Jrawnulr,fSourh

Amrimn Eurrh Scirncrs. Vol. Y. Nos l/2. pp. 5Y-68. IYYh

Copyright 0 1996 Elsevier Science Lti B Earth Sciences B Resourw lnslituw
Pergamon
Primal in Greal Britain. All rights mc~cd
OXYS-981 I/Y6 sls.trl + O.(Kl
PII: SO895-9811(%)00027-2

The Bolivar mafic-ultramafic complex, SW Colombia: the base of

an abducted oceanic plateau

ALVARO NIVIA

INGEOMlNAS - Regional Pacifico, A.A. 9724, Cali, Colombia

Abstract - The Bolivar mat%-uhramafic complex (Valle Department) is a late Cretaceous banded sequence of dunites,
lhenolites. pyroxenites and gabbtonorites. The gabbronorites. which can be layered or isotropic, are extensively amphibolitized
and have suffered pervasive deformation and local migmatisation, indicating that these reeks wete affected by a HaO-rich
metasomatic event that eventually led to their partial fusion. The complex is cut by pegmatitic dykes. which vary
petrographically according to their level of emplacement. In the lower levels plagioclase-hornblende dykes ate found, wheteas in
the upper parts the dykes are rich in plagioclase, quartz, muscovite, and &cite. ‘Ihe dunites have been altered to serpentinite
and fotm the host rock of a stcckwork deposit of cryptocrystalline magnesite veins. Opal veins deposited a&r the magnesite
veins occur in the upper levels of the deposit. The mechanism producing this type of deposit is commatly believed to involve
the alteration of olivine (dunite) by CO,-rich fluids generated at depth. Interestingly, the world’s more impottant deposits of this
type, located in a discontinuous belt through the Dinaric Alps and Hellenides have geological characteristics very similar to
those of the Bolivar deposit. These characteristics are consistent with these deposits having formed during the process of
abduction of anomalous (?plateau) ocean floor. It is proposed that the metasomatic event, the generation of the pegmatitic dykes,
and the fotrnation of the magnesite vents ate all genetically related to an abduction prccess during which the igneous complex
was accteted to the margin of Colombia. During abduction, geothenns beneath the ovetthrust lithospheric block rose, distilling
the Ha0 and boron contained in the terrigenous sediments found in the upper (undetthrust) part of the block The resulting fluids
migrating through the overthrust block led to the formation of the migmatites, amphibolites and pegmatite dykes. A second
generation of CO*-rich fluids, created in a similar fashion ftwn calcarecats sediments, may have been responsible for the
magnesite mineralization. Copyright 0 1996 Elsevier Science Ltd & Earth Sciences & Resources Institute

Remmen - El complejo m4ficoultramafico de Bolfvar (Departamento de1 Valle) esta fomrado por una secuencia bandeada de1
Cretaceo superior, confomtada par dunitas, lhetzolitas. piroxenitas y gabronotitas. Las gabrcnotitas, que pueden ser bandeadas o
isotr6picas. estan ampliamente anfibolitizadas y presentan defomuG5n penetrativa conspicua y texturas migmatfticas locales
indicativas de un evento de metasomatismo tico en HsO, que condujo a su fusi6n parcial. Bl complejo esta mttuido por diques
pegmatfticos, cuya composici6n varfa dependiendo de su nivel de emplazamiertto. Bn las panes inferiores estos diques e&n
constituidos esencialmente por plagioclasa y homblenda. mientras que en las supetiores consisten cn plagioclasa. cuatzo.
muscovita y sericita. Las dunitas fueton serpentinizadas y actdan coma tcca encajante de un dep6sito en enrejado de venas de
magnesita cripoctistalina. Hacia la parte superior de1 dep6sito ocurten venas de 6palo depositadas con posterioridad a las de
magnesita. Se cme que el mecanfsmo de fomraci6n de estos dep6sitos involucra la alteraci6n de olivino (dtmita) por fluidos rices
en CO, generados en profundidad. Adicionalmente, 10s dep6sitos de este tipo mas importantes de1 mundo, localizados en un
cintur6n discontinue a lo Iargo de 10s Alpes Dinaricos y Helettidos, presentan caracterfsticas geol6gicas muy similarzs a las de1
yacimiento de Bolfvar y que penniten interpretarlos coma fortnados por pr~esos de obducci6n de suelo oceanic0 an6malo. Se
postula que el evento de metasomatismo. la generaci6n de 10s diques pegmatfticos y la formaci6n de1 yacimiento de magnesita
de Bolfvar e&n geneticamente relacionados a un proceso de obducci6n. durante el cual se acrecion6 el Complejo Igneo al
margen Colombiano. Durante la obducci6n el bloque subyacente sufri6 un inctemento de temperatttra que produjo la destilaci6n
de1 Hz0 y bon, contenidos en 10s sedimentos tertfgenos que cubtian su parte superior. La migraci6n de 10s fluidos test&antes a
naves de1 bloque obducido condujo a la formacidn de migmatitas, anfibolitas y diques de pegmatita. Una segunda generaci6n de
fluidos rices en CO,, pmducidos de manera similar por sedimentos calc&os, pudo haber sido rcsponsable de la mineralzaci6n
de la magnesita.

