Abstract

The Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

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