Petrology and geochemistry of Neoproterozoic volcanic arc terranes beneath the Atlantic Coastal Plain, Savannah River Site, South Carolina

Allen J. Dennis² Department of Biology and Geology, University of South Carolina, Aiken, South Carolina 29801-6309, USA John W. Shervais Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA Joshua Mauldin Department 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. Wright Department of Geology, University of Georgia, Athens, Georgia 30602, USA

ABSTRACT tuffs, all metamorphosed under greenschist- the basis of their compositions and ages, we facies conditions), (2) the Deep Rock tentatively correlate these rocks with the The Piedmont of South Carolina and Metaigneous Complex (ma®c to felsic vol- Hyco Formation in southern Virginia and Georgia is a complex mosaic of exotic ter- canic and plutonic rocks metamorphosed central North Carolina. The Hyco Forma- ranes of uncertain provenance. Farther under lower amphibolite±facies conditions), tion constitutes the infrastructure of the south and east, these terranes form the (3) the Pen Branch Metaigneous Complex Carolina terrane in Virginia and North basement beneath several kilometers of (amphibolites, garnet amphibolites, garnet- Carolina, where it was affected by the ca. Cretaceous and Cenozoic sedimentary biotite schists, and gneiss), and (4) the Tri- 600 Ma ``Virgilina'' orogeny. The rocks of rocks, commonly referred to as the Atlantic assic Dunbarton Basin Group, a sedimen- the Deep Rock and Pen Branch Metaig- Coastal Plain. The distribution and geolog- tary unit that ®lls a northeast-trending neous Complexes may have formed the arc ic history of this hidden crystalline base- graben beneath younger sedimentary rocks infrastructure of the Carolina Slate belt in ment can be inferred only on the basis of of the Atlantic Coastal Plain. South Carolina, detached by later tectonic limited exposures at the margins of the All of the metaplutonic and metavolcanic events, or may have formed the Late Pro- Coastal Plain onlap, aeromagnetic linea- rocks have calc-alkaline fractionation terozoic arc infrastructure at another lo- ments that de®ne basement trends in the trends, consistent with formation in sub- cation in the arc that has been moved into subsurface, and core data from wells that duction-related arc terranes at convergent its current location by transcurrent mo- penetrate basement. margins. Zircon U-Pb crystallization ages tions. Limited age and isotopic data suggest During the past 40 years, basement cores of ca. 626 Ma to 619 Ma, however, show that none of these rocks correlate with the aggregating more than 6 miles (10,000 m) that the Deep Rock and Pen Branch com- Suwannee terrane of North Florida and have been recovered from 57 deep wells at plexes do not correlate with the younger southern Georgia. the Department of Energy's Savannah Riv- Carolina terrane (570±535 Ma) or Suwan- er Site. These cores provide the only known nee terrane (ca. 550 Ma). The Deep Rock Keywords: Neoproterozoic, peri-Gondwana, samples of basement terranes that lie south- and Pen Branch Metaigneous Complexes arc volcanism, Carolina terrane, geochem- east of the Fall Line in central South Car- may be a continuation of Proterozoic base- istry, petrology. olina. Cores from the 57 deep wells, along ment that forms the older infrastructure of with structural trends de®ned by aeromag- the Carolina arc. The contact between the INTRODUCTION ؍) netic lineaments, allow us to de®ne four dis- Crackerneck Metavolcanic Complex tinct units within the basement beneath the Persimmon Fork Formation?) and the Deep The hinterland of the southern Appala- Coastal Plain: (1) the Crackerneck Meta- Rock and Pen Branch Metaigneous Com- chians, which lies southeast of Grenville base- volcanic Complex (greenstones and felsic plexes thus may be equivalent to the an- ment and an attached cover sequence exposed gular unconformity between the Uwharrie in the Blue Ridge province, comprises a com- ²E-mail: [email protected]. Formation and the Virgilina sequence. On plex mosaic of exotic or ``suspect'' tectono-

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

For permission to copy, contact [email protected] 572 ᭧ 2004 Geological Society of America NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

their petrologic evolution, and explore their subsequent tectonic evolution.

GEOLOGIC FRAMEWORK OF THE SAVANNAH RIVER SITE

Crystalline basement at the Savannah River Site is entirely covered by onlap of the Atlan- tic Coastal Plain. This basement is separated from well-characterized rocks of the Carolina Slate belt (Carolina terrane) by three major fault zones (Secor et al., 1986a, 1986b; Maher et al., 1991, 1994; Horton et al., 1991): the Modoc zone, the Augusta/Belair fault sys- tems, and an inferred fault zone indicated by a strong aeromagnetic lineament that de®nes the southern edge of the Belair belt (Ascauga fault zone; Fig. 2). South of this aeromagnetic Figure 1. Regional map of the southern Appalachians, showing the distribution of major lineament, high-grade sillimanite-bearing tectonic subdivisions of the Laurentian margin (Blue Ridge, Piedmont, Carolina terrane) gneisses of the Belvedere belt are intruded by and the extent of post-Jurassic sedimentary onlap onto the continental margin (Atlantic the Carboniferous Graniteville pluton (e.g., Coastal Plain). Previous areas studied by the authors in the Appalachian Piedmont of Samson et al., 1995a); both are exposed in South Carolina are shown as polygons: (1) western Carolina terrane/Charlotte belts, Cen- erosional windows through the coastal-plain tral Piedmont suture (Dennis and Shervais, 1991, 1996; Dennis and Wright, 1995, 1997; sediments (Fig. 2). The limits of the Granite- Dennis, 1995; Dennis et al. 1995); (2) eclogite/high-P granulites of the Charlotte belt± ville pluton may be estimated from its asso- Carolina Slate belt boundary and Carolina Slate belt in central South Carolina (Shervais ciated gravity anomaly; gravity anomalies also et al., 2003; Dennis et al., 2000b); and (3) the Slate belt±Kiokee belt±Belair belt (Maher de®ne the extent of the Devonian Spring®eld et al., 1981, 1991, 1994; Maher, 1987a, 1987b; Dennis et al. 1987; Shervais et al., 1996). pluton (Speer, 1982) and smaller, unnamed Location of current study shown as circle (4) Savannah River Site. AFÐAugusta fault, granitic stocks related to the larger bodies BZÐBrevard zone, CPSÐcentral Piedmont suture, GHÐGold Hill fault, GSFÐGreat (Fig. 2). The Graniteville pluton is penetrated Smoky Fault, H-FFSÐHayesville-Fries fault system, MZÐModoc Zone. and sampled by a single borehole, the C-2 well. stratigraphic terranes that range in age from years, basement cores totaling more than 6 Magnetic and gravity potential ®eld data late Neoproterozoic through middle Paleozoic. miles (Ͼ10,000 m) long have been recovered from the Savannah River Site, coupled with These terranes were accreted to Laurentia dur- from 57 deep wells at the Department of En- core logs from over 30 locations, show that ing the middle to late Paleozoic and now form ergy's Savannah River Site. Southeast of the the subsurface geology can be divided into all of the exposed crystalline rocks east of the eastern Piedmont in central South Carolina four main units (from north to south): (1) Blue Ridge (e.g., Williams and Hatcher, 1982, and Georgia, these cores provide most of the Crackerneck Metavolcanic Complex: low- 1983; Secor et al., 1983; Maher et al., 1981, known samples of crystalline basement and, grade (greenschist-facies) metavolcanic rocks 1991; Horton et al., 1989, 1991; Samson et along with structural trends de®ned by aero- (tuffs, lapilli tuffs, lavas), named for a small al., 1990; Hibbard et al., 2002). Farther to the magnetic lineaments, allow us to de®ne four creek that drains the surface above this unit; southeast, terranes accreted to and overthrust distinct units within the basement beneath the (2) Deep Rock Metaigneous Complex: upper onto crystalline basement of the Laurentian Coastal Plain. Three of these units represent greenschist± to lower amphibolite±facies me- margin are hidden beneath several kilometers crystalline basement; they comprise ma®c to tavolcanic rocks, metaplutonic rocks, and of Mesozoic and Tertiary sedimentary rocks, felsic metavolcanic rocks and dioritic to gra- crosscutting dikes, strongly deformed or my- commonly referred to as the Atlantic Coastal nitic metaplutonic rocks, all metamorphosed lonitized in places, named for its type expo- Plain (Colquhoun, 1995; Fallaw and Price, at greenschist- to amphibolite-facies condi- sures in the Deep Rock Borehole (DRB) series 1995). Deciphering the origin and provenance tions. The fourth unit represents the clastic of wells; (3) Pen Branch Metaigneous Com- of this buried crystalline basement is central sedimentary ®ll of a northeast-trending Trias- plex: upper amphibolite to granulite-facies to our understanding of terrane accretion dur- sic graben, the Dunbarton Basin. metagranitoids and metavolcanic rocks, ing the Paleozoic and has implications for col- This paper examines metaigneous rocks named for a small creek that drains the surface lisional orogenesis in the hinterland of the sampled by these deep core holes and com- above this unit and for its type exposures in southern Appalachians. pares them to rocks exposed in surface out- the ``Pen Branch fault'' (PBF) series of wells; The U.S. Department of Energy Savannah crops throughout the Piedmont of the Caroli- and (4) Dunbarton Basin: a deep Triassic rift River Site is located near the northwest mar- nas, Georgia, and Virginia; a parallel basin with fault-controlled margins that was gin of the Atlantic Coastal Plain in central investigation of much of this core material buried by coastal-plain sediments (Marine, South Carolina (Fig. 1), where up to 2 km of was carried out by Roden et al. (2002). For 1974; Marine and Siple, 1974; Cumbest et al., Cretaceous and younger sedimentary deposits these rocks we present new whole-rock geo- 1992; Fallaw and Price, 1995). These units are overlie crystalline rocks of the basement and chemistry, mineral chemistry, and isotopic shown in Figure 2. South of the site, geo- a major Triassic rift basin. During the past 40 data, including two U-Pb zircon ages, examine physical potential ®eld data have been inter-

Geological Society of America Bulletin, May/June 2004 573 DENNIS et al.

