The amphibolites from the Ossa–Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation

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Lithos 68 (2003) 23–42

The amphibolites from the Ossa–Morena / Central Iberian

Variscan suture (Southwestern Iberian Massif):

geochemistry and tectonic interpretation

M.T. Gomez-Pugnairea, A. Azorb,*, J.M. Fernandez-Solera,V. Lopez Sanchez-Vizcaınoc

aDepartamento de Mineralogıa y Petrologıa, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s/n,

E-18002 Granada, SpainbDepartamento de Geodinamica, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s/n, E-18002 Granada, Spain

cDepartamento de Geologıa, Universidad de Jaen, E.U. Politecnica de Linares, C/ Alfonso X el Sabio, 28, E-23700 Linares, Spain

Received 15 March 2002; accepted 27 January 2003

Abstract

Basic rocks included in the Ossa–Morena/Central Iberian Variscan suture of SW Iberia have been studied in order to

decipher the pre-orogenic evolution along this major boundary. These rocks appear as garnet-free amphibolites and garnet-

bearing amphibolites. Previous geochronological data on these basic rocks suggest the existence of two different protolith ages:

Upper Precambrian and Lower Palaeozoic (Ordovician). Upper Precambrian amphibolites form small-size (up to several metres

thick), lense-, dike- or elongated-shaped bodies, while Lower Palaeozoic ones form elongated bodies with thicknesses up to

several hectometres.

The chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic

magmas. As for the originally basaltic rocks, their geochemical characteristics indicate a shallow melting with different

proportions of at least three end-members, namely (i) a MORB-like asthenospheric source, (ii) a more enriched (plume-derived

or subcontinental lithosphere) mantle source, and (iii) a continental crust component. Moreover, Upper Precambrian and Lower

Palaeozoic amphibolites have different geochemical affinities. In the former group, we envisage an advanced intra-continental

rifting situation or an intra-continental back-arc setting related to the Cadomian orogeny, while for the latter, we propose an

environment of oceanic crust formation predating Variscan subduction and collision. The recognition of these remains of

oceanic-affinity basic rocks along the OMZ/CIZ boundary allows us to propose that some sort of ocean may well have existed

between the two zones during Early Palaeozoic times, and that this ocean was presumably closed at a later stage, probably

during the Devonian, by oceanic subduction under the CIZ crust.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: MORB magmas; Late Precambrian intra-continental rifting; Early Palaeozoic oceanic crust; Variscan belt

0024-4937/03/$ - see front matter D 2003 Elsevier Science B.V. All right

doi:10.1016/S0024-4937(03)00018-5

* Corresponding author. Tel.: +34-958-242900; fax: +34-958-

248527.

E-mail address: [email protected] (A. Azor).

1. Introduction

Metamorphosed basic and ultrabasic rocks are of

substantial importance in establishing the tectonic

s reserved.

M.T. Gomez-Pugnaire et al. / Lithos 68 (2003) 23–4224

evolution of mountain belts. These rocks usually

mark those places where subduction of oceanic litho-

sphere took place before the collision of two con-

tinents. Four different aspects need to be addressed in

order to understand the significance of basic rocks in

the orogens: (i) ages of protolith and metamorphism,

(ii) geochemistry, (iii) metamorphic evolution, and

(iv) structural evolution. In particular, the geochem-

ical features of these rocks, together with the protolith

age, can be used to characterize the pre-collisional

history of the orogens in terms of the tectonic

scenario where the basic rocks were formed, e.g.

large or narrow ocean, back-arc basin, continental

rift, subduction zone, etc. In this respect, the recog-

nition of remains of ophiolitic sequences is of partic-

ular interest. The study of the metamorphic and

structural evolution of basic rocks provides infor-

mation on possible subduction events responsible

for the closure of an intervening ocean, as well as

on the subsequent exhumation of these rocks after the

collision.

In the case of Palaeozoic and older orogens, the use

of basic rocks in the reconstruction of pre-collisional

tectonic settings is much more difficult than in the case

of more recent ones. The difficulties in question arise

from a number of uncertainties concerning the age of

the protoliths along with complicated metamorphic and

structural evolutions—sometimes polyorogenic— that

may obliterate the primary geochemical signatures of

the rocks. Despite these limitations, careful and system-

atic geochemical studies on basic rocks, together with

other geological and geophysical data, can be regarded

as the basic tools to produce well-supported and

coherent plate tectonic reconstructions.

Basic rocks in the Central Unit, one of the suture

contacts of the SW Iberian Massif, are relatively well

known, especially from the standpoint of their meta-

morphic and structural evolution. However, their geo-

chemical features have not received due attention in

the literature so far. This paper aims to study the

magmatic significance of these basic rocks, which

were strongly deformed and metamorphosed under

high-pressure conditions (Abalos et al., 1991; Lopez

Sanchez-Vizcaıno et al., 2003) during the Variscan

orogeny (Azor et al., 1993, 1995; Ordonez Casado,

1998). First, we will outline the geochemical features

of these rocks, and then will go on to address their

likely significance and tectonic implications in the

context of their location along one of the main boun-

daries of the Variscan belt in the Iberian Peninsula.

2. Geological setting

The southwestern Iberian Massif comprises three

zones separated by major tectonic boundaries (Fig. 1a):

the South Portuguese Zone (SPZ), the Ossa–Morena

Zone (OMZ) and the Central Iberian Zone (CIZ). The

SPZ/OMZ contact has been considered to be a Variscan

suture according to the existence of basic igneous rocks

(amphibolites and basalts) with oceanic affinity (Bard,

1977; Dupuy et al., 1979; Munha, 1983; Munha et al.,

1986).However, the lack of high-pressuremetamorphic

assemblages within the basic rocks (Bard, 1977) may

indicate a very limited subduction along this contact.

The OMZ/CIZ contact (Fig. 1b) has been located

along an important left-lateral shear zone, namely, the

Badajoz–Cordoba Shear Zone (Burg et al., 1981). In

terms of its metamorphic and tectonic evolution, this

contact has been considered to be one of the sutures of

the Variscan orogen in the Iberian Peninsula (Matte,

1986; Azor et al., 1994; Simancas et al., 2001). The

boundary itself is marked by a complex unit, namely,

the Central Unit (Azor et al., 1994), made up of

orthogneisses, amphibolites and metasediments. More-

over, Ordovician–Devonian stratigraphic and palae-

ontological differences at both sides of this boundary

(Robardet, 1976; Robardet and Gutierrez Marco, 1990)

suggest the possible existence of an ocean between the

OMZ and CIZ in Lower Palaeozoic times. Neverthe-

less, oceanic subduction prior to collision along this

suture has not yet been proved. In this regard, the

existence of sheet-shaped strongly deformed bodies

of basic rocks included in this contact may well prove

useful to confirm or discard the hypothesis of an

oceanic subduction along this suture. Thus, the geo-

chemical characterisation of these basic rocks, which is

the target of our study, can contribute to the deciphering

of the pre-collisional Early Palaeozoic evolution of the

OMZ/CIZ suture.

The amphibolites crop out as metre- to hectometre-

thick bodies parallel to the foliation and intercalated

within orthogneisses. The rocks are garnet-free

amphibolites and garnet-bearing amphibolites (see

Lopez Sanchez-Vizcaıno et al., 2003, for a detailed

petrographic description). Both types of rocks under-

Fig. 1. (a) Schematic geological map of the Iberian Massif with the main zones and tectonic boundaries of the Variscan belt. (b) Very simplified geological map of the boundary

between the Ossa–Morena and the Central Iberian Zones; asterisks depict the localities where the amphibolites were sampled. (c) Geological cross-section across the Ossa–Morena/

Central Iberian contact, showing the lithological succession and the structural position of the Central Unit underneath the Central Iberian Zone; location in (b).

