Gabbroic xenoliths in tuff-breccia pipes from the 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 in three groups: gabbros (plagioclaseclinopyroxeneFe–Ti oxides +/- apatite +/- amphibole +/- Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene +/- Fe–Ti oxides +/- orthopyroxene) and mafic granulites (plagioclaseclinopyroxenegreen 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 process. Incompatible element ratios (Zr=Nb¼ 23 5–26; Y=Nb¼1.4–11) 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.