Palynostratigraphy and Palaeogeography of the Upper Ordovician Gorgan Schists (Southeastern Caspian Sea), Eastern Alborz Mountain Ranges, Northern Iran

Determination of the exact age and palaeogeographic position of the low grade metamorphic rocks of the so-called “Gorgan Schists” exposed in the northern Alborz Mountains of northern Iran has been long debated. Therefore, 163 samples were collected for palynological analysis throughout the entire thickness of the metasedimentary succession. In addition, three samples from the overlying non-metamorphic limestones were also analyzed micropalaeontologically. Most of the Gorgan Schists samples yielded abundant and well-preserved acritarchs (29 species belonging to 17 genera), chitinozoans (35 species belonging to 16 genera), scolecodonts (counted, but not identified to species), and some graptolite remains. Of the 35 species of chitinozoans present, two new species, Belonechitina kordkuyensis and Spinachitina aidaiae, are erected. Based on the presence of well-known chitinozoan and acritarch species, a Late Ordovician (Katian-Hirnantian) age has been assigned to the Gorgan Schists for the first time. The characteristics of the palynological assemblages also suggest a shallow marine depositional environment for these metasediments before metamorphism. Therefore, the Gorgan Schists are time-equivalent to sediments of the non-metamorphic Ghelli Formation, and are clearly older than the Soltan Maidan Basalts. Based on the presence of foraminifers in three samples, a Late Cretaceous age is suggested for the overlying non-metamorphic, fossiliferous limestones of the Gorgan Schists. The encountered chitinozoan assemblages in the Gorgan Schists permit the recognition of the Belonechitina robusta, Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongata and Spinachitina oulebsiri chitinozoan Biozones, which have been previously established in the northern Gondwanan Domain. The palynological results thus indicate that the northeastern Alborz Mountain Range was part of Gondwanan Domain during the Late Ordovician. The shallow marine volcano-sediments coupled with flood basalts of the Gorgan Schists are indicative of the Late Ordovician rift-related volcanic events affecting the northern margin of Gondwana during the opening process of the Palaeo-Tethys. This marginal rift assemblage was metamorphosed during the closure of the Palaeo-Tethys and its collision with the Laurasia during the Early Triassic (Rhaetian).