Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.2003.218.01.05p69-89.

2003, v.218;Geological Society, London, Special Publications Othmar Müntener and Giovanni B. Piccardo embryonic ocean basinsfrom Alpine-Apennine peridotites and implications for Melt migration in ophiolitic peridotites: the message

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Melt migration in ophiolitic peridotites: the message from Alpine- Apennine peridotites and implications for embryonic ocean basins

O T H M A R M U N T E N E R 1 & G I O V A N N I B. P I C C A R D O 2

llnstitute o f Geology, University o f Neuchdtel, 11 rue Emile Argand, CH-2OO7 Neuchdtel,

Switzerland (e-mail: [email protected])

2Dipteris, Universitgt di Genova, Corso Europa 26, 1-16132 Genoa, Italy

Abstract: Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene • spinel and precipitated orthopyroxene + plagioelase -4- olivine, forming orthopyroxene + plagioclase-rich peridotite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene • plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-A1 and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath mid- ocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/ or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.

Exhumation of mantle rocks and formation of (slow) spreading ridges involve two principal geological processes, tectonism and magmatism, which reflect the strain rate and temperature- dependent processes of deformation and adiabatic decompression melting within the Earth. Impor- tant extensional environments with direct expo- sures of mantle rocks are embryonic ocean basins (e.g. the Red Sea) and ocean-continent transition zones (e.g. Iberia margin). Slow-spreading mid- ocean ridges constitute an important extensional environment where mantle rocks are exposed and magma supply is limited (Dick et al. 1984; Cannat 1993, 1996). There, direct outcrop of peridotite on the ocean floor via denudation of the mantle occurs in different environments such as fracture zones, magma-starved rift segments, and oceanic core complexes (e.g. Blackman et al. 1998; Tucholke et al. 1998). Models of continental break-up (rifting) followed by sea-floor spreading (drifting) conventionally separate continental

and oceanic crustal types. Studies from the Red Sea (Bonatti et al. 1983; Piccardo et al. 1988, 1993), fieldwork in the Alps (Lagabrielle et al. 1984; Herrnann & Mfintener 1996; Manatschal & Nievergelt 1997; Desmurs et al. 2001) and Ocean Drilling Program results from the Iberian margin (Boillot et al. 1995) challenged this view, and it has been proposed (Whitmarsh et al. 2001), that the concept of an abrupt boundary should be replaced by that of a several tens-of-kilometres wide ocean-continent transition zone (OCTZ) of mainly exhumed continental mantle rocks and subordinate mafic intrusions separating thinned continental crust from oceanic crust. Both slow- spreading lithosphere and the thinned lithosphere in ocean-continent transitions are different from ophiolites as defined by the Penrose conference (Anonymous 1972) but can better account for many recent observations on ophiolites from the Alps and the Apennines (e.g. Rampone & Piccar- do 2000).

From: DILEK, Y. & ROBINSON P. T. (eds) 2003. Ophiolites in Earth History. Geological Society, London, Special Publications, 218, 69-89. 0305-8719/03/$15 �9 The Geological Society of London 2003.

at University of Texas At El Paso on October 19, 2014http://sp.lyellcollection.org/Downloaded from


70 O. MIJNTENER & G.B. PICCARDO

The peculiar stratigraphy of the Alpine-Apen- nine ophiolites, first described by Steinmann (1927) in the Central Eastern Alps and by Decandia & Elter (1972) in the Ligurian Alps, has led researchers to propose three classes of genetic models: (1) the slow-spreading ridge model (Lagabrielle & Cannat 1990; Lagabrielle & Le- moine 1997); (2) the transform fault model (Gianelli & Principi 1977; Weissert & Bernoulli 1985); (3) the low-angle detachment faulting model (Lemoine et al. 1987; Froitzheim & Eberli 1990; Piccardo et al. 1990; Froitzheim & Mana- tschal 1996). All these models have some widely accepted analogues in modern oceans. However, Dick et al. (2000) questioned whether on-land analogues of slow-spreading crust such as the peridotites in the Alps can directly be compared with the results obtained from the slow-spreading Southwest Indian Ridge (Leg 176), where more than 1500m of continuous igneous crust have been drilled and which is probably the best known slow-spreading oceanic crust from modern oceanic basins. In any case, the formation of a slow- spreading ridge is necessarily preceded by a period of continental rifting and at some point there must be a transition from (largely) amag- matic passive rifting to the formation of igneous crust and finally the establishment of a slow- spreading system.

As for the Ligurian ophiolites, the subcontinental origin of mantle peridotites was proposed earlier by some workers (Piccardo 1976), who outlined the diversity of the Alpine-Apennine ophiolites in comparison with mature oceanic lithosphere formed at modern mid-ocean ridges. Based on the atypical association of fertile subcontinental-type mantle and mid-ocean ridge basalt (MORB) magmatism, Piccardo (1977) suggested that the Ligurian ophiolites were formed during the early stages of opening of the oceanic basin, following continental rifting, thinning and break-up of the continental crust, and that they were therefore located in a marginal, pericontinen- tal position to the Jurassic oceanic basin.

The temperature field in all these 'thin crust regions' is determined by the rate of magma supply. Recent results from deep-sea drilling and dredging indicate that some of the liquids gener- ated in the asthenosphere crystallize on a conductive geotherm in the mantle, and thus the igneous crust is significantly reduced or even absent in OCTZs and slow-spreading ridges (Cannat 1996; Cannat et al. 1997; Bonatti et al. 2001; Desmurs et al. 2001). A fundamental remaining question is whether mantle peridotites from marginal settings of ancient and modern oceanic basins provide some systematic information that could shed light on the enigmatic evolution between rifting and the

formation of a slow-spreading system. Although it is well established that many of the Alpine peridotites underwent non-adiabatic uplift and subsolidus evolution from upper-mantle levels to the sea floor (Piccardo 1976; Hoogerduijn-Strating et al. 1993; Rampone et al. 1993; M/intener et al. 2000), followed by the intrusion of small volumes of gabbros and the extrusion of MORB, little is known about the interaction of peridotite with migrating liquids, which must pass through the overlying mantle to form oceanic crust.

In this paper we present observations from the Lanzo and Corsica ophiolitic peridotites, sum- marized below, which show that porous flow of melt and melt-rock reaction are widespread in exhumed ex-subcontinental and oceanic peridotites and are probably related to incipient opening of embryonic ocean basins. In contrast to previous treatments, we consider the effects of rising temperature during melt percolation and impregnation and the development ofplagioclase-enriched peridotites. We evaluate the mode and nature of viable mechanisms for plagioclase formation and examine interactions of subcontinental mantle with rising MORB melt fractions. We also discuss the implications of our findings for the highly variable pressure-temperature paths of exhumed peridotites.

