DIVISION OF RESOURCES PUBLICATION 157

BEDROCK OF THE MADISON QUADRANGLE, VIRGINIA

Christopher M. Bailey, Peter J. Berquist, Stephanie M. Mager, Brian D. Knight, Nathan L. Shotwell. and Amv K. Gilmer

COMMONWEALTH OF VIRGINIA DEPARTMENT OF MINES, AND ENERGY DIVISION OF MINERAL RESOURCES C. R. Berquist, Jr., State Geologist

CHARLOTTESVILLE, VIRGINIA 2003 VIRGINIA DIVISION OF MINERAL RESOURCES PUBLICATION 157

BEDROCK GEOLOGY OF THE MADISON QUADRANGLE, VIRGINIA

Christopher M. Bailey, Peter J. Berquist, Stephanie M. Mager, Brian D. Knight, Nathan L. Shotwell. and Amv K. Gilmer

COMMONWEALTH OF VIRGINIA DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINE,RAL RESOURCES C. R. Berquist, Jr., State Geologist

CHARLOTTESVILLE, VIRGINIA 2003 FRONT COVER: Shaded relief image of the Madison 7.5-minute quadrangle derived from 10 m USGS digital elevation model data. VIRGINIA DIV]SION OF MINERAL RESOURCES

PUBLICATION 157

BEDROCK GEOLOGY OF THE MADISON QUADRANGLE, VIRGINIA

Christopher M. Bailey, Peter J. Berquist, Stephanie M. Mager, Brian D. Knight, Nathan L. Shotwell, and Amy K. Gilmer

COMMONWEARH OF VIRGINIA DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINERAL RESOURCES C. R. Berquist, Jr., State Geologist

CHARLOTTESVILLE, VIRGINIA 2003 DEPARTMENT OF MINES. MINERALS AND ENERGY zuCHMOND. VIRGINIA O.Gene Dishner, Director

DIVISION OF MINERAL RESOURCES CHARLOTTE SVILLE, VIRGINIA C. R. Berquist, Jr., State Geologist and Division Director

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Portions of this publication may be quoted if credit is given to the Virginia Division of Mineral Resources. It is recommended that reference to this report be made in the following form:

Bailey, Christopher M., Berquist, Peter J., Mager, Stephanie M., Knight, Brian D., Shotwell, Nathan L., and Gilmer, Amy K., 2003, Bedrock geology of the Madison quadrangle, Virginia: Virginia Division of Mineral Resources Publication 157,22p. CONTENTS

Acknowledgments...... 3

Blue Ridge complex...... 3 Mesoproterozoic rocks...... 4 Garnetiferous (Ygn)...... 4 Leucocratic granitoid and leucocratic gneiss (Ylg)...... 4 Biotite-bearing granitoid gneiss and layered granitoid gneiss (Ybg)...... 5 Charnockite (Ych)...... 6 Coarse-grained alkali (Ya|...... 6 Neoproterozoic rocks...... 7 Robertson River Igneous Suite...... 7 Granite and alkali feldspar granite (Zgrl\...... 7 Alkali feldspar granite (Zgr2)...... 7 Alkali feldspar syenite (Zgr3)...... 7 Blue Ridge cover sequence...... 8 Neoproterozoic rocks...... 8 Mechum River Formation...... 8 Metaconglomerate (Zmc)...... 8 Meta-arkose and feldspathic meta-wacke (Zma)...... 8 Metasiltstone and phyllite (Zms)...... 9 Tabular intrusive rocks...... 9 Altered ultramafic rock (Zum)...... 9 Homblende metagabbro (Zhg)...... 9 Metabasalt (Zmb)...... 9 Mesozoic rocks...... 10 Diabase (Mzd)...... 10 Structural g High-shain zones and faults...... 10 Foliations...... 13 Joints...... 16 ...... 16 Geologic histor...... 17 Economic geology...... 19 Crushed stone...... 19 Gemstones ...... 20 Sand and gravel...... 20 Pyrite...... 20 Titantium-bearing minerals...... 20 ILLUSTRATIONS

PLATE 1. Bedrock geologic map of the Madison quadrangle, Virginia...... In pocket

FIGURE Shaded relief image of the Madison 7.5-minute quadrangle derived from 10 m USGS digital elevation model data.:...... Front cover 1. Generalized geologic map of the north-central Virginia Blue Ridge ...... 2 2. Schematic contact relationships in the Blue Ridge basement complex...... 3 3. Subhorizontal outcrop surface exposed in debris flow scar...... 5 4. Leucocratic granitoid gneiss inclusions in biotite-bearing granitoid gneiss...... 6 5. Coarse-grained chamockite ...... 6 6. Network of alkali feldspar syenite dikes cutting biotite-bearing granitoid gneiss...... 7 7. Mechum River Formation metaconglomerate along Hatter Run...... 8 8. Mechum River Formation meta-arkose...... 9 9. Generalized structural map of the Madison quadrangle ...... 10 10. Stereogram of fabric elements in the White Oak Run high-strain aorre...... 11 11. Polished slab from the White Oak Run high-strain 2one...... 11 12. a. Stereogram of poles to bedding in the Mechum River Formation...... 12 b. Stereogram of poles to the penetrative foliation...... 12 13. Stereogram of fabric elements in the Champlain Valley and Quaker Run high-strain 2ones...... 13 t4. a. Mylonite from the Quaker Run high-sfiainzone...... 13 b. Prototmylonite from the Quaker Run high-strainzone...... 13 15. Diagram illustrating strain integration technique for estimating displacement...... I4 16. a. Stereogram of poles to foliation for high-temperature foliation...... 14 b. Stereogram of poles to foliation for low-temperature foliation...... 14 r7. Folded pegmatitic leucogranite dike in medium-grained leucocratic granitoid gneiss...... l5 18. High temperature fabric in leucocratic granitic gneiss...... 15 t9. Stereogram of poles to joints and contoured stereogram of poles to joints in the Madison quadrangle ...... 16 20. Generalized geologic history of the Madison quadrangle and Virginia Blue Ridge...... 18 BEDROCK GEOLOGY OF THE MADISON QUADRANGLE, VIRGINIA

Christopher M. Baileyl, Peter J. Berquistr, Stephanie M. Magerl, Brian D. Knightl, Nathan L. Shotwelll. andAmv K. Gilmer I

ABSTRACT minerals. Numerous joint sets cut the bedrock, however, there are no dominant joint orientations Rocks exposed in the Madison 7.5-minute at the quadrangle scale. quadrangle, north-central Virginia, lie in the core of the Blue Ridge anticlinorium. The oldest rocks are Mesoproterozoic (-1150 to 1050 Ma) INTRODUCTION granitoid rocks and include garnetiferous gneiss, leucocratic granitoid gneiss, biotite-bearing The geology of the Madison 7.5-minute granitoid gneiss, charnockite, and alkali feldspar quadrangle was mapped from 1997 to 1999 at granite. Neoproterozoic (-730 to 700 Ma) l:24,000 scale as part of a research program granitoids of the Robertson River batholith intrude at the College of William and Mary designed Mesoprote r ozoic rocks. Metasedim entary rocks of to understand the stratigraphy and structural the Neoproterozoic Mechum River Formation crop geometry of the Blue Ridge province in north- out in a nanow northeast-southwest trending belt. central Virginia. Parts of the quadrangle had Tabular bodies of Neoproterozoic (?) hornblende been mapped at 1:62,500 scale as geologic maps metagabbro, Neoproterozoic metabasalt, and of Greene and Madison counties (Allen, 1963) Mesozoic diabase intrude the older and Shenandoah National Park (Gathright, 1976) rocks. and at l:100,000 scale by Tollo and Lowe (1994), Significant faults and ductile high-strain however, no recent detailed geology (l:24,000 zones in the Madison 7.5-minute quadrangle scale) was available. Ph.D. theses by Lukert include the White Oak Run, Quaker Run, and (1973) and Mitra (1977) reported on petrologic, Champlain Valley high-strain zones and the geochronologic, and structural aspects of the Mechum River fault system. The White Oak Run geology in the Madison quadrangle. high-strain zone is composed of steeply-dipping The Madison quadrangle includes mylonites that record extensional deformation. approximately 60 square miles in central Madison The Quaker Run and Champlain Valley high- County, Virginia, in the Blue Ridge geologic strain zones are part of an anastomosing regional province (Figure 1). The quadrangle is bounded network of mylonite zones that accommodated by 78' 15'00" and 78" 22' 30- west longitudes northwest-directed shortening during the and 38' 22' 30" and 38' 30' 00" north latitudes. Paleozoic. The Mechum River Formation is The area mapped is approximately 30 miles north- bounded on the southeast by steeply dipping northeast of Charlottesville and encompasses the reverse faults that bring basement granitoids over eastern part of the Blue Ridge Mountains and younger metasedimentary rocks. Two distinct the western part of the foothills region of the foliations are common in the Mesoproterozoic physiographic province. The western basement complex: an amphibolite to part of the quadrangle is mountainous and facies fabric defined by recrystallized feldspar Doubletop Mountain (elevation 3005 feet) forms and qtartz aggregates and a younger crosscutting the highest point. The central and eastern parts fabric defined by aligned greenschist facies of the quadrangle are characterized by rolling

rDepartment of Geology, Box 8795, College of William and Mary Williamsburg, Virginia 23187 VIRGINIA DIVISION OF MINERAL RESOURCES

'ft,,y2,,i;,n

and ductile t\ hiqh-strain zone attt .{ - 7 /a reverse fault and I ductile high-strain zone ' N 0r 10 20 kilometers I 510 ITil Mz- Mesozoic basins co- cambro-ordovician carbonates KW Zc- Neoproterozoic catoctin Formation w pz- p aleozoic metasedimentayo"n" W Zs- Neoproterozoic metasedimentary rocks tK3 Gc- Cambrian Chilhowee Group granitoid and gneiss

Figure 1. Generalized geologic map of the north-central Virginia, Blue Ridge, and the location of the Madison quadrangle. PUBLICATION 157 topography with isolated hills rising to elevations careful reviews of both the map and manuscript. of -1000 to -2150 feet. The southeast-flowing E. Campbell and M. Upchurch of the Division Robinson and Rapidan rivers drain the region, of Mineral Resources are thanked for their able and the lowest elevation (400 to 420 feet) occurs assistance with digital formatting of the map and in the channel of the Robinson River along publication the eastern boundary of the quadrangle. The quadrangle includes approximately 2 square miles STRATIGRAPITY of Shenandoah National Park. Intense rainfall on June 27,1995 caused numerous slope failures in BLUE RIDGE BASEMENT COMPLEX first-order stream valleys in the western part of the quadrangle and produced many new bedrock The Blue Ridge basement complex exposures. These newly exposed outcrops consists of a suite of Mesoproterozoic granitoids created an opportunity to study large volumes of and and granitoids of the Neoproterozoic fresh bedrock with a level of detail not generally Robertson River Igneous Suite. In this report obtainable in Virginia. the term granitoid is used to describe plutonic or The Blue Ridge geologic province metaplutonic rocks of broadly granitic composition separates the and thrust belt of the Valley (alkali feldspar granite to tonalite). Age relations and Ridge from the orogenic hinterland in the were established between units based on cross Piedmont (Figure 1). In central and northern cutting relations, inclusions of one rock type Virginia, the Blue Ridge forms an anticlinorium in another, and inferences drawn from regional with Mesoproterozoic basement rock in the core geochronologic studies. Figure 2 illustrates the and Neoproterozoic to early Paleozoic cover rock generuIizedage relations among basement units in on the flanks and in fault-bounded inliers. The the Madison quadrangle. present day structure of the Blue Ridge province is that of an imbricated stack of basement thrust sheets that experienced southeast-northwest directed shortening during Paleozoic deformation (Evans, 1989; Bailey and Simpson, 1993). Rocks exposed in the Blue Ridge province have experienced a long and complex history.

