Petrology of Gabbroic to Plagiogranitic Rocks from Southern Tuscany (Italy): Evidente for Magmatic Differentiation in an Ophiolitic Sequence

This work deals with the petrogenesis of a composite ophiolite body composed of gabbroic to plagiogranitic rocks. In particular, the sequence consists of evolved gabbros, Fe-gabbros and Fe-diorites crosscut by ferrobasaltic dikes and intruded by dikelets and lenses of plagiogranites, mainly represented by diorites and trondhjemites. Complex relationships among the different lithotypes indicate multiple melt injections into both crystal-mush and solidified rocks. Whole-rock compositions show continuously increasing FeO/(FeO + MgO) and increasing REE, Th, Nb, Y, LaN/SmN and Eu depletion from gabbros to plagiogranites. Major and trace element variations and modelling show that the whole intrusive sequence originated from the differentiation of a ferrobasaltic magma reservoir. Ferrobasaltic liquid compositions are compatible with fractional crystallization of N-MOR type basalts through olivine + plagioclase + clinopyroxene removal, whereas the evolved gabbros represent clinopyroxene + plagioclase cumulates from ferrobasaltic liquids with small amounts of trapped melt. Fegabbroic to Fe-dioritic rocks, characterized by abundant Fe-Ti oxides + apatite as liquidus phases, likely represent cumulates from evolved liquids, possibly segregated from the gabbroic crystal-mush. The attainment of Fe-Ti oxide saturation promoted silica enrichment yielding residual liquids of dioritic composition which were segregated and injected into fractures of solidified gabbroic rocks or infiltrated into the Fe-gabbroic crystal-mush. Locally, silica-rich and slightly LREE-enriched liquids of trondhjemite composition were produced through fractionation of plagioclase + clinopyroxene + amphibole + Fe-Ti oxides (+ apatite + zircon + allanite).