Melt transport and deformation history in a nonvolcanic ophiolitic section, northern Apennines,...

Article

Volume 12, Number 7

28 July 2011

Q0AG04, doi:10.1029/2010GC003429

ISSN: 1525‐2027

Melt transport and deformation history in a nonvolcanicophiolitic section, northern Apennines, Italy: Implicationsfor crustal accretion at slow spreading settings

Alessio SanfilippoDipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Via Ferrata 1, I‐27100Pavia, Italy ([email protected])

Riccardo TribuzioDipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Via Ferrata 1, I‐27100Pavia, Italy

Also at Unità Operativa di Supporto Pavia, Istituto di Geoscienze e Georisorse, CNR, Via Ferrata 1,I‐27100 Pavia, Italy ([email protected])

[1] Field observations and petrological and geochemical data are used to constrain a conceptual model forthe formation of a gabbro‐peridotite section from Ligurian ophiolites (Italy). The studied section is attrib-uted to an intraoceanic domain of the Jurassic Ligurian‐Piedmontese basin and is characterized by the lackof a basalt layer, similar to nonvolcanic segments from (ultra)slow spreading ridges. The proposed modelshows a “hot” lithospheric evolution in which melt transport in the mantle under spinel to plagioclase faciesconditions occurred mostly in the form of grain‐scale porous flow. We recognize a series of melt/peridotiteinteraction events, either diffuse or channeled, which modified the composition of the moderately depletedprecursor mantle. In particular, localized infiltrations of MORB‐type melts gave rise to formation of spinelwebsterite layers close to the lithosphere‐asthenosphere boundary. The peridotite‐websterite associationwas involved in a spinel facies deformation attributed to emplacement of asthenospheric material at thebase of the lithosphere. The “hot” lithospheric evolution is followed by an evolution characterized by melttransport through fractures, which started with crystallization of melt into troctolite to olivine gabbro dikes.Both mantle structures and gabbroic dikes are locally crosscut by gabbroic sills. As the mantle sectioncooled significantly, the dip of the melt migration structures evolved from subvertical to subhorizontal.The growth of a gabbroic pluton (up to ∼400 m thick) that is intruded into the mantle sequence is attributedto accretion of gabbroic sills. The tectonomagmatic history recorded by the gabbroic pluton after its solid-ification is characterized by ductile shearing developed from near‐solidus to amphibolite facies conditions.

Components: 19,400 words, 10 figures, 11 tables.

Keywords: gabbros; mantle peridotites; mantle pyroxenites; melt transport; oceanic lithosphere; ophiolites.

Index Terms: 1030 Geochemistry: Geochemical cycles (0330); 1032 Geochemistry: Mid-oceanic ridge processes (3614,8416); 1038 Geochemistry: Mantle processes (3621).

Received 15 November 2010; Accepted 18 April 2011; Published 28 July 2011.

Sanfilippo, A., and R. Tribuzio (2011), Melt transport and deformation history in a nonvolcanic ophiolitic section, northernApennines, Italy: Implications for crustal accretion at slow spreading settings, Geochem. Geophys. Geosyst., 12, Q0AG04,doi:10.1029/2010GC003429.

Copyright 2011 by the American Geophysical Union 1 of 34

http://dx.doi.org/10.1029/2010GC003429

1. Introduction

[2] Ophiolites offer the opportunity for 3‐D in-vestigations of ancient oceanic lithosphere. Tounravel the architecture of the oceanic lithosphereand to constrain the tectonomagmatic processesthrough which the oceanic lithosphere is generated,the study of ophiolites provides complementaryinformation to that acquired by ocean floor drillingand dredging. For instance, the processes of meltextraction, melt‐peridotite reactions and building ofgabbroic crust inferred from studies of the Omanophiolite are considered to be relevant to theformation of modern fast spreading ridges [e.g.,Nicolas and Boudier, 1995; Kelemen et al., 1995,1997]. However, models of the creation of oceaniclithosphere at fast spreading ridges are notappropriate for (ultra)slow spreading ridges [e.g.,Cannat, 1993; Cannat et al., 1997; Kelemen et al.,2007] and ophiolites representing fossil analogs of(ultra)slow spreading ridges have not been clearlyidentified yet.

[3] Most Jurassic ophiolites from the Alpine‐Apennine belt are considered to be either remnantsof an embryonic ocean, or analogs of an ocean‐continent transition developed in a magma poorsystem [e.g., Marroni et al., 1998; Manatschaland Müntener, 2009; Piccardo and Guarnieri,2009]. However, a few ophiolitic sequences fromthe Alpine‐Apennine belt, considered to repre-sent oceanward paleogeographic domains of theJurassic basin, bear lithostratigraphic, structuraland compositional similarities to modern (ultra)slowspreading ridges. These successions are essentiallyrepresented by the Chenaillet ophiolite from thewestern Alps [Lagabrielle and Cannat, 1990; Caby,1995; Manatschal et al., 2011], the M. Maggioremantle section from Corsica [Rampone et al., 2008,2009] and by the Internal Ligurian ophiolites fromthe northern Apennines [Tribuzio et al., 2000;Principi et al., 2004].

[4] In this study, we wish to understand the tecto-nomagmatic evolution experienced by a lesserknown gabbro‐peridotite association from theInternal Ligurian ophiolites, exposed near theScogna and Rocchetta Vara localities. The Scogna‐Rocchetta Vara ophiolite is characterized by theabsence of a basalt layer, similar to nonvolcanicsegments from (ultra)slow spreading ridges [e.g.,Kelemen et al., 2007], and by preservation of pri-mary relationships between the gabbro‐peridotitebasement and overlying sedimentary cover [Barret

and Friedrichsen, 1989; Principi et al., 2004]. Onthe basis of new field data, analysis of micro-structures and major and trace element micro-analyses of minerals, we propose a compositeconceptual model for the formation and evolution ofthe studied ophiolite. In particular, we show (1) thecompositional and structural modifications thatoccurred in the mantle section in conjunction withits exhumation from spinel to plagioclase faciesconditions, (2) the formation of oceanic gabbroiccrust, and (3) the tectonomagmatic evolutionleading to exposure of the gabbro‐peridotite asso-ciation at the seafloor. In addition, we emphasizethe similarities between the studied ophiolite andthe melt‐poor sections from modern (ultra)slowspreading settings.

2. Geological Setting

[5] Jurassic ophiolitic sequences representing dif-ferent paleogeographic domains of the Ligurian‐Piedmontese basin are exposed in the northernApennines. The ophiolites from theExternal Ligurianunits contain mantle sequences of subcontinentallithospheric origin [Beccaluva et al., 1984; Ramponeet al., 1995; Montanini et al., 2006] and are locallyassociated with continental crust rocks [Molli, 1996;Montanini and Tribuzio, 2001; Renna and Tribuzio,2009]. The association of ophiolites and continentalcrust material from the External Ligurian units isconsidered to represent a fossil ocean‐continenttransition such as the magma‐poor continentalmargin of western Iberia [Marroni et al., 1998;Tribuzio et al., 2004]. The ophiolites from theInternal Ligurian units do not show relationshipswith continental material and are attributed to adistal domain of the basin. They are characterizedby a gabbro‐peridotite basement discontinuouslycovered by a volcanosedimentary sequence thatcommonly exhibits interlayering among MORB‐type lava flows, sedimentary breccias and Middle‐Upper Jurassic radiolarian cherts [Cortesogno et al.,1987; Principi et al., 2004].

[6] Three major ophiolite bodies are present in theInternal Ligurian units (Figure 1a), the Bracco‐Levanto, Val Graveglia‐Bargonasco and Scogna‐Rocchetta Vara. The Bracco‐Levanto ophioliteprovides evidence for the occurrence of a mor-phological high in the Ligurian‐Piedmontese basin[Cortesogno et al., 1987]. This is shown by asequence in which the Jurassic volcanosedimentarycover is absent, apart from the local occurrence ofthin breccia levels, and the gabbro‐peridotite asso-

GeochemistryGeophysicsGeosystems G3G3 SANFILIPPO AND TRIBUZIO: EVOLUTION OF A NONVOLCANIC OPHIOLITE 10.1029/2010GC003429

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Figure 1


3 of 34

ciation is commonly directly overlain by Cretaceousshaly pelagites [see also Principi et al., 2004]. Thispaleomorphological high is made up of a gabbroicpluton intruded into mantle peridotites, as observedfor oceanic core complexes from the Mid AtlanticRidge, such as the Atlantis Massif [e.g., Cann et al.,1997] and the Kane Megamullion [Dick et al.,2008]. In the Val Graveglia‐Bargonasco ophiolite,the gabbro‐peridotite basement is covered by avolcanosedimentary sequence characterized by anoverall continuous basalt flow layer [Principi et al.,2004]. Conversely, in Scogna‐Rocchetta Varaophiolite, the basalt layer is absent [see also Barretand Friedrichsen, 1989].

[7] The mantle sequence from the Val Graveglia‐Bargonasco ophiolite consists mainly of depletedspinel peridotites showing reequilibration underplagioclase facies conditions [Beccaluva et al.,1984; Rampone et al., 1996]. These peridotitesare isotopically variably depleted relative to typicaldepleted mantle reservoirs [see also Rampone et al.,1998], similar to what is observed for modern oce-anic lithosphere [Salters and Dick, 2002;Cipriani etal., 2004]. The Val Graveglia‐Bargonasco perido-tites represent either asthenospheric material thatascended in response to oceanic spreading, orexhumed subcontinental lithosphere that underwentthermochemical erosion by the upwelling astheno-sphere during the rifting [see also Tribuzio et al.,2004; Piccardo and Guarnieri, 2009].

[8] Gabbros and basalts from the Bracco‐Levantoophiolite have trace element and initial Nd isotopiccompositions similar to those of modern NMORB[Tribuzio et al., 1995, 2000; Rampone et al.,1998]. The gabbro‐peridotite association of Bracco‐

Levanto ophiolite records a polyphase tectonic evo-lution in ductile to brittle shear zones, which wascorrelated with its exhumation at the seafloor[Treves and Harper, 1994; Molli, 1995, 1996;Tribuzio et al., 1995, 2000; Menna, 2009]. Thegabbroic rocks from Bracco‐Levanto and Scogna‐Rocchetta Vara ophiolites are associated with oliv-ine‐rich troctolites [Bezzi and Piccardo, 1971].These olivine‐rich troctolites are texturally andcompositionally similar to those from oceanic corecomplexes at the Mid Atlantic Ridge [Suhr et al.,2008; Drouin et al., 2009; Dick et al., 2010]. Theolivine‐rich troctolites from Internal Ligurianophiolites have been recently shown to have formedby interaction of an olivine‐rich matrix with infil-trating MORB‐type melts [Renna and Tribuzio,2011]. This interaction most likely occurred inmantle melt conduits of replacive origin, which weresubsequently impregnated by MORB‐type meltssaturated in plagioclase + clinopyroxene to producethe olivine‐rich troctolites [Renna and Tribuzio,2011].

3. Field Relationships

[9] The studied ophiolite is exposed for ∼15 km2

and consists mainly of two tectonic successions,namely the Scogna and Rocchetta Vara (Figure 1b).The Scogna succession consists mostly of an alteredgabbroic section that is locally overlain by gabbroicbreccias. The Rocchetta Vara succession occurs inan orogenic‐related tight syncline. In particular, theRocchetta Vara exposure constitutes the overturnedlimb of this fold and stretches along the NW‐SEdirection for ∼6 km. The Rocchetta Vara successionis characterized by a gabbro‐peridotite basement

Figure 1. (a) Location of ophiolites from the northern Apennines. The major ophiolitic bodies of the Internal Ligurianophiolites are indicated as follows: SRV, Scogna‐Rocchetta Vara; VGB, Val Graveglia‐Bargonasco; BL, Bracco‐Levanto. (b) Geological sketch map of Scogna‐Rocchetta Vara ophiolite and simplified tectonic scheme of thenorthern Apennines. IL, Internal Ligurian units (Middle‐Upper Jurassic ophiolitic sequences); EL, External Ligur-ian units (Upper Cretaceous turbitites and breccias containing ophiolitic blocks); GO, Gottero Unit (Maastrichtian‐early Paleocene sandstones); TU, Tuscany nappe (late Oligocene–early Miocene turbiditic sandstones). (c) Detail ofthe geological sketch map of Figure 1b and cross section of Rocchetta Vara succession. The orientation of the tec-tonitic foliation in the mantle peridotites is N95°–70°, 30°–45°; the orientation of the magmatic layering in thegabbroic pluton is N250°–290°, 40°–75°. Another ophiolitic tectonic slice (TS) is exposed to the west of RocchettaVara succession and consists mostly of serpentinized peridotites of mantle origin that are overlain by radiolariancherts. The absence of gabbroic breccias in the sedimentary cover makes the paleogeographic attribution of thistectonic slice uncertain, which has thus not been considered in this work. Maps of Figure 1b and Figure 1c arecompiled after the 1:50,000 geological map scale from the ISPRA Web site (http://www.apat.gov.it/MEDIA/carg/233_PONTREMOLI/Foglio.html) and this work. (d) Detail of cross section in Figure 1c. The contact betweenthe mantle sequence and the underlying gabbroic pluton is characterized by the occurrence of olivine‐rich trocto-lites. Gabbroic sills show sharp contacts with respect to the mantle peridotites, crosscutting at high angles theirtectonitic foliation, and the olivine‐rich troctolites. A gabbroic sill within the mantle peridotites crosscuts an olivinegabbro dike at high angle.


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https://www.researchgate.net/publication/235900391_The_pre-orogenic_volcano-sedimentary_covers_of_the_Western_Tethys_Oceanic_basin_A_review?el=1_x_8&enrichId=rgreq-d1463594-6c2f-4b76-91f3-905868fb17b9&enrichSource=Y292ZXJQYWdlOzI1MTQyOTk5NTtBUzoxMDAwMzkxNjY3OTE3MDJAMTQwMDg2MjYwNDgzNg==

https://www.researchgate.net/publication/245032081_Structural_features_of_the_Ligurian_ophiolites_petrologic_evidence_for_the_oceanic_floor_of_the_Northern_Apennine_geosyncline_a_contribution_to_the_problem_of_the_Alpine-type_gabbro-peridotite_associa?el=1_x_8&enrichId=rgreq-d1463594-6c2f-4b76-91f3-905868fb17b9&enrichSource=Y292ZXJQYWdlOzI1MTQyOTk5NTtBUzoxMDAwMzkxNjY3OTE3MDJAMTQwMDg2MjYwNDgzNg==

overlain by a sedimentary cover made up of gab-broic breccias, Middle‐Upper Jurassic radiolariancherts and Cretaceous shaly pelagites [Barret andFriedrichsen, 1989; Chiari et al., 2000; Principiet al., 2004].

