Mafic Pegmatites Intruding Oceanic Plateau Gabbros and Ultramafic Cumulates from Bolı ´var, Colombia: Evidence for a ‘Wet’ Mantle Plume? ANDREW C. KERR 1 * , JOHN TARNEY 2 , PAMELA D. KEMPTON 3 y, MALCOLM PRINGLE 4 AND ALVARO NIVIA 5 1 DEPARTMENT OF EARTH SCIENCES, CARDIFF UNIVERSITY, PARK PLACE, CARDIFF CF10 3YE, UK 2 DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, UNIVERSITY ROAD, LEICESTER LE1 7RH, UK 3 NERC ISOTOPE GEOSCIENCES LABORATORY, c/o BRITISH GEOLOGICAL SURVEY, KEYWORTH, NOTTINGHAM NG12 5GG, UK 4 SCOTTISH UNIVERSITIES ENVIRONMENTAL RESEARCH CENTRE, EAST KILBRIDE, GLASGOW G75 0QF, UK 5 INGEOMINAS—REGIONAL PACIFICO, AA 9724, CALI, COLOMBIA RECEIVED JULY 22, 2003; ACCEPTED APRIL 6, 2004 ADVANCE ACCESS PUBLICATION JULY 29, 2004 The fault-bounded Bolı ´var Ultramafic Complex (BUC) on the eastern fringes of the Western Cordillera of Colombia was tectonic- ally accreted onto the western coast of South America in the late Cretaceous–early Tertiary, along with pillow basalts of the Caribbean–Colombian Oceanic Plateau (CCOP). The complex consists of a lower sequence of ultramafic cumulates, successively overlain by layered and isotropic gabbroic rocks. The gabbros grade into, and are intruded by, mafic pegmatites that consist of large magnesiohornblende and plagioclase crystals. These pegmatites yield a weighted mean 40 Ar– 39 Ar step-heating age of 905 09 Ma and thus coincide with the timing of peak CCOP volcanism. The chemistry of the BUC is not consistent with a subduction-related origin. However, the similarity in Sr–Nd–Pb–Hf isotopes between the CCOP and the BUC, in conjunction with their indistinguishable ages, suggests that the BUC is an integral part of the plume-derived CCOP. The parental magmas of the Bolı ´var complex were probably hydrous picrites that underwent 20–30% crystallization. The residual magmas from this fractionation contained 70 wt % MgO and enough water to permit fractionation of magnesiohorn- blende (and bytownite) on the sidewalls of veins, forming horn- blende-rich pegmatites. As the residual magmas migrated upwards through the complex, they became more evolved and, thus, fractio- nated less magnesiohornblende along with a greater proportion of more sodic plagioclase. At the highest preserved levels of the complex the pegmatites consist predominantly of andesine plagioclase with quartz and biotite. During crystallization of the pegmatites some of the residual magma was tapped or erupted and intruded at higher levels to form breccias and dykes that are exposed nearby at Vijes. Our calculations suggest that the BUC resulted from melting of a mantle plume source region that contained at least 400 ppm H 2 O. We propose that mantle plumes, as well as being heterogeneous in terms of radiogenic isotopes and trace elements, are also heterogeneous with regard to their water content, and so can have portions that are more hydrous than others. This has fundamental implications for mantle rheology and the environmental impact of Large Igneous Province eruptions. KEY WORDS: mantle plume; Caribbean; Colombia; wet melting; oceanic plateau; pegmatite INTRODUCTION The accretion of the Cretaceous Caribbean–Colombian Oceanic Plateau (CCOP) around the margins of the Caribbean and along the NW margin of South America has resulted in the CCOP being arguably one of the best JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 PAGES 1877–1906 2004 DOI: 10.1093/petrology/egh037 * Corresponding author. E-mail: [email protected]yPresent address: Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, UK. Journal of Petrology 45(9) # Oxford University Press 2004; all rights reserved
Transcript
Mafic Pegmatites Intruding Oceanic PlateauGabbros and Ultramafic Cumulates fromBolıvar, Colombia: Evidence for a ‘Wet’Mantle Plume?
ANDREW C. KERR1*, JOHN TARNEY2, PAMELA D. KEMPTON3y,MALCOLM PRINGLE4 AND ALVARO NIVIA5
1DEPARTMENT OF EARTH SCIENCES, CARDIFF UNIVERSITY, PARK PLACE, CARDIFF CF10 3YE, UK
2DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, UNIVERSITY ROAD, LEICESTER LE1 7RH, UK
3NERC ISOTOPE GEOSCIENCES LABORATORY, c/o BRITISH GEOLOGICAL SURVEY, KEYWORTH, NOTTINGHAM
NG12 5GG, UK
4SCOTTISH UNIVERSITIES ENVIRONMENTAL RESEARCH CENTRE, EAST KILBRIDE, GLASGOW G75 0QF, UK
5INGEOMINAS—REGIONAL PACIFICO, AA 9724, CALI, COLOMBIA
RECEIVED JULY 22, 2003; ACCEPTED APRIL 6, 2004ADVANCE ACCESS PUBLICATION JULY 29, 2004
The fault-bounded Bolıvar Ultramafic Complex (BUC) on the
eastern fringes of the Western Cordillera of Colombia was tectonic-
ally accreted onto the western coast of South America in the late
Cretaceous–early Tertiary, along with pillow basalts of the
Caribbean–Colombian Oceanic Plateau (CCOP). The complex
consists of a lower sequence of ultramafic cumulates, successively
overlain by layered and isotropic gabbroic rocks. The gabbros grade
into, and are intruded by, mafic pegmatites that consist of large
magnesiohornblende and plagioclase crystals. These pegmatites yield
a weighted mean 40Ar–39Ar step-heating age of 90�5 � 0�9Ma
and thus coincide with the timing of peak CCOP volcanism. The
chemistry of the BUC is not consistent with a subduction-related
origin. However, the similarity in Sr–Nd–Pb–Hf isotopes between
the CCOP and the BUC, in conjunction with their indistinguishable
ages, suggests that the BUC is an integral part of the plume-derived
CCOP. The parental magmas of the Bolıvar complex were probably
hydrous picrites that underwent 20–30% crystallization. The
residual magmas from this fractionation contained �7�0 wt %
MgO and enough water to permit fractionation of magnesiohorn-
blende (and bytownite) on the sidewalls of veins, forming horn-
blende-rich pegmatites. As the residual magmas migrated upwards
through the complex, they became more evolved and, thus, fractio-
nated less magnesiohornblende along with a greater proportion of
more sodic plagioclase. At the highest preserved levels of the complex
the pegmatites consist predominantly of andesine plagioclase with
quartz and biotite. During crystallization of the pegmatites some of
the residual magma was tapped or erupted and intruded at higher
levels to form breccias and dykes that are exposed nearby at Vijes.
Our calculations suggest that the BUC resulted from melting of a
mantle plume source region that contained at least 400 ppm H2O.
We propose that mantle plumes, as well as being heterogeneous in
terms of radiogenic isotopes and trace elements, are also heterogeneous
with regard to their water content, and so can have portions that are
more hydrous than others. This has fundamental implications for
mantle rheology and the environmental impact of Large Igneous
The accretion of the Cretaceous Caribbean–ColombianOceanic Plateau (CCOP) around the margins of theCaribbean and along the NW margin of South Americahas resulted in the CCOP being arguably one of the best
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 PAGES 1877–1906 2004 DOI: 10.1093/petrology/egh037
yPresent address: Natural Environment Research Council, Polaris
House, North Star Avenue, Swindon SN2 1EU, UK.
Journal of Petrology 45(9) # Oxford University Press 2004; all rights
reserved
exposed and understood oceanic plateaux (Donnelly et al.,1990; Kerr et al., 1997b; Sinton et al., 1998). Detailedstudies of these accreted sections have shed new light onthe mantle source region, formation and structure of theCCOP in particular, and oceanic plateaux in general (e.g.Kerr et al., 1998; Walker et al., 1999; Hauff et al., 2000a).The mid-Cretaceous period (85–125Ma) is marked by
a major phase of anomalous, mantle plume-related vol-canism in the ocean basins (Larson, 1991; Eldholm &Coffin, 2000). The largest of these events are representedby the Ontong–Java and Manihiki plateaux (�122Maand possibly 90Ma; Mahoney et al., 1993), the southernKerguelen Plateau (110–118Ma; Coffin et al., 2002), thecentral Kerguelen Plateau–Broken Ridge (95–100Ma;Coffin et al., 2002) and the Caribbean–ColombianOceanic Plateau (88–93Ma; Sinton et al., 1998). Theseoceanic plateaux have anomalously thick (>10 km) crustand cover areas in excess of 1�0 � 106 km2 (Coffin &Eldholm, 1994); their eruption in submarine conditionsmay well have contributed to anoxia and mass extinctionevents in the Cretaceous oceans (Vogt, 1989; Sinton &Duncan, 1997; Kerr, 1998).It is now generally recognized that the CCOP formed
in the Pacific at c. 90Ma, on the eastward-movingFarallon plate, and possibly represents melting duringthe start-up phase of the Galapagos hotspot (Duncan &Hargraves, 1984; Richards et al., 1991; Thompson et al.,2004). Shortly after its formation, the northern part of theplateau appears to have collided with the eastward-dipping subduction zone of the Caribbean ‘Great Arc’(Burke, 1988). The hot, thick and consequently buoyantplateau was relatively unsubductable and so clogged thesubduction zone, resulting in a reversal in subductionpolarity from east to west. This reversal allowed thenorthern portion of the CCOP to move into the seawaybetween North and South America along strike-slip faultsto its present-day location (Burke, 1988).The CCOP was drilled during Deep Sea Drilling Pro-
ject (DSDP) Leg 15 in the Caribbean Sea (Bence et al.,1975), and portions of it are exposed around its marginsas a result of tectonic processes (Kerr et al., 1997b; Sintonet al., 1998). In particular, the southern portion of theplateau collided with a continental subduction zone and,unable to subduct, was partially accreted in a series oftectonic slices along the northwestern margin of SouthAmerica (Millward et al., 1984; Kerr et al., 1997a; Hauffet al., 2000b).Although the majority of the accreted sections of the
CCOP in northwestern South America consist of basalticlavas and intrusive sheets (Kerr et al., 1997a), picritesand/or komatiites (sensu Kerr & Arndt, 2001) alsooccur; for example, Gorgona and in the CentralCordillera (Arndt et al., 1997; Kerr et al., 2002b). Inaddition to the extrusive and hypabyssal components,accreted gabbroic and ultramafic plutonic rocks
interpreted as part of the plateau are also exposed inColombia and Ecuador (Kerr et al., 1998, 2002a;Lapierre et al., 2000; R�eevillon et al., 2000).One of the largest of these plutonic complexes is
located west of the town of Bolıvar (4�200N, 76�150W;Fig. 1); this complex predominantly comprises layeredand isotropic gabbros and gabbronorites, underlain bymore ultramafic rocks. The plutonic suite is knowncollectively as the Bolıvar Ultramafic Complex (BUC)(Aspden, 1984). Intruding the BUC, and in places grada-tional to the gabbros, is a suite of predominantly mafic,hornblende-bearing pegmatite dykes, which are the mainfocus of this study. A limited amount of chemical data forgabbros from the BUC was reported by Kerr et al. (1998).In this paper we present a more extensive dataset for thegabbros and discuss 40Ar–39Ar ages and elemental andSr–Nd–Pb–Hf isotope data for the pegmatites. We usethese data to constrain models for the origin of theBolıvar pegmatites and their petrogenetic relationship tonearby felsic dykes and breccias. Our conclusions onthe origin of the hydrous rocks in the Bolıvar complexenable us to contribute to the important debate on thewater content of mantle plumes and the associated impli-cations for both mantle and magmatic processes.
