the structural evolution of the asinara island (nw sardinia, italy)

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The structural evolution of the Asinara Island (NW Sardinia, Italy)R. Carosi a , A. Di Pisa b , D. Iacopini a , C. Montomoli a & G. Oggiano ba Dipartimento di Scienze della Terra , Università di Pisa , via S.Maria 53, 56126 , Pisa , Italyb Istituto di Scienze Geologico-Mineralogiche , Università di Sassari , Corso Angioj 10, 07100 , Sassari , ItalyPublished online: 13 Apr 2012.

To cite this article: R. Carosi , A. Di Pisa , D. Iacopini , C. Montomoli & G. Oggiano (2004) The structural evolution of the Asinara Island (NW Sardinia,Italy), Geodinamica Acta, 17:5, 309-329, DOI: 10.3166/ga.17.309-329

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© 2004 Lavoisier SAS. All rights reserved.

Geodinamica Acta 17/5 (2004) 309–329

The structural evolution of the Asinara Island (NW Sardinia, Italy)

R. Carosi

a,

*, A. Di Pisa

b

, D. Iacopini

a

, C. Montomoli

a

, G. Oggiano

b

a

Dipartimento di Scienze della Terra, Università di Pisa via S.Maria 53, 56126 Pisa, Italy

b

Istituto di Scienze Geologico-Mineralogiche, Università di Sassari, Corso Angioj 10, 07100 Sassari, Italy.

*

Istituto di Geoscienze e Georisorse, CNR Pisa, via Moruzzi 1, 56124 Pisa, Italy.

Received: 20/11/2003, accepted: 23/07/22004

Abstract

The metamorphic basement of the Asinara island represents a key area of the Sardinia Variscan segment, because it displays an almostcomplete cross-section through the inner part of the Sardinia Variscan belt, where different tectono-metamorphic complexes have beenjuxtaposed along narrow belts of high-strain concentration. Detailed field mapping coupled with preliminary studies on the structuraland metamorphic features of this small island, allow to draw a better picture of the structural frame issued from the Variscan collision inthe inner zone of the belt. Three deformation phases related to crustal thickening in a compressive and transpressive, partitioned tectonicregime, followed by a later phase of extensional deformation have been recognised. In spite of a general HT/LP metamorphic overprint,linked to the post-collisional deformation phases, a relic Barrovian zoneography is still detectable. The Barrovian assemblages are pre-to syn-kinematic with respect to the D2 deformation phase, and pre-date the third, contractional tectonic event.

The HT/LP assemblage indicates a static growth of weakly deformed by the last deformation events. The complex geometry of thefabric associated to the D2 and D3 deformation events suggests an heterogenous deformation history with a monoclinic geometry char-acterized by switching of the stretching lineation orientation and a contrasting sense of displacement, probably controlled by a northwardpartitioned pure shear.

© 2004 Lavoisier SAS. All rights reserved.

Keywords:

Structural geology; Sardinia; Variscan orogeny; Polyphase deformation

1. Introduction

The Variscan belt in Sardinia (Fig. 1) is a suitable settingto investigate the tectonic mechanisms affecting the middleand the lowest portions of the crust during the evolution of acollisional-type orogen. In the last twenty years a growingnumber of structural and petrographic studies performed inthe Sardinian Variscan basement described a polyphaseevolution in which three or four main deformation eventswere recognised [1-10]. All these authors agree that the crus-tal thickening was accommodated by southward directedthrusting, folding and nappe emplacement (the D1-D2 defor-

mation phases described in [1, 5], or the Gerrei and Meanaphase of Conti

et al.

[6]) followed by Westward-directed tec-tonics (in southern and central part, the “Sarrabus phase” ofConti

et al.

[7]; and the D3 of Carosi and Pertusati [5]).These events were eventually followed by a deformationepisode related to the exhumation stage [9, 10] correspond-ing to the D4 of Carosi and Pertusati [8]. Recently, someauthors [8, 9, 10] have argued that the second deformationphase was characterized, in the inner part of the chain, by aW-NW directed transpressive kinematic event that contrib-uted to the exhumation of medium-pressure metamorphicrocks at 315-320 Ma [11]. Very few data are available on the

* Corresponding author.R. Carosi, Dipartimento di Scienze della Terra, Via S. Maria, 53, 56126 Pisa, Italy. Tel. 0039 050 2212747, Fax. 0039 050 2215800

E-mail address:

[email protected]

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Fig. 1 Tectonic sketch-map of the Variscan belt in Sardinia and location of the study area in Fig. 2. 1: Post-Variscan cover deposits; 2: Variscanbatholith; 3: High Grade Metamorphic Complex; 4: Internal nappes (low- to medium-grade metamorphism); 5: Internal nappes (low-grademetamorphism); 6: External nappes; 7: External Zone; 8: Thrusts (a: main thrusts; b: minor thrusts); 9: faults; 10: PAL: Posada-Asinara Line.

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geology of the Asinara island, due to a former limitation ofthe access, while it represents an almost continuous outcropof Variscan rocks that can be traced from the Nurra penin-sula northwards (Fig. 1). In order to provide additionalconstraints on the tectonic evolution of the inner zone of theSardinian Variscides, we describe new structural and petro-graphical features of the medium and high-grademetamorphic complexes outcropping in Asinara island.

The new data highlight a complex three-dimensional fab-ric that we interpret to be associated with a partitioned D2-D3 transpression-related deformation phase. This seems tobe strictly related to the exhumation of middle crustal rocks,with a peculiar late HT/LT overprint. These findings help topropose a new model of the structural and metamorphic evo-lution of the region.

2. Regional setting

The Sardinia basement is a segment of the South Variscanbelt [4, 12, 14]

which, after the Cainozoic drifting of theisland, exhibits a NW-SE trend and outcrops continuouslyfrom south to north. Three main tectonic zones have beendistinguished ([4]; Fig. 1):

–a foreland “thrust-and-fold” belt, consisting of a sedi-mentary sequence ranging in age from (?) upperVendian-Lower Cambrian to lower Carboniferous, thatextensively crops out in the SW part of the island;

–a SW-verging stack of piled nappes, that equilibrated forthe most part under greenschist facies conditions, con-sisting of Palaeozoic sedimentary sequences hosting athick continental arc-related volcanic suite;

–an inner zone, characterised by medium—to high-grademetamorphic rocks. The inner zone comprises two dif-ferent metamorphic complexes:

• a complex made up by metapelites, metasandstones andquartzites metamorphosed under high—to-intermedi-ate P amphibolite conditions, with scattered lenses ofN-MORB-derived metabasalts retaining eclogiticassemblages [14, 16];

• a high-grade metamorphic complex, mainly consistingof migmatites, characterised by the assemblage Sil+ Kfs and containing large amounts of orthogneissesand scattered mafic bodies retaining granulitic and/oreclogitic relic assemblages [16-19].

