ELSEVIER Earth and Planetary Science Letters 150 (1997) 427-441

EPSL

Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: new constraints for Late Proterozoic-Early

Paleozoic paleocontinental reconstructions of the Americas

Pedro A. Restrepo-Pace a,‘, Joaquin Ruiz a3 * , George Gehrels a, Michael Costa b a Department of Geosciences, Unioersity of Arizona, Tucson. AZ 85721, USA

b Institute du Mine’ralogie, Unioersit6 de Lmsanne, 1015 Lausanne. Switzerland

Received 25 February 1997; revised 8 May 1997; accepted 8 May 1997

Abstract

New U-Pb zircon crystallization ages and “Ar/ 39Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. eNd (t = 1.1 Ga) for

these rocks range from - 3.9 to + 0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga erogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (T,,) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions. 0 1997 Elsevier Science B.V.

Keywords: Northern Andes: Grenvillian Orogeny; geochronology; neodymium; reconstruction

1. Introduction

Recent paleocontinental reconstructions of the

Americas based on correlating age, isotopic tracer, lithostratigraphic, fauna1 and paleomagnetic data [ l- 81 have focused on the possible tectonic interactions

Corresponding author. Tel.: + 1 602 621 2365. Fax: + 1 602

621 2672. E-mail: jruiz@geo.arizona.edu

’ Current address: Conoco Inc., Advance Exploration Organiza-

tion, 600 N. Dairy Ashford, Houston, TX 77252-2197, USA.

between western South America and northeastern

North America in the late Proterozoic-Early Paleo- zoic. Recently [9,10] geological affinities between

basement rocks of northern South America and east-

em Mexico have also been suggested. However, these affinities need further support. The presence of

Grenville age crust in eastern Mexico has been well

documented [9,11,12], yet its possible counterpart in northwestern South America has been less con- strained because of the strong Mesozoic-Cenozoic tectonic overprints on older rocks. The purpose of this paper is to review the existing age data for the

0012-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZI s0012-821x(97)0009l-5

428 P.A. Restrepo-Pace et al./Earth and Planetarv Science Letters 150 (1997) 427-441

basement rocks of the Colombian Andes and to report new ages which confirm the existence of a N 1.1 Ga Orinoquian tectonometamorphic episode in northern South America (after Martin-Bellizzia [ 131). We also provide Nd isotopic data, which, together with regional geologic relationships, constrain the Late Proterozoic-Early Paleozoic paleogeography of the Americas.

2. Regional setting

The northern termination of the Andes of South America, in Colombia, branch into three ranges trending north-northeast and separated by narrow valleys (Fig. 1). Here the Andean system was built

by successive discrete erogenic episodes which be- gan in the late Cretaceous and climaxed during the late Tertiary Andean Orogeny. Uplift initiated with the collision of terranes with oceanic affinity, de- rived from the Pacific, and continues today as a result of the ongoing convergence between the South American and Nazca plates and by the collision of the Panama arc with South America [14-161. As a result of the Andean uplift, Mesozoic rift and peri- cratonic sedimentary sequences were inverted and partially removed unveiling the pre-Mesozoic core of the northern Andes, mainly along the eastern and central range, of Colombia. The Andean domain is separated from the eastern Guiana shield domain by the NE-SW trending Borde Llanero fault system. Crystalline rocks of the Guiana shield are exposed in

ibbean

_&B

Santa Marta Massif

/ Guajira

Fig. 1. Andean physiography of Colombia. A = Eastern range; B = Magdalena Valley; C = Central range; D = Western range; E = Merida

Andes; F = Perija range; G = Santa Marta uplift. Main map shows the location of basement exposures in the northern Andes and major

fault systems: I = Borde-Llanero fault system; 2 = Santa Marta-Bucaramanga fault; 3 = Pamplona-Cubugon-Mercedes thrust-front and T&chira depression; 4 = Oca fault system.

P.A. Restrepo-Pace et al./Earth and Planetary Science L.etter.7 150 (1997) 427-441 429

the vicinity of the Colombian-Venezuelan-Brazilian

border. The upthrusted basement rocks of the Colombian

Andes consist primarily of high grade metamorphic

rocks of Precambrian age, low grade metamorphic pelitic rocks of Early Paleozoic age and marine

sedimentary rocks of Cambro-Ordovician, Devonian

and Permo-Carboniferous age. MacDonald and Hur-

ley [ 171, Goldsmith et al. [18], Ward et al. [ 191 and Tschanz et al. [20,21] have reported the existence of

Precambrian rocks. K/Ar and Rb/Sr ages of base- ment rocks from the Garz6n Massif led Alvarez and

Cordani [22], Alvarez [23,24], Kroonenberg [25] and

Priem et al. [26] to suggest that sporadic basement exposures in the Colombian Andes constituted rem- nants of a Grenville age metamorphic belt. A compi-

lation of previously obtained ages is summarized in

Table 1.

2. I. Geology of Andean basement exposures

Three significant exposures of Andean basement, known as the Garzbn, Santander and Santa Marta

massifs, are shown in Fig. 2.

The Garzcin Massif is the most extensive and best

known Andean basement exposure in Colombia (Figs. 1 and 2a). It is bounded on the east by the

east-verging Borde-Llanero fault system that places the massif over Tertiary elastics of the Andean fore-

land. On the west, the massif is bounded by the west

verging Garzbn-Suaza thrust fault placing the massif

basement over Meso-Cenozoic sedimentary fill of the Upper Magdalena Valley. Triassic-Jurassic plu-

tons intrude the basement complex at its western

margin. Garz6n basement metamorphic rocks extend west beneath the Magdalena Valley fill and into the

eastern margin of the Central Andean range. Expo-

sures everywhere are limited to road cuts and river gorges since the area is covered with lush vegetation.

Detailed petrology of the Garz6n basement was con-

ducted by Radelli [27] and later by Kroonenberg [25,28], who divided the massif’s metamorphic rocks

into two petrotectonic units: the dominant (about

80%) Garz6n Group, consisting of banded felsic and

mafic granulites and the less extensive Guapot6n and

Mancagua gneisses consisting of pegmatitic augen ortho-gneisses. In detail, Garz6n Group rocks are

Table I Previously reported ‘Grenvillian’ ages in the Colombian Andes (data from Maya [61])

Location Lithology Method and type Age (Ma)

Reference

Garz6n Massif

Garz6n Massif

Garzdn Massif

Car&n Massif

Car&n Massif

Garzdn Massif

Garzbn Massif

Garz6n Massif

Car&n Massif

Garz6n Massif

Garz6n Massif

Garz6n Massif

Garz6n Massif

Santander Massif

Santa Marta Massif

Santa Marta Massif

Santa Marta Massif

Santa Marta Massif

Santa Marta Massif

Guajira

Granulite

Pegmatitic granitoid

Pegmatitic granitoid

Marble

Granulite

Mafic granulite

Clinopx. amphibolite

Banded amphibolitic granulite

Granulite

Granulite

Granulite

Granulite

Granitic augen-gneiss

Amphibolitic gneiss

Granulite

Granulite

Potasic granite

Quartz-perthite gneiss

Granulite

Leuco granite

Rb/Sr whole rock

Rb/Sr whole rock

Rb/Sr K-spar

K/Ar phlogopite

Rb/Sr K-spar

K/Ar hornblende

K/Ar hornblende

K/Ar hornblende

Rb/Sr whole rock

Rb/Sr whole rock

Rb/Sr whole rock

Rb/Sr whole rock

Rb/Sr whole rock

K/Ar hornblende

Rb/Sr whole rock

K/Ar hombiende

Rb/Sr whole rock

Rb/Sr whole rock

Rb/Sr whole. rock

U-Pb zircon

601 + 56 [231

847 + ? [261 895 + 16 [26] 912 + 35 [261 918 + 27 [Xl 925 f 50 [241 971+ 19 [261 lOOOk [Xl 1150+ 180 1231 1150+70 1231 1172+90 1261 1180+? [23] 1595 f 300 [Xl 945 + 40 [191 752 + 70 El1 940 f 30 [211 1300&? [67] 1300+ 100 [211 1400 f ? [I71 1250 f ? @I

430 PA Reswepo-Pace et al. /Earrh and Planetary Science Letters 150 (1997) 427-441

varied and comprise banded chamockites and pelitic

gneisses alternating with mafic granulites and/or marbles, metacalcsilicate layers and cross-cut by

aplite dykes [28]. Subordinate amphibolites and lo- tally othopyroxene homblendite and meta-ultramafic lenses are also found. Ubiquitous orthopyroxene and

i L

=I Catibbam Sm - Dibutta

73ow Y --.-.

