Download Petrology and geochemistry of Neoproterozoic volcanic arc terranes

Petrology and geochemistry of Neoproterozoic volcanic arc terranes
beneath 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. 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
The Piedmont of South Carolina and
Georgia is a complex mosaic of exotic terranes of uncertain provenance. Farther
south and east, these terranes form the
basement beneath several kilometers of
Cretaceous and Cenozoic sedimentary
rocks, commonly referred to as the Atlantic
Coastal Plain. The distribution and geologic history of this hidden crystalline basement can be inferred only on the basis of
limited exposures at the margins of the
Coastal Plain onlap, aeromagnetic lineaments that define basement trends in the
subsurface, and core data from wells that
penetrate basement.
During the past 40 years, basement cores
aggregating more than 6 miles (10,000 m)
have been recovered from 57 deep wells at
the Department of Energy’s Savannah River Site. These cores provide the only known
samples of basement terranes that lie southeast of the Fall Line in central South Carolina. Cores from the 57 deep wells, along
with structural trends defined by aeromagnetic lineaments, allow us to define four distinct units within the basement beneath the
Coastal Plain: (1) the Crackerneck Metavolcanic Complex (greenstones and felsic
E-mail: [email protected].
†
tuffs, all metamorphosed under greenschistfacies conditions), (2) the Deep Rock
Metaigneous Complex (mafic to felsic volcanic and plutonic rocks metamorphosed
under lower amphibolite–facies conditions),
(3) the Pen Branch Metaigneous Complex
(amphibolites, garnet amphibolites, garnetbiotite schists, and gneiss), and (4) the Triassic Dunbarton Basin Group, a sedimentary unit that fills a northeast-trending
graben beneath younger sedimentary rocks
of the Atlantic Coastal Plain.
All of the metaplutonic and metavolcanic
rocks have calc-alkaline fractionation
trends, consistent with formation in subduction-related arc terranes at convergent
margins. Zircon U-Pb crystallization ages
of ca. 626 Ma to 619 Ma, however, show
that the Deep Rock and Pen Branch complexes do not correlate with the younger
Carolina terrane (570–535 Ma) or Suwannee terrane (ca. 550 Ma). The Deep Rock
and Pen Branch Metaigneous Complexes
may be a continuation of Proterozoic basement that forms the older infrastructure of
the Carolina arc. The contact between the
Crackerneck Metavolcanic Complex (5
Persimmon Fork Formation?) and the Deep
Rock and Pen Branch Metaigneous Complexes thus may be equivalent to the angular unconformity between the Uwharrie
Formation and the Virgilina sequence. On
the basis of their compositions and ages, we
tentatively correlate these rocks with the
Hyco Formation in southern Virginia and
central North Carolina. The Hyco Formation constitutes the infrastructure of the
Carolina terrane in Virginia and North
Carolina, where it was affected by the ca.
600 Ma ‘‘Virgilina’’ orogeny. The rocks of
the Deep Rock and Pen Branch Metaigneous Complexes may have formed the arc
infrastructure of the Carolina Slate belt in
South Carolina, detached by later tectonic
events, or may have formed the Late Proterozoic arc infrastructure at another location in the arc that has been moved into
its current location by transcurrent motions. Limited age and isotopic data suggest
that none of these rocks correlate with the
Suwannee terrane of North Florida and
southern Georgia.
Keywords: Neoproterozoic, peri-Gondwana,
arc volcanism, Carolina terrane, geochemistry, petrology.
INTRODUCTION
The hinterland of the southern Appalachians, which lies southeast of Grenville basement and an attached cover sequence exposed
in the Blue Ridge province, comprises a complex mosaic of exotic or ‘‘suspect’’ tectono-
GSA Bulletin; May/June 2004; v. 116; no. 5/6; p. 572–593; 14 figures; 4 tables; Data Repository item 2004074.
572
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q 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
Figure 1. Regional map of the southern Appalachians, showing the distribution of major
tectonic subdivisions of the Laurentian margin (Blue Ridge, Piedmont, Carolina terrane)
and the extent of post-Jurassic sedimentary onlap onto the continental margin (Atlantic
Coastal Plain). Previous areas studied by the authors in the Appalachian Piedmont of
South Carolina are shown as polygons: (1) western Carolina terrane/Charlotte belts, Central 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 (Shervais
et al., 2003; Dennis et al., 2000b); and (3) the Slate belt–Kiokee belt–Belair belt (Maher
et 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—Great
Smoky Fault, H-FFS—Hayesville-Fries fault system, MZ—Modoc Zone.
stratigraphic terranes that range in age from
late Neoproterozoic through middle Paleozoic.
These terranes were accreted to Laurentia during the middle to late Paleozoic and now form
all of the exposed crystalline rocks east of the
Blue Ridge (e.g., Williams and Hatcher, 1982,
1983; Secor et al., 1983; Maher et al., 1981,
1991; Horton et al., 1989, 1991; Samson et
al., 1990; Hibbard et al., 2002). Farther to the
southeast, terranes accreted to and overthrust
onto crystalline basement of the Laurentian
margin are hidden beneath several kilometers
of Mesozoic and Tertiary sedimentary rocks,
commonly referred to as the Atlantic Coastal
Plain (Colquhoun, 1995; Fallaw and Price,
1995). Deciphering the origin and provenance
of this buried crystalline basement is central
to our understanding of terrane accretion during the Paleozoic and has implications for collisional orogenesis in the hinterland of the
southern Appalachians.
The U.S. Department of Energy Savannah
River Site is located near the northwest margin of the Atlantic Coastal Plain in central
South Carolina (Fig. 1), where up to 2 km of
Cretaceous and younger sedimentary deposits
overlie crystalline rocks of the basement and
a major Triassic rift basin. During the past 40
years, basement cores totaling more than 6
miles (.10,000 m) long have been recovered
from 57 deep wells at the Department of Energy’s Savannah River Site. Southeast of the
eastern Piedmont in central South Carolina
and Georgia, these cores provide most of the
known samples of crystalline basement and,
along with structural trends defined by aeromagnetic lineaments, allow us to define four
distinct units within the basement beneath the
Coastal Plain. Three of these units represent
crystalline basement; they comprise mafic to
felsic metavolcanic rocks and dioritic to granitic metaplutonic rocks, all metamorphosed
at greenschist- to amphibolite-facies conditions. The fourth unit represents the clastic
sedimentary fill of a northeast-trending Triassic graben, the Dunbarton Basin.
This paper examines metaigneous rocks
sampled by these deep core holes and compares them to rocks exposed in surface outcrops throughout the Piedmont of the Carolinas, Georgia, and Virginia; a parallel
investigation of much of this core material
was carried out by Roden et al. (2002). For
these rocks we present new whole-rock geochemistry, mineral chemistry, and isotopic
data, including two U-Pb zircon ages, examine
Crystalline basement at the Savannah River
Site is entirely covered by onlap of the Atlantic 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 systems, and an inferred fault zone indicated by
a strong aeromagnetic lineament that defines
the southern edge of the Belair belt (Ascauga
fault zone; Fig. 2). South of this aeromagnetic
lineament, high-grade sillimanite-bearing
gneisses of the Belvedere belt are intruded by
the Carboniferous Graniteville pluton (e.g.,
Samson et al., 1995a); both are exposed in
erosional windows through the coastal-plain
sediments (Fig. 2). The limits of the Graniteville pluton may be estimated from its associated gravity anomaly; gravity anomalies also
define the extent of the Devonian Springfield
pluton (Speer, 1982) and smaller, unnamed
granitic stocks related to the larger bodies
(Fig. 2). The Graniteville pluton is penetrated
and sampled by a single borehole, the C-2
well.
Magnetic and gravity potential field data
from the Savannah River Site, coupled with
core logs from over 30 locations, show that
the subsurface geology can be divided into
four main units (from north to south): (1)
Crackerneck Metavolcanic Complex: lowgrade (greenschist-facies) metavolcanic rocks
(tuffs, lapilli tuffs, lavas), named for a small
creek that drains the surface above this unit;
(2) Deep Rock Metaigneous Complex: upper
greenschist– to lower amphibolite–facies metavolcanic rocks, metaplutonic rocks, and
crosscutting dikes, strongly deformed or mylonitized in places, named for its type exposures in the Deep Rock Borehole (DRB) series
of wells; (3) Pen Branch Metaigneous Complex: upper amphibolite to granulite-facies
metagranitoids and metavolcanic rocks,
named for a small creek that drains the surface
above this unit and for its type exposures in
the ‘‘Pen Branch fault’’ (PBF) series of wells;
and (4) Dunbarton Basin: a deep Triassic rift
basin with fault-controlled margins that was
buried by coastal-plain sediments (Marine,
1974; Marine and Siple, 1974; Cumbest et al.,
1992; Fallaw and Price, 1995). These units are
shown in Figure 2. South of the site, geophysical potential field 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 mafic igneous complex in the subsurface (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 Paleozoic 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 Complex; (2) the Crackerneck fault, a northeasttrending feature that offsets the subCretaceous 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
defined northeast-trending feature that forms
the southern margin of the Dunbarton Basin
and also offsets the sub-Cretaceous unconformity (Fig. 2: Domoracki, 1995; Snipes et al.,
1993).
METHODS
Core was logged for 26 wells drilled in and
around the Westinghouse/Savannah River
Site, including wells drilled between approximately 1960 (DRB series) and the mid-1990s
(C series, MMP series). Figure 2 is a geologic
map based on subcrop lithology; well locations are shown in Figure 3. Cores from all
wells studied here were logged for lithology,
structural relationships, fractures, folds, foliations, layering, and relict primary intrusive
contacts. Over 400 samples were collected,
and from this collection ;150 samples were
selected for further petrologic study, including
petrographic, mineral-chemical, and/or whole-
Figure 3. Outline map of Savannah River Site and surrounding area showing locations of
deep wells that intersect basement studied for this project.
rock geochemical analysis. Petrographic and
mineral assemblage data for each unit are
summarized in Table 1; formal descriptions
and type sections for all new units defined in
this publication are available (Table DR-11).
Whole-rock samples were analyzed for 10
major elements (SiO2, TiO2, Al2O3, total Fe as
Fe2O3*, MnO, MgO, CaO, Na2O, K2O, P2O5)
1
GSA Data Repository item 2004074, formal description of new units described herein, whole rock
major and trace element analyses by x-ray fluoresence, whole rock REE and other analyses by ICPMS, representative plagioclase and amphibole mineral chemistry, is available on the Web at http://
www.geosociety.org/pubs/ft2004.htm. Requests
may 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 University
of South Carolina’s Philips PW-1400 X-ray
fluorescence spectrometer. Major elements
were analyzed on fused glass disks by using
a method similar to that of Taggart et al.
(1987) with selected U.S. Geological Survey
and international standards prepared identically to the samples. Accepted concentrations
were taken from the compilation of Potts et
al. (1992). Matrix corrections were carried out
within the Philips X41 software package,
which uses the fundamental parameters approach (Rousseau, 1989) to calculate theoretical alpha coefficients for the range of standards. 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 mafic 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 Springfield pluton and the Carboniferous Graniteville pluton are based on gravity anomalies. The Graniteville pluton is sampled by core from well C-2 and in limited surface exposures (outlined on map). The Springfield
pluton is sampled by well SAL-1. EPFS—Eastern Piedmont fault system. DRB—Deep 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
Crackerneck Metavolcanic Complex
Deep Rock Metaigneous Complex
Deep Rock Metavolcanic Suite
DRB1 Metaplutonic Suite
Pen Branch Metaigneous Complex
Pen Branch Metavolcanic Suite
Lithologies
Mineral assemblages
Relict igneous
phases
Felsic tuffs and lapilli tuffs, mafic tuffs and
greenstones
Qtz, Alb, Chl, Epi, Ms,
Opq
None
Greenschist to
subgreenschist
Plg An3–8, Qtz, Chl,
Epi, Bi, 6BGA, Gt
(rare)
Plg An9–20, Qtz, Chl,
Epi, Act, BGA
Hbl, Plg An29–43
Epidote amphibolite
Hbl, Plg An20–40
Epidote amphibolite
None
Garnet amphibolite
None
Garnet amphibolite
None
Gt amphibolite with
greenschist
overprint
Aphyric mafic to intermediate tuffs,
plagioclase-phyric tuffs and schists,
hornblende-phyric tuffs and schists
Diorite–quartz diorite gneiss,
porphyroclastic gneiss
PBF7 Metaplutonic Suite, normal
Mafic to intermediate amphibolite, garnet
amphibolite
Metagranite, granodiorite gneiss
PBF7 Metaplutonic Suite, pinked
Metagranite, granodiorite gneiss
Plg An35–45, BGA, Qtz,
Bi, Epi, Gt
Plg An24–38, BGA, Qtz,
Ksp, Epi, Bi, Chl
Plg An4, Ksp, Qtz, Gt,
Chl, rare BGA, Bi
Metamorphic facies
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 abundant
major elements, and ;1%–6% for all trace elements except Cr, which is slightly higher
(13%). Thirty whole-rock samples were also
analyzed for rare earth element (REE) and
other trace element concentrations by inductively coupled argon plasma–mass spectrometry (ICP-MS) at the University of New Mexico. Selected major and trace element data are
presented in Table 2; the complete data set of
major and trace element data are available (Tables DR-2 and DR-3 [see footnote 1]).
Quantitative electron-microprobe analyses
of major and minor elements in minerals were
obtained with a Cameca SX50 electron microprobe at the University of South Carolina.
Analyses were made at 20 kV accelerating
voltage, 30 nA probe current, and counting
times of 20–100 s; both natural and synthetic
mineral standards were used. Analyses were
corrected for instrumental drift and dead time,
and electron beam/matrix effects by using the
‘‘PAP’’ f(rz) correction procedures provided
with the Cameca microprobe automation system; these correction procedures are based on
the model of Pouchou and Pichoir (1991). Analytical precision is ;1% of the amount present for oxide concentrations greater than 10
wt%, 1%–2% for oxide concentrations between 1 and 10 wt%, and 5%–10% for oxide
concentrations between 0.01 and 1 wt%. Representative mineral analyses are available (Tables DR-4 and DR-5 [see footnote 1]).
Zircon was separated by conventional techniques using a Wilfley Table, heavy liquids,
and a Franz magnetic separator. The least
magnetic zircons from each sample were split
into size fractions and then handpicked to remove any contaminating grains. Zircon dis-
576
solution and ion-exchange chemistry for separation of uranium and lead followed
procedures modified from Krogh (1973). Isotope ratios were measured with the MAT 262
multicollector instrument at Rice University.
Analytical uncertainties, blanks, and common
lead corrections are outlined in Table 3.
DESCRIPTION OF THE LITHOLOGIC
UNITS
Crackerneck Metavolcanic Complex
The Crackerneck Metavolcanic Complex
(named herein as described in Table DR-1 [see
footnote 1]) underlies the northwestern corner
of the Savannah River Site and continues at
least as far north as the Graniteville pluton
(Fig. 2). The complex is represented by cores
from wells C-1, C-3, P30, P6R, P8R, GCB-1,
GCB-2, GCB-3, and MMP4 (Fig. 3). Rocks
in the northern part of this unit are dominantly
intermediate-composition to felsic tuffs and
lapilli tuffs, whereas cores from the southern
part of the subcrop area are dominated by
greenstone and mafic tuff (Table 1). Lowgrade metavolcanic rocks of the complex are
penetrated by younger granitic intrusions, including the Devonian Springfield granite and
the Carboniferous Graniteville granite (Fig. 2).
Felsic tuffs of the Crackerneck Metavolcanic Complex are generally composed of quartz
and plagioclase and have sparse plagioclase
phenocrysts (Fig. 4A). Mafic tuffs are composed of chlorite, plagioclase, epidote, and
opaque minerals. All plagioclase grains were
metamorphosed to nearly pure albite (An2 to
An5: Fig. 5A), but relict igneous textures are
typically well preserved. Lapilli tuffs in core
MMP4 display large pumice lapilli that were
flattened and elongated, giving these rocks an
appearance similar to lapilli tuffs of the Persimmon Fork Formation.
Deep Rock Metaigneous Complex
The Deep Rock Metaigneous Complex
(named herein as described in Table DR-1 [see
footnote 1]) comprises most of the Savannah
River Site basement north of the Triassic Dunbarton Basin (Fig. 2). It consists of two subunits: the Deep Rock Metavolcanic Suite and
the DRB1 Metaplutonic Suite (Table 1).
Deep Rock Metavolcanic Suite
The Deep Rock Metavolcanic Suite was
sampled most extensively by the 4-inch-diameter (10.16-cm-diameter) Deep Rock Borehole (DRB) cores, but is also penetrated by a
number of other, more widely spaced wells
(SSW-1, SSW-2, SSW-3, GCB-5.1; Fig. 3).
Metavolcanic rocks of the Deep Rock
metavolcanic complex display a wide array
of textures (aphyric, plagioclase-phyric,
hornblende-phyric), compositions, and minerals. Mineral assemblages are consistent with
lower amphibolite–facies metamorphism. All
rocks have been overprinted with a penetrative
fabric (foliation) that dips ;408 to 558; based
on regional correlations, the orientation of
strike is probably northeast and the direction
of dip is to the southeast.
Plagioclase phenocrysts commonly form
subhedral to euhedral crystals (Fig. 4B) that
preserve primary igneous compositions and
zoning (An29 to An43). Other plagioclase
grains are anhedral, albitized (An3 to An8) porphyroclasts (Fig. 5A) that have been partially
to extensively epidotized. Hornblende phenocrysts range from blocky, subhedral crystals
Geological Society of America Bulletin, May/June 2004
NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA
Fig. 6). Many intermediate-composition to felsic 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 compositions, and (3) aphyric with rhyolitic compositions. Plagioclase-phyric mafic 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 mafic dikes are identical to the plagioclase-phyric dikes, but lack
the conspicuous large phenocrysts. The felsic
dikes consist of fine-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 plagioclase porphyroclasts in a mylonitic groundmass. Many of these rocks contain unaltered
biotite, and others contain abundant muscovite
and aluminosilicate minerals; the rocks texturally resemble button schists. These characteristics support a sedimentary protolith for
these samples.
Figure 4. Photomicrographs of Crackerneck Metavolcanic Complex and Deep Rock Metaigneous 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 Metavolcanic
Suite (cross-polarized light). (C) Relict igneous hornblende (brown) surrounded by mantle
of 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, relict
igneous hornblende phenocrysts (brown in
thin section) are characterized by lower Al2O3
and FeO contents and by higher SiO2, MgO,
and TiO2 contents and Mg/Fe ratios. Hornblende phenocrysts and groundmass hornblende are commonly metamorphosed to bluegreen tschermakitic amphibole that is low in
Ti and high in Al (e.g., Roden et al., 2002;
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
rock. The DRB-1 metadiorite are metamorphosed to epidote-amphibolite–facies assemblages and range texturally from equigranular
to porphyritic gneisses (Table 1).
