Basamento Cortical Andes Colombianos Oswaldo 2006

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Geochronological and isotopical review of pre-Devoniancrustal basement of the Colombian Andes

Oswaldo Ordóñez-Carmona a,*, Jorge Julián Restrepo Álvarez a, Márcio Martins Pimentel b

a Universidad Nacional de Colombia, Facultad de Minas, A.A. 1027 Medellı́ n-Colombia, Colombiab Instituto de Geociê ncias, Universidade de Brası́ lia, CEP: 70910-900 Brasilia, D.F., Brazil

Received 1 October 2004; accepted 1 March 2006

Abstract

Under the flatlands east of the Andes, the crustal basement is exposed in a few places, composed mainly of the Mitú migmatitic com-plex and the Parguaza granite, whose ages range between 1.78 and 1.45 Ga. Extensive outcrops of high-grade metamorphic rocks arefound in several places. Two metamorphisms are dated between 1.2–1.1 and 1.0–0.9 Ga. They are considered blocks that formed duringthe Grenville orogeny and have Sm–Nd T DM model ages of 1.87–1.47. The Andaquı́ terrane is formed mainly by the Garzón Massif,composed of granulites, migmatites, and granites, and the metamorphic rocks of the Sierra de la Macarena, which are covered by unde-formed Cambrian sediments. It is believed that after the Grenville orogeny, this unit remained attached to the Amazonic Craton. All theother areas grouped in the Chibcha terrane, though they formed during the Grenville orogeny, are believed to have remained either aspart of another continental block or dispersed islands to be amalgamated to the Amazonic Craton during the Lower Paleozoic orogeny,which in the Quetame Massif is dated between the Silurian and Devonian and is named the Quetame orogenic event. 2006 Elsevier Ltd. All rights reserved.

Keywords: Colombian Andes; Grenville orogeny; Quetame event

1. Introduction

The Colombian Andes are divided into three mainbranches, known as the Eastern, Central, and Westerncordilleras, and include minor orographic systems such asthe Sierra Nevada de Santa Marta and the Serranı́a deBaudó. There is not a complete coincidence between thegeology and the orography, because the Colombian Andes

are composed of allochtonous terranes accreted to theAmazonian Craton (Restrepo and Toussaint, 1988; Tous-saint, 1993; Toussaint and Restrepo, 1994, 1996). The East-ern Cordillera, the eastern flank of the Central Cordillera,and parts of the Sierra Nevada de Santa Marta and theGuajira Peninsula constitute the Chibcha terrane, whereasthe western flank of the Central Cordillera and the NWpart of the Sierra Nevada de Santa Marta constitute the

Tahamı́ terrane. These two terranes are composed of con-tinental crust. The Western Cordillera and Serranı́a deBaudó are composed of oceanic crust and are grouped inthe Calima, Cuna and Gorgona terranes, respectively.The Andaquı́ terrane lies between the Amazonian Cratonand the Chibcha terrane (Fig. 1).

The geological history of current Colombian territorycan be traced back to the Paleoproterozoic, with the gener-

ation of approximately 1.8 Ga old rock units presentlyexposed in easternmost Colombia, along the borders withBrazil and Venezuela. The last important tectonic eventwas the collision of the Panama-Baudó arc during the lateCenozoic. Aleman and Ramos (2000) provide a brief review of the geological history of the northern part of the Andes (Ecuador, Colombia, and Venezuela) in the past2.0 Ga, and Ramos and Aleman (2000) provide greaterunderstanding of the Andean orogeny.

During the Proterozoic and Paleozoic, evolution wasrelated to a Wilson cycle, with continental collisions at

0895-9811/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jsames.2006.07.005

* Corresponding author.E-mail address: [email protected] (O. Ordóñez-Carmona).

www.elsevier.com/locate/jsames

Journal of South American Earth Sciences 21 (2006) 372–382

mailto:[email protected]:[email protected]


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approximately 1000, 380(?), and 270 Ma, followed by thesubsequent opening of oceanic basins. During the Mesozo-ic and Cenozoic, the regime changed to a North Andeantype of orogen, characterized by the accretion of oceanicterranes. The collision of the Panamá-Baudo block duringthe late Cenozoic produced the final uplift of the Colombi-an Andes.

In this work, a review of the geochronological and iso-topic data of the pre-Devonian basement of the ColombianAndes is developed and complemented by recent data. Amodel of the geodynamic evolution of the area betweenthe Mesoproterozoic and the Silurian is presented.

2. Isotopic data and analytical methods

Sr, Sm, and Nd isotopic analyses were performed at theGeochronology Laboratory of the University of Brasilia,using standard ion-exchange chromatography for the sepa-ration of Sr, Sm, and Nd with a multicollector FinniganMAT-262 mass spectrometer. Sm and Nd concentrations

were obtained by isotope dilution using a mixed

149Sm– 150Nd spike. Sr and Nd isotopic ratios were correct-ed for mass fractionation to 86Sr/88Sr = 0.1194 and146Nd/144Nd = 0.7219. Two sigma uncertainties for the87Sr/86Sr and 143Nd/144Nd ratios are smaller than 0.01%and 0.005%, respectively.

