysis of zircons from the Flint Hill Gneiss are considered unreliable due to the isotopic complexities that have been documented in zircons from other Grenvillian rocks in the area (Aleinikoff and others, 2000; Tollo and others, in press a).

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Turn around and proceed east on Va. 630.
103.1   1.9 Turn left to continue on Va. 630.
103.9   2.7 Turn right onto U.S. 522 and proceed south through the town of Flint Hill.
108.7   7.5 Turn right onto U.S. 522/U.S. 211 and proceed west.
116.7   15.5 Turn left onto U.S. 522/Va. 231 in Sperryville and proceed south.
      Cross bridge over the Thornton River.
116.8   15.6 Turn left to remain on U.S. 522 and proceed southeast.
117.4   16.2 Turn right onto Va. 231 and proceed south.
126.6   25.4 Old Rag Mountain is visible to the west (right). The mountain, formerly
      called "Old Raggedy," is characterized by steep rocky slopes and is a
      popular local recreational destination. The mountain is underlain by
      typically coarse-grained, locally garnet-bearing leucogranite of the Old
      Rag magmatic suite, defining a fault-bounded pluton.
132.5   31.3 Turn right onto Va. 670 and proceed west. View toward the west (straight
      ahead) of Double Top Mountain, which is underlain primarily by
      amphibole-bearing low-silica charnockite (Yfqj).
133.3   32.1 Stop 13. Follow Trip Leaders. PRIVATE PROPERTY

Stop 13. Biotite granitoid gneiss (Ybg) containing xenoliths of medium-grained foliated leucocratic granitoid (Ylg).

Medium- to coarse-grained biotite granitoid gneiss and layered granitoid gneiss, mapped together within a single lithologic unit (Ybg), underlie a large area east of the Quaker Run fault zone in the northern part of the Madison 7.5-minute quadrangle (fig. 3). This mineralogically distinctive unit, and other similarly biotite-rich rocks mapped east of the Rockfish Valley fault zone in areas of the Blue Ridge province located to the south (Hughes and others, in press), contrast sharply with the more typical granites, leucogranites, and charnockitic rocks that underlie much of the area. Pegmatites associated with the Old Rag Granite pluton appear to cut biotite granitoid gneiss along the southeastern margin of the body southeast of Old Rag Mountain (Hackley, 1999), indicating that the peraluminous leucogranitoids of the Old Rag magmatic suite are likely younger than the Ybg rocks. Dikes of probable Ybg affinity intrude leucogranite gneiss (Ylg) at Stop 15 and elsewhere in the Madison area, indicating that Ybg is younger than the Ylg lithologic complex.

The Ybg unit includes gray to grayish-black, medium- to coarse-grained, massive to foliated, biotite granitoid containing a dominant mineral assemblage of alkali feldspar+plagioclase+quartz+biotite. The unit also contains medium- to coarse-grained, layered granitic gneiss with 1- to 3-cm (0.4–1.2-in)-thick felsic domains composed of plagioclase+alkali feldspar+quartz separated by weakly developed layers of biotite+epidote+quartz (Bailey and others, 2003). Biotite typically constitutes 15 to 25 percent of the rock. This outcrop exposes protomylonitic biotite granitoid gneiss containing porphyroclasts of white feldspar and elongate quartz lenses. Abundant biotite+epidote bands define a foliation that strikes 020° and dips steeply to the east. The mineralogic assemblage defining this foliation is indicative of formation at greenschist-facies conditions and suggests that the fabric was developed during Paleozoic orogenesis.

Geochemical data indicate that the Ybg lithologic unit is characterized by a bimodal range in silica contents, with compositions corresponding to SiO2 values of 61 to 63 and 67 to 68 weight percent (fig. 5). The rocks are metaluminous to mildly peraluminous (fig. 5D), and display trace-element characteristics such as high concentrations of high-field-strength elements and modestly high Ga/Al ratios that suggest affinity to A-type granites (figs. 6, 7). These compositional characteristics and the overall tholeiitic and subalkaline nature of the rocks suggest that Ybg rocks were likely igneous in origin.

Xenoliths of foliated medium- to coarse-grained leucocratic granitoid, interpreted to be derived from the Ylg lithologic unit, occur within the foliated biotite granitoid gneiss at this locality (fig. 11). Foliation defined by alignment of feldspar megacrysts and discontinuous compositional banding involving recrystallized quartz within the xenoliths is oriented at an angle to foliation in the surrounding biotite granitoid gneiss. This relation, and the likely higher-temperature ductile origin of foliation within the xenoliths, suggests that the fabric in the leucogranite formed during Grenvillian orogenesis. This field relation is consistent with the presence of dikes of Ybg affinity that cut Ylg rocks elsewhere in the vicinity, and thus provides a maximum age constraint of 1,183±11 Ma (age of emplacement for part of the Ylg lithologic complex) for emplacement of protolith magmas for the Ybg rocks.