Plagioclase porphyroclasts may show relict
igneous features, including oscillatory zoning
and albite twinning. Individual porphyroclasts
may have cores as calcic as An40 and rims as
sodic as An9 (Fig. 5B). The calcic cores are
interpreted to represent relict igneous plagioclase compositions; the sodic rims formed
during metamorphism along with the bluegreen amphibole (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.:
Box:
Sample no.:
Rock type:
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
SUM
LOI
Nb
Zr
Y
Sr
Rb
Zn
Cu
Ni
Cr
Sc
V
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y
Pb
Th
U
C1
none
574
Felsic
tuff
C3
none
540
Felsic
tuff
68.32
71.55
0.70
0.66
16.00
14.24
5.96
5.04
0.13
0.09
1.93
1.54
2.09
1.34
2.87
4.15
2.52
1.52
0.15
0.16
100.66
100.29
1.93%
2.47%
12
206
22
350
69
88
17
34
42
14
97
1241
28.1
66.5
8.1
29.4
7.2
1.44
4.7
0.84
4.3
0.82
1.9
0.30
1.3
0.20
17.5
11.2
6.7
1.6
9
159
19
106
30
76
5
9
10
7
45
753
13.6
34.4
4.7
18.3
4.2
0.95
3.4
0.57
3.1
0.57
1.5
0.25
1.2
0.17
16.7
8.1
1.1
1.1
P30
MMP4
none
none
781
817
Felsic
Felsic
lapilli tuff lapilli tuff
74.29
0.23
12.40
4.29
0.14
0.71
0.62
5.16
1.31
0.03
99.18
0.86%
14
287
72
64
27
134
2
42
79
nd
7
119
11.6
31.3
3.8
16.2
3.6
0.59
5.7
1.12
6.0
1.40
4.5
0.67
3.2
0.57
68.3
2.8
0.6
0.6
76.98
0.23
11.63
2.96
0.13
1.11
0.39
4.41
2.01
0.03
99.88
9
296
65
35
32
138
2
43
160
nd
5
438
16.5
46.7
6.3
24.6
5.9
0.87
7.4
1.50
7.5
1.54
4.7
0.65
3.7
0.50
64.0
11.1
0.8
0.8
net, consistent with metamorphism at relatively low temperatures and pressures (lower
amphibolite–facies conditions). Biotite is rare
to absent in the DRB-1 metadiorites; almost
all primary biotite has been retrograded to
chlorite.
Most amphibole porphyroclasts, and essentially all groundmass amphiboles, are metamorphosed to blue-green amphibole (Roden et
al., 2002). Some amphibole porphyroclasts
preserve cores of brown hornblende, interpreted to represent relict igneous hornblende,
with higher Mg/Fe ratios than the metamorphic blue-green amphibole. These differences
are shown clearly in a plot of tetrahedral Al
vs. Ti, where metamorphic amphibole has AlIV
. 1.4 afu (atoms per formula unit on the basis
of 23 oxygens) and Ti , 0.06 afu (Fig. 6).
Coexisting igneous hornblende has higher Ti
578
DRB2
17M
1081M
Felsic
MV
DRB2
17
1081
Mafic
MV
63.44
0.96
16.41
7.44
0.19
1.90
4.83
4.27
1.59
0.43
101.45
0.34%
51.74
1.06
16.98
10.82
0.188
4.61
9.44
2.91
1.186
0.335
99.27
1.90%
10
175
45
634
38
60
17
5
6
13
80
625
11.9
29.1
3.9
15.0
3.7
0.79
4.1
0.71
3.8
0.88
2.4
0.41
2.1
0.36
22.5
7.1
3.0
0.8
6
148
31
715
52
106
174
18
47
30
309
500
19.2
42.5
4.8
18.2
3.9
0.78
3.9
0.69
3.9
0.87
2.8
0.42
2.2
0.32
26.2
8.4
8.1
1.8
DRB3
6
988
Felsic
MV
DRB3
59
1346
Mafic
MV
65.59
49.94
0.80
1.13
16.81
20.85
4.65
10.56
0.12
0.18
1.58
3.70
3.47
10.20
4.38
3.49
3.18
1.53
0.26
0.40
100.85
101.98
2.55%
2.00%
11
188
35
365
62
75
4
nd
3
7
74
1440
21.6
48.6
5.8
21.0
5.9
1.50
4.6
0.80
4.6
1.11
2.9
0.46
2.7
0.39
29.5
10.0
8.2
2.0
5
102
25
782
28
94
148
18
28
20
287
607
19.8
50.7
7.2
29.2
6.9
1.81
5.4
0.84
4.2
0.76
2.2
0.32
1.6
0.23
25.5
7.1
0.8
0.8
DRB4
76
1450.7
Felsic
gneiss
DRB4
131
1831
Felsite
MV
DRB4
101
1613
Inter MV
DRB4
123
1758.4
Mafic
MV
DRB4
145
1912
Mafic
MV
78.32
0.10
11.74
1.21
0.02
0.15
0.77
5.82
0.91
0.01
99.05
0.72%
70.98
0.74
13.98
4.20
0.11
1.60
3.44
2.05
2.37
0.14
99.59
1.17%
68.05
0.80
13.90
5.07
0.13
1.54
5.04
1.39
2.25
0.16
98.32
1.74%
51.47
2.12
15.12
13.26
0.21
6.95
7.46
2.36
0.54
0.26
99.74
1.81%
54.51
72.89
50.48
1.71
0.38
0.97
14.57
14.34
18.88
11.21
2.94
11.13
0.19
0.04
0.19
6.10
0.87
5.68
7.63
2.17
9.30
2.23
5.87
3.29
1.55
0.96
0.57
0.23
0.08
0.24
99.905 100.54
100.75
1.01%
0.75%
0.97%
29
190
76
48
40
30
11
3
nd
4
19
244
30.3
70.4
8.2
30.0
7.4
0.34
6.8
1.34
7.2
1.62
4.2
0.66
3.4
0.53
41.3
13.8
15.2
3.2
16
176
41
116
78
59
nd
14
48
nd
56
317
16
182
42
146
92
71
22
27
63
15
109
440
26.4
59.9
7.7
26.9
5.8
0.96
6.2
1.15
5.0
1.07
3.5
0.48
2.3
0.31
46.2
13.3
1.6
1.6
(0.03 to 0.45 afu; most samples have .0.1 afu
Ti) and lower AlIV (,1.35 afu; Fig. 6). Some
blue-green metamorphic amphiboles contain
cores of fibrous actinolite (AlIV ø 0.4 afu; Fig.
6) that formed under retrograde greenschistfacies conditions.
The DRB1 Metaplutonic Suite contains xenoliths and screens of amphibolite-grade metavolcanic rocks that are deformed and contain
fabric elements similar to those of rocks of the
Deep Rock Metavolcanic Suite, with which
they are assumed to correlate.
Pen Branch Metaigneous Complex
The Pen Branch Metaigneous Complex
(named herein for the Pen Branch fault, as described in Table DR-1 [see footnote 1]) forms
a thin slice of crystalline basement between
29.8
68.2
8.5
28.4
6.1
1.19
6.2
1.08
5.4
1.02
2.8
0.47
2.3
0.35
43.6
11.8
1.7
1.7
16
185
33
310
23
105
70
165
329
32
304
233
12.7
30.8
4.2
17.1
4.4
1.05
4.5
0.82
4.6
0.96
2.5
0.34
1.8
0.24
25.9
6.7
2.1
0.4
11
170
28
209
55
93
76
178
384
29
226
447
11.7
29.6
3.6
12.2
3.2
0.62
3.1
0.62
2.5
0.51
1.9
0.31
1.3
0.25
27.9
7.1
0.6
0.6
DRB5
73
1802
Felsite
MV
11
149
30
331
17
28
27
2
2
7
38
301
12.3
28.7
3.5
13.1
3.2
0.47
3.2
0.68
3.8
0.98
2.9
0.48
2.4
0.36
23.9
8.5
4.7
1.2
DRB5
15M
1324
Mafic
MV
4
78
21
637
11
98
120
14
22
18
221
274
7.8
19.1
2.4
10.2
2.4
0.59
2.8
0.55
2.7
0.61
1.5
0.23
1.2
0.20
14.4
6.7
1.2
0.3
the Triassic Dunbarton Basin to the south and
the more extensive Deep Rock Metaigneous
Complex to the north (Fig. 2). It also appears
to continue on the south side of the Dunbarton
Basin, but few drill holes penetrate basement
there. The Pen Branch Metaigneous Complex
consists of two subunits: the PBF7 Metavolcanic Suite and the PBF7 Metaplutonic Suite.
