Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores

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Geochemical evidence concerningsources and petrologic evolution ofFaial Island, Central AzoresEvandro Fernandes de Lima a , Adriane Machado b , LauroValentim Stoll Nardi a , Dejanira Luderitz Saldanha a , José ManuelMartins Azevedo b , Carlos Augusto Sommer a , Breno LeitãoWaichel c , Farid Chemale Jr d & Delia del Pilar Montecinos deAlmeida ea Instituto de Geociências, Universidade Federal do Rio Grande doSul, Porto Alegre, RS, Brazilb Centro de Geofísica da Universidade de Coimbra-CGUC,Coimbra, Portugalc Universidade Estadual do Oeste do Paraná (UNIOESTE), RuaUniversitária, Jardim Universitário, Cascavel, PR, Brazild Universidade Federal de Sergipe DEAPE/Universidade Federal deSergipe – Cidade Universitária Prof. José Aloísio de Campos s/n,Jardim Rosa Elze, São Cristóvão-Se, Brazile Universidade Federal do Pampa UNIPAMPA, Campus Caçapava doSul Av. Pedro Anunciação, s/n, Vila Batista, Caçapava do Sul, RS,Brazil

Available online: 28 Jul 2010

To cite this article: Evandro Fernandes de Lima, Adriane Machado, Lauro Valentim Stoll Nardi,Dejanira Luderitz Saldanha, José Manuel Martins Azevedo, Carlos Augusto Sommer, Breno LeitãoWaichel, Farid Chemale Jr & Delia del Pilar Montecinos de Almeida (2011): Geochemical evidenceconcerning sources and petrologic evolution of Faial Island, Central Azores, International GeologyReview, 53:14, 1684-1708

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International Geology ReviewVol. 53, No. 14, December 2011, 1684–1708

ISSN 0020-6814 print/ISSN 1938-2839 online© 2010 Taylor & FrancisDOI: 10.1080/00206814.2010.496248http://www.informaworld.com

TIGR0020-68141938-2839International Geology Review, Vol. 1, No. 1, Jun 2010: pp. 0–0International Geology ReviewGeochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores

International Geology ReviewE.F. de Lima et al.Evandro Fernandes de Limaa*, Adriane Machadob, Lauro Valentim Stoll Nardia, Dejanira Luderitz Saldanhaa, José Manuel Martins Azevedob,

Carlos Augusto Sommera, Breno Leitão Waichelc, Farid Chemale Jrd

and Delia del Pilar Montecinos de Almeidae

aInstituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil; bCentro de Geofísica da Universidade de Coimbra-CGUC,

Coimbra, Portugal; cUniversidade Estadual do Oeste do Paraná (UNIOESTE), Rua Universitária, Jardim Universitário, Cascavel, PR, Brazil; dUniversidade

Federal de Sergipe DEAPE/Universidade Federal de Sergipe – Cidade UniversitáriaProf. José Aloísio de Campos s/n, Jardim Rosa Elze, São Cristóvão-Se, Brazil;

eUniversidade Federal do Pampa UNIPAMPA, Campus Caçapava do Sul Av. Pedro Anunciação, s/n, Vila Batista, Caçapava do Sul, RS, Brazil

(Accepted 17 May 2010)

Volcanic rocks that make up Faial Island, Central Azores, consist of four volcano-stratigraphic units, with ages between 730 ka and the present. Lavas range from alkalibasalts to trachyandesites and belong to the alkaline-sodic series. The oldest unit is theRibeirinha Volcanic Complex, generally characterized by low MgO contents. TheCedros Volcanic Complex is composed of basalts to benmoreites with low MgOcontents. The Almoxarife Formation represents fissure flows, containing MgO contentssimilar to to slightly higher than those of the underlying Cedros Volcanic Complex.The youngest unit, the Capelo Formation, consists of mafic rocks with MgO valueshigher than those of the other units. Bulk-rock major and trace element trends suggestthat differentiation of the three earliest units were dominated by fractional crystalliza-tion of plagioclase ± clinopyroxene ± olivine ± titanomagnetite. Capelo bulk-rockcompositions are the most primitive, and are related to a period when volcanic activitywas fed by deep magmatic chambers, and melts ascended more rapidly. Comparisonamong geochemical patterns of the trace elements suggests a strong similarity betweenthe lavas from Faial and Pico islands. Corvo Island volcanism contrasts with thegeochemistry of Faial and Pico lavas, reflecting its strong K and Rb depletion, and Th,U, Ta, Nb, La, and Ce enrichment. Absence of the Daly gap in the Faial volcanics isattributed to early crystallization of Ti-Fe oxides. The probable source of the Faialmagma coincides with the MORB-FOZO array, which implies the presence of ancientrecycled oceanic crust in the mantle source. Ratios of incompatible trace elementssuggest the similarity of Corvo volcanic rocks with magmas derived from HIMUsources, whereas the Faial and Pico volcanic rocks could have been produced fromsources very close to EMII-type OIB.

Keywords: Azores archipelago; oceanic island volcanism; mantle sources; FOZO;Daly gap

*Corresponding author. Email: [email protected]

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mailto:[email protected]

International Geology Review 1685

1. IntroductionThe most common mafic lavas of the Azores are alkaline basalts, associated with minorvolumes of transitional types. Basic rocks predominate in all islands, occurring also inter-mediately – mugearites, benmoreites, and trachytes, as well as acid rocks in some islands.Schmincke and Weibel (1972) and Schmincke (1973) reported the occurrence of comenditictrachytes in Terceira and São Miguel islands, and comendites and pantelerites in TerceiraIsland. According to these authors, São Miguel Island shows the most K2O-rich rocks in thearchipelago, whereas Santa Maria Island contains the most sodic basalts (White et al. 1979).

Recently, many scientists have discussed the volcano-stratigraphy of the Azoreanarchipelago and investigated possible magmatic sources, chemistry, and evolution (e.g.França 1993; Almeida and Rodrigues 1993; França and Rodrigues 1993/1994; Françaet al. 1995a, 1995b, 2006a, 2006b, 2008; Widom and Shirey 1996; Widom 2002;Azevedo and Portugal Ferreira 2006; Machado and Azevedo 2006). Available bulk-rockgeochemical data on Faial rocks, however, are scarce and little information has beenpreviously published (White et al. 1979; Machado et al. 2008).

This article presents a large dataset consisting of whole-rock analyses, major and traceelements, including rare earth elements (REEs), as well as isotope data, to discuss andcharacterize the geochemistry of Faial rocks and to investigate the possible sources andmagmatic evolution of Faial Island.

2. Sample preparation and analytical techniquesForty-eight samples representing the four volcanic stages on Faial Island were studied.The whole-rock chemical analyses were performed at the Actlabs (Canada) using theWRA + trace 4 Litho-research routine. Major elements were determined by ICP-AES andthe trace elements (including REEs) by ICP-MS.