ANALYTICAL METHODS
Major element mineral compositions were determinedusing a JEOL 8600 electron microprobe at the Universityof Leicester. A 30 nA beam current, 15 kV acceleratingvoltage and 10 mm spot size were used for all analyses. AZAF correction procedure was used to reduce the rawdata. The major element data are reported in Tables 1and 2 and summarized in Fig. 2, which also shows 2s errorbars for the analyses. The trace element abundances inmagnesiohornblende and plagioclase were analysed onmineral separates by solution inductively coupled plasmamass spectrometry (ICP-MS) at Cardiff (see below) afterhand-picking of crystals (Table 3).
40Ar–39Ar dating was carried out at the ScottishUniversities Environmental Research Centre, EastKilbride. Hand-picked magnesiohornblende grains werewrapped in copper foil packets and placed in quartz vials.Samples were irradiated for 18 h at 1 MW in the Cd-shielded CLICIT facility at the Oregon State University(OSU) TRIGA reactor. The flux monitor Taylor CreekRhyolite sanidine 85G003 (28�34� 0�28Ma, Renne et al.,1998) was loaded into copper packets and placed every7mm in each of the vials; the neutron flux monitor J isconservatively known to better than 0�3% at any givenposition in the vials. On the basis of previous analyses ofoptical grade CaF2 and degassed, Fe-doped K-silicateglass irradiated in the CLICIT facility at OSU, correc-tions for interfering neutron-induced reactions on 40K and
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1878
40Ca are [40Ar/39Ar]K ¼ 0�00086, [36Ar/37Ar]Ca ¼0�000264 and [39Ar/37Ar]Ca ¼ 0�000673.Individual rock cores were loaded into a glass side-arm
over a double vacuum furnace attached to a gas clean-upsystem. Incremental-heating experiments consisted ofbetween five and 17 steps. Following a 15min heatingstep and 10min additional clean-up stage, the purified
gas was analysed on an EMI secondary electron multi-plier operated at a gain of �5000, with a MAP 215 raregas mass spectrometer. For each analysis, the peaks overthe mass range 36–40 were measured over nine cycles.Blanks were analysed at least every six steps, usually at theend of each experiment. Analyses were then corrected forblanks, which typically were 3�0 � 10�17 moles for 36Ar,
Fig. 1. Geological sketch map showing the location of the Bolıvar Ultramafic Complex and the Vijes felsites. Major regional faults are shown asbold lines.
2� 10�15 moles for 39Ar and 2�4� 10�15 moles for 40Ar.Monitor samples were analysed and their J curves calcu-lated from their positions within the vial. These curveswere interpolated and J values were obtained for eachsample, which was then used as a correction factor. Thegoodness of fit [MSWD, or SUMS/(n � 2) where n is thenumber of steps] indicates if the scatter about theisochron regression line is solely analytical [e.g. SUMS/(n � 2) <2�5] or implies geological disturbance of themeasured argon isotopes (McDougall & Harrison, 1988).40Ar–39Ar data are reported in Table 4.Weathered surfaces were removed from the whole-rock
samples using a clipper saw and the samples crushedusing a fly press. In the case of the coarse-grainedpegmatites a large volume (�1000 cm3) of sample was
crushed and thoroughly mixed to ensure that the sampleswere representative of the whole rock. After crushing, thesamples were powdered in an agate Tema1 mill. Themajor elements along with Zr, Cr, Ni, Sc and V wereanalysed by X-ray fluorescence (XRF) at LeicesterUniversity using conventional techniques; further detailshave been given by Tarney &Marsh (1991) and Kerr et al.(1996a). The XRF data are reported in Tables 5–7.Most samples have also been analysed by ICP-MS
using a Thermo Elemental Series X at Cardiff for theelements Ba, Ga, Nb, Rb, Sr, Th, Y, Hf and rare earthelements (REE). Rh and Re were used as internal stand-ards for drift correction, and the calibration was carriedout using laboratory standards and selected internationalstandards with similar matrices. The samples were
Fig. 2. (a–d) Plots of magnesiohornblende compositions in mafic, intermediate and felsic pegmatites. Plotted compositions (cations per formulaunit) are individual microprobe analyses. Cation proportions have been calculated on the basis of 23 oxygens (Table 1). The core (c) and rim (r)analyses of individual crystals are linked by a tie-line. (e) Plot of the An content of plagioclases in mafic, intermediate and felsic pegmatites.
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1882
prepared for analysis by dissolution of 0�1 g powder withHF and HNO3, spiked by internal standards and run in anitric acid matrix at a 0�1 g/dm3 concentration of totaldissolved solids. The correction procedure involvesblank-subtraction, drift-monitoring and correction foroxide–hydroxide interferences and isotopic overlaps.Sr, Pb, Nd and Hf isotope ratios were measured on
unleached samples and determined at the NERC IsotopeGeosciences Laboratory, Keyworth (NIGL). The dataare presented in Table 8. Procedures used in the analysisof Sr, Pb, Nd and Hf isotopes have been described byRoyse et al. (1998), Kempton et al. (2000) and Kempton &McGill (2002). Sr and Nd were run as the metal specieson single Ta filaments and double Re–Ta filamentsrespectively, using a Finnigan MAT 262 multicollectormass spectrometer in static mode. The effects of fractio-nation during runs were eliminated by normalizing87Sr/86Sr to a 86Sr/88Sr value of 0�1194 and143Nd/144Nd to a 146Nd/144Nd value of 0�7219. Sample
values for 143Nd/144Nd are reported relative to anaccepted value of 0�71024 for NBS 987 and 0�51186 forLa Jolla. Minimum uncertainty is derived from theexternal precision of standard measurements that overthe course of analysis average 29 ppm (2s) for 143Nd/144Nd and 35 ppm (2s) for 87Sr/86Sr. Blanks for Sr andNd are typically less than 300 and 200 pg, respectively.Pb isotopes were analysed by multicollector (MC)-ICP-
MS on a VG P54 system. Mass fractionation was cor-rected for during the run using a 203Tl/205Tl value of0�41876, which was determined empirically by cross calib-ration with NBS 981. All Pb isotope ratios have beencorrected relative to the NBS 981 composition of Todtet al. (1996). On the basis of repeated runs of NBS 981,the reproducibility of whole-rock Pb isotope measure-ments is better than �0�01% (2s). Hf was also analysedon the VG P54. Within-run standard error for Hf isotopemeasurements is normally less than 22 ppm (2s). Mini-mum uncertainties are derived from the external preci-sion of standard measurements, which average 44 ppm(2s). Replicate analysis of our internal rock standard, pk-G-D12, over the course of analysis yielded 0�283050� 12(2s, n ¼ 45), which is indistinguishable from our pre-viously reported values (Nowell et al., 1998; Kemptonet al., 2000). The data are corrected for mass fractiona-tion during the run by normalization to 179Hf/177Hf of0�7325 and are reported relative to an accepted value ofJMC 475 of 0�282160, as recommended by Nowell et al.(1998). In all cases, within-run precision is less thanexternal reproducibility, and the procedural blank is lessthan 200 pg.Sr, Nd and Hf isotope ratios have been age corrected to
90Ma.
