Liberi Piluso 2009

ABSTRACT

Ophiolites crop out discontinuously in the Northern CalabrianArc (Southern Italy). They consist of high pressure/low temperaturemetamorphic ophiolitic sequences of Late Jurassic-?Early Creta-ceous age, in which a metabasic and metaultramafic association isthe base of a complex metasedimentary cover ranging from pelagicto flyschoid type sediments. These ophiolitic sequences, interpretedas slices of oceanic lithosphere belonging to the Jurassic Tethysrealm, occupy an intermediate position in the northern CalabrianArc nappe pile, between the overlying Hercynian continental lithos-phere (Calabride nappe) and the underlying Apenninic carbonate units.

In the literature, these ophiolitic sequences are subdivided intoseveral tectonometamorphic units; some authors distinguishedbetween an upper slightly metamorphic ophiolitic unit and a lowerHP-metamorphic ophiolitic unit. This subdivision contrasts withnew petrological data and geothermobarometric modelling. In fact,the overall P-T evolution for several ophiolitic sequences from thenorthern Calabrian arc describes comparable paths, characterizedby a HP-LT metamorphism followed by retrogression under green-schist facies conditions. The metamorphic climax is calculated atpressures ranging between 0,9 and 1,1 GPa and a temperaturearound 380°C.

Moreover, structural analysis of rocks characterized by HP syn-metamorphic ductile deformation suggests that tectonic evolution isquite homogeneous and similar, although different degrees of defor-mation can be observed. The high-pressure mineral assemblagedefines a pervasive foliation developed during a compressive tectonicevent (D1) that transposed the earlier structures. A second tectonicevent (D2) which occurred during decompression at 0,4 GPa,produced millimetre to decametre scale asymmetric folds. Laterextensional brittle structures are responsible for final exhumation ofthe HP rocks.

The tectonometamorphic evolution of the ophiolitic sequencesof Northern Calabrian Arc is well explained in a context in which theoceanic-derived rocks underwent subduction and exhumation astectonic slices inside an accretionary wedge.

KEY WORDS: Ophiolites, Calabrian Arc, HP-LT metamorphism,Jurassic Tethys.

RIASSUNTO

Evoluzione tettonometamorfica delle sequenze ofioliticheaffioranti nell’arco Calabro settentrionale.

Nell’Arco Calabro settentrionale affiorano sequenze ofioliticheriferibili alla Tetide giurassica coinvolte nelle orogenesi meso-ceno-zoiche. L’assetto tettonico dell’Arco Calabro settentrionale è caratte-rizzato dall’impilamento di tre elementi tettonici principali, riferibilia diversi domini paleogeografici, che dal basso verso l’alto sono: leunità Appenniniche, la falda Liguride, comprendente i terreni di de-rivazione oceanica, e la falda Calabride. Quest’ultimo rappresenta,nel quadro paleogeografico pre-alpino, un frammento di litosfera

continentale ercinica appartenente ad uno dei margini della TetideGiurassica. Le sequenze ofiolitiche, di età Giurassico superiore-?Cre-tacico inferiore, sono costituite da serpentiniti, basalti, rari gabbri eduna copertura sedimentaria caratterizzata da radiolariti, Calcari aCalpionella e sequenze calcareo-peliticoarenacee, caratterizzate daun metamorfismo di alta pressione/bassa temperatura.

Le evoluzioni tettonometamorfiche ricostruite per le diverse se-quenze ofiolitiche possono essere rappresentate attraverso dei per-corsi Pressione-Temperatura Deformazione che risultano pienamen-te confrontabili e sovrapponibili. Il P-T-D (fig. 5.1) può esseresuddiviso in tre fasi principali:

i) il segmento progrado segue un tipico gradiente geotermicoalpino (14-15°C/Km) fino al raggiungimento del climax metamorficostimato, in condizioni di pressione variabile tra 0.9-1.1 GPa ad unatemperatura di 380°C. La blastesi della paragenesi di alta pressioneavviene lungo una foliazione tettonica pervasiva S1, sviluppatasi inun contesto tettonico compressivo (fase deformativa D1);

ii) la fase di retrocessione può essere suddivisa in due segmenti:il primo è caratterizzato da una decompressione pressoché isotermafino a valori di pressioni di circa 0,4 GPa; il secondo segmento è ca-ratterizzato soprattutto dalla diminuzione delle temperature. Le pa-ragenesi mineralogiche relative alla retrocessione sono sincinemati-che con l’evento deformativo D2, durante il quale si sviluppanopieghe asimmetriche a diversa scala (F2);

iii) infine, l’esumazione finale delle rocce ed il raggiungimentodell’attuale assetto geologico avviene attraverso lo sviluppo di diver-se generazioni di strutture fragili legate agli eventi geodinamicidell’orogenesi appenninica e dell’apertura del bacino tirrenico.

