Geología en la corteza de la tierra en la zona de Colombia

1. Mantle diversity beneath the Colombian Andes, Northern
Volcanic Zone: Constraints from Sr and Nd isotopes
A. Rodriguez-Vargasa
, E. Koesterb,*, G. Mallmannb,c
, R.V. Conceiçãob,d
,
K. Kawashitab
, M.B.I. Webera
a
Escuela de Geociencias y Medio Ambiente, Universidad Nacional de Colombia, Bogotá 4452, Colombia
b
Laboratório de Geologia Isotópica, Centro de Estudos em Petrologia e Geoquı́mica, Instituto de Geociências,
Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, CP 15001, Porto Alegre, RS 91501-970, Brazil
c
Programa de Pós-graduação em Geociências, Universidade Federal do Rio Grande do Sul,
Av. Bento Gonçalves 9500, CP 15001, Porto Alegre, RS 91501-970, Brazil
d
Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul,
Av. Bento Gonçalves 9500, CP 15001, Porto Alegre, RS 91501-970, Brazil
Received 24 February 2004; accepted 4 January 2005
Available online 19 March 2005
Abstract
In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies
were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the
Granatifera Tuff, a deposit of Cenozoic age, in which mantle- and crustal-derived xenoliths are present in bombs and fragments
of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but
websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites,
granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together
with field, petrographic, mineral, and whole-rock chemistry and pressure–temperature estimates, suggest three main sources for
these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 8C) and
MORB isotopic signature, 87
Sr/86
Sr ratio of 0.7030, and 143
Nd/144
Nd ratio of 0.5129. Garnet-bearing peridotite and websterite
xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29–35 kbar, 1250–1295 8C)
conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature (87
Sr/86
Sr ratio of 0.7043 and 143
Nd/144
Nd
ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (N38 kbar, 1140–
1175 8C) and OIB isotopic characteristics (87
Sr/86
Sr ratio=0.7041 and 143
Nd/144
Nd ratio=0.5135).
D 2005 Elsevier B.V. All rights reserved.
Keywords: Colombian Andes; Mantle xenoliths; Crustal xenoliths; Mantle diversity; Continental accretion; Subduction zone; Sr and Nd
isotopes
0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2004.09.027
T Corresponding author. Tel.: +55 51 33167193; fax: +55 51 33167270.
E-mail address: koester@ufrgs.br (E. Koester).
Lithos 82 (2005) 471–484
www.elsevier.com/locate/lithos

2. 1. Introduction
Mantle and crustal xenoliths have been described
in the Andean region associated with alkaline
magmatism related to the subduction of Nazca and
Antarctic plates beneath the South American Plate.
Crustal xenoliths present in these areas are mainly
granulites, hornblendites, pyroxenites, and gneisses,
and represent the lower crustal rocks (e.g. Selvertsone
and Stern, 1983; Weber et al., 2002). Mantle
xenoliths, representing the lower and upper litho-
spheric mantle, have also been described. They are
generally spinel-bearing peridotite xenoliths (e.g.
Gobernador Gregores; Gorring and Kay, 2000;
Laurora et al., 2001), whereas garnet-bearing perido-
tite xenoliths are restricted to a few localities (e.g. Pali
Aike, Stern et al., 1999; Praguaniyeu, Ntaflos et al.,
2002) (Fig. 1).
Mineralogical and geochemical data of crustal
lithologies constitute a powerful tool to the under-
standing of crustal growth models. Mineralogy,
chemistry, geophysics, and petrology of mantle
lithologies, on the other hand, allow the knowledge
of pressure and temperature conditions for the stability
of mineral phases, the characterisation of the sources
of mantle-derived magmas, and the detection of
possible enrichment processes, all of which still need
to be better studied and constrained.
The Mercaderes region in SW Colombia (Fig. 2A
and B), located in the Northern Volcanic Zone (NVZ;
Thorpe, 1982), is a key area used to provide
information for the understanding of the mantle
evolution model in the NVZ, once garnet-bearing
peridotite and pyroxenite xenoliths are common in
this area (Weber, 1998). New Sr and Nd isotope data,
together with field, petrographic, and geochemical
whole-rock and mineral geochemical data, are used in
a discussion of mantle and crustal models for the
region.
2. Geological setting
The geological evolution of the Colombian
Andes (Fig. 2A and B) has been interpreted as a
composite margin made up of successively accreted
terranes and oceanic island arc sequences from
Palaeozoic to Miocene (McCourt et al., 1984; Weber
et al., 2002). The most important events took place
during Devonian–Carboniferous and Cretaceous
times (Restrepo and Toussaint, 1988). At least five
igneous episodes were proposed by Aspden et al.
(1987) for the Central Cordillera of the Colombian
Andes, of which the Jurassic, Cretaceous, and
Neogene episodes are well-represented, and con-
tributed to major crustal and lithospheric growth of
the region. Thus, this region was interpreted as an
edge of a periodically active convergent margin
since the Palaeozoic, where different events of
Nazca
Plate
South American
Plate
Scotia Plate
Antarctic
Plate
9 cm/y
7,8
cm/y
1,5
cm/y
2 cm/y
South American Platform
A
n
d
e
a
n
c
ordillera
AR
AR
500 km
Patagonia
NVZ
CVZ
SVZ
AVZ
Volcanic gap
Flat-slab segment
40
˚W
60
˚W
0
˚
20
˚S
40
˚S
Flat-slab segment
Flat-slab segment
80
˚W
Fig. 2
a
b
c
d
Fig. 1. Present geodynamic configuration of the South American
continent. AVZ—Austral Volcanic Zone; SVZ—Southern Volcanic
Zone; CVZ—Central Volcanic Zone; NVZ—Northern Volcanic
Zone; AR—aseismic ridge. Circles represent some mantle xenolith
occurrences: (a) Mercaderes, (b) Praguaniyeu, (c) Gobernador
Gregores, (d) Pali Aike (modified from Ramos, 1999).
