Gondwana Research 17 (2010) 44–58

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Multiple early Paleozoic volcanic events at the northern Gondwana margin:U–Pb age evidence from the Southern Variscan branch (Sardinia, Italy)

Oggiano Giacomo a,⁎, Gaggero Laura b, Funedda Antonio c, Buzzi Laura b, Tiepolo Massimo d

a Department of Botanical, Ecological and Geological Sciences, University of Sassari, Via Muroni 25, 07100, Italyb Department for the Study of the Territory and its Resources, University of Genoa, Corso Europa 26, 16132 Genoa, Italyc Department of Earth Sciences, University of Cagliari, Via Trentino 51, 09127 Cagliari, Italyd Institute of Geosciences and Georesources, CNR Pavia, Via Ferrata 1, 27100 Pavia, Italy

⁎ Corresponding author. Former Institute of GeologicUniversity of Sassari, Corso Angjoi 10, 07100 Sassari (Botanical, Ecological and Geological Sciences since 2002006628; fax: +39 079 231250.

E-mail address: giacoggi@uniss.it (G. Oggiano).

1342-937X/$ – see front matter © 2009 International Adoi:10.1016/j.gr.2009.06.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 January 2009Received in revised form 3 June 2009Accepted 3 June 2009Available online 12 June 2009

Keywords:Cambrian OrdovicianSardic phaseVolcanismU–Pb zircon geochronologyNorth Gondwana marginSardinia

In Sardinia, one of the southernmost remains of the European Variscan belt, a crustal section throughdifferent Gondwanan paleodomains is largely preserved. Laser ablation ICP-MS U–Pb ages on undoubtedlyigneous sites of the zircons were determined on the Lower Palaeozoic volcanic rocks, constrained by definedfield relationships, thus evidencing three subsequent volcanic events:

I. Intermediate and felsic (491.7±3.5 Ma÷479.9±2.1 Concordia ages) transitional volcanic rocksembedded within a Cambro–Ordovician terrigenous succession, that occurs with continuity in externaland inner nappes, bounded to the top by the Sardic unconformity.

II. This Cambrian–Lower Ordovician succession is cut by calc-alkalic rhyodacites, which yielded a Concordiaage of 465.4±1.4 Ma, confirming their pertinence to the huge, bimodal Mid-Ordovician arc volcanism,commonly interpreted as the widespread marker of the Rheic ocean subduction.

III. Alkalic metaepiclastites in the external nappe, within the post-Caradocian transgressive sequence, datedat 440±1.7 Ma, likely related to rifting and collapse of the Mid-Ordovician volcanic arc.

In the reshaped Lower Palaeozoic stratigraphy of Sardinia, the timing of the early steps of the Variscan Wilsoncycle can be inferred.

© 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Elements of the northern margin of Gondwana were incorporated inseveral segments of the European Variscides. In spite of terrane dis-ruption, the evidence of Lower Palaeozoic magmatismwithin the orogenhas been widely reported from the French, (Alexandrov et al., 2001;Guillot et al., 2002;Rogeret al., 2004;Alexandre, 2007)Spanish (Sánchez-Garcíaet al., 2003)Bohemian, CarpathianandAlpineareas (Stampfli et al.,2002; von Raumer et al., 2003; Teipel et al., 2004; Féménias et al., 2008).

Sardinia, as one of the best-exposed Variscan section in the southernEuropean Variscides, preserves several occurrences of volcano–sedi-mentary complexes, at present stacked in thenappe zone (Carmignani etal., 1994; Di Pisa et al., 1992). Throughout the Palaeozoic successions ofthe external nappes (Fig. 1; Carmignani et al., 1994) the sedimentaryrecord with its fossil content is sometimes preserved. Hence all thevolcanic products, which are associated with widespread psammitic topelitic siliciclastic successions (Fig. 2), can be placed within a sequence

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of chrono-stratigraphically verified episodes. This task grows to bedifficult where the primary relationships are obscured by polyphasedeformation and recrystallization, particularlywhere a late VariscanHT/LP metamorphism (Casini and Oggiano, 2008) affects the Palaeozoicsuccession.

Thewidespread space–time occurrence and the volumes of volcanicrocks in the different tectonic units, sometimes not associatedwith age-constrained clastic sediments, account for the interest in dating and incharacterising both sediments, as palaeoenvironmental indicators, andigneous rocks as geodynamic markers. With the aim to contribute indeciphering the Lower Palaeozoic geologic processes at the northernGondwana margin we undertook the reappraisal of the stratigraphiccontext, combined with new U–Pb in situ analyses obtained throughLA ICP-MS (Table 1), on zircons from felsic volcanic rocks.

2. Geological setting

The nappe zone of the Sardinia segment of the Variscan orogenconsists of several tectonic units, characterised by slightly differentlitshostratigraphic successions, equilibrated under greenschist facies,embricated and emplaced with a general top to south-west transport.This structural zone (Fig. 1) is the richest of the volcanic products

Published by Elsevier B.V. All rights reserved.

Fig. 1. A) Major tectonic and metamorphic zones of the Variscan basement of Sardinia. B) Geological sketch map of the Variscan basement of Sardinia, the squares indicate thelocation of Figs. 3 and 4.

45G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

(Beccaluva et al., 1981); to the north it is comprised between anallochtonous, high-grade, metamorphic complex consisting of mig-matite retaining relic granulite to HT eclogite bodies with OrdovicianMORB-like protolith (Cortesogno et al., 2004). To the south the nappestack overrides a foreland severely deformed but unmetamorphosedor weakly affected by metamorphic recrystallization. In the forelandzone the succession involved in the Variscan collision is made up ofearly Cambrian–Lower Ordovician epicontinental, terrigenous andcarbonate sediments suggesting a continental passive margin, likelypertinent to Gondwana. During the entire Middle Ordovician awidespread emergence occurred, associated with a key angularunconformity — the still debated Sardic unconformity — and withsyntectonic alluvial deposits (Martini et al., 1991).

After the Middle Ordovician, a large-scale transgression occurredat the Upper Ordovician time, and marked the onset of a new sedi-mentary cycle, through the Silurian and Devonian systems until theearly Carboniferous. Coeval and similar successions, with a compar-able unconformity, are present in the different tectonic units stackedin the nappe zone. Nevertheless these differ from the succession ofthe foreland for the widespread occurrence of volcanic rocks, likelythe markers of geodynamic steps within theWilson Cycle culminatingin the Variscan collision. Beyond the Mid-Ordovician age of the calc-alkaline metavolcanites (porphyroids), well constrained on strati-graphic bases, no definitive ages are available for the othersubvolcanic, effusive and pyroclastic rocks, which are embedded indifferent metasediments.