INTRODUCTION and can be traced throughout Colombia and Ecuador

(Meissner et al., 1976; Duque-Caro, 1979; Feininger and
Geological evidence, in conjunction with gravimetric Bristow, 1980; McCourt, 1984; Aspden et al., 1992).
and seismic studies, suggest that the Colombian Andes can Gravimetric and seismic profiles traversing the Andes
be described in terms of two different crustal provinces: an confirm that to the east of the fault gmvimetric anomaly
eastern province of continental affinity and a western values are strongly negative (as low as -220 mgals, Case et
province of oceanic affinity that broadly corresponds to the al., 1973), whereas to the west, Bouguer anomalies are
Basic Igneous Complex as defined by Goosens et al., positive with values as high as +135 mgals (Case et al.,
(1977) in Ecuador. he-Cenozoic rocks of the eastern 1971). The distribution of such values and of those
province consist of igneous and metamorphic Precambrian calculated for the seismic velocities (Meyer er al., 1977;
and Palaeozoic rocks, intruded by Mesozoic granitoid Ckola et al., 1977) suggest that to the east of the fault there
plutons. In Central Colombia (Fig. 1) the western province exists sialic crust with the Moho located at a depth of 40-50
comprises the basic volcanics and locally associated mafic- km (Case et al., 1984). To the west, the Andes are made of
ultramafic rocks of the post-Lower Cretaceous basic high density, high velocity materials, probably oceanic
igneous complex and also includes deformed marine crust (Case et al., 1971, 1973; Meissner et al., 1976) with
sedimentary strata. The Cauca-Almaguer Fault (former the Moho located at a depth of 25-30 km (Case et al.,
Romeral Fault sensu Case et al., 1971; McCourt, 1984) 1984).
which marks the boundary between the two provinces Interpretations of the evolution of the crust in Colombia
parallels the western flank of the Andean Central Cordillera (Barrero, 1979; Bourgois et al., 1982; McCourt et al., 1984;

59
60 A. NIVIA

LEGEND
CENOZOIC ROCKS

CRETACEOUS ROCKS
BugaBatholith
(109 Ma RbSr)

BASIC IGNEOUS COMPLE>

Tholeiilic basalis
a fermasalls

Gabbronorites

Layered norles
8 gabbmnorhes

Serpentinites
PWldolH~S

Bluescmts 8
* eclogites
n

PALEOZOIC ROCKS

metavolcanic.5

‘CARIBBEAN SEA

.\
ECLIN ------\

BOGOTA
@ COLOMBIA

Fig. 1. Geological sketch map of central South-Western Colombia (simplified from Vergara, 1983; De Armas, 1985; McCourt et al.,
1985; Nivia et al., 1992), showing the location of the main ultramafic complexes: 1 = Bolivar mafic-ultramafic complex; 2 = Venus
ultramafic body; 3 = Ginebra ultramafic mnassif.

Aspden and McCourt, 1986) consider that the Basic considered that the fault represents the location of a
Igneous Complex was accreted to the continental margin of paleosubduction zone, i.e. the continental margin of South
South America over the period Late Cretaceous to Early America, during the Ctetaceous.
Tertiary. The presence of a 125 Ma blueschist belt (Orrego Associated with the Basic Igneous Complex up to 30
er al., 1980b. Feininger 1982), and eclogite lenses km from the Cauca-Almaguer Fault, there are several
associated with tectonised ultramafic rocks emplaced some ultramafic massifs (Barrero, 1979; Vergara. 1983;
15-20 km to the east of the Cauca-Almaguer fault Espinosa, 1980, 1985). The northernmost one, the Bolivar
(Feininger, 1980; McCourt and Feininger, 1984; Orrego er ultramafic complex, exhibits zones of amphibolitisation
al., 1980a), has played an important role in the and deformation, is cross-cut by a series of homblende-
interpretation of the evolution of the Northern Andes. It is pegmatitic dykes and include layers of serpentinite that host
 Evidence for Obduction in the Bolivar Ultramafic Complex, SW Colombia 61

