Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy
Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T=590-630°C, P=2.7-2.9 GPa) of the Zermatt-Saas ophiolite from Lago di Cignana, Italy, may be made through combined Lu-Hf and Sm-Nd garnet geochronology in conjunction with petrologic estimates of the prograde P-T path. We report a Lu-Hf garnet-omphacite-whole-rock isochron age of 48.8±2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm-Nd age of 40.6±2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425-438] obtained from the same sample and mineral material. The Sm-Nd and Lu-Hf ages, as well as other ages determined on metamorphic garnet, zircon and white mica [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425-438; Mayer et al., Eur. Union Geosci. 10 (1999) Abstr. 809; Rubatto et al., Contrib. Mineral. Petrol. 132 (1998) 269-287; Dal Piaz et al., Int. J. Earth Sci. 90 (2001) 668-684] from Lago di Cignana and elsewhere in the Zermatt-Saas ophiolite, lie within a range of ∼50-38 Ma, which we interpret to encompass the duration of prograde metamorphism, and possibly the duration of garnet growth. The difference in measured Sm-Nd and Lu-Hf ages from Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally following a profile that is predicted by Rayleigh fractionation. Preferential enrichment of Lu in the core produces a Lu-Hf age that is weighted toward the older garnet core. Sm-Nd ages, as predicted by Rayleigh fractionation of Sm and Nd during garnet growth, however, reflect later grown garnet as compared to Lu-Hf ages. The difference in Sm-Nd and Lu-Hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as Sm-Nd and Lu-Hf blocking temperatures were not exceeded for a long period of time. Based on the 12 Myr duration of prograde garnet growth estimated in this study, burial rates for rocks at Lago di Cignana were on the order of 0.23-0.47 cm/yr. These values correlate with continuous shortening rates of 0.4-1.4 cm/yr between the European plate and the African-Adriatic promontory between 50 and 38 Ma, which is on the order of that calculated for plate velocities from plate reconstructions, suggesting that the Zermatt-Saas ophiolite may have remained well-coupled to the down-going slab to UHP conditions.