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Petrochronological close-up on the thermal structure of a paleo-subduction zone (W. Alps)

Abstract : We present the results from a study combining zoned geochronology, mineral trace element geochemistry and phase equilibria modeling of a mafic lithology from the Zermatt Saas Ophiolite in order to elucidate the tectonic rates and thermal structure of the Western Alps paleo-subduction interface. The Zermatt-Saas Ophiolite represents dismembered slices derived from an eclogitized, 60-km wide coherent fragment of Tethyan oceanic lithosphere. Two cm-sized garnet crystals from a pyrite-rich chlorite-talcschist (representative of sub-seafloor hydrothermally-altered basalts) from the Servette mine locality (St. Marcel Valley, Italy) were microdrilled to separate core and rim growth generations and dated using Sm-Nd geochronology to determine the overall duration of garnet growth. Garnet cores were dated to 46.9 ± 1.6Ma, signifying the timing of the initiation of garnet growth. Garnet rims were dated to 43.5 ± 1.3Ma, signifying the timing of burial to peak depths (constrained to ∼75 km). Major and trace element zoning in garnet suggest two distinct generations of garnet growth. Garnet crystal cores display evidence for rapid nucleation and growth, while garnet crystal rims suggest growth at relatively slow (tectonic) rates. The duration of garnet growth (3.4 ± 2.1Ma), when coupled to constraints from phase equilibria modeling and Zr-in-rutile thermometry, provides estimates on the burial and heating rate of 4.7 (+ 7.9
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Besim Dragovic, Samuel Angiboust, Michael J. Tappa. Petrochronological close-up on the thermal structure of a paleo-subduction zone (W. Alps). EARTH AND PLANETARY SCIENCE LETTERS, 2020, 547, p. 54-71. ⟨10.1016/j.epsl.2020.116446⟩. ⟨insu-03584473⟩



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