FIELD RELATIONSHIPS,
PETROGRAPHY AND MINERALOGY
Field relationships in the BolıvarUltramafic Complex
The BUC forms part of the eastern foothills of theWesternCordillera. The complex is bounded to the east by theCauca–Patia Fault and to the west by the Roldanillo Fault(Fig. 1). TheRoldanillo Faultmarks the contact of the BUCwith the basaltic, fault-bounded blocks of the CCOP (Kerret al., 1997a). These accreted basaltic slices alternate withfault blocks of deformed late Cretaceous marine shales andgreywackes (Fig. 1). The detailed geology of the Bolıvarultramafic rocks has already been discussed in some detail(Barrero, 1979; Bourgois et al., 1982; Nivia et al., 1992;Nivia, 1996) and will be summarized only briefly here.The ultramafic rocks consist of intercalations of �10m
thick layers of serpentinized dunite with 10–30 cm thickunits of serpentinized olivine websterite, clinopyroxenite,wehrlite and olivine gabbronorite. The BUC also
Rock type: intermediate mafic intermediate mafic mafic
Ba 70.4 8.0 8.8 23.7 8.1
Nb 0.064 0.025 3.492 3.316 3.076
Rb 4.563 0.068 0.634 0.382 0.137
Sc 0.3 7.0 48.8 83.0 87.3
Sr 369 270 58 19 18
Th 0.018 0.004 0.096 0.073 0.038
Y 0.2 0.2 39.3 41.1 38.1
Zr 1.7 0.8 86.5 89.4 83.7
La 1.926 0.129 2.991 2.669 1.922
Ce 2.831 0.240 12.582 10.440 7.599
Pr 0.230 0.029 2.323 1.852 1.430
Nd 0.688 0.157 13.465 11.278 9.054
Sm 0.080 0.036 4.648 4.607 3.843
Eu 0.307 0.111 1.142 1.337 1.167
Gd 0.048 0.035 5.113 5.630 4.920
Tb 0.007 0.006 0.995 1.145 0.958
Dy 0.042 0.036 6.466 7.272 6.357
Ho 0.008 0.010 1.422 1.603 1.384
Er 0.029 0.033 4.710 4.732 4.015
Tm 0.005 0.004 0.763 0.748 0.631
Yb 0.033 0.028 5.100 4.615 3.744
Lu 0.004 0.005 0.867 0.718 0.573
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1883
comprises an extensive sequence of layered and isotropicgabbros. The lower gabbro horizons are dominated byinter-banded (0�3–1�0m thick) gabbro, olivine gabbro,gabbronorite and occasionally anorthosite, whereas theupper horizons consist of isotropic gabbro, which in placesgrades into anorthosite. A significant proportion of theBUC rocks are extensively foliated and sheared, and theclinopyroxene in some of the gabbro horizons has beenreplaced by secondary amphibole (Nivia, 1996).Historically, the elucidation of the structure of the BUC
has proved problematic, not least because of poorexposure. Barrero (1979) proposed that the BUC was aconcentrically zoned body, whereas Bourgois et al. (1982)suggested that the general form of the complex was anti-clinal. In the 1990s the construction of a new roadthrough the complex opened up new road-cut sectionsthat revealed structural evidence suggesting that the com-plex comprises a series of faulted–imbricated blocksrather than being a concentrically zoned body or ananticlinal structure (Nivia et al., 1992). On the basis ofthis new structural evidence, it is clear that the ultramaficrocks generally occupy the lower layers of the complexand grade upwards into the layered gabbro, with theisotropic gabbro occurring at high structural levels withinthe complex (Nivia et al., 1992).The most conspicuous feature of the BUC is abundant
plagioclase–hornblende-bearing pegmatite dykes andveins (Fig. 3), most of which trend east–west. Theseveins are, on average, 50 cm wide, although they rangefrom a few centimetres to 70 cm wide. On the basis offield observations, three main pegmatite types can be
identified: (1) mafic pegmatites, containing a largeproportion of hornblende (>50%) with plagioclase; (2)intermediate pegmatites, containing between 50 and10% hornblende with abundant plagioclase; (3) felsicpegmatites comprising predominantly plagioclase with<10% hornblende and small, but variable, amounts ofquartz and biotite. Euhedral to subhedral hornblendecrystals are up to 10 cm in length, and many grow per-pendicular to the walls of the veins (Fig. 3).Some of the veins were intruded after solidification of
the host rocks, as evidenced by sharp contacts betweenthe veins and the host rock. However, the absence ofchilled margins suggests that the veins were intrudedonly shortly after the solidification of the host rock. Inseveral places the pegmatites show a progressive grada-tion to more gabbroic compositions. Although many ofthe pegmatites are essentially undeformed, some aresheared and ptygmatically folded. The close temporalrelationship between the BUC and the pegmatites isfurther evidenced by the occurrence of rounded blocksof leucrocratic gabbro within some of the pegmatites.Observations in a quarry just outside Bolıvar town
(Nivia et al., 1992; Nivia, 1996) suggest that the mineral-ogy of the pegmatites is controlled, at least in part, bytheir level of emplacement within the complex; that is,the pegmatites found in the lower parts of the BUC areof the hornblende-rich variety, whereas at higher struc-tural levels hornblende-poor, quartz and biotite-bearingpegmatites predominate. Furthermore, many of the veinspossess a much greater modal percentage of hornblendeat their margins than in their centre.
Table 4: 40Ar/39Ar plateau and inverse isochron age calculations for magnesiohornblendes from the Bolivar
The ages are reported relative to 28.34Ma for Taylor Creek Rhyolite sanidine 85G003 (Renne et al., 1998).
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1884
Table5:MajorandtraceelementanalysesofBolıvar
basalts,gabbrosandpyroxenites
e-gab
brosan
dpyroxenites
d-gab
brosan
dpyroxenites
Sam
ple:
Bol94-25
Bol94-26
Bol94-24
Bol94-17
Bol94-23
Bol94-28
Bol94-35
JTBol-8
Bol94-5
Bol94-11
Bol94-7
Bol94-8
Bol94-10
Bol94-6
Typ
e:basalt
basalt
gab
bro
gab
bro
gab
bro
gab
bro
pyroxenite
gab
bro
gab
bro
gab
bro
gab
bro
gab
bro
pyroxenite
foliatedgab
bro
SiO
252. 21
51. 10
50. 06
48. 00
49. 95
55. 82
46. 47
50. 99
47. 66
49. 42
52. 98
43. 89
50. 43
44. 96
Al 2O3
13. 47
12. 02
14. 36
13. 43
13. 97
15. 21
14. 10
16. 97
20. 76
17. 46
4.57
5.21
10. 37
3.57
Fe2O3(t)
14. 99
17. 36
8.71
9.74
11. 75
8.61
10. 90
7.76
7.90
4.88
6.24
17. 47
7.17
12. 07
MgO
4.42
4.25
9.79
9.00
8.09
4.12
10. 00
10. 37
8.67
11. 24
17. 50
28. 50
17. 73
31. 54
CaO
8.71
8.63
13. 74
12. 95
12. 18
9.95
13. 38
13. 67
13. 86
16. 34
18. 91
5.07
14. 27
7.30
Na 2O
3.04
2.45
1.46
1.25
1.41
4.27
1.32
0.50
0.51
0.55
0.14
0.00
0.21
0.00
K2O
0.094
0.088
0.129
0.051
0.041
0.060
0.018
0.034
0.004
0.005
0.005
0.001
0.010
0.001
TiO
21.22
2.41
0.71
0.74
1.07
1.02
0.96
0.15
0.07
0.16
0.19
0.15
0.18
0.14
MnO
0.197
0.222
0.134
0.148
0.165
0.109
0.158
0.077
0.133
0.092
0.127
0.214
0.134
0.174
P2O5
0.118
0.208
0.065
0.060
0.080
0.014
0.058
0.019
0.011
0.014
0.012
0.011
0.012
0.011
Total
98. 46
98. 74
99. 17
95. 36
98. 71
99. 18
97. 37
100.53
99. 58
100.16
100.66
100.57
100.51
99. 75
LOI
0.38
0.09
0.30
2.60
0.50
0.34
0.26
0.29
0.20
0.54
0.24
1.15
0.48
0.84
Ba
2927
2216
1241
426
610
43
243
Cr
1015
768
462
357
23477
370
311165
2675
3069
1564
2848
Ga
16. 9
22. 6
14. 2
14. 8
17. 0
19. 1
15. 3
14. 7
14. 4
10. 2
4.9
5.7
7.5
4.1
Nb
3.22
8.28
2.71
2.33
3.26
4.84
2.13
0.52
0.18
0.21
0.63
0.16
0.09
0.15
Ni
1526
129
133
8329
143
145
35114
171
999
227
689
Rb
0.17
0.18
1.89
0.58
0.46
0.51
0.18
0.58
0.09
0.11
0.08
0.07
0.50
0.05
Sc
41. 7
41. 7
43. 4
44. 4
44. 8
38. 3
49. 0
35. 6
31. 9
38. 4
81. 6
24. 1
89. 7
34. 5
Sr
123
9675
7379
132
9395
119
9125
3849
22
Th
0.230
0.569
0.173
0.147
0.179
0.179
0.160
0.055
0.013
0.026
0.090
0.015
0.006
0.004
V402
643
261
267
323
165
352
329
85116
182
161
126
96
Y22. 3
47. 6
17. 2
16. 8
23. 3
28. 6
19. 9
3.7
1.5
4.0
5.1
2.1
4.9
2.9
Zr
39. 7
142.0
40. 0
39. 4
53. 0
55. 3
49. 6
11. 0
3.6
8.1
9.2
3.9
8.6
5.5
Hf
1.01
3.91
1.17
1.03
1.56
1.63
1.38
0.34
0.09
0.22
0.25
0.12
0.22
0.16
La
1.82
8.86
2.49
2.07
2.94
1.60
1.62
0.57
0.11
0.19
0.54
0.09
0.12
0.20
Ce
4.51
23. 04
6.48
5.50
7.39
4.17
5.35
1.48
0.24
0.53
1.21
0.22
0.35
0.55
Pr
0.70
3.31
0.97
0.80
1.11
0.72
0.91
0.22
0.05
0.10
0.18
0.04
0.09
0.10
Nd
4.06
17. 87
5.22
4.29
5.95
4.19
5.37
1.32
0.26
0.70
0.99
0.30
0.55
0.57
Sm
1.46
5.92
1.74
1.61
2.08
1.77
2.02
0.42
0.10
0.29
0.41
0.14
0.30
0.23
Eu
0.54
1.86
0.64
0.60
0.75
0.82
0.80
0.17
0.09
0.16
0.17
0.08
0.15
0.10
Gd
1.96
7.12
2.09
2.03
2.55
2.44
2.54
0.46
0.12
0.40
0.52
0.20
0.47
0.27
Tb
0.43
1.29
0.42
0.39
0.50
0.55
0.47
0.10
0.03
0.08
0.11
0.03
0.09
0.05
Dy
2.91
8.30
2.67
2.45
3.08
3.57
3.32
0.65
0.17
0.61
0.71
0.24
0.63
0.36
Ho
0.70
1.79
0.60
0.56
0.67
0.83
0.69
0.14
0.05
0.13
0.16
0.07
0.15
0.09
Er
2.11
5.22
1.77
1.71
2.15
2.58
2.07
0.47
0.15
0.36
0.52
0.21
0.43
0.25
Tm
0.32
0.82
0.27
0.27
0.33
0.43
0.32
0.07
0.02
0.06
0.07
0.03
0.07
0.04
Yb
2.10
4.93
1.97
1.68
2.03
2.63
2.13
0.45
0.14
0.34
0.45
0.21
0.43
0.22
Lu
0.29
0.74
0.29
0.28
0.34
0.44
0.33
0.07
0.03
0.06
0.07
0.04
0.07
0.05
e-gab
bro,en
rich
edgab
bro;d-gab
bro,dep
letedgab
bro.