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Table 1

Major (wt.%) and trace element (ppm) contents of Upper Precambrian amphibolites (Group 1)

Sample AA-274 AA-364 AA-367 AA-9B AA-9A BCA-1C BCA-1D BCA-2B CA-2 CM-1 CM-2 CM-4 CM-5b CM-7b CPR-2 CPR-12 CPR-14

Group 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

SiO2 48.21 44.48 43.28 49.55 49.30 46.97 49.98 47.85 43.92 50.87 48.80 48.64 47.79 49.45 41.00 49.31 51.03

TiO2 3.14 3.34 1.40 2.38 2.48 3.06 2.84 2.66 1.56 1.54 2.47 1.90 2.08 1.46 3.43 2.88 2.54

Al2O3 14.06 12.73 19.59 15.68 15.70 14.47 15.07 14.87 18.89 16.59 15.57 16.81 16.15 14.90 12.56 14.85 14.54

Fe2O3 14.83 15.77 15.31 12.83 12.75 14.41 11.01 13.58 14.61 11.33 11.97 11.65 12.13 12.33 20.11 11.99 13.63

MnO 0.27 0.22 0.18 0.25 0.22 0.27 0.24 0.22 0.18 0.18 0.15 0.20 0.19 0.17 0.42 0.18 0.25

MgO 4.96 5.98 5.98 6.22 6.81 5.89 5.70 5.86 6.24 5.62 6.34 7.16 7.21 8.23 4.22 5.74 5.35

CaO 9.02 11.79 11.75 8.51 7.43 9.46 9.56 10.07 11.63 7.89 10.46 9.92 9.62 10.58 11.41 8.25 7.43

Na2O 4.57 3.03 1.89 3.54 3.66 4.21 4.41 3.99 2.20 4.23 3.63 3.05 4.12 2.65 4.44 4.78 4.33

K2O 0.54 0.71 0.57 0.77 1.38 0.78 0.75 0.48 0.71 1.52 0.33 0.48 0.46 0.21 0.73 1.63 0.60

P2O5 0.42 1.96 0.06 0.27 0.25 0.47 0.43 0.41 0.06 0.24 0.28 0.20 0.23 0.01 1.68 0.39 0.29

Total 101.47 100.39 100.00 100.12 99.48 100.26 100.88 100.56 100.04 99.41 100.34 99.55 99.64 99.48 100.47 100.97 100.31

XMg 0.40 0.43 0.44 0.49 0.51 0.45 0.51 0.46 0.46 0.50 0.51 0.55 0.54 0.57 0.29 0.49 0.44

Li 3.5 4.3 16.6 22.9 46.8 14.4 17.8 7.6 18.2 12.3 3.1 9.9 7.6 5.4 10.3 27.8 11.3

Rb 10.6 18.5 11.2 20.4 41.1 15.3 17.7 8.6 16.6 57.2 5.9 20.0 10.8 6.4 11.7 41.6 13.6

Cs 0.2 0.2 0.6 1.8 3.6 0.6 2.0 0.3 0.8 1.4 0.4 0.8 1.0 1.4 0.2 0.5 0.4

Be 1.7 0.4 0.5 1.1 1.5 1.1 1.5 1.6 0.7 1.2 1.3 1.2 0.9 0.3 3.1 1.4 1.8

Sr 232.8 175.6 564.1 256.5 274.0 255.9 432.3 207.5 487.2 321.1 371.9 317.7 275.7 183.9 230.1 360.9 212.8

Ba 151.1 159.3 124.5 201.1 289.1 146.5 160.5 117.5 110.6 332.6 106.5 112.6 112.2 41.9 172.1 538.7 1777.6

Sc 45.8 32.1 34.9 39.2 40.7 39.7 40.6 47.2 36.1 31.3 43.7 40.0 34.6 45.0 35.5 42.5 39.9

V 385.8 437.3 401.7 281.8 277.1 324.7 330.2 346.5 384.0 235.2 325.2 254.8 246.6 461.1 234.4 333.0 307.5

Cr 52.2 19.8 3.7 65.0 65.4 53.5 69.9 104.8 8.1 68.2 135.7 103.3 126.9 256.4 0.0 104.9 50.3

Co 48.2 72.1 60.4 40.6 39.2 39.2 32.1 49.8 47.9 31.0 42.2 46.3 38.9 52.6 31.3 25.5 39.0

Ni 24.7 23.4 104.2 11.6 11.9 26.9 26.0 33.3 8.1 39.7 43.0 30.5 28.4 85.3 22.9 31.7 28.2

Cu 47.2 84.6 58.0 20.1 20.4 50.1 13.5 48.1 59.2 83.4 31.2 30.9 21.8 41.4 31.8 14.2 40.5

Zn 124.5 90.7 107.3 109.6 105.3 110.6 83.8 111.3 93.7 74.2 95.9 94.1 95.3 61.4 163.9 258.9 143.2

Ga 22.6 16.6 23.6 21.7 21.7 19.7 21.5 21.8 22.4 18.3 21.6 20.4 18.9 16.5 27.2 20.6 22.7

Y 60.7 34.1 24.9 38.6 39.5 51.2 47.0 45.1 23.3 33.0 42.4 35.4 35.6 9.0 116.0 49.6 46.5

M.T.Gomez-P

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Nb 7.2 4.6 3.0 3.6 3.5 10.8 9.0 8.8 3.1 4.2 4.6 2.8 3.7 0.4 21.1 9.1 5.5

Ta 0.6 0.5 0.3 0.4 0.4 0.8 0.8 0.8 0.3 0.4 0.4 0.3 0.4 0.1 1.5 0.7 0.5

Zr 246.4 49.4 56.4 175.8 170.3 317.2 310.7 252.5 113.2 127.7 167.6 153.5 157.4 14.1 543.0 212.2 232.2

Hf 7.5 2.2 4.1 5.1 5.5 10.2 8.7 7.6 6.5 3.7 6.6 6.3 7.4 0.7 15.5 6.8 6.0

Mo 1.5 1.9 1.0 1.9 1.5 1.3 1.2 3.5 0.6 0.0 0.6 0.3 0.8 1.7 0.8 0.5 1.3

Sn 3.1 1.5 2.7 1.6 1.3 9.6 1.2 3.0 2.3 5.1 2.0 6.0 3.5 1.2 4.5 1.1 1.7

Tl 0.1 0.1 0.1 0.1 0.3 0.1 0.1 0.1 0.1 0.4 0.1 0.1 0.1 0.0 0.1 0.2 0.1

Pb 4.4 3.7 7.1 6.9 4.2 2.6 4.5 2.6 5.4 2.7 3.1 3.9 2.1 2.1 6.8 42.9 24.9

U 0.5 0.3 0.2 0.3 0.3 0.4 0.6 0.4 0.3 0.8 0.2 0.2 0.1 0.0 2.2 0.4 0.9

Th 1.8 1.0 0.4 0.6 0.6 1.4 1.2 1.1 0.4 3.6 0.5 0.4 0.5 0.1 6.5 0.9 2.8

La 17.1 11.9 6.7 9.4 9.1 16.5 15.3 14.7 6.5 12.8 11.7 6.7 7.6 1.4 44.7 14.6 11.6

Ce 42.6 29.5 18.9 25.0 24.8 43.5 38.0 38.2 18.5 28.8 27.0 20.5 21.2 3.2 118.1 35.7 30.3

Pr 6.6 4.5 3.2 4.0 3.9 6.4 5.6 5.8 3.1 4.2 4.6 3.4 3.4 0.5 16.7 5.4 4.5

Nd 31.4 23.2 15.9 19.5 19.3 29.6 25.5 27.0 15.9 18.8 22.4 17.3 16.7 2.6 77.1 25.9 20.3

Sm 8.8 6.4 4.6 5.7 5.8 8.1 6.8 7.2 4.4 5.4 6.4 5.1 5.1 1.0 20.5 7.3 6.1

Eu 2.8 2.2 1.5 2.0 2.1 2.6 2.5 2.3 1.5 1.6 2.3 2.0 1.8 0.6 6.3 2.5 2.1

Gd 10.0 7.2 4.7 6.5 6.5 9.2 8.0 7.8 4.5 6.0 7.2 6.5 5.9 1.2 23.0 8.1 7.1

Tb 1.7 1.1 0.7 1.1 1.1 1.5 1.3 1.3 0.7 1.0 1.2 1.1 1.0 0.2 3.8 1.5 1.3

Dy 10.8 6.3 4.6 7.0 7.1 9.3 8.3 8.2 4.2 6.0 7.5 7.2 6.5 1.6 22.7 8.5 8.1

Ho 2.4 1.3 1.0 1.5 1.5 2.0 1.8 1.7 0.9 1.3 1.7 1.5 1.4 0.4 4.7 1.8 1.8

Er 6.2 3.1 2.5 4.0 3.9 5.5 4.8 4.6 2.4 3.4 4.2 3.9 3.9 0.9 12.2 4.7 4.7

Tm 1.0 0.4 0.4 0.6 0.6 0.8 0.7 0.7 0.4 0.5 0.6 0.6 0.6 0.1 1.8 0.7 0.7

Yb 6.0 2.6 2.4 3.8 3.6 5.0 4.6 4.3 2.2 3.2 3.8 3.5 3.7 0.9 11.1 4.5 4.5

Lu 0.9 0.4 0.4 0.6 0.5 0.8 0.7 0.6 0.4 0.5 0.6 0.5 0.6 0.2 1.7 0.6 0.7

Ne 4.8 2.3 3.6 0.0 0.2 6.4 3.0 4.0 4.7 2.3 1.0 0.0 5.6 0.0 15.8 7.6 0.0

Di 21.9 22.1 12.1 13.1 10.4 21.3 21.1 21.8 14.6 12.7 20.5 14.0 18.0 19.9 28.7 19.9 13.7

Hy 0.0 0.0 0.0 6.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.2 0.0 10.7 0.0 0.0 7.6

Ol 16.0 18.2 23.5 15.1 20.8 17.1 13.0 16.5 21.9 17.2 15.8 16.7 19.0 13.2 18.3 14.5 13.0

XMg=[(Mg/(Mg + Fe2 +)] assuming Fe2O3/FeO= 0.15.

