A VERY CLOSE LOOK AT A VERY CLASSIC UHP WHITESCHIST FROM DORA MAIRA, ITALIAN ALPS: EMP MONAZITE DATING AND PHASE EQUILIBRIA

Preliminary EMP age results for monazite (mnz) in a phengitic schist from the Brossasco Zone, Dora Maira UHP massif, yield two discrete populations of 64 Ma (± 2, 2σ) and 55 Ma (± 2). Additional minerals in the schist include garnet (prp₉₂)(grt), kyanite, Mg-phengite, phlogopite, Na-phlogopite, jadeitic cpx (65% jd), talc, rutile, zircon, coesite (coe), and quartz (qz). EMP dating is typically limited to events >100 Ma, but we report mnz ages for grains with up to 35 wt% Th and 2 wt% U, yielding 250-1200 ppm Pb, and emphasize that the higher than normal Th and U content allow for EMP dating of such young monazite. 200 EMP analyses were performed and processed through a refined data synthesis protocol based on Cocherie & Albarede (2001). Major-element data for each analysis spot showed a natural separation into two discrete groups based on Th/Y vs Th/U and Ca/Y ratios. Each compositional group corresponded to rim and core locations, as documented by WDS imaging of 5 grains. The low-Y group correlated with mnz core regions of ~64 Ma age, whereas the higher-Y population represented rim areas of ~55 Ma. All ages were calculated using a weighted average, CHIME plots, and U/Pb vs Th/Pb plots, with a high level of consistency among the separate calculations. We plan soon to perform LA-MC-ICPMS mnz dating to complement and corroborate the EMP ages.

Characteristic UHP textures and evidence of decompression (e.g., development of "palisades" textured qz surrounding coe inclusions in grt, complete inversion of matrix coe to qz, and resorption of grt rims) can be directly correlated to specific domains of mnz growth or recrystallization. The older age of ~64 Ma appears to represent the timing of UHP metamorphism and grt growth (two mnz grains occur as inclusions in Mg-rich grt), whereas the younger ~55 Ma population may represent the timing of initiation of exhumation and the beginning of decompression-driven grt-out reactions. The correlation of a ~55 Ma population to Y-enriched rim domains suggests that mnz growth or recrystallization occurred during grt breakdown and recorded the early stages of decompression. In one key texture, a mnz grain inside grt lies in a Ca-enriched euhedral garnet growth band that must have formed at UHP conditions. This mnz shows the 64 Ma age but no younger ages, indicating that timing of UHP conditions must be > 55 Ma and ≤ 64 Ma.