Fig. 2. Litho-stratigraphic sketch of the relationships within the Lower Paleozoic successions across the Variscan nappes in Sardinia. The volcanic products are highlighted (after Carmignani et al., 2001; Oggiano and Mameli, 2006, modified).

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Table 1Technical and data acquisition parameters.

Inductively coupled plasma mass spectrometry (ICP-MS)Model Element I, ThermoFinningan MatType Single-collector double-focusing magnetic sector fieldICP torch Capacitive decoupling CD-2RF power 1200 WGas flowCooling 13.5 I mm−1

Auxiliary 1.5 I mm−1

Carrier I (Ar) 1.0 I mm−1

Carrier II (He) 1.0 I mm−1

Laser ablation (LA)Model Geolas 200 Q-MicrolasType Excimer ARFWavelength 193 nmRepetition rate 5 HzMaximal pulse energy c. 0.4 mJEnergy density 12 J cm−2

Spot diameter (depth) 25 and 10 μm

Data acquisitionDetermined isotopes 202Hg, 204(Pb+Hg), 206Pb, 207Pb, 208Pb, 232Th, 235U, 238UData reduction software GLITTER (van Achterbergh et al., 2001)

47G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

2.1. The distribution of the volcanic activity in the Sardinian Variscides

The Middle Ordovician, andesite to rhyolite calc-alkaline suite isthemost extensive and best-known product of the Palaeozoic volcanicactivity. Despite the scanty availability of radiometric data, its age is

Fig. 3. Geological sketch map of the Minderrì area, in

well constrained on biostratigraphic bases for it being sandwichedbetween the Sardic unconformity at the base and theUpperOrdoviciantransgressive deposits at the top. Carmignani et al. (1994), Cappelliet al. (1992) and Dl Pisa et al. (1991) ascribe the products of the Mid-Ordovician activity to an Andean-type arc, originated in the subduction of the Rheic Ocean beneath the NorthGondwana margin before the early Carboniferous nappe stacking.

More challenging is the chronological and geodynamic interpreta-tion of other volcanic products with alkaline signature, formerlyreferred to either as Silurian (Lehman, 1975; Ricci and Sabatini, 1978)or as Upper Ordovician, for being interbedded, as pillow-lavas, withina supposed Caradocian metagreywake (Gattiglio and Oggiano, 1990).Ricci and Sabatini (1978) supposedly attributed to a Silurian riftingstage this volcanic activity, which Oggiano and Mameli (2006)rather refer to an Upper Ordovician rifting that affected the NorthernGondwanamargin and, in the Silurian, ended in the detachment of theArmorica Terranes Assemblage. Di Pisa et al. (1992) considered anearly Carboniferous age for similar volcanic rocks on the base of thelithostratigraphic correlations.

Other volcanic rocks within the internal and external nappes,although lacking in stratigraphic constraints and radiometric data-tions, were referred to the post Sardic Phase magmatic activities.

The recent field mapping carried out for the new geological map ofItaly (CARG Project) evidenced new outcrops of volcanic formations.Moreover, a sharper distinction between intrusive-subvolcanic rocks,volcanic flows and pyroclastic flows was possible, as well as a fine-tuning of stratigraphy, particularly in relation to the Sardic uncomfor-mity, in spite of deformation and metamorphism.

the Sarrabus tectonic unit, see location in Fig. 1.

48 G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

2.2. Volcanic products in the pre-Sardic sequence

Within the pre-Sardic sequence of the external nappe (Cambrian–Arenigian), no volcanic activity was known, as some alkaline doleriticdykes crosscutting the Cambrian lower Ordovician metasandstones(Solanas Fm.) were considered to be the feeders of the late Ordovicianalkaline basalts embedded within the late Ordovician metasediments(Di Pisa et al., 1992).

We sampled a rhyolitic ignimbrite (Samples ORD17–19, Fig. 3),which caps the Cambrian–early Ordovician sedimentary sequence ofthe Sarrabus Unit. This pyroclastic flow is slightly discordant with theoverlying late Ordovician sequence that includes pillow basalt flowstilted vertically; the field relationships between volcanic and sedi-mentary host point to a syn-sedimentary emplacement.

The welded rhyolitic ignimbrite includes a crystal-, fiamma-richand a glassy eutaxitic facies, that macroscopically results in a bandedtexture alternating quartz+feldspar and biotite-rich bands. The crystalfragments are K-feldspar and quartz in silicic fine-grained and/orglassy devitrified groundmass. Quartz phenoclasts and K-feldspar frag-ments, overgrown by microcline, are cracked and quartz also sub-rounded, likely as the effect of thermal corrosion during emplacement.The biotite is largely altered to chlorite+oxides. Zircon is a commonaccessory phase.

The fiammae are transformed to flattened aggregates of micro-crystalline quartz and feldspar, from recrystallized devitrified lenti-cular glassy fragments. The very low-grade metamorphic imprintresults in sericite replacing K-feldspar.

Fig. 4. Geological sketch map of the Nurra region

In the inner nappes of north-western Sardinia (Li Trumbetti andCanaglia Units, Fig. 4) we sampled peraluminous metarhyolite andmetadacite flows within a metasandstone sequence, of unknown age,unconformably capped by metaepiclastites, phyllites associated withalkalic metadolerites–metabasalts and oolitic ironstones referred tothe late Ordovician (Oggiano and Mameli, 2006). The Mt. Geisgiametadacite (Samples ORD34–35, Fig. 2) is a two meter-thick sheet-like body characterised by flattened augens of formerly lobate quartzand plagioclase porphyroclasts in a banded quartz-feldspar andphyllosilicate matrix. White mica+oxide are pseudomorphs afterbiotite, whereas plagioclase is partially albitised and silicified.

In the Li Trumbetti unit, themetarhyolite (SamplesORD45–47, Fig. 2)derives from a few meter thick flow, characterised by blastoporphyriticfabric, flattened and rotated eyes of plagioclase, K-feldspar and quartzporphyroclasts in a fine-grained recrystallized groundmass. Theseacidic metavolcanic rocks were referred to the calc-alkaline activity,well constrained at the Middle Ordovician in the external nappes(Carmignani et al., 1978).

2.3. Mid-Ordovician volcanites

The Mid-Ordovician sub-alkalic calc-alkalic volcanic suite is wellknown in the external nappes (e.g. in Sarrabus and Sarcidano) sincethe 60's and has been divided into lithologic subunits (e.g. grey-,white-porphyries and “porphyroids”, Calvino, 1972, 1963; M. CorteCerbos, Manixeddu and Serra Tonnai Formations in Bosellini andOgniben,1968). Thewhole volcanic suite consists of basaltic-andesites

in the inner nappe zone, see location in Fig. 1.

