Rapid transition from MORB-type to SSZ-type oceanic crust generation following subduction initiation: insights from the mafic dikes and metamorphic soles in the Pozanti-Karsanti ophiolite, SE Turkey

The Pozanti-Karsanti ophiolite is one of the well-preserved supra-subduction-zone oceanic lithosphere slices. It offers a unique opportunity to study oceanic crust formation, and to specify subduction initiation processes of the Inner Tauride ocean in the Neo-Tethyan subduction system. The Pozanti-Karsanti ophiolite is tectonically underlain by a subophiolitic metamorphic sole and melanges. The metamorphic sole is mostly amphibolites that are enriched in light rare earth elements and large-ion lithophile elements, low eNd(t) values (+ 5.4 to + 8.3) and high 207Pb/204Pb ratios, suggesting an OIB-like geochemical affinity. Secondary ion mass spectrometry (SIMS) zircon and titanite U–Pb ages reveal a protolith age at ~ 122.2 ± 1.3 Ma and metamorphic age at 92–90 Ma for the amphibolites. Gabbro and diabase dikes intruded the metamorphic sole rocks and ophiolitic units at ~ 91 to 90 Ma. Their elemental and Sr–Nd–Pb isotopic characteristics indicate a MORB-like mantle source, different from subduction fluid-altered dikes formed at 86.9 ± 3.1 Ma. The temporal-geochemical sequence therefore suggests that the intra-oceanic subduction initiated at 91.9 ± 0.8 Ma within an older lithosphere (> 122 Ma) of the Inner Tauride ocean. Incipient subduction led to forearc (ultra-)slow-spreading seafloor with mafic dikes of 91–90 Ma. The fast switch from MORB- to SSZ-type oceanic spreading might have occurred in less than 3 Myr and marks the inception of a mature subduction zone.