reconstruction of igneous, tectonic and sedimentary events...

ABSTRACT

Based on a detailed geological mapping, drilling data and struc-tural-petrological interpretations, the processes controlling the evo-lution of the latest Carboniferous-Early Permian Seui Basin (Sardi-nia) are reconstructed and interpreted in the post-Variscangeodynamic context. The basin represents a good exposure wherethe tectonic, sedimentary and igneous processes are recorded. Theevolutionary picture is analogous to that occurring in many intra-montane basins developed as a consequence of the transtensionaland transpressional wrench tectonics active from the Maghrebidesto a large part of Europe.

The opening of the basin structure was associated with ande-sitic volcanism interlayered with detrital sedimentation in a fluvio-lacustrine to marsh environment. Abundant flora and repeated flowsof rhyolitic pyroclastics from distal sources, probably to the NW, arerecorded within the sedimentary succession. The basin was boundedto the N and to the W by morphological highs due to concomitantbowing of the metamorphic basement and intrusion of diorite dykesfeeding dacite cryptodomes. The growth of the cryptodomes trig-gered the gravitational collapse of basement and cover slices. Con-tact metamorphism and hydrothermal processes along fractures alsoaffected both basement and sediments.

The andesites and the diorite-dacite suite, as well as theoverlying rhyolitic ignimbrites have a common calc-alkaline signa-ture, but different geochemical trends. Hybridisation of the magmasdue to the complex interaction of mantle-derived and crustal melts,through different stages of assimilation, fractionation and mixing,probably accounts for the geochemical and petrographic character-istics.

KEY WORDS: wrench tectonics, late orogenic magmatism,intracontinental basin, latest Carboniferous-EarlyPermian.

RIASSUNTO

Ricostruzione degli eventi magmatici, tettonici, sedimenta-ri e modello evolutivo del Bacino tardo carbonifero-permianoinferiore di Seui (Sardegna, Italia).

Gli studi condotti sul Bacino di Seui hanno portato ad una rico-struzione delle condizioni geodinamiche che hanno interessato, tra iltardo Carbonifero ed il Permiano inferiore, questo settore centro-orientale della Sardegna. Il bacino presenta un quadro relativamentechiaro dei processi tettonici, sedimentari e magmatici. Il quadroevolutivo è analogo a quello che si evince in altri bacini intramontani

generati da una tettonica transtensile-transpressionale attiva dalleMagrebidi fino a settori est-europei. L’apertura di un semi-graben ini-ziale è associata a vulcanismo andesitico intercalato a sedimentazio-ne detritica in ambiente fluvio-lacustre e palustre. Nella successionesedimentaria sono inoltre localmente presenti, soprattutto nella por-zione basale, ripetuti episodi piroclastici distali a composizione rioli-tica derivanti da centri di emissione probabilmente localizzati a NO.Il bacino era limitato a N e ad O da alti morfologici prodotti dal con-temporaneo inarcamento del basamento metamorfico e dall’intrusio-ne di filoni dioritici che alimentano criptodomi dacitici. L’accresci-mento dei criptodomi ha causato il collasso gravitativo di scaglie dibasamento e delle associate coperture. Fenomeni di metamorfismo dicontatto e processi idrotermali lungo fratture si sono sviluppati nelbasamento e nei sedimenti.

Le andesiti e la sequenza dioritica-dacitica, nonché le sovra-stanti ignimbriti riolitiche a Seui hanno una comune impronta calc-alcalina, ma diversi trend geochimici. La natura ibrida dei magmi,dovuta ad una interazione complessa di magmi di derivazione man-tellica e crostale attraverso diversi episodi polifasici di assimilazio-ne, frazionamento e mixing, verosimilmente rende conto delle lorocaratteristiche geochimiche e petrografiche.

TERMINI CHIAVE: tettonica transtensile, magmatismo tardoorogenico, bacini intracontinentali, Carbonifero ter-minale, Permiano inferiore.

INTRODUCTION

All over Europe, a predominantly wrench-inducedcollapse followed the collisional phases and crustal thick-ening of the Variscan orogeny during Late Carboniferousand Early Permian times. The transtensional and trans-pressional wrench tectonics, associated with predomi-nantly calc-alkaline magmatic vents, gave rise to thedevelopment of intracontinental basins (ARTHAUD &MATTE, 1977; BONIN et alii, 1993; HENK, 1997; CORTE-SOGNO et alii, 1998; ROTTURA et alii, 1998; ZIEGLER &STAMPFLI, 2001).

The basins were infilled by short range sedimentationand by locally predominant intra- and extrabasinal vol-canic products associated with subvolcanic intrusions(FRANCIS, 1988; BENEK et alii, 1996; AWDANKIEWICZ, 1999;CORTESOGNO et alii, 1988; LAGO et alii, 1994, 2001; MARTI,1996; VALERO GARCÉS, 1993; STOLLHOFEN et alii, 1999).

The Seui Basin (Barbagia di Seulo – figs. 1 and 2)provides evidence for a detailed reconstruction of theintersecting tectonic, sedimentary and magmatic events.The Seui and Seulo late Paleozoic sedimentary outcropswere investigated from the perspective of anthraciteexploitation (LAURO, 1970; ACCARDO et alii, 1984; SARRIA,1987; SARRIA & SERRI, 2000). In the present study, an

Boll. Soc. Geol. It., Volume speciale n. 2 (2003), 99-117, 12 ff., 3 tabb., 1 pl. n.t., 1 tav. f.t.

Reconstruction of igneous, tectonic and sedimentary eventsin the latest Carboniferous-Early Permian Seui basin (Sardinia, Italy),

and evolutionary model

G. CASSINIS (*), L. CORTESOGNO (**), L. GAGGERO (**), A. RONCHI (*),E. SARRIA (***), R. SERRI (***) & P. CALZIA (****)

(*) Dipartimento di Scienze della Terra dell’Università diPavia, via Ferrata 1, 27100 Pavia.

(**) Dipartimento per lo Studio del Territorio e delle sueRisorse dell’Università di Genova, C.so Europa 26, 16132 Genova.

(***) PROGEMISA, Via Contivecchi 7, 09122 Cagliari.(****) Progetto CARG-Sardegna, Via Dolcetta 19, 09122 Cagliari.

evolutionary model for the basin is inferred after reap-praisal of the structural, stratigraphical and petrologicaldata based on 1:5,000 scale mapping. Furthermore, therelated PROGEMISA drillings gave considerable insights onthe subsurface geometry of the basin, thus supporting theinterpretation of field data.

STRATIGRAPHIC OUTLINE

Even though a complete stratigraphical log of theSeui Basin is generally inhibited by the lack of clear keybeds and by the overwhelming volcano-tectonic effects,its succession is supported by detailed field mapping andENSAE and PROGEMISA drilling data (LAURO et alii, 1963;LAURO, 1970; SARRIA, 1987). The inferred lithological col-umn is made up (base to top) as follows.

The metamorphic basement («Postgotlandiano» Auct.;«Filladi Grigie del Gennargentu»: CAVINATO, 1976) is for-med by a complex including terrigenous successions (VAI

& COCOZZA, 1974) whose ages range between Cambrianand Lower Carboniferous (PILI & SABA, 1975; DESSAU etalii, 1982).

