The Pb isotopic systematics during crustal contamination of subcrustal magmas: the Hercynian magmatism in the Serie dei Laghi (Southern Alps, Italy)

Abstract

The Pb isotopic composition and Pb, U and Th concentrations of a series of Hercynian granite plutons and mafic-intermediate stocks and dykes belonging to the Serie dei Laghi (Western Southern Alps) have been analyzed. The initial Pb isotope ratios of both the granites and the dykes are typical of the lower crust, and are spread over a significant range [( 206 Pb 204 Pb ) 280=17.883–18.530 , ( 207 Pb 204 Pb ) 280=15.597–15.710 and ( 208 Pb 204 Pb ) 280=38.077–38.708 ]. Four dykes, all at the same locality, display evidence of post-emplacement fluid circulation, with mobilization of U and Pb. In spite of the Pb isotopic crustal signature of the studied rock series, the 87 Sr 86 Sr ratios reach values as low as 0.704 in the dykes and as low as 0.707 in the granites, suggesting a mantle contribution in the genesis of both lithologies. Indeed, when compared with the metasediments from the same area, the magmatic rocks have somewhat lower initial 207 Pb 204 Pb ratios, which weakly trend towards the less radiogenic Pb isotopic composition of the mantle. The relationships between the Pb isotopic composition and the Pb content, along with the Pb and Sr isotopic covariation, suggest that the suite under examination was generated from a mantle-derived primary magma that underwent extensive crustal contamination. A primary magma similar to the neighbouring Ivrea mafic complex has been inferred. Further assimilation of crustal material is particularly evident in the dykes, while the granites appear to have been generated mainly by crystal fractionation processes from the intermediate magma, already contaminated.