For the Sr, Nd, and Pb analyses, whole-rock powders were spiked with mixed 87Rb-84Sr and 149Sm-150Nd tracer and dissolved in a Teflon vial using an HF–HNO3 mixtureand 6N HCl until complete material dissolution. The Sr and REE were separated usingcation exchange columns (resin AG-50×-X8/200–400 mesh). The Sm and Nd wereseparated from other REEs in cation exchange columns prepared with Teflon LN-B50-A (100–150 mesh) following Patchet and Ruiz (1987). Each sample was dried to asolid and then loaded with 0.25N H3PO4 onto an appropriated filament (single Ta for Rb,Sr, and Sm; and triple Ta-Re-Ta for Nd). Isotopic ratios were measured in static modewith a VG Sector 54 multi-collector mass spectrometer at the Isotope Geology Laboratoryof Universidade Federal do Rio Grande do Sul (Brazil). We normally collected 100–120ratios with a 1 V 88Sr beam and a 0.5–1 V 144Nd beam. Sr and Nd ratios were normalizedto 87Sr/86Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. All analyses were adjustedfor variations instrumental bias due to periodic adjustment of collector positions as moni-tored by measurements of our internal standards: 87Sr/86Sr = 0.710270 ± 0.000019 (NBS987 value for 87Sr/86Sr = 0.71025) and for the JNdi-1 standard 143Nd/144Nd = 0.512110 ± 0.00007 (literature value for 143Nd/144Nd = 0.512115 ± 0.00007,Tanaka et al. 2000). Total blanks average were <100 pg for Sr and Sm and <500 pg forNd. Correction for blank was insignificant for Sr and Nd isotopic compositions and gener-ally insignificant for Rb/Sr and Sm/Nd ratios. Neodymium crustal residence ages (TDM)were calculated following the depleted mantle model of DePaolo (1981).

For the Pb isotopic measurements, an aliquot of 1 ml from dissolved WR samples usedfor Rb-Sr and Sm-Nd analysis has been taken. Pb was extracted using 2 ml micro-columns

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1686 E.F. de Lima et al.

with AG-1 × 8, 200–400 mesh, anion resin. Each sample was dried to a solid and addedto a solution of HNO3 with 50 ppb Tl to correct the Pb fractionation during the analyses assuggested by Tanimizu and Ishikawa (2006). Pb isotope analyses were carried out at theIsotope Geology Laboratory of UFRGS with a Neptune MC-ICP-MS in static mode, bycollecting 60 ratios of Pb isotopes. The obtained values of NBS 981 common Pb standardduring the analyses were in agreement with the NIST values.

3. Geological settingThe Azores archipelago lies in the area limited by coordinates 37°–40°N and 25°–31°W inthe North Atlantic Ocean, close to the Mid-Atlantic Ridge (MAR) and to the triple junctionbetween American, African, and Eurasian lithospheric plates (Figure 1).

Faial Island is situated in the westernmost part of the Central Group of Azores archi-pelago, limited by coordinates 38°30′56″–38°38′40″N and 28°35′55″–28°50′06″W. Thisisland is located about 120 km east of the MAR, on the seismically active Faial–Pico Frac-ture Zone (Figure 1); it is 21 km long, 14 km wide, and covers 173 km2.

The volcanic history of the island started with the formation of the Ribeirinha volcano,with an age of 730 ka (Féraud 1977), whose volcanic products compose the RibeirinhaVolcanic Complex. This volcano-stratigraphic unit consists of basalt, hawaiite, mugearite,and benmoreite lava flows and pumice deposits, welded scoria, ignimbrites, and basalticpyroclasts. The second volcanic stage (410 ka) led to the formation of a polygeneticvolcano with caldera, the Central Volcano (Serralheiro et al. 1989), which would haveemerged in confluence with several tectonic structures and corresponds to the CedrosVolcanic Complex unit. This complex was divided into Lower and Upper Groups(Pacheco 2001), reflecting a variation of chemical composition and of volcanism type.The Upper Group consists of basaltic to benmoreitic products (16 ka). The Upper Groupshows predominantly explosive eruptions (16–1.2 ka), which produced more evolvedrocks, as fall and surge deposits, besides pyroclastic flows of trachytic to benmoreitic com-position (Pacheco 2001). The Horta Platform (Almoxarife Formation) was constructed by

Figure 1. Localization map of Faial Island in the Azores archipelago.

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an 11,000–year–old fissural volcanism, constituted by lava flows that vary from alkaline-olivine basalts to hawaiites-mugearites and mugearites-benmoreites (Serralheiro et al.1989). The lava flows are interbedded with pumice levels and subaerial–submarine basal-tic pyroclasts. The youngest volcano-stratigraphic unit, the Capelo Formation (1.2 ka tothe present), includes all products that are present on the Capelo Peninsula and is com-posed of basaltic rocks and more rarely of hawaiite. The last eruption on Capelo Peninsulawas in 1957–1958, the so-called Capelinhos surtseyan eruption. That eruption added1.5 km2 of new land to the island, and caused a 1.5 m maximum subsidence during theMay 1958 seismic crisis (Machado 1959; Tazieff 1959; Machado et al. 1962).

The Pedro Miguel Graben is the most prominent feature in the eastern part of FaialIsland. Its fault scarps face SW and NE, with maximum height of about 170 m. The lowestblock of that structure is located near Pedro Miguel Village.

4. Petrographic aspectsThe Faial rocks exhibit glomeroporphyritic, porphyritic, intergranular, pilotaxitic, andintersertal textures. All the stratigraphic units show plagioclase, olivine, and clinopyroxenephenocrysts, and the groundmass is composed of plagioclase, clinopyroxene, olivine, andtitanomagnetite microcrystals. Apatite occurs as an accessory phase. Subhedral to euhedralolivine grains are in general fractured and altered to bowlingite, iddingsite, and serpentine.Some crystals show undulose extinction and can be interpreted as mantle xenocrysts.Clinopyroxene grains are light green, subhedral to anhedral, and in some cases show twin-ning and compositional zoning. Plagioclase shows Carlsbad-Albite twinning and oscillatoryzoning.

Basalts and hawaiites share very similar petrographic features, some hawaiites, never-theless, contain dark-brown amphibole. Hyalopilitic textures are widespread in mugearitesand benmoreites, where phenocrysts and mycrolites of plagioclase, engulfed by a glassygroundmass, are oriented. In the Ribeirinha Volcanic Complex, the porphyritic texturesand plagioclase phenocrysts are predominant. The Cedros Volcanic Complex and theAlmoxarife Formation show plagioclase and clinopyroxene phenocrysts, with minoramounts of olivine. The youngest Capelos Volcanic Complex shows glomeroporphyritictextures, with larger concentrations of clinopyroxene, olivine, and Ti-magnetite generallyforming aggregates. Disequilibrium textures are registered as rounded olivine and pyroxenephenocrysts, which are interpreted as cumulate phases.

The trachytic rocks are constituted by plagioclase, sanidine, amphibole, biotite,opaque phases, and apatite. Phenocrysts of biotite occur as flakes disseminated in thematrix.

5. Geochemistry of the effusive rocks from Faial IslandForty samples of effusive rocks, which are representative of the four different volcano-stratigraphic units (Figure 2) from Faial Island, were selected for geochemical studies.The set of selected samples includes 6 from Ribeirinha Volcanic Complex, 18 fromCedros Volcanic Complex, 8 from Almoxarife Formation, and 8 from Capelo Formation.The samples plot on the TAS diagram (Le Maitre 1989) following a trend that varies frombasalt to trachyandesite (Figure 3). The rocks that plot on the trachybasalt, basaltic trachy-andesite, and trachyandesite fields show (Na2O-2) > K2O, and so are classified as hawaiites,mugearites, and benmoreites (except for sample FA-04/07).

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CIPW calculations show that 56% of the studied samples have Nen (<6.5%) and Oln,37% show Hyn and Oln, and only 7% have Hyn and Qzn. The alkaline nature of Faialvolcanism is confirmed on the classificatory diagrams based on trace elements.

Figure 2. Stratigraphic distribution of volcanic units on Faial Island.

410,000 years

730,000 years

16,000 years

RibeirinhaVolcanicComplex

AlmoxarifeFormation

LowerGroup

UpperGroup

Ced

ros

Vol

cani

c C

ompl

ex

CapeloFormation

Figure 3. Classification of effusive rocks from Faial Island on a TAS diagram. Symbols: circle,Cedros Volcanic Complex; triangle, Capelo Formation; diamond, Almoxarife Formation; square,Ribeirinha Volcanic Complex.

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5.1. Major elementsThe SiO2 contents, which were recalculated on an anhydrous base, vary from 45.9 wt.% to56.9 wt.% (Table 1). The MgO contents vary from 3.02 wt.% to 6.03 wt.% in the RibeirinhaVolcanic Complex, except for one sample which shows higher MgO content and containsolivine xenocrysts. In the Cedros Volcanic Complex, the MgO contents vary from 1.68 wt.% to8.26 wt.%, whilst in the Almoxarife Formation, they are between 3.34 wt.% and 9.15 wt.%.The Capelo Formation shows the highest MgO contents, ranging from 5.31 wt.% to 14.23 wt.%and represents the less-evolved magmatic compositions. The highest MgO contents (>10 wt.%)observed in some samples are caused by accumulation of pyroxene and olivine phenocrysts, asobserved in the petrographic studies. That is also confirmed by their high Cr and Ni contents,therefore, these samples are not considered as representing magmatic liquids.

The solidification index (SI) = 100 MgO/(MgO+FeO+Fe2O3+Na2O+K2O) wasselected as the differentiation index (Kuno 1960), considering the alkaline nature of theFaial intermediate and basic rocks.

Two different trends are well illustrated in SI versus SiO2 diagram (Figure 4). Forrocks with SI > 25, there is a liquid line of descent (LLD) that can be explained byfractional crystallization, mostly of clinopyroxene olivine and magnetite. For the more-differentiated rocks with SI < 25, a different trend is clearly defined. It is ascribed to accu-mulation and segregation of phenocrysts, mainly of plagioclase and Ti-magnetite, duringthe magmatic flow of a melt which is co-magmatic with those that generated the liquidscorresponding to rocks with SI > 25. The porphyritic and glomeroporphyritic texture ofrocks with SI < 25 corroborates this hypothesis.

The major element distribution patterns relative to SI show a well-defined trend, with awide dispersion in the more-differentiated samples (SI < 25). This feature probably resultsfrom segregation of plagioclase phenocrysts during the magmatic flow. The effect of plagio-clase segregation is well illustrated by samples with SI in between 15 and 20, andAl2O3 > 19 wt.% or <17 wt.% (Figure 4). If these samples are not considered, a probable LLDis clearly defined, which shows SiO2, Al2O3, K2O, Na2O, MnO, and P2O5 contents increase(Figure 4). The CaO and MgO contents show decrease with the differentiation, whereas theFeOt and TiO2 show a distinct pattern: they are incompatible in the less-differentiated rocks(SI = 25–50) and are strongly compatible in the more differentiated ones (SI < 25). The largedispersion of Fe and Ti values observed for the lower SI values (Figure 5) are probably causedby segregation of Fe–Ti oxides during magmatic flow, as previously assumed.

The predominant linear trend defined by major elements relative to the SI can beexplained by fractional crystallization, mostly of clinopyroxene and olivine for sampleswith SI > 25, with a more intense contribution of the plagioclase + titanomagnetite + apatitefractionated assemblage for liquids with SI < 25.

The Faial lavas show a continuous compositional range of basalt-hawaiite-mugearite-benmoreite, and the bimodality (basalt-trachyte), which is common in within-plate volca-noes (e.g. Tenerife; Ablay et al. 1998; Bryan et al. 2002), is not observed. The bimodalityin oceanic island volcanism can be attributed to the increase of iron concentration in theintermediate magmas, which determines a density barrier, preventing the ascent of theseliquids to the surface (Stolper et al. 1981).

The origin of the Daly gap can be explained, too, in terms of magma plumbing, assumingthat mafic magmas when ascending through the crust, loose heat and start to differentiate.This results in higher viscosity and density due to increasing iron content in the liquidbefore the onset of magnetite crystallization (e.g. Mann 1983). The increase in density is

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Tabl

e 1.

Maj

or a

nd tr

ace

elem

ent c

ompo

sitio

n of

the

effu

sive

rock

s fro

m F

aial

Isla

nd.

Sam

ple

unit

FA-0

2 A

FFA

-03

AF

FA-0

4 C

VC

FA-0

5 C

VC

FA-0

6 C

VC

FA-0

7 C

VC

FA-0

8 C

VC

FA-0

9 C

VC

FA-1

0 C

VC

FA-1

1 R

VC

FA-1

3 A

FFA

-14

RV

CFA

-15

RV

CFA

-16

RV

CFA

-17

RV

CFA

-18

CV

C

SiO

248

.56

47.1

047

.11

47.1

849

.85

50.0

655

.16

48.5

847

.42

46.6

848

.01

44.8

746

.87

48.8

546

.11

54.1

6Ti

O2

3.00

3.53

2.88

2.65

3.22

2.96

1.86

3.62

2.54

3.65

2.87

2.85

3.14

3.44

2.54

1.83

Al 2O

316

.26

16.0

818

.09

15.5

416

.00

16.8

417

.64

15.8

419

.59

16.7

116

.86

21.8

520

.36

16.0

314

.86

17.5

7Fe

O9.

099.

798.

668.

408.

968.

556.

509.

928.

499.

549.

587.

428.

5410

.46

9.47

7.49

Fe2O

32.

753.

022.

622.

552.

712.

591.

963.

002.

572.

892.

902.

252.

583.

172.

872.

27M

nO0.

167

0.17

30.

156

0.15

70.

183

0.17

40.

186

0.20

00.

146

0.18

60.

180

0.13

10.

155

0.18

40.

150

0.19

1M

gO6.

354.

444.

068.

103.

683.

362.

493.

963.

594.

205.

603.

023.

804.

009.

322.

28C

aO9.

348.

378.

449.

577.

947.

805.

378.

4310

.24

8.15

6.74

9.60

8.72

8.46

10.5

25.

22N

a 2O

3.78

4.01

3.47

3.26

4.30

4.62

5.59

4.34

2.90

3.51

3.51

2.49

3.80

4.25

2.90

5.36

K2O

1.60

1.53

1.59

1.52

1.92

2.12

2.77

1.61

1.12

1.40

1.45

0.95

1.22

1.83

1.09

2.62

P 2O

50.

580.

710.

610.

480.

840.

870.

720.

670.

390.

640.

880.

400.

500.

740.

410.

70LO

I−0

.46

1.42

1.78

−0.0

70.

570.

200.

170.

462.

591.

851.

584.

391.

680.

520.

120.

77Su

m10

1.02

100.

1799

.47

99.3

410

0.17

100.

1410

0.42

100.

6310

1.59

99.4

110

0.16

100.

2210

1.37

101.

9310

0.36

100.

46R

b33

3332

3144

4249

3231

2327

2018

4023

57C

s<0

.1<0

.10.

10.

30.

4<0

.1<0

.10.

30.

40.

2<0

.10.

1<0

.10.

40.

20.

2Sr

706

663

673

677

646

651

588

631

611

636

513

661

571

670

527

656

Ba

460

456

455

433

505

561

770

448

330

484

443

329

364

507

294

760

Sc22

2017

2317

1610

1618

2216

1721

2128

7V

272

221

236

257

258

227

9726

527

831

117

424

727

128

925

978

Cr

140

<20

3024

0<2

0<2

0<2

0<2

020

<20

100

<20

30<2

038

0<2

0C

o32

3018

3820

1911

2828

2930

2328

2443

9N

i<2

0<2

0<2

010

0<2

0<2

0<2

0<2

030

<20

<20

0<2

0<2

014

0<2

0G

a21

2420

2025

2524

2421

2523

2523

2419

26Y

29.7

37.5

31.5

23.7

38.6

42.4

38.2

37.8

24.9

35.5

4025

.430

.536

.923

.942

.9N

b54

.563

.245

.242

.159

.870

.278

.655

.631

.658

.761

.133

.739

.758

32.7

82.9

Zr28

633

627

323

935

338

441

430

817

232

633

620

223

833

418

947

8H

f6.

57.

65.

95.

37.

68.

99

6.7

3.7

7.1

7.8

4.5

5.4

7.3

4.3

9.8

Sn1

32

23

22

21

22

12

21

3Pb

<513

<57

8<5

<59

<57

6<5

<57

<56

U1.

281.

411.

391.

061.

681.

781.

491.

380.

521.

51.

540.

961.

11.

530.

812.

09Th

4.31

4.64

4.74

3.69

5.71

5.66

6.44

4.63

2.41

5.04

4.67

3.13

3.71

5.34

2.79

8.03

Dow

nloa

ded

by [

b-on

: Bib

liote

ca d

o co

nhec

imen

to o

nlin

e U

C]

at 1

2:24

03

Febr

uary

201

2


Ta3.

694.

183.

452.

974.

34.

775.

223.

962.

24.

124.

112.

332.

834.

122.

35.

88La

41.7

46.2

47.6

37.6

53.4

52.3

63.5

45.1

26.1

53.2

46.1

28.5

3949

.727

.470

.5C

e87

.295

.392

73.5

107

109

128

90.1

51.3

107

93.4

56.4

70.8

99.7

55.8

137

Pr10

.211

.610

.88.

4712

.613

14.7

10.9

6.36

12.3

11.5

6.87

9.19

11.9

6.78

15.5

Nd

37.8

42.8

40.7

31.2

48.7

48.3

50.6

42.8

24.8

47.9

43.4

26.2

35.7

45.5

26.5

57.4

Sm8.

079.

578.

827.

0210

.910

.710

.59.

925.

9310

.510

.16.

277.

9510

.16.

4211

.7Eu

2.65

3.14

32.

43.

623.

424.

013.

342.

13.

483.

322.

342.

773.

362.

153.

8G

d7.

28.

747.

86.

349.

7310

98.

975.

559.

239.

445.

977.

299.

015.

8610

Tb1.

111.

321.

150.

941.

461.

521.

371.

350.

861.

391.

430.

91.

111.

360.

881.

55D

y5.

797.

186.

294.

948.

118.

247.

337.

794.

857.

547.

835

6.18

7.5

4.78

8.25

Ho

1.05

1.32

1.14

0.89

1.46

1.51

1.36

1.43

0.9

1.33

1.43

0.92

1.12

1.36

0.87

1.55

Er2.

853.

583.

082.

393.

984.

123.

713.

872.

513.

63.

92.

543.

063.

632.

414.

3Tm

0.38

40.

476

0.42

80.

325

0.53

50.

565

0.51

10.

531

0.34

0.48

40.

520.

348

0.41

70.

496

0.33

0.59

Yb

2.22

2.77

2.48

1.87

3.09

3.22

3.02

2.99

1.98

2.8

3.03

22.

42.

841.

883.

57Lu

0.31

10.

388

0.35

10.

272

0.42

90.

460.

428

0.43

70.

296

0.38

80.

432

0.27

80.

356

0.41

40.

257

0.53

3

Sam

ple

Uni

tFA

-19

CV

CFA

-20

CV

CFA

-26

CV

CFA

-28

AF

FA-2

9 A

FFA

-30

CV

CFA

-31

CF

FA-3

2 C

FFA

-33

CV

CFA

-34

CV

CFA

-36

CV

CFA

-37

CV

CFA

-38

CF

FA-3

9 C

VC

FA-4

0 C

VC

FA-4

1 C

VC

SiO

255

.77

48.6

546

.38

48.0

047

.13

48.9

548

.34

48.0

553

.22

51.5

150

.37

47.5

647

.93

46.2

846

.37

47.6

0Ti

O2

1.44

2.90

2.11

3.19

2.72

2.66

2.62

2.63

2.16

2.08

2.68

2.68

2.68

2.71

2.57

2.74

Al 2O

318

.58

17.5

711

.76

15.3

615

.41

18.0

715

.93

16.1

718

.25

17.5

617

.28

16.1

216

.49

21.0

721

.07

19.8

2Fe

O5.

538.

299.

1510

.20

9.01

7.90

8.37

8.29

6.33

7.17

7.61

8.04

8.02

7.61

6.91

7.90

Fe2O

31.

682.

512.

773.

082.

732.

392.

532.

512.

112.

172.

312.

542.

512.

302.

092.

38M

nO0.

209

0.16

20.1

550.

197

0.15

70.1

610.1

570.1

560.

193

0.18

60.1

810.1

590.

159

0.14

10.1

350.

140

MgO

1.68

3.23

14.4

73.

348.

102.

678.

128.

062.

622.

643.

598.

107.

783.

052.

883.

09C

aO3.

878.

1510

.92

7.40

9.05

7.79

9.35

9.22

5.90

5.88

7.37

9.73

9.56

9.27

8.65

8.88

Na 2

O5.

793.

972.

294.

203.

344.

313.

513.

675.

404.

784.

773.

303.

603.

293.

503.

51K

2O2.

921.

740.

712.

341.

421.

861.

521.

542.

532.

332.

171.

541.

631.

191.

141.

29P 2

O5

0.53

0.67

0.32

1.05

0.54

0.70

0.52

0.52

0.84

0.78

0.93

0.48

0.49

0.53

0.54

0.55

LOI

2.27

1.59

−0.1

01.

340.

162.

20−0

.06

−0.0

90.

691.

910.

240.

17−0

.27

4.21

2.33

2.96

Sum

100.

2799

.43

100.

9499

.70

99.7

799

.66

100.

9110

0.73

100.

2499

.00

99.5

010

0.42

100.

5810

1.65

98.1

910

0.86

(Con

tinue

d)

Dow

nloa

ded

by [

b-on

: Bib

liote

ca d

o co

nhec

imen

to o

nlin

e U

C]

at 1

2:24

03

Febr

uary

201

2


Tabl

e 1.