[10] The gabbroic breccias from Scogna and Roc-chetta Vara successions are poorly sorted and clastsupported. The clasts are commonly nearly angularand up to meter scale; they consist of medium tocoarse‐grained olivine‐ to clinopyroxene‐richgabbro, in places showing porphyroclastic fabric.Olivine and clinopyroxene from the gabbro clastsare locally replaced by reddish patches that are madeup of fine‐grained hematite + calcite ± quartz. Nearthe contact with the gabbroic breccias, the mantleperidotites are transformed into calcite‐veinedhematite‐bearing serpentinites (ophicalcites). Cal-cite‐ and hematite‐bearing brittle structures are alsofound in the gabbroic rocks along the contact withoverlying breccias. The calcite‐ and hematite‐bearing brittle structures probably formed whenthe gabbro‐peridotite basement was exposed at theseafloor. The contacts between the gabbro‐peridotiteassociation and gabbroic breccias are frequentlytectonically reworked.

[11] The Rocchetta Vara mantle sequence mostlyconsists of peridotites commonly showing extensiveserpentinization. The original fabric is generallypreserved and the peridotites show porphyroclasticto tectonitic foliation (Figures 2a and 2b), charac-terized by alignment of porphyroclastic orthopyr-oxene and spinel. The peridotites locally include upto 3 cm thick pyroxene‐rich layers that are gener-ally boudinaged and elongated nearly concordantlywith respect to the foliation of the host rocks.Porphyroclastic pyroxene and spinel from thepyroxenite layers and the host peridotites show thesame alignment. The peridotites frequently containsubparallel plagioclase‐rich veinlets (up to 2 mmthick and up to 2 cm long) that broadly follow thefoliation of the host rocks (Figure 2c).

[12] The Rocchetta Vara mantle sequence locallyalso includes dunite bodies, that are up to meterscale in thickness. These bodies are elongatednearly parallel to the tectonitic foliation and includemm‐scale aggregates made up of euhedral spinel,which generally form trails reaching up to 0.3 m inlength (Figure 2d). The orientation of the spineltrails is geometrically nearly concordant with thecontact with the host rocks and their foliation. Thecontacts between the tectonized peridotites and thedunites are characterized by a gradual inward

decrease in modal proportions of orthopyroxene,thus suggesting a replacive origin for the dunites.The contact zone commonly also displays a gradualinward decrease of the subparallel plagioclase‐richveinlets, thus indicating that the replacive dunitesformed after the plagioclase‐rich veinlets. Plagio-clase rarely occurs within the dunites and developsirregular and thin veinlets (<1 mm) containing, inplaces, grains of pyroxene.

[13] The peridotite foliation and the dunite bodiesare locally crosscut by troctolite to olivine gabbrodikes (Figure 2c). These dikes are up to meter scalein thickness and commonly show diffuse contactswith the host rocks; the angle between the perido-tite foliation and the gabbroic dikes is <10°. In thetroctolite to olivine gabbro dikes, euhedral olivineand plagioclase grains show parallel alignment,thus leading to a magmatic foliation that is sub-parallel to the contacts with the host mantle rocksand their foliation. The thickest dikes also displaysubparallel modal and/or grain size layering.

[14] In the NW sector of the Rocchetta Vara suc-cession, a gabbroic intrusion (up to 400 m thick) isexposed below a ∼150 m thick mantle sequence(Figure 1c). In particular, the mantle sequenceoverlying the gabbroic intrusion contains sills up to3 m thick made up of coarse‐grained clinopyrox-ene‐rich gabbro. These gabbroic sills are elongatednearly perpendicular to the peridotite foliation. Thegabbroic sills display sharp planar boundaries tothe host peridotites, without grain size reduction.We also found one gabbroic sill crosscutting anolivine‐rich gabbro dike at a high angle (∼70°;Figure 1d). The thickest sills exhibit modal and/orgrain size layering that is subparallel to the contactswith the host mantle rocks.

[15] The Rocchetta Vara gabbroic intrusion con-sists broadly of coarse‐grained clinopyroxene‐richgabbros, associated with minor amounts of mediumto coarse‐grained olivine gabbros to troctolites.Poor exposure conditions, and locally the markedalteration of rocks, obscure the contacts betweenthe gabbros, olivine gabbros and troctolites. Note,however, that the troctolites form lenticular bodies(up to a few tens of meters in thickness) occurringat different distances from the contact with over-lying sediments. Troctolites to olivine gabbros andgabbros are locally characterized by a weak modaland/or grain size layering, which is subparallel tothe orientation of the gabbroic sills. In addition, thetroctolites commonly show a magmatic foliationproduced by alignment of euhedral olivine and


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plagioclase grains, which is at a low angle to themodal/grain size layering. The clinopyroxene‐richgabbros from the Rocchetta Vara gabbroic intru-sion are in places characterized by a ductile shearfoliation that forms a low angle with respect to themagmatic layering (Figure 2e). The shear zoneshave a width of several meters and were found nearthe contacts with the olivine‐rich troctolites and the

mantle peridotite lenses. Sheared gabbros exhibitporphyroclastic and, rarely, mylonitic fabric.

[16] Within the Rocchetta Vara gabbroic intrusion,there are a few mantle bodies (up to 50 m inthickness) that are elongated subparallel to themagmatic layering of the host gabbros. The mantleperidotites from these lenses retain a tectonitic to

Figure 2. Main mesostructures of Scogna‐Rocchetta Vara ophiolite: (a) Porphyroclastic peridotite with pyroxenitebanding (M. Sovrani area). (b) Peridotite‐pyroxenite association showing a tectonitic foliation (M. Gruzzo area).(c) Tectonized peridotite showing plagioclase‐rich veinlets elongated concordantly with the foliation of the host rock.At the top of the photograph, an olivine gabbro dike displays diffuse contacts with the host peridotite (M. Gruzzoarea). (d) Replacive dunite with spinel trails showing diffuse contacts with the host tectonized peridotite. The orien-tation of these spinel trails is nearly concordant with the contact with the host rock and its foliation (M. Gruzzo area).(e)Ductile shear foliation in clinopyroxene‐rich gabbros forming a low anglewith respect tomodal layering (M. Sovrani area).


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porphyroclastic foliation and show subparallelplagioclase‐rich veinlets. The mantle lenses withinthe gabbroic pluton also contain meter‐scale dunitebodies as well as gabbroic dikes showing diffusecontacts with the host peridotites. The structures ofthese mantle lenses and those of the mantlesequence enclosing the gabbroic intrusion aregeometrically concordant. In the Rocchetta Varasuccession, the contact between the gabbroicintrusion and overlying mantle sequence is char-acterized by occurrence of olivine‐rich troctolites(Figure 1d). These rocks are exposed for a thick-ness of ∼75 m and contain sills (up to meter scale inthickness) made up of clinopyroxene‐rich gabbros,which show sharp planar contacts with the hostolivine‐rich troctolites.

[17] The Scogna gabbroic section is mostly madeup of clinopyroxene‐rich gabbros, olivine gabbrosand troctolites, similar to that from Rocchetta Varasuccession. In addition, the Scogna gabbroic sectioncontains two bodies of olivine‐rich troctolite (∼50 mthick), lying at different distances from the contactwith the overlying gabbroic breccias. Furthermore,the Scogna gabbroic section locally include basaltdikes displaying chilled margins and reaching acouple of meters in thickness. The basalt dikes arecommonly porphyritic, with up to 10 vol % pla-gioclase phenocrysts, and crosscut the fabric of thehost gabbros (i.e., modal/grain size layering andtroctolite foliation) at a high angle.

[18] Figure 3 displays a paleotectonic reconstruc-tion of the Scogna‐Rocchetta Vara ophiolite inUpper Jurassic. The mantle peridotites display afoliation that is at a high angle with respect to themagmatic layering in the gabbroic pluton. These

peridotites include diffuse plagioclase‐rich veinletsand, locally, dunite bodies that are both elongatedconcordantly with the mantle foliation. In addition,the peridotites are crosscut by olivine‐rich gabbroicdikes that are subparallel to the mantle structuresand commonly display diffuse contacts with thehost peridotites. Near the contact with the gabbroicpluton, the gabbroic dikes are postdated by clin-opyroxene‐rich gabbroic sills, displaying sharpplanar boundaries with the host peridotites. Thegabbroic pluton consists of clinopyroxene‐richgabbros, olivine gabbros and troctolites and locallycontains mantle peridotite lenses as well as bodiesmade up of olivine‐rich troctolites. The intrusivefabric of the gabbroic pluton is subparallel to theorientation of the gabbroic sills. The gabbroicpluton locally includes ductile shear zones andbasalt dikes at low and high angles, respectively,relative to the intrusive fabric.

4. Petrography and Major ElementMineral Compositions

[19] The main petrographic features and localitiesof Scogna‐Rocchetta Vara samples selected forchemical analyses are reported in Tables 1a–1c.Major element analyses of mineral cores (Tables 2–7) were carried out using a JEOL JXA‐8200 elec-tron microprobe located at Dipartimento di Scienzedella Terra, Università degli Studi di Milano (Italy);conditions of analyses were 15 kV and 15 nA, andnatural standards were utilized. For comparativepurposes, new chemical analyses were also carriedout on dunite, gabbro and basalt samples from othersuccessions of the Internal Ligurian ophiolites.

Figure 3. Paleotectonic reconstruction of Scogna‐Rocchetta Vara ophiolite in the Middle Jurassic. The scale ofreplacive dunite bodies, gabbroic dikes, gabbroic sills, and basalt dikes is exaggerated. The contact between the gabbro‐peridotite basement and overlying gabbroic breccias is arbitrarily depicted as subhorizontal. However, the original geo-metric relationships between the fabrics of the gabbro‐peridotite basement and the overlying sediments are most likelyobliterated by the orogenic tectonics (see text for further details).


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Tab

le1a

.LocationandMainPetrographicFeaturesof

Ultram

afic

Sam

ples

SelectedforChemical

Analysesa

Sam

ple

Unit

Locality

Latitu

deand

Lon

gitude

RockTyp

eb

Oliv

ine

Ortho

pyroxene

Clin

opyrox

ene

Spinel

Plagioclase

Rem

arks

Mod

alAmou

nts

(vol

%)

Degreeof

Alteratio

n(vol

%)

Mod

alAmou

nts

(vol

%)

Degreeof

Alteratio

n(vol

%)

Mod

alAmou

nts

(vol

%)

Degreeof

Alteratio

n(vol

%)

Mod

alAmou

nts

(vol

%)

Degreeof

Alteratio

n(vol

%)

Mod

alAmou

nts

(vol

%)

Degreeof

Alteratio

n(vol

%)

MG3

Scogn

a–R.Vara

M.Gruzzo

44°13′34

.59″N,

9°47′48.66″E

perido

titec

5970

2210

610

25

1390

MG4

Scogn

a–R.Vara

M.Gruzzo

44°13′33

.65″N,

9°47′49.92″E

perido

titec

6460

1915

515

25

1010

0no

Spl

porph

CC2

Scogn

a–R.Vara

M.Sov

rani

44°15′6.29″N

,9°44′53.02″E

perido

titec

6880

2030

450

115

710

0mantle

lens

with

ingabb

roic

pluton

;no

Cpx

porph

MG2a

Scogn

a–R.Vara

M.Gruzzo

44°13′33

.96″N,

9°47′51.31″E

perido

tited

6360

1610

410

25

1590

with

20mm

thick

pyroxenite

layer

MG2a

Scogn

a–R.Vara

M.Gruzzo

44°13′33

.96″N,

9°47′51.31″E

websterite

d5

7053

028

07

07

100

MG20

Scogn

a–R.Vara

M.Gruzzo

44°13′33

.76″N,

9°47′50.15″E

perido

tited

6470

1810

410

15

1390

with

15mm

thick

pyroxenite

layer

MG20

Scogn

a–R.Vara

M.Gruzzo

44°13′33

.76″N,

9°47′50.15″E

websterite

d7

7048

035

05

05

100

MG5a

Scogn

a–R.Vara

M.Gruzzo

44°13′34

.36″N,

9°47′50.05″E

dunitee

9010

0_

__

_10

5_

_spinel

trails

MG5b

Scogn

a–R.Vara

M.Gruzzo

44°13′34

.34″N,

9°47′50.35″E

dunitee

8510

0_

_<5

100

<5

5<5

100

alteredPxs

with

inPlveinlets

VF10

2Bracco‐

Levanto

Bon

assola

44°10′32

.60″N,

9°35′08.65″E

dunitee

9010

0_

__

_10

10_

_spinel

trails

a Mineral

abbreviatio

nsafterKretz[198

3].

bMod

esareestim

ated

usingpo

intcoun

ting.

c One

thou

sand

pointsin

each

standard

size

section.

dFivehu

ndredpo

intsin

each

standard

size

section.

e Mod

esarevisually

estim

ated.


8 of 34

Tab

le1b

.LocationandMainPetrographicFeaturesof

GabbroicSam

ples

SelectedforChemical

Analysesa

Sam

ple

Unit

Locality

Latitu

deand

Lon

gitude

Rock

Typ

eOl

(vol

%)

Degree

ofOl

Alteratio

n(vol

%)

PlIgneou

sPlNeoblasts

Degree

ofPl

Alteratio

n(vol

%)

Cpx

Igneou

sCpx

Neoblasts

Amph

ibole

Rem

arks

Volum

e%

Grain

(mm)

Volum

e%

Grain

(mm)

Volum

e%

Grain

(mm)

Volum

e%

Grain

(mm)

Volum

e%

Grain

(mm)

MG2b

Scogn

a–R.Vara

M.Gruzzo

44°13′33

.95″N,

9°47′51.30″E

troctolite

dike

4510

050

3.5–5.0

_30

52.5–

5.0

__

foliatio

nin

Oland

Plgrains,30

mm

thick

MG12

Scogn

a–R.Vara

M.Gruzzo

44°14′52

.99″N,

9°44′55.78″E

Olgabb

rodike

2510

045

2–8.5.0

_40

303.5–

10.0

__

2m

thick

RV37

aScogn

a–R.Vara

M.Sov

rani

46°85′18

6″N,

9°48′138″E

gabb

roic

sill

_50

2.5–10

.0_

6050

3.5–

15.0

__

50mm

thick

CC11

Scogn

a–R.Vara

M.Sov

rani

44°15′26

.85″N,

9°44′33.53″E

sheared

gabb

ro5

100

302.5–10

300.5–1.0

3020

4.0–10

.010

0.1–0.2

accessory

Ti‐prg

neob

lastic

<0.05

CC12

aScogn

a–R.Vara

M.Sov

rani

44°15′26

.82″N,

9°44′33.46″E

sheared

gabb

ro5

100

201.2–7.5

300.1–0.2

3035

4.0–8.0

<5

0.05–0.1

10,Hbl

neob

lastic

0.2–0.7

with

relicsof

neob

lastic

Cpx

CC12

bScogn

a–R.Vara

M.Sov

rani

44°15′26

.82″N,

9°44′33.46″E

sheared

gabb

ro5

100

101.0–3.6

400.1–0.2

20<5

1.0–2.0

<5

0.01–0.2

40,Hbl

neob

lastic

0.2–0.8

with

relicsof

neob

lastic

Cpx

FG1/1

Internal

Ligurian

ophiolites

Bon

assola

44°10′50

.01″N,

9°34′27.73″E

Cpx‐rich

gabb

ro_

503.5 –15

.0_

100

502.5–

20.5

_accessory

Ti‐prg

interstitial

0.05

a Mineral

abbreviatio

nsafterKretz[198

3];mod

esarevisually

estim

ated.


