THE BLUE RIDGE
TRANSECT COORDINATES 550 - 586 KM
BLUE RIDGE GRENVILLE BASEMENT
Transect Coordinates 550-575 km
Felsic and intermediate gneisses of the Grenville province comprise the oldest rocks in the central and southern Appalachians. These billion- year- old rocks, the basement upon which the Laurentian part of the Appalachian orogenic system was assembled, crop out along the axis of the Blue Ridge in Virginia [Sinha and Bartholomew, 1984] and reappear in the eastern Piedmont where they are known as the Goochland "terrane" [Glover et al., 1978; Farrar, 1984]. Although it is well established that the Grenville lies with profound unconformity [Nelson, 1932] below Late Proterozoic conglomeratic sandstones (rift facies of Wehr and Glover [1985]) that comprise the oldest strata of the Appalachian system, these basement rocks remain the least understood of all geologic units exposed within the Appalachians. The recent state of Appalachian Grenville knowledge was summarized in a symposium volume [Bartholomew, 1984].
In the central Virginia Blue Ridge the Rockfish Valley fault divides the Grenville basement into two massifs of contrasting lithology [Bartholomew, 1977; Bartholomew et al., 1981]: the Pedlar massif west of the fault and the Lovingston massif east of the fault. The Pedlar contains granulite facies, massive pyroxene granofels and layered gneisses with a slight overprint of low grade metamorphic minerals. The Lovingston appears to represent a similar suite of rocks with a more intense greenschist metamorphic and deformational overprint of Paleozoic age. Bartholomew et al. [1981], and Sinha and Bartholomew [1984] consider the Lovingston and Pedlar to represent massifs metamorphosed during the Proterozoic at shallower, and deeper P- T conditions respectively, and to have been juxtaposed at their present structural levels during Paleozoic orogenesis. Evans [1984, 1991] made the interpretation that both massifs were originally at granulite facies during the Late Proterozoic, and that the Lovingston massif was retrograded, largely to greenschist facies, during the Paleozoic.
Pettingill et al. [1984] report Rb/Sr whole rock ages of orthogneisses in central Virginia ranging from about 1009 to 1021 Ma. Sinha and Bartholomew [1984] give zircon U/Pb ages for the Grenville orthogneisses in central Virginia ranging from 1130 to 1070 Ma. Detrital zircons suggest an older sediment source of 1870 Ma. The final metamorphism may have culminated at about 920 Ma. Following the peak of Grenville metamorphism, uplift and erosion deeply dissected the orogen over a period of about 230 m.y. During this time the Grenville was denuded to a depth of about 25 km [Herz, 1984], exposing granulite facies rocks.
RIFT FACIES AND THE CAMBRIAN RIFT- TO- DRIFT TRANSITION
TRANSECT COORDINATES 575-586 KM
Crossnore Volcanic- Plutonic Suite. Between 690 +/- 10 m.y. ago [Odom and Fullagar, 1984], 729 Ma [Tollo et al., 1991], and probably as old as 760 Ma [Aleinikoff et al., 1991], continental rifting began coevally with emplacement of the fluorite and sodic amphibole- bearing, peralkaline Crossnore plutonic- volcanic suite [Rankin, 1976]. The coeval (734 +/- 26 Ma) granite/gabbro Bakersville suite in North Carolina also belongs to this event [Goldberg et al., 1986]. Non- marine and marine volcanic rocks and arkosic sandstones accumulated in rift grabens. The youngest age (Rb/Sr whole- rock) of Crossnore plutonism in that area is 646 +/- 9 m.y. for the Crossnore Granite itself. Odom and Fullagar found that earlier zircon U/Pb ages [Rankin et al., 1969] gave falsely older ages (820 Ma) because of contamination from old Grenville gneisses which they assimilated.
Several members of the Crossnore suite occur in the Blue Ridge of central and northern Virginia:
1) One of these, the fluorite- bearing Mobley Mountain Granite near Roseland, about 40 km south of Charlottesville, gives a Rb/Sr whole- rock age of 652 +/-22 m.y. [Herz and Force, 1984].
2) Another, the Rockfish River pluton, located about 30 km south of Charlottesville, has yielded ages of 646 +/- 55 m.y. [Mose and Nagel, 1984], and 630 Ma, [Mose and Kline, 1986].