574 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA preted to show the presence of a Triassic± Jurassic ma®c igneous complex in the subsur- face (e.g., Petty et al., 1965; Daniels, 1974; Stevenson and Talwani, 1996), but limited core data from this area (from the C-10 well) show that much of this basement is similar to that found in the Pen Branch Metaigneous Complex. The most prominent tectonic feature of the Savannah River Site subsurface is the Pen Branch fault, a Triassic border fault along the northern margin of the Dunbarton Basin that was reactivated after the Cretaceous (Fig. 2; Snipes et al., 1993). Other tectonic features include (1) the Tinker Creek nappe, a Paleo- zoic structure adjacent to the Pen Branch fault that places high-grade rocks of the Pen Branch Metaigneous Complex against lower-grade rocks of the Deep Rock Metaigneous Com- plex; (2) the Crackerneck fault, a northeast- trending feature that offsets the sub- Cretaceous unconformity; (3) the Upper Three Runs fault, a northeast-trending lineament in the aeromagnetic map forms the southeast margin of the Crackerneck Metavolcanic Complex; and (4) the Martin fault, a poorly de®ned northeast-trending feature that forms the southern margin of the Dunbarton Basin Figure 3. Outline map of Savannah River Site and surrounding area showing locations of and also offsets the sub-Cretaceous unconfor- deep wells that intersect basement studied for this project. mity (Fig. 2: Domoracki, 1995; Snipes et al., 1993).

METHODS rock geochemical analysis. Petrographic and and 12 trace elements (Nb, Zr, Y, Sr, Rb, Zn, mineral assemblage data for each unit are Cu, Ni, Cr, Sc, V, Ba) by using the University Core was logged for 26 wells drilled in and summarized in Table 1; formal descriptions of South Carolina's Philips PW-1400 X-ray around the Westinghouse/Savannah River and type sections for all new units de®ned in ¯uorescence spectrometer. Major elements Site, including wells drilled between approx- this publication are available (Table DR-11). were analyzed on fused glass disks by using imately 1960 (DRB series) and the mid-1990s Whole-rock samples were analyzed for 10 a method similar to that of Taggart et al. (C series, MMP series). Figure 2 is a geologic major elements (SiO2,TiO2,Al2O3, total Fe as (1987) with selected U.S. Geological Survey map based on subcrop lithology; well loca- Fe2O3*, MnO, MgO, CaO, Na2O, K2O, P2O5) and international standards prepared identical- tions are shown in Figure 3. Cores from all ly to the samples. Accepted concentrations wells studied here were logged for lithology, 1GSA Data Repository item 2004074, formal de- were taken from the compilation of Potts et structural relationships, fractures, folds, folia- scription of new units described herein, whole rock al. (1992). Matrix corrections were carried out tions, layering, and relict primary intrusive major and trace element analyses by x-ray ¯uores- within the Philips X41 software package, contacts. Over 400 samples were collected, ence, whole rock REE and other analyses by ICP- which uses the fundamental parameters ap- MS, representative plagioclase and amphibole min- and from this collection ϳ150 samples were eral chemistry, is available on the Web at http:// proach (Rousseau, 1989) to calculate theoret- selected for further petrologic study, including www.geosociety.org/pubs/ft2004.htm. Requests ical alpha coef®cients for the range of stan- petrographic, mineral-chemical, and/or whole- may also be sent to [email protected]. dards. Replicate analyses of selected standards

Figure 2. Geologic map showing the distribution of crystalline basement lithologic units in and around the Savannah River Site (dashed outline), 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 include the Crackerneck Metavolcanic Complex, the Deep Rock Metaigneous Complex, the Pen Branch Metaigneous Complex, and clastic sedimentary rocks of the Triassic Dunbarton basin. Triassic±Jurassic ma®c igneous complex south of the Dunbarton basin is based on aeromagnetic anomalies; core data from wells C-7 and C-10 indicate that much of this basement is pinked granite of the PBF7 intrusive complex. Subsurface extents of the Devonian Spring®eld 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 Spring®eld pluton is sampled by well SAL-1. EPFSÐEastern Piedmont fault system. DRBÐDeep Rock Borehole series of wells. N

Geological Society of America Bulletin, May/June 2004 575 DENNIS et al.

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

Unit Lithologies Mineral assemblages Relict igneous Metamorphic facies phases Crackerneck Metavolcanic Complex Felsic tuffs and lapilli tuffs, ma®c tuffs and Qtz, Alb, Chl, Epi, Ms, None Greenschist to greenstones Opq subgreenschist Deep Rock Metaigneous Complex

Deep Rock Metavolcanic Suite Aphyric ma®c to intermediate tuffs, Plg An3±8, Qtz, Chl, Hbl, Plg An29±43 Epidote amphibolite plagioclase-phyric tuffs and schists, Epi, Bi, ϮBGA, Gt hornblende-phyric tuffs and schists (rare)

DRB1 Metaplutonic Suite Diorite±quartz diorite gneiss, Plg An9±20, Qtz, Chl, Hbl, Plg An20±40 Epidote amphibolite porphyroclastic gneiss Epi, Act, BGA Pen Branch Metaigneous Complex

Pen Branch Metavolcanic Suite Ma®c to intermediate amphibolite, garnet Plg An35±45, BGA, Qtz, None Garnet amphibolite amphibolite Bi, Epi, Gt

PBF7 Metaplutonic Suite, normal Metagranite, granodiorite gneiss Plg An24±38, BGA, Qtz, None Garnet amphibolite Ksp, Epi, Bi, Chl

PBF7 Metaplutonic Suite, pinked Metagranite, granodiorite gneiss Plg An4, Ksp, Qtz, Gt, None Gt amphibolite with Chl, rare BGA, Bi greenschist overprint Note: QtzÐquartz, PlgÐplagioclase, AlbÐalbite, KspÐmicrocline, ChlÐchlorite, EpiÐepidote, ActÐactinolite, BGAÐblue-green amphibole, HblÐhornblende (igneous), BiÐbiotite, MsÐmuscovite, GtÐgarnet, OpqÐopaques.

as unknowns suggest percent relative errors solution and ion-exchange chemistry for sep- ¯attened and elongated, giving these rocks an

ϳ1% for SiO2, ϳ2%±4% for less abundant aration of uranium and lead followed appearance similar to lapilli tuffs of the Per- major elements, and ϳ1%±6% for all trace el- procedures modi®ed from Krogh (1973). Iso- simmon Fork Formation. ements except Cr, which is slightly higher tope ratios were measured with the MAT 262 (13%). Thirty whole-rock samples were also multicollector instrument at Rice University. Deep Rock Metaigneous Complex analyzed for rare earth element (REE) and Analytical uncertainties, blanks, and common other trace element concentrations by induc- lead corrections are outlined in Table 3. The Deep Rock Metaigneous Complex tively coupled argon plasma±mass spectrom- (named herein as described in Table DR-1 [see etry (ICP-MS) at the University of New Mex- DESCRIPTION OF THE LITHOLOGIC footnote 1]) comprises most of the Savannah ico. Selected major and trace element data are UNITS River Site basement north of the Triassic Dun- presented in Table 2; the complete data set of barton Basin (Fig. 2). It consists of two sub- major and trace element data are available (Ta- Crackerneck Metavolcanic Complex units: the Deep Rock Metavolcanic Suite and bles DR-2 and DR-3 [see footnote 1]). the DRB1 Metaplutonic Suite (Table 1). Quantitative electron-microprobe analyses The Crackerneck Metavolcanic Complex of major and minor elements in minerals were (named herein as described in Table DR-1 [see Deep Rock Metavolcanic Suite obtained with a Cameca SX50 electron micro- footnote 1]) underlies the northwestern corner The Deep Rock Metavolcanic Suite was probe at the University of South Carolina. of the Savannah River Site and continues at sampled most extensively by the 4-inch-di- Analyses were made at 20 kV accelerating least as far north as the Graniteville pluton ameter (10.16-cm-diameter) Deep Rock Bore- voltage, 30 nA probe current, and counting (Fig. 2). The complex is represented by cores hole (DRB) cores, but is also penetrated by a times of 20±100 s; both natural and synthetic from wells C-1, C-3, P30, P6R, P8R, GCB-1, number of other, more widely spaced wells mineral standards were used. Analyses were GCB-2, GCB-3, and MMP4 (Fig. 3). Rocks (SSW-1, SSW-2, SSW-3, GCB-5.1; Fig. 3). corrected for instrumental drift and dead time, in the northern part of this unit are dominantly Metavolcanic rocks of the Deep Rock and electron beam/matrix effects by using the intermediate-composition to felsic tuffs and metavolcanic complex display a wide array ``PAP'' ␾(␳z) correction procedures provided lapilli tuffs, whereas cores from the southern of textures (aphyric, plagioclase-phyric, with the Cameca microprobe automation sys- part of the subcrop area are dominated by hornblende-phyric), compositions, and miner- tem; these correction procedures are based on greenstone and ma®c tuff (Table 1). Low- als. Mineral assemblages are consistent with the model of Pouchou and Pichoir (1991). An- grade metavolcanic rocks of the complex are lower amphibolite±facies metamorphism. All alytical precision is ϳ1% of the amount pres- penetrated by younger granitic intrusions, in- rocks have been overprinted with a penetrative ent for oxide concentrations greater than 10 cluding the Devonian Spring®eld granite and fabric (foliation) that dips ϳ40Њ to 55Њ; based wt%, 1%±2% for oxide concentrations be- the Carboniferous Graniteville granite (Fig. 2). on regional correlations, the orientation of tween 1 and 10 wt%, and 5%±10% for oxide Felsic tuffs of the Crackerneck Metavolcan- strike is probably northeast and the direction concentrations between 0.01 and 1 wt%. Rep- ic Complex are generally composed of quartz of dip is to the southeast. resentative mineral analyses are available (Ta- and plagioclase and have sparse plagioclase Plagioclase phenocrysts commonly form bles DR-4 and DR-5 [see footnote 1]). phenocrysts (Fig. 4A). Ma®c tuffs are com- subhedral to euhedral crystals (Fig. 4B) that Zircon was separated by conventional tech- posed of chlorite, plagioclase, epidote, and preserve primary igneous compositions and niques using a Wil¯ey Table, heavy liquids, opaque minerals. All plagioclase grains were zoning (An29 to An43). Other plagioclase and a Franz magnetic separator. The least metamorphosed to nearly pure albite (An2 to grains are anhedral, albitized (An3 to An8) por- magnetic zircons from each sample were split An5: Fig. 5A), but relict igneous textures are phyroclasts (Fig. 5A) that have been partially into size fractions and then handpicked to re- typically well preserved. Lapilli tuffs in core to extensively epidotized. Hornblende pheno- move any contaminating grains. Zircon dis- MMP4 display large pumice lapilli that were crysts range from blocky, subhedral crystals

576 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

Fig. 6). Many intermediate-composition to fel- sic metavolcanic rocks contain biotite, which is commonly partially chloritized, and a few contain garnet. Garnet-bearing rocks are most common in DRB-3, which may sample higher-grade metamorphic rocks than other cores from this unit. Postkinematic dikes are common in cores DRB-2, DRB-3, DRB-5, and DRB-6. These dikes, which crosscut the metavolcanic rocks, are generally undeformed (not folded) and lack the fabric elements of the wall rocks, but are metamorphosed to similar grade. 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 ma®c dikes are characterized by large, euhedral plagioclase phenocrysts or glomerocrysts that preserve relict primary zoning, within a groundmass of amphibole, plagioclase, epidote, chlorite, and biotite. The aphyric ma®c dikes are iden- tical to the plagioclase-phyric dikes, but lack the conspicuous large phenocrysts. The felsic dikes consist of ®ne-grained plagioclase and quartz and have minor opaque oxides, biotite, and hornblende. Core from DRB-4 contains a number of

rocks with extremely high SiO2 values (78%± 86%) and high modal mica; these rocks may represent sedimentary protoliths. These rocks are extremely quartz rich and carry sparse pla- gioclase porphyroclasts in a mylonitic ground- mass. Many of these rocks contain unaltered biotite, and others contain abundant muscovite and aluminosilicate minerals; the rocks tex- turally resemble button schists. These char- acteristics support a sedimentary protolith for these samples.

DRB1 Metaplutonic Suite The DRB1 Metaplutonic Suite is sampled exclusively by core DRB-1. The suite consists dominantly of diorite and quartz diorite sheets that intrude preexisting metavolcanic wall Figure 4. Photomicrographs of Crackerneck Metavolcanic Complex and Deep Rock Me- rock. The DRB-1 metadiorite are metamor- taigneous Complex. (A) Subhedral feldspar phenocrysts within a matrix of plagioclase, phosed to epidote-amphibolite±facies assem- quartz, and minor biotite, Crackerneck Metavolcanic Complex (cross-polarized light). (B) blages and range texturally from equigranular Subhedral plagioclase phenocryst in ma®c metavolcanic rock, Deep Rock Metavolcanic to porphyritic gneisses (Table 1). Suite (cross-polarized light). (C) Relict igneous hornblende (brown) surrounded by mantle Plagioclase porphyroclasts may show relict of metamorphic blue-green amphibole, DRB1 Metaplutonic Suite (plane-polarized light). igneous features, including oscillatory zoning Width of ®eld of view represents 2.3 mm in all. and albite twinning. Individual porphyroclasts may have cores as calcic as An40 and rims as

sodic as An9 (Fig. 5B). The calcic cores are to elongated, prismatic crystals (Fig. 4C). and TiO2 contents and Mg/Fe ratios. Horn- interpreted to represent relict igneous plagio- Compared to metamorphic amphiboles, relict blende phenocrysts and groundmass horn- clase compositions; the sodic rims formed igneous hornblende phenocrysts (brown in blende are commonly metamorphosed to blue- during metamorphism along with the blue- thin section) are characterized by lower Al2O3 green tschermakitic amphibole that is low in green amphibole (e.g., Roden et al., 2002). and FeO contents and by higher SiO2, MgO, Ti and high in Al (e.g., Roden et al., 2002; Metaplutonic rocks of DRB-1 contain no gar-

Geological Society of America Bulletin, May/June 2004 577 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 DRB5 Box: none none none none 17M 17 6 59 76 131 101 123 145 73 15M Sample no.: 574 540 781 817 1081M 1081 988 1346 1450.7 1831 1613 1758.4 1912 1802 1324 Rock type: Felsic Felsic Felsic Felsic Felsic Ma®c Felsic Ma®c Felsic Felsite Inter MV Ma®c Ma®c Felsite Ma®c tuff tuff lapilli tuff lapilli tuff MV MV MV MV gneiss MV MV MV MV MV

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 50.48

TiO2 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 0.97

Al2O3 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 18.88

Fe2O3 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 11.13 MnO 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 0.19 MgO 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 5.68 CaO 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 9.30

Na2O 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 3.29

K2O 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 0.57

P2O5 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 0.24 SUM 100.66 100.29 99.18 99.88 101.45 99.27 100.85 101.98 99.05 99.59 98.32 99.74 99.905 100.54 100.75 LOI 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 4 Zr 206 159 287 296 175 148 188 102 190 176 182 185 170 149 78 Y 221972654531352576414233283021 Sr 350 106 64 35 634 715 365 782 48 116 146 310 209 331 637 Rb 69 30 27 32 38 52 62 28 40 78 92 23 55 17 11 Zn 88 76 134 138 60 106 75 94 30 59 71 105 93 28 98 Cu 17 5 2 2 17 174 4 148 11 nd 22 70 76 27 120 Ni 34 9 42 43 5 18 nd 18 3 14 27 165 178 2 14 Cr 42 10 79 160 6 47 3 28 nd 48 63 329 384 2 22 Sc 14 7 nd nd 13 30 7 20 4 nd 15 32 29 7 18 V 97 45 7 5 80 309 74 287 19 56 109 304 226 38 221 Ba 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 7.8 Ce 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 19.1 Pr 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 2.4 Nd 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 10.2 Sm 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 2.4 Eu 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 0.59 Gd 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 2.8 Tb 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 0.55 Dy 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 2.7 Ho 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 0.61 Er 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 1.5 Tm 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 0.23 Yb 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 1.2 Lu 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 0.20 Y 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 14.4 Pb 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 6.7 Th 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 1.2 U 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- (0.03 to 0.45 afu; most samples have Ͼ0.1 afu the Triassic Dunbarton Basin to the south and ly low temperatures and pressures (lower Ti) and lower AlIV (Ͻ1.35 afu; Fig. 6). Some the more extensive Deep Rock Metaigneous amphibolite±facies conditions). Biotite is rare blue-green metamorphic amphiboles contain Complex to the north (Fig. 2). It also appears to absent in the DRB-1 metadiorites; almost cores of ®brous actinolite (AlIV ഠ 0.4 afu; Fig. to continue on the south side of the Dunbarton all primary biotite has been retrograded to 6) that formed under retrograde greenschist- Basin, but few drill holes penetrate basement chlorite. facies conditions. there. The Pen Branch Metaigneous Complex Most amphibole porphyroclasts, and essen- The DRB1 Metaplutonic Suite contains xe- consists of two subunits: the PBF7 Metavol- tially all groundmass amphiboles, are meta- noliths and screens of amphibolite-grade me- canic Suite and the PBF7 Metaplutonic Suite. morphosed to blue-green amphibole (Roden et tavolcanic rocks that are deformed and contain al., 2002). Some amphibole porphyroclasts fabric elements similar to those of rocks of the Pen Branch Metavolcanic Suite preserve cores of brown hornblende, inter- Deep Rock Metavolcanic Suite, with which Rocks of the Pen Branch Metavolcanic preted to represent relict igneous hornblende, they are assumed to correlate. Suite were recovered from eight wells: PBF- with higher Mg/Fe ratios than the metamor- 7, PBF-8, SSW-1, SSW-2, SSW-3, GCB-4, C- phic blue-green amphibole. These differences Pen Branch Metaigneous Complex 5, and the seismic attenuation (SA) well (lo- are shown clearly in a plot of tetrahedral Al cated adjacent to GCB-4 in Fig. 3). Ma®c vs. Ti, where metamorphic amphibole has AlIV The Pen Branch Metaigneous Complex metavolcanic rocks in the lowermost part of Ͼ 1.4 afu (atoms per formula unit on the basis (named herein for the Pen Branch fault, as de- PBF-7 are separated from the overlying me- of 23 oxygens) and Ti Ͻ 0.06 afu (Fig. 6). scribed in Table DR-1 [see footnote 1]) forms tagranitoids by a fault ϳ1100 m below the Coexisting igneous hornblende has higher Ti a thin slice of crystalline basement between surface. Higher up in this same well, ma®c

578 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

TABLE 2. (Continued)

Core no.: DRB6 DRB6 DRB2 DRB2 DRB1 DRB1 DRB1 DRB1 C5 PBF7 PBF7 PBF7 PBF7 C5 PBF7 Box: 4M 103 108 12 18 25 41 60 none 103 107 108 139 none 171 Sample no.: 1125 1810 1687 1053.5 1005 1047 1151 1276 1084 2550 2602 2629 3086 1080 3568 Rock type: Ma®c Ma®c Ma®c Plg Quartz Diorite Quartz Tonalite Ma®c Grano- Grano- Grano- Grano- Felsic Pink MV MV dike porph diorite diorite schist diorite diorite diorite diorite dike granite dike

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 75.31

TiO2 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 0.14

Al2O3 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 13.23

Fe2O3 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 1.36 MnO 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 0.04 MgO 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 0.28 CaO 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 0.98

Na2O 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 4.03

K2O 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 5.25

P2O5 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 0.02 SUM 100.70 101.01 99.64 100.03 100.49 101.56 100.91 101.15 100.33 101.06 100.90 101.27 100.97 100.33 100.64 LOI 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%

Nb76567354 919182353040 Zr 164 163 148 132 141 65 107 85 192 275 305 298 134 246 115 Y 332834322518271630425344215382 Sr 711 885 617 645 316 263 297 381 458 421 347 311 341 205 27 Rb 59 108 21 30 19 7 20 3 61 49 97 70 55 72 106 Zn 125 136 62 98 81 91 69 59 72 179 89 66 103 40 34 Cu 230 150 178 162 81 87 26 6 18 81 117 51 61 15 nd Ni 19 18 7 19 12 41 6 24 35 19 22 14 21 6 5 Cr 35 38 12 49 16 142 19 22 29 42 32 12 47 2 nd Sc 22 26 29 30 15 38 30 24 19 20 24 11 17 4 4 V 283 316 273 310 142 316 352 172 144 157 147 96 148 38 3 Ba 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 17.0 Ce 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 48.2 Pr 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 7.2 Nd 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 26.8 Sm 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 8.3 Eu 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 0.34 Gd 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 9.9 Tb 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 2.05 Dy 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 11.3 Ho 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 2.19 Er 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 6.9 Tm 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 1.02 Yb 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 5.4 Lu 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 0.80 Y 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 82.5 Pb 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 14.1 Th 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 1.4 U 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 ␮g/g. LOIÐloss on ignition; MVÐmetavolcanic rock; porphÐporphyry.

