Abstract

Olivine calcium concentrations have been used to obtain details of the final emplacement history of the Lanzo massif, northern Italy. The olivine-clinopyroxene geobarometer was applied to core CaO concentrations (0.08 wt%) in olivine. When coupled with two-pyroxene temperature estimates of 1000 °C, an equilibration pressure of 20 ±4 kbar is obtained. Olivine from some samples shows a systematic decrease in CaO to 0.04 wt% at rims in contact with clinopyroxene grains. These concentration profiles have been modeled by assuming that they result from diffusion during ascent of the Lanzo from the above equilibrium conditions. Results from a one-peridotite dimensional diffusion algorithm indicate that a two-stage uplift model, in which the massif is first brought adiabatically from 20 kbar to between 3 and 8 kbar and then allowed to cool isobarically, produces calculated diffusion profiles that are in excellent agreement with observations. The second stage of uplift occurs after the massif has cooled to below the 750 °C blocking temperature of Ca diffusion, and terminates with emplacement at the surface. These results indicate that the Lanzo massif originated in the upper mantle and was emplaced in the crust while solid at a rate consistent with plate velocities.

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