Spinel-facies mantle xenoliths from Cerro Redondo, Argentine Patagonia: Petrographic, geochemical, and isotopic evidence of interaction between xenoliths and host basalt

Mantle diversity beneath the Colombian Andes, NorthernVolcanic Zone: Constraints from Sr and Nd isotopes

A. Rodriguez-Vargasa, E. Koesterb,*, G. Mallmannb,c, R.V. Conceicaob,d,K. Kawashitab, M.B.I. Webera

aEscuela de Geociencias y Medio Ambiente, Universidad Nacional de Colombia, Bogota 4452, ColombiabLaboratorio de Geologia Isotopica, Centro de Estudos em Petrologia e Geoquımica, Instituto de Geociencias,

Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, CP 15001, Porto Alegre, RS 91501-970, BrazilcPrograma de Pos-graduacao em Geociencias, Universidade Federal do Rio Grande do Sul,

Av. Bento Goncalves 9500, CP 15001, Porto Alegre, RS 91501-970, BrazildDepartamento de Geologia, Instituto de Geociencias, Universidade Federal do Rio Grande do Sul,

Av. Bento Goncalves 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 studieswere performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in theGranatifera 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, butwebsterites (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, togetherwith 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) andMORB isotopic signature, 87Sr/86Sr ratio of 0.7030, and 143Nd/144Nd ratio of 0.5129. Garnet-bearing peridotite and websteritexenoliths 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 (87Sr/86Sr ratio of 0.7043 and 143Nd/144Ndratio 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 (87Sr/86Sr ratio=0.7041 and 143Nd/144Nd 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: [email protected] (E. Koester).

Lithos 82 (2005) 471–484

www.elsevier.com/locate/lithos

1. Introduction

Mantle and crustal xenoliths have been describedin the Andean region associated with alkalinemagmatism related to the subduction of Nazca andAntarctic plates beneath the South American Plate.Crustal xenoliths present in these areas are mainlygranulites, hornblendites, pyroxenites, and gneisses,and represent the lower crustal rocks (e.g. Selvertsoneand Stern, 1983; Weber et al., 2002). Mantlexenoliths, representing the lower and upper litho-spheric mantle, have also been described. They aregenerally 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. PaliAike, Stern et al., 1999; Praguaniyeu, Ntaflos et al.,2002) (Fig. 1).

Mineralogical and geochemical data of crustallithologies constitute a powerful tool to the under-standing of crustal growth models. Mineralogy,chemistry, geophysics, and petrology of mantlelithologies, on the other hand, allow the knowledgeof pressure and temperature conditions for the stabilityof mineral phases, the characterisation of the sourcesof mantle-derived magmas, and the detection ofpossible enrichment processes, all of which still needto be better studied and constrained.

The Mercaderes region in SW Colombia (Fig. 2Aand B), located in the Northern Volcanic Zone (NVZ;Thorpe, 1982), is a key area used to provideinformation for the understanding of the mantleevolution model in the NVZ, once garnet-bearingperidotite and pyroxenite xenoliths are common inthis area (Weber, 1998). New Sr and Nd isotope data,together with field, petrographic, and geochemicalwhole-rock and mineral geochemical data, are used ina discussion of mantle and crustal models for theregion.

2. Geological setting

The geological evolution of the ColombianAndes (Fig. 2A and B) has been interpreted as acomposite margin made up of successively accretedterranes and oceanic island arc sequences fromPalaeozoic to Miocene (McCourt et al., 1984; Weber

et al., 2002). The most important events took placeduring Devonian–Carboniferous and Cretaceoustimes (Restrepo and Toussaint, 1988). At least fiveigneous episodes were proposed by Aspden et al.(1987) for the Central Cordillera of the ColombianAndes, of which the Jurassic, Cretaceous, andNeogene episodes are well-represented, and con-tributed to major crustal and lithospheric growth ofthe region. Thus, this region was interpreted as anedge of a periodically active convergent marginsince the Palaeozoic, where different events of

NazcaPlate

South AmericanPlate

Scotia PlateAntarctic

Plate

9 cm/y

7,8cm/y

1,5cm/y

2 cm/y

South American PlatformAndean

cordillera

AR

AR

500 km

Patagonia

NVZ

CVZ

SVZ

AVZ

Volcanic gap

Flat-slab segment40˚W60˚W

0˚

20˚S

40˚S

Flat-slab segment

Flat-slab segment

80˚W

Fig. 2a

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) 471–484472

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 Gonzalez et al., 1988).

A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 473

continental growing and mantle modification wererecognised.

To the west of the Mercaderes area (Fig. 2A andB) lies the Cauca–Almaguer Fault (Maya andGonzalez, 1995), which separates rocks of oceanicaffinity to the west from rocks of continental affinityto the east. The oceanic rocks formed by extensiveand continuous igneous activity during the Creta-ceous, resulting in the formation of the Caribbeanoceanic plateau (Kerr et al., 1996, 1997). This suturepossibly represents a subduction zone of continentalocean type, which was possibly bjammedQ by thethick, buoyant plateau, enabling it to migrate oncemore to the west.

The Mercaderes region (Fig. 2A and B) ischaracterised by late Cenozoic to Pleistocene volcanicactivity, where the Mercaderes Tableland comprisesPleistocene volcanic and volcano-sedimentary flows.On the south-eastern part of this tableland lies theGranatifera Tuff, which is possibly a small, partiallyeroded tuff cone or tuff ring, containing xenoliths ofboth crustal (e.g. diorite, granulite, hornblendite) andmantle (e.g. garnet-bearing peridotites) origin (Weberet al., 2002).

The oldest rocks in this region are metasedimentsfrom the Arquıa Complex (Maya and Gonzalez,1995). Their metamorphic age (K/Ar dates onhornblende from amphibolites and the metagabbros)is Cretaceous, but two thermal events of 120 and 95Ma are indicated, which leads to two interpretations:a) they are Mesozoic rocks that have sufferedsubsequent metamorphism (Restrepo and Toussaint,1988); or b) they are Palaeozoic rocks that werethermally affected in the Cretaceous (McCourt et al.,1984; Maya, 2001). Metavolcanic and metasedimen-tary rocks from Diabasico Group showing Cretaceousage, based on fossiliferous associations and fieldrelations, overlie this unit (Murcia and Cepeda, 1991;Kerr et al., 1997; Nivia et al., 1997). The thick, foldedsequences of marine and continental Esmita andMosquera formations discordantly overlie the Diaba-sico Group. They have, respectively, an UpperOligocene age and a Middle Eocene up to LowerMiocene age, defined by fossil record (Murcia andCepeda, 1991; Martınez and Garcıa, 1989; Gonzalezet al., 1988 and references therein). Dacitic andandesitic rocks of 13F3 Ma (whole rock, K/Ar)intrude these Tertiary sedimentary rocks (Murcia and

Cepeda, 1991). Overlying the Esmita and MosqueraFormations are the pyroclastic rocks of the GaleonFormation (Martınez and Garcıa, 1989; Murcia andCepeda, 1991).

Three main subdivisions were proposed for theGranatifera Tuff (Martınez and Garcıa, 1989): i) UnitA, the basal unit, 200 m thick, formed by breccias,agglomerates, and tuffs, with clasts of porphyriticandesites, quartzites, schists, amphibolites, garnetgranulites, and eclogites; ii) Unit B, less than 45 mthick, 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), debrisflow (40 m), and tuffs (5 m), containing clasts ofdiabase, andesite, schists, quartzites, and pumice,which are b1 m in size.

3. Petrography

The Granatifera Tuff in the Mercaderes regionhosts mantle and crustal xenoliths, with up to 20 cm indiameter (Weber, 1998). Garnet-bearing rocks, rang-ing from peridotite to websterite, are the mostcommon mantle xenoliths. Garnet-free websteriteand spinel-bearing peridotite mantle xenoliths arepresent in minor amounts. Lower crustal xenolithscomprise a variety of amphibolites, pyroxenites,granulites, and gneisses, metamorphosed into theamphibolite to granulite facies. The modal composi-tion of the studied mantle and crustal xenoliths ispresented in Table 1, and photomicrographs areshown in Fig. 3.