Ne, Di, Hy, Ol =CIPW-normative nepheline, diopside, hypersthene and olivine.

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Table 2

Major (wt.%) and trace element (ppm) contents of Precambrian cumulates (Group 2) and Lower Palaeozoic amphibolites (Group 3)

Sample AA-275 AA-368 BCA-6 CM-6 LC-2 LC-3 AA-217 PC-1 PC-4 PC-5

Group 2 2 2 2 2 2 3 3 3 3

SiO2 49.69 49.04 51.49 42.90 50.63 51.09 47.75 49.95 46.99 46.79

TiO2 1.75 0.39 0.39 6.54 0.72 1.17 1.12 1.63 1.78 1.70

Al2O3 15.20 18.71 16.97 15.67 17.58 13.84 15.33 12.89 17.03 14.47

Fe2O3 tot 12.91 7.76 8.47 13.55 9.27 10.65 12.03 14.92 13.37 13.63

MnO 0.17 0.13 0.13 0.23 0.20 0.18 0.25 0.27 0.20 0.20

MgO 7.19 8.67 8.76 6.42 8.02 9.36 7.47 6.84 5.32 4.77

CaO 9.86 13.02 11.55 9.71 9.69 10.30 13.04 10.04 11.39 14.99

Na2O 2.72 0.92 1.92 4.42 3.38 2.65 1.88 3.00 3.06 2.88

K2O 0.48 1.35 0.31 0.54 0.46 0.72 1.00 0.30 0.68 0.40

P2O5 0.04 0.01 0.01 0.03 0.05 0.04 0.12 0.16 0.20 0.17

Total 99.57 99.80 99.47 101.08 99.27 99.78 99.18 100.44 100.05 99.77

XMg 0.52 0.69 0.67 0.48 0.63 0.63 0.55 0.48 0.44 0.41

Li 3.2 17.4 5.5 15.5 11.4 7.2 22.2 9.3 11.7 15.3

Rb 16.6 42.8 11.1 18.7 9.5 15.9 30.0 6.4 15.2 9.0

Cs 0.6 1.3 0.4 6.0 0.5 0.7 3.6 0.1 0.4 0.4

Be 0.6 0.3 0.4 0.6 0.2 0.6 1.0 0.8 0.9 0.8

Sr 215.2 176.5 263.6 261.5 234.3 212.0 207.8 86.9 341.6 327.1

Ba 104.8 119.3 80.6 88.1 58.4 156.5 321.8 117.6 305.3 220.7

Sc 46.0 44.8 39.3 44.8 28.3 46.7 43.4 46.3 35.4 40.1

V 577.6 149.9 182.7 497.1 143.7 365.3 262.6 366.9 240.0 291.3

Cr 222.0 369.1 422.4 18.7 146.6 376.2 342.3 201.2 325.1 179.3

Co 48.1 51.5 46.2 49.7 38.3 42.1 49.9 51.6 50.9 50.3

Ni 79.7 112.0 97.7 58.9 58.0 83.3 141.8 59.4 115.2 81.0

Cu 32.4 142.5 34.2 15.7 34.3 7.4 50.5 67.2 85.5 77.5

Zn 78.2 41.8 36.9 84.2 56.3 163.6 157.3 122.7 110.0 139.9

Ga 17.6 16.0 16.2 16.8 16.4 16.2 18.7 17.3 19.3 18.6

Y 12.6 9.0 8.1 12.0 10.9 13.1 28.5 36.6 28.9 30.5

Nb 1.1 0.3 0.4 5.3 1.5 1.0 3.9 3.0 10.5 6.9

Ta 0.2 0.1 0.2 0.6 0.2 0.1 0.3 0.3 0.8 0.6

Zr 22.5 12.9 13.0 56.2 30.4 22.1 57.6 106.0 137.5 115.0

Hf 1.0 0.7 0.7 2.6 1.4 1.1 3.0 5.6 7.4 5.2

Mo 0.6 0.3 0.6 1.1 1.3 0.5 0.8 2.2 0.7 0.8

Sn 0.7 0.7 0.3 1.5 3.6 1.2 4.8 1.2 4.1 1.4

Tl 0.1 0.2 0.1 0.1 0.1 0.1 0.3 0.1 0.1 0.1

Pb 4.5 2.6 3.2 2.8 2.5 2.8 9.4 5.4 6.1 19.3

U 0.2 0.0 0.1 0.2 0.2 0.2 0.6 0.2 0.5 0.3

Th 0.5 0.1 0.1 0.3 0.6 0.3 0.4 0.3 1.3 0.7

La 3.8 1.0 1.4 3.0 3.3 1.9 4.0 4.5 10.8 7.8

Ce 6.4 2.6 3.2 6.5 7.4 4.8 10.1 12.9 25.8 19.5

Pr 1.2 0.5 0.5 1.0 1.0 0.8 1.7 2.1 3.7 2.9

Nd 5.2 2.5 2.3 4.9 4.6 4.0 8.8 11.5 16.4 14.5

Sm 1.5 0.9 0.8 1.5 1.4 1.4 2.9 3.9 4.6 4.1

Eu 0.7 0.6 0.5 0.8 0.6 0.7 1.1 1.3 1.7 1.5

Gd 1.9 1.3 1.2 1.8 1.6 1.9 3.9 5.0 5.4 5.0

Tb 0.3 0.2 0.2 0.3 0.3 0.3 0.7 0.9 0.9 0.8

Dy 2.2 1.6 1.3 2.1 1.9 2.3 4.8 6.3 5.7 5.5

Ho 0.5 0.4 0.3 0.5 0.4 0.5 1.1 1.4 1.1 1.2

Er 1.3 1.0 0.8 1.3 1.2 1.4 3.1 4.0 3.2 3.2

Tm 0.2 0.2 0.1 0.2 0.2 0.2 0.5 0.6 0.5 0.5


Table 2 (continued )

Sample AA-275 AA-368 BCA-6 CM-6 LC-2 LC-3 AA-217 PC-1 PC-4 PC-5

Group 2 2 2 2 2 2 3 3 3 3

Yb 1.2 0.9 0.7 1.3 1.2 1.4 3.0 3.7 3.1 2.9

Lu 0.2 0.1 0.1 0.2 0.2 0.2 0.5 0.6 0.5 0.4

Ne 0.0 0.0 0.0 12.1 0.0 0.0 0.9 0.0 3.5 7.1

Di 17.0 17.3 16.4 21.9 13.1 21.9 27.2 23.3 20.2 39.8

Hy 13.7 13.3 25.3 0.0 3.9 11.8 0.0 12.0 0.0 0.0

Ol 10.4 8.5 1.1 12.7 17.1 11.8 17.0 11.8 16.5 8.5

XMg=[(Mg/(Mg+ Fe2 +)] assuming Fe2O3/FeO= 0.15.

Ne, Di, Hy, Ol =CIPW-normative nepheline, diopside, hypersthene and olivine.

M.T. Gomez-Pugnaire et al. / Lithos 68 (2003) 23–42 29

went a common metamorphic evolution, characterized

by an initial eclogite-facies event with peak pressures

above 1.9 GPa and temperatures around 750 jCfollowed by a strong decompression accompanied by

cooling to amphibolite facies (T= 700F 50 jC, P=1.4F 0.2 GPa) and greenschist facies (Lopez Sanchez-

Vizcaıno et al., 2003). Estimates of the ages of the

protoliths of these amphibolites were obtained by the

Sensitive High Resolution Ion Microprobe (SHRIMP)

U–Pb method on zircons (Ordonez Casado, 1998).

Two groups with different ages and geochemical fea-

tures will be considered here for geochemical descrip-

tion: (i) Lower Palaeozoic amphibolites and (ii) Upper

Precambrian amphibolites. The Lower Palaeozoic ages

obtained by Ordonez Casado (1998) are 483F 13 Ma

for the Pocico outcrop (Fig. 1b), 490F 17 Ma for a

sample taken in Portugal near Arronches, and 525F 13

Ma for a sample from Arroyo Argallon (Fig. 1b). The

Upper Precambrian ages provided by this author are

577F 26 Ma for a sample from La Cardenchosa,

596F 14Ma for a sample from the Cerro de las Mesas,

and 566F 9 Ma for another sample from La Carden-

chosa (Fig. 1b). We must emphasize that these two

groups of amphibolites appear in the field with very

different dimensions. Upper Precambrian amphibolites

crop out as small-size bodies (some centimetres to

several metres thick) parallel to the main foliation.

By contrast, Lower Palaeozoic amphibolites at Pocico

form an 800-m-thick strongly deformed, elongated

body with lateral dimensions of several kilometres,

lacking any intercalation of orthogneissic rocks. This

means that Upper Precambrian amphibolites are likely

to represent former dikes or sills intruded in the

gneissic lithologies, while Lower Palaeozoic amphib-

olites may well represent former gabbro stocks

emplaced in the middle or lower crust.

