THE ISLAND OF PANTELLERIA - A CASE FOR THE DEVELOPMENT OF DMM-HIMU ISOTOPIC COMPOSITIONS IN A LONG-LIVED EXTENSIONAL SETTING

The Sr, Nd and Pb isotopic and trace element compositions of basalts a nd pantellerites from the island of Pantelleria (Strait of Sicily) hav e been used to constrain the evolution of these magmas and their sourc es. All Pantelleria products have isotopic compositions that plot in S r-Pb, Nd-Pb and Sr-Nd space between DMM and HIMU oceanic mantle source s. The Pantelleria basalts have variable Ce/Pb, Nb/U and Th/U, and som e have elevated Pb-207/Pb-204 and Pb-208/Pb-204, which is indicative o f the addition of an older component that resembles old enriched litho sphere sampled by potassic magmas in the circum-Tyrrhenian Sea. It is proposed that the relatively homogeneous pantellerites derive their Ce /Pb, Nb/U, and GIB-like Sr, Nd and Pb isotopic compositions from a wel l-mixed crustal magma chamber supplied by heterogeneous basalts. We pr opose further that the radiogenic Pb-206/Pb-204 and Nd-143/Nd-144, cou pled with the unradiogenic Sr isotopic composition of some of the Pant elleria basalts, are inherited in a process involving chemical modific ation of the thinned lithospheric crust-mantle source by addition of b asaltic (MORB) materials, a process that may have occurred intermitten tly at least since the Permo-Triassic. This process is responsible for lowering the Pb concentration and changing the U/Pb and Th/Pb ratios of portions of the lithospheric crust-mantle boundary. It is envisaged that the increase in mu and omega of the modified lithospheric mantle was caused by the addition of phases such as clinopyroxene and amphib ole, crystallized in equilibrium with MORB. After chemical exchange wi th the asthenospheric mantle ceased during the Hercynian orogeny, this young (or rejuvenated) lithosphere evolved a radiogenic Pb isotopic c omposition. It is proposed that the isotopic differences observed betw een the volcanic rocks erupted in the Strait of Sicily and those erupt ed in the southern margin of the Tyrrhenian abyssal plain are primaril y the result of mixing between magmas derived from a variably modified lithospheric source, some of it with GIB-type isotopic characteristic s, that were developed by intermittent interaction with asthenospheric melts in a geodynamic regime dominated by extension.