Mid-Cretaceous Olistostromal Ophiolitic Mélange Developed in the Back-arc Basin of the Eastern Pontide Magmatic Arc, Northeast Turkey

Abstract

Geological and geochemical characteristics of the Mid-Cretaceous olistostromal ophiolitic mélange exposed in the southern part of the eastern Pontide magmatic arc indicates that it originally formed during the drifting stage of a back-arc basin (Neotethys). This view conflicts with the popular idea that this ophiolitic mélange represents a typical tectonic mélange or an accretionary prism developed in the fore-arc of the eastern Pontide magmatic arc by northward subduction of Neotethys or Paleotethys during the Cretaceous. This Mid-Cretaceous ophiolitic mélange is composed of the three distinct lithological facies that may reflect a strike-slip cycle—from transtensional to transpressive tectonic regimes—in the deep spreading troughs of pull-apart basins. The lower part of the mélange is represented by redeposited carbonate rocks that occur as an upward thinning and fining sequence. The middle level of the mélange comprises basaltic pillow lavas, hyaloclastics, and intercalated pelagic sediments such as radiolarite, mudstone, and red pelagic limestone. The upper part of the mélange is represented by an olistolith-olistostromal facies that includes sandstone, siltstone, marl, and intervening olistoliths and olistostromes—mainly limestone, peridotite-gabbro, and metamorphic blocks derived from the continental shelf and a nearby peridotitic-metamorphic ridge. Trace-element and REE contents of the basaltic rocks belonging to the Mid-Cretaceous ophiolitic mélange show distinct geotectonic settings (IA, WP, MOR) with various enrichment and depletion trends with respect to MORB and chondrite. Moreover, this ophiolitic mélange is overlain by analcime-bearing Upper Cretaceous-Paleocene(?) shoshonitic basalts in the Bayburt-Maden area. These geological and geochemical data imply that mélange formed in the back-arc basin of the eastern Pontide magmatic arc.