Magmatic Evolution and Tectonic Setting of the Iberian Pyrite Belt

JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 PAGES 727755 1997

Magmatic Evolution and Tectonic Setting ofthe Iberian Pyrite Belt Volcanism

J. MITJAVILA, J. MARTI AND C. SORIANOINSTITUTE OF EARTH SCIENCES JAUME ALMERA, CSIC, LLUIS SOLE I SABARIS S/N, 08028 BARCELONA, SPAIN

RECEIVED ON JUNE 1, 1996 REVISED TYPESCRIPT ACCEPTED FEBRUARY 4, 1997

result of oblique continental collision that followed the subductionThe Iberian Pyrite Belt, which extends from Portugal to Spain inof the South Portuguese plate beneath the Ossa Morena plate. Thissouthwest Iberia, constitutes one of the worlds largest reservoirstectonically driven magmatism does not have a modern analogue,of massive sulphide deposits. Volcanic-hosted massive sulphidebut it is not inconsistent with the proposed geodynamic evolution ofmineralization occurs at several stratigraphic horizons within anthe studied area. This model gives insights into the petrology andEarly Carboniferous volcano-sedimentary package formed of tur-geochemistry of strike-slip settings in the continental part of sub-biditic siliciclastic deposits and basaltic, intermediate and silicicducting plates, a region usually poorly constrained from a petrologicalvolcanic rocks. Volcanic rocks do not show significant temporal orpoint of view.spatial variations in the stratigraphic sequence of the Iberian Pyrite

Belt and mainly occur as shallow intrusions into wet marinesediments with some minor lavas, hydroclastic rocks and volcanogenicsediments. A geochemical study, including major, trace and rareearth elements, and Sr and Nd isotopes, of the least altered volcanic

KEY WORDS: calc-alkaline volcanism; isotope geochemistry; strike-sliprocks has been carried out to determine the primary magmatic affinitytectonics; Iberian Pyrite Beltand tectonic setting of the Iberian Pyrite Belt volcanism. Most of

the basaltic rocks are continental tholeiites, but a few samples showan alkaline affinity. The origin of the basaltic rocks and theirdiversity of compositions are explained by a single mixing modelbetween E- and N-MORB (mid-ocean ridge basalt) and as- INTRODUCTIONsimilation of crustal material. Calc-alkaline intermediate and silicic Volcanic-hosted massive sulphide deposits are mainlyrocks include basaltic andesites, andesites, dacites and rhyolites. associated world-wide with calc-alkaline submarine vol-Volumetrically, dacites and rhyolites are the most abundant. Inter- canism. Petrological and geochemical data, together withmediate and silicic rocks are not related by fractional crystallization, stratigraphic and structural studies, have been of majornor is there a relationship between the basaltic and calc-alkaline importance in constraining geological models of the vol-rocks by the same process. We suggest that in the Iberian Pyrite canism associated with massive sulphide deposits. In-Belt silicic calc-alkaline magmas were generated on a large scale tegrated studies in areas such as the Kuroko province inby the invasion of continental crust by mafic magmas generated in Japan (Ohmoto, 1983), the Mount Read Volcanics inthe underlying upper mantle. The diversity of compositions shown Tasmania (Crawford et al., 1992), and the Mount Windsorby dacites and rhyolites can mainly be explained either by differences Volcanics in northwestern Australia (Stolz, 1995), havein the composition of the source rocks or by different degrees of revealed that this volcanism may be developed duringpartial melting of upper-crust rocks. Andesites, however, formed by different stages of the subduction process, always beingmixing between basaltic magmas and upper-crust material. The new located on the overriding plate.geochemical data agree with previously published tectonostratigraphic This paper documents the petrology and geochemistrydata which suggest that the Iberian Pyrite Belt volcanism formed of the volcanism of the Iberian Pyrite Belt, an Earlyon the South Portuguese plate owing to strike-slip tectonics. This Carboniferous metallogenic province that extends from

Portugal to Spain in southwest Iberia and constituteslocal extensional tectonic setting was related to transtension as a

Corresponding author. Telephone: 34-3-330 27 16. Fax: 34-3-411 0012. e-mail: [email protected] Oxford University Press 1997

JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997

one of the worlds largest reservoirs of massive sulphide thrust that emplaces the Ossa Morena Zone structurallyabove the South Portuguese Zone (Fig. 1) and is linkeddeposits. Despite the economic significance of this area,

the volcanic rocks have not been studied extensively and to a major geological boundary of similar characteristicsin the southern British Isles (Crespo-Blanc & Orozco,most of the existing geodynamic models proposed to

explain the origin of volcanism and associated massive 1991). Thus, the South Portuguese Zone was accretedto the rest of the Iberian Massif during the Variscansulphide deposits are not based upon petrological and

geochemical data. Thus, in spite of the existence of Orogeny (Ribeiro et al. 1990) and represents the con-tinental crust of a tectonic plate whose oceanic crust wasseveral studies aimed at characterizing the geochemistry

of volcanic rocks (Rambaud, 1969; Strauss, 1970; Hamet totally subducted under the continental crust of the Ossa& Delcey, 1971; Lecolle, 1977; Routhier et al., 1977; Morena Zone (Quesada et al., 1994).Priem et al., 1978; Soler, 1980; Strauss et al., 1981; The Iberian Pyrite Belt is one of the four VariscanMunha, 1983; Moller et al., 1983; Schutz et al., 1988), structural units distinguished by Quesada (1991) in theand some studies focused on the stratigraphy and tectonic South Portuguese Zone (Figs 1 and 2). It is bounded tosetting of the Iberian Pyrite Belt (Schermerhorn, 1971; the north by the Pulo do Lobo oceanic terrane and toStrauss & Madel, 1974; Ribeiro et al., 1983; Oliveira, the south by the Baixo Alentejo Flysch Group. The main1990; Silva et al., 1990; Quesada et al., 1994; Giese et al., feature of the Iberian Pyrite Belt is the occurrence of1994), the nature and evolution of the volcanism are polymetallic sulphide deposits associated with basic andstill poorly known. Geochemical data (elementary and silicic volcanic rocks which are interbedded with Earlyisotopic) are mainly concerned with the Portuguese sector Carboniferous turbiditic siliciclastic deposits. Sedi-of the Iberian Pyrite Belt. In contrast, petrological and mentation of these deposits is continuous through thegeochemical data relating to the volcanic rocks from the stratigraphic sequence of the Iberian Pyrite Belt and noSpanish sector, which includes the eastern part of the internal unconformities have been observed. DepositionIberian Pyrite Belt, are relatively scarce. of these rocks took place in a submarine continental

In this paper, we describe and interpret the magmatic platform environment (Oliveira, 1983, 1990) from bottomevolution and tectonic setting of the Iberian Pyrite Belt and turbidity currents.volcanism. The volcanic rocks comprise basalts, andesites, Volcanic rocks interbedded with marine sediments aredacites and rhyolites which mainly appear as shallow concentrated in the central part of the Iberian Pyriteintrusions into wet marine sediments with some minor Belt stratigraphic sequence, whereas they are lacking in itslavas, hydroclastic rocks and volcanogenic sediments. upper and lower parts. Strauss (1970) and SchermerhornThe detailed stratigraphy established by Soriano (1997) (1971) distinguished three lithostratigraphic units ac-shows a lack of significant temporal and spatial variations cording to the presence or absence of volcanic rocks andin the distribution of volcanic rocks. We report new volcanism-related hydrothermal alteration. From the basemajor, trace and rare earth element (REE), and SrNd to the top, these stratigraphic units are as follows:isotope data from the Spanish sector of the Iberian (1) The Phyllite and Quartzite unit is composed ofPyrite Belt which have been integrated with previous phyllites, quartzites and conglomerates deposited on apetrological and geochemical data, mainly from the Por- continental platform (Oliveira, 1990). Rare limestonetuguese sector. A comprehensive geodynamic model, lenses bearing conodont fauna are located at the top ofwhich differs from those deduced from other volcanic- the unit and indicate a Lower to Upper Famennian agehosted massive sulphide deposit areas, is proposed to (Fig. 3). Its base never crops out throughout the Iberianexplain the origin and evolution of the Iberian Pyrite Pyrite Belt.Belt volcanism. We also discuss the genetic relationship (2) The Volcano-Sedimentary Complex is the strati-of the different rock types found in this volcanism. graphic unit which contains the ore deposits. Most of

the volcanic rocks are shallow intrusive bodies whichshow irregular shapes and contacts with the host rocks.Therefore, the lower and upper boundaries and the

GEOLOGICAL SETTING thickness of the Volcano-Sedimentary Complex are vari-able. The sediments interbedded with the volcanic rocksThe sector of the European Variscan Orogen that crops

out in the western Iberian Peninsula is known as the are mainly turbiditic platform deposits with oceanic faunasuch as radiolaria. Rare limestone lenses bearing con-Iberian Massif (Lotze, 1945), where five major Variscan

geological units were distinguished by Julivert et al. (1974) odont fauna are located at the top of the unit and indicatea Lower to Upper Visean age (Fig. 3).(Fig. 1). Recent studies on the boundary between the

South Portuguese Zone and the Ossa Morena Zone have (3) The Culm Group is the uppermost stratigraphicunit in the Iberian Pyrite Belt. Its top has been erodedinterpreted it as a major suture of the European Variscan