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Continue west on Va. 670 through Criglersville.
134.1   0.8 Outcrops within and adjacent to the riverbed on the left expose contact
      relations between the Ylg and Ybg lithologic units (Bailey and others, 2003).
134.3   1.0 Turn left onto Va. 649 (Double Top Road.)
136.8   3.5 Becomes dirt road.
137.3   4.0 Stop 14. Pull over to the right at hairpin turn.

Stop 14. Low-silica charnockite (Yfqj) and mylonite in the Quaker Run high-strain zone.

Amphibole-bearing low-silica charnockite (Yfqj) ranging from monzogranite (farsundite) to quartz monzodiorite (quartz jotunite) in normative composition (fig. 4) is widespread throughout the field trip area, constituting a large pluton located east of the crest of the Blue Ridge. Similar rocks also occur as dikes and small plutons in areas located to the north. The characteristic nonfoliated to weakly foliated fabric of this rock unit indicates that it is part of the youngest group of Grenvillian plutonic rocks in this part of the Blue Ridge.

The outcrop at the sharp bend in the road is an outstanding example of a fresh, massive, coarse-grained charnockite. This lithologic unit is composed of dark-gray to dark-gray-green, medium- to very coarse-grained, equigranular to inequigranular, massive to weakly foliated monzogranite, granodiorite, and quartz monzodiorite (farsundite, opdalite, and quartz jotunite) composed of 9 to 30 percent alkali feldspar microperthite (chiefly microcline), 30 to 49 percent plagioclase, and 14 to 26 percent quartz with 10 to 17 percent orthopyroxene, 0 to 7 percent amphibole, and rare clinopyroxene. Accessory minerals include apatite, ilmenite, magnetite, zircon, epidote, and actinolite. Rare, typically weakly developed foliation is defined locally by planar alignment of ferromagnesian minerals. Lenticular, magnetite-rich enclaves, which range in length to 0.5 m (20 in), occur locally and are aligned parallel to foliation. Subhedral to euhedral, monocrystalline alkali feldspar megacrysts range up to 10 cm (4 in) in length and are best observed on weathered surfaces. The rock typically weathers to form a distinct orange rind and similarly colored, clay-rich soils that are useful in mapping. This outcrop is located within the Quaker Run high-strain zone (fig. 3) (Bailey and others, 2003), as indicated by fabric observed in other exposures along the road where undeformed charnockite grades into mylonite with multigranular feldspar porphyroclasts and elongate quartz ribbons. The occurrence of undeformed rock within highly strained rocks illustrates the heterogeneous nature of strain within the Quaker Run high-strain zone. The northwestern boundary of the high-strain zone is clearly defined by massive charnockite; however, the southeastern boundary is more difficult to define due to the abundance of phyllosilicates and the foliated nature of the biotite granitoid gneiss (Ybg).

The compositionally diverse Yfqj lithologic unit is characterized by relatively low silica contents (51 to 65 weight percent SiO2) that distinguish the unit from most of the other, typically high-silica Grenvillian rocks in the area (fig. 5). The rocks are generally metaluminous and exhibit subalkaline tholeiitic characteristics (fig. 5D–F). The Yfqj rocks are also distinguished compositionally by relatively high Ga/Al ratios and concentrations of high-field-strength elements, including Y, Nb, and Zr (figs. 6, 7). These features, especially the high concentrations of both Zr and Nb, distinguish this unit from nearly all other rocks studied in the area, and suggest geochemical affinity to A-type granitoids (Eby, 1990; Whalen and others, 1987) and within-plate granitoids (Pearce and others, 1984). These features suggest that Yfqj magmas were derived from dominantly crustal sources (Landenberger and Collins, 1996), in contrast to the typically mixed heritage that characterizes most other units in the region (Tollo and others, in press a).

