Abstract

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 206Pb/238U age pattern of U–Pb concordant dates from the Adamello mafic rock shows a major crystallization event at c. 41 Ma and older age peaks at c. 50 and c. 45 Ma. Hornblendite and amphibole gabbro samples of the Adamello batholith have zircon with initial ϵHf of c. +9.0 and c. +7.0, respectively. Amphibole gabbro and diorite samples of the Bergell pluton yield a younger age of c. 31 Ma and have zircon with lower initial ϵHf (c. +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.

Supplementary material:

(1) A map showing the location of analysed samples and table with GPS coordinates of the samples, (2) data tables for U–Pb in zircon determined by LA-ICP-MS, (3) data tables for Hf isotopes in zircon, (4) U–Pb and Hf isotope analytical methods, (5) cathodoluminescence images of analysed zircon from the amphibole-rich mafic rocks of the Adamello and Bergell intrusions, and (6) a U–Pb concordia diagram for zircon of the Adamello amphibole gabbro are available at www.geolsoc.org.uk/SUP18763.

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