(Con

tinue

d)

Sam

ple

Uni

tFA

-19

CV

CFA

-20

CV

CFA

-26

CV

CFA

-28

AF

FA-2

9 A

FFA

-30

CV

CFA

-31

CF

FA-3

2 C

FFA

-33

CV

CFA

-34

CV

CFA

-36

CV

CFA

-37

CV

CFA

-38

CF

FA-3

9 C

VC

FA-4

0 C

VC

FA-4

1 C

VC

Rb

5739

2447

2940

3535

5843

4236

3413

1627

Cs

0.2

0.2

0.2

<0.1

<0.1

0.2

0.2

0.3

0.5

0.3

<0.1

0.4

0.3

<0.1

<0.1

0.2

Sr59

869

339

957

757

770

864

064

570

067

972

469

270

582

479

176

2B

a78

451

622

366

037

952

642

442

871

868

661

344

145

245

539

741

1Sc

416

3416

2314

2323

88

1324

2515

1515

V39

230

242

226

241

190

255

252

115

108

196

264

264

195

170

212

Cr

<20

<20

810

<20

160

<20

180

230

<20

<20

<20

250

240

<20

<20

<20

Co

520

6025

4018

3038

1211

1640

3922

2016

Ni

20<2

028

0<2

0<2

0<2

0<2

010

0<2

0<2

0<2

010

010

0<2

0<2

0<2

0G

a24

2516

2720

2523

2026

2423

2020

2324

22Y

34.7

36.4

20.6

5227

.440

.330

.624

.842

4041

.424

.325

30.1

29.3

28.8

Nb

85.5

6026

.880

.244

.861

.751

.741

.177

73.8

70.4

43.9

44.6

47.6

45.2

46.6

Zr50

034

116

246

025

934

828

424

642

941

535

724

725

226

626

826

8H

f10

.37.

53.

910

.66.

17.

56

5.2

8.6

8.3

8.2

5.4

5.4

6.2

5.7

5.7

Sn3

22

32

22

23

22

22

22

1Pb

66

5<5

<510

57

116

<57

<519

<5<5

U1.

931.

760.

682.

081.

161.

711.

291.

12.

152.

021.

621.

141.

181.

211.

181.

24Th

8.55

5.34

2.36

6.89

3.75

5.6

4.63

3.79

7.12

6.91

5.51

3.8

3.9

3.84

3.93

4.14

Ta6.

054.

191.

915.

393.

134.

323.

652.

875.

265.

024.

823.

063.

113.

193.

083.

25La

58.7

50.4

22.3

62.1

34.5

54.5

43.4

35.8

65.7

65.1

54.8

37.5

38.3

39.5

37.8

39C

e12

199

.646

.313

071

.510

686

.170

.712

912

711

574

.275

.279

.872

.977

.4Pr

13.6

11.8

5.61

15.8

8.44

12.8

10.2

8.41

14.9

14.5

13.8

8.75

8.78

9.49

8.82

9.23

Nd

48.2

44.5

21.9

57.5

31.8

48.5

37.6

30.8

55.6

53.2

5131

.732

.735

.233

.333

.7Sm

10.1

105.

312

.97

118.

466.

8911

.711

.411

.17.

037.

47.

657.

697.

96Eu

3.36

3.4

1.87

4.04

2.29

3.51

2.84

2.32

3.76

3.67

3.59

2.37

2.36

2.55

2.68

2.67

Gd

8.62

9.04

5.04

12.2

6.46

9.66

7.32

6.03

9.93

9.89

106.

36.

187.

047.

017.

07Tb

1.28

1.37

0.76

1.85

11.

441.

10.

881.

521.

461.

480.

950.

941.

091.

051.

09D

y7.

027.

484.

2210

.15.

367.

846.

064.

888.

187.

888.

144.

975

5.87

5.54

5.91

Ho

1.31

1.35

0.79

1.88

0.98

1.41

1.09

0.88

1.48

1.45

1.48

0.89

0.9

1.08

1.01

1.05

Er3.

743.

652.

145.

022.

723.

872.

952.

454.

033.

884.

042.

392.

392.

942.

742.

78Tm

0.53

10.

497

0.283

0.66

40.

374

0.526

0.391

0.335

0.56

50.

541

0.53

80.

327

0.32

0.4

0.38

20.

39Y

b3.

222.

791.

623.

922.

173.

112.

321.

973.

333.

243.

181.

921.

862.

32.

272.

29Lu

0.47

60.

401

0.231

0.57

30.

310.4

460.3

350.2

810.

478

0.47

40.

460.

256

0.27

0.33

0.32

20.

323

Dow

nloa

ded

by [

b-on

: Bib

liote

ca d

o co

nhec

imen

to o

nlin

e U

C]

at 1

2:24

03

Febr

uary

201

2


Sam

ple

Uni

tFA

-44

CF

FA-4

5 C

FFA

-46

CV

CFA

-48

CV

CFA

-49

CV

CFA

-50

AF

FA-5

2E

AF

FA-5

2H

AF

FA-6

1 A

FFA

-62

AF

FA-6

3 C

VC

FA-6

5 C

FFA

-66

CF

FA-6

7 C

FFA

-68

RV

CFA

-69

RV

C

SiO

246

.53

46.2

248

.01

48.8

149

.10

46.8

747

.85

46.8

148

.96

45.1

449

.13

47.2

946

.75

45.6

848

.69

47.2

4Ti

O2

2.60

2.43

2.69

2.73

2.35

2.83

2.90

2.90

3.35

3.37

2.95

3.24

3.28

2.34

2.95

3.47

Al 2O

314

.04

14.3

615

.95

17.2

520

.92

16.0

215

.91

16.1

816

.76

14.4

418

.50

17.6

417

.00

11.9

617

.59

16.6

8Fe

O8.

528.

518.

948.

476.

909.

548.

939.

135.

937.

634.

565.

275.

526.

375.

326.

48Fe

2O3

2.58

2.58

2.71

2.56

2.09

2.89

2.70

2.76

4.85

4.87

4.45

4.74

4.78

3.84

4.45

4.97

MnO

0.15

20.

149

0.15

80.

191

0.12

90.

166

0.16

40.

162

0.19

00.

180

0.16

00.

150

0.16

00.

150

0.17

00.

170

MgO

10.8

110

.52

8.26

3.34

3.04

7.40

6.39

6.60

3.78

9.15

2.93

5.31

5.52

14.2

34.

366.

03C

aO10

.80

10.6

79.

917.

059.

6810

.01

9.13

9.98

8.26

10.7

48.

359.

1510

.26

11.2

37.

418.

43N

a 2O

2.68

2.82

3.46

4.22

3.69

2.96

3.74

3.44

3.83

2.67

4.35

4.47

3.86

2.46

4.09

3.92

K2O

0.83

0.87

1.60

2.07

1.35

1.18

1.54

1.32

1.84

1.15

1.93

1.69

1.50

0.83

2.22

1.38

P 2O

50.

360.

350.

471.

150.

470.

520.

580.

500.

610.

500.

670.

560.

580.

290.

740.

60LO

I−0

.51

−0.1

4−0

.43

2.54

1.01

1.13

−0.6

50.

070.

900.

201.

40−0

.20

0.10

−0.3

01.

30−0

.20

Sum

99.3

999

.34

101.

7310

0.38

100.

7310

1.52

99.1

899

.85

99.2

610

0.04

99.3

899

.31

99.3

199

.08

99.2

999

.17

Rb

1917

2937

2522

3326

32.8

2136

.331

.828

.716

.542

.327

.3C

s<0

.1<0

.1<0

.1<0

.1<0

.1<0

.1<0

.1<0

.10.

20.

20.

30.

30.

30.

10.

20.

2Sr

523

517

700

763

881

631

689

671

593.

655

6.2

785.

377

5.9

745.

742

8.4

741.

278

9.9

Ba

281

281

435

600

437

382

445

378

445.

230

5.3

506.

941

8.3

392.

921

4.8

619

402.

6Sc

3128

2412

1224

2123

1828

1519

2235

1417

V28

625

726

615

617

026

526

726

925

629

717

926

728

123

419

523

7C

r48

044

021

0<2

0<2

022

014

015

0<2

039

0<2

0<2

080

520

<20

60C

o44

4240

1619

4034

3429

.660

.830

.250

.444

.971

.738

.143

.6N

i<2

0<2

0<2

0<2

0<2

0<2

0<2

0<2

0<5

138

<533

4235

624

59G

a17

1719

2423

2021

2022

.719

.424

.621

.721

.414

.723

.622

.4Y

2322

.924

.746

.527

.528

28.9

27.7

36.5

27.9

38.3

26.7

3020

.235

.131

.6N

b33

.731

.744

.777

.645

.547

53.4

47.4

50.1

41.8

63.3

50.3

48.7

26.3

66.9

57.4

Zr18

017

323

837

825

323

628

623

626

8.1

195.

931

8.8

225.

823

1.6

141

354.

627

1H

f4.

44.

25.

58.

55.

95.

66.

75.

76.

55.

47.

95.

75.

93.

98.

96.

6Sn

11

22

22

21

22

22

21

22

Pb<5

<56

<5<5

5<5

<51.

91.

20.

90.

81.

81

0.8

1.3

U0.

740.

721.

011.

591.

131.

051.

321.

060.

81

1.5

1.1

1.1

0.6

1.7

1.4

Th2.

392.

293.

325.

513.

623.

634.

333.

613.

73

5.1

3.8

3.4

2.1

5.8

4.1

Ta2.

282.

192.

985.

083.

113.

173.

663.

163.

32.

93.

93.

43.

22

4.2

3.8

(Con

tinue

d)

Dow

nloa

ded

by [

b-on

: Bib

liote

ca d

o co

nhec

imen

to o

nlin

e U

C]

at 1

2:24

03

Febr

uary

201

2


Tabl

e 1.

(Con

tinue

d)

Sam

ple

Uni

tFA

-44

CF

FA-4

5 C

FFA

-46

CV

CFA

-48

CV

CFA

-49

CV

CFA

-50

AF

FA-5

2E

AF

FA-5

2H

AF

FA-6

1 A

FFA

-62

AF

FA-6

3 C

VC

FA-6

5 C

FFA

-66

CF

FA-6

7 C

FFA

-68

RV

CFA

-69

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C: C

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ion;

AF:

Alm

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n; R

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: Rib

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nic

Com

plex

.

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Figure 4. Diagrams for major elements versus SI (solidification index) for the effusive rocks fromFaial Island. Symbols as shown in Figure 3.

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likely to cause the stagnation of the Fe-rich intermediate liquids within the upper crust wherethey fractionate and differentiate to trachyte. The early crystallization of Ti-magnetite in thebasic magmas of Faial magmatism prevented the density increase and the generation of Fe-richliquids. As is shown in the geochemical diagrams, Fe does not increase in the basic to interme-diate compositions, and, consequently, the Daly gap is not developed. The usual compositionaltrend of less-oxidized magmas, evolving gradually from basalts to benmoreites, is observed.

5.2. Trace elementsThe Faial samples have low Ni, Sc, Cr, Co contents and, frequently, they are lower thanthe analytical detection limit. The relatively low Ni and Cr concentrations and low magne-sium number (100 MgO/(MgO+FeO+Fe2O3)) of the Faial basic rocks (23–54) indicatethat these liquid compositions are not in equilibrium with a peridotitic mantle, and are,therefore, interpreted as evolved magmas. None of the samples was identified as repre-sentative of primary liquids (Mg > 0.7; Ni > 400–500 ppm; Cr > 1000 ppm, and SiO2<50%) according to Wilson (1989) and Albarede (1992) criteria. Therefore, these results

taken along with the MgO content variations, suggest a strong differentiation of the Faialbasic and intermediate magmas before the extrusion. The transition elements show astrong positive correlation with SI (Figure 6). The decrease of the Ni concentrations withthe differentiation is coherent with olivine plus clinopyroxene fractionation, whichexplains the decreasing of Cr, Co, and Sc values.

The large ion Lithophile element (LILE) distribution patterns in relation to the SI(Figure 7) shows the incompatible behaviour of Ba and high field strength elements(HFSEs) (U, Nb, Y, Ta, Zr, Hf, Th), and a clear dispersion of Sr and Rb for rocks withSI < 25. As admitted for the major elements, this wide dispersion can be ascribed to plagi-oclase and Fe–Ti oxide segregation caused by magmatic flow.

The similarity of the studied basalts with the within-plate magmatism is confirmed inthe Meschede (1986) discrimination diagram, which uses the Zr/4, Nb/0.5, and Y system(Figure 8), and on the Wood (1980) diagram, based on the low-mobility HFSEs: Th, Hf,and Ta (Figure 9).

Figure 5. TiO2 versus FeOt diagram for the effusive rocks from Faial Island. Symbols as shown inFigure 3.

6 7 8 9 10 11 12 13 14

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

FeOt

TiO

2

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The REE patterns normalized by Haskin et al. (1968) show light rare earth element(LREE) enrichment relative to the heavy rare earth element (Figure 10), which is commonin the volcanic rocks generated by low melt degree from a garnet-bearing mantle peridot-ite. The REE patterns of all samples are very similar and the REEs behave as incompatibleelements, as observed for LILEs and HFSEs. La/YbN ratios are around 10 and the largedispersion of REEs, mainly in the more-differentiated rocks, is probably explained by seg-regation of apatite, which is also supported by P2O5 behaviour in the same samples. Theslight positive Eu-anomalies, shown mainly by samples with high-Al2O3 contents, may becaused by plagioclase concentration during magmatic flow.

Consistent ratios of incompatible trace elements such as Ce/Zr, Nb/Zr, and Rb/Zr areoften used to test Whether fractional crystallization has occurred (Wilson 1989). The Faialvolcanic rocks plot has consistent ratios of Zr/Nb, Ce/Zr, and Rb/Zr of 5, 0.30, and 0.10,respectively, which confirms that evolution through fractional crystallization has dominatedin the three volcanic units.

The Figure 11 spidergram shows incompatible-element abundances, normalized byOIB values (Sun and McDonough 1989). The patterns are very similar to OIB, particu-larly for samples with MgO contents in the 6–10 wt.% range. There is a very slight nega-tive-Nb anomaly (La/NbNratios < 1), which can be related to a previously consumedlithosphere, since this kind of anomaly is usually found in magmas produced from mantlesegments affected by lithosphere-subduction-related metasomatism.

6. Magmatic evolution of the effusive rocks from Faial IslandMajor and trace element data suggest that fractional crystallization was the major processin the magmatic differentiation of the three earliest Faial volcano-stratigraphic units. The

Figure 6. Trace element contents versus SI for the effusive rocks from Faial Island. Symbols asshown in Figure 3.

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Cedros Volcanic Complex evolution can be explained by clinopyroxene + calcic plagi-oclase + titanomagnetite fractionation, whereas in the Almoxarife and Capelo formations thedominant fractionation of olivine + clinopyroxene generated the more evolved compositions.

The role of clinopyroxene as a fractionating phase is also evident from the decreasingCaO/Al2O3 ratio with decreasing MgO (Figure 12). Clinopyroxene is the only phase in

Figure 7. LILEs and HFSEs elements versus SI for the effusive rocks from Faial Island. Symbolsas shown in Figure 3.

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basalts that can cause lowering in CaO over Al2O3, and is often associated with crystalli-zation at high pressures (Fisk et al. 1982; Schilling et al. 1983; Le Roex et al. 1996). Thefractionated mineral phases are coherent with textural evidence and mineral chemistrydata obtained in the Azores region by França et al. (2006c).

The compositional diversity observed in the samples, the low values of the transitionelements and MgO can be explained by fractionation in a shallow magmatic chamber anda long magmatic history. The Capelo Formation compositions are the most primitive, asindicated by the highest MgO, Cr, Ni, Sc, and Co contents. The preservation of these moreprimitive patterns can be explained by a faster ascension of these liquids generated in deepmagma chambers.

We made a comparison among the geochemical data of Faial, Pico, and Corvo islands,considering that Pico Island is the youngest of the archipelago (300 ka) and the closer tothe Faial Island. Both of the islands (Pico and Faial) are localized in the same geotectoniccontext, the Eurasiatic Plate, and their major faults show a NW trend. On the other hand,Corvo Island was built on the American Plate and shows an orientation that is coincidentwith the MAR.

The effusive rocks of Faial Island represent four volcano-stratigraphic units, whoseages vary from 730 ka to 16 ka. These rocks belong to the alkaline-sodic series and defineon the TAS diagram a trend that varies from basalt to benmoreite. All the units are SiO2undersaturated, and rarely the basalts show normative hypersthene. The older volcano-stratigraphic unit is Ribeirinha Volcanic Complex, which shows low MgO contents(=6%). The Cedros Volcanic Complex is widespread on the island and is composed of

Figure 8. Meschede’s (1986) diagram for the effusive rocks from Faial Island. Legend: AI, within-plate alkali basalts; AII, within-plate alkali basalts and tholeiites; B, E-type MORB; C, within-platetholeiites and volcanic-arc basalts; D, N-type MORB and volcanic-arc basalts. Symbols as shown inFigure 3.

AI

AII

B

C

D

Nb*2

Zr/4 Y

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rocks that vary from basalts to benmoreites, generally with low MgO, except for the Fa-26sample which shows olivine large xenocrystals. The Almoxarife Formation is a fissuralmanifestation, which shows dominantly basic compositions, with MgO values that aresimilar to slightly higher than those of the Cedros Volcanic Complex. The youngest

Figure 9. Wood’s (1980) diagram for the effusive rocks from Faial Island. Legend: A, N-typeMORB; B, E-type MORB and within-plate tholeiites; C, alkaline within-plate basalts; D, volcanic-arcbasalts. Symbols as shown in Figure 3.

A

B

D

Hf/3

Th Ta

C

Figure 10. REE patterns normalized by chondrite values (Haskin et al. 1968) for the effusive rocksfrom Faial Island. Symbols as shown in Figure 3.

103

100

101

102

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

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volcano-stratigraphic unit is the Capelo Formation, composed of basic rocks with MgOrelatively higher than the other units.

Three volcanic complexes were recognized in Pico Island (França et al. 2006c), whichare less differentiated than the extrusive rocks from Faial. At Pico Island, the basalts arepredominant over hawaiites, with rare occurrences of mugearites and benmoreites. Theserocks are generally alkaline (Ne normative) and rarely show transitional and subalkalinecharacteristics. Fractional crystallization in shallow magma chambers was the majordifferentiation process.

At Corvo Island (1.5–1.0 Ma), two volcanic complexes were identified (Zbyszewski et al.1967; Azevedo et al. 2003; França et al. 2003). The proto-island volcanism, whichincludes pyroclastic and basic effusive rocks, is defined as the Basal Complex. The

Figure 11. Spidergram for incompatible trace elements from the effusive rocks from Faial Island,normalized by OIB values (Sun and McDonough 1989). Symbols as shown in Figure 3.

Figure 12. (CaO/Al2O3) versus MgO diagram for the effusive rocks from Faial Island. Symbols asshown in Figure 3.

–2 0 2 4 6 8 10 12 14 16 18

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

MgO

CaO

/Al 2

O3

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Upper Complex is composed of basic effusive rocks and fall and flow pyroclasticsequences. On the TAS diagram, the Corvo rocks show an alkaline-sodic trenddefined by basalt-hawaiite-mugearite-benmoreite followed by rare picrobasalts and bas-anites. This compositional variation is similar to the Faial Island and attributed by Françaet al. (2006c) to the fractional crystallization.

A comparison of the spider diagram patterns normalized by primordial mantle(McDonough 1992) values shows a strong similarity between the Faial and Pico islands(Figure 13). The more enriched patterns of Faial Island rocks is due to their higher differ-entiation. The Corvo Island magmatic rocks show a different pattern (Figure 13) with astrong K and Rb depletion and Th, U, Ta, Nb, La, and Ce enrichment.

7. Magma sourcesThe Azorean magmatism according to Widom and Shirey (1996) and Silveira et al. (2006)has heterogeneous mantle sources including a MORB component, a regional plume thatincorporated a depleted Archaean harzburgitic mantle (Schaefer et al. 2002) and anenriched mantle component (Turner et al. 1997). Sleep (1990) proposed that the plume inthis region would be one of lower-buoyancy flux (1–2 Mg/s), different from the high-buoyancy flux of the Hawaiian plume (∼5–8∼7 Mg/s).

According to these authors, the 187Os/188Os isotope ratios identified on Faial and Picoislands are the lowest determined in oceanic islands (187Os/188Os 0.110), despite the Azo-rean lithosphere being very young to generate this subchondritic pattern, which would cor-respond to the Arquean harzburgitic component. This combination among the variable Osisotope ratios, reaching subchondritic values, joined to the positive eNd, contrasts with thegeochemistry of the lavas, which indicates enrichment in incompatible elements, which istypical of the metasomatic additions, including volatiles (Turner et al. 1997).

Figure 13. Incompatible-element spidergram normalized by primordial mantle (McDonough1992). Grey field, Faial volcanic rocks; pale-grey field, Pico volcanic rocks; dark-grey field, Corvovolcanic rocks.

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Ratios of incompatible elements have been used in the identification of magmatic sourcein the heterogenous mantle by Weaver (1991). The comparison of these ratios in the less-differentiated rocks from Faial volcanic association with the values from other AzoresIslands (Pico and Corvo), and with those of mantle sources characterized by Weaver (1991),makes clear the similarity of volcanic rocks from Corvo with HIMU sources, whilst the Faialand Pico volcanic rocks could be produced from sources very close to EMII (Figure 14).Some of these ratios have been used by Pilet et al. (2005) for the mantle source identificationof Cantal basalts (Massif Central, France). When plotted in the diagrams presented by thoseauthors, most Faial rocks are situated in the field of basalts produced from EM sources.

Nd and Sr isotopic ratios for the volcanic rocks from Faial and Pico (França et al.2006c) islands are relatively homogeneous (Table 2). Since volcanic rocks are young,their radiogenic in-growth is insignificant, and their isotope compositions represent theirsource regions. The rocks are characterized by low 87Sr/86Sr (0.70306–0.70339) and high143Nd/144Nd (0.51294–0.513048) ratios, typical of what is usually registered in OIB(HIMU)-type rocks from both oceanic and continental settings (Zindler and Hart 1986;Wilson 1993). According to the mantle-reservoir model proposed by Zindler and Hart

Figure 14. Incompatible trace element ratios after Weaver (1991) normalized by N-MORB values.Double line, OIB EMI; dashed line, OIB EMII; full line, Primitive mantle; dotted line, HIMU.

Table 2. Sr, Nd, and Pb isotopic data of Faial (FA) and Pico (Pix) islands volcanic rocks.

