Per. Mineral. (2001), 70, 2, 147-192 http: //go. to/permin
An InternationalJournal of PERIOD!CO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY
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Evolution of the Ligurian Tethys: inference from petrology and geochemistry of the Ligurian Ophiolites
GIOVANNI B. PICCARD01*, ELISABETTA RAMPONE2, ANNA ROMAIRONE1, MARCO SCAMBELLURI1, RICCARDO TRIBUZI03 and CLAUDIO BERETTA1
' Dipartimento per lo Studio del Territorio e delle sue Risorse. Universita di Genova, Corso Europa 26, I- 16132 Genova, Italy 2 Dipartimento di Scienze Geologiche e Geotecnologie. Universita di Milano- Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy 3 Dipartimento di Scienze della Terra. Universita di Pavia, Via Ferrata 1, I-27 100 Pavia, Italy
Submitted, October 2000 -Accepted, March 2001
ABSTRACT. - Ophiolites exposed along the equilibrium with their clinopyroxenes indicates a Western Alpine - Northern Apennine (WA-NA) clear MORB affinity. Geochronological data on NA orogenic chain represent the oceanic lithosphere of ophiolitic gabbros yield ages of intrusion in the the Ligurian Tethys which separated, during Late range 185-160 Ma: Triassic ages of intrusion are Jurassic - Cretaceous times, the Europe and Adria documented for gabbroic rocks from the plates. W A-NA ophiolites show peculiar Montgenevre ophiolites (Western Alps) (212-192 or compositional, structural and stratigraphic 185 Ma). The intrusion ages of the ophiolitic characteristics: 1) mantle peridotites are dominantly gabbroic rocks are significantly older than the Late fertile, clinopyroxene(cpx)-rich lherzolites, while Jurassic (160-150 Ma) opening of the Ligurian depleted, cpx-poor peridotites are subordinate; 2) Tethys and the basaltic extrusion. gabbroic intrusives and basaltic volcanites have a Basaltic volcanites are widespread in the WA-NA MORB affinity; 3) gabbroic rocks were intruded into ophiolites: petrological and geochemical studies mantle peridotites. The Jurassic Ligurian Tethys was have provided clear evidence of their overall floored by a peridotite-gabbro basement, tholeiitic composition and MORB affinity. Zircon subsequently covered by extrusion of discontinuous U/Pb dating on acidic differentiates yield ages in the basaltic flows and by sedimentation of radiolarian range 160-150 Ma for the basaltic extrusion: these cherts, i.e. the oldest oceanic sediments. In the ages are consistent with the palaeontological ages of whole Ligurian Tethys the inception of the oceanic the radiolarian cherts (160-150 Ma). stage, that followed rifting and continental breakup, The External Liguride (EL) mantle peridotites are occurred during Late Jurassic. fertile spinel lherzolites and display a complete The Ligurian ophiolites (Voltri Massif of the recrystallization under spinel-facies conditions, that Ligurian Alps (LA) and Liguride Units of the NA) is interpreted as the stage of annealing are a representative sampling of the diversity of the recrystallization at the conditions of the regional oceanic lithosphere which floored the Jurassic geotherm, after accretion of the EL mantle section to Ligurian Tethys. the conductive lithosphere (i.e. isolation from the In the W A-NA ophiolites the gabbroic rocks convective asthenospheric mantle). Nd model ages occur as km-scale bodies intruded in mantle indicate Proterozoic times for the lithospheric peridotites. REE composition of computed liquids in accretion. The Internal Liguride (IL) mantle ultramafics are depleted peridotites, i.e. refractory residua after low-degree fractional melting on a * Corresponding author, E-mail: [email protected]. it MORB-type asthenospheric mantle source, 148 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TR!BUZIO and C. BERETIA
producing MORB-type melts. The IL peridotites decompressional partial melting of asthenospheric display a complete equilibrium recrystallization that mantle sources recorded by the IL residual is related to accretion of the IL residual mantle to the peridotites; 3) the post-Variscan Permian MORE conductive lithosphere, after partial melting. Nd derived gabbroic bodies, which were intruded into model ages indicate Permian times (275 Ma) for the the extending lithosphere of the Adria margin depletion event. The Erro-Tobbio (ET) mantle (Austroalpine Units of the Western Alps); 4) the peridotites of the Voltri Massif (LA) are spinel Triassic-Jurassic ophiolitic MORB-type gabbros, lherzolites, and represent refractory residua after intruded into the subcontinental mantle, which were variable degrees of incremental partial melting exposed at the sea-floor during Late Jurassic starting from a MORB-type asthenospheric mantle opening of the Ligurian Tethys. source: they show granular to tectonite-mylonite The Ligurian ophiolites represent, therefore, the fabrics, these latter occur in km-scale shear zones spatial association of: where plagioclase- and amphibole-facies - Proterozoic and Permian subcontinental assemblages were developed during deformation. lithospheric mantle peridotites, which are locally (as The Ligurian peridotites show records of a in the EL Units) linked to continental crust tectonic-metamorphic evolution, after the accretion granitoids and granulites; to the conductive subcontinental lithosphere, i.e. 1) - Triassic to Jurassic gabbroic rocks, intruded in development of km-scale shear zones, 2) partial the peridotites; reequilibration at plagioclase-facies and amphibole - Late Jurassic MORB volcanites, interlayered facies conditions, and 3) later sea-water interaction with radiolarian cherts. and partial serpentinization, which indicates their This peculiar association cannot be reconciled progressive upwelling from subcontinental with present-day mature oceanic lithosphere, where lithospheric depths to the ocean floor. Plagioclase the mantle peridotites and the associated gabbroic facies reequilibration developed at 273-313 Ma in basaltic crust are linked by a direct cogenetic the ET peridotites and 165 Ma in the EL peridotites relationship and are almost coeval. In addition, the (Sm-Nd systematics). These data indicate that the large exposure of mantle peridotites to the sea-floor, decompressional evolution of the lithospheric mantle and the long history of extensional upwelling of the Europe-Adria system was already active since recorded by peridotites agree with a geodynamic Late Carboniferous Permian times and continued evolution driven by the passive extension of the till the Late Jurassic ·opening of the Ligurian Tethys. Europe-Adria continental lithosphere. The passive Further evidence of the extensional decompressional extension of the lithosphere is the most suitable evolution of the Europe-Adria lithosphere in the geodynamic process to account for the tectonic Ligurian sector is given by the continental crust denudation at the sea-floor of large sectors of material (the gabbro-derived granulites): their subcontinental mantle, as deduced from analogue gabbroic protoliths were intruded during Lower geophysical modelling for mantle exhumation at Carboniferous - Upper Permian times (about 290 continent-ocean boundary. Structural, metamorphic Ma, Sr-Nd systematics), and underwent and magmatic features recorded by the Austroalpine decompressional retrogression from granulite to (Sesia-Lanzo) and Southalpine (lvrea-Verbano) amphibolite facies between Permian and Middle units (the marginal units of the future Adria plate) Triassic times. suggest that the lithosphere extension was Geological-structural knowledge on the Western asymmetric, with eastward dipping of the Alps indicates that the Europa-Adria system, detachment zones. following Variscan convergence, underwent Late The subduction history of mafic-ultramafic Palaeozoic onset of lithosphere extension through associations of the Western NA and WA ophiolites simple shear mechanisms along deep low-angle was accompanied by prograde reactions, detachment zones, evolving to asymmetric culminating in one main high pressure event. It continental rift and Late Jurassic oceanic opening. caused eclogitization (i.e. development of This may account for the partial melting under metamorphic assemblages characterized by the decompression of the asthenospheric mantle and the association of sodic clinopyroxene and almandine gabbroic intrusions. This post-Variscan evolution is rich garnet, in the absence of plagioclase) of mafic evidenced by: 1) the Late Carboniferous to Jurassic rocks and partial recrystallization and dewatering subsolidus decompressional evolution (spinel- to (i.e. formation of metamorphic olivine in plagioclase- to amphibole-facies transition to late equilibrium with antigorite, diopside, Ti-clinohumite oceanic serpentinization) recorded by the and fluids) of ultramafites, previously variably subcontinental lithospheric mantle sections of the hydrated (serpentinized) during the oceanic EL and ET peridotites; 2) the Permian evolution. The high pressure ultramafic rocks still Evolution of the Ligurian Tethys: inference from petrology and geochemistJy of the Ligurian Ophiolites 149
preserve oxygen isotope signatures of the oceanic e interpretata come lo stadia di equilibratura alle settings, indicating that the fluid recycled at the condizioni del locale gradiente geotermico, dopo eclogitic stage was the one incorporated during 1' accrezione del mantello delle EL alla litosfera exposure close to the oceanic floor. conduttiva (cioe l'isolamento dal mantello astenosferico convettivo). Eta modello del Nd
RIASSUNTO. - Le ofioliti che affiorano lungo la indicano tempi Proterozoici per 1' accrezione alla catena orogenica delle Alpi Occidentali (WA), delle litosfera. Le peridotiti delle Liguri Interne (IL) sono Alpi Liguri (LA) e dell' Appennino Settentrionale impoverite, e rappresentano residui refrattari da (NA) rappresentano la litosfera oceanica della Tetide ricollegare ad una fusione frazionata di basso grado Ligure, un bacino oceanico che sepan), durante il su una sorgente di mantello astenosferico di tipo Giurassico Superiore e il Cretaceo, le placche Europa MORB, che ha prodotto fusi MORB. Le peridotiti e Adria. Le ofioliti W A-NA mostrano peculiari IL mostrano una ricristallizzazione completa che e caratteri stiche composizionali, strutturali e riferita all' accrezione del mantello residuale IL all a stratigrafiche: 1) le peridotiti di mantello so no litosfera conduttiva, dopo la fusione parziale. Eta prevalentemente lherzoliti fertili, ricche in modello del Nd indicano tempi Permiani (275 Ma) clinopirosseno (cpx), mentre le peridotiti impoverite per 1' even to di fusione. Le peridotiti ET (Erro sono subordinate; 2) le rocce gabbriche intrusive e le Tobbio) del Massiccio di Voltri (LA) sono lherzoliti vulcaniti basaltiche hanno affinita MORB; 3) i gabbri a spinello, e rappresentano residui refrattari da sono intrusi nelle peridotiti di mantello. Il bacino ricollegare a variabili gradi di fusione parziale della Tetide Ligure Giurassica fu pavimentato da un incrementale, a partire da un sorgente di mantello basamento gabbro-peridotitico, successivamente astenosferico di tipo MORB: esse mostrano strutture ricoperto dall' effusione di colate basaltiche e tessiture sia granulari che tettonitiche-milonitiche, discontinue e dalla sedimentazione dei diaspri, cioe i queste ultime sono sviluppate lungo zone di shear di primi sedimenti oceanici. In tutta la Tetide Ligure, dimensioni chilometriche in cui si sviluppano l'inizio dell'apertura del bacino oceanico, che segul paragenesi a plagioclasio e ad anfibolo durante la stadi di rifting e di rottura continentale, avvenne deformazione. durante il Giurassico Superiore. Le peridotiti Liguri mostrano chiare evidenze Le ofioliti liguri (Massiccio di Voltri delle LA e di una evoluzione tettonico-metamorfica, dopo Unita Liguridi del NA) sono un campionamento la loro accrezione alla litosfera conduttiva rappresentativo delle variabilita della litosfera di tipo sottocontinentale, cioe: 1) lo sviluppo di zone di oceanico del bacino della Tetide Ligure. shear di dimensioni chilometriche; 2) la parziale Nelle ofioliti W A-NA le rocce gabbriche ricristallizzazione in facies a plagioclasio e ad affiorano come corpi di dimensioni chilometriche anfibolo; 3) la tardiva interazione con acqua di mare intrusi nelle peridotiti di mantello. La composizione e la parziale serpentinizzazione; questi processi in REE dei fusi calcolati all'equilibrio con i loro indicano la loro progressiva risalita da profondita di clinopirosseni indicano una chiara affinita MORB mantello litosferico sottocontinentale fino al fondo per i fusi capostipiti. Dati geocronologici sui gabbri oceanico della Tetide Ligure. La ricristallizzazione a ofiolitici NA danno eta di intrusione nell'intervallo plagioclasio si sviluppo a 273-313 Ma nelle 185- 160 Ma: eta Triassiche di intrusione sono peridotiti ET e a 165 Ma nelle peridotiti EL documentate nei gabbri delle ofioliti W A di (sistematica Sm-Nd). Questi dati indicano che Monginevro (212-192 o 185 Ma). Le eta di 1' evoluzione decompressionale del mantello intrusione dei gabbri ofiolitici sono litosferico del sistema Europa-Adria era gia attiva da significativamente piu' vecchie rispetto all' apertura tempi Tardo Carboniferi - Permiani e prosegul fino al Giurassico Superiore (160-150 Ma) della Tetide all'apertura della Tetide Ligure (Giurassico Ligure ed alle effusioni basaltiche. Superiore). Ulteriori evidenze dell'evoluzione Le vulcaniti basaltiche sono diffuse nelle ofioliti estensionale decompressionale della litosfera del W A-NA : le attuali conoscenze petrologiche e sistema Europa-Adria nel settore Ligure sono fornite geochimiche indicano composizioni tholeiitiche ed dal materiale di crosta continentale (le granuliti di affinita MORB . Datazioni U/Pb su zirconi di derivazione gabbrica): i loro protoliti gabbrici differenziati acidi hanno fornito eta nell'intervallo furono intrusi durante il Carbonifero-Permiano 160-150 Ma per le effusioni basaltiche: queste eta (circa 290 Ma, sistematiche Sr e Nd), e subirono, sono in buon accordo con le eta paleontologiche dei dopo una ricristallizzazione in facies granulitica, una diaspri. retrogressione decompressionale da facies Le peridotiti di mantello delle Liguridi Esterne granulitica a facies anfibolitica fra il Permiano e il (EL) sono lherzoliti fertili a spinello e mostrano una Triassico Medio. completa ricristallizzazione in facies a spinello, che Le conoscenze geologico-strutturali sulle Alpi 150 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
