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

Zircon has long been used as a geochronological tool for determining the age of rock formations. However, our discovery of anomalously ‘young’ zircon grains (&lt;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 gabbros, diabases and pillow lavas. 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 to 95 Ma. Notably, the &lt;81 Ma zircon grains show a progressive decrease in trace element concentrations (except for U and Hf) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="IM1" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf(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 modification of the characteristics inherited from detrital precursors during the earliest stages of subduction initiation. Based on the chondrite-like rare earth element 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 pre-existing 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 the ancient signatures (e.g. Proterozoic to Archean Os model ages and inherited zircon in peridotites and chromitites) documented in many Neotethyan ophiolites, offering a unifying framework for reconciling disparate geochronological records in supra-subduction zone settings.