TABLE 3A. U-Pb ISOTOPE DATA

Sample² U 206Pb³ Measured ratios§ Atomic ratios Apparent ages# (Ma) (ppm) (ppm) 206Pb 207Pb 208Pb 206Pb³ 207Pb³ 207Pb³ 206Pb³ 207Pb³ 207Pb³ 204Pb 206Pb 206Pb 238U 235U 206Pb³ 238U 235U 206Pb³ PBF-7 ϩ150A 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 Ϯ 0.9 PBF-7 ϩ150 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 Ϯ 1.4 PBF-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 Ϯ 1.7 PBF-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 Ϯ 1.2 DRB-1 ϩ150A 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 Ϯ 1.2 DRB-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 Ϯ 1.1 Note: Error analysis for individual zircon fractions follows Mattinson (1987); errors are shown in parentheses and refer to the least signi®cant digit. Total Pb blanks ranged 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 each sample. ²Sample masses between 0.05 and 0.5 mg; ϩ100, 100±200, etc., refer to size fractions in mesh. ³Denotes radiogenic Pb, corrected for common Pb by using the isotopic composition of 206Pb/204Pb ϭ 18.6 and 207Pb/204Pb ϭ 15.6. Sample dissolution and ion-exchange chemistry modi®ed 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 ϭ 9.8485 ϫ 10Ϫ10 yrϪ1 and 1.55125 ϫ 10Ϫ10 yrϪ1, respectively; 238U/235U ϭ 137.88.

Geological Society of America Bulletin, May/June 2004 579 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 87 86 87 86 ²² 87 86 147 144 143 144 ³³ (ppm) (ppm) Rb/ Sr Sr/ Sr Sr/ Sr(624) (ppm) (ppm) Sm/ Nd Nd/ Nd PBF-7 99 243 1.1774 0.715526(9) ± 10.01 43.01 0.14069 0.512512(4) ϩ2.0 (626 Ma) DRB-1 10 344 0.0840116 0.704028(6) 0.70193851 3.44 14.34 0.14487 0.512606(6) ϩ3.5 (619 Ma) Note: Values used for CHUR (chondritic uniform reservoir) are 143Nd/144Nd ϭ 0.512638 and 147Sm/144Nd ϭ 0.1967. Decay constants: Sm ϭ 6.54 ϫ 10Ϫ12 yrϪ1;Rbϭ 1.42 ϫ 10Ϫ11 yrϪ1. Sm and Nd concentrations determined by isotope dilution by addition of a mixed 149Sm-150Nd spike prior to sample dissolution. Rb and Sr concentrations determined by XRF (X-ray ¯uorescence) at University of South Carolina. Repeated analysis of SRM-987 yielded 87Sr/86Sr ϭ 0.710247. Repeated analysis of BCR-1 yielded 143Nd/144Nd ϭ 0.512633. All errors are at the 95% con®dence limit; errors are shown in parentheses and refer to the least signi®cant digit. ²²Corrected for mass fractionation by normalizing to 86Sr/88Sr ϭ 0.1194. ³³Corrected for mass fractionation by normalizing to 146Nd/144Nd ϭ 0.72190. §§ 143 144 143 144 ␧Nd(T) ϭ [( Nd/ Nd(T)sample/ Nd/ Nd(T)CHUR) Ϫ 1] ϫ 10,000.

metavolcanic rocks form screens of wall rock Gneissic metagranitoids. Gneissic meta- cline, and chlorite (Table 1). Microcline com- that were intruded by granodiorites of the granitoids are porphyroclastic gneisses with monly occurs as megacrysts up to ϳ5mm PBF7 Metaplutonic Suite. In the SA well, maf- porphyroclasts of amphibole and plagioclase across and may account for as much as 40% ic to intermediate-composition metavolcanic set in a groundmass of amphibole, plagioclase, of the mode (Fig. 7C). Partially pinked rocks rocks were sampled by spot coring below 580 microcline, biotite, quartz, and epidote (Table have plagioclase An25 to An33, whereas pla- m; continuous coring to depths of ϳ370 m 1, Fig. 7B). Plagioclase porphyroclasts within gioclase in the thoroughly pinked granites has sampled metagranitoids similar to those in the the Pen Branch gneisses range from An24 to been completely albitized (An4) and, subse- lower part of core PBF-7. Well C-5 is char- An38 and have an average composition of An35 quently, partially sericitized (Fig. 5D). Am- acterized by intermediate-composition meta- (Fig. 5D). Amphibole porphyroclasts are un- phibole and biotite are almost completely volcanic rocks at depths of ϳ330 m that un- zoned and have Mg/Fe ratios similar to the chloritized, but some relict blue-green amphi- derlie sedimentary deposits of the coastal metamorphic amphiboles in the DRB-1 me- bole is preserved. plain with no intercalated metagranitoids. Fo- tadiorites, but TiO2 values range from 0.7% to liation dips ϳ45Њ±60Њ, similar to foliation in 1.0%, higher than metamorphic amphibole in GEOCHEMISTRY the adjacent Deep Rock Metavolcanic Suite. the DRB-1 metadiorites. Blue-green amphi- Rocks of the Pen Branch Metavolcanic bole in the PBF7 Metaplutonic Suite is clearly Whole-rock geochemical analyses were ob- Suite are ma®c to intermediate-composition distinguished from the relict igneous horn- tained by X-ray ¯uorescence for 59 metavol- amphibolites and garnet amphibolites that are blende of the DRB1 Metaplutonic Suite, how- canic rocks, 35 metaplutonic rocks, 15 post- distinct from metavolcanic rocks of the adja- ever, by its higher AlIV contents (Ͼ1.42 afu; kinematic dikes, and 3 metasedimentary cent Deep Rock Metavolcanic Suite (Table 1). Fig. 6). rocks; 31 of these samples were analyzed for There is no relict igneous amphibole, and all Pen Branch gneissic metagranitoids are dis- additional trace elements by ICP-MS. Repre- metamorphic amphibole is tschermakitic, with tinguished from DRB-1 metadiorite by their sentative whole-rock data are presented in Ta- low Si and high Al. Ti is much higher than in lack of relict igneous hornblende, the relative- ble 2. The complete data set of major and metamorphic amphiboles of the Deep Rock ly high Ti in blue-green metamorphic amphi- trace element data are available (Tables DR-2 Metaigneous Complex, possibly in response bole, higher An-content plagioclase, the com- and DR-3 [see footnote 1]). to higher metamorphic grade (Fig. 6). The rel- mon occurrence of microcline, and the atively calcic plagioclase is also considered to preservation of biotite (Table 1). Some of Crackerneck Metavolcanic Complex re¯ect higher metamorphic grade (An35 to these characteristics may result from higher

An45; Fig. 5C). Garnet is a common meta- metamorphic grade in the Pen Branch meta- Low-grade metavolcanic rocks of the morphic mineral in many of these amphibo- granitoids, but others (e.g., common micro- Crackerneck Metavolcanic Complex may be lites, but garnet-biotite pairs are relatively rare cline) re¯ect fundamental compositional dif- classi®ed as basalts, dacites, and rhyolites by because biotite is commonly retrograded to ferences between the two intrusive series. using their silica vs. alkali relationships (Cox chlorite. Garnets, which range up to 1 cm Pinked metagranitoid. Much of the granitic et al., 1979). Most are high in SiO2 (65±77 across, are typically elongated parallel to fo- core material recovered from wells PBF-7, wt%) and exhibit decreasing MgO, Fe2O3*, liation and have spongy, inclusion-rich tex- PBF-8, C-10, and SA had been subjected to TiO2, CaO, and Al2O3 with increasing SiO2; tures (Fig. 7A). All of the garnet studied here partial to extensive hydrothermal alteration only one metabasalt has been analyzed (Fig. is unzoned, re¯ecting annealing during or after and potassium metasomatism after formation 8). Na2O increases with increasing SiO2 con- peak thermal metamorphism. of foliation. We refer to this alteration as the tent, but surprisingly, K2O is highest in the ``pinking'' event because of the salmon-pink two dacites, C-1±570 and C-1±574 (Table DR- PBF7 Metaplutonic Suite color imparted to rocks affected by the hydro- 2 [see footnote 1]). These rocks are also high The PBF7 Metaplutonic Suite dominates thermal ¯uids. The effects of this event range in Ba (1240±1350 ppm vs. 120±750 in the the upper part of the cores recovered from from millimeter-scale selvages on fractures to other samples), Rb, Sr, and Cu, suggesting wells PBF-7, PBF-8, C-10, GCB-4, and the pervasive alteration of hundreds of meters of that K2O and the other mobile trace elements SA well. The PBF7 Metaplutonic Suite com- cored rock (Dennis et al., 2000a). were enriched by secondary hydrothermal prises two dominant lithologies: gneissic me- ``Pinked'' Pen Branch metagranites com- processes. The overall characteristics of these tagranitoids and ``pinked'' metagranitoids, monly contain large plagioclase, microcline, rocks are calc-alkaline on an AFM diagram formed by hydrothermal alteration of the and garnet porphyroblasts set in a ®ner- (Fig. 9A), covering much the same composi- gneissic metagranitoids (Dennis et al., 2000a). grained matrix of quartz, plagioclase, micro- tional range as similar low-grade metavolcanic

580 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

Figure 5. Plagioclase ternary plots showing compositions of igneous and metamorphic plagioclase porphyroclasts. (A) Crackerneck and Deep 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.

rocks in the Belair belt and in the Persimmon Fork 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 ϳ32 to 80 times chondritic abundances, and La/Lu ratios are ϳ2toϳ14 times the chondritic ratio (Fig. 10A).The highest La/Lu ratios are observed in the two K-rich dacites, suggesting that the LREEs may have been enriched, or the HREEs depleted, by the same hydrothermal solutions that enriched these samples in K, Rb, and Ba. All four samples have small but well-de®ned negative Eu anomalies. MORB- normalized spider diagrams show that all four samples are enriched in low ®eld strength el- ements and depleted in high ®eld strength el- ements, consistent with subduction-zone en- Figure 6. Tetrahedral aluminum atoms per formula unit (afu, on the basis of 23 oxygens) richment processes (Fig. 11A). vs. titanium afu in metamorphic and relict igneous amphiboles of the Deep Rock and Pen Branch Metaigneous Complexes. Relict igneous hornblendes in the Deep Rock Metavol- Deep Rock Metaigneous Complex canic Suite (␯) and DRB1 Metaplutonic Suite (␪) have less AlIV compared to the meta- morphic amphiboles in either the metadiorites of the Deep Rock Metaigneous Complex Deep Rock Metavolcanic Suite (␷) or the metavolcanic rocks of the Deep Rock Metavolcanic Suite (x); relict igneous Metavolcanic rocks of the Deep Rock Me- hornblendes of the DRB1 Metaplutonic Suite are also higher in Ti. Metagranitoids of the tavolcanic Suite may be classi®ed as basalts, PBF7 Metaplutonic Suite contain no relict igneous hornblende, but the metamorphic blue- andesites, dacites, and rhyolites. Major and green amphibole (q) is higher in Ti than metamorphic amphiboles in the Deep Rock trace element trends on Harker diagrams are Metavolcanic Suite and the DRB1 Metaplutonic Suite, suggesting higher equilibration typical of calc-alkaline volcanic suites (Fig. temperatures. Actinolite is plotted for reference. 8). The alkalis show signi®cant scatter, sug-

