This article was downloaded by: [University of Genova], [Giovanni B. Piccardo] On: 30 November 2012, At: 06:56 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tigr20 Subduction of a fossil slow ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif, Ligurian Alps, Northwest Italy Giovanni B. Piccardo a a DISTAV, University of Genova, Corso Europa 26, I-16132, Genova, Italy Version of record first published: 30 Nov 2012. To cite this article: Giovanni B. Piccardo (2012): Subduction of a fossil slow ultraslow spreading ocean: a petrologyconstrained geodynamic model based on the Voltri Massif, Ligurian Alps, Northwest Italy, International Geology Review, DOI:10.1080/00206814.2012.746806 To link to this article: http://dx.doi.org/10.1080/00206814.2012.746806 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
International Geology Review ifirst, 2012, 1 17 Subduction of a fossil slow ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif, Ligurian Alps, Northwest Italy Giovanni B. Piccardo* DISTAV, University of Genova, Corso Europa 26, I-16132 Genova, Italy (Accepted 29 November 2012) Slow ultraslow spreading oceans are mostly floored by mantle peridotites and are typified by rifted continental margins, where subcontinental lithospheric mantle is preserved. Structural and petrologic investigations of the high-pressure (HP) Alpine Voltri Massif ophiolites, which were derived from the Late Jurassic Ligurian Tethys fossil slow ultraslow spreading ocean, reveal the fate of the oceanic peridotites/serpentinites during subduction to depths involving eclogite-facies conditions, followed by exhumation. The Ligurian Tethys was formed by continental extension within the Europe Adria lithosphere and consisted of sea-floor exposed mantle peridotites with an uppermost layer of oceanic serpentinites and of subcontinental lithospheric mantle at the rifted continental margins. Plate convergence caused eastward subduction of the oceanic lithosphere of the Europe plate and the uppermost serpentinite layer of the subducting slab formed an antigorite serpentinite-subduction channel. Sectors of the rather unaltered mantle lithosphere of the Adria extended margin underwent ablative subduction and were detached, embedded, and buried to eclogite-facies conditions within the serpentinite-subduction channel. At such P T conditions, antigorite serpentinites from the oceanic slab underwent partial HP dehydration (antigorite dewatering and growth of new olivine). Water fluxing from partial dehydration of host serpentinites caused partial HP hydration (growth of Ti-clinohumite and antigorite) of the subducted Adria margin peridotites. The serpentinite-subduction channel (future Beigua serpentinites), acting as a low-viscosity carrier for high-density subducted rocks, allowed rapid exhumation of the almost unaltered Adria peridotites (future Erro Tobbio peridotites) and their emplacement into the Voltri Massif orogenic edifice. Over in the past 35 years, this unique geologic architecture has allowed us to investigate the pristine structural and compositional mantle features of the subcontinental Erro Tobbio peridotites and to clarify the main steps of the pre-oceanic extensional, tectonic magmatic history of the Europe Adria asthenosphere lithosphere system, which led to the formation of the Ligurian Tethys. Our present knowledge of the Voltri Massif provides fundamental information for enhanced understanding, from a mantle perspective, of formation, subduction, and exhumation of oceanic and marginal lithosphere of slow ultraslow spreading oceans. Keywords: Jurassic Ligurian Tethys; fossil slow ultraslow spreading ocean; subduction; exhumation and tectonic emplacement; Voltri Massif; Beigua serpentinites; Erro Tobbio peridotites; HP eclogite-facies dehydration; HP eclogitefacies hydration 1. Introduction Modern and fossil slow ultraslow spreading oceans are characterized by rifted continental margins and sea-floor exposure of serpentinized mantle peridotites. They generally exhibit a discontinuous cover of basaltic lavas, volcanic and non-volcanic sectors, and a lack of gabbroic layer 3. In present-day oceanic basins, the direct exposure at the sea floor of crust-free mantle lithosphere is more common than previously recognized (Dick et al. 1984, 2006; Cannat 1993, 1996; Cannat et al. 1997; Bonatti et al. 2001; Michael et al. 2003): it has been suggested that nearly half of the global mid-ocean ridge system is made of mantle peridotites. Recent investigations of the Southwest Indian Ridge and the Arctic Ridge have revealed an ultraslowspreading class of ocean ridges that is characterized by intermittent volcanism and continuous emplacement of mantle to the sea floor over large regions, whereas the spreading rate is lower than 20 mm year 1 (Dick et al. 2003 and references therein). Slow ultraslow spreading oceans are characterized by the presence of extended continental margins and ocean continent transition (OCT) zones, where the lithospheric subcontinental mantle of the rifted system is preserved (e.g. Manatschal et al. 2007; Péron-Pinvidic and Manatschal 2009; and references therein). *Email: piccardo@dipteris.unige.it; giovanni.b.piccardo@gmail.com ISSN 0020-6814 print/issn 1938-2839 online 2012 Taylor & Francis http://dx.doi.org/10.1080/00206814.2012.746806 http://www.tandfonline.com
