The Santa Giusta ignimbrite (NW Sardinia): a clue for the magmatic, structural and sedimentary evolution of a Variscan segment between Early Permian and Triassic

ABSTRACT

The main volcanic rhyolitic event in NW Sardinia (Nurra) andPermo-Triassic successions outcrop between Punta Capparoni andCala Viola, where they attain the maximum thickness (> 300 m),northwards undergo sudden reduction. In particular at Mt. SantaGiusta, the «Autunian» succession was not deposited, and thethickness of the Permo-Triassic red beds decreases to few metresabove the ignimbrite that rests directly on the basement. New40Ar-39Ar step heating technique on biotite separates gives291.5±1.5 Ma for this event; this age is considerably older thanexisting radiometric data. The corresponding Lower Sakmarianage and the field relationships with the Permo-Triassic successionsin Nurra to the S, allow to assess that north of Porto Ferro theNurra basement was a structural high bounded by E-W trendingfaults since the Late Carboniferous-Lower Permian, and that thesame faults that controlled the post-collisional Variscan extensionalso controlled the development of Mid Permian and Lower Trias-sic successions.

KEY WORDS: Rhyolite, 40Ar-39Ar geochronology, litho-spheric delamination, Permo-Triassic, Sardinia.

RIASSUNTO

L’ignimbrite di Santa Giusta (Sardegna NW): evoluzionemagmatica, strutturale e sedimentaria di una porzione dellevariscidi tra il Permiano Inferiore e il Trias.

Nella Sardegna nordoccidentale il più cospicuo evento vulcani-co a composizione riolitica e le successioni Permo-Triassiche affio-rano presso Punta Capparoni e Cala Viola nella massima potenza(>300 m), che si riduce rapidamente verso N. In particolare a M.Santa Giusta, la successione «Autuniana» non si è depositata ed i redbeds permo-triassici si riducono a pochi metri sopra l’ignimbrite,che poggia direttamente sul basamento. Nuovi dati 40Ar-39Ar, ottenu-ti col metodo del riscaldamento a intervalli di temperatura su sepa-rati di biotite, forniscono una età di 291.5±1.5 Ma significativamentepiù antica rispetto ai dati di letteratura. La corrispondente età sak-mariana e le relazioni geometriche con le successioni Permo-Triassi-che della Nurra più a Sud, portano a interpretare il basamento dellaNurra a nord di Porto Ferro come un alto strutturale controllato dafaglie ad andamento E-W a partire dal Carbonifero Superiore-Per-miano Inferiore. Tali lineamenti, che hanno controllato l’estensionepost-collisionale varisica, hanno anche controllato lo sviluppo dellesuccessioni permo-triassiche.

TERMINI CHIAVE: Riolite, geocronologia 40Ar-39Ar, delami-nazione litosferica, Permo-Triassico, Sardegna.

INTRODUCTION

The reconstructions of the late-Variscan orogenichistory in southern Europe based on the integration ofstratigraphic, paleontological, petrologic and geochemicaldatasets, correspond to a regional scenario characterisedby:

i) A conspicuous accretion, mostly by crust recycling,e.g. the Sardinia-Corsica batholith, whose southern sec-tion developed between 330 and 280 Ma (COCHERIE etalii, 2005).

ii) The orogenic collapse associated with tectonicunroofing, within a wrench tectonic regime active at theEuropean scale that gave rise between the Late Carboni-ferous and the Early Permian to transpressional andtranstensional structures.

iii) Intramontane basins (Ligurian Briançonnais, Sar-dinia, Southalpine, Balkan Terrane, Provençe, MontaigneNoire, Central Morocco, Pyrenees, Corsica) developedcoeval with sub-intrusive to effusive acidic to basic-inter-mediate volcanic activity and fluvial-lacustrine sedimen-tation.

The igneous calc-alkaline rocks of the volcano-sedi-mentary sequences, and the overall stratigraphic features,generally define a first tectonic-volcanic-sedimentarycycle, developed from the Appalachian range to most ofsouthern Europe (ARTHAUD & MATTE, 1977; CASSINIS etalii, 1996; CORTESOGNO et alii, 1998; ZIEGLER et alii,2001). The southern branch of the Variscan chain is theresult of oceanic convergence followed by continental col-lision and underthrusting of thinned continental crust. Asa consequence, magmas were originated in partial melt-ing at the crust-mantle boundary, followed by mixingbetween mantle-derived and crustal anatectic melts. Theyemplaced at shallow crust levels or in effusive conditionswithin extensional, syn-collapse structures that mighthave favoured the ascent of magmas from the middlecrust.

In the Late Permian-Early Triassic time interval(260-245 Ma), a post-Variscan (or eo-Alpine) cycle definedas «Permo-Triassic Western Mediterranean province»(COCHERIE et alii, 2005), developed as an igneous cyclewith transitional to alkaline serial affinity. The most rele-vant occurrences are in Corsica (Rb-Sr and K-Ar ages forCorsican A-type granite BONIN, 1989; BONIN et alii, 1993,1998; COCHERIE et alii, 2005), Sardinia (CORTESOGNO etalii, 1998; TRAVERSA et alii, 2003; GAGGERO et alii, 2007),Southalpine (CASSINIS et alii, 2007), the Pyrenees (LEROY

Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 127, No. 3 (2008), pp. 683-695, 8 figs., 2 tabs.

The Santa Giusta ignimbrite (NW Sardinia): a clue for the magmatic,structural and sedimentary evolution of a Variscan segment between

Early Permian and Triassic

LAURA BUZZI (*), LAURA GAGGERO (*) & GIACOMO OGGIANO (**)

(*) Dip.Te.Ris., Corso Europa, 26 - I-16132 Genova, [email protected], [email protected]

(**) Istituto di Scienze Geologico-Mineralogiche, Università diSassari, Corso Angjoi, 10 - I-07100, [email protected]

BUZZI (Boll. Vol. 127 Fasc.3-2008)

Queste bozze, corrette e accompagnate dall’al-legato preventivo firmato e dal buono d’ordine,debbono essere restituite immediatamente allaSegreteria della Società Geologica Italianac/o Dipartimento di Scienze della TerraPiazzale Aldo Moro, 5 – 00185 ROMA

& CABANIS, 1993), Provençe (LAPIERRE et alii, 1999),Morocco (AIT CHAYEB et alii, 1998); in particular the alka-line products have been generally assumed as evidence ofan anorogenic environment.

The transition from late Variscan processes to thePangaea break-up and rifting, has been classically associ-ated with the occurrence (BONIN, 1989) of the secondigneous cycle. The change of igneous affinity from calc-alkaline to alkaline has been in turn interpreted as associ-ated with the reactivation of older tectonic lineaments(BONIN, 1989, 2007), corresponding to the deepening ofmagmatic sources in the asthenospheric mantle (LEROY

& CABANIS, 1993; TRAVERSA et alii, 2003), within a newpurely extensional regime. More recently, the magmasissued from the partial melting of subcontinental litho-spheric mantle since the intermediate phases of theVariscan collapse, were interpreted as affected by variousextents of contamination in the lower and intermediatecrust. The tectonic unroofing of the Variscan chainaccompanied and followed by delamination of the conti-nental lithosphere, could have triggered the partial melt-ing of lithospheric mantle at depth (ROSSI & COCHERIE,1991; GAGGERO et alii, 2007). As a fact, the switch of ser-ial affinity of volcanic rocks from calc-alkaline in thePermo-Carboniferous to alkaline in the Triassic is not

sharp or easily detected. Even direct relations of changesin serial affinity with sedimentary breaks and a new tec-tonic regime are not explicit.

For instance alkaline volcanic rocks emplaced in Cor-sica between 290 and 286 Ma (COCHERIE et alii, 2005);conversely, in Sardinia rocks of the same age within theEarly Permian basins in Barbagia, and at minor extentwithin a Permo-Carboniferous basin in Gallura, showcalc-alkaline fingerprint. In Sardinia, alkaline (MACERA etalii, 1989) and transitional basalt dykes (GAGGERO et alii,2007) were emplaced in the Upper Permian-Triassic timeinterval. To this event have been also referred the acidiclavas and pyroclastic flows cropping at Nurra (NW Sar-dinia) up to now considered as Triassic alkaline product(LOMBARDI et alii, 1974).

As a whole, the post-Variscan scenario in the south-ern Europe suggests a complex volcanic pattern, not com-parable with present day analogues. This investigationwas addressed to the volcanic episodes in a key sector ofthe southern European Variscides as for the magmaticevidence, i.e. north-western Sardinia. Here, the remnantof a late Variscan basin preserved a volcano-sedimentarysequence that goes back, at least, to Lower Sakmarianand continues up to the Lower Triassic. In particular, inthe Santa Giusta area, the contact between the Variscanbasement and its Upper Palaeozoic cover is exposed.Hence this area is favourable to investigate the onset ofsedimentary and magmatic activity between the Variscancollapse and the Triassic geodynamic setting, by charac-terizing the petrology, geochemistry and geochronologyof the ignimbrite, and its field relations with other vol-canic rocks. We aimed to constrain I) the birth and evolu-tion of a Permo-Triassic succession over the collapsingorogen, and II) the early emplacement of magmas withalkaline characters, i.e. long before the beginning of con-ventional «anorogenic» activity.

THE POST-OROGENIC EVOLUTIONOF THE VARISCAN BELT IN SARDINIA

After collision and thickening the Variscan segmentof Sardinia experienced widespread extension (OGGIANO,1994; CARMIGNANI et alii, 1994). This extensional regimewas contemporaneous to HT/LP (sillimanite, cordierite,K-feldspar, andalusite) metamorphism, anatexis (MACERA

et alii, 1989) and emplacement of the batholith, andfavoured the onset of intracratonic basins characterisedby volcano-sedimentary infillings (CASINI & OGGIANO,2008; CORTESOGNO et alii, 1998). At 338, 305 and 280 Ma,over a 50 Ma interval the Sardinia-Corsica batholithintruded the Lower Paleozoic polymetamorphic basementthrough distinct short-lived episodes (Corsica; PAQUETTE

et alii, 2003).In the external and nappe zones of Sardinia, Permo-

Carboniferous basins developed as strike-slip or pull-apart basins filled with fluvio-lacustrine or fluvio-palu-strine sediments, effusive and sub-intrusive products (SanGiorgio, Mulargia, Seui, Perdasdefogu, Escalaplano, fig. 1;CORTESOGNO et alii, 1998). In the nappe zone, the poly-metamorphic basement is cut by NE-SW trending, calc-alkaline rhyolite dyke swarms (Note illustrative Foglio549-Muravera, 2004). Probably still at the Early Permian,a large region (Nurra, Gallura, Ogliastra, Iglesiente) wasaffected by important explosive acid volcanism, likely

684 L. BUZZI ET ALII

Fig. 1 - Schematic tectonic map of Sardinia: 1) Tertiary and Quater-nary covers; 2) Late Variscan batholith; 3) High-grade metamorphiccomplex; 4) Internal nappes; 5) External nappes; 6) External zone;7) Upper Carboniferous-Permian continental sediments and volcanicrocks; 8) Major and minor thrusts; 9) Posada-Asinara Line.– Schema tettonico della Sardegna: 1) Coperture terziarie e quaternarie;2) batolite tardo-varisico; 3) complesso metamorfico di alto grado;4) falde interne; 5) falde esterne; 6) zona esterna; 7) Successioni vulcano-sedimentarie continentali Carbonifere superiori-Permiane; 8) thrusts diordine maggiore e minore; 9) linea Posada-Asinara.

associated with an extensional phase (CASSINIS et alii,2003a). Afterwards, the deposition in the basins wassutured by one or more regional unconformities. Evi-dence for a Mid-Permian hiatus, marked by an unconfor-mity throughout the Variscan outcrops in Europe, is pro-vided by tectonic activity, high heat flow and subsequentigneous activity (DEROIN & BONIN, 2003).

On the whole, the structural evolution can thus beenvisaged as switching from a transtensional to exten-sional tectonic regime. Across the belt, the Lower Sak-marian-«Autunian» igneous, sedimentary, tectonic andmetamorphic features are consistent with the orogeniccollapse and disruption.

THE POST-OROGENIC SEDIMENTARY SUCCESSIONS IN NW SARDINIA

The sedimentary successions lying on the basement innorth-western Sardinia are not affected by pervasivedeformation or by metamorphic recrystallisation. Onlythe volcano-sedimentary deposits of Late Carboniferous-Early Permian age in the Gallura basins are affected bycontact metamorphism at 289 Ma, owing to the intrusionof small plutons (DEL MORO et alii, 1996). In Nurra, theRb/Sr blocking ages of biotite (DEL MORO et alii, unpub-lished data) within the Variscan basement just below thePermo-Triassic cover, show a spread from Upper Car-boniferous up to Jurassic time. Rather than to the well-known thermal event linked to the opening of the Li-gurian-Piedmont rift, documented in the Southalpinedomain (DI PAOLA & SPALLA, 1999), the re-opening of theVariscan isotopic blocking ages are best referable to theonset of the Pyrenean rift (DAUTEUIL & RICOU, 1989).

Open folds with NE axial trend, thrusts with west-wards tectonic transport and normal faults characterisethe present day setting of the post-Variscan sequences.This tectonics of uncertain age affects the carbonatic cov-ers of Mid-Upper Triassic age but is less developed in theunderlying clastic and volcanic Permian succession, prob-ably due to detachment surfaces favoured by the Triassicevaporites.

In Nurra (NW Sardinia), two main sedimentary cycleswere distinguished (CASSINIS et alii, 2003b): I) an olderone, exposed in a condensed, few tens of metres thicksuccession, shows typical grey «Autunian» facies andflora, and like the correlative Les Pellegrins Fm. inProvençe it is associated with volcanic rocks of presumedcalc-alkaline affinity, i.e. the dacites close to the BaratzLake at Pedru Siligu farmstead; II) a younger one, furtherdivided into minor continental sequences.

Northeast of the line Capparoni-Baratz Lake, theexposed thickness of both sequences decreases to fewtens of meters. South of this lineament the deposits of thesecond cycle, mostly red beds, are exposed for at least 300meters in thickness. They represent an alluvial megacycle,further subdivided into three minor continental sequences.The megacycle begins with tens of meters of canalisedquartz-conglomerates and sandstones, deposited in analluvial setting and includes wind-worn grains, alluvialbraided rivers and meandering deposits. Fine flood-plaindeposits and palaeosols characterise the top of the minorsequence (CASSINIS et alii, 2003b). The paleoclimaticinferences are debated: according to CASSINIS et alii(2003b) the wind-worn clasts in some deposits testify arid

condition, whereas SHELDON (2005) suggests that weath-ering occurred under humid climate on the base of thepalaeosol features. Alkaline volcanic rocks in Provençeare hosted within Les Salettes Fm., which is thought cor-relative of the Porto Ferro Formation of Nurra i.e. thesecond minor sequence of the second megacycle.

At Mt. Santa Giusta, few meters of red beds, referableto the second megacycle, are capped by the dolostones inMuschelkalk facies and rest on a rhyolitic ignimbrite. The«Autunian» and Permo-Triassic successions with Saxono-Thuringian facies crop out between Punta Capparoniand Cala Viola, where they attain the maximum thickness(> 300 m), which abruptly decreases northwards. In par-ticular at Mt. Santa Giusta, the «Autunian» successionwas not deposited, and the Permo-Triassic red bedsdecrease to few metres above the Sakmarian ignimbrite,that rests directly on the basement. Thus, north of PortoFerro the Nurra basement was a structural high boundedby WNW trending faults since the Late Carboniferous-Lower Permian (fig. 2A).

Sub-volcanic bodies of dacite, hosted in the red con-glomerates and sandstones, cut the basement and theignimbrite; in turn they are cut by the transitional basaltdolerites dated at 253-248 Ma (fig. 2B, GAGGERO et alii2007). As a whole, it appears that north of the WNW-trending Baratz lineament both the first and secondmega-cycle are condensed in few metres of deposits,suggesting that this area acted as a structural high alsoduring the Permian up to the Early Triassic.

THE PERMIAN VOLCANIC RECORD IN NW SARDINIA:OCCURRENCE AND PETROGRAPHY

The post-Variscan volcanic record in Nurra is mostlyrepresented by pyroclastic deposits and by reworkedpyroclastic rocks (CASSINIS et alii, 1996, 2003b). Herein,the Mt. Santa Giusta ignimbrite is the major occurrenceand consists of a tabular body, some tens of metres thick,including fiamma (fig. 3A), pumice-rich or lava-rich levelsor layers of fine-grained breccias of volcanic and meta-morphic clasts. Millimetre to tens of centimetres in sizemetamorphic xenoliths (polycrystalline quartz, micas-chists and gneisses), volcanic ortholiths and xenoliths(pyroclastites and lavas, sometimes pumiceous) and gran-itoid rocks are present (fig. 3B). The volcanic conglomer-ate facies are characterised by scarce volcanic and cry-stalline rock clasts (Ø ~0.5 cm) in a glassy medium- tofine- grained groundmass (~80% in volume), affected bydiffuse alteration to phyllosilicates. The crystalline rockfragments have dacite composition.

On the whole the ignimbrite is fresh, the groundmassvarying from welded glass shards to eutaxitic and to com-pacted microcrystalline aggregates or granophyric inter-growths of quartz and feldspar (fig. 4A, B). Phenoclastsare sanidine, sometimes showing vitrified rims (fig. 4C),quartz and biotite, sometimes kinked (fig. 4D). Dehydra-tion partially affects biotite during the magma ascent. Inaccordance with the modal mineralogy, the Mt. SantaGiusta rhyolite is characterised by K2O wt% higher thanother Lower Permian felsic rocks of Sardinia but lowerthan the Trinità ignimbrite that suffered widespreadalteration (TRAVERSA, 1967). The alteration is sparse andnot pervasive; K-feldspar and biotite are well preserved.Only locally the groundmass is replaced by kaolinite, illite

THE SANTA GIUSTA IGNIMBRITE (NW SARDINIA) 685

and Fe-hydroxides. The porosity is filled by aggregates ofradiating kaolinite, chalcedony, fibrous zeolites andmicrocrystalline quartz. At places, whitish rock portionscorresponding to silicified groundmass are likely theeffect of elutriation (fig. 3C, D). The hydrothermal alter-ation produced zeolitisation of alkali feldspar, celadoniticalteration of biotite and iron oxidation of glass. The upperpart of the deposit is tuffaceous, often oxidised and car-bonated, probably from pedogenic alteration.

Contrasting K-Ar literature data (197±6 and 204±6Ma on whole rock LOMBARDI et alii, 1974; 249±9 and303±9 Ma on the biotite-feldspar pair EDEL et alii, 1981)and the association with the red beds, inferred as Trias-sic, induced to consider a Triassic age for the ignimbrite.

At Case Satta, 2 kilometres west of Baratz Lake, tuffsfrom fall deposits are prevalent and moderately resedi-mented. Some pyroclast-rich deposits contain basement

clasts of quartz-micaschists, angular fragments of coarsemosaic calcite and Fe-hydroxides. The biotite is widelyaltered to white mica and illite, and the K-feldspar tokaolinite. The matrix contains cuspate X and Y-shapedshards from glass vesicle fragmentation. They tend to bereplaced by aggregates of microcrystalline quartz, chal-cedony, kaolinite and Fe-hydroxide.

About two kilometres north east of Baratz lake, thesuccessions include well-sorted tuffaceous sandstone withclasts of volcanic quartz, volcaniclastic vitric fragmentsand basement lithoclasts.

The Torre Negra, Torre Bianca and Baratz Lake con-glomerates contain volcanic lithoclasts of consolidatedtuffs with quartz phenoclasts, biotite and K-feldspar.Alteration of K-feldspar to kaolinite, and of vitric frag-ments to quartz, chalcedony and Fe-hydroxide is wide-spread and kaolinite, illite, celadonite and barite are dif-fuse. Coarse tuffs from fall out contain quartz, K-feldsparand biotite phenoclasts. The vitric fraction (clasts andmatrix) is altered to kaolinite and microcrystalline quartz.

ANALYTICAL METHODS

Whole-rock major and trace element abundances forvolcanic products from northwestern Sardinia were car-ried out with XRF techniques at the X-RAL Laboratories,Canada. Losses on ignition (LOI) were determined withthe gravimetric method. The RE elements were analyzedwith ICP-MS at the X-RAL Laboratories, Canada.

The mineral phases were analysed at the University ofGenoa using a Philips SEM 515 scanning electron micro-scope, equipped with an EDAX PV9100 spectrometer inthe energy dispersive mode. Operating conditions were15 kV accelerating voltage and 2.1 nA of beam current.Reference standards for the elements (in brackets) were:jadeite (Na), forsterite (Mg), albite (Al), augite (Si, Ca),microcline (K), ilmenite (Ti), chromite (Cr), rhodonite(Mn) and fayalite (Fe). Other elements were below detec-tion limits. The natural standards were analysed by WDSmicroprobe at Modena University. Na2O and MgO con-tents analysed in silicates by an EDS microprobe are gen-erally underestimated if the analysis is processed withcurrent automatic methods. To overcome this problem,the background for Na (1.040 keV) and Mg (1.252 keV)was manually corrected and considered to be between 0.9


Fig. 2 - B) Mt. Santa Giusta area. Cross-section and basement-cover relationships.– Area del M. Santa Giusta. Profilo geologico e relazioni basamento-copertura.

Fig. 2 - A) Mt. Santa Giusta. Schematic geological map.– Area del M. Santa Giusta. Schema geologico.

A

B


Fig. 3 - A) Fine-grained, patchy-leached tuffitic layer; B) web-shaped elutriation paths in the tuffites; C) eutaxitic rheomorphic texture inmassive, crystal-, lithic- and fiamma- rich matrix. Correlation with model ignimbrite stratigraphy from BRANNEY & KOKELAAR (2002).– A) Livelli tufitici a grana fine affetti da lisciviazione irregolarmente distribuita a chiazze; B) camini di elutriazione con andamento reticolare nelletufiti; C) tessitura reomorfica eutassitica in matrice massiva ricca in cristalli, litici e fiamme. Correlazione con la stratigrafia di una ignimbritemodello secondo BRANNEY & KOKELAAR (2002).

and 4.2 keV. The manual background was corrected alsofor the natural standards. Raw data were reduced usingthe ZAF algorithm and the standard software of the EDAXPV9100. The major element composition of biotites wasalso analysed by a Cameca SX50 electron microprobeequipped with wavelength-dispersive spectrometer at theBRGM (Orléans), under the following operating condi-tions: acceleration voltage 15 kV, beam diameter 10-12 nA,counting time 6 s and correction by the ZAF method.Detection limits are c. 0.2% and concentrations of lessthan 0.3 wt.% are considered qualitative. The biotite analy-ses were recast to 22 oxygens and 14 cations excludingCa, Na and K. The feldspar analyses were recalculated tototal cations = 5 on the basis of eight oxygens.

The 40Ar-39Ar age determinations were carried out onbiotite phenocryst separate. The separated fraction wasanalysed with the 40Ar-39Ar incremental method at theActlab Laboratories (Canada). The sample wrapped in Alfoil was loaded in an evacuated and sealed quartz vialwith K and Ca salts and packets of LP-6 biotite inter-spersed with the samples to be used as a flux monitor.

LP-6 biotite has an assumed age of 128.1 Ma. The samplewas irradiated in a nuclear reactor for 48 hours. The neu-tron gradient did not exceed 0.5% of the sample size. TheAr isotope composition was measured with a Micromass5400 static mass spectrometer. The 1200ºC blank of 40Ardid not exceed n*10-10 STP.

MINERAL CHEMISTRY

BIOTITE

The TiO2 content of Mt. Santa Giusta biotites is gen-erally high, ranging between 2.3 and 4.2 wt% (tab. 1); thehighest Ti contents occur in phenocrysts from the con-glomeratic facies. In the Altot vs. Mg diagram (NACHIT etalii, 1985), subalkaline compositions are evidenced (fig.5A). In the FeOtot-Al2O3-MgO (ABDEL-RAHMAN, 1994)discrimination diagram (fig. 5B), most biotites fall at theboundary between the peraluminous and calc-alkalineorogenic series.


Fig. 4 - Representative transmitted light microphotographs of ignimbrite textures and composition. Bar scale in photos. A) Plane polarisedlight. Stretched vitric fragment in welded eutaxitic texture (centre). B) Plane polarised light. Vitrophyric fragment surrounded by crystalclasts in welded eutaxitic texture. C) Crossed polars. Sanidine phenoclasts with radiating, devitrified rim in glassy matrix. D) Plane polarisedlight. Detail of a biotite phenoclast «inflated» along cleavages, likely during the surge blast.– Microfotografie in luce trasmessa rappresentative delle tessiture dell’ignimbrite e della sua composizione. La barra della scala è riportata nellefoto. A) Luce polarizzata. Frammento vetroso stirato in tessitura eutassitica saldata (centro). B) Luce polarizzata. Frammento vetrofiricocircondato da clasti minerali in una tessitura eutassitica saldata. C) Nicols incrociati. Fenoclasti di sanidino con bordi devetrificati, a strutturaradiale, in matrice vetrosa. D) Luce polarizzata. Dettaglio di un fenoclasto di biotite deformato «a fisarmonica» lungo i clivaggi, probabilmentedurante l’evento esplosivo.


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0.05

0.10

0.15

0.10

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0.00

0.00

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Nd

12.2

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0.17

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0.11

29.9

129

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29.8

430

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27.2

528

.75

28.9

329

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2.6

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0.00

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5.51

5.59

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5.26

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0.86

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060.

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420.

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380.

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550.

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2.94

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0.40

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0.39

0.40

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1.5

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0.00

0.02

0.06

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Yb

1.7

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13.1

311

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16.9

612

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8.90

9.10

8.94

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8.52

8.77

8.71

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1.63

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00

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26

0.31

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00

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13

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70

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1.77

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0.35

K-FELDSPAR

Alkali feldspar phenocrysts are sanidine characterisedby albite contents in the range 25-32 mole % (tab. 1; fig.5C). In the groundmass, feldspars in eutectic with quartzhave orthoclase composition. An overall low anorthitecontent (1.1-2.6 mole %) is common to K-feldspar fromphenocrysts and groundmass.

BULK ROCK COMPOSITION

Alteration processes

The interpretation of primary composition of effusiverocks, and particularly of an ignimbrite, should take intoaccount the possibility of element mobilisation during theearly diagenesis. The petrographic features evidence thatthe pyroclastic products intercalated in the sedimentarysuccession of Nurra and the welded lower horizon of theMt. Santa Giusta ignimbrite can be assumed as fairly rep-resentative of the primary outpoured products (fig. 6A),based on the cross-reference of modal, normative andchemical compositions. On the contrary, the tuffaceouslayer of the ignimbrite, or the resedimented pyroclastitesgive petrographic and chemical evidence of more or lessextensive compositional changes, e.g. SiO2 increase in the

pumiceous matrix, alkali loss and consequent kaolinisa-tion at the expenses of K-feldspar. Analyzed samples wereselected as fresh as possible, taking into account the alter-ation pattern. To assess the possibility of K2O secondaryenrichment and the reliability of the trace element abun-dances for the ignimbrite, the compositions of Mt. Santa


Fig. 5 - Mineral chemistry of the Mt. Santa Giusta ignimbrite and py-roclastic products. A) Altot-Mg diagram (NACHIT et alii, 1985) and B)FeO*-MgO-Al2O3 discrimination diagram (ABDEL-RAHMAN, 1994)for biotite phenoclasts. Fields represent biotite compositions in: A,anorogenic alkaline suites; P, peraluminous suites and C, calc-alkali-ne orogenic suites. C) Composition of alkali-feldspar phenoclasts.Symbols: full diamond = ignimbrite; open diamond = pyroclastites.– Minerochimica dell’ignimbrite e dei prodotti piroclastici di M. SantaGiusta. A) diagramma Altot-Mg (NACHIT et alii, 1985) e B) diagrammadiscriminante FeO*-MgO-Al2O3 (ABDEL-RAHMAN, 1994) per i fenocla-sti di biotite. I campi in figura B rappresentano composizioni di biotitein: A, serie alcaline anorogeniche; P, serie peralluminose; C, serie oro-geniche calcalcaline. C) Composizione di fenoclasti di alcali-feldspato.Simboli: rombo pieno = ignimbrite; rombo aperto = piroclastite.

Fig. 6 - A) (Na2O+CaO)-Al2O3-K2O diagram of NESBITT & YOUNG(1984) showing the weathering trends for average granite. B) Oceanridge granite (ORG) normalised spiderdiagrams for Mt. Santa Giustaignimbrite and pyroclastic products. For comparison are reportedthe patterns of post-collision and volcanic arc granites (data fromPEARCE et alii, 1984).– A) Il diagramma (Na2O+CaO)-Al2O3-K2O di NESBITT & YOUNG(1984) evidenzia il vettore di alterazione a partire da una composizionegranitica media. B) Diagramma spider normalizzato agli ocean ridgegranite (ORG) per l’ignimbrite e i prodotti piroclastici di M. SantaGiusta. Per confronto sono riportati le composizioni di granitoidi diarco e post-collisionali (dati da PEARCE et alii, 1984).

Giusta were compared with those of significant orogenicand post-orogenic granitoid compositions, normalised inthe Rock/ORG spiderdiagrams, and resulted within theircompositional interval (fig. 6B).

Primary features

The Permian volcanites of Nurra (tab. 2) fall along theandesite-dacite-rhyolite line in the WINCHESTER & FLOYD

(1977) classification diagram (fig. 7A), where the Mt.Santa Giusta ignimbrite however displays high Nb/Y andZr/TiO2.

Two groups of REE patterns arise for the Nurra vol-canites (fig. 7B): the Casa Satta ignimbrites and tuffitesare comparable with the calc-alkaline dacites from south-eastern Sardinia, whereas the Mt. Santa Giusta ignim-brite and pyroclastic products differ by less fraction-ated LREE and more pronounced negative Eu anomaly(Eu/Eu*= 0.28-0.30); conversely the HREE are respec-tively more fractionated and enriched than the LowerPermian Baratz Lake pyroclastic products (fig. 7B). TheRock/MORB normalised spiderdiagrams do not allow asignificant discrimination between the volcanites of thelower and upper sedimentary cycles, except LILE enrich-


TABLE 2

Bulk rock analyses of Mt. Santa Giusta ignimbrites and pyroclastites, and Permian volcanics from Nurra region.– Analisi chimiche su roccia totale delle ignimbriti e piroclastiti di M. Santa Giusta e delle vulcaniti permiane della Nurra.

Sample SG5 SG6 SG23 SG24 SG26 ARS01/7 CS22 ARS01/3 SG29 PC14Provenance Mt. Santa Mt. Santa Mt. Santa Mt. Santa Mt. Santa Lu Caparoni Casa Satta Casa Satta Bonaita PonteLithology Giusta Giusta Giusta Giusta Giusta Rhyolite Tuffite Upper Epiclastite Crabolu

Ignimbrite Ignimbrite Pyroclastite Pyroclastite Pyroclastite tuffite Volcanite

Oxide (wt%)

SIO2 77.40 87.80 72.81 70.67 69.98 82.60 78.81 77.09 63.80 64.70TIO2 0.09 0.21 0.19 0.26 0.25 0.22 0.16 0.10 0.93 0.58AL2O3 11.50 6.80 14.02 15.60 16.19 7.86 10.72 9.07 17.00 21.59Fe2O3 1.64 0.54 1.94 2.45 3.10 0.75 2.27 1.35 6.21 2.78Cr2O3 <0.01 <0.01 0.00 0.00 0.00 <0.01 - 0.01 <0.01 -MnO 0.02 <0.01 0.03 0.03 0.02 <0.01 0.14 <0.01 0.12 0.01MgO 0.28 0.27 0.36 0.75 0.51 0.15 0.90 0.37 2.56 1.25CaO 0.10 0.12 0.34 0.56 0.28 0.06 1.79 1.06 0.80 0.03Na2O 0.27 0.19 0.12 0.49 0.37 0.07 0.04 0.06 5.79 0.12K2O 8.05 2.68 8.38 6.65 7.21 1.70 0.88 1.66 0.88 3.85P2O5 0.02 0.02 0.26 0.24 0.08 0.04 0.03 0.08 0.22 0.06LOI 1.00 1.55 1.55 2.31 1.99 3.40 4.25 4.90 2.10 5.12Sum 100.40 100.20 100.00 100.01 99.98 96.91 99.99 95.87 100.40 100.09

Trace element (ppm)

Cr <5 <5 7 8 7 n.a 22 n.a n.a 39Ni 3 3 <2 <2 23 6 7 6 <5 17Ba 270 184 649 655 550 174 253 673 192 269Rb 103 81 132 142 171 64 47 91.7 92 230Sr 13 32 23 33 55 123 33 119 90 89Nb 9 27 6 6 7 <10 6 11 8 17Th 19 6.4 12 8 8 4.4 4 7.3 n.a 15U 2.3 2.9 3 <2 <2 4.06 5 1.45 n.a 5Y 59 7 19 17 21 21 17 25 n.a 39Zr 72 53 125 158 157 123 72 121 158 275Sr/Y 0.2 4.6 1.2 1.9 2.6 5.9 1.9 4.8 2.3

REE (ppm)

La 28.7 34.3 29 32 28 18.9 14 34.4 n.a 44Ce 66.8 58.6 55 56 49 37.8 30 74 n.a 87Pr 7.4 7.3 n.a n.a n.a 4.65 n.a 10.4 n.a n.aNd 25.9 23.1 23 24 18 17.4 13 40.4 n.a 39Sm 6.9 4.5 n.a n.a n.a 3 n.a 8 n.a n.aEu 0.64 0.38 n.a n.a n.a 0.57 n.a 1.65 n.a n.aGd 7.1 2.9 n.a n.a n.a 3.12 n.a 7.07 n.a n.aTb 1.6 0.4 n.a n.a n.a 0.57 n.a 1.05 n.a n.aDy 9.9 2 n.a n.a n.a 3.69 n.a 5 n.a n.aHo 2.14 0.39 n.a n.a n.a 0.82 n.a 0.98 n.a n.aEr 6.3 1.3 n.a n.a n.a 2.41 n.a 2.57 n.a n.aTm 1 0.2 n.a n.a n.a 0.36 n.a 0.34 n.a n.aYb 6 1.5 n.a n.a n.a 2.4 n.a 2.5 n.a n.aLu 0.96 0.24 n.a n.a n.a 0.34 n.a 0.35 n.a n.aLaN/YbN 3.20 15.29 5.27 9.20LaN/SmN 2.57 4.70 3.89 2.65GdN/YbN 0.94 1.54 1.04 2.25Eu/Eu* 0.28 0.30 0.57 0.67ΣREE 171.3 137.1 96.0 188.7


Fig. 7 - A) Zr/TiO2 versus Nb/Y classification diagram (WINCHESTER & FLOYD, 1977) for Permian volcanic rocks from Nurra. B) Chondrite-normalised REE patterns (NAKAMURA, 1974). For comparison, the shaded field corresponds to the calc-alkalic dacites from southeasternSardinia (Perdasdefogu and Seui basins, CORTESOGNO et alii, 1998; CASSINIS et alii, 2003a). C) MORB-normalised spiderdiagram (PEARCE &PARKINSON, 1993). Shaded field as in fig. 7B. D) Rb/100-Nb/16-Y/44 diagram (THIÉBLEMONT & CABANIS, 1990). Symbols: empty triangle: LuCapparoni tuffites and cinerites; filled triangle: Lu Capparoni volcanite; filled inverted triangle: Lu Capparoni porphyroid; filled circle: CasaSatta ignimbrite; empty circle: Casa Satta tuffite; filled diamond: Ponte Crabolu volcanite; filled square: Mt. Santa Giusta ignimbrite; emptysquare: Mt. Santa Giusta pyroclastite; open diamond: Bonaita epiclastites; stars: calc-alkalic dacites from Nurra and from southeasternSardinia (as for the shaded field in fig. 7B).– A) Diagramma classificativo Zr/TiO2-Nb/Y (WINCHESTER & FLOYD, 1977) per le rocce vulcaniche permiane della Nurra. B) Pattern delle REEnormalizzate a chondrite (NAKAMURA, 1974). Per confronto, il campo ombreggiato corrisponde alle daciti calc-alcaline della Sardegna sud-orientale (bacini di Perdasdefogu e Seui, CORTESOGNO et alii, 1998; CASSINIS et alii, 2003a). C) Diagrammi spider normalizzati MORB (PEARCE& PARKINSON, 1993). Campo ombreggiato come in fig. 7B. D) Diagramma Rb/100-Nb/16-Y/44 (THIÉBLEMONT & CABANIS, 1990). Simboli:triangolo vuoto: tufiti e cineriti di Lu Capparoni; triangolo pieno: vulcanite di Lu Capparoni; triangolo pieno rovesciato: porfiroide di LuCapparoni; cerchio pieno: ignimbrite di Casa Satta; cerchio vuoto: tufite di Casa Satta; rombo pieno: vulcanite di Ponte Crabolu; quadrato pieno:ignimbrite di M. Santa Giusta; quadrato vuoto: piroclastite di M. Santa Giusta; rombo vuoto: epiclastiti di Bonaita; stelle: daciti calc-alcalinedella Nurra e Sardegna sud-orientale (come per il campo ombreggiato in fig. 7B).

A

B

C

D

ment, for both groups (fig. 7C). This feature however,cannot be considered conclusive due to the pyroclasticand / or reworked origin of the volcanic rocks. Both«anorogenic» and post-collisional characters arise in theY/44-Rb/100-Nb/16 diagram (fig. 7D; THIÉBLEMONT &CABANIS, 1990).

40AR-39AR DATING

One 40Ar-39Ar age determination was carried out onbiotite separates from the Mt. Santa Giusta ignimbrite(sample SG5; Lat. 40°48’37.49’’ N, Long. 8°14’30.23’’ E).The separated fraction was analysed with the 40Ar-39Arincremental methods. The incremental age spectrum isgiven in fig. 8. The biotite fraction gives a plateau age of291.5±1.5 Ma for the 89.7% of the 39Ar released. The ini-tial increments represent the youngest and apparent agesincrease of the first 2.9% of the released gas to approachthe total fusion ages. The latter is 290.2±2.7 Ma and, con-sequently, concordant with the plateau age. The age spec-trum for biotite fraction indicates a homogeneous argonisotopic content and implies that the biotite was essen-tially closed to argon diffusion after cooling. The plateauage of 291.5±1.5 Ma likely represents the time at whichthe sample cooled through the biotite Ar-retention tem-perature.

DISCUSSION

Throughout northern Sardinia, high T/P post-kine-matic mineral assemblages record a high thermal flow inCarboniferous times. They overprint the older barrovianones related with crustal thickening at Asinara island andAnglona, where the blocking age of muscovite is about300 Ma, whereas the biotite, due to the lower blockingtemperature, is constantly younger of about 10-15 Ma(DEL MORO et alii, 1991). In lower Gallura, the calc-alka-line volcanic activity and the emplacement of thebatholith are sub-contemporaneous; as a fact some grani-toids intruded sediments and volcaniclastic rocks origi-nating a contact aureole dated at 290 Ma (Rb-Sr on ther-mometamorphic micas; DEL MORO et alii, 1996). In thePermian basin of Guardia Pisano (SW Sardinia), the earlyAsselian age inferred by means of sporomorphs is in goodagreement with the mean radiometric age of the volcanicrocks obtained by the U-Pb on zircon dating at 297±5 Ma(PITTAU et alii, 2002; SHRIMP and Pb-zircon evaporationmethods). As a whole, in Sardinia, the igneous-sedimen-tary, structural and metamorphic record is consistentwith the geodynamic pattern assessed in contiguoussegments as the Montaigne Noire (HECHTLER &MALAVIEILLE, 1990), the Central massif and the Pyrenees.At larger scale however, the late-orogenic regional uplift,erosion and crustal thinning developed in the SouthernEurope as late as in the foreland (PRAEG, 2004).

The stratigraphic setting suggests that the emplace-ment of the Mt. Santa Giusta ignimbrite was tightly con-trolled by the local topography, as in BRANNEY & KOKE-LAAR (2002). The ignimbrite outpoured from the vent (notoutcropping, but likely located to the SW), generating adeposit that laps onto tilted basement blocks, developingdensity stratification and segregation of lithic clasts. Theelutriation pipes should develop in the sediments, as

observed at Santa Giusta, and parts of the currents, lessdense than ambient air, originate a co-ignimbrite plume.This emplacement pattern accounts for the condensedLate Permian-Early Triassic record, i.e. the lack of the«Autunian» succession and the rhyolite sandwichedbetween the Variscan basement and the Permo-Triassicsuccession (red beds and the Muschelkalk limestone).

On the whole, in Nurra at Mt. S. Giusta a Permo-Triassic succession is preserved, and deposited within anasymmetric basin; in particular the geometry of Permiansedimentary record implies a half-graben structure, cutby the calc-alkaline volcanic rocks, after the emplacementof the ignimbrite.

CONCLUSIONS

The Mt. Santa Giusta ignimbrite, up to now regardedas Triassic for its two-folded radiometric ages and theassociation with red beds, emplaced at about 291.5±1.5Ma, as pointed out by 40Ar-39Ar age on biotite. The struc-tural setting of the tilted blocks likely represents themechanism for the rise of deep magmas in the uppercrust during Late Carboniferous-Early Permian times.

COCHERIE et alii (2005) report a comparable occur-rence in the Corsica section, where alkalic intrusions havebeen dated at the Lower Permian. The same faults thatcontrolled the post-collisional Variscan extension alsoplayed a role in the development of Mid Permian andLower Triassic successions.

At Mt. Santa Giusta, the Early Permian age of theignimbrite, which rests on the basement, suggests thatthe overlying few meters of red conglomerates and sand-stones, capped by the Muschelkalk deposits, is a con-densed succession comprising the Mid Permian to Trias-sic time span. Southward of the NE-SW trending lowangle grow fault that bounds the subsiding crustal blocks,a thicker succession deposited (fig. 2A). It started with


Fig. 8 - Apparent age spectrum 40Ar-39Ar for the biotite fraction sepa-rated from the Mt. Santa Giusta ignimbrite.– Spettro 40Ar-39Ar di età apparente per la frazione di biotite estrattadall’ignimbrite di M. Santa Giusta.

grey «Autunian» siltites and sandstones, followed by withat least 300 metres of red beds, ranging from PermianRotliegende-type deposits to typical Bundsandstein sand-stones and siltites (Cala Viola formation). On the wholeat local scale, the geometry, stratigraphy and nature ofthe volcano-sedimentary succession suggest a NW-SEoriented polarity of the orogenic collapse.

As a whole, the lowermost Permian age of emplace-ment for the Mt. Santa Giusta ignimbrite points to a step-wise evolution for this sector: tectonic unroofing of theVariscan range accompanied and followed by delamina-tion of the continental lithosphere, that could have trig-gered the partial melting of crustal/subcrustal sources. Asa result, the sub-alkalic magmas were originated andemplaced mostly as intrusions. In this phase a conspicu-ous crustal melting should have occurred due to the riseof asthenospheric mantle to accommodate erosion andthe related decompression of the continental roots. As awhole, this late orogenic phase results in volcanic-domi-nated successions.

ACKNOWLEDGMENTS

We would like to thank Antonio Funedda and Philippe Rossi forconstructive reviews of the manuscript. A particular acknowledg-ment is due to Philippe Rossi for providing biotite analyses by WDSmicroprobe. Bernard Bonin helped us improving a preliminary ver-sion of the manuscript, This work is dedicated to Luciano Corte-sogno, who greatly contributed to the geology of the Paleozoic base-ments. The research has been supported by the PRIN 2004 funds toL. Cortesogno and G. Oggiano.

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Received 2 November 2007; revised version accepted 17 June 2008.

The Santa Giusta ignimbrite (NW Sardinia): a clue for the magmatic, structural and sedimentary evolution of a Variscan segment between Early Permian and Triassic

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