9 , 74aw I t

hta Mmta mass$

Map units ;

l?ldz.2 5fii@* Jurassic continenial series

CII : : : : Triass-Jur plutonic rocks

Mid-Late Paleazoic marine sediments

= ------Early Paleozoic peliiic schists

Precambrian metixnorphic rocks

Fig. 2. Simplified geologic maps of (a) Garz6n Massif; (b) Santander Massif and (c) Santa Marta Massif. Maps also show sample location and names of localities quoted in text.

P.A. Restrepo-Pace et al/Earth and Planetary Science Letters 150 (1997) 427-441 431

strongly perthitic and/or anthiperthitic feldspars in-

dicate granulite grade P-T conditions. The absence of cordierite in the pelitic rocks and the presence of olivine plus plagioclase in the mafic suite suggest

intermediate pressures [28]. Retrogression is evident in some samples from the presence of hornblende

rims around orthopyroxene. Gneisses of the

Guapoton-Mancagua unit consist of gray to pink

colored, hornblende- and biotite-bearing, foliated granitic pegmatitic gneisses. The Guapoton-

Mancagua augen gneisses are concordantly foliated with the hosting Garzon Group rocks and are re-

garded as being syntectonic granitoids [25,28]. As a whole, the Garzon Massif lithologies are quartz-

feldspathic rich, which, together with the calcareous and talc-silicate rocks. suggest a continental sedi-

mentary and volcanic origin. On the eastern flank of

the Garzon Massif, a Cambrian shelfal sequence is

exposed at La Uribe and Duda River [29-311. The basal contact of this sequence is not exposed but it is

assumed to be discordantly overlying the Garzon metamorphics [30,31]. The Cambrian sequence is

1500 m and consists of limestone, dolomite, serpen-

tinized diabase and towards the top, sandstone and shale beds; Ehmmlia [29] and Paradoxides [32] trilo- bites have been recovered from this sequence.

North of the Garzon Massif is the Santander

Massif (Figs. 1 and 2b), which has one the most complete late Proterozoic and Phanerozoic rock

records of the northern Andes [33]. The Santander

Massif is bounded on the west by the left-lateral Bucaramanga-Santa Marta fault and on the east by

the east vergent Pamplona-Cubugon-Mercedes

thrust system that places the Santander Massif over the Tachira depression, the southern Venezuelan MCrida Andes and also over the petroleum-rich Cata-

tumbo basin to the north. The oldest exposed rocks are the Bucaramanga gneisses, which consist of quartz-feldspathic gneisses with subordinate interlay-

ered amphibolitic gneisses and diopside-tremolite- epidote-bearing metacalcsilicate rocks, and the Sil- gara schists. The Bucaramanga gneiss also com-

monly contains the association quartz-plagioclase- biotite-andalusite-cordierite f sillimanite or quartz-plagioclase-biotite-sillimanite-potassium feldspar + garnet, indicating high metamorphic grade

[19]. Given the overall preponderance of pelitic gneisses and metacalcsilicate rocks, the Bucara-

manga gneisses are considered to be mainly of sedi-

mentary origin. The contact between the Bucara- manga gneisses and the low to medium metamorphic grade Silgara schists remains unclear and currently appears to be defined by the biotite-sillimanite iso-

grad. Ages obtained from synkinematic plutons em-

placed within the schists suggest that they formed

during the Late Ordovician Caparonensis regional

metamorphic event [33]. The overlying sedimentary cover rocks include Silurian (?&Devonian and

Permo-Carboniferous marine sediments, a thick

Jurassic continental molasse, Cretaceous marine sed- iments and Tertiary continental sediments. Calc-al- kaline plutons of Triassic-Jurassic age intrude the

basement complex and have strongly perturbed all

pre-Jurassic isotopic systems [19,33].

The third exposure of basement is the Santa Marta

Massif (Figs. 1 and 2c), which is an isolated, triangu- lar-based uplift bounded on the north by the right-

lateral Oca fault. on the west by the left-lateral Santa

Marta-Bucaramanga fault and on the southeast by the Cesar Valley. The massif consists of three north-

east-southwest trending tectonostratigraphic belts

[20,21]. The two younger northwestern belts are low grade metamorphic pelitic schists intruded by Trias-

sic-Jurassic and Tertiary talc-alkalic plutonic rocks.

The southeastern belt consists of granulite facies migmatites. Exposed gneisses in the Santa Marta

massif consist of interlayered pelitic and quartz-

feldspathic garnet-orthopyroxene-biotite granulites, which is the dominant lithology, and

orthopyroxene-clinopyroxene metabasites and hom-

blende-clinopyroxene mafic gneisses. Similar litho-

logical associations and equivalent metamorphic grade rocks are found in the Garzon massif. How-

ever, Santa Marta also has anorthosites, which are exposed at the western portion of the high metamor-

phic grade belt [34,20].

3. Analytical methods

Sampling was carried out according to guidelines described in Yafiez et al. [9], and analytical methods are described in detail in Restrepo-Pace [33]. Many of the samples analyzed were collected at the same sites as previous studies (see Table 1). This proce-

432 P.A. Restrepo-Pace et al./Earth and Planetayv Science Letters 150 (1997) 427-441

dure allowed us to compare our results with those previously obtained.

Mineral separations were done at the University

of Arizona, using heavy liquids only for the zircon separates. All minerals were hand picked and in- spected to ensure purity. U/PI, analyses were per-

S;arron Masslf

i I

Santander Massif

OT-1 Hornblende Amphibolitic @Wiss

Santa Marta Massif

hegrated Age = 174 f3 Mn

cumuLtiw %39Ar Released

Chamickilic Gtiss

Fig. 3. @Ar/ 3gAr apparent age spectra for selected basement rocks of the Colombian Andes.

0.14 I I I , I , ,

1.4 1.6 1.8 2.0 Z.2

0.32

Upper intercept = 1088 rt 6 Ma - Lower intercept = 238 + 79 Ma ’ RGl

P.A. Restrepo-Pace et al./Earth and Planetary Science Letters 150 (1997) 427-441

0.20

“W 0.18

=IJ cl.17

0.28

0.170 I I I I I, I I I t I, J

1.74 1.78 1.8Z 1.86 I.‘)0 1.94 1.98 2.02

20’Pb*P)5U

0.24

0.0s II 1 2 3 4

“‘Pb*/=W

Fig. 4. U/Pb zircon concordia diagrams for selected basement rocks of the Colombian Andes.

formed as described in Gehrels [35] and Sm/Nd

analyses as described in Patchett and Ruiz [ 121.

“‘Ar/ ‘9Ar analyses were carried out at the Univer- sity of Lausanne, Switzerland, as described by Costa and O’Nions [36].

4. Geochronology: results and discussion

“‘Ar/ 39Ar analyses were performed on mineral

separates from ten samples; six were from the Garz6n massif, two from the Santander massif and two from

the Santa Marta massif (Fig. 3) (for 40Ar/ 39Ar iso- topic compositions, see the EPSL Online Back- ground Dataset *). With the exception of Garzon

’ URL: http://www.elsevier.nl/locate/epsl. mirror site:

http://www.eIsevier.com/locate/epsl

sample HP-3, all the samples depict complex de-

gassing spectra, which was expected, as these rocks experienced an intense tectonothermal event during a

Triassic-Jurassic event [21,33,68]. Hornblende separates from rock sample SnAnKr-

1, a pegmatitic biotite-hornblende augen-gneiss

from the Guapoton-Mancagua unit of the Garz6n

massif, yields a staircase “‘Ar/ 39Ar apparent age spectrum (Fig. 3). Approximately 45% of 39Ar was released at N 180 Ma apparent age in the first steps of the heating experiment. During the following

steps the gas was released gradually, with the final gas discharged at an N 890 Ma apparent age. The

K/Ca ratio mimics the age spectrum. suggesting the presence of two mineral phases within the hom- blendes. No scattered electron microscopy analyses were made to determine if such was the case. By following the criteria outlined by McDougall et al. [37], this sample’s “‘Ar/ 39Ar spectra may be inter-

434 P.A. Restrepo-Pace et al/Earth and Planetary Science Letters 150 (1997) 427-441

preted as an older N 890 Ma cooling age, which approximates its metamorphic age, and a younger thermal overprint at N 180 Ma. The older cooling age is interpreted here to be related to the Orino- quiense metamorphic event at N 1.1 Ga, while the younger age is related to partial resetting due to me intrusion of the Triassic-Jurassic Suaza-Altamira granitoid exposed along the western margin of the massif.

Samples HP-3, G-20, G-17 and G-2 are from the Garzon Group and HP-3 from the Higado creek, in the eastern flank of the central Andean range (Fig. 2a). In the Higado creek, the gneisses of the Garzon Group underlie fossiliferous sedimentary rocks of Llanvimian age (- 475 Ma) [38]. A hornblende separate from sample HP-3, an amphibolitic gneiss, yields a 40Ar/ 39Ar plateau age of 911 + 2 Ma, which is a cooling age related to the Orinoquiense orogeny. Very dark green euhedral homblendes from sample G-20, an orthopyroxene-bearing homblendite, dis- played a “OAr/ 39Ar saddle-shaped spectrum indicat- ing the presence of excess argon. Approximately

Table 2

50% of the 39Ar was released in the middle stages of the heating experiment. The central portion of the spectra defines a plateau age at 1074 Ma, interpreted as the cooling metamorphic age related to the Orino- quiense orogeny. Inverse isochron plots yield poor correlations, due to the overwhelming radiogenic “‘Ar component in the samples. Feldspar and biotite separates from sample G-17 and biotite from sample G-2, both garnet-orthopyroxene charnockites, yield complex 40Ar/ 39Ar apparent age spectra, which are probably a result of the relatively lower retaining temperatures for the argon system of these minerals.

Zircons were separated from sample SnAnK-1 to further corroborate the cooling ages obtained by 40Ar/ 39Ar. Under the binocular microscope the sepa- rates were crystalline euhedral and transparent pink colored crystals. U-Pb analyses of six abraded sin- gle zircons yielded an age of 1088 f 6 Ma based on two concordant grains, and a lower intercept of 238 f 79 Ma for the four discordant grains (Fig. 4, Table 2). The upper intercept is interpreted as record- ing the Orinoquiense orogeny. The lower intercept

U/PI, zircon comoosition and aee data for selected basement samnles of the Colombian Andes

Sample Weight Pb U Measured ratios Apparent ages

(ILg) (ppm) (ppm) 206/204 206/207 206/208 206 ’ /238 207 ‘/235 207 */206 *

SnAnKr-1 25 104.3 600 4200 12.79 12.5 1036 + 4 1046*4 1067+4

21 21.5 112 700 10.53 7.6 1062 + 6

34 35.9 195 10600 12.99 11.3 1088 * 4

23 72.9 385 990 11.09 11.5 1088 + 4

SMR-4 53 82.8 567 7870 13.44 19.7 906+3

14 107.5 690 4200 12.42 14.6 942 f 3

24 91.8 584 5500 12.39 18.8 963 f 3

9 120.1 723 1580 11.29 11.7 975 * 4

24 205.8 1290 13600 13.00 23.5 989 + 3

9 291.3 1600 8800 12.11 13.3 1082 * 4

33 27.6 145 6100 12.19 16.9 1146+5

RG-1 19 190.6 1750 1850 11.99 32.9 691 k3

23 117.8 1550 3060 12.11 12.9 697 & 3

26 235.2 2030 3700 12.89 46.5 743 * 4

26 301.5 2214 1290 11.31 23.9 834 4 4

13 40.0 146 450 8.10 6.1 1384 + 13

36 106.2 377 7490 10.26 20.3 1620+7

51 129.8 416 1197 9.37 13.5 1707 + 7

1066 + 8

1089 + 5

1091 f 6

936 + 3

1000+4

1023 f 4

1045 + 5

1023 k 4

1130f4

1164f5

793 f 4

815 & 4

820 + 5

921 &6

1427 + 14

1587f8

1631 f 8

1074 * 10

1092 f 4

1098 + 8

1007+3

1130+5

1155*5

1194*5

1096+4

1224 + 3

1197+4

1092+5

115oi3

1037 * 3

1137*8

1492 f 15

1543 + 2

1534 + 3

* Radiogenic Pb.

Measured ratios are uncorrected for blank, spike, or initial Pb.

Constants used: h235 = 9.8485 X lo-“, A238 = 1.55125 X lo-“, 238/235 = 137.88.

Data reduction from [69], concordia diagrams from [70]. Analytical methods described by Gehrels [35].

Samples corrected for: (1) fractionation factors of 0.14 4 O.O6%/amu for Pb and 0.04 f O.O6%/amu for U; (2) blank values of 5 pg for Pb

and 1 pg for U; and (3) initial Pb values interpreted from Stacey and Framers 1711.

P.A. Restrepo-Pace et al./ Earth and Planetary Science Letters 150 (1997) 427-441 435

may record an event associated with the late Paleo-

zoic consolidation of Pangea, although the high MSWD of 15 for the regression suggests that zircons

may have experienced a complex Pb-loss history. A

cooling curve has been constructed for the Garzon basement rocks with the most reliable data thus far gathered (Fig. 5). From this curve a simplified cool-

ing history is derived relating it to distinctive tec- tonic episodes. A protracted cooling period followed

the Orinoquian erogenic episode ( N 1.1 Ga) at an

approximate rate of 1.5-2”C/Ma then, after an

episode of plutonism and back-arc extension, a rapid Jurassic cooling period at rates above S”C/Ma fol-

lowed and finally, by an accelerated cooling period at a rate exceeding lO”C/Ma, since about 10 Ma,

related to the final uplift of the massif during the

Andean orogen y. The samples collected from the Santander Massif

were picked at the same localities sampled by Ward

et al. [ 191 who reported K/Ar mineral ages of 945 + 40 Ma. 4”Ar/ 39Ar apparent age spectra for two hornblende separates from samples OT-1 and

OT-2, amphibolitic gneisses, obtained in this study

resulted in complex patterns (Fig. 3). Both samples experienced partial argon loss at - 200 Ma, as

interpreted from the initial stages of their spectra, which is probably related to the Triassic-Jurassic

plutonic arc along the eastern range [19,33]. Both spectra also exhibit a stair-case 39Ar release pattern

with increasing apparent ages up to N 800 Ma for sample OT-1 and N 850 Ma for sample OT-2 (at 1100°C). During the final stages of the heating ex-

periment apparent ages decrease gradually. K/Ca

ratios for these samples mimic their age spectra

overall shape, implying again that perhaps more than

one mineral phase was present in the degassed sam- ples. In thin section a few hornblende grains were

associated with chlorite. The resulting age spectra,

therefore, could represent a mixing age between hornblende and younger phyllosilicates ([37], pp.

lOO- 101). These samples reveal an older argon com-

ponent with an apparent initial cooling age of at least 800 Ma that, when added to stratigraphic relation-

ships, suggest that these rocks may represent an

1000 ,

800 -

u 600 -

;i

400 -

200 -,

optTTy,. , , , , ,I 0 200 400 600 800 1000 1200

0

Age Ma = ranges associated with closure temperature and age.

Fig. 5. Temperature-time curve for the Garz6n massif. Data from [61,62] and this work. Estimated closure temperature ranges for U-Pb

zircon from [63] and all others from [64].

436 P.A. Restrepo-Pace et al. /Earth and Planetay Science Letters 150 (1997) 427-441

extension of Orinoquian rocks exposed in the Garzon Massif. Supporting this interpretation are ages for the Lajas Granite in the Perija range [39] and the Paramo Rico pluton in the Santander Massif (Grosser, writ- ten commun., 1994), which yield Orinoquian ( N 1.1 Gal U/Pb upper discordia age intercepts.

The Santander basement lithologies differ in their mineral assemblages from those of Garz6n, the for- mer attaining peak upper amphibolite low pressure- high temperature PT conditions [19,33], while the latter depicts characteristic granulite facies PT condi- tions. The high temperature-low pressure conditions of Santander prevailed during a Late Triassic-Early Jurassic intrusive-related regional metamorphic episode [33].

Samples RG-3 and RG-6, quartz-pyroxene- garnet-biotite gneisses from the Santa Marta massif yield complex saddle-shaped “‘Ar/ 39Ar apparent age spectra for their biotite separates. Inter-layered or included chlorite could account for the samples’ degassing behavior. U/PI> zircon analyses were con- ducted for two Santa Marta samples to improve the above age constraints. For sample RG-1, the nine abraded zircon grains lie along the discordia line with intercepts of 15 13 + 35 Ma and 456 f 60 Ma (Fig. 3). The significance of these intercepts is not clear, however, because of the large uncertainty of the regression (MSWD = 390), the reverse discor- dance of two grains, and the occurrence of grains near both the lower and upper intercepts. Our inter- pretation is that the upper intercept records crystal-

Table 3

lization and that the lower intercept is approximately the age of Pb loss and perhaps new zircon growth during high-grade metamorphism. The reverse dis- cordance in two grains could be explained by diffu- sion of radiogenic Pb from a high-U rim into a low-Pb core during high grade metamorphism, fol- lowed by removal of some or all of the rim material during laboratory abrasion. A crystallization age of 15 13 Ma is older than Orinoquian ages but is consis- tent with crystallization ages for rocks along the western edge of the Guiana shield [40]. Regional metamorphism at N 456 Ma during the Caparonen- sis orogeny [41,33] may relate to the lower intercept.

Dark red and rounded zircons from sample SMR- 4, a quartz-plagioclase-hornblende gneiss yield U/Pb ages between 1.0 and 1.3 Ga. The detrital character of the zircons from sample SMR-4 pre- cluded a definition of a unique crystallization or Pb-loss age.

5. Nd isotopic data, results and discussion

Nd model ages were calculated for samples from the Garzon and Santa Marta massifs and the results are summarized in Table 3 and plotted in Fig. 6. The Garzon samples show a consistent average model age of = 1.55 Ga, excluding sample G-20 from a mafic granulite with a Nd model age of 2.7. The high Sm/Nd ratio of the mafic sample, however, yields an unreliable model age.

Neodymium isotope composition data for selected basement rocks of the Colombian Andes

Sample Unit Age Sm Nd ‘47Sm/ ‘44Nd ‘43Nd/ ‘44Nd b &Nd ’ &Nd c d

(Ma) (mm) (mm) measured present initial

G-2

G-11

G-20

HP-3

HP-5

SnAnKr- 1

RG-3

RG-6

Garzon group 1100 1.37 16.78 0.1325

Garz6n group 1100 1.941 16.68 0.1885

Garz6n group 1100 1.14 3.62 0.1896

Garzdn group 1100 9.25 32.95 0.1697

Garz6n group 1100 0.18 1.587 0.18801

Guapotdn gneiss 1100 14.93 78.97 0.1143

S. Marta Massif 1100 7.38 40.42 0.1104

S. Marta Massif 1100 1.13 9.68 0.1883

0.511770 + 7

0.512032 k 5

0.5 12626 + 7

0.512472 + 10

0.512085 It. 6

0.512062 + 7

0.511872 + 8

0.511879 * 8

- 16.93

- 11.82

- 0.24

- 3.23

- 10.79

- 11.2

- 14.9

- 14.8

-0.51 1.45

-0.16 1.54

+0.6 2.71

+0.2 1.97

0.91 1.46

-0.7 1.50 - 3.9 1.72 -3.13 1.77

a Uncertainties at 20 are +0.5%.

b Ratios normalized to ‘46Nd/ ‘44Nd = 0.7219 (2~ errors reflect in-run precision).

’ &Nd = 104[(‘43Nd/ ‘44Nd(t)CHUR) - 111, using 14’Nd/ ‘44 Nd = 0.512638 as present day CHUR value, and 14’Sm/ ‘44NdCHUR =

0.1966. d Model ages calculated using equation of DePaolo [72].

P.A. Restrepo-Pace et al. / Earth and Planetary Science Letters 150 (1997) 427-441 431

Samples from the Santa Marta Massif appear to

have slightly older ( N 1.7 Ga) model ages relative to the Garzon samples (- 1.55 Ga). ,sNdtt= ,_, oa) range from - 3.9 to +0.6, suggesting mixing of older

crustal with juvenile material N 1.1 Ga ago. Our

calculated model ages closely match the crystalliza- tion ages obtained from rocks of the western Guiana shield [40,42]. These data, when added to the

8

4

0

eNd -4

-8

-12

-16

I I I I I 2.5 2.0 1.5 1.0 0.5 0.0

Nd Crustal Residence Age (Ga)

a ) n Colombian Andean basement samples

0 0.5 1.0 1.5 2.0 2.5

T DMmodd ages, Ga

cl Southwest U.S. & Mexicu

Eastem U.S.

EJ GIenviue~vinloe

w l Andes - Colombia

nobilebelt mobilebelt

Fig. 6. (a) Nd crustal residence ages for selected - 1.1 Ga age rocks of Colombia. (b) Comparison with crustal residence ages for the Grenvillian rocks of North America (modified from [65]). (c) Distribution of Nd crustal residence ages for the Grenvillian basement of

Colombia in relation to age provinces from western Guiana shield (modified from Teixeira et al., [40]).

438 P.A. Restrepo-Pace et al. / Earth and Planetary Science Letters 1.50 (1997) 427-441

quartzo-feldspathic rich lithological associations of the Orinoquiense basement rocks, suggests that these samples are a metamorphosed pericratonal sequence of mostly reworked Guiana shield rocks.

6. Tectonic implications

The geochronological data indicate unequivocally that the Garz6n basement rocks were metamor- phosed during a regional tectonothermal event which took place 1.1 Ga ago. The data still does not uniquely constrain the age of the Santa Marta and Santander massifs’ basement rocks. However, re- gional geologic relationships suggest that the Garz6n Massif extends north towards the Santander and Santa Marta massifs, representing the northernmost

exposure of Grenvillian-age basement in South America.

In South America the Orinoquiense erogenic event (ca. I. 1 Ga) is represented by an extensive metamor- phic belt sporadically exposed along the entire An- des from Colombia, continuing in Peru in the coastal Arequipa Massif [43], in the Eastern Cordillera of Peru [44,45], in eastern Bolivia at the Sunsas Belt [46], and into the northwestern Argentinean Pre- cordillera [47].

Relative paleocontinental positions following the Orinoquiense erogenic event may be inferred from the close biostratigraphic affinities between the Cam- brian shelf sequences that overlie the Grenville age basement rocks of Colombia [48,30,3 11, eastern North America, and northwestern Argentina. In Colombia

Fig. 7. Paleocontinental reconstruction for the 1.2-0.5 Ga period, showing the position of A = Amazonia with respect to L = Laurentia and

B = Baltica, modified from [2,8,66].

P.A. Restrepo-Pace et al. /Earth and Planetary Science Letters 150 (1997) 427-441 439

these rocks are marked by the presence of the trilo-

bites Ehmania [29], akin to the Argentinean Pre- cordillera Amecephalina zone (cf. [29,49]) and Paradoxides [32] of Acado-Baltic affinity, present in several Appalachian terranes [50-531. Addition-

ally, the overlying Ordovician strata in Colombia contains fauna of the Olenid-Ceratopygid province

of Whittington et al. [54], present at El Bail in

eastern Venezuela and akin to southern Mexico’s TiiiG Formation, which overlies the Oaxaca

Grenville-age basement [55-581. These rocks are

correlated by the presence of Parabolina argentina,

a zonal index fossil for the Lower Tremadoc in

northwestern Argentina. The Clarenville Fm. in Ran- dom Island-Eastern Newfoundland also yields Parabolina argentina, together with Angelina [59].

In the Ordovician sequence at the Macarena uplift, an eastern prong of the Garzdn Massif, fauna also

relate to northern Argentina-southern Bolivia and recall the Kaianella fauna of Argentina-Bolivia

[46,29]. Previous models postulating intercontinental linkages based on this type of data (e.g. [6,7]), have

failed to incorporate data from Mexico and Colom- bia.

Given the sparse paleomagnetic data for the Late Precambrian to Early Paleozoic, many scenarios can

explain the regional geological relationships outlined

here. One model, illustrated in Fig. 7, shows the South American Grenville-age basement of colli- sional origin marking the closure of a Wilson Cycle,

as suggested recently by Hoffman [2] and Dalziel [3]. Characteristic high metamorphic grade basement,

similar petrologic assemblages and matching isotopic

signatures of Colombian and North American

Grenvillian-age rocks argue in favor of a collisional

origin. In South America, the latter is reinforced by the magnitude of the Orinoquiense event, as evi-

denced by 0.9-l .2 Ga cooling ages obtained along

the western edge of the Guiana shield in northwest- em South America, deep inland from the mobile Andean basement [25,60].

Acknowledgements

We thank Peter Coney, Rob van der Voo, Sa- lomon Kroonenberg and William MacDonald for the revision of previous versions of this manuscript. This

work was partly supported by The National Science

Foundation through grant EAR 935061, the W.M. Keck Foundation, and The National Hispanic Schol-

arship Fund. [RF’]

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ill

121

[31

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El

b1

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