Occidentali indicano che il sistema Europa-Adria, L' estensione passiva della litosfera e il processo dopo la convergenza V arisica, inizio a subire, dal geodinamico piu' idoneo per giustificare il Tardo Paleozoico, un processo di estensione della denudamento tettonico sui fondo oceanico di larghi litosfera attraverso meccanismi di shear semplice settori di mantello sottocontinentale, come e stato lungo zone profonde di faglie a basso angolo, che dedotto anche da modelli geofisici analogici per evolvette in un r{fting continentale asimmetrico ed l' esumazione del mantello lungo i limiti oceano infine nell' apertura Tardo Giurassica del bacino continente. Le caratteristiche geologico-strutturali, oceanico. Questo processo rende anche conto della metamorfiche e magmatiche registrate dalle Unita fusione parziale sotto decompressione della Austroalpine (Sesia-Lanzo) e Sudalpine (Ivrea sottostante astenosfera, della produzione di fusi Verbano) (cioe le unita marginali dell a futura placca MORB e della loro intrusione nella soprastante Adria) suggeriscono che l'estensione della litosfera litosfera in estensione sotto forma di corpi gabbrici. fu asimmetrica, con l'approfondimento verso est Questa evoluzione post-V arisica e mess a in evidenza della zona di faglie a basso angolo. da: 1) l' evoluzione decompressionale di subsolidus La storia di subduzione delle rocce femiche ed (transizione da facies a spinello a facies a ultrafemiche delle ofioliti W A e LA fu plagioclasio e ad anfibolo, fino alla tardiva accompagnata da reazioni prograde, che serpentinizzazione oceanica), dal Carbonifiero culminarono in un principale evento di alta Superiore al Giurassico, registrata dal mantello pressione. Esso produsse la eclogitizzazione della litosferico sottocontinentale rappresentato dalle rocce femiche (sviluppo di paragenesi caratterizzate peridotiti EL e ET; 2) la fu sione parziale dall' associazione di pirosseno sodico e grana to decompressionale Permiana di sorgenti di mantello almandinico, in assenza di plagioclasio) e astenosferico, che e registrata dalla peridotiti delle ricristallizzazione, con parziale disidratazione IL; 3) le intrusioni gabbriche post-Varisiche (sviluppo di paragenesi caratterizzate da olivina, Permiane derivanti da fusi astenosferici ad affinita antigorite, diopside, Ti-clinohumite e fluidi), delle MORB, che furono intruse nella litosfera in peridotiti di mantello, che erano state variamente estensione del margine Adria (le Unita Austroalpine idratate (serpentinizzate) durante 1' evoluzione delle Alpi Occidentali); 4) le intrusioni gabbriche, ad oceanica. Queste rocce ultrafemiche di alta pressione affinita MORB che intrusero, dal Triassico al ancora conservano nell' impronta isotopica Giurassico, il mantello litosferico sottocontinentale dell' Ossigeno il segno dell' evoluzione oceanica, ad successivamente denudato ed esposto sul fondo indicare che i fluidi riciclati durante lo stadio marino durante I' apertura Giurassico Superiore della Tetide Ligure. eclogitico sono quelli incorporati durante Le ofioliti liguri rappresentano, quindi, l'esposizione sul fondo oceanico. 1' associazione spaziale di: 1) peridotiti di mantello lito sferico KEY WoRDS: Ligurian Teth ys, rifting, oceanization, sottocontinentale, accrete alia litosfera nel subduction, Ligurian ophiolites, mantle Proterozoico e nel Permiano, che sono ancora peridotites, serpentinites, eclogites. associate (come nelle Unita EL) a materiale di crosta continentale, granitoidi e granuliti derivanti da gabbri Permiani; 2) gabbri intrusi nelle peridotiti da tempi Triassici INTRODUCTION a Giurassici; 3) vulcaniti basaltiche ad affinita MORB, Ophioli tes exposed along the Western interstratificate con i primi sedimenti (diaspri a Alpine-Northern Apennine (and Corsica) (WA radiolari) del bacino oceanico. NA) orogenic system are thought to represent Questa associazione peculiare non puo essere confrontata con l'attuale litosfera oceanica di oceani the oceanic lithosphere of the Ligurian Tethys, which separated, during Late Jurassic maturi, dove le peridotiti di mantello e I' associata crosta oceanica, gabbrica e basaltica, sono legate da Cretaceous times, the Europe and Adria dirette relazioni cogenetiche e sono coeve. Inoltre, la continental blocks. grande esposizione di peridotiti di mantello sui In the last decades, numerous contributions fondo del bacino, e la lunga storia di risalita in concerning the WA-NA ophiolites evidenced regime estensionale registrata dalle peridotiti, bene si inquadrano in un modello di evoluzione that: geodinamica guidata dall'estensione passiva della i) rather fertile, cpx-rich lherzolites are litosfera continentale del sistema Europa-Adria. dominant, while depleted, cpx-poor peridotites Evolution of the Ligurian Tethys: inference.fl·om petrology and geochemistry of the Ligurian Ophiolites 151 are scarce (Bezzi and Piccardo, 1971a; Nicolas P ALAEOGEOGRAPHY OF and Jackson, 1972); THE LIGURIAN OPHIOLITES ii) both gabbroic intrusives and basaltic volcanites have MORB affinity (Serri, 1980; The Ligurian Tethys is believed to have Beccaluva et al., 1980). developed by progressive divergence of the WA-NA ophiolites show peculiar structural Europe and Adria blocks, in connection with petrographic features, which indicate that: the pre-Jurassic rifting and Late Jurassic i) mantle rocks underwent a composite opening of the Northern Atlantic (Dewey et al., subsolidus evolution after depletion by partial 1973; Lemoine et al., 1987), (fig. 1). melting; Plate tectonic and palinspastic ii) gabbroic rocks were intruded into mantle reconstructions suggest that this basin was peridoti tes; limited in size and that it never reached the iii) peridotites and gabbroic intrusions were exposed at the sea-floor prior to extrusion of pillowed basalts and deposition of radiolarian cherts. NORTH Moreover, sheeted dyke complexes are AMERICA lacking and comagmatic relations did not exist between the gabbro bodies and the basaltic dykes and flows. Accordingly, a general consensus exists on the idea that the Jurassic Ligurian Tethys was floored by an older peridotite-gabbro basement (Decandia and Elter, 1969; Piccardo, 1983; Lemoine et al., 1987), subsequently covered by extrusion of a discontinuous layer of younger SOUTH pillowed basaltic flows and by deposition of AMERICA radiolarian cherts, i.e. the first oceanic sedimentation. The radiolarian cherts, which are coeval to the basaltic extrusions, are not older than Late EUROPE Jurassic (160-150 Ma) (De Wever and Caby, 19 81; Marcucci and Passerini, 1991) in the whole Ligurian Tethys: accordingly, the inception of the oceanic stage, following the continental breakup, is considered not older than Late Jurassic. The time gap between most of the gabbro intrusions (Triassic to Middle Jurassic) (Bigazzi et al., 1973; Carpena and Caby, 1984) and the basalt extrusion I chert deposition (Late Jurassic) (see also the following discussion) SOUTH LATE JURASSIC, reinforces the structural evidence of the AMERICA EARLY CRETACEOUS decoupling between the early pre-oceanic intrusion (i.e. the gabbroic bodies) and the late Fig. 1 - Mesozoic evolution of Central Atlantic and oceanic extrusion (i.e. the pillowed basalts) of Ligurian Tethys oceans, from rifting to ocean formation MORB-type magmas. (redrawn and modified after Lemoine et al., 1987). 152 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
ophiolites (Erro-Tobbio in fig. 2), which underwent deep evolution in the subduction zone and recrystallized at eclogite facies conditions, were located west of the subduction zone, close to the European margin; the NA ophiolites (Internal and External Ligurides in fig. 2), which underwent solely low-grade oceanic and orogenic metamorphism, were located east of the subduction zone, closer to the Adria margin (fig. 2).
Erro Tobbio Internal Ligurides External Ligurides THE VOLTRI MASSIF THE LIGURIAN TETHYS DURING LATE JURASSIC - CRETACEOUS Introduction to the geology of the Voltri Massif Fig. 2 - Generalized paleogeographic restoration of Upper Jurassic Ligurian Tethys, with location of the different Ligurian domains (redrawn and modified after Dal Piaz, The Voltri Massif (fig. 3) occupies the 1995). southeastern end of the W A and is structurally ascribed to the Internal Pennidic Units of the Alps. Its tectonic units record a widespread dimensions of present-day oceans. In addition, recrystallization at eclogite-facies conditions as age data indicate a narrow time span between the result of subduction zone metamorphism. cessation of divergence and the onset of The V oltri Massif is prevalently composed of convergence and subduction. Oceanic ophiolitic materials, associated with slices of accretion in the Ligurian Tethys started during continental crystalline rocks of the European Late Jurassic and continued for approximately margin (Chiesa et al., 1975; Messiga et al., 25 Ma (cf. Winterer and Bosellini, 1981). Plate 1992). To the east it is separated from the convergence leading to subduction of oceanic Ligurian Apennines by the Sestri-Voltaggio lithosphere started during the Earl y zone, a composite terrain consisting of Cretaceous, about 25 Ma after cessation of the Triassic-Liassic carbonatic rocks and of oceanic spreading (Hunziker, 1974 ). The dismembered Mesozoic ophiolites showing subduction zone had a south-west trending, peak blueschists metamorphic assemblages with the Europe plate underthrusting the Adria related to the alpine subduction. Klippen of plate, and it was, most probably, intra analogous blueschist terrains (Cravasco continental in the northern sector of the W A Voltaggio-Montenotte units) also overlie the and progressively intra-oceanic towards the eclogitic terrains of the Voltri Massif. To the Ligurian sector of the basin. west the Voltri Massif overlies the Savona The Ligurian Tethys was completely closed continental basement of Hercynian gneisses in the Early Tertiary, when fragments of its (granitoids with associated amphibolites, oceanic lithosphere were emplaced as west showing a greenschist to blueschist facies vergent thrust units in the Alps and east Alpine metamorphism). Towards the north it is vergent thrust units in the Apennine. bounded by Tertiary sediments of the Piemonte Depending on their stratigraphic, structural Basin, whose basal breccias include clasts of and metamorphic characteristics, the different high-pressure metaophiolites (metagabbros and ophiolitic sequences of the Ligurian sector metaperidotites) and thereby give a 35-38 Ma have been ascribed to different age for the exhumation and erosion of the high palaeogeographic settings in the Jurassic pressure terrains of the Voltri Massif. Cretaceous Ligurian Tethys. The Voltri Massif Several studies (Chiesa et al., 1975; Messiga Evolution of the Ligurian Te thys: inference fr om petrology and geochemistry of the Ligurian Ophio!ites 153
in the lithologies of the Voltri Massif, and are related to lithospheric-scale tectonic processes: 1) pre-oceanic rifting; 2) oceanic evolution; 3) Cretaceous subduction and pre-Oligocene exhumation. The Voltri Massif includes the following lithologic and tectonic units (fig. 3): 1) the Beigua Unit consists of antigorite serpentinites and eclogitic metagabbros, which derived from previous mantle peridotites and MORB-type gabbroic intrusions (dominant Fe Ti-rich gabbros); 2) the Voltri-Rossiglione, Ortiglieto and Alpicella Units consists of metavolcanics and calcschists, which derived from oceanic MORB-type basalts and their sedimentary cover: they locally preserve primary stratigraphic relationships with the ultramafic .-vM-, sv NA BTP and mafic rocks of the Beigua Unit;
3 4 DD5 6 7 3) the Erro-Tobbio Unit (ET) consists of partly recrystallized antigori te-bearing Fig. 3 - Geological sketch map of the Voltri Massif metaperidotites, preserving km-scale volumes showing location of the Erro-Tobbio peridotite (after of the lherzolite mantle protolith, and Chiesa et al., 1975). Major units as follows: I =Arenzano Crystalline Massif (SB =Gran San Bermu·do Nappe); VM associated rodingitic and eclogitic mafic rocks = Voltri Massif: 2 = Alpicella, Voltri-Rossiglione and (mostly after Mg-Al-rich olivine gabbros and Ortiglieto Units: metavolcanics and paraschists with relict subordinate basaltic dykes with MORB-type eclogitic assemblages; 3 = Beigua Unit: antigoritic affinity). serpentinites with eclogitic metagabbros; 4 =Erro-Tobbio Unit: mantle peridotites; 5 = Sestri Voltaggio Zone (SV); 6 As a whole, the ET peridotites are less = Northern Apennine Unit (NA); 7 = Tertiary molasses (BTP). intensely serpentinized than the ultramafic rock of the Beigua Unit. The latter displays about 90% serpentinization. The most important and Piccardo, 1974; Piccardo, 1977) have difference between the Beigua and Erro-Tobbio shown that in the Voltri Massif slices from tectonic units is the survival of composite different lithospheric levels (subcontinental mantle, oceanic and prograde, subduction mantle, Mesozoic oceanic lithosphere and related structures inside the Erro-Tobbio sediments, continental margin units) are peridotites. These structures are no longer tectonically coupled. preserved in the Beigua mafic-ultramafic The present-day geometric relationships association: it mostly records an eclogite-facies among the different tectonic units of the Voltri metamorphism, that affected the precursor Massif are the result of the collisional tectonic oceanic rocks, overprinted by retrograde under greenschist-facies conditions, which metamorphism and tectonics during caused late-stage folding of the high-pressure exhumation of the high pressure rocks towards nappes (post-nappe folding) and enhanced the surface. formation of shallow thrusts that masked the Preservation of such a long geological original nappe setting. In spite of such memory inside the ET peridotites is likely the grenschist overprint, the relics of previous results of less intense hydration and tectonic-metamorphic evolutions are preserved hydrothermal alteration during exposure close 154 G.B. PrCCARDO, E. RAMPONE, A. RoMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
to the floor of the Mesozoic Ligurian Tethys indicate further emplacement of more ocean. differentiated melts. Such intrusion episodes were followed by the emplacement of basaltic dikes at shallow Eclogitic metagabbros and high pressure depths. This stage was likely coeval with setpentinites of the Beigua Unit effusion of MORE basaltic lava flows that are presently associated with metasediments of the The Beigua unit mostly consists of V ol tri-Rossiglione, Ortiglieto and Alpicella serpentinites enclosing lenses of eclogites and Units. Exposure of the Eeigua gabbro-peridotite minor bodies of metasomatic rocks consisting association at the ocean f1oor was accompanied of metarodingite and Ti-clinohumite dykelets. by local metasomatic exchanges between Intrusive relations between eclogitized mafic ultramafic and mafic rocks. Metasomatic rocks rocks and host ultramafites are still locally of the Beigua Unit are represented by: preserved within the Beigua Unit. Generally, 1) metarodingi te boudins contmmng the eclogites form lenses several tens of meters garnet+diopside+ zoisi te/ clinozoisi te+chlorite+ long and boudins of smaller dimensions. magnetite±apatite; 2) brownish dykelets Previous petrological studies have shown that containing Ti-clinohumite+diopside+ the eclogites formed by metamorphism (at 13 magnetite+chlorite+apatite. These rocks have kbar pressure and 450-500°C) of original been described by Piccardo et al. ( 1980),
gabbroic material, represented by dominant Cimmino et al. ( 1981 ) , Scambelluri and iron- and titanium-rich varieties, subordinated Ram pone ( 1999), and interpreted as the result Mg-Al-rich gabbros and basalts, and rare of high-pressure metamorphism of gabbros diorites and plagiogranites (Mottana and which underwent pre-subduction Ca- and Bocchio, 1975; Ernst et al., 1982; Piccardo et metasomatism. al., 1979; Piccardo, 1984; Messiga and The classic garnet, pyroxene, rutile eclogites Scambelluri, 1991). of the Eeigua Unit mostly derive from original Based on major and trace element data Fe-Ti gabbros (superferrian eclogites; Mottana metavolcanites have been interpreted as and Eocchio, 1975), these rocks have been tholeiitic basalts with N-MORB affinity (Ernst thoroughly described and analyzed by a et al., 1982; Piccardo, 1984). Mafic intrusives number of papers (Ernst, 1976; Messiga, 1987; correspond to crystal cumulates at different Messiga and Scambelluri, 1988; 1991; Messiga fractionation steps from a common primary et al., 1995a). Since subduction deformation tholeiitic N-MORB magma. On the whole, the was localized within ductile shear zones, the different types of intrusives (M g-Al to Fe-Ti Eeigua eclogites can be subdivided in coronitic metagabbros) show flat REE patterns and mylonitic types. Relics of igneous minerals fractionated in LREE. Absolute REE (mostly clinopyroxene) are locally preserved in concentrations are 2-3 times x chondrite in Mg coronitic eclogites, where the high pressure Al rich rocks, and rise to > 100 times x assemblages develop either as pseudomorphic chondrite in the most differentiated Fe-Ti-rich replacements of the igneous minerals, or as types (Ernst et al., 1982; Morten et al., 1979; reaction coronas among the various igneous Piccardo, 1984). The intrusive relations and the mineral sites. The igneous augite is compositional variability discussed above, topotactically replaced by omphacite±fine indicate that upper mantle peridotites were grained garnet and rutile. Plagioclase is intruded by MORE melts. Low pressure pseudomorphed by fine-grained omphacite + crystallization and differentiation firstly garnet ± paragonite and epidote. The igneous produced olivine-p1agioclase cumulates and ilmenite is replaced by rutile locally associated Mg-Al rich gabbros; presence of Fe-Ti with pyroxene. Coronas of almandine-rich gabbros, rare diorites and plagiogranites, garnet develop along interfaces separating the Evolution of the Ligurian Te thys: inf'erencef/·om petrology and geochemist1y of the Ligurian Ophiolites 155
igneous minerals sites. The eclogitic minerals The post eclogitic evolution of the Beigua of coronitic metagabbros display eclogites consists of the complex superposition heterogeneous major and trace element of metamorphic events driven by interaction compositions, which reflect the compositional with fluids at decreasing pressure conditions. variability of the precursor igneous mineral, An early study by Ernst (1976) showed that the thereby reflecting global chemical eclogitic minerals were firstly overgrown by disequilibrium at hand sample scale. glaucophanic amphiboles, followed by Mylonitic and tectonitic eclogites consist of barroisite-bearing assemblages and finally by omphacite porphyroclasts in a finegrained actinolite+al bi te+chlorite greenschis t matrix of omphacite, garnet and rutile. Mylonitic pm·ageneses. This superposition of metamorphic omphacite is compositionally different from assemblages pointed to an adiabatic uplift of the porphyroclastic one, being richer in jadeite end deeply subducted eclogites. A later study by member (Messiga and Scambelluri, 1991 Messiga and Scambelluri ( 1991) pointed out the Messiga et al., 1995a). Rutile is strongly early development of amphibole+plagioclase± elongated parallel to the foliation and garnet diopside symplectites that predated displays inclusion-rich core and neoblastic rims. glaucophane formation. This stage was Transition from the garnet cores to their rims is interpreted as due to an initi al post-peak accompanied by an increase in pyrope and increase in temperature, followed by rock almandine components and decrease in grossular cooling and glaucophane formation as the result and spessartine. The cores of garnet crystals of deep underthrusting of cold nappes during from tectonitic and mylonitic eclogites, locally exhumation of the Beigua eclogites. still enclose inclusions of sodic clinopyroxene Development of amphibole + plagioclase + (Tribuzio, 1992), crossite, paragonite diopside symplectite was accompanied by representing relics of a prograde blueschists reequilibration and internal cycling of eclogitic facies matrix present in these rocks. The above fluid inclusions that catalized mineral reactions textural and compositional features were and kynetics (Vallis and Scambelluri, 1996). intepreted to result from blueschist to eclogite This process of internal redistribution of facies transition and from increasing pressure eclogitic fluids within these eclogite facies temperature conditions during eclogite-facies rocks continued at lower grades to produce recrystallization. greenschist facies parageneses (Scambelluri, Trace element analyses of minerals from the 1992). eclogites were carried out by Messiga et al. The serpentinites forming large part of the (1995a) and Tribuzio et al. (1996). These data Beigua Unit display an antigorite+chlorite+ show that the omphacite contribution to the magnetite assemblage overgrown by whole-rock REE inventory is negligible. In olivine+antigorite due to partial particular, the omphacites have a bell-shaped deserpentinization of the previous pm·agenesis (Cimmino et al., 1979). They also contain few REE pattern, which was related to the mineralogical relics of oceanic serpentine coexistence with garnet and accessory allanite, (mostly chrysotile) and rare mantle respectively incorporating almost all the LREE clinopyroxene (Piccardo et al., 1980). The and HREE of the rock (Tribuzio et al., 1996). olivine+antigorite-bearing assemblage has been The transformati on of original gabbroic shown to develop at eclogite-facies conditions protoliths into eclogites was not accompanied and represents the highest metamorphic grade by significant REE mobilization, and REE achieved in the area (Cimmino et al., 1981). were redistributed among newly formed minerals. This implies that the subduction of The Erro-Tobbio mantle peridotites mafic oceanic crust to 65 km depths did not release significant amounts of LREE (Tribuzio The ET mantle peridotite were involved in et al., 1996). the Alpine orogenesis, due to convergence of 156 G. B. PICCARDO, E. RAMPONE, A. RoMAIRONE, M. ScAMBELLURI, R. TRIBUZIO and C. BERETIA
CaO (wt0/o ) 5
Primordial Mantle 4 - • 4- Primordial Mantle 0 • 3 - 3 - 0�
- 2 ' 21- • 11 (ISp- Granular peridotites • .� 11 Sp-Tectonites Jti.. • 1 - <� 1 r- � Plg-Tectonites .AAbyssal peridotites 0 J I I I I o �--��-----._�----��--��-----��--� 35 37 39 41 43 45 47 35 37 39 41 43 45 47 MgO (wt0/o ) MgO (wt0/o )
Ti02 (wt0/o ) Se 0.3 20 (ppm)
Primordial Primordial Mantle Mantle 0.2 15 ,.... • • � 0.1 � 10 - 11 •• �-. •
s • A A .. 0 I I I I . I 5 I I I I I 35 37 39 41 43 45 47 35 37 39 41 43 45 47 MgO (wto/o) MgO (wto/o)
Fig. 4- Bulk rock variations of MgO (wt%) vs CaO, Al203, Ti02 (wt%) and Se (ppm) in the Erro-Tobbio peridotites (Romairone, 1999). Abbreviations: Sp-Granular peridotites = Granular spinel peridotites; Sp-Tectonites = Spinel peridotite tectonites; Plg-Tectonites = Plagioclase peridotite tectonites. Primordial mantle estimates are from Hofmann (1988). The representative compositions of abyssal peridotites are from Dick (1989). Evolution of the Ligurian Tethys: inference fr om petrology and geochemistry of the Ligurian Ophiolites 157 the European and Adriatic plates, but the fractionated in the LREE (CeN/SmN=0.05-0. ll) extreme localization of alpine deformation (fig. 5). Particularly, the granular spinel-facies along mylonite shear zones preserves km-scale peridotites, according to their major element volumes of coherent unaltered lherzolites chemistry, show bulk REE absolute which almost completely retain mantle textures concentrations covering a significant range (M and assemblages. This fact has allowed detailed to H-REE from 0.5 to 1.5xC l); bulk REE structural and petrological investigations of the composition of the deformed rocks generally magmatic, tectonic and metamorphic upper falls within the above compositional range. mantle evolution which predated their sea-floor Spinel-facies clinopyroxenes show, emplacement during the opening of the accordingly, low concentration of fusible Ligurian Tethys (Bezzi and Piccardo, 1971a; elements (AI, Ti, Sr, Zr, Y) and significant Ernst and Piccardo, 1979; Ottonello et al., depletion in LREE: their REE patterns are 1979; Piccardo et al., 1990,1992; Hoogerduijn almost flat from HREE to MREE (at 7-llxCl) Strating et al., 1990,1993; Vissers et al., 1991). and strongly fractionated in LREE (CeN/SmN =0.06-0.09). Moreover, the peridotites show increasing forsterite contents in olivine with The mantle protolith increasing Mg* = Mg/(Mg+Fe101) in the bulk The ET mantle peridotites consist of partly rock. serpentinized lherzolites, with variable All the above features indicate that the clinopyroxene contents, which commonly different samples represent refractory residua show spinel-bearing assemblage. Texturally, after variable degrees of partial melting starting they vary from granular types to highly from a compositionally homogeneous mantle deformed peridotite mylonites. Detailed field source: this event most probably predated the mapping (Hoogerduijn Strating, 1991) has complete equilibrium recrystallization under documented that the oldest structures preserved spinel-facies conditions. are represented by the spinel-facies The ET peridotites show rather assemblages granular textures. These latter are homogeneous Nd isotopic compositions overprinted by spinel-, to plagioclase-, to (143Ndfl44Nd=0.513183-0.513385), while the hornblende-bearing peridotite tectonites and Sr isotopic ratios are more variable mylonites forming composite km-scale shear (S7SrfS6Sr=0.703019-0.704769) (Romairone, zones (Vissers et al., 1991). 1999; 2000; Romairone et al., 2001) (fig.6). The ET peridotites exhibit heterogeneous Both I43Ndfl44Nd and I47Smfl44Nd (0.38-0.45) bulk-rock chemistry, varying from moderately ratios are higher than those typical of a MORE to significantly depleted compositions (Ernst upper mantle, confirming the residual nature of and Piccardo, 1979; Ottonello et al., 1979; the peridotites (fig. 7). The rather high 87Srf86Sr Romairone, 1999; 2000; Romairone et al., ratios most likely resulted from oceanic 2001). Decrease in modal clinopyroxene (from alteration, as commonly recognized for oceanic 10 to 3 vol%) and increase in MgO (from 41.3 and ophiolitic rocks. to 45.9 wt%) are accompanied by decrease in Based on REE geochemical modelling (using A12 03 (from 3.3 to 1.1 wt%) and CaO _(from both bulk rock and clinopyroxene data) it has 2. 7 to 1.1 wt% ), (fig. 4 ). Similarly, the Se been stressed that the compositional features of content decreases at decreasing modal the Erro-Tobbio peridotites are consistent with clinopyroxene and increasing MgO refractory residua after variable degrees (5- abundances. Bulk rock C !-normalized REE 14%) of incremental melting, most likely patterns for granular and tectonite-mylonite occurred at spinel-facies conditions peridotites (Romairone, 1999; 2000; (Romairone, 1999; 2000; Romairone et al., Romairone et al., 2001) are rather flat (at less 2001). than 2xC 1) for the HREE, and significantly The spinel-lherzolites commonly show 158 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
Sample/Chondrite Sample/Chondrite 10 �------·
1
0.1
Spinel Granular Spinel Tectonite Peridotites Peridotites
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb
Sample/Chondrite 10 �------�
0.1
Plagioclase Tectonite Peridotites
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb
Fig. 5- REE patterns (normalization to Cl of Anders and Ebihara, 1982) of the Erro-Tobbio peridotites (Romairone, 1999).
pyroxenite bands bounded by strongly depleted in the upper mantle, similarly to the mafic peridotite walls. The dominant rock types are layers in orogenic peridotite massifs and spinel-bearing Al-augite clinopyroxenites and pyroxenite xenoliths in alkaline basalts (Morten Cr-diopside websterites. They have been and Obata, 1983; Bodinier et al., 1987, interpreted as the result of magmatic events Menzies and Hawkesworth, 1987; Fabries et (melt intrusion and crystallization) deep-seated al., 1991). Evolution of the Ligurian Tethys: inference fr om petrology and geochemist1y of the Ligurian Ophiolites 159
143 144 Nd/ Nd
SEA WATER ALTERATION 0.5138
0.5135
0.5132
0.5129
0.5126
0 EtTO Tobbio
0.5123 • EL 0 IL
0.5120 0.701 0.702 0.703 0.704 0.705 0.706 0.707
87 86 Sr/ Sr
Fig. 6- Present-day I43Nd1144Nd vs 87Sr/S6Sr diagram for the Erro-Tobbio clinopyroxenes (Romairone, 1999). Also reported, for comparison: i) the data of IL and EL peridotites (Rampone et al., 1995, 1996); ii) the fields for MORB and OIB (Zindler and Hart, 1986) and for Ronda peridotites (Reisberg and Zindler, 1986; Reisberg et al., 1989); iii) field L refers to the Lanzo (Western Alps) peridotites (Bodinier et al., 1991); iv) field Prefers to the Pyrenean peridotites (Downes et al., 1991). The early spinel-facies equilibration. maximum temperatures below 1100°C (Hoogerduijn Strating et al., 1993), i.e. at much The granular spinel-facies recrystallization lower temperatures than the dry solidus for commonly overprints both peridotites and fertile upper mantle compositions. Such P-T pyroxenite layers. Locally, the pyroxenite layers exhibit isoclinal folds lacking of any estimates are consistent with equilibration axial-plane foliation and showing hinge along an intermediate (neither cratonic, nor structures completely recovered by granular mid-oceanic) lithospheric geothermal gradient. spinel-bearing equilibrium textures The early stability of a hydrous phase (most (Hoogerduijn Strating et al., 1990, 1993; probably a Ti-rich pargasitic amphibole) in the Vissers et al., 1991). This evidence points out spinel-bearing granular assemblage is testified that the peridotites have been plastically by widespread pseudomorphs and rare reddish, deformed after the pyroxenite formation, and deeply chloritized, amphibole relics. It is prior to the static, spinel-facies equilibrium noteworthy that stability of pargasitic recrystallization. amphibole in mantle peridotites implies Thermobarometry of the spinel-facies temperature below 1100°C (Jenkins, 1983). recrystallization yields a rough pressure This temperature limit is consistent with the T estimate of about 20 Kb with corresponding estimates made for the spinel-facies 162 G. B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETIA facies porphyroclasts within tectonite-mylonite the Ligurian Tethys. It is, moreover, consistent peridotites has the highest AI and Mg contents with mechanisms of tectonic unroofing (Al203 up to 59 wt%, MgO up to 20 wt%); following the passive and asymmetric relict spinels in tectonites and mylonites are extension of the lithosphere (Hoogerduijn progressively enriched in Cr and Fe. The most Strating et al., 1990,1993; Vissers et al., 1991; Cr-Fe-rich spinel relics occur in the Piccardo et al., 1992, 2001; Romairone et al., plagioclase- and hornblende-bearing mylonites. 2001). Plagioclase has 80-90% anorthite end-member. Amphibole in the amphibole-bearing mylonites The intrusion of basic dykes is edenitic to pargasitic hornblende The ET peridotites are intruded by mafic (Hoogerduijn Strating et al., 1993). dykes: they frequently retain textural, Thermobarometry made on the various mineralogical and geochemical characteristics assemblages recognized (Hoogerduijn Strating which allow recognition of troctolites, olivine et al., 1993) indicate that, starting from the P-T bearing clinopyroxene-gabbros and basalts as conditions of the early spinel-facies complete the magmatic protoliths. equilibration, the ET peridotites underwent a Major element compositiOns and progressive temperature decrease through metamorphic mineral assemblages of the mafic spinel-facies (T ranging 1000-1100°C) to dykes indicate that slight to significant chemical plagioclase-facies (T ranging 900-100 0°C), to changes occurred after their magmatic hornblende-facies (T lower than 900°C) crystallization and prior to the Alpine conditions. The composite tectono recrystallization. REE absolute concentrations metamorphic history indicates a subsolidus, and spectra of both rodingitic and eclogitic non-adiabatic evolution during exhumation meta-basaltic dykes in the ET peridotites from lithospheric mantle depths (Hoogerduijn indicate a clear MORB affinity for the parental Strating et al., 1993), (fig. 8). magmas (Cazzante, 1991 ). They closely The sequence of tectonic and metamorphic resemble REE compositions of basaltic rocks events and the variation in the mineral from the ophiolites of the EL Units of the NA. compositions indicate that the ET peridotites, These evidence are suggestive of the shallow after equilibration within the conductive emplacement of the ET peridotites close to the subcontinental lithosphere, underwent a ocean floor, where they were intruded by decompressional evolution towards shallow MORB-type magmas deriving from deeper crusta} levels. So far, no precise informations asthenospheric mantle sources. are available about the timing of the tectono metamorphic evolution. The oceanic evolution Sm-Nd isotope data, performed on two samples of plagioclase-bearing tectonites, have The seafloor hydration led to widespread yielded two essentially parallel isochrons serpentinization of peridotites and to partial giving ages of 273 and 313 Ma (± 16 Ma) for rodingitization of the mafic dykes (Cimmino et the plagioclase-bearing recrystallization (fig. al., 1979; Piccardo et al., 1988). Hydrous 9): these data indicate that the lithospheric assemblages including several generations of extension and the mantle decompressional serpentine minerals, chlorite , brucite and evolution were already active since Late mixed-layer phyllosilicates statically replace Carboniferous-Permian times (Romairone, the mantle assemblages and are in turn 1999; 2000; Romairone et al., 2001; Piccardo overgrown by subduction-related antigorite et al., 2001). bearing assemblages (Scambelluri et al., 1991; The decompressional path of the ET 1995). Several generations of serpentine are subcontinental lithospheric peridotites has been present: chrysotile, lizardite and antigorite, related to the early pre-oceanic rifting stage in which show variable Al203 content (max 0.7 Evolution of the Ligurian Tethys: inference from petrology and geochemistry of the Ligurian Ophiolites 163 wt% in chrysotile; 2.5 to 7 wt% in lizardite; 0.5 the igneous clinopyroxene is overgrown by to 3.2 wt% in antigorite). omphacite with minor garnet and talc, and The oceanic serpentine minerals are mostly plagioclase is replaced by finegrained chrysotile and lizardite whereas antigorite is aggregates of jadeite , zoisite and garnet, the serpentine phase stable at high-pressure. whereas olivine is pseudomorphed by tremolite Chrysotile and lizardite, in association with and talc. Coronas between pseudomorphs after magnetite and locally brucite, replace the olivine and plagioclase consist of early layered mantle olivine and pyroxenes: they can contain coronas of orthopyroxene+chlorite+garnet, slight amounts of Cl, which is absent in the which appear progressively overgrown hy high-pressure antigorite. talc+chloritoid+garnet. Coronas between The oceanic stage is also marked by pyroxene and plagioclase pseudomorphs development of mixed-layer phyllosilicates mostly consist of garnet and omphacite. mainly at the expence of mantle spinel both as Tectonitic and mylonitic eclogitized coronitic layers and as pseudomorphic metagabbros display an equilibrium replacements. These phyllosilicates can contain assemblage of omphacite+zoisite+garnet+ up to 5 wt% K20, 0.5 wt% Na20 and 0.35 wt% chloritoid+talc. The pressure and temperature Cl; they are overgrown by high-pressure estimates suggest that the eclogitic parageneses chlorite lacking chlorine and alkalies. in metagabbros formed at 18-25 Kbar and 500- The low-grade nature of such a metamorphic 650 °C (Messiga et al., 1995b ). essamblage and the mineral compositios, point Several structural studies (Scambelluri et al., to an early stage of peridotite interaction with 1991; 1995) have shown that eclogitization of Cl- and alkali-bearing solutions, presumably metagabbros is coeval with formation of sea-water-derived (Scambelluri et al., 1997). metamorphic oli vine in the associated Compositional data on oceanic mantle sampled ultramafic rocks as the result of their partial during DSDP and ODP reveal the occurrence deserpentinization. Oceanic serpentine is infact of oceanic serpentine minerals and mixed layer cut by prograde antigorite veins and foliations, phyllosilicates whose compositions are which are in turn cut by olivine structures. comparable to the one described here for the Formation of peak metamorphic olivine is
ET peridotite (Agrinier et al., 1988; Bassias caused by the reaction antigorite+brucite = and Triboulet, 1982). olivine+fluid, which brings to stability of olivine+antigorite+diopside+Ti The alpine evolution clinohumite+chlorite and to liberation of a metamorphic fluid phase. The most obvious Studies on the alpine history of the ET Unit evidence of fluidrelease is the development of demonstrate that peridotites and mafic dykes widespread vein systems containing olivine, underwent common subduction and eclogite Ti-clinohumite , diopside and chlorite facies recrystallization (Piccardo et al., 1988; (Scambelluri et al., 1991; 1995). During Hoogerduijn Strating et al., 1990; Scambelluri subduction and eclogitization, deformation was et al., 1991). Peak metamorphic assemblages extremely channelled within intensely consist of omphacite+garnet+Mg-chloritoid+ serpentinized domains (serpentinite mylonites), zoisite+talc+chlorite in the metagabbros whereas large volumes of ultramafic rocks did (Messiga et al., 1995b), of olivine+titanian not undergo significant plastic deformation and clinohumite+antigorite+diopside in the recrystallized statically into the new peridotite. olivine+antigorite - bearing assemblage Eclogitic minerals in the metagabbros ( metaperidoti tes). develop both as static assemblages in massive One main implication of antigorite coronitic rocks, and in tectonitic and mylonitic serpentine stability in the ultramafites at structures. In massive coronitic metagabbros, eclogite facies is that these rocks represent the 164 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
143 144 Nd/ Nd 0.5134
0.5132
0.5130 313±16Ma
0.5128
• ETR13 () ETR22
0.5126
0.1 0.2 0.4 0.5
Fig. 9 - l43Nd/l44Nd vs l47Sm/l 44Nd diagram for clinopyroxene and plagioclase separates and whole rock from two samples (ETR13 and ETR22) of the Erro-Tobbio plagioclase tectonites (Romairone, 1999).