A Variscan shear zone (Posada-Asinara Line) [14, 20]separates these two complexes. This line is regarded bysome authors as part of a “South Hercynian Suture Zone”between the Armorica and Gondwana continental margins[14]. A complete Wilson cycle was suggested on the basis ofthe following evidence [4]: (1) the N-MORB type geochem-ical and isotope signature of the amphibolites along thePosada-Asinara Line; (2) their relic eclogitic assemblagewhich seems to indicate a subduction zone thermal gradient;(3) the structure of the nappe zone. This consists of a stackof different tectonic units which, from bottom to top, are

characterized by a large amount of Ordovician calcalkalinemetavolcanic rocks (interpreted as a disactivated Andeantype margin), and a Low—to Medium-Grade MetamorphicComplex (hereafter LMGMC).

Within this framework, the basement of Asinara islandrepresents the northwesternmost part of the Sardinian Varis-cides (Figs. 1, 2, 3) characterised by the transition from low-grade metamorphic Palaeozoic sequences of the Nurra InnerNappes ([2, 8]; Figs. 1, 2) to the High-Grade MetamorphicComplex (hereafter HGMC). This transition is through azone where prograde Barrovian metamorphic assemblagesrapidly change to oligoclase + garnet and garnet + staurolite± kyanite assemblages going north [19]. The main differencebetween Asinara island and other basement sectors close tothe Posada Asinara line, is in the late HT/LP metamorphicassemblages, that overprint the older Barrovian suites. Anal-ogous relationships were previously documented only in theAnglona region (north-central Sardinia; [23]).

2.1. Metamorphic evolution of the Asinara island

The P and T conditions of the Barrovian metamorphicstage in the LMGMC were estimated at about 8-10 Kb and560°C, respectively, on the basis of mineral core-core com-positional relationships in garnet-staurolite micaschists fromCampo Perdu and from La Reale orthogneiss; this stage wasfollowed by low P re-equilibration at P = 2-3 Kb and a min-imum T of about 500°C [19, 21, 22].

In the HGMC, the Punta Scorno amphibolites are envel-oped within migmatites so that both suffered the same P-Tmetamorphic evolution. The former contain a garnet+ plagioclase + clinopyroxene relic assemblage, that pointsto an early granulitic stage (minimum P = 7-8 Kbar, T = 720-740°C; [19]). This was followed by low-P amphibolitefacies equilibration to T and P conditions of 500-600°C and3-4 Kb through reactions such as: Cpx + Grt + Qtz + H2O= Hbl + An and Cpx + An + H2O = Zo + Hbl + Qtz [20].Migmatites originally equilibrated within the kyanite stabil-ity field and then suffered partial melting duringdecompression.

On the basis of the petrogenetic grid based on observedequilibria, and of available thermo-barometric estimates [19,21], some preliminary P-T paths have been proposed for thetwo metamorphic complexes of Asinara (Fig. 4; [19, 21]). Inthis picture, the metamorphic history of the two metamor-phic complexes outcropping in the Asinara island onlypartially matches the evolution of other sectors close to andacross the Posada-Asinara Line, with remarkable differencesin the final stages of their evolution. In the NE sector of thebelt, Variscan metamorphism of the LMGMC describes acomplete clockwise P-T loop consisting of a Barrovian pro-grade evolution followed by a retrograde evolution [19, 22]with no evidence of low-pressure metamorphic minerals[21]. The recognizable portion of the P-T-t path as far estab-lished in the HGMC of NE Sardinia conversely points to anexhumative trajectory [19, 21, 23], like those in Asinara.


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Fig. 2 Structural-geological sketch map of the Asinara island. 1) Post Variscan cover; 2) post-tectonic Variscan granitoid; 3) pegmatites and quartzdykes; 4) migmatitic gneiss; 5) Cala d‘Oliva orthogneiss; 6) Cala Reale orthogneiss; 7) andalusite and sillimanite bearing mylonitic micaschist; withbody of augen gneiss 8) andalusite bearing micaschist and paragneiss with body of quartzites; 9) garnet and albite/oligoclase bearing micaschist andparagneiss; 10) amphibolite; 11) migmatitic complex: metatexite, diatexite and agmatite; 12) Peraluminous granitoid bearing stromatic enclave; 13) S2schistosity; 14) A2 fold axes 15) A3 fold axes 16) L2 stretching lineation 17) faults; 18) trace of the geological cross-sections in Fig. 3.


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3. The metamorphic complexes

The lower LMGMC complex and the upper HGMC com-plex are separated by a tight belt of mylonites (Figs. 2, 3). Inthis tight belt orthogneisses and augen gneisses have beenrecognized and mapped (Fig. 2). Both complexes areintruded by late Variscan granitoids.

3.1. The LMGMC

In the LMGMC the following lithotypes, from bottom totop, have been recognized:

–fine-grained (“minuti”) paragneisses and porphyroblasticparagneisses (cropping out in the northern Nurra penin-sula and in the southern part of Asinara island; Fig. 2).The mineral assemblage includes biotite, white mica,quartz, plagioclase (oligoclase) and garnet. The acces-sory minerals are epidote, zircon, monazite and oxides.

Chloritoid has been found only in the Stintino peninsula.Close to the Mt. Castellaccio intrusion, garnet is trans-formed to spheroidal aggregates of decussate biotite,muscovite and quartz. Franceschelli

et al.

[22] estimatedthe temperature and pressure conditions of this Barrovianassociation in northern Nurra at 480°C and 8 Kb;

–andalusite and sillimanite bearing porphyroblastic parag-neisses and micaschists: they outcrop in the central part ofthe island up to Punta s’Arrocu and in the La Reale—Punta Trabuccato zone (Fig. 2). In the more pelitic levels,exceptional modal proportions of andalusite, often inprismatic porphyroblasts can be recognised (Fig. 5). Gar-net and staurolite relics are abundant and oftendestabilised in a mineral association defined by biotite,andalusite and oxides. Fibrolite derives both by the desta-bilisation of biotite and by muscovite and quartz reaction.The accessory components are mainly defined by tourma-line, ilmenite apatite, epidote and zircons;

Fig. 3 Cross sections of the Asinara island. 1) Post Variscan cover; 2) post-tectonic Variscan granitoid; 3) garnet and albite/oligoclase bearingmicaschist and paragneiss; 4) andalusite bearing micaschist and paragneiss with body of quartzites; 5) Cala Reale orthogneiss; 6) andalusite andsillimanite bearing mylonitic micaschist with body of augen gneiss; 7) Cala d‘Oliva orthogneiss; 8) migmatitic gneiss; 9) amphibolite; 10) migmatiticcomplex: metatexite, diatexite and agmatite; 11) Peraluminous granitoid bearing stromatic enclave; 12) S2 Foliation plane 13) S1 Foliatiom plane14) A3 axial plane. LMGMC: Low—to Medium Grade Metamorphic Complex; HGMC: High Grade Metamorphic Complex.


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Fig. 4 Petrogenetic grid and P-T path for the HGMC and the LMGMC of Asinara. Pressure and temperature are inferred from combination ofassemblage reaction constraints and geothermobarometry (modified after Di Pisa

et al.

[20] and Ricci [22]).

(1)

: Staurolite + Quartz stability field from Richardson [24];

(2a)

: Holdaway, [25];

(2b)

: from Richardson

et al.

