THE PB ISOTOPIC SYSTEMATICS DURING CRUSTAL CONTAMINATION OF SUBCRUSTAL MAGMAS - THE HERCYNIAN MAGMATISM IN THE SERIE DEI LAGHI (SOUTHERN ALPS, ITALY)

The Pb isotopic composition and Pb, U and Th concentrations of a serie s of Hercynian granite plutons and mafic-intermediate stocks and dykes belonging to the Serie dei Laghi (Western Southern Alps) have been an alyzed. The initial Pb isotope ratios of both the granites and the dyk es are typical of the lower crust, and are spread over a significant r ange [(Pb-206/Pb-204)(280)=17.883-18.530, (Pb-207/Pb-204)(280)=15.597- 15.710 and (Pb-208/Pb-204)(280)=38.077-38.708]. Four dykes, all at the same locality, display evidence of postemplacement fluid circulation, with mobilization of U and Pb. In spite of the Pb isotopic crustal si gnature of the studied rock series, the Sr-87/Sr-86 ratios reach value s as low as 0.704 in the dykes and as low as 0.707 in the granites, su ggesting a mantle contribution in the genesis of both lithologies. Ind eed, when compared with the metasediments from the same area, the magm atic rocks have somewhat lower initial Pb-207/Pb-204 ratios, which wea kly trend towards the less radiogenic Pb isotopic composition of the m antle. The relationships between the Pb isotopic composition and the P b content, along with the Pb and Sr isotopic covariation, suggest that the suite under examination was generated from a mantle-derived prima ry magma that underwent extensive crustal contamination. A primary mag ma similar to the neighbouring Ivrea mafic complex has been inferred. Further assimilation of crustal material is particularly evident in th e dykes, while the granites appear to have been generated mainly by cr ystal fractionation processes from the intermediate magma, already con taminated.