Fe-Ti-Zr metasomatism in the oceanic mantle due to extreme differentiation of tholeiitic melts (Moa-Baracoa ophiolite, Cuba)

Abstract

Ti-rich amphibole, Mg-rich ilmenite, baddeleyite, zirconolite, srilankite, and zircon are important high-field-strength elements (HFSE) bearing phases in the Potosí chromitite bodies located in the Moho Transition Zone of the Cretaceous Moa-Baracoa suprasubduction zone ophiolite (eastern Cuba). Such HFSE-bearing phases were found in the interaction zone between gabbroic intrusions and chromitite pods. In addition to HFSE-bearing minerals, the studied samples are composed of Fe3+ and Ti-rich chromite, olivine (Fo86-90), clinopyroxene (En44-49), plagioclase (An51-56), orthopyroxene (En84-94), F-rich apatite, and Fe-Cu-Ni sulfides. The studied ilmenite hosting Zr oxides (baddeleyite, zirconolite, and srilankite) contains up to 13 wt.% MgO. The Potosí zirconolite is the first record of this mineral in ophiolitic chromitites and non-metamorphic ophiolite units, and it has relatively high REE contents (up to 10 wt.% of REE2O3) and the highest concentrations in Y2O3 (up to 11 wt.%) reported so far in zirconolite from terrestrial occurrences. Zircon is observed forming coronas surrounding ilmenite grains in contact with silicate minerals, and is characterized by very low U and Pb contents. The zircons formed after high temperature Zr diffusion in ilmenite (exsolution) and a subsequent reaction along grain boundaries following crystallization. Finally, U-Pb dating of baddeleyite exsolutions within ilmenite yielded an average age of 134.4 ± 14 Ma, which provides the first ever dating for a metasomatic event in Potosí that matches well (within uncertainty) the formation age of the oceanic crust of the eastern Cuba ophiolite. We propose that the occurrence of HFSE- and REE-bearing minerals in the Potosí chromite deposit is the result of a two stage process: first, water-rich and HFSE-rich residual melts are produced by intercumulus crystal fractionation after an evolved MORB (BABB)-like melt; and secondly, these residual melt fractions escaped the solidifying mush and extensively reacted and metasomatized the surrounding chromitites, crystallizing HFSE- and REE-bearing minerals and Fe-Cu-Ni sulfides.