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484
472

3. Fig. 2. Main tectonic geological framework of the Colombian Andes showing the convergence of the Nazca Plate beneath the South American
Plate. Triangles represent the active volcanoes related to the Northern Volcanic Zone (NVZ) and the location of the Mercaderes region (modified
from González et al., 1988).
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 473

4. continental growing and mantle modification were
recognised.
To the west of the Mercaderes area (Fig. 2A and
B) lies the Cauca–Almaguer Fault (Maya and
Gonzalez, 1995), which separates rocks of oceanic
affinity to the west from rocks of continental affinity
to the east. The oceanic rocks formed by extensive
and continuous igneous activity during the Creta-
ceous, resulting in the formation of the Caribbean
oceanic plateau (Kerr et al., 1996, 1997). This suture
possibly represents a subduction zone of continental
ocean type, which was possibly bjammedQ by the
thick, buoyant plateau, enabling it to migrate once
more to the west.
The Mercaderes region (Fig. 2A and B) is
characterised by late Cenozoic to Pleistocene volcanic
activity, where the Mercaderes Tableland comprises
Pleistocene volcanic and volcano-sedimentary flows.
On the south-eastern part of this tableland lies the
Granatifera Tuff, which is possibly a small, partially
eroded tuff cone or tuff ring, containing xenoliths of
both crustal (e.g. diorite, granulite, hornblendite) and
mantle (e.g. garnet-bearing peridotites) origin (Weber
et al., 2002).
The oldest rocks in this region are metasediments
from the Arquı́a Complex (Maya and Gonzalez,
1995). Their metamorphic age (K/Ar dates on
hornblende from amphibolites and the metagabbros)
is Cretaceous, but two thermal events of 120 and 95
Ma are indicated, which leads to two interpretations:
a) they are Mesozoic rocks that have suffered
subsequent metamorphism (Restrepo and Toussaint,
1988); or b) they are Palaeozoic rocks that were
thermally affected in the Cretaceous (McCourt et al.,
1984; Maya, 2001). Metavolcanic and metasedimen-
tary rocks from Diabasico Group showing Cretaceous
age, based on fossiliferous associations and field
relations, overlie this unit (Murcia and Cepeda, 1991;
Kerr et al., 1997; Nivia et al., 1997). The thick, folded
sequences of marine and continental Esmita and
Mosquera formations discordantly overlie the Diaba-
sico Group. They have, respectively, an Upper
Oligocene age and a Middle Eocene up to Lower
Miocene age, defined by fossil record (Murcia and
Cepeda, 1991; Martı́nez and Garcı́a, 1989; González
et al., 1988 and references therein). Dacitic and
andesitic rocks of 13F3 Ma (whole rock, K/Ar)
intrude these Tertiary sedimentary rocks (Murcia and
Cepeda, 1991). Overlying the Esmita and Mosquera
Formations are the pyroclastic rocks of the Galeón
Formation (Martı́nez and Garcı́a, 1989; Murcia and
Cepeda, 1991).
Three main subdivisions were proposed for the
Granatifera Tuff (Martı́nez and Garcı́a, 1989): i) Unit
A, the basal unit, 200 m thick, formed by breccias,
agglomerates, and tuffs, with clasts of porphyritic
andesites, quartzites, schists, amphibolites, garnet
granulites, and eclogites; ii) Unit B, less than 45 m
thick, containing black and green schists, quartzites,
amphibolites, gneisses, hornblendites, pyroxenites,
and andesites; and iii) Unit C, at the top, 50 m thick,
comprises pseudostratified ash material (5 m), debris
flow (40 m), and tuffs (5 m), containing clasts of
diabase, andesite, schists, quartzites, and pumice,
which are b1 m in size.
3. Petrography
The Granatifera Tuff in the Mercaderes region
hosts mantle and crustal xenoliths, with up to 20 cm in
diameter (Weber, 1998). Garnet-bearing rocks, rang-
ing from peridotite to websterite, are the most
common mantle xenoliths. Garnet-free websterite
and spinel-bearing peridotite mantle xenoliths are
present in minor amounts. Lower crustal xenoliths
comprise a variety of amphibolites, pyroxenites,
granulites, and gneisses, metamorphosed into the
amphibolite to granulite facies. The modal composi-
tion of the studied mantle and crustal xenoliths is
presented in Table 1, and photomicrographs are
shown in Fig. 3.
3.1. Host rocks
The host rocks consist of breccias, and tuffs of the
Granitifera Tuff. The xenoliths are found as clasts
(up to 12 cm for mantle and up to 20 cm for crustal
xenoliths) immersed in the tuffaceous matrix or
inside lamprophyre and andesite fragments and
bombs in the breccias. Lamprophyres are character-
ized by porphyritic texture given by up to 0.8 mm
amphiboles, and a groundmass composed of plagio-
clase, amphibole, and pyroxene. Andesites are
massive with light gray color, containing up to 0.5
cm long phenocrysts of plagioclase, and up to 0.4
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484
474

5. cm long amphibole immersed in a matrix of
plagioclase, amphibole, and pyroxene. Next to some
contacts between xenoliths and host andesite, and
around some altered pyroxene xenocrysts, the
andesite presents nepheline aggregates, which are
products of interaction between these two rock types.
Diorites and schist are also found as centimetric
fragments in the Mercaderes region, but they contain
neither mantle nor lower crustal xenoliths.