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1885
Field relationships of the Vijes felsites
Just north of the town of Vijes, and �50 km south of theBUC (Fig. 1), a series of felsites (rhyodacites) are asso-ciated with the oceanic plateau basalts of the Western
Cordillera. The Vijes felsites occur along strike and in thesame fault block as the BUC. The field relationships andpossible origin of these felsites have previously been dis-cussed by Millward (1983), Aspden & McCourt (1986)
Table 6: Major and trace element analyses of Bolıvar pegmatites
Sample: JTBol-9 Bol 94-22 JTBol-04 Bol 94-15 Bol 94-18 Bol 94-19 JTBol-14 JTBol-10 JTBol-12 JTBol-13 Bol 94-12 Bol 94-14
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1888
and Kerr et al. (1996b), and they were originally observedto comprise a sequence of plugs (up to 3�5 km2 in area),sheets and dykes (up to 3–4mwide). Construction of a newroad through the area has allowed access to deeper levelsof this rock association and uncovered evidence of magmamixing in the form of volcanic rocks comprising felsicblebs in a basaltic matrix (Fig. 3c). Additionally, the newroad cuts have exposed bedded deposits of felsitic volcanicash and breccia, comprising felsite clasts (up to 5 cm indiameter) set in a fine-grained felsitic matrix (Fig. 3d).Some of the larger felsite breccia clasts have been sampledand analysed during the present study. For the purposes ofthis study, the Vijes rocks can be considered in terms ofthree broad groups: felsite dykes, felsite breccias andbasalts–andesites from the mixed magma units.
Petrography and mineral chemistryof the BUC and Vijes felsites
In thin section the plagioclase and hornblende crystals inthe pegmatites show virtually no evidence of zoning.Unlike many pegmatites, accessory minerals are uncom-mon, and apatite is the only accessory phase that has
been positively identified. Many of the plagioclase crys-tals have undergone variable degrees of late-stage seri-citization, particularly around their margins, and alongcracks. Magnesiohornblende crystals have in places beenaltered to chlorite and an oxide mineral.To fully characterize the chemistry of the pegmatite
minerals, representative samples from the mafic, inter-mediate and felsic pegmatite groups were analysed byelectron microprobe at Leicester University. Representa-tive data are listed in Tables 1 and 2 and summarizedgraphically in Fig. 2. All amphiboles can be classified asmagnesiohornblendes according to the revised amphi-bole classification scheme of the International Mineralo-gical Association (Leake et al., 1997); that is, they haveCaB � 1�5, (Na þ K)A < 0�5, Mg/(Mg þ Fe2þ) > 0�5,and Si in the formula ranges from 7�5 to 6�5 (Fig. 2).Magnesiohornblendes in the mafic pegmatites (those witha high modal content of amphibole) have generallyhigher Ca and Ti and lower Mn than magnesiohorn-blendes from the intermediate pegmatites (Fig. 2).Although there is no petrographic evidence of zoning, corevs rim analyses of several amphiboles indicate a small butsystematic variation in chemistry; that is, Si, Ca, Mg and
Fig. 3. Field photographs. (a) An intermediate Bolıvar pegmatite intruding isotropic gabbro. The pegmatite contains large magnesiohornblendecrystals growing perpendicular to the side of the vein. (b) A block of mafic pegmatite with large aligned magnesiohornblende crystals. (c) A basalticsheet from Vijes, which preserves evidence of magma mixing in the form of globular masses of felsic material. (d) Vijes volcanic breccia, consistingof felsite fragments set in a matrix of welded, but altered, felsitic ash.
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1889
Mn are higher, whereas Al, Fe and K are lowerin cores relative to rims of crystals (Fig. 2 and Table 1).Plagioclase varies widely in composition from An84 to
An34. Mafic pegmatites contain plagioclases that fallbroadly into two compositional groups: An84–82 (i.e.bytownite) and An54–47 (i.e. labradorite). Intermediateand felsic pegmatites contain andesine plagioclases,which are much more restricted in composition(i.e. An50–41 and An41–34, respectively; Fig. 2 andTable 2). As a result of alteration it was difficult to analysethe rims of many of the plagioclase crystals. However,where this was possible, cores and rims were usuallyfound to have very similar An contents (Table 2). Smalldecreases in An content (up to An3–4) were observedbetween core and rim in only a couple of intermediatepegmatites.The gabbros, olivine gabbros and gabbronorites of the
BUC are equigranular and commonly have ophitic tex-tures. In the lower layered part of the sequence, crystalsare oriented with their long axis parallel to the composi-tional banding. Olivines are anhedral and are partiallyaltered to serpentine, whereas clinopyroxenes are mainlysubhedral to anhedral, show hypersthene exolutionlamellae and are variably altered to amphibole. Ortho-pyroxenes are subhedral to euhedral with a stubby pris-matic habit. Plagioclases are variably altered to sericiteand in the cumulitic levels exhibit zonation. Moredetailed descriptions have been given by Nivia (1996).Petrographically, the felsites consist of 5–10% plagio-
clase and (strain shadowed) quartz microphenocrysts in afine-grained or glassy groundmass, comprising quartz,plagioclase and minor hornblende. The groundmass hasbeen extensively modified by low-grade metamorphismto chlorite, clays, epidote and pumpellyite. A similargrade of alteration and metamorphism is observed inthe oceanic plateau basalts in the Western Cordillera,suggesting that the felsites formed prior to continentalaccretion of the CCOP.
40Ar–39Ar AGES OF THE BOLIVAR
PEGMATITES
Previous attempts at radiometric dating of the Bolıvarpegmatites have applied the K–Ar method to the magne-siohornblendes. These have yielded ages of 106 � 18,102 � 18, 78 � 18 and 70 � 14Ma (Barrero, 1979;Brook, 1984). However, the Bolıvar pegmatites haveundergone variable degrees of sub-solidus alteration andthis, combined with their low contents of K2O, hasresulted in the wide range of reported K–Ar ages withlarge errors.Our 40Ar–39Ar step-heating experiments on magnesio-
hornblendes from the Bolıvar pegmatites have yieldedmore precise ages with much smaller errors than the
K–Ar dating (Table 4); step-age spectra and inverse iso-chron correlation diagrams are given in Fig. 4. Althoughsome experiments yielded (unreported) discordant ages,the four experiments reported in Table 4 all gave con-cordant ages with low SUMS/(n – 2) values, and near-atmospheric 36Ar/40Ar intercepts with between 68 and98% of the released 39Ar comprising the age-determiningplateau (Fig. 4). The four concordant step-heatingage determinations give a weighted mean age of90�5 � 0�9Ma.Of the four concordant age determinations, two were
for mafic pegmatites and two for intermediate pegma-tites. The two mafic pegmatites, Bol 94-22 and JTBol-4,give slightly older step-heated plateau ages (weightedmean 92�5 � 1�8Ma) than the two intermediate pegma-tites, Bol 94-18 and Bol 94-15 (weighted mean 89�7 �1�1Ma). It is significant that the age range of the pegma-tites, in particular the older ages, are consistent withthe proposed connection between the Cenomanian–Turonian (93Ma) anoxic event and CCOP volcanism(Sinton & Duncan, 1997; Kerr, 1998).
GEOCHEMISTRY
Major elements
The Bolıvar gabbros range in composition from 4�1 to31�5 wt % MgO (Table 5; Fig. 5), largely reflecting themodal abundance of olivine. For samples with MgO<12 wt %, two groups of gabbros can be distinguishedin terms of TiO2 contents: enriched (e-) gabbros (TiO2
>0�6 wt %) and depleted (d-) gabbros (TiO2 <0�2 wt %).These groups can be distinguished on the basis of incom-patible trace element abundances (see below).The petrographic division of the pegmatites into mafic,
intermediate and felsic types can be more rigorouslyquantified on the basis of major element composition.Mafic pegmatites contain >7 wt % MgO, >11 wt %CaO and <53 wt % SiO2, whereas intermediate pegma-tites range from 7 to �2 wt % MgO, from 11 to 5 wt %CaO and from 53 to 75 wt % SiO2. Felsic pegmatitescontain <2 wt % MgO, <5 wt % CaO and >75 wt %SiO2. (Table 6; Fig. 5). These chemical differences areconsistent with the mineralogical changes on going frommafic through intermediate to felsic pegmatites, i.e.increasing modal percentage of more Na-rich plagioclaseat the expense of magnesiohornblende.On the basis of their field relationships, the rocks from
Vijes can be divided into three broad groups: felsitedykes, felsite breccias and basalts–andesites from themixed magma bodies. Although the Vijes felsites andbasalts are altered, containing abundant secondary clayminerals, these three groups can also be distinguished onthe basis of major element chemistry (Table 7; Fig. 5).Felsite dykes generally have lower MgO (�1 wt %) and
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1890
Fe2O3(t) (4–5 wt %) than breccias [MgO 1–4 wt %;Fe2O3(t) 5–13 wt %]. Although some of the scatter inNa2O and K2O concentrations (Table 7; Fig. 5) may bedue to sub-solidus alteration, it is noteworthy that thedykes all have higher Na2O and generally higher K2Othan the breccia clasts. The basaltic–andesitic portion ofthe mixed magma unit possesses broadly higher MgO,
TiO2, Fe2O3 and CaO and lower SiO2 than the morefelsitic rocks (Table 7; Fig. 5).
Trace elements
The major element division of Bolıvar gabbros into twogroups can also be observed in trace elements, with the
Fig. 4. Ar–Ar step-heating plateau experiments and inverse isochron plots. &, Steps that have been used in the calculation of the inverse isochronage (Table 4); &, steps not used in the regression.
