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The Lanzo peridotites (Western Italian Alps) were involved in the geodynamic processes related to extension of the Europe-Adria continental lithosphere which led to rifting and opening of the Jurassic Western Tethys basin. They record both tectonic-metamorphic evolution, caused by their exhumation from the mantle lithosphere, and melt-rock interaction, caused by the upward porous flow migration of asthenospheric melts.
South-Central Lanzo peridotites: (i) underwent reactive percolation of fractional MORB-type melts at spinel(sp)-peridotite facies conditions, (ii) were impregnated by fractional MORB-type melts at plagioclase(plg)-peridotite facies conditions, and (iii) underwent focused percolation of aggregate MORB melts and, sporadically, alkaline melts.
North Lanzo peridotites: (i) were exhumed from sp-facies conditions, (ii) were locally impregnated at plg-facies conditions by aggregate MORB melts, and (iii) underwent focused percolation of alkaline melts. Accordingly, MORB-type fractional melts infiltrated and modified the South-Central Lanzo peridotite protoliths, forming both depleted and refertilized peridotites, while the North Lanzo peridotite protoliths escaped these melt-peridotite interaction events. The North Lanzo peridotites were locally infiltrated by aggregate MORB melts which were related to the melts migrating through the South-Central Lanzo peridotites within replacive harzbugite-dunite channels. This fact favours the interpretation that the Nord Lanzo peridotite protoliths were located at shallower lithospheric levels than the South-Central Lanzo peridotite protoliths, at the axial zone of the extensional system above the melting asthenosphere. Accordingly, North Lanzo peridotites escaped diffuse percolation/interaction of fractional MORB-type melts, which is recorded by the South-Central Lanzo peridotites, and were solely infiltrated by later aggregate MORBs.
Subsequent alkaline melts were formed by partial melting of garnet-pyroxenites-bearing sections of the lower lithosphere which were delaminated and sank into the melting asthenosphere.
Further lithosphere extension and stretching by means of trans-lithospheric shear zones caused the last exhumation of the Lanzo peridotites, the breakup of the continental crust and the peridotite exposure at the sea-floor. North Lanzo peridotites, deriving from shallow mantle sectors, were exposed at the sea-floor at more External Ocean-Continent Transition (OCT) zones of the basin, close to the Adria continental margin, whereas South Lanzo peridotites were exposed at more Internal Oceanic (MIO) settings of the basin. Accordingly, the petrologic characteristics of mantle peridotites along the section from the North through the Central to the South Lanzo bodies unravel the geodynamic evolution of the lithosphere/asthenosphere system during rifting from a-magmatic lithosphere extension to asthenosphere decompressional melting to lithosphere percolation of the asthenospheric melts.
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