Grupo 3. CretaceousBasalticTerranesWesternColombia

8/3/2019 Grupo 3. CretaceousBasalticTerranesWesternColombia

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JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 PAGES 677702 1997

Cretaceous Basaltic Terranes in Western

Colombia: Elemental, Chronologicaland SrNd Isotopic Constraints onPetrogenesis

A. C. KERR1, G. F. MARRINER2, J. TARNEY1, A. NIVIA3,A. D. SAUNDERS1, M. F. THIRLWALL2 AND C. W. SINTON4

1DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, UNIVERSITY ROAD, LEICESTER LE1 7RH, UK

2DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK

3INGEOMINASREGIONAL PACIFICO, AA 9724, CALI, COLOMBIA

4COLLEGE OF OCEANOGRAPHY, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331, USA

RECEIVED AUGUST 19, 1996 REVISED TYPESCRIPT ACCEPTED JANUARY 10, 1997

Accreted terranes comprising Mid to Late Cretaceous picrites, basalts an oceanic spreading centre, and valuable comparisons can be madebetween Iceland and the CaribbeanColombian plateau.and dolerites occur in three northsouth trending belts in western

Colombia, in the Central Cordillera, Western Cordillera and along

the Pacific coast. The geochemistry of these rocks is consistent withan oceanic plateau (plume-related) origin, and they most probably

formed in the Pacific as part of the Caribbean oceanic plateau.KEY WORDS: basalt; Colombia; geochemistry; mantle plume; oceanicThese igneous rocks display small but significant inter-cordillera

plateauvariations, being younger and more depleted in incompatible traceelement ratios (and with more positive Nd values) to the west. Theigneous rocks of the Pacific coast (Serrana de Baudo) are dated at7378 Ma (40Ar/39Ar), and those of the Western Cordillera at

INTRODUCTION~90 Ma, whereas the volcanics of the Central Cordillera areThe relative contribution of lithospheric vs as-believed to be older than 100 Ma. Most of the igneous rocks arethenospheric (plume) mantle sources to continental floodbasaltic, and it is suggested that they have fractionated from picriticbasalt volcanism is a contentious subject in the pet-primary magmas, generated by partial melting within a hot mantlerological literature (e.g. Gallagher & Hawkesworth, 1992;plume. Variable and positiveNd values reveal that the plume must

Saunders et al., 1992; Arndt et al., 1993; Thirlwall et al.,have been heterogeneous, originating from a mantle source with a1994; Gibson et al., 1995). One thing seems relativelylong-term history of depletion. Partial melt modelling suggests thatclear: only a small percentage of plume-derived con-the composition of the basalts requires at least some input from atinental flood basalts pass cleanly through the continentalmantle source region containing garnet and that the extent of partiallithosphere, with the rest being contaminated. The naturemelting required to reproduce the composition of the erupted basaltsof this contamination and the discussions bearing on itsis of the order of ~20%. Mixing of melts from different depths,relative importance have tended to divert attention awayeither in the mantle melting column or during fractionation infrom the primary cause of the volcanism, the plume itself.lithospheric magma chambers, can explain the relative homogeneityIncreasingly, however, it has been realized that mantleof basaltic lavas erupted to form this (and other) oceanic plateaux.

The CaribbeanColombian oceanic plateau may have formed at plumes form not only continental flood basalts but also

Corresponding author. Telephone: +44 116 2523639. Personal fax:

+44 116 2523639. Department fax:

+44 116 2523918.e-mail: [email protected]

Present address: Graduate School of Oceanography, Rhode Island

University, Narragansett, RI 02882, USA Oxford University Press 1997


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KERR et al. CRETACEOUS COLOMBIAN BASALTS

Fig. 1. Map showing the location of Cretaceous basalts of the CaribbeanColombian oceanic plateau. Also shown are Deep Sea Drilling Project(DSDP) Leg 15 drill sites.

1974; Mooney, 1980; Bourgois et al., 1982). However, as CRETACEOUS COLOMBIANNivia (1987) has pointed out, typical ophiolitic sequences,

BASALTIC TERRANESFIELDmost of which have sheeted dyke complexes, indicativeRELATIONS AND SAMPLINGof spreading ridges, are not found in the Cretaceous

accreted terranes of Colombia. Here we evaluate more The Cretaceous mafic sequences form three main beltsfully the suggestion by Millward et al. (1984), Nivia (1987), (Fig. 2) which trend approximately NNESSW, namelyStorey et al. (1991) and Kerr et al. (1996c ) that the the Central Cordillera, the Western Cordillera and theCretaceous Colombian mafic terranes formed as part of Serrana de Baudo along the Pacific coast.

an oceanic plateau, resulting from partial melting of amantle plume, which generated thicker than normaloceanic crust.

Central CordilleraPrevious studies (e.g. Millward et al., 1984; Spadea etal., 1987; Storey et al., 1991) have tended to focus on the The mafic igneous rocks of the western flank of themajor and trace element composition of basalts from Central Cordillera occur in several discontinuous lenses,relatively small areas. This study, however, is the first to from Medelln in the north to Pasto in the south. Thereport comprehensive major and trace element data, exposures are bounded to the east by the Romeral FaultSrNd isotopic analyses, and precise 40Ar/39 Ar ages, (Fig. 2). Cretaceous volcanics of the Central Cordillerafor the Cretaceous basalts throughout a wide area of also extend along the trace of the Romeral Fault intoColombia. In doing this we hope to shed some new light northern Ecuador (Lebras et al., 1987).on the petrogenesis and original tectonic setting of the The largest continuous outcrop of mafic volcanics in

the Central Cordillera, known as the Amaime FormationCretaceous Colombian basalts.

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JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997

Fig. 2. Map of western Colombia showing the three main belts of Cretaceous igneous rocks, along with sample localities mentioned in the text.

(Fig. 3b) (McCourt et al., 1984), is 140 km long, 520 Ultramafic Complex (Nivia, 1987) (Fig. 3b). This bodyconsists of a sequence of dunites, wehrlites and layeredkm wide and bounded by the Romeral Fault to the east

and by the GuabasPradera Fault to the west. The and isotropic gabbros, which are overlain by am-phibolitized basalts. Aspden & McCourt (1986) proposedformation consists of both massive and pillowed tholeiitic

basalts and occasional cumulate picrites which occur in that this sequence represents a lower-crustal level of the Amaime Formation. Other exposures of the igneousseveral fault-bounded blocks of 510 km width. The

occurrence of picrites within the Amaime Formation has rocks of the Central Cordillera shown in Fig. 2 includethe pillowed picritic and picritic-basalts and tuff-brecciasalso been noted by McCourt et al. (1984) and Spadea et

al. (1989). found at El Encenillo, 20 km SSW of Popayan, and theLos Azules complex, which consists mostly of a series ofTo the west of the GuabasPradera Fault an elongate

(northsouth) body of mafic and ultramafic rocks (30 km ultramafic cumulates with some massive and pillowedpicritic to basaltic lavas (Spadea et al., 1989). Somelong 7 km wide) is exposed, known as the Ginebra

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Fig. 3. Close-up maps of (a) the Serrana de Baudo and (b) the area around Cali [after Nivia (1987)] detailing sample locations. The areascovered by these maps are outlined in Fig. 2.

