x:r

r b

This phenomenon was common in the Peruvian and Chilean Andes during the Uppermost Jurassic and Lower Cretaceous. The marginalbasin was trapped during the collision of the CaribbeanColombian Cretaceous oceanic plateau, which accreted west of the Arqua Com-

Atwater, 1978; Tarney et al., 1981; Saunders and Tarney,1982). Such basins commonly occur in intra-oceanic set-tings, but examples also are associated with continental-

Dalziel, 1981; Atherton et al., 1983; Saunders and Tarney,1984; Miller et al., 1994). During the Uppermost JurassicLower Cretaceous, the continental margin was on the brinkof splitting from the continent, and basin formation withthe eruption of mantle-derived basalts was an outstandingfeature. A rosary of NS-elongated, ensialic marginal

* Corresponding author.E-mail address: anivia@ingeominas.gov.co (A. Nivia).

Journal of South American Earth Sciplex in the Early Eocene. Dierences in the geochemical characteristics of basalts of the oceanic plateau and those of the QuebradagrandeComplex indicate these units were generated in very dierent tectonic settings. 2006 Elsevier Ltd. All rights reserved.

Keywords: Continental active margin; Back arc basin; Extensional tectonics; Ophiolitic complexes

1. Introduction

A characteristic feature of many convergent plate mar-gins, especially those aected by the subduction of old,dense, oceanic lithosphere, is the development of backarcbasins resulting from extensional tectonics (Molnar and

based magmatic arcs. Throughout the Mesozoic and prob-ably much of the Paleozoic, the western side of SouthAmerica and the Antarctic Peninsula formed a semicontin-uous magmatic arc along the margin of Gondwana, wherethe formation of volcanic arcs and marginal basins clearlyplayed an important evolutionary role (Dalziel et al., 1974;a Instituto Colombiano de Geologa y Minera INGEOMINAS, Unidad Operativa Cali, A.A. 9724, Cali, Colombiab Department of Geology, Royal Halloway University of London, Egham, Surrey TW20 0EX, UK

c School of Earth, Ocean and Planetary Sciences, Cardi University, Main Building, Park Place, Cardi CF1 3YE, UKd University of Leicester, Department of Geology, Leicester LE1 7RH, UK

Received 1 October 2004; accepted 1 January 2006

Abstract

The Quebradagrande Complex of Western Colombia consists of volcanic and AlbianAptian sedimentary rocks of oceanic anityand outcrops in a highly deformed zone where spatial relationships are dicult to unravel. BerriasianAptian sediments that displaycontinental to shallow marine sedimentary facies and mac and ultramac plutonic rocks are associated with the Quebradagrande Com-plex. Geochemically, the basalts and andesites of the Quebradagrande Complex mostly display calc-alkaline anities, are enriched inlarge-ion lithophile elements relative to high eld strength elements, and thus are typical of volcanic rocks generated in supra-subductionzone mantle wedges. The Quebradagrande Complex parallels the western margin of the Colombian Andes Central Cordillera, forming anarrow, discontinuous strip fault-bounded on both sides by metamorphic rocks. The age of the metamorphic rocks east of the Quebra-dagrande Complex is well established as Neoproterozoic. However, the age of the metamorphics to the west the Arqua Complex ispoorly constrained; they may have formed during either the Neoproterozoic or Lower Cretaceous. A Neoproterozoic age for the ArquaComplex is favored by both its close proximity to sedimentary rocks mapped as Paleozoic and its intrusion by Triassic plutons. Thus, theQuebradagrande Complex could represent an intracratonic marginal basin produced by spreading-subsidence, where the progressivethinning of the lithosphere generated gradually deeper sedimentary environments, eventually resulting in the generation of oceanic crust.The Quebradagrande Complemarginal basin in the Central Co

Alvaro Nivia a,*, Giselle F. Marrine0895-9811/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.jsames.2006.07.002A Lower Cretaceous ensialicdillera of the Colombian Andes

, Andrew C. Kerr c, John Tarney d

www.elsevier.com/locate/jsames

ences 21 (2006) 423436

basins of Tithonian to Albian age from Cape Horn andSouth Georgia Island to southwestern Mexico the lattercontinuous with South America in most TriassicJurassicPangea reconstructions (Coney and Evenchick, 1994) remain as scars of a generalized extensional episode.

In Colombia, a sequence of pillow lavas, diabases, andassociated volcaniclastic materials, the Diabase and Daguagroups (Nelson, 1957; Barrero, 1979) is postulated to con-tinue this belt of marginal basins northward (Aberg et al.,1984; Aguirre, 1987; Aguirre and Atherton, 1987). Howev-er, these materials correspond to younger (Upper Creta-ceous) accreted terranes of oceanic plateau anity(Millward et al., 1984; Nivia, 1987; Kerr et al., 1996,1997a,b, 2001). The missing link in the chain of marginal

basins along the Northern Andes, according to the geo-chemical results we present herein, is represented by theQuebradagrande Complex that outcrops east of the accret-ed plateau in a more ensialic position and has been errone-ously considered part of the latter (Bourgois et al., 1982,1985, 1987; Toussaint and Restrepo, 1993; Kammer,1995; Kammer and Mojica, 1996).

2. Regional geology and stratigraphic nomenclature

On the western ank of the Central Cordillera of theColombian Andes, a sequence of intermediate to basic vol-canic and sedimentary rocks of lower Cretaceous ageappears. In northern Colombia, this sequence was originally

.

