Pre-Oceanic History of An Alpine Peridotite: The Lanzo Massif of the Western Alps (NW Italy)

The Lanzo Peridotite Massif belongs to the internal part of the High-Pressure, Low-Temperature metamorphic belt of the Western Alps (NW Italy) and it is divided in Southern (~ 55km2), Central (~ 90km2) and Northern (~ 5km2) bodies.

After the advent of Plate Tectonics it was considered (e.g. Nicolas, 1986) an asthenospheric diapir that was emplaced at shallow levels in the early Mesozoic, during opening of the Ligurian-Piemontese oceanic basin. Recent investigations (e.g. Piccardo et al., 2007) document that the Lanzo peridotites derived from the sub-continental lithospheric mantle of the Europe-Adria system. They were exhumed towards shallow levels during the passive lithospheric extension driving pre-oceanic rifting of the Jurassic Ligurian-Piemontese basin. They were strongly modified by melt-rock interaction operated by MORB-type asthenospheric melts which percolated the extending mantle lithosphere during exhumation.

The North Lanzo protoliths were located at shallower lithospheric levels than the South Lanzo protoliths with respect to the axial zone of the extensional system. During lithosphere extension, early fractional MORB melts from the asthenosphere infiltrated the South Lanzo lithospheric peridotites, causing widespread melt/rock interaction and melt impregnation. Subsequently, aggregate MORB melts migrated through the South Lanzo peridotites within replacive harzburgite/dunite channels and impregnated/refertilized the shallower North Lanzo peridotites.

Following lithosphere stretching and break-up of the continental crust, the Lanzo peridotites were exposed at the sea-floor. The North Lanzo peridotites, deriving from shallower, strongly sheared, lithospheric mantle levels, were exposed earlier and located at more External (continent-ward) Ocean-Continent Transition (OCT) zones of the basin, whereas the South Lanzo peridotites, deriving from deeper, strongly modified lithospheric mantle levels, were exposed later and located at more Internal (ocean-ward) Oceanic (MIO) settings of the basin.

Present knowledge on the Lanzo peridotites constraints from a mantle perspective the geodynamic evolution of the Europe-Adria extensional system during pre-oceanic rifting in the Ligurian-Piemontese system.