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

ABSTRACT

Multiscale structural analysis and petrological modelling were used to establish the pressure-peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt-Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure-peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti-clinohumite and chlorite. Three rodingite types can be defined: epidote-bearing, garnet–chlorite–clinopyroxene-bearing and vesuvianite-bearing rodingite. In these, the pressure-peak assemblages coeval with S2 development involve: (i) epidoteII + clinopyroxeneII + Mg-chloriteII + garnetII ± rutile ± tremoliteI in the epidote-bearing rodingite; (ii) Mg-chloriteII + garnetII clinopyroxeneII ± vesuvianiteII ± ilmenite in the garnet–chlorite–clinopyroxene-bearing rodingite; (iii) vesuvianiteII + Mg-chloriteII + clinopyroxeneII + garnetII ± rutile ± epidote in vesuvianite-bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre-Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr-rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn-S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low-P phase, could also be stable in favourable chemical systems under high-P/ultra-high-pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure-peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.