Reconstruction of igneous, tectonic and sedimentary events in the

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 events
in 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 (****)
ABSTRACT
Based on a detailed geological mapping, drilling data and structural-petrological interpretations, the processes controlling the evolution of the latest Carboniferous-Early Permian Seui Basin (Sardinia) are reconstructed and interpreted in the post-Variscan
geodynamic context. The basin represents a good exposure where
the tectonic, sedimentary and igneous processes are recorded. The
evolutionary picture is analogous to that occurring in many intramontane basins developed as a consequence of the transtensional
and transpressional wrench tectonics active from the Maghrebides
to a large part of Europe.
The opening of the basin structure was associated with andesitic volcanism interlayered with detrital sedimentation in a fluviolacustrine to marsh environment. Abundant flora and repeated flows
of rhyolitic pyroclastics from distal sources, probably to the NW, are
recorded within the sedimentary succession. The basin was bounded
to the N and to the W by morphological highs due to concomitant
bowing of the metamorphic basement and intrusion of diorite dykes
feeding dacite cryptodomes. The growth of the cryptodomes triggered the gravitational collapse of basement and cover slices. Contact metamorphism and hydrothermal processes along fractures also
affected both basement and sediments.
The andesites and the diorite-dacite suite, as well as the
overlying rhyolitic ignimbrites have a common calc-alkaline signature, but different geochemical trends. Hybridisation of the magmas
due 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 characteristics.
KEY WORDS: wrench tectonics, late orogenic magmatism,
intracontinental basin, latest Carboniferous-Early
Permian.
RIASSUNTO
Ricostruzione degli eventi magmatici, tettonici, sedimentari e modello evolutivo del Bacino tardo carbonifero-permiano
inferiore di Seui (Sardegna, Italia).
Gli studi condotti sul Bacino di Seui hanno portato ad una ricostruzione delle condizioni geodinamiche che hanno interessato, tra il
tardo Carbonifero ed il Permiano inferiore, questo settore centroorientale della Sardegna. Il bacino presenta un quadro relativamente
chiaro dei processi tettonici, sedimentari e magmatici. Il quadro
evolutivo è analogo a quello che si evince in altri bacini intramontani
(*) Dipartimento di Scienze della Terra dell’Università di
Pavia, via Ferrata 1, 27100 Pavia.
(**) Dipartimento per lo Studio del Territorio e delle sue
Risorse dell’Università di Genova, C.so Europa 26, 16132 Genova.
(***) PROGEMISA, Via Contivecchi 7, 09122 Cagliari.
(****) Progetto CARG-Sardegna, Via Dolcetta 19, 09122 Cagliari.
generati da una tettonica transtensile-transpressionale attiva dalle
Magrebidi fino a settori est-europei. L’apertura di un semi-graben iniziale è associata a vulcanismo andesitico intercalato a sedimentazione detritica in ambiente fluvio-lacustre e palustre. Nella successione
sedimentaria sono inoltre localmente presenti, soprattutto nella porzione basale, ripetuti episodi piroclastici distali a composizione riolitica derivanti da centri di emissione probabilmente localizzati a NO.
Il bacino era limitato a N e ad O da alti morfologici prodotti dal contemporaneo inarcamento del basamento metamorfico e dall’intrusione di filoni dioritici che alimentano criptodomi dacitici. L’accrescimento dei criptodomi ha causato il collasso gravitativo di scaglie di
basamento e delle associate coperture. Fenomeni di metamorfismo di
contatto e processi idrotermali lungo fratture si sono sviluppati nel
basamento e nei sedimenti.
Le andesiti e la sequenza dioritica-dacitica, nonché le sovrastanti ignimbriti riolitiche a Seui hanno una comune impronta calcalcalina, ma diversi trend geochimici. La natura ibrida dei magmi,
dovuta ad una interazione complessa di magmi di derivazione mantellica e crostale attraverso diversi episodi polifasici di assimilazione, frazionamento e mixing, verosimilmente rende conto delle loro
caratteristiche geochimiche e petrografiche.
TERMINI CHIAVE: tettonica transtensile, magmatismo tardo
orogenico, bacini intracontinentali, Carbonifero terminale, Permiano inferiore.
INTRODUCTION
All over Europe, a predominantly wrench-induced
collapse followed the collisional phases and crustal thickening of the Variscan orogeny during Late Carboniferous
and Early Permian times. The transtensional and transpressional wrench tectonics, associated with predominantly calc-alkaline magmatic vents, gave rise to the
development of intracontinental basins (ARTHAUD &
MATTE, 1977; BONIN et alii, 1993; HENK, 1997; CORTESOGNO et alii, 1998; ROTTURA et alii, 1998; ZIEGLER &
STAMPFLI, 2001).
The basins were infilled by short range sedimentation
and by locally predominant intra- and extrabasinal volcanic 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 the
intersecting tectonic, sedimentary and magmatic events.
The Seui and Seulo late Paleozoic sedimentary outcrops
were investigated from the perspective of anthracite
exploitation (LAURO, 1970; ACCARDO et alii, 1984; SARRIA,
1987; SARRIA & SERRI, 2000). In the present study, an
100
G. CASSINIS ET ALII
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NUORO
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BAG
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(LATE) POST-VARISCAN
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Mesozoic and
Cenozoic covers
Permian volcanic
rocks
Permian and Lower Triassic
continental deposits
G
ER
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SARDINIAN VARISCAN
BASEMENT
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ES
Intrusive complex
(Upper Carbonif.-Permian)
IE N T E
CAGLIARI
High to low grade
metamorphic complex
(?Precambr.-Lower Carbonif.)
S UL
C
IS
Main faults
39¡
0
50
Km
evolutionary model for the basin is inferred after reappraisal of the structural, stratigraphical and petrological
data based on 1:5,000 scale mapping. Furthermore, the
related PROGEMISA drillings gave considerable insights on
the subsurface geometry of the basin, thus supporting the
interpretation of field data.
STRATIGRAPHIC OUTLINE
Even though a complete stratigraphical log of the
Seui Basin is generally inhibited by the lack of clear key
beds and by the overwhelming volcano-tectonic effects,
its succession is supported by detailed field mapping and
ENSAE and PROGEMISA drilling data (LAURO et alii, 1963;
LAURO, 1970; SARRIA, 1987). The inferred lithological column is made up (base to top) as follows.
The metamorphic basement («Postgotlandiano» Auct.;
«Filladi Grigie del Gennargentu»: CAVINATO, 1976) is formed by a complex including terrigenous successions (VAI
& COCOZZA, 1974) whose ages range between Cambrian
and Lower Carboniferous (PILI & SABA, 1975; DESSAU et
alii, 1982).
Specifically, the basement of the Seui area is ascribed
to the Barbagia tectonic unit in the Internal Zone of the
Fig. 1 - Regional and geological
sketch of Sardinia with location of
the Seui Permian Basin. Box area
enlarged in fig. 2.
– Schema geologico e regionale della Sardegna, con localizzazione del
bacino permiano di Seui. L’area riquadrata è ingrandita in fig. 2.
Variscan nappes (CARMIGNANI et alii, 1992). It tectonically overlies the Meana Sardo Unit with an estimated
thickness exceeding 1000 m. The unit is formed of alternating layers of quartz-micaceous metasandstones,
quartzites, quartzitic phyllites and phyllites, from metres
to tens of metres thick (LAURO et alii, 1963).
The foliated texture is often anequigranular for relict
quartz and detrital micas. The neoblastic micas are very
finegrained giving a lepidoblastic texture. The main foliation developed under a low-metamorphic grade, with a
muscovite, chlorite and albite assemblage (FERRARA et
alii, 1978; FRANCESCHELLI et alii, 1982; CAROSI et alii,
1991; FADDA et alii, 1991).
The inferred Permian sequence (fig. 3) begins with reddish coarse-to-finegrained breccia/conglomerate («Basal
Conglomerate» corresponding to the «complesso inferiore
o 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 increasing thickness (up to 100 m) towards the centre of the
basin (SARRIA & SERRI, 1986; SARRIA, 1987), and unconformably overlies the Variscan basement (Barbagia Unit,
Internal Nappes; CARMIGNANI et alii, 1992). The breccia/conglomerate is clast-supported; the lithic fragments
are mainly angular and heterometric (up to 20 cm), with
M. Perdaliana
M. Perdedu
SEULO
BA
RB
A GI
M. Arbo
SEUI
A
SEULO
USSASSAI
0
2
Km
Variscan
metamorphic units
Permian alluvial-lacustrine
deposits
Permian igneous
rocks
Jurassic dolostones and
clastics
Main faults
Fig. 2 - Simplified geological map of the Seui Basin and surrounding
areas.
– Carta geologica semplificata del bacino di Seui e delle aree adiacenti.
predominant quartz- and micaceous-schists reworked
from the underlying metamorphic substrate.
The «Conglomerate» is poorly organised; however, a
fining-upward trend occurs as well as crude bedding
towards the top. Coarse- to medium-grained sandstone
beds and rarer blackish pelitic lenses are interfingered in
the conglomerates. Pollen analysis revealed only poorly
preserved and carbonised remains (PITTAU, pers. comm.).
On the whole, the deposit mirrors an alluvial fan or footslope environment.
To the top of this basal sedimentary unit, thin rhyolite pyroclastics, interbedded and in part reworked in the
overlying conglomerates, support the existence of early
extrabasinal igneous acidic activity.
The bulk of the deposition in the basin is represented
by alluvial-to-lacustrine, fine- to coarse-grained terrigenous sediments over 300 m thick, with intercalated intermediate lavas. Different units were defined in the past, on
the basis of geometric relationships with the volcanics:
«complesso antracitifero intermedio» («Antracitifero
s.s.») and «complesso superiore» (LAURO et alii, 1963);
«sequenza clastica intermedia», «sequenza clastica superiore di letto», and «sequenza clastica superiore di tetto»
(ACCARDO et alii, 1984).
The sedimentary succession shows a discontinuous
coarsening-upward trend. Irregular alternations of dark
grey to black pelites with subordinate medium- to finegrained, well-bedded sandstones occur at the bottom, and
grade to coarse sandstones and conglomerates towards
the top. The resulting coarsening upward trend reflects
the change from a prevalent lacustrine-marsh environment to a higher-energy fluvial setting characterised by
coarser stream deposits.
EFFUSIVE PRODUCTS
M. Alastria
Rhyolitic ignimbrites
Andesites
Acidic pyroclastics
Volcaniclastic conglomerates
SEDIMENTARY FACIES
NARGENTU
Flu m endosa
UPPERMOST CARBONIFEROUS - LOWER PERMIAN p.p.
GEN
101
Riv
er
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
Coarse alluvial facies
Medium-to-fine
alluvial facies
Lacustrine facies and
anthracite layers
"Basal Conglomerate"
Metamorphic basement
Macrofloras
Wood stems
20
10 m
Algal oncoids
Nonconformity
0
Fig. 3 - Representative section of the volcanic and sedimentary
deposits in the central sector of Seui Basin.
– Sezione rappresentativa dei depositi vulcanici e sedimentari nel
settore centrale del bacino di Seui.
Layers with coarse-grained basement clasts and/or
quartz-rich conglomerates (Plate 1d) in the medium to
upper part of the succession suggest repeated changes in
the deposition. It is not clear whether the transition from
the fine to the coarse deposition is gradual, or abrupt and
controlled by tectonics.
Abundant macrofloral remains occur in the finegrained
sediments (fig. 3; Plate 1b and 1c) throughout the Seui Basin
as well in the adjacent Seulo outcrops. Since the first finds
of LAMARMORA (1857), examined by MENEGHINI (1857), a
large number of fossil plants have been collected from these
deposits. In particular, ARCANGELI (1901) (see also the
review of COMASCHI CARIA, 1959) discovered and determined 51 species, mostly attributed to the Late Carboniferous-Early Permian. From a recent collection undertaken by
the authors (BROUTIN et alii, 2000), BROUTIN recognised the
presence of the following forms: Annularia sphenophylloides,
G. CASSINIS ET ALII
Plate 1
102
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
cf. Pecopteris arborescens, ?P. cyathea, Pecopteris sp., Pecopteris sp. aff. hemitelioides, P. unita, Cordaites sp., Sigillaria
brardii, and Artisia sp. (Plate 1b and 1c), and generally ascribed the association to an interval spanning from latest Carboniferous to basal Early Permian («Autunian»).