Specifically, the basement of the Seui area is ascribedto the Barbagia tectonic unit in the Internal Zone of the

Variscan nappes (CARMIGNANI et alii, 1992). It tectoni-cally overlies the Meana Sardo Unit with an estimatedthickness exceeding 1000 m. The unit is formed of alter-nating layers of quartz-micaceous metasandstones,quartzites, quartzitic phyllites and phyllites, from metresto tens of metres thick (LAURO et alii, 1963).

The foliated texture is often anequigranular for relictquartz and detrital micas. The neoblastic micas are veryfinegrained giving a lepidoblastic texture. The main folia-tion developed under a low-metamorphic grade, with amuscovite, chlorite and albite assemblage (FERRARA etalii, 1978; FRANCESCHELLI et alii, 1982; CAROSI et alii,1991; FADDA et alii, 1991).

The inferred Permian sequence (fig. 3) begins with red-dish coarse-to-finegrained breccia/conglomerate («BasalConglomerate» corresponding to the «complesso inferioreo di base»: SBARACCANI, 1963; LAURO et alii, 1963; LAURO,1970 or «complesso clastico di base»: ACCARDO et alii,1984). The breccia extends over the basin, with increas-ing thickness (up to 100 m) towards the centre of thebasin (SARRIA & SERRI, 1986; SARRIA, 1987), and uncon-formably overlies the Variscan basement (Barbagia Unit,Internal Nappes; CARMIGNANI et alii, 1992). The brec-cia/conglomerate is clast-supported; the lithic fragmentsare mainly angular and heterometric (up to 20 cm), with

100 G. CASSINIS ET ALII

Mesozoic andCenozoic covers

Permian and Lower Triassiccontinental deposits

Permian volcanicrocks

Intrusive complex(Upper Carbonif.-Permian)

High to low grademetamorphic complex(?Precambr.-Lower Carbonif.)

SARDINIAN VARISCANBASEMENT

(LATE) POST-VARISCANCOVER

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Fig. 1 - Regional and geologicalsketch of Sardinia with location ofthe Seui Permian Basin. Box areaenlarged in fig. 2.– Schema geologico e regionale del-la Sardegna, con localizzazione delbacino permiano di Seui. L’area ri-quadrata è ingrandita in fig. 2.

predominant quartz- and micaceous-schists reworkedfrom the underlying metamorphic substrate.

The «Conglomerate» is poorly organised; however, afining-upward trend occurs as well as crude beddingtowards the top. Coarse- to medium-grained sandstonebeds and rarer blackish pelitic lenses are interfingered inthe conglomerates. Pollen analysis revealed only poorlypreserved and carbonised remains (PITTAU, pers. comm.).On the whole, the deposit mirrors an alluvial fan or foot-slope environment.

To the top of this basal sedimentary unit, thin rhyo-lite pyroclastics, interbedded and in part reworked in theoverlying conglomerates, support the existence of earlyextrabasinal igneous acidic activity.

The bulk of the deposition in the basin is representedby alluvial-to-lacustrine, fine- to coarse-grained terrige-nous sediments over 300 m thick, with intercalated inter-mediate lavas. Different units were defined in the past, onthe basis of geometric relationships with the volcanics:«complesso antracitifero intermedio» («Antracitiferos.s.») and «complesso superiore» (LAURO et alii, 1963);«sequenza clastica intermedia», «sequenza clastica supe-riore di letto», and «sequenza clastica superiore di tetto»(ACCARDO et alii, 1984).

The sedimentary succession shows a discontinuouscoarsening-upward trend. Irregular alternations of darkgrey to black pelites with subordinate medium- to fine-grained, well-bedded sandstones occur at the bottom, andgrade to coarse sandstones and conglomerates towardsthe top. The resulting coarsening upward trend reflectsthe change from a prevalent lacustrine-marsh environ-ment to a higher-energy fluvial setting characterised bycoarser stream deposits.

Layers with coarse-grained basement clasts and/orquartz-rich conglomerates (Plate 1d) in the medium toupper part of the succession suggest repeated changes inthe deposition. It is not clear whether the transition fromthe fine to the coarse deposition is gradual, or abrupt andcontrolled by tectonics.

Abundant macrofloral remains occur in the finegrainedsediments (fig. 3; Plate 1b and 1c) throughout the Seui Basinas well in the adjacent Seulo outcrops. Since the first findsof LAMARMORA (1857), examined by MENEGHINI (1857), alarge number of fossil plants have been collected from thesedeposits. In particular, ARCANGELI (1901) (see also thereview of COMASCHI CARIA, 1959) discovered and deter-mined 51 species, mostly attributed to the Late Carbonife-rous-Early Permian. From a recent collection undertaken bythe authors (BROUTIN et alii, 2000), BROUTIN recognised thepresence of the following forms: Annularia sphenophylloides,

IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY) 101

SEUI

SEULO

M. Alastria

M. Perdedu

USSASSAI

BARBAGIASEULO

M. Arbo

M. Perdaliana

Flumendosa

Riv

er

Variscanmetamorphic units

Permian alluvial-lacustrinedeposits

Permian igneousrocks

Jurassic dolostones andclastics

GENNARGENTU

Main faults

0 2Km

Fig. 2 - Simplified geological map of the Seui Basin and surroundingareas.– Carta geologica semplificata del bacino di Seui e delle aree adiacenti.

0

10

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Metamorphic basement

"Basal Conglomerate"

Lacustrine facies andanthracite layers

Medium-to-finealluvial facies

Volcaniclastic conglomerates

Andesites

Rhyolitic ignimbrites

Coarse alluvial facies

Nonconformity

Acidic pyroclastics

EF

FU

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Wood stems

Fig. 3 - Representative section of the volcanic and sedimentarydeposits in the central sector of Seui Basin.– Sezione rappresentativa dei depositi vulcanici e sedimentari nelsettore centrale del bacino di Seui.


Pla

te 1

cf. Pecopteris arborescens, ?P. cyathea, Pecopteris sp., Pecop-teris sp. aff. hemitelioides, P. unita, Cordaites sp., Sigillariabrardii, and Artisia sp. (Plate 1b and 1c), and generally ascri-bed the association to an interval spanning from latest Car-boniferous to basal Early Permian («Autunian»).

The northern and western sections of the basin arebounded by a structural high largely represented by dacitedomes (see map; Plate 1f and 1g). The emplacement ofthese domes within the basement was associated with thesuperposition of basement slices above the sedimentarysequence (Plate 1a). This feature was related to «brittleextensional tectonics» (LAURO, 1970). The lack of kine-matic indicators at the base of the slices, and the evidentcorrelation with dome geometry, allow us to infer an ori-gin from gravitational collapse induced by dome growth.

THE IGNEOUS ROCKS: FIELD OCCURRENCE,PETROGRAPHY AND MINERAL CHEMISTRY

AndesitesIn the SE area andesites («Porphyrites» Auct.) occur

as plugs cutting the basement and the breccias, and asthin layers of medium- to fine grained volcanic brecciaswithin the sedimentary sequence (fig. 3 and map). Theplugs have broadly elliptical sections with SW-NE elonga-tion. The texture is porphyric seriate, often glomeropor-phyric with an intersertal to felsitic groundmass. Thephenocrysts are plagioclase, biotite, hornblende and rarepyroxene, with accessory zircon and apatite.

Xenocrysts and xenoliths are relatively frequent:quartzite xenoliths (up to 1 cm) show lobate boundariesand sometimes reaction rims of acicular pyroxene. Garnetxenocrysts, sometimes in clusters of small grains, occurrarely. Both the quartzite and the garnet probably repre-sent the refractory products of the assimilation of meta-morphic crust during the early stages of liquid ascent.Embayed grains of volcanic quartz, commonly polycrystal-line due to deformation and growth, likely indicate mixingwith acidic magmas. Compared with metamorphic quartzxenoliths, the lack of pyroxene reaction rims suggests thatthey were involved later in the ascending magma.

Hydrothermal veining of quartz, chalcedony and car-bonates is diffuse inside the plugs.

Two main outcrops of intermediate effusives occur atthe centre of the paleobasin (fig. 3).

Along the eastern margin, controlled by SW-NE tec-tonic lineaments, the lava flow attains its maximumthickness (> 60 m) and is characterised by evident colum-nar joints. The joints are curved with an E-verging dip

progressively decreasing from bottom to the top. As thecolumnar joints propagate away from the cooling surface,their orientation identifies the original location of theridge margin.

To the NW, andesites are represented by prevalentvolcanic breccias with intercalated lavas. The total thick-ness can locally attain 140 metres in the easternmostzones and tends to decrease to the W, where it rangesbetween 0 and 30 metres.

Very finegrained quartz and chalcedony occur as cm-thick beds or as breccia matrix, sometimes with silicifiedlava fragments. They possibly originated by silica precip-itation from hydrothermal vents.

The wedge-shaped geometry of the lavas supports theasymmetry of the basin. The lava ascent occurred alongfractures at the eastern margin; magmas then flowed tothe NW in the subsiding half-graben tectonic trough.

In the NE sector, andesites mostly occur as thin brec-cia layers. A submetric layer occurring near the bottom ofthe terrigenous cover includes an originally glassygroundmass, with rare plagioclase phenocrysts as the vol-canic component and clasts of quartz and metamorphicrocks as the sedimentary component. The clasts arecomparable to those in the basal breccia. The verticaldistribution of clasts within the layer shows a stronglyheterogeneous zoning. Very finegrained devitrified matterconstitutes the bottom of the layer for 5-10 cm. Above,the clasts appear in the glassy groundmass and increaseup to more than 80% of the volume, resulting in a grain-supported texture. The clasts decrease sharply upwards todisappear for some tens of centimetres at the top (Plate1e). An emplacement mechanism like that described forthe origin of «peperite» can be assumed: a magma risingalong a fracture in the underlying basement stalls at thecontact with unconsolidated sediments and induces boil-ing of the interstitial water. The process gives rise to acomplex mixture of magma and sediments with a softplastic or viscous fluid behaviour, which spreads laterallyin the sediment at shallow levels (BATIZA & WHITE, 2000).The mechanism shows that the magma emplaced intounconsolidated sediments. Thin beds of black silicifiedhyaloclastites are also related to localised explosive intra-basinal volcanic activity.

Andesite lavas show a porphyric seriate texture (P.I. ≈20) with zoned plagioclase and pyroxene phenocrysts; thegroundmass varies from holocrystalline intersertal, some-times fluidal, to felsitic. Medium-grained ortholiths arediffuse, as well as quartzite xenoliths, showing acicularpyroxene reaction rims, and embayed quartz xenocrysts.


Plate 1 - a) Panorama and geological sketch of the Seui Basin from San Sebastiano area. VB: Variscan basement; “VB”: Allochtonous base-ment slices; BC: Basal Conglomerate; Sed: Alluvial-to-lacustrine deposits; And: Andesitic lavas and breccias; PQ: Rhyolitic and dacitic domes;Ign: Rhyolitic ignimbrites. Dotted areas: Quaternary cover. b) Annularia sphenophylloides and fructified Pecopteris (cyatheoid type). c) Artisiasp. (medullar cast of Cordaites stem). d) Quartz- conglomerates in the upper portion of the volcano-sedimentary succession (SE of Mt.Marigosu). e) Polished slab of “peperite” (sample height: 31 cm). Evident sorting of clasts deriving from the host conglomerate, with rareplagioclase phenoclasts. f) Is Poddazzu from Arcu e Tradalei. Elliptical sections of dacite cryptodomes intruding the basement. g) Alignement of domes from Mt. Tradalei to the E. The lineament ends in the Mt. Taddi-Forada and Taddi dome structure, elevating abovea flat area of basement, terrigenous sediments and andesites. The basement also outcrops in the small flat wooded area within closer domes.– a) Panorama e schema geologico del Bacino di Seui dalla zona di San Sebastiano. VB: Basamento varisico; “VB”: scaglie di basamentoalloctono; BC: Conglomerato Basale; Sed: depositi alluvio-lacustri; And: Lave andesitiche e brecce; PQ: Domi riolitici e dacitici; Ign: Ignimbritiriolitiche. Area punteggiata: copertura quaternaria. b) Annularia sphenophylloides e Pecopteris. c) Artisia sp. (frammento di ramo di Cordaites).d) Conglomerati quarzosi nella zona superiore della successione vulcano-sedimentaria. e) Lastra levigata di “peperite” (altezza del campione:31 cm). Evidente classazione dei clasti che derivano dal conglomerato, con rari fenocristalli di plagioclasio. f) Is Poddazzu da Arcu e Tradalei.Sezioni ellittiche di criptodomi dacitici intrusi nel basamento. g) Allineamento di domi da M. Tradalei verso E. Il lineamento finisce nella strut-tura a domo di M. Taddi-Forada e Taddi che si eleva al di sopra di un’area pianeggiante costituita da basamento, sedimenti terrigeni e andesiti.Il basamento affiora anche in corrispondenza della piccola area boscosa all’interno dei domi più vicini.


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The widespread alteration of mineral phases and ofthe glassy groundmass could depend on autohydrother-malism under at least temporarily subaqueous condi-tions, and possibly activated by thermal flows.

Diorite dykes (Tratallas Diorite Auct.)Diorites occur as a system of E-W trending dykes (see

map). The major dyke, whose thickness ranges between250 and 500 metres, cuts the basement and the lower ter-rigenous sediments to the N of the basin. Two minordykes outcrop to the south below the domes.

The texture is medium-grained hypidiomorphic, withdiffuse, finegrained, aplite-like, sometimes granophyricinterstitial patches due to the concentration of residualeutectic liquids. Porphyritic textures develop towards thecontact with the host rock, mostly in the minor dykes,where they represent transitional features to the dacite.Compositions range from dioritic to quartz- and monzo-dioritic; the most evolved compositions occur at the topof the major dyke and in the minor dykes. In the moreprimitive rocks, large euhedral plagioclases (An45-67) showcomplex zoning (tab. 1b; fig. 4a); the cores, often sur-rounded by sieve textures, have an anorthite content upto 83-87%, but exceptionally up to 94%. The rims, as wellas the granophyric and interstitial grains, have An con-tents in the range 18-23%, with Or1-5.