Pen Branch Metavolcanic Suite
Rocks of the Pen Branch Metavolcanic
Suite were recovered from eight wells: PBF7, PBF-8, SSW-1, SSW-2, SSW-3, GCB-4, C5, and the seismic attenuation (SA) well (located adjacent to GCB-4 in Fig. 3). Mafic
metavolcanic rocks in the lowermost part of
PBF-7 are separated from the overlying metagranitoids by a fault ;1100 m below the
surface. Higher up in this same well, mafic
Geological Society of America Bulletin, May/June 2004
NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA
TABLE 2. (Continued)
Core no.:
Box:
Sample no.:
Rock type:
DRB6
4M
1125
Mafic
MV
DRB6
103
1810
Mafic
MV
DRB2
108
1687
Mafic
dike
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
SUM
LOI
51.87
49.44
1.32
1.29
17.74
17.68
11.52
12.39
0.21
0.23
4.15
5.03
8.23
8.90
2.79
1.73
2.45
3.94
0.42
0.39
100.70
101.01
1.08%
1.67%
Nb
Zr
Y
Sr
Rb
Zn
Cu
Ni
Cr
Sc
V
Ba
7
164
33
711
59
125
230
19
35
22
283
710
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y
Pb
Th
U
14.0
33.3
4.4
17.5
4.5
0.97
4.6
0.91
4.8
1.12
2.8
0.46
2.4
0.33
27.4
11.1
3.4
0.9
6
163
28
885
108
136
150
18
38
26
316
660
DRB2
12
1053.5
Plg
porph
dike
DRB1
18
1005
Quartz
diorite
DRB1
25
1047
Diorite
DRB1
41
1151
Quartz
diorite
DRB1
60
1276
Tonalite
51.76
51.89
60.04
50.00
54.34
70.37
1.19
1.05
0.78
1.09
1.17
0.58
18.00
17.18
17.05
17.39
16.14
15.10
10.77
11.06
8.05
12.10
12.08
3.61
0.18
0.19
0.15
0.20
0.15
0.05
3.70
4.78
3.15
6.72
4.25
1.00
9.57
9.33
5.89
9.60
8.37
3.22
3.18
3.06
4.28
3.84
3.21
6.78
0.95
1.18
0.93
0.47
1.03
0.29
0.35
0.33
0.18
0.14
0.16
0.15
99.64
100.03
100.49
101.56
100.91
101.15
1.05%
0.96%
1.51%
1.50%
0.69%
1.33%
5
148
34
617
21
62
178
7
12
29
273
425
19.1
44.9
5.7
22.9
5.9
1.43
5.3
0.93
4.9
0.94
2.6
0.33
1.9
0.29
26.6
17.1
2.8
0.9
14.7
36.4
4.8
18.3
4.5
0.91
4.9
0.93
5.0
1.20
3.3
0.59
2.9
0.48
27.5
5.3
3.5
1.1
6
132
32
645
30
98
162
19
49
30
310
388
7
141
25
316
19
81
81
12
16
15
142
579
15.0
36.7
5.1
19.3
4.9
1.10
4.7
0.82
4.4
0.79
2.6
0.38
1.9
0.32
30.9
5.3
1.0
1.0
3
65
18
263
7
91
87
41
142
38
316
126
10.4
25.7
3.2
13.1
2.9
0.60
3.3
0.59
3.2
0.64
2.1
0.37
1.8
0.28
26.5
4.5
0.6
0.6
5
107
27
297
20
69
26
6
19
30
352
346
4.6
11.8
1.8
8.0
2.5
0.75
2.7
0.51
2.8
0.54
1.7
0.26
1.3
0.20
18.4
3.4
0.4
0.4
4
85
16
381
3
59
6
24
22
24
172
106
7.9
21.7
3.3
14.1
4.1
0.92
4.1
0.72
4.3
0.84
2.5
0.39
2.0
0.31
27.8
16.6
0.9
0.9
C5
none
1084
Mafic
schist
PBF7
103
2550
Granodiorite
PBF7
108
2629
Granodiorite
PBF7
139
3086
Granodiorite
C5
none
1080
Felsic
dike
PBF7
171
3568
Pink
granite
56.79
58.16
54.69
64.24
59.30
73.65
75.31
1.33
1.39
1.76
1.07
0.83
0.46
0.14
16.76
16.74
16.54
15.66
17.54
13.43
13.23
8.25
8.46
9.93
6.18
8.37
2.88
1.36
0.152
0.15
0.18
0.12
0.18
0.07
0.04
4.91
3.46
3.52
1.97
2.97
0.64
0.28
6.26
6.94
7.69
4.70
5.82
1.58
0.98
3.81
3.93
2.84
3.90
4.11
3.67
4.03
1.871
1.48
3.15
3.22
1.69
3.85
5.25
0.192
0.34
0.60
0.21
0.13
0.10
0.02
100.33
101.06
100.90
101.27
100.97
100.33
100.64
2.29%
1.60%
1.26%
0.63%
1.78%
1.44%
0.71%
9
192
30
458
61
72
18
35
29
19
144
1647
6.5
15.5
1.9
7.5
1.8
0.43
2.2
0.42
2.4
0.53
1.8
0.25
1.6
0.27
14.5
3.4
1.8
0.4
PBF7
107
2602
Granodiorite
19
275
42
421
49
179
81
19
42
20
157
486
11.7
26.2
3.2
13.5
10.3
4.69
3.7
0.76
4.1
0.96
2.5
0.39
2.0
0.30
24.7
19.8
4.5
1.6
25.8
63.7
8.6
32.6
7.6
1.77
7.2
1.25
6.9
1.34
4.0
0.57
3.1
0.48
42.1
16.6
1.4
1.4
18
305
53
347
97
89
117
22
32
24
147
995
23
298
44
311
70
66
51
14
12
11
96
960
29.8
69.4
8.6
34.2
9.0
2.54
7.9
1.48
8.0
1.80
4.6
0.68
3.6
0.59
46.5
12.7
4.4
1.4
5
134
21
341
55
103
61
21
47
17
148
427
22.2
51.1
5.8
22.7
5.4
1.30
5.5
1.19
6.4
1.52
4.2
0.66
3.4
0.58
40.7
13.0
6.9
1.5
30
246
53
205
72
40
15
6
2
4
38
550
11.3
28.3
3.9
15.1
4.3
1.05
3.6
0.62
3.4
0.69
2.2
0.36
1.7
0.31
24.4
14.8
1.0
1.0
32.4
79.8
10.3
38.2
10.3
2.08
8.7
1.50
8.1
1.59
5.1
0.72
3.9
0.61
58.8
18.9
2.1
2.1
40
115
82
27
106
34
nd
5
nd
4
3
115
17.0
48.2
7.2
26.8
8.3
0.34
9.9
2.05
11.3
2.19
6.9
1.02
5.4
0.80
82.5
14.1
1.4
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
†
PBF-7 1150A
PBF-7 1150
PBF-7 150–210
PBF-7 –210
DRB-1 1150A
DRB-1 –210
U
(ppm)
200.5
395.6
416.9
432.5
421.6
405.7
Pb
(ppm)
206
Measured ratios§
‡
17.53
33.57
33.51
34.52
36.44
32.08
Pb
Pb
Pb
Pb
Atomic ratios
Pb
Pb
206
207
208
204
206
206
22,222
4,808
3,559
11,494
8,696
10,989
0.06128
0.06362
0.06464
0.06188
0.06210
0.06179
0.16555
0.17199
0.17898
0.18036
0.35902
0.24565
Pb
U
206
‡
238
0.10179(51)
0.09878(49)
0.09354(47)
0.09291(46)
0.10062(50)
0.09203(46)
Pb
U
207
‡
235
0.85095(427)
0.82554(416)
0.78123(395)
0.77651(391)
0.83848(422)
0.76740(386)
Apparent ages# (Ma)
Pb
Pb‡
207
‡
206
0.06063(3)
0.06061(4)
0.06057(5)
0.06062(3)
0.06044(3)
0.06048(3)
Pb
U
206
‡
238
624.9
607.3
576.5
572.7
618.1
567.6
Pb‡
U
207
235
625.2
611.1
586.5
583.5
618.3
578.3
Pb‡
Pb‡
207
206
626.2
625.5
624.0
625.6
619.1
620.6
6
6
6
6
6
6
0.9
1.4
1.7
1.2
1.2
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 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.
†
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.
Geological Society of America Bulletin, May/June 2004
579
DENNIS et al.
TABLE 3B. Rb-Sr AND Sm-Nd ISOTOPE DATA
Sample
PBF-7
DRB-1
Rb
(ppm)
Sr
(ppm)
Atomic ratio
87
Rb/86Sr
Measured ratio
87
Sr/86Sr††
99
10
243
344
1.1774
0.0840116
0.715526(9)
0.704028(6)
Initial ratio
Sr/86Sr(624)
Sm
(ppm)
Nd
(ppm)
Atomic ratio
147
Sm/144Nd
Measured ratio
143
Nd/144Nd‡‡
«Nd§§
–
0.70193851
10.01
3.44
43.01
14.34
0.14069
0.14487
0.512512(4)
0.512606(6)
12.0 (626 Ma)
13.5 (619 Ma)
87
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 1.42
3 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 concentrations
determined 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 yielded
143
Nd/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 rock
that were intruded by granodiorites of the
PBF7 Metaplutonic Suite. In the SA well, mafic to intermediate-composition metavolcanic
rocks were sampled by spot coring below 580
m; continuous coring to depths of ;370 m
sampled metagranitoids similar to those in the
lower part of core PBF-7. Well C-5 is characterized by intermediate-composition metavolcanic rocks at depths of ;330 m that underlie sedimentary deposits of the coastal
plain with no intercalated metagranitoids. Foliation dips ;458–608, similar to foliation in
the adjacent Deep Rock Metavolcanic Suite.
Rocks of the Pen Branch Metavolcanic
Suite are mafic to intermediate-composition
amphibolites and garnet amphibolites that are
distinct from metavolcanic rocks of the adjacent Deep Rock Metavolcanic Suite (Table 1).
There is no relict igneous amphibole, and all
metamorphic amphibole is tschermakitic, with
low Si and high Al. Ti is much higher than in
metamorphic amphiboles of the Deep Rock
Metaigneous Complex, possibly in response
to higher metamorphic grade (Fig. 6). The relatively calcic plagioclase is also considered to
reflect higher metamorphic grade (An35 to
An45; Fig. 5C). Garnet is a common metamorphic mineral in many of these amphibolites, but garnet-biotite pairs are relatively rare
because biotite is commonly retrograded to
chlorite. Garnets, which range up to 1 cm
across, are typically elongated parallel to foliation and have spongy, inclusion-rich textures (Fig. 7A). All of the garnet studied here
is unzoned, reflecting annealing during or after
peak thermal metamorphism.
PBF7 Metaplutonic Suite
The PBF7 Metaplutonic Suite dominates
the upper part of the cores recovered from
wells PBF-7, PBF-8, C-10, GCB-4, and the
SA well. The PBF7 Metaplutonic Suite comprises two dominant lithologies: gneissic metagranitoids and ‘‘pinked’’ metagranitoids,
formed by hydrothermal alteration of the
gneissic metagranitoids (Dennis et al., 2000a).
580
Gneissic metagranitoids. Gneissic metagranitoids are porphyroclastic gneisses with
porphyroclasts of amphibole and plagioclase
set in a groundmass of amphibole, plagioclase,
microcline, biotite, quartz, and epidote (Table
1, Fig. 7B). Plagioclase porphyroclasts within
the Pen Branch gneisses range from An24 to
An38 and have an average composition of An35
(Fig. 5D). Amphibole porphyroclasts are unzoned and have Mg/Fe ratios similar to the
metamorphic amphiboles in the DRB-1 metadiorites, but TiO2 values range from 0.7% to
1.0%, higher than metamorphic amphibole in
the DRB-1 metadiorites. Blue-green amphibole in the PBF7 Metaplutonic Suite is clearly
distinguished from the relict igneous hornblende of the DRB1 Metaplutonic Suite, however, by its higher AlIV contents (.1.42 afu;
Fig. 6).
