New evidence of Pre-Hercynian volcanics from Southern ...

ABSTRACT

Several occurrences of metamorphosed pre-Hercynian volcanicrocks crop out in the Paleozoic basement of Southern Sulcis (SWSardinia), hosted in Cambrian-Early Carboniferous sedimentarysequences belonging to two different structural units (Sulcis-Igle-siente and Arburese tectonic units). Basic lava flows and intermedi-ate to acid sub-volcanic bodies may be related to different Paleozoicvolcanic events. Geological, petrographical, chemical and U-Pbgeochronological data suggest three distinct volcanic episodes: (1)an Early Ordovician episode, (475±10 Ma; U-Pb standard determina-tions on zircons), consisting of intermediate to acid calcalkalinevolcanics, presumably related to an ancient active continentalmargin/volcanic arc setting; (2) a Middle Devonian episode (387±2Ma; U-Pb standard determinations on zircons), with calcalkalineacid products of orogenic affinity; (3) a Devonian(?)-Early Carbonif-erous (?) episode, consisting of within-plate alkaline basalts (WPB),derived from a depleted upper mantle source. Early Ordovician cal-calkaline metavolcanics and Devonian(?)-Early Carboniferous (?)metabasites may be correlated to similar magmatic products typicalof the External Nappe Zone of SE Sardinia. Regional-scale correla-tions for the Middle Devonian acid metavolcanics, here defined forthe first time in Sardinia, are still problematic; age determinationssuggest to date back the lower part of the hosting lithostratigraphicunit, interpreted as Hercynian flysch of SW Sardinia.

KEY WORDS: metavolcanics, SW Sardinia, paleozoic base-ment, pre-hercynian magmatic events, geochronology.

RIASSUNTO

Nuove evidenze di attività vulcanica pre-ercinica nel Sulcis(Sardegna Sud-occidentale).

Il rilevamento geologico di una vasta area del basamento paleo-zoico del Sulcis meridionale (Sardegna sud-occidentale: ProgettoCARG Sardegna - Carta Geologica d’Italia alla scala 1:50000, Foglio565 Capoterra, SERRI et alii, 2004, in stampa), ha consentito nonsolo la verifica della presenza di diversi affioramenti di metavulcani-ti e metavulcanoclastiti già descritte, ma ha anche permesso di sco-prire nuove manifestazioni, a carattere sia acido sia basico, che testi-moniano eventi vulcanici paleozoici distinti nel tempo.

Le metavulcaniti sono state campionate e studiate in due settoriprincipali (Villa San Pietro e Capoterra), nei quali affiorano le suc-cessioni appartenenti all’unità autoctona dell’Iglesiente-Sulcis, cui,nei settori più orientali, è tettonicamente sovrapposta l’Unità dell’Ar-burese, appartenente alle Falde Esterne del basamento paleozoicosardo (CONTI et alii, 2001). I dati geologici, petrografici, geochimici,

nonché quelli geocronologici (determinazioni standard U-Pb su zir-coni), permettono di delineare un quadro composito degli eventimagmatici, nel quale è possibile distinguere:

1) un evento dell’Ordoviciano inferiore (475±10 Ma), individua-to esclusivamente nelle successioni appartenenti all’Unità dell’Arbu-rese (settore di Capoterra) e costituito da metavulcaniti intermedio-acide calcalcaline (metadaciti), del tutto analoghe, da un punto divista petrografico e geochimico, ai prodotti del magmatismo di arcodell’Ordoviciano inferiore-medio della Sardegna centrale e sud-orientale, tipiche di un’ambientazione geodinamica di arco magma-tico/margine continentale attivo (GARBARINO et alii, 1981; MEMMI etalii, 1982); questi prodotti sono inquadrabili in un più vasto eventoche caratterizza il Paleozoico inferiore della catena ercinica nell’Eu-ropa sud-occidentale;

2) un evento del Devoniano medio (387±2 Ma), per la prima vol-ta descritto nel basamento sardo, individuato, nel settore di VillaSan Pietro, nell’unità litostratigrafica sommitale delle successioniautoctone dell’Iglesiente-Sulcis (Formazione di Pala Manna: flyschercinico della Sardegna meridionale: BARCA, 1991; BARCA et alii,1998); esso è caratterizzato da metavulcaniti acide calcalcaline (me-tariodaciti) di affinità orogenica;

3) un evento di presunta età tardo devoniana o carbonifera infe-riore, anch’esso ospitato nella Formazione di Pala Manna nell’unitàautoctona del Sulcis-Iglesiente, rappresentato da prodotti basici ef-fusivi a carattere alcalino analoghi a quelli definiti in passato in altrisettori del basamento sardo (Sarrabus: DI PISA et alii, 1992) riferibiliad ambientazioni geodinamiche di tipo estensionale.

Nel complesso, i dati che emergono dallo studio permettono diampliare il quadro delle conoscenze sull’evoluzione magmatica e tet-tonica del basamento paleozoico nella Sardegna SW. L’attribuzioneal Devoniano medio delle metavulcaniti riodacitiche del settore diVilla San Pietro (metariodaciti di Monte Ninnixeddu) ed il loro rap-porto spaziale con l’incassante, consentono di ipotizzare una retro-datazione della parte inferiore della Formazione di Pala Manna nelSulcis. La successione di eventi magmatici che viene ad essere deli-neata nell’area studiata si accorda, in generale, con le conoscenze ac-quisite sia a livello regionale, sia a livello della catena ercinicadell’Europa SW; la collocazione dell’evento magmatico di MonteNinnixeddu nel quadro dell’evoluzione tettonica e magmatica del ba-samento sardo è tuttavia ancora problematica.

TERMINI CHIAVE: metavulcaniti, Sardegna SW, basamentopaleozoico, ciclo magmatico pre-ercinico, geocronologia.

INTRODUCTION

The Paleozoic basement of southern Sulcis (SW Sar-dinia) is characterized by thick Early Cambrian-EarlyCarboniferous carbonatic and silicoclastic successions.Metavolcanics and meta-epiclastites have been previouslyevidenced by BARCA et alii (1986, 1998) in the Capoterraand Villa San Pietro sectors. Recent geological surveys ofthe area (Servizio Geologico d’Italia – Regione Sardegna –Progetto CARG: SERRI et alii, 2004, in press) defined new

Boll. Soc. Geol. It., 124 (2005), 69-85, 12 ff., 4 tabb.