Decay constants used are those recommended by Steigerand Jäger (1977), and ages are reported at the 95% confi-dence interval. Analysis of the NBS-987 Sr standard givesvalues between 0.71024 and 0.71029, and the La Jolla Ndstandard yields values between 0.511828 and 0.511842 dur-ing the period when the analyses were performed. Isochronages were calculated using Ex-Isoplot program 2.05 version(Ludwig, 1999).

3. Precambrian basement

The Precambrian rocks in Colombia are exposed intwo main regions (Figs. 1 and 2): (1) as the sialic base-ment of the Chibcha terrain in the Andes and (2) inthe eastern flatlands, comprising part of the Amazonian

Craton.

Fig. 1. Suspect terrains of Colombia (modified from Toussaint and Restrepo, 1994, 1996).

O. Ordó ñez-Carmona et al. / Journal of South American Earth Sciences 21 (2006) 372–382 373


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3.1. Eastern flatlands

The western/northwestern part of the Amazonic Cratonforms the basement of the flatlands (Llanos) east of theColombian Andes. Most of it is covered by Tertiary sedi-mentary rocks, though to the east, close to the borders withVenezuela and Brazil, some outcrops of Precambrian rocksappear. In the border region between Colombia, Venezue-la, and Brazil, these metamorphic rocks are included in theMitú migmatitic complex (Fig. 1), which comprises mainlybiotite gneisses and migmatites of sedimentary origin withlow-pressure mineral assemblages that normally includeandalusite and cordierite. The sequence is closely associat-

ed with granitoids that likely correspond to anatectic mag-

mas. Descriptions of these poorly studied rocks appear inGalvis et al. (1979) and Bruneton et al. (1983).

Gneisses and granites yield Rb–Sr isochron ages rangingbetween 1780 and 1450 Ma (Priem et al., 1982), interpretedas reflecting the formation of a metamorphic basement atapproximately 1780 Ma superimposed by a magmaticevent at approximately 1450 Ma. One U–Pb zircon dateindicates an age of 1480 Ma (Priem et al., 1982), and vari-ous ages between 1447 and 1215 Ma were obtained by K– Ar and Rb–Sr methods in mineral separates (hornblende,biotite, and muscovite).

The Parguaza granite, a body with a rapakivi texturethat seems to intrude the Mitú Complex, has a U–Pb zircon

age of 1545 ± 20 Ma and a Rb–Sr whole-rock isochron age

Fig. 2. Simplified geological map of the Colombian Andes (after Toussaint, 1993).

374 O. Ordó ñez-Carmona et al. / Journal of South American Earth Sciences 21 (2006) 372–382


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of 1499 ± 39 Ma (Gaudett et al., 1978). These rocks arepart of the Paleo-Mesoproterozoic Rio Negro-Juruenaprovince that evolved in a magmatic-arc environmentbetween 1.8 and 1.55 Ga (Tassinari, 1984; Teixeira et al.,1989; Tassinari et al., 1996).

3.2. Garzón Massif

Extensive outcrops of high-grade metamorphic rocksare exposed in the Garzón Massif. Although this zone isgeographically part of the Eastern Cordillera, it is dis-cussed in this section because it includes metamorphic rockunits east of the Andes (Figs. 1 and 2).

Two main units are recognized, one formed by the Gua-potón and Mancagua granitic augen gneisses in the westand the other, the Garzón Group, consisting of granulitefacies rocks, including charnockites, enderbites, migma-tites, mafic granulites, pyroxene amphibolites, and ultra-mafic rocks (Álvarez, 1981; Kroonenberg, 1982a;

Rodrı́guez, 1995). The anatectic granite of El Recreo alsois part of this massif (Rodrı́guez, 1995), and most rockunits of the Garzón Massif are cut by pegmatites with largemagnetite and biotite crystals.

The most detailed geochronological work in the area isthat by Cordani et al. (2005). Many U–Pb SHRIMP agesshow an igneous event at 1158 ± 23 Ma, with high-grademetamorphism at 1000 ± 25 Ma for the Guapotón-Manca-gua orthogneiss. From Las Margaritas gneiss, two garnetwhole-rock Sm–Nd isochrones were obtained with agesof 1034 ± 6 Ma and 990 ± 8 Ma, which indicate the ageof cooling under 600 C after peak metamorphism. Similar-

ly, a U–Pb zircon age of 1015 ± 7.8 Ma for leucosomes of migmatites indicates metamorphism at the end of the Mes-oproterozoic. Other younger ages, ranging from 1000 to905 Ma and obtained by several methods, probably reflecteither a second metamorphism or cooling after the mainmetamorphic event.

Other areas that may be part of this block are the Sierrade la Macarena to the northeast and the granitic gneisses,amphibolites, and migmatites that crop out to the south-west, along the Eastern Cordillera from Garzón to theEcuatorian border (Maya, 2001). These units are knownas La Cocha-Rı́o Tellez migmatitic complex (Murcia andCepeda, 1991) and an unnamed unit in the Sierra de LaMacarena. The metamorphic rocks (gneiss and amphibo-lites) of the Sierra de la Macarena are covered by unde-formed Cambrian sediments, which supports aPrecambrian age for these rocks.

