Rifting and Arc-Related Early Paleozoic Volcanism Along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy)

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Rifting and Arc-Related Early Paleozoic Volcanism along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy)

Laura Gaggero,1,* Giacomo Oggiano,2 Antonio Funedda,3 and Laura Buzzi1

1. Department for the Study of Territory and Its Resources, University of Genoa, Corso Europa 26, 16132 Genoa, Italy; 2. Department of Botanics, Ecology, and Geology, University of Sassari, Via Piandanna 4, 07100 Sassari, Italy; 3. Department of Earth Sciences, University of Cagliari, Via Trentino 51, 09127 Cagliari, Italy

ABSTRACT Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian–Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (ca. 492–480 Ma), is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the ␧ Ϫ Ϫ external Variscan nappes, overlies the Sardic Unconformity. Negative Ndi values ( 3.03 to 5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician–Early Silurian (ca. 440 Ma) volcano-sedimentary cycle consists of an alkalic maﬁc suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta and have Zr/Nb ratios in the range 4.20–30.90 ␧ (typical of a rift environment) and positive Ndi values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination.

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Introduction An exceptional geodiversity in the Earth history accompanied by a complex magmatic evolution re- has been evidenced at the Paleozoic, between the corded by a bimodal intraplate volcanism associ- two major events of continental accretion that pro- ated with terrigenous sedimentation (Etxebarria et duced Gondwana first and then Pangea (Stampfli al. 2006; Chichorro et al. 2008; Linnemann et al. et al. 2002; von Raumer and Stampfli 2008; Nance 2008), mid-ocean ridge basalt (MORB)-type mag- et al. 2010). Thus, the Paleozoic Era begins and ends matism associated with lithospheric rifting and with a similar geographic configuration, in that one oceanization (between Cambrian and Devonian; supercontinent is inferred both in the Neoprote- Murphy et al. 2008, 2011), arc magmatism during rozoic and in the Permian. Between the extremes subduction and continental collision (Middle Or- of this Wilson cycle ranging over an interval of 300 dovician; Sańchez-Garcıá et al. 2003), and post- million years, the supercontinent Rodinia progres- orogenic alkalic magmatism (Upper Ordovician– sively broke up into macro-microcontinents, which Silurian boundary; Lo´ pez-Moro et al. 2007; Keppie later further fragmented, collided, and subse- et al. 2008). quently reassembled in Pangea. This process was In particular, the significance of the Paleozoic Rheic Ocean to the building of Laurussia and Gond- wana continental blocks before the Variscan- Manuscript received February 17, 2011; accepted October 19, 2011. Alleghanian-Ouachita collision has been recently * Author for correspondence; e-mail: [email protected]. emphasized and its Cambrian-Paleozoic evolution

273 274 L. GAGGERO ET AL. analyzed and dissected along the western, central, southern European Variscides (Casini et al. 2010), and eastern Gondwana margins (Nance et al. 2010). and it can be divided into the following tectono- Despite the occurrence in Sardinia of an almost metamorphic zones: a foreland zone in the SW, complete cross section of the Variscan orogen, this with either very low-grade or no metamorphism; a segment has barely been included in comprehen- nappe zone in the SE and central parts of the island sive reconstructions of the precollisional history of (subdivided into external and internal nappes, with the Variscan belt, probably due to the scarcity of several stacked tectonic units), affected by low- geochronological and geochemical data on the Sar- grade metamorphism; and an inner zone in the dinian pre-Variscan basement. A set of recent U-Pb north, with medium- to high-grade metamorphism geochronological data (obtained using excimer laser (fig. 1). ablation–inductively coupled plasma mass spec- The foreland and nappe zones are also charac- trometry; Oggiano et al. 2010) provides evidence terized by a Middle Ordovician angular unconfor- that in Sardinia the precollisional volcanic activity mity (Sardic Unconformity; Carmignani et al. 2001, along the North Gondwana margin, or in related and references therein), which is also recognized in terrane assemblages, developed in at least three the Eastern Iberian Plate (Casas et al. 2010; Navidad stages, each stage being characterized by a different et al. 2010). Along the southern boundary of the geodynamic environment: (1) a Late Cambrian– inner zone, an eclogite-bearing belt is exposed, Early Ordovician episode of volcanism (ca. 492–480 which was interpreted as a suture zone (Cappelli Ma) within a stratigraphically well-constrained et al. 1992; Carmignani et al. 1994). The protolith Ma; the 2 ע Cambro-Ordovician clastic sequence, (2) Middle of the eclogite has been dated at457 Ordovician calc-alkalic activity ascribed to the high-pressure event is Devonian (Cortesogno et al. Dapingian-Sandbian on the basis of paleontology 2004; Giacomini et al. 2005; Franceschelli et al. and now dated radiometrically at ca. 465 Ma, and 2007). These eclogites have MORB signatures and (3) an uppermost Ordovician (ca. 440 Ma) volcanic are embedded within a metapelitic-metarenaceous event of alkaline affinity that is widespread in all complex hosting also homogeneous quartzite beds the tectonic units of the Sardinian Variscides. In (metacherts?), orthogneisses, and metabasite with general, however, the various tectonic units are high- to medium-P granulite metamorphic imprint characterized by wide variations in these volcanic (Franceschelli et al. 2007). The deformation, local- rocks in space, time, and volume, and this is typ- ized in low-strength shear zones, and the geometric ically combined with a lack of adequate age control association of rock bodies with different metamor- on the associated clastic sediments. Our interest, phic records point to a me´lange of rocks tectoni- therefore, was raised in obtaining more data, valid cally sampled from diverse crustal levels within a for reconstruction of the paleogeography and geo- channel flow, probably linked to the subduction of dynamic events (rifting, breakup, drifting, accre- a lower Paleozoic ocean (Cappelli et al. 1992; tion/hypercollision) related to the northern Gond- Stampfli et al. 2002; von Raumer et al. 2003). wana margin and its derived “terranes” over a time Throughout the external nappes (Carmignani et period ranging from the Cambro-Ordovician up to al. 1994), the sedimentary record and fossil content the precollisional setting that gave rise to the Var- is generally preserved, and several volcano-sedi- iscan configuration of the Mesoeuropean crust. mentary complexes are stacked in the nappe zone Moreover, the petrology of Paleozoic processes is (Di Pisa et al. 1992; Carmignani et al. 1994; fig. 2). fundamental to understanding the influence on the The Sardinia-Corsica Microplate at the Cambro- lithospheric setting until the Alpine cycle. Diverse Ordovician Boundary. In the foreland, the pre- sectors of Sardinian Variscides were therefore stud- Sardic sedimentary history is dominated by the de- ied and comparison made with adjacent paleogeo- position of epicontinental sediments (Nebida graphic areas. Group), including carbonate shelf deposits (Gon- The aim of this article is to characterize the geo- nesa Group), which are inferred to grade laterally chemical features of the volcanic rocks in order to into deeper siliciclastic sequences (Iglesias Group), constrain the source region and crustal evolution, all of which are topped by the Sardic Unconformity. as well as the nature of the geodynamic setting. The sedimentary rocks of the shelf-slope transition indicate passive continental margin conditions during the Late Cambrian–Early Ordovician (Cocozza Geological Setting 1979; Galassi and Gandin 1992; Pillola et al. 1995, Present-Day Geological Framework of Sardinia. and references therein). Only the Capo Spartivento The Sardinia-Corsica Microplate exhibits one of the orthogneiss, basement of the foreland, is referred most complete and best-preserved transects of the to this setting, although the error of the available JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 275

