Rb-Sr and U-Th-Pb systematics of alkaline rocks: the alkaline rocks from Italy

The isotopic composition of Pb and Sr and the abundance of Rb, Sr, U, Th, and Pb were determined for whole rock samples from all major volcanic centres of the Genozoic alkaline volcanism of Central and South Italy, together with some samples from the contemporaneous anatectic Tuscan volcanism. The Sr and Pb isotopic compositions of the alkaline rocks show a negative correlation combined with a regional trend: the 87Sr/86Sr ratios decrease from 0.711 in the north-west to 0.704 in the south-east, while the 206Pb/204Pb ratios increase from 18.7 to 20.0. Variations in both isotopic compositions are generally small throughout erupted rock sequences for any volcanic centre. The Pb and Sr isotopic abundance variations are interpreted on the basis of two alternative models, which correspond to two groups of geological processes: variations can result (i) from a time dependent development in subsystems with different Rb/Sr or U(Th)/Pb ratios or, (ii) from mixing of Sr or Pb with different isotopic compositions. Combining both Pb and Sr isotope abundance measurements it is shown that the source of each volcanic centre is formed by various degrees of mixing between two components. One component and the most southern Tuscan anatectic rocks most likely have a common source, whereas the other component of the mixing process is suggested to be a liquid fraction derived frm a small degree of partial fusion of a hydrous mantle. Thus at least a two-stage evolution of the Italian alkaline rocks is indicated: first a mixing process leading to the formation of the parental material followed by differentiation processes leading to the formation of the erupted rock sequences. The geodynamic model which explains the data best is that of a lateral inhomogeneous mantle. (author)