[26];

(3)

: from Richardson [24]; (

4a)

:Ms

ss

+ Bt

ss

+ Kfs + Qtz + V

→

L;

(4b)

: Ms

ss

+ Bt

ss

+ Qtz + V

→

L + Als;

(4c)

: Bt

ss

+ Als + Kfs + Qtz + V

→

L;

(4d)

: Bt

ss

+ Als + Qtz + V

→

L + Crd;

(4e)

: Ms

ss

+ Qtz

→

Bt

ss

+ Kfs + Als + V;

(4f)

: Bt

ss

+ Als + Qtz

→

Crd + Kfs + L; (

4g

): Bt

ss

+ Crd + Kfs + Qtz + V

→

L; (

4h

): Bt

ss

+ Als + Qtz

→

Crd

+ Kfs + V (reactions 4 from Vilzeuf and Holloway [27]); (

5

): Hbl + An + Qtz + H

2

O

→

Cpx + L [28];

(6)

: Cpx

(Di50)

+ Grt

(Gross50 Alm50)

+ Qtz + H

2

O

→

Hbl

(XMg=0.45)

+ An;

(7)

: Cpx + An + H

2

O

→

Czo + Hbl + Qtz (a

CaPl

= 1, Mg end-member) (6 and 7 mineral reactions in the Na

2

O-CaO-FeO-MgO-

Al

2

O

3

-SiO

2

-H

2

O system modelized using the Holland and Powell [29], dataset).

Ruled boxes: T and P metamorphic conditions in the LMGMC, estimated on the basis of core compositions of mineral pairs or assemblages for theBarrovian metamorphic stage and on the basis of rim compositions for the HT/LP stage; T were estimated by using the calibration of Perchuck [30] forGrt-St pairs and the calibrations of Ferry and Spear [31], Perchuck

et al.

[32] and Indares and Martignole [33] for Grt-Bt pairs; P was estimated by usingthe calibration of Ghent and Stout [34] for Grt-Pl-Bt-Ms assemblages and of Perchuck

et al.

[32] and Newton and Haselton [35] for Grt-Pl-And-Qtzassemblages. The dotted line represents the P-T trajectory for the studied rocks.Dotted box: minimum T and P metamorphic conditions in the HGMC, estimated on the basis of core compositions of mineral pairs or assemblages fromthe Punta Scorno amphibolites; T was estimated by using the calibration of Ellis and Green [36] for Grt-Cpx pairs and of Graham and Powell [37] forGrt-Hbl pairs; P was estimated by using the calibration of Newton and Perkins [38] for Grt-Cpx-Pl-Qtz assemblage. The dashed line represents the P-Ttrajectory for the studied rocks. The P-T evolutions of migmatites and amphibolites are considered similar to each other because the former envelopethe latter (for example, the P-T conditions for the partial crustal melting of the meta-sedimentary rocks described in the text were suffered also by theamphibolites; on the other hand, the high pressure extrapolation of the line represents the inferred original presence of kyanite within the migmatiteprotoliths and is legal also for amphibolites).


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–massive amphibolites. They outcrop as boudins or len-ticular bodies of metric to decametric size, expecially inthe Stretti zone (Fig. 2). They are surrounded by quartz-ites in the paragneisses complex which wrap around theboudins. The mineral assemblage mainly consists ofhornblende and plagioclase with minor biotite, cloriteand oxides. According to Cappelli

et al.

[14], the prot-holith of the amphibolites is a basalt with N-MORBsignature of probable Precambrian age;

–quartzites. These crop out between Punta s’Arrocu andthe Campu Perdu area (Fig. 2). They are weakly foliatedor massive quartzites, showing a thickness of few tens ofmeters. The paragenesis is defined by quartz, with aclear shape preferred orientation, and minor amount ofwhite mica nearly parallel to the granoblastic levels ofquartz;

–la Reale orthogneiss. This orthogneiss crops out in theLa Reale zone (Fig. 2). It is a L tectonite with, a weakmain foliation. The linear fabric is defined by laths ofalkali-feldspar in a matrix characterised by a shape pre-ferred orientation of quartz, plagioclase, microcline, andweakly fibrolitized muscovite;

–augen gneisses, fibrolite, andalusite and cordierite bear-ing mylonitic micaschists. They crop out in the centraland northern part of the island, between Punta s’Arrocuand Punta Trabuccato (Fig. 2). The mylonitic micas-chists are made by an alternation of weakly deformedquartzo-feldspatic centimetric bands and dark-violetdecimetric portions constituted by biotite, andalusite—muscovite, fibrolite, cordierite and oxides (Fig. 6). Theaugen gneisses are defined by centimentric-size augenclasts constituted of aggregates of feldspar, quartz andbiotite, in a matrix of quartz, biotite and muscovite. Bothrocks could be derived by an intense non-coaxial defor-mation and a leaching of the alkaline and alkaline-earthelement (which caused an enrichment in aluminium)under amphibolite facies condition at the expense of anoriginal granitoid, giving rise to an alternation of mylo-nitic-micaschist, augen gneisses and orthogneisses.

3.2. The HGMC

The HGMC is composed of migmatites and migmatiticorthogneisses.

–Migmatites are mainly composed of diatexites andmetatexites (nomenclature after Ashworth [39]) withsillimanite (both prismatic and fibrolitic) + alkali-feld-spar (microcline) + cordierite and

in situ

garnet. Thediatexites are mainly composed by bodies of anatecticgranitoids (tonalitic and trondhjemitoid composition,Fig. 2) with refractory bodies or enclave and by stro-matic diatexite. The stromatic diatexites arecharacterised by either nebulitic or agmatitic textures[39] showing concordant and discordant granitic veins(sensu Vanderhaeghe [40]) and centimeter to meter K-feldspar or plagioclase, quartz rich leucosomes withbiotite-rich schlieren. In the northernmost area theseleucosome are deformed indicating flattening and non-coaxial deformation. In the diatexites it is possible torecognize restitic bodies of polydeformed amphibolitesand refractory rocks. The paleosome and restitic bodiessuggests that the protolith of the diatexites was consti-tuted of both sedimentary rocks (paragneisses) andorthogneisses with a granitic composition. The metatex-ites are principally characterised by syn-migmatiticmesosome layers (biotite + sill + quartz + cord ± K-feld-spar) having a continuous gneissic framework, showinga clear subsolidus fabric with a principal schistosity and,at place, scattered neosome patches with scarce anatec-tic differentiation.

–Cala d’Oliva and Punta Scorno orthogneisses. The Calad’Oliva orthogneiss constitutes a sheet body with varia-ble thickness (up to 100 metres) and shows a typicalplanar-linear structure with down-dip stretching linea-tions. These are defined by polycristalline aggregates ofquartz, plagioclase-antiperthite and microcline-perthiteinterlayered with discontinuous micaceous domainswhere biotite and minor muscovite show reciprocalequilibrium relationships [41]. Garnet and fibrolite are

Fig. 5 Static large-size andalusite crystals in the micaschist (LMGMC,Stretti zone). Pencil for scale.

Fig. 6 Example of andalusite and fibrolite bearing mylonitic micaschist(HGMC, Punta Soriana, central Asinara).


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also present in accessory proportions enclosed withinfeldspars and quartz. Del Moro

et al.