Recently, Jiménez and Cordani (2003) challenged thevalidity of the correlation of La Cocha-Rı́o Tellez migmat-itic complex with the Garzon Massif on the basis of the U– Pb zircon age of 166 ± 3.8 Ma for a granodiorite. Theysuggest that the tectonomagmatic evolution of the complexmay be related to the emplacement of granitoids withinparaschists and paragneisses, which formed the migmatites.However, the Jurassic magmatic belt that extends from

Ecuador to the Sierra Nevada de Santa Marta does not

show any relation to the main regional metamorphicevent(s). Also, in Ecuador, these granites, known as Aza-frán, have no relation with any known orogenic metamor-phic event (Noble et al., 1997). Only in the SantanderMassif did the Mesozoic intrusives cause thermal metamor-phism of the Paleozoic metamorphic and sedimentary

rocks (Restrepo-Pace, 1995).According to Kroonenberg (1982a), the rocks from theGarzón Massif, as well as those of the Sierra Nevada deSanta Marta, are part of a granulitic belt that probablyformed during the collision between Amazonia and Laur-entia. The alternative model of Toussaint (1993) proposesthat the Garzón Massif, together with the Sierra de LaMacarena, constitute the Andaquı́ terrane, allochtonouswith respect to the basement of the Amazonian Cratonand accreted as an isolated block to South America duringthe Grenvillian orogeny. Restrepo-Pace (1995) believes thatthe granulitic belt formed during a continent–continentcollision, with the rocks in the Garzón Massif being derived

from the Guyana Shield. A very different model, implyingan autochtonous origin for the Andes, was put forward byPriem et al. (1989), who state that ‘‘not much, if any, con-tinental accretion occurred in the Andes between 1.6 Gaand the Cretaceous’’.

The ages found for the Garzon Massif are comparableto those of the Sunsás orogeny, which took place in thewestern region of the Amazonic Craton between 1.25 and1.0 Ga (Tassinari et al., 2000).

3.3. Andean region

Consensus among geologists indicates the presence of aPrecambrian basement beneath the Andes in the EasternCordillera and the eastern flank of the Central Cordillera.However, the extension of this basement to the centraland western parts of the Central Cordillera is still debated.According to Kroonenberg (1982b), the Precambrian base-ment underlies the entire Central Cordillera. However,Restrepo and Toussaint (1988) argue that it only exists— or at least crops out—east of the Otú-Pericos fault, whichis taken as the boundary between the Chibcha terrane tothe east and the Tahamı́ (or Central Andean) terrane tothe west. Currently no reliable dating of Precambrian rockshas been obtained from the Tahamı́ terrane, so the subse-quent discussion relates only to the Chibcha terrane.

3.3.1. Santander Massif

It is located in the northern part of the Eastern Cordil-lera (Fig. 2), which is formed predominantly by sedimenta-ry rocks. In this area, metamorphic and plutonic rockscrop out, with higher-grade metamorphic rocks includedin the Bucaramanga gneiss and lower-grade rocks consti-tuting the Silgará Formation. The first unit is consideredPrecambrian, whereas the second probably formed duringthe Caledonian orogeny.

The Bucaramanga gneiss consists of biotite gneisses (some

with cordierite and sillimanite), migmatites, quartzites, and



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amphibolites. The predominant rock, banded biotitegneiss, is considered derived from a pelitic sediment (Wardet al., 1973; Restrepo-Pace et al., 1997). In addition, thepresence of alternating bands of amphibolites and amphi-bole gneisses suggest some contribution of volcanic materi-al or sills of basic rocks.

On the basis of K–Ar (945 ± 40 Ma) and Rb–Sr modelages, Ward et al. (1973) consider that the Bucaramangagneiss metamorphosed during the Proterozoic. 40Ar/39Ardates by Restrepo-Pace (1995) give apparent ages between668 ± 9 and 574 ± 8 Ma, though the argon spectrum showsolder ages on the order of 850–800 Ma. Restrepo-Paceattributes this divergence to rejuvenation of Grenvillianmetamorphic rocks by later thermal effects. Cordani et al.(2005) obtain 40Ar/39Ar ages close to 200 Ma in biotitefrom the gneisses, which reflects the loss of argon duringthe intrusion of the large batholiths in that area. They alsopresent SHRIMP U–Pb zircon ages between 1558 and

864 Ma. The older ages are interpreted as representingthe sources of the sedimentary protolith, whereas an ageof 1057 ± 28 Ma is believed to correspond to a Grenvillemetamorphic event. Two zircon grains yield an age of 864 ± 66 Ma, interpreted as representing a late metamor-phism episode. This latter age is similar to that detected

in the eastern flank of the Central Cordillera, which sup-ports the idea that both regions belong to the same terrane.For the present study, two samples of the Bucaramanga

gneiss were collected in the same location that Ward et al.(1973) obtained samples for K–Ar dating, specifically,along the highway between the towns of Aguachica andOcaña. In that area, gneisses have banded structure, withpredominant quartz–feldspar gneiss and minor amphibolegneisses and amphibolites. Samples NB-1 and NB-3, select-ed for Sr and Nd isotopic analyses (Table 1), correspond toquartz–feldspar–biotite gneiss and hornblende gneiss,respectively.