ACKNOWLEDGMENTS

The U.S. Geological Survey's Educational Mapping program (EDMAP), the Jeffress Memorial Trust, the College ofWilliam and Mary and the Virginia Division of Mineral Resources supported this research. The authors appreciate discussions with N. H. Evans, R. P. To11o, W. C. Burton, W. S. Henika, D. B. Spears, L. S. Eaton, and C. R. Berquist Jr. concerning the geology of the E. S. Madisonarea. L. Coiner, Eyster, Giorgis, Figure 2. Schematic contact relationships between the A. Griffith, C. Koteas, C. Lamofl, M.DeSmedt, Blue Ridge basement complex, Madison quadrangle. J. Relyea, and H. Wadman are thanked for their Zgr- Robertson River lgneous Suite, Yaf- Coarse- assistance in the fleld. State Geologists S. S. grained alkali feldspar granite, Ybg- biotite granitoid gneiss, Ych- charnockite, Ygn- garnetiferous gneiss, Johnson and C. R. Berquist Jr. provided support Ylc- leucocratic granitoid gneiss. Note the transition and encouragement for this project. R. P. Tollo, region between charnockite and biotite granitoid C. S. Southworth, and I. J. Duncan provided gneiss. VIRGINIA DIVISION OF MINERAL RESOURCES

Mesoproterozoic Rocks orthopyroxene, K-feldspar, biotite, and accessory titanite andFe-Ti oxides. Orthopyroxene is rimmed Until the 1980s rock units in the Blue by uralitic amphibole and fine-grained biotite. Ridge basement were traditionally mapped Layeingis defined by 0. 1 to approximately I inch as formations. Allen (1963) recognized four (.25 to 2.5 cm) felsic-rich and mafic-rich domains. formations in the Mesoproterczoic rocks of This unit crops out in debris flow scars on the Madison County: these included the Lovingston, southern side of Blakey Ridge and is intruded Marshall, Old Rag, and Pedlar Formations. In by leucogranite (Ylg). Small elongate bodies of Nelson County, approximately 80 miles (-120 gametiferous gneiss (less than 100 square feet) km) southwest of Madison, Bartholomew (1977) also occur within charnockitic rocks (Ych) in the documented that within individual formations western part of the Madison quadrangle and in the there were older gneisses intruded by younger Fletcher quadrangle. These bodies are interpreted plutons. Bartholomew and others (1981) divided to be inclusions included within younger Mesoproterozoic rocks in the central Virginia Mesoproterozoic plutons. Blue Ridge into two massifs: the Pedlar massif to the northwest and the Lovingston massif to the southeast. Each massif included a variety of individual rock types, but the individual Leucocratic Granitoid and Leucocratic Gneiss massifs were interpreted to have been distinct (Ylg): crustal blocks during the Grenville Leucocratic granitoid and gneiss crops out In this model the high-pressure granulite facies in a large area in the central part of the Madison Pedlar massif and the lower-pressure granulite quadrangle. This unit is comprised of a fine- to to amphibolite facies Lovingston massif were medium-grained leucocratic gneiss, coarse-grained juxtaposed along the regionally extensive Rockfish granitic gneiss, and leucogranite pegmatite. The Valley fault zone during Paleozoic Appalachian medium-grained leucocratic gneiss is composed orogenesis (Bartholomew and others, 1931). of white perthitic feldspar and gray-blue , Evans (1991) argued that Mesoproterozoic rocks with minor plagioclase, biotite, titanite, and in the eastern Blue Ridge (Lovingston massif) opaque minerals. Compositional bands are rare, experienced a retrograde metamorphic event that but layers up to 3 feet (1 m) thick with up to 15 altered original , such that the massifs percent biotite are present. The coarse-grained were not necessarily different Grenvillian crustal granite, and leucogranite pegmatite are composed blocks. Bailey and Simpson (1993) noted that the predominately (-90 percent) of white perthitic movement history and displacement across the feldspar and gray-blue quartz. Feldspar forms Rockfish Valley fault zone was inconsistent with euhedral to subhedral grains up 2 inches (5 cm) in the Grenvillian massif model. Recent geologic diameter. Accessory minerals include plagioclase, studies in the central and northern Virginia Blue biotite, epidote, titanite, apatite, magnetite, and Ridge have not employed the formation or massif ilmenite. The fine- to medium-grained gneiss is terminology, rather Mesoproterozoic units are commonly weakly foliated. The coarse-grained distinguished on the basis of rock type and modern leucogranite ranges from poorly foliated to strongly geochronology (Virginia Division of Mineral foliated. All three rock types are commonly found Resources, 1993; Southworth and others 2000: together in the same outcrop (Figure 3). The this study). leucogranite pegmatites consistently cross cut the other leucocratic rocks (Figure 3). The leucocratic granitoid gneiss is interpreted to be a composite Garnetiferous Gneiss (Ygn): felsic pluton. This suite (Ylg) is intruded by dikes The oldest rock recognized in the Madison of biotite-bearing granitoid gneiss (Ybg) (Figure quadrangle is a dark {ay, fine- to coarse- 3), hornblende metagabbro (Zhg), fine-grained layered gneiss with plagioclase, quartz, gamet, metabasalt (Zmb), and diabase (Mzd). PUBLICATION 157

Figure 3. Photograph (a.) and sketch (b.) of subhorizontal outcrop surface exposed in debris flow scar, western Deep Hollow (38.4216" N, 78.318'1' W). Rock types: Ybg- medium-grained biotite-bearing granitoid, Ylgl- coarse- grained foliated leucogranite,Ylg2- medium-grained leucocratic gneiss, Yl93- leucogranite pegmatite. S,- trace of high temperature foliation. Leucogranite pegmatite dike is folded and the S, foliation is axial planar to fold. Biotite- bearing granitoid dikes postdate the deformation that produced the S, foliation. All rock types are cut by narrow deformation zones and faults with apparent dextral offset. Hammer is approximately 1B inches (50 cm) in length.

Biotite-bearing Granitoid Gneiss and Layered (1 to 3 cm) thick felsic layers of plagioclase, Granitoid Gneiss (Ybg): alkali feldspar, and quartz. separated by weakly Areally extensive biotite-bearing granitoid developed layers of biotite, epidote, and quartz. gneiss crops out to the east and west of the Epidote-rich pods and veins up to 3 feet (1 m) Mechum River Formation. This unit is gray to wide are common in the biotite-bearing granitoid grayish black, medium to coarse grained, and gneiss. These epidotized zones are sulrounded by composed of biotite, quartz, alkali feldspar, haloes of biotite granitoid gneiss depleted in mafic plagioclase, epidote, and muscovite, with minor phases (Wadman and others, 1998). titanite, apatite,zircon. and opaque minerals. The Dikes ofbiotite-bearing granitoid (6 inches unit is charucteized by abundant biotite (15 to 25 to more than 5 feet wide) intrude the leucocratic percent) and is distinguished from charnockite granitoid gneiss (Ylg) at a number of locations (Ych) by the absence of pyroxene. Subhedral (Figure 3) and inclusions of leucocratic gneiss are to euhedral alkali feldspar augen (originally common in the biotite-bearing granitoid (Figure megacrysts) are common. Blue to gray quartz 4). No distinct contacts or cross cutting relations forms irregular lenses. The rock is commonly were observed between the charnockite exposed in well foliated and exposures of massive nonfoliated the northwestern part of the Madison quadrangle biotite-bearing granitoid are rare. Biotite-bearing and the biotite-bearing granitoid gneiss. Field granitoids contain 58 to 67 percent SiO, and based and petrographic data indicate that a l.5-mile- on the modal mineralogy and bulk chemistry (-2 km) wide zone of transitional mineralogy have a granitic to granodioritic bulk composition separates the charnockite (Ych) from the biotite- (Berquist, 2000). A medium- to coarse-grained, bearing granitoid (Figure 2). In this transitional layered granitic gneiss underlies Dulaney and zone, orthorhombic pyroxene is progressively Adam Yowell mountains in the northern part of altered to fine-grained uralitic amphibole and the Madison quadrangle and occurs as isolated biotite (Berquist, 2000). For mapping purposes, bodies surrounded by biotite-bearing granitoid. all granitic rocks with modal orthopyroxene were This rock is characterized by 0.5- to l.5-inch- designated charnockite. PUBLICATION 157

Figure 3. Photograph (a.) and sketch (b.) of subhorizontal outcrop surface exposed in debris flow scar, western Deep Hollow (38.4216" N, 78.3181 ' W). Rock types: Ybg- medium-grained biotite-bearing granitoid, Ylgl- coarse- grained foliated leucogranite, Ylg2- medium-grained leucocratic gneiss, Ylg3- leucogranite pegmatite. S,- trace of high temperature foliation. Leucogranite pegmatite dike is folded and the S, foliation is axial planar to fold. Biotite- bearing granitoid dikes postdate the deformation that produced the S, foliation. All rock types are cut by narrow deformation zones and faults with apparent dextral offset. Hammer is approximately 18 inches (50 cm) in length.

Biotite-bearing Granitoid Gneiss and Layered (1 to 3 cm) thick felsic layers of plagioclase, Granitoid Gneiss (Ybg): alkali feldspar, and quartz, separated by weakly Areally extensive biotite-bearing granitoid developed layers of biotite, epidote, and quartz. gneiss crops out to the east and west of the Epidote-rich pods and veins up to 3 feet (l m) Mechum River Formation. This unit is gray to wide are common in the biotite-bearing granitoid grayish black, medium to coarse grained, and gneiss. These epidotized zones are sulrounded by composed of biotite, quartz, alkali feldspar, haloes of biotite granitoid gneiss depleted in mafic plagioclase, epidote, and muscovite, with minor phases (Wadman and others, 1998). titanite, apatite, zircon, and opaque minerals. The Dikes ofbiotite-bearing granitoid (6 inches unit is characterrzed by abundant biotite (15 to 25 to more than 5 feet wide) intrude the leucocratic percent) and is distinguished from charnockite granitoid gneiss (Ylg) at a number of locations (Ych) by the absence of pyroxene. Subhedral (Figure 3) and inclusions of leucocratic gneiss are to euhedral alkali feldspar augen (originally common in the biotite-bearing granitoid (Figure megacrysts) are common. Blue to gray quartz 4). No distinct contacts or cross cutting relations forms irregular lenses. The rock is commonly were observed between the charnockite exposed in well foliated and exposures of massive nonfoliated the northwestern part of the Madison quadrangle biotite-bearing granitoid are rare. Biotite-bearing and the biotite-bearing granitoid gneiss. Field granitoids contain 58 to 67 percent SiO, and based and petrographic data rndicate that a l.5-mile- on the modal mineralogy and bulk chemistry (-2 km) wide zone of transitional mineralogy have a granitic to granodioritic bulk composition separates the chamockite (Ych) from the biotite- (Berquist, 2000). A medium- to coarse-grained, bearing granitoid (Figure 2). In this transitional layered granitic gneiss underlies Dulaney and zone, orthorhombic pyroxene is progressively Adam Yowell mountains in the northern part of altered to fine-grained uralitic amphibole and the Madison quadrangle and occurs as isolated biotite (Berquist, 2000). For mapping pulposes, bodies surrounded by biotite-bearing granitoid. all granitic rocks with modal orthopyroxene were This rock is characterized by 0.5- to l.5-inch- desisnated charnockite. VIRGINIA DIVISION OF MINERAL RESOURCES

Figure 5. Coarse-grained charnockite exposed along Doubletop Mountain (38.4750" N, 78.3658"W).

charnockite cut coarse-grained charnockite. Coarse-grained charnockites in the Madison quadrangle contain 58-61 percent SiO, and have a granitic to granodioritic (farsundite to quartz jotunite for orthopyroxene-bearing granitoids) bulk composition (Berquist, 2000). A l.5-mile- (- 2 km) wide zone of transitional mineralogy, where orthopyroxene is altered to uralitic amphibole and biotite, separates Figure 4. Leucocratic granitoid gneiss (Ylg) inclusions the chamockite from the biotite granitoid gneiss in biotite-bearing granitoid gneiss (Ybg) Note foliation (Ybg) to the southeast. For mapping pu{poses granitoid in leucocratic gneiss is truncated. Length of all granitic rocks with modal orthopyroxene were hammer is approximately 14 inches (35 cm). Outcrop designated charnockite. Tollo and others (2003) is located beside driveway of the North Madison United Methodist Parsonage, off State Route 670, 0.1 mile obtained a U/Pb zircon age of 1,050 + 8 Ma on east of Criglersville (38.4549' N, 78.2974' W). a similar low-silica charnockite exposed near Kinsey Run approximately 1 mile (2 km) west of the Madison quadrangle.

