Yo-yo subduction recorded by accessory minerals in the Italian Western Alps

High-pressure metamorphic rocks form during subduction of Earth's crust to mantle depths at convergent plate margins. Their exposure at the surface of Earth today provides a record of the subduction zone process. In general, such metamorphic rocks record only a single cycle of subduction and exhumation, yet tectonic models suggest that individual rock units should undergo multiple subduction-exhumation cycles. Here we investigate the microstructure and chemical composition of metamorphic minerals in high-pressure rocks exposed in the Sesia zone in the Italian Western Alps. We find that the minerals white mica, garnet, allanite and zircon each exhibit multiple generations of mineral overgrowths. In particular, two generations of white mica with high-silicon content, indicative of formation at high pressure, are separated by an overgrowth with low-silicon content that formed during exhumation at low pressures. Furthermore, the trace-element signatures of distinct zones within zircon and allanite also reveal two episodes of high-pressure metamorphism, separated by a period of rapid exhumation. We use uranium-lead dating of zircon and allanite overgrowths to constrain the timing of this subduction-exhumation-subduction cycle to 79-65 Myr ago. We conclude that slices of the Sesia zone crust experienced two cycles of burial to mantle depths in less than 20 million years. The yo-yo subduction occurred during oblique convergence between the African and European plates, which involved a continental margin.