Clinopyroxene, subhedral to poikilitic on plagioclase,has relatively homogeneous compositions from salite to augite (En37-41Fs12-18Wo37-47; tab. 1a; fig. 4a) and low TiO2

(0.17-0.59 wt%), Cr2O3 (0.15-0.36 wt%) and Al2O3 (0.0-2.11wt%). Orthopyroxene (En59-60Fs34-36Wo2-3; fig. 4a; tab. 1a) isdiffuse as euhedral to subhedral grains, but is generally alte-red to chlorite and only preserved as small grains within pla-gioclase. Late magmatic brown hornblende (Mg# 0.71-0.90,AlIV = 0.98-1.18 atoms per formula unit (a.p.f.u.), AlVI = 0.0-0.025 a.p.f.u., Ti = 0.10-0.24 a.p.f.u.) widely replaces the cli-nopyroxene or is developed as a poikilitic to interstitialphase. The polyphase growth of igneous brown hornblendesuggested by textural evidence is also consistent with poly-baric crystallisation constrained by barometric estimatesbased on conventional calibrations (HAMMARSTROM & ZEN,1985; HOLLISTER et alii, 1987; JOHNSON & RUTHERFORD,1988; BLUNDY & HOLLAND, 1990). Calculated pressures,between 0.4-0.3 GPa for the largest hornblende grains, and0.1-0.06 GPa for interstitial grains (tab. 2), are consistentwith a progressive ascent to the surface. Apatite and zirconare diffuse and more abundant in quartz-diorites.

Biotite (Mg#: 0.52-0.77, AlIV: 0.963-1.17, AlVI: 0.0-0.06, Ti: 0.27-0.33 a.p.f.u.; fig. 4b) generally occurs assmall interstitial grains in diorites and as large euhedralgrains in quartz- and monzodiorites, where granophyricor interstitial K-feldspar (Or96-98) also occurs. In the


Contact metamorphism assemblage

Hercynite Corundum Cordierite High Ti Biotite

SiO2 0,29 SiO2 0,15 SiO2 47,15 SiO2 34,90TiO2 0,19 TiO2 0,19 TiO2 0,27 TiO2 4,92

Cr2O3 0,08 Cr2O3 0,11 Cr2O3 0,10 Cr2O3 0,15Al2O3 62,91 Al2O3 98,87 Al2O3 33,03 Al2O3 16,87Fe2O3 0,00 Fe2O3 0,00 Fe2O3 0,00 Fe2O3 3,13FeO 0,00 FeO 0,42 FeO 6,93 FeO 10,54MnO 0,00 MnO 0,01 MnO 0,16 MnO 0,02MgO 7,50 MgO 0,00 MgO 8,99 MgO 12,53NiO 0,00 NiO 0,02 NiO 0,00 NiO 0,00CaO 0,09 CaO 0,11 CaO 0,15 CaO 0,14Na2O 0,00 Na2O 0,00 Na2O 0,00 Na2O 0,00K2O 0,16 K2O 0,12 K2O 0,22 K2O 10,09

H2O 3,95

Total 71,22 Total 100,00 Total 97,00 Total 97,23

Si 0,010 Si 0,003 Si 4,910 Si 2,651Ti 0,005 Ti 0,002 AlIV 1,090 6,000 Ti 0,281Cr 0,002 Cr 0,002 Cr 0,009Al 2,583 Al 1,985 Al 4,054 Al 1,510

Fe3+ 0,000 Fe3+ 0,000 AlVI 2,964 Fe3+ 0,179Fe2+ 0,000 Fe2+ 0,006 Ti 0,021 2,993 Fe2+ 0,669Mn 0,000 Mn 0,000 Fe3+ 0,000 Mn 0,001Mg 0,389 Mg 0,000 Cr 0,008 Mg 1,419Ni 0,000 Ni 0,000 Ni 0,000Ca 0,003 Ca 0,002 Fe2+ 0,604 Ca 0,011Na 0,000 Na 0,000 Mn 0,014 2,013 Na 0,000K 0,007 K 0,003 Mg 1,395 K 0,978

Ni 0,000 OH 2,000Ca 0,017 0,046Na 0,000K 0,029

Tab. 1b


E F

Diopsid

Augite

Pigeonit

Enstatit

✕✕✕

SG5

5

2

45

E F

Diopsid

Augite

Pigeonit

Enstatit

✕✕

✕

CP8

5

2

45

E F

Diopsid

Augite

Pigeonit

Enstatit ✕

✕✕

✕

✕✕ ✕✕

✕

✕

5

2

45

SG5

E F

Diopsid

Augite

Pigeonit

Enstatit

✕

✕

✕

5

2

45✕

✕

SG6

E F

Diopsid

Augite

Pigeonit

Enstatit

✕ ✕

P13A

5

2

45

✕

E F

Diopsid

Augite

Pigeonit

Enstatit

✕✕

SG5

5

2

45

Ab

❍❍

❍❍

❍❍❍ ❍

An Ab

OrSG53

❍❍

AnAb

Or

❍

An Ab

OrSG55

❍❍

Or

An Ab

❍❍ ❍

An

Or

SG54

❍❍ ❍

❍ ❍❍

An Ab

OrSG50

❍ ❍❍

❍❍

An Ab

OrP13A

❍

An Ab

OrCP85

Ab

❍

An

OrSG52

✕

✕✕

TiO2

FeO. Ti 2

2FeOTi 2

F2O3.Ti 2

CP95

✕✕

✕✕

TiO2

FeO. Ti 2

2FeOTi 2

F2O3.Ti 2

SG54

✕ ✕✕✕✕ ✕

✕✕✕

TiO2

FeO. Ti 2

Ti 2

F2O3.Ti 2

2FeO SG50

✕✕✕✕

✕✕

✕

✕

Fe F2O3

TiO2

FeO. Ti 2

2FeOTi 2

FeOTi 2F

2O3.Ti 2

FeOF2O

3

SG53

●

❍

■■■

▲

MgO

FeO Al 2O3

● SG60

❍ SG50

■ CP91 contactmetamorphism

▲ CP97 contact

Calc-alkaline

PeraluminousAlkaline

Fig. 4 - a) Above. Compositional variability of pyroxene and feldspars in diorites. b) Centre. Coexisting ilmenites and Ti-magnetites indiorites and in contact metamorphosed metasediment CP95 on the FeO-Fe2O3-TiO2 space. c) Below. Composition of biotite from dioritesand contact metamorphism assemblage; tectonic pertinence after ROTTURA et alii, 1998.– a) Sopra. Variabilità composizionale dei pirosseni e dei feldspati nelle dioriti. b) Al centro. Ilmeniti e Ti-magnetiti coesistenti nelle dioriti e nelsedimento CP95 metamorfico per contatto nel diagramma FeO-Fe2O3-TiO2. c) Sotto. Composizione delle biotiti in dioriti e nella paragenesimetamorfica per contatto; pertinenza tettonica secondo ROTTURA et alii, 1998.

near-surface, porphyritic portion of the dykes, biotitephenocrysts break down to pseudomorphic aggregatesof clinopyroxene and opaques. The lack of biotite andhornblende in the groundmass, where clinopyroxene isstable, suggests the degassing of the magma.

