Geochemical and NdSrPb isotopic composition of Alleghanian granites of the southern Appalachians: Origin, tectonic setting, and source characterization

EPSL EISEVIER

Earth and Planetary Science Letters 134 (1995) 359-376

Geochemical and Nd-Sr-Pb isotopic composition of Alleghanian granites of the southern Appalachians: Origin, tectonic setting, and source characterization Scott D. Samson a, David G. Coler a, J. Alexander Speer b a Department of Earth Sciences, Syracuse University, Syracuse, NY13244-1070, USA b Mineralogical Society ofAmrica, 1130 17th Street N. W., Suite 330, Washington, DC 20036, USA

Received 13 April 1995; accepted 30 June 1995

Abstract We report major element, trace element, and Nd and Sr isotopic ratios for 35 samples of Late Mississippian-Early Permian AIleghanian granites that intrude the Carolina terrane, Eastern slate belt, Inner Piedmont, and Kiokee belt of the southern Appalachians. Major element compositions indicate that most plutons are high-potassium (K,O 14%) metaluminous granites. Initial eNd, depleted mantle model ages, and initial S7Sr/ *‘?Srratios, respectively, of the plutons range from - 8.2 to -3.4, 940-1020 Ma, and 0.70642-0.72798 for the Inner Piedmont; - 2.3 to + 2.0, 660-1130 Ma, and 0.70421-0.71035 for the Kiokee belt; -2.7 to +2.4, 660-870 Ma, and 0.70353-0.70816 for the Eastern slate belt; and - 6.7 to + 1.9, 690-1140 Ma, and 0.70391-0.70739 for the Carolina terrane. The chemical and isotopic data are most consistent with formation of the granites by anatexis of continental crust, rather than derivation from a depleted mantle source. Rocks of the Carolina terrane have e,,(300 Ma) values and Pb isotopic ratios indistinguishable from those of most Alleghanian granites, suggesting it is a likely source. Some Alleghanian granites within the Carolina terrane require a more evolved source than exposed Carolina terrane rocks, suggesting that evolved crust, possibly Grenville basement, occurs structurally below or within the terrane. The chemical and isotopic compositions of granites intruding the Kiokee belt, Eastern slate belt, and Carolina terrane are similar, suggesting that these terranes are eochemicaIly similar. In contrast, Alleghanian granites within the Inner Piedmont $ have higher initial 87Sr/ “Sr and lower ’ 3Nd/ tUNd ratios, indicating that their sources are isotopically more evolved. The virtual absence of AlIeghanian plutons with mafic-intermediate compositions, and the lack of geographic trends in chemical and isotopic composition across the orogen, makes a subduction origin for the granites unlikely. The consistency of the geochemical data with a crustal anatectic origin of the granites, together with the observation that they intruded synchronously with Alleghanian thrusting in the Valley and Ridge Province, suggests strongly that they are collisional in origin. The crustal heating events that produced the granites might have been caused by delamination of mantle lithosphere during collision of Laurentia with Gondwana [1,2]. Alternatively, adequate heating may have occurred by crustal thickening during emplacement of thrust sheets [3].

1. Introduction The Allegbanian orogeny, culminating with the collision of Gondwaua and Laurentia during the late

Paleozoic, is the dominant erogenic event that affected the southern Appalachians. Granites of midMississippian to Permian age throughout the central and southern Appalachians are one of the major

0012-821X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00124-7

360

S.D. Samson et al. /Earth and Planetary Science Letters 134 (1995) 359-376

manifestations of Alleghanian orogenesis. The petrogenesis of these granites, and their tectonic setting, remain poorly understood. It has been suggested that the granites are remnants of a continental margin magmatic arc formed during west-dipping subduction of Theic ocean lithosphere beneath eastern Laurentia [4,5]. Others have suggested that they are collisional granites formed during the terminal Late Paleozoic collision between Laurentia and Gondwana [6]. Recent proposals of the collision hypothesis have suggested that the heat necessary to produce crustal melting might have resulted from delamination of the mantle lithosphere during continental collision (e.g., [1,2]). Another hypothesis is that the granites formed by crustal arching associated with strike-slip faulting in a transpressional tectonic regime [71. One difficulty in evaluating models of Alleghanian granite petrogenesis is lack of data that place limits on the ratio of evolved crust to depleted mantle involved in granite formation. Neodymium isotopic ratios are well suited to this purpose, yet remarkably few Nd data are available for southern Appalachian granites of any age. Initial Sr isotopic ratios also provide important constraints on the nature of source materials. Many southern Appalachian Alleghanian plutons have been dated by the wholerock Rb-Sr isochron technique (e.g., [8]), thus initial 87Sr/ 86Sr ratios are available. However, the accuracy of initial ratios determined from whole-rock isochrons depends upon all samples being cogenetic, a requirement often difficult to meet. Furthermore, the small spread in Rb/Sr ratios of many of the Alleghanian plutons makes it difficult to obtain precise initial 87Sr/ 86Sr ratios [8]. Major element contents and Nd and Sr isotopic ratios are reported here from petrographically and mineralogically well-characterized samples of Alleghanian granites emplaced within several lithotectonic belts in the southern Appalachians. The goals of this study are twofold. The first is to use chemical and isotopic data to aid in evaluating models of granite petrogenesis, and thereby produce a clearer understanding of the tectonic setting of the eastern margin of Laurentia during the late Paleozoic. The second goal is to use granite compositions to constrain the geochemical nature of the unexposed portions of southern Appalachian terranes. The latter

goal is part of a larger effort to determine the volume of new continental crust produced during the Phanerozoic.

2. Geologic background Timing of the initial stage of the Alleghanian Orogeny is constrained by the Middle Mississippian age of the earliest synorogenic elastic sediments deposited in Appalachian foreland basins [9]. The end of the erogenic event is given by the Early Permian age of the youngest deformed sedimentary units, the Dunkard Group [9]. Because most available radiometric ages of Alleghanian plutons are between 325 and 280 Ma (e.g., [8,10]), magmatic activity may have occurred throughout all stages of the orogeny. The low precision of the emplacement ages, however, precludes determining whether magmatic activity occurred during discrete intervals or was essentially continuous [7]. In the southern Appalachians, Alleghanian plutons occur from central Georgia to southern Virginia, a distance of approximately 900 km (Fig. 1). Plutons were emplaced into five lithotectonic zones, from west to east, the Inner Piedmont, the Carolina terrane (composed of the Charlotte and Carolina slate belts), the Kiokee belt, the GoochIand terrane/Raleigh belt, and the Eastern slate belt (Fig. 1). Drilling and geophysical studies [ll] show that other plutons occur further east, below the Atlantic Coastal Plain. The latter intrude either the covered portion of the Eastern slate belt or other terranes that have been identified from geophysical data [12]. Alleghanian magmatism resulted overwhelmingly in the production of granites: less than 5% of the number of plutons have mafic-intermediate compositions, and more than 99% of the area1 extent of the plutons are granites [7]. General features of the plutons have been summarized by Speer et al. [7], and details of individual plutons may be found in their extensive bibliography. The chemical composition of the Carolina terrane is well-characterized (e.g., [13]) as is the Nd isotopic composition of its older portion [14]. In contrast, few geochemical data are available for other Appalachian terranes, precluding detailed modelling of Alleghanian granite petrogenesis. Generalized models, how-

S.D. Samson et al. /Earth and Planetary Science Letters 134 (1995) 359-376

ever, can be made using the new geochemical data. The geochemical data can also be used in an inverse way: granite compositions can place limits on possible compositions of the unexposed sources of the granites and thus on the nature of the terranes through which the plutons ascended.