Orogen (Munha et al., 1986; Crespo-Blanc & Orozco, and cannot be observed. Shales and sandstones depositedfrom turbidity currents are the main lithologies. Facies1988; Quesada, 1991). This suture is interpreted as a

728

MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE IBERIAN PYRITE BELT

Fig. 1. Tectonostratigraphic terrane map of the South Portuguese Zone and the southern part of the Ossa Morena Zone. Adapted fromQuesada (1991). Inset: Variscan geological units of the Iberian Massif ( Julivert et al., 1974). CZ, Cantabrian Zone; ALZ, AsturianLeonese

Zone; CIZ, Central Iberian Zone; OMZ, Ossa Morena Zone; SPZ, South Portuguese Zone. Adapted from Julivert et al. (1974).

distributions strongly suggest flysch sedimentation (Olive- north to south polarity (Dallmeyer et al., 1993; De LaRosa et al., 1993).ira, 1990). Goniatite fauna found at the base of the unit

Variscan tectonics in the Iberian Pyrite Belt is char-indicate an Upper Visean age (Fig. 3).acteristic of a fold and thrust belt with a southwardHydrothermal alteration took place during and shortlyvergence. Thrust sequences followed a piggy-back pro-after the volcanism of the Volcano-Sedimentary Complexpagation mode, and related folds developed an axial plane(Munha, 1990). Oxygen, hydrogen and sulphur isotopiccleavage sometimes transecting the fold axes (Ribeiro etcompositions (Munha & Kerrich, 1980; Barriga & Ker-al., 1983). An increase in the cleavage intensity is seenrich, 1984) demonstrate that these hydrothermal pro-from south to north across the entire South Portuguesecesses involved the interaction of magmatic fluids withZone (Silva et al., 1990), and is probably related to aseawater.thermal control on the development of ductile structuresAll of the above-mentioned rocks of the Iberian Pyriteby large intrusives in the northeastern part of the SouthBelt were metamorphosed and deformed during thePortuguese Zone. The detachment level of the thrusts isVariscan Orogeny. A regional low-grade metamorphismfound at the base of the Palaeozoic sequence of the Southof zeolite to greenschist facies increases from south toPortuguese Zone (Ribeiro et al., 1983).north across the Iberian Pyrite Belt and the South Por-

tuguese Zone, and towards the base of the stratigraphicsequence (Munha, 1990). Large intrusive bodies of gran-

STRATIGRAPHY OF THE VOLCANO-itic, tonalitic and dioritic composition, which appear atSEDIMENTARY COMPLEXthe northeastern part of the South Portuguese Zone and

the southern part of the Ossa Morena Zone, are thought The Volcano-Sedimentary Complex is characterized bythe presence of a monotonous sequence (500800 mto be responsible for this regional metamorphism and its

729


Fig. 2. Geological map of the Spanish sector of the Iberian Pyrite Belt showing the location of the stratigraphic sections in Fig. 4 [modifiedafter Instituto Geologico y Minero de Espana (1982)].

thick) of Late DevonianEarly Carboniferous sandstones the Iberian Pyrite Belt silicic rocks are mostly found inthe lower part of the Volcano-Sedimentary Complexand shales with shallow sub-horizontal intrusions of ba-sequence, whereas in the north and northeastern partsaltic, intermediate and silicic compositions, and minorvery shallow silicic intrusives are located at its top (Fig.interbedded lavas, hydroclastic rocks and volcaniclastic4). Despite this trend, silicic intrusives also appear at thesediments (Fig. 4). A detailed revision of the stratigraphytop of the Phyllite and Quartzite unit in the north andof the Volcano-Sedimentary Complex (Soriano, 1997)east of the Iberian Pyrite Belt.has revealed that the emplacement of these intrusives

Andesitic rocks do not crop out in the southwesternmosttook place at different levels in the stratigraphic sequencepart of the studied area. They mainly appear as shallowand that they are interfingered with the host sediments.intrusives above and below felsic volcanics in the centralThis has caused some misinterpretations of the volcanicpart of the Iberian Pyrite Belt, and can also intrude eitherevents represented in the Volcano-Sedimentary Com-the top of the Phyllite and Quartzite Unit or the highestplex, such as the distinction of several well-separatedlevels of the Volcano-Sedimentary Complex. In thevolcanic episodes made by Instituto Geologico y Mineronortheasternmost part of the Iberian Pyrite Belt andesiticde Espana (1982), which we will discuss in a later section.rocks are always below silicic volcanics in the stratigraphicPrevious studies of the Iberian Pyrite Belt volcanismrecord (Fig. 4). On the other hand, basaltic rocks aresuggested the occurrence of primary pyroclastic depositswidely distributed and do not show any specific strati-(Schermerhorn, 1976; Lecolle, 1977). However, most ofgraphic or palaeogeographic position.the volcaniclastic deposits found in the studied area

are not of pyroclastic origin. They were formed byhydroclastic fragmentation and erosion of submarinelavas and domes (Soriano, 1997).

PETROGRAPHY OF VOLCANICThe palaeogeographic and stratigraphic distribution ofROCKSvolcanic rocks in the Volcano-Sedimentary Complex is

relatively haphazard (Fig. 4), but some trends can be Felsic rocks range in composition from dacite to rhyolite.inferred from stratigraphic sections throughout the Iber- Most of them appear as shallow intrusives which show

peperitic textures developed at the contacts betweenian Pyrite Belt. In the south and southwestern part of

730


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731


Fig. 4. Stratigraphic sections of the Volcano-Sedimentary Complex in the Iberian Pyrite Belt (see Fig. 2 for location).

intrusions and wet sediments (Boulter, 1993a, b). Oc- hydroclastic deposits. Other field structures such as flow-banding foliation, flow autobrecciation and soft-sedimentcasionally, felsic rocks reached the oceanic floor as ex-

trusive domes giving rise to short-length lava flows and intrusions through columnar jointing of volcanic rocks

732


also support a shallow intrusive emplacement for most procedures used were inductively coupled plasma massspectrometry (ICP-MS) after fusion for major elementsof the silicic volcanic rocks.

Rhyolitic rocks are mainly composed of albite and analysis plus Ba, Sr, Y and Zr; ICP-MS after HF digestionfor trace metals (Cu, Pb, Zn, Ag, Ni, Cd, Bi, V and Be);quartz phenocrysts and minor biotite phenocrysts (par-

tially or totally replaced by chlorite). Occasionally, K- X-ray fluorescence (XRF) pressed pellet for the elementsGa, Sn, S, Nb and Rb; and instrumental neutron ac-feldspar may also be present. A felsitic groundmass is

characteristic of rhyolites, which also show perlitic and tivation analysis (INAA) for Au, As, Co, Cr, Cs, Hf, Sb,Sc, Ta, Th, U and REE. All the analyses were performedspherulitic textures. Plagioclase phenocrysts and those

parts of the groundmass enriched with plagioclase mi- at ACTLABS (Canada) following standard proceduresfor each method. The detection limits are indicated incrolites are often altered to calcite, muscovite and sericite.

Accessory minerals in dacites and rhyolites include ap- Table 1. The standards used by ACTLABS were:CCRMP SY-2, MRG-1, SY-3, USGS G-2, W-2 andatite. Dacites usually show porphyritic and glomero-

porphyritic to massive coherent textures. Embayed and AGV-1. Some of the standards have been run in duplicateand the results obtained do not differ from the certifiedcurviplanar quartz phenocrysts, subhedral albitized pla-

gioclase, and rare biotite phenocrysts and clino- values. The selection of the samples was done with theaim of covering all the Spanish sector of the Iberianpyroxene microphenocrysts are set in a micro-

crystalline quartzfeldspar groundmass. Pyrite Belt and all the volcanic rock types present. Therocks show varying degrees of alteration, therefore careIntermediate volcanic rocks are porphyritic to glo-

meroporphyritic andesites with subhedral prismatic pla- was taken in selecting the less altered rocks after detailedpetrographic study.gioclase, occasionally albitized, clinopyroxene

phenocrysts, Fe-oxides, and rare quartz and biotite Twelve of these samples were selected for analysis ofRbSr and SmNd isotope geochemistry (Table 2). These(mostly chloritized), embedded in a microcrystalline

groundmass of microlitic plagioclase and microcrystalline 12 samples are the least altered and cover all the rocktypes recognized in the Iberian Pyrite Belt. The samplesquartz. They usually have amygdales filled with chlorite,

calcite, epidote and quartz. Plagioclase phenocrysts are were ground to fine powder and acid washed for 45 minat 50C to remove alteration products. Sample dissolutionoften altered to calcite, muscovite and epidote, and the

groundmass can be altered to chlorite. Field exposures and chemical separation methods for Rb, Sr, Sm andNd followed standard procedures. Blank levels averagedof andesites can show highly irregular, sometimes chilled,

contacts with the host sediments, and frequently have 01 ng for Nd and 02 ng for Sr. Samples were analysedusing a Finnigan MAT 262 mass spectrometer at thecolumnar and spheroidal jointing. Two of the studied

samples contained olivine phenocrysts, together with University of Oslo (Norway). Sm and Nd were loadedonto Re filaments of a double Re filament assembly andclinopyroxene and a Ca-rich plagioclase. This suggests

the presence of restricted basaltic andesites in the vol- run as metal ions. Sr was run in a single Ta filament.Nd isotopes were measured using a single-jump triple-canism of the Iberian Pyrite Belt.

Basaltic rocks show massive, intergranular, equi- collector dynamic routine. Sr was measured in staticmulti-collection mode. Analyses of inter-laboratory stand-granular and minor porphyritic textures. Subhedral clino-

pyroxene and plagioclase (mostly albitized) frequently ards gave the values of 143Nd/144Nd=051109210 forthe Johnson & Matthey JMC321 Nd standard and ofshow intergrowing of crystals. In most of the rocks the

clinopyroxene is a pale-coloured augite. However, a 87Sr/86Sr=071017712 for the NBS 987 Sr standard.few basaltic samples contain a titaniferous salite, whichsuggests an alkaline affinity. In the porphyritic varietiesthe groundmass contains plagioclase microlites and mi- Alteration of volcanic rockscrocystalline augite. Minor subhedral crystals of olivine The volcanic rocks of the Iberian Pyrite Belt were affectedand FeTi oxides are also present in all basaltic rocks. by several alteration and modification processes, whichChlorite, epidote and calcite usually fill original vesicles include low-temperature hydration of glass (probablybut also appear as secondary minerals replacing original caused by seawater percolation), hydrothermal alteration,components of the rock. Field exposures occasionally and regional low-grade metamorphism (greenschist fa-show minor pillows and chilled margins developed at the cies). All these processes have changed the primary chem-contacts with host sediments. istry of the rocks, with significant gains and losses of

several elements and oxidation of Fe.Mobility of chemical components of volcanic rocks

affected by hydrothermal alteration and low-grade meta-GEOCHEMISTRYMethods morphism has been extensively documented (Wood et

al., 1976; Floyd & Winchester, 1978; MacLean & Kran-Fifty-nine new samples have been analysed for majorand trace elements, and REE (Table 1). The analytical idiostis, 1987; MacLean & Barret, 1993). To better

733


Tabl

e1:

Maj

or,tra

cean

dra

reea

rthele

men

tana

lyse

sof

volca

nic

rock

san

don

ese

dim

ent(

FP-5

2)fro

mth

eSp

anish

secto

rof

the

Iber

ian

Pyrit

eB

elt;t

hetra

ceele

men

tdete

ction

limits

are

3r

Met

ho

dD

etec

.lim

its

FP-1

09FP

-80

FP-9

1FP

-34

FP-9

1AFP

-20

FP-1

18FP

-114

FP-7

FP-4

0FP

-105

FP-3

1FP

-117

FP-1

5FP

-16

FP-5

7FP

-119

FP-1

02FP

-116

FP-1

50

Fusi

on

ICP

SiO

20

0544

06

450

446

23

462

846

44

467

847

12

474

148

17

481

748

19

486

649

04

518

651

96

522

653

38

547

355

32

560

9

TiO

20

051

450

641

641

661

621

831

621

401

091

362

061

421

651

310

971

551

470

911

200

67

Al 2

O3

002

141

818

32

152

615

06

155

013

20

156

815

43

165

615

76

145

915

83

158

416

04

136

211

97

160

516

99

164

717

32

Fe2O

30

002

107

98

3310

40

128

010

22

110

19

8810

24

919

807

108

59

9310

83

852

109

09

348

928

617

755

26

Mn

O0

020

130

130

160

180

160

170

170

120

140

120

200

170

140

090

150

160

170

150

140

07

Mg

O0

036

1011

11

461

643

448

428

662

107

48

195

616

637

687

195

876

066

615

844

724

212

03

CaO

002

789

102

717

25

923

172

18

859

115

618

0813

45

103

49

677

868

109

828

774

815

694

827

52

Na 2

O0

022

571

380

464

010

243

013

884

232

592

262

862

902

963

913

373

655

022

894

510

95

K2O

000

50

320

380

05


Tota

ld

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tio

nIC

PC

u1

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.m.

1220

3643

3920

3028

3553

2024

3425

3634

188

2914

Pb

5p

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.


Tabl

e1:

cont

inue

d

Met

ho

dD

etec

.lim

its

FP-8

4FP

-144

FP-8

3FP

-101

FP-1

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-97

FP-1

54FP

-95

FP-9

4FP

-93

FP-1

9FP

-134

FP-2

7FP

-17

FP-5

FP-4

9FP

-137

FP-2

2FP

-133

FP-7

9

Rec

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late

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0%

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ee

SiO

260

99

613

561

83

633

163

70

649

265

51

638

164

63

653

568

33

691

369

36

704

371

64

758

076

10

755

574

96

747

1

TiO

20

850

890

780

650

820

860

650

840

810

770

470

340

540

360

700

120

330

300

190

20

Al 2

O3

168

616

79

171

715

69

176

014

84

159

813

93

154

115

78

169

415

57

133

315

49

115

714

17

126

713

01

122

213

29

Fe2O

36

568

286

466

076

626

933

576

846

415

223

134

436

283

614

911

642

561

801

962

69

Mn

O0

050

030

090

090

170

080

040

110

100

070

020

050

090

030

080

020

030

020

030

03

Mg

O4

532

094

533

122

003

070

804

312

601

761

711

590

901

981

283

092

830

380

400

78

CaO

363

319

453

785

197

269

676

385

422

611

199

235

370

115

160

028

017

094

025

181

Na 2

O5

596

523

942

706

465

266

455

254

064

543

134

753

893

754

831

941

255

090

923

93

K2O

080

067

058

043

050

124

012

093

163

025

420

167

177

313

323

290

399

283

904

251

P2O

50

140

190

110

090

160

120

120

130

130

140

100

120

130

080

140

030

060

080

040

06

Tota

l99

99

100

0010

002

100

0110

000

100

0010

000

100

0010

000

100

0010

001

100

0010

000

100

0199

99

100

0010

000

100

0110

002

100

00

Ba

3p

.p.m

.14

612

519

694

8917

337

201

351

6446

737

834

739

040

862

724

642

041

366

3

Sr

1p

.p.m

.18

222

028

818

015

310

267

581

200

126

119

153

158

7618

535

3012

528

148

Y1

p.p

.m.

2716

2627

2925

2228

3530

2946

8230

7249

2535

7442

Zr

3p

.p.m

.12

591

114

115

167

133

100

115

148

154

114

172

195

7317

158

150

7027

316

9

XR

Fp

ress

edp

elle

tG

a5

p.p

.m.

1910

2517

1620

1514

2022

2821

2625

2123

1613

2128

Sn

5p

.p.m

.


INA

AA

u(p

.p.b

.)5

p.p

.b.

13


Tabl

e1:

cont

inue

d

Met

ho

dD

etec

.lim

its

FP-7

8FP

-128

FP-2

9FP

-130

FP-1

11FP

-122

FP-1

31FP

-121

FP-4

6FP

-127

FP-1

12FP

-113

FP-4

5FP

-129

FP-2

5FP

-107

FP-1

10FP

-28

FP-5

2

Rec

alcu

late

d10

0%

wat

erfr

ee

SiO

276

50

772

976

87

776

477

23

771

477

37

770

177

58

786

978

51

781

878

94

790

778

98

798

882

29

836

868

57

TiO

20

190

140

390

180

060

140

160

120

260

140

050

060

260

130

140

120

100

100

40

Al 2

O3

127

112

02

106

313

23

117

814

24

116

412

57

122

811

31

121

612

44

120

610

68

114

811

12

124

69

3616

12

Fe2O

33

523

032

122

021

180

932

191

361

711

230

410

541

461

641

090

370

501

024

35

Mn

O0

020

030

020

010

010

010

010

040

030

010

010

010

020

030

010

010

010

010

03

Mg

O2

923

212

252

431

520

500

100

240

340

230

600

370

310

250

050

220

740

074

33

CaO

032

056

071

196

008

016

042

020

197

009

006

009

034

035

001

022

020

042

088

Na 2

O1

021

222

241

411

034

653

764

334

621

891

122

224

431

194

353

170

265

000

60

K2O

275

241

469

103

711

220

431

409

113

636

706

607

212

660

387

486

342

032

467

P2O

50

060

080

060

090

020

040

040

040

070

060

030

030

060

070

020

030

030

020

05

Tota

l10

000

100

0099

99

100

0010

002

100

0110

000

100

0110

000

100

0110

001

100

0199

99

100

0210

000

100

0110

001

100

0110

000

Ba

3p

.p.m

.26

554

941

415

742

067

303

412

158

605

431

442

205

385

486

329

134

144

424

Sr

1p

.p.m

.42

3875

141

2323

5527

157

5427

2339

4537

7411

8613

Y1

p.p

.m.

3064

3165

5158

6539

3565

5650

3757

5028

3762

77

Zr

3p

.p.m

.16

413

384

176

7811

026

196

6988

8083

7116

012

013

588

4820

2

XR

Fp

ress

edp

elle

tG

a5

p.p

.m.

1923

1326

2220

2420

1722

2215

1523

208

2210

23

Sn

5p

.p.m

.9

88

14


INA

AA

u(p

.p.b

.)5

p.p

.b.


Tabl

e2:

Isot

opic

com

posit

ions

ofSr

and

Nd

for

the

selec

ted12

sam

ples

Sam

ple

Rb

(p.p

.m.)

Sr

(p.p

.m.)

87S

r/86

Sr (0

)2r

87R

b/8

6 Sr

e Sr(

0)f(

Rb

/Sr)

87S

r/86

Sr (3

68M

a)e S

r(36

8)

FP-9

1B

asal

t0

2741

278

070

3884

180

0019

0

874

2

097

700

7038

74

274

FP-4

0B

asal

t16

94

591

930

7037

7518

008

275

10

30

0006

070

3342

10

30

FP-2

0B

asal

t69

71

864

070

7045

2618

023

333

037

181

8214

070

3304

10

82

FP-1

5B

asal

t4

7438

691

070

4491

160

0354

6

012

2

057

120

7043

053

39

FP-1

02A

nd

esit

e47

81

342

790

7066

8716

040

347

310

473

8787

070

4573

721

FP-1

54A

nd

esit

e1

6475

937

070

4237

160

0062

4

373

2

092

450

7042

041

95

FP-8

3A

nd

esit

e19

17

358

460

7054

802

015

468

139

070

8704

070

4670

856

FP-5

Dac

ite

113

9918

955

071

3245

161

7407

712

413

200

492

070

4125

091

FP-2

7D

acit

e46

60

173

270

7083

5018

077

818

546

438

4097

070

4273

296

FP-4

6R

hyo

lite

463

615

051

071

0941

160

8914

591

428

977

930

7062

7031

33

FP-1

30R

hyo

lite

344

615

79

070

9951

180

6316

377

375

663

760

7066

4236

58

FP-7

9R

hyo

lite

345

015

727

070

8287

160

6347

453

752

667

520

7049

6112

73

Sam

ple

Sm

(p.p

.m.)

Nd

(p.p

.m.)

143 N

d/1

44N

d(0

)2r

145 N

d/1

44N

d2r

147 N

d/1

44N

de N

d(0

)f(

Sm

/Nd

)14

3 Nd

/144

Nd

(368

Ma)

e Nd

(368

)

FP-9

1B

asal

t3

7713

27

051

2789

130

3483

931

017

182

938

0

1263

051

2375

410

FP-4

0B

asal

t1

906

030

5128

097

034

8441

30

1904

333

7

003

150

5123

503

63

FP-2

0B

asal

t0

922

050

5127

1311

034

8527

50

2713

145

90

3797

051

2059

2

05

FP-1

5B

asal

t2

258

680

5126

8628

034

8454

30

1569

093

8

020

190

5123

082

80

FP-1

02A

nd

esit

e4

7020

66

051

2461

90

3484

495

013

76

345

4

030

010

5121

29

068

FP-1

54A

nd

esit

e2

2610

86

051

2511

220

3484

366

012

57

247

6

036

060

5122

080

85

FP-8

3A

nd

esit

e4

1418

68

051

2425

10

3484

3213

013

39

415

6

031

890

5121

02

121

FP-5

Dac

ite

510

189

00

5124

971

034

8391

60

1632

2

75

017

010

5121

04

118

FP-2

7D

acit

e9

8939

12

051

2649

80

3484

213

015

280

207

0

2226

051

2281

226

FP-4

6R

hyo

lite

279

139

40

5123

798

034

8416

30

1209

5

048

0

3852

051

2088

1

49

FP-1

30R

hyo

lite

426

164

10

5124

626

034

8426

30

1571

3

433

0

2008

051

2083

1

58

FP-7

9R

hyo

lite

449

205

50

5124

539

034

8418

30

1320

3

601

0

3285

051

2135

0

57

740


constrain the changes caused by the alteration processes that these basalts do not show calc-alkaline fractionationon the primary geochemistry of the Iberian Pyrite Belt trends. Munha (1983) found the same characteristics involcanic rocks, and to evaluate the relative mobility the basalts from the Portuguese sector of the Iberianof chemical elements, factor analysis of the principal Pyrite Belt. The rest of the major elements show a highcomponents has been used. This method has been largely dispersion of values, mainly those which are more mobileused to reveal the elemental association that, in this during secondary alteration (Na, Ca, Al, etc.), whereascase, could be an indication of remobilization owing to K and P values are less varied. Some of the observedsecondary processes (Howarth & Sinding-Larsen, 1983). variations may be primary, such as the contents in TiThe factors obtained give some clues that confirm the and Mg, which may reflect the fractionation of somemobility of some elements, already deduced by pet- ferromagnesian minerals. These variations in Ti and Mgrography and geochemistry. Thus, Na, Pb, Co, Si, Al, are accompanied by strong variations in Ni and Cr, andK, Ba and Cu, and to a smaller degree Rb, Mg, Ni, Sr, to a lesser degree Co, which agree with ferromagnesianCr and Th, should be considered as mobile elements, mineral fractionation.i.e. elements redistributed during secondary alteration in Basaltic andesites and andesites can be distinguished bythe Iberian Pyrite Belt. The mobility of some of these their major element contents along with the petrographicelements differs depending on the rock type. A similar analysis. Only one sample analysed in this study is asituation is found in other palaeovolcanic areas [see, e.g. basaltic andesite: FP-119. It shows the highest Ti, Fe andThorpe et al. (1993)]. Mn contents of the andesitic group and the lowest K

content, whereas it shows intermediate concentrations ofAl, Ca and Na. The rest of the andesites show a cleargeneral decreasing trend for Ti, Fe and Mg, whereas theGeneral classification and description ofsilica content increases. Two groups of andesites can begeochemical dataconsidered depending on their MgO content, which show

The chemical compositions of the studied volcanic rocks two divergent correlations with Ti, Fe and V contents.are shown in Table 1. Major element analyses have been Also, the Fe2O3t/MgO ratios vs Fe2O3t, Ti, Ti/V, etc.recalculated on a 100% water free basis to minimize the show the same divergent correlations. Munha (1983)effects of alteration by removing variations owing to found similar subdivisions for the Portuguese andesites.different loss on ignition values. The Nb/Y vs Zr/TiO2 These two groups of andesites do not show any pref-variation diagram (Fig. 5; Winchester & Floyd, 1977) has erential stratigraphic or geographic position.been used to establish a broad classification of the studied The most evolved rocks in the Iberian Pyrite Beltvolcanics. This diagram shows the existence of different volcanism, which are volumetrically dominant, are rep-types of rocks, which have already been deduced in the resented by dacites and rhyolites. A gradual geochemicalfield and from the petrographical studies. transition exists between the two groups of rocks. Despite

the fact that secondary alteration has caused a significantMajor elements change in the original composition of these rocks, with

a high perturbation of the Al, K, Na and Si contents,Major element compositions of basaltic rocks from theSpanish sector of the Iberian Pyrite Belt (see Table 1) several general tendencies can be established. Ti, Fe, Caindicate that most of them are sub-alkaline basalts and and P show an increasing depletion towards the mostthat a few samples show a slightly alkaline affinity. This evolved rocks, whereas no tendency is apparent for Nais suggested by the relatively high TiO2 (>12) and P2O5 and K. Mg is higher for the Si-poor rhyolites than for(~02) contents (see Table 1), the low contents in Zr and the dacites or the Si-rich rhyolites. These differences inthe low (


Fig. 5. Classification of the studied rocks on a Winchester & Floyd (1977) diagram.

crust normalized values. They also have the highestcontents in Ni, Cr and Mg, suggesting that these samplesrepresent the most primitive basalts in the studied area,and that they formed by high degrees of partial meltingof a peridotitic mantle source. Less primitive basalts (FP-15, FP-16, FP-1097 and FP-20) have similar con-centrations of compatible elements to the continentalcrust but show higher normalized concentrations of theincompatible elements than the average crust, withoutany possible correlation. Their low contents of compatibleelements suggest that they originated by lower degreesof partial melting, or that they suffered fractionation ofprimitive minerals during their ascent. The incongruent

Fig. 6. TiO2 vs Y/Nb diagram. Two samples fall in the alkaline field, enrichment in incompatible elements suggests that thesewhereas the rest fall on the continental tholeiites and mid-ocean ridge basalts could also have assimilated continental crust. In

basalt (MORB) fields. Fields are after Floyd & Winchester (1975).addition, secondary alteration may have had a moderateinfluence on the content of some incompatible elements.

Mg, Co, Fe) show significant variations and positive For instance, all the basaltic rocks show a negativecorrelations among the different samples. On the other anomaly in Cu, which is clearly associated with a sec-hand, the variations observed in the normalized con- ondary origin related to the ore deposition process.centration of the incompatible elements (Rb, Pb, U, Th, Trace element contents of andesitic rocks do not showBa, K) do not show any correlation among the different typical intermediate values between basalts and the moresamples of this group of rocks. The variation observed evolved rocks. On the contrary, their trends appear toin the compatible elements suggests differences in the be independent of the rest of the volcanic rocks from theprimary source of these basalts (differences in the source studied area. In some cases, trace element contents ofor different degrees of partial melting), or fractionation andesites are similar to the contents either of the basalticof ferromagnesian minerals which affects the contents of or the dacitic rocks. This suggests that andesites are notNi, Mg, Cr, etc. Thus, in Fig. 7, FP-80 and FP-114 plot related either to basalts or to dacites or rhyolites by

fractional crystallization.between the primitive mantle values and the continental

742


Fig

.7.

743


Fig. 7. Elements from all the studied rocks normalized to primitive mantle in order of incompatibility in the continental crust (Hofmann, 1988).Average continental crust after Taylor & MacLennan (1985) has also been plotted for comparison as shaded circles. The basalts and rhyolites

have been plotted in separate diagrams to increase clarity.