3) A third, questionably part of the Robertson River granite, gave an age of ca. 650 Ma on zircon 207Pb/206Pb analysis by T. Stern (reported in Rankin [1976]). According to Lukert and Banks [1984], Stern's analysis was done on a riebeckite granite that intrudes the main body of the Robertson River pluton, which lies about 100 km north of Charlottesville. Lukert and Banks determined an age of 732 +/- 5 m.y from a zircon U/Pb concordia intercept for the main body of the Roberston River. The zircon samples of Lukert and Banks did not appear to contain inherited older cores. Mose and Nagel [1984] determined a Rb/Sr whole- rock age of 646 +/- 55 m.y. from samples spread over most of the length of the Roberston River, excluding the area of the Stern riebeckite granite. Subsequently, a refined Rb/Sr whole- rock age of about 650 Ma was reported [Mose and Kline, 1986]. Therefore, U/Pb and Rb/Sr ages were in disagreement by about 80 m.y. A more detailed zircon and Rb- Sr analysis of the Robertson River suite is now underway. Tollo et al. [1991] reported that the Robertson River suite is 729 to 640 m.y. old (confirming Lukert and Banks [1984]), and may have extended intermittently over the range 729 to 570 Ma, the younger age being that of the Catoctin. It is also worth remembering that Rb/Sr ages of deeply emplaced and slowly cooled plutons are commonly as much as 25 m.y. younger than the emplacement age because of late closure of the isotopic system. Thus, the youngest Crossnore granitoid plutons in the region of the transect are thought to be about 650 Ma. We do not include the Catoctin in the same suite.
Although the Catoctin and all post 650 Ma igneous rocks were originally included in the Crossnore volcanic- plutonic suite by Rankin [1976], it now seems that an older granite and rhyolite- bearing suite of rocks lies unconformably below the Lynchburg and its southern equivalent, the Ashe Formation. This granite- bearing suite is about 760- 650 Ma, and it may be best to confine usage of the term Crossnore to these older rocks. A similar argument has been made by Badger and Sinha [1988], and Glover [1989]. Badger and Sinha [1988] and Alinikoff et al. [1991] point out that the pre 650 Ma Late Proterozoic Mount Rogers and associated rocks may belong to an early aborted rift sequence.
Erosion has removed volcanic rocks associated with the Crossnore volcanic- plutonic suite over much of the Blue Ridge in Virginia, and cobbles of the Robertson River (northern equivalent of the Crossnore) members may now be found in the basal conglomerates of the overlying Mechums River and Fauquier (Lynchburg) Formations [Lukert and Banks, 1984]. This provides an older age limit of about 650 Ma for the Lynchburg Group..
Lynchburg Group. The Lynchburg Formation was named by Jonas [1927] for exposures along the James River near Lynchburg, Virginia. This sequence of rift- related clastic rocks, basaltic volcanic rocks, and shallowly emplaced ultramafic segregations in dikes and sills, crops out along the east flank of the Blue Ridge in Virginia. It nonconformably overlies Grenville basement or, locally, rocks of the Crossnore Volcanic- Plutonic suite. It has been recently subdivided into five formations by Wehr [1985] in the Culpeper- Charlottesville region. In the Culpeper area, the Lynchburg Group comprises a terrestrial, alluvial outwash deposit (Bunker Hill Formation) at its base. The overlying formations, Monumental Mills, Thorofare Mountain, Ball Mountain, and Charlottesville include a deep water retrogradational fan sequence [Wehr, 1983]. These formations are briefly characterized here.
Bunker Hill Formation. This formation consists of 0- 1000 m of poorly sorted, medium- grained to granule feldspathic arenite with minor siltstone and mudstone. It is absent in the Rockfish River area south of Charlottesville. Facies analysis indicates deposition as a braided outwash plain adjacent to glaciated highlands composed largely of Grenville basement.