Figure 1. Generalized tectonic map of the Variscan basement of Sardinia and tectonic and metamorphic zones of the Variscan basement of Sardinia (a). metamorphism (Arenarie di San Vito). The ages of ע date does not allow a clear age attribution (478 13 Ma; Delaperrie`re and Lancelot 1989). the metasedimentary deposits, based on acritarch The pre-Sardic lithostratigraphic succession of biostratigraphy, range from Middle Cambrian to the external nappes consists of metasandstones, Lower Ordovician (Naud and Pittau Demelia 1987). phyllites, and quartzites (ﬁg. 2) affected by Variscan Interbedded in the uppermost part of the succession pumpellyite-actinolite and lower-greenschist facies is a volcanic suite that predates the Sardic Uncon- 276 L. GAGGERO ET AL.

Figure 2. Lithostratigraphic sketch of the relationships within the Lower Paleozoic successions, across the Variscan nappes of Sardinia. The volcanic products are highlighted (after Carmignani et al. 2001; Oggiano and Mameli 2006; Oggiano et al. 2010). formity (Oggiano et al. 2010); it consists of welded Pb dating (Oggiano et al. 2010) has now clearly es- -Ma, U-Pb zircon tablished their Early Ordovician age. This new dat 3.5 ע rhyolitic ignimbrites (491 age), trachyandesitic pyroclastic fall deposits, and ing also better constrains on the ages of the dacite to trachyte lava flows (fig. 3A). In the Meana Cambrian–Lower Ordovician host metasedimen- Sardo tectonic unit, the volcanic rocks within the tary rocks (fig. 2). pre-Sardic sequence occur as scarce epiclastic crys- The Middle Ordovician Succession. The post- tal-rich tuffites. Sardic succession in the foreland starts with huge In the innermost tectonic units (Li Trumbetti, amounts of continental, alluvial, fan-related con- NW Sardinia), significant volumes of Early Ordo- glomerates (M. Argentu Formation; Leone et al. vician metavolcanic rocks are preserved, despite 1991; Martini et al. 1992), and these grade upward the strong deformation and upper-greenschist fa- into a trangressive sequence made up mostly of cies metamorphism (Oggiano and Mameli 2006), sandstone and silt of Katian-Hirnantian age. No and they form meter-thick augen-textured metavolcanic rocks are found in this highstand episode dacite and metarhyolite lava flows within meta- of the post-Sardic phase. sandstones and slates of unknown age. The petro- Conversely, in various tectonic units within the graphic similarity of these volcanic rocks to the nappe zone, an arc-related post-Sardic volcanic Middle Ordovician porphyroids led to mistakenly suite occurs immediately above thin metaconglom- ascribe them to this younger date (Carmignani et erates (metaconglomerati di Muravera) or directly al. 1979; Di Pisa and Oggiano 1984), but recent U- on top of the pre-Sardic metamorphosed deposits. JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 277

Figure 3. Zr/TiO2 versus Nb/Y diagram (Winchester and Floyd 1977) for volcanic rocks of Early Ordovician (A), Middle Ordovician (B), and Upper Ordovician (C) ages. For comparison, the compositional trends (arrows) of Ararat high-Y (1), Easter Island (2), and Dunedin Volcano (3). In A, the compositions that fall in the comendite/pantellerite

ﬁeld on a Zr/TiO2 versus Nb/Y diagram are also reported on the Al2O3-FeOt classiﬁcation diagram (Macdonald et al. 1974).