The northern and western sections of the basin are
bounded by a structural high largely represented by dacite
domes (see map; Plate 1f and 1g). The emplacement of
these domes within the basement was associated with the
superposition of basement slices above the sedimentary
sequence (Plate 1a). This feature was related to «brittle
extensional tectonics» (LAURO, 1970). The lack of kinematic indicators at the base of the slices, and the evident
correlation with dome geometry, allow us to infer an origin from gravitational collapse induced by dome growth.
THE IGNEOUS ROCKS: FIELD OCCURRENCE,
PETROGRAPHY AND MINERAL CHEMISTRY
Andesites
In the SE area andesites («Porphyrites» Auct.) occur
as plugs cutting the basement and the breccias, and as
thin layers of medium- to fine grained volcanic breccias
within the sedimentary sequence (fig. 3 and map). The
plugs have broadly elliptical sections with SW-NE elongation. The texture is porphyric seriate, often glomeroporphyric with an intersertal to felsitic groundmass. The
phenocrysts are plagioclase, biotite, hornblende and rare
pyroxene, with accessory zircon and apatite.
Xenocrysts and xenoliths are relatively frequent:
quartzite xenoliths (up to 1 cm) show lobate boundaries
and sometimes reaction rims of acicular pyroxene. Garnet
xenocrysts, sometimes in clusters of small grains, occur
rarely. Both the quartzite and the garnet probably represent the refractory products of the assimilation of metamorphic crust during the early stages of liquid ascent.
Embayed grains of volcanic quartz, commonly polycrystalline due to deformation and growth, likely indicate mixing
with acidic magmas. Compared with metamorphic quartz
xenoliths, the lack of pyroxene reaction rims suggests that
they were involved later in the ascending magma.
Hydrothermal veining of quartz, chalcedony and carbonates is diffuse inside the plugs.
Two main outcrops of intermediate effusives occur at
the centre of the paleobasin (fig. 3).
Along the eastern margin, controlled by SW-NE tectonic lineaments, the lava flow attains its maximum
thickness (> 60 m) and is characterised by evident columnar joints. The joints are curved with an E-verging dip
103
progressively decreasing from bottom to the top. As the
columnar joints propagate away from the cooling surface,
their orientation identifies the original location of the
ridge margin.
To the NW, andesites are represented by prevalent
volcanic breccias with intercalated lavas. The total thickness can locally attain 140 metres in the easternmost
zones and tends to decrease to the W, where it ranges
between 0 and 30 metres.
Very finegrained quartz and chalcedony occur as cmthick beds or as breccia matrix, sometimes with silicified
lava fragments. They possibly originated by silica precipitation from hydrothermal vents.
The wedge-shaped geometry of the lavas supports the
asymmetry of the basin. The lava ascent occurred along
fractures at the eastern margin; magmas then flowed to
the NW in the subsiding half-graben tectonic trough.
In the NE sector, andesites mostly occur as thin breccia layers. A submetric layer occurring near the bottom of
the terrigenous cover includes an originally glassy
groundmass, with rare plagioclase phenocrysts as the volcanic component and clasts of quartz and metamorphic
rocks as the sedimentary component. The clasts are
comparable to those in the basal breccia. The vertical
distribution of clasts within the layer shows a strongly
heterogeneous zoning. Very finegrained devitrified matter
constitutes the bottom of the layer for 5-10 cm. Above,
the clasts appear in the glassy groundmass and increase
up to more than 80% of the volume, resulting in a grainsupported texture. The clasts decrease sharply upwards to
disappear for some tens of centimetres at the top (Plate
1e). An emplacement mechanism like that described for
the origin of «peperite» can be assumed: a magma rising
along a fracture in the underlying basement stalls at the
contact with unconsolidated sediments and induces boiling of the interstitial water. The process gives rise to a
complex mixture of magma and sediments with a soft
plastic or viscous fluid behaviour, which spreads laterally
in the sediment at shallow levels (BATIZA & WHITE, 2000).
The mechanism shows that the magma emplaced into
unconsolidated sediments. Thin beds of black silicified
hyaloclastites are also related to localised explosive intrabasinal volcanic activity.
Andesite lavas show a porphyric seriate texture (P.I. ≈
20) with zoned plagioclase and pyroxene phenocrysts; the
groundmass varies from holocrystalline intersertal, sometimes fluidal, to felsitic. Medium-grained ortholiths are
diffuse, as well as quartzite xenoliths, showing acicular
pyroxene 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 basement 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) Artisia
sp. (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 rare
plagioclase 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 above
a 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 basamento
alloctono; BC: Conglomerato Basale; Sed: depositi alluvio-lacustri; And: Lave andesitiche e brecce; PQ: Domi riolitici e dacitici; Ign: Ignimbriti
riolitiche. 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 struttura 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.
Tab. 1a
a) Representative EMP analyses of pyroxenes and plagioclases from diorites; b) representative EMP analyses of the contact metamorphism assemblage.
a) Microanalisi rappresentative di pirosseni e plagioclasi da dioriti; b) microanalisi rappresentative della paragenesi metamorfica di contatto.
