The history of a continent from UPb ages of zircons from Orinoco River sand and SmNd isotopes in Orinoco basin river sediments

CHEMICAL GEOLOGY IAttTgt/O//qa

ELSEVIER

ISOTOPE GEOSCIENCE

Chemical Geology 139 (1997) 271-286

The history of a continent from U-Pb ages of zircons from Orinoco River sand and Sm-Nd isotopes in Orinoco basin river sediments S.L. Goldstein

a, *

,1 N.T. Arndt b R.F. Stallard c

a Max-Planck-lnstitutfiir Chemie, Postfach 3060, D-55020 Mainz, Germany b lnstitut de G~ologie, Universit~ de Rennes, Avenue du G~nJral Leclerc F-35042 Rennes, France e US Geological Survey, 3215 Marine Street, Boulder CO 80303, USA

Received 15 September 1996; accepted 24 January 1997

Abstract We report SHRIMP U-Pb ages of 49 zircons from a sand sample from the lower Orinoco River, Venezuela, and Nd model ages of the fine sediment load from the main river and tributaries. The U-Pb ages reflect individual magmatic or metamorphic events, the Sm-Nd model ages reflect average crustal-residence ages of the sediment sources. Together they allow delineation of the crust-formation history of the basement precursors of the sediments. The U-Pb ages range from 2.83 to 0.15 Ga, and most are concordant or nearly so. Discrete age groupings occur at ~ 2.8, ~ 2.1, and ~ 1.1 Ga. The oldest group contains only three samples but is isolated from its closest neighbors by a ~ 600 Ma age gap. Larger age groupings at ~ 2.1 and ~ 1.1 Ga make up about a third and a quarter of the total number of analyses, respectively. The remaining analyses scatter along concordia, and most are younger than 1.6 Ga. The ~ 2.8 and ~ 2.1 Ga ages correspond to periods of crust formation of the Imataca and Trans-Amazonian provinces of the Guyana Shield, respectively, mid record intervals of short but intensive continental growth. These ages coincide with ~ 2.9 and ~ 2.1 Ga Nd model ages of sediments from tributaries draining the Archean and Proterozoic provinces of the Guyana Shield, respectively, indicating that the U-Pb ages record the geological history of the crystalline basement of the Orinoco basin. Zircons with ages corresponding to the major orogenies of the North Atlantic continents (the Superior at ~ 2.7 Ga and Hudsonian at 1.7--1.9 Ga) were not found in the Orinoco sample. The age distribution may indicate that South and North America were scparated throughout their history. Nd model ages of sediments from the lower Orinoco River and Andean tributaries are ~ 1.9 Ga, broadly within the range displayed by major rivers and dusts. This age does not coincide with known thermal events in the region and reflects mixing of sources with different crust-formation ages. The igneous and metamorphic history of these sources, as recorded by the detrital zircons, is that of the Orinoco basin basement. This implies that, despite evidence of fast sedimentary recycling, global similarities in Nd crustal-residence ages, and the probability of cross-continent mixing through continental drift, the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin. The global

* Correspondingauthor. FAX: 914-365-8155;E-mail: [email protected]. i Current address: Lamont-Doherty Earth Observatory and Department of Earth and EnvironmentalSciences, Columbia University, Palisades, NY 10964, USA 0009-2541/97/$17.00 ¢) 1997 Elsevier ScienceB.V. All rights reserved. PH S0009-2541 (97)00039-9

272

S.L Goldstein et al. / Chemical Geology 139 (1997) 271-286

semblance in Nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large-scale sampling of the continents, which are heterogeneous in age on smaller regional scales. A large portion of the continental crust in the Orinoco basin formed during the Trans-Amazonian orogeny at 2.0--2.1 Ga, and smaller portions formed both earlier, at ~ 2.8 Ga, and later, after 1.6 Ga. These observations, which are consistent with the relative sizes of crustal age provinces in the Orinoco basin, indicate that sediments from the lower Orinoco and Andean tributaries contain 25--35% of material added to the crust since Trans-Amazonian times. Nd model ages of these sediments underestimate the average crust-formation age of the basement of the Orinoco basin by only about 10%. If this relationship holds in other river basins, then Nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is ~ 1.9--2 Ga. © 1997 Elsevier Science B.V. Keywords: U - P b ages; zircons; S m - N d isotopes; Orinoco River; sediments

1. Introduction Uncertainties regarding the rate and mechanism of continental growth remain among the outstanding problems of continent--mantle interactions. There is still no consensus as to whether the continents grew throughout geological time (Moorbath, 1977; Moorbath and Taylor, 1981), or whether the continental mass ~ 3.8 Ga ago was comparable to today's (Armstrong, 1981, 1991). It is also unclear whether continental growth was continuous, or episodic, as suggested by peaks of orogenic activity at ~ 3.5, ~2.7, ~2.1, ~1.9, ~ 1 . 1 Ga, and ~ 0 . 7 Ga in most of North America, Europe, Australia, and Africa (Condie, 1976; Moorbath, 1977; Gurnis and Davies,

1986). A related question is whether these peaks are consistent with continent growth at convergent plate margins, or if they imply alternative mechanisms (Fyfe, 1974; Tarney et al., 1976; Reymer and Schubert, 1984, 1986; Taylor and McLennan, 1985; Abouchami et al., 1990; Samson and Patchett, 1991; Boher et al., 1992; Seng~ir et al., 1993; Stein and Goldstein, 1996). Many of these questions could be addressed if the distribution of crust-formation ages were known in sediments derived from large areas of the continental crust. Goldstein et al. (1984) showed that Nd model ages of major river sediments and wind-blown dust display a relatively small range of ~ 1.7 + 0.35 Ga, and this was confirmed by later studies (Grousset et

Fig. 1. Map of Orinoco River basin showing major tectonostratigraphic divisions. The river flows for much of its course near the boundary of the Guyana Shield and the plain comprised of alluvium derived from the Andes.

S.L, Goldstein et al. / Chemical Geology 139 (1997) 271-286

al., 1988; Goldstein and Jacobsen, 1988). In these cases the Nd model ages represent mixtures of basement and sedimentary precursors with a wide range of 'crust-formation' ages. They give no information on the history of crustal growth because the Nd is mainly in clay minerals and the different age components cannot be separated. They provide information about the average 'crustal-residence' age but nothing about the distribution of crust-formation ages in the components of a sediment. It has been recognized for decades that a record of crustal thermal events is preserved in some detrital minerals. The classic: early studies of U - P b ages in detrital zircons (Ledent et al., 1964; Tatsumoto and Patterson, 1964) used large sample concentrates which were probably mixtures of grains having different ages. Later studies using small concentrates with similar color and morphology, and eventually single grains, represented major advances (e.g. Gaudette et al., 1981), but traditional mass spectrometry and chemistry is labor intensive, and this limits the possibility of acquiring an appropriate number of age data. The SHRIMP ion-probe at ANU has allowed rapid analysis of single zircon grains and even different overgrowths, and has been used in recent studies of detrital zircons to unravel complex crustal histories (Gebauer e'L al., 1989; Huhma et al., 1991; Ireland, 1992; Zhao et al., 1992). U - P b zircon ages record individual thermal events related to crystallization of intermediate to silicic magmas and crustal metamorphism. Rather than providing direct information on the timing of juvenile crustal additions, they give the ages of crustal accretion and reworking. Here we use the different records provided by U - P b zircon ages and Nd crustal-residence ages to piece together the crustal-growth history of the basement precursors of a sediment. The Orinoco River drains a large area of northern South America, ranking third on Earth in annual runoff and eighth in sediment discharge (Milliman and Meade, 1982). Its catchment basin consists of two main tectono-stratigraphic regions (Fig. 1). The Andean province comprises portions of the Venezuelan and Colombian Andes, and a Tertiary-Quaternary sedimentary platform. The Guyana Shield comprises the Imataca Complex of Archean age (Hurley et al,, 1976; Montgomery and Hurley, 1,978; Montgomery, 1979) and the Trans-Amazonian Province which un-