[41] pointed outthe peraluminous character of the orthogneiss, inter-preted as a syn-collisional Variscan anatectic granitoid.The Punta Scorno orthogneiss, normatively a syenogranite,consists of mylonitic augen gneiss (Fig. 6) characterised bylarge laths of alkali-feldspar which exhibit both oblate andprolate shapes. The mineral assemblage is the same of theCala d’Oliva orthogneiss even though mica, garnet and sil-limanite are comparatively more abundant.

–Banded amphibolites. They mainly outcrop near PuntaScorno and consist either of centimetric to decimetric darkamphibolites alternated with leptinitic layers, or a massivehornblende-plagioclase amphibolites. The mineral assem-blages sometime include calcic-pyroxene and garnet (bothas relics of a previous granulite metamorphic stage; [20])and zoisite. Previous studies [42] indicated that the geo-chemical features of the Punta Scorno amphibolites aresimilar to the amphibolites of a suprasubduction zone (lowTi and HFSE and REE concentrations and moderate selec-tive enrichment in LILE, Ce and P).

3.2.1. Intrusive complex

The granitoids that outcrop in the southern—centralmetamorphic unit of the Asinara island (Mt. Castellaccio)can be considered as single intrusion. It consists of inequi-granular granodiorite—monzogranite, characterised bylarge laths of K-feldspar defining a magmatic fluidality witha mean N020° orientation in the northern contact, and rang-ing from N010° to N180°E in the southern part. Small bodiesof nearly equigranular and muscovite rich monzograniticfacies also occur within the main intrusion (Cala S. Andrea,Fig. 2) and in the northernmost part of the island.

Pegmatites can be found all over the island, but are partic-ularly abundant near Punta Rumasinu, Crastu Biancu, PuntaTumbarinu, and Punta Sabina (Fig. 2) where they reach dec-ametric thickness. They are leucocratic dykes withcentimetric-size crystals. Their mineral association is mainlydefined by quartz, plagioclase, muscovite, biotite, tourma-line and allanite. They show either a fracturation or a roughspaced foliation (subparallel to the S2 foliation in the coun-try rocks), and sometimes a weak, spaced crenulationcleavage (Campo Perdu, Fig. 2).

4. Structural analysis

Four deformation phases have been identified: a firstdeformation phase D1 associated to a relic foliation S1, asecond deformation phase D2 that produces the main fabricin both the metamorphic complexes, a third deformationphase D3 related to open folds and a fourth brittle/ductiledeformation phase (D4). The sequence of deformation is thesame in the two complexes and the deformations show sim-ilar geometric and structural features all over the study area.

4.1. The earlier D1 deformation

F1 folds have rarely been observed, because of the D2deformation phase transposition. Only in the central part ofthe island, close to Punta sa Nave, interference structuresbetween F1 and F2 folds, referable to “domes and basinfolds” (type E sensu Ramsay [43]), have been recognised.An S1 foliation has only been observed as relics in D2microlithons or in F2 fold hinges and as inclusion trails ingarnet and plagioclase porphyroblasts. In the granoblasticportions of porphyroblastic paragneisses the S1 foliation ismarked by oblique mica grains. It is worth to note that relicsof S1 foliation have also been recognised in D2 microlithonsin the andalusite bearing micaschists, north of the graniticintrusion. In the HGMC, both in the migmatitic gneisses andin the metatexitic migmatites, relics of the first deformationphase have been found in some feldspar and quartz porphy-roblasts (inter D1-D2 or syn-D2), where inclusion patterns(discontinuous with the external foliation), defined by relicsof S1 microlithons (biotite + muscovite) have beenobserved. In the LMGMC, in some M-domains, defined bybiotite and muscovite, locally chloritized or partly fibroli-tized, it is possible to observe some stacks of biotiteobliquely disposed respect to the main schistosity S2(Fig. 7). In the amphibolite complex, at Punta Scorno, thealternance of elongate feldspar-quartz rich layers and amphi-bole-piroxene rich layers could be attributed to the S1foliation. In the diatexite complex, a first deformation phasehas been preserved in some paleosomes constituted by anda-lusite-bearing micaschists and paragneisses that behavedlike “resisters” [39, and see also 44]. Here we can observetight folds affecting an earlier schistosity, possibly the S1foliation. Anyway, the resisters have randomly distributeddirections, with no continuity with the surrounding migma-tites. Only the amphibolites seem to preserve an S1 foliationparallel to an original primary layering defined by an alter-nation of quartz-plagioclase and feldspar rich layers, wherethe quartz and the feldspars show intracristalline deforma-tions (subgrain domains, bulging structures, deformationbands and, in some cases, relics of recrystallised grains ofquartz) and amphibolite-pyroxene rich layers that show anundulose extinction and a preferred orientation in the XZsection of finite strain ellipsoid. Big clasts of amphibolites,embedded in the neosome (metatexitic neoesome) with thesame microstructural features were also found in the north-ern part of the migmatite complex.

4.2. The second deformation phase D2

4.2.1. F2 folds

F2 folds constitute the principal fold system, and they arecharacterized by a well-developed axial plane schistosity (S2).In the southern portion of the island (south of the granitoidintrusion), the folds are tight to isoclinal [45] and quartz rodsare frequently found. North of the intrusion, in the central partof the island, between Punta Tumbarinu and Pian degli Stretti,


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the fold geometry is more complex and some polyharmonicfolds, with stretched limbs and thickened hinges, have beenobserved. F2 folds are upright (sensu Fleuty [45]) or weaklyoverturned toward the SW and often show a cuspate geome-try. Their interlimb angles vary between 20° and 30° (tight toopen folds according to Fleuty [45]; Fig. 8). In the Strettizone, some decametric SW overturned folds are of the closedtype [45], with an interlimb angle higher than 40°. North ofPian degli Stretti, the folds become closed to isoclinal. In theandalusite fibrolite bearing milonitized micaschist the F2folds are transposed by the axial plane mylonitic schistosity.In the northern and central part of the island, F2 fold axestrend from N120° to N 130° (Figs. 2a, 9). Their plunge is quitescattered: In the north towards the Castellaccio intrusion F2fold axes plunge toward the WNW in the western part of theisland; in the eastern part of the island they plunge toward theESE (Figs. 2a, 9). In the southern part ofthe island, F2 foldaxes show a mean trend of N160 and plunge moderatelytowards the NNW.

4.2.2. S2 schistosity

In the southern part of the island the S2 schistosity showsa mean direction at N100°-120°and dips 35°-45° toward the

SSW (Figs. 2, 10). North of the granitoid intrusion, thedirection is scattered from NE to NW and the dip is about30°-45°. Stereographic projection of the S2 schistosity(Figs. 2, 10) highlights a kilometre-scale antiform in thePunta Rumasinu and M. Marcutzeddu with an axis plungingtoward the NW. North of Pian degli Stretti (central sector,Figs. 2, 2a, 10), the S2 schistosity strikes N100°-120° anddips 30°-40° toward the NE. Only in the northern part of theisland, close to the contact between the Cala d’Oliva orthog-neiss and the migmatitic complex, the direction of the S2schistosity changes to N40E-N50E (Figs. 2, 2a) and some-times it is shallowly dipping. In the HGMC, mainly in themetatexitic gneisses, the S2 schistosity is the main fabric andit is nearly parallel to the leucosome—melanosomeboundaries.