Andesites are characterized by an irregular distribution silica content variation, i.e. samples with the same silicacontent have large differences (hundreds of p.p.m.) ofof highly incompatible elements (Fig. 7), absence of K

and Na anomalies (except for sample FP-150, which has Ba, Zr, Th, U, Hf, etc., in both rock types. Dacites arethe volcanic rocks from the Iberian Pyrite Belt whicha positive anomaly in K and a negative one in Na), lower

Mg and Ni contents than the basalts, and a strong positive show normalized concentrations (Fig. 7) of trace elementscloser to the average continental crust (see Taylor &anomaly of Sn and a negative anomaly of Cu. As with the

basaltic rocks, the behaviour of the highly incompatible McLennan, 1985). However, many dacites are slightlyenriched in some incompatible elements with respect toelements may indicate different source characteristics.

However, the original contents of Pb, U, Th, Rb and the continental crust. Samples FP-154, FP-93 and FP-17, however, show extremely low values of Pb, Ba, K,Nb are clearly masked by secondary alteration. The

absence of Na and K anomalies implies that these ele- Nb, Sn and Sr. Most of the dacites show negativeanomalies in Ti and Cu, and a positive anomaly in Sn.ments were not highly mobilized. The anomalies in Sn

and Cu, which are found in nearly all rock types, are All the rhyolites show a very similar trend, with increasingnegative anomalies in Sr, Eu, Ti, Ca and Fe, and positiveclearly related to the ore-forming event.

Incompatible element contents of dacites and rhyolites anomalies in K and Sn as silica content increases (Fig.7).are extremely variable and they do not depend on the

744


the primary character of these rocks. The parallelismREEshown by most of the normalized patterns of the dacitesIn the REE normalized to chondrites (Nakamura, 1974)and rhyolites indicates the existence of different degreesdiagram (Fig. 8) it is possible to observe that most of theof partial melting in the formation of these two groupsnormalized concentrations of the basaltic rocks show aof rocks, and precludes mixing or assimilation betweenrelatively flat pattern and are limited by two extremethem. Only the previously mentioned anomalous rhy-samples (FP-20 and FP-80). Sample FP-20 shows theolites may have undergone such processes, as is reflectedhighest fractionation between light REE (LREE) andby the REE normalized pattern.heavy REE (HREE) [(La/Lu)n=578], i.e. it has the

maximum La content and the minimum Lu content ofall the group. Sample FP-80, however, shows an almostflat pattern with a positive Tb anomaly and a fractionation Isotope geochemistry and age of thevalue between Lan and Lun of 137. As has been indicated Iberian Pyrite Belt volcanismbefore, FP-80 represents the most primitive basalt of the Sr and Nd isotopic compositions have been obtained instudied area. The rest of the basalts show (La/Lu)n values 12 selected samples comprising 4 basalts, 2 andesites, 3ranging from 168 to 495. This, together with the Eu/ dacites and 3 rhyolites. The results are shown in TableSm values (026042), and the relative flat trends of the 2. Initial ratios, as well as e values, have been calculatedREE normalized to chondrite, suggests that most of the for the age of 368 Ma, as we will discuss below. Thebasaltic rocks from the Spanish sector of the Iberian constants used for all the calculations are: 87Sr/86SrURPyrite Belt have the characteristics of tholeiites from (0)=07045, 87Rb/86SrUR (0)=00827, kRb=1421011/continental settings [see Cullers & Graff (1984) and year for the Rb/Sr method, and 143Nd/144NdCHUR (0)=references therein]. Samples which show high LREE/ 0511847, 143Sm/144NdCHUR (0)=01967, kSm=HREE ratios are those with a slightly alkaline affinity. 6541012/year for the Sm/Nd couple.

Basaltic andesites and andesites show, in general, To constrain the age of the Iberian Pyrite Belt vol-homogeneous patterns with low fractionation between canism, several methods have been applied by differentLREE and HREE. (La/Lu)n is 342 for the basaltic workers (see Fig. 3 for details). The difficulty of datingandesite, and is slightly higher (391483) for the andes- these rocks by isotopic means is that they have undergoneites (Fig. 8). The main difference among the REE patterns a low-grade metamorphism. Some of these radiometricof these rocks is the Eu anomaly. The basaltic andesite ages are those of the metamorphism, and others (see Fig.and one andesite show no Eu anomaly (Fig. 8), whereas 3 for references) correspond to the age of the plutonicthe rest of the andesites show an increasing Eu anomaly rocks responsible for such metamorphism. Hamet &not related to any other geochemical feature, thus in- Delcey (1971) obtained an Rb/Sr age of 376 Ma for adicating the existence of different original plagioclase group of six rhyolites from Portugal. This age has alreadycompositions in the andesitic group, or variations on f (O2) been recalculated with the new decay constant (kRb=during crystallization. These changes in the plagioclase 1421011/year) and is statistically indistinguishablecomposition are not reflected in the other REE. Sample from our result (see below). Priem et al. (1978) dated theFP-144 shows the same fractionation between LREE and metamorphism which affects the Volcano-SedimentaryHREE but with very low values and with a straight Complex in Portugal using the Rb/Sr method and ob-pattern (except for Lu). This feature may also indicate tained an age of 30810 Ma, after recalculating it withdifferences in the source, but factor analysis suggests that the new decay constant. Dallmeyer et al. (1993), with thethis sample has been significantly altered. use of the 40Ar/39Ar method, have obtained a cooling

REE normalized diagrams of dacites and rhyolites (Fig. age of 335342 Ma for the intrusive bodies and the8) show parallel patterns except for three samples of associated low-grade metamorphism in the Ossa Morenarhyolite, which have irregular patterns. The fractionation Zone. Finally, De la Rosa et al. (1993) have obtained anbetween LREE and HREE is (La/Lu)n=314693 for errorchrone for pluton emplacement at the north of thethe dacites, and (La/Lu)n=248526 for the rhyolites. South Portuguese Zone by including in their data theTwo extreme rhyolitic samples have a value of 073 age obtained by Dallmeyer et al. (1993) in adjacent units.(sample FP-113) and 703 (sample FP-122). The Eu In this study, whole-rock KAr dating has been per-anomaly is clearly defined and reaches a maximum value formed on some samples to check the possibility of usingin the intermediate members of the group. One dacitic this method with the less altered samples or at least tosample shows low normalized contents in Yb and Lu date the metamorphic phase. All the ages obtained are(Fig. 8) with respect to the rest. These differences in the younger than the age of the metamorphism itself. ThisYb and Lu normalized contents have no equivalent in suggests that Ar has been removed (and probably K hasthe other elements, at least for the same samples, sug- been added) also after the metamorphic phase, as a

continuous process. Thus KAr ages in this case havegesting that secondary alteration has probably masked

745


Fig

.8.

RE

Eno

rmal

ized

toch

ondr

ites

afte

rN

akam

ura

(197

4)fo

rth

est

udie

dro

cks.

746


no geological meaning and no further discussion of these below the sea-floor up to 1000 m (Soriano, 1997). Thus,results will be presented here. their apparent position in the stratigraphic sequence does

Using the isochron method with the Sr isotopic com- not necessarily indicate a time relation with other volcanicposition on whole-rock samples, divided into basaltic and rocks in the same section. Also, many of these shallowcalc-alkaline rocks, we have obtained an age of 36864 intrusions show interfingering with the host sediments(1r error) Ma for the calc-alkaline suite (andesites, dacites and give the appearance, in the same stratigraphic profile,and rhyolites). This isochron age was calculated using of different intrusive events. In addition, some of thethe Model 3 solution of the Yorkfit calculation, which former volcanic episodes were defined based on theassumes that scatter is due to analytical error plus nor- general assumption that some of them were representedmally distributed error in initial 87Sr/86Sr, i.e. it takes by primary pyroclastic rocks. However, a detailed studyinto account not only the analytical error, but also the of the field relationships and textures of the volcanicgeological error. The basaltic rocks alone, however, give rocks has revealed that nearly all the volcanic rocks fromages inconsistent with the stratigraphy and with very the Spanish sector of the Iberian Pyrite Belt are intrusivehigh 1r errors. with some minor extrusive silicic domes which have

Using the Nd isotopic composition, the isochron age produced small lavas and associated hydroclastic vol-is 369150 Ma. This calculation assumes Model 3, as canogenic deposits (Soriano, 1997).before, and is the isochron for all the samples studied Field characteristics of the calc-alkaline and basalticindependently of their composition. If the age is calculated intrusives indicate that they were emplaced nearly alwaysby groups of rocks (calc-alkaline and basalts separately) at very shallow depths into wet sediments (Boulter, 1993a,the ages obtained are meaningless and the 1r errors are 1993b; Soriano, 1997). Volcanic intrusions reached rel-larger than the age itself. In contrast to the use of two ative higher positions in the stratigraphic succession asdifferent isochrons calculated with the Rb/Sr method, sedimentation progressed and, consequently, as the thick-the use of a single Sm/Nd age for all samples can be ness of sediments increased (Soriano, 1997). This fact,accepted, because the decay of Rb into Sr is twice as together with the presence of some lavas and volcanogenicfast as the decay of Sm into Nd. Thus, for the age that

sediments interbedded at different levels of the Volcano-we are studying (368 Ma) and for the Sm/Nd method,Sedimentary Complex, suggests that volcanism was moreit is possible to consider all the samples as members ofor less continuous during the entire deposition of thethe same group.volcano-sedimentary package, which covers a time spanThe age obtained, 36862 Ma, is statistically in agree-of ~30 m.y. (see Fig. 3).ment with the ages obtained by other workers (Fig. 3)

