Mg- and Fe-gabbroids from Northern Apennine ophiolites: Parental liquids and igneous differentiation process

This paper deals with a sequence of olivine cumulates, troctolites, olivine-bearing gabbros, gabbronorites, Fe/Ti-oxide diorites and amphibole-bearing albitites from Northern Apennine ophiolites. Mineral and whole-rock compositions point to significant chemical covariations. In particular, the variation trend of anorthite content of plagioclase versus the Fe/(Fe+Mg) ratio in clinopyroxene closely fits with that of present-day oceanic gabbroids from Mid Cayman Rise and from South West Indian Ridge. Trace element compositions of representative minerals were determined by ion microprobe. The tract element compositions of parental liquids, calculated applying experimentally determined partition coefficients to clinopyroxenes of Mg-rich rocks (Mg-gabbroids), closely resemble those of present-day N-MORB. We thus suggest that the gabbroic complex from Northern Apennine ophiolites formed in a slow-spreading ridge. The petrogenesis of Fe-rich rocks (Fe-gabbroids) is not completely established. The major element variation trends are indicative of a continuous differentiation process, controlled by fractional crystallization, from olivine cumulates to highly evolved amphibole-bearing albitites. However, the variations in La-N/Sm-N of clinopyroxenes from Mg-gabbroids to Fe-gabbroids suggest that some other differentiation process than simple fractional crystallization was involved. The parental liquids of Fe-gabbroids were most likely nor basaltic and characterized by a relatively high grade of polymerization. Realistic REE liquid compositions were obtained applying mineral/liquid partition coefficients for evolved liquids (dacitic to andesitic compositions) to clinopyroxene and apatite. Textural and microchemical investigations evidence for the occurrence of Ti-rich igneous amphibole in both Mg- and Fe-gabbroids. Its modal proportions increase with the differentiation degree of the rock, thus indicating that amphibole origin was related to progressive volatile enrichment in the residual liquids during the igneous differentiation.