Gabbroic xenoliths in tuff-breccia pipes from Hyblean Plateau: insights into the nature and composition of the lower crust underneath South-eastern Sicily, Italy

Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase, clinopyroxene, Fe –Ti oxides, apatite, amphibole, Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene, Fe – Ti oxides, orthopyroxene) and mafic granulites (plagioclase, clinopyroxene, green spinel, orthopyroxene, Fe –Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe –Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well- known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.