most effective carriers of water into deep levels dewatering of these rocks, as well as the scales of subduction zones and that they maintain of fluid migration at depth (Vallis, 1997; Frueh extremely low densities at mantle depths Green et al., 2001). (Scambelluri et al., 1995; Hermann et al., As a whole, the ultramafites have 2001). heterogeneous (5180 values, ranging from 180- enriched to 180-depleted compositions with Geochemical inference on fluid and element respect to unaltered reference mantle values cycling during subduction of the Erro (fig. 10). Serpentinized mantle peridotites are Tobbio peridotite generally enriched in bulk 180 (5.7 to 8.1%o ), The shallow hydration of the ET peridotite high pressure metaperidotites and serpentinite had relevant consequences on deep transport of mylonites cover the same range of bulk-rock water and element during subsequent (5180 values (4.4 to 7.6%o) and show a slight subduction, and on element cycling into the 180 depletion compared with the serpentinized mantle via production of deep eclogitic mantle (fig. lOA). The lowest bulk rock (5180 deserpentinization fluids. Stable isotope studies values pertain to high pressure veins (3.5 to of the ET ultramafic rocks have clarified the 5. 7%o) and to eclogitized metagabbros (3 .1 to fluid-rock interactions during shallow 5.3%o). The (5180 variability of clinopyroxene hydration and subsequent subduction and serpentine reflects the same general Evolution of the Ligurian Tethys: inference fr om petrology and geochemistJ)I of the Ligurian Ophiolites 165
-� �l [-Eclogitic metaga-b-b ros- - ! ® @ ® ® A High-pressure veins Bulk-rocks ®®� fi High-pressure serpentinite mylonites ®
I High-pressure I' metaperidotites
il il Serpentinized mantle I peridotites ® � ® ®® N ((ID ® ______-- ______I L l L -+- +------J L_ i : Eclogitic met�bbros ® ' @ I I B ! 1 High pressure veins i {;linopyroxenes �- � ® l i I : High-pressure � €iD � � serpentinite mylonites - i1 High-pressure @ metaperidotites ��@�I i I i 1 Serpentinized mantle peridotites ® i- -· ______L...... L ...... 1 ..... ······· ·· ····� i
High pressure veins I � ® Serpentines High-pressure serpentinite mylonites (high strain domains) High-pressure metaperidotites (low strain domains) ® Serpentinized mantle peridotites ® @ @)® @®• ® � ______j______L______-·------·------�- ______j__ __ !3 4 5 6 7 8 b180(%o)
Fig. 10 - Oxygen isotope variability of bulk rocks (A), clinopyroxene (B) and serpentine separates (C) from the Erro Tobbio metaperidotite and eclogitized metagabbro. The shaded vertical fields represent the primary isotopic ratios in unaltered mantle rocks and in mantle clinopyroxenes (C hazot et al., 1993 ) (redrawn and modified after Frueh-Green et al. 2001). 166 G. B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA variations as the bulk-rock compositions. The ratios, closed system behaviour during high ISQ compositions of mantle clinopyroxene (5 pressure metamorphism, local-scale exchange to 7%o) and of serpentine (chrysotile and with compositionally heterogeneous eclogitic lizardite, 4.2 to 7 .6%o) preserved in the fluids and absence of large-scale fluid flushing serpentinized mantle peridotites, are closely causing resetting of pre-subduction isotopic comparable with the ones of metamorphic signatures. Frueh Green et al. (2001) have thus diopside (4.1 to 6.5%o) and antigorite (5.0 to interpreted the heterogeneity of oxygen isotope 7.1%o) of the high pressure ultramafic rocks signatures as evidence for centimeter-scale (fig. lOB, C). In general the oxygen isotope recycling of internally-derived fluids during composition of high pressure phases are eclogitization of the ET Unit. Fluid uptake in slightly depleted in ISQ (less than 1 %a) with these rocks most likely occurred during shallow respect to the pre-eclogitic ones. (crus tal) interaction processes; inflow of These variations are comparable to the ones externally-derived f1uids along shear zones at measured in mafic and ultramafic rocks from high pressure conditions seems to be unlikely. modern oceanic environments and from Based on the hydrogen isotope signature of ophiolites, which record both low temperature the various serpentine minerals in the and high temperature alteration and varying serpentinized mantle peridotites ( chrysotile and fluid fluxes (e.g. Wenner and Taylor 1973; lizardite) and in the high pressure ultramafites Gregory and Taylor 1981; Agrinier et al., 1988; (antigorite), Frueh Green et al. (2001) have Fruh-Green et al., 1996; Agrinier et al., 1995). concluded that fluid uptake in the ET peridotite In particular, a large number of oxygen isotope occurred at oceanic conditions and during the ratios of clinopyroxene in serpentinized mantle early (shallow) stages of subduction. peridotites, high pressure metaperidotites, The trace element compositions of serpentinite mylonites, high pressure veins and serpentinized mantle peridotites and of high omphacite of metagabbros, have d I SQ depleted pressure ultramafites of the ET peridotites are compositions ( < 5%o), thereby suggesting exchange with seawater at T >300°C. In Whole-rock/Chondrite contrast, most of the serpentine oxygen isotope lO r------� compositions are greater than 5%o: these values Profile from serpentinized are similar to serpentine compositions of the peridotite core to Iberian passive margin (5 to 13%o, Agrinier et serpentinite mylonite al., 1988; Agrinier et al., 1995; Plas, 1997) and the Tyrrhenian Sea (3 to 8%o, Plas, 1997), reflecting lower temperature fluid/rock interaction at crustal levels. Similarities in the oxygen isotope signatures of oceanic and eclogite-facies rocks have been pointed out in a number of stable isotope 0.1 studies, and have been interpreted as an indication of the preservation of oceanic + Serpentinized mantle peridotites signatures and thus a lack of isotopic 8 Serpentinized mantle peridotites overprinting during eclogitization (e.g. 0 Serpentinite mylonites Matthews and Schliestedt, 1984; Nadeau et al.,
1993; B arnicoat and Cartwright, 1997; � &NdbSm�GdTI � Er Yb Philippot et al., 1998; Putlitz et al., 2000). Preservation of pre-eclogitic dl8Q signatures of Fig. 11- Bulk trace-element compositions of samples in profiles from serpentinized mantle peridotite to serpentinite the ET ultramafic rocks and metagabbros mylonite from the Erro-Tobbio peridotite (redrawn and implies local-scale fluid flow at low water/rock modified after Scambelluri et al. , 2001). Evolution of the Ligurian Tethys: inference fr om petrology and geochemistl)' of the Ligurian Ophiolites 167 shown in fig. 11. Main features of the shallow melting event, responsible of its variable serpentinization are immobility of rare earth depletion, and melt intrusion (pyroxenite elements, considerable water increase, local dykes). CaO decrease and uptake of trace amounts of The diffuse and complete reequilibration Sr, probably as a function of intensity of the under spinel-facies conditions, at temperature shallow alteration (Scambelluri et al., 2001 ). not higher than 1100°C, evidences that the ET Comparable rare earth element behaviour and peridotite, after an early upwelling and partial Sr enrichment were observed by Menzies et al. melting (asthenosphere evolution) was accreted (1993) after experimental peridotite-seawater to the subcontinental conductive lithosphere interactions at 300°C, and were reported for and reequilibrated to the regional geotherm. abyssal serpentinites by Bonatti et al. (1970). The subsequent composite retrograde The alkalies and chlorine early stored in evolution from lithospheric mantle depth oceanic hydrous phases are no longer present in towards shallow levels is well constrained on the high pressure pm·ageneses, as they likely the basis of structural and petrological data. partitioned in the synmetamorphic fluid drained Sm/Nd isotopic data indicate that the in the veins. lithospheric extension and the decompressional A fluid inclusions study (Scambelluri et al., evolution were already active since Late 1997) has pointed to the presence of Carboniferous-Permian times. hypersaline fluids containing NaCl, KCI and During upwelling, the ET peridotite was MgC12 in the vein filling minerals. The fluid intruded by masses and dykes of gabbroic and inclusion compositions strongly suggest that later basaltic dykes showing clear MORB the fluid incorporated at the seafloor, was affinity. It is indicative that the peridotite released at high pressure and its composition reached shallow levels during the opening of was mostly controlled by the host peridotite the Jurassic Ligurian Tethys. (Scambelluri et al. , 1997). Trace element Sea-floor exposure is, moreover, testified by analyses of mantle clinopyroxenes and high pressure diopsides (in country ultramafites and Clinopyroxene/Chondrite veins), highlight the close similarity in the REE 100 �-.....:;__::...______, compositions of the various clinopyroxenes (fig. 12), again indicating rock control on the vein fluids and lack of exotic components carried by externally-derived fluids. Presence 10 of appreciable Sr contents in vein-forming diopside indicate cycling of oceanic Sr in the high-pressure fluid. The aqueous fluid equilibrated with such a clinopyroxene lacks HFSE, has Sr contents of about 1.5 x chondrite (i.e. in the range of normal mantle values) and is Cl- and alkalies-rich. The recognition that 0.1 pre-subduction water, chlorine, alkalies and stronti urn were carried by the vein fluid, indicate closed system behaviour during eclogitization and internal cycling at 80 km 0.01 depth of exogenic components. Ce Sr Nd Zr Sm Eu Gd Ti Dy Er Yb Fig. 12 Trace element compositions of clinopyroxenenes Geodynamic il�ference from the Erro-Tobbio perid otite. Grey dots: high-pressure diopside from host rocks and veins. Shaded fie ld : reference The ET peridotite is presently interpreted as mantle clinopyroxe ne s (redrawn and modified after upper mantle which underwent an early partial Scambelluri et al. , 2001). 168 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA the diffuse serpentinization of the peridotite THE NORTHERN APENNINE and rodingitization of the mafic dykes. The oceanic origin of the intervening fluids is Introduction to the geology of the Northern indicated by the chlorine contents of early Apennine serpentine minerals and the formation of potassium-bearing phyllosilicates consisting of The NA is formed by the Late Oligocene mixed chlorite-biotite layers. At the ocean Miocene stacking of tectonic units belonging to floor, heterogeneity of fluid infiltration and two main systems: peridotites alteration caused the spatial - the lowermost Tuscan units, derived from association of extremely serpentinized volumes continental domains and formed by close to less altered peridotites domains. This sedimentary and metasedimentary sequences variability in water distribution inside the originally deposited on a Paleozoic basement; oceanic lithosphere strongly controlled strain - the uppermost Liguride units, derived from localization during later subduction. The oceanic and ocean-continent transitional intensely serpentinized domains worked as soft domains. The Liguride units occupy the highest horizons during convergence and closure of the position in the nappe pile. oceanic basin, and became the main shear The stratigraphic and structural features of zones and detachment horizons which the Tuscan Domain allowed the reconstruction accomplished deformation during subduction. of the evolution of the Adriatic continental During alpine subduction and collision, the margin from the Hercynian orogen, to the Late ET peridotites recrystallized to antigorite Paleozoic trans-extensional setting. During the serpentinites and antigorite-bearing Middle Triassic, evidence of further crustal metaperidotites. The peak metamorphic attenuation was provided by the Anisian assemblage in the peridotite consists of the Ladinian extensional basins (Punta Bianca antigorite+olivine+diopside+Ti-cl inohumite sequence) in which marine platform sediments association which is coeval with the are associated with alkaline basaltic flows and omphacite+garnet - bearing assemblage in the breccias. mafic dykes, which indicates peak Early to Middle Jurassic block faulting and metamorphic conditions in the range 18-25 kb progressive subsidence of the continental and 500-650°C. margin led to the dismembering of the Survival at eclogite-facies conditions of carbonate platforms and the formation of the coerent tectonic units containing dominant low ocean basin of the Ligurian Tethys (Malm). density antigorite serpentinites, provides a low The Liguride Units have been subdivided density buoyant medium driving the into two main groups of units on the basis of exhumation of deeply subducted rocks and of stratigraphic and structural features, i.e. the eclogite bodies present inside alpine tectonic Internaland the External Liguride Units. units with dominant serpentinite (Hermann et The IL Units consist of serpentinized mantle al., 2001). peridotites, generally covered by ophicarbonate In conclusion, structural and petrologic breccias ( ophicalcite ), and gabbroic bodies. studies on the ET peridotite indicate an early Ultramafic and gabbroic rocks are locally asthenosphere upwelling and lithosphere covered by ophiolitic sedimentary breccias accretion, followed by progressive uprise of interlayered with MORB-type basaltic flows. this subcontinental lithosphere starting from Their sedimentary cover consists of Late Carboniferous-Permian time and sea-floor Radiolarian Cherts (Callovian-Oxfordian), exposure during the Jurassic opening of the Calpionella Limestone (Berriasian) and Ligurian Tethys: these peridotites were later Palombini Shales (Berriasian-Aptian). subducted during closure of the oceanic basin The EL Units are characterized by the and exhumed during alpine collision. presence of «basal complexes» (pre-flysch Evolution of the Ligurian Tethys: inference jl-om petrology and geochemistry of the Ligurian Ophiolites 169 formations) overlain by the typical involvement in the overthrusting (Late Helminthoid Flysch (Cretaceous-Paleocene Oligocene-Miocene) onto the easternmost calcareous-dominant sequences). According to continental margin (i.e. the Tuscan Domain). their internal stratigraphy, two main groups of The Cretaceous evolution produced the units have been recognized and referred to formation of the basal complexes, resulting in different domains: i) the Western External slicing and inversion of the External Ligurian Liguride Domain, characterized by units Domain. The Mesoalpine deformation in the containing ophiolites and ophiolite-derived Liguride Units involved early west-verging debris and ii) the Eastern External Liguride large scale folding and thrusting. This tectonic Domain, characterized by units containing processes were associated with metamorphic fragments of mesozoic sedimentary sequences recrystallizations ranging from very low- to and conglomerates with Austro-Sudalpine or low blueschists grade (Van Wamel, 1987; Insubrian affinity, without ophiolites. Hoogerdujing Strating, 1994; Marroni and In Western External Liguride Domain Pandolfi, 1996) in the IL Units and very low ophiolite rocks are mainly represented by to anchimetamorphic grade in the EL Units. MORB-type basalts and mantle peridotites The Mesoalpine deformation was followed by occurring as olistoliths, slide blocks and large scale backthrusting, gravitational tectonic slices in the basal complex (i.e. the spreading and extensional tectonics associated Casanova complex, a Santonian-Early with east-directed tectonic transport. The Campanian sedimentary melange). MORE subsequent large scale deformational history of type basalts and mantle peridotites are the Apennine involved northeastward nappe associated with continental mafic and felsic transport and progressive deformation of the granulites, and granitoids. The basal complex westernmost sector of the Adriatic continental (sedimentary melange) grades upward to the margin, when the Liguride Units became parts Late Cretaceous-Paleocene Helminthoid Flysch of the Apenninic accretionary wedge in (Marroni et al., 1998 and references therein). relationship with the underthrusting of the In the Eastern External Liguride Domain the External Tuscan Domain. basal complex is locally characterized by slices of Middle Triassic to Early Cretaceous The NorthernApennine Ophiolites e carbonate sequences which represent th pre A representative sampling of the diversity of Early Cretaceous base (Vercesi and Cobianchi, the oceanic lithosphere which floored the 1998) of the basal complex. This evidence Jurassic Ligurian Tethys is shown by the NA indicates the presence of a thinned continental ophiolites, which crop out in two distinct crust representing the westermost domain of geological units, the IL and EL Units (fig. 13). the Adria continental margin (Sturani, 1973; In the IL Units , ophiolitic rocks show Zanzucchi, 1988; Elter and Marroni, 1991; structural relationships similar to those Molli, 1996). The differences between the described for the whole Ligurian basin; they basal complexes in the two External Liguride consist of a peridotite-gabbro basement Domains and the ubiquitous presence of the stratigraphically covered by ophiolitic breccias, Late Cretaceous-Paleocene Helmi nthoid pillowed basaltic lava flows, and oceanic Flysch, suggest the presence of a transition sediments (Abbate et al., 1994, and quoted between a thinned continental crust (Eastern references). Field and structural evidences External Liguride Domain) and an ocean indicate that mantle peridotites were intruded continent transition area (Western External by gabbroic bodies at depth. Subsequently, the Liguride Domain). peridotite-gabbro association experienced a The Liguride Units bear evidence of tectono-metamorphic decompressional Eoalpine (Cretaceous) and Mesoalpine evolution as testified by deformation and (Eocene) deformation predating their recrystallization along shear zones: this 170 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETIA
Ligurian Sea
Fig . 13 - Geologic sketch map of the Northern Apennines (Eastern Liguria) with indication of the main ophiolitic bodies. VG: Voltri Group. SV: Sestri-Voltaggio Units. MA: Mt. Antola Formation and Mt. Caio tlysch (calcareous sandy turbidites with interbedded shales). 1) Ophiolites (peridotites. gabbros and basalts); field A: Internal Lig uride ophiolites; field B I and B2: External Liguride (Mt.Ragola-Mt.Aiona and Suvero, respectiv ely) ophiolites. 2) Internal Liguride sedimentary sequences: Radiolarian Cherts, Calpionella Limestones, Palombini Shales, Lavag na and Giaiette Shales, Gottero Sandstones. 3) External Liguride tlyschoid sequences: Sopralacroce Shales and Casanov a Complex, (redrawn and modified after Ram pone and Piccardo, 2001). indicates progressive uplift and fi nal exposure as well as the overlying tectonic and at the sea-floor, where the peridotites were sedimentary breccias. extensively serpentinized. The uppermost part The NA ophiolites also crop out in the EL of the serpentinites suffered intensive Units, where they are associated with fracturing with development of tectonic continental crust material (Marroni et al., 1998, breccias (ophicalcites), which were partially and quoted references). In these units, covered by sedimentary ophiolitic breccias. ophiolites mostly consist of mantle lherzolites Ophicalcites and sedimentary breccias were and pillowed basaltic lavas which occur as discontinu ously covered by MORB-type huge slide blocks ( olistoliths) within the basal pillowed basaltic lava flows and Oxfordian complexes of the Cretaceous-Eocene flysch Callovian radiolarian cherts, i.e. the oldest sequences. Due to the association with pelagic sediments (Marcucci and Passerini, continental crust, the source area for the EL 1991). Discrete basaltic dikes, related to the lherzolites and basalts has been located close to basaltic extrusions, commonly cut the Adria continental margin (Piccardo et al., serpentinized peridotites and foliated gabbros, 1990, and quoted references). Accordingly, the Evolution of the Ligurian Tethys: il�ference fr om petrology and geochemistry of the Ligurian Ophiolites 171
5 5 Primordial Mantle estimates Primordial Mantle 4 estimates 4 � dij 3 3
2 � 2 �. D External Ligurides 1 - 1 0 Internal Ligurides A Abyssal peridotites
0 0 35 37 39 41 43 45 47 35 37 39 41 43 45 47 MgO (wt0/o ) MgO (wt0/o )
Fig. 14 - Whole rock abundances of CaO and Al203 vs MgO for the Internal and External Liguride peridotites (d ata from Rampone et al., 1995, 1996; all data on anhydrous basis in wt%). Primordial mantle estimates (open triangl e and asterisk) are from Hofmann (1988) and Jagoutz et al. (1979), respectively). The representative compositions of abyssal peridotites are from Dick (1989), (redrawn after Rampone and Piccardo, 200 1 ).
EL units have been regarded as a fossil Piccardo et al. ( 1994) and Ramp one and example of the ocean to continent transition Piccardo (2001). (Marroni et al., 1998). Peridotites record a composite tectonic-metamorphic evolution, The mantle protoliths developed prior to widespread serpentinization, The EL peridotites are dominantly fertile which indicates progressive upwelling toward spinel lherzolites which crop out as km-scale the sea-floor (Piccardo et al., 1990; Rampone bodies largely preserving mantle textures and et al. , 1995). Chilled basaltic dikes intruding assemblages, despite widespread peridotites and crosscutting all mantle serpentinization. Pyroxenite bands structures are abundant. The continental crust (spinel-bearing Al-augite clinopyroxenites and consist of late Hercynian granitoids, granulites Cr-diopside websterites) are common within and paragneisses (e.g., Eberhardt et al., 1962; the peridotites, and represent high-pressure Marroni et al., 1998). The primary cumulates from basaltic melts (Rampone et al., relationships between granitoids, ophiolitic 1995, and quoted references). basalts and radiolarian cherts are locally The EL peridotites exhibit rather fertile preserved. composition, with only slight depletion in fusible components. This is indicated by: 1) the The nwntle peridotites lherzolitic modal compositi ons, 2) the As outlined above, the dominant lithotype in relatively high bulk rock Al203 (2.86-4.00%) the NA ophiolites is mantle peridotite. Quite and CaO (2.33-3.39%) contents (fig. 14), 3) the distinct petrologic and isotope features clinopyroxene REE patterns, showing only characterize peridotites belonging to the EL moderate LREE depletion (CeN/SmN=0.6-0.8) and IL Units, as recently summarized by and absolute concentrations at 1 0-16xC 1 (fig. 172 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
Sample/Chondrite 100 �--�------.Sample/Chondrite 100 A B amph cpx
10 10 cpx
1 Whole Rocks ExternalLigurides Whole Rocks Lanzo North Internal Ligurides 0.1 • 0.1 e External Ligurides &.. Lanzo South • cpx * Dinaric Belt D amph o cpx (Int. Lig.)
Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb
Fig. 15- Representative chondrite-normalized REE compositions for: (A) whole rock, clinopyroxenes (cpx) an d pargasitic amphiboles (amph) from the External Liguride peridotites (data from Rampone et al., 1995); als o reported, for comparison, a representative whole rock composition for the Lanzo North (Western Alps) (Bodinier, 1988) peridotites; (B) whole rock and clinopyroxenes from the Internal Liguride (IL) peridotites; the IL data are compared with those of the Lanzo South (Bodinier, 1988) and Central Dinaric Ophiolite Belt (Yugoslavia) (Lugovic et al.,l99l) peridotites. Normalizing values are from An ders and Ebihara (1982), (redrawn after Rampone and Piccardo, 2001).
15A) (Emst and Piccardo, 1979; Beccaluva et Present-day Sr and Nd isotope ratios of the al., 1984; Ottonello et al., 1984; Rampone et EL peridotites (fig. 16) plot within the depleted al., 1993; 1995). end of the MORB field; this feature is common The EL lherzolites are characterized by a to many subcontinental orogenic spinel complete static equilibrium recrystallization lherzolites from the western Mediterranean under spinel-facies conditions. Disseminated area (e.g. the Pyrenees and Lanzo North kaersutite/Ti-pargasite amphiboles, in peridotites). Most EL peridotites display Nd ·equilibrium with the spinel-bearing model ages (assuming a CHUR mantle source) assemblage, show LREE-depleted REE spectra in the range 1.9-1.7 Ga (fig. 17), and consistent (fig. 15A) and very low Sr, Zr and Ba contents. results are obtained with Rb-Sr systematics The stability of pargasitic amphibole constrains (Rampone et al., 1995). In particular, one the spinel-facies equilibration to temperatures single sample with extremely depleted isotopic lower than 1100°C (e.g. Jenkins, 1983), in composition (87Srf86Sr=0.701736; agreement with thermometric estimates on the I43Ndfl44Nd=0.513543) yields Sr and Nd spinel-facies assemblage (T = 1000-1100°C). model ages of 2.1 and 2.4 Ga (assuming a DM The spinel-facies reequilibration in a rather and a CHUR mantle source respectively) (fig. «cold» (T<1100°C) thermal regime has been 17): these can be considered as minimum ages interpreted as a stage of annealing of differentiation form the asthenospheric recrystallization after accretion of the EL mantle. mantle to the conductive lithosphere (Piccardo The IL peridotites consist of clinopyroxene et al., 1994; Rampone et al., 1995). poor (5-10 vol%) spinel-lherzolites showing Information about the timing of lithospheric depleted compositions: their bulk rock MgO, accretion has been derived from Rb-Sr and Sm CaO and Al203 contents are comparable to Nd isotope studies (Rampone et al., 1995). those of abyssal peridotites (see fig.14). The Evolution of the Ligurian Tethys: inference from petrology and geochemistJ)' of the Ligurian Ophiolites 173
143 144 Nd/ Nd
External Ligurides Internal Ligurides peridotites 0.5138 Ill peridotites 0 () basalts basalts L gabbroic rocks 0.5135
0.5132
0.5129
0.5126
0.5123
0.5120 0.701 0.702 0.703 0.704 0.705 0.706 0.707
87 s6 Sr/ Sr Fig. 16- Present-day l43Nd/l44Nd vs 87Sr/86Sr diagram for the Northern Apennines (IL and EL) peridotites and basal ts, and for the IL gabbroic rocks (data from Rampone et al ., 1995, 1996, 1998). Also show n are the fields for MORB and OIB (Hofmann, 1997), as well as for the Ronda peridotites (Reisberg et al., 1989, and quoted references). Field L refers to the Lanzo (Western Alps) peridotites (data from Bodinier et al., 1991 ). Fiel d P refers to the Pyrenean peridotites (data from Downes et al., 1991) (redrawn after Rampone and Piccardo, 2001).
REE spectra are characterized by strong Tethys. A Middle- to Late-Jurassic age of fractionation (CeN/YbN=0.015-0.028), and very shallow emplacement and sea-floor exposure low LREE absolute concentrations. These for the IL peridotites is indicated by the compositions are among the most depleted of Oxfordian-Callovian age of the overlying ophiolitic peridotites from the Western radiolarian cherts (Marcucci and Passerini, Mediterranean area (see fig. 15B). 1991). However, isotope investigations have Geochemical modeling indicates that the IL provided unexpected results about the age of peridotite compositions are consistent with the partial melting event. mantle residua left after low-degree ( < 10%) The present-day 87SrJ86Sr ratios (0.702203- fractional melting, presumably initiated in the 0.702285) of the IL peridotites are consistent garnet-facies stability field (Rampone et al., with a MORB-type mantle, but the l43Ndfl44Nd 1996). ratios (0.513619-0.51 3775) are very high and The IL peridotites exhibit strong chemical plot significantly above the MORB field (fig. similarity to present-day oceanic mantle, and 16). The high Nd isotope ratios are coupled to could therefore represent depleted oceanic very high I47SmJI44Nd ratios (0.54-0.56) (fig. mantle which produced the Jurassic MORE 18A). As discussed in Rampone et al., (1996) type gabbros and basalts of the Ligurian (1998), such compositions are not consistent 174 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA 43 44 1 Nd/ 1 Nd 0.5140 Old 0.5138 V' /.depletion
0.5136 111111 • 0.5134
0.5132 0 Ext. Lig.
0.5130 e Lanzo N 111111 Ronda 0.5128 .A. Pyrenees t = 1.9-1.7 Ga 0.5126 *CHUR
0.5124 0.15 0.20 0.25 0.30 0.35 4 4 1 7Sm/ 1 Nd
Fig. 17 - J43Nd/144Nd vs 147Sm/144Nd diagram for the External Liguride (Ext. Lig. ) peridotites (data from Rampone et al., 1995), compared with data from other subcontinental orogenic peridotites (Ronda, Reisberg et al., 1989; Pyrenees, Downes et al., 1991, Mukasa et al., 1991; Lanzo North, Bodinier et al., 1991 ). All data are from clinopyroxene separates. The Depleted Mantle (DM) and CHUR source ratios are, respectively, 143Nd/1 44N d = 0.513114, 147Sm/144Nd = 0.222 and 143Nd/1 44Nd = 0.512638, J47Sm/1 44Nd 0.1967 (see text and Rampone et al., 1995 for more explanation) (redrawn after Rampone and Piccardo, 2001). with a Jurassic partial melting event of a frame of the Permian extension-related mantle MORB -type asthenosphere: Nd model ages partial melting and magma production in the calculated for an average IL peridotite Alpine realm (Dal Piaz, 1993; and quoted composition and assuming a depleted mantle bibliography). (DM) source (see fig. 18A) yield a Permian The IL peridotites, after the melting event, time of depletion (t=275 Ma). A Permian age were completely recrystallized under spinel (about 270 Ma) of partial melting has been also facies conditions; the spinel-facies assemblages inferred for the Balmuccia peridotites (Voshage record equilibration temperatures in the range et al., 1988) (see fig. 18A). Asthenospheric 1150- 1 250°C (Beccaluva et al., 1984; mantle upwelling and melting during the Rampone et al., 1996). Permian is well documented by the widespread occurrence, in the Alpine belt, of Permian The decompressional rift evolution gabbroic bodies intruded beneath or within the Adria thinned continental crust (see discussions The EL peridotites and associated in Piccardo et al., 1994; Rampone et al., 1996, pyroxenites show a composite tectono 1998; Rampone and Piccardo 2001, and quoted metamorphic evolution, i.e. partial references). Thus, the Permian age of depletion recrystallization from spinel- to plagioclase recorded by the IL ultramafics is surprising as bearing assemblages, and progressive they represent the upper mantle of the Jurassic deformation leading to the development of oceanic lithosphere, but it is reasonable in the porphyroclastic textures and tectonite to Evolution of the Ligurian Te thys: inference from petrology and geochemistry of the Ligurian Ophiolites 175
143 144
0.5138 Nd/ Nd A t = 275 Ma 0.5136 (DM model age)
0.5134
0.5132 0 Int. Ligurides
� Lanzo South 0.5130 B Balmuccia
0.5128 DM source
0.5126
0.10 0.20 0.30 0.40 0.50 0.60
0.5138 InternalLiguride B ophiolites 0.5136 b. intrusive rocks
0.5134
0.5132
t = 164 ± 14 Ma 0.5130 (gabbro isochron)
0.5128
0.5126 0 0.10 0.20 0.30 0.40 0.50
Fig. 18 I43Nd/I44Nd vs I47S m/I 44Nd diagram for: (A) the Internal Liguride peridotites (data from Rampone et al., 1996), compared with data from the Balmuccia (Ivrea Zone, Voshage et al., 1988) and Lanzo South (Bodinier et al., 1991) peridotites. All data refer to clinopyroxene separates (see fig. 5 for definition of the DM source, and text for explanation on the model age calculation); (B) the gabbroic rocks (whole rocks [wr], clinopyroxene [cpx] and plagioclase [plag] separates) of the Internal Liguride ophiolites (data from Rampone et al., 1998) (redrawn after Rampone and Piccardo, 2001). 178 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA
Available geochronologic data on the NA response to the Triassic-Jurassic rifting of the ophiolitic gabbroids commonly yield ages of Ligurian Tethys (Lemoine et al., 1987, and intrusion older than the age of basaltic quoted references). extrusion and chert deposition,. i.e. older than the opening of the Ligurian Tethys (Piccardo et The basaltic rocks al., 1994). Moreover, relatively younger ages Basaltic rocks mostly occur as pillow or are recorded by the IL intrusives (160-167 Ma; massive lava fl ows, and as discrete dikes Bigazzi et al., 1972, 1973; Rampone et al., intruding deformed gabbros and mantle 1998), with respect to gabbroic rocks from the peridotites. Petrologic and geochemical studies EL (185 Ma; Bigazzi et al. , 1973). Triassic highlighted that the Alpine-Apennine ophiolitic ages of magmatic emplacement are also basalts have overall tholeiite composition and documented for gabbroic rocks from the MORB affinity (Ferrara et al., 1976; Venturelli Montgenevre ophiolites (Western Alps) ( 192- et al., 1981; Beccaluva et al., 1984; Ottonello 212 Ma, Carpena and Caby, 1984; about 185 et al., 1984; Vannucci et al., 1993a; Rampone Ma, Costa and Caby, 1997). Thus, the gabbroic et al., 1998). The IL and EL basaltic rocks rocks were mostly intruded in a pre-oceanic show a large degree of differentiation, as subcontinental environment, prior to the Late indicated by their REE compositional variation Jurassic oceanic break-up (Dal Piaz and (from about 1 OxC 1 to more than 40xC 1 Lombardo, 1985; Piccardo et al., 1994). absolute values) (see the field in fig. 19B; data The gabbroic rocks, like the peridotites, from Venturelli et al., 1981, Beccaluva et al., record a low-pressure tectono-metamorphic 1984, Ottonello et al., 1984, Marroni et al., evolution, characterized by progressive 1998). The most primitive IL basalts show temperature decrease (Cortesogno et al., 1975). moderate LREE fractionation (CeN/SmN=0.6) The early high-temperature (T about 900°C) and HREE abundances at about 1 OxC 1, the metamorphic recrystallization is localized least differentiated EL basalts display almost along ductile shear zones (Molli, 1994; 1995) flat or slightly LREE-enriched REE spectra and develops an assemblage of clinopyroxene, (CeN/SmN=0.9-1) (fig. 19B). Vannucci et al. plagioclase, titanian pargasite and ilmenite. ( 1993) have shown that the compositions of EL Trace element mineral compositions indicate and IL basalts could be derived by varying that such a metamorphic event occuned in the degrees of fractional melting (totalling less absence of seawater-derived fluids (Tribuzio et than 10%) of a M ORB-type asthenospheric al., 1995). The high temperature ductile shear spinel-facies mantle source. Sr-Nd isotope zones are commonly postdated by a retrograde study stresses out the MORB affinity of the NA metamorphic event, from amphibolite- to ophiolitic basalts, which show, in spite of a subgreenschist-facies conditions (Messiga and large 87Srf86Sr variability caused by seawater Tribuzio, 1991; Tribuzio et al., 1997). This alteration, a fairly homogeneous 143Ndfl44Nd event was most likely related to interaction ratios ranging 0.51 3046-0.51 3098 (Rampone et with seawater-derived fluids and was al., 1998). frequently accompanied by the development of Information concerning the age of basaltic brittle deformations. In the IL ophiolites, in activity in the Ligurian Tethys has been mostly particular, parallel swarms of hornblende derived by U/Pb dating of zircons from acidic bearing veins are locally widespread, thus differentiates which are considered, on the indicating the development of active, high basis of their field occurrence and structural temperature hydrothermal system (Cortesogno relationships, to be almost contemporaneous et al., 1975; Tribuzio et al., 1995; 1997). with the basaltic volcanism. These U/Pb data Similar metamorphic histories are documented have yielded ages in the range 150-160 Ma, for many Alpine ophiolitic gabbros, and have which are interpreted as the age of the basaltic been related to the exumation of these rocks, in volcanism (Bortolotti et al., 1991, 1995; Borsi Evolution of the Ligurian Te thys: inference fr om petrology and geochemistry of the Ligurian Ophiolites 179 et al., 1996; Ohnenstetter et al., 1981; Costa 1962; Marroni et al., 1998). These rocks show and Caby, 1997), consistently with peraluminous chemical character, which is palaeontological data on the coeval radiolarian consistent with the local occurrence of Mn-rich cherts (not older than 150-160 Ma: De Wever garnet. Mineral and whole-rock compositions and Caby, 1981; Lemoine, 1983; Marcucci and point to orogenic affinity and shallow level Passerini, 1991) which are frequently emplacement (Marroni et al., 1998). interlayered with the basaltic volcanites. Radiometric data indicate that the granitoids were emplaced during late Hercynian times The continental crust material (280-3 10 Ma; Ferrara and Tonarini, 1985). After the Middle Triassic, the granitoids Ophiolitic rocks cropping out in the EL Units experienced cataclastic deformation and were are associated with materials from the lower finally exposed at shallow levels during (mafic and felsic granulite) and upper Jurassic, as testified by the intrusion of basaltic (granitoids) continental crust (Marroni et al., dikes, and by the occurrence of primary 1998, and quoted references). Marroni and stratigraphic contacts between granitoids and Tribuzio ( 1996), based on petrologic and Oxfordian-Callovian radiolarian cherts geochronologic studies, have inferred that the (Marroni et al., 1998, and quoted references). mafic granulites derived from gabbroic protoliths which were emplaced at low crustal Geodynamic inference levels (P about 8 kb) during Lower Carboniferous-Upper Permian times (about 290 The NA ophiolites represent the spatial Ma; Meli et al., 1996). The gabbroic protoliths association of late Jurassic MOR basalts with of the granulites were most likely formed by Proterozoic and Permian mantle peridotites and fractional crystallization of tholeiite primary Jurassic gabbros with MORB affinity: melts and concomitant assimilation of 1) mantle peridotites derived from the sub continental crust material (Montanini et al., continental mantle of the Europe-Adria plate; 1998; Montanini and Tribuzio, 2001). The 2) gabbros were formed by intrusion of felsic granulites are mainly represented by asthenospheric MORB melts into the sub quartz-feldspathic anatectic rocks (Balestrieri continental mantle during lithospheric et al., 1997), which probably resulted from extension, prior to the Jurassic oceanic stage of multi-stage melting of lower crustal basement the Ligurian Tethys; gneisses (Montanini and Tribuzio, 2001). The mafic granulites experienced a 3) basalts were produced by late Jurassic retrograde metamorphic evolution from partial melting of the upwelling asthenospheric granulite- to subgreenschist-facies conditions. mantle during the oceanic opening. Radiometric data (Ar-Ar amphibole age of 228 Structural and petrological evidence does not Ma; Meli et al., 1996) indicate that the support relating the formation of this peculiar granulite- to amphibolite-facies recrys ophiolitic association to active asthenospheric tallization occurred between Permian and upwelling and mid-ocean ridge - transform Middle Triassic times. A similar retrograde fault systems, as common at divergent plate evolution has been also inferred for the felsic margins of mature oceanic basins. granulites that are primarily associated with More properly, these ophiolitic sequences mafic granulites (Marroni et al., 1998). In both represent the lithological associations which mafic and felsic granulites, retrogression is are expected to develop after the break up of commonly accompanied by deformations the continental crust, driven by the passive progressively changing from ductile to brittle. extension of the continental lithosphere. The granitoids are mostly represented by Most probably the NA ophiolites were two-mica granodiorites to leucogranites with formed when the passive, extensional pertithic alkali feldspars, (Ebherardt et al., mechanisms were still dominant, prior to the 180 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA inception of the sea-floor spreading driven by (as recognized in the EL units), it was argued active asthenospheric mantle upwelling. This, that these ophiolites were formed during early more mature, oceanic stage is not recorded by stages of opening of the oceanic basin, to known ophiolite sequences from the NA as following rifting, thinning, and break-up of the well as the whole Alpine-Apennine system. continental crust, and were therefore located in a marginal, pericontinental, position of the oceanic basin (Beccaluva and Piccardo, 1978). Further petrologic investigations on the NA ORIGINAL TECTONIC SETTING OF THE NORTHERN ophiolitic ultramafics, demonstrated the APENNINE 0PHIOLITES depleted compositions of the IL peridotites, and inferred the existence of a residua-melt In the past, different interpretations of the relationship between the peridotites and main structural and/or petrological features of associated MORB magmatism (Ottonello et al., the Alpine-Apennine Jurassic ophiolites have 1984). It was therefore asserted that the IL led to the development of different models for ophiolites represent oceanic lithosphere their original tectonic setting. Besides the produced during a more evolved stage of common interpretation as sections of a MORE evolution of the Ligurian Tethys (Beccaluva et type oceanic lithosphere, mainly based on the al., 1984). The comparison with peridotites MORB affinity of the magmatic rocks, the from different settings in modern oceanic widespread occurrence of peridotites exposed basins emphasized close similarities between on the sea-floor has led some authors to the EL peridotites and the marginal and pre-rift suggest that these ophiolites were formed in a lherzolites, and between the IL ultramafics and transform-zone setting (Gianelli and Principi, mid-ocean ridge peridotites (Piccardo et al., 1977; Lemoine, 1980; Weissert and Bernoulli, 1990). 1985). However, recent petrologic and isotope Investigations on the structure of present-day investigations of the NA ophiolitic mantle oceanic lithosphere have provided increasing ultramafics (as reported in the previous evidence that slow-spreading ridges are chapters) have shown that none of the Ligurian frequently characterized by the direct exposure peridotites can be considered as typical oceanic of serpentinized mantle peridotites on the sea mantle, and that a simple mantle residua floor. These latter are intruded by discrete basaltic melt genetic relationship does not exist gabbroic bodies and only partially covered by in the IL ophiolites. basaltic lava flows. Based on these features, it As recently discussed by Rampone and has been argued by some authors that Alpine Piccardo (2001), it has been definitely Apennine ophiolites represent mature oceanic demonstrated that the EL peridotites consist of lithosphere formed in a slow-spreading ridge fertile subcontinental lithospheric mantle setting (Barrett and Spooner, 1977; Lagabrielle (presumably Proterozoic), whereas the IL and Cannat, 1990; Lagabrielle and Lemoine, mantle ultramafics are depleted peridotites 1997). which experienced MORB-type partial melting Other contributions, however, stressed the during the Permian, i.e. well before production probable subcontinental origin of many mantle of the associated Jurassic basaltic crust. Both rocks flooring the Ligurian ocean (Decandia the EL and IL peridotites display a composite and Elter, 1969, 1972; Piccardo, 1976), and subsolidus retrograde evolution, which reflect opened the discussion on the diversity of the their uplift from lithospheric mantle depths, Alpine-Apennine ophiolites compared with and emplacement on the ocean floor. mature oceanic lithosphere. Based on the The NA ophiolites therefore represent the atypical association of fertile subcontinental spatial association of older (Proterozoic and type lherzolitic mantle and MORB magmatism Permian) subcontinental mantle peridotites, Evolution of the Ligurian Tethys: inference from petrology and geochemist1y of the Ligurian Ophio_lites 181
Lithospheric A Mantle
Asthenosphere I Permo-Carboniferous
B Lower plate
c External
Late Jurassic
Fig . 20 - Model for the evolution of the Ligurian Te thys from late Palaeozoic extension to Jurassic continental break-up and ocean opening (redrawn and updated after Piccardo et al., 1994). The model only considers the Penn ian asthenospheric partial melting and the intrusio n of the manle-derived melts (in black) into the Adria lithosphere; Late Variscan crusta! magmatism and Permian calcalkaline magmatism are not considered. A) Inception of the asymmetric passive extension of the continental lithosphere and of the decompressio nal evolution of the litho spheric mantle (Erro-Tobbio peridotites): I) Upwelling and subsolidus reequil ibration to plagiocl ase -facies of the Erro-Tobbio peridotites; 2) Upwell ing and partial melting of the asthe nospheric mantle (the mantle protoliths for the Internal Liguride peridotites); 3) Permian gabbroic intrusio ns from mantle-derived basaltic melts into the marginal units of the Adria plate. B) Accretion of Permian residual mantle (the Internal Liguride peridotites) 4), after asthenospheric partial melting, to the subco ntinental lithospheric mantle; C) Late Jurassic opening of the Ligurian Tethys and sea-floor exposure of the subcontinental lithospheric mantle. 182 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA which are partly still linked to continental crust Other contributions have proposed the material, and younger (mostly Jurassic) eastward dipping of the detachment zone, i.e. unrelated MORB-type magmatism. This Adria as the upper plate of the extensional peculiar association cannot be reconciled with system. This interpretation is based on present-day mature oceanic lithosphere, where structural, metamorphic and magmatic features the mantle and the associated mafic crust are recorded by the Austroalpine (Sesia-Lanzo) linked by a direct genetic relationship. In and South Alpine (Ivrea-Verbano) units (i.e. contrast, the NA ophiolites most likely the marginal units of the Adria plate), which represent lithological associations which are indicate their involvement in the pre-oceanic expected to develop after the break-up of ex tensional regime (Dal Piaz, 1993; continental crust in response to passive Trommsdorff et al., 1993; Piccardo et al., extension of the lithosphere. This latter is the 1994; Dal Piaz and Martin, 1998). According most suitable geodynamic process to account to these authors, the occurrence of post for the tectonic denudation of large sectors of Hercynian gabbroic bodies in the Austroalpine subcontinental mantle. The exumation of units of the Western-Central Alps testifies the lithospheric mantle in response to passive early intrusion into the extending Adria lithosphere extension has been recently lithosphere of basaltic magmas produced by confirmed by analogue geophysical modelling partial melting of the passively upwelling (Brun and Beslier, 1996) performed to study asthenosphere. Moreover, the original location the modes of passive-margin formation and of the gabbroic bodies indicates that related mantle exumation at continent-ocean asthenospheric partial melting started boundary. asymmetrically with respect to the future axis Mechanisms of passive extension of the of the continental break-up, and that the lithosphere were already invoked in pioneering melting zone shifted progressively towards the studies on the NA ophiolites, to explain the location of the future oceanic basin (fig. 20) peculiar structural features of the Ligurian (Piccardo et al., 1994; Rampone et al., 1998; Tethys seafloor (Decandia and Elter, 1969, Rampone and Piccardo, 2001; Piccardo et al., 1972; Elter, 1972). More recently, models of 2001; and quoted references). asymmetric passive extension of the lithosphere by means of normal simple-shear mechanisms (according to the model of SUMMARY AND CONCLUSION Wernicke, 1985), have been proposed to account for some asymmetrical features which Ophiolites exposed along the WA-NA have been recognized on both sides of the pre orogenic belt represent the oceanic lithosphere oceanic continental rift of the Ligurian Tethys. of the Ligurian Tethys ocean which separated, Some authors have inferred the large-scale during Late Jurassic-Cretaceous times, the extensional geometry of the oceanic rift of the Europe and Adria continental blocks. These Ligurian Tethys by the comparison of the ophiolites are characterized by: i) dominant tectono-sedimentary evolution of the conjugate fertile, cpx-rich mantle lherzolites, while more (Europe and Adria) continental margins. Based depleted peridotites are scarce; ii) gabbroic on the contrast between syn-extensional (Lias intrusives and basaltic volcanites with MORB Dogger) subsidence of the Adria margin and affinity. uplift of the Brianc;onnais ridge of the WA-NA ophiolites show peculiar structural European margin, it has been argued that Adria and petrological characteristics which indicate and Europe were, respectively, the lower and that: i) mantle ultramafics underwent a upper plate margin during simple-shear composite subsolidus evolution from extension (Lemoine et al., 1987; Hoogerduijn subcontinental lithospheric mantle depths et al., 1993; Vissers et al., 1991). towards the sea-floor; ii) the gabbroic rocks Evolution of the Ligurian Te thys: inferencefi·om petrology and geochemistJy of the Ligurian Ophiolites 183 were intruded into mantle peridotites during Sr/Nd model ages indicate that this melting their decompressional evolution; iii) mantle event occurred during Permian (275 Ma). peridotites and gabbros were exposed at the Intrusives rocks (ultramafic cumulates, Mg sea-floor before basaltic extrusion and Al-gabbros, Fe- Ti -gabbroids and radiolarian chert deposition. plagiogranites) represent the crystallization Stratigraphic-structural evidence points out products, during low pressure fractionation, of that the Ligurian Tethys was floored by a tholeiitic MORB-type basaltic magmas, peridotite-gabbro basement, subsequently similarly to the whole intrusive rocks of covered by a discontinuous layer of pillowed the WA-NA ophiolites. Available basaltic flows and radiolarian cherts, i.e. the geochronological data on the NA gabbroic first oceanic sediments. Palaeontological ages rocks indicate vmiable intrusion ages, ranging of the radiolarian cherts and isotopic (U/Pb) from about 185 M a (some EL gabbros) to zircon ages of acidic differentiates, linked to about 160 Ma (some IL gabbros): it must be the basaltic volcanites, concordantly indicate mentioned that some W A ophiolitic gabbros Late J urassic (160-150 M a) ages for the (i.e. the Chenaillet body) record Triassic age of inception of the oceanic stage, along the whole intrusion. Accordingly, the gabbroic rocks are Ligurian Tethys. significantly younger with respect to the partial In the Ligurian ophiolites, primary melting events recorded by the associated stratigraphic-structural relationships are mantle peridotites and, moreover, they are preserved in the IL Units, while in the EL significantly older than the true oceanic Units, which are considered to derive from a magmatism, represented by the Late Jurassic paleogeographic realm transitional between the basaltic extrusion. continental (Adria) and oceanic (IL) domains, The continental mafic granuli tes were mantle peridotites and basalts crop out as giant derived from gabbroic protoliths which were olistholiths within Cretaceous/Eocene flysch emplaced at low crustal levels (P about 8 kb) sequences, where they are associated to during Lower Carboniferous-Upper Permian continental crust material (Permian granulites times (about 290 Ma). The gabbro-derived and Hercynian granitoids). mafic granulites experienced a retrograde Mantle peridotites from the EL Units are metamorphic evolution from granulite- to fertile, cpx-rich lherzolites: Sr/Nd models ages subgreenschist-facies conditions. Radiometric indicate minimum Proterozoic ages of data (Ar-Ar amphibole age of 228 Ma) suggest differentiation. This is interpreted as the that the granulite- to amphibolite-facies accretion age of the EL mantle to the Europe recrystallization occurred between Permian and Adria subcontinental lithosphere, where the Middle Triassic times. The continental ultramafics underwent complete equilibration granitoids were emplaced at shallow levels at temperature below 1100°C, under spinel during late Hercynian times (about 310 M a). facies conditions. Sm/Nd pl-cpx isochrons After the Middle Triassic, the granitoids indicate that transition from spinel- to experienced cataclastic deformation and were plagioclase-facies conditions during the finally exposed at shallow levels during decompressional evolution of the Jurassic, as testified by the intrusion of basaltic subcontinental mantle started from Late dikes, and by the occurrence of primary Palaeozoic (273-313 Ma: ET peridotites of the stratigraphic contacts between granitoids and Voltri Massif) and continued up to Jurassic Oxfordian-Callovian radiolarian cherts. (165 Ma: EL peridotites). Mantle peridotites The composite decompressional evolution of from the IL Units are significantly depleted, the peridotites and the associated continental cpx-poor, lherzolites: they are interpreted as gabbro-derived granulites and granitoids, refractory residua formed by partial melting of deriving from the continental lithosphere (crust an asthenospheric MORB-type mantle source. and upper mantle) of the Europe-Adria system, 184 G.B. PICCARDO, E. RAMPONE, A. ROMAIRONE, M. SCAMBELLURI, R. TRIBUZIO and C. BERETTA is related to the pre-oceanic rifting processes deformation and thus became detachments and which were active within the Europe-Adria shear zones which allowed delamination of the lithosphere prior to the ocean opening. subducting oceanic lithosphere. The Permian age of partial melting of the IL Serpentinization did not produce significant peridotites and the Late Palaeozoic to Jurassic changes in the major and rare earth element subsolidus decompression of the ET and EL compositions of the primary ultramafic rocks, peridotites, and the associated continental but was accompanied by uptake of trace rocks, indicate that lithospheric extension of amounts of marine Sr, Cl and alkalis. Mafic the Europe-Adria continental lithosphere and rocks at this stage locally underwent asthenospheric upwelling were already active metasomatic exchanges with their host since late Palaeozoic times. This interpretation ultramafites leading first to a stage of Mg is strongly supported by the presence of huge enrichment and chloritization of the mafic post-Variscan gabbroic bodies, derived by rocks, followed by a stage of ea-enrichment parental MORB magmas, which are intruded and rodingitization. Serpentinization played a into the extending lithosphere of the Adria relevant control on the composition of fluid continental margin (Austroalpine Units of the phases evolved during subduction burial of Western-Central Alps) and by the presence of ultramafic rocks. Subduction of the Ligurian residual mantle peridotites (IL peridotites) ultramafic rocks was accompanied by prograde which underwent partial melting during reactions, culminating in one main high Permian and were accreted to the sub pressure event of partial dewatering, which led continental lithosphere of the Europe-Adria to fluid production and to formation of plate. metamorphic alpine olivine in equilibrium with The peculiar oceanic lithosphere of the antigorite, diopside and Ti-clinohumite. Main Jurassic Ligurian Tethys (i.e. the association of evidence of this fact is the widespread Proterozoic and Permian subcontinental mantle formation of olivine-bearing metamorphic vein peridotites, Triassic to J urassic gabbroic systems coeval with eclogitization of the intrusives and Late Jurassic MORB volcanites) associated mafic rocks. developed after the Jurassic break-up of the Mafic rocks at this stage developed different continental crust in response to passive peak assemblages depending on their bulk rock extension of the Europe-Adria continental compositions and on their pre-subduction lithosphere. This is the most suitable evolutions stories: garnet-rutile - omphacite geodynamic process to account for the tectonic (Fe-Ti metagabbros), garnet-chloritoid denudation of large sectors of subcontinental omphaci te-zoisite-tal c (Mg-metagabbr os), mantle. grossula r-zoi si te-chlori te-di op side One main effect of mantle exposure at ( metarodingi te), Ti -clinoh umite-di opside superficial oceanic (and suboceanic) settings is chlorite-magnetite (Mg-enriched the widespread hydration of peridotites, leading metagabbros). The pressures and temperatures to a significant ductility change. The oceanic of eclogitic recrystallization range from P > 13 serpentinization of the ET peridotite was Kbar and T 450-500°C in the Beigua Unit, to P heterogeneous, and brought to the spatial 20-25 Kbar and T 550-650°C in the ET Unit. association of extremely serpentinized Despite subduction led in both Units to ultramafit�s close to mantle peridotites less partial dewatering of the ultramafic rocks, affected by serpentinization. Such a antigorite serpentine survived high pressure heterogeneity in water distribution in the metamorphism as a stable mineral phase in the oceanic lithosphere played a major control on new high-pressure olivine assemblage. its behaviour during later Alpine convergence Persistence of large volumes of low-density and subduction. Serpentinites worked as soft buoyant serpentinites in the deep roots of the horizons which accomplished ductile Alpine orogeny provides a mechanism for the Evolution of the Ligurian Tethys: inference from petrology and geochemist1y of the Ligurian Ophiolites 185 exhumation of eclogites and other high to thermochronological constraints from zircon ultrahigh presure rocks from mantle depths. fission-track data. Comptes Rendus de 1' Academie The eclogitized ultramafic rocks still des Sciences, 324, 87-94. BARNICOAT A.C. and CARTWRIGHT I. (1997) - The preserve oxygen isotope signatures acquired at gabbro-eclogite transition: an oxygen isotope and oceanic settings, indicating that the fluid petrographic study of west Alpine ophiolites. J. recycled at this stage was the one incorporated Metam. Geol., 15, 93-104. during exposure close to the oceanic floor. BARRETT J.J. and SPOONER E.F.C. 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