the pre-oceanic evolution of the erro-tobbio peridotite (voltri massif, ligurian alps, italy)

Journal of Geodynamics 43 (2007) 417449

The pre-oceanic evolution of the Erro-Tobbio peridotite(Voltri Massif, Ligurian Alps, Italy)

G.B. Piccardo a,, R.L.M. Vissers ba DIPTERIS, Universita di Genova, 16132 Genova, Italy

b Department of Earth Sciences, Utrecht University, Utrecht, The NetherlandsReceived 29 May 2006; received in revised form 31 August 2006; accepted 18 October 2006

Abstract

This paper presents the results of field, structural, petrologic and geochemical investigations on the Erro-Tobbio (E-T) ophioliticperidotite (Voltri Massif, Ligurian Alps, Italy). This massif represents a mantle section equilibrated at spinel-facies conditionsin the subcontinental lithosphere of the Europe-Adria system prior to the Early Jurassic that has been exhumed and emplacedat the sea-floor during rifting and opening of an ocean basin. The E-T massif comprises km-scale volumes of peridotites withstructural and compositional characteristics pointing to meltperidotite interaction. Their formation is thought to result from theinteraction of pristine lithospheric peridotites with MORB-type melts ascending by porous flow, leading to the development ofreactive spinel harzburgites, impregnated plagioclase peridotites and replacive spinel dunites. The melt-related events were fol-lowed by MORB melt intrusion. Field relationships between sheared lithospheric peridotites, including coarse tectonites as well asfine-grained mylonites developed during lithosphere extension, and melt-modified peridotites suggest that melt-related processesoccurred during exhumation of the E-T mantle. These melt-related processes likely included both diffuse percolation and focusedintrusion and are considered to be a consequence of MORB-forming partial melting of the asthenosphere induced by near-adiabaticdecompressional upwelling related to lithosphere extension and thinning. Field, structural and petrological data allow us to concludethat the entire pre-oceanic evolution of deformation, metamorphism and magmatism recorded by the E-T mantle started duringthe Early-Middle Jurassic and was related to lithospheric extension leading to the Late Jurassic opening of the Ligurian Tethysocean.

2006 Published by Elsevier Ltd.

Keywords: Lithosphere extension; Mantle exhumation; Meltperidotite interaction; Erro-Tobbio peridotite; Ligurian Tethys

1. Introduction

The Erro-Tobbio (E-T) peridotite in the Ligurian Alps of N Italy represents the mantle section of the Voltri Massif, thelargest ophiolite exposure in the European Alps (Fig. 1). The E-T mantle peridotite was (i) exhumed from lithosphericmantle depths during lithosphere extension, (ii) exposed at the sea-floor of the Jurassic Ligurian Tethys and (iii)subducted during closure of the basin, reaching eclogite-facies conditions prior to final exhumation during the later

Paper presented at the Peridotite Workshop 2005, held in Lanzo, 2730 September 2005, organized by Alessandra Montanini and Giovanni B.Piccardo, and sponsored by the Italian Working Group on Mediterranean Ophiolites.

Corresponding author. Tel.: +39 010 3538308; fax: +39 010 352169.E-mail address: [email protected] (G.B. Piccardo).

0264-3707/$ see front matter 2006 Published by Elsevier Ltd.doi:10.1016/j.jog.2006.11.001

418 G.B. Piccardo, R.L.M. Vissers / Journal of Geodynamics 43 (2007) 417449

Fig. 1. Sketch map of Western Alps and Northern Apennines showing main ophiolite exposures (redrawn and modified after Piccardo et al. (2004a,b)).

stages of Alpine collision (Piccardo et al., 1988; Scambelluri et al., 1991; Hoogerduijn Strating et al., 1993; andreferences therein).

Previous studies (Drury et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al., 1993) of the pre-oceanicevolution of the E-T mantle rocks have demonstrated that, in response to lithospheric extension leading to openingof the Ligurian Tethys ocean, the E-T peridotites were uplifted along a subsolidus P-T trajectory, starting from sub-continental lithospheric mantle depths (T < 10001100 C and spinel-facies conditions). Peridotites were deformed inkm-scale extensional shear zones, forming peridotite tectonites and mylonites. The presence of discrete MORB gab-broic intrusions and gabbroic/basaltic dikes cutting deformed peridotites strongly suggests that lithosphere extensionand thinning was accompanied by emplacement of melts formed by decompression partial melting of the underlyingasthenosphere.

Recent investigations (Piccardo et al., 2004a,b) reveal that, following the onset of extensional deformation and priorto intrusion of aggregate MORB melts, pristine spinel-facies lithospheric peridotites were diffusely transformed tocoarse granular spinel and plagioclase peridotites which show structural and compositional characteristics suggestingmeltrock interaction. This evidence indicates that melt fractions from the upwelling asthenosphere migrated upwardsby diffuse porous flow while reacting with the overlying extending lithosphere, causing significant modification of thelithospheric mantle peridotites.

This paper aims to present: (i) the structural and compositional characteristics of the E-T lithospheric mantleprotoliths and of peridotites within the E-T massif modified by diffuse melt percolation and meltrock interaction;(ii) the mutual relationships between deformed and melt-modified rocks; (iii) the composite scenario of tectono-metamorphic and melt-related events recorded in the E-T peridotite; (iv) an improved and more complete scenario forthe evolution from lithosphere extension to continental breakup and real ocean spreading in the Ligurian Tethys realm.

G.B. Piccardo, R.L.M. Vissers / Journal of Geodynamics 43 (2007) 417449 419

In this study we focus on field, petrographic-structural and petrologic-geochemical characteristics of reactive spinelperidotites, impregnated plagioclase peridotites and replacive spinel dunites, well exposed in few key areas of theE-T massif. These new data allow a better understanding of the mutual relationships between the different rock-typesand the related mantle processes and, in particular, of the sequence of melt-related events recorded in the E-T massif.In addition, they shed further light on the composition and migration mode of melts rising from deep asthenosphericsources through the overlying lithospheric mantle and indicate the role that thermo-mechanical erosion of the extendingmantle lithosphere may have played in the transition from lithospheric extension to oceanic spreading (Corti et al.,2007; Ranalli et al., 2007).

Previous structural and petrologic investigations on the E-T peridotite (e.g. Ernst and Piccardo, 1979; Piccardoet al., 1990; Drury et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al., 1993) provided evidence that theperidotite represents a fragment of subcontinental lithospheric mantle, emplaced at high crustal levels during riftingand ocean opening. These mantle peridotites mainly comprise spinel lherzolites, showing spinel pyroxenite banding(Plate 1A), spinel harzburgites, plagioclase-bearing peridotites and minor dunites. Locally, the pyroxenite banding inspinel lherzolites is deformed by early folds (Plate 1B) that predate a stage of complete spinel-facies recrystallizationascribed to accretion of the E-T peridotite to the thermal lithosphere (Hoogerduijn Strating et al., 1993).

The granular peridotites are transected by five generations of shear zone structures forming porphyroclastic spinel-bearing peridotite tectonites (Plate 1C and D), plagioclase-, hornblende- and chlorite-bearing peridotite mylonites,and serpentinite mylonites. There is a systematic correlation between the microstructures and mineral compositions,related to changing P and T conditions during the pertinent stages of syntectonic recrystallization in the shear zones.On the basis of geothermobarometric data, it has been suggested (Hoogerduijn Strating et al., 1993) that the shearzones structures developed at progressively lower P and T conditions. The P-T path obtained for the E-T peridotite hasbeen interpreted (Hoogerduijn Strating et al., 1993) as indicative of subsolidus uplift, from deep levels (at spinel-faciesconditions) in the subcontinental lithospheric mantle towards the ocean floor. This subsolidus trajectory is consistentwith the thermal history expected for the footwall of a lithosphere-scale, dipping extensional shear zone. The shearzone structures in the E-T peridotites have, therefore, been interpreted as fragments of an extensional detachmentsystem. It was suggested (Hoogerduijn Strating et al., 1993) that the uplift of the E-T peridotite occurred by tectonicdenudation, in a slightly to strongly asymmetric oceanic rift.

Discrete gabbro bodies and dikes are present in the E-T peridotite (Piccardo, 1984; Hoogerduijn Strating et al.,1990), which show clear MORB affinity of their primary parental melts. These mafic intrusive rocks frequently recordmetamorphic overprints related to both the oceanic and the orogenic evolution (Piccardo, 1984; Piccardo et al., 1989).Some gabbroic dikes show both fresh centers, which indicate that their primary compositional features were preservedtill the subduction environment and altered (rodingitized, i.e. alkali-silica-depleted and CaMg-enriched) borders,which indicate that they reacted with the ambient peridotite during oceanic serpentinization. This evidence indicatesthat the gabbroic dikes intruded before serpentinization of the ambient peridotite at oceanic conditions.

Occasionally, fine-grained basaltic dikes are found which preserve a porphyritic texture and geochemical MORBaffinity (Piccardo, 1984; Hoogerduijn Strating et al., 1990). They frequently intrude along the serpentinite myloniteshear zones. They are transformed to both eclogitic and rodingitic rocks. This fact has been interpreted to indicatethat the intruded basaltic rocks maintained or modified their primary compositions in response to reaction with thehost peridotite (Piccardo et al., 1980) in the oceanic environment, and that basaltic intrusion both preceded andfollowed the oceanic serpentinization. Likewise, chrysotile-lizardite-bearing serpentinization of the ultramafic rocksand rodingitization of the mafic rocks have been related to the near-seafloor metamorphism of the E-T peridotites(Piccardo et al., 1980).

Recent investigations (Piccardo et al., 2004a,b) have revealed that km-scale domains of the E-T peridotite massifconsist of coarse granular spinel and plagioclase peridotites, which show structural and compositional characteristicsmore properly interpreted in terms of meltrock interaction rather than of partial melting or subsolidus recrystallization.These studies have shown the presence and abundance of: (1) reactive spinel peridotites, formed by meltperidotiteinteraction (i.e. pyroxene-dissolution and olivine-precipitation processes) caused by melts percolating via diffuse porousflow, (2) impregnated plagioclase peridotites, formed by meltperidotite interaction and interstitial melt crystallization,and (3) replacive spinel dunites, formed by complete dissolution of pyroxenes by focused percolating melts.

Moreover, petrologic and geochemical studies on cpx-poor spinel lherzolites and harzburgites of the E-T massif(Rampone et al., 2004, 2005a,b) [including the granular spinel peridotites of Panorama Point (samples ETR1, ETR2,ETR3, ETR4 of Romairone (1999))] suggest that their contrasting bulk and mineral chemistries are evidence for reactive


Plate 1. Lithospheric spinel peridotites(A) Rio Eremiti: Lithospheric spinel lherzolite showing parallel pyroxenite bands and a weak foliation;(B) Gorzente River: Lithospheric spinel lherzolite with folded pyroxenite bands; (C) Rio Eremiti: Lithospheric spinel tectonite-mylonite, with aboudinaged pyroxenite band; (D) Piani di Praglia: Lithospheric spinel peridotite tectonite transected by spinel peridotite mylonite. Reactive spinelperidotites(E) Rio Eremiti: Coarse granular, reactive spinel peridotite showing dissolved and vanishing pyroxenite bands, where Cpx crystals arestrongly corroded and dissolved, and partly replaced by new unstrained olivine; (F) Panorama Point: Coarse granular, reactive spinel peridotite.


Fig. 2. Structural map of the North-East Voltri Massif showing pertinent locations referred to in the text.

porous flow. Some E-T plagioclase-bearing peridotites [including the plagioclase peridotite tectonites from GorzenteRiver (samples ETR6, ETR7, ETR9, ETR10 of Romairone (1999))] have been recently interpreted by Rampone et al.(2005a,b) as products of low pressure (plagioclase-facies) subsolidus recrystallization of pristine spinel peridotites. Ithas been shown that textural features that could be related to melt impregnation are present in the studied rocks: thissheds uncertainty whether some magmatic plagioclase was present before extension-related deformation (Ramponeet al., 2005a,b).

Discrete bodies of stratified cumulate mafic and ultramafic rocks, which intrude impregnated plagioclase peridotitesof the E-T massif, have been recently studied by Borghini et al. (2006). Their geochemical characteristics clearlyindicate a MORB affinity whilst geobarometric estimates suggest that their intrusion occurred at 0.30.5 GPa. TheseMORB intrusives have yielded SmNd ages of 180 14 Ma for their intrusion (Rampone et al., 2005a,b).

2. The melt-related rocks

2.1. Field relationships and petrographic features

Field work has been focused in few key areas where previous investigations (Piccardo et al., 2004a) have documentedthe presence and abundance of rocks recording melt-related processes (Figs. 2 and 3). Detailed field work was dedicatedto the identification of diagnostic structural features of the different rock-types related to meltperidotite interaction and


Fig. 3. Structural sketch map of the northeastern Voltri Massif (modified after Vissers et al. (1991)) showing localities of detailed study.

to the recognition of their mutual relationships. Particular attention was devoted to relationships between melt-relatedrocks and deformation pertaining to the km-scale shear zones, and between intrusive mafic rocks and melt-related anddeformed rock-types. The selected key areas are (Figs. 2 and 3):

(1) The Mt. Pennello-Punta Martin area, where km-scale bodies of plagioclase peridotites and decametre-scale bandsof spinel dunites are embedded within spinel peridotites.

(2) The Piani di Praglia area, where granular-porphyroclastic spinel lherzolites, passing into low- and high-strainspinel-facies peridotite tectonites and mylonites, are cut by decimetre- to metre-wide granular spinel dunite bands,sometimes bordered by decimetre-wide zones of coarse granular spinel harzburgite.

(3) The Mt. Tobbio-Mt. Tugello area, and particularly the Panorama Point, Rio Eremiti zone, where granular spinelperidotites occupy km-scale areas, consisting both of lithospheric granular to porphyroclastic lherzolites and coarsegranular reactive spinel harzburgites. In this area clear relationships are visible between plagioclase-rich peridotitesand plagioclase-free spinel peridotites, frequently with sharp contacts.

(4) The Gorzente River (north of Guado) area, where both pyroxenite-bearing, lithospheric spinel lherzolites, reactivespinel peridotites, plagioclase-rich peridotites and dunite bodies are present, showing clear mutual relationships.Part of the riverbed pertains to a km-scale shear zone and the lithospheric and melt-related peridotites are stronglydeformed and transformed to high-strain tectonites.

In addition to the above key areas, a detailed petrographic-microstructural survey has been carried out on samplesinvestigated in previous studies (i.e. Robbiano, 1994; Romairone, 1999; Rampone et al., 2005a,b), to identify the pres-ence of structures and microstructures indicative of melt-related processes. This work has revealed, in particular, that:

(1) The granular spinel peridotites (ETR1, ETR2, ETR3, ETR4) from Panorama Point (Romairone, 1999) showmicrostructures indicative of mineralogical reactions related to meltrock interaction processes. This supports theinterpretation, on a chemical basis, that these rocks formed by reactive porous flow (Rampone et al., 2004).

(2) The plagioclase peridotites (ETR6, ETR7, ETR9, ETR10) from the Gorzente River (Romairone, 1999) (re-locatedat Costa Lavezzara by Rampone et al. (2005a,b)) show microstructures clearly indicating mineral reactions relatedto meltrock interaction and interstitial crystallization from a melt, suggesting that plagioclase is a newly introduced


magmatic mineral. This contrasts with the previous interpretation that plagioclase in these rocks was formed bya metamorphic reaction, i.e. that these rocks were formed by closed system recrystallization at plagioclase-faciesconditions of previous spinel peridotites (Rampone et al., 2005a,b).

In this paper, different names are adopted to indicate the different rock-types, and the mantle processes they record(the discriminant structural and compositional features of the different rock-types are described below):

(1) Lithospheric spinel peridotites (LSP): the oldest rock-types, which preserve relics of metamorphic reactions andbulk rock and mineral compositions indicating refractory residual characteristics and a stage of high pressurehightemperature equilibration in the mantle lithosphere.

(2) Reactive spinel peridotites (RSP): pyroxene-depleted harzburgitic rocks, which show meltperidotite reactiontextures and bulk rock, modal and mineral compositions that are significantly modified, as compared with thelithospheric peridotites, by reaction with percolating pyroxene-undersaturated melts.

(3) Impregnated plagioclase peridotites (IPP): plagioclase-rich peridotites, which show meltperidotite reaction tex-tures and are enriched in gabbro-noritic microgranular aggregates by the interstitial crystallization of percolating,orthopyroxene(-silica)-saturated melts.

(4) Replacive spinel dunites (RSD): almost pyroxene-free granular rocks, which preserve textural features supportingtheir replacive origin; the reactive percolation of a pyroxene-undersaturated melt along compositional or structuraldiscontinuities caused complete pyroxene dissolution of the pre-existing peridotitic rocks.

2.1.1. Lithospheric spinel lherzolitesThe lithospheric spinel peridotites preserved in the Erro-Tobbio massif have granular and porphyroclastic microstruc-

tures (e.g. Ernst and Piccardo, 1979; Hoogerduijn Strating et al., 1993). They are rather fertile to moderately depletedspinel lherzolites with ubiquitous pyroxenite bands consisting of spinel-bearing websterite (Plate 1AC).

The less deformed spinel lherzolites show complete spinel-facies equilibration, i.e. spinel-facies olivine(Ol) + orthopyroxene (Opx) + clinopyroxene (Cpx) + spinel (Sp) assemblages and protogranular to porphyroclastictextures, and preserve: (i) Cpx porphyroclast cores showing relatively high NaAl contents, (ii) subsolidus microtex-tures consisting of broadly rounded intergrowths (clusters) of Opx + Sp Cpx and micro-symplectites of vermicularSp in Opx, located along the outer border of the Opx porphyroclasts. The interpretation of these features shed light onthe early evolution of these mantle peridotites as follows.

The Opx + Sp clusters have been interpreted as breakdown products of preexisting mantle garnets (Gnt) (HoogerduijnStrating et al., 1993): this indicates that the pristine mantle peridotites underwent spinel-facies equilibration afterupwelling from deeper levels and accretion to the spinel-facies mantle lithosphere (Piccardo et al., 2004a). More-over, the Opx + Sp micro-symplectites represent (Piccardo et al., 2004b) the subsolidus exsolution of an Al-rich,Mg-Tschermakitic component, in the form of Sp, from a preexisting, more aluminous Opx which was stable at highertemperature under spinel-facies conditions. These features, accordingly, could indicate that the pristine mantle peri-dotites upwelled from garnet- to spinel-facies conditions during convection in the asthenosphere, were isolated by theconvecting asthenosphere and were accreted to the thermal lithosphere, where they completely recrystallized whilecooling, at spinel-facies conditions.

The pyroxenite bands are locally folded and boudinaged. As previously evidenced by Hoogerduijn Strating et al.(1993), lherzolite and pyroxenite in the hinges of these folds show coarse-grained, spinel-bearing equilibrium textures(i.e. curvilinear to straight grain boundaries and 120 triple junction), suggesting that folding occurred before a stageof annealing recrystallization in the spinel lherzolite stability field. Accordingly, although garnet is notably absentin the primary mineral assemblage, it can be inferred that deformation recorded by the pyroxenite bands represent anolder event, preceeding the complete spinel-facies recrystallization of the pyroxenite-bearing lherzolites subsequentto accretion to the thermal lithosphere. The recent finding in some E-T spinel pyroxenites of Cpx preserving unusualtrace element signatures which are interpreted as inherited from a precursor garnet-bearing magmatic assemblage(Rampone and Borghini, 2006) further confirms the interpretation that the E-T spinel pyroxenites were formed at higherpressure conditions, subsequently folded and finally recrystallized at spinel-facies conditions.

As evidenced by previous studies (e.g. Vissers et al., 1991; Hoogerduijn Strating et al., 1993), these spinel-faciespristine lherzolites are affected by extensional deformation along km-scale shear zones, giving rise to spinel-facies


peridotite tectonites and mylonites, which mark the onset of extension of the continental lithosphere of the Europe-Adriasystem, leading to the Jurassic opening of the Ligurian Tethys.

2.1.2. Reactive spinel peridotitesIn the region of Panorama Point and Rio Eremiti (Fig. 3), in particular, hectometre- to kilometre-wide areas of coarse

granular, pyroxene-depleted, spinel harzburgites (Plate 1F) crop out showing replacive relationships on pre-existinggranular/porphyroclastic spinel lherzolites and spinel peridotite tectonites. The deformation fabrics of the pre-existingrocks are almost completely obliterated and solely testified by aligned trains of Sp grains, whilst the pyroxenite bandsare dissolved and frequently vanish, being solely marked by alignments of spinel grains and corroded relics of Cpxporphyroclasts (Plate 1E). These granular harzburgites show a strongly reduced pyroxene content as compared to thelithospheric spinel lherzolites.

In thin section, these modified peridotites show conspicuous microtextural features including: (i) broad undeformedolivine rims replacing corroded, deformed and exsolved pyroxene porphyroclasts (Plate 2A), (ii) euhedral crystals ofundeformed olivine grown inside strongly deformed/exsolved Opx porphyroclasts, and (iii) new interstitial grains ofunstrained Ol between the strongly deformed mantle porphyroclasts (Plate 2C), all suggesting that mantle pyroxeneswere partly dissolved by melt-mineral reaction and new magmatic olivine crystallized from the percolating melt(Piccardo et al., 2004a). In some cases, late interstitial crystallization of small pyroxene grains is evident.

The above microstructures indicate that liquids, reactively migrating via intergranular flow, initially dissolvedpyroxenes and precipitated olivine. Progressive pyroxene dissolution and olivine crystallization led, most probably, thepercolating melts to attain pyroxene saturation: as a consequence, they precipitated new interstitial pyroxene grains.

Usually, the coarse granular spinel harzburgites preserve structural-mineralogical relics of the precursor peridotites;they consist of both (i) rounded Opx + Sp clusters and (ii) micro-symplectites of vermicular Sp in Opx, located alongthe outer border of the Opx porphyroclasts or inside new unstrained Ol patches in form of small structural relics. Thisevidence indicates that the microstructure and modal composition of the pristine mantle protolith were significantlymodified by an early melt-related event: the pre-existing spinel lherzolites were transformed into coarse granular,reactive spinel harzburgites.

The absence of plagioclase (Plg) between the crystallizing minerals, the lack of melt-induced, Plg-forming min-eralogical reactions and of Plg exsolution from Cpx suggests that Plg was not stable under the prevailing pressureconditions. It can be deduced that this first melt-related process occurred under spinel-peridotite facies conditions, atpressure conditions most probably higher than 1.0 GPa.

Field and microstructural evidence indicates that similar microstructural changes also affected strongly deformedspinel peridotite tectonites pertaining to the extensional shear zones. In fact, pyroxene-dissolving, olivine-formingmineral reactions are frequent in strongly deformed spinel peridotite tectonites, where unstrained grains of newlyformed Ol partly replace strongly elongated, deformed (kinked) and exsolved, pyroxene porphyroclasts (Plate 2DF).Consequently, the early melt-related event (i.e. the formation of granular reactive harzburgites) occurred when thedeep mantle lithosphere was already affected by extensional deformation, under spinel-facies conditions, in responseto lithosphere extension. This relationships between deformation and melt percolation processes has an importantgeodynamic consequence since they link the first appearance of melts with MORB affinity to the extension andthinning of the continental lithosphere. Inception of partial melting of the asthenosphere and MORB-type melt formationwere, most probably, related to the near-adiabatic upwelling and decompression of the asthenosphere in response tolithospheric extension and thinning (see below).

2.1.3. Impregnated plagioclase peridotitesIn the Mt. Pennello-Punta Martin and Gorzente River North (Rio Eremiti) areas (Figs. 2 and 3), up to km-scale

domains of spinel peridotites are transformed to coarse granular plagioclase-enriched peridotites. The exceptionallyhigh concentration of Plg (frequently over 1015 vol.%) in these peridotites (Plate 3E and F) clearly precludes an originby subsolidus recrystallization of pristine fertile spinel-facies lherzolites to metamorphic plagioclase-facies peridotite,but instead suggests an intervening metasomatic enrichment by interstitial melt crystallization (Piccardo et al., 2004a,2005).

The contact between plagioclase-rich and plagioclase-free peridotites is usually sharp (Plate 3A and B) and gen-tly curved. Moreover, dm-wide elongated bands of plagioclase-rich peridotite intrude, with sharp contacts, theplagioclase-free spinel peridotites (Plate 3C and D).


Plate 2. Microtextures in reactive spinel peridotites: (A) Rim of unstrained olivine between mantle olivine ad exsolved orthopyroxene; (B) vermicularolivine replacing orthopyroxene; (C) unstrained olivine between exsolved mantle pyroxenes; (D) unstrained olivine replacing strongly deformed andkinked orthopyroxene porphyroclast in a strongly deformed, lithospheric spinel peridotite tectonite; (E) unstrained olivine growing on a stronglydeformed and kinked orthopyroxene porphyroclast in a lithospheric spinel peridotite tectonite; (F) unstrained olivine growing along the contact ofthe kink bands of an orthopyroxene porphyroclast in a lithospheric spinel peridotite tectonite.


Plate 3. Impregnated plagioclase peridotites(A) Rio Eremiti: Sharp contact between strongly plagioclase-enriched, impregnated peridotite andcoarse granular, reactive spinel peridotite; (B) Rio Eremiti: Similar sharp contact between strongly plagioclase-enriched, impregnated peridotite andcoarse granular, reactive spinel peridotite; (C) Gorzente River: Metre-wide band of strongly plagioclase-enriched, impregnated peridotite withincoarse granular, plagioclase-free and cpx-poor, reactive harzburgite; (D) Rio Eremiti: Dm-wide band of strongly plagioclase-enriched, impregnatedperidotite within coarse granular, plagioclase-free reactive harzburgite; (E) Gorzente River: Plagioclase-enriched impregnated peridotite, evidencingthe diffuse and unoriented growth of the magmatic plagioclase; (F) Gorzente River: Plagioclase-enriched impregnated peridotite, evidencing diffuseinterstitial growth of magmatic plagioclase.


In thin section, the plagioclase peridotites show the following microtextural features: (i) Opx rims and Opx + Plgsymplectitic coronas around corroded Cpx porphyroclasts (Plate 4AD), (ii) replacement of kinked Ol by undeformedOpx patches, (iii) mm-size Opx-rich noritic veins and pods, and symplectitic Opx + Plg aggregates interstitial to themantle minerals (Plate 4E), (iv) rims of plagioclase surrounding spinel (Plate 4F).

These data suggest that the Plg enrichment of the rock, as evident in the field, is linked to addition of gabbro-noriticmaterial and is coupled to Opx( Plg) replacement on Cpx and Ol.

Plagioclase rims surrounding spinel are usually formed in between spinel and olivine and, frequently, olivine isrimmed by a thin band of Opx against the Plg (Plate 4F). This texture can be confused with the rim of Plg + Ol inbetween Sp and Px, which classically indicates the metamorphic reaction (Sp + Pxs Ol + Plg) to the plagioclase-facies assemblage. The present texture, however, more properly indicates that a silica-saturated melt percolated alongthe spinel rim, forming magmatic plagioclase at the expenses of mantle Sp and magmatic Opx at the expenses of mantleOl (Piccardo et al., 2004b). It is thus important to note that a plagioclase rim around spinel can as such not be simplyinterpreted as indicative of subsolidus recrystallization at lower pressure conditions because melt percolation in spinelperidotite can induce a melt-assisted transition to a plagioclase-facies mineral assemblage.

The above microtextures have been interpreted as evidence for the interaction of the mantle peridotite with dif-fusely percolating reactive melts, which were Opx(-silica)-saturated and Cpx-undersaturated (Piccardo et al., 2004a),and, moreover, underwent crystallization of cumulus minerals (Opx and Plg) interstitially to the mantle minerals.Accordingly, plagioclase enrichment is connected to a melt-related event which produced significant modificationof the microstructure and composition of precursor spinel-facies peridotites, which were transformed into coarsegranular, impregnated plagioclase peridotites, refertilized by the addition of basaltic components in the form ofplagioclase-rich, gabbro-noritic material.

Usually, the plagioclase peridotites preserve structural-mineralogical relics of previous reactive peridotites, consist-ing of Ol coronas surrounding pyroxene porphyroclasts and euhedral Ol crystals inside Opx porphyroclasts. In somecases, double coronas are present, surrounding Cpx porphyroclasts: (i) an inner one, formed by unstrained Ol and (ii)an outer one, composed by Opx + Plg, this latter partly replacing both the mantle Cpx and the Ol corona. Accordingly,different textures are superimposed, which are related to first a reaction with a silica-undersaturated melt, followed ata later stage by reaction with a silica-saturated melt. The abundance of Plg and the presence of plagioclase-formingmineral reactions suggest that this event occurred under plagioclase-peridotite facies conditions, most probably atpressures lower than 1.0 GPa.

2.1.4. Replacive spinel dunitesAll previously described rocks types [i.e. (i) protogranular-porphyroclastic spinel lherzolites and spinel peridotite

tectonite-mylonites; (ii) coarse granular, reactive spinel peridotites and (iii) impregnated plagioclase peridotites] arecut by elongate bands and bodies of coarse granular spinel dunites. These dunites occur as metre- to decametre-widediscordant dunite masses crosscutting the foliation of previous rock types, and as elongated, dm- to metre-wide bandsin sheared peridotites, with the bands being concordant with the main foliation of the surrounding peridotite tectonitesand mylonites (Plate 5A and B).

In the first case, the spinel foliation and pyroxenite banding in the host peridotite crosscut the contact between theperidotite and dunite and continue within the dunite as elongated Sp trains. This clearly suggests a replacive origin ofthe spinel dunites (see also Boudier and Nicolas, 1972; Boudier, 1978), i.e. these dunite channels have been formedby the focused and reactive migration of pyroxene-undersaturated melts across preexisting compositional or structuraldiscontinuities (Piccardo et al., 2004a).

The transition from host peridotite to granular dunite is marked by change in both rock fabric and mineral modalcomposition. Discordant dunite bands are sometimes bordered by dm-wide zones where the fabric of the host peridotiteis depleted in Cpx and Plg (in the case of a former plagioclase peridotite) and the fabric is almost completely transformedinto a coarse granular texture and the mineral association (Plate 5C). These dunite bands, accordingly, have transitionalborder zones of coarse granular harzburgites which are replacive with respect to the host peridotite and solely preserveoriented trains of Sp.

The occurrence of the replacive spinel dunites suggests that reactive pyroxene-undersaturated melts were forced tomigrate by focused percolation, producing both structural and modal-compositional modifications of the percolatedrock. In fact, these melts completely dissolved all pyroxenes in the dunitic center of the band and mostly Cpx inthe harzburgitic borders, whilst the preexisting deformed fabrics recrystallized to coarse granular textures. This phe-


Plate 4. Microtextures in impregnated plagioclase peridotites: (A) Symplectitic Opx + Plg rim surrounding a mantle Cpx porphyroclast; (B) coarseOpx border replacing a mantle Cpx porphyroclast; (C) wide Opx border replacing a mantle Cpx porphyroclast; (D) rim of new unstrained Opxbetween mantle Cpx and Ol; (E) magmatic Opx + Plg symplectitic aggregate in between mantle porphyroclasts; (F) Plg (altered to prehnite) rimsurrounding mantle spinel. Note that plagioclase is formed in between Sp and Ol and, particularly, Ol is rimmed by a thin band of Opx againstthe Plg. This texture indicates that a silica-saturated melt percolated along the spinel rim, forming plagioclase at the expense of Sp and Opx at theexpense of Ol.


Plate 5. Replacive spinel dunites(A) Piani di Praglia: Concordant dm-wide, replacive spinel dunite band within strongly deformed, lithosphericspinel peridotite tectonite-mylonite; (B) Piani di Praglia: Detail of the concordant spinel dunite band (in A): note the aligned trains of Sp grains,preserving the trend of the spinel foliation of the precursor spinel peridotite tectonite-mylonite; (C) Gorzente River: Part of a metre-wide replacivespinel dunite body, showing a dm-wide border of ambient peridotite, completely recovered and transformed to coarse granular spinel harzburgite;(D) Gorzente River: Metre-wide band of replacive spinel dunite within impregnated plagioclase peridotite; (E) Gorzente River: Part of a metre-wideband of replacive spinel dunite in contact with a strongly impregnated, plagioclase peridotite, still preserving boudinaged pyroxenite bands withdepleted dunitic borders; (F) Piani di Praglia: Concordant dm-wide, replacive spinel dunite band in strongly deformed, lithospheric spinel peridotitemylonite.

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Table 1Bulk rock major element compositions (oxides vt%)

Sample (rock-type)ET4/3a (LSP) ET11/27a (LSP) ETA20a (LSP) ETR1b (RSP) ETR2b (RSP) ETR3b (RSP) ETR4b (RSP) ETR6c (IPP) ETR7c (IPP) ETR9c (IPP) ETR10c (IPP)

SiO2 45.63 45.44 45.14 43.16 42.91 43.75 42.51 43.80 43.76 43.56 43.94TiO2 0.10 0.07 0.06 0.05 0.04 0.07 0.04 0.07 0.06 0.09 0.08Al2O3 3.13 2.71 2.62 1.54 1.16 1.97 1.12 2.20 2.65 3.29 2.69FeOa 8.24 8.10 8.12 8.58 8.63 8.28 8.71 8.22 8.23 8.15 8.23MnO 0.13 0.12 0.12 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14MgO 39.68 40.97 41.72 44.92 45.78 43.61 46.32 43.44 42.94 41.95 42.31CaO 3.09 2.59 2.22 1.62 1.34 2.18 1.15 2.14 2.16 2.77 2.61Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.05 0.00

Mga bulk 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90Total 100 100 100 100 100 100 100 100 100 100 100

Modal composition (%) calculated with Petmix4cpx (vol.%) 10.9 10.3 7.9 5.7 4.4 8.6 2.9 7 7.4 10.6 9.1ol (vol.%) 57.5 61.1 64.4 80.7 82.5 74.4 85.6 72.6 69.7 67.6 68.7opx (vol.%) 30.3 27.2 27.7 13 13 16.1 10.9 19 20.6 18.9 20.2sp (vol.%) 1.4 1.4 0.1 0.6 0.2 0.9 0.6 plg (vol.%) 3.9 5.9 7.6 5.7

LSP: lithospheric spinel peridotite; RSP: reactive spinel peridotite; IPP: impregnated plagioclase peridotite; cpx: clinopyroxene; opx: orthopyroxene; ol: olivine. Petmix4 does not calcolate sp inplagioclase peridotite: petrographic observations indicate that they have sp in the range 0.51.0%.

a Data from Robbiano (1994).b Data from Rampone et al. (2004).c Data from Romairone (1999).

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Table 2Bulk rock trace element compositions (element in ppm)

Sample (rock-type)ET4/3a (LSP) ET11/27a (LSP) ETA20a (LSP) ETR1b (RSP) ETR2b (RSP) ETR3b (RSP) ETR4b (RSP) ETR6c (IPP) ETR7c (IPP) ETR9c (IPP) ETR10c (IPP)

Rb 0.10 0.12 0.20 Sr 4.55 3.98 2.47 1.94 1.12 1.56 1.42 2.83 5.83 3.81 6.70Y Zr 0.94 0.96 0.91 0.67 0.31 0.94 0.52 1.12 1.13 1.47 1.49Nb 0.03 0.02 0.02 Ba 1.48 3.55 0.86 La 0.036 0.018 0.036 0.006 0.007 0.017 0.007 0.014 0.014 0.017 0.020Ce 0.092 0.043 0.065 0.020 0.021 0.038 0.021 0.049 0.045 0.060 0.081Pr 0.028 0.018 0.022 0.011 0.010 0.019 0.010 0.022 0.022 0.030 0.035Nd 0.224 0.154 0.155 0.107 0.091 0.166 0.074 0.175 0.187 0.266 0.272Sm 0.145 0.098 0.097 0.076 0.057 0.118 0.048 0.117 0.114 0.173 0.160Eu 0.062 0.044 0.041 0.033 0.025 0.051 0.023 0.053 0.053 0.077 0.072Gd 0.283 0.209 0.193 0.156 0.122 0.238 0.103 0.244 0.231 0.344 0.308Tb 0.056 0.042 0.039 0.032 0.024 0.047 0.021 0.049 0.047 0.067 0.060Dy 0.436 0.326 0.303 0.262 0.198 0.383 0.170 0.395 0.367 0.528 0.462Ho 0.099 0.077 0.072 0.058 0.045 0.083 0.038 0.088 0.081 0.112 0.100Er 0.293 0.233 0.224 0.178 0.138 0.246 0.121 0.268 0.247 0.330 0.296Tm 0.046 0.036 0.035 0.028 0.022 0.039 0.020 0.041 0.037 0.050 0.045Yb 0.303 0.241 0.237 0.190 0.156 0.250 0.130 0.270 0.246 0.329 0.297Lu 0.303 0.241 0.237 0.033 0.027 0.043 0.024 0.047 0.042 0.055 0.050Hf 0.077 0.066 0.060 0.048 0.039 0.066 0.035 0.069 0.070 0.095 0.093Ta 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

a Data from Robbiano (1994).b Data from Rampone et al. (2004).c Data from Romairone (1999).


nomenon is particularly clear in the case of the strongly deformed peridotite mylonites in which the fabric is completelyrecrystallized to a very coarse granular texture in dunite: the granular texture consists of largely overgrown (over 1 cmin size) Ol crystals, which show interfingering and lobate contacts and enclose elongated trains of Sp grains, parallel tothe foliation of the deformed country rocks. Deformation textures (kink bands and tabular subgrains) of mantle olivineare, moreover, progressively recrystallized and, locally, kinked mantle olivine is replaced by granular aggregates ofrelatively unstrained, fine-grained olivine.

Aside the discordant dunites, concordant bands of dunite occur in sheared peridotites where replacive dunite bandsappear to follow pre-existing mylonitic bands (Plate 5F). This may indicate that, within the shear zones, focused meltmigration was enhanced by the deformation fabrics (defined by thin bands of strongly reduced grain size, and bythe preferred shape orientation of the constituent grains and grain aggregates) of the percolated peridotite mylonites;accordingly, focused melt percolation was in such cases controlled by the pre-existing deformational structure in themylonites.

In some places, in strict relation with the replacive dunite bands, euhedral clinopyroxenes and interstitial plagioclasesform microgabbroic veins, a few cm-wide, which show fuzzy contacts with the host rock. Moreover, thin, cm-widegabbro-noritic dikelets cut across dunites and surrounding peridotites. These gabbroic veins and dikelets could indicatethat the last melt circulating through the dunites has been squeezed out as a consequence of compaction of the dunitechannels which prevented further melt migration by focused porous flow.

In summary, the mutual relationships observed in clear outcrops indicate that the replacive dunite bands crosscutprotogranular and porphyroclastic lithospheric lherzolites, that they both cut discordant and run concordant to the tec-tonite/mylonite fabric of the extensional shear zones and that they replace reactive spinel harzburgites and impregnatedplagioclase peridotites. Accordingly, the focused melt percolation forming the dunite channels was a younger eventpostdating the diffuse porous flow percolation events responsible for both the reactive and the impregnated peridotites.

2.2. Main compositional features

In Tables 15, bulk rock and mineral major and trace element compositions are reported of some representativesamples of the E-T peridotites, recognized, on the basis of their structure and composition as lithospheric (LSP), reactive(RSP) and impregnated (IPP) peridotites. Most of the analyses are taken from published previous works, whilst somemineral trace element data presented below are new.

2.2.1. Analytical methodsBulk rock major element compositions were determined by XRF analysis carried out at the Dipartimento di Scienze

della Terra dellUniversita` degli Studi di Modena e Reggio Emilia, Italy, by wavelength dispersive X-Ray fluorescence(Philips PW1480) on pressed pellets, using the methods of Franzini et al. (1975) and Leoni and Saitta (1976). Analysesare considered accurate within 25% for major elements. The total iron oxide content is reported as FeO.

The measurement of the bulk rock trace element composition was performed by conventional nebulization ICP-MSusing a VG Plasmaquad II turbo (+ Option S) housed at the University of Montpellier II. Outlines of the analyticalprocedures have been reported by Ionov et al. (1992).

Major element mineral compositions of plagioclase, pyroxenes, olivine and spinel have been determined with aWDS and EDS electron microprobe JEOL JXA-8600, at CNR-Istituto di Geoscienze e Georisorse, Section of Firenze,according to the analytical procedure described by Vaggelli et al. (1999).

Trace elements in clinopyroxene, orthopyroxene and plagioclases were analysed by laser-ablation microprobe LA-ICP-(SF)MS installed at the CNR-Istituto di Geoscienze e Georisorse, Section of Pavia. The LA-ICP-(SF)MS usedis composed of a double focussing sector field analyser (Finnigan Mat Element) coupled with a Q-switched Nd:YAGlaser source (Quantel Brilliant). Full details of the analytical parameters and quantification procedures can be foundin Tiepolo et al. (2002).

2.2.1.1. Bulk rock major and trace element composition. Table 1 shows bulk rock major element compositionsof a number of selected samples representative of: (i) the more fertile, lithospheric spinel-facies lherzolites (LSP)from Praglia, (ii) the reactive cpx-bearing spinel harzburgites (RSP) from Panorama Point and (iii) the impregnatedplagioclase peridotites (IPP) from the Gorzente River.

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Table 3Mineral major element compositions (oxides vt%) clinopyroxenes

Sample (rock-type)ETA20a(LSP)

ETA20a(LSP)

ETR1b(RSP)

ETR1b(RSP)

ETR2b(RSP)

ETR3b(RSP)

ETR3b(RSP)

ETR4b(RSP)

ETR4b(RSP)

ET22(IPP)

ET22(IPP)

ET22(IPP)

ET24(IPP)

ET24(IPP)

ET24(IPP)

SiO2 50.78 50.31 50.38 50.19 50.79 50.05 49.83 49.95 50.48 50.79 50.87 51.18 50.77 50.75 51.80TiO2 0.37 0.34 0.29 0.32 0.42 0.38 0.40 0.38 0.36 0.44 0.43 0.45 0.52 0.60 0.47Cr2O3 0.77 0.92 1.30 1.49 1.38 1.12 0.98 1.30 1.25 1.30 1.33 1.31 1.31 1.63 1.27Al2O3 7.41 7.25 7.18 7.19 6.24 7.03 6.56 6.53 6.55 4.69 3.98 4.42 5.10 4.60 3.95FeO 2.90 3.07 2.77 2.95 2.80 3.03 3.16 2.91 2.97 3.74 3.44 3.13 3.03 2.94 3.76MnO 0.14 0.09 0.07 0.14 0.21 0.10 0.11 0.13 0.12 0.11 0.10 0.14 0.14 0.14 0.09MgO 14.94 15.03 14.92 14.49 14.89 15.12 16.51 15.87 15.52 18.32 17.37 16.85 16.04 15.42 19.26NiO 0.19 0.21 0.11 0.00 0.03 0.10 0.42 0.00 0.00 0.00 0.00 0.00 0.00CaO 21.83 21.48 22.56 22.20 22.04 22.57 21.92 21.66 21.96 20.14 21.19 21.91 22.10 23.05 18.31Na2O 1.02 1.03 0.54 0.46 0.55 0.60 0.49 0.64 0.41 0.26 0.29 0.25 0.30 0.38 0.29K2O 0.12 0.11 0.04 0.01 0.07 0.09 0.00 0.01 0.01 0.00 0.00 0.01

Total 100.16 99.52 100.32 99.75 99.47 100.01 99.99 99.54 100.13 99.77 99.01 99.65 99.27 99.51 99.21a Data from Robbiano (1994).b Data from Rampone et al. (2004).


Table 4Mineral major element compositions (oxide vt%) spinels

Sample (rock-type)ETA20a (LSP) ETR1b (RSP) ETR2b (RSP) ETR3b (RSP) ETR4b (RSP) ET22 (IPP) ET22 (IPP) ET24 (IPP) ET24 (IPP)

Si2O 0.13 0.06 0.01 0.04TiO2 0.08 0.16 0.11 0.08 0.09 0.94 0.77 0.75 0.69Cr2O3 13.20 15.31 15.82 13.79 15.83 37.58 37.77 38.09 38.62Al2O3 54.38 53.17 52.49 54.14 52.35 25.04 26.07 25.58 24.75Fe2O3 1.98 1.60 0.97 1.62 0.99 6.40 5.67 5.61 5.81FeO 11.10 11.10 11.70 10.90 12.71 16.45 15.91 16.69 17.41MnO 0.19 0.02 0.00 0.12 0.01 0.27 0.19 0.26 0.22MgO 19.45 19.28 18.53 19.19 17.91 13.02 13.41 12.80 12.29CaO 0.06 0.12 0.12 0.00 0.05 0.01 0.17 0.17 0.12NiO 0.19 0.17 0.17

Total 100.56 100.76 99.74 99.84 99.94 99.96 100.13 100.13 99.95

Mg# 0.76 0.75 0.74 0.76 0.71 0.58 0.60 0.57 0.55Cr# 0.14 0.16 0.17 0.15 0.17 0.50 0.49 0.49 0.51

a Data from Robbiano (1994).b Data from Rampone et al. (2004).

As a whole, the lithospheric lherzolites show rather fertile compositions. In fact they have relatively highCpx (8.011.0 vol.%) and Opx (27.230.3 vol.%), and relatively low Ol (57.564.4 vol.%) modal contents, andrather constant Cpx/Opx ratios (0.280.38). Their bulk rock major element compositions show relatively highCaO (2.223.09 wt%) and Al2O3 (2.623.13 wt%) and SiO2 (45.1445.63 wt%) contents, and relatively low MgO(39.741.7 wt%) contents.

The reactive spinel peridotites are poor in Cpx and show significantly depleted compositions, as com-pared with the lithospheric lherzolites. They have highly variable Cpx (2.98.6 vol.%), significantly low Opx(10.916.1 vol.%), and significantly high Ol (74.485.6 vol.%) modal contents, and high variable Cpx/Opx ratios(0.270.53). Accordingly, their bulk rock major element compositions show relatively low CaO (1.152.18 wt%),Al2O3 (1.121.97 wt%) and SiO2 (42.5143.75 wt%) contents, and relatively high MgO (43.646.3 wt%)contents.

The impregnated plagioclase peridotites have silica (43.5643.94 wt%), Al2O3 (2.203.29 wt%) and CaO(2.142.77 wt%) contents slightly higher than those of the reactive spinel peridotites.

Table 2 shows the bulk rock trace element compositions of a number of selected samples again representative ofthe lithospheric spinel-facies lherzolites (LSP) from Praglia, the reactive cpx-bearing spinel harzburgites (RSP) fromPanorama Point and the impregnated plagioclase peridotites (IPP) from Gorzente River.

The bulk rock trace element data show fractionated REE patterns for all the rock types, with different absoluteconcentration and LREE fractionation (Fig. 4).

Lithospheric lherzolites are characterized by almost flat HREE patterns, in the HoNYbN region (maximum at YbN1.90 C1, sample ET4/3), progressive fractionation from MREE to LREE and a well evident LREE fractionation(CeN/SmN in the range 0.100.16).

Consistent with their modal composition, reactive harzburgites show overall lower REE concentrations than thelithospheric lherzolites. They are characterized, moreover, by very slight HREE fractionation (HoN/YbN in the range0.840.97, with a maximum at YbN 1.57 C1, sample ETR3), progressive fractionation from MREE to LREE and aclear LREE fractionation (CeN/SmN in the range 0.060.10).

The plagioclase peridotites, in good agreement with their modal composition, show overall REE concentrationshigher than the reactive peridotites and similar to higher than the lithospheric lherzolites. They are characterized,moreover, by slightly humped REE patterns (with a maximum at DyN, up to about 2.15 C1, sample ETR9), withslight HREE fractionation, progressive fractionation from MREE to LREE and a marked LREE fractionation (CeN/SmNin the range 0.080.12) significantly higher than that of the lithospheric lherzolites and rather similar to that one of thereactive peridotites.

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Table 5Mineral trace element compositions (element in ppm) clinopyroxenes

Sample (rock-type)ET4/1a(LSP)

ET4/1a(LSP)

ETR1b(RSP)

ETR3b(RSP)

ETR3b(RSP)

ETR4b(RSP)

ETR4b(RSP)

ETR6c(IPP)

ETR9c(IPP)

ETR9c(IPP)

ETR9c(IPP)

PIC1(IPP)

PIC2d(IPP)

ET22(IPP)

ET24(IPP)

PIC3d(dikelet)

La 0.08 0.06 0.06 0.07 0.04Ce 0.49 0.41 0.30 0.33 0.32 0.41 0.41 0.67 0.35 0.39 0.54 1.12 1.15 0.88 1.28 0.28Pr 0.17 0.15 0.20 0.17 0.42 0.44 0.36 0.46 0.12Nd 1.63 1.3 1.69 1.59 1.70 1.77 1.66 3.30 2.37 2.99 3.50 3.65 4.14 3.13 3.94 1.33Sm 1.16 1.04 1.13 0.95 0.98 1.01 1.11 2.35 1.77 2.04 2.27 2.11 2.38 2.03 2.09 1.09Eu 0.44 0.46 0.55 0.51 0.50 0.41 0.52 0.96 0.60 0.82 0.73 0.77 0.91 0.74 0.74 0.46Gd 1.79 1.99 2.11 1.87 1.88 4.60 2.74 3.10 3.20 3.22 3.94 2.68 2.92 2.13Tb 0.40 0.36 0.41 0.39 0.65 0.79 0.52 0.56 0.47Dy 2.22 2.26 2.54 2.86 2.89 2.66 2.48 5.70 3.50 4.70 5.00 4.19 4.99 3.63 3.80 3.20Ho 0.60 0.63 0.56 0.52 0.95 1.05 0.77 0.78 0.71Er 1.32 1.49 1.45 1.83 1.92 1.57 1.70 3.50 2.08 2.66 2.65 2.52 2.58 1.98 2.07 1.86Tm 0.25 0.25 0.23 0.23 0.36 0.34 0.25 0.28 0.26Yb 1.09 1.23 1.36 1.63 1.55 1.57 1.29 3.30 2.41 3.55 3.20 2.27 2.19 1.66 1.94 1.71Lu 0.22 0.22 0.20 0.18 0.31 0.31 0.23 0.22 0.25Sr 2.50 2.66 2.70 3.20 3.30 4.70 5.50 0.66 1.20 1.80 0.80 1.35 1.19 0.42 0.87 0.62Zr 7.20 7.438 6.00 5.50 5.30 7.10 6.80 14.00 9.00 12.00 12.00 14.83 15.00 12.70 15.76 3.98Y 15 15 16 14 14 32 21 29 28 23 24 19 20 18Sc 48 51 42 66 67 73 70 62 53 67 75 60 57 55 58 60V 192 185 247 268 288 273 267 322 280 346 399 364 379 365 407 402Cr 4920 7088 7334 8293 7581 7528 6804 5305 7104 7466 10,328 10,689 10,041 11,537 9426

a Data from Robbiano (1994).b Data from Rampone et al. (2004).c Data from Romairone (1999).d Data from Piccardo et al. (2004a).


Fig. 4. Bulk rock C1-normalised REE patterns of lithospheric, reactive and impregnated peridotites.

2.2.1.2. Mineral major and trace element composition. In this section, we present and discuss new mineral chemistrydata and data from previous papers (i.e. Robbiano, 1994; Romairone, 1999; Piccardo et al., 2004a,b; Rampone et al.,2004, 2005a,b). Tables 34 report the Cpx and Sp major element compositions of samples representating the threegroups: the LSP from Praglia, the RSP from Panorama Point and the IPP from Gorzente River.

The Praglia LSP preserve rather Al- and Na-rich (Al2O3 ca. 7.3 wt%, Na2O 1.0 wt%) clinopyroxenes, which arerelatively poor in Cr (Cr2O3 ca. 0.9 wt%) and Ti (TiO2 0.340.37 wt%), and aluminous spinels (54.4 wt%) showinglow Ti contents (TiO2 0.08 wt%) (Figs. 5 and 6). The RSP from Panorama Point have clinopyroxenes showing asignificant decrease of Na (Na2O 0.40.6 wt%) and a slight decrease of Al (Al2O3 6.27.2 wt%) contents, coupledwith a significant increase of Cr (Cr2O3 1.01.5 wt%) content, and spinels enriched in Cr (Cr2O3 13.815.8 wt%)and Ti (TiO2 0.080.16 wt%), compared with the corresponding minerals of the LSP (Figs. 5 and 6). The IPP fromthe Gorzente river bed and Mt. Pennello have clinopyroxenes showing relatively low Al (Al2O3 3.95.1 wt%) andNa (Na2O 0.30.4 wt%) contents, and high Cr (Cr2O3 1.31.6 wt%) and Ti (TiO2 0.40.6 wt%) contents, and spinelssignificantly depleted in Al (Al2O3 24.726.1 wt%) and strongly enriched in Ti (TiO2 0.70.9 wt%), again comparedwith the corresponding minerals of both LSP and RSP (Figs. 5 and 6).

Fig. 5. (A) Ti vs. Al (apfu) of clinopyroxenes from lithospheric, reactive and impregnated peridotites. Note that Ti content increases, at decreasingAl, from lithospheric to impregnated peridotites, most probably as a consequence of equilibration with the percolating basaltic melts; (B) Na vs.Al (apfu) of clinopyroxenes from lithospheric, reactive and impregnated peridotites. Note that the former preserve relatively high Na- and Al-richcompositions, whereas Cpx in reactive peridotites are significantly depleted in Na and Cpx in impregnated peridotites are strongly depleted in bothNa and Al, because of their equilibration with magmatic plagioclase.


Fig. 6. (A) Ti (apfu) vs. Mg#x100 of spinels from lithospheric, reactive and impregnated peridotites. Note that Ti content strongly increases, atstrongly decreasing Mg#x100, from lithospheric to impregnated peridotites; (B) TiO2 (wt%) vs. Cr#x100 of spinels from lithospheric, reactive andimpregnated peridotites. Note that TiO2 content strongly increases, at strongly increasing Cr#x100, from lithospheric to impregnated peridotites:this can be related to Ti enrichment of spinels during equilibration with the percolating basaltic liquids. TiO2 contents of spinel from impregnatedperidotites is remarkably higher than those of the refractory abyssal peridotites and fall inside the field of spinel compositions from MORB (Dickand Bullen, 1984).

Clinopyroxenes from both reactive and impregnated peridotites do not show significant and systematic compositionalvariations in major element contents (Na, Ti and Al, in particular) with microstructural site (i.e. porphyroclasts coresand rims, interstitial grains). This may point to attainment of chemical equilibration with the percolating melts, duringmeltperidotite interaction.

Table 5 shows trace element compositions of Cpx from samples from the three groups LSP, RSP and LPP. Cpx ofLSP has C1-normalized REE patterns (Fig. 7) rather flat in the MREE-HREE region and marked LREE fractionation(CeN/SmN = 0.10). Likewise, RSP Cpx has C1-normalized REE patterns (Fig. 7) again rather flat in the MREE-HREEregion and a marked LREE fractionation (CeN/SmN = 0.08). Cpx shows remarkably similar trace element contents,irrespectively of microtextural site (i.e. porphyroclast core and rim, interstitial grain): this strongly suggests thattrace element equilibration of the whole rock was attained during meltperidotite interaction. Opx shows a markedfractionation from HREE to LREE (CeN/YbN = 0.02). IPP Cpx has significantly humped C1-normalized REE patterns(Fig. 7) with maxima at TbN: they show a marked LREE fractionation (CeN/SmN in the range 0.150.11). Opxshows a significant fractionation from HREE to LREE (CeN/YbN = 0.410.29) and, sometimes, a weak negativeEuN anomaly. Like in the RSP, Cpx shows closely similar trace element contents irrespectively of microtexturalsite.

Trace elements, such as REE, Ti, Sc, V, Zr, Y, of Cpx and Opx of IPP are significantly higher than those of Cpxfrom LSP and RSP (Fig. 7). Similar trace element enrichment has been described in pyroxenes of the IPP from mostof the AlpineApennine ophiolitic peridotites (e.g. Piccardo et al., 2004a,b, and references therein).

Thin gabbro-noritic dikelets associated with some replacive spinel dunites at Piani di Praglia have Cpx showingalmost flat REE patterns, in the M-HREE region and strong negative LREE fractionation (LaN/SmN = 0.002) (Fig. 7).They have very low Sr contents (0.6 ppm).

2.2.1.3. Inferences on the origin of the different rock types. The chemical data summarized above may provide clues tothe processes responsible for the formation of the different rock-types using representative compositions on a bulk rockMgO versus SiO2 diagram (Fig. 8) as proposed by Niu (1997). The LSP plot along the melting trends, as calculatedby Niu, consistent with their origin as refractory residua after low degrees of partial melting. In contrast, the RSP havelower SiO2 contents compared to the refractory residua, confirming that they were formed by silica-depletion (i.e.pyroxene-dissolution and olivine-precipitation) during meltperidotite reaction.

The fact that the Praglia LSP plot along the refractory residua trends whilst the Gorzente River plagioclase peridotitesplot at lower SiO2 contents with respect to these trends suggest that these plagioclase peridotites cannot represent simpleproducts of closed system subsolidus recrystallization at plagioclase-facies conditions of precursor (rather fertile) spinel


Fig. 7. (A) C1 normalized REE patterns of representative Cpx from lithospheric spinel peridotites; (B) C1 normalized REE patterns of representativeCpx from reactive spinel peridotites; (C) C1 normalized REE patterns of representative Cpx from impregnated plagioclase peridotites. Mean Cpxfrom reactive spinel peridotites are also reported for comparison. Note Cpx in impregnated plagioclase peridotites have significantly higher REEconcentration than those from the reactive spinel peridotites; (D) C1 normalized REE patterns of mean Cpx from gabbro-noritic dikelets.

peridotites (as suggested for the same samples by Rampone et al., 2005a,b). Moreover, the Gorzente River plagioclaseperidotites plot at slightly lower MgO and higher SiO2 contents than the majority of the Panorama Point reactiveperidotites: this is consistent with addition of SiO2 (i.e. transformation of part of Ol to Opx) and plagioclase (i.e.dilution of the MgO content).

Similar information can be obtained by plotting bulk rock Mg# versus modal olivine of the different samples (Fig. 9).The LSP fall along the trends calculated by Bedini et al. (2002) for refractory residua after different kinds of partialmelting, confirming their fertile to residual character, whereas the RSP fall along the calculated reactive porous flowtrends. Plagioclase peridotites fall outside the refractory residua trends but are aligned with the reactive peridotitetrends at similar Mg# values but relatively low modal olivine contents. This supports our petrographic evidence thatpreviously RSP underwent a reduction of modal olivine due to partial replacement of Ol by Opx.

The plagioclase peridotites show a marked Sr enrichment, coupled with higher Ca and Al contents, comparedwith the RSP. This evidence suggests addition of plagioclase and further supports the interpretation that these plagio-clase peridotites represent initially RSP transformed to plagioclase peridotites by melt impregnation (i.e. addition ofplagioclase-rich gabbro-noritic material). Consequently, the petrographic-structural and compositional characteristicsof the plagioclase peridotites from the Gorzente River, previously interpreted as products of subsolidus, plagioclase-facies recrystallization of pristine spinel peridotites (Rampone et al., 2005a,b), are recognized here as impregnatedplagioclase peridotites formed via interaction with, and interstitial crystallization of a percolating, silica-saturated meltaffecting a reactive spinel peridotite precursor.


Fig. 8. Bulk rock SiO2 vs. MgO plot of the melt-related peridotites of the E-T massif. Melting trends of refractory residua compositions afterany kind of partial melting are reported after Niu (1997). Note that the lithospheric spinel lherzolites plot along the melting trends, confirmingtheir residual characters after partial melting, whereas reactive spinel peridotites plot at significantly lower SiO2 concentrations, confirming theirsilica-depleted character, in consequence of the pyroxene-dissolving olivine-forming reactions. Impregnated plagioclase peridotites plot at lowerSiO2 concentrations than refractory residua, and at slightly increasing silica, and decreasing MgO, than reactive spinel peridotites, in good agreementwith introduction of gabbroic material in reactive spinel peridotites.

The significant Cr# and Ti enrichment of spinels and the Al and Na depletion coupled to Cr and Ti enrichmentof clinopyroxenes in the IPP (compared with both lithospheric and reactive peridotites) suggest that, whatever thepre-impregnation rock-type was, meltperidotite interaction during impregnation significantly modified mineral majorelement compositions. These mineral compositions from the E-T plagioclase peridotites are consistent with those ofplagioclase peridotites formed by meltrock interaction and interstitial crystallization of percolating melts documentedfrom other localities (e.g. Rampone et al., 1997; Hellebrand et al., 2001; Muntener and Piccardo, 2003; Piccardo et al.,2004a,b, 2005, 2006).

Fig. 9. Bulk rock Mg# vs. modal olivine plot of the melt-related peridotites of the E-T massif. Melting trends of refractory residua compositionsafter any kind of partial melting and reactive porous flow trends are reported after Bedini et al. (2002). Note that the lithospheric spinel lherzolitesplot along the melting trends, confirming their residual characters after partial melting. Reactive spinel peridotites plot outside the melting trendsand along the reactive porous flow trends, at significantly higher modal olivine contents and slightly higher Mg# than lithopsheric spinel peridotites,confirming significant olivine addition in consequence of the pyroxene-dissolving olivine-forming reactions. Impregnated plagioclase peridotitesplot outside the melting trends, at significantly higher modal olivine contents and slightly higher Mg# than refractory residua, and at similar or lowermodal olivine contents than reactive spinel peridotites, in good agreement with introduction of gabbroic material in reactive spinel peridotites.


Fig. 10. (A) Calculated chondrite-normalized REE abundances of liquid in equilibrium with average Cpx composition of reactive spinel peridotites,using melt/cpx KD of Hart and Dunn (1993). (Fractional melting model from Johnson et al. (1990), mantle source from Ionov et al. (2002)). (B)Calculated chondrite-normalized REE abundances of liquid in equilibrium with average Cpx composition of gabbro-noritic dikelets, using melt/cpxKD of Hart and Dunn (1993). (Fractional melting model from Johnson et al. (1990), mantle source from Ionov et al. (2002)). (C) Calculated chondrite-normalized REE abundances of liquid in equilibrium with average Cpx composition of impregnated plagioclase peridotites, using melt/cpx KD ofHart and Dunn (1993). (D) Calculated chondrite-normalized REE abundances of liquid in equilibrium with average Cpx composition of impregnatedplagioclase peridotites, using melt/cpx KD of Vannucci et al. (1998). (Fractional melting model from Johnson et al. (1990), mantle source fromIonov et al. (2002).)

2.2.1.4. Inferences on the geochemical composition of the migrating melts. The reactive spinel harzburgites. Pet-rographic evidence indicates that the main stage of melt percolation forming the coarse granular, reactive spinelharzburgites around Panorama Point was accompanied by pyroxene-dissolving, olivine-forming reactions. This indi-cates that the percolating melts had silica-undersaturated compositions. Cpx of these reactive spinel harzburgites show aremarkable trace element homogeneity, characterized by strongly fractionated, LREE-depleted patterns, with coherentLREE concentration (CeN/SmN = 0.08, CeN = 0.490.68), and only slightly variable HREE content and fractionation.The homogeneous trace element composition of Cpx in the different samples, irrespective of their Cpx contents, suggestthat all samples were percolated by similar melts and that all achieved Cpx-melt trace element equilibration duringmeltrock interaction.

In this case, information on the geochemical signature of the equilibrium melts can be obtained on the basisof the trace element composition of clinopyroxenes, using Cpx-liquid partition coefficients appropriate for high-T,silica-undersaturated systems (e.g. Hart and Dunn, 1993; Ionov et al., 2002).

This approach indicates that the trace element distributions in Cpx reflect virtually complete chemical equilibriumwith the composition of primary melts (Fig. 10A). In particular, a remarkable match is shown by the liquids calculatedin equilibrium with Cpx and the melt increments modelled by 4% fractional melting of a spinel-facies asthenospheric


DM source (melting model of Ionov et al. (2002), KD from Hart and Dunn (1993)). Thus, it can be deduced that thePanorama Point RSP were formed by interaction of spinel peridotites with a reactively percolating melt, correspondingto a silica-undersaturated, low degree (4%) single melt increment with MORB affinity; this event affected the extendingmantle lithosphere at spinel-facies conditions.

The impregnated plagioclase peridotites. The IPP from the Mt. Pennello and Gorzente River exposures showwidespread replacement of kinked mantle Ol by newly formed Opx and abundance of Opx in the magmatic patches.This indicates that the parental melts of the plagioclase-bearing assemblages were silica-saturated, i.e. more silica-richthan typical MORBs. Cpx are enriched, at least in the moderately incompatible elements, with respect to the sameminerals from the LSP and RSP and minerals in equilibrium with MORB. In particular, regarding REE fractionation, theabsolute concentrations of the incompatible trace elements are significantly higher than those modelled in equilibriumwith fractional melt increments using solidliquid partition coefficients relevant for SiO2-undersaturated systems athigh TP conditions (e.g. Hart and Dunn, 1993; Ionov et al., 2002) (Fig. 10C).

Such estimates are strongly biased, however, by the adopted solid-melt partition coefficients. Increasing polymeri-sation of melts, due to silica-saturation, can lead to an increase of Solid/LiquidDREE (Vannucci et al., 1998; Green etal., 2000). The REE composition of liquids in equilibrium with Cpx of the plagioclase peridotites have, therefore,been calculated using the Cpx/LiquidDREE proposed by Vannucci et al. (1998) applicable to silica-saturated systems;the obtained REE compositions and patterns are closely similar to those of liquids modelled by low degrees (4%) ofnear-fractional melting of an asthenospheric DM source (as proposed by Ionov et al. (2002)) (Fig. 10D).

In conclusion, it can be deduced that the Mt. Pennello-Gorzente IPP record melt percolation and melt interstitialcrystallization of low degree (4%) single melt increments showing MORB affinity, which attained silica saturation byreactive percolation (pyroxene dissolution, olivine precipitation processes) during upwelling in the lithospheric mantle,as suggested by the RSP. Melt impregnation presumably affected the extending mantle lithosphere at plagioclase-faciesconditions.

The gabbroic dikelets. Cpx of the gabbro-noritic dikelets of Piani di Praglia show a strong LREE fractionation.Liquids calculated in equilibrium with Cpx are consistent with depleted melt increments modelled by 4% fractionalmelting of spinel-facies asthenospheric DM source (melting model of Ionov et al. (2002), KD from Hart and Dunn(1993)) (Fig. 10B).

3. Discussion

3.1. Relationships between melt-related features and deformation history

The present data confirm previous inferences (Drury et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al.,1993) that the oldest recognizable rock-types in the E-T peridotite are coarse-grained protogranular to porphyroclasticspinel lherzolites showing widespread, almost parallel pyroxenite banding, which derive from the subcontinentalspinel-facies mantle lithosphere. These pristine LSP frequently show a well-developed tectonite foliation associatedwith a km-scale shear zone and defined by the shape-preferred orientation of deformed pyroxenes, spinel and olivinegrains (Drury et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al., 1993). Previous studies evidenced that thisdeformation event formed spinel peridotite tectonites whereas younger spinel peridotite mylonites were developed underdecreasing pressure and temperature conditions, in up to 100 m scale shear zones transecting the peridotite tectonites.

Our field and petrographic data confirm previous suggestions by Piccardo et al. (2004a) that both pristineprotogranular-porphyroclastic spinel peridotites and spinel peridotite tectonites (Plate 1AC) were transformed, in upto km-scale domains, to reactive granular spinel peridotites formed by meltperidotite reaction processes. Previouslydeformed lithospheric spinel lherzolites underwent structural and microstructural resetting to granular textures alliedwith compositional and modal modification to pyroxene-depleted spinel harzburgites (Plate 1E and F). These trans-formations occurred in response to reactive percolation of pyroxene(-silica)-undersaturated, depleted melt fractionsshowing MORB affinity. Subsequently, both lithospheric and reactive spinel peridotites were diffusely transformedinto plagioclase-enriched peridotites (Plate 3A and B) by melt impregnation, which modified their composition bythe introduction of basaltic components in the form of plagioclase-rich, gabbro-noritic microgranular aggregates. Thisrefertilization occurred in response to pervasive percolation by silica-saturated melts derived from primary depletedmelt fractions with MORB affinity. All pre-existing lithotypes were subsequently cut by bodies and bands of replacivespinel dunites (Plate 5A, B, E and F), which were formed by the focused migration of pyroxene-undersaturated melts


across and along compositional and/or structural discontinuities: we envisage that this process represents the last meltmigration event by porous flow.

The strongly deformed rock-types in the Gorzente River (Plate 6AE) consist of coarse-grained deformed/recrystallized porphyroclasts, preserving microtextural records of pristine Opx- and Plg-forming mineral reactions (i.e.altered Plg crystals both interstitial and corroding deformed mantle minerals, Opx patches on mantle Ol, unstrainedOpx + Plg rims around deformed Cpx porphyroclasts), pointing to previous impregnated peridotites, and of fine-grainedelongated granoblastic domains surrounding the porphyroclasts, which are composed by grains of pyroxenes, Ol andbrownish altered Plg, in textural equilibrium. Usually, Plg is altered, but rarely fresh Plg grains are found within thefine-grained granoblastic mineral association in the form of microgranular aggregates, in textural equilibrium withthe other mineral phases (Opx, Cpx, Ol). Moreover, all of the samples of plagioclase-rich peridotite tectonites fromthe Guado-Gorzente riverbed with abundant (altered) plagioclase in outcrop and hand specimen, have abundant dustyand brownish grains in thin sections, certainly replacing former plagioclase, in precisely the same microtextural set-ting, i.e. within the equilibrium granoblastic aggregates. The textures and metamorphic mineral associations in thesegranoblastic aggregates strongly suggest syntectonic recrystallization under plagioclase-facies conditions, whilst thestructure and composition of the strongly deformed, plagioclase-rich tectonites clearly suggest intense deformation ofa plagioclase-impregnated peridotite precursor.

The discrete gabbroic bodies and dikes show primary, intrusive relationships with most of the previous rock-types.In some cases they cut strongly deformed, plagioclase-rich peridotite tectonites, thus indicating that they were intrudedinto deformed impregnated peridotites. Thus, they intruded at pressures compatible with plagioclase-facies conditions,which is confirmed by the estimated 0.30.5 GPa pressures of intrusion for some of the gabbroic bodies (Borghini etal., 2006). Locally, the coarse tectonites as well as the spinel- and plagioclase-bearing mylonites were transformedto amphibole- and chlorite-bearing peridotite mylonites, and finally, serpentinite mylonites. They locally preserveprimary contacts and structural-compositional features which allow the recognition of their protoliths and their mutualrelationships (Plate 6F). This evidence strongly suggests that extension reflected in the development of the differenttypes of shear zone structures (tectonites and mylonites) was active during and after the melt percolation events, andthat the rocks modified by meltrock interaction were uplifted by subsolidus extension towards the sea-floor of thegrowing Jurassic basin.

3.2. The Erro-Tobbio mantle protolith

On the basis of major and trace element bulk-rock and mineral compositions of samples of granular spinel peri-dotites and spinel peridotite tectonites from the E-T massif, and particularly of samples of coarse granular cpx-poorharzburgites from Panorama Point, Rampone et al. (2005a,b) have recently concluded that: (1) the E-T mantle protolithsrecord a composite history of partial melting and meltrock interaction by reactive porous flow (following the meltingand melt extraction models beneath oceanic ridges proposed by Niu, 1997) and (2) the inferred reactive porous flowevent preceded the exhumation-related lithospheric history of the E-T mantle. In particular, Rampone et al. (2005a,b)stated that the earliest step in the evolution of the E-T mantle peridotites was the Late Palaeozoic melting and interac-tion with MORB-type melts. These melts migrated through the peridotites by reactive porous flow, caused the modalvariability (from cpx-poor lherzolites to harzburgites) and the contrasting bulk-rock and mineral chemistry recordedby the mantle protoliths. Thus, partial melting and meltrock interaction by reactive porous flow have been relatedto a single composite asthenospheric stage which occurred prior to, and/or concomitant with, the incorporation ofthe E-T mantle in the lithosphere (documented by the annealing recrystallization in the spinel-facies stability field,at T < 1100 C) and preceded the Late Palaeozoic onset of exhumation from spinel-facies mantle depths to shallowerlithospheric levels (Rampone et al., 2005a,b).

Our study indicates that, on the basis of their different structural and compositional characteristics and their clearlyrecognizable mutual relationships in the field, two different types of spinel peridotites can be distinguished whichrecord different evolution stages.

The older type, the LSP, is typically represented by protogranular to porphyroclastic spinel lherzolites. They showrather fertile compositions, having relatively high pyroxenes and low olivine contents, and relatively high CaO, Al2O3,SiO2, and low MgO bulk rock concentrations; their bulk rock compositions are consistent with partial melting residua.Their Cpx are significantly rich in Al2O3 and Na2O, in good agreement with the rather fertile composition of the bulkrocks. Distinctive textural features are the Opx + Sp clusters and the vermicular micro-symplectites of Sp at the outer


Plate 6. Plagioclas peridotite tectonites(A) Gorzente River: Strongly foliated, plagioclase-rich peridotite tectonite; (B) Gorzente River: Plagioclase-rich, cpx-poor peridotite tectonite, showing a plagioclase-rich harzburgitic composition; plagioclase occur in strongly deformed and recrystallized,elongated granular aggregates. Ol and Cpx porphyroclasts still preserve reaction borders of Opx (i.e. relics of reaction with a silica saturatedmelt), closely similar to what observed in impregnated plagioclase peridotites. The pre-deformation rock type was, most probably, an impregnatedplagioclase peridotite; (C) Gorzente River: Plagioclase-rich, cpx-poor peridotite tectonite, showing an overall harzburgitic composition; plagioclase,particularly, in strongly deformed and recrystallized to elongated granular aggregates. Structural relics suggest that the pre-deformation rocktype was an impregnated plagioclase peridotite; (D) Gorzente River: Foliated plagioclase-rich, harzburgite tectonite; (E) Gorzente River: Detailsof previous plagioclase-rich, harzburgite tectonite: note that very high concentration of plagioclase, suggesting that plagioclase was originallyintroduced by a percolating melt; (F) Gorzente River: Strongly deformed chlorite-peridotite mylonite, still preserving evidence of the pre-deformationrock types, both a spinel peridotite, transformed to a chlorite + olivine mylonite, and an impregnated plagioclase peridotite, transformed to anolivine + amphibole(diopside) + chlorite mylonite.


border of large Opx porphyroclasts. These textures indicate complete spinel-facies recrystallization of a precursorgarnet-bearing assemblage under decreasing P-T conditions. LSP frequently have cm- to dm-wide spinel pyroxenitebands most probably derived from garnet-bearing magmatic precursors (Rampone and Borghini, 2006) which areisoclinally folded and completely recrystallized at spinel-facies conditions (Hoogerduijn Strating et al., 1993).

The younger type, the RSP, typically consists of coarse granular, cpx-poor spinel harzburgites. These reactive spinelperidotites have significantly depleted compositions. In fact they have relatively low Cpx and Opx, and relatively highOl modal contents, highly variable Cpx/Opx ratios and relatively low CaO, Al2O3, SiO2, and high MgO bulk rockconcentrations. These bulk rock compositions are not consistent with partial melting residua but resemble productsof meltrock reaction. Compared with the lithospheric spinel peridotites, their Cpx have significantly lower Na2Ocontents. They show replacive relationships on the former lithospheric spinel lherzolites. In fact, deformation fabricsof the pristine peridotites are transformed to coarse granular textures, the pyroxene contents are drastically reducedand the pre-existing pyroxenite banding is almost completely dissolved, being solely recorded by elongated trainsof spinel grains and strongly corroded pyroxene relics. Distinctive textural features of these rocks are Ol coronassurrounding corroded Px porphyroclasts, indicative of Px-dissolving, Ol-forming melt-mineral reactions, whereasrelict textural features (i.e. Opx + Sp clusters and vermicular micro-symplectites of Sp in Opx) of the pristine mantleprotoliths are locally preserved.

Thus, the early stages recorded in the LSP indicate that: (i) pristine garnet-bearing lherzolites, most probably residuaafter low-degree partial melting, upwelled from garnet- to spinel-facies conditions; (ii) early during this evolution stagethese lherzolites were intruded by basaltic melts giving rise to garnet-bearing pyroxenite bands; (iii) lherzolites andpyroxenites underwent plastic deformation and were, later on, completely recrystallized at spinel facies conditions underdecreasing temperature. These early evolution stages should be related to the isolation of the E-T mantle protolith fromthe convecting asthenosphere and to the accretion with annealing recrystallization in the thermal lithosphere, prior toany meltperidotite interaction.

The above data suggest that a composite history of magmatic (i.e. pyroxenite intrusion) and tectono-metamorphic(plastic deformation and spinel-facies recrystallization) events separates the early asthenospheric stage (i.e. thedeep-seated partial melting of the mantle protolith in the convecting asthenosphere before accretion to the mantlelithosphere) from the later lithospheric stage (i.e. the meltperidotite interaction after accretion and annealingrecrystallization in the conductive lithosphere). The coarse granular RSP were also formed at the expense of stronglydeformed, lithospheric spinel peridotite tectonites of the km-scale extensional shear zones. This suggests, moreover,that the reactive percolation of MORB-type melts followed, and not preceded, the early, spinel-facies stages of theexhumation-related shear zone deformation in the E-T mantle. We therefore suggest that the early stage of meltrockinteraction forming the RSP did not precede the incorporation of the E-T mantle in the lithosphere (as proposed byRampone et al. (2005a,b)) but that it was related to the exhumation itself. Consequently, we envisage that the percolatingMORB-type melts were formed by partial melting of the asthenosphere due to passive, near-adiabatic upwelling of theasthenosphere as a consequence of lithosphere extension and thinning.

3.3. Timing of whole-lithosphere evolution

A major problem concerns the timing of whole-lithosphere extension leading to opening of the Ligurian Tethys, asrecorded by the E-T peridotite. Our field and structural data clearly suggest that all of the recognized melt-related eventsoccurred during lithospheric extension. They started with the melt reactive percolation of MORB-type melt increments,which occurred during the extensional deformation at spinel-facies conditions, continued with the intrusion of MORBgabbroic rocks at plagioclase-facies conditions and culminated with intrusion/extrusion of MORB basaltic rocks closeor at the sea-floor. The chemical data clearly indicate a MORB affinity of both percolating and intruding melts,suggesting a similar asthenospheric DM mantle source. This evidence supports the idea that the different melt-relatedprocesses were related to the same melting cycle of the asthenosphere, connected to the ongoing geodynamic evolutionof the Europe-Adria system, leading to Jurassic ocean opening. Moreover, SmNd ages of 180 14 Ma for some E-TMORB gabbroic intrusions (Borghini et al., 2006) suggest an Early-Middle Jurassic age for the asthenosphere partialmelting. It is, therefore, highly plausible that melt percolation and melt intrusion in the E-T peridotite were indeedrelated to this Early-Middle Jurassic, MORB-producing partial melting of the upwelling asthenosphere.

Additional support for the hypothesis that MORB-type melt percolation and intrusion in the ophiolitic peridotites ofthe Ligurian Tethys were related to the same magmatic cycle comes from the geochronological data on the Monte Mag-


giore (Corsica) peridotites which show a closely similar pre-oceanic tectonic and magmatic evolution (Piccardo, 2003;Muntener and Piccardo, 2003). In fact, at Monte Maggiore, MORB melt impregnation has been dated at 155 6 Ma(Rampone and Piccardo, 2003), and MORB gabbro dike intrusion at 162 10 Ma (Rampone et al., 2005a,b). Spinel peri-dotites from the same body, interpreted as refractory residua after oceanic partial melting (Rampone et al., 2005a,b), or asreactively percolated lithospheric peridotites (Piccardo et al., 2004a), have yielded SmNd model ages of 165 14 Ma.Thus, in this peridotite body MORB melt percolation/impregnation and intrusion occurred very close in time and wererelated to the same Middle Jurassic cycle of MORB melt formation by asthenosphere decompressional melting.

Sm/Nd isotope data on plagioclase and clinopyroxene separates (and corresponding whole rock) from two E-Tplagioclase-bearing peridotite mylonites have yielded ages of 273 and 313 16 Ma, interpreted by Rampone et al.(2005a,b) as records of Late Palaeozoic lithospheric extension. On this basis, it was concluded that the Erro-Tobbioperidotites represent subcontinental lithospheric mantle that was tectonically exhumed from spinel-facies depths toshallower lithospheric levels during Late CarboniferousPermian times (Rampone et al., 2005a,b). Although thesedata indeed indicate that the E-T peridotite could occasionally preserve records of Permian extension, our evidencestrongly suggests that most of E-T tectonic metamorphic and magmatic evolution is related to rifting and opening ofthe Piemonte-Ligurian ocean (Hoogerduijn Strating et al., 1993), whose fundamental tectonic and magmatic stagesstarted in Early-Middle Jurassic times.

3.4. Transition from lithospheric rifting to oceanic drifting

Palaeogeographic evidence suggests that the E-T peridotite was located in a pericontinental position of the Euro-pean margin, whilst the Internal Liguride (and Corsica) terranes pertain to a more internal part of the basin, and theExternal Liguride terranes in a pericontinental position of the Adria continental margin (Muntener and Piccardo, 2003,and references therein). Geochronological data give some age constraints on the geodynamic evolution of the basinand suggest that the former Europe-Adria continental lithosphere underwent lithosphere extension leading to wholelithosphere failure during Late Jurassic and formation of more or less symmetric ocean-continent transition (OCT)zones (Manatschal, 2004, and references therein).

The following scenario is mostly based on radiometric ages obtained from magmatic rocks with MORB affinityintruded in ophiolitic sequences where MORB intrusion was preceded by percolation and impregnation by fractionalmelts with MORB affinity. In this scenario we assume first that lithosphere extension and thinning led to asthenospherenear-adiabatic upwelling and decompressional partial melting, forming melts with MORB affinity, and secondly thatMORB fractional melt increments percolated via diffuse porous flow through the extending lithosphere, and thatMORB aggregate magmas intruded the extending lithosphere at shallow levels, forming discrete gabbroic bodies anddikes and, finally, extruded on the sea-floor forming basaltic lava flows.

Bearing in mind the paleogeographic distribution of the geological-structural units from the Ligurian sector of thefuture Ligurian Tethys basin, prior to oceanic opening, SmNd mineral isochron ages indicate very similar intrusion agesof gabbroic rocks from the OCT zones of both the European and the Adria margins: 180 14 Ma for the E-T gabbros(Borghini et al., 2006) and 179 9 in the External Liguride gabbros (Tribuzio et al., 2004), whereas slightly youngerages (170 13 and 173 4.8 Ma) have been yielded by gabbros from the more internal units of the Cecina Valley,Tuscany (Tribuzio et al., 2004). The MORB gabbroic intrusions of the internal sequences of the Internal Liguridesand Monte Maggiore (Corsica) show very similar intrusion ages: 162 10 Ma at Monte Maggiore (Rampone et al.,2005a,b) and 165 14 Ma in the Internal Ligurides (Rampone et al., 1998). As argued before, these data suggest thatMORB melt percolation and impregnation and MORB melt intrusion occurred very close in time, hence that thesevarious magmatic events were related to the same Middle Jurassic process of MORB melt formation by asthenospherepartial melting. Therefore, in the ophiolitic sections deriving from the Ligurian Tethys oceanic lithosphere, the intrusivemagmatic processes appear to have been distributed in a broadly symmetric way: slightly older, Early-Middle Jurassic(about 180 Ma) ages are obtained in the lithospheric mantle units from the OCT settings of both the European andAdria paleomargins, and slightly younger, Middle Jurassic (about 164 Ma) ages occur in the lithospheric mantle fromthe more Internal units of the basin.

Moreover, UPb zircon geochronology on plagiogranites from the Voltri Massif and I

the pre-oceanic evolution of the erro-tobbio peridotite (voltri massif, ligurian alps, italy)

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