Nancy J. Mahlen

Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy

Abstract 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.

Provenance of Jurassic Tethyan sediments in the HP/UHP Zermatt-Saas ophiolite, western Alps

Rubidium-Sr and Sm-Nd isotope data and rare earth element (REE) concentrations of the metasedimentary rocks within the Zermatt-Saas (ZS) ophiolite complex of the western Alps are used to investigate element mobility and to determine the provenance of the metasediments in order to place constraints on the precollisional paleogeography of the Piemont-Ligurian portion of the neo-Tethys ocean. Present-day 8 7 Sr/ 8 6 Sr variations for the ZS metasediments scatter about an early Tertiary Alpine metamorphic age, whereas local nappes that have been interpreted to reflect African/Apulian and European basements scatter about a Variscan-like age; this suggests 8 7 Sr/ 8 6 Sr isotope systematics were nearly completely homogenized for most of the ZS metasediments during early Tertiary metamorphism, probably because they were relatively wet prior to metamorphism. In contrast to the Sr isotope data, REE data and Nd isotope compositions of the ZS metasediments overlap those of average upper continental crust, average shale, and the local nappes, and Nd model ages of the ZS metasediments overlap with those of Variscan-age rocks. These relations suggest that the REEs of the ZS metasediments were not disturbed during high- to ultra-high pressure Alpine metamorphism. Based on REE data, Nd isotope compositions, and mixing models, the ZS metasediments comprise two groups that require distinct source terranes: one group (Group I) seems to be mixing of an old, crustal component, such as the paragneissic basement nappe samples, with metasediments similar to the second group (Group II). The source material for Group II is dominated by homogenization of the Variscan-like orthogneissic basement nappe samples. The provenance of Group I samples is interpreted to be local, where source nappes must have been proximal to the Piemont-Ligurian basin prior to Alpine convergence. The similarity in dispersion of Nd isotope compositions of the metasediments and likely source terranes suggests that the metasediments reflect deposition in small, isolated basins early in the formation of the Piemont-Ligurian ocean.