Evidence for protracted prograde metamorphism followed by rapid exhumation of the Zermatt‐Saas Fee ophiolite

Abstract

Major and trace-element zoning in garnet, in combination with Rb-Sr, Sm-Nd and Lu-Hf geochronology, provide evidence for a protracted garnet growth history for the Zermatt-Saas Fee (ZSF) ophiolite, western Alps. Four new Lu-Hf ages from Pfulwe (c. 52-46 Ma) and one from Chamois (c. 52 Ma) are very similar to a previously published Lu-Hf age from Lago di Cignana. Overall, the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history, commensurate with their similar structural position near the top of the ZSF. Samples from the lower part of the ZSF at Saas Fee and St. Jacques, however, produced much younger Lu-Hf ages (c. 41-38 Ma). Neither differences in whole-rock geochemistry, which might produce distinct garnet growth histories, nor rare-earth-element zoning in garnet, can account for the age differences in the two suites. This suggests a much later prograde history for the lower part of the ZSF, supporting the idea that it was subducted diachronously. Such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions, where early oblique subduction, which produced long prograde garnet growth, changed to more orthogonal subduction, which corresponds to shorter prograde garnet growth. Six new Rb-Sr phengite ages range from c. 42 to 39 Ma and, in combination with previously published Rb-Sr ages, constrain the timing of the transition from eclogite to upper greenschist facies P-T conditions. The proximity of the ZSF in the Saas Fee region to the underlying continental Monte Rosa unit and the similarity of peak-metamorphic ages suggest these two units were linked for part of their tectonic history. This in turn indicates that the Monte Rosa may have been partly responsible for rapid exhumation of the ZSF unit.

Ocean floor and subduction record in the Zermatt‐Saas rodingites, Valtournanche, Western Alps

“…Recently, part of the Monviso ophiolite has been interpreted as forming a single unit with the ZSZ, that would consist of a continuous slab recording a coherent metamorphic evolution (Angiboust et al ., ). In contrast with the last interpretation, the age data indicate a wide time interval of re‐equilibration during subduction of the ZSZ rocks and associated continental slivers between 80 and 40 Ma (with ages interpreted as referred to peak conditions comprised between 58 and 38 Ma; Skora et al ., , ; Springer et al ., ; Weber et al ., and references therein) consistent with indications of a heterogeneous metamorphic evolution.…”

“…For temperatures <550°C, lawsonite, which was detected in the Valtournanche metabasite as relicts replaced by paragonite, epidote and minor albite (e.g. Skora et al, 2015), has been never observed in the rodingites, even if it is predicted to be stable at these temperatures. For temperatures >580°C and up to 680°C, most of the rodingite compositions are characterized by assemblages involving different modal amounts of diopside, chlorite, garnet, epidote and rutile.…”

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

“…Exhumation of the subducted metaophiolites of the Western Alps occurred in a time span ranging between ca. 42 and 36 Ma (e.g., Beltrando et al, ; Gouzu et al, ; Reddy et al, ; Skora et al, ).…”

Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

“…The Zermatt-Saas zone has been affected by eclogite facies metamorphism (P~25-30 kbar and T~550-600°C: Bucher et al, 2005) dated at ca. 40-50 Ma (Amato et al, 1999;Dal Piaz et al, 2001;Lapen et al, 2003;Rubatto, Gebauer, & Fanning, 1998;Skora et al, 2015). Moreover, in the Lago di Cignana area, it recorded ultrahigh pressure conditions (i.e., Lago di Cignana unit; T = 615 ± 15°C and P = 28 ± 10 kbar: Reinecke, 1998; T = 600°C and P ≥ 32 kbar: Frezzotti, Selverstone, Sharp, & Compagnoni, 2011).…”