Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite-facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), is, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of the eastern Tibet. In‐situ core‐rim 40Ar/39Ar biotite and U‐Th/Pb allanite data show that amphibolite‐facies conditions (~10‐11 kbar, 530 °C to 6‐7 kbar, 580 °C) were reached at 210‐180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140‐120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist‐overprint (7 ± 1 kbar, 370 ± 35 °C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low to medium grade rocks seems to be mainly controlled by dissolution‐reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasediment