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Proceedings of the Geologists' Association, 2025 (SCI-Expanded)
New U–Pb zircon geochronology and geochemical data from slightly folded gabbro dykes intruding pre-Devonian strata in the İstanbul Terrane (İstanbul–Zonguldak Tectonic Unit, NW Turkey) constrain a Silurian magmatic episode (434.3 ± 4.0 Ma and 428.5 ± 2.2 Ma) associated with syn-collisional alkaline magmatism. Despite within-plate trace element signatures (notably positive Nb anomalies), tectonic discrimination diagrams indicate emplacement within a syn-collisional regime, consistent with slab break-off following soft docking between Far East Avalonia and Baltica. High-intensity zircon overgrowths yield concordant Early Devonian ages (c.416–415 Ma), interpreted as a distal Caledonian tectono-thermal overprint. The Th/U ratios and cathodoluminescence textures of these domains support deformation-related recrystallisation rather than magmatic or metamorphic zircon growth. Structurally, folded geometries and axial-plane cleavage in the dykes are consistent with this event. These data imply that the İstanbul Terrane records a previously unrecognised disconformity at the Silurian–Devonian boundary, correlating with the established unconformity in the Zonguldak Terrane. This stratigraphical gap, masked by urbanisation in the İstanbul region, aligns with regional deformations of the Caledonian orogeny across Avalonian fragments.