Role of mantle source heterogeneity and melt/rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)

This study reports a geochemical investigation of two thick basalt lava 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 a Jurassic slow or ultraslow spreading basin. Trace elements and Nd isotopic compositions were examined to achieve information about: (i) mantle source materials, and (ii) melt-rock reactions during basalt ascent. Whole-rock and clinopyroxene analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb and Ta with respect to the Bracco-Levanto counterparts. For instance, clinopyroxene from the Balagne basalts has higher CeN/SmN and ZrN/YN than clinopyroxene from the Bracco-Levanto basalts. The basalts from the two ophiolites conversely have homogeneous initial Nd isotopic compositions, within typical depleted mantle values. We propose that the Bracco-Levanto and the Balagne basalts formed by partial melts of a mantle source containing a garnet-bearing enriched component. Similar conclusions were reported for basalts from other ophiolites of the Alpine-Apennine belt (e.g., Montanini et al., 2008). The decoupling between incompatible elements and Nd isotopic signature is explained by reaction of ascending melts with a lower crustal crystal mush. Montanini A., Tribuzio R., Vernia L. (2008). Petrogenesis of basalts and gabbros from an ancient continent-ocean transition (External Liguride ophiolites, Northern Italy). Lithos, 101, 453-479.