40 km north of Pasto a sequence of pillowed and massive town of Sevilla and due west of Buga (AMA112), aswell as west of Florida (FLO14); Fig. 3]. Basaltic sampleslavas is exposed along a new road cut traversed by thewere collected along the Pan American Highway nearPan American Highway, near the village of Taminango.Taminango (YAN18), and at El Encenillo (Fig. 2) blocksAlong the eastern margin of the Romeral Fault a 510in a picrite breccia (ROM4ii) and a picritic flow (ROM2)km wide belt of high-pressure lawsoniteglaucophanewere sampled. The volcanic section of the Los Azulesschists and eclogites can be found stretching from south-complex (Fig. 2) yielded basalts and cumulate and non-ern Ecuador to northcentral Colombia. These high-cumulate picrites. Only the non-cumulate picrites andgrade metamorphic rocks are associated with highly

basalts (LER4, AZU2 and -3 and ROM510) will betectonized and serpentinized ultramafic rocks and gab- discussed in this paper. A basaltic sample (BAR2) wasbros (McCourt & Feininger, 1984), and are known asalso collected from a minor fault-bounded sliver 30 kmthe Arqua Complex (Fig. 2). Exposed to the east of theSW of Medelln, which is believed to be part of theArqua Complex is a discontinuous (510 km wide) beltbasaltic sequence of the Central Cordillera.of basalts, andesites and tuffs, tectonically intermixed

with Palaeozoic low-grade schiststhe QuebradagrandeComplex (Fig. 2). Preliminary geochemical studies (A.Nivia et al., unpublished data, 1996) of lavas and tuffs

Western Cordillerafrom the Quebradagrande Complex suggests that theyhave been formed in a subduction-related tectonic setting. The most extensive outcrops of Cretaceous basalt in

During the present study, basalts and several cumulate Colombia occur in the Western Cordillera (Figs 2 andpicrites were sampled from the main outcrop of the 3), which is separated from the Central Cordillera by the

CaucaPatia Graben, and from the coastal Serrana de Amaime Formation east of Cali [SW and NE of the

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Table 1: Stratigraphic nomenclature of the rocks of the Western Cordillera

Northern Western Cordillera SouthCentral Western Cordillera

Barroso Formation (volcanic) Volcanic Formation (volcanic)

Can asgordas Group

Penderisco Formation (sedimentary) } Espinal Formation (sedimentary)Cisneros Formation (sedimentary)

Baudo sequence by the San JuanAtrato Trough (Fig. and have undergone low-grade (zeolite and prehnitepumpellyite facies) metamorphism (Barrero, 1979). As-2). The igneous rocks crop out in NNESSW trending

fault-bounded slices which can be up to 15 km wide, sociated sedimentary lenses (chert and grey shales) areusually 3 km). The presence of a few tuffswithin the lava succession of the Western CordilleraEspinal Formations (Barrero, 1979; Aspden, 1984). The

Cisneros Formation may be up to 2000 m thick (Barrero, suggests that as the lava pile accumulated, eruptionsmay have occurred in shallower water; nevertheless, no1979; Millward et al., 1984); it consists of a sequence of

strongly deformed slates and phyllites with thin lenses of evidence of subaerial eruptions has so far been foundIn the eastern part of the Western Cordillera, at 415Nmeta-limestones, cherts and greywackes, and is in faultedcontact with igneous rocks of the Western Cordillera. In (Fig. 3b) a suite of ultramaficmafic rocks crops out

(Barrero, 1979; Nivia, 1996), known as the BolvarRocontrast, the ~700 m thick Espinal Formation is com-posed of a series of relatively unmetamorphosed cherts, Frio ultramafic complex (Kerr et al., 1997). This complex

consists of both layered and isotropic gabbros and norites,shales and pebbly sandstones, and appears to be con-formable with the volcanics of the Western Cordillera structurally underlain by serpentinized dunite containing

bands of both clinopyroxenite and olivine gabbronorite.(McCourt et al., 1984; Millward et al., 1984). It hasrecently been suggested by Nivia et al. (1996) that the The composition of the rocks from the Bolvar complex

strongly suggests that they are genetically related to thedifference in deformation between the two formations isdue to the relative competence of each formation, with the basalts of the Western Cordillera (Nivia, 1996; Kerr et

al., in preparation) and that they could represent thegenerally finer-grained Cisneros sediments being moredeformed than the coarser Espinal Formation sediments. lower section of the CaribbeanColombian oceanic plat-

eau (Kerr et al., 1997).Palaeontological evidence (Barrero, 1979), suggests thatthe Cisneros Formation is older than the Espinal (and In the northern section of the Western Cordillera,sediments make up more than half of the outcrop. TheseVolcanic) Formation.

The Volcanic Formation (formerly the Diabase Group) sediments, the Penderisco Formation, consist of asequence of cherts, black micritic limestones and depositsconsists of a >5 km thick (Barrero, 1979) sequence of

both pillowed and massive basaltic lavas, dolerites, local with possible turbiditic characteristics (Alvarez & Gon-zalez, 1978). The associated volcanic rocks, the Barrosogabbros and rare tuffs. The pillow lavas can be up to 50

m thick, and hyaloclastite breccia is commonly found Formation, include basalts (with some pillows), dolerites,hyaloclastites, thin tuffs, and volcanic breccias, with oc-between the individual pillows, which sometimes preserve

chilled skins and occasionally concentrically arranged casional lenses of pelites and cherts.Along the western periphery of the Western Cordillera,amygdales. Coarser-grained basalts and dolerites form

either massive flows or intrusive sheets (200300 m thick). a sequence of Early Tertiary subduction-related volcanicscrop out, the Dabeiba Volcanic Arc (Tistl & Salazar,The basalts and dolerites are highly sheared in places

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Table 2: 40Ar39Ar plateau and isochron age calculations from Western Colombia

Sample Location Material Plateau age by 39Ar % of Isochron age N SUMS/ 40Ar/36Ar

1/2

(m.y.) total ( Ma) (N 2) inte rc ept1

SDB11 Serr ana de Baudo whole rock 73608 98 72908 5 260 304429

SDB11 Serr ana de B audo plagiocla se 72 504 95 71814 10 055 324243

SDB16 Serr ana de B audo plagioclas e 77910 35 76211 3 058 3106118

AN1464 Dabeiba Volcanic Arc plagioclase 43104 72 41486 5 1703 3232137

PAN6 West er n Cor di ll er a whole rock 91 727 89 94764 3 025 2924156

Ages calculated using the following decay constants: e=05811010 yr1, b=496310

10 yr1. Isotopic interferences for theOSU TRIGA reactor are: ( 36Ar/37Ar)Ca=26410

4, ( 39Ar/37Ar)Ca=673104 and ( 40Ar/39Ar)K=10010

3. Reported errors are 2.Sample PAN6 is from Sinton (1996).

1994). The basalts, andesites and pyroclastic rocks of this not reported by previous workers (Goossens et al., 1977;Macia, 1985), intercalations of fine-grained sandstone,arc are best developed to the west of Medelln (Fig. 2),chert and limestones were found between some of theand the rocks appear to be significantly younger thanbasalts. The nature of these intercalated sediments impliesthe basalts of the Western Cordillera. 40Ar/39 Ar datinga shallower eruption depth for these basalts than forof a basalt from the Dabeiba Volcanic Arc (AN1464),those of the Western Cordillera, and this is perhaps ausing step heating techniques, yielded a plateau age ofreflection of their younger age (see below). The basalts43104 Ma (i.e. Eocene) (Table 2).are unconformably overlain by a poorly exposed sequenceTo the south, in Ecuador, the Macuchi Formation hasof subduction-related basalts which are intercalated withbeen widely correlated with the Volcanic FormationEocene limestones (Gansser, 1973).(McCourt et al., 1984; Lebras et al., 1987; Wallrabe-

