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

The Pozant-Karsant 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 Pozant-Karsant 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 Nd(t) values (+5.4 to +8.3) and high 207Pb/204Pb ratios, suggesting an OIB-like geochemical affinity. Secondary ion mass spectrometry zircon and titanite U-Pb ages reveal a protolith age at ~122.2 1.3 Ma and metamorphic age at 9290 Ma for the amphibolites. Gabbro and diabase dikes intruded the metamorphic sole rocks and ophiolitic units at ~9190 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 9190 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.