Table 1Sr and Nd isotopic results for the Precambrian and Early Paleozoic rocks

Sample Sm Nd 143Nd/144Nd 147Sm/144Nd eNd(T) T DM (Ga) 87Sr/86Sr 87Rb/86Sr 87Sr/86Sri T (Ma)

Bucaramanga Gneiss

NB-1 4.39 16.84 0.512370 ± 25 0.1576 0.50 1.76 0.71398 ± 6 0.5551 0.70648 945NB-3 4.96 25.53 0.511939 ± 37 0.1175 4.07 1.71 0.70506 ± 5 0.1062 0.70363 945

Los Mangos Granulite

GRM-1 5.18 22.52 0.512290 ± 12 0.1391 0.37 1.51 0.72607 ± 6 971GRM-2 6.15 39.56 0.511882 ± 15 0.0940 1.99 1.47 0.72302 ± 7 971GRM-10 WR 9.26 44.48 0.511930 ± 11 0.1259 5.02 1.87 971GRM-10 Gr 8.47 5.02 0.517620 ± 15 1.0195 971RG-3a 7.38 40.42 0.511872 ± 08 0.1104 4.23 1.69 971

RG-6a 1.13 9.68 0.511879 ± 08 0.0706 0.87 971

El Vapor Mylonitic Gneisses

B-4 7.48 37.15 0.511931 ± 14 0.1218 5.23 1.79 0.74861 ± 5 2.55 0.71603 894B-22 7.99 30.79 0.512370 ± 13 0.1560 0.57 1.71 0.77792 ± 5 4.40 0.72171 894

Garzón Group

G-2a 1.37 16.78 0.511770 ± 07 0.0494 5.34 1180G-11a 1.94 16.68 0.512032 ± 05 0.0703 7.30 1180G-20a 1.14 3.62 0.512626 ± 07 0.1904 0.72 1180

Guapotón Granite

SnAnkr-1a 14.9 78.97 0.512062 ± 07 0.1143 0.24 1.50 1088

El H ı́g ado Amphibolite

HP-3a 9.25 32.95 0.512472 ± 10 0.1697 0.09 911HP-5a 0.18 1.587 0.512085 ± 06 0.0686 4.15 911

Ocañ a Batholith

BOC-1 5.90 27.61 0.512341 ± 32 0.1291 2.23 1.27 0.73750 ± 6 9.6723 413BOC-2 5.42 25.93 0.512318 ± 16 0.1264 2.54 1.27 0.72874 ± 7 7.4091 413BOC-3 5.04 24.92 0.512274 ± 12 0.1222 3.17 1.28 0.73118 ± 8 7.8942 413BOC-4 5.29 26.98 0.512292 ± 16 0.1186 2.63 1.21 0.72917 ± 7 7.4416 413

Sanı́ n Villa Diorite

DSV-1 5.98 31.73 0.512190 ± 13 0.1139 4.38 1.30 0.70714 ± 6 0.1907 0.70602 413

2r uncertainties for the isotopic ratio 87Rb/86Sr < 1%, 87Sr/86Sr < 0.01%, 143Nd/144Nd < 0.003%, and 147Sm/144Nd < 0.1. Data for other Precambrianunits of the Colombian Andes are also included (Restrepo-Pace et al., 1997). The right column indicates the most accepted age of the correspondinggeological unit.

Note: WR = whole-rock, Gr = garnet; eNd = [{(143Nd/144Nd)i /(

143Nd/144Nd)t-CHUR}1] · 104, using 143Nd/144Nd = 0.512638 as present-day CHUR

value. T DM = k1ln[1 + (143Nd/144Nd)i (

143Nd/144Nd)DM/(147Sm/144Nd)i (

147Sm/144Nd)DM], using 143Nd/144Nd = 0.513114 as present-day DM

value, and (147Sm/144Nd)DM = 0.222.

a Restrepo-Pace et al. (1997).



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T DM model ages of 1.76 and 1.71 Ga represent averagemodel ages of the original sediment source areas. There-fore, the maximum age for the protolith sedimentationmust be approximately 1.71 Ga.

3.3.2. Sierra Nevada de Santa Marta

These mountains are not part of the main Andeanmountain chain; however, their sialic basement presentssimilarities with other high-grade areas of the cordillera(Fig. 2). The eastern part of the Sierra Nevada de SantaMarta contains the most extensive exposures of high-graderocks, called Los Mangos granulites (Fig. 2, N4) and Dib-ulla gneiss. The first Precambrian metamorphic age inColombia was obtained by MacDonald and Hurley(1969) for the Dibulla gneiss, represented by a Rb–Sr iso-chron of 1400 Ma with an initial 87Sr/86Sr ratio of 0.703.Los Mangos granulite consists of a banded sequence of mafic and granitic gneisses, amphibolites, anorthosites,and ultramafic and calcareous bands, containing mineral

assemblages that indicate granulite facies conditions(Gansser, 1955; MacDonald and Hurley, 1969; Tschanzet al., 1974; Ordóñez-Carmona et al., 2002).

Restrepo-Pace et al. (1997) date detrital zircons from asample of the Dibulla gneiss, obtaining ages between 1.0and 1.3 Ga, possibly associated with the Grenville event.They also report a U–Pb (zircon) upper intercept age of 1.5 Ga, interpreted as the age of crystallization of part of the protolith of the granulites, and a lower intercept ageof 0.45 Ga interpreted as Pb loss related to an Andeanorogenic event. The Ar–Ar analyses of biotites from twogranulites produce complex age spectra with an integrated

age of 561 ± 6 Ma for one of the samples and a totalfusion age of approximately 845 Ma for the other. A prov-enance pattern of detrital zircon grains (Cordani et al.,2005) documents a Mesoproterozoic (1375 Ma) source,as well as an early metamorphic event between 1190 and1140 Ma and a later metamorphic event at approximately990 Ma.

Samples in the present study were collected at the con-fluence of the Guatapurı́ and Los Mangos rivers, approxi-mately 20 km northwest of Valledupar (Fig. 2, N4).Additional details about the petrographic and field charac-teristics of these rocks may be found in Ordóñez-Carmonaet al. (2002).

The samples analyzed belong to three of the most abun-dant associations of the banded sequence exposed in thisarea. Sample GRM-1 (amphibolite) presents the lowestSm and Nd concentrations, and the T DM model ages calcu-lated for these rocks are 1.47–1.87 Ga, suggesting that theprotoliths of the volcanosedimentary sequence are at least1.47 Ga old. Garnet crystals from sample GRM-10 wereseparated and analyzed, and the resulting garnet whole-rock isochron indicates an age of 971 ± 8 Ma, consideredto be the age of granulite facies metamorphism and corre-lated chronologically with the Grenville orogeny. It alsoagrees within error with the K/Ar hornblende age of

940 ± 30 Ma reported by Tschanz et al. (1974).

3.3.3. Guajira Peninsula

The Guajira Peninsula lies to the northeast of the SierraNevada de Santa Marta, separated by the right-lateral Ocafault (Fig. 2). Pb- age of zircons from the Jojoncito gran-ite (Alvarez, 1967) gave an age of 1250 Ma (Banks, 1975);so far, it is the only Proterozoic unit identified in the area.

According to Cardona (2003), this unit is not a granite buta paragneiss.SHRIMP U–Pb ages of 17 zircons indicate ages of

1529 ± 43, 1342 ± 25, and 1236 ± 16 Ma for three nuclei.Two overgrowths yield ages of 1167 ± 17 and1165 ± 37 Ma, and younger overgrowths of other grainsgive an average age of 916 ± 19 Ma (Cordani et al.,2005). The younger ages likely represent metamorphicevents between 1165 and 916 Ma, whereas the older agesindicate the age of Mesoproterozoic sources that, accord-ing to Cordani et al. (2005), were located in the AmazonianCraton.

3.3.4. Eastern flank of the Central CordilleraSince the geological mapping project carried out by

Feininger et al. (1972), the gneisses exposed east of theOtú fault have been considered Precambrian retrogradedgranulites, similar to those in the Sierra Nevada de SantaMarta, though diagnostic minerals of this facies have notbeen found. It is worth noting that the fault that marksthe western limit of the Precambrian outcrops, accordingto the map of Feininger et al. (1972), is the Otú fault, notthe Palestina fault as shown in some publications ( Cedielet al., 2002).

Geological evidence of the age is available, because

Ordovician sedimentary rocks (Harrison, 1930; Botero-Arango, 1940), metamorphosed at low-grade conditions,rest unconformably on the medium-grade gneisses. Also,a Rb–Sr isochron of El Vapor mylonitic gneisses (Fig. 2,N1), close to the town of Puerto Berrı́o (latitude 630 0N),indicates an age of 894 ± 36 Ma (Ordóñez-Carmonaet al., 1999) that can be related to ductile deformation of the gneisses and therefore represents a minimum age of metamorphism. In addition, the T DM model ages calculat-ed for these rocks are between 1.71 and 1.79 Ga, suggestingthat the protoliths are at least 1.71 Ga old. The extensionof these rocks to the north might occur in the poorlyknown Serranı́a de San Lucas, where the basement isthought to be formed by Precambrian rocks (González-Ire-gui, 1996). Recently, Ordóñez-Carmona and Restrepo (inpress) found granulites in the northeastern region of Serra-nı́a de San Lucas, which allows correlation and associationwith other Precambrian high-grade metamorphic rocks inthe Chibcha terrain.

Restrepo-Pace et al. (1997) report an Ar–Ar hornblendeage of 911 ± 2 Ma for the El Hı́gado amphibolite (Fig. 2,N3) located in the eastern flank of the Central Cordilleraat latitude 211 0N. Similarly, the Tierradentro Amphibolite(Fig. 2, N2), at latitude 5N, is dated at 1360 ± 270 Ma byK–Ar on hornblende (Vesga and Barrero, 1978), though

large error casts doubt on the exact age.



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3.3.5. Central Cordillera west of the Otú fault

Although no Precambrian radiometric ages have beenfound west of the Otú fault (i.e., in the Tahamı́ terrane),the latest maps by Ingeominas show two areas where gran-ulitic rocks are exposed: the Puquı́ Complex and El RetiroGroup, indicated as probably Proterozoic (González-Ire-

gui, 1996, 2001). Recent radiometric dates do not supportthis assumption (Ordóñez-Carmona et al., 2001; Ord-óñez-Carmona and Pimentel, 2002) but indicate ages of 306 ± 11 Ma (Puquı́ Complex, Rb–Sr isochron) and226 ± 17 Ma (Retiro Group, garnet whole-rock Sm–Ndisochron).

A Proterozoic age for the Cajamarca Group, the mainmetamorphic unit in the Central Cordillera, has beenproposed by Gómez and Núñez (2003), on the basis of the presence of clasts of metamorphic rocks that seemto be derived from this group and are found in the SantaTeresa metasediments, dated paleontologically as Ordovi-cian. This unit is located along the Pericos fault, consid-

ered the southern extension of the Otú fault, between theCajamarca Group to the west and the TierradentroAmphibolites to the east. As discussed previously, theOtú-Pericos fault has been considered the limit betweentwo different terranes. Thus, an allocthtonous origin forthe Santa Teresa metasediments cannot be discarded,and their original position with respect to both the Caja-marca Group and the Tierradentro Amphibolites remainsuncertain.

4. Early Paleozoic basement

Although no radiometric ages in this interval are cur-rently available for the rock units west of the Otú -Pericosfault, La Miel orthogneiss, dated by a Rb–Sr isochron at388 ± 12 Ma, and the Samaná orthogneiss, dated by thesame method at 346 ± 26 Ma (Restrepo et al., 1991), con-tain xenoliths of the country rocks composed of low- tomedium-grade metamorphic rocks. Therefore, a metamor-phic event older than 388 Ma is indicated. Restrepo et al.(1991) and Toussaint (1993) suggest that one of the meta-morphic events that formed the basement of the northernpart of the Central Cordillera is associated with a ‘‘Cale-donian orogeny,’’ but geochronological confirmation isstill lacking. With respect to the protholith, a Late Prote-rozoic–Early Paleozoic age is inferred for at least part of the metamorphic sequence. In marbles from the easternand western flanks of the Central Cordillera, an Ediaca-ran age has been obtained by C-isotope stratigraphy forthe deposition of the limestone that formed marbles (Silvaet al., 2004). Also, locally in the eastern flank of the Cen-tral Cordillera, fossils in low-grade metasediments indi-cate Lower Ordovician deposition (González-Iregui,2001).

In the eastern block (Chibcha terrane), important igne-ous, metamorphic, and sedimentary events took place dur-ing the Early Paleozoic. Metamorphic rocks generally

considered to be of this age include the Perijá and Silgará

formations, the Quetame Group, and La Cristalina metase-diments. The main plutonic unit is the Santander PlutonicGroup, which presents K–Ar ages of 457–413 Ma, whereasLa Cristalina, Amoyá, and El Hı́gado formations representthe sedimentary record for the Early Paleozoic.

4.1. Santander Plutonic Group

The only dated magmatic unit of Early Paleozoic age inthe Colombian Andes is found in the Santander Massif of the Eastern Cordillera (Fig. 2). This unit comprises bothgranitic and gabbroic rocks that intrude the metamorphicbasement, consisting of the Bucaramanga gneiss and Sil-gará Formation.

In the massif, the metamorphic basement and intru-sions are covered by Devonian and Carboniferous sedi-ments, thus indicating a pre-Devonian age for thebasement rocks. This age relationship is confirmed byK–Ar data that yield ages for the intrusions in the range

of 456 ± 23 Ma (whole-rock) to 413 ± 30 Ma (horn-blende) (Goldsmith et al., 1971; Boinet et al., 1985). How-ever, some igneous rocks considered related to the EarlyPaleozoic event yield Mesozoic K–Ar and U–Pb ages(Goldsmith et al., 1971; Boinet et al., 1985; Dörr et al.,1995), which suggests Mesozoic intrusions are present inthe area or an important younger tectonothermal eventhas somehow reset the isotopic systems, yielding Mesozoicages.

Four samples of the Ocaña Batholith (Fig. 2, N5), com-posed of medium- to coarse-grained pink granites, withquartz, k-feldspar, plagioclase, and biotite, were collected

for the analyses. In the study area, the batholith intrudesthe metamorphic rocks of the Bucaramanga gneiss. A N– S–elongated diorite body that intrudes the Bucaramangagneiss in this region also is known as the Sanin Villa dio-rite. A sample was collected in the Sanin Villa sector of the Aguachica–Ocaña road. It is composed of a medium-to fine-grained diorite with hornblende, plagioclase, andsome K-feldspar and quartz.

Sr and Nd isotopic analyses were carried out on thesefive samples (Table 1). Sm and Nd concentrations, as wellas the isotopic ratios, are quite uniform in the five samplesanalyzed. eNd(T) values were calculated for a tentativeintrusion age of 413 Ma. The Rb/Sr data are linear(MSWD of 1.04), but the initial Sr87/Sr86 ratio of 0.7007for the isochron makes the obtained age of 268 ± 26 Mameaningless. Also, the Ocaña Batholith is covered by theMiddle Devonian Las Mercedes Formation (Bayer et al.,1973). The T DM ages are as old as 1.30 Ga for the dioriteand between 1.21 and 1.28 Ga for the batholith, which sug-gests the magma crustal contaminants are at least Meso-proterozoic in age. In the case of the Ocaña Batholith,the uniform model age values indicate magma homogenei-ty and perhaps contaminant homogeneity as well.

The eNd(413Ma) values for these rocks are negative (2.23to 4.38), indicating an important crustal component in

the parental magma.



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4.2. Silgará Formation

Stratigraphically the Silgará Formation rests on top of the Bucaramanga Gneiss. However, the nature of the con-tact between them remains unclear, so an unconformitybetween the two units cannot be ruled out. It comprises

slates, phyllites, schists, metasandstones, marbles, andortoamphibolites (Ward et al., 1973; Shäfer et al., 1998).The metamorphic conditions vary from greenschist tolow amphibolite facies, but some local upper amphibolitefacies associations have been described (Shäfer et al.,1998). These rocks show fine-banded structures and abun-dant phyllites and schists compared with the Bucaramangagneiss rocks. Where contact is covered, the thickness of thebanding and rock association is useful to distinguishbetween the two rock units. In other cases, the contactoften is defined by the biotite–sillimanite isograd (Wardet al., 1973; Restrepo-Pace et al., 1997), though this differ-entiation method seems to imply a common metamorphism

for both units.For the Silgará Formation, a pre-Devonian age is cer-

tain, because the fossiliferous La Floresta Formation, of Middle Devonian age, rests unconformably on it. In addi-tion, dykes and granitoids intrude the metamorphic rocksof the Santander Massif, and some K–Ar ages for thesebodies range between 457 and 413 Ma, as previously dis-cussed. According to these dates, the ages of metamor-phism of the Bucaramanga gneiss and Silgará Formationare older than 460 Ma. With respect to the protholith, C-isotope stratigraphy obtained from some marbles of theSilgará Formation indicates an Ediacaran–Early Cambrian

deposition of the original limestone (Silva et al., 2005);therefore, a common age for metamorphism of the Bucara-manga gneiss and Silgará Formation is not possible, con-trary to the point of view of Ward et al. (1973) andShäfer et al. (1998). These authors propose that the ageof metamorphism is the same for both units, but the gneisssimply represents the lower, more strongly metamorphosedportion of a single volcanosedimentary sequence, whereasthe Silgará Formation represents the upper, less metamor-phosed part.

In contrast, Toussaint (1993) considers the possibility of an unconformity between the Bucaramanga gneiss and theSilgará Formation, as well as a difference in the metamor-phic grade, which enables the separation between the twounits, with the gneiss Precambrian in age and the SilgaráFormation Early Paleozoic. Presently, the Early Paleozoicage for the Silgará Formation is widely accepted by Colom-bian geologists, as reaffirmed recently by Rı́os et al. (2003).

4.3. Perijá Formation

According to Forero (1970), this formation is composedof a sequence of phyllites and metasandstones in whichsome sedimentary structures are preserved. These rocksrepresent the basement of the Serranı́a de Perijá, one of

the branches of the Eastern Cordillera (Fig. 2) for which

no absolute ages are available. Undeformed sediments withMiddle Devonian (390 Ma) fossils cover this sequence,indicating that the metamorphism is pre-Devonian. Thus,the rocks may be either Lower Paleozoic or represent thenorthern continuity of the Precambrian metamorphic rocksof the Santander Massif, metamorphosed at a lower grade.

4.4. Quetame Group

The Quetame Massif (Fig. 2) comprises a sequence of low-grade metamorphic rocks that attain the biotite zone.No absolute ages are available, but stratigraphic relation-ships show that they are pre-Devonian, because they arecovered by the undeformed fossiliferous sediments of theLutitas (shales) de Portachuelo Formation of lower Devo-nian age and the Areniscas (sandstones) de Gutierrez For-mation of Middle to Upper Devonian age (Renzoni, 1962,1968; Stibane, 1969).

These rocks also contain palynomorphs of Ludlovian

age (Gröser and Prossl, 1991) and are the only rocks withSilurian fossil material in Colombia. Therefore, the meta-morphic event that affected the Quetame Group ispost-Ludlovian and pre-Middle Devonian, chronologicallycorrelated with the Caledonian orogeny (425–390 Ma).

4.5. La Cristalina metasediments

Along the eastern flank of the Cordillera Central, in thePuerto Berrı́o area, a sequence of low-grade metamorphicrocks is exposed (Feininger et al., 1972), covering the base-ment rocks represented by El Vapor mylonitic gneisses

(Fig. 2). These rocks, known as La Cristalina metasedi-ments, include preserved fossil fauna that corresponds toLower Ordovician graptolites (Harrison, 1930; Botero-Arango, 1940). Consequently, the age of metamorphismis considered post-Lower Ordovician, though due to a lackof geochronological data, regional correlations remainuncertain.

4.6. El Hı́gado and Amoyá formations

South of La Cristalina rocks and in faulted contact withEl Hı́gado Amphibolites (Fig. 2, N3), a sequence of shales,sandstones, and limestone corresponds to El Hı́gado For-mation (Mojica et al., 1987). The sedimentary structuresand substantial fossil fauna are preserved in this unmeta-morphosed sequence, indicating Middle Ordoviciansedimentation.

North of the type section of El Hı́gado Formation,another set of sedimentary rocks is known and may be cor-related with it. These rocks were affected by low-grademetamorphism and constitute the Amoyá Formation(Núñez et al., 1982).

Considering that these formations, together with LaCristalina metasediments, are located in the eastern flankof the Central Cordillera and contain Middle Ordovician

fossils, they might be interpreted as a sedimentary belt that



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covers the Grenvillian basement of the eastern flank of theCentral Cordillera. Subsequent to the sedimentation, partof this belt was affected by a low-grade metamorphic eventthat is more evident in the north (La Cristalina metasedi-ments) and not detected in southern rocks (El Hı́gado For-mation). This metamorphic event is post-Middle

Ordovician.

5. Geological evolution

The rocks of the Mitú migmatitic complex and Parguazagranite are the oldest rocks dated in Colombia. They wereincorporated into the Amazonic Craton during the RioNegro-Juruena event. Although they crop out in limitedexposures at the limits with Venezuela and Brazil, theyprobably form most of the basement of the Colombianeastern flatlands, in that they are covered by mostly Tertia-ry sediments.

For other Proterozoic rocks in Colombia, according to

the data reviewed previously, the presence of a Grenvillianbelt in the Colombian Andes, as proposed initially byKroonenberg (1982a), is generally confirmed. The high-grade metamorphic rocks of the Andaquı́ and Chibchaterranes formed during this orogeny, though some differ-ences emerge between the rocks in the two terranes.

In the Andaquı́ terrane, the Garzón Massif underwent afirst metamorphic event between approximately 1050 and1015 Ma, with other ages in the range of 1000–905 Ma,which reflects either cooling of the rocks or, more proba-bly, a second metamorphism. Ages around 1100 Ma maycorrespond to the intrusion of the granites. An important

characteristic of this terrane alone is that after the Gren-ville orogeny, the block remained attached to the Amazon-ic Craton, covered in the Sierra de la Macarena byunmetamorphosed Cambrian sediments. In this way, dur-ing the Phanerozoic, it constituted part of the Autochton-ous Block, though part of it was uplifted during theAndean orogeny as a component of the Eastern Cordillera.

The other Grenvillian blocks, including the SantanderMassif, Sierra Nevada de Santa Marta, Guajira Peninsula,and eastern flank of the Central Cordillera, all grouped inthe Chibcha terrane, followed a different evolution. Thereis no certitude that a continuous belt of Neoproterozoicrocks links these massifs, in part because little is knownabout the basement that underlies the thick sedimentarybasins that separate them. In all, there was a first metamor-phism between 1200 and 1100 Ma, followed by a secondone in the range 1000–900 Ma. Little is known about theevolution of these massifs between 900 and 500 Ma, butduring Early Paleozoic times, the region initially underwentsedimentation followed by metamorphism and plutonism(Quetame event), probably as a collision between Gondw-ana and eastern North America. The exact position of these blocks at the moment is unknown. Several differencesin tectonic styles, sedimentation, and the presence orabsence of metamorphism led Toussaint (1993) to propose

that the Chibcha terrane followed a different evolution

than that of the Autochtonous Block, at least until the Ear-ly Paleozoic. The Chibcha terrane probably was locatedsomewhere within the Silurian–Devonian collision zonebut not connected to the Autochtonous Block in its presentposition, being displaced, possibly by faults (Paleo-Guaicá-ramo fault), during the Late Paleozoic to its present loca-

tion. Thus, the eastern part of the Colombian Andes(‘‘Oriente Andino’’) was completely amalgamated to theAmazonic Craton by the beginning of the Mesozoic.

Acknowledgments

This study was partially supported by the Institute of Geosciences of the University of Brası́lia, Brazil, byCNPq, the Brazilian Research Council, and the NationalUniversity of Colombia through DIME and DINAIN.The authors thank the staff of the Geochronology Labo-ratory of the University of Brasilia and the National Uni-versity (GEMMA Group) for logistic support duringfieldwork. They also appreciate the concepts and correc-tions made by the reviewers, which were very relevantto structure this article.

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