Faial (FA) and Pico islands (Pix)

Sample 87Sr/86Sr 143Nd/144Nd 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

FA-61 0.70306 ± 0.00001 0.51302 ± 14 19.20214 15.60920 38.53222FA-62 0.70339 ± 0.00001 0.51302 ± 15 19.37173 15.68336 38.69045FA-63 0.70313 ± 0.00002 0.51299 ± 7 19.44523 15.69448 38.81818FA-65 0.70324 ± 0.00001 0.51299 ± 9 19.27754 15.66009 38.74445FA-66 0.70331 ± 0.00001 0.51299 ± 11 19.18108 15.66802 38.70613FA-67 0.70327 ± 0.00001 0.51299 ± 25 19.04136 15.64996 38.70744FA-68 0.70322 ± 0.00001 0.51294 ± 8 18.90069 15.63194 38.75207FA-69 0.70329 ± 0.00001 0.51305 ± 5 19.2363 15.62882 38.68488Pix244 0.70369 ± 0.00001 0.51289 ± 0.0 20.445 ± 0.03 15.726 ± 0.03 39.798 ± 0.03Pix231 0.70386 ± 0.00002 0.51286 ± 0.0 19.834 ± 0.02 15.717 ± 0.03 39.555 ± 0.03

Data from Machado et al. (2008) and França et al. (2006c).

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(1986), the 87Sr/86Sr and 143Nd/144Nd ratios of Faial and Pico Island samples are compatiblewith the values from the Cape Verde archipelago, which has the HIMU and MORB as themain mantle components (White 1998). Pico Island shows Pb isotopic ratios with astronger influence of HIMU component, which is similar to those found in rocks fromSt Helena, whose source is HIMU-type mantle (Rollinson 1996). Following Stracke et al.(2005), these isotope signatures are closer to sources of FOZO type, since they showlower 206,207,208Pb/204Pb and higher 87Sr/86Sr ratios than those of typical HYMU-typerocks (Figure 15). These authors suggest that ‘the apparent ubiquity of FOZO in the man-tle and the calculated isotopic evolution of compositionally diverse MORB suggest thatnormal mantle melting and continuous subduction and aging of that crust during recyclingthrough the mantle are the dominant causes of the MORB-FOZO array. . .’.

As proposed by Hart et al. (1992), FOZO was a mantle composition component wide-spread in OIB produced from plume activity. FOZO represented a source where EMI,EMII, HIMU, and DMM were present in variable proportions. Stracke et al. (2005) redefinedthe FOZO-type source and excluded the participation of HIMU components, therefore,this source only implies a mantle modified by previous crustal subduction.

The isotope data plot inside the fields of prevalent mantle reservoirs (PREMA) asmost of the basaltic magmas, with the Pico Island samples slightly displaced towards theBSE field (Bulk Silicate Earth reservoir, Figure 15a). The samples show positive values ofεNd and negative values of εSr, like most of the ocean island basalts (Rollinson 1996).The Pb ratios range between 18.90069 and 19.44523 for 206Pb/204Pb, from 15.60920 to15.69448 for 207Pb/204Pb, and from 38.53222 to 38.81818 for 208Pb/204Pb (Table 2). Thesamples plot close to the EMII reservoir in the 206Pb/204Pb versus 207Pb/204Pb diagram(Figure 15b), which suggests the participation of the EMII component. In the 143Nd/144Ndversus 206Pb/204Pb and 87Sr/86Sr versus 206Pb/204Pb diagrams (Figure 15c, d), the data plotclose to the Atlantic MORB and EMII mantle components. The available Sr, Nd, and Pb

Figure 15. Sr versus Nd (a), Pb versus Pb (b), Pb versus Nd (c), and Pb versus Sr (d) relationshipsfor Faial (circles) and Pico (×) islands rocks. Mantle reservoirs identified by Zindler and Hart (1986)and Rollinson (1996).

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isotopic data are consistent with multiple mantle sources including a MORB-type orDMM, with slighter influence of EMII components. Compared with isotope data from SãoMiguel rocks, the Faial and Pico lavas are similar to the Sete Cidades volcanic sequence, whichis formed from mantle sources which include ancient recycled oceanic crust (Beier et al. 2007).

The relatively small variation in the trace element (such as Zr/Nb) and isotopic ratios sug-gest that melt generation occurred by variable degrees of partial melting of an isotopicallyheterogeneous source in the mantle.

The 87Sr/86Sr versus 143Nd/144Nd diagram shows that the samples plot within thedepleted field of mantle array and extend from PREMA compositions towards the BSE(Figure 15a). Systematic changing proportions of mixing between melts produced by var-iable degrees of partial melting of at least two compositionally distinct sources in the man-tle is generally invoked to explain both the isotopically depleted, but LREE-enrichednature of many OIB-type alkaline suites and the quasi-linear patterns of highly incompati-ble elements observed in OIB-type rocks. Several recent studies regarding trace elementand isotope compositional variations on relatively short time scales and length scales havesuggested binary mixing as a dominant source of chemical variation in alkali primarysuites (e.g. Class and Goldstein 1997; Kamber and Collerson 2000).

Pb/Ce, Ba/Nb, and 207Pb/204Pb ratios, as discussed by White (1998) and Pilet et al.(2005), of less-differentiated liquids from Faial Island, particularly from Capelo Formation,indicate the presence of small percentages of modern marine sediments in the mantle source.

8. ConclusionsFaial lavas vary from basalts to benmoreite, with ages from Present – Capelinho Volcano –to 730 ka. These rocks belong to the alkaline-sodic series, and constitute four volcano-stratigraphic units, from the base to the top: (1) the Ribeirinha Volcanic Complex; (2) theCedros Volcanic Complex; (3) the Almoxarife Formation, dominantly fissure eruptionsprincipally of basic composition; and (4) the Capelo Formation, displaying the most primitivecomposition. Compared with Faial Island magmatism, Pico Island rocks are less differentiatedand basalt predominates over hawaiite, whereas on Corvo Island, volcanism is composi-tionally equivalent to the former.

Major and trace element data from the upper three volcano-stratigraphic units indicatethat fractional crystallization of plagioclase ± clinopyroxene ± olivine ± titanomagnetite,was the major mechanism of the differentiation. Olivine and clinopyroxene fractionationexplains the decreasing Ni, Cr, Co, and Sc values; whereas wide dispersion of Sr and Rbare ascribed to plagioclase and Ti-magnetite segregation–accumulation caused by magmaticflow. The absence of the Daly gap, typically present in ocean–island magmatism, isascribed to early crystallization of Ti-magnetite under relatively oxidized conditions.

Based on isotope and trace element data, the probable source of Faial and Pico islandmagmas reflects DMM and EMII components or a FOZO-type source as redefined byStracke et al. (2005), like the Sete Cidades lavas (Beier et al. 2007); this implies the pres-ence of ancient recycled oceanic crust in the source of the magmatism, as also concludedby Schaefer et al. (2002) based upon 187Os/188Os data. Ratios of incompatible traceelements suggest the compatibility of volcanic rocks from Corvo with HIMU sources,whereas the Faial and Pico volcanic rocks could have been produced from sources verysimilar to OIB–EMII or FOZO. Following proposals by Stracke et al. (2005), it seemspossible that Faial and Pico magmatism were produced by normal melting of DMMsources modified by continuous subduction and ageing of subducted crust during recy-cling through the mantle.

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AcknowledgementsThis work was made possible by a fellowship granted by CAPES (Brazil) and Fundação para a Ciência e aTecnologia-FCT (Portugal) – Bilateral Cooperation Project. We also thank the Institute of Geo-sciences from Federal University of Rio Grande do Sul and Centre for Geophysics of the Universityof Coimbra.

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Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores

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Transcript of Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores