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 Ar-39-Ar-40 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 (approximate to 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 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-Ar-40 dating combined with detailed microprobe
analysis can exploit the potential to relate conditions of amphibole
formation to their respective ages.