magnesite mineralization of economic importance. The forming a concentrically zoned body consisting of a core of
hypothesis that the genesis of these features is related to the dunite sum>unded by successive rims of olivine-
suture event of the two provinces, during which part of the clinopyroxenite, clinopyroxenite and peridotite that permits
western province was abducted on top of the Cretaceous its comparison with the Alaskan ultramafic zoned complex
continental border of South America, is proposed in this described by Irvine in 1974 (Banero, 1979). This
paper. concentric zoning is, however, not obvious so it is
The field work for this study was completed during undesirable to include this genetic connotation in the name
regional rock sampling to assess the geochemistry of the of the complex. In different studies this unit has been called
basalts from the Basic Igneous Complex (Nivia 1987). the Bolivar ultramafic complex (Aspden, 1984;
known in Colombia as the Diabase Group (Nelson, 1957) INGEOMINAS-BGS, 1991), the Bolivar ultrabasic and
or the Amaime and Volcanic Formations (Aspden et al., basic massif (Bourgois er al.. 1982) or the Bolivar
1985; McCourt et al., 1985); and also during the ultrabasic massif (Espinosa, 1987). The last two papers
INGEOMINAS regional mapping project “Geology of the propose that the general structure of the Bolivar complex is
sheet 242 - Zarzal” (Nivia et al., 1992). Mineral anticlinal (i.e. not zoned). Nivia et al. (1992) interpreted the
compositions were determined at the University of complex as faulted/imbricated blocks showing different
L&ester using a Cambridge Microscan V electron levels of oceanic layer 3 in an ophiolitic sequence, and
microprobe (energy dispersive system). Whole rock described it in terms of the horizons usually recognized in
analysis for major and trace elements, using a Philips PW this layer: ultramafic rocks, cumulus gabbros and an upper
1400 automatic X-ray fluorescence spectrometer (XRF , horizon of isotropic gabbros. Here the latter interpretation
was also performed at Leicester. is adopted as a criteria for polarity, with the inference that
the ultramafic rocks represent the lower part of the
GEOLOGY AND PETROGRAPHY sequence which grades upwards to more differentiated
rocks.
The Bolivar ultramafic complex forms the eastern
foothills of the Western Cordillera to the north of the Valle LJIrroqfic Rocks
Department and it extends westwards to the Roldanillo
Fault which marks its contact with the basalts of the The ultramafic rocks consist of an intercalation of very
Volcanic Formation (Fig. 1). Barrero (1979) defined this thick, ca. lOm., layers of serpentinised dunite and
unit as the ‘Bolivar M&c-Ultramafic Zoned Complex’ and interbanded with units (lo-30 cm thick) of serpentinised
postulated that it diapirically intrudes the basalts and lherzolites, olivine websterites and olivine gabbronorites.
dolerites of the Volcanic Formation (sic. Diabase Group) The dunites are usually serpentinised and when they

Table 1. Representative olivine and clinopyroxene compositions, Bolivar ultramafic complex

Olivines Clinopyroxenes
AN50 AN120 AN50 AN125 AN121

SiO, 39.19 39.57 38.64 39.26 51.68 52.16 51.64 52.10 51.96
TiO, 0.14 - 0.26 0.19 0.34 0.21 0.21
42% 0.06 0.14 - 0.29 2.59 1.57 1.81 1.30 1.15
Cr20, - 0.16 - - 0.92 0.13 0.12 0.17 0.02
Fe0 18.76 18.85 20.16 19.81 4.00 12.08 9.20 9.60 9.93
MnO 0.34 0.28 0.20 0.38 0.13 0.21 0.30 0.17 0.37
MgO 41.15 41.27 40.00 40.38 15.67 15.11 14.23 13.33 12.95
23.61 17.34 22.12 23.27 22.72
NazG
CaO 0.04
1.02 0.04
0.51 0.56 :*!: 0.78 0.14 0.59 0.08 0.60
K,O - - 0.02 0:04 0.13 0.11 0.02 - -
Total 100.56 100.96 99.58 100.48 99.77 99.04 100.37 100.23 99.91

Cation proportions on the basis of 4 oxygens Cation proportions on the basis of 6 oxygens

Si 1.000 1.003 1.000 1.004 1.910 1.%7 1.929 1.953 1.959

Ti 0.003 - - 0.007 0.005 0.010 0.006 0.006
Al 0.002 0.004 - 0.009 0.113 0.070 0.080 0.058 0.05 1
Cr 0.003 0.027 0.004 0.004 0.005 -
Fe2+ 0.400 0.400 i.436 0.424 0.123 0.381 0.287 0.301 0.313
Mn 0.007 0.006 0.004 0.008 0.004 0.007 0.009 0.005 0.012
Mg 1.564 1.559 1.544 1.539 0.863 0.849 0.793 0.745 0.728
Ca 0.001 0.001 0.935 0.701 0.886 0.935 0.918
Na 0.050 0.025 0.028 0.016 0.056 0.010 0.042 0.006 0.044
K 0.001 0.001 0.006 0.005 0.001 - -

Fo 80.5 80.6 78.0 78.4 Ca 48.67 36.20 45.07 47.20 46.86

Mg 44.92 43.96 40.35 37.60 37.16
Fe 6.40 19.73 14.60 15.19 15.97
AN50: olivine websterite; AN120: olivine gabbronorite;
AN125 and AN121: gabbronorite.
62 A. NIVIA

occur intercalated with peridotites and pyroxenites, have a Fe +

mylonitic foliation. Fresh fine grained, dark green to black
melanocratic dunite is locally present. In thin section the
dunites are holocrystalline, equigranular, exhibit mosaic
textures, and are composed of olivine (>90% mode) and
opaque chromite. The Cr content of the dunites varies
between 1800 and 7000 ppm.
Serpentinites are light brown in colour, strongly
fractured and relatively soft consisting of ca. 50% by
volume altered olivine. Alteration products exhibit fibrous
habits and patches which are difficult to resolve but suggest
the presence of chrysotile and lizardite. Associated with the
latter, fresh opaque minerals (probably chromite) can also
be found.
a a
.:““< \
In the field, the lherzolite, olivine websterite and olivine

