Upper mantle and lower crust xenoliths of Quaternary basanitoid lavas and Miocene tuff-breccias of the Hyblean Volcanic Plateau (HVP), southeastern Sicily, have been studied for their REE contents and Sr-Nd isotope composition. Mantle rocks mainly consist of spinel-harzburgites, spinel-websterites and monomineralic assemblages of Al-augite. Plagioclase-bearing xenoliths are two-pyroxene mafic granulites and subordinate anorthosites. Pressure estimates point to a depth ranging from the lower crust down to the spinel-peridotite field (at least 45 km); temperature varies between 800 and 1050°C. Mantle xenoliths have LREEn > HREEn, high Nd (0.51289-0.51299) and low Sr (0.70298-0.70334) isotopic ratios, suggesting the occurrence of LREE enrichment in relatively recent times. The possibility that contamination is induced in the xenoliths by host lavas is discussed against the hypothesis of a Permo-Triassic age for the metasomatic event. Latter model is preferred due to the occurence of igneous activity in SE Sicily since Trias, to whole-rock TDM (200-270 Ma) and to the linear correlation of mantle samples in the Sm/Nd vs. 143Nd/144Nd diagram pointing to an "age" of 170 ± 110 Ma. In this case, initial 8Nd = 6.8 ± 0.9 denotes an asthenospheric origin for the metasomatic agent. Plagioclase-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" ranging between 370 ± 110 and 190 ± 40 Ma. This suggests that, since Hercynian time, products of subalkaline and alkaline affinity have been 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 isotopic 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 (143Nd/144Nd = 0.51315- 0.51306; 87Sr/86Sr = 0.70273-0.70285) and alkaline rocks (143Nd/144Nd = 0.51299-0.51293; ^ S Γ Λ S Γ = 0.70302- 0.70307) suggest that the genesis of HVP lavas is controlled by both a component akin to the source of MORB and a relatively enriched source similar to mantle xenoliths. A model is proposed in which both sources are present in the Hyblean lithosphere due to veining of the Thermal Boundary Layer.
|Titolo:||Geochemical features of eastern Sicily lithosphere as probed by hyblean xenoliths and lavas|
|Data di pubblicazione:||1996|
|Appare nelle tipologie:||1.1 Articolo in rivista|