HOW ARE SEDIMENTS INCORPORATED INTO LOWER CONTINENTAL CRUST? A DETRITAL ZIRCON CASE STUDY FROM THE IVREA-VERBANO ZONE, ITALY (Invited Presentation)

Numerous exhumed exposures and xenoliths of lower continental crust contain significant quantities of metasedimentary rocks, yet how these sediments were incorporated into the deep continental crust remains uncertain. The Ivrea-Verbano Zone (IVZ) is a coherent lower continental crustal section in the southernmost Italian Alps that is primarily composed of metamorphosed sediments. We analyzed detrital zircon cores from the IVZ metasediments to link the depositional history of these units to the tectonic processes responsible for their emplacement in the lower continental crust. The dataset comprises ~6,500 concordant zircon U-Pb dates and associated trace-element compositions (REEs, Ti, Hf) from 33 samples across three IVZ transects. Comparisons of date vs. Th/U reveal a clear trend of decreasing Th/U with younging date in analyses younger than ~450 Ma, with Th/U approaching a minimum at ~300 Ma. Critically, dates younger than ~450 Ma are consistent with mechanical mixing of detrital cores (>450 Ma, high Th/U) and metamorphic rims (~300 Ma, low Th/U); there is no demonstrably robust population of detrital zircon grains younger than ~450 Ma in the metamorphosed sediments of the IVZ. This interpretation is inconsistent with prior work suggesting that the IVZ sediments were deposited no earlier than ~350 Ma in a Variscan subduction zone and emplaced at the base of a waning arc. Instead, the IVZ sediments more likely represent an Orodovician-Silurian section that did not reach peak metamorphic conditions until more than ~100 Myr after their initial deposition. Such lengthy timescales between deposition and metamorphism are similar to other localities where continental underthrusting (Chin et al., 2013) and sedimentary accretion in subduction zones (Ewing et al., 2023) have been proposed to explain the incorporation of sediments into the lower continental crust.