Pen Branch gneissic metagranitoids are distinguished from DRB-1 metadiorite by their
lack of relict igneous hornblende, the relatively high Ti in blue-green metamorphic amphibole, higher An-content plagioclase, the common occurrence of microcline, and the
preservation of biotite (Table 1). Some of
these characteristics may result from higher
metamorphic grade in the Pen Branch metagranitoids, but others (e.g., common microcline) reflect fundamental compositional differences between the two intrusive series.
Pinked metagranitoid. Much of the granitic
core material recovered from wells PBF-7,
PBF-8, C-10, and SA had been subjected to
partial to extensive hydrothermal alteration
and potassium metasomatism after formation
of foliation. We refer to this alteration as the
‘‘pinking’’ event because of the salmon-pink
color imparted to rocks affected by the hydrothermal fluids. The effects of this event range
from millimeter-scale selvages on fractures to
pervasive alteration of hundreds of meters of
cored rock (Dennis et al., 2000a).
‘‘Pinked’’ Pen Branch metagranites commonly contain large plagioclase, microcline,
and garnet porphyroblasts set in a finergrained matrix of quartz, plagioclase, micro-
cline, and chlorite (Table 1). Microcline commonly occurs as megacrysts up to ;5 mm
across and may account for as much as 40%
of the mode (Fig. 7C). Partially pinked rocks
have plagioclase An25 to An33, whereas plagioclase in the thoroughly pinked granites has
been completely albitized (An4) and, subsequently, partially sericitized (Fig. 5D). Amphibole and biotite are almost completely
chloritized, but some relict blue-green amphibole is preserved.
GEOCHEMISTRY
Whole-rock geochemical analyses were obtained by X-ray fluorescence for 59 metavolcanic rocks, 35 metaplutonic rocks, 15 postkinematic dikes, and 3 metasedimentary
rocks; 31 of these samples were analyzed for
additional trace elements by ICP-MS. Representative whole-rock data are presented in Table 2. The complete data set of major and
trace element data are available (Tables DR-2
and DR-3 [see footnote 1]).
Crackerneck Metavolcanic Complex
Low-grade metavolcanic rocks of the
Crackerneck Metavolcanic Complex may be
classified as basalts, dacites, and rhyolites by
using their silica vs. alkali relationships (Cox
et al., 1979). Most are high in SiO2 (65–77
wt%) 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 content, but surprisingly, K2O is highest in the
two dacites, C-1–570 and C-1–574 (Table DR2 [see footnote 1]). These rocks are also high
in Ba (1240–1350 ppm vs. 120–750 in the
other samples), Rb, Sr, and Cu, suggesting
that K2O and the other mobile trace elements
were enriched by secondary hydrothermal
processes. The overall characteristics of these
rocks are calc-alkaline on an AFM diagram
(Fig. 9A), covering much the same compositional range as similar low-grade metavolcanic
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.
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 Pen
Branch Metaigneous Complexes. Relict igneous hornblendes in the Deep Rock Metavolcanic Suite (n) and DRB1 Metaplutonic Suite (u) have less AlIV compared to the metamorphic amphiboles in either the metadiorites of the Deep Rock Metaigneous Complex
(y) or the metavolcanic rocks of the Deep Rock Metavolcanic Suite (x); relict igneous
hornblendes of the DRB1 Metaplutonic Suite are also higher in Ti. Metagranitoids of the
PBF7 Metaplutonic Suite contain no relict igneous hornblende, but the metamorphic bluegreen amphibole (q) is higher in Ti than metamorphic amphiboles in the Deep Rock
Metavolcanic Suite and the DRB1 Metaplutonic Suite, suggesting higher equilibration
temperatures. Actinolite is plotted for reference.
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 enriched in the light rare earth elements (LREEs)
relative to the heavy rare earth elements
(HREEs): La contents are ;32 to 80 times
chondritic abundances, and La/Lu ratios are
;2 to ;14 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-defined negative Eu anomalies. MORBnormalized spider diagrams show that all four
samples are enriched in low field strength elements and depleted in high field strength elements, consistent with subduction-zone enrichment processes (Fig. 11A).
Deep Rock Metaigneous Complex
Deep Rock Metavolcanic Suite
Metavolcanic rocks of the Deep Rock Metavolcanic Suite may be classified as basalts,
andesites, dacites, and rhyolites. Major and
trace element trends on Harker diagrams are
typical of calc-alkaline volcanic suites (Fig.
8). The alkalis show significant scatter, sug-
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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 elements (HREEs): La contents are ;22 to 90
times chondritic abundances, and La/Lu ratios
are ;3.5 to ;8.8 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 Metavolcanic Complex. All of the samples have
moderate to deeply negative Eu anomalies,
which seem to be deepest in rocks with SiO2
. 65 wt% (Fig. 10B), consistent with extensive plagioclase fractionation under relatively
low oxygen fugacity conditions. MORBnormalized spider diagrams show that all are
enriched in low field strength elements and
depleted in high field strength elements, consistent with subduction-zone enrichment processes (Fig. 11B).
Figure 7. Photomicrographs of Pen Branch Metaigneous Complex: (A) Typical
amphibolite-facies assemblage of garnet, biotite, plagioclase, and lesser amounts of hornblende in the Pen Branch Metavolcanic Suite (cross-polarized light); note the elliptical
nature of the garnets. (B) Microcline with characteristic tartan twinning in quartzplagioclase-amphibole granite (cross-polarized light). (C) Metamorphic amphibole (extinct) intergrown with quartz, plagioclase, and K-feldspar (cross-polarized light). Width
of field of view represents 2.3 mm in all.
gesting either complex petrogenetic processes
involving mixing or assimilation, or (more
likely) element mobility during amphibolitefacies metamorphism. These rocks are calc-
582
alkaline on an AFM plot (Fig. 9B), consistent
with their observed lack of Fe or Ti enrichment on Harker diagrams.
Fourteen samples were chosen for REE
Younger Dike Rocks (DRB-2, DRB-3,
DRB-5, and DRB-6)
Postkinematic mafic dikes (aphyric and plagioclase 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/calcalkaline dividing line of Irvine and Baragar
(1971), but they have higher Fe/Mg ratios than
most Deep Rock Metavolcanic Suite amphibolites. These younger mafic dikes are also
characterized by higher Sr concentrations
(;550 to ;700 ppm Sr) than most amphibolites in the suite. Postkinematic felsic dikes
include dacites and rhyolites that exhibit typical calc-alkaline fractionation trends. The felsic dikes exhibit the same characteristic high
Sr as the younger mafic dikes. Only two postkinematic dikes were analyzed for REE concentrations: one aphyric mafic dike and one
plagioclase megaphyric mafic dike. Both dikes
have moderate LREE enrichment: La contents
are ;42 times chondritic abundances, and La/
Lu ratios are ;3 to ;4 times the chondritic
ratio (Fig. 10C). The La/Lu ratios are low
compared to the host metavolcanic rocks.
Both samples have small negative Eu anomalies, showing that even the plagioclase megaphyric samples crystallized from melts in
which the plagioclase was being removed
rather than accumulating. MORB-normalized
spider diagrams are similar to those of the volcanic rocks (Fig. 11B).
DRB1 Metaplutonic Suite
Metaplutonic rocks sampled from the DRB1 core may be classified as diorites or quartz
diorites on the basis of their normative min-
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
typical of calc-alkaline intrusive suites (Figs.
8, 9B). At any given weight percent silica, the
DRB-1 metadiorite samples are lower in mafic
elements (Mg, Fe, Ti, Cr, Ni), K2O, and Rb
than the Pen Branch metagranitoids and high-
er in plagiophile elements (Ca, Na, Al). Four
samples were chosen for REE analysis (Table
DR-3 [see footnote 1]). They are slightly enriched 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 observed in the Deep Rock Metavolcanic Suite,
but all have small negative Eu anomalies.
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.
9C).
Figure 9. AFM diagrams with the Irvine
and Baragar (1974) dividing line for tholeiitic and calc-alkaline trends. (A) Crackerneck Metavolcanic Complex, (B) Deep
Rock Metaigneous Complex, (C) Pen
Branch Metaigneous Complex.
sistent with subduction-zone enrichment processes (Fig. 11B).
Pen Branch Metaigneous Complex
Pen Branch Metavolcanic Suite
Amphibolites and garnet amphibolites of
the Pen Branch Metavolcanic Suite can be
classified geochemically as metamorphosed
basalts, basaltic andesites, and andesites. They
are slightly lower in Fe and higher in Ca than
the Deep Rock metavolcanic rocks at any given weight percent silica, and they are restricted to SiO2 contents of #60% SiO2 (Fig. 8).
They plot in the calc-alkaline field on an AFM
diagram, where they exhibit typical calcalkaline fractionation trends similar to those
584
PBF7 Metaplutonic Suite
Metagranitoids. The gneissic metagranitoids may be classified as quartz monzodiorites and granodiorites by using their normative mineral contents; their major and trace
element trends are similar to those of DRB-1
(Fig. 8). At any given weight percent silica,
the Pen Branch metagranitoids are higher in
mafic elements (Mg, Fe, Ti, Cr, Ni), K2O, and
Rb than the metadiorites of DRB-1 and lower
in plagiophile elements (Ca, Na, Al). Like the
DRB1 metadiorite samples, the Pen Branch
metagranitoids are strongly calc-alkaline, as
seen on an AFM plot (Fig. 9C).
Six ‘‘unpinked’’ metagranitoids of the
PBF7 Metaplutonic Suite were analyzed for
REE concentrations (Table DR-3 [see footnote
1]). All are enriched in light rare earth elements (LREEs) relative to the heavy rare earth
elements (HREEs): La contents are ;30–100
times chondritic abundances, and La/Lu ratios
are ;3.6 to ;5.4 times the chondritic ratio
(Fig. 10D). Both the La/Lu ratios and total
REEs are higher than those observed in the
DRB1 Metaplutonic Suite. Five of these samples have small negative Eu anomalies, suggesting plagioclase fractionation, whereas one
has a significant positive Eu anomaly, suggesting plagioclase accumulation (Fig. 10D).
