The Corno Alto complex (Adamello batholith): A modern analogue of the high Ba/K sanukitoids

The Corno Alto complex represents the onset of the intrusive igneous activity during the Alpine Orogen and for this reason has particular importance in the interpretation of the geodynamic evolution of the Alps. Whole-rock chemistry of the granitoid rocks of the Corno Alto complex reveals peculiar features with respect to the other units of the Adamello batholith, and to typical I-type and S-type granitoids. In particular, the Corno Alto rocks are enriched in Ba, Sr and La/Yb at any given SiO2 content. U–Pb geochronology on zircon suggests an incremental assembly of the Corno Alto complex by multiple and possibly discrete magma injections in a time span of about 5 Myr starting from 44 Ma. The different zircon domains have significantly distinct Hf isotopic signature (up to 18 εHf units of variation) with some values trending towards the isotopic composition of the depleted mantle (DM). Bulk major and trace element geochemistry together with in-situ Hf isotope composition of zircon allow to distinguish at least two geochemically components in the Corno Alto rocks: i) a high Ba component characterized by high Sr and La/Yb ratios, likely derived from melting of carbonate sediments of slab origin; ii) a juvenile component with Hf isotopic signature close to the DM and capable to crystallize plagioclase with An90, which is interpreted as the primitive mantle signal. This work constrains for the first time the occurrence of a slab derived carbonate input in the mantle-derived melts during the onset of the Alpine magmatism (dated at 44 Ma). The anomalously high thermal conditions required to induced carbonate melting may reflect the rise of astheno- spheric material near the torn edge of the European slab where the Corno alto is located. Remarkably, these high thermal conditions parallel those at the Archean-Proterozoic transition and the Corno Alto complex could thus represent a kind of modern analogues of the high Ba/K sanukitoids.