The succession of metandesites that overlies the Middle Ordovician emersion of this sector of the Sardic Unconformity is associated with subordi- North Gondwana margin, a large-scale transgres- nate felsic metavolcanics, interlayered coarse sion marked the onset of a new sedimentary cycle meta-epiclastic rocks, and volcanoclastics (Monte in the Upper Ordovician (Katian-Hirnantian), Santa Vittoria Formation; Carmignani et al. 2001; which then continued through the Silurian and De- inclusive of the Corte Cerbos, Manixeddu, and vonian into the Early Carboniferous. Three main Serra Tonnai formations of Bosellini and Ogniben types of volcanic rock (fig. 3C) are distinguished 1968). In the decimeter-thick alluvial deposits of within the Upper Ordovician terrigenous succes- the Gerrei tectonic unit, micaceous metasand- sion: (1) effusive products (pillow lavas and hyalo- stones and mature quartz-arenites (Su Muzzioni clastics), concordant with the host sediments; (2) Formation; Funedda 2000) cap the volcano-sedi- sills and larger intrusions; and (3) dykes that cut mentary succession, indicating high erosion rates the entire Lower and Middle Ordovician on the volcanic edifice. In the Sarrabus, Gerrei, and successions. Ozieri tectonic units, the end of this volcanic ac- In the foreland, epiclastic rocks with pebbles of tivity is marked by rhyolite-rhyodacite lavas, ig- alkaline metabasite (Beccaluva et al. 1981) are as- nimbrites, and tuffs, with a combined thickness of sociated with the Hirnantian glacio-marine depos- !100 m (Porfidi grigi del Sarrabus and Porphyroid its of the San Marco Formation (Leone et al. 1991; formation; Calvino 1972). In the outermost nappe Ghienne et al. 2000). zone (i.e., Sarrabus), metadacite occurs as sparse The uppermost Ordovician terrigenous deposits ,Ma, U-Pb zircon age; in the external nappes are associated with dykes 1.4 ע dykes and sills (465.4 Oggiano et al. 2010) within the pre-Sardic se- sills, and basaltic lavas that locally have pillow quences, and they are inferred to be the feeder structures. Minor layers of epiclastic volcanites dykes of the Porfidi grigi del Sarrabus. crop out close to the Ordovician-Silurian boundary Ma; Oggiano et al. 2010). The alkalic 1.7 ע The Late Ordovician–Silurian Succession. After the (440 278 L. GAGGERO ET AL. volcanism is best developed in the inner nappe ized by white mica, epidote, chlorite, Fe oxide, and units (fig. 2), where relatively abundant sills of me- aggregates of albite. tadolerite, gabbroic stocks, and meta-epiclastic Middle Ordovician (ca. 465 Ma) Bimodal Volcanism. rocks are widespread within metasiltstones. Ages In the Gerrei tectonic unit, the andesite lavas (fig. are constrained by the oolitic ironstones and dia- 3B) exhibit blasto-porphyritic to glomeroporphyri- mictite associated with the Hirnantian glaciation tic textures with plagioclase and biotite pheno- (Oggiano and Mameli 2006). crysts set in a fine-grained groundmass. Minor secondary growths of chlorite, white mica, and Fe oxides developed at the expense of biotite, and al- Petrography of the Volcanic Suites bite ϩ sericite aggregates at the expense of plagio- Early Ordovician (492–480 Ma) Bimodal Volcanic clase. Generally, the andesites are leucocratic and Episode. In the Sarrabus Unit, the volcanic prod- weakly vesicular, and there are also some horizons ucts are pyroclastites, lava flows, and welded ig- of dacitic tuff. The andesites contain abundant pla- nimbrites. The pyroclastic fall deposits are char- gioclase and quartz phenocrysts and minor biotite acterized by abundant quartz fragments and rare K-feldspar fragments. (porphyritic index [PI] p 6–14; Ø p 0.6–1.0 mm) In the Sarrabus tectonic unit, sparse tabular bod- in a welded tuff matrix. Millimeter-thick quartzo- ies, intruded into the Cambro-Ordovician succes- feldspathic laminae suggest sorting by transport. sion, are characterized by porphyritic textures with