3. Samples and analytical techniques All samples analyzed were from petrographically and mineralogically well-characterized plutons [7,15]. Many samples are from drill cores and are less weathered than surface samples. Surface samples were collected from quarries and from the freshest available outcrops. The ages and characteristics of samples are listed in Table 1. Descriptions of the field relations and petrographic features of the plutons are given elsewhere [7,15]. Major element concentrations were determined by

Alleghanian

Ia

X-ray fhrorescence techniques at Syracuse University. Whole-rock powders were fused in Pt-Au CNcibles using lithium tetraborate flux (flux:rock ratio = 9:l). Glass disks were analyzed using a fully automated Fisons/ARL 8410 XRF spectrometer. Additional analytical details are given in [25]. Details of sample preparation, dissolution, chemical, and mass spectrometric procedures for Sm-Nd analysis are given in [14]. Depleted mantle model ages (TDM) are reported using the model of [26]. Model ages based on the depleted mantle model of 1271 are approximately 150-200 Ma greater. Strontium was separated from dissolved wholerock samples by standard cation exchange techniques using BioRad AG50-12X resin in 8.9 ml quartz columns with 2.5 M HCl as the elutriant. Sr was loaded onto tungsten filaments using TaF solution as an emitter. Sr isotopic ratios were measured on a fully automated VG Sector 54 mass spectrometer using four of the seven available faraday cups. The

Plutonic Rocks

Analyzed for Nd isotopes Proterozoic crust known/inferred

ATLANTIC

361

-_--__ c4

/

‘t, !

f-0

,- I’, 6L -I_ -%___--_-_f

C u p -

Fig. 1. Map of the major lithotectonic belts in the southern Appalachians (modified from [121) showing the locations Alleghanian plutons. Those with dashed outlines are covered by Atlantic Coastal Plian sediments. Plutons analyzed in this study are shown in black. Numbers correspond to samples listed in Tables l-3. ESB = Eastern slate belt.

362

SD. Samson et al. / Earih and Planetary Science Letters 134 (I 995) 359-376

intensity of the a8Sr ion beam was typically 3-3.5 X lo-‘* amperes. Reported ratios are based on an average of 120 dynamic ratio measurements, each one the average of two separate ratios determined by measuring the ‘*Sr, 87Sr, and 86Sr peaks in two different collectors. Interference from 87Rb were monitored by measuring mass 85 during every cycle.

Table 1 Ages and characteristics

of Alleghanian

Pluton

CABOL3NA SIiAl’B 1 L.iIesvlIIe

Measured 87Sr/86Sr ratios were corrected for fractionation using an exponential fractionation law and a value of 86Sr/ @%r= 0.1194. Rb and Sr concentrations were measured from pressed powder pellets using an automated Phillips X-ray fluorescence spectrometer at Franklin and Marshall College. Rock powder was mixed with a

plutons of the southern Appalachians

Station No.

BELT CB7-29 2 PeeDee S7-47 3 PageIaad sa1-pg4 4 Pageiand S81-pgl K3-72 5 Liberty Hill 6 Liberty fill F6-24 7 Liberty Hill S6-54 8 Wirmsboro Winl-890 9 Winnsboro S76-34 10 Harbisoo Sal-Hbl 11 Columbia CB7-20 12 Columbia CB7-21 13 Cuffytown Creek SlO-SOA 14 Siloam SMl-594 15 Danburg AS7-220 16 Lowry WRO-8 17 Catawba cw-1 18 York WRO-04 19 Clover F-28 20 Gastoaia WRO-10 21 Landis S8LD-6A EASlERN SLATEBELT 22 Sims x-27 23 SiiS SI-3 24 Castalia 147-151 CS-10 25 Castalia 57-61 cs-12 26 Portsmouth PO-10 KIOICBB BELT 27 Clouds Creek CC4A 28 GrmlitevilIe SlGV4 29 Edgefield Sl-EF2A 30 Appling F7-33 31 Town Creek ASP-151 INNER PIEDMONT 32 Elberton FI-37 33 StoneMtn CE7-1 34 Panola SB9-PNl 35 Palmetto PMl-187

Lithology

M&hod

Age* Ma)

Cgr biotite granite Gabbro Cgr amphibole biotite granite Cgr biotite granite Fgr biotite granite Cgr hornblende biotite granite Cgr biotite granite Fgr biotite granite Mgr biotite granite Mgr biotite granodiorite Mgr biotite gmnodiorite Mgr biotite granodiorite Mgr garnet muscovite biotite granite Cgr biotite granite Mgr biotite granite Cgr hornblende biotite granite Mgr garnet muscovite biotite granite Cgr biotite granite Mgr biotite granite Mgr biotite granite Cgr biotite granite

326 f 54 326’ 296 f 10 296 f 10 299 f 30 299 f 30 299 f 30 301 f 8 301 f 8 309 f 6 285 f 14 285 f 14 293 f 28 264 f 6 294 f 1.2’ undated 323 f 28 322 f 12 306 f 8 undated 292 f 58

Cgr biotite granite Megacrst. biotite granite Cgr biotite graoite Cgr biotite granite Cgr muscovite biotite granite

288 288 313 313 263

Mgr cordierite biotite granite Cgr biotite granite Cgr garnet muscovite biotite granite Cgr biotite granite Cgr biotite granite Mgr biotite granite Mgr muscovite biotite granite Mgr muscovite biotite granite Cgr biotite granite

Abbreviations: Cgr = coarse grained; Mgr = medium grained; mineral + whole-rock. a Uncertainty in ages are f20. b Pluton is assumed to be coeval with the Lilesville pluton. ’ D. Dvoracek, pers. commun. [8,10,16-241.

Fgr = fine grained;

f f f f f

13 13 26 26 25

313 f

4

Reference

RbSr wr RbSr RbSr RbSr RbSr RbSr Rb-Sr RbSr RbSr Rb-Sr RbSr RbSr RbSr Rb-Sr

w m m m m m wr min-wr m wr m WT wr

Rb-Sr wr RbSr wr RbSr min-wr Rb-Sr wr RbSr RbSr RbSr RbSr RbSr

wr wr wr wr wr

RbSr mia-wr

IS1 I81 1181 PI 1191 H91 [201 [201 [211 181

UUdakd

317 f 4 undated undated

U-Pb zircon

1221

320 f 40 291 f 7

U-Pb zircon RbSrm

1231 1241

Undated UIldati

Megacrst.

= megacrystic;

w = whole-rock;

min-wr =

S.D. Samson et al. /Earth and Planetary Science Letters 134 (1995) 359-376

cellulose binder in a ratio of 1:1.4. Precision of Rb/Sr ratios is estimated to be better than + 1%.

4. Results and discussion Major element concentrations of the Alleghanian plutons are given in Table 2. Rb, Sr, Sm, and Nd concentrations, 143Nd/ 144Nd and 87Sr/ 86Sr ratios, and TDM ages are given in Table 3. Because of the high Rb/Sr ratios of most plutons, uncertainties in age of 2 10 Ma have a large effect on the calculated initial 87Sr/ 86Sr ratio. Consequently, a range of calculated initial 87Sr/ 86Sr ratios is given in Table 3 incorporating the 2rr uncertainties of the published ages. Plutons that have not been radiometrically

Table 2 Major element chemistry

of Alleghanian

CAROLINATBRRANE 1 Lilesville 65.72 2 PeeDee 48.75 3 Pageland 47.48 4 P&and 71.49 5 Liberty Hill 70.93 6 Liberty Hill 65.84 7 Liberty Hill 70.12 8 Winnsbm 71.24 9 winnsbom 70.13 IO Herbison 66.99 11 Columbia 69.67 12 Columbia 72.27 13 CutQtownCk 75.36 14 Siloam 70.32 15 Danburg 72.80 16 Lmy 63.40 17 catawba 71.16 18 York 73.74 19 Clover 72.18 20 Gastonia 70.17 21 Landis 71.36 EASTERN SLATE BELT 22 Sims 70.85 23 Sims 72.35 24 Cestelia 69.23 25 C&ah 71.90 26 Portsmouth 71.09 KIOKEE BELT 27 CloudsCk 67.15 28 Gmniteville 72.43 29 Edgefield 72.83 30 Ap~liiS 70.76 31 Town Creek 66.72 INNER PIEDMONT 32 Elbaton 71.24 33 StoneMtn 72.89 34 Panala 67.45 35 Palmetto 62.13