McGeary & Ben-Avraham (1986) proposed that the Adams, 1990; Van Thournout et al., 1992). This for-volcanic succession on Gorgona Island (Fig. 2) representsmation consists of a more westerly subduction-relateda continuation of the basalts of the Serrana de Baudo.sequence of lavas and volcaniclastic sediments (Hend-

Gorgona Island is the location of the worlds only knownerson, 1979; Lebras et al., 1987). A scattered discontinuous Phanerozoic komatiites (Echeverra, 1980; Kerr et al.,belt of basalts intercalated with silicified shale, along with1996a ). However, despite extensive sampling along theperidotites and layered gabbros, crops out along thecoast from 710N to 530N (Fig. 3a), no evidence ofeastern margin of the belt, and has been termed the mid-lavas with spinifex (komatiitic) textures was found. Basaltsocean ridge basalt of the Macuchi Formation (Lebras etand some gabbros were collected from the Serrana deal., 1987).Baudo coast (SDB125; Fig. 3a). Dense jungle and deepWestern Cordillera igneous rocks were sampled fromtropical weathering meant that sampling was restrictednew road cuts along the Cali to Buenaventura roadto coastal exposures.(CBU217; Fig. 3b). Additionally, volcanic rocks were

sampled in the Vijes area, 30 km NNE of Cali (VIJ 14,PAN211 and 19) and at Calima, 50 km north of Cali

(COL-1; Fig. 3b). A suite of lavas have also been collectedANALYTICAL METHODSfrom the Barroso Formation type-section 4060 km SW

After powdering in an agate Tema

mill, major and traceof Medelln (Fig. 2), on the Remolino to El Barroso road, elements were analysed by X-ray fluorescence (XRF) atand from the Barroso valley along the road to JardnRoyal Holloway University of London and Leicester(BAR311).University using conventional techniques [see Tarney &Marsh (1991) and Kerr et al. (1996b ) for further details].The 2 SD errors are as follows: SiO2, 02; TiO2, 003;

Serrana de Baudo Al2O3, 01; Fe2O3, 01; CaO, 005; MgO, 01; Na2O,The westernmost belt of Cretaceous volcanic rocks out- 01; K2O, 003; MnO, 003; P2O5, 003; Ba, 20; Cr,crops principally along the Pacific coast of NW Colombia, 1%; Ga, 10; Nb, 02; Ni, 1%; Rb, 03; Sr, 1%; V, 15%;known as the Serrana de Baudo (Figs 2 and 3a). The Y, 04; Zr, 05. The XRF data are reported in Table 3.coastal exposures in the region consist of pillowed and The rare earth elements (REE) and Th, U, Co, Sc,massive basalts, with some basaltic breccias (with clasts Ta and Hf ( Table 4) have been analysed by instrumental

neutron activation analysis (INAA) at the University ofup to 30 cm in diameter), dolerites and gabbros. Although

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Leicester [see Fitton et al. (1997) for analytical details]. poorly determined step ages and the particular dis-tribution of Ar isotopic compositions in 36Ar/40Ar vs 39Ar/Additionally, several samples were analysed for REE by40Ar plots. We will therefore restrict our discussion to theinductively coupled plasma-atomic emission spectroscopyplateau ages.at Royal Holloway [see Walsh et al. (1981) for analytical

Samples CBU11 and CBU12 from the Western Cor-details].dillera displayed erratic step ages such that no age plat-Sr and Nd isotope ratios (Table 5) were measured ateaux or isochrons could be discerned. This can beRoyal Holloway, on a VG354 mass spectrometer fittedattributed to a combination of alteration, the low K2Owith five Faraday collectors. The isotopic analysis pro-contents of the rocks and interferences from hydro-cedures have been described by Thirlwall (1991a, b). Thecarbons.powders used for Sr isotope analysis were leached in 6M

HCl for 1 h before commencing the chemical procedures.

The more altered samples were leached a second time in

6M HCl. During the period of the analyses, internationalGEOCHRONOLOGYreference material SRM987 averaged 071024317 (2Different environments of formation as well as differentSD, n=64) and laboratory standard Low Aldrich aver-relative ages have been proposed for the three belts ofaged 05114219 (2 SD, n=14). [See Thirlwall (1991a)Cretaceous igneous rocks:

for comparison of Low Aldrich with international stand- (1) The igneous rocks of the three belts form part ofards.]one near-synchronous province (Goossens & Rose, 1973; Age determinations for whole-rock and plagioclaseGoossens et al., 1977; Marriner & Millward, 1984).mineral separates from the least altered samples were

(2) The volcanic and intrusive rocks of the Westernperformed at Oregon State University using standardand Central Cordillera form one province, whereas those40Ar39 Ar incremental heating techniques (Duncan &of the Serrana de Baudo constitute a younger provinceHargraves, 1990; Duncan & Hogan, 1994) Whole-rock(Barrero, 1979; Feininger & Bristow, 1980).

samples were either crushed in bulk in a ceramic jaw(3) The Cretaceous volcanic belts young westwards

crusher (PAN6) and sieved to a uniform 0510 mmfrom the Central Cordillera, through the Western Cor-

grain size or made into mini-cores. Plagioclase crystalsdillera, to the Serrana de Baudo (McCourt et al., 1984;

were magnetically separated, checked for purity under aAspden et al., 1987; Lebras et al., 1987).

binocular microscope, briefly washed in 5% HF andThese previous interpretations are unfortunately heav-

ultrasonically cleaned in distilled water. Samples were

ily dependent on unreliable (owing to Ar and K loss, orsealed in evacuated quartz glass vials and irradiated for K gain) K/Ar ages, and on palaeontological evidence6 h at the OSU TRIGA nuclear reactor facility. Neutron

from associated sediments that are only in tectonic contactflux during irradiation was monitored by FCT-3 biotite

with the volcanic rocks. We have dated two basalts from(277 Ma, Hurfurd & Hammerschmidt, 1985). Ar isotopes

the Serrana de Baudo, one from the Western Cordillera,of the whole-rock samples (~500 mg) were determined

and one from the Dabeiba Volcanic Arc by 40Ar/39Arusing an AEI MS-10S mass spectrometer. The plagioclase step heating. We have also restricted palaeontologicalseparates (~100 mg) were analysed using an MAP 215- evidence to those fossils that are derived from sediments50 mass spectrometer. which are clearly stratigraphically intercalated (rather