9 of 34

4.1. Tectonized Peridotites

[20] The sampled peridotites have ∼5 vol % clin-opyroxene (Tables 1a–1c). In all samples, olivine ispartly replaced by serpentine and minor Fe oxidephases, and plagioclase is altered into fine‐grainedaggregates of prehnite ± epidote ± hydrogrossular ±chlorite. The peridotites hosting the gabbroic plutonand those within the gabbroic pluton have similarmodal compositions and microstructures.

[21] The tectonitic foliation is shown by the align-ment of porphyroclastic olivine, orthopyroxene,clinopyroxene and spinel. Porphyroclastic olivine isup to 3 mm in length and have 89.3 to 90.4 mol %forsterite (Table 2). Porphyroclastic orthopyroxeneand clinopyroxene (up to 7 mm and 3 mm in length,respectively) are exsolved and commonly showundulose extinction and kink bands. They are man-tled by polygonal aggregates made up of unstrainedorthopyroxene and clinopyroxene (both 0.3–0.5mm),which are associated with accessory amounts ofspinel (Figure 4a) and olivine. In these aggregates,neoblastic olivine and spinel are smaller (both≤0.2 mm) than the associated pyroxenes.

[22] Porphyroclastic orthopyroxene have low TiO2

concentrations (Figure 5) and relatively high Cr2O3

abundances (0.8–0.9 wt %). Porphyroclastic clin-opyroxene have low amounts of Na2O and TiO2

(Figure 6), and high Cr2O3 concentrations (1.3–1.4 wt %). Neoblastic orthopyroxene and clinopyr-oxene have commonly slightly lower concentrationsof Al2O3 and Cr2O3 than their porphyroclasticcounterparts (Tables 3 and 4), most likely as aresult of formation of synkinematic spinel, whichincorporates a high amount of these elements(Table 5). Figure 6 also shows that analyzed por-phyroclastic clinopyroxenes are chemically similarto those from another mantle section of the InternalLigurian ophiolites [Rampone et al., 1996].

[23] Porphyroclastic spinel (up to 3 mm in length)occurs in accessory modal amounts (Tables 1a–1c).Spinel is commonly rimmed by a plagioclase coronareaching up to 0.2 mm in thickness (Figure 4b).Orthopyroxene grains, locally up to 0.1 mm thick,are commonly associated with the coronitic plagio-clase, at the contact with porphyroclastic olivine.Neoblastic spinel from the pyroxene‐dominatedpolygonal aggregates is chemically similar to theporphyroclastic spinel (Figure 7). We applied theCa‐in‐Opx geothermometer [Brey and Köhler,1990] to the polygonal aggregates, assuming aconfining pressure of 1.0 GPa; neoblastic ortho-pyroxene has 1.3–1.4 wt % CaO, which correspondto temperature estimates of 1090–1130°C.T

able

1c.

LocationandMainPetrographicFeaturesof

Basaltic

Sam

ples

SelectedforChemical

Analysesa

Sam

ple

Unit

Locality

Latitu

deand

Lon

gitude

RockTyp

e

Groun

dmass

PlPheno

crystals

Degreeof

PlAlteratio

n(vol

%)

Cpx

Pheno

crystals

Volum

e%

Grain

(mm)

Volum

e%

Grain

(mm)

Volum

e%

Grain

(mm)

SC1

Scogn

a–R.Vara

Scogn

a44

°17′38

.55″N,

9°42′11.66″E

basaltdike

850.5–

1.5

151.6–

5.0

605

1

BR10

0Bracco‐Levanto

Bracco

44°16′23

.81″N,

9°33′46.35″E

basaltdike

950.5–

2.0

52.0–

8.0

90_

BR10

1Bracco‐Levanto

Bracco

44°15′00

.97″N,

9°34′06.43″E

basaltdike

950.1–

2.5

54.5–

10.8

80_

VF10

0Bracco‐Levanto

Bon

assola

44°10′34

.06″N,

9°35′06.08″E

basaltdike

100

0.01

–0.75

_90

_

a Mineral

abbreviatio

nsafterKretz[198

3];mod

esarevisually

estim

ated.


10 of 34

[24] The tectonized peridotites commonly containsubparallel plagioclase‐rich veinlets that are com-monly 0.5 to 2 mm thick and up to 20 mm long.The plagioclase‐rich veinlets commonly crosscutthe porphyroclastic olivines and follow the edgesof the pyroxene dominated neoblastic aggregates(Figure 4c). Within the neoblastic aggregates, thereare sinuous apophyses of these veinlets (up to150 mm thick). Note that the plagioclase‐rich vein-lets show concave contacts against the peridotitematrix. The plagioclase‐rich veinlets sometimescontain euhedral orthopyroxene grains that are upto 0.5 mm in length. Plagioclase‐rich veinlets withsimilar microstructures were documented for anearly undeformed mantle section from the InternalLigurian ophiolites and interpreted to reflect meltimpregnation [Rampone et al., 1997].

4.2. Pyroxenite Layers

[25] These rocks consist of deformed orthopyroxeneporphyroclasts (up to 5 mm in length) rimmed byunstrained aggregates made up of clinopyroxene,orthopyroxene and minor spinel. In particular, por-phyroclastic orthopyroxene commonly shows clin-opyroxene exsolution lamellae, undulose extinctionand kink bands. In the neoblastic aggregates, pyr-oxenes (0.3–0.5 mm) show 120° triple junctions(Figure 4d); neoblastic spinel is smaller (≤0.2 mm)than associated pyroxenes, and olivine is locallypresent as small (≤0.1 mm) serpentinized grains.Minor amounts of porphyroclastic spinel are alsocommonly present and elongated parallel to por-phyroclastic orthopyroxene. On the basis of themeasured modes (Tables 1a–1c), these rocks arehereafter referred to as websterites.

[26] Plagioclase‐rich veinlets are commonly foundalong the contacts between the websterites and thehost peridotites. These veins show sinouos apophyses(up to 0.2 mm thick) within the websterites, whichalso contain altered plagioclase as coronas aroundthe porphyroclastic spinel. Orthopyroxene, clin-opyroxene and spinel from the websterites haverelatively high concentrations of TiO2 (Figures 5–7).Clinopyroxene also has high concentrations ofNa2O. Equilibrium temperatures for the neoblasticmineral assemblage were calculated by applying theCa‐in‐Opx geothermometer [Brey and Köhler,1990], assuming P = 1.0 GPa; the CaO content oforthopyroxene is ∼1.5 wt %, which gave estimatesof ∼1150°C.

[27] In the tectonized peridotite enclosing thewebsterite layer, at the thin section scale, theminerals are chemically distinct from those ofthe other tectonized peridotites considered in thepresent study. Porphyroclastic orthopyroxene andspinel are enriched in TiO2 with respect to por-phyroclastic orthopyroxene and spinel from theother peridotites (Figures 5 and 7). In addition,porphyroclastic olivine, orthopyroxene and spinelfrom the peridotite hosting the websterite layer hasslightly lower Mg # with respect to the other peri-dotites (see also Tables 2 and 5). Porphyroclasticorthopyroxene and spinel from the websterite layerand adjacent tectonized peridotite are thereforechemically similar. Furthermore, we analyzed aclinopyroxene grain (∼250 mm) rimming a por-phyroclastic orthopyroxene at a distance of 5 mmfrom the websterite layer. This clinopyroxene hashigher TiO2 and Na2O than clinopyroxenes fromthe other peridotites and chemically resembles

Table 2. Major Element Olivine Compositionsa

Sample

MG3 MG4 CC2 MG2a MG20

Rock type perid perid perid perid peridMineral Ol porph Ol porph Ol porph Ol porph Ol porphNumber 5 3 6 5 7SiO2 41.22 (0.20) 41.49 (0.09) 41.48 (0.27) 41.29 (0.38) 40.68 (0.20)TiO2 0.01 (0.01) 0.01 (<0.01) 0.01 (0.02) 0.01 (0.01) 0.01 (0.01)Al2O3 0.01 (0.01) 0.02 (<0.01) <0.01 (<0.01) 0.02 (0.01) <0.01 (<0.01)Cr2O3 0.02 (0.02) 0.01 (0.010 0.01 (0.02) <0.01 (<0.01) <0.01 (<0.01)FeO 9.42 (0.37) 9.66 (0.02) 10.22 (0.08) 10.53 (0.03) 10.29 (0.09)MnO 0.13 (0.02) 0.14 (0.03) 0.15 (0.02) 0.15 (0.01) 0.14 (0.03)NiO – (–) 0.37 (0.03) 0.37 (0.04) 0.33 (0.03) 0.34 (0.04)MgO 49.54 (0.08) 49.89 (0.03) 49.58 (0.15) 49.11 (0.38) 48.89 (0.48)CaO 0.05 (0.02) 0.03 (0.01) 0.06 (0.01) 0.05 (0.02) 0.05 (0.01)Na2O <0.01 (0.01) 0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01)K2O <0.01 (<0.01) 0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01)Sum 100.42 (0.51) 101.61 (0.04) 101.89 (0.40) 101.49 (0.70) 100.42 (0.56)Forsterite (mol %) 90.4 (0.3) 90.2 (0.0) 89.6 (0.1) 89.3 (0.1) 89.4 (0.2)

aAverage values (wt %); perid, peridotite. Dash indicates not analyzed. SD values are in parentheses.


11 of 34

Tab

le3.

Major

ElementOrtho

pyroxene

Com

positio

nsa

Sam

pleandMineral

MG3

MG4

CC2

MG2a

Mg2

0

Opx

porph

Opx

neo

Opx

porph

Opx

neo

Opx

porph

Opx

neo

Opx

porph

Opx

neo

Opx

Opx

neo

Opx

Rocktype

perid

perid

perid

perid

perid

perid

web

web

hostlherz

web

hostlherz

Num

ber

38

312

613

35

63

4SiO

256

.23(0.76)

55.29(1.15)

57.37(0.23)

55.64(1.05)

56.63(0.32)

55.85(0.34)

56.35(0.31)

56.68(0.83)

56.58(0.61)

54.95(0.57)

55.47(0.21)

TiO

20.11

(0.01)

0.12

(0.04)

0.12

(0.01)

0.13

(0.05)

0.11

(0.03)

0.13

(0.03)

0.18

(0.02)

0.20

(0.01)

0.22

(0.22)

0.24

(0.03)

0.20

(0.02)

Al 2O3

3.23

(0.170

2.99

(0.45)

2.96

(0.21)

2.78

(0.30)

2.90

(0.20)

2.75

(0.17)

2.73

(0.12)

2.62

(0.17)

2.63

(0.23)

2.73

(0.15)

2.60

(0.24)

Cr 2O3

0.89

(0.07)

0.78

(0.05)

0.84

(0.02)

0.78

(0.06)

0.83

(0.03)

0.85

(0.06)

0.84

(0.01)

0.79

(0.06)

0.83

(0.09)

0.85

(0.05)

0.76

(0.06)

FeO

6.23

(0.06)

6.23

(0.13)

6.26

(0.10)

6.20

(0.14)

6.61

(0.09)

6.41

(0.10)

6.75

(0.03)

6.69

(0.09)

6.77

(0.15)

6.60

(0.09)

6.57

(0.09)

MnO

0.15

(0.01)

0.15

(0.03)

0.14

(0.00)

0.14

(0.02)

0.15

(0.03)

0.13

(0.03)

0.14

(<0.01

)0.17

(0.02)

0.15

(0.02)

0.18

(0.01)

0.16

(0.02)

NiO

0.02

(0.03)

0.06

(0.05)

0.11

(0.02)

0.09

(0.02)

0.09

(0.03)

0.10

(0.04)

0.11

(0.01)

0.09

(0.02)

0.09

(0.03)

0.06

(0.04)

0.06

(0.06)

MgO

32.70(0.05)

32.66(0.42)

32.65(0.32)

32.70(0.51)

32.88(0.18)

32.72(0.31)

32.65(0.15)

32.57(0.32)

32.69(0.61)

33.01(0.66)

32.33(0.34)

CaO

1.32

(0.11)

1.44

(0.11)

1.39

(0.15)

1.38

(0.07)

1.29

(0.14)

1.29

(0.18)

1.27

(0.04)

1.53

(0.28)

1.24

(0.12)

1.53

(0.07)

1.26

(0.28)

Na 2O

0.01

(<0.01

)0.02

(0.02)

0.01

(0.01)

0.02

(0.01)

0.01

(<0.01

)0.01

(0.02)

0.02

(<0.01

)0.03

(0.01)

0.04

(0.04)

0.03

(0.01)

0.05

(0.05)

K2O

0.01

(<0.01

)0.01

(<0.01

)0.01

(0.01)

0.00

(<0.01

)<0.01

(<0.01

)<0.01

(<0.01

)0.01

(0.01)

<0.01

(<0.01

)<0.01

(<0.01

)0.01

(<0.01

)0.02

(0.01)

Sum

100.88

(0.90)

99.74(0.93)

101.86

(0.23)

99.86(1.41)

101.49

(0.33)

100.25

(0.35)

101.04

(0.32)

101.51

(0.83)

101.25

(0.99)

100.19

(0.19)

99.47(0.38)

Cr#

15.6

(0.7)

15.1

(1.9)

15.9

(0.7)

15.9

(1.4)

16.2

(1.1)

17.1

(1.1)

17.1

(0.5)

16.8

(0.9)

17.5

(0.4)

17.3

(0.4)

16.3

(0.2)

Mg#

90.3

(0.1)

90.3

(0.3)

90.3

(0.1)

90.4

(0.2)

89.9

(0.1)

90.1

(0.1)

89.6

(0.0)

89.7

(0.1)

89.6

(0.1)

89.9

(0.2)

89.8

(0.1)

a Average

values

(wt%);perid,

perido

tite;

web,websterite.Mg#=Mg/(M

g+Fe3

++Fe2

+)*10

0;Cr#=Cr/(Cr+Al)*1

00.SD

values

arein

parentheses.