The calc-alkaline rocks show a preferential distributionusing radiometric and palaeontological methods. Thein the stratigraphic sequence depending on their pa-high 1r error on the final age is probably related to thelaeogeographic position. They are preferentially con-high dispersion of isotopic values owing to the use ofcentrated towards the top of the stratigraphic series indifferent rock types from different stratigraphic units.the north and northeast of the Iberian Pyrite Belt, whereasintermediate and silicic volcanics appear mainly at thebase of the stratigraphic sequence in the south and

DISCUSSION OF RESULTS southwest of the Spanish sector of the Iberian Pyrite Belt.The different positions in the stratigraphic sequence ofTemporal and spatial evolution ofcalc-alkaline rocks do not necessarily indicate the ex-volcanismistence of different volcanic episodes affecting the wholeAccording to Instituto Geologico y Minero de EspanaIberian Pyrite Belt. More probably, these differences(1982) and Oliveira (1990), the volcanism of the Iberianrepresent a migration of the focus of calc-alkaline vol-Pyrite Belt has been divided into five volcanic episodes:canism to the northeast, as suggested by Monteiro &(1) initial acid volcanism; (2) basic volcanism; (3) middleCarvalho (1987), probably related to the asynchronicityacid volcanism; (4) upper acid volcanism; (5) intrusivein the opening of the main fractures that controlleddiabases. This division is based on the stratigraphicthe subsidence of the sedimentary basins. This is alsoposition of volcanic rocks, their petrographic texture, andindicated by the irregular structure of the basin and bythe occurrence of sediments between volcanic packages.the migration of the depocentres (Soriano, 1997).Thus, two volcanic rocks separated by a sedimentary

The basaltic rocks, which are most commonly intrusive,interval in any stratigraphic section have, until now, beendo not show any stratigraphic or palaeogeographic con-interpreted as two different volcanic events in time,trol and appear throughout the stratigraphic sequencethe highest stratigraphic position corresponding to theof the Volcano-Sedimentary Complex. In the Portugueseyoungest volcanic event.sector, Munha (1983) found a stratigraphic control inMost of the volcanics in the Iberian Pyrite Belt are

shallow intrusives, ranging in depth from tens of metres the distribution of tholeiitic and alkaline basalts, which

747


were concentrated at the base and the top of the strati- Petrological and geochemical data presented in pre-graphic sequence, respectively. This relationship, how- vious sections demonstrate that most of the basaltic rocksever, has not been observed in the Spanish sector, where from the Iberian Pyrite Belt have a tholeiitic affinity andbasaltic rocks with both affinities appear as intrusives in that a few samples show a slightly alkaline affinity. Thisany part of the stratigraphic sequence. difference between the two types of basalts is indicated

Therefore, the previous division of the Iberian Pyrite by the Y/Nb values, by their TiO2 and P2O5 contentsBelt volcanism into several volcanic episodes should be (see Table 1), by the LREEHREE fractionation, theabandoned, at least for the Spanish sector, as it has no lack of mantle xenoliths or cumulates, and the absencemeaning in terms of magmatic and volcanic evolution. of normative Ne. The presence of both types of basaltsBased on palaeogeographic distribution, stratigraphic po- is common in different geodynamic settings and thesition, petrography and field textures (Soriano, 1997), we distinctive chemical features of tholeiites and alkalineconsider the volcanism of the Iberian Pyrite Belt as a basalts are related to different degree of partial meltingcontinuous, long-lived, volcanic episode which resulted from the same source region (Moore et al., 1995).from a magmatic event coeval with the tectonic processes The mixing model between E- and N-MORB used tothat controlled the formation of basins where the Iberian explain the origin of the most primitive basaltic rocksPyrite Belt sedimentary succession was deposited. Differ- does not discriminate between the origin of both tholeiiticent pulses may logically be distinguished in this volcanism, and alkaline affinities in the same basaltic volcanism.suggesting the existence of alternating active and less However, the existence of tholeiitic and alkaline affinitiesactive periods. However, this does not indicate any may be explained by different degrees of partial meltingchange in the tectonic conditions that caused this vol- of a peridotitic mantle (see Carmichael et al., 1974; Hall,canism or in the nature of its products. The geodynamic 1987; Wilson, 1989), as is suggested by the REE patternsframework of this volcanism and the origin of the as- shown by the basaltic rocks studied here. The highersociated sedimentary basins are discussed in a later sec- degree of partial melting is reflected in the lower LREE/tion. HREE ratio of tholeiites with respect to that of the basalts

with a slightly alkaline affinity and by the flat patternsof the normalized REE.

Origin of basaltsThe trace elements and REE compositions and the high

Origin of intermediate and silicic rocksSr/Nd values (>20) shown by some of the basaltic rocksfrom the Iberian Pyrite Belt suggest that they originated Trace elements and REE compositions of intermediatein the asthenospheric mantle (see Zindler et al., 1981). and silicic rocks from the Iberian Pyrite Belt demonstrateThese basalts are the most primitive of the studied area that they are not related by fractional crystallizationand derive from different mantle sources that can be processes. Munha (1983), using classical geochemicalinternally related to the E- and N-MORBs by a single data, also concluded that no fractional crystallization ismixing model calculation (Figs 9 and 10). The rest of the involved in the formation of the evolved rocks. Thatbasaltic rocks may be explained by assimilationfractional researcher suggested that andesites directly derived fromcrystallization (AFC) processes or more likely by mixing of partial melting of the mantle, but their high Sr contentthe mantle-derived basaltic magmas with crustal materials precludes that possibility. Isotopic compositions reveal(Figs 9 and 10). They have lower Sr/Nd values, which that andesites (samples FP-83 and FP-102) fall in a moreindicate that they fractionated plagioclase, assimilated evolved position than the dacites (Figs 12 and 13), havingcrustal material or assimilated magmas derived from very similar Nd isotope ratios to rhyolites but different

Sr isotope ratios. This suggests that the andesites maycrustal melts.The influence of continental crust on the primary have formed by direct mixing of mantle-derived basalts,

which follow the mantle array, with young upper-crustmagmas of some of the basaltic rocks from the IberianPyrite Belt is also shown on the Ce/NbTh/Nb diagram material. Also, the Sr isotopic and absolute compositions,

and the Zr/Nb, La/Sm and Ce/Th (Figs 9 and 10), andand by the ratio Th/Nb (Figs 10 and 11). The Ce/NbTh/Nb diagram (Fig. 10) shows how almost all the the low Sm/Nd ratios, confirm this point, as andesites

are numerically between FP-40 (or even FP-91) andbasaltic samples plot inside an area that represents mixingbetween E-MORB, N-MORB and continental crust. upper-crust average. The fact that andesites have higher

eSr and lower eNd than dacites implies that there is noIsotopic compositions confirm mixing as the most plaus-ible mechanism to explain the diversity of compositions relation by fractional crystallization between andesites

and rhyolites through the dacites.shown by the basaltic rocks from the Iberian Pyrite Beltand their relationship with the other volcanic rocks (Figs The high variation in eNd, Sr (p.p.m.), Rb (p.p.m.) and

Nd (p.p.m.), together with the low variation in eSr shown12 and 13).

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Fig. 9. La/Sm normalized to chondrites vs Zr/Nb. The hyperbola represents a single mixing model with the end-members in sample FP-20and N-MORB. The mixing line passes through E-MORB, and samples FP-40, FP-34, FP-31 and FP-80. Sample FP-20 is the most LREEHREEfractionated basalt (Fig. 12), with clear tholeiitic characteristics, whereas FP-80 is the more primitive of the studied rocks and has a flat REEpattern (Fig. 12). From FP-80 and FP-31, two mixing or AFC lines depart towards sample FP-7 (one clearly contaminated basalt which also

falls in the field occupied by the rest of the samples, the continental crust, and the analysed sediment).

by the dacites, and the high variation on eSr with almost and REE (mainly Sr), and the high variation of the eNd,demonstrates that not all of the dacitic samples supportno variation of eNd, Sr (p.p.m.) and Nd (p.p.m.) shown

by the rhyolites (Fig. 13), suggests that they are derived fractional crystallization as their genetic process. Thus,most of the dacites, like the rhyolites, can each beby different degrees of partial melting of different crustal

rocks. Moreover, despite the fact that sample FP-5 (dacite) considered as an individual case of partial melting ofcrustal material of different type (variations in Sr andhas coincident Nd (p.p.m.) and eNd with FP-83 (andesite),

dacite samples appear not to be related by fractional Nd concentrations) and age (differences in eSr and eNd).Only samples FP-5 and FP-27 could be considered to becrystallization to any of the andesites. This last point is

demonstrated by the lower values in eSr of the dacites generated by fractional crystallization processes from amixture between basalts and rhyolites.with respect to the andesites and [for two dacites (FP-

27 and FP-154)] by the higher values of eNd with respect The presence of high-silica content rhyolites is char-acteristic of both sectors of the Iberian Pyrite Belt (Munha,to the andesites (Fig. 13). Furthermore, there is no relation

between dacites and rhyolites by fractional crystallization 1983; this work). Despite the fact that this high silicacontent may be due to a secondary silicification [most(see Fig. 12). In contrast, the eSr of the analysed dacites