Charnockite (Ych): Coarse-grained Alkali Feldspar Granite (YaI): The mountainous region in the Two small bodies of coarse-grained alkali northwestern part of the Madison quadrangle is feldspar granite occur between Ruth and White underlain by a massive to foliated, coarse-grained Oak Lake in the southern part of the Madison charnockite. This rock is gray-green to dark quadrangle and along the crest of Gaar Mountain. green on fresh surfaces and weathers to a dark This rock is a gray-white, coarse grained, gray and white (Figure 5). The charnockite is composed of alkali feldspar, quartz, plagioclase, equigranular to porphyritic, alkali feldspar granite composed of 0.5- to 2-inch (1-5 cm) white and orthopyroxene with minor amounts of biotite, alkali feldspar and blue-gray qtartz titanite, amphibole, ilmenite, and clinopyroxene. with minor plagioclase, biotite, titanite, and are both perthitic and antiperthitic. At opaque minerals. The alkali feldspar granite is commonly massive, some locations, a weak compositional layering is anastomosing developed. Dikes of medium-grained porphyritic but cut by mica-rich high-strain zones that arc less than 1 inch to 2 foot (2 cm to VIRGINIA DIVISION OF MINERAL RESOURCES

Figure 5. Coarse-grained charnockite exposed along Doubletop Mountain (38.4750' N, 78.3658'W).

chamockite cut coarse-grained charnockite. Coarse-grained charnockites in the Madison quadrangle contain 58-61 percent SiO, and have a granitic to granodioritic (farsundite to quartz jotunite for orlhopyroxene-bearing granitoids) bulk composition (Berquist, 2000). A l.5-mile- (- 2 km) wide zone of transitional mineralogy, where orthopyroxene is altered to uralitic amphibole and biotite, separates Figure 4. Leucocratic granitoid gneiss (Ylg) inclusions the chamockite from the biotite granitoid gneiss in biotite-bearing granitoid gneiss (Ybg) Note foliation (Ybg) to the southeast. For mapping pulposes in leucocratic granitoid gneiss is truncated. Length of all granitic rocks with modal orthopyroxene were hammer is approximately 14 inches (35 cm). Outcrop designated chamockite. Tollo and others (2003) is located beside driveway of the North Madison United Methodist Parsonage, off State Route 670, 0.1 mile obtained a U/Pb zicon age of 1,050 + 8 Ma on east of Criglersville (38.4549' N, 78.2974' W). a similar low-silica chamockite exposed near Kinsey Run approximately I mile (2 km) west of the Madison quadrangle.

Charnockite (Ych): Coarse-grained Alkali Feldspar Granite (Yaf): The mountainous region in the Two small bodies of coarse-grained alkali northwestern part of the Madison quadrangle is feldspar granite occur between Ruth and White underlain by a massive to foliated, coarse-grained Oak Lake in the southem part of the Madison charnockite. This rock is gray-green to dark quadrangle and along the crest of Gaar Mountain. green on fresh surfaces and weathers to a dark This rock is a gray-white, coarse grained, gray and white (Figure 5). The charnockite is equigranular porphyritic, feldspar composed of alkali feldspaq quartz, plagioclase, to alkali granite composed of 0.5- to 2-inch (1-5 cm) white and orlhopyroxene with minor amounts of biotite, alkali feldspar and blue-gray quarlrz with minor titanite, amphibole, ilmenite, and clinopyroxene. plagioclase, biotite, titanite, and opaque minerals. Feldspars are both perthitic and antiperthitic. At The alkali feldspar granite is commonly massive, some locations, a weak compositional layering is anastomosing high-strain developed. Dikes of medium-grained porphyritic but cut by mica-rich zones that are less than 1 inch to 2 foot (2 cm to PUBLICATION 157

0.6 m) wide. Alkali feldspar granite dikes 4 inches to 10 feet (0.1 to 3 m) wide cut the leucocratic granitoid gneiss and biotite-bearing granitoid. Dikes of hornblende metagabbro (Zhg intrude this unit. The coarse-grained alkali feldspar granite is similar to the granite exposed on ,3 miles (5 k-) to the north of the Madison quadrangle. Tollo and others (2003) report a U/Pb zircon age of 1,060 + 5 Ma for the Old Rag granite.

Neoproterozoic Rocks

Robertson River Isneous Suite Figure 6. Network of alkali feldspar syenite dikes (Zgr3) cutting biotite-bearing granitoid gneiss (Ybg). Outcrop located in the channel of the Robinson River The Robertson a River Igneous Suite is (38.4457" N, 78.2667" W). Field of view is 6 by 10 feet compositionally distinct group of 705 to 735 Ma (2 by 3 m). granitoid plutons that intrude Mesoproterozoic granitoid gneisses in central and northern Virginia (Tollo and Aleinikoff, 1996). These plutons are of Tollo and Lowe (1994). Tollo and Aleinikoff A-type granitoids (a chemically distinct group (1996) obtained a UlPb zircon age of 730 + 4 of anorogenic igneous intrusive rocks (Whalen Ma from exposures of the Arrington Mountain and others, 1987)). The main Robertson River granite approximately 0.1 mile (0.2 km) east of batholith is an elongate body, 70 miles (120 km) the Madison quadrangle. in length, extending from near Charlottesville in the south to near Ashby Gap in the north. These Alkali Feldspar Granite (Zgr2): granitic rocks were originally named and described A distinctive alkali feldspar granite crops by Allen (1963) for exposures along the Robinson out in a naffow northeast-southwest trending belt River in Madison County (note: the Robinson in the southeastern part of the Madison quadrangle. River was misprinted as "Robertson River" on This rock is intruded by alkali feldspar syenite older topographic maps). The Robertson River (Zgr3) and dikes of hornblende metagabbro granitic rocks are associated with an early (700 to (Zhg. This unit is a light-gray to gray, fine- to 750 Ma) phase of continental rifting that affected coarse-grained, inequigranular alkali feldspar eastern North America (Rankin and others, 1989; granite and composed of alkali feldspar, qtartz, Tollo and Aleinikoff. 1 996). plagioclase, amphibole, and biotite. Much of this unit is strongly deformed and forms awell-foliated Granite and Alkali Feldspar Granite (Zgrl)z mylonitic rock in the White Oak Run high-strain A Eray, medium- to coarse-grained, zone. This unit is equivalent to the White Oak generally massive, equigranular granite and alkali feldspar granite of Tollo and Lowe (1994).

alkali feldspar granite crops out near Haywood Tollo and Aleinikoff ( 1 996) obtained a U/Pb zir con and is extensively exposed in the Brightwood age of 724 + 3 Ma from exposures of the White quadrangle to the east of the Madison quadrangle. Oak alkali feldspar granite approximately 0.5 mile This unit is composed of alkali feldspar, quartz, (0.8 km) east of the Madison quadrangle. plagioclase, and amphibole with accessory biotite, allanite, and Fe-Ti oxides. The contact between Alkali Feldspar Syenite (Zgr3)z this unit (Zgrl) and the alkali feldspar syenite Alkali feldspar syenite crops out in two (Zgr3) is not exposed. This unit is equivalent areas in the southeastern part of the Madison to the Arrington Mountain alkali feldspar granite quadrangle. This rock is light-gray, coarse- PUBLICATION 157

0.6 m) wide. Alkali feldspar granite dikes 4 inches to 10 feet (0.1 to 3 m) wide cut the leucocratic granitoid gneiss and biotite-bearing granitoid. Dikes of hornblende metagabbro (Zhg intrude this unit. The coarse-grained alkali feldspar granite is similar to the granite exposed on Old Rag Mountain, 3 miles (5 km) to the north of the Madison quadrangle. Tollo and others (2003) report a U/Pb zircon age of 1,060 + 5 Ma for the Old Rag granite.

Neoproterozoic Rocks

Robertson River Isneous Suite Figure 6. Network of alkali feldspar syenite dikes (Zgr3) cutting biotite-bearing granitoid gneiss (Ybg) Outcrop located in the channel of the Robinson River Robertson The River Igneous Suite is a (38.4457" N, 78.2667' W). Field of view is 6 by 10 feet compositionally distinct group of 705 to 735 Ma (2 by 3 m). granitoid plutons that intrude Mesoproterozoic granitoid gneisses in central and northerrr Virginia (Tollo and Aleinikoff, 1996). These plutons are of Tollo and Lowe (1994). Tollo and Aleinikoff A-type granitoids (a chemically distinct group (1996) obtained a UlPb zfucon age of 730 + 4 of anorogenic igneous intrusive rocks (Whalen Ma from exposures of the Arrington Mountain and others, 1987)). The main Robertson River granite approximately 0.1 mile (0.2 km) east of batholith is an elongate body, 70 miles (120 km) the Madison quadrangle. in length, extending from near Charlottesville in the south to near Ashby Gap in the north. These Alkali Feldspar Granite (Zgr2): granitic rocks were originally named and described A distinctive alkali feldspar granite crops by Allen (1963) for exposures along the Robinson out in a nalrow northeast-southwest trending belt River in Madison County (note: the Robinson in the southeastem part of the Madison quadrangle. River was misprinted as "Robertson River" on This rock is intruded by alkali feldspar syenite older topographic maps). The Robertson River (Zgr3) and dikes of hornblende metagabbro granitic rocks are associated with an early (700 to (ZhS). This unit is a lighrgray to gray, fine- to 750 Ma) phase of continental rifting that affected coarse-grained, inequigranular alkali feldspar eastern North America (Rankin and others, 1989; granite and composed of alkali feldspar, quartz,

Tollo and Aleinikoff. 1 996). plagioclase, amphibole, and biotite. Much of this unit is strongly deformed and forms a well-foliated Granite and Alkali Feldspar Granite (Zgrl): mylonitic rock in the White Oak Run high-strain A Eray, medium- to coarse-grained, zone. This unit is equivalent to the White Oak generally massive, equigranular granite and alkali feldspar granite of Tollo and Lowe (1994). alkali feldspar granite crops out near Haywood Tollo andAleinikoff (1996) obtained a U/Pb zircon and is extensively exposed in the Brightwood age of 124 + 3 Ma from exposures of the White quadrangle to the east of the Madison quadrangle. Oak alkali feldspar granite approximately 0.5 mile This unit is composed of alkali feldspar, quartz, (0.8 km) east of the Madison quadrangle. plagioclase, and amphibole with accessory biotite, allanite, and Fe-Ti oxides. The contact between Alkali Feldspar Syenite (Zgr3)z this unit (Zgrl) and the alkali feldspar syenite Alkali feldspar syenite crops out in two (Zgr3) is not exposed. This unit is equivalent areas in the southeastern part of the Madison to the Arrington Mountain alkali feldspar granite quadrangle. This rock is light-gray, coarse- VIRGINIA DIVISION OF MINERAL RESOURCES

grained to locally pegmatitic and composed Metaconglomerate (Zmc) : of alkali feldspar, quartz, amphibole, and Green-gray, gravel and cobble plagioclase, with minor biotite and fluorite. The conglomerate crops out as two lenticular bodies in alkali feldspar syenite is commonly unfoliated the Madison quadrangle. The metaconglomerate and massive. Dikes of alkali feldspar syenite cut contains subrounded to subangular clasts (up to biotite-bearing granitoid gneiss (Ybg) (Figure 6) 4 inches (10 cm) in diameter) of leucogranitic and dikes of hornblende metagabbro (Zhg) cut gneiss and granitoid in a medium-grained sandy the alkali feldpsar syenite. This unit is equivalent matrix of arkosic composition (Figure 7). Beds to the Hitt Mountain alkali feldspar syenite of are 4 to 40 inches (10 cm to 1 m) thick. The Tollo and Lowe (1994). Tollo and Aleinikoff metaconglomerate crops out at the base of the (1996) obtained a U/Pb zircon age of 706 + 2 Ma Mechum River Formation and nonconformablv from exposures of the Hitt Mountain syenite near overlies Mesoprote r ozoic basement. Ruckersville, 10 miles (16 km) southwest of the Madison quadrangle.