363

dated have been assigned an age of 300 Ma, and an uncertainty of f25 Ma was chosen to estimate the range in initial a7Sr/ 86Sr. Uncertainties in age of up to 50 Ma affects the initial eNd by only half an E unit and thus this assumption does not introduce significant error in calculated initial Nd isotopic ratios. 4.1. Major element compositions The major element chemical compositions of the plutons are remarkably homogeneous, both within and between lithotectonic belts (Table 2). Exclusive of the only known Alleghanian gabbro, most plutons can be described as high-potassium (K,O typically 2 4%) metaluminous to slightly peraluminous gran-

plutons of the southern Appalachians

2.38 8.58 2.37 1.56 1.55 2.77 I .65 1.61 1.73 2.57 2.31 1.67 0.51 1.84 1.47 2.98 0.60 1.47 1.87 2.31 1.72

3.74 330 3.84 3.58 3.50 4.72 3.46 3.40 3.26 4.31 4.09 3.74 4.16 3.49 3.58 3.95 3.49 3.53 3.81 3.70 3.32

4.49

14.21 14.42 15.34 14.57 14.41 14.36 15.41 15.38 14.32 13.48 14.43 13.41 16.38 15.06 13.77 14.44 15.06 14.20

4.71 12.80 3.89 2.66 2.29 4.25 2.27 2.17 2.60 3.47 2.39 2.00 0.96 2.74 2.09 3.95 2.37 1.45 1.52 2.40 1.76

1.28 4.31 4.90 5.20 4.60 5.65 5.12 5.31 3.91 3.89 4.36 4.63 4.99 4.55 5.42 5.59 4.74 4.05 4.12 5.11

1.20 6.42 1.01 0.46 0.39 1.02 0.47 0.44 0.57 1.24 0.64 0.51 0.04 0.69 0.43 1.90 0.51 0.29 0.45 0.74 0.57

0.74 1.96 0.60 0.38 0.34 0.71 0.37 0.30 0.43 0.57 0.30 0.27 0.09 0.50 0.35 0.75 0.22 0.23 0.24 0.41 0.32

0.20 0.43 0.18 0.09 0.09 0.18 0.09 0.10 0.12 0.21 0.10 0.081 0.01 0.18 0.09 0.30 0.07 0.06 0.08 0.12 0.08

0.079 0.179 0.077 0.056 0!035 0.075 0.041 0.066 0.040 0.060 0.053 0.038 0.166 0.049 0.062 0.075 0.12 0.042 0.027 0.033 0.030

0.70 0.65 0.57 0.43 0.51 0.64 0.89 0.54 0.39 0.58 0.46 0.73 0.65 0.41 0.47 0.41 0.90 0.29 0.33 0.65 0.78

100.03 99.23 99.81 99.26 100.14 99.59 99.39 98.94 99.34 99.22 100.00 100.07 99.64 99.31 99.39 100.08 99.60 98.99 99.71 99.25

1.037 0.720 0.974 1.013 1.015 0.863 0.983 1.024 1.008 0.%3 1.017 1.029 1.053 0.996 0.994 0.933 1.170 1.012 1.028 1.022 1.006

14.75 14.17 14.98 14.66 14.43

1.84 1.56 2.80 I .75 2.07

I.52 1.19 2.80 1.58 1.35

4.07 3.71 4.00 3.87 3.85

4.71 4.91 3.03 4.60 4.56

0.45 0.40 0.92 0.42 0.67

0.27 0.24 0.38 0.23 0.32

0.09 0.09 0.11 0.07 0.11

0.054 0.058 0.049 0.027 0.063

0.93 0.93 0.74 0.46 I .09

99.54 99.30 99.04 99.57 99.60

1.014 1.043 1.003 1.031 1.051

15.67 14.10 15.04 14.10 15.71

4.64 2.13 1.03 2.96 4.28

1.81 1.47 1.45 2.11 2.49

3.46 3.84 4.23 3.59 3.83

4.10 4.28 4.48 4.12 3.92

1.12 0.54 0.22 0.73 1.16

0.66 0.29 0.12 0.44 0.65

0.17 0.09 0.05 0.16 0.22

0.069 0.059 0.032 0.06 0.086

0.92 0.43 0.56 0.12 0.75

99.76 99.66 100.03 99.15 99.83

1.167 1.034 1.040 0.992 1.041

14.41 14.98 14.97 17.15

2.08 0.82 3.86 4.56

1.57 1.02 2.26 3.57

3.38 3.93 3.33 4.02

5.37 4.70 4.76 4.37

0.47 0.16 0.92 1.74

0.37 0.09 0.80 0.84

0.10 0.11 0.33 0.33

0.024 0.019 0.066 0.066

0.37 0.66 0.65 0.99

99.38 99.38 99.40 99.77

1.013 I.117 1.014 O.%l

15.91 15.89 14.89

All values in wt%. LOI = Loss on ignition.

A/CNK

= Al,O,/(CaO

+ Na,O + K,O).

99.87

364

S.D. Samson et al. /Earth

and Planetary Science Letters I34 (1995) 359-376

ites. A few bodies, such as the Clouds Creek, Palmetto and Town Creek, have elevated alumina to silica ratios compared to most of the other granites, possibly indicating an origin by anatexis of a sedimentary parent. Some of the peraluminous plutons contain muscovite, garnet and/or cordierite [7], have high Rb/Sr ratios (Table 31, and have S’*O values greater than 9%0 [28] - features consistent with a sedimentary origin. Table 3 Rb-Sr and Sm-Nd

isotopic data for Alleghanian IRbl mm

CAROLINA

tsrl ppm

!I& “6sr

plutons of the southern Appalachians

w

%r %

%t

9

WI DJdj !%a ppm ppm Ysrn

%iW u=W)

!%I

l”Nd,,,

G,&’ er+,m ‘L’

l”Nd,

TRRRANE

1 Lilcpviue

133

2 PCS Dee Gabbro

32.0

277

1.385

0.711500

f

7

0.70507

0.7042XI.7062

8.90

52.4

0.1027

0.512498 f 6

0.512280

-2.7

+1.2

757

605

0.1530

0.704617

f

9

0.7o391

0.70380.7O40

s.n

39.3

0.1347

0.512565 f 7

0.512301

-1.4

+l.O

927

0.710567 0.717716

f f

10 10

0.70463 0.70504

0.7o44-o.7o4a 0.7046-0.7M5

9.40 6.46

54.4 39.3

0.1045

0.512470 f 5

-3.3

+0.3

807

0.09947

0.512485 f 4

0.512265 0.512290

-3.0

+0.7

753

9.22

64.7

0.08613

0.512084 f 5

0.511920

-10.8

6.7

1144

56.3

0.1096

0.512416 f 4

0.512206

-4.3

-1.1

922

. 27.4

0.1192

0.512203 0.512111

-4.1 -6.8

-1.2 -2.7

992 937

3 Pagchnd

150

307

4 Pagchd 5 Libctty Hill Fgr.

172

166

1.411 3.010

267

212

3.657

0.722370

* 12

0.70682

0.70524.7083

6 Liberty Hi

Cgr.

123

307

1.162

0.710156

f

10

0.70521

0.7o47-0.7057

7 Liberty Hi a wmn&lru

Cgr.