Geological Society of America Bulletin, May/June 2004 581 DENNIS et al.

analysis (Table DR-3 [see footnote 1]). All are enriched in the light rare earth elements (LREEs) relative to the heavy rare earth ele- ments (HREEs): La contents are ϳ22 to 90 times chondritic abundances, and La/Lu ratios are ϳ3.5 to ϳ8.8 times the chondritic ratio (Fig. 10B). The La/Lu ratios are higher than those in the DRB-1 metadiorite but in the same range as those observed in the low-grade metavolcanic rocks of the Crackerneck Meta- volcanic Complex. All of the samples have moderate to deeply negative Eu anomalies,

which seem to be deepest in rocks with SiO2 Ͼ 65 wt% (Fig. 10B), consistent with exten- sive plagioclase fractionation under relatively low oxygen fugacity conditions. MORB- normalized spider diagrams show that all are enriched in low ®eld strength elements and depleted in high ®eld 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 ma®c dikes (aphyric and pla- gioclase megaphyric) are all basalt (or rarely, basaltic andesite) in composition, with low

TiO2 (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 than most Deep Rock Metavolcanic Suite amphib- olites. These younger ma®c dikes are also characterized by higher Sr concentrations (ϳ550 to ϳ700 ppm Sr) than most amphib- olites in the suite. Postkinematic felsic dikes include dacites and rhyolites that exhibit typ- ical calc-alkaline fractionation trends. The fel- sic dikes exhibit the same characteristic high Sr as the younger ma®c dikes. Only two post- kinematic dikes were analyzed for REE con- centrations: one aphyric ma®c dike and one plagioclase megaphyric ma®c dike. Both dikes have moderate LREE enrichment: La contents are ϳ42 times chondritic abundances, and La/ Lu ratios are ϳ3toϳ4 times the chondritic Figure 7. Photomicrographs of Pen Branch Metaigneous Complex: (A) Typical ratio (Fig. 10C). The La/Lu ratios are low amphibolite-facies assemblage of garnet, biotite, plagioclase, and lesser amounts of horn- compared to the host metavolcanic rocks. blende in the Pen Branch Metavolcanic Suite (cross-polarized light); note the elliptical Both samples have small negative Eu anom- nature of the garnets. (B) Microcline with characteristic tartan twinning in quartz- alies, showing that even the plagioclase me- plagioclase-amphibole granite (cross-polarized light). (C) Metamorphic amphibole (ex- gaphyric samples crystallized from melts in tinct) intergrown with quartz, plagioclase, and K-feldspar (cross-polarized light). Width which the plagioclase was being removed of ®eld of view represents 2.3 mm in all. rather than accumulating. MORB-normalized spider diagrams are similar to those of the vol- canic rocks (Fig. 11B). gesting either complex petrogenetic processes alkaline on an AFM plot (Fig. 9B), consistent DRB1 Metaplutonic Suite involving mixing or assimilation, or (more with their observed lack of Fe or Ti enrich- Metaplutonic rocks sampled from the DRB- likely) element mobility during amphibolite- ment on Harker diagrams. 1 core may be classi®ed as diorites or quartz facies metamorphism. These rocks are calc- Fourteen samples were chosen for REE diorites on the basis of their normative min-

582 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

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

eralogy. Major and trace element trends are er in plagiophile elements (Ca, Na, Al). Four chondritic abundances, and La/Lu ratios are typical of calc-alkaline intrusive suites (Figs. samples were chosen for REE analysis (Table ϳ2.3±3.8 times the chondritic ratio (Fig. 8, 9B). At any given weight percent silica, the DR-3 [see footnote 1]). They are slightly en- 10C). La/Lu ratios are lower than those ob- DRB-1 metadiorite samples are lower in ma®c riched in the light rare earth elements (LREEs) served in the Deep Rock Metavolcanic Suite, elements (Mg, Fe, Ti, Cr, Ni), K2O, and Rb relative to the heavy rare earth elements but all have small negative Eu anomalies. than the Pen Branch metagranitoids and high- (HREEs): La contents are ϳ10±30 times MORB-normalized spider diagrams are con-

Geological Society of America Bulletin, May/June 2004 583 DENNIS et al.

of the Deep Rock Metavolcanic Suite (Fig. Age Constraints and Tracer Isotopes 9C). Two samples of metaplutonic rock were dated by using the U-Pb zircon method (Table PBF7 Metaplutonic Suite 3). Four zircon fractions from a quartz mon- Metagranitoids. The gneissic metagrani- zodiorite of the Pen Branch Metaigneous toids may be classi®ed as quartz monzodio- Complex (PBF-7) gave a concordia-intercept rites and granodiorites by using their norma- date of 626.1 Ϯ 4.2 Ma (Fig. 12A). The tive mineral contents; their major and trace weighted mean of the 207Pb*/206Pb* dates is element trends are similar to those of DRB-1 625.6 Ϯ 1.3 Ma. Two zircon fractions from a (Fig. 8). At any given weight percent silica, quartz diorite of the Deep Rock Metaigneous the Pen Branch metagranitoids are higher in Complex (DRB-1) yield identical 207Pb*/ ma®c elements (Mg, Fe, Ti, Cr, Ni), K2O, and 206Pb* dates with a weighted mean of 619.9 Rb than the metadiorites of DRB-1 and lower Ϯ 0.8 Ma; the concordia intercept for this pair in plagiophile elements (Ca, Na, Al). Like the is 619 Ϯ 3.4 Ma (Fig. 12B). These Late Pro- DRB1 metadiorite samples, the Pen Branch terozoic ages are too old to correlate with the metagranitoids are strongly calc-alkaline, as Carolina Slate belt (Lincolnton metadacite± seen on an AFM plot (Fig. 9C). Persimmon Fork Formation±Uwharrie For- Six ``unpinked'' metagranitoids of the mation at ca. 550 Ma; Whitney et al., 1978; PBF7 Metaplutonic Suite were analyzed for Wright and Seiders, 1980; Carpenter et al., REE concentrations (Table DR-3 [see footnote 1982; Dallmeyer et al., 1986; Barker et al., 1]). All are enriched in light rare earth ele- 1998), the western Carolina terrane (Charlotte ments (LREEs) relative to the heavy rare earth belt at ca. 580 Ma to 535 Ma; Dennis and elements (HREEs): La contents are ϳ30±100 Wright, 1997), or North Florida Volcanic Se- times chondritic abundances, and La/Lu ratios ries of the Suwannee terrane (ca. 552 Ma; are ϳ3.6 to ϳ5.4 times the chondritic ratio Heatherington et al., 1996). Similar ages have (Fig. 10D). Both the La/Lu ratios and total been determined for the Hyco Formation in REEs are higher than those observed in the central North Carolina (ca. 633 Ma to 612 Ma; DRB1 Metaplutonic Suite. Five of these sam- Wright and Seiders, 1980; Harris and Glover, ples have small negative Eu anomalies, sug- 1988; Mueller et al., 1996; Wortman et al., gesting plagioclase fractionation, whereas one 2000; Mueller et al., 1996) and for an isolated has a signi®cant positive Eu anomaly, sug- granodiorite of the Suwannee terrane in south- gesting plagioclase accumulation (Fig. 10D). ern Alabama (ca. 625 Ma; Heatherington et Figure 9. AFM diagrams with the Irvine MORB-normalized spider diagrams show that al., 1996, p. 263). and Baragar (1974) dividing line for tholei- all six samples are enriched in low ®eld Age-corrected initial isotope ratios were itic and calc-alkaline trends. (A) Cracker- strength elements and depleted in high ®eld calculated by using the measured parent/ neck Metavolcanic Complex, (B) Deep strength elements, consistent with subduction- daughter ratios and the U-Pb ages of the sam- Rock Metaigneous Complex, (C) Pen zone enrichment processes (Fig. 11A). ples. These ratios indicate derivation of the Branch Metaigneous Complex. Pinked metagranitoids. Pinked metagrani- DRB1 and PBF7 Metaplutonic Suites from similar, relatively primitive sources, with ␧ toids are characterized by higher SiO ,KO, Nd 2 2 ϭϩ2.0 to ϩ3.5 (Table 3). The DRB-1 me- and Rb contents than their unaltered equiva- tadiorite sample has a calculated initial 87Sr/ lents and by lower MgO, Fe O *, TiO ,AlO , 2 3 2 2 3 86Sr ratio of 0.70194, but an initial 87Sr/86Sr CaO, Na O, and Sr (Fig. 8). As a result, these sistent with subduction-zone enrichment pro- 2 ratio could not be calculated for PBF-7 be- rocks are classi®ed as granites by using their cesses (Fig. 11B). cause the Rb-Sr system was disturbed by the normative mineral contents. They extend the Triassic pinking event. These values of ␧ are strong calc-alkaline trend of the unpinked me- Nd Pen Branch Metaigneous Complex similar to those that have been measured on tagranitoids (Fig. 9C), but this apparent calc- metaigneous rocks of the Carolina terrane and Pen Branch Metavolcanic Suite alkaline ``fractionation trend'' results from hy- other peri-Gondwana ``Avalonian'' terranes Amphibolites and garnet amphibolites of drothermal alteration, not igneous processes. (e.g., Nance and Murphy, 1996), but are high- the Pen Branch Metavolcanic Suite can be Only one sample of pinked metagranitoid was er than those found in the Suwannee terrane classi®ed geochemically as metamorphosed analyzed for REE concentrations (PBF-7± (Heatherington et al., 1996; Fig. 12C). basalts, basaltic andesites, and andesites. They 3568). This sample is characterized by modest are slightly lower in Fe and higher in Ca than LREE enrichment (La/Lu ratio is ϳ2.1 times DISCUSSION the Deep Rock metavolcanic rocks at any giv- the chondritic ratio) and by a pronounced neg- en weight percent silica, and they are restrict- ative Eu anomaly not seen in the unpinked Crackerneck Metavolcanic Complex ed to SiO2 contents of Յ60% SiO2 (Fig. 8). metagranitoids (Fig. 10D). This negative Eu They plot in the calc-alkaline ®eld on an AFM anomaly suggests that Eu2ϩ was mobilized Volcanic rocks of the Crackerneck Meta- diagram, where they exhibit typical calc- along with Ca2ϩ during the pinking event and volcanic Complex are the least deformed and alkaline fractionation trends similar to those was removed from the system. least metamorphosed of the units studied here.