49G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

and andesites to rhyolites (Carmignani et al., 2001). In the forelandarea a Mid-Ordovician volcanic event could be only inferred from theoccurrence of volcanic clasts at the base of the Hirnantian succession(Leone et al., 1991).

The Mid Ordovician sub-alkalic volcanic suite has reliable strati-graphic and palaeontological constraints. In fact this continentalclastic and volcanic sequence unconformably (Sardic Unconformity)lies on metasandstones, which, based on acritarchs assemblages,ranges from middle Cambrian to early (Arenig) Ordovician (Naud andPittau Demelia, 1987; Di Milia et al., 1993). The transgressive sedi-mentary covers of the volcanic suite have been attributed to the UpperOrdovician (Katian-Hirnantian age), on the basis of fossil content (Loiet al., 1992; Naud, 1979; Barca and Di Gregorio, 1979). Also the U-Pbage of 460±1Ma on ametarhyolite in the external nappes (Giacominiet al., 2006) is consistent with this time span. Furthermore, refiningprevious data (Ferrara et al., 1978), Helbing and Tiepolo (2005)obtained a 456±14 Ma protolith age by in situ U–Pb zircon for theLodè granodiorite orthogneiss in the internal nappe, interpreted as theintrusive counterpart of the Mid-Ordovician “porphyroids”.

In the external nappes (Fig. 2, column A), at Porto Corallo, the sub-intrusive dacite hosted in the Cambrian Lower-Ordovician metasand-stones has been considered representative of this volcanic activity(Sample ORD12). It is a fresh, poorly deformed body, whose schis-tosity developed only at the rim, close to the host metasediments. It ischaracterised by porphyritic texture (porphyritic index: P.I.≈35) withplagioclase, biotite and quartz phenocrysts in a fine-grained felsicgroundmass. Biotite phenocrysts often include plagioclase. The subor-dinate quartz phenocrysts are subrounded and sometimes lobate.Minor secondary growth of chlorite, white mica and oxides, and ofalbite+sericite aggregates developed at the expense of biotite andplagioclase respectively.

2.4. Late Ordovician volcanites

The upper Ordovician transitional to alkalic volcanic activity isdocumented both in the foreland, and in the external and internalnappes.

Table 2Representative trace element contents (ppm) of zircons analysed by LA ICP-MS.

Sample-zircon ident. Ordl7–7 Ordl7–13 Ord34–59 Ord34–65 Ord45–2

Concordant age (Ma) 491 483 478 491 481

Element (ppm)Ti 40.58 b25.32 13.23 b7.23 b21.25Sr 0.44 0.39 0.52 0.531 b0.36Y 1123 1683 2578 2330 1457Nb 2.69 1.92 3.11 2.1 47.45Ba b1.28 b0.98 b0.43 b0.56 b0.89La b0.00 b0.173 0.083 0.619 b0.20Ce 0.86 0.37 4.77 1.103 9.25Pr 0.065 b0.072 0.118 0.257 b0.094Nd b0.67 b0.44 1.07 1.57 2.23Sm 2.02 b0.56 2.3 2.26 5.66Eu b0.00 b0.16 0.268 0.311 0.23Gd 10.58 9.17 29.18 20.71 30.93Tb 5.95 8.02 13.81 11.33 12.72Dy 85.48 122 218 176 135Ho 33.88 57.00 81.43 76.52 53.13Er 199 281 404 389 213Tm 37.92 67.96 93.86 86.15 42.14Yb 404 652 844 843 351Lu 74.71 131 153 159 65.5Hf 9583 9492 9991 9382 10986Ta 0.30 0.48 0.81 0.525 13.98Pb 1.07 5.34 5.21 5.57 8.45Th 21.66 16.33 75.03 38.65 186U 172 280 374 339 402Th/U 0.1 0.1 0.2 0.1 0.5∑REE 854 1329 1845 1768 921

In the internal nappes, the volcano-sedimentary lithostratigraphicunit consists of some meters of thick sills of dark hornblende-bearinggabbro affected by greenschist facies re-equilibration. They showorthocumulus texture, with euhedral seriate plagioclase and accessoryFe-oxide. The coarser plagioclase grains are broken and sutured bynewly precipited feldspar, or deformed, thus evidencing syn-mag-matic deformation likely during emplacement. Pl-rich dioriteveins cut the gabbro body. The late Ordovician age is constrained onlithostratigraphic and palaeoclimatic bases (Hirnantian glaciation;Oggiano and Mameli, 2006). In Asinara island, the Cala d'Olivaorthogneiss resulted as old as 439±6 Ma (Rossi et al., 2009).

In the external nappes (Meana Sardo, Gerrei and Sarrabus units;Di Pisa et al., 1992), the volcanic products are embedded within thetransgressive Upper Ordovician sequence. They consist of epiclastitesand lava flows (often with well preserved pillow lava structures,Fig. 4). The volcanic unit addressed for geochronology is made ofepiclastites (sample ORD30), characterised by subrounded quartz andsubordinate plagioclase phenoclasts in a fine-grained quartz-feldspa-tic groundmass. Biotite is replaced by chlorite pseudomorphs. Scarcecalcite fills a localised cataclastic vein network.

3. Analytical methods

The described metavolcanites were addressed for U–Pb geochronol-ogy. Zircon grains were firstly separated at the University of Genovausing standard magnetic techniques and heavy liquids. The grainswithout fractures and as free from inclusions as possible were thenselected by hand picking, embedded in epoxy resin and polished to0.25 μm using diamond paste. Prior to the age determination, theinternal structure of the zircons was investigated with back-scatteredelectron (BSE)microscopyand cathodoluminescence (CL)with a PhilipsXL30 electron microscope, at the Earth Science Dept., Siena University,Italy. The in-situ U–Pb geochronology and trace element abundanceswere determined with excimer laser ablation (ELA) ICP-MS at CNR —

Istituto di Geoscienze e Georisorse (IGG) — Unità di Pavia, the laserablation instrument consisting of an ArF excimer laser microprobe at193 nm (Geolas200Q-Microlas) and a high-resolution sector-field ICP-

Ord45–29 Ord12–13 Ord12–62 Ord30–22 Ord30–50 Ord30–4

479 467 464 437 443 416

b38.55 b7.63 b8.29 b29.37 b8.17 b8.490.63 0.40 0.21 0.79 0.447 0.3232452 975 1197 2834 2980 269016.29 3.08 4.11 2.01 2.25 2.3b1.10 b0.30 b0.37 b2.11 b0.303 0.84b0.175 0.04 b0.039 0.169 0.022 0.19813.18 4 4.95 1.85 0.787 1.77b0.117 0.01 b0.038 0.181 b0.045 0.2955.19 0.74 0.82 1.1 1.36 2.439.7 2.40 2.32 2.83 2.75 3.27b0.38 0.18 0.39 0.27 0.036 0.31770.13 13.97 15.49 25.27 24.87 21.1923.92 5.38 6.12 15.88 15.53 14.1239 72.84 96.29 227 221 20597.97 31.34 41.82 93.06 93.7 86.6325 161 194 474 488 48366.97 35.17 42.68 105 107 115569 348 426 967 1002 112091.22 71.10 84.63 187 189 2339472 9147 8786 9057 10899 122684.78 0.64 0.67 0.62 0.515 0.68712.03 3.67 5.09 3.59 1.95 3.06211 72.06 99.86 36.27 36.97 37.56396 318 417 358 378 7100.5 0.2 0.2 0.1 0.1 0.11512 746 915 2100 2147 2286

Table 3Selected LA ICP-MS U–Pb–Th isotope data and calculated ages for zircons from the studied samples.