3.2. Mantle xenoliths
Garnet-bearing mantle xenoliths are the most
common mantle rocks in the Mercaderes region. They
are hydrous garnet-bearing websterite and hydrous
garnet-bearing lherzolite xenoliths, presenting proto-
granular texture according to the classification of
Mercier and Nicolas (1975). They are characterized
by the presence of coarse-grained pyroxenes (in
general, Cr-endiopside [cpx] and enstatite [opx]),
garnet (b5 mm, Cr-pyrope), and minor olivine
(Fo89–92) and Cr-spinel (b4 mm). Some samples
(XM-2 and XM4) show porphyroclastic texture, in
which these minerals are immersed in a fine-grained
matrix (b1 mm) composed by the same mineral
assemblage. Vermicular spinel (b1 mm), amphibole,
pyroxenes, and serpentine are secondary minerals
present as kelyphytic rims surrounding pyroxenes and
olivines in these xenoliths as a result of metasomatism
(fluid percolation), while garnet presents comminu-
tion of grains in their borders. Millimetric veins filled
with serpentine are present, cutting all primary
minerals. In the xenolith–host rock contact, some
olivine and pyroxene crystals are zoned possibly due
to diffusion processes, and some of them are recrystal-
lised into fine-grained aggregates with mosaic shapes.
Garnet-free websterite xenoliths are also present in
minor amounts in the Mercaderes region. They
comprise pyroxenes (in general, Cr-endiopside and
enstatite) showing protogranular textures, following
the Mercier and Nicolas (1975) classification, olivine
(Fo89–92), and Cr-spinel in minor amounts. Amphib-
oles (pargasite and pargasite–hornblende) and Fe-
oxides are secondary minerals. Generally, the pres-
ence of amphibole in the mantle characterizes a modal
metasomatic event.
In this paper, we work just with the garnet
(Fspinel)-bearing and spinel-bearing websterite, with
or without amphibole.
3.3. Crustal xenoliths
Lower crustal xenoliths have a wide composi-
tional variation in the Mercaderes region, and
include amphibolites as the most abundant rock
type, with subordinate pyroxenites, granulites, and
orthogneisses. Garnet-bearing amphibolites and
pyroxenites, containing felsic phases such as feldspar
and/or scapolite, are the dominant crustal xenoliths.
They have brown hornblende or clinopyroxene and
garnets as the main mineral phases, and titanite and
apatite as accessory minerals, showing granoblastic
textures. Granulites comprise garnet, clinopyroxene,
plagioclase, and/or scapolite and quartz, with apatite,
rutile, and titanite as accessory phases, all showing
Table 1
Modal composition of host rocks, mantle, and crustal xenoliths from
the Mercaderes region, Colombia
Mantle xenoliths
Sample number XM1 XM2 XM3 XM4 XM5 XM6 XM7 XM8
Orthopyroxene 86.5 35.8 18.4 63.5 27.1 10.6 32.2 11.4
Clinopyroxene 2.8 3.6 67.3 9.2 65.4 31.9 42.3 72.8
Garnet 4.4 59.3 – 22.3 – 54.0 19.0 10.5
Spinel 5.5 – 8.2 – 7.5 0.2 3.5 2.5
Olivine – 0.6 – 2.7 – – – –
Amphibole – – 6.0 0.6 – 2.8 3.0 2.8
Opaques 0.2 – – – – – – –
Veins – 0.7 – 1.7 – – – –
Host rocks and crustal xenoliths
Sample number L1 L2 L3 XC1 XC2 XC3 XC4 XC5
Matrix 34.9 72.7 63.6 – – – – –
Amphibole 25.2 10.1 7.9 45.1 95.8 28.4 92.6 –
Plagioclase 37.7 – 25 53.1 – 27.7 – 34.8
Orthopyroxene – 14.5 2.9 – – – 2.7 25.0
Clinopyroxene – – – – 4.1 43.4 2.9 30.0
Garnet – – – – – – – 10.0
Spinel – – – – – – 1.7 –
Biotite – – 0.3 1.5 – – – –
Opaques 4.2 2.7 0.3 – – – – –
Accessory minerals – – – 0.3 0.1 0.5 0.1 0.2
XM1, XM2, XM4, XM6=garnet-bearing websterite xenoliths;
XM3, XM5=spinel-bearing websterite xenoliths; XM7, XM8=spi-
nel- and garnet-bearing websterite xenoliths; L1, L3=andesites;
L2=lamprophyre (host rocks); XC1, XC3=diorite gneisses; XC2,
XC4=amphibolites; XC5=granulite (crustal xenoliths).
Modes were calculated after counting more than 1000 points under
a petrographic microscope.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 475

6. granoblastic texture. Gneisses are mainly banded
orthogneisses, with felsic and mafic millimetre-sized
bands composed of plagioclase and quartz, and
biotite and amphibole, respectively. Garnet, epidote,
and scapolite are also present in a few samples.
Apatite, zircon, titanite, and opaque are the main
accessory minerals.
4. Whole-rock chemistry
Whole-rock major and trace elements concentra-
tions were determined by X-ray Fluorescence at the
Laboratório de Geoquı́mica of the Instituto de Geo-
ciências, Universidade de São Paulo (Brazil). Rare
earth elements (REE) and some trace element analyses
of mantle xenoliths were performed by ICP-MS at the
Activation Laboratories—Actlabs (Canada). Results
are listed in Table 2 and shown in Figs. 4 and 5, in which
analyses from Weber (1998) are also plotted for
comparison.
4.1. Host rocks
Lamprophyres and andesites are characterised by
similar Al2O3 and CaO (around 17 wt.% and around 7
wt.%, respectively), MgO around 3.12 wt.% for
andesite and 5.87 wt.% for lamprophyre, and Na2O/
K2O ratios between 2.37 for andesite and 4.17 for
lamprophyre. The Na2O ratios for the lamprophyre
suggest that it is alkaline following Rock’s (1990)
classification. High Ba contents (around 669 and 336
ppm for andesite and lamprophyre, respectively) are
also characteristic of these rocks. Cr is enriched in
lamprophyre (183 ppm) when compared to the
andesite (17 ppm). Andesite and lamprophyre also
show strong fractionated REE patterns, with LaN from
0.02 to 9 and LuN from 0.6 to 10.
4.2. Mantle xenoliths
The garnet-bearing mantle xenoliths from the
Mercaderes region are characterised by two distinct
D)
B)
A)
C)
Grt
Cpx
Cpx
Pl
Amph
Cpx
Cpx
Spl
Fig. 3. Photomicrographs (crossed-polarized light) of host rocks, mantle, and crustal xenoliths from Mercaderes region, Colombia. (A) diorite
gneiss; (B) spinel-bearing peridotite xenolith; (C) andesite; (D) garnet-bearing peridotite xenolith. Amph=amphibole; Cpx=clinopyroxene;
Spl=spinel; Pl=plagioclase; Grt=garnet. Scale bars correspond to 0.5 mm.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484
476

7. chemical groups: Group I encompasses the high-
MgO-content (N32 wt.%) garnet-bearing websterite
xenoliths, with high modal contents of orthopyrox-
ene (N60 vol.%), and Group II encompasses low-
MgO-content (b32 wt.%) garnet-bearing websterite
xenoliths with low modal contents of orthopyroxene
(b40 vol.%). The garnet-free mantle xenoliths
present moderate MgO contents c18 wt.%. The
Table 2
Whole-rock major (wt.%) and trace element (ppm) composition of host rocks, mantle, and crustal xenoliths from the Mercaderes region
Sample number Host rocks Mantle xenoliths Crustal xenoliths
L1 L2 XM 1 XM 2 XM 3 XM 4 XM 5 XM 6 XM 7 XM 8 XC 1 XC 3 XC 4 XC 5
SiO2 55.77 52.68 55.04 45.81 51.99 47.48 47.95 52.02 50.45 46.93 41.26
Al2O3 17.86 16.39 3.47 2.80 4.26 15.74 12.73 19.05 12.50 8.17 18.82
MnO 0.13 0.14 0.13 0.15 0.13 0.16 0.26 0.12 0.12 0.20 0.30
MgO 3.12 5.87 32.53 39.33 17.20 19.02 21.73 5.24 5.78 17.80 5.62
CaO 7.04 7.60 1.55 2.45 20.00 9.51 6.77 8.90 15.94 8.49 14.04
Na2O 3.45 2.63 0.03 0.08 0.42 0.61 0.15 4.99 2.72 1.67 1.39
K2O 1.45 0.63 0.01 0.01 0.01 0.02 0.01 0.45 0.29 0.40 0.55
TiO2 0.88 1.07 0.05 0.10 0.12 0.23 0.18 0.46 1.62 1.97 1.50
P2O5 0.28 0.25 0.01 0.01 0.01 0.02 0.16 0.35 0.34 0.11 0.60
Fe2O3 7.54 9.56 6.04 8.82 4.93 7.25 10.11 6.91 10.39 13.72 15.59
LOI 1.86 2.76 0.01 0.18 0.27 0.01 0.01 0.96 0.26 0.75 0.89
Total 99.39 99.59 98.84 99.73 99.33 100.07 99.90 99.45 100.41 100.21 100.56
Ba 669 336 8 6 6 14 86 31 63 24 287 358 58 315
Cl 360 628 b15 57 b15 b15 b15 b15 64 161 151
Co 53 59 46 30 31 69 25 31 42 27 24 54 81 59
Cr 17 183 7345 1310 2400 3033 4960 1792 2992 384 189 316 1955 62
Cu 7 23 9 26 10 19 14 16 9 16 3 13 30 7
Ga 20 20 3 4 2 2 3 8 5 13 22 17 16 39
Nb 11 8 4 3 4 4 4 4 31 15 20
Ni 8 120 631 81 980 2061 299 289 458 62 53 161 572 59
Pb 11 12 6 11 9 18 15 17 22 20 25
Rb 29.8 11.3 0.2 0.3 0.9 0.6 0.2 67.7 6.1 4.6 2.5
Sc 19 25 11 13 51 50 75 23 25 31 33
Sr 576 457 7 27 117 7 26 24 20 85 1262 568 114 713
Th 6 b3 b3 b3 b3 b3 b3 12 7 b3 19
U b3 b3 b3 b3 b3 b3 b3 b3 b3 4 5
V 173 223 44 163 35 39 133 157 105 267 173 146 326 317
Y 19 25 1 29 4 7 20 45 11 26 21 17 38
Zn 100 92 32 31 61 16 19 38 100 90 212 110 259
Zr 114 107 9 13 10 22 24 37 205 128 80 302
Hf b0.2 0.2 b0.2 0.5 b0.2 0.4 0.5 1.7
La 30 22 0.1 0.5 0.3 0.1 0.4 0.4 0.7 2.7 36 37 b14 56
Ce 40 31 0.2 1.5 1.2 0.3 1 1.3 2.2 11.3 65 56 b18 99
Pr b0.05 0.35 0.19 0.79 0.19 0.2 0.36 2.23
Nd 20.8 18.8 0.1 2.2 1.1 0.4 1.2 1.3 2.4 12.9 39.2 23.9 13.8 65.1
Sm 4.3 4.5 0.5 0.9 0.3 0.2 0.5 0.5 1.0 4.1 7.9 4.9 3.5 12.4
Eu b0.05 0.37 0.1 0.07 0.19 0.25 0.4 1.16
Gd b0.1 1.7 0.3 0.3 0.7 1 2 3.7
Tb b0.1 0.4 b0.1 b0.1 0.1 0.3 0.5 0.6
Dy b0.1 3.9 0.2 0.4 1 2.3 4.5 2.6
Ho b0.1 1 b0.1 b0.1 0.2 0.6 1.2 0.4
Er b0.1 3.9 b0.1 0.3 0.7 2.5 4.6 1
Tm b0.05 0.66 b0.05 b0.05 0.1 0.45 0.78 0.13
Yb 0.1 4.5 b0.1 0.3 0.6 3.3 5.3 0.8
Lu b0.04 0.71 b0.04 0.05 0.09 0.56 0.86 0.11
Regular—X-ray fluorescence analysis; italics—ICP-MS analysis.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 477

8. 0 10 20 30 40
MgO (wt%)
40
45
50
55
60
SiO
2
(wt%)
0 10 20 30 40
MgO (wt%)
0
0.5
1.0
1.5
2.0
K
2
O
(wt%)