3. Geochemistry of amphibolites

In this work we study amphibolites from seven

different outcrops (Fig. 1b) belonging to the lower part

of the sequence of the Central Unit. Five of these

outcrops occur in the southeastern part of the unit (Fig.

1b), near La Cardenchosa (samples BCA, CA and LC),

Arroyo Argallon (samples AA) and Cerro de las Mesas

(samples CM). The other two outcrops appear more

than 50 km to the northwest at Cortijo de Penarresbala

(samples CPR) and Pocico (samples PC).

Major-element determinations for the bulk rocks

were performed by X-ray fluorescence (XRF) (Philips

PW1404), calibrated against international standards of

appropriate composition, after fusion with lithium te-

traborate. Precision was typically better than F 1.5%

for a concentration of 10 wt.%. Zirconium was deter-

mined by XRF on pressed pellets, with a precision

better than F 4% at a 100 ppm level. Trace-element

determinations were done by ICP-mass spectrometry

(ICP-MS) after HNO3 + HF digestion of 100 mg

sample powder in a Teflon-lined vessel at f 180 jCand f 14 bar during 30 min, evaporation to dryness,

and subsequent dissolution in 100 ml of 4 vol.%

HNO3. Instrument measurements were carried out in

triplicate with a PE SCIEX ELAN-5000 spectrometer

using Rh as internal standard. Precision was better

than F 2% and F 5% relative for concentrations of 50

and 5 ppm, respectively.87Sr/86Sr and 143Nd/144Nd analyses were per-

formed using a Finnigan MAT 262 RPQ spectrometer

after separation by ion-exchange resins. External pre-

cision (2r) measured in 10 replicates of the standard

WS-E (Govindaraju et al., 1994) was approximately

F 0.003% relative for 87Sr/86Sr and F 0.0026% rel-

ative for 143Nd/144Nd. 87Rb/86Sr and 147Sm/144Nd

Fig. 2. (a) Geochemical classification (TAS) diagram (after Le Bas et

al., 1992) for the samples studied. The dashed line that separates

alkaline from subalkaline series is from Irvine and Baragar (1971).

(b) Zr/P2O5 vs. Nb/Y discrimination diagram (after Floyd and

Winchester, 1975); in this diagram most of the samples plot in the

tholeiitic basalt field. (c) XMg vs. TiO2 diagram, showing the

distinction between low Ti (TiO2 < 2%) and high Ti basaltic samples,

as well as the position of the samples interpreted as cumulates.


proportions were obtained by ICP-MS, with an exter-

nal precision (2r) better than F 1.2% and F 0.8%

relative, respectively (Montero and Bea, 1998).

3.1. Geochemical classification

According to previous geochronological data

(Ordonez Casado, 1998), two groups of amphibolites

can be differentiated: (i) Lower Palaeozoic amphib-

olites and (ii) Upper Precambrian amphibolites.

Tables 1 and 2 show the chemical composition (major

and trace elements) of these amphibolites. For classi-

fication and plotting purposes, major element data

have been recalculated on an anhydrous basis, with

FeO/Fe2O3 = 0.15.

On the basis of their SiO2 and alkaline contents, and

their CIPW norms, most of the protoliths of these

metabasites may be classified as basalts and trachyba-

salts (Fig. 2a), with subalkaline to moderately alkaline

affinity (with up to 5% normative nepheline). How-

ever, special care should be taken with this classifica-

tion because, as will be discussed below, in many of

the rocks analysed, substantial evidence can be found

that these can be cumulate rocks. In addition, since the

rocks have undergone at least one metamorphic event,

it can be expected that they should have suffered

significant element mobility, especially involving the

alkali and LIL elements. Therefore, the alkaline char-

acter of some of the igneous protoliths should also be

regarded with caution. A better classification can be

attempted drawing on those elements less sensible to

the metamorphic mobility, such as the high field

strength elements (HFSE). Accordingly, the low Nb/

Y ratios ( < 0.7) can be concluded to reveal a clear

tholeiitic affinity in the samples studied (Floyd and

Winchester, 1975; Fig. 2b).

3.2. Major elements

Most of the samples are characterized by moderate

to high Al2O3 (12.56–19.59 wt.%), and CaO (7.43–

13.09 wt.%) contents, and variable TiO2 (0.39–3.34

wt.%) and P2O5 contents (0.01–0.25 wt.%). On a Mg-

number [i.e. XMg =molar MgO/(MgO + FeOt)] vs.

TiO2 plot, three groups of samples can be distin-

guished (Fig. 2c): (1) a group with high Mg-number

and low to high TiO2, interpreted as cumulates (see

below), (2) high Ti metabasaltic samples (TiO2>2%),

and (3) low Ti metabasaltic samples (TiO2 < 2%). In

general, a neat correlation can be established between

TiO2 and P2O5 contents. Two samples show very high


P2O5 (1.96 wt.%) and TiO2 (6.54 wt.%) contents,

which can be explained in terms of an extreme accu-

mulation of apatite and ilmenite (and/or hornblende),

respectively. In fact, abnormal abundance of apatite

and a TiO2-rich phase (rutile) is petrographically

observed in some of these amphibolites. The Na2O

and K2O contents are variable and range between

0.92–4.78 and 0.01–1.96 wt.%, respectively. The

MgO and Fe2O3t contents range from 4.77 to 9.36,

and from 7.75 to 15.75 wt.%, respectively. There is not

Fig. 3. Selected major and trace element bivariate plots for the studied roc

different outcrops. Light shaded area marks the position of Group 2 rocks, i

of Group 3 (Lower Palaeozoic) amphibolites.

any clear regular variation of major elements with any

common fractionation index, although a rough increase

in Fe2O3, Na2O, TiO2 and P2O5, and a decrease in

Al2O3 and CaO contents with increasing Zr or decreas-

ingXMgmay be observed (Fig. 3), with the exception of

the Pocico samples (Lower Palaeozoic amphibolites)

and other samples interpreted here as cumulates. A

better positive correlation can be observed between Zr

and TiO2 and P2O5, which indicates an effective

ilmenite and/or Ti-rich amphibole fractionation except

ks using Zr as differentiation index. Symbols indicate samples from

nterpreted as cumulates (see text). Dark grey line highlights the trend


for cumulates samples. These observations highlight

the importance of fractional crystallization in the petro-

genesis of the rocks studied, but also imply the exis-

tence of several magmatic lineages, deriving from

different parental magmas, and probably originated in

different scenarios (see below).

3.3. Trace and rare earth elements

There cannot be said to exist a good correlation

between most minor elements and Zr (or XMg). As a

matter of fact, this is to be expected if one takes into

account the wide extent of ages of the different Upper

Precambrian amphibolites. The best linear correlation

is between REE, Ta (Nb) and Y with respect to Zr

(Fig. 3). According to the chondrite-normalized REE

Fig. 4. Chondrite-normalized REE plots for the rocks under examinatio

corresponding to Group 1 samples. (c) Plot for Group 2 samples, interpr

amphibolites). (d) Comparison between the three groups of patterns.

plots shown in Fig. 4, three groups of rocks can be

distinguished: one corresponds to Lower Palaeozoic

amphibolites and the other two (liquids and cumu-

lates, see below) to Upper Precambrian amphibolites.

Some of the Upper Precambrian amphibolites

(Group 1 in the following) represent magmatic liquids

and display roughly parallel, slightly LREE-enriched

patterns [(La/Yb)N = 1.35–3.07)], with the REE global

content ranging between 20 and 70 chondrite, and

commonly a relative depletion in La and Ce (Fig. 4a

and b) that may be explained by the extraction of

LREE-rich minerals such as apatite. There is not any

apparent Eu anomaly. This fact indicates that the

generation of this group cannot simply be explained

in terms of different degrees of fractional crystalliza-

tion. The uniform increase in global REE contents may

n here (normalization values after Nakamura, 1974). (a, b) Plots

eted as cumulates, and for the Group 3 samples (Lower Palaeozoic


be the result of shallow magmatic differentiation.

However, there cannot be established any clear corre-

lation between total REE content and XMg, neither in

the same outcrop nor in all the sampled outcrops. In

fact, the lowest REE contents are not always repre-

sented by samples with the lowest XMg (CA-2, Fig. 4b)

or, alternatively, almost identical REE contents are

displayed by samples with different XMg (BCA sam-

ples, Fig. 4b). This, together with the slightly sigmoi-

dal LREE patterns, can be taken to indicate that there is

not a simple fractionation relationship between the

different samples. Therefore, the samples concerned

do not represent a real liquid-line of descent (see Fig.

3), probably reflecting that most of them must have

undergone different degrees of crystal accumulation.

In addition, most of the samples are high Ti metaba-

saltic rocks, but low Ti ones are also present, thus

implying the existence of several lines of magmatic

evolution from different magmatic parental batches.

The remaining Upper Precambrian amphibolites

samples (Group 2 in the following) display the lowest

total REE contents (6–11 chondritic values, Fig. 4c),

almost flat to slightly LREE-depleted patterns (La/

Yb)N = 0.730–1.823) and a distinct positive Eu anom-

aly (Eu/Eu* = 1.20–1.64). These patterns may be pro-

duced by the accumulation of a mixture of plagioclase

(La >Yb), pyroxene (La <Yb) and minor olivine (La =

Yb), in liquids slightly LREE richer than E-MORB.

The high Mg-number and SiO2 content, the compatible

element contents, such as Cr and Ni, and the lower

contents in all the incompatible elements (see Fig. 3

and Table 2) are consistent with a cumulate assemblage

of pyroxene, plagioclase, and minor olivine.

Lower Palaeozoic amphibolites (Group 3 hereafter)

were sampled in the Pocico outcrop. Other samples

from the Arroyo Argallon outcrop (AA-217) show

similar geochemical characteristics and have been

tentatively considered as belonging to the same group.

These rocks are mostly low Ti (TiO2 < 2%) metaba-

saltic rocks. Two samples are slightly LREE-depleted

[(La/Yb)N = 0.82–0.90], and have 18–20 chondritic

total REE contents. HREE patterns are flat, and

crosscut some of the patterns of Group 1 amphibolites

(Fig. 4d). A minor negative Eu anomaly can be

observed in sample PC-1. These features are similar

to transitional MORB patterns (Saunders, 1984). The

other PC samples display a similar geochemical

signature, despite their LREE-enriched patterns [(La/

Yb)N = 1.77–2.35; Fig. 4c)]. These differences in (La/

Yb)N, however, can easily be explained by a variable

degree of melting of the same mantle source.

MORB-normalized diagrams of trace element

abundances are displayed in Fig. 5. The three groups

defined on the basis of REE data show also distinctive

trace element patterns.

Samples from Group 1 display gradually increas-

ing contents from La to Rb, with values always

exceeding MORB abundances (Fig. 5a and b). LIL

elements (Li, Rb, Ba, Cs, Th, U and Pb) are variably

enriched, and there is a minor but significant negative

anomaly in Nb. Compatible element contents are

lower than typical MORB values. Sample AA-364

(Fig. 5a) shows a strong positive P anomaly and high

Ti content. This amphibolite is very rich in apatite

and, to a lesser extent, ilmenite, which were probably

accumulated together with plagioclase, according to

its high Al2O3 and CaO and low MgO, Cr and Ni

contents (Table 1).

The patterns of Group 2 samples are more variable

than the other amphibolites but commonly display

more moderate enrichments in alkaline metals and Ba.

U and Th are relatively depleted with respect to other

LIL elements (Fig. 5c and d). Incompatible HFS and

other immobile elements are strongly depleted. Com-

patible element contents (Cr, Ni and Sc) are variable,

some of these samples showing higher values than in

Group 1 samples. The Sr contents are similar to Group

1 amphibolites, while P outlines a clearly marked

negative peak. There is also a remarkable negative Nb

anomaly. The differences in chemical composition

among samples of this group can be explained in

terms of variable plagioclase, olivine, pyroxene, and

magnetite modal contents in the original protolith.

The samples of Lower Palaeozoic amphibolites

(Group 3) show patterns rather similar to those of

Group 1 (Fig. 5c), with a somewhat lower enrichment

in LILE and flat patterns for the immobile elements.

There is also a very small negative Nb anomaly in

some of the samples. Compatible elements are similar

to N-MORB, and higher than in Group 1.

In short, the trace element contents may be

explained in terms of the different modal proportion

of cumulate phases (probably plagioclase, pyroxene,

and minor olivine, apatite and ilmenite) in the original

rocks. Thus, Groups 1 and 3 probably correspond to

liquid-rich, porphyritic rocks with widely variable, but

Fig. 5. MORB-normalized plot for the samples under study (normalizing values modified after Pearce (1982)). (a, b) Plots for the Group 1

samples. (c) Plot for the Group 2 samples, interpreted as cumulates, and for the Group 3 samples (Lower Palaeozoic amphibolites). (d)

Comparison of the three groups.


low, phenocryst contents, while Group 2 samples are

clearly cumulate rocks of the phases in question with

minor trapped liquid. The Ba, Sr, Rb and other LIL

elements are always relatively enriched and their

abundances do not define any regular trend with

common differentiation indexes (either using Zr or

Mg-number), thus indicating the post-magmatic mobi-

lity of these elements.

3.4. Sr–Nd isotope geochemistry

Sr and Nd isotopes have been measured on sam-

ples from the three groups of rocks previously

defined. Table 3 shows the analytical results. Geo-

chronological and Nd isotope data from samples of

the same outcrops have been reported by Ordonez

Casado (1998). The ages proposed by this author have

been used in Table 3 to compute initial isotopic ratios

and epsilon values.

Fig. 6 shows the variation in Sr and Nd initial

values. eNd varies within the range 2.12–7.53, cor-

responding to a slightly to moderately depleted

source. Sample CPR-12 from Group 1 has an initial

eNd of 6.31. Sample PC-1 from Group 3 amphibolites

shows the most depleted character (eNd= 7.53), a

value close to the Depleted Mantle (DM) component

for the considered age. The eNd values for Group 2

range from 2.12 to 5.64. The widespread Nd initial

ratios indicate the participation of a more enriched

component in their genesis. All the samples have

relatively high Sr initial ratios and plot slightly to

the right of the mantle array, recalculated for the range

of ages considered in this work (483–595 Ma). Two

samples show very high eSr values, and plot in the

Table 3

Isotope data for selected samples

Sample LC-3 AA-368 BCA-6 CM-7b CPR-12R PC-1

Rb 15.94 39.34 9.04 6.17 42.75 4.03

Sr 215 168 275 197 374 9187Rb/86Sr 0.2149 0.6776 0.0952 0.0906 0.3304 0.128587Sr/86Sr 0.7089 0.7084 0.7049 0.7046 0.7089 0.704687Sr/86Srinitial 0.7072 0.7033 0.7066 0.7041 0.7038 0.7037

eSr 47.76 � 7.66 37.48 4.11 � 0.3 � 3.05

Sm 1.36 0.82 0.79 0.96 7.35 3.85

Nd 3.68 2.19 2.38 2.67 26.8 0.68147Sm/144Nd 0.2241 0.2266 0.2018 0.2166 0.1659 0.2177143Nd/144Nd 0.5129 0.5130 0.5128 0.5129 0.5129 0.5131143Nd/144Ndinitial 0.5120 0.5122 0.5123 0.5120 0.5121 0.5124

eNd 2.16 5.64 6.31 2.6 3.6 7.53

eSr and eNd are calculated assuming ages of 483–599 Ma.


upper right quadrant of the 143Nd/144Nd–87Sr/86Sr

diagram. These high eSr values may be due either to

crustal contamination with a low-Nd high-Sr contam-

inant, or most probably to post-magmatic alteration. A

likely contamination prevents the use of their Sr

isotope composition as a mantle source petrogenetic

indicator. However, their Nd initial ratios cannot be

taken to be seriously affected by this alteration and,

what is more, they plot within the range depicted by

the rest of samples.

Fig. 6. eNd isotope compositions vs. 87Sr/86Sr initial ratio plot.

Samples CPR-12 and LC-3 plot on the upper right quadrant of the

diagram; this fact can be attributed to a significant secondary

modification of Sr composition. Sample PC-1 shows the most

depleted composition, while the rest of samples plot slightly to the

right of the Mantle Array, in the mixing line between Depleted

Mantle and Bulk Earth at their original protolith ages.

3.5. Petrogenesis

In the previous sections, we have shown that the

compositional features of some of the Upper Precam-

brian amphibolites indicate a cumulative origin for

these rocks. For this reason, in what follows we will

concentrate on the origin of the rest of Upper Pre-

cambrian amphibolites (Group 1) and all of the Lower

Palaeozoic ones (Group 3), which are more represen-

tative of the original basaltic melts.

The (La/Yb)N values for both groups of liquids are

low (Upper Precambrian samples = 1.28–3.07, Lower

Palaeozoic samples = 0.82–2.35) and show a narrow

variation. This is consistent with a generation by

partial melting of a mantle source in which garnet

does not remain as a residual phase. Therefore, the

magma generation depth should have been at < 60–

80 km, within the spinel peridotite field (Watson and

McKenzie, 1991). This is also consistent with the

narrow ranges in Sc/V and Ti/Y (see Tables 1 and 2).

Fractional crystallization is unable to produce sig-

nificant variations in incompatible element ratios.

Instead, such variations can be produced in response

to different degrees of partial melting from a homo-

geneous source. Accordingly, the positive slope in

Fig. 7a displayed by Group 3 samples can be attrib-

uted to variable degrees of partial melting from a

common source, with higher melting degree in the

lower end of this array, where the most depleted

samples plot (PC-1 and AA-217). For Group 1 sam-

ples, the overall tendency is horizontal, which can be

attributed to the predominance of low pressure frac-

Fig. 7. (a) Ce vs. Ce/Yb diagram for Groups 1 and 3; see text for

further explanation. (b) Y vs. Cr diagram (Pearce, 1982). Most

samples plot in the MORB and within-plate basalt (WPB) fields,

with the exception of three very differentiated samples which are

outside the volcanic arc basalt (VAB) field.

Fig. 8. (a) Zr vs. Ce/Zr diagram for the samples studied. (b) Sm vs.

Ce/Yb diagram. Arrows show the trends of Groups 1 and 3.

Composition of E-MORB, N-MORB, PM, OIB and Continental

Crust according to Sun and McDonough (1989) and Taylor and

MacLennan (1985). The effects of fractional crystallization and

zircon addition are also shown.


tional crystallization. Nevertheless, there is a slight

spread of Ce/Yb ratio, which could reflect minor

differences in the degree of melting or source hetero-

geneity. The Cr vs. Y plot in Fig. 7b (Pearce, 1982;

Pearce and Parkinson, 1993) also shows that the

variations among Group 1 samples are better

explained in terms of variable degrees of fractional

crystallization, while Group 3 samples may have been

produced by a slightly higher degree of melting (likely

at shallower depths). Both groups plot in the over-

lapping field of MORB and/or within-plate (WP)

basalts proposed by Pearce (1982), and outside the

volcanic arc basalt (VAB) field.

The variations observed in Fig. 8a for Group 1

amphibolites can also be better explained in terms of

an evolution dominated by low pressure fractional

crystallization from MORB-like asthenospheric melts,

comparable to Group 3 amphibolites, the most

depleted samples. These variations in the Group 1

rocks are also compatible with addition of crustal

material to MORB-like, tholeiitic basalt. Those crustal

materials which are likely to supply this component

could be similar to some of the nearby Upper Pre-

cambrian orthogneisses, which appear intercalated

with the amphibolites in the lithostratigraphic

sequence (Azor et al., 1995). The addition of this

Zr-rich material could be taken to be responsible for

Fig. 9. (a) Ti/Y vs. Zr/Y diagram for Groups 1 and 3; plate margin

basalt and within-plate basalt (WPB) fields after Pearce and Gale

(1977); arrows depict the trends for Groups 1 and 3. (b) Zr/Nb vs.

Y/Nb plot; Upper Precambrian amphibolites show a marked

increase in Zr. (c) Ta/Yb vs. Th/Yb plot, in which the mantle array

field is after Pearce (1982); for arrows labelled ‘‘a’’ and ‘‘b’’, see

explanation in the text. E-MORB, N-MORB, PM, OIB, Continental

Crust (CC), Lower Continental Crust (LCC) and Upper Continental

Crust (UCC) compositions from Sun and McDonough (1989) and

Taylor and MacLennan (1985).


the rather negative slope of the observed trend. The

same effect is also observed in Fig. 8b.

Additional information regarding the mantle source

of the basaltic magmas can be obtained from the

diagrams in Fig. 9. In these diagrams, the observed

trends can be explained by considering at least three

components:

(1) A depleted, N-MORB-like asthenospheric source

is predominant in Group 3. Many geochemical

features of these rocks match those of average N-

MORB magmas, such as their high initial Nd

isotope ratio (eNd(483) = 7.53). In Fig. 10 we have

represented the immobile element composition

normalized to the fertile MORB mantle (FMM) of

Pearce and Parkinson (1993). The plot shows a

negative slope with very highly incompatible

(VHI) elements > highly incompatible (HI) ele-

ments >moderately incompatible (MI) elements.

These patterns are usually produced by a

moderate melting degree from an undepleted

FMM source (Pearce and Parkinson, 1993).

(2) A more enriched source is required to explain the

shift towards higher Ce/Yb, Ce/Nb, Ti/Y, Zr/Y and

Ta/Yb, and to lower Zr/Nb and Y/Nb ratios (Figs.

7–9). The influence of a more enriched compo-

nent is more evident in the Group 1 rocks than in

those of Group 3. This enriched source could be a

(plume-related?) OIB, an E-MORB component

present in the asthenosphere source or, alterna-

tively, a subcontinental lithospheric mantle

(SCLM). The isotopic variation towards lower

eNd values in the analysed samples is also

consistent with a mixing of melts from depleted

and enriched reservoirs. This fact is more evident

again in the Group 1 amphibolites.

(3) A limited amount of crustal material is required to

produce the variation towards higher Th, Zr, Zr/Y,

Th/Nb and Th/Yb, and lower Ce/Zr values

observed in some samples (Figs. 8 and 9). The

nature of this crustal material is unknown, but we

have argued previously that it could be similar to

some of the nearby Upper Precambrian orthog-

neisses. In Fig. 9c, we have represented in a Ta/Yb

vs. Th/Yb plot the average upper continental crust

value from Taylor and MacLennan (1985).

Interestingly, most of the Groups 1 and 3 samples

plot on the mantle array, between the composi-

Fig. 10. Fertile MORB mantle (FMM)-normalized plots after Pearce

and Parkinson (1993). (a) Group 1 amphibolites. (b) Group 3

amphibolites. See text for further explanations. VHI, very highly

incompatible; HI, highly incompatible; MI, moderately incompat-

ible; SI, slightly incompatible; C, compatible.


tions of typical N- and E-MORB, while some

Group 1 samples show an increase in the Th/Yb

ratio, a feature commonly attributed to the

introduction of a ‘‘subduction’’ component. The

same variation could alternatively be obtained by

mixing asthenospheric basalts with acidic rocks

(arrow ‘‘a’’ in Fig. 9c) or with average upper crust

(arrow ‘‘b’’ in Fig. 9c). The slightly negative Nb

anomaly could also be produced in the same way

(Wilson, 1989). The enrichment in LIL elements

displayed in the MORB-normalized diagrams

(Fig. 5) could also be attributed to a ‘‘subduction

component’’ or to mixing with crustal melts.

However, this should be considered with caution,

because the original LILE contents could have

been largely modified by metamorphism, due to

the mobility of these elements.

To conclude, the different geochemical features in-

dicate that Lower Palaeozoic amphibolites (Group 3)

have numerous features similar to present-day MORB

magmas and derive from a more depleted source than

Upper Precambriam amphibolites (Group 1). Further-

more, some of the Group 1 amphibolites are more

affected by crustal contamination than Group 3 ones.

4. Discussion

Despite the limited outcrop extent of amphibolites

in the Central Unit, their origin and tectonic signifi-

cance are key aspects to fully understand the signifi-

cance of the OMZ/CIZ boundary. According to

previous geochronological data (Ordonez Casado,

1998), two groups of amphibolites can be differenti-

ated: (i) Upper Precambrian amphibolites and (ii)

Lower Palaeozoic amphibolites. Both types of

amphibolites underwent a common tectono-metamor-

phic evolution during the Variscan orogeny. Subduc-

tion and continental collision were followed by fast

exhumation owing to extensional processes (Azor et

al., 1994; Lopez Sanchez-Vizcaıno et al., 2003).

Our geochemical data have revealed that the two

groups of amphibolites also have different origins. As

for Upper Precambrian amphibolites (Group 1), we

envisage an advanced intra-continental rifting situa-

tion or an intra-continental back-arc setting related to

the Cadomian orogeny, while for Lower Palaeozoic

ones (Group 3), we propose an environment of

oceanic crust formation related to the Variscan orog-

eny. Ordonez Casado (1998) proposed a similar sce-

nario for both groups of amphibolites drawing on the143Nd/144Nd isotopic values.

Basalts ranging from depleted to slightly enriched,

such as those of Group 1, can be generated in an intra-

continental rifting stage (including intra-continental

back-arc settings) (cf. Holm, 1985). In modern ana-

logues, this setting usually includes alkaline as well as

tholeiitic associations, depending on the degree of

extension in the rifting process and the source compo-

sition. In areas of advanced degree of extension,

asthenospheric melting would produce depleted tho-

leiitic suites, similar to those from Group 3. However,

the passage of basaltic magmas through thinned sub-

continental lithosphere and continental crust is usually

reflected in the composition of the final magmas,

which then incorporates an enriched and/or crustal

component. As described above, these enriched and

crustal components, superimposed on an overall

MORB chemistry, are clearly present in the trace

element and isotopic composition of Upper Precam-

brian amphibolites. In this respect, these amphibolites

are regionally related to felsic rocks of alkaline affinity

located near to La Cardenchosa, although their age is

poorly constrained [690F 134 Ma, Rb–Sr/whole rock

method; Azor et al. (1995)]. It is clear, however, that

magmas generated in this setting incorporate compo-

nents from far too many sources, which cannot be

easily discriminated by their geochemical signature.


The geochronological data available (cf. Ordonez

Casado, 1998) record long-lasting, likely pulsating

magmatic activity in the Upper Precambrian. Thus,

the overlapping of temporally different magmatic

events further contribute to the complexity of the

chemical features, and explain the scatter observed in

the geochemical plots.

Lower Palaeozoic amphibolites (Group 3) have

compositions similar to N- or E-MORB recent basalts.