General features of the Piedmont Ligurian ophiolites

The Ligurian Tethys is believed to have developed by progressive divergence of the European and Adriatic continents, in connection with the Early to mid-Jurassic rifting and the Cretaceous opening of the Northern Atlantic (Dewey et al. 1973). Palaeotectonic reconstructions of the Ligurian Tethys suggest that the Piedmont Ligurian ocean was not wider than 400-500 km (Stampfli 1993) and that the Late Cretaceous to Tertiary plate convergence led to complete closure of the Ligur- ian Tethys in the Early Tertiary, resulting in the emplacement of fragments of the oceanic lithosphere as west-vergent thrust units in the Alps and east-vergent thrust units in the Apennines. De- pending on their stratigraphic, structural and metamorphic characteristics, the ophiolitic sequences have been related to different palaeogeographical settings in the Jurassic-Cretaceous Ligurian Tethys. The Voltri Massif, the Piemontese and Saas Zermatt ophiolites (Fig. 1), which were subducted and recrystallized at eclogite facies conditions, were located west of the subduction zone, whereas some of the Northern Apennine ophiolites (External Ligurides) as well as the Eastern Central Alpine ophiolites (Davos-Platta- Malenco), which underwent low-grade oceanic



A L P I N E - A P E N N I N E PERIDOTITES 71

Southern AI

Western AI

i ' i ' i ' i - i ' i ~ ' i ' i ' i P ~ o B a s i n i ' i "

Adriatic Sea

ET IL

Ligurian Sea

• l Tertiary basins ~ European units

~ dangonnais & Valais units

~ Liguria-Piemonte units ophiolites

F ~ I Adriatic units

Monte Maggiore I ~ ~

0

Tyrrhenian Sea

Fig. 1. Generalized tectonic overview of the Alpine and Northern Apennine ophiolites. Modified from Schaltegger et al. (2002). D, Totalp peridotite; P1, Platta ophiolite; M, Malenco peridotite; ZE, Saas Zermatt ophiolite; Ch, Chenaillet ophiolite; ET, Erro Tobbio peridotite; EL, External Ligurides; IL, Internal Ligurides; T, Tuscany peridotites; CO, Corsica ophiolite; GE, Nappe des gets ophiolites.

and orogenic metamorphism, were located east of the subduction zone, close to the Adria margin.

Field relations and petrography o f

serpentinites and peridoti tes

The ophiolites of the Piedmont Ligurian ocean show a predominance of a largely serpentinized peridotite basement intruded and/or covered by small or moderate volumes of mafic rocks, and the lack of a 'complete' ophiolite stratigraphy (Fig. 2). Serpentinized peridotites are commonly overlain by ophicalcites, which represent tectono- sedimentary breccias related to mantle exhumation (Lemoine et al. 1987; Desmurs et al. 2001). In the Eastern Central Alps extensional allochthons of

continental basement rocks and their pre- and synrift sedimentary cover locally overlie the exhumed mantle rocks (Froitzheim & Manatschal 1996; Manatschal & Nievergelt 1997). More importantly, the serpentinized mantle rocks are in places stratigraphically overlain by Jurassic radi- olarites, indicating that they must have been uplifted from mantle depth to the sea floor in Mesozoic times. From field and geochemical evidence it appears that in many areas of the Alps and the Apennines (Piccardo 1976; Piccardo et al. 1990; Trommsdorff et al. 1993; Rampone et al. 1995, 1998; Mtintener & Hermann 1996; Rampone & Piccardo 2000), the serpentinized peridotite basement represents former subcontinental mantle; however, the chemical composition and the conditions of equilibration of these peri-



72 O. MONTENER & G.B. PICCARDO

to ocean to continent

. . . . . . . . . . . . . . . . . . . . . Argille a Palombini [

~ ~ _ _ i _ . _ _ i _ . ~ Calpionella Limestone[ f I ,

~ ~ ~ ~ . ~ ~ . . . . . . . . . . . . . Radiolarite Formation JU_....ex~...-t-~..~.---"-2"-"A-__-_"_-_--_-.-

~(-~. , ,J"~ se-~pent,nite _~,,o., ~ ~ . / ~ - ~ ~ " '~'.-.~ / / ~ . \ \ \ . . . . . breccia ~ "'" // ,v, .#,v,. 7. - /o--PhicXa~ci;e '~~ / / /// ~ '"_,~7 / &basaltic ,, / Z ~ /~

/ / /~ ~ ' ~ / / / / extensional allochthon v ~ dyke v / / . /.// 7. / / " basaltic'~-~ " // / / detacnmem// dyke serpentinized peridotite Fig. 2. Generalized structural and stratigraphic relationships of an ancient ocean-continent transition in the Alps (Platta-Totalp-Malenco area in Eastern Switzerland and Northern Italy; after Manatschal & Nievergelt 1997 and Desmurs et el. 2001). This area is characterized by an incomplete ophiolite sequence typical for many Alpine ophiolites, with basalts and post-rift sediments covering the top of exhumed subcontinental mantle. In the proximal (continentward) part of the ocean-continent transition, the exhumed subcontinental mantle rocks are locally overlain by extensional allochthons, stranded klippen of continental basement, pre-rift sediments and synrift marine breccias, emplaced along extensional detachment faults. In the distal (oceanward) part of the ocean-continent transition the mantle rocks are intruded by gabbros and stratigraphically overlain by pillow laves and breccias. JU, Jurassic; TP,, Tries: JL, lower Jurassic; JM, middle Jurassic.

dotites are different from place to place. Many of these ophiolitic peridotite massifs show areas where plagioclase is present and, frequently, rather abundant. Two microtextural settings might be established: (1) granular to porphyroclastic spinel peridotites with only incipient recrystallization of plagioclase; (2) granular to porphyroclastic peridotites with abundant, granoblastic aggregates of plagioclase, interstitial to deformed spinel-facies mantle minerals.

The majority of the mantle peridotites of the first group (e.g. Davos, Malenco, Upper Platte, some of the Extemal Ligurides, Erro Tobbio, Tuscany) are fertile, clinopyroxene-rich lherzolites, whereas depleted, clinopyroxene-poor peridotites are subordinate. Most of these massifs are composed of amphibole-bearing spinel peridotite with abundant (garnet) pyroxenite layers and locally phlogopite-homblendite veins (Peters 1963; Piccardo 1976; Rampone et el. 1995; Mtintener & Hermann 1996; Desmurs 2001). They display a static equilibrium recrystallization under spinel-facies conditions, and the presence of light rare earth element (LREE)-depleted titanian par- gasite, in structural and chemical equilibrium with the spinel-bearing assemblage (Vannucci et el. 1995). Thermobarometric estimates on the spinel- facies assemblages yield temperatures in the range of 900-1100 ~ and equilibration pressures of 1.0-1.5 GPa. Plagioclase recrystallization is rare or non-existent and restricted to porphyroclastic to mylonitic peridotites. There, olivine-plagioclase

symplectites form as subsolidus reaction between pyroxenes and spinel. The second group of peridotites (e.g. Lanzo, Corsica, Lower Platte, Internal Ligurides, Chenaillet) are in general made of clinopyroxene-poor lherzolites similar to abyssal peridotites; however, fertile peridotites are common. Most of these peridotites contain little or no Ti-amphibole and pyroxenite layers are locally transformed into olivine gabbro. They are strongly enriched in plagioclase within the granoblastic aggregates. Thermobarometric estimates yield high temperatures in the range of 1100-1280 ~ and equilibration pressures of less than about 1 GPa. Plagioclase is abundant and forms granoblastic aggregates, interstitial to deformed spinel- facies mineral assemblages.

