POSSIBLE EDIACARAN AGE FOR BASAL TERRESTRIAL DEPOSITS IN THE ISTANBUL ZONE: RADIOMETRIC AGES FROM CROSSCUTTING DYKES

Paleozoic sequence in the İstanbul Zone termed as depositional rocks of the Rheic ranges from virtually uninterrupted terrestrial clastics to platform deposits with Ordovician to Carboniferous ages, and occurs in İstanbul, Çamdağ, Düzce, Karadere areas. Basement of the Paleozoic is exposed in the Armutlu peninsula, Karadere area, and the Sünnice Massif as high-grade amphibolite-facies quartz-plagioclase-biotite-hornblende gneiss (Demirci paragneisses) and greenschist-facies meta-volcanic association-lavas and pyroclastics (Yellice meta-volcanics; 1782±130 Ma), intruded by the Dirgine granite (576-565 Ma). Tectonically overlain terrestrial deposits of the Paleozoic are represented by fluvial fine-coarse grained clastics (Kurtköy Formation; t>3000 m) that conformably overlies lacustrine deposits (Kocatöngel Formation; t≈2000 m).

Depositional ages of the lacustrine-fluvial deposits are accepted as the early-middle Ordovician without any fossil data for two reasons: (a) the fluvial deposits are overlain by the beach deposits (Aydos formation) which have an upper age of Darriwilian, and (b) in the Karadere area, the basement made up of high-grade meta-deposits is intruded by the granitic bodies-dykes (590-560 Ma) and is unconformably overlain by the fossiliferous strata of fluvial fine grained deposits representing a fluvial-beach setting that have an upper age of Tremadocian.

For this study, two folded dykes have been mapped in 1/1000 scale in the Sünnice Massif and its surroundings. Geochemical data suggest both folded dykes have calc-alkaline magma with a subduction signature. Fold axis of the dacite dykes cross-cutting the Dirgine granite is N74⁰E with a plunge of 76⁰NE, and fold axis of the dioritic diabase dykes cross-cutting the Kurtköy formation is N74⁰E with a plunge of 81⁰NE. U-Pb dating of the zircons yield an age of 555.9±2.6 Ma for the folded dacite dyke and 548.8±2.8 Ma for the folded dioritic diabase dyke. Field and analytical data suggest that these folded dykes were folded after the host rocks had been intruded, and age of the basal terrestrial deposits is older than previously suggested for the İstanbul Zone.