Monumental Mills Formation. Wehr [1985] divided this formation into a lower sandstone member of thin bedded fine- to medium- grained, well- sorted sandstone and siltstone, and an upper member of thin bedded to laminated siltstone and mudstone. The outcrop belt is 0- 1500 m wide and thins toward the south. It is absent or represented only by the Rockfish Conglomerate in the Rockfish River area south of Charlottesville. Facies analysis suggests a slope environment. The Rockfish Conglomerate is a pebbly, feldspathic sandstone with conglomerate lenses. It makes up the basal unit of the Lynchburg Group in the Rockfish River area south of Charlottesville, Virginia. It is about 500 m thick and consists of cobble conglomerate in the lower part grading upward into coarse- grained pebbly sandstone. The upper 20 m is graded thin- bedded sandstone with local occurrences of outsized clasts interpreted to be ice- rafted dropstones. The lower contact is with a mylonitic zone separating basement from the Rockfish. The upper contact is gradational into the lower Thorofare Mountain Formation. According to Wehr [1985], most of the larger clasts are very coarse- grained light colored basement gneiss. The Rockfish also contains clasts of granite, biotite gneiss, fine- grained aplite (?), and dark siltstone. In thin section, Rockfish sandstones contain detrital quartz and feldspar in a schistose matrix of quartz, plagioclase, mica and magnetite. Facies analysis [Wehr, 1985] of outcrops along the Rockfish River has shown that the outsized clasts are ice- rafted dropstones and indicates that the conglomerate was deposited as subaqueous glacial outwash.
Thorofare Mountain Formation. The Thorofare Mountain Formation is recognized from the Culpeper area to the Rockfish River. This formation consists of medium- grained to pebbly, poorly sorted feldspathic sandstone with minor conglomerate, siltstone and graphitic mudstone. The sandstones are massive to faintly stratified in beds a few centimeters to more than 8 m thick. Interbeds of thin bedded to laminated siltstone and graphitic mudstone are common, and these lithologies also occur locally as rip- up clasts in intraformational conglomerate. Facies analysis indicates that this sequence was formed in a deep water submarine fan.
Ball Mountain Formation. This sequence extends throughout the area of study by Wehr [1985] and occupies a belt 1- 4 km in width. It consists of coarse- grained to pebbly quartz wackes and quartzites interbedded with laminated siltstone and graphitic mudstone. The upper 100 m is locally a graphitic schist named the Johnson Mill Member [Nelson, 1962]. Over much of the area between Culpeper and the Rockfish River, the Ball Mountain truncates underlying units and is either in unconformable or fault contact with them. In some places it is in conformable, and gradational, contact with the Thorofare Mountain. Facies analysis of the Ball Mountain shows that it has sedimentary characteristics similar to the underlying Ball Mountain Formation and was deposited by sediment gravity flows [Wehr, 1983,1985]. The Johnson Mill Member at the top of the formation is euxinic which suggests abrupt cessation of influx of clastic material and basin- wide starvation following Ball Mountain sandstone deposition [Wehr, 1985].
Charlottesville Formation. The Charlottesville formation extends throughout the Culpeper- Rockfish River area. According to Wehr [1985], it comprises schistose siltstone and mudstone with isolated outcrops of medium to coarse- grained, commonly amalgamated sandstone beds. Sandstone beds range from a few millimeters to approximately a meter in thickness. They tend to be massive, although grading, horizontal stratification, and complete Bouma T(a- e) sequences occur. The lower 1000 m of the formation in the Rockfish area is characterized by coarsely laminated to very thin bedded, fine grained sandstone and siltstone with prominent biotite porphyroblasts. Similar rocks occur more locally near Culpeper. Primary textures and sedimentary structures indicate deposition by turbidity currents in deep water.
Swift Run Formation. This formation occurs throughout most of the Culpeper- Rockfish River area, ranging from 0- 5 km in width of outcrop belt. On the west side of the Blue Ridge [Stose and Stose, 1946; Bloomer, 1950; Werner, 1966; Brown, 1970], the Swift Run occurs in lenses as much as 400 m thick, unconformable upon basement and grading upward by interleaving with the overlying Catoctin Basalt. Here it consists of cross- bedded arkose, conglomerate, mudstone and intercalations of mafic tuffs and lavas and is interpreted as alluvial in depositional environment [Gathright, 1976].
On the east side of the Blue Ridge, in the Culpeper- Rockfish River area, the Swift Run is conformable with the underlying Charlottesville Formation and is gradational over a short distance by interleaving with the base of the Catoctin Formation. In this area it contains, at the base, coarse- grained feldspathic sandstone; in the middle, greenstone, rare felsic volcanic rock, fine- grained sandstone, and graphitic mudstone; and at the top, coarse- grained blue- quartz sandstone and arkose interbedded with pale green mudstone and a few thin greenstone beds. In the Culpeper area many Swift Run sandstones are calcareous, and along the Hazel River tabular marble clasts as much as 45 cm in length occur in a coarse- grained sandstone matrix [Wehr, 1985]. To the north of the Culpeper area thin lenses of marble are present below the Catoctin Formation [Furcron, 1939; Parker 1968], and these may be correlative with the limestone conglomerate in the Culpeper area. Turbidites suggest that the Swift Run on the east side of the Blue Ridge is probably a deep water sedimentary gravity- flow deposit, in contrast to its non- marine nature to the west.