273

derwent pervasive deformation between 2.2 and 1.7 Ga (Gibbs and Barron, 1983; Teixeira et al., 1989; Onstott et al., 1989). Crystallization ages within the catchment area range from Archean through Phanerozoic, and a large range of ages can be expected in detrital minerals. The river flows for much of its course along the boundary between the Andean and Shield provinces, and thus should receive contributions from both. It is therefore well-suited to investigate the relationship between the ages of detrital components of a major river sediment and the growth and evolution of its basement precursors. Here we report U - P b ages of 49 zircons separated from a sample of Orinoco River sand, and Sm-Nd isotopic compositions of fine-particulate sediments from the main river and tributaries draining the Andean and Shield provinces. The Shield tributaries chosen are located entirely either in the Imataca Complex or the Trans-Amazonian Province.

2. Results

Zircons were separated from a sand sample collected from a sand bar in the middle of the lower Orinoco River. U - P b ages (Table 1) of cores of 49 individual zircons were measured on the SHRIMP ion-probe at Australian National University, using procedures of Compston et al. (1986). Determinations of the P b / U and P b / T h ratios are based on the revised value of 2°6pb/Z38U = 0.0928, equivalent to an age of 572 Ma, for standard zircon SL3. Sm-Nd isotope ratios (Table 2) were measured on samples of fine sediment from near the mouths of several tributaries that drain the tectonostratigraphic provinces shown in Fig. 1. Sm and Nd were separated from the samples at MPI-Mainz using procedures modified from White and Patchett (1984), and measured on a Finnegan MAT 261 mass spectrometer. ratios are normalized to 146Nd/144Nd -- 0.7219, and corrected to 143Nd/144Nd = 0.511860 based on replicates of the La Jolla standard (measured 143Nd/144Nd = 0.511850, n = 15, 2o" (external) =0.000022). In Table 2 the in-run errors are listed. Errors on Sm and Nd concentrations are ~ 0.2%. Nd model ages (in Ga) are calculated based on a linear evolution for the

143Nd/144Nd

Morpboi.

(c)

Subrounded, colourless 20.1 5.1 106.1 105.1 z 7.1

26.1

Euhedral, colourless 1.1 17.1 inc (c) 28.1 4.1 116.1 3.1 115.1 z, inc 107.1 19.1 Euhedral, brown 27.1 z 23.1 102.1 24.1 29.1 z 26.2 z (m)

Sample

20 38 55 41 121

96 1928 1452 796 131 206 228

547 4849 3567 2114 366 522 569

106 219 198 105 273

18 25 75 12 5 43 86 11 14

Pb * (ppm)

555 468 1293 225 176 226 340 53 52

U (ppm)

24 9 2 13 25

28 37 27 43 48 33 69

13 14 35 17 4 27 439 1 20

Vb (ppb)

Common

0.16436 0.17535 0.26687 0.33944 0.35890

O.164 19 0.40712 0A1670 0.38446 0.33094 0.36896 0.39591

0.02727 0.05109 0.06193 0.04878 0.02572 0.18387 0.25175 0.19486 0.25912

2°6Pb/23Su

1.65614 1.94835 3.65113 5.48596 5.91208

1.88473 7.15857 7.38695 6.82608 5.69272 6.43398 7.68803

0.00205 0.00493 0.00836 0.00470 0.00432 0.00479 0.00518 0.00251 0.00233 0.00576 0.00746 0.00484

0.18991 0.36212 0.45814 0.37608 0.20278 2.04998 2.98500 2.39172 3.64828

2°Tpb/235U

0.00106 0.00063 0.00075 0.0O062 0.00055 0.00244 0.00524 0.00456 0.00487

Error

Table 1 Characteristics of Orinoco detrital zircons and U-Pb analytical data and ages

0.07308 0.08059 0.09922 0.11722 0.11947

0.08325 0.12753 0.12857 0.12877 0.12476 0.12647 0.14084

0.04219 0.09106 0.15043 0.09228 0.11755 0.11533 0.14087 0.11476 0.06144 0.11742 0.20259 0.12648

0.05051 0.05141 0.05365 0.05592 0,05719 0.08086 0.08599 0.08902 0.10211

2°7pb/2°6pb

0.01870 0.01950 0.01235 0.03269 0.02181 0.06794 0.09766 O.11422 0.21081

Error

2°6pb/

980 2202 2245 2097 1843 2025 2150

0.00142 0.00034 0.00023 0.00057 0.00180 0.00137 0.00159

981 1042 1525 1884 1977

173 321 387 307 164 1088 1448 1148 1485

0.00480 0.00217 0.00211 0.00317 0.00179

23S U

Age (Ma)

0.00485 0.00262 0.00122 0.00472 0.00588 0.00232 0.00196 0.00346 0.00534

Error

1016 1211 1610 1914 1948

1275 2064 2078 2081 2025 2050 2238

218 259 356 449 499 1218 1338 1405 1663

age (Ma)

2°7pb/ 206pb

I

to

r"

68 53 883 678 1042 1053 811 63 788 1243 20 5 28 10 29 16 26 46 10 18 10 24 38 29 60 120 497 811

293 146 2376 1740 2735 2838 2072 2618 1299 2166

359 36 168 47 140 94 137 236 40 110 60 113 126 89 167

445 1492 1424

12 27 18

9 16 6 5 4 6 13 21 14 21 23 13 16 11 35

90 58 21 36 39 31 223 320 46 24

0.2628 0.33998 0.5466

0.05063 0.14122 0.17105 0.16848 0.19214 0.16341 0.18048 0.19404 0.21968 0.16133 0.15423 0.21188 0.25398 0.2982 0.35832

0.21707 0.36159 0.38061 0.39405 0.38541 0.38038 0.37890 0.01772 0.56752 0.54.604

0.00337 0.00688 0.00673

0.00063 0.00268 0.00355 0.00313 0.00409 0.00370 0.00257 0.00258 0.00423 0.00233 0.00254 0.00314 0.00376 0.00671 0.00528

0.00456 0.00539 0.00465 0.00482 0.00468 0.00463 0.00763 0.00036 0.01169 0.00665

3.54618 6.00076 15.0288

0.39127 1.38559 1.77395 1.75099 2.06523 1.77588 1.96998 2.13350 2.45562 1.80786 1.85103 2.59719 3.19046 4.18165 6.24307

3.38654 6.23486 6.73074 6.99349 6.84488 6.76204 u.lu91~ 0.42582 14.8179 14.8802

0.07112 0.12892 0.19776

0.02465 0.16101 0.05247 0.19841 0.08464 0.10109 0.08678 0.06485 0.19661 0.08795 0.13567 0.09990 0.13586 0.17747 0.18480

0.10220 0.22319 0.09011 0.09495 0.08908 0.08935 u.~4~, 0.01518 0.32245 0.18996

0.09787 0.12801 0.19941

0.05605 0.07116 0.07522 0.07538 0.07796 0.07882 0.07917 0.07974 0.08107 0.08127 0.08704 0.08890 0.09111 0.10170 0.12636

0.11315 0.12506 0.12826 0.12872 0.12881 0.12893 ~.,~,~o 0.17427 0.18937 0.19765

Notes. Morphology: z is zoned; inc is inclusions; pits is pitted surface; (c) is core of grain; (m) is margin of grain. Pb * is radiogenic Pb. Errors + ltr. Common Pb corrected using 2°4pb for ages > 1.8 Ga and 2°spb for younger ages.

