Triassic limestone, turbidites and serpentinite-the Cimmeride orogeny in the Central Pontides

摘要

The basement of the Central Pontides, and by implication that of Crimea, consists of pre-Permian low-grade metaclastic rocks intruded by latest Permian-Early Carboniferous (305290 Ma) granitoids. Further up in the stratigraphic sequence are Triassic limestones, which are now preserved as olistoliths in the deformed Upper Triassic turbidites. New conodont and foraminifera data indicate an Anisian to Carnian (Middle to Late Triassic) age for these hemi-pelagic Hallstatt-type limestones. The siliciclastic turbidites surrounding the Triassic limestone contain the Norian (Late Triassic) bivalve Monotis salinaria; the same species is also found in the Tauric series in Crimea. The Upper Triassic flysch in the Central Pontides is locally underlain by basaltic pillow lavas and includes kilometre-size tectonic slices of serpentinite. Both the flysch and the serpentinite are cut by an undeformed acidic intrusion with an Ar-Ar biotite age of 162 +/- 4 Ma (Callovian-Oxfordian). This indicates that the serpentinite was emplaced into the turbidites before Middle Jurassic time, most probably during latest Triassic or Early Jurassic time, and that the deformation of the Triassic sequence pre-dates the Middle Jurassic. Regional geological data from the circum-Black Sea region, including widespread Upper Triassic flysch, Upper Triassic eclogites and blueschists of oceanic crustal affinity, and apparent absence of a 'Cimmerian continent' between the Cretaceous and Triassic accretionary complexes indicate that the latest Triassic Cimmeride orogeny was accretionary rather than collisional and is probably related to the collision and accretion of an oceanic plateau to the southern active margin of Laurasia.