,A$
gabbronorite bands contrast with the serpentinite bands in
ANg0?
..
MCI
that they display a lack of both alteration and magnesite
mineralization. These rocks are melanocratic and vary in V k
colour from dark grey to black. The colour index is
90 AN120 ‘O 50
dependent on the plagioclase content, that increases from
1 I I
-5% in websterites from the lower sequence to -60% in
some layers of cumulitic gabbro. Texturally the rocks are Mel% Forsterite
holocrystalline and equigranular, having medium to small
crystals usually arranged to form xenomorphic granular Fig. 2. Pyroxene and olivine compositions in the Bolivar
textures. Their mineralogy consist mainly of ortho- and ultramafic complex rocks. Tie-lines indicate analyses in the
clino-pyroxenes, variable amounts of olivine, and accessory same sample.
plagioclase and opaque phases. Orthopyroxenes exhibit
fine cpyx exsolution lamellae, needles or bubbles. An exsolved orthopyroxene, and have compositions close to
exsolved opaque phase associated with the cleavage planes Wo4@n45FsS (Table 1; Fig. 2). Olivines have compositions
is present. Poikilitic orthopyroxene encloses euhedral near Fo,,, (Table 1; Fig. 2) and sometimes have
clinopyroxenes and plagioclases; the orthopyroxenes have incomplete kelyphitic pyroxene rims. Plagioclase is present
compositions close to En,, (Table 2, Fig. 2). as small interstitial crystals that are close to An,, in
Clinopyroxenes are smaller, mostly interstitial, have composition (Table 3; Fig, 3).

Table 2. Representative orthopyroxene compositions, Bolivar ultramafic complex.

AN50 AN120 AN90 AN125 AN121

SiO, 54.46 54.84 55.48 52.84 52.49 53.09 53.04 52.50 52.25
TiO, 0.05 0.08 0.11 0.12 0.17 0.08 0.25 0.14
A1203 2.34 1.67 1.30 1.25 1.06 1.07 1.03 0.52 0.92
Cr203 0.41 0.31 0.08 0.12 0.02 0.10 0.04 0.08
Fe0 11.95 12.48 12.92 21.09 20.60 21.38 20.75 24.83 25.41
MnO 0.36 0.24 0.34 0.40 0.62 0.47 0.54 0.80 0.83
MgO 29.03 28.73 29.54 23.26 22.73 23.03 23.05 20.05 19.17
CaO 0.57 0.39 0.33 1.11 2.24 0.77 0.87 0.68 0.83
Na20 0.71 0.37 0.44 0.36 0.09 0.34 0.08 0.35 0.41
K,O - - - - 0.06
Total 99.88 99.11 100.54 100.55 100.02 100.23 99.77 99.91 99.90

Cation proportions on the basis of 6 oxygens.

Si 1.939 1.966 1.965 1.952 1.953 1.967 1.969 1.986 1.984

Ti 0.001 0.002 0.003 0.003 0.005 0.002 0.007 0.004 -
Al 0.098 0.070 0.054 0.055 0.047 0.047 0.045 0.023 0.041
Cr 0.012 0.009 0.002 0.004 0.001 0.003 0.001 0.002
Fez+ 0.356 0.374 0.383 0.652 0.641 0.662 0.644 0.786 0.807
Mn 0.011 0.007 0.010 0.013 0.019 0.015 0.017 0.026 0.027
Mg 1.541 1.536 1.560 1.281 1.261 1.272 1.276 1.131 1.085
Ca 0.022 0.015 0.012 0.044 0.089 0.03 1 0.035 0.028 0.034
Na 0.049 0.026 0.030 0.026 0.007 0.025 0.006 0.025 0.030
K 0.003

En 79.81 80.41 80.28 66.27 66.30 65.77 66.45 59.00 57.34

AN50 = olivine websterite: AN120 = olivine gabbronorite; AN 90 = norite;

AN125 and AN121 = gabbronorite.
 Evidence for Obduction in the Bolivar Ultramafic Complex, SW Colombia 63

MO196 ANORTHITE two notable features: a conspicuous pervasive deformation,

parallel to subparallel with the banding, and variable
100 90 80 70 60 50
alteration of pyroxenes to amphibole. These rocks, and to
I I I I some extent the ultramafics below, have suffered local
) AN1211 minor remobilization that led to the formation of coarse
L-__l_lIl pegmatitic leucogabbros that display clear intrusive
1 AN1251 contacts with the banded finer grained melagabbros.
L_Ld 1 Petrographically, the rocks of these two horizons are
I I I ! #AN901 holoctystalline, equigranular, medium to coarse grained
L_J_LdG.. ! I
with xenomorphic textures. Crystals are oriented with their
I AN1201 long axis parallel to the compositional banding in the lower
L___L& 1
part of the sequence. Ophitic textures between
1 AN501 clinopyroxenes and plagioclase are common. Olivines are
U$%l. 1
anhedral, of medium grain size, and seldom exhibit
I I I I I 1 kelyphitic rims, being partially altered to serpentine along
100 90 80 70 60 50 fractures. Orthopyroxenes are subhedral to euhedral with
stubby prismatic habit; clinopyroxenes mainly subhedral to
Fig. 3. Plagioclase compositions in the Bolivar ultramafic anhedral, showing hypersthenic exsolution lamellae. Both
complex rocks. Tie-lines in sample AN90 link analyses from the exhibit exsolution of an opaque phase (“Schiller texture”)
core and rim of the same crystal. that in extreme cases can replace up to 50% of the
pyroxenes leaving an aggregate of rounded opaque
crystallites. Plagioclases, some with corroded borders, are
Ban&d and isotropic gabbros clouded and/or schillerised parallel to the cleavage planes.
In the cumulitic levels there is clear zonation of plagioclase
The Bolivar ultramafic complex gabbros have been crystals (Fig. 3). Sometimes plagioclase exhibits undulatory
mapped separately as either banded or isotropic gabbro extinction and filling of fractures by very fine grainecl
(Nivia et al., 1992). The lower banded gabbro horizon is amphiboles. Two opaque exsolved phases have been
relatively homogeneous and consists of interbanded olivine identified: interstitial ilmenite (53wt% TiO,, 42wt% FeO)
norites, norites, gabbronorites and occasionally and magnetite (88wt% FeO, 3wt% TiO&
anorthosites; the upper isotropic gabbro horizon is The mineral chemistry of the crystalline phases in the
composed essentially of gabbronorites. The banded horizon gabbros follow trends expected of differentiated tholeiitic
consist of alternating bands of variable thickness (0.3-lm), liquids showing increasing FeO/MgO ratio with
but with gradational contacts. Them are also bands showing fractionation. Olivines, restricted to the lower part of the
cumulate Fe-Mg minerals at the base and plagioclase near sequence, have compositions of Fo,,.s in olivine norites
the top, the latter almost pure anorthosite. These rocks have and Fo,,, in olivine web&rites (Table 1; Fig. 2).