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1891
e-gabbros having higher contents of incompatible traceelements than the d-gabbros (Figs 6 and 7). Thee-gabbros are characterized by essentially flat primitive-mantle-normalized trace element patterns and the simi-larity to the basalts of the Western Cordillera is striking(Fig. 7a). In contrast, the d-gabbros are more composi-tionally variable, with the majority showing depletion in
the most incompatible trace elements (Figs 6 and 7). It isinteresting to note that the d-gabbros are those from themiddle of the complex west of Bolıvar, which are mostclosely associated with the pegmatites; that is, thesegabbros either grade into, or are intruded by, the pegma-tites. In contrast, the e-gabbros are rarely associated withpegmatites and predominate to the south of the BUC.
Fig. 5. (a–e) Plots of major elements vs SiO2, and (f) CaO vs MgO for the Bolıvar gabbros and pegmatites and the Vijes rocks. The field labelledWC represents the range of compositions of CCOP basalts of the Western Cordillera of Colombia [data from Kerr et al. (1997)]. Dotted curveswith arrows represent modelled (MIXFRAC; Nielsen, 1988) fractionation trends, with ticks indicating 20 and 40% crystallization of a picriticparental CCOP magma from Curacao [data from Kerr et al. (1996a)].
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1892
Fig. 6. (a–g) Plots of selected trace elements vs SiO2, and (h) Nb vs MgO, for the Bolıvar gabbros and pegmatites and the Vijes rocks. The fieldlabelled WC represents the range of compositions displayed by the CCOP basalts of the Western Cordillera of Colombia [data from Kerr et al.(1997)]. Also shown are modelled (MIXFRAC; Nielsen, 1988) fractionation trends (dotted curves). Ticks along the curve indicate 20 and 40%crystallization of a picritic parental CCOP magma from Curacao [data from Kerr et al. (1996a)]. Continuous-line trends with arrows are modelledfractionation vectors using average Vijes basalt as a starting composition, for incompatible and relatively immobile elements. (See Table 9 for furtherdetails.) Labels on these trends indicate the proportion of plagioclase and magnesiohornblende that needs to be removed to model the composition ofboth the Vijes felsic dykes and the felsic breccia clasts. Dashed tie-lines join the composition of plagioclase and magnesiohornblende.
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1893
Mafic, intermediate and felsic pegmatites have distinc-tive primitive-mantle-normalized trace element patterns(Fig. 7b). Mafic pegmatites are moderately enriched inincompatible trace elements (2–8 times primitive mantle);they show a marked depletion in the most incompatibletrace elements and a small but distinct negative Ti anom-aly (Fig. 7b). Unsurprisingly, the trace element pattern formagnesiohornblende (hand-picked and analysed by solu-tion ICP-MS) is very similar to the pattern for the maficpegmatites. In contrast, felsic pegmatites are markedlydepleted in the heavy rare earth elements (HREE;�0�1 times primitive mantle) relative to the most incom-patible trace elements, although the latter are similar tothe values for mafic pegmatites (Fig. 7b). Felsic pegma-tites also show negative Nb and Ti anomalies, combinedwith a positive Eu anomaly. The patterns of these felsicpegmatites are similar to the trace element pattern forplagioclase (andesine) in the Bolıvar pegmatites (Fig. 7d).As with major elements, the trace element contents of the
intermediate pegmatites lie between the compositions ofthe mafic and felsic pegmatites (Fig. 7b).The Vijes felsic dykes are slightly enriched in the most
incompatible trace elements relative to the HREE andhave negative Ti anomalies (Figs 6 and 7c). Of the threerock groups observed at Vijes, the felsite dykes have thehighest abundances of incompatible trace elements,whereas these elements are significantly less abundantin felsite breccia clasts (Figs 6 and 7c). Although dataare limited, breccia clasts also appear to show a progres-sive decrease in most incompatible trace element con-tents as major element compositions become moreevolved (Fig. 6). The basaltic–andesitic part of the Vijesmixed magma has a flat to slightly light REE (LREE)-depleted primitive-mantle-normalized pattern, with traceelement contents intermediate between those of the felsitedykes and the breccia clasts (Fig. 7c). As will be discussedbelow, the lack of a significant Nb anomaly, positive ornegative, for all the Vijes rock types is significant.
Fig. 7. Primitive-mantle-normalized (Sun & McDonough, 1989) multi-element plots showing: (a) Bolıvar depleted (d-gabbros) and enrichedgabbros (e-gabbros), with a field for Western Cordillera CCOP basalts (Kerr et al., 1997); (b) Bolıvar mafic, intermediate and felsic pegmatites; (c)felsite breccias, felsite dykes and the basaltic portion of a mixed magma, all from Vijes; (d) plagioclase and magnesiohornblende mineralcompositions from the Bolıvar pegmatites.
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1894
Radiogenic isotopes
Given that all the samples collected during the presentstudy have undergone moderate degrees of sub-solidusalteration, the most robust isotope systems are likely to beSm–Nd and Lu–Hf. With the exception of one sample, allthe pegmatites andVijes rocks show only limited variationin eHfi and eNdi (calculated here and elsewhere for an ageof 90Ma), ranging fromþ12�8 toþ14�4 and fromþ6�5 toþ7�3, respectively (Fig. 8b). The exceptional sample, inter-mediate pegmatite Bol 94-19, has a similar eNdi to theother pegmatites but a higher eHfi value (þ15�2). Two ofthe gabbros have slightly lower eHfi values (þ12�4 toþ12�5) than Vijes rocks and pegmatites but are similar tomost rocks from the Western Cordillera. One of thee-gabbros (Bol 94-28) and one of the d-gabbros (Bol94-11) have a eHfi values of 16�1 and 14�6, respectively, but
have eNdi values that are within the range of the othergabbros and pegmatites (Fig. 8b). Paradoxically, one ofthe e-gabbros (i.e. those which are more enriched inincompatible trace elements) has higher eNdi than one ofthe d-gabbros analysed ( JTBol-8). Most basalts of theWestern Cordillera (including Bol 94-25, which was col-lected near the margin of the BUC) range from þ6�5 toþ7�6 and fromþ12�1 toþ12�6 for eNdi and eHfi, respec-tively. One exception is Cbu 94-13. This sample hasan eNdi value similar to other basalts of the WesternCordillera, but its eHfi is significantly higher (þ18�5). ItsHf isotope composition is comparable to that of Gorgonakomatiites (Fig. 8b), although it is displaced to a signifi-cantly lower eNdi value.The Bolıvar pegmatites and gabbros display a relatively
restricted range in (87Sr/86Sr)i from 0�7033 to 0�7038. In
Fig. 8. Radiogenic isotope plots showing: (a) eNdi vs (87Sr/86Sr)i; (b) eHfi vs eNdi; (c)208Pb/204Pb vs 206Pb/204Pb; (d) 207Pb/204Pb vs 206Pb/204Pb.
i, initial values age corrected to 90Ma. Data sources: East Pacific Rise (EPR) MORB from Mahoney et al. (1994), Regelous et al. (1999) andCastillo et al. (2000); Ontong–Java Plateau (OJP) from Mahoney et al. (1993); Gorgona from Aitken & Echeverrıa (1984), Dupr�ee & Echeverrıa(1984), Arndt et al. (1997) and R�eevillon et al. (2000); Indian Ocean MORB from Mahoney et al. (1989, 1992). Hf isotope data for MORB andIndian–Pacific MORB discriminant boundary from Kempton et al. (2002) and references therein. Galapagos data from Blichert-Toft & White(2001) and Thompson et al. (2004). Fields for Gorgona basalts and the Caribbean Plateau from Thompson et al. (2004).
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1895
contrast, (87Sr/86Sr)i for the Vijes rocks ranges from0�7045 to 0�7055. Given the advanced state of alterationof these rocks, and the fact that their eNdi and eHfi valuessubstantially overlap with the compositions of the pegma-tites and gabbros, it is likely that these high 87Sr/86Srvalues are the result of alteration. Some the CCOPbasalts of the western Cordillera also have (87Sr/86Sr)i>0�704 (Fig. 8a), as do rocks from elsewhere in theCCOP (Kerr et al., 1996a, 1997a; Hauff et al., 2000b).These high (87Sr/86Sr)i signatures have, for the most part,been ascribed to interaction with seawater (e.g. R�eevillonet al., 2002; Thompson et al., 2004).Measured 206Pb/204Pb ratios of the Bolıvar pegmatites
range from 18�6 to 18�9. These values overlap the206Pb/204Pb ratios of the d-gabbros, whereas e-gabbroshave higher 206Pb/204Pb ratios (19�1–19�4), similar tobasalts of the Western Cordillera (Fig. 8c and d). Mea-sured 208Pb/204Pb ratios show a similar spread of databetween the different sample types (Fig. 8c). With theexception of one sample, the pegmatites show a relativelylimited variation in 207Pb/204Pb (15�533–15�546); inter-mediate pegmatite Bol 94-19 has a higher ratio of 15�556,similar to the d-gabbros. The rocks from Vijes showmuch more variable Pb isotope ratios than either theBolıvar pegmatites or the gabbros. For example, thefelsite dyke Pan 94-23 has the most radiogenic Pb isotopecomposition of any Vijes or Bolıvar rock (Fig. 8c),with a 206Pb/204Pb of 19�8. However, there is evidencethat Pb isotope systematics can be disturbed by sub-solidus alteration (Hauff et al., 2000a; Kempton et al.,2002; Thompson, 2002), particularly in older rocks. It issignificant that Pan 94-23 is the most altered of the Vijessamples analysed for isotopes, containing as it does,highly sericitized feldspars set in a groundmass that hasbeen largely altered to clay minerals. Thus it may be thatthe Pb isotope systematics of Pan 94-23 have been dis-turbed by interaction with a fluid with a relatively radio-genic Pb isotope composition. Although the source ofsuch a fluid remains unclear, the elevated 87Sr/86Srratio of Pan 94-23 is consistent with this interpretation,as are Nd and Hf isotope ratios that are virtually indis-tinguishable from those of other Vijes rocks (Fig. 8b).Therefore, Pb and Sr isotope compositions of many ofthe rocks may be able to tell us only about the alterationhistory.The Pb isotope ratios of the e-gabbro(s) (Bol 94-24 and
Bol 94-28) and the Bolıvar basalt (Bol 94-25) are similarto those of the basalts from theWestern Cordillera (Fig. 8cand d). The Bolıvar pegmatites and the d-gabbros gen-erally have lower Pb isotope ratios than the basalts of theWestern Cordillera and are, in fact, most similar to thepicrites and komatiites from Gorgona Island. However,the Nd isotope compositions of Gorgona rocks indicatederivation from a significantly more depleted sourcemantle source region (Fig. 8b).
