Radiolarian evidence for the age of chert blocks from the Upper Cretaceous ophiolitic mélange unit of the Erakh area, Armenia

Abstract

Abstract Two new and relatively well-preserved radiolarian faunas were extracted from blocks included in the Cretaceous ophiolitic mélange unit that crops out in the Erakh area south of Yerevan, Armenia. One of these radiolarian assemblages was extracted from cherts that are in stratigraphic contact with variolitic lavas. This radiolarian assemblage suggests a late Aalenian to middle Bajocian age (Unitary Association Zones 2–3) based on the co-occurrence of the species Parasaturnalis diplocyclis (Yao) and Transhsuum hisuikyoense (Isozaki & Matsuda). A second radiolarian assemblage was obtained from a block, several metres thick, of alternating pelagic limestones and cherts. It was characterized by the co-occurrence of the species Hemicryptocapsa capita Tan and Dicerosaturnalis dicranacanthos (Squinabol). The presence of these two species within the same assemblage correlates with Unitary Association Zone 17 and these samples are therefore assigned to the late Valanginian. These results suggest that submarine volcanic activity took place during the late Aalenian–middle Bajocian interval and that mixed carbonate and siliceous ooze accumulated during the late Valanginian. Previous biochronological data on Radiolaria from the Erakh mélange and the Vedi ophiolite are reviewed and their implications for the geodynamic and palaeoenvironmental evolution of the Tethyan ocean in Armenia are discussed.