Catoctin Formation. This formation was named by Keith [1894]. Metabasalts and minor intercalated siliciclastic rocks of the Catoctin Formation are abundant across the northward plunging nose of the Blue Ridge anticlinorium in southern Pennsylvania. From there to the south the Catoctin forms two belts of outcrop along the east and west flanks of the anticlinorium into central and southern Virginia where it pinches out. Thus, the Catoctin is a key unit in relating the stratigraphy of the Great Valley, and Valley and Ridge, with the rift facies of the eastern Blue Ridge.
The Catoctin is a sequence of greenschist- facies tholeiitic basalt lavas and minor breccias and tuffs intercalated with quartzose feldspathic sandstone and mudstone. The Catoctin is gradational over a short interval by interleaving with both overlying and underlying formations. In Pennsylvania, minor rhyolite is intercalated with the basalt lavas. The total thickness of the formation may reach 1000 m [Gathright et al., 1977]. Along the west flank of the Blue Ridge, the Catoctin overlies the Swift Run Formation and is overlain by the Lower Chilhowee Group Unicoi/Weverton Formation. Rocks of the Unicoi and Swift Run are similar, and it is probable that in southern Virginia where the Catoctin is absent the Unicoi and Swift Run have been mapped together as Unicoi. On the west flank of the Blue Ridge, the Catoctin (including the enveloping Swift Run and Unicoi formations) is non- marine [Reed, 1955], and on the east flank the Catoctin and overlying Candler and underlying Swift Run formations are marine [Wehr and Glover, 1985]. The transition from non- marine to marine takes place north of Culpeper, Virginia, along the east side of the Blue Ridge anticlinorium.
The Catoctin is a member of the Albemarle- Nelson suite as defined below. Blackburn and Brown [1976] and Bland [1978] have suggested by trace element geochemistry and petrochemistry that the Catoctin is a tholeiite related to rifting during the formation of Iapetus. Badger and Sinha [1988] dated the Catoctin by Rb/Sr whole rock and mineral isochron methods at 570 +/-36 Ma. This age is Late Proterozoic, possibly overlapping the Early Cambrian according to recent dating of the basal Cambrian at 544 Ma. by Bowring et al. [1993]. It is consistent with the older part of the Early Cambrian and Early Cambrian (?) age-range deduced by Werner [1966] in central Virginia, and by Simpson and Eriksson [1989] in southern and south- central Virginia from studies of the sedimentology and fauna [Simpson and Sundberg, 1987] of the rift-related, basalt- bearing Unicoi Formation of the basal Chilhowee Group.
Evington Group. Rocks of the Evington Group overlie the Catoctin Formation (Main Display), or where that is absent, the Lynchburg Group. The Evington sequence comprises the youngest Laurentian sequence known in the Piedmont of Virginia. Some of the most important earlier work may be found in Espenshade [1954], Brown [1958, 1970], and Redden [1963]. These authors were uncertain about the order of stratigraphic succession in this complexly deformed and metamorphosed group of rocks. Patterson [1987a, 1987b, 1989] revised the stratigraphic ordering based on mapping and structural studies in the Lynchburg, Virginia, area. Three facies sequences, proximal, distal and an eastern allochthon, were recognized. The Slippery Creek and Mount Athos Quartzite pinch out eastward toward the distal facies where Joshua Schist was deposited directly on Candler. The Slippery Creek Greenstone and Mount Athos Quartzite pinch out to the northeast. Along strike to the southwest , the Slippery Creek pinches out, and the Mount Athos quartzite is underlain by the "Moon Mountain Greenstone" (informal name by Patterson [1987b]). Still farther east, in the eastern allochthon, only Candler lithologies with interbedded greenstone and quartzite are present [Brown, 1958; Patterson, 1987a, 1987b, 1989].
Candler Formation. This unit may be as much as 1.7 km thick and is dominantly siliciclastic. The basal contact of the Candler is gradational over a short distance by interleaving with the underlying Catoctin Formation, or, where that is absent, is gradational with the Lynchburg Group. In the proximal facies sequence, the upper contact is gradational with the Mount Athos Quartzite. In the distal facies sequence pelites of the Joshua Schist overlie the Candler. The top of the Candler Formation in the eastern allochthon is not known. The western, proximal, facies is composed of (fine grained units given in metamorphic rock type because protoloths are more difficult to discern):
1) Sandy laminated schist: These contain sand laminae 1- 2 mm thick, composed of quartz and minor feldspar. Pelite laminae are as much as 5 mm thick. Local quartz- rich beds vary from 2.5 to 10 cm in thickness and rarely reach 50 cm.