4.2.3. L2 lineations

The L2 object lineation grain (sensu Piazolo and Passch-ier [46] and references therein) is represented both by grainand stretching lineations, that are well defined in thegneisses and quartzites. It is usually defined by quartz, gar-net, biotite and by sillimanite in the migmatitic gneisses.Pre-D2 garnet and biotite, recrystallized during the D2 tec-tonic phase, have been rotated parallel to L2 stretchinglineation. It is worth to note that a change in the attitude ofthe L2 stretching lineation is clearly observable in the studyarea. In the southern and central portion of the island the L2stretching lineation trends N090°-N110°, parallel to the F2fold axes (Figs. 2a, 10), and plunges few degrees toward theNW and the SE. The L2 lineation switches from sub-hori-zontal, near Punta Gruzitta, to down dip in the northern partof the island (Fig. 11), whereas the S2 foliation maintains thesame attitude (Fig. 10). This geometric distribution assumes

Fig. 7 a) Thin section showing some microlithons with relics of S1 insidea principal S2 schistosity. Micaschist, LMGMC. Nicol x; magnification20x. b) Thin section showing a micafish with a top to the NW sense ofshear. Oligoclase bearing micaschist. Nicol x; magnification 20 x.

Fig. 8 F2 open fold in paragneiss of the LMGMC; central Asinaraisland.


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Fig. 9 A2 fold axes orientation (Schmidt equal area projection, lower hemisphere). Data contoured at 1, 4, 7%.

Fig. 10 Attitude of S2 schistosity and L2 stretching lineation (Schmidt equal area projection, lower hemisphere). m : migmatites ; o: ortogneisses. Datacontoured at 1, 4, 7, 10%.


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a fundamental meaning for constraining the kynematic his-tory of the D2 deformation phase.

4.2.4. D2 microstructures

In the porphyroblastic paragneiss and garnet bearingmicaschist the S2 schistosity is defined by an alternation ofan irregular spaced cleavage, constituted by M domaindefined by white mica, biotite alternated to Q domain (sensuPasschier & Trouw [47]) with granoblastic texture definedby quartz and plagioclase (albite).

Garnet and plagioclase porphyroblasts often contain inclu-sion trails defined by biotite and quartz, obliquely arranged withrespect to the external schistosity. Nevertheless, both the garnetand plagioclase are badly preserved and both their pressureshadows and the recrystallisation tails have been fractured anddamaged by the post-D3 static recrystallisation. Therefore, theinclusion trails could represent an early, (S1) schistosity, rotatedwith the porphyroblast during the D2 deformation phase. Micafishes are partially annealed by the latest static recrystallization.

Inside the porphyroblastic paragneisses outcropping inthe central-southern area, it is possible to observe stair-step-ping (till sigmoidal structure) and mica-fish structures(Fig. 7b), often partially disrupted by static recrystallization.All these structures are reliable kinematic indicators inducedby non-coaxial deformation

.

In andalusite-bearing paragneisses and micaschists thecleavage domains are defined by biotite, muscovite, anda-lusite and sillimanite. Biotite and muscovite are metastablewith respect to fibrolite and andalusite. The biotite and mus-covite are lined-up along the S2 foliation, while thegranoblastic portions are characterized by a seriate and ineq-uigranular texture with their boundary defined by polygonalto polylobate shapes.

In several samples, the granoblastic portions is affectedby incipient dynamic recrystallization. In sections parallel tothe lineation and normal to the foliation (XZ plane of thefinite strain ellipsoid) it is possible to observe a weaklyoblique or asymmetric orientation of the quartz-plagioclaseaggregate with respect to the S2 foliation. Mica fish in the Mdomains (sensu Passchier & Trouw [47]) point to a top-to-the-NW sense of shear (Fig. 7b). In this case, though the fab-ric in the andalusite-bearing micaschists is affected by laterstatic recrystallizzation, relic asymmetric structurs anddynamic recrystallization identify them as myloniticmicaschists.

In the quartzites, the S2 schistosity is poorly defined bywhite mica often showing a discontinuous and spaced cleav-age domain and by the preferred orientation of quartz andplagioclase.

In the Punta Scorno orthogneiss the shear sense indica-tors, mainly sigma type mantled porphyroclasts, shows a topto the SW sense of shear. Here the mantled porphyroclastgeometry could be observed both along the XY and the XZplanes (Fig. 11). This allows to observe the 3-D geometry ofthe recrystallized tail (Fig. 11b) that, in a steady statehomogenous flow, approximates the principal flow apophy-sis of the local system within a monoclinic shear zone [48,49]. Nevertheless, the augen portion defined by the k-feld-spar shows a geometry varying within few meters fromspherical to elongate. All these asymmetric structures andkinematic indicators point to a deformation geometry ofmonoclinic type, probably partitioned [50].

In the metatexites and migmatitic gneisses, S2 schistosityis defined by white mica and biotite, the latter often fibroli-tized or chloritized, showing a pattern of discontinouswriggly and spaced cleavage domain.

In all lithotypes, quartz shows undulose extinction, sub-grain boundaries and deformation lamellae. Interlobate grainboundaries and subgrains indicate that intracrystalline defor-mation was associated with a recovery process and, in somecases, with incipient recrystallization with the developmentof subgrain boundaries and subgrain boundary migrationrecrystallization.

In the quartzites two different quartz domains have beendistinguished. In sections oriented normal to the S2 foliation

Fig. 11 a) D2 shear sense indicators in the Punta Scorno orthogneisses(HGMC) observed on a plane parallel to the lineation and orthogonal tothe foliation. Shear sense is top to the SW. b) Recrystallized tails on theS2 foliation. The perturbation of the recrystallized tails indicates aninhomogeneous distribution of strain at the local scale. Orthogneiss,HGMC. Scale bar is 2 cm.


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and parallel to the L2 lineation, we observe two domains ofsubgrain boundaries: one obliquely oriented with respect tothe stretching lineation; the other, that shows subgrains,commonly strain-free, oriented orthogonal to the stretchinglineation. It is possible that these two families of domains aredue to different deformation phases and the first oblique onecould be related to a non-coaxial deformation whereas thelast one to a final static recrystallization event. Also in thiscase, it is possible to consider the rocks as mylonites [47].

Plagioclase is always showing undulose extinction andminor internal microfracturing. Twinning deformation andsome phenomena of flame-perthite are observed.

Biotite and muscovite show undulose extinction andweak microkinks. The microlithons are constituted by grain

aggregates of seriate-polygonal shape with boundaries ofteninterlobated and oriented parallel to the foliation.

The microstructures associated to the second deformationphase point out that the principal deformation mechanismsare represented by intracrystalline deformation in competi-tion with recovery and incipient dynamic recrystallizationprocesses in a metamorphic context of medium-high grade.From a rheological point of view, we could assess that dur-ing the D2 deformation phase rocks behaved in a crystal-plastic way [47, 51, 52].