Individual ages for each 40Ar39 Ar temperature step than tectonically intercalated) with the volcanic rocks.were calculated after corrections for background, mass The Serrana de Baudo rocks yield 40Ar/39 Ar step-fractionation, isotopic interferences and atmospheric ar- heating plateaux which range from 72504 togon contamination ( 40Ar/36Ar=2955). Plateau ages were 77910 Ma (Table 2), consistent with the occurrencecalculated from consecutive steps that are concordant

of Upper Cretaceous bivalves in intercalated sedimentswithin 2 error using the procedure described by Dal- (Gansser, 1973). 40Ar/39 Ar dates for the basalts andrymple et al. (1988), in which step ages were weighted by gabbros of Gorgona Island, the proposed offshore con-the inverse of their variance (Table 2). Isotope correlation tinuation of the Serrana de Baudo (Kerr et al., 1996a),diagrams ( 36Ar/40 Ar vs 39Ar/40 Ar) were made for each have yielded significantly older ages (86046 toanalysis, in which the slope is proportional to the age 88319 Ma) than the basalts of the Serrana de Baudo.(isochron) and the inverse of the y-intercept gives the The implications of this will be discussed below.initial 40Ar/36 Ar composition. The isochron ages are The basalts of the Western Cordillera have provedcalculated using the same steps as in the plateau ages. more problematical to date, owing to both their alteredPlateau ages almost invariably have smaller errors be- nature and low K2O contents. Nevertheless, one samplecause the weighting procedure emphasizes the most pre- (PAN6) has been successfully dated (Table 2) and hascisely determined age steps, whereas uncertainties in the yielded a plateau age of 91727 Ma. This age is

also consistent with palaeontological evidence from theisochron ages reflect the analytical errors of the more

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Table4:Additionaltrac

eelementdataforselectedCretaceousC

olombianintrusiveandvolcanicrocks

Sample

Method

Sc

Co

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

Lu

Hf

Ta

Th

U

SerranadeBaudo

SDB8

INAA

452

470

34

107

84

27

12

33

07

24

040

225

029

033

bdl

SDB10

INAA

572

567

15

55

541

062

032

131

021

008

029

bdl

SDB11

INAA

496

466

27

62

61

19

08

06

21

032

140

023

040

bdl

SDB13

INAA

517

522

15

41

35

13

06

52

04

14

020

091

012

012

bdl

SDB16

INAA

550

474

34

82

72

24

10

06

24

033

181

024

026

bdl

SDB18

INAA

462

499

38

98

93

29

11

08

28

043

205

030

030

bdl

SDB20

INAA

545

540

32

80

64

24

11

06

25

036

168

020

031

bdl

SDB25

INAA

344

503

126

298

2189

53

196

582

096

26

038

459

095

119

bdl

WesternCordillera

CBU5

INAA

485

505

23

73

58

20

08

04

20

031

165

018

025

012

CBU8

INAA

482

517

28

76

620

224

096

326

058

22

035

170

031

bdl

CBU9

INAA

443

461

27

75

69

22

09

05

22

026

165

018

023

011

CBU11

INAA

482

480

59

150

122

39

14

09

41

062

310

045

059

025

CBU12

INAA

423

422

49

145

131

38

14

58

10

40

067

308

046

061

bdl

CBU13

INAA

524

483

96

274

223

72

24

96

18

99

132

860

161

120

bdl

CBU14

INAA

464

497

31

85

73

23

09

30

07

26

034

174

024

029

bdl

PAN5

ICP-AES

27

73

09

58

17

07

29

06

19

19

029

PAN11

ICP-AES

39

111

15

89

27

11

46

10

31

30

048

COL1

INAA

463

492

25

61

65

21

08

06

22

026

143

017

027

bdl

VIJ1

INAA

476

480

61

166

124

40

15

09

43

063

314

048

062

021

VIJ4

INAA

483

395

58

164

1210

377

142

551

095

40

065

297

046

059

bdl

CentralCordillera

AMA1

INAA

369

390

32

86

66

21

07

24

05

21

035

173

041

060

bdl

AMA8

INAA

396

455

119

280

184

45

16

54

08

21

030

343

092

119

bdl

AMA12

INAA

526

482

05

17

15

bdl

03

02

13

020

048

bdl

bdl

bdl

YAN1

INAA

507

502

37

101

79

27

10

44

08

29

046

165

029

043

bdl

YAN8

INAA

491

457

35

85

78

24

09

06

28

039

175

031

043

017

SouthernCentral

Cordillera

ROM4iiXRF

305

395

198

451

2619

588

21

638

085

19

027

501

171

212

bdl

ROM5

ICP-AES

56

143

19

105

30

11

37

36

07

21

18

026

Standards(INAA)

BOB-1

335

563

48

125

102

31

12

38

08

26

039

246

044

040

measured

BOB-1

47

138

107

33

13

41

07

25

044

253

051

045

recomended

INAA,instrumentalneutronactivationanalysis;XRF,X-rayfluorescence;ICP-AES,inductivelycoupledplasma-atomicemissionspectroscopy.

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Table 5: Radiogenic isotope data for Cretaceous Colombian intrusive and volcanic rocks

87Sr/86Sr m (87Sr/86Sr)i143Nd/144Nd m (Nd)i

Serrana de Baudo

SDB8 0703039 10 0703023 0513081 5 866

SDB11 0703854 11 0703687 0513048 7 806

SDB13 0703073 11 0702991 0513073 7 816

SDB16 0703128 13 0703065 0513070 5 836

SDB18 0703429 9 0703380 0513067 8 848

SDB20 0702998 11 0702918 0513073 7 818

Western Cordillera

VlJ1 0703224 11 0703207 0513053 11 813

VlJ4 0703246 11 0703227 0513047 14 801

CBU4 0703855 11 0703751

CBU9 0704572 9 0704500 0513020 20 754

CBU11 0704714 12 0704650 0513019 5 746

CBU12 0704909 10 0704606 0513014 5 756

CBU13 0703686 11 0703610 0513063 5 831

CBU14 0703297 12 0703208 0513049 5 814

COL1 0703795 10 0703753

Central Cordillera

AMA1 0704404 11 0704376 0512953 4 619

AMA8 0703420 10 0703292 0512909 5 601

AMA12 0705033 10 0704919 0513056 13 777

YAN1 0703856 11 0703461 0513027 6 740

YAN8 0703563 13 0703527 0513078 4 877

m, measured isotope ratio.i= 75 Ma for Serrana de Baudo;

i= 90 Ma for Western Cordillera;

i= 120 Ma for CentralCordillera.

intercalated sediments. For example, Bourgois et al. (1987) metasediments of which have yielded poorly preservedfossils ranging from Hauterivian to Albian [13597 Ma;noted the presence of Cenomanian to Turonian (9788

Ma) microfaunas in limestones and cherts interbedded summarized by Nivia (1987)] will be discussed in a latersection.with basalts, 50 km NW of Medell n, whereas Barrero

(1979) reported the occurrence of TuronianConiacian Unfortunately, no Central Cordillera basalts or dol-erites were fresh enough or contained enough K2O (>01(9187 Ma) microfossils and ammonites from the Espinal

Formation. wt %) to yield reliable 40Ar/39Ar ages. Additionally, nofossils have been found within the intercalated sedimentsAs was noted above, the Macuchi Formation in Ecua-

dor can be divided into two petrological provinces: an (Aspden & McCourt, 1986). One of the few constraints

on the age of the Amaime Formation comes from theeastern, more basaltic province, and a western tuff

