Jurassic and Cretaceous radiolarian age constraints for the Mesozoic evolution of the Tethyan realm in the Lesser Caucasus ; new data from Armenia

The Lesser Caucasus represents a key component of the Alpine-Himalayan mountain belt, as it preserves the remnants of a Tethyan oceanic realm that continues westwards into northeastern Turkey. Their spatial continuation to the south-southeast, into Iranian territory, still remains poorly understood. Radiolarian data obtained during the last twenty years have been particulalry useful for the establishment of an accurate chronostratigraphic framework for the depositional and geodynamic history of the Tethyan realm in this region. We here present recently obtained radiolarian age data from northern and eastern Armenia that enrich the existing dataset by dating radiolarites most of which are in stratigraphic contact with submarine lavas. In the Bazoum region of northern Armenia, in a very tectonized and chaotic state, radiolarites and submarine lavas crop out in the Sevguet valley, which are part of the Stepanavan-Amasia sector of the Amasia-Sevan-Hakari (or Sevan-Akera) ophiolite zone considered a suture. A large lense of radiolarian cherts incorporated in submarine lavas (basaltic trachyandesite, ‘Within-plate’ affinity) yielded a late Bajocian – early Bathonian radiolarian assemblage that can be correlated with the zones 5-6 of Baumgartner et al. (1995). East of the Sevan lake, in the Daranak locality, submarine basaltic flows that correspond to basanites (OIB-type, alkaline affinity ; Galoyan et al. 2009) are intercalated by radiolarites that are now dated as late Oxfordian/early Kimmerdidgian to late Kimmeridgian/early Tithonian in age (zones 10-11 of Baumgartner et al. 1995).

References

Baumgartner P.O., Bartolini A., Carter E.S., Conti M., Cortese G., Danelian T., De Wever P., Dumitrica P., Dumitrica-Jud R., Gorican S., Guex J., Hull D.M., Kito N., Marcucci M., Matsuoka A., Murchey B., O’Dogherty L., Savary J., Vishnevskaya V., Widz D. and Yao A., 1995. Middle Jurassic to Early Cretaceous Radiolarian biochronology of Tethys based on Unitary Associations. In: Baumgartner et al. (Eds.), Middle Jurassic to Lower Cretaceous Radiolaria of Tethys: Occurrences, Systematics, Biochronology. Mém. Géol. (Lausanne), 23: 1013-1048.

Galoyan G., Rolland Y., Sosson M., Corsini M., Billo S., Verati C. and Melkonian R., 2009. Geology, geochemistry and ⁴⁰Ar/³⁹Ar dating of Sevan ophiolites (lesser Caucasus, Armenia): evidence for Jurassic back-arc opening and hot spot event between the south Armenian block and Eurasia. J. Asian Earth Sci., 34: 135-153.