PANAMA

OCEANPACIFIC

4oN

6oN

78 oW

Medelln

12

3

76oW

Armenia

80W 40W60W

0

20S

40S

LEGEND

Cenozoic rocks and deposits

Manizales

AtlanticOcean

SOUTH AMERICA

PacificOcean

AN1416

AN1417

AN1419

QBG95-7

14251426

QBG95-2QBG95-3

QBG95-10QBG95-11

AN1412AN1410AN1410A

0

424 A. Nivia et al. / Journal of South American Earth Sciences 21 (2006) 423436ECUADOR

2oN

Popayn

Cali

1

2

3

Pasto

ANAN

0 5Fig. 1. Geological sketch map of western Colombia to show the distribution,and location of samples listed in Table 2.MAIN STRUCTURAL ELEMENTSSan Jernimo Fault

Cauca-Almaguer FaultSilvia-Pijao Fault

123

Western Lithospheric OceanicCretaceous Province

Triassic plutons

Arqua Complex

Cajamarca Complex

Quebradagrande Complex - QGC

100 Kmspatial relationships of the Lower Cretaceous Quebradagrande Complex,

ricadescribed as the Quebradagrande Formation (Botero,1963). Although regionally exposed (Fig. 1), it is coveredin places by recent volcanic and volcanoclastic deposits thatmake correlations dicult. Parts of this sequence have beenmapped as the Aranzazu-Manizales sedimentary Complex(Gomez et al., 1995) and the Aranzazu-Manizales metasedi-mentary volcanic Complex (Mosquera, 1978); in other loca-tions, it was mapped (Paris and Marn, 1979) within theDiabase Group (Nelson, 1957, 1962). The Quebradagranderocks consist of imbricated slices of strongly deformeddynamometamorphic rocks, with crenulation cleavage andAndean milonitic foliation that bears NNE and dips 5070 to the east (Lozano et al., 1984a; Kammer, 1995; Gomezet al., 1995). Deformation in these rocks has caused them tobe described erroneously as schists and included within theCajamarca belt of metamorphic rocks (Nelson, 1957, 1962;Mosquera, 1978). Moreover, deformation has preventedthe identication of sedimentary sequences within theQuebradagrande rocks (Rodrguez and Rojas, 1985);though they were originally dened as a formation with dis-tinct sedimentary and volcanic members, these lithostrati-graphic units lack precisely dened limits.

To solve the problem of stratigraphic nomenclaturecaused by the deformation of the Quebradagrande rocks,Maya and Gonzalez (1996) propose a stratigraphic schemebased on lithodemic units (North American Commission onStratigraphic Nomenclature, 1983). They assign the rank ofstructural complexes to the former Cajamarca, Quebrada-grande, and Arqua units (Table 1). Because the names ofthe regional faults that bound these complexes also varyalong their length, new names for the faults have been pro-posed. Thus, the fault separating the Cajamarca Complexto the east and the Quebradagrande Complex to the westhas become known as the San Jeronimo fault, whereas thatwhich separates the Quebradagrande Complex to the eastand the Arqua Complex to the west is called the SilviaPijao fault (Fig. 1, Table 1). Another important fault isthe CaucaAlmaguer fault that bounds the Arqua Com-plex to the west and, according to some petrogenetic models(McCourt et al., 1984; Aspden and McCourt, 1986; Aspdenet al., 1987), marks the limit between Palaeozoic metamor-phic rocks of continental anity and Cretaceous accretedterranes of oceanic character. The latter include the Canas-gordas, Diabase, and Dagua groups, the Amaime and Vol-canic formations, and so forth. For these, Nivia (1997) usesthe term Western Oceanic Cretaceous Lithospheric Prov-ince (Fig. 1, Table 1), which corresponds to the southernextreme of the CaribbeanColombian Cretaceous IgneousProvince (Kerr et al., 1996, 1997a,b) that has been accretedonto the Northern Andes.

The Quebradagrande Complex is composed of anassemblage of metavolcanic and metasedimentary rocks.The protoliths of the metavolcanic rocks were basaltic toandesitic lavas and pyroclastics aected by the metamor-phism of zeolite, prhenitepumpellyite, and greenschist

A. Nivia et al. / Journal of South Amefacies. The metasedimentary rocks display a wide variationin grain size, from breccias and conglomerates to coarsesandstones with clasts of cobbles and pebbles of both vol-canic rocks and chert (Gomez et al., 1995). The presence ofthese rocks suggests underwater volcanoclastic sedimenta-tion produced by mass movements. The metasedimentaryhorizons also contain lithic sandstones and volcanoclasticarkoses. In the lithic sandstones, Gonzalez (1980a) reportsbasic volcanic rock fragments as the main components,with smaller quantities of mudstones and chert fragments,whereas the clastic arkoses are dominated by plagioclase.Lozano et al. (1984b) report black and grey graphitic meta-greywackes. Milonite slices, up to 1 km thick, formed fromclay-rich carbonaceous mudstones, intercalated with thinbeds of limestone and cherts (Gonzalez, 1980a).