(Fig. 12) is nearly the same. This could suggest that of these rocks have been classified as quartz keratophyresby previous workers (Soler, 1980)], it may also be adacites are genetically related to them by fractional

crystallization and, as shown in Fig. 12, they can be primary feature. This is also suggested by the fact thatthe high-silica content rhyolites show the highest negativederived by the same process from a mixing product

between basalts and rhyolites. However, as we have seen, Eu anomaly. Silica-rich rhyolites are typical of areaswith bimodal volcanism rather than being generatedthe differences in concentration of some trace elements

749


Fig. 11. Nb vs. Th plot for the studied basalts. There is a clear positiveratio between Th and Nb that relates the two end-members FP-80 andFP-20 with other samples which plot on the same line. It seems atypical fractional crystallization (FC) line, but it passes over the lower-crust composition, suggesting that it may be coincident with a mixingor contamination line. There are three other trends: the first showsincreasing Nb and constant Th and relates samples FP-31, FP-34 andFP-40 by partial melting processes (these samples occupy intermediateFig. 10. Ce/Nb vs Th/Nb. This diagram demonstrates the influencepositions on the hyperbola in Fig. 16); the second, which is not as clearof continental crust on the evolution of some basaltic rocks from theas the others, relates some of the basalts to the basaltic andesites andIberian Pyrite Belt. Most of the basaltic samples plot inside a mixingto the sediment sample; the third line shows a high increase in Th andtriangle with the three end-members E-MORB, N-MORB and averageconstant Nb and connects samples FP-16, FP-15 and FP-7. These threecontinental crust (Taylor & MacLennan, 1985). Three samples (FP-samples always follow the same pattern, which suggests mixing with16, FP-15 and FP-7) fall out of the triangle towards an external end-source materials that are rich in Th but poor in Nb (see also Fig. 10).member, which for samples FP-15 and FP-16 could be the analysedAn increase in Th with no modification in Nb may be explained bysediment, and for sample FP-7 could be an artefact owing to its

secondary alteration or sediment assimilation.secondary modification.

by fractional crystallization processes from calc-alkalineandesites (Christiansen & Lipman, 1972). We suggestthat the high silica content that characterizes some ofthe Iberian Pyrite Belt rhyolites is a primary characterprobably owing to differences in the source, consistentwith an origin by partial melting of upper crust drivenby basaltic magmas.

Therefore, geochemical and isotopic data presented inthis paper negate any relationship by pure fractionalcrystallization between basalts, intermediate rocks andsilicic rocks. This fact, together with the volumetricpredominance of silicic rocks and the widespread oc-currence of basaltic volcanism, suggests that in the IberianPyrite Belt calc-alkaline silicic magmas were generatedon a large scale by the invasion of continental crust bymafic magmas generated in the underlying upper mantle.

Fig. 12. Diagram of 87Sr/86Sr vs SiO2. A three end-member mixingMantle-derived magmas can provide large quantities of (trend 1) can be established taking as end-members FP-91, FP-20 andheat for partial melting and assimilation of lower- and the rhyolite FP-130 (taken as an isotopic equivalent of the continental

crust in this area). It is also feasible to consider the rhyolite FP-79 asupper-crustal rocks (Huppert & Sparks, 1988; Kaczor etthe evolved end-member (trend 2). From these mixing fields a regularal., 1988; Grunder, 1995). The diversity of compositions FC pattern can be observed for the dacites, but it is not consistent with

shown by dacites and rhyolites can be explained either Nd isotope data nor with trace element contents. Andesites can beconsidered as mixing products.by differences in the composition of the source rocks or

by different degrees of partial melting of upper-crustrocks. In contrast, andesites formed by direct mixing ofmantle-derived basalts with young upper-crust material, the intrusion of mantle-derived magmas or by direct

partial melting of the upper mantle.rather than by partial melting of lower crust induced by

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(4) The Baixo Alentejo Flysch Group, which is com-posed of a thick sequence of syntectonic turbiditic depositsof Upper Visean to Namurian age (Oliveira, 1990) andis bounded to the south by the Iberian Pyrite Belt terranewith a thrust plane that dips to the north (Quesada,1991). At the north of the Baixo Alentejo Flysch Groupthe southern part of the Ossa Morena Zone is consideredto represent a continental terrane with calc-alkaline arc-related volcanism associated with the subduction of theBejaAcebuches ophiolite (Santos et al., 1987).

The geological characteristics and boundaries of thesetectonostratigraphic domains and the occurrence of awell-defined suture between the South Portuguese andthe Ossa Morena Zones (Munha et al., 1986; Silva et al.,1990; Crespo-Blanc & Orozco, 1991; Dallmeyer et al.,

Fig. 13. eNd isotope composition vs. eSr isotope composition diagram. 1993; Giese et al., 1994; Quesada et al., 1994) have ledSample FP-40 falls on the mantle array and is very close to E-MORB.to a general consensus on the tectonic evolution of theSample FP-91 is slightly deviated towards the pattern defined by the

other recent continental volcanism [see the review by Worner et al. area. From Upper Devonian (Frasnian and Famennian)(1986)] which is followed by FP-15 (a basalt with high influence of the through the entire Carboniferous Period the South Por-upper crust or sediment). Sample FP-20 falls between HIMU and EMI.

tuguese and the Ossa Morena plates were continuouslyThis sample coincides with the zone equivalent to the North Atlanticconverging. This plate convergence evolved with timeScottish Tertiary volcanic provinces (Carter et al., 1978). These authors

explain this field as being produced by contamination of granulites from a subduction of South Portuguese oceanic lith-from the lower crust. With this distribution of basalts on the eNd vs. eSr osphere beneath continental Ossa Morena crust to aplot, one can deduce mixing and evolution lines towards the rhyolitic

continental collision of both plates (Monteiro & Carvalho,end-members to explain the origin of all the studied rocks.1987; Silva et al. 1990; Giese et al. 1994; Quesada et al.,1994). In such a moving plates scenario the Iberian PyriteBelt volcanism took place. The tectonic style of theGeodynamic setting and origin of theIberian Pyrite Belt terrane (Soriano, 1996, 1997), as withIberian Pyrite Belt volcanismthe rest of the South Portuguese Zone terranes (Ribeiro

Tectonostratigraphic terrane analysis of the South Por- et al., 1983), also agrees with such a geodynamic evolution,tuguese Zone and the southern part of the Ossa Morena as most of the structures clearly show an eastwest trendZone (Oliveira, 1990; Eden, 1991; Quesada, 1991) has and south vergence.revealed the occurrence of several well-differentiated Such a consensus, however, does not surround thegeological domains which have a specific significance in interpretation of the tectonic setting of the Iberian Pyriteterms of Variscan plate tectonics evolution (Fig. 1). Thus, Belt, which has been considered to have formed in afrom north to south the tectonostratigraphic terranes of variety of settings. These include: (1) a convergent platethe South Portuguese Zone are: boundary, in an island arc generated over a subduction

(1) The BejaAcebuches ophiolitic complex, which has zone (Schutz et al., 1988); (2) extensional tectonics as-recently been interpreted as an Early Devonian oceanic sociated with back-arc spreading developed on the Southcomplex (Munha et al., 1986; Quesada et al., 1994) and Portuguese continental plate and involving a subductionwhich represents an oceanic crust of the South Portuguese of the Ossa Morena Zone (Soler, 1973); (3) extensionalplate subducted beneath the continental crust of the Ossa tectonics associated with the initial stages of a back-arcMorena plate in Upper Famennian to Lower Car- spreading developed on a continental plate (Munha,boniferous times (Silva et al., 1990). 1983); (4) a forearc basin developed on the continental

(2) The Pulo do Lobo oceanic accretionary prism, crust of an overriding plate, the Ossa Morena Zonewhich is an oceanic terrane composed of siliciclastic (Monteiro & Carvalho, 1987).sediments of Lower to Upper Devonian age (Oliveira et These different interpretations of the tectonic settingal., 1986) deposited on an oceanic lithosphere. of the Iberian Pyrite Belt are in part due to the lack of

(3) The intracontinental Iberian Pyrite Belt volcanic a precise interpretation of the nature of the associatedterrane, which is composed of marine turbiditic deposits volcanism. Most of the previous studies concerning theand shallow calc-alkaline and basaltic sill-complexes that tectonic setting of the Iberian Pyrite Belt have con-intruded over a continental well-differentiated crust centrated on the interpretation of its stratigraphical and(Munha, 1983). Fauna of turbiditic deposits and isotopic structural features, but have not considered in detaildating of shallow intrusions indicate a Late Devonian to whether the characteristics of volcanism were compatible

with the various proposed tectonic settings. To determineEarly Carboniferous age (Fig. 3).

751


the validity of the previous interpretations of the Iberian Quesada et al., 1994) preclude its location in a fore-arcsetting.Pyrite Belt tectonic setting, we compare them with the

Recently, Silva et al. (1990), Giese et al. (1994) andpetrological and geochemical data presented in this study.Quesada et al. (1994) have proposed a new interpretationMineralogical and geochemical data of the Iberianfor the Iberian Pyrite Belt volcanism based on terranePyrite Belt volcanics (Routhier et al., 1977; Soler, 1980;analysis. They suggest that local extensional tectonics,Munha, 1983; this paper) indicate a clear continentalrelated to transtension owing to oblique collision, de-crust affinity for the calc-alkaline rocks. This precludesveloped in the continental crust of the South Portuguesea certain number of tectonic settings for the Iberianplate during a continental collision of the South Por-Pyrite Belt volcanism. On the basis of this evidence, andtuguese and the Ossa Morena plates. This strike-slipin accordance with previous workers (Schermerhorn,tectonics caused the opening of pull-apart basins, where1975; Munha, 1983), an island arc setting must besubmarine terrigenous sedimentation occurred, as wellrejected, as the volume of rhyolites and dacites in theas the bimodal volcanism which characterizes the IberianIberian Pyrite Belt is too high compared with modernPyrite Belt. The location of the Iberian Pyrite Belt as-analogues of island arc environments (Carmichael et al.,sumed by this model is consistent with the geodynamic1974; Baker, 1982; Gill, 1981; Wilson, 1989). A well-framework described above, and also with the palaeo-developed lower and upper continental crust is necessarygeographic reconstruction of the area (Soriano, 1997).to explain the geochemistry of the Iberian Pyrite BeltHowever, there is not a clear modern analogue that canvolcanism. In addition, basaltic rocks do not show anybe used to define the main geochemical guidelines ofsubduction-related affinity. They have higher TiO2 andsuch a volcanism. Previous models of collision-zone mag-Fe2O3t contents, and lower K, Sr and Zr contents than matism do not consider this possibility (see Harris et al.,typical subduction-derived basaltic magmas (Carmichael1986) and no indications of the geochemical signatureet al., 1974; Wilson, 1989), and they do not have hydrousof this magmatism exist. Despite this fact, we considerminerals. In addition, andesitic rocks also differ min-that the geochemical and isotopic data presented in thiseralogically from typical island-arc andesites, which nor-study are compatible with this tectonic setting.mally have orthopyroxene.

The ultimate source for the Iberian Pyrite Belt vol-Basalt and rhyolitic associations such as those observedcanism is the asthenospheric mantle, where large volumesin the Iberian Pyrite Belt are commonly found in ex-of tholeiitic, and occasionally alkaline, basaltic magmastensional tectonic setting such as continental rifts ororiginated. Extensive melting of the asthenospheric

continental back-arcs. However, mineralogical and geo- mantle was probably induced by rapid decompressionchemical compositions of the Iberian Pyrite Belt volcanics caused by the effect of regional strike-slip tectonics whichdo not show any evidence of a subducting slab de- affected the entire continental crust and probably thehydration such as invariably occurs in back-arc volcanism lithospheric mantle, thus causing a restricted lithospheric(Tarney et al., 1977; Weaver et al., 1979; Wilson, 1989). thinning. The large LREE depletion and the absence ofCalc-alkaline basalts are also a common feature in mod- Eu anomaly observed in most of the studied basalticern back-arcs (Weaver et al., 1979; Wilson, 1989), but rocks are compatible with a continental breakup settingnone of the basalts in the Iberian Pyrite Belt have (see Cullers & Graf, 1984). It is important to note,this calc-alkaline pattern. Moreover, the sequence of however, that the Iberian Pyrite Belt did not form in atectonostratigraphic domains described above is clearly post-collisional extensional setting. The strike-slip tec-inconsistent with a back-arc setting for the Iberian Pyrite tonics responsible for the opening of the basin and theBelt volcanism. The geochemical features of this vol- associated volcanism developed as a direct response tocanism also differ from the distinctive characteristics of the oblique continental collision between the Ossa Mo-modern and ancient intracontinental rift zones [alkaline rena and the South Portuguese plates, which lastednature, enrichment in large ion lithophile elements from Middle Devonian until Late Carboniferous, and(LILE)] (see Bailey, 1983; Wilson, 1989). culminated with the Variscan orogeny in that area. Thus,

A fore-arc location should be also rejected in the light in such a scenario the existence of significant lithosphericof several features: some influence of a subducting slab stretching caused by the relaxation of compressionalshould be observed in the geochemistry of the Iberian stresses is unlikely as the compression had not ceased.Pyrite Belt volcanics if they had formed in a fore-arc Basaltic magmas invaded a relatively thick continentalzone developed above the continental crust that overrides crust causing both assimilation of crustal materials anda subducting oceanic crust. On the other hand, the nature extensive melting of upper-crust rocks. This gave rise to(an accretionary prism) and actual position of the Pulo the formation of andesites and silicic rocks, respectively.do Lobo oceanic terrane and the Baixo Alentejo Flysch The magmatic process that gave rise to the IberianGroup with respect to the Iberian Pyrite Belt (Monteiro Pyrite Belt volcanism was, thus, intimately related to

the tectonic regime which controlled the opening and& Carvalho, 1987; Giese et al., 1988; Silva et al., 1990;

752


evolution of the sedimentary basin. This process was comments by Cecilio Quesada and an anonymous re-viewer are also gratefully acknowledged. This researchpulsating rather than continuous, as is indicated by

changes in the structure of the basin with time and in was (in part) supported by the research project entitled:Proyecto de investigacion paleogeografica y vol-the position of the depocentres (Soriano, 1997). These

pulses caused different intensities of decompression in canologica en la Faja Pirtica del SW de Espana (ITGEFundacio Bosc i Gimpera). We thank the Institutothe mantle and this would explain the existence of

differences in the degree of partial melting of the as- Tecnologico y Geo Minero for giving us permission topublish part of the results from that project.thenospheric mantle, which would explain the existence

of tholeiitic and alkaline affinities for the basaltic magmas.However, in the Spanish sector there is no evidence tojustify a sequence of events that could explain the tem-

REFERENCESporal evolution from tholeiites to alkaline basalts thatBailey, D. K., 1983. The chemical and thermal evolution of rifts.Munha (1983) suggested for the Portuguese sector.

Tectonophysics 94, 585597.Baker, P. E., 1982. Evolution and classifications of orogenic volcanic

rocks. In: Thorpe, R. S. (ed.) Andesites: Orogenic Andesites and RelatedCONCLUSIONS Rocks. New York: John Wiley, pp. 1123.

Barriga, F. J. A. S. & Kerrich, R., 1984. Extreme 18O-enrichedThe Iberian Pyrite Belt volcanism is mainly representedvolcanics and 18O-evolved marine water, Aljustrel, Iberian Pyrite Belt:by shallow intrusives emplaced into wet turbiditictransition from high to low Rayleigh number convective regimes.siliciclastic deposits of early Carboniferous age.Geochimica et Cosmochimica Acta 48, 10211031.

Volumetrically, this volcanism can be considered as bi- Boulter, C. A., 1993a. High level peperitic sills at Ro Tinto, Spain:modal, with a predominance of basaltic rocks of tholeiitic implications for stratigraphy and mineralization. Transactions, In-affinity and calc-alkaline silicic rocks, despite the presence stitution of Mining and Metallurgy, Section B: Applied Earth Sciences 102,

B30B38.of some basalts with alkaline affinity and intermediateBoulter, C. A., 1993b. Comparison of Ro Tinto, Spain, and Guaymascalc-alkaline rocks. Differences in composition shown by

Basin, Gulf of California: an explanation of a supergiant massivethe basaltic rocks can be explained by a single mixingsulfide deposit in an ancient sillsediment complex. Geology 21,model involving three end-members: E-MORB, N- 801804.

MORB and continental crust. Silicic calc-alkaline rocks Carmichael, I. S. E., Turner, F. J. & Verhoogen, J., 1974. Igneousoriginated by large-scale partial melting of upper con- Petrology. New York: McGrawHill.tinental crust induced by the intrusion of basaltic magmas Carter, S. R., Evensen, N. M., Hamilton, P. J. & ONions, R. K.,

1978. Neodymium and strontium isotope evidence for crustal con-generated in the underlying upper mantle. In contrast,taminations of continental volcanics. Science 202, 743747.andesites originated by direct mixing of mantle-derived

Christiansen, R. L. & Lipman, P. W., 1972. Cenozoic volcanism andbasalts with young upper-crustal material. The volcanismplate-tectonic evolution of the western United States. IILate

covers a long time span (15 m.y.) and does not show any Cenozoic. Philosophical Transactions of the Royal Society of London A271,significant temporal or spatial variation. The existence 249284.of tholeiitic and alkaline affinities in the basaltic rocks is Crawford, A. J., Corbett, K. D. & Everard, J. L., 1992. Geochemistry

of the Cambrian volcanic-hosted massive sulfide-rich Mount Readinterpreted to be due to different degrees of partialVolcanics, Tasmania, and some tectonic implications. Economic Geo-melting in the asthenospheric mantle caused by differentlogy 87, 597619.intensities of tectonically driven decompression. The new

Crespo-Blanc, A. & Orozco, M., 1988. The Southern Iberian Sheargeochemical and isotopic data are consistent with theZone: a major boundary in the Hercynian folded belt. Tectonophysics

stratigraphic and structural data, which suggest that 148, 221227.the Iberian Pyrite Belt developed in a complex setting, Crespo-Blanc, A. & Orozco, M., 1991. The boundary between theinvolving local extensional tectonics within the South OssaMorena and South Portugues

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Magmatic Evolution and Tectonic Setting of the Iberian Pyrite Belt