4.2.5. High -temperatures shear zones

In the diatexitic and mesosome complex in the northernpart of the HGMC some shear zones (Fig. 12), showing a

Fig. 12 Example of high-temperature shear zone in diatexite. a) Extensional foliation fish (in the leucosome) provides a top-down-to-the NE sense ofshear. Nearly XZ sections. HGMC, Punta Scorno. b) Sigmoidal structure in the leucosome provides a top-down-to-the-NE sense of shear in sectionsclose to the XZ plane. HGMC, Punta Scorno. c) Leucosome asymmetric pressure shadows around a melanosome body, embedded in the diatexite:apparent shear sense is top-to-the-SE. Nearly orthogonal plane view with respect to Figs. 12a and b. HGMC, Punta Scorno. Scale bar is 5 cm.d) Concordant and boudinated diffuse granitic and pegmatitic dyke in diatexitic rocks. View orthogonal to the diatexitic folation plane, HGMC, PuntaScorno. Scale bar is 5 cm.


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decametric thickness, have been detected. They strike nearlyNW-SE and are steeply dipping to the NE. No clear stretch-ing lineation can be observed in this sheared diatexiticcomplex and only the foliation plane and the recrystallizedporphyroclast tails have been used as reference system. Inthe sections nearly orthogonal to the XY plane (Fig. 12) andsubparallel to the recrystallized tails kinematic indicators aremainly represented by foliation fish (Fig. 12a), sigma anddelta-like recrystallized tails on clasts (Fig. 12b, c) and stair-stepping of wings on amphibolite lenses, indicating a top-down-to-the-ENE sense of shear. Some recrystallized tailsare made of both leucocratic and melanocratic composition(Fig. 12c). In sections close to the YZ plane it is possible toobserve both meter-size pegmatite and leucosome portionsin the magmatic foliation, strongly boudinaged (Fig. 12d)and weakly sheared with pegmatitic and melanosomic clastsand leucocratic patches in the pressure shadows (Fig. 12c).Therefore, the overall shear sense is top-(down)-to-the-NE.The amphibolite clasts show a relic principal foliation (oftencontinuously linked to the external foliation registered in theneosome matrix) or folds that have been interpreted (seemicrostructural description) as D2 structures (Fig. 13). Thisis due to the fact that approaching the shear zone we canobserve the progressive involvement of D2 structures in theshear zone deformation.

On a mesoscopic scale, the magmatic foliation of diatex-ites including the amphibolite and leucosome boudins isdeformed by F3 open folds (Fig. 14) and overprinted by D4brittle/ductile structures suggesting that the high-tempera-ture shear zones developed late or post-D2 and before D3deformation.

4.3. D3 deformation

The D3 deformation phase developed mainly in the cen-tral and northern part of the island. F3 folds have beenobserved starting from Punta Rumasinu, north of the granitic

intrusion. No mesoscopic F3 folds have been recognized upto the northern limit of the granite intrusions area (Fig. 2)and all over the study area D3 deformation is associated tothe development of a centimetre-size crenulation cleavagerecognizable both at the outcrop and at the microscopicscale. F3 folds are predominantly upright [45] and A3 axestrend nearly E-W and plunge 20°-30° both toward the E andthe W (Fig. 15).

The attitude of S2 foliation around the Mt. Castellacciointrusion describes a large-scale antiform trending nearlyN100°E and plunging 24°NW (Fig. 15). The calculatedaxis fits well with the measured minor A3 fold axes, sug-gesting that the observed S2 trend variation could berelated to the D3 deformation phase. This antiform couldbe related to the Mt. Castellaccio intrusion. We cannotexclude that also the Mt. Castellaccio intrusion couldhave contributed, later in the tectonic history, to this rota-tion, enhancing the antiformal shape. In the northern partof the island the D3 deformation phase affected also theHGMC. A3 fold axes trend N080-110° (Fig. 15). Outcropscale fold interference patterns of Type 3 (divergent-con-vergent pattern; Ramsay [43]) have been found nearPunta Sa Nave and in the western coast of the Stretti zone(Fig. 16).

In the HGMC the D3 deformation is well recorded both inthe migmatitic ortogneisses and in the migmatitic complex(Fig. 14).

In the LMGMC the S3 cleavage domains are defined bybiotite, white mica, fibrolite and oxides. These minerals donot show syn-D3 recrystallization, but they are associated tomechanical rotation and kinking. In some thin sections it ispossible to observe a solution transfer mechanism ([48] andreference therein). The microlithon domains are defined bygranoblastic portions of quartz, plagioclase, apatite and epi-dote, and they seem to maintain all the intracrystallinedeformation inherited from the D2 deformation phase.

Fig. 13 F2 fold in amphibolite embedded in the neosome component ofthe diatexitic complex. HGMC, Punta Scorno.

Fig. 14 F3 open fold affecting the amphibolite bearing diatexitic(stromatic) complex.


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4.4. D4 deformation phase

In the LMGMC, the D4 deformation phase is defined byfractures, extensional veins (Fig. 17) and by kinks whose axialplanes trend NE-SW and dip toward the NW. In the HGMC

the D4 phase gives rise to a fold system with an horizontalaxial plane while in the migmatitic gneiss it is associated to thedevelopment of asymmetric folds with subhorizontal axialplanes. In some places it is possible to observe also F4 uprightfolds with axes trending N-S and plunging 50°-70° toward the

Fig. 15 A3 fold axes (Schmidt equal area projection, lower hemisphere). Data contoured at 1, 4, 7, 10% (a and b). C: calcultated A3 fold axis (big blackdot) from scattering of S2 foliation in southern-central sector of Asinara island.


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North (Fig. 18). In few cases we have observed that F4 foldsaffect andalusite porphyroblasts.

In some outcrops extensional veins are filled by leucosomethat testify for a melt enhanced embrittlement (Fig. 17b) as sug-gested by [54] that affected the diatexites during the exhumation.

4.5. Magmatic fabric of the granitoids

The intrusive complex outcropping in the central Asinaraisland does not show field evidence of deformation struc-ture. A magmatic fluidality, defined by the (001) plane of theK-feldspars in the granitoids was measured and comparedwith the orientation of tectonic fabrics of the surroundingbasement. Well-oriented portions of granitoids outcropalong the southern contact between the Mt. Castellaccio andPunta Rumasinu zones and between Punta Marcutza andPunta s’Arrocu and also in the La Reale—Periodo Secondoand Punta Trabuccato. In these areas magmatic fluidality isdiscordant with respect to the general trend of the fabric inthe host rocks.

Magmatic lineation in the granitoids is also discordantwith respect to the attitude of the L2 stretching lineation ofthe host rocks (Fig. 19).

Only in Punta s’Arrocu and Trabuccato zones granitoidsshow a magmatic fabric sub-parallel to the L2 lineationdeveloped in the gneissic rocks. The prevailing discordantrelations between granitoids and sourrounding basementrocks highlight the intrusive nature of the contact and indi-cates that the granitoids were unaffected by the deformationphases recorded in the metamorphic basement.

4.6. Pegmatitic dykes and veins

Pegmatitic intrusions are quite abundant all over the studyarea. Foliated and non- foliated pegmatites have been distin-guished according to their mesoscopic fabrics.