-richandesitic province. The only palaeontological ages come fact that it is intruded by the Buga tonalitic batholith.This tonalite has yielded an Rb/Sr mineral isochron (onfrom intercalated sediments in the western arc-derived

province, where foraminifera of Eocene age have been biotite and hornblende) of 994 Ma (McCourt et al.,1984), suggesting that the formation of the Amaimereported (Henderson, 1979). This suggests that the more

andesitic portion of the Macuchi Formation, like the basaltic crust and its accretion onto the margin of NWColombia must have occurred in the Early Cretaceous,Dabeiba Volcanic Arc sequence (sample AN1464 dated

at 43104 Ma; see above), is significantly younger well before 100 Ma. Thus the igneous rocks of the CentralCordillera would appear to be appreciably older thanthan, and should not be correlated with, the basalts of

the Western Cordillera. The tectonic significance of these those of the Western Cordillera, which are in turn younger than the volcanic and intrusive rocks of theEocene subduction-related volcanics along the western

periphery of the Western Colombian and Ecuadorian Serrana de Baudo. Nevertheless, the compositions ofbasalts from the Central Cordillera are similar to thoseCordillera, and the Quebradagrande Complex, the

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of basalts from the other two Cordilleras, thus implying zeolites occur in small amounts within the groundmassand also in amygdales and sometimes in veinlets. Ina similar petrogenetic history.several samples altered olivine has been recognized inthe groundmass, and occasionally altered (to chlorite andiddingsite) subhedral olivine microphenocrysts can be

PETROGRAPHY AND MINERALfound. Ophitic textures are common, and these lavas

CHEMISTRY can contain pyroxene and plagioclase phenocrysts, withina granular groundmass of plagioclase, pyroxene andCentral Cordillerasecondary minerals. Occasionally, plagioclase and pyr-The basalts and dolerites consist of plagioclase, clino-oxene form glomeroporphyritic clusters.pyroxene and FeTi oxides, with secondary chlorite, iron

oxides, zeolites, pumpellyite and quartz. Euhedral tosubhedral olivine phenocrysts, which are altered to ser-pentine, are more common than in the Western Cor-

GEOCHEMISTRYdillera (see below), but aphyric basalts still predominate.

Alteration and elemental mobilityThe clinopyroxenes are relatively fresh and range inThe altered nature (up to prehnitepumpellyite grade)composition from diopside to augite (Nivia, 1987),

of the rocks from all three cordilleras means that beforewhereas the fresher plagioclase crystals vary between any petrological inferences can be drawn from the chem-An67 and An50 (Nivia, 1987). Textures range from pla-istry of the rocks, the possible chemical effects of sub-gioclase laths and euhedral clinopyroxene micro-solidus mobility of elements must be considered.phenocrysts in a groundmass of plagioclase and clino-Zirconium is widely regarded as being essentially im-pyroxene spherulites, to ophitic to subophitic clino-mobile during low-grade alteration of basaltic rocks bypyroxene enclosing or partially enclosing plagioclasehydrothermal fluids (e.g. Humphris & Thompson, 1978;laths, or occasionally a very fine-grained granular texture.Gibson et al., 1982; Kerr, 1995) and so has been plotted An excellent summary of the petrography and mineralagainst all the other minor and trace elements. A selectionchemistry of the picrites and basalts from El Encenilloof these diagrams is shown in Fig. 4. Nb, Y and TiO2,and Los Azules has been given by Spadea et al. (1989).which are also believed to be relatively immobile, producegood correlations against Zr. The minor differences inratio seen in the Y and Nb vs Zr plots are significant,Western Cordillera

and are due either to variable degrees of partial mantleMost of the basalts and dolerites are fine- to medium- melting or to a heterogeneous mantle source region.grained holocrystalline rocks, with altered glass only being

In contrast, Ba, Sr, Rb and K2O display virtually nofound in the outer skin of pillows. Textures vary fromcorrelation with Zr, which strongly implies that, as in

ophitic to subophitic with plagioclase laths poikiliticallyother altered basalts, these large ion lithophile elements

enclosed within anhedral clinopyroxenes, to intergranularhave been extensively mobilized. Accordingly, variation

and intersertal. The basalts and dolerites are generallyin these elements will not be discussed further. All the

aphyric but occasionally microphenocrysts of clino-other minor and trace elements are, like TiO2, Y andpyroxene can be found, set in a groundmass of variablyNb, well correlated with Zr contents, implying relative

altered plagioclase laths (An7250; Nivia, 1987) and an- immobility.hedral to occasionally euhedral diopside and augite (En50-35Wo4435Fs630; Nivia, 1987), with


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Fig. 4. Plots of Y, TiO2, Nb, Sr, K2O, Ba and Rb vs Zr. (Note that the large ion lithophile elements display virtually no correlation with Zrcontent.)

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major element composition [480517 wt % SiO2; 62101 wt % MgO (Fig. 6); 92146 wt % Fe2O3(total)].In terms of trace element contents, two basalts (SDB10

and -13) and one gabbro (SDB15) have slightly lowerZr, Y, Nb and TiO2 values than the rest of the basaltsand gabbros, and lie slightly below the main fractionationtrend (Fig. 6). This depletion is characteristic of all

the other incompatible trace elements, as the primitivemantle-normalized pattern of SDB13 shows (Fig. 5a).These three samples (SDB10, -13 and -15) have lowerNb/Zr ratios (Fig. 7b) and higher 147Sm/144Nd (Fig. 8)

than the rest of the Serrana de Baudo rocks. The restof the basalts and gabbros of the Serrana de Baudopossess relatively flat primitive mantle-normalized pat-terns (Fig. 5) ranging from 2 to 65 times primitive mantle,

with the more evolved basalts having generally higherlevels of incompatible trace elements. All the rocks (exceptSDB2225) are slightly depleted in light REE (LREE)[(La/Nd)pn 07510; Fig. 7a] and have (Sm/Yb)pn ratios

ranging from 10 to 12 (pn denotes primitive mantle-normalized; Sun & McDonough, 1989).

The isotopic data for the Serrana de Baudo samplesare given in Table 5 and plotted in Fig. 8. (Nd)i (wherei=75 Ma age corrected value) span a narrow range from+80 to +87. Four of the samples also display arestricted range in ( 87Sr/86Sr)i from 070292 to 070307;

however, SDB18 and SDB11 have elevated ( 87Sr/86Sr)iof 070338 and 070369, respectively.

Western CordilleraThe lavas and intrusives of the Western Cordillera are

tholeiitic in character. Most of the rocks are basaltic incomposition, and range from 48 to 53 wt % SiO2, 101

to 147 wt % Fe2O3(total) (Table 3) and 60 to 104 wt

% MgO (Fig. 6). In addition, two basaltic andesites(PAN2 and -3) and an andesite PAN19, occur near the

town of Vijes (Fig. 3b) in an area where rhyolitic dykesof uncertain origin cut the lava succession (Kerr et al.,1996c). These more evolved lavas have SiO2>53wt%andFig. 5. Primitive mantle-normalized (Sun & McDonough, 1989) multi-

element plots for the igneous rocks from (a) the Serran a de Baudo, (b) MgO60 wt % MgOthat the more enriched (e-)basalts from Gorgona Island(VIJ1, CBU11 and CBU12) have higher abundances of(Fig. 2), also part of the CaribbeanColombian oceanic

plateau (Kerr et al., 1996a ), possess similar incompatible incompatible trace elements. All the basalts shown inFig. 5 have essentially parallel and flat primitive mantle-trace element contents and ratios to SDB2325 (Fig. 7).