MORB-normalized spider diagrams show that
all six samples are enriched in low field
strength elements and depleted in high field
strength elements, consistent with subductionzone enrichment processes (Fig. 11A).
Pinked metagranitoids. Pinked metagranitoids are characterized by higher SiO2, K2O,
and Rb contents than their unaltered equivalents and by lower MgO, Fe2O3*, TiO2, Al2O3,
CaO, Na2O, and Sr (Fig. 8). As a result, these
rocks are classified as granites by using their
normative mineral contents. They extend the
strong calc-alkaline trend of the unpinked metagranitoids (Fig. 9C), but this apparent calcalkaline ‘‘fractionation trend’’ results from hydrothermal alteration, not igneous processes.
Only one sample of pinked metagranitoid was
analyzed for REE concentrations (PBF-7–
3568). This sample is characterized by modest
LREE enrichment (La/Lu ratio is ;2.1 times
the chondritic ratio) and by a pronounced negative Eu anomaly not seen in the unpinked
metagranitoids (Fig. 10D). This negative Eu
anomaly suggests that Eu21 was mobilized
along with Ca21 during the pinking event and
was removed from the system.
Age Constraints and Tracer Isotopes
Two samples of metaplutonic rock were
dated by using the U-Pb zircon method (Table
3). Four zircon fractions from a quartz monzodiorite of the Pen Branch Metaigneous
Complex (PBF-7) gave a concordia-intercept
date of 626.1 6 4.2 Ma (Fig. 12A). The
weighted mean of the 207Pb*/206Pb* dates is
625.6 6 1.3 Ma. Two zircon fractions from a
quartz diorite of the Deep Rock Metaigneous
Complex (DRB-1) yield identical 207Pb*/
206
Pb* dates with a weighted mean of 619.9
6 0.8 Ma; the concordia intercept for this pair
is 619 6 3.4 Ma (Fig. 12B). These Late Proterozoic ages are too old to correlate with the
Carolina Slate belt (Lincolnton metadacite–
Persimmon Fork Formation–Uwharrie Formation 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 (Charlotte
belt at ca. 580 Ma to 535 Ma; Dennis and
Wright, 1997), or North Florida Volcanic Series of the Suwannee terrane (ca. 552 Ma;
Heatherington et al., 1996). Similar ages have
been determined for the Hyco Formation in
central 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 isolated
granodiorite of the Suwannee terrane in southern Alabama (ca. 625 Ma; Heatherington et
al., 1996, p. 263).
Age-corrected initial isotope ratios were
calculated by using the measured parent/
daughter ratios and the U-Pb ages of the samples. These ratios indicate derivation of the
DRB1 and PBF7 Metaplutonic Suites from
similar, relatively primitive sources, with «Nd
5 12.0 to 13.5 (Table 3). The DRB-1 metadiorite sample has a calculated initial 87Sr/
86
Sr ratio of 0.70194, but an initial 87Sr/86Sr
ratio could not be calculated for PBF-7 because the Rb-Sr system was disturbed by the
Triassic pinking event. These values of «Nd are
similar to those that have been measured on
metaigneous rocks of the Carolina terrane and
other peri-Gondwana ‘‘Avalonian’’ terranes
(e.g., Nance and Murphy, 1996), but are higher than those found in the Suwannee terrane
(Heatherington et al., 1996; Fig. 12C).
DISCUSSION
Crackerneck Metavolcanic Complex
Volcanic rocks of the Crackerneck Metavolcanic Complex are the least deformed and
least metamorphosed of the units studied here.
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 (subgreenschist to greenschist facies) and lack of
penetrative deformation indicate that (1) these
rocks could not have been buried deeply subsequent to their formation, and (2) they were
not overridden by thick thrust sheets during
the Alleghenian orogeny. Their geochemical
characteristics (moderate to high SiO2, calcalkaline fractionation trends, enrichment in
low field strength elements, and depletion in
high field strength elements) all point to an
origin as volcanic rocks associated with an
island-arc volcanic system. The dominance of
more felsic members of this suite suggests that
the crust upon which this volcanic arc was
built consisted either of mature island-arc
crust or a thinned continental margin. The
abundance of felsic tuffs and pumice lapilli
tuffs resembles many continental-margin arcs
(e.g., Mexico: Luhr and Carmichael, 1980).
The Crackerneck Metavolcanic Complex
correlates most closely with the Persimmon
Fork Formation of the Carolina Slate belt (Secor et al., 1986a, 1986b; Shervais et al., 1996).
Both units are dominated by felsic tuffs, were
metamorphosed under low-grade conditions,
and have comparable trace element concentrations. The Crackerneck Metavolcanic Complex may also correlate with rocks of the Belair belt. These rocks are dominantly
intermediate in composition, but include more
mafic and more felsic compositions as well
(Shervais et al., 1996). Differences in the proportions of mafic and felsic rocks may relate
more to the limited areal extent of the Belair
belt, which has been sampled systematically
in only one traverse along the banks of the
Savannah River. The Crackerneck is separated
from the Belair belt by a strand of the aeromagnetically defined Eastern Piedmont fault
system and by high-grade metamorphic rocks
of the Belvedere belt that are exposed in erosional windows through the coastal-plain sedimentary rocks near Graniteville (Fig. 2).
Thus, the Crackerneck Metavolcanic Complex
may be stratigraphically equivalent to the Belair belt, but is not contiguous with that belt.
Deep Rock Metaigneous Complex
The Deep Rock Metaigneous Complex exhibits 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
that are consistent with formation in a mature
island arc that formed over an extended period
of time. In contrast, associated metaplutonic
rocks of the DRB1 Metaplutonic Suite are
dominated by intermediate-composition metadiorites, suggesting a primitive arc origin.
The relatively high «Nd value of the DRB-1
metadiorite sample is also consistent with an
origin in a more primitive intraoceanic arc
setting.
The data presented here suggest that the
Deep Rock Metavolcanic Suite and the DRB1
586
Metaplutonic Suite are unrelated petrogenetically. This observation is supported by observations of the core: (1) DRB-1 metadiorites
are found only in core DRB-1, (2) DRB-1 metadiorites include screens of deformed metavolcanic wall rocks that are chemically and
petrologically similar to rocks of the Deep
Rock Metavolcanic Suite and contain fabric
elements that are not observed in the DRB-1
metadiorites, and (3) metavolcanic rocks exposed in cores DRB-2 through DRB-7 are cut
by three distinct dike series (aphyric mafic,
plagioclase porphyry, and felsic dikes) that are
not seen in DRB-1. These relationships suggest that the Deep Rock Metavolcanic Suite
represents an older volcanic arc assemblage
(possibly equivalent to the PBF7 Metaplutonic
Suite) that was intruded in part by the DRB1 metadiorite at ca. 619 Ma.
Pen Branch Metaigneous Complex
The Pen Branch Metaigneous Complex exhibits compositional ranges, trace element systematics, and phase assemblages consistent
with formation in a volcanic arc, similar to
Geological Society of America Bulletin, May/June 2004
NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA
Figure 12. Concordia diagrams for zircon
fractions from (A) Pen Branch (PBF-7) and
(B) Deep Rock (DRB-1) Metaigneous Complexes. (C) «Nd vs. age for Pen Branch (PBF)
and Deep Rock (DRB) Metaigneous Suite
rocks compared to published data for Carolina terrane plutonic rocks (filled squares)
and volcanic, metamorphic, and sedimentary rocks (dashed field): data from Wortman 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 Metaigneous
Complex. Metavolcanic rocks of the Pen
Branch Metavolcanic Suite exhibit a more restricted range of compositions; the lack of Fe
and Ti enrichment and the restricted range in
SiO2 contents suggest that the Pen Branch Metavolcanic Suite represents a relatively primitive island arc compared to the Deep Rock
Metavolcanic Suite. In contrast, the PBF7 Metaplutonic Suite is dominated by granodiorites
and granites with intermediate-composition to
high SiO2 contents. The low «Nd value (12.0)
of a Pen Branch gneissic granitoid (compared
to «Nd 5 13.5 for the DRB-1 metadiorite sample) implies a more evolved provenance for
these intrusive rocks, in contrast to the more
primitive characteristics of the Pen Branch
Metavolcanic Suite.
Rocks of the Pen Branch Metavolcanic
Suite appear to represent an older volcanic
framework into which the Pen Branch granitoids were intruded at ca. 626 Ma. The intrusive relationship between the Pen Branch metagranitoids and the Pen Branch metavolcanic
rocks and the differences in their geochemical
character suggest that these two rock suites are
unrelated petrogenetically. There is some suggestion, however, that granitoids of the PBF7
Metaplutonic Suite may represent the intrusive equivalent of the Deep Rock Metavolcanic Suite. This suggestion is based on chemical
similarities between the two suites, including
their compositional ranges, their rare earth element concentrations, and other trace element
similarities (Figs. 10 and 11).
ples are from the Pen Branch Metavolcanic
Suite (PBF-7–3648 and SA-4002). We cannot
calculate a pressure for these rocks because
they do not contain an aluminosilicate mineral, but we can estimate a nominal pressure
of at least 3 kbar (10 km depth), which is reasonable for rocks of this grade. Table 4 presents the results of these calculations, which
are based on the exchange of Fe and Mg between garnet and biotite (Ferry and Spear,
1978). Both samples preserve evidence for an
early high-temperature event at 700 to 800 8C,
consistent with uppermost amphibolite–facies
conditions.
An intermediate-composition metavolcanic
rock from the Deep Rock Metavolcanic Suite
(DRB-2–1449) also preserves metamorphic
biotite and garnet. Equilibrium biotite-garnet
temperatures for this sample average ;650
8C, consistent with lower to middle
amphibolite–facies conditions (Table 4). Thus,
the Deep Rock and Pen Branch Metavolcanic
Suites may represent different levels of exposure that were juxtaposed by later faulting.
