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

Mafic rocks of a Permian crust to mantle section in Val Malenco (Italy) dis play a multi-stage evolution: pre-Alpine exhumation to the ocean floor, fol lowed 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 a nd two to the Alpine metamorphic P-T evolution. The different amphiboles ha ve distinct Na-M4 Ca, K and Cl contents according to different P-T conditio ns and fluid chemistry. Analysing these mixed amphiboles by the Ar-39-Ar-40 stepwise heating technique yielded very complex age spectra. However, by c orrelating amphibole compositions directly obtained from the electron micro probe with the components deduced from the release of Ar isotopes during st epwise heating, obtained ages were consistent with the geological history d educed from field and petrological studies. The two generations of pre-Alpi ne amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceo us ages (approximate to 225 and 130-140 Ma respectively). High-Na-M4 amphib oles have higher isotopic ages than low-Na-M4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle s ection is represented by the former age. The latter age concerns Cl-dominat ed amphibole related to an Early Cretaceous oceanic stage. For the early Al pine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83 -91 Ma). The later, temperature-dominated overprint is significantly younge r, as indicated by Ar-39-Ar-40 ages of 67-73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that Ar-39-A r-40 dating combined with detailed microprobe analysis can exploit the pote ntial to relate conditions of amphibole formation to their respective ages.