Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy

An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied in order to refine the protolith, provenance and metamorphic ages of the Eclogitic Micaschist Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz inline-formula+ phengite inline-formula+ K-feldspar inline-formula± garnet inline-formula+ rutile inline-formula+ zircon, whereas the paragneiss hosts garnet inline-formula+ jadeite inline-formula+ phengite inline-formula± glaucophane inline-formula+ epidote inline-formula+ rutile inline-formula+ quartz. Phase diagram modeling of two representative samples yields minimum equilibration conditions of 550 inline-formula± 50 inline-formula^∘C and 18 inline-formula± 2 kbar. Cathodoluminescence images of zircon from the orthogneiss show oscillatory-zoned cores that are embayed and overgrown by complex, oscillatory-zoned rims. Four concordant secondary ion mass spectrometry analyses from the cores give a weighted mean inline-formula²⁰⁶Pb inline-formula $M16inlinescrollmathml/$ 8pt14ptsvg-formulamathimgc6f00d13d95b9183e3e2526db4298e27 ejm-35-645-2023-ie00001.svg8pt14ptejm-35-645-2023-ie00001.png  inline-formula²³⁸U age of 457 inline-formula± 5 Ma. The cores have inline-formula $M19inlinescrollmathmlchem\mathrm{normal\; Th}/\mathrm{normal\; U}$ 29pt14ptsvg-formulamathimg66336532033b9fae90efac8c3cb90ed2 ejm-35-645-2023-ie00002.svg29pt14ptejm-35-645-2023-ie00002.png  inline-formula= 0.1 and negative Eu anomalies indicative of an igneous protolith, which we interpret to have crystallized in the Ordovician at 780 inline-formula^∘C, based on Ti-in-zircon measurements. Zircon rims yield a range of inline-formula²⁰⁶Pb inline-formula $M23inlinescrollmathml/$ 8pt14ptsvg-formulamathimg265e2a7d42d09da6c1e252e5649f9787 ejm-35-645-2023-ie00003.svg8pt14ptejm-35-645-2023-ie00003.png  inline-formula²³⁸U dates from 74 to 86 Ma, and four concordant analyses define a weighted mean inline-formula²⁰⁶Pb inline-formula $M26inlinescrollmathml/$ 8pt14ptsvg-formulamathimg6ab83fe49e60fc98c8f39a665eb0d5b9 ejm-35-645-2023-ie00004.svg8pt14ptejm-35-645-2023-ie00004.png  inline-formula²³⁸U age of 78 inline-formula± 2 Ma. The rims are interpreted to have grown in the eclogite facies based on their lower inline-formula $M29inlinescrollmathmlchem\mathrm{normal\; Th}/\mathrm{normal\; U}$ 29pt14ptsvg-formulamathimg9fc4a0c98ca077ea326c889213881ea1 ejm-35-645-2023-ie00005.svg29pt14ptejm-35-645-2023-ie00005.png (0.01), less negative Eu anomalies and steeper heavy rare earth element (HREE) patterns at inline-formula<600 inline-formula^∘C. The paragneiss yielded a detrital zircon population with major peaks at 575–600, 655 and 765 Ma; minor older components; and a maximum depositional age of approximately 570 Ma. The prominent Neoproterozoic zircon population and Ediacaran depositional age suggest derivation from the Gondwana margin. The metamorphic zircon is consistent with the oldest eclogite-facies event in the Sesia Zone; it does not show evidence of multiple periods of rim growth or any pre-Alpine (e.g., Variscan) metamorphism.