2. Mesoproterozoic Geology of the Blue Ridge Province in North-Central Virginia: Petrologic and Structural Perspectives on Grenvillian Orogenesis and Paleozoic Tectonic Processes
1Department of Earth and Environmental Sciences, The George Washington University,
Washington, DC 20052.
2Department of Geology, College of William and Mary, Williamsburg, VA 23187.
3U.S. Geological Survey, Denver, CO 80225.
This field trip examines the geology of Grenvillian basement rocks located within the core of the Blue Ridge anticlinorium in north-central Virginia over a distance of 64 kilometers (km) (40 miles (mi)), from near Front Royal at the northern end of Shenandoah National Park southward to the vicinity of Madison. This guide presents results of detailed field mapping, structural analysis, petrologic and geochemical studies, and isotopic investigations of Mesoproterozoic rocks directed toward developing an understanding of the geologic processes involved in Grenvillian and Paleozoic orogenesis in the central Appalachians. Stops included in this field guide illustrate the lithologic and structural complexity of rocks constituting local Blue Ridge basement, and demonstrate the type of integrated, multidisciplinary studies that are necessary to decipher the protracted Mesoproterozoic through Paleozoic geologic history of the region.
The field trip traverses the crest of the Blue Ridge along Skyline Drive and the adjacent foothills located east of the mountains. This largely rural area is characterized by locally steep topography and land-use patterns that are dominated by agriculture and recreation. In late June 1995, a series of tropical storms affected parts of the central Virginia Piedmont and adjacent Appalachian Mountains. These storms produced abundant rainfall, ranging from 75 to 175 millimeters (mm) (3–6.9 inches (in)) throughout the region, which increased the moisture content of the relatively thin soils and shallow rock debris that cover the mountains (Wieczorek and others, 2000). On June 27, following this extended interval of rainfall, an exceptionally intense, localized period of precipitation, resulting from the interaction of tropical moisture and a cold front that stalled over the region, produced up to 770 mm (30.3 in) of rain in the vicinity of Graves Mills (near Stops 18 and 19) in northwestern Madison County. During this storm, more than 1,000 shallow rock, debris, and soil slides mobilized into debris flows that were concentrated in northwestern Madison County (Morgan and others, 1999). The debris flows removed large volumes of timber, soil, and rock debris, resulting in locally widened channels in which relatively unweathered bedrock commonly was exposed. Stops 18 and 19 are located within such channels, and are typical of the locally very large and unusually fresh exposures produced by the event. Materials transported by debris flows were typically deposited at constrictions in the valley pathways or on top of prehistoric fans located at the base of many of the valleys that provided passageways for the flows. Water emanating from the debris flows typically entered streams and rivers located in the valley floors bordering the mountains, greatly increasing flow volume and resulting in flooding and scoured channels. The enormous exposures at Stops 17 and 20 were enlarged and swept clean by scouring during this flooding event. Analysis of this and other events suggests that such high-magnitude, low-frequency events are a significant means of delivering coarse-grained regolith from mountainous hollows and channels to the lowland floodplains (Eaton and others, 2003). Such events may happen in the Appalachian mountains of Virginia and West Virginia with a recurrence interval of 10 to 15 years (Eaton and others, 2003).
Geology of the Blue Ridge Anticlinorium
The Blue Ridge province is one of a series of thrustbounded inliers that expose Laurentian basement within the Appalachians (Rankin and others, 1989a). The province consists of two massifs: the Shenandoah, located mostly in Virginia, and the French Broad, which extends from western North Carolina to southwestern Virginia (fig. 1). The Shenandoah massif, where the field trip area is located, constitutes part of an allochthonous, northwest-vergent, thrust-bounded sheet that, in central and northern Virginia, defines a northeast-trending anticlinorium that is overturned toward the northwest with Mesoproterozoic basement rocks constituting most of the core and a Neoproterozoic to lower Paleozoic cover sequence defining the limbs (Virginia Division of Mineral Resources, 1993) (fig. 2).
The igneous and high-grade metamorphic rocks of the basement preserve evidence of tectonic events associated with Grenvillian orogenesis at 1.2 to 1.0 Ga. These events resulted from a series of dominantly convergent tectonic events marking accretion of Laurentia and eventual assembly of the supercontinent Rodinia (Dalziel and others, 2000). Locally within the Blue Ridge massifs, as throughout much of the Grenville province of North America, Grenvillian orogenesis involved polyphase metamorphism, high-temperature deformation, and both synorogenic and postorogenic magmatism. In the Blue Ridge, these processes resulted in formation of a Mesoproterozoic terrane composed of a wide range of plutonic rocks of largely granitic composition that contain screens and inliers of preexisting country rocks.
Eastern Laurentia and the Grenvillian orogen experienced two episodes of crustal extension during the Neoproterozoic (Badger and Sinha, 1988; Aleinikoff and others, 1995). Magmatic rocks formed during the first episode include granitoids and associated volcanic deposits of the 730- to 700-Ma Robertson River batholith (Tollo and Aleinikoff, 1996) and other smaller plutons that occur throughout the Blue Ridge province of Virginia and North Carolina. Collectively, these plutons were emplaced across the region during crustal extension at 760 to 680 Ma (Fetter and Goldberg, 1995; Su and others, 1994; Bailey and Tollo, 1998; Tollo and others, 2004). This earlier episode of encratonic rifting resulted in development of local-scale rift basins in which terrestrial and marine sedimentary deposits of the Fauquier, Lynchburg, Mechum River, and Swift Run Formations were deposited (Wehr, 1988; Tollo and Hutson, 1996; Bailey and Peters, 1998). However, this episode of rifting did not lead to development of an ocean.
A second episode of Neoproterozoic extension at about 570 Ma produced extensive basaltic and relatively minor rhyolitic (only in the northern part of the anticlinorium) volcanism, ultimately resulting in creation of the pre-Atlantic Iapetus Ocean (Aleinikoff and others, 1995). The basaltic rocks produced during this latter episode constitute the Catoctin Formation, which includes a locally thick series of basaltic (now greenstone) lava flows and thin interlayered sedimentary deposits (Badger, 1989, 1999). Both the Catoctin basalts and sedimentary strata of the underlying and discontinuous Swift Run Formation were produced in a dynamic tectonic environment characterized by locally steep topography and local interaction between lava flows and stream systems, producing a series of complexly interlayered deposits that are well exposed on the western limb of the Blue Ridge anticlinorium (Simpson and Eriksson, 1989; Badger, 1999). Sedimentary deposits of the Lynchburg and Fauquier Formations, which underlie the Catoctin Formation on the east limb of the Blue Ridge anticlinorium, preserve a regional transition from braided-alluvial facies in the west to deeper-water facies in the east that is interpreted to result from a late Neoproterozoic hinge zone developed in response to extension-related crustal thinning (Wehr, 1988). The Catoctin Formation is overlain by rocks of the late Neoproterozoic to Early Cambrian Chilhowee Group and younger Paleozoic strata that were deposited on the rifted Laurentian margin during Iapetan onlap and represent local development of a tectonically passive margin (Simpson and Eriksson, 1989).
Blue Ridge basement rocks include igneous and metamorphic rocks containing orthopyroxene-bearing mineral assemblages (Rankin and others, 1989b; Bailey and others, 2003) that, in the latter, indicate that ambient metamorphic conditions reached granulite facies during Mesoproterozoic orogenesis (Spear, 1993). Many of these basement rocks and most of the overlying cover sequence display mineralogic evidence of metamorphism at upper greenschist-facies conditions that occurred as a result of Paleozoic orogenesis (Kunk and Burton, 1999). Paleozoic metamorphism is responsible for development of greenstone and chlorite-rich phyllite throughout the Catoctin Formation and some of the related cover rocks, and for production of retrograde minerals in the basement rocks such as biotite, chlorite, and uralitic amphibole.
Mesoproterozoic Basement Rocks
Basement rocks of the Virginia Blue Ridge include a diverse assemblage of granitoids and gneissic lithologies that were emplaced in thickened crust and locally metamorphosed at high-grade conditions at about 1.2 to 1.0 Ga (Aleinikoff and others, 2000; Tollo and others, in press a). The oldest rocks, which include a compositionally variable group of gneisses and deformed granitoids, typically display widespread evidence of penetrative ductile deformation. These rocks occur both as regional map units and as smaller inliers that form screens and probable roof pendants within younger intrusive bodies (fig. 3). The younger, more areally extensive group is composed mostly of compositionally diverse granitoids that are variably deformed. Granitoids throughout the area vary widely in mineralogic composition, ranging from quartz monzonite to leucocratic alkali feldspar granite (fig. 4; table 1). Both age groups include orthopyroxene-bearing charnockitic types (table 2). Most rocks exhibit geochemical characteristics that indicate derivation from crustal sources and show compositional similarities to granitic rocks produced in within-plate tectonic settings (Tollo and others, in press a).
Blue Ridge basement rocks were historically divided into two regional suites based largely on mineralogy and inferred metamorphic grade. Bloomer and Werner (1955) grouped a wide spectrum of orthopyroxene-bearing rocks into the Pedlar Formation, distinguishing these mostly high-grade rocks from lower grade, biotite±amphibole-bearing varieties designated as the Lovingston Formation by Jonas (1928). Bartholomew and others (1981) extended this classification through definition of the areally extensive Pedlar and Lovingston massifs, wherein the former occurs west of the north-south-trending Rockfish Valley fault zone (and its along-strike extensions) and the latter occurs east of this structural feature. According to this model, rocks of the lower-grade Lovingston massif were juxtaposed against the Pedlar massif as a result of movement along the Rockfish Valley fault zone (Bartholomew and others, 1981; Sinha and Bartholomew, 1984). In proposing an alternative explanation for this lithologic juxtaposition, Evans (1991) suggested that the biotite-bearing assemblages of the Lovingston terrane were produced through fluid-enhanced retrograde metamorphic recrystallization of original orthopyroxene- bearing rocks. However, results from recent studies suggest that neither model adequately accounts for the observed regional distribution of rocks. For example, recent detailed mapping and related structural studies indicate that the areal distribution of lithologies is more heterogeneous than indicated by the reconnaissance mapping that formed the basis of the earlier studies (Bailey and others, 2003; Tollo and others, in press b). Recent field-based studies also indicate that other ductile fault zones may have played a more significant role in the tectonic evolution of the north-central and northern Blue Ridge, thus diminishing the structural significance of the Rockfish Valley fault zone, especially in the north-central Blue Ridge (Bailey, 2003). Additionally, recent advances regarding the petrology of charnockitic rocks indicate that formation of orthorhombic pyroxene in the igneous systems that constitute the protoliths of most of the Blue Ridge Mesoproterozoic rocks is controlled by numerous characteristics of the original melt, such as aH2O, pCO2, Fe/(Fe+Mg), and depth of crystallization, and is thus not necessarily a reflection of ambient metamorphic grade (Frost and others, 2000).
Zircons were extracted from seven samples of Mesoproterozoic basement rocks from the field area for U-Pb geochronology. In all samples, zircon generally is medium to dark brown, subhedral to euhedral, stubby to elongate prisms. Zircons in all samples contain multiple age components, a characteristic illustrated by igneous zircons in rocks from other studies of Grenvillian basement in the Blue Ridge and elsewhere (McLelland and others, 2001; Aleinikoff and others, 2000; Carrigan and others, 2003; Hamilton and others, in press). As a result, we decided to analyze the zircons using the high-spatial-resolution capabilities of spot analysis by the sensitive high-resolution ion microprobe (SHRIMP).
Areas on zircons about 25 microns in diameter by 1 micron in depth were dated using SHRIMP. Prior to isotopic analysis, all zircons were photographed in transmitted and reflected light, and imaged in cathodoluminescence (CL) on a scanning electron microscope. Analysis locations were chosen on the basis of crystal homogeneity (lack of cracks, inclusions, and other imperfections as shown by transmitted light photographs) and apparent age homogeneity (cores and overgrowths can usually be distinguished using CL). In all samples except SNP-02-197 (garnetiferous syenogranite, Ygg), igneous cores are distinct and obvious. Observed under CL illumination, cores typically contain concentric, euhedral, oscillatory, compositional zoning, consistent with crystallization in a magma (Hanchar and Miller, 1993; Hanchar and Rudnick, 1995). Most cores typically have Th/U of about 0.3 to 0.6. Metamorphic overgrowths usually are unzoned and have distinctive, very low Th/U of 0.01 to 0.1. Rare xenocrystic cores can be distinguished by locally rounded morphology and truncated oscillatory zoning.