Subhedral to skeletal large ilmenite grains have highFe2O3 contents (up to 20.31wt%) and coexist with low-Timagnetite, whereas interstitial ilmenites have low Fe2O3

(7.41-11.41wt%) and coexist with low-Ti magnetite (fig. 4b).Melanocratic ortholiths (Cpx, Plg, Hbl) are found, as

well as large (up to 1 cm) quartz and garnet xenocrysts.Quartz xenocrysts are rounded and affected by microfrac-turing with slight rotation of the grains. Along the cracks,radiating aggregates of pyroxene developed by reactionwith the liquid. Garnet xenocrysts are almandine (Alm0.68-

0.71 Prp0.18-0.23Sps0.03-0.05Adr0.02-0.04Grs0.0-0.02) with weakcompositional zoning (fig. 5).

Secondary alteration is moderate and consists of i)chloritisation of orthopyroxene, ii) alteration of pyroxeneand hornblende to fibrous aggregates of actinolitic horn-blende or actinolite and chlorite, and iii) alteration of bio-tite to chlorite + epidote + magnetite + rutile.

Rhyolitic ignimbrite

Rhyolitic pyroclastic rocks, showing evident ignimbritefeatures at least locally, crop out as metre-thick levels tothe N of the basin, overlying andesites and terrigenous sed-iments (fig. 3). They are locally covered by basement slicescollapsed by gravitational sliding (see map).

Their peripheral setting with respect to the dome bod-ies can be assumed as a primary feature, therefore deriv-ing from the preferential deposition of the ignimbriteflows in the lowlands surrounding the domes. On the otherhand, the superposition of gravitational slices supports theprogressive and subsequent dome growth. If so, the ignim-brite vent was active during the long-lasting dome build-ing. The ignimbrite shows a poorly welded texture, withplagioclase, quartz, K-feldspar, and biotite phenoclasts,with accessory zircon, apatite, ilmenite and magnetite.

The pyroclastic layers represent the distal products ofan ignimbrite flow whose origin can be located some kilo-

metres to the NW, at Mt. Perdedu summit, formed bysome tens of metres of ignimbrites (COZZUPOLI & LOM-BARDI, 1969). The ignimbrite overlies i) subvolcanic bod-ies genetically related to the diorite dykes, and ii) a pyro-clastic complex formed by ignimbrite layers, volcanicbreccias, tuffs and hyaloclastites. In particular the fre-quent occurrence of hyaloclastite and accretionary lapillibeds in the tuffs suggests that the vent for part of the pyro-clastic products was located in a small basin already filledby volcaniclastic products prior to the ignimbrite vent.

Dacite-rhyolite domes

To the N and W the basin is bounded by acidic cryp-todomes. To the W the domes crop out only in part belowthe basement along a N-S lineament; to the northern mar-gin, the domes are grouped along three major parallelE-W alignments (see map). The emplacement occurred atshallow levels (some tens of metres) within the basement.Domes display a concentric «onion skin» foliation patternproduced by multiple magma batches inflating the outerrind. The grain variation within each batch results in alight and dark zoning. The pattern of the concentric foli-ation in the major dome of Bruncu Cintoni shows alobate structure resulting from three growth nuclei. Thetextural relationships indicate that the central body, com-positionally more acidic (Pissu Isili, Bruncu Cintoni), fol-lowed the dacitic bodies of Genna Isili (to the E and NE)and Is Incrastos (to the S) (map and fig. 6).

In the other northern domes, the structures are broadlyelliptical, with an E-W elongation. In some domes (SennaSu Monti) more evolved compositions up to rhyolite areobserved towards the centre.

The relationships between cryptodomes and theunderlying diorite dykes suggest feeding from an en-eche-lon series of dykes, which probably rotated to the southtowards the surface. The progressive ballooning of domeslifted the overlying basement and sedimentary cover, pro-ducing mounds in the palaeosurface, and in the case ofthe largest bodies, the gravitational collapse of the base-ment above the Permian sequence.

Dacites show porphyric, often glomeroporphyric,structures; phenocrysts comprise plagioclase, quartz, bio-tite, rare hornblende and apatite, ilmenite and zircon. Pla-gioclase (An20-25) shows porphyric seriate to microphyrictextures and zoning of the largest grains. Quartz pheno-


TABLE 2

Results of the Al in igneous amphibole geobarometersapplied to early coarse- and to fine grained interstial

hornblendes.Valori di pressione riferiti ad anfiboli ignei di cristallizzazio-ne precoce ed interstiziale, calcolati con i geobarometri

basati sul contenuto di Al nell’orneblenda.

Fig. 5 - Compositional zoning across garnet xenocryst.– Zonatura composizionale attraverso xenocristallo di granato.

crysts, rarely polycrystalline, are embayed and sometimesrimmed by a granophyric overgrowth. K-feldspar pheno-crysts are restricted to rhyolitic compositions. Garnet xen-ocrysts occur as inclusions in plagioclase phenocrysts. Thegroundmass texture varies from poikilomosaic (feldsparmicroliths in quartz), or rarely spherulitic, to felsitic.

The groundmass and the mineral grains are widelyaffected by secondary alteration preventing bulk rock anal-ysis, except for partially preserved plagioclase phenocrysts.

GEOCHEMICAL CHARACTERISTICSOF THE IGNEOUS ROCKS

The igneous activity in the Permian Seui Basin includesintermediate to acidic members showing a common calc-alkaline affinity (fig. 7). The intermediate volcanic rocks

range from basaltic andesites to andesites of weakly metalu-minous (less evolved members) to peraluminous character.The main dyke has diorite to quartz-diorite compositionsvarying from weakly metaluminous to Al-saturated, whereasthe domes consist of dacites and rhyolites showing Al-satu-rated to peraluminous characteristics (fig. 8A,B,C). Thebinary covariances (figs. 9A,B) are consistent with mediumto high-K calc-alkaline features. The comparison of majorand trace elements in andesites and diorites shows signifi-cantly different trends in spite of similar SiO2 ranges. Andes-ites have a higher maximum MgO content than diorites,with more evident fractionation. Ni and Cr contents are lowwith similar behaviour, whereas contrasting trends result forP2O5 and Sr (tab. 3). On the whole, diorites and dacites, sep-arated by a compositional gap (SiO2 63.43 and 67.17 wt% onanhydrous basis) (fig. 9A,B; tab. 3), show aligned covariation


Pissu Isili

Genna IsiliIs Incrastos

Bruncu Cintoni

α

δ

d

Fig. 6 - Bruncu Cintoni: composite three-folded structure of the lobatedacite dome (δδ). Centre, bottom: andesite (αα) lavas; left: detritalcovers at Fondu Corongiu (d).– Bruncu Cintoni: Struttura composita del domo dacitico (δδ) a tre lobi.Al centro in basso: lave andesitiche (αα); a sinistra: copertura detritica aFondu Corongiu (d).