Metaigneous rocks of both the DRB1 and
PBF7 Metaplutonic Suites preserve evidence
for metamorphic histories that are similar to
their adjacent metavolcanic suites. Thus, the
DRB-1 metadiorites have metamorphic amphiboles similar to those in the Deep Rock
Metavolcanic Suite, but still retain relic cores
of igneous hornblende. In contrast, the Pen
Branch metagranitoids lack relict igneous
hornblende and contain only metamorphic
amphibole.
Metamorphism and Pressure-Temperature
Estimates
Potassium Metasomatism: The Big Pink
Metavolcanic rocks of both the Deep Rock
and Pen Branch Metavolcanic Suites preserve
evidence for distinct metamorphic histories,
best illustrated by changes in amphibole composition (Roden et al., 2002; Fig. 6). Relict
igneous hornblende compositions are preserved in the cores of many volcanic phenocrysts in the Deep Rock Metavolcanic Suite.
These compositions are overprinted by the
pervasive growth of blue-green metamorphic
amphibole, consistent with amphibolite-facies
metamorphism. The high-temperature bluegreen amphibole was subsequently retrograded to the greenschist-facies minerals actinolite
and chlorite. In contrast, metavolcanic rocks
of the Pen Branch Metavolcanic Suite contain
no relict igneous minerals, and all amphiboles
are metamorphic.
Three samples that preserve appropriate
mineral assemblages were chosen to calculate
metamorphic temperatures. Two of these sam-
Many metaplutonic rocks of the PBF7 Metaplutonic Suite were affected by a postmetamorphic hydrothermal alteration event whose
characteristic signature is a pervasive discoloration of the normally gray rocks to various
shades of bright salmon-pink (Dennis et al.,
2000a). The metamorphic phase assemblage
associated with this event (actinolite, chlorite,
albite, K-feldspar) indicates greenschist-facies
metamorphism at temperatures of ;350 to
;450 8C. The primary geochemical characteristic of this event is Si, K, and Rb metasomatism of the affected rocks by hydrothermal fluids and concomitant leaching of Ca, Sr,
and Eu21. We suggest that the breakdown of
primary biotite to chlorite, which is nearly
complete in the pinked samples and affects
much of the DRB1 Metaplutonic Suite, is the
most likely source of these alkalis.
Data presented by Dennis et al. (2000a)
show that the ‘‘pinking’’ event predated the
formation of most filled 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
CALCULATED GT-BI TEMPERATURES
Core
Sample
Mineral
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Si
Ti
Al
Fe
Mn
Mg
Ca
Na
K
Pyr
Alm
Spes
Gros
Aliv
Alvi
Bio-Tot
Ann
Phl
Ti-Bi
Al-Bi
Mn-Bi
F and S
H and S
G and S
I and M
Average T (8C)
SA
4002
Garnet
PBF7
3648
Garnet
37.63
0.06
21.32
26.46
2.94
5.57
5.30
–
–
99.28
36.96
0.01
21.00
28.14
2.23
3.10
7.66
–
–
99.10
5.956
0.007
3.977
3.502
0.394
1.314
0.899
–
–
0.215
0.573
0.065
0.147
SA
4002
807
870
781
825
820
DRB-2
1449
Garnet
36.40
0.04
20.69
29.68
4.05
3.66
3.63
98.15
5.941
0.001
3.978
3.783
0.304
0.743
1.319
–
–
0.121
0.615
0.049
0.215
5.941
0.005
3.980
4.052
0.559
0.890
0.635
–
–
0.145
0.660
0.091
0.103
PBF7
3648
648
723
699
667
685
DRB-2
1449
639
674
657
614
645
timate for the absolute age of this event is the
Rb-Sr age presented by Kish (1992) for
pinked granite from core C-10. This two-point
pseudo-isochron, based on the whole rock and
on a K-feldspar separate, must represent the
age of the ‘‘pinking’’ event because the pervasive K and Rb metasomatism characterizing
this event would thoroughly reset the isotopic
systematics of the K-feldspar, which controls
the slope of the isochron. The ‘‘pseudoisochron’’ yields an age of ca. 220 Ma (Early
Triassic), which implies that the K metasomatism of the ‘‘pinking’’ event was associated
with early Mesozoic rifting of North America
from Africa and the formation of the Dunbarton Basin.
Are the Deep Rock and Pen Branch
Metavolcanic Suites Related?
Despite their overall similarity, metavolcanic rocks of the Deep Rock and Pen Branch
588
Are the DRB1 and PBF7 Metaplutonic
Suites Related?
SA
4002
Biotite
PBF7
3648
Biotite
DRB-2
1449
Biotite
36.43
1.17
17.94
17.75
0.27
12.15
0.08
0.12
9.65
95.54
36.03
1.94
17.73
20.16
0.15
10.14
0.00
0.05
9.94
96.13
35.60
1.98
16.96
19.38
0.18
11.15
0.27
0.10
9.24
94.85
5.495
0.132
3.189
2.239
0.034
2.731
0.012
0.034
1.856
5.474
0.222
3.175
2.561
0.019
2.295
0.000
0.015
1.926
5.461
0.228
3.066
2.486
0.023
2.549
0.044
0.030
1.808
2.505
0.684
5.821
0.385
0.469
0.023
0.118
0.006
2.526
0.649
5.746
0.446
0.399
0.039
0.113
0.003
2.539
0.527
5.815
0.428
0.438
0.039
0.091
0.004
Ferry and Spear (1978)
Hodges and Spear (1982)
Ganguly and Saxena (1984)
Indares and Martignole (1985)
Average of all calculated T
Metavolcanic Suites exhibit significant differences that have led us to conclude that they
do not represent the same rock series. The
Deep Rock Metavolcanic Suite ranges from
basalt to rhyolite in composition; the Pen
Branch Metavolcanic Suite is restricted to mafic to intermediate compositions. In addition,
postkinematic mafic dikes are common in the
Deep Rock, but are conspicuously absent from
the Pen Branch. The plagioclase porphyry
dikes commonly found in the Deep Rock are
distinctive and would stand out clearly if they
were present in the PBF cores. Finally, Pen
Branch rocks have been metamorphosed under
higher-grade conditions than those of the Deep
Rock Metavolcanic Suite. Metavolcanic rocks
of the Deep Rock and Pen Branch Metavolcanic Suites may have formed within the same
arc in different places or at different times;
alternately, they may have formed in unrelated
arc terranes and only been juxtaposed later.
Despite their gross similarity, plutonic
rocks of the DRB1 and PBF7 Metaplutonic
Suites exhibit significant differences that have
led us to conclude that they do not represent
the same rock series. Major differences include the following: (1) The DRB1 Metaplutonic Suite is dominated by quartz diorite,
whereas the PBF7 Metaplutonic Suite is dominated by quartz monzodiorite (unpinked) and
granite (pinked). (2) Metadiorite of the DRB1
Metaplutonic Suite and quartz monzodiorites
of the PBF7 Metaplutonic Suite have distinct,
subparallel trends on Harker diagrams that are
offset from one another. At any given weight
percent silica, DRB-1 metadiorite samples are
lower in mafic elements (Mg, Fe, Ti, Cr, Ni),
K2O, and Rb than Pen Branch metagranitoids
and higher in plagiophile elements (Ca, Na,
Al). These differences must reflect different
parent magmas and different magma source
regions. (3) DRB-1 metadiorite samples have
lower REE concentrations and lower La/Lu
ratios than the plutonic rocks of the PBF7 Metaplutonic Suite. These differences are too
large to result from fractionation processes
and must reflect different parent-magma compositions and different magma source regions.
(4) The DRB-1 metadiorite has a higher «Nd
value (13.5) than the Pen Branch granodiorite
sample («Nd 5 12.0); these data indicate an
oceanic affinity for the DRB1 Metaigneous
Suite and a weak continental influence on the
PBF7 Metaigneous Suite.
The evidence listed above suggests that the
DRB1 Metaplutonic Suite and PBF7 Metaplutonic Suite are not equivalent. The low normative quartz contents, relatively primitive
major element compositions, low total REEs,
low La/Lu ratios, and high «Nd of the DRB-1
metadiorites are interpreted to indicate that
this plutonic suite formed in a magmatic arc
that was built on older oceanic or arc crust
and that continental crust was not part of its
autochthonous basement. In contrast, granodiorites of the PBF7 Metaplutonic Suite are
more potassic than the DRB-1 metadiorites
and are enriched in SiO2 (at similar mafic element contents), have higher total REEs, higher La/Lu ratios, and a lower «Nd value. These
characteristics suggest derivation from a more
evolved source, such as an older, more mature
oceanic arc terrane or a continental-margin arc
built on transitional crust.
These differences may be analogous to the
‘‘quartz diorite line’’ in the western Sierra Nevada arc (Moore, 1959), which separates arc
plutons intruded through accreted oceanic or
Geological Society of America Bulletin, May/June 2004
NEOPROTEROZOIC VOLCANIC ARC TERRANES, SAVANNAH RIVER SITE, SOUTH CAROLINA
Origin of the Deep Rock/Pen Branch
Volcanic Arc(s)
arc-derived crust on one side (quartz diorites)
from arc plutons intruded through older cratonic crust on the other side (granites, granodiorites). In the western Sierra Nevada arc, the
quartz diorite line is essentially coincident
with the 87Sr/86Srinitial 5 0.706 line of Kistler
(1974). We suggest that these metaplutonic
suites may represent collapse of a continentalmargin arc that straddled the oceanic crust–
continental crust transition when the arc was
first constructed.
