Late Cretaceous hybrid melt sources and flare-up magmatism in the eastern Sakarya Zone, NE Türkiye: Zircon Hf-O constraints and implications for VMS fertility

Understanding melt source evolution during arc-back-arc magmatism is essential for resolving crustal growth processes and the development of felsic-hosted volcanogenic massive sulfide (VMS) systems. Here we integrate new zircon HfO isotope data with published geochronological and whole-rock geochemical datasets to reassess the petrogenesis of Late Cretaceous arc and back-arc magmatic flare-ups in the eastern Sakarya Zone (ESZ), NE T & uuml;rkiye. Two high-flux magmatic episodes are recognized: a Turonian-Santonian pulse (similar to 92-86 Ma) forming the lower volcanic sequence (LVS), and a younger Campanian pulse (similar to 84-75 Ma) forming the upper volcanic sequence (UVS), accompanied by contemporaneous arc-related granitoids. Zircons from both LVS and UVS display predominantly positive epsilon Hf(t) values (+2 to +12) coupled with mantle-like delta O-18 (5.0-5.6 parts per thousand), but contain abundant inherited Mesoproterozoic-Paleozoic cores and show variable two-stage model ages (T-DM (c) = 0.4-1.4 Ga). Together with the peraluminous affinity of many felsic units, these features indicate that the dacite-rhyolite suites hosting VMS deposits were generated by melting of compositionally heterogeneous lower crust variably hybridized by juvenile basaltic inputs. Arc granitoids likewise record a transition from dominantly juvenile, I-type magmatism during the Coniacian-Santonian to more evolved, crust-influenced signatures during the Campanian. We propose that both flare-ups reflect repeated episodes of basaltic underplating and deep-crustal heating, producing voluminous felsic magmas capable of sustaining long-lived hydrothermal systems. The spatial and temporal coincidence between felsic-dominated flare-ups and VMS mineralization suggests that hybridized felsic magmas, rather than purely juvenile mantle melts, played a central role in generating the physiochemical conditions required for metal-bearing hydrothermal fluids in the ESZ.