Yfqj charnockite is locally cut by dikes of coarse-grained leucogranite that are likely related to the 1,060±5-Ma Old Rag Granite (Tollo and others, in press a,b). The characteristic unfoliated to weakly foliated fabric of both units indicates that the rocks were emplaced after the major episode of deformation that affected the 1,078±9-Ma leucogranite gneiss (Ylgg) unit. Analysis of a sample of the Yfqj pluton collected from the Fletcher 7.5-minute quadrangle indicates that the euhedral to subhedral prismatic zircons contain (1) cores distinguished by oscillatory zoning that likely reflects compositional variation resulting from igneous crystallization and (2) unzoned rims that typically contain higher U concentrations than the cores (Tollo and others, in press a). Tollo and others (in press a) reported results from 18 analyses of cores that yielded a weighted average of the 207Pb/206Pb ages of 1,050±8 Ma, which they interpreted as the time of igneous crystallization of the charnockite. Five analyses of the overgrowths indicated a subsequent episode of thermal flux related to metamorphism at 1,018±14 Ma. The 1,060±5-Ma emplacement age of the Old Rag Granite overlaps the emplacement age of the charnockite and, considering the crosscutting field relations documented in the area, suggests that the magmatic protoliths of these rocks intruded over possibly extended, penecontemporaneous intervals.

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Turn around and proceed downhill along Va. 649.
140.1   2.8 Turn right onto Va. 670 and proceed southeast.
141.4   4.1 Outcrops in river are biotite granitoid gneiss (Ybg).
142.0   4.7 Turn right onto Va. 231 and proceed southeast.
142.5   5.2 Intersection with Va. 609. Bridge across the Robinson River.
142.7   5.4 Outcrop to the south exposes the eastern contact of the Mechum River
      Formation and Ylg lithologic unit.
143.0   5.7 Turn right onto Va. 651 (Aylor Road).
      Outcrops in the field to the left expose biotite granitoid gneiss (Ybg)
      near the intrusive contact with alkali feldspar syenite of the
      Neoproterozoic Robertson River batholith (Bailey and others, 2003).
145.9   8.6 Bear right to remain on Va. 651.
146.5   9.2 Stop 15.
      Follow Trip Leaders
      PRIVATE PROPERTY

Stop 15. Ylg basement complex exposed in debris-flow scar.

The dominant rock types in this exposure constitute a composite felsic pluton composed of multiple lithologies including (from oldest to youngest): (1) foliated, coarsegrained to megacrystic leucocratic granite (Ylg1), (2) medium-grained, equigranular, weakly to moderately foliated leucogranite (Ylg2), and (3) coarse-grained to very coarsegrained, inequigranular, nonfoliated to weakly foliated leucogranite pegmatite (Ylg3). Because of similarities in mineral assemblage and geochemical composition, these distinctive rocks are collectively mapped as the Ylg lithologic unit in the southern part of the field trip area (Bailey and others, 2003; Tollo and others, in press b).

Although dominantly leucogranitoids, constituent lithologic units of the Ylg plutonic complex exhibit minor differences in mineral assemblage. The strongly foliated, megacrystic leucocratic granite is composed of alkali feldspar mesoperthite (both microcline and orthoclase), plagioclase, quartz, and intergrown biotite+chlorite, whereas the medium-grained, equigranular, weakly to moderately foliated leucogranite is composed of alkali feldspar (chiefly microcline), plagioclase, and quartz with minor secondary biotite; and the nonfoliated to weakly foliated leucogranite pegmatite is composed of alkali feldspar (microcline), minor plagioclase, and quartz. These mineralogic differences, especially the progressive decrease in biotite content, reflect a weak trend toward more evolved chemical compositions.

This large exposure illustrates crosscutting relations among several Blue Ridge basement units. Field relations indicate that the foliated coarse-grained to megacrystic leucocratic granite is the oldest rock at this outcrop. Dikes exposed in some of the steep ledges indicate that the megacrystic granite was intruded by medium-grained leucogranite that was subsequently folded with axial planes that are nearly parallel to foliation in the coarsegrained megacrystic granitoid (fig. 12). Very coarse-grained leucogranite pegmatite occurs as locally boudinaged dikes cutting both the coarse-grained leucocratic granitoid (Ylg1) and medium-grained equigranular leucogranite (Ylg2). Local development of leucogranitic pegmatite within medium-grained leucogranite (fig. 12) further suggests that the pegmatite was derived through differentiation of medium-grained leucogranite magma. A 30- to 50- cm (12–20-in)-wide dike of fine- to medium-grained biotite granodiorite intrudes all of the leucocratic granitoid units in ledges located near the base of the exposure. This dike, which contains a mineral assemblage that is similar to biotite granitoid gneiss (Ybg), clearly postdates the deformation recorded in the leucocratic rocks, and suggests that Ybg is younger than Ylg, a relation that is consistent with field relations observed at Stop 14.

