Abstract

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 87Sr/86Sr 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 87Sr/86Sr 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.

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