The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm

he extensive study of a great number of deep- seated xenoliths from Tortonian tuff-breccia pipes in the Hyblean area (Sicily) revealed the following fundamental evidence: (1) typical continental crust rocks are completely absent in the entire xenolith suite; (2) mantle ultramafics are more abundant than gabbroids; (3) sheared oxide–gab- bros, closely resembling those from oceanic fracture zones, are relatively common; (4) secondary mineral assemblages, compatible with alteration processes in serpentinite-hosted hydrothermal systems, occur both in peridotites and gab- bros. Among the products of this hydrothermal activity, organic compounds, having abiotic origin via Fischer– Tropsch synthesis, occur in some hydrothermally altered gabbro and ultramafic xenoliths, as well as in hydrother- mal clays. Moreover, the U–Pb dating of hydrothermal zir- con grains, hosted in a xenolith of metasomatized tectonic breccia, indicated an Early–Middle Triassic age of the fos- sil hydrothermal system. Another line of evidence for the oceanic nature of the Hyblean–Pelagian basement is the complete absence of continental crust lithologies (granites, felsic metaigneous, and metasedimentary rocks) in out- crops and in boreholes, and the oceanic affinity of the Ter- tiary volcanic rocks from the Hyblean Plateau and the Sic- ily Channel (Pantelleria and Linosa Islands), which lack of any geochemical signature for continental crust contamina- tion. A reappraisal of existing geophysical data pointed out that serpentinites form the dominant lithologies in the lith- ospheric basement of the Hyblean–Pelagian area down to a mean depth of 19 km, which represents the regional Moho considered as the serpentinization front, marking the tran- sition from serpentinites to unaltered peridotites. On these grounds, we confirm that Hyblean xenoliths contain miner- alogical, compositional, and textural evidence for tectonic, magmatic, and hydrothermal processes indicating the exist- ence of fossil oceanic core complexes, in the geotectonic framework of the Paleo–Mesozoic, ultra-slow spreading, Ionian–Tethys Ocean forming the present Ionian–Hyblean– Pelagian domain.