Igneous crystallization ages of the seven dated samples range from 1,183±11 Ma to 1,050±8 Ma (table 3). All samples contain younger overgrowths with ages of about 1,040 to 1,020 Ma; a few overgrowths that apparently formed during a younger, post-Grenville event(s) have ages of about 1,000 to 980 Ma. Compared to most zircons of igneous origin, zircons in the garnetiferous syenogranite (Ygg) have very unusual CL zoning patterns. Although crosscutting relations are observed, it is impossible to determine which zone formed during igneous crystallization because the typical igneous concentric zoning patterns are lacking. Four age groups were resolved for this sample (table 3). On the basis of field relations, we conclude that the age group at 1,064±7 Ma is the most likely time of crystallization of the garnetiferous syenogranite. Two older dates (1,135±6 Ma and 1,099±9 Ma) are interpreted as ages of inherited material; a younger age of 1,028±14 Ma probably represents the time of regional metamorphism, as indicated by overgrowth ages in other samples.
The age range of basement rocks in the study area is similar to ages of Grenville rocks elsewhere in the central and southern Appalachians. For example, in the northern Blue Ridge, Aleinikoff and others (2000) dated a series of gneisses and granitoids at about 1,150 to 1,050 Ma and noted three pulses of magmatic activity occurring at about 1,150 to 1,140 Ma, 1,110 Ma, and 1,075 to 1,050 Ma. In the southern Appalachians, Carrigan and others (2003) documented a regional pulse of granitic magmatism that occurred at ~1,165 to 1,150 Ma and presented evidence for nearly ubiquitous metamorphism at ~1,030 Ma. In the Adirondacks, McLelland and others (1996) recognized a similar geochronologic sequence for tectonomagmatic activity associated with Grenvillian orogenesis but including an older period, identifying episodes at 1,350 to 1,190 Ma, about 1,090 Ma, and 1,090 to 1,030 Ma. The Adirondack ages overlap episodes of major magmatic and metamorphic events recognized in the Grenville province of Canada (Gower and Krogh, 2002) and suggest that the two areas share some aspects of orogenic activity. However, extrapolation of these tectonic events to the Blue Ridge is tempered by the allochthonous nature of the Blue Ridge and the possibility that the terrane was separated from the Adirondack-New England region by a major tectonic boundary during Grenvillian orogenesis (Bartholomew and Lewis, 1988).
Lithologies and age relations
Textures, compositional characteristics, and field relations suggest that nearly all basement rocks within the study area were originally igneous in origin. Nomenclature was determined using normative compositions and standard procedures recommended by the International Union of Geological Sciences (IUGS) because the locally very coarse grain size and strongly preferred orientation of fabrics in some rocks hindered direct application of standard modal-based procedures. General petrologic nomenclature using parameters calculated from normative data are presented in table 1, with IUGS-recommended names for orthopyroxene-bearing varieties included where appropriate. The basement rocks collectively display a wide range of normative compositions; however, in most cases, individual lithologic units are characterized by relatively restricted compositional variation (fig. 4; table 4). Syenogranite to monzogranite compositions are most common, especially for the older rocks that predate local Grenvillian deformation. The low-silica charnockite (Yfqj), which includes numerous chemically consanguineous dikes of both similar and less-evolved composition, is characterized by the greatest internal variation, ranging from orthopyroxene-bearing monzogranite (farsundite1) to quartz monzodiorite (quartz jotunite). Older rocks that predate regional Grenvillian deformation are dominated by orthopyroxene-bearing, charnockitic granitoids, whereas rocks that postdate deformation include both leucocratic granitoids ranging from alkali feldspar granite to syenogranite and low-silica charnockite that is likely unrelated to the contemporaneous leucocratic rocks. Results from U-Pb isotopic analysis of zircons indicate that mineralogically similar leucocratic granitoids were emplaced during each of the three magmatic intervals presently identified within the Blue Ridge study area. The ~1,180-Ma leucocratic granitoid and megacrystic leucocratic granite gneiss (Ylg), which also includes abundant leucogranite pegmatite, constitutes an intrusive complex characterized by multiple generations of igneous activity and represents the oldest dated rock in the region. Pervasively deformed screens and xenoliths of leucogranite gneiss (Ylgg) of ~1,080-Ma age occur within low-silica charnockite and represent the youngest intrusive unit presently recognized to predate regional Grenvillian deformation. Following deformation, magmatic activity continued to produce leucocratic granitoids represented by the garnetiferous syenogranite (Ygg), Old Rag magmatic suite (Ygr), and two small bodies of coarse-grained alkali feldspar granite (Yaf) exposed in the Madison quadrangle (fig. 3) which may be correlative with the larger Old Rag magmatic suite.
Geologic mapping and related studies throughout five contiguous 7.5-minute quadrangles in the north-central Blue Ridge (fig. 3) indicate that Mesoproterozic basement rocks define three groups based on age and field characteristics: (1) foliated rocks of about 1,180- to 1,160-Ma age, (2) foliated rocks of 1,115- to 1,080-Ma age, and (3) largely nonfoliated rocks of <1,060-Ma age. Each group is characterized by the following features.
Foliated rocks of 1,180- to 1,160-Ma age: This group comprises a compositionally diverse assemblage of lithologies, including leucocratic granitoids and granitoid gneisses, layered gneiss, and foliated charnockite. All rocks display evidence of pervasive, typically ductile deformation that is interpreted as Grenvillian in origin. Within the mapped quadrangles, this group includes the following lithologic units: (1) leucocratic granitoid and megacrystic leucocratic granite gneiss (Ylg); (2) high-silica charnockite (Ycf, charnockite and farsundite); (3) garnetiferous granite gneiss (Yg); and (4) garnetiferous gneiss (Ygn). Charnockitic layered granodiorite gneiss (Ylgn) may also belong to this group.
Foliated rocks of 1,115- to 1,080-Ma age: Rocks within this group also predate the major deformational event believed to be responsible for developing pervasive, high-temperature fabrics in many of the basement units. Two dated lithologic units are placed within this group: (1) foliated pyroxene granite (farsundite, Yfpg) and (2) leucogranite gneiss (Ylgg). Similar to the older high-silica charnockite (Ycf), the foliated pyroxene granite (Yfpg) contains abundant orthopyroxene of likely magmatic origin and is likewise also charnockitic.
Nonfoliated rocks of <1,060-Ma age: Nonfoliated lithologic units are interpreted as postorogenic with respect to the main period of Grenvillian deformation in the Blue Ridge. All rocks in this group within the field trip area are broadly granitic and include the following: (1) garnetiferous syenogranite (Ygg); (2) the Old Rag magmatic suite (Hackley, 1999), which includes pyroxene-bearing leucogranites of varying grain size and mineralogical composition (Ygr); (3) alkali feldspar granite (Yaf); and (4) low-silica charnockite (Yfqj, farsundite and quartz jotunite). The compositionally similar granitic units (including Ygg, Ygr, and Yaf) are distinctive leucocratic rocks with high potassium feldspar to plagioclase ratios and likely constitute a regional suite.
In summary, results from recent field, petrologic, and geochronological studies indicate that basement rocks of the northern Blue Ridge preserve evidence of tectonomagmatic events that spanned over 160 m.y. of Grenvillian orogenesis (Aleinikoff and others, 2000; Tollo and others, in press a). Presently, ages derived from high-precision U-Pb isotopic analyses of zircons indicate that an early interval of magmatic activity occurred at about 1,180 to 1,140 Ma and involved granitoids (now gneisses and deformed megacrystic leucogranites) of considerable compositional diversity. This episode was followed at about 1,110 Ma by a second period of magmatism presently defined by two compositionally dissimilar plutons within the northern Blue Ridge. Following another hiatus, plutonism resumed at about 1,080 Ma and was rapidly followed by a significant period of deformation that occurred within the interval 1,080 to 1,060 Ma. Most of the ductile fabrics developed in many of the older plutonic rocks were likely formed during this episode. Following deformation, plutonism continued to about 1,050 Ma, producing granitoids of considerable compositional diversity, including charnockite of A-type affinity. Isotopic evidence further indicates that thermal disturbances occurred throughout the region at 1,020 to 980 Ma. This temporal framework is similar to the sequence of Grenvillian events documented in the Adirondacks and Canadian Grenville province (McLelland and others, 1996; Rivers, 1997), suggesting the possibility of tectonic correlations between the Blue Ridge and these Laurentian terranes.
Geochemical data indicate that basement rocks within the field trip area are characterized by diverse chemical compositions and a range in silica content of nearly 30 weight percent (figs. 5A–F). Most lithologic units are felsic, containing >65 weight percent SiO2, a compositional characteristic that is reflected in the abundance of granitoids throughout the field area (figs. 3, 4). Mafic to intermediate rock types are represented only by the low-silica charnockite (Yfqj) and foliated pyroxene quartz diorite (Ypqd), which form a large pluton and small inlier, respectively (fig. 3). Although variation for the region as a whole is extensive, most individual lithologic units are characterized by relatively modest chemical diversity, suggesting that significant differentiation did not occur at or near the emplacement level of most individual plutons. In contrast to most lithologic units, the low-silica charnockite (Yfqj) and biotite granitoid gneiss (Ybg) exhibit trends in compositional and normative variation that are both considerable and petrologically distinctive. The internally differentiated low-silica charnockite, which includes abundant compositionally related dikes and fractionated pegmatite (Tollo and others, in press a), ranges in silica content from 50 to 65 weight percent, defining about half of the variation documented to date in the study area (fig. 5). Chemical variations in the biotite granitoid gneiss are bimodal, with compositions clustering at about 62 and 67 weight percent SiO2 (fig. 5). This compositional diversity corresponds to normative compositions ranging from syenogranite to quartz monzonite (fig. 4), and is likely a reflection of compositional layering that characterizes this lithologic unit in the field (Bailey and others, 2003; Hackley, 1999).
Most lithologic units are transitional metaluminous to peraluminous in composition, consistent with the typical pyroxene±amphibole mineral assemblages (fig. 5D); however, all leucogranitic rocks are characteristically peraluminous, as reflected in their locally biotite- and garnet-bearing compositions. Collectively, basement rocks within the field trip area are subalkaline and tholeiitic (figs. 5E and F, respectively), and thus share many compositional features with granitoids and charnockites of the classic anorthosite-mangerite-charnockite-granite (AMCG) suites documented in the Adirondacks and other Precambrian massifs that include abundant intrusive rocks (McLelland and Whitney, 1990; Frost and Frost, 1997). Trace-element concentrations of most of the Blue Ridge granitoids plot in a region of the source-sensitive Nb+Y versus Rb diagram of Pearce and others (1984) (fig. 6) that is characteristic of granitoids emplaced in broadly defined postorogenic geologic settings. Such compositional characteristics suggest that magmas were derived from mixed sources that included both crustal and arc-related components (Sylvester, 1989; Förster and others, 1997). As noted by previous studies (Pearce and others, 1984; Sylvester, 1989; Maniar and Piccoli, 1989; Eby, 1990, 1992; Förster and others, 1997), granites associated with postorogenic processes and (or) within-plate tectonic environments include both anorogenic and postorogenic types. Such suites typically include A-type granitoids characterized by broad compositional variation that reflects derivation from sources of mixed origin (Sylvester, 1989; Eby, 1990, 1992; Förster and others, 1997). Geochemical data indicate that the low-silica charnockite (Yfqj) displays considerable similarity to A-type granitoids, whereas most other rocks within the study area exhibit compositional features that are transitional between I- and A-types (fig. 7) (Tollo and others, in press a). The contemporaneous low-silica charnockite and leucogranitoids (Ygr, Ygg, and Yaf) display compositional characteristics, such as comparable FeOt/MgO (fig. 5F) and similar Eu/Eu* (not shown), which suggest that these contrasting rock types are unlikely to define a continuous liquid line of descent. The occurrence of A-type and relatively evolved I-type granitoids that are closely related in both space and time is not unusual in orogenic belts worldwide, as illustrated by examples from Australia (Landenberger and Collins, 1996). The Blue Ridge rocks thus appear to have been derived through melting of mixed sources present in the evolving Grenvillian orogenic belt. Compositionally transitional intermediate and felsic rocks, including peraluminous leucogranites, were emplaced episodically over a 100-m.y. time span that largely predated local orogenesis at 1,080 to 1,060 Ma. Peraluminous leucogranitoids and low-silica charnockite, sources, postdated orogenesis and marked the termination of local magmatic activity.
The older Mesoproterozoic units (>1,060 Ma) commonly display foliations and compositional layers that developed under high-grade metamorphic conditions during Grenvillian orogenesis. Foliation is defined by aligned feldspars and quartz aggregates, and individual grains have a very weak grain-shape preferred orientation with straight grain boundaries. Microstructures and certain mineral assemblages that define this foliation, such as orthopyroxene+garnet, are consistent with high-temperature (>600°C) upper amphibolite- to granulite-facies conditions (Passchier and Trouw, 1996). Regionally, this foliation generally strikes approximately eastwest and dips steeply to both the north and south. Folded foliation is only rarely observed, but at some outcrops it is axial planar to folds developed in competent layers such as coarsegrained leucogranitic dikes. The kinematics of this high-grade deformation are unclear; however, based on foliation and fold orientations, the Blue Ridge basement experienced significant ~north-south (in present-day geometry) shortening during this event. The high-temperature fabric is best preserved in units with crystallization ages of >1,080 Ma (Yg, Ygn, Ylg, and Yfpg) and generally absent in units ~1,060 Ma. Grenvillian fabrics are cut and overprinted by a lower temperature foliation defined by greenschist-facies metamorphic minerals and microstructures of probable Paleozoic age. Such low-grade fabrics are present in numerous mafic dikes of late Neoproterozoic age that locally intrude basement but are absent in mafic dikes of probable Mesozoic age. Such dikes are also compositionally distinguishable on the basis of TiO2 content (fig. 8).
The identification and interpretation of structural elements of Paleozoic age has evolved greatly during the past three decades, leading to recent recognition of the important role of high-strain zones in the structural development of the Blue Ridge. Indeed, recent discoveries regarding Paleozoic structures have called into question some longstanding ideas regarding the structural genesis of the terrane and have served as the basis for new developmental models of the Blue Ridge. Nevertheless, because of the complex, multi-stage structural evolution of the Blue Ridge, precise determination of timing relations characterizing such structural features remains in its infancy.
A younger fabric of probable Paleozoic age, defined by aligned phyllosilicates, elongate quartz, and fractured feldspars, is common in many Mesoproterozoic units. The foliation generally strikes to the northeast and dips moderately toward the southeast, and is commonly associated with a downdip mineral lineation. The microstructures and minerals that define this fabric are indicative of deformation that occurred at greenschist-facies conditions. In the northern Virginia Blue Ridge, Burton and others (1992) obtained late Paleozoic (~300 Ma) 40Ar/39Ar cooling ages on similar fabrics, whereas Furcron (1969) reported early Paleozoic (~450 Ma) K-Ar ages for metamorphic minerals in the Mechum River Formation in central Virginia.
Mitra (1977, 1979) was among the first to demonstrate the kinematic and mechanical significance of high-strain zones (ductile deformation zones) in the northern Virginia Blue Ridge. In central Virginia, Bartholomew and others (1981) named the Rockfish Valley fault zone and suggested that it forms a major tectonic boundary that separates Mesoproterozoic basement massifs (Pedlar and Lovingston) of distinctly different character. Bartholomew and others (1981) and Bartholomew and Lewis (1984) extended the Rockfish Valley fault zone northward from central Virginia to northern Virginia and linked it southward with the Fries fault zone in the southern Virginia Blue Ridge. The temporal, kinematic, and tectonic significance of the Rockfish Valley zone has been discussed by a number of workers (Bartholomew and others, 1981; Conley, 1989; Simpson and Kalaghan, 1989; Bailey and Simpson, 1993; Burton and Southworth, in press). Different workers have variously interpreted the Rockfish Valley fault zone to be a reverse, normal, and strike-slip structure.
Recent mapping in the central and northern Virginia Blue Ridge (at both 1:24,000 and 1:100,000 scale) indicates that basement rocks are cut by a series of anastomosing high-strain fault zones (fig. 9) rather than by a single fault zone (for example, the Rockfish Valley fault zone). Individual high-strain zones form northeast-southwest-striking belts of mylonitic rock, 0.5 to 3 km (0.3–1.9 mi) thick, that dip moderately to the southeast. Mineral elongation lineations plunge directly downdip to obliquely downdip. Deformation is heterogeneous and associated penetrative fabrics diminish away from the high-strain zones. From north to south, these zones include the Sperryville, Champlain Valley, Quaker Run, White Oak, and Rockfish Valley zones (fig. 9). Collectively, these zones display an en-echelon map pattern. Individual zones extend 30 to 100 km (19–60 mi) and pinch out along strike. Mylonitic rocks are characterized by microstructures consistent with deformation occurring at greenschist-facies (~350–400°C) conditions with abundant fluids (Bailey and others, 1994).
Blue Ridge high-strain zones are characterized by both monoclinic and triclinic deformation symmetries (fig. 10) (Bailey and others, in press; Bailey, 2003). Triclinic symmetries are revealed by the geometry of fabric elements with respect to high-strain zone boundaries and fabric asymmetries on planes both parallel and normal to elongation lineations. Elongation lineations plunge to the southeast and kinematic indicators on XZ sections record a northwest-directed (reverse) sense of shear. Mylonitic rocks with a triclinic symmetry also record a component of strike-parallel sinistral shear. Strain ratios, measured with quartz grain shapes and boudinaged feldspars, range from 4 to 20 in XZ sections. Three-dimensional strains are moderately to strongly oblate (K=0.8–0.0). Vorticity analysis indicates that these high-strain zones experienced bulk general shear deformation (Wm=0.6–0.9).