0 10 20 30 40
MgO (wt%)
5
10
15
20
25
Al
2
O
3
(wt%)
0
0 10 20 30 40
MgO (wt%)
0.5
1.0
1.5
2.0
2.5
TiO
2
(wt%)
0
0 10 20 30 40
MgO (wt%)
5
10
15
20
25
CaO
(wt%)
0
0 10 20 30 40
MgO (wt%)
0.1
0.2
0.3
0.4
0.5
P
2
O
5
(wt%)
0
0.6
0.7
0 10 20 30 40
MgO (wt%)
1
2
3
4
5
Na
2
O
(wt%)
0
6
0 10 20 30 40
MgO (wt%)
4
8
12
16
20
Fe
2
O
3
(wt%)
0
0 10 20 30 40
MgO (wt%)
0
400
800
1200
1600
Sr
(ppm)
0 10 20 30 40
MgO (wt%)
0
2000
4000
6000
8000
Cr
(ppm)
Fig. 4. Whole-rock major and trace elements against MgO diagrams of host rocks (triangle), mantle (squares), and crustal (diamonds) xenoliths.
Fields of mantle (continuous line) and crustal xenoliths (dashed line) from the same region compiled of Weber (1998) are presented for
comparison.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484
478

9. other major element contents are similar in all
xenoliths, except for the garnet-bearing websterite
xenoliths of Group I that have lower CaO contents
(b3 wt.%) compared with those for the garnet-
bearing websterite xenoliths Group II (CaON4 wt.%)
and the garnet-free xenoliths (CaOc20 wt.%).
Trace element contents are similar in all rocks;
most samples have low Sr (b117 ppm), Rb (b0.9
ppm), Nb (b4 ppm), and Zr (b37 ppm) contents,
and moderate Pb contents (6–18 ppm). Some
variations are mainly related to the presence of
certain minerals such as orthopyroxene, which
increases whole-rock contents of Cr. Three patterns
(Fig. 5) of chondrite-normalised REE are observed
for the mantle xenolith samples: i) strong enrich-
ment of heavy REE related to the light REE
(samples XM2, XM6, and XM7); ii) light enrich-
ment of heavy REE related to light REE (samples
XM4 and XM5); and iii) enrichment of middle REE
related to light and heavy REE (samples XM3 and
XM8). These patterns partially reflect mineralogical
composition. Enrichment of heavy REE is related to
the presence of garnet, while the enrichment of
middle REE is related to the presence of amphibole.
These relations are not straightforward, but the
garnet/amphibole proportion seems to define the
REE pattern.
4.3. Crustal xenoliths
The lower crustal xenoliths from the Mercaderes
region are characterised, when compared to mantle
xenoliths, by lower MgO contents (b14 wt.%), and
higher TiO2 (N0.25 wt.%), Na2O (N1 wt.%), and P2O5
(N0.20 wt.%) contents, and similar contents for the
other major oxides. They present higher Sr (N400
ppm), Nb (N5 ppm), and Zr (N100 ppm) contents and
lower Cr (b10 ppm) and Ni (b5 ppm) contents than
the mantle xenoliths. Chondrite-normalised REE
patterns (data from Weber, 1998) for lower crustal
xenoliths (Fig. 5) are variable and depend on the
lithology. Crustal xenoliths with garnet–pyroxenite
composition are expressively enriched in heavy REE
and display a pattern similar to some of the garnet-
bearing mantle xenoliths. However, the light REE
contents of the crustal xenoliths are also expressively
lower than the one of the mantle xenoliths. Amphib-
olites and diorites display similar REE patterns;
however, amphibolites are enriched in middle REE.
Garnet gneisses show depletion of heavy REE related
0.1
1
10
100
La Ce Pr Nd Sm Eu Gd Tb Dy
Sample/Chondrite
Ho Er Tm Yb Lu
XM2
XM1
XM4
XM5
XM8
XM6
XM7
XM3
XM1
XM3
XM2
XM4
XM5
XM8
XM6
XM7
dacite
garnet gneiss
Mantle xenoliths
Crustal xenoliths
Host rocks
garnet pyroxenites
amphibolites
diorites
andesites
lamprophyre
Fig. 5. Chondrite-normalized (Sun and McDonough, 1989) REE diagram for the studied mantle xenoliths. Data on crustal samples from Weber
(1998) are also shown for comparison.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 479

10. to light REE and a small positive anomaly of Eu,
compared to Sm.