Fig. 7 - AFM diagram. Symbols. Full circles: andesite plugs; open cir-cles: andesite lavas; squares: andesite sills; full triangles: diorites;open triangles: dacites.– Diagramma AFM. Simboli. Cerchi pieni: condotti andesitici; cerchiaperti: lave andesitiche; quadrati: sill; triangoli pieni: dioriti; triangolivuoti: daciti.

Fig. 8 - A) Total alkali-silica diagram (COX et alii, 1979); B) A/CNK; C) Nb/Y-Zr/TiO2 (WINCHESTER & FLOYD, 1977). Symbols. Full circles:andesite plugs; open circles: andesite lavas; squares: andesite sills; full triangles: diorites; open triangles: dacites.– A) Diagramma alcali totale-silice (COX et alii, 1979); B) A/CNK; C) Nb/Y - Zr/TiO2 (WINCHESTER & FLOYD, 1977). Simboli. Cerchi pieni:condotti andesitici; cerchi aperti: lave andesitiche; quadrati: sill; triangoli pieni: dioriti; triangoli vuoti: daciti.


TA

BL

E3

Rep

rese

nta

tive

wh

ole

rock

com

pos

itio

ns

of t

he

Seu

i an

des

ites

, d

iori

tes

and

dac

ites

.C

ompo

sizi

oni

rapp

rese

nta

tive

di

rocc

ia t

otal

e di

an

desi

ti,

dior

iti

e da

citi

di

Seu

i.

trends for most major elements as well as for Ti and Nb(decreasing) and for Sr (increasing). In accordance withfield evidence, this suggests a cogenetic origin for the dio-rites and dacites. As part of this hypothesis, the sharp flex-ure in K2O corresponding to the SiO2 gap is probably rela-ted to K-feldspar and biotite fractionation in quartz- andmonzodiorites, which is also consistent with the flexure forBa at SiO2 ≈ 62-63 wt% (fig. 9B). Analogously, the zirconprecipitation observed in more evolved diorites possiblyaccounts for the lower abundances in dacites for Zr and Th.

Chondrite normalised REE patterns are characterisedby LREE-enrichment (fig. 10; tab. 3); the negative Euanomaly is low to moderate in andesites (Eu/Eu*=0.59-0.86) and diorites (Eu/Eu*=0.52-0.81) and moderate in daci-tes (Eu/Eu*=0.38-0.45). Diorites and dacites show very lowHREE concentrations, and small degrees of M- and HREEfractionation (GdN/YbN= 1.45-1.59 in diorites; 1.44-1.39 indacites), comparable to ratios produced by the melting of

garnet-free sources. In andesites, the HREE fractionationis relatively strong and tends to increase in more evolvedcompositions (GdN/YbN=1.71-1.74 and GdN/YbN=1.97-2.15respectively; tab. 3). The ratios are consistent with values inliquids from garnet-bearing sources (≥3).

The decreasing ΣREE and increasingly negative Euanomaly towards more evolved compositions in dioritesand dacites are consistent with the fractionation of pla-gioclase, pyroxene and hornblende.

MORB-normalised spidergrams show LILE- and HREE-enriched incompatible element patterns (fig. 11). The lowNb and negative TiO2 and Ba anomalies are common tomost post-Variscan Permian volcanics (CORTESOGNO et alii,1998; ROTTURA et alii, 1998; LAGO et alii, 2001). Negativeanomalies for Ta, Nb and Ti, but not for Ba, are also com-mon to magmas from subduction-related environments(BAILEY, 1981; PEARCE, 1983). The low Rb in andesite(CP32) is probably caused by secondary mobilisation.


Fig. 9a

Zircon typology and magmatic affinity

Four samples (two diorites and two dacites) from theSeui Basin have been processed for zircon extraction. Indiorites (SG 50 and 51) the zircons are light green tocolourless. The largest subpopulation is rounded and gen-erally inclusion-free; however, small apatite needles maybe included. In sample SG50 some crystals show zoningand crystalline or glass inclusions. The mean A index is600 and the mean T index is 451 (fig. 12A).

Zircons from dacites (SG47 and 48) are light yellowto colourless and often include cooling cavities. The meanA index is 523 and the mean T index is 408 (fig. 12B).

The parameters are consistent with crystallisationwithin calc-alkaline series.

CONTACT METAMORPHISM

Localised thermometamorphic processes were trig-gered by the emplacement of subvolcanic magmas. Alongthe contact with the main diorite dyke, thermal recrystal-lisation affected the basement and the terrigenous coverover some tens of metres. The metamorphic peak wasrecorded by the assemblage K-feldspar + Fe-rich biotite(fig. 4b; tab. 1b) + Al-Mg and Al-Fe spinels (tab. 1) + corun-dum (tab. 1) + cordierite (Mg# 0.698; tab. 1) + ilmenite(fig. 4b), developed in metapelite layers of the basement.Chlorite + epidote replacing biotite and gibbsite replacingcorundum developed during retrograde phases.

In the Si-poor, Al-rich bulk rock compositions suggestedby corundum and spinel occurrence, the muscovite, biotite


Fig. 9 - A) Correlation of major elements vs. SiO2; B) correlation of trace elements vs. SiO2. Symbols. Full circles: andesite plugs; opencircles: andesite lavas; squares: andesite sills; full triangles: diorites; open triangles: dacites.– A) Correlazione degli elementi maggiori con SiO2; B) correlazione degli elementi in traccia con SiO2. Simboli. Cerchi pieni: condotti andesitici;cerchi aperti: lave andesitiche; quadrati: sill; triangoli pieni: dioriti; triangoli vuoti: daciti.

Fig. 9b

and K-feldspar equilibria (Ms=Kf+Cor+H2O, CHATTERJIE

& JOHANNES, 1974; Bt=Hypersthene+Kf+H2O, YODER &KUSHIRO, 1969) and the low Mg/Fe ratio in cordierite areconsistent with temperatures ≥ 600°C and very low pressures(<0.05 GPa), which are compatible with the nature of theheating magma and with stratigraphic constraints.

Thermal recrystallisation, affecting the terrigenoussediments entrapped at the base of the domes over a fewmetres, is indicated by localised blastesis of biotite, mus-covite and chlorite flakes.

HYDROTHERMAL QUARTZ DYKES

These are represented by microcrystalline quartzlocally associated with calcite-filled fractures.

In places, sulphides (sphalerite, galena, chalcopyrite)and iron oxides occur with massive, brecciated and cock-ade textures. Quartz shows abundant fluid inclusionsarranged along growth surfaces.

In the SW sector, the dykes form part of a WNW-ESEtrending fracture swarm, with subvertical dip and 1-2 m

thick dykes, between Genna Lioni and Arcu Spineddai; rareoutcrops in the NE sector have an approximately N-S trend.The trends are respectively subparallel and at right angles tothe collisional Variscan lineaments. Both trends match thestress field generated by the gravitational collapse of the belt.


Fig. 10 - REE patterns (normalized according to NAKAMURA, 1974) forandesites (squares), diorites (full triangles) and dacites (open triangles).– Pattern degli elementi delle terre rare (normalizzati secondo NAKA-MURA, 1974) per andesiti (quadrati), dioriti (triangoli pieni) e daciti(triangoli vuoti).

Fig. 11 - Rock/MORB spidergram. Symbols: squares: andesites, fulltriangles: diorites; open triangles: dacites.– Spidergramma roccia/MORB. Simboli. Quadrati: andesiti; triangolipieni: dioriti, triangoli vuoti: daciti.

In a general excursus, similar quartz dykes, locallywith Cu- and Fe-sulphides, crop out in the Val TrompiaCollio Basin (Brescian Prealps) with NNW-SSE and NW-SE trends, infilling transtensile faults in the eastern-cen-tral areas (CASSINIS, 1988; CASSINIS & PEROTTI, 1994,1997), which cutting basement and volcanic-siliciclasticdeposits, the latter dated between 283±1 and 280.5±2 Ma(SCHALTEGGER & BRACK, 1999).

Quartz dykes often associated with uranium ores cutthe Lower Permian successions in the Maritime Alps(MITTEMPERGER, 1958).

A Permian event is generally recognised; DEROIN &BONIN (this volume) point out a thermal event between theLower and the Upper Permian, associated with importanthydrothermal activity and metallogeny throughout Europe.

DATING

Radiometric K/Ar analyses (COZZUPOLI et alii, 1971)on the dacite domes (Bruncu Cintoni, Mt. Marigosu, Mt.Tradalei and Bruncu Scusorgiu), the diorite and rhyoliteignimbrite (Mt. Perdedu-Mt. Alastria) yielded values rang-ing between 250 and 265 Ma. EDEL et alii (1981) alsoobtained ages of 259±7 Ma for the ignimbrites and 261±8Ma for the ignimbritic tuffs in the Seui Basin. These datesare in contrast to the «Stephano-Autunian» age suggestedby paleontological and stratigraphic evidence of the Seuiand other Sardinian basins (such as San Giorgio, Guar-dia Pisano, Escalaplano, Mulargia, Perdasdefogu, CalaViola-Punta Lu Caparoni: PECORINI, 1962, 1974; BARCA etalii, 1992, 1995; AA.VV., 2000; BARCA & COSTAMAGNA,2001), as well as the Early Permian age of the calc-alka-line magmatism in Sardinia (COZZUPOLI et alii, 1971,1984; LOMBARDI et alii, 1974; EDEL et alii, 1981; DEL

MORO et alii, 1996; etc.) and across Europe.Secondary alteration during early diagenetic phases

and/or induced by the Mid-Permian thermal anomaly,can be assumed to have rejuvenated the radiometric ages.The hypothesis of early diagenetic alteration, developedunder at least a temporarily subaqueous environment, issupported by more pervasive alteration in the volcanicrocks compared with the diorites. On the other hand, evi-dence for later alteration arises from the comparison withPermian-Triassic alkaline dykes occurring to the north ofthe basin which almost lack secondary processes.

IGNEOUS PETROGENESIS WITHINTHE PERMIAN EUROPEAN FRAMEWORK

Over large sectors of Paleoeurope the Permian andCarboniferous volcanic activity is represented by alternat-ing andesites-dacites-rhyolites that show a common calc-alkaline affinity, and frequently with normal to high K.

A crustal anatectic provenance is likely for most rhyo-lites, whereas a prevalent mantle contribution can beenvisaged for basic-intermediate compositions; mixingprocesses were proposed for the origin of the dacite com-positions (MACERA et alii, 1994; ROTTURA et alii, 1998;CORTESOGNO et alii, 1998).

The genesis of andesite magmas within a non-subduc-tion-related geodynamic environment is problematic; amongthe hypotheses, partial melting of a mantle modified by sub-duction during pre- to early Variscan events was proposed(ROMER et alii, 2001). Although the transpressive (late-) post-

Variscan tectonics frequently overprint the collapse of Varis-cide structures, the model hardly accounts for homogeneousfeatures of the igneous activity throughout the belts extend-ing at least from the Maghrebides to Central and EasternEurope (DEROIN & BONIN, this volume). Therefore, petrog-enetic models should involve melts generated in the uppermantle and in the lower and intermediate continental crust;a striking regional example is given by the partial meltingduring Permian-Carboniferous times in adjacent mantle andlower continental crust in the Ivrea-Verbano Zone.

The genesis of the Permian-Carboniferous volcanismin Sardinia, Briançonnais and the Southern Alps wasassociated with the post-orogenic tectonic setting repre-sented by the collapse of the orogenic belt and with trans-tensile tectonics throughout southern Europe. This set-ting could favour the upwelling of hot asthenosphere andpartial melting in the mantle and at different crustal lev-els. The localised occurrence of melts with tholeiitic fea-tures may account for the ascent of mantle-derived melts(Western and Southern Alps: BRAGA et alii, 2001; ROT-TURA et alii, 1998; and Permian-Carboniferous Briançon-nais Zone: CORTESOGNO et alii, 1988).

The ascent of liquids from the mantle to the middlecrust was favoured by the progressive evolution towardsextensional-transtensile tectonics. The induced anatecticprocesses within the intermediate crust, and the emplace-ment of granitoid intrusions (Sardinia: POLI et alii, 1989;and Liguria: CORTESOGNO et alii, 1998) allowed the genesisof magmas with hybrid features whose eruption was favou-red by the tectonic regime (CORTESOGNO et alii, 1998).

The hypothesis of a hybrid nature for the Early Perm-ian magmatism due to complex interactions betweenmantle-derived and crustal melts is shared by mostauthors; two-stage models were proposed, includingassimilation processes, fractionation and mixing in mag-matic chambers deep-seated in the crust, followed by


G1

P1

P2

S5

S8

S13

S18 S19

SG48, dacite

P1

P2

P3

P4

S1

S1 S1

S1 S1

P1

P2

P4

S10

S13 S14

S19 S20

SG51, diorite

A

B

P1

P2

P3

S5

S8 S10

S12 S13 S14

S18 S19 S20

5-10%

10-20%

2-5%

0-2%

20-40%

> 40%

SG47, dacite

SG50, dacite

Fig. 12 - Frequence of zircon types in the A-T index diagram (PUPIN,1980); A) Diorites, B) Dacites.– Fraquenza tipologica degli zirconi nel diagramma indice A-indice T(PUPIN, 1980); A) Dioriti, B) Daciti.

fractionation, mixing and mingling at higher crustal lev-els (STILLE & BULETTI, 1987; VOSHAGE et alii, 1990; BO-RIANI et alii, 1992; BARTH et alii, 1993; PINARELLI et alii,1993; MACERA et alii, 1994; SINIGOI et alii, 1995; VISONÀ,1995; ROTTURA et alii, 1998). In this framework, the con-ditions for extensive crustal contamination of liquidsderived from enriched lithospheric or asthenosphericmantle sources are produced, and the general calc-alka-line affinity of the resulting Permian magmatism is con-sistent with the occurrence of melts showing strong sim-ilarities with subduction-related liquids, coeval with meltsof strictly anatectic origin.

On the whole, the stratigraphy, composition andpetrology of the igneous rocks in Seui match the hypoth-esis of a complex polystage origin.

