Radiolarian biostratigraphy and geochemistry of the Koziakas massif ophiolites (Greece).

The Koziakas massif, located at the western boundary of the Thessaly plain, consists of a nappe pile, which includes from bottom to top: 1 - the Paleocene-Eocene Pindos Flysch; 2- the Western Thessaly Units (WTU); 3- the Ophiolitic Nappe, which consists of four minor tectonic units: a) the ophiolite-bearing Koziakas Melange; b) the Fourka Unit; c) a thin discontinuous level of amphibolites (Metamorphic sole); d) a "Peridotite Unit". This work focuses on the study of both the radiolarian assemblages of cherts situated at the top of basalts and the geochemistry of the latter. Our data show that in the Fourka Unit the cherts overlying the various basalts have diverse ages: a) latest Anisian (associated to WPB-OIB), almost coeval to the trachyandesites effused onto the continental rifts to the west; b) Early Carnian-Middle Norian and Late Carnian-Early Norian (associated to WPB-OIB); c) Early Norian (associated to E-MORB). In the Koziakas Melange the age is Early-Middle Bathonian (associated to WPB-OIB). These data confirm the geodynamic evolution hypothesized for all the other ophiolitic massifs of the southern portions of the Dinaric-Hellenic belt, which can be synthesized as follows: a) birth of the Vardar-"Maliac" ocean in the Middle Triassic, coeval to continental basins to the west (Pindos basin, etc.); b) ocean spreading until Late Jurassic with intraoceanic subduction at the beginning of Middle Jurassic, followed by intraoceanic obduction; c) closure of the ocean (latest Jurassic-earliest Cretaceous) and thrusting of the Ophiolitic Nappe onto the eastern continental margin of Adria (the Pelagonian); d) westward displacement of the Ophiolitic Nappe testified by the presence of ophiolitic detritus first in the WTU: (latest Jurassic to Late Cretaceous) and then, to the west, in the Pindos Flysch (Paleocene-Eocene).