Radiolarian dating of the sedimentary cover of Sevan ophiolite (Armenia)

Dating radiolarites overlying ancient oceanic crust preserved in the Lesser Caucasus is of key importance to understand the geodynamic evolution of the greater area between Eurasia and the South-Armenian Block, a micro-continent detached from Gondwana during Late Palaeozoic–Early Mesozoic time. Micropalaeontological data are few and/or obtained before the development of a modern taxonomic framework for Mesozoic Radiolaria.
   Two main ophiolitic zones are recognized in the Lesser Caucasus and they are linked to the evolution of Tethys: 1) the Sevan-Akera zone, situated in the East and SE of Lake Sevan, constitutes the main suture zone of Neo-Tethys ocean in the Lesser Caucasus, and 2) the Vediophiolitic unit, in the SE of the capital city Yerevan, is considered as a folded klippe sequence thrusted on the South-Armenian Block. We focus here on biostratigraphic results obtained recently on the sedimentary cover of the Sevan ophiolite, considered to have been formed in a low spreading back-arc oceanic basin. Amongst the various localities studied, three yielded identifiable radiolaria. Radiolarian assemblages obtained from the Sarinar section allow to investigate the sedimentary and volcanic history recorded in an ca. 30 m-thick radiolarite sequence associated with spilitic lavas of the Sevan ophiolitic suture zone. Three distinct Radiolarian assemblages were recognized and they establish that the studied sequence is tectonically reversed. The younger assemblage can be assigned to the Unitary Association Zones (U.A.Z.) 19-22 of Baumgartner et al. (1995) and correlated with the Early Hauterivian/late Barremain to early Aptian interval, based on the co-occurrence of species “Sethocapsa” (?) orca, Tethysetta boesii and Hiscocapsa uterculus. The latter two species last occur in the lower Aptian Verbeeki subzone of O’Dogherty (1994). The intermediate assemblage is Late Jurassic in age (middle Oxfordian to early Tithonian; U.A.Z. 9-11), based essentially on the presence of Zhamoidellum ovum. Finally, the oldest assemblage may be correlated with the late Bajocian-early Bathonian, based on the presence of “Tricolocapsa” sp. M sensu Baumgartner et al. 1995. Several tuff levels are intercalated within the Upper Jurassic part of the radiolarite sequence. They are the evidence for a subaerial volcanic activity that took place in the oceanic realm of Tethys preserved in the ophiolites of Sevan-Akera zone.
   In the Dali section, radiolarites overly spilitic lavas and are intercalated with tuffites. The co-occurrence of Cinguloturris cylindra and Emiluvia pessagnoi multipora allows the assemblage to be correlated with the Late Tithonian-Berriasian (U.A.Z. 12-14). This age proves that oceanic crust was being formed at the Jurassic/Cretaceous transition being accompanied by subaerial volcanic activity.
   At the locality Tsegnaged, situated north of the town Sevan, two chert samples associated with lavas yielded Early Cretaceous radiolarian assemblages: the first can be assigned to U.A.Z 13-17 (latest Tithonian to late Valanginian) based on the co-occurrence of Archaeospongoprunum patricki and Obesacapsula cetia, while the other to U.A.Z. 18-22 (latest Valanginian/Hauterivian to early Aptian) based on the presence of Aurisaturnalis carinatus perforatus.