P l u t o n i c rocks

Gabbroic rocks occur mainly as small intrusive bodies and dykes in the peridotites and are usually discordant to the high-temperature mantle structures. Their compositions range from ultramafic cumulates to highly differentiated plagiogranites and represent the crystallization products of a typical low-pressure, tholeiitic fractionation of MORB-type parental magmas (Serri 1980; Heb~rt et el. 1989; Tiepolo et el. 1997; Tribuzio et el. 2000; Desmurs et el. 2002). The gabbros show the crystallization sequence olivine ~ plagioclase clinopyroxene, and covariations of forsterite (fo) content in olivine, anorthite (an) content in plagi-



ALPINE-APENNINE PERIDOTITES 73

oclase and Mg-number in clinopyroxene, which are typical of low-pressure crystallization of olivine tholeiites. Clinopyroxenes of primitive cumulates have rather flat heavy REE (HREE) to middle REE (MREE) patterns, at about (9- 10) • C1, and LREE depletion (CeN/SmN---- 0.21~).29). Calculated liquid compositions from the most primitive samples indicate a clear MORB affinity, in agreement with the Sr and Nd isotope ratios of some ol-gabbros and their clinopyroxenes (Rampone et al. 1998; Bill et al. 2000). U-Pb ages of zircons from highly differentiated Fe-Ti gabbros exhibit a surprisingly narrow window of crystallization ages from 166 to 160Ma (Schaltegger et al. 2002), whereas some plagiogranites of the Western Alps and Apennines are distinctly younger (153-148Ma, Borsi et al. 1996; Costa & Caby 2001). This suggests that regional-scale upwelling and partial melting of a MORB-type asthenospheric source started in the Mid-Jurassic.

E x t r u s i v e r o c k s

Basaltic rocks are common in Alpine ophiolites and occur as pillows or massive flows and as discrete dykes intruding deformed gabbros and mantle peridotites (Fig. 2). Petrological and geochemical studies have provided evidence of their overall tholeiitic composition and MORB affinity, ranging from T-MORB to N-MORB (Venturelli et al. 1981; Beccaluva et al. 1984; Ottonello et al. 1984; Rampone et al. 1998; Bill et al. 2000; Desmurs et al. 2002). The most primitive basalts show either moderate LREE fractionation (CeN/SmN = 0.6) or almost flat to slightly LREE- enriched REE spectra, and HREE abundances at about 10 • C 1. They have fairly homogeneous Nd isotopic ratios, consistent with their MORB affinity, but variable Sr isotopic ratios (up to 0.7085), which are related to oceanic sea-water altera- tion (Rampone et al. 1998; Bill et al. 2000; Schaltegger et al. 2002). Geochemical modelling indicates that the most primitive T-MORB and N- MORB-type basalts are consistent with melts gen- erated by low to moderate degrees of fractional melting of a MORB-type asthenospheric mantle source (Vannucci et al. 1993a); however, the source of some basalt is enriched in incompatible elements (Desmurs et al. 2002). This composi- tional variation seems to correlate with the spatial distribution of the mafic rocks within the ocean- continent transition whereby mafic rocks with T- MORB signatures occur close to the continental margin whereas N-MORB signatures are predomi- nantly found oceanwards (Desmurs et al. 2002).

Observations from the Lanzo and Corsica peridotites

F i e l d o b s e r v a t i o n s

The Lanzo and Monte Maggiore (Corsica) peridotites comprise the mantle section of partially dismembered ophiolites exposed in the Western Alps and the Northern Apennines (Fig. 1). The peridotites are composed mainly of massive plagioclase peridotites, and minor spinel peridotites, harzburgites and dunites.

At Lanzo, plagioclase peridotites were thought to be formed either as the residuum of low degrees of partial melting (Bodinier 1988) or as a result of melt formation and incomplete melt extraction and crystallization in an upwelling diapir (Boudier & Nicolas 1972; Boudier 1978; Nicolas 1986). The peridotite is rich in plagioclase and clinopyroxene, and spinels commonly have high Cr/(Cr + A1) ratios. These characteristics are similar to some plagioclase peridotites dredged from slow-spreading ridges and along fracture zones (Dick 1989; Cannat et al. 1997; Seyler & Bonatti 1997; Tartarotti et al. 2002). Isotopic studies of the Lanzo peridotites have pointed out important differences between the Northern and Southern bodies (Bodinier et aL 1991). The South- ern body of the Lanzo Massif has been interpreted as an asthenospheric diapir that rose from the garnet stability field and was emplaced in the early Mesozoic, during the opening of the Pied- mont Ligurian basin. The Northern body has been considered a fragment of the subcontinental lithosphere that became isolated by the convective mantle at 400-700 Ma (Bodinier et al. 1991).

In Lanzo, the peridotites are cut by an older suite of spinel websterites and a younger suite of discordant dunite, followed by various sets of gabbroic veins and dykes. Dunite cuts and locally replaces earlier spinel websterites (Fig. 3a), as indicated by trains of Cr-spinel that are continuous with the surrounding spinel websterites (see also Boudier & Nicolas 1972; Boudier 1978). New field observations show that some discordant dunites locally contain small interstitial clinopyroxene (Fig. 3b), and large clinopyroxene megacrysts (crystals of more than 1 cm in diameter, Fig. 3d) sometimes associated with plagioclase (Fig. 3c). In places, large, euhedral clinopyroxenes form aggregates a few millimetres wide or (deformed) gabbroic veinlets (Fig. 3e and f), similar to the 'indigenous' dykes described by Boudier & Nicolas (1972) and Boudier (1978). Locally, med- ial pyroxenite dykes in dunite have also been observed. The gabbroic dykes can be separated into two groups (Boudier & Nicolas 1972; Boudier 1978). Type 1 is an older 'indigenous'




Fig. 3. Field aspects of plagioclase peridotites and dunites from Lanzo South (Northern Italy), showing plagioclase peridotites, cut by spinel dunites. (a) Harzburgite-dunite contact (southern flank of Monte Musine'). It should be noted that the foliation is discordant to the dunite-harzburgite contact. (b) Interstitial clinopyroxene (green Cr- diopside) within dunite. (c) Clinopyroxene (cpx) + plagioclase (pig) cluster in dunite (Monte Musine'). (d) Clinopyroxene megacryst in dunite (Monte Musine'). (e) 'Indigenous' microgabbroic vein and a clinopyroxene megacryst within dunite (Monte Arpone). (f) Weakly deformed 'indigenous' Mg-gabbro dykelet cutting dunite (Mt Arpone). At the lower right, the gabbro is discordant to the peridotite-dunite contact. Subsequent high-temperature ductile deformation formed dunite mylonite, du, dunite; hz, harzburgite; pig thz, plagioclase lherzolite.

group of olivine gabbronorite, frequently occur- ring in en echelon fuzzy contacts with the surrounding lherzolites and dunites. This type is restricted to the southwestern part of the massif

(Compagnoni et al. 1984; Pognante et al. 1985). Type 2 is an intrusive group of troctolites, (olivine) gabbros, gabbronorites to oxide gabbros, with sharp contacts and chilled margins towards




the peridotite. These dykes can be found all over the massif (Compagnoni et al. 1984; Pognante et al. 1985), cut across dunites and plagioclase peridotites, and are generally undeformed.

Thus it seems that penetrative high-temperature deformation of the peridotite ceased between the formation of type 1 and 2; however, both are locally mylonitized and partially hydrated under upper amphibolite- to granulite-facies conditions (Compagnoni et al. 1984; Pognante et al. 1985). The geochemistry of the mafic rocks reveals that most gabbros represent cumulates with little or no trapped liquid, indicating efficient extraction of derivative liquids (Bodinier et al. 1986). In addition, late porphyritic basaltic dykes of N-MORB affinity (Pognante et al. 1985) cut the peridotites and gabbros. The extracted melts had a T-MORB and a T- to N-MORB composition in the north and south, respectively, (Bodinier 1988), similar to basalts from the Ligurian Alps (Beccaluva et al. 1984).

In Corsica, the Monte Maggiore peridotites are strongly depleted, with a spinel-facies granular assemblage: they are clinopyroxene-poor, refrac- tory spinel lherzolites, which are interpreted as mantle residua after MORB-type partial melting processes (Jackson & Ohnenstetter 1981; Rampone et al. 1997). Preliminary Sm/Nd isotope data provide a mid-Jurassic (165 Ma) DM (depleted mantle) model age of depletion (Rampone 2002). In places these peridotites contain plagioclase and show evidence for trapped melt crystallization (Rampone et al. 1997). In the Monte Maggiore region, peridotites with oriented and diffuse impregnation (Fig. 4a and b) are cut by discordant dunites (Fig. 4e), followed by the intrusion of gabbroic veins (Fig. 4c) and metre- scale pockets of mafic-ultramafic cumulates, composed mainly of olivine gabbronorites (Fig. 4d). The cumulates cut dunite-peridotite contacts and the existing oriented impregnation. Locally these pockets dominate volumetrically and the former peridotite is completely replaced by gabbronorite mineral assemblages consisting of euhedral ortho- and clinopyroxene and interstitial plagioclase (Fig. 4d). Another common feature at Monte Maggiore is the formation of coarse- grained and undeformed late gabbroic dykes (with crystals exceeding 5 cm in diameter), which cross- cut deformed peridotites and gabbronorite cumulates (Fig. 4f).

Res idua l mant le mineral assemblages

The studied spinel- and plagioclase-bearing lherzolites are mainly porphyroclastic peridotites com- prising a deformed mantle assemblage and less deformed or undeformed interstitial igneous as-

semblages. Rare samples are nearly plagioclase- free spinel lherzolites and show textures typical of common spinel peridotites. Olivine occurs in large grains (up to 1 cm) and pyroxenes form millimetre- to centimetre-scale porphyroclasts. Spinel is brown and Al-rich, and commonly shows hollyleaf shapes (Fig. 5a). Deformation-induced undulatory extinction and gliding in olivine is common. Pyroxenes are commonly deformed and show fine exsolution lamellae of the complementary pyroxene. In samples not affected by mek impregnation, Al-rich spinel is locally intergrown with orthopyroxene, producing a texture similar to that of garnet breakdown (Vannucci et al. 1993b). In the same sample, a small rim of olivine + plagioclase is locally developed between orthopyroxene and spinel, according to the reaction orthopyroxene + spinel ~ olivine + plagioclase (Fig. 5b) This microstructure probably formed during decompression from the spinel peridotite to the plagioclase peridotite field before melt impregnation and melt-rock reaction, and provides rare evidence for a metamorphic origin of the plagioclase in the Lanzo peridotite.

Impregnat ion textures

The sequence of igneous microstructures in the spinel peridotite is well established. Early melt- rock reaction dissolved clinopyroxene along grain boundaries and precipitated orthopyroxene + plagioclase around and within clinopyroxene (Fig. 5c and d). Textural relationships indicate cotectic crystallization of plagioclase + orthopyroxene (Fig. 5d). These intergrowths are not deformed, contrary to the original mantle clinopyroxene. A similar structure can be observed in spinel websterites. These features indicate that the migrating liquid crystallized clinopyroxene-free, orthopyroxene-rich gabbronoritic microgranular aggregates. Orthopyroxene partially replaced mantle minerals, showing concave contacts against the peridotite clinopyroxene (Fig. 5e). However, in many samples crystallization of two pyroxenes and plagioclase is also common. This is illustrated in Figure 5e and f, where undeformed and interstitial clino- and orthopyroxene crystallized between large mantle minerals. Large kinked mantle olivine is recrystallized close to the interstitial orthopyroxene (Fig. 5f), supporting the general observation that the impregnating assemblages are less deformed than the precursor mantle assemblage. In places the igneous domains consist of plagioclase patches, replacing spinel, together with granular orthopyroxene + olivine + clinopyroxene (Fig. 5g). These microgabbroic aggregates form xeno- morphic granoblastic mosaic textures between mantle minerals generally several millimetres to



76 O. M U N T E N E R & G.B. P I C C A R D O

Fig. 4. Field aspects of plagioclase peridotite and gabbronorite from the Monte Maggiore peridotite (Corsica). (a) Impregnated peridotites with interstitial plagioclase surrounding mantle minerals. Locally plagioclase (pig) + orthopyroxene (opx) coalesce, forming gabbronorite veinlets. (b) Oriented plagioclase-rich impregnation in peridotite discordantly cut by a cpx-rich gabbronorite dykelet, related to the cumulate pods. It should be noted that in the upper portion of the outcrop the gabbronorite dyke ends in a millimetre-scale veinlet. (e) Impregnated peridotite intruded by irregular gabbroic veins or pods related to the cumulate suite. Euhedral green cpx and interstitial pig in the gabbroic veins or pods should be noted. (d) Euhedral olivine + opx + cpx and anhedral plagioclase in a gabbronorite cumulate. (e) Discordant dunite-plagioclase peridotite contact cut by a gabbro dykelet. The euhedral clinopyroxene megacrysts in the dykelet should be noted. The foliation in both peridotite and dunite runs approximately perpendicular to the contact. Locally, millimetre-scale plagioclase seams of the gabbro intrude the surrounding dunite and plagioclase peridotite. (f) Coarse-grained Mg-gabbro dyke with chilled margins cuts spinel peridotite.



A L P I N E - A P E N N I N E P E R I D O T I T E S 77

Fig. 5. Selected thin section specimens from the Lanzo South area. (a) Intergrowth of orthopyroxene (opx) and spinel (spl) in a spinel peridotite from Mt Arpone. Of particular note is the overall shape of this intergrowth, recalling former garnet. It should be noted also that spl-opx contacts are fresh without any sign of reaction. (b) Opx-spl contacts separated by a small rim of olivine + plagioclase (completely altered). This indicates that orthopyroxene and spinel are unstable and transform into a (metamorphic) assemblage of olivine (ol) + plagioclase (pig). (c) Corrosion of exsolved and deformed mantle clinopyroxene by opx + pig intergrowths. (d) Intergrowth of opx + pig indicating cotectic crystallization of the two phases. (e) Undeformed interstitial opx + cpx separating large, kinked mantle olivine. (f) Rim of interstitial opx replacing mantle cpx. Of particular note are the concave opx-cpx contacts. It should be noted also that kinked olivine is replaced by undeformed olivine in the left part of the photomicrograph. This demonstrates that cpx + liquid reacted to form ol + opx. (g) Undeformed micro-gabbronorite aggregate between deformed mantle minerals. The anhedral shape of plg and opx between granular euhedral olivine should be noted. Cpx forms small interstitial grains. These textural relationships demonstrate the crystallization sequence ol opx + plag ~ cpx and indicate that migrating liquids were saturated in opx before cpx. (h) anhedral cpx along triple point of olivine in discordant dtmite.



78 O. MUNTENER & G.B. PICCARDO

centimetres wide. The granoblasts do not show a preferred orientation and form troctolitic to gabbronoritic assemblages. Interstitial pyroxenes do not show exsolution of the complementary pyroxene, are virtually undeformed and show concave contacts to the host peridotite minerals (Fig. 5e and f). In other samples olivine, clinopyroxene and orthopyroxene form euhedral crystals sur- rounded by interstitial plagioclase. Mantle clinopyroxene is seemingly unreacted. Microstructures in the dunites are characterized by coarse-grained olivine (up to 2 cm in size) and more or less rounded spinels, as described previously (Boudier & Nicolas 1972; Boudier 1978; Nicolas 1986). In addition, many dunites contain interstitial clinopyroxene, which is exclusively found along olivine triple junctions (Fig. 5h).

Geochemical data

We analysed crystals of peridotite and gabbroic samples by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Preliminary results are shown in Tables 1 and 2 and illustrated in Figures 6 and 7. In terms of major element compositions, the main variation in clinopyroxenes in peridotites is re- flected in the A1 and Ti contents, with high A1 and low Ti in spinel peridotites, and low AI and higher Ti in plagioclase peridotite. Mg numbers (molar Mg/(Mg + Fetot)) range from 0.89 to 0.92 (Table 1). Na20 contents are invariably low (<0.5 wt%). Plagioclase in impregnated peridotites is generally calcic (Al188-94) for Monte Maggiore peridotites (Rampone et al. 1997), whereas it is much more variable in the Lanzo peridotite, ranging from An90 to An67.

Figure 5 illustrates that Monte Maggiore minerals have similar REE slopes to those of Lanzo South, but generally lower concentrations. There is a first-order difference between spinel and plagioclase peridotite in both clinopyroxene and orthopyroxene compositions. Spinel peridotite clinopyroxene has a slightly to strongly LREE- depleted, chondrite-normalized pattern, with es- sentially flat MREE to HREE and no negative Eu anomaly. These characteristics are similar to those of some spinel peridotites in other Alpine peridotites (Rampone et al. 1996; Miintener et al. 2002) and of abyssal peridotites (Johnson et al. 1990). Plagioclase peridotite clinopyroxene REE patterns (both porphyroclastic relics and igneous grains in granular aggregates) are generally convex upward with a significant MREE enrichment (up to 20 • C1), GdN/YbN >1 and a weak to significant negative Eu anomaly (Fig. 6), which increases from core to rim, indicating equilibration with plagioclase. In addition, most of the trace ele-

ments (i.e. Ti, Sc, V, Zr, Y) in clinopyroxene from plagioclase peridotites are enriched with respect to the precursor clinopyroxene in spinel lherzolites (Table 1). Orthopyroxene follows the trends given by clinopyroxene. Plagioclase from both Lanzo and Monte Maggiore peridotites show similar REE chondrite-normalized patterns with significant LREE fractionation (CeN/NdN < 0.5) and very low Sr (<10ppm) and Na (Na20 <1.00 wt%) contents (see also Rampone et al. 1997); however, samples with nearly flat or slightly enriched LREE, and relatively higher Sr (up to 150 ppm) and Na (Na20 up to 4.00 wt%) contents are found in Lanzo. Within-sample variations and within-group variations are much smal- ler than variations between spinel peridotites and plagioclase peridotites.

Simple modelling indicates that about 6% of polybaric fractional melting is sufficient to produce the depleted REE signatures of the Lanzo spinel peridotites, whereas slightly higher degrees (about 8%) are necessary for the Monte Maggiore peridotites. The composition of clinopyroxene thus reflects near-fractional melting processes in the spinel peridotite field before impregnation and melt-rock reaction at lower pressure. It is more difficult to evaluate the liquid composition, which modified the peridotite mineral compositions and formed the igneous plagioclase-pyroxene-bearing assemblages. Rampone et al. (1997) assumed that the trace element signatures of plagioclase peridotites from Monte Maggiore resulted from variable proportions of trapped liquid within the peridotites. Modelling of the liquids that impregnate the peridotites suggests that they most probably con- sisted of unmixed single melt increments that originated from deeper levels of the mantle by a near-fractional melting process (Rampone et al. 1997).

In striking contrast to the clinopyroxenes from plagioclase peridotite, interstitial clinopyroxene in dunite from Lanzo shows no Eu anomaly, is much less depleted in LREE (Fig. 6), and shows lower MREE and HREE contents (at <10 • C1). It also has higher incompatible element contents (Table 1). Calculated liquids in equilibrium with clinopyroxenes have REE slopes and concentrations similar to MORB crystallized from low percentage aggregate liquids (<5%). In addition, spinels in Lanzo dunite (Boudier & Nicolas 1972; Boudier 1978; Nicolas 1986) are similar to spinels from MORB (Dick & Bullen 1984). Thus both spinel and interstitial clinopyroxene compositions suggest that Lanzo dunite formed in equilibrium with liquids similar to MORB, whereas the plagioclase peridotites formed from single melt increments that were trapped within the peridotites.




C~

r

�9

~J

o

~J

�9

~J

. . . . . . . . . . . . . .

