Radiolarian ages and geochemical data on the ophiolites from the Koziakas massif (Greece)

The Koziakas massif, located at the western boundary of the Thessaly plain, consists of a stack of thrust units emplaced westward onto the Pelagonian (s.l.), which in turn thrusts onto the Eocene Pindos Flysch. The dismembered units of the Koziakas are unconformably overlain by the Oligocene-Miocene molasse of the Mesohellenic trough.
   In the Koziakas massif, at the top of the “Pelagonian” succession, three ophiolitic tectonic units crop out:
   a) the “Mélange and Fourka Units”. At the base of the Fourka Unit scattered outcrops of ophiolite-bearing mélange are exposed. The Fourka Unit consists of thrust sheets and blocks of pillow lavas locally covered by radiolarian cherts.
   b) an “Ophiolite Unit”, consists of slivers of sheared serpentinites, locally containing dunite bodies, plagiogranite and boninite dykes.
   All volcanic rocks studied herein come from the “Fourka Unit” and consist of basalts and basaltic andesites. Six samples display a clear alkaline affinity and are similar to the alkaline within-oceanic plate (WPB) and are interpreted to have generated in a seamount setting. Two samples display similarities with enriched MORB (E-MORB) and are interpreted as formed from a N-MORB type mantle source slightly enriched in a plume component during the early stage of oceanic spreading or in an off-axis oceanic setting.
   We examined 32 samples for radiolarian analyses. The assemblages of the samples collected near the WPBs indicate Middle and Late Triassic age, while the radiolarites collected near the E-MORBs indicate Late Triassic age.
   The occurrence of Late Triassic WPBs and E-MORBs points to the existence of an oceanic setting in which the N-MORB asthenospheric source was influenced by a plume-type component and resulted in the off-axis eruption of enriched alkaline basalts and enriched MORB-type basalts. This conclusion is in agreement with similar results obtained from other sectors of the Hellenide ophiolites. During the post-Late Jurassic compressive tectonic phase, which affected the Internal Hellenides, the Mélange and Ophiolitic Units tectonically overthrusted the “Pelagonian” continental margin represented by the sedimentary units of the Koziakas Massif. During the post-Late Eocene compressive tectonic phase all these units were refolded and thrusted southwestwards onto the Eocene Pindos Flysch.