BLUE RIDGE COVER SEQUENCE

Neoproterozoic Rocks

Mechum River Formation

The Mechum River Formation crops out in a narrow belt that extends from Batesville, in Albemarle County, 65 miles (100 km) to the northeast into Rappahannock County. The Mechum River Formation nonconformably overlies Mesoproterozoic basement (Ylc, Ybg), Figure 7. Mechum River Formation metaconglomerate contains clasts of Mesoproterozoic basement along Hatter Run (38.3888' N, 78.3354" W). Knife is 3 inches (8 cm) long. and 730 to 705 Ma Robertson River granitoid, and is intruded by dikes and sills of hornblende metagabbro (Zhg and metabasalt (Zmb). These relations are consistent with a depositional age of approximately 700 Ma for the Mechum River Meta-arkose and Feldspathic Meta-wacke Formation. Bailey and Peters (1998) interpreted (Zma)z the basal sedimentary rocks of the Mechum River Meta-arkose and feldspathic meta-wacke Formation inAlbemarle County to be deposited in crop out in the south-central part of the Madison a glaciomarine setting; to the north of Albemarle quadrangle. These rocks are gray-brown, thin to County they interpreted the Mechum River massively bedded, medium- to coarse-grained. Formation to be of fluvial origin. In the Madison Detrital clasts are predominantly white alkali quadrangle, the Mechum River Formation crops feldspar and blue-gruy quartz with accessory out in two northeast-southwest trending belts, titanite and Fe-Ti oxides in a matrix composed 0.1 to 0.8 mile (0.25 to 1.2 km) wide, that join of metamorphic biotite, muscovite, quartz, and near Ruth. Three lithotypes were mapped in the epidote. Sedimentary structures include planar Mechum River Formation and the total thickness cross beds, graded beds, parallel laminations, and ofthis unit is estimated to be at least 1,400 feet (400 scoured beds (Figure 8). Cross-stratified meta- meters). The top of the Mechum River Formation arkoses are well exposed in Polecat Branch near is not exposed in the Madison quadrangle. Ruth. VIRGINIA DIVISION OF MINERAL RESOURCES

grained to locally pegmatitic and composed Metacon glom erate (Zmc): of alkali feldspar, quartz, amphibole, and Green-gray, gravel and cobble plagioclase, with minor biotite and fluorite. The conglomerate crops out as two lenticular bodies in alkali feldspar syenite is commonly unfoliated the Madison quadrangle. The metaconglomerate and massive. Dikes of alkali feldspar syenite cut contains subrounded to subangular clasts (up to biotite-bearing granitoid gneiss (Ybg) (Figure 6) 4 inches (10 cm) in diameter) of leucogranitic and dikes of hornblende metagabbro (Zhg) cut gneiss and granitoid in a medium-grained sandy the alkali feldpsar syenite. This unit is equivalent matrix of arkosic composition (Figure 7). Beds to the Hitt Mountain alkali feldspar syenite of are 4 to 40 inches (10 cm to 1 m) thick. The Tollo and Lowe (1994). Tollo and Aleinikoff metaconglomerate crops out at the base of the (1996) obtained a U/Pb zircon age of 706 + 2 Ma Mechum River Formation and nonconformably from exposures of the Hitt Mountain syenite near overlies Mesoproterozoic basement. Ruckersville, 10 miles (16 km) southwest of the Madison quadrangle.

BLUE RIDGE COVER SEQUENCE

Neoproterozoic Rocks

Mechum River Formation

The Mechum River Formation crops out in a narrow belt that extends from Batesville, in Albemarle County, 65 miles (100 km) ro the northeast into Rappahannock County. The Mechum River Formation nonconformably overlies Mesoproterozoic basement (Ylc, Ybg), Figure 7. Mechum River Formation metaconglomerate contains clasts of Mesoproterozoic basement along Hatter Run (38.3888' N, 78.3354' W). Knife is 3 inches (8 cm)long. and 130 to 705 Ma Robertson River granitoid, and is intruded by dikes and sills of hornblende metagabbro (Zhg and metabasalt (Zmb). These relations are consistent with a depositional age of approximately 700 Ma for the Mechum River Meta-arkose and Feldspathic Meta-wacke Formation. Bailey and Peters (1998) interpreted (Zma): the basal sedimentary rocks of the Mechum River Meta-arkose and feldspathic meta-wacke Formation inAlbemarle County to be deposited in crop out in the south-central part of the Madison a glaciomarine setting; to the north of Albemarle quadrangle. These rocks are gray-brown, thin to County they interpreted the Mechum River massively bedded, medium- to coarse-grained. Formation to be of fluvial origin. In the Madison Detrital clasts are predominantly white alkali quadrangle, the Mechum River Formation crops feldspar and blue-gray quartz with accessory out in two noftheast-southwest trending belts, titanite and Fe-Ti oxides in a matrix composed 0.1 to 0.8 mile (0.25 to 1.2 km) wide, that join of metamorphic biotite, muscovite, quartz, and near Ruth. Three lithotypes were mapped in the epidote. Sedimentary structures include planar Mechum River Formation and the total thickness cross beds, graded beds, parallel laminations, and ofthis unit is estimated to be at least 1,400 feet (400 scoured beds (Figure 8). Cross-stratified meta- meters). The top of the Mechum River Formation arkoses are well exposed in Polecat Branch near is not exposed in the Madison quadrangle. Ruth. PUBLICATION 157

form a differentiated sill that intruded both the Mesoproterozoic basement and the Mechum River Formation. The ultramaifc rock is considered a cumulate that formed towards the base of this body.

Hornblende Metagabbro (Zhg): Homblende metagabbro occurs as dikes and sills 2 to 500 feet (0.5 to 150 meters) in thickness that intrude the Mechum River Formation, Robertson River Igneous Suite, and Mesoproterozoic granitoids and gneisses. The hornblende metagabbro is greenish-black, Figure 8. Mechum River Formation meta-arkose south medium- to fine-grained, and ranges from of Burnt Ridge (38.3775' N, 78.3364" W). Knife is 3 massive to foliated. Hornblende and plagioclase inches (B cm) long. are the dominant minerals in this rock with accessory qtarlz, biotite, chlorite, titanite, and opaque minerals. Homblende metagabbros in Metasiltstone and Phyllite (Zms): the Madison area contain 47 to 51 percent SiO, This unit is composed of gray-brown and are tholeiitic in bulk chemical composition (Weiss, laminated to thinly bedded metasiltstone and 2000). The hornblende metagabbros are phyllite with a penetrative foliation. The rock mineralogically distinct from metabasalt (Zmb) is composed of quartz, alkah feldspar, titanite, dikes exposed in the westem part of the Madison opaque minerals, and plagioclase clasts with quadrangle. Unpublished A10/Ar3e dating (by the elongate muscovite, biotite, quatliz, and chlorite authors) of blocky homblende grains indicates defining a penetrative foliation. Rocks with that hornblende metagabbros may be 680 to field biotite and porphyroblasts are common at 700 Ma; these ages are consistent with (2000) some locations. Thin beds (less than I foot thick) relations. Weiss interpreted the hornblende pulse of feldspathic meta-wacke occur throughout this metagabbro to be associated with an early of unit. Iapetan -related magmatism and not related to Catoctin-age magmatism (-570 Ma). TABULAR INTRUSIVE ROCKS

Altered Ultramafic Rock (Zum): Metabasalt (Zmb): An altered ultramafic rock crops out on Grayish-green to blue-green, fine- to Gallihugh Mountain beneath a tabular body medium-grained, foliated to massive, porphyritic of homblende metagabbro (Zhg. The altered to equigranular metabasalt: composed of chlorite, ultramafic rock is greenish-gray, medium to fine epidote, actinolite, and plagioclase. Metabasalt grained and weakly foliated and composed of occurs as dikes I to ll feet (0.3 to 5 meters) actinolite, epidote, chlorite, and magnetite. This in thickness with a wide range of orientations. rock contains 45 percent SiO2, 14 percent MgO, Metabasalt dikes in the Madison quadrangle and has a normative composition of an olivine crop out within and to the west of the Mechum websterite (Weiss, 2000). The altered ultramafic River Formation and may be feeder dikes for the rock occurs at the base of a concordant body Neoproterozoic Catoctin Formation in Shenandoah 0.4 mile (600 meters) in length and 300 feet (90 National Park located to the west and north of the (1988) meters) in thickness. We interpret the hornblende Madison quadrangle. Badger and Sinha + metagabbro and altered ultramafic rock on obtained a Rb/Sr age of 510 36 Ma on Catoctin (65 Gallihugh Mountain to be genetically related and metabasalt about 40 miles km) southwest of PUBLICATION 157

form a differentiated sill that intruded both the Mesoproterozoic basement and the Mechum River Formation. The ultramaifc rock is considered a cumulate that formed towards the base of this body.

