Corundum-bearing veins in chloritite from the Etirol-Levaz Austroalpine continental slice (Valtournenche, Aosta, Italy)
Keywords:corundum-bearing chloritite; metagabbronorite; green spinel; magnesiobeltrandoite-2N3S, petrogenetic modeling; metasomatism; continental Etirol-Levaz slice; Valtournenche, Aosta valley; Western Italian Alps
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.