Abstract

A Ba-Ti rich oxymica occurs in an olivine nephelinite from S. Demetrio High in the northern margin of the Hyblean Plateau (Sicily, Italy). The rock sample exhibits a porphyritic texture formed by olivine, clinopyroxene, nepheline, titanian magnetite, apatite, and rare subhedral tabular mica crystals, which have a perfect cleavage on {001}, and a strong pleochroism with X pale brown and Y≈Z brown. The studied oxykinoshitalite, characterized by micro-Raman spectroscopy, electron microprobe WDS (wavelength-dispersive system), and secondary-ion mass spectrometry (SIMS), was compared with the type material from the Fernando de Noronha island (Pernambuco, Brazil). Structural formula of the Hyblean oxykinoshitalite, calculated on the basis of 7 (Si, Al, Fe, Mg, Ti), is (Ba0.51K0.41Na0.04Ca0.01)∑0.97(Mg1.98Fe0.55Ti0.48)∑3.01(Si2.42Al1.56)∑3.98 O10[O1.17F0.62(OH)0.21]∑2.00. The lack of chemical zoning and the enrichment in Zr and Nb in the groundmass crystals of the Hyblean oxykinoshitalite suggest formation during the final crystallization stage of a basaltoid magma with ocean island basalt (OIB) affinity. Most likely, the magma originated by partial melting of metasomatized ultramafic rocks in the Hyblean crustal basement.

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