At the base of the outcrop a ~15-cm (6-in)-thick dike of coarse-grained pegmatite cuts medium-grained leucogranite and is deformed into a series of tight folds with rounded hinges (fig. 13). The medium-grained leucogranite displays weak foliation that strikes ~070°, dips steeply to the northwest, and is axial planar to the folds in the leucogranite dike. Line-length restoration of the dike indicates ~70 percent shortening in a north-north-west–south-southeast direction, but the fabric in the enclosing leucogranite is barely visible. We interpret the medium-grained leucogranite to have statically recrystallized at high temperatures after deformation, and thus it does not faithfully record the total strain history. The high-temperature deformation recorded in the pegmatite is interpreted to be Mesoproterozoic in age and is commonly overprinted by a foliation defined by aligned micas. Crosscutting relations observed throughout the exposure are interpreted to indicate multiple episodes of magmatic injection occurring throughout an extended interval of time.

Basement units are cut by a northeast-striking, ~5-m (16-ft)-wide dike of porphyritic hornblende metagabbro. The metagabbro is composed of hornblende, extensively altered plagioclase, epidote, and chlorite. The dike is part of a suite of mafic to ultramafic igneous rocks that intrude Blue Ridge basement and Neoproterozoic metasedimentary units and which are interpreted to be associated with an early, pre-Catoctin pulse of Neoproterozoic rifting (Bailey and others, 2003).

A set of discrete high-strain zones cuts all basement units in this exposure. These zones range from a few centimeters in thickness to millimeter-scale. Where discernible, mineral elongation lineations plunge obliquely downdip. The apparent offset, as illustrated on the subhorizontal outcrop surface, is dextral. However, there clearly was out-of-section movement as well. The mineralogy and microstructures in the high-strain zones are consistent with formation at greenschist-facies conditions and are interpreted to result from Paleozoic contraction.

U-Pb isotopic analysis of zircons from foliated megacrystic granitoid collected from this exposure indicate igneous crystallization at 1,183±11 Ma, followed by metamorphic recrystallization at 1,043±16 Ma and ~1,110 Ma (table 3, sample SNP-02-189). These data indicate that Ylg is currently the oldest lithologic unit recognized in the area. Chemical analyses of samples collected from both the medium-grained and megacrystic leucocratic granitoids indicate that the rocks are characterized by SiO2 contents of 69 to 77 weight percent, with most compositions in the range of 74 to 76 weight percent (table 4, fig. 5). Such chemically evolved compositions are similar to those of the much younger, ~1,060-Ma leucocratic granitoids of the Old Rag magmatic suite (Ygr) and garnetiferous alkali feldspar granite (Ygg), and thus suggest that production of leucogranite magmas was cyclic. Compositions of Ylg samples differ from the younger leucocratic rocks in having lower concentrations of Y+Nb (fig. 6), a characteristic that suggests derivation from different sources.

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Turn around and return east along Va. 651.
147.0   0.5 Turn right onto Va. 652 (Gaar Mountain Road).
149.3   2.8 Turn right onto Va. 656 (Ruth Road).
149.7   3.2 First intersection (Ruth Road and Courtney Hollow Lane).
150.0   3.5 Stop 16.
      Follow Trip Leaders
      PRIVATE PROPERTY

Stop 16. Nonfoliated coarse-grained alkali feldspar granite (Yaf).

Nonfoliated, coarse-grained alkali feldspar granite (Yaf) defines two small (<1 km2; 0.4 mi2) plutons located within foliated biotite granitoid gneiss (Ybg) east of the Mechum River Formation in the southern part of the Madison 7.5-minute quadrangle (fig. 3). This lithologic unit includes gray-white, coarse-grained, equigranular to porphyritic alkali feldspar granite composed of 1- to 5-cm (0.5–2.0-in) white alkali feldspar+blue-gray quartz with minor plagioclase, biotite, titanite, and Fe-Ti oxide minerals (Bailey and others, 2003).

Anastomosing high-strain zones (up to 30 cm (12 in) thick) composed of well-foliated, mica-rich mylonite and ultramylonite locally cut the coarse-grained leucogranite. Detailed kinematic, fabric, and chemical analysis of the largest high-strain zone suggests bulk isochemical (isovolumetric?) behavior, modest flattening strains, and a general shear deformation with monoclinic symmetry (Bailey and others, in press).

The nonfoliated fabric and abundant alkali feldspar+quartz mineral assemblage of the Yaf unit indicates that these bodies are likely correlative with the group of postorogenic leucocratic granitoids documented to the north that includes the Old Rag magmatic suite (Ygr) and garnetiferous syenogranite (Ygg), which have emplacement ages of 1,060±5 Ma and 1,064±7 Ma, respectively (table 3). Consistent with this correlation, alkali feldspar granite dikes of Yaf affinity that range in width from 0.1 to 3 m (0.3–10 ft) cut both the leucocratic granitoid complex (Ylg) and the biotite granitoid gneiss (Ybg) (Bailey and others, 2003).

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Turn around and proceed east on Va. 656.
      Continue straight at intersection with Va. 652.
154.0   4.0 Turn right onto Main Street.
154.1   4.1 Intersection with Va. 634 (Washington Street); continue straight.
155.1   5.1 Merge onto U.S. 29 (Seminole Trail) and proceed south.
157.0   7.0 Turn right onto Va. 230 (Wolftown-Hood Road) and proceed west.
160.6   10.6 Turn right onto Va. 622 (Graves Mill Road).
162.1   12.1 Turn left onto Va. 665 (Garth Run Road).
162.9   12.9 Turn right to remain on Va. 665.
      Kirtley Mountain, visible to the west, was the site of numerous debris
      flows in the 1995 storm that affected this area. The large exposures of
      bedrock in the denuded channels visible from the highway are composed
      dominantly of amphibole-bearing low-silica charnockite (Yfqj) (Tollo and
      others, in press b).
164.3   14.3 Stop 17.
      Pull over south of the bridge over Garth Run.
      Proceed to exposures in stream valley.

Stop 17. Mylonitic rocks of the Garth Run high-strain zone.

The Garth Run high-strain zone is well exposed in a 200-m (650-ft)-long outcrop scoured out along the channel of Garth Run during the June 1995 storm. This exposure has proven to be seminal to our understanding of ductile deformation in the Blue Ridge. The high-strain zone strikes north-northwest to north-northeast and joins the 1- to 2-km (0.6–1.2-mi)-wide Quaker Run high-strain zone to the north and tips out to the south (Berquist and Bailey, 2000). Although the high-strain-zone boundary is not exposed, outcrop data indicate the zone is approximately 125 m (410 ft) thick). The zone is bounded to the west by medium- to coarse-grained, equigranular to porphyritic charnockite (Yfqj), and to the east by rocks of the leucogranite complex (Ylg). Medium-grained layered granitic gneiss is the dominant rock type east of the Garth Run high-strain zone. The gneiss is intruded by a series of fine- to coarse-grained leucogranite dikes ranging from 0.3 to 5 m (1–16 ft) in thickness.

Rocks exposed in the Garth Run high-strain zone are dominantly porphyroclast-bearing protomylonites and mylonites. Finely-layered mylonitic leucogneiss, leucogranite, and well-foliated fine-grained metabasalt occur as tabular to lenticular bodies that are 0.2 to 2 m (0.6–6.5 ft) thick throughout the high-strain zone. These tabular bodies are both subparallel and slightly discordant to the foliation. Foliation, defined by mica-rich surfaces, elongate quartz grains, and fractured feldspars, strikes 345° to 010° and dips 30 to 50° to the east. A mineral elongation lineation plunges downdip or obliquely to the east-northeast; however, mineral elongation lineations are not present everywhere.

Sheath folds occur in the finely-layered mylonitic leucogneiss and sheath axes plunge moderately to the east, parallel to the mineral elongation lineation. Coarse-grained leucogranitic bodies display pinch-and-swell structures, and are commonly isolated as lozenge-shaped boudins surrounded by porphyroclastic mylonites. Leucogranitic boudins record elongation both parallel and normal to the east-northeast-plunging elongation lineation, indicating bulk extension in both the Y and X directions. Leucogranites have a weak foliation and are cut by numerous transgranular fractures. Slightly discordant, tabular, boudinaged leucogranitic dikes are locally folded. At a few locations, the mylonitic foliation is kinked into narrow bands that are 2 to 5 cm (0.8–2.0 in) wide.

Asymmetric structures such as sigma and delta porphyroclasts, shear bands, and asymmetric boudins are common in the Garth Run high-strain zone. Asymmetric structures, both parallel (reverse sense of shear) and perpendicular (sinistral sense of shear) to the mineral elongation lineation, are consistent with triclinic deformation symmetry. Sectional strains estimated from discrete quartz lenses and ribbons range from 3:1 to 23:1. Three-dimensional strains record apparent flattening. The kinematic vorticity number was estimated using the porphyroclast hyperbolic distribution method of Simpson and De Paor (1993) on ultramylonite thin sections and joint faces with well-exposed porphyroclasts. Wn- values range from 0.6 to 0.4, indicating general shear deformation. Shear strain is conservatively estimated at 4 and integrates to a total displacement of ~500 m (1,640 ft) across the Garth Run high-strain zone. Discrete brittle faults with displacements of 0.2 to 2 m (8–79 in) cut the mylonitic fabrics.

Folded leucogranite boudins may offer a clue about the progressive deformation history of the Garth Run high-strain zone. These structures formed when competent leucogranite dikes were first elongated and then shortened. During progressive steady-state deformation, material rotated from the field of shortening into the field of extension, a progression that did not fold boudins. Folded boudins are generally interpreted to develop by polyphase deformation, such that material elongated during the first deformation is shortened by a second deformation having a different orientation. However, a change in the incremental vorticity will cause some material that was originally deformed in the field of extension tomove into the field of shortening. There are no crosscutting ductile fabrics in the Garth Run high-strain zone. Bailey and others (in press) proposed a model in which the dominant mechanism of deformation changed from simple shear to pure shear over time.

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Turn around and proceed south on Va. 665.
165.7   1.4 Turn left to remain on Va. 665.
166.4   2.1 Turn left onto Va. 622 (Graves Mill Road) and proceed north.
167.4   3.1 View of debris-slide scars to the east on German Mountain
170.2   5.9 Graves Mill.
      Continue straight onto Va. 615 (Bluff Mountain Road).
171.4   7.1 Stops 18 and 19.
      Pull into parking area on right side of Va. 615.

Stop 18. Kinsey Run, west branch: low-silica amphibole-bearing charnockite (Yfqj) and associated pegmatite; mylonite, and protomylonite.

Stops 18 and 19 are located about 2 km (1.2 mi) uphill from the road and can be reached by hiking along a logging trail to the upper reaches of Kinsey Run. The stops include the denuded channels caused by mass-wasting processes resulting from the intense rainfall that affected the area in June 1995. The stops are located on two branches of Kinsey Run that were significantly widened by the debris flows. Stop 19 can be reached from the logging trail by hiking eastward from the west branch over the drainage divide to the east branch.

Both stops are located within medium- to very coarse-grained, equigranular to inequigranular, massive to weakly foliated, amphibole-bearing low-silica charnockite (Yfqj) that defines a large pluton in the southern part of the field trip area (fig. 3). The generally nonfoliated fabric of the charnockite indicates postorogenic emplacement and is consistent with the 1,050±8-Ma age determined by U-Pb isotopic analysis (Tollo and others, in press a; table 3). Microstructural characteristics also indicate emplacement within a post-deformation environment. Quartz grains (2–7 mm), which compose 15 to 25 modal percent of the rock, are bleb-like and display no grain-shape preferred orientation, attesting to the relatively undeformed nature of the charnockite. Orthopyroxene exhibits partial alteration to a mixture of fine-grained uralitic amphibole+secondary biotite that is localized along grain boundaries and most likely indicative of Paleozoic retrograde metamorphism.

The Yfqj charnockite in the main area of exposure contains several decimeter- to meter-size pods of coarse-grained amphibole+magnetite-bearing pegmatite that generally grades into the surrounding medium- to coarse-grained charnockite. The mineral assemblage within these pegmatite pods is similar to the surrounding charnockite. Moreover, amphiboles in these pegmatites are characterized by edenitic (nomenclature after Leake and others, 1997) compositions that are similar to amphiboles within the surrounding charnockite, indicating that the pegmatites are most likely derived from the charnockitic magmas through local fluid saturation and differentiation.

Three subvertical mafic dikes, striking ~350°, are exposed intruding the charnockite (fig. 14). Contacts are well exposed and display both chilled margins and fragments of country rocks that indicate emplacement under brittle conditions. The dikes are composed of clinopyroxene+plagioclase±pigeonite, with minor amounts of magnetite, chlorite, actinolite, biotite, and quartz. The presence of pigeonite is especially characteristic of dikes of Mesozoic age (Wilson and Tollo, 2001), and is consistent with the whole-rock chemical compositions that indicate high SiO2 and relatively low TiO2 contents (table 5, samples SNP-99-7, -8, and -9) compared to metabasalt of the Neoproterozoic Catoctin Formation. These dikes are part of a widespread suite of Jurassic (~200 Ma) igneous bodies that intruded Blue Ridge, Piedmont, and Valley and Ridge country rocks during early Mesozoic rifting. In contrast, an east-west-striking greenstone dike that occurs in outcrops located below the main bedrock exposure contains highly altered orthopyroxene phenocrysts and abundant retrograde chlorite+epidote+actinolite. The low SiO2 and high TiO2 content of this dike (sample SNP-99-6 in table 5) is similar to the composition of Catoctin greenstones and suggests a Neoproterozoic age.

An approximately 30-m (98-ft)-wide high-strain zone is exposed in the channel above the diabase dikes. This zone strikes ~080° and dips steeply to the northwest (fig. 15). The zone is bounded by charnockite that passes into protomylonite and mylonite within the high-strain zone. Two deformed metabasalt dikes occur within the zone and strike subparallel to the zone boundary (fig. 15). Foliation internal to the high-strain zone dips steeply to both the northwest and southeast, and a downdip mineral elongation lineation occurs at a few locations. Asymmetric kinematic indicators generally display a reverse sense of shear on foliation normal and lineation parallel sections. Protomylonite and mylonite developed within the charnockite are composed of 30 to 50 percent quartz, 10 to 25 percent fine-grained muscovite, 10 to 20 percent alkali feldspar, 10 to 15 percent epidote, and 10 percent biotite. Feldspars are extensively fractured with quartz-filled cracks and muscovite mantles along grain boundaries. Quartz grains form monocrystalline lenses and ribbons with a moderate to strong crystallographic preferred orientation. The general lack of recrystallized quartz and the abundance of brittle microstructures in feldspar are consistent with greenschist-facies deformation conditions (T ~300–400°C). Undeformed charnockite, protomylonite, and mylonite are composed of approximately 61 percent SiO2 and have nearly identical concentrations of other major elements. Although significant mineralogical changes occurred between the undeformed charnockite and mylonitic rocks, volume loss appears to be minimal (Bailey and others, in press).

Strain was estimated from monocrystalline quartz lenses and ribbons using standard Rf/f techniques: 16 to 40 grains were measured per sample, and strain in XZ sections averaged 3.8 (3.6–4.0, n=2) for the protomylonites and 5.8 (5.5–6.2, n=3) for the mylonites. Three-dimensional strains for all samples plot in the field of apparent flattening on a Flinn diagram, and mylonitic samples consistently have lower K-values than protomylonites (K=0.6 versus 0.8). In the YZ section (which approximates the outcrop surface), both metabasalt and epidotized leucocharnockite dikes exhibit pinch-and-swell structure or form boudins, consistent with true flattening strains and elongation parallel to Y. The kinematic vorticity number was estimated using the Rs/Q method. In both the protomylonite and mylonite, Wm=0.65. Integrating shear strains across the Kinsey Run high-strain zone yields a total displacement of 60±10 m (197±33 ft). These results indicate that the Kinsey Run high-strain zone experienced a weak triclinic flow characterized by flattening strains that developed under general shear conditions (Bailey and others, in press).

Stop 19. Kinsey Run, east branch: Quaker Run high-strain zone.

This debris-flow scar, which exposes the base of the Quaker Run high-strain zone, is one of the largest created by the rainstorm in June 1995 (Morgan and others, 1999). Medium- to coarse-grained, massive to weakly foliated, low-silica charnockite (Yfqj) passes uphill into protomylonite and mylonite. The contact between undeformed charnockite and mylonitic rocks strikes to the northeast and dips moderately to the southeast. The angle between the foliation and high-strain-zone boundary is <10°. Mylonitic rocks are characterized by a southeast-plunging, downdip elongation lineation. Kinematic indicators, especially visible at this outcrop because of the vertical exposure, are plentiful and record top-to-the-northwest sense of shear. There are, however, numerous back-rotated porphyroclasts (mostly feldspar megacrysts), suggesting that this zone experienced general rather than simple shear. Leucopegmatite dikes are commonly oriented subparallel to foliation and exhibit pinch-and-swell structures along their margins. These competent dikes are cut by fibrous quartz-filled extension fractures. A 2- to 4-m (6.5–13.1-ft)-wide zone of fine-grained chlorite-bearing mylonite may be derived from a Neoproterozoic metabasalt dike.

The Quaker Run high-strain zone was defined by Berquist and Bailey (2000) for exposures a few kilometers to the north. At this latitude, the Quaker Run high-strain zone is the thickest (0.5–1.5 km; 0.3–0.9 mi) Paleozoic mylonite zone in the north-central Blue Ridge and can be traced for over 30 km (19 mi) parallel to the regional trend. Strain throughout the zone is very heterogeneous. On the basis of estimates of shear strain using values determined from other Blue Ridge mylonite zones, Berquist and Bailey (2000) estimated a total displacement of 1.5±0.5 km (4,900±1,650 ft) across the Quaker Run highstrain zone.