TECTONIC AND MAGMATIC EVENTS DURINGTHE BASIN DEVELOPMENT

The structural and sedimentary framework, and thesequence of magmatic events in the Seui Basin and sur-rounds, are comparable to those of most southern Euro-pean Permian-Carboniferous basins (e.g. CORTESOGNO etalii, 1998; CASSINIS et alii, 2000; LAGO et alii, 2001;BREITKREUZ et alii, 2001; DEROIN et alii, 2001).

As mentioned, the abundant floras from the terrige-nous strata allow its dating to the latest Carboniferous-Early Permian (mainly Autunian) interval. Rapid evolu-tion of the basin could be suggested by comparison withother basins; for example, in the circum-Mediterraneanregion, the Collio Basin attained a much greater verticaland areal extent through similar tectono-magmatic eventsin about 5 Ma (SCHALTEGGER & BRACK, 1999).

The following sequence of events is suggested by thedata.

– A basal clastic wedge («Conglomerato Basale») wasunconformably deposited on a discontinuous morpholog-ical surface of the Variscan metamorphic basement.Above this unit, thin pyroclastic levels, and conglomer-ates containing acidic volcanic clasts, suggest early occur-rences of distal ignimbrite flows into the basin.

– Early extensional phases in the southern part of thebasin are indicated by i) the deposition in an embryonicdepression, probably not exceeding 1 km2, of sands, claysand localised channelised conglomeratic intercalationsand coal beds; ii) emplacement of small andesite plugsaligned SW-NE, feeding thin lava flows.

– Terrigenous deposition in alluvial-lacustrine andlocally marsh environments, associated with huge andes-ite flows preserving a general SW-NE alignment, per-sisted in the subsiding basin. The asymmetric geometryof major andesite bodies mirrored the tectonic control onbasin development. To the SE side, the lavas filling tec-tonic troughs attained thicknesses of up to 100 m, andflowed towards the NW in units attaining up to some tensof metres thick. The stratigraphic relationships betweenterrigenous sediments and lavas through time suggest aprogressive temporal propagation of the extensional tec-tonics towards the NE. Possibly at the same period, tec-tonic highs with an E-W alignment limited the basin tothe north, while highs with a N-S alignment limited thebasin to the west, and were associated with the emplace-ment of magma bodies that intruded the basement up tothe sedimentary cover.

The distal products of a conspicuous ignimbrite flowfrom the NW were deposited in the NE sectors of thebasin above andesites and terrigenous sediments, whichwere at least temporarily exposed.

The Seui-Seulo (trans)tensional basins rotated from aNE-SW to an approximately ESE-WNW trend, empha-sised by subvertical NW-SE trending faults in the SaCanna-Genna Aussa area.

A diorite intrusion marked the northern boundary ofthe basin. The intrusion has an E-W alignment and showsprogressive fractionation from diorite to quartz-diorite.The main diorite body intruded both the basement and theoverlying terrigenous cover, inducing localised thermalmetamorphism. A structural high induced by the emplace-ment of cryptodomes within the basement, limited theSeui Basin to the west, which was thus separated from theSeulo Basin. The progressive growth of the domes fed byprogressively more acidic magma batches, developed acomplex morphology characterised by mounds and trig-gered the gravitational collapse of basement slices.

TECTONIC EVOLUTION OF THE BASIN

The evolutionary model of the «Stephano-Autunian»basins shows a common, polyphase history, characterisedby rotation of the main horizontal compressional stresstrajectories (ZIEGLER & STAMPFLI, 2001). In the investi-gated area, the tectonic framework can be depicted as fol-lows. At the eastern margin of the Seui Basin, the paralleltrend of andesite troughs and of some major faults (Tra-dalei fault, northern and southern Don Mestia faults; seemap) indicates a NE-SW direction for the early openingof the basin.

To the north, the basin was bounded from the earli-est phases by a structural high rising up from the base-ment and defined by an extensional tectonic lineament.Along this long-lasting lineament repeated, progressivelymore acidic magma batches erupted.

During relatively later stages, the basin records thedeposition of the distal products of a significant mag-matic activity whose vent can be located some km to theNW at Mt. Perdedu (COZZUPOLI & LOMBARDI, 1969). Inthis volcanic sequence the intercalation of hyaloclastitesand ignimbrites with lavas and tuffs suggests that bothsubaerial and subaqueous eruptions occurred. Therefore,structural lows, at least temporarily water-filled, alsooccurred in this area.

ANALYTICAL METHODS

Whole rock major and trace element abundances forandesites, diorites and dacites were carried out by XRFtechniques at the X-RAL Laboratories Canada. Losses onignition (LOI) were determined by the gravimetricmethod. RE elements were analysed by ICP-MS at theX-RAL Laboratories, Canada.

Quantitative electron microprobe analyses of mineralphases were acquired by a SEM-EDS microprobe instal-led at the Dipartimento per lo Studio del Territorio e dellesue Risorse, Università di Genova, equipped with andX-ray dispersive analiser (EDAX PV 9100). Operative con-ditions were 15 kV accelerating voltage and 2.20 nA beamcurrent. Natural standards were used. Na2O and MgO


contents analysed in silicates by means of an EDAXmicroprobe are generally underestimated if the analysis isprocessed with current automatic methods. To overcomethis problem, the background for nA (1.040 keV) and Mg(1.252 keV) was manually corrected and consideredbetween 0.9 and 4.2 keV.

Orthopyroxene and clinopyroxene analyses were cal-culated according to the stoichiometric method of simul-taneous normalization to 4.00 cations and 6.00 oxygens,and Fe3+=12-total cation charge was considered for cli-nopyroxene. The allocation of cations to sites T, M1 andM2 was performed according to MORIMOTO et alii (1988).

End members were calculated in the sequence: wol-lastonite, enstatite, ferrosilite, aegirine, jadeite, CaAl2SiO6,CaFeAlSiO6, CaCrAlSiO6, CaTiAl2O6. MORIMOTO et alii(1988) and ROCK (1990) nomenclature was adopted. TheCa-amphibole cation sum was normalised to 13-(Ca-Na+K), as suggested by LAIRD & ALBEE (1981); Fe3+=46-total cation charge and Fe2+=Fetot – Fe3+ ; AlVI=8-Si; AlIV=Altot-AlVI. LEAKE (1978) and ROCK & LEAKE (1984)nomenclature was adopted.

Plagioclase analyses, on the basis of eight oxygens,were recalculated to total cations = 5.

Ilmenites were recast on the basis of three oxygens;magnetite and Ti-magnetites were recast on the basis offour oxygens.

ACKNOWLEDGEMENTS

The authors acknowledge PROGEMISA for making available thedata on geological-mining prospections and for drillings carried outfrom 1983-87. Special thanks are due for the preparation of the geo-logical map issued with this article. The authors are also indebted toSebastiano Barca and Bernard Bonin for the careful review of thedraft. This work was carried out thanks to CNR and MURST (Cofin.1998) grants to G. Cassinis, and to CNR funds to L. Cortesogno(CNRC005D7B_005; G. Gosso Co-ordinator).

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Manoscritto pervenuto il 15 aprile 2002; testo approvato per la stampa il 30 Maggio 2002; ultime bozze restituite il 11 Marzo 2003.

reconstruction of igneous, tectonic and sedimentary events...

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