The total displacement across individual high-strain zones, estimated by integrating shear strains over zone thickness, range from 1 to 4 km (0.6–2.5 mi) (fig. 10). Total displacement estimates are in accord with field relations demonstrating that, at many locations, the same basement unit occurs in both the footwall and hanging wall of the high-strain zone. These modest offsets are incompatible with tectonic models that suggest Blue Ridge high-strain zones separate distinctly different Grenvillian massifs. The kinematic significance of Blue Ridge high-strain zones indicates that (1) these zones accommodated significant crustal shortening, (2) displacement on these zones is on the order of a few kilometers, and (3) widespread flattening strains require significant strike-parallel (orogen-parallel) material movement (fig. 10).
The absolute age of movement on Blue Ridge high-strain zones is not precisely known. Field relations from the central Virginia Blue Ridge indicate that mylonitic high-strain zones are cut by brittle thrusts of Alleghanian (~320–280 Ma) age (Bailey and Simpson, 1993). Polvi (2003) reported an 40Ar/39Ar plateau age of 355±3 Ma for muscovite from a greenschist-facies contractional high-strain zone in Nelson County, located approximately 100 km (60 mi) southwest of the field trip area. This cooling age is incompatible with an Alleghanian age for ductile deformation, but could reflect cooling from either a Taconian (Ordovician) or Devonian event possibly synchronous with the Acadian orogeny in New England.
Summary and Regional Implications
The new mapping and supporting investigations of basement in the study area illustrate the lithologic complexity and protracted geologic evolution of rocks associated with Mesoproterozoic orogenesis. The structural studies indicate the importance of multiple high-strain zones in accommodating the effects of Paleozoic deformation within the Blue Ridge core, and suggest that the role of the Rockfish Valley fault zone was not as significant as previously suggested. This finding necessitates a re-evaluation of models for Paleozoic structural development of the Blue Ridge that involve the Rockfish Valley fault zone as the dominant internal tectonic element responsible for the present distribution of major rock types in the anticlinorium. The lithologic variation documented in Blue Ridge basement rocks by these studies refines previous models of the province that were based largely on reconnaissance-scale mapping by demonstrating the complex juxtaposition of rocks of different composition, age, and tectonic significance. The dominance of granitic rocks in the study area is similar to the compositional characteristics documented elsewhere in the Appalachian massifs (for example, Rankin and others, 1989b; Carrigan and others, 2003). However, the lack of rocks generally associated with orogeny is noteworthy and contrasts with the presence of lithologies of calc-alkaline affinity in the Adirondacks (McLelland and others, 1996) and New Jersey Highlands (Volkert, in press). Results from geochronologic studies completed to date indicate that the northern Blue Ridge also differs from the well-documented Adirondacks in lacking rocks of >1,200-Ma age. Correspondingly, there is at present no evidence of Elzevirian orogenic processes in the Blue Ridge of Virginia, suggesting either that the province did not undergo this earlier pulse of Grenvillian orogenic activity that is otherwise well documented in the Adirondacks and some parts of the southeastern Canadian Grenville province (Rivers, 1997; Wasteneys and others, 1999; Gower and Krogh, 2002), or that the lithologic signature of this event was destroyed by subsequent Mesoproterozoic orogenesis. Nevertheless, trace-element geochemical characteristics of many of the Mesoproterozoic granitic rocks of the northern Virginia Blue Ridge indicate the presence of components of calc-alkaline affinity in the magmatic sources, suggesting that pre-existing Laurentian crust contains evidence of pre-1,200-Ma tectonic processes (Tollo and others, in press a). The geochronology of basement rocks from the northern Blue Ridge indicates that the province shares many temporal characteristics with other Appalachian massifs (Aleinikoff and others, 1990; Carrigan and others, 2003; Hatcher and others, in press) and supports definition of the Appalachian outliers as outposts of Laurentian crust affected by Grenvillian orogenic processes (Rankin and others, 1989b). Nevertheless, the petrologic and compositional heterogeneity and substantial range in emplacement ages that characterize Blue Ridge basement rocks suggest that detailed correlation of events and processes recorded within individual massifs will depend on continued scientific advances.
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