Petrology and geochemistry of Neoproterozoic volcanic arc ... - my articles... · Joshua Mauldin Department of Geological Sciences, University of South Carolina, Columbia, South Carolina

For permission to copy, contact [email protected] 2004 Geological Society of America572

GSA Bulletin; May/June 2004; v. 116; no. 5/6; p. 572–593; 14 figures; 4 tables; Data Repository item 2004074.

Petrology and geochemistry of Neoproterozoic volcanic arc terranesbeneath the Atlantic Coastal Plain, Savannah River Site,

South Carolina

Allen J. Dennis†

Department of Biology and Geology, University of South Carolina, Aiken, South Carolina 29801-6309, USA

John W. ShervaisDepartment of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA

Joshua MauldinDepartment of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA

Harmon D. Maher, Jr.Department of Geology and Geography, University of Nebraska Omaha, Omaha, Nebraska 68182-0199, USA

James E. WrightDepartment of Geology, University of Georgia, Athens, Georgia 30602, USA

ABSTRACT

The Piedmont of South Carolina andGeorgia is a complex mosaic of exotic ter-ranes of uncertain provenance. Farthersouth and east, these terranes form thebasement beneath several kilometers ofCretaceous and Cenozoic sedimentaryrocks, commonly referred to as the AtlanticCoastal Plain. The distribution and geolog-ic history of this hidden crystalline base-ment can be inferred only on the basis oflimited exposures at the margins of theCoastal Plain onlap, aeromagnetic linea-ments that define basement trends in thesubsurface, and core data from wells thatpenetrate basement.

During the past 40 years, basement coresaggregating more than 6 miles (10,000 m)have been recovered from 57 deep wells atthe Department of Energy’s Savannah Riv-er Site. These cores provide the only knownsamples of basement terranes that lie south-east of the Fall Line in central South Car-olina. Cores from the 57 deep wells, alongwith structural trends defined by aeromag-netic lineaments, allow us to define four dis-tinct units within the basement beneath theCoastal Plain: (1) the Crackerneck Meta-volcanic Complex (greenstones and felsic

†E-mail: [email protected].

tuffs, all metamorphosed under greenschist-facies conditions), (2) the Deep RockMetaigneous Complex (mafic to felsic vol-canic and plutonic rocks metamorphosedunder lower amphibolite–facies conditions),(3) the Pen Branch Metaigneous Complex(amphibolites, garnet amphibolites, garnet-biotite schists, and gneiss), and (4) the Tri-assic Dunbarton Basin Group, a sedimen-tary unit that fills a northeast-trendinggraben beneath younger sedimentary rocksof the Atlantic Coastal Plain.

All of the metaplutonic and metavolcanicrocks have calc-alkaline fractionationtrends, consistent with formation in sub-duction-related arc terranes at convergentmargins. Zircon U-Pb crystallization agesof ca. 626 Ma to 619 Ma, however, showthat the Deep Rock and Pen Branch com-plexes do not correlate with the youngerCarolina terrane (570–535 Ma) or Suwan-nee terrane (ca. 550 Ma). The Deep Rockand Pen Branch Metaigneous Complexesmay be a continuation of Proterozoic base-ment that forms the older infrastructure ofthe Carolina arc. The contact between theCrackerneck Metavolcanic Complex (5Persimmon Fork Formation?) and the DeepRock and Pen Branch Metaigneous Com-plexes thus may be equivalent to the an-gular unconformity between the UwharrieFormation and the Virgilina sequence. On

the basis of their compositions and ages, wetentatively correlate these rocks with theHyco Formation in southern Virginia andcentral North Carolina. The Hyco Forma-tion constitutes the infrastructure of theCarolina terrane in Virginia and NorthCarolina, where it was affected by the ca.600 Ma ‘‘Virgilina’’ orogeny. The rocks ofthe Deep Rock and Pen Branch Metaig-neous Complexes may have formed the arcinfrastructure of the Carolina Slate belt inSouth Carolina, detached by later tectonicevents, or may have formed the Late Pro-terozoic arc infrastructure at another lo-cation in the arc that has been moved intoits current location by transcurrent mo-tions. Limited age and isotopic data suggestthat none of these rocks correlate with theSuwannee terrane of North Florida andsouthern Georgia.

Keywords: Neoproterozoic, peri-Gondwana,arc volcanism, Carolina terrane, geochem-istry, petrology.

INTRODUCTION

The hinterland of the southern Appala-chians, which lies southeast of Grenville base-ment and an attached cover sequence exposedin the Blue Ridge province, comprises a com-plex mosaic of exotic or ‘‘suspect’’ tectono-

Geological Society of America Bulletin, May/June 2004 573

NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

Figure 1. Regional map of the southern Appalachians, showing the distribution of majortectonic subdivisions of the Laurentian margin (Blue Ridge, Piedmont, Carolina terrane)and the extent of post-Jurassic sedimentary onlap onto the continental margin (AtlanticCoastal Plain). Previous areas studied by the authors in the Appalachian Piedmont ofSouth Carolina are shown as polygons: (1) western Carolina terrane/Charlotte belts, Cen-tral Piedmont suture (Dennis and Shervais, 1991, 1996; Dennis and Wright, 1995, 1997;Dennis, 1995; Dennis et al. 1995); (2) eclogite/high-P granulites of the Charlotte belt–Carolina Slate belt boundary and Carolina Slate belt in central South Carolina (Shervaiset al., 2003; Dennis et al., 2000b); and (3) the Slate belt–Kiokee belt–Belair belt (Maheret al., 1981, 1991, 1994; Maher, 1987a, 1987b; Dennis et al. 1987; Shervais et al., 1996).Location of current study shown as circle (4) Savannah River Site. AF—Augusta fault,BZ—Brevard zone, CPS—central Piedmont suture, GH—Gold Hill fault, GSF—GreatSmoky Fault, H-FFS—Hayesville-Fries fault system, MZ—Modoc Zone.

stratigraphic terranes that range in age fromlate Neoproterozoic through middle Paleozoic.These terranes were accreted to Laurentia dur-ing the middle to late Paleozoic and now formall of the exposed crystalline rocks east of theBlue Ridge (e.g., Williams and Hatcher, 1982,1983; Secor et al., 1983; Maher et al., 1981,1991; Horton et al., 1989, 1991; Samson etal., 1990; Hibbard et al., 2002). Farther to thesoutheast, terranes accreted to and overthrustonto crystalline basement of the Laurentianmargin are hidden beneath several kilometersof Mesozoic and Tertiary sedimentary rocks,commonly referred to as the Atlantic CoastalPlain (Colquhoun, 1995; Fallaw and Price,1995). Deciphering the origin and provenanceof this buried crystalline basement is centralto our understanding of terrane accretion dur-ing the Paleozoic and has implications for col-lisional orogenesis in the hinterland of thesouthern Appalachians.

The U.S. Department of Energy SavannahRiver Site is located near the northwest mar-gin of the Atlantic Coastal Plain in centralSouth Carolina (Fig. 1), where up to 2 km ofCretaceous and younger sedimentary depositsoverlie crystalline rocks of the basement anda major Triassic rift basin. During the past 40

years, basement cores totaling more than 6miles (.10,000 m) long have been recoveredfrom 57 deep wells at the Department of En-ergy’s Savannah River Site. Southeast of theeastern Piedmont in central South Carolinaand Georgia, these cores provide most of theknown samples of crystalline basement and,along with structural trends defined by aero-magnetic lineaments, allow us to define fourdistinct units within the basement beneath theCoastal Plain. Three of these units representcrystalline basement; they comprise mafic tofelsic metavolcanic rocks and dioritic to gra-nitic metaplutonic rocks, all metamorphosedat greenschist- to amphibolite-facies condi-tions. The fourth unit represents the clasticsedimentary fill of a northeast-trending Trias-sic graben, the Dunbarton Basin.

This paper examines metaigneous rockssampled by these deep core holes and com-pares them to rocks exposed in surface out-crops throughout the Piedmont of the Caroli-nas, Georgia, and Virginia; a parallelinvestigation of much of this core materialwas carried out by Roden et al. (2002). Forthese rocks we present new whole-rock geo-chemistry, mineral chemistry, and isotopicdata, including two U-Pb zircon ages, examine

their petrologic evolution, and explore theirsubsequent tectonic evolution.

GEOLOGIC FRAMEWORK OF THESAVANNAH RIVER SITE

Crystalline basement at the Savannah RiverSite is entirely covered by onlap of the Atlan-tic Coastal Plain. This basement is separatedfrom well-characterized rocks of the CarolinaSlate belt (Carolina terrane) by three majorfault zones (Secor et al., 1986a, 1986b; Maheret al., 1991, 1994; Horton et al., 1991): theModoc zone, the Augusta/Belair fault sys-tems, and an inferred fault zone indicated bya strong aeromagnetic lineament that definesthe southern edge of the Belair belt (Ascaugafault zone; Fig. 2). South of this aeromagneticlineament, high-grade sillimanite-bearinggneisses of the Belvedere belt are intruded bythe Carboniferous Graniteville pluton (e.g.,Samson et al., 1995a); both are exposed inerosional windows through the coastal-plainsediments (Fig. 2). The limits of the Granite-ville pluton may be estimated from its asso-ciated gravity anomaly; gravity anomalies alsodefine the extent of the Devonian Springfieldpluton (Speer, 1982) and smaller, unnamedgranitic stocks related to the larger bodies(Fig. 2). The Graniteville pluton is penetratedand sampled by a single borehole, the C-2well.

Magnetic and gravity potential field datafrom the Savannah River Site, coupled withcore logs from over 30 locations, show thatthe subsurface geology can be divided intofour main units (from north to south): (1)Crackerneck Metavolcanic Complex: low-grade (greenschist-facies) metavolcanic rocks(tuffs, lapilli tuffs, lavas), named for a smallcreek that drains the surface above this unit;(2) Deep Rock Metaigneous Complex: uppergreenschist– to lower amphibolite–facies me-tavolcanic rocks, metaplutonic rocks, andcrosscutting dikes, strongly deformed or my-lonitized in places, named for its type expo-sures in the Deep Rock Borehole (DRB) seriesof wells; (3) Pen Branch Metaigneous Com-plex: upper amphibolite to granulite-faciesmetagranitoids and metavolcanic rocks,named for a small creek that drains the surfaceabove this unit and for its type exposures inthe ‘‘Pen Branch fault’’ (PBF) series of wells;and (4) Dunbarton Basin: a deep Triassic riftbasin with fault-controlled margins that wasburied by coastal-plain sediments (Marine,1974; Marine and Siple, 1974; Cumbest et al.,1992; Fallaw and Price, 1995). These units areshown in Figure 2. South of the site, geo-physical potential field data have been inter-

574 Geological Society of America Bulletin, May/June 2004

DENNIS et al.



Figure 3. Outline map of Savannah River Site and surrounding area showing locations ofdeep wells that intersect basement studied for this project.

Figure 2. Geologic map showing the distribution of crystalline basement lithologic units in and around the Savannah River Site (dashedoutline), based on our work and that of Petty et al. (1965), Daniels (1974), Speer (1982), Cumbest et al. (1992), Stevenson and Talwani(1996), and Snipes (1996, personal commun.). Inset shows location of main map. Major units beneath the Savannah River Site includethe Crackerneck Metavolcanic Complex, the Deep Rock Metaigneous Complex, the Pen Branch Metaigneous Complex, and clasticsedimentary rocks of the Triassic Dunbarton basin. Triassic–Jurassic mafic igneous complex south of the Dunbarton basin is based onaeromagnetic anomalies; core data from wells C-7 and C-10 indicate that much of this basement is pinked granite of the PBF7 intrusivecomplex. Subsurface extents of the Devonian Springfield pluton and the Carboniferous Graniteville pluton are based on gravity anom-alies. The Graniteville pluton is sampled by core from well C-2 and in limited surface exposures (outlined on map). The Springfieldpluton is sampled by well SAL-1. EPFS—Eastern Piedmont fault system. DRB—Deep Rock Borehole series of wells.N

preted to show the presence of a Triassic–Jurassic mafic igneous complex in the subsur-face (e.g., Petty et al., 1965; Daniels, 1974;Stevenson and Talwani, 1996), but limitedcore data from this area (from the C-10 well)show that much of this basement is similar tothat found in the Pen Branch MetaigneousComplex.

The most prominent tectonic feature of theSavannah River Site subsurface is the PenBranch fault, a Triassic border fault along thenorthern margin of the Dunbarton Basin thatwas reactivated after the Cretaceous (Fig. 2;Snipes et al., 1993). Other tectonic featuresinclude (1) the Tinker Creek nappe, a Paleo-zoic structure adjacent to the Pen Branch faultthat places high-grade rocks of the Pen BranchMetaigneous Complex against lower-graderocks of the Deep Rock Metaigneous Com-plex; (2) the Crackerneck fault, a northeast-trending feature that offsets the sub-Cretaceous unconformity; (3) the Upper ThreeRuns fault, a northeast-trending lineament inthe aeromagnetic map forms the southeastmargin of the Crackerneck MetavolcanicComplex; and (4) the Martin fault, a poorlydefined northeast-trending feature that formsthe southern margin of the Dunbarton Basinand also offsets the sub-Cretaceous unconfor-mity (Fig. 2: Domoracki, 1995; Snipes et al.,1993).

METHODS

Core was logged for 26 wells drilled in andaround the Westinghouse/Savannah RiverSite, including wells drilled between approx-imately 1960 (DRB series) and the mid-1990s(C series, MMP series). Figure 2 is a geologicmap based on subcrop lithology; well loca-tions are shown in Figure 3. Cores from allwells studied here were logged for lithology,structural relationships, fractures, folds, folia-tions, layering, and relict primary intrusivecontacts. Over 400 samples were collected,and from this collection ;150 samples wereselected for further petrologic study, includingpetrographic, mineral-chemical, and/or whole-

rock geochemical analysis. Petrographic andmineral assemblage data for each unit aresummarized in Table 1; formal descriptionsand type sections for all new units defined inthis publication are available (Table DR-11).