Subrounded, brown 114.1 z 32.1 25.2 z 31.1 z 33.1 z 25.1 z ilia z 112.1 z 110.1 z 30.1 z Rounded, colourless 17.2 inc, z ( m ) 22.1 103.1 8.1 pi~ 108.1 109.1 13.1 pim 9.1 pits 14.1 pits 10.1 pits 16.1 ine 18.1 inc 6.1 101.1 21.1 Rounded, brown 15.1 113.1 z 11.1 inc 0.00136 0.00065 0.00067

0.00337 0.00799 0.00140 0.00825 0.00253 0.00390 0.00316 0.00205 0.00611 0.00363 0.00604 0.00300 0.00347 0.00338 0.00302

0.00217 0.00384 0.00053 0.00059 0.00044 0.00049 0.00059 O.00464 0.00092 0.00051

1504 1887 2811

318 852 1018 1004 1133 976 1070 1143 1280 964 925 1239 1459 1682 1974

1266 1990 2079 2142 2102 2078 297! 113 2898 2809

1584 2071 2821

454 962 1074 1079 1146 1168 1176 1191 1223 1228 1361 1402 1449 1655 2048

1851 2030 2074 2081 2082 2083 2092 2599 2737 2807

to

I

3

276

S.L Goldstein et a L / Chemical Geology 139 (1997) 271-286

Table 2 Nd isotopes and Sm and Nd abundances of Orinoco tributary and Orinoco River sediments Sample River Sm (ppm) Nd (ppm) |47Sm/144Nd

143Nd/ 144Nd

TNa (Ga)

OR 514 OR 523 OR 532 OR 552 OR 528 OR 551 OR 501

0.511926:5:22 0.512040 + 17 0.511949 4- 12 0.511632 4- 15 0.5116174- 13 0.511587 + 16 0.511063 4- 12

1.88 1.77 1.89 2.25 2.11 2.19 2.88

Lower Orinoco Apure Meta Upper Orinoco Suapure Ventuari Aro

6.620 7.662 5.964 3.996 10.00 4.088 8.244

35.08 39.35 30.86 21.78 58.68 23.52 47.97

0.1142 0.1179 0.1168 0.1109 0.1030 0.1050 0.1039

whereas those younger than ,-, 1.6 Ga scatter along concordia. Age does not appear to correlate with U or Th content (Table 1). Some older grains have high U abundance (2000-5000 ppm in some ~ 2.0 Ga grains), but others have lower abundances identical to younger grains. It is probable that most of the Orinoco River sand is derived from reworking of sediment from the Andes, which form the northern and western limits to the basin (Fig. 1) and where outcropping rocks are mostly sediments. This inference is supported by the zircon morphology, because many grains are almost spherical and have moderately to heavily pitted surfaces (Table 1) indicating significant residence time in a sedimentary environment. Overall, grain sizes range from > 100 /zm to several /zm, color from

depleted mantle from ~r~a = 0 at 4.55 Ga to ~rqd= 10 today, based on Goldstein et al. (1984):TNd 1 -

-

-

0.00654 [

Xln

( 0 . 5 1 3 1 6 -- 143Nd//144Ndsample) ]

1 + (0.2134 - 1475m//144Ndsample) The U - P b data are unevenly distributed and generally lie along concordia (Fig. 2). Three grains have ages of 2.74-2.83 Ga, and there are large populations at 2.0-2.1 Ga and at 1.0-1.2 Ga. Less than half of the zircon ages fall outside these three groups. There are no ages greater than 2.82 Ga, and none between 2.24 and 2.74 Ga. Only one near-concordant zircon has an age between 1.90 and 1.57 Ga. Ages older than ~ 1.6 Ga form isolated groups,

L

0.6

i

i

i

i

I

t

Orinoco Detrital Zircons

I

I

2800

o.4I 0.2

1000

0.0

"

2000

i

0

4

i

l

8

12

16

207pb/235u Fig. 2. Concordia diagram for the Orinoco detritat zircons analyzed. The inset shows a blowup of the area around 2.0 Oa.

277

S.L. Goldstein et al. / Chemical Geology 139 (1997) 271-286

colorless to deep brown, and habit from euhedral prismatic to well-rounded. We classified the zircons on the basis of morphology and color, to see if they could be separated into age groups. Two of the oldest zircons ( ~ 2.8 Ma) are deep brown, moderately elongate (3:1), and well-rounded with pitted surfaces, but zircons with similar appearance were also found in the 2.0-2.2 Ga age group (Table 1). Similarly, although five of six of the youngest ~,ircons ( < 500 Ma) were colorless, euhedral, with internal zoning and sharp crystal faces, grains with sitmilar characteristics have ages between 1.1 and 1.5 Ga. The sixth Phanerozoic grain is colorless, but strongly zoned, with numerous inclusions and a well-rounded form. A majority of the 2.0-2.2 Ga grains are relatively large and deep brown, but the degree of rounding varies and some grains are pale brown to colorless. Most grains in the 1.0-1.2 Ga group are highly rounded, pitted, clear, colorless grains and have low U concentrations. Most of the Phanerozoic grains are enhedral and colorless. The grains that scatter between ~ 1.6 and 1.2 Ga along concordia show a variety of sizes, colors, and shapes. The zircons were not evaluated by cathodo-

luminescence, which would enhance the ability to recognize overgrowths; such an evaluation would help to determine if the scatter along concordia reflects mixed ages (Gebauer et al., 1989; Gebauer, 1993). In summary, although there is some relationship between zircon appearance and age, separation of zircons into populations based solely on color and habit for the purpose of conventional multi-grain U - P b analysis would have resulted in mixed-age populations, and would not yield true crystallization ages. The Nd model ages of the fine clastic sediments display clear relationships to the tectono-stratigraphic provinces within the basin (Table 2, Fig. 3). The 1.8-1.9 Ga model ages of samples from the lower Orinoco and Andean tributaries are within the range found in other major rivers and wind-blown dusts, but are slightly older than average (Goldstein et al., 1984; Goldstein and Jacobsen, 1988; Grousset et al,, 1988). The Nd model age of ~ 2.9 Ga for the Aro River sample confirms an Archean age for the Imataca Complex and is effectively the same as the oldest U - P b zircon ages found in the region. The ~ 2.2 Ga Nd model ages from tributaries draining

IO~N

-

8 Ga

5ON

Fig. 3. Drainagebasins and Nd model ages of the lowerOrinocoRiver and the tributariesWhereSm-Nd ratios of sedimentwere analyzed as part of this study. Locationsof the sample sites are shown.

278

S.L Goldstein et aL / Chemical Geology 139 (1997) 271-286

the Trans-Amazonian province are effectively the same as the U-Pb ages of the largest zircon population.

3. Detrital zircon ages and Guyana Shield geochronology The Guyana Shield records a long history of igneous activity (summarized in: Cordani and de Brito Neves, 1982; Gibbs and Barron, 1983; Gibbs and Wirth, 1985; Teixeira et al., 1989). An Archean age for the Imataca Complex has been inferred from Sr and Pb isotopes (Hurley et al., 1976; Montgomery and Hurley, 1978; Montgomery, 1979). Magmatism in the Guyana Shield has been reported at 2.7-2.8 Ga (in the Imataca Complex), 2.0-2.3 Ga (in the Trans-Amazonian Province), 2.0-1.5 Ga (the Roraima-Uatun~ Group and Rio-Negro-Juruena mobile belt), and 1.55-1.45 Ga (the Parguazan Batholith). The entire region was affected by a major thermal event which reset K - A r ages to 1.9-1.7 Ga (Onstott et al., 1989). Small anorogenic granitic bodies with ages of 1.3-0.92 Ga are scattered throughout the Shield. Phanerozoic activity in the Orinoco basin is limited to the intrusion of Paleozoic and Mesozoic dike swarms and the development of the Andean orogenic margin. The coincidence of U-Pb zircon ages and Sm-Nd model ages of the Shield tributaries suggests that zircons with ages of ~ 2.7-2.8 Ga and 2.0-2.2 Ga formed during Imataca and Trans-Amazonian crustal-accretion events. We infer that the ~ 2.8 Ga zircons were derived from the Imataca Complex, because (a) they are the oldest zircons in the sample, (b) the age is consistent with ,-, 2.7 Ga Imataca events previously referred to in the literature (Montgomery, 1979; Gibbs and Wirth, 1985), and (c) the age is compatible with the ~ 2.9 Ga Nd model age of sediment from the Aro River, whose catchment basin is completely within the Imataca Complex (Table 2). In the Trans-Amazonian Province, granite-gneiss and greenstone belt lithologies closely resemble those of Archean terrains such as the 3.0-2.6 Ga Superior Province in the Canadian Shield (Gibbs and Barton, 1983). However, whole rock Sm-Nd and Rb-Sr isochron ages of 2.0-2.1 Ga together with positive initial end values and low

initial SVSr/86Sr (Gruau et al., 1985) indicate that this province formed through juvenile crustal addition in the Early Proterozoic. The Nd and Sr isochron ages are in substantial agreement with other geochronological data, including (a) previously published U-Pb zircon ages of 2.25 Ga (Gibbs and Olszewski, 1982), (b) Nd model ages of river sediments in the Trans-Amazonian portion of the Guyana Shield, which fall between 2.0 and 2.3 Ga (Table 2), and (c) Sm-Nd and U-Pb zircon ages of 2.0-2.1 Ga in the adjacent Birimian Province of West Africa (Abouchami et al., 1990; Boher et al., 1992). The large fraction of zircons in the Orinoco sand with 2.0-2.1 Ma ages (Table 1, Fig. 2) were almost certainly formed as part of this orogeny. Taken together, the Sm-Nd, Rb-Sr and U-Pb data show that this was an intense period of crustal accretion in the Guyana and Birimian Shields. This period appears as the major peak in the zircon age spectrum, which, in total, appears to fingerprint the geological history of the basement in the basin. U-Pb zircon ages younger than 1.9 Ga do not fall in isolated groups but scatter unevenly along concordia (Fig. 2). There are no concordant zircons with ages corresponding to the Trans-Amazonian metamorphic events in the Guyana Shield at ~ 1.7 Ga: we interpret this as evidence that zircon crystallization associated with metamorphism was of minor importance. Grains with ages of ~ 1.5 Ga might be associated with the emplacement of the large Parguazan Batholith, but the origin of grains with ages between 1.5 and 1.2 Ga remains unclear. Most probably they came from small, scattered anorogenic intrusions. The source of the relatively large population at 1.2-1.0 Ga is also uncertain. Major orogenies during this period include the Grenville in the northern continents, but tectonic events during this period in the southern parts of the Orinoco basin have been considered to be mainly deformational and metamorphic along with some scattered anorogenic bodies (Gibbs and Barron, 1983). Low U contents of these grains (Table 1) may indicate a metamorphic origin. The cluster may be an artifact of small-scale Pb loss, a possibility that could be tested by conventional single-grain analysis (Davis et al., 1990, 1994). In spite of the uncertainties in interpreting the post-1.9 Ga zircon ages, it is evident that significant crustal growth occurred since 2.0 Ga, because the Sm-Nd

S.L. Goldstein et al. / Chemical Geology 139 (1997) 271-286

model ages of the Andean tributary and lower Orinoco sediments are younger than the age of the Trans-Amazonian orogeny.

earlier Archean continental crust in the Orinoco basin. The correspondence of Nd model ages in the Guyana Shield tributaries to the ages of the Imataca and Trans-Amazonian provinces indicates that the amount of continent that accreted during the Archean and Early Proterozoic events was roughly proportional to the sizes of these provinces. Because the Imataca Province is small ( < 20% of the area of the TransAmazonian Province), the mean age of the Guyana Shield must be less than 2.5 Ga (which would correspond to 50 : 50 Imataca : Trans-Amazonian), and is probably closer to ~ 2.2 Ga. Sediments from the lower Orinoco and the Andean tributaries have Nd model ages of ~ 1.9 Ga (Fig. 3) and are mixtures of sources with different crust-formation ages. The fraction of post-TransAmazonian basement can be estimated from the Nd isotopic data, if the average crust-formation ages of the Guyana Shield and post-Trans-Amazonian components are known (Fig. 4). The older the average crust-formation age of either component, the larger the fraction of post-Trans-Amazonian basement in the sediment. The average crustal-residence age of the post-Trans-Amazonian component is probably bounded by 1.6 and 0.9 Ga, since only two of our

4. Crustal growth in the greater Orinoco Basin Crustal growth in the Guyana Shield was dominated by the accretion of the Trans-Amazonian province between 2.0 and 2.2 Ga, as these ages are pervasive throughout the region, extending beyond the Orinoco basin into the Guyana and Brazilian shields, well as to the Birimian in West Africa (Gibbs and Olszewski, 1982; Gruau et al., 1985; Gibbs et al., 1986; Abouchami et al., 1990; Boher et al., 1992). The coincidence of these U - P b zircon ages with Sm-Nd isochron ages and model ages of ~ 2.0-2.3 Ga in the Trans-Amazonian tributaries (Table 2 and Fig. 3) indicates that the TransAmazonian Province contains only minor amounts of re-worked older basement. The Imataca Complex formed at 2.8-2.9 Ga, as indicated by the coincidence of U - P b zircon ages with the Nd model age of the sediment from Are River which drains the Complex (Figs. 1 and 3). Our data give no indication of

1.0

'

I

279

I

I

I

Post-Guyana Shield Basement

TNd = 1.9 Ga

in Lower Orinoco Sediment

Q)

E 0.8

g 0.6 (0 O

15 0.4 C 0

- Range limits

= 2.4 Ga

TGS

0.2

._._.....--

Preferred range

2.2 Ga

U.

0.0 0.0

,

I

0.4

,

I

,

I

,

I

0.8 1.2 1.6 Mean age of post-Guyana Shield basement (Ga)

2.0

Fig. 4. Estimate of the maximum amounts of post-Trans-Amazonian basement in the lower Orinoco sediment sample, with a Nd model age of 1.9 Ga. The result varies depending upon the mean crust-formation ages of the Guyana Shield and the post-Trans-Amazonian basement. Curves are shown for mean ages of 2.2 and 2.4 Ga for the Guyana Shield. The field for the 'probable range' is based on the assumption that the mean age of the Guyana Shield is roughly approximated by the relative sizes of the Imataca and Trans-Amazonian provinces, and that the average age of post-Guyana Shield basement is 1.0-1.2 Ga. GS refers to Guyana Shield.

