The transition from alkaline to tholeiitic magmas: a case study from the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy)

Lithos 63 (2002) 83 – 113 www.elsevier.com/locate/lithos

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 b

a Dipartimento di Scienze della Terra, Universita` degli Studi di Roma La Sapienza, P.le A. Moro 5, I-00185 Roma, Italy Dipartimento 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/ 86 Sr = 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 complex region which has been characterized during the last 30 Ma by magmatic activity with a wide range of chemical compositions, from strongly alkaline (with *

Corresponding author. E-mail address: [email protected] (M. Lustrino).

sodic to potassic and ultrapotassic character) to subalkaline 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: S 0 0 2 4 - 4 9 3 7 ( 0 2 ) 0 0 11 3 - 5

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

1991; Pe´cskay 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 western Mediterranean area for several reasons: (1) the mafic rocks show compositions typical of mantlederived undifferentiated melts; (2) they are often associated 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 (Hercynian 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 developed at the same time as the present-day Ligurian – Provencß al basin and eventually formed ocean crust. The formation of this basin (30 –15 Ma; Se´ranne, 1999) is thought to be related to NW-directed subduction of the Mesogean oceanic lithosphere (Gueguen 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 developed 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 lamproitic and kamafugitic affinity together with anatectic crustal melts; e.g., Conticelli, 1998). Potassic to ultrapotassic 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’Antonio et al., 1999). Further south, volcanic activity generally younger than 1 Ma produced calc-alkaline, potassic and ultrapotassic volcanic rocks of the Aeolian 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 geodynamic scenario, characterized by coeval formation of extensional basins (Ligurian – Provencßal 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 Mediterranean 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) 83–113 85

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).

86

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 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 TiO2 Al2O3 Fe2O3XRF Fe2O3 FeO MnO MgO CaO Na2O K2 O P2O5 LOI Mg# qz hy ol ne V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Nd

49.91 2.05 16.28 10.07 3.13 6.25 0.13 7.26 7.97 3.40 2.08 0.42 1.12 0.62 0.00 0.00 16.82 0.04 194 280 45 181 46 97 35 771 18 205 36 833 32 65 30

49.94 1.85 15.82 10.02 3.75 5.65 0.13 8.18 7.81 3.04 2.06 0.42 1.36 0.65 0.00 5.96 14.71 0.00 187 382 44 264 46 95 33 721 19 209 33 808 32 69 32

50.06 1.75 16.62 9.27 1.17 7.30 0.12 8.91 7.48 3.23 2.10 0.44 0.81 0.68 0.00 2.12 18.98 0.00 177 353 45 231 45 88 33 701 17 196 31 796 34 66 30

49.89 1.79 15.89 9.31 8.52 0.71 0.13 8.79 8.12 3.44 1.97 0.36 0.39 0.68 0.00 0.00 18.54 1.34 186 363 47 212 47 89 44 649 19 165 29 660 29 68 31

52.66 1.66 16.32 9.44 2.24 6.49 0.12 7.00 7.65 3.67 1.04 0.26 0.90 0.62 0.00 19.56 2.71 0.00 161 246 40 140 41 105 19 476 15 123 15 396 15 31 17

52.63 1.77 16.27 9.79 2.38 6.68 0.13 6.57 7.88 3.27 1.00 0.23 1.20 0.60 1.27 22.67 0.00 0.00 165 250 47 140 41 97 21 492 20 142 16 408 17 46 24

51.27 1.65 15.57 10.22 6.83 3.05 0.12 7.57 7.16 3.59 1.44 0.33 1.41 0.62 0.00 12.65 9.54 0.00 162 419 43 328 44 104 25 657 17 135 25 608 21 43 22

54.03 1.43 16.23 9.53 3.03 5.86 0.12 6.50 7.37 3.62 0.56 0.15 1.10 0.61 2.85 23.32 0.00 0.00 140 243 40 134 37 101 9 475 17 95 11 296 12 26 17

54.61 1.36 16.37 9.35 2.98 5.74 0.12 6.35 7.37 3.60 0.69 0.18 0.64 0.60 3.35 22.93 0.00 0.00 143 250 40 133 39 100 10 478 17 96 12 279 15 28 16

54.67 1.47 16.12 9.70 3.42 5.66 0.12 5.65 7.39 3.64 0.57 0.14 1.15 0.57 4.58 21.10 0.00 0.00 150 247 42 132 38 104 11 464 19 96 11 265 13 27 16

55.38 1.34 16.29 8.81 2.92 5.31 0.12 5.80 7.26 3.41 0.48 0.14 1.55 0.60 6.95 21.47 0.00 0.00 132 231 35 129 37 99 6 445 18 87 8 224 12 20 14

55.82 1.24 16.23 8.83 2.24 5.94 0.11 5.96 7.07 3.59 0.56 0.16 1.08 0.60 6.22 21.99 0.00 0.00 129 267 38 148 35 96 8 463 17 83 8 422 9 30 19

55.72 1.43 16.32 9.21 3.10 5.51 0.11 5.54 7.19 3.67 0.50 0.14 0.77 0.57 6.24 20.86 0.00 0.00 139 251 38 131 36 102 8 447 18 92 9 227 11 24 16

55.14 1.36 15.64 9.04 5.46 3.23 0.11 6.78 6.92 3.66 0.53 0.14 1.03 0.63 4.50 23.65 0.00 0.00 155 263 37 134 40 110 9 494 16 96 12 325 12 26 15

48.85 1.88 16.68 9.79 5.11 4.22 0.13 8.24 7.92 2.69 2.08 0.43 1.78 0.65 0.00 7.13 14.65 0.00 189 362 46 240 47 87 40 763 23 184 40 891 45 72 32

52.93 1.60 16.78 10.05 3.53 5.88 0.13 6.08 7.88 3.41 0.72 0.19 0.87 0.58 1.86 22.58 0.00 0.00 157 235 44 138 46 105 8 505 19 113 15 387 18 40 19

50.23 1.99 16.68 9.73 3.06 6.01 0.13 7.16 7.83 3.36 2.16 0.43 0.98 0.62 0.00 1.86 15.56 0.00 194 255 45 170 45 93 34 766 18 203 35 850 32 68 33

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.