3.1. Host rocks

The host rocks consist of breccias, and tuffs of theGranitifera Tuff. The xenoliths are found as clasts(up to 12 cm for mantle and up to 20 cm for crustalxenoliths) immersed in the tuffaceous matrix orinside lamprophyre and andesite fragments andbombs in the breccias. Lamprophyres are character-ized by porphyritic texture given by up to 0.8 mmamphiboles, and a groundmass composed of plagio-clase, amphibole, and pyroxene. Andesites aremassive with light gray color, containing up to 0.5cm long phenocrysts of plagioclase, and up to 0.4

A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484474

cm long amphibole immersed in a matrix ofplagioclase, amphibole, and pyroxene. Next to somecontacts between xenoliths and host andesite, andaround some altered pyroxene xenocrysts, theandesite presents nepheline aggregates, which areproducts of interaction between these two rock types.Diorites and schist are also found as centimetricfragments in the Mercaderes region, but they containneither mantle nor lower crustal xenoliths.

3.2. Mantle xenoliths

Garnet-bearing mantle xenoliths are the mostcommon mantle rocks in the Mercaderes region. Theyare hydrous garnet-bearing websterite and hydrousgarnet-bearing lherzolite xenoliths, presenting proto-granular texture according to the classification of

Mercier and Nicolas (1975). They are characterizedby the presence of coarse-grained pyroxenes (ingeneral, 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, inwhich these minerals are immersed in a fine-grainedmatrix (b1 mm) composed by the same mineralassemblage. Vermicular spinel (b1 mm), amphibole,pyroxenes, and serpentine are secondary mineralspresent as kelyphytic rims surrounding pyroxenes andolivines in these xenoliths as a result of metasomatism(fluid percolation), while garnet presents comminu-tion of grains in their borders. Millimetric veins filledwith serpentine are present, cutting all primaryminerals. In the xenolith–host rock contact, someolivine and pyroxene crystals are zoned possibly dueto diffusion processes, and some of them are recrystal-lised into fine-grained aggregates with mosaic shapes.

Garnet-free websterite xenoliths are also present inminor amounts in the Mercaderes region. Theycomprise pyroxenes (in general, Cr-endiopside andenstatite) showing protogranular textures, followingthe 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 modalmetasomatic event.

In this paper, we work just with the garnet(Fspinel)-bearing and spinel-bearing websterite, withor without amphibole.

3.3. Crustal xenoliths

Lower crustal xenoliths have a wide composi-tional variation in the Mercaderes region, andinclude amphibolites as the most abundant rocktype, with subordinate pyroxenites, granulites, andorthogneisses. Garnet-bearing amphibolites andpyroxenites, containing felsic phases such as feldsparand/or scapolite, are the dominant crustal xenoliths.They have brown hornblende or clinopyroxene andgarnets as the main mineral phases, and titanite andapatite as accessory minerals, showing granoblastictextures. 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

granoblastic texture. Gneisses are mainly bandedorthogneisses, with felsic and mafic millimetre-sizedbands composed of plagioclase and quartz, andbiotite and amphibole, respectively. Garnet, epidote,and scapolite are also present in a few samples.Apatite, zircon, titanite, and opaque are the mainaccessory minerals.

4. Whole-rock chemistry

Whole-rock major and trace elements concentra-tions were determined by X-ray Fluorescence at theLaboratorio de Geoquımica of the Instituto de Geo-ciencias, Universidade de Sao Paulo (Brazil). Rareearth elements (REE) and some trace element analysesof mantle xenoliths were performed by ICP-MS at theActivation Laboratories—Actlabs (Canada). Resultsare listed in Table 2 and shown in Figs. 4 and 5, inwhichanalyses from Weber (1998) are also plotted forcomparison.

