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

Abstract. An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied order to refine protolith, provenance metamorphic ages of Eclogitic Micaschist Complex Sesia Zone. The contains jadeite with quartz + phengite K-feldspar ± garnet rutile zircon, whereas hosts glaucophane epidote quartz. Phase diagram modeling two representative samples yields minimum equilibration conditions 550 50 ∘C 18 2 kbar. Cathodoluminescence images zircon show oscillatory-zoned cores that are embayed overgrown by complex, rims. Four concordant secondary ion mass spectrometry analyses give weighted mean 206Pb / 238U age 457 5 Ma. have Th/U = 0.1 negative Eu anomalies indicative an igneous which we interpret crystallized Ordovician at 780 ∘C, based on Ti-in-zircon measurements. Zircon rims yield range dates 74 86 Ma, four define 78 interpreted grown eclogite facies their lower (0.01), less steeper heavy rare earth element (HREE) patterns <600 ∘C. yielded detrital population major peaks 575–600, 655 765 Ma; minor older components; maximum depositional approximately 570 prominent Neoproterozoic Ediacaran suggest derivation Gondwana margin. is consistent oldest event Zone; it does not evidence multiple periods rim growth or any pre-Alpine (e.g., Variscan) metamorphism.