Spot Spot size(μm)

206Pb/238U 207Pb/235U 207Pb/206Pb 208Pb/232Th 206Pb/238U 207Pb/235U 207Pb/206Pb 208Pb/232Th Conc. age ±2σ(Ma)Ratio RSD

(%)Ratio RSD

(%)Ratio RSD

(%)Ratio RSD

(%)Age(Ma)

±2σ(Ma)

Age(Ma)

±2σ(Ma)

Age(Ma)

±2σ(Ma)

Age(Ma)

±2σ(Ma)

a

Minderri metarhyolite (Sample ORD17)a2 10 0.0808 2.18% 0.6337 4.96% 0.0575 5.13% 0.0300 7.59% 501 11 498 25 512 26 598 45 501 21a6 10 0.0791 1.89% 0.6272 3.10% 0.0576 3.29% 0.0332 3.84% 491 9 494 15 516 17 660 25 491 18a7 10 0.0791 2.25% 0.6259 5.26% 0.0583 5.45% 0.0321 5.44% 490 11 494 26 540 29 638 35 491 21a9 10 0.0781 1.73% 0.6110 2.75% 0.0569 3.00% 0.0275 3.69% 485 8 481 13 486 15 548 20 484 16a12 10 0.0801 2.21% 0.8315 5.08% 0.0571 5.28% 0.0237 3.70% 497 11 497 25 495 26 473 17 497 21a13 10 0.0777 1.85% 0.6121 3.45% 0.574 3.67% 0.0359 4.49% 483 9 485 17 508 19 712 32 483 17a17 10 0.0809 1.93% 0.6468 3.43% 0.0575 3.61% 0.0282 4.22% 502 10 507 17 511 18 542 24 502 18

b

Monti di Geisgia metadacite (Sample ORD34)al 25 0.0783 1.32% 0.6135 1.51% 0.0569 1.49% 0.0258 1.74% 486 6 486 7 485 7 515 9 486 12a3 25 0.0762 1.40% 0.5978 2.05% 0.0568 2.04% 0.0244 3.17% 474 7 476 10 483 10 487 15 474 13a5 25 0.0772 1.34% 0.6028 1.62% 0.0566 1.60% 0.0252 1.95% 479 6 479 8 474 8 503 10 479 12a6 25 0.0780 1.34% 0.6128 1.59% 0.0569 1.56% 0.0255 1.79% 484 6 485 8 486 8 508 9 485 12a7 25 0.0779 1.25% 0.6051 1.31% 0.0563 1.34% 0.0248 1.41% 483 6 481 6 464 6 496 7 478 9.6a11 25 0.0767 1.35% 0.6034 1.75% 0.0570 1.75% 0.0368 2.65% 476 6 479 8 492 9 731 19 477 12a10 25 0.0774 1.32% 0.6032 1.57% 0.0565 1.57% 0.0265 1.94% 481 6 479 8 471 7 530 10 480 12a9 25 0.0767 1.35% 0.6005 1.55% 0.0567 1.51% 0.0254 1.76% 476 6 478 7 479 7 506 9 477 12a14 25 0.0805 1.29% 0.6328 1.46% 0.0570 1.47% 0.0295 1.39% 499 6 498 7 491 7 587 8 498 11a15 25 0.0793 1.33% 0.6209 1.48% 0.0567 1.45% 0.0265 1.64% 492 7 490 7 481 7 528 9 491 11a19 25 0.0799 1.30% 0.6240 1.33% 0.0566 1.32% 0.0265 1.41% 495 6 492 7 476 6 529 7 487 8.8a20 25 0.0798 1.31% 0.6290 1.37% 0.0571 1.35% 0.0354 1.52% 495 6 496 7 496 7 703 11 496 10a26 25 0.0771 1.34% 0.6062 1.53% 0.0570 1.50% 0.0246 1.77% 479 6 481 7 491 7 491 9 480 12a25 25 0.0777 1.52% 0.6091 2.36% 0.0571 2.31% 0.0303 2.69% 483 7 483 11 496 11 603 16 483 14a31 25 0.0788 1.34% 0.6178 1.61% 0.0568 1.60% 0.0260 1.94% 489 7 489 8 484 8 519 10 489 12a34 25 0.0801 1.36% 0.6309 1.55% 0.0571 1.51% 0.0245 1.77% 497 7 497 8 494 7 490 9 497 12a37 25 0.0806 1.32% 0.6395 1.39% 0.0576 1.37% 0.0274 1.44% 500 7 502 7 512 7 547 8 503 11a42 25 0.0771 1.38% 0.6051 1.68% 0.0569 1.64% 0.0246 2.10% 479 7 481 8 487 8 491 10 479 12a49 25 0.0785 1.37% 0.6137 1.47% 0.0567 1.42% 0.0268 1.56% 487 7 486 7 479 7 534 8 486 11a52 25 0.0790 1.40% 0.6265 1.73% 0.0575 1.69% 0.0304 1.94% 490 7 494 9 512 9 605 12 492 13a59 25 0.0769 1.43% 0.6007 1.77% 0.0567 1.71% 0.0271 1.97% 478 7 478 8 479 8 540 11 478 13a65 25 0.0785 1.31% 0.6177 1.33% 0.0571 1.32% 0.0291 1.43% 487 6 488 7 495 7 580 8 491 8.6

Li Trumbetti metarhyolite (Sample ORD45)a2 10 0.0775 1.53% 0.6038 3.55% 0.0565 3.04% 0.0245 9.98% 481 7 480 17 471 14 489 49 481 14a8 10 0.0792 1.47% 0.6236 3.86% 0.0574 3.46% 0.0225 10.58% 492 7 492 19 507 18 449 47 491 14a10 10 0.0756 1.64% 0.5943 4.06% 0.0573 3.65% 0.0496 10.34% 470 8 474 19 501 18 979 101 470 15a11 10 0.0783 1.49% 0.6134 3.63% 0.0568 3.17% 0.0528 10.39% 486 7 486 18 484 15 1040 108 486 14a19 10 0.0771 1.51% 0.5995 3.53% 0.0564 3.03% 0.0506 10.04% 479 7 477 17 465 14 998 100 479 14a18 10 0.0773 1.87% 0.6053 5.02% 0.0568 4.73% 0.0283 11.19% 480 9 481 24 484 23 564 63 480 17a16 10 0.0760 1.43% 0.6029 3.44% 0.0576 2.96% 0.0305 10.13% 472 7 479 16 515 15 608 62 472 13a15 10 0.0761 1.39% 0.5937 3.35% 0.0566 2.86% 0.0229 9.93% 473 7 473 16 477 14 458 45 473 13a21 10 0.0765 1.64% 0.6043 3.99% 0.0574 3.57% 0.0260 10.55% 475 8 480 19 508 18 518 55 475 15a23 10 0.0760 1.45% 0.5862 3.47% 0.0561 2.99% 0.0256 10.11% 472 7 468 16 454 14 511 52 472 13a29 10 0.0772 1.47% 0.6012 3.42% 0.0566 2.91% 0.0234 9.95% 479 7 478 16 474 14 468 47 479 13a28 10 0.0779 1.43% 0.6063 3.44% 0.0565 2.96% 0.0253 10.04% 484 7 481 17 473 14 504 51 484 13a30 10 0.0809 1.47% 0.6343 3.84% 0.0571 3.45% 0.0322 10.51% 502 7 499 19 493 17 640 67 502 14a32 10 0.0753 1.49% 0.5857 3.49% 0.0565 2.99% 0.0229 10.21% 468 7 468 16 473 14 458 47 468 13a35 10 0.0781 1.43% 0.6153 3.48% 0.0572 3.00% 0.0253 10.16% 485 7 487 17 500 15 505 51 485 13a36 10 0.0757 1.51% 0.5921 4.19% 0.0568 3.85% 0.0330 10.05% 470 7 472 20 483 19 657 66 470 14

c

Porto Corallo dacite (Sample ORD12)a1 25 0.0750 1.67% 0.5832 2.29% 0.0564 2.28% 0.0242 1.91% 466 8 467 11 466 11 483 9 466 15a2 25 0.0761 1.66% 0.5942 2.16% 0.0567 2.14% 0.0241 1.73% 473 8 474 10 478 10 481 8 473 15a4 25 0.0776 1.74% 0.6057 2.28% 0.0570 2.22% 0.0226 1.72% 482 8 481 11 490 11 451 8 482 16a7 25 0.0754 1.72% 0.5927 2.27% 0.0571 2.23% 0.0241 1.82% 469 8 473 11 495 11 482 9 470 15a15 25 0.0768 1.62% 0.6033 2.06% 0.0569 2.05% 0.0238 1.63% 477 8 479 10 489 10 475 8 478 14a13 25 0.0753 1.62% 0.5815 2.05% 0.0560 2.05% 0.0213 1.59% 468 8 465 10 451 9 425 7 467 14a9 25 0.0772 1.61% 0.6078 2.15% 0.0571 2.17% 0.0262 1.74% 479 8 482 10 496 11 523 9 480 15a33 25 0.0737 1.56% 0.5738 2.11% 0.0565 2.15% 0.0224 1.70% 459 7 461 10 469 10 447 8 459 14a30 25 0.0773 1.74% 0.6079 2.38% 0.0570 2.35% 0.0276 2.02% 480 8 482 11 491 12 550 11 481 16a25 25 0.0752 1.59% 0.5807 2.00% 0.0560 2.01% 0.0233 1.56% 467 7 465 9 453 9 465 7 466 14a34 25 0.0738 1.68% 0.5776 2.46% 0.0569 2.46% 0.0238 2.02% 459 8 463 11 487 12 475 10 460 15a35 25 0.0724 1.56% 0.5634 2.24% 0.0566 2.30% 0.0212 1.71% 450 7 454 10 476 11 424 7 451 13a36 25 0.0744 1.80% 0.5878 3.05% 0.0573 3.05% 0.0238 2.92% 463 8 469 14 502 15 476 14 464 16a37 25 0.0725 1.57% 0.5653 2.00% 0.0566 2.02% 0.0212 1.53% 451 7 455 9 474 10 423 6 452 13a43 25 0.0713 1.58% 0.5505 2.00% 0.0560 2.02% 0.0211 1.53% 444 7 445 9 451 9 423 6 445 13a42 25 0.0724 1.57% 0.5597 1.98% 0.0561 2.00% 0.0241 1.54% 450 7 451 9 456 9 481 7 451 13a47 25 0.0744 1.58% 0.5780 2.07% 0.0564 2.09% 0.0237 1.63% 463 7 463 10 465 10 474 8 463 14a55 25 0.0724 1.63% 0.5592 2.03% 0.0560 2.02% 0.0207 1.54% 451 7 451 9 453 9 414 6 451 14

50 G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

Table 3 (continued)

Spot Spot size(μm)

206Pb/238U 207Pb/235U 207Pb/206Pb 208Pb/232Th 206Pb/238U 207Pb/235U 207Pb/206Pb 208Pb/232Th Conc. age ±2σ(Ma)Ratio RSD

(%)Ratio RSD

(%)Ratio RSD

(%)Ratio RSD

(%)Age(Ma)

±2σ(Ma)

Age(Ma)

±2σ(Ma)

Age(Ma)

±2σ(Ma)

Age(Ma)

±2σ(Ma)

c

a56 25 0.0762 1.74% 0.5856 2.49% 0.0563 2.46% 0.0254 2.07% 473 8 468 12 464 11 508 10 472 16a59 25 0.0781 1.74% 0.6095 2.45% 0.0572 2.42% 0.0257 1.99% 485 8 483 12 499 12 513 10 484 16a61 25 0.0783 1.60% 0.6064 2.01% 0.0562 2.01% 0.0261 1.52% 486 8 481 10 459 9 520 8 484 14a62 25 0.0743 1.59% 0.5823 2.01% 0.0568 2.01% 0.0229 1.54% 462 7 466 9 485 10 457 7 464 14a63 25 0.0755 1.77% 0.5806 2.49% 0.0556 2.44% 0.0256 2.09% 469 8 465 12 438 11 512 11 468 16a64 25 0.0740 1.76% 0.5787 2.46% 0.0566 2.42% 0.0241 2.04% 460 8 464 11 475 12 480 10 461 15a51 25 0.0747 1.74% 0.5797 2.25% 0.0562 2.19% 0.0239 1.74% 465 8 464 10 458 10 477 8 464 15a50 25 0.0755 1.76% 0.5927 2.49% 0.0569 2.45% 0.0245 2.08% 469 8 473 12 485 12 489 10 470 16a52 25 0.0751 1.76% 0.5844 2.52% 0.0564 2.49% 0.0287 2.23% 467 8 467 12 467 12 571 13 467 16