TABLE 1
104
G. CASSINIS ET ALII
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
105
Tab. 1b
Contact metamorphism assemblage
Hercynite
Corundum
SiO2
TiO2
Cr2O3
Al2O3
Fe2O3
FeO
MnO
MgO
NiO
CaO
Na2O
K 2O
0,29
0,19
0,08
62,91
0,00
0,00
0,00
7,50
0,00
0,09
0,00
0,16
SiO2
TiO2
Cr2O3
Al2O3
Fe2O3
FeO
MnO
MgO
NiO
CaO
Na2O
K 2O
Total
71,22
Total
100,00
Si
Ti
Cr
Al
Fe3+
Fe2+
Mn
Mg
Ni
Ca
Na
K
0,010
0,005
0,002
2,583
0,000
0,000
0,000
0,389
0,000
0,003
0,000
0,007
Si
Ti
Cr
Al
Fe3+
Fe2+
Mn
Mg
Ni
Ca
Na
K
0,003
0,002
0,002
1,985
0,000
0,006
0,000
0,000
0,000
0,002
0,000
0,003
Cordierite
0,15
0,19
0,11
98,87
0,00
0,42
0,01
0,00
0,02
0,11
0,00
0,12
SiO2
TiO2
Cr2O3
Al2O3
Fe2O3
FeO
MnO
MgO
NiO
CaO
Na2O
K 2O
47,15
0,27
0,10
33,03
0,00
6,93
0,16
8,99
0,00
0,15
0,00
0,22
SiO2
TiO2
Cr2O3
Al2O3
Fe2O3
FeO
MnO
MgO
NiO
CaO
Na2O
K 2O
H2O
34,90
4,92
0,15
16,87
3,13
10,54
0,02
12,53
0,00
0,14
0,00
10,09
3,95
Total
97,00
Total
97,23
AlIV
4,910
1,090
Si
Ti
Cr
Al
Fe3+
Fe2+
Mn
Mg
Ni
Ca
Na
K
OH
2,651
0,281
0,009
1,510
0,179
0,669
0,001
1,419
0,000
0,011
0,000
0,978
2,000
Si
Al
Ti
Fe3+
Cr
4,054
2,964
0,021
0,000
0,008
Fe2+
Mn
Mg
Ni
Ca
Na
K
0,604
0,014
1,395
0,000
0,017
0,000
0,029
AlVI
The widespread alteration of mineral phases and of
the glassy groundmass could depend on autohydrothermalism under at least temporarily subaqueous conditions, 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 between
250 and 500 metres, cuts the basement and the lower terrigenous sediments to the N of the basin. Two minor
dykes outcrop to the south below the domes.
The texture is medium-grained hypidiomorphic, with
diffuse, finegrained, aplite-like, sometimes granophyric
interstitial patches due to the concentration of residual
eutectic liquids. Porphyritic textures develop towards the
contact with the host rock, mostly in the minor dykes,
where they represent transitional features to the dacite.
Compositions range from dioritic to quartz- and monzodioritic; the most evolved compositions occur at the top
of the major dyke and in the minor dykes. In the more
primitive rocks, large euhedral plagioclases (An45-67) show
complex zoning (tab. 1b; fig. 4a); the cores, often surrounded by sieve textures, have an anorthite content up
to 83-87%, but exceptionally up to 94%. The rims, as well
as the granophyric and interstitial grains, have An contents in the range 18-23%, with Or1-5.
High Ti Biotite
6,000
2,993
2,013
0,046
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 (0.0-2.11
wt%). Orthopyroxene (En59-60Fs34-36Wo2-3; fig. 4a; tab. 1a) is
diffuse as euhedral to subhedral grains, but is generally altered to chlorite and only preserved as small grains within plagioclase. Late magmatic brown hornblende (Mg# 0.71-0.90,
AlIV = 0.98-1.18 atoms per formula unit (a.p.f.u.), AlVI = 0.00.025 a.p.f.u., Ti = 0.10-0.24 a.p.f.u.) widely replaces the clinopyroxene or is developed as a poikilitic to interstitial
phase. The polyphase growth of igneous brown hornblende
suggested by textural evidence is also consistent with polybaric crystallisation constrained by barometric estimates
based 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, and
0.1-0.06 GPa for interstitial grains (tab. 2), are consistent
with a progressive ascent to the surface. Apatite and zircon
are diffuse and more abundant in quartz-diorites.
Biotite (Mg#: 0.52-0.77, AlIV: 0.963-1.17, AlVI: 0.00.06, Ti: 0.27-0.33 a.p.f.u.; fig. 4b) generally occurs as
small interstitial grains in diorites and as large euhedral
grains in quartz- and monzodiorites, where granophyric
or interstitial K-feldspar (Or96-98) also occurs. In the
106
G. CASSINIS ET ALII
Diopsid
5
4
✕
✕
✕✕
Augite
Augite
✕
E
2
SG5
Pigeonit
Diopsid
F
5
4
✕
✕
✕
✕
✕
✕ ✕✕
SG6
Pigeonit
Enstatit
F
5
4
Diopsid
Augite
E
SG50
SG5
5
E
✕
F O3.Ti 2
2
✕
✕
✕
❍ ❍
Or
Ab
Ab
Or
❍
An
Ab
Or
❍
❍
SG53
Ab
Or
❍
❍
Ab
TiO2
TiO2
FeO. Ti 2
FeO. Ti 2
F O3.Ti 2
2
An
❍
❍❍
❍❍
❍
An
✕
✕
2FeOTi 2
2FeOTi 2
Ab
Or
SG55 An
F
FeO. Ti 2
Ab
Or
❍
❍
2
TiO2
FeO. Ti 2
❍
❍
An
Ab
Or
❍
❍
SG54
5
Enstatit ✕
TiO2
✕
✕
✕
CP8
Pigeonit
5
F
FeOTi 2
2
Augite
Enstatit
❍
❍
An
F
5
4
✕
✕
2
❍
❍
❍
An
5
Diopsid
✕✕✕
Pigeonit
P13A
Enstatit
E
❍ ❍
❍❍
SG52
Pigeonit
5
Ab
Or
An
Augite
2
❍
P13A
2
F
5
4
Diopsid
Augite
E
SG5
Enstatit
E
Or
CP85
An
✕✕
Pigeonit
5
Enstatit ✕✕
5
4
Diopsid
F O3.Ti 2
2
✕
✕
2FeOTi 2
SG50
2FeOTi
SG54 2
✕✕
✕
✕✕
✕
F O3.Ti 2
2
CP95
SG53
✕✕
✕
Fe
FeOF O
2
✕
✕
✕
F O3
2
MgO
3
● SG60
❍ SG50
■ CP91 contact
metamorphism
▲ CP97 contact
Calc-alkaline
❍
●
Alkaline
■
■
■
Peraluminous
▲
FeO
Al
2 O3
Fig. 4 - a) Above. Compositional variability of pyroxene and feldspars in diorites. b) Centre. Coexisting ilmenites and Ti-magnetites in
diorites and in contact metamorphosed metasediment CP95 on the FeO-Fe2O3-TiO2 space. c) Below. Composition of biotite from diorites
and 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 nel
sedimento CP95 metamorfico per contatto nel diagramma FeO-Fe2O3-TiO2. c) Sotto. Composizione delle biotiti in dioriti e nella paragenesi
metamorfica per contatto; pertinenza tettonica secondo ROTTURA et alii, 1998.