Table 3. Representative plagioclase compositions, Bolivar ultramafic complex.

AN50 AN120 AN90 AN125 AN121

SiO, 44.66 44.63 44.99 45.82 47.25 45.08 45.23 45.99 46.41
TiO, 0.03 0.15 0.11 -- 32.77 34.65
0.02 0.04 __ 0.03
A12o3 34.48 34.56 34.57 33.63 34.32 33.99 33.44
Cr203 0.04 0.03 0.05 - 0.03 -
Fe0 0.13 0.19 a.10 0.46 0.57 0.67 0.67 0.32 0.32
MnO 0.03 - 0.07 0.01 -
MgO 0.04 - 0.05 0.13
-
CaO 18.90 18.82 19.36 18.15 17.03 19.31 18.95 18.34 17.96
Na20 O.% 0.80 0.95 1.34 2.04 0.88 0.84 1.28 1.49
K,O - 0.20 0.02 - 0.04 0.03 0.01 -
Total 99.23 99.39 100.10 99.45 99.76 100.78 100.08 99.97 99.65

Cation proportions on the basis of 32 oxygens

Si 8.322 8.315 8.320 8.512 8.727 8.303 8.372 8.492 8.587

Ti 0.004 0.021 0.015 - 0.002 0.005 0.004
Al 7.575 7.591 7.537 7.361 7.134 7.521 7.487 7.399 7.293
Cr 0.004 0.008 0.005 -
Fe?+ 0.021 :::: 0.016
- 0.07 1 0.089 0.102 0.104 0.049 0.049
Mn 0.005 - - 0.010 0.020 0.001 -
0.014 -
zg 0.012
3.774 3.758
- 3.837 3.612 3.371 3.812 3.758 3.630 3.561
Na 0.345 0.288 0.339 0.482 0.730 0.315 0.302 0.460 0.536
K 0.005 0.005 - 0.009 0.007 0.001 -

An 91.64 92.76 91.77 88.26 82.19 92.16 92.40 88.73 86.91

Ab 8.37 7.11 8.11 11.77 17.80 7.62 7.42 11.24 13.08
or 0.12 0.12 - 0.22 0.17 0.02 -
64 A. NIVIA