155 285

218 231

2.058 3.574

0.713805 0.721220

* 13 * 13

0.70505 0.7M91

0.7o42-0.7059 0.7055-0.7063

10.2 5.39 6.97

46.3

0.09087

0.512429 f 5 0.5122% * 6

9 Wmnsbom

259

209

3.590

0.722764

f

9

0.70739

0.7070-0.7O78

79.5

0.1X869

0.512137 f 4

0.511966

-9.8

-5.7

1104

10 Hahiion

134

790

0.492

0.7O6586 f

13

0.70442

0.7044-0.7044

4.61

2a.9

0.09647

0.512463 f 4

0.512274

-3.4

+0.4

762

11 Columbh 12 Columbia

172 238

280 120

1.773 5.739

0.711760 0.709926

f f

9 13

0.70457 0.68543

0.7042~.7049 0.6855-0.6878

5.70 3.33

33.7 20.8

O.lo21 0.09654

0.512548 f 5 0.512527 f 4

0.512348 0.512352

-1.7 -2.1

+1.9 +I.6

687 676

241

291

2.399

0.715952

f

9

0.70677

0.7067~.7071

5.90 9.11

27.5 62.4

0.1298 0.08827

0.512486 f 5 0.512236 f 4

0.512237

14 sioam

-3.0 -7.8

-0.5 -3.6

1013 982

IS Danburg 16 l.4wly

252 111

199 1513

3.664 0.212

0.720697 f 0.7o542B f

9 9

0.70537

0.7053-0.7054 0.7o44-o.7046

-5.8

-1.7

848

17 catnwba

260

231

3.252

0.719187

f

7

4.4 -1.0

0.0 +1.2

808 943

18 York 19 clover 20 Gastonia

169

263

1.863

0.712972

f

14

131 148

713 522

0.531 0.82O

0.706569 0.708447

f f

9 9

0.70443 0.7O426 0.70495

0.7o410.7048 0.7W2-0.7O43

21 Landii EASTERN SLATE

115

712

0.467

0.706405

f

11

0.70446

13 C&town

CL

0.70452 0.70425

11.7

6.00

41.6

0.08708

71.0 15.3

0.09375

0.512343 f 5 0.512412 f 4

0.512175 0.512227

20.8

0.512589 f 4 0.512360 * 5

0.512314

3.22

0.1400 0.09349

-1.2

870

0.70470.7052

27.8 40.1

0.08801 0.07938

0.512345 f 5 0.512291 f 5

0.512164 0.512172 0.512135

-5.4

4.04 5.26

-5.7 6.8

-1.6 -2.3

851 858

0.7o410.7o49

3.99

27.0

0.08923

0.512353 f 5

0.512183

-5.6

-1.6

850

0.512350 0.512319

-1.9 -2.5

+1.6 +l.O

686 733

0.7029-o.7055

BELT 209

248

2.434

0.714098

f

I1

269

219

3.558

0.719668

f

9

0.70413 0.70509

0.7037-0.7o46 0.7044-0.7057

5.15 4.44

31.1 26.4

0.1001 0.1016

0.512539 f 0.512511 f

5 5

cgr.

0.512063

11.1 3.54

23 Sims Mclyst.

22 siia

24 Cartilia 147’-151’

127

325

1.126

0.709408

& 13

0.70438

0.7o4o.o.7048

4.23

23.4

0.1095

0.512588 f

5

0.512373

-1.0

i2.4

678

25 Chtilia 57’61’

174

270

1.865

0.711837

zt 9

0.70353

0.7028-0.7042

3.76

23.0

0.09873

0.512518 f

4

0.512324

-2.3

+1.4

7%

26 Portsmouth

303

219

4.016

0.723182

f

11

0.70816

0.7067-0.7096

3.60

20.4

0.1065

0.512432 f

5

0.512249

-4.0

-1.0

874

KIOKEE BELT 27 Clouds Creek

144

202

2.065

0.719543

f

9

0.71035

0.7102-0.7105

9.19

44.8 0.1241

0.512378 f

6

0.512134

-5.1

-2.3

1129

28 Gmnitcvillc

153

216

2.049

f f f

9 9 10

0.70421

0.7035-0.7049

3.89 3.33 7.56

23.3 15.0 44.8

0.1012 0.1342 0.1020

0.512552 f 0.512471 f 0.512480 f

4 4 5

0.512353 0.512195 0.512280

-1.7

+2.0

656

-3.3 -3.1

-0.7 +0.5

lo91 776

6.59

42.3

0.09407

0.512446 f

5

0.512261

-3.7

+0.2

768

8.77 3.30

65.9 12.7

0.08w6 0.1599

0.512150 0.512147

5 5

0.511982 0.511843

-9.5 -9.6

4.8 -8.2

1021 **a*

29 EdBefietd 30 Ap~ling

150

278

1.561

0.712955 0.722090 0.710971

31 TownCreek

142

339

1.212

0.710338

f

10

INNER PIEDMONT 32 Elbaton

249

246

33 stone hall

238

120

2.939 5.762

0.719761 0.751844

f f

13 7

34 Parlola

167

332

1.459

0.715383

f

9

0.70915

0.7o86-o.7097

15.2

35 Palmcuo

108

8%

0.349

0.710176

f

13

0.70869

0.7086-0.7088

13.4

0.70430 0.70516

0.70380.7049 0.7047-0.7O56

0.7-2

0.70530.7075 0.7274-0.7286

0.72798

’ Uncertainties are f 2 (+ of the mean and refer to least significant ’ Data normalized to %Sr/ ss Sr = 0.1194. 3 Data normalized to ‘46Nd/ 144Nd = 0.7219. 4

It has been argued [4] that K,O contents of the granites, at constant silica, increase across the strike of the orogen. The data from this study show no spatial trends in granite major element composition, either within individual lithotectonic belts or throughout the whole orogen (Table 2). A complete absence of spatial trends is also clearly evident in other major element data sets (e.g., [29]). No temporal trends in major element compositions can be

ENd(0)

=

107 96.0

f f

0.08572

0.512191

f

4

0.512012

-8.7

-4.2

1016

0.08459

0.512246

f

6

0.512080

-7.6

-3.4

944

digit.

[143W “4~l~~~,,~~~~~~~NdlB”‘kBar’h x 104. Present day parameters

‘47Sm/ 144Nd = 0.1966.L ’ ’ 5 TDM = Depleted mantle model age, based on model of [26]. Samples shownby ****.

with ‘47Sm/‘44Nd

for Bulk Earth:

143Nd/ 144Nd = 0.512638,

> 0.15 have meaningless

TDM ages and are

SD. Samson et al. /Earth and Planetary Science Letters 134 (199.5) 359-376

ascertained either, although this may be due in part to the imprecision of most available radiometric dates. 4.2. Sr isotopic compositions The majority of Alleghanian granites in this study have initial *‘Sr/ 86Sr ratios I 0.7050, in general agreement with previous Rb-Sr studies of the granites (e.g., [SJO]). Some plutons have obviously not remained closed systems with respect to Rb-Sr as their calculated initial “Sr/ 86Sr ratios are unrealistically low (i.e., < 0.7001, even when taking uncertainties in age into account (Table 3). Most of the plutons have such high Rb/Sr ratios that even slight Rb mobility has a large effect on calculated initial ratios. Moreover, many of the plutons are synorogenie and have thus been subjected to some deformation. Discounting samples with unrealistically low ratios, approximately 60% of the granites have initial “Sr/ 86Sr ratios I 0.7050. If these ratios are accurate estimates of the true initial ratios, an important proportion of their source region must have had low time-integrated Rb/Sr ratios. Although the Sr isotopic ratios reported here are in general agreement with published values, there are some discrepancies. For instance, the Liberty Hill pluton has a reported initial “Sr/ 86Sr of 0.7046 [lo]. This pluton is a composite body composed of coarse-grained biotite and biotite-amphibole facies, and a fine-grained biotite facies. Samples from all three facies were included in the Rb-Sr whole-rock isochron that is the basis of the 0.7046 initial ratio [lo]. If considered as three separate samples, “Sr/ 86Sr ratios (calculated at 299 Ma) are 0.70521 and 0.70505 for the coarse-grained facies and 0.70682 for the fine-grained sample, demonstrating that the Liberty Hill is a composite body and the coarse-and fine-grained facies are genetically unrelated. The Castalia pluton, a granitic body near the boundary of the Raleigh belt and Eastern slate belt in southern Virginia (Fig. 11, has a reported initial ratio of 0.7141 [8], an unusually high value for an Alleghanian pluton. The samples analyzed in that study were taken from the north rim of the pluton where magmatic interaction with the country rock produced a muscovite-biotite-garnet granite with abundant epidote, carbonate, and chlorite. Two drill-core sam-

