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Geodynamic implications of deep mantle upwelling in thesource of Tertiary volcanics from the Veneto region

(South-Eastern Alps)

Patrizia Maceraa,*, Daniela Gasperinia, Claudia Piromallob,Janne Blichert-Toftc, Delphine Boschd, Aldo Del Moroe, Silvana Martinf

aDipartimento di Scienze della Terra, Universita’ di Pisa, Via S. Maria, 53, 56126 Pisa, ItalybIstituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata, 605, 00143 Rome, Italy

cEcole Normale Superieure, 46, Allee d’Italie, 69364 Lyon Cedex 07, FrancedUniversite ‘‘Montpellier 200, 22, Place E. Bataillon, 34095 Montpellier Cedex 05, FranceeIstituto di Geoscienze e Georisorse, C.N.R., Via Moruzzi, 1, 56010 Ghezzano-Pisa, Italy

fDipartimento di Chimica Fisica Matematica, Universita Insubria, via Lucini 3, 22100 Como, Italy

Received 7 May 2003; received in revised form 5 August 2003; accepted 7 August 2003

Abstract

Major and trace element and Sr–Nd–Hf–Pb isotopic data for the most primitive Tertiary lavas from theVeneto region (South-Eastern Alps, Italy) show the typical features of HIMU hotspot volcanism, variablydiluted by a depleted asthenospheric mantle component (87Sr/86Sri=0.70306–0.70378; "Ndi=+3.9 to+6.8; "Hfi=+6.4 to +8.1,

206Pb/204Pbi=18.786–19.574). P-wave seismic tomography of the mantle belowthe Veneto region shows the presence of low-velocity anomalies at depth, which is consistent with possibleupwellings of plume material. Between the depths of 100–250 km the velocity anomalies are approximately2–2.5% slower than average, implying a temperature excess of about 220–280 K, in agreement with esti-mates for other mantle plumes in the world. In this context, the Veneto volcanics may represent the shal-low expression of a mantle upflow. The presence of a HIMU-DM component in a collision environmenthas significant geodynamic implications. Slab detachment and ensuing rise of deep mantle material into thelithospheric gap is proposed to be a viable mechanism of hotspot magmatism in a subduction zone setting.# 2003 Elsevier Ltd. All rights reserved.

0264-3707/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jog.2003.08.004

Journal of Geodynamics 36 (2003) 563–590

* Corresponding author. Tel.: +39-050-847292.

E-mail address: macera@dst.unipi.it (P. Macera).

1. Introduction

During Tertiary time the Veneto region was affected by extensive volcanic activity of primarilybasaltic composition. Over a time span of 30–35 million years, from late Paleocene to late Oli-gocene, with minor magmatic events occurring during the early Miocene, four main volcaniccenters developed in the eastern sector of the Southern Alps: Mts. Lessini, Berici, and the Euga-nean Hills to the west of the Schio-Vicenza fault line, and the Marostican Hills to the east (Fig. 1).The magmatic activity took place discontinuously in 12 main stages beginning in North-WesternLessini at Paleocene time. During early Eocene, the volcanism migrated south-eastwards intoCentral-Western Lessini, Berici, and the Euganean Hills, manifesting itself by the extrusion ofmafic volcanics (alkaline basalts and basanites). At Oligocene time, the magmatic activity, now ofprevailing transitional character, resumed again in the easternmost part of the Lessini mountainsand the Marostica Hills, as well as in the southern part of the Euganean Hills. Differentiatedvolcanics (trachytes and rhyolites) were erupted only in the Euganean Hills during early Oligo-cene. Finally, in early Miocene, minor magmatism took place in the Marostican and EasternLessini centers. The climax of the volcanic activity in the whole area, however, occurred in middleEocene (De Vecchi and Sedea, 1995; Barbieri et al., 1982, 1991).On the basis of petrological and geochemical data (De Vecchi and Sedea, 1974, 1995), threemain volcanic suites have been identified in the Veneto region: alkaline, transitional, and tholeii-tic. The most representative rock types are alkali basalts, basanites, and transitional basalts.Hawaiites, trachybasalts, tephrites, basaltic andesites, and differentiated rocks are either missingor scarce. All the volcanics show the typical features of within-plate basalts, the petrogenesis ofwhich usually has been ascribed to partial melting of a metasomatized, amphibole-bearing litho-spheric mantle source (Milani et al., 1999). According to De Vecchi and Sedea (1995), the alkalibasalts, basanites, and transitional basalts are closely related in space and time, but as evidentfrom the description above, their relative abundances vary with time within the different volcaniccenters. The initiation of the mafic alkaline magmatic activity in the South-Eastern Alps precededby only a few million years the widespread calc-alkaline magmatism of the whole Alpine chainthat took place along the Periadriatic Lineament at Oligocene time.In attempting to understand the petrogenesis of the Alpine magmatism, two important pointswhich must be considered are the relationship between the two contrasting magmatic series (maficalkaline and felsic calc-alcaline) and the nature of the tectonic events giving rise to them.According to some authors (Zampieri, 1995; De Vecchi and Sedea, 1995; Milani et al., 1999), themafic volcanism outcropping in the Veneto region is strictly related to the extensional tectonics thatdeveloped in the Southern Alps foreland in response to the WNW–SSE compression of the Adriaand European plates during the Alpine orogenesis. Here we propose an alternative hypothesis con-ceptualized in the light of recent geochemical investigations of the Eastern Alpine area and combinedwith the results of a new high-resolution tomographic model (Piromallo and Morelli, 2003).This paper presents new geochemical (major and trace elements) and isotopic (Sr–Nd–Hf–Pb)data for the most primitive magmas (alkaline basalts and basanites) from the Marostican, Lessi-nian, and Berici districts, as well as trachybasalts and basaltic trachyandesites from the EuganeanHills. With the help of the additional information provided by seismic tomographic profilesextracted from the south-eastern sector of the Alpine region, the geochemical and isotopic datawill be used to re-evaluate the following questions:

564 P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590

Fig. 1. (top): Sketch map of the South-Alpine foreland (Veneto region) after De Vecchi and Sedea (1995) showingsample locations: MAR (Marostican); SG (Lessinian); BE (Berici); CB (Euganean); (bottom): Sketch map of the

Alpine chain after Dal Piaz et al. (1988) displaying (in black) the main intrusive bodies that outcrop along the Peria-driatic (Insubric) Lineament. The box contoured area represents the Veneto Volcanic Province. Aa and Bb indicate thelocation of the seismic tomographic profiles in the central-eastern Alps.

P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590 565

1. What is the nature of the mantle source of the Veneto volcanics?

2. What is the relationship between the mafic alkaline magmatism of the Southalpine foreland (the Veneto region) and the felsic, calc-alkaline, mostly intrusive, Periadriatic magmatismof the central-southern Alps?

3. Which geodynamic scenario was responsible for the production of the two contrasting

(mafic alkaline and felsic calc-alkaline) magma series?

2. Sample description

Most of the mafic samples are fine-grained hypocrystalline or porphyritic basalts and basaniteswith variable amounts of glass. The samples are usually low in phenocrysts or phenocryst-free,with olivine being the main mineral phase. Olivine is present either as rounded microphenocrystsor sub-millimetric elongated sticks (magmatic olivine), or as large, zoned, occasionally corrodedand recrystallized clasts, likely inherited from mantle xenoliths and not in equilibrium with thehost magma. The groundmass is microcrystalline or glassy and includes aggregates of clinopyr-oxene, plagioclase, magnetite, and rare amphibole. The trachyandesites from the Euganean Hillsare porphyritic rocks containing phenocrysts of clinopyroxene and plagioclase, and minoramounts of opaque minerals (usually Ti-magnetite), amphibole, and biotite.