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

Rock Haw type (TAS)

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

Mug

B And

Haw

52.63 1.51 16.93 9.79 1.91 7.10 0.13 6.38 7.84 3.44 0.70 0.21 1.22 0.59 1.10 23.39 0.00 0.00 150 245 41 142 44 100 9 502 18 107 13 362 17 27 13

50.66 1.93 16.59 10.13 6.24 3.50 0.15 5.80 7.25 3.54 1.85 0.46 2.02 0.56 0.00 10.93 7.84 0.00 188 276 49 205 45 101 34 761 22 176 32 842 33 68 33

51.49 1.89 16.17 10.39 5.72 4.20 0.12 5.92 7.77 3.57 1.61 0.35 1.19 0.56 0.00 12.43 6.08 0.00 168 237 42 168 43 101 29 635 20 159 27 638 29 58 29

53.58 1.57 16.47 9.72 1.30 7.59 0.12 6.76 7.44 3.30 1.01 0.24 0.62 0.61 2.02 24.41 0.00 0.00 154 267 42 171 45 103 17 508 17 130 15 402 21 39 20

49.07 49.06 1.82 2.11 16.08 15.52 9.78 10.09 1.09 2.93 7.83 6.45 0.14 0.13 9.71 8.27 8.02 8.39 3.41 3.31 1.80 2.28 0.41 0.43 0.63 1.13 0.69 0.65 0.00 0.00 0.00 0.00 21.20 17.37 2.14 2.69 187 187 376 280 47 45 228 196 43 48 91 99 33 40 682 829 20 19 174 201 35 39 743 1105 36 38 60 89 28 39

Haw

Alk B

Mug

Mug

Alk B

Haw

Haw

47.60 1.75 15.53 10.14 2.46 6.92 0.14 10.82 8.42 3.58 0.81 0.38 1.59 0.71 0.00 0.00 23.18 2.64 193 386 54 295 47 91 26 681 18 156 31 689 27 60 28

51.26 1.73 16.49 10.17 4.40 5.20 0.11 6.35 7.39 3.64 1.81 0.40 1.23 0.58 0.00 8.67 10.22 0.00 168 230 39 178 41 102 32 737 19 159 34 746 28 56 27

51.77 1.72 16.51 9.22 3.28 5.35 0.12 7.02 7.56 3.57 1.67 0.40 1.03 0.63 0.00 11.83 8.20 0.00 166 249 41 156 45 91 29 621 21 159 28 646 31 57 27

47.48 2.04 15.57 9.93 2.17 6.99 0.13 10.69 8.24 3.95 0.84 0.45 1.46 0.71 0.00 0.00 22.28 4.41 201 403 54 293 41 93 40 806 17 167 42 883 40 71 34

51.48 1.61 15.83 9.69 3.43 5.64 0.12 8.26 7.10 3.66 1.44 0.31 1.13 0.66 0.00 10.57 11.98 0.00 159 382 46 343 47 98 26 617 17 134 24 556 22 47 22

51.23 50.12 1.65 1.83 15.31 17.02 10.15 9.60 5.68 5.62 4.03 3.59 0.12 0.12 7.90 6.54 7.05 7.51 3.66 3.24 1.42 2.23 0.30 0.50 1.65 1.68 0.64 0.60 0.00 0.00 11.24 6.61 10.92 11.99 0.00 0.00 157 163 419 235 46 40 343 168 46 41 109 96 24 34 625 836 27 21 135 173 26 39 580 1000 33 43 45 71 24 33

Haw

B And

Haw

52.22 49.50 1.84 2.06 16.69 15.87 10.90 9.68 6.04 4.46 4.37 4.71 0.14 0.13 3.56 8.02 8.69 7.14 3.39 4.39 1.30 1.10 0.30 0.52 1.46 2.10 0.42 0.65 2.15 0.00 14.75 0.00 0.00 17.24 0.00 2.02 164 188 263 350 47 48 180 282 43 35 98 98 21 15 568 993 20 16 149 235 22 50 548 1329 29 54 41 98 19 43

Haw

B And

Haw

Haw

50.82 1.95 16.02 9.07 5.08 3.60 0.13 6.95 6.62 4.55 1.53 0.53 2.23 0.63 0.00 0.00 16.30 0.42 158 325 39 229 38 101 22 1119 17 286 59 1143 62 119 51

52.22 1.86 16.39 10.43 3.84 5.94 0.14 5.87 8.20 3.49 0.94 0.25 0.86 0.56 0.31 20.84 0.00 0.00 167 240 46 150 44 103 11 496 22 146 19 425 19 37 17

49.63 1.78 16.11 9.51 1.10 7.58 0.13 10.04 7.39 3.34 1.86 0.39 0.65 0.70 0.00 0.00 22.44 0.35 165 414 46 283 47 91 33 666 20 185 36 721 35 69 29

50.91 1.98 14.20 11.00 5.53 4.84 0.16 7.03 8.18 2.94 2.05 0.45 1.74 0.59 0.00 9.66 9.19 0.00 200 283 40 205 43 100 36 695 24 182 38 758 31 63 30

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

B And

87

88

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 Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba Hf Ta Pb Th U Sc

180 351 44.0 211 n.a n.a 48.3 670 19.1 171.8 31.9 0.76 657 4.14 1.99 3.3 3.9 0.79 19.2

156 268 39.3 150 30.0 107.4 13.8 465 15.6 89.5 12.1 0.23 264 2.35 0.9 2.6 1.9 0.43 n.a

161 251 42.0 140 41.0 123.0 13.0 513 16.4 107.0 14.0 n.a. 346 2.70 0.9 3.4 2.2 0.40 n.a.

132 212 31.0 123 30.3 124.0 8.1 457 16.0 85.4 8.1 n.a. 242 2.60 0.6 6.7 1.5 0.32 15.0

192 307 37.4 223 36.0 140.5 37.9 719 21.0 179.8 39.4 n.a. 902 4.78 2.7 8.3 4.8 n.a. 21.0

152 190 35.2 119 36.3 128.7 9.0 515 19.0 108.4 14.1 n.a. 406 3.09 1.1 7.9 2.3 0.64 18.0

145 209 33.8 167 34.2 112.9 17.0 520 17.0 123.3 15.0 n.a. 415 3.43 1.1 6.4 2.7 0.83 16.0

204 404 50.1 231 32.1 112.0 39.5 722 17.7 185.0 37.1 0.59 691 4.07 2.4 4.6 4.8 0.94 n.a

177 227 38.2 166 34.6 131.1 39.7 856 18.0 199.7 40.7 0.64 1114 5.41 2.9 14.8 4.6 1.11 17.0

192 427 51.7 298 33.7 104.3 21.2 690 17.0 158.8 31.2 0.82 648 3.46 2.1 3.0 3.4 0.79 n.a

165 279 38.8 168 36.3 107.8 20.9 536 18.1 134.0 19.7 0.53 507 3.31 1.4 4.0 2.9 0.64 n.a

164 392 44.6 280 n.a n.a 36.6 679 19.8 187.8 37.3 0.52 710 4.46 2.27 4.6 4.5 0.94 18.9

179 266 44.4 193 n.a n.a 35.3 668 20.1 175.9 36.0 0.24 703 4.13 2.17 4.0 3.8 0.84 19.3

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

Table 2 ICP-MS trace element and Sr, Nd and Pb isotopic data for Orosei-Dorgali volcanic rocks

32.15 62.89 7.55 30.04 5.67 1.89 5.13 0.72 3.86 0.71 1.69 0.23 1.33 0.20 0.70447 0.512558
1.6

12.08 24.64 3.30 15.68 3.98 1.45 3.76 0.55 3.12 0.54 1.36 0.19 1.21 0.19 0.70446 0.512465
3.4 17.837 15.598 37.989

14.20 29.80 4.02 18.00 4.28 1.57 4.15 0.54 3.01 0.55 1.37 0.19 1.05 0.16 0.70449 0.512524
2.3

11.42 22.34 2.72 14.56 4.43 1.64 4.42 0.67 3.59 0.59 1.66 0.21 1.23 0.16 0.70455 0.512518
2.4

38.32 68.88 6.81 29.50 6.38 2.00 5.81 0.78 4.29 0.74 2.00 0.27 1.57 0.20

16.67 31.66 3.40 17.08 4.77 1.67 4.57 0.70 3.72 0.64 1.77 0.23 1.33 0.17 0.70465 0.512470
3.3 17.826 15.594 38.016

18.72 36.89 3.84 18.61 4.79 1.65 4.71 0.65 3.58 0.60 1.60 0.21 1.25 0.16

33.29 64.63 7.70 26.80 5.76 1.81 4.46 0.66 3.84 0.65 1.72 0.24 1.53 0.20 0.70451 0.512550
1.8

42.54 79.45 7.58 32.78 6.77 2.15 6.21 0.79 4.12 0.67 1.78 0.22 1.31 0.15

26.51 51.98 6.16 25.17 4.87 1.63 4.36 0.60 3.22 0.55 1.47 0.20 1.16 0.18 0.70442 0.512571
1.3 17.860 15.596 37.942

20.35 39.37 4.71 19.81 4.37 1.57 4.42 0.60 3.30 0.66 1.59 0.23 1.36 0.22 0.70453 0.512538
2.0 17.738 15.531 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.

34.08 66.62 7.82 30.65 5.71 1.70 5.28 0.73 4.06 0.72 1.82 0.25 1.38 0.21 0.70446 0.512528
2.2

32.96 63.65 7.49 29.66 5.69 1.69 5.11 0.73 3.91 0.73 1.70 0.27 1.36 0.20 0.70442 0.512510
2.5

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

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 87 Sr/86Sr 143 Nd/144Nd eNd 206 Pb/204Pb 207 Pb/204Pb 208 Pb/204Pb

89

90

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

basaltic plateaux (Orosei-Dorgali; Planargia – Abbasanta 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, andesite, 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 comprise f 80% alkaline and 20% tholeiitic lavas (Lustrino 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.

tion. REEs and other trace elements were determined by ICP-MS at CRPG (Nancy, France), Actlabs (Ontario, Canada) and Dipartimento di Scienze della Terra of Pisa (Italy). Bias among trace element concentrations obtained using three different ICP-MS is generally within 5%, whereas bias between XRF and ICPMS is generally within 10%. Electron microprobe analyses have been carried out at CSQEA, CNR, Rome, utilizing a CAMECA SX50 electron microprobe 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/HNO3 mixtures 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 automated, 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.

3. Analytical techniques 4. Results Major and trace element analyses have been performed 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-

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 isotopic 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) 83–113

sified in the order of abundance as: hawaiite, basaltic andesite, alkali basalt and rare mugearite (Fig. 2). Alkaline rocks are silica-saturated to slightly silica-undersaturated character; they are porphyritic with euhedral olivine and plagioclase F clinopyroxene and oxides in a pilotaxitic and/or hyalopilitic matrix. Abundant disrupted mantle xenoliths and rare crustal xenocrysts (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 clinopyroxene F iddingsitized olivine phenocrysts. The groundmass is made up of microlites of plagioclase, clinopyroxene and oxides. Rare groundmass orthopyroxene 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 subhedral to euhedral phenocrysts, as xenomorphic groundmass crystals of Fo83-56 and as xenocrysts

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 (Esperancß 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).

91

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 monticellite 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%. Clinopyroxene 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, respectively). The higher AlVI in clinopyroxenes from alkaline 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 (Ulvo¨spinel content from 26% to 86%), while ilmenite show much less solid solution (ilmenite content from 90% to 99%) indicating 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

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Table 3a Selected analyses of olivine from Orosei-Dorgali volcanic rocks Olivine Sample MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV

26 26 60 60 60 60 60 71 71 81 81 81 81 81 83 83 83 83 83 83 83

gm mf-c gm gm mf-c mf-r gm gm gm gm gm gm gm gm mf gm gm gm mf-c mf-r gm

SiO2

MnO

FeO

MgO

CaO

NiO

Sum

Fo

36.80 39.96 36.11 35.60 39.00 38.13 37.07 40.13 39.50 38.19 38.30 38.19 38.58 37.29 38.30 37.80 37.44 37.81 39.15 38.77 36.60

0.37 0.14 0.53 0.31 0.06 0.33 0.44 0.20 0.28 0.44 0.44 0.47 0.34 0.52 0.17 0.41 0.39 0.44 0.30 0.15 0.37

31.62 17.86 37.96 37.34 19.39 26.28 35.58 15.98 17.39 23.79 26.34 25.71 23.84 29.34 23.89 27.64 28.80 26.66 19.81 21.89 29.09

30.42 42.36 27.40 27.23 41.96 36.32 29.81 44.24 42.78 36.98 35.86 35.07 36.94 32.38 38.52 34.90 34.44 36.09 41.70 39.90 32.46

0.27 0.20 0.25 0.36 0.34 0.32 0.33 0.20 0.29 0.21 0.27 0.27 0.20 0.25 0.32 0.28 0.29 0.28 0.24 0.27 0.29

0.11 0.26

99.58 100.77 102.24 101.25 101.34 101.94 103.23 100.74 100.23 99.75 101.42 99.87 100.09 99.78 101.67 101.54 101.82 101.73 101.79 101.52 99.28

63.17 80.87 56.27 56.52 79.42 71.13 59.90 83.15 81.44 73.48 70.83 70.86 73.42 66.30 74.19 69.24 68.07 70.70 78.96 76.47 66.55

0.41 0.59 0.56

0.13 0.21 0.15 0.20 0.47 0.52 0.45 0.45 0.59 0.54 0.47

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

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-normative have been classified as transitional (Lustrino, 1999). SiO 2 ranges from 47.5 to 55.8 wt.% while Mg# [Mg# = Mg/(Mg + Fe 2 + ), assuming Fe 2 O 3 /

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

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

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Table 3b Selected analyses of clinopyroxene from Orosei-Dorgali volcanic rocks Clinopyroxene Sample MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV

26 26 26 26 58 58 60 60 60 60 60 71 71 71 71 71 71 83 93e 93e 95S 95S 95S 95S 95S 95S 95S 95S 95S 95S 95S 97 97 97 97 97 97 97 97 97

glom-c gm gm gm gm gm gm gm gm gm gm f-c glom-r glom-m glom-c glom-c glom-r in pl gm gm gm mf-r gm gm f-c f-r f-r f-c spon spon-c rim-q glom-c glom-r glom-r glom-c gm gm gm in pl gm

SiO2

TiO2

Al2O3

Cr2O3

MnO

FeO

MgO

CaO

Na2O

Sum

Wo

En

Fs

53.74 51.17 52.09 51.15 49.76 53.08 53.04 52.38 50.04 52.48 51.13 48.11 47.78 48.34 50.27 47.95 47.90 51.66 47.13 47.47 47.56 51.97 47.43 47.60 49.72 47.65 47.84 48.00 52.68 53.94 54.16 53.15 52.97 52.45 52.51 52.15 51.16 50.62 50.37 52.01

0.63 1.52 1.00 1.59 2.15 0.99 0.90 1.07 1.82 1.04 1.35 2.23 2.19 2.05 1.34 2.14 2.28 0.88 2.77 2.68 2.81 1.22 2.61 2.63 1.91 2.66 2.04 2.53 0.91 0.20 0.41 0.70 0.80 0.63 0.71 1.15 1.47 1.74 1.45 1.35

0.86 2.11 1.78 1.83 2.91 1.71 1.36 2.14 3.81 1.90 2.96 6.27 6.33 6.09 3.88 6.23 6.38 1.33 6.98 6.22 5.78 3.23 5.78 5.97 4.03 5.79 3.82 5.81 1.34 0.97

0.30 0.23 0.57 0.08 0.05 0.25 0.35 0.37 0.27 0.25 0.65 0.78 0.51 0.52 0.33 0.54 0.48 0.06 0.40 0.30 0.38 0.21 0.17 0.52 0.15 0.19 0.05 0.64 0.05 0.13 0.16 0.51 0.44 0.65 0.54 0.24 0.07 0.08 0.62 0.30

0.29 0.18 0.15 0.39 0.41 0.11 0.24 0.21 0.30 0.24 0.15 0.14 0.18 0.16 0.23 0.15 0.08 0.40 0.18 0.14

11.81 12.66 9.77 11.29 13.55 7.99 10.40 9.80 10.75 9.43 9.84 6.83 6.92 6.71 6.84 6.83 7.22 10.09 7.18 7.22 7.19 6.28 7.69 7.02 7.54 7.16 7.14 7.14 7.07 8.14 8.32 7.74 8.66 7.09 7.20 9.69 14.16 15.52 8.22 9.74

19.82 16.21 17.34 14.76 12.33 16.29 17.35 16.59 15.07 16.99 15.71 13.76 13.65 14.00 15.31 13.70 13.52 14.51 13.54 13.50 13.02 15.75 13.24 13.32 14.57 13.32 13.84 13.53 14.55 15.27 14.36 18.33 17.27 17.15 17.42 16.57 13.36 14.17 16.10 16.30

12.98 15.84 16.20 18.46 19.01 20.39 17.03 18.37 18.57 18.78 18.95 21.48 21.80 21.54 21.00 21.54 21.61 19.57 21.44 22.14 22.28 21.99 22.15 22.31 21.51 22.43 21.37 22.06 22.44 20.33 22.81 17.79 18.57 18.79 17.78 18.03 17.99 15.69 18.20 18.18

0.19 0.28 0.27 0.30 0.53 0.32 0.29 0.32 0.36 0.37 0.38 0.48 0.49 0.43 0.41 0.51 0.44 0.51 0.52 0.46 0.52 0.47 0.48 0.46 0.42 0.44 0.48 0.49 0.56 0.91 0.45 0.34 0.30 0.34 0.40 0.31 0.36 0.35 0.44 0.30

100.61 100.20 99.16 99.86 100.70 101.12 100.96 101.24 100.98 101.49 101.12 100.07 99.85 99.82 99.61 99.59 99.91 98.99 100.14 100.13 99.53 101.26 99.53 99.99 99.98 99.88 96.69 100.31 99.76 100.25 100.67 100.95 100.73 100.17 99.60 100.19 100.93 100.49 100.53 100.53

25.96 32.72 33.71 38.36 40.39 41.30 34.43 37.29 38.56 37.59 38.99 46.64 47.05 46.45 43.93 46.79 46.85 40.83 46.59 47.45 48.44 44.95 47.56 48.03 45.04 48.01 46.17 47.41 46.43 42.18 46.29 36.08 37.50 38.91 37.19 36.94 37.56 32.74 38.55 37.39

55.14 46.57 50.18 42.68 36.45 45.90 48.79 46.84 43.53 47.30 44.96 41.55 40.98 41.99 44.53 41.38 40.78 42.09 40.92 40.24 39.36 44.77 39.56 39.88 42.44 39.64 41.59 40.44 41.88 44.07 40.53 51.68 48.51 49.39 50.68 47.21 38.80 41.12 47.42 46.62

18.91 20.71 16.11 18.96 23.16 12.80 16.79 15.87 17.91 15.11 16.04 11.81 11.97 11.56 11.55 11.83 12.36 17.07 12.48 12.31 12.20 10.28 12.88 12.09 12.52 12.34 12.24 12.15 11.69 13.75 13.18 12.24 13.99 11.70 12.12 15.85 23.65 26.14 14.03 15.99

2.40 1.51 2.93 2.81 1.83 2.02 1.78 4.88 2.15

0.16 0.17 0.13 0.22 0.12 0.10 0.17 0.35

0.22 0.14 0.23 0.23 0.35 0.53 0.26 0.22

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.

(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 compatible (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

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Table 3c Selected analyses of plagioclase and alkali feldspar from Orosei-Dorgali volcanic rocks Sample Plagioclase MGV 5 MGV 5 MGV 7 MGV 7 MGV 7 MGV 26 MGV 26 MGV 26 MGV 26 MGV 26 MGV 26 MGV 26 MGV 26 MGV 58 MGV 60 MGV 60 MGV 60 MGV 60 MGV 60 MGV 60 MGV 71 MGV 71 MGV 81 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 83 MGV 93e MGV 97 MGV 97 MGV 97 MGV 97 MGV 97 MGV 97 MGV 97 MGV 97

SiO2 gm f-c gm f-c f-r gm f-c f-r glom-c gm glom glom-c glom-r gm gm gm gm gm f-c f-r gm gm mf-c gm mf-c gm gm gm dus-r dus-c corr corr corr dus gm f-r f-c corr-c gm f-r f-c int in xq glom-c

Alkali Feldspar MGV 58 gm MGV 81 gm MGV 81 mf-r MGV 81 gm MGV 95S gm

52.89 51.01 52.01 51.96 52.16 53.26 53.97 54.79 54.33 54.62 54.42 54.37 60.02 53.98 55.41 55.50 58.18 54.25 55.05 56.79 51.86 52.99 61.62 55.78 52.96 60.87 58.55 56.33 57.23 56.55 59.91 58.73 55.80 52.97 53.21 57.06 52.56 53.64 55.49 53.13 53.74 61.72 53.76

