Triassic-Jurassic arc magmatism in the pontides as revealed by the U-Pb detrital zircon ages in the Jurassic sandstones of northeastern Turkey

The Eastern Pontides were located at the southern active margin of Laurasia during Mesozoic time. Jurassic volcaniclastic sediments and volcanic rocks of the Pontides represent products of the arc magmatism above a north-dipping subduction zone. Despite the wide distribution of the Jurassic volcaniclastic/volcanic succession, the precise age of the Jurassic volcaniclastic sequence and that of the synsedimentary magmatism are poorly constrained. Here we present U-Pb detrital zircon ages from two Jurassic sandstones belonging to the Şenköy Formation of the Eastern Pontides. One sample is taken from the base of the Şenköy Formation unconformably overlying the late Carboniferous Gümüşhane granite. The depositional age of this sandstone is constrained as late Sinemurian-Pliensbachian based on the faunal assemblage of the overlying Ammonitico Rosso type carbonates. Detrital zircon grains from this sample yielded an unexpected component of 203.4 ± 0.2 Ma (Latest Triassic, Rhaetian) U-Pb age, indicating the existence of Late Triassic magmatic activity in the region that has not been reported yet from the exposed magmatic bodies or from the detrital zircon ages. The sample taken from the upper part of the Jurassic succession yielded a youngest U-Pb age component of 155.9 ± 1.8 Ma, indicating that the depositional age of the Jurassic volcaniclastic succession extends from the Early Jurassic (Sinemurian), as revealed by the fossil content and abundant U-Pb detrital zircon ages, to the Late Jurassic (Oxfordian-Kimmeridgian). The detrital zircon ages from this study together with those from the literature indicate arc magmatism on the southern margin of Laurasia during the Triassic and Late Jurassic (250–156 Ma).