584 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

Figure 10. Chondrite-normalized REE plots for (A) Crackerneck Metavolcanic Complex, (B) Deep Rock Metavolcanic Suite, (C) DRB1 Metaplutonic Suite and dikes within the Deep Rock Metavolcanic Suite, and (D) PBF7 Metaplutonic Suite.

Their low grade of metamorphism (sub- abundance of felsic tuffs and pumice lapilli more to the limited areal extent of the Belair greenschist to greenschist facies) and lack of tuffs resembles many continental-margin arcs belt, which has been sampled systematically penetrative deformation indicate that (1) these (e.g., Mexico: Luhr and Carmichael, 1980). in only one traverse along the banks of the rocks could not have been buried deeply sub- The Crackerneck Metavolcanic Complex Savannah River. The Crackerneck is separated sequent to their formation, and (2) they were correlates most closely with the Persimmon from the Belair belt by a strand of the aero- not overridden by thick thrust sheets during Fork Formation of the Carolina Slate belt (Se- magnetically de®ned Eastern Piedmont fault the Alleghenian orogeny. Their geochemical cor et al., 1986a, 1986b; Shervais et al., 1996). system and by high-grade metamorphic rocks of the Belvedere belt that are exposed in ero- characteristics (moderate to high SiO2, calc- Both units are dominated by felsic tuffs, were alkaline fractionation trends, enrichment in metamorphosed under low-grade conditions, sional windows through the coastal-plain sed- low ®eld strength elements, and depletion in and have comparable trace element concentra- imentary rocks near Graniteville (Fig. 2). high ®eld strength elements) all point to an tions. The Crackerneck Metavolcanic Com- Thus, the Crackerneck Metavolcanic Complex origin as volcanic rocks associated with an plex may also correlate with rocks of the Be- may be stratigraphically equivalent to the Be- island-arc volcanic system. The dominance of lair belt. These rocks are dominantly lair belt, but is not contiguous with that belt. more felsic members of this suite suggests that intermediate in composition, but include more Deep Rock Metaigneous Complex the crust upon which this volcanic arc was ma®c and more felsic compositions as well built consisted either of mature island-arc (Shervais et al., 1996). Differences in the pro- The Deep Rock Metaigneous Complex ex- crust or a thinned continental margin. The portions of ma®c and felsic rocks may relate hibits phase assemblages, calc-alkaline frac-

Geological Society of America Bulletin, May/June 2004 585 DENNIS et al.

Figure 11. MORB-normalized spider diagram for (A) Crackerneck and Pen Branch Metaigneous Complexes, (B) the Deep Rock Metaigneous 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 systematics Metaplutonic Suite are unrelated petrogeneti- not seen in DRB-1. These relationships sug- that are consistent with formation in a mature cally. This observation is supported by obser- gest that the Deep Rock Metavolcanic Suite island arc that formed over an extended period vations of the core: (1) DRB-1 metadiorites represents an older volcanic arc assemblage of time. In contrast, associated metaplutonic are found only in core DRB-1, (2) DRB-1 me- (possibly equivalent to the PBF7 Metaplutonic rocks of the DRB1 Metaplutonic Suite are tadiorites include screens of deformed meta- Suite) that was intruded in part by the DRB- dominated by intermediate-composition me- volcanic wall rocks that are chemically and 1 metadiorite at ca. 619 Ma. tadiorites, suggesting a primitive arc origin. petrologically similar to rocks of the Deep

The relatively high ␧Nd value of the DRB-1 Rock Metavolcanic Suite and contain fabric Pen Branch Metaigneous Complex metadiorite sample is also consistent with an elements that are not observed in the DRB-1 origin in a more primitive intraoceanic arc metadiorites, and (3) metavolcanic rocks ex- The Pen Branch Metaigneous Complex ex- setting. posed in cores DRB-2 through DRB-7 are cut hibits compositional ranges, trace element sys- The data presented here suggest that the by three distinct dike series (aphyric ma®c, tematics, and phase assemblages consistent Deep Rock Metavolcanic Suite and the DRB1 plagioclase porphyry, and felsic dikes) that are with formation in a volcanic arc, similar to

586 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA

adjacent rocks of the Deep Rock Metaigneous ples are from the Pen Branch Metavolcanic Complex. Metavolcanic rocks of the Pen Suite (PBF-7±3648 and SA-4002). We cannot Branch Metavolcanic Suite exhibit a more re- calculate a pressure for these rocks because stricted range of compositions; the lack of Fe they do not contain an aluminosilicate min- and Ti enrichment and the restricted range in eral, but we can estimate a nominal pressure

SiO2 contents suggest that the Pen Branch Me- of at least 3 kbar (10 km depth), which is rea- tavolcanic Suite represents a relatively primi- sonable for rocks of this grade. Table 4 pre- tive island arc compared to the Deep Rock sents the results of these calculations, which Metavolcanic Suite. In contrast, the PBF7 Me- are based on the exchange of Fe and Mg be- taplutonic Suite is dominated by granodiorites tween garnet and biotite (Ferry and Spear, and granites with intermediate-composition to 1978). Both samples preserve evidence for an

high SiO2 contents. The low ␧Nd value (ϩ2.0) early high-temperature event at 700 to 800 ЊC, of a Pen Branch gneissic granitoid (compared consistent with uppermost amphibolite±facies

to ␧Nd ϭϩ3.5 for the DRB-1 metadiorite sam- conditions. ple) implies a more evolved provenance for An intermediate-composition metavolcanic these intrusive rocks, in contrast to the more rock from the Deep Rock Metavolcanic Suite primitive characteristics of the Pen Branch (DRB-2±1449) also preserves metamorphic Metavolcanic Suite. biotite and garnet. Equilibrium biotite-garnet Rocks of the Pen Branch Metavolcanic temperatures for this sample average ϳ650 Suite appear to represent an older volcanic ЊC, consistent with lower to middle framework into which the Pen Branch grani- amphibolite±facies conditions (Table 4). Thus, toids were intruded at ca. 626 Ma. The intru- the Deep Rock and Pen Branch Metavolcanic sive relationship between the Pen Branch me- Suites may represent different levels of ex- tagranitoids and the Pen Branch metavolcanic posure that were juxtaposed by later faulting. rocks and the differences in their geochemical Metaigneous rocks of both the DRB1 and character suggest that these two rock suites are PBF7 Metaplutonic Suites preserve evidence unrelated petrogenetically. There is some sug- for metamorphic histories that are similar to gestion, however, that granitoids of the PBF7 their adjacent metavolcanic suites. Thus, the Metaplutonic Suite may represent the intru- DRB-1 metadiorites have metamorphic am- sive equivalent of the Deep Rock Metavolcan- phiboles similar to those in the Deep Rock ic Suite. This suggestion is based on chemical Metavolcanic Suite, but still retain relic cores similarities between the two suites, including of igneous hornblende. In contrast, the Pen their compositional ranges, their rare earth el- Branch metagranitoids lack relict igneous ement concentrations, and other trace element hornblende and contain only metamorphic similarities (Figs. 10 and 11). amphibole.

Metamorphism and Pressure-Temperature Potassium Metasomatism: The Big Pink Estimates Many metaplutonic rocks of the PBF7 Me- Metavolcanic rocks of both the Deep Rock taplutonic Suite were affected by a postmeta- and Pen Branch Metavolcanic Suites preserve morphic hydrothermal alteration event whose evidence for distinct metamorphic histories, characteristic signature is a pervasive discol- Figure 12. Concordia diagrams for zircon best illustrated by changes in amphibole com- oration of the normally gray rocks to various fractions from (A) Pen Branch (PBF-7) and position (Roden et al., 2002; Fig. 6). Relict shades of bright salmon-pink (Dennis et al., (B) Deep Rock (DRB-1) Metaigneous Com- igneous hornblende compositions are pre- 2000a). The metamorphic phase assemblage plexes. (C) ␧Nd vs. age for Pen Branch (PBF) served in the cores of many volcanic pheno- associated with this event (actinolite, chlorite, and Deep Rock (DRB) Metaigneous Suite crysts in the Deep Rock Metavolcanic Suite. albite, K-feldspar) indicates greenschist-facies rocks compared to published data for Car- These compositions are overprinted by the metamorphism at temperatures of ϳ350 to olina terrane plutonic rocks (®lled squares) pervasive growth of blue-green metamorphic ϳ450 ЊC. The primary geochemical charac- and volcanic, metamorphic, and sedimen- amphibole, consistent with amphibolite-facies teristic of this event is Si, K, and Rb meta- tary rocks (dashed ®eld): data from Wort- metamorphism. The high-temperature blue- somatism of the affected rocks by hydrother- man et al. (1995; W) and Mueller et al. green amphibole was subsequently retrograd- mal ¯uids and concomitant leaching of Ca, Sr, (1996; M); North Florida Volcanic Series ed to the greenschist-facies minerals actinolite and Eu2ϩ. We suggest that the breakdown of (NFVS): data from Heatherington et al. and chlorite. In contrast, metavolcanic rocks primary biotite to chlorite, which is nearly (1996; H). of the Pen Branch Metavolcanic Suite contain complete in the pinked samples and affects no relict igneous minerals, and all amphiboles much of the DRB1 Metaplutonic Suite, is the are metamorphic. most likely source of these alkalis. Three samples that preserve appropriate Data presented by Dennis et al. (2000a) mineral assemblages were chosen to calculate show that the ``pinking'' event predated the metamorphic temperatures. Two of these sam- formation of most ®lled fractures. The best es-

Geological Society of America Bulletin, May/June 2004 587 DENNIS et al.

TABLE 4. GARNET AND BIOTITE COMPOSITIONS IN METAVOLCANIC ROCKS, ALONG WITH Are the DRB1 and PBF7 Metaplutonic CALCULATED GT-BI TEMPERATURES Suites Related? Core SA PBF7 DRB-2 SA PBF7 DRB-2 Sample 4002 3648 1449 4002 3648 1449 Mineral Garnet Garnet Garnet Biotite Biotite Biotite Despite their gross similarity, plutonic rocks of the DRB1 and PBF7 Metaplutonic SiO 37.63 36.96 36.40 36.43 36.03 35.60 2 Suites exhibit signi®cant differences that have TiO2 0.06 0.01 0.04 1.17 1.94 1.98 Al2O3 21.32 21.00 20.69 17.94 17.73 16.96 led us to conclude that they do not represent FeO 26.46 28.14 29.68 17.75 20.16 19.38 MnO 2.94 2.23 4.05 0.27 0.15 0.18 the same rock series. Major differences in- MgO 5.57 3.10 3.66 12.15 10.14 11.15 clude the following: (1) The DRB1 Metaplu- CaO 5.30 7.66 3.63 0.08 0.00 0.27 tonic Suite is dominated by quartz diorite, Na O ± ± 0.12 0.05 0.10 2 whereas the PBF7 Metaplutonic Suite is dom- K2O ± ± 9.65 9.94 9.24 Total 99.28 99.10 98.15 95.54 96.13 94.85 inated by quartz monzodiorite (unpinked) and granite (pinked). (2) Metadiorite of the DRB1 Si 5.956 5.941 5.941 5.495 5.474 5.461 Ti 0.007 0.001 0.005 0.132 0.222 0.228 Metaplutonic Suite and quartz monzodiorites Al 3.977 3.978 3.980 3.189 3.175 3.066 of the PBF7 Metaplutonic Suite have distinct, Fe 3.502 3.783 4.052 2.239 2.561 2.486 Mn 0.394 0.304 0.559 0.034 0.019 0.023 subparallel trends on Harker diagrams that are Mg 1.314 0.743 0.890 2.731 2.295 2.549 offset from one another. At any given weight Ca 0.899 1.319 0.635 0.012 0.000 0.044 percent silica, DRB-1 metadiorite samples are Na ± ± ± 0.034 0.015 0.030 K ± ± ± 1.856 1.926 1.808 lower in ma®c elements (Mg, Fe, Ti, Cr, Ni), Pyr 0.215 0.121 0.145 K2O, and Rb than Pen Branch metagranitoids Alm 0.573 0.615 0.660 and higher in plagiophile elements (Ca, Na, Spes 0.065 0.049 0.091 Gros 0.147 0.215 0.103 Al). These differences must re¯ect different Aliv 2.505 2.526 2.539 parent magmas and different magma source Alvi 0.684 0.649 0.527 Bio-Tot 5.821 5.746 5.815 regions. (3) DRB-1 metadiorite samples have Ann 0.385 0.446 0.428 lower REE concentrations and lower La/Lu Phl 0.469 0.399 0.438 ratios than the plutonic rocks of the PBF7 Me- Ti-Bi 0.023 0.039 0.039 Al-Bi 0.118 0.113 0.091 taplutonic Suite. These differences are too Mn-Bi 0.006 0.003 0.004 large to result from fractionation processes and must re¯ect different parent-magma com- SA PBF7 DRB-2 4002 3648 1449 positions and different magma source regions.

F and S 807 648 639 Ferry and Spear (1978) (4) The DRB-1 metadiorite has a higher ␧Nd H and S 870 723 674 Hodges and Spear (1982) value (ϩ3.5) than the Pen Branch granodiorite G and S 781 699 657 Ganguly and Saxena (1984) sample (␧Nd ϭϩ2.0); these data indicate an I and M 825 667 614 Indares and Martignole (1985) oceanic af®nity for the DRB1 Metaigneous Average T (ЊC) 820 685 645 Average of all calculated T Suite and a weak continental in¯uence on the PBF7 Metaigneous Suite. The evidence listed above suggests that the DRB1 Metaplutonic Suite and PBF7 Metaplu- timate for the absolute age of this event is the Metavolcanic Suites exhibit signi®cant differ- tonic Suite are not equivalent. The low nor- Rb-Sr age presented by Kish (1992) for ences that have led us to conclude that they mative quartz contents, relatively primitive pinked granite from core C-10. This two-point do not represent the same rock series. The major element compositions, low total REEs, pseudo-isochron, based on the whole rock and Deep Rock Metavolcanic Suite ranges from low La/Lu ratios, and high ␧Nd of the DRB-1 on a K-feldspar separate, must represent the basalt to rhyolite in composition; the Pen metadiorites are interpreted to indicate that age of the ``pinking'' event because the per- Branch Metavolcanic Suite is restricted to maf- this plutonic suite formed in a magmatic arc vasive K and Rb metasomatism characterizing ic to intermediate compositions. In addition, that was built on older oceanic or arc crust this event would thoroughly reset the isotopic postkinematic ma®c dikes are common in the and that continental crust was not part of its systematics of the K-feldspar, which controls Deep Rock, but are conspicuously absent from autochthonous basement. In contrast, grano- the slope of the isochron. The ``pseudo- the Pen Branch. The plagioclase porphyry diorites of the PBF7 Metaplutonic Suite are isochron'' yields an age of ca. 220 Ma (Early more potassic than the DRB-1 metadiorites dikes commonly found in the Deep Rock are Triassic), which implies that the K metaso- and are enriched in SiO (at similar ma®c el- distinctive and would stand out clearly if they 2 matism of the ``pinking'' event was associated ement contents), have higher total REEs, high- were present in the PBF cores. Finally, Pen with early Mesozoic rifting of North America er La/Lu ratios, and a lower ␧Nd value. These from Africa and the formation of the Dunbar- Branch rocks have been metamorphosed under characteristics suggest derivation from a more ton Basin. higher-grade conditions than those of the Deep evolved source, such as an older, more mature Rock Metavolcanic Suite. Metavolcanic rocks oceanic arc terrane or a continental-margin arc Are the Deep Rock and Pen Branch of the Deep Rock and Pen Branch Metavol- built on transitional crust. Metavolcanic Suites Related? canic Suites may have formed within the same These differences may be analogous to the arc in different places or at different times; ``quartz diorite line'' in the western Sierra Ne- Despite their overall similarity, metavolcan- alternately, they may have formed in unrelated vada arc (Moore, 1959), which separates arc ic rocks of the Deep Rock and Pen Branch arc terranes and only been juxtaposed later. plutons intruded through accreted oceanic or

588 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA arc-derived crust on one side (quartz diorites) Origin of the Deep Rock/Pen Branch from arc plutons intruded through older cra- Volcanic Arc(s) tonic crust on the other side (granites, grano- diorites). In the western Sierra Nevada arc, the The data presented here show that rocks of quartz diorite line is essentially coincident the DRB1 and PBF7 Metaplutonic Suites 87 86 with the Sr/ Srinitial ϭ 0.706 line of Kistler formed in subduction-related volcanic arcs (1974). We suggest that these metaplutonic during the Late Proterozoic, ca. 625±620 Ma. suites may represent collapse of a continental- The rocks share Nd isotope systematics that margin arc that straddled the oceanic crust± are similar to those of other peri-Gondwana continental crust transition when the arc was arc terranes (e.g., Carolina, Avalon; Nance et ®rst constructed. al., 1991; Nance and Murphy, 1996). The closely similar ages and isotopic compositions Comparison with Adjacent Proterozoic argue against formation in different arcs. The Arc Terranes arc represented by the Deep Rock/PBF7 Me- taplutonic Suites cannot be correlated with the Arc rocks of the Savannah River Site lie Figure 13. Chondrite-normalized La con- proximal Carolina Slate belt (Persimmon Fork ratio of projected Eu Formation) in central South Carolina because ؍) *between rocks of similar character and age to centrations vs. Eu both the north (Carolina terrane) and south concentration to actual Eu concentration) the Persimmon Fork Formation (1) is too (Suwannee terrane). Metaigneous rocks of the for rocks of Savannah River Site (SRS), young at ca. 550 Ma, and (2) is dominated by Suwannee terrane are characterized by nega- Carolina terrane, and Suwannee terrane. felsic to intermediate-composition volcanic Data sources as in Figure 11. tive to low positive ␧Nd values (Heatherington rocks, not the basalt to dacite volcanic rocks et al., 1996) that are in stark contrast to the that dominate the Deep Rock/Pen Branch arc.

positive ␧Nd values determined for rocks of the The rocks of the Deep Rock/Pen Branch vol- Deep Rock/Pen Branch arc (Fig. 12C). In con- canic arc are also older than the western Car- trast, the Carolina terrane and other Avalonian ative anomalies. Metaigneous rocks of the Sa- olina terrane (e.g., Dennis and Wright, 1997), peri-Gondwana arcs have similar positive ␧Nd vannah River Site all have moderate to large although rocks of the DRB1 and PBF7 Me- values (e.g., Wortman et al., 1995; Mueller et negative Eu anomalies at all La concentrations taplutonic Suites are similar lithologically to al., 1996; Nance and Murphy, 1996; Fig. except for one rock with a large positive rocks of the western Carolina terrane (Dennis 12C). Thus, although arc rocks of the central anomaly (average Eu* ϭ 0.64, range 0.11 to and Shervais, 1991, 1996). and eastern Piedmont may have a similar 1.87; Fig. 13). In contrast, rocks of the Su- The Deep Rock/Pen Branch volcanic arc crustal provenance, arc rocks of the Suwannee wannee terrane have small negative anomalies may correlate with the Hyco Formation in terrane re¯ect a crustal substrate that differed at high La (average Eu* ϭ 0.83, range 0.63 central North Carolina and southern Virginia, signi®cantly from that of the other terranes. to 0.94), and rocks of the Carolina terrane which represents the infrastructure of the slate Reconstructions that place the Suwannee ter- have small positive anomalies at low La (av- belt arc in North Carolina and Virginia (Fig. rane adjacent to the Carolina terrane or the erage Eu* ϭ 1.05) and small negative anom- 14). The Hyco Formation and its correlatives Savannah River Site during the Late Protero- alies at high La (average Eu* ϭ 0.78; Fig. 13). consist of ma®c to felsic metavolcanic rocks zoic are suspect. Rocks of all suites are inferred to have dated at ca. 620 Ma (Glover and Sinha, 1973; MORB-normalized trace element diagrams evolved in part by plagioclase fractionation. Wortman et al., 2000). These rocks were de- (Fig. 11) show that all of these rocks are en- The small negative Eu anomalies in the Car- formed and metamorphosed during the ``Vir- riched in low ®eld strength elements (eg., Rb, olina and Suwannee terrane rocks imply frac- gilina orogeny'' (Glover and Sinha, 1973; Ba, Th, La) and depleted in high ®eld strength tionation under relatively high oxygen fugac- Hibbard and Samson, 1995; Wortman et al., elements (e.g., Nb, Ta, Ti), consistent with ity conditions, where Eu is largely oxidized to 2000) between 612 ϩ5.2/Ϫ1.7 Ma and 586 Ϯ subduction-zone enrichment processes. In de- Eu3ϩ and thus excluded from plagioclase (e.g., 10 Ma in they type area (Wortman et al., tail, however, there are some signi®cant dif- Gill, 1981; Shah and Shervais, 1999). In con- 2000). An angular unconformity between ferences. Rocks of the Carolina terrane tend trast, the larger negative anomalies in the Sa- Hyco/Aaron rocks and younger arc rocks of to have deeper negative Nb anomalies than the vannah River Site rocks implies fractionation the Uwharrie Formation and Albermarle other suites (possibly because of greater en- under relatively lower oxygen fugacity con- Group (Harris and Glover, 1988) provides the richment in Th and K) and small negative or ditions, where more Eu is reduced to Eu2ϩ and minimum age for this deformation (Wortman positive Eu anomalies (Fig. 11). Rocks of the partitioned into plagioclase (Gill, 1981). et al., 2000). The Uwharrie Formation was af- Deep Rock Metaigneous Complex have small- These differences suggest distinct conditions fected by minor deformation and low-grade er Ti anomalies than the other suites and larger in the source region of each terrane, similar to metamorphism, but lacks fabric elements of negative Eu anomalies than the Carolina or variations in H2O contents observed in arc the older Virgilina event. However, the Hyco Suwannee terranes. rocks of central Mexico (Cervantes and Wal- Formation was metamorphosed at low meta- Figure 13 compares Eu* (ratio of projected lace, 2003). The occurrence of signi®cant neg- morphic grade (greenschist) and cannot be di- Eu concentration to actual Eu concentration) ative Eu anomalies, especially in the more maf- rectly equivalent to Deep Rock and Pen and chondrite-normalized La in rocks of the ic rocks, is generally characteristic of tholeiitic Branch Metavolcanic Suites, which record Savannah River Site to those of rocks from volcanic suites, not calc-alkaline ones, and middle to upper amphibolite facies conditions. the Carolina and Suwannee terranes (Shervais con®rms other evidence that suggests that the It could be that the Deep Rock and Pen et al., 1996; Heatherington et al., 1996); Eu* Deep Rock/Pen Branch arc is distinct from Branch metavolcanic rocks represent the same is Ͼ1 for positive anomalies and Ͻ1 for neg- both of its adjacent neighbors. rocks found in the Hyco Formation metamor-

Geological Society of America Bulletin, May/June 2004 589 DENNIS et al. Formation, Figure 14. Correlation chartAlbemarle for district column, Carolina are terrane for rocks pre-Uwharrie of units North described and and South dated Carolina by and Wortman et eastern al. Georgia, (2000). adapted from Dennis (1995). Ages beside Hyco

590 Geological Society of America Bulletin, May/June 2004 NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA phosed to higher grades in a different location; the Brasiliano event in South America, the Ca- Appalachian-Caledonian orogen: Geological Associa- tion of Canada Special Paper 41, p. 173±190. alternatively, the postulated correlation with domian event in Europe, and the Petermann- Dennis, A.J., and Shervais, J.W., 1991, Arc rifting of the the Hyco Formation may be spurious. Paterson orogens of Australia (e.g., Rogers et Carolina terrane in northwestern South Carolina: Ge- The Crackerneck Metavolcanic Complex al., 1995; Mallard and Rogers, 1997; Veevers, ology, v. 19, p. 226±229. may correlate with the Persimmon Fork 2003). In general, deformation and metamor- Dennis, A.J., and Shervais, J.W., 1996, The Carolina ter- rane in northwestern South Carolina: Insights into the Formation of central South Carolina, the Lin- phism were contemporaneous with arc mag- development of an evolving island arc, in Nance, D., colnton metadacite of northeastern Georgia, matism in these terranes, which implies that ed., Avalonian and related terranes of the circum- and the Uwharrie Formation of North these events are driven not by continental col- Atlantic: Geological Society of America Special Paper 304, p. 237±256. Carolina±Virginia (Fig. 14). All are dominated lisions or the closure of ocean basins, but by Dennis, A.J., and Wright, J.E., 1995, Mississippian (ca. by felsic to intermediate-composition tuffs, the collapse and collision of arc terranes in 326±323 Ma) U-Pb crystallization ages for two gran- have ages that range from ca. 590 to 550 Ma, response to complex plate motions associated itoids in Spartanburg and Union Counties, South Car- and were affected by minor deformation and with the rifting and breakup of Rodinia and olina, in Dennis, A.J., Butler, J.R., Garihan, J.M., Ran- son, W.A., and Sargent, K.A., eds., Geology of the low-grade metamorphism, but lack fabric el- the subsequent assembly of Gondwana. western part of the Carolina terrane in northwestern ements of older, Virgilina-age events. South Carolina: Columbia, South Carolina Geological Thus, one possible interpretation of the ACKNOWLEDGMENTS Survey, Carolina Geological Society Field Trip Guide- book for 1995, South Carolina Geology, v. 37, Deep Rock±Pen Branch volcanic arc is that it p. 23±28. represents the infrastructure of the Carolina We thank Randy Cumbest and Sharon Lewis of Dennis, A.J., and Wright, J.E., 1997, The Carolina terrane terrane in South Carolina but has been de- Westinghouse/SRC for their moral and logistical in northwestern South Carolina, USA: Late support and Westinghouse/SRC for access to their tached by later tectonic events. More likely, Precambrian±Cambrian deformation and metamor- core-storage facility. Sample preparation for XRF phism in a peri-Gondwana oceanic arc: Tectonics, given the distinct Eu* characteristics of the analyses was carried out by Hampton Uzzelle, Mor- v. 16, p. 460±473. Savannah River Site rocks, is that the Deep ris Jones, and Neil Wicker; ICP-MS analyses were Dennis, A.J., Sacks, P.E., and Maher, H.D., 1987, Nature Rock±Pen Branch arc represents Late Prote- performed by Scott Vetter, Centenary College. This of the late Alleghanian strike-slip deformation in the work was funded by the U.S. Department of Energy eastern South Carolina Piedmont: The Irmo shear rozoic arc infrastructure from another location zone, in Secor, D.T., Jr., ed., Anatomy of the Allegha- through Westinghouse/SRC, SCUREF Task 170. in the arc that has been moved into its present nian orogeny as seen from the Piedmont of South Car- Partial support for the preparation of this manuscript location by transcurrent motions. In either olina and Georgia: Columbia, South Carolina Geolog- has come from the I.W. Marine Fund, University of case, the low-grade Crackerneck Metavolcanic ical Survey, Carolina Geological Society Field Trip South Carolina, Aiken. Guidebook for 1987, p. 49±66. Complex, which seems to correlate with the Dennis, A.J., Butler, J.R., Garihan, J.M., Ranson, W.A., and Uwharrie±Persimmon Fork±Lincolnton as- Sargent, K.A., 1995, Geology of the Carolina terrane semblage, may sit unconformably on top of REFERENCES CITED in northwestern South Carolina: South Carolina Ge- ology, v. 37, 29 p. the older Deep Rock±Pen Branch arc rocks, Dennis, A.J., Shervais, J.W., and Maher, H.D., Jr., 2000a, Barker, C.A., Secor, D.T., Jr., Pray, J.R., and Wright, J.E., much as the Uwharrie Formation sits uncon- Outline of the geology of Appalachian basement rocks 1998, Age and deformation of the Longtown Meta- underlying the Savannah River Site, Aiken, South formably on the Hyco Formation in central granite, South Carolina Piedmont: A possible con- North Carolina (Harris and Glover, 1988). straint on the origin of the Carolina terrane: Journal Carolina, in Wyatt, D.E., and Harris, M.K., eds., Sa- of Geology, v. 106, p. 713±725. vannah River Site: Environmental remediation sys- Carpenter, R.H., Odom, A.L., and Hartley, M.E., III, 1982, tems in unconsolidated upper coastal plain sedi- CONCLUSIONS Geochronological investigation of the Lincolnton me- mentsÐStratigraphic and structural considerations: tadacite, Georgia and South Carolina, in Bearce, D.N., Carolina Geological Society, 2000 Annual Field Trip Black, W.W., Kish, S.A., and Tull, J.F., eds., Tectonic Guidebook, WSRC-MS-2000-00606 (CD-ROM), Neoproterozoic volcanic and plutonic rocks p. 95±113. that underlie the Savannah River Site in cen- studies in the Talladega and Carolina Slate belts, southern Appalachian orogen: Geological Society of Dennis, A.J., Shervais, J.W., and Secor, D.T., Jr., 2000b, tral South Carolina re¯ect igneous and meta- America Special Paper 191, p. 145±152. 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Veevers, J.J., 2003, Pan-African is Pan-Gondwanaland; Williams, H., and Hatcher, R.D., Jr., 1983, Appalachian sus- history of the Carolina terrane: Journal of Geology, oblique convergence drives rotation during 650±500 pect terranes, in Hatcher, R.D., Jr., Williams, H., and v. 108, p. 321±338. Ma assembly: Geology, v. 31, p. 501±504. Zietz, I., eds., Contributions to the tectonics and geo- Wright, J.E., and Seiders, V.M., 1980, Age of zircon from Whitney, J.A., Paris, T.A., Carpenter, R.H., and Hartley, physics of mountain chains: Geological Society of volcanic rocks of the central North Carolina Piedmont M.E., III, 1978, Volcanic evolution of the southern America Memoir 158, p. 33±53. and tectonic implications for the Carolina volcanic slate belt of Georgia and South Carolina: A primitive Wortman, G.L, Samson, S.D., and Hibbard, J.P., 1996, Dis- slate belt: Geological Society of America Bulletin, oceanic island arc: Journal of Geology, v. 86, crimination of the Milton belt and the Carolina terrane v. 91, p. 287±294. p. 173±192. in the southern Appalachians; an Nd isotopic ap- MANUSCRIPT RECEIVED BY THE SOCIETY 6JUNE 2003 Williams, H., and Hatcher, R.D., Jr., 1982, Suspect terranes proach: Journal of Geology, v. 104, p. 239±247. MANUSCRIPT ACCEPTED 11 JULY 2003 and accretionary history of the Appalachian orogen: Wortman, G.L., Samson, S.D., and Hibbard, J.P., 2000, Pre- Geology, v. 10, p. 530±536. cise U-Pb zircon constraints on the earliest magmatic Printed in the USA

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