Rare plagioclase phenoclasts (An54–65) do not exceed plagioclase, biotite, and embayed quartz pheno- 0.3 mm in size. The lava flows have abundant crysts set in a fine-grained groundmass that has ע quartz, plagioclase (An35–60) K-feldspar as the been partly recrystallized to muscovite, chlorite, main phenocrystic phases (PI p 8–10; Ø p 1.2–1.6 albite, and aggregates of Fe oxides. Dacitic and mm) in a fine-grained sericitized groundmass. Some rhyolitic ignimbrites have porphyritic textures polycrystalline quartz xenocrysts also occur. The with embayed quartz, K-feldspar, and plagioclase, pyroclastic fall deposits and the lava flows have as well as biotite in the dacitic ignimbrites. The been pervasively deformed and partially replaced welded eutaxitic matrix of the ignimbrites includes by sericite, chlorite, and subordinate albite ϩ epi- pumice fragments recrystallized to sericite, devit- dote. The welded rhyolitic ignimbrites include rified glass shards, chalcedony-filled amygdales, crystal and fiamma-rich and glassy eutaxitic facies, quartz, albite, and aggregates of mica. which are visible macroscopically as alternating Upper Ordovician–Silurian (440 Ma) Basic-Interme- bands of quartz ϩ feldspar and biotite-rich material. diate Volcanism.In the external nappes, dykes and The crystal fragments are K-feldspar and quartz, sills embedded within the transgressive Upper Or- and these are set in a silicic fine-grained and/or dovician sequence are characterized by subrounded glassy devitrified groundmass. Quartz and K-feld- quartz and subordinate plagioclase phenocrysts set spar clasts, overgrown by microcline, are cracked, in a fine-grained quartzo-feldspathic groundmass. and the quartz is also subrounded, possibly as a Chlorite forms pseudomorphs after biotite. Locally, result of thermal corrosion during emplacement. the rocks have been fractured and cemented by a The biotite is largely altered to chlorite ϩ iron ox- network of calcite veins. Alkali basalt dykes cut ides. The originally glassy fiammae have been across the pre-Sardic sequence; these were inferred transformed to flattened aggregates of microcrys- to be the feeder dykes of the Upper Ordovician vol- talline quartz and feldspar. The very low-grade canism (Di Pisa et al. 1992). The dykes display in- metamorphic overprint resulted in sericite replac- tersertal to porphyritic textures with plagioclase ing K-feldspar. phenocrysts. Skeletal ilmenite and apatite occur, In the Meana Sardo Unit, rare dacite flows con- titano-magnetite has replaced ilmenite, and mafic tain phenocrysts of plagioclase set in a dark ground- phases and the groundmass are altered to chlorite, mass that contains minor quartz. The internal epidote, and Fe oxides. In the outermost unit (the nappes dacite and rhyolite flows are characterized Sarrabus Unit), plagioclase-bearing (An45–48) pillow by flattened augen of lobate quartz, plagioclase, and lavas contain olivine microphenocrysts preserving K-feldspar phenocrysts (PI p 4–5; Ø p 0.8 mm), the pristine mesh texture that are replaced by chlo- set in a fine-grained groundmass of quartz, feldspar, rite, diopside that is rimmed by hornblende and and phyllosilicates. The flows stratigraphically un- ilmenite, and a glassy to fine-grained intersertal derlie metabasites with a pervasive schistosity (e.g., groundmass with sparse calcite-filled vacuoles. Fi- the Li Trumbetti Unit). The fine-grained recrystal- brous blue-green amphibole develops in the outer lized groundmass of the metabasites is character- shell of the pillows and in fractures and veins. The JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 279 pillow lavas are in contact with metamorphic strata determining the pristine chemistry of an altered that are spotted as a result of a pre-Variscan thermal igneous rock, it is important to consider the pos- overprint. The spots were probably andalusite orig- sibility of chemical transformations that resulted inally but are now stilpnomelane. from contact with the host metasedimentary rocks In the internal nappes, the Ordovician succession or from early diagenetic processes, especially in ef- in the Canaglia Unit is cut by meter-thick sills of fusive rocks. In addition, the effects of element mo- hornblende-gabbro (fig. 3C), in part reequilibrated bility during tectonism and metamorphism (which under the greenschist facies. The sills show ortho- here includes pumpellyite to amphibolite facies cumulus textures, with euhedral seriate plagioclase overprints) have been considered. Primary com- and accessory Fe oxides with intercumulus horn- positional heterogeneities, observed in textures and blende. The coarser plagioclase grains are broken the modal distribution of minerals, were avoided and sutured by newly precipitated feldspar or bent, in sampling, as well as the occurrence of carbonates thus providing evidence of a syn-magmatic defor- and hydrous phases, which indicate fluid-induced mation. Plagioclase-rich diorite veins cut the gab- modifications. Nevertheless, meso- and micro- bro body. scopic observations generally enable the correlation of mineral phases and bulk rock compositions. The samples analyzed were selected with these Geochemistry of the Volcanic Suites points in mind, but caution is still required when Analytical Methods. Forty-four rock samples interpreting the data. were collected from the tectonic units described Late Cambrian–Early Ordovician Volcanism. above, and they were selected to represent different The Early Ordovician metavolcanic rocks are occurrences and timing of events. The samples mainly silicic (52.4–84.1 wt% SiO2 on an anhy- were analyzed for major and trace elements using drous basis) and are weakly to strongly peralumi- the X-ray fluorescence facilities at SGS Laborato- nous with an aluminium saturation index (ASI p ries. Loss on ignition was determined with the grav- (Al/Ca Ϫ 1.67)(P ϩ Na ϩ K); Zen 1988) in the range imetric method. The rare earth elements (REEs) 1.1–3.4. The nature of their protolith is indicated were analyzed using inductively coupled plasma by using a Zr/TiO2 versus Nb/Y classification di- mass spectrometry at SGS Laboratories. The bulk- agram (Winchester and Floyd 1977; fig. 3A), where rock compositions for Sardinian volcanic rocks of the data define a mildly alkaline trend. The lava Early, Middle, and Upper Ordovician ages are in- flows consist of subalkalic basalt, dacite, trachyan- cluded in the repository data. desite, trachyte, and comendite. The ignimbrites Twenty-seven of the rock samples were selected have rhyolitic and comenditic trachyte composi- and analyzed for Sr and Nd isotopes at the Geo- tions (Al2O3 vs. FeOtot; Macdonald et al. 1974), chemistry Laboratory of Trieste University. Sam- whereas the pyroclastic fall deposits are trachyan- ples were dissolved in Teflon vials for isotopic anal- desitic. Overall, they define linear fractionation ysis using a mixture of purified HF-HNO3 and HCl trends with negative correlations of TiO2,Al2O3, reagents. Sr and Nd were collected after ion ex- K2O, and Rb with SiO2 (fig. 4). The scatter for K2O change and reversed-phase chromatography, re- suggests that mica is the major component of the spectively; the total blank for Sr was !20 pg. The fractionating assemblage, because of the generally Sr and Nd isotopic compositions were obtained us- incompatible behavior of Rb in K-feldspar within ing a VG 54E mass spectrometer and Analyst soft- peraluminous melts (Icenhower and London 1996). ware (Ludwig 1994) for data acquisition and reduc- Most lava flows and ignimbrites are character- tion. The 87Sr/86Sr and 143Nd/144Nd ratios were ized by a slight fractionation of light REEs (LREEs; 86 88 p p fractionation corrected to Sr/ Sr 0.1194 and LaCN/SmCN 1.14–6.71) and negative Eu anoma- 146Nd/144Nd p 0.7219, respectively, and the mea- lies, which become more pronounced toward sured ratios were corrected for instrumental bias to evolved compositions (fig. 5A). In trachyandesitic NBS 987 and JNdi-1 standard values of 0.71025 and pyroclastic rocks and trachyte lava, the negative Eu 0.512100. Repeated analyses of the NBS 987 and anomaly is absent, and there is significant heavy p JNdi-1 standards gave average values of REE (HREE) fractionation (GdCN/YbCN 0.67– ע p ע 0.71025 0.00002 (n 15 ) and 0.51211 14.31). The positive correlation of REE with SiO2 0.00002 (n p 10 ), and no corrections were applied suggests minor fractionation of REE-rich accessory to the measured data for instrumental bias. The phases. reported errors represent the 95% confidence level. In the primitive mantle-normalized multiele- Geochemical Features of the Volcanic Products. In ment diagram, the trace element patterns of the 280 L. GAGGERO ET AL.

Figure 4. Al2O3,Fe2O3t,K2O, P2O5,TiO2, and Rb versus silica diagrams for Early Ordovician volcanic rocks. Filled symbols correspond to the external Variscan nappe zone (circle, Sarrabus tectonic unit; star, Meana Sardo tectonic unit). Open symbols correspond to the Internal Variscan nappe zone (circle, Li Trumbetti tectonic unit; star, Canaglia tectonic unit). See also figure A1, available in the online edition or from the Journal of Geology office. subalkali basalts are characterized by an overall with negative Eu anomalies (Eu/Eu∗ p 0.53–0.79 ) large ion lithophile element (LILE) and LREE en- are evident (fig. 5B). richment, fractionation between LILEs, high field In the primitive mantle-normalized multiele- strength elements (HFSEs) and LREEs/HREEs (fig. ment diagram, the trace element patterns show en- 6A), Ba depletion, a Ta-Nb trough, a marked neg- richment in LILEs and LREEs, associated with ative Sr anomaly, and a negative Eu anomaly LILE/HFSE and LREE/HREE fractionation, and Ta- (Eu/Eu∗ up to 0.63), all of which suggest that the Nb, Sr, P, and Ti troughs (fig. 6B). Andesitic tuffs melts experienced some degree of plagioclase are the least LILE enriched. fractionation. Upper Ordovician–Silurian Volcanism.The Middle Ordovician Volcanism. The Middle Or- Upper Ordovician lava flows, sills, and dykes are dovician volcanic rocks consist of a calc-alkaline mostly alkali basalt, but there are also some trachy- suite of andesites, dacites, and rhyolites (fig. 3B). andesite pillow lavas (fig. 3C). The alkali basalts

