MULTISCALE GEOCHEMICAL STUDY OF THE CORNO ALTO COMPLEX (ADAMELLO BATHOLITH)

Granitoid batholiths are important end products of the Earth’s long-term compositional differentiation, representing either the addition of juvenile material to the crust or the reworking of older crustal components. The nature and composition of crustal granitoids significantly changed at the Archean-Proterozoic boundary highlighting major changes in the mechanisms of continental crust formation/differentiation throughout the Earth’s history. In particular, the transition from the typical Archean TTG rocks to the post-Archean calc-alkaline granitoids is marked, in all cratons, by the occurrence of distinctive rocks called sanukitoids. These latter have been linked to subduction-related partial melting of enriched mantle and hence signal the onset of the modern plate-tectonic regime. In this context, modern analogues of the Archean granitoids provide valuable clues for investigating continental crust formation/differentiation and its secular evolution. This study focuses on the Corno Alto complex, the oldest intrusion of the Adamello batholith and also the oldest known intrusive complex of the whole Periadriatic magmatism. The Corno Alto intrusion is crucial to shed light on the tectono-magmatic conditions active at the onset of alpine magmatism; conditions that are not yet fully understood. In the field, three distinct types of granitoid rocks have been recognized, ranging in composition from tonalite to granodiorite and without encompassing the trondhjemitic terms described in the literature. Whole-rock chemistry reveals peculiar features with respect to the other units of the Adamello batholith, and to typical I-type and S-type granitoid rocks. The Corno Alto rocks exhibit the highest SiO2 contents, K2O+Na2O up to 7.2%, a strong enrichment in Ba and to a minor extent in Sr (Ba + Sr ≈ 1100-1900 ppm). Other geochemical features include a moderately-to-strong enrichment in LREE over HREE (LaN/YbN > 20) and Y (Sr/Y > 40). U-Pb geochronology on zircon shows an east-west trend of decreasing ages in the intrusive complex, with three main recurring age peaks, at c. 44 Ma, c. 42 Ma, and c. 39 Ma. The different zircon domains have significantly distinct Hf isotopes (up to 18 ƐHf units of variation) with some values trending towards the isotopic composition of the depleted mantle (DM). The evidence of multiple components in the petrogenesis of the Corno Alto is also supported by the occurrence of multiple plagioclase populations as suggested by their crystals zoning and the intense chemical disequilibria for major and trace elements, as well as for Sr isotopes (determined in situ by LA-ICP-MS). The new geochronological data presented in this study indicate an incremental assembly of the Corno Alto complex by multiple and possibly discrete magma injections in a time span of about ~5 Myr. These results refine the current knowledge of the Corno Alto complex emplacement as a single event at 43 Ma. Noticeably, the observed age trend parallel that observed on a larger scale by Ji et al (2019) being perpendicular to the direction of the Giudicarie line, which is located right above the European slab edge (Sun et al., 2019; Zhao et al. 2016). The new data suggest that the NW migration of the magmatism observed at the orogen scale as a consequence of the Eocene-Oligocene slab steepening is also evident at the scale of Corno Alto complex. Major-trace element mineral chemistry, Hf isotope on zircon and Sr isotope on plagioclase suggest that the Corno Alto is the product of a multi-stage and multi-component process involving a high Ba component with high Sr and La/Yb ratios and a juvenile component. The overall geochemical signature of the Corno Alto rocks resembles that of a peculiar group of Phanerozoic rocks known as high Ba-Sr granites which are considered as modern analogues of the Archean sanukitoids. The relatively high thermal gradient required to generate such peculiar melts, typical of the Archean, are likely ensured during the Eocene by the thermal perturbation induced by rising asthenospheric material along the slab tear in response to slab edge effects.