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

Abstract

This paper presents new radiolarian biostratigraphic and igneous/metamorphic geochemical data for a Mesozoic volcanic–sedimentary mélange on the island of Evia (Euboea or Evvoia), eastern Greece. This mélange includes dismembered thrust sheets and blocks of radiolarian chert and basalt. Biostratigraphic age data show that radiolarites interbedded with basalt-derived, coarse clastic sediments near the base of a coherent succession were deposited in Middle and Late Triassic time (Late Ladinian–Carnian, Norian?). Geochemical evidence shows that associated extrusive rocks, of inferred Triassic age, range from ‘enriched’ alkaline basalts, to ‘transitional’ basalts, and more ‘depleted’ mid-ocean ridge-type basalts. Amphibolite facies meta-basalts from the metamorphic sole of the over-riding Evia ophiolite exhibit similar chemical compositions. Both the basalts and the meta-basalts commonly show an apparent subduction-related influence (e.g. relative Nb depletion) that may have been inherited from a previous subduction event in the region. The 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 stratigraphically higher in the coherent succession studied are dated as Middle Jurassic (Late Bathonian–Early Callovian). Similar-aged radiolarites are depositionally associated with ophiolitic rocks (including boninites), in some other areas of Greece and Albania. During initial ocean basin closure (Bajocian–Bathonian) the adjacent shallow-water carbonate platform (Pelagonian zone) disintegrated to form basins in which siliceous sediments were deposited and highs on which shallow-water carbonates continued to accumulate. This facies differentiation is seen as a response to crustal flexure as the Neotethyan ocean began to close. The over-riding Pagondas Mélange and other similar units in the region are interpreted as accretionary prisms related to subduction of Neotethyan oceanic crust in Middle–Late Jurassic time. These mélanges were emplaced, probably diachronously during Oxfordian–Kimmeridgian time, when the passive margin collapsed, creating a foredeep ahead of advancing thrust sheets of mélange and ophiolites.