L’evoluzione tettonometamorfica delle sequenze ofioliticheappartenenti al complesso Liguride della Calabria settentrionale,precedentemente suddivise in varie unità tettonometamorfiche, ènell’insieme comparabile sia sotto l’aspetto termobarometrico siaper quel che riguarda l’evoluzione tettonica. Le differenze nei valoridi massima pressione raggiunti durante il climax metamorfico, ilgradiente geotermico ed i pattern strutturali che caratterizzano lafase prograda e retrograda sono ben inquadrabili in un contesto geo-dinamico nel quale le rocce ofiolitiche studiate sono coinvolte inprocessi di subduzione e successiva esumazione come scaglie all’in-terno di un cuneo d’accrezione.

TERMINI CHIAVE: Ofioliti, Arco Calabro, Metamorfismo diHP-LT, Tetide Giurassica.

INTRODUCTION

The Calabria-Peloritani Arc is an arc-shaped orogenicbelt located between the subducting Ionian plate and theTyrrhenian basin (fig. 1). Since the Lower Miocene, it wasinvolved in the Africa-verging Apenninic-Maghrebian oro-genic system (TORTORICI, 1982; BONARDI et alii, 2005),but its nature and tectonic position seems to be the con-sequence of a more complex history. In the literature, infact, the Calabria-Peloritani Arc, from the occurrence of acrystalline basement nappe and high pressure/low tem-perature metamorphic rocks, is interpreted as a fragmentof an earlier Alpine chain later involved in the Apenninicphase orogenesis («double-verging model»).

Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 128, No. 2 (2009), pp. 483-493, 6 figs., 2 tabs. (DOI: 10.3301/IJG.2009.128.2.483)

(*) Dipartimento di Scienze della Terra, Università della Calabria,Via P. Bucci, Cubo, 15b - 87036 Arcavacata di Rende (CS), telephone:+39 0984 493630, fax: +39 0984 493601. E-mail of correspondingauthor: [email protected].

Tectonometamorphic evolution of the ophiolitic sequencesfrom Northern Calabrian Arc

FRANCESCA LIBERI (*) & EUGENIO PILUSO

24 249-Liberi&Piluso 483-494 27-07-2009 9:18 Pagina 483

Nevertheless, the relationships with other Alpinechain segments is not clear due to the drifting of the Cala-brian Arc during the opening of the Tyrrhenian basin(ALVAREZ, 1974; ALVAREZ et alii, 1974; BOCCALETTI et alii,1984; BOUILLON, 1984; CARMINATI & DOGLIONI, 2004).However, a different geodynamic model (KNOTT, 1987,1994; DIETRICH, 1988; WALLIS et alii, 1993; THOMPSON,1998; ROSSETTI et alii, 2000, 2004) suggests a single-ver-gence Apenninic orogenic phase, associated with a widesyn-convergence extension.

In this framework, the study of ophiolites has signifi-cant implications for testing the different geodynamicinterpretations. Ophiolite-bearing units cropping out inSouthern Italy between Cilento and the Catanzaro grabenhave been referred to as Liguride Complex by OGNIBEN

(1973). Nevertheless, the ophiolites cropping out withinthe Calabrian Arc and those exposed in the LucanianApennines occur in different lithostratigraphic and tec-tonic settings (BONARDI et alii, 2001 and quoted ref.).In fact, Lucanian ophiolite-bearing units consist ofOligocene to Middle Miocene turbiditic successions andMiddle Jurassic to Eocene epimetamorphic and sedimen-tary successions which include volumes of oceanic andcontinental crust and upper mantle rocks, which are inpart or totally interpreted as tectonic melange (VEZZANI,

1969; SPADEA, 1982, 1994; BONARDI et alii, 1988, 2001,2004; MONACO & TORTORICI, 1995).

These units have been the subject of detailed struc-tural studies (KNOTT, 1988, 1994; MONACO & TORTORICI,1995) and the proposed geodynamic models, althoughdifferent, are framed in the Africa-verging Apenninic oro-genesis. Besides, these units recorded only the Apenninictectonic phase, and their relationship with the CalabriaHercynian basement rocks are not clear. On the otherside, ophiolitic sequences of Northern Calabria, for theirstructural position within the Catena Costiera and Silamassif nappe pile, just beneath the Hercynian basement,and for their marked metamorphic imprint, are the bestcandidates for showing possible inheritance of an Eo-Alpine stage. In this paper we present new petrologicaldata on ophiolite sequences from the Northern CalabrianArc in order to characterize the metamorphic evolution ofthe Upper and Lower Ophiolitic Units as defined first byDIETRICH & SCANDONE (1972).

GEOLOGICAL SETTING OF NORTHERN CALABRIAN ARC

In the Calabria-Peloritani Arc, pre-Mesozoic crys-talline basement rocks outcrop extensively.

484 F. LIBERI & E. PILUSO

Fig. 1 - Tectonic sketch map of the northern Calabrian Arc, simplified after AMODIO-MORELLI et alii (1976).– Schema tettonico dell’Arco Calabro settentrionale, semplificato da AMODIO-MORELLI et alii (1976).


By contrast, the Apenninic-Maghrebian orogenic sys-tem is characterized by thrust sheets of Meso-Cenozoicpassive margin sedimentary successions and Neogeneforedeep sediments. The boundaries between the Cala-bria-Peloritani Arc and the Apenninic domains aredefined by a fault systems with left-lateral strike slip com-ponent, known as Sangineto line, to the north and by theTaormina line to the south, where the Peloritan sectoroverthrusts the Maghrebide chain (fig. 1).

In the geological literature, the Calabrian Arc is sub-divided into two sectors, separated by the Mesimagraben. The Northern Calabrian Arc and Southern Cala-brian Arc are interpreted as two terranes, with differentpaleogeographic histories, accreted during Middle Miocene(BONARDI et alii, 2004). One main difference is the pre-sence of ophiolites only in the northern sector.

The Northern Calabria nappe pile can be described asconsisting of three major structural elements (fig. 1)derived from different palaeogeographic domains:

i) the lowermost Apenninic units, constituted byMeso-Cenozoic sedimentary and metasedimentary suc-cessions, derived from Miocene to early Pleistocene defor-mation of the innermost sector of the Apulia passive mar-gin (IANNACE et alii, 2007). An alternative interpretationenvisages that its original position may have been on theCalabride palaeomargin (PERRONE, 1996);

ii) the intermediate ophiolitic nappe represents rem-nants of Jurassic Tethys separating the Africa and Europeplates;

iii) the uppermost Calabride nappe consists of crys-talline basement rocks with Hercynian metamorphic andstructural imprint intruded by late-Hercynian magma-tism, on which a Mesozoic sedimentary cover wasdeposited. The Calabride nappe was subdivided into sev-eral tectonometamorphic units (AMODIO-MORELLI et alii,1976) but, more recently, it is interpreted as a nearlycontinuous Hercynian continental lithospheric section(GRAESSNER & SCHENK, 2001; PILUSO & MORTEN, 2004)with a Mesozoic syn-rift sedimentary cover (Santantonioand Teale, 1987). The Calabride nappe is therefore a con-tinental lithosphere fragment whose pre-Alpine paleogeo-graphic position is, however, still matter of debate:

a) it might be part of the Australpine domain piledup with the ophiolitic nappes during the Eo-Alpine stage;in this case the ophiolitic and continental crust nappesare considered to have been a Europe-verging Alpinechain segment overthrust onto the Appeninic units duringLower Miocene (HACCARD et alii, 1972; AMODIO-MORELLI

et alii, 1976; SCANDONE, 1982; BONARDI et alii, 1982);b) it might be part of the European palaeomargin

overthrust onto the Apenninic domain with an Africanpolarity from the Paleogene (OGNIBEN, 1973; BOUILLIN,1984; DIETRICH, 1988; KNOTT, 1987, 1994; WALLIS et alii,1993; MONACO & TORTORICI, 1995);

c) it might be part of a microcontinent placed betweenAfrica and Europe (GUERRERA et alii, 1993; CELLO et alii,1996).

The ophiolitic nappe has been subdivided into severaltectonometamorphic units by AMODIO-MORELLI et alii(1976, with references therein) (tab. 1). In particular, pet-rographic studies performed by different authors on theophiolitic units suggest differences in maximum pres-sures reached during metamorphic peak. These differ-ences are well explained by the tectonic scheme proposed

by DIETRICH & SCANDONE (1972) and SPADEA (1976,1994): in these models the tectonometamorphic ophioliteunits distinguished by AMODIO-MORELLI et alii (1976) arearranged into an Upper Ophiolitic Unit, characterized byprehnite-pumpellyite to lawsonite-albite facies paragene-ses (DE ROEVER, 1972), a Lower Ophiolitic Unit, charac-terized by blueschist (often retrogressed to greenschist)facies assemblages (CELLO et alii, 1991), and the lower-most Frido unit (tab. 1).

In fact, the lowermost tectonic rocks, which areexposed in a few tectonic windows of Catena Costiera andSila Piccola, are correlated with the Frido unit, whichcrops out extensively in the Lucanian Apennines (VEZ-ZANI, 1969). However, on the basis of petrological andbiostratigraphic data, this correlation needs to be revised(BONARDI et alii, 1993; SPADEA, 1994).

Recently, ROSSETTI et alii (2001, 2004) proposed arevised nappe structure for the Northern Calabrian Arc.In their tectonic model, an unmetamorphosed or slightlymetamorphosed Upper Plate Complex overlies ablueschist facies Lower Plate Complex, constituted by anEocene-Oligocene Africa-verging accretionary wedge. TheLower Plate Complex was exhumed through a lateOligocene-middle Miocene extensional detachment faultsystem with top-to-the-west sense of shear that marks thecontact between the Upper and Lower Complexes. If sup-ported by future work, this interpretation represents asignificant revision of the Northern Calabrian Arc nappearchitecture, because some of the previously recognizedstructural relationships are completely reversed.