Andesites have low Mg# values (32–24) and very have SiO2 contents between 44.5 and 51.9 wt% and low Ni (7–48 ppm) and Cr contents (66–137 ppm). Mg# values in the range 43–15. Fractionated REE p The positive correlations of K2O, Ba, and Rb versus patterns (LaCN/YbCN 3.23–12.99) with positive or silica (fig. 4B) reflect the role of K-feldspar and/or no Eu anomalies are evident (fig. 5C). The high REE mica in fractionation. Fractionated chondrite-nor- contents and positive Eu anomalies could represent p malized REE patterns (LaCN/YbCN 6.22–12.44) melts that experienced significant hornblende/cli- JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 281

samples ORD32 and ORD44). In general, there is an overall Ta and Nb enrichment relative to La, a fractionation between LREEs and HREEs, and a negative Sr anomaly (except for sample ORD42). Trachyandesite and dacite lavas have REE abundances that lie within the range for alkali basalts

Figure 5. Rare earth element patterns normalized to primitive mantle (Sun and McDonough 1989) for volcanic rocks of Early Ordovician (A), Middle Ordovician (B), and Upper Ordovician (C) ages. nopyroxene fractionation, while negligible Eu anomalies could be produced by equal proportions of plagioclase and hornblende/clinopyroxene fractional crystallization (Hanson 1980). However, the primary composition of the 440- Ma metavolcanic rocks could have been affected by element mobilization during metamorphism or, to some extent, during weathering, as indicated by petrography and loss on ignition values up to 10%. Weathering could affect the concentration of the most incompatible elements (e.g., Rb, Ba, and K), which are known to be mobile under surface al- teration. This possibility is indicated for alkali- basalts, which display a considerable scatter of LILEs in the primitive mantle-normalized multi- Figure 6. Multielement diagrams normalized to prim- element diagram (ﬁg. 6C). The patterns reveal itive mantle (Sun and McDonough 1989) for maﬁc vol- mainly Rb and K troughs relative to Ba and Th canic rocks of Early Ordovician (A), Middle Ordovician (however, Rb and K enrichments are observed in (B), and Upper Ordovician (C) ages. 282 L. GAGGERO ET AL.

Figure 7. Early, Middle, and Upper Ordovician mafic volcanics from Sardinia plotted on the Zr-Nb-Y discrimination diagram (Meschede 1986). A1, within-plate alkali basalts; A2, within-plate alkali basalts and within-plate tholeiites; B, E-type mid-ocean ridge basalt (MORB); C, within-plate tholeiites and volcanic-arc basalts; D, N-MORB and volcanic arc basalts. with negative Eu anomalies (∼0.56–0.59), but sam- relationships indicate an enriched mantle source, ϩ ple B8 has an almost flat REE pattern (LaCN/SmCN although compositions with CaO MgO exceed- p 1.2). The lavas are depleted in Ba, Nb, Sr, P, and ing 12–20 wt% are to be considered with caution Ti in the primitive mantle-normalized multiele- in the discriminant diagram of Meschede (1986; fig. ment diagram, while sample B8 shows Hf and Zr 7). enrichments relative to LREEs and a positive Sr The Nb negative anomaly, typical of subalkalic anomaly. basalts in multielement diagrams (fig. 8), is considered to indicate subduction-related magmas (e.g., Pearce 1983), but it is also found in many Tracking the Sources continental flood basalts. The intermediate com- Late Cambrian–Early Ordovician Volcanism. Be- positions (dacite, trachyandesite, and trachyte) cause of their very low Mg# values (20–14) and low have lower contents of Ta and Nb, corresponding Ni (∼45 ppm) and Cr contents (∼147 ppm), the Early to contamination by—or anatexis in—the middle- Ordovician subalkali-basalts cannot be considered lower continental crust, where their abundances as primitive melts. are lower than in upper crust and MORB (Weaver In the Early Ordovician volcanites, the Nb/Ta and Tarney 1984). High-silica rocks reveal LILE, Ta, ratio approaches 11, representative of crustal- and Nb enrichment (relative to HFSEs) in the ocean derived magmas (Green 1995). Overall, the patterns ridge granite–normalized multielement diagram in figure 6 compare favorably with the middle crust (fig. 8A), and this is probably evidence of contam- from rifted continental margins (Rudnick and ination in the upper crust. The Th/Ta ratios in the Fountain 1995). The high Th/Ta (11.5) and La/Nb felsic volcanics are generally lower than those in (3.39) values suggest a significant contamination by magmas from active continental margins (Gorton the upper continental crust rather than melting at and Schandl 2000), and there is a considerable over- middle-crustal levels, although this possibility can- lap with the within-plate volcanic zones field (fig. not be ruled out entirely. However, the Zr-Nb-Y 9). JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 283

