³⁹Ar−⁴⁰Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)

Abstract

Mafic rocks of a Permian crust to mantle section in Val Malenco (Italy) display a multi-stage evolution: pre-Alpine exhumation to the ocean floor, followed by burial and re-exhumation during Alpine convergence. Four prominent generations of amphiboles were formed during these stages. On the basis of microstructural investigations combined with electron microprobe analyses two amphibole generations can be assigned to the pre-Alpine decompression and two to the Alpine metamorphic P–T evolution. The different amphiboles have distinct Na^M4, Ca, K and Cl contents according to different P–T conditions and fluid chemistry. Analysing these mixed amphiboles by the ³⁹Ar−⁴⁰Ar stepwise heating technique yielded very complex age spectra. However, by correlating amphibole compositions directly obtained from the electron microprobe with the components deduced from the release of Ar isotopes during stepwise heating, obtained ages were consistent with the geological history deduced from field and petrological studies. The two generations of pre-Alpine amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceous ages (≈225 and 130–140 Ma respectively). High-Na^M4 amphiboles have higher isotopic ages than low-Na^M4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle section is represented by the former age. The latter age concerns Cl-dominated amphibole related to an Early Cretaceous oceanic stage. For the early Alpine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83–91 Ma). The later, temperature-dominated overprint is significantly younger, as indicated by ³⁹Ar−⁴⁰Ar ages of 67–73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that ³⁹Ar−⁴⁰Ar dating combined with detailed microprobe analysis can exploit the potential to relate conditions of amphibole formation to their respective ages.