Foliated pegmatites have been found in Punta Rumasinuand in the Stretti zone. They show a foliation and fracturesystem parallel to S2 schistosity, and locally they are crenu-lated by the later D3 deformation phase. Their emplacementcan be confined partly during the D2 tectonic phase andpartly between D2 and D3 tectonic phases. Non foliatedpegmatites crosscut the D2 fabric recognized in the base-ment rocks so that their development can be confined later inthe tectonic evolution, after the main tectonic phases.

Structural orientations are quite scattered: foliated pegm-atites show a mean attitude at N090°, while non foliatedpegmatites strike from N040° to N110°.

5. Textural relations of the HT/LP assemblage

The metamorphic features of the Asinara island are differ-ent from those occurring in other sectors of the inner zone of

Fig. 16 Example of type 2 interference pattern between F2 and F3 foldsystems in the paragneiss outcropping in the LMGMC in central Asinara.

Fig. 17 Examples of D4 brittle ductile deformation phase. a) Pseudoflanking fold with a top-to-the-South sense of shear. HGMC; PuntaScorno. b) Enhanced melt embrittlement testified by tension gashesfilled by leucoosme in diatexite; (top-to-the-South) sense of shear.HGMC, Punta Scorno.


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the Variscan chain in northern Sardinia [10, 21, 22]. In fact,both in Asinara and in the Anglona region, regional HT/LPmineral associations overprint previous Barrovian metamor-phic assemblages observed by [19]. This is evident in thecentral part of the island, north of Mt. Castellaccio. This sectorbelongs to the LMGMC and is characterized by the abundant

growth of andalusite

±

cordierite in rocks of pelitic-psammiticcomposition. This mineral association appears of late crystalli-zation as it overprints and overgrowths the D3 crenulationmicrostructure and results from the combination of the stauro-lite- and garnet-consuming reactions (abbreviations after [55]):

–St + Qtz + And = Crd + H2O;–St + Grt + Qtz = Crd + H2O;–Grt + Ms + H2O = And + Bt + Qtz;–St + Ms + Qtz = And + Bt + H2O

as suggested by staurolite and/or garnet (pre-D2) relicswithin andalusite porphyroblasts (Fig. 20a).

Furthermore, in the central part of the island, evidence offibrolitization of biotite, probably due to base-cation leaching[56, 57], has been observed (Fig. 20b). The growth of fibroliteat the expense of biotite according to the reaction [57]:

–2K(Mg, Fe) AlSi3O10(OH) 2 + 14H+ = Al2SiO5+ 2K+ + 6(Mg, Fe) 2+ + 9H2O + 5SiO2 generatesfibrolite and quartz intergrowths as shown in Fig. 20b.

Andalusite sometimes encloses fibrolite and/or prismaticsillimanite of likely early crystallization.

In the HGMC, prismatic sillimanite pseudomorphs occurafter kyanite laths and appear as the so-called intermediatetextural arrangement of “disth-sillimanite” [58]. It consistsin the disappearence of the typical (100) cleavage of kyaniteand in the fragmentation of the original kyanite laths intoseveral parallel sillimanite crystals, separated by quartz rimsand showing perfect (010) cleavage (Fig. 20c). This kyanite/sillimanite transition may represents the first sillimanite iso-grad of several authors [13, 22, 23]. Furthermore, theHGMC is characterized by anatexis. In migmatites the mainmelt-producing reaction is inferred to be the:

Fig. 18 D4 fractures and kink axial planes distributions (Schmidt equalarea projection, lower hemisphere). Data contoured at 1, 4, 7, 10%.

Fig. 19 Attitude of magmatic lineation; Mt. Castellaccio and Periodo Secondo intrusions (Schmidt equal area projection, lower hemisphere). Datacontoured at 1, 4, 7 %.


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–Bt + Sil + Qtz + V = Melt + Crd;or possibly also the vapour-absent (V) equivalent:

–Bt + Sil + Qtz = Melt + Crd + Kfs,owing to the presence of cordierite either in leucosomes orin restites.

In places, sillimanite (both prismatic and fibrolitic) isclosely associated to biotite (Fig. 20b) and texturally occu-pies the site usually occupied by muscovite in normallepidoblastic intergrowths of the two micas withinmetapelites and metapsammites. Alkali-feldspar and silli-manite close to the biotite-fibrolite intergrowths, togetherwith relic primary muscovite, testify for the previous con-sumption of the latter, via reactions such as:

–Ms + Pl + Qtz + V = Melt + Silor

–Ms + Bt + Qtz + V = Melt + Sil(where at least part of the melt remained

in situ

),–Ms + Pl + Qtz = Melt + Sil + Kfs.Peritectic garnet-producing reactions were not observed.In places, the above picture is complicated by further reac-

tions which are apparently due to a high-strain concentrationand/or to fluid circulation. For example, in the central part of

the island the andalusite/sillimanite transition has beenobserved. These reactions appear to be tied to “strain induced”processes [58], as they are restricted to late shear zones wherealso quartz and micas show deformative effects.

From field data, available literature and petrografical evi-dence, it could be argued that the two metamorphiccomplexes of Asinara sustained different starting metamor-phic conditions before sharing an incomplete equilibrationunder low-P conditions. These low-P conditions are ofregional character appearing to be genetically unrelated tothe post-tectonic monzogranitic intrusions as well as to anyother type of localised external heath supply that was notobserved in the field. In the southern contact of the mainintrusion of Mt. Castellaccio the LP/HT mineral assem-blages do not occur, and only Barrovian assemblages arepreserved.

The HT/LP metamorphic evolution was syn- to post-kin-ematic with respect to post-collisional deformation (D3 andD4? phases) which gave rise to refolding and localization ofbrittle/ductile shear zones. It could be related to strongdecompression during the exhumative stages up to 3-4- Kbarof pressure as suggested by [42].

Fig. 20 a) Andalusite overgrowth on the staurolite relics in andalusite bearing micaschist. Nicol x; magnification 20x. b) Fibrolitization of biotite inandalusite and fibrolite bearing micaschists. LMGMC. Nicol x; magnification. 20x. c) Disthene-sillimanite showing subdivision of single kyanite blastinto several sillimanite crystals. Migmatitic gneiss. HGMC. Nicol x; magnification 20x.


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6. Discussion

Geological mapping and structural analysis of the Sardin-ian Variscan basement of the Asinara island, allowed todistinguish two different metamorphic complexes outcrop-ping respectively in the southern and central-northern part ofthe island. Both complexes were affected by four deforma-tion phases and by three metamorphic events. The southernportion shows the same tectonic and metamorphic evolutionas the basement outcropping in northern Nurra and is char-acterised by garnet and plagioclase paragneisses that,according to Franceschelli

et al.

[22], indicate a retrogrademetamorphism, with a thermal peak developed after thepressure peak, of respectively 6-7 kbar and 480-500°C [23,44]. In the middle and northern portions, on the contrary, thebasement is characterised by the presence of the stauroliteand by the destabilisation of biotite into fibrolite. The occur-rence of late-D2 fibrolitic sillimanite shows that thetemperature during the D2 deformation was higher in thiszone with respect to the southern portion. During the seconddeformation phase a Barrovian metamorphic regime wasestablished in a decompressive context [21, 22]. For whatconcerns the structural relations between the granitoid intru-sion and the country rocks, it has been observed that thegranitoid shows an oblique magmatic lineation, both withrespect to the principal lineation of the metamorphic rocksand to the main schistosity and does not record the deforma-tion events observed in the host rocks. This is in agreementwith a post-D2 age for the intrusion of the granitoids. Thepegmatite dykes, otherwise, show a complex intrusive his-tory, with some of them showing a foliation parallel to the S2foliation in the country rocks locally folded by the D3 defor-mation phase and others, crosscutting the main fabric of thebasement. This suggests that at least two main generations ofintrusive pegmatitic dykes have been intruded into the base-ment: a syn-tectonic and a post- tectonic group.

A prominent third deformation phase, developing uprightfolds, produced a spaced crenulation cleavage without blastesis.Even if the strain associated to this crenulation phase increasednorthward, it never gave rise to a penetrative foliation. Moreo-ver, in the F3 fold domains the attitude of the stretchinglineation L2 has not been perturbed. The change in trend andplunge of L2 lineation is not due to the F3 folding effect.

Relics of the first deformation phase have been recog-nised both in the southern and in the central part of theAsinara island. In the HGMC, some paleosome “resisters”embedded in the neosome part of the migmatitic complex ofdiatexites, show few relics of the D1 phase. The seconddeformation phase gives rise to the main fabric all over theisland. A general change in its attitude has been observed. Tothe south it strikes WNW, dips to the SSW and bears a sub-horizontal L2 stretching lineation. The change in dip of theS2 schistosity around the Mt. Castellaccio intrusion is attrib-uted to a kilometre-scale F3 fold.

In the middle and southern areas of the island A2 foldaxes are sub-parallel to the L2 mineral and stretching linea-

tion with a mean NW-SE trend. The peculiarity of the D2deformation in the Asinara island, compared to the other sec-tions of metamorphic rocks of NE Sardinia, is the rotation ofthe L2 stretching lineation from sub-horizontal and sub-par-allelel to the A2 axes to down-dip on the S2 schistosity in thenorthern part (Fig. 2, 2a). In sections parallel to L2 lineationand perpendicular to S2 foliation, different D2 shear senseindicators have been recognized in the two metamorphiccomplexes. In the LMGMC the D2 shear sense is top-to-theW and NW whereas in the orthogneiss of Punta Scorno(HGMC) it is top to SW, showing a “thrusting” motion.These structural and kinematic features suggest a spatial par-titioning of the D2 deformation.

The complex D2 deformation pattern could be explainedby a general steady-state ductile deformation, with pureshear and simple shear components acting simultaneously,in which the pure shear component is partitioned in space(Fig. 21). In fact, as suggested by theoretical consideration[60-62] and by similar field examples [50], the increasingcomponent of pure shear toward the north could be respon-sible for both the down-dip attitude of the L2 stretchinglineation and the top-to-SW thrusting observed in theHGMC. As a consequence, assuming a steady state strainframework, the complex spatial distribution of the D2 fab-ric could be explained by a partitioned transpressivedeformation kinematic history that affected the two meta-morphic complexes after the collision-related D1deformation.

Late (or post?) D2 high-grade shear zones in the diatexiticcomplex (HGMC) show a complex deformation pattern.Shear sense indicators [47] suggest an extensional shearmovement with a top-to-the-NE displacement. The normalcomponent of shear in the high-grade shear zones is relatedto the exhumation of the HGMC durings late- or post-D2times.

Moreover, the occurrence of F3 folds suggest that thebasement continued to undergo sub-horizontal shortening upto the final stages of collision. The D2 ductile transpressivedeformation, followed by the D3 contractional-relateddeformation at higher structural levels, testifies that theexhumation of the medium-grade rocks was mainly drivenby transpression/shortening. F3 folds could be produced bythe nearly N-S direction of shortening in the later evolutionof the D2-D3 transpressional system.

After the D3 deformation phase, the tectonic and meta-morphic evolution of Asinara island differs from the othersectors of the inner zone of the chain [13, 21, 22] because ofthe occurrence of a HT/LP mineralogical assemblage thatoverprints the previous Barrovian one within the LMGMC.This overprint has been interpreted as due to a strong decom-pression occurring during the exhumation phase [19].

The D4 deformation phase gave rise to kink folds, asym-metric folds with sometimes a reverse sense of movementand to micro-pull—apart structures (crosscutting andalusitecrystals) in central—southern part of the island and to foldswith subhorizontal axial planes in the HGMC. These struc-


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tures are linked to the post-collisional collapse of the chain,after the thickening stage.

Overprinting criteria at microscopic and mesoscopicscale show that extensional tectonics and HT/LP over-print occurred only in the final stages of the collisionalhistory.

7. Conclusions

Asinara island belongs to the inner zone of the Variscanbelt, that recorded a syn-collisional history controlled by asyn-thickening stage of deformation (D1) followed by anorogen-parallel D2 transpressive tectonics, in a decompres-

Fig. 21 Sketch of the D2 and late-D2 fabric of the Asinara basement. 1) trace of the S2 foliation; 2) attitude of the L2 stretching lineation.


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sive regime, that gave rise to the exhumation of medium-pressure metamorphic rocks. This is in agreement with thetectonic and metamorphic history described both in the inter-nal nappes and in the northern zone of the SardinianVariscan belt [9-11].

Exhumation continued up to higher structural levelswhere Barrovian metamorphic assemblages were over-printed by HT-LP minerals in an extensional setting andintruded by late Variscan granitoids. The field structuralinvestigations on D2 and late-D2 strain geometry on the Asi-nara basement indicate a partitioned monoclinic kinematicD2 history. This is characterized both by (1) a

rotation of the(L2) stretching lineation from parallel to strike to down-dipon the (S2) foliation during the same deformation event,with a deformation concentrated in high-strain zones sepa-rated by zones of prevailing folding, partially reworked bythe D3 folding, and (2) by an increasing component of pureshear responsible for change of the D2 sense of shear fromoblique top-to-WNW in southern and central Asinara, to areverse top-to-the South in the northern portion.

As suggested by some authors [62, 63] at regional scalethis kinematic history could be related to a post-collisionaltranspressive heterogeneous tectonic. This kinematicdescription is in agreement to similar tectonic and metamor-phic history described both in the internal nappes and in thenorthern zone of the Sardinian Variscan belt [9-11].

Acknowledgement

This research was supported by University of Pisa funds andMIUR Cofin 2001 (Resp. R. Carosi and G. Oggiano). We wishto thank “Ente Parco” for provided access to the Asinara island,“Azienda Demaniale” and “Corpo Forestale” for logistic helpduring the permanence on the island. Part of this work has beenrealized during the PhD thesis of D. Iacopini funded by Univer-sity of Pisa. We acknowledge P.C. Pertusati, R.A. Trouw andC.W. Passchier for interesting and stimulating discussion onsome critic aspects of the Asinara geology and C.W. Passchierfor stimulating observations in the field. We thank E. Campusfor provided access to the topographic maps. O. Vanderhaegheand E. Tavarnelli are thanked for constructive criticism.

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the structural evolution of the asinara island (nw sardinia, italy)

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