CONSTRAINING THE AGE OF GARNET BREAKDOWN IN THE DORA MAIRA, WESTERN ALPS: IMPLICATIONS OF U-TH-PB AGES AND REE CONCENTRATIONS IN PHOSPHATES

Hollister (1982) proposed high exhumation rates for the accreted margin of British Columbia as he was able to link several stages of the Central Gneiss pressure-temperature path with geochronologic information. He understood that it is important to precisely link the geochronologic ages to the metamorphic conditions. This study presents U-Th-Pb and rare earth element data collected by LA-ICPMS for monazite, monazite-apatite symplectite, and florencite in pyrope quartzites from Parigi, Dora Maira nappe, western Alps, Italy. U-Pb and Th-Pb isochron ages of the monazite are 31.48 ± 0.22 and 32.67 ± 0.70 Ma, respectively (all uncertainties are reported at 2σ). The monazite-apatite symplectite and florencite yielded U-Pb and Th-Pb isochron ages of 31.3 ± 1.7 and 31.8 ± 4.3 Ma, respectively; these phosphate mineral assemblages are indistinguishable in age and initial Pb isotope composition from the monazite. The monazite ages of this study are younger than ages of peak metamorphic conditions (e.g., 34.1 ± 1.0 Ma Lu-Hf garnet-matrix age and 35.1 ± 0.9 Ma U-Pb age of UHP titanite and zircon). Monazite ages of this study are consistent with, but slightly younger than U-Th-Pb monazite ages from Tilton et al. (1991) and in agreement with U-Pb ages of retrograde titanite (Rubatto and Hermann 2001). Given that the monazite symplectite from this study likely records REE liberated by garnet break-down associated with the reaction phengite+pyrope+H₂O = phlogopite+kyanite+talc, ages from this study likely reflect the age of this reaction. The P-T conditions of this reaction are estimated to lie above the quartz-to-coesite transition (e.g., Chopin 1984; Schertl et al. 1991), so the monazite ages may record UHP conditions of the retrograde path. Given these constraints, exhumation rates of the Dora Maira nappe were initially 0.42 and 1.13 cm/yr from 120 km to 105-85 km and increased to 15 to 21.25 cm/yr during retrograde from 105-85 km to 40 km (32.9 ± 0.9 Ma) and 20 km (31.8 ± 0.5 Ma).