Whole-rock samples were analyzed for 10major elements (SiO2, TiO2, Al2O3, total Fe asFe2O3*, MnO, MgO, CaO, Na2O, K2O, P2O5)

1GSA Data Repository item 2004074, formal de-scription of new units described herein, whole rockmajor and trace element analyses by x-ray fluores-ence, whole rock REE and other analyses by ICP-MS, representative plagioclase and amphibole min-eral chemistry, is available on the Web at http://www.geosociety.org/pubs/ft2004.htm. Requestsmay also be sent to [email protected].

and 12 trace elements (Nb, Zr, Y, Sr, Rb, Zn,Cu, Ni, Cr, Sc, V, Ba) by using the Universityof South Carolina’s Philips PW-1400 X-rayfluorescence spectrometer. Major elementswere analyzed on fused glass disks by usinga method similar to that of Taggart et al.(1987) with selected U.S. Geological Surveyand international standards prepared identical-ly to the samples. Accepted concentrationswere taken from the compilation of Potts etal. (1992). Matrix corrections were carried outwithin the Philips X41 software package,which uses the fundamental parameters ap-proach (Rousseau, 1989) to calculate theoret-ical alpha coefficients for the range of stan-dards. Replicate analyses of selected standards


DENNIS et al.

TABLE 1. SUMMARY OF LITHOLOGIES, MINERAL ASSEMBLAGES, AND PETROGRAPHY OF UNITS DISCUSSED

Unit Lithologies Mineral assemblages Relict igneousphases

Metamorphic facies

Crackerneck Metavolcanic Complex Felsic tuffs and lapilli tuffs, mafic tuffs andgreenstones

Qtz, Alb, Chl, Epi, Ms,Opq

None Greenschist tosubgreenschist

Deep Rock Metaigneous ComplexDeep Rock Metavolcanic Suite Aphyric mafic to intermediate tuffs,

plagioclase-phyric tuffs and schists,hornblende-phyric tuffs and schists

Plg An3–8, Qtz, Chl,Epi, Bi, 6BGA, Gt

(rare)

Hbl, Plg An29–43 Epidote amphibolite

DRB1 Metaplutonic Suite Diorite–quartz diorite gneiss,porphyroclastic gneiss

Plg An9–20, Qtz, Chl,Epi, Act, BGA

Hbl, Plg An20–40 Epidote amphibolite

Pen Branch Metaigneous ComplexPen Branch Metavolcanic Suite Mafic to intermediate amphibolite, garnet

amphibolitePlg An35–45, BGA, Qtz,

Bi, Epi, GtNone Garnet amphibolite

PBF7 Metaplutonic Suite, normal Metagranite, granodiorite gneiss Plg An24–38, BGA, Qtz,Ksp, Epi, Bi, Chl

None Garnet amphibolite

PBF7 Metaplutonic Suite, pinked Metagranite, granodiorite gneiss Plg An4, Ksp, Qtz, Gt,Chl, rare BGA, Bi

None Gt amphibolite withgreenschist

overprint

Note: Qtz—quartz, Plg—plagioclase, Alb—albite, Ksp—microcline, Chl—chlorite, Epi—epidote, Act—actinolite, BGA—blue-green amphibole, Hbl—hornblende (igneous),Bi—biotite, Ms—muscovite, Gt—garnet, Opq—opaques.

as unknowns suggest percent relative errors;1% for SiO2, ;2%–4% for less abundantmajor elements, and ;1%–6% for all trace el-ements except Cr, which is slightly higher(13%). Thirty whole-rock samples were alsoanalyzed for rare earth element (REE) andother trace element concentrations by induc-tively coupled argon plasma–mass spectrom-etry (ICP-MS) at the University of New Mex-ico. Selected major and trace element data arepresented in Table 2; the complete data set ofmajor and trace element data are available (Ta-bles DR-2 and DR-3 [see footnote 1]).

Quantitative electron-microprobe analysesof major and minor elements in minerals wereobtained with a Cameca SX50 electron micro-probe at the University of South Carolina.Analyses were made at 20 kV acceleratingvoltage, 30 nA probe current, and countingtimes of 20–100 s; both natural and syntheticmineral standards were used. Analyses werecorrected for instrumental drift and dead time,and electron beam/matrix effects by using the‘‘PAP’’ f(rz) correction procedures providedwith the Cameca microprobe automation sys-tem; these correction procedures are based onthe model of Pouchou and Pichoir (1991). An-alytical precision is ;1% of the amount pres-ent for oxide concentrations greater than 10wt%, 1%–2% for oxide concentrations be-tween 1 and 10 wt%, and 5%–10% for oxideconcentrations between 0.01 and 1 wt%. Rep-resentative mineral analyses are available (Ta-bles DR-4 and DR-5 [see footnote 1]).

Zircon was separated by conventional tech-niques using a Wilfley Table, heavy liquids,and a Franz magnetic separator. The leastmagnetic zircons from each sample were splitinto size fractions and then handpicked to re-move any contaminating grains. Zircon dis-

solution and ion-exchange chemistry for sep-aration of uranium and lead followedprocedures modified from Krogh (1973). Iso-tope ratios were measured with the MAT 262multicollector instrument at Rice University.Analytical uncertainties, blanks, and commonlead corrections are outlined in Table 3.

DESCRIPTION OF THE LITHOLOGICUNITS

Crackerneck Metavolcanic Complex

The Crackerneck Metavolcanic Complex(named herein as described in Table DR-1 [seefootnote 1]) underlies the northwestern cornerof the Savannah River Site and continues atleast as far north as the Graniteville pluton(Fig. 2). The complex is represented by coresfrom wells C-1, C-3, P30, P6R, P8R, GCB-1,GCB-2, GCB-3, and MMP4 (Fig. 3). Rocksin the northern part of this unit are dominantlyintermediate-composition to felsic tuffs andlapilli tuffs, whereas cores from the southernpart of the subcrop area are dominated bygreenstone and mafic tuff (Table 1). Low-grade metavolcanic rocks of the complex arepenetrated by younger granitic intrusions, in-cluding the Devonian Springfield granite andthe Carboniferous Graniteville granite (Fig. 2).

Felsic tuffs of the Crackerneck Metavolcan-ic Complex are generally composed of quartzand plagioclase and have sparse plagioclasephenocrysts (Fig. 4A). Mafic tuffs are com-posed of chlorite, plagioclase, epidote, andopaque minerals. All plagioclase grains weremetamorphosed to nearly pure albite (An2 toAn5: Fig. 5A), but relict igneous textures aretypically well preserved. Lapilli tuffs in coreMMP4 display large pumice lapilli that were

flattened and elongated, giving these rocks anappearance similar to lapilli tuffs of the Per-simmon Fork Formation.

Deep Rock Metaigneous Complex

The Deep Rock Metaigneous Complex(named herein as described in Table DR-1 [seefootnote 1]) comprises most of the SavannahRiver Site basement north of the Triassic Dun-barton Basin (Fig. 2). It consists of two sub-units: the Deep Rock Metavolcanic Suite andthe DRB1 Metaplutonic Suite (Table 1).

Deep Rock Metavolcanic SuiteThe Deep Rock Metavolcanic Suite was

sampled most extensively by the 4-inch-di-ameter (10.16-cm-diameter) Deep Rock Bore-hole (DRB) cores, but is also penetrated by anumber of other, more widely spaced wells(SSW-1, SSW-2, SSW-3, GCB-5.1; Fig. 3).Metavolcanic rocks of the Deep Rockmetavolcanic complex display a wide arrayof textures (aphyric, plagioclase-phyric,hornblende-phyric), compositions, and miner-als. Mineral assemblages are consistent withlower amphibolite–facies metamorphism. Allrocks have been overprinted with a penetrativefabric (foliation) that dips ;408 to 558; basedon regional correlations, the orientation ofstrike is probably northeast and the directionof dip is to the southeast.

Plagioclase phenocrysts commonly formsubhedral to euhedral crystals (Fig. 4B) thatpreserve primary igneous compositions andzoning (An29 to An43). Other plagioclasegrains are anhedral, albitized (An3 to An8) por-phyroclasts (Fig. 5A) that have been partiallyto extensively epidotized. Hornblende pheno-crysts range from blocky, subhedral crystals



Figure 4. Photomicrographs of Crackerneck Metavolcanic Complex and Deep Rock Me-taigneous Complex. (A) Subhedral feldspar phenocrysts within a matrix of plagioclase,quartz, and minor biotite, Crackerneck Metavolcanic Complex (cross-polarized light). (B)Subhedral plagioclase phenocryst in mafic metavolcanic rock, Deep Rock MetavolcanicSuite (cross-polarized light). (C) Relict igneous hornblende (brown) surrounded by mantleof metamorphic blue-green amphibole, DRB1 Metaplutonic Suite (plane-polarized light).Width of field of view represents 2.3 mm in all.

to elongated, prismatic crystals (Fig. 4C).Compared to metamorphic amphiboles, relictigneous hornblende phenocrysts (brown inthin section) are characterized by lower Al2O3

and FeO contents and by higher SiO2, MgO,

and TiO2 contents and Mg/Fe ratios. Horn-blende phenocrysts and groundmass horn-blende are commonly metamorphosed to blue-green tschermakitic amphibole that is low inTi and high in Al (e.g., Roden et al., 2002;

Fig. 6). Many intermediate-composition to fel-sic metavolcanic rocks contain biotite, whichis commonly partially chloritized, and a fewcontain garnet. Garnet-bearing rocks are mostcommon in DRB-3, which may samplehigher-grade metamorphic rocks than othercores from this unit.

Postkinematic dikes are common in coresDRB-2, DRB-3, DRB-5, and DRB-6. Thesedikes, which crosscut the metavolcanicrocks, are generally undeformed (not folded)and lack the fabric elements of the wallrocks, but are metamorphosed to similargrade. There are three groups of dikes:(1) aphyric with basaltic compositions, (2)plagioclase-phyric with basaltic composi-tions, and (3) aphyric with rhyolitic compo-sitions. Plagioclase-phyric mafic dikes arecharacterized by large, euhedral plagioclasephenocrysts or glomerocrysts that preserverelict primary zoning, within a groundmassof amphibole, plagioclase, epidote, chlorite,and biotite. The aphyric mafic dikes are iden-tical to the plagioclase-phyric dikes, but lackthe conspicuous large phenocrysts. The felsicdikes consist of fine-grained plagioclase andquartz and have minor opaque oxides, biotite,and hornblende.

Core from DRB-4 contains a number ofrocks with extremely high SiO2 values (78%–86%) and high modal mica; these rocks mayrepresent sedimentary protoliths. These rocksare extremely quartz rich and carry sparse pla-gioclase porphyroclasts in a mylonitic ground-mass. Many of these rocks contain unalteredbiotite, and others contain abundant muscoviteand aluminosilicate minerals; the rocks tex-turally resemble button schists. These char-acteristics support a sedimentary protolith forthese samples.

DRB1 Metaplutonic SuiteThe DRB1 Metaplutonic Suite is sampled

exclusively by core DRB-1. The suite consistsdominantly of diorite and quartz diorite sheetsthat intrude preexisting metavolcanic wallrock. The DRB-1 metadiorite are metamor-phosed to epidote-amphibolite–facies assem-blages and range texturally from equigranularto porphyritic gneisses (Table 1).

Plagioclase porphyroclasts may show relictigneous features, including oscillatory zoningand albite twinning. Individual porphyroclastsmay have cores as calcic as An40 and rims assodic as An9 (Fig. 5B). The calcic cores areinterpreted to represent relict igneous plagio-clase compositions; the sodic rims formedduring metamorphism along with the blue-green amphibole (e.g., Roden et al., 2002).Metaplutonic rocks of DRB-1 contain no gar-


DENNIS et al.

TABLE 2. REPRESENTATIVE ANALYSES OF METAIGNEOUS ROCKS FROM THE SAVANNAH RIVER SITE, SOUTH CAROLINA

Core no.: C1 C3 P30 MMP4 DRB2 DRB2 DRB3 DRB3 DRB4 DRB4 DRB4 DRB4 DRB4 DRB5 DRB5Box: none none none none 17M 17 6 59 76 131 101 123 145 73 15MSample no.: 574 540 781 817 1081M 1081 988 1346 1450.7 1831 1613 1758.4 1912 1802 1324Rock type: Felsic

tuffFelsic

tuffFelsic

lapilli tuffFelsic

lapilli tuffFelsic

MVMaficMV

FelsicMV

MaficMV

Felsicgneiss

FelsiteMV

Inter MV MaficMV

MaficMV

FelsiteMV

MaficMV

SiO2 68.32 71.55 74.29 76.98 63.44 51.74 65.59 49.94 78.32 70.98 68.05 51.47 54.51 72.89 50.48TiO2 0.70 0.66 0.23 0.23 0.96 1.06 0.80 1.13 0.10 0.74 0.80 2.12 1.71 0.38 0.97Al2O3 16.00 14.24 12.40 11.63 16.41 16.98 16.81 20.85 11.74 13.98 13.90 15.12 14.57 14.34 18.88Fe2O3 5.96 5.04 4.29 2.96 7.44 10.82 4.65 10.56 1.21 4.20 5.07 13.26 11.21 2.94 11.13MnO 0.13 0.09 0.14 0.13 0.19 0.188 0.12 0.18 0.02 0.11 0.13 0.21 0.19 0.04 0.19MgO 1.93 1.54 0.71 1.11 1.90 4.61 1.58 3.70 0.15 1.60 1.54 6.95 6.10 0.87 5.68CaO 2.09 1.34 0.62 0.39 4.83 9.44 3.47 10.20 0.77 3.44 5.04 7.46 7.63 2.17 9.30Na2O 2.87 4.15 5.16 4.41 4.27 2.91 4.38 3.49 5.82 2.05 1.39 2.36 2.23 5.87 3.29K2O 2.52 1.52 1.31 2.01 1.59 1.186 3.18 1.53 0.91 2.37 2.25 0.54 1.55 0.96 0.57P2O5 0.15 0.16 0.03 0.03 0.43 0.335 0.26 0.40 0.01 0.14 0.16 0.26 0.23 0.08 0.24SUM 100.66 100.29 99.18 99.88 101.45 99.27 100.85 101.98 99.05 99.59 98.32 99.74 99.905 100.54 100.75LOI 1.93% 2.47% 0.86% 0.34% 1.90% 2.55% 2.00% 0.72% 1.17% 1.74% 1.81% 1.01% 0.75% 0.97%