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
107
TABLE 2
Results of the Al in igneous amphibole geobarometers
applied to early coarse- and to fine grained interstial
hornblendes.
Valori di pressione riferiti ad anfiboli ignei di cristallizzazione 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.
near-surface, porphyritic portion of the dykes, biotite
phenocrysts break down to pseudomorphic aggregates
of clinopyroxene and opaques. The lack of biotite and
hornblende in the groundmass, where clinopyroxene is
stable, suggests the degassing of the magma.
Subhedral to skeletal large ilmenite grains have high
Fe2O3 contents (up to 20.31wt%) and coexist with low-Ti
magnetite, 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 microfracturing with slight rotation of the grains. Along the cracks,
radiating aggregates of pyroxene developed by reaction
with the liquid. Garnet xenocrysts are almandine (Alm0.680.71 Prp0.18-0.23Sps0.03-0.05Adr0.02-0.04Grs0.0-0.02) with weak
compositional zoning (fig. 5).
Secondary alteration is moderate and consists of i)
chloritisation of orthopyroxene, ii) alteration of pyroxene
and hornblende to fibrous aggregates of actinolitic hornblende or actinolite and chlorite, and iii) alteration of biotite to chlorite + epidote + magnetite + rutile.
Rhyolitic ignimbrite
Rhyolitic pyroclastic rocks, showing evident ignimbrite
features at least locally, crop out as metre-thick levels to
the N of the basin, overlying andesites and terrigenous sediments (fig. 3). They are locally covered by basement slices
collapsed by gravitational sliding (see map).
Their peripheral setting with respect to the dome bodies can be assumed as a primary feature, therefore deriving from the preferential deposition of the ignimbrite
flows in the lowlands surrounding the domes. On the other
hand, the superposition of gravitational slices supports the
progressive and subsequent dome growth. If so, the ignimbrite vent was active during the long-lasting dome building. The ignimbrite shows a poorly welded texture, with
plagioclase, quartz, K-feldspar, and biotite phenoclasts,
with accessory zircon, apatite, ilmenite and magnetite.
The pyroclastic layers represent the distal products of
an ignimbrite flow whose origin can be located some kilo-
metres to the NW, at Mt. Perdedu summit, formed by
some tens of metres of ignimbrites (COZZUPOLI & LOMBARDI, 1969). The ignimbrite overlies i) subvolcanic bodies genetically related to the diorite dykes, and ii) a pyroclastic complex formed by ignimbrite layers, volcanic
breccias, tuffs and hyaloclastites. In particular the frequent occurrence of hyaloclastite and accretionary lapilli
beds in the tuffs suggests that the vent for part of the pyroclastic products was located in a small basin already filled
by volcaniclastic products prior to the ignimbrite vent.
Dacite-rhyolite domes
To the N and W the basin is bounded by acidic cryptodomes. To the W the domes crop out only in part below
the basement along a N-S lineament; to the northern margin, the domes are grouped along three major parallel
E-W alignments (see map). The emplacement occurred at
shallow levels (some tens of metres) within the basement.
Domes display a concentric «onion skin» foliation pattern
produced by multiple magma batches inflating the outer
rind. The grain variation within each batch results in a
light and dark zoning. The pattern of the concentric foliation in the major dome of Bruncu Cintoni shows a
lobate structure resulting from three growth nuclei. The
textural relationships indicate that the central body, compositionally more acidic (Pissu Isili, Bruncu Cintoni), followed 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 broadly
elliptical, with an E-W elongation. In some domes (Senna
Su Monti) more evolved compositions up to rhyolite are
observed towards the centre.
The relationships between cryptodomes and the
underlying diorite dykes suggest feeding from an en-echelon series of dykes, which probably rotated to the south
towards the surface. The progressive ballooning of domes
lifted the overlying basement and sedimentary cover, producing mounds in the palaeosurface, and in the case of
the largest bodies, the gravitational collapse of the basement above the Permian sequence.
Dacites show porphyric, often glomeroporphyric,
structures; phenocrysts comprise plagioclase, quartz, biotite, rare hornblende and apatite, ilmenite and zircon. Plagioclase (An20-25) shows porphyric seriate to microphyric
textures and zoning of the largest grains. Quartz pheno-
108
G. CASSINIS ET ALII
Bruncu Cintoni
Pissu Isili
Is Incrastos
Genna Isili
δ
d
α
Fig. 6 - Bruncu Cintoni: composite three-folded structure of the lobate
α) lavas; left: detrital
dacite dome (δδ). Centre, bottom: andesite (α
covers at Fondu Corongiu (d).
– Bruncu Cintoni: Struttura composita del domo dacitico (δ) a tre lobi.
Al centro in basso: lave andesitiche (α); a sinistra: copertura detritica a
Fondu Corongiu (d).
Fig. 7 - AFM diagram. Symbols. Full circles: andesite plugs; open circles: andesite lavas; squares: andesite sills; full triangles: diorites;
open triangles: dacites.
– Diagramma AFM. Simboli. Cerchi pieni: condotti andesitici; cerchi
aperti: lave andesitiche; quadrati: sill; triangoli pieni: dioriti; triangoli
vuoti: daciti.
crysts, rarely polycrystalline, are embayed and sometimes
rimmed by a granophyric overgrowth. K-feldspar phenocrysts are restricted to rhyolitic compositions. Garnet xenocrysts occur as inclusions in plagioclase phenocrysts. The
groundmass texture varies from poikilomosaic (feldspar
microliths in quartz), or rarely spherulitic, to felsitic.
The groundmass and the mineral grains are widely
affected by secondary alteration preventing bulk rock analysis, except for partially preserved plagioclase phenocrysts.
range from basaltic andesites to andesites of weakly metaluminous (less evolved members) to peraluminous character.