2 G.B. Piccardo The Jurassic Ligurian Tethys (or Ligurian-Piedmont oceanic basin) has been recognized as the fossil analogue of a modern slow ultraslow spreading ocean (i.e. Tribuzio et al. 2000; Piccardo 2008; and quoted references). After ocean closure, ophiolites from the Ligurian Tethys were tectonically emplaced into the Alpine and Apennine orogenic belts, having undergone high-pressure (HP) eclogitefacies subduction (the westward-emplaced Western and Ligurian Alps ophiolites) or shallow level obduction (the eastward-emplaced Northern Apennine ophiolites). Studies of HP Alpine ophiolites, derived from the Late Jurassic Ligurian Tethys fossil slow ultraslow spreading basin, provide important constraints on the petrologic and geodynamic processes involved in their evolution during subduction and exhumation. Both marginal peridotites (Erro Tobbio) and oceanic serpentinites (Beigua) from the HP Alpine Voltri Massif ophiolites (Ligurian Alps, Northwest Italy) bear the petrologic record of HP recrystallization under eclogitefacies conditions. Our field and petrologic investigations provide significant information to unravel the fate of these ultramafic rocks during the subduction/exhumation cycle. We suggest that the uppermost hydrated level of the oceanic lithosphere (serpentinized peridotites and their volcanicsedimentary cover) formed an antigorite serpentinitesubduction channel which became detached and enclosed subcontinental mantle sections from the Adria margin, carrying them to eclogite-facies P T conditions and allowing their rapid exhumation during subsequent continental collision. The Erro Tobbio peridotites, moreover, mostly preserve records of their pristine mantle characteristics and furnish the unique opportunity to unravel the mantle lithosphere evolution during pre-oceanic continental extension and rifting of a slow ultraslow spreading basin, from a mantle perspective. 1.1. Goal of this work This article aims at providing new detailed field and petrologic investigations into the ultramafic units of the Voltri Massif and a comprehensive discussion of the fundamental characteristics recorded in its different ultramafic units, taking into account the pertinent bibliography. We hope to improve significantly the pertinent evidence base which is relevant to reconstruction of the entire evolution of the Voltri Massif, starting from the pre-oceanic continental extension, documenting the palaeogeographic setting of the future Voltri Massif units, and a deepening of the knowledge of the mechanism of subduction and exhumation during closure of the Ligurian Tethys basin. We intend to provide important constraints regarding the understanding of subduction of peridotite/serpentinitefloored oceanic lithosphere from slow ultraslow spreading oceans, and in particular, concerning: (1) the petrologic and geodynamic processes which involve marginal peridotites and oceanic serpentinites subducted to eclogite-facies P T conditions; (2) the role of the antigorite serpentinite-subduction channel in both underflow and exhumation; and (3) the evolution of the oceanic basin, from continental extension and ocean opening, to plate convergence (subduction and exhumation) and orogenic emplacement. 1.2. Case study of the Voltri Massif The Alpine-Apennine chain is characterized by the widespread exposure of ophiolites. Modern studies on ophiolites from the Ligurian Alps and the Northern Apennines were set up at the end of the 1960s. It was recognized that they are characterized by the association of basic extrusive rocks, gabbro-peridotite cumulates, and ultramafic tectonites, which were reinterpreted as tectonic fragments of the ocean floor of the Northern Apennine oceanic basin (Bezzi and Piccardo 1971). The Voltri Massif is the largest metamorphic ophiolite Massif in the Alpine Apennine chain (Figure 1). It occupies the easternmost part of the Ligurian Alps, and it is separated from the Ligurian Apennines to the east by the Sestri Voltaggio Zone. The Voltri Massif consists of calcschists (metamorphic oceanic sediments), prasinites (metamorphic MORB-type oceanic volcanites), HP metagabbros, eclogites and rodingites (metamorphic MORB-type oceanic intrusives), and huge masses of mantle peridotites and antigorite serpentine schists (e.g. Messiga and Piccardo 1974). A combined structural and petrologic study of the Voltri Massif (Chiesa et al. 1975) and the surrounding Units recognized different structural units, among which are: (i) the Voltri Rossiglione Unit, consisting of calc-schists and meta-volcanics with relics of HP assemblages; (ii) the Beigua Unit, consisting of antigorite serpentinites with relics of HP assemblages and eclogitic/rodingitic metagabbros; and (iii) the Erro Tobbio Unit, consisting of almost unaltered mantle peridotites. Chiesa et al. (1975), inspired by the papers of Dal Piaz (1974a, 1974b), depicted the fundamental architecture of the Voltri Massif and the most important constraints to its geodynamic evolution during closure of the Late Jurassic Ligure Piemontese basin. They suggest that the oceanic lithosphere of the Europe plate underwent eastward subduction under the insubrian (Adria) margin, and that the tectonic units of the Voltri Massif were tectonically exhumed and were emplaced onto the Briançonnaise (Europe) margin. Chiesa et al. (1975) considered the Erro Tobbio peridotites as deriving from the insubrian (Adria) subcontinental lithosphere, which was thrusted above the HP Units of the Voltri Massif during continental collision, without having
International Geology Review 3 44 20 I 44 40 I 8 32 I 9 01 I Varazze Mt Tobbio Voltri LIGURIAN SEA Genova 5 km N Erro Tobbio peridotite Beigua serpentinite Voltri Rossiglione calcschists and metavolcanics Tertiary sediments Savona basement Sestri Voltaggio Zone Liguride Units Genova urban area Figure 1. Sketch map of the Voltri Massif and surrounding terranes [redrawn and modified after Chiesa et al. (1975)]. undergone subduction. The Voltri Massif was overridden by tectonic units from the insubrian plate, such as: (i) the Montenotte Unit which consists of blueschist rocks derived from the insubrian oceanic lithosphere they have been recognized to be related to the Voltaggio Unit, and some authors have evidenced that sectors of this overridden unit pertained to the Cravasco(Voltaggio) Montenotte Unit (e.g. Messiga et al. 1995) (in fact, remnants of these blueschist ophiolites are widespread onto the Erro Tobbio Unit); (ii) granitic rocks, which are actually represented by klippens in the Savona region, which bear records of blueschist metamorphism (i.e. Haccard et al. 1972); and (iii) the Triassic Liassic unit of the Sestri