Comparison with Adjacent Proterozoic
Arc Terranes
Arc rocks of the Savannah River Site lie
between rocks of similar character and age to
both the north (Carolina terrane) and south
(Suwannee terrane). Metaigneous rocks of the
Suwannee terrane are characterized by negative to low positive «Nd values (Heatherington
et al., 1996) that are in stark contrast to the
positive «Nd values determined for rocks of the
Deep Rock/Pen Branch arc (Fig. 12C). In contrast, the Carolina terrane and other Avalonian
peri-Gondwana arcs have similar positive «Nd
values (e.g., Wortman et al., 1995; Mueller et
al., 1996; Nance and Murphy, 1996; Fig.
12C). Thus, although arc rocks of the central
and eastern Piedmont may have a similar
crustal provenance, arc rocks of the Suwannee
terrane reflect a crustal substrate that differed
significantly from that of the other terranes.
Reconstructions that place the Suwannee terrane adjacent to the Carolina terrane or the
Savannah River Site during the Late Proterozoic are suspect.
MORB-normalized trace element diagrams
(Fig. 11) show that all of these rocks are enriched in low field strength elements (eg., Rb,
Ba, Th, La) and depleted in high field strength
elements (e.g., Nb, Ta, Ti), consistent with
subduction-zone enrichment processes. In detail, however, there are some significant differences. Rocks of the Carolina terrane tend
to have deeper negative Nb anomalies than the
other suites (possibly because of greater enrichment in Th and K) and small negative or
positive Eu anomalies (Fig. 11). Rocks of the
Deep Rock Metaigneous Complex have smaller Ti anomalies than the other suites and larger
negative Eu anomalies than the Carolina or
Suwannee terranes.
Figure 13 compares Eu* (ratio of projected
Eu concentration to actual Eu concentration)
and chondrite-normalized La in rocks of the
Savannah River Site to those of rocks from
the Carolina and Suwannee terranes (Shervais
et al., 1996; Heatherington et al., 1996); Eu*
is .1 for positive anomalies and ,1 for neg-
Figure 13. Chondrite-normalized La concentrations vs. Eu* (5 ratio of projected Eu
concentration to actual Eu concentration)
for rocks of Savannah River Site (SRS),
Carolina terrane, and Suwannee terrane.
Data sources as in Figure 11.
ative anomalies. Metaigneous rocks of the Savannah River Site all have moderate to large
negative Eu anomalies at all La concentrations
except for one rock with a large positive
anomaly (average Eu* 5 0.64, range 0.11 to
1.87; Fig. 13). In contrast, rocks of the Suwannee terrane have small negative anomalies
at high La (average Eu* 5 0.83, range 0.63
to 0.94), and rocks of the Carolina terrane
have small positive anomalies at low La (average Eu* 5 1.05) and small negative anomalies at high La (average Eu* 5 0.78; Fig. 13).
Rocks of all suites are inferred to have
evolved in part by plagioclase fractionation.
The small negative Eu anomalies in the Carolina and Suwannee terrane rocks imply fractionation under relatively high oxygen fugacity conditions, where Eu is largely oxidized to
Eu31 and thus excluded from plagioclase (e.g.,
Gill, 1981; Shah and Shervais, 1999). In contrast, the larger negative anomalies in the Savannah River Site rocks implies fractionation
under relatively lower oxygen fugacity conditions, where more Eu is reduced to Eu21 and
partitioned into plagioclase (Gill, 1981).
These differences suggest distinct conditions
in the source region of each terrane, similar to
variations in H2O contents observed in arc
rocks of central Mexico (Cervantes and Wallace, 2003). The occurrence of significant negative Eu anomalies, especially in the more mafic rocks, is generally characteristic of tholeiitic
volcanic suites, not calc-alkaline ones, and
confirms other evidence that suggests that the
Deep Rock/Pen Branch arc is distinct from
both of its adjacent neighbors.
The data presented here show that rocks of
the DRB1 and PBF7 Metaplutonic Suites
formed in subduction-related volcanic arcs
during the Late Proterozoic, ca. 625–620 Ma.
The rocks share Nd isotope systematics that
are similar to those of other peri-Gondwana
arc terranes (e.g., Carolina, Avalon; Nance et
al., 1991; Nance and Murphy, 1996). The
closely similar ages and isotopic compositions
argue against formation in different arcs. The
arc represented by the Deep Rock/PBF7 Metaplutonic Suites cannot be correlated with the
proximal Carolina Slate belt (Persimmon Fork
Formation) in central South Carolina because
the Persimmon Fork Formation (1) is too
young at ca. 550 Ma, and (2) is dominated by
felsic to intermediate-composition volcanic
rocks, not the basalt to dacite volcanic rocks
that dominate the Deep Rock/Pen Branch arc.
The rocks of the Deep Rock/Pen Branch volcanic arc are also older than the western Carolina terrane (e.g., Dennis and Wright, 1997),
although rocks of the DRB1 and PBF7 Metaplutonic Suites are similar lithologically to
rocks of the western Carolina terrane (Dennis
and Shervais, 1991, 1996).
The Deep Rock/Pen Branch volcanic arc
may correlate with the Hyco Formation in
central North Carolina and southern Virginia,
which represents the infrastructure of the slate
belt arc in North Carolina and Virginia (Fig.
14). The Hyco Formation and its correlatives
consist of mafic to felsic metavolcanic rocks
dated at ca. 620 Ma (Glover and Sinha, 1973;
Wortman et al., 2000). These rocks were deformed and metamorphosed during the ‘‘Virgilina orogeny’’ (Glover and Sinha, 1973;
Hibbard and Samson, 1995; Wortman et al.,
2000) between 612 15.2/21.7 Ma and 586 6
10 Ma in they type area (Wortman et al.,
2000). An angular unconformity between
Hyco/Aaron rocks and younger arc rocks of
the Uwharrie Formation and Albermarle
Group (Harris and Glover, 1988) provides the
minimum age for this deformation (Wortman
et al., 2000). The Uwharrie Formation was affected by minor deformation and low-grade
metamorphism, but lacks fabric elements of
the older Virgilina event. However, the Hyco
Formation was metamorphosed at low metamorphic grade (greenschist) and cannot be directly equivalent to Deep Rock and Pen
Branch Metavolcanic Suites, which record
middle to upper amphibolite facies conditions.
It could be that the Deep Rock and Pen
Branch metavolcanic rocks represent the same
rocks found in the Hyco Formation metamor-
Geological Society of America Bulletin, May/June 2004
589
Figure 14. Correlation chart for Carolina terrane rocks of North and South Carolina and eastern Georgia, adapted from Dennis (1995). Ages beside Hyco Formation,
Albemarle district column, are for pre-Uwharrie units described and dated by Wortman et al. (2000).
DENNIS et al.
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;
alternatively, the postulated correlation with
the Hyco Formation may be spurious.
The Crackerneck Metavolcanic Complex
may correlate with the Persimmon Fork
Formation of central South Carolina, the Lincolnton metadacite of northeastern Georgia,
and the Uwharrie Formation of North
Carolina–Virginia (Fig. 14). All are dominated
by felsic to intermediate-composition tuffs,
have ages that range from ca. 590 to 550 Ma,
and were affected by minor deformation and
low-grade metamorphism, but lack fabric elements of older, Virgilina-age events.
Thus, one possible interpretation of the
Deep Rock–Pen Branch volcanic arc is that it
represents the infrastructure of the Carolina
terrane in South Carolina but has been detached by later tectonic events. More likely,
given the distinct Eu* characteristics of the
Savannah River Site rocks, is that the Deep
Rock–Pen Branch arc represents Late Proterozoic arc infrastructure from another location
in the arc that has been moved into its present
location by transcurrent motions. In either
case, the low-grade Crackerneck Metavolcanic
Complex, which seems to correlate with the
Uwharrie–Persimmon Fork–Lincolnton assemblage, may sit unconformably on top of
the older Deep Rock–Pen Branch arc rocks,
much as the Uwharrie Formation sits unconformably on the Hyco Formation in central
North Carolina (Harris and Glover, 1988).
CONCLUSIONS
Neoproterozoic volcanic and plutonic rocks
that underlie the Savannah River Site in central South Carolina reflect igneous and metamorphic activity that occurred in an island-arc
terrane far from Laurentia, where they are now
located. These rocks correlate in general with
metaigneous rocks of the Carolina and Avalon
terranes, which are inferred to have originated
as fringing arcs marginal to Gondwana (e.g.,
Secor et al., 1983; Shervais et al., 1996;
Nance and Murphy, 1996). Peri-Gondwana
arc terranes range in age from ca. 630 Ma to
535 Ma, coincident with the rifting of Laurentia along both its eastern and western margins
(e.g., Dennis and Wright, 1997; Barker et al.,
1998).
Many of these arcs contain evidence for deformation and metamorphism during this
same time period (e.g., Dennis and Wright,
1997; Strachan et al., 1996; Shervais et al.,
2003). Deformation and metamorphism of
volcanic arc terranes at ca. 590 6 50 Ma were
widespread throughout Gondwana, e.g., the
Pan-African events in West Africa and Arabia,
the Brasiliano event in South America, the Cadomian event in Europe, and the PetermannPaterson orogens of Australia (e.g., Rogers et
al., 1995; Mallard and Rogers, 1997; Veevers,
2003). In general, deformation and metamorphism were contemporaneous with arc magmatism in these terranes, which implies that
these events are driven not by continental collisions or the closure of ocean basins, but by
the collapse and collision of arc terranes in
response to complex plate motions associated
with the rifting and breakup of Rodinia and
the subsequent assembly of Gondwana.
ACKNOWLEDGMENTS
We thank Randy Cumbest and Sharon Lewis of
Westinghouse/SRC for their moral and logistical
support and Westinghouse/SRC for access to their
core-storage facility. Sample preparation for XRF
analyses was carried out by Hampton Uzzelle, Morris Jones, and Neil Wicker; ICP-MS analyses were
performed by Scott Vetter, Centenary College. This
work was funded by the U.S. Department of Energy
through Westinghouse/SRC, SCUREF Task 170.
Partial support for the preparation of this manuscript
has come from the I.W. Marine Fund, University of
South Carolina, Aiken.
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