Accordingly, the trace element and isotopic data point

to a moderate melting of depleted asthenosphere as the

main source of this group of samples. The transition

from positive-slope, depleted REE patterns (e.g. sam-

ple PC-1, Table 2) to negative, slightly LREE-enriched

patterns (Fig. 5), may be effectively related to variable

melting degrees from a homogeneous source, or by

involving an enriched mantle component, which can be

either a plume component present in the asthenosphere

(OIB or E-MORB), or subcontinental lithospheric

mantle (SCLM). The trace element and isotopic data

available are unfortunately, not conclusive enough so

as to distinguish between these possibilities. In addi-

tion, both the REE pattern and the depleted isotopic

signature of sample PC-1 indicate that the amount of

crustal addition, if present at all, should be very low. A

number of geodynamic scenarios are coherent with the

features of the Early Palaeozoic basic magmas: a mid-

ocean ridge, a very extended ensialic rift, and a back-

arc rift. Nevertheless, other geological data (mainly

stratigraphical) point to a mid-ocean ridge as the most

likely situation. In this respect, the Lower Palaeozoic

succession of the OMZ denotes the beginning of a

rifting process in Cambrian times with rift-related

sediments and widespread basaltic volcanism in Mid-

dle Cambrian (cf. Linan and Quesada, 1990; Giese and

Buhn, 1993). In this context of progression of the rift

during Ordovician times, the formation of an oceanic

crust of this age seems to be quite plausible (cf. Pin,

1990; Crowley et al., 2000; Floyd et al., 2000),

although a very extended continental rift should not

be completely ruled out. Moreover, the relatively large

dimensions of the Pocico basic body and the presence

of dismembered ultramafic bodies in nearby areas

(Apalategui et al., 1988) are also in agreement with

this hypothesis. In this respect, the presence of a

magnetic anomaly just to the NE of the OMZ/CIZ

boundary in the Hornachos area (Fig. 1b) has been

related to a 3- to 6-km-thick body of basic and/or

ultramafic rock located at depth (Aller, 1996). This

anomaly may well indicate the existence at depth of

huge volumes of basic and ultramafic rocks similar to

those cropping out in the Central Unit. Their present

position to the NE of the OMZ/CIZ has been related to

subduction of the OMZ crust underneath the CIZ one

(Azor et al., 1994; Aller, 1996; Simancas et al., 2001).

The recognition of remains of oceanic-affinity

basic rocks along the OMZ/CIZ boundary changes

the interpretation of this major contact of the Variscan

belt in SW Iberia. This contact marks a change in

vergence where high-pressure metamorphism and

intense ductile deformations took place during the

Variscan orogeny. On these grounds, Simancas et al.

(2001) propose that this boundary is a suture contact

where significant intra-continental subduction of

OMZ lower crust under CIZ crust took place in

Devonian–Early Carboniferous times, thus accommo-

dating large amounts of shortening in the OMZ upper

crust. Drawing on the new geochemical data, we can

propose in this respect that some sort of incipient

ocean between the OMZ and CIZ blocks may prob-

ably have existed during Early Palaeozoic times and

that the ocean in question was closed later, probably

during the Devonian, by oceanic subduction under the

CIZ crust.

The chronology favoured here, with ocean gener-

ation in Ordovician times and ocean subduction in

Devonian times, fits in neatly with the chronology re-

cently proposed by Ordonez Casado et al. (2001) for

the Cabo Ortegal complex in NW Iberia. This com-

plex, as the Central Unit, includes oceanic-affinity

rocks having undergone a high-pressure/high-temper-

ature metamorphism during the Variscan orogeny. In

addition to this similar chronology, it must be stressed

that the Central Unit (Badajoz–Cordoba Shear Zone in

previous works) has been considered by some authors

as the root zone of the allochthonous complexes with

oceanic rocks outcropping in NW Iberia (e.g. Matte,

1986). However, the different subduction polarities in

NW and SW Iberia have led Simancas et al. (2002) to

propose that both domains were connected in pre-

orogenic times by a transfer fault coincident with the

present position of the Porto–Tomar Fault. This hypo-

thesis is reinforced now with the new data provided

here, which are consistent with the existence of an

oceanic domain between OMZ and CIZ in Lower

Palaeozoic times.


The situation described, with basic-dominated

magmatism associated to rifted continental margins,

eventually leading to ocean production, has also been

recognized in other sectors of the Variscides, such as

the N Bohemian Massif, the Massif Central, the

Maures Massif, and NW Iberia (cf. Piboule and

Briand, 1985; Pin and Marini, 1993; Briand et al.,

1995, 2002; Santos Zalduegui et al., 1996; Crowley et

al., 2000; Floyd et al., 2000; Kryza and Pin, 2002),

being commonly interpreted as a consequence of an

episode of considerable rifting and fragmentation of

the northern Gondwana margins in Early Palaeozoic

times. In these regions, the metabasites observed in

different crustal blocks include variably depleted and

enriched tholeiitic basalts as well as alkali basalts.

These rocks are often associated with felsic rocks,

resulting in bimodal suites in some of the aforemen-

tioned Variscan massifs. The geochemical variations

of the metabasites have been attributed to a number of

factors, such as variable participation of a sediment-

contaminated lithospheric mantle, a possible contri-

bution of a plume component, crustal contamination,

and different degrees of lithospheric thinning and

ocean-floor production.

5. Conclusion

Amphibolites included in the OMZ/CIZ contact

appear as small-size dike or lens-shaped bodies of

Upper Precambrian age and an elongated hectometre-

thick body of Lower Palaeozoic age. According to

their geochemical features, Upper Precambrian

amphibolites seem to be generated in an advanced

intra-continental rift or an intra-continental back-arc

setting related to the Cadomian orogeny. Lower Palae-

ozoic amphibolites were formed, more likely than not,

in an oceanic scenario which predated Variscan sub-

duction and collision. A situation of widely extended

continent rift cannot, however, be completely dis-

carded for these amphibolites. In Devonian times,

the oceanic domain was consumed by subduction

underneath the CIZ, as attested by eclogite facies

assemblages preserved in the amphibolites. Finally,

exhumation of the high-pressure rocks must have

taken place in the Late Devonian–Early Carbonifer-

ous by means of an intense ductile shearing. The

Lower Palaeozoic oceanic domain, represented by

some of the amphibolites studied, was probably con-

nected with the one represented by the oceanic-affin-

ity rocks included in the allochthonous complexes of

NW Iberia.

Acknowledgements

The research presented in this paper was funded by

the Spanish Ministry of Science and Technology

through the projects BTE2000-1489 and BTE2000-

1490-C02-01. We are indebted to P. O’Brien and J.

Winchester for their insightful reviews, which resulted

in considerable improvement of the manuscript.

Finally, Francisco Gonzalvez Garcıa helped us to im-

prove our English text.

References

Abalos, B., Gil Ibarguchi, J.I., Eguiluz, L., 1991. Cadomian sub-

duction/collision and Variscan transpression in the Badajoz–

Cordoba shear belt, southwest Spain. Tectonophysics 199,

51–72.

Aller, J., 1996. 2.5D modelling and structural implications of mag-

netic anomalies associated with the boundary between the Cen-

tral-Iberian and Ossa–Morena zones in the Hornachos sector

(Hercynian belt, southwest Spain). Rev. Soc. Geol. Esp. 9,

197–204.

Apalategui, O., Higueras, P., Contreras, F., Arriola, A., Garrote, A.,

Eguiluz, L., Sanchez Carretero, R., 1988. Sheet number 830

(Hornachos) of the geological map of Spain. Instituto Geologico

y Minero de Espana, Madrid.

Azor, A., Gonzalez Lodeiro, F., Simancas, J.F., 1993. Cadomian

subduction/collision and Variscan transpression in the Bada-

joz–Cordoba shear belt (SW Spain)—a discussion on the age

of the main tectonometamorphic events. Tectonophysics 217,

343–346.

Azor, A., Gonzalez Lodeiro, F., Simancas, J.F., 1994. Tectonic

evolution of the boundary between the Central Iberian and Os-

sa –Morena Zones (Variscan Belt, SW Spain). Tectonics 13,

45–61.

Azor, A., Bea, F., Gonzalez Lodeiro, F., Simancas, J.F., 1995. Geo-

chronological constraints on the evolution of a suture: the Os-

sa–Morena/Central Iberian contact (Variscan Belt, south-west

Iberian Peninsula). Geol. Rundsch. 84, 375–383.

Bard, J.P., 1977. Signification tectonique des metatholeites d’affin-

ite abyssale de la ceinture metamorphique de basse pression

d’Aracena (Huelva, Espagne). Bull. Soc. Geol. Fr. 19, 385–393.

Briand, B., Bouchardon, J.L., Ouali, H., Piboule, M., Capiez, P.,

1995. Geochemistry of bimodal amphibolitic – felsic gneiss

complexes from eastern Massif Central, France. Geol. Mag.

132, 321–337.


Briand, B., Bouchardon, J.L., Capiez, P., Piboule, M., 2002. Felsic

(A-type)–basic (plume-induced) Early Palaeozoic bimodal mag-

matism in the Maures Massif (southeastern France). Geol. Mag.

139, 291–311.

Burg, J.P., Iglesias, M., Laurent, Ph., Matte, Ph., Ribeiro, A.,

1981. Variscan intracontinental deformation: the Coimbra–

Cordoba Shear Zone (SW Iberian Peninsula). Tectonophysics

78, 161–177.

Crowley, Q.G., Floyd, P.A., Winchester, J.A., Franke, W., Holland,

J.G., 2000. Early Palaeozoic rift-related magmatism in Variscan

Europe: fragmentation of the Armorican Terrane Assemblage.

Terra Nova 12, 171–180.

Dupuy, C., Dostal, J., Bard, J.P., 1979. Trace element geochemistry

of Paleozoic amphibolites of SW Spain. Tschermak’s Mineral.

Petrogr. Mitt. 26, 87–93.

Floyd, P.A., Winchester, J.A., 1975. Magma-type and tectonic set-

ting discrimination using immobile elements. Earth Planet. Sci.

Lett. 27, 211–218.

Floyd, P.A., Winchester, J.A., Seston, R., Kryza, R., Crowley, Q.G.,

2000. Review of geochemical variation in Lower Palaeozoic

metabasites from the NE Bohemian Massif: intracratonic rifting

and plume–ridge interaction. In: Franke, W., Haak, V., Oncken,

O., Tanner, D. (Eds.), Orogenic Processes: Quantification and

Modelling in the Variscan Belt. Geol. Soc. London, Spec. Pub.,

vol. 179, pp. 155–174.

Giese, U., Buhn, B., 1993. Early Paelozoic rifting and bimodal

volcanism in the Ossa–Morena Zone of south-west Spain. Geol.

Rundsch. 83, 143–160.

Govindaraju, K., Potts, P.J., Webb, P.C., Watson, J.S., 1994. Report

on Whin Sill Dolerite WS-E from England and Pitscurrie Micro-

grabbro PM-S from Scotland: assessment by one hundred and

four international laboratories. Geostand. Newsl. 18, 211–300.

Holm, P.E., 1985. The geochemical fingerprints of different tecto-

nomagmatic environments using hygromagmatophile element

abundances of tholeiitic basalts and basaltic andesites. Chem.

Geol. 51, 303–323.

Irvine, N., Baragar, W.R.A., 1971. A guide to chemical classifica-

tion of common volcanic rocks. Can. J. Earth Sci. 8, 523–548.

Kryza, R., Pin, C., 2002. Mafic rocks in a deep-crustal segment of

the Variscides (the Gory Sowie, SW Poland): evidence for

crustal contamination in an extensional setting. Int. J. Earth

Sci. 91, 1017–1029.

Le Bas, M.J., Le Maitre, T.W., Woolley, A.R., 1992. The construc-

tion of the total alkali-silica chemical classification of volcanic

rocks. Mineral. Petrol. 48, 1–22.

Linan, E., Quesada, C., 1990. Ossa–Morena Zone. Rift phase

(Cambrian). In: Dallmeyer, R.D., Martınez Garcıa, E. (Eds.),

Pre-Mesozoic Geology of Iberia. Springer, Berlin, pp. 259–266.

Lopez Sanchez-Vizcaıno, V., Gomez-Pugnaire, M.T., Azor, A.,

Fernandez-Soler, J.M., 2003. Phase diagram sections applied

to amphibolites: a case study from the Ossa–Morena/Central

Iberian Variscan suture (Southwestern Iberian Massif). Lithos

68, 1–21.

Matte, Ph., 1986. Tectonics and plate tectonic model for the Varis-

can Belt of Europe. Tectonophysics 126, 331–334.

Montero, P., Bea, F., 1998. Accurate determination of 87Rb/86Sr and147Sm/144Nd ratios by inductively-coupled-plasma mass spec-

trometry in isotope geoscience: an alternative to isotope dilution

analysis. Anal. Chim. Acta 358, 227–233.

Munha, J., 1983. Hercynian magmatism in the Iberian Pyrite Belt.

In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carbo-

niferous of Portugal. Mem. Serv. Geol. Portugal, vol. 29,

pp. 39–81.

Munha, J., Oliveira, J.T., Ribeiro, A., Oliveira, V., Quesada, C.,

Kerrick, R., 1986. Beja–Acebuches Ophiolite: characterization

and geodynamic significance. Maleo 2, 31.

Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K

in carbonaceous and ordinary chondrites. Geochim. Cosmo-

chim. Acta 38, 757–775.

Ordonez Casado, B., 1998. Geochronological studies of the Pre-

Mesozoic basement of the Iberian Massif: the Ossa–Morena

zone and the Allochthonous Complexes within the Central Iber-

ian zone. Diss. EHZ-Zurich. 235 pp.

Ordonez Casado, B., Gebauer, D., Schafer, H.J., Gil Ibarguchi, J.I.,

Peucat, J.J., 2001. A single Devonian subduction event for the

HP/HT metamorphism of the Cabo Ortegal complex within the

Iberian Massif. Tectonophysics 332, 359–385.

Pearce, J.A., 1982. Trace elements characteristic of laves from de-

structive plate boundaries. In: Thorpe, R.S. (Ed.), Andesites.

Wiley, New York, pp. 525–528.

Pearce, J.A., Gale, G.H., 1977. Identification of ore-deposition en-

vironment from trace element geochemistry and associated igne-

ous host rocks. Geol. Soc. London, Spec. Publ. 7, 14–24.

Pearce, J.A., Parkinson, I.J., 1993. Trace elements model for

the mantle melting: application to volcanics arc petrogenesis.

In: Princhard, H.M., Alabaster, T., Harris, N., Band Neary, C.R.

(Eds.), Magmatic Processes and Plate Tectonics. Geol. Soc. Spec.

Publ., vol. 76, pp. 373–404.

Piboule, M., Briand, B., 1985. Geochemistry of eclogites and asso-

ciated rocks of the southeastern area of the French Massif Cen-

tral: origin of the protoliths. Chem. Geol. 50, 189–199.

Pin, C., 1990. Variscan oceans: ages, origins and geodynamic im-

plications inferred from geochemical and radiometric data. Tec-

tonophysics 177, 215–227.

Pin, C., Marini, F., 1993. Early Ordovician continental break-up in

Variscan Europe: Nd–Sr isotope and trace element evidence

from bimodal igneous associations of Southern Massif Central,

France. Lithos 29, 177–196.

Robardet, M., 1976. L’originalite du segment hercynien sud-iberi-

que au Paleozoıque inferieur: Ordovicien, Silurien et Devonien

dans le Nord de la Province de Seville (Espagne). C. R. Acad.

Sci. Paris, Ser. D 283, 999–1002.

Robardet, M., Gutierrez Marco, J.C., 1990. Sedimentary and Faunal

Domains in the Iberian Peninsula during Lower Paleozoic

Times. In: Dallmeyer, R.D., Martınez Garcıa, E. (Eds.), Pre-

Mesozoic Geology of Iberia. Springer, Berlin, pp. 383–395.

Santos Zalduegui, J.F., Scharer, U., Gil Ibarguchi, J.I., Girardeau, J.,

1996. Origin and evolution of the Paleozoic Cabo Ortegal ultra-

mafic–mafic complex (NW Spain): U–Pb, Rb–Sr and Pb–Pb

isotope data. Chem. Geol. 29, 281–304.

Saunders, A.D., 1984. The rare earth element characteristics of

igneous rocks from the ocean basins. In: Henderson, P.

(Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam,

pp. 205–236.


Simancas, J.F., Martınez Poyatos, D., Exposito, I., Azor, A., Gonza-

lez Lodeiro, F., 2001. The structure of a major suture zone in the

SW Iberian Massif: the Ossa–Morena/Central Iberian contact.

Tectonophysics 332, 295–308.

Simancas, J.F., Gonzalez Lodeiro, F., Exposito Ramos, I., Azor, A.,

Martınez Poyatos, D., 2002. Opposite subduction polarities con-

nected by transform faults in the Iberian Massif and western

European Variscides. In: Martınez Catalan, J.R., Hatcher, R.D.,

Arenas, R., Dıaz Garcıa, F. (Eds.), Variscan–Appalachian Dy-

namics: The building of the Late Paleozoic basement. Geol. Soc.

Amer. Spec. Paper, vol. 364, pp. 253–262.

Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic system-

atics of oceanic basalts: implications for mantle composition and

processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in

Ocean Basins. Geol. Soc. Spec. Publ., vol. 42, pp. 312–345.

Taylor, S.T., MacLennan, S.M., 1985. The Continental Crust: Its

Composition and Evolution. Blackwell, London. 312 pp.

Watson, S., McKenzie, D., 1991. Melt generation by plumes: a

study of Hawaiian volcanism. J. Petrol. 32, 501–537.

Wilson, M., 1989. Igneous Petrogenesis. Unwin Hyman, London.

466 pp.

Documents

The amphibolites from the Ossa–Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation