GEOCHEMICAL FEATURES OF EASTERN SICILY LITHOSPHERE AS PROBED BY HYBLEAN XENOLITHS AND LAVAS

Upper mantle and lower crust xenoliths of Quaternary basanitoid lavas and Miocene tuff-breccias of the Hyblean Volcanic Plateau (HVP), south eastern Sicily, have been studied for their REE contents and Sr-Nd iso tope composition. Mantle rocks mainly consist of spinel-harzburgites, spinel-websterites and monomineralic assemblages of Al-augite, Plagioc lase-bearing xenoliths are two-pyroxene mafic granulites and subordina te anorthosites. Pressure estimates point to a depth ranging from the lower crust down to the spinel-peridotite field (at least 45 km); temp erature varies between 800 and 1050 degrees C. Mantle xenoliths have L REE(n) > HREE(n), high Nd (0.51289-0.51299) and low Sr (0.70298-0.7033 4) isotopic ratios, suggesting the occurrence of LREE enrichment in re latively recent times. The possibility that contamination is induced i n the xenoliths by host lavas is discussed against the hypothesis of a Permo-Triassic age for the metasomatic event. Latter model is preferr ed due to the occurence of igneous activity in SE Sicily since Trias, to whole-rock T-DM(Nd) (200-270 Ma) and to the linear correlation of m antle samples in the Sm/Nd vs. Nd-143/Nd-144 diagram pointing to an '' age'' of 170 +/- 110 Ma. In this case, initial epsilon(Nd) = 6.8 +/- 0 .9 denotes an asthenospheric origin for the metasomatic agent. Plagioc lase-bearing xenoliths have Nd and Sr isotope ratios ranging between 0 .51297-0.51288 and 0.70278-0.70292, respectively. In these lower crust mafic granulites, Sm/Nd whole-rock and mineral pairs yield ''ages'' r anging between 370 +/- 110 and 190 +/- 40 Ma. This suggests that, sinc e Hercynian time, products of subalkaline and alkaline affinity have b een adding to the crust and merges the evolution of the lower crust of SE Sicily to the cycle of igneous activity linked to the extensional tectonics affecting the northern margin of the African Plate. Sr-Nd is otopic and trace element data on Plio-Quaternary primitive lavas from the HVP provide a further sampling of the upper mantle of southeastern Sicily. Nd-Sr isotopic data of tholeiitic (Nd-143/Nd-144 = 0.51315-0. 51306; Sr-87/Sr-86 = 0.70273-0.70285) and alkaline rocks (Nd-143/Nd-14 4 = 0.51299-0.51293; Sr-87/Sr-86 = 0.70302-0.70307) suggest that the g enesis of HVP lavas is controlled by both a component akin to the sour ce of MORB and a relatively enriched source similar to mantle xenolith s. A model is proposed in which both sources are present in the Hyblea n lithosphere due to veining of the Thermal Boundary Layer.