PETROLOGY OF GABBROIC TO PLAGIOGRANITIC ROCKS FROM SOUTHERN TUSCANY (ITALY): EVIDENCE FOR MAGMATIC DIFFERENTATION IN AN OPHIOLITIC SEQUENCE

Abstract

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).