Tectonic Evolution of the Izmir-Ankara Suture Zone in Northwest Turkey using Zircon U-Pb Geochronology and Zircon Lu-Hf Isotopic Tracers

摘要

Detrital zircons from the Late Cretaceous Murdunu-Goynuk forearc basin and the Paleogene Saricakaya foreland basin; part of the greater Central Sakarya Basin located along the Sakarya Zone of the Western Pontides were analyzed to better understand the closure history of the Tethyan oceans. In northwest Turkey, the Variscan Orogeny is characterized by abundant 350-300 Ma zircon U-Pb ages and epsilonHf values that plot within the minimally to highly evolved domains. In epsilonHf vs. age space no distinct trends are apparent, consistent with a north dipping subduction zone that emplaced plutons into a southward growing, heterogeneous accretionary margin. From 300-250 Ma epsilonHf values trend from highly to minimally evolved, interpreted as crust thinning, a result of slab roll-back and rifting of the Intra-Pontide Ocean. The Cimmerian Orogeny is characterized by a decrease in magmatism from 250-230 Ma associated with minimally to moderately evolved epsilonHf evolution, followed by a 230-200 Ma magmatic gap consistent with crustal thickening followed by flat-slab underthrusting of the Karakaya Complex. Zircons with 200-115 Ma U-Pb ages are all but absent, interpreted as a magmatic lull. The Alpine Orogeny in northwest Turkey is characterized by an increase in magmatism from 115-85 Ma, associated with minimally intermediate to moderately evolved epsilonHf evolution of Late Cretaceous Murdunu-Goynuk forearc zircons. At 100 Ma, Late Cretaceous zircons only found within Paleogene Saricakaya foreland basin sediments deviate from similar aged epsilonHf evolution in forearc basin sediments and plot in both the juvenile and intermediate domains. Minor zircon U-Pb age peaks and contrasting inter-basinal epsilonHf evolution are interpreted to represent onset of Andean-style subduction along the southern margin of the Sakarya Zone at ∼115 Ma followed by 100 Ma initiation of intra-oceanic subduction within the Izmir-Ankara Ocean. Epsilon Hf values from zircons with 85-75 Ma U-Pb ages sampled from forearc basin sediments trend from moderately evolved to moderately intermediate, interpreted as crustal thinning, a result of slab roll-back along the southern margin of the Sakarya Zone, responsible for final rifting of the Western Black Sea. Foreland basin zircon with U-Pb ages of 85-80 Ma deviate towards highly evolved epsilonHf values. These highly evolved and deviant epsilonHf values are interpreted to represent synchronous melting of the Tav?anli Zone and intra-oceanic slab break-off. A single concordant ∼66 Ma pre-collisional zircon grain collected from Late Cretaceous forearc basin flysch located directly beneath a regional unconformity is defined by a moderately evolved epsilonHf value prior to complete absence of young detrital grains and is interpreted to represent incipient collision between the Sakarya and Tavsanli zones followed by total arc shut-off. Syn-collisional tuffs yield minimally evolved epsilonHf values that trend toward minimally intermediate epsilonHf values from 55-50 Ma and from minimally intermediate to highly intermediate from 50-46 Ma, interpreted to represent a second episode of slab break-off followed by crustal thickening, a result of renewed syn-collisional underthrusting.