THE DURATION OF PROGRADE METAMORPHISM OF A UHP TERRANE: AN EXAMPLE FROM LAGO DI CIGNANA, ZERMATT-SAAS OPHIOLITE, ITALY

Estimation of burial rates and duration of prograde metamorphism of ultrahigh-pressure rocks (T=590-630°C, P=2.7-2.9 GPa; Reinecke, 1991, 1998) 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 two Lu-Hf gt-cpx-amph-wr and gt-wr isochron ages of 48.8 ± 2.1 and 55.5 ± 6.1 Ma respectively 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., 1999) obtained from the same sample and mineral material as that of the 48.8 Ma age above. The Sm-Nd and Lu-Hf ages are consistent with recent metamorphic age estimates from elsewhere in the Zermatt-Saas ophiolite.

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 provide a minimum estimate for the duration of garnet growth and prograde metamorphism at Cignana.

Based on a 12 m.y. duration of prograde garnet growth determined from data presented here and data from elsewhere in the Zermatt-Saas ophiolite, burial rates for rocks at Lago di Cignana were on the order of 0.23 to 0.47 cm/yr. These values correlate to continuous shortening rates of 0.4 to 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.