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Thetransitionfromalkalinetotholeiiticmagmas:AcasestudyfromtheOrosei-DorgaliPliocenevolcanicdistrict(NE...

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DOI:10.1016/S0024-4937(02)00113-5

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The transition from alkaline to tholeiitic magmas: a case study from

the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy)

Michele Lustrino a,*, Leone Melluso b, Vincenzo Morra b

aDipartimento di Scienze della Terra, Universita degli Studi di Roma La Sapienza, P.le A. Moro 5, I-00185 Roma, ItalybDipartimento di Scienze della Terra, Universita degli Studi di Napoli Federico II, Via Mezzocannone 8, I-80134 Napoli, Italy

Received 13 June 2001; accepted 22 March 2002

Abstract

During the Pliocene, simultaneously with the opening of the Tyrrhenian Sea, mafic magmas were erupted in NE Sardinia

(Orosei-Dorgali area). These range from mildly alkaline with sodic affinity (about 80% of exposure) to tholeiitic (about 20%).

The tholeiitic rocks (basaltic andesite) are slightly more evolved than the alkaline ones and show geochemical features (e.g.,

Mg# < 63; Ni < 150 ppm and Cr < 270 ppm) different from typical primitive mantle liquids, suggesting low pressure fractional

crystallization processes. Alkaline lavas (mainly hawaiite plus rare alkali basalt and mugearite) are commonly characterized by

the presence of mantle xenoliths and have higher Mg# (up to 71), Ni (up to 340 ppm) and Cr (up to 420 ppm) than the tholeiitic

rocks. Both alkaline and tholeiitic lavas show sub-parallel patterns in primitive mantle-normalized diagrams, with peaks at Ba,

Pb and Sr and relatively low abundances of Nb and Ta, resulting in high Ba/Nb ratios (generally > 20). Similar incompatible

element ratios for both alkaline and tholeiitic rocks suggest different degrees of melting of a single mantle source. Mathematical

modeling indicates f 4–6% and f 10–15% partial melting for alkaline and tholeiitic lavas, respectively. Trace element

abundances of the Orosei-Dorgali volcanic rocks are typical of Plio-Pleistocene volcanic rocks of Sardinia but differ strongly

from other Cenozoic anorogenic volcanic rocks of Europe. Similarly, Sr (87Sr/86Sr = 0.70442–0.70455), Nd

(143Nd/144Nd = 0.512465–0.512558) and Pb (206Pb/204Pb = 17.74–17.86; 207Pb/204Pb = 15.53–15.60; 208Pb/204Pb = 37.89–

38.02) isotopic ratios are very unusual when compared with other Cenozoic European volcanic rocks. Trace element

abundances and isotopic composition of the Orosei-Dorgali volcanic rocks suggest a lithospheric mantle origin. D 2002

Elsevier Science B.V. All rights reserved.

Keywords: Sardinia; Pliocene; Alkaline; Tholeiitic; Partial melting; EMI; CEVP

1. Introduction

The Mediterranean area is a geodynamically com-

plex region which has been characterized during the

last 30 Ma by magmatic activity with a wide range of

chemical compositions, from strongly alkaline (with

sodic to potassic and ultrapotassic character) to sub-

alkaline character (both tholeiitic and calc-alkaline)

(Conticelli, 1998; D’Antonio et al., 1999; Turner et

al., 1999; Cebria et al., 2000; Lustrino et al., 2000a;

Downes et al., 2001). The Plio-Pleistocene Sardinian

volcanic rocks (hereafter called PSV) belong to the

well studied Cenozoic European Volcanic Province

(hereafter CEVP) for which a large set of chemical

data is currently available (e.g., Wilson and Downes,

0024-4937/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0024 -4937 (02 )00113 -5

* Corresponding author.

E-mail address: michele.lustrino@uniroma1.it (M. Lustrino).

www.elsevier.com/locate/lithos

Lithos 63 (2002) 83–113

1991; Pecskay et al., 1995; Liotard et al., 1999; Jung

and Hoernes, 2000; Wedepohl, 2000). Many products

of the PSV (e.g., Orosei-Dorgali area) currently lack

high-quality geochemical data.

The PSV allows us to investigate the geochemical

signature of this section of the European subcontinental

mantle, as well as the magmatic evolution of the west-

ern Mediterranean area for several reasons: (1) the

mafic rocks show compositions typical of mantle-

derived undifferentiated melts; (2) they are often asso-

ciated with mantle xenoliths that provide direct insights

into the subcontinental mantle beneath the island; (3)

their geographic position is critical as the region was

involved in the last two major tectonic events that

reworked the European subcontinental mantle (Hercy-

nian and Alpine orogenies; Lustrino, 2000b).

The Orosei-Dorgali area in NE Sardinia is one of

the largest outcrops of the PSV (Lustrino, 1999;

Lustrino et al., 2000b). In this paper, major and trace

element and Sr–Nd–Pb isotopic data of the volcanic

rocks are discussed. A comparison with other volcanic

rocks of the circum-Mediterranean area and CEVP is

given in order to provide evidence for the different

styles of enrichment of the mantle source.

2. Geological setting

The continental crust of Sardinia–Corsica was in

contact to that of southern France and Spain up to

Oligocene time when a continental rift system devel-

oped at the same time as the present-day Ligurian–

Provenc�al basin and eventually formed ocean crust.

The formation of this basin (30–15 Ma; Seranne,

1999) is thought to be related to NW-directed sub-

duction of the Mesogean oceanic lithosphere (Gue-

guen et al., 1998). This subduction system produced

the formation of a back-arc basin in response to a

retreat of the trench and a SE shift of the subduction

hinge (Doglioni et al., 1999). The Sardinia–Corsica

block firstly moved toward SE and then suffered a

counterclockwise rotation of about 40j (Speranza,

1999). A volcanic cycle from about 28–15 Ma devel-

oped in the island in response to subduction. This

activity peaked at 21–19 Ma (Beccaluva et al., 1985)

and marks the final stages of subduction. It formed a

huge amount of effusive and explosive products,

ranging from arc-tholeiites to high-K calc-alkaline

rocks, with rare evolved peralkaline eruptions (Morra

et al., 1994, 1997; Brotzu, 1997; Downes et al., 2001).

From about 15 to about 5 Ma no volcanism is

recorded in Sardinia; instead the formation of the

Tyrrhenian Sea east of Sardinia took place. The 15

Ma old Sisco lamproite in NE Corsica (Civetta et al.,

1978) and f 12 Ma old shoshonitic to lamproitic

Cornacya seamount (SE margin of Sardinia in the

Tyrrhenian Sea; Mascle et al., 2001) are considered to

be the first magmatic products related to the opening

of this basin (Fig. 1a). The opening of the Tyrrhenian

Sea was associated with widespread magmatism of

variable character (potassic to ultrapotassic with lamp-

roitic and kamafugitic affinity together with anatectic

crustal melts; e.g., Conticelli, 1998). Potassic to ultra-

potassic products and rarer calc-alkaline volcanic

rocks were emplaced mainly during the last 1 Ma

forming the so-called Roman Magmatic Province

(e.g., Beccaluva et al., 1991; Conticelli, 1998; D’An-

tonio et al., 1999). Further south, volcanic activity

generally younger than 1 Ma produced calc-alkaline,

potassic and ultrapotassic volcanic rocks of the Aeo-

lian archipelago, related to the Calabrian subduction

system (Francalanci et al., 1993; De Astis et al.,

2000). Tholeiitic to sodic alkaline volcanic rocks crop

out at Mt. Etna and Hyblean Mts. in Sicily (D’Orazio

et al., 1997; Beccaluva et al., 1998; Schiano et al.,

2001). Mildly alkaline sodic and tholeiitic rocks occur

also at Pantelleria, Linosa and Ustica islands (Rossi et

al., 1996; Civetta et al., 1998). Enriched MORB to

calc-alkaline basaltic andesites represent the more

abundant volcanic rocks of the Tyrrhenian abyssal

plain and of the main seamounts (Argnani and Savelli,

1999).

The PSV developed within this complex geody-

namic scenario, characterized by coeval formation of

extensional basins (Ligurian–Provenc�al and Algerian

basins, Tyrrhenian Sea) and mountain chains (Alps,

Apennine, Betic, Rif and Maghrebide chains). The

magmatic activity erupted over continental crust

approximately 30 km thick which changes eastward

and westward into a thinned continental crust and

finally into an oceanic crust. Thus, Sardinia represents

a continental lithospheric slice which is about 70 km

thick and isolated during the boudinage of the Med-

iterranean region (e.g., Gueguen et al., 1998).

PSV magmatic activity occurs throughout the

entire island (Fig. 1b) and is mainly represented by

M. Lustrino et al. / Lithos 63 (2002) 83–11384

Fig. 1. Simplified geologic sketch map of: (a) Neogene to present volcanic rocks related to the opening of the Tyrrhenian Sea; (b) Oligo-Miocene and Plio-Pleistocene volcanic

outcrops of Sardinia; (c) Pliocene volcanic outcrops of the Orosei-Dorgali area (NE Sardinia; modified after Beccaluva and Macciotta, 1983).

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Table 1

Major and trace element compositions of selected volcanic rocks from Orosei-Dorgali determined by XRF

Label MGV1 MGV2 MGV5 MGV7 MGV10 MGV12 MGV15 MGV22 MGV24 MGV25 MGV26 MGV28 MGV33 MGV34 MGV35 MGV50 MGV51

Rock

type (TAS)

Haw Haw Haw Haw Haw B And B And Haw B And B And B And B And B And B And B And Alk B B And

SiO2 50.23 49.91 49.94 50.06 49.89 52.66 52.63 51.27 54.03 54.61 54.67 55.38 55.82 55.72 55.14 48.85 52.93

TiO2 1.99 2.05 1.85 1.75 1.79 1.66 1.77 1.65 1.43 1.36 1.47 1.34 1.24 1.43 1.36 1.88 1.60

Al2O3 16.68 16.28 15.82 16.62 15.89 16.32 16.27 15.57 16.23 16.37 16.12 16.29 16.23 16.32 15.64 16.68 16.78

Fe2O3XRF 9.73 10.07 10.02 9.27 9.31 9.44 9.79 10.22 9.53 9.35 9.70 8.81 8.83 9.21 9.04 9.79 10.05

Fe2O3 3.06 3.13 3.75 1.17 8.52 2.24 2.38 6.83 3.03 2.98 3.42 2.92 2.24 3.10 5.46 5.11 3.53

FeO 6.01 6.25 5.65 7.30 0.71 6.49 6.68 3.05 5.86 5.74 5.66 5.31 5.94 5.51 3.23 4.22 5.88

MnO 0.13 0.13 0.13 0.12 0.13 0.12 0.13 0.12 0.12 0.12 0.12 0.12 0.11 0.11 0.11 0.13 0.13

MgO 7.16 7.26 8.18 8.91 8.79 7.00 6.57 7.57 6.50 6.35 5.65 5.80 5.96 5.54 6.78 8.24 6.08

CaO 7.83 7.97 7.81 7.48 8.12 7.65 7.88 7.16 7.37 7.37 7.39 7.26 7.07 7.19 6.92 7.92 7.88

Na2O 3.36 3.40 3.04 3.23 3.44 3.67 3.27 3.59 3.62 3.60 3.64 3.41 3.59 3.67 3.66 2.69 3.41