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S.L. Goldstein et al. ~Chemical Geology 139 (1997) 271-286

8F O n°co River Sand J 6

f

U-Pb zircon

7

.o-2.t

,[

II

t

1.1 Ga

that accreted since the Trans-Amazonian orogeny constitutes a significant fraction of the sediment,

5. Implications for sedimentary mass dynamics

f

•

t

I

INorth America--T--]

2.7 Ga

1.9 aa

1

Z 2o [ [North Amedca ] I lad model ages ]

:

0

3.5

3.0

Data

2.5

2.0 1.5 Age (Ga)

1.0

o.s 0.0

Fig. 5. Comparison of U - P b zircon ages in the Orinoco sand sample, and zircon ages and Nd model ages in North America. Top: histogram of Orinoco zircon U - P b ages. Samples whose 2°7pb/2°6pb and 2°6pb/23su ages differed by > 20% (6 of the 49 zircons) are not plotted. For samples less than 600 Ma old the 2°6pb//238U age is used. Center: North American zircon U - P b ages. Bottom: North American Nd model ages. Orinoco zircon ages are from Table 1; other data are from the literature.

nearly concordant post-Trans-Amazonian zircons have ages greater than 1.56 Ga, and zircons with ages younger than 0.9 Ga are only a small fraction ( < 10%) of the total. A reasonable estimate is 1.01.2 Ga, near the middle of the period and coincident with a large zircon age population (Figs. 2 and 5). Using these estimates, the fraction of post-TransAmazonian Shield component is 25-35%. Therefore, although the mean crustal-residence ages of the lower Orinoco and Andean tributary sediments of ~ 1.9 Ga are not very much younger the average age of ~ 2.2 Ga for the Guyana Shield, continental crust

Evaluations of the relationship between the global sediment mass and depositional age have shown that ~ 50% of existing sediments were deposited since 250-600 Ma (Garrels and MacKenzie, 1971; Veizer and Jansen, 1979). The contrast between these young depositional ages and the much older mean age of the continental crust of 1.9-2.3 Ga (All~gre and Rousseau, 1984; Goldstein et al., 1984; Miller et al., 1986) reflects efficient sediment recycling, mainly through erosion and re-deposition. Veizer and Jansen (1979, 1985) estimate that ~ 90% of sedimentary detritus is derived from erosion of older sediments. The material in wind-blown dust and sediments of major rivers with Nd crustal-residence ages averaging ~ 1.75 Ga (Goldstein et al., 1984; Grousset et al., 1988; Goldstein and Jacobsen, 1988) may have experienced three to six erosion-deposition cycles. The Orinoco data allow us to investigate how the Nd model ages of sediments from a major river are related to the history of magmatic and metamorphic events recorded in detfital zircons, and to the basement ages in the catchment area. A complicating factor is the averaging effect of fast sediment recycling. Although this process makes sediments useful for investigating the history of continental growth, it may obscure the relationship between fiver sediments and basement rocks. Sediments are often deposited thousands of kilometers from their source. As a consequence of orogenic uplift, or continental rift and drift, a sediment may end up in a different drainage basin or on a continental block that no longer contains the original basement precursors. In such cases, a sediment is likely to contain components that are unrelated to the crystalline basement in its fiver basin. These considerations lead to the possibility that the restricted global range of Nd isotope ratios in river sediments and wind-borne particulates reflects a well-mixed sedimentary mass (as a consequence of the efficiency of sedimentary mixing due to rapid recycling and erosion combined with continental drift) rather than to similar average basement ages on different continents.

S.L Goldstein et al. / Chemical Geology 139 (1997) 271-286

The Nd model age of 1.9 Ga of the lower Orinoco and Andean tributary sediments is within the normal range for major rivers (Goldstein et al., 1984; Goldstein and Jacobsen, 1988). The 1.0-1.2 Ga age of about 25% of Orinoco River zircons (Figs. 2 and 4) is absent from the age spectrum of orogenic events in South America but does correspond t o the Grenville orogeny in North America. The presence of these zircons might indicate a substantial North American component in the sources of the Orinoco sand. The presence of a North America-derived component would be consistent with reconstructions of Pangea in which South America is juxtaposed with eastern or southern North America in the late Paleozoic (Van Der Voo and French, 1974; Irving, 1977; Morel and Irving, 1981). If zircons are derived from the Grenville Province, then it is intriguing that other major crust-forming events of the North Atlantic continents are not represented. The Orinoco zircon ages are also distinc~ from Hercynian metasediments from Central Europe, which have significant age populations at 2.6-2.5 and 1.9-1.8 Ga (Gebauer et al., 1989). There are no Orinoco zircons with U - P b ages corresponding to the prominent 1.7-1.9 Ga Hudsonian and 2.7 Ga Superior orogenies (Fig. 5). On the other hand, absence of Hudsonian and Superior age zircons doe,,; not preclude a North American component. In certain flysch and mollasse sediments in the New England Appalachians, Grenville-age zircons are abundant and Superior and Hudsonian ages are missing (McLennan et al., 1995). 1.0-1.2 Ga zircons are also abundant in other major rivers in South America and Africa, such as the Paran~ and Congo (Goldstein and Arndt, unpublished data). On balance, it appears that the zircons from the Orinoco River were derived from a source distinct from the North Atlantic continents. If this is true, it supports alternative reconstructions of Pangea that have South America separated from Laurasia (Rowley and Pindell, 1989). We tentatively conclude that the zircons from the Orinoco River were derived from sources in the present southeru continents, but a finn conclusion awaits further work. If these results are valid for other major rivers, our investigation leads to the following implications for global sedimentary dynamics. (1) Sediments in different major rivers have different crust-formation histories, despite global similarities in Nd crustal-res-

281

idence ages and the probability of some cross-continent mixing. (2) Despite a relatively young mean depositional age which reflects efficient recycling through erosion and re-deposition, the sedimentary mass is not well-mixed on a global scale. (3) The sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin. 6. Nd model ages of shales and basement ages

Nd model ages of shales have been used to investigate the evolution of the continental crust (McCulloch and Wasserburg, 1978; O'Nions et al., 1983; All~gre and Rousseau, 1984; Miller and O'Nions, 1984; Michard et al., 1985; Miller et al., 1986), following the assumption that such sediments provide an estimate of the average age of continental crust available for erosion at any point in geological time. Because sediments are primarily derived from areas of high topography in active orogenic belts (rocks that are generally assumed to be younger than those in low-lying shield regions), these studies have led to the assumption that the mean age of the continental crust is greater than the crustal-residence ages of shale (McCulloch and Wasserburg, 1978; O'Nions et al., 1983; All~gre and Rousseau, 1984; Miller and O'Nions, 1984; Michard et al., 1985; Miller et al., 1986). Some studies have suggested extreme differences between the crustal-residence ages of the shales and the crystalline basement (e.g. Taylor and McLennan, 1985; Harris et al., 1987). The results of our study of the Orinoco basin shed light on the relationship between sediment crustal, residence ages and average basement ages. The average basement age of the whole catchment is bracketed by the ages of the Andean Province ( ~ 1.9 Ga as indicated by the Andean tributaries) and the Guyana Shield ( ~ 2.2 Ga as discussed above). A reasonable estimate is ~ 2.0-2.1 Ga, only ~ 10% older than the Nd model age of the lower Orinoco sediment. If the Orinoco is typical of large river systems, and Nd model ages of sediments from major rivers underestimate the mean basement age of the continental crust by about 10%, then the global mean of ~ 1.75 Ga for major river sediments (Goldstein et al., 1984) implies a mean continental age of ~ 1.9-2.0 Ga.

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7. Nd modal ages and crust-formation ages The pitfalls of interpreting Nd model ages as 'crust-formation' ages were discussed by Amdt and Goldstein (1987), who emphasized that Nd model ages often do not date real events, but are rather imprecise estimates of the average of crustal-residence time. Granitoids, metamorphic rocks and sediments often contain reworked older crust, and the Nd is usually derived from mixed-age sources. Concordant zircon U - P b ages, on the other hand, date specific magmatic or metamorphic events. Nd model ages should therefore be considered as 'crust-formation' ages only if there is independent evidence, in the form of U - P b zircon ages or other reliable geochronological data, that they correspond to crustformation events. The distinction between 'real' events, as documented by U - P b zircon ages, and the 'average' crustal-residence ages provided by Nd isotopes is well-illustrated by the Orinoco data. The Nd model age of ~ 1.9 Ga in the lower Orinoco and Andean tributary samples is not found in the U - P b zircon data: it probably results from admixture of detritus from the dominant 2.0-2.1 Ga terrains with smaller inputs from Archean and post-l.6 Ga basement. In contrast, the ~ 2.9 Ga Nd model age of the particulate sample from the Aro River, which drains the Archean Imataca complex, is essentially the same as the oldest U - P b zircon ages in the Orinoco sand, and the ~ 2.1-2:3 Ga Nd model ages from tributaries draining the Trans-Amazonian Shield are similar to the large population of 2.0-2.3 Ga zircons. In sediments from the lower Orinoco and Andean tributaries, the Nd model ages are average crustal-residence ages. In the Shield tributaries they coincide with crust-formation events. A compilation of literature data for North America (Fig. 5) shows many Nd model ages between 2.0 and 2.5 Ga but very few zircon ages in this range. Bowring et ai. (1989) have argued for a large amount of continent formation in North America between 2.0 and 2.4 Ga, primarily on the basis of the large number of Nd model ages in this range. A strong case for significant continental growth between 2.0 and 2.4 Ga in North America can be made only when significant numbers of zircons with these ages are found. Until then, these Nd model ages are best

interpreted as the average age of sources with different crust-formation ages (Nelson and DePaolo, 1985; Patchett and Amdt, 1986).

8. Zircon age distributions and episodic continental growth It has been a matter of debate whether the continents grew mainly through arc magmatism at convergent plate margins or by other mechanisms (Hargraves, 1976; Windley, 1981; Kr~iner, 1984; Taylor and McLennan, 1985; Ellam and Hawkesworth, 1988; Stein and Hofmann, 1994; Stein and Goldstein, 1996). Present rates of continent addition at convergent plate margins extrapolated to 4 Ga account for less than half of the mass of continental crust (Reymer and Schubert, 1984). The problem of accommodating arc accretion rates and continental growth is a general one that cannot be resolved by appealing to faster rates of sea-floor spreading and convergence in the past; although a progressive decrease in plate convergence rates through geological time might account for the present mass of the continents, the mean age of the continental crust would then be older than current estimates (All~gre and Rousseau, 1984; Goldstein et al., 1984; Miller et al., 1986; and this study) of 1.9-2.3 Ga. Evidence that large portions of Early Precambrian continents formed during short periods of < 200 Ma duration, combined with the apparent existence of age gaps in the geological record, have been taken to indicate that intensive episodes of continent growth alternated with global periods of tectonic quiescence during the Archean and Early Proterozoic (Condie, 1976; Moorbath, 1977; Reymer and Schubert, 1984, 1986; Taylor and McLennan, 1985). On the other hand, Gurnis and Davies (1986) showed that continuous continental accretion combined with large-scale recycling to the mantle could leave a record of irregular age distributions within individual continents and a record of apparently episodic crustal growth. Although U - P b zircon ages do not provide direct information on the timing of continent formation, they record the durations and patterns of orogenic events, and thus provide information that can be used to address issues relating to crustal addition rates. The Trans-Amazonian-Birimian orogeny, at

S.L Goldstein et al. / Chemical Geology 139 (1997) 271-286

~ 2.1 Ga, lasted ~ 200 Ma and produced a continent similar in size to the Superior Province in North America. This interval falls within the gap between ~ 2.7 to ~ 1.9 Ga in the age spectrum from North America, Europe, Southern Africa, and Australia (Moorbath, 1977; Patchett and Amdt, 1986; McCulloch, 1987). Other periods of intensive crust-formation occurred between 1.0 and 1.2 Ga in eastern North America, between 0.7 and 0.9 Ga in the Middle East (Reymer and Schubert, 1986; Stein and Goldstein, 1996), and during the Phanerozoic in the North American Cordillera (Samson and Patchett, 1991). It is possible, however, that the existing record of episodic growth is derived largely from an incomplete data set, and that further studies would fill the gaps. Consideration of accretion rates in individual crustal-age provinces suggests that much of the continental crust formed during discrete, intensive crust-formation episodes, even if it transpires that there are no global gaps in the geological record. The North Atlantic and Australian cratons, which accreted primarily during two ~ 200 Ma intervals at 2.6-2.8 Ga and 1.7-1.9 Ga, comprise more than 35% of all the continental crust known to have formed between 3.0 :rod 1.5 Ga. Cratons having the Guyana-Birimian shield pattern with peaks at ~ 2.8 Ga and 2.0-2.2 Ga comprise a further ~ 25%. The total duration of crustal accretion represented by cratons with North Atlantic-Australian or GuyanaBirimian shield patterns is 500-600 Ma, leaving 900-1000 Ma to grow the remaining 40%. If this remainder grew at a rate similar to the Guyana-Birimian or North Athmtic shields, they require only 200-400 Ma, leaving 600-800 Ma of apparent quiescence. This suggests that even when the coverage is extended to all continents, episodic crust-formation will remain the distinguishing characteristic of continental accretion. Crustal growth mechanisms consistent with episodic crustal additions include lateral accretion of multiple island arcs (Samson and Patchett, 1991; SengiSr et al., 1993), or accretion of oceanic plateaus and continental flood basalts which erupt when 'plume heads' reach the surface of the Earth (e.g. Campbell and Griffiths, 1990). Recent studies concluded that much of the Superior, Birimian, and Pan-African provinces, covering an age range from

283

2.7 to 0.7 Ga, formed originally as oceanic plateaus (Abouchami et al., 1990; Boher et al., 1992; Kimura et al., 1993; Desrochers et al., 1993; Stein and Goldstein, 1996). The Orinoco zircons display two very different U - P b age patterns, indicating episodic orogeny prior to 1.6 Ga ~and semi-continuous orogeny thereafter (Fig. 2). A Middle Proterozoic change in zircon age patterns appears global in scope. In North America, a change from episodic to semi-continuous patterns occurs at ~ 1.9 Ga (Fig. 5). This shift in zircon age patterns may indicate a change in orogenic style between the Archean-Early Proterozoic and the Middle Proterozoic and more recent times, perhaps reflecting a greater influence of plume-related tectonic events during the earlier period.

9. Conclusions The combination of SmLNd isotopic analyses of river sediments and U - P b dating of detrital zircons provides information about crustal-residence ages and the timing of thermal events, thereby furnishing a means to address some important questions surrounding the evolution of the continents. Our analyses of minerals and bulk sediments from the Orinoco River and tributaries show that: (1) The zircon age pattern fingerprints the tectonic history of the basement in the Orinoco basin. The major portion formed during the Trans-Amazonian orogeny at 2.0-2.1 Ga ago; smaller portions formed earlier, at ~ 2.8 Ga, and later, at < 1.6 Ga. These ages differ from those of North America where ~ 2.7 and ~ 1.8 Ga were times of major continent growth. The absence of U - P b ages fitting the North American pattern may indicate isolation of the two continents throughout geological history. (2) The distinctly different zircon age patterns from different continents shows that, although the Nd model ages of sediments from major rivers on many continents are similar to that of the Orinoco sediment, they reflect distinctly different crustal growth histories. (3) Nd model ages of the lower Orinoco and Andean tributary sediments have no direct relationship to any orogenic event, but are the average crustal-residence times o f detritus derived from

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sources having a range of crust-formation ages. In tributaries draining the Archean Imataca and the Early Proterozoic Trans-Amazonian provinces, on the other hand, the Nd model ages coincide with U - P b zircon ages, which indicates that these are 'crust-formation' ages. (4) Nd model ages of the lower Orinoco and Andean tributary sediments are similar to the average crustal-residence age of the basement of the Orinoco basin, underestimating it by only about 10%. If this holds in other river basins, Nd model ages of clastic sediments provide a close estimate of the average age of the continental crust, which is ~ 2.0 Ga. (5) Discrete zircon U - P b age populations at ~ 2.8 Ga and 2.0-2.2 Ga can be attributed to major orogenic events, each of which lasted less than 200 Ma. These appear to coincide with major crust-formation events and indicate that intense, short episodes appear to be a distinguishing feature of continent accretion.

Acknowledgements This paper is dedicated to AI Hofmann in honor of his receipt of the 1996 V.M. Goldschmidt Medal and in appreciation for his support and encouragement of geochemical research on a wide range of subjects in the Geochemistry Division of MPI-Mainz. We thank Bill Compston for access to the SHRIMP ion probe, and Ian Williams, Trevor Ireland, and the late Robert Hill for help at ANU. The manuscript benefited from reviews by Scott McLennan, Urs Schaltegger, and an anonymous referee of a previous version of this paper. SLG and RFS thank R. Herrera and others of the Instituto Venezolano de Investigaci6nes Cientfficas (IVIC) in Caracas for their hospitality and logistical help during fieldwork.

References Abouchami, W., Boher, M., Michard, A., Albar&le, F., 1990. A major 2.1 Ga old event of mafic magmatism in West Africa: an early stage of crustal accretion. J. Geophys. Res. 95, 17606-17629. Ali~gre, C.J., Rousseau, D., 1984. The growth of the continent through geological time studied by bid isotope analysis of shales. Earth Planet Sci. Lett. 67, 19-34.

Armstrong, R.L., 1981. Radiogenic isotopes: the case for crustal recycling on a near-steady-state no-continental-growth earth. Philos. Trans. R. Soc. London A301, 433-471. Armstrong, R.L., 1991. The persistent myth of crustal growth. Aust. J. Earth Sci. 38, 613-630. Arndt, N.T., Goldstein, S.L., 1987. Use and abuse of Nd model ages. Geology 15, 893-895. Boher, M., Abouchami, W., Michard, A., Albar~de, F., Amdt, N., 1992. Crustal growth in West Africa at 2.1 Ga. J. Geophys. Res. 97, 345-369. Bowring, S.A., King, J.E., Housh, T.B., Isachsen, C.E., Podosek, F.A., 1989. Neodymium and lead isotope evidence for enriched early Archean crust in North America. Nature 340, 222-225. Campbell, I.H., Griffiths, R.W., 1990. Implications of mantle plume structure for the evolution of flood basalts. Earth Planet. Sci. Lett. 99, 79-93. Compston, W., Kinny, P.D., Williams, I.S., Foster, J.J., 1986. The age and Pb loss behaviour of zircons from the Isua supracrustal belt as determined by ion microprobe. Earth Planet. Sci. Lett. 80, 71-81. Condie, K.C., 1976. Plate Tectonics and Crustal Evolution. Pergamon, New York, 288 pp. Cordani, U.G., de Brito Neves, B.B., 1982. The geologic evolution of South America during the Archean and early Proterozoic. Rev. Bras. Geocienc. 12, 78-88. Davis, D.W., Hirdes, W., Schaltegger, U., Nunoo, E.A., 1994. U-Pb age constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa. Precambrian Res. 67, 89-107. Davis, D.W., Pezzutto, F., Ojakangas, R.W., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses; implications for Archean sedimentation and tectonics in the Superior Province. Earth Planet. Sci. Lett. 99, 195-205. Desrochers, J.-P., Hubert, C., Ludden, J.N., Pilote, P., 1993. Accretion of Archean oceanic plateau fragments in the Abitibi greenstone belt, Canada. Geology 21,451-454. Ellam, R.M., Hawkesworth, C.J., 1988. Is average continental crust generated at subduction zones?. Geology 16, 314-317. Fyfe, W.F., 1974. Archean tectonics. Nature 249, 338. Garrels, R.M., MacKenzie, F.T., 1971. Evolution of the Sedimentary Rocks. W.W. Norton and Co., New York, 394 pp. Gandette, H.E., Vitrac-Michard, A., All~gre, C.I., 1981. North American Precambrian history recorded in a single sample: high-resolution U-Pb systematics of Potsdam sandstone detrital zircons, New York State. Earth Planet. Sci. Lett. 54, 248-260. Gebaner, D., 1993. Ultramafic rocks in the Pre-Mesozoic basement of the central and external Western Alps. In: J.F. Von Raumer and F. Neubauer (Editors), Pre-Mesozoic Geology in the Alps. Springer-Verlag, Berlin. Gebaner, D., Williams, I.S., Compston, W., GrUnenfelder, M., 1989. The development of the Central European continental crust since the Early Archean based on conventional and ion-microprobe dating of up to 3.84 Ga old detrital zircons. Tectonophysics 157, 81-96.

S.L Goldstein et al. / Chemical Geology 139 (1997) 271-286 Gibbs, A.K., Barton, C.N., 1983. The Guiana Shield reviewed. Episodes 2, 7-14. Gibbs, A.K., Olszewski Jr., W.J., 1982. Zircon U-Pb ages of Guyana greenstone gneiss terrane. Precambrian Res. 17, 199214. Gibbs, A.K., Wirth, K.IL, 1985. Origin and evolution of the Amazonian craton (abstract). In: LPSI (Editor), Early Crustal Genesis: The World's Oldest Rocks, Godth~b, Greenland. pp. 87-91. Gibbs, A.K., Montgomery, C.W., O'Day, P.A., Erslev, E.A., 1986. The Archean-Proterozoic transition: evidence from the geochemistry of me~tsedimentary rocks from Guyana and Montana. Geochim. Cosmochim. Acta 50, 2125-2141. Goldstein, S.J., Jacobsen, S.B., 1988. Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution. Earth Planet. Sci. Lett. 87, 249-265. Goldstein, S.L., O'Nions, R.K., Hamilton, P.J., 1984. A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems. F,arth Planet. Sci. Lett. 70, 221-236. Grousset, F.E., Biscaye, P.E., Zindler, A., Prospero, J., Chester, R., 1988. Neodymium isotopes as tracers in marine sediments and aerosols: North Atlantic. Earth Planet. Sci. Left. 87, 367-378. Grnau, G., Martin, H., L~veque, B., Capdevila, R., 1985. Rb-Sr and Sm-Nd geochronology of Lower Proterozoic granitegreenstone terrains in French Guiana, South America. Precambrian Res. 30, 63-80. Gurnis, M., Davies, G.F., 1986. Apparent episodic crustal growth arising from a smoothly evolving mantle. Geology 14, 396399. Hargraves, R.B., 1976. Precambrian geologic history. Science 193, 363-371. Harris, N.B.W., Hawkesworth, C.J., Van Calsteren, P., McDermott, F., 1987. Evollution of continental crust in southern Africa. Earth Planet. ~ci. Lett. 83, 85-93. Huhma, H., Claesson, S., FAnny, P.D., Williams, I.S., 1991. The growth of Early Proterozoic crust: new evidence from Svecofennian delrital zircons. Terra Nova 3, 175-178. Hurley, P.M., Fairbairn, H.W., Gandette, H.E., 1976. Progress report on Early Archean rocks from Liberia, Sierra Leone, and Guyana, and their general stratigraphic setting. In: B.F. Windley (Editor), The Early History of the Earth. Wiley-Interscience, London, pp. 511-535. Ireland, T.R., 1992. Crns~tal evolution of New Zealand: evidence from age distributions of detrital zircons in Western Province paragneisses and Torltesse greywacke. Geochim. Cosmochim. Acta 56, 911-920. Irving, E., 1977. Drift of major continental blocks since the Devonian. Nature 270, 304-309. FAmura, G., Ludden, J.N., Desrochers, J.-P., Hori, R., 1993. A model of ocean-crnst accretion for the Superior province, Canada. Lithos 30, 337-355. Krtner, A. (Editor), 1984. Evolution, Growth, and Stabilization of the Precambrian Lithosphere. Physics and Chemistry of the Earth, 15. Pergamon Press, Oxford, pp. 69-106. I.edent, D., Patterson, C., Triton, G.R., 1964. Ages of zircon and

285

feldspar concentrates from Northern American beach sand. J. Geol. 72, 112-122. McCulloch, M.T., 1987. Sm-Nd isotopic constraints on the evolution of Precambrian crust in the Australian continent. In: A. Krb'ner (Editor), Proterozoic Lithospheric Evolution. Am. Geophys. Union, Geophys. Monogr., pp. 115-130. McCulioch, M.T., Wasserburg, G.J., 1978. Sm-Nd and Rb-Sr chronology of continental crust formation. Science 200, 10031011. McLennan, S.M., Compston, W., Bock, B., Hemming, S.R., 1995. Age distribution of detrital zircon in North American foreland sedimentary rocks of the Taconian and Acadian orogenies. V.M. Goldschmidt Conf. Abstr., p. 72. Michard, A., Gurriet, P., Soudant, M., Albar~de, F., 1985. Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta 49, 601-610. Miller, R.G., O'Nions, R.K., 1984. The provenance and crustal residence ages of British sediments in relation to paleogeographic reconstructions. Earth Planet Sci. Lett. 68, 459-470. Miller, R.G., O'Ni0ns, R.K., Hamilton, PJ., Welin, E., 1986. Crustal residence ages of clastic sediments, orogeny, and continental evolution. Chem. Geol. 57, 87-99. Milliman, LD., Meade, R.H., 1982. World-wide delivery of river sediment to the oceans. J. Geol. 91, 1-21. Montgomery, C.W., 1979. U-Ph geochronology of the Arcbean Imataca Series, Venezuelan Guyana Shield. Contrib. Mineral. Petrol. 69, 167-176. Montgomery, C.W., Hurley, P.M., 1978. Total-rock U-Pb and Rb-Sr systematics in the Imataca Series, Guyana Shield, Venezuela. Earth Planet. Sci. Lett. 39, 281-290. Moorbath, S., 1977. Ages, isotopes, and evolution of Precambrian continental crust. Chem. Geol. 20, 151-187. Moorbath, S., Taylor, P.N., 1981. Isotopic evidence for continental growth in the Precambrian. In: A. Kri~ner (Editor), Precambrian Plate Tectonics. Elsevier, Amsterdam, pp. 491-525. Morel, P., Irving, E., 1981. Paleomagnetism and the evolution of Pangea. J. Geophys. Res. 86, 1858-1872. Nelson, B.K., DePaolo, D.J., 1985. Rapid production of continental crust 1.7-1.9 Ga ago: Nd and Sr isotopic evidence from the basement of the North American mid-continent. Geol. Soc. Am. Bull. 96, 746-754. O'Nions, R.K., Hamilton, P.J., Hooker, P.J., 1983. A Nd-isotope investigation of sediments related to crustal development in the British Isles. Earth Planet. Sci. Lett. 63, 229-240. Onstott, T.C., Hall, C.M., York, D., 1989. 4°Ar/39Ax Thermochronometry of the Imataca Complex, Venezuela. Precambrian Res. 42, 255-291. Patchett, P.J., Arndt, N.T., 1986. Nd isotopes and tectonics of 1.9-1.7 Ga crustal genesis. Earth Planet. Sci. Lett. 78, 329338. Reymer, A., Schubert, G., 1984. Phanerozoic addition rates to the continental crust and crustal growth. Tectonics 3, 63-67. Reymer, A., Schubert, G., 1986. Rapid growth of some major segments of continental crust. Geology 14, 299-302. Rowley, D.B., Pindell, J.L., 1989. End Paleozoic-early Mesozoic

286

S.L Goldstein et a L / Chemical Geology 139 (1997) 271-286

western Pangean reconstruction and its implications for the distribution of Precambrian and Paleozoic rocks around mesoAmerica. Precambrian Res. 42, 4l 1-444. Samson, S.D., Patchett, P.J., 1991. The Canadian Cordillera as a modem analogue of Proterozoic crustal growth. Aust. J. Earth Sci. 38, 595-611. Seng&, A.M.C., Natal'in, B.A., Burtman, V.S., 1993. Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia. Nature 364, 299-307. Stein, M., Hofmann, A.W., 1994. Mantle plumes and episodic crustal growth. Nature 372, 63-68. Stein, M., Goldstein, S.L., 1996. From plume head to continental lithosphere in the Arabian-Nubian Shield. Nature 282, 773778. Tarney, J., Dalziel, I.W,D., de Wit, M.J., 1976. Marginal basin 'Rocas Verdes' complex from S. Chile: a model for Archean greenstone belt formation. In: B.F. Windley (Editor), The Early History of the Earth. Wiley-Interscience, London, pp. 131-146. Tatsumoto, M., Patterson, C., 1964. Age studies of zircon and feldspar concentrates from the Franconia sandstone. J. Geol. 72, 232-242. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp.

Teixeira, W., Tassinari, C.C.G., Cordani, U.G., Kawashita, K., 1989. A review of the geochronology of the Amazonian Craton: tectonic implications. Precambrian Res. 42, 213-227. Van Der Voo, R., French, R.B., 1974. Apparent polar wandering for the Atlantic-bordering continents: late Carboniferous to Eocene. Earth Sci. Rev. 10, 99-119. Veizer, J., Jansen, S.L., 1979. Basement and sedimentary recycling and continental evolution. J. Geol. 87, 341-370. Veizer, J., Jansen, S.L., 1985. Basement and sedimentary recycling-2: time dimension to global tectonics. J. Geol. 93, 625643. White, W.M., Patchett, P.J., 1984. Hf-Nd-Sr and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution. Earth Planet. Sci. Lett. 67, 167-185. Windley, B.F., 1981. Precambrian rocks in the light of the plate tectonic concept. In: A. KrSner (Editor), Precambrian Plate Tectonics. Elsevier, Amsterdam, pp. 1~-20. Zhao, J.X., McCulloch, M.T., Bennett, V.C., 1992. Sm-Nd and U-Pb isotopic constraints on the provenance of sediments from the Amadeus Basin, central Australia: evidence for REE fractionation. Geochim. Cosmochim. Acta 56, 921-940.