Geochemical and palaeontological insights into Middle Triassic Intra-Pontide ocean island fragments: Elmadağ Olistostrome, Central Anatolia

Abstract

The Late Cretaceous closure of the Neotethyan Intra-Pontide Ocean generated mélanges in front of southward-advancing nappes. These include the Elmadağ Olistostrome, which consists of blocks embedded in a Coniacian (Upper Cretaceous) siliciclastic to calcareous-siliciclastic matrix. This olistostrome locally contains basaltic blocks alternating with platform limestones. Benthic foraminifera from these limestones yielded Anisian ages in two localities east/southeast of Ankara (Central Anatolia). At the Taşönü locality, the Anisian to lower Ladinian basalt-limestone succession is overlain by an upper Ladinian sequence of volcanogenic clastic rocks, chert, and detrital limestone. At all localities, the basalts display alkaline compositions with typical OIB-like trace element signatures. High abundances of incompatible elements, coupled with high Nb/Zr, Zr/Y, and Nb/Y ratios, indicate a contribution from enriched/recycled components in their petrogenesis. Geological and geochemical evidence suggests that the basalt-limestone assemblage represents fragments of Anisian ocean islands in the Intra-Pontide Ocean. These islands were accreted to the southern Pontide margin during the late Ladinian. Following the closure of the Intra-Pontide Ocean during the Late Cretaceous, the Elmadağ Olistostrome was emplaced onto the Middle Triassic basement and the Jurassic-Cretaceous carbonate cover of the Sakarya Continent, as well as on the Izmir-Ankara-Erzincan ophiolitic mélange.