Quartz wacke: These beds may contain coarse to lower very coarse, blue quartz grains in a pelitic matrix. At one location, graded beds were found with sandstones averaging 5 mm thick, grading into pelites 5- 10 mm thick. Compositionally this rock type varies from 30- 50% quartz, 0- 15% plagioclase, 20- 30% biotite/chlorite, with local calcite.
Green chloritic phyllites and schists: These vary with increasing quartz content to a green sandy laminated lithology, which may contain as much as 50% chlorite. Thin (5 cm) layers of white marble are rare.
2) Blue weathering phyllite: This rock is composed of 10- 45% chlorite, 25- 50% muscovite, 5- 35% quartz, and as much as 5% albite. Very rare millimetre thick sand laminations occur, and 1- 2 mm diameter nodes of quartz sand are found locally.
3) Quartz arenite and subarkosic quartz arenite: This lithology may occur as lenses (75 x 530 m to 150 x 3700 m) and pods (as much as 8 m thick). Laminated lower- to middle- fine- grained quartz arenite occurs with micaceous quartzite and quartz muscovite schist. Fine grained arkosic quartz arenite occurs in medium to thin beds. Massive arkosic and subarkosic quartz arenites and matrix supported wackes contain feldspars (generally potassic) as much as 3 mm and blue quartz clasts as much as 4 mm in diameter.
4) Impure marbles and calc- silicate schists occur in lenses (75 x 115 m to 190 x 650 m), commonly along fault zones. The marble is dark grey to bluish, has a phyllitic sheen, and contains nodes and laminae of white calcite. Mica- quartz- feldspar, calcite, and mixed mica- quartz- feldspar- calcite layers, 1.5 to 5 m. thick, are gradationally interlaminated.
Still farther east, in rocks equivalent to the eastern allochthon of Patterson [1987b], Brown [1958] mapped dominantly Candler facies lying above a domal structure cored by Lynchburg Group. Greenstones, commonly associated with quartz arenite, are scattered throughout this sequence which is truncated at it's top by a fault. Patterson suggested that the alternating compositional laminations in the sandy laminated schist resulted from variable flow conditions. Phyllites (mudstones) may have been deposited from dilute turbidity currents or fair weather suspension fallout. Map-scale lenses of massive quartzose sandstones may have been deposited from sediment gravity flow. Some smaller lenses could be channel fill cut into the underlying muds.
Mount Athos Quartzite. This quartz arenite is the most resistant formation in the Evington Group and forms ridges which flank the James River Valley. The formation decreases in thickness and grain size toward the east. The Mount Athos is about 300 m thick and occurs only in the proximal facies of the Evington Group. The lower contact of the Mount Athos is gradational over a short distance by interleaving with the Candler, the upper contact is also gradational over a short distance into greenstone. Two lithologies are predominant in the western most proximal facies fault block:
1) coarse granule conglomerate with clasts of blue quartz and subordinate potassium feldspar, and
2) clast to matrix supported quartz wacke.
Primary sedimentary structures occur throughout the length of the Evington belt in central and northern Virginia. Bedding is planar to irregular, very thin to very thick. Cross stratification occurs in ripples 3 to 5 cm high. Small- scale, tabular- tangential cross stratification occurs. Trough cross strata 3 cm thick and 12 cm wide are developed in pebbly quartz arenite, with stratification by mica- rich layers. Graded bedding occurs in thicknesses varying from less than 5 to 30 cm thick.
According to Patterson [1987a], the graded bedding points to turbidity flow deposition. Massive coarse granule sandstone is structureless and may have been deposited from high density sediment gravity flow. Parallel- laminated sandstones with internal discordances relative to bedding planes, and planar bedded to lenticular sand bodies, are structures which typify hummocky cross stratification. Such structures are formed above storm wave base and below fairweather wave base by combined or oscillatory and unidirectional flow, under storm wave conditions. Tabular, tangential, trough cross stratification is produced by migration of dunes or ripples. Flowing currents which generate cross stratification are not restricted to any environment. Therefore, the Mount Athos is inferred to have been deposited by sediment gravity flows, with possibly minor sediment transport and reworking by storm generated currents.