3. Geochemistry

3.1. Analytical procedures

Fifteen basaltic and basanitic rocks of different ages from the Lessini, Berici, and Marosticavolcanic centers, and two Oligocene volcanics (trachyandesites) from the Euganean Hills wereanalyzed for major and trace elements and Sr, Nd, Pb, and Hf isotopic compositions for some ofthe most representative samples.Major element concentrations (Table 1) were determined by XRF at the Pisa Earth ScienceDepartment following the analytical procedures of Franzini et al. (1975) and Leoni and Saitta(1976), while trace element analyses (Table 2) were carried out at the C.R.P.G. in Nancy (Centrede Recherches Petrographiques et Geochimiques: Service d’Analyse des Roches et des Minerauxdu C.N.R.S., Nancy, France) (Alibert et al., 1983). Precision for the trace element analyses is 5–20% at the 1 ppm level, 2–10% at the 10 ppm level, and 2–5% at the 100 ppm level. Ferrous/ferric iron ratios were determined by titration. Sr and Nd isotopic compositions (Table 3) wereanalyzed by the multi-collector Finnigan Mat 262 mass spectrometer at the I.G.G.-C.N.R. facil-ity in Pisa (Istituto Geoscienze e Georisorse, C.N.R., Pisa, Italy), following the elementalseparation procedure of Clocchiatti et al. (1994), while Hf (Table 3) and Pb (Table 4) isotopecompositions were determined by multi-collector magnetic-sector inductively-coupled-plasmamass spectrometry, using the VG model Plasma 54 in Lyon (Ecole Normale Superieure de Lyon,Lyon, France; Blichert-Toft et al., 1997; White et al., 2000). Hf and Pb chemical purificationswere done according to Blichert-Toft et al. (1997) and a slightly modified procedure of Manhes etal. (1978) and White and Dupre (1986), respectively. Total procedural blanks for Sr, Nd, Hf, and

566 P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590

Pb were better than 2 ng, 2 ng, 25 pg, and 40 pg, respectively. Sample to blank ratios for all fourelements for all samples analyzed were >>1000. Uncertainties reported on Sr, Nd, and Hfmeasured isotope ratios are 2�/n1/2 analytical errors in the last decimal place, where n is thenumber of measured isotope ratios. 87Sr/86Sr of the NBS-987 Sr standard=0.710226�12 (n=11);143Nd/144Nd of the La Jolla Nd standard=0.511855�5 (n=12); 176Hf/177Hf of the JMC-475 Hfstandard=0.282160�10 (Hf standard run every second sample and before and after first and lastsample). 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf were normalized for mass fractionation relativeto 86Sr/88Sr=0.1194, 146Nd/144Nd=0.7219, and 179Hf/177Hf=0.7325, respectively. Additionaldetails on the Pb isotope analytical technique are given in White et al. (2000) and Marechal et al.

Table 1Major element concentrations for Veneto volcanics

Sample

Locality Lithotype SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K20 P2O5 LOI Tot Mg#

MAR1

Marostica Basanite 42.83 3.07 13.05 2.98 7.66 0.17 11.13 10.66 2.90 1.78 1.07 1.86 99.16 65.73

MAR2

Marostica Basanite 43.34 3.03 13.05 2.95 7.60 0.16 11.06 10.72 3.26 1.41 1.06 1.52 99.16 65.78

MAR3

Marostica Basanite 43.10 3.07 13.04 2.90 7.79 0.17 11.19 10.61 3.02 1.78 1.03 1.44 99.14 65.73

MAR4

Marostica Basanite 42.87 3.04 12.66 2.86 7.80 0.17 12.60 10.42 3.01 1.82 0.83 1.05 99.13 68.40

SG1

Lessini Basanite 41.81 2.50 13.36 2.06 7.69 0.16 16.33 8.63 3.33 1.24 1.11 0.94 99.16 75.31

SG2

Lessini Basanite 43.39 2.63 14.37 2.79 8.12 0.18 10.96 9.64 2.88 1.60 1.24 1.29 99.09 64.76

SG3

Lessini Basanite 43.03 2.82 14.22 2.77 8.10 0.17 11.64 9.24 3.29 1.73 1.20 0.89 99.10 66.20

SG4

Lessini Basanite 42.83 2.63 13.97 2.38 8.03 0.17 13.68 8.66 3.26 1.61 1.20 0.70 99.12 70.56

SG5

Lessini Basalt 44.69 3.05 14.91 2.87 8.03 0.16 9.80 9.22 3.10 1.55 1.27 0.46 99.11 62.21

SG6

Lessini Basalt 44.93 2.99 15.11 2.37 8.32 0.16 9.82 8.94 3.05 1.52 1.28 0.58 99.07 62.61

SG7

Lessini Basalt 45.07 3.09 15.02 2.97 8.07 0.16 9.20 9.15 3.10 1.54 1.30 0.43 99.10 60.42

SG8

Lessini Basalt 44.59 3.00 14.77 2.66 8.36 0.16 9.87 9.11 2.93 1.57 1.25 0.80 99.07 62.06

BE1a

Berici Basalt 42.24 2.22 13.79 4.36 6.73 0.17 8.59 10.95 2.11 0.92 0.65 5.44 98.17 58.97

BE2a

Berici Basalt 43.98 2.28 14.33 3.85 7.10 0.15 10.03 9.66 1.93 1.25 0.66 4.38 99.60 62.86

BE3a

Berici Basalt 44.74 2.33 14.03 4.47 6.57 0.16 9.21 10.21 2.31 1.30 0.71 3.66 99.70 60.78

BE4a

Berici Basanite 41.11 2.68 13.50 4.72 6.98 0.18 10.78 11.47 2.51 1.50 1.10 3.07 99.60 63.12

BE5a

Berici Basalt 42.56 2.60 12.30 5.70 6.11 0.12 12.30 12.20 1.16 0.32 1.21 3.20 99.78 66.11

BE6a

Berici Basanite 41.21 2.59 13.20 3.30 7.99 0.18 10.31 11.47 2.55 1.03 1.13 4.67 99.63 62.64

BE12a

Berici Basalt 45.60 1.97 13.69 5.52 6.60 0.13 9.29 9.82 2.66 1.48 0.50 2.93 100.19 58.87

BE13a

Berici Basalt 46.93 1.84 12.31 7.86 6.06 0.13 7.92 8.54 3.72 0.93 0.71 2.54 99.49 51.81

BE15a

Berici Tephrite 45.21 2.85 14.95 2.72 8.45 0.17 7.61 8.77 3.81 2.20 0.81 2.01 99.56 55.45

B8907b

Berici Basanite 41.57 2.89 12.72 4.95 7.10 0.18 10.04 10.80 3.22 0.58 1.20 3.00 98.25 60.77

B8910b

Berici Basalt 40.99 2.67 12.33 5.39 5.65 0.14 9.09 14.50 1.77 0.44 1.05 3.97 97.99 60.68

B8905b

Berici Basalt 45.04 2.65 13.54 4.06 7.30 0.16 8.77 9.52 2.77 1.56 1.11 2.17 98.65 58.80

B8908b

Berici Basalt 45.06 2.27 13.65 3.47 7.55 0.16 9.21 9.59 2.15 1.32 0.68 3.23 98.34 60.60

VIZ3b

Berici Basalt-

trachyandesite

46.70

2.80 14.50 3.59 7.55 0.16 7.09 7.95 3.50 1.90 0.85 1.99 98.58 53.97

CB1

Euganei Trachyandesite 56.69 1.78 16.23 2.75 4.35 0.11 4.01 5.30 4.13 2.67 0.89 0.60 99.51 51.16

CB2

Euganei Trachyandesite 56.55 1.84 16.27 2.55 4.69 0.11 3.81 5.45 4.06 2.58 0.89 0.68 99.48 49.30

E8964b

Euganei Basalt 50.26 2.35 14.78 4.46 5.25 0.13 5.52 6.98 3.71 2.33 0.83 2.00 98.60 51.51

E8926b

Euganei Basalt 51.30 2.02 15.24 2.51 6.80 0.14 5.38 7.19 3.84 1.82 0.58 0.85 97.67 51.42

E8948b

Euganei Basalt 52.58 2.07 14.46 4.18 5.45 0.11 5.33 7.72 3.30 1.00 0.28 2.18 98.66 50.77

CN4Ab

Euganei Basalt 53.70 2.12 14.80 10.50 0.12 5.86 8.23 2.79 0.56 0.27 1.48 100.43

E8950b

Euganei Basalt 51.44 1.99 14.46 4.01 6.00 0.13 6.13 8.06 3.32 0.93 0.31 2.15 98.93 53.21

E8947b

Euganei Basalt 51.03 1.97 14.02 5.07 4.75 0.09 5.80 8.13 3.10 0.81 0.24 2.49 97.50 52.61

VO8b

Euganei Basalt 49.50 3.13 13.24 11.50 0.13 6.68 9.35 2.46 1.52 0.64 1.09 99.24

a Umpublished data from De Vecchi.b Data from Tuchschmid (1992).

