Crustal xenoliths from the Hyblean Miocenic diatremes (Sicily, Italy): inference on the nature of the underneath crust.

Miocenic basaltic diatreme pipes from the Hyblean Plateau (southeastern Sicily) carry a large number of crust-derived xenoliths. They include gabbroic (SiO2=36-51 wt%, MgO=4-14 wt%) and dioritic (SiO2=49-57 wt%, MgO=2-4 wt%) rocks. On textural basis, the gabbro xenoliths can be subdivided into two groups: (a) xenoliths with still preserved igneous-like cumulitic texture defined by euhedral plagioclase (An57-82), + clinopyroxene (mg#=70-80, TiO2=0.4-4 wt%, Al2O3=2-6 wt%) + Fe-Ti oxides ± apatite ± Fe-rich green spinel. The Fe-Ti oxides occur as (I) rare inclusions in the silicate phases, as (II) isolated and interstitial grains (2-5%), or, most commonly, (III) they form opaque patches (8-16%) infiltrated through the grain matrix of the rocks in places along shear zones: a correlation between deformation and abundance of Fe-Ti oxide minerals are observed. (b) Gabbroic granulites show granoblastic texture defined by plagioclase (An40-75) + clinopyroxene (mg#=71-79, TiO2=0.4-1.5 wt%, Al2O3=4-9 wt%) + green spinel (Al2O3=46-65 wt%, FeO=15-39 wt%) ± orthopyroxene (mg#=65-68, Al2O3=2-6.5 wt%) ± Fe-Ti oxides. The diorite xenoliths are coarse- to medium-grained leucocratic rocks consisting of plagioclase (An19-35) + clinopyroxene (mg#=70-73, TiO2=0.4-0.7 wt%, Al2O3=2-4 wt%) ± Fe-Ti oxides. Albite and K-feldspar micro-veins occur within plagioclase. In both cumulitic gabbros and diorites secondary phases such as chlorites, zeolites, smectites, Fe-oxide/hydroxide and carbonates are observed. Trace element patterns are characterised by Eu and Sr positive anomaly, Ba and Rb enrichment over the LREE and the LREE/HREE ratio always >1 (LaN/YbN=2-20). Such features are likely due to their enriched source and/or to hydrothermal alteration. The comparison with gabbros from different geodynamic environments (e.g., oceanic crust: SWIR; ocean-continent transition: Platta; continental crust: Calabria) indicates that the Hyblean xenoliths match some geochemical characteristics of both continental and oceanic gabbros. Nevertheless, some features as the Fe-Ti impregnation in many xenoliths, which are similar to that shown by some present day abyssal gabbros, the alteration phases and the lack of felsic crystalline rocks among the xenoliths suggest that the Hyblean crustal basement has most likely an oceanic or an ocean-continent transition nature.