Defining the Cimmerian orogeny in the West Pontides: geochronological and geochemical data from Permo-Triassic dykes in the İstanbul-Zonguldak Tectonic Unit

Abstract

ABSTRACT Permo-Triassic intrusive rocks and back-arc basins are exposed in the İstanbul-Zonguldak Tectonic Unit in the West Pontides. Their tectono-magmatic history, thus to the evolution of the back-arc basins, is poorly understood. Here, I present on olivine-bearing basalt and andesite dykes and granite dykes and pyroxene-bearing andesite dykes of Permo-Triassic crystallization and their Pb-loss ages from the basement rocks and sedimentary rocks in the İstanbul-Zonguldak Tectonic Unit (NW Turkey). They have holocrystalline and porphyritic to vitrophyric textures. U-Pb dating of igneous zircon grains from olivine-bearing basalt and andesite dykes and granite dykes yielded Middle Permian ages of ca. 270.1 ± 1.1 to 261.4 ± 1.7 Ma (2σ), and pyroxene-bearing andesite dykes obtained Early Triassic ages of ca. 252.1 ± 2.4 to 250.1 ± 2.0 Ma (2σ). Their young ages of the dykes yielded Late Triassic ages of ca. 228 to 220 Ma (2σ). Geochemically, the Middle Permian dykes have a calc-alkaline affinity and are notable for subduction components, as indicated by the presence of distinct negative Nb anomalies. Early Triassic dykes show alkaline signatures and show within-plate character, corresponding to prominent positive Nb anomalies. Combined with data from the literature, the Middle Permian dykes are magmatic products associated with the Tethys Ocean subducting under the Pontides and the Early Triassic dykes are magmatic pulses related to the rifting of the Permo-Triassic back-arc basins. The Late Triassic young ages of the Permo-Triassic dykes in the İstanbul-Zonguldak Tectonic Unit represent a deformation event corresponding to the Cimmerian Orogeny, and this implies that the Permo-Triassic back-arc basins closed during the late Triassic.