Slippery Creek Greenstone. This metabasalt, approximately 2 km thick, contains the upper greenschist, epidote- amphibolite faces, mineral assemblage albite (15- 35%) + quartz (0- 14%) + hornblende (8- 45%) + clinozoisite and/or zoisite (1- 20%) + epidote (1- 20%) + minor; titanite, magnetite and biotite. Relict plagioclase phenocrysts and amygdales are locally present. Volcaniclastic layers and quartz muscovite schist occur locally interlayered with the metabasalts. The lava sequence is depositionally conformable with the underlying Mount Athos. The upper contact was not observed in Patterson's area. Bland [1978] gave trace element abundance data to support a rift origin and extrusion through continental crust for the Slippery Creek. The formation is therefore interpreted as a submarine lava related to rifting. Just south of Patterson's area, near Oxford Furnace, the writer has seen enclaves that appear to be xenoliths of coarse- grained granite in the Slippery Creek. These are probably fragments of Grenville basement, suggesting that this part of the Evington Group was deposited on the continent of Laurentia, and not on oceanic crust.
Joshua Schist. This formation contains several siliciclastic lithologies and shows many well developed sedimentary structures. The lower contact with the Slippery Creek was not seen in Patterson's area. In places where the Slippery Creek is absent, the Joshua overlies the Candler gradationally. The upper contact is gradational into the overlying Arch Marble. The formation may be as much as 0.7 km thick in the area. The following rock types occur:
1) Quartz mica schist and phyllite with graded bedding. This facies is commonly graphitic. Graded bedding occurs with quartz sandstone laminae 1 mm thick capped by mica schist 0.2- 0.3 mm thick. The mineralogy is; quartz (30- 65%), muscovite (30- 60%), graphite (0- 25%), biotite (1- 25%) with trace amounts of pyrite, apatite, zircon, plagioclase, tourmaline and titanite.
Soft sediment slump structures are preserved in the coarser graded beds.
2) Dark phyllite is very schistose and shows no primary sedimentary structures aside from bedding. Mineralogy is similar to that of the quartz- mica schist except that there is a lower quartz content. In thin section quartz rich laminae can be seen in the phyllite.
3) Green schist was found at two locations in the Joshua. This lithology contains: hornblende (39%), epidote (15%), biotite (15%), quartz (10%), plagioclase (15%), and minor amounts of clinozoisite and magnetite.
4) Conglomerate occurs locally in the Joshua. Angular to rounded clasts range from 1 mm to 5 cm x 1.5 cm. The clasts consist of quartz, plagioclase and potassium feldspar, micaceous quartz wacke, arkosic wacke, phyllite and dolomite. The conglomerate bodies are matrix to clast supported, with a matrix of fine grained quartz and mica.
5) Quartz wacke, quartz arenite, and calcareous quartz wacke occur in areally restricted lenses throughout the formation. Quartz arenite is rare, and occurs as small isolated outcrops of very fine grained quartzite.
Arch Marble. The Arch is generally laminated to thin bedded and locally massive. Color banding is dark and light depending on the amount of siliciclastic material in the layer. Layering commonly ranges from 0.2 mm to 5 mm in thickness. Locally graded bedding is preserved. The formation is about 200 m thick in Patterson's area. The Arch is considered a deep water carbonate facies deposited by turbidity currents from sources nearer the shore face [Patterson, 1987b; Read, 1989].
ALBEMARLE- NELSON SUITE ULTRAMAFIC INTRUSIVE ROCKS, AND
MAFIC DIKES, SILLS, LAVAS AND TUFFS (POST 650, PRE 570 MA)
A suite of mafic and ultramafic sills and dikes, including mafic lavas with minor felsic derivatives occur in the Lynchburg, Swift Run, Catoctin, Unicoi (Cambrian of western Blue Ridge), and upper Evington Group (Slippery Creek Greenstone). Ultramafic rocks are confined to the sequence below the Catoctin, and a set of hornblende gabbros may also be confined to the pre- Catoctin sequence. Most of these rocks were originally included in the Crossnore Plutonic- Volcanic suite of Rankin and considered to be about 820 Ma [Rankin et al., 1969; Rankin et al., 1973]. Since then additional isotopic dating indicates that the felsic peralkaline plutonic and volcanic rocks of the type Crossnore are older than about 650 Ma. and unconformably underlie the Lynchburg/Ashe formations (see section on Crossnore Plutonic-Volcanic suite.). This suggests that the Crossnore is distinct from the younger ultramafic and basaltic rocks and can be considered a sub- suite of the Late Proterozoic - Early Cambrian rift- related igneous rocks in the Blue Ridge and western Piedmont. In this report the younger mafic- ultramafic suite will be referred to the Albemarle- Nelson suite as herein modified from Burfoot [1930].
