Ophiolites in the Dinarides and Hellenides: the contribution of radiolarian biochronology to the understanding of their formation and emplacement

Mesozoic radiolarian biochronology has been essential in the understanding of the timing of formation and emplacement of remnants of ancient ocean basins in the Alpine-Mediterranean orogens. The first descriptions and biochronologic assessments of radiolarian faunas of the late 1970ies in the Helledides and Dinarides depended on biostratigraphic calibrations from Deep Sea Drilling Sites and on the first zonations established in Western North America, that were not adequate for the area. In the early 1980’s, as the first European Jurassic-Cretaceous radiolatian zonations were established, the dating of radiolarian-bearing sediments associated with basalts and ophiolitic mélanges became possible. The age assignments have been continuously refined since. The discovery of Triassic radiolarites associated with MORB-like basalts in the late 1980’s considerably changed the interpretations. Now, a wealth of biochronologic work has been published in the last 3 decades. For this report we have revised data from NW-Croatia, Serbia, Albania, Northern Greece, Othris, Evvia, Argolis, in an attempt to produce a coherent picture of all this data.
   Radiolarian biochronology established in oceanic sediments associated with ophiolite belts in the Dinarides and Hellenides reveal 3 age clusters: Middle to Late Triassic, Middle Jurassic and Late Middle to Late Jurassic. Early Jurassic ages are extremely rare. Triassic ages have been found in oceanic sediments, chiefly radiolarites, associated with MORB-like and within-plate basalts, while the majority of Middle Jurassic ages have been found in sediments associated with basalts that geochemically are related to an intraoceanic convergent margin setting. Middle Jurassic radiolarites and radiolarian mudstones are also associated with ophiolite mélanges that are allochthonous with respect to the continental margins. Late Middle to Late Jurassic ages are found in synorogenic deepwater pelagic and ophiolitebearing detrital sediments that stratigraphically overly marginal series. These deposits formed during the obduction of the ophiolites onto the adjacent continental margin. Exposure/erosion and emplacement of the ophiolites is largely diachronous along the Pelagonian-Korab-Durmitor margin and in part synchronous with an ongoing formation of Vardar (suprasubduction) oceanic crust. Westward younging of ophiolite detritus on the Pelagonian margin implies an eastern (Vardar) origin of the ophiolites in Eastern Greece.
   In our simplest geodynamic scenario the Triassic ophiolite components are interpreted as remnants of the Maliac-Meliata Ocean that formed NE of the Pelagonian microcontinent, during the detachment of the latter from Eurasia. During the Middle Jurassic an intra-oceanic subduction zone developed in the Maliac-Meliata Ocean outboard of the Pelagonian-Korab-Durmitor-Drina-Ivanjica margin. Pieces of Triassic Maliac-Meliata ocean floor and seamounts became ripped off the lower plate and accreted in this subduction zone together with very young (0-10 my, supra-subduction) oceanic basalts of the upper plate attributed to the Vardar (backarc) Ocean. When the subduction zone reached the Pelagonian- Korab-Durmitor-Kuci margin, the latter became the lowermost unit of the accretionary wedge. The intraoceanic arc collided with the trench and was overthrust by the young back-arc Vardar crust just before subduction ceased. Further westward thrusting (mostly during Late Cretaceous-Early Tertiary) emplaced this composite ophiolite unit onto the more external Pindos-Cukali zones.