12 of 34

the neoblastic clinopyroxene from the websterite(Figure 6).

4.3. Replacive Dunites

[28] In these rocks, olivine is altered into serpentineand minor Fe oxide phases, and is associated with

minor amounts of euhedral spinel. Spinel is charac-terized by low Cr # and TiO2 concentrations, similarto spinel from replacive dunites from another mantlesection of the Internal Ligurian ophiolites (Figure 7).Plagioclase films (up to 0.5 mm thick) are com-mon among the olivine grains and locally containeuhedral pyroxene grains. Both plagioclase and

Table 4 (Sample). Major Element Clinopyroxene Compositionsa [The full Table 4 is available in the HTML version of thisarticle]

Sample and Mineral

MG3 MG4

CC2: Cpx neo

MG2a

Cpx porph Cpx neo Cpx porph Cpx neo Cpx neo Cpx diss

Unit SRV SRV SRV SRV SRV SRV SRVRock type perid perid perid perid perid web host peridNumber 6 11 5 8 8 10 3SiO2 53.04 (0.71) 52.51 (0.80) 52.99 (0.18) 52.39 (0.18) 52.69 (0.94) 52.77 (0.55) 52.56 (0.65)TiO2 0.25 (0.03) 0.28 (0.03) 0.28 (0.02) 0.29 (0.02) 0.31 (0.02) 0.42 (0.04) 0.50 (0.01)Al2O3 4.16 (0.11) 3.79 (0.69) 5.00 (0.28) 4.13 (0.28) 4.08 (0.70) 4.03 (0.16) 3.58 (0.44)Cr2O3 1.33 (0.01) 1.23 (0.21) 1.38 (0.02) 1.36 (0.02) 1.26 (0.09) 1.47 (0.04) 1.40 (0.15)FeO 3.08 (0.09) 2.93 (0.21) 3.16 (0.25) 2.84 (0.25) 2.88 (0.14) 3.06 (0.13) 3.08 (0.04)MnO 0.09 (0.03) 0.09 (0.01) 0.08 (0.02) 0.09 (0.02) 0.09 (0.02) 0.10 (0.02) 0.10 (0.02)NiO 0.06 (0.03) 0.06 (0.03) 0.04 (0.04) 0.04 (0.04) 0.05 (0.02) 0.05 (0.02) 0.07 (0.01)MgO 17.22 (0.26) 17.36 (0.63) 16.68 (0.83) 16.63 (0.83) 16.40 (0.57) 16.58 (0.48) 16.42 (0.58)CaO 21.52 (0.51) 22.27 (0.77) 21.26 (1.08) 22.90 (1.08) 22.97 (0.36) 21.99 (0.61) 22.32 (0.07)Na2O 0.19 (0.01) 0.19 (0.02) 0.21 (0.01) 0.22 (0.01) 0.16 (0.01) 0.48 (0.03) 0.52 (0.04)K2O <0.01 (0.01) 0.01 (0.01) 0.00 (<0.01) 0.00 (<0.01) <0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01)Sum 100.95 (0.39) 100.70 (0.43) 101.09 (0.22) 100.90 (0.22) 100.89 (0.62) 100.95 (0.50) 100.56 (0.53)Cr # 17.7 (0.4) 17.9 (0.8) 15.6 (0.8) 18.1 (0.8) 17.4 (1.7) 19.6 (0.6) 20.8 (0.3)Mg # 90.9 (0.2) 91.4 (0.4) 90.4 (0.3) 91.3 (0.3) 91.0 (0.5) 90.6 (0.2) 90.5 (0.3)

aAverage values (wt %). SRV‐IL, Scogna‐Rocchetta Vara ophiolite; BL, other bodies of Internal Ligurian ophiolites; perid, peridotite; web,websterite; troc‐dike, troctolitic dike; gabb‐sill, gabbroic sill; Ol‐gab dike, olivine gabbro dike; cpx‐gab, Cpx‐rich gabbro; HT1‐gab, plagioclase +clinopyroxene assemblage; HT2‐gab, plagioclase + hornblende assemblage; B‐dike, basalt dike. Mg # = Mg/(Mg + Fe3+ + Fe2+) * 100; Cr # =Cr/(Cr + Al) * 100. SD values are in parentheses.

Table 5 (Sample). Major Element Spinel Compositionsa [The full Table 5 is available in the HTML version of this article]

Sample, Rock Type, and Mineral

MG3: perid

MG4: perid, spl neo

CC2: perid

Spl porph spl neo Spl Rimmed by Pl Spl porph Spl Rimmed by Pl

Unit SRV SRV SRV SRV SRV SRVNumber 7 4 7 3 2 5SiO2 0.05 (0.06) 0.03 (0.00) 0.05 (0.02) 0.02 (0.02) 0.00 (0.00) 0.17 (0.32)TiO2 0.35 (0.02) 0.38 (0.05) 0.32 (0.04) 0.36 (0.11) 0.43 (0.02) 0.37 (0.03)Al2O3 31.39 (0.63) 30.33 (0.47) 27.38 (1.16) 29.67 (1.10) 27.56 (0.09) 27.44 (0.78)Cr2O3 36.13 (0.94) 38.41 (0.65) 37.28 (0.95) 39.04 (0.49) 38.22 (0.06) 37.63 (1.04)FeO 17.18 (0.74) 18.35 (0.10) 19.99 (1.44) 18.19 (1.01) 21.16 (0.13) 20.90 (0.42)MnO 0.09 (0.02) 0.09 (0.00) 0.11 (0.03) 0.14 (0.06) 0.11 (0.11) 0.12 (0.02)NiO 0.02 (0.02) 0.22 (0.01) 0.11 (0.02) 0.13 (0.06) 0.16 (0.01) 0.13 (0.02)MgO 15.10 (0.35) 14.25 (0.51) 13.36 (1.24) 13.80 (1.18) 13.32 (0.05) 12.94 (0.35)CaO 0.01 (0.01) 0.01 (0.00) 0.01 (0.01) 0.01 (0.02) 0.01 (0.01) 0.01 (0.01)Na2O 0.01 (0.01) 0.02 (0.02) 0.05 (0.12) 0.02 (0.03) 0.01 (0.01) 0.01 (0.01)K2O <0.01 (<0.01) 0.01 (0.01) <0.01 (<0.01) 0.01 (0.01) <0.01 (<0.01) <0.01 (<0.01)Sum 100.33 (0.95) 102.09 (0.23) 98.66 (1.14) 101.38 (0.72) 100.96 (0.06) 99.72 (0.47)Mg # 65.3 (1.6) 0.6 (<0.1) 60.5 (4.2) 0.6 (<0.1) 58.9 (0.1) 57.8 (2.0)Cr # 43.6 (1.1) 45.9 (0.8) 57.7 (1.5) 46.9 (1.1) 48.2 (<0.1) 57.8 (1.3)

aAverage values (wt %). SRV‐IL, scogna rocchetta Vara ophiolite; BL, other bodies of Internal Ligurian Ophiolites; perid, peridotite; web,websterite; dun, dunite. Mg # = Mg/(Mg + Fe2+) * 100; Cr # = Cr/(Cr + Al) * 100. SD values are in parentheses.


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pyroxene are replaced by fine‐grained aggregates ofprehnite ± epidote ± hydrogrossular and serpentine ±minor Fe oxide phases, respectively.

4.4. Gabbroic Rocks

[29] Troctolite to olivine gabbro dikes intruding themantle sequence are generally medium grained(Tables 1a–1c) and show nearly equigranularstructure. In these rocks, plagioclase (anorthite =60–61 mol % (Table 6)) and olivine are euhedral,and clinopyroxene (up to 20 vol %) is anhedral tosubhedral. In the gabbroic sills, olivine is <10 vol %.The gabbros from the sills are commonly coarsegrained; olivine and plagioclase are euhedral tosubhedral, and clinopyroxene is subophitic. Inthese rocks, plagioclase and olivine are frequentlyreplaced by microaggregates made up of epidote +albite and serpentine + Fe oxide phases, respectively.

[30] Clinopyroxene‐rich gabbros from the plutonare modally and texturally similar to the gabbrosfrom the sills (Tables 1a–1c). Clinopyroxenes fromthe plutonic gabbros have slightly lower Mg # thanclinopyroxenes from the dikes and sills (Figure 6),and plagioclase with anorthite component rangingfrom 57 to 61 mol % (Table 6). Clinopyroxenesfrom gabbros of other Internal Ligurian plutonicsequences have similar major element composi-tions [Tribuzio et al., 1995; Rampone et al., 1998;this work].

[31] The gabbroic sequences from the studiedophiolite are locally associated with olivine‐rich(commonly 80–90 vol %) troctolites [Renna and

Tribuzio, 2011]. These rocks have olivine (Fo =87–88 mol %) and accessory spinel with roundedto embayed morphology. The high Mg # valuesand the high Cr2O3 concentrations of accessorypoikilitic clinopyroxene (88–90 and 1.3–1.5 wt %,respectively) from the olivine‐rich troctolites werecorrelated with a reaction between an olivine‐spinelmatrix and infiltrating MORB‐type melts [Rennaand Tribuzio, 2011]. Conversely, in the troctoliteseither from the dikes or the gabbroic sequences,

Table 6. Major Element Plagioclase Compositionsa

Sample and Mineral

MG2b: Pl Core MG12: Pl Core

CC11 CC12a

Pl porph Pl neo Pl porph Pl neo

Rock type troc‐dike Ol‐gab‐dike HT1‐gab HT1‐gab HT2‐gab HT2‐gabNumber 6 5 7 5 10 10SiO2 52.72 (2.00) 52.07 (0.29) 53.77 (0.14) 54.07 (0.29) 54.00 (0.29) 53.93 (1.32)TiO2 0.07 (0.04) 0.06 (0.01) 0.06 (0.02) 0.04 (0.01) 0.05 (0.02) 0.01 (0.02)Al2O3 30.39 (0.40) 29.31 (0.17) 29.15 (0.26) 29.31 (0.47) 28.52 (0.13) 29.82 (1.39)Cr2O3 <0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01) <0.01 (<0.01) 0.01 (0.01)FeO 0.21 (0.10) 0.22 (0.04) 0.17 (0.03) 0.14 (0.03) 0.18 (0.02) 0.11 (0.04)MnO 0.01 (0.01) 0.01 (0.01) 0.01 (0.01) 0.01 (0.01) 0.01 (<0.01) 0.01 (0.01)NiO 0.01 (0.01) 0.01 (0.02) 0.01 (0.02) 0.01 (0.01) 0.00 (0.01) 0.01 (0.02)MgO 0.16 (0.20) 0.05 (0.01) 0.02 (0.01) 0.01 (0.00) 0.02 (0.01) 0.04 (0.10)CaO 12.73 (0.25) 11.88 (0.16) 11.83 (0.15) 11.87 (0.24) 12.49 (0.20) 12.45 (0.73)Na2O 4.41 (0.23) 4.43 (0.05) 4.88 (0.06) 4.91 (0.13) 4.36 (0.13) 4.38 (0.56)K2O 0.02 (0.01) 0.02 (0.01) 0.03 (0.00) 0.02 (0.01) 0.03 (0.00) 0.11 (0.29)Sum 100.72 (1.78) 98.05 (0.60) 99.93 (0.35) 100.40 (0.33) 99.67 (0.11) 100.90 (1.03)An (mol %) 61.4 (1.7) 59.7 (0.5) 57.2 (0.6) 57.1 (1.2) 61.2 (1.1) 60.7 (3.8)

aAverage values (wt %); troc‐dike, troctolitic dike; HT1‐gab, plagioclase + clinopyroxene assemblage; HT2‐gab, plagioclase + hornblendeassemblage. SD values are in parentheses.

Table 7. Major Element Amphibole Compositionsa

Sample

CC11 CC12a CC12b

Rock type HT1‐gab HT2‐gab HT2‐gabMineral Ti‐prg neo Hbl neo Hbl neoNumber 9 9 10SiO2 45.35 (0.54) 48.21 (1.08) 49.13 (0.68)TiO2 2.61 (0.230 0.80 (0.43) 0.81 (0.29)Al2O3 11.29 (0.38) 9.49 (0.42) 8.90 (0.46)Cr2O3 0.40 (0.07) 0.54 (0.48) 0.58 (0.35)FeO 6.85 (0.35) 6.54 (0.25) 6.60 (0.21)MnO 0.09 (0.02) 0.09 (0.01) 0.10 (0.02)NiO 0.04 (0.03) 0.07 (0.03) 0.05 (0.02)MgO 17.16 (0.22) 18.05 (0.42) 17.80 (0.34)CaO 12.07 (0.07) 12.09 (0.13) 11.85 (0.14)Na2O 2.81 (0.13) 2.16 (0.22) 2.16 (0.22)K2O 0.03 (0.01) 0.08 (0.01) 0.08 (0.01)Cl <0.01 (<0.01) 0.21 (0.03) – (–)Sum 98.71 (0.23) 98.34 (0.22) 98.06 (0.23)Mg # 81.7 (0.9) 83.1 (0.7) 82.8 (0.7)

aAverage values (wt %). HT1‐gab, plagioclase + clinopyroxeneassemblage; HT2‐gab, plagioclase + hornblende assemblage;Mg # =Mg/(Mg + Fe2+) *1 00; dash indicates not analyzed. SD values are inparentheses.


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olivine and plagioclase occur in nearly equal modalamounts and are both euhedral, thereby suggestinga cumulate origin.