The main dyke has diorite to quartz-diorite compositions
varying from weakly metaluminous to Al-saturated, whereas
the domes consist of dacites and rhyolites showing Al-saturated to peraluminous characteristics (fig. 8A,B,C). The
binary covariances (figs. 9A,B) are consistent with medium
to high-K calc-alkaline features. The comparison of major
and trace elements in andesites and diorites shows significantly different trends in spite of similar SiO2 ranges. Andesites have a higher maximum MgO content than diorites,
with more evident fractionation. Ni and Cr contents are low
with similar behaviour, whereas contrasting trends result for
P2O5 and Sr (tab. 3). On the whole, diorites and dacites, separated by a compositional gap (SiO2 63.43 and 67.17 wt% on
anhydrous basis) (fig. 9A,B; tab. 3), show aligned covariation
GEOCHEMICAL CHARACTERISTICS
OF THE IGNEOUS ROCKS
The igneous activity in the Permian Seui Basin includes
intermediate to acidic members showing a common calcalkaline affinity (fig. 7). The intermediate volcanic rocks
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.
Representative whole rock compositions of the Seui andesites, diorites and dacites.
Composizioni rappresentative di roccia totale di andesiti, dioriti e daciti di Seui.
TABLE 3
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
109
110
G. CASSINIS ET ALII
Fig. 9a
trends for most major elements as well as for Ti and Nb
(decreasing) and for Sr (increasing). In accordance with
field evidence, this suggests a cogenetic origin for the diorites and dacites. As part of this hypothesis, the sharp flexure in K2O corresponding to the SiO2 gap is probably related to K-feldspar and biotite fractionation in quartz- and
monzodiorites, which is also consistent with the flexure for
Ba at SiO2 ≈ 62-63 wt% (fig. 9B). Analogously, the zircon
precipitation observed in more evolved diorites possibly
accounts for the lower abundances in dacites for Zr and Th.
Chondrite normalised REE patterns are characterised
by LREE-enrichment (fig. 10; tab. 3); the negative Eu
anomaly is low to moderate in andesites (Eu/Eu*=0.590.86) and diorites (Eu/Eu*=0.52-0.81) and moderate in dacites (Eu/Eu*=0.38-0.45). Diorites and dacites show very low
HREE concentrations, and small degrees of M- and HREE
fractionation (GdN/YbN= 1.45-1.59 in diorites; 1.44-1.39 in
dacites), comparable to ratios produced by the melting of
garnet-free sources. In andesites, the HREE fractionation
is relatively strong and tends to increase in more evolved
compositions (GdN/YbN=1.71-1.74 and GdN/YbN=1.97-2.15
respectively; tab. 3). The ratios are consistent with values in
liquids from garnet-bearing sources (≥3).
The decreasing ΣREE and increasingly negative Eu
anomaly towards more evolved compositions in diorites
and dacites are consistent with the fractionation of plagioclase, pyroxene and hornblende.
MORB-normalised spidergrams show LILE- and HREEenriched incompatible element patterns (fig. 11). The low
Nb and negative TiO2 and Ba anomalies are common to
most post-Variscan Permian volcanics (CORTESOGNO et alii,
1998; ROTTURA et alii, 1998; LAGO et alii, 2001). Negative
anomalies for Ta, Nb and Ti, but not for Ba, are also common to magmas from subduction-related environments
(BAILEY, 1981; PEARCE, 1983). The low Rb in andesite
(CP32) is probably caused by secondary mobilisation.
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
111
Fig. 9b
Fig. 9 - A) Correlation of major elements vs. SiO2; B) correlation of trace elements vs. SiO2. Symbols. Full circles: andesite plugs; open
circles: 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.
Zircon typology and magmatic affinity
Four samples (two diorites and two dacites) from the
Seui Basin have been processed for zircon extraction. In
diorites (SG 50 and 51) the zircons are light green to
colourless. The largest subpopulation is rounded and generally inclusion-free; however, small apatite needles may
be included. In sample SG50 some crystals show zoning
and crystalline or glass inclusions. The mean A index is
600 and the mean T index is 451 (fig. 12A).
Zircons from dacites (SG47 and 48) are light yellow
to colourless and often include cooling cavities. The mean
A index is 523 and the mean T index is 408 (fig. 12B).
The parameters are consistent with crystallisation
within calc-alkaline series.
CONTACT METAMORPHISM
Localised thermometamorphic processes were triggered by the emplacement of subvolcanic magmas. Along
the contact with the main diorite dyke, thermal recrystallisation affected the basement and the terrigenous cover
over some tens of metres. The metamorphic peak was
recorded by the assemblage K-feldspar + Fe-rich biotite
(fig. 4b; tab. 1b) + Al-Mg and Al-Fe spinels (tab. 1) + corundum (tab. 1) + cordierite (Mg# 0.698; tab. 1) + ilmenite
(fig. 4b), developed in metapelite layers of the basement.
Chlorite + epidote replacing biotite and gibbsite replacing
corundum developed during retrograde phases.
In the Si-poor, Al-rich bulk rock compositions suggested
by corundum and spinel occurrence, the muscovite, biotite
112
G. CASSINIS ET ALII
Fig. 10 - REE patterns (normalized according to NAKAMURA, 1974) for
andesites (squares), diorites (full triangles) and dacites (open triangles).
– Pattern degli elementi delle terre rare (normalizzati secondo NAKAMURA, 1974) per andesiti (quadrati), dioriti (triangoli pieni) e daciti
(triangoli vuoti).
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 are
consistent with temperatures ≥ 600°C and very low pressures
(<0.05 GPa), which are compatible with the nature of the
heating magma and with stratigraphic constraints.
Thermal recrystallisation, affecting the terrigenous
sediments entrapped at the base of the domes over a few
metres, is indicated by localised blastesis of biotite, muscovite and chlorite flakes.
HYDROTHERMAL QUARTZ DYKES
These are represented by microcrystalline quartz
locally associated with calcite-filled fractures.
In places, sulphides (sphalerite, galena, chalcopyrite)
and iron oxides occur with massive, brecciated and cockade textures. Quartz shows abundant fluid inclusions
arranged along growth surfaces.
In the SW sector, the dykes form part of a WNW-ESE
trending fracture swarm, with subvertical dip and 1-2 m
Fig. 11 - Rock/MORB spidergram. Symbols: squares: andesites, full
triangles: diorites; open triangles: dacites.
– Spidergramma roccia/MORB. Simboli. Quadrati: andesiti; triangoli
pieni: dioriti, triangoli vuoti: daciti.
thick dykes, between Genna Lioni and Arcu Spineddai; rare
outcrops in the NE sector have an approximately N-S trend.
The trends are respectively subparallel and at right angles to
the collisional Variscan lineaments. Both trends match the
stress field generated by the gravitational collapse of the belt.
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
In a general excursus, similar quartz dykes, locally
with Cu- and Fe-sulphides, crop out in the Val Trompia
Collio Basin (Brescian Prealps) with NNW-SSE and NWSE trends, infilling transtensile faults in the eastern-central areas (CASSINIS, 1988; CASSINIS & PEROTTI, 1994,
1997), which cutting basement and volcanic-siliciclastic
deposits, the latter dated between 283±1 and 280.5±2 Ma
(SCHALTEGGER & BRACK, 1999).
Quartz dykes often associated with uranium ores cut
the 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 the
Lower and the Upper Permian, associated with important
hydrothermal activity and metallogeny throughout Europe.
SG51, diorite
S13
IGNEOUS PETROGENESIS WITHIN
THE PERMIAN EUROPEAN FRAMEWORK
Over large sectors of Paleoeurope the Permian and
Carboniferous volcanic activity is represented by alternating andesites-dacites-rhyolites that show a common calcalkaline affinity, and frequently with normal to high K.
A crustal anatectic provenance is likely for most rhyolites, whereas a prevalent mantle contribution can be
envisaged for basic-intermediate compositions; mixing
processes were proposed for the origin of the dacite compositions (MACERA et alii, 1994; ROTTURA et alii, 1998;
CORTESOGNO et alii, 1998).
The genesis of andesite magmas within a non-subduction-related geodynamic environment is problematic; among
the hypotheses, partial melting of a mantle modified by subduction during pre- to early Variscan events was proposed
(ROMER et alii, 2001). Although the transpressive (late-) post-
P1
P2
S1
S14
S19
S20
P4
P2
S1
S1
P3
S1
S1
P4
SG48, dacite
SG47, dacite
B
0-2%
G1
2-5%
S8
S5
S5
P1
S10
P2
S8
P3
S13
P1
P2
10-20%
S12
Radiometric K/Ar analyses (COZZUPOLI et alii, 1971)
on the dacite domes (Bruncu Cintoni, Mt. Marigosu, Mt.
Tradalei and Bruncu Scusorgiu), the diorite and rhyolite
ignimbrite (Mt. Perdedu-Mt. Alastria) yielded values ranging between 250 and 265 Ma. EDEL et alii (1981) also
obtained ages of 259±7 Ma for the ignimbrites and 261±8
Ma for the ignimbritic tuffs in the Seui Basin. These dates
are in contrast to the «Stephano-Autunian» age suggested
by paleontological and stratigraphic evidence of the Seui
and other Sardinian basins (such as San Giorgio, Guardia Pisano, Escalaplano, Mulargia, Perdasdefogu, Cala
Viola-Punta Lu Caparoni: PECORINI, 1962, 1974; BARCA et
alii, 1992, 1995; AA.VV., 2000; BARCA & COSTAMAGNA,
2001), as well as the Early Permian age of the calc-alkaline 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, developed
under at least a temporarily subaqueous environment, is
supported by more pervasive alteration in the volcanic
rocks compared with the diorites. On the other hand, evidence for later alteration arises from the comparison with
Permian-Triassic alkaline dykes occurring to the north of
the basin which almost lack secondary processes.
SG50, dacite
P1
S10
5-10%
DATING
A
113
S13
S14
S18
S19
20-40%
S20
S18
S19
> 40%
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.
Variscan tectonics frequently overprint the collapse of Variscide structures, the model hardly accounts for homogeneous
features of the igneous activity throughout the belts extending at least from the Maghrebides to Central and Eastern
Europe (DEROIN & BONIN, this volume). Therefore, petrogenetic models should involve melts generated in the upper
mantle and in the lower and intermediate continental crust;
a striking regional example is given by the partial melting
during Permian-Carboniferous times in adjacent mantle and
lower continental crust in the Ivrea-Verbano Zone.
The genesis of the Permian-Carboniferous volcanism
in Sardinia, Briançonnais and the Southern Alps was
associated with the post-orogenic tectonic setting represented by the collapse of the orogenic belt and with transtensile tectonics throughout southern Europe. This setting could favour the upwelling of hot asthenosphere and
partial melting in the mantle and at different crustal levels. The localised occurrence of melts with tholeiitic features may account for the ascent of mantle-derived melts
(Western and Southern Alps: BRAGA et alii, 2001; ROTTURA et alii, 1998; and Permian-Carboniferous Briançonnais Zone: CORTESOGNO et alii, 1988).
The ascent of liquids from the mantle to the middle
crust was favoured by the progressive evolution towards
extensional-transtensile tectonics. The induced anatectic
processes within the intermediate crust, and the emplacement of granitoid intrusions (Sardinia: POLI et alii, 1989;
and Liguria: CORTESOGNO et alii, 1998) allowed the genesis
of magmas with hybrid features whose eruption was favoured by the tectonic regime (CORTESOGNO et alii, 1998).
The hypothesis of a hybrid nature for the Early Permian magmatism due to complex interactions between
mantle-derived and crustal melts is shared by most
authors; two-stage models were proposed, including
assimilation processes, fractionation and mixing in magmatic chambers deep-seated in the crust, followed by
114
G. CASSINIS ET ALII
fractionation, mixing and mingling at higher crustal levels (STILLE & BULETTI, 1987; VOSHAGE et alii, 1990; BORIANI 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 conditions for extensive crustal contamination of liquids
derived from enriched lithospheric or asthenospheric
mantle sources are produced, and the general calc-alkaline affinity of the resulting Permian magmatism is consistent with the occurrence of melts showing strong similarities with subduction-related liquids, coeval with melts
of strictly anatectic origin.
On the whole, the stratigraphy, composition and
petrology of the igneous rocks in Seui match the hypothesis of a complex polystage origin.
TECTONIC AND MAGMATIC EVENTS DURING
THE BASIN DEVELOPMENT
The structural and sedimentary framework, and the
sequence of magmatic events in the Seui Basin and surrounds, are comparable to those of most southern European Permian-Carboniferous basins (e.g. CORTESOGNO et
alii, 1998; CASSINIS et alii, 2000; LAGO et alii, 2001;
BREITKREUZ et alii, 2001; DEROIN et alii, 2001).
As mentioned, the abundant floras from the terrigenous strata allow its dating to the latest CarboniferousEarly Permian (mainly Autunian) interval. Rapid evolution of the basin could be suggested by comparison with
other basins; for example, in the circum-Mediterranean
region, the Collio Basin attained a much greater vertical
and areal extent through similar tectono-magmatic events
in about 5 Ma (SCHALTEGGER & BRACK, 1999).
The following sequence of events is suggested by the
data.
– A basal clastic wedge («Conglomerato Basale») was
unconformably deposited on a discontinuous morphological surface of the Variscan metamorphic basement.
Above this unit, thin pyroclastic levels, and conglomerates containing acidic volcanic clasts, suggest early occurrences of distal ignimbrite flows into the basin.
– Early extensional phases in the southern part of the
basin are indicated by i) the deposition in an embryonic
depression, probably not exceeding 1 km2, of sands, clays
and localised channelised conglomeratic intercalations
and coal beds; ii) emplacement of small andesite plugs
aligned SW-NE, feeding thin lava flows.
– Terrigenous deposition in alluvial-lacustrine and
locally marsh environments, associated with huge andesite flows preserving a general SW-NE alignment, persisted in the subsiding basin. The asymmetric geometry
of major andesite bodies mirrored the tectonic control on
basin development. To the SE side, the lavas filling tectonic troughs attained thicknesses of up to 100 m, and
flowed towards the NW in units attaining up to some tens
of metres thick. The stratigraphic relationships between
terrigenous sediments and lavas through time suggest a
progressive temporal propagation of the extensional tectonics towards the NE. Possibly at the same period, tectonic highs with an E-W alignment limited the basin to
the north, while highs with a N-S alignment limited the
basin to the west, and were associated with the emplacement of magma bodies that intruded the basement up to
the sedimentary cover.
The distal products of a conspicuous ignimbrite flow
from the NW were deposited in the NE sectors of the
basin above andesites and terrigenous sediments, which
were at least temporarily exposed.
The Seui-Seulo (trans)tensional basins rotated from a
NE-SW to an approximately ESE-WNW trend, emphasised by subvertical NW-SE trending faults in the Sa
Canna-Genna Aussa area.
A diorite intrusion marked the northern boundary of
the basin. The intrusion has an E-W alignment and shows
progressive fractionation from diorite to quartz-diorite.
The main diorite body intruded both the basement and the
overlying terrigenous cover, inducing localised thermal
metamorphism. A structural high induced by the emplacement of cryptodomes within the basement, limited the
Seui Basin to the west, which was thus separated from the
Seulo Basin. The progressive growth of the domes fed by
progressively more acidic magma batches, developed a
complex morphology characterised by mounds and triggered the gravitational collapse of basement slices.
TECTONIC EVOLUTION OF THE BASIN
The evolutionary model of the «Stephano-Autunian»
basins shows a common, polyphase history, characterised
by rotation of the main horizontal compressional stress
trajectories (ZIEGLER & STAMPFLI, 2001). In the investigated area, the tectonic framework can be depicted as follows. At the eastern margin of the Seui Basin, the parallel
trend of andesite troughs and of some major faults (Tradalei fault, northern and southern Don Mestia faults; see
map) indicates a NE-SW direction for the early opening
of the basin.
To the north, the basin was bounded from the earliest phases by a structural high rising up from the basement and defined by an extensional tectonic lineament.
Along this long-lasting lineament repeated, progressively
more acidic magma batches erupted.
During relatively later stages, the basin records the
deposition of the distal products of a significant magmatic activity whose vent can be located some km to the
NW at Mt. Perdedu (COZZUPOLI & LOMBARDI, 1969). In
this volcanic sequence the intercalation of hyaloclastites
and ignimbrites with lavas and tuffs suggests that both
subaerial and subaqueous eruptions occurred. Therefore,
structural lows, at least temporarily water-filled, also
occurred in this area.
ANALYTICAL
METHODS
Whole rock major and trace element abundances for
andesites, diorites and dacites were carried out by XRF
techniques at the X-RAL Laboratories Canada. Losses on
ignition (LOI) were determined by the gravimetric
method. RE elements were analysed by ICP-MS at the
X-RAL Laboratories, Canada.
Quantitative electron microprobe analyses of mineral
phases were acquired by a SEM-EDS microprobe installed at the Dipartimento per lo Studio del Territorio e delle
sue Risorse, Università di Genova, equipped with and
X-ray dispersive analiser (EDAX PV 9100). Operative conditions were 15 kV accelerating voltage and 2.20 nA beam
current. Natural standards were used. Na2O and MgO
IGNEOUS, TECTONIC AND SEDIMENTARY EVENTS IN THE LATEST CARBONIFEROUS-EARLY PERMIAN SEUI BASIN (SARDINIA, ITALY)
contents analysed in silicates by means of an EDAX
microprobe are generally underestimated if the analysis is
processed with current automatic methods. To overcome
this problem, the background for nA (1.040 keV) and Mg
(1.252 keV) was manually corrected and considered
between 0.9 and 4.2 keV.
Orthopyroxene and clinopyroxene analyses were calculated according to the stoichiometric method of simultaneous normalization to 4.00 cations and 6.00 oxygens,
and Fe3+=12-total cation charge was considered for clinopyroxene. The allocation of cations to sites T, M1 and
M2 was performed according to MORIMOTO et alii (1988).
End members were calculated in the sequence: wollastonite, enstatite, ferrosilite, aegirine, jadeite, CaAl2SiO6,
CaFeAlSiO6, CaCrAlSiO6, CaTiAl2O6. MORIMOTO et alii
(1988) and ROCK (1990) nomenclature was adopted. The
Ca-amphibole cation sum was normalised to 13-(CaNa+K), as suggested by LAIRD & ALBEE (1981); Fe3+=46total 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 of
four oxygens.
ACKNOWLEDGEMENTS
The authors acknowledge PROGEMISA for making available the
data on geological-mining prospections and for drillings carried out
from 1983-87. Special thanks are due for the preparation of the geological map issued with this article. The authors are also indebted to
Sebastiano Barca and Bernard Bonin for the careful review of the
draft. 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.