PETROGENESIS
Negative Nb anomaly in thepegmatites—evidence of subduction?
Felsic and intermediate pegmatites have negative Nbanomalies on primitive-mantle-normalized multi-elementplots (Fig. 7b). This feature, in a rock that represents anoriginal liquid composition, is usually taken as evidencefor a subduction-related origin or contamination bycontinental crust. However, as will be discussed inmore detail below, it is unlikely that these pegmatitesrepresent original liquid compositions. Alternatively, thenegative Nb anomaly may be due to the accumulation ofa particular mineral phase. We discuss this possibility forthe Bolıvar pegmatites below.Figure 9a shows that (Nb/La)n correlates positively
with MgO for the pegmatites. The samples with thelowest MgO and (Nb/La)n are the felsic and intermediatepegmatites. These rocks have a high modal abundance ofplagioclase and thus it may be accumulation of this phasethat is the source of the low (Nb/La)n. Felsic and inter-mediate pegmatites also have positive Eu anomalies—a
Fig. 9. Variation of (a) MgO vs (La/Nb)n for Bolıvar pegmatites andplagioclase from the pegmatites; (b) Eu* vs (Nb/La)n for Bolıvar peg-matites and plagioclase from the pegmatites. Eu* ¼ [Eun/(Smn þGdn)/2)] � 1, where n indicates chondrite-normalized.
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
1896
result of their high plagioclase content. We can thereforeuse the size of the positive Eu anomaly as a proxy for theamount of plagioclase in the pegmatite. A plot of (Nb/La)n vs Eu* (i.e. a measure of the Eu anomaly; Fig. 9b),shows that, although a strong linear correlation is absent,samples with low (Nb/La)n also tend to have a larger Euanomaly and hence more plagioclase. Thus, the sampleswith the largest negative Nb anomaly also contain thehighest modal percentage of plagioclase. This occursbecause the average Kd for Nb in plagioclase in a basalticmelt is 0�009, whereas the average Kd for La inplagioclase in a basaltic melt is 0�156. (Partition coeffi-cient data from the GERMwebsite http://www.earthref.org/databases/KDD/main.htm.) La will, therefore, bepresent in higher concentrations than Nb in plagioclase.Our two analyses of plagioclase from the Bolıvarpegmatites (Fig. 7d) confirm this conclusion. Thus the low(Nb/La)n values for the more felsic Bolıvar pegmatitessimply reflect their high modal plagioclase content and donot indicate a subduction-related origin for these rocks.
Magmatic processes
To understand the petrogenesis of the Bolıvar pegma-tites, we must first assess whether they represent original,closed-system, magmatic compositions. Several lines ofevidence suggest that they do not. First, the relative lackof compositional zoning in both the magnesiohornblendeand plagioclase crystals argues against closed-system evo-lution. If crystallization occurred in a closed system, theliquid remaining after early crystallization would bedepleted in CaO and MgO. This magma would precipi-tate more sodic feldspars and less MgO-rich hornblendes,both at the margins of the crystals and in the centre of thepegmatite veins, neither of which is observed. The morefelsic pegmatites also tend to occur at higher stratigraphiclevels in the BUC, whereas mafic pegmatites predomi-nate nearer the base. This suggests that some form ofdifferentiation has occurred within the pegmatites asmelts migrated upwards. Thus, a possible model for themagmatic evolution of the pegmatites is differentiation byside-wall crystallization from mafic magma migratingupwards through the Bolıvar complex. If the magmafrom which the crystals grew was constantly replenishedfrom below, the pegmatite veins would consist of crystalsof broadly uniform composition. In such a model themore evolved crystals would grow from magmas thathad migrated to a higher level within the complex.The lack of zoning in the most felsic pegmatites sug-
gests that these rocks do not represent the final crystal-lization products of a residual melt and that hydrousmagma also passed through these conduits en route tosomewhere else. The similarity in Nd and Hf isotoperatios for most Bolıvar pegmatites and Vijes felsites sug-gests that they may be genetically related. Thus themodelling presented below will test the proposal that
the Vijes felsites (both dykes and breccia clasts) mayrepresent the magmas residual from the crystallizationof the Bolıvar pegmatites. Vijes dykes and breccias dis-play two distinct compositional groups on variation dia-grams, particularly those involving trace elements (Fig. 6).When the compositions of magnesiohornblende and pla-gioclase (labradorite) from the mafic pegmatites areplotted on the variation diagrams, it becomes clear thatthe lower concentrations of Zr, Nb, LREE and Rb in theVijes felsite breccia clasts could be due to the fractiona-tion of magnesiohornblende from a basaltic magma.The composition of the Vijes felsite breccia clasts can
be successfully modelled by 85% fractionation of amineral assemblage comprising 71% magnesiohorn-blende and 29% plagioclase, from a parental magma withthe composition of the basaltic portion of the Vijes mixedmagma (VBM) (Fig. 6; Table 9). In contrast, the higherconcentrations of Zr, Nb and LREE in the Vijes felsitedykes (Fig. 6) can be modelled by 85% fractionation of acrystal assemblage that contains a higher proportion ofplagioclase (42% plagioclase; 58% magnesiohornblende),again using VBM as a starting composition (Fig. 6;Table 9). As magnesiohornblende contains higher con-centrations of incompatible trace elements than the pos-tulated parental magma, removal of an assemblagecomprising a significant proportion of magnesiohorn-blende will result in a reduction in the incompatibleelement trace element content of the felsitic daughtermagma. Figure 10 is a primitive-mantle-normalizedmulti-element plot showing results of calculations, assum-ing 80% fractionation of assemblages with variable pro-portions of plagioclase and magnesiohornblende andVBM as the starting composition. Although these calcu-lations give slightly different results relative to the massbalancemodelling described above, theynone the less illus-trate the influence of fractionation of magnesiohornblende
in reducing the concentration of the incompatible traceelements in the residual magma. This modelling clearlyshows that the composition of the felsite breccia clastsrequires removal of a crystal assemblage comprising ahigher proportion of hornblende than is necessary toexplain the composition of the less incompatible-element-depleted Vijes dykes.In contrast, the major element chemistry of the felsite
breccia clasts and dykes shows less compositional diver-gence at 70–80 wt% SiO2 than the trace elements (Fig. 5).There are several possible reasons for this. First, magne-siohornblende and plagioclase, although the main frac-tionating minerals, are not the only fractionating phases,as small amounts (<10%) of biotite and quartz are alsofound in the felsic pegmatites. The mismatch for TiO2
(Fig. 10) values (i.e. neither the magnesiohornblende northe plagioclase contain enough TiO2 to explain thedepletion with increasing SiO2; Fig. 5a) can be explainedby invoking fractionation of Fe–Ti oxide from the felsitemagma. Additionally, the small negative Nb anomaliesobserved in the primitive-mantle-normalized patterns forthe felsite dykes (Fig. 7c) can also be explained by theremoval of a small amount of Fe–Ti oxide, as Nb ismoderately compatible in magnetite and ilmenite. Sec-ond, the major element compositions (particularly K2Oand Na2O) of the felsites and, to a lesser extent, ofthe pegmatites may have been modified by sub-solidusalteration. Evidence for elemental mobility is alsoobserved in the variable Rb contents of the Bolıvar andVijes rocks, in contrast to generally more immobile ele-ments such as Zr and Nb (Fig. 6).The d-gabbros of the BUC are in some cases transi-
tional to the pegmatites, grading mineralogically from
hornblende-bearing gabbro to pegmatite. This suggeststhat the pegmatites (and ultimately the felsites) may havecrystallized from magmas similar to those that crystal-lized the gabbros. However, although the Nd isotopesystematics suggest that the pegmatites and gabbrosmay be genetically related, two out of the four gabbroshave lower eHfi values than the pegmatites and compar-able eNdi values to most of the basalts of the WesternCordillera. As there are few (if any) feasible crustalcontaminants within the oceanic lithosphere that wouldincrease eHfi while leaving eNdi unaffected for thepegmatites and some of the gabbros, it is probable thatthis signature is derived from the mantle source region.Salters & Zindler (1995) have reported eHfi values of>20for abyssal peridotites from the SW Indian Ridge. Thus,melting of a heterogeneous mantle source region contain-ing such a high eHfi component may explain the differ-ence in eHfi between the Bolıvar pegmatites and some ofthe gabbros. In this scenario, the generally slightlyyounger pegmatites crystallized from heterogeneousmagmas that had already undergone significant fractio-nation from a high-MgO parental magma (Fig. 11).To assess the validity of this proposed petrogenetic
scheme, we need to start with a suitable parentalmagma composition. Kerr et al. (1997a) have shown thatthe basalts of the Western Cordillera have undergone�20% crystallization of an assemblage dominated byolivine and clinopyroxene. Therefore, none of theWestern Cordillera basalts can be used directly as aparental magma in our modelling. We have thereforechosen to use a picrite from Curacao (also a part of theCCOP; Kerr et al., 1996a) as the starting composition.We have used the MIXFRAC program of Nielsen (1990)to model the fractionation of the parental magma, andthese results are shown in Figs 5 and 6. Although theMIXFRAC parameterization is primarily designed tomodel the fractionation of dry magmas, in the early stagesof fractionation of a picritic magma olivine is likely to bethe dominant fractionating phase regardless of whetherthe magma is wet or dry. The results show that thecomposition of the more mafic (6�7 wt % MgO) of thetwo VBM samples (Pan 94-28) can be modelled by20–30% fractionation of a crystal assemblage dominatedby olivine with smaller amounts of pyroxene and plagio-clase (Figs. 5 and 6). Back-projection of the fractionationtrend intersects the compositions of some d-gabbros(Figs 5 and 6), thus providing support for the contentionthat the d-gabbros represent the cumulates fractionatedfrom the parental magmas that form the VBM magmas.In contrast, the more evolved (3�61 wt % MgO) VBM
sample (Pan 94-29) has lower incompatible trace elementconcentrations (Figs 6 and 10) and so does not fit themodelled trend as well. These lower concentrations canbe explained by fractionation of magnesiohornblendefrom the magma. The fact that Pan 94-29 fits the ‘dry’
Fig. 10. Primitive-mantle-normalized multi-element plot showing aver-age compositions of Vijes felsite, Vijes breccia and Vijes basaltic mixedmagma. The dashed lines are modelled compositions representing 80%fractional crystallization of the composition of the average basalticmixed magma shown in the diagram. The curves illustrate the effectof progressively increasing the proportion of magnesiohornblende from40% to 100% of the total fractionating assemblage.
