Impact-induced ‘young’ zircon in old rocks from Troodos ophiolite, Cyprus

Zircon has long been utilized as a geochronological tool for determining the age of rock formations. However, our recent discovery of anomalously ‘young’ zircon grains (<81 Ma) within igneous rocks from the Troodos ophiolite in Cyprus challenges conventional interpretations. These chronologically younger zircon grains, hosted in rocks dated to 95–81 Ma, are ubiquitously distributed across all mafic lithologies, including gabbro, diabase, and pillow lava. Their morphology, trace element signatures, and Hf isotope compositions differ markedly from the magmatic zircon grains formed between 95 and 81 Ma, as well as the hydrothermal zircons. Instead, they exhibit inherited characteristics akin to older detrital zircon grains dating from 3300 and 95 Ma. Notably, the <81 Ma zircon grains show a progressive decline in trace element concentrations (except for U and Hf) and εHf(t) values correlating with younger apparent ages. These trends suggest that their anomalously ‘young’ U–Pb ages reflect secondary processes involving Pb loss and U gain. We propose that these zircon grains underwent variable degrees of modifications inherited from detrital precursors during the earliest stages of subduction initiation. Drawing upon chondrite-like rare earth element (REE) patterns, Hf isotope heterogeneities, mineral inclusions, and the ophiolite’s circular outcrop pattern, we further hypothesize that a meteorite impact event occurred during this modification phase. The associated high-pressure, high-temperature conditions extensively reset the U–Pb system while minimally affecting the Lu–Hf system of preexisting zircon grains in the oceanic crust. These modified grains were subsequently incorporated into magmatic systems during subduction-related melting. The resultant mixture of meteoritic components, marine sediments, and mantle materials explains the unique geochemical signatures observed in Troodos magmatic zircon grains and their host rocks. This model may also account for ancient signatures (e.g. Proterozoic to Archean Os model ages, inherited zircon in peridotites and chromitites) documented in many Neo-Tethyan ophiolites, offering a unifying framework for reconciling disparate geochronological records in suprasubduction zone settings.