5. Sr–Nd isotopes
5.1. Analytical procedures
Sixteen whole-rock xenoliths and one mineral
(garnet) were powdered in agate mortar down to
b200 mesh. Before dissolution, the mineral sample
was washed in warm 2.5 N HCl to remove surface
contamination. Each sample was properly spiked
(with mixed 87
Rb/84
Sr and 149
Sm/150
Nd tracers) and
processed using standard dissolution procedures with
HF, HNO3, and HCl in Teflon vial, and warmed on a
hot plate until complete material dissolution. Column
procedures used cationic AG50W-X8 resin (200–400
mesh) in order to separate Rb, Sr, and REE,
followed by Sm and Nd separation using anionic
LN-B50-A resin (100–150 Am). Each sample was
dried to a solid and then loaded with 0.25 N H3PO4
on appropriate filament; single Ta for Rb, Sr, and
Sm; and triple Ta–Re–Ta for Nd. The samples were
run in a VG Sector 54 thermal ionisation mass
spectrometer at the Laboratório de Geologia Iso-
tópica, Universidade Federal do Rio Grande do Sul
(Brazil), in static mode. Nd and Sr ratios were
normalised to 86
Sr/88
Sr=0.1194 and 146
Nd/144
Nd=
0.7219 respectively. Measurements for the Sr NIST
standard NBS-987 gave 87
Sr/86
Sr=0.710260F
0.000014, and for the Nd La Jolla, standard values of
143
Nd/144
Nd=0.511859F0.000010. Total blanks aver-
aged b750 pg for Nd and Rb and b150 for Sm and Sr.
Standard errors percentual (1dm%) for 87
Rb/86
Sr and
147
Sm/144
Nd were F1% or smaller, based on inter-
active sample analysis and spike recalibration, and
b0.0057% for 87
Sr/86
Sr and 143
Nd/144
Nd ratios. The
errors are presented as standard deviation for 87
Sr/86
Sr
ratios and in parts per million for 143
Nd/144
Nd ratios.
Results are listed in Tables 3 and 4, and illustrated in
Fig. 6.
5.2. Results
A lower crustal xenolith sample with dioritic
composition shows the highest Rb (68 ppm) and Sr
(1262 ppm) contents among the analysed rocks. Sm
and Nd values for this xenolith are 8 and 39 ppm,
the 87
Sr/86
Sr ratio=0.749, and the 143
Nd/144
Nd ratio=
0.5128. The other lower crustal xenoliths plot close
to the garnet- and spinel-bearing mantle xenoliths
Table 3
Rb–Sr isotope data for host rocks, mantle, and crustal xenoliths from the Mercaderes area
Sample number Rb (ppm) Sr (ppm) Rb/Sr 87
Rb/86
Sr 87
Sr/86
Sra
S.D. (1r)
XM-1 0.2 6.9 0.030798 0.089675 0.704104 0.000122
XM-2 0.1 26.4 0.002228 0.006483 0.704378 0.000104
XM-3 0.1 96.1 0.001452 0.004224 0.703000 0.000137
XM-4 0.3 6.7 0.037683 0.109726 0.704104 0.000122
XM-5 0.9 25.8 0.035638 0.103725 0.705342 0.000106
XM-6 0.6 23.6 0.026321 0.076603 0.704227 0.000128
XM-7 0.2 19.9 0.008089 0.023541 0.704320 0.000131
XM-8 0.4 81.4 0.005180 0.015075 0.704458 0.000097
L1 29.8 576.4 0.051677 0.150367 0.704346 0.000174
L2 11.3 456.7 0.024834 0.072272 0.705904 0.000551
L3 18.6 562.0 0.033063 0.096208 0.704553 0.000173
XC-1 67.7 1262.2 0.053603 0.155979 0.704872 0.000149
XC-2 2.3 186.9 0.000000 0.012521 0.704471 0.000155
XC-3 6.1 567.6 0.010824 0.031500 0.705402 0.000099
XC-4 4.6 113.5 0.040574 0.118075 0.704681 0.000143
XC-5 2.5 713.1 0.003535 0.010286 0.704425 0.000166
Normalised to 86
Sr/88
Sr=0.1194, fitted to bias with base on SrCO3 NBS-987, using 87
Sr/86
Sr=0.71025 and correction in order of the presence of
spike. NBS values during analyses were 0.71026F0.000014.
a
Whole-rock average of F130 isotopic ratios, 1.0 V of ionic intensity for 88
Sr, and multicollection with 86
Sr in the axial collector.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484
480

11. from the Mercaderes region. However, crustal
xenoliths are richer in radiogenic Sr compared to
mantle xenoliths.
Two host-rock volcanic samples (andesite and
lamprophyre) show Rb values b18 ppm, Srb561
ppm, Smb4.5 ppm, and Ndb19 ppm having 87
Sr/86
Sr
0.708
0.707
0.706
0.705
0.704
0.703
0.702
0.5120
0.5122
0.5124
0.5126
0.5128
0.5130
0.5132
0.5134
0.5136
87Sr/86Sr
143
Nd/
144
Nd
Sp-Grt-PA
mantle xenoliths
Sp-PA
mantle xenoliths
PA mantle
xenoliths
Mercaderes
mantle xenoliths
PA basalts
Crustal xenoliths
Garnet-bearing mantle xenoliths
Spinel-bearing mantle xenoliths
Atl. MORB
Continental Plateau Basalts
(Paraná Province)
OIB
Kerguelen
Pac.
MORB
EM I EM II
HIMU
Mercaderes
crustal xenoltihs
BSE
BSE
BSE
Lamprophyres and andesites
Lamprophyre
Andesite
Fig. 6. Sr and Nd isotopic composition for host rocks, mantle, and crustal xenoliths from the Mercaderes region. Fields compiled in the georoc
database (http://www.georoc.mpch-mainz.gwdg.de/). OIB field includes Hawaii, La Palma, Azores, St. Helena, and Easter and Ascension
Islands. Pali Aike (PA) fields from Stern et al. (1999); Mercaderes fields from Weber (1998). Pac.=Pacific; Atl.=Atlantic.
Table 4
Sm–Nd isotope data for host rocks, mantle, and crustal xenoliths from the Mercaderes area
Sample number Sm (ppm) Nd (ppm) 147
Sm/144
Nd 143
Nd/144
Nda
Error (ppm) Epsilon Nd (0)
XM-1 0.2 0.8 0.155950 0.513157 13 10.1
XM-2 0.7 1.5 0.260118 0.512879 16 4.7
XM-3 0.4 1.4 0.1552762 0.513082 25 8.7
XM-4 0.1 0.1 0.240371 0.513485 57 16.5
XM-5 1.0 2.5 0.249552 0.512927 18 5.6
XM-6 0.4 0.9 0.269386 0.512869 56 4.5
XM-7 0.8 1.8 0.260720 0.512945 14 6.0
XM-7b
1.1 7.9 0.086210 0.511764 33 17.1
XM-8 3.3 3.2 0.625961 0.512761 12 2.4
L1 3.7 14.7 0.152254 0.512808 9 3.3
L2 4.5 18.8 0.144955 0.512802 25 3.2
L3 3.2 18.0 0.107889 0.512596 15 0.8
XC-1 7.9 39.2 0.121824 0.512761 11 2.4
XC-2 6.9 28.7 0.144522 0.512837 13 3.9
XC-3 4.9 23.9 0.123742 0.512838 12 3.9
XC-4 3.5 13.8 0.154907 0.512884 13 4.8
XC-5 12.4 65.1 0.114979 0.512947 15 6.0
Normalised to 146
Nd/144
Nd=0.7219, fitted to bias with base on the Nd SPEX using suggested 143
Nd/144
Nd=0.511110, and calibrated against Nd
La Jolla using a value of 143
Nd/144
Nd of 0.511859F0.000010.
a
Whole-rock average of F100 isotopic ratios, 1.0 V of ionic intensity for 146
Nd, and multicollection with 146
Nd in the axial colector.
b
- garnet sample from XM-7 mantle xenolith.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 481

12. values of 0.7045 and 0.7059, and143
Nd/144
Nd of
0.5125 and 0.5128. The lamprophyre has more
evolved radiogenic Sr than andesite and the garnet-
and spinel-bearing mantle xenoliths.
The studied mantle xenoliths have low Rb (b0.9
ppm),Sr (b96ppm),Sm(b3.2ppm),andNd(b3.3ppm)
contents. Five garnet-bearing peridotite xenoliths and
two websterite xenoliths show 87
Sr/86
Sr ratios between
0.7029 and 0.7044, and 143
Nd/144
Nd ratios between
0.5127 and 0.5134. Three lower crustal xenoliths
(hornblendite, granulite, and pyroxenite) present
Rbb6.1 ppm, Srb713 ppm, Smb12 ppm, and Ndb65
ppm. Their 87
Sr/86
Sr ratios range between 0.7044 and
0.7054, and 143
Nd/144
Nd ratios from 0.5128 to 0.5130.
A garnet sample from a spinel garnet-bearing
websterite xenolith (sample XM-7) shows high Sm
and Nd contents of 1.1 and 7.9 ppm, respectively,
comparedwiththecontentsintheothermantlexenoliths
(except for the sample XM-8). A Sm–Nd isochron age
of 1031F130 Ma was obtained for this sample (not
shown).ThisageisolderthanTDM agesforcrustalrocks
(b700Ma)inthisregionandsotheinterpretationforthis
age is unclear. It could be interpreted either as mantle-
growingageortheageofasecondaryeventthataffected
this garnet, such as metasomatism or melting percola-
tion. Furthers studies in other mantle xenoliths in the
Mercaderes region will provide more information that
can shed some light into this problem.
The Mercaderes garnet-bearing peridotite xenoliths
plot within the oceanic basalt field (OIB) in the
87
Sr/86
Sr vs. 143
Nd/144
Nd diagram (Fig. 6), towards
the Bulk Silicate Earth (BSE) values, or more radio-
genic Sr isotopic compositions. Only one sample has a
distinct signature as it has higher 143
Nd/144
Nd values
compared to other garnet-bearing peridotite xenoliths.
The isotopic composition of one spinel-bearing
peridotite xenolith plots in the field of MORB, while
another sample plots away from this field. The high
radiogenic Sr in this sample is probably related to its
high CaO contents (20%). Thus, an MORB signature
is suggested for spinel-bearing peridotite xenoliths,
while an OIB signature is evidenced by the garnet-
bearing peridotite xenoliths.
Lower crustal xenoliths show more radiogenic Sr
compositions compared to those for the mantle
xenoliths. All analysed samples plot near the field of
crustal xenoliths from Mercaderes studied by Weber et
al. (2002). Large variation in the 87
Sr/86
Sr ratios for
these xenoliths suggests that the lower crust under the
Mercaderes region is isotopically heterogeneous.
The values of qNd (t=0) for the studied mantle
xenoliths range from 2.4 to 16.5 and confirm the
depleted isotopic composition of these rocks. Crustal
xenoliths have positive qNd (t=0) values, ranging
from 2.4 to 6.0, suggesting the presence of ortho-
derived material in the lower crust, while for the host
volcanic rocks, qNd (t=0) values are 0.3 and 3.2. The
higher value is given by a lamprophyric sample.
Nd model ages (TDM; De Paolo, 1981) for the
lower crustal xenoliths indicate an extraction age
varying from 0.3 to 0.4 Ga, which attests to crustal
growth in this area at this time. However, TDM for the
lamprophyre and andesite xenoliths ranges from 0.6 to
0.7 Ga, older than that for the lower crustal xenoliths,
suggesting distinct events of mantle extraction.
6. Discussion and conclusions
Mantle and crustal xenoliths from the Granatifera
Tuff, Colombia, provide valuable information useful
to the discussion of the lithospheric mantle and the
crustal evolution of the Mercaderes region. Sm–Nd
and Rb–Sr isotopic systems integrated with field
relationships, geochemistry data, and pressure–tem-
perature estimates for the garnet-bearing mantle
xenoliths are compatible with two distinct mantle
reservoirs, which reflect the mantle diversity beneath
the Northern Colombian Andes.