4.5. Sheared Gabbros

[32] Two distinct mineral assemblages were recog-nized in sheared gabbros. The first is characterized

by recrystallization of clinopyroxene and plagio-clase. In porphyroclastic metagabbros, in particular,polygonal aggregates of neoblastic clinopyroxeneand plagioclase occur at the edges of deformedclinopyroxene and plagioclase porphyroclasts,respectively. In addition, fine‐grained neoblasticTi pargasite (Table 7) is commonly present as anaccessory within the aggregates rimming the por-

Figure 4. Main microstructures of the studied samples. (a) Tectonized peridotite MG4: neoblastic assemblage oforthopyroxene (Opx II) + clinopyroxene (Cpx) + spinel (Spl) at the edge of a porphyroclastic orthopyroxene (Opx I).The peridotite matrix is crosscut by serpentine (Srp) microveins. (b) Tectonized peridotite MG3: porphyroclastic spinel(Spl) rimmed by plagioclase (Pl) and orthopyroxene (Opx). (c) Tectonized peridotite MG3: plagioclase‐rich veinlet (Pl)crosscutting a porphyroclastic olivine (Ol) and clinopyroxene (Cpx). A spinel (Spl) occurs at the boundary with the hostperidotite. Plagioclase is replaced by fine‐grained aggregates of prehnite ± epidote ± hydrogrossular. (d) Websteritelayer MG2a: 120° triple junctions between neoblastic clinopyroxene (Cpx), orthopyroxene (Opx), and spinel (Spl).(e) Sheared gabbro CC11: neoblastic clinopyroxene (Cpx II) associated with minor amounts of Ti pargasite (Ti‐Prg)at the edge of porphyroclastic clinopyroxene (Cpx I). The Cpx II + Ti‐Prg aggregates are in contact with polygonalaggregates made up of plagioclase (Pl). (f) Thin section of sheared gabbro CC12b: mylonitic structure made up ofbands rich in neoblastic hornblende (Hbl) and plagioclase (Pl).


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phyroclastic clinopyroxene (Figure 4e). Withinindividual samples, neoblastic clinopyroxene andplagioclase are chemically similar to the porphyr-oclastic counterparts (Tables 4 and 6).

[33] The second mineral assemblage is representedby the hornblende + plagioclase amphibolite faciesassociation (Figure 4f). In the amphibolite faciessheared gabbros, medium‐ to fine‐grained aggregatesof neoblastic hornblende are present at the rimsof porphyroclastic clinopyroxene and fine‐grainedplagioclase aggregates are found at the margin ofporphyroclastic plagioclase. Neoblastic hornblendehas lower TiO2, Al2O3 and Na2O than neoblasticTi pargasite (Table 7) and contains significantamounts of Cl (∼0.2 wt %). Associated neoblasticplagioclase is chemically similar to porphyroclasticplagioclase of igneous origin (Table 6). Locally,the hornblende + plagioclase assemblage mantlesthe association of neoblastic clinopyroxene +plagioclase + Ti pargasite.

[34] Sheared gabbros showing crystallization ofclinopyroxene + plagioclase + Ti pargasite and, inplaces, of hornblende + plagioclase were reportedfor other gabbroic bodies from the Internal Ligurian

ophiolites [i.e.,Cortesogno et al., 1975;Molli, 1994,1995, 1996; Tribuzio et al., 1995, 2000; Menna,2009]. These shear zones bear similar structural,microstructural and compositional features to thosedocumented in the present study. Temperature con-ditions of the ductile shearing events recorded by thegabbros from Scogna‐Rocchetta Vara ophiolite wereevaluated applying the amphibole‐plagioclase geo-thermometer of Holland and Blundy [1994], assum-ing pressure conditions of 0.2 GPa. Temperatureestimates of ∼850°C were obtained for the plagio-clase + Ti pargasite (+ clinopyroxene) assemblage.The hornblende + plagioclase pairs yielded tem-perature values of ∼710°C.

4.6. Basalt Dikes

[35] These rocks commonly include phenocrysticplagioclase in an aphanitic groundmass made up ofeuhedral plagioclase, ophitic clinopyroxene andaccessory ilmenite. Phenocrystic olivine and clin-opyroxene are also locally present. Plagioclase isreplaced by microaggregates consisting of epidoteand albite, and olivine is altered into serpentine andminor Fe oxide phases. Phenocrystic clinopyroxene

Figure 5. Variation of TiO2 versus Mg # [Mg/(Mg+Fe2++Fe3+) × 100] for orthopyroxene cores from tectonized peri-dotites and websterite layers of Scogna‐Rocchetta Vara ophiolite. Data are averaged per sample; the error bars rep-resent one standard deviation of the mean value.


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shows slightly higher Mg # and Cr2O3, and slightlylower TiO2 than groundmass clinopyroxene(Figure 6 and Table 4).

5. Trace Element Compositionsof Clinopyroxenes

[36] Clinopyroxene cores from tectonized peridotites,websterites, gabbroic rocks and basalts were ana-

lyzed for trace element concentrations (Table 8) bylaser ablation ICP‐MS at C.N.R.–Istituto di Geos-cienze e Georisorse, Unità di Pavia. This instrumentcouples a Nd:YAG laser source (Brilliant, Quantel)operating at 266 nm with a quadrupole ICP‐MS(Drc‐e, Perkin Elmer). Analyses were carried outwith a spot diameter of ∼40 mm. Data reduction wasperformed using the “Glitter” software package[Van Achterbergh et al., 2001]. Ablation signal and

Figure 6. Variation of TiO2 and Na2O versus Mg # [Mg/(Mg+Fe2++Fe3+) × 100] for clinopyroxene cores frommantleand crustal rocks of Scogna‐Rocchetta Vara ophiolite. Data are averaged per sample; the error bars represent one stan-dard deviation of the mean value. Grey field indicates compositions of porphyroclastic clinopyroxenes frommantle peri-dotites of Val Graveglia‐Bargonasco ophiolite (Internal Ligurian units [Rampone et al., 1996]). The compositions ofclinopyroxenes from the olivine‐rich troctolites of Internal Ligurian ophiolites are from Renna and Tribuzio [2011]. Thecompositions of clinopyroxenes from gabbroic and basaltic rocks of Bracco‐Levanto ophiolite (Internal Ligurian units)are also shown [Rampone et al., 1998; this work].


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integration intervals were selected by inspecting thetime‐resolved data to ensure that no inclusions werepresent in the analyzed volume. NIST SRM 612 and44Ca were used as external and internal standards,respectively. Accuracy was tested on the BCR2‐g(USGS) reference glass and is estimated to be betterthan ±5% (1s).

5.1. Tectonized Peridotites

[37] Porphyroclastic and neoblastic clinopyroxenesfrom the peridotites of Scogna‐Rocchetta Varaophiolite have similar trace element compositions(Figure 8a). Their chondrite‐normalized REE pat-terns are characterized by a marked LREE depletion(LaN/SmN = 0.01–0.02) relative to MREE andHREE, which are nearly flat at ∼9 times chondrite.Incompatible elements normalized to chondriteshow depletions of Sr, Zr and Ti relative to neigh-boring REE. Zr is also depleted relative to Hf (ZrN/HfN = 0.2–0.3). Note that clinopyroxene from theperidotites within the gabbroic pluton has the sametrace element signature of clinopyroxene from the

peridotites hosting the gabbroic pluton. A similargeochemical fingerprint was observed for porphyr-oclastic clinopyroxenes from peridotites of anothermantle sequence of the Internal Ligurian ophiolites[Rampone et al., 1996; Piccardo et al., 2004].

5.2. Websterite Layers

[38] Clinopyroxene from the selected websterite isless depleted in LREE (LaN/SmN = 0.14) thanclinopyroxene from the tectonized peridotites(Figure 8a). The REE pattern of clinopyroxenefrom the websterite is characterized by nearly flatMREE and HREE at about 13 times chondrite anda slight negative Eu anomaly (Eu/Eu* = 0.7). Sr isdepleted relative to LREE, with higher absoluteconcentrations than in clinopyroxenes from theperidotites. Zr and Ti are depleted with respect toneighboring REE. In particular, the Zr depletion isless marked than in clinopyroxenes from the peri-dotites. This is also shown by the relatively highZrN/HfN value (0.6) of clinopyroxene from the

Figure 7. Variation of TiO2 versus Cr # [Cr/(Cr+Al) × 100] for spinel cores from tectonized peridotites, websteritelayers, and replacive dunites of Scogna‐Rocchetta Vara ophiolite. Data are averaged per sample; the error bars representone standard deviation of the mean value. Compositions of porphyroclastic spinels from peridotites of Val Graveglia‐Bargonasco ophiolite (Internal Ligurian units [Rampone et al., 1996]) and of spinels trails from replacive dunites ofanother section of Alpine ophiolites [Piccardo et al., 2007] are displayed as field A and field B, respectively. Thecompositions of spinels from replacive dunites (this work) and basalt dikes [Cortesogno and Gaggero, 1992] from otherbodies of the Internal Ligurian ophiolites are also shown.


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Table 8 (Sample). Trace Element Clinopyroxene Compositions of Ultramafic and Mafic Rocks Obtained With LA‐ICP‐MSa

[The full Table 8 is available in the HTML version of this article]

Sample and Mineral

MG4

CC2: Cpx neo

MG2a

MG20: Cpx neo MG2b: Cpx CoreCpx porph Cpx neo Cpx diss Cpx neo

Unit SRV SRV SRV SRV SRV SRV SRVRock type perid perid perid host perid web web troc‐dikeNumber 3 4 2 2 4 4 3V 323 (2) (7) 358 (23) 380 (13) 377 (9) 378 (13) 411 (23)Cr 11,055 (177) (248) 10,686 (631) 10,905 (109) 11,122 (139) 12,017 (634) 10,340 (193)Co 34.8 (1.5) 33.8 (0.6) 34.3 (6.6) 26.7 (1.6) 32.7 (1.8) 31.7 (1.8) 30.8 (5.6)Ni 491 (17) 480 (13) 453 (32) 380 (9) 398 (80 422 (34) 424 (32)Sc 65.5 (0.9) 61.3 (1.3) 67.9 (0.7) 76.8 (1.1) 74.5 (1.2) 71.6 (3.3) 75.4 (1.8)Ti 2,085 (53) 2,162 (29) 2,302 (114) 3,286 (30) 3,059 (60) 3,238 (196) 4,478 (40)Sr 0.26 (0.04) 0.49 (0.24) 0.29 (0.08) 3.91 (0.74) 1.49 (0.10) 2.79 (0.23) 13.9 (0.8)Zr 3.29 (0.11) 2.95 (0.30) 2.23 (0.23) 16.93 (0.11) 13.26 (0.40) 16.47 (0.63) 16.95 (1.15)Nb 0.02 (0.02) 0.02 (0.01) <0.01 <0.1 0.10 (0.01) 0.09 (0.03) 0.05 (0.01)Y 15.8 (0.4) 13.6 (0.7) 16.8 (1.5) 18.8 (0.1) 19.9 (1.1) 19.6 (0.5) 17.0 (0.6)Hf 0.27 (0.06) 0.33 (0.07) 0.26 (0.03) 0.64 (0.14) 0.56 (0.12) 0.60 (0.14) 0.66 (0.04)Ta <0.01 <0.01 <0.01 0.01 (0.01) 0.02 (0.02) 0.02 (<0.01) 0.01 (0.01)La 0.01 (0.01) 0.01 (0.00) <0.04 0.33 (0.03) 0.36 (0.05) 0.39 (0.05) 0.32 (0.03)Ce 0.17 (0.02) 0.18 (0.01) 0.10 (<0.01) 1.86 (0.19) 1.82 (0.04) 1.87 (0.08) 2.08 (0.05)Pr 0.12 (0.01) 0.09 (0.02) 0.05 (<0.01) 0.42 (0.01) 0.41 (0.03) 0.39 (0.06) 0.52 (0.01)Nd 1.14 (0.11) 1.27 (0.14) 1.16 (0.15) 2.98 (0.03) 2.65 (0.18) 3.42 (0.42) 3.76 (0.22)Sm 0.86 (0.12) 0.83 (0.05) 0.90 (0.08) 1.81 (0.11) 1.56 (0.09) 1.51 (0.14) 1.48 (0.07)Eu 0.33 (0.02) 0.36 (0.06) 0.37 (0.06) 0.46 (0.03) 0.48 (0.05) 0.61 (0.13) 0.56 (0.09)Gd 1.79 (0.47) 1.71 (0.21) 1.72 (0.21) 2.59 (0.37) 2.58 (0.32) 2.66 (0.18) 2.37 (0.05)Tb 0.38 (0.06) 0.32 (0.04) 0.38 (0.02) 0.54 (0.06) 0.50 (0.03) 0.54 (0.09) 0.44 (0.01)Dy 2.94 (0.33) 2.63 (0.13) 3.08 (0.17) 3.62 (0.23) 3.82 (0.32) 3.61 (0.12) 3.28 (0.29)Ho 0.63 (0.02) 0.50 (0.09) 0.61 (0.05) 0.75 (0.00) 0.79 (0.06) 0.75 (0.04) 0.70 (0.04)Er 1.70 (0.16) 1.50 (0.08) 1.69 (0.03) 2.11 (0.06) 2.20 (0.24) 1.92 (0.14) 1.71 (0.24)Tm 0.22 (0.04) 0.19 (0.01) 0.24 (0.01) 0.32 (0.02) 0.32 (0.01) 0.29 (0.05) 0.27 (0.02)Yb 1.77 (0.08) 1.35 (0.11) 1.13 (0.01) 1.82 (0.04) 1.58 (0.17) 1.96 (0.19) 1.67 (0.18)Lu 0.20 (0.03) 0.17 (0.02) 0.23 (0.05) 0.27 (0.03) 0.27 (0.03) 0.19 (0.03) 0.25 (0.02)LaN/SmN 0.01 0.01 0.02 0.12 0.15 0.16 0.14Eu/Eu* 0.81 0.91 0.91 0.64 0.73 0.92 0.91Sr/Sr* 0.04 0.07 0.06 0.11 0.05 0.07 0.33Zr/Zr* 0.22 0.19 0.14 0.48 0.43 0.47 0.47Ti/Ti* 0.45 0.51 0.50 0.54 0.49 0.52 0.80ZrN/HfN 0.32 0.23 0.23 0.69 0.63 0.72 0.68

aAverage values (ppm). SRV‐IL, Scogna‐Rocchetta Vara ophiolite; BL, other bodies of Internal Ligurian ophiolites; perid, peridotite; web,websterite; troc‐dike, troctolitic dike; Eu/Eu* = EuN/√(GdN * SmN); Sr/Sr* = SrN/√(CeN * NdN); Zr/Zr* = ZrN/√(NdN * SmN); Ti/Ti* = TiN/√(GdN * DyN); normalized to C1 chondrite [Anders and Ebihara, 1982]; Cpx‐gab, clinopyroxene‐rich gabbro; B‐dike, basalt dike. SD values(1 sigma) are in parentheses.

Figure 8. REE and incompatible element compositions of clinopyroxene cores from mantle and crustal rocks ofScogna‐Rocchetta Vara ophiolite, normalized to C1 chondrite [Anders and Ebihara, 1982]. (a) Clinopyroxenesfrom mantle rocks. Note that clinopyroxene from the peridotites enclosed within the gabbroic pluton has the sametrace element signature as clinopyroxene from the peridotites hosting the gabbroic pluton. Clinopyroxenes fromperidotites of Val Graveglia‐Bargonasco ophiolite (Internal Ligurian units [Piccardo et al., 2004]) are also reported.Compositions of clinopyroxenes from peridotites of Mid Atlantic Ridge [Brunelli et al., 2006] and from westerites ofSoutheast Indian Ridge [Dantas et al., 2007] are displayed as field A and field B, respectively. (b) Clinopyroxenesfrom gabbroic dikes and sills. Clinopyroxenes from the olivine‐rich troctolites of Internal Ligurian ophiolites [Rennaand Tribuzio, 2011] and the gabbroic dikes intruding the mantle sequences of Val Graveglia‐Bargonasco [Piccardo etal., 2004] are also reported. (c) Clinopyroxenes from plutonic gabbroic rocks. Clinopyroxene of a gabbro fromBracco‐Levanto ophiolite (Internal Ligurian units (this work)) is also reported. (d) Clinopyroxenes from basalt dikes.Clinopyroxenes from basalt dikes of Bracco‐Levanto ophiolite (this work) are also reported. The incompatible ele-ment compositions of clinopyroxene at the equilibrium with NMORB were calculated through average NMORBcompositions of Hofmann [1988] and experimentally derived coefficients for clinopyroxene/basalt partitioning ofHart and Dunn [1993].


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Figure 8


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websterite. In the thin section including thewebsterite layer, we analyzed a clinopyroxenegrain from the host peridotite; the clinopyroxenefrom the peridotite has trace element composi-tions resembling those of clinopyroxene from thewebsterite.

5.3. Gabbroic Rocks

[39] Igneous clinopyroxenes from the gabbroicdikes, gabbroic sills and clinopyroxene‐rich gabbrosof the gabbroic pluton display nearly parallel REEand incompatible element patterns (Figures 8b and8c). In particular, the LREE are depleted relative toMREE and HREE (LaN/SmN = 0.12–0.23), whichare nearly flat and range from about 7 to 11 timeschondrite. The incompatible element patterns dis-play negative Sr, Zr and Ti anomalies relative toneighboring REE and show relatively high ZrN/HfNvalues (0.6–0.9). These patterns mimic the patternof clinopyroxene at equilibrium with NMORB,calculated using average NMORB compositions[Hofmann, 1988] and experimentally derived parti-tion coefficients [Hart and Dunn, 1993]. Note thatthe incompatible element signature of clinopyrox-ene from the gabbroic rocks is similar to that ofclinopyroxene from the websterite layers. A similarincompatible element fingerprint was also found forclinopyroxenes from the olivine‐rich troctolites[Renna and Tribuzio, 2011] and other gabbroicbodies of the Internal Ligurian ophiolites [Tribuzioet al., 1995; Rampone et al., 1996, 1997; Piccardoet al., 2004; this work].

5.4. Basalt Dikes

[40] Phenocrystic and groundmass clinopyroxenesdisplay nearly parallel REE and incompatible ele-ment patterns (Figure 8d). The REE patterns arecharacterized by LREE depletion (LaN/SmN =0.11–0.12) and the incompatible element patternsshow negative Sr, Zr and Ti anomalies relative toneighboring REE, similar to clinopyroxenes fromthe gabbroic rocks; the ZrN/HfN value of clinopyr-oxenes from the basalts is relatively high 0.5–0.6.Phenocrystic clinopyroxene has lower concentra-tions of incompatible elements than groundmassclinopyroxene. For instance, HREE are ∼10 and∼16 times chondrite for phenocrystic and ground-mass clinopyroxene, respectively. Groundmass clin-opyroxene also differs in a barely appreciablenegative Eu anomaly (Eu/Eu* = 0.8) and the mostmarked negative Sr anomaly. Figure 8d also showsthat groundmass clinopyroxenes from basalt dikesintruding other gabbroic bodies of the Internal

Ligurian ophiolites have a similar incompatibleelement signature.

6. Whole‐Rock Compositions of Basalts

[41] One basalt dike from Scogna gabbroic sectionwas selected for whole‐rockmajor and trace elementanalyses (Table 9). For comparative purposes, newanalyses were also carried out on two basalt dikesintruding other gabbroic bodies from the InternalLigurian ophiolites. The chemical analyses werecarried out by ICP‐MS spectrometry at “ActivationLaboratories” (Ancaster, Ontario). Precision andaccuracy of ICP‐MS analyses are commonly within10%.

[42] The selected sample has a Mg # value [molarMg/(Mg+Fetot

2+) × 100] of 69, relatively high Al2O3

and TiO2 concentrations (16.3 and 1.3 wt %,respectively). The concentrations of Cr and Ni are260 and 100 ppm, respectively. Its chondrite nor-malized REE pattern is characterized by slightLREE depletion (Figure 9) relative to MREE andHREE (LaN/SmN = 0.6, for SmN = 20). With respectto average NMORB compositions [Hofmann,1988], the basalt dike is slightly LREE enrichedand HREE depleted. In addition, normalization ofincompatible trace elements to average NMORBcompositions [Hofmann, 1988] reveals that Zr isslightly enriched over Y (Zr/Y = 4.1) and LREE.Basalt dikes intruding other gabbroic bodies of theInternal Ligurian ophiolites [Rampone et al., 1998;this work] display a similar incompatible elementsignature (Figure 9).

7. Discussion

7.1. Compositional and StructuralModifications in the Mantle Section

[43] The data presented allowed us to reconstruct acomposite tectonomagmatic evolution for the spi-nel plagioclase mantle section of Scogna‐RocchettaVara ophiolite before the intrusion of gabbroicrocks, which commenced under spinel facies con-ditions. In particular, this section deals with theexhumation of the mantle section from spinel toplagioclase facies conditions, which is associatedwith repeated events of interaction with migratingmelts.

7.1.1. The Peridotite Precursor

[44] The studied peridotites contain 4–6 vol % ofclinopyroxene (Tables 1a–1c), which is character-


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ized by marked depletions in LREE and Zr andnearly flat MREE and HREE (Figure 8a). Thesefeatures imply a moderately depleted geochemicalsignature of the spinel facies mantle precursor,which was similarly documented for anothermantle sequence of the Internal Ligurian ophiolites[Rampone et al., 1996; Piccardo et al., 2004] andfor other mantle bodies of the Alpine‐Apennineophiolites [Rampone et al., 1997, 2008; Piccardoet al., 2007; Tribuzio et al., 2004; Müntener et al.,2004, 2010]. Taken as a whole, these peridotitesshare many compositional similarities with moder-ately depleted abyssal peridotites from modernocean lithosphere [e.g., Johnson et al., 1990;Hellebrand et al., 2001; Brunelli et al., 2006].

[45] The mantle peridotites from Scogna‐RocchettaVara ophiolite could represent residues after a lowdegree partial melting of an asthenospheric source[see also Rampone et al., 1996; Tribuzio et al., 2004].Alternatively, their geochemical signature couldhave been produced by interaction of the peridotiteprotoliths with highly depleted, olivine‐saturatedmelts derived from the underlying asthenosphere, asrecently proposed for other depleted peridotites fromthe Alpine‐Apennine ophiolites [Piccardo et al.,2007; Rampone et al., 2008]. Note that percolationand crystallization of highly depleted melts in mantlesequences from slow and ultraslow spreading set-tings have been also recognized on the basis ofcompositions of clinopyroxenes from peridotites[Seyler et al., 2001] and websterite layers [Dantaset al., 2007].

7.1.2. Origin of the Websterite Layers

[46] The depleted tectonized peridotites fromScogna‐Rocchetta Vara ophiolite locally include thin layersmade up of spinel websterites. Pyroxenite layeringhas been documented in other mantle sections fromthe Alpine‐Apennine ophiolites [e.g., Montaniniet al., 2006; Rampone and Borghini, 2008]. In par-ticular,Montanini et al. [2006] analyzed pyroxenitelayers from the External Ligurian mantle section,which retains a subcontinental origin [Beccaluvaet al., 1984; Rampone et al., 1995]. The ExternalLigurian pyroxenites in places show relics of garnetfacies assemblages, thus providing evidence forequilibration at deep lithospheric levels, most likelyin pre‐Jurassic times [Montanini et al., 2006]. Con-versely, the websterites from Scogna‐RocchettaVara ophiolites do not have garnet relics and do notretain a geochemical signature indicating a forma-tion by precursor garnet‐bearing assemblages.

Table 9. Whole‐RockMajor and Trace Element Compositionsof Basaltsa

Unit

SRV BL BL

Sample SC1 VF100 BR101Rock type b‐dike b‐dike b‐dike

Major Elements (wt %)SiO2 49.71 50.4 49.16TiO2 1.28 1.42 1.514Al2O3 16.33 15.9 15.89Fe2O3 7.71 7.07 8.7MnO 0.141 0.172 0.143MgO 8.81 7.6 8.78CaO 7.3 8.93 7.95Na2O 3.76 4.15 3.75K2O 0.82 0.17 0.5P2O5 0.12 0.13 0.16L.O.I 3.60 2.91 3.09Total 99.6 98.9 99.6Mg # 69.4 68.0 66.7

Trace Elements (ppm)V 220 236 250Cr 260 290 210Co 32 39 34Ni 100 110 90Cu 20 80 60Zn <30 110 <30Ga 13 19 14Rb 5 5 4Sr 191 208 188Ba 6 17 10Zr 99 136 102Nb 1.6 3.4 2.5Y 24.1 34.3 29.8Hf 2.4 3 2.6Ta 0.09 0.21 0.13Pb <5 <4 <5Th 0.21 0.26 0.21U 0.07 0.2 0.08La 3.01 4.25 3.96Ce 10.0 13.4 12.7Pr 1.75 2.34 2.19Nd 9.06 11.3 11.4Sm 3.01 3.34 3.75Eu 1.06 1.23 1.3Gd 3.92 4.42 4.87Tb 0.72 0.8 0.9Dy 4.44 5.04 5.46Ho 0.97 1.06 1.16Er 2.7 3.27 3.27Tm 0.416 0.484 0.512Yb 2.71 2.96 3.3Lu 0.37 0.41 0.48Zr/Y 4.12 3.92 3.42

aB‐dike, basalt dike; SRV, Scogna‐Rocchetta Vara ophiolite; IL,other bodies of Internal Ligurian ophiolites; LOI, loss on ignition.Mg # = molar Mg/(Mg + Fetot

2+).


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[47] Clinopyroxene from the studied websteriteshas relatively high concentrations of Na2O andTiO2 and a REE pattern characterized by slightLREE fractionation and nearly flat MREE andHREE, similar to clinopyroxene from the associatedgabbroic rocks (Figures 5 and 8a). Clinopyroxenefrom the websterites is therefore geochemicallyenriched with respect to clinopyroxene from thetectonized peridotites. In addition, the clinopyrox-enes disseminated in the peridotite close to thewebsterite layer are chemically similar to the clin-opyroxenes from the websterite layer. Furthermore,in the peridotite samples containing the websteritelayer, porphyroclastic orthopyroxene and spinel areenriched in TiO2 and depleted in Mg # with respectto the other peridotites. The MORB‐type meltsproducing the websterites therefore led to localrefertilization of the host peridotite, thus implyingthat the websterites formed after the depletion eventrecorded by the tectonized peridotites.

[48] Pyroxenites are also locally found in mantlesections from slow and ultraslow spreading ridges[Fujii, 1990; Juteau et al., 1990; Kempton andStephens, 1997; Dantas et al., 2007]. In particular,the main petrological features of websterites fromScogna‐Rocchetta Vara ophiolite are similar tothose of the spinel websterite layers dredged fromthe Southwest Indian Ridge Oblique Supersegment[Dantas et al., 2007]. The websterites from South-west Indian Ridge Oblique Supersegment differ inthe highly depleted incompatible element signatureof the clinopyroxenes (Figure 8). These websteriteswere interpreted as cumulates produced by meltssegregated into veins under spinel facies conditions,although an origin through melt‐peridotite reaction

could not be excluded [Dantas et al., 2007]. Sobolevet al. [2007] envisaged that olivine‐free pyroxenitelayers in the upper mantle are produced by reactionbetween peridotites and silica‐rich melts, in turnformed by melting of recycled basalts and gabbrosof the oceanic crust transformed to eclogite. How-ever, this petrogenetic process does not match theMORB‐type incompatible element signature ofclinopyroxenes from the studied websterites, whichargues against a high proportion of garnet compo-nent in the hypothetical melts reacting with theperidotites.

[49] Experimental determinations shows that crys-tallization of clinopyroxene + orthopyroxene ±olivine ± spinel from MORB‐type melts occur atP ≥ 0.9 GPa [e.g., Elthon, 1993], i.e., under pres-sure conditions that are compatible with the pres-ence of spinel in the host peridotites [Gasparik,1984]. The formation of Scogna‐Rocchetta Varawebsterite layers is therefore attributed to infiltrationof MORB‐type melts under spinel facies mantleconditions, thus implying the existence of a thickconductive lid. These melts led to local refertiliza-tion of the host tectonized peridotites and were mostlikely produced by the underlying asthenosphere.

7.1.3. Spinel Facies Deformation

[50] The peridotites and websterite layers fromScogna‐Rocchetta Vara ophiolite show a foliationproduced by the alignment of porphyroclasticorthopyroxene and spinel. In particular, porphyro-clastic orthopyroxenes are rimmed by polygonalaggregates consisting of orthopyroxene and clino-pyroxene plus minor spinel and olivine (Figures 4a

Figure 9. REE and incompatible compositions of the basalt dikes Scogna‐Rocchetta Vara and Bracco‐Levantoophiolites (Internal Ligurian units), normalized to C1 chondrite [Anders and Ebihara, 1982] and average NMORBcompositions [Hofmann, 1988], respectively. One of the basalt dikes from Bracco‐Levanto ophiolite is from Ramponeet al. [1998].


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and 4d). CaO compositions of neoblastic ortho-pyroxene from both peridotites and websterites(1.3–1.5 wt % (Table 3)) yield temperature evalua-tions of 1090–1150°C on the basis of the Ca‐in‐Opxgeothermometer [Brey and Köhler, 1990]. There-fore, we propose that the spinel facies deformationoccurred close to the asthenosphere‐lithosphereboundary, which is classically fixed at 1200°C[e.g., Ceuleneer and Rabinowicz, 1992].

[51] Spinel tectonized peridotites are reported forother mantle sections of the Alpine‐Apennineophiolites attributed to ocean‐continent transitions[e.g., Vissers et al., 1991; Montanini et al., 2006;Kaczmarek and Müntener, 2008]. They locallyconstitute up to km‐scale thick extensional shearzones that were correlated with uplift of subconti-nental mantle in response to the lithospheric exten-sion that led to opening of the Ligurian‐Piedmontesebasin [see also Vissers et al., 1995]. In particular, thespinel facies deformation in these mantle shearzones was inferred to occur before the interactionof the peridotites with melts derived from theasthenosphere [Piccardo and Vissers, 2007]. In theScogna‐Rocchetta Vara mantle sequence, the spi-nel facies deformation affected both the depletedperidotites and the included websterite layers, thusimplying that it followed the injection of theMORB‐type melts that gave rise to the websteritelayers (i.e., a depletion event in the underlyingasthenosphere).

[52] Mantle sections characterized by a widespreadspinel tectonitic foliation are uncommon at slowand ultraslow spreading ridges [e.g., Kelemen et al.,2007; Achenbach et al., 2011]. Nevertheless, theoccurrence of spinel facies deformation events inthe mantle sections from these settings is docu-mented by grain size reduction of pyroxenes inassociation with neoblastic spinel, similar to theperidotites of the present study [Jaroslow et al.,1996; Ceuleneer and Cannat, 1997; Brunelli et al.,2006; Dick et al., 2010]. In addition, the websteritelayers from Southwest Indian Ridge Oblique Super-segment [Dantas et al., 2007] and those fromScogna‐Rocchetta Vara ophiolite show similarspinel‐bearing deformation structures. In the oce-anic settings, these structures were interpreted toreflect recrystallization and static recovery in con-junction with the mantle flow that followed theaccretion of the asthenosphere into the oceaniclithosphere [Jaroslow et al., 1996; Ceuleneer andCannat, 1997].

[53] The development of the spinel tectonitic foli-ation in the Scogna‐Rocchetta Vara mantle section

is attributed to an exhumation process that involvedthe lower lithospheric mantle. We propose that thespinel facies deformation followed the emplace-ment of asthenospheric material at the base of thelithosphere, most likely in the Jurassic. We cannotdefine, however, whether the studied mantle wasaccreted to a thick oceanic lithosphere or it wasincorporated beneath an extending subcontinentallithosphere.

7.1.4. Melt Impregnation Under PlagioclaseFacies Conditions

[54] The tectonized peridotites from Scogna‐Rocchetta Vara ophiolite contain a high modal per-centage of plagioclase (7–15 vol % (Tables 1a–1c)),which suggests the addition of a Al2O3‐rich com-ponent (i.e., a melt) to the peridotite. The plagioclasecommonly develop orthopyroxene‐bearing veinletsthat are subparallel to the spinel facies foliationplanes (Figure 2c). These veinlets frequently have alength exceeding the length of the porphyroclasticminerals. It is therefore unlikely that the plagioclaseformed by subsolidus reaction betweenmineral grains(i.e., orthopyroxene, clinopyroxene and spinel) inmutual contact. Similar veinlets were reported for anearly undeformed mantle section from the InternalLigurian ophiolites and were interpreted as crystal-lization products of orthopyroxene‐saturated meltspercolating through the peridotites [Rampone et al.,1997]. Infiltration and impregnation of peridotitesby orthopyroxene‐saturated melts in the plagioclasestability field was also proposed for other mantlesections of the Alpine‐Apennine ophiolites [e.g.,Piccardo et al., 2007; Rampone et al., 2008;Müntener et al., 2010]. A similar petrogenetic pro-cess explains the formation of the plagioclase‐richveinlets in the Scogna‐Rocchetta Vara mantlesection. In particular, the studied tectonized peri-dotites show that the spinel facies foliation planesfavored the infiltration of these melts.

[55] The process of melt impregnation under pla-gioclase facies conditions is also shown by a fewgeochemical features of the studied peridotites. Forinstance, the high Cr # of porphyroclastic and neo-blastic spinel (Figure 7) and of associated pyroxenes(Tables 3–5) hypothetically correspond to a highlyrefractory mantle peridotite [Hellebrand et al.,2001; Brunelli et al., 2006], thus arguing againstthe moderately depleted geochemical signatureindicated by the clinopyroxene. We attribute thehigh Cr # of spinel, orthopyroxene and clino-pyroxene from Scogna‐Rocchetta Vara peridotitesto chemical modifications occurring in response to


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a reaction with infiltrating melts in the plagioclasestability field, as typically documented for plagio-clase‐bearing peridotites from (ultra)slow spreadingridges [e.g., Kelemen et al., 2007;Dick et al., 2010].For instance, the moderately depleted plagioclase‐bearing peridotites from the Romanche FractureZone (equatorial Atlantic) have spinel and pyrox-enes with high Cr # [Tartarotti et al., 2002], similarto the peridotites of the present study.

[56] The plagioclase‐rich veinlets are common alongthe contacts between the tectonized peridotitesand the websterite layers, with frequent apophyseswithin the websterites. We thus also argue thatalso the websterites were subjected to melt infil-tration in the plagioclase stability field and led tochemical reequilibration of both porphyroclasticand neoblastic minerals. A chemical reequilibrationof clinopyroxene from the peridotites and includedwebsterites is consistent with its marked negativeSr anomaly, and with the fact that the deepest Srnegative anomaly is associated with the developmentof a negative Eu anomaly (Figure 8a). In addition,the low concentrations of Na2O in the clinopyroxenefrom the peridotites (0.2 wt %), which hypotheticallyimply a highly refractory nature for these mantlerocks [Hellebrand and Snow, 2003], are most likelypartly related to the incorporation of Na2O intonewly formed plagioclase [see also Tribuzio et al.,2004; Müntener et al., 2010].

[57] A thorough geochemical study of plagioclase‐rich veinlets from the tectonized peridotites ofScogna‐Rocchetta Vara ophiolite is hampered by theextensive alteration of primary minerals. Ramponeet al. [1997] showed that the orthopyroxene‐saturated melts impregnating another mantle sec-tion of the Internal Ligurian ophiolites were depletedin incompatible elements with respect to typicalNMORB compositions [Rampone et al., 1997]. Wepropose that the melts impregnating Scogna‐Rocchetta Vara mantle section had a similar geo-chemical fingerprint, which is consistent with thedepleted signature retained by the spinel faciesclinopyroxenes. If the impregnating melts had aMORB‐type signature, the depleted clinopyroxeneswould record a significant increase in the concen-trations of most incompatible trace elements (e.g.,the LREE), thus leading to a chemical refertilizationof the host peridotite [Müntener et al., 2010]. Themelts impregnating the studied peridotites could begenetically related to silica‐undersaturated, highlydepleted melts that had previously reacted with themantle sequence, thus producing residual melts

enriched in silica by preferential dissolution ofpyroxenes. In particular, the geochemical depletionof the percolating melts may be attributed to lowdegree (5%–7%) fractional melting of a slightlydepleted spinel‐bearing mantle source [Piccardoet al., 2007; Rampone et al., 2008].

7.1.5. Formation of Dunitic Conduits

[58] The tectonized plagioclase peridotites are locallyreplaced by meter‐scale dunite bodies with spineltrails. These bodies are subparallel to the spinelfacies foliation planes and the plagioclase‐richveinlets in the host rocks (Figures 2d and 3). There isa general consensus that replacive dunites form bypyroxene reactive dissolution by olivine‐saturatedmelts migrating in the peridotites [e.g., Kelemenet al., 1997]. The spinel concentrations in replacivedunites are commonly attributed to precipitationfrom hybrid melts formed by mixing between prim-itive basaltic melts and secondary melts enriched inSiO2 andCr2O3 [Irvine, 1977;Dick andBullen, 1984;Arai and Yurimoto, 1994; Zhou et al., 1994]. Thesecondary melts are produced in response to theformation of the replacive dunite, when primitivebasalts migrate upward and dissolve pyroxenes fromthe peridotites. The dunite bodies from Scogna‐Rocchetta Vara ophiolite may thus be considered asconduits of olivine‐saturated melt.

[59] Dunite bodies crosscutting plagioclase‐impregnated peridotites were found in other depletedmantle sections from the Alpine‐Apennine ophiolitesand related to channeled porous flow of olivine‐saturated melts [Müntener and Piccardo, 2003;Piccardo et al., 2007; Rampone et al. 2008]. Inparticular, on the basis of clinopyroxene incompat-ible element compositions, Piccardo et al. [2007]showed that the replacive dunites from one ofthese sections formed by infiltration of melts withMORB‐type geochemical signature. These MORB‐derived dunites are characterized by spinels withrelatively high Cr # and TiO2 concentrations(Figure 7), as also reported for the replacive dunitesin equilibrium with MORB‐type melts from theOman ophiolite [Kelemen et al., 1997]. The spinelsfrom the dunites considered in this study differin the lower Cr # and TiO2 concentrations, withrespect to the spinels from the MORB‐type dikesof the Internal Ligurian ophiolites [Cortesognoand Gaggero, 1992]. These data argue against theinvolvement of typical MORB‐type melts for theformation of the replacive dunites from Scogna‐Rocchetta Vara ophiolite. In particular, the low


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concentrations of TiO2 in the spinels from thereplacive dunites indicate an equilibration withmelts depleted in incompatible trace elementsrelative to MORB.

[60] In the Scogna‐Rocchetta Vara mantle section,the pyroxene‐dissolving melts forming the dunitesand the orthopyroxene‐saturated, highly depletedmelts that led to plagioclase crystallization in thetectonized peridotites could be attributed to thesame event. However, the replacive structuralrelationships indicate that the plagioclase‐formingmelts in the tectonized peridotites are not gen-erated by the physically associated dunite bodies.Impregnation of the observed peridotites is attrib-uted to the occurrence of dunitic conduits deeper inthe mantle section. These high‐permeability con-duits may have prevented new infiltrating meltsfrom reacting with the host tectonized peridotites[see also Kelemen et al., 1995]. The prolongedmigration of these olivine‐saturated melts presum-ably allowed the interaction with the plagioclase‐impregnated peridotites.

7.2. Formation of the Gabbroic Rocks

[61] We recognized a composite evolution for themigration of melts that led to the building of thegabbroic crust from Scogna‐Rocchetta Vara ophio-lite, which started with the intrusion of troctolite toolivine gabbro dikes into the mantle sequence.

7.2.1. Intrusion of Gabbroic Dikes and SillsInto the Mantle Sequence

[62] The gabbroic dikes crosscut at a low angle thefoliation planes in mantle peridotites and range incomposition from troctolites to olivine gabbros.Locally, the dikes also crosscut the replacive dunitebodies. The troctolite to olivine gabbro dikes com-monly show diffuse boundaries with respect to thehost peridotites, thus indicating that the melt injec-tions occurred when the mantle section was underhigh‐temperature conditions, presumably slightlylower than the crystallization temperature of thetroctolites (∼1200°C) [e.g., Grove et al., 1992].

[63] The gabbroic sills occur near the contactbetween the mantle sequence and the gabbroicpluton (Figure 1d) and are olivine free to olivinepoor. The sills show sharp contacts against thehost tectonized peridotites, thus attesting that theyformed within a mantle sequence that was coolerthan their crystallization temperature. In addition,the gabbroic sills in places crosscut the troctolite

to olivine gabbro dikes. The temperature of themantle sequence hence progressively decreasedfrom the intrusion of the troctolitic dikes to theformation of the gabbroic sills.

[64] We propose that the dikes and sills documentthe progressive transfer from a “hot lithosphericregime,” where melt migration and crystallizationis controlled by reaction with the mantle peridotites,to a “cold lithospheric regime,” in which meltmigration occurs in fractures and melts evolvethrough fractional crystallization. This change wasassociated with a rotation of the dip of the meltmigration structures, which evolved from sub-vertical to subhorizontal.

7.2.2. Growth of the Gabbroic Pluton

[65] The gabbroic pluton consists mostly of coarse‐grained clinopyroxene‐rich gabbros, associatedwithminor, medium grained olivine gabbros to trocto-lites. The different gabbro types constituting thepluton do not show systematic modal layering.However, they locally display a weak modal and/orgrain size layering, which is at a high angle withrespect to the mantle structures and subparallel tothe orientation of the gabbroic sills. In addition,the troctolites show a magmatic foliation producedby alignment of olivine and plagioclase grains,which is nearly concordant with the modal/grainsize layering. Trace element compositions of clin-opyroxene from the gabbroic rocks show formationfrom MORB‐type melts, similar to the gabbroicrocks from the dikes and sills (Figures 8b and 8c).The Scogna‐Rocchetta Vara gabbroic pluton alsocontains up to 75 m thick bodies of olivine‐richtroctolites, which were interpreted to result from areaction process driven by the infiltration of MORB‐type melts saturated in plagioclase + clinopyroxeneinto an olivine‐rich, spinel‐bearing matrix [Rennaand Tribuzio, 2011]. In addition, we found up to50 m thick lens‐like mantle bodies within the gab-broic pluton.

[66] The main structural and compositional featuresof Scogna‐Rocchetta Vara gabbroic pluton aresimilar to those of other gabbroic bodies from theInternal Ligurian ophiolites [Cortesogno et al.,1987; Rampone et al., 1998; Tribuzio et al., 2000;Menna, 2009], as well as of those of many gabbroicsequences exposed along slow and ultraslowspreading ridges. Notable examples are the gabbroicsequences from (1) Kane Fracture Zone of MidAtlantic Ridge 23°N [Ross and Elthon, 1997; Dicket al., 2008; Lissenberg and Dick, 2008], (2) 15°N


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section of Mid Atlantic Ridge [Kelemen et al.,2007], (3) Atlantis Massif (Mid Atlantic Ridge,30°N) [Blackman et al., 2006], (4) Atlantis Bank atSouthwest Indian Ridge [Robinson et al., 1989;Dicket al., 2000], and (5) Mid Cayman Rise [Elthon,1987]. In particular, we wish to emphasize that thegabbroic complexes of Atlantis Massif [Suhr et al.,2008; Tamura et al., 2008; Drouin et al., 2009]and Scogna Rocchetta Vara share the inclusionof olivine‐rich troctolites and mantle peridotitebodies.

[67] The most primitive gabbroic rocks of theScogna‐Rocchetta Vara pluton (Mg # in clinopyr-oxene cores ranging between 85 and 87) have highconcentrations of Na2O in plagioclase (An = 60–61 mol % (Table 6)). Plagioclase with a low pro-portion of anorthite component was also reported forprimitive olivine gabbros and troctolites fromanother gabbroic sequence of the Internal Ligurianophiolites [Tiepolo et al., 1997; Rampone et al.,1998]. This chemical feature was also documentedfor abyssal gabbroic rocks from Mid‐Cayman Rise[Elthon, 1987], where high Na2O abundances arefurther recognized in associated glasses [Thompsonet al., 1980]. Conversely, many abyssal gabbroicsequences have primitive olivine gabbros and troc-tolites with plagioclase displaying 65–75 mol % ofanorthite component [e.g., Suhr et al., 2008]. Wethus propose that the parental melts of ScognaRocchetta Vara gabbroic pluton had high Na2Oconcentrations, which may be correlated with a lowdegree of melting of an asthenospheric source [seealso Dick et al., 1984; Meyer et al., 1989; Kemptonand Casey, 1997].

[68] The mantle bodies from Scogna‐RocchettaVara gabbroic pluton show the same origin andtectonomagmatic evolution of the mantle sequenceenclosing the gabbroic pluton (Figure 1c). In par-ticular, the structures of the mantle lenses and theenclosing mantle sequence are geometrically con-cordant. The occurrence of these mantle remnantstogether with the presence of the gabbroic sills inthe mantle sequence overlying the gabbroic pluton(Figure 1d) may be reconciled with a process ofpluton growth through a series of sill‐like separateintrusions. The process of sill accretion is alsoconsistent with the occurrence of troctolite layers atdifferent depths within the gabbroic pluton.

[69] The sill accretion model is currently proposedfor the building of gabbroic plutons exposed alongslow and ultraslow spreading ridges, where a steadystate magma chamber is considered to be absent[e.g., Kelemen et al., 2007; Godard et al., 2009].

In particular, field observations (troctolite layers upto a few tens meters in thickness) are consistentwith the model of Grimes et al. [2008] for theconstruction of the gabbroic complex from AtlantisMassif, which imply that single sills are on theaverage ten meters thick. However, we cannotdefine whether the sills represent melt injectionsintruded at random depths [Schwartz et al., 2005;Grimes et al., 2008], or they formed at nearlyconstant depths beneath an exhuming sequence[Dick et al., 2000, 2002; Suhr et al., 2008]. Notethat the composition of the gabbroic pluton (i.e.,made up of clinopyroxene‐rich gabbros and minortroctolites) requires the presence of primitivecumulates deeper in the mantle section.

7.2.3. Origin of the Olivine‐Rich Troctolites

[70] Renna and Tribuzio [2011] proposed that theolivine‐spinel matrix of the olivine‐rich troctolitesformed in mantle melt conduits of replacive nature.One of the arguments against a cumulate originof the olivine‐rich troctolites was furnished bythe Rocchetta Vara section (Figure 1c), where theolivine‐rich troctolites are exposed along the contactbetween the gabbroic complex and the overlyingmantle section. There are no evolved gabbroic rocksoverlying the olivine‐rich troctolites, thus indicatingthe absence of the hypothetical residual melts afterthe cumulus process [Renna and Tribuzio, 2011].

[71] The new data reported in this work are consis-tent with the interpretation proposed by Renna andTribuzio [2011]. We observed that the replacivedunites from the studied ophiolite are locallycrosscut by gabbroic dikes with diffuse contacts andthat the olivine‐rich troctolites are locally crosscutby gabbroic sills with sharp planar contacts(Figure 1d). The geochemical signature of clino-pyroxene from the gabbroic dikes and the olivine‐rich troctolites similarly show a formation byMORB‐type melts (Figure 8), whereas the dunite‐forming melts and the melts impregnating themantle section were most likely depleted in incom-patible elements with respect to typical NMORBcompositions. Therefore, the olivine‐rich troctolitebodies may have formed in conjunction with theonset of the gabbroic dyking, most likely whenlarge amounts of MORB‐type melts were injectedinto dunitic conduits. Note that the mantle sectionoverlying the olivine‐rich troctolites in the RocchettaVara section contains meter‐scale replacive dunitebodies, which could be originally connected to thelarge dunitic conduits (order of tens of meters inscale) presently represented by the olivine‐rich


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troctolites. We suggest that these large duniticconduits impregnated by MORB type melts weredissected by the gabbroic sills, similar to the expla-nation of the occurrence of mantle lenses within thegabbroic complex.

7.3. The Tectonomagmatic EvolutionLeading to Exposure of the Gabbro‐Peridotite Association at the Seafloor

[72] This section considers the tectonomagmaticevolution recorded by the gabbroic pluton after itssolidification, which comprises ductile to brittleshearing. The brittle deformation was associatedwith injections of basalt dikes and led to exposureof the gabbro‐peridotite association at the seafloor,as shown by the calcite‐ and hematite‐bearingstructures in both gabbroic and mantle rocks at thecontact with the sedimentary cover.

7.3.1. Ductile Deformation in the GabbroicPluton

[73] The gabbroic pluton was affected by ductiledeformation along localized shear zones. Theshearing foliation crosscuts at a low angle themodal/grain size layering of the gabbros and the magmaticfoliation of the troctolites. The shear zones showa retrograde evolution characterized by earlyrecrystallization of clinopyroxene + plagioclase(±accessory Ti pargasite) at ∼850°C, followed by ahornblende + plagioclase amphibolite facies event at∼710°C. Neoblastic hornblende occurs in highmodal proportions and contains significant amountsof Cl (0.2 wt % (Table 7)), thus suggesting thatdownward infiltration of seawater‐derived fluidsoccurred along the amphibolite facies shear zones.Sheared gabbros with similar microstructural andcompositional features were reported for othergabbroic bodies from the Internal Ligurian ophio-lites [Molli, 1995, 1996; Tribuzio et al., 1995; 2000;Menna, 2009]. In a few cases, the high‐temperatureshearing affecting the gabbros was shown to alsoinvolve the associated mantle peridotites, withdevelopment of plagioclase‐bearing mylonitic peri-dotites [see also Cortesogno et al., 1987].

[74] In gabbroic sections from modern (ultra)slowspreading ridges, sheared gabbros are commonlylocalized in discrete zones that overprint the mag-matic foliation [e.g., Mével, 1987; Cortesognoet al., 2000; Dick et al., 2000; Escartín et al.,2003; Ildefonse et al., 2007]. Most of these gabbrosare characterized by neoblastic aggregates of pyrox-

ene (±accessory Ti pargasite) and plagioclase, thatare considered to have been formed under high‐temperature conditions (T > 800°C) before seawaterpenetration (just after magmatic crystallization). Atthe 15°N section of Mid Atlantic Ridge, the ductileshearing locally affected both the gabbroic andmantle sequences, and formed plagioclase‐bearingmylonitic peridotites in the mantle section [Kelemenet al., 2007; Achenbach et al., 2011]. Amphibolitefacies shearing was observed in some of these oce-anic sections, e.g., Hole 735B at Atlantis II Bank atSouthwest Indian Ridge [MacLeod et al., 1999;Dicket al., 2000], Mid Cayman Rise [Ito and Anderson,1983] and Vema fracture zone of Mid AtlanticRidge [Honnorez et al., 1984] and interpreted to actas pathways for seawater‐derived fluids.

[75] The foliated sheared gabbros from the Scogna‐Rocchetta Vara ophiolites therefore show strikingstructural and compositional similarities with theircounterparts from modern (ultra)slow spreadingsettings. We recognized a retrograde evolution fromnear solidus to amphibolite facies conditions, whichis consistent with the idea proposed by Manatschalet al. [2011] that the ductile shear zones act as adecoupling horizon at the ductile‐brittle transition.The sheared gabbros thus probably formed at theinterface between a magma‐rich ductile layer and afluid‐rich brittle layer.

7.3.2. Intrusion of Basalt Dikes

[76] The Scogna‐Rocchetta Vara gabbroic pluton islocally crosscut by chilled basalt dikes forming ahigh angle with respect to the intrusive fabric ofenclosing gabbros. Dikes were therefore injectedinto a solidified gabbroic pluton, i.e., when thegabbro‐peridotite association was subjected to abrittle deformation regime. Similar relationshipswere reported for another gabbroic pluton of theInternal Ligurian ophiolites [Cortesogno et al.,1987; Menna, 2009]. Taken as a whole, the basaltdikes from the Internal Ligurian ophiolites share acommon incompatible element signature (see alsoFigure 9).

[77] Clinopyroxene phenocrysts from the basaltsdikes and clinopyroxene cores from gabbroic rocksshow the same trace element signature, which impliesa formation by MORB‐type melts with a similargeochemical fingerprint (Figures 8c and 8d). Clino-pyroxene from the basalt groundmass shows anincompatible element pattern that is nearly parallelto that of the clinopyroxene phenocrysts, but athigher absolute concentrations (Figure 8d). We


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attribute these variations to fractional crystallizationcontrolled by plagioclase, olivine and clinopyroxene,as indicated by the occurrence of these mineralsas phenocrysts. In particular, the involvement ofplagioclase in the crystallization process is confirmedby the development of negative Eu and Sr anomaliesin the clinopyroxene from the groundmass.

[78] Whole‐rock compositions of the InternalLigurian basalts are slightly LREE‐ and Zr‐enrichedrelative to typical NMORB (Figure 9). Similarcompositions [Kempton and Casey, 1997] werereported for basalt dikes crosscutting mantleperidotites from Site 920 at Kane Fracture Zone(Mid Atlantic Ridge). This incompatible elementsignature was attributed to low degree partial melt-ing of a spinel peridotite mantle source [Kemptonand Casey, 1997]. The composition of basalt thedikes and the host gabbros (section 7.2.2) from theScogna‐Rocchetta Vara mantle ophiolite thereforeconcordantly indicate a formation by a low degreeof melting of an asthenospheric mantle source.Note that the slight depletion of HREE relative toMREE of the Internal Ligurian basalts may indicatethe involvement of a minor garnet‐bearing compo-

nent in their mantle sources [see Hirschmann andStolper, 1996].

8. Conclusions

[79] The gabbro‐peridotite association from Scogna‐Rocchetta Vara Jurassic ophiolite share many struc-tural and compositional similarities with melt‐poorsections from modern (ultra)slow spreading settings,which are characterized by an elevated lithosphericthickness [e.g., Cannat et al., 1997, 2006; Kelemenet al., 2007; Schroeder et al., 2007]. The series ofmagmatic and tectonic events recorded by thestudied gabbro‐peridotite association have allowedus to propose a conceptual model (Figure 10) for itsformation and evolution, which is summarized asfollows:

[80] 1. The mantle sequence was made up ofmoderately depleted peridotites and scattered spinelwebsterite layers formed by infiltrations of MORB‐type melts close to the lithosphere‐asthenosphereboundary.

[81] 2. A deformation event affected the mantlesection after the formation of the websterite layers.

Figure 10. Conceptual model for the tectonomagmatic evolution of Scogna‐Rocchetta Vara ophiolite. Websterites,dunites, gabbroic dikes, and sills are exaggerated in scale. The transition between spinel and plagioclase peridotite isassumed to be 0.7 GPa [Gasparik, 1984]. See text for further details.


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This deformation led to uplift of the mantlesequence and is probably related to the emplace-ment of asthenospheric material at the base of thelithosphere.

[82] 3. At shallower levels (in the plagioclasestability field), the mantle sequence was subjectedto further infiltrations of melts, which were mostlikely depleted with respect to typical MORB com-positions. Melt transport occurred in form of grain‐scale porous flow, either channeled or diffuse, andwas favored by the spinel facies foliation planes.Reactive channeling of primitive olivine‐saturatedmelts formed replacive dunitic conduits, whereasresidual orthopyroxene‐saturated melts led to meltimpregnation of the mantle section.

[83] 4. New injections of melts displaying aMORB‐type geochemical signature formed troc-tolite to olivine gabbro dikes, documenting thatmelt migration started to occur through fracture‐controlled mechanisms. This diking event is pre-sumably correlated with the formation of theolivine‐rich troctolite bodies, by infiltration ofMORB‐type melts within large dunitic conduits.

[84] 5. As the mantle section cooled significantly,the dip of the melt migration structures evolvedfrom subvertical to subhorizontal. Injected meltsthus developed sill‐like gabbroic bodies.

[85] 6. The growth of the gabbroic pluton (up to∼400 m thick) is attributed to a process of accretionof gabbroic sills. Hence, the mantle section wasdissected by the gabbroic sills and partly incorpo-rated by the gabbroic intrusions.

[86] 7. The gabbroic pluton records ductile shearingat a low angle with respect to the intrusive fabric. Itshows a retrograde evolution from near solidus toamphibolite facies conditions; the latter stage wasmost likely associated with penetration of seawater‐derived fluids. This deformation most likelyoccurred close to the ductile‐brittle transition.

[87] 8. The basalt diking represents the last event ofmelt injection and occurred during exhumation ofthe gabbro‐peridotite association to the seafloor,when it was under brittle regime conditions.

[88] The conceptual model proposed shows a “hot”lithospheric evolution in which melt migration isassociated with reactive crystallization (steps 1 to 3).This “hot” evolution implies that uprising meltsmay be trapped within the lithospheric mantle. Thefollowing “cold” lithospheric evolution is charac-terized by melt transport through fractures and isassociated with fractional crystallization (steps 4

to 8). Taken as a whole, the melt transport evo-lution recognized for the Scogna‐Rocchetta Varaophiolite is consistent with the petrogenetic modelrecently reported by Collier and Kelemen [2010]for the melt migration mechanisms under oce-anic spreading ridges.

Acknowledgments

[89] We would like to acknowledge R. Vannucci for construc-tive discussions and for the support in this study.We are gratefulto M. Tiepolo for suggestions and assistance during LA‐ICP‐MS analyses and to M. R. Renna for the discussions onthe origin of the dunites and troctolites. Conversations withG. B. Piccardo enhanced our understanding of the petrologyof the mantle sequences from the Alpine‐Apennine ophiolites.The paper was improved by insightful reviews by K. Achenbach,E. Hellebrand, and T. Morishita. We also wish to thank theAssociate Editor M. Cheadle for his recommendations thatenhanced the revision of the manuscript. This work was finan-cially supported by Programma di Ricerca di Interesse Nazio-nale of Italian Ministero dell’Università e della Ricerca andFondi di Ateneo per la Ricerca of Università di Pavia.

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