K2O 2.16 2.08 2.06 2.10 1.97 1.04 1.00 1.44 0.56 0.69 0.57 0.48 0.56 0.50 0.53 2.08 0.72

P2O5 0.43 0.42 0.42 0.44 0.36 0.26 0.23 0.33 0.15 0.18 0.14 0.14 0.16 0.14 0.14 0.43 0.19

LOI 0.98 1.12 1.36 0.81 0.39 0.90 1.20 1.41 1.10 0.64 1.15 1.55 1.08 0.77 1.03 1.78 0.87

Mg# 0.62 0.62 0.65 0.68 0.68 0.62 0.60 0.62 0.61 0.60 0.57 0.60 0.60 0.57 0.63 0.65 0.58

qz 0.00 0.00 0.00 0.00 0.00 0.00 1.27 0.00 2.85 3.35 4.58 6.95 6.22 6.24 4.50 0.00 1.86

hy 1.86 0.00 5.96 2.12 0.00 19.56 22.67 12.65 23.32 22.93 21.10 21.47 21.99 20.86 23.65 7.13 22.58

ol 15.56 16.82 14.71 18.98 18.54 2.71 0.00 9.54 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14.65 0.00

ne 0.00 0.04 0.00 0.00 1.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

V 194 194 187 177 186 161 165 162 140 143 150 132 129 139 155 189 157

Cr 255 280 382 353 363 246 250 419 243 250 247 231 267 251 263 362 235

Co 45 45 44 45 47 40 47 43 40 40 42 35 38 38 37 46 44

Ni 170 181 264 231 212 140 140 328 134 133 132 129 148 131 134 240 138

Cu 45 46 46 45 47 41 41 44 37 39 38 37 35 36 40 47 46

Zn 93 97 95 88 89 105 97 104 101 100 104 99 96 102 110 87 105

Rb 34 35 33 33 44 19 21 25 9 10 11 6 8 8 9 40 8

Sr 766 771 721 701 649 476 492 657 475 478 464 445 463 447 494 763 505

Y 18 18 19 17 19 15 20 17 17 17 19 18 17 18 16 23 19

Zr 203 205 209 196 165 123 142 135 95 96 96 87 83 92 96 184 113

Nb 35 36 33 31 29 15 16 25 11 12 11 8 8 9 12 40 15

Ba 850 833 808 796 660 396 408 608 296 279 265 224 422 227 325 891 387

La 32 32 32 34 29 15 17 21 12 15 13 12 9 11 12 45 18

Ce 68 65 69 66 68 31 46 43 26 28 27 20 30 24 26 72 40

Nd 33 30 32 30 31 17 24 22 17 16 16 14 19 16 15 32 19

Haw=Hawaiite; B And =Basaltic Andesite; Alk B=Alkali Basalt; Mug =Mugearite. The complete list of the samples is available upon request from the first author.

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MGV53 MGV54 MGV58 MGV60 * MGV62 MGV71 MGV76 MGV79 MGV80 MGV81 MGV83 MGV87 MGV88 MGV89 MGV93 MGV95 MGV97 MGV98 MGV215

B And Mug Mug B And Haw Haw Alk B Mug Mug Alk B Haw Haw Haw B And Haw Haw B And Haw Haw

52.63 50.66 51.49 53.58 49.07 49.06 47.60 51.26 51.77 47.48 51.48 51.23 50.12 52.22 49.50 50.82 52.22 49.63 50.91

1.51 1.93 1.89 1.57 1.82 2.11 1.75 1.73 1.72 2.04 1.61 1.65 1.83 1.84 2.06 1.95 1.86 1.78 1.98

16.93 16.59 16.17 16.47 16.08 15.52 15.53 16.49 16.51 15.57 15.83 15.31 17.02 16.69 15.87 16.02 16.39 16.11 14.20

9.79 10.13 10.39 9.72 9.78 10.09 10.14 10.17 9.22 9.93 9.69 10.15 9.60 10.90 9.68 9.07 10.43 9.51 11.00

1.91 6.24 5.72 1.30 1.09 2.93 2.46 4.40 3.28 2.17 3.43 5.68 5.62 6.04 4.46 5.08 3.84 1.10 5.53

7.10 3.50 4.20 7.59 7.83 6.45 6.92 5.20 5.35 6.99 5.64 4.03 3.59 4.37 4.71 3.60 5.94 7.58 4.84

0.13 0.15 0.12 0.12 0.14 0.13 0.14 0.11 0.12 0.13 0.12 0.12 0.12 0.14 0.13 0.13 0.14 0.13 0.16

6.38 5.80 5.92 6.76 9.71 8.27 10.82 6.35 7.02 10.69 8.26 7.90 6.54 3.56 8.02 6.95 5.87 10.04 7.03

7.84 7.25 7.77 7.44 8.02 8.39 8.42 7.39 7.56 8.24 7.10 7.05 7.51 8.69 7.14 6.62 8.20 7.39 8.18

3.44 3.54 3.57 3.30 3.41 3.31 3.58 3.64 3.57 3.95 3.66 3.66 3.24 3.39 4.39 4.55 3.49 3.34 2.94

0.70 1.85 1.61 1.01 1.80 2.28 0.81 1.81 1.67 0.84 1.44 1.42 2.23 1.30 1.10 1.53 0.94 1.86 2.05

0.21 0.46 0.35 0.24 0.41 0.43 0.38 0.40 0.40 0.45 0.31 0.30 0.50 0.30 0.52 0.53 0.25 0.39 0.45

1.22 2.02 1.19 0.62 0.63 1.13 1.59 1.23 1.03 1.46 1.13 1.65 1.68 1.46 2.10 2.23 0.86 0.65 1.74

0.59 0.56 0.56 0.61 0.69 0.65 0.71 0.58 0.63 0.71 0.66 0.64 0.60 0.42 0.65 0.63 0.56 0.70 0.59

1.10 0.00 0.00 2.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.15 0.00 0.00 0.31 0.00 0.00

23.39 10.93 12.43 24.41 0.00 0.00 0.00 8.67 11.83 0.00 10.57 11.24 6.61 14.75 0.00 0.00 20.84 0.00 9.66

0.00 7.84 6.08 0.00 21.20 17.37 23.18 10.22 8.20 22.28 11.98 10.92 11.99 0.00 17.24 16.30 0.00 22.44 9.19

0.00 0.00 0.00 0.00 2.14 2.69 2.64 0.00 0.00 4.41 0.00 0.00 0.00 0.00 2.02 0.42 0.00 0.35 0.00

150 188 168 154 187 187 193 168 166 201 159 157 163 164 188 158 167 165 200

245 276 237 267 376 280 386 230 249 403 382 419 235 263 350 325 240 414 283

41 49 42 42 47 45 54 39 41 54 46 46 40 47 48 39 46 46 40

142 205 168 171 228 196 295 178 156 293 343 343 168 180 282 229 150 283 205

44 45 43 45 43 48 47 41 45 41 47 46 41 43 35 38 44 47 43

100 101 101 103 91 99 91 102 91 93 98 109 96 98 98 101 103 91 100

9 34 29 17 33 40 26 32 29 40 26 24 34 21 15 22 11 33 36

502 761 635 508 682 829 681 737 621 806 617 625 836 568 993 1119 496 666 695

18 22 20 17 20 19 18 19 21 17 17 27 21 20 16 17 22 20 24

107 176 159 130 174 201 156 159 159 167 134 135 173 149 235 286 146 185 182

13 32 27 15 35 39 31 34 28 42 24 26 39 22 50 59 19 36 38

362 842 638 402 743 1105 689 746 646 883 556 580 1000 548 1329 1143 425 721 758

17 33 29 21 36 38 27 28 31 40 22 33 43 29 54 62 19 35 31

27 68 58 39 60 89 60 56 57 71 47 45 71 41 98 119 37 69 63

13 33 29 20 28 39 28 27 27 34 22 24 33 19 43 51 17 29 30

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Table 2

ICP-MS trace element and Sr, Nd and Pb isotopic data for Orosei-Dorgali volcanic rocks

Label MGV10 MGV25 MGV26 MGV 35 MGV 50 MGV 51 MGV 60 MGV62 MGV 71 MGV76 MGV89 MVG98 MGV215

Rock type

(TAS)

Haw B And B And B And Alk B B And B And Haw Haw Alk B B And Haw Haw

V 180 156 161 132 192 152 145 204 177 192 165 164 179

Cr 351 268 251 212 307 190 209 404 227 427 279 392 266

Co 44.0 39.3 42.0 31.0 37.4 35.2 33.8 50.1 38.2 51.7 38.8 44.6 44.4

Ni 211 150 140 123 223 119 167 231 166 298 168 280 193

Cu n.a 30.0 41.0 30.3 36.0 36.3 34.2 32.1 34.6 33.7 36.3 n.a n.a

Zn n.a 107.4 123.0 124.0 140.5 128.7 112.9 112.0 131.1 104.3 107.8 n.a n.a

Rb 48.3 13.8 13.0 8.1 37.9 9.0 17.0 39.5 39.7 21.2 20.9 36.6 35.3

Sr 670 465 513 457 719 515 520 722 856 690 536 679 668

Y 19.1 15.6 16.4 16.0 21.0 19.0 17.0 17.7 18.0 17.0 18.1 19.8 20.1

Zr 171.8 89.5 107.0 85.4 179.8 108.4 123.3 185.0 199.7 158.8 134.0 187.8 175.9

Nb 31.9 12.1 14.0 8.1 39.4 14.1 15.0 37.1 40.7 31.2 19.7 37.3 36.0

Cs 0.76 0.23 n.a. n.a. n.a. n.a. n.a. 0.59 0.64 0.82 0.53 0.52 0.24

Ba 657 264 346 242 902 406 415 691 1114 648 507 710 703

Hf 4.14 2.35 2.70 2.60 4.78 3.09 3.43 4.07 5.41 3.46 3.31 4.46 4.13

Ta 1.99 0.9 0.9 0.6 2.7 1.1 1.1 2.4 2.9 2.1 1.4 2.27 2.17

Pb 3.3 2.6 3.4 6.7 8.3 7.9 6.4 4.6 14.8 3.0 4.0 4.6 4.0

Th 3.9 1.9 2.2 1.5 4.8 2.3 2.7 4.8 4.6 3.4 2.9 4.5 3.8

U 0.79 0.43 0.40 0.32 n.a. 0.64 0.83 0.94 1.11 0.79 0.64 0.94 0.84

Sc 19.2 n.a n.a. 15.0 21.0 18.0 16.0 n.a 17.0 n.a n.a 18.9 19.3

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La 32.15 12.08 14.20 11.42 38.32 16.67 18.72 33.29 42.54 26.51 20.35 34.08 32.96

Ce 62.89 24.64 29.80 22.34 68.88 31.66 36.89 64.63 79.45 51.98 39.37 66.62 63.65

Pr 7.55 3.30 4.02 2.72 6.81 3.40 3.84 7.70 7.58 6.16 4.71 7.82 7.49

Nd 30.04 15.68 18.00 14.56 29.50 17.08 18.61 26.80 32.78 25.17 19.81 30.65 29.66

Sm 5.67 3.98 4.28 4.43 6.38 4.77 4.79 5.76 6.77 4.87 4.37 5.71 5.69

Eu 1.89 1.45 1.57 1.64 2.00 1.67 1.65 1.81 2.15 1.63 1.57 1.70 1.69

Gd 5.13 3.76 4.15 4.42 5.81 4.57 4.71 4.46 6.21 4.36 4.42 5.28 5.11

Tb 0.72 0.55 0.54 0.67 0.78 0.70 0.65 0.66 0.79 0.60 0.60 0.73 0.73

Dy 3.86 3.12 3.01 3.59 4.29 3.72 3.58 3.84 4.12 3.22 3.30 4.06 3.91

Ho 0.71 0.54 0.55 0.59 0.74 0.64 0.60 0.65 0.67 0.55 0.66 0.72 0.73

Er 1.69 1.36 1.37 1.66 2.00 1.77 1.60 1.72 1.78 1.47 1.59 1.82 1.70

Tm 0.23 0.19 0.19 0.21 0.27 0.23 0.21 0.24 0.22 0.20 0.23 0.25 0.27

Yb 1.33 1.21 1.05 1.23 1.57 1.33 1.25 1.53 1.31 1.16 1.36 1.38 1.36

Lu 0.20 0.19 0.16 0.16 0.20 0.17 0.16 0.20 0.15 0.18 0.22 0.21 0.2087Sr/86Sr 0.70447 0.70446 0.70449 0.70455 0.70465 0.70451 0.70442 0.70453 0.70446 0.70442143Nd/144Nd 0.512558 0.512465 0.512524 0.512518 0.512470 0.512550 0.512571 0.512538 0.512528 0.512510

eNd � 1.6 � 3.4 � 2.3 � 2.4 � 3.3 � 1.8 � 1.3 � 2.0 � 2.2 � 2.5206Pb/204Pb 17.837 17.826 17.860 17.738207Pb/204Pb 15.598 15.594 15.596 15.531208Pb/204Pb 37.989 38.016 37.942 37.894

Haw=Hawaiite; B And =Basaltic Andesite; Alk B=Alkali Basalt. Samples MGV51 and MGV76 from Lustrino et al. (2000a). n.a. = not analyzed.

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basaltic plateaux (Orosei-Dorgali; Planargia–Abba-

santa Plains, Gerrei area) with rarer volcanic piles

(Montiferro and Mt. Arci), cinder cones (Logudoro),

small necks (Rio Girone, Guspini) and small lava

flows (Barisardo, Baunei, Capo Ferrato, Tharros) (Di

Battistini et al., 1990; Montanini et al., 1994; Lustrino

et al., 1996, 2000a; Lustrino, 1999, 2000c; Gasperini

et al., 2000; Fig. 1b). These rocks are mainly mildly

alkaline mafic to evolved products (alkali basalt,

basanite, trachibasalt, hawaiite, mugearite, with rarer

trachyte, rhyolite and phonolite). Tholeiitic rocks form

about 30% of the PSV and are slightly more evolved

than the alkaline counterparts (basaltic andesite, ande-

site, dacite and rhyolite).

The Orosei-Dorgali area (Fig. 1c) is a Pliocene

volcanic district which overlies Paleozoic basement

and Mesozoic limestones and dolostones, cropping

out over 150 km2 along a roughly NNE–SSW and

NW–SE trending fissure system. These rocks com-

prise f 80% alkaline and 20% tholeiitic lavas (Lus-

trino et al., 2000b). There is no apparent correlation

between time of emplacement, silica saturation and

geographic position of the erupted rocks, with time

overlap between tholeiitic and alkaline lavas. With the

exception of two samples reported by Lustrino et al.

(2000a), no isotopic data are available and major and

trace element analyses have not yet published for the

Pliocene volcanic rocks from Orosei-Dorgali.

3. Analytical techniques

Major and trace element analyses have been per-

formed at the Dipartimento di Scienze della Terra of

Naples and Florence on pressed powder pellets using

PW1400 (CISAG, Naples) and PW1440 (Florence)

XRF spectrometers, Rh and W anodes and the data

reduction methods of Franzini et al. (1975) and Leoni

and Saitta (1976). Calibration curves were obtained

using 35 international standards. Precision is better

than 3% (relative) for major elements, 5% for Zn, Cu,

Sr, Zr and Ba and 10% for the other trace elements.

Na2O and MgO have been determined by atomic

absorption spectrophotometry (AAS) at Naples. FeO

has been obtained by colorimetry (KMnO4 titration).

Loss on ignition (LOI) has been determined with

standard gravimetric techniques, after igniting the

powder at f 1100 jC, and corrected for FeO oxida-

tion. REEs and other trace elements were determined

by ICP-MS at CRPG (Nancy, France), Actlabs (On-

tario, Canada) and Dipartimento di Scienze della Terra

of Pisa (Italy). Bias among trace element concentra-

tions obtained using three different ICP-MS is gener-

ally within 5%, whereas bias between XRF and ICP-

MS is generally within 10%. Electron microprobe

analyses have been carried out at CSQEA, CNR,

Rome, utilizing a CAMECA SX50 electron microp-

robe and mixed EDS-WDS acquisition procedure

operating at 15 kV and 15 nA and an electron beam

variable from 1 Am (olivine, pyroxene and opaque

minerals) to 5 Am (feldspar). The data were reduced

according to the PAP correction method. Sr, Nd and Pb

isotope ratios have been measured at the SOEST

(University of Hawaii at Manoa). Small pieces of

samples were leached with HF/HNO3 mixtures for

about half an hour and then rinsed twice with ultrapure

distilled water. The chips were ground in a boron

carbide mortar, dissolved using hot HF/HNO3mixtures

in Teflon bombs for at least 48 h and converted to

chloride form using HCl. Sr and Nd were separated by

standard ion exchange procedures using cation resin

columns. Sr and Nd were loaded on single Ta filaments

and analyzed with a VG Micromass 354 fully auto-

mated, multiple collector mass spectrometer. Pb was

run on single Re filaments with silica gel evaporator

and H3PO4. Total procedure blanks: < 200 pg for Sr,

< 20 pg for Nd, < 30 pg for Pb.

4. Results

Seventy-eight samples have been analyzed for

major and trace element abundances; on a selected

subset of samples, Sr–Nd–Pb isotope systematics

have been carried out. Samples with LOI>2.3 wt.%

and/or with interstitial secondary calcite and clear

evidence of deuteric alteration have been excluded

from the discussion. XRF major and trace element

data of representative samples are shown in Table 1;

ICP-MS trace element analyses and Sr–Nd–Pb iso-

topic ratios are shown in Table 2.

4.1. Petrography

According to the TAS diagram (Le Bas et al.,

1992), the volcanic rocks of Orosei-Dorgali are clas-

M. Lustrino et al. / Lithos 63 (2002) 83–11390

sified in the order of abundance as: hawaiite, basaltic

andesite, alkali basalt and rare mugearite (Fig. 2). Al-

kaline rocks are silica-saturated to slightly silica-un-

dersaturated character; they are porphyritic with

euhedral olivine and plagioclaseF clinopyroxene and

oxides in a pilotaxitic and/or hyalopilitic matrix. Abun-

dant disrupted mantle xenoliths and rare crustal xen-

ocrysts (mainly quartz with clinopyroxene rims) have

been observed.

The subalkaline rocks are silica-saturated to slightly

silica-oversaturated. They are sparsely porphyritic

mainly with plagioclase and minor intergranular cli-

nopyroxeneF iddingsitized olivine phenocrysts. The

groundmass is made up of microlites of plagioclase,

clinopyroxene and oxides. Rare groundmass orthopyr-

oxene has also been observed. No mantle xenoliths

occur in the basaltic andesites.

Selected electron microprobe analyses of olivine

are presented in Table 3a. Olivine is almost ubiquitous

in the alkaline rocks. It occurs as iddingsitized sub-

hedral to euhedral phenocrysts, as xenomorphic

groundmass crystals of Fo83-56 and as xenocrysts

derived from disrupted mantle assemblages (Fo92-88).

Groundmass olivine in the alkaline rocks has higher

Fo (Fo83-66) compared to the tholeiitic group (Fo79-56).

In general, olivine of the tholeiites has a larger mon-

ticellite fraction (CaO>0.3 wt.%) than that of the

alkaline rocks (generally CaO < 0.3 wt.%), indicating

shallower depths of equilibration (e.g., Kohler and

Brey, 1990).

Clinopyroxene is ubiquitous in the tholeiitic rocks,

but rarer in the alkaline group. It mainly occurs in the

interstices of plagioclase laths and as rare glomerules

together with plagioclase xenocrysts. Clinopyroxene

composition ranges from diopside and salite to augite

(Table 3b). In general, clinopyroxene from alkaline

rocks has < 14% of ferrosilite component, whereas

that of tholeiites has generally FeSiO3>15%. Clino-

pyroxene of glomerules is characterized by higher Al/

Ti ratios with respect to both the phenocrysts and

groundmass phases (4.4–7.3 vs. 1.8–5.2, respec-

tively). The higher AlVI in clinopyroxenes from alka-

line lavas agree with the lower Ca content of olivine,

possibly suggesting greater depth of equilibration with

the host lava, compared to the mafic phases of

tholeiitic lavas (e.g., Nimis, 1999).

Plagioclase is the most common phase in both the

alkaline and tholeiitic rocks. Its composition ranges

from labradorite (f 70% of analyses) to andesine

(f 25%), with minor (f 5%) oligoclase (Table 3c).

No differences exist in terms of major elements

between plagioclase from alkaline and tholeiitic

liquids. Rare sanidine and anorthoclase also occur as

groundmass phases in both alkaline and tholeiitic

samples.

Opaques are present exclusively in groundmass

and in interstitial position. Both magnetite s.s. and

ilmenite s.s. are present (Table 3d). Magnetite shows a

wide range of composition (Ulvospinel content from

26% to 86%), while ilmenite show much less solid

solution (ilmenite content from 90% to 99%) indicat-

ing weakly oxidizing conditions of formation, plotting

between the Nickel –Nickel Oxide (NNO) and

Quartz–Fayalite–Magnetite (QFM) buffers (Lustrino,

1999).

4.2. Major and trace element composition

The Pliocene volcanic rocks from Orosei-Dorgali

have chemical characters similar to those of other

Fig. 2. Total alkali vs. silica (TAS) diagram (Le Bas et al., 1992) for

the Pliocene volcanic rocks of Orosei-Dorgali. Filled circles:

alkaline; half-filled circles: transitional; open circles: tholeiitic. Also

shown for comparison the field of Italian mafic anorogenic volcanic

rocks: Mt. Etna (D’Orazio et al., 1997), Hyblean Mts. (Beccaluva et

al., 1998), Pantelleria (Esperanc�a and Crisci, 1995; Civetta et al.,

1998) and Linosa islands (Rossi et al., 1996) and Plio-Pleistocene

volcanic rocks from Sardinia: Gerrei (Lustrino et al., 1996; Lustrino,

2000c), Mt. Arci (Cioni et al., 1982; Montanini et al., 1994),

Montiferro (Di Battistini et al., 1990), Guspini, Rio Girone,

Barisardo, Abbasanta–Planargia–Paulilatino plains (Lustrino et

al., 2000a) and Logudoro (Gasperini et al., 2000).

M. Lustrino et al. / Lithos 63 (2002) 83–113 91

PSV. In particular, subalkaline rocks show a tholeiitic

character with low K2O (average 0.70 wt.%), iron

enrichment during initial stages of evolution and late

appearance of opaque minerals. The alkaline rocks are

sodic (Na2O/K2O= 1.14–4.70; average 2.11), similar

to Neogene–Quaternary alkaline volcanic rocks from

Sicily (D’Orazio et al., 1997; Beccaluva et al., 1998;

Trua et al., 1998; Schiano et al., 2001) and southern

Mediterranean Sea (Cinque et al., 1988; Rossi et al.,

1996; Civetta et al., 1998; Fig. 1a). With a few

exceptions (MGV49 and MGV79), tholeiitic rocks

are CIPW quartz-normative (norm. quartz = 1.1–

6.9%; CIPW norm calculated assuming a Fe2O3/FeO

ratio = 0.15) whereas the alkaline ones are all CIPW

olivine-normative (norm. olivine = 2.7–23.2%), with

only few (about 20%) characterized by nepheline in

the norm (normative nepheline = 0.1–4.4%). Rocks

with mineralogical and chemical affinity to alkaline

rocks but which are hypersthene- and/or quartz-nor-

mative have been classified as transitional (Lustrino,

1999).

SiO2 ranges from 47.5 to 55.8 wt.% while

Mg# [Mg# =Mg/(Mg + Fe2 + ), assuming Fe2O3/

FeO = 0.15] varies from 0.71 to 0.57 (average 0.62;

Table 1). The alkaline rocks are more mafic than the

transitional and tholeiitic rocks; their SiO2 ranges

from 47.5 to 52.9 wt.% and MgO from 5.8 to 10.8

wt.%, while Al2O3 (15.3–17.0 wt.%), CaO (7.0–8.6

wt.%) and Na2O (2.7–3.9 wt.%) show less variation.

The Orosei-Dorgali alkaline rocks are enriched both in

compatible and incompatible trace elements when

compared to the transitional and tholeiitic ones. A

striking feature is their high Ba content (408–1105

ppm) coupled with relatively low Nb (15–43 ppm);

these are near the most extreme values reported for the

roughly coeval Italian sodic alkaline within-plate (i.e.

not related to subduction processes) volcanic rocks at

the same level of evolution (see Lustrino, 2000a for a

review). High Cr and Ni contents (maximum values

419 and 343 ppm, respectively), coupled with high

Mg#, indicate a mantle-derived origin for some of

these melts.

The tholeiitic rocks are slightly more evolved than

the alkaline ones; SiO2 ranges from 51.3 to 55.8 wt.%

(average 54.2 wt.%), MgO from 5.5 to 6.8 wt.%

(average 6.1 wt.%) and Mg# from 0.63 to 0.57

Table 3a

Selected analyses of olivine from Orosei-Dorgali volcanic rocks

Olivine

Sample SiO2 MnO FeO MgO CaO NiO Sum Fo

MGV 26 gm 36.80 0.37 31.62 30.42 0.27 0.11 99.58 63.17

MGV 26 mf-c 39.96 0.14 17.86 42.36 0.20 0.26 100.77 80.87

MGV 60 gm 36.11 0.53 37.96 27.40 0.25 102.24 56.27

MGV 60 gm 35.60 0.31 37.34 27.23 0.36 0.41 101.25 56.52

MGV 60 mf-c 39.00 0.06 19.39 41.96 0.34 0.59 101.34 79.42

MGV 60 mf-r 38.13 0.33 26.28 36.32 0.32 0.56 101.94 71.13

MGV 60 gm 37.07 0.44 35.58 29.81 0.33 103.23 59.90

MGV 71 gm 40.13 0.20 15.98 44.24 0.20 100.74 83.15

MGV 71 gm 39.50 0.28 17.39 42.78 0.29 100.23 81.44

MGV 81 gm 38.19 0.44 23.79 36.98 0.21 0.13 99.75 73.48

MGV 81 gm 38.30 0.44 26.34 35.86 0.27 0.21 101.42 70.83

MGV 81 gm 38.19 0.47 25.71 35.07 0.27 0.15 99.87 70.86

MGV 81 gm 38.58 0.34 23.84 36.94 0.20 0.20 100.09 73.42

MGV 81 gm 37.29 0.52 29.34 32.38 0.25 99.78 66.30

MGV 83 mf 38.30 0.17 23.89 38.52 0.32 0.47 101.67 74.19

MGV 83 gm 37.80 0.41 27.64 34.90 0.28 0.52 101.54 69.24

MGV 83 gm 37.44 0.39 28.80 34.44 0.29 0.45 101.82 68.07

MGV 83 gm 37.81 0.44 26.66 36.09 0.28 0.45 101.73 70.70

MGV 83 mf-c 39.15 0.30 19.81 41.70 0.24 0.59 101.79 78.96

MGV 83 mf-r 38.77 0.15 21.89 39.90 0.27 0.54 101.52 76.47

MGV 83 gm 36.60 0.37 29.09 32.46 0.29 0.47 99.28 66.55

gm=groundmass; mf-c =microphenocrystal core; mf-r =microphenocrystal rim.

M. Lustrino et al. / Lithos 63 (2002) 83–11392

(average 0.59). Compared to the alkaline rocks, the

tholeiitic group has similar contents of Al2O3 (15.6–

16.9 wt.%) and CaO (6.9–7.9 wt.%), together with

lower incompatible (e.g. Ba, Nb, Zr, REE) and com-

patible (e.g. Ni) trace element abundances (Table 1).

The transitional rocks share more geochemical

similarities with the alkaline rocks and so they were

grouped together. The presence of a transitional group

with major and trace element features intermediate

between alkaline and tholeiitic rocks was already

Table 3b

Selected analyses of clinopyroxene from Orosei-Dorgali volcanic rocks

Clinopyroxene

Sample SiO2 TiO2 Al2O3 Cr2O3 MnO FeO MgO CaO Na2O Sum Wo En Fs

MGV 26 glom-c 53.74 0.63 0.86 0.30 0.29 11.81 19.82 12.98 0.19 100.61 25.96 55.14 18.91

MGV 26 gm 51.17 1.52 2.11 0.23 0.18 12.66 16.21 15.84 0.28 100.20 32.72 46.57 20.71

MGV 26 gm 52.09 1.00 1.78 0.57 0.15 9.77 17.34 16.20 0.27 99.16 33.71 50.18 16.11

MGV 26 gm 51.15 1.59 1.83 0.08 0.39 11.29 14.76 18.46 0.30 99.86 38.36 42.68 18.96

MGV 58 gm 49.76 2.15 2.91 0.05 0.41 13.55 12.33 19.01 0.53 100.70 40.39 36.45 23.16

MGV 58 gm 53.08 0.99 1.71 0.25 0.11 7.99 16.29 20.39 0.32 101.12 41.30 45.90 12.80

MGV 60 gm 53.04 0.90 1.36 0.35 0.24 10.40 17.35 17.03 0.29 100.96 34.43 48.79 16.79

MGV 60 gm 52.38 1.07 2.14 0.37 0.21 9.80 16.59 18.37 0.32 101.24 37.29 46.84 15.87

MGV 60 gm 50.04 1.82 3.81 0.27 0.30 10.75 15.07 18.57 0.36 100.98 38.56 43.53 17.91

MGV 60 gm 52.48 1.04 1.90 0.25 0.24 9.43 16.99 18.78 0.37 101.49 37.59 47.30 15.11

MGV 60 gm 51.13 1.35 2.96 0.65 0.15 9.84 15.71 18.95 0.38 101.12 38.99 44.96 16.04

MGV 71 f-c 48.11 2.23 6.27 0.78 0.14 6.83 13.76 21.48 0.48 100.07 46.64 41.55 11.81

MGV 71 glom-r 47.78 2.19 6.33 0.51 0.18 6.92 13.65 21.80 0.49 99.85 47.05 40.98 11.97

MGV 71 glom-m 48.34 2.05 6.09 0.52 0.16 6.71 14.00 21.54 0.43 99.82 46.45 41.99 11.56

MGV 71 glom-c 50.27 1.34 3.88 0.33 0.23 6.84 15.31 21.00 0.41 99.61 43.93 44.53 11.55

MGV 71 glom-c 47.95 2.14 6.23 0.54 0.15 6.83 13.70 21.54 0.51 99.59 46.79 41.38 11.83

MGV 71 glom-r 47.90 2.28 6.38 0.48 0.08 7.22 13.52 21.61 0.44 99.91 46.85 40.78 12.36

MGV 83 in pl 51.66 0.88 1.33 0.06 0.40 10.09 14.51 19.57 0.51 98.99 40.83 42.09 17.07

MGV 93e gm 47.13 2.77 6.98 0.40 0.18 7.18 13.54 21.44 0.52 100.14 46.59 40.92 12.48

MGV 93e gm 47.47 2.68 6.22 0.30 0.14 7.22 13.50 22.14 0.46 100.13 47.45 40.24 12.31

MGV 95S gm 47.56 2.81 5.78 0.38 7.19 13.02 22.28 0.52 99.53 48.44 39.36 12.20

MGV 95S mf-r 51.97 1.22 3.23 0.21 0.16 6.28 15.75 21.99 0.47 101.26 44.95 44.77 10.28

MGV 95S gm 47.43 2.61 5.78 0.17 7.69 13.24 22.15 0.48 99.53 47.56 39.56 12.88

MGV 95S gm 47.60 2.63 5.97 0.52 0.17 7.02 13.32 22.31 0.46 99.99 48.03 39.88 12.09

MGV 95S f-c 49.72 1.91 4.03 0.15 0.13 7.54 14.57 21.51 0.42 99.98 45.04 42.44 12.52

MGV 95S f-r 47.65 2.66 5.79 0.19 0.22 7.16 13.32 22.43 0.44 99.88 48.01 39.64 12.34

MGV 95S f-r 47.84 2.04 3.82 0.05 0.12 7.14 13.84 21.37 0.48 96.69 46.17 41.59 12.24

MGV 95S f-c 48.00 2.53 5.81 0.64 0.10 7.14 13.53 22.06 0.49 100.31 47.41 40.44 12.15

MGV 95S spon 52.68 0.91 1.34 0.05 0.17 7.07 14.55 22.44 0.56 99.76 46.43 41.88 11.69

MGV 95S spon-c 53.94 0.20 0.97 0.13 0.35 8.14 15.27 20.33 0.91 100.25 42.18 44.07 13.75

MGV 95S rim-q 54.16 0.41 0.16 8.32 14.36 22.81 0.45 100.67 46.29 40.53 13.18

MGV 97 glom-c 53.15 0.70 2.40 0.51 7.74 18.33 17.79 0.34 100.95 36.08 51.68 12.24

MGV 97 glom-r 52.97 0.80 1.51 0.44 0.22 8.66 17.27 18.57 0.30 100.73 37.50 48.51 13.99

MGV 97 glom-r 52.45 0.63 2.93 0.65 0.14 7.09 17.15 18.79 0.34 100.17 38.91 49.39 11.70

MGV 97 glom-c 52.51 0.71 2.81 0.54 0.23 7.20 17.42 17.78 0.40 99.60 37.19 50.68 12.12

MGV 97 gm 52.15 1.15 1.83 0.24 0.23 9.69 16.57 18.03 0.31 100.19 36.94 47.21 15.85

MGV 97 gm 51.16 1.47 2.02 0.07 0.35 14.16 13.36 17.99 0.36 100.93 37.56 38.80 23.65

MGV 97 gm 50.62 1.74 1.78 0.08 0.53 15.52 14.17 15.69 0.35 100.49 32.74 41.12 26.14

MGV 97 in pl 50.37 1.45 4.88 0.62 0.26 8.22 16.10 18.20 0.44 100.53 38.55 47.42 14.03

MGV 97 gm 52.01 1.35 2.15 0.30 0.22 9.74 16.30 18.18 0.30 100.53 37.39 46.62 15.99

gm= groundmass; f-c = phenocrystal core; f-r = phenocrystal rim; glom-c = glomerule core; glom-r = glomerule rim; glom-m= glomerule mantle;

in pl = in plagioclase.

M. Lustrino et al. / Lithos 63 (2002) 83–113 93

Table 3c

Selected analyses of plagioclase and alkali feldspar from Orosei-Dorgali volcanic rocks

Sample SiO2 TiO2 Al2O3 FeO MgO CaO K2O Na2O BaO SrO Sum An Ab Or

Plagioclase

MGV 5 gm 52.89 29.69 0.61 0.26 12.23 0.14 4.40 100.21 60.08 39.13 0.79

MGV 5 f-c 51.01 0.06 31.89 0.60 0.33 14.14 0.11 3.31 101.44 69.77 29.58 0.65

MGV 7 gm 52.01 0.11 29.09 0.81 12.55 0.26 4.11 0.06 0.10 99.11 61.82 36.64 1.54

MGV 7 f-c 51.96 0.18 29.78 0.54 12.86 0.22 3.84 0.06 0.04 99.46 64.10 34.60 1.30

MGV 7 f-r 52.16 0.19 29.28 0.74 0.10 12.35 0.31 4.18 0.10 0.05 99.45 60.91 37.29 1.80

MGV 26 gm 53.26 0.12 28.34 0.34 11.98 0.10 4.63 98.76 58.49 40.94 0.56

MGV 26 f-c 53.97 0.18 28.63 0.34 12.08 0.11 4.72 100.02 58.21 41.17 0.63

MGV 26 f-r 54.79 28.02 0.58 0.06 11.34 0.11 5.09 99.98 54.84 44.53 0.63

MGV 26 glom-c 54.33 0.08 28.34 0.56 0.07 11.61 0.15 4.81 0.05 99.99 56.67 42.46 0.87

MGV 26 gm 54.62 0.12 28.43 0.75 0.09 11.68 0.12 4.85 100.65 56.70 42.63 0.66

MGV 26 glom 54.42 28.37 0.09 11.77 0.10 4.78 0.02 99.54 57.34 42.10 0.56

MGV 26 glom-c 54.37 27.77 0.33 0.08 11.30 0.15 4.77 0.03 98.79 56.21 42.91 0.88

MGV 26 glom-r 60.02 0.15 24.70 0.70 7.47 0.27 7.12 0.06 100.47 36.13 62.32 1.55

MGV 58 gm 53.98 0.26 28.85 0.61 0.15 11.26 0.40 4.75 0.03 0.04 100.32 55.40 42.25 2.35

MGV 60 gm 55.41 0.14 28.38 0.80 0.08 10.79 0.31 5.27 0.10 101.28 52.11 46.09 1.79

MGV 60 gm 55.50 27.87 0.95 10.50 0.32 5.35 0.06 100.54 51.09 47.07 1.83

MGV 60 gm 58.18 0.09 26.53 0.98 8.50 0.51 6.55 0.04 101.37 40.55 56.54 2.91

MGV 60 gm 54.25 0.08 29.65 0.55 0.17 11.87 0.24 4.60 0.05 101.45 57.96 40.66 1.38

MGV 60 f-c 55.05 0.18 28.66 0.53 0.22 10.94 0.25 5.20 0.09 101.11 52.98 45.60 1.42

MGV 60 f-r 56.79 0.17 28.14 0.81 0.16 10.04 0.36 5.66 0.08 102.20 48.49 49.44 2.07

MGV 71 gm 51.86 0.12 29.29 0.75 0.12 12.78 0.36 3.97 0.11 0.07 99.43 62.70 35.21 2.10

MGV 71 gm 52.99 0.06 28.78 0.42 11.82 0.42 4.32 0.06 0.04 98.91 58.70 38.81 2.49

MGV 81 mf-c 61.62 0.17 24.02 0.36 5.14 1.04 7.41 0.80 0.04 100.60 25.95 67.78 6.27

MGV 83 gm 55.78 0.13 28.12 1.01 10.45 0.51 5.37 0.04 0.10 101.51 50.30 46.79 2.91

MGV 83 mf-c 52.96 0.08 30.63 0.55 0.27 12.75 0.28 4.14 0.05 0.08 101.79 61.97 36.39 1.64

MGV 83 gm 60.87 0.24 23.38 0.96 5.68 1.94 7.29 0.20 100.56 26.83 62.25 10.91

MGV 83 gm 58.55 0.21 26.42 1.01 8.37 0.57 6.55 0.12 0.02 101.81 40.05 56.70 3.25

MGV 83 gm 56.33 28.06 0.95 0.13 10.11 0.42 5.59 0.09 0.08 101.75 48.80 48.81 2.40

MGV 83 dus-r 57.23 25.76 0.18 9.14 0.36 6.29 98.96 43.64 54.31 2.05

MGV 83 dus-c 56.55 25.76 9.33 0.61 5.64 97.88 46.05 50.38 3.57

MGV 83 corr 59.91 0.25 23.08 0.61 6.14 1.89 6.53 0.11 98.52 30.37 58.49 11.15

MGV 83 corr 58.73 24.51 7.62 0.74 6.61 98.21 37.25 58.43 4.32

MGV 83 corr 55.80 0.08 26.55 0.09 9.86 0.41 5.79 0.05 98.62 47.36 50.29 2.35

MGV 83 dus 52.97 0.05 27.29 0.76 0.06 11.65 0.34 4.60 0.07 0.03 97.83 57.19 40.82 1.99

MGV 93e gm 53.21 0.12 29.35 0.72 0.11 11.40 0.44 4.84 0.04 0.24 100.46 55.11 42.34 2.55

MGV 97 f-r 57.06 0.18 27.37 0.65 0.15 9.52 0.41 5.86 0.04 101.22 46.19 51.45 2.36

MGV 97 f-c 52.56 0.07 30.51 0.67 0.29 12.93 0.20 4.03 0.04 101.29 63.21 35.65 1.14

MGV 97 corr-c 53.64 29.35 0.70 11.88 0.26 4.55 0.06 100.44 58.20 40.30 1.49

MGV 97 gm 55.49 27.83 0.67 10.54 0.30 5.22 0.05 100.09 51.81 46.43 1.77

MGV 97 f-r 53.13 0.10 30.19 0.44 0.08 12.66 0.17 4.07 100.83 62.56 36.42 1.02

MGV 97 f-c 53.74 29.07 0.61 0.25 11.69 0.21 4.67 100.23 57.36 41.44 1.20

MGV 97 int in xq 61.72 0.17 23.66 0.73 0.06 5.30 0.93 7.81 0.03 100.40 25.78 68.81 5.41

MGV 97 glom-c 53.76 29.31 0.53 0.18 11.72 0.20 4.57 100.27 57.95 40.90 1.15

Alkali Feldspar

MGV 58 gm 67.58 0.38 19.04 0.92 0.46 6.65 6.81 101.83 2.21 59.54 38.25

MGV 81 gm 63.83 0.23 21.44 0.22 2.73 4.04 6.65 0.77 99.91 13.93 61.48 24.59

MGV 81 mf-r 64.32 0.19 21.10 0.26 2.07 5.12 6.09 0.74 99.89 10.76 57.47 31.76

MGV 81 gm 65.15 0.40 20.51 0.40 1.86 4.79 6.68 0.20 99.99 9.47 61.49 29.04

MGV 95S gm 65.74 0.38 20.39 0.25 1.63 4.68 6.59 0.20 0.05 99.92 8.50 62.36 29.14

gm=groundmass; f-c = phenocrystal core; f-r = phenocrystal rim; glom-c = glomerule core; glom-r = glomerule rim; dus-r = dusty rim; dus-

c = dusty core; corr-c = corrose core; mf-r =microphenocrystal rim.

M. Lustrino et al. / Lithos 63 (2002) 83–11394

observed in Pliocene volcanic rocks from central

Sardinia (Lustrino et al., 1996).

Selected major and trace element variations in

alkaline, transitional and tholeiitic volcanic rocks

from Orosei-Dorgali are plotted in Fig. 3. TiO2,

Fe2O3 and CaO show slightly negative correlations

with SiO2, without any change in slope from the

alkaline to the tholeiitic group, whereas MgO and

K2O show such a change. Na2O has a slightly positive

correlation with SiO2; Na2O/K2O ratio varies from

about 2 (in the alkaline group) to about 8 (in the

tholeiitic rocks). In contrast with other mafic PSV

(e.g., Di Battistini et al., 1990; Lustrino et al., 1996),

the Orosei-Dorgali rocks show a negative correlation

of incompatible trace elements such as Ba, Sr, Nb, Zr

and LREE with SiO2.

ICP-MS REE analyses have been carried out on a

selected set of samples (Table 2) and their concen-

trations, normalized to chondrite values, are plotted in

Fig. 4. The alkaline group shows higher light to heavy

rare earth element ratios (La/Yb)N (23.7–14.7) than

the tholeiitic rocks [(La/Yb)N = 10.1–6.3]. All the

Table 3d

Selected analyses of Fe–Ti oxides

Rhomboedral phase

Sample TiO2 Al2O3 Cr2O3 MnO FeOtot MgO Sum Ilm

MGV 7 gm 48.70 0.79 0.54 42.17 4.13 96.32 0.927

MGV 26 gm 48.90 0.06 0.07 0.47 47.31 0.89 97.71 0.941

MGV 26 gm 47.99 0.06 0.42 47.56 0.62 96.66 0.934

MGV 26 gm 49.80 0.05 0.45 47.29 1.13 98.72 0.946

MGV 60 gm 48.12 0.13 0.33 50.61 1.11 100.29 0.897

MGV 81 gm 50.35 0.14 0.07 0.55 42.07 3.94 97.12 0.950

MGV 81 gm 51.06 0.75 43.28 3.33 98.42 0.954

MGV 83 gm 51.87 0.14 0.13 0.28 39.22 4.17 95.80 0.997

MGV 93e gm 50.89 0.04 0.06 0.64 43.51 4.59 99.73 0.926

MGV 93e gm 51.31 0.05 0.70 41.89 5.22 99.17 0.934

MGV 95S gm 50.75 0.08 0.16 0.58 40.48 5.19 97.24 0.945

MGV 95S gm 51.17 0.08 0.21 0.48 39.70 5.62 97.26 0.951

Spinel group

Sample TiO2 Al2O3 Cr2O3 MnO FeOtot MgO Sum Usp

MGV 7 gm 20.40 1.49 0.35 0.39 66.89 2.85 92.36 0.615

MGV 7 gm 23.66 0.99 0.44 0.49 64.89 2.58 93.05 0.707

MGV 26 gm 18.20 2.23 14.06 0.54 59.86 1.72 96.61 0.685

MGV 60 in ol 19.90 2.23 7.04 0.07 65.26 2.87 97.36 0.643

MGV 60 gm 17.98 0.47 0.08 0.29 77.61 0.43 96.86 0.513

MGV 71 gm 27.36 1.90 2.78 0.54 59.12 2.44 94.14 0.859

MGV 71 gm 25.01 2.23 0.74 0.79 66.10 1.99 96.86 0.741

MGV 81 mf-c 2.20 34.46 16.85 0.49 34.48 10.16 98.63 0.260

MGV 81 mf-r 17.20 4.36 9.62 0.54 60.58 4.55 96.86 0.602

MGV 81 mf-r 15.88 6.50 13.66 0.48 54.35 6.30 97.17 0.618

MGV 81 gm 19.07 1.93 0.07 0.73 70.25 2.30 94.35 0.566

MGV 81 gm 17.34 1.83 0.97 0.37 72.58 1.84 94.92 0.519

MGV 83 gm 9.68 1.81 0.66 0.31 81.96 1.62 96.05 0.283

MGV 83 gm 8.73 1.76 1.34 0.22 81.46 1.20 94.71 0.262

MGV 83 gm 11.74 1.61 0.97 76.71 1.10 92.13 0.361

MGV 83 gm 9.04 1.72 0.53 0.08 80.38 1.72 93.47 0.270

MGV 83 gm 10.01 1.82 1.42 0.17 76.40 1.16 90.97 0.315

MGV 83 gm 9.16 1.69 0.88 0.32 81.21 1.23 94.50 0.273

MGV 95S gm 2.89 24.05 21.38 0.41 37.80 9.29 95.81 0.257

gm= groundmass; in ol = in olivine; mf-c =microphenocrystal core; mf-r =microphenocrystal rim.

M. Lustrino et al. / Lithos 63 (2002) 83–113 95

Fig. 3. Variation diagrams of major (wt.%) and trace elements (ppm) vs. SiO2 (wt.%) for the Pliocene volcanic rocks of Orosei-Dorgali. Open circles: tholeiitic volcanic rocks; filled

circles: alkaline and transitional volcanic rocks.

M.Lustrin

oet

al./Lith

os63(2002)83–113

96

Orosei-Dorgali volcanic rocks have Eu/Eu* ratios

close to unity, although tholeiitic rocks have higher

values than the alkaline samples (1.08–1.14 vs. 1.00–

1.08, respectively). In Fig. 4, Orosei-Dorgali alkaline

rocks plot close to the lower part of the field of the

other alkaline PSV, whereas tholeiitic rocks from

Orosei extend the field of tholeiitic rocks toward

higher values. Overall, the other PSV display a

smooth pattern with relatively stronger LREE/HREE

fractionation in the alkaline group [average (La/

Yb)Nf 21]. LREE enrichment is inversely correlated

with the degree of silica saturation: the alkaline rocks

have higher LaN than the tholeiitic group at roughly

similar YbN (Fig. 4).

Primitive mantle-normalized (Sun and McDo-

nough, 1989) diagrams for selected Orosei-Dorgali

mafic rocks are shown in Fig. 5, together with the

field of other alkaline (n = 14) and tholeiitic (n = 4)

mafic PSV (Lustrino, 1999; Gasperini et al., 2000;

Lustrino et al., 2000a). The patterns are smooth, with

positive peaks at Ba, Pb and Sr and small troughs at

Nb. Alkaline volcanic rocks of Orosei-Dorgali have

compositions similar to the other alkaline PSV, with

positive Ba, Pb and Sr peaks and small trough at Nb.

Sample MGV89, classified as transitional and shown

in Fig. 3 with the same symbol of alkaline rocks,

displays an intermediate composition of incompatible

trace elements between alkaline and tholeiitic groups.

The tholeiitic volcanic rocks of Orosei-Dorgali also

show peaks at Ba, Pb and Sr and troughs at Nb. The

Orosei-Dorgali rocks reflect the general pattern of

PSV with the Ba and Pb peaks as prominent features

(Di Battistini et al., 1990; Lustrino et al., 1996).

4.3. Sr and Nd isotope compositions

New Sr–Nd isotopic data for Orosei-Dorgali rocks

plus two other previously published analyses (Lus-

trino et al., 2000a) are shown in Fig. 6, together with

the field of the PSV (Lustrino, 1999; Gasperini et al.,

2000; Lustrino et al., 2000a). 87Sr/86Sr ranges from

0.70442 to 0.70455 while 143Nd/144Nd varies from

0.512465 to 0.512558. Tholeiitic rocks are slightly

more depleted in radiogenic Nd and more enriched in

radiogenic Sr with respect to the alkaline group. The

new data plot within the published isotopic field of

the PSV (Gasperini et al., 2000; Lustrino et al.,

2000a).

Fig. 4. Chondrite-normalized REE patterns for the Orosei-Dorgali volcanic rocks. Filled circles: alkaline and transitional; open circles: tholeiitic.

Also shown for comparison the field of mafic Plio-Pleistocene alkaline and tholeiitic volcanic rocks of Sardinia (Lustrino, 1999; Gasperini et al.,

2000; Lustrino et al., 2000a).

M. Lustrino et al. / Lithos 63 (2002) 83–113 97

4.4. Pb isotope compositions

Two published data (Lustrino et al., 2000a) plus

two new Pb isotopic ratios of samples from Orosei-

Dorgali are listed in Table 2. 206Pb/204Pb ranges

from 17.74 to 17.86, 207Pb/204Pb from 15.53 to

15.60 and 208Pb/204Pb from 37.89 to 38.02. No

substantial differences exist among the Orosei-Dorgali

rocks and most of the PSV, all the samples having206Pb/204Pb < 18 and 207Pb/204Pb (15.54–15.62)

(Gasperini et al., 2000; Lustrino et al., 2000a). The

high 208Pb/206Pb and 207Pb/206Pb ratios (>2.10

Fig. 5. Primitive mantle-normalized trace element patterns for the Orosei-Dorgali volcanic rocks. (a) Orosei-Dorgali mafic alkaline volcanic

rocks. Sample MGV 89 is transitional between alkaline and tholeiitic in term of major and trace element abundance but shares more similarities

with the alkaline group. (b) Orosei-Dorgali mafic tholeiitic volcanic rocks.

M. Lustrino et al. / Lithos 63 (2002) 83–11398

and >0.86, respectively) differentiate the Orosei-Dor-

gali rocks and most PSV from the other CEVP

products (208Pb/206Pb < 2.10 and 207Pb/206Pb < 0.86)

(Fig. 7).

5. Discussion

Among major elements, the behaviour of K2O is

anomalous, as it shows a negative correlation with

Fig. 6. Nd vs. Sr isotope ratios for the Orosei-Dorgali volcanic rocks compared with the other PSV and a lower crustal xenolith borne by alkali

basalt from Gerrei (Lustrino, 1999) (a) and compared with Italian mafic anorogenic (filled triangles) and orogenic (open triangles) mafic rocks

and UPV rock group of Lustrino et al. (2000a) (Guspini hawaiite, Rio Girone basanite and Capo Ferrato trachyte) (b).

M. Lustrino et al. / Lithos 63 (2002) 83–113 99

SiO2 (Fig. 3). Due to the absence of K-bearing

mineral phases, this oxide is to be considered incom-

patible, so fractional crystallization would produce a

positive rather than negative correlation with SiO2.

Positive correlations between K2O and SiO2 have

been reported for other PSV (Lustrino et al., 1996;

Lustrino, 1999). To a lesser extent, P2O5 also shows a

similar behaviour. Furthermore, the behaviour of

incompatible trace elements in Orosei-Dorgali vol-

canic rocks contrasts with that of other PSV suites. In

other PSV, positive correlations between incompatible

trace elements and SiO2 are apparent and explicable

with fractionation of gabbroic cumulate (plagiocla-

seF clinopyroxeneF olivineF opaque minerals)

from a basaltic (s.l.) parental magma, plus variable

extents of crustal contamination (Cioni et al., 1982; Di

Battistini et al., 1990; Lustrino et al., 1996).

On the basis of trace element patterns (Figs. 4 and 5)

and the constancy of average incompatible interele-

ment ratios (Ba/Nb = 23.7–24.9, Ba/La = 22.6–23.3,

Th/U = 4.55–4.53, La/Nb = 1.09–1.08, Ti/Zr = 92–

93, Rb/Nb = 1.0–0.9 for alkaline and tholeiitic rocks,

respectively; Fig. 8) a single mantle source for the

entire spectrum of the Orosei-Dorgali volcanic rocks

could be suggested. Alkaline rocks would represent

lower degrees of partial melting (equilibrated at higher

pressure), whereas tholeiitic rocks could be related to

higher degrees of melting (formed at shallower depths)

of a similar source.

5.1. Constraints on the degrees of partial melting of

the Orosei-Dorgali mantle source

In order to test the hypothesis of different degrees

of partial melting, the composition of the volcanic

rocks from Orosei-Dorgali has been modeled for

batch, fractional and dynamic melting. A detailed

description of these methods is given in Appendix A.

The concentration ratio method of Maaloe (1994)

has been used to calculate the approximate degree of

melting of the Orosei-Dorgali magmas. Samples

MGV1 (alkali basalt) and MGV24 (basaltic andesite)

have been selected to represent melts formed at low and

high degree of partial melting of the same source,

respectively. The two elements used are La (highly

incompatible element) and Zr (moderately incompat-

ible element). The calculated D and P are 0.002 and

0.009 for MGV1 and 0.021 and 0.085 for MGV24,

respectively; the values of Qa and Qb are 2.1 and 2.7,

respectively. Transferring these values in Eqs. (A3) and

(A4), the values for f1 and f2 are 4.2% and 11.5%,

respectively; these values are taken as representative of

the degree of partial melting for the alkaline and

tholeiitic series. On this basis, and starting from Eq.

(A1), the compositions of the Orosei-Dorgali rocks

have been modeled using a REE inversion method.

The absolute abundance and the pattern of chon-

drite-normalized REE have been used to constrain the

mantle source mineralogy and the degree of partial

melting of the Orosei-Dorgali magmas. In Fig. 9, REE

abundances of the Orosei-Dorgali rocks are shown

together with the compositions of hypothetical liquids

derived from partial melting of spinel-bearing mantle

sources at various degrees of f ( f = degree of partial

melting). The calculated REE abundance of the peri-

dotitic source is plotted in Fig. 9.

Melts obtained from this calculated mantle source

at 2%, 4%, 6%, 10% and 15% partial melting, using

Shaw’s equation, are also shown in Fig. 9. The results

show: (a) transitional sample MGV89 reflects slightly

higher degrees of melting ( < 10% f; not shown); (b)

the tholeiitic mafic rocks lie between 10% and 15%

partial melting intervals and thus would reflect higher

degree partial melts of a source similar to that one that

generated alkaline rocks.

Fig. 7. 208Pb/206Pb vs. 207Pb/206Pb diagram for Orosei-Dorgali

volcanic rocks compared with the CEVP anorogenic and orogenic

rocks and the field of the other PSV. References given in the text.

European Asthenospheric Reservoir (EAR) from Granet et al.

(1995); Enriched Mantle I and II (EMI and EMII) composition from

Zindler and Hart (1986).

M. Lustrino et al. / Lithos 63 (2002) 83–113100

The estimated degree of partial melting for alkaline

(f 4–6%) and tholeiitic (f 10–15%) magmas of

Orosei-Dorgali obtained with Eqs. (A1) and (A2) have

been compared with the results obtained with the

Dynamic Inversion Melting method (Eq. (A9)), as

proposed by Zou and Zindler (1996) and Zou et al.

Fig. 8. Interelemental ratios for alkaline (filled circles) and tholeiitic (open circles) PSV. Circles =Orosei-Dorgali rocks; squares = other PSV

(UPG of Lustrino et al., 2000a); triangles (RPG of Lustrino et al., 2000a).

M. Lustrino et al. / Lithos 63 (2002) 83–113 101

(2000). Assuming a / value (volume porosity) = 1%,

qs (density of the residue) = 3.3 g/cm3, and qf (density

of the melt) = 2.8 g/cm3, Eq. (A9) gave estimates of

the degree of partial melting equal to 5.6% and 13.7%

for the alkaline (MGV1) and tholeiitic (MGV24)

magmas, respectively. These results roughly agree

with the above estimates obtained using two different

methods.

Thus the proposal that a single source partially

melted to variable degrees and at variable depths to

give the entire spectrum of the Orosei-Dorgali mag-

mas seems to be correct.

5.2. Sources for Orosei-Dorgali rocks

The two main evolutionary processes (fractional

crystallization and variable partial melting) can be

shown on diagrams, such as Zr vs. La and Nb vs.

Ba (Fig. 10). Open system modifications, such as

crustal contamination or AFC-type processes are not

considered here mainly because of the presence of

mantle xenoliths in many of the alkaline rocks, which

indicates rapid rise of the host magma en route to the

surface, thus reducing the possibility of crustal con-

tamination (e.g., Lustrino et al., 1999). Crustal con-

tamination can be also considered an unlikely process

for the tholeiitic magmas (and the Orosei-Dorgali

rocks in general) on the basis of the absence of

correlation between Cr and 87Sr/86Sr. The Orosei-

Dorgali volcanic rocks show a relatively large Cr

variation (f 350–140 ppm) coupled with the rela-

tively constant 87Sr/86Sr ratio (0.70442–0.70453);

crustal assimilation would produce cooling effect in

the magmas with subsequent crystal fractionation.

This process, therefore, would result in negative

correlation between Cr (and other compatible ele-

ments) and 87Sr/86Sr ratio. The absence of a correla-

tion between Cr and 87Sr/86Sr (R2 < 0.09) therefore

relates to crystal fractionation of mantle-derived melts

without crustal assimilation. In the La vs. Zr diagram,

fractional crystallization-related paths might lead to an

enrichment of both the incompatible elements; on the

other hand, decreasing depth of melt segregation

(resulting in an increasing degree of partial melting,

due to adiabatic decompression) would result in a

depletion of the same elements, these being diluted in

larger batches of melt. The most differentiated PSV

(phonolites from Montiferro) plot toward high La and

Fig. 9. REE inversion batch melting modelization for Orosei-Dorgali volcanic rocks. To obtain the hypothetical liquid compositions, it has been

assumed that the average mafic alkaline PSV formed after 5% partial melting, as discussed in the text. From this assumption, and using the

equation of Shaw (1970) for batch melting and the average composition of mafic alkaline volcanic rocks from Sardinia, the REE composition of

the source (C0) has been calculated. The olivine/clinopyroxene/orthopyroxene/spinel ratio in the source and as phases entering in the melt

adopted in the calculations (to obtain D and P) is 0.6 Ol:0.1 Cpx:0.25 Opx:0.05 Sp and 0.1 Ol:0.6 Cpx:0.2 Opx:0.1 Sp, respectively.

M. Lustrino et al. / Lithos 63 (2002) 83–113102

Fig. 10. Zr vs. La (a) and Nb vs. Ba (b) diagrams for Orosei-Dorgali alkaline (filled circles) and tholeiitic (open circles) volcanic rocks. Thick

marks and italicized numbers indicate % of partial melting of a source with Zr = 21.6 ppm, La = 2 ppm, Nb = 2.1 ppm and Ba = 44 ppm. These

values lie within the range of the lithospheric mantle estimate of McDonough (1990) and the mantle xenoliths carried by alkaline lavas of

Sardinia (Lustrino et al., 1999). Assuming a unique source that melts to variable degrees, tholeiitic rocks cluster towards higher degrees of

partial melting compared with alkaline group. From the composition of the liquid formed at 3% (Fig. 9a) and 5% (Fig. 9b) of melting, a

cumulate made up of olivine, clinopyroxene, plagioclase and spinel in the ratio 0.30:0.15:0.40:0.15, has been subtracted at every 10%.

Compositions akin to those of the most evolved rocks (phonolites from Montiferro; not shown) have been obtained after the removal of f 60%

of such a cumulate from the liquid formed after 3–5% of partial melting of the hypothesized source previously described.

M. Lustrino et al. / Lithos 63 (2002) 83–113 103

Zr (not shown), while tholeiitic rocks from Orosei-

Dorgali cluster towards low La and Zr compositions.

Both the processes of fractional crystallization and

partial melting have been modeled quantitatively

using Shaw’s (1970) equations, and are presented in

the insets of Fig. 10.

The starting material chosen has La = 2 ppm,

Zr = 21.6 ppm, Ba = 44 ppm and Nb = 2.1 ppm, reli-

able values of a lithospheric mantle. Partial melts at

1–15% degrees of melting of such a source are shown

in Fig. 10. Orosei-Dorgali alkaline volcanic rocks

cluster between the 2–10% and 3–8% melting inter-

vals of the calculated source for the Zr vs. La and Nb

vs. Ba, respectively; conversely, the tholeiitic group

clusters towards higher values (11–17% and 9–18%

for the Zr vs. La and Nb vs. Ba, respectively).

The results of the modeling need careful consid-

erations: (1) the composition of the source, as well as

its mineralogy, are only theoretical and are not directly

constrained; (2) the ratio of the mineral phases

involved in the melting process, even if petrologically

sound, are also hypothesized; (3) the values of D and

P are thought to remain constant throughout the entire

process of partial melting. Assuming a single source

for all the PSV, it is possible that fractional crystal-

lization and variable degrees of partial melting can

buffer almost all the compositions of these products.

The tholeiitic rocks from Orosei-Dorgali plot close

the f 9–15% range of partial melting, while the

alkaline rocks of the same area cluster around f 3–

8% of partial melting. These melting degrees roughly

overlap the range of the estimated approximate

degrees of partial melting obtained above. In conclu-

sion, it is possible to hypothesize a single source for

the PSV, all the compositional variability being buf-

fered by varying degrees of partial melting and vary-

ing degrees of fractional crystallization.

6. Relationships with neighboring igneous

provinces

In this section, a comparison between the Orosei-

Dorgali mafic rocks, the rest of the PSV and other

Neogene to Recent mafic anorogenic volcanic rocks

from the circum-Mediterranean area is addressed. The

unusual trace element and Sr–Nd–Pb isotopic fea-

tures of the Orosei-Dorgali rocks and the entire PSV

within the Cenozoic European Volcanic Province will

be discussed.

6.1. Trace elements

When compared to the anorogenic mafic rocks of

the CEVP, the PSV have generally lower high field

strength element (HFSE) contents. This characteristic

strongly contrasts with the general trend of the circum-

Mediterranean rocks and is similar only to the Hyblean

Mts. rocks (Beccaluva et al., 1998; Trua et al., 1998).

Neither the alkaline nor the tholeiitic PSV have the

high Nb and the relatively low Ba concentrations

typical of anorogenic rocks, such as Calatrava Prov-

ince (central Spain; Cebria and Lopez-Ruiz, 1995),

French Massif Central (Wilson and Downes, 1991),

Bas-Languedoc (southern France; Liotard et al., 1999),

Hessian Depression and Rhon (Germany, Wedepohl et

al., 1994; Jung and Hoernes, 2000), Pannonian Basin

(Hungary and Slovakia, Dobosi et al., 1995; eastern

Rhodopes, Bulgaria, Marchev et al., 1998; and recent

alkaline igneous activity of Turkey; Polat et al., 1997;

Parlak et al., 2001). Compared to the other Neogene–

Quaternary Italian mafic anorogenic rocks, the Orosei-

Dorgali volcanic rocks, together with most PSV, have

generally higher (Ba/Nb) (>20) and the lower Ce/Pb

( < 20) and Nb/U ( < 40).

The Ba/Nb ratio and the absolute abundance of Nb

can be used to discriminate the PSV from anorogenic

volcanic rocks of the CEVP (Fig. 11). In Fig. 11a, the

Nb/Nb* parameter is plotted against Ba/Nb ratio. The

Nb/Nb* parameter [ = NbR/NbPM/((KR/KPM)*(LaR/

LaPM))0.5]; where subscripts R and PM stand for rock

and primitive mantle values) reflects the Nb anomaly

in primitive mantle-normalized diagrams. Almost all

the CEVP mafic volcanic rocks display Nb/Nb*>1

and Ba/Nb < 20, whereas almost all the Orosei-Dor-

gali rocks have Nb/Nb * < 1 and Ba/Nb>20. High Ba/

Nb does not mean high Ba: in Fig. 11b, it is apparent

that Ba of the Orosei-Dorgali rocks roughly overlaps

with that of the other anorogenic European rocks; the

only exceptions are Linosa and Pantelleria islands and

some Hyblean basalts that show slightly lower Ba

(down to f 90 ppm).

The relatively homogeneous composition of most

CEVP anorogenic rocks is also evident in primitive

mantle-normalized diagrams (not shown). Most CEVP

anorogenic rocks show negative peaks in Pb and high

M. Lustrino et al. / Lithos 63 (2002) 83–113104

Fig. 11. Ba/Nb vs. Nb/Nb* (a) and Ba vs. Nb/Nb* (b) diagrams for Orosei-Dorgali volcanic rocks compared with other PSV, mafic anorogenic

volcanic rocks from Italy (Mt. Etna, Hyblean Mts, Linosa and Pantelleria islands) and Cenozoic European Volcanic Province such as the Massif

Central and Provence (France), the Calatrava and Olot volcanic districts (Spain), the central European rocks from Rhenish and Bohemian

Massifs, Vosges and Poland and rocks from the Pannonian and Transylvanian Basins. References in the text and in Lustrino (2000a). Nb/

Nb * =NbR/NbPM/((KR/KPM)*(LaR/LaPM)0.5); where subscripts R and PM stay for rock and primitive mantle values. This parameter reflects the

Nb anomaly in primitive mantle-normalized diagrams. The Orosei-Dorgali volcanic rocks and the majority of PSV have low Nb/Nb* ( < 1),

coupled with higher Ba/Nb (>20), but roughly similar Ba compared with mafic anorogenic volcanic rocks from Italy and Europe.

M. Lustrino et al. / Lithos 63 (2002) 83–113 105

Ce/Pb. This feature, shared also by a few PSV (the RPV

group of Lustrino et al., 2000a), is typical of HIMU-

OIB magmas. HIMU-OIB end member is also charac-

terized by a relatively LILE-depleted, HFSE-enriched

composition, with positive peaks at Nb, Zr and Ti,

troughs in Ba, K, and Pb and (Nb/Ba)N and (Ce/

Pb)NH1 and high Nb/U (>40) (Chauvel et al., 1997;

Kogiso et al., 1997).

When compared to the anorogenic volcanic rocks

of the Cenozoic European Volcanic Province, the

peculiar trace element character of the Orosei-Dorgali

volcanic rocks and most PSV becomes clear. They are

characterized by anomalous trace element abundance

when compared to the great majority of European

Cenozoic mafic anorogenic volcanic rocks: they have

higher Ba/Nb and lower Zr/Ba, Nb/U and Ce/Pb.

6.2. Radiogenic isotopes

The PSV also have peculiar and almost unique Sr–

Nd–Pb isotope ratios. These rocks show 87Sr/86Sr

close to the model bulk Earth estimate (0.70423–

0.70474, avg. 0.70449) and unradiogenic 143Nd/143Nd

(0.51235–0.51258, avg. 0.51250) and DUPAL-like

Pb (i.e., compositions above the Northern Hemisphere

Reference Line of Hart, 1984). In particular, they have2 0 6 Pb / 2 0 4Pb = 17 . 37 – 18 . 0 1 ( a vg . 17 . 68 ) ,207Pb/204Pb = 15.54 – 15.62 (avg. 15.58) and208Pb/204Pb = 37.44–38.03 (avg. 37.82), with average

D7/4 and D8/4 = 17.5 and 82.7, respectively.

Neogene–Quaternary Italian anorogenic volcanic

rocks define a narrow field in the depleted quadrant

(Fig. 6), partially overlapping the MORB and HIMU-

OIB field and are quite distinct from the PSV. The

only Sardinian rocks that fall in the Sr–Nd field of the

Italian anorogenic volcanic rocks are Rio Girone and

Guspini samples (Lustrino et al., 2000a; Fig. 6). The

interpretation of differences between the PSV and the

Italian anorogenic volcanic rocks is still a matter of

debate. Notwithstanding the debate for these latter as

derived from lithospheric or asthenospheric melts

(e.g., Esperanc�a and Crisci, 1995; Civetta et al.,

Fig. 12. 87Sr/86Sr vs. 143Nd/144Nd isotopic ratios for Orosei-Dorgali volcanic rocks compared to other PSV (UPV and RPV of Lustrino et al.,

2000a), CEVP rocks and north-Africa rocks. Pantelleria (Esperanc�a and Crisci, 1995; Civetta et al., 1998), Hyblean Mts. (Beccaluva et al., 1998;

Trua et al., 1998; Bianchini et al., 1999), Mt. Etna (D’Orazio et al., 1997), Poland (Alibert et al., 1987; Blusztajn and Hart, 1989), Provence

(Liotard et al., 1999), Bulgaria (Marchev et al., 1998), Spain (Neumann et al., 1999; Cebria et al., 2000), French Massif Central (Chauvel and

Jahn, 1984; Briot et al., 1991; Wilson and Downes, 1991), Germany (Worner et al., 1986; Kramm and Wedepohl, 1990; Wedepohl et al., 1994;

Jung and Masberg, 1998; Jung and Hoernes, 2000; Wedepohl, 2000), Carpatho–Pannonian Region (Embey-Istzin et al., 1993; Harangi et al.,

1994; Downes et al., 1995); Morocco and Algeria (Maza et al., 1998; El Azzousi et al., 1999; Ait-Hamour et al., 2000).

M. Lustrino et al. / Lithos 63 (2002) 83–113106

1998), a general HIMU-DM character of the sources,

with only limited EM composition involvement and

carbonatitic metasomatism, is now generally accepted

(e.g. Esperanc�a and Crisci, 1995; Beccaluva et al.,

1998; Civetta et al., 1998; Trua et al., 1998; Bianchini

et al., 1999). On the other hand, the geochemical

features of the Orosei-Dorgali and their Sr–Nd–Pb

isotopic ratios are difficult to reconcile without taking

into account some external compositions. The pecu-

liarity of the PSV in terms of Sr and Nd isotopes (low143Nd/144Nd coupled with bulk Earth 87Sr/86Sr) are

apparent also when compared with the other anoro-

genic CEVP rocks and with the volcanic products of

the Maghrebian Margin (Fig. 12). The European and

African Miocene–Pleistocene within-plate products

have high 143Nd/144Nd and low 87Sr/86Sr (partially

overlapping the MORB-HIMU field), while the sub-

duction-related rocks (linked to the Alpine Orogeny;

Carpathian Arc and Aegean Arc; not shown) are

characterized by wider compositional range mainly

toward radiogenic (Pb and Sr) compositions (see Fig.

2 of Lustrino et al., 2000a).

7. Asthenospheric or lithospheric sources for the

Cenozoic European Volcanic Province?

The similarity of the Cenozoic European anoro-

genic volcanic rocks with Ocean Island Basalts (OIBs)

in term of major and trace elements, as well as Sr–Nd–

Pb isotopic compositions, has allowed many authors to

propose asthenospheric rather than lithospheric mantle

sources for these rocks (e.g., Wilson and Downes,

1991; Wedepohl and Baumann, 1999; Wedepohl,

2000; Wilson and Patterson, 2001). Moreover, seismic

tomography underneath Europe traced plume channels

down to 250 km depth (Hoernle et al., 1995; Granet et

al., 1995; Sobolev et al., 1997; Ritter et al., 2001) or,

possibly, down to 2000 km (Goes et al., 1999) rein-

forcing the possibility of a derivation from astheno-

sphere or lower mantle, excluding major contributions

from the lithosphere (Wedepohl and Baumann, 1999).

This convecting mantle reservoir has been alterna-

tively called European Asthenospheric Reservoir

(EAR; Granet et al., 1995), Low Velocity Zone

(LVZ; Hoernle et al., 1995), Central European Anom-

aly (CEA; Goes et al., 1999) or Low Velocity Anomaly

(LVA; Ritter et al., 2001) and has been possibly related

to Canary islands plume (Hoernle et al., 1995; Oyarzun

et al., 1997; Ritter et al., 2001) or to Iceland plume

(Bijwaard and Spakman, 1999; Wilson and Patterson,

2001). In particular, Wilson and Patterson (2001)

evidenced the possibility of the existence of a low-

velocity structure linking the Iceland plume, the central

European velocity anomaly and the Canary Islands

plume between 900 and 1200 km depth.

The French Massif Central, the Vosges–Black

Forest dome, the Rhenish and Bohemian Massifs

and the Pannonian Basin Tertiary–Quaternary vol-

canic fields should be related to thermal anomalies

(called ‘‘finger-like plumes’’) linked to a common

asthenospheric reservoir at the base of the upper

mantle (670 km discontinuity) (Granet et al., 1995;

Wilson and Patterson, 2001).

7.1. The PSV compared with other CEVP rocks

The CEVP rocks composition suggests strong evi-

dence of an asthenospheric source, sometimes conta-

minated by lithospheric melts (Worner et al., 1986;

Alibert et al., 1987; Wedepohl et al., 1994; Cebria and

Lopez-Ruiz, 1995; Hoernle et al., 1995; Downes et al.,

1995; Rosenbaum et al., 1997; Wedepohl and Bau-

mann, 1999; Wedepohl, 2000). Within this contest, the

EMI-like composition of the vast majority of the PSV

has been related to lithospheric sources modified dur-

ing the previous orogenies with digestion of lower

crustal lithologies (Lustrino et al., 2000a).

The European subcontinental lithospheric mantle is

heterogeneous on a relatively small scale (e.g. Worner

et al., 1986; Zangana et al., 1999; Downes, 2001).

Much of these evidences for this come from radio-

genic and stable isotopic studies and modal metaso-

matism of mantle and crustal xenoliths commonly

found in Cenozoic to Quaternary rocks in the Massif

Central (France), Rhine Graben (Germany), Panno-

nian Basin (Austria, Hungary, Romania and Poland)

and Sardinia (Rosenbaum et al., 1997; Lustrino et al.,

1999; Zangana et al., 1999; Downes, 2001). The

heterogeneity often recorded in the mantle xenoliths

(but rarely in the host lavas), especially in terms of

radiogenic isotope ratios, in the Massif Central, Rhine

Graben and surrounding areas, can be explained only

by considering Paleozoic (or possibly older) subduc-

tion systems, when the central Europe was the sand-

wiched hinterland squeezed between a roughly North-

M. Lustrino et al. / Lithos 63 (2002) 83–113 107

dipping and a roughly South-dipping orogenic belt

(Lorenz and Nicholls, 1984). The metasomatism

(mainly observed in central Europe) cannot be a

consequence of the Alpine orogeny, because during

this orogeny, central Europe was the foreland of the

subduction system whose polarity was S- to SW-

dipping (Carpathian Arc and Apennines).

The heterogeneity of the European subcontinental

lithosphere contrasts with the roughly homogeneous

major and trace element and isotope geochemistry of

the anorogenic products of the CEVP. Indeed, these

products (mainly alkaline and tholeiitic rocks) have

typical OIB pattern in primitive mantle-normalized

plots, show positive anomalies in Nb, variously K-

depleted compositions, high HFSE/LILE ratios, quite

depleted Sr and Nd isotopic ratios (87Sr/86Sr = 0.7031–

0.7046; 143Nd/144Nd = 0.51264–0.51305) and radio-

genic Pb (206Pb/204Pb = 18.3–19.7, with many sam-

ples >19; 208Pb/204Pb = 38.4–39.6, with many samples

>38.8). Deviations from this typical HIMU-OIB (i.e.,

asthenosphere-derived) geochemical character, some-

times found in tholeiitic products, have been related to

lithospheric contamination of mantle melts during

residence in lithospheric mantle or crustal magma

chambers (e.g., Jung and Masberg, 1998; Wedepohl,

2000). This substantial geochemical and isotopic

homogeneity is hard to reconcile with lithospheric

sources. Alpine subduction-related mafic rocks differ

in their strong HFSE negative anomalies and the peaks

at Rb and Ba, the absence of troughs in K and the

higher LILE/HFSE and K2O/Na2O ratios. Isotopically,

the European (comprising the Italian) orogenic rocks

plot mainly in the enriched Sr–Nd quadrant (with

eSr>0 and eNd < 0), with slightly higher 207Pb/204Pb

for a given 206Pb/204Pb, but with 206Pb/204Pb and208Pb/204Pb roughly overlapping the field of anoro-

genic rocks (i.e. >18.3 and >38.4, respectively).

Notwithstanding this quite uniform isotopic sce-

nario, Wilson and Downes (1991) hypothesized for

the CEVP HIMU-DM sources, which variably inter-

acted with EM compositions (not specifying if EMI or

EMII). The EM imprint was related by these authors

to modifications that occurred during Paleozoic sub-

duction. The relatively short period elapsed since this

event (f 300–400 Ma) could be, according to Wil-

son and Downes (1991), at the base of the lack of the

homogenization of the asthenospheric mantle by con-

vective flow. This unhomogenized (Hercynian sub-

duction-related) asthenospheric mantle would be the

origin of the HIMU-DM-EM transitional character of

the CEVP.

In strong contrast with the asthenosphere (plume)-

related origin of the CEVP, the Orosei-Dorgali rocks

are more likely to represent lithospheric melts, whose

geochemical and isotopic characteristics reflect the

heterogeneities of their source. These rocks, rather

than the CEVP rocks, can be the most appropriate

evidence of modifications of the lithosphere related to

the Hercynian orogeny. In fact, the other European

volcanic rocks resemble asthenosphere-derived melts

(with strong geochemical and structural plume con-

trol) and show little or no memory of the ancient

modifications.

8. Concluding remarks

Pliocene volcanic rocks of the Orosei-Dorgali area

consist of hawaiite, basaltic andesite, alkali basalt and

mugearite. The similarity of strongly incompatible

element ratios between alkaline and tholeiitic rocks

suggests a single mantle source which variably melted

to give the entire spectrum of the Orosei-Dorgali

rocks. Alkaline rocks would represent lower degrees

of partial melting ( ff 4–6%), whereas tholeiitic

rocks could be related to a higher degree of melting

( ff 10–15%).

The Orosei-Dorgali rocks represent an extremely

unusual trace element and Sr–Nd–Pb isotopic com-

positions within the Cenozoic European Volcanic

Province and share extreme similarities with the

EMI-type mantle end-member.

Acknowledgements

This study was mostly derived from the distil-

lation of the PhD thesis of the first author at the

University of Naples Federico II. Special thanks to:

Enrica Mascia for help in database acquisition,

Sandro Conticelli (Florence) for his kind help during

XRF analyses, Piero Brotzu (Naples) for comments

on an early version of the manuscript, Vincenzo

Monetti for AAS measurements, John Mahoney

(Hawaii) for his hospitality at the SOEST, Marcello

Serracino and Giuseppe Cavarretta (Rome) for the

M. Lustrino et al. / Lithos 63 (2002) 83–113108

skilled assistance during electron microprobe work,

Samuele Agostini and Massimo D’Orazio (Pisa) for

high quality ICP-MS analyses, Gianfranco Secchi

(Sassari) for the help during field trip and Lucio

Morbidelli (Rome) for logistic assistance during the

preparation of this manuscript. Special thanks also to

Steve Harris, Bruce Dickinson, Nicko McBrian,

Dave Murray and Adrian Smith. This work benefited

of a thorough review of Hilary Downes and Karl H.

Wedepohl and was granted by the Italian agency

CNR ‘‘Agenzia 2000’’ (ML and VM) and by the

University of Rome La Sapienza ‘‘Progetto Giovani

Ricercatori’’ (ML).

Appendix A

Three methods are commonly used to model the

trace element behaviour during partial melting pro-

cesses: batch, fractional and dynamic melting.

Because batch melting assumes a continuos equili-

brium between the melt and the residual solid and

fractional melting requires that the melt is extracted

from the residual solid as soon as it is formed, both are

extreme conditions to occur in nature (e.g., Zou,

1998).

The equations of Shaw (1970) for the batch and

fractional (Rayleigh) melting are listed below

Cl ¼ C0=ðDþ f ð1� PÞÞ ðBatch MeltingÞ ðA1Þ

Cl ¼ ðC0=DÞð1� f Þexpðð1=DÞ � 1ÞðRayleigh MeltingÞ ðA2Þ

Where Cl and C0 are the concentrations of an

element in the melt and in the initial solid, respec-

tively; D is the bulk solid/melt distribution coefficient

(calculated from the weight proportions of each min-

eral in the source assemblage); f represents the degree

of partial melting and P is the bulk solid/melt distri-

bution coefficient during non-modal partial melting

(calculated from the weight proportions of each min-

eral which is involved in the melting processes).

To calculate the approximate degree of partial

melting using the above equations, it is important to

know roughly the values of C0, D and P. This is the

more difficult point in resolving these equations, and

particularly C0, which is variable by several orders of

magnitude (Maaloe, 1994; Zou and Zindler, 1996).

For this reason, Maaloe (1994) proposed a simple

graphical method (the concentration ratio method) to

calculate the degree of partial melting without assum-

ing any value of C0. This method is based on the

enrichment ratio of two strongly incompatible ele-

ments in two rocks formed at different degrees of

partial melting. The landmark requirement of this

method is that two volcanic rocks must be cogenetic

and that fractional crystallization processes did not

modified substantially the interelemental ratios. This

graphical method has been numerically solved by Zou

and Zindler (1996) as follows

f1 ¼ ðDað1� PbÞð1� QaÞ þ Dbð1� PaÞðQb � 1ÞÞ=ððQa � QbÞð1� PaÞð1� PbÞÞ ðA3Þ

f2 ¼ ðQbðDb þ f1ð1� PbÞÞ � DbÞ=ð1� PbÞ ðA4Þ

where f1 and f2 are the lower and the higher degrees of

partial melting, subscripts 1 and 2 refer to the rock

formed by the lower and the higher degree of partial

melting, respectively (i.e. the rocks which have the

higher and the lower concentration of a strongly

incompatible element); subscripts a and b refer to

extremely incompatible (e.g., La) and the less incom-

patible element (e.g., Nd), respectively; D and P are

the bulk coefficient in the source assemblage and

during non-modal partial melting, respectively; Qa

and Qb represent the enrichment ratio and are equal

to C1/C2 for elements a and b. Assuming that D and P

approach zero (Maaloe, 1994), f1 and f2 can be

calculated independently from C0.

The model of Dynamic melting is somewhat inter-

mediate between the two extreme possibilities.

According to this assumption the first drops of melts

remain in equilibrium with the residue until the space

porosity is filled; the melt will start to be extracted

only after the threshold value (i.e., when the melt

fraction is greater than the porosity of the residual

solid, which in a peridotitic media is f 1%; see Zou

and Zindler, 1996). In this case, the f value (i.e., the

degree of partial melting) is calculated as the sum of

M. Lustrino et al. / Lithos 63 (2002) 83–113 109

the mass fraction of the extracted liquid and residual

liquid.

According to Zou and Zindler (1996), the concen-

tration of a trace element in the extracted dynamic

melt is

Cl ¼ ð1=X ÞC0Gð1� ð1� X ÞexpðGð1� DÞ þ 1ÞÞ=ðGð1� DÞ þ 1Þ ðA5Þ

where

G ¼ ðqf/ þ qsð1� /ÞÞ=ðqf/ þ qsð1� /ÞDÞ ðA6Þ

C0 is the initial concentration of the element in the

source, D is the bulk distribution coefficient, qf is the

density of melt, qs is the density of solid matrix and /is the volume porosity.

If the degree of partial melting increases from stage

1 ( f1) to stage 2 ( f2) and the mass fraction of liquid

extracted increases from X1 to X2, the enrichment ratio

Q for the highly incompatible element a is (Zou and

Zindler, 1996)

Qa ¼ C1a=C

2a

¼ ðX2ð1� ð1� X1ÞexpðGað1� DaÞ þ 1ÞÞÞ=ðX1ð1� ð1� X2ÞexpðGað1� DaÞ þ 1ÞÞÞ ðA7Þ

Similarly, for the less-so-highly incompatible ele-

ment b

Qb ¼ C1b=C

2b

¼ ðX2ð1� ð1� X1ÞexpðGbð1� DbÞ þ 1ÞÞÞ=ðX1ð1� ð1� X2ÞexpðGbð1� DbÞ þ 1ÞÞÞ ðA8Þ

It is important to note that both Qa and Qb are

independent of the source concentration C0. After

obtaining X1 and X2 (which can be solved by New-

ton’s method for a system of nonlinear equations), the

degrees of partial melting can be calculated as follows

(Zou and Zindler, 1996)

f ¼ X ððqsð1� /ÞÞ=ðqf/ � qsð1� /ÞÞÞ

þ ðqf/=ðqf/ þ qsð1� /ÞÞÞ ðA9Þ

Where the first and second terms in Eq. (A9)

represent the mass fraction of extracted liquid and

residual liquid.

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