Marine fossils found in these metasedimentary rocksinclude ammonites, gastropods, bivalves, radiolarians, bra-chiopods, and residues of plants (Gomez et al., 1995).According to Gonzalez (1980a), faunas within the Quebra-dagrandeComplexwould have lived in epineritic to brackishwaters. However, Gonzalez (1980a) interprets these rocks aspart of a turbiditic sequence, whereas Lozano et al. (1984a),on the basis of the lack of maturity of the sedimentary com-ponents, suggest they accumulated in deep trenches. The fos-sils range in age from Valanginian to Albian (14097 Ma)(Gonzalez, 1980a; Gomez et al., 1995). Toussaint and Rest-repo (1978) report a KAr (whole-rock) age of 105 10Mafrom a basalt of the Quebradagrande Complex. AlthoughKAr dating is notoriously unreliable in volcanic rocks asaltered as those of the Quebradagrande Complex, the agereported by Toussaint and Restrepo (1978) nonethelessagrees well with paleontological ages.

Areno-rudaceous clastic sequences also are associatedwith the Quebradagrande Complex. In northern Colombia,these rocks are known as the Abejorral (Burgl and Radelli,1962), Valle Alto (Gonzalez, 1980a), and La Soledad (Hallet al., 1972) formations; to the south, they are known as theSan Francisco (Orrego et al., 1976) and Rojiza (Orrego,1993) sedimentary sequences (Table 1). The stratigraphicrelationships among these units are dicult to establish,but their discordant deposition on top of the CajamarcaComplex and general transgressive character have beendescribed in several localities (Burgl and Radelli, 1962; Hallet al., 1972; Gonzalez, 1980a). Within the Valle Alto andAbejorral formations, Rodrguez and Rojas (1985) recog-nize sedimentary facies that vary with time from continen-tal to oshore marine to brackish, marine-brackish, andlittoral-marine. The fossils in these rocks indicate theyare not older than Berriasianmiddle Albian (Etayo, 1985).

Imbricated slices of gabbro and ultramac rocks areclosely associated with the Quebradagrande Complex inseveral localities and often show the same degree of defor-mation. The most studied outcrops are the Liborina andSucre peridotites, the Pereira gabbro, the Pacora and Cor-doba complexes, and a series of small bodies mapped as theRomeral gabbros (Calle et al., 1980; Gonzalez, 1980b,c;Meja et al., 1983a,b). Toussaint and Restrepo (1974)

n Earth Sciences 21 (2006) 423436 425group some of these intrusive rocks and volcanic rocks ofthe Quebradagrande Complex within the Cauca ophiolitic

Table 1Stratigraphic units of western Colombia

Western OceanicCretaceous LithosphericProvince

Arquia Complex Quebradagrande Complex Cajamarca Complex

(Upper Cretaceous) CaucaAlmaguerFault

(Neoproterozoic - ?) SilviaPijaoFault

(Berriasian to middle Albian) San JeronimoFault

(Neoproterozoic)

N

S

Marine sediments Areno-rudaceous clastic sequences

Penderisco Fm.: Abejorral Fm. Cajamarca SeriesUrrao Member Valle Alto Fm. Cajamarca GroupNutibara Member La Soledad Fm.

Dagua Group: San Francisco sed. sequenceCisneros Fm. Rojiza sedimentary sequenceEspinal Fm.

Rio Piedras Fm.Ampudia Fm.Marilopito Fm.Aguaclara Fm.

N

S

Plateau volcanics Mainly meta-volcanics and meta-sediments Lavas and pyroclastics

Barroso Fm. Arquia Group Quebradagrande Fm.Diabase Group Bugalagrande Schists Aranzazu-Manizales

(meta)-Sedimentary ComplexAmaime Fm. La Mina GreenschistsVolcanic Fm.

N

S

Mac-ultramac rocks Metamorphic basic plutonics Mac- ultramac rocks

Bolivar Ultramac Complex Rosario Amphibolites Liborina peridotiteGinebra Ophiolitic Massif Bolo Azul Metagabbroids Sucre peridotite

San Antonio Amphibolitesand Metagabbroids

Pacora Complex

Romeral GabbrosPereira gabbro

426A.Nivia

etal./JournalofSouth

America

nEarth

Scien

ces21(2006)423436

as Th. The scatter of the Quebradagrande Complex sam-ples on a variation diagram (Fig. 2) suggests that the alkalishave been relatively mobile in the rocks, which might havedisplaced some samples to the mugearite eld in the totalalkali-silica diagram (Fig. 3). Similarly, the only samplethat contains greater than 62 wt% SiO2 is petrographicallyidentical to the andesites but looks far more altered in thinsection. It is generally considered that large-ion lithophileelements (LILE), such as K, Ba, Sr, and Rb, are relativelymore mobile during low-grade metamorphism than higheld strength elements (HFSE), such as Ti, P, Nb, Y, Zr,and rare earth elements (REE) (Wood et al., 1979; Pearce,1983). Consequently, we place more emphasis on the geo-chemical behavior of the relatively immobile, trace HFSE.Five samples that show petrographical evidence of alter-ation and scatter on variation diagrams that include the

Fig. 2. Th versus K2O diagram for Quebradagrande Complex volcanicrocks. Solid triangles, Group 1; stars, Group 2; solid circles, Group 3;solid diamonds, Group 4.

rican Earth Sciences 21 (2006) 423436 427Complex and suggest a possible cogenetic relationshipbetween them (see also Gonzalez, 1980a).

3. Geochemistry

3.1. Sampling localities

The volcanic rocks of the Quebradagrande Complexwere sampled on a regional basis (Fig. 1) between latitudes635 0N (Santa Fe de Antioquia) and 145 0 N (El RosalCauca). Sampling was performed along main roads thatcut the Quebradagrande Complex outcrop areas, such asSanta Fe de AntioquiaPlanadas (635 N), MedellnEbej-ico (622 0N), ItaguHeliconia (613 0N), FredoniaSantaBarbara (555 0N), ArmaAguadas (537 0N), AguadasPacora (535 0N), PacoraSan Bartolome (533 0N), Pac-oraSalamina (527 0N), SalaminaLa Merced (524 0N),ArmeniaCajamarca (430 0N), PijaoCordoba (420 0N),and BolvarEl Rosal (145 0N).

3.2. Analytical methods

Twenty-six samples from the Quebradagrande Complexwere analyzed for major and trace elements at the geo-chemistry laboratories of Royal Halloway University ofLondon and University of Leicester. Samples were brokeninto chips using manual and hydraulic jaw splinters toremove weathered surfaces and thin veins of altered mate-rial. The samples were further crushed using a ypress of asteel die and shoe. Some 150 g of the coarse material wasground into a ne powder (

latter elements were discarded for geochemicalconsiderations.

3.4. Major and trace element geochemistry

Representative major and trace elements analysis of theQuebradagrande Complex samples appear in Table 2. Ana-lyzed samples were separated into four groups on the basisof their trace element characteristics. On a total alkali-silicadiagram (Le Bas et al., 1986), most samples plot in thebasaltic andesite eld, but some also fall in the basalt,mugearite, and andesite elds (SiO2 = 48.2561.7 wt%),and one sample is a dacite (Fig. 3) whose SiO2 content(66.35 wt%) might be modied by alteration, as indicatedpreviously. The diagram suggests the eld dened by 186samples from the Amaime and Volcanic formations ofthe Western Oceanic Cretaceous Lithospheric Provincereported by Nivia (1987) and Kerr et al. (1997b, 2001).

Compared with these samples, the Quebradagrande Com-plex rocks display a greater range in SiO2 contents.

In an AFM diagram (Fig. 4), the Quebradagrande Com-plex samples straddle the tholeiiticcalk-alkaline boundary;most follow a calk-alkaline dierentiation trend, but someevolved along a tholeiitic trend, as indicated by iron enrich-ment. The eld denedby the samples from theWesternOce-anic Cretaceous Lithospheric Province also indicates adierence in the geochemical evolution of the two provinces.In a variation diagram includingTi (Fig. 5), the higher degreeof dierentiation comparedwith the eld denedby theWes-tern Oceanic Cretaceous Lithospheric Provinces samplescontrast with the low TiO2 (1.30.42 wt%) concentration,which shows that the Quebradagrande Complex samplesare not dierentiated components of tholeiitic mid-oceanridge or plateau series, which show higher Ti contents.

Fig. 6 shows primordial mantle-normalized multielementdiagrams (Sun and McDonough, 1989) for representative

Table 2Representative XRF analyses of Quebradagrande Complex volcanic rocks

Sample QBG95-1 QBG95-3 AN1410A AN1425 AN1426 AN1409 AN1414 AN1412 AN1416

SiO2 52.03 52.48 49.29 53.54 48.25 57.64 61.72 52.04 54.11Al2O3 20.77 17.11 19.59 16.8 18.42 16 16.53 15.67 18.13Fe2O3

a 9.33 11.52 13.07 11.7 13.16 11.91 7.76 8.01 10.07MgO 3.33 5.13 7.16 7.93 7.97 5.22 2.77 10.54 4.02CaO 10.83 8.57 8.33 7.31 9.79 2.07 6.06 9.73 9.15Na2O 2.96 3.04 0.74 1.33 1.02 4.08 3.26 2.01 2.49K2O 0.3 1.29 0.38 0.4 0.35 1.46 0.7 0.39 0.81TiO2 0.66 0.78 1.05 0.6 0.74 0.96 0.82 1.03 0.85MnO 0.18 0.16 0.08 0.14 0.19 0.18 0.18 0.13 0.21P2O5 0.29 0.25 0.1 0.08 0.08 0.14 0.16 0.14 0.12

Total 100.67 100.34 99.94 100.02 100.12 99.94 100.07 100.01 100.14

LOI% 4.38 3.2 4.54 2.14 6.2 3.11 2.26 3.93 2.42

Trace elements in ppm

Ni 1.9 2.2 18.5 28.9 21.1 3.1 4.9 295.1 7.2Cr 8 23.4 35.4 39.2 28.8 4 5.8 851.8 13.7

428 A. Nivia et al. / Journal of South American Earth Sciences 21 (2006) 423436V 210.4 336 399.5 315.5Sc 27.4 33.5 39.9 39.6Cu 88.7 114.3Zn 90.8 77.4Ga 19 17.2 17.6 14.2Pb 8.5 4.7 3.1 4.4Sr 268.5 483.5 261.6 336.5Rb 7 28.2 4 8.3Ba 212 553.8 122.5 209.2Zr 63.1 86.7 46 19.4Nb 0.7 1 1 2.5Th 4.2 3.3 0.4 0.4Y 17.6 17.3 17.9 11.4La 8.6 9.4 1.9 2.2Ce 18.7 22.8 8.7 7.9Nd 13 15.1 6.5 4.6

Interelement selected ratios

La/Nb 12.3 9.4 1.9 0.9Ba/Zr 3.4 6.4 2.7 10.8CeN/YN

b 2.9 3.7 1.3 1.9

Major elements recalculated to a volatile-free total.

a Total iron reported as Fe2O3. LOI Losses on ignition.b Chondrite-normalized values.398 266.5 214 239.5 27141 37.9 26.8 36.8 38.8147.5 29.4 91.5 53.3 71.894.9 112.5 116.2 60.2 112.916.2 17.9 14.3 13.5 18.23.3 2.6 6.1 0.8 6.6

189.3 125.7 242.5 445.3 325.76.8 11.6 12.2 5.2 15.1

143.1 1658.7 175.8 199.2 443.327.7 67.8 71.8 72.6 64.30.9 1.3 4.7 2 1.41.1 1.8 1.9 0.8 112.4 29 24 23.2 26.72.9 6 5.7 2.6 3.610.7 16.4 15.1 9.3 11.76.3 11.8 10.7 10.7 9.3

3.2 4.6 1.2 1.3 2.65.2 24.5 2.4 2.7 6.92.4 1.6 1.7 1.1 1.2

ricaFig. 4. AFM diagram for Quebradagrande Complex volcanic rocks.Symbols as in Figs. 2 and 3.

A. Nivia et al. / Journal of South Amesamples of the Quebradagrande Complex. All the diagramsdisplay LILE enrichment relative to HFSE, particularlyNb. Also, Ba, K, and Sr enrichments are conspicuous bytheir pronounced peaks in the diagrams that may reach,as in the case of Ba and Sr, up to 237 and 58 times themantle-normalized concentrations (1659 ppm Ba and1228 ppm Sr). Conversely, the depletion in Nb is outstand-ing by the throat it displays in all diagrams. The LILEenrichment relative to HFSE is usually coupled with LREEenrichment relative to HREE. The degree of enrichment inLREE can be monitored using the chondrite-normalized(Nakamura, 1974) CeN/YN ratio that, in the Quebrada-grande Complex, varies between 1 and 4 times the chon-dritic values, suggesting at to enriched REE patterns(Fig. 7).

According to the shape of their multielement-normal-ized pattern, the samples can be divided into four dierentgroups (Fig. 6). This separation is based mainly on inter-HFSE ratios, which we believe represent inherited featuresfrom the source of the magmas. For example, Group 1 ischaracterized by its low Y/P ratio values (Group 1 = 0.20.5; Group 2 = 0.30.9; Group 3 = 0.71; Group 4 = 0.61.1), and Group 2 is distinguished by its high Ti/Zr ratios(Group 1 = 0.30.7; Group 2 = 12.1; Group 3 = 0.71;Group 4 = 0.71.2). However, groups are also homoge-neous in their LILE and major element characteristics.

Fig. 5. Th versus TiO2 diagram for Quebradagrande Complex volcanicrocks. Symbols as in Figs. 2 and 3.Group 1 possesses the highest LILE enrichments relativeto HFSE, as indicated by their highest Th contents (2.64.2 ppm) and La/Nb ratio values (112.3) compared withthe other dened groups (Group 2: Th = 0.42.7, La/Nb = 0.93.8; Group 3: Th = 1.31.9, La/Nb = 0.74.6;Group 4: Th = 0.51.4, La/Nb = 0.84.4). Fig. 7 suggeststhat Group 1 also displays the most enriched LREE pat-terns and a general intergroup trend of increasing CeN/YN ratios with increasing fractionation (Group 1 = 2.63.8; Group 2 = 12.7; Group 3 = 1.41.7; Group 4 = 0.61.3). According to the FeO* and TiO2 contents, Group 1exhibits the most calc-alkaline behavior, as also is indicatedby its position on the AFM diagram (Fig. 4) and thedecreasing TiO2 content with increasing fractionation(Group 1 = 0.420.78; Group 2 = 0.61.3; Group3 = 0.70.9; Group 4 = 0.61.3 wt%).

4. Petrogenesis

4.1. Fractional crystallization FeTi oxide arguments

Most of the analyzed Quebradagrande Complex sam-ples seem to have evolved along two crystallizationtrends: a calc-alkaline and a more tholeiitic (Fig. 4).The occurrence of calc-alkaline characteristics supportsa supra-subduction zone environment of origin for theQuebradagrande Complex. Although tholeiitic crystalliza-tion trends are present in most environments where basicvolcanic igneous rocks are generated, calc-alkaline trendsare exclusive to magmatic environments with a conver-gent margin (Wilson, 1987). Both trends are interpretedas the result of fractional crystallization of olivine, pla-gioclase, and clinopyroxene. The main dierence betweenthe two trends is the control exerted over the FeTi oxi-des during crystallization (Gill, 1981). Experimental evi-dence (Grove and Baker, 1984) shows that subalkaline,anhydrous magmas crystallizing in the crust under geo-logically reasonable oxygen fugacity follow tholeiitic dif-ferentiation trends. In hydrated basaltic magmas,dissolved water reduces olivine, pyroxene, and plagio-clase stability without aecting the thermal stability ofFeTi oxides (Sisson and Grove, 1993), which results inthe early crystallization of FeTi oxides from a calc-alka-line magmatic system and lower Fe and Ti contents inthe resultant magmas than in a tholeiitic crystallizationsequence where the fractionation of FeTi oxide isdelayed, which increases the Fe and Ti contents of themagma. Thus, water activity inuences the early or latecrystallization of titaniferous magnetite, and subalkalinemagmas with high water contents follow calc-alkalinedierentiation trends, whereas those with low water con-tents display tholeiitic trends.

4.2. Trace element arguments

n Earth Sciences 21 (2006) 423436 429The LILE enrichment in volcanic rocks can be producedby several processes, such as oceanic mantle contaminated

Fig. 6. Normalized multielement plot of Quebradagrande Complex volc

Fig. 7. CeN/YN versus Th diagram for Quebradagrande Complexvolcanic rocks. Symbols as in Fig. 2.

430 A. Nivia et al. / Journal of South American Earth Sciences 21 (2006) 423436by deeper, uncirculated mantle plumes; low percentages offusion in the source; mantle metasomatism by subductingplate-derived uids; or contamination. The characteristicfeature of magmas originated from supra-subduction zonemantle wedges is the LILE enrichment relative to HFSE(Saunders et al., 1980; Tarney et al., 1981; Saunders andTarney, 1982; Pearce, 1983). This feature, produced bychemical fractionation between LILE and HFSE in thehydrous environment associated with subduction zones, isoutstanding in the pronounced Nb negative anomaly dis-played by the Quebradagrande Complex in the spider-grams. The strong chemical behavior anity (i.e., similarincompatibility with the original mantle and basic volcanicrocks mineralogies) of elements like La (LILE), Nb, and Ta(HFSE) is well known, such that these elements usually areplaced at adjacent positions in multielement-normalized

anic rocks. Normalizing values from Sun and McDonough (1989).

diagrams (Bougault et al., 1985). These elements are notfractionated by processes like partial fusion or fractionalcrystallization. However, metamorphic processes actingon the subduction zone produce dehydration of the sub-ducting oceanic crust with consequent production ofhydrous supercritical uids that transport water and solu-ble LILE (e.g., La) to the supra-subduction zone mantlewedge (Saunders et al., 1980; Tarney et al., 1981). TheHFSE (e.g., Nb) can be retained by the subducting oceaniccrust (then an eclogitic assemblage) and returned to themantle (Saunders and Tarney, 1982). The latter can beascribed to the hydrous environment in the subductionzone, which favors the stability of mineral phases such asrutile or ilmenite that retain HFSE (Tarney et al., 1981).Fig. 8 shows a plot of La/Y versus Nb/Y (La and Nb nor-malized to Y to eliminate the eects of fractional crystalli-zation) for the Quebradagrande Complex samples. Thisdiagram is useful for separating samples that contain a sub-

A. Nivia et al. / Journal of South Americaduction-related component (low La/Nb ratios) from thosederived from an oceanic mantle source region, far from theinuence of a subduction zone (e.g., mid-ocean ridge bas-alts, mantle plume-generated oceanic plateau basalts). Thisdiagram also shows elds encompassing the basalts fromthe Amaime and Volcanic formations of the Western Oce-anic Cretaceous Lithospheric Province (Nivia, 1987; Kerret al., 1997b, 2001) and lavas of the recent Andean volca-noes of Colombia (Marriner and Millward, 1984). Theenvironment of formation for the former rocks was unre-lated to a subduction zone (Millward et al., 1984; Nivia,1987; Kerr et al., 1996, 1997a,b, 2001), whereas the latterare modern examples of rocks produced in a destructiveplate margin. In this diagram, the chemical dierencesbetween the basalts of the Western Oceanic CretaceousLithospheric Province and the subduction-related Quebra-dagrande Complex are evident. It is thus highly unlikelythat the Quebradagrande Complex lavas and pyroclasticsrepresent part of the same tectonomagmatic event.Fig. 8. Nb/Y versus La/Y diagram for Quebradagrande Complexvolcanic rocks. Symbols as in Fig. 2.5. Discussion

On the basis of major element chemical analyses, Gon-zalez (1980a) proposes that the Quebradagrande Complexis composed of tholeiitic rocks generated in an oceanic rift.Bourgois et al. (1982, 1985, 1987) and Toussaint and Rest-repo (1993) propose the same origin but also suggest thatthe Quebradagrande Complex had been thrust from thewest onto the continental margin, along with basic rocksthat outcrop west of the Cauca-Almaguer fault (i.e., theWestern Oceanic Cretaceous Lithospheric Province).According to this model, a single common mantle sourceregion is responsible for both the Quebradagrande Com-plex and the volcanic rocks of the Western Oceanic Creta-ceous Lithospheric Province (Barroso, Amaime, Volcanicformations, Table 1). Similarly, Kammer (1995) and Kam-mer and Mojica (1996) consider a close anity between therocks of the accreted Western Oceanic Cretaceous Litho-spheric Province and the Quebradagrande Complex. Theresults presented herein clearly show a subduction zone-de-rived component in the geochemical composition of theQuebradagrande Complex. However, the geochemicalcharacteristics of the Quebradagrande Complex dier fun-damentally from those of the basic volcanic rocks of theWestern Oceanic Cretaceous Lithospheric Provinceexposed in western Colombia (Millward et al., 1984; Nivia,1987, 1989; Kerr et al., 1996, 1997a,b). In addition, thereported ages of the Western Oceanic Cretaceous Litho-spheric Province indicate these rocks were formed duringthe Late Cretaceous, whereas evidence for the Quebrada-grande Complex indicates an Early Cretaceous age. Thus,the volcanic rocks of the Quebradagrande Complex andthe Western Oceanic Cretaceous Lithospheric Provincewere generated from two dierent, unrelated mantle sourceregions; in turn, the geotectonic models and interpretationsof a common genesis for both are inherently incorrect.

Geochemical data suggest that the volcanic rocks of theQuebradagrande Complex formed during the Lower Creta-ceous in a supra-subduction zone environment, in an islandarc, marginal basin, or active continental margin. To eval-uate these possibilities, the regional relationships betweenthe Quebradagrande Complex and adjacent rocks mustbe considered. The Quebradagrande Complex outcropsare bounded by the metamorphic Cajamarca and Arquacomplexes (Fig. 1, Table 1). The age of the CajamarcaComplex, to the east, is constrained as Neoproterozoic(Gomez and Nunez, 2003), but the age of the Arqua Com-plex to the west is controversial. Some researchers believethe Arqua Complex was formed during the Lower Creta-ceous (Toussaint and Restrepo, 1989, 1993; Restrepo et al.,1991; Gonzalez and Nunez, 1991; Gonzalez, 1993, 2001),whereas others think it Paleozoic in age (McCourt andAspden, 1983; McCourt et al., 1984; Aspden et al., 1987).However, both the presence of Triassic granitoid plutons,such as the Santa Barbara Batholith and Amaga and Cam-

n Earth Sciences 21 (2006) 423436 431bumbia stocks, intruding schists west of the Quebrada-grande Complex (Fig. 1; McCourt et al., 1984; Meja

ricaet al., 1983b; Restrepo et al., 1991) and the occurrence ofPaleozoic rocks west of the Quebradagrande Complex(Mosquera, 1978; Calle et al., 1980; Meja et al., 1983a,b;Gonzalez, 2001) are strong arguments in favor of a Neo-proterozoic age for the Arqua Complex. Assuming aPaleozoic age for the Arqua Complex, McCourt et al.(1984) present an evolutionary model for the northernAndes. According to this model, during the Carbonifer-ous, the continental margin of the northern Andes wascomposed of an autochthonous block (Cajamarca Com-plex) and accreted island arc (Bugalagrande schists,Rosario amphibolites, and Bolo Azul metagabbroids;Arqua Complex, Table 1). We concur with this modeland believe that the data presented here support a modelin which the Quebradagrande Complex is related to amagmatic environment associated with a continentalactive margin.

Processes operating in subduction zones result in dehy-dration of the subducting oceanic crust and transport ofgenerated uids into the overlying mantle wedge. Theinux of such volatile-rich uids into the mantle wedgelowers the solidus of the mantle, resulting in meltingand the formation of hydrated magmas. However, thewater (volatile) content of the magmas may vary accord-ing to the distance between the centers of volcanic erup-tion and the trench. For marginal basins, it depends onthe degree of evolution, that is, the development of thebasin. As marginal basins (sensu stricto) open andthe expansion center separates from the island arc, thehydrous (subduction) component becomes less pro-nounced (Saunders and Tarney, 1984; Atherton and Agu-irre, 1992). In this way, the calc-alkaline and tholeiitictrends and the LILE/HFSE ratio in the QuebradagrandeComplex could be controlled by the preeruptive contentsof the hydrous component in the mantle. As we notedpreviously, the samples can be separated into four dier-ent chemical types (Fig. 6), which may be interpreted asheterogeneities produced in the mantle during the evolu-tion of the basin. Thus, Group 1 samples, which appearto contain more of the subduction component(s) (higherLILE/HFSE ratios), could have been generated duringthe initial stages of basin opening. During latter stagesof basin opening, progressive dilution of this componentin the source region, as reected in the lower LILE/HFSEratios of Groups 2 and 3, culminates in the Group 4 sam-ples, which display the lowest La/Nb and CeN/YN ratios(Figs. 7 and 8). Parallel trends between groups in bivari-ate diagrams for the same degree of dierentiation favorthis hypothesis. Furthermore, increasing values of theCeN/YN ratios with increasing Th in a Th versus CeN/YN diagram (Fig. 7) could be related to intersample var-iation due to dierentiation processes, whereas parallelintergroup trends suggest dierent LILE enrichment inthe mantle. However, the strong deformation of theQuebradagrande Complex inhibits any reconstruction of

432 A. Nivia et al. / Journal of South Amethe basin that might help evaluate this hypothesis in termsof geochemical characteristics.To explain the Quebradagrande Complex genesis, wepropose a petrogenetic model in which the rocks formedduring the opening of a marginal ensialic basin. Aberget al. (1984) and Aguirre (1987) propose that these basinsmight form by ensialic expansion or subsidence mecha-nisms promoted by the rollback action of the subductinglithosphere on the active continental margin. These mech-anisms may lead to crustal thinning with consequent adia-batic decompression melting of the mantle, whichproduced both basins and magmatism. The volcanic prod-ucts erupt over the continental crust or, in cases of extremeextension, result in the complete rupture of the continentalmargin and generation of oceanic crust (Aberg et al., 1984;Aguirre, 1987; Atherton and Aguirre, 1992).

Ophiolitic sequences proposed as generated by theseprocesses are common to the Uppermost Jurassic andLower Cretaceous of the South American Pacic margin,extending from its southernmost extreme in South Geor-gia Island (Chile) to central-western Mexico; the latter iscontinuous with eastern Colombia in most TriassicJu-rassic Pangea reconstructions (Coney and Evenchick,1994). Well-documented ensialic marginal basins arefound in the ophiolitic complexes of the Larsen Peninsu-la, Tortuga and Sarmiento at South Georgia Island,Tierra de Fuego, and Patagonia (Tarney et al., 1981;Saunders and Tarney, 1982; Miller et al., 1994; Sternet al., 1976); the Rocas Verdes of central and southernChile (Aberg et al., 1984); the Puente Piedra and Casmaformations in Peru (Atherton et al., 1985; Aguirre andOer, 1985; Atherton and Aguirre, 1992); the Celica For-mation of southern Ecuador and northern Peru (Aguirreand Atherton, 1987; Lebras et al., 1987); and the inter-bedded calc-alkaline volcanics and sedimentary stratacontaining shallow marine fauna and imbricated withserpentinite bodies, ultramac cumulates, and podiformchromite of the Siuna Terrane of Nicaragua and Hondu-ras, which extends northward into the Guerrero Terraneof southern Mexico. In the latter, Elias-Herrera and Ort-ega-Gutierrez (1998) posit the upper volcanoclasticsequence was generated in a continental margin backarcbasin setting. The spatial, chronological characteristicsand geochemical composition of the QuebradagrandeComplex suggest they belong to this belt.

The petrogenetic model proposed for the origin of theQuebradagrande Complex is illustrated in Fig. 9. In thismodel, subduction along the proto-Pacic Colombian mar-gin produced distension and crustal thinning over the con-tinental margin. The thinning resulted in the formation of abasin, as evidenced by the accumulation of transgressivesedimentary sequences (e.g., Valle Alto, Abejorral; Table1). The genesis of these arenorudaceous clastic sequenceswould agree with the ensialic marginal basin model if theyrepresent and show the rst stages of basin opening. On theoceanic plate, the progressive increase of temperature andpressure as subduction processes produced metamorphism

n Earth Sciences 21 (2006) 423436with consequent dehydration. The corresponding subduc-tion-liberated uids moved upward, adding H2O, LILE,

AB

C

D

Fig. 9. Diagrammatic sketch illustrating evolution of the Quebradagrande ComplexQGC marginal ensialic basin. (A) Subduction and consequentmetasomatism of subcontinental mantle under Paleozoic crust of an oceanic crustal block (Arqua Complex) accreted to the continental crustal block(Cajamarca Complex + shield). (B) Rollback of the continental margin leads to lithospheric thinning and subsequent generation of basins and adiabaticmantle melting. (C) Formation of marginal basin by generation of oceanic oor in the zone of backarc spreading. (D) Closure of basin, probably due toforces produced on the continental plate during the aperture of the South Atlantic.

A. Nivia et al. / Journal of South American Earth Sciences 21 (2006) 423436 433

Rodrguez, C., Munosz, J. Duran, J., 1980. Mapa Geologico de

ricaand LREE to the subcontinental mantle wedge. The addi-tion of these uids lowered the solidus; combined with adi-abatic decompression induced by crustal thinning, itresulted in mantle melting. The magmas produced werecalc-alkaline basalts that dierentiated at crustal levels toform andesites and dacites (Quebradagrande ComplexGroups 1 and 2). Basin opening culminated in the genera-tion of oceanic crust, represented today by ophiolitic com-plexes. According to this petrogenetic model, more basicvolcanic rocks (Groups 2 and 3) and sedimentary rocksof the Quebradagrande Complex accumulated on top ofthe basin, whereas the associated mac and ultramac plu-tonic rocks (Table 1) represent deeper horizons of theophiolitic complexes developed by ocean oor formationduring the opening of the basin.

The opening of the basin led to the gradual movement ofthe locus of magmatism away from the trench, which mayhave resulted in the dilution of the subduction zone-derivedcomponent and a change in the magmatic products fromcalc-alkaline to tholeiitic. According to Saunders and Tar-ney (1984), marginal basins are short-life geotectonic fea-tures. Furthermore, Dalziel (1981) suggests that inmarginal basins of southern Chile, collapse or closure coin-cides with the opening of the South Atlantic. Thus, thechange in the velocity of plate displacement induced bythe South Atlantic opening also may have promoted theclosure of the Quebradagrande Complex marginal basin.However, the accretion of the CaribbeanColombian oce-anic plateau in the late Cretaceousearly Tertiary likelycontributed signicantly to the closure of the basin anddeformation of this unit.

6. Conclusions

Regional sampling of the Quebradagrande Complexbetween 635 0N and 145 0N in Colombia shows that basal-tic andesites and andesites have geochemical characteristicstypical of rocks formed in supra-subduction zone magmat-ic environments. These rocks follow two contrasting dier-entiation trends: One is calc-alkaline, the other tholeiitic.

The geochemical characteristics of the QuebradagrandeComplex rocks and their spatial and chronological rela-tionships with the Cajamarca and Arqua complexes, theultramac and mac Cretaceous rocks, and the arenoruda-ceous Lower Cretaceous related sequences can be integrat-ed into an evolutionary petrogenetic model of ensialicmarginal basin opening.

The model has global tectonic implications, in that itforms an important link, through the northern Andes, ofthe chain of marginal basins that extended from Tierrade Fuego to Mexico in the Early Cretaceous. During thisstratigraphic interval, subduction was active along theSouth American margin.

The results we present clearly demonstrate that thereis no genetic relationship between the Quebradagrande

434 A. Nivia et al. / Journal of South AmeComplex and the Upper Cretaceous volcanic rocks thatoutcrop west of the Cauca-Almaguer fault rocks thatColombia Escala 1:100.000, Plancha 166 Jerico: INGEOMINAS.Bogota.

Coney, P.J., Evenchick, C.A., 1994. Consolidation of the Americanare well documented to have formed in an oceanic pla-teau setting.

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The Quebradagrande Complex: A Lower Cretaceous ensialic marginal basin in the Central Cordillera of the Colombian AndesIntroductionRegional geology and stratigraphic nomenclatureGeochemistrySampling localitiesAnalytical methodsAlterationMajor and trace element geochemistry

PetrogenesisFractional crystallization ldquo Fe-Ti oxide arguments rdquo Trace element arguments

DiscussionConclusionsReferences