Hornblende Metagabbro (Zhg) : Hornblende metagabbro occurs as dikes and sills 2 to 500 feet (0.5 to 150 meters) in thickness that intrude the Mechum River Formation, Robertson River Igneous Suite, and Mesoproterozoic granitoids and gneisses. The hornblende metagabbro is greenish-black, Figure 8. Mechum River Formation meta-arkose south medium- to fine-grained, and ranges from of Burnt Ridge (38.3775' N, 78.3364" W). Knife is 3 massive to foliated. Hornblende and plagioclase inches (8 cm)long. are the dominant minerals in this rock with accessory quartz, biotite, chlorite, titanite, and opaque minerals. Hornblende metagabbros in Metasiltstone and Phyllite (Zms): the Madison area contain 47 to 51 percent SiO, This unit is composed of gray-brown and are tholeiitic in bulk chemical composition laminated to thinly bedded metasiltstone and (Weiss, 2000). The hornblende metagabbros are phyllite with a penetrative foliation. The rock mineralogically distinct from metabasalt (Zmb) is composed of quartz, alkali feldspar, titanite, dikes exposed in the western part of the Madison opaque minerals, and plagioclase clasts with quadrangle. Unpublished Ara0lAr3e dating (by the elongate muscovite, biotite, quartz, and chlorite authors) of blocky hornblende grains indicates defining a penetrative foliation. Rocks with that hornblende metagabbros may be 680 to biotite and garnet porphyroblasts are common at 700 Ma; these ages are consistent with field some locations. Thin beds (less than I foot thick) relations. Weiss (2000) interpreted the hornblende of feldspathic meta-wacke occur throughout this metagabbro to be associated with an early pulse of urut. Iapetan rift-related magmatism and not related to Catoctin-age magmatism (-570 Ma). TABULAR INTRUSIVE ROCKS

Altered Ultramafic Rock (Zum)z Metabasalt (Zmb): An altered ultramafic rock crops out on Grayish-green to blue-green, fine- to Gallihugh Mountain beneath a tabular body medium-grained, foliated to massive, porphyritic of hornblende metagabbro (Zhg). The altered to equigranular metabasalt: composed of chlorite, ultramafic rock is greenish-gray, medium to fine epidote, actinolite, and plagioclase. Metabasalt grained and weakly foliated and composed of occurs as dikes 1 to 17 feet (0.3 to 5 meters) actinolite, epidote, chlorite, and magnetite. This in thickness with a wide range of orientations. rock contains 45 percent SiO2, 14 percent MgO, Metabasalt dikes in the Madison quadrangle and has a normative composition of an olivine crop out within and to the west of the Mechum websterite (Weiss, 2000). The altered ultramafic River Formation and may be feeder dikes for the rock occurs at the base of a concordant body Ne oprotero zoic C ato ctin F ormation in Shenando ah 0.4 mile (600 meters) in length and 300 feet (90 National Park located to the west and north of the meters) in thickness. We interpret the hornblende Madison quadrangle. Badger and Sinha (1988) + metagabbro and altered ultramafic rock on obtained a Rb/Sr age of 510 36 Ma on Catoctin (65 Gallihugh Mountain to be genetically related and metabasalt about 40 miles km) southwest of 10 VIRGINIA DIVISION OF MINERAL RESOURCES

the Madison quadrangle. Aleinikoff and other high-strain zones. From southeast to northwest (1995) obtained a U/Pb age for zircon of 564 + 12 these include the 1) White Oak Run high-strain Ma in rhyolitic rocks from the Catoctin Formation zone, 2) Mechum River fault system, and 3) in Maryland. Quaker Run and Champlain Valley high-strain Mesozoic Rocks zones (Figure 9). These structures parallel the regional Blue Ridge trend and strike northeast- Diabase (Mzd): southwest. Mitra (1977, 1979) first studied the Diabase occurs as dikes ranging from 5 to kinematic and mechanical significance of high- 50 feet (2 to 15 meters) in thickness. Diabase dikes strain zones (ductile deformation zones) in the generally strike in a north-northwesterly direction Virginia Blue Ridge. Minor transverse faults and have subvertical contacts. At many locations, (with possible components of strike-slip and dikes are not traceable beyond a single outcrop. dip-slip displacement) strike northwest-southeast This rock is grayish black, weathers to orange- and are subvertical. These faults are evident brown, and is fine to medium grained. Diabase where the Mechum River Formation is offset, but is massive and unfoliated. Plagioclase and augite presumably cut the basement complex. (with an ophitic texture) are the dominant minerals; magnetite is a common accessory mineral. Dike intrusion is inferred to have occurred during the early Jurassic (-200 Ma) based on a0Ar/3eAr ages of related igneous rocks in the Culpeper Mesozoic basin 20 miles (30 km) east of the Madison quadrangle (Kunk and others, 1992).

STRUCTURAL GEOLOGY

The Madison quadrangle is located in the core of the Blue Ridge anticlinorium, a regional scale structure that extends from near Roanoke, Virginia, northeastward into south-central . In northern Virginia and Maryland, the anticlinorium plunges gently to the northeast. The contactbetweenthe Mesoproterozoic basement rocks and Neoproterozoic metasedimentary rocks of the Lynchburg Group on the southeast limb of the anticlinorium is located approximately 3 miles (5 km) southeast of the Madison quadrangle in the Brightwood quadrangle (Figure 1). The contact between the Mesoproterozoic basement and overlying Neoproterozoic Catoctin Formation on the northwest limb of the anticlinorium is located approximately 3 miles (5 km) northwest of the Madison quadrangle in the Fletcher and Big Meadows quadrangles (Figure 1). Figure 9. Generalized structural map of the Madison quadrangle. Y- Mesoproterozoic basement complex, HIGH-STRAIN ZONES AND FAULTS Zgr- Robertson River granitoids, Zs- Mechum River Formation, CVHSZ- Champlain Valley high-strain zone, MRFS- Mechum River fault system, QRHSZ- Rocks in the Madison quadrangle are Quaker Run high-strain zone, and WORHSZ- White affected by a number of major faults and ductile Oak Run high-strain zone. PUBLICATION 157 t1

o

8/.g

poles to foliation (n = 9; u elongation lineations (n = 3)

Figure 10. Stereogram of fabric elements in the White Oak Run high-strain zone.

Figure '1'1 . Polished slab of narrow mylonite zone from The White Oak Run high-strain zone, in the White Oak Run high-strain zone. Mylonite derived the southeastern part of the Madison quadrangle, from hornblende-bearing alkali feldspar syenite. The slab is a near-vertical section. Asymmetry between is a northeast-southwest striking zone of mylonitic foliation and zone boundary indicates toptothe- rocks that dips steeply to the northwest (Figures 9 northwest (hanging wall down/extensional) sense of and 10). The White Oak Run high-strain zone cuts shear. granitoids of the Roberston River Igneous Suite (Zgr2,, Zgr3), homblende metagabbros (Zhg), and Mesoproterozoic basement (Ybg). Mylonitic movement) (Figure 11). Microstructural textures rocks are exposed in a belt up to 1,000 feet (300 and garnet-biotite geothermometry indicate that meters) wide, butthe deformation is heterogeneous the White Oak Run high-strain zone developed at and lenses of weakly deformed to undeformed amphibolite facies conditions (-500' C) (Knight, rock occur within this zone. Allen (1963) mapped 1999). Knight and Bailey (1999) and Bailey and a gravrty (normal) fault along the topographic Simpson (1993) suggested that the White Oak lineament defined by the northeast-southwest Run high-strainzone is a Neoproterozoic structure trending White Oak Run valley. The topographic associated with Iapetan rifting. lineament is approximately parallel to the zone of The Mechum River fault system places mylonitic rock. Mylonitic rocks are characterized Mesoproterozorc basement units and Robertson by a strong foliation and down-dip mineral River granitoids on the Neoproterozoic Mechum lineation defined by needle-like hornblende grains River Formation (Figure 9). These faults dip and elongate quartzand feldspar grains. Kinematic steeply to the southeast (75" to 80' based on field indicators consistently record top-to-the-northwest observations) and regionally cut bedding in the sense of shear (hanging wall dowrVextensional Mechum River Formation. The contact between PUBLICATION 157 ll

I poles to foliation (n = 9) o elongation lineations (n = 3)

Figure 10. Stereogram of fabric elements in the White Oak Run high-strain zone.

Figure 11 . Polished slab of narrow mylonite zone from The White Oak Run high-strain zore, rn the White Oak Run high-strain zone. Mylonite derived the southeastern part of the Madison quadrangle, from hornblende-bearing alkali feldspar syenite. The slab is a near-vertical section. Asymmetry between is a norlheast-southwest striking zone of mylonitic foliation and zone boundary indicates top-tothe- rocks that dips steeply to the norlhwest (Figures 9 northwest (hanging wall down/extensional) sense of and l0). The White Oak Run high-strain zone cuts shear. granitoids of the Roberston River Igneous Suite (Zgr2, Zgr3), hornblende metagabbros (Zhg), and Mesoproterozoic basement (Ybg). Mylonitic movement) (Figure 11). Microstructural textures rocks are exposed in a belt up to 1,000 feet (300 and gamet-biotite geothermometry indicate that meters) wide, but the deformation is heterogeneous the White Oak Run high-strain zone developed at and lenses of weakly deformed to undeformed amphibolite facies conditions (-500' C) (Ituight, rock occur within this zone. Allen (1963) mapped 1999). Knight and Bailey (1999) and Bailey and a graviry (normal) fault along the topographic Simpson (1993) suggested that the White Oak lineament defined northeast-southwest by the Run hi gh- s tr ain zone i s a Neoproterozoic structure trending White Oak Run valley. The topographic associated with Iapetan rifting. lineament is approximately parallel to the zone of The Mechum River fault system places mylonitic rock. Mylonitic rocks are characterrzed Mesoproterozoic basement units and Robertson by a strong foliation and down-dip mineral River granitoids on the Neoproterozoic Mechum lineation defined by needle-like hornblende grains River Formation (Figure 9). These faults dip and elongate quartzand feldspar grains. Kinematic steeply to the southeast (75" to 80' based on field indicators consistently record top-to-the-nofthwe st observations) and regionally cut bedding in the sense of shear (hanging wall down/extensional Mechum River Formation. The contact between t2 VIRGINIA DIVISION OF MINERAL RESOURCES

Mechum River metasiltstones and structurally This foliation strikes northeast-southwest, dips overlying leucocratic basement rocks is exposed moderately to steeply to the southeast, and is axial in the Robinson River approximately 20 feet (6 m) planar with respect to folds in the Mechum River downstream from the Route 23Ibridge. Shotwell Formation (Figure 12b). and Bailey (2000) reported that metasedimentary The 0.3- to 0.4-mile- (0.5 to 0.7 km) rocks of the Mechum River Formation were wide Champlain Valley high-strain zone and the folded prior to movement along these faults. 0.6- to l.0-mile- (1 to 1.6 km) wide Quaker Run Mylonitic rocks are not cofitmon and movement high-strain zone cut the basement complex in on this predominantly brittle fault system may the northwestem part of the Madison quadrangle post-date deformation associated with the ductile (Figure 9). These zones strike northeast- high-strain zones. southwest, dip moderately to the southeast, and In the Ruth area, two en-echelon faults are charccterized by protomylonite, mylonite, separate the belts exposing the Mechum River and rare ultramylonite derived primarily from Formation. Gooch (1958) first recognized this massive charnockitic rocks. Lenses of weakly structure and interpreted it to be an anticline foliated to massive charnockite occur within the exposing basement in its core. Our mapping does high-strain zones. The Champlain Valley high- not support the anticline interpretation; rather strain zone was first described by Mitra and Lukert faulting duplicates the Mechum River Formation. (1982) from exposures in the Old Rag Mountain Within the Mechum River belt, bedding is folded quadrangle approximately 1 mile (1.6 km) north into steeply dipping panels with different overall of the Madison quadrangle. The Champlain strikes (Figure 12a). Folds are open to tight and Valley and Quaker Run high-strain zones link commonly have southeast dipping axial planes up to other anastomosing high-strain zones in (Shotwell and Bailey, 1999), Metasedimentary the Blue Ridge and are interpreted to be part of rocks of the Mechum River Formation commonly a group of Paleozoic high-strain zones across display a penetrative foliation defined by aligned which the Blue Ridge basement was shortened biotite, muscovite, and elongate detrital grains. (Mitra, 1979;' Balley and Simpson, 1993; Bailey

i: .' l"g' /rV + /6/,P ,4a" (d)/s- ,

Figure 12a. Stereogram of poles to bedding in the Mechum River Formation. b. Stereogram of poles to the penetrative foliation defined by aligned biotite, muscovite, and elongate detrital grains in the Mechum River Formation. PUBLICATION 157 13 and others, 1994). Foliations dip moderately to FOLIATIONS the southeast and a downdip elongation lineation is commonly present (Figure 13). Asymmetric Rocks exposed in the Madison quadrangle structures consistently record top-to-the-northwest have been affected by multiple episodes of (hanging wall up/ contractional movement) sense deformation. The oldest structures are foliations of shear (Figure 14). Mylonitic rocks in the and compositional bands developed in the Quaker Run high-strain zone record sectional Mesoproterozoic gneissic units. This fabric strain ratios of 3:1 to 15:1 (based on quartz grain is defined by aligned feldspars and quartz shape measurements) and strain integration across aggregates. Individual grains have a very weak the zone indicates a total displacement of 1.0 + grain shape preferred orientation with straight 0.3 mile (1.5 + 0.5 km) (Figure 15) (Berquist and grain boundaries. Microstructures that define Bailey, 2000). Mylonitic rocks in the Champlain this foliation are consistent with high temperature Valley and Quaker Run zones are charactefizedby conditions (greater than 600' C) at the upper ribbons of quartz that preserve evidence of ductile deformation and feldspars that are pervasively fractured and altered to micas along grain boundaries. These microstructures are consistent with abundant solution transfer under greenschist facies (-350' to 400" C) conditions during deformation (Bailey and others, 1994; Passchier and Trouw. 1996).

1d** ** * &! * & w!*ax#u*% !!r ;-. $$n.t4FS n "* ou tE o - * fltrkE $: o *l 5r+ - , n o x@6@

Figure 14a. Near vertical surface of outcrop that exposes mylonite from the Quaker Run high- x poles to mylonitic foliation (n = 76 ) strain zone. Note well-developed foliation and o elongation lineations (n 38 feldspar porphyroclasts with toptothe-northwest = ) (contractional) sense of shear (38.4432" N, 78.3686" W). Mylonite derived from a charnockitic protolith. b. Figure 13. Stereogram of fabric elements in the Slab of protomylonite from the Quaker Run high-strain Champlain Valley and Quaker Run high-strain zones. zone with fractured feldspars porphyroclasts (f), quartz Mean foliation (great circle) 036' 59" SE. lenses (q), and mica-rich domains (m). PUBLICATION 157 13

and others, 1994). Foliations dip moderately to FOLIATIONS the southeast and a downdip elongation lineation is commonly present (Figure l3). Asymmetric Rocks exposed in the Madison quadrangle structures consistently record top-to-the-northwe st have been affected by multiple episodes of (hanging wall up/ contractional movement) sense deformation. The oldest structures are foliations of shear (Figure 14). Mylonitic rocks in the and compositional bands developed in the Quaker Run high-strain zone record sectional Mesoproterozoic gneissic units. This fabric strain ratios of 3:1 to 15:1 (based on quartz grain is defined by aligned feldspars and quartz shape measurements) and strain integration across aggregates. Individual grains have a very weak the zone indicates a total displacement of 1.0 + grain shape preferred orientation with straight 0.3 mile (1.5 + 0.5 km) (Figure 15) (Berquist and grain boundaries. Microstructures that define Bailey, 2000). Mylonitic rocks in the Champlain this foliation are consistent with high temperature Valley and Quaker Run zones are characterizedby conditions (greater than 600" C) at the upper ribbons of quartz that preserve evidence of ductile deformation and feldspars that are pervasively fractured and altered to micas along grain boundaries. These microstructures are consistent with abundant solution transfer under greenschist facies (-350" to 400' C) conditions during deformation (Bailey and others, 1994; Passchier and Trouw. 1996).

K *xs lltt

'X r-@ ns @ -o"*o m6is+

66 @

IB

Figure 14a. Near vertical surface of outcrop that exposes mylonite from the Quaker Run high- * poles to mylonitic foliation (n = 76 ) strain zone. Note well-developed foliation and o elongation lineations (n 38 feldspar porphyroclasts with toptothe-northwest = ) (contractional) sense of shear (38.4432" N, 78.3686' W). Mylonite derived from a charnockitic protolith. b. Figure 13. Stereogram of fabric elements in the Slab of protomylonite from the Quaker Run high-strain Champlain Valley and Quaker Run high-strain zones. zone with fractured feldspars porphyroclasts (f), quarlz Mean foliation (great circle) 036' 59' SE. lenses (q), and mica-rich domains (m). T4 VIRGINIA DIVISION OF MINERAL RESOURCES

Quaker Run high-strain zone Mylonitic Northwest Southeast hand sample

D-dydw 0 D - (1.5) (0,7 mi) which simpllfies to D-YW using mean shear strain Displacement = -1 mi

Figure 15. Diagram illustrating strain integration technique for estimating displacement (D) across the Quaker Run high-strain zone. w = width of high-strain zone normal to its boundaries, y = shear strain. strain ratios were estimated from quartz grain shapes in mylonites. Strain ratios are used to determine shear strain. Shear strain is integrated over the width of the zone to determine displacement.

a. b. '.t \, .I I r I rl r r ,tt

+a{ oV I .dz -rll - rr I {ry

(n = 55) (n = 73)

Figure 16a. Stereogram of poles to foliation for hightemperature (amphibolite to greenschist facies) foliation. b. Stereogram of poles to foliation for low-temperature (greenschist facies) foliation developed in Mesoproterozoic basement rocks away from the high-strain zones. PUBLICATION 157 15 amphibolite to granulite facies (Passchier and Trouw, 1996). Regionally this fabric has been interpreted to be associated with deformation associated with Grenvillian orogenesis (1,200- 1,000 Ma) (Bailey and Simpson,L993;Aleinikoff and others, 2000). In the Madison quadrangle, this foliation generally strikes approximately east- west and dips steeply to both the north and south (Figure 16a). Folding of this foliation is rarely observed, but at some outcrops is axial planar to folds developed in competent layers such as coarse-grained leucogranitic dikes (Figure l1). This foliation is cut and overprinted by a lower Figure 18. Photograph of hightemperature fabric temperature foliation defined by greenschist (S,, foliation and compositional bands) in leucocratic facies metamorphic minerals and microstructures granitic gneiss (Ylg1) and leucrocratic gneiss (Ylg2) (Figure 18). cut and offset by 2-inch- (5 cm) wide deformation zone The younger foliation, defined by aligned (Sr) defined by aligned greenschist faces minerals. phyllosilicates, elongate quartz, and fractured Outcrop in debris flow scar in western Deep Hollow (38.4226" N, 78.31 89' W). feldspars, is common in many Mesoproterozoic units. In the layered gneisses this foliation cross- cuts or overprints the older fabrics (Figure 18). This foliation strikes northeast-southwest and obtained latePaleozoic (-300 Ma) AriAr cooling dips moderately to the southeast (Figure 16b). ages on similar fabrics, whereas Furcron (1969) The microstructures and minerals that define reports early Paleozoic (-450 Ma) K/Ar ages this foliation are indicative of greenschist facies for metamorphic minerals in the Mechum River conditions during deformation. In the northern Formation 35 miles (55 km) southwest of the Virginia Blue Ridge, Burton and others (1992) Madison quadrangle.

Figure 17. Photograph (a.) and sketch (b.) of folded pegmatitic leucogranite (Ylg3) dike in medium-grained Ieucocratic granitoid gneiss (Ylg2) with a weak high-temperature foliation that is axial planar to folds. Knife is 3 inches (8 cm) long. Outcrop at base of debris flow scar in western Deep Hollow (38.4216' N, 78.3178" W). PUBLICATION I57 15 amphibolite to granulite facies (Passchier and Trouw, 1996). Regionally this fabric has been interpreted to be associated with deformation associated with Grenvillian orogenesis (1,200- 1,000 Ma) (Bailey and Simpson,1993; Aleinikoff and others, 2000). In the Madison quadrangle, this foliation generally strikes approximately east- west and dips steeply to both the north and south (Figure 16a). Folding of this foliation is rarely obseled, but at some outcrops is axial planar to folds developed in competent layers such as coarse-grained leucogranitic dikes (Figure ll). This foliation is cut and overprinted by a lower Figure 18. Photograph of hightemperature fabric temperature foliation defined by greenschist (S,, foliation and compositional bands) in leucocratic facies metamorphic minerals and microstructures granitic gneiss (Ylgl) and leucrocratic gneiss (Ylg2) (Figure 18). cut and offset by 2-inch- (5 cm) wide deformation zone The younger foliation, defined by aligned (Sr) defined by aligned greenschist faces minerals. phyllosilicates, elongate quartz, and fractured Outcrop in debris flow scar in western Deep Hollow (38.4226' N, 78.31 89' W). feldspars, is common in many Mesoproterozorc units. In the layered gneisses this foliation cross- cuts or overprints the older fabrics (Figure 18). This foliation strikes norlheast-southwest and obtained IatePaleozoic (-300 Ma) ArlAr cooling dips moderately to the southeast (Figure 16b). ages on similar fabrics, whereas Furcron (1969) The microstructures and minerals that define repofts early Paleozoic (-450 Ma) K/Ar ages this foliation are indicative of greenschist facies for metamorphic minerals in the Mechum River conditions during deformation. In the northern Formation 35 miles (55 km) southwest of the Virginia Blue Ridge, Burton and others (1992) Madison quadrangle.

Figure '17. Photograph (a.) and sketch (b.) of folded pegmatitic leucogranite (Ylg3) dike in medium-grained leucocratic granitoid gneiss (Ylg2) with a weak hightemperature foliation that is axial planar to folds. Knife is 3 inches (8 cm) long. Outcrop at base of debris flow scar in western Deep Hollow (38.4216" N, 78.3178' W). 16 VIRGINIA DIVISION OF MINERAL RESOURCES

JOINTS the chamockite (Ych) and garnetiferous gneiss (Ygn) have a two pyroxene * garnet mineral Ubiquitous joints cut the bedrock in the assemblage consistent with granulite facies Madison quadrangle. At any given exposure one metamorphism. Penekative fabrics associated or two distinct fracture sets may be present, but at with high-grade metamorphism occur are the quadrangle scale there are no dominant joint common in the garnetiferous gneiss (Ygn) and sets (Figure 19). Given the complex geologic leucocratic granitic gneiss (Ylg). It is likely that history of the Blue Ridge there are fractures of biotite-bearing granitoid gneiss (Ybg) and coarse- many different ages. Mager and Bailey (2000) grained alkali feldspar granite (Yaf ) experienced reported that fracture densities tend to be greatest similar Grenvillian metamorphic conditions, but in the tabular igneous intrusions and lowest in the preserve evidence of only a younger greenschist Robertson River granitoids. facies event. Tollo and others (2003) report U- Pb metamorphic recrystallization ages of -1,030 to 980 Ma and interpret these ages to record a METAMORPHISM thermal disturbance associated at the end of the Grenville event. Mesoproterczoic rocks in the Madison Rocks deformed in the White Oak Run quadrangle show evidence of at least two episodes high-strain zone developed at lower amphibolite of metamorphism. The earlier episode (perhaps facies conditions (-500' C) (Knight,1999). The multiple episodes) is associated with the Grenville zone of amphibolite facies metamorphism appears orogeny. Mineral assemblages (in some units) and to be restricted to the fault zone and may reflect microstructures suggest that this event reached the deformation during Neoproterozoic cooling of the upper amphibolite to granulite facies. Locally, Robertson River plutons.

ttt. -J'a-- a a -A-aottr'. - 4.c--a-a a ! aa,ta..' . t'l. ':b a a t 7. g'.t t -.4 l'a' 3c a la.. aa: %ot' at a_at ai.,ta 13o .f t - .l .1. ..s a to ' oa ot 'j' att i.. :.: . et3 lt a oaa :'n;i, t -r aa '- r''t I ... o'!3 -r e ! . l.o -art-r-1" . .r-i!r t'_. al t \ o- t -rr a.'c-^ 'o \ .a I-- ,.o' ' \- 9.r t-t-g .',r..:/ t :'rro-tr r - ---..-z (n = 438), Contour interval 1o/o pat 1"h area.

Figure 19. Stereogram of poles to joints and contoured stereogram of poles to joints in the Madison quadrangle. PUBLICATION 157 T7

Proterozoic rocks in the Madison Ma, granitic and charnockitic plutons intruded quadrangle experienced a greenschist facies older gneissic rocks (Figure 20) (Aleinikoff and metamorphic event during the Paleozoic. others, 2000; Tollo and others, 2A$). In the However, not all rock units are equally affected Madison quadrangle, the leucocratic granitoid by this metamorphism. Charnockite (Ych) and gneiss (Ylg) is interpreted to have formed during Robertson River granitoids show only minor this early magmatic interval. Small bodies of evidence of this event, whereas the biotite-bearing garnetiferous gneiss (Ygn) are interpreted to be granitoid gneiss (Ybg), Mechum River Formation remnants of older crust. A second major episode (Zms, Zma, Zmc), hornblende metagabbro of igneous activity occurred between 1,080 Ma (Zhg), and metabasalt (Zmb) are extensively and 1,050 Ma. Plutonic rocks formed during this altered. Diagnostic greenschist facies minerals interval include charnockite (Ych), biotite-bearing include biotite, muscovite, chlorite, epidote, * granitoid gneiss (Ybg), and coarse-grained alkali garnet i calcite + actinolite. The greenschist feldspar granite (Yafl (Figure 20). Rocks of facies metamorphic event is interpreted to be of the older magmatic interval (Ylg in particular) mid-Paleozoic age and associated with regional were ductily deformed prior to the intrusion of ductile deformation in the Blue Ridse. the younger plutons (Fig. 3, 4, 17, and 20). The attitude of foliation and folds is consistent with significant north-south crustal shortening during GEOLOGIC HISTORY this Grenvillian deformation event (Figure 17). Metamorphic overgrowths on zircon grains Geologic studies of the bedrock and indicate Mesoproterozoic rocks were thermally structures exposed in the Madison quadrangle disturbed between 1.030 and 980 Ma at the end of and surrounding region provide evidence for a the (Tollo and others, 2003). long and complex geologic history. The geologic The next evidence of geologic activity history of the Madison quadrangle and more in the Blue Ridge occurred with the intrusion of broadly, the Virginia Blue Ridge is summarized the Robertson River Igneous Suite between 735 in Figure 20. The temporal overview presented andl05 Ma (Figure 20) (Tollo and others, 1996). here is based in part on information gathered in These granitic plutons are interpreted as the this study but is primarily the result of the efforts thermal pulse associated with the early stages of made by numerous geoscientists over the past few continental rifting. During the late Neoproterozoic, decades. rocks of the basement complex were exposed Mesoproterozoic granitoids and gneisses at the surface and being shed as sediment into were formed in the middle and lower crust during a basin that would become the Mechum River the Grenville orogeny. This long-lived event Formation. In the Madison quadrangle, arkoses resulted from complex tectonic plate interactions (Zma) and conglomerates (Zmc) of the Mechum along the eastem margin of North America River Formation were deposited in braided stream and affected rocks from Texas to northeastern environments relatively close to uplands underlain Quebec (Hoffrnan, 1988; Dalziel, 1994). The by the Blue Ridge basement complex (Bailey Grenville orogeny led to the amalgamation of the and Peters, 1998). Although it is likely that the Rodinian supercontinent (which included much Mechum River Formation accumulated in a rift- of North America, South America, Australia, basin, the present day geometry of the Mechum and Antarctica) by 1,000 Ma (Moores, l99l;' River belt is not a rift graben as suggested by Dalziel, 1994). Recent geochronologic studies previous workers (Schwab, 1974). Dikes and in the northern Virginia Blue Ridge (including the sills of hornblende metagabbro (Zllrg) intruded Madison area) reveal distinct pulses of igneous the basement complex and the Mechum River activity during the Grenville orogeny (Aleinikoff Formation during this period. Ductile shearing and others, 2000; To1lo and others, 2003). During along the extensional White Oak Run high-skain the early magmatic eventbetween 1,160 and 1,140 zone is interpreted to have occurred dming rifting 18 VIRGINIA DIVISION OF MINERAL RESOURCES

0 differential erosion and development of modern Blue Ridge topography 100 continued erosion

+t 2AO Intrusion of diabase dikes (Jd) c uplift, exhumation, and erosion o f_thrusting and emplacement of Btue Ridge thrust sheet over o 300 I Valley & Ridge sedimentary rocks Lo o. 400 o uplift, exhumation, and erosion Lo )_contractional movement on the Champlain Valley and Quaker Run ', high-strain zones, greenschist facies metamorphism and deformation rFo o 500 continual burial cl Deposition of Chilhowee Group o emplacment of Catoctin Formation basaltic dikes (Zmb) and lavas L 600 2 (5 V extensional movement on the White Oak Run high-strain zone ? o i Intrusion of Hornblende MetaqabbroMetagabbro (Zhg)(Zhq) rts 700 Deposition of Mechum River Formation o Intrusion of Robertson River Granitoids co 800 exhumation and erosion E 900 = 1,000 n ? ^' Intrusioni!:::,?::.?*?::ry:r. of Charnockite (Ych), Biotite-bearingBiotite-beari Granitoid Gneiss (Ybg), g and Coarse-grained Alkali Feldspar Granite (Yaf) 1,100 p granulite c defomration and facies metamorphism o Intrusion Leucocratic (Ylg) L of Granitic Gneiss (5 1,200 ?-fe1;"nation of Garnetiferous Gneiss (Ygg)

Figure 20. Generalized geologic history of the Madison quadrangle and Virginia Blue Ridge. Data from numerous sources.

the opening of the Iapetus Ocean in the early (Bailey and Simpson, 1993; Knight and Bailey, Cambrian (Simpson and Erikkson, 1989). Rocks reeg). in the Blue Ridge were progressively buried under A second phase of rifting began at a sequence of sedimentary rocks deposited on approximately 570 Ma with the intrusion ofbasaltic the North American continental margin. These dikes in the Madison quadrangle and extrusion of Cambrian and Ordovician rocks form a package the volcanic Catoctin Formation, a regionally in excess of 5 miles thick and are exposed today in extensive unit exposed along the Blue Ridge the Shenandoah Valley. from central Virginia to southern Pennsylvania By the middle to late Ordovician, a (Badger and Sinha, 1988; Aleinikoff and others, collisional tectonic event had begun in eastern 1995). Metabasalt dikes in the Madison area are North America. Rocks in the Blue Ridge were interpreted to be feeder dikes for the overlying metamorphosed to the greenschist facies and volcanic flows of the Catoctin Formation. In the brought to the surface during this orogenic event western Blue Ridge, the Catoctin Formation is (Taconic orogeny) as illustrated by clasts of overlain by a sequence of non-marine to marine metamorphosed basement and Chilhowee Group siliciclastic rocks (the Chilhowee Group) that quartzite clasts in Ordovician conglomerates record the transition from rifting to the develop exposed in the Valley and Ridge (Karpa, 1974; of a passive continental margin created by Glover and others, 1989). Ductile faults such PUBLICATION 157 19 as the Quaker Run and Champlain Valley zones to the east of the Madison quadrangle (Figure 1), is probably developed duing this period. In high- a major structure into which terrestrial sediments strain zones basement rocks were transformed were deposited in the Triassic. Rifting generated into mylonites with penetrative fabrics that basaltic and the diabase dikes exposed developed when minerals were stretched, rotated, in the Madison quadrangle are a product of that and recrystallized. Sedimentary rocks of the magmatism. Apatite fission-track ages reveal that Mechum River Formation were folded into a rocks currently exposed in the Blue Ridge cooled series of tight to open anticlines and synclines. A below approximately 120" F (-60" C) during penetrative foliation defined by greenschist facies the Jurassic and Cretaceous indicating that Blue minerals developed in many Mesoproterozoic Ridge rocks have been near the surface for over and Neoproterozoic units (Figures 12b and 16b). 100 million years (Naeser and others, 1999). The These high-strain zones, folds, and foliations modern topographic expression of the Blue Ridge accommodated significant northwest-southeast Mountains developed during the late Cenozoic directed crustal shortening of Blue Ridge rocks. as differential erosion by Atlantic flowing rivers Rocks in the Blue Ridge province were (e.g. Potomac, Rappahannock, and James) eroded thrust over Paleozoic rocks of the Valley & Ridge weaker rocks in the Valley and Ridge (Harbor, during the Late Paleozoic (Alleghanian orogeny) 1996). As dramatically revealed by debris flows (Harris, 1979; Evans, 1989). The Blue Ridge in1995,modern surficial processes continue to act anticlinorium developed as a basement thrust on the rocks of the Blue Ridge. sheet because of movement over a ramp underlain Although the geological history of the by Cambrian to Ordovician sedimentary rocks. Blue Ridge is relatively well understood there The frontal Blue Ridge thrust is exposed in many remain a number of unanswered questions. The places on the western flank of the Blue Ridge age of many of the rock units and the timing of (Virginia Division of Mineral Resources, 1993). Paleozoic deformation and metamorphism needs In central Virginia, mylonitic high-strain zones better resolution. Continued bedrock mapping, are truncated by latePaleozoic brittle thrust faults structural analysis, quantitative , and (Bailey and Simpson, 1993). Zfucon fission track geochronology will refine our understanding of ages (from Mesoproterozoic charnockite) of 285 the geological evolution of the Blue Ridge. to 300 Ma indicate rapid cooling of rocks in the Madison area at the end of Alleghanian orogeny ECONOMIC GEOLOGY (Naeser and others, 1999). Quantitative estimates of deformation in the Blue Ridge province during Currently, there are no active mineral Appalachian mountain building indicate 50 to extraction sites in the Madison quadrangle. 70%o total shortening (Mitra, 1977; Evans, 1989; However, there may be economic potential for the Bailey, T994). Today the Blue Ridge province is production of crushed stone, sand and gravel, and approximately 30 miles (45 km) wide (northwest gemstones. The most comprehensive discussion to southeast), prior to Paleozoic shortening the of the mineral resources of this area can be found province was 60 to 100 miles (90 to 160 km) in Rhesa Allen's Geology and Mineral Resources wide. The Alleghanian orogeny was the result of of Greene and Madison Counties (1963). the tectonic collision between North America and Africa, leading to the formation of the Pangean CRUSHED STONE supercontinent at the end of the Paleozoic. During the early Mesozoic (-200 Ma), In the past, several quarries have been eastern North America and northwestern Africa active in Madison County. Generally, these rifted apart opening theAtlantic Ocean. This event were roadside quarries where granitoid rocks resulted in the formation of a series of sedimentary were mined for road material and fiIl. Alkali rift basins in the Virginia Piedmont and eastern feldspar granite of the Robertson River Igneous Blue Ridge. The Culpeperbasin,20 miles (30 km) Suite was quarried for road material just east 20 VIRGINIA DIVISION OF MINERAL RESOURCES of State Highway 231 and south of White Oak rocks" a concentrated occurrence of these minerals Run (186A-901). A small quarry, along the Rose within the alkali feldspar syenite of the Robertson River approximately 0.5 mile northwest of Syna, River Igneous Suite was observed along State extracted charnokite of the Blue Ridge Basement Highway 231,2 miles north of Madison (1864- Complex for road material (1864-101) (Allen, 902). There is a zone of concentration of ilmenite 1963). and titanite that extends from this location in the alkali feldspar syenite westward to the southeast GEMSTONES slope of Yager Mountain in the biotite granitoid and layered granitic gneiss (Allen, 1963). Unakite is an epidote-bearing granitoid commonly used for jewelry. It is composed of green epidote, pink potassium feldspar, and REFERENCES quiartz. Unakite was observed in outcrop in Madison quadrangle along State Route 649 inthe Aleinikoff, J. N., Zartman, R.E., Walter, M., Rankin, D. W., LWle, P. T., and Burton, W. C., 1995, U-Pb ages gap of Chapman Mountain near the junction of the of metarhyolites of the Catoctin and Mount Rogers fire road to the Blakey Ridge lookout tower. Formations, central and southern Appalachians: evidence for two pulses of Iapetan rifting: American Journal of Science, v. 295, p. 428-454. SAND AND GRAVEL Aleinikofl J. N., Burton, W. C., Lyttle, P. T., Nelson, A. E., Southworth, C. S., 2000, U-Pb geochronology of Historically, plain the the flood at zircon and monazite from Mesoprorerozoic granitic confluence of the Rose and Robinson rivers, gneisses of the northem Blue Ridge, Virginia and approximately I mile southeast of Syria, has been Maryland, USA: Precambrian Research, v. 99, p. utilized as a source ofsand and gravel. The gravel tt3-146. consists primarily of granitoid and gneissic rock, Allen, R. M., 1963, Geology and mineral resources of with minor amounts of greenstone (Allen, 1963). Greene and Madison counties, Virginia: Virginia Division of Mineral Resources Bulletin 81, 78 p.

PYRITE Badger, R. L. and Sinha, A. K., 1988, Age and Sr isotopic signature of the Catoctin volcanic province: Implications for subcrustal mantle evolution: A historical prospect for pyrite, known as Geology, v. 16,p. 692-695. the Alger Prospect (1864-801), was reported by Allen (1963). This prospect is located 0.5 mile Bailey, C. M. and Simpson, C., 1993, Extensional and from the end of State Route 651 on Blakey Ridge. contractional deformation in the Blue Ridge The pyrite occurs as large clusters of crystals province, Virginia: Geological Society of America Bulletin, v. 105, p.4ll-422. within 2 to 3 footthick qtartz veins in a shear zone of leucocratic granitoid gneiss. Occurring Bailey, C. M., Simpson C., and De Paor, D. G., 1994, alongside the pyrite are smaller veinlets of Volume loss and tectonic flattening shain in granitic specular hematite (Allen, 1963). mylonites from the Blue Ridge province, Central Appalachians. Journal of Structural Geology, v. 16, 1403-1416.

TITANruM-BEARING MINERALS Bailey, C. M., 1994, Temporal, kinematic, and strain analysis of granitic tectonites from the central Titanium-bearing minerals such as ilmenite Appalachians: unpublished Ph.D. thesis, Johns (FeTiOr), titanite (CaTiSiOr), and rutile (TiOr) Hopkins University, 256 p. are present in the granitoid and gneissic rocks of Bailey, C. M. and Peters, S. E., 1998, Glaciogenic this quadrangle. Although the titanium-bearing sedimentation in the late Neoproterozoic Mechum minerals are primarily accessory minerals in these River Formation, Virginia: Geology, p. 623 -626. PUBLICATION I57 21

Bartholomew, M. J., 1977, Geology of the Greenfield and Ridge anticlinorium in Virginia: Geological Society Sherando quadrangles, Virginia: Virginia Division of America Bulletin, v. 69, p. 569-57 4. of Mineral Resources Publication 4,43 p. Harbor, D. J., 1996, Non-uniform erosion pattems in the Bartholomew, M. J., Gathright, T. M., and Henika, W. S., of Virginia: Geological 1981, A tectonic model for the Blue Ridge in Society of America Abstracts with Programs, v. 28 central Virginia: American Joumal of Science, v. n.7,pg. 116. 281,p.1164-1183. Harris, L. D., 1979, Similarities between the thick-skinned Berquist, P. J., 2000, High-strain zones and contact relations Blue Ridge anticlinorium and the thin-skinned in the basement complex of the Blue Ridge in Powell Valley anticline: Geological Society of Madison County, Virginia: unpublished B. S. America Bulletin, v. 90, p. 525-539. thesis, College of William & Mary 63 p. Hoffrnan, P. F., 1988, Unitedplates ofAmerica, the birth of a Berquist, P. J. and Bailey, C. M., 2000, Displacement across cratonic Early Proterozoic assembly and growth of Paleozoic high-strain zones in the Blue Ridge Laurentia: Annual Reviews in Earth and Planetary province, Madison County, Virginia: The Pedlar Science, v. 16 543-603. and Lovingston massifs reconsidered: Geological Society ofAmericaAbstracts with Programs, v. 32, Karpa, J. 8., 1974, The middle Ordovician Fincastle n.2,p.4. Conglomerate north of Roanoke, Virginia and its implications for Blue Ridge tectonism: Burton, W. C., Kunk, M. J., and LWle, P. T., 1992, Age unpublished Ph.D. thesis, Virginia Polytechnic and constraints on the timing of regional cleavage State Universily, 107 p. formation in the Blue Ridge anticlinorium, northernmost Virginia: Geological Society of Knight, B. D., 1999, The White Run Oak fault zone: a America Abstracts with Programs, v.24, n.2, p. 5. Neoproterozoic extensional structure in the eastem Blue Ridge, Madison County Virginia: r,npublished Dalziel, I. W. D., 1995, Earth before Pangea: Scientific B. S. thesis, College ofWilliam &Mary,70p. American v. 272,pg. 58-63. Knight, B. D. and Bailey, C. M., 1999, The White Oak Run Evans, M. 4., 1989, Skuctural geometry and evolution fault zone: a Neoproterozoic extensional skucfure of foreland thrust systems, northem Virginia: in the eastern Blue Ridge, Madison County, Geological Society of America Bulletin, v. 101, p. Virginia: Geological Society of America Abstracts 339-354. with Programs, v. 3 l, n. 3, p. 26.

Evans, N. H., 1991, Latest Precambrian to Ordovician Kunk, M. J., Froelich, A. J., and Gottfried, D.,1992, Timing metamorphism in the Virginia Blue Ridge: origin of emplacement of diabase dikes and sheets in the of the contrasting Lovingston and Pedlar basement Culpeper Basin and vicinity, Virginia and Maryland: terranes: American Joumal of Science, v. 291, p. Ar/Ar age spectrum results from hornblende and 425-452. K-feldspar in granophyres: Geological Society of America Abshacts with Programs, v. 25, n. 2, p. Furcron, A. S., 1969, Late Precambrian and Early Paleozoic 31. erosional and depositional sequences of northem and central Virginia: Georgia Geological Survey Lukert, M.T.,1973, The petrology and geochronology of the Bulletin, p. 57-88. Madison area, Virginia: unpublished Ph.D thesis, Case Westem Reserve University, 218 p. Gathright, T. M. II, 1976, Geology of the Shenandoah National Park, Virginia: Virginia Division of Lukert, M. T. and Mitra, G., 1982, Field trip guide to the Mineral Resources Bulletin 86, 93 p. northern Virginia Blue Ridge: Geological Society ofAmerica. Glover, L. III, Evans, N. H., Patterson, J. G., and Brown, W. R., 1989, Tectonics of the Virginia Blue Ridge and Mager, S. M. and Bailey, C. M., 2000, Fracture analysis Piedmont: 28ft International Geological Congress, in the Blue Ridge province, Madison County, Field Trip Guidebook T363,p.59. Virginia: Geological Society of America Abstracts with Programs, v. 32, n. 2, p. 59. Gooch, E. O., 1958, Infolded metasedimentary rocks near the axial zone of the - Blue Mitra, G., 1977, The mechanical processes of deformation 22 VIRGINIA DIVISION OF MINERAL RESOURCES

of granitic basement and the role of ductile Southworth, S., Burton, W.C., Schindler, J.S., and Froelich, deformation zones in the deformation of Blue A.J., 2000, Digital geologic map of Loudoun Ridge basement in northern Virginia: unpublished County, Virginia: U.S. Geological Survey Open Ph.D. thesis, Johns Hopkins University, 219 p. File Report 99-150, scale l:50,000.

Mitra, G., 1977, Ductlle deformation zones in Blue Tollo, R. P. and Lowe, T. K., 1994, Geologic map of Ridge basement and estimation of finite strains: the Robertson River Igneous Suite, Blue Ridge Geological Society of America Bulletin, v. 90, p. province, northem and central Virginia: United 935-95r. States Geological Survey Miscellaneous Field Studies Map,MF-2229. Moores, E. M., 1991, Southwest U.S.-East Antarctic (SWEAT) connection; a hypothesis: Geology v. 19, Tollo, R. P. and Aleinikoff, J. N., 1996, Petrology and U- pg.425-428. Pb geochronology ofthe Robertson River Igneous Suite, Blue Ridge province, Virginia: evidence for Naeser, N. D., Naeser, C.W., Morgan, B. A., Schultz, A.P., multistage magmatism associated with an early and Southworth, C. S., 1999, Paleozoic to recent episode of Laurentian rifting: American Journal of cooling history ofthe Blue Ridge province, Virginia, Science, v. 296,p. 1045-1090. , and , from apatite and zircon fission-track analysis: Geological Society Tollo, R. P., Aleinikoff, J. N., Borduas, E. A., and Hackley, of America Abstracts with Programs, v.31, p. ll7. P. C., 2003, Petrologic and geochronologic evolution of the Grenville orogen, northern Blue Passchier, C. W. and Trouw, R. A. J., 1996, Microtectonics, Ridge province, Virginia, in Proterozoic tectonic Springer Verlag. 283 p. evolution of the Grenville orogen in NorthAmerica, eds. Tollo, R. P., Corriveau L., Mclelland, J., and Rankin, D. W., Drake, A. A. Jr., Glover, L, III, Goldsmith, Bartholomew, M. J., Geological Society ofAmerica R., Hall, L. M., Murray, D. P., Ratcliffe, N. M., Memoir, in press. Read, J. F., Secor, D. T., Jr., and Stanley, R. S., 1989, Pre-orogenic terranes: in The Appalachian - Wadman, H. M., Owens, B. E., and Bailey, C. M., Ouachita Orogen in the United States, eds.Hatcher, 1998, Petrological analysis of the White Oak R. D., Thomas, W. A., and Viele, G.W., v.F-2,p. Dam exposure, Blue Ridge province, Virginia: 7-100. Geological Society of America Abstracts with Progtams, v. 30, n. 4,p.64. Schwab, F. L., Mechum River Formation: Late Precambrian (?) alluvium in the Blue Ridge province ofVirginia: Weiss, J. R., 2000, The origin of mafic and ultramafic Journal of Sedimentary Petrology, v. 44, p. 862- rock bodies in the Blue Ridge province, Madison 871. County, Virginia: unpublished B. S. thesis, College of William andMary 38 p. Shotwell, N. L. and Bailey, C. M., 2000, Structural geometry and strain in the Neoproterozoic Mechum River Whalen, J. B., Currie, K. L., and Chappell, B. W, 1987, Formation, Blue Ridge province, Virginia: A-type : geochemical characteristics, Geological Society of America Abstracts with discrimination, and petrogenesis : Contributions to Programs, v. 32, n. 2, p. 73. Mineralogy and Petrology, v. 95, p. 407-419.

Simpson, E. L. and Erikkson, K. A., 1989, Sedimentology of the Unicoi Formation in southem and central Virginia: Evidence for Late Proterozoic to Early Cambrian rift-to-passive margin transition: Geological Society of America Bulletin, v. 101, p.42-54.

Sinha, A. K. and Bartholomew, M. J., 1984, Evolution in the Grenville terrane in the central Virginia Appalachians, in Bartholomew, M. J., Force, E. R., Sinha, A. K., and Herz, N., eds. The Grenville event in the Appalachians and other related topics: Geological Society of America Special Paper 194, p.175-186.