4.1. Host rocks

Lamprophyres and andesites are characterised bysimilar Al2O3 and CaO (around 17 wt.% and around 7wt.%, respectively), MgO around 3.12 wt.% forandesite and 5.87 wt.% for lamprophyre, and Na2O/K2O ratios between 2.37 for andesite and 4.17 forlamprophyre. The Na2O ratios for the lamprophyresuggest that it is alkaline following Rock’s (1990)classification. High Ba contents (around 669 and 336ppm for andesite and lamprophyre, respectively) arealso characteristic of these rocks. Cr is enriched inlamprophyre (183 ppm) when compared to theandesite (17 ppm). Andesite and lamprophyre alsoshow strong fractionated REE patterns, with LaN from0.02 to 9 and LuN from 0.6 to 10.

4.2. Mantle xenoliths

The garnet-bearing mantle xenoliths from theMercaderes region are characterised by two distinct

D)

B)A)

C)

Grt

Cpx

CpxPl

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) 471–484476

chemical groups: Group I encompasses the high-MgO-content (N32 wt.%) garnet-bearing websteritexenoliths, with high modal contents of orthopyrox-ene (N60 vol.%), and Group II encompasses low-

MgO-content (b32 wt.%) garnet-bearing websteritexenoliths with low modal contents of orthopyroxene(b40 vol.%). The garnet-free mantle xenolithspresent 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

0 10 20 30 40MgO (wt%)

40

45

50

55

60

SiO

2 (w

t%)

0 10 20 30 40MgO (wt%)

0

0.5

1.0

1.5

2.0

K2O

(wt%

)

0 10 20 30 40MgO (wt%)

5

10

15

20

25

Al2O

3 (w

t%)

00 10 20 30 40

MgO (wt%)

0.5

1.0

1.5

2.0

2.5

TiO

2 (w

t%)

0

0 10 20 30 40MgO (wt%)

5

10

15

20

25

CaO

(wt%

)

00 10 20 30 40

MgO (wt%)

0.10.20.30.40.5

P2O

5 (w

t%)

0

0.60.7

0 10 20 30 40MgO (wt%)

123

45

Na2

O (w

t%)

0

6

0 10 20 30 40MgO (wt%)

4

8

12

16

20

Fe2O

3 (w

t%)

0

0 10 20 30 40MgO (wt%)

0

400

800

1200

1600

Sr (p

pm)

0 10 20 30 40MgO (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) 471–484478

other major element contents are similar in allxenoliths, except for the garnet-bearing websteritexenoliths 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 (b0.9ppm), Nb (b4 ppm), and Zr (b37 ppm) contents,and moderate Pb contents (6–18 ppm). Somevariations are mainly related to the presence ofcertain minerals such as orthopyroxene, whichincreases whole-rock contents of Cr. Three patterns(Fig. 5) of chondrite-normalised REE are observedfor 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 (samplesXM4 and XM5); and iii) enrichment of middle REErelated to light and heavy REE (samples XM3 andXM8). These patterns partially reflect mineralogicalcomposition. Enrichment of heavy REE is related tothe presence of garnet, while the enrichment ofmiddle REE is related to the presence of amphibole.These relations are not straightforward, but the

garnet/amphibole proportion seems to define theREE pattern.

4.3. Crustal xenoliths

The lower crustal xenoliths from the Mercaderesregion are characterised, when compared to mantlexenoliths, by lower MgO contents (b14 wt.%), andhigher TiO2 (N0.25 wt.%), Na2O (N1 wt.%), and P2O5

(N0.20 wt.%) contents, and similar contents for theother major oxides. They present higher Sr (N400ppm), Nb (N5 ppm), and Zr (N100 ppm) contents andlower Cr (b10 ppm) and Ni (b5 ppm) contents thanthe mantle xenoliths. Chondrite-normalised REEpatterns (data from Weber, 1998) for lower crustalxenoliths (Fig. 5) are variable and depend on thelithology. Crustal xenoliths with garnet–pyroxenitecomposition are expressively enriched in heavy REEand display a pattern similar to some of the garnet-bearing mantle xenoliths. However, the light REEcontents of the crustal xenoliths are also expressivelylower 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

