Rb/Sr geochronology of Lower Permian plutonism in Massiccio dei Laghi, southern Alps (NW Italy)

Based on Rb/Sr whole-rock isochrons and Rb/Sr biotite determinations, the emplacement age of the Montorfano, Baveno-Mottarone and Pella intrusives is 275-280Ma. The Quarna, Roccapietra granites and Sacromonte di Brissago, Pedemonte, M. Zuccaro, Quarna and Valsesia calcalkaline intermediate-mafic rocks, which lie along or near the Cossato-Mergozzo-Brissago (CMB) lineament, give significantly younger Rb/Sr biotite ages. These biotite dates are considered a result of the post-Permian reactivation of this fault. The initial Sr isotopic ratios of the granitic bodies, around 0.7100, are generally higher than those of the appinites, which are between 0.7051 and 0.7103. As regards the protholith of the parental magmas, the two series of Sr-isotopic values do not permit an univocal interpretation. The suggested hypotheses are: a) a heterogeneous anomalized mantle; b) a "normal' mantle, with later contamination by crustal material; or c) a lower crust. -Authors