~ d 2 ~ d d d M d d ~ = _ ~ ~ d d & d d d s "~

+



80 O. MI]NTENER & G.B. PICCARDO

Table 2. Representative analyses of minerals from Lanzo South replacive dunite, gabbro veinlets and Monte Maggiore gabbronorite cumulates

Lanzo Lanzo gabbro veinlets Monte Maggiore cumulates dunites

Sample: La70 La71 La71 La71 CR5/2 CR5/2 CR5/2 CR5/2 Mineral: Cpx Cpx core Cpx rim Pig Cpx core Cpx rim Opx core Plg

wt% SiO2 51.88 50.41 51.66 52.58 50.21 52.3 54.67 45.41 TiO2 0.34 0.37 0.45 0.11 0.31 0.28 0.18 0 Cr203 1.64 1.56 1.66 0.14 1.48 1.23 0.91 0 A1203 4.64 4.53 3.10 29.44 5.91 2.93 4.01 34.65 Fe203 0.25 FeO 3.11 3.41 3.34 0.20 3.25 2.89 6.73 0 MnO 0.15 0.02 0.12 0.00 0.11 0.19 0.16 0 MgO 16.07 16.59 17.30 0.00 15.96 16.98 32.89 0 NiO 0 0.00 0.06 0.23 0.06 0.21 0.05 0 CaO 22.3 22.19 22.20 12.70 22.79 23.3 1.13 17.75 Na20 0.58 0.80 0.00 3.62 n.d. n.d. n.d. 0.81 K20 0.12 0.12 0.10 0.19 n.d. n.d. n.d. 0.08

Total 100.83 100.00 99.99 99.21 100.08 100.31 100.73 98.95 REE + Y (ppm) La 0.232 0.220 0.224 0.317 0.020 0.020 0.020 Ce 1.383 1.407 1.313 0.796 0.190 0.240 0.020 0.090 Pr 0.334 0.384 0.335 0.114 0.080 0.090 0.010 0.020 Nd 2.100 2.237 2.272 0.403 0.780 0.980 0.070 0.110 Sm 1.058 1.140 1.276 0.075 0.720 0.860 0.065 0.020 Eu 0.434 0.555 0.494 0.380 0.330 0.410 0.030 0.090 Gd 1.573 1.877 1.665 0.069 1.310 1.590 0.170 0.020 Tb 0.303 0.367 0.359 0.012 0.290 0.320 0.050 0.005 Dy 2.127 2.324 2.362 0.054 2.070 2.530 0.390 Ho 0.481 0.550 0.504 0.009 0.510 0.540 0.090 Y 12.643 14.560 13.650 0.201 12.700 15.000 2.590 0.130 Er 1.273 1.570 1.367 0.019 1.320 1.550 0.290 0.013 Tm 0.202 0.215 0.215 0.000 0.200 0.240 0.050 Yb 1.266 1.405 1.324 0.017 1.340 1.490 0.450 Lu 0.172 0.202 0.179 0.003 0.190 0.200 0.070

n.d., not determined

The clinopyroxene and plagioclase data of megacrysts and indigenous gabbroic dykelets from Lanzo are compared with those of the Monte Maggiore gabbroic veinlets and cumulates in Figure 7 and listed in Table 2. Pyroxene and olivine Mg number ranges from 0.89 to 0.91, indicating that the intruding liquids were rather primitive. Plagioclase in indigenous gabbronorite veins and dykelets in Lanzo is relatively sodic (An54-66) and enriched in Sr (500-750ppm) . Plagioclase in the Monte Maggiore maf ic - ultramafic cumulates and gabbronorite dykelets is in turn extremely poor in Sr (20-30 ppm) and Ca- rich (An88-96). Lanzo clinopyroxenes have REE concentrations and slopes strikingly different from those for the Monte Maggiore area (Fig. 7). Liquids in equilibrium with clinopyroxene from these dykelets and cumulate pods indicate that: (1) the Lanzo dykelets have REE slopes and concentrations corresponding to low-degree ( 2 -

3%), single melt increments after fractional melting of an asthenospheric mantle source; (2) the primary liquids of the Monte Maggiore dykelets and mafic-ultramafic cumulates are compatible with crystallization from single melt fractions produced by moderate degrees (6 -7%) of fractional melting. Thus, whereas the Lanzo data are generally compatible with crystallization from low percentage (less than 5%) aggregate MORB, the data from Monte Maggiore suggest that single melt fractions remained isolated during transport and crystallized strongly depleted gabbroic cumulates, similar to those from the Mid-Atlantic Ridge, Deep Sea Drilling Project Site 334 (Ross & Elthon 1993).

Discussion and conclusions

Our field observations and geochemical data suggest that ex-subcontinental and oceanic litho-




100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I Monte Maggiore (a) ::

10 ~ cpx. i

__r ~2 _ 0.1 --"-3 ~

= ~ - 4 ~ ~ --~-5 !

0.01 , ~ , " ~ / , , , ~ 3 - 8 1

Z 100 ....................................................................................................................................................................................................................

Lanzo South (b) cpx

r 10

o p x

1 - ~ - 1 ~ - - 2 ~ - 3

0.1

0 .01 "

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu

Fig. 6. Representative REE concentrations in minerals from the plagioclase-free spinel lherzolites and plagioclase-enriched impregnated lherzolites from (a) Monte Maggiore, Corsica and (b) Lanzo South. Data are normalized to C1 chondrite of Anders & Ebihara (1982). (a) Monte Maggiore (Corsica). (i) Spinel lherzolite: 1, cpx porphyroclast core; 2, opx porphyroclast core. (2) Impregnated lherzolite: 3, cpx porphyroclast core; 4, opx porphyroelast core; 5, opx interstitial grain; 6, cpx interstitial grain; 7 and 8, interstitial plg. (b) Lanzo South. (i) Spinel lherzolite: 1, epx porphyroclast core; 2, opx porphyroclast core. (2) Impregnated lherzolite: 3, cpx porphyroclast core; 4, cpx porphyroclast rim; 5, cpx interstitial grain; 6, opx porphyroclast core; 7, opx porphyroclast rim; 8, opx interstitial grain; 9, interstitial plg.

spheric mantle may be substantially modified by migrating magmas during opening of embryonic ocean basins. The occurrence of melt-rock reaction and trapped liquids within peridotite is well known from xenoliths (Menzies et al. 1987), peridotite massifs (Van Der Wal & Bodinier 1996) and studies of present-day oceanic settings (Dick 1989; Elthon 1992; Cannat & Casey 1995; Cannat et al. 1997; Tartarotti et al. 2002), and it has been suggested that upwelling hot asthenosphere causes

100_

10-

1

0.1

0 Z 0.01

lOO

Monte Maggiore (a) cpx

y ... / ' / ~ +~lg / ~ 1 \ + ~,~

�9 / ~ 2 :x p ig + opx

MORB (b) 1 calculated liquids

, , i i ,

..~anzo South

0.1

0.01

-e-cpx in replacive dunite calculated liquid

.m~MORB cpx core, gabbro dykelet cpx rim, gabbro dykelet

--e- pig, gabbro dykelet

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu

Fig. 7. Representative REE concentrations in minerals from (a) gabbroic cumulates of Monte Maggiore and (b) replacive dunites and gabbro veinlets in Lanzo South.

chemical and thermal modifications of the overlying lithosphere. However, in the past, the im- portance of melt migration processes has been underestimated in explaining the evolution of the Piedmont Ligurian ophiolites, and only a few studies have addressed this topic (Rampone et al. 1997; Dijkstra et al. 2001). The reason was that the origin of plagioclase was explained by subsolidus formation during non-adiabatic decompression in a closed system (Piccardo 1976; Hoogerduijn-Strating et al. 1993; Rampone et al. 1993, 1995). This might be the case in some places; however, lithospheric extension was accompanied by almost adiabatic upwelling of the underlying asthenosphere, which underwent decompression melting and produced MORB-type liquids of mid-Jurassic age (e.g. Schaltegger et al. 2002). As shown in the previous sections, liquids produced by the upwelling asthenosphere migrated into the overlying peridotite, reacted with, and impregnated the lithospheric mantle. Melt migration evolved from diffuse porous flow to focused porous flow and finally to dyking. This is schema-



82 O. MrUNTENER & G.B. PICCARDO

tically illustrated in Figure 8. Below, we discuss the compositions of the primary liquids and present some explanations for the variable melt migration mechanisms. These explanations are presented as a group of alternative hypotheses, which are not mutually exclusive.

Porous flow of melt during incipient opening

of the Piedmont Ligurian ocean

Field and microstructural relationships demonstrate that migrating melts produced nearly pervasive impregnation of the lithospheric mantle in the form of small irregular veins and microgranular aggregates of undeformed igneous minerals between deformed mantle minerals. The length scale over which the magmas continued to ascend by porous flow might be several kilometres, as suggested by the ubiquitous occurrence of plagio-

clase-bearing peridotite in the Lanzo massif (e.g. Boudier 1978). The impregnating melts either: (1) reacted with mantle clinopyroxenes, forming sym- plectitic orthopyroxene + plagioclase reaction rims, partially replacing the deformed mantle clinopyroxenes, and crystallized as clinopyroxene- free, orthopyroxene-rich gabbroic micro-granular aggregates, or (2) did not react with mantle clinopyroxene and crystallized as orthopyroxene- rich, clinopyroxene-bearing gabbroic microgranular aggregates.

Fig. 8. Generalized evolution of subcontinental peridotites from embryonic ocean basins above an upwelling asthenosphere, as evidenced from the Piedmont Ligurian ophiolitic peridotites. (a) Summary of the situation before exhumation. Peridotite shows high-temperature foliation and several generations of pyroxenite dykes. Temperatures are well below the solidus of peridotites and correspond to a conductively cooled mantle. Model ages of such peridotites range from Proterozoic to Permian (Bodinier et al. 1991; Rampone et al. 1995, 1996). (b) Pervasive melt infiltration and melt-rock reaction at rising temperatures. Ascending liquid from below reacted extensively with its surrounding peridotite, producing orthopyroxene-saturated, cpx-undersaturated compositions, followed by saturation of both pyroxenes. Crystallization products are millimetre- to centimetre- scale depleted olivine gabbronorites. (c) Focused porous melt flow in dunite channels at a thermal maximum close to the peridotite solidus. Dunites are discordant and replace plagioclase peridotites and spinel websterites. The transition from divergent to focused flow of melt might indicate that the competing effects between heating of the mantle by ascending magmas from the underlying hot asthenosphere and cooling by exhumation are dominated by the rising isotherms. (d) Beginning of cooling would gradually fill the dunite conduits with interstitial cpx, cpx and plagioclase megacrysts, and eventually change the melt migration mechanism from focused porous flow to cracks. These cracks are probably represented by the small 'indigenous' gabbroic veinlets. It should be noted that the cpx crystallizing in dunite is close to equilibrium with MORB. (e) Continued cooling and exhumation leads to the formation of kilometre-scale gabbroic dykes, discordant to the previous igneous features. These gabbroic dykes range from primitive troctolite to Fe-Ti- gabbros, indicating the increasing contribution of crystaI fractionation on the evolution of magmas at falling temperatures.




The porphyroclastic mantle pyroxenes in the impregnated Monte Maggiore, Lanzo and Internal Ligurides peridotites have trace element compositions unlike residues of fractional melting and are enriched in many trace elements (i.e. REE, Ti, Sc, V, Zr, Y) with respect to porphyroclastic pyroxenes from spinel lherzolites. Clinopyroxene shows convex-upward REE spectra, with a significant MREE enrichment, and both pyroxenes generally show a negative EuN anomaly. Core-rim analyses of deformed and reacted porphyroclastic clinopyroxene and the interstitial undeformed, granular igneous pyroxene have very similar trace element compositions. This indicates that (1) pyroxenes in the impregnated peridotites attained trace element equilibrium with the migrating liquid; (2) the impregnating liquids were significantly enriched in many trace elements.

Early crystallization and abundance of orthopyroxene in the interstitial magmatic micro-gabbroic aggregates suggest that the impregnating melts were silica-saturated. Thus, the primary liquids must have interacted with the ambient peridotite during upward migration in the mantle column, where they dissolved mantle pyroxenes and crystallized olivine, attaining orthopyroxene saturation. Magmatic plagioclase and clinopyroxene in the interstitial granular aggregates have high Mg number (Mg/(Mg + Fe)), are strongly LREE depleted, and have very low Sr contents, significantly different relative to aggregated MORB. These features indicate that the parental melts were strongly depleted in the most incompatible elements. This interpretation suggests that the primary liquids represent depleted single melt increments, produced during fractional melting of an asthenospheric mantle source (Rampone et al. 1997). A likely alternative is that the depleted compositions of the crystallizing phases record their open-system provenance. The trace element composition of both plagioclase and clinopyroxene might be attained by liquid rising adiabatically through the mantle and reacting with the surrounding host rocks. Such liquids would dissolve pyroxenes and crystallize olivine. As discussed by Kelemen et al. (1995), the crystallization sequence of liquids that reacted with mantle peridotite depends on the relative effects of reaction with the surrounding host rocks and cooling. For relatively rapid reactions and slow cooling, liquids might quickly become saturated in orthopyroxene. More rapid cooling, or slower pyroxene dissolution would produce less silica-rich liquid compositions. Continued melt-rock reaction and/or cooling finally led to saturation in two pyroxenes.

Thermometric estimates based on trace element (Sc, V) distribution between coexisting ortho- and clinopyroxene in impregnated peridotites (Seitz

et al. 1999) indicate high temperatures (c. 1200- 1300~ close to the peridotite dry solidus, during melt percolation and impregnation.

The transition from diffuse porous flow to focused porous flow is accompanied by the formation of discordant dunite (Fig. 8c). Textural evidence for a replacive origin of dunite in the Lanzo peridotite was first given by Boudier & Nicolas (1972). Dunite cuts and locally replaces earlier spinel websterites, as indicated by trains of Cr- spinel that are continuous with the surrounding spinel websterites. Melt percolating in the dunite channels sporadically crystallized small interstitial clinopyroxenes and, later, trails of clinopyroxene megacrysts (up to 2cm in size), which are precursors to the early gabbroic veins and dykelets. Calculated liquids in equilibrium with clinopyroxene are similar to normal MORB. Both field evidence and geochemical data indicate that dunite formed from liquids that were significantly different from those that impregnated the peridotites.

Extraction o f melt in dykes

The development of interstitial clinopyroxene to megacrysts and finally the formation of centimetre-scale gabbroic veins and dykelets, which are characterized by euhedral clinopyroxene and interstitial ptagioclase, indicate that the intercon- nected melt network in dunite channels becomes progressively clogged with crystallization products (Fig. 8d). This suggests that cooling was important and melt migration changed from focused porous flow in dunite channels to intrusion into narrow conduits. Cooling and crystallization might produce substantial hydrostatic overpressure, which might initiate the formation of cracks, and liquid might be expelled in dykes (Nicolas 1986; Kelemen et al. 1997; Kelemen & Aharonov 1998). In both peridotite massifs, the gabbroic veins and dykelets, and the mafic-ultramafic cumulate pods are rather primitive, with Mg number of olivine and pyroxenes of about 90. How- ever, the trace element composition of liquids in equilibrium with clinopyroxene from early dykelets and cumulate pods at Monte Maggiore indicates that: (1) the primary melts of the Lanzo dykelets most probably correspond to low-degree (2-3%) melts, after fractional melting of an asthenospheric mantle source; (2) the primary melts of the Monte Maggiore dykelets and mafic- ultramafic cumulates correspond to higher-degree (6-7%), depleted single melt increments after fractional melting. This indicates that the earliest dyke intrusions allowed both depleted and enriched single melt increments to migrate in the lithospheric mantle without undergoing significant




melt-peridotite reaction within the mantle column.

The Western Alpine-Northern Apennine ophiolitic peridotites are intruded by metre-scale gabbroic dykes and kilometre-scale gabbroic bodies, which have sharp contacts with the country peridotite and which cut across all the previous mantle and magmatic structures (Fig. 8e). They vary in composition from rather primitive troctolite to Mg-gabbros to Fe-Ti-gabbros, and rare plagiogranites. In addition, they show chilled

margins against their country rocks, indicating that the surrounding peridotites were substantially colder than the intruding liquids. Calculated liquids in equilibrium with clinopyroxenes from the most primitive olivine gabbros from all peridotite massifs have almost fiat to slightly LREE- depleted, chondrite-normalized patterns: they are closely similar to those of average aggregated normal MORB (Beccaluva e t al. 1984; Bodinier 1988). The intrusion of MORB-type fractionated Mg-rich and Fe-Ti-rich magmas most probably

Fig. 9. Possible scenario of mantle exhumation and melt-rock reaction in the framework of the tectonic evolution of the Piedmont Ligurian ocean (modified from Whitmarsh et al. 2001). Whether or not rift-related melt infiltration and heating are recorded by exhumed lithospheric mantle along ocean-continent transitions and/or slow-spreading ridges depends on the position of each peridotite relative to the underlying upwelling asthenosphere. Peridotites in the eastern Central Alps (Malenco, Upper Platta) are still associated with the lower continental crust and show a 'cold' exhumation history during opening of the Piedmont Ligurian ocean (Mfintener et al. 2000; Mfintener & Hermann 2001), indicating a considerable distance to the upwelling asthenosphere. Lanzo, Corsica and the Ligurides may illustrate the other extreme of a 'hot' exhumation, where melt infiltration and melt-rock reaction by asthenosphere- type liquids substantially modified the mantle peridotites.




occurred when the lithospheric mantle became more brittle at shallower levels in the conductive lithosphere.

Chemica l refertil ization and thermal erosion

o f the l i thosphere

An important effect of melt impregnation was significant heating of the impregnated peridotites; estimates indicate raised temperatures in the impregnated lithospheric mantle from the 1000- 1100 ~ of the spinel-facies annealing recrystallization, attained during lithosphere accretion and subsequent cooling, to > 1250 ~ reached during impregnation, close to the dry peridotite solidus. Impregnation added basaltic components (e.g. the gabbroic microgranular aggregates) to the lithospheric peridotites, whereas mantle minerals, and particularly clinopyroxenes, were significantly enriched in most of the trace elements. Accordingly, the lithospheric mantle was significantly enriched and refertilized by the impregnating melts (Miintener et al. 2002). The heating combined with the chemical modification of the mantle rocks is similar to that proposed for asthenosphere-lithosphere interaction during early continental rifting (Menzies et al. 1987; Bedini et al. 1997) and along slow-spreading ridges (Elthon 1992; Cannat & Casey 1995; Cannat et al. 1997). Such a model of asthenosphere-lithosphere interaction could reconcile the highly contrasting inter- pretations with respect to the Lanzo peridotites. Whereas Nicolas, Boudier and coworkers interpreted the various igneous rocks as a consequence of dynamic melting of a rising mantle diapir during formation of the Piedmont Ligurian ocean, Pognante et al. (1985) stated that the Lanzo ultramafic rocks might be a section of subcontinental lithosphere with a polyphase history of partial melting and decompression during rifling that was later intruded in the Jurassic by N-MORB type liquids. Bodinier et al. (1991) provided some convergence of ideas in that they showed a transition from continental to oceanic mantle, based on Sr and Nd isotopes.

Our interpretation of the Lanzo and Corsica peridotites as a product of refertilized and ther- mally modified lithospheric peridotite (Fig. 8), which was exhumed during the formation of the Piedmont Ligurian ocean, might serve as a general model to explain the highly contrasting evolution of different peridotite bodies along ocean-continent transitions and (ultra-)slow-spreading ridges. Whether or not rift-related melt infiltration and heating are recorded by exhumed lithospheric mantle along ocean-continent transitions and/or slow-spreading ridges depends on the relative

position to the underlying upwelling asthenosphere (Fig. 9). For example, peridotites in the eastern Central Alps are still associated with the lower continental crust and show a 'cold' exhumation history during opening of the Piedmont Ligurian ocean (Mfintener et al. 2000; Mfintener & Her- mann 2001), indicating a considerable distance from the upwelling asthenosphere. Lanzo, Corsica and the Ligurides might illustrate the other extreme of a 'hot' exhumation, where modification of lithospheric peridotite by asthenospheric-type liquids was dominant. During the late rifting stage of embryonic oceans, the thermochemical erosion of mantle lithosphere above the upwelling asthenosphere could have played a fundamental role in the dynamics of the rifting system and in the transition from passive lithospheric extension to active oceanic spreading. Melt infiltration into mantle peridotite may be one of the reasons for the absence of well-developed layers of oceanic crust in the Alpine-Apennine system, which caused problems in interpreting these ophiolites in the sense of the 1972 Penrose conference defini- tion.

We gratefully acknowledge the financial support by the Swiss National Science Foundation (Grant 21-66923.01) and Italian MURST and the University of Genova. We thank A. Zanetti for LA-ICP-MS analyses, A. Romair- one and S. Bruzzone for assistance in the field and in the laboratory, G. Manatschal for comments, and Y. Dilek and E T. Robinson for constructive reviews.

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Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins