Constraining the age of metamorphism in the Dora Maira, Western Alps: Implications of U-Th-Pb ages and REE concentrations in phosphates

Understanding the rates of subduction, burial, and exhumation requires accurate and precise age determinations of various stages along a unit’s pressure-temperature path. Advances in geochronology result in age uncertainties that are significantly less than the duration of many geological processes such as prograde metamorphism and exhumation. Thus, it is more important than ever to precisely link the measured 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 symplectite (+florencite) yielded U-Pb and Th-Pb isochron ages of 31.3 ± 1.7 and 31.8 ± 4.3Ma, 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 as represented by a Lu-Hf garnet-matrix age of 34.1 ± 1.0, recalculated with 176Lu decay constant, and a 35.1 ± 0.9 Ma U-Pb age of UHP titanite. Monazite ages of this study are consistent with, but slightly younger than U-Th-Pb monazite ages from previous studies and in agreement with U-Pb ages of retrograde titanite. Given that the monazite 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 timing of this process. The P-T conditions of this reaction are estimated to lie above the quartz-to-coesite transition, so the monazite ages mayrecord UHP conditions of the retrograde path. Given these constraints, initial exhumation rates of the Dora Maira nappe may be significantly greater than the 3.4 cm/yr previously proposed.