Nb 12 9 14 9 10 6 11 5 29 16 16 16 11 11 4Zr 206 159 287 296 175 148 188 102 190 176 182 185 170 149 78Y 22 19 72 65 45 31 35 25 76 41 42 33 28 30 21Sr 350 106 64 35 634 715 365 782 48 116 146 310 209 331 637Rb 69 30 27 32 38 52 62 28 40 78 92 23 55 17 11Zn 88 76 134 138 60 106 75 94 30 59 71 105 93 28 98Cu 17 5 2 2 17 174 4 148 11 nd 22 70 76 27 120Ni 34 9 42 43 5 18 nd 18 3 14 27 165 178 2 14Cr 42 10 79 160 6 47 3 28 nd 48 63 329 384 2 22Sc 14 7 nd nd 13 30 7 20 4 nd 15 32 29 7 18V 97 45 7 5 80 309 74 287 19 56 109 304 226 38 221Ba 1241 753 119 438 625 500 1440 607 244 317 440 233 447 301 274

La 28.1 13.6 11.6 16.5 11.9 19.2 21.6 19.8 30.3 26.4 29.8 12.7 11.7 12.3 7.8Ce 66.5 34.4 31.3 46.7 29.1 42.5 48.6 50.7 70.4 59.9 68.2 30.8 29.6 28.7 19.1Pr 8.1 4.7 3.8 6.3 3.9 4.8 5.8 7.2 8.2 7.7 8.5 4.2 3.6 3.5 2.4Nd 29.4 18.3 16.2 24.6 15.0 18.2 21.0 29.2 30.0 26.9 28.4 17.1 12.2 13.1 10.2Sm 7.2 4.2 3.6 5.9 3.7 3.9 5.9 6.9 7.4 5.8 6.1 4.4 3.2 3.2 2.4Eu 1.44 0.95 0.59 0.87 0.79 0.78 1.50 1.81 0.34 0.96 1.19 1.05 0.62 0.47 0.59Gd 4.7 3.4 5.7 7.4 4.1 3.9 4.6 5.4 6.8 6.2 6.2 4.5 3.1 3.2 2.8Tb 0.84 0.57 1.12 1.50 0.71 0.69 0.80 0.84 1.34 1.15 1.08 0.82 0.62 0.68 0.55Dy 4.3 3.1 6.0 7.5 3.8 3.9 4.6 4.2 7.2 5.0 5.4 4.6 2.5 3.8 2.7Ho 0.82 0.57 1.40 1.54 0.88 0.87 1.11 0.76 1.62 1.07 1.02 0.96 0.51 0.98 0.61Er 1.9 1.5 4.5 4.7 2.4 2.8 2.9 2.2 4.2 3.5 2.8 2.5 1.9 2.9 1.5Tm 0.30 0.25 0.67 0.65 0.41 0.42 0.46 0.32 0.66 0.48 0.47 0.34 0.31 0.48 0.23Yb 1.3 1.2 3.2 3.7 2.1 2.2 2.7 1.6 3.4 2.3 2.3 1.8 1.3 2.4 1.2Lu 0.20 0.17 0.57 0.50 0.36 0.32 0.39 0.23 0.53 0.31 0.35 0.24 0.25 0.36 0.20Y 17.5 16.7 68.3 64.0 22.5 26.2 29.5 25.5 41.3 46.2 43.6 25.9 27.9 23.9 14.4Pb 11.2 8.1 2.8 11.1 7.1 8.4 10.0 7.1 13.8 13.3 11.8 6.7 7.1 8.5 6.7Th 6.7 1.1 0.6 0.8 3.0 8.1 8.2 0.8 15.2 1.6 1.7 2.1 0.6 4.7 1.2U 1.6 1.1 0.6 0.8 0.8 1.8 2.0 0.8 3.2 1.6 1.7 0.4 0.6 1.2 0.3

net, consistent with metamorphism at relative-ly low temperatures and pressures (loweramphibolite–facies conditions). Biotite is rareto absent in the DRB-1 metadiorites; almostall primary biotite has been retrograded tochlorite.

Most amphibole porphyroclasts, and essen-tially all groundmass amphiboles, are meta-morphosed to blue-green amphibole (Roden etal., 2002). Some amphibole porphyroclastspreserve cores of brown hornblende, inter-preted to represent relict igneous hornblende,with higher Mg/Fe ratios than the metamor-phic blue-green amphibole. These differencesare shown clearly in a plot of tetrahedral Alvs. Ti, where metamorphic amphibole has AlIV

. 1.4 afu (atoms per formula unit on the basisof 23 oxygens) and Ti , 0.06 afu (Fig. 6).Coexisting igneous hornblende has higher Ti

(0.03 to 0.45 afu; most samples have .0.1 afuTi) and lower AlIV (,1.35 afu; Fig. 6). Someblue-green metamorphic amphiboles containcores of fibrous actinolite (AlIV ø 0.4 afu; Fig.6) that formed under retrograde greenschist-facies conditions.

The DRB1 Metaplutonic Suite contains xe-noliths and screens of amphibolite-grade me-tavolcanic rocks that are deformed and containfabric elements similar to those of rocks of theDeep Rock Metavolcanic Suite, with whichthey are assumed to correlate.

Pen Branch Metaigneous Complex

The Pen Branch Metaigneous Complex(named herein for the Pen Branch fault, as de-scribed in Table DR-1 [see footnote 1]) formsa thin slice of crystalline basement between

the Triassic Dunbarton Basin to the south andthe more extensive Deep Rock MetaigneousComplex to the north (Fig. 2). It also appearsto continue on the south side of the DunbartonBasin, but few drill holes penetrate basementthere. The Pen Branch Metaigneous Complexconsists of two subunits: the PBF7 Metavol-canic Suite and the PBF7 Metaplutonic Suite.

Pen Branch Metavolcanic SuiteRocks of the Pen Branch Metavolcanic

Suite were recovered from eight wells: PBF-7, PBF-8, SSW-1, SSW-2, SSW-3, GCB-4, C-5, and the seismic attenuation (SA) well (lo-cated adjacent to GCB-4 in Fig. 3). Maficmetavolcanic rocks in the lowermost part ofPBF-7 are separated from the overlying me-tagranitoids by a fault ;1100 m below thesurface. Higher up in this same well, mafic



TABLE 2. (Continued)

Core no.: DRB6 DRB6 DRB2 DRB2 DRB1 DRB1 DRB1 DRB1 C5 PBF7 PBF7 PBF7 PBF7 C5 PBF7Box: 4M 103 108 12 18 25 41 60 none 103 107 108 139 none 171Sample no.: 1125 1810 1687 1053.5 1005 1047 1151 1276 1084 2550 2602 2629 3086 1080 3568Rock type: Mafic

MVMaficMV

Maficdike

Plgporphdike

Quartzdiorite

Diorite Quartzdiorite

Tonalite Maficschist

Grano-diorite

Grano-diorite

Grano-diorite

Grano-diorite

Felsicdike

Pinkgranite

SiO2 51.87 49.44 51.76 51.89 60.04 50.00 54.34 70.37 56.79 58.16 54.69 64.24 59.30 73.65 75.31TiO2 1.32 1.29 1.19 1.05 0.78 1.09 1.17 0.58 1.33 1.39 1.76 1.07 0.83 0.46 0.14Al2O3 17.74 17.68 18.00 17.18 17.05 17.39 16.14 15.10 16.76 16.74 16.54 15.66 17.54 13.43 13.23Fe2O3 11.52 12.39 10.77 11.06 8.05 12.10 12.08 3.61 8.25 8.46 9.93 6.18 8.37 2.88 1.36MnO 0.21 0.23 0.18 0.19 0.15 0.20 0.15 0.05 0.152 0.15 0.18 0.12 0.18 0.07 0.04MgO 4.15 5.03 3.70 4.78 3.15 6.72 4.25 1.00 4.91 3.46 3.52 1.97 2.97 0.64 0.28CaO 8.23 8.90 9.57 9.33 5.89 9.60 8.37 3.22 6.26 6.94 7.69 4.70 5.82 1.58 0.98Na2O 2.79 1.73 3.18 3.06 4.28 3.84 3.21 6.78 3.81 3.93 2.84 3.90 4.11 3.67 4.03K2O 2.45 3.94 0.95 1.18 0.93 0.47 1.03 0.29 1.871 1.48 3.15 3.22 1.69 3.85 5.25P2O5 0.42 0.39 0.35 0.33 0.18 0.14 0.16 0.15 0.192 0.34 0.60 0.21 0.13 0.10 0.02SUM 100.70 101.01 99.64 100.03 100.49 101.56 100.91 101.15 100.33 101.06 100.90 101.27 100.97 100.33 100.64LOI 1.08% 1.67% 1.05% 0.96% 1.51% 1.50% 0.69% 1.33% 2.29% 1.60% 1.26% 0.63% 1.78% 1.44% 0.71%

Nb 7 6 5 6 7 3 5 4 9 19 18 23 5 30 40Zr 164 163 148 132 141 65 107 85 192 275 305 298 134 246 115Y 33 28 34 32 25 18 27 16 30 42 53 44 21 53 82Sr 711 885 617 645 316 263 297 381 458 421 347 311 341 205 27Rb 59 108 21 30 19 7 20 3 61 49 97 70 55 72 106Zn 125 136 62 98 81 91 69 59 72 179 89 66 103 40 34Cu 230 150 178 162 81 87 26 6 18 81 117 51 61 15 ndNi 19 18 7 19 12 41 6 24 35 19 22 14 21 6 5Cr 35 38 12 49 16 142 19 22 29 42 32 12 47 2 ndSc 22 26 29 30 15 38 30 24 19 20 24 11 17 4 4V 283 316 273 310 142 316 352 172 144 157 147 96 148 38 3Ba 710 660 425 388 579 126 346 106 1647 486 995 960 427 550 115

La 14.0 19.1 14.7 15.0 10.4 4.6 7.9 6.5 11.7 25.8 29.8 22.2 11.3 32.4 17.0Ce 33.3 44.9 36.4 36.7 25.7 11.8 21.7 15.5 26.2 63.7 69.4 51.1 28.3 79.8 48.2Pr 4.4 5.7 4.8 5.1 3.2 1.8 3.3 1.9 3.2 8.6 8.6 5.8 3.9 10.3 7.2Nd 17.5 22.9 18.3 19.3 13.1 8.0 14.1 7.5 13.5 32.6 34.2 22.7 15.1 38.2 26.8Sm 4.5 5.9 4.5 4.9 2.9 2.5 4.1 1.8 10.3 7.6 9.0 5.4 4.3 10.3 8.3Eu 0.97 1.43 0.91 1.10 0.60 0.75 0.92 0.43 4.69 1.77 2.54 1.30 1.05 2.08 0.34Gd 4.6 5.3 4.9 4.7 3.3 2.7 4.1 2.2 3.7 7.2 7.9 5.5 3.6 8.7 9.9Tb 0.91 0.93 0.93 0.82 0.59 0.51 0.72 0.42 0.76 1.25 1.48 1.19 0.62 1.50 2.05Dy 4.8 4.9 5.0 4.4 3.2 2.8 4.3 2.4 4.1 6.9 8.0 6.4 3.4 8.1 11.3Ho 1.12 0.94 1.20 0.79 0.64 0.54 0.84 0.53 0.96 1.34 1.80 1.52 0.69 1.59 2.19Er 2.8 2.6 3.3 2.6 2.1 1.7 2.5 1.8 2.5 4.0 4.6 4.2 2.2 5.1 6.9Tm 0.46 0.33 0.59 0.38 0.37 0.26 0.39 0.25 0.39 0.57 0.68 0.66 0.36 0.72 1.02Yb 2.4 1.9 2.9 1.9 1.8 1.3 2.0 1.6 2.0 3.1 3.6 3.4 1.7 3.9 5.4Lu 0.33 0.29 0.48 0.32 0.28 0.20 0.31 0.27 0.30 0.48 0.59 0.58 0.31 0.61 0.80Y 27.4 26.6 27.5 30.9 26.5 18.4 27.8 14.5 24.7 42.1 46.5 40.7 24.4 58.8 82.5Pb 11.1 17.1 5.3 5.3 4.5 3.4 16.6 3.4 19.8 16.6 12.7 13.0 14.8 18.9 14.1Th 3.4 2.8 3.5 1.0 0.6 0.4 0.9 1.8 4.5 1.4 4.4 6.9 1.0 2.1 1.4U 0.9 0.9 1.1 1.0 0.6 0.4 0.9 0.4 1.6 1.4 1.4 1.5 1.0 2.1 1.4

Note: Compound analyses in weight percent oxide; elemental analyses in mg/g. LOI—loss on ignition; MV—metavolcanic rock; porph—porphyry.

TABLE 3A. U-Pb ISOTOPE DATA

Sample† U 206Pb‡ Measured ratios§ Atomic ratios Apparent ages# (Ma)(ppm) (ppm)

206Pb 207Pb 208Pb 206Pb‡ 207Pb‡ 207Pb‡ 206Pb‡ 207Pb‡ 207Pb‡

204Pb 206Pb 206Pb 238U 235U 206Pb‡ 238U 235U 206Pb‡

PBF-7 1150A 200.5 17.53 22,222 0.06128 0.16555 0.10179(51) 0.85095(427) 0.06063(3) 624.9 625.2 626.2 6 0.9PBF-7 1150 395.6 33.57 4,808 0.06362 0.17199 0.09878(49) 0.82554(416) 0.06061(4) 607.3 611.1 625.5 6 1.4PBF-7 150–210 416.9 33.51 3,559 0.06464 0.17898 0.09354(47) 0.78123(395) 0.06057(5) 576.5 586.5 624.0 6 1.7PBF-7 –210 432.5 34.52 11,494 0.06188 0.18036 0.09291(46) 0.77651(391) 0.06062(3) 572.7 583.5 625.6 6 1.2DRB-1 1150A 421.6 36.44 8,696 0.06210 0.35902 0.10062(50) 0.83848(422) 0.06044(3) 618.1 618.3 619.1 6 1.2DRB-1 –210 405.7 32.08 10,989 0.06179 0.24565 0.09203(46) 0.76740(386) 0.06048(3) 567.6 578.3 620.6 6 1.1

Note: Error analysis for individual zircon fractions follows Mattinson (1987); errors are shown in parentheses and refer to the least significant digit. Total Pb blanksranged from 10 to 30 pg. U and Pb concentrations determined by isotope dilution via the addition of a mixed 208Pb-235U tracer added to a solution aliquot (HCl) of eachsample.

†Sample masses between 0.05 and 0.5 mg; 1100, 100–200, etc., refer to size fractions in mesh.‡Denotes radiogenic Pb, corrected for common Pb by using the isotopic composition of 206Pb/204Pb 5 18.6 and 207Pb/204Pb 5 15.6. Sample dissolution and ion-exchange

chemistry modified from Krogh (1973).§Isotopic compositions corrected for mass fractionation (0.11% per atomic mass unit).#Ages calculated by using the following constants: decay constants for 235U and 238U 5 9.8485 3 10210 yr21 and 1.55125 3 10210 yr21, respectively; 238U/235U 5 137.88.


DENNIS et al.

TABLE 3B. Rb-Sr AND Sm-Nd ISOTOPE DATA

Sample Rb Sr Atomic ratio Measured ratio Initial ratio Sm Nd Atomic ratio Measured ratio «Nd§§

(ppm) (ppm) 87Rb/86Sr 87Sr/86Sr†† 87Sr/86Sr(624) (ppm) (ppm) 147Sm/144Nd 143Nd/144Nd‡‡

PBF-7 99 243 1.1774 0.715526(9) – 10.01 43.01 0.14069 0.512512(4) 12.0 (626 Ma)DRB-1 10 344 0.0840116 0.704028(6) 0.70193851 3.44 14.34 0.14487 0.512606(6) 13.5 (619 Ma)

Note: Values used for CHUR (chondritic uniform reservoir) are 143Nd/144Nd 5 0.512638 and 147Sm/144Nd 5 0.1967. Decay constants: Sm 5 6.54 3 10212 yr21; Rb 5 1.423 10211 yr21. Sm and Nd concentrations determined by isotope dilution by addition of a mixed 149Sm-150Nd spike prior to sample dissolution. Rb and Sr concentrationsdetermined by XRF (X-ray fluorescence) at University of South Carolina. Repeated analysis of SRM-987 yielded 87Sr/86Sr 5 0.710247. Repeated analysis of BCR-1 yielded143Nd/144Nd 5 0.512633. All errors are at the 95% confidence limit; errors are shown in parentheses and refer to the least significant digit.

††Corrected for mass fractionation by normalizing to 86Sr/88Sr 5 0.1194.‡‡Corrected for mass fractionation by normalizing to 146Nd/144Nd 5 0.72190.§§«Nd(T) 5 [(143Nd/144Nd(T)sample/143Nd/144Nd(T)CHUR) 2 1] 3 10,000.

metavolcanic rocks form screens of wall rockthat were intruded by granodiorites of thePBF7 Metaplutonic Suite. In the SA well, maf-ic to intermediate-composition metavolcanicrocks were sampled by spot coring below 580m; continuous coring to depths of ;370 msampled metagranitoids similar to those in thelower part of core PBF-7. Well C-5 is char-acterized by intermediate-composition meta-volcanic rocks at depths of ;330 m that un-derlie sedimentary deposits of the coastalplain with no intercalated metagranitoids. Fo-liation dips ;458–608, similar to foliation inthe adjacent Deep Rock Metavolcanic Suite.

Rocks of the Pen Branch MetavolcanicSuite are mafic to intermediate-compositionamphibolites and garnet amphibolites that aredistinct from metavolcanic rocks of the adja-cent Deep Rock Metavolcanic Suite (Table 1).There is no relict igneous amphibole, and allmetamorphic amphibole is tschermakitic, withlow Si and high Al. Ti is much higher than inmetamorphic amphiboles of the Deep RockMetaigneous Complex, possibly in responseto higher metamorphic grade (Fig. 6). The rel-atively calcic plagioclase is also considered toreflect higher metamorphic grade (An35 toAn45; Fig. 5C). Garnet is a common meta-morphic mineral in many of these amphibo-lites, but garnet-biotite pairs are relatively rarebecause biotite is commonly retrograded tochlorite. Garnets, which range up to 1 cmacross, are typically elongated parallel to fo-liation and have spongy, inclusion-rich tex-tures (Fig. 7A). All of the garnet studied hereis unzoned, reflecting annealing during or afterpeak thermal metamorphism.

PBF7 Metaplutonic SuiteThe PBF7 Metaplutonic Suite dominates

the upper part of the cores recovered fromwells PBF-7, PBF-8, C-10, GCB-4, and theSA well. The PBF7 Metaplutonic Suite com-prises two dominant lithologies: gneissic me-tagranitoids and ‘‘pinked’’ metagranitoids,formed by hydrothermal alteration of thegneissic metagranitoids (Dennis et al., 2000a).

Gneissic metagranitoids. Gneissic meta-granitoids are porphyroclastic gneisses withporphyroclasts of amphibole and plagioclaseset in a groundmass of amphibole, plagioclase,microcline, biotite, quartz, and epidote (Table1, Fig. 7B). Plagioclase porphyroclasts withinthe Pen Branch gneisses range from An24 toAn38 and have an average composition of An35

(Fig. 5D). Amphibole porphyroclasts are un-zoned and have Mg/Fe ratios similar to themetamorphic amphiboles in the DRB-1 me-tadiorites, but TiO2 values range from 0.7% to1.0%, higher than metamorphic amphibole inthe DRB-1 metadiorites. Blue-green amphi-bole in the PBF7 Metaplutonic Suite is clearlydistinguished from the relict igneous horn-blende of the DRB1 Metaplutonic Suite, how-ever, by its higher AlIV contents (.1.42 afu;Fig. 6).

Pen Branch gneissic metagranitoids are dis-tinguished from DRB-1 metadiorite by theirlack of relict igneous hornblende, the relative-ly high Ti in blue-green metamorphic amphi-bole, higher An-content plagioclase, the com-mon occurrence of microcline, and thepreservation of biotite (Table 1). Some ofthese characteristics may result from highermetamorphic grade in the Pen Branch meta-granitoids, but others (e.g., common micro-cline) reflect fundamental compositional dif-ferences between the two intrusive series.

Pinked metagranitoid. Much of the graniticcore material recovered from wells PBF-7,PBF-8, C-10, and SA had been subjected topartial to extensive hydrothermal alterationand potassium metasomatism after formationof foliation. We refer to this alteration as the‘‘pinking’’ event because of the salmon-pinkcolor imparted to rocks affected by the hydro-thermal fluids. The effects of this event rangefrom millimeter-scale selvages on fractures topervasive alteration of hundreds of meters ofcored rock (Dennis et al., 2000a).

‘‘Pinked’’ Pen Branch metagranites com-monly contain large plagioclase, microcline,and garnet porphyroblasts set in a finer-grained matrix of quartz, plagioclase, micro-

cline, and chlorite (Table 1). Microcline com-monly occurs as megacrysts up to ;5 mmacross and may account for as much as 40%of the mode (Fig. 7C). Partially pinked rockshave plagioclase An25 to An33, whereas pla-gioclase in the thoroughly pinked granites hasbeen completely albitized (An4) and, subse-quently, partially sericitized (Fig. 5D). Am-phibole and biotite are almost completelychloritized, but some relict blue-green amphi-bole is preserved.

GEOCHEMISTRY

Whole-rock geochemical analyses were ob-tained by X-ray fluorescence for 59 metavol-canic rocks, 35 metaplutonic rocks, 15 post-kinematic dikes, and 3 metasedimentaryrocks; 31 of these samples were analyzed foradditional trace elements by ICP-MS. Repre-sentative whole-rock data are presented in Ta-ble 2. The complete data set of major andtrace element data are available (Tables DR-2and DR-3 [see footnote 1]).


Low-grade metavolcanic rocks of theCrackerneck Metavolcanic Complex may beclassified as basalts, dacites, and rhyolites byusing their silica vs. alkali relationships (Coxet al., 1979). Most are high in SiO2 (65–77wt%) and exhibit decreasing MgO, Fe2O3*,TiO2, CaO, and Al2O3 with increasing SiO2;only one metabasalt has been analyzed (Fig.8). Na2O increases with increasing SiO2 con-tent, but surprisingly, K2O is highest in thetwo dacites, C-1–570 and C-1–574 (Table DR-2 [see footnote 1]). These rocks are also highin Ba (1240–1350 ppm vs. 120–750 in theother samples), Rb, Sr, and Cu, suggestingthat K2O and the other mobile trace elementswere enriched by secondary hydrothermalprocesses. The overall characteristics of theserocks are calc-alkaline on an AFM diagram(Fig. 9A), covering much the same composi-tional range as similar low-grade metavolcanic



Figure 5. Plagioclase ternary plots showing compositions of igneous and metamorphic plagioclase porphyroclasts. (A) Crackerneck andDeep Rock metavolcanic rocks. (B) DRB-1 metaplutonic rocks. (C) Pen Branch metavolcanic rocks. (D) Pen Branch metagranitoids:(unpinked) porphyroclastic gneiss, partially pinked gneiss, and thoroughly pinked granitic gneiss.

Figure 6. Tetrahedral aluminum atoms per formula unit (afu, on the basis of 23 oxygens)vs. titanium afu in metamorphic and relict igneous amphiboles of the Deep Rock and PenBranch Metaigneous Complexes. Relict igneous hornblendes in the Deep Rock Metavol-canic Suite (n) and DRB1 Metaplutonic Suite (u) have less AlIV compared to the meta-morphic amphiboles in either the metadiorites of the Deep Rock Metaigneous Complex(y) or the metavolcanic rocks of the Deep Rock Metavolcanic Suite (x); relict igneoushornblendes of the DRB1 Metaplutonic Suite are also higher in Ti. Metagranitoids of thePBF7 Metaplutonic Suite contain no relict igneous hornblende, but the metamorphic blue-green amphibole (q) is higher in Ti than metamorphic amphiboles in the Deep RockMetavolcanic Suite and the DRB1 Metaplutonic Suite, suggesting higher equilibrationtemperatures. Actinolite is plotted for reference.

rocks in the Belair belt and in the PersimmonFork Formation of the Carolina Slate belt(Shervais et al., 1996).

Four samples were chosen for REE analysis(Table DR-3 [see footnote 1]). All are en-riched in the light rare earth elements (LREEs)relative to the heavy rare earth elements(HREEs): La contents are ;32 to 80 timeschondritic abundances, and La/Lu ratios are;2 to ;14 times the chondritic ratio (Fig.10A).The highest La/Lu ratios are observed inthe two K-rich dacites, suggesting that theLREEs may have been enriched, or theHREEs depleted, by the same hydrothermalsolutions that enriched these samples in K,Rb, and Ba. All four samples have small butwell-defined negative Eu anomalies. MORB-normalized spider diagrams show that all foursamples are enriched in low field strength el-ements and depleted in high field strength el-ements, consistent with subduction-zone en-richment processes (Fig. 11A).


Deep Rock Metavolcanic SuiteMetavolcanic rocks of the Deep Rock Me-

tavolcanic Suite may be classified as basalts,andesites, dacites, and rhyolites. Major andtrace element trends on Harker diagrams aretypical of calc-alkaline volcanic suites (Fig.8). The alkalis show significant scatter, sug-


DENNIS et al.

Figure 7. Photomicrographs of Pen Branch Metaigneous Complex: (A) Typicalamphibolite-facies assemblage of garnet, biotite, plagioclase, and lesser amounts of horn-blende in the Pen Branch Metavolcanic Suite (cross-polarized light); note the ellipticalnature of the garnets. (B) Microcline with characteristic tartan twinning in quartz-plagioclase-amphibole granite (cross-polarized light). (C) Metamorphic amphibole (ex-tinct) intergrown with quartz, plagioclase, and K-feldspar (cross-polarized light). Widthof field of view represents 2.3 mm in all.

gesting either complex petrogenetic processesinvolving mixing or assimilation, or (morelikely) element mobility during amphibolite-facies metamorphism. These rocks are calc-

alkaline on an AFM plot (Fig. 9B), consistentwith their observed lack of Fe or Ti enrich-ment on Harker diagrams.

Fourteen samples were chosen for REE

analysis (Table DR-3 [see footnote 1]). All areenriched in the light rare earth elements(LREEs) relative to the heavy rare earth ele-ments (HREEs): La contents are ;22 to 90times chondritic abundances, and La/Lu ratiosare ;3.5 to ;8.8 times the chondritic ratio(Fig. 10B). The La/Lu ratios are higher thanthose in the DRB-1 metadiorite but in thesame range as those observed in the low-grademetavolcanic rocks of the Crackerneck Meta-volcanic Complex. All of the samples havemoderate to deeply negative Eu anomalies,which seem to be deepest in rocks with SiO2

. 65 wt% (Fig. 10B), consistent with exten-sive plagioclase fractionation under relativelylow oxygen fugacity conditions. MORB-normalized spider diagrams show that all areenriched in low field strength elements anddepleted in high field strength elements, con-sistent with subduction-zone enrichment pro-cesses (Fig. 11B).

Younger Dike Rocks (DRB-2, DRB-3,DRB-5, and DRB-6)

Postkinematic mafic dikes (aphyric and pla-gioclase megaphyric) are all basalt (or rarely,basaltic andesite) in composition, with lowTiO2 (0.7% to 1.3%) and high Al2O3 (16.5%to 19%). They straddle the tholeiitic/calc-alkaline dividing line of Irvine and Baragar(1971), but they have higher Fe/Mg ratios thanmost Deep Rock Metavolcanic Suite amphib-olites. These younger mafic dikes are alsocharacterized by higher Sr concentrations(;550 to ;700 ppm Sr) than most amphib-olites in the suite. Postkinematic felsic dikesinclude dacites and rhyolites that exhibit typ-ical calc-alkaline fractionation trends. The fel-sic dikes exhibit the same characteristic highSr as the younger mafic dikes. Only two post-kinematic dikes were analyzed for REE con-centrations: one aphyric mafic dike and oneplagioclase megaphyric mafic dike. Both dikeshave moderate LREE enrichment: La contentsare ;42 times chondritic abundances, and La/Lu ratios are ;3 to ;4 times the chondriticratio (Fig. 10C). The La/Lu ratios are lowcompared to the host metavolcanic rocks.Both samples have small negative Eu anom-alies, showing that even the plagioclase me-gaphyric samples crystallized from melts inwhich the plagioclase was being removedrather than accumulating. MORB-normalizedspider diagrams are similar to those of the vol-canic rocks (Fig. 11B).

DRB1 Metaplutonic SuiteMetaplutonic rocks sampled from the DRB-

1 core may be classified as diorites or quartzdiorites on the basis of their normative min-



Figure 8. Harker diagrams for Crackerneck, Deep Rock, and Pen Branch Metaigneous Complexes.

eralogy. Major and trace element trends aretypical of calc-alkaline intrusive suites (Figs.8, 9B). At any given weight percent silica, theDRB-1 metadiorite samples are lower in maficelements (Mg, Fe, Ti, Cr, Ni), K2O, and Rbthan the Pen Branch metagranitoids and high-

er in plagiophile elements (Ca, Na, Al). Foursamples were chosen for REE analysis (TableDR-3 [see footnote 1]). They are slightly en-riched in the light rare earth elements (LREEs)relative to the heavy rare earth elements(HREEs): La contents are ;10–30 times

chondritic abundances, and La/Lu ratios are;2.3–3.8 times the chondritic ratio (Fig.10C). La/Lu ratios are lower than those ob-served in the Deep Rock Metavolcanic Suite,but all have small negative Eu anomalies.MORB-normalized spider diagrams are con-


DENNIS et al.

Figure 9. AFM diagrams with the Irvineand Baragar (1974) dividing line for tholei-itic and calc-alkaline trends. (A) Cracker-neck Metavolcanic Complex, (B) DeepRock Metaigneous Complex, (C) PenBranch Metaigneous Complex.

sistent with subduction-zone enrichment pro-cesses (Fig. 11B).


Pen Branch Metavolcanic SuiteAmphibolites and garnet amphibolites of

the Pen Branch Metavolcanic Suite can beclassified geochemically as metamorphosedbasalts, basaltic andesites, and andesites. Theyare slightly lower in Fe and higher in Ca thanthe Deep Rock metavolcanic rocks at any giv-en weight percent silica, and they are restrict-ed to SiO2 contents of #60% SiO2 (Fig. 8).They plot in the calc-alkaline field on an AFMdiagram, where they exhibit typical calc-alkaline fractionation trends similar to those

of the Deep Rock Metavolcanic Suite (Fig.9C).

PBF7 Metaplutonic SuiteMetagranitoids. The gneissic metagrani-

toids may be classified as quartz monzodio-rites and granodiorites by using their norma-tive mineral contents; their major and traceelement trends are similar to those of DRB-1(Fig. 8). At any given weight percent silica,the Pen Branch metagranitoids are higher inmafic elements (Mg, Fe, Ti, Cr, Ni), K2O, andRb than the metadiorites of DRB-1 and lowerin plagiophile elements (Ca, Na, Al). Like theDRB1 metadiorite samples, the Pen Branchmetagranitoids are strongly calc-alkaline, asseen on an AFM plot (Fig. 9C).

Six ‘‘unpinked’’ metagranitoids of thePBF7 Metaplutonic Suite were analyzed forREE concentrations (Table DR-3 [see footnote1]). All are enriched in light rare earth ele-ments (LREEs) relative to the heavy rare earthelements (HREEs): La contents are ;30–100times chondritic abundances, and La/Lu ratiosare ;3.6 to ;5.4 times the chondritic ratio(Fig. 10D). Both the La/Lu ratios and totalREEs are higher than those observed in theDRB1 Metaplutonic Suite. Five of these sam-ples have small negative Eu anomalies, sug-gesting plagioclase fractionation, whereas onehas a significant positive Eu anomaly, sug-gesting plagioclase accumulation (Fig. 10D).MORB-normalized spider diagrams show thatall six samples are enriched in low fieldstrength elements and depleted in high fieldstrength elements, consistent with subduction-zone enrichment processes (Fig. 11A).

Pinked metagranitoids. Pinked metagrani-toids are characterized by higher SiO2, K2O,and Rb contents than their unaltered equiva-lents and by lower MgO, Fe2O3*, TiO2, Al2O3,CaO, Na2O, and Sr (Fig. 8). As a result, theserocks are classified as granites by using theirnormative mineral contents. They extend thestrong calc-alkaline trend of the unpinked me-tagranitoids (Fig. 9C), but this apparent calc-alkaline ‘‘fractionation trend’’ results from hy-drothermal alteration, not igneous processes.Only one sample of pinked metagranitoid wasanalyzed for REE concentrations (PBF-7–3568). This sample is characterized by modestLREE enrichment (La/Lu ratio is ;2.1 timesthe chondritic ratio) and by a pronounced neg-ative Eu anomaly not seen in the unpinkedmetagranitoids (Fig. 10D). This negative Euanomaly suggests that Eu21 was mobilizedalong with Ca21 during the pinking event andwas removed from the system.

Age Constraints and Tracer Isotopes

Two samples of metaplutonic rock weredated by using the U-Pb zircon method (Table3). Four zircon fractions from a quartz mon-zodiorite of the Pen Branch MetaigneousComplex (PBF-7) gave a concordia-interceptdate of 626.1 6 4.2 Ma (Fig. 12A). Theweighted mean of the 207Pb*/206Pb* dates is625.6 6 1.3 Ma. Two zircon fractions from aquartz diorite of the Deep Rock MetaigneousComplex (DRB-1) yield identical 207Pb*/206Pb* dates with a weighted mean of 619.96 0.8 Ma; the concordia intercept for this pairis 619 6 3.4 Ma (Fig. 12B). These Late Pro-terozoic ages are too old to correlate with theCarolina Slate belt (Lincolnton metadacite–Persimmon Fork Formation–Uwharrie For-mation at ca. 550 Ma; Whitney et al., 1978;Wright and Seiders, 1980; Carpenter et al.,1982; Dallmeyer et al., 1986; Barker et al.,1998), the western Carolina terrane (Charlottebelt at ca. 580 Ma to 535 Ma; Dennis andWright, 1997), or North Florida Volcanic Se-ries of the Suwannee terrane (ca. 552 Ma;Heatherington et al., 1996). Similar ages havebeen determined for the Hyco Formation incentral North Carolina (ca. 633 Ma to 612 Ma;Wright and Seiders, 1980; Harris and Glover,1988; Mueller et al., 1996; Wortman et al.,2000; Mueller et al., 1996) and for an isolatedgranodiorite of the Suwannee terrane in south-ern Alabama (ca. 625 Ma; Heatherington etal., 1996, p. 263).

Age-corrected initial isotope ratios werecalculated by using the measured parent/daughter ratios and the U-Pb ages of the sam-ples. These ratios indicate derivation of theDRB1 and PBF7 Metaplutonic Suites fromsimilar, relatively primitive sources, with «Nd

5 12.0 to 13.5 (Table 3). The DRB-1 me-tadiorite sample has a calculated initial 87Sr/86Sr ratio of 0.70194, but an initial 87Sr/86Srratio could not be calculated for PBF-7 be-cause the Rb-Sr system was disturbed by theTriassic pinking event. These values of «Nd aresimilar to those that have been measured onmetaigneous rocks of the Carolina terrane andother peri-Gondwana ‘‘Avalonian’’ terranes(e.g., Nance and Murphy, 1996), but are high-er than those found in the Suwannee terrane(Heatherington et al., 1996; Fig. 12C).

DISCUSSION


Volcanic rocks of the Crackerneck Meta-volcanic Complex are the least deformed andleast metamorphosed of the units studied here.



Figure 10. Chondrite-normalized REE plots for (A) Crackerneck Metavolcanic Complex, (B) Deep Rock Metavolcanic Suite, (C) DRB1Metaplutonic Suite and dikes within the Deep Rock Metavolcanic Suite, and (D) PBF7 Metaplutonic Suite.

Their low grade of metamorphism (sub-greenschist to greenschist facies) and lack ofpenetrative deformation indicate that (1) theserocks could not have been buried deeply sub-sequent to their formation, and (2) they werenot overridden by thick thrust sheets duringthe Alleghenian orogeny. Their geochemicalcharacteristics (moderate to high SiO2, calc-alkaline fractionation trends, enrichment inlow field strength elements, and depletion inhigh field strength elements) all point to anorigin as volcanic rocks associated with anisland-arc volcanic system. The dominance ofmore felsic members of this suite suggests thatthe crust upon which this volcanic arc wasbuilt consisted either of mature island-arccrust or a thinned continental margin. The

abundance of felsic tuffs and pumice lapillituffs resembles many continental-margin arcs(e.g., Mexico: Luhr and Carmichael, 1980).

The Crackerneck Metavolcanic Complexcorrelates most closely with the PersimmonFork Formation of the Carolina Slate belt (Se-cor et al., 1986a, 1986b; Shervais et al., 1996).Both units are dominated by felsic tuffs, weremetamorphosed under low-grade conditions,and have comparable trace element concentra-tions. The Crackerneck Metavolcanic Com-plex may also correlate with rocks of the Be-lair belt. These rocks are dominantlyintermediate in composition, but include moremafic and more felsic compositions as well(Shervais et al., 1996). Differences in the pro-portions of mafic and felsic rocks may relate

more to the limited areal extent of the Belairbelt, which has been sampled systematicallyin only one traverse along the banks of theSavannah River. The Crackerneck is separatedfrom the Belair belt by a strand of the aero-magnetically defined Eastern Piedmont faultsystem and by high-grade metamorphic rocksof the Belvedere belt that are exposed in ero-sional windows through the coastal-plain sed-imentary rocks near Graniteville (Fig. 2).Thus, the Crackerneck Metavolcanic Complexmay be stratigraphically equivalent to the Be-lair belt, but is not contiguous with that belt.


The Deep Rock Metaigneous Complex ex-hibits phase assemblages, calc-alkaline frac-


DENNIS et al.

Figure 11. MORB-normalized spider diagram for (A) Crackerneck and Pen Branch Metaigneous Complexes, (B) the Deep RockMetaigneous Complex, (C) the Carolina terrane, and (D) the Suwannee terrane. Data for Carolina terrane from Shervais et al. (1996);data for Suwannee terrane from Heatherington et al. (1996).

tionation trends, and trace element systematicsthat are consistent with formation in a matureisland arc that formed over an extended periodof time. In contrast, associated metaplutonicrocks of the DRB1 Metaplutonic Suite aredominated by intermediate-composition me-tadiorites, suggesting a primitive arc origin.The relatively high «Nd value of the DRB-1metadiorite sample is also consistent with anorigin in a more primitive intraoceanic arcsetting.

The data presented here suggest that theDeep Rock Metavolcanic Suite and the DRB1

Metaplutonic Suite are unrelated petrogeneti-cally. This observation is supported by obser-vations of the core: (1) DRB-1 metadioritesare found only in core DRB-1, (2) DRB-1 me-tadiorites include screens of deformed meta-volcanic wall rocks that are chemically andpetrologically similar to rocks of the DeepRock Metavolcanic Suite and contain fabricelements that are not observed in the DRB-1metadiorites, and (3) metavolcanic rocks ex-posed in cores DRB-2 through DRB-7 are cutby three distinct dike series (aphyric mafic,plagioclase porphyry, and felsic dikes) that are

not seen in DRB-1. These relationships sug-gest that the Deep Rock Metavolcanic Suiterepresents an older volcanic arc assemblage(possibly equivalent to the PBF7 MetaplutonicSuite) that was intruded in part by the DRB-1 metadiorite at ca. 619 Ma.


The Pen Branch Metaigneous Complex ex-hibits compositional ranges, trace element sys-tematics, and phase assemblages consistentwith formation in a volcanic arc, similar to



Figure 12. Concordia diagrams for zirconfractions from (A) Pen Branch (PBF-7) and(B) Deep Rock (DRB-1) Metaigneous Com-plexes. (C) «Nd vs. age for Pen Branch (PBF)and Deep Rock (DRB) Metaigneous Suiterocks compared to published data for Car-olina terrane plutonic rocks (filled squares)and volcanic, metamorphic, and sedimen-tary rocks (dashed field): data from Wort-man et al. (1995; W) and Mueller et al.(1996; M); North Florida Volcanic Series(NFVS): data from Heatherington et al.(1996; H).

adjacent rocks of the Deep Rock MetaigneousComplex. Metavolcanic rocks of the PenBranch Metavolcanic Suite exhibit a more re-stricted range of compositions; the lack of Feand Ti enrichment and the restricted range inSiO2 contents suggest that the Pen Branch Me-tavolcanic Suite represents a relatively primi-tive island arc compared to the Deep RockMetavolcanic Suite. In contrast, the PBF7 Me-taplutonic Suite is dominated by granodioritesand granites with intermediate-composition tohigh SiO2 contents. The low «Nd value (12.0)of a Pen Branch gneissic granitoid (comparedto «Nd 5 13.5 for the DRB-1 metadiorite sam-ple) implies a more evolved provenance forthese intrusive rocks, in contrast to the moreprimitive characteristics of the Pen BranchMetavolcanic Suite.

Rocks of the Pen Branch MetavolcanicSuite appear to represent an older volcanicframework into which the Pen Branch grani-toids were intruded at ca. 626 Ma. The intru-sive relationship between the Pen Branch me-tagranitoids and the Pen Branch metavolcanicrocks and the differences in their geochemicalcharacter suggest that these two rock suites areunrelated petrogenetically. There is some sug-gestion, however, that granitoids of the PBF7Metaplutonic Suite may represent the intru-sive equivalent of the Deep Rock Metavolcan-ic Suite. This suggestion is based on chemicalsimilarities between the two suites, includingtheir compositional ranges, their rare earth el-ement concentrations, and other trace elementsimilarities (Figs. 10 and 11).

Metamorphism and Pressure-TemperatureEstimates

Metavolcanic rocks of both the Deep Rockand Pen Branch Metavolcanic Suites preserveevidence for distinct metamorphic histories,best illustrated by changes in amphibole com-position (Roden et al., 2002; Fig. 6). Relictigneous hornblende compositions are pre-served in the cores of many volcanic pheno-crysts in the Deep Rock Metavolcanic Suite.These compositions are overprinted by thepervasive growth of blue-green metamorphicamphibole, consistent with amphibolite-faciesmetamorphism. The high-temperature blue-green amphibole was subsequently retrograd-ed to the greenschist-facies minerals actinoliteand chlorite. In contrast, metavolcanic rocksof the Pen Branch Metavolcanic Suite containno relict igneous minerals, and all amphibolesare metamorphic.

Three samples that preserve appropriatemineral assemblages were chosen to calculatemetamorphic temperatures. Two of these sam-

ples are from the Pen Branch MetavolcanicSuite (PBF-7–3648 and SA-4002). We cannotcalculate a pressure for these rocks becausethey do not contain an aluminosilicate min-eral, but we can estimate a nominal pressureof at least 3 kbar (10 km depth), which is rea-sonable for rocks of this grade. Table 4 pre-sents the results of these calculations, whichare based on the exchange of Fe and Mg be-tween garnet and biotite (Ferry and Spear,1978). Both samples preserve evidence for anearly high-temperature event at 700 to 800 8C,consistent with uppermost amphibolite–faciesconditions.

An intermediate-composition metavolcanicrock from the Deep Rock Metavolcanic Suite(DRB-2–1449) also preserves metamorphicbiotite and garnet. Equilibrium biotite-garnettemperatures for this sample average ;6508C, consistent with lower to middleamphibolite–facies conditions (Table 4). Thus,the Deep Rock and Pen Branch MetavolcanicSuites may represent different levels of ex-posure that were juxtaposed by later faulting.

Metaigneous rocks of both the DRB1 andPBF7 Metaplutonic Suites preserve evidencefor metamorphic histories that are similar totheir adjacent metavolcanic suites. Thus, theDRB-1 metadiorites have metamorphic am-phiboles similar to those in the Deep RockMetavolcanic Suite, but still retain relic coresof igneous hornblende. In contrast, the PenBranch metagranitoids lack relict igneoushornblende and contain only metamorphicamphibole.

Potassium Metasomatism: The Big Pink

Many metaplutonic rocks of the PBF7 Me-taplutonic Suite were affected by a postmeta-morphic hydrothermal alteration event whosecharacteristic signature is a pervasive discol-oration of the normally gray rocks to variousshades of bright salmon-pink (Dennis et al.,2000a). The metamorphic phase assemblageassociated with this event (actinolite, chlorite,albite, K-feldspar) indicates greenschist-faciesmetamorphism at temperatures of ;350 to;450 8C. The primary geochemical charac-teristic of this event is Si, K, and Rb meta-somatism of the affected rocks by hydrother-mal fluids and concomitant leaching of Ca, Sr,and Eu21. We suggest that the breakdown ofprimary biotite to chlorite, which is nearlycomplete in the pinked samples and affectsmuch of the DRB1 Metaplutonic Suite, is themost likely source of these alkalis.

Data presented by Dennis et al. (2000a)show that the ‘‘pinking’’ event predated theformation of most filled fractures. The best es-


DENNIS et al.

TABLE 4. GARNET AND BIOTITE COMPOSITIONS IN METAVOLCANIC ROCKS, ALONG WITHCALCULATED GT-BI TEMPERATURES

Core SA PBF7 DRB-2 SA PBF7 DRB-2Sample 4002 3648 1449 4002 3648 1449Mineral Garnet Garnet Garnet Biotite Biotite Biotite

SiO2 37.63 36.96 36.40 36.43 36.03 35.60TiO2 0.06 0.01 0.04 1.17 1.94 1.98Al2O3 21.32 21.00 20.69 17.94 17.73 16.96FeO 26.46 28.14 29.68 17.75 20.16 19.38MnO 2.94 2.23 4.05 0.27 0.15 0.18MgO 5.57 3.10 3.66 12.15 10.14 11.15CaO 5.30 7.66 3.63 0.08 0.00 0.27Na2O – – 0.12 0.05 0.10K2O – – 9.65 9.94 9.24Total 99.28 99.10 98.15 95.54 96.13 94.85

Si 5.956 5.941 5.941 5.495 5.474 5.461Ti 0.007 0.001 0.005 0.132 0.222 0.228Al 3.977 3.978 3.980 3.189 3.175 3.066Fe 3.502 3.783 4.052 2.239 2.561 2.486Mn 0.394 0.304 0.559 0.034 0.019 0.023Mg 1.314 0.743 0.890 2.731 2.295 2.549Ca 0.899 1.319 0.635 0.012 0.000 0.044Na – – – 0.034 0.015 0.030K – – – 1.856 1.926 1.808Pyr 0.215 0.121 0.145Alm 0.573 0.615 0.660Spes 0.065 0.049 0.091Gros 0.147 0.215 0.103Aliv 2.505 2.526 2.539Alvi 0.684 0.649 0.527Bio-Tot 5.821 5.746 5.815Ann 0.385 0.446 0.428Phl 0.469 0.399 0.438Ti-Bi 0.023 0.039 0.039Al-Bi 0.118 0.113 0.091Mn-Bi 0.006 0.003 0.004

SA PBF7 DRB-24002 3648 1449

F and S 807 648 639 Ferry and Spear (1978)H and S 870 723 674 Hodges and Spear (1982)G and S 781 699 657 Ganguly and Saxena (1984)I and M 825 667 614 Indares and Martignole (1985)Average T (8C) 820 685 645 Average of all calculated T

timate for the absolute age of this event is theRb-Sr age presented by Kish (1992) forpinked granite from core C-10. This two-pointpseudo-isochron, based on the whole rock andon a K-feldspar separate, must represent theage of the ‘‘pinking’’ event because the per-vasive K and Rb metasomatism characterizingthis event would thoroughly reset the isotopicsystematics of the K-feldspar, which controlsthe slope of the isochron. The ‘‘pseudo-isochron’’ yields an age of ca. 220 Ma (EarlyTriassic), which implies that the K metaso-matism of the ‘‘pinking’’ event was associatedwith early Mesozoic rifting of North Americafrom Africa and the formation of the Dunbar-ton Basin.

Are the Deep Rock and Pen BranchMetavolcanic Suites Related?

Despite their overall similarity, metavolcan-ic rocks of the Deep Rock and Pen Branch

Metavolcanic Suites exhibit significant differ-ences that have led us to conclude that theydo not represent the same rock series. TheDeep Rock Metavolcanic Suite ranges frombasalt to rhyolite in composition; the PenBranch Metavolcanic Suite is restricted to maf-ic to intermediate compositions. In addition,postkinematic mafic dikes are common in theDeep Rock, but are conspicuously absent fromthe Pen Branch. The plagioclase porphyrydikes commonly found in the Deep Rock aredistinctive and would stand out clearly if theywere present in the PBF cores. Finally, PenBranch rocks have been metamorphosed underhigher-grade conditions than those of the DeepRock Metavolcanic Suite. Metavolcanic rocksof the Deep Rock and Pen Branch Metavol-canic Suites may have formed within the samearc in different places or at different times;alternately, they may have formed in unrelatedarc terranes and only been juxtaposed later.

Are the DRB1 and PBF7 MetaplutonicSuites Related?

Despite their gross similarity, plutonicrocks of the DRB1 and PBF7 MetaplutonicSuites exhibit significant differences that haveled us to conclude that they do not representthe same rock series. Major differences in-clude the following: (1) The DRB1 Metaplu-tonic Suite is dominated by quartz diorite,whereas the PBF7 Metaplutonic Suite is dom-inated by quartz monzodiorite (unpinked) andgranite (pinked). (2) Metadiorite of the DRB1Metaplutonic Suite and quartz monzodioritesof the PBF7 Metaplutonic Suite have distinct,subparallel trends on Harker diagrams that areoffset from one another. At any given weightpercent silica, DRB-1 metadiorite samples arelower in mafic elements (Mg, Fe, Ti, Cr, Ni),K2O, and Rb than Pen Branch metagranitoidsand higher in plagiophile elements (Ca, Na,Al). These differences must reflect differentparent magmas and different magma sourceregions. (3) DRB-1 metadiorite samples havelower REE concentrations and lower La/Luratios than the plutonic rocks of the PBF7 Me-taplutonic Suite. These differences are toolarge to result from fractionation processesand must reflect different parent-magma com-positions and different magma source regions.(4) The DRB-1 metadiorite has a higher «Nd

value (13.5) than the Pen Branch granodioritesample («Nd 5 12.0); these data indicate anoceanic affinity for the DRB1 MetaigneousSuite and a weak continental influence on thePBF7 Metaigneous Suite.

The evidence listed above suggests that theDRB1 Metaplutonic Suite and PBF7 Metaplu-tonic Suite are not equivalent. The low nor-mative quartz contents, relatively primitivemajor element compositions, low total REEs,low La/Lu ratios, and high «Nd of the DRB-1metadiorites are interpreted to indicate thatthis plutonic suite formed in a magmatic arcthat was built on older oceanic or arc crustand that continental crust was not part of itsautochthonous basement. In contrast, grano-diorites of the PBF7 Metaplutonic Suite aremore potassic than the DRB-1 metadioritesand are enriched in SiO2 (at similar mafic el-ement contents), have higher total REEs, high-er La/Lu ratios, and a lower «Nd value. Thesecharacteristics suggest derivation from a moreevolved source, such as an older, more matureoceanic arc terrane or a continental-margin arcbuilt on transitional crust.

These differences may be analogous to the‘‘quartz diorite line’’ in the western Sierra Ne-vada arc (Moore, 1959), which separates arcplutons intruded through accreted oceanic or



Figure 13. Chondrite-normalized La con-centrations vs. Eu* (5 ratio of projected Euconcentration to actual Eu concentration)for rocks of Savannah River Site (SRS),Carolina terrane, and Suwannee terrane.Data sources as in Figure 11.

arc-derived crust on one side (quartz diorites)from arc plutons intruded through older cra-tonic crust on the other side (granites, grano-diorites). In the western Sierra Nevada arc, thequartz diorite line is essentially coincidentwith the 87Sr/86Srinitial 5 0.706 line of Kistler(1974). We suggest that these metaplutonicsuites may represent collapse of a continental-margin arc that straddled the oceanic crust–continental crust transition when the arc wasfirst constructed.

Comparison with Adjacent ProterozoicArc Terranes

Arc rocks of the Savannah River Site liebetween rocks of similar character and age toboth the north (Carolina terrane) and south(Suwannee terrane). Metaigneous rocks of theSuwannee terrane are characterized by nega-tive to low positive «Nd values (Heatheringtonet al., 1996) that are in stark contrast to thepositive «Nd values determined for rocks of theDeep Rock/Pen Branch arc (Fig. 12C). In con-trast, the Carolina terrane and other Avalonianperi-Gondwana arcs have similar positive «Nd

values (e.g., Wortman et al., 1995; Mueller etal., 1996; Nance and Murphy, 1996; Fig.12C). Thus, although arc rocks of the centraland eastern Piedmont may have a similarcrustal provenance, arc rocks of the Suwanneeterrane reflect a crustal substrate that differedsignificantly from that of the other terranes.Reconstructions that place the Suwannee ter-rane adjacent to the Carolina terrane or theSavannah River Site during the Late Protero-zoic are suspect.

MORB-normalized trace element diagrams(Fig. 11) show that all of these rocks are en-riched in low field strength elements (eg., Rb,Ba, Th, La) and depleted in high field strengthelements (e.g., Nb, Ta, Ti), consistent withsubduction-zone enrichment processes. In de-tail, however, there are some significant dif-ferences. Rocks of the Carolina terrane tendto have deeper negative Nb anomalies than theother suites (possibly because of greater en-richment in Th and K) and small negative orpositive Eu anomalies (Fig. 11). Rocks of theDeep Rock Metaigneous Complex have small-er Ti anomalies than the other suites and largernegative Eu anomalies than the Carolina orSuwannee terranes.

Figure 13 compares Eu* (ratio of projectedEu concentration to actual Eu concentration)and chondrite-normalized La in rocks of theSavannah River Site to those of rocks fromthe Carolina and Suwannee terranes (Shervaiset al., 1996; Heatherington et al., 1996); Eu*is .1 for positive anomalies and ,1 for neg-

ative anomalies. Metaigneous rocks of the Sa-vannah River Site all have moderate to largenegative Eu anomalies at all La concentrationsexcept for one rock with a large positiveanomaly (average Eu* 5 0.64, range 0.11 to1.87; Fig. 13). In contrast, rocks of the Su-wannee terrane have small negative anomaliesat high La (average Eu* 5 0.83, range 0.63to 0.94), and rocks of the Carolina terranehave small positive anomalies at low La (av-erage Eu* 5 1.05) and small negative anom-alies at high La (average Eu* 5 0.78; Fig. 13).

Rocks of all suites are inferred to haveevolved in part by plagioclase fractionation.The small negative Eu anomalies in the Car-olina and Suwannee terrane rocks imply frac-tionation under relatively high oxygen fugac-ity conditions, where Eu is largely oxidized toEu31 and thus excluded from plagioclase (e.g.,Gill, 1981; Shah and Shervais, 1999). In con-trast, the larger negative anomalies in the Sa-vannah River Site rocks implies fractionationunder relatively lower oxygen fugacity con-ditions, where more Eu is reduced to Eu21 andpartitioned into plagioclase (Gill, 1981).These differences suggest distinct conditionsin the source region of each terrane, similar tovariations in H2O contents observed in arcrocks of central Mexico (Cervantes and Wal-lace, 2003). The occurrence of significant neg-ative Eu anomalies, especially in the more maf-ic rocks, is generally characteristic of tholeiiticvolcanic suites, not calc-alkaline ones, andconfirms other evidence that suggests that theDeep Rock/Pen Branch arc is distinct fromboth of its adjacent neighbors.

Origin of the Deep Rock/Pen BranchVolcanic Arc(s)

The data presented here show that rocks ofthe DRB1 and PBF7 Metaplutonic Suitesformed in subduction-related volcanic arcsduring the Late Proterozoic, ca. 625–620 Ma.The rocks share Nd isotope systematics thatare similar to those of other peri-Gondwanaarc terranes (e.g., Carolina, Avalon; Nance etal., 1991; Nance and Murphy, 1996). Theclosely similar ages and isotopic compositionsargue against formation in different arcs. Thearc represented by the Deep Rock/PBF7 Me-taplutonic Suites cannot be correlated with theproximal Carolina Slate belt (Persimmon ForkFormation) in central South Carolina becausethe Persimmon Fork Formation (1) is tooyoung at ca. 550 Ma, and (2) is dominated byfelsic to intermediate-composition volcanicrocks, not the basalt to dacite volcanic rocksthat dominate the Deep Rock/Pen Branch arc.The rocks of the Deep Rock/Pen Branch vol-canic arc are also older than the western Car-olina terrane (e.g., Dennis and Wright, 1997),although rocks of the DRB1 and PBF7 Me-taplutonic Suites are similar lithologically torocks of the western Carolina terrane (Dennisand Shervais, 1991, 1996).

The Deep Rock/Pen Branch volcanic arcmay correlate with the Hyco Formation incentral North Carolina and southern Virginia,which represents the infrastructure of the slatebelt arc in North Carolina and Virginia (Fig.14). The Hyco Formation and its correlativesconsist of mafic to felsic metavolcanic rocksdated at ca. 620 Ma (Glover and Sinha, 1973;Wortman et al., 2000). These rocks were de-formed and metamorphosed during the ‘‘Vir-gilina orogeny’’ (Glover and Sinha, 1973;Hibbard and Samson, 1995; Wortman et al.,2000) between 612 15.2/21.7 Ma and 586 610 Ma in they type area (Wortman et al.,2000). An angular unconformity betweenHyco/Aaron rocks and younger arc rocks ofthe Uwharrie Formation and AlbermarleGroup (Harris and Glover, 1988) provides theminimum age for this deformation (Wortmanet al., 2000). The Uwharrie Formation was af-fected by minor deformation and low-grademetamorphism, but lacks fabric elements ofthe older Virgilina event. However, the HycoFormation was metamorphosed at low meta-morphic grade (greenschist) and cannot be di-rectly equivalent to Deep Rock and PenBranch Metavolcanic Suites, which recordmiddle to upper amphibolite facies conditions.It could be that the Deep Rock and PenBranch metavolcanic rocks represent the samerocks found in the Hyco Formation metamor-


DENNIS et al.

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phosed to higher grades in a different location;alternatively, the postulated correlation withthe Hyco Formation may be spurious.

The Crackerneck Metavolcanic Complexmay correlate with the Persimmon ForkFormation of central South Carolina, the Lin-colnton metadacite of northeastern Georgia,and the Uwharrie Formation of NorthCarolina–Virginia (Fig. 14). All are dominatedby felsic to intermediate-composition tuffs,have ages that range from ca. 590 to 550 Ma,and were affected by minor deformation andlow-grade metamorphism, but lack fabric el-ements of older, Virgilina-age events.

Thus, one possible interpretation of theDeep Rock–Pen Branch volcanic arc is that itrepresents the infrastructure of the Carolinaterrane in South Carolina but has been de-tached by later tectonic events. More likely,given the distinct Eu* characteristics of theSavannah River Site rocks, is that the DeepRock–Pen Branch arc represents Late Prote-rozoic arc infrastructure from another locationin the arc that has been moved into its presentlocation by transcurrent motions. In eithercase, the low-grade Crackerneck MetavolcanicComplex, which seems to correlate with theUwharrie–Persimmon Fork–Lincolnton as-semblage, may sit unconformably on top ofthe older Deep Rock–Pen Branch arc rocks,much as the Uwharrie Formation sits uncon-formably on the Hyco Formation in centralNorth Carolina (Harris and Glover, 1988).

CONCLUSIONS

Neoproterozoic volcanic and plutonic rocksthat underlie the Savannah River Site in cen-tral South Carolina reflect igneous and meta-morphic activity that occurred in an island-arcterrane far from Laurentia, where they are nowlocated. These rocks correlate in general withmetaigneous rocks of the Carolina and Avalonterranes, which are inferred to have originatedas fringing arcs marginal to Gondwana (e.g.,Secor et al., 1983; Shervais et al., 1996;Nance and Murphy, 1996). Peri-Gondwanaarc terranes range in age from ca. 630 Ma to535 Ma, coincident with the rifting of Lauren-tia along both its eastern and western margins(e.g., Dennis and Wright, 1997; Barker et al.,1998).

Many of these arcs contain evidence for de-formation and metamorphism during thissame time period (e.g., Dennis and Wright,1997; Strachan et al., 1996; Shervais et al.,2003). Deformation and metamorphism ofvolcanic arc terranes at ca. 590 6 50 Ma werewidespread throughout Gondwana, e.g., thePan-African events in West Africa and Arabia,

the Brasiliano event in South America, the Ca-domian event in Europe, and the Petermann-Paterson orogens of Australia (e.g., Rogers etal., 1995; Mallard and Rogers, 1997; Veevers,2003). In general, deformation and metamor-phism were contemporaneous with arc mag-matism in these terranes, which implies thatthese events are driven not by continental col-lisions or the closure of ocean basins, but bythe collapse and collision of arc terranes inresponse to complex plate motions associatedwith the rifting and breakup of Rodinia andthe subsequent assembly of Gondwana.

ACKNOWLEDGMENTS

We thank Randy Cumbest and Sharon Lewis ofWestinghouse/SRC for their moral and logisticalsupport and Westinghouse/SRC for access to theircore-storage facility. Sample preparation for XRFanalyses was carried out by Hampton Uzzelle, Mor-ris Jones, and Neil Wicker; ICP-MS analyses wereperformed by Scott Vetter, Centenary College. Thiswork was funded by the U.S. Department of Energythrough Westinghouse/SRC, SCUREF Task 170.Partial support for the preparation of this manuscripthas come from the I.W. Marine Fund, University ofSouth Carolina, Aiken.

REFERENCES CITED

Barker, C.A., Secor, D.T., Jr., Pray, J.R., and Wright, J.E.,1998, Age and deformation of the Longtown Meta-granite, South Carolina Piedmont: A possible con-straint on the origin of the Carolina terrane: Journalof Geology, v. 106, p. 713–725.

Carpenter, R.H., Odom, A.L., and Hartley, M.E., III, 1982,Geochronological investigation of the Lincolnton me-tadacite, Georgia and South Carolina, in Bearce, D.N.,Black, W.W., Kish, S.A., and Tull, J.F., eds., Tectonicstudies in the Talladega and Carolina Slate belts,southern Appalachian orogen: Geological Society ofAmerica Special Paper 191, p. 145–152.

Cervantes, P., and Wallace, P.J., 2003, Role of H2O in sub-duction-zone magmatism: New insights from melt in-clusions in high-Mg basalts from central Mexico: Ge-ology, v. 31, p. 235–238.

Colquhoun, D.J., 1995, A review of Cenozoic evolution ofthe southeastern United States Atlantic coast north ofthe Georgia Trough, in Brown, L.J., ed., Diversity incoastal evolution in the Quaternary (a contribution toIGCP Project 274): Quaternary International, v. 26,p. 35–41.

Cox, K.G., Bell, J.D., and Pankhurst, R.J., 1979, The In-terpretation of Igneous Rocks: London, Allen and Un-win, 450 p.

Cumbest, R.J., Price, V., and Anderson, E.E., 1992, Gravityand magnetic modeling of the Dunbarton Triassic ba-sin, South Carolina: Southeastern Geology, v. 33,p. 37–51.

Dallmeyer, R.D., Wright, J.E., Secor, D.T., Jr., and Snoke,A.W., 1986, Character of the Alleghanian orogeny inthe southern Appalachians: Part II. Geochronologicalconstraints on the tectonothermal evolution of theeastern Piedmont in South Carolina: Geological So-ciety of America Bulletin, v. 97, p. 1329–1344.

Dennis, A.J., 1995, The Carolina terrane in northwesternSouth Carolina: Relative timing of events and recenttectonic models, in Hibbard, J.P., van Staal, C.R.,and Cawood, P.A., eds., New perspectives in the

Appalachian-Caledonian orogen: Geological Associa-tion of Canada Special Paper 41, p. 173–190.

Dennis, A.J., and Shervais, J.W., 1991, Arc rifting of theCarolina terrane in northwestern South Carolina: Ge-ology, v. 19, p. 226–229.

Dennis, A.J., and Shervais, J.W., 1996, The Carolina ter-rane in northwestern South Carolina: Insights into thedevelopment of an evolving island arc, in Nance, D.,ed., Avalonian and related terranes of the circum-Atlantic: Geological Society of America Special Paper304, p. 237–256.

Dennis, A.J., and Wright, J.E., 1995, Mississippian (ca.326–323 Ma) U-Pb crystallization ages for two gran-itoids in Spartanburg and Union Counties, South Car-olina, in Dennis, A.J., Butler, J.R., Garihan, J.M., Ran-son, W.A., and Sargent, K.A., eds., Geology of thewestern part of the Carolina terrane in northwesternSouth Carolina: Columbia, South Carolina GeologicalSurvey, Carolina Geological Society Field Trip Guide-book for 1995, South Carolina Geology, v. 37,p. 23–28.

Dennis, A.J., and Wright, J.E., 1997, The Carolina terranein northwestern South Carolina, USA: LatePrecambrian–Cambrian deformation and metamor-phism in a peri-Gondwana oceanic arc: Tectonics,v. 16, p. 460–473.

Dennis, A.J., Sacks, P.E., and Maher, H.D., 1987, Natureof the late Alleghanian strike-slip deformation in theeastern South Carolina Piedmont: The Irmo shearzone, in Secor, D.T., Jr., ed., Anatomy of the Allegha-nian orogeny as seen from the Piedmont of South Car-olina and Georgia: Columbia, South Carolina Geolog-ical Survey, Carolina Geological Society Field TripGuidebook for 1987, p. 49–66.

Dennis, A.J., Butler, J.R., Garihan, J.M., Ranson, W.A., andSargent, K.A., 1995, Geology of the Carolina terranein northwestern South Carolina: South Carolina Ge-ology, v. 37, 29 p.

Dennis, A.J., Shervais, J.W., and Maher, H.D., Jr., 2000a,Outline of the geology of Appalachian basement rocksunderlying the Savannah River Site, Aiken, SouthCarolina, in Wyatt, D.E., and Harris, M.K., eds., Sa-vannah River Site: Environmental remediation sys-tems in unconsolidated upper coastal plain sedi-ments—Stratigraphic and structural considerations:Carolina Geological Society, 2000 Annual Field TripGuidebook, WSRC-MS-2000-00606 (CD-ROM),p. 95–113.

Dennis, A.J., Shervais, J.W., and Secor, D.T., Jr., 2000b,Newberry, South Carolina eclogite: Structural settingand style of occurrence, in Wyeth, A., ed., A com-pendium of field trips of South Carolina geology:South Carolina Department of Natural Resources,Geological Survey, p. 29–38.

Domoracki, W.J., 1995, Geophysical investigation of geo-logic structure and regional tectonic setting at the Sa-vannah River Site [Ph.D. thesis]: Blacksburg, VirginiaPolytechnic and State University, 125 p.

Fallaw, W.C., and Price, V., 1995, Stratigraphy of the Sa-vannah River site and vicinity: Southeastern Geology,v. 35, p. 21–58.

Ferry, J.M., and Spear, F.S., 1978, Experimental calibrationof partitioning of Fe and Mg between biotite and gar-net: Contributions to Mineralogy and Petrology, v. 66,p. 113–117.

Ganguly, J., and Saxena, S.K., 1984, Mixing properties ofaluminosilicate garnets: Constraints from natural andexperimental data, and applications to geothermo-barometry: American Mineralogist, v. 69, p. 88–97.

Gill, J.B., 1981, Orogenic andesites and plate tectonics:Berlin, Springer-Verlag, 390 p.

Glover, L., III, and Sinha, A.K., 1973, The Virgilina de-formation, a late Precambrian to Early Cambrian (?)orogenic event in the central Piedmont of Virginia andNorth Carolina: American Journal of Science, v. 273-A,p. 234–251.

Harris, C.W., and Glover, L., III,1988, The regional extent


DENNIS et al.

of the ca. 600 Ma Virgilina deformation: Implicationsfor stratigraphic correlation in the Carolina terrane:Geological Society of America Bulletin, v. 100,p. 200–217.

Heatherington, A.L., and Mueller, P.A., 1996, Geochemis-try and origin of Floridan crustal basement terranes,in Randazzo, A., and Jones, D., eds., Geology of Flor-ida: Gainesville, University of Florida Press.

Heatherington, A.L., Mueller, P.A., and Nutman, A.P., 1996,Neoproterozoic magmatism in the Suwannee terrane:Implications for terrane correlations, in Nance, R.D.,and Thompson, M., eds., Avalonian and related peri-Gondwanan terranes of the circum–North Atlantic:Geological Society of America Special Paper 304,p. 219–236.

Hibbard, J.P., and Samson, S.D., 1995, Orogenesis exoticto the Iapetan cycle in the southern Appalachians, inHibbard, J.P., van Staal, C.R., and Cawood, P.A.: As-sociation of Canada Special Paper 41, p. 191–205.

Hibbard, J.P., Stoddard, E.F., Secor, D.T., and Dennis, A.J.,2002, The Carolina Zone: Overview of the Neopro-terozoic to Early Paleozoic peri-gondwana terranesalong the eastern flank of the southern Appalachians:Earth Science Reviews, v. 57, p. 299–339.

Hodges, K.V., and Spear, F.S., 1982, Geothermometry, geo-barometry and the Al2SiO5 triple point at Mt. Moosi-lauke, New Hampshire: American Mineralogist, v. 67,p. 1118–1134.

Horton, J.W., Jr., Drake, A.A., Jr., and Rankin, D.W., 1989,Tectonostratigraphic terranes and their Paleozoicboundaries in the central and southern Appalachians,in Dallmeyer, R.D., ed., Terranes in the circum-Atlan-tic Paleozoic orogens: Geological Society of AmericaSpecial Paper 230, p. 213–245.

Horton, J.W., Jr., Drake, A.A., Jr., Rankin, D.W., and Dall-meyer, R.D., 1991, Preliminary tectonostratigraphicterrane map of the central and southern Appalachians:U.S. Geological Survey Miscellaneous InvestigationsSeries Map I-2163, scale 1:2,000,000, 1 sheet.

Indares, A., and Martignole, J., 1985, Biotite-garnet geo-thermometry in the granulite facies: The influence ofTi and Al in biotite: American Mineralogist, v. 70,p. 272–278.

Irvine, T.N., and Baragar, W.R.A., 1971, A guide to thechemical classification of the common volcanic rocks:Canadian Journal of Earth Sciences, v. 8, p. 523–548.

Kish, S.A., 1992, An initial geochemical and isotopic studyof granite from core C-10, Savannah River Site, SouthCarolina, in Fallaw, W., and Price, V., eds., Geologicalinvestigations of the central Savannah River area,South Carolina and Georgia: Columbia, Carolina Geo-logical Society Field Trip Guidebook, p. B-IV-1–B-IV-4.

Kistler, R.W., 1974, Phanerozoic batholiths in westernNorth America: A summary of some recent work onvariations in time, space, chemistry, and isotopic com-positions: Annual Review of Earth and Planetary Sci-ences, v. 2, p. 403–418.

Kozuch, M., Mueller, P., Heatherington, A., Wooden, J.,Loeppen, R., and Klein, T., 1993, Isotopic evidence inthe Carolina slate and Charlotte belts (North Carolina)for reactivated Grenvillian and older sources: Geolog-ical Society of America Abstracts with Programs,v. 25, p. 35.

Krogh, T.E., 1973, A low-contamination method for hydro-thermal decomposition of zircon and extraction of Uand Pb for isotopic age determinations: Geochimicaet Cosmochimica Acta, v. 37, p. 485–494.

Luhr, J.F., and Carmichael, I.S.E., 1980, The Colima Vol-canic Complex, Mexico: Contributions to Mineralogyand Petrology, v. 71, p. 343–372.

Maher, H.D., Jr., 1987a, Kinematic history of myloniticrocks from the Augusta fault zone, South Carolina andGeorgia: American Journal of Science, v. 287,p. 795–816.

Maher, H.D., Jr., 1987b, D3 folding in the eastern Piedmontassociated with Alleghanian thrusting, in Secor, D.T.,

Jr., ed., Anatomy of the Alleghanian orogeny as seenfrom the Piedmont of South Carolina and Georgia:Columbia, South Carolina Geological Survey, Caro-lina Geological Society Guidebook, p. 35–48.

Maher, H.D., Palmer, A.R., Secor, D.T., and Snoke, A.W.,1981, New trilobite locality in the Piedmont of SouthCarolina, and its regional implications: Geology, v. 9,p. 34–36.

Maher, H.D., Sacks, P.E., and Secor, D.T., Jr., 1991, Theeastern Piedmont in South Carolina, in Horton, J.W.,Jr., and Zullo, V.A., eds., The geology of the Caroli-nas: Knoxville, University of Tennessee Press, p. 93–108.

Maher, H.D., Jr., Dallmeyer, R.D., Secor, D.T., Jr., andSacks, P.E., 1994, 40Ar/39Ar constraints on chronologyof Augusta fault zone movement and late Alleghanianextension, southern Appalachian Piedmont, SouthCarolina and Georgia: American Journal of Science,v. 294, p. 428–448.

Mallard, L.D., and Rogers, J.J.W., 1997, Relationship ofAvalonian and Cadomian terranes to Grenville andPan-African events: Journal of Geodynamics, v. 23,p. 197–221.

Marine, I.W., 1974, Geohydrology of buried Triassic basinat Savannah River Plant, South Carolina: AmericanAssociation of Petroleum Geologists Bulletin, v. 58,p. 1825–1837.

Marine, I.W., and Siple, G.E., 1974, Buried Triassic basinin the central Savannah River area, South Carolinaand Georgia: Geological Society of America Bulletin,v. 85, p. 311–320.

Mattison, J.M., 1987, U-Pb ages of zircons: A basic ex-amination of error propagation: Chemical Geology,v. 66, p. 151–162.

Milton, D.J., 1984, Revision of the Albermarle Group,North Carolina: U.S. Geological Survey Bulletin1537-A, p. 69–72.

Moore, J.G., 1959, The quartz diorite boundary line in thewestern United States: Journal of Geology, v. 67,p. 198–210.

Mueller, P.A., Heatherington, A.L., Wooden, J.L., Shuster,R.D., Nutman, A.P., and Williams, I.S., 1994, Precam-brian zircons from the Florida basement: A Gondwanaconnection: Geology, v. 22, p. 119–122.

Mueller, P.A., Kozuch, M., Heatherington, A.L., Wooden,J.L., Offield, T.W., Koeppen, R.P., and Klein, T.L.,1996, Evidence for Mesoproterozoic basement in theCarolina terrane and speculations on its origin, inNance, R.D., and Thompson, M.D., eds., Avalonianand related peri-Gondwanan terranes of the circum–North Atlantic: Geological Society of America SpecialPaper 304, p. 207–217.

Nance, R.D., and Murphy, J.B., 1996, Basement isotopicsignatures and Neoproterozoic paleogeography of theAvalonian-Cadomian and related terranes in the cir-cum North Atlantic, in Nance, R.D., and Thompson,M., eds., Avalonian and related peri-Gondwanan ter-ranes of the circum–North Atlantic: Geological Soci-ety of America Special Paper 304, p. 333–346.

Nance, R.D., Murphy, J.B., Strachan, R.A., D’Lemos, R.S.,and Taylor, G.K., 1991, Late Proterozoic tectonostra-tigraphic evolution of the Avalonian and Cadomianterranes: Precambrian Research Geology, v. 53,p. 41–78.

Potts, P.J., Tindle, A.G., and Webb, P.C., 1992, Geochem-ical reference material compositions: Boca Raton,Florida, CRC Press, 313 p.

Pouchou, J.-L., and Pichoir, F., 1991, Quantitative analysisof homogeneous or stratified microvolumes applyingthe model ‘‘PAP,’’ in Heinrich, K.F.J., Newbury, D.E.,eds., Electron probe quantitation: New York, PlenumPress, p. 31–76.

Roden, M., La Tour, T.E., Whitney, J., Anderson, M., andCapps, R.C., 2002, Geochemistry and petrology ofcrystalline basement beneath Coastal Plain sedimentsat the Savannah River Site, South Carolina, USA:Southeastern Geology, v. 41, p. 37–62.

Rogers, J.J.W., Unrug, R., and Sultan, M., 1995, Tectonicassembly of Gondwana: Journal of Geodynamics,v. 19, p. 1–34.

Rousseau, R.M., 1989, Concepts of influence coefficientsin XRF analysis and calibration, in Ahmedali, S.T.,ed., X-ray fluorescence analysis in the geological sci-ences: Advances in methodology: Geological Asso-ciation of Canada GAC-MAC Short Course 7, p. 141–220.

Samson, S., Palmer, A.R., Robison, R.A., and Secor, D.T.,Jr., 1990, Biogeographical significance of Cambriantrilobites from the Carolina Slate belt: Geological So-ciety of America Bulletin, v. 102, p. 1459–1470.

Samson, S.D., Coler, D.G., and Speer, J.A., 1995a, Geo-chemical and Nd-Sr-Pb isotopic composition of Al-leghanian granites of the southern Appalachians: Or-igin, tectonic setting, and source characterization:Earth and Planetary Science Letters, v. 134,p. 359–376.

Samson, S.D., Hibbard, J.P., and Wortman, G.L., 1995b, Ndisotopic evidence for juvenile crust in the Carolinaterrane, southern Appalachians: Contributions to Min-eralogy and Petrology, v. 121, p. 171–184.

Secor, D.T., Jr., Samson, S.L., Snoke, A.W., and Palmer,A.R., 1983, Confirmation of the Carolina Slate belt asan exotic terrane: Science, v. 221, p. 649–650.

Secor, D.T., and Snoke, A.W., 2002, Geologic map of theBatesburg and Emory 7.59 quadrangles, Lexingtonand Richland counties with explanatory notes: Geo-logical Society of America Map and Chart Series,MCH-091.

Secor, D.T., Jr., Snoke, A.W., Bramlett, K.W., Costello, O.P.,and Kimbrell, O.P., 1986a, Character of the Allegha-nian orogeny in the southern Appalachians: Part I. Al-leghanian deformation in the eastern Piedmont ofSouth Carolina: Geological Society of America Bul-letin, v. 97, p. 1319–1328.

Secor, D.T., Jr., Snoke, A.W., and Dallmeyer, R.D., 1986b,Character of the Alleghanian orogeny in the southernAppalachians: Part III. Regional tectonic relations:Geological Society of America Bulletin, v. 97,p. 1345–1353.

Shah, M.T., and Shervais, J.W., 1999, The Dir-Utror me-tavolcanic sequence, Kohistan arc terrane, northernPakistan: Journal of Asian Earth Sciences, v. 17/4,p. 459–475.

Shervais, J.W., Shelley, S.A., and Secor, D.T., Jr., 1996,Geochemistry of volcanic rocks of the Carolina andAugusta terranes in central South Carolina: An exoticrifted volcanic arc?, in Nance, R.D., and Thompson,M., eds., Avalonian and related peri-Gondwanan ter-ranes of the circum–North Atlantic: Geological Soci-ety of America Special Paper 304, p. 219–236.

Shervais, J.W., Dennis, A.J., McGee, J.J., and Secor, D.T.,2003, Deep in the heart of Dixie: Pre-Alleghanianeclogite and HP granulite metamorphism in the Car-olina terrane, South Carolina, USA: Journal of Meta-morphic Geology, v. 21, p. 65–80.

Snipes, D.S., Fallaw, W.C., Price, V., and Cumbest, R.J.,1993, The Pen Branch fault: Documentation of LateCretaceous–Tertiary faulting in the coastal plain ofSouth Carolina, Southeastern Geology, v. 33,p. 195–218.

Speer, J.A., 1982, Description of granitoid rocks associatedwith two gravity minima in Aiken and Barnwell coun-tries, South Carolina: South Carolina Geology, v. 26,15–24.

Stevenson, D.A., and Talwani, P., 1996, August 8, 1993Aiken, South Carolina earthquake: Seismological Re-search Letters, v. 67, p. 43–50.

Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel,A.J., and Stewart, K.C., 1987, Analysis of geologicmaterials by wavelength dispersive X-ray fluorescencespectrometry: U.S. Geological Survey ProfessionalPaper 1770, p. E1–E19.



Veevers, J.J., 2003, Pan-African is Pan-Gondwanaland;oblique convergence drives rotation during 650–500Ma assembly: Geology, v. 31, p. 501–504.

Whitney, J.A., Paris, T.A., Carpenter, R.H., and Hartley,M.E., III, 1978, Volcanic evolution of the southernslate belt of Georgia and South Carolina: A primitiveoceanic island arc: Journal of Geology, v. 86,p. 173–192.

Williams, H., and Hatcher, R.D., Jr., 1982, Suspect terranesand accretionary history of the Appalachian orogen:Geology, v. 10, p. 530–536.

Williams, H., and Hatcher, R.D., Jr., 1983, Appalachian sus-pect terranes, in Hatcher, R.D., Jr., Williams, H., andZietz, I., eds., Contributions to the tectonics and geo-physics of mountain chains: Geological Society ofAmerica Memoir 158, p. 33–53.

Wortman, G.L, Samson, S.D., and Hibbard, J.P., 1996, Dis-crimination of the Milton belt and the Carolina terranein the southern Appalachians; an Nd isotopic ap-proach: Journal of Geology, v. 104, p. 239–247.

Wortman, G.L., Samson, S.D., and Hibbard, J.P., 2000, Pre-cise U-Pb zircon constraints on the earliest magmatic

history of the Carolina terrane: Journal of Geology,v. 108, p. 321–338.

Wright, J.E., and Seiders, V.M., 1980, Age of zircon fromvolcanic rocks of the central North Carolina Piedmontand tectonic implications for the Carolina volcanicslate belt: Geological Society of America Bulletin,v. 91, p. 287–294.

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