Upper Ordovician–Silurian Volcanism. The alkali basalts have low Th/Ta ratios in the range 1–2 and Ta/Yb ratios that are !2, in the field of within-plate volcanism. La/Nb values that are !1 in primitive mantle–normalized within-plate basalts compositions have been interpreted as related to (1) an “orogenic” component, possibly a hydrous metasomatized mantle that preserved a record of older subduction events (Cabanis and Thieblemont 1988; Cabanis et al. 1990), or (2) the involvement of lower continental crust (Innocent et al. 1994) or some segments of the continental lithospheric mantle (Hooper and Hawkesworth 1993, and references within) in the mantle source. Sr-Nd Isotopic Compositions. The positive ␧ Nd490 Ma values of Early Ordovician rhyolitic ignimbrites (ϩ1.15 to ϩ2.42) signify that their pre- ∼ cursors, with a crustal residence age of 1Ga(TDM), were derived from a long-term depleted mantle source. 87 87 86 p The high radiogenic Sr content ( Sr/ Sr490 Ma 0.71169) and the Nd isotope fingerprint for one Early Ordovician dacite lava (corresponding to an ␧ Nd490 Ma value of –6.54) provide evidence for a role of lower crust in the source region, alternatively (1) a single stage mantle melt and fractional crystallization or (2) a mixing of primary basaltic melts Figure 8. Multielement diagrams normalized to ocean issued from the mantle with some amounts of re- ridge granites (Pearce et al. 1984) for felsic volcanic rocks melted material either from the crust or at earlier of Early Ordovician (A) and Middle Ordovician (B) ages. phases of magmatism. The high radiogenic 87Sr content could imply a time-integrated LREE-enriched The strong HREE depletion and high values of source, consistent with the anatexis of a metased- Ce/Yb and Nb/Y in the trachyandesitic pyroclastic imentary component. The latter could be a Cadom- rocks and trachyte lava are consistent with small ian basement, which is well known in the hinter- melt fractions and/or a melt fraction in equilibrium land of Corsica (Barca et al. 1996; Rossi et al. 2009), with a garnet residuum, possibly derived from a felsic granulites from the European Variscides lower-crustal source. (Liew and Hofmann 1988), or crustal contamina- Middle Ordovician Volcanism. The Middle Ordo- tion of mantle-derived magmas (fig. A2, available vician volcanic rocks in both the internal and ex- in the online edition or from the Journal of Geology ternal Variscan tectonic units consist of interme- office). In effect, the data match the isotopic data diate to fractionated rhyolitic compositions. The reported by Pin and Marini (1993) for coeval felsic andesites are characterized by higher Mg# values volcanics from the southern Massif Central ␧ and lower Ni and Cr contents than the Early Or- (France), where Nd480 Ma lies between –3.5 and 87 86 dovician subalkalic basalts, consistent with a sub- –6.0, and Sr/ Sr480 Ma is in the range 0.70889 to alkalic calc-alkalic affinity. HFSE (La, Ce, Hf, Th, 0.70901 (fig. 10). Zr, and Sm) enrichments are evident, when com- The Sr and Nd isotopic compositions of Middle pared with the primitive mantle normalization, Ordovician andesite lava and dacitic tuff are con- and this confirms a crustal contribution; however, sistent with a mantle source that is less depleted although LILEs (Rb, K) are generally enriched, they than the source of the Early Ordovician rhyolitic ␧ p Ϫ Ϫ can also be affected by secondary mobilizations. ignimbrites ( Nd465 Ma 3.03 to 5.75; 87 86 p Most significantly, the Th/Ta and Zr-Nb-Y rela- Sr/ Sr465 Ma 0.70931–0.71071; fig. A2). The data tionships within the Middle Ordovician effusives make a good match with the compositions of the ␧ p are indicative of arc volcanic rocks and, in partic- Lode` and Golfo Aranci orthogneisses ( Nd465 Ma ular, with arc rocks from an active continental mar- Ϫ4.32 to Ϫ5.11; Di Vincenzo et al. 1996), though 87 86 87 86 p gin (figs. 7, 9). with a scatter for Sr/ Sr ( Sr/ Sr465 Ma 0.69965– 284 L. GAGGERO ET AL.

Figure 9. Th/Yb-Ta/Yb diagram (Gorton and Schandl 2000) for felsic volcanics from Sardinia. ACM, active continental margin; WPVZ, within-plate volcanic zone; WPB, within-plate basalts.