365

ples of biotite granite, taken from the center of the Castalia pluton far from the border facies, were analyzed in this study. Using the reported Rb-Sr age of 313 Ma [19], initial “Sr/ ‘?lr ratios of 0.70353 and 0.70438 were determined for the samples, suggesting that the body is either composite or the samples still record some magmatic interaction with wall rock. Both initial “Sr/ 86Sr ratios reported here are considerably lower than 0.7141 and are more typical of other Alleghanian pluton values. The discrepancy between the drill-core samples and the quarry sample underscores the importance of carefully chosen sample sites. Most Alleghanian granites have moderate initial Sr isotopic ratios. This observation has been emphasized in previous studies and has been cited as evidence that the granites may have had a depleted mantle source [8,10]. The Sr data reported here, however, indicate that several plutons have initial “Sr/ 86Sr > 0.706, requiring an evolved source. Some of these more evolved granites are the largest bodies, thus they represent a significant percentage of the total volume of magmatic material generated during the Alleghanian Orogeny. 4.3. Nd isotopic compositions Initial 143Nd/ 144Ndratios of the plutons are more robust than initial Sr isotopic ratios because rare earth elements are less mobile than Rb. Additionally, even large uncertainties in the age of the granites has little effect on calculated initial Nd isotopic ratios because of the low Sm/Nd ratio of the granites and the 106 Ga half-life of 14’Sm. The initial eNd values of the granites range from -8.2 to +3.0 (Fig. 2). Ranges for granites from individual terranes or belts are more restricted; Carolina terrane granites range from -6.7 to + 1.9, Eastern slate belt granites range from - 2.7 to + 2.4, granites in the Kiokee belt range from -2.3 to + 2.0, Raleigh belt granites range from -4.3 to +3.0 [25], and the granites in the Inner Piedmont range from - 8.2 to - 3.4. The span in TDM ages for the granites is 660-1140 Ma (Table 3). It has been suggested [7] that many Alleghanian granites are composite bodies composed of genetically distinct plutons. Nd isotopic data show this for the Winnsboro and Liberty Hill granites. Two facies

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and Planetary Science Letters 134 (1995) 359-376

+n

. +6 -

Depleted Mantle 0

Eastern Slate beii

+4-

375

350

325

300

275

250

225

AGE (Ma)

Fig. 2. Initial en,, of the Alleghanian granites (20 errors are smaller than symbol size). Shown for reference is average depleted mantle [26], and the bulk earth (CHUR). The labeled points are samples of different facies of the named pluton. The two coarse-grained facies of the Liberty Hill pluton have identical lNd, suggesting a genetic relationship. The fine-grained facies has substantially lower lNd, demonstrating that this pluton is a composite body. Two facies of the Winnsboro pluton also have significantly different initial en,, , indicating that is also a composite body. Two facies of the Sims and Columbia plutons have almost identical eNd values and thus these bodies may be single, differentiated plutons. Data from Goochland terrane/Raleigh belt from [25].

of the Wimrsboro body have initial eNd values of -2.7 and -5.7 (Fig. 21, documenting that two distinct plutons are present. A fine-grained facies of the Liberty Hill pluton has an initial Q, of -6.7 whereas two coarse-grained facies have indistinguishable lNd values of - 1.1 and - 1.2. These Nd data document that the Liberty Hill and Whmsboro granites are composed of plutons that are spatially related but do not have common sources, or do not share the same proportions of similar source materials. Thus, distinct magma batches apparently took the same pathways of ascent. Two facies each from the Pageland, Columbia, and Sims granites have initial cNd values that are within 0.4, 0.3, and 0.6 e units, respectively, of one another, thus these facies could be genetically related (Fig. 2). The difficulty in predicting which Alleghanian granites are composite bodies helps explain the large scatter observed in many Rb-Sr whole-rock isochrons of Alleghanian granites. The bulk of the initial +,, values for the Allegha-

nian granites range between - 3 and + 3, indicating that a significant proportion of their source material cannot be old, evolved crust. Given the isotopic composition of many of the granites the depleted mantle is an obvious source candidate, as suggested previously [8,10]. A depleted mantle source is also implied in models that call for a subduction origin of the granites (e.g., [4]) and in tectonic models that show closing of the ocean basin between Laurentia and Gondwana via west-dipping subduction beneath Laurentia (e.g., [5]). Models of granite production via the depleted mantle do not assume that the granites were directly formed by partial melting of peridotite, but rather through a series of closelyspaced partial melting and fractionation events of successive source material, the first of which is mantle peridotite. The fact that even the highest initial eNd values of the granites are lower than that estimated for average depleted mantle at 300 Ma (Er.&j= +7.5) indicates that some evolved material must have mixed with the mantle-derived magmas to reduce even the granites with the highest P 43Nd/ 144Nd ratios. The proportion of this evolved component would have to be even greater to explain the initial Nd isotopic composition of bodies like the Stone Mountain, Winnsboro and Liberty Hill granites (Table 3). Whereas a depleted-mantle source with variable amounts of crustal contamination is not excluded by the isotopic data presented here, there are more plausible sources. For instance, crust of the Carolina terrane itself could be the main source of most of the granites that intrude the terrane. Most Alleghanian plutons within the Carolina terrane have initial lNd values between - 2 and + 2 (Table 3); the e,,(300 Ma) of most Carolina terrane rocks ranges from - 2 to +3 [14] (Fig. 3). Thus these granites could have formed by wholesale melting of Carolina terrane crust; a depleted-mantle component is not required. Many of the Alleghanian granites that intrude the other tectonic belts have cNd values similar to the l,,(300 Ma) values of the Carolina terrane. Those granites could also have been produced by anatexis of terrane crust if the crust in those belts is similar to that in the Carolina terrane. Supporting this idea are the Nd isotopic compositions of two Eastern slate belt volcanic rocks (Fig. 3). The values are slightly lower than those of the Carolina terrane, and are

SD. Samson et al. /Earth

and Planetary Science Letters 134 (I 995) 359-376

367

Carolina terrane at the time of Alleghanian magmatism (Fig. 4). The calculated ratios significantly overlap with the ratios of the Alleghanian granites. Choosing different p and w ratios for average Carolina terrane crust, or different estimates of the timing of ore formation, would produce different ‘06Pb/ 204Pb and 208Pb/ 204Pb ratios of the Carolina terrane, but using any combination of realistic parameters still produces overlap between the ratios of the granites and the Carolina terrane. The similarity between Pb isotopic compositions of Alleghanian I 600

I

I 700

I

I

600

I

I, 500

I

‘ml

F---t.., 300

1 200

AGE (Ma)

Fig. 3. Initial end values of the Alleghanian granites compared to fields for Grenville crust from North America [30,32], the Carolina terrane [14], and two samples of the Eastern slate belt [25]. Symbols for the granites as in Fig. 2. The granites have the appropriate isotopic compositions to have been formed by anatexis of Carolina-like crust, Grenville-like crust, or a combination of both. The low en,, values for some of the Alleghanian granites intruding the Carolina terrane, combined with the high lnd of the terrane itself, suggests that the Carolina terrane was sitting structurally above mid-Proterozoic basement in late Paleozoic time. Granites within the Inner Piedmont also appear to have a significant, Grenville-like component.

q

Alleghanian granites

0 Carolina terrane Mantle (300 Ma) ,

I

,

,

consistent with the Eastern slate belt as a potential source for the lower ENd granites within this belt. 4.4. Pb isotopic compositions The Pb isotopic ratios of Alleghanian granites emplaced within the Carolina terrane and Raleigh belt, determined on acid leached K-feldspars, are similar to the ratios of average crustal Pb at 200-300 Ma, but are considerably higher than estimated Pb isotopic ratios of the mantle at 300 Ma (Fig. 4). These data provide additional evidence of the importance of a continental crustal source of the granites. The Pb isotopic composition of the Carolina terrane at N 300 Ma can be estimated from the Pb isotopic composition of galena ores thought to have formed from Carolina crust between 450-550 Ma [30,31]. Projecting the Pb isotopic compositions of the ores forward by 200 Ma, using typical crustal p cz8U/ 204Pb) and w (232Th/ 204Pb) ratios, provides an estimate of the Pb isotopic composition of the

q

Allegbanian granites

0 Carolina femme

Mantle (300 Ma) ‘am 18.0

185

190

206Pb/204Pb

Fig. 4. Pb isotopic ratios of acid leached K-feldspars from Alleghaniaa granites [33-381 and estimated ratios for crust of the Carolina terrane at 300 Ma (based on the Pb isotopic ratios of _ 500 Ma Pb ores [33,34] and the assumption that average terrane crust had p = 8.5 and w = 36). Also shown is a growth curve of average continental crust [39], and a - 300 Ma mantle field (calculated using the mantle parameters of [40]). The Pb ratios for Alleghanian granites substantially overlap the calulated values for the Carolina terrane and for average 200-300 Ma crust, providing strong evidence for a crustal source for the granites.

