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 +
plagioclase +/- 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-Al 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.