THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

The Erro-Tobbio (ET) ophiolitic peridotite (Voltri Massif – Liguria, Italy) represents a sector of subcontinental lithospheric mantle that has been emplaced at crustal, sub-oceanic levels during rifting and opening of the Jurassic Ligurian Tethys (Ernst and Piccardo, 1979). Structural and petrologic works (e.g. Drury et al., 1990; Vissers et al., 1991; Hoogerduijn Strating et al., 1993) have demonstrated that the Erro-Tobbio peridotites were uplifted along a subsolidus P-T trajectory, characterized by progressively decreasing temperature. Pristine granular mantle protoliths, completely equilibrated at subcontinental lithospheric mantle depths (i.e. 1000-1100°C and spinel-facies conditions), were deformed along km-scale extensional shear zones, where they were transformed to spinel peridotite tectonites, spinel- and plagioclase-bearing mylonites, hornblende/ chlorite peridotite mylonites and, finally, serpentine mylonites. This composite P-T evolution has been interpreted as the exhumation trajectory of mantle sections evolving as the footwall of an asymmetric extensional system dominated by simple shear mechanisms. Recent contributions have revealed the presence of: i) reactive spinel peridotites, that were formed by melt-peridotite interaction under spinel-facies conditions (Piccardo et al., 2004; Rampone et al., 2004); ii) large areas of melt impregnated, plagioclase-rich peridotites, that are cut by a network of replacive spinel dunite channels and gabbroic dikelets (Piccardo et al., 2004). Our ongoing field, microstructural and geochemical investigations allow: i) to evidence and document some main steps in the composite evolution of the Erro-Tobbio peridotite, which are characterized by significant melt-peridotite interaction, and ii) to reconstruct the time-space relationships between these melt-related events and the subsolidus tectonicmetamorphic evolution of the Erro-Tobbio peridotite. Pristine mantle protoliths are sporadically preserved in the Erro-Tobbio massif: they are moderately depleted lherzolites, showing complete recrystallization under spinel-facies conditions (T below 1100°C; P below 2.5 GPa, according to Hoogerduijn Strating et al., 1993), which has been related to the accretion of pristine asthenospheric masses to the mantle lithosphere. They have relatively Na-Al-rich clinopyroxenes and preserve structural relics of their previous evolution, i.e. rounded opx+sp clusters, suggesting spinel-facies breakdown of a precursor garnet, i.e. a pristine garnet-bearing protolith. These spinel-facies lithospheric peridotites were subsequently affected by extensional deformation which formed km-scale shear zones, consisting of spinel peridotite tectonites and mylonites (Hoogerduijn Strating et al., 1993). In the field, spinel tectonite peridotites are replaced by coarse granular spinel peridotites, which: i) show microstructural (i.e. pyroxene dissolution and olivine precipitation) and compositional features indicating their reactive origin, due to interaction with percolating pyroxene-undersaturated melts; ii) preserve microstructural features (i.e. the opx+sp clusters), which indicate that they were formed by almost complete recovering of pristine lithospheric peridotites. Available clinopyroxene trace element compositions suggest complete trace element equilibration with the percolating melts, consisting of depleted melt increments formed by 6% of fractional melting of a DMM asthenospheric mantle source. Both tectonite and coarse granular spinel peridotites are replaced by plagioclase-rich peridotites, showing rather sharp contacts with the spinel peridotites. Plagioclase peridotites show microtextural and compositional characteristics [i.e.: i) opx replacement on mantle olivine, ii) mm-size noritic pods and veins, iii) opx+plg coronas replacing mantle cpx] which indicate melt/peridotite interaction and interstitial crystallization of pervasively percolating melts, having orthopyroxene(-silica)-saturated, clinopyroxene-undersaturated characteristics. Plagioclase peridotites frequently preserve microstructural features (i.e. olivine coronas replacing pyroxenes) which indicate that the pre-impregnation spinel peridotites frequently were represented by reactive peridotites. Geochemical modeling indicates that melts which percolated and impregnated the Erro-Tobbio spinel peridotites had a strongly depleted signature: they, most probably, were formed as depleted melt increments by fractional melting and attained orthopyroxene(-silica)-saturation during reactive migration in the lithospheric mantle column. Presence of channels of replacive spinel dunites, cutting both spinel and plagioclase peridotites, indicates that: i) further upwelling melts were forced to migrate within focused channels where both ortho- and clinopyroxenes were completely dissolved by reaction with pyroxene-undersaturated melts, and ii) these high permeability channels allowed more “rapid” migration of melts. Frequently coarse granular dunites replace spinel peridotite mylonite bands, suggesting the close relationships between active deformation and focused melt migration. A subsequent compaction of the dunite channels squeezed out the migrating melts, forming cm-size gabbronoritic dikelets. Geochemical modeling indicates that melts which crystallized in the gabbroic dikelets have a strongly LREE depleted signature, and have been formed as depleted melt increments by fractional melting of a DMM asthenospheric mantle source. Subsequently, the Erro-Tobbio peridotite underwent a composite tectonic-metamorphic evolution under lowered pressure conditions, were intruded by shallow MORB gabbroic bodies and basaltic dikes and, finally, were exposed at the sea-floor of the Jurassic Ligurian Tethys basin. This composite scenario of tectonic-metamorphic and melt-related events suggests the the Erro-Tobbio peridotites, after their accretion to the thermal lithosphere, were progressively exhumed to shallower levels, during lithospheric extension. As a whole, field relationships between rocks recording melt-peridotite interaction and subsolidus tectonic- metamorphic evolution indicate that the melt percolation processes, forming reactive spinel peridotites, impregnated plagioclase peridotites and replacive dunites, postdate the early stages of extension, under spinel-facies conditions, of the lithospheric mantle. During extension, when the rising asthenosphere began to melt, the overlain extending mantle lithosphere were percolated by depleted melt fractions. Early reactive percolation formed spinel-facies reactive peridotites, subsequent interstitial crystallization at shallower conductive levels formed impregnate plagioclase peridotites, whereas further upward migration of depleted melt fractions was forced within high permeability channels of replacive dunites. Abundance of reactive spinel peridotites and impregnated plagioclase peridotites suggest that asthenosphere/lithosphere interaction by melt percolation modified the compositional and rheological characteristics of large sectors of the lithospheric mantle during exhumation related to pre-oceanic lithosphere extension. The thermal softening of the mantle lithosphere could be an important factor in the dynamics of the extensional system during transition from passive lithosphere stretching to active oceanic rifting (Ranalli et al., 2004).

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