d

Silius metaepiclastite (Sample ORD30)a12 10 0.0733 1.82% 0.5721 4.55% 0.0558 4.81% 0.0291 7.86% 456 8 459 21 445 21 580 46 456 16a9 10 0.0725 1.65% 0.5721 4.12% 0.0576 4.44% 0.0355 7.68% 451 7 459 19 515 23 704 54 451 14a15 10 0.0710 1.74% 0.5531 3.87% 0.0564 4.15% 0.0231 7.93% 442 8 447 17 466 19 461 37 443 15a26 10 0.0693 1.42% 0.5270 2.62% 0.0551 3.00% 0.0417 6.87% 432 6 430 11 418 13 825 57 432 12a24 10 0.0719 1.48% 0.5607 2.55% 0.0566 2.91% 0.0286 6.77% 448 7 452 12 476 14 570 39 448 13a22 10 0.0700 1.56% 0.5476 2.89% 0.0567 3.22% 0.0251 7.01% 436 7 443 13 481 15 501 35 437 13a21 10 0.0715 1.58% 0.5461 3.37% 0.0554 3.70% 0.0274 7.16% 445 7 442 15 427 16 546 39 445 14a29 10 0.0702 1.76% 0.5316 4.88% 0.0559 5.19% 0.0263 8.45% 437 8 433 21 447 23 524 44 437 15a38 10 0.0709 1.53% 0.5546 3.04% 0.0566 3.37% 0.0263 7.09% 442 7 448 14 477 16 525 37 442 13a39 10 0.0699 1.45% 0.5266 2.53% 0.0546 2.91% 0.0247 6.84% 436 6 430 11 394 11 493 34 435 12a37 10 0.0689 1.48% 0.5297 2.75% 0.0557 3.11% 0.0250 6.92% 430 6 432 12 440 14 499 35 430 12a36 10 0.0690 1.50% 0.5286 2.67% 0.0555 3.02% 0.0276 6.95% 430 6 431 11 430 13 550 38 430 12a45 10 0.0708 1.67% 0.5475 4.03% 0.0562 4.34% 0.0237 8.09% 441 7 443 18 458 20 473 38 441 14a50 10 0.0712 1.50% 0.5475 2.82% 0.0557 3.17% 0.0258 7.26% 443 7 443 12 439 14 515 37 443 13a59 10 0.0723 1.45% 0.5672 2.89% 0.0567 3.25% 0.0226 7.15% 450 7 456 13 480 16 453 32 451 13a60 10 0.0691 1.54% 0.5533 3.13% 0.0579 3.47% 0.0206 6.89% 431 7 447 14 524 18 412 28 431 13

Spot size is given in µm; RSD, relative standard deviation given in percent; age is given in Ma; 2σ given in Ma; MSWD, mean square of weighted deviates.

Porto Corallo dacite (Sample ORD12)

51G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

MS (Element I from ThermoFinnigan). The analytical method followedthat described by Tiepolo (2003) and considering the recent discussionin Cocherie and Robert (2008) about the spot size and sensitivity of themethods. The instrumental and laser-induced U–Pb fractionationswerecorrected adopting the 1065Ma 91500 zircon (Wiedenbeck et al., 1995)as an external standard. The spot size was set to 25 or 10 μm and laserfluence to 12 J/cm′. The data reductionwasperformedusing the “Glitter”software package (van Achterbergh et al., 2001). Time resolved isotoperatioswere carefully inspected inorder todetectperturbations related toinclusions, fractures ormixingof different age domains. In all these casesablation intervalswere set to represent only the initial unalteredportionof the signal. The reproducibility of the standards was propagated to alldeterminations according to the equation in Horstwood et al. (2003),after this procedure analyses are retained accuratewithin quoted errors.The 295 Ma zircon 02123 (Ketchum et al., 2001) was however analysedin every analytical run together with unknowns in order to monitor theinstrumental accuracy. The mean accuracy can be evaluated at about1.7%. Inmanycases, however, isotopically concordantportions cannot beresolved. To avoid mixed ages from such discordant portions, onlyconcordant data were considered. The Isoplot/EX 3.0 software (Ludwig,2000)wasused to calculate Concordia ages and drawConcordia plots. Inthe text all errors are given at 2σ level.

The analytical conditions and data are provided in Tables 1, 2 and 3.

4. Textural features of the magmatic zircon grains

4.1. Pre-Sardic phase volcanic sequence

The investigated zircons occur as euhedral, short-prismatic to fairlyelongated crystals with a width-to-length ratio from 1:1.4 to 1:2.8, andthe highest length frequency in the range of 100÷300 μm, exceptionally

exceeding 400 μm. The BSE/CL images (Fig. 5) reveal complex innerstructures in the investigated crystals. All the zircons exhibit xenocrysticcores rimmed by oscillatory zoning. The xenocrystic cores may becharacterised by oscillatory, convolute or chaotic zoning, and also byhigh luminescence domains. Often the cores are subrounded orbounded by irregular surfaces, likely due to igneous resorption. Theovergrowths show well-developed oscillatory zoning defined byvariable degrees of thickness and luminescence. Occasionally, almostuniform cores mantled by thin-zoned rims are also found. We focusedthe analysis on certain igneous textures preserved in the outer domains,in order to obtain the most likely ages of crystallization.

4.2. Mid-Ordovician volcanites (sub-intrusive dacite)

The euhedral zircon grains are strongly elongated to short prismaticwith a width-to-length ratio from 1:4 to 1:1.9 and length between 185and 380 μm, disregarding the size parameters of the broken grains. Theyhave inner regular zoning, evidenced by variable luminescence (Fig. 5;ORD12–62 and –47) sometimes interrupted by textural discontinuities(ORD12–25) due to resorption followed by the crystallization of zonedzircon. A few grains exhibit also a small xenocrystic core (ORD12–33).

4.3. Late Ordovician volcanites

The euhedral zircon grains are short prismatic to strongly elongatedwith a width-to-length ratio from 1:2 to 1:3 and the highest lengthfrequency in the range of 180–320 μm. The inner growth zoning isregular (ORD30–7) and sometimes interrupted by convolute zoning(ORD30–9), likely due to resorption. The almost uniform coresare mantled by low luminescent zoned rims (ORD30–24) whereassector zoned grains (ORD30–22) rarely occur. Sector zoning is generally

Fig. 5. Representative CL microphotographs of zircons, the location and size of LA ICP-MS spot analyses, their concordant ages and 1σ uncertainties are indicated. tr: ablation pits fortrace and RE element determinations.

52 G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

ascribed to slow growth and low diffusivity in fluid-enriched subsolidusregime (Watson and Liang, 1995).

5. Trace and REE zircon compositions

In order to assess the unaltered igneous origin and correctlyinterpret the U–Pb age data for the zoned rims, the trace and rare

earth element (REE) composition was measured. On the whole, theTh/U ranges between 0.1 and 0.7 (Table 2) and is consistent with anigneous origin (Williams et al., 1996; Rubatto and Gebauer 2000).Furthermore, the rare earth patterns and the very low concentrationsof volatile, LIL and HFS elements, often below the detection limit(Table 2), suggest unaltered igneous zircons (Hoskin and Schaltegger2003).

Fig. 6. C1 chondrite-normalised (after Sun and McDonough, 1989) REE patterns for representative analyzed zircons. A) Pre-Sardic Lower Ordovician metavolcanites B) Mid-Ordovician metadacite C) Late Ordovician metavolcanite. D) Nb/Y vs. ΣHREE evidencing the significant element partitioning in zircons from the three volcanic cycles.

53G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

5.1. Pre-Sardic phase volcanic sequence

The zircon rims share C1-chondrite normalised REE patterns (Sunand McDonough, 1989; Fig. 6) characterised by steeply rising slopefrom the LREE to the HREE with positive Ce-anomaly and negative-Eu anomaly (Eu/Eu⁎=0.01–0.3) consistent with zircon growthunder igneous conditions (e.g. Hoskin and Schaltegger, 2003;Rubatto, 2002; Rubatto and Gebauer, 2000). LREE abundances arehighly variable and someweak negative Ce-anomalies are consistent

with a value of 1.0 within analytical uncertainty. The Th/U isrelatively low (0.1–0.2). The structureless light grey domains of a fewgrains from Li Trumbetti rhyolite, reveal higher Th/U (0.46–0.68, 4spots).

5.2. Mid-Ordovician volcanites (sub-intrusive dacite)

The C1-chondrite normalised REE patterns (Sun andMcDonough,1989; Fig. 6) corresponding to the sites of igneous oscillatory

Fig. 7. U–Pb mean Concordia age for the analysed zircons, representative of A) Lower Ordovician, pre-Sardic unconformity volcanism (ORD17, external nappes, ORD34 and 45,internal nappes); B) ORD12, feeder dyke of Mid Ordovician Porphyroids, cutting the Cambro–Ordovician metasediments in the external nappes and C) ORD30, Late Ordovicianintermediate volcanic product.

54 G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

zoning, display regular and steep increase from the LREE to theHREE with positive Ce-anomaly (Ce/Ce⁎=4.1–42.5) and negativeEu-anomaly (Eu/Eu⁎=0.03–0.23), indicating unaltered igneouszircons.

5.3. Late Ordovician volcanites

The C1-chondrite normalised REE patterns (Sun and McDonough,1989; Fig. 6) for the magmatic sites increase regularly and steeply

55G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

from the LREE to the HREE with generally positive Ce-anomaly (Ce/Ce⁎=1.8–3.2) and negative Eu-anomaly (Eu/Eu⁎=0.01–0.12) sug-gesting unaltered igneous zircons.

6. Geochronological results

Among the several felsic volcanic units addressed for zirconseparation, high common lead content precluded reliable agedetermination for many of the samples, in spite of the favourablecathodoluminescence (CL) appearance. In any case five samplesyielded concordant U–Pb ages. Isotopic data are presented in Table 3and plotted on conventional U–Pb Concordia diagram in Fig. 7.

6.1. Pre-Sardic sequence (Cambro–Ordovician acid volcanites)

Eighteen spot analyses were carried out by LA ICP-MS on zircongrains extracted from the sample ORD17. Twelve spots gaveconcordant ages ranging from 587±18 to 449±17 Ma. The oldestage of 587±18 Ma was determined on an inherited zircon core;however a cluster of seven concordant ages, obtained mostly in thezircon rims, defines a mean Concordia age at 491.7±3.5 Ma.

Thirty-five spot analyses were performed on zircon grains ex-tracted from sample ORD34. Twenty-four spots give concordant agesranging from 503±11 to 474±13Ma and define a mean Concordia ageat 486±1.2 Ma. Two older ages at 578±18 and 617±16 Ma wereobtained on weakly zoned and highly luminescent zircon domains.

Twenty-nine spot analyses were carried out on zircon grainsextracted from the sample ORD45. Sixteen spots give concordant agesranging from 502±13 to 468±13Ma and define amean Concordia ageat 479.9±2.1 Ma (Fig. 7, Table 3). A cluster of five concordant agesyielded scattered younger ages between 452±14 and 415±13 Ma,likely related to a lead loss event. Furthermore, in spite of the short timeinterval between the emplacement and the isotope perturbation event,a veritable time gap precludes the acquisition of one concordant age.

6.2. Mid-Ordovician volcanites (sub-intrusive dacite)

Thirty-six spot analyses were conducted on euhedral and generallyunbroken zircon grains extracted from the sample ORD12. Twenty-nine spots yielded concordant ages ranging from 527±15 to 435±13 Ma. A cluster of twenty-seven concordant ages defines a meanConcordia age at 465.4±1.4 Ma.

6.3. Late Ordovician volcanites

Thirty-five spot analyses were performed on zircon grains fromsample ORD30. Sixteen concordant ages ranging from 456±16 to430±12Ma allowed calculating amean Concordia age at 440±1.7Ma(MSWD=0.83). One concordant age at 824±21 Ma represents aninherited zircon. A cluster of younger concordant age (6 spot analyses)between 424±15 and 406±11 Ma is likely related to an isotopeperturbation event, as suggested by the zircon zonation patterns andthe consistently higher U and Hf (e.g. U=710 ppm, Hf=12,768 ppmon zircon 4; Table 2). This is in good agreement with the obser-vations of Pan (1997) and Hoskin and Black (2000) of Hf- and U-richrecrystallized rims on igneous zircon cores. Four further spot analyseson structureless or faded oscillatory, zoned domains provided agesbetween 469±16 and 485±16 Ma.

7. Tentative geodynamic significance of the igneous events

7.1. Pre-Sardic phase

The radiometric dating on the metavolcanites of the SardiniaVariscan branch, sampled with stratigraphic and structural criteria,

evidences a more complex picture of the volcanic events that affectedthe northern Gondwana margin.

The previous data referred to two cycles, both developed after theSardic Phase, i.e. during the Middle and late Ordovician. Before thistectonic event no Cambrian–Lower Ordovician volcanic activity wasyet evidenced. This time interval was considered as dominated bypassive margins that experienced only terrigenous and carbonateshelf sedimentation (Cocozza, 1979; Pillola et al., 1995, and referencestherein). The Cambrian age issued from the metarhyolite ORD17–19(Fig. 2) allows assessing the same age for the metasediments of theexternal nappe (Sarrabus unit) earlier considered as Early Carbonifer-ous, due to the lack of fossil records. Besides this consideration of thelocal interest, the emplacement of acidic metavolcanic rocks withinthe pre-Sardic sequence of the nappes zone represents a new event thatsuggests some possible scenarios so far not considered, following theCambrian amalgamation of Gondwana (Santosh et al., 2009): i) a long-lasting continental volcanism from the Upper Cambrian, as e.g. inFernández et al. (2008), to Middle Ordovician to be referred tosubductionof Rheic oceanic crust beneath theNorthGondwanamargin;ii) an aborted rifting stage during late Cambrian; iii) the onset of passivevolcanic margin (e.g. as the Camerun line, Fitton, 1987) that precededthe calc-alkaline, subduction-related, Middle Ordovician suite.

In the adjacent Ossa–Morena Zone (Sánchez-García et al., 2003;Valverde-Vaquero and Dunning, 2000), as well as in Massif Central(Roger et al., 2004; Pin and Lancelot, 1982; Pin and Marini, 1993) andeven in the farthest western Gondwana (Alonso et al., 2008) thepassive volcanic margin is inferred to evolve through several tectonicand sedimentary stages. In Sardinia, as well, a polyphase passivemargin can be established at the Cambro–Ordovician boundary. Eventhough the tectonic units are involved in an orogenic prism theirprovenance from crustal blocks affected by diachronous, shallow levelvolcanism is now apparent.

As a fact, in Sardinia, diachronous emplacement ages arise for therhyolites from the external (491.7±3.59) Ma to the internal (481±1.2 Ma and 479.9±2.1 Ma) nappes, associated with changingcompositional features (HREE fractionation, Ba, Sr, Rb, Fe2O3t andMnO decreasing, Nb, Th, Y, K2O increasing from older to youngermetavolcanites, Gaggero et al., in prep.). To this regard, modernanalogues (e.g. the Faroer–Shetland, Norway, western central Africamargins) of the Cambrian Gondwanan margin display pre-, syn- topost-sedimentary acidic-intermediate volcanism (Rocchi et al., 2007;Hansen and Cartwright, 2006; Bell and Butcher, 2002; Berndt et al.,2000). A chemical variability for volcanic products of Cambrian riftingenvironments was nevertheless evidenced also at the scale of volcanicapparatus (Ossa–Morena rift Zone, Etxebarria et al., 2006; Castiñeiraet al., 2008). Moreover, in the internal nappe, crustal recycling isevidenced by the common occurrence of xenocrystic zircon coresand concordant ages at ~590 Ma for some inherited domains, in ac-cordancewith coevalmetavolcanic rocks fromCentral Iberia (Bea et al.,2007).

The available geochemical and isotopic data on the newlyevidenced pre-Sardic phase volcanism are not yet exhaustive,however a geodynamic setting of volcanic passive margin is in betteragreement with the local and regional geological frame than anaborted rift branch.

7.2. Mid Ordovician

The subintrusive dacite aged 465.4±1.4 Ma, rarely preservingxenocrystic cores, despite its occurrence within the Cambrian–LowerOrdovician metasandstones, postdates the pre-Sardic sequence, thusresulting as coeval and cogenetic with the subaerial arc volcanism thatled to the emplacement of the bimodal calc-alkaline suite constrainedbetween Arenigian and Caradocian metasediments. As the metarhyo-lite and metarhyodacite of inner nappes yielded late Cambrian–earlyOrdovician ages it can be argued that the Mid Ordovician bimodal

56 G. Oggiano et al. / Gondwana Research 17 (2010) 44–58

suite was not developed in the internal nappes successions thatconsequently pertained to a forearc basin.

7.3. Late Ordovician

The terrigenous sequences that followed the upper Ordoviciantransgression is associated with the epiclastite at 440±1.7 Ma, thusbetter marking the onset of this volcano-sedimentary cycle at theOrdovician–Silurian boundary. The post-orogenic alkalic volcanism ishowever best represented by relatively abundant subintrusive-effusive alkali–basalts in the internal nappes (Di Pisa et al., 1992;Gaggero et al., in prep). Similar products were described from theCentral Iberia zone ascribed to a hybrid magma source encompassinga few OIB-mantle and depleted end-member mantle components(López Moro et al., 2007).

8. Conclusions

The stratigraphic and radiometric investigations of the LowerPalaeozoic volcanites in the nappe zone of the South Variscan Branch

Fig. 8.Geodynamic sketchof thepre-Variscan evolutionof thenorthernGondwanamarginbased on the volcano-stratigraphic evidence carried out in the Sardinian transect. A:Passive margin evolution of Gondwana. B: Development of a volcanic arc originated in asubduction of oceanic below continental crust. C: Incipient rifting stage that affected theMid-Ordovician volcanic arc. D: Siluro-Devonian back-arc spreading and oceanization.

provided clues useful to re-define the history of the volcanic events inthe northern Gondwana margin during the Lower Palaeozoic. Threemain volcanic cycles can be distinguished on the base of radiometricand field evidence (Fig. 8):

I. A felsic Upper Cambrian–Lower Ordovician volcanic cycle olderthan the Sardic phase. The occurrence as dykes cutting theepicontinental clastic arenitic sediments and rare ignimbritessuggest a volcanic passive margin context.

II. The calc-alkaline volcanic cycle (Memmi et al., 1983; Dl Pisaet al., 1991) embracing theMiddle Ordovician. It is referred to asan Andean type arc, developed at the northern Gondwanamargin. The volcanic suite pertaining to this cycle is repre-sented in almost all the tectonic units except for those of theinner nappe zone and in the foreland.

III. An alkaline (Lehman, 1975; Ricci and Sabatini, 1978), volcanicactivity, at the Ordovician–Silurian boundary, the products ofwhich are associated with the glacial diamictites widespread inthe late Ordovician of North Gondwana (Le Heron et al., 2008).This volcanism, which lasted until early Silurian, is the witnessof a probable rifting stage superimposed and subsequent to theMiddle Ordovician Andean-type arc.

Acknowledgments

We wish to dedicate this work to the late Luciano Cortesogno(Genoa University) and Mimmo Di Pisa (Sassari University). N. Bonev,Y. Dilek, and P. Rossi are thanked for their constructive reviews. Thefinancial support was provided by PRIN 2004 and 2006 (L. Cortesogno,G. Oggiano), Ateneo 2006 (University of Genoa) grant to L. Gaggero,and University of Cagliari (Ex 60%) grant to A. Funedda.

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