Voltaggio Unit, which shows marked affinity with the Tuscan Noric Retic units, and represents isolated klippens onto the Voltri Massif metamorphic units. Accordingly, also the Adria margin, both oceanic and continental, was involved in subduction. From the beginning of the 1970s, modern petrologic studies have been developed on the Voltri Massif eclogites in antigoritic serpentinites (e.g. Bocchio and Mottana 1974; Mottana and Bocchio 1975; Ernst 1976; Cortesogno et al. 1977) and on the Erro Tobbio peridotites (Ernst and Piccardo 1979; Ottonello et al. 1979; Ernst et al. 1983; Messiga et al. 1983). Cimmino and Lucchetti (1975) studied Ti-clinohumite in the Voltri serpentinites. Cimmino et al. (1979), Cimmino et al. (1981), and Piccardo et al. (1980) recognized in the antigoritic serpentinites of the Voltri Massif the presence of olivine + antigorite + Ti-clinohumite ± diopside mineral assemblages and suggested that the Voltri antigoritic serpentinites recrystallized at the highest P T conditions reached during subduction, comparable to the eclogitic recrystallization of the mafic rocks. Piccardo et al. (1989) and Scambelluri et al. (1991) described the presence of olivine + antigorite + Ti-clinohumite assemblages in the Erro Tobbio peridotites, suggesting that also these peridotites underwent subduction to HP (eclogitic) conditions. In the last 20 years, several studies on the Voltri Massif have been dedicated to petrology and geochemistry of the Erro Tobbio peridotites in order to investigate both (i) their pre-oceanic evolution (e.g. Drury et al. 1990; Hoogerduijn Strating et al. 1990, 1993; Vissers et al. 1991; Rampone et al. 2004, 2005; Piccardo and Vissers 2007) and (ii) their subduction history (e.g. Scambelluri et al. 1991, 1995, 1997; Messiga et al. 1995; Hermann et al. 2000; Früh-Green et al. 2001; Malatesta et al. 2012a, 2012b). Nonetheless, since the study by Chiesa et al. (1975), little attention has been dedicated to the whole geodynamic evolution, from continental extension to orogenic emplacement, of the sector of the Ligure Piemontese basin, which originated the different units of the Voltri Massif. Recently, Vignaroli et al. (2008) recognized in the Voltri Massif and surrounding domains (i) a lower tectonic complex (LTC), constituted by polymetamorphic eclogitic mega-boudins equivalent to serpentinites with eclogites (i.e. the Beigua Unit) and to metasediments (i.e. the Voltri Rossiglione Unit) and (ii) an upper tectonic complex (UTC), composed of pre-alpine rocks locally affected by Alpine eclogitic metamorphism (the Erro Tobbio Unit), the blueschist units (Cravasco Voltaggio Unit), and a very low-grade Alpine tectonic mélange. This model recalls in part the model earlier proposed by Chiesa et al. (1975). Recently, Malatesta et al. (2012a) suggested that the eastern sector of the Voltri Massif did not act as a group of massive tectonic units but can be instead interpreted as a tectonic subduction mélange in which deformed serpentinites and metasediments enclose variably deformed lenses of metagabbro, metabasite, and
4 G.B. Piccardo peridotite. Piccardo (2012) recognized that the Voltri Massif terrains went down to eclogite-facies conditions inside an antigorite subduction channel and were rapidly exhumed and emplaced in the European margin as a subduction mélange. This was mostly composed of the Beigua antigorite serpentinites (former oceanic serpentinites) enclosing huge bodies of eclogitic metagabbros and almost unaltered mantle peridotites (former subcontinental peridotites from the Adria Margin). 2. Palaeogeographic pertinence of the Voltri Massif ultramafics Tectonic-metamorphic units from oceanic lithosphere (the Beigua ultramafic rocks and mafic intrusives), associated with their volcanic-sedimentary cover (the Voltri Rossiglione calc-schists and meta-volcanics) and from the insubrian (Adria) subcontinental mantle (the Erro Tobbio (A) (B) Continental lithosphere EUROPE PLATE VM LIGURIAN TETHYS Oceanic lithospher Voltri Massif traverse LA IL ET EL Future subduction peridotites) of the Late Jurassic Ligure Piemontese basin, were earlier recognized within the Voltri Massif by Chiesa et al. (1975). The pertinence of the Beigua serpentinites to the lithospheric mantle peridotites exhumed and exposed to the sea floor has been frequently sustained (see Piccardo and Vissers 2007 and references therein). On the basis of the observations carried out by Piccardo et al. (1977), the Beigua unit was thought to represent subducted and dismembered oceanic crust of the Ligurian Tethys, whereas the Erro Tobbio peridotites were interpreted as mantle material from the hanging wall of the subduction zone (Drury et al. 1990) (Figure 2). Recently Scambelluri and Tonarini (2012) suggested, on the basis of B, O H, and Sr isotope systems, that the Erro Tobbio peridotites were earlier serpentinized when resident in the mantle wedge of the subduction system, confirming the pertinence of the Erro Tobbio peridotites to the subcontinental mantle of the Adria rifted margin. Jurassic Shear Zone Southern Alps Ivrea Zone Future obduction of Ligurian Units Erro-Tobbio protolith ADRIA PLATE Austroalpine 1 2 Beigua protolith Figure 2. The Jurassic Ligurian Tethys at its maximum extension [Redrawn and modified after Dal Piaz (1974a, 1974b)]. (A) The broadly E W section connects the present position of the Voltri-Massif and the palaeo-geographic position of the Erro Tobbio peridotites subcontinental mantle of the Adria (extended margin). The sea-floor along the traverse was mostly represented by the uppermost level of the exhumed and sea-floor-exposed oceanic mantle lithosphere, which underwent significant sea-floor alteration (the future Beigua serpentinites). LA, Lanzo; VM, Voltri Massif; ET, Erro Tobbio; IL, Internal Ligurides; EL, External Ligurides. (B) Section along the eastern part of the traverse, after inception of eastward subducion. (1) Oceanic mantle lithosphere, (2) subcontinental mantle lithosphere of the Adria margin.