Estimates of pressure and temperature (Weber,
1998) show three main P–T conditions for the mantle
xenoliths. The spinel-bearing peridotite xenoliths
were formed at low P (16 kbar, 1065 8C), whereas
the garnet-bearing peridotite xenoliths were formed at
high P and T at two different conditions: sub-oceanic
geotherm (29–35 kbar, 1250–1295 8C) and sub-
continental geotherm (N38 kbar, 1140–1175 8C).
Pressure and temperature estimates for the lower
crustal indicate that they were formed at 730–830
8C at 9–14 kbar for amphibolites, and at 950–1050 8C
at 13–15 kbar for all other rocks (Fig. 7).
The garnet-bearing xenoliths represent deeper
fragments (around 90 km) and spinel-bearing peri-
dotite xenoliths are fragments of upper lithospheric
mantle (40 km), as suggested by Weber (1998). Some
of the garnet-bearing peridotite xenoliths derived from
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484
482

13. a mantle source with OIB isotopic signature in high
P–T conditions, similar to that of a sub-continental
geothermal gradient. This is compatible with the
convergent tectonic setting of the region that is the
active subduction of the Nazca Plate under the South
American plate. Other groups of garnet-bearing peri-
dotite xenoliths derived from a source with lower P and
higher T, similar to that of a sub-oceanic geothermal
gradient; the source has Sr isotopic ratios similar to
those of the other reservoir, but distinct Nd isotopic
signature. This enrichment in radiogenic Nd, sugges-
tive of a different isotopic reservoir, could be related to
a process of chromatographic isotopic separation that
would lead to an increase in the Nd concentrations
without disturbance in Sr values. Spinel-bearing
peridotite xenoliths derived from a source with con-
ditions of lower P–T similar to that of sub-ocegeother-
mal gradient, and a MORB isotopic signature.
The lower crust xenoliths comprising heteroge-
neous materials, recorded by distinct isotopic signa-
tures, formed at 0.3–0.4 Ga. No similar age has been
reported for rocks outcropping in the area.
The andesite volcanic host rocks resulted from
partial melting of a source that has isotopic signature
similar to that of the BSE continental plateau basalts,
but the position of the lamprophyre sample in Fig. 6
suggests some contributions of a subducted slab that
has contaminated the mantle source.
Mantle xenoliths from the Mercaderes region and
from the Pali Aike region, southernmost Chilean
Andes (Stern et al., 1999), include garnet- and spinel-
bearing xenoliths, but their isotopic signatures are
quite distinct. Garnet-bearing mantle xenoliths of Pali
Aike present lower 87
Sr/86
Sr ratios and less depleted
143
Nd/144
Nd ratios, compared with the Mercaderes
xenoliths, approaching the Nd and Sr isotopic
compositions of HIMU. The Pali Aike spinel-bearing
peridotite xenoliths are Sr-enriched in comparison
with similar rocks from the Mercaderes area, except
for the sample XM-5, which is the most enriched in
CaO, suggesting some contamination by fluids or
alteration. Pressure and temperature estimates for the
garnet- and spinel-bearing mantle xenoliths in Pali
Aike area are also distinct, with temperatures ranging
from 970 to 1160 8C and pressures between 19 and 24
kbar (Stern et al., 1999). Thus, in terms of lithosphere
mantle evolution, these two regions present a MORB-
like signature (for spinel xenoliths), but an additional
OIB-like region is suggested in the Mercaderes area.
An important mantle event has occurred at 1.0 Ga,
as suggested by the Sm–Nd garnet and whole-rock
isochron age. This age is older than other mantle
600 700 800 900 1000 1100 1200
5
10
15
20
P
(kbar)
T (˚C)
60
45
30
15
km
Grt+Cpx+Qtz
Grt+Cpx+Plg+Qtz
Grt-in
Plg-out
Gart-in
Cpx+Opx+Plg+Qtz
Grt+Cpx+Opx
+Plg+Qtz
D
r
y
P
e
r
i
d
o
t
i
t
e
s
o
l
i
d
u
s
Sub-oceanic geotherm
Sub-continental geotherm
800 1000 1200 1400
5
10
15
20
25
30
35
40
T (˚C)
P
(kbar)
Garnet
peridotites/pyroxenites
Spinel
peridotites
Mantle
xenoliths
Lower crustal
xenoliths
a
b
Pali Aike
Fig. 7. Pressure–temperature diagrams for crustal and mantle
xenoliths from the Mercaderes region (Weber, 1998). (a) Mantle
xenoliths show three distinct patterns. The spinel-bearing peridotite
xenoliths formed at low P (16 kbar, 1065 8C), whereas the garnet-
bearing peridotite xenoliths formed at higher P and T. The high-PT
mantle xenoliths plot close to and parallel to the sub-oceanic
geotherm (29–35 kbar, 1250–1295 8C) and to the sub-continental
geotherm (N38 kbar, 1140–1175 8C). (b) Crustal xenoliths show P
varying from 10 to 15 kbar and T from 800 to 1100 8C.
A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 483

14. extraction ages (TDM) in the Mercaderes region and
suggests a metasomatic or melting event, demonstrat-
ing a long history of mantle evolution in this area.
Santos et al. (2000) described a geological event of
this age (1.33–0.99 Ga) in the nearby Sunsás
Province, and interpreted as an event of recycling of
continental crust during the Greenvillian Orogeny.
Acknowledgements
We gratefully acknowledge Farid Chemale Junior
for his support and comments on various aspects of
laboratory studies. The manuscript benefited from
constructive reviews by A. Giret and an anonymous
reviewer. R. Rupp is warmly thanked for the English
reviews, and V.P. Ferreira, A.N. Sial, and I. McReath
for editorial improvements. This work was funded by
PROSUL-CNPq (project AC-74).
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