Orthopyroxenes, change from En,,, in olivine norites, serpentinite bands of the ultramafic lower horizon. The
through En, in norites and gabbronohtes from the cumulus mineralization consists of magnesite and small number of
horizons to En,, in the more differentiated isotropic opal veins that cut through the serpentinite. The higher
gabbronorites (Table 2, Fig. 2). Clinopyroxenes, salites and concentration of opals veins in the higher parts of the
augites range similarly from Ca,,MgMFe,, to quarry suggests some type of emplacement level control.
Ca,,Mg,sFe,, (Table 1, Fig. 2). Plagioclase compositional The thickness of the veins varies over short distances from
variation is less well defined, but is mostly near An, l-2 mm up to a maximum of 30 cm. The length of the
(Table 3, Fig. 3). However, microprobe analysis of the individual veins is difficult to measure because they are
centre and rim of the same crystal suggest crystallization of very irregular and lack preferred orientation, forming a
intercumulus fractionated liquids around An-rich random network. This suggests filling of fractures in a rock
primocrysts (Fig. 3). affected by cataclastic deformation. The contact between
the magnesite veins and the host rock is sharp, and
Amphibolitisation sometimes shows a thin film of chlorite, talc and tremolite
in the walls of the veins. Both veins and dykes were
A conspicuous feature of the gabbro horizons is the affected by cataclastic deformation as indicated by the
development of amphibolitisation, present as variable abundance of microfaults, however the main deformation
recrystallization of the margins of clinopyroxenes to uralite, of the veins described as “ptygmatic-type” (Nivia et al.,
cummingtonite and hornblende. It is possible to observe 1992) seems to be due to an increase in volume after the
two types of recrystallization: clusters of needle-like formation as veins, perhaps during the serpentinization
cummingtonite and green hornblende crystals growing into event.
all spaces available (i.e. borders, fractures and cleavage The magnesite is cryptocrystalline, white, earthy. with
planes) in the pyroxene and plagioclase crystals: and opaque lustre; porosity variation is about 10%. The average
amphiboles growing in optical continuity forming a single MgO content is 49 wt%, but with an important component
crystal that occupies all the thin section, poikilitically of silica (9.5%) and small amounts of Ca and Fe. The high
enclosing all the phases not susceptible to silica may represent disseminated opal within the
amphibolitisation. The associated plagioclase crystals are magnesite itself. Silica, in the form of opal, forms separate
An-rich, and this rules out the possibility that the original veins: this opal is cryptocrystalline, white, massive, and has
textures were ophitic. Very coarse skeletal crystals of porcellanous lustre and conchoidal fracture.
hornblende, associated with needle-like microlites of
plagioclase, have been described by Baldion er al. (1991) DISCUSSION
as a spinifex texture. The recrystallization seems to be
accompanied by the exsolution of a very fine grained Genesis of the Bolivar ultramqfii complex
opaque phase.
Although Barrero (1979) compared the Bolivar complex
Hornblende-Plagioclase-Quartz Pegmatitic Dykes with the zoned ultramafic complexes of Alaska (Irvine,
1974). the concentric zoning - superficially also a feature
The rocks of the Bolivar ultramafic complex are of some up-domed ophiolite complexes such as the classic
intruded by a suite of dykes that are on average 50 cm thick Troodos Massif, Cyprus (Hall, 1987) - is not well
and show E-W preferred orientation. They also occur in developed and, in places, the dunites and gabbros are in
other ultramafic bodies in western Colombia (Tistl er al., contact (Baldion er al.. 1991). Alaskan-type ultramafics are
1994). These dykes consist of large crystals of hornblende considered to have formed in erogenic belts from andesitic
and variable amounts of plagioclase and quartz, and contain or hydrated basaltic magmas (Murray, 1972; Hall, 1987)
serpentinised dunite, peridotite and gabbro xenoliths. Most and both the Bolivar Ultramafic Complex (Fig. 2), and the
dykes have large euhedral to subhedral hornblendes (up to Volcanic Formation basalts - considered as consanguinous
50 cm in extreme cases) growing perpendicular to the dyke (Barrero, 1979, Nivia, 1987) - are tholeiitic, and probably
margins or to contacts with xenoliths. Late-crystallising generated in a tectonic environment unrelated to erogenic
plagioclases show albite and pericline twining and have belts (Nivia, 1987).
graphic intergrowths with quartz. The rocks also contain The characteristics of the complex pennit comparison
very coarse biotite. muscovite and a mineral phase with layer 3 of ophiolitic sequences. Models proposed to
petrographically identified dumortierite, explain these sequences (Coleman, 1977; Gass et al., 1984)
(Al,Fe3+),03(Si0J3(B03). The latterlihase is present in consider that the ultramafic rocks and banded gabbros are
crystals up to 25 cm long and exhibit a chlorite-muscovite formed by accumulation of crystals at the base of magma
reaction rim. Andalusite and spessartite have also been chambers, whereas the isotropic gabbros are produced by
reported in these dykes (Ortiz and Gomez, 1971: Baldion er cooling of the magma against the walls of the chamber. The
al., 1991). liquids contained in the chamber erupted forming an upper
Observations in the “Sociedad Magnesios Bolivalle layer of pillow lavas, basaltic flows and sheeted dykes (=
Ltda.” magnesite qua”y, Bolivar. shows that the layer 2 of ocean crust). The trace element data reported for
composition of these pegmatoids varies according to their the gabbros and amphibolites of the complex exhibit the
level of emplacement, the dykes being hornblende- same general characteristics as the basalts of the Volcanic
plagioclase-rich and anorthositic below, but are quartz- Formation, and so can also be considered in the model as
feldspar-rich (+biotite, muscovite, chlorite f dumortierite) layer 2. Consequently it is possible to infer consanguinity
in the dykes exposed in the higher levels of the quarry. This between the two groups of rocks (Nivia, 1987), the layered
feature suggests a relatively rapid and effective in situ gabbros and ultramafic rocks of the Bolivar Complex
fractionation process during dyke emplacement. representing the crystal cumulates from magma chambers
where the liquids represent the basalts of the Volcanic
Magnesite Mineralization Formation. The conspicuous magmatic banding. the
abundance of cumulus textures, the presence of plagioclase
Magnesite mineralization is hosted in the thick associated with the rocks of the three levels of the complex
 Evidence for Obduction in the Bolivar Ultramafic Complex, SW Colombia 65

XDUCTION FEATURE!
h-lagnesite 8 opal
veins lz3

Dolerite sills and

q Isotropic &
layered gabbro

Peridotite
w
CONTINENT
Marine shale 8 ..‘.
carbonate seds a
.:.:.:.::.,:
Continental crust $$;,i
•II

Fig. 4. Schematic model for the generation of hornblende pegmatites during abduction of the Bolivar ophiolite: (a)
Recently formed oceanic plateau approaches NW margin of S. America, (b) Imbrication and abduction of “hot”
plateau, (c) hydrous fluids and CO2 released where plateau thrust over elastic sediments and limestone then rise up
to generate hornblende-gabbro and magnesite veins respectively (circled region in (b)).

and the lack of rocks that can be compared with the block, increasing its temperature, and resulting in
tectonised harzburgite typical of other ophiolitic dehydration of hydrous phases in the underthrust
complexes, indicate that the complex was formed in a high continental metasediments. The expelled fluids are then
level cnrstal magma chamber immediately above the available to permeate the overthrusted block producing
petrological Moho and so lacks the associated mantle recrystallisation and fusion of some portions of the
tectonites. Ophiolitic sequences have generally been complex leading to the coarse migmatitic textures and
interpreted as fragments of oceanic crust that have accreted associated amphibolitisation of the gabbros. Although the
or abducted onto continental margins. The geochemical pegmatitic textures had initially been interpreted as
characteristics of the basalts of the Volcanic Formation produced by sedimentary-type processes during
however are more consistent with an origin as an oceanic accumulation in a magmatic chamber (Barrero, 1979;
plateau - part of which may now be represented by the Baldion et al., 1991) the presence of clear intrusive
floor of the Caribbean (Millward et al., 1984; Nivia, 1987, relationships suggest local fusion resulting from the
1989). It is possible, given the enhanced thickness and migration of fluids. The depression of the solidus of
buoyancy of young oceanic plateaus when compared with basaltic systems at high pH,O is well known (Yoder, 1979;
normal oceanic crust, that arrival of such a plateau at an Cox et al., 1984).
active margin will jam the subduction zone and cause the
accretion/abduction of the plateau. The lack of tectonised Genesis of the Pegtnatitic Dykes
harzburgite in the Bolivar complex, and in other ophiolitic
massifs located in the same structural position (Espinosa, An interesting feature relating the ophiolitic massifs of
1980, 1985; Vergara, 1983; Tistl et al., 1994) suggests that the Basic Igneous Complex in Colombia is the common
the decollement of the plateau might have occurred at the presence of coarse hornblende pegmatite dykes. In the Los
level of the Moho. Azules ultramafic massif their presence is conspicuous,
Thrusting of hot plateau oceanic crust over continental Vergara (1983) reported such dykes associated with the
crust will raise the geotherm in the underlying crustal Venus ophiolitic body and Espinosa (1985) described, in
66 A. NIVIA

--I
-.
/
I , _ ~
/- y
-T hochfilzen 0 000 1 I
r- Ra&zheinp _ _ !?Titanau
L )_ /y r L
, -
K
--/ \

\
/

,
‘---- - - ” ‘I Gsejgrade
_- - ( Black Sea
OSumadija _ c -.

Sarbinia !

------.-

v ‘endow
Sant Antioco &uJ

Mediterranean Sea

TX Sicily
Syracuse
0

Fig. 5. (a) Location of the magnesite deposits associated with

ultramafic rocks in Europe and Asia Minor (close triangles:
modified from Harben and Bates, 1984, Fig. 84); other types
also shown for completeness. (b) These can be correlated with
ophiolite formation during the closure of a Cretaceous ocean
basin located between the African and Euroasian plates
(modified from Michard. 1987, Fig. 7).

origin of the dykes is uncertain, because there are no

intrusive bodies of granitic composition in the region.
Although they clearly favoured the dykes possibly
representing a late-crystallizing phases of granite intrusion,
the lack of sulphate mineral phases into the paragenesis of
the dykes, often associated with the late stages of
crystallization of granite magmas, is noteworthy.
According to the proposed model, water is added to the
base of the abducted crust by dehydration of the underlying
block. Moreover, if the abduction is produced over
sediments laid on top of the latter, connate waters can also
be distilled and injected upwards through fractures.
Observations in Bay of Islands and Semail ophiolites
suggest that the crystallization and emplacement of
ophiolites are reiatively contemporaneous and in
consequence they are abducted as complete slices of hot,
immature lithosphere, in close vicinity to their places of
the Ginebra Ophiolitic Massif, “plagiogranite” dykes which generation (Spray, 1984; Case and Dewey, 1984; Michard,
may well correspond to those described here. Tistl et al. 1987). The input of water to the base of the ophiolite and
(1994) noted hornblende pegmatite dykes in Viravira- the additional heat produced by friction during thrusting
Condoto ultramafic complexes, NW Colombia. might facilitate its partial fusion by the increase of
According to Barrero (1979) the pegmatitic dykes that temperature and lowering the solidus, or by its reaction
cutting the present complex were generated as the result of with residual fluids. The liquids produce are injected
the reaction between residual liquids and water which upwards crystallizing and fractionating in the fractures
penetrated through fractures produced by its diapiric present in the abducted block, so giving rise to the dykes.
emplacement. Ortiz and Gomez (1971) indicated that the The reported occurrence of dumortierite into the mineral
 Evidence for Obduction in the Bolivar Ultramafic Complex, SW Colombia 67

assemblage of tbe dykes favours the model of distillation sediments that underlay and enveloped the abducted
by abduction. According to the ideal formula of the ophiolite were expelled and migrated through it when the
mineral, boron represents 2.8% of its weight. A ophiolite reached a semi-brittle state. He also indicated that
consequence of this is that high concentrations of boron are the fluids derived from these sediments, and the sediments
needed in the melt or fluid for dumortierite crystallization. themselves, acted as lubricant in the shear zones so
However, boron is extremely dispersed in nature having on reducing the frictional resistance of the ophiolite to
the average concentrations of 3.3 ppm in the Barth crust movement. Dabitzias (1982) reacted for&rite with
and 4.6 on the sea water (Harben and Bates, 1984). Its H@+CO&NaCI solutions at temperatures up to 300°C,
concentration varies from 3 ppm in ultramafic rocks pressures up to 1 kb and with different CO, concentrations,
through 5 ppm in mafic rocks, 10 ppm in granites, 20 ppm and concluded that with increasing CO, in the solutions
in limestones, 35 ppm in sandstones to 100 ppm in marine and with decreasing temperature the Mg content of the
shales (Rose et al., 1979). It can be therefore be postulated solution increases while that of silica decreases drastically:
that the boron contained in the dumortierite was distilled this might explain the absence of silica from the magnesite
from marine shales and concentrated in a fluid veins.
pneumatolitic phase during the process of abduction. Potentially, any ages obtained from pegmatitic dykes or
magnesite veins could, on the basis of the above model,
Genesis of the Magnesite Deposit help constrain the date of the abduction event itself. K/Ar
dating of homblendes from the pegmatitic dykes by Brook
The origin of cryptocrystalline magnesite vein deposits (1984) gave ages of 70 f14, 78 f14 and 102 f18 Ma, and
associated with ultramafic rocks has been ascribed to (1) in by Barrero (1979) gave 106 *18 Ma. Future studies
situ me&somatic replacement of serpentine, and (2) filling hopefully could narrow this range.
of fractures by mineralizing solutions. Textural and mass
balance considerations relating the experimental interaction CONCLUSIONS
between forsterite-H@-CO2 f NaCl have been used to
demonstrated the non-viability of the first process The layered gabbroic and ultramafic rocks of the
(Dabitzias, 1980). The second hypothesis considered two Bolivar Complex probably formed through crystallization
alternatives: deposition in open fractures by the action of in a high level magma chamber in an oceanic plateau (now
either meteoric or hypogenic CO,-rich waters. Both the Caribbean-Colombian Plateau) near the Galapagos
mechanism have been considered in the generation of the hotspot, where the tholeiites of the Volcanic Formation in
Bolivar magnesite deposit; Cuevas (1968) proposed a western Colombia represent the associated extrusives,
supergene origin for the deposit, whereas Ortiz and Gomez During the attempted subduction of this plateau beneath the
(1971) proposed that the magnesite veins had been formed South American continent the leading edge of the plateau
by ascending hydrothermal hypogenic waters. was overthrust onto the continental margin. During this
The hypogenic origin of the magnesite is accepted today process, the water trapped in the terrigenous and calcareous
as the most feasible (Harben and Bates, 1984), and its rocks and sediments that lay on top of the continental
formation mechanism seem to involve CO#ch waters, platform was expelled along with boron, and later, CO*.
produced by steam distillation at depth. This produces The introduction of water into the hot base of the abducted
alteration of the magnesium-rich serpentinite to magnesium segment of plateau crust, together with the frictional heat
carbonate in a process that can be progressive via talc- produced during thrusting of the mass, contributed to the
magnesite and quark-magnesite (Harben and Bates, 1984). partial fusion of the deeper sections of the plateau, the
The appropriate mechanism for the distillation at depth of hydrous melts later crystallising in low-strain zones as
water contained in hydrated minerals and marine shales and hornblende pegmatoid dykes. The action of CO,-rich
muds, has been indicated in the abduction model proposed. hydrothermal waters on serpentinites previously formed by
If associated with the latter sediments, there are also the alteration of the basal dunites of the ophiolite, increased
calcareous rocks from which to distil the CO, needed to the magnesium content of the solutions that then
explain satisfactorily the genesis of the Bolivar magnesite precipitated magnesite close to the surface as pressure
deposit, according to the hypogenic theory. changed from lithospheric toward hydrospheric and
The genesis of the more important magnesite deposits wherever void were created as a result of tectonic uplift or
associated with ultramafic rocks of the world has also been hydrofracture. The depth of precipitation of the magnesite
explained using a model of ophiolite emplacement. These veins, inferred from the cataclastic fracturing during vein
deposits are located in a belt that extends through the deposition, suggest a range in the initial states of the brittle
Dinaric Alps (Fallick et al., 1991), Greece (Dabitzias, deformation. The mechanical behaviour of the rocks was
1980) and Turkey and to the east through Pakistan (see clearly an important factor, necessary to account for
Harben and Bates, 1984). The main producing sites from difference between the highly magnesite-mineralized
ultramafic rocks include Bosnia, Zlativor, Kapaonik, and dunites and the barren pyroxenites.
Kosova (Fig. Sa). The main deposits are located in Greece
particularly in the Chalkidiki Peninsula and in the Eubea Acknowledgments: I would like to thank Prof. J. Tarney for
and Lesbos Islands in the Aegean Sea (Ha&en and Bates, assistance in producing this manuscript, to he, Dr. Andrew
1984). Magnesite is absent from other ophiolites in the Kerr, Dr. Spyros Dabitzias and MS Marion Weber for
region, and this led to the suggestion (Dabitzias, 1980) that helpful improvements to the manuscript, and to Drs. John
there should be a relationship between magnesite veining Aspden and Bill McCourt for constructive reviews. This
and the mechanism of ophiolite emplacement. In order to paper is published with permission of the General Director,
explain the Chalkidiki Peninsula (Vavdos) magnesite Ingeominas.
deposit, Dabitzias (1980) proposed that the ultramafic rocks
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