Tracing metamorphism, magmatism and tectonics in the southern Alps (Italy): constraints from Rb-Sr and Pb-Pb geochronology, and isotope geochemistry.

The Variscan metamorphic basement of the westernmost part of the Southern Alps consists of the Ivrea-Verbano Zone and Serie dei Laghi separated by the Cossato-Mergozzo-Brissago Line, a subvertical, late-Variscan ductile shear zone. The paper deals with the complex history of magmatism, deformation, and metamorphism during pre-Alpine and Alpine evolution of the Serie dei Laghi, a metasedimentary sequence hosting older and younger granites. Rb-Sr whole-rock (WR) isochron (466±5 Ma) and Pb-Pb single zircon evaporation ages (458±6 Ma and 463±4 Ma) on metagranites date the emplacement of the older intrusive series, whereas Rb-Sr muscovite ages (311-325 Ma) approach the Carboniferous metamorphism (331-340 Ma). Rb-Sr WR isochrons (277±8 Ma) and biotite ages (276-281 Ma) on granitic plutons date the emplacement of the younger intrusive series. However, both intrusive series have some rejuvenated biotite ages (up to 170 Ma). Such ages suggest opening of the Sr isotope system, though the timing of this event cannot be determined precisely via Rb-Sr systematics. The metasediments reached incomplete whole- rock Sr isotopic homogenisation at 288±99 Ma, very close to the age of the Permian event recorded by the granites. Ordovician U-Pb zircon ages have been ascribed to infiltration of residual granitic melts. Moreover, zircons of 1003±2 Ma reflect the presence of old detrital components. Finally, most of the intrusive, metaintrusive, and metasedimentary rocks plot on well-defined linear correlations in both the initial Pb-Pb and the 206Pb/ 204 Pb(i) vs. 238U/204Pb diagrams. These correlations, as well as 238U/204Pb ratios far higher than those reported for crustal rocks, are interpreted as due to recent opening of the Pb isotope system, which caused either an increase in the U/Pb ratios and/or a rough homogenisation of the Pb isotopes. However, this process affected the Sr isotope system only at the mineral scale (post-Variscan biotite ages), while whole-rock isochrons are still preserved in both metaintrusive and intrusive rocks. An estimated age of 26±10 Ma is obtained by applying an "inverse" approach to the Pb data. Such a widespread process, which involved most of the igneous, metaigneous, and metasedimentary rocks of the Variscan belt, implies a large-scale mobilisation of fluids that could be attained only through the reactivation of the many pre-Alpine faults dissecting the Serie dei Laghi.