Corundum-bearing veins in chloritite from the Etirol-Levaz Austroalpine continental slice (Valtournenche, Aosta, Italy)

Authors

  • Roberto Compagnoni Dept. of Earth Sciences, University of Torino, 35 Via Valperga Caluso, 10125 Torino, Italy
  • Roberto Cossio Università di Torino
  • Daniele Regis Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

Keywords:

corundum-bearing chloritite; metagabbronorite; green spinel; magnesiobeltrandoite-2N3S, petrogenetic modeling; metasomatism; continental Etirol-Levaz slice; Valtournenche, Aosta valley; Western Italian Alps

Abstract

The high-pressure (HP) continental Etirol-Levaz slice (ELS) is exposed on the right side of the Valtournenche (Aosta) at the contact between the overlying blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone. The ELS contains a layered metagabbro showing a complex evolution, which includes a pre-Alpine high-temperature (HT) recrystallization of the igneous mineralogy, a polyphase Alpine high-pressure (HP) quartz eclogite-facies and a late greenschist-facies (GS) retrogression. In this paper, an unusual corundum-bearing ultramafic rock associated with the metagabbro is described. The studied sample is a Mg-chloritite with relict green spinel partly replaced by corundum, Mg-beltrandoite-2N3S (a new mineral of the högbomite supergroup), Mg-chlorite and dolomite. The rock is cut by mm- to cm-thick veins composed of coarse-grained corundum + Mg-chlorite + dolomite. P-T phase-diagram projections indicate that the corundum-bearing assemblages formed in the presence of a water-rich fluid (X(CO2) ≤ 0.04), during a retrograde decompressional evolution in continuity with the prograde to HP-peak path estimated from the associated eclogites. The new estimated prograde-to-retrograde P-T path suggests that the ELS and the underlying Zermatt-Saas Zone shared a common Alpine metamorphic evolution. A detailed study of the relict minerals preserved in the chloritite allowed to infer its pre-Alpine protolith, a green spinel websterite, and evolution, characterized by a high-T recrystallization of the original igneous minerals. During the Alpine orogeny, the spinel websterite experienced a metasomatic process that converted the original igneous rock into a chloritite.

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Published

2021-06-02

How to Cite

Compagnoni, R., Cossio, R., & Regis, D. (2021). Corundum-bearing veins in chloritite from the Etirol-Levaz Austroalpine continental slice (Valtournenche, Aosta, Italy). Ofioliti, 46(2). Retrieved from https://www.ofioliti.it/index.php/ofioliti/article/view/630