Sam

ple/

Cho

ndrit

e

Ho Er Tm Yb Lu

XM2

XM1XM4

XM5

XM8

XM6

XM7

XM3

XM1

XM3

XM2

XM4

XM5XM8

XM6XM7

dacitegarnet gneiss

Mantle xenolithsCrustal xenolithsHost rocksgarnet pyroxenitesamphibolitesdiorites

andesiteslamprophyre

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

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 tob200 mesh. Before dissolution, the mineral samplewas washed in warm 2.5 N HCl to remove surfacecontamination. Each sample was properly spiked(with mixed 87Rb/84Sr and 149Sm/150Nd tracers) andprocessed using standard dissolution procedures withHF, HNO3, and HCl in Teflon vial, and warmed on ahot plate until complete material dissolution. Columnprocedures used cationic AG50W-X8 resin (200–400mesh) in order to separate Rb, Sr, and REE,followed by Sm and Nd separation using anionicLN-B50-A resin (100–150 Am). Each sample wasdried to a solid and then loaded with 0.25 N H3PO4

on appropriate filament; single Ta for Rb, Sr, andSm; and triple Ta–Re–Ta for Nd. The samples wererun in a VG Sector 54 thermal ionisation massspectrometer at the Laboratorio de Geologia Iso-

topica, Universidade Federal do Rio Grande do Sul(Brazil), in static mode. Nd and Sr ratios werenormalised to 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219 respectively. Measurements for the Sr NISTstandard NBS-987 gave 87Sr/86Sr=0.710260F0.000014, and for the Nd La Jolla, standard values of143Nd/144Nd=0.511859F0.000010. Total blanks aver-aged b750 pg for Nd and Rb and b150 for Sm and Sr.Standard errors percentual (1dm%) for 87Rb/86Sr and147Sm/144Nd were F1% or smaller, based on inter-active sample analysis and spike recalibration, andb0.0057% for 87Sr/86Sr and 143Nd/144Nd ratios. Theerrors are presented as standard deviation for 87Sr/86Srratios and in parts per million for 143Nd/144Nd ratios.Results are listed in Tables 3 and 4, and illustrated inFig. 6.

5.2. Results

A lower crustal xenolith sample with dioriticcomposition shows the highest Rb (68 ppm) and Sr(1262 ppm) contents among the analysed rocks. Smand Nd values for this xenolith are 8 and 39 ppm,the 87Sr/86Sr ratio=0.749, and the 143Nd/144Nd ratio=0.5128. The other lower crustal xenoliths plot closeto 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 87Rb/86Sr 87Sr/86Sra 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 86Sr/88Sr=0.1194, fitted to bias with base on SrCO3 NBS-987, using87Sr/86Sr=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 88Sr, and multicollection with 86Sr in the axial collector.

A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484480

from the Mercaderes region. However, crustalxenoliths are richer in radiogenic Sr compared tomantle xenoliths.

Two host-rock volcanic samples (andesite andlamprophyre) show Rb values b18 ppm, Srb561ppm, Smb4.5 ppm, and Ndb19 ppm having 87Sr/86Sr

0.7080.7070.7060.7050.7040.7030.7020.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.5134

0.5136

87Sr/86Sr

143

Nd/14

4 Nd

Sp-Grt-PAmantle xenoliths

Sp-PAmantle xenoliths

PA mantlexenoliths

Mercaderesmantle 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

BSEBSEBSE

Lamprophyres and andesites

LamprophyreAndesite

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) 147Sm/144Nd 143Nd/144Nda 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 146Nd/144Nd=0.7219, fitted to bias with base on the Nd SPEX using suggested 143Nd/144Nd=0.511110, and calibrated against Nd

La Jolla using a value of 143Nd/144Nd of 0.511859F0.000010.a Whole-rock average of F100 isotopic ratios, 1.0 V of ionic intensity for 146Nd, and multicollection with 146Nd in the axial colector.b - garnet sample from XM-7 mantle xenolith.

A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484 481

values of 0.7045 and 0.7059, and143Nd/144Nd of0.5125 and 0.5128. The lamprophyre has moreevolved radiogenic Sr than andesite and the garnet-and spinel-bearing mantle xenoliths.

The studied mantle xenoliths have low Rb (b0.9ppm), Sr (b96ppm), Sm (b3.2 ppm), andNd (b3.3 ppm)contents. Five garnet-bearing peridotite xenoliths andtwo websterite xenoliths show 87Sr/86Sr ratios between0.7029 and 0.7044, and 143Nd/144Nd ratios between0.5127 and 0.5134. Three lower crustal xenoliths(hornblendite, granulite, and pyroxenite) presentRbb6.1 ppm, Srb713 ppm, Smb12 ppm, and Ndb65ppm. Their 87Sr/86Sr ratios range between 0.7044 and0.7054, and 143Nd/144Nd ratios from 0.5128 to 0.5130.

A garnet sample from a spinel garnet-bearingwebsterite xenolith (sample XM-7) shows high Smand Nd contents of 1.1 and 7.9 ppm, respectively,comparedwith the contents in theothermantlexenoliths(except for the sample XM-8). A Sm–Nd isochron ageof 1031F130 Ma was obtained for this sample (notshown). This age is older thanTDMages for crustal rocks(b700Ma) in this region and so the interpretation for thisage is unclear. It could be interpreted either as mantle-growing ageor the ageof a secondary event that affectedthis garnet, such as metasomatism or melting percola-tion. Furthers studies in other mantle xenoliths in theMercaderes region will provide more information thatcan shed some light into this problem.

The Mercaderes garnet-bearing peridotite xenolithsplot within the oceanic basalt field (OIB) in the87Sr/86Sr vs. 143Nd/144Nd diagram (Fig. 6), towardsthe Bulk Silicate Earth (BSE) values, or more radio-genic Sr isotopic compositions. Only one sample has adistinct signature as it has higher 143Nd/144Nd valuescompared to other garnet-bearing peridotite xenoliths.The isotopic composition of one spinel-bearingperidotite xenolith plots in the field of MORB, whileanother sample plots away from this field. The highradiogenic Sr in this sample is probably related to itshigh CaO contents (20%). Thus, an MORB signatureis suggested for spinel-bearing peridotite xenoliths,while an OIB signature is evidenced by the garnet-bearing peridotite xenoliths.

Lower crustal xenoliths show more radiogenic Srcompositions compared to those for the mantlexenoliths. All analysed samples plot near the field ofcrustal xenoliths from Mercaderes studied by Weber etal. (2002). Large variation in the 87Sr/86Sr ratios for

these xenoliths suggests that the lower crust under theMercaderes region is isotopically heterogeneous.

The values of qNd (t=0) for the studied mantlexenoliths range from 2.4 to 16.5 and confirm thedepleted isotopic composition of these rocks. Crustalxenoliths have positive qNd (t=0) values, rangingfrom 2.4 to 6.0, suggesting the presence of ortho-derived material in the lower crust, while for the hostvolcanic rocks, qNd (t=0) values are 0.3 and 3.2. Thehigher value is given by a lamprophyric sample.

Nd model ages (TDM; De Paolo, 1981) for thelower crustal xenoliths indicate an extraction agevarying from 0.3 to 0.4 Ga, which attests to crustalgrowth in this area at this time. However, TDM for thelamprophyre and andesite xenoliths ranges from 0.6 to0.7 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 GranatiferaTuff, Colombia, provide valuable information usefulto the discussion of the lithospheric mantle and thecrustal evolution of the Mercaderes region. Sm–Ndand Rb–Sr isotopic systems integrated with fieldrelationships, geochemistry data, and pressure–tem-perature estimates for the garnet-bearing mantlexenoliths are compatible with two distinct mantlereservoirs, which reflect the mantle diversity beneaththe Northern Colombian Andes.

Estimates of pressure and temperature (Weber,1998) show three main P–T conditions for the mantlexenoliths. The spinel-bearing peridotite xenolithswere formed at low P (16 kbar, 1065 8C), whereasthe garnet-bearing peridotite xenoliths were formed athigh P and T at two different conditions: sub-oceanicgeotherm (29–35 kbar, 1250–1295 8C) and sub-continental geotherm (N38 kbar, 1140–1175 8C).Pressure and temperature estimates for the lowercrustal indicate that they were formed at 730–8308C at 9–14 kbar for amphibolites, and at 950–1050 8Cat 13–15 kbar for all other rocks (Fig. 7).

The garnet-bearing xenoliths represent deeperfragments (around 90 km) and spinel-bearing peri-dotite xenoliths are fragments of upper lithosphericmantle (40 km), as suggested by Weber (1998). Someof the garnet-bearing peridotite xenoliths derived from

A. Rodriguez-Vargas et al. / Lithos 82 (2005) 471–484482

a mantle source with OIB isotopic signature in highP–T conditions, similar to that of a sub-continentalgeothermal gradient. This is compatible with theconvergent tectonic setting of the region that is the

active subduction of the Nazca Plate under the SouthAmerican plate. Other groups of garnet-bearing peri-dotite xenoliths derived from a source with lower P andhigher T, similar to that of a sub-oceanic geothermalgradient; the source has Sr isotopic ratios similar tothose of the other reservoir, but distinct Nd isotopicsignature. This enrichment in radiogenic Nd, sugges-tive of a different isotopic reservoir, could be related toa process of chromatographic isotopic separation thatwould lead to an increase in the Nd concentrationswithout disturbance in Sr values. Spinel-bearingperidotite 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 beenreported for rocks outcropping in the area.

The andesite volcanic host rocks resulted frompartial melting of a source that has isotopic signaturesimilar to that of the BSE continental plateau basalts,but the position of the lamprophyre sample in Fig. 6suggests some contributions of a subducted slab thathas contaminated the mantle source.

Mantle xenoliths from the Mercaderes region andfrom the Pali Aike region, southernmost ChileanAndes (Stern et al., 1999), include garnet- and spinel-bearing xenoliths, but their isotopic signatures arequite distinct. Garnet-bearing mantle xenoliths of PaliAike present lower 87Sr/86Sr ratios and less depleted143Nd/144Nd ratios, compared with the Mercaderesxenoliths, approaching the Nd and Sr isotopiccompositions of HIMU. The Pali Aike spinel-bearingperidotite xenoliths are Sr-enriched in comparisonwith similar rocks from the Mercaderes area, exceptfor the sample XM-5, which is the most enriched inCaO, suggesting some contamination by fluids oralteration. Pressure and temperature estimates for thegarnet- and spinel-bearing mantle xenoliths in PaliAike area are also distinct, with temperatures rangingfrom 970 to 1160 8C and pressures between 19 and 24kbar (Stern et al., 1999). Thus, in terms of lithospheremantle evolution, these two regions present a MORB-like signature (for spinel xenoliths), but an additionalOIB-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-rockisochron age. This age is older than other mantle

600 700 800 900 1000 1100 1200

5

10

15

20

P (k

bar)

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

Dry Peridotite solidus

Sub-oceanic geotherm

Sub-continental geotherm

800 1000 1200 1400

5

10

15

20

25

30

35

40

T (˚C)

P (k

bar)

Garnet peridotites/pyroxenites

Spinelperidotites

Mantlexenoliths

Lower crustalxenoliths

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

extraction ages (TDM) in the Mercaderes region andsuggests a metasomatic or melting event, demonstrat-ing a long history of mantle evolution in this area.Santos et al. (2000) described a geological event ofthis age (1.33–0.99 Ga) in the nearby SunsasProvince, and interpreted as an event of recycling ofcontinental crust during the Greenvillian Orogeny.

Acknowledgements

We gratefully acknowledge Farid Chemale Juniorfor his support and comments on various aspects oflaboratory studies. The manuscript benefited fromconstructive reviews by A. Giret and an anonymousreviewer. R. Rupp is warmly thanked for the Englishreviews, and V.P. Ferreira, A.N. Sial, and I. McReathfor editorial improvements. This work was funded byPROSUL-CNPq (project AC-74).

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