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.

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.

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 FeO_t/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.

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.

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.

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+FeO_t) (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+FeO_t) 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).

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.

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 FeO_t/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, FeO_t, and CaO contents (table 4). Compositionally, Yg differs from the Ycf unit that encloses it in having generally higher SiO₂ contents, higher FeO_t/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.

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).

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).

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 FeO_t/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.

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).

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).

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.

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).