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Studies on Alpine-Apennine ophiolitic peridotites (i.e. Lanzo, Western Alps; Erro-Tobbio, Ligurian Alps; External Ligurides, Northern Apennines) allow to unravel the mantle processes which accompanied rifting of the continental lithosphere, formation of non volcanic margins and opening of the Jurassic Ligurian Tethys oceanic basin.
Ophiolite stratigraphy, peridotite petrology and palaeogeographic restorations evidence that some of these peridotite bodies were exhumed from the subcontinental mantle and exposed at the sea-floor of the ancient basin along ocean-continent transition (OCT) zones. These peridotites show: i) pre-rift equilibration under continental geothermal conditions (i.e. spinel-peridotite facies assemblages at T of about 1000°C); ii) syn-rift deformation-recrystallization under decreasing T-P conditions (i.e. plagioclase- and amphibole-chlorite-peridotite facies conditions), before diffuse near sea-floor serpentinization.
The more proximal (i.e. continent-ward) peridotite massifs, which were exposed at the sea-floor of the basin close to the continental margins, mostly record the subsolidus tectonic-metamorphic evolution related to their exhumation. The more distal (i.e. ocean-ward) peridotite massifs show increasing effects of diffuse and focused percolation of melts, as evidenced by formation of km-size bodies of reactive spinel harzburgites, impregnated plagioclase peridotites and m- to decam-scale replacive harzburgite-dunite channels.
Appearance of asthenospheric melts which percolated the mantle lithosphere evidences that the underlying asthenosphere began melting under adiabatic decompression. The relationships between lithosphere stretching and asthenosphere partial melting are clearly evidenced by field, structural and petrologic features. In fact, strongly deformed spinel-plagioclase tectonite-mylonite peridotites which were formed during ongoing exhumation of mantle lithosphere were transformed to coarse-granular reactive and impregnated peridotites by melt-rock interaction with MORB-type asthenospheric melts. The occurrence at OCT settings of large exposures of subcontinental mantle which escaped both partial melting and melt-rock interaction processes indicates that: i) the early stages of continental extension were not simply "non-volcanic" but most probably "a-magmatic", in the sense that melting conditions (i.e. significant adiabatic decompression) in the underlying asthenosphere were not jet reached, and, conversely, that ii) asthenosphere partial melting started when the overlying lithosphere was significantly thinned and stretched to allow appropriate adiabatic decompression of the mantle asthenosphere.
Diffuse melt percolation and melt-peridotite interaction during extension of the continental lithosphere caused significant melt entrapment and storage in the extending continental mantle enhancing the "non-volcanic" (but not "a-magmatic") nature of the ocean-continent transition zones of the passive margins.
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