International Geology Review 5 In the following, we will mostly focus on the ultramafic rocks (i.e. the Beigua serpentinite and the Erro Tobbio peridotite), which better preserve records of the pre-oceanic, oceanic, subduction-related, and orogenic evolution and provide more reliable constraints for the geodynamic evolution of the Voltri Massif. 3. Field relationships During the field work, we thoroughly revisited the real areal extension of the different ultramafic rock types, the position and characteristics of their contacts, and their structural relationships. Petrologic investigations were concentrated on structural and compositional features of the different rock types, the mineral reactions related to their P T evolution, and the geodynamic relevance of the pertinent mineral assemblages. Our field investigations evidence that pluri-km-scale volumes of almost unaltered mantle ultramafics of the Erro Tobbio peridotites are partially embedded within a carapace of the Beigua rodingite-bearing antigorite serpentinite-schists. In some places of both the eastern and western parts of the Voltri Massif, km-scale bodies of unaltered Erro Tobbio peridotites are completely embedded into the Beigua rodingite-bearing antigorite serpentinites. The contacts between the two rock types are generally sharp and frequently tectonized. In some outcrops, the outer margins of the Erro Tobbio peridotite bodies show metre- to decametre-wide areas of meta-peridotites (i.e. HP eclogite-facies-recrystallized mantle peridotites, see below) and both peridotites and meta-peridotites are cut by mm- to cm-wide olivine + Ti-clinohumite veins which propagate for some metres inside the unaltered Erro Tobbio peridotite. As a rule, the Erro Tobbio peridotite bodies show a subsequent antigorite serpentinization for a few to several metres from the contacts. The discordant directions of the serpentineschists foliation of the Beigua serpentinites and of the structural features (i.e. pyroxenite bands, mantle shear zones, etc.) preserved in the Erro Tobbio peridotites are frequently clearly recognizable. In most cases, the Beigua serpentinite schistosity envelopes the Erro Tobbio peridotite bodies, which represent mega-boudins within the Beigua serpentinite-schists. When considering the whole Voltri Massif units and, in particular, the structural relationships between the serpentinite units, the peridotite units and the calc-schists + prasinites units, as already depicted by Chiesa et al. (1975), the composite edifice generally consists of (from bottom to top) (i) Voltri Rossiglione-type calcschists + prasinites, (ii) Beigua-type serpentinites and serpentineschists, and (iii) Erro Tobbio-type peridotites. An important exception is represented by a discontinuous lowermost level of Beigua-type serpentinites lying directly on top of the continental basement rocks (the San Luca- Colma and Ponzema antigorite serpentinite units of Chiesa et al. 1975). Accordingly, the Beigua-type antigorite serpentinites frequently represent (i) the lowermost level over the continental basement and (ii) the uppermost level over the Erro Tobbio peridotites. Moreover, huge bodies of eclogitic meta-gabbros and peridotites are completely and chaotically enclosed in the Beigua antigorite serpentinites, suggesting the emplacement of all these rocks as a tectonic subduction melange (Figures 3E and 3F). This indicates that the Voltri Massif architecture was rather composite after accretion onto the European continental margin. 4. Main structural petrologic characteristics 4.1. The Beigua serpentinite The mantle protoliths of the Beigua serpentinites derive from the more internal, oceanic settings of the basin and they were mostly represented by exhumed and sea-floor exposed, melt-reacted peridotites (i.e. reactive spinel harzburgites and plagioclase-enriched peridotites, see Piccardo and Vissers 2007 and references therein), with widespread gabbroic intrusions and dikes. The Beigua antigorite serpentinites show, in some outcrops, the close association with metamorphic oceanic ophicalcites (tectonic and sedimentary ocean-floor serpentinite breccias) and metamorphic basaltic breccias (Figures 3A and 3B). This evidence confirms the origin of the Beigua serpentinites from the uppermost level of the sea-floor exhumed and exposed lithospheric peridotites of the basin. Accordingly, the Beigua serpentinites represent, together with the discontinuous cover of basalts and oceanic sediments (HP meta-basites and calc-schists of the Voltri Rossiglione Unit), the uppermost level of the oceanic lithosphere, which underwent significant low-grade alteration (i.e. serpentinization and rodingitization) when it was exposed at, or close to, the sea floor. Large bodies of eclogitic meta-gabbros (Figures 3E and 3F) and widespread rodingitic meta-gabbroic dikes are primarily associated to the Beigua serpentinites. 4.1.1. The antigorite serpentinites Serpentinites consist of antigorite and magnetite (±chlorite) assemblages, which, in places, show textural and mineralogical relics (e.g. Al Cr-rich clinopyroxene porphyroclasts) of the lithospheric mantle protolith. Due to effects of strain partitioning, some undeformed domains within the serpentine-schists preserve antigorite mesh textures marked by magnetite trails (Figures 4A and 4B), fine-grained cm-scale iso-oriented antigorite aggregates (Figure 4C), pseudomorphs of Ti-free metamorphic diopside after Ti-bearing mantle clinopyroxene (Figure 4D), and chlorite coronas surrounding magnetite aggregates.
6 G.B. Piccardo (A) (B) (C) (D) (E) 1 2 1 200 m 100 m Figure 3. Field evidence. (A and B) Eclogitic basaltic breccias primarily related to the Beigua serpentinites. (B and C) Field relationships between Beigua serpentinites and Erro Tobbio peridotites. (C) Carapace of Beigua serpentinites (grey) (1) partially enveloping a km-scale body of unaltered Erro Tobbio peridotites (reddish) (2). (D) Km-scale body of unaltered Erro Tobbio peridotites (reddish) (2) completely embedded into Beigua serpentinites (grey) (1). (E and F) Beigua serpentinite melange enclosing huge ellipsoidal bodies of eclogitic meta-gabbros. The mesh textures are structural relics inherited by previous oceanic serpentinization of olivine (i.e. previous mesh textures of lizardite + chrysotile). The antigorite + Al-poor chlorite + diopside fine-grained iso-oriented aggregates are pseudomorphs after previous cleavage mantle pyroxenes, whereas the Al-rich chlorite + magnetite aggregates replace pristine mantle spinel (e.g. Piccardo et al. 1980). Accordingly, the Beigua serpentinites underwent significant low-grade sea-floor hydrous alteration, which was followed by an almost complete antigorite recrystallization. These ultramafics are characterized by the subsequent widespread crystallization of HP olivine (as acicular aggregates) + antigorite + diopside ± chlorite ± magnetite assemblages, formed after serpentine minerals and primary mineralogical relics, which post-date the early antigorite formation (Figures 4E 4H). The Beigua HP-recrystallized serpentinites are completely free of Ti-clinohumite within the HP olivine + antigorite assemblages of the bulk rocks. This indicates that Ti of the mantle clinopyroxene and (F) 2 spinel (particularly Ti rich in plagioclase impregnated peridotites), which are the most important repository of Ti in mantle assemblages, was released during oceanic alteration prior to subduction, since mantle clinopyroxene and spinel recrystallized to Ti-poor/free diopside and magnetite. Moreover, olivine of the acicular aggregates has sistematically lower in Mg# (80.5 82.5) and higher in MnO (0.35 0.80 wt.%) contents with respect to the mantle olivine (MnO ranging between 0.1 and 0.2 wt.%), most probably related to Mn uptake by the system during oceanfloor recrystallization. In fact, Edmonds et al. (2003), studying the Gakkel Ridge, recently suggested that several chemical tracers such as manganese are diagnostic of a hydrothermal origin. 4.1.2. The eclogitic metagabbros Eclogitic metagabbroic lenses are embedded within the Beigua serpentinites. They frequently preserve the primary structural and compositional characteristics of the
International Geology Review 7 (A) (B) 1 mm 1 mm (C) (D) (E) (G) 1 mm 500 µm 1 cm (F) (H) 0.5 cm 500 µm 500 µm Figure 4. Beigua antigorite serpentinite. Early antigorite recrystallization after ocean-floor alteration. (A and B) Thin section (parallel and crossed nicols) of isotropic antigorite aggregates (B) which shows magnetite trails marking the previous shape of the oceanic serpentinite mesh textures (A). (C) Fine-grained, iso-oriented antigorite crystal pseudomorphs after previous cleavaged mantle orthopyroxene. (D) Former Ti-bearing mantle clinopyroxene recrystallized to Ti-free diposide and antigorite + chlorite flakes. HP eclogite-facies partial de-hydration. (E and F) Field evidence of radial aggregate of acicular olivine growing from antigorite. (G and H) Thin section (parallel and crossed nicols): acicular olivine aggregate growing at the expense of isotropic antigorite. Mg Al or Fe Ti gabbroic protoliths. Structural and chemical features suggest that they mostly escaped oceanic alteration (i.e. Na Si depletion and Ca Mg enrichment) and were recrystallized to eclogite facies conditions during subduction (e.g. Ernst 1976; Cortesogno et al. 1977; Ernst et al. 1983; Messiga et al. 1983). The two different protoliths developed different HP assemblages: (i) Na-clinopyroxene + zoisite + Alm-rich garnet + talc, in the Mg Al gabbros (e.g. Messiga et al. 1983); and (ii) Almrich garnet + omphacite + rutile, in the Fe Ti gabbros (e.g. Cortesogno et al. 1977). More recently, eclogite rocks from the Beigua serpentinites have been studied by Liou et al. (1998) and Federico et al. (2005). 4.1.3. The mafic dikes The Beigua serpentinites are frequently cut by mafic dikes, which show primary intrusive relationships. On the basis of some textural, structural, and geochemical characters (i.e. their Al, Fe, Ti, Ni,Cr, Y, and Zr contents), both Mg-gabbroic and Fe-gabbroic protoliths have been recognized (Cortesogno et al. 1977; Ernst et al. 1983; Messiga et al. 1983). Their whole rock compositions show the peculiar characteristics of sea-floor metasomatism and rodingitization (i.e. silica alkali depletion, Ca Mg enrichment) (Piccardo et al. 1980; Scambelluri and Rampone 1999). The dominant metamorphic minerals of the Beigua rodingites are Ca-rich garnet and diopside with minor chlorite, replacing the whole hydrous Ca-bearing phases
8 G.B. Piccardo (hydrogrossular, vesuvianite, prehnite, and epidote), which were formed during the oceanic alteration. Accordingly, the Beigua rodingites, characterized by the mineral assemblage composed by Na-poor clinopyroxene + Ca-rich garnet, were recrystallized to meta-rodingites during the subsequent subduction. In some cases, garnets preserve cores with relatively high Almandine contents, which have been interpreted as records of the highest pressure conditions reached by these rocks (Cimmino et al. 1981) during subduction. In a few places, thin cm-wide gabbroic dikelets of pristine Fe-gabbroic and dioritic rocks preserve textural and mineralogical relics of Ti-augite porphyroclasts, Fe Ti-ores, and apatite. They show sea-floor alteration (bulk rock Si-alkali depletion and Mg-enrichment) with respect to the corresponding unaltered rocks. They have been subsequently almost completely recrystallized to HP diopside + antigorite + olivine assemblages, which are rich in Ticlinohumite (Cimmino et al. 1979; Piccardo et al. 1980; Scambelluri and Rampone 1999). In this case, Ti-bearing phases (i.e. Ti-augite and Fe Ti ores) should have survived during subduction and released Ti at the highest pressure conditions to form the very local Ti-clinohumite enrichments in the Beigua HP-recrystallized serpentinites. The presence and abundance within the Beigua serpentinites of meta-rodingite dikes, which are products of HP eclogite-facies recrystallization of former oceanic lowgrade rodingites, represent very useful markers to identify the oceanic pertinence of the host serpentinites. Moreover, the local preservation of metamorphic Tifree, Mn-enriched olivine acicular aggregates within low strain domains in antigorite serpentinites help us to recognize and ascribe the host serpentinite to the Beigua antigorite serpentinites. 4.2. The Erro Tobbio peridotite The protoliths of the Erro Tobbio peridotites were the lithospheric peridotites from the Adria rifted margin, which mostly escaped low-grade sea-floor alteration and maintained the Ti-bearing mantle minerals (i.e. clinopyroxene and spinel). The Erro Tobbio ultramafics mostly consist of almost unaltered to significantly serpentinized mantle spinel peridotites, which preserve structural and compositional features derived from their composite evolution within the subcontinental mantle lithosphere during continental extension, rifting, and opening of the Ligurian Tethys oceanic basin. Their mantle characteristics and the records of the pre-alpine (pre-oceanic) evolution have been studied from the end of the 1970s to the present (e.g. Ernst and Piccardo 1979; Drury et al. 1990; Hoogerduijn Strating et al. 1990, 1993; Vissers et al. 1991; Piccardo and Vissers 2007; and references therein). Field, petrologic, and geochemical studies have evidenced that, similarly to other ophiolitic peridotites from the Jurassic Ligurian Tethys, the Erro Tobbio peridotites are characterized by mineral modal, micro-textural, and compositional features that indicate (i) melt/peridotite interaction during reactive porous flow and focused percolation of asthenospheric melts and (ii) interstitial crystallization (impregnation and refertilization) of the upward migrating MORBtype melts (Müntener and Piccardo 2003; Piccardo 2003; Piccardo et al. 2004; Rampone et al. 2004; Piccardo and Vissers 2007; Rampone and Borghini 2008; and references therein). The frequent presence of melt-reacted rocks hosted within the mantle shear zones as granular undeformed bands concordant to the tectonite-mylonite foliation suggests that the shear zones have channelled the upwards focused migration of the asthenospheric melts (Padovano, personal communication). As recently remarked by Piccardo (2012), field and petrologic evidence do not allow us to sustain that The eclogite-facies metamorphic imprint (i.e. HP metamorphic recrystallization) in the Erro Tobbio...affects the entire volume of the Erro Tobbio peridotites (Capponi et al. 2009). We think that it should be more correct to suggest that the presence of widespread HP eclogitefacies records suggests that the whole Erro Tobbio bodies went down to HP eclogite-facies conditions. Similarly (Piccardo 2012), we think that it is not correct to extend to all Erro Tobbio peridotite bodies the term Erro Tobbio serpentinites (Scambelluri and Tonarini 2012 and references therein), particularly in the case where these geochemical studies were developed on limited outcrops not representative of the whole ultramafics of the Voltri Massif. This definition is clearly in contrast with the field occurrence of large, km-scale and mountain-wide, bodies of almost unaltered or moderately serpentinized Erro Tobbio peridotites. 4.2.1. The gabbroic rocks Gabbroic bodies and dikes in the Erro Tobbio peridotite consist of both bodies of unaltered ultramafic/mafic cumulates (e.g. Borghini et al. 2007) and gabbroic bodies/dikes recrystallized under HP conditions, which mostly escaped sea-floor alteration (i.e. rodingitization) (e.g. Messiga et al. 1995). The latter are primarily associated in the field to hydrous HP meta-peridotites. It must be noted that the mafic intrusions in the Erro Tobbio peridotite either escaped sea-floor alteration and even HP recrystallization (e.g. Borghini et al. 2007) or remained unaltered till eclogite-facies conditions (e.g. Messiga et al. 1995). Accordingly, the presence in the Erro Tobbio peridotites of unaltered gabbroic rocks which reached HP eclogite-facies conditions before hydrous HP recrystallization is a clear marker that most of the Erro Tobbio peridotites were subducted unaltered to eclogite-facies conditions.
International Geology Review 9 4.2.2. The records of subduction in peridotites Mostly along the contacts with the Beigua serpentinites, the Erro Tobbio peridotites show records of a composite tectonic metamorphic history of subsequent recrystallizations leading to formation of static HP antigorite + olivine + diopside + Ti-clinohumite + chlorite assemblages, thus testifying their subduction evolution (Figure 5). Incipient recrystallization to HP conditions is marked by development of fine-grained granoblastic veins of new HP olivine (also a few antigorite and Ticlinohumite crystals) (Figures 5E 5G) on mantle olivine. The term meta-peridotite, frequently used and abused in the recent bibliography, has been introduced to indicate (A) (C) (E) (G) 2 cm 500 µm completely recrystallized mantle peridotites to static HP antigorite + olivine + Ti-clinohumite + diopside assemblages (Figure 5H). In some cases, this term has been erroneously used to indicate the whole Erro Tobbio peridotite, which, on the contrary, mostly maintains the whole structural and compositional characteristics of the mantle protoliths. Our field and petrologic investigations evidence that the HP meta-peridotite bodies in the Erro Tobbio peridotite are characterized by abundant Ti-clinohumite in the eclogitic assemblages (Figure 5A), in contrast to the Beigua serpentinites, which are almost completely free of Ti-clinohumite within the olivine-bearing HP assemblages, (B) (D) (F) 5 cm 500 µm 1 mm 1 mm (H) 1 mm 500 µm Figure 5. Erro Tobbio peridotite. HP eclogite-facies partial hydration. (A) Field evidence of radial aggregate of acicular Ti-clinohumite growing in rather unaltered peridotite. (B) Olivine + Ti-clinohumite (brown crystals) vein cutting the host peridotite. (C and D) Thin section (parallel and crossed nicols) of a former Ti-bearing mantle clinopyroxene replaced by a rim of Ti-free diopside and by an outer border of Ti-clinohumite. (E and F) Fine-grained granoblastic HP eclogite-facies olivine cutting huge mantle olivine crystals. (G) Granoblastic HP eclogite-facies olivine in association with soma antigorite and Ti-clinohumite crystals. (H) HP eclogite-facies assemblages of olivine + antigorite + Ti-clinohumite in a completely recrystallized Erro Tobbio meta-peridotite.
10 G.B. Piccardo because of Ti loss during their sea-floor alteration and antigorite recrystallization. Ti-bearing mantle clinopyroxenes underwent breakdown at HP eclogite-facies conditions, recrystallizing Ti-free diopside and releasing Ti, which concurred to Ti-clinohumite formation (Figures 5C and 5D). Moreover, the meta-peridotites and the HP antigorite shear zones of the Gorzente river, and other localities of the Erro Tobbio peridotite unit, are cut by a framework of olivine veins rather rich in Ti-clinohumite (Figure 5B), thus suggesting circulation of Ti-bearing/rich fluids. This implies that these limited domains were characterized by: (i) abundance of water at eclogite facies conditions; (ii) Ti availability (i.e. Ti release from the mantle Ti-bearing mineral phases); and (iii) Ti mobility within the HP fluids (e.g. Gao et al. 2007). Moreover, metamorphic olivines (and Ti-clinohumites) are characterized by systematically higher Mg# (0.84 0.88) and lower MnO (0.2 0.4 wt.%) with respect to acicular olivine in the Beigua serpentinites. 5. Discussion 5.1. Continental extension to ocean opening The Beigua serpentinites and the Erro Tobbio peridotites derive from the Ligurian Tethys oceanic basin, which is considered a narrow oceanic basin formed by continental extension of the Adria Europe lithosphere by far field tectonic forces. Continental extension was driven by lithosphere scale, km-wide shear zones which propagated from the deep mantle lithosphere (garnet-peridotite facies conditions) to shallower levels (amphibole-chlorite peridotite facies conditions), during mantle lithosphere extension and thinning (Drury et al. 1990; Hoogerduijn Strating et al. 1990, 1993; Vissers et al. 1991). After significant thinning of the lithosphere, asthenosphere near-adiabatic upwelling led to its decompressional partial melting and MORB-type melts percolated upwards within the lithospheric mantle by reactive porous flow, frequently using the shear zones for a faster migration (Padovano et al., in preparation). The extending lithospheric mantle underwent important melt-peridotite interaction processes and was mostly transformed to reactive spinel harzburgites and plagioclase-enriched refertilized peridotites (Piccardo and Vissers 2007). Passive continental extension led to the formation of extended continental margins (OCT settings), where the lithospheric mantle preserved significant records of the extensional evolution (i.e. shear zones, decompression evolution, infiltration of MORB-type melts from the melting asthenosphere). More internal settings of the basin (MIO settings) were, on the contrary, almost characterized by sea-floor exposure of melt-reacted peridotites (i.e. reactive spinel harzburgites and plagioclase-enriched peridotites) (Piccardo 2008, 2010; and references therein) that underwent sea-floor alteration. 5.2. Oceanic closure During the Late Cretaceous, the oceanic expansion ended and the geodynamic evolution of the western sector of the Tethys led to convergence of the Europe and Adria plates, originating thus an eastward subduction. The subduction was intra-continental in the northern sector of the basin and became progressively intra-oceanic moving to the south (e.g. Dal Piaz 1974a, 1974b; Chiesa et al. 1975). Accordingly, the geometry of the plates changed as a consequence of the subduction plane. In the northern sector of the basin (corresponding to the Western Alps), the whole oceanic lithosphere pertained to the Europe plate and has been subducted under the Adria plate. In the southern sector (corresponding to the Northern Apennine), the oceanic lithosphere of the Europe plate has been subducted under the oceanic lithosphere of the Adria plate. At the level of the almost E W Voltri traverse, the subduction was most probably intra-oceanic and located close to the Adria rifted margin. Within this frame, large portions of the oceanic lithosphere, represented by the Beigua serpentinites, subducted beneath the Adria rifted margin. The mantle precursors of the Beigua serpentinites derived from the more internal oceanic lithosphere and underwent intense oceanic alteration (serpentinization and rodingitization). The mantle precursors of the Erro Tobbio peridotites derived from the subcontinental mantle exhumed at the ocean continent transition of the extended margin of the Adria plate and, also subjected to localized serpentinization, mostly escaped interaction with sea-water-derived fluids, thus preserving unaltered mantle structural and petrologic features. Present structural, petrologic and geochemical knowledge indicates that Erro Tobbio peridotites: (i) went down within the subduction zone; (ii) reached eclogite facies conditions; and (iii) underwent rapid exhumation. The presence of Ti-clinohumite in the olivine + antigorite HP assemblage indicates metamorphic conditions characterized by temperatures of 500 600 C and pressures ranging between 18 and 25 kbars (Scambelluri et al. 1995). 5.3. Formation of a serpentinitic subduction channel Several models and natural occurrences have defined that the exhumation of HP rocks involved in intraoceanic subduction often occurs within serpentinite channels (Gerya et al. 2002; Gorczyk et al. 2007; Agard et al. 2009; Guillot et al. 2009; Blanco-Quintero et al. 2011; Krebs et al. 2011). Our field and petrologic data on the Beigua serpentinites and, in particular, (1) the relicts of oceanic rock assemblages (ophicalcites and basaltic breccias) and (2) the preserved sea-floor alteration textures (meshtextured serpentine and pseudomorphs on pyroxenes), indicate that most of the subducted serpentinized peridotites derived from the oceanic subducting slab, which
International Geology Review 11 was characterized by an uppermost level of hydrated oceanic peridotites. We envisage that the formation of the serpentinitic subduction channel was strongly favoured by the presence of these oceanic serpentinites (i.e. the future Beigua serpentinites), even though it is known that the channel can progressively widen by progressive hydration of the mantle wedge (Gerya et al. 2002). The presence of a small-scale serpentinite subduction channel in the Voltri Massif was first described by Federico et al. (2007). Recently, Malatesta et al. (2012a, 2012b), on the basis of numerical simulations, proposed that a suitable mechanism that allows the exhumation of HP rocks in the Voltri Massif is represented by the formation of a serpentinitic subduction channel and proposed, for the eastern sector of the Voltri Massif, the formation of a low-viscosity serpentinite channel during subduction. 5.4. Ablative subduction and exhumation In the case of intra-oceanic subduction, it has been highlighted that slices of the overriding plate can undergo ablative subduction (e.g. Tao and O Connel 1992). This model is supported by several authors (e.g. Platt 1986; Clift and Vannucchi 2004), who have suggested that slices of the overriding plate are torn away and buried together with the subducting slab. Our field and petrologic investigation on both the Beigua serpentinites and the Erro Tobbio peridotites, and, particularly, (1) the common evolution at HP eclogitefacies of the oceanic serpentinites (Beigua) and subcontinental peridotites (Erro Tobbio), (2) the clear field relationships between the two lithotypes, where rather unaltered Erro Tobbio peridotite bodies are embedded within Beigua antigorite serpentinites, and (3) the formation of Erro Tobbio eclogite-facies meta-peridotites close to the contacts with the Beigua serpentinites, clearly indicate that the Erro Tobbio peridotites, after their enclosure into the antigorite serpentinite channel, mostly escaped alteration and where rapidly subducted and exhumed within the subduction channel, together with bodies of eclogitic metagabbro and meta-rodingites. Accordingly, we suggest that sectors of the subcontinental mantle from the Adria extended margin (e.g. the future Erro Tobbio peridotite) were detached and embedded within the serpentinite-subduction channel. This particular feature has been recently observed in numerical simulations performed by Malatesta et al. (2012b), who recognized that during convergence, slices of the overriding oceanic plate can be scraped off and dragged to great depth in the channel. Most of the HP rocks presently embedded within the Beigua serpentinites (i.e. the eclogitic meta-gabbros, which locally present records of sea-floor alteration, and the meta-rodingites) should derive from the oceanic gabbroic intrusions from the shallow hydrated levels of the oceanic lithosphere. It cannot be excluded that some HP and unaltered mafic rocks presently enclosed in the Erro Tobbio peridotite (Messiga et al. 1995; Borghini et al. 2007) derive from the former gabbroic intrusions of the exhumed lithospheric mantle of the rifted margin of the Adria plate. Accordingly, we propose (Figure 6) that the serpentinite-subduction channel was mostly made of the uppermost serpentinized layer (i.e. the future Beigua serpentinites) and their volcanic-sedimentary cover (i.e. the future Voltri Rossiglione meta-sediments and metavolcanics) of the oceanic lithosphere of the Ligurian Tethys basin. The Erro Tobbio peridotites were detached from the mantle lithosphere of the Adria extended margin, were carried down to eclogite-facies conditions within the oceanic serpentinite-subduction channel, and were rapidly exhumed. 5.5. The carrier of water to eclogite-facies conditions It is well established (e.g. Scambelluri et al. 1995) that subduction of serpentinized peridotites from the oceanic lithosphere slabs is the most effective mechanism of bringing water to great depths within the mantle. Concerning the Voltri Massif, Scambelluri et al. (1997) suggested that the Erro Tobbio peridotites were exposed to the sea-floor, underwent hydrothermal alteration by sea-water-derived fluids, and were responsible for deep recycling of seawater-derived fluids. Früh-Green et al. (2001) suggested that fluids may have been released by the Erro Tobbio antigorite shear zones, which formed at the sea-floor and carried ocean-floor signatures to the deepest levels of the subduction zone. As a consequence, Früh-Green et al. (2001) suggested that the Erro Tobbio Unit represents an example of cycling of internally derived fluids. Hermann et al. (2000) focused their structural and geochemical studies on a 2 km-long section in the Gorzente riverbed that they referred to as pristine Erro Tobbio peridotites, and evidenced that antigorite mylonites document structures formed during a whole tectonic cycle from subduction to exhumation of hydrated mantle rocks. Since the early structural study of Drury et al. (1990), this section was recognized as a rather composite structure being formed by Erro Tobbio peridotites and mylonites and Beigua serpentinites and serpentinite mylonites. Our revisitation of this section evidences that, besides subduction/exhumation-related antigorite shear zones, the low strain domains consist of 10 100 m-wide deformed lenses of (i) almost unaltered Erro Tobbio peridotite, showing pre-oceanic plagioclase impregnation and dunite channels, (ii) HP eclogite-facies meta-peridotites bearing olivine + Ti-clinohumite veins, and (iii) HP eclogite-facies olivine-bearing antigorite serpentinites.
12 G.B. Piccardo 5 6 1 2 4 3 7 (A) (B) 8 (C) 3 3 3 9 8 (D) 8 Figure 6. Convergence evolution of a slow ultraslow spreading ocean. (A) Maximum basin extension: (1) oceanic lithosphere and (2) uppermost level of the sea-floor-exposed mantle lithosphere, which underwent sea-floor alteration (serpentinization and rodingitization); (3) and (5) subcontinental mantle of the extended rifted margins; and (4) and (6) extended rifted margins. (B) Formation of the serpentine-subduction channel at the expenses of the uppermost hydrated level of the oceanic lithosphere, (8) serpentinite-subduction channel. (C) Sections from the marginal subcontinental mantle lithosphere (3) are detached by the subducting slab and enclosed and embedded within the serpentinite-subduction channel (8). (D) Exhumation of HP rocks within the serpentinite-subduction channel, (3) peridotite sectors from the subcontinental rifted margin that reached HP eclogite-facies conditions within the subduction channel, (9) eclogitic and rodingitic meta-gabbros that reached HP eclogite-facies conditions within the subduction channel (8). In the light of our recent investigations, we realize that Beigua antigoritic serpentinites and Erro Tobbio peridotites reached together HP eclogite-facies conditions, where the Beigua serpentinites underwent partial dehydration and the Erro Tobbio peridotites underwent partial hydration. This scenario indicates that (i) the Beigua serpentinites and their sea-floor derived geochemical imprints, which were derived from the oceanic lithosphere of the basin, were the most important component of the subduction channel and, accordingly, the main carrier of water to depth, (ii) the Erro Tobbio peridotites reached HP conditions almost unaltered or moderately serpentinized, and (iii) HP eclogite-facies partial dehydration of Beigua antigorite serpentinites and HP eclogite-facies partial hydration of Erro Tobbio peridotites were strictly related. Since the paper of Scambelluri et al. (1995), Scambelluri and his co-workers (i.e. Hermann et al. 2000; Früh-Green et al. 2001) defined that so-called Erro Tobbio antigorite serpentinites, interpreted as derived from a slice of subcontinental mantle, (i) were exposed to the sea-floor and underwent sea-floor hydration and (ii) were considered the main carrier of water and sea-floor geochemical signatures to HP eclogite-facies conditions. The Beigua serpentinites, which bear clear evidence of provenance from the sea-floor exposed oceanic peridotites (i.e. ophicalcites bodies and meta-volcanic breccias), and certainly represented the uppermost hydrated level of the subducted slab, were completely disregarded by these authors. Recently, Scambelluri and Tonarini (2012) presented bulk-rock B and B isotope data on high pressure serpentinites, using the Erro Tobbio as the key example for subduction of serpentinized mantle. They found that the B composition documented for Erro Tobbio serpentinites are surprising for slabs and better fit the behaviour of hydrated mantle wedge. They conclude that the alteration of the Erro Tobbio occurred in environments such as the forearc mantle or the slab-mantle interface and that the Erro Tobbio serpentinization was driven by fluids that infiltrated the slab-mantle interface early in the subduction history (shallow level transfer across the subduction zone). We described Ti loss and Mn uptake during ocean-floor alteration of the Beigua serpentinites. Recently, Alt et al. (2012) presented evidence that sulphur and carbon contents and isotope compositions of Beigua HP antigorite serpentinites in the Beigua unit of the Voltri massif were uptaken during the sea-floor alteration. Recrystallization of chrysotile and lizardite to antigorite and partial dehydration to olivine during HP eclogite facies metamorphism result in no significant changes in the contents or isotope compositions of sulphur and carbon in the serpentinites. Summing up, these data imply that the sea-floor derived serpentinites from the uppermost oceanic lithosphere (the future Beigua antigorite serpentinites) were a major component of the subducted slab. After recrystallization to antigorite serpentinites, they did not participate in the release of fluids at shallow level because of the stability of antigorite to HP eclogite-facies conditions. Accordingly, we support that the Beigua antigorite serpentinites are the best candidates to bring water and sea-floor derived geochemical signatures into the mantle. 5.6. The carrier of Ti to eclogite-facies conditions A major difference between the HP assemblages in the Beigua serpentinites and in the meta-peridotite masses within the Erro Tobbio peridotite is the presence and abundance of Ti-clinohumite. As already outlined, Ticlinohumite is rare in the Beigua serpentinites, since the