Role of melting process and melt–rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)

This study reports a geochemical investigation of two thick basalt sequences, exposed in the Bracco–Levanto ophiolite (northern Apennine, Italy) and in the Balagne ophiolite (central-northern Corsica, France). These ophiolites are considered to represent an oceanward and a continent-near paleogeographic domain of the Jurassic Liguria–Piedmont basin. Trace elements and Nd isotopic compositions were examined to obtain information about: (1) mantle source and melting process and (2) melt–rock reactions during basalt ascent. Whole-rock analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb, and Ta with respect to the Bracco–Levanto counterparts. These variations are paralleled by clinopyroxene chemistry. In particular, clinopyroxene from the Balagne basalts has higher CeN/SmN (0.4–0.3 vs. 0.2) and ZrN/YN (0.9–0.6 vs. 0.4–0.3) than that from the Bracco–Levanto basalts. The basalts from the two ophiolites have homogeneous initial Nd isotopic compositions (initial εNd from + 8.8 to + 8.6), within typical depleted mantle values, thereby excluding an origin from a lithospheric mantle source. These data also reject the involvement of contaminant crustal material, as associated continent-derived clastic sediments and radiolarian cherts have a highly radiogenic Nd isotopic fingerprint (εNd at the time of basalt formation = − 5.5 and − 5.2, respectively). We propose that the Bracco–Levanto and the Balagne basalts formed by partial melts of a depleted mantle source, most likely containing a garnet-bearing enriched component. The decoupling between incompatible elements and Nd isotopic signature can be explained either by different degrees of partial melting of a similar asthenospheric source or by reaction of the ascending melts with a lower crustal crystal mush. Both hypotheses are reconcilable with the formation of these two basalt sequences in different domains of a nascent oceanic basin.