P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590 567

568

Table 2Trace element concentrations for Veneto volcanics

Sample Locality Lithotype Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Ti Dy Y Ho Yb Lu Ni Cr

MAR1 Marostica Basanite 0.79 43.86 597 8.21 2.03 76.16 5.04 14 778 58.14 114.57 3.99 12.92 954 4669 49.94 264 5.49 9.23 2.76 18 405 5.85 25.50 0.95 1.79 0.25 197 437

MAR2 Marostica Basanite 0.76 37.35 599 7.33 2.00 72.42 4.87 11 706 55.66 107.84 4.10 12.81 1047 4626 50.78 268 6.32 9.60 3.09 18 165 5.31 26.04 0.92 1.60 0.23 207 447

MAR3 Marostica Basanite 0.61 44.35 604 7.42 1.99 72.92 4.98 14 778 56.93 111.79 3.70 12.59 942 4495 51.04 261 6.22 9.66 3.01 18 405 5.36 25.23 0.89 1.71 0.26 216 445

MAR4 Marostica Basanite 0.66 42.81 580 7.47 1.96 72.42 4.87 15 110 58.18 109.99 3.78 13.37 816 3622 51.14 260 6.36 9.78 3.07 18 225 5.67 25.30 0.95 1.64 0.26 221 473

SG1 Lessini Basanite 0.74 27.32 785 9.97 2.55 59.95 3.85 10 294 66.46 125.32 11.62 14.14 972 4844 54.25 240 5.00 10.57 3.23 14 988 5.83 28.24 1.06 2.03 0.29 242 340

SG2 Lessini Basanite 0.63 36.07 756 10.20 2.39 57.51 3.69 13 283 63.38 120.34 15.05 13.35 904 5411 53.36 226 4.77 9.91 2.97 15 767 6.02 27.21 1.02 2.08 0.30 212 294

SG3 Lessini Basanite 0.60 42.48 770 10.80 2.67 63.42 4.18 14 362 67.29 129.38 9.55 14.52 930 5237 57.46 263 5.70 10.80 3.27 16 906 6.44 29.51 1.03 2.09 0.29 224 284

SG4 Lessini Basanite 0.58 37.10 735 9.83 2.43 63.05 3.98 13 366 63.82 118.37 9.57 13.82 949 5237 53.76 255 5.21 10.45 3.11 15 767 6.11 28.97 1.02 2.03 0.29 271 303

SG5 Lessini Basalt 0.37 32.12 471 5.97 1.49 55.23 3.30 12 868 52.73 104.53 2.90 12.13 858 5542 46.18 235 4.67 9.29 3.09 18 285 6.16 28.11 0.99 2.08 0.28 168 294

SG6 Lessini Basalt 0.37 28.88 458 4.81 1.29 51.66 3.04 12 619 48.41 96.72 2.94 10.95 826 5586 43.89 232 4.36 8.96 3.04 17 925 6.19 28.47 0.93 1.80 0.27 166 269

SG7 Lessini Basalt 0.36 30.63 456 5.03 1.39 50.27 2.97 12 785 49.63 98.42 4.55 11.49 836 5673 44.18 223 4.63 9.42 2.97 18 525 5.87 27.80 0.98 1.86 0.25 159 254

SG8 Lessini Basalt 0.46 31.07 464 5.75 1.61 51.03 3.27 13 034 51.19 103.43 3.39 11.57 856 5455 46.33 218 4.90 9.97 3.12 17 985 6.22 27.76 1.06 1.97 0.28 171 273

BE12 Berici Basanite 0.14 26.41 437 3.38 0.74 37.93 2.37 12 287 28.82 58.33 2.29 6.75 667 2182 29.09 158 4.30 6.17 1.99 11 810 4.51 23.95 0.86 1.73 0.26

BE13 Berici Basalt 0.15 10.05 256 2.76 0.68 23.09 1.42 7721 19.46 40.75 2.27 4.94 399 3098 22.24 138 3.97 5.16 1.73 11 031 4.24 22.66 0.81 1.64 0.24

BE4 Berici Basanite 0.43 29.00 588 5.37 1.35 60.67 3.58 12 453 54.25 110.02 2.24 12.82 857 4800 54.17 217 5.59 10.19 3.20 16 067 5.93 30.01 1.08 1.94 0.29

B8907a Berici Basanite 7.68 518 5.89 1.57 70.60 10 045 60.90 120.00 13.70 739 5280 54.40 228 5.95 10.60 4.15 15 587 6.00 28.50 1.02 1.32 0.18 183 237

B8910a Berici Basalt 12.20 421 4.33 2.09 64.10 9381 57.90 112.00 12.80 737 4931 50.90 259 6.34 10.20 3.89 15 527 5.70 27.60 0.99 1.38 0.18 163 235

B8905a Berici Basalt 31.00 850 5.39 1.09 80.60 4151 62.50 116.00 13.60 1386 2182 53.50 467 5.78 10.80 4.17 11 810 5.71 25.30 1.01 1.36 0.18 146 241

B8908a Berici Basalt 32.70 367 4.40 0.94 44.60 5894 38.20 67.60 8.83 604 3098 37.20 160 4.16 7.95 2.71 11 031 5.25 25.90 0.96 1.90 0.26 191 240

VIZ3a Berici Basalt 43.80 428 6.28 1.59 70.00 6725 50.90 96.10 11.90 811 3535 49.30 290 8.07 10.50 3.58 17 086 6.62 29.20 1.15 1.95 0.27 135 170

CB 1 Euganei Trach. 1.95 78.88 622 12.46 3.63 66.37 5.14 22 166 57.87 111.40 6.69 12.41 749 3884 48.04 372 8.46 9.61 2.90 10 671 5.60 24.03 0.94 1.77 0.25 41 53

CB 2 Euganei Trach. 1.84 77.63 576 11.94 3.59 63.52 5.02 21 419 54.17 108.01 5.33 12.09 745 3884 47.65 358 8.30 9.50 2.81 11 031 5.11 24.00 0.91 1.80 0.26 42 59

E8964a Euganei Basalt 61.40 473 7.28 1.89 67.00 8717 56.00 98.80 11.50 744 4582 45.00 304 7.94 8.84 2.99 16 007 5.39 26.40 0.94 1.78 0.26 84 142

E8926a Euganei Basalt 48.20 433 5.79 1.49 47.60 9215 43.30 83.10 9.47 566 4844 37.40 246 6.78 8.05 2.65 15 887 5.22 25.90 0.93 1.77 0.27 98 136

E8948a Euganei Basalt 30.40 166 3.36 0.77 22.20 5645 16.10 30.60 3.96 358 2968 18.80 153 4.50 5.52 2.07 13 609 5.12 25.00 0.93 1.68 0.26 129 201

CN4Aa Euganei Basalt 16.50 117 3.01 0.70 18.60 7057 13.50 26.40 3.57 327 3709 17.50 136 4.20 4.86 1.83 16 786 4.66 22.00 0.83 1.43 0.24 128 191

E8950a Euganei Basalt 13.50 240 3.81 1.12 29.60 6891 17.50 34.90 4.28 428 3622 20.40 174 4.89 5.51 2.03 14 088 4.33 23.00 0.77 1.43 0.22 173 250

E8947a Euganei Basalt 21.00 163 2.79 0.87 17.40 4815 12.80 26.20 3.46 377 2531 17.00 142 4.08 5.16 1.96 12 110 4.61 22.40 2.27 1.49 0.23 191 225

VO8a Euganei Basalt 43.50 353 4.64 1.37 40.80 2325 34.30 67.30 9.04 654 1222 38.40 245 6.21 9.12 2.90 12 410 5.74 28.60 0.97 1.79 0.26 211 331

Trach., trachyandesite.a Data from Tuchschmid (1992).