Within the region between Culpeper and Charlottesville mapped by Wehr [1985] amphibolite dikes and sills are abundant in the basement and lower part of the Lynchburg and Swift Run sequence, but are found as well, though less commonly, in the upper part of the clastic sequence to a level just below the Catoctin greenstone. Mineralogy [Evans, 1984] of the dikes along the Rockfish River is:
plagioclase (An 25- 35 )+ epidote + hornblende + magnetite + quartz
All minerals are considered to be metamorphic. This would appear to be a medium grade amphibolite facies rock consistent with temperatures above 500deg.C. However, according to Evans [1984] the metamorphic facies of the surrounding Grenville biotite gneiss is greenschist, with garnet- biotite pairs implying a temperature of about 400deg. C. Evans noted that the grade of Paleozoic metamorphism decreases up section into the Lynchburg, Catoctin and Evington, none of which are reported to contain garnet. Davis [1974], working near the same area, described a coarse- grained "hornblende metagabbro" with nearly equidimensional aggregates of metamorphic hornblende in a matrix of highly saussuratized plagioclase. Relict pyroxenes are altered partially or totally to hornblende, zoisite, magnetite and chlorite. Other metamorphic minerals are epidote, titanite, calcite, garnet, and rarely biotite. Davis did not discuss the conditions of regional metamorphism.
Reed and Morgan [1971] analyzed dikes of metabasalt in the Blue Ridge, northwest of the Rockfish area, and concluded that the compositions of the dikes were similar to those of the overlying Catoctin Formation. In Reed and Morgan's area as in the Rockfish area the Catoctin is a greenschist facies basalt derived from a dry pyroxene- bearing protolith with no evidence of hornblende in either mineral assemblage. Since Reed and Morgan's study all subsequent workers seem to have accepted that the amphibolite dikes on the southeast side of the Blue Ridge are also feeders to the Catoctin. Several geologic maps [Wehr, 1983; Brown, 1958] of segments of the belt over a distance of 120 km along strike, from Culpeper to Lynchburg, Virginia, consistently show amphibolite dikes and sills throughout the Lynchburg to within a few tens of meters of the Catoctin, yet the Catoctin is chlorite- and actinolite- bearing, and is without hornblende in these outcrops. Hornblende amphibolite occurs within 500 m, stratigraphically below the Catoctin near Lynchburg, and the Catoctin is a biotite- bearing albite- actinolite schist of probable middle greenschist facies (Ping Wang, personal communication to L. Glover, 1988). If the amphibolite dikes are feeders of the Catoctin, why don't they have a similar mineralogy where they are at levels of emplacement just below the Catoctin?
The Schuyler ultramafic body, in the Rockfish River area is one of a number of thin, tabular ultramafic units emplaced dominantly in the upper part of the Lynchburg and Swift Run formations along the east side of the Blue Ridge in Virginia. They comprise the Albemarle- Nelson soapstone belt [Burfoot, 1930]. The ultramafic association includes, in decreasing order of abundance, amphibolite- chlorite schist, serpentinite, soapstone, and altered peridotite [Burfoot, 1930; Hess, 1933; Brown, 1958; Nelson, 1962; Misra and Keller, 1978]. Hess [1933] concluded that the parent material was peridotite and feldspathic peridotite (picrite). An intrusive contact is visible between thin- bedded Charlottesville Formation and the Schuyler ultramafic sill at Schuyler. Hess [1933] found the following sequence of rock types in the Schuyler body: 1) at the base, ultramafic rock, talc- chlorite- actinolite- calcite; 2) in the middle, gabbroic rock with hornblende and actinolite assemblages; and 3) silicic rocks with quartz- albite- microcline- chlorite- hornblende assemblages. This suggested, to Hess, that the sill had differentiated in place. Brown [1958] thought that the ultramafics might be extrusive rocks, as this would explain the localization parallel to bedding, and association with the Catoctin lavas.
In the Culpeper to Rockfish River region, the ultramafic rocks are confined to the upper part of the Lynchburg Group and Swift Run Formation. None are found above the base of the overlying Catoctin Formation or within the still younger Evington Group. In the Lynchburg area, 100 km south of Charlottesville, ultramafic rocks occur throughout all but the lowest part of the Lynchburg [Brown, 1958] where the contact relations reveal them to be intrusive sills [Wang, 1991]. Differentiation relations between hornblende gabbro and ultramafic rocks suggested for the Schuyler sill [Hess, 1933] and confinement of hornblendite dikes and sills as well as the ultramafics to stratigraphic levels no higher than the Catoctin Formation, suggest that the hornblende- actinolite gabbro and ultramafics may be differentiates of a distinct subcrustal magma (see also Bloomer and Werner [1955]) of upper Lynchburg and/or Swift Run age, that is 650- 570 Ma.
Glover and Wang [1992], Wang and Glover [1992] and Wang [1991] have shown that the ultramafic- mafic assemblages described above are part of the Late Proterozoic Iapetan rift sequence which extends upward and includes the Slippery Creek basalt of the Evington Group, as well as basalts in the Unicoi Formation in the western Blue Ridge. The mafic- ultramafic rocks have been injected into the enclosing Lynchburg sedimentary rocks, and geochemical discriminant diagrams previously used to support an ophiolite origin fail when used on continental basalts. They are not part of an ophiolitic mélange as implied by Hatcher [1987], Hatcher et al. [1989], Rankin et al. [1989], Horton et al. [1989], and Keppie and Dallmeyer [1989], therefore they cannot be used to support a suture in the Blue Ridge.
CORRELATIONS WITH THE GREAT VALLEY, AND THE VALLEY AND RIDGE
The Catoctin and Swift Run formations form a common stratigraphic datum on both sides of the Blue Ridge in central Virginia. This has long been recognized as an important starting point for correlation of the strata across the Blue Ridge [Bloomer and Werner, 1955, Brown, 1970, Patterson, 1987b].
Glover and Costain [1984] and Wehr and Glover [1985] have shown that the Blue Ridge province is the thrust- decapitated crest of the early Paleozoic hinge zone of Laurentia. Late- to-Early Cambrian through Early Ordovician strata west of the Blue Ridge crest belong to the shallow water drift sequence, and correlative rocks east of the Blue Ridge are deep water distal shelf and slope deposits [Brown, 1970; Wehr and Glover, 1985; Patterson, 1987; Glover, 1989].
On the west side of the Blue Ridge, the Catoctin is overlain by the Chilhowee Group, which includes, from oldest to youngest, the Unicoi/Weverton, Hampton/Harpers and Erwin/Antietam formations. The second named in each couplet is the commonly used term for equivalent formations north of central Virginia. In southern Virginia, the Unicoi Formation, at the base of the Chilhowee, is a sequence of dominantly non- marine feldspathic sandstones, conglomerates and basalts formed during the later part of the rift stage that led to the development of Iapetus [Simpson, 1987; Simpson and Eriksson, 1989]. As noted previously, the Unicoi south of the Catoctin pinch out on the west side of the Blue Ridge, probably includes rocks equivalent to the Swift Run which occurs below the Catoctin north of that pinch out.
The Unicoi is overlain by the dominantly progradational [Simpson, 1987], and compositionally more mature, quartz arenitic Hampton and Erwin Formations of the middle and upper Chilhowee. Simpson [1987] and Simpson and Eriksson [1989] place the rift to drift transition at the top of the Unicoi. Overlying the Chilhowee is the Shady Dolomite which records continued progradation of the drift sequence culminating in development of a rimmed shelf [Read, 1989].
This information can be used to provide an improved correlation with the deeper water facies of the Evington Group, which lies above the Catoctin on the east side of the Blue Ridge. In the Patterson [1987] preferred model of this correlation, the rift- to- drift transition is placed just above the Slippery Creek Greenstone, the youngest lava in the sequence. Thus, the Swift Run (?), Catoctin, Candler, Mount Athos, and Slippery Creek should all be approximately correlative with the Cambrian and Late Proterozoic Unicoi Formation. The Joshua Schist should be correlative with the Hampton and Erwin, the Slippery Creek, and the Arch Marble with the Shady Dolomite. The Chilhowee and Shady are Early Cambrian (excepting the older Unicoi Formation of the Chilhowee Group), therefore the Evington Group above the Slippery Creek should be Early Cambrian also. The upper part of the Evington Group is truncated by faults and no younger Laurentian strata are known in the western Piedmont.