Neotethyan evolution of eastern Greece (Pagondas Melange, Evia island) inferred from radiolarian biostratigraphy and the geochemistry of associated extrusive rocks

This Paper presents new radiolarian biostratigraphic and igneous/metamorphi c geochemical data for a Mesozoic volcanic-sedimentary melange on the islan d of Evia (Euboea or Evvoia), eastern Greece. This mi lange includes dismem bered thrust sheets and blocks of radiolarian chert and basalt. Biostratigr aphic age data show that radiolarites interbedded with basalt-derived, coar se elastic sediments near the base of a coherent succession were deposited in Middle and Late Triassic lime (Late Ladinian-Carnian, Norian?). Geochemi cal evidence shows that associated extrusive rocks, of inferred Triassic ag e, range from 'enriched' alkaline basalts, to 'transitional' basalts, and m ore 'depleted' midocean ridge-type basalts. Amphibolite facies meta-basalts from the metamorphic sole of the over-riding Evia ophiolite exhibit simila r chemical compositions. Both the basalts and the meta-basalts commonly sho w an apparent subduction-related influence (e.g. relative Nb depletion) tha t may have been inherited from a previous subduction event in the region. T he basalts are interpreted to have erupted during Middle-Late Triassic time (Late Ladinian-Carnian), related to initial opening of a Neotethyan ocean basin adjacent to a rifted continental margin. Radiolarites located stratig raphically higher in the coherent succession studied are dated as Middle Ju rassic (Late Bathonian-Early Callovian). Similar-aged radiolarites are depo sitionally associated with ophiolitic rocks (including boninites), in some other areas of Greece and Albania. During initial ocean basin closure (Bajo cian-Bathonian) the adjacent shallow-water carbonate platform (Pelagonian z one) disintegrated to form basins in which siliceous sediments were deposit ed and highs on which shallow-water carbonates continued to accumulate. Thi s facies differentiation is seen as a response to crustal flexure as the Ne otethyan ocean began to close. The over-riding Pagondas Melange and other s imilar units in the region are interpreted as accretionary prisms related t o subduction of Neotethyan oceanic crust in Middle-Late Jurassic time. Thes e melanges were emplaced, probably diachronously during Oxfordian-Kimmeridg ian time, when the passive margin collapsed, creating a foredeep ahead of a dvancing thrust sheets of melange and ophiolites.