How do intra-oceanic arcs recycle ancient continental crust? Insights from Archean-Mesozoic zircons of the Kohistan arc

Intra-oceanic arcs have been considered as predominantly juvenile crustal systems, yet the occurrence of ancient recycled zircons in arc magmas challenges this perspective. The Cretaceous Kohistan Island Arc (KIA), northern Pakistan, shows a complete crust-mantle section that provides a unique opportunity to explore how continental material is recycled into oceanic arcs. This study integrates zircon U-Pb geochronology, Lu-Hf isotopic analysis, and trace element geochemistry from chromitites (forearc—Jijal and Sapat) and gabbronorites (arc-root—Chilas) of the KIA, to constrain the sources and pathways of incorporation of ancient crustal material in intra-oceanic arcs.U-Pb dating on the zircon grains recovered from these mantle- and lower-crustal rocks shows xenocrystic ages spanning ~2523 Ma to 122 Ma (Archean-Mesozoic), all older than their host lithologies. Significant age clusters at ~775 Ma, ~400 Ma, ~215 Ma, and ~122 Ma are consistent with magmatic and/or orogenic events across peri-Gondwanan terranes. These zircons are characterized by oscillatory zoning, high Ti-in-zircon crystallization temperatures (>700 ℃), and Type-1 REE (steep HREE-enriched) patterns with positive Ce and negative Eu anomalies, indicating that they are of magmatic origin within older continental crustal sources rather than the KIA itself.The presence of these recycled magmatic xenocrysts in mantle-derived rocks of the KIA is consistent with the incorporation of Gondwanan-derived zircons through sediment subduction, subsequent processing within a metasomatized mantle wedge, and eventual entrainment into arc magmas. Our results indicate that the KIA represents a composite system formed from juvenile and recycled material. Ancient continental material can be transferred into the deepest, most primitive levels of an intra-oceanic arc via sediment subduction and mantle-wedge processing.