67.58 63.83 64.32 65.15 65.74

TiO2

0.06 0.11 0.18 0.19 0.12 0.18 0.08 0.12

0.15 0.26 0.14 0.09 0.08 0.18 0.17 0.12 0.06 0.17 0.13 0.08 0.24 0.21

0.25 0.08 0.05 0.12 0.18 0.07

0.10 0.17

0.38 0.23 0.19 0.40 0.38

Al2O3

FeO

MgO

CaO

K2O

Na2O

29.69 31.89 29.09 29.78 29.28 28.34 28.63 28.02 28.34 28.43 28.37 27.77 24.70 28.85 28.38 27.87 26.53 29.65 28.66 28.14 29.29 28.78 24.02 28.12 30.63 23.38 26.42 28.06 25.76 25.76 23.08 24.51 26.55 27.29 29.35 27.37 30.51 29.35 27.83 30.19 29.07 23.66 29.31

0.61 0.60 0.81 0.54 0.74 0.34 0.34 0.58 0.56 0.75 0.09 0.33 0.70 0.61 0.80 0.95 0.98 0.55 0.53 0.81 0.75 0.42 0.36 1.01 0.55 0.96 1.01 0.95 0.18

0.26 0.33

12.23 14.14 12.55 12.86 12.35 11.98 12.08 11.34 11.61 11.68 11.77 11.30 7.47 11.26 10.79 10.50 8.50 11.87 10.94 10.04 12.78 11.82 5.14 10.45 12.75 5.68 8.37 10.11 9.14 9.33 6.14 7.62 9.86 11.65 11.40 9.52 12.93 11.88 10.54 12.66 11.69 5.30 11.72

0.14 0.11 0.26 0.22 0.31 0.10 0.11 0.11 0.15 0.12 0.10 0.15 0.27 0.40 0.31 0.32 0.51 0.24 0.25 0.36 0.36 0.42 1.04 0.51 0.28 1.94 0.57 0.42 0.36 0.61 1.89 0.74 0.41 0.34 0.44 0.41 0.20 0.26 0.30 0.17 0.21 0.93 0.20

4.40 3.31 4.11 3.84 4.18 4.63 4.72 5.09 4.81 4.85 4.78 4.77 7.12 4.75 5.27 5.35 6.55 4.60 5.20 5.66 3.97 4.32 7.41 5.37 4.14 7.29 6.55 5.59 6.29 5.64 6.53 6.61 5.79 4.60 4.84 5.86 4.03 4.55 5.22 4.07 4.67 7.81 4.57

19.04 21.44 21.10 20.51 20.39

0.92 0.22 0.26 0.40 0.25

0.46 2.73 2.07 1.86 1.63

6.65 4.04 5.12 4.79 4.68

6.81 6.65 6.09 6.68 6.59

0.10

0.06 0.07 0.09 0.08 0.15 0.08

0.17 0.22 0.16 0.12

0.27

0.13

0.61 0.09 0.76 0.72 0.65 0.67 0.70 0.67 0.44 0.61 0.73 0.53

0.06 0.11 0.15 0.29

0.08 0.25 0.06 0.18

BaO

0.06 0.06 0.10

SrO

0.10 0.04 0.05

0.05 0.02 0.03 0.06 0.03 0.10 0.06 0.04 0.05 0.09 0.08 0.11 0.06 0.80 0.04 0.05 0.20 0.12 0.09

0.04

0.07 0.04 0.04 0.10 0.08 0.02 0.08

0.11 0.05 0.07 0.04 0.04 0.04 0.06 0.05

0.03 0.24

0.03

0.77 0.74 0.20 0.20

0.05

Sum

An

Ab

Or

100.21 101.44 99.11 99.46 99.45 98.76 100.02 99.98 99.99 100.65 99.54 98.79 100.47 100.32 101.28 100.54 101.37 101.45 101.11 102.20 99.43 98.91 100.60 101.51 101.79 100.56 101.81 101.75 98.96 97.88 98.52 98.21 98.62 97.83 100.46 101.22 101.29 100.44 100.09 100.83 100.23 100.40 100.27

60.08 69.77 61.82 64.10 60.91 58.49 58.21 54.84 56.67 56.70 57.34 56.21 36.13 55.40 52.11 51.09 40.55 57.96 52.98 48.49 62.70 58.70 25.95 50.30 61.97 26.83 40.05 48.80 43.64 46.05 30.37 37.25 47.36 57.19 55.11 46.19 63.21 58.20 51.81 62.56 57.36 25.78 57.95

39.13 29.58 36.64 34.60 37.29 40.94 41.17 44.53 42.46 42.63 42.10 42.91 62.32 42.25 46.09 47.07 56.54 40.66 45.60 49.44 35.21 38.81 67.78 46.79 36.39 62.25 56.70 48.81 54.31 50.38 58.49 58.43 50.29 40.82 42.34 51.45 35.65 40.30 46.43 36.42 41.44 68.81 40.90

0.79 0.65 1.54 1.30 1.80 0.56 0.63 0.63 0.87 0.66 0.56 0.88 1.55 2.35 1.79 1.83 2.91 1.38 1.42 2.07 2.10 2.49 6.27 2.91 1.64 10.91 3.25 2.40 2.05 3.57 11.15 4.32 2.35 1.99 2.55 2.36 1.14 1.49 1.77 1.02 1.20 5.41 1.15

101.83 99.91 99.89 99.99 99.92

2.21 13.93 10.76 9.47 8.50

59.54 61.48 57.47 61.49 62.36

38.25 24.59 31.76 29.04 29.14

gm = groundmass; f-c = phenocrystal core; f-r = phenocrystal rim; glom-c = glomerule core; glom-r = glomerule rim; dus-r = dusty rim; dusc = dusty core; corr-c = corrose core; mf-r = microphenocrystal rim.

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

95

Table 3d Selected analyses of Fe – Ti oxides Rhomboedral phase Sample MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV

7 26 26 26 60 81 81 83 93e 93e 95S 95S

gm gm gm gm gm gm gm gm gm gm gm gm

TiO2

Al2O3

48.70 48.90 47.99 49.80 48.12 50.35 51.06 51.87 50.89 51.31 50.75 51.17

0.79 0.06

0.08 0.08

0.13 0.06 0.05 0.16 0.21

TiO2

Al2O3

Cr2O3

20.40 23.66 18.20 19.90 17.98 27.36 25.01 2.20 17.20 15.88 19.07 17.34 9.68 8.73 11.74 9.04 10.01 9.16 2.89

1.49 0.99 2.23 2.23 0.47 1.90 2.23 34.46 4.36 6.50 1.93 1.83 1.81 1.76 1.61 1.72 1.82 1.69 24.05

0.35 0.44 14.06 7.04 0.08 2.78 0.74 16.85 9.62 13.66 0.07 0.97 0.66 1.34 0.97 0.53 1.42 0.88 21.38

0.05 0.13 0.14 0.14 0.04

Cr2O3 0.07 0.06

0.07

MnO

FeOtot

MgO

Sum

Ilm

0.54 0.47 0.42 0.45 0.33 0.55 0.75 0.28 0.64 0.70 0.58 0.48

42.17 47.31 47.56 47.29 50.61 42.07 43.28 39.22 43.51 41.89 40.48 39.70

4.13 0.89 0.62 1.13 1.11 3.94 3.33 4.17 4.59 5.22 5.19 5.62

96.32 97.71 96.66 98.72 100.29 97.12 98.42 95.80 99.73 99.17 97.24 97.26

0.927 0.941 0.934 0.946 0.897 0.950 0.954 0.997 0.926 0.934 0.945 0.951

MnO

FeOtot

MgO

Sum

Usp

0.39 0.49 0.54 0.07 0.29 0.54 0.79 0.49 0.54 0.48 0.73 0.37 0.31 0.22

66.89 64.89 59.86 65.26 77.61 59.12 66.10 34.48 60.58 54.35 70.25 72.58 81.96 81.46 76.71 80.38 76.40 81.21 37.80

2.85 2.58 1.72 2.87 0.43 2.44 1.99 10.16 4.55 6.30 2.30 1.84 1.62 1.20 1.10 1.72 1.16 1.23 9.29

92.36 93.05 96.61 97.36 96.86 94.14 96.86 98.63 96.86 97.17 94.35 94.92 96.05 94.71 92.13 93.47 90.97 94.50 95.81

0.615 0.707 0.685 0.643 0.513 0.859 0.741 0.260 0.602 0.618 0.566 0.519 0.283 0.262 0.361 0.270 0.315 0.273 0.257

Spinel group Sample MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV MGV

7 7 26 60 60 71 71 81 81 81 81 81 83 83 83 83 83 83 95S

gm gm gm in ol gm gm gm mf-c mf-r mf-r gm gm gm gm gm gm gm gm gm

0.08 0.17 0.32 0.41

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

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 concentrations, 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

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

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97

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).

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)N f 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 McDonough, 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 (Lustrino 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).

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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.

4.4. Pb isotope compositions Two published data (Lustrino et al., 2000a) plus two new Pb isotopic ratios of samples from OroseiDorgali 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 having 206 Pb/ 204 Pb < 18 and 207 Pb/ 204 Pb (15.54 – 15.62) (Gasperini et al., 2000; Lustrino et al., 2000a). The high 208 Pb/ 206 Pb and 207 Pb/ 206 Pb ratios (>2.10

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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).

and >0.86, respectively) differentiate the Orosei-Dorgali 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

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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

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).

SiO2 (Fig. 3). Due to the absence of K-bearing mineral phases, this oxide is to be considered incompatible, 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 volcanic 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 (plagioclase F clinopyroxene F olivine F 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 interelement 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

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 incompatible 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 chondrite-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 peridotitic 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.

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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).

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.

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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.

(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 magmas 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 contamination 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 OroseiDorgali volcanic rocks show a relatively large Cr variation ( f 350– 140 ppm) coupled with the relatively 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 elements) and 87Sr/86Sr ratio. The absence of a correlation 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

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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.

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Zr (not shown), while tholeiitic rocks from OroseiDorgali 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, reliable 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 intervals 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 considerations: (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 crystallization 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 conclusion, it is possible to hypothesize a single source for the PSV, all the compositional variability being buffered by varying degrees of partial melting and varying degrees of fractional crystallization.

6. Relationships with neighboring igneous provinces In this section, a comparison between the OroseiDorgali 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 features 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 circumMediterranean 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 Province (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 Rho¨n (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 OroseiDorgali 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-Dorgali 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

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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.

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Ce/Pb. This feature, shared also by a few PSV (the RPV group of Lustrino et al., 2000a), is typical of HIMUOIB magmas. HIMU-OIB end member is also characterized 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 have 206 Pb/204Pb = 17.37 – 18.01 (avg. 17.68), 207 Pb/ 2 0 4 Pb = 15.54 – 15.62 (avg. 15.58) and 208 Pb/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 HIMUOIB 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., Esperancßa 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 (Esperancßa 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 (Wo¨rner 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).

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1998), a general HIMU-DM character of the sources, with only limited EM composition involvement and carbonatitic metasomatism, is now generally accepted (e.g. Esperancßa 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 peculiarity of the PSV in terms of Sr and Nd isotopes (low 143 Nd/144Nd coupled with bulk Earth 87Sr/86Sr) are apparent also when compared with the other anorogenic 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 subduction-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 anorogenic 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) reinforcing the possibility of a derivation from asthenosphere or lower mantle, excluding major contributions from the lithosphere (Wedepohl and Baumann, 1999). This convecting mantle reservoir has been alternatively called European Asthenospheric Reservoir (EAR; Granet et al., 1995), Low Velocity Zone (LVZ; Hoernle et al., 1995), Central European Anomaly (CEA; Goes et al., 1999) or Low Velocity Anomaly (LVA; Ritter et al., 2001) and has been possibly related

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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 lowvelocity 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 volcanic 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 evidence of an asthenospheric source, sometimes contaminated by lithospheric melts (Wo¨rner 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 Baumann, 1999; Wedepohl, 2000). Within this contest, the EMI-like composition of the vast majority of the PSV has been related to lithospheric sources modified during 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. Wo¨rner et al., 1986; Zangana et al., 1999; Downes, 2001). Much of these evidences for this come from radiogenic and stable isotopic studies and modal metasomatism of mantle and crustal xenoliths commonly found in Cenozoic to Quaternary rocks in the Massif Central (France), Rhine Graben (Germany), Pannonian 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) subduction systems, when the central Europe was the sandwiched hinterland squeezed between a roughly North-

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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 SWdipping (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 Kdepleted 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 radiogenic Pb (206Pb/204Pb = 18.3– 19.7, with many samples >19; 208Pb/204Pb = 38.4 – 39.6, with many samples >38.8). Deviations from this typical HIMU-OIB (i.e., asthenosphere-derived) geochemical character, sometimes 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 and 208 Pb/204Pb roughly overlapping the field of anorogenic rocks (i.e. >18.3 and >38.4, respectively). Notwithstanding this quite uniform isotopic scenario, Wilson and Downes (1991) hypothesized for the CEVP HIMU-DM sources, which variably interacted with EM compositions (not specifying if EMI or EMII). The EM imprint was related by these authors to modifications that occurred during Paleozoic subduction. The relatively short period elapsed since this event ( f 300– 400 Ma) could be, according to Wilson and Downes (1991), at the base of the lack of the homogenization of the asthenospheric mantle by convective 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 control) 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 ( f f 4 – 6%), whereas tholeiitic rocks could be related to a higher degree of melting ( f f 10 –15%). The Orosei-Dorgali rocks represent an extremely unusual trace element and Sr –Nd – Pb isotopic compositions 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 distillation 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) 83–113

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 processes: batch, fractional and dynamic melting. Because batch melting assumes a continuos equilibrium 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 =ðD þ f ð1
PÞÞ

ðBatch MeltingÞ

Cl ¼ ðC0 =DÞð1
f Þexpðð1=DÞ
1Þ ðRayleigh MeltingÞ

ðA1Þ

ðA2Þ

Where Cl and C0 are the concentrations of an element in the melt and in the initial solid, respectively; D is the bulk solid/melt distribution coefficient (calculated from the weight proportions of each mineral in the source assemblage); f represents the degree of partial melting and P is the bulk solid/melt distribution coefficient during non-modal partial melting (calculated from the weight proportions of each mineral which is involved in the melting processes). To calculate the approximate degree of partial melting using the above equations, it is important to

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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 assuming any value of C0. This method is based on the enrichment ratio of two strongly incompatible elements 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 ðDa ð1
Pb Þð1
Qa Þ þ Db ð1
Pa ÞðQb
1ÞÞ =ððQa
Qb Þð1
Pa Þð1
Pb ÞÞ ðA3Þ f2 ¼ ðQb ðDb þ f1 ð1
Pb ÞÞ
Db Þ=ð1
Pb Þ

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 incompatible 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 intermediate 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

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the mass fraction of the extracted liquid and residual liquid. According to Zou and Zindler (1996), the concentration of a trace element in the extracted dynamic melt is Cl ¼ ð1=X ÞC0 Gð1
ð1
X ÞexpðGð1
DÞ þ 1ÞÞ =ðGð1
DÞ þ 1Þ

ðA5Þ

where G ¼ ðqf / þ qs ð1
/ÞÞ=ðqf / þ qs ð1
/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 Ca1 =Ca2 ¼ ðX2 ð1
ð1
X1 ÞexpðGa ð1
Da Þ þ 1ÞÞÞ =ðX1 ð1
ð1
X2 ÞexpðGa ð1
Da Þ þ 1ÞÞÞ ðA7Þ

Similarly, for the less-so-highly incompatible element b Qb ¼ Cb1 =Cb2 ¼ ðX2 ð1
ð1
X1 ÞexpðGb ð1
Db Þ þ 1ÞÞÞ =ðX1 ð1
ð1
X2 ÞexpðGb ð1
Db Þ 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 Newton’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 ððqs ð1
/ÞÞ=ðqf /
qs ð1
/ÞÞÞ þ ðqf /=ðqf / þ qs ð1
/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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