Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif – Western Alps)

摘要: The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean-continent transition context and eclogitized during alpine collision. Despite the subduction history, has preserved its sedimentary cover, suggesting that it may have structure. It exceptional case for studying evolution from oceanization to then exhumation. We present field petrological study retracing different serpentinization episodes their impact on composed slightly serpentinized peridotites (<20% serpentinization) surrounded by envelope foliated serpentinites (100% bordered metabasalts metasedimentary rocks. limit between defines front serpentinization. This sharp: marked presence massive (80% and, locally, dykes metagabbros mylonitic gabbros. deformation these gabbros contemporaneous with emplacement magma. early lizardite testifies began oceanization, which confirmed meta-ophicarbonates bordering serpentinite envelope. Two additional generations serpentine occur ultramafic first prograde antigorite partially replaced relict primary minerals subduction, indicating active process at ridge zone. Locally, this episode followed deserpentinization peak P-T (estimated 2-2.5 GPa 550-620 °C): crystallization secondary olivine associated chlorite and/or clinopyroxene, amphibole assemblages. A second formed exhumation completely obliterating previous textures serpentinites. Serpentinites are important component slow ultraslow spreading settings, there gradient depth upper mantle. seismic Moho could correspond affecting zone constitutes less competent level where, exhumation, fluid circulation localized. In zone, reaction kinetics increased later steps obliterate evidence progressive massif, fully recrystallized into Thus, represents crust as defined geophysicists, sharp corresponds palaeo-Moho.