Alpine Tethys closure as revealed by amphibole-rich mafic and ultramafic rocks from the Adamello and the Bergell intrusions (Central Alps)

<p>U–Pb ages and Hf isotope data were obtained for zircon from amphibole-rich mafic to ultramafic rocks from the Adamello batholith and the Bergell pluton, the largest Palaeogene intrusions of the Alpine Orogen. The <sup>206</sup>Pb/<sup>238</sup>U age pattern of U–Pb concordant dates from the Adamello mafic rock shows a major crystallization event at <em>c</em>. 41 Ma and older age peaks at <em>c</em>. 50 and <em>c</em>. 45 Ma. Hornblendite and amphibole gabbro samples of the Adamello batholith have zircon with initial ϵHf of <em>c</em>. +9.0 and <em>c</em>. +7.0, respectively. Amphibole gabbro and diorite samples of the Bergell pluton yield a younger age of <em>c</em>. 31 Ma and have zircon with lower initial ϵHf (<em>c</em>. +4.0). We propose that the amphibole-rich rocks from the Adamello batholith originated from a depleted mantle source activated by the subduction of the Ligurian–Piedmontese Basin. The amphibole-rich rocks from the Bergell pluton formed 10–15 Ma later than the Adamello counterparts by melts derived from a mantle sector metasomatized by the subduction of the Valais Basin. The enriched Hf isotopic signature of the amphibole-rich rocks from the Bergell pluton is therefore interpreted to reflect the peculiar lithostratigraphy of the Valais Basin or a primary feature of the newly activated mantle source. </p>