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
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modelled fractionation trend suggests that relatively littlemagnesiohornblende fractionated from this magma,thus validating the use of a dry magma modellingprogram. Magnesiohornblende obviously begins tofractionate from the VBM magmas at MgO contents<6�5 wt %.The proposed petrogenetic scheme for the Bolıvar–
Vijes complex, discussed in this section, is summarizedschematically in Fig. 11.
Water content of the primary (parental)magma
Mantle melt modelling has been carried out for manyCCOP magmas having very similar trace element chem-istry to the Bolıvar rocks (Kerr et al., 1996a, 1997a, 2002b;
Arndt et al., 1997; R�eevillon et al., 2000; Herzberg &O’Hara, 2002); consequently, similar modelling will notbe repeated here. Instead, our focus is on assessing theamount of water in the parental magma(s) and the impli-cations this has for their petrogenesis. Mantle melt model-ling formagmas of theCCOP shows that theMgOcontentof the parental magmas was on average �15 wt % MgO(e.g. Kerr et al., 1996a, 1996b; R�eevillon et al., 2000). There-fore, if we can estimate the amount of H2O amagmamustcontain before it begins to fractionate hornblende, we cancalculate the water content of the parental magma.The concentration of H2O necessary for amphibole to
fractionate from a magma has been estimated by severalstudies. The threshold H2O contents obtained by thesestudies range from 6 wt % (Merzbacher & Eggler, 1984)through 5 wt % (Naney, 1983) to 2 wt % (Luhr, 1992). As
Fig. 11. Flow diagram to illustrate the proposed petrogenetic scheme and the genetic interrelationships between the rock types observed at Bolıvarand Vijes.
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
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two of these studies suggest that the threshold abovewhich amphibole will fractionate is 5–6 wt % H2O, theaverage water content at which amphibole will begin tofractionate is likely to be>4 wt %. Therefore, the magmathat VBM sample Pan 94-28 crystallized from (6�7 wt %MgO), probably originally contained between 4 and6 wt % H2O. In contrast to VBM sample Pan 94-29,Pan 94-28 shows little chemical evidence for hornblendefractionation and was probably just saturated in water.Assuming (1) 25% fractional crystallization, (2) a partitioncoefficient (D) for water of 0�01 (in a magma not fractio-nating amphibole; Michael, 1995) and (3) a parentalmagma containing 15 wt % MgO, the amount of waterin the parental magma can be calculated from theassumed water content of Pan 94-28. These calculationssuggest that if the magma that Pan 94-28 crystallizedfrom contained between 4 and 6 wt % H2O, then theparental magma would have contained between 3�0 and4�5 wt % water.In addition to water content, the temperature of the
magma will also control whether amphibole will fraction-ate from a basic magma. Experiments by Helz (1982) andAllen & Boettcher (1983) have shown that amphibole isunlikely to be stable in a magma at temperatures in excessof 1000�C, even at high water contents. This, therefore,supports the results of our geochemical modelling, whichsuggest that hornblende did not fractionate fromthe Bolıvar magmas at high MgO contents (i.e. hightemperatures).
POSSIBLE MODELS FOR THE
TECTONIC SETTING OF THE BUC
Arc-related setting
The most obvious tectonic environment of formation forigneous rocks containing a large proportion of amphibole(implying a volatile-rich mantle source region) is in asubduction-related setting. This is a conclusion reachedby some previous workers on the BUC (e.g. Barrero,1979; Bourgois et al., 1982). However, as we haveshown above, the trace element patterns of the Bolıvarrocks do not support a subduction-related origin.Furthermore, the residual felsitic magmas (the Vijes fel-sites) derived from crystallization of the Bolıvar ultrama-fic rocks, gabbros and pegmatites, show no evidence of asubduction-related origin, i.e. they do not have high La/Nb ratios. Another line of evidence that argues against asubduction-related origin for the BUC is weighted meanage of the Bolıvar pegmatites. This mean age almostexactly coincides with the main peak of ages for theCCOP (weighted mean age 90�6 � 0�3; Fig. 12). Thesestatistically indistinguishable ages mean that if the BUC issubduction-related, the arc had to form at the edge of theplateau at the same time as the plateau lavas were
erupting. Although this is geodynamically possible, it isunlikely and, when combined with the lack of evidencefor a subduction signature, makes this scenario highlyimprobable.
Melting during accretion of the CCOP
On the basis of the limited geochemical data then avail-able, Nivia (1996) and Kerr et al. (1998) proposed that theBolıvar dunites and gabbros were generated in a magmachamber within the CCOP. In their models, the pegma-tites were proposed to represent fluid-aided melting at thebase of the accreting plateau as slices of it were obductedonto the margin of NW South America in the lateCretaceous. Our new dates and radiogenic isotope datamean that we are now able to test this model morerigorously. The lack of an obvious geochemical inputfrom sediment, although not precluding the obductionmodel, does make it less likely. However, it is the closesimilarity in ages between the CCOP and the Bolıvarpegmatites that firmly rules out an origin for the pegma-tites by melting during obduction. In short, the ages andchemistry of the Bolıvar pegmatites mean that they hadto have formed at the same time as the plateau, in a non-subduction-related environment, well away from acontinental margin.
Derivation from a mantle plume
The geochemical and geochronological data presented inthis paper suggest that the BUC is an integral part of theCCOP. This being the case, the question that arises is:‘Where does the water come from?’ Wet melting of litho-sphere heated from below by a mantle plume has beeninvoked as a model for the generation of some continentalflood basalt provinces (Gallagher & Hawkesworth, 1992;Turner et al., 1996). Therefore in such provinces thecontinental lithosphere could act as a source region for‘wet’ melts. However, the hydrous or metasomatizedlithosphere that may be present below continental floodbasalt provinces is relatively ancient cratonic lithospherethat has existed for time periods in excess of 0�5 Gyr. Incontrast, the oldest in situ oceanic crust at the present dayis Jurassic in age. The oceanic crust of the Farallon plateon which the CCOP was built is believed to be EarlyCretaceous, or possibly Jurassic, in age (Nakanishi &Winterer, 1998). This oceanic lithosphere was thereforeprobably too young to be a realistic source of the higherwater contents in the Bolıvar magmas. Furthermore,oceanic lithosphere that has been altered and hydratedby circulating seawater will possess elevated 87Sr/86Srratios, because Jurassic and Early Cretaceous seawaterhas an average 87Sr/86Sr in excess of 0�707 (Jones et al.,1994). Thus contamination of plume-derived magmas bypartial melting of altered oceanic crust will yield magmas
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
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with significantly higher 87Sr/86Sr. However, none of theBUC rocks possess (87Sr/86Sr)i values >0�7039, suggest-ing that they have not significantly interacted with alteredoceanic crust. The Vijes rocks have elevated (87Sr/86Sr)i(0�7045–0�7055); however, they are much more alteredthan the BUC rocks and so these elevated values aremore likely to be the result of seawater alteration andnot contamination by a previously altered oceanic crust.Therefore, if it is unlikely that the water in the BUC
magmas is derived from the lithosphere, we need to assessthe possibility that the water in these magmas had itsultimate origin as an integral part of the mantle plumesource region of the CCOP. Over the last few years theissue of whether mantle plumes, the source regions oflarge igneous provinces (LIPs), contain significant(>300 ppm) quantities of water (and/or other volatiles)has been extensively discussed (Arndt et al., 1998;Michael, 1999; Jamtveit et al., 2001; Dixon et al., 2002;Nichols et al., 2002; Wallace, 2002).Water in mantle plumes can ultimately come from one
of two sources: it may be either juvenile (i.e. left over fromaccretion of the Earth) or recycled (i.e. derived fromsubducted slabs; Dixon et al., 2002). A key issue is theamount of water that survives subduction and is trans-ported to the deep mantle. Dixon et al. (2002) havecalculated that water is extracted from the lithosphereduring subduction with >92% efficiency.High-pressure experiments have shown that dense
hydrous magnesian silicate phases can exist at high pres-sures in the mantle (Thompson, 1992; Kohlstedt et al.,1996; Ohtani et al., 1997). Data from mid-ocean ridgebasalt (MORB), ocean islands and LIPs reveal that thewater content of mantle plume source regions is generallyhigher than that of MORB. For example, the Sala yGomez plume source region contains 750 � 210 ppm
water, whereas nearby depleted MORB-source mantlecontains �120 ppm water (Simons et al., 2002). The sameappears to be true of the Iceland hotspot, which has beencalculated to contain between 620 and 920 ppm water. Incontrast, farther south along the Reykyanes Ridge, thewater content of depleted MORB mantle is �165 ppm(Nichols et al., 2002). Jamtveit et al. (2001) have analysedthe water content of olivines from the North AtlanticIgneous Province and have used these to infer that theplume source region has (and had) a water content of>300 ppm.The mantle melt modelling carried out by Kerr et al.
