Triassic gypsum layers in Keban Metamorphics, eastern Tauride Belt, Southeastern Turkey: New data about Triassic rifting in northern Tethys margin

Abstract

A gypsum interlayer with 100 m thick was identified within the Early-Middle Triassic shelf type clastic and carbonates of the Keban Metamorphics, along the shore of the Firat River, similar to 500 m west of Keban district (Elazig, southeast Turkey). Gypsum layers have thickness of 15-50 cm, and exhibit the original sedimentary bedding intercalated with dolomitic carbonate rocks. They were subjected to hydrothermal alteration related to late Cretaceous magmatic intrusions. Hydrothermal activity caused the dissolution and recrystallization of gypsum, as well as occurrences of secondary gypsum, elemental sulphur and quartz (amethyst). The geochemical (trace element and sulphur and oxygen isotope) compositions of gypsum and dolomites indicate typical marine evaporite compositions. The delta S-34 and delta O-18 values of the Keban gypsum correspond to Early Triassic evaporites, based on Early-Middle Triassic seawater sulphate composition. Keban gypsum represents Triassic rifting along the northeastern margin of Gondwana and exhibits similar age (Early-Middle Triassic) and palaeogeographic setting to Kemer (Antalya, Turkey) gypsum, associated with opening of the Neotethyan Ocean between Gondwana (Pangea) and the Cimmerian microcontinent. However, the Keban gypsum is different from the late Triassic Honaz-Denizli and Nusaybin-Gaziantep gypsum representing the southern margin of the Paleo-Tethys Ocean (Tauride Belt) and southern margin of the Neo-Tethys (Arabian Plate). The presence of gypsum is observed on both passive continental margins of Neo-Tethys means that their deposition took place before the subduction of the Neo-Tethys Ocean during Late Triassic-Jurassic period.