Long-lasting (25 Ma) re-crystallization during exhumation recorded by white mica in situ 40Ar/39Ar ages (Sesia-Lanzo Zone, Western Alps)?

Since the interpretation of apparent 40Ar/39Ar dates in polymetamorphic terranes is difficult due to incomplete re-crystallization of minerals and loss of daughter and/or parent isotopes by volume diffusion [1], the combination of in situ 40Ar/39Ar analyses with textural, petrologic and geochemical data can provide information that is critical to understand the geological relevance of 40Ar/39Ar dates. Here, we attempt to link effects of fluid flux to 40Ar/39Ar dates by using concentrations and isotopic compositions of the fluid-mobile elements boron (B) and lithium (Li) in phengites from partially overprinted rocks from the Western Alpine Sesia-Lanzo zone (SLZ). The Gneiss Minuti (GM) unit of the SLZ comprises basement crust of Hercynian metamorphic rocks, which was subuducted to eclogite-facies conditions during the Cretaceous. During uplift, the rocks experienced almost complete greenschist-facies re-setting. Gneisses from the GM unit contain phengites with significant major element differences between pristine cores and overprinted rims. In contrast, concentrations of Li, B, Rb and Ba as well as δ11B (-4 to -7 ‰) and δ7Li (-8 to +1 ‰) values overlap between core and rims. The lack of any difference in both B concentrations and isotopic composition points to the lack of an external fluid infiltration and shows that B was not leached in significant amounts from the overprinted parts as a result of fluid percolation. The relict phengite cores yield apparent 40Ar/39Ar ages between 58 and 67 Ma. These ages can be broadly related to fluid flux in an adjacent km-scale shear zone at 65×3 Ma. This interpretation is supported by the overlap in δ11B values between relict phengite of the GM unit and mylonitic phengite from the shear zone. Phengite rims yield younger ages with a large range from 33 to 58 Ma. Previous geochronological data revealed a similar age range (37-60 Ma) based on Rb-Sr and 40Ar/39Ar data, from which a preferred age of ~39 Ma for re-crystallization under greenschist-facies conditions was derived [2]. However, the relatively continuous distribution of 40Ar/39Ar ages and the lack of isotopic and trace elemental changes during rim growth indicates continuous re-crystallization rather than a discrete event. Although we cannot rule out that the range in apparent ages reflects the variable influence of a discrete greenschist-facies event that caused incomplete rejuvenation, our preferred interpretation is that the samples record greenschist-facies re-crystallization at different times during exhumation, which lasted for about 25 Ma. References: [1] Bröcker, M. et al., 2013, J. metamorph. Geol. 31:629-646 [2] Inger, S. et al., 1996, Contrib. Mineral. Petrol. 126:152-168