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and Planetary Science Letters 134 (1995) 359-376

granites and average Carolina terrane crust lends further support to the conclusion that the main sources of the granites are the terranes they intrude. 4.5. Proterozoic

crustal contributions

Because most Carolina terrane samples have Ma) 2 - 2, a more evolved source is required to produce the granites with initial lNdI - 2. It is possible that the Carolina terrane is heterogeneous and evolved crust occurs at depth, or in areas not yet sampled. Another possibility is that older crust structurally lies beneath the Carolina terrane and the more evolved bodies were produced by melting a combination of this crust and the Carolina terrane. The eastern extent of Grenville basement in southern North America is not known, but some tectonic interpretations suggest that it extends beneath the full width of the Carolina terrane (e.g., [38]). Typical e&300 Ma) values of Grenville igneous rocks are - 10 to -5 [39-411. The Nd isotopic composition of granites like the Liberty Hill and Winnsboro are consistent with their derivation by anatexis of Grenville crust (Fig. 3). The Pb isotopic composition of Grenville crust at N 300 Ma overlaps the compositions of the least radiogenic Alleghanian granites [35], consistent with an anatectic origin. If the Carolina terrane was thrust over Grenville basement as an intact sheet, then bodies like the Liberty Hill and Winnsboro must have originated in the lower plate, below the dCcollement. In this scenario, Alleghanian granites would have formed at variable crustal depths. Alternatively, pieces of Grenville crust could have been tectoni-

l,,(300

-

Inner Piedmont -

/-

cally interleaved within the Carolina terrane during thrusting. In this case, all of the granites could have formed within the upper plate at mid-crustal levels, but still have incorporated evolved Proterozoic crust (Fig. 5).

5. Thermal/ tectonic models of magma generation The isotopic composition of the Alleghanian granites can place important constraints on possible sources of the plutons. By identifying probable and improbable (or impossible) source materials, the likelihood of various contradictory models relating to the generation of Alleghanian magmas can be evaluated. Several of these tectonic models are examined below in light of the new geochemical data. 5.1. generation by subduction

One of the main models for Alleghanian granite generation is the production of magmas in response to subduction of oceanic crust beneath eastern Laurentia (e.g., [4]). It has been argued that there is a systematic increase in K,O contents (at constant SiO, content) and initial 87Sr/s6Sr ratios of the granites towards the craton. Such a trend is similar to that observed for the Peninsular and Sierra Nevada batholiths [42-441, Cordilleran batholiths thought to have been generated in continental arc settings. There are several problems with a subduction model. First, the new geochemical data reported here show that there is no geographic trend in

Carolinaterrane-

j Kiokee j , belt 4

Fig. 5. Hypothetical cross-section of eastern Laurentia in late Paleozoic time showing possible source regions of Alleghanian granites. Most of the granites (black) are shown forming by anatexis of crust of Appalachian accreted terranes. A few plutoas, however, have a Grenville-like crustal source, shown here as a duplex beneath the Kiokee belt and Carolina terrane. In this model, the granites formed at similar, mid-crustal, depths. Thickness of section is approximately 60 km. CPS = Central Piedmont Suture. Modified from cross-section of [411.

SD. Samson et al. /Earth

and Planetary Science Letters 134 (1995) 359-376

143Nd/ 144Nd, 87Sr/ 86Sr, K,O, or Sr compositions across the strike of the orogen. Second, generation of the plutons by flux-melting of the mantle is difficult to reconcile with the almost complete absence of plutons with mafic-intermediate compositions. The majority of plutons in Mesozoic Cordilleran batholiths have dioritic, tonalitic, or granodioritic compositions (e.g., [42,45,46]], whereas most Alleghanian plutons are granites sensu stricto. The area1 ratio of Alleghanian granites to gabbros + intermediate rocks in the southern Appalachians is 18O:l [7], a ratio completely atypical of continental margin arcs. If the plutons were generated in response to subduction of oceanic crust, resulting in closure of the ocean basin between Laurentia and Gondwana, then the end of Alleghanian magmatism should coincide with the beginning of continental collisional orogenesis. Published radiometric dates (Table 1) range between 325 and 280 Ma, thus it appears that magmatism may have occurred throughout the entire erogenic period. However, because most of the dates have large uncertainties (up to +50 Ma in some cases) a precise estimate of the duration of magmatism cannot yet be made. If early magmatism was a result of subduction, but later magmatism was a result of continental collision, the geochemical character of the younger granites could reflect this change in origin. No discernable differences in isotopic composition between the youngest and oldest granites are observed (Table 3). 5.2. Magma generation by crustal thickening Several models have been developed which show that crustal thickening during orogenesis can cause sufficient heating of the crust to induce melting (e.g., [3,47-491). Most of these thermal models predict that crustal anatexis will occur during crustal stacking, if the volume of thrust sheets is adequate, although the melting event is predicted to lag behind initiation of thrusting by several tens of millions of years. Part of the earliest phase of the Alleghanian orogeny involved transport of the Blue RidgePiedmont thrust sheet, a composite of multiple imbricate thrusts (e.g., [50]). Emplacement of these thrust sheets may have provided adequate thermal insulation to cause crustal melting [51].

369

It has been argued that formation of Alleghanian magmas by crustal thickening is doubtful because early-formed magmas should be water-saturated, later magmas should be undersaturated, and melting by crustal thickening should be a gradual process -features of Alleghanian granites which are not observed [7]. However, in the models described by Patiiio Deuce et al. [3], melting of mid-level crust under water-absent conditions occurs by the breakdown of hydrous minerals as a result of crustal thickening. Minimum-melts would be the first to be generated, but the volume of such melts would be too small for ascent to occur [3]. Thus, older, water-saturated bodies and younger, unsaturated bodies should not be observed at similar crustal levels, in accordance with the observation for Alleghanian granites. Another aspect of mid-crust water-absent dehydration melting models is that once biotite dehydration-melting conditions are attained the time for the lowest kilometer of a fertile crustal layer to reach melt percentages of 40%, taken as a maximum percentage of melt in the model of [3], is between 0.5-1.5 Ma. Melting larger thicknesses of a fertile crustal layer would take longer, but the main point is that a gradual increase in melt volume is not predicted in these thermal models. A significant problem with an origin of the older Alleghanian granites by crustal thickening is the requirement of an extensive period of time between initiation of thrusting and the onset of anatexis. Most thermal models predict a time interval in the range of 20-40 Myr [3,47-491. The amount of time depends upon the number and thickness of thrust sheets, the duration of the erogenic/thickening event, the initial crustal temperatures, and the amount of water present. Given that the earliest estimate of the onset of thrusting is pre-Middle Mississippian (based on the stratigraphic ages of the oldest portions of thrustgenerated elastic wedges) [9], it is difficult to explain the presence of Alleghanian granites as old as 330320 Ma. Unless initiation of crustal thickening was considerably earlier than mid-Mississippian, or the geothermal gradient was unusually high, this aspect of the model makes it difficult to explain the generation of all of the Alleghanian plutons. However, because the Alleghanian Orogeny was a protracted event, possibly lasting 2 50 Ma, it is possible that some of the syn-kinematic, and all of the post-