The second group comprises the remaining volcanics normalized patterns, and thus have relatively small ranges

in incompatible trace element ratios (Figs 7 and 8c). Asand intrusives from the Serrana de Baudo. These thole-iitic basalts and gabbros exhibit a moderate range in Fig. 7a shows, the basalts of the Western Cordillera are

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Fig. 6. Plots of Zr, Nb, TiO2 and Y vs MgO, showing the compositions of the Cretaceous igneous rocks of the three cordilleras in Colombia.(The oblong field represents the El Encenillo samples.) Also shown are modelled (using the TRACE3 program; Nielsen, 1988) fractionalcrystallization trends for three starting compositions: a, CUR7 (picrite); b, ROM7 (picrite); c, SDB13 (basalt). Ticks on the fractional crystallization

trends represent 10% crystallization intervals.

moderately depleted in the LREE [(La/Nd)pn 075095], and dolerites of the Amaime Formation and the Yanawith essentially flat heavy REE (HREE) patterns [(Sm/ area.Yb)pn 1011]. Lavas from Los Azules are tholeiitic, whereas those

The rocks of the Western Cordillera have (Nd)i (i=90 from El Encenillo are more alkaline. The non-cumulateMa) in the range +75 to +81, whereas the diorite is lavas range from 71 to 166 wt % MgO (Fig. 6), theslightly higher at +83. Three of the samples have higher values falling within the range of calculated MgO( 87Sr/86Sr)i in the range 0703207033, the diorite being content (128173 wt %) (Spadea et al., 1989) for a liquiddistinctly higher at 07036 (Fig. 8a). However, like the in equilibrium with Fo89 olivine (maximum in Los Azulesbasalts of the Serrana de Baudo, three of the basalts picrites). Therefore ROM7, the sample containing 166have elevated ( 87Sr/86Sr)i (>07045) with only a small wt % MgO, may represent a potential primary magmaconcomitant decrease in (Nd)i values. for the Los Azules complex. In terms of trace elements,

the two analysed lavas from El Encenillo are significantly

more enriched in incompatible trace elements (at equi-Central Cordillera valent MgO content) than the lavas from Los Azules

(Fig. 6). This is also reflected in the steeper primitiveThe igneous rocks of the Central Cordillera are the mostmantle-normalized trace element patterns for the Elcompositionally diverse of the three cordilleras, rangingEncenillo lavas (Fig. 5) and incompatible trace elementfrom moderately enriched (relative to proposed primitiveratio plots (Fig. 7), with the El Encenillo lavas having,mantle compositions) in incompatible trace elements tofor example, higher La/Y and Nb/Zr than the Losrelatively depleted (Fig. 5). The rocks can be divided into

Azules lavas.two broad groups: the moderately incompatible traceThe tholeiitic basalts and dolerites of the Amaimeelement-enriched basalts and picrites from Los Azules

and El Encenillo, and the mostly less enriched basalts Formation and Yana area range in MgO content from

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exception to this is a group of basalts from the moresoutherly parts of the Amaime Formation (AMA36 andFLO14), which possess higher Nb/Zr ratios than therest of the Amaime and Yana basalts (Fig. 7). Thisenrichment seems only to affect Nb contents, as ratiosnot involving Nb (e.g. Ti/Zr) are the same as for therest of the basalts. Two of the basalts from the AmaimeFormation (AMA8 and -11) are relatively enriched inincompatible trace element contents and ratios (Figs 5,6 and 7), whereas one basalt (AMA12) has been foundwhich possesses more depleted levels of incompatibletrace elements (Figs 5, 6 and 7).

In terms of (Nd)i (i=120 Ma), the lavas span a com-paratively wide range from +60 in AMA8, one of themost enriched Amaime basalts, to +81 in YAN-8 (Fig.8). The most trace element depleted Amaime basalt(AMA12) has an (Nd)i of+77. As with the basalts ofthe Western Cordillera, ( 87Sr/86Sr)i appears be decoupledfrom (Nd)i, with several of the basalts having (

87Sr/86Sr)i>07043, but most lie in the range 0703207035.

DISCUSSION

Original tectonic setting and relative agesof the Colombian basalts

Tectonic setting

The chemical data presented can help to resolve thelong-standing controversy regarding the tectonic setting

of the Colombian mafic volcanic terranes, namely,whether they represent a subduction zone environment,ocean floor or oceanic plateau. This problem can beaddressed in two ways:

(1) Mature subduction zone volcanic sequences are

characterized by abundant explosion-derived tuffs andlavas of andesitic composition, as seen in recent Co-lombian subduction-related volcanoes (Marriner & Mill-ward, 1984). However, the Cretaceous volcanic terranes

are predominantly basaltic, and although pyroclastic de-posits do occur, they are relatively rare and are mostlybasaltic in composition.

Fig. 7. Incompatible element ratio plots displaying the CretaceousColombian igneous rocks along with fields for lavas from Gorgona,Curacao, Iceland and the Ontong Java plateau: (a) (Sm/Yb)pn vs (La/Nd)pn; (b) Ti/Zr vs Nb/Zr; (c) Nb/Y vs La/Y [also shown in (c) areCretaceous arc lavas from Bonaire Island in the southern Caribbeanand Recent Colombian arc-derived lavas]. Symbols are as in Fig. 4,apart from those which have been separately denoted. Data sources:

50 to 103 wt % (Fig. 6). Most of the lavas possess GorgonaAitken & Echeverra (1984), Arndt et al. (1997), Kerr et al.(1996a ); CuracaoKerr et al. (1996b ); IcelandHemond et al. (1993);relatively flat primitive mantle-normalized trace elementOntong JavaMahoney et al. (1993); BonaireG. Klaver (unpublishedpatterns (Fig. 5), and their incompatible trace elementdata, 1979); Recent Colombian lavasMarriner & Millward (1984);

ratios are similar to those from the Western Cordillera the field for Cretaceous Colombian lavas in (c) is taken from Nivia(1987).and from Serrana de Baudo (Fig. 7). An interesting

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Fig. 8 . (a) (Nd)i vs (87Sr/86Sr)i, (b) (

143Nd/144Nd)i vs (Sm/Yb)pn, (c) (143Nd/144Nd)i vs

147Sm/144Nd. i denotes initial age corrected values. Symbolsare as in Fig. 4, except those separately marked. Data sources: HaitiSen et al. (1988); GalapagosWhite et al. (1993); all other data sources

are as for Fig. 7.

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(2) As Fig. 5 shows, these Cretaceous volcanic rocks Leg 165, at Site 1001 (40 km WSW of Site 152), en-countered Middle Campanian limestone mixed with claydo not possess the characteristic Nb depletion (withand basaltic ashlapilli overlying, and grading downcorresponding LREE enrichment), which results in ainto, plateau basalt (Pearce & Pearson, 1996). Thus thenegative Nb anomaly on multi-element normalized plotspresence of Campanian limestone intimately associatedof subduction-related lavas. Figure 7c confirms that thewith the uppermost volcanics at both these sites placesCretaceous Colombian basalts and picrites have con-a maximum age of 83 Ma on the underlying basalt.sistently higher Nb/La ratios (relative to Y) than both

Additionally, a dolerite sill intruding the upper part ofRecent Colombian volcanic arc rocks (Marriner & Mill-the Curacao lava succession has recently been dated,ward, 1984) and Cretaceous subduction-related lavasusing 40Ar/39Ar step-heating, at 75819 Ma (C. Sinton,from Bonaire in the southern Caribbean (G. Klaver,unpublished data, 1996).unpublished data, 1979).

It is interesting to note that, as in Colombia, whereTherefore, both field and geochemical evidence mil-the westernmost basalts (Serrana de Baudo) are theitate strongly against a subduction-related origin for the

youngest (7278 Ma), so it is the westernmost drilledCretaceous Colombian volcanic terranes. Moreover, theholes in the Caribbean Sea that have also produced thepredominance of basalts with relatively flat primitive

youngest (Campanian) ages. These observations supportnormalized trace element patterns, and the not un-a younger episode of CaribbeanColombian plateau vol-common occurrence of lavas with more picritic com-canism mostly in the west of the province, consistentpositions, are more consistent with an origin in plume-with eastward movement of the plate(s) above a stationaryderived oceanic plateau, rather than in normal oceanplume (possibly Galapagos).floor. This is further supported by Figs 7 and 8, which

The CaribbeanColombian oceanic plateau is notreveal that lavas drilled from the Ontong Java plateauunique in this regard, because many of the worlds plume-(Mahoney et al., 1993) possess similar trace element andrelated LIPs display at least two distinct periods of majorisotopic ratios to most of the basalts and picrites fromeruptive eruptions, separated by between 20 and 90 m.y.Colombia. However, there is a greater chemical vari-(Bercovici & Mahoney, 1994). The Ontong Java plateau,ability within the Colombian lavas, and possible ex-like the CaribbeanColombian oceanic plateau, appearsplanations for this will be discussed in a later section.to have peaks of volcanic activity at about 120 and 90The ~90 Ma basalts and picrites from Curacao are partMa (Mahoney et al., 1993). However, perhaps a moreof the CaribbeanColombian oceanic plateau (Kerr etsuitable analogue for the CaribbeanColombian Cre-al., 1996b), and the close similarity in isotopic and tracetaceous oceanic plateauwhich appears to have been

element ratios between these Curacao lavas and the volcanically active over ~40 Mais the long-lived vol-Colombian basalts and picrites (Figs 7 and 8) supportscanism associated with the Icelandic plume. Interestingly,an origin within the Caribbean oceanic plateau.there are other similarities in both chemistry and tectonicsetting between the CaribbeanColombian and Icelandic

Timing of volcanism basalts, which will be explored in a later section.The volcanism associated with the Colombian portion Finally, whereas it is possible that the earlier (>100of the Caribbean oceanic plateau appears to be of three Ma) oceanic plateau forming event in the Caribbeandistinct ages: two well-dated events ( 40Ar/39 Ar; fossils Colombian province is linked with the same plume asfrom intercalated sediments), one of Late Cenomanian that which produced the 90 and 75 Ma events, weTuronian (8892 Ma) and another of Late Campanian caution that there is no way of discounting the possibilityEarly Maastrichtian (7278 Ma), plus another eruptive that the earlier event was linked to an entirely differentepisode older than 100 Ma. Other fragments of Early plume.

Cretaceous oceanic plateau material have been reportedfrom elsewhere in the Caribbean region, for example, inthe Duarte Complex of Hispaniola (Lapierre et al., 1997)

Petrogenetic aspectsand Cuba (Iturralde-Vinent, 1994). Similarly, more evi-The high 87Sr/86Sr puzzledence for the younger plateau eruptive event within the

Caribbean region (possibly contemporaneous with the Arguably, the most enigmatic feature of the chemistryformation of the Serrana de Baudo rocks) is also be- of these lavas is their tendency to high ( 87Sr/86Sr)i valuesginning to accumulate. The westernmost Caribbean site relative to (Nd)i (Fig. 8a), which do not appear to correlatedrilled during DSDP Leg 15, Site 152 (Fig. 1) encountered with any other chemical parameter. Repeated leachingbasalt with the chemical characteristics of an oceanic of powders from several of these samples failed to sig-plateau which contained fragments of Campanian (8374 nificantly reduce the high 87Sr/86Sr more than just a singleMa) limestone (Donnelly et al., 1973). Recent Ocean leaching. Within the CaribbeanColombian Cretaceous

igneous province these high 87Sr/86Sr values are notDrilling Program (ODP) drilling in the Caribbean during

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unique, and they have been found in well-leached ko- have used three different starting magma compositions:a picrite (CUR7) containing 20% MgO from Curacaomatiites and picrites from Gorgona Island (Aitken &(Fig. 1); a slightly more enriched picrite (ROM7) fromEcheverra, 1984), basalts from Nicoya in Costa Rica (F.Los Azules (166% MgO); and one of the more depleted,Hauff, personal communication, 1996) and basalts fromhigher-MgO basalts from the Serrana de Baudo (SDB13;Curacao (Kerr et al., 1996b ). The last leaching ex-101 wt % MgO). Not surprisingly, the first phase toperiments support the view that these high 87Sr/86Srcrystallize in all three models is olivine (Cr-spinel); values are magmatic in origin and are not caused bythen, when 89 wt % MgO is reached, plagioclase andsub-solidus hydrothermal alteration (Kerr et al., 1996b).clinopyroxene join the fractionating assemblage, to be Accordingly, Kerr et al. (1996b ) proposed that the highfollowed by FeTi oxide at


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appear to have a long-term history of depletion, withone component being slightly more enriched (lower Nd)than the other.

(2) The Colombian basalts are not primary melts fromthe mantle, but may have been trapped and fractionatedfrom picritic melts in magma chambers en route to thesurface. However, as well as fractionation, the entrapmentof picritic liquids in magma chambers will also serve tohomogenize individual heterogeneous melt fractions.

This suggestion is reinforced by the fact that the Co-lombianCaribbean Cretaceous high-MgO lavas are, asa group, significantly more heterogeneous than theirassociated basalts (Kerr et al., 1996a, c). Thus, althoughmany of the Cretaceous basalts in Colombia (and in

other oceanic plateaux) are relatively homogeneous, thiscould just reflect mixing of compositionally distinctmagmas derived from a heterogeneous plume sourceregion by variable degrees of melting.

An LREE-depleted mantle source composition wasused in the modelling of pooled fractional melting. Threedifferent mineral assemblages were useda garnet lher-zolite, a spinel lherzolite and a 50:50 mixture of these two

mineralogiesto simulate melting in the garnetspineltransition zone [source compositions, mineral proportionsand partition coefficients are from McKenzie & ONions(1991)].

The results of the mantle melt modelling are presentedFig. 9. (a) Log plot of Nb/Y vs Zr/Y [after Fitton et al. (1997)], in Fig. 9b, along with fractional crystallization trendsshowing the compositions of the Cretaceous Colombian igneous rocks.

that show a slight increase in Nb/Y ratios as the magmasLavas from the neovolcanic zones of Iceland fall between the twoparallel lines, whereas East Pacific Rise MORBs [from Mahoney et al.

fractionate. Thus, when fractional crystallization has been(1994)] plot below this field. (b) Nb/Y vs Zr with modelled partial accounted for, it can be inferred that many of thefractional melting curves for a garnet lherzolite (Gnt lz), spinel lherzolite

Colombian basalts result from fairly extensive degrees(Sp lz) and a 50:50 spinelgarnet mixture (SpGnt lz). Numbered ticks

(~20%) of partial mantle melting, or that they wereon the melting curves indicate percentage of partial melting (see textfor more details). Near-horizontal arrows represent several possible derived from a more depleted source than that used infractional crystallization trajectories (based on the modelling shown in the melt modelling. As Fig. 9b shows, at 20% meltingFig. 6). Numbered ticks on the lowermost trajectory indicate the

the differences in incompatible trace element contentscomposition of the residual liquid after 50 and 70% crystallization froma picritic parent. and ratios between the three mantle mineral assemblages

are significantly less than at smaller degrees of melting.It thus becomes more difficult to assess the approximatebasalts from North Atlantic MORB. In Fig. 9a, our newdepth of melting of the erupted lavas, a difficulty whichdata from Colombia are plotted along with the tramlinesis compounded by the possibility of mixing melts derivedof Fitton et al. (1997) (between which all the plume-from both the garnet and spinel lherzolite stability fields.derived Icelandic lavas plot), and fields for East Pacific

However, melting seems to have occurred over a ratherRise MORB and the Ontong Java plateau (Mahoney et wide depth range (polybaric melting) from relatively deep,al., 1993, 1994). Virtually all the Colombian basaltsmostly within the garnet lherzolite field, to shallowerfall within the Icelandic tramlines, strongly implyingmelting of spinel lherzolite.derivation from a plume source region. Even the most

Figure 9b shows that basalts with Nb/Y>02 probablydepleted Colombian rocks (SDB13 and AMA12) dohad a melt input from a mantle source region containingnot appear to originate from a depleted upper-mantlesome garnet. These more incompatible element-enriched(MORB-source) region; rather, they seem to be derivedbasalts also generally have lower ( 143Nd/144Nd)i valuesfrom a depleted (relative to Bulk Earth) source region(Fig. 8b and c), implying that they are derived from a morewithin the plume.enriched source region, or contain a higher proportion ofIn modelling the mantle melting processes severalenriched component. The negative correlation betweenfactors need to be considered:( 143Nd/144Nd)i and (Sm/Yb)pn displayed by the samples(1) The plume source region was isotopically hetero-

geneous, comprising at least two components which both from the Western and Central Cordilleras reveals that

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the more enriched melts are derived from deeper source +78 and for the Serrana de Baudo +83. In additionregion and vice versa. This source heterogeneity and to this, the younger lavas drilled by DSDP Leg 15 at Sitenegative correlation between the degree of enrichment 152 are similarly the most elementally and isotopicallyand depth of melting support the recent models of Kerr depleted lavas sampled during Leg 15 (Kerr et al., 1997;et al. (1995) and Arndt et al. (1997). These models have G. F. Marriner & A. D. Saunders, unpublished data,proposed that most mantle plumes are heterogeneous and 1987). Thus if one single plume is responsible for theconsist predominantly of a relatively refractory depleted volcanism in the CaribbeanColombian oceanic plateau,matrix component, with a small proportion (p10%) of then it appears to have become more depleted with time.more enriched and more fusible streaks or blobs. Thus

deeper, and therefore smaller, extents of mantle meltingwill preferentially sample these more enriched streaks,

Is the Caribbean Plateau an Icelandicwhereas at shallower depths, more extensive meltinganalogue?means that a greater proportion of the magma producedRecently, linear NESW and eastwest long-wavelengthwill be derived from the more depleted matrix.magnetic anomalies have been discovered over the Ven-A feature worth stressing is that many of the Colombianezuelan and Colombian Basins (Fig. 1) in the Caribbeanbasalts, and indeed basalts from many other oceanic(Hall, 1995). It has been suggested by Hall (1995) thatplateaux, particularly Ontong Java, possess positive Ndthese anomalies may have resulted from an Early Cre-values but paradoxically, have near-chondritic ratios oftaceous phase of seafloor spreading at the Farallonincompatible trace elements. This problem has becomePacificPhoenix triple junction, at or near which theknown as the plateau paradox (Babbs et al., 1996). OneCaribbeanColombian Oceanic Plateau formed in thepossible explanation for this paradox may be that mixingLate Cretaceous. It is interesting to note that the Ga-of small degree (0) from the garnet lherzolite stability field, withexpression of the plume responsible for the Caribbeanlarger degree (~20%) LREE-depleted melts, pre-Colombian oceanic plateau, is currently situated close todominantly derived from the spinel lherzolite field, hasthe Galapagos Spreading Centre. Indeed, the occurrenceoccurred. Such mixing of different degree melts couldof a plume track on both the Cocos and Nazca Platesoccur either in the mantle plume source region, orsuggests that it was only within the last 5 m.y. thatperhaps more likely within crustal or lithospheric magmathe ridge moved northwards away from the Galapagoschambers en route to the surface, and has the potentialhotspot (Hay, 1977), so for a large proportion of its

to produce magmas with near-chondritic incompatible history the Galapagos plume may have impinged on thetrace element ratios but with positive Nd values.base of thelithosphere at, or close to, an oceanic spreadingA key issue is whether the sequence of late Cretaceouscentre.komatiites, picrites and basalts on Gorgona are not only

Thus, as well as both the Galapagos and Icelandican integral part of the CaribbeanColombian Cretaceousplumes having a long history of activity, it also seemsoceanic plateau (Storey et al., 1991; Kerr et al., 1996a)that the two plumes may have been situated below abut can be correlated with, and are also a southwardmid-ocean ridge for a considerable part of their histories.continuation of, the basaltic sequence of the SerranaIt is therefore a constructive exercise to amplify the initialde Baudo (McGeary & Ben-Avraham, 1986). The newstudies of Nivia (1987) and compare the chemistry of thechemical data presented above support this latter pro-CaribbeanColombian oceanic plateau with that of lavasposal, in that the three more enriched basalts (from theproduced by the Icelandic plume. In terms of tracesouthernmost exposure of the Serrana de Baudo ) areelement and radiogenic ratios, the Icelandic lavas [from very similar to the most enriched e-basalts (Kerr et al.,

Hemond et al. (1993)] span a very similar range to the1996a ) from Gorgona, whereas the rest of the Serrana Cretaceous Colombian lavas and intrusives reported inde Baudo samples possess incompatible trace elementthis paper (Figs 7, 8 and 9). The data in Fig. 8b and cratios which are within the range displayed by the restshow that, at a given incompatible trace element ratio,of the Gorgona basalts. However, the older ages (8688the Icelandic lavas possess slightly higher (Nd)i. Never-Ma; Kerr et al., 1996a ) for the Gorgona lavas andtheless, the basic similarity between the Icelandic andintrusives, compared with the Serrana de Baudo (7278Colombian lavas adds further weight to the suppositionMa) appear to rule out the idea that the basalts of thethat the CaribbeanColombian oceanic plateau formedSerrana de Baudo are a continuation of the sequenceat, or near an oceanic spreading centre. Like present-found on Gorgona.day Iceland, the more enriched lavas in the Caribbean As well as becoming younger towards the west, theCretaceous oceanic plateau could have been producedlavas and intrusives of the three cordilleras also generallyby smaller degrees of melting below thicker lithosphere,become more depleted; for example, the average Nd for

the Central Cordillera is+72, for the Western Cordillera further away from the spreading centre (see Hards et al.,

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