(1996a, 1996b, 1997a, 2002b), Arndt et al. (1997), Hauffet al. (1997), Herzberg &O’Hara (2002) and R�eevillon et al.(2000) has shown that the extent of partial mantle meltingresponsible for the CCOP magmas was of the order of20%. Assuming fractional melting of a mantle sourceregion and a partition coefficient of water of 0�01 duringmantle melting, we have calculated the water content ofthe mantle source region at variable degrees of melting forprimary magmas containing between 1�5 and 4�5 wt %water. These results are summarized in Fig. 13, and revealthat at 20% melting the amount of water in the sourcevaries between 188 and 565 ppm when the primary meltcontains 1�5 and 4�5 wt % water, respectively. However,as was discussed above, the average threshold water con-tents for the crystallization of amphibole are greater than4 wt %, which equates to a primary magma water contentof 3 wt %. Thus, the water content of the mantle sourceregion of the Bolıvar and Vijes magmas is likely to havebeen >380 ppm. The estimated water content of theplume source region would be higher if we assumed agreater degree of mantle melting, a higher primarymagma water content, a lower Kd for water in the mantle,or if we had used a pooled fractional melting model.The evidence from volcanic rocks erupted in different
tectonic settings seems to indicate that the mantle sourcesof ocean island basalts and LIPs contain higher levels ofwater than the depleted MORB-source mantle. How-ever, as discussed by Dixon et al. (2002), this evidencesuggests that mantle plume source regions, particularlythose that are ultimately derived from subducted slabs,are more ‘damp’ than ‘wet’. Certainly, this is supportedby the calculated levels of water in some mantle plumesand the general lack of evidence for the eruption ofhydrous magmas in LIPs in general, and oceanicplateaux in particular. Nevertheless, our evidence impliesthat the plume head source region of the CCOP didcontain a region (or regions) wet enough to producelocalized hydrous melts.The mantle plume source region of oceanic plateaux,
particularly for well-exposed plateaux such as the CCOP,appears to be markedly heterogeneous in terms of itstrace element and radiogenic isotope characteristics(Arndt et al., 1997; Hauff et al., 2000a; Kerr et al., 2002b;
Fig. 12. Histogram showing the spread of 40Ar/39Ar ages for theCCOP and the Bolıvar pegmatites. 40Ar/39Ar ages for the CCOPare from Kerr et al. (1997a, 2002b), Sinton et al. (1998), Lapierre et al.(1999) and Hauff et al. (2000b). All data have been normalized to anage of 28�34 � 0�28Ma for Taylor Creek Rhyolite sanidine 85G003(Renne et al., 1998).
KERR et al. BOLIVAR ULTRAMAFIC COMPLEX, COLOMBIA
1901
R�eevillon et al., 2002; Mamberti et al., 2003). This hetero-geneity is not surprising when one considers the nature ofthe subducted materials believed to descend into the deepmantle. Although the calculations of Dixon et al. (2002)suggest that >92% of the water is extracted from slabsduring subduction, �8% of the water is transported todepth. It is therefore not inconceivable that mantle plumesource regions are also heterogeneous in terms of watercontent.
IMPLICATIONS OF LOCALIZED
HIGHER WATER CONTENTS
IN MANTLE PLUMES
There are several important implications of localizedhigher water contents in mantle plumes. The first ofthese will be a lowering of the solidus temperature,which will result in either a higher percentage of meltgeneration (at a fixed mantle potential temperature) or, ifthe percentage of mantle melting remains the same, alowering of the mantle temperature. These effects havebeen quantified by the hydrous melting experiments ofHirose & Kawamoto (1995) on KLB-1 at 1Ga, whichrevealed that at a mantle temperature of 1350�C theamount of melting increased from 20 to 30% upon addi-tion of 1000 ppmwater to the source. Likewise, the experi-ments showed that addition of 1000 ppm water to thesource led to a reduction in the temperature needed toform the same amount of melt as an anhydrous source. At20% melting this temperature was reduced by 35�C from1350�C to 1315�C. The slightly more incompatible traceelement-depleted nature of the Vijes basalt Pan 94-28
(i.e. the one that has not fractionatedmagnesiohornblende)in comparison with other basalts of equivalent Mg-number from the Western Cordillera may be a reflectionof higher degrees of melting. An obvious implication isthat if all mantle plumes contain significant hydrouspatches, then calculations that estimate the potentialtemperature (Tp) of mantle plumes based on the volumeand chemistry of the erupted melt, which assume ananhydrous source (e.g. McKenzie & Bickle, 1988), maywell be overestimates and thus in error.Mantle plumes that contain wet patches also have
implications for the rheology of the mantle. Experimentalresults reported by Hirth & Kohlstedt (1996) have shownthat water, if contained in mantle olivine, can decreasethe viscosity of the mantle by at least two orders ofmagnitude relative to anhydrous mantle. They also esti-mated that partial melting causes a large increase in theviscosity of residual mantle after melt extraction, as aresult of dehydration. This reduction in viscosity hasbeen proposed to create a viscosity discontinuity, whichoccurs at depths of �60–70 km below mid-ocean ridges(Hirth & Kohlstedt, 1996) or between �70 and 100 kmdepths for mantle plumes (Wallace, 1998). This viscositydiscontinuity may be important in focusing melts from abroad melting region towards a localized eruptive centre(Wallace, 1998). The reduced viscosity may also meanthat hydrous plumes (or hydrous patches in essentiallyanhydrous plumes) can rise through the mantle at a fasterrate than totally anhydrous plumes.Finally, if mantle plumes are more hydrous than has
hitherto been realized, this has important ramificationsfor the environmental impact of LIPs. A key componentof manymodels of the climatic impact of LIP eruption is of
Fig. 13. Diagram showing how the calculated water content of a mantle source region varies with increased fraction of melting in parentalmagmas (15wt % MgO) of different water contents. (See text for discussion.)
JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 SEPTEMBER 2004
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sulphuric acid aerosols and ash in blocking sunlightfrom reaching the Earth, so causing a ‘volcanic winter’(e.g. Courtillot et al., 1996; Wignall, 2001). For these aero-sols and ash to be distributed around the globe, and so havea global rather than a local influence on climate, theymustbe injected into the stratosphere (Strothers et al., 1986).The tropopause, the boundary between the troposphereand the stratosphere, is �9�5 km above sea level at polarlatitudes and 17 km above sea level at equatorial latitudes.Therefore, eruptions from LIPs need to be explosiveenough to enable them to inject ash and aerosols to heightsof between 10 and 17 km, if LIP eruptions are to have aneffect on global climate.Obviously, ifmantle plume sourceregions are more hydrous than has previously beenthought, this will mean that at least some of their eruptiveepisodes will be more explosive, thus making it morelikely that ash and aerosols will reach the stratosphere.The presence of ash and volcanic breccias at Vijes is testa-ment to explosive eruptions during the formation ofthe CCOP, as a result of higher than usual mantle watercontents.
CONCLUSIONS
(1) The Bolıvar mafic pegmatites are indistinguishable inage from, and overlap in terms of radiogenic isotopecomposition with, the basalts of the CCOP. The chem-istry of the Bolıvar mafic pegmatites is inconsistent with asubduction-related origin for the Bolıvar UltramaficComplex.(2) The Bolıvar ultramafic rocks and gabbros represent
the olivine and pyroxene crystal cumulates from thefractionation of a hydrous plume-derived magma. Inthe later stages of crystallization, when the MgO contentreached 7 wt % MgO, the magma began to crystallizemagnesiohornblende on the sidewalls of thin conduits asit migrated upwards. Continual replenishment of magmafrom below resulted in the formation of pegmatites with ahigh modal proportion of unzoned magnesiohornblendein the lower levels of the complex.(3) At higher levels in the complex, felsic residual mag-
mas crystallized hornblende-poor pegmatites with a highmodal proportion of essentially unzoned plagioclasealong with quartz and biotite.(4) Tapping of the magma at different stages during
pegmatite crystallization resulted in the emplacementand eruption of variably incompatible trace element-depleted felsites. The extent of incompatible trace elementdepletion is dependent on the amount of magnesiohorn-blende, relative to plagioclase, that is removed from themagma during pegmatite crystallization.(5) A hydrous component appears to be an integral
part of the mantle plume source region of the CCOP,and modelling calculations suggest that the mantle plume
source region of the BUC contained at least 400 ppmwater.(6) We propose that, in addition to their heterogeneity
in trace elements and radiogenic isotopes, mantle plumesare also heterogeneous in terms of their water contentand may well possess localized hydrous regions.(7) Mantle plumes that are wetter than previously rea-
lized probably melted at lower temperatures (or togreater extents at comparable temperatures), possessedlower viscosity and thus rose more quickly, and producedmagmas with more explosive eruptions than equivalentdry compositions. The last factor has implications inparticular for links between LIPs and mass extinctionevents.
ACKNOWLEDGEMENTS
Discussions with Andy Saunders, Giz Marriner, PatThompson, Roz White, Bernard Leake, John Aspdenand Dave Millward helped clarify many of the ideascontained in this paper. The analytical support of RobWilson and Nick Marsh at Leicester and Iain McDonaldat Cardiff is greatly appreciated. The paper has benefitedconsiderably from constructive reviews from BobDuncan, Dominique Weiss and Henriette Lapierre, andthe editorial advice of Marjorie Wilson. These studieswere supported by the Natural Environment ResearchCouncil (UK) through Grants GR3/8984 and GR9/583A (to J.T.). Additional ICP-MS trace element analyseswere supported by an NERC–HEFCE Joint Infrastruc-ture Fund grant (NER/H/S/200/00862) to J. A. Pearceet al. Isotope analyses were funded by a block grant fromthe NERC to NIGL. We wish to thank INGEOMINASfor logistical support during fieldwork in Colombia. Thispaper represents NERC Isotope Geosciences Laboratorypublication number 635.
REFERENCES
Aitken, B. G. & Echeverrıa, L. M. (1984). Petrology and geochemistry
of komatiites and tholeiites from Gorgona Island, Colombia.
Contributions to Mineralogy and Petrology 86, 94–105.
Allen, J. C. & Boettcher, A. L. (1983). The stability of amphibole in
andesite and basalt at high pressures. American Mineralogist 68,
307–314.
Arndt, N., Ginibre, C., Chauvel, C., Albar�eede, F., Cheadle, M.,
Herzberg, C., Jenner, G. & Lahaye, Y. (1998). Were komatiites wet?
Geology 26, 739–742.
Arndt, N. T., Kerr, A. C. & Tarney, J. (1997). Dynamic melting in
plume heads: the formation of Gorgona komatiites and basalts. Earth
and Planetary Science Letters 146, 289–301.
Aspden, J. A. (1984). The geology of the Western Cordillera,
Department of Valle, Colombia (sheets 261, 278, 280 and 299).