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kinematic (280-300 Ma) plutons, formed due to crustal thickening caused by emplacement of the Blue Ridge-Piedmont thrust sheets. 5.3. Magma generation by decompression melting Decompressional melting of the mantle and lower crust has also been suggested as a model for the formation of the granites [52]. In this model, rapid lowering of pressure occurred in response to crustal arching, which was caused by transpressional strikeslip faulting. Such a model helps explain the spatial relationship of Alleghanian granites and strike-slip faults, an association not accounted for by other models. The decompression model is also consistent with the immediate appearance of magmatism and the lack of temporal or spatial trends in the granite compositions [7]. The decompression model is consistent with the long duration of magmatic activity, if arching and strike-slip faulting occurred throughout most of the erogenic event. The model is dependent upon the assumption that crustal uplift associated with strikeslip faulting would be sufficient to allow warm upper mantle material to induce melting of surrounding lower crust, or that lower crust itself could begin melting because of the reduced pressure. But given even an unusually high subcontinental geotherm (e.g., [3]), an unrealistic amount of uplift (at least several kilometers) would be required for lower crustal rocks to rise past their dry solidi. Given these inherent difficulties, this model for granite petrogenesis cannot be fully evaluated until the likelihood of lithospheric melting by uplift is more rigorously demonstrated. 5.4. Magma generation by lithospheric delamination Delamination of the southern North American lithosphere has been suggested to have occurred as a consequence of detachment of an oceanic slab subducting beneath Gondwana during the initial stages of Laurentia-Gondwana collision [l]. A model in which delamination of mantle lithosphere occurs after collision due to collapse of the thickened orogen by its own weight has also been proposed [2]. In both scenarios a main consequence of mantle lithospheric delamination is emplacement of hot asthenosphere

next to cooler continental lithosphere. The rise in thermal conditions could cause extensive lower crustal melting and heat from these extensive melts could then be convected or advected to mid-crustal levels causing further crustal anatexis. In both of the above delamination models crustal anatexis produces silicic magmas. The volume of silicic magma produced and the timing of the initiation of that magmatism, may be different in these two delamination scenarios. It has been estimated [53] that if delamination occurs by lithosphere breaking off a subducting slab the time required for the lithosphere at 40 km depth to reach N 750°C would be N 20 Ma (assuming an instantaneous rise in temperature of the lithosphere at 80 km depth, the point of slab break off, to 1370°C). A heat pulse associated with slab breakoff might be limited spatially and in intensity, thus only relatively small volumes of melt might be generated [53]. This is consistent with the limited amount of magma produced during the Alleghanian erogenic event compared to other major granitic terrains (the area1 extent of Alleghanian granites is N 10,500 km2 [7]; the area1 extent of Sierra Nevada plutons is > 50,000 km*). In delamination models where the entire lithospheric mantle root founders into the asthenosphere a more spatially extensive heat source is present and the volume of melt generated should be considerably greater. Furthermore, if the asthenosphere was emplaced in more direct contact with the lower crust, a more rapid onset of crustal melting would occur. In this latter scenario, wholesale melting of the lower crust can occur. Current geophysical techniques cannot document if wholesale melting of the lower southern Appalachian crust occurred (e.g., [2]), but such a scenario appears inconsistent with the limited area1 extent of Alleghanian plutons observed at present exposure levels. An attraction of lithospheric delamination models is that they can account for the high temperatures required to produce water-undersaturated granitic melts in both lower-and mid-crustal regions. If only some of the melt generated in the lower crust ascended to mid-crustal levels then only a few Alleghanian bodies would have the isotopic signature of that lower crust. This is consistent with the interpretation presented above that Grenville basement may be present beneath the Carolina terrane, but

S.D. Samson et al. /Earth and Planetary Science Letters 134 (1995) 359-376

only a few Alleghanian granites within the terrane were produced by melting a Grenville-like source. The lack of a crustal “root” in the southern Appalachians is also consistent with an Alleghanian delamination event [2]. Recent continental collision zones, like the Himalayas and Alps, have well developed crustal roots. The lack of Late Paleozoic mafic rocks in the southern Appalachians may be inconsistent with Alleghanian mantle lithosphere delamination. The onset of mafic volcanism = 2-3 Myr ago in the Puna Plateau region of Argentina has been emphasized as key evidence for Andean lithospheric delamination, and is considered an important criterion for identifying other delamination events [54,55]. However, the evidence of mafic volcanism in the Puna region of the Andes will be destroyed by future erosion. This is an important point with respect to the southern Appalachians because much of the crust has been removed since u 300 Ma. Aluminum-in-hornblende paleobarometric data indicates many Alleghanian granites were emplaced at pressures of 3-5 kbar, or approximate depths of 11-19 km [56], thus any mafic volcanic rocks produced would no longer be preserved. If gabbros were generated they must have been emplaced at depths greater than the granites, as their area1 extent is less than 1% of exposed plutons. The timing of a proposed delamination event is an important aspect of this model of granite petrogenesis. If a delamination event occurred after the main phase of collision-driven compression then most plutons should be younger than collision-related thrusting events. If delamination by mantle foundering occurred during continental collision then the ages of most granites should coincide with this event. There is a clear need for more precise estimates of the timing of initiation of continent collision and for high-precision crystallization ages of the Alleghanian granites.

6. Chemical nature of southern Appalachian terranes Little is known about the isotopic composition of exposed regions of southern Appalachian terranes and virtually nothing is known about the composition of unexposed Appalachian crust. If the Allegha-

371

nian granites were generated by crustal anatexis, as suggested here, their isotopic composition provides direct information about the isotopic composition of their source regions. Those regions could be the terranes the granites intrude, or the crust beneath the terranes if the terranes are large-scale thrust sheets. 6.1. The Carolina terrane The oldest portion of the Carolina terrane consists of isotopically juvenile crust [14]. Because many Alleghanian granites that intrude the Carolina terrane have initial lNd values identical to the e,,(300 Ma) values of exposed Carolina crust (Fig. 3) it is likely that much of the unexposed portion of the Carolina terrane is also composed of isotopically juvenile material. There is no systematic difference in isotopic composition between Alleghanian granites within the western part of the Carolina terrane (Charlotte belt), versus those emplaced within the eastern part of the Carolina terrane (Carolina slate belt), strengthening the argument that the terrane has a broadly homogeneous Nd isotopic composition, Some granites have lNd values lower than those of exposed Carolina terrane crust suggesting the terrane is evolved at depth or that different crust lies below or embedded within the terrane (see Fig. 5). It is unlikely that deep crust native to the Carolina terrane is isotopically evolved because the oldest known rocks of the Carolina terrane have the highest initial 143Nd/ 144Nd ratios [14]. If the deepest part of the terrane was evolved then the earliest magmas passing through this evolved region would be the least “shielded”, and the oldest rocks should thus have the lowest 143Nd/ 144Ndratios. 6.2. The Eastern slate belt The Eastern slate belt (ESB) is composed predominantly of greenschist-facies volcanic and volcanielastic rocks, thus sharing many lithological similarities to the Carolina terrane [57]. The ESB is bounded to the west by the Raleigh belt/Goochland terrane and to the east by sediments of the Atlantic Coastal Plain (Fig. 1). Nd and Sr isotopic compositions have been analyzed for six Alleghanian plutons from exposed ESB crust and two plutons buried by Atlantic Coastal

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Plain sediments (Table 3; [25]). The latter two samples were obtained from drill cores from a region referred to as the Roanoke Rapids terrane [58]. The initial lNd values of these granites cluster tightly ( - 2.7 to + 2.4) and are similar to those of the more radiogenic Alleghanian granites intruding the Carolina terrane. Two volcanic rocks of the ESB have l,,(300 Ma) values of -4.2 and - 1.3 [25] and therefore the least radiogenic Alleghanian granites in the ESB could have been produced by melting similar rocks (Fig. 3). Melting of such crust might also have produced the more radiogenic granites, but the current database is too limited to determine if ESB crust could be the sole source material. If no ESB rocks have positive l ,,(300 Ma) values, then a more juvenile component must have been an additional source. One possibility is that the Carolina terrane extends structurally beneath the ESB and is a source of the more radiogenic granites in this belt. The two plutons buried by Atlantic Coastal Plain sediments have similar values to the plutons within the exposed ESB. This suggests that the Roanoke Rapids terrane is isotopically similar to the ESB. 6.3. Raleigh belt / Goochland terrane The Raleigh belt, as defined by [59], consists of biotite gneisses and pelitic schists bounded on the west by the Nutbush Creek mylonite zone and on the east by the volcanic units of the Eastern slate belt and the Macon mylonite zone. The metamorphic rocks of the Raleigh belt have been petrographically correlated with similar metamorphic rocks in Virginia that are considered to be part of the Goochland terrane [60], a terrane characterized by rocks that experienced granulite-facies metamorphism. One of the main units of the Goochland terrane, the State Farm Gneiss, has a reported whole-rock Rb-Sr date of 1031 + 94 Ma [61]. The entire Goochland terrane has been considered to be Grenville in age because the whole terrane experienced the same granulitefacies metamorphism as the State Farm Gneiss [60]. Many Alleghanian granites within the Raleigh belt (Fig. 1) have the same range in Nd isotopic compositions and trace element abundances [25] as the granites in the Eastern slate belt (Figs. 2 and 3). If the Raleigh belt was composed of N 1 Ga metamorphic rocks it is likely that the Alleghanian gran-

ites in this belt would reflect the isotopic composition of that Grenville crust. That is, if the Raleigh belt was composed of Grenville basement, and the granites formed by melting such crust, they would have lNd values of about -5 to - 8. Instead, the Raleigh belt granites are isotopically indistinguishable from those in adjoining terranes, and the Alleghanian granite with the highest lNd value yet determined (eNd = +3) is in the Raleigh belt. It is therefore unlikely that the belt is composed of Grenville crust. In fact, because the isotopic compositions of Raleigh belt granites are so similar to those in adjoining terranes it is suggested that the Raleigh belt is the metamorphic equivalent of the Carolina terrane and/or Eastern slate belt. A more detailed discussion of the characteristics of the Raleigh belt, as inferred from the isotopic and trace element chemistry of Alleghanian granites, is given in [25]. 6.4. Kiokee belt The Kiokee belt (Fig. 11, like the Raleigh belt, is composed of metamorphic rocks of unknown age and affinity. It has been speculated that the Kiokee belt might be equivalent to the Raleigh belt (e.g., [56]). The range in initial eNd values of the Alleghanian plutons that intrude this belt is - 2.3 to + 0.5. The Graniteville pluton, which occurs just east of the exposed Kiokee belt in the Belair belt, is covered by Atlantic Coastal Plain sediments. If it is included with the Kiokee belt suite then the range in initial lNd values is - 2.3 to + 2.0. This range is comparable to that observed for plutons in both the Raleigh belt and Eastern slate belt. Thus the suggestion that the Kiokee and Raleigh belts might be equivalent [56] is supported by the isotopic data, and it is speculated that the Kiokee belt is a more intensely deformed part of either the Carolina terrane or Eastem slate belt. 6.5. Inner Piedmont The Inner Piedmont is an extensive lithotectonic belt (Fig. 1) made up of a group of thrust sheets, including the Chauga-Walhalla, Paris Mountain, Six Mile, and Laurens sheets [61]. In general, the Inner Piedmont is composed of pelitic to quartzofeldspathic metasedimentary rocks, gneisses, amphibo-

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lites, minor ultramafic bodies, and numerous Paleozoic intrusive rocks. The origin of this belt, and its affinity to other terranes, is poorly understood. Some models suggest that much of the Inner Piedmont is a block containing late Proterozoic continental slope and rise deposits that rifted from Laurentia during the latest Proterozoic, but then collided back into Laurentia during the Ordovician (e.g., [5]). A controversial model is that the Inner Piedmont is basement to the Carolina terrane and was separated from its cover rocks by normal movement along the Central Piedmont Suture [62]. Four AIleghanian plutons have been analyzed from the Inner Piedmont. The initial lNd values of these f;anites range from - 8.2 to -3.4 and initial Sr/ 86Sr ratios range from 0.7064 to 0.7280 (Table 3). If the granites in the Inner Piedmont formed by wholesale melting of crust, then at least the southern part of the Inner Piedmont appears to be composed of more evolved crust than the other southern Appalachian tectonic belts studied. Although the evidence is circumstantial, the composition of the Inner Piedmont granites casts doubt on the idea [62] that the Carolina terrane was built upon Inner Piedmont basement. If both belts were part of a single crustal block it would be likely that the plutons intruding both pieces of this block would have similar isotopic characteristics. A full comparison of the two belts will be possible only when the Inner Piedmont is thoroughly characterized, but based on differences in the isotopic compositions of the granites in the belts, it is predicted that the isotopic nature of the two belts will be shown to be considerably different.

7. Conclusions The Nd, Sr, and Pb isotopic compositions of most Alleghanian granites in the southern Appalachians are consistent with formation of the granites by melting crust of the juvenile terranes they intrude. An intracrustal origin is also consistent with the chemical compositions of the granites (high SiO,, K,O, Nd, Sm, and Rb contents) and with the tiny area1 percentage ( N 0.6%) of gabbroic-intermediate bodies. Some Alleghanian plutons have Q,,, values too low to be accounted for by melting of crust like that

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of the accreted Carolina terrane. Either the Carolina terrane is isotopically more evolved with depth or the terrane structurally overlies crust with an evolved isotopic composition. A possibility is that Grenville crust underlies the entire width of the Carolina terrane and that melting of this deep source, or melting of thrusted slices of Grenville crust within the Carolina terrane, produced the least radiogenic granites. There is no geographic trend in either the chemical or isotopic composition of the Alleghanian granites. This is in conflict with an earlier suggestion [4] that initial 87Sr/ 86Sr ratios and K,O contents vary systematically across the strike of the orogen, suggestive of a subduction origin for the granites. The lack of geographic chemical trends, combined with the paucity of mafic-intermediate compositions, renders a subduction origin unlikely. Thus tectonic reconstructions showing west-dipping subduction of ocean crust beneath Laurentia in the late Paleozoic should be reconsidered. Thermal models that are consistent with a wholly intracrustal origin of the granites include crustal heating due to the insulating effect of stacked thrust sheets and crustal anatexis caused by mantle lithospheric delamination. Crustal melts can form at mid-crustal levels by the breakdown of hydrous minerals due to radioactive heat build up if stacked thrust sheets are present to act as an insulator (e.g., [3]). Such a model may account for the production of the younger Alleghanian granites but may not be able to account for the generation of older plutons because of the lack of time available for crustal temperatures to reach sufficiently high levels. Lithospheric delamination during continent-continent collision (e.g., [1,2]) could have produced sufficient temperatures to melt crust by juxtaposing hot deep mantle against or near the base of the continental crust. If most Alleghanian granites formed by melting the crust they intrude, the isotopic compositions of the granites reflect that of the surrounding terranes. Because most granites in the Ralei h, Kiokee and 8? Eastern slate belts have low initial Sr/ 86Sr ratios and positive, or only slightly negative, lNd values it is likely that these belts contain a considerable amount of juvenile crust. Thus these belts, like the Carolina terrane, may be composed of a large percentage of isotopically juvenile crust. This is the first

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evidence that much of the crust at depth in the southern Appalachians may be composed of juvenile, mantle-derived material. Documenting the chemical nature of Appalachian crust will be an important part of determining the volume of new crust generated in the Phanerozoic.

Acknowledgements We thank K.D. Nelson for reviewing an earlier version of this manuscript and thank Alan Brandon and Eirik Krogstad for providing insightful reviews. This work was supported in part by NSF grants EAR-9219583 and EAR-9220708 to SDS. [CL]

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