New evidence of Pre-Hercynian volcanics from Southern Sulcis(Southwestern Sardinia)

CARLO GARBARINO (*), STEFANO NAITZA (*), ROBERTO RIZZO (*), SANDRO TOCCO (*),SEBASTIANO BARCA (**), ANGELO FARCI (***), ALESSANDRO FORCI (****) & ROBERTO SERRI (****)

(*) DIGITA - Dipartimento di Geoingegneria e TecnologieAmbientali, Università degli Studi di Cagliari, Piazza d’Armi, Caglia-ri. [email protected].

(**) DISTER - Dipartimento di Scienze della Terra, Universitàdegli Studi di Cagliari. Via Trentino n. 51, Cagliari.

(***) Sardinia Gold Mining S.p.A., S. Miali, Furtei (Ca).(****) Progetto CARG-Sardegna, Via Dolcetta n. 19, Cagliari.

acid, intermediate and basic volcanic bodies. This work isfocused on a geological study and a geochemical charac-terization of these new discovered magmatic rocks. Stan-dard U-Pb isotopic ages on zircons were performed toachieve a chronological framework of the Paleozoic mag-matic events in the area.

REGIONAL GEOLOGY

The Sardinian Paleozoic massif is part of the south-ern European Hercynian collisional chain. Collisionalevents were associated with MP barrovian metamor-phism, dated at 350 Ma (DEL MORO et alii, 1991): defor-mation and metamorphic grade increase from SW to NESardinia, from lower greenschist to amphibolite facies.According to CARMIGNANI et alii (1994), Early Carbonifer-ous crustal collision and thickening, followed by LateCarboniferous-Early Permian extension, led to a differenttectonic evolution in the northern, central-southeasternand southwestern part of the island (fig. 1). High- andmedium-grade metamorphic complexes characterize theAxial Zone of northern Sardinia; a low-to very low-grademetamorphic complex, structured in a South-vergingnappe stack, occurs in central and SE Sardinia (NappeZone); a very low-grade metamorphic complex is presentin SW Sardinia (External Zone, or Hercynian foreland).Late Hercynian times were also marked by a major mag-matic event, leading to the emplacement of the intrusivecalc-alkaline magmatic complexes of northern, centraland southern Sardinia (fig. 1).

The Paleozoic of the Iglesiente-Sulcis region (SW Sar-dinia) consist of two very low-grade metamorphic tec-tonic units: the autochthonous Iglesiente-Sulcis Unit andthe allochthonous Arburese Unit. The Iglesiente-SulcisUnit (fig. 4A) includes an Early Cambrian-Early Ordovi-cian silicoclastic and carbonatic succession. It was cov-ered, through a Middle Ordovician angular unconformity(STILLE, 1939), by a Late Ordovician-Late Devonian pre-vailing silicoclastic succession. In the Sulcis area, EarlyDevonian carbonates are unconformably covered by athick silicoclastic unit (Pala Manna Formation, BARCA,1991), which is interpreted as Hercynian flysch deposit(MAXIA, 1984; BARCA et alii, 1998). This assumption issupported by close stratigraphic and structural affinitieswith the Early Carboniferous Hercynian synorogenicdeposits of SE Sardinia (BARCA, 1991; BARCA & OLIVIERI,1991; BARCA et alii, 1992). The Arburese tectonic Unit (fig.4B) is part of a West-directed nappe fold that overthruststhe Iglesiente-Sulcis Unit (CONTI et alii, 2001). It isformed by only one lithostratigraphic unit, a >500 m -thick silicoclastic succession. This unit was slightlydeformed in the Middle Ordovician, therefore consideredof Middle Cambrian-Early Ordovician age (BARCA et alii,1982) and referred to the Arenarie di San Vito Formation(CALVINO, 1959).

As delineated in the recent work of CONTI et alii(2001), different deformational phases are known in SWSardinia, related both to Caledonian (Ordovician)«Sardic Phase» and to two main Hercynian foldingphases. The first Hercynian deformational phase is char-acterized by East-West trending folds; the second Her-cynian phase is marked by NS trending folds and thedevelopment of a widespread axial plane penetrativefoliation.

THE VILLA SAN PIETRO SECTOR

The Villa San Pietro sector includes the southern partof the Sulcis survey area (figs. 2, 4A). In this sector, thesilicoclastic Pala Manna Formation covers the EarlyDevonian (Pragian-Emsian: GNOLI et alii, 1990) MasonPorcus carbonatic Formation. The stratigraphic boundarybetween the two formations is distinctly tectonized andsometimes marked by polygenic metaconglomerates (fig.4A,a). The Pala Manna Formation consists of alternatingmetargillites, metarenites and quartzites, with intercala-tions of metaconglomerates and resedimented materials(Silurian lyditic olistolithes, breccias and olistostromes,polygenic metaconglomerates). The succession is presum-ably over 350-400 m thick. In the studied area, the Her-cynian tectonics caused isoclinal folding, evolved throughshearing in a complex of tectonic wedges verging towardsWSW. Late tectonic phases determined gentle foldingand, usually, the reactivation of shear surfaces as normalfaults. Late Hercynian leucogranites are the most spreadmagmatic rock type in the area. Their contact with theenclosing rocks is usually marked by intense thermalmetamorphism and hydrothermal alteration, locally gen-erating skarn phenomena.

The field survey of the Villa San Pietro sector recog-nized mafic (metabasites) and felsic (metarhyodacites)volcanic rocks, embedded in the Pala Manna Formation.Metabasites were discovered in the Punta Brunchizzolu-s’Olioni area (fig. 2), in a >100 m thick litostratigraphicsequence located in the lowest part of the Pala MannaFormation. From the bottom to the top, this sequenceconsists of polygenic metaconglomerates, followed byquartzites, metasiltites, fine-grained metarenites andmetaepiclastites, mostly derived from acid volcanics. Fre-quent olistolithes of Silurian lydites, lyditic breccias andgraptolitic black shales are included at different levelsalong the sequence (BARCA et alii, 1998; STORCH et alii,2002). Two main outcrops of metabasites were recog-nized (fig. 2). These lava flow bodies are up to 10 m thick,N-S in strike and dip eastward. The middle part of thelava flow bodies is massive. Dark grey scoriaceous andvesiculated layers characterize the lowermost and upper-most parts. The Punta Brunchizzolu-s’Olioni metabasitesare deeply altered. Basal contacts show small occurrencesof Fe-(Zn-Cu) sulfides and oxides; calcite veinlets and fill-ings of vesicles are characteristic of the lower and upperpart of the lava bodies.

Metarhyodacites are mainly represented by the out-crop of Monte Ninnixeddu (fig. 2). They consists of a>100 m thick felsic criptodome intruded into the PalaManna Formation metasediments. This sub-volcanicbody is distinctly massive, but characterized by a zone ofwidespread brecciation at the endocontact (figs. 5c, 5d).At the mesoscopic scale, the metarhyodacites do not showany evidence of foliation or tectonic stress. The emplace-ment of the metarhyodacites caused country rocks defor-mation and weak thermal metamorphism. Ingestion ofcountry rocks along the contacts with the magmatic bodywas also frequently observed.

THE CAPOTERRA SECTOR

The Capoterra sector (figs. 3, 4B) is located in thenortheastern part of the area of survey. In this sector, the

70 C. GARBARINO ET ALII

PRE-HERCYNIAN VOLCANICS FROM SOUTHERN SULCIS (SW SARDINIA) 71

Fig. 1 - Simplified geo-structural map of the Paleozoic basement of Sardinia (after SERRI et alii, in press). In the box the studied area.– Schema geologico-strutturale del basamento paleozoico della Sardegna (modificato da SERRI et alii, in stampa). Nel riquadro l’area studiata.

autochthonous succession of the Sulcis-Iglesiente Unit(fig. 4B), several hundreds of meters thick, consists ofLate Ordovician siliciclastic metasediments (Domusnovasand Rio San Marco Formations: LEONE et alii, 1991),Early Silurian black shales and lydites (Genna MuxerruFormation LEONE et alii, 1991), Early Devonian metal-

imestones (Mason Porcus Formation: GNOLI et alii, 1990)and Early Carboniferous siliciclastic deposits (PalaManna Formation: BARCA, 1991). The autochthonoussuccession is tectonically overlain by the siliciclasticmetasediments (prevailing metasandstones, fine metacon-glomerates and metasiltites of the Arenarie di San Vito


Fig. 2 - Geological sketch map of the Villa San Pietro sector. 1) Metapelites and metasandstones (Upper Ordovician); 2) Graptolitic blackshales and lydites (Early Silurian); 3) Nodular massive metalimestones alternating to metapelites, metasiltites and levels of carbonaticmetapelites (Silurian?-Early Devonian; 4) Metasiltites, metasandstones and layered quartzites containing olistolithes of Silurian black shalesand lydites, lenses of metaconglomerates (a), metabasite lava-flows (b) and a metarhyodacite subvolcanic body (c) (Pala Manna Formation:Devonian-Early Carboniferous); 5) Late Paleozoic biotite leucosyenogranites; 6) Late Paleozoic rhyolitic dykes; 7) Alluvial deposits(Holocene); 8) Extensional faults; 9) Generic faults. Stars: localities of geochemical and geochronological sampling.– Schema geologico semplificato del settore di Villa San Pietro. 1) Metapeliti e metarenarie (Ordoviciano superiore); 2) Black shales graptolitici eliditi (Siluriano inferiore); 3) Metacalcari nodulari massivi in alternanza a metapeliti, metasiltiti e livelli di metapeliti carbonatiche (Siluriano?-Devoniano inferiore); 4) Metasiltiti, metarenarie e quarziti stratificate contenenti olistoliti di black shales e liditi siluriani, lenti di metaconglome-rati (a), metabasiti in flussi lavici (b), metariodaciti in corpi subvulcanici (c) (Formazione di Pala Manna: Devoniano-Carbonifero inferiore);5) Leucosienograniti a biotite tardo-paleozoici; 6) Dicchi riolitici tardo-paleozoici; 7) Depositi alluviali (Olocene); 8) Faglie estensionali; 9) Faglie;Stellette: località di campionature geochimiche.

Formation: CALVINO, 1959), which are about 300 m thickand belong to the Arburese tectonic Unit. In theCapoterra sector, the Paleozoic basement is intruded byLate Hercynian granitic plutons that induced thermalmetamorphism over a large area.

Paleozoic intermediate metavolcanics have been rec-ognized at South (Riu Gutturu Mannu Valley, Punta suAingiu Mannu) and NE (Punta Gennasoli) of the aban-doned San Leone Mine, where several N-S trendingmetadacite dikes cross cut the metasediments of the Are-narie di San Vito Formation (fig. 4B,b). NE of the Gut-turu Mannu Valley, these metavolcanics crop out asswarms of widespread <1m thick dikes, always N-S trend-ing. The largest metadacite outcrop was discovered in theRiu Gutturu Mannu Valley, about 1 km SW from the SanLeone mine. In this area, an up to 10 m thick dike cropsout for about 500 m, cutting the layered metasandstonesof Arenarie di S.Vito, near to the tectonic contact with theunderlying autochthonous sequence. At the mesoscopicscale, these magmatic (sub-volcanic) rocks show a slighttectonic foliation.

PETROGRAPHY OF METAVOLCANICS

The Punta Brunchizzolu-s’Olioni metabasites show adark-grey to blackish color. They have an intergranularand faintly microporphyritic texture, with sparse relictsof plagioclase microphenocrysts, completely replaced byalbite. The intergranular groundmass is made of polycry-stalline neoblastic biotite, chlorite, fine-grained aggre-gates of hematite and sulphides. Primary mafic mineralphases are strongly chloritized (fig. 6c). Frequent slightlydeformed vesicles and microveins, widely variable in size,are filled by calcite, polysynthetically twinned. Chloriteand/or prehnite are typical of the vesicles core zones.

The Monte Ninnixeddu metarhyodacites are pale-greyporphyritic rocks, with about 40% of sub-millimetric phe-nocrysts, consisting of prevalently albitized and sericit-ized plagioclase, subordinate dark orthoclase, and scarceeuhedral quartz and chloritized biotite, found in a micro-cristalline groundmass. Frequent polycrystalline biotiteaggregates have been interpreted as mineralogical re-equilibration phases, as products of contact metamor-phism induced by granitoid plutons (SERRI et alii, 2004,in press). Fine aggregates of opaque minerals (mostlymagnetite and iron sulphides) and secondary chlorite andepidote are also present. Apatite and zircon are primarycommon accessory phases. The devitrified groundmass ofthe border facies is marked by lower contents of phe-nocrysts, by a prevailing glassy fluidal texture and byrelicts of perlitic textures (fig. 6a and 6b). Some xenolithinclusions are present and related to wall rocks ingestion.

The metadacites from the Capoterra sector (Riu Gut-turu Mannu dike) are porphyritic grey rocks, character-ized by an oriented fabric, with abundant (about 40%)albitized and partly sericitized plagioclase phenocrysts.Some quartz crystals were detected in a quartz-felds-pathic and biotitic sericitized microcristalline ground-mass (fig. 6d). Mafic minerals are almost completelyreplaced by frequently chloritized biotite, marked by alepidoblastic texture and interpreted as related to contactmetamorphism. Zircon is a common accessory phase.Some darker microgranular magmatic enclaves (MME)are also present.

PETROCHEMICAL FEATURES

Major and trace element chemical analyses ofselected metavolcanic rock samples are reported in tab.1and tab. 2, respectively.

The Monte Ninnixeddu metarhyodacite and the SanLeone metadacite show quite different silica contents(Monte Ninnixeddu ≤ 61.45 wt% SiO2; San Leone 64.01wt% SiO2), and appreciable variations in Ca, Na, K, aswell as Al.

The Punta Brunchizzolu-S’Olioni metabasites evi-dence high K (> 6 wt% K2O) contents. These rocks havelow Na (≤ 1.27 wt% Na2O), Ca (≤ 2.42 wt% CaO) and Mg(≤ 3.22 wt% MgO) contents.

As regards as the trace elements, the metabasitesshow relatively low Cr, Ni and Co contents. The variabil-ity in LILE element concentration (Rb > 235 ppm, Cs > 44ppm, and Li > 66 ppm) could be explained by the cristal-lization of brown mica and sericite. Very high contents ofHFSE elements (Nb-Ta 121 and 14 ppm respectively)have been recognized in samples from the S’Olioni out-crop (F7 sample). The total REE contents in metabasitesrange from 54 to 190 ppm. LREE (46-165 ppm) are rela-tively more abundant than HREE (7-25 ppm).

The resulting geochemical framework is probably dueto new mineralogical equilibria induced by thermal meta-morphism, as evidenced by the growth of fine hematiteaggregates and neoblastic brown mica lamellae wide-spread in the overall groundmass. This element redistrib-ution was considered when petrographic and chemicaldata were used to define the chemical classification of themetavolcanic rocks and their regional tectonic setting. Asthe standard chemical parameters cannot be applied,pairs of specific elements (and their ratios), which arewell known not to be susceptible to the effects of super-imposed alteration processes, have been used for a betterchemical classification (FLOYD & WINCHESTER, 1975).

In fig. 7 the Nb/Y-Zr/Ti classification diagram (WIN-CHESTER & FLOYD, 1977) confirms the differences in thebasic and intermediate to acid character of the studiedmetavolcanic rocks. In this diagram, two distinct groupsare clearly identified: 1) the Punta Brunchizzolu-s’Olionimetabasites, 2) the more evolved rocks of the metadacitedykes from San Leone and the metarhyodacites fromMonte Ninnixeddu.

In the chondrite-normalized REE plots (fig. 8), themetabasite REE patterns (fig. 8a) evidence a distinct frac-tionation (normalized La/Yb ranging from 6 to 14) amongthe samples. Only two metabasite samples (SAR3 and F8)show a slight negative Eu anomaly, while the others aremarked by a slight positive Eu anomaly. REE patterns ofthe dacite (fig. 8b) and rhyodacite (fig. 8c) metavolcanicsevidence characteristic features of chemically evolvedrocks with a more pronounced total REE content (up to353 ppm, tab. 2). As a whole, the REE pattern of the SanLeone metadacite is typical of intermediate (andesitic todacitic) rocks without indicative Eu anomaly. The MonteNinnixeddu metarhyodacite, instead, exhibits REE pat-terns distinctive of acid rocks, with a typical negative Euanomaly and a more fractionated LREE trend (normal-ized La/Yb ranging from 8 to 17).

Metabasites show very low Y/Nb ratios, ranging from0.27 to 0.8. These values are typical of alkaline and/ortransitional magmatic series. Cr, Ni and Co values may bealso indicative of an origin of these metabasites from



Fig. 3 - Geological sketch map of the Capoterra sector. Iglesiente-Sulcis External Zone: 1) Metasiltites and metasandstones (Upper Ordovi-cian); 2) Graptolitic black shales and lydites (Early Silurian); 3) Silicified metalimestones and skarn bodies (Devonian); 4) Layered metasand-stones with coarse metaconglomerates, lyditic breccias, lyditic and rare metalimestone olistolithes (Pala Manna Formation: Devonian-EarlyCarboniferous); Nappe Zone: 5) Layered metasandstones with lenses of fine metaconglomerates belonging to the «Arenarie di San Vito»Formation (Middle Cambrian-Early Ordovician) of the Arburese tectonic Unit, injected by Middle Ordovician metadacite dykes (a); 6) LatePaleozoic biotite leucomonzogranites; 7) Quartz bearing lodes; 8) Alluvial deposits (Holocene); 9) Main Hercynian overthrusts; 10) Com-pressional faults; 11) Extensional faults; 12) Generic faults. Star: localities of geochemical and geochronological sampling.– Schema geologico semplificato del settore di Capoterra. Zona Esterna (Iglesiente-Sulcis): 1) Metasiltiti e metarenarie (Ordoviciano superiore);2) Black shales graptolitici e liditi (Siluriano inferiore); 3) Metacalcari silicizzati e skarn (Devoniano); 4) Metarenarie stratificate con metacon-glomerati grossolani, brecce a liditi, olistoliti di liditi e metacalcari (Formazione di Pala Manna: Devoniano-Carbonifero inferiore); Zona a Falde:5) Metarenarie stratificate con lenti di metaconglomerati minuti appartenenti alla Formazione delle «Arenarie di San Vito» (Cambriano medio-Ordoviciano inferiore) dell’Unità tettonica dell’Arburese, iniettate da dicchi di metadaciti (a) dell’Ordoviciano medio; 6) Leucomonzograniti abiotite tardo-paleozoici; 7) Filoni quarzosi; 8) Depositi alluviali; 9) Principali sovrascorrimenti ercinici; 10) Faglie compressive; 11) Faglieestensive; 12) Faglie. Stellette: località di campionature geochimiche.


Fig. 4 - Schematic logs of studied SE Sulcis sectors; A) Villa San Pietro; B) Capoterra.– Colonne stratigrafiche schematiche dei settori studiati nel Sulcis sud-orientale: A) Villa San Pietro; B) Capoterra.

mantle-derived magmas, via olivine, spinel and clinopy-roxene fractionation. As a whole, the Southern Sulcismetabasites show close affinities with alkaline within-plate basalts (WPB). The metadacites and metarhyo-dacites, instead, may be easily related to metaluminouscalc-alkaline magmatic series.

Considering Ta, Th and Yb as immobile elementsunder most geological conditions, the Ta/Yb-Th/Yb valuesare expected to remain constant during metamorphism(cfr., GORTON & SCHANDL, 2000, p. 1071). Thus, all theprevious geochemical indications are well evidenced inthe Ta/Yb vs. Th/Yb diagram of fig. 9. Two distinct groups


Fig. 5 - Main outcrops of studied metavolcanics of the Villa San Pietro sector; a) view of the main outcrop of Punta Brunchizzolu-s’Olionimetabasites, the white line is the boundary between the metabasites (β) at the top, and the Pala Manna Formation metasediments (ms) at thebottom; b) detail showing the vacuolar texture in the metabasites; c) view of the outcrops of Monte Ninnixeddu metarhyodacites; d) detail ofthe autoclastic breccia in the apical zone of the magmatic body.– Affioramenti delle metavulcaniti del settore di Villa San Pietro; a) panorama dell’affioramento delle metabasiti di Punta Brunchizzolu-s’Olioni,la linea bianca materializza il contatto fra le metabasiti (β) al tetto ed i metasedimenti della Formazione di Pala Manna (ms) a letto; b) particolaredelle metabasiti, in chiara evidenza la tessitura vacuolare; c) panorama dell’affioramento delle metariodaciti di Monte Ninnixeddu; d) particolaredella breccia autoclastica nella zona apicale del corpo magmatico.

can be clearly defined: the metabasites plot in the within-plate basalts field and the metadacites and metarhyo-dacites plot in the destructive plate margins (orogenicareas) field.

The within plate petrogenetic affinity of the studiedmetabasites is evidenced in the Ti-V diagram (fig. 10a),where other SW Sardinia metabasite samples from litera-ture (BECCALUVA et alii, 1981) are plotted for comparison.Moreover, in the La-Y-Ce diagram of fig. 10b metabasitesshow LREE enrichments similar to continental andoceanic alkaline basalts, as previously envisaged by BEC-CALUVA et alii (1981) for basic metavolcanics from othersectors of SW Sardinia. Furthermore, the 143Nd/144Nd iso-topic ratios (tab. 4), and calculated εNd values (from +6.6to +7.4) indicate, for the Punta Brunchizzolu-s’Olionimetabasites, an origin from a depleted upper mantlesource.

Felsic and intermediate metavolcanic rocks (fig. 11)delineate a typical geochemical affinity of magmasderived from active continental margins (orogenic areas).

GEOCHRONOLOGY

Isotopic age determinations were accomplished atGeospec Consultants Ltd. Laboratories (Edmonton,Canada). Descriptions regarding the analytical methodsare reported in appendix.

Metarhyodacites from Monte Ninnixeddu (sampleNIN-4) and metadacites from San Leone (sample SLE-1)were submitted for the U-Pb zircon geochronology.

The NIN-4 sample from Monte Ninnixeddu has a simi-lar zircon assemblage as the SLE-1 sample of San Leone,but few of the zircon crystals are non-magnetic. In NIN-4sample, zircon crystals occur in a variety of shapes, fromequant to 3:1 (length: width) prismatic habits. Crystal frag-ments and reabsorbed prisms are also present. Zircon nee-dles (>8:1 length:width ratio) prevail in the more magneticfractions. These zircon crystals show an overall slight varia-tion in color, from colorless to tan. Visible core/overgrowthrelationships are abundant, with cores typically showing agray turbidity. The overgrowths are commonly colorless.


Fig. 6 - Thin section microphotographs showing some textural characteristics of the studied metavolcanics: a) Monte Ninnixeddumetarhyodacite: perlitic devetrification textures (natural light); b) Monte Ninnixeddu metarhyodacite: partly resorbed feldspar phenocrystin a devitrified groundmass with magmatic fluidality (polarized light); c) Punta Brunchizzolu-s’Olioni metabasites: Albite-replaced relicsof plagioclase phenocrysts in a microcrystalline groundmass with calcite-filled vugs (natural light); d) San Leone metadacite: relics ofplagioclase phenocrysts and neoblastic biotite lamellae in a microcrystalline groundmass (natural light). (Magnification 2X).– Microfotografie in sezione sottile mostranti le caratteristiche tessiturali delle metavulcaniti studiate: a) Metariodacite di Monte Ninnixeddu:tessiture perlitiche di devetrificazione (luce naturale); b) Metariodacite di Monte Ninnixeddu: fenocristalli di feldspato parzialmente riassorbitiin massa di fondo devetrificata e mostrante una fluidalità magmatica (luce polarizzata); c) Metabasiti di Punta Brunchizzolu-s’Olioni: relitti difenocristalli di plagioclasio sostituiti da albite in massa di fondo con vacuoli riempiti di calcite (luce naturale); d) Metadacite di San Leone:relitti di fenocristalli di plagioclasio e lamelle neoblastiche di biotite in massa di fondo microcristallina (luce naturale) (2X).

The U-Pb results for five zircon multi-grain fractionsfrom the Monte Ninnixeddu sample are reported in tab. 3and shown on a concordia diagram in fig. 12a. The recog-nized zircon fractions have a slight range in U content(121-405 ppm) and Th/U ratios (0.15-0.30). There is alarge range in 207Pb/206Pb ages between 485 and 1368 Ma.

As can be seen from fig. 12a, most of the analyses arequite discordant. A reference line constructed to passthrough analyses n. 3-5 yields a lower intercept age of387±2 Ma (MSWD=0.23), which is interpreted as the bestestimate for the time of igneous zircon crystallization andis considered a good approximation for the emplacementage (Middle Devonian). The Paleoproterozoic upper inter-cept age of 1869±12 Ma is interpreted to reflect the age ofzircon inheritance and indicates the presence, perhaps atdepth, of a Paleoproterozoic basement. The large amountof discordance observed in these data is consistent withthe observation that there are abundant visible core/over-growth relationships in this zircon population. One frac-tion of zircon needles (n. 4) has the youngest 207Pb/206Pbage and the highest Th/U ratio (0.30), which is compatiblewith the general absence of low Th/U zircon cores in suchcrystals. Two zircon analyses (n. 1 and 2) plot below thereference discordia line indicating that even older (>1.9Ga) zircon xenocrysts/cores must exist in this population.

The SLE-1 sample from San Leone contained abun-dant zircon on to perform U-Pb geochronology. Most ofthe crystals are colorless to slight tan, euhedral, andmulti-faceted bi-pyramids. A few contain tiny mineralinclusions. The grains vary in size from 40 to 250 mm andhave a range in length:width ratios from 2:1 to 8:1 (mostare 3:1). A few grains have visible core/overgrowth rela-tionships that are difficult to detect with a binocularmicroscope because both are colorless.

The U-Pb results for five zircon fractions are reportedin tab. 3 and displayed on a concordia diagram in fig.12b. Four of the zircon analyses are multi-grain fractionsconsisting of between 23 and 62 crystals. One analysis(n. 4 on fig. 12b) represents a single zircon grain. All analy-ses indicate compatible zircon chemistry, with U ranging


TABLE 1

Representative wt % major elements chemical composition of SE Sulcis metavolcanics.– Analisi chimiche (%) rappresentative degli elementi maggiori delle metavulcaniti del Sulcis SE.

Fig. 7 - Rock classification diagrams of Paleozoic metavolcanics fromSE Sulcis using trace element pairs of WINCHESTER & FLOYD (1977).1: Monte Ninnixeddu; 2: San Leone; 3: Punta Brunchizzolu-s’Olioni.Shaded area: field of metabasites from BECCALUVA et alii (1981).– Diagramma di classificazione delle metavulcaniti paleozoiche delSulcis SE che fa uso di coppie di elementi in tracce secondo WINCHE-STER & FLOYD (1977). 1: Monte Ninnixeddu; 2: San Leone; 3: PuntaBrunchizzolu-s’Olioni. L’area ombreggiata indica il campo delle meta-basiti da BECCALUVA et alii (1981).


TABLE 2

Representative trace elements chemical composition (ppm) of SE Sulcis metavolcanics.– Analisi chimiche rappresentative degli elementi in traccia (ppm) delle metavulcaniti del Sucis SE.


Fig. 9 - Ta/Yb vs. Th/Yb plot (after PEARCE, 1983, and revised byGORTON & SCHANDL, 2000) of the Palezoic metabasites and interme-diate to acid metavolcanics from SE Sulcis. ACM: active continentalmargins; WPVZ: within plate volcanic zones; WPB: within platebasalts; MORB: middle ocean ridge basalts. Symbols are as fig. 7.– Diagramma Ta/Yb-Th/Yb (secondo PEARCE, 1983, modificato daGORTON & SCHANDL, 2000) per le metabasiti e le metavulcaniti inter-medio-acide del Paleozoico del Sulcis SE. ACM: margini continentaliattivi; WPVZ: zone vulcaniche intraplacca; WPB: basalti intraplacca;MORB: basalti di dorsale medio-oceanica. Simboli come in fig. 7.

Fig. 10 - Tectonomagmatic discrimination diagrams for Paleozoicmetabasites from SE Sulcis. a: after SHERVAIS (1982); b: afterSCHILLING (1971) and RICCI & SERRI (1975). Symbols are as fig. 7.Shaded area field of metabasites from BECCALUVA et alii (1981). – Discriminanti tettonomagmatiche per le metabasiti paleozoiche delSulcis SE. a: secondo SHERVAIS (1982); b: secondo SCHILLING (1971)e RICCI & SERRI (1975). L’area ombreggiata indica il campo dellemetabasiti da BECCALUVA et alii (1981). I simboli sono come nella fig. 7.

Fig. 8 - Spider diagrams of chondrite-normalized REE-patterns ofPaleozoic metavolcanics from SE Sulcis. Chondrite-normalizationvalues are after SUN & MCDONOUGH (1989).– Diagrammi REE normalizzati alle condriti delle metavulcaniti paleo-zoiche del Sulcis SE (normalizzazioni secondo SUN & MCDONOUGH,1989).

from 124 to 317 ppm and the Th/U ratio between 0.27and 0.29 (tab. 3). The U-Pb analyses evidence some slightscatter in fig. 12b, with the analysis n. 2 clearly having anolder 207Pb/206Pb age (667 Ma). This may likely reflect thepresence of an inherited Pb component in this fraction.The remaining four analyses have very similar 207Pb/206Pbages between 464-470 Ma (Early Ordovician) and a best-fit regression line constructed to pass through analyses3-5 yields an upper intercept age of 475±10 Ma, which isinterpreted to reflect the emplacement age of themetadacite dikes.

The isotopic dating of the metabasites from the VillaSan Pietro sector was attempted by whole-rock Sm-Ndmethod. Measured isotopic ratios, Sm and Nd concentra-tions, and other calculated parameters of the analyzedsamples are presented in tab. 4. The analyzed samplesexhibit a very narrow range of 143Nd/144Nd and147Sm/144Nd isotopic ratios, i.e. too narrow range of val-ues for derivation of a precise isochron age yielding aclearly too approximate emplacement age of 429±80 Ma(MSWD=1.3).

Nevertheless, as previously stated, determined143Nd/144Nd ratios are more useful for their petrologicalsignificance.

DISCUSSION

In the Capoterra sector (Arburese Unit), the petro-chemical features of the San Leone metadacites clearlysuggest an orogenic affinity. U-Pb isotopic data (475±10Ma), allow to outline an Early-Middle Ordovician calc-alkaline magmatic event. Isotopic age determinations,along with geological, petrographical, and geochemicaldata, strongly suggest that the San Leone metadacitescould be associated to the Ordovician calc-alkaline mag-matic products typical of the Nappe Zone in Central and

SE Sardinia («Porfidi grigi del Sarrabus»: CALVINO, 1959;«Porfiroidi» Auct.; Formazione di Monte Santa Vittoria:CARMIGNANI et alii, 2001). In particular, the San Leonemetadacites are similar to calc-alkaline small dikes, sillsand domes, widespread in the Sarrabus region (SE Sar-dinia: CARMIGNANI et alii, 2001), which cross cut themetasediments of the Arenarie di S. Vito Formation. TheMiddle Ordovician magmatism of Central and SE Sar-dinia was defined in the geodynamic framework of anancient active plate margin, linked to oceanic crust sub-duction and magmatic arc development (GARBARINO etalii, 1981, 1984; MEMMI et alii, 1982; CAROSI et alii, 1992).


Fig. 11 - Yb vs.Th/Ta plot (after GORTON & SCHANDL, 2000) of theintermediate to acid Paleozoic metavolcanics from SE Sulcis. Symbolsare as fig. 7.– Diagramma Yb-Th/Ta (secondo GORTON & SCHANDL, 2000) per lemetavulcaniti intermedio-acide del Paleozoico del Sulcis SE. I simbolisono come in fig. 7.

Fig. 12 - Concordia diagrams of U-Pb zircon geochronology ofmetarhyodacite from Monte Ninnixeddu (a) and metadacite fromSan Leone (b) samples.– Diagrammi concordia (geocronologia U-Pb su zirconi) per la meta-riodacite di Monte Ninnixeddu (a) e la metadacite di San Leone (b).


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The Ordovician isotopic ages determined for the SanLeone metadacites are in substantial agreement with: 1)The isotopic age of the «Capo Spartivento-Monte Filauorthogneiss» (478±16 Ma, by conventional U-Pb on zir-cons: DELAPERRIERE & LANCELOT, 1989; 449±6 Ma byion-microprobe U-Pb on zircons: LUDWIG & TURI, 1989);2) the isotopic age (453±10 Ma: FERRARA et alii, 1978)established for the emplacement of the amphiboliticorthogneiss cropping out in NE Sardinia (InternalNappes). Analogous calc-alkaline magmatic products arewidespread throughout the Hercynian belt of SouthernEurope, all referred to an Early to Late Ordovician mag-matic activity (i.e., Alpine chain: LOESCKE, 1989; SASSI etalii, 1994; PEZZOTTA & PINARELLI, 1994; SCHALTEGGER &GEBAUER, 1999; MELI & KLÖTZLI, 2001; LÄUFER et alii,2001; Tuscany: DEL MORO et alii, 1993; Aspromonte-Peloritani Mts: ACQUAFREDDA et alii, 1992; FERLA, 1994;TROMBETTA et alii, 2002; Corse: MENOT & ORSINI, 1990;Southern French Central Massif: PIN & MARINI, 1993;Eastern Pyrenees: DELOULE et alii, 2002; Central IberianMassif: VALVERDE-VAQUERO & DUNNING, 2000).

In the Villa San Pietro sector (Iglesiente-Sulcis Unit),the geological, petrographical and geochemical data ofthe studied metavolcanics suggest a more problematictectonomagmatic evolution.

The Punta Brunchizzolu-s’Olioni metabasites havebeen outlined as pre-Hercynian basic lava flows, set intothe siliciclastic metasediments of the Pala Manna Forma-tion. Due to the strong alteration, no reliable Sm-Nd iso-topic age was established for these basic rocks. Geologicalevidences, however, seem to imply that metabasites couldhave the same age of the Pala Manna Formation. Conse-quently, the Villa San Pietro metabasites may be tenta-tively ascribed to Early Carboniferous. The specific chem-ical features of Punta Brunchizzolu-s’Olioni metabasitessuggest that they may be ascribed to alkaline within-plateand rift-related basic volcanism. In SW Sardinia, similarfeatures are also typical of the Late Ordovician metaba-site occurrences described by BECCALUVA et alii (1981).Thus, in SW Sardinia (Iglesiente-Sulcis Unit), the exis-tence of two different Paleozoic magmatic alkaline WPBcycles (Late Ordovician and Early Carboniferous) shouldbe conceived. This framework appears to be coherentwith the Paleozoic magmatic and geotectonic evolutionsuggested in the Nappe Zone of Central and SE Sardiniaby DI PISA et alii (1992), which envisaged different alka-line WPB cycles associated with Late Ordovician andEarly Carboniferous rifting stages. Early Paleozoic basicalkaline to tholeiitic and rift-related volcanics are wellknown in different parts of the European Hercynides,from Polish Sudetes to Iberian Massif. They are widelyinterpreted as products of continental break-up of theNorthern Gondwana platform during Early Cambrian-Early Devonian times (PIN & MARINI, 1993, and refer-ences therein). Basic metavolcanics of supposed LateDevonian-Early Carboniferous age have been recognizedin the Eastern Alps (Carnic Alps), where they have beeninterpreted as related to pre- to syn-collisional extensionaltectonic phases (LÄUFER et alii, 2001).

Isotopic ages with geological and geochemical data forthe Monte Ninnixeddu metarhyodacites involve differentkinds of problems. Standard U-Pb geochronology on zir-cons defined an age of 387±2 Ma. Moreover, a pre-hercyn-ian age is coherent with the petrographic indications ofthermal metamorphism induced by the Late Hercynian

granitoids. The Monte Ninnixeddu criptodome showscross cutting relationships with the enclosing rocks of thePala Manna Formation. Thus, the Monte Ninnixedducriptodome should be considered the product of a Devon-ian calc-alkaline magmatic event in the Hercynian fore-land. Nevertheless, these geological and analytical datacontrast with the Eocarboniferous stratigraphic age attrib-uted to the Pala Manna Formation (BARCA et alii, 1998),suggesting that, at least in the Sulcis area, part of this suc-cession could have deposited before the Early Carbonifer-ous. In the Sulcis area, a relevant siliciclastic sedimenta-tion, unconformably following the deposition of the LowerDevonian carbonates, may have developed during Middleand Late Devonian times, preceding and being coeval tothe calc-alkaline magmatism of Monte Ninnixeddu. Con-sequently, also the age of the metabasites, which arelocated in the lower Pala Manna sequences (SERRI et alii,2004, in press) could be shifted to Devonian.

The petrochemical features of the Monte Ninnixeddumetarhyodacites suggest an orogenic affinity. Regional-scale correlations for the Monte Ninnixeddu metarhyo-dacites are still undetermined: up to now, no evidence ofsimilar magmatic occurrences have been recognized inthe Devonian of Sardinia. Late Devonian to Early Car-boniferous felsic calc-alkaline rocks are present in differ-ent portions of the Hercynian belt of Europe (Northeast-ern French Central Massif, Renohercynian Massif,Austroalpine Cristalline Complex, Eastern Carnic Alps),usually interpreted as subduction-related products, asso-ciated to hercynian active margins (PIN & PAQUETTE,2002; NEUBAUER & FRITSCH, 1993; SASSI et alii, 1994;LÄUFER et alii, 2001).

CONCLUSIVE REMARKS

1) In the Arburese Tectonic Unit, a 475±10 Ma calc-alkaline volcanic episode is represented by the San Leonedacite dikes, which may be related to coeval volcanicproducts of the same affinity occurring in Central and SESardinia.


TABLE 4

Sm-Nd whole rock isotopic analyses of metabasites fromSE Sulcis.

– Analisi isotopiche Sm-Nd su roccia totale delle metabasitidel Sulcis SE.

2) In the Iglesiente-Sulcis Unit, alkaline-WPB metaba-sites are present at the bottom of the Pala Manna Forma-tion. In spite of the current lacking of correct isotopicages, the geological and geochemical affinities of themetabasites with similar SE Sardinia products, mayallow to ascribe them to Late Devonian-Early Carbonifer-ous. Thus, the occurrence of a possible Late Devonian-Early Carboniferous WPB cycle in SW Sardinia is docu-mented for the first time.

3) In the Iglesiente-Sulcis Unit, felsic calc-alkalinemetavolcanics were recognized for the first time in the PalaManna Formation. The 387±2 Ma U-Pb isotopic age on zir-cons for the Monte Ninnixeddu calc-alkaline metarhyo-dacites requires a partial stratigraphic redefinition of thePala Manna Formation, which, in southern Sulcis, mayhave been deposited starting from the Middle Devonian.The Monte Ninnixeddu metarhyodacites are the onlyDevonian volcanic episode dated at this time in Sardinia.

Appendix: analytical methods

Whole rock samples were analyzed for major andminor elements by XRF spectrometry, using powder pel-lets. X-ray determinations were performed with an auto-matic Philips spectrometer (PW 1400), set with an Rhtube. REE and other trace elements were determined byICP spectrometry, using an ICP-MS Perkin-Elmer SCIEXElan Model 5000, after acid decomposition (HF, HNO3

and HClO4) in microwave oven. Samples for U-Pb age determinations were processed

trough standard crushing; Wilfley Table and heavy liquidand magnetic mineral separation techniques accom-plished isolation of zircons. Selected mineral grains orfractions were put in a somewhat improved Krough typepressure bombs and dissolved in mixture of concentrateHF and HNO3 acids in the bombs that are kept at temper-ature of 220 °C for a week. Purified aliquots of uraniumand lead were obtained using anion exchange chromatog-raphy. The isotopic composition of U and Pb were deter-mined on a solid source thermal ionization mass spec-trometer. All ages were calculated using the followingdecay constants: 238U-1.55125 × 10-10 yr-1 and 235U-9.8485× 10-10 yr-1.

Samples for Nd-Sm age were milled and dissolved inHF:HNO3 at 160 °C for 3 days. Nd and Sm were separatedby standard cation chromatography and HDEHP-basedchromatography. Nd or Sm was loaded on a side of theRhenium double filament bead and isotopic compositionwas measured in a VG 354 mass spectrometer. Overallreproducibility of the isotopic measurements were deter-mined from large number of measurement of La Jollastandard. Reproducibility of the concentration measure-ment was determined from repeated measurements of theSiberian Trap ST-2 local laboratory standard. At 1 s errorlevel the reproducibility for the 143Nd/144Nd and147Sm/144Nd are 0.001% to 0.002% and 0.3% to 0.5%,respectively. 143Nd/144Nd ratios of the analyzed sampleshave been normalized to La Jolla standard value of0.511858.

ACKNOWLEDGEMENTS

This report is the result of a scientific cooperation between theProgetto CARG Sardegna and the DIGITA (Dipartimento diGeoIngegneria e Tecnologie Ambientali, Università di Cagliari). The

research was also partially supported by MURST (grants 60% S.Tocco; 40% and 60% S. Barca) and by Servizio Geologico d’Italia-Regione Sardegna program «CARG L.385\89». Authors are gratefulto A. Rivoldini (DIGITA, Università di Cagliari) for the ICP analyses.Isotope determination were made under contract by Geospec Con-sultants Limited (Canada).

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Manoscritto pervenuto il 9 Luglio 2003; testo approvato per la stampa il 4 Ottobre 2004; ultime bozze restituite il 10 Gennaio 2005.

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