MELT IMPREGNATION BY STRONGLY DEPLETED MORB MELTS IN THE OPHIOLITIC PERIDOTITES FROM THE LIGURIAN TETHYS (ALPS, APENNINES AND CORSICA)

Melt impregnation and peridotite-melt interaction are ubiquitous phenomena in the oceanic mantle: they are documented in peridotites from present oceans and ophiolite massifs. Peridotites from Western Alps, Northern Apennines and Corsica ophiolites, which derive from the Jurassic Ligurian Tethys, show evidence of impregnation (i.e. plagioclase blebs and/or veins along grain boundaries and crosscutting mantle minerals), melt/rock interaction (i.e. orthopyroxene + plagioclase intergrowth replacing mantle clinopyroxenes) and intrusion (i.e. gabbroic dykelets and mafic-ultramafic cumulate pods) by percolating melts.

Impregnating plagioclase shows unusual compositions, i.e. exceptionally low Sr (<20 ppm) and LREE-depleted REE spectra (CeN/SmN < 0.20): this LREE- and Sr-depleted signature is significantly different relative to the LREE-enriched, high-Sr composition of plagioclase in equilibrium with an average MORB.

Mafic-ultramafic cumulates and gabbroic dykelets at Mt. Maggiore (Corsica) are characterized by: 1) clinopyroxenes, with flat REE patterns, from HREE to MREE, at 10xC1, strong negative LREE fractionation (CeN/SmN=0.04-0.08, LaN 0.1) and very low Sr (1.06-2.77 ppm) and Zr (2.54-8.57 ppm) contents; 2) plagioclases, with low REE concentrations, extremely low Sr (20.4-30.5 ppm) and Ba (<0.2 ppm) contents, and REE patterns with a strong negative LREE fractionation (LaN/SmN=0.03-0.43, LaN=0.05). These minerals show geochemical signature strongly depleted in the most incompatible elements: the impregnating melts are characterized, accordingly, by a strongly depleted geochemical signature. Computed liquids in equilibrium with clinopyroxenes of these cumulates show REE patterns strongly fractionated for the LREE (CeN/SmN=0.17, CeN/YbN=0.17, LaN=1.49), relative to any erupted MORB.

Geochemical modelling indicates that the impregnating liquids probably consisted of unmixed depleted melt increments produced by 6-7% fractional melting. Studied cumulates are similar to the unique cumulate suite from strongly depleted MOR basalts from MAR (DSDP Site 334). Ultra-depleted melts (UDM) have been also found as melt inclusions in olivine phenocrysts from oceanic tholeiites, and interpreted as instantaneous melt increments of a continuous melting process.