P.Macera

etal./

JournalofGeodynamics

36(2003)563–590

Table 3Sr–Nd–Hf isotopic compositions of Veneto volcanics

Sample Locality Lithotype Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr�2s IRSr48Ma Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd�2s IRNd48Ma ENd48Ma 176Hf/177Hf EHf48MaSG1 Lessini Basanite 27.32 971.6 0.081 0.703778�6 0.70372�1 10.57 48.04 0.133 0.512850�15 0.512809 4.52�0.30 0.283001�7 8.10

SG2 Lessini Basanite 36.07 903.8 0.115 0.703859�6 0.70378�1 9.91 53.36 0.112 0.512849�7 0.512814 4.62�0.14

SG3 Lessini Basanite 42.48 930.5 0.132 0.703747�7 0.70366�1 10.8 57.46 0.114 0.512852�6 0.512817 4.67�0.13

SG4 Lessini Basanite 37.10 949.3 0.113 0.703753�8 0.70368�1 9.29 53.76 0.105 0.512863�5 0.512830 4.95�0.11

SG5 Lessini Basalt 32.12 858.0 0.108 0.703281�6 0.70321�1 9.91 46.18 0.130 0.512947�5 0.512906 6.44�0.11 0.28299�3 7.71

SG6 Lessini Basalt 28.88 826.2 0.101 0.703209�8 0.70314�1 8.96 43.89 0.123 0.512946�8 0.512907 6.46�0.16

SG8 Lessini Basalt 31.07 855.7 0.105 0.703215�8 0.70314�1 9.97 46.33 0.130 0.512944�11 0.512903 6.38�0.21

Sample Locality Lithotype Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr�2s IRSr33Ma Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd�2s IRNd33Ma ENd33Ma 176Hf/177Hf EHf33MaMAR1 Marostica Basanite 43.86 953.8 0.133 0.703459�5 0.70339�1 9.23 49.94 0.112 0.512932�6 0.512908 6.09�0.12 0.282952�11 6.37

MAR2 Marostica Basanite 37.35 1047.4 0.103 0.703521�6 0.70347�1 9.60 50.78 0.114 0.512931�5 0.512906 6.06�0.10

MAR3 Marostica Basanite 44.35 942.0 0.136 0.703448�9 0.70338�1 9.66 51.04 0.114 0.512934�5 0.512909 6.14�0.10

MAR4 Marostica Basanite 42.81 815.9 0.152 0.703435�8 0.70336�1 9.78 51.14 0.116 0.512896�13 0.512871 5.37�0.26 0.282969�4 6.97

Sample Locality Lithotype Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr�2s IRSr40Ma Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd�2s IRNd40Ma ENd40Ma 176Hf/177Hf EHf40MaCB1 Euganei Trach. 78.88 749.4 0.304 0.703687�8 0.70354�1 9.61 48.04 0.121 0.512880�7 0.512848 5.11�0.14 0.282974�3 7.14

CB2 Euganei Trach. 77.63 745.4 0.301 0.703614�6 0.70344�1 9.50 47.65 0.121 0.512873�6 0.512841 4.97�0.12

Sample Locality Lithotype Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr�2s IRSr48Ma Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd�2s IRNd48Ma ENd48Ma 176Hf/177Hf EHf48MaBE4 Berici Basanite 29.00 857.4 0.098 0.703125�7 0.70306�1 10.20 54.17 0.114 0.512958�7 0.512922 6.75�0.15

Sample Locality Lithotype Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr�2s IRSr33Ma Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd�2s IRNd33Ma ENd33Ma 176Hf/177Hf EHf33MaBE12 Berici Basanite 26.41 667.7 0.115 0.703468�5 0.70341�1 6.18 29.09 0.128 0.512914�7 0.512886 5.67�0.14

BE13 Berici Basalt 10.05 398.9 0.073 0.703520�6 0.70349�1 5.16 22.24 0.140 0.512823�10 0.512793 3.85�0.20

Trach., trachyandesite.

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Table 4Pb isotopic compositions on Veneto volcanics

Sample Locality Lithotype U

(ppm)

Th

(ppm)

Pb

(ppm)

238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)48Ma (207Pb/204Pb)48Ma (208Pb/204Pb)48Ma

SG1 Lessini Basanite 2.55 9.97 11.62 14.142 0.103 57.134 18.808 15.637 38.878

SG2 Lessini Basanite 2.39 10.20 15.05 10.216 0.074 45.052 18.786 15.632 38.836

SG4 Lessini Basanite 2.43 9.83 9.57 16.396 0.119 68.536 18.856 15.648 38.925

SG5 Lessini Basalt 1.49 5.97 2.90 33.357 0.242 138.106 19.138 15.595 38.807

SG6 Lessini Basalt 1.29 4.81 2.94 28.443 0.204 109.587 19.111 15.604 38.814

Sample Locality Lithotype U Th Pb 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)33Ma (207Pb/204Pb)33Ma (208Pb/204Pb)33Ma

MAR1 Marostica Basanite 2.03 8.21 3.99 33.403 0.242 139.592 19.574 15.643 39.350

MAR4 Marostica Basanite 1.96 7.47 3.78 34.036 0.247 134.039 19.573 15.636 39.347

Sample Locality Lithotype U Th Pb 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)40Ma (207Pb/204Pb)40Ma (208Pb/204Pb)40Ma

CB1 Euganei Trachyandesite 3.63 12.46 6.69 35.271 0.256 125.101 19.077 15.641 39.025

CB2 Euganei Trachyandesite 3.59 11.94 5.33 43.767 0.317 150.415 19.020 15.633 38.958

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(1999). �Nd and �Hf values were calculated using143Nd/144NdCHUR(0)=0.512638 and

176Hf/177HfCHUR(0)=0.282772, respectively.

3.2. Geochemical data

Although the Tertiary magmatism of the Veneto region gave rise to both tholeiitic, transitional,and alkaline volcanic products, we here focus exclusively on the alkaline volcanic rocks and, inparticular, the most primitive among them, from each of the four volcanic centers describedabove. The aim is to establish the nature and composition of the mantle source of the maficalkaline magmas. The geochemical features of the felsic calc-alkaline rocks outcropping along thePeriadriatic Lineament have been described in Macera et al. (2003) and will not be repeated here.Major and trace element data for the mafic volcanic rocks from the main eruptive centers of theVeneto region are reported in Tables 1 and 2, respectively. Except for the Euganean and some ofthe Berici samples, which display relatively evolved compositions (basaltic andesite and tra-chyandesite), the remaining volcanics are basalts, trachybasalts, and basanites, many of whichshow the typical features of primary magmas, i.e., high Mg# (550), low FeOT/MgO (usually<1.6), low Al2O3 (<15 wt.%), and relatively high Ni (160–270 ppm) and Cr (250–470 ppm)contents. In contrast to the mafic magmas from the nearby Periadriatic Lineament, which areoften enriched in K2O (Macera et al., 2003, and references therein), most of the samples analyzedhere have alkaline affinities with Na2O>K2O (usually Na2O/K2O>1.5) and low SiO2 (<45wt.%) contents, features they share with some OIBs (Oceanic Island Basalts; Chauvel et al., 1992;Kogiso et al., 1997). In primitive mantle-normalized spiderdiagrams (normalization values fromGERM, http://www.earthref.org/, Fig. 2), the Veneto volcanics show a progressive (from rightto left) increase of the most incompatible trace elements up to Ta. From Nb to Cs, the nor-malized abundances decrease progressively, defining a bell-shaped trend in the left corner of thediagram. The leftward descending path of the most highly incompatible elements is typical ofdepleted mantle-derived magmas, i.e., mid-ocean-ridge basalts (MORB) and HIMU basalts(High �=high 238U/204Pb; Hofmann, 1997). In addition, most of these samples show a deple-tion in Rb and K (Fig. 2a). Both of these features are specific to HIMU magmas (Weaver, 1991;Kogiso et al., 1997). Even if some continental basalts may occasionally display negative Rb andK anomalies in primitive mantle-normalized spiderdiagrams, they never show a bell-shapedpattern in the left corner of the diagram, and thus are not akin to Veneto volcanics and HIMUbasalts. By contrast, the Lessinian basanites clearly stand out from the general pattern (Fig. 2b)because of their higher concentrations of the most incompatible trace elements from Pb toCs, their positive spikes for Pb, and their slightly negative anomalies for Nb, features that allsignal a contribution of a crustal component by some contamination process to either theirmantle source or to the magmas themselves while en route to the surface. The rare earthelements (REE) are uniformly distributed within the different volcanic centers, show stronglyfractionated patterns with (La/Yb)n=22–25, and lack Eu anomalies (Eu/Eu*=0.94–1.06).Despite these overall first-order similarities between the various primitive Veneto magmas, theynevertheless differ from each other in terms of several critical major and trace element ratios. TheMarostican and Berici basanites are characterized by higher CaO/Al2O3 ratios than Lessinianand Euganean volcanics at a given FeOT/MgO ratio. The CaO/Al2O3 and FeOT/MgO ratios areinversely correlated only for the Berici and Euganean basalts (Fig. 3), indicating that clinopyr-

P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590 571

Fig. 2. Primitive mantle-normalized (GERM, http://www.earthref.org/) incompatible element-spider diagrams for the

Veneto Volcanic Province: (a) Lessinian basalts and Marostican basanites, and (b) Lessinian basanites, Euganean, andBerici basalts. These patterns show a progressive increase in the abundances of the most incompatible elements up toTa. By contrast, the normalized abundances decrease gradually from Nb to Cs, with negative Rb and K anomalies,

similar to the pattern of OIB-HIMU magmas. Despite their similar geochemical compositions, the purest HIMU sig-nature is shown by the Marostican basanites and Lessianian basalts (a), whereas the Lessinian basanites and Euga-nean, and Berici basalts record a more diluted character, particularly evident in the former (e.g. positive Pb anomaly).

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oxene is an important fractionating phase in these volcanic centers. Furthermore, Ni and Crdecrease with increasing FeOT/MgO for Lessini, Berici, and Euganean basalts, suggesting thatcombined olivine and clinopyroxene fractionation is responsible for the chemical variationobserved in these basalts. On the other hand, alkaline basalts and basanites from the variousVeneto volcanic centers are characterized by significantly different highly incompatible trace ele-ment ratios (Ce/Pb, Nd/Pb, Ba/Nb, La/Nb, Nb/U, and Th/La) that reflect different or hetero-geneous mantle sources. In fact, many trace element ratios, such as Ba/Nb, Ce/Pb, and Nd/Pb(Fig. 4), in the Veneto volcanics signal the presence of a DM-HIMU-type mantle for the Mar-ostican basanites, the Lessinian, and some of the Berici basalts, whereas the Lessinian basanitesand Euganean basalts show Nb/U and Nd/Pb ratios that are shifted towards the mean values(Nb/Uffi 8.45; Nd/Pb=1.59) of the continental crust (GERM, http://www.earthref.org), com-parable to those of EMII lavas from the Society Islands (White and Duncan, 1996).The crustal component can be identified by the help of geochemical tracers, such as the Ce/Pband Hf/Sm ratios, that are useful indicators of the presence of sediments in the source of OIBs(White and Duncan, 1996; Blichert-Toft et al., 1999a; Gasperini et al., 2002). While the Ce/Pbratio is capable of revealing the presence of sediments in the mantle source of hotspot basalts, theHf/Sm ratio is capable of discriminating between the type of sediment involved, i.e., pelagic vs

Fig. 3. FeOT/MgO vs CaO/Al2O3 ratios for Veneto volcanics. Clinopyroxene fractionation is recorded only by theBerici and Euganean basalts.

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terrigenous. Whereas oceanic basalts (MORB and OIB) are characterized by virtually constantHf/Sm with a value similar to that of chondrites of �0.7 (Blichert-Toft and Albarede, 1999; Bli-chert-Toft et al., 1999b; Albarede and Blichert-Toft, 2000; Chauvel and Blichert-Toft, 2001),sediments display a large variation, with terrigenous sediments showing higher-than-chondriticHf/Sm (due to an excess of zircons) and pelagic sediments showing lower-than-chondritic Hf/Sm(due to overall lack of zircons) (Albarede and Blichert-Toft, 2000; Gasperini et al., 2002). InFig. 5, where the Hf/Sm ratio is plotted against the 147Sm/144Nd ratio, some of the Berici andEuganean volcanics are seen to plot above the chondritic value, towards the field of terrigenoussediments. Other Euganean basalts plot closer to or within the mantle array. By contrast, Lessi-nian basanites fall below the chondritic value, along with the rest of the Veneto volcanics andHIMU-type lavas, within the field of pelagic sediments. These characteristics indicate that thenature of the crustal component in the Veneto basalts differs for different volcanic centers, somecenters being affected by terrigenous sediments (e.g., Berici and Euganean volcanics) and others(e.g., Lessinian basanites) by pelagic sediments.

3.3. Isotopic data

Sr, Nd, Hf, and Pb isotopic data are listed in Tables 3 and 4 and plotted in Fig. 6 along withdata from the literature. Since the samples have different ages, from 48 Ma to 33 Ma (Barbieri etal., 1991; Borsi et al., 1969), the isotopic ratios have been back-calculated to their respectiveeruption ages. All the analyzed samples have Sr, Nd, and Hf initial isotopic compositions whichare, respectively, lower, higher, and higher than bulk silicate earth, reflecting their origin from atime-integrated depleted mantle source. Whereas the Lessinian basalts, on average, show thelowest initial Sr isotopic compositions (87Sr/86Sr48Ma=0.70314–0.70321) and the highest initial�Nd values (�Nd48Ma=+6.4 to +6.5), the Lessinian basanites are characterized by the highestinitial Sr isotopic compositions (87Sr/86Sr48Ma=0.70366–0.70378) and the lowest initial �Nd values(�Nd48Ma=+4.5 to +5.0). The mafic rocks from Berici show a relatively wide range of initial Srand Nd isotopic compositions (87Sr/86Sr=0.70306–0.70349; �Nd=+3.9 to +6.8), whereas theMarostican basanites display rather uniform Sr and Nd isotopic compositions(87Sr/86Sr33Ma=0.70336–0.70347; �Nd33Ma=+5.4 to +6.1), similar to those of the Euganeantrachybasalts (87Sr/86Sr40Ma=0.70344–0.70354; �Nd40Ma=+5.0 to +5.1).The initial Pb isotopic compositions of the volcanic rocks show variable 206Pb/204Pb (18.786–19.574) and 208Pb/204Pb (38.836–39.350) at almost constant 207Pb/204Pb (15.595–15.648) with theMarostican basanites having the most radiogenic values. Only five samples among the Venetovolcanics dicussed here were analyzed for Hf isotopes. The most depleted isotopic signature wasfound for the Lessinian basanites (�Hf48Ma=+8.1), whereas less depleted compositions werefound for the Marostican lavas (�Hf33Ma=+6.4).In Sr–Nd isotope space (Fig. 6a), the Veneto volcanics plot in the depleted mantle field betweenHIMU lavas (Tubuai, Mangaia, Rurutu, and St. Helena) and most other ocean island basalts(http://petdb.Ideo.columbia.edu/petdb/ and courtesy A.W. Hofmann). The HIMU character-istics of the analyzed magmas are further accentuated by their Hf and Pb isotopic compositions.In Hf–Nd isotope space (Fig. 6b), the Veneto volcanics plot at an angle to the mantle array, akinto HIMU basalts. Though to a lesser extent, Pb isotopes also confirm a HIMU fingerprint in theVeneto lavas, in that these rocks are characterized by relatively high 206Pb/204Pb (generally519)

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Fig. 4. (a) Mg# vs Ba/Nb ratios, (b) Ce vs Ce/Pb ratios, and (c) Nb/U vs Nd/Pb ratios for Veneto volcanics along with

data for DM (Depleted Mantle), HIMU (high �=high 238U/204Pb), EMI (Enriched Mantle I), and EMII (EnrichedMantle II). Data are from http://petdb.Ideo.columbia.edu/petdb/; http://georoc.mpch-mainz.gwdg.de/Start.asp; Wood-head and McCulloch, 1989; Gasperini et al., 2000, 2002). Data for the continental crust (C.C.) are from GERM (http://

www.earthref.org/).

P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590 575

and 208Pb/204Pb (39–39.5) and intermediate 207Pb/204Pb (15.60–15.65). The fact that Pb in theVeneto volcanics is not as radiogenic as in typical ‘‘pure’’ HIMU lavas suggests either mixing ofthe HIMU component with some other material or a younger than 2 Ga HIMU source. Ifassuming the former, the position of the Veneto lavas in Fig. 6c (206Pb/204Pb vs 208Pb/208Pb) and6d (206Pb/204Pb vs 207Pb/204Pb) on the mantle array defined by MORB and OIB, relatively closeto the HIMU end-member, indicates that the most primitive material (e.g. Marostican basanitesand Lessinian basalts) with which the HIMU component mixed was depleted mantle (DM)material. However, although the general isotopic features of these lavas point to a HIMU mantlereservoir variably diluted by MORB-type material, the small but significant shift of some of thesesamples towards more crustal isotopic compositions also calls for a contribution of a crustalcomponent to the source of some of the Veneto volcanics, or, alternatively, crustal contaminationof the rising mafic magmas. The lack of correlation between isotopic and chemical variables (e.g.SiO2) of the analyzed magmas favors the first hypothesis.

Fig. 5. 147Sm/144Nd vs Hf/Sm ratios for Veneto volcanics. Data for sediments, HIMU, DM, EMI, and EMII, and themantle array are from numerous literature sources (http://petdb.Ideo.columbia.edu/petdb/; White and Hofmann, 1982;

and courtesy J. Vervoort). CHUR: Chondritic Uniform Reservoir, Hf/Sm=0.7. Most of the samples fall within thefield of HIMU rocks. The Euganean and a few Berici basalts plot above the chondritic value and within the field ofterrigenous sediments, whereas the Lessinian basanites fall below the field for HIMU, in the field of pelagic sediments.

576 P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590

Fig. 6. (a) 87Sr/86Sr vs 143Nd/144Nd, (b) �Nd vs �Hf, (c)206Pb/204Pb vs 208Pb/204Pb, and (d) 206Pb/204Pb vs 207Pb/204Pb

for Veneto volcanics. The mantle array and the DM, HIMU, EMI, and EMII fields are based on numerous literaturedata (http://petdb.Ideo.columbia.edu/petdb/; http://georoc.mpch-mainz.gwdg.de/Start.asp; Hofmann, 1997; Gasperiniet al., 2002; and courtesy A.W. Hofmann). The isotopic characteristics of the Veneto volcanics, particularly evident in

Sr–Nd isotope space (a), signal the presence of a HIMU component in their source.

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Fig. 6. (continued)

578 P. Macera et al. / Journal of Geodynamics 36 (2003) 563–590

4. Seismic tomography

4.1. Imaging procedures

The detection through tomography of cold subducted lithosphere or upwellings of hot material,imaged as high- or low-seismic velocity regions, respectively, provides a useful contribution to theinvestigation of the geodynamic environment characteristic of a given area. We focus here on alimited region of the recent high-resolution tomographic model for the whole Euro-Mediterra-nean area proposed by Piromallo and Morelli (2003). The model (PM0.5) is obtained through theinversion of a large dataset of P-wave delay times provided by the International SeismologicalCenter (1964–1995). After accurate data selection and relocation, summary residuals of bothregional (epicentral distance �428�) and teleseismic (�528�) rays from shallow earthquakes(focal depth h 450 km) are used as input data to the inversion (Piromallo and Morelli, 1997,2003). The velocity model is parameterized with linear splines over a three-dimensional cartesiangrid, with horizontal node spacing of roughly 55 km in both directions and 50 km vertical spac-ing, down to 1000 km depth. The perturbation to the velocity field is computed and displayedwith respect to the global reference velocity model sp6 (Morelli and Dziewonski, 1993), and a 1Dray-tracing algorithm in the background model is adopted to solve the forward problem. Theinverse problem is solved by least-squares minimization using the iterative LSQR method (Nolet,1987) combined with the additional constraint of an explicit roughness damping.The actual resolution of the retrieved velocity model can be hampered by several factors, suchas the limitation of sampling in the region of interest, lesser data quality, and the way in whichthe inversion is implemented. A common, though not exhaustive, way to assess the reliability of amodel is through sensitivity analyses performed by means of synthetic tests (i.e. Spakman andNolet, 1988; van der Hilst and Engdahl, 1991; Leveque et al., 1993). Such analyses address thespatial resolution of the inversion scheme in terms of its efficiency in retrieving an input model(exactly known in spatial location, geometrical shape, and amplitude of anomalies), given thesame ray coverage and technique which will be used for the real data inversion. These tests, per-formed with input velocity anomalies of different spatial wavelength and shape, indicate thatstructures with spatial extent ranging from �100 to �600 km are reasonably well resolved overlarge parts of the inversion domain. In the best sampled regions, features of sizes smaller than 100km (and larger than 55 km, the nominal resolution) can also be detected. However, small-scaledetails may be unstable in the inversion and care must be taken to avoid overinterpretation of theimaged features (Piromallo and Morelli, 2003). Crustal structure is not modeled and thereforepoorly or not at all resolved, implying that features at crustal depths may give unreliable results.A thorough description of the tomographic model used below is given in Piromallo and Morelli(2003).

4.2. Velocity model

Overall, the distribution of deep, large-scale velocity anomalies in the Alpine-Adriatic areasuggests the presence of seismically slow material in the mantle above the transition zone belowthe Adriatic plate, bordered by fast material related to (i) the Apenninic subduction to the west,(ii) the Dinaric subduction to the east, and (iii) the subduction of Eurasian lithosphere to the

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north. Fig. 7 shows map views of four layers (50, 100, 150, 200 km) extracted from model PM0.5.The Alpine foreland is marked by high velocities in the shallow layer at 50 km depth, while lowvelocities appear right below the belt. In the Western Alps a sharp boundary between low-velo-city heterogeneities along the northwestern border of the chain and a strong fast anomaly to theeast is detected. At deeper levels, the whole belt, from west to east, is underlain by high velocitiesin a continuous stripe down to 150 km. Moving to greater depths (200–250 km) anomaliesbecome weaker and the deep roots of the Western and Eastern Alps appear to be horizontallydisconnected from each other and from the fast anomaly below the Dinaric chain. Fast velocity

Fig. 7. Map views at 50, 100, 150, and 200 km depth of tomographic model PM0.5 (Piromallo and Morelli, 2003).

Velocity anomalies are displayed in percentages with respect to the reference model sp6 (Morelli and Dziewonski,1993). The traces of cross sections of Fig. 8 are plotted for reference.

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features rejoin at depth (starting at about 300 km) and connect to the whole Alpine fast beltanomalies (Piromallo and Morelli, 2003). A consistent slow anomaly is present from depths ofabout 100 km down to 400 km in the area of the north Adriatic, Southern Alps, and eastern PoPlain, and is slightly shifted to the north in the topmost layer at a depth of 50 km.Seismic tomographic profiles in the Eastern sector of the Alps are shown in Fig. 8 (compare toFig. 1 for the exact location of profiles in the Alpine geologic setting). The vertical cross-section

Fig. 8. Seismic tomographic profiles of PM0.5 (Piromallo and Morelli, 2003) across the Eastern Alps (left panels). Forcross-section locations refer to Fig. 7. Each cross-section has the same length (approximately ten degrees along a great

circle) and its vertical extent reaches the bottom of the model. Tick marks along the line segments indicate the distancefrom the extremities by 5� intervals. The right panels illustrate a slice, along the same trace, resulting from a three-dimensional box-car recovery test, to estimate spatial resolution. Here, input box-car heterogeneities of opposite sign

(�4%), approximately 220 km in each direction, are alternately superimposed on the ambient velocity profile. Gaus-sian random noise is added to the data. Input is shown by thin lines (positive) and dashed lines (negative), with con-touring at 1, 2 and 3%; output is shown in colors. The simulated input is well resolved in the inversion, both in sign

and shape of the anomaly, and the edges of the box-car are also reasonably well detected in the best resolved part. Atthe base of the model some smearing of the anomalies occurs.

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Aa, centered on the Veneto region, shows the fast-velocity anomaly of the European lithospheredeepening from the Eurasian foreland below the Alps, approximately down to 200 km depth, andthe shallow Adriatic lithosphere lying horizontally in the topmost 50 km. A broad, high-ampli-tude, low-velocity anomaly resides below the Adriatic Sea, extending down to 400 km (top of thetransition zone) and possibly reaching the surface north of the tomographically imaged Adriaticlithosphere. Fast-velocity material is accumulated in the transition zone. Cross-section Bb illus-trates a profile roughly transverse to the previous one. Again it is possible to identify the Eur-opean (to the north) and Adriatic (to the south) lithospheres and the slow anomaly beneath thenorth Adriatic Sea. The quasi-vertical high-velocity anomaly imaged at sub-lithospheric depths,that can be followed down to 350 km, does not show robust connections with either of the platesalong this section. On the basis of tomographic images only, it is impossible to establish its origin(Eurasian or Adriatic) unambiguously.

4.3. Comparison with other models

The large-scale difference between the deep structures of the Western and Eastern Alps, withthe western sector exhibiting a deeper, stronger, and broader high-velocity anomaly than theeastern sector, was already recognized in earlier studies of the lithosphere–asthenosphere struc-ture (i.e. Panza and Mueller, 1979; Babuska et al., 1990). During the late 1980s, the Western Alpswere the site of intensive research using seismic profiling methods, some of which formed theEuropean Geotraverse (Blundell et al., 1992), and gave impulse also to several studies concerningtheir deeper structure and evolution (see Kissling and Spakman, 1996, for a review). Tomo-graphic images of the western and central portions of the Alpine belt are consistent in their grossfeatures among various studies based on different data and techniques: a high-velocity body inthis portion of the chain, interpreted as the lithospheric root of the Alps, is detected down to 300–350 km (Spakman et al., 1993; Solarino et al., 1996; Lucente et al., 1999; Piromallo and Morelli,1997). The high-velocity structure represents the Eurasian lithosphere subducted southwardlybelow the Adriatic promontory and exhibits a clear shallow dip, particularly evident in the centraland western parts (Piromallo and Morelli, 2003).In comparison with the western sector, little is known so far about the deep structure of theEastern Alps, which also exhibit significantly different surface features. In the Eastern Alps, thefast velocity anomaly imaged by regional-scale models from surface to depth is not very pro-nounced. The high-velocity features of the Eurasian and Adriatic lithospheres both seem to beonly weakly connected with the deep-seated vertical, or slightly north-dipping, body (Amato etal., 1993; Piromallo and Morelli, 2003). While the model of Amato et al. (1993) and Lucenteet al. (1999) may have a limited resolution in this region, because it is located at theboundary of their inversion volume and makes use only of teleseismic data, imaging cap-abilities of model PM0.5, according to our tests, should not be crucially different betweenthe western and eastern arcs. Within the framework of the recent multidisciplinary projectTRANSALP (TRANSALP Working Group, 2001), focused on the eastern sector of theAlps, the high-resolution teleseismic tomographic study of Lippitsch (2002) reveals a steep,fast, slab-like body beneath the Alpine orogen in the depth range of 150–300 km, borderedby a slow anomaly southward, in good agreement with the findings of PM0.5 of comparablenominal horizontal resolution.

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5. Discussion and geodynamic implications

Whereas the distribution of high-velocity anomalies at depth below the Eastern Alpine region isunanimously recognized as relating to subducted oceanic lithosphere (i.e. Spakman et al., 1993;Amato et al., 1993; Lucente et al., 1999; Lippitsch, 2002), no consensus has been reached for theinterpretation of low-velocity anomalies. We propose here, on the basis of our geochemical andisotopic data for the Veneto volcanics, that these anomalies could have a plume-related origin.Such an origin is in general not obvious for intraplate volcanic centers with small eruptionvolumes, discontinuous magma production, and lack of well-defined hotspot tracks. However,during the last several decades, the hypothesis of the existence of several weaker and smallerplumes, in addition to major hotspot centers, has gained increasing interest (i.e. Dalrymple et al.,1987; Malamud and Turcotte, 1999; Davis et al., 2002). Moreover, studies based on tomographicand geochemical data analysis have suggested the existence of small intra-continental magmaticfields probably fed by mantle upwellings. A noteworthy example is given by the numerous Cen-ozoic volcanic centers characterizing central Europe and proposed to have originated from aseries of individual diapir-like mantle upwellings, arising either from the top of a continuous layer(fossil plume-head?) within the upper mantle (Granet et al., 1995; Hoernle et al., 1995) or from adeeper mantle reservoir (Goes et al., 1999; Ritter et al., 2001).In this context, the Veneto volcanics could represent the shallow expression of mantle upflow.In view of discussing a possible plume-related origin for the detected low-velocity heterogeneities,the temperature contrast of the anomalies should be quantified in order to verify whether it iscompatible with the geochemically favored estimates for the excess temperature of a plume,which are in the range of 75–300 K for the upper mantle according to, among others, McKenzie(1984), Sleep (1990), and Shen et al. (1996). However, providing reliable estimates of the tem-perature responsible for the observed velocity anomalies retrieved from tomographic inversions isa difficult task. On the one hand, seismic velocities are sensitive to a number of parametersbesides just temperature, such as chemical composition, anisotropy, anelasticity, phase transi-tions, presence of partial melt, and water content, and these parameters are in turn affected bytheir own large uncertainties (i.e. Karato, 1993; Goes et al., 2000). Therefore, the contribution ofvarious effects can result in a large range of variability of the temperature estimated from velocityanomalies (Allen et al., 1999). On the other hand, amplitudes of velocity anomalies as derivedfrom tomography are overall largely underestimated, due to the effect of insufficient data cover-age, smoothing and damping of the inversion scheme on the resulting model (Spakman andNolet, 1988), and wavefront healing (Wielandt, 1987; Allen et al., 1999; Nolet and Dahlen, 2000).For the velocity model adopted here, Piromallo and Morelli (2003) have demonstrated, throughsynthetic tests, that the reconstructed amplitudes can be heavily underestimated, with the outputvalue being reduced to about 60% of the input amplitude.The above considerations imply that only a rough determination of bulk temperature anoma-lies from velocity heterogeneities is possible, and these will likely yield only minimum estimates(Karato, 1993; Granet et al., 1995; Allen et al., 1999; Goes et al., 1999). Assuming that in theupper mantle the main source for lateral variation in seismic velocities is the lateral variation intemperature (Ranalli, 1996), accounting for first-order effects such as anharmonicity and anelas-ticity (Karato, 1993; Sobolev et al., 1997) and neglecting other parameters as second-order effects,we may compute the excess temperature from temperature derivatives calculated from laboratory

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experiments. Between the depths of 100 and 250 km, the velocity anomalies detected below thepresent study region are approximately 2–2.5% slower than average, implying a temperatureexcess of about 220–280 K, which is consistent with estimates for other mantle plumes.Upwelling of hot mantle material beneath the South-Eastern Alps at Tertiary time may havebeen the cause of the alkaline basaltic volcanism in the Veneto region. Though partially dilutedby a DM component, these basalts show geochemical features that are characteristic of HIMUmagmas. The existence of a HIMU-type component in the source of Tertiary and Quaternaryalkaline basalts throughout Central and Western Europe has already been recognized by a num-ber of authors (Wilson and Downes, 1991; Vaselli et al., 1995; Rosembaum et al., 1997; Jung andHoernes, 2000) and ascribed to the presence of an enriched asthenospheric reservoir (EAR) var-iously interacting with the overlying depleted lithospheric mantle. In addition, Beccaluva et al.(2000) called upon a sub-lithospheric HIMU-component to explain the isotopic characteristics ofVeneto volcanics.It is noteworthy that recent studies of Tertiary-Quaternary alkaline basalts from the easternAtlantic (Serra de Monchique-Mt. Ormonde, Madeira, Canary and Cape Verde Islands),northern Africa (Morocco, Algeria, Tunisia), and central Europe (French Massif Central,Rhon-Eifel, Austria, Poland, Czech Republic, Hungary, Italy) seem to indicate a commonmantle reservoir with a typical OIB-HIMU signature beneath the entire area (Hoernle et al.,1995; Gasperini et al., 2003; Maffei et al., 2003b, work in progress), whose tail may be locatedin the Cape Verde–Madeira–Canary Islands region. Here, the HIMU reservoir appears to bethe purest and becomes progressively more diluted by a crustal component upon going east-wards (increasing from Morocco–Algeria to Tunisia, and from the French Massif Central toPoland). Based on geochemical, geological, and geophysical data, it has been suggested(Gasperini et al., 2003) that the head of this large-scale swell has been frayed by Eurasia while itwas moving at this latitude towards the northeast, and has been trapped in the shallow mantle(above the transition zone) by subducted slabs. At different points in time, local extensional tec-tonics and slab rupture may have favored plume-related volcanism, even in areas dominated byplate convergence and subduction.Seismic tomographic profiles seem to support this view, but they only provide snapshots of thepresent-day situation, which may have been very different during the Tertiary. Currently, we see abroad, seismically fast velocity anomaly below Central and Western Europe in the transition zone(Fig. 8), that clearly could constitute an obstacle to deep mantle material upwelling (Goes et al.,1999; Ritter et al., 2001). However, if this body represents the detached slab of the Piedmontoceanic lithosphere, as suggested by Marchant and Stampfli (1995) and von Blanckenburg andDavies (1995, 1996), estimated to have begun its subduction about 65 Ma ago (Schmid et al.,1997), it is likely that 60 Ma ago, i.e., at the beginning of OIB magmatic activity in the Venetoregion, it would not yet have reached the transition zone.According to a recent model based on tectonic reconstruction, plate motion, and tomo-graphic images combined to unravel the evolution of the Alps-Apenninic slabs and theirinteraction with the alleged plume at the scale of the upper mantle (Maffei et al., 2003a,b,work in progress), it appears that the plume could have been captured and detached from itsdeeper roots by the Alpine-Apennine subducting slab during the Paleogene. The latest evolu-tionary stage of the system is manifested by the break of the Alpine slab that is envisaged to havetrapped the plume material. We propose that slab detachment and ensuing upwelling of deep

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mantle material through the lithospheric gap can be a viable mechanism for HIMU-type basaltextrusions in the South-Eastern Alps as well as in other subduction zones of the westernMediterranean area.According to Goes et al. (1999), a low-velocity structure, interpreted as lower mantle upwelling,is present between 660 and 2000 km depth under central Europe and is responsible for weak,smaller, upper-mantle plumes which have fed localized alkaline volcanism from the Tertiary up tothe present in several European centers. They suggested that mantle upwelling in these regionswas in part hindered by subduction processes, implying that the resulting volcanism could nothave the same signature as in OIB environments.On the basis of models that attribute HIMU sources to ancient recycled oceanic crust processedin the deep mantle (Kogiso et al., 1997, and references therein), we propose that the occurrence ofHIMU-type basalts of either Paleocene-Eocene or Oligo-Miocene age in the Veneto region couldbe due to the existence of a deep mantle plume in the Veneto region which, during those times,was able to rise to shallower levels than presently possible. Alternatively, the source of Venetobasalts may have been a plume head, whose tail was probably located at some far distance(Gasperini et al., 2003). Such plume-type material may have interacted with the overlying deple-ted asthenospheric mantle (DM), so as to attain the geochemical features of the EAR component.It is of fundamental importance to understand how plume material can be associated withmagmas erupted in a collisional environment, such as that of the Tertiary Alpine convergence orother similar settings (e.g. Columbia River Basalts; Hooper, 1997). One plausible explanation isthat a mantle diapir may rise to near the surface, provided there are no lithospheric obstacles todeep mantle upwelling, that is as long as the subducting slab does not intersect the upflow of hotdiapirs. Alternatively, slab detachment may create an opening to mantle upwelling in the form ofa plate window. According to this hypothesis, the oldest (60 Ma) HIMU-type basalts erupted inthe Lessinian center at Paleocene time may be associated with a mantle diapir, whereas thewidespread magmatic activity of Eocene age may be related to slab detachment. We suggest that,after the Penninic (European) slab breakoff took place, estimated at mid-Eocene time (vonBlanckenburg and Davies, 1995, 1996), blobs of lower mantle diapirs were dragged/sucked intothe lithospheric gap, upwelled towards shallower levels, and induced partial melting of both theHIMU and the surrounding asthenospheric material. Counterflows (to the downgoing movementof the descending slab) of deep mantle material may also have migrated through the lithosphericgap towards the top of the subducting slab and conductively heated the overriding lithosphericplate to the point of triggering partial melting (Albarede et al., 2000). The subsequent ascent ofthe resulting mafic magmas and their ponding at the mantle-lower crust interface, favored partialmelting of the latter and production of the tonalitic–granodioritic–trondhjemitic plutons (Maceraet al., 1998; 2003; Maffei et al., 2003c, work in progress) found throughout the Periadriatic mag-matic province. New geochemical and isotope (Sr, Nd, Pb) data on mafic and felsic rocks fromthe Adamello batholith seem to support this hypothesis (Macera et al., 2003; Maffei et al. 2003c,work in progress). In Sr–Nd and Pb–Pb isotope space (see Macera et al., 2003), the two con-trasting magmatic suites define binary mixing relationships, the end-members of which arerepresented by a lower crustal component and a HIMU-DM component (e.g. Veneto volcanicbasalts). According to this model, the OIB magmatism of the Alpine foreland would be closelyrelated to the calc-alkaline magmatism of the Periadriatic Lineament and caused by the sameTertiary Alpine convergence tectonics.

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6. Conclusions

Geochemical and Sr–Nd–Hf–Pb isotopic data for some of the most primitive Tertiary magmasfrom the Veneto region, the Lessinian, Berici, Euganean, and Marostican volcanic districts,indicate a mixed HIMU-DM mantle reservoir for most of these lavas. In addition, evidence forthe presence in this mantle source of a shallow crustal component of either pelagic (Lessinianbasanites) or terrigenous (Euganean basalts) origin was also found.Results from a recent high-resolution seismic tomographic model show a distribution ofanomalously low velocities at depth below the Eastern Alpine region, which could be ascribed todeep upwelling. The Veneto volcanics may thus be included among the numerous magmaticprovinces of Central Europe to which a possible plume-related origin has been ascribed.The observed HIMU component, the hallmark of hotspot basalts, is suggested to have risenfrom the deep mantle twice: first at Paleocene time, i.e. before the subducted European litho-spheric slab became an obstacle to deep mantle upwelling, and again during middle Eocene,through a gap created in the subducting slab by its breakoff.Vertical mixing of blobs of material from deep mantle plumes with the shallower astheno-spheric depleted mantle led to the main mantle reservoir of Veneto basalts, while lateral migra-tion of hotspot basalts along the bottom of the overriding plate may have triggered partialmelting in the lithospheric wedge, thus giving rise to the widespread calc-alkaline magmatismdeveloped along the entire Periadriatic Lineament.

Acknowledgements

We thank G. De Vecchi for advice on sample selection and for providing most of the Bericisamples, and G. Ranalli and B. Cousens for helpful and constructive reviews. C.P. acknowledgesE. Boschi for continuous encouragement and A. Morelli for contributing to the development ofthe tomographic model. E. Davini is thanked for help with supplementary sample collection. Wefurther thank P. Ulmer for a helpful review of an earlier version of this paper. This work wassupported by the Italian MIUR (Cofinanziamento-2001) and the French Institut National desSciences de l’Univers.

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