GEOLOGICAL OUTLINE OF THE OPHIOLITES

Ophiolitic sequences are widely exposed in central andnorthern Catena Costiera, where they constitute a thicknappe (figs. 1 and 2), as evident from data achieved fromthe Cosenza-Paola railway tunnel (RIZZO & BOZZO, 1998).In fact, the ophiolites disappear to the south below theCalabride nappe and re-appear again in the Sila Piccola.

TECTONOMETAMORPHIC EVOLUTION OF THE OPHIOLITIC SEQUENCES 485

TABLE 1

Summary of tectonic subdivision proposed for ophiolites from northern Calabria.

– Confronto tra le suddivisioni tettoniche proposte in lette-ratura per le ofioliti dell’Arco Calabro settentrionale.


Ophiolites from the Northern Calabrian Arc appearquite heterogeneous, as pointed out by the subdivisionsproposed by AMODIO-MORELLI et alii (1976). In fact, thedifferent ophiolitic sequences appear macroscopicallyquite different as to the metamorphic mineralogicalassemblages, the degree of deformation, the structures ofigneous protolith and the nature and characteristics oftheir original sedimentary cover.

Nevertheless, the major stratigraphical features of theophiolites can be reconstructed on the basis of the obser-vations collected from the different tectonic units (fig. 3).The primary stratigraphy of the ophiolites may be sum-marized as follows (from bottom to top):

– the oceanic basement consists of mantle-derivedultramafics and a meta volcanic complex. The mantle ultra-mafics, cropping out only in the Gimigliano and MonteReventino area, are serpentinites from a lherzolitic-harzbur-gitic protolith, capped by ophicarbonate rocks. Volcaniccomplex consists of major metabasalts, metadolerites andmetahyaloclastites. Metabasites may be either weaklydeformed and preserving the pillow structures, or showingvarious degree of deformation up to strongly foliated. Geo-chemical data indicate that the metabasites derive fromsubalkaline basalts with tholeiitic affinity (T-MORB type);no remarkable or systematic geochemical differencesamong metabasites from the various outcrops are evident(SPADEA, 1994; BECCALUVA et alii, 1982; LIBERI et alii, 2006);

– a complex metasedimentary cover, which includespelagic to flyschoid type sediments.

In the southernmost and more complete ophioliticsequence of Gimigliano the cover consists of poly-chrome schists, marbles and alternating metapelites,matacarbonates and metarenites. Toward the north,this sequence is replaced by meta-radiolarites andcalcschists. The calcschists, from their age and thelithological characters, are correlated with the Calpio-nella Limestone Fm. of the northern Apennines. Thislithology, dated as Early Cretaceous (LANZAFAME &ZUFFA, 1976), provides the only biostratigraphic ageavailable.

Stratigraphic relationships between the northern andsouthern sequences are not clear.

In the central sector of the Catena Costiera, alternat-ing metapelites, metacarbonates and metarenites rest ontop of the ophiolitic basement, and LANZAFAME & ZUFFA

(1976) report interfingering contacts between thissequence and the calcschists.

PETROLOGY

In order to obtain new petrologic data on the meta-morphic evolution, new sampling of the most representa-tive ophiolitic sequence has been carried out.


Fig. 2 - a) Panoramic view looking north of the northern Catena Costiera (Malvito area) where ophiolites are widely exposed and constitute acontinuous and thick nappe sheet overlain by the Calabride nappe; b) Detail of the Malvito sequence: from top to bottom left are visiblecalcschists, metaradiolarites and c) pillow metabasalts.– a) Visione panoramica della Catena Costiera settentrionale nell’area di Malvito. In questa zona le ofioliti affiorano estesamente e costituisconoun elemento strutturale spesso e continuo al di sotto della falda Calabride; b) Particolare della sequenza ofiolitica di Malvito: procedendo versol’angolo in basso a sinistra si attraversano i calcescisti, le metaradiolariti e c) i matabasalti a pillow.


In the Gimigliano ultramafics, the protogranular toporphyroclastic structure of the harzburgitic protolith isstill recognizable, in spite of a severe serpentinization.

Serpentine after olivine shows the typical mesh struc-ture and the bastitic serpentine has replaced the originalorthopyroxene. A few relics of a brown spinel are preservedas both small idiomorphic grains and large interstitialgrains almost completely transformed to magnetite. Themineral assemblage is composed of antigorite + lizardite +magnetite.

Ophicarbonate rocks show fabrics varying from brec-ciated to mylonitic. The latter is characterized by a perva-sive foliation defined by crystallization of tremolite and talc.The mineral assemblage is composed of antigorite + tremo-lite + calcite + chlorite + talc + magnetite.

The foliated, and often banded, metabasalts are char-acterized by the assemblage Na-amphibole + pistacite+ clinozoisite + brown epidote + albite + phengite + acti-nolite + chlorite + titanite + prehnite + calcite + ma-gnetite. The rare Na-amphibole has a crossitic composi-tion (tab. 2). The metadoleritic dykes are fine grained andshow ophitic to subophitic structures. Their mineralassemblage consists of: augitic pyroxene + albite + clino-zoisite + phengite + titanite + prehnite + calcite. Smallvolumes of epidosites, most likely derived from formerveins, occur as rods and ribbons of pistacite and the asso-ciation of pumpellyite + prehnite + calcite.

In the metasedimentary cover, the mm- to dm-thickalternating layers of metapelite and meta-arenite consistof: phengite + paragonite + magnetite and albite + quartz+ clinozoisite + chloritoid + phengite + calcite + tourmaline,respectively. The carbonatic layers consist of calcite andsubordinate phengite + clinozoisite + albite + magnetite.

The chemical composition of phengites from Gimi-gliano indicates the presence of two generations of phen-gites having different compositions (SiIV max=3.5 andSiIV max=3.32) (tab. 2), defining two different microstruc-tural positions.

In the Malvito, S. Agata di Esaro and Rose outcrops,porphyritic and aphyric pillow-lavas can be observed. Themagmatic plagioclase phenocrysts are pseudomorphosed

by a fine-grained aggregate of albite, lawsonite, clino-zoisite and rare phengite. The mineral assemblage of thegroundmass consists of lawsonite + albite + brown epi-dote + chlorite + prehnite + pumpellyite + titanite + phen-gite + calcite.

In the Rose-Fuscaldo transect, already studied by DE

ROEVER (1972), metabasalts and metahyaloclastites arewidely exposed. The magmatic porphyritic structure isstill recognizable in metabasalts, in spite of the deforma-tion that stretched the original phenocrysts according tothe main foliation. Petrographic and petrological cahar-acteristics are comparable with those of the Malvitometabasalts.

The calcschists show the mineral assemblage: calcite+ quartz + albite + lawsonite + phengite + paragonite+ titanite.

The Diamante-Terranova metabasites are glauco-phane-bearing. In the San Lorenzo del Vallo outcrops,strongly deformed pillows with porphyritic structure,pillow breccias and massive fine-grained mafic rocksare exposed. The deformed pillows are slightly foliated,with the mineralogical assemblage made of Na-amphi-bole + lawsonite + Na-pyroxene + augitic px + clinozoisite+ phengite + albite + actinolite + brown epidote + chlorite+ pumpellyite + calcite + magnetite. The Na-amphiboleranges in composition from glaucophane to Mg-riebeckite(tab. 2) and the Na-pyroxene is characterized by aegirinicto omphacitic compositions (tab. 2).

Metabasalts from the Diamante outcrop show welldeveloped foliation and a variety of textures: 1) bandedstructure given by alternating greenish and deep-bluebands, the blue bands have a porphyroblastic textureand consist of lawsonite + Na-amphibole + Na-pyroxene+ augitic pyroxene + clinozoisite + phengite1 + titanite,whereas the greenish bands are the greenschist-faciesretrogression products consisting of albite + actinolite+ phengite2 + pumpellyite + prehnite + pistacite + chlo-rite; 2) massive deep blue galaucophanites, the texture issimilar to the blue portions of the banded type, while ret-rogression is limited to veins consisting of brown epidoteand pumpellyite; 3) a small volume of medium- to coarse-


Fig. 3 - Sketch of the stratigraphical relationships in the ophiolitic sequence from different areas of the northern Calabrian Arc.– Schema dei rapporti stratigrafici esistenti all’interno delle sequenze ofiolitiche affioranti nelle varie zone dell’Arco Calabro settentrionale.



TABLE 2

Representative electron microprobe analyses of Na-amphiboles, Na-pyroxenes and phengites from different ophioliticsequences. Structural formulae were calculated on the basis of 23 O for Na-amphiboles, 6 O for Na-pyroxenes and

22 O for phengites.– Analisi rapressentative, ottenute tramite microsonda elettronica, di Na-anfiboli, Na-pirosseni e fengiti provenienti dallesequenze ofiolitiche. Le formule strutturali sono state ricalcolate sulla base di 23 O per Na-anfiboli, 6 O per Na-pirosseni e

22 O per fengiti.

Fig. 4 - P-T-D paths for the Gimigliano, Malvi-to-Rose and Diamante-S. Lorenzo del Valloophiolitic sequences. Clinozoisite + tremolite+ H2O=pumpellyite + chlorite + quartz (LIOUet alii, 1985); heulandite=lawsonite + quartz(NITSCH, 1971); laumontite=lawsonite + quartz(LIOU, 1971); wairackite=lawsonite + quartz(LIOU, 1971); lawsonite=clinozoisite + margarite+ quartz (NITSCH, 1978, 1972); glaucophane+ clinozoisite + quartz + H2O=tremolite + chlo-rite + albite (MARUYAMA et alii, 1986); aragoni-te=calcite (CARLSON, 1980); K-mica’s SiIV iso-pleths (MASSONNE, 1992, 1995); Jd isopleth byPX-NOM (STURM, 2002).– Diagramma Pressione-Temperatura-Deforma-zione ricostruito per le sequenze ofiolitiche diGimigliano, Malvito-Rose e Diamante-S. Lorenzodel Vallo.


grained rocks probably deriving from diabase dikes,which locally display relics of igneous texture withclinopyroxenes partly replaced by metamorphic aegirine-rich pyroxene, in turn rimmed by blue amphibole (CELLO

et alii, 1991). Lawsonite and albite occur as pseudo-morphs after plagioclase, blue amphibole after magmaticclinopyroxene, and titanite aggregates after magmaticilmenite or Ti-magnetite.

The mineral assemblage of this rock type is made ofaugite + Na-amphibole + lawsonite + albite + clinozoisite+ titanite; and 4) coarse grained epidotite lenses.

Na-amphibole compositions from the Diamante out-crop range from crossite to glaucophane (tab. 2) and theNa-pyroxenes from omphacite to aegirine-augite composi-tion (tab. 2). The SiIV max content of phengite is 3.5 (tab. 2).

The calcschists consist of calcite + quartz + albite+ lawsonite + phengite + paragonite + titanite ± aragonite.

P-T PATH

This new petrological data and the new thermobaro-metric estimates, based on conventional methods,allowed us to reconstruct in more details the metamor-phic evolution of northern Calabria ophiolites, whichappear to follow similar P-T trajectories (fig. 4).

The presence of chloritoid and the absence of biotiteare in agreement with this temperature estimate. Thehigh pressure paragenesis in metabasites consists ofcrossite/glaucophane ± clinopyroxene (Jd50) + lawsonite+ clinozoisite + sphene + phengite1 (SiIV max=3.5). Thepresence of aragonite relics in the calcschists agrees withthe inferred conditions of the metamorphic peak. Chlori-toid + phengite1 + paragonite + albite + quartz + clino-zoisite are the high pressure mineral assemblage in theflyschoid-type metasedimentary cover.

The metabasites experienced first an ocean floormetamorphism responsible for the formation of epidotite,calcite-filled veins and the pseudomorphic replacent of

plagioclase by pistacite, albite and calcite, and later theorogenic HP metamorphism.

The physical conditions for the metamorphic climaxhas been calculated on the basis of the jadeite content inclinopyroxene (Jd50) and the celadonitic content in phen-gite (SiIV=3.5), at about 1.0-1.1 GPa (as a minimum pres-sure) and 380°C (fig. 4). The HP metamorphic peak is fol-lowed by a nearly isothermal decompression at 0.4GPa-380°C, with destabilization of the blueschist facies(EVANS, 1990) into a greenschist facies association. Aphengite2, with SiIV max=3.3-3.2 and decussate geometry,developed during this stage. Crystallization of chlorite,pumpellyite and prehnite marks the transition to lower-greenschist facies conditions at lower pressure with sig-nificant temperature decrease (fig. 4).

DEFORMATION

Meso and microstructural analyses of the metabasitesand metasedimentary cover rocks of the ophioliticsequences have allowed us to reconstruct a polyphase,although unevenly distributed, deformation history com-mon to all ophiolitic sequences, although various grade ofdeformation can be observed. Our study focused on thesynmetamorphic deformation in order to correlate defor-mation and mineral growth events for reconstructing thePressure-Temperature-Deformation history. Two syn-metamorphic deformation events are distinguished, char-acterized by different structures. These structures wereoverprinted by brittle deformation features referred bothto the Apenninic orogenesis and to the opening of theTyrrhenian Sea that involved the Calabrian Arc from theEarly Miocene onwards (TORTORICI, 1982; BONARDI etalii, 2005).

D1, during this phase the oceanic basement rocks andtheir sedimentary covers underwent severe isoclinal fold-ing (F1), with development of an S1 axial plane foliationthat is the main structural feature (fig. 5). Relicts of iso-


Fig. 5 - Stereoplot (equal area, lower hemisphere) of: a) Rose sequence; b) Gimigliano sequence and c) Diamante sequence. Symbols: emptycircles: stretching lineation L1; full diamonds: pole to planar anisotropies S1; cross: F2 fold axes; filled circles: crenulation lineation relative toF2; triangle: pole to shear plane associated with reverse sense of shear of D2 phase in Gimigliano area.– Proiezione stereografica (equal area, emisfero inferiore) degli elementi strutturali che caratterizzano la sequenza ofiolitica di: a) Rose; b) Gimiglianoe c) Diamante. Simboli: poli della foliazione S1 (rombo nero); poli dei piani di taglio T2 (triangolo); assi F2 (croce); lineazione da crenulazione(pallino pieno); asse π (rombo vuoto); stretching lineation L1 (pallino vuoto).


clinal F1 fold hinges are visible with axial planes parallelto the foliation. S1 completely transposed previous struc-tures and is often underlined by compositional bandingin metabasites (fig. 6). This is visible at micro- and meso-scale and characterized by alternating levels with differ-ent grain sizes and mineralogical assemblages. As dis-cussed by PICCARETA & ZIRPOLI (1969) and ALVAREZ

(2005) for the Monte Reventino greenschists, the bandedstructure may be derived from metamorphism of basicvolcanic tuffs or severe deformation of pillow lavas. Inaddition, the transposition of the epidotite volumes, veinsand sediment layers, formed during the ocean floor stage,emphisizes the banded structure.

The high-pressure mineral assemblage occurs alongS1 foliation. In fact, in metabasalts the foliation ismarked by the preferred orientation of Na-amphibole,clinozoisite, lawsonite and phengite1 (SiIV max=3.5).Similarly, in the metasedimentary cover the S1 foliation isdefined by the preferred orientation of phengite1, chlori-toid and clinozoisite crystals. These elements allow us to

place the D1 deformation event at physical conditionsclose to the metamorphic climax.

In the less deformed ophiolites cropping out in theRose-Fuscaldo transect, central Catena Costiera, the mag-matic porphyritic structure is still recognizable inmetabasalts, in spite of the strong deformation thatstretched the igneous phenocrysts producing a well devel-oped E-W trending lineation (L1) (figs. 5a and 6a) (DE

ROEVER, 1976; CARRARA & ZUFFA, 1976; DIETRICH, 1976).The ophiolites exposed in the northern sector of CatenaCostiera between Fagnano Castello, Malvito (fig. 2) andS. Agata di Esaro, appear weakly deformed during the D1

event and high pressure paragenesis developed staticallyon the magmatic minerals.

– D2, during this event the previous foliation S1 isaffected by an F2 asymmetric folding locally associatedwith mesoscopic reverse shear planes (fig. 6b). It isresponsible for the formation of folds at different scale,which include S- and Z parasitic folds distributed alongthe major fold limbs. In the phyllosilicate-rich portion,


Fig. 6 - a) Thin section of metabasalt from Rose showing original plagioclase phenocrysts, completely recristallyzed under HP-LT metamorphicconditions, strongly deformed and stretched, and crenulated by D2. The iso-orientation defines a well developed E-W trending lineation (L1)(fig. 5a); b) Thin section of metabasalt from Gimigliano showing the S1 foliation crenulated by F2, with development of the S2 foliation onlyin the phengite rich levels; c) F2 mesoscopic folds deforming S1 in metabasalts from Diamante. Compositional banding parallel to S1 is welldeveloped; d) Thin section of metabasalt from Diamante showing the crenulation F2 of S1 anisotropy marked by iso-orientation of Na-amphibole,lawsonite, phengite1 and titanite crystals.– a) Sezione sottile di metabasalto affiorante a Rose, in cui si osservano i fenocristalli di plagioclasio relitto interessati da intenso stiramento esuccessiva crenulazione F2. L’isorientazione dei fenocristalli relitti definisce una stretching lineation L1 orientata E-W (fig. 5a); b) Sezione sottiledi metabasalto affiorante a Gimigliano, in cui si osserva la crenulazione F2 che interessa la foliazione S1 con lo sviluppo di un incipiente clivaggiodi crenulazione S2; c) Pieghe mesoscopiche F2 che deformano la foliazione S1 nei metabasalti di Diamante. Una struttura a bande parallela ad S1è ben visibile; d) Sezione sottile di metabasalto affiorante a Diamante, in cui si osserva la crenulazione F2 che deforma la foliazione S1.


particularly abundant in some lithotypes of the sedimen-tary cover, the development of a discontinuous and dis-crete crenulation cleavage S2 can be observed (figs. 6aand b). Fig. 5 shows attitudes of F2 axes and crenulationlineation L2 for the Gimigliano, Rose and Diamantesequences. A large-scale F2 structure is evident atGimigliano, with a SE plunging axis which results fromπ-diagram show to be consistent with the trend of minorF2 axes and L2 crenulation lineation. Dissection and rela-tive rotation of blocks at a large scale, caused by later tec-tonic events, may be partially responsible for differencesin the orientation of the main structural features.

At the microscale the crenulation of Na-amphibole,lawsonite and phengite1 layers in metabasalts and ofphyllosilicate-rich portions in metasediments may beobserved (figs. 6b and d). Moreover, a syn-kinematicphengite2 (SiIV max=3.30 a.p.f.u.) grew along the S2

crenulation cleavage, suggesting a lower pressure valuefor the D2 deformation phase with respect to D1.

CONCLUDING REMARKS

The northern Calabria Late Jurassic-?Early Creta-ceous ophiolitic suite constitutes a thick and continuousthrust sheet.

The sequence is incomplete from the lack of the gab-bro and a sheeted dike complex. In the sedimentarycover, a clear increase in terrigenous supply is noticedmoving southwards (fig. 3). Nevertheless, the stratigraphyof the sedimentary covers needs further investigation tounravel the age of the upper portions of the sequences,their correlations with the ophiolitic units of the Lucan-ian Apennines, and the architecture of the oceanic basin.

Regarding the tectonometamorphic evolution of Cala-brian ophiolites, the subdivision into an upper slightlymetamorphosed and a lower HP-metamorphic ophioliticunit is not supported by the new collected petrologicaldata provided in this paper. In fact, the Pressure-Temper-ature-Deformation paths of the ophiolites from differentoutcrops display comparable features and may be subdi-vided into two main portions (fig. 4):

i) the prograde portion follows a typical Alpine sub-duction geothermal gradient (14-15° C/Km) up to the meta-morphic climax, calculated as between 0.9 and 1.1 GPaand 380°C. The high-pressure mineral assemblage definesa pervasive foliation developed during a compressive tec-tonic event (D1) that transposed the primary magmatic andsedimentary structures. An E-W trending stretching lin-eation is associated with this deformation event;

ii) the retrogression path can in turn be subdividedinto two portions: the first one is characterized by anearly isothermal decompression to about 0.4 GPa and iscoeval with a second tectonic event (D2), as indicated by asyn-kinematic phengite2. This deformation event pro-duced millimetre to decametre scale NW-SE trendingasymmetric folds locally associated with mesoscopicreverse shear planes; the second portion of the path fol-lows the typical cooling trajectory.

In conclusion, the overall P-T evolution of the differ-ent ophiolitic suites from northern Calabria describescomparable P-T paths: the oceanic lithospheric sectionunderwent a HP metamorphism followed by retrogres-sion under greenschist facies conditions.

Structural analysis indicates a polyphase deformationhistory. The syn-metamorphic ductile deformation sug-gests that tectonic evolution is quite homogeneous andsimilar, in spite of the different degrees of deformationobserved. This is probably due to strain concentrationalong shear zones during subduction, which led to thepreservation of large flakes of weakly deformed sequences(such as Malvito sequence) during D1 phase. It is impor-tant to stress that D1 and D2 are peculiar to the ophioliticnappe and thus occurred before the nappe emplacement.The first deformation phase and the HP metamorphismgive evidence for the involvement of the oceanic lithos-phere in a subduction zone. The second deformationevent, which occurred at decreasing pressure conditions,is consistent with exhumation and accretion of HP oceanderived rocks.

Later brittle structures, induced by Apenninic orogen-esis and the opening of the Tyrrhenian Sea, involved theoverall nappe stack and may were responsible for finalexhumation of the HP rocks.

Differences in the peak pressure reached during meta-morphic climax, in the range of 0.2 GPa, are wellexplained in a context in which the ophiolitic sequencesunderwent subduction and exhumation as tectonic slicesinside an accretionary wedge.

The detected deformation and metamorphic featuresare well explained either within an Alpine east-dipping sub-duction, or within an Appeninic west-dipping subduction.

Nevertheless, shear sense indicators regarding ophio-lite accretion and nappe emplacement, provided by sev-eral papers (CELLO et alii, 1996; ROSSETTI et alii, 2001,2004; ALVAREZ, 2005 are the most recent), conflict witheach other. These data are framed in different paleotec-tonic models to corroborate either the «double-vergingmodel» and the single-verging Apenninic model. In thisdebate, however, the better constrained tectonic evolutionof Lucanian ophiolites, provided by KNOTT (1988, 1994)and MONACO & TORTORICI (1995), cannot be consideredas a definitive proof to unravel the sense of tectonic trans-port during accretion of the northern Calabria ophiolites.In fact, ophiolites exposed in the Lucanian Apenninesdid not record the same tectonometamorphic evolutiondescribed above and occur in a different lithostrati-graphic and tectonic setting.

Systematic dating of the described HP metamorphicevents may provide some constraints essential to make amore reliable reconstruction of the whole tectonic evolu-tion for paleotectonic models.

ACKNOWLEDGEMENTS

We dedicate this work to Prof. Lauro Morten. We tank R. Carampinfor his assistance during microprobe analyses. Constructive revi-sions by two anonymous reviewer are greatly appreciated. Thisresearch was supported by MIUR.

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Received 30 March 2008; revised version accepted 10 January 2009.


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