Mileage
Trip
Cumulative
  Stop-to-Stop
Cumulative
 
    0.0 Turn around and proceed east and south on Va. 615.
172.6   1.2 Intersection with Va. 622; continue south on Va. 622.
176.5   5.1 Bridge across the Rapidan River.
177.9   6.5 Turn right onto Va. 230 (Wolftown-Hood Road) and proceed west.
179.2   7.8 Bridge across the Rapidan River.
181.5   10.1 Bridge across the Conway River.
181.9   10.5 Turn right onto Va. 667 (Middle River Road) and proceed northwest.
185.2   13.8 View on the right of home built in flood plain.
185.6   14.2 Kinderhook.
      View on the right of the dredged riverbed and rocks piled on the
      embankment. Dredging was undertaken after the June 1995 flooding
      in order to deepen and widen the debris-choked channels.
187.5   16.1 Va. 667 becomes unpaved; continue north.
187.9   16.5 Large outcrops in Conway River are leucogranite gneiss (Ylgg).
188.4   17.0 Stop 20.
      Follow Trip Leaders
      PRIVATE PROPERTY

Stop 20. Leucogranite gneiss (Ylgg) xenoliths enclosed within nonfoliated low-silica charnockite (Yfqj).

Outcrops exposed at the base of the hill within the scoured channel of the Conway River provide critical field relations and age information bearing on the timing of local Grenvillian deformation. The hill located on the east side of the river is underlain by light-greenish-gray, strongly foliated leucogranite gneiss (Ylgg) that defines an isolated screen within low-silica charnockite (fig. 3) (Tollo and others, in press a,b). The coarse- to very coarse-grained, inequigranular, strongly foliated leucogranite gneiss is composed of 80 percent alkali feldspar mesoperthite (chiefly microcline), <1 percent plagioclase, and 20 percent gray quartz with <1 percent biotite. Accessory minerals include ilmenite, magnetite, zircon, epidote, and muscovite. Locally prominent gneissic layering is defined by interlayered quartz+feldspar-rich and quartz-rich domains ranging from less than 3 cm (1.2 in) to greater than 13 cm (5 in). Foliation is locally defined by planar alignment of tabular mesoperthite megacrysts. Subhedral to euhedral, monocrystalline mesoperthite megacrysts range up to 10 cm (4 in) in length. Chemical analyses of the leucogranite gneiss collected from this riverside exposure indicate very high SiO2 contents (74–75 weight percent) (table 4); however, low Y+Nb concentrations in one sample suggest that chemical alteration resulting from interaction between the xenoliths and surrounding charnockitic magma likely occurred. Although extensively altered in the outcrop, the nonfoliated charnockite contains the same mineral assemblage that is typical of the Yfqj lithologic unit. Two samplesof charnockite collected from this exposure (samples SNP-99-91 and SNP-99-92 in table 4) generally plot with other data points from the Yfqj lithologic unit but are characterized by significantly different SiO2 contents (table 4) that suggest that local chemical exchange occurred between the charnockite magma and leucogranite gneiss xenoliths.

The outcrops along the river channel display a crosscutting relation that is similar to the map pattern: xenoliths of strongly foliated leucogranite gneiss are enclosed within dark-gray-green, nonfoliated to weakly foliated, amphibole-bearing charnockite (Yfqj) that locally truncates the foliation of the leucogranite gneiss at the contacts of individual xenoliths (fig. 16). This relation is confirmed by U-Pb SHRIMP isotopic analyses of zircons. Subhedral, typically prismatic zircons in the leucogranite gneiss have broad cores charaterized by concentric, oscillatory zoning surrounded by distinct, unzoned rims. Thirteen analyses of cores yield a weighted average of the 207Pb/206Pb ages of 1,078±9 Ma, which is interpreted as the crystallization age of the igneous protolith (table 3) (Tollo and others, in press a). Analyses of a limited number of overgrowths suggest two periods of metamorphism at 1,028±10 Ma and 997±19 Ma (table 3). The occurrence of these foliated leucogranite gneiss xenoliths within the 1,050±8-Ma charnockite indicates that local deformation took place within the interval 1,078 to 1,050 Ma. The age of this Blue Ridge deformation partly overlaps the period of tectonomagmatic activity associated with Ottawan orogenesis in the Adirondacks that was constrained to the interval 1,090 to 1,035 Ma by McLelland and others (2001). As a result, ductile fabrics and gneissosity developed in the leucogranite gneiss and older units are interpreted to result from a possibly correlative period of orogenesis in the Blue Ridge.

End of Field Trip.


U.S. Department of the Interior, U.S. Geological Survey
URL: https:// pubs.usgs.gov /circ/2004/1264/html/trip2/log.html
For more information, contact Richard Tollo
Maintained by Reston Publications Service Center
Last modified: 12:24:14 Wed 23 Nov 2016
Privacy statement | General disclaimer | Accessibility