Ma suggests 1.7 ע The negative Nd values suggest the melt- closeness of this value to440 .(0.70839 ing of a sub-arc mantle that was variably enriched that the Sm-Nd isotope system has not been deeply by the recycling of continental material, possibly perturbed since the crystallization of the magmas. during subduction events that predate the Middle The ␧Nd values for the alkali basalts (calculated Ordovician arc. fort p 440 Ma) range from ϩ1.60 to ϩ4.14, reflect- Crustal residency ages, indicated by Nd model ing an origin in a depleted mantle source, while the 87 86 age data, suggest that (1) this continental block un- Sr/ Sr440 Ma values vary from 0.70520 to 0.71321 derwent orogenesis at 1.0 and 1.4–1.6 Ga, incor- (fig. A2). One very low 87Sr/86Sr value of 0.69434 porating crustal components from different source (sample ORD32) is probably due to Rb and Sr mo- areas, or (2) there was a juxtaposition of Mesopro- bilization during the Variscan metamorphic event. terozoic crust above a Neoproterozoic subconti- On the whole, the results are consistent with the Ma- 17 ע nental mantle lithosphere (Murphy et al. 2008). Nd isotopic compositions of the436 The similar Nd isotopic compositions of the mafic sills from the Central Iberian Zone (Sm-Nd Early and Middle Ordovician dacite lavas should be isochron age; Lo´ pez-Moro et al. 2007). The negative ␧ considered as an inherited feature that resulted Nd440 Ma values of –4.76 and –4.62 for the trachy- from the repeated extraction of melts from a com- andesite pillow lavas, coupled with the high Th/Ta p mon basement source (TDM 1.4–1.6 Ga). This in- values (3.4–6.8), indicate a less depleted mantle 87 86 terpretation is supported by the population of Neo- source, while the values of Sr/ Sr440 Ma (0.70511– proterozoic inherited zircons, which is conspicuous 0.70694) and Sm/Nd (up to 0.36) extend along the in the Early Ordovician felsic volcanic rocks but mantle array. less so in the Middle Ordovician andesites (Oggiano et al. 2010). Discussion According to the 143Nd/144Nd vs. 147Sm/144Nd correlation, the slope of the best-fit line of the eight According to von Raumer and Stampfli (2008), the Upper Ordovician alkali basalt samples corre- northward dispersal of crustal blocks derived from sponds to 440 Ma, though with a large scatter the North Gondwana margin occurred between the (MSWD p 10.1), and an initial 143Nd/144Nd ratio of Late Cambrian and Devonian under a variety of -In spite of the uncertainty, the kinematic regimes, including local back-arc spread .0.00052 ע 0.51221 JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 285 ing, intracontinental rifting, and even the spreading light an ongoing extension along the northern mar- of a mid-ocean ridge (e.g., in the Chamrousse area ginal areas of Gondwana during the Early of the western Alps). Ordovician. Late Cambrian–Early Ordovician. It is widely re- There exists uncertainty in terms of correlating ported that Late Cambrian–Early Ordovician bi- the stratigraphic and timing relationships of vol- modal magmatism within terrigenous and carbon- canic events in the pre-Sardic succession of Sar- ate shelf deposits is related to the diachronous dinia with the Armorican (Balle`vre et al. 2009) or northward rifting-to-drifting of microplates from Iberian domains (Lo´ pez–Guijarro et al. 2008). Over- the western (ca. 500 Ma, Avalonian terranes) to the all, the different domains of Late Cambrian–Early eastern Gondwana margins (ca. 460 Ma, Hun and Ordovician volcanism seem to retain distinctive Galatian terranes; e.g., Carmignani et al. 1994; geochemical features. Stampfli et al. 2002; Sańchez-Garcıá et al. 2003; Taken together, the geochemical and isotopic Etxebarria et al. 2006; Murphy et al. 2006; Montero data for the felsic volcanics suggest (1) an origin by et al. 2007; Pin et al. 2007; Castin˜ eiras et al. 2011; anatexis of a crustal source, probably as small melt Chichorro et al. 2008). fractions and/or with a garnet control (i.e., thick In the French Massif Central, the Late Cambrian– lithosphere). However, in the alternative of (2) re- Lower Ordovician volcanic rocks are the protoliths melting in the crust and mixing, the melt fraction of the leptynite-amphibolite complexes (487–478 can vary over a wide range, and metamorphic gar- Ma; Pin and Lancelot 1982). In the Bohemian mas- net can occur in the crustal source region without sif, Pin et al. (2007) reported a 500-Ma bimodal suite need of an unusual thick lithosphere. Afterward, that is referred to as anorogenic, supposedly related the transition from early felsic to later more mafic to extension along the margin of North Gondwana, magmatism points to the gradual opening of the although no oceanic lithosphere was generated. In system. The middle-crustal signature of the sub- Iberia, rhyolitic to dacitic tuffs and later alkaline alkalic basalts could be produced by crustal melting basaltic lava flows with ages of 515–490 Ma (Ossa fed by the intrusion of small batches of hot magma Morena Zone and Iberian Chain; A´ lvaro et al. 2008; from the mantle, probably linked to the onset of a Chichorro et al. 2008) match both the ages and the volcanic passive margin. signatures of the “pre-Sardic phase” volcanic rocks The structural style of the Cambro-Ordovician of the Sardinia nappe zone. Because of their sub- rift at upper-crustal and syn-rift sedimentary levels alkaline to alkaline signatures and the association as well as the geochemical signature of the volcanic with terrigenous sedimentation, an ensialic rift en- rocks suggests the lithosphere was relatively thick vironment preliminary to oceanization has been in- and cold at depth. However, the volumes of the ferred (Sańchez-Garcıá et al. 2003; Etxebarria et al. intrusive and effusive volcanic rocks are much 2006; Chichorro et al. 2008). Dıéz Montes et al. smaller than those found in present-day volcanic (2010) referred the Ollo de Sapo volcanic rocks to passive margins (e.g., Tsikalas et al. 2008; Hirsch an incipient rift that evolved by the necking and et al. 2009; Voss et al. 2009). Therefore, the overall crustal-scale boudinage of the wide passive margin tectonic setting of the Cambro-Ordovician rift in of North Gondwana. Mafic magmas that intrude Sardinia better fits the features of an aborted rift. or underplate the lower crust were proposed as heat In general, it seems that a diversity of geological sources for crustal melting. settings existed along the North Gondwana margin The occurrence of pre-Sardic Late Cambrian– at the time of the Cambrian-Ordovician boundary. Early Ordovician volcanic rocks in Sardinia sug- Mid-Ordovician. The conspicuous Mid-Ordovi- gests the following possible scenarios: (1) a conti- cian Andean-type arc volcanism, which marks the nental volcanic arc that lasted from the Upper onset of subduction beneath the southern Rheic Cambrian to the Middle Ordovician and that can margin after the Lower Ordovician (Carmignani et be related to the subduction of a proto-Tethyan oce- al. 1994), ceased in the Upper Ordovician. This sub- anic crust beneath the northern peripheries of alkalic calc-alkalic suite developed after the Sardic Gondwana, (2) a back-arc region that was subjected phase, which is widespread over all the external to extension (e.g., Iberia; Fernańdez et al. 2008), (3) nappes but absent in the internal nappes and in the an aborted rifting stage during the Late Cambrian, foreland, can be bracketed stratigraphically be- and (4) the onset of a passive volcanic margin (e.g., tween the Floian and the Katian. In fact, in the the Cameroon line; Fitton 1987) that preceded the external nappes, the volcanism formerly ascribed calc-alkaline, subduction-related, Middle Ordovi- to the Middle Ordovician cycle has been recently and 1.2 ע cian suite. Felsic and mafic volcanic activities high- reassessed as Early Ordovician (486

JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 287

Ma; Oggiano et al. 2010). In the internal or the Rheic and Paleotethys oceans in this region 2.1 ע 479.9 nappes, volcanic rocks may possibly have been the during the Ordovician and Silurian. protoliths of the migmatites, as inferred by Cru- ciani et al. (2001, 2008). The lack of Middle Or- Concluding Remarks dovician volcanic rocks in the foreland suggests that the this region acted as an Andean-type back- The new data presented here on pre-Variscan vol- arc, which experienced only continental clastic canic rocks of Sardinia improve our understanding sedimentation and which was subjected to constant of the geodynamic setting of the North Gondwana uplift and renewal of relief as a result of faulting margin before the Variscan collision within the (Martini et al. 1992). The distribution of Middle Southern Variscan Realm. In fact, despite good ex- Ordovician volcanic products is restricted to the posures, the Sardinian segment of the Variscan external nappes, where mostly felsic and inter- chain has barely been considered in reconstructions mediate products occur. of the Variscan orogeny since the first attempts of Late Ordovician. Late Ordovician–Early Silurian Carmignani et al. (1994). Broadly speaking, previ- mafic lavas, sills (Lehman 1975; Ricci and Sabatini ous reconstructions of the South Variscan Branch 1978; Oggiano and Mameli 2006), and their feeder have envisaged a complex pattern of local back-arc dykes (Di Pisa et al. 1992) have been described and basins or intracontinental rifts during the Early dated (Oggiano et al. 2010) in the Sardinia base- Cambrian, followed by Upper Cambrian–Early Or- ment. On the regional scale, Silurian mafic sills dovician closure (von Raumer and Stampfli 2008). have been documented from the Central Iberian However, these scenarios do not match the geo- -Ma, Sm-Nd isochron; Lo´ pez-Moro logical evolution of Sardinia, as indicated by its vol 17 ע Zone (436 et al. 2007). Only a few works have focused on this canic activity. In fact, the following are the main volcanism in Sardinia, and Di Pisa et al. (1992) have findings indicated by the geochemistry of the vol- described the field relationships, petrography, and canic rocks and their spatial and temporal rela- elemental geochemistry. These rocks were inferred tionships (fig. 11): to be of Lower Carboniferous age (Di Pisa et al. 1. In Sardinia, the mildly alkaline, pre-Sardic vol- 1992). The bulk compositions of the mafic lavas, canic activity represents an intracontinental exten- sills, and dykes are homogeneous, thus supporting sional setting such as a rift. This rifting is coeval the concept of a common volcanic event that over- with the breakup that led to the opening of the printed the internal and external nappes at the time Rheic Ocean and the drift of Avalonia to the west of the Ordovician-Silurian boundary. By analogy of Sardinia crust, which did not propagate to this with modern equivalents of postorogenic volca- part of the North Gondwana margin. There is no nism, an origin by partial melting at minimum evidence in the Late Cambrian–Early Ordovician temperatures could be envisaged. volcanic rocks for the convergence that started later However, the geochemical data for the Upper Or- in the Middle Ordovician. dovician alkalic suite in Sardinia suggest a conti- 2. During the Middle Ordovician, the Sardinian nental rift geodynamic setting, most probably an crust experienced arc volcanism that is well con- early phase of the major rifting event that led to strained by geochronology, geochemistry, and field expansion of the Paleotethys. Nevertheless, a re- evidence. This volcanism differed from the large- construction along the northeastern Gondwana scale, extension-related volcanic activity (reported margin (von Raumer and Stampfli 2008; Rossi et elsewhere from North Gondwana) that led to the al. 2009) at this time points to back-arc spreading. opening of the Paleotethys at the same time as the We consider that the radiometric, geochemical, and eastern Gondwana margin experienced the start of isotopic data for contiguous terranes along the the drift of the future Hun superterrane. North Gondwana margin suggest a variety of geo- 3. The Upper Ordovician–Early Silurian meta- dynamic settings rather than one continuous ex- basalts show unquestionable alkaline signatures tensional regime related to the opening of the Rheic that constrain the onset of an important period of

Figure 10. A, ␧Ndi versus Ti/Yb # 10Ϫ4 and MORB ﬁeld, source enrichment, and crustal contamination vectors. B, ␧ 87 86 Ndi versus Sr/ Sri. For comparison with Early Ordovician volcanism, felsic volcanics from the southern Massif Central (Pin and Marini 1993) are reported. Lode` and Golfo Aranci orthogneisses (Di Vincenzo et al. 1996) are reported to be similar to the Middle Ordovician volcanic rocks. 288 L. GAGGERO ET AL.

Figure 11. Schematic palinspastic reconstruction of the possible evolution of northern Gondwana between the early and middle Paleozoic, as inferred from geochemical and geological evidence in Sardinia. rift dynamics along the North Gondwana margin. wide enough to support faunal and/or climatic dif- If this rift had led to the detachment of the Ar- ferentiation. Assuming the average spreading rates morica Terrane Assemblage or Galatian from North of present-day back-arc basins and oceanic rifts, Gondwana, the onset of the drift of terranes should North Gondwana and its detached terranes—at have occurred in the Early Silurian, possibly driven least those involved in the Southern Variscan by slab retreat after an aborted rifting episode at Realm—display similar climatic environments and the Late Cambrian–Early Ordovician boundary, and must be set at high latitudes until the Early Silu- after the onset of oceanic (Rheic) subduction be- rian. Although paleomagnetic data are scant and neath the North Gondwana margin in the Middle debated when available (Robardet 2003), the Ordovician (ﬁg. 11). It has yet to be established Ordovician-Silurian diamictite of glacio-marine or- whether the expanding oceanic basin and conse- igin of Sardinia (Oggiano and Mameli 2006) provide quent latitudinal gap between the shelves of Gond- the lithostratigraphic evidence of eustatic emer- wana and the Armorica Terrane Assemblage were gence, ﬁt in well with the glacio-eustatic record of JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 289

North Gondwana (Loi et al. 2010), and are com- ACKNOWLEDGMENTS parable with the coeval glacial deposits of Brittany This research was carried out with the aid of PRIN (Pic¸arra et al. 2002), Corsica (Barca et al. 1996), Can- 2004 (L. Cortesogno, G. Oggiano), PRIN 2008 (G. tabria (Gutie´rrez-Marco et al. 2010), and Thuringia Oggiano, L. Gaggero), Ateneo 2006 (University of (Erdtmann 1991). All these crustal sectors are re- Genoa), a grant to L. Gaggero, PRIN 2007, and ex lated to the Armorica Terrane Assemblage, which 60% University of Cagliari grants to A. Funedda. detached from Gondwana and were accreted on We thank F. Slejko and R. Petrini for help with Laurussia. acquiring isotope data.

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Appendix from L. Gaggero et al., “Rifting and Arc-Related Early Paleozoic Volcanism along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy)” (J. Geol., vol. 120, no. 3, p. 273)

1 APPENDIX FROM L. GAGGERO ET AL., PALEOZOIC VOLCANISMS IN NORTH GONDWANA

Supplemental Figures

Figure A1. Occurrences of Ordovician volcanic rocks in the Sardinian nappes and tectonic units, and sampling locations.

2 APPENDIX FROM L. GAGGERO ET AL., PALEOZOIC VOLCANISMS IN NORTH GONDWANA Rb-Sr and Sm-Nd concentrations and isotopic analyses for selected samples from the external and internal nappes of Sardinia. Figure A2.